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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4287

Network Working Group                                 M. Nottingham, Ed.
Internet-Draft                                             R. Sayre, Ed.
Expires: October 20, 2005                                 April 18, 2005


                      The Atom Syndication Format
                      draft-ietf-atompub-format-08

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 20, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

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








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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1  Examples . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2  Notational Conventions . . . . . . . . . . . . . . . . . .   5
   2.   Atom Documents . . . . . . . . . . . . . . . . . . . . . . .   6
   3.   Common Atom Constructs . . . . . . . . . . . . . . . . . . .   8
     3.1  Text Constructs  . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1  The "type" Attribute . . . . . . . . . . . . . . . . .   8
     3.2  Person Constructs  . . . . . . . . . . . . . . . . . . . .  11
       3.2.1  The "atom:name" Element  . . . . . . . . . . . . . . .  11
       3.2.2  The "atom:uri" Element . . . . . . . . . . . . . . . .  11
       3.2.3  The "atom:email" Element . . . . . . . . . . . . . . .  11
     3.3  Date Constructs  . . . . . . . . . . . . . . . . . . . . .  12
   4.   Atom Element Definitions . . . . . . . . . . . . . . . . . .  13
     4.1  Container Elements . . . . . . . . . . . . . . . . . . . .  13
       4.1.1  The "atom:feed" Element  . . . . . . . . . . . . . . .  13
       4.1.2  The "atom:entry" Element . . . . . . . . . . . . . . .  15
       4.1.3  The "atom:content" Element . . . . . . . . . . . . . .  17
     4.2  Metadata Elements  . . . . . . . . . . . . . . . . . . . .  20
       4.2.1  The "atom:author" Element  . . . . . . . . . . . . . .  20
       4.2.2  The "atom:category" Element  . . . . . . . . . . . . .  20
       4.2.3  The "atom:contributor" Element . . . . . . . . . . . .  20
       4.2.4  The "atom:copyright" Element . . . . . . . . . . . . .  21
       4.2.5  The "atom:generator" Element . . . . . . . . . . . . .  21
       4.2.6  The "atom:icon" Element  . . . . . . . . . . . . . . .  21
       4.2.7  The "atom:id" Element  . . . . . . . . . . . . . . . .  22
       4.2.8  The "atom:image" Element . . . . . . . . . . . . . . .  23
       4.2.9  The "atom:link" Element  . . . . . . . . . . . . . . .  24
       4.2.10   The "atom:published" Element . . . . . . . . . . . .  26
       4.2.11   The "atom:source" Element  . . . . . . . . . . . . .  26
       4.2.12   The "atom:subtitle" Element  . . . . . . . . . . . .  27
       4.2.13   The "atom:summary" Element . . . . . . . . . . . . .  27
       4.2.14   The "atom:title" Element . . . . . . . . . . . . . .  27
       4.2.15   The "atom:updated" Element . . . . . . . . . . . . .  27
   5.   Securing Atom Documents  . . . . . . . . . . . . . . . . . .  29
   6.   Extending Atom . . . . . . . . . . . . . . . . . . . . . . .  30
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  33
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  35
   9.   References . . . . . . . . . . . . . . . . . . . . . . . . .  36
     9.1  Normative References . . . . . . . . . . . . . . . . . . .  36
     9.2  Informative References . . . . . . . . . . . . . . . . . .  37
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  38
   A.   Contributors . . . . . . . . . . . . . . . . . . . . . . . .  39
   B.   RELAX NG Compact Schema  . . . . . . . . . . . . . . . . . .  40
   C.   Change Log . . . . . . . . . . . . . . . . . . . . . . . . .  48
        Intellectual Property and Copyright Statements . . . . . . .  51




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1.  Introduction

   Atom is an XML-based document format that describes 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.

1.1  Examples

   A minimal, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-08">

     <title>Example Feed</title>
     <link href="http://example.org/"/>
     <updated>2003-12-13T18:30:02Z</updated>
     <author>
       <name>John Doe</name>
     </author>

     <entry>
       <title>Atom-Powered Robots Run Amok</title>
       <link href="http://example.org/2003/12/13/atom03"/>
       <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
       <updated>2003-12-13T18:30:02Z</updated>
       <summary>Some text.</summary>
     </entry>

   </feed>

















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   A more extensive, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-08">
     <title type="text">dive into mark</title>
     <subtitle type="html">
       A &lt;em&gt;lot&lt;/em&gt; of effort
       went into making this effortless
     </subtitle>
     <updated>2005-04-02T12:29:29Z</updated>
     <id>tag:example.org,2003:3</id>
     <link rel="alternate" type="text/html"
      hreflang="en" href="http://example.org/"/>
     <copyright>Copyright (c) 2003, Mark Pilgrim</copyright>
     <generator uri="http://www.example.com/" version="1.0">
       Example Toolkit
     </generator>
     <entry>
       <title>Atom draft-07 snapshot</title>
       <link rel="alternate" type="text/html"
        href="http://example.org/2005/04/02/atom"/>
       <link rel="enclosure" type="audio/mpeg" length="1337"
        href="http://example.org/audio/ph34r_my_podcast.mp3"/>
       <id>tag:example.org,2003:3.2397</id>
       <updated>2005-04-02T12:29:29Z</updated>
       <published>2003-12-13T08:29:29-04:00</published>
       <author>
         <name>Mark Pilgrim</name>
         <uri>http://example.org/</uri>
         <email>f8dy@example.com</email>
       </author>
       <contributor>
         <name>Sam Ruby</name>
         <uri>http://intertwingly.net/blog/</uri>
       </contributor>
       <contributor>
         <name>Joe Gregorio</name>
         <uri>http://bitworking.org/</uri>
       </contributor>
       <content type="xhtml" xml:lang="en"
        xml:base="http://diveintomark.org/">
         <div xmlns="http://www.w3.org/1999/xhtml">
           <p><i>[Update: The Atom draft-07 snapshot is out.]</i></p>
         </div>
       </content>
     </entry>
   </feed>




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1.2  Notational Conventions

   This specification describes conformance in terms of two artifacts;
   Atom Feed Documents and Atom Entry documents.  Additionally, it
   places some requirements on Atom Processors.

