Network Working Group J. Snell
Internet-Draft May 12, 2006
Expires: November 13, 2006
Atom Threading Extensions
draft-snell-atompub-feed-thread-10.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 . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
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. 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.
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 and 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 MUST conform to the same
construction and comparison rules as the value of the atom:id
element. 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 "ref" attribute.
The "href" attribute specifies the 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 entries MAY contain any number of "in-reply-to" extension
elements.
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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
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, it's value
is assumed to be "application/atom+xml".
Atom feed, entry and source elements MAY contain any number of
replies links.
A "replies" link appearing as a child of the Atom feed or source
element indicates that the linked resource may contain responses to
any of that feeds entries. A "replies" link appearing as a child of
an Atom entry element indicates that the linked resource may contain
responses specific to that entry.
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
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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,
<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="2005-07-28T12:10:00Z" />
<summary>This is my original entry</summary>
</entry>
</feed>
The use 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
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
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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 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.
[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>.
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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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