WWW Distributed Authoring and B. Korver
Versioning (webdav) Xythos
Internet-Draft L. Dusseault
Expires: January 5, 2005 OSAF
July 7, 2004
Quota and Size Properties for DAV Collections
draft-ietf-webdav-quota-03
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
WebDAV servers are frequently deployed with quota (size) limitations.
This Internet-Draft discusses the properties and minor behaviors
needed for clients to interoperate with quota implementations on
WebDAV repositories.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Notational Conventions . . . . . . . . . . . . . . . . . . 3
1.2 Requirement for quotas . . . . . . . . . . . . . . . . . . 3
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. DAV:quota-available-bytes . . . . . . . . . . . . . . . . . . 4
4. DAV:quota-used-bytes . . . . . . . . . . . . . . . . . . . . . 5
5. DAV:quota-assigned-bytes . . . . . . . . . . . . . . . . . . . 6
5.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Example PROPFIND request and response . . . . . . . . . . . . 7
7. Error reporting . . . . . . . . . . . . . . . . . . . . . . . 8
8. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Internationalization Considerations . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
13.2 Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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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].
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. This specification requires WebDAV because
it requires PROPFIND support and relies on the WebDAV definition of
collections and properties, including the definitions for live and
protected properties.
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. www.example.com
and 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 quota-available
on collections. Further, client software should have some way of
fairly reliably determining how much storage space is already counted
towards that quota.
In addition to displaying the quota-available and quota-used on
collections, this specification does not forbid these properties on
any resource.
2. Solution Overview
The approach to meeting the requirements and scenarios outlined above
is to define three live properties. This specification can be met on
a server by implementing both quota-available and quota-used on
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collections only. Implementing both quota-available and quota- used
on all resources is RECOMMENDED.
None of these properties need be returned in a <DAV:allprop> request
though the server may include them. 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 quota-available and quota-used definitions below borrow heavily
from the quota definitions in the NFS [RFC3010] 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 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.
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 quota-available on a file or
directory approaches 0, further allocations to that file or directory
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 quota-
available, the server is at liberty to choose any of 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
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Found response in Multi-Status), although this specification
RECOMMENDS that servers return some appropriate value (e.g. the
amount of free disc 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. A 403 Forbidden response is
RECOMMENDED for attempts to write a protected property.
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 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 count against the quota-available.
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 quota-available.
Note that there may be a number of distinct but overlapping sets of
files or directories for which a quota-used 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.
Support for this property enhances the client experience, because
together with DAV:quota-available-bytes, 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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5. DAV:quota-assigned-bytes
Name: quota-assigned-bytes
Namespace: DAV:
Purpose: Indicates the amount of storage assigned.
DTD: <!ELEMENT quota-bytes (#PCDATA) >
The DAV:quota-assigned-bytes property value is the amount of storage
space potentially either assigned to or requested for this file or
directory, measured in octets. DAV:quota-assigned-bytes is primarily
intended to support implementations that allow quota to be
PROPPATCHed or configured by some other means.
The value of this property will usually be protected, although a user
with sufficient privileges may be permitted to change the value. The
property is useful even if it is protected. A 403 Forbidden response
is RECOMMENDED for attempts to write a protected property. This
property will usually be read-only for instance because a user's
quota should generally not configurable by other users.
Support for this property is OPTIONAL.
Note that a resource may show more quota-used than its quota-
assigned appears to allow, and that quota-available MUST never be
greater than the value of quota-assigned.
5.1 Example 1
As an example of quota-assigned, imagine a quota system where each
collection may have a quota assigned and where a resource contained
in a collection is subject to only the quota constraints of the
nearest collection which has a quota assigned. Assume the
administrator creates a collection A and gives it a quota-assigned of
300KB and then creates a subcollection B which is given
quota-assigned of 8000KB. In this case, the initial quota-available
for B is 8000KB, not 3000KB, since the constraint on A is ignored in
favor of the constraint on B.
DAV:quota-assigned-bytes DAV:quota-available-bytes
/A 300KB 300KB
/A/B 8000KB 8000KB
Note that this is only one example quota system, and that other quota
systems are possible, such as one described in the example below.
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5.2 Example 2
As another example of quota-assigned, imagine a quota system where
each collection may have a quota assigned and where a resource
contained in a collection is subject to the quota constraints of all
parent collections. Assume the administrator creates a collection A
and gives it a quota-assigned of 500KB and then creates a
subcollection B which is given quota-assigned of 1000KB. In this
case, the initial quota-available for B is 500KB, not 1000KB, since
the constraint on A applies to B as well.
DAV:quota-assigned-bytes DAV:quota-available-bytes
/A 500KB 500KB
/A/B 1000KB 500KB
6. Example PROPFIND request and response
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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</quota-used-bytes>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
</D:multistatus>
7. 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) is forbidden because it would exceed
their allotted quota. 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 <DAV:storage-quota-reached/>
precondition tag.
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Example error response:
HTTP/1.1 507 Insufficient Storage
Content-Length: 100
Content-Type: text/xml
<?xml version="1.0">
<error xmlns="DAV:">
<storage-quota-reached/>
</error>
8. 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.
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
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systems ([RFC3253]).
o The total size of a collection, DAV:quota-used-bytes, is not
necessarily a sum of the DAV:getcontentlength properties for
resources stored in the collection.
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
its 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.
9. 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.
If a server chooses to make the DAV:quota-assigned-bytes writable by
clients with sufficient authorization, then it is opening up a
certain amount of near-administration functionality to clients.
However, it is not required for the DAV:quota-assigned-bytes property
to be writeable by any clients, so a server can easily avoid this
consideration.
10. Internationalization Considerations
Quota is counted in Arabic numerals expressed in strings. There are
no internationalization considerations.
11. IANA Considerations
There are no IANA considerations.
12. Acknowledgements
Stefan Eissing, Jim Luther, Julian Reschke, and Jim Whitehead and
provided valuable comments on this document.
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13. References
13.1 Normative References
[RFC2026] Bradner, S., "The Internet Standards Process - Revision
3", RFC 2026, October 1996.
[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.
13.2 Informative References
[RFC3010] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M. and D. Noveck, "NFS version 4
Protocol", RFC 3010, December 2000.
Authors' Addresses
Brian Korver
Xythos Software
One Bush Street
Suite 600
San Francisco, CA 94104
US
Phone: +1 415 248-3800
EMail: briank@xythos.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
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