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML element names.  It uses the following
   namespace prefix for the indicated namespace URI;

   "atom": http://purl.org/atom/ns#draft-ietf-atompub-format-08

   [[anchor4: This paragraph to be removed by the RFC Editor.  The
   namespace here is a temporary one and will be changed when the IESG
   approves this document as a standard.  At that time, the namespace
   will be drawn from W3C URI space.  The choice of that namespace will
   be coordinated between the IETF and W3C through their respective
   liaisons.]]

   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-20040204].  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
   referring to an Element Information Item in Infoset terms.  Likewise,
   when it uses the term "attribute," it is referring to an Attribute
   Information Item.

   Some sections of this specification are illustrated with fragments of
   a non-normative RELAX NG Compact schema [RELAX-NG].  However, the
   text of this specification provides the definition of conformance.  A
   complete schema appears in Appendix B.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.









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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 root is the atom:feed element.

   An Atom Entry Document represents exactly one Atom entry, outside of
   the context of an Atom feed.  Its root is the atom:entry element.

   namespace atom =
      "http://purl.org/atom/ns#draft-ietf-atompub-format-08"
   start = atomFeed | atomEntry

   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.  This specification does not
   define a DTD for Atom Documents, and hence does not require them to
   be valid (in the sense used by XML).

   Any element defined by this specification MAY have an xml:base
   attribute.  XML Base [W3C.REC-xmlbase-20010627] processing MUST be
   applied to any relative reference [RFC3987] 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 defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its children.  The language context is only significant
   for elements and attributes declared to be "language-sensitive" by
   this specification.  Requirements regarding the content and
   interpretation of xml:lang are specified in XML 1.0
   [W3C.REC-xml-20040204], Section 2.12.

   atomCommonAttributes =
      attribute xml:base { atomUri }?,
      attribute xml:lang { atomLanguageTag }?

   Atom allows the use of IRIs [RFC3987], as well as URIs [RFC3986].
   IRIs can easily be converted to URIs.  Every URI is an IRI, so any
   URI can be used where an IRI is needed.  When comparing IRIs serving
   as atom:id values, they MUST NOT be converted to URIs.

   Atom is an extensible format.  See Section 6 of this document for a
   full description of how Atom Documents can be extended.



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   Atom Processors MAY keep state (e.g., metadata in atom:feed, entries)
   sourced from Atom Feed Documents and combine them with other Atom
   Feed Documents, in order to facilitate a contiguous view of the
   contents of a feed.  The manner in which Atom Feed Documents are
   combined in order to reconstruct a feed (e.g., updating entries and
   metadata, dealing with missing entries) is out of the scope of this
   specification.












































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3.  Common Atom Constructs

   Many of Atom's elements share a few common structures.  This section
   defines those 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 small
   quantities.  The content of Text constructs is language-sensitive.

   atomPlainTextConstruct =
      atomCommonAttributes,
      attribute type { "text" | "html" }?,
      text

   atomXHTMLTextConstruct =
      atomCommonAttributes,
      attribute type { "xhtml" },
      xhtmlDiv

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct

3.1.1  The "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, Atom Processors MUST behave as though it were present
   with a value of "text".  MIME media types [MIMEREG] MUST NOT be used
   as values for the "type" attribute.

















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3.1.1.1  Text

   Example atom:title with text content:

   ...
   <title type="text">
     Less: &lt;
   </title>
   ...

   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, Atom Processors MAY collapse
   white-space (including line-breaks), and display the text using
   typographic techniques such as justification and proportional fonts.

3.1.1.2  HTML

   Example atom:title with HTML content:

   ...
   <title type="html">
     Less: &lt;em> &amp;lt; &lt;/em>
   </title>
   ...

   If the value of "type" is "html", the content of the Text construct
   MUST NOT contain child elements, and SHOULD be suitable for handling
   as HTML [HTML].  Any markup within MUST be escaped; for example,
   "<br>" as "&lt;br>".  HTML markup within SHOULD be such that it could
   validly appear directly within an HTML <DIV> element, after
   unescaping.  Atom Processors that display such content MAY use that
   markup to aid in its display.


















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3.1.1.3  XHTML

   Example atom:title with XHTML content:

   ...
   <title type="xhtml" xmlns:xhtml="http://www.w3.org/1999/xhtml">
     <xhtml:div>
       Less: <xhtml:em> &lt; </xhtml:em>
     </xhtml:div>
   </title>
   ...

   If the value of "type" is "xhtml", the content of the Text construct
   MUST be a single XHTML div element [XHTML].  The XHTML div MUST
   contain XHTML text and markup that could validly appear within an
   XHTML div element.  The XHTML div element itself MUST NOT be
   considered part of the content.  Atom Processors which display the
   content MAY use the markup to aid in displaying it.  Escaped
   characters, such as "&" and ">", represent those characters, not
   markup.

   Examples of valid XHTML content:

   ...
   <summary type="xhtml">
      <div xmlns="http://www.w3.org/1999/xhtml">
         This is <b>XHTML</b> content.
      </div>
   </summary>
   ...
   <summary type="xhtml">
      <xhtml:div xmlns:xhtml="http://www.w3.org/1999/xhtml">
         This is <xhtml:b>XHTML</xhtml:b> content.
      </xhtml:div>
   </summary>
   ...

   The following example assumes that the XHTML namespace has been bound
   to the "xh" prefix earlier in the document:

   ...
   <summary type="xhtml">
      <xh:div>
         This is <xh:b>XHTML</xh:b> content.
      </xh:div>
   </summary>
   ...




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3.2  Person Constructs

   A Person construct is an element that describes a person,
   corporation, or similar entity (hereafter, 'person').

   atomPersonConstruct =
      atomCommonAttributes,
      (element atom:name { text }
       & element atom:uri { atomUri }?
       & element atom:email { atomEmailAddress }?
       & extensionElement*)

   This specification assigns no significance to the order of appearance
   of the child elements in a Person construct.  Person constructs allow
   extension Metadata Elements (see Section 6.4).

