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Versions: 00 01

Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                           July 27, 2007
Expires: January 28, 2008


       The Hypertext Transfer Protocol (HTTP) GET-Location header
                   draft-reschke-http-get-location-00

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   This Internet-Draft will expire on January 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Several hypertext transfer protocol (HTTP) extensions use methods
   other than GET to expose information.  This has the drawback that
   this kind of information is harder to identify (missing a URL to
   which a GET request could be applied) and to cache.

   This document specifies a simple extension header through which a
   server can advertise a substitute URL that an HTTP client
   subsequently can use with the GET method.



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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-get-location>.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  The 'GET-Location' Header  . . . . . . . . . . . . . . . . . .  3
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  5
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  5
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . .  6
     A.1.  WebDAV Collection Membership . . . . . . . . . . . . . . .  6
     A.2.  WebDAV Custom Properties . . . . . . . . . . . . . . . . .  9
     A.3.  DeltaV Version History Report  . . . . . . . . . . . . . . 11
   Appendix B.  Related HTTP features . . . . . . . . . . . . . . . . 12
     B.1.  Status 303 . . . . . . . . . . . . . . . . . . . . . . . . 12
     B.2.  Content-Location Header  . . . . . . . . . . . . . . . . . 13
     B.3.  Location header  . . . . . . . . . . . . . . . . . . . . . 13
   Appendix C.  Open Issues . . . . . . . . . . . . . . . . . . . . . 13
     C.1.  Content Negotiation on GET-Location  . . . . . . . . . . . 13
     C.2.  Using URI Templates rather than URIs . . . . . . . . . . . 14
     C.3.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15












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1.  Introduction

   Several HTTP ([RFC2616]) extensions use methods other than GET to
   expose information.  This has the drawback that this kind of
   information is harder to identify (missing a URL to which a GET
   request could be applied) and to cache.

   This document specifies a simple extension header through which a
   server can advertise a substitute URL that an HTTP client
   subsequently can use with the GET method.


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].


3.  The 'GET-Location' Header

   The GET-Location entity header identifies a substitute resource that
   can be used in subsequent requests for the same information, but
   using the GET method.

   Note that, by definition, the GET-Location header can only used on
   responses to safe methods.





















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   Syntax (using the the augmented Backus-Naur Form (BNF) defined in
   Section 2.1 of [RFC2616]):

   GET-Location = "GET-Location" ":" "<" Simple-ref ">"
                                     *( ";" location-directive ) )

   location-directive = "etag=" entity-tag
                      | "max-age" "=" delta-seconds
                      | location-extension

   location-extension = token [ "=" ( token | quoted-string ) ]

   Simple-ref     = absolute-URI | ( path-absolute [ "?" query ] )

   absolute-URI   = <defined in [RFC3986], Section 4.3>
   delta-seconds  = <defined in [RFC2616], Section 3.3.2>
   entity-tag     = <defined in [RFC2616], Section 3.11>
   path-absolute  = <defined in [RFC3986], Section 3.3>
   quoted-string  = <defined in [RFC2616], Section 2.2>
   query          = <defined in [RFC3986], Section 3.4>
   token          = <defined in [RFC2616], Section 2.2>

   Where:

   Simple-ref  Contains either the URI or the absolute path of the
      location.

   etag  The server can include the entity tag for the returned entity,
      if it would have been retrieved by a GET request to the substitute
      resource.  Note that this is different from the value of the
      "ETag" header which could also be included in the response, but
      which would apply to the resource identified by the Request-URI.

   max-age  Specifies a lifetime for the information returned by this
      header.  A client MUST discard any information related to this
      header after the specified amount of time.

   The freshness lifetime for the information obtained from the GET-
   Location header does not depend on the cacheability of the response
   it was obtained from (which, in general, may not be cacheable at
   all).  The "max-age" directive allows the server to specify after how
   many seconds a client should discard knowledge about the alternate
   resource.  In absence of that header, clients SHOULD discard the
   information after 3600 seconds.







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4.  Security Considerations

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


5.  IANA Considerations

   This document specifies the new HTTP header listed below, to be added
   to the permanent registry (see [RFC3864]).

   Header field name:  GET-Location

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  Section 3 of this specification


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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              January 2005.

6.2.  Informative References

   [RFC3253]  Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
              Whitehead, "Versioning Extensions to WebDAV", RFC 3253,
              March 2002.

