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Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Updates: 2616 (if approved)                             October 25, 2007
Intended status: Experimental
Expires: April 27, 2008


       The Hypertext Transfer Protocol (HTTP) Entity Tag ("ETag")
                  Response Header in Write Operations
                  draft-reschke-http-etag-on-write-08

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Hypertext Transfer Protocol (HTTP) specifies a state identifier,
   called "Entity Tag", to be returned in the "ETag" response header.
   However, the description of this header for write operations such as
   PUT is incomplete, and has caused confusion among developers and
   protocol designers, and potentially interoperability problems.




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   This document explains the problem in detail and suggests both a
   clarification for a revision to the HTTP/1.1 specification (RFC2616)
   and a new header for use in responses, making HTTP entity tags more
   useful for user agents that want to avoid round-trips to the server
   after modifying a resource.

Editorial Note (To be removed by RFC Editor before publication)

   Distribution of this document is unlimited.  Please send comments to
   the Hypertext Transfer Protocol (HTTP) mailing list at
   ietf-http-wg@w3.org [1], which may be joined by sending a message
   with subject "subscribe" to ietf-http-wg-request@w3.org [2].

   Discussions of the HTTP working group are archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   XML versions, latest edits and the issues list for this document are
   available from
   <http://greenbytes.de/tech/webdav/#draft-reschke-http-etag-on-write>.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Questions out of Scope . . . . . . . . . . . . . . . . . .  4
     1.2.  The Interoperability Problem . . . . . . . . . . . . . . .  4
     1.3.  Analysis of RFC2616's Definitions  . . . . . . . . . . . .  6
     1.4.  Prior Work . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  8
   3.  Clarifications on the Behavior of the 'ETag' Response
       Header . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  The 'Entity-Transform' Header  . . . . . . . . . . . . . . . .  9
     4.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  How This Helps . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Use Cases . . . . . . . . . . . . . . . . . . . . . . 12
     A.1.  Simple Authoring . . . . . . . . . . . . . . . . . . . . . 12
     A.2.  Simple Authoring, with Entity-Transform header . . . . . . 14
     A.3.  Setting both Content and Metadata  . . . . . . . . . . . . 15
     A.4.  Setting both Content and Metadata, new style . . . . . . . 17
   Appendix B.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 18
     B.1.  Since draft-reschke-http-etag-on-write-00  . . . . . . . . 18
     B.2.  Since draft-reschke-http-etag-on-write-01  . . . . . . . . 18
     B.3.  Since draft-reschke-http-etag-on-write-02  . . . . . . . . 19
     B.4.  Since draft-reschke-http-etag-on-write-03  . . . . . . . . 19
     B.5.  Since draft-reschke-http-etag-on-write-04  . . . . . . . . 19
     B.6.  Since draft-reschke-http-etag-on-write-05  . . . . . . . . 19
     B.7.  Since draft-reschke-http-etag-on-write-06  . . . . . . . . 19
     B.8.  Since draft-reschke-http-etag-on-write-07  . . . . . . . . 19
   Appendix C.  Open issues (to be removed by RFC Editor prior to
                publication)  . . . . . . . . . . . . . . . . . . . . 19
     C.1.  edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21













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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) specifies a state identifier,
   called "Entity Tag", to be returned in the "ETag" response header
   (see [RFC2616], Section 14.19).  However, the description of this
   header for write operations such as PUT is incomplete, and has caused
   confusion among developers and protocol designers, and potentially
   interoperability problems.

   This document explains the problem in detail and suggests both a
   clarification for a revision to [RFC2616] and a new header for use in
   responses, making HTTP entity tags more useful for user agents that
   want to avoid round-trips to the server after modifying a resource.

1.1.  Questions out of Scope

   Note that there is a related problem: modifying content-negotiated
   resources.  Here the consensus seems to be simply not to do it.
   Instead, the origin server should reveal specific URIs of content
   that is not content-negotiated in the Content-Location response
   header ([RFC2616], Section 14.14), and user agents should use this
   more specific URI for authoring.  Thus, the remainder of this
   document will focus on resources for which no content negotiation
   takes place.

   Another related question is the usability of the weak entity tags for
   authoring (see [RFC2616], Section 13.3.3).  Although this document
   focuses on the usage of strong entity tags, it is believed that the
   changes suggested in this document could be applied to weak entity
   tags as well.

