Internet Engineering Task Force (IETF) D. Noveck, Ed.
Request for Comments:
8881 NetApp
Obsoletes:
5661 C. Lever
Category: Standards Track ORACLE
ISSN: 2070-1721 August 2020
Network File System (NFS) Version 4 Minor Version 1 Protocol
Abstract
This document describes the Network File System (NFS) version 4 minor
version 1, including features retained from the base protocol (NFS
version 4 minor version 0, which is specified in
RFC 7530) and
protocol extensions made subsequently. The later minor version has
no dependencies on NFS version 4 minor version 0, and is considered a
separate protocol.
This document obsoletes
RFC 5661. It substantially revises the
treatment of features relating to multi-server namespace, superseding
the description of those features appearing in
RFC 5661.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in
Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8881.
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Table of Contents
1. Introduction
1.1. Introduction to This Update
1.2. The NFS Version 4 Minor Version 1 Protocol
1.3. Requirements Language
1.4. Scope of This Document
1.5. NFSv4 Goals
1.6. NFSv4.1 Goals
1.7. General Definitions
1.8. Overview of NFSv4.1 Features
1.9. Differences from NFSv4.0
2. Core Infrastructure
2.1. Introduction
2.2. RPC and XDR
2.3. COMPOUND and CB_COMPOUND
2.4. Client Identifiers and Client Owners
2.5. Server Owners
2.6. Security Service Negotiation
2.7. Minor Versioning
2.8. Non-RPC-Based Security Services
2.9. Transport Layers
2.10. Session
3. Protocol Constants and Data Types
3.1. Basic Constants
3.2. Basic Data Types
3.3. Structured Data Types
4. Filehandles
4.1. Obtaining the First Filehandle
4.2. Filehandle Types
4.3. One Method of Constructing a Volatile Filehandle
4.4. Client Recovery from Filehandle Expiration
5. File Attributes
5.1. REQUIRED Attributes
5.2. RECOMMENDED Attributes
5.3. Named Attributes
5.4. Classification of Attributes
5.5. Set-Only and Get-Only Attributes
5.6. REQUIRED Attributes - List and Definition References
5.7. RECOMMENDED Attributes - List and Definition References
5.8. Attribute Definitions
5.9. Interpreting owner and owner_group
5.10. Character Case Attributes
5.11. Directory Notification Attributes
5.12. pNFS Attribute Definitions
5.13. Retention Attributes
6. Access Control Attributes
6.1. Goals
6.2. File Attributes Discussion
6.3. Common Methods
6.4. Requirements
7. Single-Server Namespace
7.1. Server Exports
7.2. Browsing Exports
7.3. Server Pseudo File System
7.4. Multiple Roots
7.5. Filehandle Volatility
7.6. Exported Root
7.7. Mount Point Crossing
7.8. Security Policy and Namespace Presentation
8. State Management
8.1. Client and Session ID
8.2. Stateid Definition
8.3. Lease Renewal
8.4. Crash Recovery
8.5. Server Revocation of Locks
8.6. Short and Long Leases
8.7. Clocks, Propagation Delay, and Calculating Lease Expiration
8.8. Obsolete Locking Infrastructure from NFSv4.0
9. File Locking and Share Reservations
9.1. Opens and Byte-Range Locks
9.2. Lock Ranges
9.3. Upgrading and Downgrading Locks
9.4. Stateid Seqid Values and Byte-Range Locks
9.5. Issues with Multiple Open-Owners
9.6. Blocking Locks
9.7. Share Reservations
9.8. OPEN/CLOSE Operations
9.9. Open Upgrade and Downgrade
9.10. Parallel OPENs
9.11. Reclaim of Open and Byte-Range Locks
10. Client-Side Caching
10.1. Performance Challenges for Client-Side Caching
10.2. Delegation and Callbacks
10.3. Data Caching
10.4. Open Delegation
10.5. Data Caching and Revocation
10.6. Attribute Caching
10.7. Data and Metadata Caching and Memory Mapped Files
10.8. Name and Directory Caching without Directory Delegations
10.9. Directory Delegations
11. Multi-Server Namespace
11.1. Terminology
11.2. File System Location Attributes
11.3. File System Presence or Absence
11.4. Getting Attributes for an Absent File System
11.5. Uses of File System Location Information
11.6. Trunking without File System Location Information
11.7. Users and Groups in a Multi-Server Namespace
11.8. Additional Client-Side Considerations
11.9. Overview of File Access Transitions
11.10. Effecting Network Endpoint Transitions
11.11. Effecting File System Transitions
11.12. Transferring State upon Migration
11.13. Client Responsibilities When Access Is Transitioned
11.14. Server Responsibilities Upon Migration
11.15. Effecting File System Referrals
11.16. The Attribute fs_locations
11.17. The Attribute fs_locations_info
11.18. The Attribute fs_status
12. Parallel NFS (pNFS)
12.1. Introduction
12.2. pNFS Definitions
12.3. pNFS Operations
12.4. pNFS Attributes
12.5. Layout Semantics
12.6. pNFS Mechanics
12.7. Recovery
12.8. Metadata and Storage Device Roles
12.9. Security Considerations for pNFS
13. NFSv4.1 as a Storage Protocol in pNFS: the File Layout Type
13.1. Client ID and Session Considerations
13.2. File Layout Definitions
13.3. File Layout Data Types
13.4. Interpreting the File Layout
13.5. Data Server Multipathing
13.6. Operations Sent to NFSv4.1 Data Servers
13.7. COMMIT through Metadata Server
13.8. The Layout Iomode
13.9. Metadata and Data Server State Coordination
13.10. Data Server Component File Size
13.11. Layout Revocation and Fencing
13.12. Security Considerations for the File Layout Type
14. Internationalization
14.1. Stringprep Profile for the utf8str_cs Type
14.2. Stringprep Profile for the utf8str_cis Type
14.3. Stringprep Profile for the utf8str_mixed Type
14.4. UTF-8 Capabilities
14.5. UTF-8 Related Errors
15. Error Values
15.1. Error Definitions
15.2. Operations and Their Valid Errors
15.3. Callback Operations and Their Valid Errors
15.4. Errors and the Operations That Use Them
16. NFSv4.1 Procedures
16.1. Procedure 0: NULL - No Operation
16.2. Procedure 1: COMPOUND - Compound Operations
17. Operations:
REQUIRED,
RECOMMENDED, or
OPTIONAL 18. NFSv4.1 Operations
18.1. Operation 3: ACCESS - Check Access Rights
18.2. Operation 4: CLOSE - Close File
18.3. Operation 5: COMMIT - Commit Cached Data
18.4. Operation 6: CREATE - Create a Non-Regular File Object
18.5. Operation 7: DELEGPURGE - Purge Delegations Awaiting
Recovery
18.6. Operation 8: DELEGRETURN - Return Delegation
18.7. Operation 9: GETATTR - Get Attributes
18.8. Operation 10: GETFH - Get Current Filehandle
18.9. Operation 11: LINK - Create Link to a File
18.10. Operation 12: LOCK - Create Lock
18.11. Operation 13: LOCKT - Test for Lock
18.12. Operation 14: LOCKU - Unlock File
18.13. Operation 15: LOOKUP - Lookup Filename
18.14. Operation 16: LOOKUPP - Lookup Parent Directory
18.15. Operation 17: NVERIFY - Verify Difference in Attributes
18.16. Operation 18: OPEN - Open a Regular File
18.17. Operation 19: OPENATTR - Open Named Attribute Directory
18.18. Operation 21: OPEN_DOWNGRADE - Reduce Open File Access
18.19. Operation 22: PUTFH - Set Current Filehandle
18.20. Operation 23: PUTPUBFH - Set Public Filehandle
18.21. Operation 24: PUTROOTFH - Set Root Filehandle
18.22. Operation 25: READ - Read from File
18.23. Operation 26: READDIR - Read Directory
18.24. Operation 27: READLINK - Read Symbolic Link
18.25. Operation 28: REMOVE - Remove File System Object
18.26. Operation 29: RENAME - Rename Directory Entry
18.27. Operation 31: RESTOREFH - Restore Saved Filehandle
18.28. Operation 32: SAVEFH - Save Current Filehandle
18.29. Operation 33: SECINFO - Obtain Available Security
18.30. Operation 34: SETATTR - Set Attributes
18.31. Operation 37: VERIFY - Verify Same Attributes
18.32. Operation 38: WRITE - Write to File
18.33. Operation 40: BACKCHANNEL_CTL - Backchannel Control
18.34. Operation 41: BIND_CONN_TO_SESSION - Associate Connection
with Session
18.35. Operation 42: EXCHANGE_ID - Instantiate Client ID
18.36. Operation 43: CREATE_SESSION - Create New Session and
Confirm Client ID
18.37. Operation 44: DESTROY_SESSION - Destroy a Session
18.38. Operation 45: FREE_STATEID - Free Stateid with No Locks
18.39. Operation 46: GET_DIR_DELEGATION - Get a Directory
Delegation
18.40. Operation 47: GETDEVICEINFO - Get Device Information
18.41. Operation 48: GETDEVICELIST - Get All Device Mappings for
a File System
18.42. Operation 49: LAYOUTCOMMIT - Commit Writes Made Using a
Layout
18.43. Operation 50: LAYOUTGET - Get Layout Information
18.44. Operation 51: LAYOUTRETURN - Release Layout Information
18.45. Operation 52: SECINFO_NO_NAME - Get Security on Unnamed
Object
18.46. Operation 53: SEQUENCE - Supply Per-Procedure Sequencing
and Control
18.47. Operation 54: SET_SSV - Update SSV for a Client ID
18.48. Operation 55: TEST_STATEID - Test Stateids for Validity
18.49. Operation 56: WANT_DELEGATION - Request Delegation
18.50. Operation 57: DESTROY_CLIENTID - Destroy a Client ID
18.51. Operation 58: RECLAIM_COMPLETE - Indicates Reclaims
Finished
18.52. Operation 10044: ILLEGAL - Illegal Operation
19. NFSv4.1 Callback Procedures
19.1. Procedure 0: CB_NULL - No Operation
19.2. Procedure 1: CB_COMPOUND - Compound Operations
20. NFSv4.1 Callback Operations
20.1. Operation 3: CB_GETATTR - Get Attributes
20.2. Operation 4: CB_RECALL - Recall a Delegation
20.3. Operation 5: CB_LAYOUTRECALL - Recall Layout from Client
20.4. Operation 6: CB_NOTIFY - Notify Client of Directory
Changes
20.5. Operation 7: CB_PUSH_DELEG - Offer Previously Requested
Delegation to Client
20.6. Operation 8: CB_RECALL_ANY - Keep Any N Recallable Objects
20.7. Operation 9: CB_RECALLABLE_OBJ_AVAIL - Signal Resources
for Recallable Objects
20.8. Operation 10: CB_RECALL_SLOT - Change Flow Control Limits
20.9. Operation 11: CB_SEQUENCE - Supply Backchannel Sequencing
and Control
20.10. Operation 12: CB_WANTS_CANCELLED - Cancel Pending
Delegation Wants
20.11. Operation 13: CB_NOTIFY_LOCK - Notify Client of Possible
Lock Availability
20.12. Operation 14: CB_NOTIFY_DEVICEID - Notify Client of Device
ID Changes
20.13. Operation 10044: CB_ILLEGAL - Illegal Callback Operation
21. Security Considerations
22. IANA Considerations
22.1. IANA Actions
22.2. Named Attribute Definitions
22.3. Device ID Notifications
22.4. Object Recall Types
22.5. Layout Types
22.6. Path Variable Definitions
23. References
23.1. Normative References
23.2. Informative References
Appendix A. The Need for This Update
Appendix B. Changes in This Update
B.1. Revisions Made to
Section 11 of RFC 5661 B.2. Revisions Made to Operations in
RFC 5661 B.3. Revisions Made to Error Definitions in
RFC 5661 B.4. Other Revisions Made to
RFC 5661 Appendix C. Security Issues That Need to Be Addressed
Acknowledgments
Authors' Addresses
1. Introduction
1.1. Introduction to This Update
Two important features previously defined in minor version 0 but
never fully addressed in minor version 1 are trunking, which is the
simultaneous use of multiple connections between a client and server,
potentially to different network addresses, and Transparent State
Migration, which allows a file system to be transferred between
servers in a way that provides to the client the ability to maintain
its existing locking state across the transfer.
The revised description of the NFS version 4 minor version 1
(NFSv4.1) protocol presented in this update is necessary to enable
full use of these features together with other multi-server namespace
features. This document is in the form of an updated description of
the NFSv4.1 protocol previously defined in
RFC 5661 [66].
RFC 5661 is obsoleted by this document. However, the update has a limited
scope and is focused on enabling full use of trunking and Transparent
State Migration. The need for these changes is discussed in
Appendix A.
Appendix B describes the specific changes made to arrive
at the current text.
This limited-scope update replaces the current NFSv4.1 RFC with the
intention of providing an authoritative and complete specification,
the motivation for which is discussed in [36], addressing the issues
within the scope of the update. However, it will not address issues
that are known but outside of this limited scope as could be expected
by a full update of the protocol. Below are some areas that are
known to need addressing in a future update of the protocol:
* Work needs to be done with regard to
RFC 8178 [67], which
establishes NFSv4-wide versioning rules. As
RFC 5661 is currently
inconsistent with that document, changes are needed in order to
arrive at a situation in which there would be no need for
RFC 8178 to update the NFSv4.1 specification.
* Work needs to be done with regard to
RFC 8434 [70], which
establishes the requirements for parallel NFS (pNFS) layout types,
which are not clearly defined in
RFC 5661. When that work is done
and the resulting documents approved, the new NFSv4.1
specification document will provide a clear set of requirements
for layout types and a description of the file layout type that
conforms to those requirements. Other layout types will have
their own specification documents that conform to those
requirements as well.
* Work needs to be done to address many errata reports relevant to
RFC 5661, other than errata report 2006 [64], which is addressed
in this document. Addressing that report was not deferrable
because of the interaction of the changes suggested there and the
newly described handling of state and session migration.
The errata reports that have been deferred and that will need to
be addressed in a later document include reports currently
assigned a range of statuses in the errata reporting system,
including reports marked Accepted and those marked Hold For
Document Update because the change was too minor to address
immediately.
In addition, there is a set of other reports, including at least
one in state Rejected, that will need to be addressed in a later
document. This will involve making changes to consensus decisions
reflected in
RFC 5661, in situations in which the working group
has decided that the treatment in
RFC 5661 is incorrect and needs
to be revised to reflect the working group's new consensus and to
ensure compatibility with existing implementations that do not
follow the handling described in
RFC 5661.
Note that it is expected that all such errata reports will remain
relevant to implementors and the authors of an eventual
rfc5661bis, despite the fact that this document obsoletes
RFC 5661 [66].
* There is a need for a new approach to the description of
internationalization since the current internationalization
section (
Section 14) has never been implemented and does not meet
the needs of the NFSv4 protocol. Possible solutions are to create
a new internationalization section modeled on that in [68] or to
create a new document describing internationalization for all
NFSv4 minor versions and reference that document in the RFCs
defining both NFSv4.0 and NFSv4.1.
* There is a need for a revised treatment of security in NFSv4.1.
The issues with the existing treatment are discussed in
Appendix C.
Until the above work is done, there will not be a consistent set of
documents that provides a description of the NFSv4.1 protocol, and
any full description would involve documents updating other documents
within the specification. The updates applied by
RFC 8434 [70] and
RFC 8178 [67] to
RFC 5661 also apply to this specification, and will
apply to any subsequent v4.1 specification until that work is done.
1.2. The NFS Version 4 Minor Version 1 Protocol
The NFS version 4 minor version 1 (NFSv4.1) protocol is the second
minor version of the NFS version 4 (NFSv4) protocol. The first minor
version, NFSv4.0, is now described in
RFC 7530 [68]. It generally
follows the guidelines for minor versioning that are listed in
Section 10 of RFC 3530 [37]. However, it diverges from guidelines 11
("a client and server that support minor version X must support minor
versions 0 through X-1") and 12 ("no new features may be introduced
as mandatory in a minor version"). These divergences are due to the
introduction of the sessions model for managing non-idempotent
operations and the RECLAIM_COMPLETE operation. These two new
features are infrastructural in nature and simplify implementation of
existing and other new features. Making them anything but
REQUIRED would add undue complexity to protocol definition and implementation.
NFSv4.1 accordingly updates the minor versioning guidelines
(
Section 2.7).
As a minor version, NFSv4.1 is consistent with the overall goals for
NFSv4, but extends the protocol so as to better meet those goals,
based on experiences with NFSv4.0. In addition, NFSv4.1 has adopted
some additional goals, which motivate some of the major extensions in
NFSv4.1.
1.3. Requirements Language
The key words "
MUST", "
MUST NOT", "
REQUIRED", "
SHALL", "
SHALL NOT",
"
SHOULD", "
SHOULD NOT", "
RECOMMENDED", "
MAY", and "
OPTIONAL" in this
document are to be interpreted as described in
RFC 2119 [1].
1.4. Scope of This Document
This document describes the NFSv4.1 protocol. With respect to
NFSv4.0, this document does not:
* describe the NFSv4.0 protocol, except where needed to contrast
with NFSv4.1.
* modify the specification of the NFSv4.0 protocol.
* clarify the NFSv4.0 protocol.
1.5. NFSv4 Goals
The NFSv4 protocol is a further revision of the NFS protocol defined
already by NFSv3 [38]. It retains the essential characteristics of
previous versions: easy recovery; independence of transport
protocols, operating systems, and file systems; simplicity; and good
performance. NFSv4 has the following goals:
* Improved access and good performance on the Internet
The protocol is designed to transit firewalls easily, perform well
where latency is high and bandwidth is low, and scale to very
large numbers of clients per server.
* Strong security with negotiation built into the protocol
The protocol builds on the work of the ONCRPC working group in
supporting the RPCSEC_GSS protocol. Additionally, the NFSv4.1
protocol provides a mechanism to allow clients and servers the
ability to negotiate security and require clients and servers to
support a minimal set of security schemes.
* Good cross-platform interoperability
The protocol features a file system model that provides a useful,
common set of features that does not unduly favor one file system
or operating system over another.
* Designed for protocol extensions
The protocol is designed to accept standard extensions within a
framework that enables and encourages backward compatibility.
1.6. NFSv4.1 Goals
NFSv4.1 has the following goals, within the framework established by
the overall NFSv4 goals.
* To correct significant structural weaknesses and oversights
discovered in the base protocol.
* To add clarity and specificity to areas left unaddressed or not
addressed in sufficient detail in the base protocol. However, as
stated in
Section 1.4, it is not a goal to clarify the NFSv4.0
protocol in the NFSv4.1 specification.
* To add specific features based on experience with the existing
protocol and recent industry developments.
* To provide protocol support to take advantage of clustered server
deployments including the ability to provide scalable parallel
access to files distributed among multiple servers.
1.7. General Definitions
The following definitions provide an appropriate context for the
reader.
Byte: In this document, a byte is an octet, i.e., a datum exactly 8
bits in length.
Client: The client is the entity that accesses the NFS server's
resources. The client may be an application that contains the
logic to access the NFS server directly. The client may also be
the traditional operating system client that provides remote file
system services for a set of applications.
A client is uniquely identified by a client owner.
With reference to byte-range locking, the client is also the
entity that maintains a set of locks on behalf of one or more
applications. This client is responsible for crash or failure
recovery for those locks it manages.
Note that multiple clients may share the same transport and
connection and multiple clients may exist on the same network
node.
Client ID: The client ID is a 64-bit quantity used as a unique,
short-hand reference to a client-supplied verifier and client
owner. The server is responsible for supplying the client ID.
Client Owner: The client owner is a unique string, opaque to the
server, that identifies a client. Multiple network connections
and source network addresses originating from those connections
may share a client owner. The server is expected to treat
requests from connections with the same client owner as coming
from the same client.
File System: The file system is the collection of objects on a
server (as identified by the major identifier of a server owner,
which is defined later in this section) that share the same fsid
attribute (see
Section 5.8.1.9).
Lease: A lease is an interval of time defined by the server for
which the client is irrevocably granted locks. At the end of a
lease period, locks may be revoked if the lease has not been
extended. A lock must be revoked if a conflicting lock has been
granted after the lease interval.
A server grants a client a single lease for all state.
Lock: The term "lock" is used to refer to byte-range (in UNIX
environments, also known as record) locks, share reservations,
delegations, or layouts unless specifically stated otherwise.
Secret State Verifier (SSV): The SSV is a unique secret key shared
between a client and server. The SSV serves as the secret key for
an internal (that is, internal to NFSv4.1) Generic Security
Services (GSS) mechanism (the SSV GSS mechanism; see
Section 2.10.9). The SSV GSS mechanism uses the SSV to compute
message integrity code (MIC) and Wrap tokens. See
Section 2.10.8.3 for more details on how NFSv4.1 uses the SSV and
the SSV GSS mechanism.
Server: The Server is the entity responsible for coordinating client
access to a set of file systems and is identified by a server
owner. A server can span multiple network addresses.
Server Owner: The server owner identifies the server to the client.
The server owner consists of a major identifier and a minor
identifier. When the client has two connections each to a peer
with the same major identifier, the client assumes that both peers
are the same server (the server namespace is the same via each
connection) and that lock state is shareable across both
connections. When each peer has both the same major and minor
identifiers, the client assumes that each connection might be
associable with the same session.
Stable Storage: Stable storage is storage from which data stored by
an NFSv4.1 server can be recovered without data loss from multiple
power failures (including cascading power failures, that is,
several power failures in quick succession), operating system
failures, and/or hardware failure of components other than the
storage medium itself (such as disk, nonvolatile RAM, flash
memory, etc.).
Some examples of stable storage that are allowable for an NFS
server include:
1. Media commit of data; that is, the modified data has been
successfully written to the disk media, for example, the disk
platter.
2. An immediate reply disk drive with battery-backed, on-drive
intermediate storage or uninterruptible power system (UPS).
3. Server commit of data with battery-backed intermediate storage
and recovery software.
4. Cache commit with uninterruptible power system (UPS) and
recovery software.
Stateid: A stateid is a 128-bit quantity returned by a server that
uniquely defines the open and locking states provided by the
server for a specific open-owner or lock-owner/open-owner pair for
a specific file and type of lock.
Verifier: A verifier is a 64-bit quantity generated by the client
that the server can use to determine if the client has restarted
and lost all previous lock state.
1.8. Overview of NFSv4.1 Features
The major features of the NFSv4.1 protocol will be reviewed in brief.
This will be done to provide an appropriate context for both the
reader who is familiar with the previous versions of the NFS protocol
and the reader who is new to the NFS protocols. For the reader new
to the NFS protocols, there is still a set of fundamental knowledge
that is expected. The reader should be familiar with the External
Data Representation (XDR) and Remote Procedure Call (RPC) protocols
as described in [2] and [3]. A basic knowledge of file systems and
distributed file systems is expected as well.
In general, this specification of NFSv4.1 will not distinguish those
features added in minor version 1 from those present in the base
protocol but will treat NFSv4.1 as a unified whole. See
Section 1.9 for a summary of the differences between NFSv4.0 and NFSv4.1.
1.8.1. RPC and Security
As with previous versions of NFS, the External Data Representation
(XDR) and Remote Procedure Call (RPC) mechanisms used for the NFSv4.1
protocol are those defined in [2] and [3]. To meet end-to-end
security requirements, the RPCSEC_GSS framework [4] is used to extend
the basic RPC security. With the use of RPCSEC_GSS, various
mechanisms can be provided to offer authentication, integrity, and
privacy to the NFSv4 protocol. Kerberos V5 is used as described in
[5] to provide one security framework. With the use of RPCSEC_GSS,
other mechanisms may also be specified and used for NFSv4.1 security.
To enable in-band security negotiation, the NFSv4.1 protocol has
operations that provide the client a method of querying the server
about its policies regarding which security mechanisms must be used
for access to the server's file system resources. With this, the
client can securely match the security mechanism that meets the
policies specified at both the client and server.
NFSv4.1 introduces parallel access (see
Section 1.8.2.2), which is
called pNFS. The security framework described in this section is
significantly modified by the introduction of pNFS (see
Section 12.9), because data access is sometimes not over RPC. The
level of significance varies with the storage protocol (see
Section 12.2.5) and can be as low as zero impact (see
Section 13.12).
1.8.2. Protocol Structure
Unlike NFSv3, which used a series of ancillary protocols (e.g., NLM,
NSM (Network Status Monitor), MOUNT), within all minor versions of
NFSv4 a single RPC protocol is used to make requests to the server.
Facilities that had been separate protocols, such as locking, are now
integrated within a single unified protocol.
Minor version 1 supports high-performance data access to a clustered
server implementation by enabling a separation of metadata access and
data access, with the latter done to multiple servers in parallel.
Such parallel data access is controlled by recallable objects known
as "layouts", which are integrated into the protocol locking model.
Clients direct requests for data access to a set of data servers
specified by the layout via a data storage protocol which may be
NFSv4.1 or may be another protocol.
Because the protocols used for parallel data access are not
necessarily RPC-based, the RPC-based security model (
Section 1.8.1)
is obviously impacted (see
Section 12.9). The degree of impact
varies with the storage protocol (see
Section 12.2.5) used for data
access, and can be as low as zero (see
Section 13.12).
1.8.3. File System Model
The general file system model used for the NFSv4.1 protocol is the
same as previous versions. The server file system is hierarchical
with the regular files contained within being treated as opaque byte
streams. In a slight departure, file and directory names are encoded
with UTF-8 to deal with the basics of internationalization.
The NFSv4.1 protocol does not require a separate protocol to provide
for the initial mapping between path name and filehandle. All file
systems exported by a server are presented as a tree so that all file
systems are reachable from a special per-server global root
filehandle. This allows LOOKUP operations to be used to perform
functions previously provided by the MOUNT protocol. The server
provides any necessary pseudo file systems to bridge any gaps that
arise due to unexported gaps between exported file systems.
As in previous versions of the NFS protocol, opaque filehandles are
used to identify individual files and directories. Lookup-type and
create operations translate file and directory names to filehandles,
which are then used to identify objects in subsequent operations.
The NFSv4.1 protocol provides support for persistent filehandles,
guaranteed to be valid for the lifetime of the file system object
designated. In addition, it provides support to servers to provide
filehandles with more limited validity guarantees, called volatile
filehandles.
The NFSv4.1 protocol has a rich and extensible file object attribute
structure, which is divided into
REQUIRED,
RECOMMENDED, and named
attributes (see
Section 5).
Several (but not all) of the
REQUIRED attributes are derived from the
attributes of NFSv3 (see the definition of the fattr3 data type in
[38]). An example of a
REQUIRED attribute is the file object's type
(
Section 5.8.1.2) so that regular files can be distinguished from
directories (also known as folders in some operating environments)
and other types of objects.
REQUIRED attributes are discussed in
Section 5.1.
An example of three
RECOMMENDED attributes are acl, sacl, and dacl.
These attributes define an Access Control List (ACL) on a file object
(
Section 6). An ACL provides directory and file access control
beyond the model used in NFSv3. The ACL definition allows for
specification of specific sets of permissions for individual users
and groups. In addition, ACL inheritance allows propagation of
access permissions and restrictions down a directory tree as file
system objects are created.
RECOMMENDED attributes are discussed in
Section 5.2.
A named attribute is an opaque byte stream that is associated with a
directory or file and referred to by a string name. Named attributes
are meant to be used by client applications as a method to associate
application-specific data with a regular file or directory. NFSv4.1
modifies named attributes relative to NFSv4.0 by tightening the
allowed operations in order to prevent the development of non-
interoperable implementations. Named attributes are discussed in
Section 5.3.
1.8.3.3. Multi-Server Namespace
NFSv4.1 contains a number of features to allow implementation of
namespaces that cross server boundaries and that allow and facilitate
a nondisruptive transfer of support for individual file systems
between servers. They are all based upon attributes that allow one
file system to specify alternate, additional, and new location
information that specifies how the client may access that file
system.
These attributes can be used to provide for individual active file
systems:
* Alternate network addresses to access the current file system
instance.
* The locations of alternate file system instances or replicas to be
used in the event that the current file system instance becomes
unavailable.
These file system location attributes may be used together with the
concept of absent file systems, in which a position in the server
namespace is associated with locations on other servers without there
being any corresponding file system instance on the current server.
For example,
* These attributes may be used with absent file systems to implement
referrals whereby one server may direct the client to a file
system provided by another server. This allows extensive multi-
server namespaces to be constructed.
* These attributes may be provided when a previously present file
system becomes absent. This allows nondisruptive migration of
file systems to alternate servers.
1.8.4. Locking Facilities
As mentioned previously, NFSv4.1 is a single protocol that includes
locking facilities. These locking facilities include support for
many types of locks including a number of sorts of recallable locks.
Recallable locks such as delegations allow the client to be assured
that certain events will not occur so long as that lock is held.
When circumstances change, the lock is recalled via a callback
request. The assurances provided by delegations allow more extensive
caching to be done safely when circumstances allow it.
The types of locks are:
* Share reservations as established by OPEN operations.
* Byte-range locks.
* File delegations, which are recallable locks that assure the
holder that inconsistent opens and file changes cannot occur so
long as the delegation is held.
* Directory delegations, which are recallable locks that assure the
holder that inconsistent directory modifications cannot occur so
long as the delegation is held.
* Layouts, which are recallable objects that assure the holder that
direct access to the file data may be performed directly by the
client and that no change to the data's location that is
inconsistent with that access may be made so long as the layout is
held.
All locks for a given client are tied together under a single client-
wide lease. All requests made on sessions associated with the client
renew that lease. When the client's lease is not promptly renewed,
the client's locks are subject to revocation. In the event of server
restart, clients have the opportunity to safely reclaim their locks
within a special grace period.
1.9. Differences from NFSv4.0
The following summarizes the major differences between minor version
1 and the base protocol:
* Implementation of the sessions model (
Section 2.10).
* Parallel access to data (
Section 12).
* Addition of the RECLAIM_COMPLETE operation to better structure the
lock reclamation process (
Section 18.51).
* Enhanced delegation support as follows.
- Delegations on directories and other file types in addition to
regular files (
Section 18.39,
Section 18.49).
- Operations to optimize acquisition of recalled or denied
delegations (
Section 18.49,
Section 20.5,
Section 20.7).
- Notifications of changes to files and directories
(
Section 18.39,
Section 20.4).
- A method to allow a server to indicate that it is recalling one
or more delegations for resource management reasons, and thus a
method to allow the client to pick which delegations to return
(
Section 20.6).
* Attributes can be set atomically during exclusive file create via
the OPEN operation (see the new EXCLUSIVE4_1 creation method in
Section 18.16).
* Open files can be preserved if removed and the hard link count
("hard link" is defined in an Open Group [6] standard) goes to
zero, thus obviating the need for clients to rename deleted files
to partially hidden names -- colloquially called "silly rename"
(see the new OPEN4_RESULT_PRESERVE_UNLINKED reply flag in
Section 18.16).
* Improved compatibility with Microsoft Windows for Access Control
Lists (
Section 6.2.3,
Section 6.2.2,
Section 6.4.3.2).
* Data retention (
Section 5.13).
* Identification of the implementation of the NFS client and server
(
Section 18.35).
* Support for notification of the availability of byte-range locks
(see the new OPEN4_RESULT_
MAY_NOTIFY_LOCK reply flag in
Section 18.16 and see
Section 20.11).
* In NFSv4.1, LIPKEY and SPKM-3 are not required security mechanisms
[39].
2. Core Infrastructure
2.1. Introduction
NFSv4.1 relies on core infrastructure common to nearly every
operation. This core infrastructure is described in the remainder of
this section.
2.2. RPC and XDR
The NFSv4.1 protocol is a Remote Procedure Call (RPC) application
that uses RPC version 2 and the corresponding eXternal Data
Representation (XDR) as defined in [3] and [2].
2.2.1. RPC-Based Security
Previous NFS versions have been thought of as having a host-based
authentication model, where the NFS server authenticates the NFS
client, and trusts the client to authenticate all users. Actually,
NFS has always depended on RPC for authentication. One of the first
forms of RPC authentication, AUTH_SYS, had no strong authentication
and required a host-based authentication approach. NFSv4.1 also
depends on RPC for basic security services and mandates RPC support
for a user-based authentication model. The user-based authentication
model has user principals authenticated by a server, and in turn the
server authenticated by user principals. RPC provides some basic
security services that are used by NFSv4.1.
2.2.1.1. RPC Security Flavors
As described in "Authentication",
Section 7 of [3], RPC security is
encapsulated in the RPC header, via a security or authentication
flavor, and information specific to the specified security flavor.
Every RPC header conveys information used to identify and
authenticate a client and server. As discussed in
Section 2.2.1.1.1,
some security flavors provide additional security services.
NFSv4.1 clients and servers
MUST implement RPCSEC_GSS. (This
requirement to implement is not a requirement to use.) Other
flavors, such as AUTH_NONE and AUTH_SYS,
MAY be implemented as well.
2.2.1.1.1. RPCSEC_GSS and Security Services
RPCSEC_GSS [4] uses the functionality of GSS-API [7]. This allows
for the use of various security mechanisms by the RPC layer without
the additional implementation overhead of adding RPC security
flavors.
2.2.1.1.1.1. Identification, Authentication, Integrity, Privacy
Via the GSS-API, RPCSEC_GSS can be used to identify and authenticate
users on clients to servers, and servers to users. It can also
perform integrity checking on the entire RPC message, including the
RPC header, and on the arguments or results. Finally, privacy,
usually via encryption, is a service available with RPCSEC_GSS.
Privacy is performed on the arguments and results. Note that if
privacy is selected, integrity, authentication, and identification
are enabled. If privacy is not selected, but integrity is selected,
authentication and identification are enabled. If integrity and
privacy are not selected, but authentication is enabled,
identification is enabled. RPCSEC_GSS does not provide
identification as a separate service.
Although GSS-API has an authentication service distinct from its
privacy and integrity services, GSS-API's authentication service is
not used for RPCSEC_GSS's authentication service. Instead, each RPC
request and response header is integrity protected with the GSS-API
integrity service, and this allows RPCSEC_GSS to offer per-RPC
authentication and identity. See [4] for more information.
NFSv4.1 client and servers
MUST support RPCSEC_GSS's integrity and
authentication service. NFSv4.1 servers
MUST support RPCSEC_GSS's
privacy service. NFSv4.1 clients
SHOULD support RPCSEC_GSS's privacy
service.
2.2.1.1.1.2. Security Mechanisms for NFSv4.1
RPCSEC_GSS, via GSS-API, normalizes access to mechanisms that provide
security services. Therefore, NFSv4.1 clients and servers
MUST support the Kerberos V5 security mechanism.
The use of RPCSEC_GSS requires selection of mechanism, quality of
protection (QOP), and service (authentication, integrity, privacy).
For the mandated security mechanisms, NFSv4.1 specifies that a QOP of
zero is used, leaving it up to the mechanism or the mechanism's
configuration to map QOP zero to an appropriate level of protection.
Each mandated mechanism specifies a minimum set of cryptographic
algorithms for implementing integrity and privacy. NFSv4.1 clients
and servers
MUST be implemented on operating environments that comply
with the
REQUIRED cryptographic algorithms of each
REQUIRED mechanism.
The Kerberos V5 GSS-API mechanism as described in [5]
MUST be
implemented with the RPCSEC_GSS services as specified in the
following table:
column descriptions:
1 == number of pseudo flavor
2 == name of pseudo flavor
3 == mechanism's OID
4 == RPCSEC_GSS service
5 == NFSv4.1 clients
MUST support
6 == NFSv4.1 servers
MUST support
1 2 3 4 5 6
------------------------------------------------------------------
390003 krb5 1.2.840.113554.1.2.2 rpc_gss_svc_none yes yes
390004 krb5i 1.2.840.113554.1.2.2 rpc_gss_svc_integrity yes yes
390005 krb5p 1.2.840.113554.1.2.2 rpc_gss_svc_privacy no yes
Note that the number and name of the pseudo flavor are presented here
as a mapping aid to the implementor. Because the NFSv4.1 protocol
includes a method to negotiate security and it understands the GSS-
API mechanism, the pseudo flavor is not needed. The pseudo flavor is
needed for the NFSv3 since the security negotiation is done via the
MOUNT protocol as described in [40].
At the time NFSv4.1 was specified, the Advanced Encryption Standard
(AES) with HMAC-SHA1 was a
REQUIRED algorithm set for Kerberos V5.
In contrast, when NFSv4.0 was specified, weaker algorithm sets were
REQUIRED for Kerberos V5, and were
REQUIRED in the NFSv4.0
specification, because the Kerberos V5 specification at the time did
not specify stronger algorithms. The NFSv4.1 specification does not
specify
REQUIRED algorithms for Kerberos V5, and instead, the
implementor is expected to track the evolution of the Kerberos V5
standard if and when stronger algorithms are specified.
2.2.1.1.1.2.1.1. Security Considerations for Cryptographic Algorithms
in Kerberos V5
When deploying NFSv4.1, the strength of the security achieved depends
on the existing Kerberos V5 infrastructure. The algorithms of
Kerberos V5 are not directly exposed to or selectable by the client
or server, so there is some due diligence required by the user of
NFSv4.1 to ensure that security is acceptable where needed.
Regardless of what security mechanism under RPCSEC_GSS is being used,
the NFS server
MUST identify itself in GSS-API via a
GSS_C_NT_HOSTBASED_SERVICE name type. GSS_C_NT_HOSTBASED_SERVICE
names are of the form:
service@hostname
For NFS, the "service" element is
nfs
Implementations of security mechanisms will convert nfs@hostname to
various different forms. For Kerberos V5, the following form is
RECOMMENDED:
nfs/hostname
2.3. COMPOUND and CB_COMPOUND
A significant departure from the versions of the NFS protocol before
NFSv4 is the introduction of the COMPOUND procedure. For the NFSv4
protocol, in all minor versions, there are exactly two RPC
procedures, NULL and COMPOUND. The COMPOUND procedure is defined as
a series of individual operations and these operations perform the
sorts of functions performed by traditional NFS procedures.
The operations combined within a COMPOUND request are evaluated in
order by the server, without any atomicity guarantees. A limited set
of facilities exist to pass results from one operation to another.
Once an operation returns a failing result, the evaluation ends and
the results of all evaluated operations are returned to the client.
With the use of the COMPOUND procedure, the client is able to build
simple or complex requests. These COMPOUND requests allow for a
reduction in the number of RPCs needed for logical file system
operations. For example, multi-component look up requests can be
constructed by combining multiple LOOKUP operations. Those can be
further combined with operations such as GETATTR, READDIR, or OPEN
plus READ to do more complicated sets of operation without incurring
additional latency.
NFSv4.1 also contains a considerable set of callback operations in
which the server makes an RPC directed at the client. Callback RPCs
have a similar structure to that of the normal server requests. In
all minor versions of the NFSv4 protocol, there are two callback RPC
procedures: CB_NULL and CB_COMPOUND. The CB_COMPOUND procedure is
defined in an analogous fashion to that of COMPOUND with its own set
of callback operations.
The addition of new server and callback operations within the
COMPOUND and CB_COMPOUND request framework provides a means of
extending the protocol in subsequent minor versions.
Except for a small number of operations needed for session creation,
server requests and callback requests are performed within the
context of a session. Sessions provide a client context for every
request and support robust replay protection for non-idempotent
requests.
2.4. Client Identifiers and Client Owners
For each operation that obtains or depends on locking state, the
specific client needs to be identifiable by the server.
Each distinct client instance is represented by a client ID. A
client ID is a 64-bit identifier representing a specific client at a
given time. The client ID is changed whenever the client re-
initializes, and may change when the server re-initializes. Client
IDs are used to support lock identification and crash recovery.
During steady state operation, the client ID associated with each
operation is derived from the session (see
Section 2.10) on which the
operation is sent. A session is associated with a client ID when the
session is created.
Unlike NFSv4.0, the only NFSv4.1 operations possible before a client
ID is established are those needed to establish the client ID.
A sequence of an EXCHANGE_ID operation followed by a CREATE_SESSION
operation using that client ID (eir_clientid as returned from
EXCHANGE_ID) is required to establish and confirm the client ID on
the server. Establishment of identification by a new incarnation of
the client also has the effect of immediately releasing any locking
state that a previous incarnation of that same client might have had
on the server. Such released state would include all byte-range
lock, share reservation, layout state, and -- where the server
supports neither the CLAIM_DELEGATE_PREV nor CLAIM_DELEG_CUR_FH claim
types -- all delegation state associated with the same client with
the same identity. For discussion of delegation state recovery, see
Section 10.2.1. For discussion of layout state recovery, see
Section 12.7.1.
Releasing such state requires that the server be able to determine
that one client instance is the successor of another. Where this
cannot be done, for any of a number of reasons, the locking state
will remain for a time subject to lease expiration (see
Section 8.3)
and the new client will need to wait for such state to be removed, if
it makes conflicting lock requests.
Client identification is encapsulated in the following client owner
data type:
struct client_owner4 {
verifier4 co_verifier;
opaque co_ownerid<NFS4_OPAQUE_LIMIT>;
};
The first field, co_verifier, is a client incarnation verifier,
allowing the server to distinguish successive incarnations (e.g.,
reboots) of the same client. The server will start the process of
canceling the client's leased state if co_verifier is different than
what the server has previously recorded for the identified client (as
specified in the co_ownerid field).
The second field, co_ownerid, is a variable length string that
uniquely defines the client so that subsequent instances of the same
client bear the same co_ownerid with a different verifier.
There are several considerations for how the client generates the
co_ownerid string:
* The string should be unique so that multiple clients do not
present the same string. The consequences of two clients
presenting the same string range from one client getting an error
to one client having its leased state abruptly and unexpectedly
cancelled.
* The string should be selected so that subsequent incarnations
(e.g., restarts) of the same client cause the client to present
the same string. The implementor is cautioned from an approach
that requires the string to be recorded in a local file because
this precludes the use of the implementation in an environment
where there is no local disk and all file access is from an
NFSv4.1 server.
* The string should be the same for each server network address that
the client accesses. This way, if a server has multiple
interfaces, the client can trunk traffic over multiple network
paths as described in
Section 2.10.5. (Note: the precise opposite
was advised in the NFSv4.0 specification [37].)
* The algorithm for generating the string should not assume that the
client's network address will not change, unless the client
implementation knows it is using statically assigned network
addresses. This includes changes between client incarnations and
even changes while the client is still running in its current
incarnation. Thus, with dynamic address assignment, if the client
includes just the client's network address in the co_ownerid
string, there is a real risk that after the client gives up the
network address, another client, using a similar algorithm for
generating the co_ownerid string, would generate a conflicting
co_ownerid string.
Given the above considerations, an example of a well-generated
co_ownerid string is one that includes:
* If applicable, the client's statically assigned network address.
* Additional information that tends to be unique, such as one or
more of:
- The client machine's serial number (for privacy reasons, it is
best to perform some one-way function on the serial number).
- A Media Access Control (MAC) address (again, a one-way function
should be performed).
- The timestamp of when the NFSv4.1 software was first installed
on the client (though this is subject to the previously
mentioned caution about using information that is stored in a
file, because the file might only be accessible over NFSv4.1).
- A true random number. However, since this number ought to be
the same between client incarnations, this shares the same
problem as that of using the timestamp of the software
installation.
* For a user-level NFSv4.1 client, it should contain additional
information to distinguish the client from other user-level
clients running on the same host, such as a process identifier or
other unique sequence.
The client ID is assigned by the server (the eir_clientid result from
EXCHANGE_ID) and should be chosen so that it will not conflict with a
client ID previously assigned by the server. This applies across
server restarts.
In the event of a server restart, a client may find out that its
current client ID is no longer valid when it receives an
NFS4ERR_STALE_CLIENTID error. The precise circumstances depend on
the characteristics of the sessions involved, specifically whether
the session is persistent (see
Section 2.10.6.5), but in each case
the client will receive this error when it attempts to establish a
new session with the existing client ID and receives the error
NFS4ERR_STALE_CLIENTID, indicating that a new client ID needs to be
obtained via EXCHANGE_ID and the new session established with that
client ID.
When a session is not persistent, the client will find out that it
needs to create a new session as a result of getting an
NFS4ERR_BADSESSION, since the session in question was lost as part of
a server restart. When the existing client ID is presented to a
server as part of creating a session and that client ID is not
recognized, as would happen after a server restart, the server will
reject the request with the error NFS4ERR_STALE_CLIENTID.
In the case of the session being persistent, the client will re-
establish communication using the existing session after the restart.
This session will be associated with the existing client ID but may
only be used to retransmit operations that the client previously
transmitted and did not see replies to. Replies to operations that
the server previously performed will come from the reply cache;
otherwise, NFS4ERR_DEADSESSION will be returned. Hence, such a
session is referred to as "dead". In this situation, in order to
perform new operations, the client needs to establish a new session.
If an attempt is made to establish this new session with the existing
client ID, the server will reject the request with
NFS4ERR_STALE_CLIENTID.
When NFS4ERR_STALE_CLIENTID is received in either of these
situations, the client needs to obtain a new client ID by use of the
EXCHANGE_ID operation, then use that client ID as the basis of a new
session, and then proceed to any other necessary recovery for the
server restart case (see
Section 8.4.2).
See the descriptions of EXCHANGE_ID (
Section 18.35) and
CREATE_SESSION (
Section 18.36) for a complete specification of these
operations.
2.4.1. Upgrade from NFSv4.0 to NFSv4.1
To facilitate upgrade from NFSv4.0 to NFSv4.1, a server may compare a
value of data type client_owner4 in an EXCHANGE_ID with a value of
data type nfs_client_id4 that was established using the SETCLIENTID
operation of NFSv4.0. A server that does so will allow an upgraded
client to avoid waiting until the lease (i.e., the lease established
by the NFSv4.0 instance client) expires. This requires that the
value of data type client_owner4 be constructed the same way as the
value of data type nfs_client_id4. If the latter's contents included
the server's network address (per the recommendations of the NFSv4.0
specification [37]), and the NFSv4.1 client does not wish to use a
client ID that prevents trunking, it should send two EXCHANGE_ID
operations. The first EXCHANGE_ID will have a client_owner4 equal to
the nfs_client_id4. This will clear the state created by the NFSv4.0
client. The second EXCHANGE_ID will not have the server's network
address. The state created for the second EXCHANGE_ID will not have
to wait for lease expiration, because there will be no state to
expire.
2.4.2. Server Release of Client ID
NFSv4.1 introduces a new operation called DESTROY_CLIENTID
(
Section 18.50), which the client
SHOULD use to destroy a client ID
it no longer needs. This permits graceful, bilateral release of a
client ID. The operation cannot be used if there are sessions
associated with the client ID, or state with an unexpired lease.
If the server determines that the client holds no associated state
for its client ID (associated state includes unrevoked sessions,
opens, locks, delegations, layouts, and wants), the server
MAY choose
to unilaterally release the client ID in order to conserve resources.
If the client contacts the server after this release, the server
MUST ensure that the client receives the appropriate error so that it will
use the EXCHANGE_ID/CREATE_SESSION sequence to establish a new client
ID. The server ought to be very hesitant to release a client ID
since the resulting work on the client to recover from such an event
will be the same burden as if the server had failed and restarted.
Typically, a server would not release a client ID unless there had
been no activity from that client for many minutes. As long as there
are sessions, opens, locks, delegations, layouts, or wants, the
server
MUST NOT release the client ID. See
Section 2.10.13.1.4 for
discussion on releasing inactive sessions.
2.4.3. Resolving Client Owner Conflicts
When the server gets an EXCHANGE_ID for a client owner that currently
has no state, or that has state but the lease has expired, the server
MUST allow the EXCHANGE_ID and confirm the new client ID if followed
by the appropriate CREATE_SESSION.
When the server gets an EXCHANGE_ID for a new incarnation of a client
owner that currently has an old incarnation with state and an
unexpired lease, the server is allowed to dispose of the state of the
previous incarnation of the client owner if one of the following is
true:
* The principal that created the client ID for the client owner is
the same as the principal that is sending the EXCHANGE_ID
operation. Note that if the client ID was created with
SP4_MACH_CRED state protection (
Section 18.35), the principal
MUST be based on RPCSEC_GSS authentication, the RPCSEC_GSS service used
MUST be integrity or privacy, and the same GSS mechanism and
principal
MUST be used as that used when the client ID was
created.
* The client ID was established with SP4_SSV protection
(
Section 18.35,
Section 2.10.8.3) and the client sends the
EXCHANGE_ID with the security flavor set to RPCSEC_GSS using the
GSS SSV mechanism (
Section 2.10.9).
* The client ID was established with SP4_SSV protection, and under
the conditions described herein, the EXCHANGE_ID was sent with
SP4_MACH_CRED state protection. Because the SSV might not persist
across client and server restart, and because the first time a
client sends EXCHANGE_ID to a server it does not have an SSV, the
client
MAY send the subsequent EXCHANGE_ID without an SSV
RPCSEC_GSS handle. Instead, as with SP4_MACH_CRED protection, the
principal
MUST be based on RPCSEC_GSS authentication, the
RPCSEC_GSS service used
MUST be integrity or privacy, and the same
GSS mechanism and principal
MUST be used as that used when the
client ID was created.
If none of the above situations apply, the server
MUST return
NFS4ERR_CLID_INUSE.
If the server accepts the principal and co_ownerid as matching that
which created the client ID, and the co_verifier in the EXCHANGE_ID
differs from the co_verifier used when the client ID was created,
then after the server receives a CREATE_SESSION that confirms the
client ID, the server deletes state. If the co_verifier values are
the same (e.g., the client either is updating properties of the
client ID (
Section 18.35) or is attempting trunking (
Section 2.10.5),
the server
MUST NOT delete state.
2.5. Server Owners
The server owner is similar to a client owner (
Section 2.4), but
unlike the client owner, there is no shorthand server ID. The server
owner is defined in the following data type:
struct server_owner4 {
uint64_t so_minor_id;
opaque so_major_id<NFS4_OPAQUE_LIMIT>;
};
The server owner is returned from EXCHANGE_ID. When the so_major_id
fields are the same in two EXCHANGE_ID results, the connections that
each EXCHANGE_ID were sent over can be assumed to address the same
server (as defined in
Section 1.7). If the so_minor_id fields are
also the same, then not only do both connections connect to the same
server, but the session can be shared across both connections. The
reader is cautioned that multiple servers may deliberately or
accidentally claim to have the same so_major_id or so_major_id/
so_minor_id; the reader should examine Sections
2.10.5 and
18.35 in
order to avoid acting on falsely matching server owner values.
The considerations for generating an so_major_id are similar to that
for generating a co_ownerid string (see
Section 2.4). The
consequences of two servers generating conflicting so_major_id values
are less dire than they are for co_ownerid conflicts because the
client can use RPCSEC_GSS to compare the authenticity of each server
(see
Section 2.10.5).
2.6. Security Service Negotiation
With the NFSv4.1 server potentially offering multiple security
mechanisms, the client needs a method to determine or negotiate which
mechanism is to be used for its communication with the server. The
NFS server may have multiple points within its file system namespace
that are available for use by NFS clients. These points can be
considered security policy boundaries, and, in some NFS
implementations, are tied to NFS export points. In turn, the NFS
server may be configured such that each of these security policy
boundaries may have different or multiple security mechanisms in use.
The security negotiation between client and server
SHOULD be done
with a secure channel to eliminate the possibility of a third party
intercepting the negotiation sequence and forcing the client and
server to choose a lower level of security than required or desired.
See
Section 21 for further discussion.
2.6.1. NFSv4.1 Security Tuples
An NFS server can assign one or more "security tuples" to each
security policy boundary in its namespace. Each security tuple
consists of a security flavor (see
Section 2.2.1.1) and, if the
flavor is RPCSEC_GSS, a GSS-API mechanism Object Identifier (OID), a
GSS-API quality of protection, and an RPCSEC_GSS service.
2.6.2. SECINFO and SECINFO_NO_NAME
The SECINFO and SECINFO_NO_NAME operations allow the client to
determine, on a per-filehandle basis, what security tuple is to be
used for server access. In general, the client will not have to use
either operation except during initial communication with the server
or when the client crosses security policy boundaries at the server.
However, the server's policies may also change at any time and force
the client to negotiate a new security tuple.
Where the use of different security tuples would affect the type of
access that would be allowed if a request was sent over the same
connection used for the SECINFO or SECINFO_NO_NAME operation (e.g.,
read-only vs. read-write) access, security tuples that allow greater
access should be presented first. Where the general level of access
is the same and different security flavors limit the range of
principals whose privileges are recognized (e.g., allowing or
disallowing root access), flavors supporting the greatest range of
principals should be listed first.
2.6.3. Security Error
Based on the assumption that each NFSv4.1 client and server
MUST support a minimum set of security (i.e., Kerberos V5 under
RPCSEC_GSS), the NFS client will initiate file access to the server
with one of the minimal security tuples. During communication with
the server, the client may receive an NFS error of NFS4ERR_WRONGSEC.
This error allows the server to notify the client that the security
tuple currently being used contravenes the server's security policy.
The client is then responsible for determining (see
Section 2.6.3.1)
what security tuples are available at the server and choosing one
that is appropriate for the client.
2.6.3.1. Using NFS4ERR_WRONGSEC, SECINFO, and SECINFO_NO_NAME
This section explains the mechanics of NFSv4.1 security negotiation.
2.6.3.1.1. Put Filehandle Operations
The term "put filehandle operation" refers to PUTROOTFH, PUTPUBFH,
PUTFH, and RESTOREFH. Each of the subsections herein describes how
the server handles a subseries of operations that starts with a put
filehandle operation.
2.6.3.1.1.1. Put Filehandle Operation + SAVEFH
The client is saving a filehandle for a future RESTOREFH, LINK, or
RENAME. SAVEFH
MUST NOT return NFS4ERR_WRONGSEC. To determine
whether or not the put filehandle operation returns NFS4ERR_WRONGSEC,
the server implementation pretends SAVEFH is not in the series of
operations and examines which of the situations described in the
other subsections of
Section 2.6.3.1.1 apply.
2.6.3.1.1.2. Two or More Put Filehandle Operations
For a series of N put filehandle operations, the server
MUST NOT return NFS4ERR_WRONGSEC to the first N-1 put filehandle operations.
The Nth put filehandle operation is handled as if it is the first in
a subseries of operations. For example, if the server received a
COMPOUND request with this series of operations -- PUTFH, PUTROOTFH,
LOOKUP -- then the PUTFH operation is ignored for NFS4ERR_WRONGSEC
purposes, and the PUTROOTFH, LOOKUP subseries is processed as
according to
Section 2.6.3.1.1.3.
2.6.3.1.1.3. Put Filehandle Operation + LOOKUP (or OPEN of an Existing
Name)
This situation also applies to a put filehandle operation followed by
a LOOKUP or an OPEN operation that specifies an existing component
name.
In this situation, the client is potentially crossing a security
policy boundary, and the set of security tuples the parent directory
supports may differ from those of the child. The server
implementation may decide whether to impose any restrictions on
security policy administration. There are at least three approaches
(sec_policy_child is the tuple set of the child export,
sec_policy_parent is that of the parent).
(a) sec_policy_child <= sec_policy_parent (<= for subset). This
means that the set of security tuples specified on the security
policy of a child directory is always a subset of its parent
directory.
(b) sec_policy_child ^ sec_policy_parent != {} (^ for intersection,
{} for the empty set). This means that the set of security
tuples specified on the security policy of a child directory
always has a non-empty intersection with that of the parent.
(c) sec_policy_child ^ sec_policy_parent == {}. This means that the
set of security tuples specified on the security policy of a
child directory may not intersect with that of the parent. In
other words, there are no restrictions on how the system
administrator may set up these tuples.
In order for a server to support approaches (b) (for the case when a
client chooses a flavor that is not a member of sec_policy_parent)
and (c), the put filehandle operation cannot return NFS4ERR_WRONGSEC
when there is a security tuple mismatch. Instead, it should be
returned from the LOOKUP (or OPEN by existing component name) that
follows.
Since the above guideline does not contradict approach (a), it should
be followed in general. Even if approach (a) is implemented, it is
possible for the security tuple used to be acceptable for the target
of LOOKUP but not for the filehandles used in the put filehandle
operation. The put filehandle operation could be a PUTROOTFH or
PUTPUBFH, where the client cannot know the security tuples for the
root or public filehandle. Or the security policy for the filehandle
used by the put filehandle operation could have changed since the
time the filehandle was obtained.
Therefore, an NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC in
response to the put filehandle operation if the operation is
immediately followed by a LOOKUP or an OPEN by component name.
2.6.3.1.1.4. Put Filehandle Operation + LOOKUPP
Since SECINFO only works its way down, there is no way LOOKUPP can
return NFS4ERR_WRONGSEC without SECINFO_NO_NAME. SECINFO_NO_NAME
solves this issue via style SECINFO_STYLE4_PARENT, which works in the
opposite direction as SECINFO. As with
Section 2.6.3.1.1.3, a put
filehandle operation that is followed by a LOOKUPP
MUST NOT return
NFS4ERR_WRONGSEC. If the server does not support SECINFO_NO_NAME,
the client's only recourse is to send the put filehandle operation,
LOOKUPP, GETFH sequence of operations with every security tuple it
supports.
Regardless of whether SECINFO_NO_NAME is supported, an NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC in response to a put filehandle
operation if the operation is immediately followed by a LOOKUPP.
2.6.3.1.1.5. Put Filehandle Operation + SECINFO/SECINFO_NO_NAME
A security-sensitive client is allowed to choose a strong security
tuple when querying a server to determine a file object's permitted
security tuples. The security tuple chosen by the client does not
have to be included in the tuple list of the security policy of
either the parent directory indicated in the put filehandle operation
or the child file object indicated in SECINFO (or any parent
directory indicated in SECINFO_NO_NAME). Of course, the server has
to be configured for whatever security tuple the client selects;
otherwise, the request will fail at the RPC layer with an appropriate
authentication error.
In theory, there is no connection between the security flavor used by
SECINFO or SECINFO_NO_NAME and those supported by the security
policy. But in practice, the client may start looking for strong
flavors from those supported by the security policy, followed by
those in the
REQUIRED set.
The NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC to a put
filehandle operation that is immediately followed by SECINFO or
SECINFO_NO_NAME. The NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC
from SECINFO or SECINFO_NO_NAME.
2.6.3.1.1.6. Put Filehandle Operation + Nothing
The NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC.
2.6.3.1.1.7. Put Filehandle Operation + Anything Else
"Anything Else" includes OPEN by filehandle.
The security policy enforcement applies to the filehandle specified
in the put filehandle operation. Therefore, the put filehandle
operation
MUST return NFS4ERR_WRONGSEC when there is a security tuple
mismatch. This avoids the complexity of adding NFS4ERR_WRONGSEC as
an allowable error to every other operation.
A COMPOUND containing the series put filehandle operation +
SECINFO_NO_NAME (style SECINFO_STYLE4_CURRENT_FH) is an efficient way
for the client to recover from NFS4ERR_WRONGSEC.
The NFSv4.1 server
MUST NOT return NFS4ERR_WRONGSEC to any operation
other than a put filehandle operation, LOOKUP, LOOKUPP, and OPEN (by
component name).
2.6.3.1.1.8. Operations after SECINFO and SECINFO_NO_NAME
Suppose a client sends a COMPOUND procedure containing the series
SEQUENCE, PUTFH, SECINFO_NONAME, READ, and suppose the security tuple
used does not match that required for the target file. By rule (see
Section 2.6.3.1.1.5), neither PUTFH nor SECINFO_NO_NAME can return
NFS4ERR_WRONGSEC. By rule (see
Section 2.6.3.1.1.7), READ cannot
return NFS4ERR_WRONGSEC. The issue is resolved by the fact that
SECINFO and SECINFO_NO_NAME consume the current filehandle (note that
this is a change from NFSv4.0). This leaves no current filehandle
for READ to use, and READ returns NFS4ERR_NOFILEHANDLE.
The LINK and RENAME operations use both the current and saved
filehandles. Technically, the server
MAY return NFS4ERR_WRONGSEC
from LINK or RENAME if the security policy of the saved filehandle
rejects the security flavor used in the COMPOUND request's
credentials. If the server does so, then if there is no intersection
between the security policies of saved and current filehandles, this
means that it will be impossible for the client to perform the
intended LINK or RENAME operation.
For example, suppose the client sends this COMPOUND request:
SEQUENCE, PUTFH bFH, SAVEFH, PUTFH aFH, RENAME "c" "d", where
filehandles bFH and aFH refer to different directories. Suppose no
common security tuple exists between the security policies of aFH and
bFH. If the client sends the request using credentials acceptable to
bFH's security policy but not aFH's policy, then the PUTFH aFH
operation will fail with NFS4ERR_WRONGSEC. After a SECINFO_NO_NAME
request, the client sends SEQUENCE, PUTFH bFH, SAVEFH, PUTFH aFH,
RENAME "c" "d", using credentials acceptable to aFH's security policy
but not bFH's policy. The server returns NFS4ERR_WRONGSEC on the
RENAME operation.
To prevent a client from an endless sequence of a request containing
LINK or RENAME, followed by a request containing SECINFO_NO_NAME or
SECINFO, the server
MUST detect when the security policies of the
current and saved filehandles have no mutually acceptable security
tuple, and
MUST NOT return NFS4ERR_WRONGSEC from LINK or RENAME in
that situation. Instead the server
MUST do one of two things:
* The server can return NFS4ERR_XDEV.
* The server can allow the security policy of the current filehandle
to override that of the saved filehandle, and so return NFS4_OK.
2.7. Minor Versioning
To address the requirement of an NFS protocol that can evolve as the
need arises, the NFSv4.1 protocol contains the rules and framework to
allow for future minor changes or versioning.
The base assumption with respect to minor versioning is that any
future accepted minor version will be documented in one or more
Standards Track RFCs. Minor version 0 of the NFSv4 protocol is
represented by [37], and minor version 1 is represented by this RFC.
The COMPOUND and CB_COMPOUND procedures support the encoding of the
minor version being requested by the client.
The following items represent the basic rules for the development of
minor versions. Note that a future minor version may modify or add
to the following rules as part of the minor version definition.
1. Procedures are not added or deleted.
To maintain the general RPC model, NFSv4 minor versions will not
add to or delete procedures from the NFS program.
2. Minor versions may add operations to the COMPOUND and
CB_COMPOUND procedures.
The addition of operations to the COMPOUND and CB_COMPOUND
procedures does not affect the RPC model.
* Minor versions may append attributes to the bitmap4 that
represents sets of attributes and to the fattr4 that
represents sets of attribute values.
This allows for the expansion of the attribute model to allow
for future growth or adaptation.
* Minor version X must append any new attributes after the last
documented attribute.
Since attribute results are specified as an opaque array of
per-attribute, XDR-encoded results, the complexity of adding
new attributes in the midst of the current definitions would
be too burdensome.
3. Minor versions must not modify the structure of an existing
operation's arguments or results.
