RFC 4331

Network Working Group                                          B. Korver
Request for Comments: 4331                             Network Resonance
Category: Standards Track                                   L. Dusseault
                                                           February 2006

                       Quota and Size Properties
       for Distributed Authoring and Versioning (DAV) Collections

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   Web Distributed Authoring and Versioning (WebDAV) servers are
   frequently deployed with quota (size) limitations.  This document
   discusses the properties and minor behaviors needed for clients to
   interoperate with quota (size) implementations on WebDAV

Table of Contents

   1. Introduction ....................................................2
      1.1. Notational Conventions .....................................2
      1.2. Requirement for Quotas .....................................2
   2. Solution Overview ...............................................3
   3. DAV:quota-available-bytes .......................................3
   4. DAV:quota-used-bytes ............................................4
   5. Example PROPFIND Request and Response ...........................5
   6. Error Reporting .................................................6
   7. Notes ...........................................................6
   8. Security Considerations .........................................8
   9. Internationalization Considerations .............................8
   10. Acknowledgements ...............................................8
   11. References .....................................................8
      11.1. Normative References ......................................8
      11.2. Informative References ....................................8

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RFC 4331                     WebDAV Quotas                 February 2006

1.  Introduction

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The definition of live property is provided in [RFC2518].  The
   definition of protected and computed properties is provided in
   [RFC3253], Section 1.4.

1.2.  Requirement for Quotas

   WebDAV servers based on [RFC2518] have been implemented and deployed
   with quota restrictions on collections and users, so it makes sense
   to standardize this functionality to improve user experience and
   client interoperability.

   The reasons why WebDAV servers frequently have quotas enforced are
   the same reasons why any storage system comes with quotas.

   o  Sometimes the storage service charges according to quota.

   o  Sometimes the storage service is provided free, but the storage
      service provider has limited storage space (e.g., university-
      provided student accounts).

   o  Even in cases where the storage can be upgraded, the storage
      managers may choose to limit quota in order to encourage users to
      limit the files they store on the system and to clean up obsolete
      files (e.g., IT departments within corporations).

   In order to work best with repositories that support quotas, client
   software should be able to determine and display the DAV:quota-
   available-bytes (defined below) on collections.  Further, client
   software should have some way of fairly reliably determining how much
   storage space is already counted towards that quota.

   Support for the properties defined in this document enhances the
   client experience, because the client has a chance of managing its
   files to avoid running out of allocated storage space.  Clients may
   not be able to calculate the value as accurately on their own,
   depending on how total space used is calculated by the server.

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RFC 4331                     WebDAV Quotas                 February 2006

2.  Solution Overview

   The approach to meeting the requirements and scenarios outlined above
   is to define two live properties.  This specification can be met on a
   server by implementing both DAV:quota-available-bytes and DAV:quota-
   used-bytes on collections only.

   A <DAV:allprop> PROPFIND request SHOULD NOT return any of the
   properties defined by this document.  However, these property names
   MUST be returned in a <DAV:propname> request for a resource that
   supports the properties, except in the case of infinite limits, which
   are explained below.

   The DAV:quota-available-bytes and DAV:quota-used-bytes definitions
   below borrow heavily from the quota definitions in the Network File
   System (NFS) [RFC3530] specification.

3.  DAV:quota-available-bytes

   Name: quota-available-bytes

   Namespace: DAV:

   Purpose: Indicates the maximum amount of additional storage available
      to be allocated to a resource.

   DTD: <!ELEMENT quota-available-bytes (#PCDATA) >

   The DAV:quota-available-bytes property value is the value in octets
   representing the amount of additional disk space beyond the current
   allocation that can be allocated to this resource before further
   allocations will be refused.  It is understood that this space may be
   consumed by allocations to other resources.

   Support for this property is REQUIRED on collections, and OPTIONAL on
   other resources.  A server SHOULD implement this property for each
   resource that has the DAV:quota-used-bytes property.