1.2.  The Interoperability Problem

   For a long time, nobody realized that there was a problem at all, or
   those who realized preferred to ignore it.

   Server implementers added code that would return the new value of the
   "ETag" header in a response to a successful PUT request.  After all,
   the client could be interested in it.

   User agent developers in turn were happy to get a new "ETag" value,
   saving a subsequent HEAD request to retrieve the new entity tag.

   However, at some point of time, potentially during a Web Distributed
   Authoring and Versioning (WebDAV, [RFC2518], obsoleted by [RFC4918])
   interoperability event, client programmers asked server programmers
   to always return "ETag" headers upon PUT, never ever to change the
   entity tag without "good reason", and - by the way - always to



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   guarantee that the server stores the new content octet-by-octet.

   From the perspective of client software that wants to treat an HTTP
   server as a file system replacement, this makes a lot of sense.
   After all, when one writes to a file one usually expects the file
   system to store what was written, and not to unexpectedly change the
   contents.

   However, in general, an HTTP server is not a file system replacement.
   There may be some that have been designed that way, and some that
   expose some parts of their namespace that have this quality.  But in
   general, HTTP server implementers have a lot of freedom in how
   resources are implemented.  Indeed, this flexibility is one of the
   reasons for HTTP's success, allowing it to be used for a wide range
   of tasks, of which replacing file systems is just one (and not
   necessarily the most interesting one).

   In particular:

   o  A server may not store a resource as a binary object - in this
      case, the representation returned in a subsequent GET request may
      just be similar, but not identical to what was written.  Good
      examples are servers that use HTTP to access XML data ([RFC4825]),
      Calendaring data ([RFC4791]) or newsfeed data ([RFC5023]).

   o  A server may change the data being written on purpose, while it is
      being written.  Examples that immediately come to mind are keyword
      substitution in a source control system, or filters that remove
      potentially insecure parts out of HTML pages.

   Furthermore:

   o  An "unrelated" method such as WebDAV's PROPPATCH (see [RFC4918],
      Section 9.2) may affect the entity body and therefore the entity
      tag in an unexpected way, because the server stores some or all of
      the WebDAV properties inside the entity body (for instance, GPS
      information inside a JPG image file).

   As long as servers store the content octet-by-octet, and return
   exactly what the client wrote, there is no problem at all.

   Things get more interesting when a server does change the content,
   such as in the "simple authoring" example given in Appendix A.1.
   Here, the server does change the content upon writing to the
   resource, yet no harm is done, because the final state of the
   resource on the server does not depend on the client being aware of
   that.




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   All of the content rewriting examples mentioned above have this
   quality: the client can safely continue to edit the entity it sent,
   because the result of the transformation done by the server will be
   the same in the end.  Formally, if we call the server-side
   transformation "t", the initial content "c", and the client-side
   editing steps "e1" and "e2", then

   t(e2(e1(c))) = t(e2(t(e1(c))))

   e.g., it is irrelevant whether the client obtained the current entity
   body before doing its second edit.

   [[example.for.non.safe.rewrite: Question: does anybody know a real-
   world example for server-side content rewriting where the above is
   not true? --julian.reschke@greenbytes.de]]

   Problems will only occur if the client uses the entity body it sent,
   and the entity tag it obtained in return, in subsequent requests that
   only transfer part of the entity body, such as GET or PUT requests
   using the "Range" request header (see [RFC2616], Section 14.35).

   Furthermore, some clients need to expose the actual contents to the
   end user.  These clients will have to ensure that they really have
   the current representation.

   Entity bodies (and thus entity tags) changing due to side effects of
   seemingly unrelated requests are indeed a problem, as demonstrated in
   Appendix A.3, and this specification proposes a way to resolve this
   in Section 3.

1.3.  Analysis of RFC2616's Definitions

   There are several places in the HTTP/1.1 specification ([RFC2616])
   mentioning the "ETag" response header.

   Let us start with the header definition in Section 14.19:

      The ETag response-header field provides the current value of the
      entity tag for the requested variant.  Sections 14.24, 14.26 and
      14.44 describe the headers used with entity tags.  The entity tag
      MAY be used for comparison with other entities from the same
      resource (see Section 13.3.3).

