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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 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. Reschke Expires January 28, 2008 [Page 1]
Internet-Draft HTTP GET-Location Header July 2007 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 Reschke Expires January 28, 2008 [Page 2]
Internet-Draft HTTP GET-Location Header July 2007 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. Reschke Expires January 28, 2008 [Page 3]
Internet-Draft HTTP GET-Location Header July 2007 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. Reschke Expires January 28, 2008 [Page 4]
Internet-Draft HTTP GET-Location Header July 2007 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 Reschke Expires January 28, 2008 [Page 5]
Internet-Draft HTTP GET-Location Header July 2007 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: Reschke Expires January 28, 2008 [Page 6]
Internet-Draft HTTP GET-Location Header July 2007 >>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 Reschke Expires January 28, 2008 [Page 7]
Internet-Draft HTTP GET-Location Header July 2007 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: Reschke Expires January 28, 2008 [Page 8]
Internet-Draft HTTP GET-Location Header July 2007 >>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> Reschke Expires January 28, 2008 [Page 9]
Internet-Draft HTTP GET-Location Header July 2007 >>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" Reschke Expires January 28, 2008 [Page 10]
Internet-Draft HTTP GET-Location Header July 2007 >>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> Reschke Expires January 28, 2008 [Page 11]
Internet-Draft HTTP GET-Location Header July 2007 >>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 Reschke Expires January 28, 2008 [Page 12]
Internet-Draft HTTP GET-Location Header July 2007 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. Reschke Expires January 28, 2008 [Page 13]
Internet-Draft HTTP GET-Location Header July 2007 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/ Reschke Expires January 28, 2008 [Page 14]
Internet-Draft HTTP GET-Location Header July 2007 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 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Reschke Expires January 28, 2008 [Page 15]