Independent Submission                                          E. Wilde
Request for Comments: 6892                               EMC Corporation
Category: Informational                                       March 2013
ISSN: 2070-1721

                   The 'describes' Link Relation Type

Abstract

   This specification defines the 'describes' link relation type that
   allows resource representations to indicate that they are describing
   another resource.  In contexts where applications want to associate
   described resources and description resources, and want to build
   services based on these associations, the 'describes' link relation
   type provides the opposite direction of the 'describedby' link
   relation type, which already is a registered link relation type.

Status of This Memo

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

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not 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/rfc6892.

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Table of Contents

   1. Introduction ....................................................2
   2. Resource Descriptions ...........................................2
   3. Use Case ........................................................3
   4. IANA Considerations .............................................4
   5. Security Considerations .........................................4
   6. Acknowledgements ................................................4
   7. References ......................................................5
      7.1. Normative References .......................................5
      7.2. Informative References .....................................5

1.  Introduction

   Resources on the web can be identified by any (registered) URI scheme
   and can be represented by any (registered) media type.  In many
   cases, applications establish specific (i.e., typed) relations
   between the resources they are concerned with, which can either be
   under their control or controlled by another authority.  A common
   usage pattern for associating resources is to have resources that are
   descriptions of other resources.  This specification registers the
   'describes' link relation, which allows applications to represent the
   fact that one resource is a description of another resource.

   RFC 5988 [1] "defines a framework for typed links that isn't specific
   to a particular serialisation or application.  It does so by
   redefining the link relation registry established by Atom to have a
   broader domain, and adding to it the relations that are defined by
   HTML".  This registration request intends to augment the link
   relation registry with a link relation that is the inverse of the
   already registered 'describedby' relation, so that links between
   described resources and describing resources can be represented in
   both directions.

2.  Resource Descriptions

   Associating resources with descriptions of these resources is a
   recurring pattern on the web.  The IANA "Link Relations" registry
   established by RFC 5988 [1] currently contains a 'describedby' link
   relation type, which has been registered by POWDER [2].  The
   definition given in the reference document for that registration is
   as follows: "The relationship A 'describedby' B asserts that resource
   B provides a description of resource A.  There are no constraints on
   the format or representation of either A or B, neither are there any
   further constraints on either resource".

   Since many scenarios using resource descriptions need to represent
   the fact that some resource describes another resource (the opposite
   of the 'describedby' relation), this document registers a 'describes'
   link relation type.  Establishing a link A 'describes' B asserts that
   the resource identified by A is a description of the resource
   identified by B, without constraining in any way the identifiers
   being used for A and B, and the media types for the representations
   being provided when those identifiers are dereferenced.
   Specifically, it is possible that identifiers A and/or B have no
   associated interaction method (they could be URNs, for example), but
   it still is valid to establish the A 'describes' B link.

   Another design freedom is to use "chains" of 'describes' (or
   'describedby') links, so that one resource is a description of
   another resource, which in turn is a description of yet another
   resource.  The "levels" of descriptions can go as deep as required by
   an application and are not constrained by this specification.

3.  Use Case

   Beginning with the POWDER document [2], which specifies the
   'describedby' link relation, the use case for the 'describedby' link
   relation is that a described resource, such as an HTML web page, can
   specify a link where clients can find a description of this resource.
   While the 'describedby' link relation is defined to be independent of
   specific formats and representations, within the context of POWDER,
   the assumption is that the linked resources most often will provide a
   description based on the Resource Description Framework (RDF), for
   example, to provide metadata about a document's author and other
   provenance information.

   The 'describes' link relation allows servers hosting description
   resources to associate those description resources with the resources
   that they are describing.  In the RDF-oriented scenario of POWDER,
   this means that a service managing description resources would use
   'describes' links to represent the fact that the description
   resources it exposes provide some description of the described
   resource, very likely in some RDF representation.  However, since
   link relations are independent of resource formats or
   representations, such an association could also be made in other
   formats such as XML or JavaScript Object Notation (JSON), allowing
   servers to use a single and consistent link relation to associate
   description resources with described resources.

   Generally speaking, the idea of the 'describes' relation is the same
   as the idea of the 'describedby' relation; to be independent of
   specific formats and representations of both described resources and
   description resources.  The 'describes' link relation (together with

   the already registered 'describedby' link relation) thus serves as a
   general foundation of how described resources and description
   resources can be associated.

4.  IANA Considerations

   The link relation type below has been registered by IANA per Section
   6.2.1 of RFC 5988 [1]:

      Relation Name: describes

      Description: The relationship A 'describes' B asserts that
      resource A provides a description of resource B.  There are no
      constraints on the format or representation of either A or B,
      neither are there any further constraints on either resource.

      Reference: [RFC6892]

      Notes: This link relation type is the inverse of the 'describedby'
      relation type.  While 'describedby' establishes a relation from
      the described resource back to the resource that describes it,
      'describes' established a relation from the describing resource to
      the resource it describes.  If B is 'describedby' A, then A
      'describes' B.

5.  Security Considerations

   Resource descriptions should never be treated as authoritative or
   exclusive without relying on additional mechanisms for trust and
   security.  Resources can have many (possibly conflicting)
   descriptions, and the 'describes' link relation type makes no claim
   whatsoever about the authority of the party providing the association
   between the two resources, or about the authority of the party
   providing the description resource.  Before making any assumptions
   about the authority of the description resource (both the accuracy of
   the description contained in the description resource, and the
   authority to provide a description of the described resource),
   clients need a context that allows them to understand both the
   authority of the description itself, and the authority to establish
   the 'describes' relation.  Nobody can stop clients from providing
   misleading unauthorized and/or descriptions, and clients need to have
   both a security and trust framework to allow them to choose between
   trusted and untrusted descriptions.

6.  Acknowledgements

   Thanks for comments and suggestions provided by Mark Nottingham.

7.  References

7.1.  Normative References

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

7.2.  Informative References

   [2]  Archer, P., Smith, K., and A. Perego, "Protocol for Web
        Description Resources (POWDER): Description Resources", World
        Wide Web Consortium Recommendation REC-powder-dr-20090901,
        September 2009,
        <http://www.w3.org/TR/2009/REC-powder-dr-20090901/>.

Author's Address

   Erik Wilde
   EMC Corporation
   6801 Koll Center Parkway
   Pleasanton, CA 94566
   U.S.A.

   Phone: +1-925-600-6244
   EMail: erik.wilde@emc.com
   URI:   http://dret.net/netdret/