   The meaning of a "response-header" in turn is defined in Section 6.2:

      The response-header fields allow the server to pass additional
      information about the response which cannot be placed in the
      Status-Line.  These header fields give information about the



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      server and about further access to the resource identified by the
      Request-URI.

   The "ETag" response header itself is mentioned mainly in the context
   of cache validation, such as in Section 13.3.2.  What is missing is a
   coherent description on how the origin server can notify the user-
   agent when the entity tag changes as result of a write operation,
   such as PUT.

   Indeed, the definition of the 201 Created status code mentions entity
   tags (Section 10.2.2):

      A 201 response MAY contain an ETag response header field
      indicating the current value of the entity tag for the requested
      variant just created, see Section 14.19.

   The "ETag" response header is mentioned again in the definition of
   206 Partial Content (Section 10.2.7) and 304 Not Modified (Section
   10.3.5), but notably missing are statements about other 2xx series
   status codes that can occur upon a successful PUT operation, such as
   200 OK (Section 10.2.1) and 204 No Content (Section 10.2.5).

   Summarizing, the specification is a bit vague about what an ETag
   response header upon a write operation means, but this problem is
   somewhat mitigated by the precise definition of a response header.  A
   proposal for enhancing [RFC2616] in this regard is made in Section 3
   below.

1.4.  Prior Work

   While working on the revision of [RFC2518], the IETF WebDAV working
   group realized that this is a generic problem that needs attention
   independently of WebDAV.  An initial attempt was made with
   [draft-whitehead-http-etag] in February 2006, but no progress was
   made since.

   At the time of this writing in May 2007, two specifications based on
   HTTP were just published by the RFC Editor, taking two opposite
   approaches:

   o  Section 8.5 of [RFC4825] makes it a MUST-level requirement to
      return an entity tag upon PUT, even though the very nature of an
      XCAP server will cause it to rewrite contents (due to its XML-
      based storage).

   o  Section 5.3.4 of [RFC4791] explicitly forbids ("MUST NOT")
      returning an entity tag upon PUT if the content was rewritten.




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   In essence, this makes it impossible to implement an HTTP resource
   that conforms to both specifications.  Due to the differing use cases
   of XCAP and CalDAV, this may not be a problem in practice, but the
   disagreement in itself is scary.  Publication of these specifications
   on the standards track will make it much harder for future protocols
   to deal with this topic in a meaningful way (comments were sent
   during IETF Last Call for CalDAV, see
   <http://www1.ietf.org/mail-archive/web/ietf/current/msg43021.html>).


2.  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].

   The terminology used here follows and extends that in the HTTP
   specification [RFC2616], notably the augmented Backus-Naur Form (BNF)
   defined in Section 2.1 of that document.


3.  Clarifications on the Behavior of the 'ETag' Response Header

   This section describes a minimal change to [RFC2616], proposed in <ht
   tp://lists.w3.org/Archives/Public/ietf-http-wg/2006JanMar/0003.html>.

   At the end of Section 10.2 of [RFC2616], add:

      The response MAY contain an ETag response header field indicating
      the current value of the entity tag (Section 14.19) for the
      requested variant.  The value SHOULD be the same as the one
      returned upon a subsequent HEAD request addressing the same
      variant.

   In Section 10.2.1 of [RFC2616], remove:

      A 201 response MAY contain an ETag response header field
      indicating the current value of the entity tag for the requested
      variant just created, see Section 14.19.

   In essence, this moves the statement about entity tags in write
   operations from the specific case of status 201 Created into the more
   generic description of the 2xx series status codes.

   Note that the term "requested variant" is somewhat misleading, in
   particular in the context of write operations (see
   <http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis/issues/#i69> on the
   RFC2616bis issues list).



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4.  The 'Entity-Transform' Header

   The 'Entity-Transform' entity header provides information about
   whether a transformation has been applied to an entity body.

   When used in an HTTP request, its meaning is undefined.  In an HTTP
   response, it provides information whether the server has applied a
   transformation when the entity was stored last.

   In general, entity headers may be stored in intermediates.  The main
   use of this header however applies to the HTTP PUT method, of which
   by default the results are not cacheable (see [RFC2616], Section
   9.6).  In addition, the value format is defined so that a client can
   reliably detect whether the information is fresh.

