Network Working Group                                 M. Nottingham, Ed.
Internet-Draft
Expires: July 27, September 13, 2005                                R. Sayre, Ed.
                                               Boswijck Memex Consulting
                                                        January 26,
                                                          March 12, 2005

                      The Atom Syndication Format
                      draft-ietf-atompub-format-05
                      draft-ietf-atompub-format-06

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, I certify each
   author represents that any applicable patent or other IPR claims of
   which I am he or she is aware have been or will be disclosed, and any of
   which I he or she become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

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

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
     3.1  Text Constructs  . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1  "type" Attribute . . . . . . . . . . . . . . . . . . .   9
     3.2  Person Constructs  . . . . . . . . . . . . . . . . . . . .  10  11
       3.2.1  The "atom:name" Element  . . . . . . . . . . . . . . .  10  11
       3.2.2  The "atom:uri" Element . . . . . . . . . . . . . . . .  10  11
       3.2.3  The "atom:email" Element . . . . . . . . . . . . . . .  11
     3.3  Date Constructs  . . . . . . . . . . . . . . . . . . . . .  11
     3.4  Service Constructs .
   4.   Atom Element Definitions . . . . . . . . . . . . . . . . . .  13
     4.1  Container Elements .  11
       3.4.1  The "href" Attribute . . . . . . . . . . . . . . . . .  11
     3.5  Identity Constructs . .  13
       4.1.1  The "atom:feed" Element  . . . . . . . . . . . . . . .  13
       4.1.2  The "atom:entry" Element . .  11
       3.5.1  Dereferencing Identity Constructs . . . . . . . . . .  12
       3.5.2  Comparing Identity Constructs . . .  14
       4.1.3  The "atom:content" Element . . . . . . . . .  12
   4.   Element Definitions . . . . .  16
     4.2  Metadata Elements  . . . . . . . . . . . . . . .  14
     4.1  The "atom:feed" Element . . . . .  19
       4.2.1  The "atom:author" Element  . . . . . . . . . . . .  14
       4.1.1  The "version" Attribute . .  19
       4.2.2  The "atom:category" Element  . . . . . . . . . . . . .  14
     4.2  20
       4.2.3  The "atom:head" "atom:contributor" Element . . . . . . . . . . . .  20
       4.2.4  The "atom:copyright Element  . . . . .  14
       4.2.1  Usage of "atom:head" within "atom:entry" . . . . . . .  16
     4.3 .  20
       4.2.5  The "atom:entry" "atom:generator" Element . . . . . . . . . . . . . . . . .  16
     4.4  21
       4.2.6  The "atom:title" "atom:icon" Element  . . . . . . . . . . . . . . . . .  18
     4.5  21
       4.2.7  The "atom:id" Element  . . . . . . . . . . . . . . . . . .  18
     4.6  21
       4.2.8  The "atom:link" "atom:image" Element . . . . . . . . . . . . . . . . .  18
       4.6.1  23
       4.2.9  The "href" Attribute . . "atom:link" Element  . . . . . . . . . . . . . . .  19
       4.6.2  23
       4.2.10   The "rel" Attribute "atom:published" Element . . . . . . . . . . . .  26
       4.2.11   The "atom:source-feed" Element . . . . .  19
       4.6.3  The "type" Attribute . . . . . .  26
       4.2.12   The "atom:subtitle" Element  . . . . . . . . . . .  20
       4.6.4 .  26
       4.2.13   The "hreflang" Attribute "atom:summary" Element . . . . . . . . . . . . .  27
       4.2.14   The "atom:title" Element . .  20
       4.6.5  The "title" Attribute . . . . . . . . . . . .  27
       4.2.15   The "atom:updated" Element . . . .  20
       4.6.6  The "length" Attribute . . . . . . . . .  27
   5.   Securing Atom Documents  . . . . . . .  20
     4.7  The "atom:updated" Element . . . . . . . . . . .  28
     5.1  Digital Signatures . . . . .  20
     4.8  The "atom:published" Element . . . . . . . . . . . . . . .  21
     4.9  The "atom:author" Element  28
     5.2  Encryption . . . . . . . . . . . . . . . .  21
     4.10   The "atom:contributor" Element . . . . . . . .  28
   6.   Extending Atom . . . . .  21
     4.11   The "atom:host" Element . . . . . . . . . . . . . . . .  21
     4.12   The "atom:copyright" Element . .  29
     6.1  Extensions From Non-Atom Vocabularies  . . . . . . . . . .  29
     6.2  Extensions To the Atom Vocabulary  . .  21
     4.13   The "atom:category" Element . . . . . . . . . .  29
     6.3  Software Processing of Foreign Markup  . . . .  22
       4.13.1   The "term" Attribute . . . . . .  29
     6.4  Extension Elements . . . . . . . . . .  22
       4.13.2   The "scheme" Attribute . . . . . . . . . . . . . . .  22
       4.13.3   The "label" attribute  . . . . . . . . . . . . . . .  22
     4.14   The "atom:summary" Element . . . . . . . . . . . . . . .  22
     4.15   The "atom:content" Element . . . . . . . . . . . . . . .  23
       4.15.1   The "type" attribute . . . . . . . . . . . . . . . .  23
       4.15.2   The "src" attribute  . . . . . . . . . . . . . . . .  23
       4.15.3   Processing Model . . . . . . . . . . . . .  29
       6.4.1  Simple Extension Elements  . . . . .  24
     4.16   The "atom:introspection" Element . . . . . . . . .  30
       6.4.2  Structured Extension Elements  . . .  25
     4.17   The "atom:post" Element . . . . . . . . .  30
   7.   IANA Considerations  . . . . . . .  25
     4.18   The "atom:edit" Element . . . . . . . . . . . . .  31
     7.1  Registry of Link Relations . . .  25
     4.19   The "atom:tagline" Element . . . . . . . . . . . . .  31
   8.   Security Considerations  . .  25
     4.20   The "atom:generator" Element . . . . . . . . . . . . . .  25
     4.21   The "atom:info" Element . .  33
     8.1  HTML and XHTML Content . . . . . . . . . . . . . .  26
   5.   Managing Feed State . . . .  33
     8.2  URIs . . . . . . . . . . . . . . . .  27
   6.   Securing Atom Documents . . . . . . . . . . .  33
     8.3  IRIs . . . . . . .  28
     6.1  Digital Signatures . . . . . . . . . . . . . . . . . . . .  28
     6.2  33
     8.4  Encryption and Signing . . . . . . . . . . . . . . . . . . . . . . . .  28
   7.   Embedding Atom in Other Formats  . . . . . . . . . . . . . .  29
   8.   Extending Atom  33
   9.   References . . . . . . . . . . . . . . . . . . . . . . .  30
   9.   IANA Considerations . .  34
     9.1  Normative References . . . . . . . . . . . . . . . . . .  31
     9.1  Registry of Link Relations .  34
     9.2  Informative References . . . . . . . . . . . . . . .  31
   10.  Security Considerations . . .  35
        Authors' Addresses . . . . . . . . . . . . . . .  33
   11.  References . . . . . .  36
   A.   Contributors . . . . . . . . . . . . . . . . . . .  34
   11.1   Normative References . . . . .  37
   B.   Collected RELAX NG Compact Schema  . . . . . . . . . . . . .  38
   C.   Change Log .  34
   11.2   Informative References . . . . . . . . . . . . . . . . . .  35
        Authors' Addresses . . . . . .  45
        Intellectual Property and Copyright Statements . . . . . . . . . . . . . . .  36
   A.   Contributors . . . . . . . . . . . . . . . . . . . . . . . .  37
   B.   Collected RELAX NG Compact Schema  . . . . . . . . . . . . .  38
   C.   Revision History . . . . . . . . . . . . . . . . . . . . . .  45
        Intellectual Property and Copyright Statements . . . . . . .  47  48

