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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4685

Network Working Group                                           J. Snell
Internet-Draft                                         February 20, 2006
Expires: August 24, 2006


             Feed Thread: Enabling Threaded Entries in Atom
                 draft-snell-atompub-feed-thread-05.txt

Status of this Memo

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   This Internet-Draft will expire on August 24, 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 . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11







































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1.  Introduction

   This document specifies a mechanism that allows the expression of
   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.

   In this specification, "entry" refers to an atom:entry element.

   In this specification, "feed" refers to an Atom Feed Document.

   In this specification, "head section" refers to the children of a
   feed's document-wide metadata container; e.g., the child elements of
   the atom:feed element in an Atom Feed Document.

   In this specification, "link" refers to the atom:link element.

   In this specification, "link relation" refers to the value of the
   atom:link element's rel attribute.

   In this specification, the term "extension element" refers to a
   structured extension element as described in [RFC4287].

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML element names.  It uses the following
   namespace prefix for the indicated namespace URI;
     "thr": "http://purl.org/syndication/thread/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 term "element,"
   it is referring to an Element Information Item in Infoset terms.


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 'id' and 'href' attributes.





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     in-reply-to = element thr:in-reply-to {
       atomCommonAttributes,
       attribute id { atomURI }?,
       attribute href { atomURI }?,
       attribute type { atomMediaType }?,
       attribute source { atomURI }?
     }

   The 'id' 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 'href' attribute specifies an IRI that may be used to retrieve a
   representation of the resource being responded to.

   The "in-reply-to" element MAY contain a "type" attribute whose value
   identifies the media type of the resource identified by the href
   attribute.

   The "in-reply-to" element MAY contain a "source" attribute whose
   value identifies the IRI of an Atom Feed Document within which an
   atom:entry representing the resource being responded to MAY be found.

   o  atom:entry elements MAY contain any number of 'in-reply-to'
      extension elements.
   o  atom:entry elements SHOULD NOT contain more than one 'in-reply-to'
      extension element having the same 'id' attribute value.
   o  atom:feed elements MAY contain any number of 'in-reply-to'
      extension elements.
   o  atom:feed elements SHOULD NOT contain more than one 'in-reply-to'
      extension elements having the same 'id' attribute value.
   o  atom:source elements MAY contain any number of 'in-reply-to'
      extension elements.
   o  atom:source elements SHOULD NOT contain more than one
      'in-reply-to' extension element having the same 'id' attribute
      value.
















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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</td>
         <title>My original entry</title>
         <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</td>
         <title>A response to the original</title>
         <link href="http://www.example.org/entries/1/1" />
         <thr:in-reply-to
           id="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>

   In order to allow Atom clients that are not familiar with the in-
   reply-to element to know that a relationship exists between the
   containing 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</td>
         <title>A response to the original</title>
         <link href="http://www.example.org/entries/1/1" />
         <thr:in-reply-to
           id="tag:example.org,2005:1,0"
           type="application/xhtml+xml"
           href="http://www.example.org/entries/1" />
         <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 entries MAY appear within the same feed as the
   entry 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 of
   a resource where responses may be found.  For this purpose, the
   'replies' link relation is used.

   The type attribute of the 'replies' link relation indicates the type
   of resource containing the responses.  A value of 'application/
   atom+xml' indicates that the resource identified by the link's href
   attribute is either an Atom Feed Document or an Atom Entry Document
   that MAY contain one or more 'in-reply-to' link relations.  If the
   type attribute is omitted, it's value is assumed to be "application/
   atom+xml".

   o  atom:entry elements MAY contain any number of 'replies' link
      relations.
   o  atom:entry elements MUST NOT contain more than one 'replies' link
      relation having the same 'type' and 'href' attribute values.




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   o  atom:feed elements MAY contain any number of 'replies' link
      relations.
   o  atom:feed elements MUST NOT contain more than one 'replies' link
      relation having the same 'type' and 'href' attribute values.
   o  atom:source elements MAY contain any number of 'replies' link
      relations.
   o  atom:source elements MUST NOT contain more than one 'replies' link
      relation having the same 'type' and 'href' attribute values.

   A 'replies' link relation appearing as a child of an atom:feed or
   atom:source element indicates that the referenced resource MAY
   contain responses to any of the Feed contained atom:entry's.  A
   'replies' link relation appearing as a child of an atom:entry element
   indicates that the referenced resource MAY contain responses to the
   containing atom:entry.

   Atom link elements specifying 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.

   The "replies" link MAY also contain a "thr:when" attribute whose
   value is a [RFC3339] date-time stamp conforming to the same
   construction rules as the Date Construct from [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.

   The presence of a 'replies' link relation 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.


















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   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</td>
         <title>My original entry</title>
         <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 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.


5.  Security Considerations

   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).  This
   risk can be mitigated by requiring user intervention after a certain
   number of requests, or by limiting requests either according to a
   hard limit, or with heuristics.


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: (see section 4)
       Security considerations: (see section 6)



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7.  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.

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


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://snellspace.com











































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Copyright Statement

   Copyright (C) The Internet Society (2006).  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.


Acknowledgment

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




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