From: http://www.ietf.org/internet-drafts/draft-ietf-atompub-format-04.txt
Title: The Atom Syndication Format
Reference: IETF Network Working Group, Internet Draft 'draft-ietf-atompub-format-04'
Date: January 10, 2005
Previous version: http://xml.coverpages.org/draft-ietf-atompub-format-03.txt
http://xml.coverpages.org/draft-ietf-atompub-format-02.txt
http://xml.coverpages.org/draft-ietf-atompub-format-01.txt
http://xml.coverpages.org/draft-ietf-atompub-format-00.txt
See: http://xml.coverpages.org/atom.html
"Atom Publishing Format and Protocol"
========================================================================
Network Working Group M. Nottingham, Ed.
Internet-Draft
Expires: July 11, 2005 R. Sayre, Ed.
Boswijck Memex Consulting
January 10, 2005
The Atom Syndication Format
draft-ietf-atompub-format-04
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on July 11, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document specifies Atom, an XML-based Web content and metadata
syndication format.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Editorial Notes . . . . . . . . . . . . . . . . . . . . . 4
1.2 Example . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Conformance . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Notational Conventions . . . . . . . . . . . . . . . . . . 5
2. Atom Documents . . . . . . . . . . . . . . . . . . . . . . . 7
3. Common Atom Constructs . . . . . . . . . . . . . . . . . . . 8
3.1 Text Constructs . . . . . . . . . . . . . . . . . . . . . 8
3.1.1 "type" Attribute . . . . . . . . . . . . . . . . . . . 8
3.2 Person Constructs . . . . . . . . . . . . . . . . . . . . 9
3.2.1 "atom:name" Element . . . . . . . . . . . . . . . . . 9
3.2.2 "atom:uri" Element . . . . . . . . . . . . . . . . . . 9
3.2.3 "atom:email" Element . . . . . . . . . . . . . . . . . 9
3.3 Date Constructs . . . . . . . . . . . . . . . . . . . . . 9
3.4 Service Constructs . . . . . . . . . . . . . . . . . . . . 9
3.4.1 "href" Attribute . . . . . . . . . . . . . . . . . . . 10
3.5 Link Constructs . . . . . . . . . . . . . . . . . . . . . 10
3.5.1 "rel" Attribute . . . . . . . . . . . . . . . . . . . 10
3.5.2 "type" Attribute . . . . . . . . . . . . . . . . . . . 11
3.5.3 "href" Attribute . . . . . . . . . . . . . . . . . . . 11
3.5.4 "hreflang" Attribute . . . . . . . . . . . . . . . . . 11
3.5.5 "title" Attribute . . . . . . . . . . . . . . . . . . 11
3.5.6 "length" Attribute . . . . . . . . . . . . . . . . . . 11
3.6 Identity Constructs . . . . . . . . . . . . . . . . . . . 12
3.6.1 Dereferencing Identity Constructs . . . . . . . . . . 12
3.6.2 Comparing Identity Constructs . . . . . . . . . . . . 12
3.7 The Category Construct . . . . . . . . . . . . . . . . . . 13
3.7.1 The "term" Attribute . . . . . . . . . . . . . . . . . 13
3.7.2 The "scheme" Attribute . . . . . . . . . . . . . . . . 13
3.7.3 The "label" attribute . . . . . . . . . . . . . . . . 13
4. The "atom:feed" Element . . . . . . . . . . . . . . . . . . 14
4.1 "version" Attribute . . . . . . . . . . . . . . . . . . . 14
4.2 The "atom:head" Element . . . . . . . . . . . . . . . . . 14
4.2.1 "atom:title" Element . . . . . . . . . . . . . . . . . 14
4.2.2 "atom:link" Element . . . . . . . . . . . . . . . . . 14
4.2.3 "atom:category" Element . . . . . . . . . . . . . . . 15
4.2.4 "atom:introspection" Element . . . . . . . . . . . . . 15
4.2.5 "atom:post" Element . . . . . . . . . . . . . . . . . 15
4.2.6 "atom:author" Element . . . . . . . . . . . . . . . . 15
4.2.7 "atom:contributor" Element . . . . . . . . . . . . . . 15
4.2.8 "atom:tagline" Element . . . . . . . . . . . . . . . . 15
4.2.9 "atom:id" Element . . . . . . . . . . . . . . . . . . 16
4.2.10 "atom:generator" Element . . . . . . . . . . . . . . 16
4.2.11 "atom:copyright" Element . . . . . . . . . . . . . . 16
4.2.12 "atom:info" Element . . . . . . . . . . . . . . . . 16
4.2.13 "atom:updated" Element . . . . . . . . . . . . . . . 16
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5. The "atom:entry" Element . . . . . . . . . . . . . . . . . . 18
5.1 "atom:title" Element . . . . . . . . . . . . . . . . . . . 18
5.2 "atom:link" Element . . . . . . . . . . . . . . . . . . . 18
5.3 "atom:category" Element . . . . . . . . . . . . . . . . . 18
5.4 "atom:edit" Element . . . . . . . . . . . . . . . . . . . 19
5.5 "atom:author" Element . . . . . . . . . . . . . . . . . . 19
5.6 "atom:contributor" Element . . . . . . . . . . . . . . . . 19
5.7 "atom:host" Element . . . . . . . . . . . . . . . . . . . 19
5.8 "atom:id" Element . . . . . . . . . . . . . . . . . . . . 19
5.9 "atom:updated" Element . . . . . . . . . . . . . . . . . . 19
5.10 "atom:published" Element . . . . . . . . . . . . . . . . 20
5.11 "atom:summary" Element . . . . . . . . . . . . . . . . . 20
5.12 "atom:content" Element . . . . . . . . . . . . . . . . . 20
5.12.1 "type" attribute . . . . . . . . . . . . . . . . . . 20
5.12.2 "src" attribute . . . . . . . . . . . . . . . . . . 21
5.12.3 Processing Model . . . . . . . . . . . . . . . . . . 21
5.13 "atom:copyright" Element . . . . . . . . . . . . . . . . 22