1.  Introduction

   Atom is an XML-based document format which 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.  However, nothing precludes it from being
   used for other purposes and kinds of content.

   Details of communication protocols between software agents using Atom
   can be found in the Atom Protocol specification [Atom-protocol].

   [[ more motivation / design principles ]]

1.1  Editorial Notes

   The Atom format is a work-in-progress, and this draft is both
   incomplete and likely to change rapidly.  As a result, THE FORMAT
   DESCRIBED BY THIS DRAFT SHOULD NOT BE DEPLOYED, either in production
   systems or in any non-experimental fashion on the Internet.

   Discussion of this draft happens in two fora;

      The mailing list [1] <http://www.imc.org/atom-syntax/index.html>
      The Atom Wiki Web site [2]
      <http://www.intertwingly.net/wiki/pie/FrontPage>

   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-05:do not deploy"
   xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-05">
     <head>
       xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-06">

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

     <entry>
     <title>Atom-Powered Robots Run Amok</title>
     <link href="http://example.org/2003/12/13/atom03"/>
       <id>vemmi://example.org/2003/32397</id>
     <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
     <updated>2003-12-13T18:30:02Z</updated>
     </entry>
   </feed>

   [[anchor4: Ask yourself: is the example atom:entry valid?]]

1.3  Conformance

   [[ talk about atom documents and atom consumers,  Notational Conventions

   This specification describes conformance in terms of two kinds of
   artefacts; Atom Feed Documents and how Atom Entry documents.
   Additionally, it places some requirements
   are placed on them ]]

1.4  Notational Conventions Atom Processors.

   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]. [RFC2119], as
   scoped to those conformance targets.

