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rfc:rfc4331

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

                                                                  OSAF
                                                         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).

Abstract

 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
 repositories.

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

Korver & Dusseault Standards Track [Page 1] RFC 4331 WebDAV Quotas February 2006

1. Introduction

1.1. Notational Conventions

 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 [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.

Korver & Dusseault Standards Track [Page 2] 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

Korver & Dusseault Standards Track [Page 3] RFC 4331 WebDAV Quotas February 2006

 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
 repository.
 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.

Korver & Dusseault Standards Track [Page 4] 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

 Request:
       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:">
         <D:prop>
           <D:quota-available-bytes/>
           <D:quota-used-bytes/>
         </D:prop>
       </D:propfind>
 Response:
       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:response>
         <D:href>http://www.example.com/~milele/public/</D:href>
         <D:propstat>
           <D:prop>
             <D:quota-available-bytes>596650</D:quota-available-bytes>
             <D:quota-used-bytes>403350</D:quota-used-bytes>
           </D:prop>
           <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
       </D:response>
       </D:multistatus>

Korver & Dusseault Standards Track [Page 5] RFC 4331 WebDAV Quotas February 2006

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.
 Preconditions:
 (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:">
      <quota-not-exceeded/>
    </error>
 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
    accounting.
 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.

Korver & Dusseault Standards Track [Page 6] RFC 4331 WebDAV Quotas February 2006

 o  Resource bodies can be compressed.
 o  Some resources may be stored for "free", not counting against
    quota.
 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.

Korver & Dusseault Standards Track [Page 7] 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
 document.

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
            2002.

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.

Korver & Dusseault Standards Track [Page 8] RFC 4331 WebDAV Quotas February 2006

Authors' Addresses

 Brian Korver
 Network Resonance, Inc.
 2483 E. Bayshore Road
 Suite 212
 Palo Alto, CA  94303
 US
 Phone: +1 650 812-7705
 EMail: briank@networkresonance.com
 Lisa Dusseault
 Open Source Applications Foundation
 543 Howard Street
 5th Floor
 San Francisco, CA  94105
 US
 Phone: +1 415 946-3040
 EMail: lisa@osafoundation.org

Korver & Dusseault Standards Track [Page 9] RFC 4331 WebDAV Quotas February 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Korver & Dusseault Standards Track [Page 10]

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