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RFC 4685 - Atom Threading Extensions

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Network Working Group                                           J. Snell
Request for Comments: 4685                                September 2006
Category: Standards Track

                       Atom Threading Extensions

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


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

Table of Contents

   1. Introduction ....................................................1
   2. Notational Conventions ..........................................2
   3. The 'in-reply-to' Extension Element .............................2
   4. The 'replies' Link Relation .....................................5
   5. The 'total' Extension Element ...................................6
   6. Considerations for Using thr:count, thr:updated, and total ......7
   7. Security Considerations .........................................8
   8. IANA Considerations .............................................9
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References ....................................10
   Appendix A.  Acknowledgements .....................................11

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",
   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:

   In this document, the namespace prefix "thr:" is used for the above
   Namespace URI.  Note that the choice of namespace prefix is arbitrary
   and not semantically significant.

   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.

   This specification allows the use of IRIs [RFC3987].  Every URI
   [RFC3986] is also an IRI, so a URI may be used wherever 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].  When an IRI is serving as an identifier, it MUST
   NOT be so mapped.

   Some sections of this specification are illustrated with a non-
   normative RELAX NG Compact schema [RELAXNG].  In those sections, this
   specification uses the atomCommonAttributes, atomMediaType, and
   atomURI patterns, defined in [RFC4287].

   However, the text of this specification provides the sole definition
   of conformance.

3.  The 'in-reply-to' Extension Element

   The "in-reply-to" element is used to indicate that an entry is a
   response to another resource.  The element MUST contain a "ref"
   attribute identifying the resource that is being responded to.

   The element is not unlike the references and in-reply-to email
   message headers, defined by [RFC2822].  However, unlike the in-
   reply-to header, the "in-reply-to" element is required to identify
   the unique identifier of only a single parent resource.  If the entry

   is a response to multiple resources, additional "in-reply-to"
   elements MAY be used.  There is no direct equivalent to the
   references header, which lists the unique identifiers of each
   preceding message in a thread.

   in-reply-to =
     element thr:in-reply-to {
       ( undefinedContent )

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

   The "ref" attribute specifies the persistent, universally unique
   identifier of the resource being responded to.  The value MUST
   conform to the same construction and comparison rules as the value of
   the atom:id element, as defined in Section 4.2.6 of [RFC4287].
   Though the IRI might use a dereferenceable scheme, processors MUST
   NOT assume that it can be dereferenced.

   If the resource being responded to does not have a persistent,
   universally unique identifier, the publisher MUST assign an
   identifier that satisfies all the considerations in Section 4.2.6 of
   [RFC4287] for use as the value of the "ref" attribute.  In that case,
   if a representation of the resource can be retrieved from an IRI that
   can be used as a valid atom:id value, then this IRI SHOULD be used as
   the value of both the "ref" and "href" attributes.

   The "source" attribute MAY be used to specify the IRI [RFC3987] of an
   Atom Feed or Entry Document containing an atom:entry with an atom:id
   value equal to the value of the "ref" attribute.  The IRI specified,
   once appropriately mapped to a corresponding URI, MUST be

   The "href" attribute specifies an IRI that may be used to retrieve a
   representation of the resource being responded to.  The IRI
   specified, once appropriately mapped to a corresponding URI, MUST be

   The "type" attribute MAY be used to provide a hint to the client
   about the media type [RFC4288] of the resource identified by the
   "href" attribute.  The "type" attribute is only meaningful if a
   corresponding "href" attribute is also provided.

   This specification assigns no significance to the order in which
   multiple "in-reply-to" elements appear within an entry.

   An example of an entry with a response follows:

   <feed xmlns="http://www.w3.org/2005/Atom"
     <title>My Example Feed</title>
     <link href="http://www.example.org/myfeed" />
       <title>My original entry</title>
         href="http://www.example.org/entries/1" />
       <summary>This is my original entry</summary>
       <title>A response to the original</title>
       <link href="http://www.example.org/entries/1/1" />
       <summary>This is a response to the original entry</summary>

   To allow Atom processors 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.  Although such links
   are unlikely to be processed as a reference to a predecessor in a
   threaded conversation, they are helpful in at least establishing a
   semantically meaningful relationship between the linked resources.

   For example,

   <feed xmlns="http://www.w3.org/2005/Atom"
     <title>My Example Feed</title>
     <link href="http://www.example.org/myfeed" />
       <title>A response to the original</title>
       <link href="http://www.example.org/entries/1/1" />
         source="http://www.example.org/myfeed" />
         href="http://www.example.org/entries/1" />
       <summary>This is a response to the original entry</summary>

4.  The 'replies' Link Relation

   An Atom link element with a rel attribute value of "replies" may be
   used to reference a resource where responses to an entry may be
   found.  If the type attribute of the atom:link is omitted, its value
   is assumed to be "application/atom+xml".

   A "replies" link appearing as a child of the Atom feed or source
   element indicates that the referenced resource likely contains
   responses to any of that feed's entries.  A "replies" link appearing
   as a child of an Atom entry element indicates that the linked
   resource likely contains responses specific to that entry.

   An atom:link element using the "replies" rel attribute value MAY
   contain a "thr:count" attribute whose value is an unsigned, non-
   negative integer, conforming to the canonical representation of the
   XML Schema nonNegativeInteger data type [W3C.REC-xmlschema-2-
   20041028], that provides a hint to clients as to the total number of
   replies contained by the linked resource.  The value is advisory and
   may not accurately reflect the actual number of replies.

   The link MAY also contain a "thr:updated" 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
   provide a hint to clients as to the date and time of the most
   recently updated reply contained by the linked resource.  The value
   is advisory and may not accurately reflect the actual date and time
   of the most recent reply.

