B. Korver
Xythos
L. Dusseault
Xythos
Internet Draft C. Warner
Document: draft-ietf-webdav-quota-01.txt Netezza
Expires: September 2003 March 2003
Quota and Size Properties for DAV Collections
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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.
Table of Contents
Introduction.......................................................2
DAV:quota-limit-bytes..............................................3
DAV:quota-used-bytes...............................................3
DAV:quota-assigned-bytes...........................................4
Example PROPFIND request and response..............................5
Error reporting....................................................6
Notes..............................................................6
Security Considerations............................................7
Internationalization Considerations................................7
IANA Considerations................................................7
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Intellectual Property..............................................7
Acknowledgements...................................................8
References.........................................................9
Author's Addresses.................................................9
Introduction
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 RFC 2119 [RFC2119].
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.
- Sometimes the storage service charges according to quota
- Sometimes the storage service is provided free, but the storage
service provider has limited storage space (e.g. www.sharemation.com
and university-provided student accounts)
- 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-limit 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-limit and quota-used on
collections, this specification does not forbid these properties on
any resource.
Solution Overview
The approach to meeting the requirements and scenarios outlined
above is to define three live properties. This specification can be
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met on a server by implementing both quota-limit and quota-used on
collections only. Implementing both quota-limit 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 definitions below for quota-limit and quota-used borrow heavily
from the definition of quota in the NFS [RFC3010] specification.
DAV:quota-limit-bytes
Name: quota-limit-bytes
Namespace: DAV:
Purpose: Indicates the total amount of storage potentially
allocated.
DTD: <!ELEMENT quota-limit-bytes (#PCDATA) >
The DAV:quota-limit-bytes property value is the total amount of
storage space potentially allocated to this file or directory,
measured in octets.
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.
A value of 0 indicates that storage is limited to 0. 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.
If a resource has no quota enforced or unlimited storage, 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 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.
DAV:quota-used-bytes
Name: quota-used-bytes
Namespace: DAV:
Purpose: Contains the amount of storage counted against the quota-
limit of a resource.
DTD: <!ELEMENT quota-used-bytes (#PCDATA) >
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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
ôsimilarö meets at least the criterion that allocating space to any
file or directory in the set will count against the quota-limit. 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-limit.
Clients SHOULD expect that once the quota-used on a file or
directory meets or exceeds the quota-limit, further allocations to
that file or directory will be refused. A resource may show more
quota-used than its quota-limit or quota-assigned appears to allow.
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. ôall
files with a given ownerö, ôall files with a given group ownerö,
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, or may be ôchoose the set with the smallest quotaö.
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-limit-bytes property.
Support for this property enhances the client experience, because
together with DAV:quota-limit-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.
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.
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.
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-assigned MUST NOT be less
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than the quota-limit. Servers which receive a request to change
quota-assigned to a value less than quota-limit MUST reduce quota-
limit to this value at the same time.
For many quota systems, quota-assigned is synonymous with quota-
limit. However, in any system, quota-limit is a hard limit. For
example, 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 1,000,000 bytes and then creates a sub-collections B
which is given quota-assigned of 10,000,000 bytes. In this case,
the quota-limit for B is 1,000,000 bytes.
Example PROPFIND request and response
Request:
PROPFIND /~milele/public/ HTTP/1.1
Depth: 0
Host: www.sharemation.com
Content-Type: text/xml
Content-Length: xxx
<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
<D:prop><D:quota-limit-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.sharemation.com/~milele/public/</D:href>
<D:propstat>
<D:prop>
<D:quota-limit-bytes>1000000</D:quota-limit-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>
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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.
Example error response:
HTTP/1.1 507 Insufficient Storage
Content-Length: 100
Content-Type: text/xml
<?xml version=ö1.0ö>
<error xmlns=öDAV:ö>
<storage-quota-reached/>
</error>
Notes
Server implementations store and account for their data in many
different ways. Some of the challenges:
- Some server implementations find it prohibitive to count storage
used for metadata, others may choose to do so for better accounting.
- Older versions of resources may be stored as well.
- Variants of one resource may exist with different content lengths
- Content may be dynamically generated.
- Resource bodies can be compressed
- Some resources may be stored for ôfreeö, not counting against
quota.
Since server storage accounting can vary so much, clients should
expect the following:
- The size of a file on the clientÆs 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.
- 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 ôtrashö
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folder or ôrecycle binö, or retained as in versioning systems
[RFC3253].
- 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.
- 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.
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.
Internationalization Considerations
Quota is counted in Arabic numerals expressed in strings. There are
no internationalization considerations.
IANA Considerations
There are no IANA considerations.
Intellectual Property
The following notice is copied from [RFC2026], and describes the
position of the IETF concerning intellectual property claims made
against this document.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
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has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Acknowledgements
Jim Whitehead and Jim Luther provided valuable comments on this
document.
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References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2026] Bradner, S., ôThe Internet Standards Process û Revision
3ö, BCP 9, RFC2026, October 1996.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and
Jensen, D., "HTTP Extensions for Distributed Authoring --
WebDAV", RFC2518, February 1999.
[RFC3010] Shepler S., B. Callaghan, D. Robinson, R. Thurlow, C.
Beame, M. Eisler, D. Noveck, ôNFS version 4 Protocolö, RFC3010,
December 2000.
Author's Addresses
Brian Korver
Xythos Software, Inc.
77 Maiden Lane, Suite 200 Phone: 1-415-248-9033
San Francisco, CA, USA Email: briank@xythos.com
Lisa Dusseault
Xythos Software, Inc.
77 Maiden Lane, Suite 200 Phone: 1-415-248-9004
San Francisco, CA, USA Email: lisa@xythos.com
Clark Warner
Netezza Corporation
200 Crossing Blvd. Phone: 1-508-665-6800 x889
Framingham, MA 01702 Email: webdav@thewarners.com
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Datatracker
draft-ietf-webdav-quota-01
Internet-Draft