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

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed



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              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [draft-gregorio-uritemplate]
              Gregorio, J., Ed., Hadley, M., Ed., Nottingham, M., Ed.,
              and D. Orchard, "URI Template",
              draft-gregorio-uritemplate-01 (work in progress),
              July 2007.

URIs

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

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


Appendix A.  Examples

A.1.  WebDAV Collection Membership

   In this example the client uses the WebDAV PROPFIND method ("HTTP
   Extensions for Web Distributed Authoring and Versioning", [RFC4918],
   Section 9.1) to get a list of all collection members, along with
   their DAV:resourcetype property ([RFC4918], Section 15.9):

   >>Request

   PROPFIND /collection/ HTTP/1.1
   Host: example.com
   Depth: 1
   Content-Type: application/xml

   <propfind xmlns="DAV:">
     <prop>
       <resourcetype/>
     </prop>
   </propfind>

   The response contains the requested information, plus the GET-
   Location header, identifying a separate resource which can provide
   the same information using the HTTP GET method:











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   >>Response

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml
   GET-Location: <http://example.com/collection/;members>; etag="123";
                 max-age=3600

   <multistatus xmlns="DAV":>
     <response>
       <href>/collection/</href>
       <propstat>
         <prop>
           <resourcetype><collection/></resourcetype>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
     <response>
       <href>/collection/member</href>
       <propstat>
         <prop>
           <resourcetype/>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   The response provided the URL of the substitute resource, so when the
   client wishes to refresh the collection information, it uses that
   URI.  The response contained the entity tag for the data being
   returned, so it can make the request conditional:

   >>Request

   GET /collection/;members HTTP/1.1
   Host: example.com
   Accept: application/xml
   If-None-Match: "123"

   The information did not change, so the server does not need to return
   new data:

   >>Response

   HTTP/1.1 304 Not Modified

   Later on, the client tries again.  This time, however, a second



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   member has been added:

   >>Request

   GET /collection/;members HTTP/1.1
   Host: example.com
   Accept: application/xml
   If-None-Match: "123"

   >>Response

   HTTP/1.1 200 OK
   Content-Type: application/xml
   ETag: "124"

   <multistatus xmlns="DAV":>
     <response>
       <href>/collection/</href>
       <propstat>
         <prop>
           <resourcetype><collection/></resourcetype>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
     <response>
       <href>/collection/member</href>
       <propstat>
         <prop>
           <resourcetype/>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
     <response>
       <href>/collection/member2</href>
       <propstat>
         <prop>
           <resourcetype/>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   Finally, the collection has been removed by somebody else.  The
   client tries a refresh:




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   >>Request

   GET /collection/;members HTTP/1.1
   Host: example.com
   Accept: application/xml
   If-None-Match: "124"

   >>Response

   HTTP/1.1 404 Not Found

   Note that it may be hard to compute strong entity tags for more
   complex PROPFIND responses.  For instance, most properties depend on
   the state of the collection member, not the state of the collection
   itself, and thus the response will change even though the state of
   the collection itself did not change.

   This is why this extension leaves it to the server whether to return
   a GET-Location at all, and if so, whether to return cache validators
   along with it.

A.2.  WebDAV Custom Properties

   Here, the client uses the WebDAV PROPFIND method ([RFC4918], Section
   9.1) to obtain a custom property:

   >>Request

   PROPFIND /collection/member HTTP/1.1
   Host: example.com
   Depth: 0
   Content-Type: application/xml

   <propfind xmlns="DAV:">
     <prop>
       <title xmlns="http://ns.example.com/"/>
     </prop>
   </propfind>













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   >>Response

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml
   GET-Location: </collection/member;prop=title>; etag="1"

   <multistatus xmlns="DAV":>
     <response>
       <href>/collection/member</href>
       <propstat>
         <prop>
           <title xmlns="http://ns.example.com/"
           >Document Title</title>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   >>Request

   GET /collection/member;prop=title HTTP/1.1
   Host: example.com
   If-None-Match: "1"

   >>Response

   HTTP/1.1 304 Not Modified

   Later, the request is repeated after the title property indeed
   changed...:

   >>Request

   GET /collection/member;prop=title HTTP/1.1
   Host: example.com
   If-None-Match: "1"














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   >>Response

   HTTP/1.1 200 OK
   Content-Type: application/xml
   ETag: "2"

   <multistatus xmlns="DAV":>
     <response>
       <href>/collection/member</href>
       <propstat>
         <prop>
           <title xmlns="http://ns.example.com/"
           >New Document Title</title>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   Although this example may look like every WebDAV property would need
   a separate entity tag, this is of course not the case.  For instance,
   a server that stores all custom properties in a single place (like a
   properties file) could use the same computation for the entity tag
   for all properties.  Also, it could implement resources representing
   multiple custom property values the same way.