3.2.1  The "atom:name" Element

   The "atom:name" element's content conveys a human-readable name for
   the person.  The content of atom:name is language-sensitive.  Person
   constructs MUST contain exactly one "atom:name" element.

3.2.2  The "atom:uri" Element

   The "atom:uri" element's content conveys an IRI 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 an IRI reference [RFC3987].

3.2.3  The "atom:email" Element

   The "atom:email" element's content conveys an e-mail address
   associated with the person.  Person constructs MAY contain an
   atom:email element, but MUST NOT contain more than one.  Its content
   MUST conform to the "addr-spec" production in [RFC2822].
















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3.3  Date Constructs

   A Date construct is an element whose content MUST conform to the
   "date-time" production in [RFC3339].  In addition, an uppercase "T"
   character MUST be used to separate date and time, and an uppercase
   "Z" character MUST be present in the absence of a numeric time zone
   offset.

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime

   Such date values happen to be compatible with the following
   specifications: [ISO.8601.1988], [W3C.NOTE-datetime-19980827], and
   [W3C.REC-xmlschema-2-20041028].

   Date values SHOULD be as accurate as possible.  For example, it would
   be generally inappropriate for a publishing system to apply the same
   timestamp to several entries which were published during the course
   of a single day.































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4.  Atom Element Definitions

4.1  Container Elements

4.1.1  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 element children consist of metadata
   elements followed by zero or more atom:entry child elements.

   atomFeed =
      element atom:feed {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContributor*
          & atomCopyright?
          & atomGenerator?
          & atomIcon?
          & atomId?
          & atomImage?
          & atomLink+
          & atomSubtitle?
          & atomTitle
          & atomUpdated
          & extensionElement*),
         atomEntry*
      }

   This specification assigns no significance to the order of atom:entry
   elements within the feed.



















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   The following child elements are defined by this specification (note
   that the presence of some of these elements is required):

   o  atom:feed 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.
   o  atom:feed elements MUST NOT contain more than one atom:author
      element.
   o  atom:feed elements MAY contain any number of atom:category
      elements.
   o  atom:feed elements MAY contain any number of atom:contributor
      elements.
   o  atom:feed elements MUST NOT contain more than one atom:copyright
      element.
   o  atom:feed elements MUST NOT contain more than one atom:generator
      element.
   o  atom:feed elements MUST NOT contain more than one atom:icon
      element.
   o  atom:feed elements MUST NOT contain more than one atom:image
      element.
   o  atom:feed elements MUST NOT contain more than one atom:id element.
   o  atom:feed elements MUST contain at least one atom:link element
      with a relation of "alternate".
   o  atom:feed elements SHOULD contain one atom:link element with a rel
      attribute value of "self".  This URI identifies the feed and a
      representation equivalent to the feed.
   o  atom:feed 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:feed elements MAY contain
      additional atom:link elements beyond those described above.
   o  atom:feed elements MUST NOT contain more than one atom:subtitle
      element.
   o  atom:feed elements MUST contain exactly one atom:title element.
   o  atom:feed elements MUST contain exactly one atom:updated element.
   o  atom:feed elements MUST NOT contain atom:entry elements with
      identical atom:id values.















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4.1.2  The "atom:entry" Element

   The "atom:entry" element represents an individual entry, acting as a
   container for metadata and data associated with the 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.

   atomEntry =
      element atom:entry {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContent?
          & atomContributor*
          & atomCopyright?
          & atomId
          & atomLink*
          & atomPublished?
          & atomSource?
          & atomSummary?
          & atomTitle
          & atomUpdated
          & extensionElement*)
      }

   This specification assigns no significance to the order of appearance
   of the child elements of atom:entry.























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   The following child elements are defined by this specification (note
   that it requires the presence of some of these elements):

   o  atom:entry elements MUST contain exactly one atom:author element,
      unless the atom:entry contains an atom:source element which
      contains an atom:author element, or, in an Atom Feed Document, the
      atom:feed element contains an atom:author element itself.
   o  atom:entry elements MUST NOT contain more than one atom:author
      element.
   o  atom:entry elements MAY contain any number of atom:category
      elements.
   o  atom:entry elements MUST NOT contain more than one atom:content
      element.
   o  atom:entry elements MAY contain any number of atom:contributor
      elements.
   o  atom:entry elements MUST NOT contain more than one atom:copyright
      element.
   o  atom:entry elements MUST contain exactly one atom:id element.
   o  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".
   o  atom:entry elements MUST NOT contain more than one atom:link
      element with a rel attribute value of "alternate" that has the
      same combination of type and hreflang attribute values.
   o  atom:entry elements MAY contain additional atom:link elements
      beyond those described above.
   o  atom:entry elements MUST NOT contain more than one atom:published
      element.
   o  atom:entry elements MUST NOT contain more than one atom:source
      element.
   o  atom:entry elements MUST contain an atom:summary element in any of
      the following cases:
      *  the atom:entry element contains no atom:content element.
      *  the atom:entry contains an atom:content that has a "src"
         attribute (and is thus empty).
      *  the atom:entry contains content that is encoded in Base64; i.e.
         the "type" attribute of atom:content is a MIME media type
         [MIMEREG] and does not begin with "text/" nor end with "+xml".
   o  atom:entry elements MUST NOT contain more than one atom:summary
      element.
   o  atom:entry elements MUST have exactly one "atom:title" element.
   o  atom:entry elements MUST contain exactly one atom:updated element.









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4.1.3  The "atom:content" Element

   The "atom:content" element either contains or links to the content of
   the entry.  The content of atom:content is language-sensitive.

   atomInlineTextContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { "text" | "html" }?,
         (text)*
      }

   atomInlineXHTMLContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { "xhtml" },
         xhtmlDiv
      }

   atomInlineOtherContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { atomMediaType }?,
         (text|anyForeignElement)*
      }

   atomOutOfLineContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { atomMediaType }?,
         attribute src { atomUri },
         empty
      }

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomInlineOtherContent
    | atomOutOfLineContent

4.1.3.1  The "type" attribute

   On the atom:content element, the value of the "type" attribute MAY be
   one of "text", "html", or "xhtml".  Failing that, it MUST be a MIME
   media type, but MUST NOT be a composite type (see Section 4.2.6 of
   [MIMEREG]).  If the type attribute is not provided, Atom Processors
   MUST behave as though it were present with a value of "text".





