Network Working Group J. Snell
Internet-Draft December 6, 2005
Expires: June 9, 2006
Feed Thread: Enabling Threaded Entries in Atom
draft-snell-atompub-feed-thread-03.txt
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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
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 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 ([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 }?
}
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.
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.
An 'in-reply-to' extension element appearing as a child of an atom:
feed element indicates that all of the atom:entry elements contained
within that atom:feed are responses to the referenced resource.
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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.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
id="tag:entries.com,2005:1" href="#"/>
<summary>This is a response to the original entry</summary>
</entry>
</feed>
When an 'in-reply-to' extension element is used to reference an
entity that does not appear within the same Atom document as the
entry containing the in-reply-to element, a "related" link relation
SHOULD be included that references the same URI as the href
attribute.
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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.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
id="tag:example.com,2005:1,0"
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.
o atom:feed elements MAY contain any number of 'replies' link
relations.
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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
element indicates that the referenced resource MAY contain responses
to any of the atom:feed elements contained atom:entry's. A 'replies'
link relation appearing as a child of an atom:entry or atom:source
element indicates that the referenced resource MAY contain responses
to the containing atom:entry.
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
sequence of) network requests, causing denial of service (either to
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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)
7. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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 the members of
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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