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Network Working Group                                           J. Snell
Internet-Draft                                           October 8, 2012
Intended status: Informational
Expires: April 11, 2013


                      HTTP Link and Unlink Methods
                       draft-snell-link-method-00

Abstract

   This specification defines the semantics of the Link and Unlink HTTP
   methods.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 11, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  LINK  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  UNLINK  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7










































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1.  Introduction

   This specification updates the HTTP LINK and UNLINK methods
   originally defined in [RFC2068].  These were originally defined as
   "additional request methods" that were later dropped entirely from
   follow-on iterations of the HTTP specification due to previous lack
   of interest or use.

   TODO: Fill in explanation as to why this is needed.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].


2.  LINK

   The LINK method is used to establish one or more Link relationships
   between the existing resource identified by the effective request URI
   and other resources.  Metadata contained within Link header fields
   [RFC5988] provide the information about which other resources are
   being linked to the target resource and the type of link being
   established.  A payload within a LINK request message has no defined
   semantics.

   The semantics of the LINK method change to a "conditional LINK" if
   the request message includes an If-Modified-Since, If-Unmodified-
   Since, If-Match, If-None-Match, or If-Range header field
   ([I-D.ietf-httpbis-p4-conditional]).  A conditional LINK requests
   that the Link be established only under the circumstances described
   by the conditional header field(s).

   LINK request messages are idempotent.  For any pair of resources,
   only a single Link of any given relation type can exist.  However,
   multiple links of different relation types can be established between
   those resources.

   LINK request messages are not safe, however, in that establishing a
   Link causes an inherent change to the state of the target resource.

   The response to a LINK request is cacheable and MAY be used to
   satisfy subsequent LINK requests (see [I-D.ietf-httpbis-p6-cache]).
   However, caching such responses is not likely to provide any
   significant benefit or be supported by existing infrastructure.

   A single LINK request message can contain multiple Link header
   fields, each of which establishes a separate Link relationship with
   the target resource.  In such cases, the server MUST accept the



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   entire set of Links atomically.  If any of the specified Link
   relationships cannot be created, the server MUST NOT create any of
   the Links.

   A successful response SHOULD be 200 (OK) if the response includes a
   representation describing the status, 201 (Created) if the action
   results in the creation of a new resource that represents the newly
   established link, 202 (Accepted) if the action has not yet been
   enacted, or 204 (No Content) if the action has been enacted but the
   response does not include a representation.


3.  UNLINK

   The UNLINK method is used to remove one of more Link relationships of
   between the existing resource identified by the effective request URI
   and other resources.  Metadata contained within Link header fields
   [RFC5988] provide the information about the resources to which links
   of a specific type are to be removed.  A payload within an UNLINK
   request message has no defined semantics.

   The semantics of the UNLINK method change to a "conditional UNLINK"
   if the request message includes an If-Modified-Since, If-Unmodified-
   Since, If-Match, If-None-Match, or If-Range header field
   ([I-D.ietf-httpbis-p4-conditional]).  A conditional UNLINK requests
   that the Link be removed only under the circumstances described by
   the conditional header field(s).

   UNLINK request messages are idempotent.

   UNLINK request messages are not safe, however, in that removing a
   Link causes an inherent change to the state of the target resource.

   The response to an UNLINK request is cacheable and MAY be used to
   satisfy subsequent UNLINK requests (see [I-D.ietf-httpbis-p6-cache]).
   However, caching such responses is not likely to provide any
   significant benefit or be supported by existing infrastructure.

   A single UNLINK request message can contain multiple Link header
   fields, each of which identifies a separate Link relationship to
   remove.  In such cases, the server MUST remove the entire set of
   Links atomically.  If any of the specified Link relationships cannot
   be removed, the server MUST NOT remove any of the Links.

   A successful response SHOULD be 200 (OK) if the response includes a
   representation describing the status, 202 (Accepted) if the action
   has not yet been enacted, or 204 (No Content) if the action has been
   enacted but the response does not include a representation.



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   The UNLINK method MAY be overridden by human intervention (or other
   means) on the origin server.  The client cannot be guaranteed that
   the operation has been carried out, even if the status code returned
   from the origin server indicates that the action has been completed
   successfully.  However, the server SHOULD NOT indicate success
   unless, at the time the response is given, it intends to remove the
   specified Links.


4.  Example

   There exists a broad range of possible use cases for the LINK and
   UNLINK methods.  The examples that follow illustrate a subset of
   those cases.

   Example 1: Creating two separate links between an image and the
   profiles of two people associated with the image:

     LINK /images/my_dog.jpg HTTP/1.1
     Host: example.org
     Link: <http://example.com/profiles/joe>; rel="tag"
     Link: <http://example.com/profiles/sally>; rel="tag"

   Example 2: Removing an existing Link relationship between two
   resources:

     UNLINK /images/my_dog.jpg HTTP/1.1
     Host: example.org
     Link: <http://example.com/profiles/sally>; rel="tag"

   Example 3: Establish a "pingback" or "trackback" style link to a blog
   entry about an article

     LINK /articles/an_interesting_article HTTP/1.1
     Host: example.org
     Link: <http://example.com/my_blog_post>; rel="about"

   Example 4: Establish a link between two semantically related
   resources:

     LINK /some-resource HTTP/1.1
     Host: example.org
     Link: <http://example.com/schemas/my_schema>; rel="describedBy"








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   Example 5: Add an existing resource to a collection:

     LINK /some-collection-resource HTTP/1.1
     Host: example.org
     Link: <http://example.com/my-member-resource>; rel="item"

   Example 6: Link one resource to another that monitors it's current
   state (e.g. pub/sub)

     LINK /my-resource HTTP/1.1
     Host: example.org
     Link: <http://example.com/my-monitor>; rel="monitor"


5.  Security Considerations

   The LINK and UNLINK methods are subject to the same general security
   considerations as all HTTP methods as described in
   [I-D.ietf-httpbis-p2-semantics].

   Implementers need to be aware of the possible ways the LINK method
   can be abused as a means of propagating inappropriate links to
   external resources.  For instance, the unregulated acceptance of LINK
   requests can be used as a vector for spam or malware distribution.

   Because the LINK and UNLINK methods cause changes to a resource's
   state, the server is responsible for determining the client's
   authorization to make such changes.


6.  Normative References

   [I-D.ietf-httpbis-p2-semantics]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-21 (work in progress),
              October 2012.

   [I-D.ietf-httpbis-p4-conditional]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Conditional Requests",
              draft-ietf-httpbis-p4-conditional-21 (work in progress),
              October 2012.

   [I-D.ietf-httpbis-p6-cache]
              Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching",
              draft-ietf-httpbis-p6-cache-21 (work in progress),



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              October 2012.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.


Author's Address

   James M Snell

   Email: jasnell@gmail.com


































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