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Individual Submission                                           J. Snell
Internet-Draft
Intended status: Standards Track                          August 1, 2014
Expires: February 2, 2015


                      HTTP Link and Unlink Methods
                       draft-snell-link-method-10

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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on February 2, 2015.

Copyright Notice

   Copyright (c) 2014 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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Table of Contents

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

1.  Introduction

   This specification updates the HTTP LINK and UNLINK methods
   originally defined in [RFC2068].

   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 Relationships

   The LINK and UNLINK methods are used to manage relationships between
   resources.  Those relationships are defined using the Link model
   established in Section 3 of [RFC5988].  For every individual link,
   the context IRI, link relation type, target IRI, and optional
   collection of target attributes MUST be considered; that is, for any
   effective request URI, there can exist at most one Link relationship
   between any context and target IRI pairing with any given combination
   of link relation type and target attributes.

   Within LINK and UNLINK requests, a [RFC5988] Link header field is
   used to describe a Link relationship to be managed.  Any single LINK
   or UNLINK request MAY contain multiple Link header fields, each of
   which describes a separate relationship between a context IRI and
   target IRI.  When a LINK request contains multiple Link header
   fields, the server MUST create all of the specified relationships or
   not create any of them.  Likewise, when an UNLINK request contains
   multiple Link header fields, the server MUST either remove all the
   specified relationship or not remove any of them.






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   The target and context IRIs of a Link relationship are determined
   following the requirements specified in Sections 5.1 and 5.2 of
   [RFC5988].

   When determining whether or not a relationship already exists between
   a context IRI and target IRI, implementations will need to compare
   the given IRIs with other, previously established relationships.  To
   do so, the implementation MUST first resolve the IRIs as required by
   [RFC5988] and then compare on a case-sensitive, character-by-
   character basis.  For instance, the IRIs "http://example.org/foo" and
   "http://example.org/Foo" MUST NOT be considered to be equivalent.

3.  LINK

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

   LINK requests are idempotent but are not safe.  Establishing a
   relationship causes an inherent change to the state of the target
   resource.

   Any successful response (using a 2xx status code) to a LINK request
   indicates that all of the Link relationships described in the request
   have been established.  No specific 2xx status code is required.

   Responses to LINK requests SHOULD contain one Link header field for
   each Link relationship established by the LINK request.

   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 [RFC7234]).

   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 ([RFC7232]).
   A conditional LINK requests that the relationship be established only
   under the circumstances described by the conditional header field(s).

4.  UNLINK

   The UNLINK method is used to remove one or more relationships between
   the resource identified by the effective request URI and other
   resources.  Metadata contained within Link header fields [RFC5988]



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   provide the information about the relationships that are to be
   removed.  A payload within an UNLINK request has no defined
   semantics.

   UNLINK request messages are idempotent but are not safe.  Removing a
   relationship causes an inherent change to the state of the target
   resource.

   Responses to UNLINK requests SHOULD contain one Link header field for
   each Link relationship removed by the UNLINK request.

   Any successful response (using a 2xx status code) to an UNLINK
   request indicates that all of the Link relationships described in the
   request have been removed.  No specific 2xx status code is required.

   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 [RFC7234]).

   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 ([RFC7232]).
   A conditional UNLINK requests that the relationship be established
   only under the circumstances described by the conditional header
   field(s).

5.  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.

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








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6.  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"

   Possible response:

     HTTP/1.1 202 Accepted
     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"

   Possible response:

     HTTP/1.1 204 No Content
     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="mention"

   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 its current
   state (e.g. pub/sub)

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

   Example 7: Using the Link anchor attribute to change the context IRI
   (in this example, a link relationship is established between the IRIs
   "acct:joe@example.org" and "acct:sally@example.org")

     LINK /my-resource HTTP/1.1
     Host: example.org
     Link: <acct:joe@example.org>; rel="follow";
       anchor="acct:sally@example.org"

7.  Security Considerations

   The LINK and UNLINK methods are subject to the same general security
   considerations as all HTTP methods as described in [RFC7231].

   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.

8.  IANA Considerations

   IANA is requested to add the LINK and UNLINK methods to the permanent
   registry at <http://www.iana.org/assignments/http-methods> (see
   Section 8.1 of [RFC7231]).

            +-------------+------+------------+---------------+
            | Method Name | Safe | Idempotent | Specification |
            +-------------+------+------------+---------------+
            | LINK        | No   | Yes        | Section 3     |
            | UNLINK      | No   | Yes        | Section 4     |
            +-------------+------+------------+---------------+








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9.  References

9.1.  Normative References

   [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.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [RFC7232]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Conditional Requests", RFC 7232, June 2014.

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
              2014.

9.2.  Informational References

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

Author's Address

   James M Snell

   Email: jasnell@gmail.com





















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