5.14 "atom:head" Element . . . . . . . . . . . . . . . . . . 22
6. Managing Feed State . . . . . . . . . . . . . . . . . . . . 23
7. Securing Atom Documents . . . . . . . . . . . . . . . . . . 24
7.1 Digital Signatures . . . . . . . . . . . . . . . . . . . . 24
7.2 Encryption . . . . . . . . . . . . . . . . . . . . . . . . 24
8. Embedding Atom in Other Formats . . . . . . . . . . . . . . 25
9. Extending Atom . . . . . . . . . . . . . . . . . . . . . . . 26
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27
10.1 Registry of Link Relations . . . . . . . . . . . . . . . 27
11. Security Considerations . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1 Normative References . . . . . . . . . . . . . . . . . . . 30
12.2 Informative References . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 32
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 33
B. Revision History . . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . 36
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1. Introduction
Atom is an XML-based document format intended to allow lists of
related information, known as "feeds". Feeds are composed of a
number of items, known as "entries", each with an extensible set of
attached metadata. For example, each entry has a title.
The primary use case that Atom addresses is the syndication of Web
content such as Weblogs and news headlines to Web sites as well as
directly to user agents. However, nothing precludes it from being
used for other purposes and kinds of content.
Details of communication protocols between software agents using Atom
can be found in the Atom Protocol specification [Atom-protocol].
[[ more motivation / design principles ]]
1.1 Editorial Notes
The Atom format is a work-in-progress, and this draft is both
incomplete and likely to change rapidly. As a result, THE FORMAT
DESCRIBED BY THIS DRAFT SHOULD NOT BE DEPLOYED, either in production
systems or in any non-experimental fashion on the Internet.
Discussion of this draft happens in two fora;
The mailing list [1]
The Atom Wiki Web site [2]
Active development takes place on the mailing list, while the Wiki is
used for issue tracking and new proposals.
This document is an early draft and known to be incomplete. Topics
marked [[like this]] indicate where additional text is likely to be
added.
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1.2 Example
A minimal, single-entry Atom Feed Document:
Example Feed2003-12-13T18:30:02ZJohn DoeAtom-Powered Robots Run Amokvemmi://example.org/2003/323972003-12-13T18:30:02Z
1.3 Conformance
[[ talk about atom documents and atom consumers, and how requirements
are placed on them ]]
1.4 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].
This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
to uniquely identify XML elements and attribute names. It uses the
following namespace prefixes for the indicated namespace URIs;
"atom": http://purl.org/atom/ns#draft-ietf-atompub-format-04
Note that the choice of any namespace prefix is arbitrary and not
semantically significant.
Atom is specified using terms from the XML Infoset
[W3C.REC-xml-infoset-20011024]. However, this specification uses a
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shorthand for two common terms; the phrase "Information Item" is
omitted when naming Element Information Items 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 it uses the term "attribute," it is referring to an Attribute
Information Item.
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2. Atom Documents
This specification describes two kinds of Atom Documents; Atom Feed
Documents and Atom Entry Documents.
An Atom Feed Document is a representation of an Atom feed, including
metadata about the feed, and some or all of the entries associated
with it. Its document element is atom:feed.
An Atom Entry Document represents exactly one Atom Entry, outside of
the context of an Atom Feed. Its document element is atom:entry.
Both kinds of Atom documents are specified in terms of the XML
Information Set, serialised as XML 1.0 [W3C.REC-xml-20040204] and
identified with the "application/atom+xml" media type. Atom
Documents MUST be well-formed XML.
[[ Validity? ]]
Atom constrains the appearance and content of elements and
attributes; unless otherwise stated, Atom Documents MAY contain other
Information Items as appropriate. In particular, Comment Information
Items and Processing Instruction Information Items SHOULD be ignored
in the normal processing of an Atom Document.
Any element in an Atom Document MAY have an xml:base attribute. XML
Base [W3C.REC-xmlbase-20010627] processing MUST be applied to any
relative reference [RFC2396bis] present in an Atom Document. This
includes such elements and attributes as specified by Atom itself, as
well as those specified by extensions to Atom.
Any element in an Atom Document MAY have an xml:lang attribute, whose
content indicates the default natural language of the element's
content. Requirements regarding the content and interpretation of
xml:lang are specified in XML 1.0 [W3C.REC-xml-20040204] Section
2.12.
