PATCH Method for HTTP
draft-dusseault-http-patch-16

Network Working Group                                       L. Dusseault
Internet-Draft                                                Linden Lab
Intended status: Standards Track                                J. Snell
Expires: May 29, 2010                                  November 25, 2009

                         PATCH Method for HTTP
                     draft-dusseault-http-patch-16

Abstract

   Several applications extending the Hypertext Transfer Protocol (HTTP)
   require a feature to do partial resource modification.  The existing
   HTTP PUT method only allows a complete replacement of a document.
   This proposal adds a new HTTP method, PATCH, to modify an existing
   HTTP resource.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on May 29, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

Dusseault & Snell         Expires May 29, 2010                  [Page 1]
Internet-Draft                 HTTP PATCH                  November 2009

   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The PATCH Method . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  A simple PATCH example . . . . . . . . . . . . . . . . . .  5
     2.2.  Error handling . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Advertising Support in OPTIONS . . . . . . . . . . . . . . . .  7
     3.1.  The Accept-Patch Header  . . . . . . . . . . . . . . . . .  7
     3.2.  Example OPTIONS Request and Response . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  The 'Accept-Patch' Response Header . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Changes . . . . . . . . . . . . . . . . . . . . . . . 10
     B.1.  Changes from -00 . . . . . . . . . . . . . . . . . . . . . 10
     B.2.  Changes from -01 . . . . . . . . . . . . . . . . . . . . . 10
     B.3.  Changes from -02 . . . . . . . . . . . . . . . . . . . . . 10
     B.4.  Changes from -03 . . . . . . . . . . . . . . . . . . . . . 11
     B.5.  Changes from -04 . . . . . . . . . . . . . . . . . . . . . 11
     B.6.  Changes from -05 . . . . . . . . . . . . . . . . . . . . . 11
     B.7.  Changes from -06 . . . . . . . . . . . . . . . . . . . . . 11
     B.8.  Changes from -07 . . . . . . . . . . . . . . . . . . . . . 11
     B.9.  Changes from -08 . . . . . . . . . . . . . . . . . . . . . 12
     B.10. Changes from -09 . . . . . . . . . . . . . . . . . . . . . 12
     B.11. Changes from -10 . . . . . . . . . . . . . . . . . . . . . 12
     B.12. Changes from -11 . . . . . . . . . . . . . . . . . . . . . 13
     B.13. Changes from -12 . . . . . . . . . . . . . . . . . . . . . 13
     B.14. Changes from -13 . . . . . . . . . . . . . . . . . . . . . 13
     B.15. Changes from -14 . . . . . . . . . . . . . . . . . . . . . 13
     B.16. Changes from -15 . . . . . . . . . . . . . . . . . . . . . 14
   Appendix C.  Notes to RFC Editor . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

Dusseault & Snell         Expires May 29, 2010                  [Page 2]
Internet-Draft                 HTTP PATCH                  November 2009

1.  Introduction

   This specification defines the new HTTP/1.1 [RFC2616] method PATCH
   that is used to apply partial modifications to a resource.

   A new method is necessary to improve interoperability and prevent
   errors.  The PUT method is already defined to overwrite a resource
   with a complete new body, and can not be reused to do partial
   changes.  Otherwise, proxies and caches and even clients and servers
   may get confused as to the result of the operation.  POST is already
   used but without broad interoperability (for one, there is no
   standard way to discover patch format support).  PATCH was mentioned
   in earlier HTTP specifications, but not completely defined.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].

   Furthermore, this document uses the ABNF syntax defined in Section
   2.1 of [RFC2616].

2.  The PATCH Method

   The PATCH method requests that a set of changes described in the
   request entity be applied to the resource identified by the Request-
   URI.  The set of changes is represented in a format called a "patch
   document" identified by a media type.  If the Request-URI does not
   point to an existing resource, the server MAY create a new resource,
   depending on the patch document type (whether it can logically modify
   a null resource) and permissions etc.

   The difference between the PUT and PATCH requests is reflected in the
   way the server processes the enclosed entity to modify the resource
   identified by the Request-URI.  In a PUT request, the enclosed entity
   is considered to be a modified version of the resource stored on the
   origin server and the client is requesting that the stored version be
   replaced.  With PATCH, however, the enclosed entity contains a set of
   instructions describing how a resource currently residing on the
   origin server should be modified to produce a new version.  The PATCH
   method affects the resource identified by the Request-URI, and also
   MAY have side effects on other resources; i.e., new resources may be
   created, or existing ones modified, by the application of a PATCH.