   Clients SHOULD expect that as the DAV:quota-available-bytes on a
   resource approaches 0, further allocations to that resource may be
   refused.  A value of 0 indicates that users will probably not be able
   to perform operations that write additional information (e.g., a PUT
   inside a collection), but may be able to replace through overwrite an
   existing resource of equal size.

   Note that there may be a number of distinct but overlapping limits,
   which may even include physical media limits.  When reporting DAV:
   quota-available-bytes, the server is at liberty to choose any of

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   those limits but SHOULD do so in a repeatable way.  The rule may be
   configured per repository, or may be "choose the smallest number".

   If a resource has no quota enforced or unlimited storage ("infinite
   limits"), the server MAY choose not to return this property (404 Not
   Found response in Multi-Status), although this specification
   RECOMMENDS that servers return some appropriate value (e.g., the
   amount of free disk space).  A client cannot entirely assume that
   there is no quota enforced on a resource that does not have this
   property, but might as well act as if there is no quota.

   The value of this property is protected (see Section 1.4.2 of
   [RFC3253] for the definition of protected properties).  A 403
   Forbidden response is RECOMMENDED for attempts to write a protected
   property, and the server SHOULD include an XML error body as defined
   by DeltaV [RFC3253] with the <DAV:cannot-modify-protected-property/>
   precondition tag.

4.  DAV:quota-used-bytes

   Name: quota-used-bytes

   Namespace: DAV:

   Purpose: Contains the amount of storage counted against the quota on
      a resource.

   DTD: <!ELEMENT quota-used-bytes (#PCDATA) >

   The DAV:quota-used-bytes value is the value in octets representing
   the amount of space used by this resource and possibly a number of
   other similar resources, where the set of "similar" meets at least
   the criterion that allocating space to any resource in the set will
   count against the DAV:quota-available-bytes.  It MUST include the
   total count including usage derived from sub-resources if
   appropriate.  It SHOULD include metadata storage size if metadata
   storage is counted against the DAV:quota-available-bytes.

   Note that there may be a number of distinct but overlapping sets of
   resources for which a DAV:quota-used-bytes 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 but
   SHOULD do so in a repeatable way.  The rule may be configured per

   Support for this property is REQUIRED on collections, and OPTIONAL on
   other resources.  A server SHOULD implement this property for each
   resource that has the DAV:quota-available-bytes property.

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RFC 4331                     WebDAV Quotas                 February 2006

   This value of this property is computed (see Section 1.4.3 of
   [RFC3253] for the definition of computed property).  A 403 Forbidden
   response is RECOMMENDED for attempts to write a protected property,
   and the server SHOULD include an XML error body as defined by DeltaV
   [RFC3253] with the <DAV:cannot-modify-protected-property/>
   precondition tag.

5.  Example PROPFIND Request and Response


         PROPFIND /~milele/public/ HTTP/1.1
         Depth: 0
         Host: www.example.com
         Content-Type: text/xml
         Content-Length: xxx

         <?xml version="1.0" ?>
         <D:propfind xmlns:D="DAV:">


         HTTP/1.1 207 Multi-Status
         Date: Tue, 16 Oct 2001 22:13:39 GMT
         Content-Length: xxx
         Content-Type: text/xml; charset=UTF-8

         <?xml version="1.0" encoding="utf-8" ?>
         <D:multistatus xmlns:D="DAV:">
             <D:status>HTTP/1.1 200 OK</D:status>

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6.  Error Reporting

   WebDAV [RFC2518] defines the status code 507 (Insufficient Storage).
   This status code SHOULD be used when a client request (e.g., a PUT,
   PROPFIND, MKCOL, MOVE, or COPY) fails because it would exceed their
   quota or physical storage limits.  In order to differentiate the
   response from other storage problems, the server SHOULD include an
   XML error body as defined by DeltaV [RFC3253] with the appropriate
   precondition tag.


   (DAV:quota-not-exceeded): the request MUST NOT cause the allocated
   quota to be exceeded.

   (DAV:sufficient-disk-space): there is sufficient physical space to
   execute the request.