[[ discussion of URI escaping and i18n, IRI ]]
[[ discussion of white space ]]
Atom is extensible. See the section titled 'Extending Atom' later in
this document for a full description of how Atom Documents can be
extended.
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3. Common Atom Constructs
Many of Atom's elements share a few common structures. This section
defines a few such structures and their requirements, for convenient
reference by the appropriate element definitions.
When an element is identified as being a particular kind of
construct, it inherits the corresponding requirements from that
construct's definition in this section.
3.1 Text Constructs
A Text construct contains human readable text, usually in fairly
small quantities.
3.1.1 "type" Attribute
Text constructs MAY have a "type" attribute. When present, the value
MUST be one of "TEXT", "HTML" or "XHTML". If the "type" attribute is
not provided, software MUST behave as though it were present with a
value of "TEXT".
Note that MIME media types [RFC2045] are not acceptable values for
the "type" attribute.
If the value is "TEXT", the content of the Text construct MUST NOT
contain child elements. Such text is intended to be presented to
humans in a readable fashion. Thus, software MAY display it using
normal text rendering techniques such as proportional fonts,
white-space collapsing, and justification.
If the value of "type" is "HTML", the content of the Text construct
MUST NOT contain child elements, and SHOULD be suitable for handling
by software that knows HTML. The HTML markup must be escaped; for
example, " " as "<br>". The HTML markup SHOULD be such that it
could validly appear directly within an HTML
element.
Receiving software which displays the content MAY use the markup to
aid in displaying it.
If the value of "type" is "XHTML", the content of the Text construct
MAY contain child elements. The content SHOULD be XHTML text and
markup that could validly appear directly within an xhtml:div
element. Receiving software which displays the content MAY use the
markup to aid in displaying it. Escaped markup is interpreted as a
text representation of markup, and MUST NOT be interpreted as markup
itself.
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3.2 Person Constructs
A Person construct is an element that describes a person,
corporation, or similar entity.
Person constructs MAY be extended by namespace-qualified element
children.
This specification assigns no significance to the order of appearance
of the child elements of a Person construct.
3.2.1 "atom:name" Element
The "atom:name" element's content conveys a human-readable name for
the person. Person constructs MUST contain exactly one "atom:name"
element.
3.2.2 "atom:uri" Element
The "atom:uri" element's content conveys a URI associated with the
person. Person constructs MAY contain an atom:uri element, but MUST
NOT contain more than one. The content of atom:uri in a Person
construct MUST be a URI reference [RFC2396bis].
xml:base [W3C.REC-xmlbase-20010627] processing MUST be applied to the
atom:uri element's content.
3.2.3 "atom:email" Element
The "atom:email" element's content conveys an e-mail address
associated with the persons. Person constructs MAY contain an
atom:email element, but MUST NOT contain more than one. Its content
MUST be an e-mail address [RFC2822].
3.3 Date Constructs
A Date construct is an element whose content MUST conform to the
date-time BNF rule in [RFC3339].
3.4 Service Constructs
A Service construct is an empty element that conveys the URI of an
Atom Publishing Protocol [Atom-protocol] service associated with an
entry or feed. The type of service is identified by the element
name.
A Service construct has the following attribute:
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3.4.1 "href" Attribute
The "href" attribute contains the a URI pointing to the endpoint of
the service named by the name attribute. atom:service elements MUST
have a "href" attribute, whose value MUST be a URI reference
[RFC2396bis].
xml:base processing MUST be applied to the "href" attribute.
3.5 Link Constructs
A Link construct is an empty element that describes a connection from
an Atom Document to another Web resource.
3.5.1 "rel" Attribute
Link constructs MAY have an optional "rel" attribute that indicates
the link relation type. If the "rel" attribute is not present, the
link construct MUST be interpreted as if the link relation type is
"alternate".
rel_attribute = segment-nz-nc / URI
The value of "rel" MUST be either a name, which is non-empty and does
not contain any colon (":") characters, or a URI [RFC2396bis]. Note
that use of a relative reference to the "rel" value URI is not
allowed. If a name is given, implementations MUST consider the link
relation type to be equivalent to the same name registered within the
IANA Registry of Link Relations Section 10, and thus the URI that
would be obtained by appending the value of the rel attribute to the
string "http://www.iana.org/assignments/relation/". The value of
"rel" describes the meaning of the link, but does not impose any
behavioral requirements on implementations.
This document defines two initial values for the Registry of Link
Relations:
The value "alternate" signifies that the URI in the value of the href
attribute identifies an alternate version of the resource described
by the containing element.
The value "related" signifies that the URI in the value of the href
attribute identifies a resource to which the resource described by
the containing atom:feed or atom:entry element is related. For
example, the feed for a site which discusses the performance of the
search engine at "http://search.example.com" might contain, as a
child of atom:head:
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An identical link might appear as a child of any atom:entry whose
content contains a discussion of that same search engine.
3.5.2 "type" Attribute
Link constructs MAY have a type attribute, whose value MUST conform
to the syntax of a MIME media type [RFC2045].
The type attribute's value is an advisory media type; it MAY be used
as a hint to determine the type of the representation which is
expected to be returned when the value of the href attribute is
dereferenced. Note that the type attribute does not override the
actual media type returned with the representation.