   PATCH is neither safe or idempotent as defined by [RFC2616], Section
   9.1.

   A PATCH request can be issued in such a way as to be idempotent,

Dusseault & Snell         Expires May 29, 2010                  [Page 3]
Internet-Draft                 HTTP PATCH                  November 2009

   which also helps prevent bad outcomes from collisions between two
   PATCH requests on the same resource in a similar timeframe.
   Collisions from multiple PATCH requests may be more dangerous than
   PUT collisions, because some patch formats need to operate from a
   known base point or else corrupt the resource.  Clients using this
   kind of patch application SHOULD acquire a strong ETag [RFC2616] for
   the resource to be modified, and use that ETag in the If-Match header
   on the PATCH request to verify that the resource is still unchanged.
   If a strong ETag is not available for a given resource, the client
   can use If-Unmodified-Since as a less-reliable safeguard.

   There are also cases where patch formats do not need to operate from
   a known base-point (e.g. appending text lines to log files, or non-
   colliding rows to database tables), in which case the same care in
   client requests is not needed.

   The server MUST apply the entire set of changes atomically and never
   provide (e.g. in response to a GET during this operation) a
   partially-modified representation.  If the entire patch document
   cannot be successfully applied then the server MUST NOT apply any of
   the changes.  The determination of what constitutes a successful
   PATCH can vary depending on the patch document and the type of
   resource(s) being modified.  For example, the common 'diff' utility
   can generate a patch document that applies to multiple files in a
   directory hierarchy.  The atomicity requirement holds for all
   directly affected files.  See Error Handling in Section 2.2 for
   details on status codes and possible error conditions.

   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale.  A response to this method is only cacheable if it
   contains explicit freshness information (such as an Expires header or
   "Cache-Control: max-age" directive) as well as the Content-Location
   header matching the request-URI, indicating that the PATCH response
   body is a resource representation.  A cached PATCH response can only
   be used to respond to subsequent GET and HEAD requests; it MUST NOT
   be used to respond to other methods (in particular, PATCH).

   Note that entity-headers contained in the request apply only to the
   contained patch document and MUST NOT be applied to the resource
   being modified.  Thus, a Content-Language header could be present on
   the request but it would only mean (for whatever that's worth) that
   the patch document had a language.  Servers SHOULD NOT store such
   headers except as trace information, and SHOULD NOT use such header
   values the same way they might be used on PUT requests.  Therefore,
   this document does not specify a way to modify a document's Content-
   Type or Content-Language value through headers, though a mechanism
   could well be designed to achieve this goal through a patch document.

Dusseault & Snell         Expires May 29, 2010                  [Page 4]
Internet-Draft                 HTTP PATCH                  November 2009

   There is no guarantee that a resource can be modified with PATCH.
   Further, it is expected that different patch document formats will be
   appropriate for different types of resources and that no single
   format will be appropriate for all types of resources.  Therefore,
   there is no single default patch document format that implementations
   are required to support.  Servers MUST ensure that a received patch
   document is appropriate for the type of resource identified by the
   Request-URI.

   Clients need to choose when to use PATCH rather than PUT.  For
   example, if the patch document size is larger than the size of the
   new resource data that would be used in a PUT, then it might make
   sense to use PUT instead of PATCH.  A comparison to POST is even more
   difficult, because POST is used in widely varying ways and can
   encompass PUT and PATCH-like operations if the server chooses.  If
   the operation does not modify the resource identified by the Request-
   URI in a predictable way, POST should be considered instead of PATCH
   or PUT.

2.1.  A simple PATCH example

   PATCH /file.txt HTTP/1.1
   Host: www.example.com
   Content-Type: application/example
   If-Match: "e0023aa4e"
   Content-Length: 100

   [description of changes]

   This example illustrates use of a hypothetical patch document on an
   existing resource.  The 204 response code is used because the
   response does not have a body (a response with the 200 code would
   have a body) but other success codes can be used if appropriate.

   Successful PATCH response to existing text file

   HTTP/1.1 204 No Content
   Content-Location: /file.txt
   ETag: "e0023aa4f"

2.2.  Error handling

   There are several known conditions under which a PATCH request can
   fail.