   Format:

   Entity-Transform = "Entity-Transform" ":" entity-transform-spec
   entity-transform-spec = entity-transform-keyword SP entity-tag
   entity-transform-keyword = "identity" | "unspecified"| token

   entity-tag = <entity-tag: defined in [RFC2616], Section 3.11>
   token = <token: defined in [RFC2616], Section 2.2>

   The entity-tag specifies the entity body to which this information
   applies.

   An entity-transform-keyword of "identity" specifies that the origin
   server has stored the entity octet-by-octet, thus the user agent MAY
   use a local copy of the entity body with the given entity-tag for
   subsequent requests that rely on octet-by-octet identity (such as a
   PUT with "Range" request header).

   Both the absence of this response header and any entity-transform-
   keyword value other than "identity" specify that the origin server
   may have transformed the entity before storage, thus a subsequent
   retrieval will not necessarily return an exact copy of the submitted
   entity.

      Note: usage of keywords other than "identity" or "unspecified" is
      reserved for future revisisions of this specification.

      Further note that the definition of the "identity" keyword is
      independant of the type of entity-tag.  Thus, when "identity" is
      returned with a weak entity-tag, the client can indeed rely on
      octet-by-octet identity.





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4.1.  Examples

   Content was stored octet-by-octet:

   HTTP/1.1 200 OK
   ETag: "1"
   Entity-Transform: identity "1"

   Content was transformed:

   HTTP/1.1 200 OK
   ETag: "2"
   Entity-Transform: unspecified "2"

   Response containing a stale "Entity-Transform" header:

   HTTP/1.1 200 OK
   ETag: "3"
   Entity-Transform: unspecified "2"

   Note that in this case the newly assigned entity tag and the entity
   tag returned in "Entity-Transform" do not match, thus the client is
   aware that the header value is stale and can't be used.


5.  How This Helps

   The clarification of [RFC2616] (see Section 3) makes it clear that
   user agents can use "ETag" headers obtained in write operations, as
   long as they do not require octet-by-octet identity.  In particular,
   a new entity tag can be returned for any method, such as a WebDAV
   PROPPATCH (see [RFC4918], Section 9.2).  This helps dealing with the
   problem described in Appendix A.3.  See Appendix A.4 for details.

   The addition of the "Entity-Transform" header (see Section 4) enables
   origin servers to signal that they stored an exact copy of the
   content, thus allowing clients not to refetch the content.  Note that
   by default (in absence of the response header), a client can not make
   any assumptions about the server's behavior in this regard.  Thus
   clients will only benefit from new servers explicitly setting the new
   header.


6.  IANA Considerations

   This document specifies the new HTTP header listed below.





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   Header field name:  Entity-Transform

   Applicable protocol:  http

   Status:  informational

   Author/Change controller:  IETF

   Specification document:  Section 4 of this specification


7.  Security Considerations

   This specification introduces no new security considerations beyond
   those discussed in Section 15 of [RFC2616].


8.  References

8.1.  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.

8.2.  Informative References

   [HTML]     Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", W3C REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/html401/>.

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WEBDAV", RFC 2518, February 1999.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

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




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   [RFC5023]  Gregorio, J., Ed. and B. de hOra, Ed., "The Atom
              Publishing Protocol", RFC 5023, October 2007.

   [draft-whitehead-http-etag]
              Whitehead, J., "Design Considerations for State
              Identifiers in HTTP and WebDAV",
              draft-whitehead-http-etag-00 (work in progress),
              February 2006.

              (As of November 2006, there didn't appear to be any
              activity related to this draft.)

URIs

   [1]  <mailto:ietf-http-wg@w3.org>

   [2]  <mailto:ietf-http-wg-request@w3.org?subject=subscribe>


Appendix A.  Use Cases

A.1.  Simple Authoring

   Let us consider a server not having the quality of preserving octet-
   by-octet identity, for instance because of SVN-style keyword
   expansion in text content (<http://svnbook.red-bean.com/en/1.2/
   svn-book.html#svn.advanced.props.special.keywords>).