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4.1.3.2  The "src" attribute

   atom:content MAY have a "src" attribute, whose value MUST be an IRI
   reference [RFC3987].  If the "src" attribute is present, Atom
   Processors MAY use the IRI to retrieve the content.  If the "src"
   attribute is present, atom:content MUST be empty.

   If the "src" attribute is present, the "type" attribute SHOULD be
   provided and MUST be a MIME media type [MIMEREG], rather than "text",
   "html", or "xhtml".  The value is advisory; that is to say, upon
   dereferencing the IRI 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.

4.1.3.3  Processing Model

   Atom Documents MUST conform to the following rules.  Atom Processors
   MUST interpret atom:content according to the first applicable rule.

   1.  If the value of "type" 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, Atom Processors
       MAY collapse white-space (including line-breaks), and display the
       text using typographic techniques such as justification and
       proportional fonts.

   2.  If the value of "type" is "html", the content of atom:content
       MUST NOT contain child elements, and SHOULD be suitable for
       handling as HTML [HTML].  The HTML markup must be escaped; for
       example, "<br>" as "&lt;br>".  The HTML markup SHOULD be such
       that it could validly appear directly within an HTML <DIV>
       element.  Atom Processors that display the content MAY use the
       markup to aid in displaying it.

   3.  If the value of "type" is "xhtml", the content of atom:content
       MUST be a single XHTML div element [XHTML], and SHOULD be
       suitable for handling as XHTML.  The XHTML div element itself
       MUST NOT be considered part of the content.  Atom Processors that
       display the content MAY use the markup to aid in displaying it.
       Escaped characters, such as "&" and ">", represent those
       characters, not markup.

   4.  If the value of "type" ends with "+xml" or "/xml"
       (case-insensitive), the content of atom:content MAY include child



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       elements, and SHOULD be suitable for handling as the indicated
       media type.  If the "src" attribute is not provided, this would
       normally mean 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/" (case-insensitive),
       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 as the indicated media type.  In this
       case, the characters in the Base64 encoding MAY be preceded and
       followed in the atom:content element by white-space, and lines
       are separated by a single newline (U+000A) character.


4.1.3.4  Examples

   XHTML inline:

   ...
   <content type="xhtml">
      <div xmlns="http://www.w3.org/1999/xhtml">
         This is <b>XHTML</b> content.
      </div>
   </content>
   ...
   <content type="xhtml">
      <xhtml:div xmlns:xhtml="http://www.w3.org/1999/xhtml">
         This is <xhtml:b>XHTML</xhtml:b> content.
      </xhtml:div>
   </content>
   ...

   The following example assumes that the XHTML namespace has been bound
   to the "xh" prefix earlier in the document:

   ...
   <content type="xhtml">
      <xh:div>
         This is <xh:b>XHTML</xh:b> content.
      </xh:div>
   </content>
   ...






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4.2  Metadata Elements

4.2.1  The "atom:author" Element

   The "atom:author" element is a Person construct that indicates the
   author of the entry or feed.

   atomAuthor = element atom:author { atomPersonConstruct }

4.2.2  The "atom:category" Element

   The "atom:category" element conveys information about a category
   associated with an entry or feed.

   atomCategory =
      element atom:category {
         atomCommonAttributes,
         attribute term { text },
         attribute scheme { atomUri }?,
         attribute label { text }?,
         empty
      }

4.2.2.1  The "term" Attribute

   The "term" attribute is a string that identifies the category to
   which the entry or feed belongs.  Category elements MUST have a
   "term" attribute.

4.2.2.2  The "scheme" Attribute

   The "scheme" attribute is an IRI that identifies a categorization
   scheme.  Category elements MAY have a "scheme" attribute.

4.2.2.3  The "label" attribute

   The "label" attribute provides a human-readable label for display in
   end-user applications.  The content of the "label" attribute is
   language-sensitive.  Category elements MAY have a "label" attribute.

4.2.3  The "atom:contributor" Element

   The "atom:contributor" element is a Person construct that indicates a
   person or other entity who contributed to the entry or feed.

   atomContributor = element atom:contributor { atomPersonConstruct }





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4.2.4  The "atom:copyright" Element

   The "atom:copyright" element is a Text construct that conveys a
   human-readable copyright statement for an entry or feed.

   atomCopyright = element atom:copyright { atomTextConstruct }

   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, is considered to apply to the entry.

4.2.5  The "atom:generator" Element

   The "atom:generator" element's content identifies the agent used to
   generate a feed, for debugging and other purposes.

   atomGenerator = element atom:generator {
      atomCommonAttributes,
      attribute uri { atomUri }?,
      attribute version { text }?,
      text
   }

   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 an IRI reference [RFC3987].  When dereferenced, that IRI
   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.

4.2.6  The "atom:icon" Element

   The "atom:icon" element's content is an IRI reference [RFC3987] which
   identifies an image which provides iconic visual identification for a
   feed.

   atomIcon = element atom:icon {
      atomCommonAttributes,
      (atomUri)
   }




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   The image SHOULD have an aspect ratio of one (horizontal) to one
   (vertical), and should be suitable for presentation at a small size.

4.2.7  The "atom:id" Element

   The "atom:id" element conveys a permanent, universally unique
   identifier for an entry or feed.

   atomId = element atom:id {
      atomCommonAttributes,
      (atomUri)
   }

   Its content MUST be an IRI, as defined by [RFC3987].  Note that the
   definition of "IRI" excludes relative references.  Though the IRI
   might use a dereferencable scheme, Atom Processors MUST NOT assume it
   can be dereferenced.

   When an Atom document is relocated, migrated, syndicated,
   republished, exported or imported, the content of its atom:id element
   MUST NOT change.  Put another way, an atom:id element pertains to all
   instantiations of a particular Atom entry or feed; revisions retain
   the same content in their atom:id elements.  It is suggested that the
   atom:id element be stored along with the associated resource.