   For example,

   <feed xmlns="http://www.w3.org/2005/Atom"
     <title>My Example Feed</title>
     <link href="http://www.example.org/myfeed" />
       <title>My original entry</title>
       <link href="http://www.example.org/entries/1" />
       <link rel="replies"
             thr:count="10" thr:updated="2005-07-28T12:10:00Z" />
       <summary>This is my original entry</summary>

   Although Atom feed, entry, and source elements MAY each contain any
   number of atom:link elements using the "replies" link relation, this
   specification assigns no significance to the presence or order of
   such links.  Multiple replies links appearing within an atom:entry
   may reference alternative representations of the same set of
   responses or may reference entirely distinct resources containing
   distinct sets of responses.  Processors MUST NOT assume that multiple
   replies links are referencing different representations of the same
   resource and MUST process each replies link independently of any

5.  The 'total' Extension Element

   The "total" element is used to indicate the total number of unique
   responses to an entry known to the publisher.  Its content MUST be an
   unsigned non-negative integer value conforming to the canonical
   representation of the XML Schema nonNegativeInteger data type

      total = element thr:total { xsd:nonNegativeInteger }

   Atom entries MAY contain a "total" element but MUST NOT contain more
   than one.

   There is no implied relationship between the value of the "total"
   element of an Atom entry and any individual or aggregate values of
   the "thr:count" attributes of its Atom link elements having a
   "replies" relation.

6.  Considerations for Using thr:count, thr:updated, and total

   The thr:count, thr:updated, and total extensions provide additional
   metadata about the thread of discussion associated with an entry.
   The values are intended to make it easier for feed consumers to
   display basic contextual information about the thread without
   requiring that those consumers dereference, parse, and analyze linked
   resources.  That said, there are a number of considerations
   implementors need to be aware of.

   First, these extensions MUST NOT be assumed to provide completely
   accurate information about the thread of discussion.  For instance,
   the actual total number of responses contained by a linked resource
   MAY differ from the number specified in the thr:count attribute.
   Feed publishers SHOULD make an effort to ensure that the values are
   accurate.  The non-authoritative nature of "external reference
   metadata", like the replies link attributes, is discussed in detail
   in Section 3.3 of the W3C document "Tag Finding 12:  Authoritative
   Metadata" [TAG12].

   Second, the values of the these extensions are volatile and may
   change at a faster rate than that of the containing entry.  Frequent
   updates to these values, or to any part of the Atom document, could
   have a detrimental impact on the cacheability of the document using
   the attributes, leading to an increase in overall bandwidth

   Feed publishers SHOULD consider a change to the values of the thr:
   count, thr:updated, and total extensions an "insignificant" update in
   terms of [RFC4287], meaning that the value of the containing feed,
   entry, or source element's atom:updated element SHOULD NOT be
   affected by a change to the values of these extensions.

   Lastly, implementors need to be aware that although the Atom
   specification [RFC4287] explicitly allows the link element to contain
   arbitrary extensions, the specification does not require that
   implementations support such extensions.  Specifically, relating to
   the use of extensions, Atom does not define any level of mandatory

   conformance on the part of feed consumers beyond a requirement that
   implementations ignore any extension the implementation does not
   understand.  As a result, some implementations MAY NOT be capable of
   fully utilizing the extensions defined by this or any specification.

7.  Security Considerations

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

   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 (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.

   The mechanisms described here can be used to construct threaded
   conversations spanning resources distributed across multiple domains.
   For example, an individual posting an entry to one weblog hosted on
   one Internet domain could mark that entry as a response to an entry
   from a different weblog hosted on a different domain.  Implementors
   should note that such distributed responses can be leveraged by an
   attacker to attach inappropriate or unwanted content to a discussion.
   Such attacks can be prevented or mitigated by allowing users to
   explicitly configure the sources from which responses may be
   retrieved, or by applying heuristics to determine the legitimacy of a
   given response source.

   Implementors should also note the potential for abuse that exists
   when malicious content publishers edit or change previously published
   content.  In closed, centralized comment systems, after-the-fact
   editing of comments is typically not an issue, as such changes are
   easily prevented, detected, or tracked.  With the form of distributed
   comments enabled through the use of the thr:in-reply-to extension,
   however, such changes become more difficult to detect, raising the
   possibility of serious attribution and repudiation concerns.  XML
   Digital Signatures, as specified in Section 5.1 of [RFC4287], present
   one possible avenue for mitigating such concerns, although the
   presence of a valid XML Digital Signature within an entry is not, by
   itself, a reliable defense against repudiation issues.

8.  IANA Considerations

   This specification defines one new Atom link relation type that has
   been registered in the IANA Registry of Link Relation, as defined by

      Attribute Value: replies
      Description: (see Section 4)
      Expected display characteristics: (see Section 4)
      Security considerations: (see Section 5)

9.  References

9.1.  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.

   [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. 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.

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

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

              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", W3C REC REC-xmlschema-2-20041028, October

9.2.  Informative References

   [RELAXNG]  Clark, J., "RELAX NG Compact Syntax", December 2001,

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April

   [TAG12]    Fielding, R. and I. Jacobs, "Tag Finding 12: Authoritative
              Metadata", <http://www.w3.org/2001/tag/doc/mime-respect-

Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from Antone Roundy,
   Aristotle Pagaltzis, Byrne Reese, David Powell, Eric Scheid, James
   Holderness, John Panzer, Lisa Dusseault, M. David Peterson, Sam Ruby,
   Sylvain Hellegouarch, and the remaining members of the Atom
   Publishing Format and Protocol working group during the development
   of this specification.  Any fault or weakness in the definition of
   this extension is solely the blame of the author.

   Some portions of text in this document have been adapted from
   [RFC4287] in order to maintain a stylistic and technical alignment
   with that specification.

Author's Address

   James M Snell

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

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