RFC 5661






Internet Engineering Task Force (IETF)                   S. Shepler, Ed.
Request for Comments: 5661                               Storspeed, Inc.
Category: Standards Track                                 M. Eisler, Ed.
ISSN: 2070-1721                                           D. Noveck, Ed.
                                                                  NetApp
                                                            January 2010


      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 3530) and
   protocol extensions made subsequently.  Major extensions introduced
   in NFS version 4 minor version 1 include Sessions, Directory
   Delegations, and parallel NFS (pNFS).  NFS version 4 minor version 1
   has no dependencies on NFS version 4 minor version 0, and it is
   considered a separate protocol.  Thus, this document neither updates
   nor obsoletes RFC 3530.  NFS minor version 1 is deemed superior to
   NFS minor version 0 with no loss of functionality, and its use is
   preferred over version 0.  Both NFS minor versions 0 and 1 can be
   used simultaneously on the same network, between the same client and
   server.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5661.












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RFC 5661                         NFSv4.1                    January 2010


Copyright Notice



   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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RFC 5661                         NFSv4.1                    January 2010


Table of Contents



   1. Introduction ....................................................9
      1.1. The NFS Version 4 Minor Version 1 Protocol .................9
      1.2. Requirements Language ......................................9
      1.3. Scope of This Document .....................................9
      1.4. NFSv4 Goals ...............................................10
      1.5. NFSv4.1 Goals .............................................10
      1.6. General Definitions .......................................11
      1.7. Overview of NFSv4.1 Features ..............................13
      1.8. Differences from NFSv4.0 ..................................17
   2. Core Infrastructure ............................................18
      2.1. Introduction ..............................................18
      2.2. RPC and XDR ...............................................19
      2.3. COMPOUND and CB_COMPOUND ..................................22
      2.4. Client Identifiers and Client Owners ......................23
      2.5. Server Owners .............................................28
      2.6. Security Service Negotiation ..............................29
      2.7. Minor Versioning ..........................................34
      2.8. Non-RPC-Based Security Services ...........................37
      2.9. Transport Layers ..........................................37
      2.10. Session ..................................................40
   3. Protocol Constants and Data Types ..............................86
      3.1. Basic Constants ...........................................86
      3.2. Basic Data Types ..........................................87
      3.3. Structured Data Types .....................................89
   4. Filehandles ....................................................97
      4.1. Obtaining the First Filehandle ............................98
      4.2. Filehandle Types ..........................................99
      4.3. One Method of Constructing a Volatile Filehandle .........101
      4.4. Client Recovery from Filehandle Expiration ...............102
   5. File Attributes ...............................................103
      5.1. REQUIRED Attributes ......................................104
      5.2. RECOMMENDED Attributes ...................................104
      5.3. Named Attributes .........................................105
      5.4. Classification of Attributes .............................106
      5.5. Set-Only and Get-Only Attributes .........................107
      5.6. REQUIRED Attributes - List and Definition References .....107
      5.7. RECOMMENDED Attributes - List and Definition References ..108
      5.8. Attribute Definitions ....................................110
      5.9. Interpreting owner and owner_group .......................119
      5.10. Character Case Attributes ...............................121
      5.11. Directory Notification Attributes .......................121
      5.12. pNFS Attribute Definitions ..............................122
      5.13. Retention Attributes ....................................123
   6. Access Control Attributes .....................................126
      6.1. Goals ....................................................126
      6.2. File Attributes Discussion ...............................128



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      6.3. Common Methods ...........................................144
      6.4. Requirements .............................................147
   7. Single-Server Namespace .......................................153
      7.1. Server Exports ...........................................153
      7.2. Browsing Exports .........................................153
      7.3. Server Pseudo File System ................................154
      7.4. Multiple Roots ...........................................155
      7.5. Filehandle Volatility ....................................155
      7.6. Exported Root ............................................155
      7.7. Mount Point Crossing .....................................156
      7.8. Security Policy and Namespace Presentation ...............156
   8. State Management ..............................................157
      8.1. Client and Session ID ....................................158
      8.2. Stateid Definition .......................................158
      8.3. Lease Renewal ............................................167
      8.4. Crash Recovery ...........................................170
      8.5. Server Revocation of Locks ...............................181
      8.6. Short and Long Leases ....................................182
      8.7. Clocks, Propagation Delay, and Calculating Lease
           Expiration ...............................................182
      8.8. Obsolete Locking Infrastructure from NFSv4.0 .............183
   9. File Locking and Share Reservations ...........................184
      9.1. Opens and Byte-Range Locks ...............................184
      9.2. Lock Ranges ..............................................188
      9.3. Upgrading and Downgrading Locks ..........................188
      9.4. Stateid Seqid Values and Byte-Range Locks ................189
      9.5. Issues with Multiple Open-Owners .........................189
      9.6. Blocking Locks ...........................................190
      9.7. Share Reservations .......................................191
      9.8. OPEN/CLOSE Operations ....................................192
      9.9. Open Upgrade and Downgrade ...............................192
      9.10. Parallel OPENs ..........................................193
      9.11. Reclaim of Open and Byte-Range Locks ....................194
   10. Client-Side Caching ..........................................194
      10.1. Performance Challenges for Client-Side Caching ..........195
      10.2. Delegation and Callbacks ................................196
      10.3. Data Caching ............................................200
      10.4. Open Delegation .........................................205
      10.5. Data Caching and Revocation .............................216
      10.6. Attribute Caching .......................................218
      10.7. Data and Metadata Caching and Memory Mapped Files .......220
      10.8. Name and Directory Caching without Directory
            Delegations .............................................222
      10.9. Directory Delegations ...................................225
   11. Multi-Server Namespace .......................................228
      11.1. Location Attributes .....................................228
      11.2. File System Presence or Absence .........................229
      11.3. Getting Attributes for an Absent File System ............230



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      11.4. Uses of Location Information ............................232
      11.5. Location Entries and Server Identity ....................236
      11.6. Additional Client-Side Considerations ...................237
      11.7. Effecting File System Transitions .......................238
      11.8. Effecting File System Referrals .........................251
      11.9. The Attribute fs_locations ..............................258
      11.10. The Attribute fs_locations_info ........................261
      11.11. The Attribute fs_status ................................273
   12. Parallel NFS (pNFS) ..........................................277
      12.1. Introduction ............................................277
      12.2. pNFS Definitions ........................................278
      12.3. pNFS Operations .........................................284
      12.4. pNFS Attributes .........................................285
      12.5. Layout Semantics ........................................285
      12.6. pNFS Mechanics ..........................................300
      12.7. Recovery ................................................302
      12.8. Metadata and Storage Device Roles .......................307
      12.9. Security Considerations for pNFS ........................307
   13. NFSv4.1 as a Storage Protocol in pNFS: the File Layout Type ..309
      13.1. Client ID and Session Considerations ....................309
      13.2. File Layout Definitions .................................312
      13.3. File Layout Data Types ..................................312
      13.4. Interpreting the File Layout ............................317
      13.5. Data Server Multipathing ................................324
      13.6. Operations Sent to NFSv4.1 Data Servers .................325
      13.7. COMMIT through Metadata Server ..........................327
      13.8. The Layout Iomode .......................................328
      13.9. Metadata and Data Server State Coordination .............329
      13.10. Data Server Component File Size ........................332
      13.11. Layout Revocation and Fencing ..........................333
      13.12. Security Considerations for the File Layout Type .......334
   14. Internationalization .........................................334
     14.1.  Stringprep profile for the utf8str_cs type ..............336
     14.2.  Stringprep profile for the utf8str_cis type .............337
     14.3.  Stringprep profile for the utf8str_mixed type ...........338
     14.4.  UTF-8 Capabilities ......................................340
     14.5.  UTF-8 Related Errors ....................................340
   15. Error Values .................................................341
      15.1. Error Definitions .......................................341
      15.2. Operations and Their Valid Errors .......................361
      15.3. Callback Operations and Their Valid Errors ..............376
      15.4. Errors and the Operations That Use Them .................379
   16. NFSv4.1 Procedures ...........................................391
      16.1. Procedure 0: NULL - No Operation ........................392
      16.2. Procedure 1: COMPOUND - Compound Operations .............392
   17. Operations: REQUIRED, RECOMMENDED, or OPTIONAL ...............403
   18. NFSv4.1 Operations ...........................................407
      18.1. Operation 3: ACCESS - Check Access Rights ...............407