   The content of an atom:id element MUST be created in a way that
   assures uniqueness.

   Because of the risk of confusion between IRIs that would be
   equivalent if dereferenced, the following normalization strategy
   SHOULD be applied when generating atom:id elements:

   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 components of the IRI are appropriately
      character-normalized, e.g.  by using NFC or NFKC.





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4.2.7.1  Comparing atom:id

   Instances of atom:id elements can be compared to determine whether an
   entry or feed is the same as one seen before.  Processors MUST
   compare atom:id elements on a character-by-character basis (in a
   case-sensitive fashion).  Comparison operations MUST be based solely
   on the IRI character strings, and MUST NOT rely on dereferencing the
   IRIs.

   As a result, two IRIs 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
      HTTP://www.example.org/thing

   are four distinct identifiers, despite their differences in case.

   Likewise,

      http://www.example.com/~bob
      http://www.example.com/%7ebob
      http://www.example.com/%7Ebob

   are three distinct identifiers, because IRI %-escaping is significant
   for the purposes of comparison.

4.2.8  The "atom:image" Element

   The "atom:image" element's content is an IRI reference [RFC3987]
   which identifies an image which provides visual identification for a
   feed.

   atomImage = element atom:image {
      atomCommonAttributes,
      (atomUri)
   }

   The image SHOULD have an aspect ratio of 2 (horizontal) to 1
   (vertical).







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4.2.9  The "atom:link" Element

   The "atom:link" element is an empty element that defines a reference
   from an entry or feed to a Web resource.

   atomLink =
      element atom:link {
         atomCommonAttributes,
         attribute href { atomUri },
         attribute rel { atomNCName | atomUri }?,
         attribute type { atomMediaType }?,
         attribute hreflang { atomLanguageTag }?,
         attribute title { text }?,
         attribute length { text }?,
         empty
      }

4.2.9.1  The "href" Attribute

   The "href" attribute contains the link's IRI.  atom:link elements
   MUST have a href attribute, whose value MUST be a IRI reference
   [RFC3987].

4.2.9.2  The "rel" Attribute

   atom:link elements MAY have a "rel" attribute that indicates the link
   relation type.  If the "rel" attribute is not present, the link
   element MUST be interpreted as if the link relation type is
   "alternate".

   The value of "rel" MUST be string that is non-empty, and matches the
   "isegment-nz-nc" or "IRI" production in [RFC3987].  Note that use of
   a relative reference 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 7, and thus the IRI 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 five initial values for the Registry of Link
   Relations:

   1.  The value "alternate" signifies that the IRI in the value of the
       href attribute identifies an alternate version of the resource
       described by the containing element.




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   2.  The value "related" signifies that the IRI in the value of the
       href attribute identifies a resource related to the resource
       described by the containing element.  For example, the feed for a
       site that discusses the performance of the search engine at
       "http://search.example.com" might contain, as a child of
       atom:feed:

       <link rel="related" href="http://search.example.com/"/>

       An identical link might appear as a child of any atom:entry whose
       content contains a discussion of that same search engine.

   3.  The value "self" signifies that the IRI in the value of the href
       attribute identifies a resource equivalent to the containing
       element.

   4.  The value "enclosure" signifies that the IRI in the value of the
       href attribute identifies a related resource which is potentially
       large in size and might require special handling by consuming
       software.  For Link elements with rel="enclosure", the length
       attribute SHOULD be provided.

   5.  The value "via" signifies that the IRI in the value of the href
       attribute identifies a resource that is the source of the
       information provided in the containing element.


4.2.9.3  The "type" Attribute

   On the link element, the "type" attribute's value is an advisory
   media type; it is a hint about the type of the representation that 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.  Link elements
   MAY have a type attribute, whose value MUST conform to the syntax of
   a MIME media type [MIMEREG].

4.2.9.4  The "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 elements MAY have an hreflang attribute, whose value MUST be a
   language tag [RFC3066].

4.2.9.5  The "title" Attribute

   The "title" attribute conveys human-readable information about the



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   link.  The content of the "title" attribute is language sensitive.
   Link elements MAY have a title attribute.

4.2.9.6  The "length" Attribute

   The "length" attribute indicates an advisory length of the linked
   content in octets; it is a hint about the content length of the
   representation returned when the IRI 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 elements MAY have a length attribute.

4.2.10  The "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.

   atomPublished = element atom:published { atomDateConstruct }

   Typically, atom:published will be associated with the initial
   creation or first availability of the resource.

4.2.11  The "atom:source" Element

   If an atom:entry is copied from one feed into another feed, then the
   source atom:feed's metadata (all child elements of atom:feed other
   than the atom:entry elements) MAY be preserved within the copied
   entry by adding an atom:source child element, if it is not already
   present in the entry, and including some or all of the source feed's
   metadata elements as the atom:source element's children.  Such
   metadata SHOULD be preserved if the source atom:feed contains any of
   the child elements atom:author, atom:contributor, atom:copyright, or
   atom:category and those child elements are not present in the source
   atom:entry.
















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   atomSource =
      element atom:source {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContributor*
          & atomCopyright?
          & atomGenerator?
          & atomIcon?
          & atomId?
          & atomImage?
          & atomLink+
          & atomSubtitle?
          & atomTitle
          & atomUpdated
          & extensionElement*)
      }

4.2.12  The "atom:subtitle" Element

   The "atom:subtitle" element is a Text construct that conveys a
   human-readable description or subtitle for a feed.

   atomSubtitle = element atom:subtitle { atomTextConstruct }

4.2.13  The "atom:summary" Element

   The "atom:summary" element is a Text construct that conveys a short
   summary, abstract or excerpt of an entry.

   atomSummary = element atom:summary { atomTextConstruct }

4.2.14  The "atom:title" Element

   The "atom:title" element is a Text construct that conveys a
   human-readable title for an entry or feed.

   atomTitle = element atom:title { atomTextConstruct }

4.2.15  The "atom:updated" Element

   The "atom:updated" element is a Date construct indicating the most
   recent instant in time when an entry or feed was modified in a way
   the publisher considers significant.  Therefore, not all
   modifications necessarily result in a changed atom:updated value.

   atomUpdated = element atom:updated { atomDateConstruct }




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   Publishers MAY change the value of this element over time.


















































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5.  Securing Atom Documents

   Because Atom is an XML-based format, existing XML security mechanisms
   can be used to secure its content.

