Network Working Group                                 M. Nottingham, Ed.
Internet-Draft
Expires: March 6, April 20, 2005                                    R. Sayre, Ed.
                                               Boswijck Memex Consulting
                                                        October 20, 2004

                      The Atom Syndication Format
                      draft-ietf-atompub-format-02
                      draft-ietf-atompub-format-03

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document specifies Atom, an XML-based Web content and metadata
   syndication format.

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 . . . . . . . . . . . . . . . . . . .   9   8
     3.1  Content  Text Constructs  . . . . . . . . . . . . . . . . . . . .   9 .   8
       3.1.1  "type" Attribute . . . . . . . . . . . . . . . . . . .   9
       3.1.2  "mode" Attribute . .   8
     3.2  Person Constructs  . . . . . . . . . . . . . . . . .   9
     3.2  Person Constructs . . .   8
       3.2.1  "atom:name" Element  . . . . . . . . . . . . . . . . .   9
       3.2.1  "atom:name"
       3.2.2  "atom:uri" Element . . . . . . . . . . . . . . . . .  10
       3.2.2  "atom:uri" .   9
       3.2.3  "atom:email" Element . . . . . . . . . . . . . . . . .   9
     3.3  Date Constructs  . .  10
       3.2.3  "atom:email" Element . . . . . . . . . . . . . . . . .  10
     3.3  Date . .   9
     3.4  Service Constructs . . . . . . . . . . . . . . . . . . . .   9
       3.4.1  "href" Attribute . . . . .  10
     3.4 . . . . . . . . . . . . . .   9
     3.5  Link Constructs  . . . . . . . . . . . . . . . . . . . . .  10
       3.4.1
       3.5.1  "rel" Attribute  . . . . . . . . . . . . . . . . . . .  10
       3.4.2
       3.5.2  "type" Attribute . . . . . . . . . . . . . . . . . . .  11
       3.4.3  10
       3.5.3  "href" Attribute . . . . . . . . . . . . . . . . . . .  11
       3.4.4  10
       3.5.4  "hreflang" Attribute . . . . . . . . . . . . . . . . .  11
       3.4.5  10
       3.5.5  "title" Attribute  . . . . . . . . . . . . . . . . . .  11
     3.5
     3.6  Identity Constructs  . . . . . . . . . . . . . . . . . . .  11
       3.5.1
       3.6.1  Dereferencing Identity Constructs  . . . . . . . . . .  11
       3.5.2
       3.6.2  Comparing Identity Constructs  . . . . . . . . . . . .  12
   4.   The "atom:feed" Element  . . . . . . . . . . . . . . . . . .  13
     4.1  "version" Attribute  . . . . . . . . . . . . . . . . . . .  13
     4.2  The "atom:head" Element  . . . . . . . . . . . . . . . . .  13
       4.2.1  "atom:title" Element . . . . . . . . . . . . . . . . .  13
       4.2.2  "atom:link" Element  . . . . . . . . . . . . . . . . .  13
       4.2.3  "atom:introspection" Element . . . . . . . . . . . . .  14
       4.2.4  "atom:post" Element  . . . . . . . . . . . . . . . . .  14
       4.2.5  "atom:author" Element  . . . . . . . . . . . . . . . .  14
       4.2.4
       4.2.6  "atom:contributor" Element . . . . . . . . . . . . . .  14
       4.2.5
       4.2.7  "atom:tagline" Element . . . . . . . . . . . . . . . .  14
       4.2.6
       4.2.8  "atom:id" Element  . . . . . . . . . . . . . . . . . .  14
       4.2.7
       4.2.9  "atom:generator" Element . . . . . . . . . . . . . . .  14
       4.2.8  15
       4.2.10   "atom:copyright" Element . . . . . . . . . . . . . . .  15
       4.2.9
       4.2.11   "atom:info" Element  . . . . . . . . . . . . . . . . .  15
       4.2.10   "atom:modified"
       4.2.12   "atom:updated" Element . . . . . . . . . . . . . .  15 .  16
   5.   The "atom:entry" Element . . . . . . . . . . . . . . . . . .  16  17
     5.1  "atom:title" Element . . . . . . . . . . . . . . . . . . .  16  17
     5.2  "atom:link" Element  . . . . . . . . . . . . . . . . . . .  16  17
     5.3  "atom:author"  "atom:edit" Element  . . . . . . . . . . . . . . . . . .  16 .  17
     5.4  "atom:contributor"  "atom:author" Element  . . . . . . . . . . . . . . . .  17
     5.5  "atom:id" Element  . . . .  18
     5.5  "atom:contributor" Element . . . . . . . . . . . . . . . .  17  18
     5.6  "atom:updated"  "atom:id" Element  . . . . . . . . . . . . . . . . . .  17 . .  18
     5.7  "atom:summary"  "atom:updated" Element . . . . . . . . . . . . . . . . . .  17  18
     5.8  "atom:content"  "atom:published" Element . . . . . . . . . . . . . . . . . .  17  18
     5.9  "atom:copyright"  "atom:summary" Element . . . . . . . . . . . . . . . . .  18 .  19
     5.10   "atom:origin"   "atom:content" Element . . . . . . . . . . . . . . . . .  18
   6.   Managing Feed State  . .  19
       5.10.1   "type" attribute . . . . . . . . . . . . . . . . . .  19
   7.   Securing Atom Documents  . .
       5.10.2   "src" attribute  . . . . . . . . . . . . . . . .  20
     7.1  Digital Signatures . .  19
       5.10.3   Processing Model . . . . . . . . . . . . . . . . . .  20
     7.2  Encryption . .
     5.11   "atom:copyright" Element . . . . . . . . . . . . . . . .  21
     5.12   "atom:origin" Element  . . . . . .  20
   8.   Embedding Atom in Other Formats . . . . . . . . . . .  21
   6.   Managing Feed State  . . .  21
   9.   Extending Atom . . . . . . . . . . . . . . . . .  22
   7.   Securing Atom Documents  . . . . . .  22
   10.  IANA Considerations . . . . . . . . . . . .  23
     7.1  Digital Signatures . . . . . . . .  23
   11.  Security Considerations . . . . . . . . . . . .  23
     7.2  Encryption . . . . . . . . . . . . . . . . . . . . . . . .  23
   8.   Embedding Atom in Other Formats  . . . . . . . . . . . . . .  24
   9.   Extending Atom . . . . . . . . . . . . . . . . . . . . . . .  25
   10.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  26
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .  27
   12.  Normative References . . . . . . . . . . . . . . . . . . . .  24
        Author's Address .  27
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  25  28
   A.   Contributors . . . . . . . . . . . . . . . . . . . . . . . .  26  29
   B.   Revision History . . . . . . . . . . . . . . . . . . . . . .  27  30
        Intellectual Property and Copyright Statements . . . . . . .  29  32