Again, the complexity of handling multiple structure definitions
for a single operation is too burdensome. New operations should
be added instead of modifying existing structures for a minor
version.
This rule does not preclude the following adaptations in a minor
version:
* adding bits to flag fields, such as new attributes to
GETATTR's bitmap4 data type, and providing corresponding
variants of opaque arrays, such as a notify4 used together
with such bitmaps
* adding bits to existing attributes like ACLs that have flag
words
* extending enumerated types (including NFS4ERR_*) with new
values
* adding cases to a switched union
4. Minor versions must not modify the structure of existing
attributes.
5. Minor versions must not delete operations.
This prevents the potential reuse of a particular operation
"slot" in a future minor version.
6. Minor versions must not delete attributes.
7. Minor versions must not delete flag bits or enumeration values.
8. Minor versions may declare an operation MUST NOT be implemented.
Specifying that an operation
MUST NOT be implemented is
equivalent to obsoleting an operation. For the client, it means
that the operation
MUST NOT be sent to the server. For the
server, an NFS error can be returned as opposed to "dropping"
the request as an XDR decode error. This approach allows for
the obsolescence of an operation while maintaining its structure
so that a future minor version can reintroduce the operation.
1. Minor versions may declare that an attribute
MUST NOT be
implemented.
2. Minor versions may declare that a flag bit or enumeration
value
MUST NOT be implemented.
9. Minor versions may downgrade features from
REQUIRED to
RECOMMENDED, or
RECOMMENDED to
OPTIONAL.
10. Minor versions may upgrade features from
OPTIONAL to
RECOMMENDED, or
RECOMMENDED to
REQUIRED.
11. A client and server that support minor version X SHOULD support
minor versions zero through X-1 as well.
12. Except for infrastructural changes, a minor version must not
introduce REQUIRED new features.
This rule allows for the introduction of new functionality and
forces the use of implementation experience before designating a
feature as
REQUIRED. On the other hand, some classes of
features are infrastructural and have broad effects. Allowing
infrastructural features to be
RECOMMENDED or
OPTIONAL complicates implementation of the minor version.
13. A client
MUST NOT attempt to use a stateid, filehandle, or
similar returned object from the COMPOUND procedure with minor
version X for another COMPOUND procedure with minor version Y,
where X != Y.
2.8. Non-RPC-Based Security Services
As described in
Section 2.2.1.1.1.1, NFSv4.1 relies on RPC for
identification, authentication, integrity, and privacy. NFSv4.1
itself provides or enables additional security services as described
in the next several subsections.
2.8.1. Authorization
Authorization to access a file object via an NFSv4.1 operation is
ultimately determined by the NFSv4.1 server. A client can
predetermine its access to a file object via the OPEN (
Section 18.16)
and the ACCESS (
Section 18.1) operations.
Principals with appropriate access rights can modify the
authorization on a file object via the SETATTR (
Section 18.30)
operation. Attributes that affect access rights include mode, owner,
owner_group, acl, dacl, and sacl. See
Section 5.
NFSv4.1 provides auditing on a per-file object basis, via the acl and
sacl attributes as described in
Section 6. It is outside the scope
of this specification to specify audit log formats or management
policies.
2.8.3. Intrusion Detection
NFSv4.1 provides alarm control on a per-file object basis, via the
acl and sacl attributes as described in
Section 6. Alarms may serve
as the basis for intrusion detection. It is outside the scope of
this specification to specify heuristics for detecting intrusion via
alarms.
2.9. Transport Layers
2.9.1. REQUIRED and RECOMMENDED Properties of Transports
NFSv4.1 works over Remote Direct Memory Access (RDMA) and non-RDMA-
based transports with the following attributes:
* The transport supports reliable delivery of data, which NFSv4.1
requires but neither NFSv4.1 nor RPC has facilities for ensuring
[41].
* The transport delivers data in the order it was sent. Ordered
delivery simplifies detection of transmit errors, and simplifies
the sending of arbitrary sized requests and responses via the
record marking protocol [3].
Where an NFSv4.1 implementation supports operation over the IP
network protocol, any transport used between NFS and IP
MUST be among
the IETF-approved congestion control transport protocols. At the
time this document was written, the only two transports that had the
above attributes were TCP and the Stream Control Transmission
Protocol (SCTP). To enhance the possibilities for interoperability,
an NFSv4.1 implementation
MUST support operation over the TCP
transport protocol.
Even if NFSv4.1 is used over a non-IP network protocol, it is
RECOMMENDED that the transport support congestion control.
It is permissible for a connectionless transport to be used under
NFSv4.1; however, reliable and in-order delivery of data combined
with congestion control by the connectionless transport is
REQUIRED.
As a consequence, UDP by itself
MUST NOT be used as an NFSv4.1
transport. NFSv4.1 assumes that a client transport address and
server transport address used to send data over a transport together
constitute a connection, even if the underlying transport eschews the
concept of a connection.
2.9.2. Client and Server Transport Behavior
If a connection-oriented transport (e.g., TCP) is used, the client
and server
SHOULD use long-lived connections for at least three
reasons:
1. This will prevent the weakening of the transport's congestion
control mechanisms via short-lived connections.
2. This will improve performance for the WAN environment by
eliminating the need for connection setup handshakes.
3. The NFSv4.1 callback model differs from NFSv4.0, and requires the
client and server to maintain a client-created backchannel (see
Section 2.10.3.1) for the server to use.
In order to reduce congestion, if a connection-oriented transport is
used, and the request is not the NULL procedure:
* A requester
MUST NOT retry a request unless the connection the
request was sent over was lost before the reply was received.
* A replier
MUST NOT silently drop a request, even if the request is
a retry. (The silent drop behavior of RPCSEC_GSS [4] does not
apply because this behavior happens at the RPCSEC_GSS layer, a
lower layer in the request processing.) Instead, the replier
SHOULD return an appropriate error (see
Section 2.10.6.1), or it
MAY disconnect the connection.
When sending a reply, the replier
MUST send the reply to the same
full network address (e.g., if using an IP-based transport, the
source port of the requester is part of the full network address)
from which the requester sent the request. If using a connection-
oriented transport, replies
MUST be sent on the same connection from
which the request was received.
If a connection is dropped after the replier receives the request but
before the replier sends the reply, the replier might have a pending
reply. If a connection is established with the same source and
destination full network address as the dropped connection, then the
replier
MUST NOT send the reply until the requester retries the
request. The reason for this prohibition is that the requester
MAY retry a request over a different connection (provided that connection
is associated with the original request's session).
When using RDMA transports, there are other reasons for not
tolerating retries over the same connection:
* RDMA transports use "credits" to enforce flow control, where a
credit is a right to a peer to transmit a message. If one peer
were to retransmit a request (or reply), it would consume an
additional credit. If the replier retransmitted a reply, it would
certainly result in an RDMA connection loss, since the requester
would typically only post a single receive buffer for each
request. If the requester retransmitted a request, the additional
credit consumed on the server might lead to RDMA connection
failure unless the client accounted for it and decreased its
available credit, leading to wasted resources.
* RDMA credits present a new issue to the reply cache in NFSv4.1.
The reply cache may be used when a connection within a session is
lost, such as after the client reconnects. Credit information is
a dynamic property of the RDMA connection, and stale values must
not be replayed from the cache. This implies that the reply cache
contents must not be blindly used when replies are sent from it,
and credit information appropriate to the channel must be
refreshed by the RPC layer.
In addition, as described in
Section 2.10.6.2, while a session is
active, the NFSv4.1 requester
MUST NOT stop waiting for a reply.
Historically, NFSv3 servers have listened over TCP port 2049. The
registered port 2049 [42] for the NFS protocol should be the default
configuration. NFSv4.1 clients
SHOULD NOT use the RPC binding
protocols as described in [43].
NFSv4.1 clients and servers
MUST support and
MUST use the session
feature as described in this section.
2.10.1. Motivation and Overview
Previous versions and minor versions of NFS have suffered from the
following:
* Lack of support for Exactly Once Semantics (EOS). This includes
lack of support for EOS through server failure and recovery.
* Limited callback support, including no support for sending
callbacks through firewalls, and races between replies to normal
requests and callbacks.
* Limited trunking over multiple network paths.
* Requiring machine credentials for fully secure operation.
Through the introduction of a session, NFSv4.1 addresses the above
shortfalls with practical solutions:
* EOS is enabled by a reply cache with a bounded size, making it
feasible to keep the cache in persistent storage and enable EOS
through server failure and recovery. One reason that previous
revisions of NFS did not support EOS was because some EOS
approaches often limited parallelism. As will be explained in
Section 2.10.6, NFSv4.1 supports both EOS and unlimited
parallelism.
* The NFSv4.1 client (defined in
Section 1.7) creates transport
connections and provides them to the server to use for sending
callback requests, thus solving the firewall issue
(
Section 18.34). Races between responses from client requests and
callbacks caused by the requests are detected via the session's
sequencing properties that are a consequence of EOS
(
Section 2.10.6.3).
* The NFSv4.1 client can associate an arbitrary number of
connections with the session, and thus provide trunking
(
Section 2.10.5).
* The NFSv4.1 client and server produce a session key independent of
client and server machine credentials which can be used to compute
a digest for protecting critical session management operations
(
Section 2.10.8.3).
* The NFSv4.1 client can also create secure RPCSEC_GSS contexts for
use by the session's backchannel that do not require the server to
authenticate to a client machine principal (
Section 2.10.8.2).
A session is a dynamically created, long-lived server object created
by a client and used over time from one or more transport
connections. Its function is to maintain the server's state relative
to the connection(s) belonging to a client instance. This state is
entirely independent of the connection itself, and indeed the state
exists whether or not the connection exists. A client may have one
or more sessions associated with it so that client-associated state
may be accessed using any of the sessions associated with that
client's client ID, when connections are associated with those
sessions. When no connections are associated with any of a client
ID's sessions for an extended time, such objects as locks, opens,
delegations, layouts, etc. are subject to expiration. The session
serves as an object representing a means of access by a client to the
associated client state on the server, independent of the physical
means of access to that state.
A single client may create multiple sessions. A single session
MUST
NOT serve multiple clients.
2.10.2. NFSv4 Integration
Sessions are part of NFSv4.1 and not NFSv4.0. Normally, a major
infrastructure change such as sessions would require a new major
version number to an Open Network Computing (ONC) RPC program like
NFS. However, because NFSv4 encapsulates its functionality in a
single procedure, COMPOUND, and because COMPOUND can support an
arbitrary number of operations, sessions have been added to NFSv4.1
with little difficulty. COMPOUND includes a minor version number
field, and for NFSv4.1 this minor version is set to 1. When the
NFSv4 server processes a COMPOUND with the minor version set to 1, it
expects a different set of operations than it does for NFSv4.0.
NFSv4.1 defines the SEQUENCE operation, which is required for every
COMPOUND that operates over an established session, with the
exception of some session administration operations, such as
DESTROY_SESSION (
Section 18.37).
2.10.2.1. SEQUENCE and CB_SEQUENCE
In NFSv4.1, when the SEQUENCE operation is present, it
MUST be the
first operation in the COMPOUND procedure. The primary purpose of
SEQUENCE is to carry the session identifier. The session identifier
associates all other operations in the COMPOUND procedure with a
particular session. SEQUENCE also contains required information for
maintaining EOS (see
Section 2.10.6). Session-enabled NFSv4.1
COMPOUND requests thus have the form:
+-----+--------------+-----------+------------+-----------+----
| tag | minorversion | numops |SEQUENCE op | op + args | ...
| | (== 1) | (limited) | + args | |
+-----+--------------+-----------+------------+-----------+----
and the replies have the form:
+------------+-----+--------+-------------------------------+--//
|last status | tag | numres |status + SEQUENCE op + results | //
+------------+-----+--------+-------------------------------+--//
//-----------------------+----
// status + op + results | ...
//-----------------------+----
A CB_COMPOUND procedure request and reply has a similar form to
COMPOUND, but instead of a SEQUENCE operation, there is a CB_SEQUENCE
operation. CB_COMPOUND also has an additional field called
"callback_ident", which is superfluous in NFSv4.1 and
MUST be ignored
by the client. CB_SEQUENCE has the same information as SEQUENCE, and
also includes other information needed to resolve callback races
(
Section 2.10.6.3).
2.10.2.2. Client ID and Session Association
Each client ID (
Section 2.4) can have zero or more active sessions.
A client ID and associated session are required to perform file
access in NFSv4.1. Each time a session is used (whether by a client
sending a request to the server or the client replying to a callback
request from the server), the state leased to its associated client
ID is automatically renewed.
State (which can consist of share reservations, locks, delegations,
and layouts (
Section 1.8.4)) is tied to the client ID. Client state
is not tied to any individual session. Successive state changing
operations from a given state owner
MAY go over different sessions,
provided the session is associated with the same client ID. A
callback
MAY arrive over a different session than that of the request
that originally acquired the state pertaining to the callback. For
example, if session A is used to acquire a delegation, a request to
recall the delegation
MAY arrive over session B if both sessions are
associated with the same client ID. Sections
2.10.8.1 and
2.10.8.2 discuss the security considerations around callbacks.
A channel is not a connection. A channel represents the direction
ONC RPC requests are sent.
Each session has one or two channels: the fore channel and the
backchannel. Because there are at most two channels per session, and
because each channel has a distinct purpose, channels are not
assigned identifiers.
The fore channel is used for ordinary requests from the client to the
server, and carries COMPOUND requests and responses. A session
always has a fore channel.
The backchannel is used for callback requests from server to client,
and carries CB_COMPOUND requests and responses. Whether or not there
is a backchannel is decided by the client; however, many features of
NFSv4.1 require a backchannel. NFSv4.1 servers
MUST support
backchannels.
Each session has resources for each channel, including separate reply
caches (see
Section 2.10.6.1). Note that even the backchannel
requires a reply cache (or, at least, a slot table in order to detect
retries) because some callback operations are non-idempotent.
2.10.3.1. Association of Connections, Channels, and Sessions
Each channel is associated with zero or more transport connections
(whether of the same transport protocol or different transport
protocols). A connection can be associated with one channel or both
channels of a session; the client and server negotiate whether a
connection will carry traffic for one channel or both channels via
the CREATE_SESSION (
Section 18.36) and the BIND_CONN_TO_SESSION
(
Section 18.34) operations. When a session is created via
CREATE_SESSION, the connection that transported the CREATE_SESSION
request is automatically associated with the fore channel, and
optionally the backchannel. If the client specifies no state
protection (
Section 18.35) when the session is created, then when
SEQUENCE is transmitted on a different connection, the connection is
automatically associated with the fore channel of the session
specified in the SEQUENCE operation.
A connection's association with a session is not exclusive. A
connection associated with the channel(s) of one session may be
simultaneously associated with the channel(s) of other sessions
including sessions associated with other client IDs.
It is permissible for connections of multiple transport types to be
associated with the same channel. For example, both TCP and RDMA
connections can be associated with the fore channel. In the event an
RDMA and non-RDMA connection are associated with the same channel,
the maximum number of slots
SHOULD be at least one more than the
total number of RDMA credits (
Section 2.10.6.1). This way, if all
RDMA credits are used, the non-RDMA connection can have at least one
outstanding request. If a server supports multiple transport types,
it
MUST allow a client to associate connections from each transport
to a channel.
It is permissible for a connection of one type of transport to be
associated with the fore channel, and a connection of a different
type to be associated with the backchannel.
Servers each specify a server scope value in the form of an opaque
string eir_server_scope returned as part of the results of an
EXCHANGE_ID operation. The purpose of the server scope is to allow a
group of servers to indicate to clients that a set of servers sharing
the same server scope value has arranged to use distinct values of
opaque identifiers so that the two servers never assign the same
value to two distinct objects. Thus, the identifiers generated by
two servers within that set can be assumed compatible so that, in
certain important cases, identifiers generated by one server in that
set may be presented to another server of the same scope.
The use of such compatible values does not imply that a value
generated by one server will always be accepted by another. In most
cases, it will not. However, a server will not inadvertently accept
a value generated by another server. When it does accept it, it will
be because it is recognized as valid and carrying the same meaning as
on another server of the same scope.
When servers are of the same server scope, this compatibility of
values applies to the following identifiers:
* Filehandle values. A filehandle value accepted by two servers of
the same server scope denotes the same object. A WRITE operation
sent to one server is reflected immediately in a READ sent to the
other.
* Server owner values. When the server scope values are the same,
server owner value may be validly compared. In cases where the
server scope values are different, server owner values are treated
as different even if they contain identical strings of bytes.
The coordination among servers required to provide such compatibility
can be quite minimal, and limited to a simple partition of the ID
space. The recognition of common values requires additional
implementation, but this can be tailored to the specific situations
in which that recognition is desired.
Clients will have occasion to compare the server scope values of
multiple servers under a number of circumstances, each of which will
be discussed under the appropriate functional section:
* When server owner values received in response to EXCHANGE_ID
operations sent to multiple network addresses are compared for the
purpose of determining the validity of various forms of trunking,
as described in
Section 11.5.2.
* When network or server reconfiguration causes the same network
address to possibly be directed to different servers, with the
necessity for the client to determine when lock reclaim should be
attempted, as described in
Section 8.4.2.1.
When two replies from EXCHANGE_ID, each from two different server
network addresses, have the same server scope, there are a number of
ways a client can validate that the common server scope is due to two
servers cooperating in a group.
* If both EXCHANGE_ID requests were sent with RPCSEC_GSS ([4], [9],
[27]) authentication and the server principal is the same for both
targets, the equality of server scope is validated. It is
RECOMMENDED that two servers intending to share the same server
scope and server_owner major_id also share the same principal
name. In some cases, this simplifies the client's task of
validating server scope.
* The client may accept the appearance of the second server in the
fs_locations or fs_locations_info attribute for a relevant file
system. For example, if there is a migration event for a
particular file system or there are locks to be reclaimed on a
particular file system, the attributes for that particular file
system may be used. The client sends the GETATTR request to the
first server for the fs_locations or fs_locations_info attribute
with RPCSEC_GSS authentication. It may need to do this in advance
of the need to verify the common server scope. If the client
successfully authenticates the reply to GETATTR, and the GETATTR
request and reply containing the fs_locations or fs_locations_info
attribute refers to the second server, then the equality of server
scope is supported. A client may choose to limit the use of this
form of support to information relevant to the specific file
system involved (e.g. a file system being migrated).
Trunking is the use of multiple connections between a client and
server in order to increase the speed of data transfer. NFSv4.1
supports two types of trunking: session trunking and client ID
trunking.
In the context of a single server network address, it can be assumed
that all connections are accessing the same server, and NFSv4.1
servers
MUST support both forms of trunking. When multiple
connections use a set of network addresses to access the same server,
the server
MUST support both forms of trunking. NFSv4.1 servers in a
clustered configuration
MAY allow network addresses for different
servers to use client ID trunking.
Clients may use either form of trunking as long as they do not, when
trunking between different server network addresses, violate the
servers' mandates as to the kinds of trunking to be allowed (see
below). With regard to callback channels, the client
MUST allow the
server to choose among all callback channels valid for a given client
ID and
MUST support trunking when the connections supporting the
backchannel allow session or client ID trunking to be used for
callbacks.
Session trunking is essentially the association of multiple
connections, each with potentially different target and/or source
network addresses, to the same session. When the target network
addresses (server addresses) of the two connections are the same, the
server
MUST support such session trunking. When the target network
addresses are different, the server
MAY indicate such support using
the data returned by the EXCHANGE_ID operation (see below).
Client ID trunking is the association of multiple sessions to the
same client ID. Servers
MUST support client ID trunking for two
target network addresses whenever they allow session trunking for
those same two network addresses. In addition, a server
MAY, by
presenting the same major server owner ID (
Section 2.5) and server
scope (
Section 2.10.4), allow an additional case of client ID
trunking. When two servers return the same major server owner and
server scope, it means that the two servers are cooperating on
locking state management, which is a prerequisite for client ID
trunking.
Distinguishing when the client is allowed to use session and client
ID trunking requires understanding how the results of the EXCHANGE_ID
(
Section 18.35) operation identify a server. Suppose a client sends
EXCHANGE_IDs over two different connections, each with a possibly
different target network address, but each EXCHANGE_ID operation has
the same value in the eia_clientowner field. If the same NFSv4.1
server is listening over each connection, then each EXCHANGE_ID
result
MUST return the same values of eir_clientid,
eir_server_owner.so_major_id, and eir_server_scope. The client can
then treat each connection as referring to the same server (subject
to verification; see
Section 2.10.5.1 below), and it can use each
connection to trunk requests and replies. The client's choice is
whether session trunking or client ID trunking applies.
Session Trunking. If the eia_clientowner argument is the same in two
different EXCHANGE_ID requests, and the eir_clientid,
eir_server_owner.so_major_id, eir_server_owner.so_minor_id, and
eir_server_scope results match in both EXCHANGE_ID results, then
the client is permitted to perform session trunking. If the
client has no session mapping to the tuple of eir_clientid,
eir_server_owner.so_major_id, eir_server_scope, and
eir_server_owner.so_minor_id, then it creates the session via a
CREATE_SESSION operation over one of the connections, which
associates the connection to the session. If there is a session
for the tuple, the client can send BIND_CONN_TO_SESSION to
associate the connection to the session.
Of course, if the client does not desire to use session trunking,
it is not required to do so. It can invoke CREATE_SESSION on the
connection. This will result in client ID trunking as described
below. It can also decide to drop the connection if it does not
choose to use trunking.
Client ID Trunking. If the eia_clientowner argument is the same in
two different EXCHANGE_ID requests, and the eir_clientid,
eir_server_owner.so_major_id, and eir_server_scope results match
in both EXCHANGE_ID results, then the client is permitted to
perform client ID trunking (regardless of whether the
eir_server_owner.so_minor_id results match). The client can
associate each connection with different sessions, where each
session is associated with the same server.
The client completes the act of client ID trunking by invoking
CREATE_SESSION on each connection, using the same client ID that
was returned in eir_clientid. These invocations create two
sessions and also associate each connection with its respective
session. The client is free to decline to use client ID trunking
by simply dropping the connection at this point.
When doing client ID trunking, locking state is shared across
sessions associated with that same client ID. This requires the
server to coordinate state across sessions and the client to be
able to associate the same locking state with multiple sessions.
It is always possible that, as a result of various sorts of
reconfiguration events, eir_server_scope and eir_server_owner values
may be different on subsequent EXCHANGE_ID requests made to the same
network address.
In most cases, such reconfiguration events will be disruptive and
indicate that an IP address formerly connected to one server is now
connected to an entirely different one.
Some guidelines on client handling of such situations follow:
* When eir_server_scope changes, the client has no assurance that
any IDs that it obtained previously (e.g., filehandles) can be
validly used on the new server, and, even if the new server
accepts them, there is no assurance that this is not due to
accident. Thus, it is best to treat all such state as lost or
stale, although a client may assume that the probability of
inadvertent acceptance is low and treat this situation as within
the next case.
* When eir_server_scope remains the same and
eir_server_owner.so_major_id changes, the client can use the
filehandles it has, consider its locking state lost, and attempt
to reclaim or otherwise re-obtain its locks. It might find that
its filehandle is now stale. However, if NFS4ERR_STALE is not
returned, it can proceed to reclaim or otherwise re-obtain its
open locking state.
* When eir_server_scope and eir_server_owner.so_major_id remain the
same, the client has to use the now-current values of
eir_server_owner.so_minor_id in deciding on appropriate forms of
trunking. This may result in connections being dropped or new
sessions being created.
2.10.5.1. Verifying Claims of Matching Server Identity
When the server responds using two different connections that claim
matching or partially matching eir_server_owner, eir_server_scope,
and eir_clientid values, the client does not have to trust the
servers' claims. The client may verify these claims before trunking
traffic in the following ways:
* For session trunking, clients
SHOULD reliably verify if
connections between different network paths are in fact associated
with the same NFSv4.1 server and usable on the same session, and
servers
MUST allow clients to perform reliable verification. When
a client ID is created, the client
SHOULD specify that
BIND_CONN_TO_SESSION is to be verified according to the SP4_SSV or
SP4_MACH_CRED (
Section 18.35) state protection options. For
SP4_SSV, reliable verification depends on a shared secret (the
SSV) that is established via the SET_SSV (see
Section 18.47)
operation.
When a new connection is associated with the session (via the
BIND_CONN_TO_SESSION operation, see
Section 18.34), if the client
specified SP4_SSV state protection for the BIND_CONN_TO_SESSION
operation, the client
MUST send the BIND_CONN_TO_SESSION with
RPCSEC_GSS protection, using integrity or privacy, and an
RPCSEC_GSS handle created with the GSS SSV mechanism (see
Section 2.10.9).
If the client mistakenly tries to associate a connection to a
session of a wrong server, the server will either reject the
attempt because it is not aware of the session identifier of the
BIND_CONN_TO_SESSION arguments, or it will reject the attempt
because the RPCSEC_GSS authentication fails. Even if the server
mistakenly or maliciously accepts the connection association
attempt, the RPCSEC_GSS verifier it computes in the response will
not be verified by the client, so the client will know it cannot
use the connection for trunking the specified session.
If the client specified SP4_MACH_CRED state protection, the
BIND_CONN_TO_SESSION operation will use RPCSEC_GSS integrity or
privacy, using the same credential that was used when the client
ID was created. Mutual authentication via RPCSEC_GSS assures the
client that the connection is associated with the correct session
of the correct server.
* For client ID trunking, the client has at least two options for
verifying that the same client ID obtained from two different
EXCHANGE_ID operations came from the same server. The first
option is to use RPCSEC_GSS authentication when sending each
EXCHANGE_ID operation. Each time an EXCHANGE_ID is sent with
RPCSEC_GSS authentication, the client notes the principal name of
the GSS target. If the EXCHANGE_ID results indicate that client
ID trunking is possible, and the GSS targets' principal names are
the same, the servers are the same and client ID trunking is
allowed.
The second option for verification is to use SP4_SSV protection.
When the client sends EXCHANGE_ID, it specifies SP4_SSV
protection. The first EXCHANGE_ID the client sends always has to
be confirmed by a CREATE_SESSION call. The client then sends
SET_SSV. Later, the client sends EXCHANGE_ID to a second
destination network address different from the one the first
EXCHANGE_ID was sent to. The client checks that each EXCHANGE_ID
reply has the same eir_clientid, eir_server_owner.so_major_id, and
eir_server_scope. If so, the client verifies the claim by sending
a CREATE_SESSION operation to the second destination address,
protected with RPCSEC_GSS integrity using an RPCSEC_GSS handle
returned by the second EXCHANGE_ID. If the server accepts the
CREATE_SESSION request, and if the client verifies the RPCSEC_GSS
verifier and integrity codes, then the client has proof the second
server knows the SSV, and thus the two servers are cooperating for
the purposes of specifying server scope and client ID trunking.
2.10.6. Exactly Once Semantics
Via the session, NFSv4.1 offers exactly once semantics (EOS) for
requests sent over a channel. EOS is supported on both the fore
channel and backchannel.
Each COMPOUND or CB_COMPOUND request that is sent with a leading
SEQUENCE or CB_SEQUENCE operation
MUST be executed by the receiver
exactly once. This requirement holds regardless of whether the
request is sent with reply caching specified (see
Section 2.10.6.1.3). The requirement holds even if the requester is
sending the request over a session created between a pNFS data client
and pNFS data server. To understand the rationale for this
requirement, divide the requests into three classifications:
* Non-idempotent requests.
* Idempotent modifying requests.
* Idempotent non-modifying requests.
An example of a non-idempotent request is RENAME. Obviously, if a
replier executes the same RENAME request twice, and the first
execution succeeds, the re-execution will fail. If the replier
returns the result from the re-execution, this result is incorrect.
Therefore, EOS is required for non-idempotent requests.
An example of an idempotent modifying request is a COMPOUND request
containing a WRITE operation. Repeated execution of the same WRITE
has the same effect as execution of that WRITE a single time.
Nevertheless, enforcing EOS for WRITEs and other idempotent modifying
requests is necessary to avoid data corruption.
Suppose a client sends WRITE A to a noncompliant server that does not
enforce EOS, and receives no response, perhaps due to a network
partition. The client reconnects to the server and re-sends WRITE A.
Now, the server has outstanding two instances of A. The server can
be in a situation in which it executes and replies to the retry of A,
while the first A is still waiting in the server's internal I/O
system for some resource. Upon receiving the reply to the second
attempt of WRITE A, the client believes its WRITE is done so it is
free to send WRITE B, which overlaps the byte-range of A. When the
original A is dispatched from the server's I/O system and executed
(thus the second time A will have been written), then what has been
written by B can be overwritten and thus corrupted.
An example of an idempotent non-modifying request is a COMPOUND
containing SEQUENCE, PUTFH, READLINK, and nothing else. The re-
execution of such a request will not cause data corruption or produce
an incorrect result. Nonetheless, to keep the implementation simple,
the replier
MUST enforce EOS for all requests, whether or not
idempotent and non-modifying.
Note that true and complete EOS is not possible unless the server
persists the reply cache in stable storage, and unless the server is
somehow implemented to never require a restart (indeed, if such a
server exists, the distinction between a reply cache kept in stable
storage versus one that is not is one without meaning). See
Section 2.10.6.5 for a discussion of persistence in the reply cache.
Regardless, even if the server does not persist the reply cache, EOS
improves robustness and correctness over previous versions of NFS
because the legacy duplicate request/reply caches were based on the
ONC RPC transaction identifier (XID).
Section 2.10.6.1 explains the
shortcomings of the XID as a basis for a reply cache and describes
how NFSv4.1 sessions improve upon the XID.
2.10.6.1. Slot Identifiers and Reply Cache
The RPC layer provides a transaction ID (XID), which, while required
to be unique, is not convenient for tracking requests for two
reasons. First, the XID is only meaningful to the requester; it
cannot be interpreted by the replier except to test for equality with
previously sent requests. When consulting an RPC-based duplicate
request cache, the opaqueness of the XID requires a computationally
expensive look up (often via a hash that includes XID and source
address). NFSv4.1 requests use a non-opaque slot ID, which is an
index into a slot table, which is far more efficient. Second,
because RPC requests can be executed by the replier in any order,
there is no bound on the number of requests that may be outstanding
at any time. To achieve perfect EOS, using ONC RPC would require
storing all replies in the reply cache. XIDs are 32 bits; storing
over four billion (2^(32)) replies in the reply cache is not
practical. In practice, previous versions of NFS have chosen to
store a fixed number of replies in the cache, and to use a least
recently used (LRU) approach to replacing cache entries with new
entries when the cache is full. In NFSv4.1, the number of
outstanding requests is bounded by the size of the slot table, and a
sequence ID per slot is used to tell the replier when it is safe to
delete a cached reply.
In the NFSv4.1 reply cache, when the requester sends a new request,
it selects a slot ID in the range 0..N, where N is the replier's
current maximum slot ID granted to the requester on the session over
which the request is to be sent. The value of N starts out as equal
to ca_maxrequests - 1 (
Section 18.36), but can be adjusted by the
response to SEQUENCE or CB_SEQUENCE as described later in this
section. The slot ID must be unused by any of the requests that the
requester has already active on the session. "Unused" here means the
requester has no outstanding request for that slot ID.
A slot contains a sequence ID and the cached reply corresponding to
the request sent with that sequence ID. The sequence ID is a 32-bit
unsigned value, and is therefore in the range 0..0xFFFFFFFF (2^(32) -
1). The first time a slot is used, the requester
MUST specify a
sequence ID of one (
Section 18.36). Each time a slot is reused, the
request
MUST specify a sequence ID that is one greater than that of
the previous request on the slot. If the previous sequence ID was
0xFFFFFFFF, then the next request for the slot
MUST have the sequence
ID set to zero (i.e., (2^(32) - 1) + 1 mod 2^(32)).
The sequence ID accompanies the slot ID in each request. It is for
the critical check at the replier: it used to efficiently determine
whether a request using a certain slot ID is a retransmit or a new,
never-before-seen request. It is not feasible for the requester to
assert that it is retransmitting to implement this, because for any
given request the requester cannot know whether the replier has seen
it unless the replier actually replies. Of course, if the requester
has seen the reply, the requester would not retransmit.
The replier compares each received request's sequence ID with the
last one previously received for that slot ID, to see if the new
request is:
* A new request, in which the sequence ID is one greater than that
previously seen in the slot (accounting for sequence wraparound).
The replier proceeds to execute the new request, and the replier
MUST increase the slot's sequence ID by one.
* A retransmitted request, in which the sequence ID is equal to that
currently recorded in the slot. If the original request has
executed to completion, the replier returns the cached reply. See
Section 2.10.6.2 for direction on how the replier deals with
retries of requests that are still in progress.
* A misordered retry, in which the sequence ID is less than
(accounting for sequence wraparound) that previously seen in the
slot. The replier
MUST return NFS4ERR_SEQ_MISORDERED (as the
result from SEQUENCE or CB_SEQUENCE).
* A misordered new request, in which the sequence ID is two or more
than (accounting for sequence wraparound) that previously seen in
the slot. Note that because the sequence ID
MUST wrap around to
zero once it reaches 0xFFFFFFFF, a misordered new request and a
misordered retry cannot be distinguished. Thus, the replier
MUST return NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or
CB_SEQUENCE).
Unlike the XID, the slot ID is always within a specific range; this
has two implications. The first implication is that for a given
session, the replier need only cache the results of a limited number
of COMPOUND requests. The second implication derives from the first,
which is that unlike XID-indexed reply caches (also known as
duplicate request caches - DRCs), the slot ID-based reply cache
cannot be overflowed. Through use of the sequence ID to identify
retransmitted requests, the replier does not need to actually cache
the request itself, reducing the storage requirements of the reply
cache further. These facilities make it practical to maintain all
the required entries for an effective reply cache.
The slot ID, sequence ID, and session ID therefore take over the
traditional role of the XID and source network address in the
replier's reply cache implementation. This approach is considerably
more portable and completely robust -- it is not subject to the
reassignment of ports as clients reconnect over IP networks. In
addition, the RPC XID is not used in the reply cache, enhancing
robustness of the cache in the face of any rapid reuse of XIDs by the
requester. While the replier does not care about the XID for the
purposes of reply cache management (but the replier
MUST return the
same XID that was in the request), nonetheless there are
considerations for the XID in NFSv4.1 that are the same as all other
previous versions of NFS. The RPC XID remains in each message and
needs to be formulated in NFSv4.1 requests as in any other ONC RPC
request. The reasons include:
* The RPC layer retains its existing semantics and implementation.
* The requester and replier must be able to interoperate at the RPC
layer, prior to the NFSv4.1 decoding of the SEQUENCE or
CB_SEQUENCE operation.
* If an operation is being used that does not start with SEQUENCE or
CB_SEQUENCE (e.g., BIND_CONN_TO_SESSION), then the RPC XID is
needed for correct operation to match the reply to the request.
* The SEQUENCE or CB_SEQUENCE operation may generate an error. If
so, the embedded slot ID, sequence ID, and session ID (if present)
in the request will not be in the reply, and the requester has
only the XID to match the reply to the request.
Given that well-formulated XIDs continue to be required, this raises
the question: why do SEQUENCE and CB_SEQUENCE replies have a session
ID, slot ID, and sequence ID? Having the session ID in the reply
means that the requester does not have to use the XID to look up the
session ID, which would be necessary if the connection were
associated with multiple sessions. Having the slot ID and sequence
ID in the reply means that the requester does not have to use the XID
to look up the slot ID and sequence ID. Furthermore, since the XID
is only 32 bits, it is too small to guarantee the re-association of a
reply with its request [44]; having session ID, slot ID, and sequence
ID in the reply allows the client to validate that the reply in fact
belongs to the matched request.
The SEQUENCE (and CB_SEQUENCE) operation also carries a
"highest_slotid" value, which carries additional requester slot usage
information. The requester
MUST always indicate the slot ID
representing the outstanding request with the highest-numbered slot
value. The requester should in all cases provide the most
conservative value possible, although it can be increased somewhat
above the actual instantaneous usage to maintain some minimum or
optimal level. This provides a way for the requester to yield unused
request slots back to the replier, which in turn can use the
information to reallocate resources.
The replier responds with both a new target highest_slotid and an
enforced highest_slotid, described as follows:
* The target highest_slotid is an indication to the requester of the
highest_slotid the replier wishes the requester to be using. This
permits the replier to withdraw (or add) resources from a
requester that has been found to not be using them, in order to
more fairly share resources among a varying level of demand from
other requesters. The requester must always comply with the
replier's value updates, since they indicate newly established
hard limits on the requester's access to session resources.
However, because of request pipelining, the requester may have
active requests in flight reflecting prior values; therefore, the
replier must not immediately require the requester to comply.
* The enforced highest_slotid indicates the highest slot ID the
requester is permitted to use on a subsequent SEQUENCE or
CB_SEQUENCE operation. The replier's enforced highest_slotid
SHOULD be no less than the highest_slotid the requester indicated
in the SEQUENCE or CB_SEQUENCE arguments.
A requester can be intransigent with respect to lowering its
highest_slotid argument to a Sequence operation, i.e. the
requester continues to ignore the target highest_slotid in the
response to a Sequence operation, and continues to set its
highest_slotid argument to be higher than the target
highest_slotid. This can be considered particularly egregious
behavior when the replier knows there are no outstanding requests
with slot IDs higher than its target highest_slotid. When faced
with such intransigence, the replier is free to take more forceful
action, and
MAY reply with a new enforced highest_slotid that is
less than its previous enforced highest_slotid. Thereafter, if
the requester continues to send requests with a highest_slotid
that is greater than the replier's new enforced highest_slotid,
the server
MAY return NFS4ERR_BAD_HIGH_SLOT, unless the slot ID in
the request is greater than the new enforced highest_slotid and
the request is a retry.
The replier
SHOULD retain the slots it wants to retire until the
requester sends a request with a highest_slotid less than or equal
to the replier's new enforced highest_slotid.
The requester can also be intransigent with respect to sending
non-retry requests that have a slot ID that exceeds the replier's
highest_slotid. Once the replier has forcibly lowered the
enforced highest_slotid, the requester is only allowed to send
retries on slots that exceed the replier's highest_slotid. If a
request is received with a slot ID that is higher than the new
enforced highest_slotid, and the sequence ID is one higher than
what is in the slot's reply cache, then the server can both retire
the slot and return NFS4ERR_BADSLOT (however, the server
MUST NOT do one and not the other). The reason it is safe to retire the
slot is because by using the next sequence ID, the requester is
indicating it has received the previous reply for the slot.
* The requester
SHOULD use the lowest available slot when sending a
new request. This way, the replier may be able to retire slot
entries faster. However, where the replier is actively adjusting
its granted highest_slotid, it will not be able to use only the
receipt of the slot ID and highest_slotid in the request. Neither
the slot ID nor the highest_slotid used in a request may reflect
the replier's current idea of the requester's session limit,
because the request may have been sent from the requester before
the update was received. Therefore, in the downward adjustment
case, the replier may have to retain a number of reply cache
entries at least as large as the old value of maximum requests
outstanding, until it can infer that the requester has seen a
reply containing the new granted highest_slotid. The replier can
infer that the requester has seen such a reply when it receives a
new request with the same slot ID as the request replied to and
the next higher sequence ID.
2.10.6.1.1. Caching of SEQUENCE and CB_SEQUENCE Replies
When a SEQUENCE or CB_SEQUENCE operation is successfully executed,
its reply
MUST always be cached. Specifically, session ID, sequence
ID, and slot ID
MUST be cached in the reply cache. The reply from
SEQUENCE also includes the highest slot ID, target highest slot ID,
and status flags. Instead of caching these values, the server
MAY re-compute the values from the current state of the fore channel,
session, and/or client ID as appropriate. Similarly, the reply from
CB_SEQUENCE includes a highest slot ID and target highest slot ID.
The client
MAY re-compute the values from the current state of the
session as appropriate.
Regardless of whether or not a replier is re-computing highest slot
ID, target slot ID, and status on replies to retries, the requester
MUST NOT assume that the values are being re-computed whenever it
receives a reply after a retry is sent, since it has no way of
knowing whether the reply it has received was sent by the replier in
response to the retry or is a delayed response to the original
request. Therefore, it may be the case that highest slot ID, target
slot ID, or status bits may reflect the state of affairs when the
request was first executed. Although acting based on such delayed
information is valid, it may cause the receiver of the reply to do
unneeded work. Requesters
MAY choose to send additional requests to
get the current state of affairs or use the state of affairs reported
by subsequent requests, in preference to acting immediately on data
that might be out of date.
2.10.6.1.2. Errors from SEQUENCE and CB_SEQUENCE
Any time SEQUENCE or CB_SEQUENCE returns an error, the sequence ID of
the slot
MUST NOT change. The replier
MUST NOT modify the reply
cache entry for the slot whenever an error is returned from SEQUENCE
or CB_SEQUENCE.
2.10.6.1.3. Optional Reply Caching
On a per-request basis, the requester can choose to direct the
replier to cache the reply to all operations after the first
operation (SEQUENCE or CB_SEQUENCE) via the sa_cachethis or
csa_cachethis fields of the arguments to SEQUENCE or CB_SEQUENCE.
The reason it would not direct the replier to cache the entire reply
is that the request is composed of all idempotent operations [41].
Caching the reply may offer little benefit. If the reply is too
large (see
Section 2.10.6.4), it may not be cacheable anyway. Even
if the reply to idempotent request is small enough to cache,
unnecessarily caching the reply slows down the server and increases
RPC latency.
Whether or not the requester requests the reply to be cached has no
effect on the slot processing. If the result of SEQUENCE or
CB_SEQUENCE is NFS4_OK, then the slot's sequence ID
MUST be
incremented by one. If a requester does not direct the replier to
cache the reply, the replier
MUST do one of following:
* The replier can cache the entire original reply. Even though
sa_cachethis or csa_cachethis is FALSE, the replier is always free
to cache. It may choose this approach in order to simplify
implementation.
* The replier enters into its reply cache a reply consisting of the
original results to the SEQUENCE or CB_SEQUENCE operation, and
with the next operation in COMPOUND or CB_COMPOUND having the
error NFS4ERR_RETRY_UNCACHED_REP. Thus, if the requester later
retries the request, it will get NFS4ERR_RETRY_UNCACHED_REP. If a
replier receives a retried Sequence operation where the reply to
the COMPOUND or CB_COMPOUND was not cached, then the replier,
-
MAY return NFS4ERR_RETRY_UNCACHED_REP in reply to a Sequence
operation if the Sequence operation is not the first operation
(granted, a requester that does so is in violation of the
NFSv4.1 protocol).
-
MUST NOT return NFS4ERR_RETRY_UNCACHED_REP in reply to a
Sequence operation if the Sequence operation is the first
operation.
* If the second operation is an illegal operation, or an operation
that was legal in a previous minor version of NFSv4 and
MUST NOT be supported in the current minor version (e.g., SETCLIENTID), the
replier
MUST NOT ever return NFS4ERR_RETRY_UNCACHED_REP. Instead
the replier
MUST return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or
NFS4ERR_NOTSUPP as appropriate.
* If the second operation can result in another error status, the
replier
MAY return a status other than NFS4ERR_RETRY_UNCACHED_REP,
provided the operation is not executed in such a way that the
state of the replier is changed. Examples of such an error status
include: NFS4ERR_NOTSUPP returned for an operation that is legal
but not
REQUIRED in the current minor versions, and thus not
supported by the replier; NFS4ERR_SEQUENCE_POS; and
NFS4ERR_REQ_TOO_BIG.
The discussion above assumes that the retried request matches the
original one.
Section 2.10.6.1.3.1 discusses what the replier might
do, and
MUST do when original and retried requests do not match.
Since the replier may only cache a small amount of the information
that would be required to determine whether this is a case of a false
retry, the replier may send to the client any of the following
responses:
* The cached reply to the original request (if the replier has
cached it in its entirety and the users of the original request
and retry match).
* A reply that consists only of the Sequence operation with the
error NFS4ERR_SEQ_FALSE_RETRY.
* A reply consisting of the response to Sequence with the status
NFS4_OK, together with the second operation as it appeared in the
retried request with an error of NFS4ERR_RETRY_UNCACHED_REP or
other error as described above.
* A reply that consists of the response to Sequence with the status
NFS4_OK, together with the second operation as it appeared in the
original request with an error of NFS4ERR_RETRY_UNCACHED_REP or
other error as described above.
If a requester sent a Sequence operation with a slot ID and sequence
ID that are in the reply cache but the replier detected that the
retried request is not the same as the original request, including a
retry that has different operations or different arguments in the
operations from the original and a retry that uses a different
principal in the RPC request's credential field that translates to a
different user, then this is a false retry. When the replier detects
a false retry, it is permitted (but not always obligated) to return
NFS4ERR_SEQ_FALSE_RETRY in response to the Sequence operation when it
detects a false retry.
Translations of particularly privileged user values to other users
due to the lack of appropriately secure credentials, as configured on
the replier, should be applied before determining whether the users
are the same or different. If the replier determines the users are
different between the original request and a retry, then the replier
MUST return NFS4ERR_SEQ_FALSE_RETRY.
If an operation of the retry is an illegal operation, or an operation
that was legal in a previous minor version of NFSv4 and
MUST NOT be
supported in the current minor version (e.g., SETCLIENTID), the
replier
MAY return NFS4ERR_SEQ_FALSE_RETRY (and
MUST do so if the
users of the original request and retry differ). Otherwise, the
replier
MAY return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or
NFS4ERR_NOTSUPP as appropriate. Note that the handling is in
contrast for how the replier deals with retries requests with no
cached reply. The difference is due to NFS4ERR_SEQ_FALSE_RETRY being
a valid error for only Sequence operations, whereas
NFS4ERR_RETRY_UNCACHED_REP is a valid error for all operations except
illegal operations and operations that
MUST NOT be supported in the
current minor version of NFSv4.
2.10.6.2. Retry and Replay of Reply
A requester
MUST NOT retry a request, unless the connection it used
to send the request disconnects. The requester can then reconnect
and re-send the request, or it can re-send the request over a
different connection that is associated with the same session.
If the requester is a server wanting to re-send a callback operation
over the backchannel of a session, the requester of course cannot
reconnect because only the client can associate connections with the
backchannel. The server can re-send the request over another
connection that is bound to the same session's backchannel. If there
is no such connection, the server
MUST indicate that the session has
no backchannel by setting the SEQ4_STATUS_CB_PATH_DOWN_SESSION flag
bit in the response to the next SEQUENCE operation from the client.
The client
MUST then associate a connection with the session (or
destroy the session).
Note that it is not fatal for a requester to retry without a
disconnect between the request and retry. However, the retry does
consume resources, especially with RDMA, where each request, retry or
not, consumes a credit. Retries for no reason, especially retries
sent shortly after the previous attempt, are a poor use of network
bandwidth and defeat the purpose of a transport's inherent congestion
control system.
A requester
MUST wait for a reply to a request before using the slot
for another request. If it does not wait for a reply, then the
requester does not know what sequence ID to use for the slot on its
next request. For example, suppose a requester sends a request with
sequence ID 1, and does not wait for the response. The next time it
uses the slot, it sends the new request with sequence ID 2. If the
replier has not seen the request with sequence ID 1, then the replier
is not expecting sequence ID 2, and rejects the requester's new
request with NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or
CB_SEQUENCE).
RDMA fabrics do not guarantee that the memory handles (Steering Tags)
within each RPC/RDMA "chunk" [32] are valid on a scope outside that
of a single connection. Therefore, handles used by the direct
operations become invalid after connection loss. The server must
ensure that any RDMA operations that must be replayed from the reply
cache use the newly provided handle(s) from the most recent request.
A retry might be sent while the original request is still in progress
on the replier. The replier
SHOULD deal with the issue by returning
NFS4ERR_DELAY as the reply to SEQUENCE or CB_SEQUENCE operation, but
implementations
MAY return NFS4ERR_MISORDERED. Since errors from
SEQUENCE and CB_SEQUENCE are never recorded in the reply cache, this
approach allows the results of the execution of the original request
to be properly recorded in the reply cache (assuming that the
requester specified the reply to be cached).
2.10.6.3. Resolving Server Callback Races
It is possible for server callbacks to arrive at the client before
the reply from related fore channel operations. For example, a
client may have been granted a delegation to a file it has opened,
but the reply to the OPEN (informing the client of the granting of
the delegation) may be delayed in the network. If a conflicting
operation arrives at the server, it will recall the delegation using
the backchannel, which may be on a different transport connection,
perhaps even a different network, or even a different session
associated with the same client ID.
The presence of a session between the client and server alleviates
this issue. When a session is in place, each client request is
uniquely identified by its { session ID, slot ID, sequence ID }
triple. By the rules under which slot entries (reply cache entries)
are retired, the server has knowledge whether the client has "seen"
each of the server's replies. The server can therefore provide
sufficient information to the client to allow it to disambiguate
between an erroneous or conflicting callback race condition.
For each client operation that might result in some sort of server
callback, the server
SHOULD "remember" the { session ID, slot ID,
sequence ID } triple of the client request until the slot ID
retirement rules allow the server to determine that the client has,
in fact, seen the server's reply. Until the time the { session ID,
slot ID, sequence ID } request triple can be retired, any recalls of
the associated object
MUST carry an array of these referring
identifiers (in the CB_SEQUENCE operation's arguments), for the
benefit of the client. After this time, it is not necessary for the
server to provide this information in related callbacks, since it is
certain that a race condition can no longer occur.
The CB_SEQUENCE operation that begins each server callback carries a
list of "referring" { session ID, slot ID, sequence ID } triples. If
the client finds the request corresponding to the referring session
ID, slot ID, and sequence ID to be currently outstanding (i.e., the
server's reply has not been seen by the client), it can determine
that the callback has raced the reply, and act accordingly. If the
client does not find the request corresponding to the referring
triple to be outstanding (including the case of a session ID
referring to a destroyed session), then there is no race with respect
to this triple. The server
SHOULD limit the referring triples to
requests that refer to just those that apply to the objects referred
to in the CB_COMPOUND procedure.
The client must not simply wait forever for the expected server reply
to arrive before responding to the CB_COMPOUND that won the race,
because it is possible that it will be delayed indefinitely. The
client should assume the likely case that the reply will arrive
within the average round-trip time for COMPOUND requests to the
server, and wait that period of time. If that period of time
expires, it can respond to the CB_COMPOUND with NFS4ERR_DELAY. There
are other scenarios under which callbacks may race replies. Among
them are pNFS layout recalls as described in
Section 12.5.5.2.
2.10.6.4. COMPOUND and CB_COMPOUND Construction Issues
Very large requests and replies may pose both buffer management
issues (especially with RDMA) and reply cache issues. When the
session is created (
Section 18.36), for each channel (fore and back),
the client and server negotiate the maximum-sized request they will
send or process (ca_maxrequestsize), the maximum-sized reply they
will return or process (ca_maxresponsesize), and the maximum-sized
reply they will store in the reply cache (ca_maxresponsesize_cached).
If a request exceeds ca_maxrequestsize, the reply will have the
status NFS4ERR_REQ_TOO_BIG. A replier
MAY return NFS4ERR_REQ_TOO_BIG
as the status for the first operation (SEQUENCE or CB_SEQUENCE) in
the request (which means that no operations in the request executed
and that the state of the slot in the reply cache is unchanged), or
it
MAY opt to return it on a subsequent operation in the same
COMPOUND or CB_COMPOUND request (which means that at least one
operation did execute and that the state of the slot in the reply
cache does change). The replier
SHOULD set NFS4ERR_REQ_TOO_BIG on
the operation that exceeds ca_maxrequestsize.
If a reply exceeds ca_maxresponsesize, the reply will have the status
NFS4ERR_REP_TOO_BIG. A replier
MAY return NFS4ERR_REP_TOO_BIG as the
status for the first operation (SEQUENCE or CB_SEQUENCE) in the
request, or it
MAY opt to return it on a subsequent operation (in the
same COMPOUND or CB_COMPOUND reply). A replier
MAY return
NFS4ERR_REP_TOO_BIG in the reply to SEQUENCE or CB_SEQUENCE, even if
the response would still exceed ca_maxresponsesize.
If sa_cachethis or csa_cachethis is TRUE, then the replier
MUST cache
a reply except if an error is returned by the SEQUENCE or CB_SEQUENCE
operation (see
Section 2.10.6.1.2). If the reply exceeds
ca_maxresponsesize_cached (and sa_cachethis or csa_cachethis is
TRUE), then the server
MUST return NFS4ERR_REP_TOO_BIG_TO_CACHE.
Even if NFS4ERR_REP_TOO_BIG_TO_CACHE (or any other error for that
matter) is returned on an operation other than the first operation
(SEQUENCE or CB_SEQUENCE), then the reply
MUST be cached if
sa_cachethis or csa_cachethis is TRUE. For example, if a COMPOUND
has eleven operations, including SEQUENCE, the fifth operation is a
RENAME, and the tenth operation is a READ for one million bytes, the
server may return NFS4ERR_REP_TOO_BIG_TO_CACHE on the tenth
operation. Since the server executed several operations, especially
the non-idempotent RENAME, the client's request to cache the reply
needs to be honored in order for the correct operation of exactly
once semantics. If the client retries the request, the server will
have cached a reply that contains results for ten of the eleven
requested operations, with the tenth operation having a status of
NFS4ERR_REP_TOO_BIG_TO_CACHE.
A client needs to take care that, when sending operations that change
the current filehandle (except for PUTFH, PUTPUBFH, PUTROOTFH, and
RESTOREFH), it does not exceed the maximum reply buffer before the
GETFH operation. Otherwise, the client will have to retry the
operation that changed the current filehandle, in order to obtain the
desired filehandle. For the OPEN operation (see
Section 18.16),
retry is not always available as an option. The following guidelines
for the handling of filehandle-changing operations are advised:
* Within the same COMPOUND procedure, a client
SHOULD send GETFH
immediately after a current filehandle-changing operation. A
client
MUST send GETFH after a current filehandle-changing
operation that is also non-idempotent (e.g., the OPEN operation),
unless the operation is RESTOREFH. RESTOREFH is an exception,
because even though it is non-idempotent, the filehandle RESTOREFH
produced originated from an operation that is either idempotent
(e.g., PUTFH, LOOKUP), or non-idempotent (e.g., OPEN, CREATE). If
the origin is non-idempotent, then because the client
MUST send
GETFH after the origin operation, the client can recover if
RESTOREFH returns an error.
* A server
MAY return NFS4ERR_REP_TOO_BIG or
NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a
filehandle-changing operation if the reply would be too large on
the next operation.
* A server
SHOULD return NFS4ERR_REP_TOO_BIG or
NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a
filehandle-changing, non-idempotent operation if the reply would
be too large on the next operation, especially if the operation is
OPEN.
* A server
MAY return NFS4ERR_UNSAFE_COMPOUND to a non-idempotent
current filehandle-changing operation, if it looks at the next
operation (in the same COMPOUND procedure) and finds it is not
GETFH. The server
SHOULD do this if it is unable to determine in
advance whether the total response size would exceed
ca_maxresponsesize_cached or ca_maxresponsesize.
Since the reply cache is bounded, it is practical for the reply cache
to persist across server restarts. The replier
MUST persist the
following information if it agreed to persist the session (when the
session was created; see
Section 18.36):
* The session ID.
* The slot table including the sequence ID and cached reply for each
slot.
The above are sufficient for a replier to provide EOS semantics for
any requests that were sent and executed before the server restarted.
If the replier is a client, then there is no need for it to persist
any more information, unless the client will be persisting all other
state across client restart, in which case, the server will never see
any NFSv4.1-level protocol manifestation of a client restart. If the
replier is a server, with just the slot table and session ID
persisting, any requests the client retries after the server restart
will return the results that are cached in the reply cache, and any
new requests (i.e., the sequence ID is one greater than the slot's
sequence ID)
MUST be rejected with NFS4ERR_DEADSESSION (returned by
SEQUENCE). Such a session is considered dead. A server
MAY re-
animate a session after a server restart so that the session will
accept new requests as well as retries. To re-animate a session, the
server needs to persist additional information through server
restart:
* The client ID. This is a prerequisite to let the client create
more sessions associated with the same client ID as the re-
animated session.
* The client ID's sequence ID that is used for creating sessions
(see Sections
18.35 and
18.36). This is a prerequisite to let the
client create more sessions.
* The principal that created the client ID. This allows the server
to authenticate the client when it sends EXCHANGE_ID.
* The SSV, if SP4_SSV state protection was specified when the client
ID was created (see
Section 18.35). This lets the client create
new sessions, and associate connections with the new and existing
sessions.
* The properties of the client ID as defined in
Section 18.35.
A persistent reply cache places certain demands on the server. The
execution of the sequence of operations (starting with SEQUENCE) and
placement of its results in the persistent cache
MUST be atomic. If
a client retries a sequence of operations that was previously
executed on the server, the only acceptable outcomes are either the
original cached reply or an indication that the client ID or session
has been lost (indicating a catastrophic loss of the reply cache or a
session that has been deleted because the client failed to use the
session for an extended period of time).
A server could fail and restart in the middle of a COMPOUND procedure
that contains one or more non-idempotent or idempotent-but-modifying
operations. This creates an even higher challenge for atomic
execution and placement of results in the reply cache. One way to
view the problem is as a single transaction consisting of each
operation in the COMPOUND followed by storing the result in
persistent storage, then finally a transaction commit. If there is a
failure before the transaction is committed, then the server rolls
back the transaction. If the server itself fails, then when it
restarts, its recovery logic could roll back the transaction before
starting the NFSv4.1 server.
While the description of the implementation for atomic execution of
the request and caching of the reply is beyond the scope of this
document, an example implementation for NFSv2 [45] is described in
[46].
2.10.7. RDMA Considerations
A complete discussion of the operation of RPC-based protocols over
RDMA transports is in [32]. A discussion of the operation of NFSv4,
including NFSv4.1, over RDMA is in [33]. Where RDMA is considered,
this specification assumes the use of such a layering; it addresses
only the upper-layer issues relevant to making best use of RPC/RDMA.
2.10.7.1. RDMA Connection Resources
RDMA requires its consumers to register memory and post buffers of a
specific size and number for receive operations.
Registration of memory can be a relatively high-overhead operation,
since it requires pinning of buffers, assignment of attributes (e.g.,
readable/writable), and initialization of hardware translation.
Preregistration is desirable to reduce overhead. These registrations
are specific to hardware interfaces and even to RDMA connection
endpoints; therefore, negotiation of their limits is desirable to
manage resources effectively.
Following basic registration, these buffers must be posted by the RPC
layer to handle receives. These buffers remain in use by the RPC/
NFSv4.1 implementation; the size and number of them must be known to
the remote peer in order to avoid RDMA errors that would cause a
fatal error on the RDMA connection.
NFSv4.1 manages slots as resources on a per-session basis (see
Section 2.10), while RDMA connections manage credits on a per-
connection basis. This means that in order for a peer to send data
over RDMA to a remote buffer, it has to have both an NFSv4.1 slot and
an RDMA credit. If multiple RDMA connections are associated with a
session, then if the total number of credits across all RDMA
connections associated with the session is X, and the number of slots
in the session is Y, then the maximum number of outstanding requests
is the lesser of X and Y.
Previous versions of NFS do not provide flow control; instead, they
rely on the windowing provided by transports like TCP to throttle
requests. This does not work with RDMA, which provides no operation
flow control and will terminate a connection in error when limits are
exceeded. Limits such as maximum number of requests outstanding are
therefore negotiated when a session is created (see the
ca_maxrequests field in
Section 18.36). These limits then provide
the maxima within which each connection associated with the session's
channel(s) must remain. RDMA connections are managed within these
limits as described in
Section 3.3 of [32]; if there are multiple
RDMA connections, then the maximum number of requests for a channel
will be divided among the RDMA connections. Put a different way, the
onus is on the replier to ensure that the total number of RDMA
credits across all connections associated with the replier's channel
does exceed the channel's maximum number of outstanding requests.
The limits may also be modified dynamically at the replier's choosing
by manipulating certain parameters present in each NFSv4.1 reply. In
addition, the CB_RECALL_SLOT callback operation (see
Section 20.8)
can be sent by a server to a client to return RDMA credits to the
server, thereby lowering the maximum number of requests a client can
have outstanding to the server.
Header padding is requested by each peer at session initiation (see
the ca_headerpadsize argument to CREATE_SESSION in
Section 18.36),
and subsequently used by the RPC RDMA layer, as described in [32].
Zero padding is permitted.
Padding leverages the useful property that RDMA preserve alignment of
data, even when they are placed into anonymous (untagged) buffers.
If requested, client inline writes will insert appropriate pad bytes
within the request header to align the data payload on the specified
boundary. The client is encouraged to add sufficient padding (up to
the negotiated size) so that the "data" field of the WRITE operation
is aligned. Most servers can make good use of such padding, which
allows them to chain receive buffers in such a way that any data
carried by client requests will be placed into appropriate buffers at
the server, ready for file system processing. The receiver's RPC
layer encounters no overhead from skipping over pad bytes, and the
RDMA layer's high performance makes the insertion and transmission of
padding on the sender a significant optimization. In this way, the
need for servers to perform RDMA Read to satisfy all but the largest
client writes is obviated. An added benefit is the reduction of
message round trips on the network -- a potentially good trade, where
latency is present.
The value to choose for padding is subject to a number of criteria.
A primary source of variable-length data in the RPC header is the
authentication information, the form of which is client-determined,
possibly in response to server specification. The contents of
COMPOUNDs, sizes of strings such as those passed to RENAME, etc. all
go into the determination of a maximal NFSv4.1 request size and
therefore minimal buffer size. The client must select its offered
value carefully, so as to avoid overburdening the server, and vice
versa. The benefit of an appropriate padding value is higher
performance.
Sender gather:
|RPC Request|Pad bytes|Length| -> |User data...|
\------+----------------------/ \
\ \
\ Receiver scatter: \-----------+- ...
/-----+----------------\ \ \
|RPC Request|Pad|Length| -> |FS buffer|->|FS buffer|->...
In the above case, the server may recycle unused buffers to the next
posted receive if unused by the actual received request, or may pass
the now-complete buffers by reference for normal write processing.
For a server that can make use of it, this removes any need for data
copies of incoming data, without resorting to complicated end-to-end
buffer advertisement and management. This includes most kernel-based
and integrated server designs, among many others. The client may
perform similar optimizations, if desired.
2.10.7.4. Dual RDMA and Non-RDMA Transports
Some RDMA transports (e.g.,
RFC 5040 [8]) permit a "streaming" (non-
RDMA) phase, where ordinary traffic might flow before "stepping up"
to RDMA mode, commencing RDMA traffic. Some RDMA transports start
connections always in RDMA mode. NFSv4.1 allows, but does not
assume, a streaming phase before RDMA mode. When a connection is
associated with a session, the client and server negotiate whether
the connection is used in RDMA or non-RDMA mode (see Sections
18.36 and
18.34).