   Example error response:

      HTTP/1.1 507 Insufficient Storage
      Content-Length: xxx
      Content-Type: text/xml

      <?xml version="1.0">
      <error xmlns="DAV:">

   Implementation note: some clients may be able to take advantage of
   the different precondition codes when mapping to operating system
   status codes, such as E_NOSPC and E_DQUOT in NFS (see [RFC3530],
   Section 12).

7.  Notes

   Server implementations store and account for their data in many
   different ways.  Some of the challenges:

   o  Some server implementations find it prohibitive to count storage
      used for metadata; others may choose to do so for better

   o  Older versions of resources may be stored as well.

   o  Variants of one resource may exist with different content lengths.

   o  Content may be dynamically generated.

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   o  Resource bodies can be compressed.

   o  Some resources may be stored for "free", not counting against

   Since server storage accounting can vary so much, clients should
   expect the following:

   o  The size of a file on the client's file system, or in a PUT
      message, may not correspond to the amount of storage required by
      the server to store the resource.  Thus, the client cannot predict
      with 100% accuracy whether a given file will be allowed given the
      storage quota.

   o  Deleting or overwriting a resource may not free up the same amount
      of storage as indicated by the DAV:getcontentlength property
      defined in [RFC2518] for the resource.  If deleting a resource
      does not free up any space, the file may have been moved to a
      "trash" folder or "recycle bin", or retained as in versioning
      systems ([RFC3253]).

   o  Since there are many factors that affect the storage used by a set
      of resources, including automatic compression, the size of
      associated metadata, and server-inserted content (such as that
      created by PHP code) in the on-the-wire representation of
      resources, clients are advised not to depend on the value of DAV:
      quota-used-bytes being the sum of the DAV:getcontentlength
      properties for resources contained by a collection.

   o  Additionally, because there may be a number of distinct but
      overlapping sets of resources for which a DAV:quota-used-bytes is
      maintained (Section 4), there may be no correlation between the
      size of the resources in a collection and DAV:quota-used-bytes.
      For example, for a server that implements user-based quotas,
      DAV:quota-used-bytes usually will be the same for a collection and
      its members.

   o  On some systems where quota is counted by collection and not by
      user, a quota on a sub-collection may be larger than the quota on
      the parent collection that contains it.  For example, the quota on
      /~milele/ may be 100 MB, but the quota on /~milele/public/ may be
      unlimited.  This allows the space used by /~milele/public/ to be
      as large as the quota on /~milele/ allows (depending on the other
      contents of /~milele/) even if the quota on /~milele/ is changed.
      Thus, even when the quota on a parent collection is changed, it is
      not necessarily required to change the quota on every child or
      descendant collection.

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RFC 4331                     WebDAV Quotas                 February 2006

8.  Security Considerations

   A hacker may prefer to store files in collections with a large quota.
   This isn't strictly a security concern because it doesn't make it any
   easier to store files.  On the other hand, the DAV:quota-used-bytes
   property may make it easier to detect tampering or misuse.

9.  Internationalization Considerations

   Quota is counted in Arabic numerals expressed in strings.  There are
   no internationalization considerations.

10.  Acknowledgements

   Stefan Eissing, Geoff Clemm, Jim Luther, Julian Reschke, and Jim
   Whitehead, among others, have provided valuable comments on this

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WebDAV", RFC 2518, February 1999.

   [RFC3253]  Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
              Whitehead, "Versioning Extensions to WebDAV (Web
              Distributed Authoring and Versioning)", RFC 3253, March

11.2.  Informative References

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.

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RFC 4331                     WebDAV Quotas                 February 2006

Authors' Addresses

   Brian Korver
   Network Resonance, Inc.
   2483 E. Bayshore Road
   Suite 212
   Palo Alto, CA  94303

   Phone: +1 650 812-7705
   EMail: briank@networkresonance.com

   Lisa Dusseault
   Open Source Applications Foundation
   543 Howard Street
   5th Floor
   San Francisco, CA  94105

   Phone: +1 415 946-3040
   EMail: lisa@osafoundation.org

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RFC 4331                     WebDAV Quotas                 February 2006

Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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