1.  Introduction

   Atom is an XML-based document format intended to allow lists of
   related 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.
   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 comunication 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.

1.2  Example

   A minimal, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="utf-8"?>
   <feed version="draft-ietf-atompub-format-02: version="draft-ietf-atompub-format-03: do not deploy"
    xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-02">
    xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-03">
     <head>
       <title>Example Feed</title>
       <link rel="alternate" type="text/html" href="http://example.org/"/>
       <modified>2003-12-13T18:30:02Z</modified>
       <updated>2003-12-13T18:30:02Z</updated>
       <author>
         <name>John Doe</name>
       </author>
     </head>
     <entry>
       <title>Atom-Powered Robots Run Amok</title>
       <link rel="alternate" type="text/html" href="http://example.org/2003/12/13/atom03"/>
       <id>tag:example.org,2003:3.2397</id>
       <issued>2003-12-13T08:29:29-04:00</issued>
       <modified>2003-12-13T18:30:02Z</modified>
       <id>vemmi://example.org/2003/32397</id>
       <updated>2003-12-13T18:30:02Z</updated>
     </entry>
   </feed>

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-02  http://purl.org/atom/ns#draft-ietf-atompub-format-03

   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
   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
   refering
   referring to an Element Information Item in Infoset terms.  Likewise,
   when it uses the term "attribute," it is refering referring to an Attribute
   Information Item.

