    Internet Engineering Task Force (IETF)                       M. Amundsen
    Request for Comments: 6573                                    April 2012
    Category: Informational
    ISSN: 2070-1721

                     The Item and Collection Link Relations

    Abstract

       RFC 5988 standardized a means of indicating the relationships between
       resources on the Web. This specification defines a pair of reciprocal
       link relation types that may be used to express the relationship
       between a collection and its members.

    Status of This Memo

       This document is not an Internet Standards Track specification; it is
       published for informational purposes.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Not all documents
       approved by the IESG are a candidate for any level of Internet
       Standard; see Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6573.

    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
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Amundsen                      Informational                     [Page 1]

    RFC 6573         The Item and Collection Link Relations       April 2012

    1.  Introduction

       RFC 5988 standardized a means of indicating the relationships between
       resources on the Web. This specification defines a pair of reciprocal
       link relation types that may be used to express the relationship
       between a collection and its members.

       These link relation types can be applied to a wide range of use cases
       across multiple media types.  For example, the 'collection' and
       'item' link relation types are used in these media types:

       1.  OpenSearch 1.1: see Section 4.5.4.1 of [OpenSearch]

       2.  Maze+XML: see [Maze]

       3.  Collection+JSON: see [CollectionJSON]

    2.  Link Relations

       The following link relations are defined.

    2.1.  'item'

       When included in a resource that represents a collection, the 'item'
       link relation identifies a target resource that represents a member
       of that collection.

       For example, if a resource represents a catalog of products, that
       same representation may include one or more links to resources that
       represent members of that catalog.

       <html>
       ...
         <h1>Product Group X Listing</h1>
         ...
         <a href="..." rel="item">View Product X001</a>
         <a href="..." rel="item">View Product X002</a>
         ...
       </html>

       or, in the case of a Link Header field

       Link: <...>; rel="item"; title="View Product X001"
       Link: <...>; rel="item"; title="View Product X002"

    Amundsen                      Informational                     [Page 2]

    RFC 6573         The Item and Collection Link Relations       April 2012

    2.2.  'collection'

       When included in a resource that represents a member of a collection,
       the 'collection' link relation identifies a target resource that
       represents a collection of which the context resource is a member.

       For example, if a resource represents a single product in a catalog,
       that same representation may include a link to a resource that
       represents a product group to which this single product belongs:

       <a href="..." rel="collection">Return to Product Group X</a>

       or, in the case of a Link Header field

       Link: <...>; rel="collection"; title="Return to Product Group X"

       Since it is possible that a resource could be a member of multiple
       collections, multiple 'collection' link relations may appear within
       the same representation:

       <a href="..." rel="collection">View other widgets</a>
       <a href="..." rel="collection">View all discontinued items</a>

       The target resource representation need not be restricted to
       representing a list.  It may simply be a document that provides
       details on the collection of which the context resource is a member:

       Link: <...>; rel="collection";
             title="Shakespeare's Collected Works - A History"

       It should also be noted that the same link might represent an 'item'
       in one collection as well as a 'collection' itself.  In this case,
       both link relation values can be applied to the same link:

       Link: <...>; rel="collection item";
             title="A Review of Issac Asimov's Collected Works - Vol. I"

    3.  IANA Considerations

       IANA has registered the 'collection' and 'item' link relations below
       as per [RFC5988].

    3.1.  'item' Link Relation Registration

       Relation Name:

          item

    Amundsen                      Informational                     [Page 3]

    RFC 6573         The Item and Collection Link Relations       April 2012

       Description:

          The target IRI points to a resource that is a member of the
          collection represented by the context IRI.

       Reference:

          See Section 2

    3.2.  'collection' Link Relation Registration

       Relation Name:

          collection

       Description:

          The target IRI points to a resource that represents the collection
          resource for the context IRI.

       Reference:

          See Section 2

    4.  Security Considerations

       The two link relation types defined in this document do not introduce
       any new security issues to those which are discussed in Section 7 of
       RFC5988 [RFC5988].

    5.  Internationalisation Considerations

       The 'item' and 'collection' link relation types do not have any
       internationalization considerations other than those which are
       discussed in Section 8 of RFC5988 [RFC5988].

    6.  References

    6.1.  Normative References

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

    6.2.  Informative References

       [CollectionJSON]  Amundsen, M., "Collection+JSON - Document Format",
                         Web Page , July 2011,
                         <http://www.iana.org/assignments/media-types>.

    Amundsen                      Informational                     [Page 4]

    RFC 6573         The Item and Collection Link Relations       April 2012

       [Maze]            Amundsen, M., "Maze+XML - Format", Web Page ,
                         December 2010,
                         <http://www.iana.org/assignments/media-types>.

       [OpenSearch]      Clinton, D., "Open Search 1.1", Work in Progress ,
                         March 2011, <http://www.opensearch.org/
                         Specifications/OpenSearch/1.1/>.

    Appendix A.  Acknowledgements

       The author gratefully acknowledges the contributions of Julian
       Reschke and Mykyta Yevstifeyev.

    Author's Address

       Mike Amundsen

       EMail: mca@amundsen.com
       URI:   http://amundsen.com

    Amundsen                      Informational                     [Page 5]