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RFC 5661                         NFSv4.1                    January 2010


      18.2. Operation 4: CLOSE - Close File .........................413
      18.3. Operation 5: COMMIT - Commit Cached Data ................414
      18.4. Operation 6: CREATE - Create a Non-Regular File Object ..417
      18.5. Operation 7: DELEGPURGE - Purge Delegations
            Awaiting Recovery .......................................419
      18.6. Operation 8: DELEGRETURN - Return Delegation ............420
      18.7. Operation 9: GETATTR - Get Attributes ...................421
      18.8. Operation 10: GETFH - Get Current Filehandle ............423
      18.9. Operation 11: LINK - Create Link to a File ..............424
      18.10. Operation 12: LOCK - Create Lock .......................426
      18.11. Operation 13: LOCKT - Test for Lock ....................430
      18.12. Operation 14: LOCKU - Unlock File ......................432
      18.13. Operation 15: LOOKUP - Lookup Filename .................433
      18.14. Operation 16: LOOKUPP - Lookup Parent Directory ........435
      18.15. Operation 17: NVERIFY - Verify Difference in
             Attributes .............................................436
      18.16. Operation 18: OPEN - Open a Regular File ...............437
      18.17. Operation 19: OPENATTR - Open Named Attribute
             Directory ..............................................458
      18.18. Operation 21: OPEN_DOWNGRADE - Reduce Open File
             Access .................................................459
      18.19. Operation 22: PUTFH - Set Current Filehandle ...........461
      18.20. Operation 23: PUTPUBFH - Set Public Filehandle .........461
      18.21. Operation 24: PUTROOTFH - Set Root Filehandle ..........463
      18.22. Operation 25: READ - Read from File ....................464
      18.23. Operation 26: READDIR - Read Directory .................466
      18.24. Operation 27: READLINK - Read Symbolic Link ............469
      18.25. Operation 28: REMOVE - Remove File System Object .......470
      18.26. Operation 29: RENAME - Rename Directory Entry ..........473
      18.27. Operation 31: RESTOREFH - Restore Saved Filehandle .....477
      18.28. Operation 32: SAVEFH - Save Current Filehandle .........478
      18.29. Operation 33: SECINFO - Obtain Available Security ......479
      18.30. Operation 34: SETATTR - Set Attributes .................482
      18.31. Operation 37: VERIFY - Verify Same Attributes ..........485
      18.32. Operation 38: WRITE - Write to File ....................486
      18.33. Operation 40: BACKCHANNEL_CTL - Backchannel Control ....491
      18.34. Operation 41: BIND_CONN_TO_SESSION - Associate
             Connection with Session ................................492
      18.35. Operation 42: EXCHANGE_ID - Instantiate Client ID ......495
      18.36. Operation 43: CREATE_SESSION - Create New
             Session and Confirm Client ID ..........................513
      18.37. Operation 44: DESTROY_SESSION - Destroy a Session ......523
      18.38. Operation 45: FREE_STATEID - Free Stateid with
             No Locks ...............................................525
      18.39. Operation 46: GET_DIR_DELEGATION - Get a
             Directory Delegation ...................................526
      18.40. Operation 47: GETDEVICEINFO - Get Device Information ...530
      18.41. Operation 48: GETDEVICELIST - Get All Device



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RFC 5661                         NFSv4.1                    January 2010


             Mappings for a File System .............................533
      18.42. Operation 49: LAYOUTCOMMIT - Commit Writes Made
             Using a Layout .........................................534
      18.43. Operation 50: LAYOUTGET - Get Layout Information .......538
      18.44. Operation 51: LAYOUTRETURN - Release Layout
             Information ............................................547
      18.45. Operation 52: SECINFO_NO_NAME - Get Security on
             Unnamed Object .........................................552
      18.46. Operation 53: SEQUENCE - Supply Per-Procedure
             Sequencing and Control .................................553
      18.47. Operation 54: SET_SSV - Update SSV for a Client ID .....559
      18.48. Operation 55: TEST_STATEID - Test Stateids for
             Validity ...............................................561
      18.49. Operation 56: WANT_DELEGATION - Request Delegation .....563
      18.50. Operation 57: DESTROY_CLIENTID - Destroy a Client ID ...566
      18.51. Operation 58: RECLAIM_COMPLETE - Indicates
             Reclaims Finished ......................................567
      18.52. Operation 10044: ILLEGAL - Illegal Operation ...........569
   19. NFSv4.1 Callback Procedures ..................................570
      19.1. Procedure 0: CB_NULL - No Operation .....................570
      19.2. Procedure 1: CB_COMPOUND - Compound Operations ..........571
   20. NFSv4.1 Callback Operations ..................................574
      20.1. Operation 3: CB_GETATTR - Get Attributes ................574
      20.2. Operation 4: CB_RECALL - Recall a Delegation ............575
      20.3. Operation 5: CB_LAYOUTRECALL - Recall Layout
            from Client .............................................576
      20.4. Operation 6: CB_NOTIFY - Notify Client of
            Directory Changes .......................................580
      20.5. Operation 7: CB_PUSH_DELEG - Offer Previously
            Requested Delegation to Client ..........................583
      20.6. Operation 8: CB_RECALL_ANY - Keep Any N
            Recallable Objects ......................................584
      20.7. Operation 9: CB_RECALLABLE_OBJ_AVAIL - Signal
            Resources for Recallable Objects ........................588
      20.8. Operation 10: CB_RECALL_SLOT - Change Flow
            Control Limits ..........................................588
      20.9. Operation 11: CB_SEQUENCE - Supply Backchannel
            Sequencing and Control ..................................589
      20.10. Operation 12: CB_WANTS_CANCELLED - Cancel
             Pending Delegation Wants ...............................592
      20.11. Operation 13: CB_NOTIFY_LOCK - Notify Client of
             Possible Lock Availability .............................593
      20.12. Operation 14: CB_NOTIFY_DEVICEID - Notify
             Client of Device ID Changes ............................594
      20.13. Operation 10044: CB_ILLEGAL - Illegal Callback
             Operation ..............................................596
   21. Security Considerations ......................................597
   22. IANA Considerations ..........................................598



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RFC 5661                         NFSv4.1                    January 2010


      22.1. Named Attribute Definitions .............................598
      22.2. Device ID Notifications .................................600
      22.3. Object Recall Types .....................................601
      22.4. Layout Types ............................................603
      22.5. Path Variable Definitions ...............................606
   23. References ...................................................609
      23.1. Normative References ....................................609
      23.2. Informative References ..................................612
   Appendix A.  Acknowledgments  ....................................615










































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RFC 5661                         NFSv4.1                    January 2010


1.  Introduction



1.1.  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 described in [30].  It generally follows the
   guidelines for minor versioning that are listed in Section 10 of RFC
   3530.  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.

1.2.  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].

1.3.  Scope of This Document



   This document describes the NFSv4.1 protocol.  With respect to
   NFSv4.0, this document does not:

   o  describe the NFSv4.0 protocol, except where needed to contrast
      with NFSv4.1.

   o  modify the specification of the NFSv4.0 protocol.

   o  clarify the NFSv4.0 protocol.









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RFC 5661                         NFSv4.1                    January 2010


1.4.  NFSv4 Goals



   The NFSv4 protocol is a further revision of the NFS protocol defined
   already by NFSv3 [31].  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:

   o  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.

   o  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.

   o  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.

   o  Designed for protocol extensions

      The protocol is designed to accept standard extensions within a
      framework that enables and encourages backward compatibility.

