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Network Working Group                                           J. Snell
Internet-Draft                                          October 27, 2007
Intended status: Experimental
Expires: April 29, 2008


               Atom Publishing Protocol Feature Discovery
                   draft-snell-atompub-feature-12.txt

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   This Internet-Draft will expire on April 29, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document introduces extensions to the Atom Publishing Protocol
   Service Document format for expressing metadata about the behaviors,
   functions and capabilities supported by an Atom Publishing Protocol
   collection.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  The f:features Element . . . . . . . . . . . . . . . . . . . .  4
   4.  The f:feature element  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Feature Documents  . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  The "supportsDraft" Feature  . . . . . . . . . . . . . . . . .  7
   7.  The "ignoresDraft" Feature . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     9.1.  Content-type registration for
           'application/atomfeature+xml'  . . . . . . . . . . . . . .  8
   10. Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11
































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1.  Introduction

   This document introduces an extension to the Atom Publishing Protocol
   (Atompub) service document [RFC5023] format that allows for the
   documentation and discovery of behaviors, functions or capabilities
   supported by an Atompub collection.  Examples of such capabilities
   can include the preservation of certain kinds of content, support for
   draft entries, support for the scheduled publication of entries, use
   of a particular set of Atom format extensions, and so on.

   Describing features using the mechanisms defined in this
   specification is currently considered to be largely experimental.
   While there are examples of such mechanisms being put to practical
   use in deployed applications, there is currently not enough
   collective implementation experience to determine whether the
   mechanism defined here is sufficient to fully address the general
   needs of Atom Publishing Protocol client and server implementations.
   Implementations and feedback are encouraged.


2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119].

   This specification uses XML Namespaces [W3C.REC-xml-names-20060816]
   to uniquely identify XML element names.  It uses the following
   namespace prefix for the indicated namespace URI;

     "f": "http://purl.org/atompub/features/1.0"

   This specification uses terms from the XML Infoset
   [W3C.REC-xml-infoset-20040204].  However, this specification uses a
   shorthand; the phrase "Information Item" is omitted when naming
   Element Information Items.  Therefore, when this specification uses
   the terms "element" and "attribute" it is referring, respectively, to
   the Element and Attribute Information Items in Infoset terms.

   This specification uses the terms "atomUri" and
   "atomCommonAttributes" from the non-normative RELAX NG Compact schema
   included in [RFC4287].  Where used, these serve the same purpose and
   have the same meaning as their use in [RFC4287].

   Atom allows the use of IRIs [RFC3987].  Every URI [RFC3986] is also
   an IRI, so a URI may be used wherever below an IRI is named.  There
   are two special considerations: (1) when an IRI that is not also a
   URI is given for dereferencing, it MUST be mapped to a URI using the



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   steps in Section 3.1 of [RFC3987] and (2) when an IRI is serving as
   an identifier, it MUST NOT be so mapped.

   Any element defined by this specification MAY have an xml:base
   attribute [W3C.REC-xmlbase-20010627].  When xml:base is used, it
   serves the function described in section 5.1.1 of [RFC3986],
   establishing the base URI (or IRI) for resolving any relative
   references found within the effective scope of the xml:base
   attribute.

   Any element defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its descendents.  The language context is only
   significant for elements and attributes declared to be "Language-
   Sensitive".  Requirements regarding the content and interpretation of
   xml:lang are specified in XML 1.0 [W3C.REC-xml-20060816], Section
   2.12.


3.  The f:features Element

   The f:features element is used in an app:collection element or as the
   root element of a Features Document as a container for zero or more
   f:feature elements.

   An f:features element MAY contain any number of f:feature elements
   but MUST NOT contain more than one with the same ref attribute value.
   The order in which f:feature elements appear within the f:features
   element is insignificant.

   An app:collection element MUST NOT contain more than one f:features
   element.

     inlineFeatures =
       element f:features {
         (feature*,
          undefinedContent)
       }

     outOfLineFeatures =
       element f:features {
         attribute href { atomURI }
       }

     features = inlineFeatures | outOfLineFeatures

   When used within an app:collection element, the f:features element
   MAY contain an "href" attribute, whose value MUST be an IRI reference



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   identifying a Features Document.  If the "href" attribute is
   provided, the f:features element MUST be empty.


