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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.

Copyright Notice

Copyright (c) 2013 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.






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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".






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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



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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.




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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,
.

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/


























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