   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-05 http://purl.org/atom/ns#draft-ietf-atompub-format-06

   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].
   [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
   collected schema appears in an informative appendix.

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 root is atom:feed. the atom:feed element.

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

   namespace
     atom =
     "http://purl.org/atom/ns#draft-ietf-atompub-format-05" ="http://purl.org/atom/ns#draft-ietf-atompub-format-06"

   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.

   Atom constrains the appearance and content of elements and
   attributes; unless otherwise stated, Atom Documents MAY contain other
   Information Items as appropriate.  In particular, Comment Information
   Items and Processing Instruction Information Items SHOULD be ignored
   in the normal processing of an Atom Document.

   Any element in an Atom Document MAY have an xml:base attribute.  XML
   Base [W3C.REC-xmlbase-20010627] processing MUST be applied to any
   relative reference [RFC3986] [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 in an Atom Document MAY have an xml:lang attribute, whose
   content indicates the natural language of for the element's content. 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] [W3C.REC-xml-20040204], Section 2.12.

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

   [[ discussion

   Atom allows the use of IRIs [RFC3987], rather than only URIs
   [RFC3986].  For resolution, IRIs can easily be converted to URIs.
   When comparing IRIs serving as Identity Constructs, they MUST NOT be
   converted to URIs.  Please note that by definition, every URI escaping and i18n, is an
   IRI, so any URI can be used where an IRI ]]

   [[ is needed.

   [[anchor7: discussion of white space ]] space]]

   Atom is extensible. an extensible format.  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 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, but may be defined by an extension to Atom.

3.  Common Atom Constructs

   Many of Atom's elements share a few common structures.  This section
   defines a few such structures and their requirements for convenient
   reference by the appropriate element definitions.

   When an element is identified as being a particular kind of
   construct, it inherits the corresponding requirements from that
   construct's definition in this section.

3.1  Text Constructs

   A Text construct contains human readable text, usually in fairly small
   quantities.  Except for the "type" attribute, the content of Text
   constructs is language-sensitive.

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

   atomXHTMLTextConstruct =
      atomCommonAttributes,
      attribute type { "XHTML" "xhtml" },
      (text|anyElement)*

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct

3.1.1  "type" Attribute

   Text constructs MAY have a "type" attribute.  [[anchor11: Some feel
   type attributes with different allowable values in different elements
   is confusing.]].  When present, the value MUST be one of "TEXT", "HTML" "text",
   "html" or "XHTML". "xhtml".  If the "type" attribute is not provided, software Atom
   Processors MUST behave as though it were present with a value of "TEXT".
   "text".

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

   If the value is "TEXT", "text", the content of the Text construct MUST NOT
   contain child elements.  Such text is intended to be presented to
   humans in a readable fashion.  Thus, software Atom Processors MAY display it
   using normal text rendering techniques such as proportional fonts,
   white-space collapsing, and justification.

   If the value of "type" is "HTML", "html", the content of the Text construct
   MUST NOT contain child elements, and SHOULD be suitable for handling
   by software that knows HTML.  The
   as HTML [W3C.REC-html401-19991224].  Any markup must within MUST be
   escaped; for example, "<br>" as "&lt;br>".  The  HTML markup within SHOULD
   be such that it could validly appear directly within an HTML <DIV> element.
   Receiving software which displays the
   element, after unescaping.  Atom Processors that display such content
   MAY use the markup to aid in displaying it.

   [[example its display.

   [[anchor12: example atom entry w/ escaped markup]]

   If the value of "type" is "XHTML", "xhtml", the content of the Text construct
   MAY contain child elements.  The content SHOULD
   MUST be a single XHTML div element [W3C.REC-xhtml-basic-20001219].
   The XHTML div MUST contain XHTML text and markup that could validly
   appear directly within an xhtml:div XHTML div element.  Receiving software  The XHTML div element itself
   MUST NOT be considered part of the content.  Atom Processors which displays
   display the content MAY use the markup to aid in displaying it.
   Escaped markup is interpreted as a text representation of markup, and
   MUST NOT be interpreted as markup itself.

   Example:

   ...
   <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>
   ...

3.2  Person Constructs

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

   Person constructs MAY be extended by namespace-qualified element
   children.

   This specification assigns no significance to the order of appearance
   of the child elements in a Person construct.

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

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 a URI 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 a URI an IRI reference [RFC3986]. [RFC3987].