1.5.  NFSv4.1 Goals



   NFSv4.1 has the following goals, within the framework established by
   the overall NFSv4 goals.

   o  To correct significant structural weaknesses and oversights
      discovered in the base protocol.

   o  To add clarity and specificity to areas left unaddressed or not
      addressed in sufficient detail in the base protocol.  However, as
      stated in Section 1.3, it is not a goal to clarify the NFSv4.0
      protocol in the NFSv4.1 specification.

   o  To add specific features based on experience with the existing
      protocol and recent industry developments.



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RFC 5661                         NFSv4.1                    January 2010


   o  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.

1.6.  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).







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RFC 5661                         NFSv4.1                    January 2010


   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 sharable 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:






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      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.

1.7.  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 1.8
   for a summary of the differences between NFSv4.0 and NFSv4.1.

1.7.1.  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




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   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 1.7.2.2), 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).

1.7.2.  Protocol Structure



1.7.2.1.  Core Protocol



   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.

1.7.2.2.  Parallel Access



   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 1.7.1)
   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).






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1.7.3.  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.

1.7.3.1.  Filehandles



   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.

1.7.3.2.  File Attributes



   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
   [31]).  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



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   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.

1.7.3.3.  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 non-disruptive transfer of support for individual file systems
   between servers.  They are all based upon attributes that allow one
   file system to specify alternate or new locations for that file
   system.

   These attributes may be used together with the concept of absent file
   systems, which provide specifications for additional locations but no
   actual file system content.  This allows a number of important
   facilities:

   o  Location 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.

   o  Location attributes may be provided for present file systems to
      provide the locations of alternate file system instances or
      replicas to be used in the event that the current file system
      instance becomes unavailable.

   o  Location attributes may be provided when a previously present file
      system becomes absent.  This allows non-disruptive migration of
      file systems to alternate servers.

1.7.4.  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.



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   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:

   o  Share reservations as established by OPEN operations.

   o  Byte-range locks.

   o  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.

   o  Directory delegations, which are recallable locks that assure the
      holder that inconsistent directory modifications cannot occur so
      long as the delegation is held.

   o  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.

1.8.  Differences from NFSv4.0



   The following summarizes the major differences between minor version
   1 and the base protocol:

   o  Implementation of the sessions model (Section 2.10).

   o  Parallel access to data (Section 12).

   o  Addition of the RECLAIM_COMPLETE operation to better structure the
      lock reclamation process (Section 18.51).

   o  Enhanced delegation support as follows.





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      *  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).

   o  Attributes can be set atomically during exclusive file create via
      the OPEN operation (see the new EXCLUSIVE4_1 creation method in
      Section 18.16).

   o  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).

   o  Improved compatibility with Microsoft Windows for Access Control
      Lists (Section 6.2.3, Section 6.2.2, Section 6.4.3.2).

   o  Data retention (Section 5.13).

   o  Identification of the implementation of the NFS client and server
      (Section 18.35).

   o  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).

   o  In NFSv4.1, LIPKEY and SPKM-3 are not required security mechanisms
      [32].

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.




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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 Section 7.2 ("Authentication") 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



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   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.

2.2.1.1.1.2.1.  Kerberos V5


   The Kerberos V5 GSS-API mechanism as described in [5] MUST be
   implemented with the RPCSEC_GSS services as specified in the
   following table:










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      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 [33].

   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.

2.2.1.1.1.3.  GSS Server Principal


   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



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   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.






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   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 reply 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:



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   struct client_owner4 {
           verifier4       co_verifier;
           opaque          co_ownerid<NFS4_OPAQUE_LIMIT>;
   };

   The first field, co_verifier, is a client incarnation verifier.  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:

   o  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.

   o  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.

   o  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 [30].)

   o  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





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      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:

   o  If applicable, the client's statically assigned network address.

   o  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.

   o  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.



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   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 [30]), 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




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   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:

   o  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





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      MUST be integrity or privacy, and the same GSS mechanism and
      principal MUST be used as that used when the client ID was
      created.

   o  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).

   o  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



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   server (as defined in Section 1.6).  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 a 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.



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   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.






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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.



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   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.



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   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.

2.6.3.1.2.  LINK and RENAME


   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





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   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:

   o  The server can return NFS4ERR_XDEV.

   o  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 [30], 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.







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   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




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        *  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.



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   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.

2.8.2.  Auditing



   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:






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   o  The transport supports reliable delivery of data, which NFSv4.1
      requires but neither NFSv4.1 nor RPC has facilities for ensuring
      [34].

   o  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:



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   o  A requester MUST NOT retry a request unless the connection the
      request was sent over was lost before the reply was received.

   o  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:

   o  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.

   o  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.



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   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.

2.9.3.  Ports



   Historically, NFSv3 servers have listened over TCP port 2049.  The
   registered port 2049 [35] for the NFS protocol should be the default
   configuration.  NFSv4.1 clients SHOULD NOT use the RPC binding
   protocols as described in [36].

2.10.  Session



   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:

   o  Lack of support for Exactly Once Semantics (EOS).  This includes
      lack of support for EOS through server failure and recovery.

   o  Limited callback support, including no support for sending
      callbacks through firewalls, and races between replies to normal
      requests and callbacks.

   o  Limited trunking over multiple network paths.

   o  Requiring machine credentials for fully secure operation.

   Through the introduction of a session, NFSv4.1 addresses the above
   shortfalls with practical solutions:

   o  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.

   o  The NFSv4.1 client (defined in Section 1.6, Paragraph 2) 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





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      callbacks caused by the requests are detected via the session's
      sequencing properties that are a consequence of EOS
      (Section 2.10.6.3).

   o  The NFSv4.1 client can associate an arbitrary number of
      connections with the session, and thus provide trunking
      (Section 2.10.5).

   o  The NFSv4.1 client and server produces 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).

   o  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.



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   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.






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   State (which can consist of share reservations, locks, delegations,
   and layouts (Section 1.7.4)) 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.

2.10.3.  Channels



   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 a decision made 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 nonidempotent.

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



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   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.

2.10.4.  Server Scope



   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 compatible values of
   otherwise opaque identifiers.  Thus, the identifiers generated by one
   server of that set may be presented to another of that 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 accept a value
   generated by another inadvertently.  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 follow identifiers:






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   o  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, and locks obtained on one server conflict with those
      requested on the other.

   o  Session ID values.  A session ID value accepted by two servers of
      the same server scope denotes the same session.

   o  Client ID values.  A client ID value accepted as valid by two
      servers of the same server scope is associated with two clients
      with the same client owner and verifier.

   o  State ID values.  A state ID value is recognized as valid when the
      corresponding client ID is recognized as valid.  If the same
      stateid value is accepted as valid on two servers of the same
      scope and the client IDs on the two servers represent the same
      client owner and verifier, then the two stateid values designate
      the same set of locks and are for the same file.

   o  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 all identical 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:

   o  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 2.10.5.

   o  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.

   o  When file system migration causes the transfer of responsibility
      for a file system between servers and the client needs to
      determine whether state has been transferred with the file system



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      (as described in Section 11.7.7) or whether the client needs to
      reclaim state on a similar basis as in the case of server restart,
      as described in Section 8.4.2.

   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.

   o  If both EXCHANGE_ID requests were sent with RPCSEC_GSS
      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 also share the same principal name.

   o  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).

2.10.5.  Trunking



   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.

   NFSv4.1 servers MUST support both forms of trunking within the
   context of a single server network address and MUST support both
   forms within the context of the set of network addresses used to
   access a single server.  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



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   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 later in this section), 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



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      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.

   The client should be prepared for the possibility that
   eir_server_owner values may be different on subsequent EXCHANGE_ID
   requests made to the same network address, as a result of various
   sorts of reconfiguration events.  When this happens and the changes
   result in the invalidation of previously valid forms of trunking, the
   client should cease to use those forms, either by dropping
   connections or by adding sessions.  For a discussion of lock reclaim
   as it relates to such reconfiguration events, see Section 8.4.2.1.