   In this case, the client has previously retrieved the representation
   for <http://example.com/test>, and the server has returned the ETag
   "1":

   >> Request (1)

   GET /test HTTP/1.1
   Host: example.com

   >> Response (1)

   HTTP/1.1 200 OK
   Content-Type: text/plain
   ETag: "1"

   # $Revision: 1 $
   Sample text.

   The client now wants to update the resource.  To avoid overwriting
   somebody else's changes, it submits the PUT request with the HTTP



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   "If-Match" request header (see [RFC2616], Section 14.24):

   >> Request (2)

   PUT /test HTTP/1.1
   Host: example.com
   If-Match: "1"

   # $Revision: 1 $
   New sample text.

   If the resource was modified in the meantime, the server will reject
   the request with a 412 Precondition Failed status:

   >> Response (2a)

   HTTP/1.1 412 Precondition Failed
   Content-Type: text/plain

   Precondition Failed: entity tag supplied in If-Match request header
   did not match current.

   In this case, the client usually has take care of merging the changes
   made locally with those made on the server ("Merge Conflict").

   If there was no overlapping update, the server will execute the
   request and return a new entity tag:

   >> Response (2b)

   HTTP/1.1 200 OK
   Content-Type: text/plain
   ETag: "2"

   Note however, that at this point the client knows the new entity tag,
   but doesn not know the current representation, which will have been
   updated by the server to:

   # $Revision: 2 $
   New sample text.

   What seems to be a problem at first may not be a real problem in
   practice.  Let us assume that the client continues editing the
   resource, using the entity tag obtained from the previous request,
   but editing the entity it last sent:






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   >> Request (3)

   PUT /test HTTP/1.1
   Host: example.com
   If-Match: "2"

   # $Revision: 1 $
   A third attempt.

   Assuming there was no overlapping update, the PUT request will
   succeed:

   >> Response (3)

   HTTP/1.1 200 OK
   Content-Type: text/plain
   ETag: "3"

   Note that the only problem here is that the client doesn't have an
   exact copy of the entity it is editing.  However, from the server's
   point of view this is entirely irrelevant, because the "Revision"
   keyword will be automatically updated upon every write anyway.

   In any case, the final contents will be:

   # $Revision: 3 $
   A third attempt.

A.2.  Simple Authoring, with Entity-Transform header

   In the example above (Appendix A.1, Response 2b), the server returns
   an ETag response header upon PUT, although the content actually was
   rewritten due to keyword substitution.

   A server implementing this specification would instead respond with:

   >> Response (2c)

   HTTP/1.1 200 OK
   Content-Type: text/plain
   ETag: "2"
   Entity-Transform: unspecified "2"

   This indicates that the content that was sent is not octet-by-octet
   identical to what a subsequent GET request would retrieve.  It is
   then up to the client to decide whether it wants to re-fetch the
   content before continuing edits.




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A.3.  Setting both Content and Metadata

   In this example, the server exposes data extracted from the HTML
   <title> element ([HTML], Section 7.4.2) as a custom WebDAV property
   ([RFC4918], Section 4), allowing both read and write access.

   In the first step, the client obtains the current representation for
   <http://example.com/test.html>:

   >> Request (1)

   GET /test.html HTTP/1.1
   Host: example.com

   >> Response (1)

   HTTP/1.1 200 OK
   Content-Type: text/html
   ETag: "A"

   <html>
     <head>
     </head>
     <body>
     </body>
   </html>

   Next, it adds one paragraph to the <body> element, and gets back a
   new entity tag:

   >> Request (2)

   PUT /test.html HTTP/1.1
   Host: example.com
   If-Match: "A"

   <html>
     <head>
     </head>
     <body>
       <p>First paragraph.</p>
     </body>
   </html>

   >> Response (2)

   HTTP/1.1 200 OK
   ETag: "B"



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   Next, the client sets a custom "title" property (see [RFC4918],
   Section 9.2):

   >> Request (3)

   PROPPATCH /test.html HTTP/1.1
   Host: example.com
   Content-Type: application/xml

   <proppatch xmlns="DAV:">
     <set>
       <prop>
         <title xmlns="http://ns.example.org/"
         >A sample document</title>
       </prop>
     </set>
   </proppatch>

   >> Response (3)

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml

   <multistatus xmlns="DAV:">
     <response>
       <href>/test.html</href>
       <propstat>
         <prop>
           <title xmlns="http://ns.example.org/"/>
         </prop>
         <status>HTTP/1.1 2OO OK</status>
       </propstat>
     </response>
   </multistatus>

   The server knows how to propagate property changes into the HTML
   content, so it updates the entity by adding an HTML title document
   accordingly.  This causes the entity tag changing to "C".













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   The new entity body is shown below, but the client does not realize
   that it did change at all.

