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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
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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.
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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
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.
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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.
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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
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.
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Korver & Dusseault Standards Track [Page 10]