2.10.5.1.  Verifying Claims of Matching Server Identity



   When two servers over two connections 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:





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   o  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 (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
      (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.


   o  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.




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      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:

   o  Non-idempotent requests.

   o  Idempotent modifying requests.

   o  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.








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   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



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   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:





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   o  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.

   o  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.

   o  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).

   o  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




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   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:

   o  The RPC layer retains its existing semantics and implementation.

   o  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.

   o  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.

   o  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 begs
   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 [37]; 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:






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   o  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.

   o  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



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      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.

   o  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.



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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 [34].
   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 results of SEQUENCE or
   CB_SEQUENCE are 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:

   o  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.

   o  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.



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   o  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.

   o  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:

   o  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).

   o  A reply that consists only of the Sequence operation with the
      error NFS4ERR_FALSE_RETRY.

   o  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.

   o  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.

2.10.6.1.3.1.  False Retry


   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



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   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_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_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_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_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



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   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" [8] 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"



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   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).



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   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 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:




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   o  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.

   o  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.

   o  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.

   o  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.

2.10.6.5.  Persistence



   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):

   o  The session ID.

   o  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



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   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:

   o  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.

   o  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.

   o  The principal that created the client ID.  This allows the server
      to authenticate the client when it sends EXCHANGE_ID.

   o  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.

   o  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




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   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 [38] is described in
   [39].

2.10.7.  RDMA Considerations



   A complete discussion of the operation of RPC-based protocols over
   RDMA transports is in [8].  A discussion of the operation of NFSv4,
   including NFSv4.1, over RDMA is in [9].  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.






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2.10.7.2.  Flow Control



   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 [8]; 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.

2.10.7.3.  Padding



   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 [8].
   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





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   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 [10]) 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).









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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 [30].

   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



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   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:

   o  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.

   o  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.

   o  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 safe guarding the per-machine credential.
   Assuming a proper safeguard using the per-machine credential for
   operations like CREATE_SESSION, BIND_CONN_TO_SESSION,





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RFC 5661                         NFSv4.1                    January 2010


   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 1.6), 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:

   o  The arguments to and results of SET_SSV include digests of the old
      and new SSV, respectively.

   o  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.




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   o  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:

   o  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:





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      *  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.

   o  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.

   o  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.




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RFC 5661                         NFSv4.1                    January 2010


   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 [11] 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).





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   /* 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.





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   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




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RFC 5661                         NFSv4.1                    January 2010


   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.




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   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
   Section 5.2.2, "Context Creation Requests", in [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 [12]) 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.



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RFC 5661                         NFSv4.1                    January 2010


   o  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.

   o  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]).

   o  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.






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RFC 5661                         NFSv4.1                    January 2010


2.10.11.2.  Obligations of the Client



   The client SHOULD honor the following obligations in order to utilize
   the session:

   o  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.

   o  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.

   o  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.

   o  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.






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   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.





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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).

2.10.13.1.2.  Connection Loss


   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.



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2.10.13.1.4.  Loss of Session


   The replier might lose a record of the session.  Causes include:

   o  Replier failure and restart.

   o  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.

   o  The server purges the session of a client that has been inactive
      for a very extended period of time.

   o  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.



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   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 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



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   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.







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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.

2.10.13.2.5.  GSS Context Loss


   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.





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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 [13].

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.

   o  NFS4_FHSIZE is the maximum size of a filehandle.

   o  NFS4_VERIFIER_SIZE is the fixed size of a verifier.

   o  NFS4_OPAQUE_LIMIT is the maximum size of certain opaque
      information.

   o  NFS4_SESSIONID_SIZE is the fixed size of a session identifier.

   o  NFS4_INT64_MAX is the maximum value of a signed 64-bit integer.




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   o  NFS4_UINT64_MAX is the maximum value of an unsigned 64-bit
      integer.

   o  NFS4_INT32_MAX is the maximum value of a signed 32-bit integer.

   o  NFS4_UINT32_MAX is the maximum value of an unsigned 32-bit
      integer.

   o  NFS4_MAXFILELEN is the maximum length of a regular file.

   o  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).                                          |



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   | 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 [14] 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.               |
   +---------------+---------------------------------------------------+

                          End of Base Data Types

                                  Table 1







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3.3.  Structured Data Types



3.3.1.  nfstime4



   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.

3.3.2.  time_how4



   enum time_how4 {
           SET_TO_SERVER_TIME4 = 0,
           SET_TO_CLIENT_TIME4 = 1
   };

3.3.3.  settime4



   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.




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3.3.4.  specdata4



   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.

3.3.5.  fsid4



   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.

3.3.7.  fattr4



   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).





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   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.

3.3.9.  netaddr4



   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 r_netid and r_addr fields respectively contain a
   netid and uaddr.  The netid and uaddr concepts are defined in [15].
   The netid and uaddr formats for TCP over IPv4 and TCP over IPv6 are
   defined in [15], specifically Tables 2 and 3 and Sections 5.2.3.3 and
   5.2.3.4.

3.3.10.  state_owner4



   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).






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3.3.10.1.  open_owner4



   This data type is used to identify the owner of OPEN state.

3.3.10.2.  lock_owner4



   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.

3.3.12.  stateid4



   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.

3.3.13.  layouttype4



   enum layouttype4 {
           LAYOUT4_NFSV4_1_FILES   = 0x1,
           LAYOUT4_OSD2_OBJECTS    = 0x2,
           LAYOUT4_BLOCK_VOLUME    = 0x3
   };




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   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 22.4; 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 [40], is to be used.  Similarly, the LAYOUT4_BLOCK_VOLUME
   enumeration specifies that the block/volume layout, as defined in
   [41], is to be used.

3.3.14.  deviceid4



   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.

3.3.15.  device_addr4



   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.






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   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.

3.3.17.  layout4



   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.

3.3.18.  layoutupdate4



   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



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   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.

3.3.19.  layouthint4



   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.

3.3.20.  layoutiomode4



   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.

3.3.21.  nfs_impl_id4



   struct nfs_impl_id4 {
           utf8str_cis   nii_domain;
           utf8str_cs    nii_name;
           nfstime4      nii_date;
   };




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   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:
























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   +-------------------------+---+---------+---------------------------+
   | 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.          |
   +-------------------------+---+---------+---------------------------+

3.3.23.  mdsthreshold4



   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.





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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 [31]), 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 [42] and RFC 2055 [43].  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.




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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



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   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:








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   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]




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   o  slot is an index in the server volatile filehandle table

   o  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:





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             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          |
        +----------+-----------+---------------------------------+



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   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



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   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
   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.






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   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:

   o  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.

   o  If OPENATTR is done on a named attribute directory or on a named
      attribute, the server MUST return NFS4ERR_WRONG_TYPE.

   o  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.

   o  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 22.1 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.

   o  The per-server attribute is:

         lease_time



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   o  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

   o  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 2.  The meaning of
   the columns of the table are:

   o  Name: The name of the attribute.

   o  Id: The number assigned to the attribute.  In the event of
      conflicts between the assigned number and [13], the latter is
      likely authoritative, but should be resolved with Errata to this
      document and/or [13].  See [44] for the Errata process.

   o  Data Type: The XDR data type of the attribute.



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   o  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).

   o  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 2

5.7.  RECOMMENDED Attributes - List and Definition References



   The RECOMMENDED attributes are defined in Table 3.  The meanings of
   the column headers are the same as Table 2; see Section 5.6 for the
   meanings.

















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   +--------------------+----+----------------+-----+------------------+
   | 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 | *              | 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   |



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   | 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 3

   * fs_locations_info4

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:

   o  NF4REG designates a regular file.

   o  NF4DIR designates a directory.

   o  NF4BLK designates a block device special file.

   o  NF4CHR designates a character device special file.

   o  NF4LNK designates a symbolic link.