2.10.8. Session Security
2.10.8.1. Session Callback Security
Via session/connection association, NFSv4.1 improves security over
that provided by NFSv4.0 for the backchannel. The connection is
client-initiated (see
Section 18.34) and subject to the same firewall
and routing checks as the fore channel. At the client's option (see
Section 18.35), connection association is fully authenticated before
being activated (see
Section 18.34). Traffic from the server over
the backchannel is authenticated exactly as the client specifies (see
Section 2.10.8.2).
2.10.8.2. Backchannel RPC Security
When the NFSv4.1 client establishes the backchannel, it informs the
server of the security flavors and principals to use when sending
requests. If the security flavor is RPCSEC_GSS, the client expresses
the principal in the form of an established RPCSEC_GSS context. The
server is free to use any of the flavor/principal combinations the
client offers, but it
MUST NOT use unoffered combinations. This way,
the client need not provide a target GSS principal for the
backchannel as it did with NFSv4.0, nor does the server have to
implement an RPCSEC_GSS initiator as it did with NFSv4.0 [37].
The CREATE_SESSION (
Section 18.36) and BACKCHANNEL_CTL
(
Section 18.33) operations allow the client to specify flavor/
principal combinations.
Also note that the SP4_SSV state protection mode (see Sections
18.35 and
2.10.8.3) has the side benefit of providing SSV-derived
RPCSEC_GSS contexts (
Section 2.10.9).
2.10.8.3. Protection from Unauthorized State Changes
As described to this point in the specification, the state model of
NFSv4.1 is vulnerable to an attacker that sends a SEQUENCE operation
with a forged session ID and with a slot ID that it expects the
legitimate client to use next. When the legitimate client uses the
slot ID with the same sequence number, the server returns the
attacker's result from the reply cache, which disrupts the legitimate
client and thus denies service to it. Similarly, an attacker could
send a CREATE_SESSION with a forged client ID to create a new session
associated with the client ID. The attacker could send requests
using the new session that change locking state, such as LOCKU
operations to release locks the legitimate client has acquired.
Setting a security policy on the file that requires RPCSEC_GSS
credentials when manipulating the file's state is one potential work
around, but has the disadvantage of preventing a legitimate client
from releasing state when RPCSEC_GSS is required to do so, but a GSS
context cannot be obtained (possibly because the user has logged off
the client).
NFSv4.1 provides three options to a client for state protection,
which are specified when a client creates a client ID via EXCHANGE_ID
(
Section 18.35).
The first (SP4_NONE) is to simply waive state protection.
The other two options (SP4_MACH_CRED and SP4_SSV) share several
traits:
* An RPCSEC_GSS-based credential is used to authenticate client ID
and session maintenance operations, including creating and
destroying a session, associating a connection with the session,
and destroying the client ID.
* Because RPCSEC_GSS is used to authenticate client ID and session
maintenance, the attacker cannot associate a rogue connection with
a legitimate session, or associate a rogue session with a
legitimate client ID in order to maliciously alter the client ID's
lock state via CLOSE, LOCKU, DELEGRETURN, LAYOUTRETURN, etc.
* In cases where the server's security policies on a portion of its
namespace require RPCSEC_GSS authentication, a client may have to
use an RPCSEC_GSS credential to remove per-file state (e.g.,
LOCKU, CLOSE, etc.). The server may require that the principal
that removes the state match certain criteria (e.g., the principal
might have to be the same as the one that acquired the state).
However, the client might not have an RPCSEC_GSS context for such
a principal, and might not be able to create such a context
(perhaps because the user has logged off). When the client
establishes SP4_MACH_CRED or SP4_SSV protection, it can specify a
list of operations that the server
MUST allow using the machine
credential (if SP4_MACH_CRED is used) or the SSV credential (if
SP4_SSV is used).
The SP4_MACH_CRED state protection option uses a machine credential
where the principal that creates the client ID
MUST also be the
principal that performs client ID and session maintenance operations.
The security of the machine credential state protection approach
depends entirely on safeguarding the per-machine credential.
Assuming a proper safeguard using the per-machine credential for
operations like CREATE_SESSION, BIND_CONN_TO_SESSION,
DESTROY_SESSION, and DESTROY_CLIENTID will prevent an attacker from
associating a rogue connection with a session, or associating a rogue
session with a client ID.
There are at least three scenarios for the SP4_MACH_CRED option:
1. The system administrator configures a unique, permanent per-
machine credential for one of the mandated GSS mechanisms (e.g.,
if Kerberos V5 is used, a "keytab" containing a principal derived
from a client host name could be used).
2. The client is used by a single user, and so the client ID and its
sessions are used by just that user. If the user's credential
expires, then session and client ID maintenance cannot occur, but
since the client has a single user, only that user is
inconvenienced.
3. The physical client has multiple users, but the client
implementation has a unique client ID for each user. This is
effectively the same as the second scenario, but a disadvantage
is that each user needs to be allocated at least one session
each, so the approach suffers from lack of economy.
The SP4_SSV protection option uses the SSV (
Section 1.7), via
RPCSEC_GSS and the SSV GSS mechanism (
Section 2.10.9), to protect
state from attack. The SP4_SSV protection option is intended for the
situation comprised of a client that has multiple active users and a
system administrator who wants to avoid the burden of installing a
permanent machine credential on each client. The SSV is established
and updated on the server via SET_SSV (see
Section 18.47). To
prevent eavesdropping, a client
SHOULD send SET_SSV via RPCSEC_GSS
with the privacy service. Several aspects of the SSV make it
intractable for an attacker to guess the SSV, and thus associate
rogue connections with a session, and rogue sessions with a client
ID:
* The arguments to and results of SET_SSV include digests of the old
and new SSV, respectively.
* Because the initial value of the SSV is zero, therefore known, the
client that opts for SP4_SSV protection and opts to apply SP4_SSV
protection to BIND_CONN_TO_SESSION and CREATE_SESSION
MUST send at
least one SET_SSV operation before the first BIND_CONN_TO_SESSION
operation or before the second CREATE_SESSION operation on a
client ID. If it does not, the SSV mechanism will not generate
tokens (
Section 2.10.9). A client
SHOULD send SET_SSV as soon as
a session is created.
* A SET_SSV request does not replace the SSV with the argument to
SET_SSV. Instead, the current SSV on the server is logically
exclusive ORed (XORed) with the argument to SET_SSV. Each time a
new principal uses a client ID for the first time, the client
SHOULD send a SET_SSV with that principal's RPCSEC_GSS
credentials, with RPCSEC_GSS service set to RPC_GSS_SVC_PRIVACY.
Here are the types of attacks that can be attempted by an attacker
named Eve on a victim named Bob, and how SP4_SSV protection foils
each attack:
* Suppose Eve is the first user to log into a legitimate client.
Eve's use of an NFSv4.1 file system will cause the legitimate
client to create a client ID with SP4_SSV protection, specifying
that the BIND_CONN_TO_SESSION operation
MUST use the SSV
credential. Eve's use of the file system also causes an SSV to be
created. The SET_SSV operation that creates the SSV will be
protected by the RPCSEC_GSS context created by the legitimate
client, which uses Eve's GSS principal and credentials. Eve can
eavesdrop on the network while her RPCSEC_GSS context is created
and the SET_SSV using her context is sent. Even if the legitimate
client sends the SET_SSV with RPC_GSS_SVC_PRIVACY, because Eve
knows her own credentials, she can decrypt the SSV. Eve can
compute an RPCSEC_GSS credential that BIND_CONN_TO_SESSION will
accept, and so associate a new connection with the legitimate
session. Eve can change the slot ID and sequence state of a
legitimate session, and/or the SSV state, in such a way that when
Bob accesses the server via the same legitimate client, the
legitimate client will be unable to use the session.
The client's only recourse is to create a new client ID for Bob to
use, and establish a new SSV for the client ID. The client will
be unable to delete the old client ID, and will let the lease on
the old client ID expire.
Once the legitimate client establishes an SSV over the new session
using Bob's RPCSEC_GSS context, Eve can use the new session via
the legitimate client, but she cannot disrupt Bob. Moreover,
because the client
SHOULD have modified the SSV due to Eve using
the new session, Bob cannot get revenge on Eve by associating a
rogue connection with the session.
The question is how did the legitimate client detect that Eve has
hijacked the old session? When the client detects that a new
principal, Bob, wants to use the session, it
SHOULD have sent a
SET_SSV, which leads to the following sub-scenarios:
- Let us suppose that from the rogue connection, Eve sent a
SET_SSV with the same slot ID and sequence ID that the
legitimate client later uses. The server will assume the
SET_SSV sent with Bob's credentials is a retry, and return to
the legitimate client the reply it sent Eve. However, unless
Eve can correctly guess the SSV the legitimate client will use,
the digest verification checks in the SET_SSV response will
fail. That is an indication to the client that the session has
apparently been hijacked.
- Alternatively, Eve sent a SET_SSV with a different slot ID than
the legitimate client uses for its SET_SSV. Then the digest
verification of the SET_SSV sent with Bob's credentials fails
on the server, and the error returned to the client makes it
apparent that the session has been hijacked.
- Alternatively, Eve sent an operation other than SET_SSV, but
with the same slot ID and sequence that the legitimate client
uses for its SET_SSV. The server returns to the legitimate
client the response it sent Eve. The client sees that the
response is not at all what it expects. The client assumes
either session hijacking or a server bug, and either way
destroys the old session.
* Eve associates a rogue connection with the session as above, and
then destroys the session. Again, Bob goes to use the server from
the legitimate client, which sends a SET_SSV using Bob's
credentials. The client receives an error that indicates that the
session does not exist. When the client tries to create a new
session, this will fail because the SSV it has does not match that
which the server has, and now the client knows the session was
hijacked. The legitimate client establishes a new client ID.
* If Eve creates a connection before the legitimate client
establishes an SSV, because the initial value of the SSV is zero
and therefore known, Eve can send a SET_SSV that will pass the
digest verification check. However, because the new connection
has not been associated with the session, the SET_SSV is rejected
for that reason.
In summary, an attacker's disruption of state when SP4_SSV protection
is in use is limited to the formative period of a client ID, its
first session, and the establishment of the SSV. Once a non-
malicious user uses the client ID, the client quickly detects any
hijack and rectifies the situation. Once a non-malicious user
successfully modifies the SSV, the attacker cannot use NFSv4.1
operations to disrupt the non-malicious user.
Note that neither the SP4_MACH_CRED nor SP4_SSV protection approaches
prevent hijacking of a transport connection that has previously been
associated with a session. If the goal of a counter-threat strategy
is to prevent connection hijacking, the use of IPsec is
RECOMMENDED.
If a connection hijack occurs, the hijacker could in theory change
locking state and negatively impact the service to legitimate
clients. However, if the server is configured to require the use of
RPCSEC_GSS with integrity or privacy on the affected file objects,
and if EXCHGID4_FLAG_BIND_PRINC_STATEID capability (
Section 18.35) is
in force, this will thwart unauthorized attempts to change locking
state.
2.10.9. The Secret State Verifier (SSV) GSS Mechanism
The SSV provides the secret key for a GSS mechanism internal to
NFSv4.1 that NFSv4.1 uses for state protection. Contexts for this
mechanism are not established via the RPCSEC_GSS protocol. Instead,
the contexts are automatically created when EXCHANGE_ID specifies
SP4_SSV protection. The only tokens defined are the PerMsgToken
(emitted by GSS_GetMIC) and the SealedMessage token (emitted by
GSS_Wrap).
The mechanism OID for the SSV mechanism is
iso.org.dod.internet.private.enterprise.Michael Eisler.nfs.ssv_mech
(1.3.6.1.4.1.28882.1.1). While the SSV mechanism does not define any
initial context tokens, the OID can be used to let servers indicate
that the SSV mechanism is acceptable whenever the client sends a
SECINFO or SECINFO_NO_NAME operation (see
Section 2.6).
The SSV mechanism defines four subkeys derived from the SSV value.
Each time SET_SSV is invoked, the subkeys are recalculated by the
client and server. The calculation of each of the four subkeys
depends on each of the four respective ssv_subkey4 enumerated values.
The calculation uses the HMAC [52] algorithm, using the current SSV
as the key, the one-way hash algorithm as negotiated by EXCHANGE_ID,
and the input text as represented by the XDR encoded enumeration
value for that subkey of data type ssv_subkey4. If the length of the
output of the HMAC algorithm exceeds the length of key of the
encryption algorithm (which is also negotiated by EXCHANGE_ID), then
the subkey
MUST be truncated from the HMAC output, i.e., if the
subkey is of N bytes long, then the first N bytes of the HMAC output
MUST be used for the subkey. The specification of EXCHANGE_ID states
that the length of the output of the HMAC algorithm
MUST NOT be less
than the length of subkey needed for the encryption algorithm (see
Section 18.35).
/* Input for computing subkeys */
enum ssv_subkey4 {
SSV4_SUBKEY_MIC_I2T = 1,
SSV4_SUBKEY_MIC_T2I = 2,
SSV4_SUBKEY_SEAL_I2T = 3,
SSV4_SUBKEY_SEAL_T2I = 4
};
The subkey derived from SSV4_SUBKEY_MIC_I2T is used for calculating
message integrity codes (MICs) that originate from the NFSv4.1
client, whether as part of a request over the fore channel or a
response over the backchannel. The subkey derived from
SSV4_SUBKEY_MIC_T2I is used for MICs originating from the NFSv4.1
server. The subkey derived from SSV4_SUBKEY_SEAL_I2T is used for
encryption text originating from the NFSv4.1 client, and the subkey
derived from SSV4_SUBKEY_SEAL_T2I is used for encryption text
originating from the NFSv4.1 server.
The PerMsgToken description is based on an XDR definition:
/* Input for computing smt_hmac */
struct ssv_mic_plain_tkn4 {
uint32_t smpt_ssv_seq;
opaque smpt_orig_plain<>;
};
/* SSV GSS PerMsgToken token */
struct ssv_mic_tkn4 {
uint32_t smt_ssv_seq;
opaque smt_hmac<>;
};
The field smt_hmac is an HMAC calculated by using the subkey derived
from SSV4_SUBKEY_MIC_I2T or SSV4_SUBKEY_MIC_T2I as the key, the one-
way hash algorithm as negotiated by EXCHANGE_ID, and the input text
as represented by data of type ssv_mic_plain_tkn4. The field
smpt_ssv_seq is the same as smt_ssv_seq. The field smpt_orig_plain
is the "message" input passed to GSS_GetMIC() (see Section 2.3.1 of
[7]). The caller of GSS_GetMIC() provides a pointer to a buffer
containing the plain text. The SSV mechanism's entry point for
GSS_GetMIC() encodes this into an opaque array, and the encoding will
include an initial four-byte length, plus any necessary padding.
Prepended to this will be the XDR encoded value of smpt_ssv_seq, thus
making up an XDR encoding of a value of data type ssv_mic_plain_tkn4,
which in turn is the input into the HMAC.
The token emitted by GSS_GetMIC() is XDR encoded and of XDR data type
ssv_mic_tkn4. The field smt_ssv_seq comes from the SSV sequence
number, which is equal to one after SET_SSV (
Section 18.47) is called
the first time on a client ID. Thereafter, the SSV sequence number
is incremented on each SET_SSV. Thus, smt_ssv_seq represents the
version of the SSV at the time GSS_GetMIC() was called. As noted in
Section 18.35, the client and server can maintain multiple concurrent
versions of the SSV. This allows the SSV to be changed without
serializing all RPC calls that use the SSV mechanism with SET_SSV
operations. Once the HMAC is calculated, it is XDR encoded into
smt_hmac, which will include an initial four-byte length, and any
necessary padding. Prepended to this will be the XDR encoded value
of smt_ssv_seq.
The SealedMessage description is based on an XDR definition:
/* Input for computing ssct_encr_data and ssct_hmac */
struct ssv_seal_plain_tkn4 {
opaque sspt_confounder<>;
uint32_t sspt_ssv_seq;
opaque sspt_orig_plain<>;
opaque sspt_pad<>;
};
/* SSV GSS SealedMessage token */
struct ssv_seal_cipher_tkn4 {
uint32_t ssct_ssv_seq;
opaque ssct_iv<>;
opaque ssct_encr_data<>;
opaque ssct_hmac<>;
};
The token emitted by GSS_Wrap() is XDR encoded and of XDR data type
ssv_seal_cipher_tkn4.
The ssct_ssv_seq field has the same meaning as smt_ssv_seq.
The ssct_encr_data field is the result of encrypting a value of the
XDR encoded data type ssv_seal_plain_tkn4. The encryption key is the
subkey derived from SSV4_SUBKEY_SEAL_I2T or SSV4_SUBKEY_SEAL_T2I, and
the encryption algorithm is that negotiated by EXCHANGE_ID.
The ssct_iv field is the initialization vector (IV) for the
encryption algorithm (if applicable) and is sent in clear text. The
content and size of the IV
MUST comply with the specification of the
encryption algorithm. For example, the id-aes256-CBC algorithm
MUST use a 16-byte initialization vector (IV), which
MUST be unpredictable
for each instance of a value of data type ssv_seal_plain_tkn4 that is
encrypted with a particular SSV key.
The ssct_hmac field is the result of computing an HMAC using the
value of the XDR encoded data type ssv_seal_plain_tkn4 as the input
text. The key is the subkey derived from SSV4_SUBKEY_MIC_I2T or
SSV4_SUBKEY_MIC_T2I, and the one-way hash algorithm is that
negotiated by EXCHANGE_ID.
The sspt_confounder field is a random value.
The sspt_ssv_seq field is the same as ssvt_ssv_seq.
The field sspt_orig_plain field is the original plaintext and is the
"input_message" input passed to GSS_Wrap() (see Section 2.3.3 of
[7]). As with the handling of the plaintext by the SSV mechanism's
GSS_GetMIC() entry point, the entry point for GSS_Wrap() expects a
pointer to the plaintext, and will XDR encode an opaque array into
sspt_orig_plain representing the plain text, along with the other
fields of an instance of data type ssv_seal_plain_tkn4.
The sspt_pad field is present to support encryption algorithms that
require inputs to be in fixed-sized blocks. The content of sspt_pad
is zero filled except for the length. Beware that the XDR encoding
of ssv_seal_plain_tkn4 contains three variable-length arrays, and so
each array consumes four bytes for an array length, and each array
that follows the length is always padded to a multiple of four bytes
per the XDR standard.
For example, suppose the encryption algorithm uses 16-byte blocks,
and the sspt_confounder is three bytes long, and the sspt_orig_plain
field is 15 bytes long. The XDR encoding of sspt_confounder uses
eight bytes (4 + 3 + 1-byte pad), the XDR encoding of sspt_ssv_seq
uses four bytes, the XDR encoding of sspt_orig_plain uses 20 bytes (4
+ 15 + 1-byte pad), and the smallest XDR encoding of the sspt_pad
field is four bytes. This totals 36 bytes. The next multiple of 16
is 48; thus, the length field of sspt_pad needs to be set to 12
bytes, or a total encoding of 16 bytes. The total number of XDR
encoded bytes is thus 8 + 4 + 20 + 16 = 48.
GSS_Wrap() emits a token that is an XDR encoding of a value of data
type ssv_seal_cipher_tkn4. Note that regardless of whether or not
the caller of GSS_Wrap() requests confidentiality, the token always
has confidentiality. This is because the SSV mechanism is for
RPCSEC_GSS, and RPCSEC_GSS never produces GSS_wrap() tokens without
confidentiality.
There is one SSV per client ID. There is a single GSS context for a
client ID / SSV pair. All SSV mechanism RPCSEC_GSS handles of a
client ID / SSV pair share the same GSS context. SSV GSS contexts do
not expire except when the SSV is destroyed (causes would include the
client ID being destroyed or a server restart). Since one purpose of
context expiration is to replace keys that have been in use for "too
long", hence vulnerable to compromise by brute force or accident, the
client can replace the SSV key by sending periodic SET_SSV
operations, which is done by cycling through different users'
RPCSEC_GSS credentials. This way, the SSV is replaced without
destroying the SSV's GSS contexts.
SSV RPCSEC_GSS handles can be expired or deleted by the server at any
time, and the EXCHANGE_ID operation can be used to create more SSV
RPCSEC_GSS handles. Expiration of SSV RPCSEC_GSS handles does not
imply that the SSV or its GSS context has expired.
The client
MUST establish an SSV via SET_SSV before the SSV GSS
context can be used to emit tokens from GSS_Wrap() and GSS_GetMIC().
If SET_SSV has not been successfully called, attempts to emit tokens
MUST fail.
The SSV mechanism does not support replay detection and sequencing in
its tokens because RPCSEC_GSS does not use those features (see
"Context Creation Requests", Section 5.2.2 of [4]). However,
Section 2.10.10 discusses special considerations for the SSV
mechanism when used with RPCSEC_GSS.
2.10.10. Security Considerations for RPCSEC_GSS When Using the SSV
Mechanism
When a client ID is created with SP4_SSV state protection (see
Section 18.35), the client is permitted to associate multiple
RPCSEC_GSS handles with the single SSV GSS context (see
Section 2.10.9). Because of the way RPCSEC_GSS (both version 1 and
version 2, see [4] and [9]) calculate the verifier of the reply,
special care must be taken by the implementation of the NFSv4.1
client to prevent attacks by a man-in-the-middle. The verifier of an
RPCSEC_GSS reply is the output of GSS_GetMIC() applied to the input
value of the seq_num field of the RPCSEC_GSS credential (data type
rpc_gss_cred_ver_1_t) (see Section 5.3.3.2 of [4]). If multiple
RPCSEC_GSS handles share the same GSS context, then if one handle is
used to send a request with the same seq_num value as another handle,
an attacker could block the reply, and replace it with the verifier
used for the other handle.
There are multiple ways to prevent the attack on the SSV RPCSEC_GSS
verifier in the reply. The simplest is believed to be as follows.
* Each time one or more new SSV RPCSEC_GSS handles are created via
EXCHANGE_ID, the client
SHOULD send a SET_SSV operation to modify
the SSV. By changing the SSV, the new handles will not result in
the re-use of an SSV RPCSEC_GSS verifier in a reply.
* When a requester decides to use N SSV RPCSEC_GSS handles, it
SHOULD assign a unique and non-overlapping range of seq_nums to
each SSV RPCSEC_GSS handle. The size of each range
SHOULD be
equal to MAXSEQ / N (see
Section 5 of [4] for the definition of
MAXSEQ). When an SSV RPCSEC_GSS handle reaches its maximum, it
SHOULD force the replier to destroy the handle by sending a NULL
RPC request with seq_num set to MAXSEQ + 1 (see Section 5.3.3.3 of
[4]).
* When the requester wants to increase or decrease N, it
SHOULD force the replier to destroy all N handles by sending a NULL RPC
request on each handle with seq_num set to MAXSEQ + 1. If the
requester is the client, it
SHOULD send a SET_SSV operation before
using new handles. If the requester is the server, then the
client
SHOULD send a SET_SSV operation when it detects that the
server has forced it to destroy a backchannel's SSV RPCSEC_GSS
handle. By sending a SET_SSV operation, the SSV will change, and
so the attacker will be unavailable to successfully replay a
previous verifier in a reply to the requester.
Note that if the replier carefully creates the SSV RPCSEC_GSS
handles, the related risk of a man-in-the-middle splicing a forged
SSV RPCSEC_GSS credential with a verifier for another handle does not
exist. This is because the verifier in an RPCSEC_GSS request is
computed from input that includes both the RPCSEC_GSS handle and
seq_num (see Section 5.3.1 of [4]). Provided the replier takes care
to avoid re-using the value of an RPCSEC_GSS handle that it creates,
such as by including a generation number in the handle, the man-in-
the-middle will not be able to successfully replay a previous
verifier in the request to a replier.
2.10.11. Session Mechanics - Steady State
2.10.11.1. Obligations of the Server
The server has the primary obligation to monitor the state of
backchannel resources that the client has created for the server
(RPCSEC_GSS contexts and backchannel connections). If these
resources vanish, the server takes action as specified in
Section 2.10.13.2.
2.10.11.2. Obligations of the Client
The client
SHOULD honor the following obligations in order to utilize
the session:
* Keep a necessary session from going idle on the server. A client
that requires a session but nonetheless is not sending operations
risks having the session be destroyed by the server. This is
because sessions consume resources, and resource limitations may
force the server to cull an inactive session. A server
MAY consider a session to be inactive if the client has not used the
session before the session inactivity timer (
Section 2.10.12) has
expired.
* Destroy the session when not needed. If a client has multiple
sessions, one of which has no requests waiting for replies, and
has been idle for some period of time, it
SHOULD destroy the
session.
* Maintain GSS contexts and RPCSEC_GSS handles for the backchannel.
If the client requires the server to use the RPCSEC_GSS security
flavor for callbacks, then it needs to be sure the RPCSEC_GSS
handles and/or their GSS contexts that are handed to the server
via BACKCHANNEL_CTL or CREATE_SESSION are unexpired.
* Preserve a connection for a backchannel. The server requires a
backchannel in order to gracefully recall recallable state or
notify the client of certain events. Note that if the connection
is not being used for the fore channel, there is no way for the
client to tell if the connection is still alive (e.g., the server
restarted without sending a disconnect). The onus is on the
server, not the client, to determine if the backchannel's
connection is alive, and to indicate in the response to a SEQUENCE
operation when the last connection associated with a session's
backchannel has disconnected.
2.10.11.3. Steps the Client Takes to Establish a Session
If the client does not have a client ID, the client sends EXCHANGE_ID
to establish a client ID. If it opts for SP4_MACH_CRED or SP4_SSV
protection, in the spo_must_enforce list of operations, it
SHOULD at
minimum specify CREATE_SESSION, DESTROY_SESSION,
BIND_CONN_TO_SESSION, BACKCHANNEL_CTL, and DESTROY_CLIENTID. If it
opts for SP4_SSV protection, the client needs to ask for SSV-based
RPCSEC_GSS handles.
The client uses the client ID to send a CREATE_SESSION on a
connection to the server. The results of CREATE_SESSION indicate
whether or not the server will persist the session reply cache
through a server that has restarted, and the client notes this for
future reference.
If the client specified SP4_SSV state protection when the client ID
was created, then it
SHOULD send SET_SSV in the first COMPOUND after
the session is created. Each time a new principal goes to use the
client ID, it
SHOULD send a SET_SSV again.
If the client wants to use delegations, layouts, directory
notifications, or any other state that requires a backchannel, then
it needs to add a connection to the backchannel if CREATE_SESSION did
not already do so. The client creates a connection, and calls
BIND_CONN_TO_SESSION to associate the connection with the session and
the session's backchannel. If CREATE_SESSION did not already do so,
the client
MUST tell the server what security is required in order
for the client to accept callbacks. The client does this via
BACKCHANNEL_CTL. If the client selected SP4_MACH_CRED or SP4_SSV
protection when it called EXCHANGE_ID, then the client
SHOULD specify
that the backchannel use RPCSEC_GSS contexts for security.
If the client wants to use additional connections for the
backchannel, then it needs to call BIND_CONN_TO_SESSION on each
connection it wants to use with the session. If the client wants to
use additional connections for the fore channel, then it needs to
call BIND_CONN_TO_SESSION if it specified SP4_SSV or SP4_MACH_CRED
state protection when the client ID was created.
At this point, the session has reached steady state.
2.10.12. Session Inactivity Timer
The server
MAY maintain a session inactivity timer for each session.
If the session inactivity timer expires, then the server
MAY destroy
the session. To avoid losing a session due to inactivity, the client
MUST renew the session inactivity timer. The length of session
inactivity timer
MUST NOT be less than the lease_time attribute
(
Section 5.8.1.11). As with lease renewal (
Section 8.3), when the
server receives a SEQUENCE operation, it resets the session
inactivity timer, and
MUST NOT allow the timer to expire while the
rest of the operations in the COMPOUND procedure's request are still
executing. Once the last operation has finished, the server
MUST set
the session inactivity timer to expire no sooner than the sum of the
current time and the value of the lease_time attribute.
2.10.13. Session Mechanics - Recovery
2.10.13.1. Events Requiring Client Action
The following events require client action to recover.
2.10.13.1.1. RPCSEC_GSS Context Loss by Callback Path
If all RPCSEC_GSS handles granted by the client to the server for
callback use have expired, the client
MUST establish a new handle via
BACKCHANNEL_CTL. The sr_status_flags field of the SEQUENCE results
indicates when callback handles are nearly expired, or fully expired
(see
Section 18.46.3).
If the client loses the last connection of the session and wants to
retain the session, then it needs to create a new connection, and if,
when the client ID was created, BIND_CONN_TO_SESSION was specified in
the spo_must_enforce list, the client
MUST use BIND_CONN_TO_SESSION
to associate the connection with the session.
If there was a request outstanding at the time of connection loss,
then if the client wants to continue to use the session, it
MUST retry the request, as described in
Section 2.10.6.2. Note that it is
not necessary to retry requests over a connection with the same
source network address or the same destination network address as the
lost connection. As long as the session ID, slot ID, and sequence ID
in the retry match that of the original request, the server will
recognize the request as a retry if it executed the request prior to
disconnect.
If the connection that was lost was the last one associated with the
backchannel, and the client wants to retain the backchannel and/or
prevent revocation of recallable state, the client needs to
reconnect, and if it does, it
MUST associate the connection to the
session and backchannel via BIND_CONN_TO_SESSION. The server
SHOULD indicate when it has no callback connection via the sr_status_flags
result from SEQUENCE.
2.10.13.1.3. Backchannel GSS Context Loss
Via the sr_status_flags result of the SEQUENCE operation or other
means, the client will learn if some or all of the RPCSEC_GSS
contexts it assigned to the backchannel have been lost. If the
client wants to retain the backchannel and/or not put recallable
state subject to revocation, the client needs to use BACKCHANNEL_CTL
to assign new contexts.
The replier might lose a record of the session. Causes include:
* Replier failure and restart.
* A catastrophe that causes the reply cache to be corrupted or lost
on the media on which it was stored. This applies even if the
replier indicated in the CREATE_SESSION results that it would
persist the cache.
* The server purges the session of a client that has been inactive
for a very extended period of time.
* As a result of configuration changes among a set of clustered
servers, a network address previously connected to one server
becomes connected to a different server that has no knowledge of
the session in question. Such a configuration change will
generally only happen when the original server ceases to function
for a time.
Loss of reply cache is equivalent to loss of session. The replier
indicates loss of session to the requester by returning
NFS4ERR_BADSESSION on the next operation that uses the session ID
that refers to the lost session.
After an event like a server restart, the client may have lost its
connections. The client assumes for the moment that the session has
not been lost. It reconnects, and if it specified connection
association enforcement when the session was created, it invokes
BIND_CONN_TO_SESSION using the session ID. Otherwise, it invokes
SEQUENCE. If BIND_CONN_TO_SESSION or SEQUENCE returns
NFS4ERR_BADSESSION, the client knows the session is not available to
it when communicating with that network address. If the connection
survives session loss, then the next SEQUENCE operation the client
sends over the connection will get back NFS4ERR_BADSESSION. The
client again knows the session was lost.
Here is one suggested algorithm for the client when it gets
NFS4ERR_BADSESSION. It is not obligatory in that, if a client does
not want to take advantage of such features as trunking, it may omit
parts of it. However, it is a useful example that draws attention to
various possible recovery issues:
1. If the client has other connections to other server network
addresses associated with the same session, attempt a COMPOUND
with a single operation, SEQUENCE, on each of the other
connections.
2. If the attempts succeed, the session is still alive, and this is
a strong indicator that the server's network address has moved.
The client might send an EXCHANGE_ID on the connection that
returned NFS4ERR_BADSESSION to see if there are opportunities for
client ID trunking (i.e., the same client ID and so_major_id
value are returned). The client might use DNS to see if the
moved network address was replaced with another, so that the
performance and availability benefits of session trunking can
continue.
3. If the SEQUENCE requests fail with NFS4ERR_BADSESSION, then the
session no longer exists on any of the server network addresses
for which the client has connections associated with that session
ID. It is possible the session is still alive and available on
other network addresses. The client sends an EXCHANGE_ID on all
the connections to see if the server owner is still listening on
those network addresses. If the same server owner is returned
but a new client ID is returned, this is a strong indicator of a
server restart. If both the same server owner and same client ID
are returned, then this is a strong indication that the server
did delete the session, and the client will need to send a
CREATE_SESSION if it has no other sessions for that client ID.
If a different server owner is returned, the client can use DNS
to find other network addresses. If it does not, or if DNS does
not find any other addresses for the server, then the client will
be unable to provide NFSv4.1 service, and fatal errors should be
returned to processes that were using the server. If the client
is using a "mount" paradigm, unmounting the server is advised.
4. If the client knows of no other connections associated with the
session ID and server network addresses that are, or have been,
associated with the session ID, then the client can use DNS to
find other network addresses. If it does not, or if DNS does not
find any other addresses for the server, then the client will be
unable to provide NFSv4.1 service, and fatal errors should be
returned to processes that were using the server. If the client
is using a "mount" paradigm, unmounting the server is advised.
If there is a reconfiguration event that results in the same network
address being assigned to servers where the eir_server_scope value is
different, it cannot be guaranteed that a session ID generated by the
first will be recognized as invalid by the first. Therefore, in
managing server reconfigurations among servers with different server
scope values, it is necessary to make sure that all clients have
disconnected from the first server before effecting the
reconfiguration. Nonetheless, clients should not assume that servers
will always adhere to this requirement; clients
MUST be prepared to
deal with unexpected effects of server reconfigurations. Even where
a session ID is inappropriately recognized as valid, it is likely
either that the connection will not be recognized as valid or that a
sequence value for a slot will not be correct. Therefore, when a
client receives results indicating such unexpected errors, the use of
EXCHANGE_ID to determine the current server configuration is
RECOMMENDED.
A variation on the above is that after a server's network address
moves, there is no NFSv4.1 server listening, e.g., no listener on
port 2049. In this example, one of the following occur: the NFSv4
server returns NFS4ERR_MINOR_VERS_MISMATCH, the NFS server returns a
PROG_MISMATCH error, the RPC listener on 2049 returns PROG_UNVAIL, or
attempts to reconnect to the network address timeout. These
SHOULD be treated as equivalent to SEQUENCE returning NFS4ERR_BADSESSION for
these purposes.
When the client detects session loss, it needs to call CREATE_SESSION
to recover. Any non-idempotent operations that were in progress
might have been performed on the server at the time of session loss.
The client has no general way to recover from this.
Note that loss of session does not imply loss of byte-range lock,
open, delegation, or layout state because locks, opens, delegations,
and layouts are tied to the client ID and depend on the client ID,
not the session. Nor does loss of byte-range lock, open, delegation,
or layout state imply loss of session state, because the session
depends on the client ID; loss of client ID however does imply loss
of session, byte-range lock, open, delegation, and layout state. See
Section 8.4.2. A session can survive a server restart, but lock
recovery may still be needed.
It is possible that CREATE_SESSION will fail with
NFS4ERR_STALE_CLIENTID (e.g., the server restarts and does not
preserve client ID state). If so, the client needs to call
EXCHANGE_ID, followed by CREATE_SESSION.
2.10.13.2. Events Requiring Server Action
The following events require server action to recover.
2.10.13.2.1. Client Crash and Restart
As described in
Section 18.35, a restarted client sends EXCHANGE_ID
in such a way that it causes the server to delete any sessions it
had.
2.10.13.2.2. Client Crash with No Restart
If a client crashes and never comes back, it will never send
EXCHANGE_ID with its old client owner. Thus, the server has session
state that will never be used again. After an extended period of
time, and if the server has resource constraints, it
MAY destroy the
old session as well as locking state.
2.10.13.2.3. Extended Network Partition
To the server, the extended network partition may be no different
from a client crash with no restart (see
Section 2.10.13.2.2).
Unless the server can discern that there is a network partition, it
is free to treat the situation as if the client has crashed
permanently.
2.10.13.2.4. Backchannel Connection Loss
If there were callback requests outstanding at the time of a
connection loss, then the server
MUST retry the requests, as
described in
Section 2.10.6.2. Note that it is not necessary to
retry requests over a connection with the same source network address
or the same destination network address as the lost connection. As
long as the session ID, slot ID, and sequence ID in the retry match
that of the original request, the callback target will recognize the
request as a retry even if it did see the request prior to
disconnect.
If the connection lost is the last one associated with the
backchannel, then the server
MUST indicate that in the
sr_status_flags field of every SEQUENCE reply until the backchannel
is re-established. There are two situations, each of which uses
different status flags: no connectivity for the session's backchannel
and no connectivity for any session backchannel of the client. See
Section 18.46 for a description of the appropriate flags in
sr_status_flags.
The server
SHOULD monitor when the number of RPCSEC_GSS handles
assigned to the backchannel reaches one, and when that one handle is
near expiry (i.e., between one and two periods of lease time), and
indicate so in the sr_status_flags field of all SEQUENCE replies.
The server
MUST indicate when all of the backchannel's assigned
RPCSEC_GSS handles have expired via the sr_status_flags field of all
SEQUENCE replies.
2.10.14. Parallel NFS and Sessions
A client and server can potentially be a non-pNFS implementation, a
metadata server implementation, a data server implementation, or two
or three types of implementations. The EXCHGID4_FLAG_USE_NON_PNFS,
EXCHGID4_FLAG_USE_PNFS_MDS, and EXCHGID4_FLAG_USE_PNFS_DS flags (not
mutually exclusive) are passed in the EXCHANGE_ID arguments and
results to allow the client to indicate how it wants to use sessions
created under the client ID, and to allow the server to indicate how
it will allow the sessions to be used. See
Section 13.1 for pNFS
sessions considerations.
3. Protocol Constants and Data Types
The syntax and semantics to describe the data types of the NFSv4.1
protocol are defined in the XDR (
RFC 4506 [2]) and RPC (
RFC 5531 [3])
documents. The next sections build upon the XDR data types to define
constants, types, and structures specific to this protocol. The full
list of XDR data types is in [10].
3.1. Basic Constants
const NFS4_FHSIZE = 128;
const NFS4_VERIFIER_SIZE = 8;
const NFS4_OPAQUE_LIMIT = 1024;
const NFS4_SESSIONID_SIZE = 16;
const NFS4_INT64_MAX = 0x7fffffffffffffff;
const NFS4_UINT64_MAX = 0xffffffffffffffff;
const NFS4_INT32_MAX = 0x7fffffff;
const NFS4_UINT32_MAX = 0xffffffff;
const NFS4_MAXFILELEN = 0xffffffffffffffff;
const NFS4_MAXFILEOFF = 0xfffffffffffffffe;
Except where noted, all these constants are defined in bytes.
* NFS4_FHSIZE is the maximum size of a filehandle.
* NFS4_VERIFIER_SIZE is the fixed size of a verifier.
* NFS4_OPAQUE_LIMIT is the maximum size of certain opaque
information.
* NFS4_SESSIONID_SIZE is the fixed size of a session identifier.
* NFS4_INT64_MAX is the maximum value of a signed 64-bit integer.
* NFS4_UINT64_MAX is the maximum value of an unsigned 64-bit
integer.
* NFS4_INT32_MAX is the maximum value of a signed 32-bit integer.
* NFS4_UINT32_MAX is the maximum value of an unsigned 32-bit
integer.
* NFS4_MAXFILELEN is the maximum length of a regular file.
* NFS4_MAXFILEOFF is the maximum offset into a regular file.
3.2. Basic Data Types
These are the base NFSv4.1 data types.
+===============+==============================================+
| Data Type | Definition |
+===============+==============================================+
| int32_t | typedef int int32_t; |
+---------------+----------------------------------------------+
| uint32_t | typedef unsigned int uint32_t; |
+---------------+----------------------------------------------+
| int64_t | typedef hyper int64_t; |
+---------------+----------------------------------------------+
| uint64_t | typedef unsigned hyper uint64_t; |
+---------------+----------------------------------------------+
| attrlist4 | typedef opaque attrlist4<>; |
| | |
| | Used for file/directory attributes. |
+---------------+----------------------------------------------+
| bitmap4 | typedef uint32_t bitmap4<>; |
| | |
| | Used in attribute array encoding. |
+---------------+----------------------------------------------+
| changeid4 | typedef uint64_t changeid4; |
| | |
| | Used in the definition of change_info4. |
+---------------+----------------------------------------------+
| clientid4 | typedef uint64_t clientid4; |
| | |
| | Shorthand reference to client |
| | identification. |
+---------------+----------------------------------------------+
| count4 | typedef uint32_t count4; |
| | |
| | Various count parameters (READ, WRITE, |
| | COMMIT). |
+---------------+----------------------------------------------+
| length4 | typedef uint64_t length4; |
| | |
| | The length of a byte-range within a file. |
+---------------+----------------------------------------------+
| mode4 | typedef uint32_t mode4; |
| | |
| | Mode attribute data type. |
+---------------+----------------------------------------------+
| nfs_cookie4 | typedef uint64_t nfs_cookie4; |
| | |
| | Opaque cookie value for READDIR. |
+---------------+----------------------------------------------+
| nfs_fh4 | typedef opaque nfs_fh4<NFS4_FHSIZE>; |
| | |
| | Filehandle definition. |
+---------------+----------------------------------------------+
| nfs_ftype4 | enum nfs_ftype4; |
| | |
| | Various defined file types. |
+---------------+----------------------------------------------+
| nfsstat4 | enum nfsstat4; |
| | |
| | Return value for operations. |
+---------------+----------------------------------------------+
| offset4 | typedef uint64_t offset4; |
| | |
| | Various offset designations (READ, WRITE, |
| | LOCK, COMMIT). |
+---------------+----------------------------------------------+
| qop4 | typedef uint32_t qop4; |
| | |
| | Quality of protection designation in |
| | SECINFO. |
+---------------+----------------------------------------------+
| sec_oid4 | typedef opaque sec_oid4<>; |
| | |
| | Security Object Identifier. The sec_oid4 |
| | data type is not really opaque. Instead, it |
| | contains an ASN.1 OBJECT IDENTIFIER as used |
| | by GSS-API in the mech_type argument to |
| | GSS_Init_sec_context. See [7] for details. |
+---------------+----------------------------------------------+
| sequenceid4 | typedef uint32_t sequenceid4; |
| | |
| | Sequence number used for various session |
| | operations (EXCHANGE_ID, CREATE_SESSION, |
| | SEQUENCE, CB_SEQUENCE). |
+---------------+----------------------------------------------+
| seqid4 | typedef uint32_t seqid4; |
| | |
| | Sequence identifier used for locking. |
+---------------+----------------------------------------------+
| sessionid4 | typedef opaque |
| | sessionid4[NFS4_SESSIONID_SIZE]; |
| | |
| | Session identifier. |
+---------------+----------------------------------------------+
| slotid4 | typedef uint32_t slotid4; |
| | |
| | Sequencing artifact for various session |
| | operations (SEQUENCE, CB_SEQUENCE). |
+---------------+----------------------------------------------+
| utf8string | typedef opaque utf8string<>; |
| | |
| | UTF-8 encoding for strings. |
+---------------+----------------------------------------------+
| utf8str_cis | typedef utf8string utf8str_cis; |
| | |
| | Case-insensitive UTF-8 string. |
+---------------+----------------------------------------------+
| utf8str_cs | typedef utf8string utf8str_cs; |
| | |
| | Case-sensitive UTF-8 string. |
+---------------+----------------------------------------------+
| utf8str_mixed | typedef utf8string utf8str_mixed; |
| | |
| | UTF-8 strings with a case-sensitive prefix |
| | and a case-insensitive suffix. |
+---------------+----------------------------------------------+
| component4 | typedef utf8str_cs component4; |
| | |
| | Represents pathname components. |
+---------------+----------------------------------------------+
| linktext4 | typedef utf8str_cs linktext4; |
| | |
| | Symbolic link contents ("symbolic link" is |
| | defined in an Open Group [11] standard). |
+---------------+----------------------------------------------+
| pathname4 | typedef component4 pathname4<>; |
| | |
| | Represents pathname for fs_locations. |
+---------------+----------------------------------------------+
| verifier4 | typedef opaque |
| | verifier4[NFS4_VERIFIER_SIZE]; |
| | |
| | Verifier used for various operations |
| | (COMMIT, CREATE, EXCHANGE_ID, OPEN, READDIR, |
| | WRITE) NFS4_VERIFIER_SIZE is defined as 8. |
+---------------+----------------------------------------------+
Table 1
End of Base Data Types
3.3. Structured Data Types
struct nfstime4 {
int64_t seconds;
uint32_t nseconds;
};
The nfstime4 data type gives the number of seconds and nanoseconds
since midnight or zero hour January 1, 1970 Coordinated Universal
Time (UTC). Values greater than zero for the seconds field denote
dates after the zero hour January 1, 1970. Values less than zero for
the seconds field denote dates before the zero hour January 1, 1970.
In both cases, the nseconds field is to be added to the seconds field
for the final time representation. For example, if the time to be
represented is one-half second before zero hour January 1, 1970, the
seconds field would have a value of negative one (-1) and the
nseconds field would have a value of one-half second (500000000).
Values greater than 999,999,999 for nseconds are invalid.
This data type is used to pass time and date information. A server
converts to and from its local representation of time when processing
time values, preserving as much accuracy as possible. If the
precision of timestamps stored for a file system object is less than
defined, loss of precision can occur. An adjunct time maintenance
protocol is
RECOMMENDED to reduce client and server time skew.
enum time_how4 {
SET_TO_SERVER_TIME4 = 0,
SET_TO_CLIENT_TIME4 = 1
};
union settime4 switch (time_how4 set_it) {
case SET_TO_CLIENT_TIME4:
nfstime4 time;
default:
void;
};
The time_how4 and settime4 data types are used for setting timestamps
in file object attributes. If set_it is SET_TO_SERVER_TIME4, then
the server uses its local representation of time for the time value.
struct specdata4 {
uint32_t specdata1; /* major device number */
uint32_t specdata2; /* minor device number */
};
This data type represents the device numbers for the device file
types NF4CHR and NF4BLK.
struct fsid4 {
uint64_t major;
uint64_t minor;
};
3.3.6. change_policy4
struct change_policy4 {
uint64_t cp_major;
uint64_t cp_minor;
};
The change_policy4 data type is used for the change_policy
RECOMMENDED attribute. It provides change sequencing indication
analogous to the change attribute. To enable the server to present a
value valid across server re-initialization without requiring
persistent storage, two 64-bit quantities are used, allowing one to
be a server instance ID and the second to be incremented non-
persistently, within a given server instance.
struct fattr4 {
bitmap4 attrmask;
attrlist4 attr_vals;
};
The fattr4 data type is used to represent file and directory
attributes.
The bitmap is a counted array of 32-bit integers used to contain bit
values. The position of the integer in the array that contains bit n
can be computed from the expression (n / 32), and its bit within that
integer is (n mod 32).
0 1
+-----------+-----------+-----------+--
| count | 31 .. 0 | 63 .. 32 |
+-----------+-----------+-----------+--
3.3.8. change_info4
struct change_info4 {
bool atomic;
changeid4 before;
changeid4 after;
};
This data type is used with the CREATE, LINK, OPEN, REMOVE, and
RENAME operations to let the client know the value of the change
attribute for the directory in which the target file system object
resides.
struct netaddr4 {
/* see struct rpcb in
RFC 1833 */
string na_r_netid<>; /* network id */
string na_r_addr<>; /* universal address */
};
The netaddr4 data type is used to identify network transport
endpoints. The na_r_netid and na_r_addr fields respectively contain
a netid and uaddr. The netid and uaddr concepts are defined in [12].
The netid and uaddr formats for TCP over IPv4 and TCP over IPv6 are
defined in [12], specifically Tables 2 and 3 and in Sections
5.2.
3.3 and
5.2.3.4.
struct state_owner4 {
clientid4 clientid;
opaque owner<NFS4_OPAQUE_LIMIT>;
};
typedef state_owner4 open_owner4;
typedef state_owner4 lock_owner4;
The state_owner4 data type is the base type for the open_owner4
(
Section 3.3.10.1) and lock_owner4 (
Section 3.3.10.2).
This data type is used to identify the owner of OPEN state.
This structure is used to identify the owner of byte-range locking
state.
3.3.11. open_to_lock_owner4
struct open_to_lock_owner4 {
seqid4 open_seqid;
stateid4 open_stateid;
seqid4 lock_seqid;
lock_owner4 lock_owner;
};
This data type is used for the first LOCK operation done for an
open_owner4. It provides both the open_stateid and lock_owner, such
that the transition is made from a valid open_stateid sequence to
that of the new lock_stateid sequence. Using this mechanism avoids
the confirmation of the lock_owner/lock_seqid pair since it is tied
to established state in the form of the open_stateid/open_seqid.
struct stateid4 {
uint32_t seqid;
opaque other[12];
};
This data type is used for the various state sharing mechanisms
between the client and server. The client never modifies a value of
data type stateid. The starting value of the "seqid" field is
undefined. The server is required to increment the "seqid" field by
one at each transition of the stateid. This is important since the
client will inspect the seqid in OPEN stateids to determine the order
of OPEN processing done by the server.
enum layouttype4 {
LAYOUT4_NFSV4_1_FILES = 0x1,
LAYOUT4_OSD2_OBJECTS = 0x2,
LAYOUT4_BLOCK_VOLUME = 0x3
};
This data type indicates what type of layout is being used. The file
server advertises the layout types it supports through the
fs_layout_type file system attribute (
Section 5.12.1). A client asks
for layouts of a particular type in LAYOUTGET, and processes those
layouts in its layout-type-specific logic.
The layouttype4 data type is 32 bits in length. The range
represented by the layout type is split into three parts. Type 0x0
is reserved. Types within the range 0x00000001-0x7FFFFFFF are
globally unique and are assigned according to the description in
Section 22.5; they are maintained by IANA. Types within the range
0x80000000-0xFFFFFFFF are site specific and for private use only.
The LAYOUT4_NFSV4_1_FILES enumeration specifies that the NFSv4.1 file
layout type, as defined in
Section 13, is to be used. The
LAYOUT4_OSD2_OBJECTS enumeration specifies that the object layout, as
defined in [47], is to be used. Similarly, the LAYOUT4_BLOCK_VOLUME
enumeration specifies that the block/volume layout, as defined in
[48], is to be used.
const NFS4_DEVICEID4_SIZE = 16;
typedef opaque deviceid4[NFS4_DEVICEID4_SIZE];
Layout information includes device IDs that specify a storage device
through a compact handle. Addressing and type information is
obtained with the GETDEVICEINFO operation. Device IDs are not
guaranteed to be valid across metadata server restarts. A device ID
is unique per client ID and layout type. See
Section 12.2.10 for
more details.
struct device_addr4 {
layouttype4 da_layout_type;
opaque da_addr_body<>;
};
The device address is used to set up a communication channel with the
storage device. Different layout types will require different data
types to define how they communicate with storage devices. The
opaque da_addr_body field is interpreted based on the specified
da_layout_type field.
This document defines the device address for the NFSv4.1 file layout
(see
Section 13.3), which identifies a storage device by network IP
address and port number. This is sufficient for the clients to
communicate with the NFSv4.1 storage devices, and may be sufficient
for other layout types as well. Device types for object-based
storage devices and block storage devices (e.g., Small Computer
System Interface (SCSI) volume labels) are defined by their
respective layout specifications.
3.3.16. layout_content4
struct layout_content4 {
layouttype4 loc_type;
opaque loc_body<>;
};
The loc_body field is interpreted based on the layout type
(loc_type). This document defines the loc_body for the NFSv4.1 file
layout type; see
Section 13.3 for its definition.
struct layout4 {
offset4 lo_offset;
length4 lo_length;
layoutiomode4 lo_iomode;
layout_content4 lo_content;
};
The layout4 data type defines a layout for a file. The layout type
specific data is opaque within lo_content. Since layouts are sub-
dividable, the offset and length together with the file's filehandle,
the client ID, iomode, and layout type identify the layout.
struct layoutupdate4 {
layouttype4 lou_type;
opaque lou_body<>;
};
The layoutupdate4 data type is used by the client to return updated
layout information to the metadata server via the LAYOUTCOMMIT
(
Section 18.42) operation. This data type provides a channel to pass
layout type specific information (in field lou_body) back to the
metadata server. For example, for the block/volume layout type, this
could include the list of reserved blocks that were written. The
contents of the opaque lou_body argument are determined by the layout
type. The NFSv4.1 file-based layout does not use this data type; if
lou_type is LAYOUT4_NFSV4_1_FILES, the lou_body field
MUST have a
zero length.
struct layouthint4 {
layouttype4 loh_type;
opaque loh_body<>;
};
The layouthint4 data type is used by the client to pass in a hint
about the type of layout it would like created for a particular file.
It is the data type specified by the layout_hint attribute described
in
Section 5.12.4. The metadata server may ignore the hint or may
selectively ignore fields within the hint. This hint should be
provided at create time as part of the initial attributes within
OPEN. The loh_body field is specific to the type of layout
(loh_type). The NFSv4.1 file-based layout uses the
nfsv4_1_file_layouthint4 data type as defined in
Section 13.3.
enum layoutiomode4 {
LAYOUTIOMODE4_READ = 1,
LAYOUTIOMODE4_RW = 2,
LAYOUTIOMODE4_ANY = 3
};
The iomode specifies whether the client intends to just read or both
read and write the data represented by the layout. While the
LAYOUTIOMODE4_ANY iomode
MUST NOT be used in the arguments to the
LAYOUTGET operation, it
MAY be used in the arguments to the
LAYOUTRETURN and CB_LAYOUTRECALL operations. The LAYOUTIOMODE4_ANY
iomode specifies that layouts pertaining to both LAYOUTIOMODE4_READ
and LAYOUTIOMODE4_RW iomodes are being returned or recalled,
respectively. The metadata server's use of the iomode may depend on
the layout type being used. The storage devices
MAY validate I/O
accesses against the iomode and reject invalid accesses.
struct nfs_impl_id4 {
utf8str_cis nii_domain;
utf8str_cs nii_name;
nfstime4 nii_date;
};
This data type is used to identify client and server implementation
details. The nii_domain field is the DNS domain name with which the
implementor is associated. The nii_name field is the product name of
the implementation and is completely free form. It is
RECOMMENDED that the nii_name be used to distinguish machine architecture,
machine platforms, revisions, versions, and patch levels. The
nii_date field is the timestamp of when the software instance was
published or built.
3.3.22. threshold_item4
struct threshold_item4 {
layouttype4 thi_layout_type;
bitmap4 thi_hintset;
opaque thi_hintlist<>;
};
This data type contains a list of hints specific to a layout type for
helping the client determine when it should send I/O directly through
the metadata server versus the storage devices. The data type
consists of the layout type (thi_layout_type), a bitmap (thi_hintset)
describing the set of hints supported by the server (they may differ
based on the layout type), and a list of hints (thi_hintlist) whose
content is determined by the hintset bitmap. See the mdsthreshold
attribute for more details.
The thi_hintset field is a bitmap of the following values:
+=========================+===+=========+===========================+
| name | # | Data | Description |
| | | Type | |
+=========================+===+=========+===========================+
| threshold4_read_size | 0 | length4 | If a file's length is |
| | | | less than the value of |
| | | | threshold4_read_size, |
| | | | then it is
RECOMMENDED |
| | | | that the client read |
| | | | from the file via the |
| | | | MDS and not a storage |
| | | | device. |
+-------------------------+---+---------+---------------------------+
| threshold4_write_size | 1 | length4 | If a file's length is |
| | | | less than the value of |
| | | | threshold4_write_size, |
| | | | then it is
RECOMMENDED |
| | | | that the client write |
| | | | to the file via the |
| | | | MDS and not a storage |
| | | | device. |
+-------------------------+---+---------+---------------------------+
| threshold4_read_iosize | 2 | length4 | For read I/O sizes |
| | | | below this threshold, |
| | | | it is
RECOMMENDED to |
| | | | read data through the |
| | | | MDS. |
+-------------------------+---+---------+---------------------------+
| threshold4_write_iosize | 3 | length4 | For write I/O sizes |
| | | | below this threshold, |
| | | | it is
RECOMMENDED to |
| | | | write data through the |
| | | | MDS. |
+-------------------------+---+---------+---------------------------+
Table 2
struct mdsthreshold4 {
threshold_item4 mth_hints<>;
};
This data type holds an array of elements of data type
threshold_item4, each of which is valid for a particular layout type.
An array is necessary because a server can support multiple layout
types for a single file.
4. Filehandles
The filehandle in the NFS protocol is a per-server unique identifier
for a file system object. The contents of the filehandle are opaque
to the client. Therefore, the server is responsible for translating
the filehandle to an internal representation of the file system
object.
4.1. Obtaining the First Filehandle
The operations of the NFS protocol are defined in terms of one or
more filehandles. Therefore, the client needs a filehandle to
initiate communication with the server. With the NFSv3 protocol (
RFC 1813 [38]), there exists an ancillary protocol to obtain this first
filehandle. The MOUNT protocol, RPC program number 100005, provides
the mechanism of translating a string-based file system pathname to a
filehandle, which can then be used by the NFS protocols.
The MOUNT protocol has deficiencies in the area of security and use
via firewalls. This is one reason that the use of the public
filehandle was introduced in
RFC 2054 [49] and
RFC 2055 [50]. With
the use of the public filehandle in combination with the LOOKUP
operation in the NFSv3 protocol, it has been demonstrated that the
MOUNT protocol is unnecessary for viable interaction between NFS
client and server.
Therefore, the NFSv4.1 protocol will not use an ancillary protocol
for translation from string-based pathnames to a filehandle. Two
special filehandles will be used as starting points for the NFS
client.
4.1.1. Root Filehandle
The first of the special filehandles is the ROOT filehandle. The
ROOT filehandle is the "conceptual" root of the file system namespace
at the NFS server. The client uses or starts with the ROOT
filehandle by employing the PUTROOTFH operation. The PUTROOTFH
operation instructs the server to set the "current" filehandle to the
ROOT of the server's file tree. Once this PUTROOTFH operation is
used, the client can then traverse the entirety of the server's file
tree with the LOOKUP operation. A complete discussion of the server
namespace is in
Section 7.
4.1.2. Public Filehandle
The second special filehandle is the PUBLIC filehandle. Unlike the
ROOT filehandle, the PUBLIC filehandle may be bound or represent an
arbitrary file system object at the server. The server is
responsible for this binding. It may be that the PUBLIC filehandle
and the ROOT filehandle refer to the same file system object.
However, it is up to the administrative software at the server and
the policies of the server administrator to define the binding of the
PUBLIC filehandle and server file system object. The client may not
make any assumptions about this binding. The client uses the PUBLIC
filehandle via the PUTPUBFH operation.
4.2. Filehandle Types
In the NFSv3 protocol, there was one type of filehandle with a single
set of semantics. This type of filehandle is termed "persistent" in
NFSv4.1. The semantics of a persistent filehandle remain the same as
before. A new type of filehandle introduced in NFSv4.1 is the
"volatile" filehandle, which attempts to accommodate certain server
environments.
The volatile filehandle type was introduced to address server
functionality or implementation issues that make correct
implementation of a persistent filehandle infeasible. Some server
environments do not provide a file-system-level invariant that can be
used to construct a persistent filehandle. The underlying server
file system may not provide the invariant or the server's file system
programming interfaces may not provide access to the needed
invariant. Volatile filehandles may ease the implementation of
server functionality such as hierarchical storage management or file
system reorganization or migration. However, the volatile filehandle
increases the implementation burden for the client.
Since the client will need to handle persistent and volatile
filehandles differently, a file attribute is defined that may be used
by the client to determine the filehandle types being returned by the
server.
4.2.1. General Properties of a Filehandle
The filehandle contains all the information the server needs to
distinguish an individual file. To the client, the filehandle is
opaque. The client stores filehandles for use in a later request and
can compare two filehandles from the same server for equality by
doing a byte-by-byte comparison. However, the client
MUST NOT otherwise interpret the contents of filehandles. If two filehandles
from the same server are equal, they
MUST refer to the same file.
Servers
SHOULD try to maintain a one-to-one correspondence between
filehandles and files, but this is not required. Clients
MUST use
filehandle comparisons only to improve performance, not for correct
behavior. All clients need to be prepared for situations in which it
cannot be determined whether two filehandles denote the same object
and in such cases, avoid making invalid assumptions that might cause
incorrect behavior. Further discussion of filehandle and attribute
comparison in the context of data caching is presented in
Section 10.3.4.
As an example, in the case that two different pathnames when
traversed at the server terminate at the same file system object, the
server
SHOULD return the same filehandle for each path. This can
occur if a hard link (see [6]) is used to create two file names that
refer to the same underlying file object and associated data. For
example, if paths /a/b/c and /a/d/c refer to the same file, the
server
SHOULD return the same filehandle for both pathnames'
traversals.
4.2.2. Persistent Filehandle
A persistent filehandle is defined as having a fixed value for the
lifetime of the file system object to which it refers. Once the
server creates the filehandle for a file system object, the server
MUST accept the same filehandle for the object for the lifetime of
the object. If the server restarts, the NFS server
MUST honor the
same filehandle value as it did in the server's previous
instantiation. Similarly, if the file system is migrated, the new
NFS server
MUST honor the same filehandle as the old NFS server.
The persistent filehandle will be become stale or invalid when the
file system object is removed. When the server is presented with a
persistent filehandle that refers to a deleted object, it
MUST return
an error of NFS4ERR_STALE. A filehandle may become stale when the
file system containing the object is no longer available. The file
system may become unavailable if it exists on removable media and the
media is no longer available at the server or the file system in
whole has been destroyed or the file system has simply been removed
from the server's namespace (i.e., unmounted in a UNIX environment).
4.2.3. Volatile Filehandle
A volatile filehandle does not share the same longevity
characteristics of a persistent filehandle. The server may determine
that a volatile filehandle is no longer valid at many different
points in time. If the server can definitively determine that a
volatile filehandle refers to an object that has been removed, the
server should return NFS4ERR_STALE to the client (as is the case for
persistent filehandles). In all other cases where the server
determines that a volatile filehandle can no longer be used, it
should return an error of NFS4ERR_FHEXPIRED.
The
REQUIRED attribute "fh_expire_type" is used by the client to
determine what type of filehandle the server is providing for a
particular file system. This attribute is a bitmask with the
following values:
FH4_PERSISTENT The value of FH4_PERSISTENT is used to indicate a
persistent filehandle, which is valid until the object is removed
from the file system. The server will not return
NFS4ERR_FHEXPIRED for this filehandle. FH4_PERSISTENT is defined
as a value in which none of the bits specified below are set.
FH4_VOLATILE_ANY The filehandle may expire at any time, except as
specifically excluded (i.e., FH4_NO_EXPIRE_WITH_OPEN).
FH4_NOEXPIRE_WITH_OPEN May only be set when FH4_VOLATILE_ANY is set.
If this bit is set, then the meaning of FH4_VOLATILE_ANY is
qualified to exclude any expiration of the filehandle when it is
open.
FH4_VOL_MIGRATION The filehandle will expire as a result of a file
system transition (migration or replication), in those cases in
which the continuity of filehandle use is not specified by handle
class information within the fs_locations_info attribute. When
this bit is set, clients without access to fs_locations_info
information should assume that filehandles will expire on file
system transitions.