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 URI reference 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.  For convenience, the most important are summarised here:

   o  The content of this attribute must be a language tag [RFC3066] or
      an empty string (e.g., xml:lang=""), which indicates that there is
      no language information available.
   o  If an element does not have an xml:lang element, the first
      xml:lang attribute in its ancestors indicates the natural language
      of its content.

   [[ feedback as to whether this listing is helpful or not would be
   appreciated; re-stating the requirements of other specifications is
   tricky.  ]]

   [[ discussion of URI escaping and i18n ]]

   [[ 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.

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  Content  Text Constructs

   A Content Text construct is an element with arbitrary child content, whose
   properties are described by the following attributes: contains human readable text, usually in fairly
   small quantities.

3.1.1  "type" Attribute

   Content

   Text constructs MAY have a "type" attribute, whose value indicates
   the media type of the content. attribute.  When present, this attribute's the value
   MUST be a media type [RFC2045]. one of "TEXT", "HTML" or "XHTML".  If this the "type" attribute is
   not present,
   processors provided, software MUST behave as if though it were present with a
   value of "text/
   plain".

3.1.2  "mode" Attribute

   Content constructs MAY have a "mode" attribute, whose value indicates "TEXT".

   Note that MIME media types [RFC2045] are not acceptable values for
   the method used to encode "type" attribute.

   If the content.  When present, this
   attribute's value is "TEXT", the content of the Text construct MUST NOT
   contain child elements.  Such text is intended to be listed below.  If not present, processors
   MUST behave presented to
   humans in a readable fashion.  Thus, software MAY display it using
   normal text rendering techniques such as if its value were "xml".

   "xml":  A mode attribute with proportional fonts,
   white-space collapsing, and justification.

   If the value "xml" indicates that of "type" is "HTML", the
      element's content is inline xml (for example, namespace-qualified
      XHTML).
   "escaped":  A mode attribute with of the value "escaped" indicates Text construct
   MUST NOT contain child elements, and SHOULD be suitable for handling
   by software that
      the element's content is escaped, knows HTML.  The HTML markup must be encoded; for
   example, "<br>" as defined in
      [W3C.REC-xml-20040204] (section 4.6).  Processors MUST unescape
      the element's content before considering "&lt;br>".  The HTML markup SHOULD be such that it as
   could validly appear directly within an HTML <DIV> element.
   Receiving software which displays the content of MAY use the
      indicated media type.
   "base64":  A mode attribute with markup to
   aid in displaying it.

   If the value "base64" indicates that
      the element's content of "type" is base64-encoded [RFC2045].  Processors
      MUST decode "XHTML", the element's content before considering it as 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 indicated media type.
   markup to aid in displaying it.

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.

   Ordering

   This specification assigns no significance to the order of appearance
   of the element children child elements of Person constructs MUST NOT be
   considered significant. atom:entry.

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 [RFC2396].

   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  Link  Service Constructs

   A Link Service construct is an empty element that MUST NOT have any child content,
   and conveys the URI of an
   Atom Publishing Protocol [Atom-protocol] service associated with an
   entry or feed.

   A Service construct has the following attributes: attribute:

3.4.1  "rel"  "href" Attribute

   The "rel" "href" attribute indicates contains the type a URI pointing to the endpoint of relationship that
   the link
   represents.  Link constructs service named by the name attribute.  atom:service elements MUST
   have a rel "href" attribute, whose value MUST be one of the values enumerated in a URI.

   xml:base processing MUST be applied to the Atom Protocol
   specification [Atom-protocol].

3.4.2  "type" Attribute

   The "type" attribute indicates an "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

   The "rel" attribute indicates the type of relationship that the link
   represents.  Link constructs MAY have a rel attribute, whose value
   MUST be a string, and MUST be one of the following values:
   "alternate", "related".