4.  The f:feature element

   The f:feature element is used within an f:features element to declare
   a collection's support for a feature.

     feature = element f:feature {
       atomCommonAttributes,
       attribute ref    { atomUri },
       attribute href   { atomUri }?,
       attribute label  { text }?,
       (anyElement)*
     }

     anyElement = element * - f:feature {
       (attribute * { text }
         | text
         | anyElement)*
     }

   The ref attribute specifies a globally unique IRI identifying a
   feature.  The value of the ref attribute MUST be compared on a case-
   sensitive, character-by-character basis.  Relative references MUST
   NOT be used.  The IRI is used strictly for identification and cannot
   be assumed to be dereferenceable.

   An optional href attribute MAY be used to specify a dereferenceable
   IRI of a human-readable description of the feature.  Relative
   references MAY be used.

   The optional label attribute MAY be used to specify a human-readable
   label for the feature.  The value of the label attribute is Language-
   Sensitive as defined by Section 2 of [RFC4287].

   The f:feature element MAY contain a mix of text and any number of
   child elements and attributes from any XML namespace, with the
   exception of the f:feature element itself.  Such markup is considered
   to be metadata applicable to the feature identified.  Software agents
   MUST NOT stop processing or signal an error or change their behavior
   as a result of encountering such markup.

   The presence of a f:feature element is an indication that the
   collection supports the feature identified.  The lack of a f:feature
   element identifying a particular feature indicates that support for
   that feature is unspecified.  This does not mean that the feature is



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   not supported, only that the server has not explicitly declared
   support.  There are no means by which a server can indicate that a
   given feature is unsupported.

4.1.  Example

   The following is an example of a collection using the f:feature
   element to indicate its support for the app:draft element as defined
   by [RFC5023].

   <service
     xmlns="http://www.w3.org/2007/app"
     xmlns:atom="http://www.w3.org/2005/Atom"
     xmlns:f="http://purl.org/atompub/features/1.0">
     <workspace>
       <atom:title>My Workspace</atom:title>
       <collection href="...">
         <atom:title>My Atom Collection</atom:title>
         <accept>application/atom+xml;type=entry</accept>
         <f:features>
           <f:feature
             ref="http://purl.org/atompub/features/1.0/supportsDraft" />
         </f:features>
       </collection>
     </workspace>
   </service>


5.  Feature Documents

   A Feature Document contains a listing of f:feature elements that can
   be referenced by multiple app:collection elements.  Separating the
   list of f:features out into a separate document enables and promotes
   the reuse of features across multiple collections.

   The root of a Feature Document is the f:features element.  The
   f:features element MUST NOT contain an href atribute.  Feature
   documents are identified with the "application/atomfeature+xml" media
   type (see Section 9.1).

5.1.  Example

  <?xml version="1.0?>
  <features xmlns="http://purl.org/atompub/features/1.0">
    <feature ref="http://purl.org/atompub/features/1.0/supportsDraft" />
    <feature ref="http://example.org/foo/supportsFoo" />
  </features>




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   This Feature Document indicates support for the app:draft publishing
   control and a hypothetical extension.

   The following is an example of a collection using the f:features
   element to reference a Feature Document.

     <service
       xmlns="http://www.w3.org/2007/app"
       xmlns:atom="http://www.w3.org/2005/Atom"
       xmlns:f="http://purl.org/atompub/features/1.0">
       <workspace>
         <atom:title>My Workspace</atom:title>
         <collection href="...">
           <atom:title>My Atom Collection</atom:title>
           <accept>application/atom+xml;type=entry</accept>
           <f:features href="http://example.org/features.xml" />
         </collection>
       </workspace>
     </service>


6.  The "supportsDraft" Feature

   The "supportsDraft" feature is used to indicate that a collection
   supports the use of the app:draft control element as defined by
   Section 13.1.1 of [RFC5023].  The IRI of the "supportsDraft" feature
   is:
     http://purl.org/atompub/features/1.0/supportsDraft


7.  The "ignoresDraft" Feature

   The "ignoresDraft" feature is used to indicate that a collection will
   ignore the use of the app:draft control element as defined by Section
   13.1.1 of [RFC5023].  The IRI of the "ignoresDraft" feature is:
     http://purl.org/atompub/features/1.0/ignoresDraft

   A server MUST NOT declare support for both the "supportsDraft" and
   "ignoresDraft" features within the a single app:collection element.