FH4_VOL_RENAME The filehandle will expire during rename. This
includes a rename by the requesting client or a rename by any
other client. If FH4_VOL_ANY is set, FH4_VOL_RENAME is redundant.
Servers that provide volatile filehandles that can expire while open
require special care as regards handling of RENAMEs and REMOVEs.
This situation can arise if FH4_VOL_MIGRATION or FH4_VOL_RENAME is
set, if FH4_VOLATILE_ANY is set and FH4_NOEXPIRE_WITH_OPEN is not
set, or if a non-read-only file system has a transition target in a
different handle class. In these cases, the server should deny a
RENAME or REMOVE that would affect an OPEN file of any of the
components leading to the OPEN file. In addition, the server should
deny all RENAME or REMOVE requests during the grace period, in order
to make sure that reclaims of files where filehandles may have
expired do not do a reclaim for the wrong file.
Volatile filehandles are especially suitable for implementation of
the pseudo file systems used to bridge exports. See
Section 7.5 for
a discussion of this.
4.3. One Method of Constructing a Volatile Filehandle
A volatile filehandle, while opaque to the client, could contain:
[volatile bit = 1 | server boot time | slot | generation number]
* slot is an index in the server volatile filehandle table
* generation number is the generation number for the table entry/
slot
When the client presents a volatile filehandle, the server makes the
following checks, which assume that the check for the volatile bit
has passed. If the server boot time is less than the current server
boot time, return NFS4ERR_FHEXPIRED. If slot is out of range, return
NFS4ERR_BADHANDLE. If the generation number does not match, return
NFS4ERR_FHEXPIRED.
When the server restarts, the table is gone (it is volatile).
If the volatile bit is 0, then it is a persistent filehandle with a
different structure following it.
4.4. Client Recovery from Filehandle Expiration
If possible, the client
SHOULD recover from the receipt of an
NFS4ERR_FHEXPIRED error. The client must take on additional
responsibility so that it may prepare itself to recover from the
expiration of a volatile filehandle. If the server returns
persistent filehandles, the client does not need these additional
steps.
For volatile filehandles, most commonly the client will need to store
the component names leading up to and including the file system
object in question. With these names, the client should be able to
recover by finding a filehandle in the namespace that is still
available or by starting at the root of the server's file system
namespace.
If the expired filehandle refers to an object that has been removed
from the file system, obviously the client will not be able to
recover from the expired filehandle.
It is also possible that the expired filehandle refers to a file that
has been renamed. If the file was renamed by another client, again
it is possible that the original client will not be able to recover.
However, in the case that the client itself is renaming the file and
the file is open, it is possible that the client may be able to
recover. The client can determine the new pathname based on the
processing of the rename request. The client can then regenerate the
new filehandle based on the new pathname. The client could also use
the COMPOUND procedure to construct a series of operations like:
RENAME A B
LOOKUP B
GETFH
Note that the COMPOUND procedure does not provide atomicity. This
example only reduces the overhead of recovering from an expired
filehandle.
5. File Attributes
To meet the requirements of extensibility and increased
interoperability with non-UNIX platforms, attributes need to be
handled in a flexible manner. The NFSv3 fattr3 structure contains a
fixed list of attributes that not all clients and servers are able to
support or care about. The fattr3 structure cannot be extended as
new needs arise and it provides no way to indicate non-support. With
the NFSv4.1 protocol, the client is able to query what attributes the
server supports and construct requests with only those supported
attributes (or a subset thereof).
To this end, attributes are divided into three groups:
REQUIRED,
RECOMMENDED, and named. Both
REQUIRED and
RECOMMENDED attributes are
supported in the NFSv4.1 protocol by a specific and well-defined
encoding and are identified by number. They are requested by setting
a bit in the bit vector sent in the GETATTR request; the server
response includes a bit vector to list what attributes were returned
in the response. New
REQUIRED or
RECOMMENDED attributes may be added
to the NFSv4 protocol as part of a new minor version by publishing a
Standards Track RFC that allocates a new attribute number value and
defines the encoding for the attribute. See
Section 2.7 for further
discussion.
Named attributes are accessed by the new OPENATTR operation, which
accesses a hidden directory of attributes associated with a file
system object. OPENATTR takes a filehandle for the object and
returns the filehandle for the attribute hierarchy. The filehandle
for the named attributes is a directory object accessible by LOOKUP
or READDIR and contains files whose names represent the named
attributes and whose data bytes are the value of the attribute. For
example:
+----------+-----------+---------------------------------+
| LOOKUP | "foo" | ; look up file |
+----------+-----------+---------------------------------+
| GETATTR | attrbits | |
+----------+-----------+---------------------------------+
| OPENATTR | | ; access foo's named attributes |
+----------+-----------+---------------------------------+
| LOOKUP | "x11icon" | ; look up specific attribute |
+----------+-----------+---------------------------------+
| READ | 0,4096 | ; read stream of bytes |
+----------+-----------+---------------------------------+
Table 3
Named attributes are intended for data needed by applications rather
than by an NFS client implementation. NFS implementors are strongly
encouraged to define their new attributes as
RECOMMENDED attributes
by bringing them to the IETF Standards Track process.
The set of attributes that are classified as
REQUIRED is deliberately
small since servers need to do whatever it takes to support them. A
server should support as many of the
RECOMMENDED attributes as
possible but, by their definition, the server is not required to
support all of them. Attributes are deemed
REQUIRED if the data is
both needed by a large number of clients and is not otherwise
reasonably computable by the client when support is not provided on
the server.
Note that the hidden directory returned by OPENATTR is a convenience
for protocol processing. The client should not make any assumptions
about the server's implementation of named attributes and whether or
not the underlying file system at the server has a named attribute
directory. Therefore, operations such as SETATTR and GETATTR on the
named attribute directory are undefined.
5.1. REQUIRED Attributes
These
MUST be supported by every NFSv4.1 client and server in order
to ensure a minimum level of interoperability. The server
MUST store
and return these attributes, and the client
MUST be able to function
with an attribute set limited to these attributes. With just the
REQUIRED attributes some client functionality may be impaired or
limited in some ways. A client may ask for any of these attributes
to be returned by setting a bit in the GETATTR request, and the
server
MUST return their value.
5.2. RECOMMENDED Attributes
These attributes are understood well enough to warrant support in the
NFSv4.1 protocol. However, they may not be supported on all clients
and servers. A client may ask for any of these attributes to be
returned by setting a bit in the GETATTR request but must handle the
case where the server does not return them. A client
MAY ask for the
set of attributes the server supports and
SHOULD NOT request
attributes the server does not support. A server should be tolerant
of requests for unsupported attributes and simply not return them
rather than considering the request an error. It is expected that
servers will support all attributes they comfortably can and only
fail to support attributes that are difficult to support in their
operating environments. A server should provide attributes whenever
they don't have to "tell lies" to the client. For example, a file
modification time should be either an accurate time or should not be
supported by the server. At times this will be difficult for
clients, but a client is better positioned to decide whether and how
to fabricate or construct an attribute or whether to do without the
attribute.
5.3. Named Attributes
These attributes are not supported by direct encoding in the NFSv4
protocol but are accessed by string names rather than numbers and
correspond to an uninterpreted stream of bytes that are stored with
the file system object. The namespace for these attributes may be
accessed by using the OPENATTR operation. The OPENATTR operation
returns a filehandle for a virtual "named attribute directory", and
further perusal and modification of the namespace may be done using
operations that work on more typical directories. In particular,
READDIR may be used to get a list of such named attributes, and
LOOKUP and OPEN may select a particular attribute. Creation of a new
named attribute may be the result of an OPEN specifying file
creation.
Once an OPEN is done, named attributes may be examined and changed by
normal READ and WRITE operations using the filehandles and stateids
returned by OPEN.
Named attributes and the named attribute directory may have their own
(non-named) attributes. Each of these objects
MUST have all of the
REQUIRED attributes and may have additional
RECOMMENDED attributes.
However, the set of attributes for named attributes and the named
attribute directory need not be, and typically will not be, as large
as that for other objects in that file system.
Named attributes and the named attribute directory might be the
target of delegations (in the case of the named attribute directory,
these will be directory delegations). However, since granting of
delegations is at the server's discretion, a server need not support
delegations on named attributes or the named attribute directory.
It is
RECOMMENDED that servers support arbitrary named attributes. A
client should not depend on the ability to store any named attributes
in the server's file system. If a server does support named
attributes, a client that is also able to handle them should be able
to copy a file's data and metadata with complete transparency from
one location to another; this would imply that names allowed for
regular directory entries are valid for named attribute names as
well.
In NFSv4.1, the structure of named attribute directories is
restricted in a number of ways, in order to prevent the development
of non-interoperable implementations in which some servers support a
fully general hierarchical directory structure for named attributes
while others support a limited but adequate structure for named
attributes. In such an environment, clients or applications might
come to depend on non-portable extensions. The restrictions are:
* CREATE is not allowed in a named attribute directory. Thus, such
objects as symbolic links and special files are not allowed to be
named attributes. Further, directories may not be created in a
named attribute directory, so no hierarchical structure of named
attributes for a single object is allowed.
* If OPENATTR is done on a named attribute directory or on a named
attribute, the server
MUST return NFS4ERR_WRONG_TYPE.
* Doing a RENAME of a named attribute to a different named attribute
directory or to an ordinary (i.e., non-named-attribute) directory
is not allowed.
* Creating hard links between named attribute directories or between
named attribute directories and ordinary directories is not
allowed.
Names of attributes will not be controlled by this document or other
IETF Standards Track documents. See
Section 22.2 for further
discussion.
5.4. Classification of Attributes
Each of the
REQUIRED and
RECOMMENDED attributes can be classified in
one of three categories: per server (i.e., the value of the attribute
will be the same for all file objects that share the same server
owner; see
Section 2.5 for a definition of server owner), per file
system (i.e., the value of the attribute will be the same for some or
all file objects that share the same fsid attribute (
Section 5.8.1.9)
and server owner), or per file system object. Note that it is
possible that some per file system attributes may vary within the
file system, depending on the value of the "homogeneous"
(
Section 5.8.2.16) attribute. Note that the attributes
time_access_set and time_modify_set are not listed in this section
because they are write-only attributes corresponding to time_access
and time_modify, and are used in a special instance of SETATTR.
* The per-server attribute is:
lease_time
* The per-file system attributes are:
supported_attrs, suppattr_exclcreat, fh_expire_type,
link_support, symlink_support, unique_handles, aclsupport,
cansettime, case_insensitive, case_preserving,
chown_restricted, files_avail, files_free, files_total,
fs_locations, homogeneous, maxfilesize, maxname, maxread,
maxwrite, no_trunc, space_avail, space_free, space_total,
time_delta, change_policy, fs_status, fs_layout_type,
fs_locations_info, fs_charset_cap
* The per-file system object attributes are:
type, change, size, named_attr, fsid, rdattr_error, filehandle,
acl, archive, fileid, hidden, maxlink, mimetype, mode,
numlinks, owner, owner_group, rawdev, space_used, system,
time_access, time_backup, time_create, time_metadata,
time_modify, mounted_on_fileid, dir_notif_delay,
dirent_notif_delay, dacl, sacl, layout_type, layout_hint,
layout_blksize, layout_alignment, mdsthreshold, retention_get,
retention_set, retentevt_get, retentevt_set, retention_hold,
mode_set_masked
For quota_avail_hard, quota_avail_soft, and quota_used, see their
definitions below for the appropriate classification.
5.5. Set-Only and Get-Only Attributes
Some
REQUIRED and
RECOMMENDED attributes are set-only; i.e., they can
be set via SETATTR but not retrieved via GETATTR. Similarly, some
REQUIRED and
RECOMMENDED attributes are get-only; i.e., they can be
retrieved via GETATTR but not set via SETATTR. If a client attempts
to set a get-only attribute or get a set-only attributes, the server
MUST return NFS4ERR_INVAL.
5.6. REQUIRED Attributes - List and Definition References
The list of
REQUIRED attributes appears in Table 4. The meaning of
the columns of the table are:
Name: The name of the attribute.
Id: The number assigned to the attribute. In the event of conflicts
between the assigned number and [10], the latter is likely
authoritative, but should be resolved with Errata to this document
and/or [10]. See [51] for the Errata process.
Data Type: The XDR data type of the attribute.
Acc: Access allowed to the attribute. R means read-only (GETATTR
may retrieve, SETATTR may not set). W means write-only (SETATTR
may set, GETATTR may not retrieve). R W means read/write (GETATTR
may retrieve, SETATTR may set).
Defined in: The section of this specification that describes the
attribute.
+====================+====+============+=====+==================+
| Name | Id | Data Type | Acc | Defined in: |
+====================+====+============+=====+==================+
| supported_attrs | 0 | bitmap4 | R |
Section 5.8.1.1 |
+--------------------+----+------------+-----+------------------+
| type | 1 | nfs_ftype4 | R |
Section 5.8.1.2 |
+--------------------+----+------------+-----+------------------+
| fh_expire_type | 2 | uint32_t | R |
Section 5.8.1.3 |
+--------------------+----+------------+-----+------------------+
| change | 3 | uint64_t | R |
Section 5.8.1.4 |
+--------------------+----+------------+-----+------------------+
| size | 4 | uint64_t | R W |
Section 5.8.1.5 |
+--------------------+----+------------+-----+------------------+
| link_support | 5 | bool | R |
Section 5.8.1.6 |
+--------------------+----+------------+-----+------------------+
| symlink_support | 6 | bool | R |
Section 5.8.1.7 |
+--------------------+----+------------+-----+------------------+
| named_attr | 7 | bool | R |
Section 5.8.1.8 |
+--------------------+----+------------+-----+------------------+
| fsid | 8 | fsid4 | R |
Section 5.8.1.9 |
+--------------------+----+------------+-----+------------------+
| unique_handles | 9 | bool | R |
Section 5.8.1.10 |
+--------------------+----+------------+-----+------------------+
| lease_time | 10 | nfs_lease4 | R |
Section 5.8.1.11 |
+--------------------+----+------------+-----+------------------+
| rdattr_error | 11 | enum | R |
Section 5.8.1.12 |
+--------------------+----+------------+-----+------------------+
| filehandle | 19 | nfs_fh4 | R |
Section 5.8.1.13 |
+--------------------+----+------------+-----+------------------+
| suppattr_exclcreat | 75 | bitmap4 | R |
Section 5.8.1.14 |
+--------------------+----+------------+-----+------------------+
Table 4
5.7. RECOMMENDED Attributes - List and Definition References
The
RECOMMENDED attributes are defined in Table 5. The meanings of
the column headers are the same as Table 4; see
Section 5.6 for the
meanings.
+====================+====+====================+=====+=============+
| Name | Id | Data Type | Acc | Defined in: |
+====================+====+====================+=====+=============+
| acl | 12 | nfsace4<> | R W | Section |
| | | | | 6.2.1 |
+--------------------+----+--------------------+-----+-------------+
| aclsupport | 13 | uint32_t | R | Section |
| | | | | 6.2.1.2 |
+--------------------+----+--------------------+-----+-------------+
| archive | 14 | bool | R W | Section |
| | | | | 5.8.2.1 |
+--------------------+----+--------------------+-----+-------------+
| cansettime | 15 | bool | R | Section |
| | | | | 5.8.2.2 |
+--------------------+----+--------------------+-----+-------------+
| case_insensitive | 16 | bool | R | Section |
| | | | | 5.8.2.3 |
+--------------------+----+--------------------+-----+-------------+
| case_preserving | 17 | bool | R | Section |
| | | | | 5.8.2.4 |
+--------------------+----+--------------------+-----+-------------+
| change_policy | 60 | chg_policy4 | R | Section |
| | | | | 5.8.2.5 |
+--------------------+----+--------------------+-----+-------------+
| chown_restricted | 18 | bool | R | Section |
| | | | | 5.8.2.6 |
+--------------------+----+--------------------+-----+-------------+
| dacl | 58 | nfsacl41 | R W | Section |
| | | | | 6.2.2 |
+--------------------+----+--------------------+-----+-------------+
| dir_notif_delay | 56 | nfstime4 | R | Section |
| | | | | 5.11.1 |
+--------------------+----+--------------------+-----+-------------+
| dirent_notif_delay | 57 | nfstime4 | R | Section |
| | | | | 5.11.2 |
+--------------------+----+--------------------+-----+-------------+
| fileid | 20 | uint64_t | R | Section |
| | | | | 5.8.2.7 |
+--------------------+----+--------------------+-----+-------------+
| files_avail | 21 | uint64_t | R | Section |
| | | | | 5.8.2.8 |
+--------------------+----+--------------------+-----+-------------+
| files_free | 22 | uint64_t | R | Section |
| | | | | 5.8.2.9 |
+--------------------+----+--------------------+-----+-------------+
| files_total | 23 | uint64_t | R | Section |
| | | | | 5.8.2.10 |
+--------------------+----+--------------------+-----+-------------+
| fs_charset_cap | 76 | uint32_t | R | Section |
| | | | | 5.8.2.11 |
+--------------------+----+--------------------+-----+-------------+
| fs_layout_type | 62 | layouttype4<> | R | Section |
| | | | | 5.12.1 |
+--------------------+----+--------------------+-----+-------------+
| fs_locations | 24 | fs_locations | R | Section |
| | | | | 5.8.2.12 |
+--------------------+----+--------------------+-----+-------------+
| fs_locations_info | 67 | fs_locations_info4 | R | Section |
| | | | | 5.8.2.13 |
+--------------------+----+--------------------+-----+-------------+
| fs_status | 61 | fs4_status | R | Section |
| | | | | 5.8.2.14 |
+--------------------+----+--------------------+-----+-------------+
| hidden | 25 | bool | R W | Section |
| | | | | 5.8.2.15 |
+--------------------+----+--------------------+-----+-------------+
| homogeneous | 26 | bool | R | Section |
| | | | | 5.8.2.16 |
+--------------------+----+--------------------+-----+-------------+
| layout_alignment | 66 | uint32_t | R | Section |
| | | | | 5.12.2 |
+--------------------+----+--------------------+-----+-------------+
| layout_blksize | 65 | uint32_t | R | Section |
| | | | | 5.12.3 |
+--------------------+----+--------------------+-----+-------------+
| layout_hint | 63 | layouthint4 | W | Section |
| | | | | 5.12.4 |
+--------------------+----+--------------------+-----+-------------+
| layout_type | 64 | layouttype4<> | R | Section |
| | | | | 5.12.5 |
+--------------------+----+--------------------+-----+-------------+
| maxfilesize | 27 | uint64_t | R | Section |
| | | | | 5.8.2.17 |
+--------------------+----+--------------------+-----+-------------+
| maxlink | 28 | uint32_t | R | Section |
| | | | | 5.8.2.18 |
+--------------------+----+--------------------+-----+-------------+
| maxname | 29 | uint32_t | R | Section |
| | | | | 5.8.2.19 |
+--------------------+----+--------------------+-----+-------------+
| maxread | 30 | uint64_t | R | Section |
| | | | | 5.8.2.20 |
+--------------------+----+--------------------+-----+-------------+
| maxwrite | 31 | uint64_t | R | Section |
| | | | | 5.8.2.21 |
+--------------------+----+--------------------+-----+-------------+
| mdsthreshold | 68 | mdsthreshold4 | R | Section |
| | | | | 5.12.6 |
+--------------------+----+--------------------+-----+-------------+
| mimetype | 32 | utf8str_cs | R W | Section |
| | | | | 5.8.2.22 |
+--------------------+----+--------------------+-----+-------------+
| mode | 33 | mode4 | R W | Section |
| | | | | 6.2.4 |
+--------------------+----+--------------------+-----+-------------+
| mode_set_masked | 74 | mode_masked4 | W | Section |
| | | | | 6.2.5 |
+--------------------+----+--------------------+-----+-------------+
| mounted_on_fileid | 55 | uint64_t | R | Section |
| | | | | 5.8.2.23 |
+--------------------+----+--------------------+-----+-------------+
| no_trunc | 34 | bool | R | Section |
| | | | | 5.8.2.24 |
+--------------------+----+--------------------+-----+-------------+
| numlinks | 35 | uint32_t | R | Section |
| | | | | 5.8.2.25 |
+--------------------+----+--------------------+-----+-------------+
| owner | 36 | utf8str_mixed | R W | Section |
| | | | | 5.8.2.26 |
+--------------------+----+--------------------+-----+-------------+
| owner_group | 37 | utf8str_mixed | R W | Section |
| | | | | 5.8.2.27 |
+--------------------+----+--------------------+-----+-------------+
| quota_avail_hard | 38 | uint64_t | R | Section |
| | | | | 5.8.2.28 |
+--------------------+----+--------------------+-----+-------------+
| quota_avail_soft | 39 | uint64_t | R | Section |
| | | | | 5.8.2.29 |
+--------------------+----+--------------------+-----+-------------+
| quota_used | 40 | uint64_t | R | Section |
| | | | | 5.8.2.30 |
+--------------------+----+--------------------+-----+-------------+
| rawdev | 41 | specdata4 | R | Section |
| | | | | 5.8.2.31 |
+--------------------+----+--------------------+-----+-------------+
| retentevt_get | 71 | retention_get4 | R | Section |
| | | | | 5.13.3 |
+--------------------+----+--------------------+-----+-------------+
| retentevt_set | 72 | retention_set4 | W | Section |
| | | | | 5.13.4 |
+--------------------+----+--------------------+-----+-------------+
| retention_get | 69 | retention_get4 | R | Section |
| | | | | 5.13.1 |
+--------------------+----+--------------------+-----+-------------+
| retention_hold | 73 | uint64_t | R W | Section |
| | | | | 5.13.5 |
+--------------------+----+--------------------+-----+-------------+
| retention_set | 70 | retention_set4 | W | Section |
| | | | | 5.13.2 |
+--------------------+----+--------------------+-----+-------------+
| sacl | 59 | nfsacl41 | R W | Section |
| | | | | 6.2.3 |
+--------------------+----+--------------------+-----+-------------+
| space_avail | 42 | uint64_t | R | Section |
| | | | | 5.8.2.32 |
+--------------------+----+--------------------+-----+-------------+
| space_free | 43 | uint64_t | R | Section |
| | | | | 5.8.2.33 |
+--------------------+----+--------------------+-----+-------------+
| space_total | 44 | uint64_t | R | Section |
| | | | | 5.8.2.34 |
+--------------------+----+--------------------+-----+-------------+
| space_used | 45 | uint64_t | R | Section |
| | | | | 5.8.2.35 |
+--------------------+----+--------------------+-----+-------------+
| system | 46 | bool | R W | Section |
| | | | | 5.8.2.36 |
+--------------------+----+--------------------+-----+-------------+
| time_access | 47 | nfstime4 | R | Section |
| | | | | 5.8.2.37 |
+--------------------+----+--------------------+-----+-------------+
| time_access_set | 48 | settime4 | W | Section |
| | | | | 5.8.2.38 |
+--------------------+----+--------------------+-----+-------------+
| time_backup | 49 | nfstime4 | R W | Section |
| | | | | 5.8.2.39 |
+--------------------+----+--------------------+-----+-------------+
| time_create | 50 | nfstime4 | R W | Section |
| | | | | 5.8.2.40 |
+--------------------+----+--------------------+-----+-------------+
| time_delta | 51 | nfstime4 | R | Section |
| | | | | 5.8.2.41 |
+--------------------+----+--------------------+-----+-------------+
| time_metadata | 52 | nfstime4 | R | Section |
| | | | | 5.8.2.42 |
+--------------------+----+--------------------+-----+-------------+
| time_modify | 53 | nfstime4 | R | Section |
| | | | | 5.8.2.43 |
+--------------------+----+--------------------+-----+-------------+
| time_modify_set | 54 | settime4 | W | Section |
| | | | | 5.8.2.44 |
+--------------------+----+--------------------+-----+-------------+
Table 5
5.8. Attribute Definitions
5.8.1. Definitions of REQUIRED Attributes
5.8.1.1. Attribute 0: supported_attrs
The bit vector that would retrieve all
REQUIRED and
RECOMMENDED attributes that are supported for this object. The scope of this
attribute applies to all objects with a matching fsid.
5.8.1.2. Attribute 1: type
Designates the type of an object in terms of one of a number of
special constants:
* NF4REG designates a regular file.
* NF4DIR designates a directory.
* NF4BLK designates a block device special file.
* NF4CHR designates a character device special file.
* NF4LNK designates a symbolic link.
* NF4SOCK designates a named socket special file.
* NF4FIFO designates a fifo special file.
* NF4ATTRDIR designates a named attribute directory.
* NF4NAMEDATTR designates a named attribute.
Within the explanatory text and operation descriptions, the following
phrases will be used with the meanings given below:
* The phrase "is a directory" means that the object's type attribute
is NF4DIR or NF4ATTRDIR.
* The phrase "is a special file" means that the object's type
attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.
* The phrases "is an ordinary file" and "is a regular file" mean
that the object's type attribute is NF4REG or NF4NAMEDATTR.
5.8.1.3. Attribute 2: fh_expire_type
Server uses this to specify filehandle expiration behavior to the
client. See
Section 4 for additional description.
5.8.1.4. Attribute 3: change
A value created by the server that the client can use to determine if
file data, directory contents, or attributes of the object have been
modified. The server may return the object's time_metadata attribute
for this attribute's value, but only if the file system object cannot
be updated more frequently than the resolution of time_metadata.
5.8.1.5. Attribute 4: size
The size of the object in bytes.
5.8.1.6. Attribute 5: link_support
TRUE, if the object's file system supports hard links.
5.8.1.7. Attribute 6: symlink_support
TRUE, if the object's file system supports symbolic links.
5.8.1.8. Attribute 7: named_attr
TRUE, if this object has named attributes. In other words, object
has a non-empty named attribute directory.
5.8.1.9. Attribute 8: fsid
Unique file system identifier for the file system holding this
object. The fsid attribute has major and minor components, each of
which are of data type uint64_t.
5.8.1.10. Attribute 9: unique_handles
TRUE, if two distinct filehandles are guaranteed to refer to two
different file system objects.
5.8.1.11. Attribute 10: lease_time
Duration of the lease at server in seconds.
5.8.1.12. Attribute 11: rdattr_error
Error returned from an attempt to retrieve attributes during a
READDIR operation.
5.8.1.13. Attribute 19: filehandle
The filehandle of this object (primarily for READDIR requests).
5.8.1.14. Attribute 75: suppattr_exclcreat
The bit vector that would set all
REQUIRED and
RECOMMENDED attributes
that are supported by the EXCLUSIVE4_1 method of file creation via
the OPEN operation. The scope of this attribute applies to all
objects with a matching fsid.
5.8.2. Definitions of Uncategorized RECOMMENDED Attributes
The definitions of most of the
RECOMMENDED attributes follow.
Collections that share a common category are defined in other
sections.
5.8.2.1. Attribute 14: archive
TRUE, if this file has been archived since the time of last
modification (deprecated in favor of time_backup).
5.8.2.2. Attribute 15: cansettime
TRUE, if the server is able to change the times for a file system
object as specified in a SETATTR operation.
5.8.2.3. Attribute 16: case_insensitive
TRUE, if file name comparisons on this file system are case
insensitive.
5.8.2.4. Attribute 17: case_preserving
TRUE, if file name case on this file system is preserved.
5.8.2.5. Attribute 60: change_policy
A value created by the server that the client can use to determine if
some server policy related to the current file system has been
subject to change. If the value remains the same, then the client
can be sure that the values of the attributes related to fs location
and the fss_type field of the fs_status attribute have not changed.
On the other hand, a change in this value does necessarily imply a
change in policy. It is up to the client to interrogate the server
to determine if some policy relevant to it has changed. See
Section 3.3.6 for details.
This attribute
MUST change when the value returned by the
fs_locations or fs_locations_info attribute changes, when a file
system goes from read-only to writable or vice versa, or when the
allowable set of security flavors for the file system or any part
thereof is changed.
5.8.2.6. Attribute 18: chown_restricted
If TRUE, the server will reject any request to change either the
owner or the group associated with a file if the caller is not a
privileged user (for example, "root" in UNIX operating environments
or, in Windows 2000, the "Take Ownership" privilege).
5.8.2.7. Attribute 20: fileid
A number uniquely identifying the file within the file system.
5.8.2.8. Attribute 21: files_avail
File slots available to this user on the file system containing this
object -- this should be the smallest relevant limit.
5.8.2.9. Attribute 22: files_free
Free file slots on the file system containing this object -- this
should be the smallest relevant limit.
5.8.2.10. Attribute 23: files_total
Total file slots on the file system containing this object.
5.8.2.11. Attribute 76: fs_charset_cap
Character set capabilities for this file system. See
Section 14.4.
5.8.2.12. Attribute 24: fs_locations
Locations where this file system may be found. If the server returns
NFS4ERR_MOVED as an error, this attribute
MUST be supported. See
Section 11.16 for more details.
5.8.2.13. Attribute 67: fs_locations_info
Full function file system location. See
Section 11.17.2 for more
details.
5.8.2.14. Attribute 61: fs_status
Generic file system type information. See
Section 11.18 for more
details.
5.8.2.15. Attribute 25: hidden
TRUE, if the file is considered hidden with respect to the Windows
API.
5.8.2.16. Attribute 26: homogeneous
TRUE, if this object's file system is homogeneous; i.e., all objects
in the file system (all objects on the server with the same fsid)
have common values for all per-file-system attributes.
5.8.2.17. Attribute 27: maxfilesize
Maximum supported file size for the file system of this object.
5.8.2.18. Attribute 28: maxlink
Maximum number of links for this object.
5.8.2.19. Attribute 29: maxname
Maximum file name size supported for this object.
5.8.2.20. Attribute 30: maxread
Maximum amount of data the READ operation will return for this
object.
5.8.2.21. Attribute 31: maxwrite
Maximum amount of data the WRITE operation will accept for this
object. This attribute
SHOULD be supported if the file is writable.
Lack of this attribute can lead to the client either wasting
bandwidth or not receiving the best performance.
5.8.2.22. Attribute 32: mimetype
MIME body type/subtype of this object.
5.8.2.23. Attribute 55: mounted_on_fileid
Like fileid, but if the target filehandle is the root of a file
system, this attribute represents the fileid of the underlying
directory.
UNIX-based operating environments connect a file system into the
namespace by connecting (mounting) the file system onto the existing
file object (the mount point, usually a directory) of an existing
file system. When the mount point's parent directory is read via an
API like readdir(), the return results are directory entries, each
with a component name and a fileid. The fileid of the mount point's
directory entry will be different from the fileid that the stat()
system call returns. The stat() system call is returning the fileid
of the root of the mounted file system, whereas readdir() is
returning the fileid that stat() would have returned before any file
systems were mounted on the mount point.
Unlike NFSv3, NFSv4.1 allows a client's LOOKUP request to cross other
file systems. The client detects the file system crossing whenever
the filehandle argument of LOOKUP has an fsid attribute different
from that of the filehandle returned by LOOKUP. A UNIX-based client
will consider this a "mount point crossing". UNIX has a legacy
scheme for allowing a process to determine its current working
directory. This relies on readdir() of a mount point's parent and
stat() of the mount point returning fileids as previously described.
The mounted_on_fileid attribute corresponds to the fileid that
readdir() would have returned as described previously.
While the NFSv4.1 client could simply fabricate a fileid
corresponding to what mounted_on_fileid provides (and if the server
does not support mounted_on_fileid, the client has no choice), there
is a risk that the client will generate a fileid that conflicts with
one that is already assigned to another object in the file system.
Instead, if the server can provide the mounted_on_fileid, the
potential for client operational problems in this area is eliminated.
If the server detects that there is no mounted point at the target
file object, then the value for mounted_on_fileid that it returns is
the same as that of the fileid attribute.
The mounted_on_fileid attribute is
RECOMMENDED, so the server
SHOULD provide it if possible, and for a UNIX-based server, this is
straightforward. Usually, mounted_on_fileid will be requested during
a READDIR operation, in which case it is trivial (at least for UNIX-
based servers) to return mounted_on_fileid since it is equal to the
fileid of a directory entry returned by readdir(). If
mounted_on_fileid is requested in a GETATTR operation, the server
should obey an invariant that has it returning a value that is equal
to the file object's entry in the object's parent directory, i.e.,
what readdir() would have returned. Some operating environments
allow a series of two or more file systems to be mounted onto a
single mount point. In this case, for the server to obey the
aforementioned invariant, it will need to find the base mount point,
and not the intermediate mount points.
5.8.2.24. Attribute 34: no_trunc
If this attribute is TRUE, then if the client uses a file name longer
than name_max, an error will be returned instead of the name being
truncated.
5.8.2.25. Attribute 35: numlinks
Number of hard links to this object.
5.8.2.26. Attribute 36: owner
The string name of the owner of this object.
5.8.2.27. Attribute 37: owner_group
The string name of the group ownership of this object.
5.8.2.28. Attribute 38: quota_avail_hard
The value in bytes that represents the amount of additional disk
space beyond the current allocation that can be allocated to this
file or directory before further allocations will be refused. It is
understood that this space may be consumed by allocations to other
files or directories.
5.8.2.29. Attribute 39: quota_avail_soft
The value in bytes that represents the amount of additional disk
space that can be allocated to this file or directory before the user
may reasonably be warned. It is understood that this space may be
consumed by allocations to other files or directories though there is
a rule as to which other files or directories.
5.8.2.30. Attribute 40: quota_used
The value in bytes that represents the amount of disk space used by
this file or directory and possibly a number of other similar files
or directories, where the set of "similar" meets at least the
criterion that allocating space to any file or directory in the set
will reduce the "quota_avail_hard" of every other file or directory
in the set.
Note that there may be a number of distinct but overlapping sets of
files or directories for which a quota_used value is maintained,
e.g., "all files with a given owner", "all files with a given group
owner", etc. The server is at liberty to choose any of those sets
when providing the content of the quota_used attribute, but should do
so in a repeatable way. The rule may be configured per file system
or may be "choose the set with the smallest quota".
5.8.2.31. Attribute 41: rawdev
Raw device number of file of type NF4BLK or NF4CHR. The device
number is split into major and minor numbers. If the file's type
attribute is not NF4BLK or NF4CHR, the value returned
SHOULD NOT be
considered useful.
5.8.2.32. Attribute 42: space_avail
Disk space in bytes available to this user on the file system
containing this object -- this should be the smallest relevant limit.
5.8.2.33. Attribute 43: space_free
Free disk space in bytes on the file system containing this object --
this should be the smallest relevant limit.
5.8.2.34. Attribute 44: space_total
Total disk space in bytes on the file system containing this object.
5.8.2.35. Attribute 45: space_used
Number of file system bytes allocated to this object.
5.8.2.36. Attribute 46: system
This attribute is TRUE if this file is a "system" file with respect
to the Windows operating environment.
5.8.2.37. Attribute 47: time_access
The time_access attribute represents the time of last access to the
object by a READ operation sent to the server. The notion of what is
an "access" depends on the server's operating environment and/or the
server's file system semantics. For example, for servers obeying
Portable Operating System Interface (POSIX) semantics, time_access
would be updated only by the READ and READDIR operations and not any
of the operations that modify the content of the object [13], [14],
[15]. Of course, setting the corresponding time_access_set attribute
is another way to modify the time_access attribute.
Whenever the file object resides on a writable file system, the
server should make its best efforts to record time_access into stable
storage. However, to mitigate the performance effects of doing so,
and most especially whenever the server is satisfying the read of the
object's content from its cache, the server
MAY cache access time
updates and lazily write them to stable storage. It is also
acceptable to give administrators of the server the option to disable
time_access updates.
5.8.2.38. Attribute 48: time_access_set
Sets the time of last access to the object. SETATTR use only.
5.8.2.39. Attribute 49: time_backup
The time of last backup of the object.
5.8.2.40. Attribute 50: time_create
The time of creation of the object. This attribute does not have any
relation to the traditional UNIX file attribute "ctime" or "change
time".
5.8.2.41. Attribute 51: time_delta
Smallest useful server time granularity.
5.8.2.42. Attribute 52: time_metadata
The time of last metadata modification of the object.
5.8.2.43. Attribute 53: time_modify
The time of last modification to the object.
5.8.2.44. Attribute 54: time_modify_set
Sets the time of last modification to the object. SETATTR use only.
5.9. Interpreting owner and owner_group
The
RECOMMENDED attributes "owner" and "owner_group" (and also users
and groups within the "acl" attribute) are represented in terms of a
UTF-8 string. To avoid a representation that is tied to a particular
underlying implementation at the client or server, the use of the
UTF-8 string has been chosen. Note that
Section 6.1 of RFC 2624 [53]
provides additional rationale. It is expected that the client and
server will have their own local representation of owner and
owner_group that is used for local storage or presentation to the end
user. Therefore, it is expected that when these attributes are
transferred between the client and server, the local representation
is translated to a syntax of the form "user@dns_domain". This will
allow for a client and server that do not use the same local
representation the ability to translate to a common syntax that can
be interpreted by both.
Similarly, security principals may be represented in different ways
by different security mechanisms. Servers normally translate these
representations into a common format, generally that used by local
storage, to serve as a means of identifying the users corresponding
to these security principals. When these local identifiers are
translated to the form of the owner attribute, associated with files
created by such principals, they identify, in a common format, the
users associated with each corresponding set of security principals.
The translation used to interpret owner and group strings is not
specified as part of the protocol. This allows various solutions to
be employed. For example, a local translation table may be consulted
that maps a numeric identifier to the user@dns_domain syntax. A name
service may also be used to accomplish the translation. A server may
provide a more general service, not limited by any particular
translation (which would only translate a limited set of possible
strings) by storing the owner and owner_group attributes in local
storage without any translation or it may augment a translation
method by storing the entire string for attributes for which no
translation is available while using the local representation for
those cases in which a translation is available.
Servers that do not provide support for all possible values of the
owner and owner_group attributes
SHOULD return an error
(NFS4ERR_BADOWNER) when a string is presented that has no
translation, as the value to be set for a SETATTR of the owner,
owner_group, or acl attributes. When a server does accept an owner
or owner_group value as valid on a SETATTR (and similarly for the
owner and group strings in an acl), it is promising to return that
same string when a corresponding GETATTR is done. Configuration
changes (including changes from the mapping of the string to the
local representation) and ill-constructed name translations (those
that contain aliasing) may make that promise impossible to honor.
Servers should make appropriate efforts to avoid a situation in which
these attributes have their values changed when no real change to
ownership has occurred.
The "dns_domain" portion of the owner string is meant to be a DNS
domain name, for example, user@example.org. Servers should accept as
valid a set of users for at least one domain. A server may treat
other domains as having no valid translations. A more general
service is provided when a server is capable of accepting users for
multiple domains, or for all domains, subject to security
constraints.
In the case where there is no translation available to the client or
server, the attribute value will be constructed without the "@".
Therefore, the absence of the @ from the owner or owner_group
attribute signifies that no translation was available at the sender
and that the receiver of the attribute should not use that string as
a basis for translation into its own internal format. Even though
the attribute value cannot be translated, it may still be useful. In
the case of a client, the attribute string may be used for local
display of ownership.
To provide a greater degree of compatibility with NFSv3, which
identified users and groups by 32-bit unsigned user identifiers and
group identifiers, owner and group strings that consist of decimal
numeric values with no leading zeros can be given a special
interpretation by clients and servers that choose to provide such
support. The receiver may treat such a user or group string as
representing the same user as would be represented by an NFSv3 uid or
gid having the corresponding numeric value. A server is not
obligated to accept such a string, but may return an NFS4ERR_BADOWNER
instead. To avoid this mechanism being used to subvert user and
group translation, so that a client might pass all of the owners and
groups in numeric form, a server
SHOULD return an NFS4ERR_BADOWNER
error when there is a valid translation for the user or owner
designated in this way. In that case, the client must use the
appropriate name@domain string and not the special form for
compatibility.
The owner string "nobody" may be used to designate an anonymous user,
which will be associated with a file created by a security principal
that cannot be mapped through normal means to the owner attribute.
Users and implementations of NFSv4.1
SHOULD NOT use "nobody" to
designate a real user whose access is not anonymous.
5.10. Character Case Attributes
With respect to the case_insensitive and case_preserving attributes,
each UCS-4 character (which UTF-8 encodes) can be mapped according to
Appendix B.2 of RFC 3454 [16]. For general character handling and
internationalization issues, see
Section 14.
5.11. Directory Notification Attributes
As described in
Section 18.39, the client can request a minimum delay
for notifications of changes to attributes, but the server is free to
ignore what the client requests. The client can determine in advance
what notification delays the server will accept by sending a GETATTR
operation for either or both of two directory notification
attributes. When the client calls the GET_DIR_DELEGATION operation
and asks for attribute change notifications, it should request
notification delays that are no less than the values in the server-
provided attributes.
5.11.1. Attribute 56: dir_notif_delay
The dir_notif_delay attribute is the minimum number of seconds the
server will delay before notifying the client of a change to the
directory's attributes.
5.11.2. Attribute 57: dirent_notif_delay
The dirent_notif_delay attribute is the minimum number of seconds the
server will delay before notifying the client of a change to a file
object that has an entry in the directory.
5.12. pNFS Attribute Definitions
5.12.1. Attribute 62: fs_layout_type
The fs_layout_type attribute (see
Section 3.3.13) applies to a file
system and indicates what layout types are supported by the file
system. When the client encounters a new fsid, the client
SHOULD obtain the value for the fs_layout_type attribute associated with the
new file system. This attribute is used by the client to determine
if the layout types supported by the server match any of the client's
supported layout types.
5.12.2. Attribute 66: layout_alignment
When a client holds layouts on files of a file system, the
layout_alignment attribute indicates the preferred alignment for I/O
to files on that file system. Where possible, the client should send
READ and WRITE operations with offsets that are whole multiples of
the layout_alignment attribute.
5.12.3. Attribute 65: layout_blksize
When a client holds layouts on files of a file system, the
layout_blksize attribute indicates the preferred block size for I/O
to files on that file system. Where possible, the client should send
READ operations with a count argument that is a whole multiple of
layout_blksize, and WRITE operations with a data argument of size
that is a whole multiple of layout_blksize.
5.12.4. Attribute 63: layout_hint
The layout_hint attribute (see
Section 3.3.19) may be set on newly
created files to influence the metadata server's choice for the
file's layout. If possible, this attribute is one of those set in
the initial attributes within the OPEN operation. The metadata
server may choose to ignore this attribute. The layout_hint
attribute is a subset of the layout structure returned by LAYOUTGET.
For example, instead of specifying particular devices, this would be
used to suggest the stripe width of a file. The server
implementation determines which fields within the layout will be
used.
5.12.5. Attribute 64: layout_type
This attribute lists the layout type(s) available for a file. The
value returned by the server is for informational purposes only. The
client will use the LAYOUTGET operation to obtain the information
needed in order to perform I/O, for example, the specific device
information for the file and its layout.
5.12.6. Attribute 68: mdsthreshold
This attribute is a server-provided hint used to communicate to the
client when it is more efficient to send READ and WRITE operations to
the metadata server or the data server. The two types of thresholds
described are file size thresholds and I/O size thresholds. If a
file's size is smaller than the file size threshold, data accesses
SHOULD be sent to the metadata server. If an I/O request has a
length that is below the I/O size threshold, the I/O
SHOULD be sent
to the metadata server. Each threshold type is specified separately
for read and write.
The server
MAY provide both types of thresholds for a file. If both
file size and I/O size are provided, the client
SHOULD reach or
exceed both thresholds before sending its read or write requests to
the data server. Alternatively, if only one of the specified
thresholds is reached or exceeded, the I/O requests are sent to the
metadata server.
For each threshold type, a value of zero indicates no READ or WRITE
should be sent to the metadata server, while a value of all ones
indicates that all READs or WRITEs should be sent to the metadata
server.
The attribute is available on a per-filehandle basis. If the current
filehandle refers to a non-pNFS file or directory, the metadata
server should return an attribute that is representative of the
filehandle's file system. It is suggested that this attribute is
queried as part of the OPEN operation. Due to dynamic system
changes, the client should not assume that the attribute will remain
constant for any specific time period; thus, it should be
periodically refreshed.
5.13. Retention Attributes
Retention is a concept whereby a file object can be placed in an
immutable, undeletable, unrenamable state for a fixed or infinite
duration of time. Once in this "retained" state, the file cannot be
moved out of the state until the duration of retention has been
reached.
When retention is enabled, retention
MUST extend to the data of the
file, and the name of file. The server
MAY extend retention to any
other property of the file, including any subset of
REQUIRED,
RECOMMENDED, and named attributes, with the exceptions noted in this
section.
Servers
MAY support or not support retention on any file object type.
The five retention attributes are explained in the next subsections.
5.13.1. Attribute 69: retention_get
If retention is enabled for the associated file, this attribute's
value represents the retention begin time of the file object. This
attribute's value is only readable with the GETATTR operation and
MUST NOT be modified by the SETATTR operation (
Section 5.5). The
value of the attribute consists of:
const RET4_DURATION_INFINITE = 0xffffffffffffffff;
struct retention_get4 {
uint64_t rg_duration;
nfstime4 rg_begin_time<1>;
};
The field rg_duration is the duration in seconds indicating how long
the file will be retained once retention is enabled. The field
rg_begin_time is an array of up to one absolute time value. If the
array is zero length, no beginning retention time has been
established, and retention is not enabled. If rg_duration is equal
to RET4_DURATION_INFINITE, the file, once retention is enabled, will
be retained for an infinite duration.
If (as soon as) rg_duration is zero, then rg_begin_time will be of
zero length, and again, retention is not (no longer) enabled.
5.13.2. Attribute 70: retention_set
This attribute is used to set the retention duration and optionally
enable retention for the associated file object. This attribute is
only modifiable via the SETATTR operation and
MUST NOT be retrieved
by the GETATTR operation (
Section 5.5). This attribute corresponds
to retention_get. The value of the attribute consists of:
struct retention_set4 {
bool rs_enable;
uint64_t rs_duration<1>;
};
If the client sets rs_enable to TRUE, then it is enabling retention
on the file object with the begin time of retention starting from the
server's current time and date. The duration of the retention can
also be provided if the rs_duration array is of length one. The
duration is the time in seconds from the begin time of retention, and
if set to RET4_DURATION_INFINITE, the file is to be retained forever.
If retention is enabled, with no duration specified in either this
SETATTR or a previous SETATTR, the duration defaults to zero seconds.
The server
MAY restrict the enabling of retention or the duration of
retention on the basis of the ACE4_WRITE_RETENTION ACL permission.
The enabling of retention
MUST NOT prevent the enabling of event-
based retention or the modification of the retention_hold attribute.
The following rules apply to both the retention_set and retentevt_set
attributes.
* As long as retention is not enabled, the client is permitted to
decrease the duration.
* The duration can always be set to an equal or higher value, even
if retention is enabled. Note that once retention is enabled, the
actual duration (as returned by the retention_get or retentevt_get
attributes; see
Section 5.13.1 or
Section 5.13.3) is constantly
counting down to zero (one unit per second), unless the duration
was set to RET4_DURATION_INFINITE. Thus, it will not be possible
for the client to precisely extend the duration on a file that has
retention enabled.
* While retention is enabled, attempts to disable retention or
decrease the retention's duration
MUST fail with the error
NFS4ERR_INVAL.
* If the principal attempting to change retention_set or
retentevt_set does not have ACE4_WRITE_RETENTION permissions, the
attempt
MUST fail with NFS4ERR_ACCESS.
5.13.3. Attribute 71: retentevt_get
Gets the event-based retention duration, and if enabled, the event-
based retention begin time of the file object. This attribute is
like retention_get, but refers to event-based retention. The event
that triggers event-based retention is not defined by the NFSv4.1
specification.
5.13.4. Attribute 72: retentevt_set
Sets the event-based retention duration, and optionally enables
event-based retention on the file object. This attribute corresponds
to retentevt_get and is like retention_set, but refers to event-based
retention. When event-based retention is set, the file
MUST be
retained even if non-event-based retention has been set, and the
duration of non-event-based retention has been reached. Conversely,
when non-event-based retention has been set, the file
MUST be
retained even if event-based retention has been set, and the duration
of event-based retention has been reached. The server
MAY restrict
the enabling of event-based retention or the duration of event-based
retention on the basis of the ACE4_WRITE_RETENTION ACL permission.
The enabling of event-based retention
MUST NOT prevent the enabling
of non-event-based retention or the modification of the
retention_hold attribute.
5.13.5. Attribute 73: retention_hold
Gets or sets administrative retention holds, one hold per bit
position.
This attribute allows one to 64 administrative holds, one hold per
bit on the attribute. If retention_hold is not zero, then the file
MUST NOT be deleted, renamed, or modified, even if the duration on
enabled event or non-event-based retention has been reached. The
server
MAY restrict the modification of retention_hold on the basis
of the ACE4_WRITE_RETENTION_HOLD ACL permission. The enabling of
administration retention holds does not prevent the enabling of
event-based or non-event-based retention.
If the principal attempting to change retention_hold does not have
ACE4_WRITE_RETENTION_HOLD permissions, the attempt
MUST fail with
NFS4ERR_ACCESS.
6. Access Control Attributes
Access Control Lists (ACLs) are file attributes that specify fine-
grained access control. This section covers the "acl", "dacl",
"sacl", "aclsupport", "mode", and "mode_set_masked" file attributes
and their interactions. Note that file attributes may apply to any
file system object.
ACLs and modes represent two well-established models for specifying
permissions. This section specifies requirements that attempt to
meet the following goals:
* If a server supports the mode attribute, it should provide
reasonable semantics to clients that only set and retrieve the
mode attribute.
* If a server supports ACL attributes, it should provide reasonable
semantics to clients that only set and retrieve those attributes.
* On servers that support the mode attribute, if ACL attributes have
never been set on an object, via inheritance or explicitly, the
behavior should be traditional UNIX-like behavior.
* On servers that support the mode attribute, if the ACL attributes
have been previously set on an object, either explicitly or via
inheritance:
- Setting only the mode attribute should effectively control the
traditional UNIX-like permissions of read, write, and execute
on owner, owner_group, and other.
- Setting only the mode attribute should provide reasonable
security. For example, setting a mode of 000 should be enough
to ensure that future OPEN operations for
OPEN4_SHARE_ACCESS_READ or OPEN4_SHARE_ACCESS_WRITE by any
principal fail, regardless of a previously existing or
inherited ACL.
* NFSv4.1 may introduce different semantics relating to the mode and
ACL attributes, but it does not render invalid any previously
existing implementations. Additionally, this section provides
clarifications based on previous implementations and discussions
around them.
* On servers that support both the mode and the acl or dacl
attributes, the server must keep the two consistent with each
other. The value of the mode attribute (with the exception of the
three high-order bits described in
Section 6.2.4) must be
determined entirely by the value of the ACL, so that use of the
mode is never required for anything other than setting the three
high-order bits. See
Section 6.4.1 for exact requirements.
* When a mode attribute is set on an object, the ACL attributes may
need to be modified in order to not conflict with the new mode.
In such cases, it is desirable that the ACL keep as much
information as possible. This includes information about
inheritance, AUDIT and ALARM ACEs, and permissions granted and
denied that do not conflict with the new mode.
6.2. File Attributes Discussion
6.2.1. Attribute 12: acl
The NFSv4.1 ACL attribute contains an array of Access Control Entries
(ACEs) that are associated with the file system object. Although the
client can set and get the acl attribute, the server is responsible
for using the ACL to perform access control. The client can use the
OPEN or ACCESS operations to check access without modifying or
reading data or metadata.
The NFS ACE structure is defined as follows:
typedef uint32_t acetype4;
typedef uint32_t aceflag4;
typedef uint32_t acemask4;
struct nfsace4 {
acetype4 type;
aceflag4 flag;
acemask4 access_mask;
utf8str_mixed who;
};
To determine if a request succeeds, the server processes each nfsace4
entry in order. Only ACEs that have a "who" that matches the
requester are considered. Each ACE is processed until all of the
bits of the requester's access have been ALLOWED. Once a bit (see
below) has been ALLOWED by an ACCESS_ALLOWED_ACE, it is no longer
considered in the processing of later ACEs. If an ACCESS_DENIED_ACE
is encountered where the requester's access still has unALLOWED bits
in common with the "access_mask" of the ACE, the request is denied.
When the ACL is fully processed, if there are bits in the requester's
mask that have not been ALLOWED or DENIED, access is denied.
Unlike the ALLOW and DENY ACE types, the ALARM and AUDIT ACE types do
not affect a requester's access, and instead are for triggering
events as a result of a requester's access attempt. Therefore, AUDIT
and ALARM ACEs are processed only after processing ALLOW and DENY
ACEs.
The NFSv4.1 ACL model is quite rich. Some server platforms may
provide access-control functionality that goes beyond the UNIX-style
mode attribute, but that is not as rich as the NFS ACL model. So
that users can take advantage of this more limited functionality, the
server may support the acl attributes by mapping between its ACL
model and the NFSv4.1 ACL model. Servers must ensure that the ACL
they actually store or enforce is at least as strict as the NFSv4 ACL
that was set. It is tempting to accomplish this by rejecting any ACL
that falls outside the small set that can be represented accurately.
However, such an approach can render ACLs unusable without special
client-side knowledge of the server's mapping, which defeats the
purpose of having a common NFSv4 ACL protocol. Therefore, servers
should accept every ACL that they can without compromising security.
To help accomplish this, servers may make a special exception, in the
case of unsupported permission bits, to the rule that bits not
ALLOWED or DENIED by an ACL must be denied. For example, a UNIX-
style server might choose to silently allow read attribute
permissions even though an ACL does not explicitly allow those
permissions. (An ACL that explicitly denies permission to read
attributes should still be rejected.)
The situation is complicated by the fact that a server may have
multiple modules that enforce ACLs. For example, the enforcement for
NFSv4.1 access may be different from, but not weaker than, the
enforcement for local access, and both may be different from the
enforcement for access through other protocols such as SMB (Server
Message Block). So it may be useful for a server to accept an ACL
even if not all of its modules are able to support it.
The guiding principle with regard to NFSv4 access is that the server
must not accept ACLs that appear to make access to the file more
restrictive than it really is.
The constants used for the type field (acetype4) are as follows:
const ACE4_ACCESS_ALLOWED_ACE_TYPE = 0x00000000;
const ACE4_ACCESS_DENIED_ACE_TYPE = 0x00000001;
const ACE4_SYSTEM_AUDIT_ACE_TYPE = 0x00000002;
const ACE4_SYSTEM_ALARM_ACE_TYPE = 0x00000003;
Only the ALLOWED and DENIED bits may be used in the dacl attribute,
and only the AUDIT and ALARM bits may be used in the sacl attribute.
All four are permitted in the acl attribute.
+==============================+==============+=====================+
| Value | Abbreviation | Description |
+==============================+==============+=====================+
| ACE4_ACCESS_ALLOWED_ACE_TYPE | ALLOW | Explicitly grants |
| | | the access |
| | | defined in |
| | | acemask4 to the |
| | | file or |
| | | directory. |
+------------------------------+--------------+---------------------+
| ACE4_ACCESS_DENIED_ACE_TYPE | DENY | Explicitly denies |
| | | the access |
| | | defined in |
| | | acemask4 to the |
| | | file or |
| | | directory. |
+------------------------------+--------------+---------------------+
| ACE4_SYSTEM_AUDIT_ACE_TYPE | AUDIT | Log (in a system- |
| | | dependent way) |
| | | any access |
| | | attempt to a file |
| | | or directory that |
| | | uses any of the |
| | | access methods |
| | | specified in |
| | | acemask4. |
+------------------------------+--------------+---------------------+
| ACE4_SYSTEM_ALARM_ACE_TYPE | ALARM | Generate an alarm |
| | | (in a system- |
| | | dependent way) |
| | | when any access |
| | | attempt is made |
| | | to a file or |
| | | directory for the |
| | | access methods |
| | | specified in |
| | | acemask4. |
+------------------------------+--------------+---------------------+
Table 6
The "Abbreviation" column denotes how the types will be referred to
throughout the rest of this section.
6.2.1.2. Attribute 13: aclsupport
A server need not support all of the above ACE types. This attribute
indicates which ACE types are supported for the current file system.
The bitmask constants used to represent the above definitions within
the aclsupport attribute are as follows:
const ACL4_SUPPORT_ALLOW_ACL = 0x00000001;
const ACL4_SUPPORT_DENY_ACL = 0x00000002;
const ACL4_SUPPORT_AUDIT_ACL = 0x00000004;
const ACL4_SUPPORT_ALARM_ACL = 0x00000008;
Servers that support either the ALLOW or DENY ACE type
SHOULD support
both ALLOW and DENY ACE types.
Clients should not attempt to set an ACE unless the server claims
support for that ACE type. If the server receives a request to set
an ACE that it cannot store, it
MUST reject the request with
NFS4ERR_ATTRNOTSUPP. If the server receives a request to set an ACE
that it can store but cannot enforce, the server
SHOULD reject the
request with NFS4ERR_ATTRNOTSUPP.
Support for any of the ACL attributes is optional (albeit
RECOMMENDED). However, a server that supports either of the new ACL
attributes (dacl or sacl)
MUST allow use of the new ACL attributes to
access all of the ACE types that it supports. In other words, if
such a server supports ALLOW or DENY ACEs, then it
MUST support the
dacl attribute, and if it supports AUDIT or ALARM ACEs, then it
MUST support the sacl attribute.
The bitmask constants used for the access mask field are as follows:
const ACE4_READ_DATA = 0x00000001;
const ACE4_LIST_DIRECTORY = 0x00000001;
const ACE4_WRITE_DATA = 0x00000002;
const ACE4_ADD_FILE = 0x00000002;
const ACE4_APPEND_DATA = 0x00000004;
const ACE4_ADD_SUBDIRECTORY = 0x00000004;
const ACE4_READ_NAMED_ATTRS = 0x00000008;
const ACE4_WRITE_NAMED_ATTRS = 0x00000010;
const ACE4_EXECUTE = 0x00000020;
const ACE4_DELETE_CHILD = 0x00000040;
const ACE4_READ_ATTRIBUTES = 0x00000080;
const ACE4_WRITE_ATTRIBUTES = 0x00000100;
const ACE4_WRITE_RETENTION = 0x00000200;
const ACE4_WRITE_RETENTION_HOLD = 0x00000400;
const ACE4_DELETE = 0x00010000;
const ACE4_READ_ACL = 0x00020000;
const ACE4_WRITE_ACL = 0x00040000;
const ACE4_WRITE_OWNER = 0x00080000;
const ACE4_SYNCHRONIZE = 0x00100000;
Note that some masks have coincident values, for example,
ACE4_READ_DATA and ACE4_LIST_DIRECTORY. The mask entries
ACE4_LIST_DIRECTORY, ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY are
intended to be used with directory objects, while ACE4_READ_DATA,
ACE4_WRITE_DATA, and ACE4_APPEND_DATA are intended to be used with
non-directory objects.
6.2.1.3.1. Discussion of Mask Attributes
ACE4_READ_DATA
Operation(s) affected:
READ
OPEN
Discussion:
Permission to read the data of the file.
Servers
SHOULD allow a user the ability to read the data of the
file when only the ACE4_EXECUTE access mask bit is allowed.
ACE4_LIST_DIRECTORY
Operation(s) affected:
READDIR
Discussion:
Permission to list the contents of a directory.
ACE4_WRITE_DATA
Operation(s) affected:
WRITE
OPEN
SETATTR of size
Discussion:
Permission to modify a file's data.
ACE4_ADD_FILE
Operation(s) affected:
CREATE
LINK
OPEN
RENAME
Discussion:
Permission to add a new file in a directory. The CREATE
operation is affected when nfs_ftype4 is NF4LNK, NF4BLK,
NF4CHR, NF4SOCK, or NF4FIFO. (NF4DIR is not listed because it
is covered by ACE4_ADD_SUBDIRECTORY.) OPEN is affected when
used to create a regular file. LINK and RENAME are always
affected.
ACE4_APPEND_DATA
Operation(s) affected:
WRITE
OPEN
SETATTR of size
Discussion:
The ability to modify a file's data, but only starting at EOF.
This allows for the notion of append-only files, by allowing
ACE4_APPEND_DATA and denying ACE4_WRITE_DATA to the same user
or group. If a file has an ACL such as the one described above
and a WRITE request is made for somewhere other than EOF, the
server
SHOULD return NFS4ERR_ACCESS.
ACE4_ADD_SUBDIRECTORY
Operation(s) affected:
CREATE
RENAME
Discussion:
Permission to create a subdirectory in a directory. The CREATE
operation is affected when nfs_ftype4 is NF4DIR. The RENAME
operation is always affected.
ACE4_READ_NAMED_ATTRS
Operation(s) affected:
OPENATTR
Discussion:
Permission to read the named attributes of a file or to look up
the named attribute directory. OPENATTR is affected when it is
not used to create a named attribute directory. This is when
1) createdir is TRUE, but a named attribute directory already
exists, or 2) createdir is FALSE.
ACE4_WRITE_NAMED_ATTRS
Operation(s) affected:
OPENATTR
Discussion:
Permission to write the named attributes of a file or to create
a named attribute directory. OPENATTR is affected when it is
used to create a named attribute directory. This is when
createdir is TRUE and no named attribute directory exists. The
ability to check whether or not a named attribute directory
exists depends on the ability to look it up; therefore, users
also need the ACE4_READ_NAMED_ATTRS permission in order to
create a named attribute directory.
ACE4_EXECUTE
Operation(s) affected:
READ
OPEN
REMOVE
RENAME
LINK
CREATE
Discussion:
Permission to execute a file.
Servers
SHOULD allow a user the ability to read the data of the
file when only the ACE4_EXECUTE access mask bit is allowed.
This is because there is no way to execute a file without
reading the contents. Though a server may treat ACE4_EXECUTE
and ACE4_READ_DATA bits identically when deciding to permit a
READ operation, it
SHOULD still allow the two bits to be set
independently in ACLs, and
MUST distinguish between them when
replying to ACCESS operations. In particular, servers
SHOULD
NOT silently turn on one of the two bits when the other is set,
as that would make it impossible for the client to correctly
enforce the distinction between read and execute permissions.
As an example, following a SETATTR of the following ACL:
nfsuser:ACE4_EXECUTE:ALLOW
A subsequent GETATTR of ACL for that file
SHOULD return:
nfsuser:ACE4_EXECUTE:ALLOW
Rather than:
nfsuser:ACE4_EXECUTE/ACE4_READ_DATA:ALLOW
ACE4_EXECUTE
Operation(s) affected:
LOOKUP
Discussion:
Permission to traverse/search a directory.
ACE4_DELETE_CHILD
Operation(s) affected:
REMOVE
RENAME
Discussion:
Permission to delete a file or directory within a directory.
See
Section 6.2.1.3.2 for information on ACE4_DELETE and
ACE4_DELETE_CHILD interact.