3.5.3 "href" Attribute
The "href" attribute contains the link's URI. Link constructs MUST
have a href attribute, whose value MUST be a URI reference
[RFC2396bis].
xml:base [W3C.REC-xmlbase-20010627] processing MUST be applied to the
href attribute's content.
3.5.4 "hreflang" Attribute
The "hreflang" attribute's content describes the language of the
resource pointed to by the href attribute. When used together with
the rel="alternate", it implies a translated version of the entry.
Link constructs MAY have an hreflang attribute, whose value MUST be a
language tag [RFC3066].
3.5.5 "title" Attribute
The "title" attribute conveys human-readable information about the
link. Link constructs MAY have a title attribute.
3.5.6 "length" Attribute
The "length" attribute indicates an advisory length of the linked
content in octets; it MAY be used as a hint to determine the content
length of the representation returned when the URI in the href
attribute is dereferenced. Note that the length attribute does not
override the actual content length of the representation as reported
by the underlying protocol.
Link constructs MAY have a length attribute.
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3.6 Identity Constructs
An Identity construct is an element whose content conveys a
permanent, universally unique identifier for the construct's parent.
Its content MUST be a URI, as defined by [RFC2396bis]. Note that the
definition of "URI" excludes relative references.
When an Atom document is relocated, migrated, syndicated,
republished, exported or imported, the content of its Identity
construct MUST NOT change. Put another way, an Identity construct
pertains to all instantiations of a particular Atom entry or feed;
revisions retain the same content in their Identity constructs.
3.6.1 Dereferencing Identity Constructs
The content of an Identity construct MAY be dereferencable (e.g. an
HTTP URI). However, processors MUST NOT assume it to be
dereferencable.
The content of an Identity construct MUST be created in a way that
assures uniqueness, and it is suggested that the Identity construct
be stored along with the associated resource.
Because of the risk of confusion between URIs that would be
equivalent if dereferenced, the following normalization strategy is
strongly encouraged when generating Identity constructs:
o Provide the scheme in lowercase characters.
o Provide the host, if any, in lowercase characters.
o Only perform percent-encoding where it is essential.
o Use uppercase A-through-F characters when percent-encoding.
o Prevent dot-segments appearing in paths.
o For schemes that define a default authority, use an empty
authority if the default is desired.
o For schemes that define an empty path to be equivalent to a path
of "/", use "/".
o For schemes that define a port, use an empty port if the default
is desired.
o Preserve empty fragment identifiers and queries.
o Ensure that all portions of the URI are UTF-8 encoded NFC form
Unicode strings.
3.6.2 Comparing Identity Constructs
Instances of Identity constructs can be compared to determine whether
an entry or feed is the same as one seen before. Processors MUST
compare Identity constructs on a character-by-character basis in a
case-sensitive fashion.
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As a result, two URIs that resolve to the same resource but are not
character-for-character identical will be considered different for
the purposes of Identifier comparison.
For example, "http://www.example.org/thing",
"http://www.example.org/Thing", "http://www.EXAMPLE.org/thing" and
"HTTP://www.example.org/thing" will all be considered different
identifiers, despite their differences in case.
Likewise, "http://www.example.com/~bob",
"http://www.example.com/%7ebob" and "http://www.example.com/%7Ebob"
will all be considered different identifiers, because URI %-escaping
is significant for the purposes of comparison.
3.7 The Category Construct
Category constructs contain information about a category to which an
Atom feed or entry is associated.
3.7.1 The "term" Attribute
The "term" attribute will be a string which identifies the category
within the categorization scheme to which the entry or feed belongs.
Category constructs MUST have a "term" attribute.
3.7.2 The "scheme" Attribute
The is a URI that identifies a categorization scheme. Category
constructs MAY have a "scheme" attribute.
3.7.3 The "label" attribute
The "label" attribute provides a human-readable label that may be
displayed in end-user applications. Category constructs MAY have a
"label" attribute.
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4. The "atom:feed" Element
The "atom:feed" element is the document (i.e., top-level) element of
an Atom Feed Document, acting as a container for metadata and data
associated with the feed. Its first element child MUST be atom:head,
which MAY be followed zero or more atom:entry child elements.
4.1 "version" Attribute
atom:feed elements MUST have a "version" attribute whose content
indicates the version of the Atom specification that the feed
conforms to. The content of this attribute is unstructured text.
The version identifier for this specification is
"draft-ietf-atompub-format-04 : do not deploy".
4.2 The "atom:head" Element
The atom:head element acts as a container for metadata about the feed
itself.
The atom:head element MAY contain any namespace-qualified
[W3C.REC-xml-names-19990114] elements as children. This
specification assigns no significance to the order of appearance of
the child elements of atom:head.
The following child elements are defined by this specification (note
that the presence of some of these elements is required):
4.2.1 "atom:title" Element
The "atom:title" element is a Text construct that conveys a
human-readable title for the feed. atom:head elements MUST contain
exactly one atom:title element.
4.2.2 "atom:link" Element
The "atom:link" element is a Link construct that conveys a URI
associated with the feed. The nature of the relationship is
determined by the construct's rel attribute.
atom:head elements MUST contain at least one atom:link element with a
rel attribute value of "alternate".
atom:head elements MUST NOT contain more than one atom:link element
with a rel attribute value of "alternate" that has the same type
attribute value.