   <html>
     <head>
       <title>A sample document</title>
     </head>
     <body>
       <p>First paragraph.</p>
     </body>
   </html>

   A subsequent attempt by the client to update the entity body will
   fail, unless it realizes that changing WebDAV properties may affect
   the entity as well.  In this case, it would have had to get the
   current entity tag before proceeding.  Of course, this introduces an
   additional round-trip, and a timing window during which overlapping
   updates by other clients would go unnoticed.

      Note that a client would be affected by this behaviour even in the
      presence of an exclusive WebDAV write lock (see [RFC4918], Section
      7).

A.4.  Setting both Content and Metadata, new style

   Below we repeat the example from above (Appendix A.3), but here the
   origin server returns entity tags for all write operations, and the
   user agent knows how to deal with them.  That is, both take advantage
   of what [RFC2616] already allows.

   >> Request (3)

   PROPPATCH /test.html HTTP/1.1
   Host: example.com
   Content-Type: application/xml

   <proppatch xmlns="DAV:">
     <set>
       <prop>
         <title xmlns="http://ns.example.org/">
           A sample document
         </title>
       </prop>
     </set>
   </proppatch>






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   >> Response (3)

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml
   ETag: "C"

   <multistatus xmlns="DAV:">
     <response>
       <href>/test.html</href>
       <propstat>
         <prop>
           <title xmlns="http://ns.example.org/"/>
         </prop>
         <status>HTTP/1.1 2OO OK</status>
       </propstat>
     </response>
   </multistatus>

   As before, this causes the entity to change, and a new entity tag to
   be assigned.  But in this case, the origin server actually notifies
   the client of the changed state by including the "ETag" response
   header.

   The client now will be aware that the requested entity change, and
   can use the new entity tag in subsequent requests (potentially after
   refreshing the local copy).


Appendix B.  Change Log (to be removed by RFC Editor before publication)

B.1.  Since draft-reschke-http-etag-on-write-00

   Add and resolves issues "entity-header" and "extensibility", by
   removing the extension hooks, and by redefining the header to it can
   be used as an Entity header.

B.2.  Since draft-reschke-http-etag-on-write-01

   Update APP and CALDAV references.  Remove RFC3986 reference (not
   needed anymore after the simplication in draft 01).  Fix typo in
   header description ("submitted entity", not "stored entity").  Remove
   comparison about how XCAP and CALDAV profile RFC2616: after all, both
   mandate a behaviour that was legal but optional before.  Add
   "Updates: RFC2616".







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B.3.  Since draft-reschke-http-etag-on-write-02

   In the references, note that there was no activitiy on
   draft-whitehead-http-etag-00 anymore.  Change intended status to
   "Experimental".  Update APP reference.  Update statements about
   current status of XCAP and CALDAV.  Add and resolve "clarify-
   extension".

B.4.  Since draft-reschke-http-etag-on-write-03

   Update XCAP reference.  Update header definition to use prose rules
   rather than comments.  Clarify "identity" keyword when appearing with
   a weak entity-tag.  In PROPPATCH/HTML example, fix whitespace so that
   the HTML title element has exactly the same value as the one that was
   PROPPATCHed.  Mention that ETags changing due to a PROPPATCH could
   even occur in presence of a WebDAV write lock.  Expand keyword
   substitution example with variant where Entity-Transform is returned.

B.5.  Since draft-reschke-http-etag-on-write-04

   Update APP reference.  Update CalDAV reference (published as
   RFC4791).  Update "prior work" (RFC4791 being published, XCAP having
   been approved).

B.6.  Since draft-reschke-http-etag-on-write-05

   Update XCAP reference (published as RFC4825).  Update "prior work"
   (RFC4825 being published).

B.7.  Since draft-reschke-http-etag-on-write-06

   Replace RFC2518 references with RFC4918 where appropriate.  Update
   APP reference.

B.8.  Since draft-reschke-http-etag-on-write-07

   Update APP reference.  Reference the "requested variant" issue on the
   RFC2616bis issues list
   (<http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis/issues/#i69>).


Appendix C.  Open issues (to be removed by RFC Editor prior to
             publication)

C.1.  edit

   Type: edit




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   julian.reschke@greenbytes.de (2006-08-10): Umbrella issue for
   editorial fixes/enhancements.


Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Email: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/




































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