ATOMPUB Working Group M. Pilgrim
Internet-Draft IBM
Expires: May 26, 2007 J. Snell, Ed.
November 22, 2006
Atom Feed Autodiscovery
draft-snell-atompub-autodiscovery-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies a machine-readable method of linking to an
Atom feed from a HyperText Markup Language (HTML) or Extensible
HyperText Markup Language (XHTML) document, using the <link> element.
Editorial Note
To provide feedback on this Internet-Draft, join the atom-syntax
mailing list (http://www.imc.org/atom-syntax/index.html) [1].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Relationship to HTML and XHTML . . . . . . . . . . . . . . . . 3
3.1. Definition of an autodiscovery element . . . . . . . . . . 3
3.2. Syntax rules inherited from HTML . . . . . . . . . . . . . 3
3.3. Syntax rules inherited from XHTML . . . . . . . . . . . . 4
4. Required attributes . . . . . . . . . . . . . . . . . . . . . 4
4.1. rel attribute . . . . . . . . . . . . . . . . . . . . . . 4
4.2. type attribute . . . . . . . . . . . . . . . . . . . . . . 5
4.3. href attribute . . . . . . . . . . . . . . . . . . . . . . 5
5. Optional attributes . . . . . . . . . . . . . . . . . . . . . 5
5.1. title attribute . . . . . . . . . . . . . . . . . . . . . 5
6. Multiple autodiscovery elements . . . . . . . . . . . . . . . 5
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. rel attribute variations . . . . . . . . . . . . . . . . . 6
7.2. type attribute variations . . . . . . . . . . . . . . . . 6
7.3. Link element variations . . . . . . . . . . . . . . . . . 7
7.4. HTML variations . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Revision History . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
Atom is an XML-based file format intended to allow lists of
information, known as "feeds", to be synchronised between publishers
and consumers. Feeds are composed of a number of items, known as
"entries", each with an extensible set of attached metadata. The
primary use case that Atom addresses is for syndicating web content
such as weblogs and news headlines to other web sites and directly to
consumers. However, nothing precludes it from being used for other
purposes and types of content.
The purpose of Atom autodiscovery is for clients who know the URI of
a web page to find the location of that page's associated Atom feed.
For example, say an end user wishes to subscribe to the Atom feed of
a site. Their Atom-aware aggregator client could prompt them to
enter the home page of the site. The client could retrieve the HTML
source of the home page, find the Atom autodiscovery element, and
then retrieve the Atom feed or cache the URI of the Atom feed for
later retrieval.
Autodiscovered Atom feeds may be presented to the user in a variety
of other ways. In the past, Atom-enabled clients have implemented
local proxies that monitor visited web sites and notify the end user
of autodiscovered Atom feeds in real time. Such notification is also
built directly into some desktop web browsers.
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 [RFC2119].
3. Relationship to HTML and XHTML
3.1. Definition of an autodiscovery element
An Atom autodiscovery element is a link element, as defined in
section 12.3 of HTML 4 [W3C.REC-html401-19991224]. As with other
types of link elements, an autodiscovery element MAY appear within
the <head> element of an HTML or XHTML document, but it MUST NOT
appear within the <body>.
3.2. Syntax rules inherited from HTML
When an Atom autodiscovery element appears in an HTML document, it
shares all the syntax rules and restrictions of other HTML elements.
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The most relevant rules are listed here:
o Element names are not case-sensitive. For example, clients MUST
treat <link ...> the same as <LINK ...>
o Attribute names are not case-sensitive. For example, clients MUST
treat <link rel="alternate" ...> the same as <link REL="alternate"
...>.
o Attributes may appear in any order; the order MUST NOT be
considered significant.
o Attribute values MUST be one of the following: enclosed in double
quotes, enclosed in single quotes, or not enclosed in quotes at
all.
o Attribute values MAY contain leading or trailing spaces, or both.
Clients MUST ignore both leading and trailing spaces in attribute
values. For example, clients MUST treat all of the following the
same way: <link rel="alternate" ...>, <link rel="alternate " ...>,
<link rel=" alternate" ...>, and <link rel=" alternate " ...>.
3.3. Syntax rules inherited from XHTML
XHTML [W3C.REC-xhtml1-20020801] is a reformulation of HTML in XML
[W3C.REC-xml-20040204]. It also defines a <link> element, so XHTML
documents are a suitable container for Atom autodiscovery elements.
When an Atom autodiscovery element appears in an XHTML document, it
shares all the syntax rules and restrictions of other XHTML elements.
The syntax rules of XHTML documents are slightly different than HTML
documents. The most relevant differences are listed here:
o Element names are case-sensitive, and MUST be lowercase.
o Attribute names are case-sensitive, and MUST be lowercase.
o Attribute values MUST be enclosed in either double quotes or
single quotes.
o Empty elements (such as link elements) MUST be closed with an end
tag.