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If a feed's atom:link element with type="alternate" resolves to an
HTML document, then that document SHOULD have a autodiscovery link
element [Atom-autodiscovery] that reflects back to the feed.
atom:head elements MAY contain additional atom:link elements beyond
those described above.
4.2.3 "atom:category" Element
A Category Construct identifying a category with which the feed is
associated. atom:head elements MAY contain any number of
atom:category elements.
4.2.4 "atom:introspection" Element
The "atom:introspection" element is a Service construct that conveys
the URI of an introspection file associated with the feed. atom:head
elements MUST NOT contain more than one atom:introspection element.
4.2.5 "atom:post" Element
The "atom:post" element is a Service construct that conveys the URI
used to add entries to the feed. atom:head elements MUST NOT contain
more than one atom:post element.
4.2.6 "atom:author" Element
The "atom:author" element is a Person construct that indicates the
default author of the feed. atom:head elements MUST contain exactly
one atom:author element, UNLESS all of the atom:feed element's child
atom:entry elements contain an atom:author element. atom:head
elements MUST NOT contain more than one atom:author element.
[[explain inheritance]]
4.2.7 "atom:contributor" Element
The "atom:contributor" element is a Person construct that indicates a
person or other entity who contributes to the feed. atom:head
elements MAY contain one or more atom:contributor elements.
4.2.8 "atom:tagline" Element
The "atom:tagline" element is a Text construct that conveys a
human-readable description or tagline for the feed. atom:head
elements MAY contain an atom:tagline element, but MUST NOT contain
more than one.
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4.2.9 "atom:id" Element
The "atom:id" element is an Identity construct that conveys a
permanent, universally unique identifier for a feed. atom:head
elements MAY contain an atom:id element, but MUST NOT contain more
than one.
4.2.10 "atom:generator" Element
The "atom:generator" element's content identifies the software agent
used to generate the feed, for debugging and other purposes.
atom:head elements MAY contain an atom:generator element, but MUST
NOT contain more than one.
The content of this element, when present, MUST be a string that is a
human-readable name for the generating agent.
The atom:generator element MAY have a "uri" attribute whose value
MUST be a URI reference [RFC2396bis]. When dereferenced, that URI
SHOULD produce a representation that is relevant to that agent.
The atom:generator element MAY have a "version" attribute that
indicates the version of the generating agent. When present, its
value is unstructured text.
4.2.11 "atom:copyright" Element
The "atom:copyright" element is Text construct that conveys a
human-readable copyright statement for the feed. atom:head elements
MAY contain an atom:copyright element, but MUST NOT contain more than
one.
The atom:copyright element SHOULD NOT be used to convey
machine-readable licensing information.
4.2.12 "atom:info" Element
The "atom:info" element is a Text construct that conveys a
human-readable explanation of the feed format itself. atom:head
elements MAY contain an atom:info element, but MUST NOT contain more
than one.
The atom:info element SHOULD NOT considered meaningful by processors;
it is a convenience to publishers in certain situations.
4.2.13 "atom:updated" Element
The "atom:updated" element is a Date construct indicating the most
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recent instant in time when the feed was modified in a way the
producer considers significant. Therefore, not all modifications
necessarily result in a changed atom:updated value.
atom:head elements MUST contain exactly one atom:updated element.
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5. The "atom:entry" Element
The "atom:entry" element represents an individual entry. This
element can appear as a child of the atom:feed element, or it can
appear as the document (i.e., top-level) element of a standalone Atom
Entry Document.
When appearing in an Atom Entry Document, atom:entry elements MUST
have a "version" attribute whose content indicates the version of the
Atom specification that the entry conforms to.
The version identifier for this specification is
"draft-ietf-atompub-format-04 : do not deploy".
The atom:entry element MAY contain any namespace-qualified
[W3C.REC-xml-names-19990114] elements as children. This
specification assigns no significance to the order of appearance of
the child elements of atom:entry.
The following child elements are defined by this specification (note
that it requires the presence of some of these elements):
5.1 "atom:title" Element
The "atom:title" element is a Text construct that conveys a
human-readable title for the entry. atom:entry elements MUST have
exactly one "atom:title" element.
5.2 "atom:link" Element
The "atom:link" element is a Link construct that conveys a URI
associated with the entry. The nature of the relationship as well as
the link itself is determined by the element's content.
atom:entry elements that contain no child atom:content element MUST
contain at least one atom:link element with a rel attribute value of
"alternate".
atom:entry elements MUST NOT contain more than one atom:link element
with a rel attribute value of "alternate" that has the same type
attribute value.
atom:entry elements MAY contain additional atom:link elements beyond
those described above.
5.3 "atom:category" Element
A Category Construct identifying a category with which the entry is
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associated. atom:entry elements MAY contain any number of
atom:category elements.
5.4 "atom:edit" Element
The "atom:edit" element is a Service construct that conveys the URI
used to retrieve and edit the source representation of the entry.
atom:entry elements MUST NOT contain more than one atom:edit element.