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

3.2.3  The "atom:email" Element

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

3.3  Date Constructs

   A Date construct is an element whose content MUST conform to the
   date-time BNF rule in [RFC3339].  I.e., the content of this element
   matches this regular expression:

   [0-9]{8}T[0-9]{2}:[0-9]{2}:[0-9]{2}(\.[0-9]+)
        ?(Z|[\+\-][0-9]{2}:[0-9]{2})

   As a result, the date values conform to the following specifications:
   [RFC3339], [W3C.REC-xmlschema-2-20041028],
   [W3C.NOTE-datetime-19980827], and [ISO.8601.1988].

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime

3.4  Service Constructs

   A Service construct is an empty

4.  Atom Element Definitions

4.1  Container Elements

4.1.1  The "atom:feed" Element

   [[anchor21: Substantially changed from format-05, review carefully.]]

   The "atom:feed" element that conveys is the URI document (i.e., top-level) element of
   an Atom Publishing Protocol [Atom-protocol] service Feed Document, acting as a container for metadata and data
   associated with an
   entry the feed.  Its element children consist of one or
   more metadata elements followed by zero or more atom:entry child
   elements.

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

   atomServiceConstruct

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

3.4.1  The "href" Attribute

   The "href" attribute contains the a URI of the service.  atom:service following child elements MUST have a "href" attribute, whose value MUST be a URI
   reference [RFC3986].

   xml:base processing MUST be applied to the "href" attribute.

3.5  Identity Constructs

   An Identity construct is an element whose content conveys a
   permanent, universally unique identifier for the construct's parent.
   Its content MUST be a URI, as are defined by [RFC3986].  Note this specification (note
   that the
   definition presence of "URI" excludes relative references.

   When an Atom document is relocated, migrated, syndicated,
   republished, exported or imported, the content some of its Identity
   construct these elements is required):

   o  atom:feed elements MUST NOT change.  Put another way, an Identity construct
   pertains to contain exactly one atom:author element,
      UNLESS all instantiations of a particular Atom entry or feed;
   revisions retain the same content in their Identity constructs.

   atomIdentityConstruct =
      atomCommonAttributes,
      (atomUri)

3.5.1  Dereferencing Identity Constructs

   The content of an Identity construct MAY be dereferencable (e.g. atom:feed element's child atom:entry elements
      contain an
   HTTP URI).  However, processors atom:author element.  atom:feed elements MUST NOT assume it to be
   dereferencable.

   The content
      contain more than one atom:author element.  [[anchor22:
      inheritance]]
   o  atom:feed elements MAY contain any number of an Identity construct MUST be created in a way that
   assures uniqueness, and it is suggested that the Identity construct
   be stored along with the associated resource.

   Because of the risk atom:category
      elements.

   o  atom:feed elements MAY contain any number of confusion between URIs that would be
   equivalent if dereferenced, the following normalization strategy is
   strongly encouraged when generating Identity constructs: atom:contributor
      elements.
   o  Provide the scheme in lowercase characters.  atom:feed elements MUST NOT contain more than one atom:copyright
      element.
   o  Provide the host, if any, in lowercase characters.  atom:feed elements MUST NOT contain more than one atom:generator
      element.
   o  Only perform percent-encoding where it is essential.  atom:feed elements MUST NOT contain more than one atom:icon
      element.
   o  Use uppercase A-through-F characters when percent-encoding.  atom:feed elements MUST NOT contain more than one atom:image
      element.
   o  Prevent dot-segments appearing in paths.  atom:feed elements MUST NOT contain more than one atom:id element.
   o  For schemes that define  atom:feed elements MUST contain at least one atom:link element
      with a default authority, use an empty
      authority if the default is desired. relation of "alternate".
   o  For schemes that define an empty path to be equivalent to  atom:feed elements SHOULD contain one atom:link element with a path rel
      attribute value of "/", use "/".
   o  For schemes that define "self" and SHOULD contain a port, use href attribute with
      an empty port if absolute URI as its value.  This URI identifies the default
      is desired.
   o  Preserve empty fragment identifiers feed and queries.
   o  Ensure that all portions of the URI are UTF-8 encoded NFC form
      Unicode strings.

3.5.2  Comparing Identity Constructs

   Instances of Identity constructs can be compared a
      representation equivalent to determine whether
   an entry or feed is the same as one seen before.  Processors feed.
   o  atom:feed elements MUST
   compare Identity constructs on NOT contain more than one atom:link
      element with a character-by-character basis in rel attribute value of "alternate" that has the
      same type attribute value.  If a
   case-sensitive fashion.

   As feed's atom:link element with
      type="alternate" resolves to an HTML document, then that document
      SHOULD have a result, two URIs autodiscovery link element [Atom-autodiscovery] that resolve
      reflects back to the same resource but are not
   character-for-character feed.  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 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.

4.  Element Definitions

4.1 atom:id values.