   If the "rel" attribute is not present, the link element MUST be
   interpreted as if the value "alternate" had been supplied.

3.5.2  "type" Attribute

   The "type" attribute indicates an advisory media type; it MAY be used
   as a hint to determine the type of the representation which should be
   returned when the URI in the href attribute is dereferenced.  Note
   that the type attribute does not override the actual media type
   returned with the representation.

   Link constructs MUST MAY have a type attribute, whose value MUST be a
   registered media type [RFC2045].

3.4.3

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 [RFC2396].

   xml:base [W3C.REC-xmlbase-20010627] processing MUST be applied to the
   href attribute's content.

3.4.4

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].

3.4.5

3.5.5  "title" Attribute

   The "title" attribute conveys human-readable information about the
   link.  Link constructs MAY have a title attribute.

3.5

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 an absolute URI [RFC2396] that is universally
   unique; i.e., it MUST NOT change over time, even if the parent feed
   or entry element [RFC2396].

   When an Atom document is relocated, migrated, syndicated,
   republished, exported or imported.

3.5.1 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.  a  an
   HTTP URI).  However, processors MUST NOT assume it to be
   dereferencable.

   If the identified resource is served dynamically, the

   The content of an Identity construct MUST be created only once in a way that
   assures uniqueness, and then it is suggested that the Identity construct
   be stored along with the associated resource.  The content of an Identity construct MUST NOT be
   created dynamically.

   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.

3.5.2

3.6.2  Comparing Identity Constructs

   Instances of Identity constructs can be compared to determine whether
   a
   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.

   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.

   XML character and entity references MUST be converted into characters
   before comparison.

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-02:
   "draft-ietf-atompub-format-03: 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.  Ordering  This
   specification assigns no significance to the order of appearance of
   the
   element children child elements of atom:head element MUST NOT be considered
   significant. 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 Content 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.

   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: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.

4.2.4  "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.

4.2.5  "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 inheritence]]

4.2.4 inheritance]]

4.2.6  "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.

4.2.5

4.2.7  "atom:tagline" Element

   The "atom:tagline" element is a Content 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.

4.2.6

4.2.8  "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.

4.2.7

4.2.9  "atom:generator" Element

   The "atom:generator" element's content indentifies 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.  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.

4.2.8

4.2.10  "atom:copyright" Element

   The "atom:copyright" element is Content 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.

   [[Is the following paragraph bogus amateur lawyering? The first
   paragraph seems sufficient.]]

   The atom:copyright element may be assumed to apply to all entries
   contained by the feed except those entries which contain
   atom:copyright elements.  The atom:copyright element MUST, if
   present, be considered to apply to the feed as a collection of
   entries.

4.2.9

4.2.11  "atom:info" Element

   The "atom:info" element is a Content 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.

4.2.10  "atom:modified"

4.2.12  "atom:updated" Element

   The "atom:modified" "atom:updated" element is a Date construct that indicates indicating the most
   recent instant in time when the state of a change to the feed was last modified, including any
   changes made that the
   publisher wishes to entries therein. bring to the attention of subscribers.  For
   example, such changes might not include minor adjustments like
   spelling and grammatical corrections.

   atom:head elements MUST contain exactly one atom:modified atom:updated element.

   The content of an atom:modified element SHOULD have a time zone whose
   value MUST be "UTC".

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-02:
   "draft-ietf-atompub-format-03: do not deploy".

   The atom:entry element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  Ordering  This
   specification assigns no significance to the order of appearance of
   the
   element children child elements of atom:entry element MUST NOT be considered
   significant. 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 Content 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 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: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.

5.4  "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.

5.4

5.5  "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.

5.5

5.6  "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.

5.6

5.7  "atom:updated" Element

   The "atom:updated" element is a Date construct indicating the most
   recent instant in time when a change to the entry was made that the
   publisher wishes to bring to the attention of subscribers.  For
   example, such changes might not include minor adjustments like
   spelling and grammatical corrections.

   atom:entry elements MUST contain exactly one atom:updated element.