4. Required attributes
4.1. rel attribute
The rel attribute MUST be present in an Atom autodiscovery element.
As defined in section 6.12 of HTML 4 [W3C.REC-html401-19991224], the
value of the rel attribute is a space-separated list of keywords.
The list of keywords MUST include the keyword "alternate" in
uppercase, lowercase, or mixed case.
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4.2. type attribute
The type attribute MUST be present in an Atom autodiscovery element.
As defined in section 12.3 of HTML 4 [W3C.REC-html401-19991224], the
value of the type attribute of any link element MUST be a registered
Internet media type [RFC2045]. The media type for Atom is
"application/atom+xml" [I-D.ietf-atompub-format], so in an Atom
autodiscovery element the value of the type attribute MUST contain
the string "application/atom+xml" in uppercase, lowercase, or mixed
case.
4.3. href attribute
The href attribute MUST be present in an Atom autodiscovery element,
and its value MUST be the URI [RFC3986] of an Atom feed. The value
MAY be a relative URI, and if so, clients MUST resolve it to a full
URI (section 5 of [RFC3986]) using the document's base URI (section
12.4 of HTML 4 [W3C.REC-html401-19991224]).
5. Optional attributes
An Atom autodiscovery element MAY contain any other attributes of the
link element defined in section 12.4 of HTML 4 [W3C.REC-html401-
19991224]. Clients MAY ignore any attributes other than rel, type,
and href.
5.1. title attribute
The title attribute MAY be present in an Atom autodiscovery element.
If so, the client MAY treat the value of the title attribute as a
human-readable label of the Atom feed. Clients MAY present this
label to the end user.
6. Multiple autodiscovery elements
An HTML or XHTML document MAY contain one or more Atom autodiscovery
elements. If multiple autodiscovery elements are present in a single
document:
o Each autodiscovery element SHOULD point to a different Atom feed.
o Each autodiscovery element SHOULD include a title attribute that
gives a human-readable label for the feed that the element points
to. Clients MAY use these titles to present a list of available
Atom feeds to the end user.
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o The order of the autodiscovery elements is significant. The first
element SHOULD point to the publisher's preferred feed for the
document.
o Clients who present a list of autodiscovered feeds to the end user
SHOULD present them in the same order as the autodiscovery
elements appear in the document.
o Clients who wish to choose exactly one feed without user input
SHOULD choose the one pointed to by the first autodiscovery
element.
7. Examples
7.1. rel attribute variations
The following is a non-exhaustive list of valid rel attributes for an
Atom autodiscovery element in an HTML or XHTML document:
rel="alternate"
rel="alternate "
rel=" alternate"
rel=" alternate "
rel="foo alternate"
rel="alternate bar"
rel="foo alternate bar"
rel="ALTERNATE"
rel="Alternate"
rel="AlTeRnAtE"
rel='alternate'
rel="Alternate"
In an HTML document, the following forms are also valid:
REL="alternate"
rel=alternate
7.2. type attribute variations
The following are examples of valid type attributes for an Atom
autodiscovery element in an HTML or XHTML document:
type="application/atom+xml"
type="application/atom+xml "
type=" application/atom+xml"
type=" application/atom+xml "
type="APPLICATION/ATOM+XML"
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type="Application/Atom+Xml"
In an HTML document, the following form is also valid:
TYPE="application/atom+xml"
7.3. Link element variations
Each of the following examples represents an Atom autodiscovery
element in an HTML document located at
http://www.example.com/index.html. Each example points to an Atom
feed located at http://www.example.com/xml/index.atom.
<link rel="alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
<link rel="alternate" type="application/atom+xml"
href="xml/index.atom">
<link rel="alternate" type="application/atom+xml"
href="/xml/index.atom">
<link rel='alternate' type='application/atom+xml'
href='http://www.example.com/xml/index.atom'>
<link rel=alternate type='application/atom+xml'
href="http://www.example.com/xml/index.atom">
<link rel="AlTeRnAtE" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
<link rel="alternate" type="APPLICATION/ATOM+XML"
href="http://www.example.com/xml/index.atom">
<LINK REL="alternate" TYPE="application/atom+xml"
HREF="http://www.example.com/xml/index.atom">
<link href="http://www.example.com/xml/index.atom"
type="APPLICATION/ATOM+XML" rel="alternate">
<link rel="alternate foo" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
<link rel="foo alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
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<link rel="foo alternate bar" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
<link rel="Alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
<link rel="alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom">
Each of the following examples represents an Atom autodiscovery
element in an XHTML document located at
http://www.example.com/index.xhtml. Each example points to an Atom
feed located at http://www.example.com/xml/index.atom.