ACE4_READ_ATTRIBUTES
Operation(s) affected:
GETATTR of file system object attributes
VERIFY
NVERIFY
READDIR
Discussion:
The ability to read basic attributes (non-ACLs) of a file. On
a UNIX system, basic attributes can be thought of as the stat-
level attributes. Allowing this access mask bit would mean
that the entity can execute "ls -l" and stat. If a READDIR
operation requests attributes, this mask must be allowed for
the READDIR to succeed.
ACE4_WRITE_ATTRIBUTES
Operation(s) affected:
SETATTR of time_access_set, time_backup,
time_create, time_modify_set, mimetype, hidden, system
Discussion:
Permission to change the times associated with a file or
directory to an arbitrary value. Also permission to change the
mimetype, hidden, and system attributes. A user having
ACE4_WRITE_DATA or ACE4_WRITE_ATTRIBUTES will be allowed to set
the times associated with a file to the current server time.
ACE4_WRITE_RETENTION
Operation(s) affected:
SETATTR of retention_set, retentevt_set.
Discussion:
Permission to modify the durations of event and non-event-based
retention. Also permission to enable event and non-event-based
retention. A server
MAY behave such that setting
ACE4_WRITE_ATTRIBUTES allows ACE4_WRITE_RETENTION.
ACE4_WRITE_RETENTION_HOLD
Operation(s) affected:
SETATTR of retention_hold.
Discussion:
Permission to modify the administration retention holds. A
server
MAY map ACE4_WRITE_ATTRIBUTES to
ACE_WRITE_RETENTION_HOLD.
ACE4_DELETE
Operation(s) affected:
REMOVE
Discussion:
Permission to delete the file or directory. See
Section 6.2.1.3.2 for information on ACE4_DELETE and
ACE4_DELETE_CHILD interact.
ACE4_READ_ACL
Operation(s) affected:
GETATTR of acl, dacl, or sacl
NVERIFY
VERIFY
Discussion:
Permission to read the ACL.
ACE4_WRITE_ACL
Operation(s) affected:
SETATTR of acl and mode
Discussion:
Permission to write the acl and mode attributes.
ACE4_WRITE_OWNER
Operation(s) affected:
SETATTR of owner and owner_group
Discussion:
Permission to write the owner and owner_group attributes. On
UNIX systems, this is the ability to execute chown() and
chgrp().
ACE4_SYNCHRONIZE
Operation(s) affected:
NONE
Discussion:
Permission to use the file object as a synchronization
primitive for interprocess communication. This permission is
not enforced or interpreted by the NFSv4.1 server on behalf of
the client.
Typically, the ACE4_SYNCHRONIZE permission is only meaningful
on local file systems, i.e., file systems not accessed via
NFSv4.1. The reason that the permission bit exists is that
some operating environments, such as Windows, use
ACE4_SYNCHRONIZE.
For example, if a client copies a file that has
ACE4_SYNCHRONIZE set from a local file system to an NFSv4.1
server, and then later copies the file from the NFSv4.1 server
to a local file system, it is likely that if ACE4_SYNCHRONIZE
was set in the original file, the client will want it set in
the second copy. The first copy will not have the permission
set unless the NFSv4.1 server has the means to set the
ACE4_SYNCHRONIZE bit. The second copy will not have the
permission set unless the NFSv4.1 server has the means to
retrieve the ACE4_SYNCHRONIZE bit.
Server implementations need not provide the granularity of control
that is implied by this list of masks. For example, POSIX-based
systems might not distinguish ACE4_APPEND_DATA (the ability to append
to a file) from ACE4_WRITE_DATA (the ability to modify existing
contents); both masks would be tied to a single "write" permission
[17]. When such a server returns attributes to the client, it would
show both ACE4_APPEND_DATA and ACE4_WRITE_DATA if and only if the
write permission is enabled.
If a server receives a SETATTR request that it cannot accurately
implement, it should err in the direction of more restricted access,
except in the previously discussed cases of execute and read. For
example, suppose a server cannot distinguish overwriting data from
appending new data, as described in the previous paragraph. If a
client submits an ALLOW ACE where ACE4_APPEND_DATA is set but
ACE4_WRITE_DATA is not (or vice versa), the server should either turn
off ACE4_APPEND_DATA or reject the request with NFS4ERR_ATTRNOTSUPP.
6.2.1.3.2. ACE4_DELETE vs. ACE4_DELETE_CHILD
Two access mask bits govern the ability to delete a directory entry:
ACE4_DELETE on the object itself (the "target") and ACE4_DELETE_CHILD
on the containing directory (the "parent").
Many systems also take the "sticky bit" (MODE4_SVTX) on a directory
to allow unlink only to a user that owns either the target or the
parent; on some such systems the decision also depends on whether the
target is writable.
Servers
SHOULD allow unlink if either ACE4_DELETE is permitted on the
target, or ACE4_DELETE_CHILD is permitted on the parent. (Note that
this is true even if the parent or target explicitly denies one of
these permissions.)
If the ACLs in question neither explicitly ALLOW nor DENY either of
the above, and if MODE4_SVTX is not set on the parent, then the
server
SHOULD allow the removal if and only if ACE4_ADD_FILE is
permitted. In the case where MODE4_SVTX is set, the server may also
require the remover to own either the parent or the target, or may
require the target to be writable.
This allows servers to support something close to traditional UNIX-
like semantics, with ACE4_ADD_FILE taking the place of the write bit.
The bitmask constants used for the flag field are as follows:
const ACE4_FILE_INHERIT_ACE = 0x00000001;
const ACE4_DIRECTORY_INHERIT_ACE = 0x00000002;
const ACE4_NO_PROPAGATE_INHERIT_ACE = 0x00000004;
const ACE4_INHERIT_ONLY_ACE = 0x00000008;
const ACE4_SUCCESSFUL_ACCESS_ACE_FLAG = 0x00000010;
const ACE4_FAILED_ACCESS_ACE_FLAG = 0x00000020;
const ACE4_IDENTIFIER_GROUP = 0x00000040;
const ACE4_INHERITED_ACE = 0x00000080;
A server need not support any of these flags. If the server supports
flags that are similar to, but not exactly the same as, these flags,
the implementation may define a mapping between the protocol-defined
flags and the implementation-defined flags.
For example, suppose a client tries to set an ACE with
ACE4_FILE_INHERIT_ACE set but not ACE4_DIRECTORY_INHERIT_ACE. If the
server does not support any form of ACL inheritance, the server
should reject the request with NFS4ERR_ATTRNOTSUPP. If the server
supports a single "inherit ACE" flag that applies to both files and
directories, the server may reject the request (i.e., requiring the
client to set both the file and directory inheritance flags). The
server may also accept the request and silently turn on the
ACE4_DIRECTORY_INHERIT_ACE flag.
6.2.1.4.1. Discussion of Flag Bits
ACE4_FILE_INHERIT_ACE
Any non-directory file in any sub-directory will get this ACE
inherited.
ACE4_DIRECTORY_INHERIT_ACE
Can be placed on a directory and indicates that this ACE should be
added to each new directory created.
If this flag is set in an ACE in an ACL attribute to be set on a
non-directory file system object, the operation attempting to set
the ACL
SHOULD fail with NFS4ERR_ATTRNOTSUPP.
ACE4_NO_PROPAGATE_INHERIT_ACE
Can be placed on a directory. This flag tells the server that
inheritance of this ACE should stop at newly created child
directories.
ACE4_INHERIT_ONLY_ACE
Can be placed on a directory but does not apply to the directory;
ALLOW and DENY ACEs with this bit set do not affect access to the
directory, and AUDIT and ALARM ACEs with this bit set do not
trigger log or alarm events. Such ACEs only take effect once they
are applied (with this bit cleared) to newly created files and
directories as specified by the ACE4_FILE_INHERIT_ACE and
ACE4_DIRECTORY_INHERIT_ACE flags.
If this flag is present on an ACE, but neither
ACE4_DIRECTORY_INHERIT_ACE nor ACE4_FILE_INHERIT_ACE is present,
then an operation attempting to set such an attribute
SHOULD fail
with NFS4ERR_ATTRNOTSUPP.
ACE4_SUCCESSFUL_ACCESS_ACE_FLAG and ACE4_FAILED_ACCESS_ACE_FLAG
The ACE4_SUCCESSFUL_ACCESS_ACE_FLAG (SUCCESS) and
ACE4_FAILED_ACCESS_ACE_FLAG (FAILED) flag bits may be set only on
ACE4_SYSTEM_AUDIT_ACE_TYPE (AUDIT) and ACE4_SYSTEM_ALARM_ACE_TYPE
(ALARM) ACE types. If during the processing of the file's ACL,
the server encounters an AUDIT or ALARM ACE that matches the
principal attempting the OPEN, the server notes that fact, and the
presence, if any, of the SUCCESS and FAILED flags encountered in
the AUDIT or ALARM ACE. Once the server completes the ACL
processing, it then notes if the operation succeeded or failed.
If the operation succeeded, and if the SUCCESS flag was set for a
matching AUDIT or ALARM ACE, then the appropriate AUDIT or ALARM
event occurs. If the operation failed, and if the FAILED flag was
set for the matching AUDIT or ALARM ACE, then the appropriate
AUDIT or ALARM event occurs. Either or both of the SUCCESS or
FAILED can be set, but if neither is set, the AUDIT or ALARM ACE
is not useful.
The previously described processing applies to ACCESS operations
even when they return NFS4_OK. For the purposes of AUDIT and
ALARM, we consider an ACCESS operation to be a "failure" if it
fails to return a bit that was requested and supported.
ACE4_IDENTIFIER_GROUP
Indicates that the "who" refers to a GROUP as defined under UNIX
or a GROUP ACCOUNT as defined under Windows. Clients and servers
MUST ignore the ACE4_IDENTIFIER_GROUP flag on ACEs with a who
value equal to one of the special identifiers outlined in
Section 6.2.1.5.
ACE4_INHERITED_ACE
Indicates that this ACE is inherited from a parent directory. A
server that supports automatic inheritance will place this flag on
any ACEs inherited from the parent directory when creating a new
object. Client applications will use this to perform automatic
inheritance. Clients and servers
MUST clear this bit in the acl
attribute; it may only be used in the dacl and sacl attributes.
The "who" field of an ACE is an identifier that specifies the
principal or principals to whom the ACE applies. It may refer to a
user or a group, with the flag bit ACE4_IDENTIFIER_GROUP specifying
which.
There are several special identifiers that need to be understood
universally, rather than in the context of a particular DNS domain.
Some of these identifiers cannot be understood when an NFS client
accesses the server, but have meaning when a local process accesses
the file. The ability to display and modify these permissions is
permitted over NFS, even if none of the access methods on the server
understands the identifiers.
+===============+==================================================+
| Who | Description |
+===============+==================================================+
| OWNER | The owner of the file. |
+---------------+--------------------------------------------------+
| GROUP | The group associated with the file. |
+---------------+--------------------------------------------------+
| EVERYONE | The world, including the owner and owning group. |
+---------------+--------------------------------------------------+
| INTERACTIVE | Accessed from an interactive terminal. |
+---------------+--------------------------------------------------+
| NETWORK | Accessed via the network. |
+---------------+--------------------------------------------------+
| DIALUP | Accessed as a dialup user to the server. |
+---------------+--------------------------------------------------+
| BATCH | Accessed from a batch job. |
+---------------+--------------------------------------------------+
| ANONYMOUS | Accessed without any authentication. |
+---------------+--------------------------------------------------+
| AUTHENTICATED | Any authenticated user (opposite of ANONYMOUS). |
+---------------+--------------------------------------------------+
| SERVICE | Access from a system service. |
+---------------+--------------------------------------------------+
Table 7
To avoid conflict, these special identifiers are distinguished by an
appended "@" and should appear in the form "xxxx@" (with no domain
name after the "@"), for example, ANONYMOUS@.
The ACE4_IDENTIFIER_GROUP flag
MUST be ignored on entries with these
special identifiers. When encoding entries with these special
identifiers, the ACE4_IDENTIFIER_GROUP flag
SHOULD be set to zero.
6.2.1.5.1. Discussion of EVERYONE@
It is important to note that "EVERYONE@" is not equivalent to the
UNIX "other" entity. This is because, by definition, UNIX "other"
does not include the owner or owning group of a file. "EVERYONE@"
means literally everyone, including the owner or owning group.
6.2.2. Attribute 58: dacl
The dacl attribute is like the acl attribute, but dacl allows just
ALLOW and DENY ACEs. The dacl attribute supports automatic
inheritance (see
Section 6.4.3.2).
6.2.3. Attribute 59: sacl
The sacl attribute is like the acl attribute, but sacl allows just
AUDIT and ALARM ACEs. The sacl attribute supports automatic
inheritance (see
Section 6.4.3.2).
6.2.4. Attribute 33: mode
The NFSv4.1 mode attribute is based on the UNIX mode bits. The
following bits are defined:
const MODE4_SUID = 0x800; /* set user id on execution */
const MODE4_SGID = 0x400; /* set group id on execution */
const MODE4_SVTX = 0x200; /* save text even after use */
const MODE4_RUSR = 0x100; /* read permission: owner */
const MODE4_WUSR = 0x080; /* write permission: owner */
const MODE4_XUSR = 0x040; /* execute permission: owner */
const MODE4_RGRP = 0x020; /* read permission: group */
const MODE4_WGRP = 0x010; /* write permission: group */
const MODE4_XGRP = 0x008; /* execute permission: group */
const MODE4_ROTH = 0x004; /* read permission: other */
const MODE4_WOTH = 0x002; /* write permission: other */
const MODE4_XOTH = 0x001; /* execute permission: other */
Bits MODE4_RUSR, MODE4_WUSR, and MODE4_XUSR apply to the principal
identified in the owner attribute. Bits MODE4_RGRP, MODE4_WGRP, and
MODE4_XGRP apply to principals identified in the owner_group
attribute but who are not identified in the owner attribute. Bits
MODE4_ROTH, MODE4_WOTH, and MODE4_XOTH apply to any principal that
does not match that in the owner attribute and does not have a group
matching that of the owner_group attribute.
Bits within a mode other than those specified above are not defined
by this protocol. A server
MUST NOT return bits other than those
defined above in a GETATTR or READDIR operation, and it
MUST return
NFS4ERR_INVAL if bits other than those defined above are set in a
SETATTR, CREATE, OPEN, VERIFY, or NVERIFY operation.
6.2.5. Attribute 74: mode_set_masked
The mode_set_masked attribute is a write-only attribute that allows
individual bits in the mode attribute to be set or reset, without
changing others. It allows, for example, the bits MODE4_SUID,
MODE4_SGID, and MODE4_SVTX to be modified while leaving unmodified
any of the nine low-order mode bits devoted to permissions.
In such instances that the nine low-order bits are left unmodified,
then neither the acl nor the dacl attribute should be automatically
modified as discussed in
Section 6.4.1.
The mode_set_masked attribute consists of two words, each in the form
of a mode4. The first consists of the value to be applied to the
current mode value and the second is a mask. Only bits set to one in
the mask word are changed (set or reset) in the file's mode. All
other bits in the mode remain unchanged. Bits in the first word that
correspond to bits that are zero in the mask are ignored, except that
undefined bits are checked for validity and can result in
NFS4ERR_INVAL as described below.
The mode_set_masked attribute is only valid in a SETATTR operation.
If it is used in a CREATE or OPEN operation, the server
MUST return
NFS4ERR_INVAL.
Bits not defined as valid in the mode attribute are not valid in
either word of the mode_set_masked attribute. The server
MUST return
NFS4ERR_INVAL if any such bits are set to one in a SETATTR. If the
mode and mode_set_masked attributes are both specified in the same
SETATTR, the server
MUST also return NFS4ERR_INVAL.
6.3. Common Methods
The requirements in this section will be referred to in future
sections, especially
Section 6.4.
6.3.1. Interpreting an ACL
6.3.1.1. Server Considerations
The server uses the algorithm described in
Section 6.2.1 to determine
whether an ACL allows access to an object. However, the ACL might
not be the sole determiner of access. For example:
* In the case of a file system exported as read-only, the server may
deny write access even though an object's ACL grants it.
* Server implementations
MAY grant ACE4_WRITE_ACL and ACE4_READ_ACL
permissions to prevent a situation from arising in which there is
no valid way to ever modify the ACL.
* All servers will allow a user the ability to read the data of the
file when only the execute permission is granted (i.e., if the ACL
denies the user the ACE4_READ_DATA access and allows the user
ACE4_EXECUTE, the server will allow the user to read the data of
the file).
* Many servers have the notion of owner-override in which the owner
of the object is allowed to override accesses that are denied by
the ACL. This may be helpful, for example, to allow users
continued access to open files on which the permissions have
changed.
* Many servers have the notion of a "superuser" that has privileges
beyond an ordinary user. The superuser may be able to read or
write data or metadata in ways that would not be permitted by the
ACL.
* A retention attribute might also block access otherwise allowed by
ACLs (see
Section 5.13).
6.3.1.2. Client Considerations
Clients
SHOULD NOT do their own access checks based on their
interpretation of the ACL, but rather use the OPEN and ACCESS
operations to do access checks. This allows the client to act on the
results of having the server determine whether or not access should
be granted based on its interpretation of the ACL.
Clients must be aware of situations in which an object's ACL will
define a certain access even though the server will not enforce it.
In general, but especially in these situations, the client needs to
do its part in the enforcement of access as defined by the ACL. To
do this, the client
MAY send the appropriate ACCESS operation prior
to servicing the request of the user or application in order to
determine whether the user or application should be granted the
access requested. For examples in which the ACL may define accesses
that the server doesn't enforce, see
Section 6.3.1.1.
6.3.2. Computing a Mode Attribute from an ACL
The following method can be used to calculate the MODE4_R*, MODE4_W*,
and MODE4_X* bits of a mode attribute, based upon an ACL.
First, for each of the special identifiers OWNER@, GROUP@, and
EVERYONE@, evaluate the ACL in order, considering only ALLOW and DENY
ACEs for the identifier EVERYONE@ and for the identifier under
consideration. The result of the evaluation will be an NFSv4 ACL
mask showing exactly which bits are permitted to that identifier.
Then translate the calculated mask for OWNER@, GROUP@, and EVERYONE@
into mode bits for, respectively, the user, group, and other, as
follows:
1. Set the read bit (MODE4_RUSR, MODE4_RGRP, or MODE4_ROTH) if and
only if ACE4_READ_DATA is set in the corresponding mask.
2. Set the write bit (MODE4_WUSR, MODE4_WGRP, or MODE4_WOTH) if and
only if ACE4_WRITE_DATA and ACE4_APPEND_DATA are both set in the
corresponding mask.
3. Set the execute bit (MODE4_XUSR, MODE4_XGRP, or MODE4_XOTH), if
and only if ACE4_EXECUTE is set in the corresponding mask.
Some server implementations also add bits permitted to named users
and groups to the group bits (MODE4_RGRP, MODE4_WGRP, and
MODE4_XGRP).
Implementations are discouraged from doing this, because it has been
found to cause confusion for users who see members of a file's group
denied access that the mode bits appear to allow. (The presence of
DENY ACEs may also lead to such behavior, but DENY ACEs are expected
to be more rarely used.)
The same user confusion seen when fetching the mode also results if
setting the mode does not effectively control permissions for the
owner, group, and other users; this motivates some of the
requirements that follow.
6.4. Requirements
The server that supports both mode and ACL must take care to
synchronize the MODE4_*USR, MODE4_*GRP, and MODE4_*OTH bits with the
ACEs that have respective who fields of "OWNER@", "GROUP@", and
"EVERYONE@". This way, the client can see if semantically equivalent
access permissions exist whether the client asks for the owner,
owner_group, and mode attributes or for just the ACL.
In this section, much is made of the methods in
Section 6.3.2. Many
requirements refer to this section. But note that the methods have
behaviors specified with "
SHOULD". This is intentional, to avoid
invalidating existing implementations that compute the mode according
to the withdrawn POSIX ACL draft (1003.1e draft 17), rather than by
actual permissions on owner, group, and other.
6.4.1. Setting the Mode and/or ACL Attributes
In the case where a server supports the sacl or dacl attribute, in
addition to the acl attribute, the server
MUST fail a request to set
the acl attribute simultaneously with a dacl or sacl attribute. The
error to be given is NFS4ERR_ATTRNOTSUPP.
6.4.1.1. Setting Mode and not ACL
When any of the nine low-order mode bits are subject to change,
either because the mode attribute was set or because the
mode_set_masked attribute was set and the mask included one or more
bits from the nine low-order mode bits, and no ACL attribute is
explicitly set, the acl and dacl attributes must be modified in
accordance with the updated value of those bits. This must happen
even if the value of the low-order bits is the same after the mode is
set as before.
Note that any AUDIT or ALARM ACEs (hence any ACEs in the sacl
attribute) are unaffected by changes to the mode.
In cases in which the permissions bits are subject to change, the acl
and dacl attributes
MUST be modified such that the mode computed via
the method in
Section 6.3.2 yields the low-order nine bits (MODE4_R*,
MODE4_W*, MODE4_X*) of the mode attribute as modified by the
attribute change. The ACL attributes
SHOULD also be modified such
that:
1. If MODE4_RGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@
SHOULD NOT be granted
ACE4_READ_DATA.
2. If MODE4_WGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@
SHOULD NOT be granted
ACE4_WRITE_DATA or ACE4_APPEND_DATA.
3. If MODE4_XGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@
SHOULD NOT be granted
ACE4_EXECUTE.
Access mask bits other than those listed above, appearing in ALLOW
ACEs,
MAY also be disabled.
Note that ACEs with the flag ACE4_INHERIT_ONLY_ACE set do not affect
the permissions of the ACL itself, nor do ACEs of the type AUDIT and
ALARM. As such, it is desirable to leave these ACEs unmodified when
modifying the ACL attributes.
Also note that the requirement may be met by discarding the acl and
dacl, in favor of an ACL that represents the mode and only the mode.
This is permitted, but it is preferable for a server to preserve as
much of the ACL as possible without violating the above requirements.
Discarding the ACL makes it effectively impossible for a file created
with a mode attribute to inherit an ACL (see
Section 6.4.3).
6.4.1.2. Setting ACL and Not Mode
When setting the acl or dacl and not setting the mode or
mode_set_masked attributes, the permission bits of the mode need to
be derived from the ACL. In this case, the ACL attribute
SHOULD be
set as given. The nine low-order bits of the mode attribute
(MODE4_R*, MODE4_W*, MODE4_X*)
MUST be modified to match the result
of the method in
Section 6.3.2. The three high-order bits of the
mode (MODE4_SUID, MODE4_SGID, MODE4_SVTX)
SHOULD remain unchanged.
6.4.1.3. Setting Both ACL and Mode
When setting both the mode (includes use of either the mode attribute
or the mode_set_masked attribute) and the acl or dacl attributes in
the same operation, the attributes
MUST be applied in this order:
mode (or mode_set_masked), then ACL. The mode-related attribute is
set as given, then the ACL attribute is set as given, possibly
changing the final mode, as described above in
Section 6.4.1.2.
6.4.2. Retrieving the Mode and/or ACL Attributes
This section applies only to servers that support both the mode and
ACL attributes.
Some server implementations may have a concept of "objects without
ACLs", meaning that all permissions are granted and denied according
to the mode attribute and that no ACL attribute is stored for that
object. If an ACL attribute is requested of such a server, the
server
SHOULD return an ACL that does not conflict with the mode;
that is to say, the ACL returned
SHOULD represent the nine low-order
bits of the mode attribute (MODE4_R*, MODE4_W*, MODE4_X*) as
described in
Section 6.3.2.
For other server implementations, the ACL attribute is always present
for every object. Such servers
SHOULD store at least the three high-
order bits of the mode attribute (MODE4_SUID, MODE4_SGID,
MODE4_SVTX). The server
SHOULD return a mode attribute if one is
requested, and the low-order nine bits of the mode (MODE4_R*,
MODE4_W*, MODE4_X*)
MUST match the result of applying the method in
Section 6.3.2 to the ACL attribute.
6.4.3. Creating New Objects
If a server supports any ACL attributes, it may use the ACL
attributes on the parent directory to compute an initial ACL
attribute for a newly created object. This will be referred to as
the inherited ACL within this section. The act of adding one or more
ACEs to the inherited ACL that are based upon ACEs in the parent
directory's ACL will be referred to as inheriting an ACE within this
section.
Implementors should standardize what the behavior of CREATE and OPEN
must be depending on the presence or absence of the mode and ACL
attributes.
1. If just the mode is given in the call:
In this case, inheritance
SHOULD take place, but the mode
MUST be
applied to the inherited ACL as described in
Section 6.4.1.1,
thereby modifying the ACL.
2. If just the ACL is given in the call:
In this case, inheritance
SHOULD NOT take place, and the ACL as
defined in the CREATE or OPEN will be set without modification,
and the mode modified as in
Section 6.4.1.2.
3. If both mode and ACL are given in the call:
In this case, inheritance
SHOULD NOT take place, and both
attributes will be set as described in
Section 6.4.1.3.
4. If neither mode nor ACL is given in the call:
In the case where an object is being created without any initial
attributes at all, e.g., an OPEN operation with an opentype4 of
OPEN4_CREATE and a createmode4 of EXCLUSIVE4, inheritance
SHOULD
NOT take place (note that EXCLUSIVE4_1 is a better choice of
createmode4, since it does permit initial attributes). Instead,
the server
SHOULD set permissions to deny all access to the newly
created object. It is expected that the appropriate client will
set the desired attributes in a subsequent SETATTR operation, and
the server
SHOULD allow that operation to succeed, regardless of
what permissions the object is created with. For example, an
empty ACL denies all permissions, but the server should allow the
owner's SETATTR to succeed even though WRITE_ACL is implicitly
denied.
In other cases, inheritance
SHOULD take place, and no
modifications to the ACL will happen. The mode attribute, if
supported,
MUST be as computed in
Section 6.3.2, with the
MODE4_SUID, MODE4_SGID, and MODE4_SVTX bits clear. If no
inheritable ACEs exist on the parent directory, the rules for
creating acl, dacl, or sacl attributes are implementation
defined. If either the dacl or sacl attribute is supported, then
the ACL4_DEFAULTED flag
SHOULD be set on the newly created
attributes.
6.4.3.1. The Inherited ACL
If the object being created is not a directory, the inherited ACL
SHOULD NOT inherit ACEs from the parent directory ACL unless the
ACE4_FILE_INHERIT_FLAG is set.
If the object being created is a directory, the inherited ACL should
inherit all inheritable ACEs from the parent directory, that is,
those that have the ACE4_FILE_INHERIT_ACE or
ACE4_DIRECTORY_INHERIT_ACE flag set. If the inheritable ACE has
ACE4_FILE_INHERIT_ACE set but ACE4_DIRECTORY_INHERIT_ACE is clear,
the inherited ACE on the newly created directory
MUST have the
ACE4_INHERIT_ONLY_ACE flag set to prevent the directory from being
affected by ACEs meant for non-directories.
When a new directory is created, the server
MAY split any inherited
ACE that is both inheritable and effective (in other words, that has
neither ACE4_INHERIT_ONLY_ACE nor ACE4_NO_PROPAGATE_INHERIT_ACE set),
into two ACEs, one with no inheritance flags and one with
ACE4_INHERIT_ONLY_ACE set. (In the case of a dacl or sacl attribute,
both of those ACEs
SHOULD also have the ACE4_INHERITED_ACE flag set.)
This makes it simpler to modify the effective permissions on the
directory without modifying the ACE that is to be inherited to the
new directory's children.
6.4.3.2. Automatic Inheritance
The acl attribute consists only of an array of ACEs, but the sacl
(
Section 6.2.3) and dacl (
Section 6.2.2) attributes also include an
additional flag field.
struct nfsacl41 {
aclflag4 na41_flag;
nfsace4 na41_aces<>;
};
The flag field applies to the entire sacl or dacl; three flag values
are defined:
const ACL4_AUTO_INHERIT = 0x00000001;
const ACL4_PROTECTED = 0x00000002;
const ACL4_DEFAULTED = 0x00000004;
and all other bits must be cleared. The ACE4_INHERITED_ACE flag may
be set in the ACEs of the sacl or dacl (whereas it must always be
cleared in the acl).
Together these features allow a server to support automatic
inheritance, which we now explain in more detail.
Inheritable ACEs are normally inherited by child objects only at the
time that the child objects are created; later modifications to
inheritable ACEs do not result in modifications to inherited ACEs on
descendants.
However, the dacl and sacl provide an
OPTIONAL mechanism that allows
a client application to propagate changes to inheritable ACEs to an
entire directory hierarchy.
A server that supports this performs inheritance at object creation
time in the normal way, and
SHOULD set the ACE4_INHERITED_ACE flag on
any inherited ACEs as they are added to the new object.
A client application such as an ACL editor may then propagate changes
to inheritable ACEs on a directory by recursively traversing that
directory's descendants and modifying each ACL encountered to remove
any ACEs with the ACE4_INHERITED_ACE flag and to replace them by the
new inheritable ACEs (also with the ACE4_INHERITED_ACE flag set). It
uses the existing ACE inheritance flags in the obvious way to decide
which ACEs to propagate. (Note that it may encounter further
inheritable ACEs when descending the directory hierarchy and that
those will also need to be taken into account when propagating
inheritable ACEs to further descendants.)
The reach of this propagation may be limited in two ways: first,
automatic inheritance is not performed from any directory ACL that
has the ACL4_AUTO_INHERIT flag cleared; and second, automatic
inheritance stops wherever an ACL with the ACL4_PROTECTED flag is
set, preventing modification of that ACL and also (if the ACL is set
on a directory) of the ACL on any of the object's descendants.
This propagation is performed independently for the sacl and the dacl
attributes; thus, the ACL4_AUTO_INHERIT and ACL4_PROTECTED flags may
be independently set for the sacl and the dacl, and propagation of
one type of acl may continue down a hierarchy even where propagation
of the other acl has stopped.
New objects should be created with a dacl and a sacl that both have
the ACL4_PROTECTED flag cleared and the ACL4_AUTO_INHERIT flag set to
the same value as that on, respectively, the sacl or dacl of the
parent object.
Both the dacl and sacl attributes are
RECOMMENDED, and a server may
support one without supporting the other.
A server that supports both the old acl attribute and one or both of
the new dacl or sacl attributes must do so in such a way as to keep
all three attributes consistent with each other. Thus, the ACEs
reported in the acl attribute should be the union of the ACEs
reported in the dacl and sacl attributes, except that the
ACE4_INHERITED_ACE flag must be cleared from the ACEs in the acl.
And of course a client that queries only the acl will be unable to
determine the values of the sacl or dacl flag fields.
When a client performs a SETATTR for the acl attribute, the server
SHOULD set the ACL4_PROTECTED flag to true on both the sacl and the
dacl. By using the acl attribute, as opposed to the dacl or sacl
attributes, the client signals that it may not understand automatic
inheritance, and thus cannot be trusted to set an ACL for which
automatic inheritance would make sense.
When a client application queries an ACL, modifies it, and sets it
again, it should leave any ACEs marked with ACE4_INHERITED_ACE
unchanged, in their original order, at the end of the ACL. If the
application is unable to do this, it should set the ACL4_PROTECTED
flag. This behavior is not enforced by servers, but violations of
this rule may lead to unexpected results when applications perform
automatic inheritance.
If a server also supports the mode attribute, it
SHOULD set the mode
in such a way that leaves inherited ACEs unchanged, in their original
order, at the end of the ACL. If it is unable to do so, it
SHOULD set the ACL4_PROTECTED flag on the file's dacl.
Finally, in the case where the request that creates a new file or
directory does not also set permissions for that file or directory,
and there are also no ACEs to inherit from the parent's directory,
then the server's choice of ACL for the new object is implementation-
dependent. In this case, the server
SHOULD set the ACL4_DEFAULTED
flag on the ACL it chooses for the new object. An application
performing automatic inheritance takes the ACL4_DEFAULTED flag as a
sign that the ACL should be completely replaced by one generated
using the automatic inheritance rules.
7. Single-Server Namespace
This section describes the NFSv4 single-server namespace. Single-
server namespaces may be presented directly to clients, or they may
be used as a basis to form larger multi-server namespaces (e.g.,
site-wide or organization-wide) to be presented to clients, as
described in
Section 11.
7.1. Server Exports
On a UNIX server, the namespace describes all the files reachable by
pathnames under the root directory or "/". On a Windows server, the
namespace constitutes all the files on disks named by mapped disk
letters. NFS server administrators rarely make the entire server's
file system namespace available to NFS clients. More often, portions
of the namespace are made available via an "export" feature. In
previous versions of the NFS protocol, the root filehandle for each
export is obtained through the MOUNT protocol; the client sent a
string that identified the export name within the namespace and the
server returned the root filehandle for that export. The MOUNT
protocol also provided an EXPORTS procedure that enumerated the
server's exports.
7.2. Browsing Exports
The NFSv4.1 protocol provides a root filehandle that clients can use
to obtain filehandles for the exports of a particular server, via a
series of LOOKUP operations within a COMPOUND, to traverse a path. A
common user experience is to use a graphical user interface (perhaps
a file "Open" dialog window) to find a file via progressive browsing
through a directory tree. The client must be able to move from one
export to another export via single-component, progressive LOOKUP
operations.
This style of browsing is not well supported by the NFSv3 protocol.
In NFSv3, the client expects all LOOKUP operations to remain within a
single server file system. For example, the device attribute will
not change. This prevents a client from taking namespace paths that
span exports.
In the case of NFSv3, an automounter on the client can obtain a
snapshot of the server's namespace using the EXPORTS procedure of the
MOUNT protocol. If it understands the server's pathname syntax, it
can create an image of the server's namespace on the client. The
parts of the namespace that are not exported by the server are filled
in with directories that might be constructed similarly to an NFSv4.1
"pseudo file system" (see
Section 7.3) that allows the user to browse
from one mounted file system to another. There is a drawback to this
representation of the server's namespace on the client: it is static.
If the server administrator adds a new export, the client will be
unaware of it.
7.3. Server Pseudo File System
NFSv4.1 servers avoid this namespace inconsistency by presenting all
the exports for a given server within the framework of a single
namespace for that server. An NFSv4.1 client uses LOOKUP and READDIR
operations to browse seamlessly from one export to another.
Where there are portions of the server namespace that are not
exported, clients require some way of traversing those portions to
reach actual exported file systems. A technique that servers may use
to provide for this is to bridge the unexported portion of the
namespace via a "pseudo file system" that provides a view of exported
directories only. A pseudo file system has a unique fsid and behaves
like a normal, read-only file system.
Based on the construction of the server's namespace, it is possible
that multiple pseudo file systems may exist. For example,
/a pseudo file system
/a/b real file system
/a/b/c pseudo file system
/a/b/c/d real file system
Each of the pseudo file systems is considered a separate entity and
therefore
MUST have its own fsid, unique among all the fsids for that
server.
7.4. Multiple Roots
Certain operating environments are sometimes described as having
"multiple roots". In such environments, individual file systems are
commonly represented by disk or volume names. NFSv4 servers for
these platforms can construct a pseudo file system above these root
names so that disk letters or volume names are simply directory names
in the pseudo root.
7.5. Filehandle Volatility
The nature of the server's pseudo file system is that it is a logical
representation of file system(s) available from the server.
Therefore, the pseudo file system is most likely constructed
dynamically when the server is first instantiated. It is expected
that the pseudo file system may not have an on-disk counterpart from
which persistent filehandles could be constructed. Even though it is
preferable that the server provide persistent filehandles for the
pseudo file system, the NFS client should expect that pseudo file
system filehandles are volatile. This can be confirmed by checking
the associated "fh_expire_type" attribute for those filehandles in
question. If the filehandles are volatile, the NFS client must be
prepared to recover a filehandle value (e.g., with a series of LOOKUP
operations) when receiving an error of NFS4ERR_FHEXPIRED.
Because it is quite likely that servers will implement pseudo file
systems using volatile filehandles, clients need to be prepared for
them, rather than assuming that all filehandles will be persistent.
7.6. Exported Root
If the server's root file system is exported, one might conclude that
a pseudo file system is unneeded. This is not necessarily so.
Assume the following file systems on a server:
/ fs1 (exported)
/a fs2 (not exported)
/a/b fs3 (exported)
Because fs2 is not exported, fs3 cannot be reached with simple
LOOKUPs. The server must bridge the gap with a pseudo file system.
7.7. Mount Point Crossing
The server file system environment may be constructed in such a way
that one file system contains a directory that is 'covered' or
mounted upon by a second file system. For example:
/a/b (file system 1)
/a/b/c/d (file system 2)
The pseudo file system for this server may be constructed to look
like:
/ (place holder/not exported)
/a/b (file system 1)
/a/b/c/d (file system 2)
It is the server's responsibility to present the pseudo file system
that is complete to the client. If the client sends a LOOKUP request
for the path /a/b/c/d, the server's response is the filehandle of the
root of the file system /a/b/c/d. In previous versions of the NFS
protocol, the server would respond with the filehandle of directory
/a/b/c/d within the file system /a/b.
The NFS client will be able to determine if it crosses a server mount
point by a change in the value of the "fsid" attribute.
7.8. Security Policy and Namespace Presentation
Because NFSv4 clients possess the ability to change the security
mechanisms used, after determining what is allowed, by using SECINFO
and SECINFO_NONAME, the server
SHOULD NOT present a different view of
the namespace based on the security mechanism being used by a client.
Instead, it should present a consistent view and return
NFS4ERR_WRONGSEC if an attempt is made to access data with an
inappropriate security mechanism.
If security considerations make it necessary to hide the existence of
a particular file system, as opposed to all of the data within it,
the server can apply the security policy of a shared resource in the
server's namespace to components of the resource's ancestors. For
example:
/ (place holder/not exported)
/a/b (file system 1)
/a/b/MySecretProject (file system 2)
The /a/b/MySecretProject directory is a real file system and is the
shared resource. Suppose the security policy for /a/b/
MySecretProject is Kerberos with integrity and it is desired to limit
knowledge of the existence of this file system. In this case, the
server should apply the same security policy to /a/b. This allows
for knowledge of the existence of a file system to be secured when
desirable.
For the case of the use of multiple, disjoint security mechanisms in
the server's resources, applying that sort of policy would result in
the higher-level file system not being accessible using any security
flavor. Therefore, that sort of configuration is not compatible with
hiding the existence (as opposed to the contents) from clients using
multiple disjoint sets of security flavors.
In other circumstances, a desirable policy is for the security of a
particular object in the server's namespace to include the union of
all security mechanisms of all direct descendants. A common and
convenient practice, unless strong security requirements dictate
otherwise, is to make the entire the pseudo file system accessible by
all of the valid security mechanisms.
Where there is concern about the security of data on the network,
clients should use strong security mechanisms to access the pseudo
file system in order to prevent man-in-the-middle attacks.
8. State Management
Integrating locking into the NFS protocol necessarily causes it to be
stateful. With the inclusion of such features as share reservations,
file and directory delegations, recallable layouts, and support for
mandatory byte-range locking, the protocol becomes substantially more
dependent on proper management of state than the traditional
combination of NFS and NLM (Network Lock Manager) [54]. These
features include expanded locking facilities, which provide some
measure of inter-client exclusion, but the state also offers features
not readily providable using a stateless model. There are three
components to making this state manageable:
* clear division between client and server
* ability to reliably detect inconsistency in state between client
and server
* simple and robust recovery mechanisms
In this model, the server owns the state information. The client
requests changes in locks and the server responds with the changes
made. Non-client-initiated changes in locking state are infrequent.
The client receives prompt notification of such changes and can
adjust its view of the locking state to reflect the server's changes.
Individual pieces of state created by the server and passed to the
client at its request are represented by 128-bit stateids. These
stateids may represent a particular open file, a set of byte-range
locks held by a particular owner, or a recallable delegation of
privileges to access a file in particular ways or at a particular
location.
In all cases, there is a transition from the most general information
that represents a client as a whole to the eventual lightweight
stateid used for most client and server locking interactions. The
details of this transition will vary with the type of object but it
always starts with a client ID.
8.1. Client and Session ID
A client must establish a client ID (see
Section 2.4) and then one or
more sessionids (see
Section 2.10) before performing any operations
to open, byte-range lock, delegate, or obtain a layout for a file
object. Each session ID is associated with a specific client ID, and
thus serves as a shorthand reference to an NFSv4.1 client.
For some types of locking interactions, the client will represent
some number of internal locking entities called "owners", which
normally correspond to processes internal to the client. For other
types of locking-related objects, such as delegations and layouts, no
such intermediate entities are provided for, and the locking-related
objects are considered to be transferred directly between the server
and a unitary client.
8.2. Stateid Definition
When the server grants a lock of any type (including opens, byte-
range locks, delegations, and layouts), it responds with a unique
stateid that represents a set of locks (often a single lock) for the
same file, of the same type, and sharing the same ownership
characteristics. Thus, opens of the same file by different open-
owners each have an identifying stateid. Similarly, each set of
byte-range locks on a file owned by a specific lock-owner has its own
identifying stateid. Delegations and layouts also have associated
stateids by which they may be referenced. The stateid is used as a
shorthand reference to a lock or set of locks, and given a stateid,
the server can determine the associated state-owner or state-owners
(in the case of an open-owner/lock-owner pair) and the associated
filehandle. When stateids are used, the current filehandle must be
the one associated with that stateid.
All stateids associated with a given client ID are associated with a
common lease that represents the claim of those stateids and the
objects they represent to be maintained by the server. See
Section 8.3 for a discussion of the lease.
The server may assign stateids independently for different clients.
A stateid with the same bit pattern for one client may designate an
entirely different set of locks for a different client. The stateid
is always interpreted with respect to the client ID associated with
the current session. Stateids apply to all sessions associated with
the given client ID, and the client may use a stateid obtained from
one session on another session associated with the same client ID.
8.2.1. Stateid Types
With the exception of special stateids (see
Section 8.2.3), each
stateid represents locking objects of one of a set of types defined
by the NFSv4.1 protocol. Note that in all these cases, where we
speak of guarantee, it is understood there are situations such as a
client restart, or lock revocation, that allow the guarantee to be
voided.
* Stateids may represent opens of files.
Each stateid in this case represents the OPEN state for a given
client ID/open-owner/filehandle triple. Such stateids are subject
to change (with consequent incrementing of the stateid's seqid) in
response to OPENs that result in upgrade and OPEN_DOWNGRADE
operations.
* Stateids may represent sets of byte-range locks.
All locks held on a particular file by a particular owner and
gotten under the aegis of a particular open file are associated
with a single stateid with the seqid being incremented whenever
LOCK and LOCKU operations affect that set of locks.
* Stateids may represent file delegations, which are recallable
guarantees by the server to the client that other clients will not
reference or modify a particular file, until the delegation is
returned. In NFSv4.1, file delegations may be obtained on both
regular and non-regular files.
A stateid represents a single delegation held by a client for a
particular filehandle.
* Stateids may represent directory delegations, which are recallable
guarantees by the server to the client that other clients will not
modify the directory, until the delegation is returned.
A stateid represents a single delegation held by a client for a
particular directory filehandle.
* Stateids may represent layouts, which are recallable guarantees by
the server to the client that particular files may be accessed via
an alternate data access protocol at specific locations. Such
access is limited to particular sets of byte-ranges and may
proceed until those byte-ranges are reduced or the layout is
returned.
A stateid represents the set of all layouts held by a particular
client for a particular filehandle with a given layout type. The
seqid is updated as the layouts of that set of byte-ranges change,
via layout stateid changing operations such as LAYOUTGET and
LAYOUTRETURN.
8.2.2. Stateid Structure
Stateids are divided into two fields, a 96-bit "other" field
identifying the specific set of locks and a 32-bit "seqid" sequence
value. Except in the case of special stateids (see
Section 8.2.3), a
particular value of the "other" field denotes a set of locks of the
same type (for example, byte-range locks, opens, delegations, or
layouts), for a specific file or directory, and sharing the same
ownership characteristics. The seqid designates a specific instance
of such a set of locks, and is incremented to indicate changes in
such a set of locks, either by the addition or deletion of locks from
the set, a change in the byte-range they apply to, or an upgrade or
downgrade in the type of one or more locks.
When such a set of locks is first created, the server returns a
stateid with seqid value of one. On subsequent operations that
modify the set of locks, the server is required to increment the
"seqid" field by one whenever it returns a stateid for the same
state-owner/file/type combination and there is some change in the set
of locks actually designated. In this case, the server will return a
stateid with an "other" field the same as previously used for that
state-owner/file/type combination, with an incremented "seqid" field.
This pattern continues until the seqid is incremented past
NFS4_UINT32_MAX, and one (not zero) is the next seqid value.
The purpose of the incrementing of the seqid is to allow the server
to communicate to the client the order in which operations that
modified locking state associated with a stateid have been processed
and to make it possible for the client to send requests that are
conditional on the set of locks not having changed since the stateid
in question was returned.
Except for layout stateids (
Section 12.5.3), when a client sends a
stateid to the server, it has two choices with regard to the seqid
sent. It may set the seqid to zero to indicate to the server that it
wishes the most up-to-date seqid for that stateid's "other" field to
be used. This would be the common choice in the case of a stateid
sent with a READ or WRITE operation. It also may set a non-zero
value, in which case the server checks if that seqid is the correct
one. In that case, the server is required to return
NFS4ERR_OLD_STATEID if the seqid is lower than the most current value
and NFS4ERR_BAD_STATEID if the seqid is greater than the most current
value. This would be the common choice in the case of stateids sent
with a CLOSE or OPEN_DOWNGRADE. Because OPENs may be sent in
parallel for the same owner, a client might close a file without
knowing that an OPEN upgrade had been done by the server, changing
the lock in question. If CLOSE were sent with a zero seqid, the OPEN
upgrade would be cancelled before the client even received an
indication that an upgrade had happened.
When a stateid is sent by the server to the client as part of a
callback operation, it is not subject to checking for a current seqid
and returning NFS4ERR_OLD_STATEID. This is because the client is not
in a position to know the most up-to-date seqid and thus cannot
verify it. Unless specially noted, the seqid value for a stateid
sent by the server to the client as part of a callback is required to
be zero with NFS4ERR_BAD_STATEID returned if it is not.
In making comparisons between seqids, both by the client in
determining the order of operations and by the server in determining
whether the NFS4ERR_OLD_STATEID is to be returned, the possibility of
the seqid being swapped around past the NFS4_UINT32_MAX value needs
to be taken into account. When two seqid values are being compared,
the total count of slots for all sessions associated with the current
client is used to do this. When one seqid value is less than this
total slot count and another seqid value is greater than
NFS4_UINT32_MAX minus the total slot count, the former is to be
treated as lower than the latter, despite the fact that it is
numerically greater.
8.2.3. Special Stateids
Stateid values whose "other" field is either all zeros or all ones
are reserved. They may not be assigned by the server but have
special meanings defined by the protocol. The particular meaning
depends on whether the "other" field is all zeros or all ones and the
specific value of the "seqid" field.
The following combinations of "other" and "seqid" are defined in
NFSv4.1:
* When "other" and "seqid" are both zero, the stateid is treated as
a special anonymous stateid, which can be used in READ, WRITE, and
SETATTR requests to indicate the absence of any OPEN state
associated with the request. When an anonymous stateid value is
used and an existing open denies the form of access requested,
then access will be denied to the request. This stateid
MUST NOT be used on operations to data servers (
Section 13.6).
* When "other" and "seqid" are both all ones, the stateid is a
special READ bypass stateid. When this value is used in WRITE or
SETATTR, it is treated like the anonymous value. When used in
READ, the server
MAY grant access, even if access would normally
be denied to READ operations. This stateid
MUST NOT be used on
operations to data servers.
* When "other" is zero and "seqid" is one, the stateid represents
the current stateid, which is whatever value is the last stateid
returned by an operation within the COMPOUND. In the case of an
OPEN, the stateid returned for the open file and not the
delegation is used. The stateid passed to the operation in place
of the special value has its "seqid" value set to zero, except
when the current stateid is used by the operation CLOSE or
OPEN_DOWNGRADE. If there is no operation in the COMPOUND that has
returned a stateid value, the server
MUST return the error
NFS4ERR_BAD_STATEID. As illustrated in Figure 6, if the value of
a current stateid is a special stateid and the stateid of an
operation's arguments has "other" set to zero and "seqid" set to
one, then the server
MUST return the error NFS4ERR_BAD_STATEID.
* When "other" is zero and "seqid" is NFS4_UINT32_MAX, the stateid
represents a reserved stateid value defined to be invalid. When
this stateid is used, the server
MUST return the error
NFS4ERR_BAD_STATEID.
If a stateid value is used that has all zeros or all ones in the
"other" field but does not match one of the cases above, the server
MUST return the error NFS4ERR_BAD_STATEID.
Special stateids, unlike other stateids, are not associated with
individual client IDs or filehandles and can be used with all valid
client IDs and filehandles. In the case of a special stateid
designating the current stateid, the current stateid value
substituted for the special stateid is associated with a particular
client ID and filehandle, and so, if it is used where the current
filehandle does not match that associated with the current stateid,
the operation to which the stateid is passed will return
NFS4ERR_BAD_STATEID.
8.2.4. Stateid Lifetime and Validation
Stateids must remain valid until either a client restart or a server
restart or until the client returns all of the locks associated with
the stateid by means of an operation such as CLOSE or DELEGRETURN.
If the locks are lost due to revocation, as long as the client ID is
valid, the stateid remains a valid designation of that revoked state
until the client frees it by using FREE_STATEID. Stateids associated
with byte-range locks are an exception. They remain valid even if a
LOCKU frees all remaining locks, so long as the open file with which
they are associated remains open, unless the client frees the
stateids via the FREE_STATEID operation.
It should be noted that there are situations in which the client's
locks become invalid, without the client requesting they be returned.
These include lease expiration and a number of forms of lock
revocation within the lease period. It is important to note that in
these situations, the stateid remains valid and the client can use it
to determine the disposition of the associated lost locks.
An "other" value must never be reused for a different purpose (i.e.,
different filehandle, owner, or type of locks) within the context of
a single client ID. A server may retain the "other" value for the
same purpose beyond the point where it may otherwise be freed, but if
it does so, it must maintain "seqid" continuity with previous values.
One mechanism that may be used to satisfy the requirement that the
server recognize invalid and out-of-date stateids is for the server
to divide the "other" field of the stateid into two fields.
* an index into a table of locking-state structures.
* a generation number that is incremented on each allocation of a
table entry for a particular use.
And then store in each table entry,
* the client ID with which the stateid is associated.
* the current generation number for the (at most one) valid stateid
sharing this index value.
* the filehandle of the file on which the locks are taken.
* an indication of the type of stateid (open, byte-range lock, file
delegation, directory delegation, layout).
* the last "seqid" value returned corresponding to the current
"other" value.
* an indication of the current status of the locks associated with
this stateid, in particular, whether these have been revoked and
if so, for what reason.
With this information, an incoming stateid can be validated and the
appropriate error returned when necessary. Special and non-special
stateids are handled separately. (See
Section 8.2.3 for a discussion
of special stateids.)
Note that stateids are implicitly qualified by the current client ID,
as derived from the client ID associated with the current session.
Note, however, that the semantics of the session will prevent
stateids associated with a previous client or server instance from
being analyzed by this procedure.
If server restart has resulted in an invalid client ID or a session
ID that is invalid, SEQUENCE will return an error and the operation
that takes a stateid as an argument will never be processed.
If there has been a server restart where there is a persistent
session and all leased state has been lost, then the session in
question will, although valid, be marked as dead, and any operation
not satisfied by means of the reply cache will receive the error
NFS4ERR_DEADSESSION, and thus not be processed as indicated below.
When a stateid is being tested and the "other" field is all zeros or
all ones, a check that the "other" and "seqid" fields match a defined
combination for a special stateid is done and the results determined
as follows:
* If the "other" and "seqid" fields do not match a defined
combination associated with a special stateid, the error
NFS4ERR_BAD_STATEID is returned.
* If the special stateid is one designating the current stateid and
there is a current stateid, then the current stateid is
substituted for the special stateid and the checks appropriate to
non-special stateids are performed.
* If the combination is valid in general but is not appropriate to
the context in which the stateid is used (e.g., an all-zero
stateid is used when an OPEN stateid is required in a LOCK
operation), the error NFS4ERR_BAD_STATEID is also returned.
* Otherwise, the check is completed and the special stateid is
accepted as valid.
When a stateid is being tested, and the "other" field is neither all
zeros nor all ones, the following procedure could be used to validate
an incoming stateid and return an appropriate error, when necessary,
assuming that the "other" field would be divided into a table index
and an entry generation.
* If the table index field is outside the range of the associated
table, return NFS4ERR_BAD_STATEID.
* If the selected table entry is of a different generation than that
specified in the incoming stateid, return NFS4ERR_BAD_STATEID.
* If the selected table entry does not match the current filehandle,
return NFS4ERR_BAD_STATEID.
* If the client ID in the table entry does not match the client ID
associated with the current session, return NFS4ERR_BAD_STATEID.
* If the stateid represents revoked state, then return
NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or NFS4ERR_DELEG_REVOKED,
as appropriate.
* If the stateid type is not valid for the context in which the
stateid appears, return NFS4ERR_BAD_STATEID. Note that a stateid
may be valid in general, as would be reported by the TEST_STATEID
operation, but be invalid for a particular operation, as, for
example, when a stateid that doesn't represent byte-range locks is
passed to the non-from_open case of LOCK or to LOCKU, or when a
stateid that does not represent an open is passed to CLOSE or
OPEN_DOWNGRADE. In such cases, the server
MUST return
NFS4ERR_BAD_STATEID.
* If the "seqid" field is not zero and it is greater than the
current sequence value corresponding to the current "other" field,
return NFS4ERR_BAD_STATEID.
* If the "seqid" field is not zero and it is less than the current
sequence value corresponding to the current "other" field, return
NFS4ERR_OLD_STATEID.
* Otherwise, the stateid is valid and the table entry should contain
any additional information about the type of stateid and
information associated with that particular type of stateid, such
as the associated set of locks, e.g., open-owner and lock-owner
information, as well as information on the specific locks, e.g.,
open modes and byte-ranges.
8.2.5. Stateid Use for I/O Operations
Clients performing I/O operations need to select an appropriate
stateid based on the locks (including opens and delegations) held by
the client and the various types of state-owners sending the I/O
requests. SETATTR operations that change the file size are treated
like I/O operations in this regard.
The following rules, applied in order of decreasing priority, govern
the selection of the appropriate stateid. In following these rules,
the client will only consider locks of which it has actually received
notification by an appropriate operation response or callback. Note
that the rules are slightly different in the case of I/O to data
servers when file layouts are being used (see
Section 13.9.1).
* If the client holds a delegation for the file in question, the
delegation stateid
SHOULD be used.
* Otherwise, if the entity corresponding to the lock-owner (e.g., a
process) sending the I/O has a byte-range lock stateid for the
associated open file, then the byte-range lock stateid for that
lock-owner and open file
SHOULD be used.
* If there is no byte-range lock stateid, then the OPEN stateid for
the open file in question
SHOULD be used.
* Finally, if none of the above apply, then a special stateid
SHOULD be used.
Ignoring these rules may result in situations in which the server
does not have information necessary to properly process the request.
For example, when mandatory byte-range locks are in effect, if the
stateid does not indicate the proper lock-owner, via a lock stateid,
a request might be avoidably rejected.
The server however should not try to enforce these ordering rules and
should use whatever information is available to properly process I/O
requests. In particular, when a client has a delegation for a given
file, it
SHOULD take note of this fact in processing a request, even
if it is sent with a special stateid.
8.2.6. Stateid Use for SETATTR Operations
Because each operation is associated with a session ID and from that
the clientid can be determined, operations do not need to include a
stateid for the server to be able to determine whether they should
cause a delegation to be recalled or are to be treated as done within
the scope of the delegation.
In the case of SETATTR operations, a stateid is present. In cases
other than those that set the file size, the client may send either a
special stateid or, when a delegation is held for the file in
question, a delegation stateid. While the server
SHOULD validate the
stateid and may use the stateid to optimize the determination as to
whether a delegation is held, it
SHOULD note the presence of a
delegation even when a special stateid is sent, and
MUST accept a
valid delegation stateid when sent.
8.3. Lease Renewal
Each client/server pair, as represented by a client ID, has a single
lease. The purpose of the lease is to allow the client to indicate
to the server, in a low-overhead way, that it is active, and thus
that the server is to retain the client's locks. This arrangement
allows the server to remove stale locking-related objects that are
held by a client that has crashed or is otherwise unreachable, once
the relevant lease expires. This in turn allows other clients to
obtain conflicting locks without being delayed indefinitely by
inactive or unreachable clients. It is not a mechanism for cache
consistency and lease renewals may not be denied if the lease
interval has not expired.
Since each session is associated with a specific client (identified
by the client's client ID), any operation sent on that session is an
indication that the associated client is reachable. When a request
is sent for a given session, successful execution of a SEQUENCE
operation (or successful retrieval of the result of SEQUENCE from the
reply cache) on an unexpired lease will result in the lease being
implicitly renewed, for the standard renewal period (equal to the
lease_time attribute).
If the client ID's lease has not expired when the server receives a
SEQUENCE operation, then the server
MUST renew the lease. If the
client ID's lease has expired when the server receives a SEQUENCE
operation, the server
MAY renew the lease; this depends on whether
any state was revoked as a result of the client's failure to renew
the lease before expiration.
Absent other activity that would renew the lease, a COMPOUND
consisting of a single SEQUENCE operation will suffice. The client
should also take communication-related delays into account and take
steps to ensure that the renewal messages actually reach the server
in good time. For example:
* When trunking is in effect, the client should consider sending
multiple requests on different connections, in order to ensure
that renewal occurs, even in the event of blockage in the path
used for one of those connections.
* Transport retransmission delays might become so large as to
approach or exceed the length of the lease period. This may be
particularly likely when the server is unresponsive due to a
restart; see
Section 8.4.2.1. If the client implementation is not
careful, transport retransmission delays can result in the client
failing to detect a server restart before the grace period ends.
The scenario is that the client is using a transport with
exponential backoff, such that the maximum retransmission timeout
exceeds both the grace period and the lease_time attribute. A
network partition causes the client's connection's retransmission
interval to back off, and even after the partition heals, the next
transport-level retransmission is sent after the server has
restarted and its grace period ends.
The client
MUST either recover from the ensuing NFS4ERR_NO_GRACE
errors or it
MUST ensure that, despite transport-level
retransmission intervals that exceed the lease_time, a SEQUENCE
operation is sent that renews the lease before expiration. The
client can achieve this by associating a new connection with the
session, and sending a SEQUENCE operation on it. However, if the
attempt to establish a new connection is delayed for some reason
(e.g., exponential backoff of the connection establishment
packets), the client will have to abort the connection
establishment attempt before the lease expires, and attempt to
reconnect.
If the server renews the lease upon receiving a SEQUENCE operation,
the server
MUST NOT allow the lease to expire while the rest of the
operations in the COMPOUND procedure's request are still executing.
Once the last operation has finished, and the response to COMPOUND
has been sent, the server
MUST set the lease to expire no sooner than
the sum of current time and the value of the lease_time attribute.
A client ID's lease can expire when it has been at least the lease
interval (lease_time) since the last lease-renewing SEQUENCE
operation was sent on any of the client ID's sessions and there are
no active COMPOUND operations on any such sessions.
Because the SEQUENCE operation is the basic mechanism to renew a
lease, and because it must be done at least once for each lease
period, it is the natural mechanism whereby the server will inform
the client of changes in the lease status that the client needs to be
informed of. The client should inspect the status flags
(sr_status_flags) returned by sequence and take the appropriate
action (see
Section 18.46.3 for details).
* The status bits SEQ4_STATUS_CB_PATH_DOWN and
SEQ4_STATUS_CB_PATH_DOWN_SESSION indicate problems with the
backchannel that the client may need to address in order to
receive callback requests.
* The status bits SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRING and
SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRED indicate problems with GSS
contexts or RPCSEC_GSS handles for the backchannel that the client
might have to address in order to allow callback requests to be
sent.
* The status bits SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED,
SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED,
SEQ4_STATUS_ADMIN_STATE_REVOKED, and
SEQ4_STATUS_RECALLABLE_STATE_REVOKED notify the client of lock
revocation events. When these bits are set, the client should use
TEST_STATEID to find what stateids have been revoked and use
FREE_STATEID to acknowledge loss of the associated state.
* The status bit SEQ4_STATUS_LEASE_MOVE indicates that
responsibility for lease renewal has been transferred to one or
more new servers.
* The status bit SEQ4_STATUS_RESTART_RECLAIM_NEEDED indicates that
due to server restart the client must reclaim locking state.
* The status bit SEQ4_STATUS_BACKCHANNEL_FAULT indicates that the
server has encountered an unrecoverable fault with the backchannel
(e.g., it has lost track of a sequence ID for a slot in the
backchannel).
8.4. Crash Recovery
A critical requirement in crash recovery is that both the client and
the server know when the other has failed. Additionally, it is
required that a client sees a consistent view of data across server
restarts. All READ and WRITE operations that may have been queued
within the client or network buffers must wait until the client has
successfully recovered the locks protecting the READ and WRITE
operations. Any that reach the server before the server can safely
determine that the client has recovered enough locking state to be
sure that such operations can be safely processed must be rejected.
This will happen because either:
* The state presented is no longer valid since it is associated with
a now invalid client ID. In this case, the client will receive
either an NFS4ERR_BADSESSION or NFS4ERR_DEADSESSION error, and any
attempt to attach a new session to that invalid client ID will
result in an NFS4ERR_STALE_CLIENTID error.
* Subsequent recovery of locks may make execution of the operation
inappropriate (NFS4ERR_GRACE).
8.4.1. Client Failure and Recovery
In the event that a client fails, the server may release the client's
locks when the associated lease has expired. Conflicting locks from
another client may only be granted after this lease expiration. As
discussed in
Section 8.3, when a client has not failed and re-
establishes its lease before expiration occurs, requests for
conflicting locks will not be granted.
To minimize client delay upon restart, lock requests are associated
with an instance of the client by a client-supplied verifier. This
verifier is part of the client_owner4 sent in the initial EXCHANGE_ID
call made by the client. The server returns a client ID as a result
of the EXCHANGE_ID operation. The client then confirms the use of
the client ID by establishing a session associated with that client
ID (see
Section 18.36.3 for a description of how this is done). All
locks, including opens, byte-range locks, delegations, and layouts
obtained by sessions using that client ID, are associated with that
client ID.
Since the verifier will be changed by the client upon each
initialization, the server can compare a new verifier to the verifier
associated with currently held locks and determine that they do not
match. This signifies the client's new instantiation and subsequent
loss (upon confirmation of the new client ID) of locking state. As a
result, the server is free to release all locks held that are
associated with the old client ID that was derived from the old
verifier. At this point, conflicting locks from other clients, kept
waiting while the lease had not yet expired, can be granted. In
addition, all stateids associated with the old client ID can also be
freed, as they are no longer reference-able.
Note that the verifier must have the same uniqueness properties as
the verifier for the COMMIT operation.
