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Network Working Group                                           J. Snell
Internet-Draft                                            April 11, 2006
Expires: October 13, 2006


                       Atom Threading Extensions
                 draft-snell-atompub-feed-thread-08.txt

Status of this Memo

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   This Internet-Draft will expire on October 13, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo presents a mechanism that allows feeds publishers to
   express threaded discussions within the Atom Syndication Format.










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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  The 'in-reply-to' extension element  . . . . . . . . . . . . .  3
   4.  The 'replies' link relation  . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11





































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1.  Introduction

   This document defines an extension for expressing threaded
   discussions within the Atom Syndication Format [RFC4287].


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], as
   scoped to those conformance targets.

   The XML Namespaces URI [W3C.REC-xml-names-19990114] for the XML
   elements and attributes described in this specification is:
     http://purl.org/syndication/thread/1.0

   In this document, the namespace prefix "thr:" is used for the above
   Namespace URI.

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

   Some sections of this specification are illustrated with a non-
   normative RELAX NG Compact schema [RELAXNG].  However, the text of
   this specification provides the definition of conformance.

   This specification uses the atomCommonAttributes, atomURI, and
   atomMediaType constructs as defined in [RFC4287].


3.  The 'in-reply-to' extension element

   The "in-reply-to" extension element is used to indicate that a feed
   entry is a response to another resource.  The element MUST contain
   either or both the "ref" and "href" attributes.










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     in-reply-to =
       element thr:in-reply-to {
         atomCommonAttributes,
        ( ref, source?
        | href, type?
        | ref, source?, href, type? )
       }

     ref = attribute ref { atomURI }
     href = attribute href { atomURI }
     type = attribute type { atomMediaType }
     source = attribute source { atomURI }


   The "ref" attribute specifies the universally unique identifier of
   the resource being responded to.  The value of the attribute MUST
   conform to the same construction and comparison rules as the atom:id
   element.  The "source" attribute MAY be used to specify the IRI
   [RFC3987] of an Atom Feed or Entry Document within which an atom:
   entry with an atom:id value equal to the value of the "ref" attribute
   may be found.

   The "href" attribute specifies an IRI that may be used to retrieve a
   representation of the resource being responded to.  The "type"
   attribute MAY be used to identify the media type [RFC4288] of the
   resource identified by the "href" attribute.

   Atom feed, entry and source elements MAY each contain any number of
   "in-reply-to" extension elements but SHOULD NOT contain more than one
   having the same "ref" value.





















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   An example entry with a response,

     <feed xmlns="http://www.w3.org/2005/Atom"
           xmlns:thr="http://purl.org/syndication/thread/1.0">
       <id>http://www.example.org/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.org/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:example.org,2005:1</id>
         <title>My original entry</title>
         <updated>2006-03-01T12:12:12Z</updated>
         <link
           type="application/xhtml+xml"
           href="http://www.example.org/entries/1" />
         <summary>This is my original entry</summary>
       </entry>
       <entry>
         <id>tag:example.org,2005:1,1</id>
         <title>A response to the original</title>
         <updated>2006-03-01T12:12:12Z</updated>
         <link href="http://www.example.org/entries/1/1" />
         <thr:in-reply-to
           ref="tag:entries.com,2005:1"
           type="application/xhtml+xml"
           href="http://www.example.org/entries/1"/>
         <summary>This is a response to the original entry</summary>
       </entry>
     </feed>

   To allow Atom clients that are not familiar with the in-reply-to
   extension to know that a relationship exists between the entry and
   the resource being responded to, publishers are advised to consider
   including a "related" link referencing a representation of the
   resource identified by the in-reply-to element.















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   For example

     <feed xmlns="http://www.w3.org/2005/Atom"
           xmlns:thr="http://purl.org/syndication/thread/1.0">
       <id>http://www.example.org/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.org/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:example.org,2005:1,1</id>
         <title>A response to the original</title>
         <updated>2006-03-01T12:12:12Z</updated>
         <link href="http://www.example.org/entries/1/1" />
         <thr:in-reply-to
           ref="tag:example.org,2005:1,0"
           type="application/xhtml+xml"
           href="http://www.example.org/entries/1"
           source="http://www.example.org/myfeed" />
         <link
           rel="related"
           type="application/xhtml+xml"
           href="http://www.example.org/entries/1" />
         <summary>This is a response to the original entry</summary>
       </entry>
     </feed>


4.  The 'replies' link relation

   While responses to an atom:entry may appear within the same feed
   document as the resource being responded to, it is common practice to
   separate responses into a separate feed document or other resource.
   In such cases, it is helpful for an atom:feed or atom:entry to
   indicate the location where responses may be found.  An atom:link
   element with the "rel" attribute value "replies" is used for this
   purpose.  If the type attribute of the atom:link is omitted, it's
   value is assumed to be "application/atom+xml".