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   o  NF4SOCK designates a named socket special file.

   o  NF4FIFO designates a fifo special file.

   o  NF4ATTRDIR designates a named attribute directory.

   o  NF4NAMEDATTR designates a named attribute.

   Within the explanatory text and operation descriptions, the following
   phrases will be used with the meanings given below:

   o  The phrase "is a directory" means that the object's type attribute
      is NF4DIR or NF4ATTRDIR.

   o  The phrase "is a special file" means that the object's type
      attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.

   o  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.







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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).





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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.




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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 11.9 for more details.

5.8.2.13.  Attribute 67: fs_locations_info



   Full function file system location.  See Section 11.10 for more
   details.

5.8.2.14.  Attribute 61: fs_status



   Generic file system type information.  See Section 11.11 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.




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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.



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   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.







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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.







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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 [16], [17],
   [18].  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.





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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 [45]
   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.




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   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.






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   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 [19].  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.




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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.




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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.



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   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>;
   };




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   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.

   o  As long as retention is not enabled, the client is permitted to
      decrease the duration.

   o  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.

   o  While retention is enabled, attempts to disable retention or
      decrease the retention's duration MUST fail with the error
      NFS4ERR_INVAL.

   o  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.







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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.

6.1.  Goals



   ACLs and modes represent two well-established models for specifying
   permissions.  This section specifies requirements that attempt to
   meet the following goals:



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   o  If a server supports the mode attribute, it should provide
      reasonable semantics to clients that only set and retrieve the
      mode attribute.

   o  If a server supports ACL attributes, it should provide reasonable
      semantics to clients that only set and retrieve those attributes.

   o  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.

   o  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.

   o  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.

   o  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.

   o  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.





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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



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   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.

6.2.1.1.  ACE Type



   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.









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   +------------------------------+--------------+---------------------+
   | 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.           |
   +------------------------------+--------------+---------------------+

   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;





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   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.

6.2.1.3.  ACE Access Mask



   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





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   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





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      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





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      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



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         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




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         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.







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   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





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      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



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         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
   [20].  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.



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   This allows servers to support something close to traditional UNIX-
   like semantics, with ACE4_ADD_FILE taking the place of the write bit.

6.2.1.4.  ACE flag



   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.



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   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

   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.






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   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.

6.2.1.5.  ACE Who



   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 4

   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.




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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.







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   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.










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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:

   o  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.

   o  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.

   o  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).

   o  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.

   o  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.

   o  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



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   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.

6.3.2.1.  Discussion



   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.





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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.




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   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.





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   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.




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   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.)



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   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



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   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





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   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



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   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






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   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.






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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)






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   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) [46].  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:

   o  clear division between client and server

   o  ability to reliably detect inconsistency in state between client
      and server

   o  simple and robust recovery mechanisms






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   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



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   (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.

   o  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.

   o  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.

   o  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.





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      A stateid represents a single delegation held by a client for a
      particular filehandle.

   o  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.

   o  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.




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   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.







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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:

   o  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).

   o  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.

   o  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.

   o  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.



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   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.

   o  an index into a table of locking-state structures.

   o  a generation number that is incremented on each allocation of a
      table entry for a particular use.

   And then store in each table entry,

   o  the client ID with which the stateid is associated.



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   o  the current generation number for the (at most one) valid stateid
      sharing this index value.

   o  the filehandle of the file on which the locks are taken.

   o  an indication of the type of stateid (open, byte-range lock, file
      delegation, directory delegation, layout).

   o  the last "seqid" value returned corresponding to the current
      "other" value.

   o  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:

   o  If the "other" and "seqid" fields do not match a defined
      combination associated with a special stateid, the error
      NFS4ERR_BAD_STATEID is returned.







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   o  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.

   o  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.

   o  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.

   o  If the table index field is outside the range of the associated
      table, return NFS4ERR_BAD_STATEID.

   o  If the selected table entry is of a different generation than that
      specified in the incoming stateid, return NFS4ERR_BAD_STATEID.

   o  If the selected table entry does not match the current filehandle,
      return NFS4ERR_BAD_STATEID.

   o  If the client ID in the table entry does not match the client ID
      associated with the current session, return NFS4ERR_BAD_STATEID.

   o  If the stateid represents revoked state, then return
      NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or NFS4ERR_DELEG_REVOKED,
      as appropriate.

   o  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.

   o  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.



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   o  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.

   o  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).

   o  If the client holds a delegation for the file in question, the
      delegation stateid SHOULD be used.

   o  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.

   o  If there is no byte-range lock stateid, then the OPEN stateid for
      the open file in question SHOULD be used.

   o  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.






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   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).






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   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:

   o  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.

   o  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.




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   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).

   o  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.

   o  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.

   o  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.

   o  The status bit SEQ4_STATUS_LEASE_MOVE indicates that
      responsibility for lease renewal has been transferred to one or
      more new servers.

   o  The status bit SEQ4_STATUS_RESTART_RECLAIM_NEEDED indicates that
      due to server restart the client must reclaim locking state.

   o  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).




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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:

   o  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.

   o  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



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   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).






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   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).

8.4.2.1.  State Reclaim



   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 11.7.7 for a discussion of the
   analogous handling lock reclamation in the case of file systems
   transitioning from server to server.







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   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 11.7.7.

   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.



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   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
   [47].  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.






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   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:

   o  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.

   o  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, such trust does not have
   any negative consequences for security.  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



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   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



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   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:








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   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.




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   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:

   o  the co_ownerid field from the client_owner4 presented in the
      EXCHANGE_ID operation.

   o  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.

   o  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.





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   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



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   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.






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   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 tradeoffs 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



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   (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.

   o  SETCLIENTID since its function has been replaced by EXCHANGE_ID.

   o  SETCLIENTID_CONFIRM since client ID confirmation now happens by
      means of CREATE_SESSION.

   o  OPEN_CONFIRM because state-owner-based seqids have been replaced
      by the sequence ID in the SEQUENCE operation.

   o  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.

   o  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.

   o  Sequence ids used to sequence requests for a given state-owner and
      to provide retry protection, now provided via sessions.





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   o  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).




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   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



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   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).

   o  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.

   o  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.

   o  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.



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   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:

   o  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.

   o  A share reservation is requested that denies reading and/or
      writing and the corresponding operation is being performed.

   o  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,



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   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



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   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




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   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



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   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;




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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



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   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.





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   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.

   o  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.

   o  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.






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   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:

   o  Compatibility with a large range of server semantics.

   o  Providing the same caching benefits as previous versions of the
      NFS protocol when unable to support the more aggressive model.

   o  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.





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   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.

   o  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.




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   o  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.

   o  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:

   o  For GETATTR, see Section 18.7.4.

   o  For OPEN, see Section 18.16.4.

   o  For READ, see Section 18.22.4.

   o  For REMOVE, see Section 18.25.4.

   o  For RENAME, see Section 18.26.4.

   o  For SETATTR, see Section 18.30.4.

   o  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



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   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:

   o  client restart

   o  server restart

   o  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



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   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:

   o  Upon reclaim, a client reporting resources assigned to it by an
      earlier server instance must be granted those resources.

   o  The server has unquestionable authority to determine whether
      delegations are to be granted and, once granted, whether they are
      to be continued.

   o  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.





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   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.






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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.

   o  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.

   o  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



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      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:

   o  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.

   o  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



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   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



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   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:

   o  If GETATTR directed to two filehandles returns different values of
      the fsid attribute, then the filehandles represent distinct
      objects.

   o  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.

   o  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



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      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.

   o  If GETATTR directed to the two filehandles returns different
      values for the fileid attribute, then they are distinct objects.

   o  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:

   o  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.

   o  The client must have responded properly to previous recalls.

   o  There must be no current OPEN conflicting with the requested
      delegation.

   o  There should be no current delegation that conflicts with the
      delegation being requested.

   o  The probability of future conflicting open requests should be low
      based on the recent history of the file.

   o  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



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   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 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:

   o  the type of delegation (OPEN_DELEGATE_READ or
      OPEN_DELEGATE_WRITE).

   o  space limitation information to control flushing of data on close
      (OPEN_DELEGATE_WRITE delegation only; see Section 10.4.1)

   o  an nfsace4 specifying read and write permissions

   o  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.