8.4.2. Server Failure and Recovery
If the server loses locking state (usually as a result of a restart),
it must allow clients time to discover this fact and re-establish the
lost locking state. The client must be able to re-establish the
locking state without having the server deny valid requests because
the server has granted conflicting access to another client.
Likewise, if there is a possibility that clients have not yet re-
established their locking state for a file and that such locking
state might make it invalid to perform READ or WRITE operations. For
example, if mandatory locks are a possibility, the server must
disallow READ and WRITE operations for that file.
A client can determine that loss of locking state has occurred via
several methods.
1. When a SEQUENCE (most common) or other operation returns
NFS4ERR_BADSESSION, this may mean that the session has been
destroyed but the client ID is still valid. The client sends a
CREATE_SESSION request with the client ID to re-establish the
session. If CREATE_SESSION fails with NFS4ERR_STALE_CLIENTID,
the client must establish a new client ID (see
Section 8.1) and
re-establish its lock state with the new client ID, after the
CREATE_SESSION operation succeeds (see
Section 8.4.2.1).
2. When a SEQUENCE (most common) or other operation on a persistent
session returns NFS4ERR_DEADSESSION, this indicates that a
session is no longer usable for new, i.e., not satisfied from the
reply cache, operations. Once all pending operations are
determined to be either performed before the retry or not
performed, the client sends a CREATE_SESSION request with the
client ID to re-establish the session. If CREATE_SESSION fails
with NFS4ERR_STALE_CLIENTID, the client must establish a new
client ID (see
Section 8.1) and re-establish its lock state after
the CREATE_SESSION, with the new client ID, succeeds
(
Section 8.4.2.1).
3. When an operation, neither SEQUENCE nor preceded by SEQUENCE (for
example, CREATE_SESSION, DESTROY_SESSION), returns
NFS4ERR_STALE_CLIENTID, the client
MUST establish a new client ID
(
Section 8.1) and re-establish its lock state (
Section 8.4.2.1).
When state information and the associated locks are lost as a result
of a server restart, the protocol must provide a way to cause that
state to be re-established. The approach used is to define, for most
types of locking state (layouts are an exception), a request whose
function is to allow the client to re-establish on the server a lock
first obtained from a previous instance. Generally, these requests
are variants of the requests normally used to create locks of that
type and are referred to as "reclaim-type" requests, and the process
of re-establishing such locks is referred to as "reclaiming" them.
Because each client must have an opportunity to reclaim all of the
locks that it has without the possibility that some other client will
be granted a conflicting lock, a "grace period" is devoted to the
reclaim process. During this period, requests creating client IDs
and sessions are handled normally, but locking requests are subject
to special restrictions. Only reclaim-type locking requests are
allowed, unless the server can reliably determine (through state
persistently maintained across restart instances) that granting any
such lock cannot possibly conflict with a subsequent reclaim. When a
request is made to obtain a new lock (i.e., not a reclaim-type
request) during the grace period and such a determination cannot be
made, the server must return the error NFS4ERR_GRACE.
Once a session is established using the new client ID, the client
will use reclaim-type locking requests (e.g., LOCK operations with
reclaim set to TRUE and OPEN operations with a claim type of
CLAIM_PREVIOUS; see
Section 9.11) to re-establish its locking state.
Once this is done, or if there is no such locking state to reclaim,
the client sends a global RECLAIM_COMPLETE operation, i.e., one with
the rca_one_fs argument set to FALSE, to indicate that it has
reclaimed all of the locking state that it will reclaim. Once a
client sends such a RECLAIM_COMPLETE operation, it may attempt non-
reclaim locking operations, although it might get an NFS4ERR_GRACE
status result from each such operation until the period of special
handling is over. See
Section 11.11.9 for a discussion of the
analogous handling lock reclamation in the case of file systems
transitioning from server to server.
During the grace period, the server must reject READ and WRITE
operations and non-reclaim locking requests (i.e., other LOCK and
OPEN operations) with an error of NFS4ERR_GRACE, unless it can
guarantee that these may be done safely, as described below.
The grace period may last until all clients that are known to
possibly have had locks have done a global RECLAIM_COMPLETE
operation, indicating that they have finished reclaiming the locks
they held before the server restart. This means that a client that
has done a RECLAIM_COMPLETE must be prepared to receive an
NFS4ERR_GRACE when attempting to acquire new locks. In order for the
server to know that all clients with possible prior lock state have
done a RECLAIM_COMPLETE, the server must maintain in stable storage a
list clients that may have such locks. The server may also terminate
the grace period before all clients have done a global
RECLAIM_COMPLETE. The server
SHOULD NOT terminate the grace period
before a time equal to the lease period in order to give clients an
opportunity to find out about the server restart, as a result of
sending requests on associated sessions with a frequency governed by
the lease time. Note that when a client does not send such requests
(or they are sent by the client but not received by the server), it
is possible for the grace period to expire before the client finds
out that the server restart has occurred.
Some additional time in order to allow a client to establish a new
client ID and session and to effect lock reclaims may be added to the
lease time. Note that analogous rules apply to file system-specific
grace periods discussed in
Section 11.11.9.
If the server can reliably determine that granting a non-reclaim
request will not conflict with reclamation of locks by other clients,
the NFS4ERR_GRACE error does not have to be returned even within the
grace period, although NFS4ERR_GRACE must always be returned to
clients attempting a non-reclaim lock request before doing their own
global RECLAIM_COMPLETE. For the server to be able to service READ
and WRITE operations during the grace period, it must again be able
to guarantee that no possible conflict could arise between a
potential reclaim locking request and the READ or WRITE operation.
If the server is unable to offer that guarantee, the NFS4ERR_GRACE
error must be returned to the client.
For a server to provide simple, valid handling during the grace
period, the easiest method is to simply reject all non-reclaim
locking requests and READ and WRITE operations by returning the
NFS4ERR_GRACE error. However, a server may keep information about
granted locks in stable storage. With this information, the server
could determine if a locking, READ or WRITE operation can be safely
processed.
For example, if the server maintained on stable storage summary
information on whether mandatory locks exist, either mandatory byte-
range locks, or share reservations specifying deny modes, many
requests could be allowed during the grace period. If it is known
that no such share reservations exist, OPEN request that do not
specify deny modes may be safely granted. If, in addition, it is
known that no mandatory byte-range locks exist, either through
information stored on stable storage or simply because the server
does not support such locks, READ and WRITE operations may be safely
processed during the grace period. Another important case is where
it is known that no mandatory byte-range locks exist, either because
the server does not provide support for them or because their absence
is known from persistently recorded data. In this case, READ and
WRITE operations specifying stateids derived from reclaim-type
operations may be validly processed during the grace period because
of the fact that the valid reclaim ensures that no lock subsequently
granted can prevent the I/O.
To reiterate, for a server that allows non-reclaim lock and I/O
requests to be processed during the grace period, it
MUST determine
that no lock subsequently reclaimed will be rejected and that no lock
subsequently reclaimed would have prevented any I/O operation
processed during the grace period.
Clients should be prepared for the return of NFS4ERR_GRACE errors for
non-reclaim lock and I/O requests. In this case, the client should
employ a retry mechanism for the request. A delay (on the order of
several seconds) between retries should be used to avoid overwhelming
the server. Further discussion of the general issue is included in
[55]. The client must account for the server that can perform I/O
and non-reclaim locking requests within the grace period as well as
those that cannot do so.
A reclaim-type locking request outside the server's grace period can
only succeed if the server can guarantee that no conflicting lock or
I/O request has been granted since restart.
A server may, upon restart, establish a new value for the lease
period. Therefore, clients should, once a new client ID is
established, refetch the lease_time attribute and use it as the basis
for lease renewal for the lease associated with that server.
However, the server must establish, for this restart event, a grace
period at least as long as the lease period for the previous server
instantiation. This allows the client state obtained during the
previous server instance to be reliably re-established.
The possibility exists that, because of server configuration events,
the client will be communicating with a server different than the one
on which the locks were obtained, as shown by the combination of
eir_server_scope and eir_server_owner. This leads to the issue of if
and when the client should attempt to reclaim locks previously
obtained on what is being reported as a different server. The rules
to resolve this question are as follows:
* If the server scope is different, the client should not attempt to
reclaim locks. In this situation, no lock reclaim is possible.
Any attempt to re-obtain the locks with non-reclaim operations is
problematic since there is no guarantee that the existing
filehandles will be recognized by the new server, or that if
recognized, they denote the same objects. It is best to treat the
locks as having been revoked by the reconfiguration event.
* If the server scope is the same, the client should attempt to
reclaim locks, even if the eir_server_owner value is different.
In this situation, it is the responsibility of the server to
return NFS4ERR_NO_GRACE if it cannot provide correct support for
lock reclaim operations, including the prevention of edge
conditions.
The eir_server_owner field is not used in making this determination.
Its function is to specify trunking possibilities for the client (see
Section 2.10.5) and not to control lock reclaim.
8.4.2.1.1. Security Considerations for State Reclaim
During the grace period, a client can reclaim state that it believes
or asserts it had before the server restarted. Unless the server
maintained a complete record of all the state the client had, the
server has little choice but to trust the client. (Of course, if the
server maintained a complete record, then it would not have to force
the client to reclaim state after server restart.) While the server
has to trust the client to tell the truth, the negative consequences
for security are limited to enabling denial-of-service attacks in
situations in which AUTH_SYS is supported. The fundamental rule for
the server when processing reclaim requests is that it
MUST NOT grant
the reclaim if an equivalent non-reclaim request would not be granted
during steady state due to access control or access conflict issues.
For example, an OPEN request during a reclaim will be refused with
NFS4ERR_ACCESS if the principal making the request does not have
access to open the file according to the discretionary ACL
(
Section 6.2.2) on the file.
Nonetheless, it is possible that a client operating in error or
maliciously could, during reclaim, prevent another client from
reclaiming access to state. For example, an attacker could send an
OPEN reclaim operation with a deny mode that prevents another client
from reclaiming the OPEN state it had before the server restarted.
The attacker could perform the same denial of service during steady
state prior to server restart, as long as the attacker had
permissions. Given that the attack vectors are equivalent, the grace
period does not offer any additional opportunity for denial of
service, and any concerns about this attack vector, whether during
grace or steady state, are addressed the same way: use RPCSEC_GSS for
authentication and limit access to the file only to principals that
the owner of the file trusts.
Note that if prior to restart the server had client IDs with the
EXCHGID4_FLAG_BIND_PRINC_STATEID (
Section 18.35) capability set, then
the server
SHOULD record in stable storage the client owner and the
principal that established the client ID via EXCHANGE_ID. If the
server does not, then there is a risk a client will be unable to
reclaim state if it does not have a credential for a principal that
was originally authorized to establish the state.
8.4.3. Network Partitions and Recovery
If the duration of a network partition is greater than the lease
period provided by the server, the server will not have received a
lease renewal from the client. If this occurs, the server may free
all locks held for the client or it may allow the lock state to
remain for a considerable period, subject to the constraint that if a
request for a conflicting lock is made, locks associated with an
expired lease do not prevent such a conflicting lock from being
granted but
MUST be revoked as necessary so as to avoid interfering
with such conflicting requests.
If the server chooses to delay freeing of lock state until there is a
conflict, it may either free all of the client's locks once there is
a conflict or it may only revoke the minimum set of locks necessary
to allow conflicting requests. When it adopts the finer-grained
approach, it must revoke all locks associated with a given stateid,
even if the conflict is with only a subset of locks.
When the server chooses to free all of a client's lock state, either
immediately upon lease expiration or as a result of the first attempt
to obtain a conflicting a lock, the server may report the loss of
lock state in a number of ways.
The server may choose to invalidate the session and the associated
client ID. In this case, once the client can communicate with the
server, it will receive an NFS4ERR_BADSESSION error. Upon attempting
to create a new session, it would get an NFS4ERR_STALE_CLIENTID.
Upon creating the new client ID and new session, the client will
attempt to reclaim locks. Normally, the server will not allow the
client to reclaim locks, because the server will not be in its
recovery grace period.
Another possibility is for the server to maintain the session and
client ID but for all stateids held by the client to become invalid
or stale. Once the client can reach the server after such a network
partition, the status returned by the SEQUENCE operation will
indicate a loss of locking state; i.e., the flag
SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED will be set in sr_status_flags.
In addition, all I/O submitted by the client with the now invalid
stateids will fail with the server returning the error
NFS4ERR_EXPIRED. Once the client learns of the loss of locking
state, it will suitably notify the applications that held the
invalidated locks. The client should then take action to free
invalidated stateids, either by establishing a new client ID using a
new verifier or by doing a FREE_STATEID operation to release each of
the invalidated stateids.
When the server adopts a finer-grained approach to revocation of
locks when a client's lease has expired, only a subset of stateids
will normally become invalid during a network partition. When the
client can communicate with the server after such a network partition
heals, the status returned by the SEQUENCE operation will indicate a
partial loss of locking state
(SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED). In addition, operations,
including I/O submitted by the client, with the now invalid stateids
will fail with the server returning the error NFS4ERR_EXPIRED. Once
the client learns of the loss of locking state, it will use the
TEST_STATEID operation on all of its stateids to determine which
locks have been lost and then suitably notify the applications that
held the invalidated locks. The client can then release the
invalidated locking state and acknowledge the revocation of the
associated locks by doing a FREE_STATEID operation on each of the
invalidated stateids.
When a network partition is combined with a server restart, there are
edge conditions that place requirements on the server in order to
avoid silent data corruption following the server restart. Two of
these edge conditions are known, and are discussed below.
The first edge condition arises as a result of the scenarios such as
the following:
1. Client A acquires a lock.
2. Client A and server experience mutual network partition, such
that client A is unable to renew its lease.
3. Client A's lease expires, and the server releases the lock.
4. Client B acquires a lock that would have conflicted with that of
client A.
5. Client B releases its lock.
6. Server restarts.
7. Network partition between client A and server heals.
8. Client A connects to a new server instance and finds out about
server restart.
9. Client A reclaims its lock within the server's grace period.
Thus, at the final step, the server has erroneously granted client
A's lock reclaim. If client B modified the object the lock was
protecting, client A will experience object corruption.
The second known edge condition arises in situations such as the
following:
1. Client A acquires one or more locks.
2. Server restarts.
3. Client A and server experience mutual network partition, such
that client A is unable to reclaim all of its locks within the
grace period.
4. Server's reclaim grace period ends. Client A has either no
locks or an incomplete set of locks known to the server.
5. Client B acquires a lock that would have conflicted with a lock
of client A that was not reclaimed.
6. Client B releases the lock.
7. Server restarts a second time.
8. Network partition between client A and server heals.
9. Client A connects to new server instance and finds out about
server restart.
10. Client A reclaims its lock within the server's grace period.
As with the first edge condition, the final step of the scenario of
the second edge condition has the server erroneously granting client
A's lock reclaim.
Solving the first and second edge conditions requires either that the
server always assumes after it restarts that some edge condition
occurs, and thus returns NFS4ERR_NO_GRACE for all reclaim attempts,
or that the server record some information in stable storage. The
amount of information the server records in stable storage is in
inverse proportion to how harsh the server intends to be whenever
edge conditions arise. The server that is completely tolerant of all
edge conditions will record in stable storage every lock that is
acquired, removing the lock record from stable storage only when the
lock is released. For the two edge conditions discussed above, the
harshest a server can be, and still support a grace period for
reclaims, requires that the server record in stable storage some
minimal information. For example, a server implementation could, for
each client, save in stable storage a record containing:
* the co_ownerid field from the client_owner4 presented in the
EXCHANGE_ID operation.
* a boolean that indicates if the client's lease expired or if there
was administrative intervention (see
Section 8.5) to revoke a
byte-range lock, share reservation, or delegation and there has
been no acknowledgment, via FREE_STATEID, of such revocation.
* a boolean that indicates whether the client may have locks that it
believes to be reclaimable in situations in which the grace period
was terminated, making the server's view of lock reclaimability
suspect. The server will set this for any client record in stable
storage where the client has not done a suitable RECLAIM_COMPLETE
(global or file system-specific depending on the target of the
lock request) before it grants any new (i.e., not reclaimed) lock
to any client.
Assuming the above record keeping, for the first edge condition,
after the server restarts, the record that client A's lease expired
means that another client could have acquired a conflicting byte-
range lock, share reservation, or delegation. Hence, the server must
reject a reclaim from client A with the error NFS4ERR_NO_GRACE.
For the second edge condition, after the server restarts for a second
time, the indication that the client had not completed its reclaims
at the time at which the grace period ended means that the server
must reject a reclaim from client A with the error NFS4ERR_NO_GRACE.
When either edge condition occurs, the client's attempt to reclaim
locks will result in the error NFS4ERR_NO_GRACE. When this is
received, or after the client restarts with no lock state, the client
will send a global RECLAIM_COMPLETE. When the RECLAIM_COMPLETE is
received, the server and client are again in agreement regarding
reclaimable locks and both booleans in persistent storage can be
reset, to be set again only when there is a subsequent event that
causes lock reclaim operations to be questionable.
Regardless of the level and approach to record keeping, the server
MUST implement one of the following strategies (which apply to
reclaims of share reservations, byte-range locks, and delegations):
1. Reject all reclaims with NFS4ERR_NO_GRACE. This is extremely
unforgiving, but necessary if the server does not record lock
state in stable storage.
2. Record sufficient state in stable storage such that all known
edge conditions involving server restart, including the two noted
in this section, are detected. It is acceptable to erroneously
recognize an edge condition and not allow a reclaim, when, with
sufficient knowledge, it would be allowed. The error the server
would return in this case is NFS4ERR_NO_GRACE. Note that it is
not known if there are other edge conditions.
In the event that, after a server restart, the server determines
there is unrecoverable damage or corruption to the information in
stable storage, then for all clients and/or locks that may be
affected, the server
MUST return NFS4ERR_NO_GRACE.
A mandate for the client's handling of the NFS4ERR_NO_GRACE error is
outside the scope of this specification, since the strategies for
such handling are very dependent on the client's operating
environment. However, one potential approach is described below.
When the client receives NFS4ERR_NO_GRACE, it could examine the
change attribute of the objects for which the client is trying to
reclaim state, and use that to determine whether to re-establish the
state via normal OPEN or LOCK operations. This is acceptable
provided that the client's operating environment allows it. In other
words, the client implementor is advised to document for his users
the behavior. The client could also inform the application that its
byte-range lock or share reservations (whether or not they were
delegated) have been lost, such as via a UNIX signal, a Graphical
User Interface (GUI) pop-up window, etc. See
Section 10.5 for a
discussion of what the client should do for dealing with unreclaimed
delegations on client state.
For further discussion of revocation of locks, see
Section 8.5.
8.5. Server Revocation of Locks
At any point, the server can revoke locks held by a client, and the
client must be prepared for this event. When the client detects that
its locks have been or may have been revoked, the client is
responsible for validating the state information between itself and
the server. Validating locking state for the client means that it
must verify or reclaim state for each lock currently held.
The first occasion of lock revocation is upon server restart. Note
that this includes situations in which sessions are persistent and
locking state is lost. In this class of instances, the client will
receive an error (NFS4ERR_STALE_CLIENTID) on an operation that takes
client ID, usually as part of recovery in response to a problem with
the current session), and the client will proceed with normal crash
recovery as described in the
Section 8.4.2.1.
The second occasion of lock revocation is the inability to renew the
lease before expiration, as discussed in
Section 8.4.3. While this
is considered a rare or unusual event, the client must be prepared to
recover. The server is responsible for determining the precise
consequences of the lease expiration, informing the client of the
scope of the lock revocation decided upon. The client then uses the
status information provided by the server in the SEQUENCE results
(field sr_status_flags, see
Section 18.46.3) to synchronize its
locking state with that of the server, in order to recover.
The third occasion of lock revocation can occur as a result of
revocation of locks within the lease period, either because of
administrative intervention or because a recallable lock (a
delegation or layout) was not returned within the lease period after
having been recalled. While these are considered rare events, they
are possible, and the client must be prepared to deal with them.
When either of these events occurs, the client finds out about the
situation through the status returned by the SEQUENCE operation. Any
use of stateids associated with locks revoked during the lease period
will receive the error NFS4ERR_ADMIN_REVOKED or
NFS4ERR_DELEG_REVOKED, as appropriate.
In all situations in which a subset of locking state may have been
revoked, which include all cases in which locking state is revoked
within the lease period, it is up to the client to determine which
locks have been revoked and which have not. It does this by using
the TEST_STATEID operation on the appropriate set of stateids. Once
the set of revoked locks has been determined, the applications can be
notified, and the invalidated stateids can be freed and lock
revocation acknowledged by using FREE_STATEID.
8.6. Short and Long Leases
When determining the time period for the server lease, the usual
lease trade-offs apply. A short lease is good for fast server
recovery at a cost of increased operations to effect lease renewal
(when there are no other operations during the period to effect lease
renewal as a side effect). A long lease is certainly kinder and
gentler to servers trying to handle very large numbers of clients.
The number of extra requests to effect lock renewal drops in inverse
proportion to the lease time. The disadvantages of a long lease
include the possibility of slower recovery after certain failures.
After server failure, a longer grace period may be required when some
clients do not promptly reclaim their locks and do a global
RECLAIM_COMPLETE. In the event of client failure, the longer period
for a lease to expire will force conflicting requests to wait longer.
A long lease is practical if the server can store lease state in
stable storage. Upon recovery, the server can reconstruct the lease
state from its stable storage and continue operation with its
clients.
8.7. Clocks, Propagation Delay, and Calculating Lease Expiration
To avoid the need for synchronized clocks, lease times are granted by
the server as a time delta. However, there is a requirement that the
client and server clocks do not drift excessively over the duration
of the lease. There is also the issue of propagation delay across
the network, which could easily be several hundred milliseconds, as
well as the possibility that requests will be lost and need to be
retransmitted.
To take propagation delay into account, the client should subtract it
from lease times (e.g., if the client estimates the one-way
propagation delay as 200 milliseconds, then it can assume that the
lease is already 200 milliseconds old when it gets it). In addition,
it will take another 200 milliseconds to get a response back to the
server. So the client must send a lease renewal or write data back
to the server at least 400 milliseconds before the lease would
expire. If the propagation delay varies over the life of the lease
(e.g., the client is on a mobile host), the client will need to
continuously subtract the increase in propagation delay from the
lease times.
The server's lease period configuration should take into account the
network distance of the clients that will be accessing the server's
resources. It is expected that the lease period will take into
account the network propagation delays and other network delay
factors for the client population. Since the protocol does not allow
for an automatic method to determine an appropriate lease period, the
server's administrator may have to tune the lease period.
8.8. Obsolete Locking Infrastructure from NFSv4.0
There are a number of operations and fields within existing
operations that no longer have a function in NFSv4.1. In one way or
another, these changes are all due to the implementation of sessions
that provide client context and exactly once semantics as a base
feature of the protocol, separate from locking itself.
The following NFSv4.0 operations
MUST NOT be implemented in NFSv4.1.
The server
MUST return NFS4ERR_NOTSUPP if these operations are found
in an NFSv4.1 COMPOUND.
* SETCLIENTID since its function has been replaced by EXCHANGE_ID.
* SETCLIENTID_CONFIRM since client ID confirmation now happens by
means of CREATE_SESSION.
* OPEN_CONFIRM because state-owner-based seqids have been replaced
by the sequence ID in the SEQUENCE operation.
* RELEASE_LOCKOWNER because lock-owners with no associated locks do
not have any sequence-related state and so can be deleted by the
server at will.
* RENEW because every SEQUENCE operation for a session causes lease
renewal, making a separate operation superfluous.
Also, there are a number of fields, present in existing operations,
related to locking that have no use in minor version 1. They were
used in minor version 0 to perform functions now provided in a
different fashion.
* Sequence ids used to sequence requests for a given state-owner and
to provide retry protection, now provided via sessions.
* Client IDs used to identify the client associated with a given
request. Client identification is now available using the client
ID associated with the current session, without needing an
explicit client ID field.
Such vestigial fields in existing operations have no function in
NFSv4.1 and are ignored by the server. Note that client IDs in
operations new to NFSv4.1 (such as CREATE_SESSION and
DESTROY_CLIENTID) are not ignored.
9. File Locking and Share Reservations
To support Win32 share reservations, it is necessary to provide
operations that atomically open or create files. Having a separate
share/unshare operation would not allow correct implementation of the
Win32 OpenFile API. In order to correctly implement share semantics,
the previous NFS protocol mechanisms used when a file is opened or
created (LOOKUP, CREATE, ACCESS) need to be replaced. The NFSv4.1
protocol defines an OPEN operation that is capable of atomically
looking up, creating, and locking a file on the server.
9.1. Opens and Byte-Range Locks
It is assumed that manipulating a byte-range lock is rare when
compared to READ and WRITE operations. It is also assumed that
server restarts and network partitions are relatively rare.
Therefore, it is important that the READ and WRITE operations have a
lightweight mechanism to indicate if they possess a held lock. A
LOCK operation contains the heavyweight information required to
establish a byte-range lock and uniquely define the owner of the
lock.
9.1.1. State-Owner Definition
When opening a file or requesting a byte-range lock, the client must
specify an identifier that represents the owner of the requested
lock. This identifier is in the form of a state-owner, represented
in the protocol by a state_owner4, a variable-length opaque array
that, when concatenated with the current client ID, uniquely defines
the owner of a lock managed by the client. This may be a thread ID,
process ID, or other unique value.
Owners of opens and owners of byte-range locks are separate entities
and remain separate even if the same opaque arrays are used to
designate owners of each. The protocol distinguishes between open-
owners (represented by open_owner4 structures) and lock-owners
(represented by lock_owner4 structures).
Each open is associated with a specific open-owner while each byte-
range lock is associated with a lock-owner and an open-owner, the
latter being the open-owner associated with the open file under which
the LOCK operation was done. Delegations and layouts, on the other
hand, are not associated with a specific owner but are associated
with the client as a whole (identified by a client ID).
9.1.2. Use of the Stateid and Locking
All READ, WRITE, and SETATTR operations contain a stateid. For the
purposes of this section, SETATTR operations that change the size
attribute of a file are treated as if they are writing the area
between the old and new sizes (i.e., the byte-range truncated or
added to the file by means of the SETATTR), even where SETATTR is not
explicitly mentioned in the text. The stateid passed to one of these
operations must be one that represents an open, a set of byte-range
locks, or a delegation, or it may be a special stateid representing
anonymous access or the special bypass stateid.
If the state-owner performs a READ or WRITE operation in a situation
in which it has established a byte-range lock or share reservation on
the server (any OPEN constitutes a share reservation), the stateid
(previously returned by the server) must be used to indicate what
locks, including both byte-range locks and share reservations, are
held by the state-owner. If no state is established by the client,
either a byte-range lock or a share reservation, a special stateid
for anonymous state (zero as the value for "other" and "seqid") is
used. (See
Section 8.2.3 for a description of 'special' stateids in
general.) Regardless of whether a stateid for anonymous state or a
stateid returned by the server is used, if there is a conflicting
share reservation or mandatory byte-range lock held on the file, the
server
MUST refuse to service the READ or WRITE operation.
Share reservations are established by OPEN operations and by their
nature are mandatory in that when the OPEN denies READ or WRITE
operations, that denial results in such operations being rejected
with error NFS4ERR_LOCKED. Byte-range locks may be implemented by
the server as either mandatory or advisory, or the choice of
mandatory or advisory behavior may be determined by the server on the
basis of the file being accessed (for example, some UNIX-based
servers support a "mandatory lock bit" on the mode attribute such
that if set, byte-range locks are required on the file before I/O is
possible). When byte-range locks are advisory, they only prevent the
granting of conflicting lock requests and have no effect on READs or
WRITEs. Mandatory byte-range locks, however, prevent conflicting I/O
operations. When they are attempted, they are rejected with
NFS4ERR_LOCKED. When the client gets NFS4ERR_LOCKED on a file for
which it knows it has the proper share reservation, it will need to
send a LOCK operation on the byte-range of the file that includes the
byte-range the I/O was to be performed on, with an appropriate
locktype field of the LOCK operation's arguments (i.e., READ*_LT for
a READ operation, WRITE*_LT for a WRITE operation).
Note that for UNIX environments that support mandatory byte-range
locking, the distinction between advisory and mandatory locking is
subtle. In fact, advisory and mandatory byte-range locks are exactly
the same as far as the APIs and requirements on implementation. If
the mandatory lock attribute is set on the file, the server checks to
see if the lock-owner has an appropriate shared (READ_LT) or
exclusive (WRITE_LT) byte-range lock on the byte-range it wishes to
READ from or WRITE to. If there is no appropriate lock, the server
checks if there is a conflicting lock (which can be done by
attempting to acquire the conflicting lock on behalf of the lock-
owner, and if successful, release the lock after the READ or WRITE
operation is done), and if there is, the server returns
NFS4ERR_LOCKED.
For Windows environments, byte-range locks are always mandatory, so
the server always checks for byte-range locks during I/O requests.
Thus, the LOCK operation does not need to distinguish between
advisory and mandatory byte-range locks. It is the server's
processing of the READ and WRITE operations that introduces the
distinction.
Every stateid that is validly passed to READ, WRITE, or SETATTR, with
the exception of special stateid values, defines an access mode for
the file (i.e., OPEN4_SHARE_ACCESS_READ, OPEN4_SHARE_ACCESS_WRITE, or
OPEN4_SHARE_ACCESS_BOTH).
* For stateids associated with opens, this is the mode defined by
the original OPEN that caused the allocation of the OPEN stateid
and as modified by subsequent OPENs and OPEN_DOWNGRADEs for the
same open-owner/file pair.
* For stateids returned by byte-range LOCK operations, the
appropriate mode is the access mode for the OPEN stateid
associated with the lock set represented by the stateid.
* For delegation stateids, the access mode is based on the type of
delegation.
When a READ, WRITE, or SETATTR (that specifies the size attribute)
operation is done, the operation is subject to checking against the
access mode to verify that the operation is appropriate given the
stateid with which the operation is associated.
In the case of WRITE-type operations (i.e., WRITEs and SETATTRs that
set size), the server
MUST verify that the access mode allows writing
and
MUST return an NFS4ERR_OPENMODE error if it does not. In the
case of READ, the server may perform the corresponding check on the
access mode, or it may choose to allow READ on OPENs for
OPEN4_SHARE_ACCESS_WRITE, to accommodate clients whose WRITE
implementation may unavoidably do reads (e.g., due to buffer cache
constraints). However, even if READs are allowed in these
circumstances, the server
MUST still check for locks that conflict
with the READ (e.g., another OPEN specified OPEN4_SHARE_DENY_READ or
OPEN4_SHARE_DENY_BOTH). Note that a server that does enforce the
access mode check on READs need not explicitly check for conflicting
share reservations since the existence of OPEN for
OPEN4_SHARE_ACCESS_READ guarantees that no conflicting share
reservation can exist.
The READ bypass special stateid (all bits of "other" and "seqid" set
to one) indicates a desire to bypass locking checks. The server
MAY allow READ operations to bypass locking checks at the server, when
this special stateid is used. However, WRITE operations with this
special stateid value
MUST NOT bypass locking checks and are treated
exactly the same as if a special stateid for anonymous state were
used.
A lock may not be granted while a READ or WRITE operation using one
of the special stateids is being performed and the scope of the lock
to be granted would conflict with the READ or WRITE operation. This
can occur when:
* A mandatory byte-range lock is requested with a byte-range that
conflicts with the byte-range of the READ or WRITE operation. For
the purposes of this paragraph, a conflict occurs when a shared
lock is requested and a WRITE operation is being performed, or an
exclusive lock is requested and either a READ or a WRITE operation
is being performed.
* A share reservation is requested that denies reading and/or
writing and the corresponding operation is being performed.
* A delegation is to be granted and the delegation type would
prevent the I/O operation, i.e., READ and WRITE conflict with an
OPEN_DELEGATE_WRITE delegation and WRITE conflicts with an
OPEN_DELEGATE_READ delegation.
When a client holds a delegation, it needs to ensure that the stateid
sent conveys the association of operation with the delegation, to
avoid the delegation from being avoidably recalled. When the
delegation stateid, a stateid open associated with that delegation,
or a stateid representing byte-range locks derived from such an open
is used, the server knows that the READ, WRITE, or SETATTR does not
conflict with the delegation but is sent under the aegis of the
delegation. Even though it is possible for the server to determine
from the client ID (via the session ID) that the client does in fact
have a delegation, the server is not obliged to check this, so using
a special stateid can result in avoidable recall of the delegation.
9.2. Lock Ranges
The protocol allows a lock-owner to request a lock with a byte-range
and then either upgrade, downgrade, or unlock a sub-range of the
initial lock, or a byte-range that overlaps -- fully or partially --
either with that initial lock or a combination of a set of existing
locks for the same lock-owner. It is expected that this will be an
uncommon type of request. In any case, servers or server file
systems may not be able to support sub-range lock semantics. In the
event that a server receives a locking request that represents a sub-
range of current locking state for the lock-owner, the server is
allowed to return the error NFS4ERR_LOCK_RANGE to signify that it
does not support sub-range lock operations. Therefore, the client
should be prepared to receive this error and, if appropriate, report
the error to the requesting application.
The client is discouraged from combining multiple independent locking
ranges that happen to be adjacent into a single request since the
server may not support sub-range requests for reasons related to the
recovery of byte-range locking state in the event of server failure.
As discussed in
Section 8.4.2, the server may employ certain
optimizations during recovery that work effectively only when the
client's behavior during lock recovery is similar to the client's
locking behavior prior to server failure.
9.3. Upgrading and Downgrading Locks
If a client has a WRITE_LT lock on a byte-range, it can request an
atomic downgrade of the lock to a READ_LT lock via the LOCK
operation, by setting the type to READ_LT. If the server supports
atomic downgrade, the request will succeed. If not, it will return
NFS4ERR_LOCK_NOTSUPP. The client should be prepared to receive this
error and, if appropriate, report the error to the requesting
application.
If a client has a READ_LT lock on a byte-range, it can request an
atomic upgrade of the lock to a WRITE_LT lock via the LOCK operation
by setting the type to WRITE_LT or WRITEW_LT. If the server does not
support atomic upgrade, it will return NFS4ERR_LOCK_NOTSUPP. If the
upgrade can be achieved without an existing conflict, the request
will succeed. Otherwise, the server will return either
NFS4ERR_DENIED or NFS4ERR_DEADLOCK. The error NFS4ERR_DEADLOCK is
returned if the client sent the LOCK operation with the type set to
WRITEW_LT and the server has detected a deadlock. The client should
be prepared to receive such errors and, if appropriate, report the
error to the requesting application.
9.4. Stateid Seqid Values and Byte-Range Locks
When a LOCK or LOCKU operation is performed, the stateid returned has
the same "other" value as the argument's stateid, and a "seqid" value
that is incremented (relative to the argument's stateid) to reflect
the occurrence of the LOCK or LOCKU operation. The server
MUST increment the value of the "seqid" field whenever there is any change
to the locking status of any byte offset as described by any of the
locks covered by the stateid. A change in locking status includes a
change from locked to unlocked or the reverse or a change from being
locked for READ_LT to being locked for WRITE_LT or the reverse.
When there is no such change, as, for example, when a range already
locked for WRITE_LT is locked again for WRITE_LT, the server
MAY increment the "seqid" value.
9.5. Issues with Multiple Open-Owners
When the same file is opened by multiple open-owners, a client will
have multiple OPEN stateids for that file, each associated with a
different open-owner. In that case, there can be multiple LOCK and
LOCKU requests for the same lock-owner sent using the different OPEN
stateids, and so a situation may arise in which there are multiple
stateids, each representing byte-range locks on the same file and
held by the same lock-owner but each associated with a different
open-owner.
In such a situation, the locking status of each byte (i.e., whether
it is locked, the READ_LT or WRITE_LT type of the lock, and the lock-
owner holding the lock)
MUST reflect the last LOCK or LOCKU operation
done for the lock-owner in question, independent of the stateid
through which the request was sent.
When a byte is locked by the lock-owner in question, the open-owner
to which that byte-range lock is assigned
SHOULD be that of the open-
owner associated with the stateid through which the last LOCK of that
byte was done. When there is a change in the open-owner associated
with locks for the stateid through which a LOCK or LOCKU was done,
the "seqid" field of the stateid
MUST be incremented, even if the
locking, in terms of lock-owners has not changed. When there is a
change to the set of locked bytes associated with a different stateid
for the same lock-owner, i.e., associated with a different open-
owner, the "seqid" value for that stateid
MUST NOT be incremented.
9.6. Blocking Locks
Some clients require the support of blocking locks. While NFSv4.1
provides a callback when a previously unavailable lock becomes
available, this is an
OPTIONAL feature and clients cannot depend on
its presence. Clients need to be prepared to continually poll for
the lock. This presents a fairness problem. Two of the lock types,
READW_LT and WRITEW_LT, are used to indicate to the server that the
client is requesting a blocking lock. When the callback is not used,
the server should maintain an ordered list of pending blocking locks.
When the conflicting lock is released, the server may wait for the
period of time equal to lease_time for the first waiting client to
re-request the lock. After the lease period expires, the next
waiting client request is allowed the lock. Clients are required to
poll at an interval sufficiently small that it is likely to acquire
the lock in a timely manner. The server is not required to maintain
a list of pending blocked locks as it is used to increase fairness
and not correct operation. Because of the unordered nature of crash
recovery, storing of lock state to stable storage would be required
to guarantee ordered granting of blocking locks.
Servers may also note the lock types and delay returning denial of
the request to allow extra time for a conflicting lock to be
released, allowing a successful return. In this way, clients can
avoid the burden of needless frequent polling for blocking locks.
The server should take care in the length of delay in the event the
client retransmits the request.
If a server receives a blocking LOCK operation, denies it, and then
later receives a nonblocking request for the same lock, which is also
denied, then it should remove the lock in question from its list of
pending blocking locks. Clients should use such a nonblocking
request to indicate to the server that this is the last time they
intend to poll for the lock, as may happen when the process
requesting the lock is interrupted. This is a courtesy to the
server, to prevent it from unnecessarily waiting a lease period
before granting other LOCK operations. However, clients are not
required to perform this courtesy, and servers must not depend on
them doing so. Also, clients must be prepared for the possibility
that this final locking request will be accepted.
When a server indicates, via the flag OPEN4_RESULT_
MAY_NOTIFY_LOCK,
that CB_NOTIFY_LOCK callbacks might be done for the current open
file, the client should take notice of this, but, since this is a
hint, cannot rely on a CB_NOTIFY_LOCK always being done. A client
may reasonably reduce the frequency with which it polls for a denied
lock, since the greater latency that might occur is likely to be
eliminated given a prompt callback, but it still needs to poll. When
it receives a CB_NOTIFY_LOCK, it should promptly try to obtain the
lock, but it should be aware that other clients may be polling and
that the server is under no obligation to reserve the lock for that
particular client.
9.7. Share Reservations
A share reservation is a mechanism to control access to a file. It
is a separate and independent mechanism from byte-range locking.
When a client opens a file, it sends an OPEN operation to the server
specifying the type of access required (READ, WRITE, or BOTH) and the
type of access to deny others (OPEN4_SHARE_DENY_NONE,
OPEN4_SHARE_DENY_READ, OPEN4_SHARE_DENY_WRITE, or
OPEN4_SHARE_DENY_BOTH). If the OPEN fails, the client will fail the
application's open request.
Pseudo-code definition of the semantics:
if (request.access == 0) {
return (NFS4ERR_INVAL)
} else {
if ((request.access & file_state.deny)) ||
(request.deny & file_state.access)) {
return (NFS4ERR_SHARE_DENIED)
}
return (NFS4ERR_OK);
When doing this checking of share reservations on OPEN, the current
file_state used in the algorithm includes bits that reflect all
current opens, including those for the open-owner making the new OPEN
request.
The constants used for the OPEN and OPEN_DOWNGRADE operations for the
access and deny fields are as follows:
const OPEN4_SHARE_ACCESS_READ = 0x00000001;
const OPEN4_SHARE_ACCESS_WRITE = 0x00000002;
const OPEN4_SHARE_ACCESS_BOTH = 0x00000003;
const OPEN4_SHARE_DENY_NONE = 0x00000000;
const OPEN4_SHARE_DENY_READ = 0x00000001;
const OPEN4_SHARE_DENY_WRITE = 0x00000002;
const OPEN4_SHARE_DENY_BOTH = 0x00000003;
9.8. OPEN/CLOSE Operations
To provide correct share semantics, a client
MUST use the OPEN
operation to obtain the initial filehandle and indicate the desired
access and what access, if any, to deny. Even if the client intends
to use a special stateid for anonymous state or READ bypass, it must
still obtain the filehandle for the regular file with the OPEN
operation so the appropriate share semantics can be applied. Clients
that do not have a deny mode built into their programming interfaces
for opening a file should request a deny mode of
OPEN4_SHARE_DENY_NONE.
The OPEN operation with the CREATE flag also subsumes the CREATE
operation for regular files as used in previous versions of the NFS
protocol. This allows a create with a share to be done atomically.
The CLOSE operation removes all share reservations held by the open-
owner on that file. If byte-range locks are held, the client
SHOULD release all locks before sending a CLOSE operation. The server
MAY free all outstanding locks on CLOSE, but some servers may not support
the CLOSE of a file that still has byte-range locks held. The server
MUST return failure, NFS4ERR_LOCKS_HELD, if any locks would exist
after the CLOSE.
The LOOKUP operation will return a filehandle without establishing
any lock state on the server. Without a valid stateid, the server
will assume that the client has the least access. For example, if
one client opened a file with OPEN4_SHARE_DENY_BOTH and another
client accesses the file via a filehandle obtained through LOOKUP,
the second client could only read the file using the special read
bypass stateid. The second client could not WRITE the file at all
because it would not have a valid stateid from OPEN and the special
anonymous stateid would not be allowed access.
9.9. Open Upgrade and Downgrade
When an OPEN is done for a file and the open-owner for which the OPEN
is being done already has the file open, the result is to upgrade the
open file status maintained on the server to include the access and
deny bits specified by the new OPEN as well as those for the existing
OPEN. The result is that there is one open file, as far as the
protocol is concerned, and it includes the union of the access and
deny bits for all of the OPEN requests completed. The OPEN is
represented by a single stateid whose "other" value matches that of
the original open, and whose "seqid" value is incremented to reflect
the occurrence of the upgrade. The increment is required in cases in
which the "upgrade" results in no change to the open mode (e.g., an
OPEN is done for read when the existing open file is opened for
OPEN4_SHARE_ACCESS_BOTH). Only a single CLOSE will be done to reset
the effects of both OPENs. The client may use the stateid returned
by the OPEN effecting the upgrade or with a stateid sharing the same
"other" field and a seqid of zero, although care needs to be taken as
far as upgrades that happen while the CLOSE is pending. Note that
the client, when sending the OPEN, may not know that the same file is
in fact being opened. The above only applies if both OPENs result in
the OPENed object being designated by the same filehandle.
When the server chooses to export multiple filehandles corresponding
to the same file object and returns different filehandles on two
different OPENs of the same file object, the server
MUST NOT "OR"
together the access and deny bits and coalesce the two open files.
Instead, the server must maintain separate OPENs with separate
stateids and will require separate CLOSEs to free them.
When multiple open files on the client are merged into a single OPEN
file object on the server, the close of one of the open files (on the
client) may necessitate change of the access and deny status of the
open file on the server. This is because the union of the access and
deny bits for the remaining opens may be smaller (i.e., a proper
subset) than previously. The OPEN_DOWNGRADE operation is used to
make the necessary change and the client should use it to update the
server so that share reservation requests by other clients are
handled properly. The stateid returned has the same "other" field as
that passed to the server. The "seqid" value in the returned stateid
MUST be incremented, even in situations in which there is no change
to the access and deny bits for the file.
9.10. Parallel OPENs
Unlike the case of NFSv4.0, in which OPEN operations for the same
open-owner are inherently serialized because of the owner-based
seqid, multiple OPENs for the same open-owner may be done in
parallel. When clients do this, they may encounter situations in
which, because of the existence of hard links, two OPEN operations
may turn out to open the same file, with a later OPEN performed being
an upgrade of the first, with this fact only visible to the client
once the operations complete.
In this situation, clients may determine the order in which the OPENs
were performed by examining the stateids returned by the OPENs.
Stateids that share a common value of the "other" field can be
recognized as having opened the same file, with the order of the
operations determinable from the order of the "seqid" fields, mod any
possible wraparound of the 32-bit field.
When the possibility exists that the client will send multiple OPENs
for the same open-owner in parallel, it may be the case that an open
upgrade may happen without the client knowing beforehand that this
could happen. Because of this possibility, CLOSEs and
OPEN_DOWNGRADEs should generally be sent with a non-zero seqid in the
stateid, to avoid the possibility that the status change associated
with an open upgrade is not inadvertently lost.
9.11. Reclaim of Open and Byte-Range Locks
Special forms of the LOCK and OPEN operations are provided when it is
necessary to re-establish byte-range locks or opens after a server
failure.
* To reclaim existing opens, an OPEN operation is performed using a
CLAIM_PREVIOUS. Because the client, in this type of situation,
will have already opened the file and have the filehandle of the
target file, this operation requires that the current filehandle
be the target file, rather than a directory, and no file name is
specified.
* To reclaim byte-range locks, a LOCK operation with the reclaim
parameter set to true is used.
Reclaims of opens associated with delegations are discussed in
Section 10.2.1.
10. Client-Side Caching
Client-side caching of data, of file attributes, and of file names is
essential to providing good performance with the NFS protocol.
Providing distributed cache coherence is a difficult problem, and
previous versions of the NFS protocol have not attempted it.
Instead, several NFS client implementation techniques have been used
to reduce the problems that a lack of coherence poses for users.
These techniques have not been clearly defined by earlier protocol
specifications, and it is often unclear what is valid or invalid
client behavior.
The NFSv4.1 protocol uses many techniques similar to those that have
been used in previous protocol versions. The NFSv4.1 protocol does
not provide distributed cache coherence. However, it defines a more
limited set of caching guarantees to allow locks and share
reservations to be used without destructive interference from client-
side caching.
In addition, the NFSv4.1 protocol introduces a delegation mechanism,
which allows many decisions normally made by the server to be made
locally by clients. This mechanism provides efficient support of the
common cases where sharing is infrequent or where sharing is read-
only.
10.1. Performance Challenges for Client-Side Caching
Caching techniques used in previous versions of the NFS protocol have
been successful in providing good performance. However, several
scalability challenges can arise when those techniques are used with
very large numbers of clients. This is particularly true when
clients are geographically distributed, which classically increases
the latency for cache revalidation requests.
The previous versions of the NFS protocol repeat their file data
cache validation requests at the time the file is opened. This
behavior can have serious performance drawbacks. A common case is
one in which a file is only accessed by a single client. Therefore,
sharing is infrequent.
In this case, repeated references to the server to find that no
conflicts exist are expensive. A better option with regards to
performance is to allow a client that repeatedly opens a file to do
so without reference to the server. This is done until potentially
conflicting operations from another client actually occur.
A similar situation arises in connection with byte-range locking.
Sending LOCK and LOCKU operations as well as the READ and WRITE
operations necessary to make data caching consistent with the locking
semantics (see
Section 10.3.2) can severely limit performance. When
locking is used to provide protection against infrequent conflicts, a
large penalty is incurred. This penalty may discourage the use of
byte-range locking by applications.
The NFSv4.1 protocol provides more aggressive caching strategies with
the following design goals:
* Compatibility with a large range of server semantics.
* Providing the same caching benefits as previous versions of the
NFS protocol when unable to support the more aggressive model.
* Requirements for aggressive caching are organized so that a large
portion of the benefit can be obtained even when not all of the
requirements can be met.
The appropriate requirements for the server are discussed in later
sections in which specific forms of caching are covered (see
Section 10.4).
10.2. Delegation and Callbacks
Recallable delegation of server responsibilities for a file to a
client improves performance by avoiding repeated requests to the
server in the absence of inter-client conflict. With the use of a
"callback" RPC from server to client, a server recalls delegated
responsibilities when another client engages in sharing of a
delegated file.
A delegation is passed from the server to the client, specifying the
object of the delegation and the type of delegation. There are
different types of delegations, but each type contains a stateid to
be used to represent the delegation when performing operations that
depend on the delegation. This stateid is similar to those
associated with locks and share reservations but differs in that the
stateid for a delegation is associated with a client ID and may be
used on behalf of all the open-owners for the given client. A
delegation is made to the client as a whole and not to any specific
process or thread of control within it.
The backchannel is established by CREATE_SESSION and
BIND_CONN_TO_SESSION, and the client is required to maintain it.
Because the backchannel may be down, even temporarily, correct
protocol operation does not depend on them. Preliminary testing of
backchannel functionality by means of a CB_COMPOUND procedure with a
single operation, CB_SEQUENCE, can be used to check the continuity of
the backchannel. A server avoids delegating responsibilities until
it has determined that the backchannel exists. Because the granting
of a delegation is always conditional upon the absence of conflicting
access, clients
MUST NOT assume that a delegation will be granted and
they
MUST always be prepared for OPENs, WANT_DELEGATIONs, and
GET_DIR_DELEGATIONs to be processed without any delegations being
granted.
Unlike locks, an operation by a second client to a delegated file
will cause the server to recall a delegation through a callback. For
individual operations, we will describe, under IMPLEMENTATION, when
such operations are required to effect a recall. A number of points
should be noted, however.
* The server is free to recall a delegation whenever it feels it is
desirable and may do so even if no operations requiring recall are
being done.
* Operations done outside the NFSv4.1 protocol, due to, for example,
access by other protocols, or by local access, also need to result
in delegation recall when they make analogous changes to file
system data. What is crucial is if the change would invalidate
the guarantees provided by the delegation. When this is possible,
the delegation needs to be recalled and
MUST be returned or
revoked before allowing the operation to proceed.
* The semantics of the file system are crucial in defining when
delegation recall is required. If a particular change within a
specific implementation causes change to a file attribute, then
delegation recall is required, whether that operation has been
specifically listed as requiring delegation recall. Again, what
is critical is whether the guarantees provided by the delegation
are being invalidated.
Despite those caveats, the implementation sections for a number of
operations describe situations in which delegation recall would be
required under some common circumstances:
* For GETATTR, see
Section 18.7.4.
* For OPEN, see
Section 18.16.4.
* For READ, see
Section 18.22.4.
* For REMOVE, see
Section 18.25.4.
* For RENAME, see
Section 18.26.4.
* For SETATTR, see
Section 18.30.4.
* For WRITE, see
Section 18.32.4.
On recall, the client holding the delegation needs to flush modified
state (such as modified data) to the server and return the
delegation. The conflicting request will not be acted on until the
recall is complete. The recall is considered complete when the
client returns the delegation or the server times its wait for the
delegation to be returned and revokes the delegation as a result of
the timeout. In the interim, the server will either delay responding
to conflicting requests or respond to them with NFS4ERR_DELAY.
Following the resolution of the recall, the server has the
information necessary to grant or deny the second client's request.
At the time the client receives a delegation recall, it may have
substantial state that needs to be flushed to the server. Therefore,
the server should allow sufficient time for the delegation to be
returned since it may involve numerous RPCs to the server. If the
server is able to determine that the client is diligently flushing
state to the server as a result of the recall, the server may extend
the usual time allowed for a recall. However, the time allowed for
recall completion should not be unbounded.
An example of this is when responsibility to mediate opens on a given
file is delegated to a client (see
Section 10.4). The server will
not know what opens are in effect on the client. Without this
knowledge, the server will be unable to determine if the access and
deny states for the file allow any particular open until the
delegation for the file has been returned.
A client failure or a network partition can result in failure to
respond to a recall callback. In this case, the server will revoke
the delegation, which in turn will render useless any modified state
still on the client.
10.2.1. Delegation Recovery
There are three situations that delegation recovery needs to deal
with:
* client restart
* server restart
* network partition (full or backchannel-only)
In the event the client restarts, the failure to renew the lease will
result in the revocation of byte-range locks and share reservations.
Delegations, however, may be treated a bit differently.
There will be situations in which delegations will need to be re-
established after a client restarts. The reason for this is that the
client may have file data stored locally and this data was associated
with the previously held delegations. The client will need to re-
establish the appropriate file state on the server.
To allow for this type of client recovery, the server
MAY extend the
period for delegation recovery beyond the typical lease expiration
period. This implies that requests from other clients that conflict
with these delegations will need to wait. Because the normal recall
process may require significant time for the client to flush changed
state to the server, other clients need be prepared for delays that
occur because of a conflicting delegation. This longer interval
would increase the window for clients to restart and consult stable
storage so that the delegations can be reclaimed. For OPEN
delegations, such delegations are reclaimed using OPEN with a claim
type of CLAIM_DELEGATE_PREV or CLAIM_DELEG_PREV_FH (see Sections
10.5 and
18.16 for discussion of OPEN delegation and the details of OPEN,
respectively).
A server
MAY support claim types of CLAIM_DELEGATE_PREV and
CLAIM_DELEG_PREV_FH, and if it does, it
MUST NOT remove delegations
upon a CREATE_SESSION that confirm a client ID created by
EXCHANGE_ID. Instead, the server
MUST, for a period of time no less
than that of the value of the lease_time attribute, maintain the
client's delegations to allow time for the client to send
CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH requests. The server
that supports CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH
MUST support the DELEGPURGE operation.
When the server restarts, delegations are reclaimed (using the OPEN
operation with CLAIM_PREVIOUS) in a similar fashion to byte-range
locks and share reservations. However, there is a slight semantic
difference. In the normal case, if the server decides that a
delegation should not be granted, it performs the requested action
(e.g., OPEN) without granting any delegation. For reclaim, the
server grants the delegation but a special designation is applied so
that the client treats the delegation as having been granted but
recalled by the server. Because of this, the client has the duty to
write all modified state to the server and then return the
delegation. This process of handling delegation reclaim reconciles
three principles of the NFSv4.1 protocol:
* Upon reclaim, a client reporting resources assigned to it by an
earlier server instance must be granted those resources.
* The server has unquestionable authority to determine whether
delegations are to be granted and, once granted, whether they are
to be continued.
* The use of callbacks should not be depended upon until the client
has proven its ability to receive them.
When a client needs to reclaim a delegation and there is no
associated open, the client may use the CLAIM_PREVIOUS variant of the
WANT_DELEGATION operation. However, since the server is not required
to support this operation, an alternative is to reclaim via a dummy
OPEN together with the delegation using an OPEN of type
CLAIM_PREVIOUS. The dummy open file can be released using a CLOSE to
re-establish the original state to be reclaimed, a delegation without
an associated open.
When a client has more than a single open associated with a
delegation, state for those additional opens can be established using
OPEN operations of type CLAIM_DELEGATE_CUR. When these are used to
establish opens associated with reclaimed delegations, the server
MUST allow them when made within the grace period.
When a network partition occurs, delegations are subject to freeing
by the server when the lease renewal period expires. This is similar
to the behavior for locks and share reservations. For delegations,
however, the server may extend the period in which conflicting
requests are held off. Eventually, the occurrence of a conflicting
request from another client will cause revocation of the delegation.
A loss of the backchannel (e.g., by later network configuration
change) will have the same effect. A recall request will fail and
revocation of the delegation will result.
A client normally finds out about revocation of a delegation when it
uses a stateid associated with a delegation and receives one of the
errors NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or
NFS4ERR_DELEG_REVOKED. It also may find out about delegation
revocation after a client restart when it attempts to reclaim a
delegation and receives that same error. Note that in the case of a
revoked OPEN_DELEGATE_WRITE delegation, there are issues because data
may have been modified by the client whose delegation is revoked and
separately by other clients. See
Section 10.5.1 for a discussion of
such issues. Note also that when delegations are revoked,
information about the revoked delegation will be written by the
server to stable storage (as described in
Section 8.4.3). This is
done to deal with the case in which a server restarts after revoking
a delegation but before the client holding the revoked delegation is
notified about the revocation.
10.3. Data Caching
When applications share access to a set of files, they need to be
implemented so as to take account of the possibility of conflicting
access by another application. This is true whether the applications
in question execute on different clients or reside on the same
client.
Share reservations and byte-range locks are the facilities the
NFSv4.1 protocol provides to allow applications to coordinate access
by using mutual exclusion facilities. The NFSv4.1 protocol's data
caching must be implemented such that it does not invalidate the
assumptions on which those using these facilities depend.
10.3.1. Data Caching and OPENs
In order to avoid invalidating the sharing assumptions on which
applications rely, NFSv4.1 clients should not provide cached data to
applications or modify it on behalf of an application when it would
not be valid to obtain or modify that same data via a READ or WRITE
operation.
Furthermore, in the absence of an OPEN delegation (see
Section 10.4),
two additional rules apply. Note that these rules are obeyed in
practice by many NFSv3 clients.
* First, cached data present on a client must be revalidated after
doing an OPEN. Revalidating means that the client fetches the
change attribute from the server, compares it with the cached
change attribute, and if different, declares the cached data (as
well as the cached attributes) as invalid. This is to ensure that
the data for the OPENed file is still correctly reflected in the
client's cache. This validation must be done at least when the
client's OPEN operation includes a deny of OPEN4_SHARE_DENY_WRITE
or OPEN4_SHARE_DENY_BOTH, thus terminating a period in which other
clients may have had the opportunity to open the file with
OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH access. Clients
may choose to do the revalidation more often (i.e., at OPENs
specifying a deny mode of OPEN4_SHARE_DENY_NONE) to parallel the
NFSv3 protocol's practice for the benefit of users assuming this
degree of cache revalidation.
Since the change attribute is updated for data and metadata
modifications, some client implementors may be tempted to use the
time_modify attribute and not the change attribute to validate
cached data, so that metadata changes do not spuriously invalidate
clean data. The implementor is cautioned in this approach. The
change attribute is guaranteed to change for each update to the
file, whereas time_modify is guaranteed to change only at the
granularity of the time_delta attribute. Use by the client's data
cache validation logic of time_modify and not change runs the risk
of the client incorrectly marking stale data as valid. Thus, any
cache validation approach by the client
MUST include the use of
the change attribute.
* Second, modified data must be flushed to the server before closing
a file OPENed for OPEN4_SHARE_ACCESS_WRITE. This is complementary
to the first rule. If the data is not flushed at CLOSE, the
revalidation done after the client OPENs a file is unable to
achieve its purpose. The other aspect to flushing the data before
close is that the data must be committed to stable storage, at the
server, before the CLOSE operation is requested by the client. In
the case of a server restart and a CLOSEd file, it may not be
possible to retransmit the data to be written to the file, hence,
this requirement.
10.3.2. Data Caching and File Locking
For those applications that choose to use byte-range locking instead
of share reservations to exclude inconsistent file access, there is
an analogous set of constraints that apply to client-side data
caching. These rules are effective only if the byte-range locking is
used in a way that matches in an equivalent way the actual READ and
WRITE operations executed. This is as opposed to byte-range locking
that is based on pure convention. For example, it is possible to
manipulate a two-megabyte file by dividing the file into two one-
megabyte ranges and protecting access to the two byte-ranges by byte-
range locks on bytes zero and one. A WRITE_LT lock on byte zero of
the file would represent the right to perform READ and WRITE
operations on the first byte-range. A WRITE_LT lock on byte one of
the file would represent the right to perform READ and WRITE
operations on the second byte-range. As long as all applications
manipulating the file obey this convention, they will work on a local
file system. However, they may not work with the NFSv4.1 protocol
unless clients refrain from data caching.
The rules for data caching in the byte-range locking environment are:
* First, when a client obtains a byte-range lock for a particular
byte-range, the data cache corresponding to that byte-range (if
any cache data exists) must be revalidated. If the change
attribute indicates that the file may have been updated since the
cached data was obtained, the client must flush or invalidate the
cached data for the newly locked byte-range. A client might
choose to invalidate all of the non-modified cached data that it
has for the file, but the only requirement for correct operation
is to invalidate all of the data in the newly locked byte-range.
* Second, before releasing a WRITE_LT lock for a byte-range, all
modified data for that byte-range must be flushed to the server.
The modified data must also be written to stable storage.
Note that flushing data to the server and the invalidation of cached
data must reflect the actual byte-ranges locked or unlocked.
Rounding these up or down to reflect client cache block boundaries
will cause problems if not carefully done. For example, writing a
modified block when only half of that block is within an area being
unlocked may cause invalid modification to the byte-range outside the
unlocked area. This, in turn, may be part of a byte-range locked by
another client. Clients can avoid this situation by synchronously
performing portions of WRITE operations that overlap that portion
(initial or final) that is not a full block. Similarly, invalidating
a locked area that is not an integral number of full buffer blocks
would require the client to read one or two partial blocks from the
server if the revalidation procedure shows that the data that the
client possesses may not be valid.
The data that is written to the server as a prerequisite to the
unlocking of a byte-range must be written, at the server, to stable
storage. The client may accomplish this either with synchronous
writes or by following asynchronous writes with a COMMIT operation.
This is required because retransmission of the modified data after a
server restart might conflict with a lock held by another client.
A client implementation may choose to accommodate applications that
use byte-range locking in non-standard ways (e.g., using a byte-range
lock as a global semaphore) by flushing to the server more data upon
a LOCKU than is covered by the locked range. This may include
modified data within files other than the one for which the unlocks
are being done. In such cases, the client must not interfere with
applications whose READs and WRITEs are being done only within the
bounds of byte-range locks that the application holds. For example,
an application locks a single byte of a file and proceeds to write
that single byte. A client that chose to handle a LOCKU by flushing
all modified data to the server could validly write that single byte
in response to an unrelated LOCKU operation. However, it would not
be valid to write the entire block in which that single written byte
was located since it includes an area that is not locked and might be
locked by another client. Client implementations can avoid this
problem by dividing files with modified data into those for which all
modifications are done to areas covered by an appropriate byte-range
lock and those for which there are modifications not covered by a
byte-range lock. Any writes done for the former class of files must
not include areas not locked and thus not modified on the client.
10.3.3. Data Caching and Mandatory File Locking
Client-side data caching needs to respect mandatory byte-range
locking when it is in effect. The presence of mandatory byte-range
locking for a given file is indicated when the client gets back
NFS4ERR_LOCKED from a READ or WRITE operation on a file for which it
has an appropriate share reservation. When mandatory locking is in
effect for a file, the client must check for an appropriate byte-
range lock for data being read or written. If a byte-range lock
exists for the range being read or written, the client may satisfy
the request using the client's validated cache. If an appropriate
byte-range lock is not held for the range of the read or write, the
read or write request must not be satisfied by the client's cache and
the request must be sent to the server for processing. When a read
or write request partially overlaps a locked byte-range, the request
should be subdivided into multiple pieces with each byte-range
(locked or not) treated appropriately.
10.3.4. Data Caching and File Identity
When clients cache data, the file data needs to be organized
according to the file system object to which the data belongs. For
NFSv3 clients, the typical practice has been to assume for the
purpose of caching that distinct filehandles represent distinct file
system objects. The client then has the choice to organize and
maintain the data cache on this basis.
