Individual Submission                                       L. Dusseault
Internet-Draft                                                      OSAF
Expires: October 22, 2004                                 April 23, 2004

            Partial Document Changes (PATCH Method) for HTTP

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
   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://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on October 22, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   Several applications extending HTTP require a feature to do partial
   resource modification.  Existing HTTP functionality only allows a
   complete replacement of a document. This proposal adds a new HTTP
   method, PATCH, to modify an existing HTTP resource.

Dusseault               Expires October 22, 2004                [Page 1]

Internet-Draft                 HTTP PATCH                     April 2004

1.  Introduction

   Three use cases initially motivated this proposal

   1.  WebDAV [2] is used by authoring applications to store and share
       files on the internet.  For example, Adobe Photoshop has a
       Workgroup feature allowing the user to browse a repository and
       save the file.  Currently, Photoshop only publishes the file to
       the repository rarely, because Photoshop files are typically
       large and upload is slow.  Worse, large uploads are more likely
       to be interrupted.  Although HTTP [3] provides byte range
       downloads, it cannot provide this simple a mechanism for uploads.
   2.  DeltaV extends WebDAV to do versioning.  In versioning
       environments, a large number of files may be updated with very
       small changes.  For example, a programmer may change the name of
       a function used in a hundred source files.  Versioning
       applications typically send deltas or 'diffs' to the server to
       modify these files, however DetaV does not yet have this
   3.  The SIMPLE WG is devising a way to store and modify configuration
       information. The biggest feature missing from HTTP is the ability
       to modify information in a very lightweight manner, so that the
       client that decides to change its presence state from "free" to
       "busy" doesn't have to upload a large document. This can be
       accomplished through changes to a HTTP resource as well.

   Other working groups (like netconf) are also considering manipulating
   large files using HTTP GET and PUT. Sometimes the files aren't that
   large but the device is small or bandwidth is limited, as when phones
   need to add a new contact to an address book file. This feature would
   allow much more efficient changes to files.

   This specification defines a new HTTP 1.1 method for patches.  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 MUST 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.

   Note that byte ranges are already used in HTTP to do partial
   downloads (GET method).   However, they are not defined for uploads,
   and there are some missing pieces for uploads. For example, the HTTP
   specification has no way for the server to send errors if the byte
   range in a PUT is invalid. Byte ranges (or some other kind of range)
   could be made to work in this specification but a more flexible
   mechanism (one that could also encompass XML diffs) was desired, as
   well as a method that would not confuse caching proxies.  Reliable
   and tested patch algorithms already exist as defined MIME [1]

Dusseault               Expires October 22, 2004                [Page 2]

Internet-Draft                 HTTP PATCH                     April 2004

   document types, and this specification takes advantage of that
   existing work.

   Other delta encodings are defined for HTTP in RFC 3229 [4]. That
   standard defines delta encodings for cache updates, not for user
   write operations.  It does mean that servers can reuse delta format
   algorithms to support both that standard and this proposal.  That
   standard does not use MIME types to identify the delta algorithm, but
   the mapping is trivial.

Dusseault               Expires October 22, 2004                [Page 3]

Internet-Draft                 HTTP PATCH                     April 2004

2.  Mechanisms

2.1  PATCH Method

   The PATCH method requests that the request body (a patch document) be
   applied to the resource named in the Request-URI.  The resource named
   in the Request-URI MUST already exist (the server MUST NOT create a
   new resource with the body of the PATCH method).  The target
   resource's content type MUST be one to which the patch format
   applies.  See error handling section for details on status codes and
   possible error conditions.

   PATCH request bodies MUST NOT be cached.  A cache MAY mark the
   resource identified in the Request-URI as stale if it sees a
   successful response to the PATCH request.

   The PATCH request MUST have a body.  It MUST include the Content-Type
   header with a value indicating what the body type is.  It MUST be a
   format that has the semantics of defining a change to an existing
   document (such as gdiff).