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   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.

   o  The access and deny bits for the request and the file as described
      in Section 9.7.

   o  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:

   o  If the nfsace4 indicates that the open may be done, then it should
      be granted without reference to the server.

   o  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,



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   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 recalled 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.



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   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.




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   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:

   o  The value of the change attribute will be obtained from the server
      and cached.  Let this value be represented by c.

   o  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.

   o  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:

   o  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.




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   o  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.

   o  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.

   o  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:












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       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:

   o  potentially conflicting OPEN request (or a READ or WRITE operation
      done with a special stateid)

   o  SETATTR sent by another client

   o  REMOVE request for the file

   o  RENAME request for the file as either the source or target of the
      RENAME





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   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:

   o  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.

   o  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.)

   o  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.

   o  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.

   o  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.

   o  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



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      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




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   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.






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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,




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   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.




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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.

   o  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.

   o  An upper time boundary is maintained on how long a client cache
      entry can be kept without being refreshed from the server.






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   o  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 [18].  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




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   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:

   o  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.

   o  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.

   o  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



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      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.

   o  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.





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   Given the above issues, the following are permitted:

   o  Clients and servers MAY deny memory mapping a file for which they
      know there are byte-range locks.

   o  Clients and servers MAY deny a byte-range lock on a file they know
      is memory-mapped.

   o  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.

10.8.1.  Name Caching



   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



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   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





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   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:

   o  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.

   o  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.





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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





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   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 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.






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   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.

   o  For CREATE, see Section 18.4.4.

   o  For LINK, see Section 18.9.4.

   o  For OPEN, see Section 18.16.4.

   o  For REMOVE, see Section 18.25.4.

   o  For RENAME, see Section 18.26.4.

   o  For SETATTR, see Section 18.30.4.







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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 RECOMMENDED 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.
   Use of multi-server namespaces can provide many advantages, however,
   by separating a file system's logical position in a namespace from
   the (possibly changing) logistical and administrative considerations
   that result in particular file systems being located on particular
   servers.

11.1.  Location Attributes



   NFSv4.1 contains RECOMMENDED attributes that allow file systems on
   one server to be associated with one or more instances of that file
   system on other servers.  These attributes specify such file system
   instances by specifying a server address target (either as a DNS name
   representing one or more IP addresses or as a literal IP address)
   together with the path 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.  This attribute
   provides to the client, in addition to information about file system
   instance locations, significant information about the various file
   system instance choices (e.g., priority for use, writability,
   currency, etc.).  It also includes information to help the client
   efficiently effect as seamless a transition as possible among
   multiple file system instances, when and if that should be necessary.








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   The fs_locations RECOMMENDED attribute is inherited from NFSv4.0 and
   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 to be preferred.

11.2.  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





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   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.

11.3.  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.

11.3.1.  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.






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   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.

11.3.2.  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:

   o  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.





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   o  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.

   o  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.

   o  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.

11.4.  Uses of Location Information



   The location-bearing attributes (fs_locations and fs_locations_info),
   together with the possibility of absent file systems, provide a
   number of important facilities in providing reliable, manageable, and
   scalable data access.

   When a file system is present, these attributes can provide
   alternative locations, 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 file system
   impossible or otherwise impractical.  Under some circumstances,
   multiple alternative locations may be used simultaneously to provide
   higher-performance access to the file system in question.  Provision
   of such alternate locations is referred to as "replication" although
   there are cases in which replicated sets of data are not in fact
   present, and the replicas are instead different paths to the same
   data.

   When a file system is present and becomes absent, clients can be
   given the opportunity to have continued access to their data, at an
   alternate location.  In this case, a continued attempt to use the
   data in the now-absent file system will result in an NFS4ERR_MOVED
   error and, at that point, the successor locations (typically only one
   although multiple choices are possible) can be fetched and used to
   continue access.  Transfer of the file system contents to the new
   location is referred to as "migration", but it should be kept in mind
   that there are cases in which this term can be used, like
   "replication", when there is no actual data migration per se.




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   Where a file system was not previously present, 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 location, in place of an absent file system, is
   called a "referral".

   Because client support for location-related attributes is OPTIONAL, a
   server may (but is not required 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).

11.4.1.  File System Replication



   The fs_locations and fs_locations_info attributes provide alternative
   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 value of the set of alternate locations
   by interrogating the fs_locations or fs_locations_info attribute,
   with the latter being preferred.

   In the event that 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.  Depending on
   specific attributes of these alternate locations, as indicated within
   the fs_locations_info attribute, multiple locations may be used
   simultaneously, to provide higher performance through the
   exploitation of multiple paths between client and target file system.

   The alternate locations may be physical replicas of the (typically
   read-only) file system data, or they may reflect alternate paths to
   the same server or 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 these different modes
   of file system transition are 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 below.

   Multiple server addresses, whether they are derived from a single
   entry with a DNS name representing a set of IP addresses or from
   multiple entries each with its own server address, may correspond to
   the same actual server.  The fact that two addresses correspond to
   the same server is shown by a common so_major_id field within the
   eir_server_owner field returned by EXCHANGE_ID (see Section 18.35.3).





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   For a detailed discussion of how server address targets interact with
   the determination of server identity specified by the server owner
   field, see Section 11.5.

11.4.2.  File System Migration



   When a file system is present and becomes absent, clients can be
   given the opportunity to have continued access to their data, at an
   alternate location, as specified by the fs_locations or
   fs_locations_info attribute.  Typically, a client will be accessing
   the file system in question, get an NFS4ERR_MOVED error, and then use
   the fs_locations or fs_locations_info attribute to determine the new
   location of 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.

   Such migration can be helpful in providing load balancing or general
   resource reallocation.  The protocol does not specify how the file
   system will be moved between servers.  It is anticipated that a
   number of different server-to-server transfer mechanisms might be
   used with the choice left to the server implementor.  The NFSv4.1
   protocol specifies the method used to communicate the migration event
   between client and server.

   The new location may be an alternate communication path to the same
   server or, in the case of various forms of server clustering, another
   server providing access to the same physical file system.  The
   client's responsibilities in dealing with this transition depend on
   the specific nature of the new access path as well as how and whether
   data was in fact migrated.  These issues will be discussed in detail
   below.

   When multiple server addresses correspond to the same actual server,
   as shown by a common value for the so_major_id field of the
   eir_server_owner field returned by EXCHANGE_ID, the location or
   locations may designate alternate server addresses in the form of
   specific server network addresses.  These can be used to access the
   file system in question at those addresses and when it is no longer
   accessible at the original address.

   Although a single successor location is typical, multiple locations
   may be provided, together with information that allows priority among
   the choices to be indicated, via information in the fs_locations_info
   attribute.  Where suitable, clustering mechanisms make it possible to
   provide multiple identical file systems or paths to them; this allows
   the client the opportunity to deal with any resource or
   communications issues that might limit data availability.




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   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.

11.4.3.  Referrals



   Referrals provide a way of placing a file system in a location within
   the namespace essentially without respect to its physical location on
   a given server.  This allows a single server or a set of servers to
   present a multi-server namespace that encompasses file systems
   located on multiple servers.  Some likely uses of this include
   establishment of site-wide or organization-wide namespaces, or even
   knitting such together into a truly global namespace.

   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 by receiving the error NFS4ERR_MOVED, the actual location
   or locations of the file system can be determined by fetching the
   fs_locations or fs_locations_info attribute.

   The locations-related 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.

   When the fs_locations_info attribute indicates that there are
   multiple possible targets listed, the relationships among them may be
   important to the client in selecting which one to use.  The same
   rules specified in Section 11.4.1 defining the appropriate standards



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   for the data propagation 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 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 location.
   However, there are facilities provided that allow different clients
   to be directed to different sets of data, so as to adapt to such
   client characteristics as CPU architecture.