<link rel="alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
<link rel="alternate" type="application/atom+xml"
href="xml/index.atom" />
<link rel="alternate" type="application/atom+xml"
href="/xml/index.atom" />
<link rel='alternate' type='application/atom+xml'
href='http://www.example.com/xml/index.atom' />
<link rel="AlTeRnAtE" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
<link rel="alternate" type="APPLICATION/ATOM+XML"
href="http://www.example.com/xml/index.atom" />
<link href="http://www.example.com/xml/index.atom"
type="APPLICATION/ATOM+XML" rel="alternate" />
<link rel="alternate foo" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
<link rel="foo alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
<link rel="foo alternate bar" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
<link rel="Alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
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<link rel="alternate" type="application/atom+xml"
href="http://www.example.com/xml/index.atom" />
7.4. HTML variations
This is an example of a complete HTML document located at
http://www.example.com/index.html. It contains an Atom autodiscovery
element that points to an Atom feed at
http://www.example.com/index.html?format=atom.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<link rel="alternate"
type="application/atom+xml"
href="?format=atom">
</head>
</html>
This is an example of a complete HTML document located at
http://www.example.com/index.html. It contains an Atom autodiscovery
element that points to an Atom feed at
http://www.example.org/index.atom. It uses a relative URI which is
relative to the base URI specified in the <base> element.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<base href="http://www.example.org/">
<link rel="alternate"
type="application/atom+xml"
href="index.atom">
</head>
</html>
This is an example of a complete HTML document located at
http://www.example.com/index.html. It contains three autodiscovery
elements that point to Atom feeds at
http://www.example.com/xml/index.atom,
http://www.example.com/xml/comments.atom, and
http://example.org/index.atom respectively. It uses the optional
title attribute to label each autodiscovery element.
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<link rel="alternate"
type="application/atom+xml"
title="Main Atom feed"
href="/xml/index.atom">
<link rel="alternate"
type="application/atom+xml"
title="Recent comments feed"
href="/xml/comments.atom">
<link rel="alternate"
type="application/atom+xml"
title="Atom feed (mirror)"
href="http://example.org/index.atom">
</head>
</html>
8. IANA Considerations
This document relies on the registration of the Atom media type,
which is defined in [I-D.ietf-atompub-format].
9. Security Considerations
Since the entire purpose of an Atom autodiscovery element is for
clients to dereference the URI given in the href attribute, the
security issues listed in section 7 of [RFC3986] should be
considered.
A publisher could construct a malicious URI in the href attribute,
such as a "javascript:" URI to include an inline script, or some
other URI scheme that typically launches an external program when
dereferenced. Clients should be aware of the security context in
which they execute scripts, and they should give the end user the
ability to disable the automatic execution of external programs when
dereferencing autodiscovery URIs.
10. References
10.1. Normative References
[I-D.ietf-atompub-format]
Sayre, R. and M. Nottingham, "The Atom Syndication
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Format", draft-ietf-atompub-format-11 (work in progress),
August 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[W3C.REC-xml-20040204]
Yergeau, F., Maler, E., Paoli, J., Bray, T., and C.
Sperberg-McQueen, "Extensible Markup Language (XML) 1.0
(Third Edition)", World Wide Web Consortium
Recommendation REC-xml-20040204, February 2004,
<http://www.w3.org/TR/2004/REC-xml-20040204>.
10.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[W3C.REC-html401-19991224]
Raggett, D., Jacobs, I., and A. Hors, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
[W3C.REC-xhtml1-20020801]
Pemberton, S., "XHTML[TM] 1.0 The Extensible HyperText
Markup Language (Second Edition)", W3C REC REC-xhtml1-
20020801, August 2002.
URIs
[1] <http://www.imc.org/atom-syntax/index.html>
Appendix A. Contributors
The following people contributed to this specification's content: Joe
Gregorio, Paul Hoffman, and the members of the atom-syntax mailing
list.
Appendix B. Revision History
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2006-11-21:
Resurrect the expired draft
2005-05-08:
Updated reference to XML 3rd ed.
Updated RFC 2396 reference to RFC 3986
Added editor
2005-05-03:
Updated boilerplate to RFC 3978
2004-08-17:
Consolidated examples
Updated author address
Added IPR
Removed reference to RFC 1808
2003-12-23:
Added security considerations
Added note about order significance with multiple autodiscovery
elements
2003-12-22:
Rearranged the "Relationship to HTML and XHTML" section
Added examples of single-quoted attribute values
Added examples of numeric entities in rel and type attributes
2003-12-21:
First draft
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Authors' Addresses
Mark Pilgrim
International Business Machines Corporation
4400 Silicon Drive
Durham, NC 27713
US
Email: pilgrim@gmail.com
James Snell (editor)
Email: jasnell@gmail.com
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Acknowledgment
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