   The PATCH request is subject to access control, which in turn may
   require authentication.  The PATCH request SHOULD be subject to the
   same access control permissions as the PUT request.

   The PATCH request MUST only be used in a context which ensures that
   only one user may apply a patch at a time.  There are two reliable
   ways to do this. The first way is to find out the resource ETag at
   the time the body is downloaded, and use that Etag in the PATCH
   request to make sure the resource is still unchanged.  The second way
   to use WebDAV LOCK/UNLOCK to reserve the file (first LOCK, then GET,
   then PATCH, then UNLOCK).  PATCH collisions from multiple users are
   more dangerous than PUT collisions, because a PATCH that is not
   operating from a known base point may corrupt the resource.
   Therefore, if neither strong ETags nor LOCKS are available from the
   server, the client MUST use If-Last-Modified as a less-reliable

Dusseault               Expires October 22, 2004                [Page 4]

Internet-Draft                 HTTP PATCH                     April 2004

   Simple PATCH example

       PATCH /file.txt HTTP/1.1
       Content-Type: application/gdiff
       If-Match: "e0023aa4e"
       Content-Length: 100

       0xd1, 0xff, 0xd1, 0xff

                                Figure 1

   This example illustrates use of the platform-portable 'gdiff'
   algorithm as one possible patch format.  In this case the resource is
   a text file.

2.2  PATCH Response

2.2.1  Success Response

   The basic success response code for PATCH is 204 No Content. For this
   new method, 200 OK is not used because 200 OK implies a body in the
   response, and 201 Created is not used because the resource must
   already exist.

   The server SHOULD provide a MD5 hash of the content after the delta
   was applied.  This allows the client to verify the success of the
   operation.  The PATCH method obviously MUST cause the ETag to change.
   So, if the server supports ETags, the server MUST return a strong
   ETag for use in future client operations.  If the server does not
   support strong ETags, then the server MUST return the Last-Modified
   header instead.

   Successful PATCH response

       HTTP/1.1 204 No Content
       Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
       ETag: "e0023aa4e"

Dusseault               Expires October 22, 2004                [Page 5]

Internet-Draft                 HTTP PATCH                     April 2004

2.2.2  Error handling

   This proposal uses the same mechanism as DeltaV to add much-needed
   info to base HTTP error responses.  Existing HTTP status codes are
   not infinitely extensible but XML elements and namespaces are more
   so, and it's simple to treat the HTTP error code as a rough category
   and put detailed error codes in the body.

   The PATCH method can return the following errors. Please note that
   the notation "DAV:foobar" is merely short form for expressing "the
   'foobar' element in the 'DAV:' namespace".  It has meaning only in
   English, not on the wire.  Also note that the string error codes are
   not meant to be displayed but instead as machine parsable known error
   codes (thus there is no language code).
   DAV:delta-format-unsupported: Used with 501 Not Supported status
      code.  Returned by the server when it doesn't support the delta
      format chosen by the client.
   DAV:delta-format-forbidden-on-resource: Used with 403 Forbidden when
      the delta format chosen by the client is supported by the server
      but not allowed on this kind of resource.
   DAV:delta-format-badly-formatted: Used with 400 Bad Request when the
      server finds that the delta document provided by the client was
      badly formatted and non-compliant.
   DAV:delta-empty-resource: Used with 409 Conflict when the resource
      addressed in the Request-URI exists but is empty, and the delta
      format cannot be applied to an empty document.  Note that some
      delta formats may be applied to an empty document, in which case
      this error wouldn't be used.
   DAV:patch-result-invalid: Used with 409 Conflict when the resource
      could be patched but the result of the patch would be a resource
      which is invalid.  This could mean, for example, that a XML
      resource would become an invalid XML file if the patch specified
      that a close element text line should be deleted.

   "404 Not Found" is used with no body/error element when the URL in by
   the Request-URI does not map to a resource.

