Network Working Group J. Snell
Internet-Draft April 11, 2006
Expires: October 13, 2006
Atom Threading Extensions
draft-snell-atompub-feed-thread-08.txt
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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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