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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                                           August 15, 2005
Expires: February 16, 2006


             Feed Thread: Enabling Threaded Entries in Atom
                 draft-snell-atompub-feed-thread-01.txt

Status of this Memo

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   This Internet-Draft will expire on February 16, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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  . . . . . . . . . . . . . . . . . .   7
   6.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   8
   7.   References . . . . . . . . . . . . . . . . . . . . . . . . .   8
        Author's Address . . . . . . . . . . . . . . . . . . . . . .   8
   A.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .   9
        Intellectual Property and Copyright Statements . . . . . . .  10







































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1.  Introduction

   This document specifies a mechanism that allows the expression of
   threaded discussions within the Atom Syndication Format ([I-D.ietf-
   atompub-format]).

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 [I-D.ietf-atompub-
   format].

   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 an atom:
   entry is a response to another resource.  The element must take one
   of two forms as described by the inReplyToDereferenceable and
   inReplyToNonDereferenceable constructs defined below.




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     inReplyToCommonAttributes = {
       attribute reflang { atomLanguageTag }?,
       attribute title { text }?
     }

     inReplyToDereferenceable = element thr:in-reply-to {
       atomCommonAttributes,
       inReplyToCommonAttributes,
       attribute href { atomURI },
       attribute type { atomMediatype }?
     }

     inReplyToNonDereferenceable = element thr:in-reply-to {
       atomCommonAttributes,
       inReplyToCommonAttributes,
       attribute idref { atomURI },
       attribute type { atomMediaType }?,
       attribute source { atomURI}?
     }

   In the inReplyToDereferenceable form, the href attribute MUST specify
   a dereferenceable URI that may be used to locate the entity that the
   containing Atom Entry is a response to.  The type attribute MAY be
   used to indicate the media type of the entity specified by the href
   attribute.

   In the inReplyToNonDereferenceable form, the idref attribute MUST
   specify a nondereferenceable URI that uniquely identifies the entity
   that the containing Atom Entry is a response to.  The type attribute
   MAY be used to indicate the media type of the entity identified by
   the id attribute.  The source attribute MAY be used to specify a
   dereferenceable URI identifying the source of the entity being
   responded to.

   In either form, the title attribute MAY be used to specify a human
   readable title for the resource referenced by the href attribute.
   The reflang attribute MAY be used to specify the language of the
   resource referenced by the href attribute.  The value of the hreflang
   MUST be a language tag [RFC3066].

   o  atom:entry elements MAY contain any number of 'in-reply-to'
      extension elements.
   o  atom:entry elements MUST NOT contain more than one 'in-reply-to'
      extension element having the same 'type' and 'href' attribute or
      'type' and 'idref' attribute values.
   o  atom:feed elements MAY contain any number of 'in-reply-to'
      extension elements.




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   o  atom:feed elements MUST NOT contain more than one 'in-reply-to'
      extension elements having the same 'type' and 'href' attribute or
      'type' and 'idref' attribute values.
   o  atom:source elements MAY contain any number of 'in-reply-to'
      extension elements.
   o  atom:source elements MUST NOT contain more than one 'in-reply-to'
      extension element having the same 'type' and 'href' attribute or
      'type' and 'idref' attribute values.
   o  If an atom:entry element does not contain any 'in-reply-to'
      extension elements, any 'in-reply-to' extension elements of the
      contained atom:source are considered to apply
   o  In an Atom Feed Document, if an atom:entry element or a contained
      atom:source element do not contain any 'in-reply-to' extension
      elements, any 'in-reply-to' extension elements in the feed head
      section are considered to apply.

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

   The behavior of 'in-reply-to' extension elements with type attributes
   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.

   When an 'in-reply-to' extension element is used to reference an
   entity that does not appear within the same Atom document as the



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   entry containing the in-reply-to element, a "related" link relation
   SHOULD be included that references the same URI as the href attribute
   in dereferenceable in-reply-to elements or the source attribute in
   non-dereferenceable in-reply-to elements.

   For example
     <feed xmlns="http://www.w3.org/2005/Atom">
       <id>http://www.example.com/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.com/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:example.com,2005:1,1</td>
         <title>A response to the original</title>
         <link href="http://www.example.com/entries/1/1" />
         <thr:in-reply-to
           xmlns:thr="http://purl.org/syndication/thread/1.0"
           href="http://www.example.com/entries/1" />
         <link rel="related"
               href="http://www.example.com/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.
   o  If an atom:entry element does not contain any 'replies' link
      relations, any 'replies' link relations of the contained atom:
      source are considered to apply
   o  In an Atom Feed Document, if an atom:entry element or a contained
      atom:source element do not contain any 'replies' link relations,
      any 'replies' link relations in the feed head section are
      considered to apply.

   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 'in-reply-to' link relations or
   responses to any entries within the containing feed.

   For example, replies contained in a separate Atom feed
     <feed xmlns="http://www.w3.org/2005/Atom">
       <id>http://www.example.com/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.com/myfeed" />
       <link rel="replies"
             type="application/atom+xml"
             href="http://www.example.com/mycommentsfeed.xml" />
       <author><name>James</name></author>
       <entry>
         <id>tag:entries.com,2005:1</td>
         <title>My original entry</title>
         <link href="http://www.example.com/entries/1" />
         <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



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   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
   [I-D.ietf-atompub-format].
       Attribute Value: replies
       Description: (see section 4)
       Expected display characteristics: (see section 4)
       Security considerations: (see section 6)

7.  References

   [I-D.ietf-atompub-format]
              Sayre, R. and M. Nottingham, "The Atom Syndication
              Format", draft-ietf-atompub-format-10 (work in progress),
              July 2005.

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

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [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.


Author's Address

   James M Snell


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




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Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from the members of
   Atom Publishing Format and Protocol working group during the
   development of this specification.














































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Acknowledgment

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




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