A.3.  DeltaV Version History Report

   Here, the client uses the DeltaV DAV:version-tree report ("Versioning
   Extensions to WebDAV", [RFC3253], Section 3.7) to obtain the members
   of the version history of a version-controlled resource.

   >>Request

   REPORT /collection/member HTTP/1.1
   Host: example.com
   Depth: 0
   Content-Type: application/xml

   <version-tree xmlns="DAV:">
     <prop>
       <resourcetype/>
     </prop>
   </version-tree>







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   >>Response

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml
   GET-Location: </version-storage/12345/;justmembers>

   <multistatus xmlns="DAV":>
     <response>
       <href>/version-storage/12345/V1</href>
       <propstat>
         <prop>
           <resourcetype><collection/></resourcetype>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
     <response>
       <href>/version-storage/12345/V2</href>
       <propstat>
         <prop>
           <resourcetype><collection/></resourcetype>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   Note that in this case, the substitute resource can be almost
   identical to the one from the PROPFIND/Depth:1 example: the only
   difference being that the report result does not contain a DAV:
   response element for the collection itself.


Appendix B.  Related HTTP features

   This section discusses some related HTTP features and explains why
   they can't be used for the given use case.

B.1.  Status 303

   Section 10.3.4 of [RFC2616] defines the status code 303 (See Other):

      The response to the request can be found under a different URI and
      SHOULD be retrieved using a GET method on that resource.  This
      method exists primarily to allow the output of a POST-activated
      script to redirect the user agent to a selected resource.  The new
      URI is not a substitute reference for the originally requested
      resource.  The 303 response MUST NOT be cached, but the response



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      to the second (redirected) request might be cacheable.

   On first glance, it may look as if this addresses exactly the given
   use case.  However:

   1.  It says: "The new URI is not a substitute reference for the
       originally requested resource.  The 303 response MUST NOT be
       cached, but the response to the second (redirected) request might
       be cacheable."  That is, the information about the alternate
       resource is not cacheable.

   2.  Servers returning a 303 status instead of the one expected by the
       client, such as 207 Multistatus, would likely break existing
       clients.

B.2.  Content-Location Header

   Section 14.14 of [RFC2616] states:

      The Content-Location value is not a replacement for the original
      requested URI; it is only a statement of the location of the
      resource corresponding to this particular entity at the time of
      the request. (...)

   However, the purpose of "GET-Location" is to enable the server to
   provide a permanent replacement URI.

B.3.  Location header

   Section 14.30 of [RFC2616] states:

      The Location response-header field is used to redirect the
      recipient to a location other than the Request-URI for completion
      of the request or identification of a new resource. (...)

   Neither of these cases ("redirect to a location for completion of the
   request" and "identification of a new resource") matches the use case
   "GET-Location" covers.


Appendix C.  Open Issues

C.1.  Content Negotiation on GET-Location

   Should it be possible to use Content Negotiation on the resource
   identified by GET-Location?  A use case could be a metadata provider
   that would support different formats, such as WebDAV's multistatus
   format (MIME type missing!), RDF, JSON, whatever.



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   This could be done using a location-extension specifying the Accept
   header for the GET operation.

C.2.  Using URI Templates rather than URIs

   Should we allow servers to return URI templates
   ([draft-gregorio-uritemplate]), so that clients can compute
   substitute URLs for other requests as well?

   For instance, this could be done by allowing a URI template instead
   of the Simple-ref, and to return another template specifying how to
   derive the template variable from the Request-URI:

   >>Request

   PROPFIND /documents/a/b HTTP/1.1
   Host: example.com
   Depth: 0
   Content-Type: application/xml

   >>Response

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml
   GET-Location: </metadata/{path};members>; path-template=</a/b/{path}>

   ...

   So in this case, the actual URI to be used would be
   <http://example.com/metadata/a/b;members>.

C.3.  Extensions

   Do we need a registry for new location-directive values?


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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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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