2.3  Delta Formats

   A set of changes for a resource is itself a document, called a change
   document or delta.  Every change document format must be a registered
   MIME type.  Servers advertise supported delta mechanisms by
   advertising these MIME types, and clients specify which one they're
   using by including the MIME type in the Content-Type header.

   This table outlines the delta format support requirements for a
   server supporting this proposal.

Dusseault               Expires October 22, 2004                [Page 6]

Internet-Draft                 HTTP PATCH                     April 2004

   Set of defined delta formats

       Format  Standard   MIME type/ Requirements
       GDIFF   [5]        application/gdiff
                           - MUST support for all document types except XML.
                           - MAY support for XML documents.
       XCAP    [6]        text/xcap+xml
                           - MAY support for XML documents.

   Note that support for all of XCAP is not required in order to do XML
   diffs.  However, XCAP is the only group defining an XML diff MIME
   document type at this time.  A more generic XML diff format could
   easily be used as long as it had a unique MIME type.

2.4  Advertising Support in OPTIONS

   The server advertises its support for the features described here
   with OPTIONS response headers.  The "Allow" OPTIONS header is already
   defined in HTTP 1.1  to contain all the allowable methods on the
   addressed resource, so it's natural to add PATCH.

   Clients also need to know whether the server supports special diff
   formats, so this document introduces a new OPTIONS response header
   "Accept-Patch". "Accept-Patch" MUST appear in the OPTIONS response
   for any resource where the PATCH method is shown as an allowed

   OPTIONS * presents a bit of a special case, as it does not address a
   resource, and does not always show all the server's features. In
   responses to OPTIONS *, a server supporting this specification SHOULD
   include the PATCH method in the "Allow" header and SHOULD show the
   union of all supported diff methods in the "Accept-Patch" header.

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

   Example: OPTIONS * request and response indicating Patch support


       OPTIONS * HTTP/1.1


       HTTP/1.1 200 OK
       Accept-Patch: application/gdiff, text/xcap+xml

Dusseault               Expires October 22, 2004                [Page 7]

Internet-Draft                 HTTP PATCH                     April 2004

   Example: OPTIONS request and response for specific resource


       OPTIONS /example/buddies.xml HTTP/1.1


       HTTP/1.1 200 OK
       Accept-Patch: text/xcap+xml

3  References

   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November

   [2]  Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. Jensen,
        "HTTP Extensions for Distributed Authoring -- WEBDAV", RFC 2518,
        February 1999.

   [3]  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.

   [4]  Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland,
        Y., van Hoff, A. and D. Hellerstein, "Delta encoding in HTTP",
        RFC 3229, January 2002.

   [5]  van Hoff, A. and J. Payne, "Generic Diff Format Specification",
        August 1997.

   [6]  Rosenberg, J., "The Extensible Markup Language (XML)
        Configuration Access Protocol (XCAP)", October 2003.

Dusseault               Expires October 22, 2004                [Page 8]

Internet-Draft                 HTTP PATCH                     April 2004

Author's Address

   Lisa Dusseault
   Open Source Application Foundation
   2064 Edgewood Dr.
   Palo Alto, CA  94303


Dusseault               Expires October 22, 2004                [Page 9]

Internet-Draft                 HTTP PATCH                     April 2004

Appendix A.  Acknowledgements

   Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm and
   Scott Lawrence for review and advice.

Dusseault               Expires October 22, 2004               [Page 10]

Internet-Draft                 HTTP PATCH                     April 2004

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 formats: 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.

Dusseault               Expires October 22, 2004               [Page 11]

Internet-Draft                 HTTP PATCH                     April 2004

Intellectual Property Statement

   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
   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

Full Copyright Statement

   Copyright (C) The Internet Society (2004). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an

Dusseault               Expires October 22, 2004               [Page 12]

Internet-Draft                 HTTP PATCH                     April 2004



   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Dusseault               Expires October 22, 2004               [Page 13]