In the NFSv4.1 protocol, there is now the possibility to have
significant deviations from a "one filehandle per object" model
because a filehandle may be constructed on the basis of the object's
pathname. Therefore, clients need a reliable method to determine if
two filehandles designate the same file system object. If clients
were simply to assume that all distinct filehandles denote distinct
objects and proceed to do data caching on this basis, caching
inconsistencies would arise between the distinct client-side objects
that mapped to the same server-side object.
By providing a method to differentiate filehandles, the NFSv4.1
protocol alleviates a potential functional regression in comparison
with the NFSv3 protocol. Without this method, caching
inconsistencies within the same client could occur, and this has not
been present in previous versions of the NFS protocol. Note that it
is possible to have such inconsistencies with applications executing
on multiple clients, but that is not the issue being addressed here.
For the purposes of data caching, the following steps allow an
NFSv4.1 client to determine whether two distinct filehandles denote
the same server-side object:
* If GETATTR directed to two filehandles returns different values of
the fsid attribute, then the filehandles represent distinct
objects.
* If GETATTR for any file with an fsid that matches the fsid of the
two filehandles in question returns a unique_handles attribute
with a value of TRUE, then the two objects are distinct.
* If GETATTR directed to the two filehandles does not return the
fileid attribute for both of the handles, then it cannot be
determined whether the two objects are the same. Therefore,
operations that depend on that knowledge (e.g., client-side data
caching) cannot be done reliably. Note that if GETATTR does not
return the fileid attribute for both filehandles, it will return
it for neither of the filehandles, since the fsid for both
filehandles is the same.
* If GETATTR directed to the two filehandles returns different
values for the fileid attribute, then they are distinct objects.
* Otherwise, they are the same object.
10.4. Open Delegation
When a file is being OPENed, the server may delegate further handling
of opens and closes for that file to the opening client. Any such
delegation is recallable since the circumstances that allowed for the
delegation are subject to change. In particular, if the server
receives a conflicting OPEN from another client, the server must
recall the delegation before deciding whether the OPEN from the other
client may be granted. Making a delegation is up to the server, and
clients should not assume that any particular OPEN either will or
will not result in an OPEN delegation. The following is a typical
set of conditions that servers might use in deciding whether an OPEN
should be delegated:
* The client must be able to respond to the server's callback
requests. If a backchannel has been established, the server will
send a CB_COMPOUND request, containing a single operation,
CB_SEQUENCE, for a test of backchannel availability.
* The client must have responded properly to previous recalls.
* There must be no current OPEN conflicting with the requested
delegation.
* There should be no current delegation that conflicts with the
delegation being requested.
* The probability of future conflicting open requests should be low
based on the recent history of the file.
* The existence of any server-specific semantics of OPEN/CLOSE that
would make the required handling incompatible with the prescribed
handling that the delegated client would apply (see below).
There are two types of OPEN delegations: OPEN_DELEGATE_READ and
OPEN_DELEGATE_WRITE. An OPEN_DELEGATE_READ delegation allows a
client to handle, on its own, requests to open a file for reading
that do not deny OPEN4_SHARE_ACCESS_READ access to others. Multiple
OPEN_DELEGATE_READ delegations may be outstanding simultaneously and
do not conflict. An OPEN_DELEGATE_WRITE delegation allows the client
to handle, on its own, all opens. Only one OPEN_DELEGATE_WRITE
delegation may exist for a given file at a given time, and it is
inconsistent with any OPEN_DELEGATE_READ delegations.
When a client has an OPEN_DELEGATE_READ delegation, it is assured
that neither the contents, the attributes (with the exception of
time_access), nor the names of any links to the file will change
without its knowledge, so long as the delegation is held. When a
client has an OPEN_DELEGATE_WRITE delegation, it may modify the file
data locally since no other client will be accessing the file's data.
The client holding an OPEN_DELEGATE_WRITE delegation may only locally
affect file attributes that are intimately connected with the file
data: size, change, time_access, time_metadata, and time_modify. All
other attributes must be reflected on the server.
When a client has an OPEN delegation, it does not need to send OPENs
or CLOSEs to the server. Instead, the client may update the
appropriate status internally. For an OPEN_DELEGATE_READ delegation,
opens that cannot be handled locally (opens that are for
OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH or that deny
OPEN4_SHARE_ACCESS_READ access) must be sent to the server.
When an OPEN delegation is made, the reply to the OPEN contains an
OPEN delegation structure that specifies the following:
* the type of delegation (OPEN_DELEGATE_READ or
OPEN_DELEGATE_WRITE).
* space limitation information to control flushing of data on close
(OPEN_DELEGATE_WRITE delegation only; see
Section 10.4.1)
* an nfsace4 specifying read and write permissions
* a stateid to represent the delegation
The delegation stateid is separate and distinct from the stateid for
the OPEN proper. The standard stateid, unlike the delegation
stateid, is associated with a particular lock-owner and will continue
to be valid after the delegation is recalled and the file remains
open.
When a request internal to the client is made to open a file and an
OPEN delegation is in effect, it will be accepted or rejected solely
on the basis of the following conditions. Any requirement for other
checks to be made by the delegate should result in the OPEN
delegation being denied so that the checks can be made by the server
itself.
* The access and deny bits for the request and the file as described
in
Section 9.7.
* The read and write permissions as determined below.
The nfsace4 passed with delegation can be used to avoid frequent
ACCESS calls. The permission check should be as follows:
* If the nfsace4 indicates that the open may be done, then it should
be granted without reference to the server.
* If the nfsace4 indicates that the open may not be done, then an
ACCESS request must be sent to the server to obtain the definitive
answer.
The server may return an nfsace4 that is more restrictive than the
actual ACL of the file. This includes an nfsace4 that specifies
denial of all access. Note that some common practices such as
mapping the traditional user "root" to the user "nobody" (see
Section 5.9) may make it incorrect to return the actual ACL of the
file in the delegation response.
The use of a delegation together with various other forms of caching
creates the possibility that no server authentication and
authorization will ever be performed for a given user since all of
the user's requests might be satisfied locally. Where the client is
depending on the server for authentication and authorization, the
client should be sure authentication and authorization occurs for
each user by use of the ACCESS operation. This should be the case
even if an ACCESS operation would not be required otherwise. As
mentioned before, the server may enforce frequent authentication by
returning an nfsace4 denying all access with every OPEN delegation.
10.4.1. Open Delegation and Data Caching
An OPEN delegation allows much of the message overhead associated
with the opening and closing files to be eliminated. An open when an
OPEN delegation is in effect does not require that a validation
message be sent to the server. The continued endurance of the
"OPEN_DELEGATE_READ delegation" provides a guarantee that no OPEN for
OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH, and thus no write,
has occurred. Similarly, when closing a file opened for
OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH and if an
OPEN_DELEGATE_WRITE delegation is in effect, the data written does
not have to be written to the server until the OPEN delegation is
recalled. The continued endurance of the OPEN delegation provides a
guarantee that no open, and thus no READ or WRITE, has been done by
another client.
For the purposes of OPEN delegation, READs and WRITEs done without an
OPEN are treated as the functional equivalents of a corresponding
type of OPEN. Although a client
SHOULD NOT use special stateids when
an open exists, delegation handling on the server can use the client
ID associated with the current session to determine if the operation
has been done by the holder of the delegation (in which case, no
recall is necessary) or by another client (in which case, the
delegation must be recalled and I/O not proceed until the delegation
is returned or revoked).
With delegations, a client is able to avoid writing data to the
server when the CLOSE of a file is serviced. The file close system
call is the usual point at which the client is notified of a lack of
stable storage for the modified file data generated by the
application. At the close, file data is written to the server and,
through normal accounting, the server is able to determine if the
available file system space for the data has been exceeded (i.e., the
server returns NFS4ERR_NOSPC or NFS4ERR_DQUOT). This accounting
includes quotas. The introduction of delegations requires that an
alternative method be in place for the same type of communication to
occur between client and server.
In the delegation response, the server provides either the limit of
the size of the file or the number of modified blocks and associated
block size. The server must ensure that the client will be able to
write modified data to the server of a size equal to that provided in
the original delegation. The server must make this assurance for all
outstanding delegations. Therefore, the server must be careful in
its management of available space for new or modified data, taking
into account available file system space and any applicable quotas.
The server can recall delegations as a result of managing the
available file system space. The client should abide by the server's
state space limits for delegations. If the client exceeds the stated
limits for the delegation, the server's behavior is undefined.
Based on server conditions, quotas, or available file system space,
the server may grant OPEN_DELEGATE_WRITE delegations with very
restrictive space limitations. The limitations may be defined in a
way that will always force modified data to be flushed to the server
on close.
With respect to authentication, flushing modified data to the server
after a CLOSE has occurred may be problematic. For example, the user
of the application may have logged off the client, and unexpired
authentication credentials may not be present. In this case, the
client may need to take special care to ensure that local unexpired
credentials will in fact be available. This may be accomplished by
tracking the expiration time of credentials and flushing data well in
advance of their expiration or by making private copies of
credentials to assure their availability when needed.
10.4.2. Open Delegation and File Locks
When a client holds an OPEN_DELEGATE_WRITE delegation, lock
operations are performed locally. This includes those required for
mandatory byte-range locking. This can be done since the delegation
implies that there can be no conflicting locks. Similarly, all of
the revalidations that would normally be associated with obtaining
locks and the flushing of data associated with the releasing of locks
need not be done.
When a client holds an OPEN_DELEGATE_READ delegation, lock operations
are not performed locally. All lock operations, including those
requesting non-exclusive locks, are sent to the server for
resolution.
10.4.3. Handling of CB_GETATTR
The server needs to employ special handling for a GETATTR where the
target is a file that has an OPEN_DELEGATE_WRITE delegation in
effect. The reason for this is that the client holding the
OPEN_DELEGATE_WRITE delegation may have modified the data, and the
server needs to reflect this change to the second client that
submitted the GETATTR. Therefore, the client holding the
OPEN_DELEGATE_WRITE delegation needs to be interrogated. The server
will use the CB_GETATTR operation. The only attributes that the
server can reliably query via CB_GETATTR are size and change.
Since CB_GETATTR is being used to satisfy another client's GETATTR
request, the server only needs to know if the client holding the
delegation has a modified version of the file. If the client's copy
of the delegated file is not modified (data or size), the server can
satisfy the second client's GETATTR request from the attributes
stored locally at the server. If the file is modified, the server
only needs to know about this modified state. If the server
determines that the file is currently modified, it will respond to
the second client's GETATTR as if the file had been modified locally
at the server.
Since the form of the change attribute is determined by the server
and is opaque to the client, the client and server need to agree on a
method of communicating the modified state of the file. For the size
attribute, the client will report its current view of the file size.
For the change attribute, the handling is more involved.
For the client, the following steps will be taken when receiving an
OPEN_DELEGATE_WRITE delegation:
* The value of the change attribute will be obtained from the server
and cached. Let this value be represented by c.
* The client will create a value greater than c that will be used
for communicating that modified data is held at the client. Let
this value be represented by d.
* When the client is queried via CB_GETATTR for the change
attribute, it checks to see if it holds modified data. If the
file is modified, the value d is returned for the change attribute
value. If this file is not currently modified, the client returns
the value c for the change attribute.
For simplicity of implementation, the client
MAY for each CB_GETATTR
return the same value d. This is true even if, between successive
CB_GETATTR operations, the client again modifies the file's data or
metadata in its cache. The client can return the same value because
the only requirement is that the client be able to indicate to the
server that the client holds modified data. Therefore, the value of
d may always be c + 1.
While the change attribute is opaque to the client in the sense that
it has no idea what units of time, if any, the server is counting
change with, it is not opaque in that the client has to treat it as
an unsigned integer, and the server has to be able to see the results
of the client's changes to that integer. Therefore, the server
MUST encode the change attribute in network order when sending it to the
client. The client
MUST decode it from network order to its native
order when receiving it, and the client
MUST encode it in network
order when sending it to the server. For this reason, change is
defined as an unsigned integer rather than an opaque array of bytes.
For the server, the following steps will be taken when providing an
OPEN_DELEGATE_WRITE delegation:
* Upon providing an OPEN_DELEGATE_WRITE delegation, the server will
cache a copy of the change attribute in the data structure it uses
to record the delegation. Let this value be represented by sc.
* When a second client sends a GETATTR operation on the same file to
the server, the server obtains the change attribute from the first
client. Let this value be cc.
* If the value cc is equal to sc, the file is not modified and the
server returns the current values for change, time_metadata, and
time_modify (for example) to the second client.
* If the value cc is NOT equal to sc, the file is currently modified
at the first client and most likely will be modified at the server
at a future time. The server then uses its current time to
construct attribute values for time_metadata and time_modify. A
new value of sc, which we will call nsc, is computed by the
server, such that nsc >= sc + 1. The server then returns the
constructed time_metadata, time_modify, and nsc values to the
requester. The server replaces sc in the delegation record with
nsc. To prevent the possibility of time_modify, time_metadata,
and change from appearing to go backward (which would happen if
the client holding the delegation fails to write its modified data
to the server before the delegation is revoked or returned), the
server
SHOULD update the file's metadata record with the
constructed attribute values. For reasons of reasonable
performance, committing the constructed attribute values to stable
storage is
OPTIONAL.
As discussed earlier in this section, the client
MAY return the same
cc value on subsequent CB_GETATTR calls, even if the file was
modified in the client's cache yet again between successive
CB_GETATTR calls. Therefore, the server must assume that the file
has been modified yet again, and
MUST take care to ensure that the
new nsc it constructs and returns is greater than the previous nsc it
returned. An example implementation's delegation record would
satisfy this mandate by including a boolean field (let us call it
"modified") that is set to FALSE when the delegation is granted, and
an sc value set at the time of grant to the change attribute value.
The modified field would be set to TRUE the first time cc != sc, and
would stay TRUE until the delegation is returned or revoked. The
processing for constructing nsc, time_modify, and time_metadata would
use this pseudo code:
if (!modified) {
do CB_GETATTR for change and size;
if (cc != sc)
modified = TRUE;
} else {
do CB_GETATTR for size;
}
if (modified) {
sc = sc + 1;
time_modify = time_metadata = current_time;
update sc, time_modify, time_metadata into file's metadata;
}
This would return to the client (that sent GETATTR) the attributes it
requested, but make sure size comes from what CB_GETATTR returned.
The server would not update the file's metadata with the client's
modified size.
In the case that the file attribute size is different than the
server's current value, the server treats this as a modification
regardless of the value of the change attribute retrieved via
CB_GETATTR and responds to the second client as in the last step.
This methodology resolves issues of clock differences between client
and server and other scenarios where the use of CB_GETATTR break
down.
It should be noted that the server is under no obligation to use
CB_GETATTR, and therefore the server
MAY simply recall the delegation
to avoid its use.
10.4.4. Recall of Open Delegation
The following events necessitate recall of an OPEN delegation:
* potentially conflicting OPEN request (or a READ or WRITE operation
done with a special stateid)
* SETATTR sent by another client
* REMOVE request for the file
* RENAME request for the file as either the source or target of the
RENAME
Whether a RENAME of a directory in the path leading to the file
results in recall of an OPEN delegation depends on the semantics of
the server's file system. If that file system denies such RENAMEs
when a file is open, the recall must be performed to determine
whether the file in question is, in fact, open.
In addition to the situations above, the server may choose to recall
OPEN delegations at any time if resource constraints make it
advisable to do so. Clients should always be prepared for the
possibility of recall.
When a client receives a recall for an OPEN delegation, it needs to
update state on the server before returning the delegation. These
same updates must be done whenever a client chooses to return a
delegation voluntarily. The following items of state need to be
dealt with:
* If the file associated with the delegation is no longer open and
no previous CLOSE operation has been sent to the server, a CLOSE
operation must be sent to the server.
* If a file has other open references at the client, then OPEN
operations must be sent to the server. The appropriate stateids
will be provided by the server for subsequent use by the client
since the delegation stateid will no longer be valid. These OPEN
requests are done with the claim type of CLAIM_DELEGATE_CUR. This
will allow the presentation of the delegation stateid so that the
client can establish the appropriate rights to perform the OPEN.
(See
Section 18.16, which describes the OPEN operation, for
details.)
* If there are granted byte-range locks, the corresponding LOCK
operations need to be performed. This applies to the
OPEN_DELEGATE_WRITE delegation case only.
* For an OPEN_DELEGATE_WRITE delegation, if at the time of recall
the file is not open for OPEN4_SHARE_ACCESS_WRITE/
OPEN4_SHARE_ACCESS_BOTH, all modified data for the file must be
flushed to the server. If the delegation had not existed, the
client would have done this data flush before the CLOSE operation.
* For an OPEN_DELEGATE_WRITE delegation when a file is still open at
the time of recall, any modified data for the file needs to be
flushed to the server.
* With the OPEN_DELEGATE_WRITE delegation in place, it is possible
that the file was truncated during the duration of the delegation.
For example, the truncation could have occurred as a result of an
OPEN UNCHECKED with a size attribute value of zero. Therefore, if
a truncation of the file has occurred and this operation has not
been propagated to the server, the truncation must occur before
any modified data is written to the server.
In the case of OPEN_DELEGATE_WRITE delegation, byte-range locking
imposes some additional requirements. To precisely maintain the
associated invariant, it is required to flush any modified data in
any byte-range for which a WRITE_LT lock was released while the
OPEN_DELEGATE_WRITE delegation was in effect. However, because the
OPEN_DELEGATE_WRITE delegation implies no other locking by other
clients, a simpler implementation is to flush all modified data for
the file (as described just above) if any WRITE_LT lock has been
released while the OPEN_DELEGATE_WRITE delegation was in effect.
An implementation need not wait until delegation recall (or the
decision to voluntarily return a delegation) to perform any of the
above actions, if implementation considerations (e.g., resource
availability constraints) make that desirable. Generally, however,
the fact that the actual OPEN state of the file may continue to
change makes it not worthwhile to send information about opens and
closes to the server, except as part of delegation return. An
exception is when the client has no more internal opens of the file.
In this case, sending a CLOSE is useful because it reduces resource
utilization on the client and server. Regardless of the client's
choices on scheduling these actions, all must be performed before the
delegation is returned, including (when applicable) the close that
corresponds to the OPEN that resulted in the delegation. These
actions can be performed either in previous requests or in previous
operations in the same COMPOUND request.
10.4.5. Clients That Fail to Honor Delegation Recalls
A client may fail to respond to a recall for various reasons, such as
a failure of the backchannel from server to the client. The client
may be unaware of a failure in the backchannel. This lack of
awareness could result in the client finding out long after the
failure that its delegation has been revoked, and another client has
modified the data for which the client had a delegation. This is
especially a problem for the client that held an OPEN_DELEGATE_WRITE
delegation.
Status bits returned by SEQUENCE operations help to provide an
alternate way of informing the client of issues regarding the status
of the backchannel and of recalled delegations. When the backchannel
is not available, the server returns the status bit
SEQ4_STATUS_CB_PATH_DOWN on SEQUENCE operations. The client can
react by attempting to re-establish the backchannel and by returning
recallable objects if a backchannel cannot be successfully re-
established.
Whether the backchannel is functioning or not, it may be that the
recalled delegation is not returned. Note that the client's lease
might still be renewed, even though the recalled delegation is not
returned. In this situation, servers
SHOULD revoke delegations that
are not returned in a period of time equal to the lease period. This
period of time should allow the client time to note the backchannel-
down status and re-establish the backchannel.
When delegations are revoked, the server will return with the
SEQ4_STATUS_RECALLABLE_STATE_REVOKED status bit set on subsequent
SEQUENCE operations. The client should note this and then use
TEST_STATEID to find which delegations have been revoked.
10.4.6. Delegation Revocation
At the point a delegation is revoked, if there are associated opens
on the client, these opens may or may not be revoked. If no byte-
range lock or open is granted that is inconsistent with the existing
open, the stateid for the open may remain valid and be disconnected
from the revoked delegation, just as would be the case if the
delegation were returned.
For example, if an OPEN for OPEN4_SHARE_ACCESS_BOTH with a deny of
OPEN4_SHARE_DENY_NONE is associated with the delegation, granting of
another such OPEN to a different client will revoke the delegation
but need not revoke the OPEN, since the two OPENs are consistent with
each other. On the other hand, if an OPEN denying write access is
granted, then the existing OPEN must be revoked.
When opens and/or locks are revoked, the applications holding these
opens or locks need to be notified. This notification usually occurs
by returning errors for READ/WRITE operations or when a close is
attempted for the open file.
If no opens exist for the file at the point the delegation is
revoked, then notification of the revocation is unnecessary.
However, if there is modified data present at the client for the
file, the user of the application should be notified. Unfortunately,
it may not be possible to notify the user since active applications
may not be present at the client. See
Section 10.5.1 for additional
details.
10.4.7. Delegations via WANT_DELEGATION
In addition to providing delegations as part of the reply to OPEN
operations, servers
MAY provide delegations separate from open, via
the
OPTIONAL WANT_DELEGATION operation. This allows delegations to
be obtained in advance of an OPEN that might benefit from them, for
objects that are not a valid target of OPEN, or to deal with cases in
which a delegation has been recalled and the client wants to make an
attempt to re-establish it if the absence of use by other clients
allows that.
The WANT_DELEGATION operation may be performed on any type of file
object other than a directory.
When a delegation is obtained using WANT_DELEGATION, any open files
for the same filehandle held by that client are to be treated as
subordinate to the delegation, just as if they had been created using
an OPEN of type CLAIM_DELEGATE_CUR. They are otherwise unchanged as
to seqid, access and deny modes, and the relationship with byte-range
locks. Similarly, because existing byte-range locks are subordinate
to an open, those byte-range locks also become indirectly subordinate
to that new delegation.
The WANT_DELEGATION operation provides for delivery of delegations
via callbacks, when the delegations are not immediately available.
When a requested delegation is available, it is delivered to the
client via a CB_PUSH_DELEG operation. When this happens, open files
for the same filehandle become subordinate to the new delegation at
the point at which the delegation is delivered, just as if they had
been created using an OPEN of type CLAIM_DELEGATE_CUR. Similarly,
this occurs for existing byte-range locks subordinate to an open.
10.5. Data Caching and Revocation
When locks and delegations are revoked, the assumptions upon which
successful caching depends are no longer guaranteed. For any locks
or share reservations that have been revoked, the corresponding
state-owner needs to be notified. This notification includes
applications with a file open that has a corresponding delegation
that has been revoked. Cached data associated with the revocation
must be removed from the client. In the case of modified data
existing in the client's cache, that data must be removed from the
client without being written to the server. As mentioned, the
assumptions made by the client are no longer valid at the point when
a lock or delegation has been revoked. For example, another client
may have been granted a conflicting byte-range lock after the
revocation of the byte-range lock at the first client. Therefore,
the data within the lock range may have been modified by the other
client. Obviously, the first client is unable to guarantee to the
application what has occurred to the file in the case of revocation.
Notification to a state-owner will in many cases consist of simply
returning an error on the next and all subsequent READs/WRITEs to the
open file or on the close. Where the methods available to a client
make such notification impossible because errors for certain
operations may not be returned, more drastic action such as signals
or process termination may be appropriate. The justification here is
that an invariant on which an application depends may be violated.
Depending on how errors are typically treated for the client-
operating environment, further levels of notification including
logging, console messages, and GUI pop-ups may be appropriate.
10.5.1. Revocation Recovery for Write Open Delegation
Revocation recovery for an OPEN_DELEGATE_WRITE delegation poses the
special issue of modified data in the client cache while the file is
not open. In this situation, any client that does not flush modified
data to the server on each close must ensure that the user receives
appropriate notification of the failure as a result of the
revocation. Since such situations may require human action to
correct problems, notification schemes in which the appropriate user
or administrator is notified may be necessary. Logging and console
messages are typical examples.
If there is modified data on the client, it must not be flushed
normally to the server. A client may attempt to provide a copy of
the file data as modified during the delegation under a different
name in the file system namespace to ease recovery. Note that when
the client can determine that the file has not been modified by any
other client, or when the client has a complete cached copy of the
file in question, such a saved copy of the client's view of the file
may be of particular value for recovery. In another case, recovery
using a copy of the file based partially on the client's cached data
and partially on the server's copy as modified by other clients will
be anything but straightforward, so clients may avoid saving file
contents in these situations or specially mark the results to warn
users of possible problems.
Saving of such modified data in delegation revocation situations may
be limited to files of a certain size or might be used only when
sufficient disk space is available within the target file system.
Such saving may also be restricted to situations when the client has
sufficient buffering resources to keep the cached copy available
until it is properly stored to the target file system.
10.6. Attribute Caching
This section pertains to the caching of a file's attributes on a
client when that client does not hold a delegation on the file.
The attributes discussed in this section do not include named
attributes. Individual named attributes are analogous to files, and
caching of the data for these needs to be handled just as data
caching is for ordinary files. Similarly, LOOKUP results from an
OPENATTR directory (as well as the directory's contents) are to be
cached on the same basis as any other pathnames.
Clients may cache file attributes obtained from the server and use
them to avoid subsequent GETATTR requests. Such caching is write
through in that modification to file attributes is always done by
means of requests to the server and should not be done locally and
should not be cached. The exception to this are modifications to
attributes that are intimately connected with data caching.
Therefore, extending a file by writing data to the local data cache
is reflected immediately in the size as seen on the client without
this change being immediately reflected on the server. Normally,
such changes are not propagated directly to the server, but when the
modified data is flushed to the server, analogous attribute changes
are made on the server. When OPEN delegation is in effect, the
modified attributes may be returned to the server in reaction to a
CB_RECALL call.
The result of local caching of attributes is that the attribute
caches maintained on individual clients will not be coherent.
Changes made in one order on the server may be seen in a different
order on one client and in a third order on another client.
The typical file system application programming interfaces do not
provide means to atomically modify or interrogate attributes for
multiple files at the same time. The following rules provide an
environment where the potential incoherencies mentioned above can be
reasonably managed. These rules are derived from the practice of
previous NFS protocols.
* All attributes for a given file (per-fsid attributes excepted) are
cached as a unit at the client so that no non-serializability can
arise within the context of a single file.
* An upper time boundary is maintained on how long a client cache
entry can be kept without being refreshed from the server.
* When operations are performed that change attributes at the
server, the updated attribute set is requested as part of the
containing RPC. This includes directory operations that update
attributes indirectly. This is accomplished by following the
modifying operation with a GETATTR operation and then using the
results of the GETATTR to update the client's cached attributes.
Note that if the full set of attributes to be cached is requested by
READDIR, the results can be cached by the client on the same basis as
attributes obtained via GETATTR.
A client may validate its cached version of attributes for a file by
fetching both the change and time_access attributes and assuming that
if the change attribute has the same value as it did when the
attributes were cached, then no attributes other than time_access
have changed. The reason why time_access is also fetched is because
many servers operate in environments where the operation that updates
change does not update time_access. For example, POSIX file
semantics do not update access time when a file is modified by the
write system call [15]. Therefore, the client that wants a current
time_access value should fetch it with change during the attribute
cache validation processing and update its cached time_access.
The client may maintain a cache of modified attributes for those
attributes intimately connected with data of modified regular files
(size, time_modify, and change). Other than those three attributes,
the client
MUST NOT maintain a cache of modified attributes.
Instead, attribute changes are immediately sent to the server.
In some operating environments, the equivalent to time_access is
expected to be implicitly updated by each read of the content of the
file object. If an NFS client is caching the content of a file
object, whether it is a regular file, directory, or symbolic link,
the client
SHOULD NOT update the time_access attribute (via SETATTR
or a small READ or READDIR request) on the server with each read that
is satisfied from cache. The reason is that this can defeat the
performance benefits of caching content, especially since an explicit
SETATTR of time_access may alter the change attribute on the server.
If the change attribute changes, clients that are caching the content
will think the content has changed, and will re-read unmodified data
from the server. Nor is the client encouraged to maintain a modified
version of time_access in its cache, since the client either would
eventually have to write the access time to the server with bad
performance effects or never update the server's time_access, thereby
resulting in a situation where an application that caches access time
between a close and open of the same file observes the access time
oscillating between the past and present. The time_access attribute
always means the time of last access to a file by a read that was
satisfied by the server. This way clients will tend to see only
time_access changes that go forward in time.
10.7. Data and Metadata Caching and Memory Mapped Files
Some operating environments include the capability for an application
to map a file's content into the application's address space. Each
time the application accesses a memory location that corresponds to a
block that has not been loaded into the address space, a page fault
occurs and the file is read (or if the block does not exist in the
file, the block is allocated and then instantiated in the
application's address space).
As long as each memory-mapped access to the file requires a page
fault, the relevant attributes of the file that are used to detect
access and modification (time_access, time_metadata, time_modify, and
change) will be updated. However, in many operating environments,
when page faults are not required, these attributes will not be
updated on reads or updates to the file via memory access (regardless
of whether the file is local or is accessed remotely). A client or
server
MAY fail to update attributes of a file that is being accessed
via memory-mapped I/O. This has several implications:
* If there is an application on the server that has memory mapped a
file that a client is also accessing, the client may not be able
to get a consistent value of the change attribute to determine
whether or not its cache is stale. A server that knows that the
file is memory-mapped could always pessimistically return updated
values for change so as to force the application to always get the
most up-to-date data and metadata for the file. However, due to
the negative performance implications of this, such behavior is
OPTIONAL.
* If the memory-mapped file is not being modified on the server, and
instead is just being read by an application via the memory-mapped
interface, the client will not see an updated time_access
attribute. However, in many operating environments, neither will
any process running on the server. Thus, NFS clients are at no
disadvantage with respect to local processes.
* If there is another client that is memory mapping the file, and if
that client is holding an OPEN_DELEGATE_WRITE delegation, the same
set of issues as discussed in the previous two bullet points
apply. So, when a server does a CB_GETATTR to a file that the
client has modified in its cache, the reply from CB_GETATTR will
not necessarily be accurate. As discussed earlier, the client's
obligation is to report that the file has been modified since the
delegation was granted, not whether it has been modified again
between successive CB_GETATTR calls, and the server
MUST assume
that any file the client has modified in cache has been modified
again between successive CB_GETATTR calls. Depending on the
nature of the client's memory management system, this weak
obligation may not be possible. A client
MAY return stale
information in CB_GETATTR whenever the file is memory-mapped.
* The mixture of memory mapping and byte-range locking on the same
file is problematic. Consider the following scenario, where a
page size on each client is 8192 bytes.
- Client A memory maps the first page (8192 bytes) of file X.
- Client B memory maps the first page (8192 bytes) of file X.
- Client A WRITE_LT locks the first 4096 bytes.
- Client B WRITE_LT locks the second 4096 bytes.
- Client A, via a STORE instruction, modifies part of its locked
byte-range.
- Simultaneous to client A, client B executes a STORE on part of
its locked byte-range.
Here the challenge is for each client to resynchronize to get a
correct view of the first page. In many operating environments, the
virtual memory management systems on each client only know a page is
modified, not that a subset of the page corresponding to the
respective lock byte-ranges has been modified. So it is not possible
for each client to do the right thing, which is to write to the
server only that portion of the page that is locked. For example, if
client A simply writes out the page, and then client B writes out the
page, client A's data is lost.
Moreover, if mandatory locking is enabled on the file, then we have a
different problem. When clients A and B execute the STORE
instructions, the resulting page faults require a byte-range lock on
the entire page. Each client then tries to extend their locked range
to the entire page, which results in a deadlock. Communicating the
NFS4ERR_DEADLOCK error to a STORE instruction is difficult at best.
If a client is locking the entire memory-mapped file, there is no
problem with advisory or mandatory byte-range locking, at least until
the client unlocks a byte-range in the middle of the file.
Given the above issues, the following are permitted:
* Clients and servers
MAY deny memory mapping a file for which they
know there are byte-range locks.
* Clients and servers
MAY deny a byte-range lock on a file they know
is memory-mapped.
* A client
MAY deny memory mapping a file that it knows requires
mandatory locking for I/O. If mandatory locking is enabled after
the file is opened and mapped, the client
MAY deny the application
further access to its mapped file.
10.8. Name and Directory Caching without Directory Delegations
The NFSv4.1 directory delegation facility (described in
Section 10.9 below) is
OPTIONAL for servers to implement. Even where it is
implemented, it may not always be functional because of resource
availability issues or other constraints. Thus, it is important to
understand how name and directory caching are done in the absence of
directory delegations. These topics are discussed in the next two
subsections.
The results of LOOKUP and READDIR operations may be cached to avoid
the cost of subsequent LOOKUP operations. Just as in the case of
attribute caching, inconsistencies may arise among the various client
caches. To mitigate the effects of these inconsistencies and given
the context of typical file system APIs, an upper time boundary is
maintained for how long a client name cache entry can be kept without
verifying that the entry has not been made invalid by a directory
change operation performed by another client.
When a client is not making changes to a directory for which there
exist name cache entries, the client needs to periodically fetch
attributes for that directory to ensure that it is not being
modified. After determining that no modification has occurred, the
expiration time for the associated name cache entries may be updated
to be the current time plus the name cache staleness bound.
When a client is making changes to a given directory, it needs to
determine whether there have been changes made to the directory by
other clients. It does this by using the change attribute as
reported before and after the directory operation in the associated
change_info4 value returned for the operation. The server is able to
communicate to the client whether the change_info4 data is provided
atomically with respect to the directory operation. If the change
values are provided atomically, the client has a basis for
determining, given proper care, whether other clients are modifying
the directory in question.
The simplest way to enable the client to make this determination is
for the client to serialize all changes made to a specific directory.
When this is done, and the server provides before and after values of
the change attribute atomically, the client can simply compare the
after value of the change attribute from one operation on a directory
with the before value on the subsequent operation modifying that
directory. When these are equal, the client is assured that no other
client is modifying the directory in question.
When such serialization is not used, and there may be multiple
simultaneous outstanding operations modifying a single directory sent
from a single client, making this sort of determination can be more
complicated. If two such operations complete in a different order
than they were actually performed, that might give an appearance
consistent with modification being made by another client. Where
this appears to happen, the client needs to await the completion of
all such modifications that were started previously, to see if the
outstanding before and after change numbers can be sorted into a
chain such that the before value of one change number matches the
after value of a previous one, in a chain consistent with this client
being the only one modifying the directory.
In either of these cases, the client is able to determine whether the
directory is being modified by another client. If the comparison
indicates that the directory was updated by another client, the name
cache associated with the modified directory is purged from the
client. If the comparison indicates no modification, the name cache
can be updated on the client to reflect the directory operation and
the associated timeout can be extended. The post-operation change
value needs to be saved as the basis for future change_info4
comparisons.
As demonstrated by the scenario above, name caching requires that the
client revalidate name cache data by inspecting the change attribute
of a directory at the point when the name cache item was cached.
This requires that the server update the change attribute for
directories when the contents of the corresponding directory is
modified. For a client to use the change_info4 information
appropriately and correctly, the server must report the pre- and
post-operation change attribute values atomically. When the server
is unable to report the before and after values atomically with
respect to the directory operation, the server must indicate that
fact in the change_info4 return value. When the information is not
atomically reported, the client should not assume that other clients
have not changed the directory.
10.8.2. Directory Caching
The results of READDIR operations may be used to avoid subsequent
READDIR operations. Just as in the cases of attribute and name
caching, inconsistencies may arise among the various client caches.
To mitigate the effects of these inconsistencies, and given the
context of typical file system APIs, the following rules should be
followed:
* Cached READDIR information for a directory that is not obtained in
a single READDIR operation must always be a consistent snapshot of
directory contents. This is determined by using a GETATTR before
the first READDIR and after the last READDIR that contributes to
the cache.
* An upper time boundary is maintained to indicate the length of
time a directory cache entry is considered valid before the client
must revalidate the cached information.
The revalidation technique parallels that discussed in the case of
name caching. When the client is not changing the directory in
question, checking the change attribute of the directory with GETATTR
is adequate. The lifetime of the cache entry can be extended at
these checkpoints. When a client is modifying the directory, the
client needs to use the change_info4 data to determine whether there
are other clients modifying the directory. If it is determined that
no other client modifications are occurring, the client may update
its directory cache to reflect its own changes.
As demonstrated previously, directory caching requires that the
client revalidate directory cache data by inspecting the change
attribute of a directory at the point when the directory was cached.
This requires that the server update the change attribute for
directories when the contents of the corresponding directory is
modified. For a client to use the change_info4 information
appropriately and correctly, the server must report the pre- and
post-operation change attribute values atomically. When the server
is unable to report the before and after values atomically with
respect to the directory operation, the server must indicate that
fact in the change_info4 return value. When the information is not
atomically reported, the client should not assume that other clients
have not changed the directory.
10.9. Directory Delegations
10.9.1. Introduction to Directory Delegations
Directory caching for the NFSv4.1 protocol, as previously described,
is similar to file caching in previous versions. Clients typically
cache directory information for a duration determined by the client.
At the end of a predefined timeout, the client will query the server
to see if the directory has been updated. By caching attributes,
clients reduce the number of GETATTR calls made to the server to
validate attributes. Furthermore, frequently accessed files and
directories, such as the current working directory, have their
attributes cached on the client so that some NFS operations can be
performed without having to make an RPC call. By caching name and
inode information about most recently looked up entries in a
Directory Name Lookup Cache (DNLC), clients do not need to send
LOOKUP calls to the server every time these files are accessed.
This caching approach works reasonably well at reducing network
traffic in many environments. However, it does not address
environments where there are numerous queries for files that do not
exist. In these cases of "misses", the client sends requests to the
server in order to provide reasonable application semantics and
promptly detect the creation of new directory entries. Examples of
high miss activity are compilation in software development
environments. The current behavior of NFS limits its potential
scalability and wide-area sharing effectiveness in these types of
environments. Other distributed stateful file system architectures
such as AFS and DFS have proven that adding state around directory
contents can greatly reduce network traffic in high-miss
environments.
Delegation of directory contents is an
OPTIONAL feature of NFSv4.1.
Directory delegations provide similar traffic reduction benefits as
with file delegations. By allowing clients to cache directory
contents (in a read-only fashion) while being notified of changes,
the client can avoid making frequent requests to interrogate the
contents of slowly-changing directories, reducing network traffic and
improving client performance. It can also simplify the task of
determining whether other clients are making changes to the directory
when the client itself is making many changes to the directory and
changes are not serialized.
Directory delegations allow improved namespace cache consistency to
be achieved through delegations and synchronous recalls, in the
absence of notifications. In addition, if time-based consistency is
sufficient, asynchronous notifications can provide performance
benefits for the client, and possibly the server, under some common
operating conditions such as slowly-changing and/or very large
directories.
10.9.2. Directory Delegation Design
NFSv4.1 introduces the GET_DIR_DELEGATION (
Section 18.39) operation
to allow the client to ask for a directory delegation. The
delegation covers directory attributes and all entries in the
directory. If either of these change, the delegation will be
recalled synchronously. The operation causing the recall will have
to wait before the recall is complete. Any changes to directory
entry attributes will not cause the delegation to be recalled.
In addition to asking for delegations, a client can also ask for
notifications for certain events. These events include changes to
the directory's attributes and/or its contents. If a client asks for
notification for a certain event, the server will notify the client
when that event occurs. This will not result in the delegation being
recalled for that client. The notifications are asynchronous and
provide a way of avoiding recalls in situations where a directory is
changing enough that the pure recall model may not be effective while
trying to allow the client to get substantial benefit. In the
absence of notifications, once the delegation is recalled the client
has to refresh its directory cache; this might not be very efficient
for very large directories.
The delegation is read-only and the client may not make changes to
the directory other than by performing NFSv4.1 operations that modify
the directory or the associated file attributes so that the server
has knowledge of these changes. In order to keep the client's
namespace synchronized with that of the server, the server will
notify the delegation-holding client (assuming it has requested
notifications) of the changes made as a result of that client's
directory-modifying operations. This is to avoid any need for that
client to send subsequent GETATTR or READDIR operations to the
server. If a single client is holding the delegation and that client
makes any changes to the directory (i.e., the changes are made via
operations sent on a session associated with the client ID holding
the delegation), the delegation will not be recalled. Multiple
clients may hold a delegation on the same directory, but if any such
client modifies the directory, the server
MUST recall the delegation
from the other clients, unless those clients have made provisions to
be notified of that sort of modification.
Delegations can be recalled by the server at any time. Normally, the
server will recall the delegation when the directory changes in a way
that is not covered by the notification, or when the directory
changes and notifications have not been requested. If another client
removes the directory for which a delegation has been granted, the
server will recall the delegation.
10.9.3. Attributes in Support of Directory Notifications
See
Section 5.11 for a description of the attributes associated with
directory notifications.
10.9.4. Directory Delegation Recall
The server will recall the directory delegation by sending a callback
to the client. It will use the same callback procedure as used for
recalling file delegations. The server will recall the delegation
when the directory changes in a way that is not covered by the
notification. However, the server need not recall the delegation if
attributes of an entry within the directory change.
If the server notices that handing out a delegation for a directory
is causing too many notifications to be sent out, it may decide to
not hand out delegations for that directory and/or recall those
already granted. If a client tries to remove the directory for which
a delegation has been granted, the server will recall all associated
delegations.
The implementation sections for a number of operations describe
situations in which notification or delegation recall would be
required under some common circumstances. In this regard, a similar
set of caveats to those listed in
Section 10.2 apply.
* For CREATE, see
Section 18.4.4.
* For LINK, see
Section 18.9.4.
* For OPEN, see
Section 18.16.4.
* For REMOVE, see
Section 18.25.4.
* For RENAME, see
Section 18.26.4.
* For SETATTR, see
Section 18.30.4.
10.9.5. Directory Delegation Recovery
Recovery from client or server restart for state on regular files has
two main goals: avoiding the necessity of breaking application
guarantees with respect to locked files and delivery of updates
cached at the client. Neither of these goals applies to directories
protected by OPEN_DELEGATE_READ delegations and notifications. Thus,
no provision is made for reclaiming directory delegations in the
event of client or server restart. The client can simply establish a
directory delegation in the same fashion as was done initially.
11. Multi-Server Namespace
NFSv4.1 supports attributes that allow a namespace to extend beyond
the boundaries of a single server. It is desirable that clients and
servers support construction of such multi-server namespaces. Use of
such multi-server namespaces is
OPTIONAL; however, and for many
purposes, single-server namespaces are perfectly acceptable. The use
of multi-server namespaces can provide many advantages by separating
a file system's logical position in a namespace from the (possibly
changing) logistical and administrative considerations that cause a
particular file system to be located on a particular server via a
single network access path that has to be known in advance or
determined using DNS.
11.1. Terminology
In this section as a whole (i.e., within all of
Section 11), the
phrase "client ID" always refers to the 64-bit shorthand identifier
assigned by the server (a clientid4) and never to the structure that
the client uses to identify itself to the server (called an
nfs_client_id4 or client_owner in NFSv4.0 and NFSv4.1, respectively).
The opaque identifier within those structures is referred to as a
"client id string".
11.1.1. Terminology Related to Trunking
It is particularly important to clarify the distinction between
trunking detection and trunking discovery. The definitions we
present are applicable to all minor versions of NFSv4, but we will
focus on how these terms apply to NFS version 4.1.
* Trunking detection refers to ways of deciding whether two specific
network addresses are connected to the same NFSv4 server. The
means available to make this determination depends on the protocol
version, and, in some cases, on the client implementation.
In the case of NFS version 4.1 and later minor versions, the means
of trunking detection are as described in this document and are
available to every client. Two network addresses connected to the
same server can always be used together to access a particular
server but cannot necessarily be used together to access a single
session. See below for definitions of the terms "server-
trunkable" and "session-trunkable".
* Trunking discovery is a process by which a client using one
network address can obtain other addresses that are connected to
the same server. Typically, it builds on a trunking detection
facility by providing one or more methods by which candidate
addresses are made available to the client, who can then use
trunking detection to appropriately filter them.
Despite the support for trunking detection, there was no
description of trunking discovery provided in
RFC 5661 [66],
making it necessary to provide those means in this document.
The combination of a server network address and a particular
connection type to be used by a connection is referred to as a
"server endpoint". Although using different connection types may
result in different ports being used, the use of different ports by
multiple connections to the same network address in such cases is not
the essence of the distinction between the two endpoints used. This
is in contrast to the case of port-specific endpoints, in which the
explicit specification of port numbers within network addresses is
used to allow a single server node to support multiple NFS servers.
Two network addresses connected to the same server are said to be
server-trunkable. Two such addresses support the use of client ID
trunking, as described in
Section 2.10.5.
Two network addresses connected to the same server such that those
addresses can be used to support a single common session are referred
to as session-trunkable. Note that two addresses may be server-
trunkable without being session-trunkable, and that, when two
connections of different connection types are made to the same
network address and are based on a single file system location entry,
they are always session-trunkable, independent of the connection
type, as specified by
Section 2.10.5, since their derivation from the
same file system location entry, together with the identity of their
network addresses, assures that both connections are to the same
server and will return server-owner information, allowing session
trunking to be used.
11.1.2. Terminology Related to File System Location
Regarding the terminology that relates to the construction of multi-
server namespaces out of a set of local per-server namespaces:
* Each server has a set of exported file systems that may be
accessed by NFSv4 clients. Typically, this is done by assigning
each file system a name within the pseudo-fs associated with the
server, although the pseudo-fs may be dispensed with if there is
only a single exported file system. Each such file system is part
of the server's local namespace, and can be considered as a file
system instance within a larger multi-server namespace.
* The set of all exported file systems for a given server
constitutes that server's local namespace.
* In some cases, a server will have a namespace more extensive than
its local namespace by using features associated with attributes
that provide file system location information. These features,
which allow construction of a multi-server namespace, are all
described in individual sections below and include referrals
(
Section 11.5.6), migration (
Section 11.5.5), and replication
(
Section 11.5.4).
* A file system present in a server's pseudo-fs may have multiple
file system instances on different servers associated with it.
All such instances are considered replicas of one another.
Whether such replicas can be used simultaneously is discussed in
Section 11.11.1, while the level of coordination between them
(important when switching between them) is discussed in Sections
11.11.2 through
11.11.8 below.
* When a file system is present in a server's pseudo-fs, but there
is no corresponding local file system, it is said to be "absent".
In such cases, all associated instances will be accessed on other
servers.
Regarding the terminology that relates to attributes used in trunking
discovery and other multi-server namespace features:
* File system location attributes include the fs_locations and
fs_locations_info attributes.
* File system location entries provide the individual file system
locations within the file system location attributes. Each such
entry specifies a server, in the form of a hostname or an address,
and an fs name, which designates the location of the file system
within the server's local namespace. A file system location entry
designates a set of server endpoints to which the client may
establish connections. There may be multiple endpoints because a
hostname may map to multiple network addresses and because
multiple connection types may be used to communicate with a single
network address. However, except where explicit port numbers are
used to designate a set of servers within a single server node,
all such endpoints
MUST designate a way of connecting to a single
server. The exact form of the location entry varies with the
particular file system location attribute used, as described in
Section 11.2.
The network addresses used in file system location entries
typically appear without port number indications and are used to
designate a server at one of the standard ports for NFS access,
e.g., 2049 for TCP or 20049 for use with RPC-over-RDMA. Port
numbers may be used in file system location entries to designate
servers (typically user-level ones) accessed using other port
numbers. In the case where network addresses indicate trunking
relationships, the use of an explicit port number is inappropriate
since trunking is a relationship between network addresses. See
Section 11.5.2 for details.
* File system location elements are derived from location entries,
and each describes a particular network access path consisting of
a network address and a location within the server's local
namespace. Such location elements need not appear within a file
system location attribute, but the existence of each location
element derives from a corresponding location entry. When a
location entry specifies an IP address, there is only a single
corresponding location element. File system location entries that
contain a hostname are resolved using DNS, and may result in one
or more location elements. All location elements consist of a
location address that includes the IP address of an interface to a
server and an fs name, which is the location of the file system
within the server's local namespace. The fs name can be empty if
the server has no pseudo-fs and only a single exported file system
at the root filehandle.
* Two file system location elements are said to be server-trunkable
if they specify the same fs name and the location addresses are
such that the location addresses are server-trunkable. When the
corresponding network paths are used, the client will always be
able to use client ID trunking, but will only be able to use
session trunking if the paths are also session-trunkable.
* Two file system location elements are said to be session-trunkable
if they specify the same fs name and the location addresses are
such that the location addresses are session-trunkable. When the
corresponding network paths are used, the client will be able to
able to use either client ID trunking or session trunking.
Discussion of the term "replica" is complicated by the fact that the
term was used in
RFC 5661 [66] with a meaning different from that
used in this document. In short, in [66] each replica is identified
by a single network access path, while in the current document, a set
of network access paths that have server-trunkable network addresses
and the same root-relative file system pathname is considered to be a
single replica with multiple network access paths.
Each set of server-trunkable location elements defines a set of
available network access paths to a particular file system. When
there are multiple such file systems, each of which containing the
same data, these file systems are considered replicas of one another.
Logically, such replication is symmetric, since the fs currently in
use and an alternate fs are replicas of each other. Often, in other
documents, the term "replica" is not applied to the fs currently in
use, despite the fact that the replication relation is inherently
symmetric.
11.2. File System Location Attributes
NFSv4.1 contains attributes that provide information about how a
given file system may be accessed (i.e., at what network address and
namespace position). As a result, file systems in the namespace of
one server can be associated with one or more instances of that file
system on other servers. These attributes contain file system
location entries specifying a server address target (either as a DNS
name representing one or more IP addresses or as a specific IP
address) together with the pathname of that file system within the
associated single-server namespace.
The fs_locations_info
RECOMMENDED attribute allows specification of
one or more file system instance locations where the data
corresponding to a given file system may be found. In addition to
the specification of file system instance locations, this attribute
provides helpful information to do the following:
* Guide choices among the various file system instances provided
(e.g., priority for use, writability, currency, etc.).
* Help the client efficiently effect as seamless a transition as
possible among multiple file system instances, when and if that
should be necessary.
* Guide the selection of the appropriate connection type to be used
when establishing a connection.
Within the fs_locations_info attribute, each fs_locations_server4
entry corresponds to a file system location entry: the fls_server
field designates the server, and the fl_rootpath field of the
encompassing fs_locations_item4 gives the location pathname within
the server's pseudo-fs.
The fs_locations attribute defined in NFSv4.0 is also a part of
NFSv4.1. This attribute only allows specification of the file system
locations where the data corresponding to a given file system may be
found. Servers
SHOULD make this attribute available whenever
fs_locations_info is supported, but client use of fs_locations_info
is preferable because it provides more information.
Within the fs_locations attribute, each fs_location4 contains a file
system location entry with the server field designating the server
and the rootpath field giving the location pathname within the
server's pseudo-fs.
11.3. File System Presence or Absence
A given location in an NFSv4.1 namespace (typically but not
necessarily a multi-server namespace) can have a number of file
system instance locations associated with it (via the fs_locations or
fs_locations_info attribute). There may also be an actual current
file system at that location, accessible via normal namespace
operations (e.g., LOOKUP). In this case, the file system is said to
be "present" at that position in the namespace, and clients will
typically use it, reserving use of additional locations specified via
the location-related attributes to situations in which the principal
location is no longer available.
When there is no actual file system at the namespace location in
question, the file system is said to be "absent". An absent file
system contains no files or directories other than the root. Any
reference to it, except to access a small set of attributes useful in
determining alternate locations, will result in an error,
NFS4ERR_MOVED. Note that if the server ever returns the error
NFS4ERR_MOVED, it
MUST support the fs_locations attribute and
SHOULD support the fs_locations_info and fs_status attributes.
While the error name suggests that we have a case of a file system
that once was present, and has only become absent later, this is only
one possibility. A position in the namespace may be permanently
absent with the set of file system(s) designated by the location
attributes being the only realization. The name NFS4ERR_MOVED
reflects an earlier, more limited conception of its function, but
this error will be returned whenever the referenced file system is
absent, whether it has moved or not.
Except in the case of GETATTR-type operations (to be discussed
later), when the current filehandle at the start of an operation is
within an absent file system, that operation is not performed and the
error NFS4ERR_MOVED is returned, to indicate that the file system is
absent on the current server.
Because a GETFH cannot succeed if the current filehandle is within an
absent file system, filehandles within an absent file system cannot
be transferred to the client. When a client does have filehandles
within an absent file system, it is the result of obtaining them when
the file system was present, and having the file system become absent
subsequently.
It should be noted that because the check for the current filehandle
being within an absent file system happens at the start of every
operation, operations that change the current filehandle so that it
is within an absent file system will not result in an error. This
allows such combinations as PUTFH-GETATTR and LOOKUP-GETATTR to be
used to get attribute information, particularly location attribute
information, as discussed below.
The
RECOMMENDED file system attribute fs_status can be used to
interrogate the present/absent status of a given file system.
11.4. Getting Attributes for an Absent File System
When a file system is absent, most attributes are not available, but
it is necessary to allow the client access to the small set of
attributes that are available, and most particularly those that give
information about the correct current locations for this file system:
fs_locations and fs_locations_info.
11.4.1. GETATTR within an Absent File System
As mentioned above, an exception is made for GETATTR in that
attributes may be obtained for a filehandle within an absent file
system. This exception only applies if the attribute mask contains
at least one attribute bit that indicates the client is interested in
a result regarding an absent file system: fs_locations,
fs_locations_info, or fs_status. If none of these attributes is
requested, GETATTR will result in an NFS4ERR_MOVED error.
When a GETATTR is done on an absent file system, the set of supported
attributes is very limited. Many attributes, including those that
are normally
REQUIRED, will not be available on an absent file
system. In addition to the attributes mentioned above (fs_locations,
fs_locations_info, fs_status), the following attributes
SHOULD be
available on absent file systems. In the case of
RECOMMENDED attributes, they should be available at least to the same degree that
they are available on present file systems.
change_policy: This attribute is useful for absent file systems and
can be helpful in summarizing to the client when any of the
location-related attributes change.
fsid: This attribute should be provided so that the client can
determine file system boundaries, including, in particular, the
boundary between present and absent file systems. This value must
be different from any other fsid on the current server and need
have no particular relationship to fsids on any particular
destination to which the client might be directed.
mounted_on_fileid: For objects at the top of an absent file system,
this attribute needs to be available. Since the fileid is within
the present parent file system, there should be no need to
reference the absent file system to provide this information.
Other attributes
SHOULD NOT be made available for absent file
systems, even when it is possible to provide them. The server should
not assume that more information is always better and should avoid
gratuitously providing additional information.
When a GETATTR operation includes a bit mask for one of the
attributes fs_locations, fs_locations_info, or fs_status, but where
the bit mask includes attributes that are not supported, GETATTR will
not return an error, but will return the mask of the actual
attributes supported with the results.
Handling of VERIFY/NVERIFY is similar to GETATTR in that if the
attribute mask does not include fs_locations, fs_locations_info, or
fs_status, the error NFS4ERR_MOVED will result. It differs in that
any appearance in the attribute mask of an attribute not supported
for an absent file system (and note that this will include some
normally
REQUIRED attributes) will also cause an NFS4ERR_MOVED
result.
11.4.2. READDIR and Absent File Systems
A READDIR performed when the current filehandle is within an absent
file system will result in an NFS4ERR_MOVED error, since, unlike the
case of GETATTR, no such exception is made for READDIR.
Attributes for an absent file system may be fetched via a READDIR for
a directory in a present file system, when that directory contains
the root directories of one or more absent file systems. In this
case, the handling is as follows:
* If the attribute set requested includes one of the attributes
fs_locations, fs_locations_info, or fs_status, then fetching of
attributes proceeds normally and no NFS4ERR_MOVED indication is
returned, even when the rdattr_error attribute is requested.
* If the attribute set requested does not include one of the
attributes fs_locations, fs_locations_info, or fs_status, then if
the rdattr_error attribute is requested, each directory entry for
the root of an absent file system will report NFS4ERR_MOVED as the
value of the rdattr_error attribute.
* If the attribute set requested does not include any of the
attributes fs_locations, fs_locations_info, fs_status, or
rdattr_error, then the occurrence of the root of an absent file
system within the directory will result in the READDIR failing
with an NFS4ERR_MOVED error.
* The unavailability of an attribute because of a file system's
absence, even one that is ordinarily
REQUIRED, does not result in
any error indication. The set of attributes returned for the root
directory of the absent file system in that case is simply
restricted to those actually available.
11.5. Uses of File System Location Information
The file system location attributes (i.e., fs_locations and
fs_locations_info), together with the possibility of absent file
systems, provide a number of important facilities for reliable,
manageable, and scalable data access.
When a file system is present, these attributes can provide the
following:
* The locations of alternative replicas to be used to access the
same data in the event of server failures, communications
problems, or other difficulties that make continued access to the
current replica impossible or otherwise impractical. Provisioning
and use of such alternate replicas is referred to as "replication"
and is discussed in
Section 11.5.4 below.
* The network address(es) to be used to access the current file
system instance or replicas of it. Client use of this information
is discussed in
Section 11.5.2 below.
Under some circumstances, multiple replicas may be used
simultaneously to provide higher-performance access to the file
system in question, although the lack of state sharing between
servers may be an impediment to such use.
When a file system is present but becomes absent, clients can be
given the opportunity to have continued access to their data using a
different replica. In this case, a continued attempt to use the data
in the now-absent file system will result in an NFS4ERR_MOVED error,
and then the successor replica or set of possible replica choices can
be fetched and used to continue access. Transfer of access to the
new replica location is referred to as "migration" and is discussed
in
Section 11.5.4 below.
When a file system is currently absent, specification of file system
location provides a means by which file systems located on one server
can be associated with a namespace defined by another server, thus
allowing a general multi-server namespace facility. A designation of
such a remote instance, in place of a file system not previously
present, is called a "pure referral" and is discussed in
Section 11.5.6 below.
Because client support for attributes related to file system location
is
OPTIONAL, a server may choose to take action to hide migration and
referral events from such clients, by acting as a proxy, for example.
The server can determine the presence of client support from the
arguments of the EXCHANGE_ID operation (see
Section 18.35.3).
11.5.1. Combining Multiple Uses in a Single Attribute
A file system location attribute will sometimes contain information
relating to the location of multiple replicas, which may be used in
different ways:
* File system location entries that relate to the file system
instance currently in use provide trunking information, allowing
the client to find additional network addresses by which the
instance may be accessed.
* File system location entries that provide information about
replicas to which access is to be transferred.
* Other file system location entries that relate to replicas that
are available to use in the event that access to the current
replica becomes unsatisfactory.
In order to simplify client handling and to allow the best choice of
replicas to access, the server should adhere to the following
guidelines:
* All file system location entries that relate to a single file
system instance should be adjacent.
* File system location entries that relate to the instance currently
in use should appear first.
* File system location entries that relate to replica(s) to which
migration is occurring should appear before replicas that are
available for later use if the current replica should become
inaccessible.
11.5.2. File System Location Attributes and Trunking
Trunking is the use of multiple connections between a client and
server in order to increase the speed of data transfer. A client may
determine the set of network addresses to use to access a given file
system in a number of ways:
* When the name of the server is known to the client, it may use DNS
to obtain a set of network addresses to use in accessing the
server.
* The client may fetch the file system location attribute for the
file system. This will provide either the name of the server
(which can be turned into a set of network addresses using DNS) or
a set of server-trunkable location entries. Using the latter
alternative, the server can provide addresses it regards as
desirable to use to access the file system in question. Although
these entries can contain port numbers, these port numbers are not
used in determining trunking relationships. Once the candidate
addresses have been determined and EXCHANGE_ID done to the proper
server, only the value of the so_major_id field returned by the
servers in question determines whether a trunking relationship
actually exists.
When the client fetches a location attribute for a file system, it
should be noted that the client may encounter multiple entries for a
number of reasons, such that when it determines trunking information,
it may need to bypass addresses not trunkable with one already known.
The server can provide location entries that include either names or
network addresses. It might use the latter form because of DNS-
related security concerns or because the set of addresses to be used
might require active management by the server.
Location entries used to discover candidate addresses for use in
trunking are subject to change, as discussed in
Section 11.5.7 below.
The client may respond to such changes by using additional addresses
once they are verified or by ceasing to use existing ones. The
server can force the client to cease using an address by returning
NFS4ERR_MOVED when that address is used to access a file system.
This allows a transfer of client access that is similar to migration,
although the same file system instance is accessed throughout.
11.5.3. File System Location Attributes and Connection Type Selection
Because of the need to support multiple types of connections, clients
face the issue of determining the proper connection type to use when
establishing a connection to a given server network address. In some
cases, this issue can be addressed through the use of the connection
"step-up" facility described in
Section 18.36. However, because
there are cases in which that facility is not available, the client
may have to choose a connection type with no possibility of changing
it within the scope of a single connection.
The two file system location attributes differ as to the information
made available in this regard. The fs_locations attribute provides
no information to support connection type selection. As a result,
clients supporting multiple connection types would need to attempt to
establish connections using multiple connection types until the one
preferred by the client is successfully established.
The fs_locations_info attribute includes the FSLI4TF_RDMA flag, which
is convenient for a client wishing to use RDMA. When this flag is
set, it indicates that RPC-over-RDMA support is available using the
specified location entry. A client can establish a TCP connection
and then convert that connection to use RDMA by using the step-up
facility.
Irrespective of the particular attribute used, when there is no
indication that a step-up operation can be performed, a client
supporting RDMA operation can establish a new RDMA connection, and it
can be bound to the session already established by the TCP
connection, allowing the TCP connection to be dropped and the session
converted to further use in RDMA mode, if the server supports that.
11.5.4. File System Replication
The fs_locations and fs_locations_info attributes provide alternative
file system locations, to be used to access data in place of or in
addition to the current file system instance. On first access to a
file system, the client should obtain the set of alternate locations
by interrogating the fs_locations or fs_locations_info attribute,
with the latter being preferred.
In the event that the occurrence of server failures, communications
problems, or other difficulties make continued access to the current
file system impossible or otherwise impractical, the client can use
the alternate locations as a way to get continued access to its data.
The alternate locations may be physical replicas of the (typically
read-only) file system data supplemented by possible asynchronous
propagation of updates. Alternatively, they may provide for the use
of various forms of server clustering in which multiple servers
provide alternate ways of accessing the same physical file system.
How the difference between replicas affects file system transitions
can be represented within the fs_locations and fs_locations_info
attributes, and how the client deals with file system transition
issues will be discussed in detail in later sections.
Although the location attributes provide some information about the
nature of the inter-replica transition, many aspects of the semantics
of possible asynchronous updates are not currently described by the
protocol, which makes it necessary for clients using replication to
switch among replicas undergoing change to familiarize themselves
with the semantics of the update approach used. Due to this lack of
specificity, many applications may find the use of migration more
appropriate because a server can propagate all updates made before an
established point in time to the new replica as part of the migration
event.
11.5.4.1. File System Trunking Presented as Replication
In some situations, a file system location entry may indicate a file
system access path to be used as an alternate location, where
trunking, rather than replication, is to be used. The situations in
which this is appropriate are limited to those in which both of the
following are true:
* The two file system locations (i.e., the one on which the location
attribute is obtained and the one specified in the file system
location entry) designate the same locations within their
respective single-server namespaces.
