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Network Working Group                                           J. Snell
Internet-Draft                                           October 9, 2012
Intended status: Informational
Expires: April 12, 2013


                      HTTP Link and Unlink Methods
                       draft-snell-link-method-01

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 12, 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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  LINK  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  UNLINK  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Relationship to other HTTP Methods and Discoverability of
       Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8








































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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 relationships
   between an 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 connected to the target resource and the type of relationship
   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 relationship 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 relationship of any given type can exist.  However,
   multiple relationships of different types can be established between
   the same pair of resources.

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

   Responses to LINK requests are not cacheable.  If a LINK request
   passes through a cache that has one or more stored responses for the
   effective request URI, those stored responses will be invalidated
   (see Section 6 of [I-D.ietf-httpbis-p6-cache]).

   A single LINK request message can contain multiple Link header
   fields, each of which establishes a separate relationship with the



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   target resource.  In such cases, the server MUST accept the entire
   set of relationships atomically.  If any of the specified
   relationships cannot be created, the server MUST NOT create any of
   them.

   A successful response to a Link request that results in either the
   creation or modification of a relationship 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 relationship, 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.

   The LINK 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 create or
   update the specified relationships.

   If the LINK request message attempts to create or update an existing
   relationship and the server does not intend to comply with the
   request for any reason other than a client or server error, the
   server can return a 304 (Not Modified) response to indicate that no
   modifications have been made.


3.  UNLINK

   The UNLINK method is used to remove one or more 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 the resources to which
   relationships 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 relationship 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



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   relationship causes an inherent change to the state of the target
   resource.

   Responses to UNLINK requests are not cacheable.  If an UNLINK request
   passes through a cache that has one or more stored responses for the
   effective request URI, those stored responses will be invalidated
   (see Section 6 of [I-D.ietf-httpbis-p6-cache]).

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

   A successful response indicating the removing of the relationship
   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.

   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 relationships.

   If the UNLINK request message attempts to remove an existing
   relationship and the server does not intend to remove or otherwise
   alter the existing relationship for any reason other than a client or
   server error, the server can return a 304 (Not Modified) response to
   indicate that no modifications have been made.


4.  Relationship to other HTTP Methods and Discoverability of Links

   The use of the LINK and UNLINK request methods to manage
   relationships between resources has no direct bearing on the use or
   appearance of Link header fields within any other HTTP request or
   response message involving the same effective request URI.  Nor do
   the methods have any direct normative impact on the use of link-like
   structures within the resource representations returned by a server
   for any particular resource.

   Whether and how to represent relationships managed using LINK and
   UNLINK is left solely at the discretion of the server implementation.




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   This specification does not define a means of discovering or
   enumerating the relationships that have been established using the
   LINK request method.


5.  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"

   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"






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

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


6.  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.


7.  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),
              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.




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   [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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