11.5.  Location Entries and Server Identity



   As mentioned above, a single location entry may have a server address
   target in the form of a DNS name that may represent multiple IP
   addresses, while multiple location entries may have their own server
   address targets that reference the same server.  Whether two IP
   addresses designate the same server is indicated by the existence of
   a common so_major_id field within the eir_server_owner field returned
   by EXCHANGE_ID (see Section 18.35.3), subject to further verification
   (for details see Section 2.10.5).

   When multiple addresses for the same server exist, the client may
   assume that for each file system in the namespace of a given server
   network address, there exist file systems at corresponding namespace
   locations for each of the other server network addresses.  It may do
   this even in the absence of explicit listing in fs_locations and
   fs_locations_info.  Such corresponding file system locations can be
   used as alternate locations, just as those explicitly specified via
   the fs_locations and fs_locations_info attributes.  Where these
   specific addresses are explicitly designated in the fs_locations_info



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   attribute, the conditions of use specified in this attribute (e.g.,
   priorities, specification of simultaneous use) may limit the client's
   use of these alternate locations.

   If a single location entry designates multiple server IP addresses,
   the client cannot assume that these addresses are multiple paths to
   the same server.  In most cases, they will be, but the client MUST
   verify that before acting on that assumption.  When two server
   addresses are designated by a single location entry and they
   correspond to different servers, this normally indicates some sort of
   misconfiguration, and so the client should avoid using such location
   entries when alternatives are available.  When they are not, clients
   should pick one of IP addresses and use it, without using others that
   are not directed to the same server.

11.6.  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




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   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.

11.7.  Effecting File System Transitions



   Transitions between file system instances, whether due to switching
   between replicas upon server unavailability or to server-initiated
   migration events, are best dealt with together.  This is so even
   though, for the server, pragmatic considerations will normally force
   different implementation strategies for planned and unplanned
   transitions.  Even though the prototypical use cases of replication
   and migration contain distinctive sets of features, when all
   possibilities for these operations are considered, there is an
   underlying unity of these operations, from the client's point of
   view, that makes treating them together desirable.

   A number of methods are possible for servers to replicate data and to
   track client state in order to allow clients to transition between
   file system instances with a minimum of disruption.  Such methods
   vary between those that use inter-server clustering techniques to
   limit the changes seen by the client, to those that are less
   aggressive, use more standard methods of replicating data, and impose
   a greater burden on the client to adapt to the transition.

   The NFSv4.1 protocol does not impose choices on clients and servers
   with regard to that spectrum of transition methods.  In fact, there
   are many valid choices, depending on client and application
   requirements and their interaction with server implementation
   choices.  The NFSv4.1 protocol does define the specific choices that
   can be made, how these choices are communicated to the client, and
   how the client is to deal with any discontinuities.

   In the sections below, references will be made to various possible
   server implementation choices as a way of illustrating the transition
   scenarios that clients may deal with.  The intent here is not to
   define or limit server implementations but rather to illustrate the
   range of issues that clients may face.

   In the discussion below, references will be made to a file system
   having a particular property or to two file systems (typically the
   source and destination) belonging to a common class of any of several
   types.  Two file systems that belong to such a class share some
   important aspects of file system behavior that clients may depend
   upon when present, to easily effect a seamless transition between
   file system instances.  Conversely, where the file systems do not





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   belong to such a common class, the client has to deal with various
   sorts of implementation discontinuities that may cause performance or
   other issues in effecting a transition.

   Where the fs_locations_info attribute is available, such file system
   classification data will be made directly available to the client
   (see Section 11.10 for details).  When only fs_locations is
   available, default assumptions with regard to such classifications
   have to be inferred (see Section 11.9 for details).

   In cases in which one server is expected to accept opaque values from
   the client that originated from another server, the servers SHOULD
   encode the "opaque" values in big-endian byte order.  If this is
   done, servers acting as replicas or immigrating file systems will be
   able to parse values like stateids, directory cookies, filehandles,
   etc., even if their native byte order is different from that of other
   servers cooperating in the replication and migration of the file
   system.

11.7.1.  File System Transitions and Simultaneous Access



   When a single file system may be accessed at multiple locations,
   either because of an indication of file system identity as reported
   by the fs_locations or fs_locations_info attributes or because two
   file system instances have corresponding locations on server
   addresses that connect to the same server (as indicated by a common
   so_major_id field in the eir_server_owner field returned by
   EXCHANGE_ID), the client will, depending on specific circumstances as
   discussed below, either:

   o  Access multiple instances simultaneously, each of which represents
      an alternate path to the same data and metadata.

   o  Access one instance (or set of instances) and then transition to
      an alternative instance (or set of instances) as a result of
      network issues, server unresponsiveness, or server-directed
      migration.  The transition may involve changes in filehandles,
      fileids, the change attribute, and/or locking state, depending on
      the attributes of the source and destination file system
      instances, as specified in the fs_locations_info attribute.

   Which of these choices is possible, and how a transition is effected,
   is governed by equivalence classes of file system instances as
   reported by the fs_locations_info attribute, and for file system
   instances in the same location within a multi-homed single-server
   namespace, as indicated by the value of the so_major_id field of the
   eir_server_owner field returned by EXCHANGE_ID.




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11.7.2.  Simultaneous Use and Transparent Transitions



   When two file system instances have the same location within their
   respective single-server namespaces and those two server network
   addresses 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), those file system instances can be treated
   as the same, and either used together simultaneously or serially with
   no transition activity required on the part of the client.  In this
   case, we refer to the transition as "transparent", and the client in
   transferring access from one to the other is acting as it would in
   the event that communication is interrupted, with a new connection
   and possibly a new session being established to continue access to
   the same file system.

   Whether simultaneous use of the two file system instances is valid is
   controlled by whether the fs_locations_info attribute shows the two
   instances as having the same simultaneous-use class.  See
   Section 11.10.1 for information about the definition of the various
   use classes, including the simultaneous-use class.

   Note that for two such file systems, 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
   instances 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
   this as a transparent transition.  The server SHOULD NOT indicate
   that these instances belong to different handle, fileid, write-
   verifier, change, and readdir classes, whether or not the two
   instances are shown belonging to the same simultaneous-use class.

   Where these conditions do not apply, a non-transparent file system
   instance transition is required with the details depending on the
   respective handle, fileid, write-verifier, change, and readdir
   classes of the two file system instances, and whether the two
   servers' addresses in question have the same eir_server_scope value
   as reported by EXCHANGE_ID.

11.7.2.1.  Simultaneous Use of File System Instances



   When the conditions in Section 11.7.2 hold, in either of the
   following two cases, the client may use the two file system instances
   simultaneously.

   o  The fs_locations_info attribute does not contain separate per-
      network-address entries for file system instances at the distinct
      network addresses.  This includes the case in which the



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      fs_locations_info attribute is unavailable.  In this case, the
      fact that the two server addresses connect to the same server (as
      indicated by the two addresses sharing the same the so_major_id
      value and subsequently confirmed as described in Section 2.10.5)
      justifies simultaneous use, and there is no fs_locations_info
      attribute information contradicting that.

   o  The fs_locations_info attribute indicates that two file system
      instances belong to the same simultaneous-use class.

   In this case, the client may use both file system instances
   simultaneously, as representations of the same file system, whether
   that happens because the two network addresses connect to the same
   physical server or because different servers connect to clustered
   file systems and export their data in common.  When simultaneous use
   is in effect, any change made to one file system instance must be
   immediately reflected in the other file system instance(s).  Locks
   are treated as part of a common lease, associated with a common
   client ID.  Depending on the details of the eir_server_owner returned
   by EXCHANGE_ID, the two server instances may be accessed by different
   sessions or a single session in common.