8.  Security Considerations

   Specific features supported by a collection may introduce security
   considerations and concerns beyond those discussed by the Atom
   Publishing Protocol and Atom Syndication Format specifications.
   Implementors must refer to the specifications and description of each
   feature to determine the security considerations relevant to each.



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   Implementations of this specification handle URIs and IRIs.  See
   Section 7 of [RFC3986] and Section 8 of [RFC3987] for security
   considerations related to their handling and use.


9.  IANA Considerations

   This specification uses a new media type that conforms to the
   registry mechanism described in [RFC4288].

9.1.  Content-type registration for 'application/atomfeature+xml'

   A Feature Document, when serialized as XML 1.0, can be identified
   with the following media type:

   MIME media type name:  application
   MIME subtype name:  atomfeature+xml
   Mandatory parameters:  None.
   Optional parameters:
      "charset":  This parameter has identical semantics to the charset
         parameter of the "application/xml" media type as specified in
         [RFC3023].
   Encoding considerations:  Identical to those of "application/xml" as
      described in [RFC3023], section 3.2.
   Security considerations:  As defined in this specification.
      In addition, as this media type uses the "+xml" convention, it
      shares the same security considerations as described in [RFC3023],
      section 10.
   Interoperability considerations:  There are no known interoperability
      issues.
   Published specification:  This specification.
   Applications that use this media type:  No known applications
      currently use this media type.

   Additional information:

   Magic number(s):  As specified for "application/xml" in [RFC3023],
      section 3.2.
   File extension:  .atomfeature
   Fragment identifiers:  As specified for "application/xml" in
      [RFC3023], section 5.
   Base URI:  As specified in [RFC3023], section 6.
   Macintosh File Type code:  TEXT
   Person and email address to contact for further information:  James
      Snell <jasnell@gmail.com>






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   Intended usage:  COMMON
   Author/Change controller:  IETF (iesg@ietf.org) Internet Engineering
      Task Force


10.  Normative References

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

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [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.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC5023]  Gregorio, J. and B. de hOra, "The Atom Publishing
              Protocol", RFC 5023, October 2007.

   [W3C.REC-xml-20060816]
              Paoli, J., Maler, E., Yergeau, F., Bray, T., and C.
              Sperberg-McQueen, "Extensible Markup Language (XML) 1.0
              (Fourth Edition)", World Wide Web Consortium
              Recommendation REC-xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [W3C.REC-xml-infoset-20040204]
              Tobin, R. and J. Cowan, "XML Information Set (Second
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-infoset-20040204, February 2004,
              <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.

   [W3C.REC-xml-names-20060816]
              Hollander, D., Tobin, R., Bray, T., and A. Layman,
              "Namespaces in XML 1.0 (Second Edition)", World Wide Web
              Consortium Recommendation REC-xml-names-20060816,
              August 2006,
              <http://www.w3.org/TR/2006/REC-xml-names-20060816>.



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   [W3C.REC-xmlbase-20010627]
              Marsh, J., "XML Base", World Wide Web Consortium
              Recommendation REC-xmlbase-20010627, June 2001,
              <http://www.w3.org/TR/2001/REC-xmlbase-20010627>.


Appendix A.  Acknowledgements

   The author acknowledges the feedback from the other members of the
   IETF Atom Publishing working group during the development of this
   specification.


Author's Address

   James M Snell


   Phone:
   Email: jasnell@gmail.com
   URI:   http://snellspace.com






























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   This document is subject to the rights, licenses and restrictions
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