5.5 "atom:author" Element
The "atom:author" element is a Person construct that indicates the
default author of the entry. atom:entry elements MUST contain
exactly one atom:author element, unless, in an Atom Feed Document,
the atom:head element contains an atom:author element itself.
atom:entry elements MUST NOT contain more than one atom:author
element.
5.6 "atom:contributor" Element
The "atom:contributor" element is a Person construct that indicates a
person or other entity who contributes to the entry. atom:entry
elements MAY contain one or more atom:contributor elements.
5.7 "atom:host" Element
The "atom:host" element's content conveys a domain name or network
address associated with the entry's origin. atom:entry elements MAY
contain a single atom:host element. Its content MUST be a domain
name [RFC1035], a dotted-decimal IPv4 address [RFC0791], or a
colon-delimited IPv6 address [RFC2460].
5.8 "atom:id" Element
The "atom:id" element is an Identity construct that conveys a
permanent, universally unique identifier for an entry. atom:entry
elements MUST contain exactly one atom:id element.
5.9 "atom:updated" Element
The "atom:updated" element is a Date construct indicating the most
recent instant in time when the entry was modified in a way the
producer considers significant. Therefore, not all modifications
necessarily result in a changed atom:updated value.
atom:entry elements MUST contain exactly one atom:updated element.
Publishers MAY change the value of this element over time.
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Processors MAY present entries sorted using this value. Processors
MAY choose not to present entries until the instant in time specified
in the atom:updated element has passed.
5.10 "atom:published" Element
The "atom:published" element is a Date construct indicating an
instant in time associated with an event early in the life cycle of
the entry. Typically, atom:published will be associated with the
initial creation or first availability of the resource.
atom:entry elements MAY contain an atom:published element, but MUST
NOT contain more than one.
Processors MAY present entries sorted using this value. Processors
MAY choose not to present entries until the instant in time specified
in the atom:published element has passed.
5.11 "atom:summary" Element
The "atom:summary" element is a Text construct that conveys a short
summary, abstract or excerpt of the entry. atom:entry elements MAY
contain an atom:summary element, but MUST NOT contain more than one.
atom:entry elements MUST contain an atom:summary element in any of
the following cases:
o the atom:entry element contains no atom:content element.
o the atom:entry contains an atom:content which has a "src"
attribute (and is thus empty).
o the atom:entry contains content which is encoded in Base64; i.e.
the "type" attribute of atom:content is a MIME media type
[RFC2045] and does not begin with "text/" nor end with "+xml".
5.12 "atom:content" Element
The "atom:content" element either contains or links to the content of
the entry. atom:entry elements MUST contain zero or one atom:content
elements.
5.12.1 "type" attribute
atom:content MAY have a "type" attribute, When present, the value MAY
be one of "TEXT", "HTML", or "XHTML". Failing that, it MUST be a
MIME media type [RFC2045] in which, to use the terminology of Section
5 of [RFC2045], the top level is a discrete type. If the type
attribute is not provided, software MUST behave as though it were
present with a value of "TEXT".
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5.12.2 "src" attribute
atom:content MAY have a "src" attribute, whose value MUST be a URI
reference [RFC2396bis]. If the "src" attribute is present, software
MAY use the URI to retrieve the content. If the "src" attribute is
present, atom:content MUST be empty. That is to say, the content may
be retrievable using "src=" URI, or it may be contained within
atom:content, but not both.
If the "src" attribute is present, the "type" attribute SHOULD be
provided and MUST be a MIME media type [RFC2045], rather than "TEXT",
"HTML", or "XHTML". The value is advisory; that is to say, upon
dereferencing the URI to retrieve the content, if the server
providing that content also provides a media type, the
server-provided media type is authoritative.
If the value of type begins with "text/" or ends with "+xml", the
content SHOULD be local; that is to say, no "src" attribute should be
provided.
5.12.3 Processing Model
Software MUST apply the following rules in succession in the order
below to ascertain the rules governing the content of "atom:content".
1. If the value is "TEXT", the content of atom:content MUST NOT
contain child elements. Such text is intended to be presented to
humans in a readable fashion. Thus, software MAY display it
using normal text rendering techniques such as proportional
fonts, white-space collapsing, and justification.
2. If the value of "type" is "HTML", the content of atom:content
MUST NOT contain child elements, and SHOULD be suitable for
handling by software that knows HTML. The HTML markup must be
escaped; for example, " " as "<br>". The HTML markup
SHOULD be such that it could validly appear directly within an
HTML
element. Receiving software which displays the
content SHOULD use the markup to aid in displaying it.
3. If the value of "type" is "XHTML", the content of atom:content
MAY contain child elements. The content SHOULD be XHTML text and
markup that could validly appear directly within an xhtml:div
element. Receiving software which displays the content SHOULD
use the markup to aid in displaying it. Escaped markup is
interpreted as a text representation of markup, and MUST NOT be
interpreted as markup itself.
4. If the value of "type" ends with "+xml" or "/xml", the content of
atom:content may include child elements, and SHOULD be suitable
for handling by software that knows the indicated media type. If
the "src" attribute is not provided, this would normally mean
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that the "atom:content" element would contain a single child
element which would serve as the root element of the XML document
of the indicated type.
5. If the value of "type" begins with "text/" the content of
atom:content MUST NOT contain child elements.