11.7.2.2.  Transparent File System Transitions



   When the conditions in Section 11.7.2.1 hold and the
   fs_locations_info attribute explicitly shows the file system
   instances for these distinct network addresses as belonging to
   different simultaneous-use classes, the file system instances should
   not be used by the client simultaneously.  Rather, they should be
   used serially with one being used unless and until communication
   difficulties, lack of responsiveness, or an explicit migration event
   causes another file system instance (or set of file system instances
   sharing a common simultaneous-use class) to be used.

   When a change of file system instance is to be done, the client will
   use the same client ID already in effect.  If the client already has
   connections to the new server address, these will be used.
   Otherwise, new connections to existing sessions or new sessions
   associated with the existing client ID are established as indicated
   by the eir_server_owner returned by EXCHANGE_ID.

   In all such transparent transition cases, the following apply:

   o  If filehandles are persistent, they stay the same.  If filehandles
      are volatile, they either stay the same or expire, but the reason
      for expiration is not due to the file system transition.

   o  Fileid values do not change across the transition.



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   o  The file system will have the same fsid in both the old and new
      locations.

   o  Change attribute values are consistent across the transition and
      do not have to be refetched.  When change attributes indicate that
      a cached object is still valid, it can remain cached.

   o  Client and state identifiers retain their validity across the
      transition, except where their staleness is recognized and
      reported by the new server.  Except where such staleness requires
      it, no lock reclamation is needed.  Any such staleness is an
      indication that the server should be considered to have restarted
      and is reported as discussed in Section 8.4.2.

   o  Write verifiers are presumed to retain their validity and can be
      used to compare with verifiers returned by COMMIT on the new
      server.  If COMMIT on the new server returns an identical
      verifier, then it is expected that the new server has all of the
      data that was written unstably to the original server and has
      committed that data to stable storage as requested.

   o  Readdir cookies are presumed to retain their validity and can be
      presented to subsequent READDIR requests together with the readdir
      verifier with which they are associated.  When the verifier is
      accepted as valid, the cookie will continue the READDIR operation
      so that the entire directory can be obtained by the client.

11.7.3.  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





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   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.

11.7.4.  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 will be consistent across the transition, while in the
   analogous multi-vendor transitions they will 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.




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   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.

11.7.5.  Fsids and File System Transitions



   Since fsids are generally only unique within a per-server basis, it
   is likely that they will change during a file system transition.  One
   exception is the case of transparent transitions, but in that case we
   have multiple network addresses that are defined as the same server
   (as specified by a common value of the so_major_id field of
   eir_server_owner).  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.

11.7.5.1.  File System Splitting



   When a file system transition is made and the fs_locations_info
   indicates that the file system in question may 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 may 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 may 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.



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11.7.6.  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.

11.7.7.  Lock State and File System Transitions



   In a file system transition, the client needs to handle cases in
   which the two servers have cooperated in state management and in
   which they have not.  Cooperation by two servers in state management
   requires coordination of client IDs.  Before the client attempts to
   use a client ID associated with one server in a request to the server
   of the other file system, it must eliminate the possibility that two
   non-cooperating servers have assigned the same client ID by accident.
   The client needs to compare the eir_server_scope values returned by
   each server.  If the scope values do not match, then the servers have
   not cooperated in state management.  If the scope values match, then
   this indicates the servers have cooperated in assigning client IDs to
   the point that they will reject client IDs that refer to state they
   do not know about.  See Section 2.10.4 for more information about the
   use of server scope.

   In the case of migration, the servers involved in the migration of a
   file system SHOULD transfer all server state from the original to the
   new server.  When this is done, it must be done in a way that is
   transparent to the client.  With replication, such a degree of common
   state is typically not the case.  Clients, however, should use the
   information provided by the eir_server_scope returned by EXCHANGE_ID
   (as modified by the validation procedures described in
   Section 2.10.4) to determine whether such sharing may be in effect,
   rather than making assumptions based on the reason for the
   transition.




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   This state transfer will reduce disruption to the client when a file
   system transition occurs.  If the servers are successful in
   transferring all state, the client can attempt to establish sessions
   associated with the client ID used for the source file system
   instance.  If the server accepts that as a valid client ID, then the
   client may use the existing stateids associated with that client ID
   for the old file system instance in connection with that same client
   ID in connection with the transitioned file system instance.  If the
   client in question already had a client ID on the target system, it
   may interrogate the stateid values from the source system under that
   new client ID, with the assurance that if they are accepted as valid,
   then they represent validly transferred lock state for the source
   file system, which has been transferred to the target server.

   When the two servers belong to the same server scope, it does not
   mean that when dealing with the transition, the client will not have
   to reclaim state.  However, it does mean that the client may proceed
   using its current client ID when establishing communication with the
   new server, and the new server will either recognize the client ID as
   valid or reject it, in which case locks must be reclaimed by the
   client.

   File systems cooperating in state management may actually share state
   or simply divide the identifier space so as to recognize (and reject
   as stale) each other's stateids and client IDs.  Servers that do
   share state may not do so under all conditions or at all times.  If
   the server cannot be sure when accepting a client ID that it reflects
   the locks the client was given, the server must treat all associated
   state as stale and report it as such to the client.

   When the two file system instances are on servers that do not share a
   server scope value, the client must establish a new client ID on the
   destination, if it does not have one already, and reclaim locks if
   allowed by the server.  In this case, old stateids and client IDs
   should not be presented to the new server since there is no assurance
   that they will not conflict with IDs valid on that server.  Note that
   in this case, lock reclaim may be attempted even when the servers
   involved in the transfer have different server scope values (see
   Section 8.4.2.1 for the contrary case of reclaim after server
   reboot).  Servers with different server scope values may cooperate to
   allow reclaim for locks associated with the transfer of a file system
   even if they do not cooperate sufficiently to share a server scope.

   In either case, when actual locks are not known to be maintained, the
   destination server may establish a grace period specific to the given
   file system, with non-reclaim locks being rejected for that file
   system, even though normal locks are being granted for other file




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   systems.  Clients should not infer the absence of a grace period for
   file systems being transitioned to a server from responses to
   requests for other file systems.

   In the case of lock reclamation for a given file system after a file
   system transition, edge conditions can arise similar to those for
   reclaim after server restart (although in the case of the planned
   state transfer associated with migration, these can be avoided by
   securely recording lock state as part of state migration).  Unless
   the destination server can guarantee that locks will not be
   incorrectly granted, the destination server should not allow lock
   reclaims and should avoid establishing a grace period.

   Once all locks have been reclaimed, or there were no locks to
   reclaim, the client indicates that there are no more reclaims to be
   done for the file system in question by sending a RECLAIM_COMPLETE
   operation with the rca_one_fs parameter set to true.  Once this has
   been done, non-reclaim locking operations may be done, and any
   subsequent request to do reclaims will be rejected with the error
   NFS4ERR_NO_GRACE.

   Information about client identity may be propagated between servers
   in the form of client_owner4 and associated verifiers, under the
   assumption that the client presents the same values to all the
   servers with which it deals.

   Servers are encouraged to provide facilities to allow locks to be
   reclaimed on the new server after a file system transition.  Often,
   however, in cases in which the two servers do not share a server
   scope value, such facilities may not be available and the client
   should be prepared to re-obtain locks, even though it is possible
   that the client may have its LOCK or OPEN request denied due to a
   conflicting lock.

   The consequences of having no facilities available to reclaim locks
   on the new server will depend on the type of environment.  In some
   environments, such as the transition between read-only file systems,
   such denial of locks should not pose large difficulties in practice.
   When an attempt to re-establish a lock on a new server is denied, the
   client should treat the situation as if its original lock had been
   revoked.  Note that when the lock is granted, the client cannot
   assume that no conflicting lock could have been granted in the
   interim.  Where change attribute continuity is present, the client
   may check the change attribute to check for unwanted file
   modifications.  Where even this is not available, and the file system
   is not read-only, a client may reasonably treat all pending locks as
   having been revoked.




Shepler, et al.              Standards Track                  [Page 247]

RFC 5661                         NFSv4.1                    January 2010


11.7.7.1.  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