   Publishers MAY change the value value of this element over time.
   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.

5.8  "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.

5.9  "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".

5.10  "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.

5.10.1  "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".

5.10.2  "src" attribute

   atom:content MAY have a "src" attribute, whose value MUST be a URI.
   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 MUST 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.

5.10.3  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
       encoded; for example, "<br>" as "&lt;br>".  The HTML markup
       SHOULD be such that it could validly appear directly within an
       HTML <DIV> 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.
   4.  If the value of "type" ends with "+xml" or "/xml", the content of this element over time.
   Processors MAY present entries sorted using this value.  Processors
   MAY choose not to present entries until
       atom:content may include child elements, and SHOULD be suitable
       for handling by software that knows the instant in time specified
   in indicated media type.  If
       the atom:updated element has passed.

5.7  "atom:summary" Element

   The "atom:summary" element "src" attribute is a Content construct not provided, this would normally mean
       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.

5.8  "atom:content" Element

   The "atom:content" element is would contain a Content construct that conveys single child
       element which would serve as the
   content root element of the entry.  atom:entry elements MAY contain one or more
   atom:content elements. XML document
       of the indicated type.
   5.  If @type="multipart/alternative", @mode MUST NOT be specified, and the value of "type" begins with "text/" the content element of
       atom:content MUST NOT contain 1 or more content child elements.  These
   content elements MUST NOT specify @type="multipart/alternative" (i.e.

   only one level of nesting is allowed).  Consumers SHOULD look at
   6.  For all
   alternative other values of "type", the content elements and determine which one is most
   suitable, based on of atom:content MUST
       be a valid Base64 encoding [RFC3548], which @type 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 @mode followed in the consumer supports, atom:content element by
       whitespace, and
   preferences specified lines are separated by the end user (if any).  Consumers SHOULD NOT
   render more than one content alternative.

5.9 a single newline (U+000A)
       character, as required by XML.

5.11  "atom:copyright" Element

   The "atom:copyright" element is a Content 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.

5.10

5.12  "atom:origin" Element

   The "atom:origin" element's content conveys the original source of
   the entry; e.g., the feed where the entry was first published.

   If the source is an Atom Feed Document, then the content of
   atom:origin MUST be the same, character-for-character, as that of the
   atom:id element in that document's atom:head section (i.e., the XPath
   expression "/atom:feed/atom:head/atom:id").

   The content of this element MUST be a URI.  atom:entry elements MAY
   contain an atom:origin element, but MUST NOT contain more than one.

6.  Managing Feed State

   [[ talk about what it means to keep a view of a feed ]]

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
   defintions
   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.

8.  Embedding Atom in Other Formats

   [[ ...  ]]

9.  Extending Atom

   [[ ...  ]]

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 <mnot@pobox.com>
   Intended usage: COMMON
   Author/Change controller: This specification's author(s).  [[update
      upon publication]]

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.

12  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.

   [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.

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 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.

   [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., Bray, T., Paoli, J., Sperberg-McQueen, C. 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]
              Cowan, J. and R.
              Tobin, R. and J. Cowan, "XML Information Set", W3C REC
              FirstEdition REC-xml-infoset-20011024, October 2001.

   [W3C.REC-xml-names-19990114]
              Bray, T.,
              Hollander, D. 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]
              Eastlake,
              Solo, D., Reagle, J. and D. Solo, Eastlake, "XML-Signature
              Syntax and Processing", W3C REC REC-xmldsig-core-20020212,
              February 2002.

   [W3C.REC-xmlenc-core-20021210]
              Eastlake, D. and J.
              Reagle, J. and D. Eastlake, "XML Encryption Syntax and
              Processing", W3C REC REC-xmlenc-core-20021210, December
              2002.

   [1]  <http://www.imc.org/atom-syntax/index.html>

   [2]  <http://www.intertwingly.net/wiki/pie/FrontPage>

Author's Address

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

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.

Appendix B.  Revision History

   [[ this section should be removed before final publication.  ]]

   -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."
      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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