5.1  Digital Signatures

   The root 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].

   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 an Atom 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.

5.2  Encryption

   The root 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].




















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6.  Extending Atom

6.1  Extensions From Non-Atom Vocabularies

   This specification describes Atom's XML markup vocabulary.  Markup
   from other vocabularies ("foreign markup") can be used in an Atom
   document.  Note that the atom:content element is designed to support
   the inclusion of arbitrary foreign markup.

6.2  Extensions To the Atom Vocabulary

   Future versions of this specification could add new elements and
   attributes to the Atom markup vocabulary.  Software written to
   conform to this version of the specification will not be able to
   process such markup correctly and, in fact, will not be able to
   distinguish it from markup error.  For the purposes of this
   discussion, unrecognized markup from the Atom vocabulary will be
   considered "foreign markup".

6.3  Software Processing of Foreign Markup

   Software processing an Atom Document which encounters foreign markup
   in a location that is legal according to this specification MUST NOT
   stop processing or signal an error.  It might be the case that the
   software is able to process the foreign markup correctly and does so.
   Otherwise, such markup is termed "unknown foreign markup".

   When unknown foreign markup is encountered as a child of atom:entry,
   atom:feed, or a Person construct, software MAY bypass the markup and
   any textual content and MUST NOT change its behavior as a result of
   the markup's presence.

   When unknown foreign markup is encountered in a Text Contruct or
   atom:content element, software SHOULD ignore the markup and process
   any text content of foreign elements as though the surrounding markup
   were not present.















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6.4  Extension Elements

   Atom allows foreign markup anywhere in an Atom document.  Child
   elements of atom:entry, atom:feed, and Person constructs are
   considered Metadata Elements, and are described below.  Child
   elements of Person constructs are considered to apply to the
   construct.  The role of other foreign markup is undefined by this
   specification.

6.4.1  Simple Extension Elements

   A Simple Extension element MUST NOT have any attributes or child
   elements.  The element MAY contain character data, or be empty.
   Simple Extension elements are not language-sensitive.

   simpleExtensionElement =
      element * - atom:* {
         text
      }

   The element can be interpreted as a simple property (or name/value
   pair) of the parent element that encloses it.  The pair consisting of
   the namespace-URI of the element and the local name of the element
   can be interpreted as the name of the property.  The character data
   content of the element can be interpreted as the value of the
   property.  If the element is empty, then the property value can be
   interpreted as an empty string.

6.4.2  Structured Extension Elements

   The root element of a Structured Extension element MUST have at least
   one attribute or child element.  It MAY have attributes, it MAY
   contain well-formed XML content (including character data), or it MAY
   be empty.  Structured Extension elements are language-sensitive.

   structuredExtensionElement =
      element * - atom:* {
         (attribute * { text }+,
            (text|anyElement)*)
       | (attribute * { text }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   The structure of a Structured Extension element, including the order
   of its child elements, could be significant.

   This specification does not provide an interpretation of a Structured
   Extension element.  The syntax of the XML contained in the element,



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   and an interpretation of how the element relates to its containing
   element is defined by the specification of the Atom extension.

















































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7.  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
         [RFC3023].
   Encoding considerations: Identical to those of "application/xml" as
      described in [RFC3023], section 3.2.
   Security considerations: As defined in this specification.
      [[anchor59: update upon publication]]
      In addition, as this media type uses the "+xml" convention, it
      shares the same security considerations as described in [RFC3023],
      section 10.
   Interoperability considerations: There are no known interoperability
      issues.
   Published specification: This specification.  [[anchor60: update upon
      publication]]
   Applications that use this media type: No known applications
      currently use this media type.

   Additional information:

   Magic number(s): As specified for "application/xml" in [RFC3023],
      section 3.2.
   File extension: .atom
   Fragment identifiers: As specified for "application/xml" in
      [RFC3023], section 5.
   Base URI: As specified in [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: IESG

7.1  Registry of Link Relations

   This registry is maintained by IANA and initially contains five
   values: "alternate", "related", "self", "enclosure", and "via".  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 use the
   following template:



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   o  Attribute Value: ( A value for the "rel" attribute that conforms
      to the syntax rule given in Section 4.2.9.2 )
   o  Description:
   o  Expected display characteristics:
   o  Security considerations:














































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8.  Security Considerations

8.1  HTML and XHTML Content

   Text constructs and atom:content allow the delivery of HTML and XHTML
   to receiving software.  Many elements in these languages are
   considered 'unsafe' in that they open clients to one or more types of
   attack.  Implementers of software which processes Atom should
   carefully consider their handling of every type of element when
   processing incoming (X)HTML in Atom documents.  See the security
   sections of [RFC2854] and [HTML] for guidance.

   Atom Processors should pay particular attention to the security of
   the IMG, SCRIPT, EMBED, OBJECT, FRAME, FRAMESET, IFRAME, META, and
   LINK elements, but other elements might also have negative security
   properties.

   (X)HTML can either directly contain or indirectly reference
   executable content.

8.2  URIs

   Atom Processors handle URIs.  See Section 7 of [RFC3986].

8.3  IRIs

   Atom Processors handle IRIs.  See Section 8 of [RFC3987].

8.4  Encryption and Signing

   Atom documents can be encrypted and signed using
   [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212],
   respectively, and are subject to the security considerations implied
   by their use.

















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9.  References

9.1  Normative References

   [HTML]     Raggett, D., Hors, A. and I. Jacobs, "HTML 4.01
              Specification", W3C REC REC-html401-19991224, December
              1999, <http://www.w3.org/TR/1999/REC-html401-19991224>.

   [MIMEREG]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures",  work-in-progress
              (draft-freed-media-type-reg-04), April 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
              2001.

   [RFC2854]  Connolly, D. and L. Masinter, "The 'text/html' Media
              Type", RFC 2854, June 2000.

   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [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,
              <http://www.w3.org/TR/2004/REC-xml-2004020>.