6. For all other values of "type", the content of atom:content MUST
be a valid Base64 encoding [RFC3548], which when decoded SHOULD
be suitable for handling by software that knows the indicated
media type. In this case, the characters in the Base64 encoding
may be preceded and followed in the atom:content element by
whitespace, and lines are separated by a single newline (U+000A)
character, as required by XML.
5.13 "atom:copyright" Element
The "atom:copyright" element is a Text construct that conveys a
human-readable copyright statement for the entry. atom:entry
elements MAY contain an atom:copyright element, but MUST NOT contain
more than one.
The atom:copyright element SHOULD NOT be used to convey
machine-readable licensing information.
If an atom:entry element does not contain an atom:copyright element,
then the atom:copyright element of the containing atom:feed element's
atom:head element, if present, should be considered to apply to the
entry.
5.14 "atom:head" Element
The atom:head element acts as a container for metadata about the feed
within which the entry was created.
atom:entry elements MAY contain at most one atom:head element. If
the atom:head element is present, it SHOULD be the first child
element of atom:entry.
If an atom:entry is copied into one feed from another feed, then the
atom:head element of the source feed SHOULD be inserted into the
atom:entry unless the entry, as copied, already contains an atom:head
element. If the atom:entry already contains an atom:head, then that
atom:head SHOULD be copied without modification.
[[ ... example ... ]]
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6. Managing Feed State
[[ talk about what it means to keep a view of a feed ]]
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7. Securing Atom Documents
Because Atom is an XML-based format, existing XML security mechanisms
can be used to secure its content.
Note that while these mechanisms are available to secure Atom
documents, they should not be used indiscriminately.
7.1 Digital Signatures
The document element of an Atom document (i.e., atom:feed in an Atom
Feed Document, atom:entry in an Atom Entry Document) MAY have an
Enveloped Signature, as described by XML-Signature and Syntax
Processing [W3C.REC-xmldsig-core-20020212]. Other XML signature
mechanisms MUST NOT be used on the document element of an Atom
document.
Processors MUST NOT reject an Atom document containing such a
signature because they are not capable of verifying it; they MUST
continue processing and MAY inform the user of their failure to
validate the signature.
In other words, the presence of an element with the namespace URI
"http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
as a child of the document element must not cause a processor to fail
merely because of its presence.
Other elements in an Atom document MUST NOT be signed unless their
definitions explicitly specify such a capability.
7.2 Encryption
The document element of an Atom document (i.e., atom:feed in an Atom
Feed Document, atom:entry in an Atom Entry Document) MAY be
encrypted, using the mechanisms described by XML Encryption Syntax
and Processing [W3C.REC-xmlenc-core-20021210]. Other XML encryption
mechanisms MUST NOT be used on the document element of an Atom
document.
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8. Embedding Atom in Other Formats
[[ ... ]]
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9. Extending Atom
[[ ... ]]
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10. IANA Considerations
An Atom Document, when serialized as XML 1.0, can be identified with
the following media type:
MIME media type name: application
MIME subtype name: atom+xml
Mandatory parameters: None.
Optional parameters:
"charset": This parameter has identical semantics to the charset
parameter of the "application/xml" media type as specified in
RFC 3023 [RFC3023]. [RFC3023].
Encoding considerations: Identical to those of "application/xml" as
described in RFC 3023 [RFC3023], section 3.2.
Security considerations: As defined in this specification. [[update
upon publication]]
In addition, as this media type uses the "+xml" convention, it
shares the same security considerations as described in RFC 3023
[RFC3023], section 10.
Interoperability considerations: There are no known interoperability
issues.
Published specification: This specification. [[update upon
publication]]
Applications which use this media type: No known applications
currently use this media type.
Additional information:
Magic number(s): As specified for "application/xml" in RFC 3023
[RFC3023], section 3.2.
File extension: .atom
Fragment identifiers: As specified for "application/xml" in RFC 3023
[RFC3023], section 5.
Base URI: As specified in RFC 3023 [RFC3023], section 6.
Macintosh File Type code: TEXT
Person and email address to contact for further information: Mark
Nottingham
Intended usage: COMMON
Author/Change controller: This specification's author(s). [[update
upon publication]]
10.1 Registry of Link Relations
This registry is maintained by IANA and initially contains the two
values: "alternate" and "related". New assignments are subject to
IESG Approval, as outlined in [RFC2434]. Requests should be made by
email to IANA, which will then forward the request to the IESG
requesting approval. The request should contain discussion of at
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least the following five topics:
o A value for the "rel" attribute that conforms to the syntax rule
given in Section 3.5.1
o Common name for link type.
o Description of link type semantics.
o Expected display characteristics.
o Security considerations.
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11. Security Considerations
Atom document can be encrypted and signed using
[W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212],
respectively, and is subject to the security considerations implied
by their use.
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12. References
12.1 Normative References
[Atom-autodiscovery]
Pilgrim, M., "Atom Feed Autodiscovery", work-in-progress,
August 2004.
[Atom-protocol]
Gregorio, J. and R. Sayre, "The Atom Publishing Protocol",
work-in-progress, July 2004.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2396bis]
Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", awaiting RFC
number, December 2004.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
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[W3C.NOTE-datetime-19980827]
Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C
NOTE NOTE-datetime-19980827, August 1998.