Dusseault & Snell         Expires May 29, 2010                  [Page 5]
Internet-Draft                 HTTP PATCH                  November 2009

   Malformed patch document:  When the server determines that the patch
      document provided by the client is not properly formatted, it
      SHOULD return a 400 (Bad Request) response.  The definition of
      badly formatted depends on the patch document chosen.
   Unsupported patch document:  Can be specified using a 415
      (Unsupported Media Type) when the client sends a patch document
      format that the server does not support for the resource
      identified by the Request-URI.  Such a response SHOULD include an
      Accept-Patch response header as described in Section 3.1 to notify
      the client what patch document media types are supported.
   Unprocessable request:  Can be specified with a 422 (Unprocessable
      Entity) ([RFC4918], Section 11.2) when the server understands the
      patch document and the syntax of the patch document appears valid,
      but the server is incapable of processing the request.  This might
      include attempts to modify a resource in a way that would cause
      the resource to become invalid: for instance, a modification to a
      well-formed XML document that would cause it to no longer be well-
      formed.  There may also be more specific errors like "Conflicting
      State" that could be signaled with this status code, but the more
      specific error would generally be more helpful.
   Resource Not Found:  Can be specified with a 404 (Not Found) status
      code, when the client attempted to apply a patch document to a
      non-existent resource, but the patch document chosen cannot be
      applied to a non-existent resource.
   Conflicting State:  Can be specified with a 409 (Conflict) when the
      request cannot be applied given the state of the resource.  For
      example, if the client attempted to apply a structural
      modification and the structures assumed to exist did not exist
      (with XML, a patch might specify changing element 'foo' to element
      'bar' but element 'foo' might not exist).
   Conflicting modification:  When a client uses either the If-Match or
      If-Unmodified-Since header to define a precondition, and that
      precondition failed, then the 412 (Precondition Failed) error is
      most helpful to the client.  However, that response makes no sense
      if there was no precondition on the request.  In cases when the
      server detects a possible conflicting modification and no
      precondition was defined in the request, the server can return a
      409 (Conflict) response.
   Concurrent modification:  Some applications of PATCH might require
      the server to process requests in the order in which they are
      received.  If a server is operating under those restrictions, and
      it receives concurrent requests to modify the same resource, but
      is unable to queue those requests, the server can usefully
      indicate this error by using a 409 (Conflict) response.

   Note that the 409 Conflict response gives reasonably consistent
   information to clients.  Depending on the application and the nature
   of the patch format, the client might be able to reissue the request

Dusseault & Snell         Expires May 29, 2010                  [Page 6]
Internet-Draft                 HTTP PATCH                  November 2009

   as is (e.g. an instruction to append a line to a log file), or it
   might have to retrieve the resource content to recalculate a patch,
   or it might have to fail the operation.

   Other HTTP status codes can also be used under the appropriate
   circumstances.

   The entity body of error responses SHOULD contain enough information
   to communicate the nature of the error to the client.  The content-
   type of the response entity can vary across implementations.

3.  Advertising Support in OPTIONS

   A server can advertise its support for the PATCH method by adding it
   to the listing of allowed methods in the "Allow" OPTIONS response
   header defined in HTTP/1.1.  The PATCH method MAY appear in the
   "Allow" header even if the Accept-Patch header is absent, in which
   case the list of allowed patch documents is not advertised.

3.1.  The Accept-Patch Header

   This specification introduces a new response header "Accept-Patch"
   used to specify the patch document formats accepted by the server.
   "Accept-Patch" SHOULD appear in the OPTIONS response for any resource
   that supports the use of the PATCH method.  The presence of the
   "Accept-Patch" header in response to any method is an implicit
   indication that PATCH is allowed on the resource identified by the
   Request-URI.  The presence of a specific patch document format in
   this header indicates that specific format is allowed on the resource
   identified by the Request-URI.

   Accept-Patch = "Accept-Patch" ":" 1#media-type

   The Accept-Patch header specifies a comma separated listing of media-
   types as defined by [RFC2616], Section 3.7.

3.2.  Example OPTIONS Request and Response

   [request]

   OPTIONS /example/buddies.xml HTTP/1.1
   Host: www.example.com

Dusseault & Snell         Expires May 29, 2010                  [Page 7]
Internet-Draft                 HTTP PATCH                  November 2009

   [response]

   HTTP/1.1 200 OK
   Allow: GET, PUT, POST, OPTIONS, HEAD, DELETE, PATCH
   Accept-Patch: application/example, text/example

   The examples show a server that supports PATCH generally using two
   hypothetical patch document formats.

4.  IANA Considerations

4.1.  The 'Accept-Patch' Response Header

   The 'Accept-Patch' response header should be added to the permanent
   registry (see [RFC3864]).

   Header field name:  Accept-Patch
   Applicable Protocol:  HTTP
   Author/Change controller:  IETF
   Specification document:  this specification

5.  Security Considerations

   The security considerations for PATCH are nearly identical to the
   security considerations for PUT ([RFC2616], Section 9.6).  These
   include authorizing requests (possibly through access control and/or
   authentication) and ensuring that data is not corrupted through
   transport errors or through accidental overwrites.  Whatever
   mechanisms are used for PUT can be used for PATCH as well.  The
   following considerations apply specially to PATCH.