4.1.2  The "atom:feed" "atom:entry" Element

   The "atom:feed" element is the document (i.e., top-level) "atom:entry" element of represents an Atom Feed Document, individual entry, acting as a
   container for metadata and data associated with the feed.  Its first entry.  This
   element can appear as a child MUST be atom:head,
   which MAY be followed zero of the atom:feed element, or more it can
   appear as the document (i.e., top-level) element of a standalone Atom
   Entry Document.

   The atom:entry element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  This
   specification assigns no significance to the order of appearance of
   the child elements.

   atomFeed elements of atom:entry.

   atomEntry =
      element atom:feed atom:entry {
         atomCommonAttributes,
         atomVersionAttribute,
         (atomHead
         (atomAuthor?
          & atomEntry* atomCategory*
          & anyElement*)
      }

4.1.1  The "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-05: do not deploy".

   atomVersionAttribute =
    attribute version {"draft-ietf-atompub-format-05 : do not deploy"}

4.2  The "atom:head" Element

   The atom:head element acts as a container for metadata about the feed
   itself.

   The atom:head element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  This
   specification assigns no significance to the order of appearance of
   the child elements of atom:head.

   atomHead =
         element atom:head {
         atomCommonAttributes,
         (atomTitle
          & atomUpdated
          & atomLink+
          & atomId?
          & atomAuthor? atomContent?
          & atomContributor*
          & atomTagline? atomCopyright?
          & atomGenerator? atomId
          & atomInfo? atomLink*
          & atomCopyright? atomPublished?
          & atomCategory* atomSourceFeed?
          & atomIntrospection? atomSummary?
          & atomTitle
          & atomPost? atomUpdated &
          anyElement*)
      }

   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, in an Atom Feed Document, the atom:feed element contains
      an atom:author element itself.  atom:entry elements MUST NOT
      contain more than one atom:author element.  [[anchor24: What if
      there's a source-feed element? This is required): busted.  We should make
      author required for atom:feed and optional for atom:entry.  No
      inheritance co-constraints required.  --R.  Sayre]]
   o  atom:head  atom:entry elements MAY contain any number of atom:category
      elements.
   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:title atom:id element.
   o  atom:head  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:head  [[anchor25: "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.  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 value." This
      requirement predates @hreflang.  Keep it? --R.  Sayre]] atom:entry
      elements MAY contain additional atom:link elements beyond those
      described above.
   o  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 atom:published
      element.  [[inheritance]]
   o  atom:head  atom:entry elements MUST NOT contain more than one an
      atom:contributor
      atom:source-feed element.

   o  atom:head  atom:entry elements MUST NOT contain more than one an
      atom:copyright element.
   o  atom:head elements MUST NOT contain more than one an atom:id
      element.
   o  atom:head elements MUST contain exactly one atom:updated element.
   o  atom:head elements MAY contain atom:summary element in any number of atom:category
      elements.
   o  atom:head elements MUST NOT contain more than one atom:post
      the following cases: [[anchor26: Do these requirements reflect the
      WG's decisions? --R.  Sayre]]
      *  the atom:entry element contains no atom:content element.

   o  atom:head elements
         [[anchor27: Net result: Atom entries MUST NOT contain more than one
      atom:introspection element. have an atom:summary
         or an 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
         [RFC2045] and does not begin with "text/" nor end with "+xml".
   o  atom:head  atom:entry elements MUST NOT contain more than one atom:tagline atom:summary
      element.
   o  atom:head  atom:entry elements MUST NOT contain more than have exactly one atom:generator "atom:title" element.
   o  atom:head  atom:entry elements MUST NOT contain more than exactly one atom:info atom:updated element.

4.2.1  Usage of "atom:head" within "atom:entry"

   In this case, the atom:head

4.1.3  The "atom:content" Element

   The "atom:content" element acts as a container for metadata
   about the feed within which the entry was created.

   If an atom:entry is copied into one feed from another feed, then either contains or links to the
   atom:head element content of
   the source feed SHOULD be inserted into the
   atom:entry unless entry.  Except for the entry, as copied, already contains an atom:head
   element.  If "type" and "src" attributes, the atom:entry already contains an atom:head, then that
   atom:head SHOULD be copied without modification.

   [[ ...  example ...  ]]

4.3  The "atom:entry" Element

   The "atom:entry" element represents an individual entry.  This
   element can appear as a child content
   of the atom:feed element, atom:content is language-sensitive.  atom:entry elements MUST
   contain zero or it can
   appear as the document (i.e., top-level) one atom:content elements.