   [W3C.REC-xml-infoset-20040204]
              Cowan, J. and R. Tobin, "XML Information Set (Second



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              Edition)", W3C REC REC-xml-infoset-20040204, February
              2004,
              <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.

   [W3C.REC-xml-names-19990114]
              Hollander, D., Bray, T. and A. Layman, "Namespaces in
              XML", W3C REC REC-xml-names-19990114, January 1999,
              <http://www.w3.org/TR/1999/REC-xml-names-19990114>.

   [W3C.REC-xmlbase-20010627]
              Marsh, J., "XML Base", W3C REC REC-xmlbase-20010627, June
              2001, <http://www.w3.org/TR/2001/REC-xmlbase-20010627>.

   [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,
              <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212>.

   [W3C.REC-xmlenc-core-20021210]
              Reagle, J. and D. Eastlake, "XML Encryption Syntax and
              Processing", W3C REC REC-xmlenc-core-20021210, December
              2002,
              <http://www.w3.org/TR/2002/REC-xmlenc-core-20021210>.

   [XHTML]    Altheim, M., Boumphrey, F., McCarron, S., Dooley, S.,
              Schnitzenbaumer, S. and T. Wugofski, "Modularization of
              XHTML[TM]", W3C REC REC-xhtml-modularization-20010410,
              April 2001,
              <http://www.w3.org/TR/2001/REC-xhtml-modularization-200104
              10>.

9.2  Informative References

   [ISO.8601.1988]
              International Organization for Standardization, "Data
              elements and interchange formats - Information interchange
              - Representation of dates and times", ISO Standard 8601,
              June 1988.

   [RELAX-NG]
              Clark, J., "RELAX NG Compact Syntax", December 2001,
              <http://www.oasis-open.org/committees/relax-ng/compact-200
              21121.html>.

   [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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   [W3C.NOTE-datetime-19980827]
              Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C
              NOTE NOTE-datetime-19980827, August 1998,
              <http://www.w3.org/TR/1998/NOTE-datetime-19980827>.

   [W3C.REC-xmlschema-2-20041028]
              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", W3C REC REC-xmlschema-2-20041028, October
              2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.


Authors' Addresses

   Mark Nottingham (editor)

   Email: mnot@pobox.com
   URI:   http://www.mnot.net/


   Robert Sayre (editor)

   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.  Norman Walsh
   provided the Relax NG schema.  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.  RELAX NG Compact Schema

   This appendix is informative.

   # -*- rnc -*-
   # RELAX NG Compact Syntax Grammar for the
   # Atom Format Specification Version 08

   namespace atom =
      "http://purl.org/atom/ns#draft-ietf-atompub-format-08"
   namespace xhtml = "http://www.w3.org/1999/xhtml"
   namespace s = "http://www.ascc.net/xml/schematron"

   start = atomFeed | atomEntry

   # Common attributes

   atomCommonAttributes =
      attribute xml:base { atomUri }?,
      attribute xml:lang { atomLanguageTag }?

   # Text Constructs

   atomPlainTextConstruct =
      atomCommonAttributes,
      attribute type { "text" | "html" }?,
      text

   atomXHTMLTextConstruct =
      atomCommonAttributes,
      attribute type { "xhtml" },
      xhtmlDiv

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct

   # Person Construct

   atomPersonConstruct =
      atomCommonAttributes,
      (element atom:name { text }
       & element atom:uri { atomUri }?
       & element atom:email { atomEmailAddress }?
       & extensionElement*)

   # Date Construct

   atomDateConstruct =
      atomCommonAttributes,



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      xsd:dateTime

   # atom:feed

   atomFeed =
      [
         s:rule [
            context = "atom:feed"
            s:assert [
               test = "atom:link[@rel='alternate'] "
               ~ "or atom:link[not(@rel)]"
               "An atom:feed must have at least one link element "
               ~ "with a rel attribute of 'alternate'."
            ]
         ]
         s:rule [
            context = "atom:feed"
            s:assert [
               test = "atom:author or not(atom:entry[not(atom:author)])"
               "An atom:feed must have an atom:author unless all "
               ~ "of its atom:entry children have an atom:author."
            ]
         ]
      ]
      element atom:feed {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContributor*
          & atomCopyright?
          & atomGenerator?
          & atomIcon?
          & atomId?
          & atomImage?
          & atomLink+
          & atomSubtitle?
          & atomTitle
          & atomUpdated
          & extensionElement*),
         atomEntry*
      }

   # atom:entry

   atomEntry =
      [
         s:rule [
            context = "atom:entry"



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            s:assert [
               test = "atom:link[@rel='alternate'] "
               ~ "or atom:link[not(@rel)] "
               ~ "or atom:content"
               "An atom:entry must have at least one link element "
               ~ "with a rel attribute of 'alternate' or content."
            ]
         ]
         s:rule [
            context = "atom:entry"
            s:assert [
               test = "atom:author or "
               ~ "../atom:author or atom:source/atom:author"
               "An atom:entry must have an atom:author "
               ~ "if its feed does not."
            ]
         ]

         # N.B. This rule doesn't test
         # for content with a non-binary type.
         s:rule [
            context = "atom:entry"
            s:assert [
               test = "atom:summary or atom:content[not(@src)]"
               "An atom:entry must have an atom:summary "
               ~ "if the atom:content element is empty."
            ]
         ]
      ]
      element atom:entry {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContent?
          & atomContributor*
          & atomCopyright?
          & atomId
          & atomLink*
          & atomPublished?
          & atomSource?
          & atomSummary?
          & atomTitle
          & atomUpdated
          & extensionElement*)
      }

   # atom:content




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   atomInlineTextContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { "text" | "html" }?,
         (text)*
      }

   atomInlineXHTMLContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { "xhtml" },
         xhtmlDiv
      }

   atomInlineOtherContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { atomMediaType }?,
         (text|anyForeignElement)*
      }

   atomOutOfLineContent =
      element atom:content {
         atomCommonAttributes,
         attribute type { atomMediaType }?,
         attribute src { atomUri },
         empty
      }