[W3C.REC-xml-20040204]
Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T. and
E. Maler, "Extensible Markup Language (XML) 1.0 (Third
Edition)", W3C REC REC-xml-20040204, February 2004.
[W3C.REC-xml-infoset-20011024]
Tobin, R. and J. Cowan, "XML Information Set", W3C
FirstEdition REC-xml-infoset-20011024, October 2001.
[W3C.REC-xml-names-19990114]
Hollander, D., Bray, T. and A. Layman, "Namespaces in
XML", W3C REC REC-xml-names-19990114, January 1999.
[W3C.REC-xmlbase-20010627]
Marsh, J., "XML Base", W3C REC REC-xmlbase-20010627, June
2001.
[W3C.REC-xmldsig-core-20020212]
Solo, D., Reagle, J. and D. Eastlake, "XML-Signature
Syntax and Processing", W3C REC REC-xmldsig-core-20020212,
February 2002.
[W3C.REC-xmlenc-core-20021210]
Reagle, J. and D. Eastlake, "XML Encryption Syntax and
Processing", W3C REC REC-xmlenc-core-20021210, December
2002.
12.2 Informative References
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
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URIs
[1]
[2]
Authors' Addresses
Mark Nottingham (editor)
EMail: mnot@pobox.com
URI: http://www.mnot.net/
Robert Sayre (editor)
Boswijck Memex Consulting
EMail: rfsayre@boswijck.com
URI: http://boswijck.com
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Appendix A. Contributors
The following people contributed to preliminary drafts of this
document: Tim Bray, Mark Pilgrim, and Sam Ruby. The content and
concepts within are a product of the Atom community and the Atom
Publishing Format and Protocol Working Group.
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Appendix B. Revision History
[[ this section should be removed before final publication. ]]
-04: Update URI terms for 2396bis.
Add Category construct (PaceCategoryRevised).
Insert paranoid XHTML interpretation guidelines.
Adjust atom:copyright, per chairs' instruction.
Add atom:host as child element of atom:entry, per chairs'
direction (PacePersonConstruct).
Add link/content co-constraint (PaceContentOrLink).
Remove atom:origin as a side effect of adding atom:head to
atom:entry (PaceHeadInEntry).
Add optional length attribute to atom:link (PaceLinkRelated).
Add Link registry to Link Construct, IANA Considerations
placeholder (PaceFieldingLinks).
Change definition of atom:updated (PaceUpdatedDefinition).
-03: Move definition of Link @rel to format spec, restrict
acceptable values (PaceMoveLinkElement, PaceLinkAttrDefaults).
Add Service Construct, head/post, head/introspection, entry/edit
(PaceServiceElement).
Add Text Construct, entry/content (PaceReformedContent3).
Add entry/published (PaceDatePublished).
Adjust definition of Identity Construct per chairs' direction to
"fix it."
Add Sayre to editors.
-02: Removed entry/modified, entry/issued, entry/created; added
entry/updated (PaceDateUpdated).
Changed date construct from W3C date-time to RFC3339
(PaceDateUpdated).
Feed links to HTML pages should be reflected back
(PaceLinkReflection).
Added Identity construct (PaceIdConstruct).
Changed feed/id and entry/id to be Identity constructs
(PaceIdConstruct).
Changed entry/origin's content so that it's the same as the feed's
id, rather than its link/@rel="alternate" (PaceIdConstruct).
Added "Securing Atom Documents" (PaceDigitalSignatures).
-01: Constrained omission of "Information Item" to just elements and
attributes.
Clarified xml:lang inheritence.
Removed entry- and feed-specific langauge about xml:lang (covered
by general discussion of xml:lang)
Changed xml:lang to reference XML for normative requirements.
Changed "... MUST be a string" to "... is unstructued text."
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Remomved langauge about DOCTYPEs, PIs, Comments, Entities.
Changed atom:url to atom:uri, @url to @uri
Introduced atom:head
Introduced "Atom Feed Document" and "Atom Entry Document".
Removed requirement for all elements and attributes to be
namespace-qualified; now children of selective elements
Added extensibility to Person constructs.
Removed requirement for media types to be registered
(non-registered media types are legal)
Added atom:origin (PaceEntryOrigin)
Added requirement for entry/id to be present and a URI
(PaceEntryIdRequired).
Clarified approach to Comments, PIs and well-formedness, as per
RFC3470.
Referenced escaping algorithm in XML.
Assorted editorial nits and cleanup, refactoring
-00: Initial IETF Internet-Draft submission.
Added optional version attribute to entry
(PaceEntryElementNeedsVersionAttribute).
Added hreflang attribute (PaceHrefLang).
Clarified inheritence of copyright element (PaceItemCopyright).
Added xml:lang to entries (PaceItemLang).
Tweaked Infoset-related language (PaceNoInfoSet).
Clarified lack of structure in version attribute
(PaceVersionAsText).
Changed approach to XML Base (PaceXmlBaseEverywhere).
Added XML Base processing to atom:id (PaceXmlBaseId).
Various editorial cleanup and adjustments for IETF publication.
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Disclaimer of Validity
This document and the information contained herein are provided on an
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Copyright Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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