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RFC 4946 - Atom License Extension

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Network Working Group                                           J. Snell
Request for Comments: 4946                                     July 2007
Category: Experimental

                         Atom License Extension

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This memo defines an extension to the Atom Syndication Format for
   describing licenses associated with Atom feeds and entries.

Table of Contents

   1. Introduction ....................................................2
      1.1. Notational Conventions .....................................2
      1.2. Terminology ................................................2
   2. The "license" Link Relation .....................................3
      2.1. Unspecified License Links ..................................3
      2.2. Comparing License Links ....................................3
      2.3. Example ....................................................3
      2.4. Inherited Licenses .........................................4
      2.5. Relationship with the atom:rights Element ..................4
   3. Security Considerations .........................................5
   4. IANA Considerations .............................................5
   5. Normative References ............................................5
   Appendix A.  Acknowledgements ......................................7

1.  Introduction

   This document defines an extension to the Atom Syndication Format
   that can be used to add links to Atom feed or entry documents
   referencing resources that describe the explicit rights that have
   been granted to consumers of those documents.

   Describing licenses using the mechanisms defined in this
   specification is currently considered to be largely experimental.
   While it is a common practice for feed publishers to associate
   licenses with syndicated content, there are still very few feed
   consumers that utilize such metadata, and the effectiveness of doing
   so is still largely unknown.  As it is not yet certain how these
   mechanisms will be used, implementations and feedback are encouraged.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, [RFC2119].

1.2.  Terminology

   The term "link relation" refers to the value of the Atom link
   elements rel attribute as defined in Section of [RFC4287].

   The term "license" refers to a potentially machine-readable
   description of explicit rights that have been granted to consumers of
   an Atom feed or entry.  Rights granted by a license may be associated
   with obligations that must be assumed by those exercising those

   Atom allows the use of Internationalized Resource Identifiers (IRIs)
   [RFC3987].  Every URI [RFC3986] is also an IRI, so a URI may be used
   wherever below an IRI is named.  When an IRI that is not also a URI
   is given for dereferencing, it MUST be mapped to a URI using the
   steps in Section 3.1 of [RFC3987].

   Atom allows the use of the xml:base attribute [W3C.REC-xmlbase-
   20010627] to establish the base URI (or IRI) used to resolve any
   relative references as described in Section 5.1.1 of [RFC3986].

   Atom allows the use of the xml:lang attribute to indicate the natural
   language of an element, its attributes, and its descendents.  The
   language context is only significant for elements and attributes
   declared to be "Language-Sensitive" as described in [RFC4287].  The
   title attribute of the atom:link element is defined as being

2.  The "license" Link Relation

   The "license" link relation can be used to associate licenses with a
   feed or entry.

   Feed and entry elements MAY contain any number of "license" link
   relations but MUST NOT contain more than one with the same
   combination of href and type attribute values.  The IRI specified by
   the link's href attribute SHOULD be dereferenceable to return a
   representation of the license.  The license representation MAY be
   machine readable.

   Feeds and entries can be dual-licensed by including multiple
   "license" link relations specifying different href attribute values.
   If multiple "license" link relations are specified, each SHOULD
   contain a title attribute specifying a human-readable label for the

2.1.  Unspecified License Links

   A publisher can use an "unspecified license link" (a "license" link
   relation with an href attribute value equal to
   "http://purl.org/atompub/license#unspecified") to indicate that a
   feed or license has not been explicitly linked to a license

   A feed or entry containing an unspecified license link MUST NOT
   contain any other "license" link relations.

2.2.  Comparing License Links

   Atom implementations that are capable of supporting "license" links
   SHOULD use the simple string-comparison method described in
   [RFC3987], Section 5.3.1, to determine whether multiple "license"
   links are equivalent.  The other comparison methods discussed in
   Section 5.1 of the IRI specification MAY be used.  If the "license"
   link is specified using a relative reference, it MUST be converted to
   its fully qualified form before comparison.