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

   atomInlineXHTMLContent =
      element atom:content {
         atomCommonAttributes,
         attribute type {
            "xhtml" |
            atomMediaType
         }?,
         (text|anyElement)*
      }

   atomOutOfLineContent =
      element atom:content {
         atomCommonAttributes,
         attribute type {
            "text" |
            "html" |
            "xhtml" |
            atomMediaType }?,
         attribute src { atomUri },
         empty
      }

   atomContent =
      atomInlineTextContent |
      atomInlineXHTMLContent |
      atomOutOfLineContent

4.1.3.1  The "type" attribute

   atom:content MAY have a standalone Atom
   Entry Document. "type" attribute.  When appearing in an present, the value
   MAY be one of "text", "html", or "xhtml".  Failing that, it MUST be a
   MIME media type [RFC2045] with a discrete top-level type (see Section
   5 of [RFC2045]).  If the type attribute is not provided, Atom Entry Document, atom:entry elements
   Processors MUST
   have behave as though it were present with a "version" value of
   "text".

4.1.3.2  The "src" attribute

   atom:content MAY have a "src" attribute, whose content indicates the version of value MUST be an IRI
   reference [RFC3987].  If the "src" attribute is present, Atom specification that
   Processors MAY use the entry conforms to.

   The version identifier for this specification IRI to retrieve the content.  If the "src"
   attribute is present, atom:content MUST be empty.  That is
   "draft-ietf-atompub-format-05: do to say,
   the content may be retrievable using "src=" IRI, or it may be
   contained within atom:content, but not deploy". both.

   If the "src" attribute is present, the "type" attribute SHOULD be
   provided and MUST be a MIME media type [RFC2045], rather than "text",
   "html", or "xhtml".  The atom:entry element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  This
   specification assigns no significance value is advisory; that is to say, upon
   dereferencing the order of appearance of IRI to retrieve the child elements of atom:entry.

   atomEntry =
         element atom:entry {
         atomCommonAttributes,
         atomVersionAttribute?,
         (atomTitle
          & atomId
          & atomLink*
          & atomUpdated
          & atomPublished?
          & atomAuthor?
          & atomContributor*
          & atomHost?
          & atomCopyright?
          & atomCategory*
          & atomEdit?
          & atomSummary?
          & atomContent?
          & atomHead?
          & anyElement*)
      }

   The following child elements are defined by this specification (note content, if the server
   providing that it requires content also provides a media type, the presence of some
   server-provided media type is authoritative.

   If the value of these elements):

   o  atom:entry elements MUST have exactly one "atom:title" element.
   o  atom:entry elements MUST contain exactly one atom:id element.
   o  atom:entry elements type begins with "text/" or ends with "+xml", the
   content SHOULD be local; that contain is to say, no child atom:content element
      MUST contain at least one atom:link element with a rel "src" attribute
      value should be
   provided.  [[anchor31: J.  Reschke: I'm not sure I understand what
   this is for.  It seems to discourage putting XML data out-of-band.
   Why? ...  Explaining the issue instead of "alternate".  atom:entry elements just trying to enforce it
   may lead to better results...]]

4.1.3.3  Processing Model

   Atom Documents MUST NOT contain more
      than one atom:link element with a rel attribute conform to the following rules.  Atom Processors
   MUST interpret atom:content according to the first applicable rule.

   1.  If the value of
      "alternate" that has "type" is "text", the same type attribute value.  atom:entry
      elements MAY contain additional atom:link elements beyond those
      described above.
   o  atom:entry elements MUST contain exactly one atom:updated element.
   o  atom:entry elements content of atom:content
       MUST NOT contain more than one atom:published
      element.
   o  atom:entry elements MUST contain exactly one atom:author element,
      unless, child elements.  Such text is intended to be
       presented to humans in an a readable fashion.  Thus, Atom Feed Document, Processors
       MAY display it using normal text rendering techniques such as
       proportional fonts, white-space collapsing, and justification.
   2.  If the atom:head element contains
      an atom:author element itself.  atom:entry elements value of "type" is "html", the content of atom:content
       MUST NOT contain more than one atom:author child elements, and SHOULD be suitable for
       handling as HTML [W3C.REC-html401-19991224].  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.
   o  atom:entry elements  Atom Processors that display the
       content SHOULD use the markup to aid in displaying it.
   3.  If the value of "type" is "xhtml", the content of atom:content
       MUST NOT contain more than one
      atom:contributor element.
   o  atom:entry elements be a single XHTML div element
       [W3C.REC-xhtml-basic-20001219].  The XHTML div MUST NOT contain more than one atom:host XHTML
       text and markup that could validly appear within an XHTML div
       element.
   o  atom:entry elements  The XHTML div element itself MUST NOT contain more than one atom:copyright
      element.
   o  atom:entry elements MAY contain any number be considered
       part of atom:category
      elements.