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomInlineOtherContent
    | atomOutOfLineContent

   # atom:author

   atomAuthor = element atom:author { atomPersonConstruct }

   # atom:category

   atomCategory =
      element atom:category {
         atomCommonAttributes,
         attribute term { text },
         attribute scheme { atomUri }?,
         attribute label { text }?,
         empty
      }



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   # atom:contributor

   atomContributor = element atom:contributor { atomPersonConstruct }

   # atom:copyright

   atomCopyright = element atom:copyright { atomTextConstruct }

   # atom:generator

   atomGenerator = element atom:generator {
      atomCommonAttributes,
      attribute uri { atomUri }?,
      attribute version { text }?,
      text
   }

   # atom:icon

   atomIcon = element atom:icon {
      atomCommonAttributes,
      (atomUri)
   }

   # atom:id

   atomId = element atom:id {
      atomCommonAttributes,
      (atomUri)
   }

   # atom:image

   atomImage = element atom:image {
      atomCommonAttributes,
      (atomUri)
   }

   # atom:link

   atomLink =
      element atom:link {
         atomCommonAttributes,
         attribute href { atomUri },
         attribute rel { atomNCName | atomUri }?,
         attribute type { atomMediaType }?,
         attribute hreflang { atomLanguageTag }?,
         attribute title { text }?,



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         attribute length { text }?,
         empty
      }

   # atom:published

   atomPublished = element atom:published { atomDateConstruct }

   # atom:source

   atomSource =
      element atom:source {
         atomCommonAttributes,
         (atomAuthor?
          & atomCategory*
          & atomContributor*
          & atomCopyright?
          & atomGenerator?
          & atomIcon?
          & atomId?
          & atomImage?
          & atomLink+
          & atomSubtitle?
          & atomTitle
          & atomUpdated
          & extensionElement*)
      }

   # atom:subtitle

   atomSubtitle = element atom:subtitle { atomTextConstruct }

   # atom:summary

   atomSummary = element atom:summary { atomTextConstruct }

   # atom:title

   atomTitle = element atom:title { atomTextConstruct }

   # atom:updated

   atomUpdated = element atom:updated { atomDateConstruct }

   # Low-level simple types

   atomNCName = xsd:string { minLength = "1" pattern = "[^:]*" }




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   # Whatever a media type is, it contains at least one slash
   atomMediaType = xsd:string { pattern = ".+/.+" }

   # As defined in RFC 3066
   atomLanguageTag = xsd:string {
      pattern = "[A-Za-z]{1,8}(-[A-Za-z0-9]{1,8})*"
   }

   # Unconstrained; it's not entirely clear how IRI fit into
   # xsd:anyURI so let's not try to constrain it here
   atomUri = text

   # Whatever an email address is, it contains at least one @
   atomEmailAddress = xsd:string { pattern = ".+@.+" }

   # Simple Extension

   simpleExtensionElement =
      element * - atom:* {
         text
      }

   # Structured Extension

   structuredExtensionElement =
      element * - atom:* {
         (attribute * { text }+,
            (text|anyElement)*)
       | (attribute * { text }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   # Other Extensibility

   extensionElement =
      simpleExtensionElement | structuredExtensionElement

   anyElement =
      element * {
         (attribute * { text }
          | text
          | anyElement)*
      }

   anyForeignElement =
      element * - atom:* {
         (attribute * { text }
          | text



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          | anyElement)*
      }

   # XHTML

   anyXHTML = element xhtml:* {
      (attribute * { text }
         | text
         | anyXHTML)*
   }

   xhtmlDiv = element xhtml:div {
      (attribute * { text }
       | text
       | anyXHTML)*
   }

   # EOF

































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Appendix C.  Change Log

   [[anchor71: This section should be removed before final
   publication.]]

   -08:  Remove BNF
      complete rather than collected schema
      Remove a couple introductory sentences
      update MIME references
      Many editorial adjustments
   -07:  Change atom:source-feed to atom:source.
      Add ABNF reference
      Many editorial tweaks
      Rework extensibility
      Adjust page breaks in txt version
   -06:  Move Identity Construct into atom:id (only place it's used)
      atom:id values must be unique within a feed.
      restore atom:copyright definition mistakenly dropped during
      alphabetizing.
      Remove atom:head, add atom:source-feed, and "Extension Construct"
      text in an effort to accurately reflect WG consensus on data model
      and extensibility, acknowledging two opinions in favor of
      atom:head.
      Note @hreflang issue.
      Add comment on atom:entry/atom:summary requirements.
      Rework atom:id text.  The dereferencing section didn't talk about
      dereferencing.
      Remove protocol reference.
      Alphabetize where appropriate (PaceOrderSpecAlphabetically).
      Add mI language (PaceExtendingAtom).
      Add atom:icon and atom:image (PaceImageAndIcon).
      Change atom:tagline to atom:subtitle
      Add inline XHTML language (PaceXHTMLNamespaceDiv).
      Change "TEXT" etc, to lowercase
      Change example id IRI to urn:uuid:...
      Add rel="self" (PaceFeedLink).
      Add Feed State text (PaceNoFeedState).
      Move to IRIs (PaceIRI).
      Add rel="via" (PaceLinkRelVia).
      Add rel="enclosure" (PaceEnclosuresAndPix).
      Remove info and host (PaceRemoveInfoAndHost)
      Clarify order of entries (PaceEntryOrder).
      Remove version attribute (PaceRemoveVersionAttr).
      Date format roundup (PaceDatesXSD).
      Remove Service construct and elements.






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      fix atom:contributor cardinality typo
      Removed motivation/design principles note; if we haven't come up
      with them by now...
      Put conformance text into notational conventions.
      Removed instances of 'software'; too specific.
      Added refs to HTML and XHTML.
      Updated ref to Infoset.
      Various editorial tweaks.
      Fix RFC 3023 refs in IANA section
      Adjust head/link requirement
      fix @version typos
   -05:  Add RNC from N.  Walsh.
      Re-organize element definitions.
      Lift the prohibition on other types of DSig and encryption.
      Remove text on "indiscriminate use" of DSig and XMLEnc.
   -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).





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      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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   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
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