2.3.  Example

     <feed xmlns="http://www.w3.org/2005/Atom">
       <title>My Example Feed</title>
       <link href="http://www.example.com/myfeed" />
       <link rel="license" type="application/rdf+xml"
         href="http://creativecommons.org/licenses/by-nc/2.5/rdf" />

         Copyright (c) 2005.  Some rights reserved.  This feed
         is licensed under a Creative Commons Attribute-NonCommercial
         Use License.  It contains material originally published by
         Jane Smith at http://www.example.com/entries/1 under the
         Creative Commons Attribute License.
         <title>Atom Powered Robots Run Amok</title>
         <link href="http://www.example.org/entries/1" />
         <summary>Atom Powered Robots Run Amok</summary>
         <author><name>Jane Smith</name></author>
         <link rel="license" type="text/html"
           href="http://creativecommons.org/licenses/by/2.5/" />

2.4.  Inherited Licenses

   "License" links specified within atom:feed or atom:source elements
   are inherited by entries.  Generally, a more specific license
   overrides a less specific license.  More specifically, if an
   atom:entry has any "license" link relations, including the
   "undefined" license, it does not inherit any license from a
   containing atom:feed element or contained atom:source element.  If an
   entry has no "license" link relations, and contains an atom:source
   element, it inherits licenses from the atom:source.  If the
   atom:source element contains no "license" links, or if the entry does
   not have an atom:source element, the entry inherits licenses from the
   containing atom:feed.

   An entry's license MAY be inherited by media resources associated
   with the entry.  Some media types have ways of including license
   information, which always overrides the entry's licensing.

   An agent searching to find entries that fall under a specific license
   (e.g. a search for entries containing pictures and the word
   "knitting" where the entry is under any Creative Commons license)
   MUST check the most granular license definition available, rather
   than assume license inheritance without checking.

2.5.  Relationship with the atom:rights Element

   "License" link relations are related to the atom:rights element in
   that both are intended to communicate information about the rights

   associated with an Atom document.  Where the two differ, however, is
   in the fact that atom:rights is intended strictly as a human-
   readable assertion of rights held over a work while the "license"
   link relation is intended to provide a potentially machine-readable
   description of what rights have been granted to others.  The two
   constructs may overlap one another and conflicts could arise if they
   are contradictory.  It is important that publishers of Atom documents
   ensure that the atom:rights and "license" link relations are
   consistent with one another.

   If an atom:entry that does not contain an atom:source is copied from
   one feed into another feed, then if the feed into which it is copied
   contains a license, an atom:source element SHOULD be added to the
   copied entry.  If a source feed contains a license, that license
   SHOULD be preserved in an atom:source element added to any entries
   copied from the source feed that do not already contain atom:source

3.  Security Considerations

   When "license" link relations are used in Atom documents, the
   security considerations specified in [RFC4287] Section 8 apply.

4.  IANA Considerations

   This specification defines one new Atom link relation type to be
   registered in the IANA Registry of Link Relations as defined by

       Attribute Value: license
       Description: Specifies the IRI of a license associated with
                    the containing feed, entry, or source element.
       Expected display characteristics: None
       Security considerations: When "license" link relations are used
                    in Atom documents, the security considerations for
                    handling links in Atom documents apply.

5.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4287]  Nottingham, M., Ed., and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

              Marsh, J., "XML Base", World Wide Web Consortium
              Recommendation REC-xmlbase-20010627, June 2001,

Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from Mike Linksvayer,
   Robin Cover, Lisa Dusseault, Bjorn Hohrmann, Frank Ellermann, and the
   members of Atom Publishing Format and Protocol working group during
   the development of this specification.

Author's Address

   James M Snell

   EMail: jasnell@gmail.com
   URI:   http://www.snellspace.com

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

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   specification can be obtained from the IETF on-line IPR repository at

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