   o  atom:entry elements MUST contain an atom:summary element the content.  Atom Processors that display the content
       MAY use the markup to aid in any displaying it.  Escaped markup is
       interpreted as a text representation of markup, and MUST NOT be
       interpreted as markup itself.
   4.  If the following cases:
      *  the atom:entry element contains no atom:content element.
      * value of "type" ends with "+xml" or "/xml"
       (case-insensitive), the atom:entry contains an content of atom:content which has a may include child
       elements, and SHOULD be suitable for handling as the indicated
       media type.  If the "src" attribute (and is thus empty).
      * not provided, this would
       normally mean that the atom:entry contains content "atom:content" element would contain a
       single child element which is encoded in Base64;
         i.e. would serve as the "type" attribute root element of atom:content is a MIME media type
         [RFC2045] and does not begin the
       XML document of the indicated type.
   5.  If the value of "type" begins with "text/" nor end with "+xml".
   o  atom:entry elements MUST NOT contain more than one atom:summary
      element.
   o  atom:entry elements (case-insensitive),
       the content of atom:content MUST NOT contain more than one atom:edit
      element.
   o  atom:entry elements child elements.
   6.  For all other values of "type", the content of atom:content MUST NOT contain more than one atom:head
      element.

4.4  The "atom:title" Element

   The "atom:title" element is a Text construct that conveys
       be a
   human-readable title valid Base64 encoding [RFC3548], which when decoded SHOULD
       be suitable for an entry or feed.

   atomTitle = 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 atom:title { atomTextConstruct }

4.5 by whitespace, 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 "atom:id" 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>
   ...

4.2  Metadata Elements

4.2.1  The "atom:author" Element

   The "atom:id" "atom:author" element is an Identity a Person construct that conveys a
   permanent, universally unique identifier for an indicates the
   author of the entry or feed.

   atomId

   atomAuthor = element atom:id atom:author { atomIdentityConstruct atomPersonConstruct }

4.6

4.2.2  The "atom:link" "atom:category" Element

   The atom:link element is an empty "atom:category" element that defines conveys information about a reference
   from category
   associated with an Atom Document to a Web resource.

   atomLink entry or feed.

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

4.6.1  The "href" Attribute

   The "href" attribute contains the link's URI.  Link constructs MUST
   have a href attribute, whose value MUST be a URI reference [RFC3986].

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

4.6.2  The "rel" Attribute

   Link elements MAY have an "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".

   rel_attribute = segment-nz-nc / URI

   The value of "rel" MUST be either a name, which is non-empty and does
   not contain any colon (":") characters, or a URI [RFC3986].  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 9, and thus the URI that would be obtained by
   appending the value of the rel attribute to the string
   "http://www.iana.org/assignments/relation/".  The value of "rel"
   describes the meaning of the link, but does not impose any behavioral
   requirements on implementations.

   This document defines two initial values for the Registry of Link
   Relations:
         empty
      }

4.2.2.1  The value "alternate" signifies that the URI in the value of the href "term" Attribute

   The "term" attribute is a string that identifies an alternate version of the resource described
   by category to
   which the containing element. entry or feed belongs.  Category elements MUST have a
   "term" attribute.

4.2.2.2  The value "related" signifies "scheme" Attribute

   The "scheme" attribute is an IRI that the URI 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 the value
   end-user applications.  The content of the href "label" attribute identifies is
   language-sensitive.  Category elements MAY have a resource related "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 resource described by
   the containing atom:feed entry or atom:entry element.  For example, the
   feed for feed.

   atomContributor = element atom:contributor { atomPersonConstruct }

4.2.4  The "atom:copyright Element

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

   atomCopyright = element atom:copyright { atomTextContstruct }
   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 performance atom:copyright element of the search engine
   at "http://search.example.com" might contain, as a child of
   atom:head:

   <link rel="related" href="http://search.example.com/">
   An identical link might appear as 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 child of any atom:entry whose feed, for debugging and other purposes.

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

   The content contains a discussion of this element, when present, MUST be a string that same search engine.

4.6.3 is a
   human-readable name for the generating agent.

   The "type" Attribute

   Link elements atom:generator element MAY have a type attribute, "uri" attribute whose value
   MUST conform 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 syntax version of a MIME media type [RFC2045].

   The type attribute's the generating agent.  When present, its
   value is an advisory media type; it unstructured text.

4.2.6  The "atom:icon" Element

   The "atom:icon" element's content is an IRI [RFC3987] which
   identifies an image which provides iconic visual identification for a hint
   about
   feed.  The image SHOULD have an aspect ratio of one (horizontal) to
   one (vertical), and should be suitable for presentation at a small
   size.

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

4.2.7  The "atom:id" Element

   [[anchor45: Substantially changed from format-05, review carefully]]

   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 type
   definition of "IRI" excludes relative references.  Though the representation which is expected to IRI
   might use a dereferencable scheme, Atom Processors MUST NOT assume it
   can be returned
   when dereferenced.