* The two server network addresses (i.e., the one being used to
obtain the location attribute and the one specified in the file
system location entry) designate the same server (as indicated by
the same value of the so_major_id field of the eir_server_owner
field returned in response to EXCHANGE_ID).
When these conditions hold, operations using both access paths are
generally trunked, although trunking may be disallowed when the
attribute fs_locations_info is used:
* When the fs_locations_info attribute shows the two entries as not
having the same simultaneous-use class, trunking is inhibited, and
the two access paths cannot be used together.
In this case, the two paths can be used serially with no
transition activity required on the part of the client, and any
transition between access paths is transparent. In transferring
access from one to the other, the client acts as if communication
were interrupted, establishing a new connection and possibly a new
session to continue access to the same file system.
* Note that for two such location entries, any information within
the fs_locations_info attribute that indicates the need for
special transition activity, i.e., the appearance of the two file
system location entries with different handle, fileid, write-
verifier, change, and readdir classes, indicates a serious
problem. The client, if it allows transition to the file system
instance at all, must not treat any transition as a transparent
one. The server
SHOULD NOT indicate that these two entries (for
the same file system on the same server) belong to different
handle, fileid, write-verifier, change, and readdir classes,
whether or not the two entries are shown belonging to the same
simultaneous-use class.
These situations were recognized by [66], even though that document
made no explicit mention of trunking:
* It treated the situation that we describe as trunking as one of
simultaneous use of two distinct file system instances, even
though, in the explanatory framework now used to describe the
situation, the case is one in which a single file system is
accessed by two different trunked addresses.
* It treated the situation in which two paths are to be used
serially as a special sort of "transparent transition". However,
in the descriptive framework now used to categorize transition
situations, this is considered a case of a "network endpoint
transition" (see
Section 11.9).
11.5.5. File System Migration
When a file system is present and becomes inaccessible using the
current access path, the NFSv4.1 protocol provides a means by which
clients can be given the opportunity to have continued access to
their data. This may involve using a different access path to the
existing replica or providing a path to a different replica. The new
access path or the location of the new replica is specified by a file
system location attribute. The ensuing migration of access includes
the ability to retain locks across the transition. Depending on
circumstances, this can involve:
* The continued use of the existing clientid when accessing the
current replica using a new access path.
* Use of lock reclaim, taking advantage of a per-fs grace period.
* Use of Transparent State Migration.
Typically, a client will be accessing the file system in question,
get an NFS4ERR_MOVED error, and then use a file system location
attribute to determine the new access path for the data. When
fs_locations_info is used, additional information will be available
that will define the nature of the client's handling of the
transition to a new server.
In most instances, servers will choose to migrate all clients using a
particular file system to a successor replica at the same time to
avoid cases in which different clients are updating different
replicas. However, migration of an individual client can be helpful
in providing load balancing, as long as the replicas in question are
such that they represent the same data as described in
Section 11.11.8.
* In the case in which there is no transition between replicas
(i.e., only a change in access path), there are no special
difficulties in using of this mechanism to effect load balancing.
* In the case in which the two replicas are sufficiently coordinated
as to allow a single client coherent, simultaneous access to both,
there is, in general, no obstacle to the use of migration of
particular clients to effect load balancing. Generally, such
simultaneous use involves cooperation between servers to ensure
that locks granted on two coordinated replicas cannot conflict and
can remain effective when transferred to a common replica.
* In the case in which a large set of clients is accessing a file
system in a read-only fashion, it can be helpful to migrate all
clients with writable access simultaneously, while using load
balancing on the set of read-only copies, as long as the rules in
Section 11.11.8, which are designed to prevent data reversion, are
followed.
In other cases, the client might not have sufficient guarantees of
data similarity or coherence to function properly (e.g., the data in
the two replicas is similar but not identical), and the possibility
that different clients are updating different replicas can exacerbate
the difficulties, making the use of load balancing in such situations
a perilous enterprise.
The protocol does not specify how the file system will be moved
between servers or how updates to multiple replicas will be
coordinated. It is anticipated that a number of different server-to-
server coordination mechanisms might be used, with the choice left to
the server implementer. The NFSv4.1 protocol specifies the method
used to communicate the migration event between client and server.
In the case of various forms of server clustering, the new location
may be another server providing access to the same physical file
system. The client's responsibilities in dealing with this
transition will depend on whether a switch between replicas has
occurred and the means the server has chosen to provide continuity of
locking state. These issues will be discussed in detail below.
Although a single successor location is typical, multiple locations
may be provided. When multiple locations are provided, the client
will typically use the first one provided. If that is inaccessible
for some reason, later ones can be used. In such cases, the client
might consider the transition to the new replica to be a migration
event, even though some of the servers involved might not be aware of
the use of the server that was inaccessible. In such a case, a
client might lose access to locking state as a result of the access
transfer.
When an alternate location is designated as the target for migration,
it must designate the same data (with metadata being the same to the
degree indicated by the fs_locations_info attribute). Where file
systems are writable, a change made on the original file system must
be visible on all migration targets. Where a file system is not
writable but represents a read-only copy (possibly periodically
updated) of a writable file system, similar requirements apply to the
propagation of updates. Any change visible in the original file
system must already be effected on all migration targets, to avoid
any possibility that a client, in effecting a transition to the
migration target, will see any reversion in file system state.
Referrals allow the server to associate a file system namespace entry
located on one server with a file system located on another server.
When this includes the use of pure referrals, servers are provided a
way of placing a file system in a location within the namespace
essentially without respect to its physical location on a particular
server. This allows a single server or a set of servers to present a
multi-server namespace that encompasses file systems located on a
wider range of servers. Some likely uses of this facility include
establishment of site-wide or organization-wide namespaces, with the
eventual possibility of combining such together into a truly global
namespace, such as the one provided by AFS (the Andrew File System)
[65].
Referrals occur when a client determines, upon first referencing a
position in the current namespace, that it is part of a new file
system and that the file system is absent. When this occurs,
typically upon receiving the error NFS4ERR_MOVED, the actual location
or locations of the file system can be determined by fetching a
locations attribute.
The file system location attribute may designate a single file system
location or multiple file system locations, to be selected based on
the needs of the client. The server, in the fs_locations_info
attribute, may specify priorities to be associated with various file
system location choices. The server may assign different priorities
to different locations as reported to individual clients, in order to
adapt to client physical location or to effect load balancing. When
both read-only and read-write file systems are present, some of the
read-only locations might not be absolutely up-to-date (as they would
have to be in the case of replication and migration). Servers may
also specify file system locations that include client-substituted
variables so that different clients are referred to different file
systems (with different data contents) based on client attributes
such as CPU architecture.
If the fs_locations_info attribute lists multiple possible targets,
the relationships among them may be important to the client in
selecting which one to use. The same rules specified in
Section 11.5.5 below regarding multiple migration targets apply to
these multiple replicas as well. For example, the client might
prefer a writable target on a server that has additional writable
replicas to which it subsequently might switch. Note that, as
distinguished from the case of replication, there is no need to deal
with the case of propagation of updates made by the current client,
since the current client has not accessed the file system in
question.
Use of multi-server namespaces is enabled by NFSv4.1 but is not
required. The use of multi-server namespaces and their scope will
depend on the applications used and system administration
preferences.
Multi-server namespaces can be established by a single server
providing a large set of pure referrals to all of the included file
systems. Alternatively, a single multi-server namespace may be
administratively segmented with separate referral file systems (on
separate servers) for each separately administered portion of the
namespace. The top-level referral file system or any segment may use
replicated referral file systems for higher availability.
Generally, multi-server namespaces are for the most part uniform, in
that the same data made available to one client at a given location
in the namespace is made available to all clients at that namespace
location. However, there are facilities provided that allow
different clients to be directed to different sets of data, for
reasons such as enabling adaptation to such client characteristics as
CPU architecture. These facilities are described in
Section 11.17.3.
Note that it is possible, when providing a uniform namespace, to
provide different location entries to different clients in order to
provide each client with a copy of the data physically closest to it
or otherwise optimize access (e.g., provide load balancing).
11.5.7. Changes in a File System Location Attribute
Although clients will typically fetch a file system location
attribute when first accessing a file system and when NFS4ERR_MOVED
is returned, a client can choose to fetch the attribute periodically,
in which case, the value fetched may change over time.
For clients not prepared to access multiple replicas simultaneously
(see
Section 11.11.1), the handling of the various cases of location
change are as follows:
* Changes in the list of replicas or in the network addresses
associated with replicas do not require immediate action. The
client will typically update its list of replicas to reflect the
new information.
* Additions to the list of network addresses for the current file
system instance need not be acted on promptly. However, to
prepare for a subsequent migration event, the client can choose to
take note of the new address and then use it whenever it needs to
switch access to a new replica.
* Deletions from the list of network addresses for the current file
system instance do not require the client to immediately cease use
of existing access paths, although new connections are not to be
established on addresses that have been deleted. However, clients
can choose to act on such deletions by preparing for an eventual
shift in access, which becomes unavoidable as soon as the server
returns NFS4ERR_MOVED to indicate that a particular network access
path is not usable to access the current file system.
For clients that are prepared to access several replicas
simultaneously, the following additional cases need to be addressed.
As in the cases discussed above, changes in the set of replicas need
not be acted upon promptly, although the client has the option of
adjusting its access even in the absence of difficulties that would
lead to the selection of a new replica.
* When a new replica is added, which may be accessed simultaneously
with one currently in use, the client is free to use the new
replica immediately.
* When a replica currently in use is deleted from the list, the
client need not cease using it immediately. However, since the
server may subsequently force such use to cease (by returning
NFS4ERR_MOVED), clients might decide to limit the need for later
state transfer. For example, new opens might be done on other
replicas, rather than on one not present in the list.
11.6. Trunking without File System Location Information
In situations in which a file system is accessed using two server-
trunkable addresses (as indicated by the same value of the
so_major_id field of the eir_server_owner field returned in response
to EXCHANGE_ID), trunked access is allowed even though there might
not be any location entries specifically indicating the use of
trunking for that file system.
This situation was recognized by [66], although that document made no
explicit mention of trunking and treated the situation as one of
simultaneous use of two distinct file system instances. In the
explanatory framework now used to describe the situation, the case is
one in which a single file system is accessed by two different
trunked addresses.
11.7. Users and Groups in a Multi-Server Namespace
As in the case of a single-server environment (see
Section 5.9), when
an owner or group name of the form "id@domain" is assigned to a file,
there is an implicit promise to return that same string when the
corresponding attribute is interrogated subsequently. In the case of
a multi-server namespace, that same promise applies even if server
boundaries have been crossed. Similarly, when the owner attribute of
a file is derived from the security principal that created the file,
that attribute should have the same value even if the interrogation
occurs on a different server from the file creation.
Similarly, the set of security principals recognized by all the
participating servers needs to be the same, with each such principal
having the same credentials, regardless of the particular server
being accessed.
In order to meet these requirements, those setting up multi-server
namespaces will need to limit the servers included so that:
* In all cases in which more than a single domain is supported, the
requirements stated in
RFC 8000 [31] are to be respected.
* All servers support a common set of domains that includes all of
the domains clients use and expect to see returned as the domain
portion of an owner or group in the form "id@domain". Note that,
although this set most often consists of a single domain, it is
possible for multiple domains to be supported.
* All servers, for each domain that they support, accept the same
set of user and group ids as valid.
* All servers recognize the same set of security principals. For
each principal, the same credential is required, independent of
the server being accessed. In addition, the group membership for
each such principal is to be the same, independent of the server
accessed.
Note that there is no requirement in general that the users
corresponding to particular security principals have the same local
representation on each server, even though it is most often the case
that this is so.
When AUTH_SYS is used, the following additional requirements must be
met:
* Only a single NFSv4 domain can be supported through the use of
AUTH_SYS.
* The "local" representation of all owners and groups must be the
same on all servers. The word "local" is used here since that is
the way that numeric user and group ids are described in
Section 5.9. However, when AUTH_SYS or stringified numeric owners
or groups are used, these identifiers are not truly local, since
they are known to the clients as well as to the server.
Similarly, when stringified numeric user and group ids are used, the
"local" representation of all owners and groups must be the same on
all servers, even when AUTH_SYS is not used.
11.8. Additional Client-Side Considerations
When clients make use of servers that implement referrals,
replication, and migration, care should be taken that a user who
mounts a given file system that includes a referral or a relocated
file system continues to see a coherent picture of that user-side
file system despite the fact that it contains a number of server-side
file systems that may be on different servers.
One important issue is upward navigation from the root of a server-
side file system to its parent (specified as ".." in UNIX), in the
case in which it transitions to that file system as a result of
referral, migration, or a transition as a result of replication.
When the client is at such a point, and it needs to ascend to the
parent, it must go back to the parent as seen within the multi-server
namespace rather than sending a LOOKUPP operation to the server,
which would result in the parent within that server's single-server
namespace. In order to do this, the client needs to remember the
filehandles that represent such file system roots and use these
instead of sending a LOOKUPP operation to the current server. This
will allow the client to present to applications a consistent
namespace, where upward navigation and downward navigation are
consistent.
Another issue concerns refresh of referral locations. When referrals
are used extensively, they may change as server configurations
change. It is expected that clients will cache information related
to traversing referrals so that future client-side requests are
resolved locally without server communication. This is usually
rooted in client-side name look up caching. Clients should
periodically purge this data for referral points in order to detect
changes in location information. When the change_policy attribute
changes for directories that hold referral entries or for the
referral entries themselves, clients should consider any associated
cached referral information to be out of date.
11.9. Overview of File Access Transitions
File access transitions are of two types:
* Those that involve a transition from accessing the current replica
to another one in connection with either replication or migration.
How these are dealt with is discussed in
Section 11.11.
* Those in which access to the current file system instance is
retained, while the network path used to access that instance is
changed. This case is discussed in
Section 11.10.
11.10. Effecting Network Endpoint Transitions
The endpoints used to access a particular file system instance may
change in a number of ways, as listed below. In each of these cases,
the same fsid, client IDs, filehandles, and stateids are used to
continue access, with a continuity of lock state. In many cases, the
same sessions can also be used.
The appropriate action depends on the set of replacement addresses
that are available for use (i.e., server endpoints that are server-
trunkable with one previously being used).
* When use of a particular address is to cease, and there is also
another address currently in use that is server-trunkable with it,
requests that would have been issued on the address whose use is
to be discontinued can be issued on the remaining address(es).
When an address is server-trunkable but not session-trunkable with
the address whose use is to be discontinued, the request might
need to be modified to reflect the fact that a different session
will be used.
* When use of a particular connection is to cease, as indicated by
receiving NFS4ERR_MOVED when using that connection, but that
address is still indicated as accessible according to the
appropriate file system location entries, it is likely that
requests can be issued on a new connection of a different
connection type once that connection is established. Since any
two non-port-specific server endpoints that share a network
address are inherently session-trunkable, the client can use
BIND_CONN_TO_SESSION to access the existing session with the new
connection.
* When there are no potential replacement addresses in use, but
there are valid addresses session-trunkable with the one whose use
is to be discontinued, the client can use BIND_CONN_TO_SESSION to
access the existing session using the new address. Although the
target session will generally be accessible, there may be rare
situations in which that session is no longer accessible when an
attempt is made to bind the new connection to it. In this case,
the client can create a new session to enable continued access to
the existing instance using the new connection, providing for the
use of existing filehandles, stateids, and client ids while
supplying continuity of locking state.
* When there is no potential replacement address in use, and there
are no valid addresses session-trunkable with the one whose use is
to be discontinued, other server-trunkable addresses may be used
to provide continued access. Although the use of CREATE_SESSION
is available to provide continued access to the existing instance,
servers have the option of providing continued access to the
existing session through the new network access path in a fashion
similar to that provided by session migration (see
Section 11.12).
To take advantage of this possibility, clients can perform an
initial BIND_CONN_TO_SESSION, as in the previous case, and use
CREATE_SESSION only if that fails.
11.11. Effecting File System Transitions
There are a range of situations in which there is a change to be
effected in the set of replicas used to access a particular file
system. Some of these may involve an expansion or contraction of the
set of replicas used as discussed in
Section 11.11.1 below.
For reasons explained in that section, most transitions will involve
a transition from a single replica to a corresponding replacement
replica. When effecting replica transition, some types of sharing
between the replicas may affect handling of the transition as
described in Sections
11.11.2 through
11.11.8 below. The attribute
fs_locations_info provides helpful information to allow the client to
determine the degree of inter-replica sharing.
With regard to some types of state, the degree of continuity across
the transition depends on the occasion prompting the transition, with
transitions initiated by the servers (i.e., migration) offering much
more scope for a nondisruptive transition than cases in which the
client on its own shifts its access to another replica (i.e.,
replication). This issue potentially applies to locking state and to
session state, which are dealt with below as follows:
* An introduction to the possible means of providing continuity in
these areas appears in
Section 11.11.9 below.
* Transparent State Migration is introduced in
Section 11.12. The
possible transfer of session state is addressed there as well.
* The client handling of transitions, including determining how to
deal with the various means that the server might take to supply
effective continuity of locking state, is discussed in
Section 11.13.
* The source and destination servers' responsibilities in effecting
Transparent State Migration of locking and session state are
discussed in
Section 11.14.
11.11.1. File System Transitions and Simultaneous Access
The fs_locations_info attribute (described in
Section 11.17) may
indicate that two replicas may be used simultaneously, although some
situations in which such simultaneous access is permitted are more
appropriately described as instances of trunking (see
Section 11.5.4.1). Although situations in which multiple replicas
may be accessed simultaneously are somewhat similar to those in which
a single replica is accessed by multiple network addresses, there are
important differences since locking state is not shared among
multiple replicas.
Because of this difference in state handling, many clients will not
have the ability to take advantage of the fact that such replicas
represent the same data. Such clients will not be prepared to use
multiple replicas simultaneously but will access each file system
using only a single replica, although the replica selected might make
multiple server-trunkable addresses available.
Clients who are prepared to use multiple replicas simultaneously can
divide opens among replicas however they choose. Once that choice is
made, any subsequent transitions will treat the set of locking state
associated with each replica as a single entity.
For example, if one of the replicas become unavailable, access will
be transferred to a different replica, which is also capable of
simultaneous access with the one still in use.
When there is no such replica, the transition may be to the replica
already in use. At this point, the client has a choice between
merging the locking state for the two replicas under the aegis of the
sole replica in use or treating these separately until another
replica capable of simultaneous access presents itself.
11.11.2. Filehandles and File System Transitions
There are a number of ways in which filehandles can be handled across
a file system transition. These can be divided into two broad
classes depending upon whether the two file systems across which the
transition happens share sufficient state to effect some sort of
continuity of file system handling.
When there is no such cooperation in filehandle assignment, the two
file systems are reported as being in different handle classes. In
this case, all filehandles are assumed to expire as part of the file
system transition. Note that this behavior does not depend on the
fh_expire_type attribute and supersedes the specification of the
FH4_VOL_MIGRATION bit, which only affects behavior when
fs_locations_info is not available.
When there is cooperation in filehandle assignment, the two file
systems are reported as being in the same handle classes. In this
case, persistent filehandles remain valid after the file system
transition, while volatile filehandles (excluding those that are only
volatile due to the FH4_VOL_MIGRATION bit) are subject to expiration
on the target server.
11.11.3. Fileids and File System Transitions
In NFSv4.0, the issue of continuity of fileids in the event of a file
system transition was not addressed. The general expectation had
been that in situations in which the two file system instances are
created by a single vendor using some sort of file system image copy,
fileids would be consistent across the transition, while in the
analogous multi-vendor transitions they would not. This poses
difficulties, especially for the client without special knowledge of
the transition mechanisms adopted by the server. Note that although
fileid is not a
REQUIRED attribute, many servers support fileids and
many clients provide APIs that depend on fileids.
It is important to note that while clients themselves may have no
trouble with a fileid changing as a result of a file system
transition event, applications do typically have access to the fileid
(e.g., via stat). The result is that an application may work
perfectly well if there is no file system instance transition or if
any such transition is among instances created by a single vendor,
yet be unable to deal with the situation in which a multi-vendor
transition occurs at the wrong time.
Providing the same fileids in a multi-vendor (multiple server
vendors) environment has generally been held to be quite difficult.
While there is work to be done, it needs to be pointed out that this
difficulty is partly self-imposed. Servers have typically identified
fileid with inode number, i.e. with a quantity used to find the file
in question. This identification poses special difficulties for
migration of a file system between vendors where assigning the same
index to a given file may not be possible. Note here that a fileid
is not required to be useful to find the file in question, only that
it is unique within the given file system. Servers prepared to
accept a fileid as a single piece of metadata and store it apart from
the value used to index the file information can relatively easily
maintain a fileid value across a migration event, allowing a truly
transparent migration event.
In any case, where servers can provide continuity of fileids, they
should, and the client should be able to find out that such
continuity is available and take appropriate action. Information
about the continuity (or lack thereof) of fileids across a file
system transition is represented by specifying whether the file
systems in question are of the same fileid class.
Note that when consistent fileids do not exist across a transition
(either because there is no continuity of fileids or because fileid
is not a supported attribute on one of instances involved), and there
are no reliable filehandles across a transition event (either because
there is no filehandle continuity or because the filehandles are
volatile), the client is in a position where it cannot verify that
files it was accessing before the transition are the same objects.
It is forced to assume that no object has been renamed, and, unless
there are guarantees that provide this (e.g., the file system is
read-only), problems for applications may occur. Therefore, use of
such configurations should be limited to situations where the
problems that this may cause can be tolerated.
11.11.4. Fsids and File System Transitions
Since fsids are generally only unique on a per-server basis, it is
likely that they will change during a file system transition.
Clients should not make the fsids received from the server visible to
applications since they may not be globally unique, and because they
may change during a file system transition event. Applications are
best served if they are isolated from such transitions to the extent
possible.
Although normally a single source file system will transition to a
single target file system, there is a provision for splitting a
single source file system into multiple target file systems, by
specifying the FSLI4F_MULTI_FS flag.
11.11.4.1. File System Splitting
When a file system transition is made and the fs_locations_info
indicates that the file system in question might be split into
multiple file systems (via the FSLI4F_MULTI_FS flag), the client
SHOULD do GETATTRs to determine the fsid attribute on all known
objects within the file system undergoing transition to determine the
new file system boundaries.
Clients might choose to maintain the fsids passed to existing
applications by mapping all of the fsids for the descendant file
systems to the common fsid used for the original file system.
Splitting a file system can be done on a transition between file
systems of the same fileid class, since the fact that fileids are
unique within the source file system ensure they will be unique in
each of the target file systems.
11.11.5. The Change Attribute and File System Transitions
Since the change attribute is defined as a server-specific one,
change attributes fetched from one server are normally presumed to be
invalid on another server. Such a presumption is troublesome since
it would invalidate all cached change attributes, requiring
refetching. Even more disruptive, the absence of any assured
continuity for the change attribute means that even if the same value
is retrieved on refetch, no conclusions can be drawn as to whether
the object in question has changed. The identical change attribute
could be merely an artifact of a modified file with a different
change attribute construction algorithm, with that new algorithm just
happening to result in an identical change value.
When the two file systems have consistent change attribute formats,
and this fact is communicated to the client by reporting in the same
change class, the client may assume a continuity of change attribute
construction and handle this situation just as it would be handled
without any file system transition.
11.11.6. Write Verifiers and File System Transitions
In a file system transition, the two file systems might be
cooperating in the handling of unstably written data. Clients can
determine if this is the case by seeing if the two file systems
belong to the same write-verifier class. When this is the case,
write verifiers returned from one system may be compared to those
returned by the other and superfluous writes can be avoided.
When two file systems belong to different write-verifier classes, any
verifier generated by one must not be compared to one provided by the
other. Instead, the two verifiers should be treated as not equal
even when the values are identical.
11.11.7. READDIR Cookies and Verifiers and File System Transitions
In a file system transition, the two file systems might be consistent
in their handling of READDIR cookies and verifiers. Clients can
determine if this is the case by seeing if the two file systems
belong to the same readdir class. When this is the case, readdir
class, READDIR cookies, and verifiers from one system will be
recognized by the other, and READDIR operations started on one server
can be validly continued on the other simply by presenting the cookie
and verifier returned by a READDIR operation done on the first file
system to the second.
When two file systems belong to different readdir classes, any
READDIR cookie and verifier generated by one is not valid on the
second and must not be presented to that server by the client. The
client should act as if the verifier were rejected.
11.11.8. File System Data and File System Transitions
When multiple replicas exist and are used simultaneously or in
succession by a client, applications using them will normally expect
that they contain either the same data or data that is consistent
with the normal sorts of changes that are made by other clients
updating the data of the file system (with metadata being the same to
the degree indicated by the fs_locations_info attribute). However,
when multiple file systems are presented as replicas of one another,
the precise relationship between the data of one and the data of
another is not, as a general matter, specified by the NFSv4.1
protocol. It is quite possible to present as replicas file systems
where the data of those file systems is sufficiently different that
some applications have problems dealing with the transition between
replicas. The namespace will typically be constructed so that
applications can choose an appropriate level of support, so that in
one position in the namespace, a varied set of replicas might be
listed, while in another, only those that are up-to-date would be
considered replicas. The protocol does define three special cases of
the relationship among replicas to be specified by the server and
relied upon by clients:
* When multiple replicas exist and are used simultaneously by a
client (see the FSLIB4_CLSIMUL definition within
fs_locations_info), they must designate the same data. Where file
systems are writable, a change made on one instance must be
visible on all instances at the same time, regardless of whether
the interrogated instance is the one on which the modification was
done. This allows a client to use these replicas simultaneously
without any special adaptation to the fact that there are multiple
replicas, beyond adapting to the fact that locks obtained on one
replica are maintained separately (i.e., under a different client
ID). In this case, locks (whether share reservations or byte-
range locks) and delegations obtained on one replica are
immediately reflected on all replicas, in the sense that access
from all other servers is prevented regardless of the replica
used. However, because the servers are not required to treat two
associated client IDs as representing the same client, it is best
to access each file using only a single client ID.
* When one replica is designated as the successor instance to
another existing instance after the return of NFS4ERR_MOVED (i.e.,
the case of migration), the client may depend on the fact that all
changes written to stable storage on the original instance are
written to stable storage of the successor (uncommitted writes are
dealt with in
Section 11.11.6 above).
* Where a file system is not writable but represents a read-only
copy (possibly periodically updated) of a writable file system,
clients have similar requirements with regard to the propagation
of updates. They may need a guarantee that any change visible on
the original file system instance must be immediately visible on
any replica before the client transitions access to that replica,
in order to avoid any possibility that a client, in effecting a
transition to a replica, will see any reversion in file system
state. The specific means of this guarantee varies based on the
value of the fss_type field that is reported as part of the
fs_status attribute (see
Section 11.18). Since these file systems
are presumed to be unsuitable for simultaneous use, there is no
specification of how locking is handled; in general, locks
obtained on one file system will be separate from those on others.
Since these are expected to be read-only file systems, this is not
likely to pose an issue for clients or applications.
When none of these special situations applies, there is no basis
within the protocol for the client to make assumptions about the
contents of a replica file system or its relationship to previous
file system instances. Thus, switching between nominally identical
read-write file systems would not be possible because either the
client does not use the fs_locations_info attribute, or the server
does not support it.
11.11.9. Lock State and File System Transitions
While accessing a file system, clients obtain locks enforced by the
server, which may prevent actions by other clients that are
inconsistent with those locks.
When access is transferred between replicas, clients need to be
assured that the actions disallowed by holding these locks cannot
have occurred during the transition. This can be ensured by the
methods below. Unless at least one of these is implemented, clients
will not be assured of continuity of lock possession across a
migration event:
* Providing the client an opportunity to re-obtain his locks via a
per-fs grace period on the destination server, denying all clients
using the destination file system the opportunity to obtain new
locks that conflict with those held by the transferred client as
long as that client has not completed its per-fs grace period.
Because the lock reclaim mechanism was originally defined to
support server reboot, it implicitly assumes that filehandles
will, upon reclaim, be the same as those at open. In the case of
migration, this requires that source and destination servers use
the same filehandles, as evidenced by using the same server scope
(see
Section 2.10.4) or by showing this agreement using
fs_locations_info (see
Section 11.11.2 above).
Note that such a grace period can be implemented without
interfering with the ability of non-transferred clients to obtain
new locks while it is going on. As long as the destination server
is aware of the transferred locks, it can distinguish requests to
obtain new locks that contrast with existing locks from those that
do not, allowing it to treat such client requests without
reference to the ongoing grace period.
* Locking state can be transferred as part of the transition by
providing Transparent State Migration as described in
Section 11.12.
Of these, Transparent State Migration provides the smoother
experience for clients in that there is no need to go through a
reclaim process before new locks can be obtained; however, it
requires a greater degree of inter-server coordination. In general,
the servers taking part in migration are free to provide either
facility. However, when the filehandles can differ across the
migration event, Transparent State Migration is the only available
means of providing the needed functionality.
It should be noted that these two methods are not mutually exclusive
and that a server might well provide both. In particular, if there
is some circumstance preventing a specific lock from being
transferred transparently, the destination server can allow it to be
reclaimed by implementing a per-fs grace period for the migrated file
system.
11.11.9.1. Security Consideration Related to Reclaiming Lock State
after File System Transitions
Although it is possible for a client reclaiming state to misrepresent
its state in the same fashion as described in
Section 8.4.2.1.1, most
implementations providing for such reclamation in the case of file
system transitions will have the ability to detect such
misrepresentations. This limits the ability of unauthenticated
clients to execute denial-of-service attacks in these circumstances.
Nevertheless, the rules stated in
Section 8.4.2.1.1 regarding
principal verification for reclaim requests apply in this situation
as well.
Typically, implementations that support file system transitions will
have extensive information about the locks to be transferred. This
is because of the following:
* Since failure is not involved, there is no need to store locking
information in persistent storage.
* There is no need, as there is in the failure case, to update
multiple repositories containing locking state to keep them in
sync. Instead, there is a one-time communication of locking state
from the source to the destination server.
* Providing this information avoids potential interference with
existing clients using the destination file system by denying them
the ability to obtain new locks during the grace period.
When such detailed locking information, not necessarily including the
associated stateids, is available:
* It is possible to detect reclaim requests that attempt to reclaim
locks that did not exist before the transfer, rejecting them with
NFS4ERR_RECLAIM_BAD (
Section 15.1.9.4).
* It is possible when dealing with non-reclaim requests, to
determine whether they conflict with existing locks, eliminating
the need to return NFS4ERR_GRACE (
Section 15.1.9.2) on non-reclaim
requests.
It is possible for implementations of grace periods in connection
with file system transitions not to have detailed locking information
available at the destination server, in which case, the security
situation is exactly as described in
Section 8.4.2.1.1.
11.11.9.2. Leases and File System Transitions
In the case of lease renewal, the client may not be submitting
requests for a file system that has been transferred to another
server. This can occur because of the lease renewal mechanism. The
client renews the lease associated with all file systems when
submitting a request on an associated session, regardless of the
specific file system being referenced.
In order for the client to schedule renewal of its lease where there
is locking state that may have been relocated to the new server, the
client must find out about lease relocation before that lease expire.
To accomplish this, the SEQUENCE operation will return the status bit
SEQ4_STATUS_LEASE_MOVED if responsibility for any of the renewed
locking state has been transferred to a new server. This will
continue until the client receives an NFS4ERR_MOVED error for each of
the file systems for which there has been locking state relocation.
When a client receives an SEQ4_STATUS_LEASE_MOVED indication from a
server, for each file system of the server for which the client has
locking state, the client should perform an operation. For
simplicity, the client may choose to reference all file systems, but
what is important is that it must reference all file systems for
which there was locking state where that state has moved. Once the
client receives an NFS4ERR_MOVED error for each such file system, the
server will clear the SEQ4_STATUS_LEASE_MOVED indication. The client
can terminate the process of checking file systems once this
indication is cleared (but only if the client has received a reply
for all outstanding SEQUENCE requests on all sessions it has with the
server), since there are no others for which locking state has moved.
A client may use GETATTR of the fs_status (or fs_locations_info)
attribute on all of the file systems to get absence indications in a
single (or a few) request(s), since absent file systems will not
cause an error in this context. However, it still must do an
operation that receives NFS4ERR_MOVED on each file system, in order
to clear the SEQ4_STATUS_LEASE_MOVED indication.
Once the set of file systems with transferred locking state has been
determined, the client can follow the normal process to obtain the
new server information (through the fs_locations and
fs_locations_info attributes) and perform renewal of that lease on
the new server, unless information in the fs_locations_info attribute
shows that no state could have been transferred. If the server has
not had state transferred to it transparently, the client will
receive NFS4ERR_STALE_CLIENTID from the new server, as described
above, and the client can then reclaim locks as is done in the event
of server failure.
11.11.9.3. Transitions and the Lease_time Attribute
In order that the client may appropriately manage its lease in the
case of a file system transition, the destination server must
establish proper values for the lease_time attribute.
When state is transferred transparently, that state should include
the correct value of the lease_time attribute. The lease_time
attribute on the destination server must never be less than that on
the source, since this would result in premature expiration of a
lease granted by the source server. Upon transitions in which state
is transferred transparently, the client is under no obligation to
refetch the lease_time attribute and may continue to use the value
previously fetched (on the source server).
If state has not been transferred transparently, either because the
associated servers are shown as having different eir_server_scope
strings or because the client ID is rejected when presented to the
new server, the client should fetch the value of lease_time on the
new (i.e., destination) server, and use it for subsequent locking
requests. However, the server must respect a grace period of at
least as long as the lease_time on the source server, in order to
ensure that clients have ample time to reclaim their lock before
potentially conflicting non-reclaimed locks are granted.
11.12. Transferring State upon Migration
When the transition is a result of a server-initiated decision to
transition access, and the source and destination servers have
implemented appropriate cooperation, it is possible to do the
following:
* Transfer locking state from the source to the destination server
in a fashion similar to that provided by Transparent State
Migration in NFSv4.0, as described in [69]. Server
responsibilities are described in
Section 11.14.2.
* Transfer session state from the source to the destination server.
Server responsibilities in effecting such a transfer are described
in
Section 11.14.3.
The means by which the client determines which of these transfer
events has occurred are described in
Section 11.13.
11.12.1. Transparent State Migration and pNFS
When pNFS is involved, the protocol is capable of supporting:
* Migration of the Metadata Server (MDS), leaving the Data Servers
(DSs) in place.
* Migration of the file system as a whole, including the MDS and
associated DSs.
* Replacement of one DS by another.
* Migration of a pNFS file system to one in which pNFS is not used.
* Migration of a file system not using pNFS to one in which layouts
are available.
Note that migration, per se, is only involved in the transfer of the
MDS function. Although the servicing of a layout may be transferred
from one data server to another, this not done using the file system
location attributes. The MDS can effect such transfers by recalling
or revoking existing layouts and granting new ones on a different
data server.
Migration of the MDS function is directly supported by Transparent
State Migration. Layout state will normally be transparently
transferred, just as other state is. As a result, Transparent State
Migration provides a framework in which, given appropriate inter-MDS
data transfer, one MDS can be substituted for another.
Migration of the file system function as a whole can be accomplished
by recalling all layouts as part of the initial phase of the
migration process. As a result, I/O will be done through the MDS
during the migration process, and new layouts can be granted once the
client is interacting with the new MDS. An MDS can also effect this
sort of transition by revoking all layouts as part of Transparent
State Migration, as long as the client is notified about the loss of
locking state.
In order to allow migration to a file system on which pNFS is not
supported, clients need to be prepared for a situation in which
layouts are not available or supported on the destination file system
and so direct I/O requests to the destination server, rather than
depending on layouts being available.
Replacement of one DS by another is not addressed by migration as
such but can be effected by an MDS recalling layouts for the DS to be
replaced and issuing new ones to be served by the successor DS.
Migration may transfer a file system from a server that does not
support pNFS to one that does. In order to properly adapt to this
situation, clients that support pNFS, but function adequately in its
absence, should check for pNFS support when a file system is migrated
and be prepared to use pNFS when support is available on the
destination.
11.13. Client Responsibilities When Access Is Transitioned
For a client to respond to an access transition, it must become aware
of it. The ways in which this can happen are discussed in
Section 11.13.1, which discusses indications that a specific file
system access path has transitioned as well as situations in which
additional activity is necessary to determine the set of file systems
that have been migrated.
Section 11.13.2 goes on to complete the
discussion of how the set of migrated file systems might be
determined. Sections
11.13.3 through
11.13.5 discuss how the client
should deal with each transition it becomes aware of, either directly
or as a result of migration discovery.
The following terms are used to describe client activities:
* "Transition recovery" refers to the process of restoring access to
a file system on which NFS4ERR_MOVED was received.
* "Migration recovery" refers to that subset of transition recovery
that applies when the file system has migrated to a different
replica.
* "Migration discovery" refers to the process of determining which
file system(s) have been migrated. It is necessary to avoid a
situation in which leases could expire when a file system is not
accessed for a long period of time, since a client unaware of the
migration might be referencing an unmigrated file system and not
renewing the lease associated with the migrated file system.
11.13.1. Client Transition Notifications
When there is a change in the network access path that a client is to
use to access a file system, there are a number of related status
indications with which clients need to deal:
* If an attempt is made to use or return a filehandle within a file
system that is no longer accessible at the address previously used
to access it, the error NFS4ERR_MOVED is returned.
Exceptions are made to allow such filehandles to be used when
interrogating a file system location attribute. This enables a
client to determine a new replica's location or a new network
access path.
This condition continues on subsequent attempts to access the file
system in question. The only way the client can avoid the error
is to cease accessing the file system in question at its old
server location and access it instead using a different address at
which it is now available.
* Whenever a client sends a SEQUENCE operation to a server that
generated state held on that client and associated with a file
system no longer accessible on that server, the response will
contain the status bit SEQ4_STATUS_LEASE_MOVED, indicating that
there has been a lease migration.
This condition continues until the client acknowledges the
notification by fetching a file system location attribute for the
file system whose network access path is being changed. When
there are multiple such file systems, a location attribute for
each such file system needs to be fetched. The location attribute
for all migrated file systems needs to be fetched in order to
clear the condition. Even after the condition is cleared, the
client needs to respond by using the location information to
access the file system at its new location to ensure that leases
are not needlessly expired.
Unlike NFSv4.0, in which the corresponding conditions are both errors
and thus mutually exclusive, in NFSv4.1 the client can, and often
will, receive both indications on the same request. As a result,
implementations need to address the question of how to coordinate the
necessary recovery actions when both indications arrive in the
response to the same request. It should be noted that when
processing an NFSv4 COMPOUND, the server will normally decide whether
SEQ4_STATUS_LEASE_MOVED is to be set before it determines which file
system will be referenced or whether NFS4ERR_MOVED is to be returned.
Since these indications are not mutually exclusive in NFSv4.1, the
following combinations are possible results when a COMPOUND is
issued:
* The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED
is asserted.
In this case, transition recovery is required. While it is
possible that migration discovery is needed in addition, it is
likely that only the accessed file system has transitioned. In
any case, because addressing NFS4ERR_MOVED is necessary to allow
the rejected requests to be processed on the target, dealing with
it will typically have priority over migration discovery.
* The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED
is clear.
In this case, transition recovery is also required. It is clear
that migration discovery is not needed to find file systems that
have been migrated other than the one returning NFS4ERR_MOVED.
Cases in which this result can arise include a referral or a
migration for which there is no associated locking state. This
can also arise in cases in which an access path transition other
than migration occurs within the same server. In such a case,
there is no need to set SEQ4_STATUS_LEASE_MOVED, since the lease
remains associated with the current server even though the access
path has changed.
* The COMPOUND status is not NFS4ERR_MOVED, and
SEQ4_STATUS_LEASE_MOVED is asserted.
In this case, no transition recovery activity is required on the
file system(s) accessed by the request. However, to prevent
avoidable lease expiration, migration discovery needs to be done.
* The COMPOUND status is not NFS4ERR_MOVED, and
SEQ4_STATUS_LEASE_MOVED is clear.
In this case, neither transition-related activity nor migration
discovery is required.
Note that the specified actions only need to be taken if they are not
already going on. For example, when NFS4ERR_MOVED is received while
accessing a file system for which transition recovery is already
occurring, the client merely waits for that recovery to be completed,
while the receipt of the SEQ4_STATUS_LEASE_MOVED indication only
needs to initiate migration discovery for a server if such discovery
is not already underway for that server.
The fact that a lease-migrated condition does not result in an error
in NFSv4.1 has a number of important consequences. In addition to
the fact that the two indications are not mutually exclusive, as
discussed above, there are number of issues that are important in
considering implementation of migration discovery, as discussed in
Section 11.13.2.
Because SEQ4_STATUS_LEASE_MOVED is not an error condition, it is
possible for file systems whose access paths have not changed to be
successfully accessed on a given server even though recovery is
necessary for other file systems on the same server. As a result,
access can take place while:
* The migration discovery process is happening for that server.
* The transition recovery process is happening for other file
systems connected to that server.
11.13.2. Performing Migration Discovery
Migration discovery can be performed in the same context as
transition recovery, allowing recovery for each migrated file system
to be invoked as it is discovered. Alternatively, it may be done in
a separate migration discovery thread, allowing migration discovery
to be done in parallel with one or more instances of transition
recovery.
In either case, because the lease-migrated indication does not result
in an error, other access to file systems on the server can proceed
normally, with the possibility that further such indications will be
received, raising the issue of how such indications are to be dealt
with. In general:
* No action needs to be taken for such indications received by any
threads performing migration discovery, since continuation of that
work will address the issue.
* In other cases in which migration discovery is currently being
performed, nothing further needs to be done to respond to such
lease migration indications, as long as one can be certain that
the migration discovery process would deal with those indications.
See below for details.
* For such indications received in all other contexts, the
appropriate response is to initiate or otherwise provide for the
execution of migration discovery for file systems associated with
the server IP address returning the indication.
This leaves a potential difficulty in situations in which the
migration discovery process is near to completion but is still
operating. One should not ignore a SEQ4_STATUS_LEASE_MOVED
indication if the migration discovery process is not able to respond
to the discovery of additional migrating file systems without
additional aid. A further complexity relevant in addressing such
situations is that a lease-migrated indication may reflect the
server's state at the time the SEQUENCE operation was processed,
which may be different from that in effect at the time the response
is received. Because new migration events may occur at any time, and
because a SEQ4_STATUS_LEASE_MOVED indication may reflect the
situation in effect a considerable time before the indication is
received, special care needs to be taken to ensure that
SEQ4_STATUS_LEASE_MOVED indications are not inappropriately ignored.
A useful approach to this issue involves the use of separate
externally-visible migration discovery states for each server.
Separate values could represent the various possible states for the
migration discovery process for a server:
* Non-operation, in which migration discovery is not being
performed.
* Normal operation, in which there is an ongoing scan for migrated
file systems.
* Completion/verification of migration discovery processing, in
which the possible completion of migration discovery processing
needs to be verified.
Given that framework, migration discovery processing would proceed as
follows:
* While in the normal-operation state, the thread performing
discovery would fetch, for successive file systems known to the
client on the server being worked on, a file system location
attribute plus the fs_status attribute.
* If the fs_status attribute indicates that the file system is a
migrated one (i.e., fss_absent is true, and fss_type !=
STATUS4_REFERRAL), then a migrated file system has been found. In
this situation, it is likely that the fetch of the file system
location attribute has cleared one of the file systems
contributing to the lease-migrated indication.
* In cases in which that happened, the thread cannot know whether
the lease-migrated indication has been cleared, and so it enters
the completion/verification state and proceeds to issue a COMPOUND
to see if the SEQ4_STATUS_LEASE_MOVED indication has been cleared.
* When the discovery process is in the completion/verification
state, if other requests get a lease-migrated indication, they
note that it was received. Later, the existence of such
indications is used when the request completes, as described
below.
When the request used in the completion/verification state completes:
* If a lease-migrated indication is returned, the discovery
continues normally. Note that this is so even if all file systems
have been traversed, since new migrations could have occurred
while the process was going on.
* Otherwise, if there is any record that other requests saw a lease-
migrated indication while the request was occurring, that record
is cleared, and the verification request is retried. The
discovery process remains in the completion/verification state.
* If there have been no lease-migrated indications, the work of
migration discovery is considered completed, and it enters the
non-operating state. Once it enters this state, subsequent lease-
migrated indications will trigger a new migration discovery
process.
It should be noted that the process described above is not guaranteed
to terminate, as a long series of new migration events might
continually delay the clearing of the SEQ4_STATUS_LEASE_MOVED
indication. To prevent unnecessary lease expiration, it is
appropriate for clients to use the discovery of migrations to effect
lease renewal immediately, rather than waiting for the clearing of
the SEQ4_STATUS_LEASE_MOVED indication when the complete set of
migrations is available.
Lease discovery needs to be provided as described above. This
ensures that the client discovers file system migrations soon enough
to renew its leases on each destination server before they expire.
Non-renewal of leases can lead to loss of locking state. While the
consequences of such loss can be ameliorated through implementations
of courtesy locks, servers are under no obligation to do so, and a
conflicting lock request may mean that a lock is revoked
unexpectedly. Clients should be aware of this possibility.
11.13.3. Overview of Client Response to NFS4ERR_MOVED
This section outlines a way in which a client that receives
NFS4ERR_MOVED can effect transition recovery by using a new server or
server endpoint if one is available. As part of that process, it
will determine:
* Whether the NFS4ERR_MOVED indicates migration has occurred, or
whether it indicates another sort of file system access transition
as discussed in
Section 11.10 above.
* In the case of migration, whether Transparent State Migration has
occurred.
* Whether any state has been lost during the process of Transparent
State Migration.
* Whether sessions have been transferred as part of Transparent
State Migration.
During the first phase of this process, the client proceeds to
examine file system location entries to find the initial network
address it will use to continue access to the file system or its
replacement. For each location entry that the client examines, the
process consists of five steps:
1. Performing an EXCHANGE_ID directed at the location address. This
operation is used to register the client owner (in the form of a
client_owner4) with the server, to obtain a client ID to be used
subsequently to communicate with it, to obtain that client ID's
confirmation status, and to determine server_owner4 and scope for
the purpose of determining if the entry is trunkable with the
address previously being used to access the file system (i.e.,
that it represents another network access path to the same file
system and can share locking state with it).
2. Making an initial determination of whether migration has
occurred. The initial determination will be based on whether the
EXCHANGE_ID results indicate that the current location element is
server-trunkable with that used to access the file system when
access was terminated by receiving NFS4ERR_MOVED. If it is, then
migration has not occurred. In that case, the transition is
dealt with, at least initially, as one involving continued access
to the same file system on the same server through a new network
address.
3. Obtaining access to existing session state or creating new
sessions. How this is done depends on the initial determination
of whether migration has occurred and can be done as described in
Section 11.13.4 below in the case of migration or as described in
Section 11.13.5 below in the case of a network address transfer
without migration.
4. Verifying the trunking relationship assumed in step 2 as
discussed in
Section 2.10.5.1. Although this step will generally
confirm the initial determination, it is possible for
verification to invalidate the initial determination of network
address shift (without migration) and instead determine that
migration had occurred. There is no need to redo step 3 above,
since it will be possible to continue use of the session
established already.
5. Obtaining access to existing locking state and/or re-obtaining
it. How this is done depends on the final determination of
whether migration has occurred and can be done as described below
in
Section 11.13.4 in the case of migration or as described in
Section 11.13.5 in the case of a network address transfer without
migration.
Once the initial address has been determined, clients are free to
apply an abbreviated process to find additional addresses trunkable
with it (clients may seek session-trunkable or server-trunkable
addresses depending on whether they support client ID trunking).
During this later phase of the process, further location entries are
examined using the abbreviated procedure specified below:
A: Before the EXCHANGE_ID, the fs name of the location entry is
examined, and if it does not match that currently being used, the
entry is ignored. Otherwise, one proceeds as specified by step 1
above.
B: In the case that the network address is session-trunkable with
one used previously, a BIND_CONN_TO_SESSION is used to access
that session using the new network address. Otherwise, or if the
bind operation fails, a CREATE_SESSION is done.
C: The verification procedure referred to in step 4 above is used.
However, if it fails, the entry is ignored and the next available
entry is used.
11.13.4. Obtaining Access to Sessions and State after Migration
In the event that migration has occurred, migration recovery will
involve determining whether Transparent State Migration has occurred.
This decision is made based on the client ID returned by the
EXCHANGE_ID and the reported confirmation status.
* If the client ID is an unconfirmed client ID not previously known
to the client, then Transparent State Migration has not occurred.
* If the client ID is a confirmed client ID previously known to the
client, then any transferred state would have been merged with an
existing client ID representing the client to the destination
server. In this state merger case, Transparent State Migration
might or might not have occurred, and a determination as to
whether it has occurred is deferred until sessions are established
and the client is ready to begin state recovery.
* If the client ID is a confirmed client ID not previously known to
the client, then the client can conclude that the client ID was
transferred as part of Transparent State Migration. In this
transferred client ID case, Transparent State Migration has
occurred, although some state might have been lost.
Once the client ID has been obtained, it is necessary to obtain
access to sessions to continue communication with the new server. In
any of the cases in which Transparent State Migration has occurred,
it is possible that a session was transferred as well. To deal with
that possibility, clients can, after doing the EXCHANGE_ID, issue a
BIND_CONN_TO_SESSION to connect the transferred session to a
connection to the new server. If that fails, it is an indication
that the session was not transferred and that a new session needs to
be created to take its place.
In some situations, it is possible for a BIND_CONN_TO_SESSION to
succeed without session migration having occurred. If state merger
has taken place, then the associated client ID may have already had a
set of existing sessions, with it being possible that the session ID
of a given session is the same as one that might have been migrated.
In that event, a BIND_CONN_TO_SESSION might succeed, even though
there could have been no migration of the session with that session
ID. In such cases, the client will receive sequence errors when the
slot sequence values used are not appropriate on the new session.
When this occurs, the client can create a new a session and cease
using the existing one.
Once the client has determined the initial migration status, and
determined that there was a shift to a new server, it needs to re-
establish its locking state, if possible. To enable this to happen
without loss of the guarantees normally provided by locking, the
destination server needs to implement a per-fs grace period in all
cases in which lock state was lost, including those in which
Transparent State Migration was not implemented. Each client for
which there was a transfer of locking state to the new server will
have the duration of the grace period to reclaim its locks, from the
time its locks were transferred.
Clients need to deal with the following cases:
* In the state merger case, it is possible that the server has not
attempted Transparent State Migration, in which case state may
have been lost without it being reflected in the SEQ4_STATUS bits.
To determine whether this has happened, the client can use
TEST_STATEID to check whether the stateids created on the source
server are still accessible on the destination server. Once a
single stateid is found to have been successfully transferred, the
client can conclude that Transparent State Migration was begun,
and any failure to transport all of the stateids will be reflected
in the SEQ4_STATUS bits. Otherwise, Transparent State Migration
has not occurred.
* In a case in which Transparent State Migration has not occurred,
the client can use the per-fs grace period provided by the
destination server to reclaim locks that were held on the source
server.
* In a case in which Transparent State Migration has occurred, and
no lock state was lost (as shown by SEQ4_STATUS flags), no lock
reclaim is necessary.
* In a case in which Transparent State Migration has occurred, and
some lock state was lost (as shown by SEQ4_STATUS flags), existing
stateids need to be checked for validity using TEST_STATEID, and
reclaim used to re-establish any that were not transferred.
For all of the cases above, RECLAIM_COMPLETE with an rca_one_fs value
of TRUE needs to be done before normal use of the file system,
including obtaining new locks for the file system. This applies even
if no locks were lost and there was no need for any to be reclaimed.
11.13.5. Obtaining Access to Sessions and State after Network Address
Transfer
The case in which there is a transfer to a new network address
without migration is similar to that described in
Section 11.13.4 above in that there is a need to obtain access to needed sessions and
locking state. However, the details are simpler and will vary
depending on the type of trunking between the address receiving
NFS4ERR_MOVED and that to which the transfer is to be made.
To make a session available for use, a BIND_CONN_TO_SESSION should be
used to obtain access to the session previously in use. Only if this
fails, should a CREATE_SESSION be done. While this procedure mirrors
that in
Section 11.13.4 above, there is an important difference in
that preservation of the session is not purely optional but depends
on the type of trunking.
Access to appropriate locking state will generally need no actions
beyond access to the session. However, the SEQ4_STATUS bits need to
be checked for lost locking state, including the need to reclaim
locks after a server reboot, since there is always a possibility of
locking state being lost.
11.14. Server Responsibilities Upon Migration
In the event of file system migration, when the client connects to
the destination server, that server needs to be able to provide the
client continued access to the files it had open on the source
server. There are two ways to provide this:
* By provision of an fs-specific grace period, allowing the client
the ability to reclaim its locks, in a fashion similar to what
would have been done in the case of recovery from a server
restart. See
Section 11.14.1 for a more complete discussion.
* By implementing Transparent State Migration possibly in connection
with session migration, the server can provide the client
immediate access to the state built up on the source server on the
destination server.
These features are discussed separately in Sections
11.14.2 and
11.14.3, which discuss Transparent State Migration and session
migration, respectively.
All the features described above can involve transfer of lock-related
information between source and destination servers. In some cases,
this transfer is a necessary part of the implementation, while in
other cases, it is a helpful implementation aid, which servers might
or might not use. The subsections below discuss the information that
would be transferred but do not define the specifics of the transfer
protocol. This is left as an implementation choice, although
standards in this area could be developed at a later time.
11.14.1. Server Responsibilities in Effecting State Reclaim after
Migration
In this case, the destination server needs no knowledge of the locks
held on the source server. It relies on the clients to accurately
report (via reclaim operations) the locks previously held, and does
not allow new locks to be granted on migrated file systems until the
grace period expires. Disallowing of new locks applies to all
clients accessing these file systems, while grace period expiration
occurs for each migrated client independently.
During this grace period, clients have the opportunity to use reclaim
operations to obtain locks for file system objects within the
migrated file system, in the same way that they do when recovering
from server restart, and the servers typically rely on clients to
accurately report their locks, although they have the option of
subjecting these requests to verification. If the clients only
reclaim locks held on the source server, no conflict can arise. Once
the client has reclaimed its locks, it indicates the completion of
lock reclamation by performing a RECLAIM_COMPLETE specifying
rca_one_fs as TRUE.
While it is not necessary for source and destination servers to
cooperate to transfer information about locks, implementations are
well advised to consider transferring the following useful
information:
* If information about the set of clients that have locking state
for the transferred file system is made available, the destination
server will be able to terminate the grace period once all such
clients have reclaimed their locks, allowing normal locking
activity to resume earlier than it would have otherwise.
* Locking summary information for individual clients (at various
possible levels of detail) can detect some instances in which
clients do not accurately represent the locks held on the source
server.
11.14.2. Server Responsibilities in Effecting Transparent State
Migration
The basic responsibility of the source server in effecting
Transparent State Migration is to make available to the destination
server a description of each piece of locking state associated with
the file system being migrated. In addition to client id string and
verifier, the source server needs to provide for each stateid:
* The stateid including the current sequence value.
* The associated client ID.
* The handle of the associated file.
* The type of the lock, such as open, byte-range lock, delegation,
or layout.
* For locks such as opens and byte-range locks, there will be
information about the owner(s) of the lock.
* For recallable/revocable lock types, the current recall status
needs to be included.
* For each lock type, there will be associated type-specific
information. For opens, this will include share and deny mode
while for byte-range locks and layouts, there will be a type and a
byte-range.
Such information will most probably be organized by client id string
on the destination server so that it can be used to provide
appropriate context to each client when it makes itself known to the
client. Issues connected with a client impersonating another by
presenting another client's client id string can be addressed using
NFSv4.1 state protection features, as described in
Section 21.
A further server responsibility concerns locks that are revoked or
otherwise lost during the process of file system migration. Because
locks that appear to be lost during the process of migration will be
reclaimed by the client, the servers have to take steps to ensure
that locks revoked soon before or soon after migration are not
inadvertently allowed to be reclaimed in situations in which the
continuity of lock possession cannot be assured.
* For locks lost on the source but whose loss has not yet been
acknowledged by the client (by using FREE_STATEID), the
destination must be aware of this loss so that it can deny a
request to reclaim them.
* For locks lost on the destination after the state transfer but
before the client's RECLAIM_COMPLETE is done, the destination
server should note these and not allow them to be reclaimed.
An additional responsibility of the cooperating servers concerns
situations in which a stateid cannot be transferred transparently
because it conflicts with an existing stateid held by the client and
associated with a different file system. In this case, there are two
valid choices:
* Treat the transfer, as in NFSv4.0, as one without Transparent
State Migration. In this case, conflicting locks cannot be
granted until the client does a RECLAIM_COMPLETE, after reclaiming
the locks it had, with the exception of reclaims denied because
they were attempts to reclaim locks that had been lost.
* Implement Transparent State Migration, except for the lock with
the conflicting stateid. In this case, the client will be aware
of a lost lock (through the SEQ4_STATUS flags) and be allowed to
reclaim it.
When transferring state between the source and destination, the
issues discussed in
Section 7.2 of [69] must still be attended to.
In this case, the use of NFS4ERR_DELAY may still be necessary in
NFSv4.1, as it was in NFSv4.0, to prevent locking state changing
while it is being transferred. See
Section 15.1.1.3 for information
about appropriate client retry approaches in the event that
NFS4ERR_DELAY is returned.
There are a number of important differences in the NFS4.1 context:
* The absence of RELEASE_LOCKOWNER means that the one case in which
an operation could not be deferred by use of NFS4ERR_DELAY no
longer exists.
* Sequencing of operations is no longer done using owner-based
operation sequences numbers. Instead, sequencing is session-
based.
As a result, when sessions are not transferred, the techniques
discussed in
Section 7.2 of [69] are adequate and will not be further
discussed.
11.14.3. Server Responsibilities in Effecting Session Transfer
The basic responsibility of the source server in effecting session
transfer is to make available to the destination server a description
of the current state of each slot with the session, including the
following:
* The last sequence value received for that slot.
* Whether there is cached reply data for the last request executed
and, if so, the cached reply.
When sessions are transferred, there are a number of issues that pose
challenges in terms of making the transferred state unmodifiable
during the period it is gathered up and transferred to the
destination server:
* A single session may be used to access multiple file systems, not
all of which are being transferred.
* Requests made on a session may, even if rejected, affect the state
of the session by advancing the sequence number associated with
the slot used.
As a result, when the file system state might otherwise be considered
unmodifiable, the client might have any number of in-flight requests,
each of which is capable of changing session state, which may be of a
number of types:
1. Those requests that were processed on the migrating file system
before migration began.
2. Those requests that received the error NFS4ERR_DELAY because the
file system being accessed was in the process of being migrated.
3. Those requests that received the error NFS4ERR_MOVED because the
file system being accessed had been migrated.
4. Those requests that accessed the migrating file system in order
to obtain location or status information.
5. Those requests that did not reference the migrating file system.
It should be noted that the history of any particular slot is likely
to include a number of these request classes. In the case in which a
session that is migrated is used by file systems other than the one
migrated, requests of class 5 may be common and may be the last
request processed for many slots.
Since session state can change even after the locking state has been
fixed as part of the migration process, the session state known to
the client could be different from that on the destination server,
which necessarily reflects the session state on the source server at
an earlier time. In deciding how to deal with this situation, it is
helpful to distinguish between two sorts of behavioral consequences
of the choice of initial sequence ID values:
* The error NFS4ERR_SEQ_MISORDERED is returned when the sequence ID
in a request is neither equal to the last one seen for the current
slot nor the next greater one.
In view of the difficulty of arriving at a mutually acceptable
value for the correct last sequence value at the point of
migration, it may be necessary for the server to show some degree
of forbearance when the sequence ID is one that would be
considered unacceptable if session migration were not involved.
* Returning the cached reply for a previously executed request when
the sequence ID in the request matches the last value recorded for
the slot.
In the cases in which an error is returned and there is no
possibility of any non-idempotent operation having been executed,
it may not be necessary to adhere to this as strictly as might be
proper if session migration were not involved. For example, the
fact that the error NFS4ERR_DELAY was returned may not assist the
client in any material way, while the fact that NFS4ERR_MOVED was
returned by the source server may not be relevant when the request
was reissued and directed to the destination server.
An important issue is that the specification needs to take note of
all potential COMPOUNDs, even if they might be unlikely in practice.
For example, a COMPOUND is allowed to access multiple file systems
and might perform non-idempotent operations in some of them before
accessing a file system being migrated. Also, a COMPOUND may return
considerable data in the response before being rejected with
NFS4ERR_DELAY or NFS4ERR_MOVED, and may in addition be marked as
sa_cachethis. However, note that if the client and server adhere to
rules in
Section 15.1.1.3, there is no possibility of non-idempotent
operations being spuriously reissued after receiving NFS4ERR_DELAY
response.
To address these issues, a destination server
MAY do any of the
following when implementing session transfer:
* Avoid enforcing any sequencing semantics for a particular slot
until the client has established the starting sequence for that
slot on the destination server.
* For each slot, avoid returning a cached reply returning
NFS4ERR_DELAY or NFS4ERR_MOVED until the client has established
the starting sequence for that slot on the destination server.
* Until the client has established the starting sequence for a
particular slot on the destination server, avoid reporting
NFS4ERR_SEQ_MISORDERED or returning a cached reply that contains
either NFS4ERR_DELAY or NFS4ERR_MOVED and consists solely of a
series of operations where the response is NFS4_OK until the final
error.
Because of the considerations mentioned above, including the rules
for the handling of NFS4ERR_DELAY included in
Section 15.1.1.3, the
destination server can respond appropriately to SEQUENCE operations
received from the client by adopting the three policies listed below:
* Not responding with NFS4ERR_SEQ_MISORDERED for the initial request
on a slot within a transferred session because the destination
server cannot be aware of requests made by the client after the
server handoff but before the client became aware of the shift.
In cases in which NFS4ERR_SEQ_MISORDERED would normally have been
reported, the request is to be processed normally as a new
request.
* Replying as it would for a retry whenever the sequence matches
that transferred by the source server, even though this would not
provide retry handling for requests issued after the server
handoff, under the assumption that, when such requests are issued,
they will never be responded to in a state-changing fashion,
making retry support for them unnecessary.
* Once a non-retry SEQUENCE is received for a given slot, using that
as the basis for further sequence checking, with no further
reference to the sequence value transferred by the source server.
11.15. Effecting File System Referrals
Referrals are effected when an absent file system is encountered and
one or more alternate locations are made available by the
fs_locations or fs_locations_info attributes. The client will
typically get an NFS4ERR_MOVED error, fetch the appropriate location
information, and proceed to access the file system on a different
server, even though it retains its logical position within the
original namespace. Referrals differ from migration events in that
they happen only when the client has not previously referenced the
file system in question (so there is nothing to transition).
Referrals can only come into effect when an absent file system is
encountered a