Internet DRAFT - draft-dusseault-dav-quota
draft-dusseault-dav-quota
L. Dusseault
Xythos
Internet Draft C. Warner
Document: draft-dusseault-dav-quota-01.txt Netezza
Expires: July 2002 January 2002
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.
Internet-Drafts are working documents of the Internet Engineering
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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].
Abstract
WebDAV servers are frequently deployed with collection quota (size)
limitations. This Internet-Draft discusses the two properties and
minor behaviors needed for clients to interoperate with quota
implementations on WebDAV repositories.
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
Requirement for quotas.............................................2
Implementation.....................................................2
DAV:quota-bytes....................................................2
DAV:space-used-bytes...............................................3
Error reporting....................................................5
Notes..............................................................5
References.........................................................7
Author's Addresses.................................................7
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Requirement for quotas
WebDAV servers based on [RFC2518] are already being implemented and
deployed with quota restrictions on collections, so it make 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 collection quotas
enforced are the same reasons why any storage system comes with
quotas.
- Sometimes the storage service charges according to quota
(XDrive.com and www.storeitonline.com have both operated under this
model)
- Sometimes the storage service is provided free, but the storage
service provider has limited storage space (www.sharemation.com has
this limitation and university-provided student accounts often do as
well)
- 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. IT departments within corporations may put quotas on
storage used by employees.
In order to work best with repositories that support quotas, client
software should be able to determine and display the quota (limit)
on any collection. Further, client software should have some way of
fairly reliably determining how much storage space is already
counted towards the quota.
Implementation
The approach to meeting the requirements and scenarios outlined
above is simply to define two live, protected properties. This
specification can be met on a server by implementing either or both
of the following two properties. Both are recommended.
Neither property need be returned in a <DAV:allprop> request though
the server may include either. However, these property names MUST
be returned in a <DAV:propname> request for a resource that supports
the properties.
DAV:quota-bytes
Name: quota-bytes
Namespace: DAV:
Purpose: Contains the amount of storage space allocated to a
collection, measured in bytes.
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DTD: <!ELEMENT quota-bytes (#PCDATA) >
The DAV:quota-bytes property value is the total amount of storage
space allocated to a collection and all its descendants. 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 value of 0 indicates that storage is limited to 0. Users will
probably not be able to add resources to the collection.
A negative value indicates that the quota is not limited on this
collection. A server SHOULD show a negative value rather than omit
this property, if it supports this property at all.
If the quota on a collection is reduced, the server MUST NOT delete
excess files except as selected by an administrator. Thus, a
collection may have more storage used than its quota allows.
Quota is not additive. A collection only has the quota assigned to
it, not (in addition) the quota assigned to sub collections or any
other collections. Thus, if a collection A contains collection B,
and both have quotas of 1,000,000 bytes, then collection A can
contain up to 1,000,000 bytes including the bytes used by resources
in B. Collection B, then, can only truly contain 1,000,000 bytes of
used storage if A is empty.
A quota on any collection must directly limit the storage used on
that collection. If a collection A contains collection B but
collection B's quota is smaller than collection A's quota, then
collection B's space is limited by the quota on B.
This property is OPTIONAL on collections and SHOULD NOT exist on
non-collection resources.
DAV:space-used-bytes
Name: space-used-bytes
Namespace: DAV:
Purpose: Contains the amount of storage space used (possibly counted
towards quota) by a collection, measured in bytes.
DTD: <!ELEMENT space-used-bytes (#PCDATA) >
The DAV:space-used-bytes value is calculated by the server to
indicate approximately how much storage space is already taken up by
this collection and all its descendants (in bytes). It MUST include
child collections and all resources inside those child collections.
It SHOULD include metadata storage size if metadata storage is
counted against quota limitations. Support for this property is
OPTIONAL on collections and SHOULD NOT exist on non-collection
resources. A server SHOULD implement this property to the best of
its ability if it implements the DAV:quota-bytes property.
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The value SHOULD be an accurate account of how the server counts
what is stored against quota. Servers MUST NOT implement this
property unless the value is at least as accurate as the client
could calculate themselves by summing values for ôgetcontentlengthö.
Support for this property enhances the client experience, because
together with DAV:quota-bytes, the client can manage its files to
avoid running out of allocated storage space. Clients may not be
able to calculate the value adequately on their own, depending on
how total space used is calculated by the server.
Note that it is entirely possible for a server to support DAV:space-
used-bytes without supporting DAV:quota-bytes or any kind of quota
system.
Example PROPFIND request and response
Request:
PROPFIND /~milele/public/ HTTP/1.1
Depth: 0
Host: www.sharemation.com
Content-Type: text/xml
Content-Length: 200
<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
<D:prop><D:quota-bytes><D:space-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-bytes>-1</D:quota-bytes>
<D:space-used-bytes>4403350</space-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 [RFC??] 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 property
value size, others may choose to close that loophole 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
Since server storage accounting can vary so much, clients should
expect the following:
- The size of a file on the client 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 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 has probably been moved to a ôtrashö folder or
ôrecycle binö.
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- The total size of a collection, DAV:space-used-bytes, is not
necessarily a sum of the DAV:getcontentlength properties for
resources stored in the collection.
- A quota on a collection may be larger than the quota on a
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 preferentially prefer to store files on 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:space-used-bytes may make it easier to detect tampering or
misuse.
If a server chooses to make the DAV:quota-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-bytes property to be writeable by
any clients, so a server can easily avoid this consideration.
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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
Jensen, D., "HTTP Extensions for Distributed Authoring --
WebDAV", RFC2518, February 1999.
[RFC??] Clemm, Geoff, Jim Amsden, Tim Ellison, Chris Kaler, Jim
Whitehead, "Versioning Extensions to WebDAV", RFC??, October
2001.
Author's Addresses
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
Suite 100
1671 Worcester Rd Phone: 1-508-665-6800 x889
Framingham, MA 01701 Email: webdav@thewarners.com
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