   A document that is patched might be more likely to be corrupted than
   a document that is overridden in entirety, but that concern can be
   addressed through the use of mechanisms such as conditional requests
   using ETags and the If-Match request header as described in
   Section 2.  If a PATCH request fails, the client can issue a GET
   request to the resource to see what state it is in.  In some cases,
   the client might be able to check the contents of the resource to see
   if the PATCH request can be resent, but in other cases the attempt
   will just fail and/or a user will have to verify intent.  In the case
   of a failure of the underlying transport channel, where a PATCH
   response is not received before the channel fails or some other
   timeout happens, the client might have to issue a GET request to see
   whether the request was applied.  The client might want to ensure
   that the GET request bypasses caches using mechanisms described in
   HTTP specifications (see for example Section 13.1.6 of [RFC2616]).

Dusseault & Snell         Expires May 29, 2010                  [Page 8]
Internet-Draft                 HTTP PATCH                  November 2009

   Sometimes an HTTP intermediary might try to detect viruses being sent
   via HTTP by checking the body of the PUT/POST request or GET
   response.  The PATCH method complicates such watch-keeping because
   neither the source document nor the patch document might be a virus,
   yet the result could be.  This security consideration is not
   materially different from those already introduced by byte-range
   downloads, downloading patch documents, uploading zipped (compressed)
   files and so on.

   Individual patch documents will have their own specific security
   considerations that will likely vary depending on the types of
   resources being patched.  The considerations for patched binary
   resources, for instance, will be different than those for patched XML
   documents.  Servers MUST take adequate precautions to ensure that
   malicious clients cannot consume excessive server resources (e.g.,
   CPU, disk I/O) through the client's use of PATCH.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

6.2.  Informative References

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

Appendix A.  Acknowledgements

   PATCH is not a new concept, it first appeared in HTTP in drafts of
   version 1.1 written by Roy Fielding and Henrik Frystyk and also
   appears in Section 19.6.1.1 of RFC 2068.

   Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm, Scott
   Lawrence, Jeffrey Mogul, Roy Fielding, Greg Stein, Jim Luther, Alex
   Rousskov, Jamie Lokier, Joe Hildebrand, Mark Nottingham, Michael

Dusseault & Snell         Expires May 29, 2010                  [Page 9]
Internet-Draft                 HTTP PATCH                  November 2009

   Balloni, Cyrus Daboo, Brian Carpenter, John Klensin, Eliot Lear and
   SM for review and advice on this document.

Appendix B.  Changes

B.1.  Changes from -00

   OPTIONS support: removed "Patch" header definition and used Allow and
   new "Accept-Patch" headers instead.

   Supported delta encodings: removed vcdiff and diffe as these do not
   have defined MIME types and did not seem to be strongly desired.

   PATCH method definition: Clarified cache behavior.

B.2.  Changes from -01

   Removed references to XCAP - not yet a RFC.

   Fixed use of MIME types (this "fix" now obsolete)

   Explained how to use MOVE or COPY in conjunction with PATCH, to
   create a new resource based on an existing resource in a different
   location.

B.3.  Changes from -02

   Clarified that MOVE and COPY are really independent of PATCH.

   Clarified when an ETag must change, and when Last-Modified must be
   used.

   Clarified what server should do if both Content-Type and IM headers
   appear in PATCH request.

   Filled in missing reference to DeltaV and ACL RFCs.

   Stopped using 501 Unsupported for unsupported delta encodings.

   Clarified what a static resource is.

   Refixed use of MIME types for patch formats.

   Limited the scope of some restrictions to apply only to usage of
   required diff format.

Dusseault & Snell         Expires May 29, 2010                 [Page 10]
Internet-Draft                 HTTP PATCH                  November 2009

B.4.  Changes from -03

   Various typographical, terminology consistency, and other minor
   clarifications or fixes.

B.5.  Changes from -04

   Moved paragraphs on ACL and RFC3229 interoperability to new section.

   Added security considerations.

   Added IANA considerations, registration of new namespace, and
   discontinued use of "DAV:" namespace for new elements.

   Added example of error response.

B.6.  Changes from -05

   Due to various concerns it didn't seem likely the application/gdiff
   registration could go through so switching to vcdiff as required diff
   format, and to RFC3229's approach to specifying diff formats,
   including use of the IM header.