   Atom feed, entry and source elements MAY each contain any number of
   "replies" atom:link elements but SHOULD NOT contain more than one
   having the same "type" and "href" attribute values.

   A "replies" link appearing as a child of the Atom feed or source
   element indicates that the referenced resource may contain responses
   to any of the feeds entries.  A "replies" link appearing as a child
   of an Atom entry element indicates that the referenced resource may
   contain responses specific to that entry.



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   Atom links using the "replies" rel attribute value MAY contain a
   "thr:count" attribute whose value is a non-negative integer
   indicating the total number of replies contained by the linked
   resource.  The value is strictly advisory and may not accurately
   reflect the actual number of replies.

   Reply links MAY also contain a "thr:when" attribute whose value is a
   [RFC3339] date-time stamp conforming to the same construction rules
   as the Atom Date Construct defined in [RFC4287], and is used to
   indicate the date and time of the most recent reply contained by the
   linked resource.  The value is strictly advisory and may not
   accurately reflect the actual date and time of the most recent reply.

   For example, replies contained in a separate Atom feed

     <feed xmlns="http://www.w3.org/2005/Atom"
           xmlns:thr="http://purl.org/syndication/thread/1.0">
       <id>http://www.example.org/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.org/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:entries.com,2005:1</id>
         <title>My original entry</title>
         <updated>2006-03-01T12:12:12Z</updated>
         <link href="http://www.example.org/entries/1" />
         <link rel="replies"
               type="application/atom+xml"
               href="http://www.example.org/mycommentsfeed.xml"
               thr:count="10"
               thr:when="2006-02-20T00:00:00Z" />
         <summary>This is my original entry</summary>
       </entry>
     </feed>

   The presence of a "replies" link is merely a hint as to where
   responses to entries may be found and does not guarantee that the
   referenced resource contains any responses to any entries within the
   containing feed.

   The behavior of "replies" link relations specifying any media type
   other than "application/atom+xml" is undefined.  Software written to
   conform to this version of the specification will not be guaranteed
   to process such links correctly.

   Implementors should note that while the Atom Syndication Format does
   not forbid the inclusion of namespaced extension attributes on the



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   Atom link element, neither does it explicitly allow for such
   extensions.  As a result, the thr:count and thr:when attributes fall
   under what [RFC4287] defines as "unknown foreign markup".


5.  Security Considerations

   As this specification defines an extension to the Atom Syndication
   Format, it is subject to the same security considerations as defined
   in [RFC4287].

   Feeds using the mechanisms described here could be crafted in such a
   way as to cause a consumer to initiate excessive (or even an unending
   sequence of) network requests, causing denial of service (either to
   the consumer, the target server, and/or intervening networks).
   Consumers can mitigate this risk by requiring user intervention after
   a certain number of requests, or by limiting requests either
   according to a hard limit, or with heuristics.

   Consumers should be mindful of resource limits when storing feed
   documents; to reiterate, they are not required to always store or
   reconstruct the feed when conforming to this specification; they only
   need inform the user when the reconstructed feed is not complete.


6.  IANA Considerations

   This specification defines one new Atom link relation type to be
   registered in the IANA Registry of Link Relation as defined by
   [RFC4287].
       Attribute Value: replies
       Description: (see section 4)
       Expected display characteristics: None
       Security considerations: (see section 5)


7.  References

7.1.  Normative References

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

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

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



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   [RFC4287]  Nottingham, M. and R. Sayre, "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.

   [W3C.REC-xml-infoset-20040204]
              Tobin, R. and J. Cowan, "XML Information Set (Second
              Edition)", W3C REC REC-xml-infoset-20040204,
              February 2004.

   [W3C.REC-xml-names-19990114]
              Hollander, D., Bray, T., and A. Layman, "Namespaces in
              XML", W3C REC REC-xml-names-19990114, January 1999.

7.2.  Informative References

   [RELAXNG]  Clark, J., "RELAX NG Compact Syntax", December 2001,
              <http://www.oasis-open.org/committees/relax-ng/
              compact-20021121.html>.


Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from James
   Holderness, Byrne Reese, Aristotle Pagaltzis, and the remaining
   members of the Atom Publishing Format and Protocol working group
   during the development of this specification.























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Author's Address

   James M Snell


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











































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Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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