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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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
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Discussions of the HTTP working group are archived at
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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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