   Clarified what header server MUST use to return MD5 hash.

   Reverted to using 501 Unsupported for unsupported delta encodings.

B.7.  Changes from -06

   The reliance on RFC 3229 defined patch documents has been factored
   out in favor of delta encodings identified by MIME media type.

   The required use of DeltaV-based error reporting has been removed in
   favor of using basic HTTP status codes to report error conditions.

   The Accept-Patch response header has been redefined as a listing of
   media-ranges, similar to the Accept request header.

   Added James Snell as a co-author.

B.8.  Changes from -07

   Terminology change from "delta encoding" to "patch document"

   Added clarification on the safety and idempotency of PATCH

   Updated the caching rules of PATCH responses

Dusseault & Snell         Expires May 29, 2010                 [Page 11]
Internet-Draft                 HTTP PATCH                  November 2009

   200 responses MUST include a representation of the modified resource.
   204 responses are used to indicate successful response without
   returning a representation.

   Suggest using 422 Unprocessable Entity to indicate that a properly
   formatted patch document cannot be processed

   Clarify the use of 412 and 409 to indicate concurrent and conflicting
   resource modifications.

   Added registration for the Accept-Patch header.

   Relaxed the requirements for the use of If-Match and If-Unmodified-
   Since.

   Add language that clarifies the difference between PUT and PATCH.

   Add language that clarifies the issues with PATCH and Content
   Negotiation.

   Use of Accept-Patch on any response implies that PATCH is supported.

   Add language advising caution when pipelining PATCH requests.

B.9.  Changes from -08

   Addition of the 209 Content Returned status code

   Addition of the Prefer header field mechanism

   Removed the paragraph discussing the use of 200+Content-Location.
   This is replaced by the 209 Content Returned status code.

B.10.  Changes from -09

   Move the prefer header to a separate document

   Restructure the document sections.

B.11.  Changes from -10

   Remove paragraph about pipelined requests.  This is covered
   adequately by RFC2616.

   Remove paragraph about content negotiation.  This is covered
   adequately by RFC2616.

   Explicitly indicate that PATCH can be used to create new resources.

Dusseault & Snell         Expires May 29, 2010                 [Page 12]
Internet-Draft                 HTTP PATCH                  November 2009

   Remove recommendation for servers to provide strong etags.  This is
   recommendation is implied and does not need to be explicitly.

   Change Allow-Patch to a listing of media-type and not media-range.

B.12.  Changes from -11

   Fix section links.

   State that this uses RFC2616-style ABNF.

   Fix grammar for Accept-Patch.

   Remove requirements for handling entity-headers on PATCH and replace
   with general discussion of issues and consequences of having no
   handling requirements.

   Update Security Considerations to make it clear what security
   considerations for PUT are, for comparison.

B.13.  Changes from -12

   Remove status 209 again.

   Add security consideration about using too much server resources.

   Remove Content-MD5 from example.

B.14.  Changes from -13

   Remove '*' value from Accept-Patch again.

   Allow caching but only if context is clear.

   Clarify how some patch formats might allow creating a new document.

   Add comparison of PATCH to POST

B.15.  Changes from -14

   Clarified that Accept-Patch header SHOULD appear in OPTIONS response
   -- it is not absolutely required

   Clarified how server can indicate that a PATCH response body is
   cachable as a resource representation.

   Removed suggestion that PATCH side-effects might be specified in the
   patch document specification -- this implied that side-effects could

Dusseault & Snell         Expires May 29, 2010                 [Page 13]
Internet-Draft                 HTTP PATCH                  November 2009

   exclusively be determined that way, but in fact side-effects are
   often determined by the server unilaterally.

B.16.  Changes from -15

   Clarifications on how conflicting PATCH requests can be avoided, and
   why not all use cases necessarily involve conflict

   Added Content-Location to example response, so the ETag would be
   legit

   Expanded security considerations on avoiding collisions, recovering
   from possible (unknown) collisions

   Very slight reordering of paragraphs in section 2, for better flow

   Clarified that the concurrent-modification status response is
   optional for servers, and explained what clients can do with that
   response

   Updated text describing conflicting modifications: when 412 is used,
   vs 409

Appendix C.  Notes to RFC Editor

   The RFC Editor should remove this section and the Changes section.

Authors' Addresses

   Lisa Dusseault
   Linden Lab
   945 Battery Street
   San Francisco, CA  94111
   USA

   Email: lisa.dusseault@gmail.com

   James M. Snell

   Email: jasnell@gmail.com
   URI:   http://www.snellspace.com

Dusseault & Snell         Expires May 29, 2010                 [Page 14]