   When an Atom document is relocated, migrated, syndicated,
   republished, exported or imported, the value 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 href attribute same content in their atom:id elements.

   The content of an atom:id element MUST be created in a way that
   assures uniqueness; it is dereferenced.  Note suggested that the
   type attribute does not override the actual media type returned atom:id element be
   stored along with the representation.

4.6.4  The "hreflang" Attribute

   The "hreflang" attribute's content describes associated resource.

   Because of the language risk of confusion between IRIs that would be
   equivalent if dereferenced, the
   resource pointed to by following normalization strategy is
   strongly encouraged when generating atom:id elements:

   o  Provide the href attribute.  When used together with scheme in lowercase characters.
   o  Provide the rel="alternate", host, if any, in lowercase characters.
   o  Only perform percent-encoding where it implies 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 translated version of default authority, use an empty
      authority if the entry.
   Link elements MAY have default is desired.
   o  For schemes that define an hreflang attribute, whose value MUST empty path to be equivalent to a
   language tag [RFC3066].

4.6.5  The "title" Attribute

   The "title" attribute conveys human-readable information about the
   link.  Link elements MAY have path
      of "/", use "/".
   o  For schemes that define a title attribute.

4.6.6  The "length" Attribute

   The "length" attribute indicates port, use an advisory length of empty port if the linked
   content in octets; it default
      is a hint about desired.
   o  Preserve empty fragment identifiers and queries.
   o  Ensure that all components of the content length IRI are appropriately
      character-normalized, e.g.  by using NFC or NFKC.

4.2.7.1  Comparing atom:id

   Instances of atom:id elements can be compared to determine whether an
   entry or feed is the
   representation returned when 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 URI in IRI character strings, and MUST NOT rely on dereferencing the href attribute is
   dereferenced.  Note
   IRIs.

   As a result, two IRIs that resolve to the length attribute does same resource but are not override
   character-for-character identical will be considered different for
   the
   actual content length 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 representation as reported by the
   underlying protocol.

   Link elements MAY have a length attribute.

4.7 purposes of comparison.

4.2.8  The "atom:updated" "atom:image" Element

   The "atom:updated" element "atom:image" element's content is a Date construct indicating the most
   recent instant in time when an entry or feed was modified in a way
   the producer considers significant.  Therefore, not all modifications
   necessarily result in IRI [RFC3987] which
   identifies an image which provides visual identification for a changed atom:updated value.

   Publishers MAY change the value feed.
   The image SHOULD have an aspect ratio of this element over time.
   Processors MAY present entries sorted using this value.  Processors
   MAY choose not 2 (horizontal) to present entries until the instant in time specified
   in the atom:updated element has passed.

   atomUpdated 1
   (vertical).

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

4.8

4.2.9  The "atom:published" "atom:link" Element

   The "atom:published" "atom:link" element is a Date construct indicating an
   instant in time associated with empty element that defines a reference
   from an event early in the life cycle of 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 entry.  Typically, atom:published will link's IRI.  atom:link elements
   MUST have a href attribute, whose value MUST be a IRI reference
   [RFC3987].

   xml:base [W3C.REC-xmlbase-20010627] processing MUST be associated with the
   initial creation or first availability of the resource.

   Processors MAY present entries sorted using this value.  Processors
   MAY choose not applied to present entries until the instant in time specified
   in the atom:published element has passed.

4.9  The "atom:author" Element
   href attribute's content.

4.2.9.2  The "atom:author" element is a Person construct "rel" Attribute

   atom:link elements MAY have an "rel" attribute that indicates the
   default author of
   link relation type.  If the an entry or feed

   atomAuthor = element atom:author { atomPersonConstruct }

4.10  The "atom:contributor" Element

   The "atom:contributor" element "rel" attribute is a Person construct that indicates a
   person or other entity who contributes to not present, the entry.

   atomContributor = link
   element atom:contributor { atomPersonConstruct }

4.11  The "atom:host" Element MUST be interpreted as if the link relation type is
   "alternate".

   rel_attribute = segment-nz-nc / IRI

   The "atom:host" element's content conveys a domain name or network
   address associated with an entry's origin.  Its content value of "rel" MUST be either a
   domain name [RFC1035], a dotted-decimal IPv4 address [RFC0791], that is non-empty and does
   not contain any colon (":") characters, or a
   colon-delimited IPv6 address [RFC2460].

4.12  The "atom:copyright" Element

   The "atom:copyright" element IRI [RFC3987].  Note
   that use of a relative reference is not allowed.  If a Text construct 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 conveys a
   human-readable copyright statement for an entry or feed.

   The atom:copyright element SHOULD NOT would be used obtained by
   appending the value of the rel attribute to convey
   machine-readable licensing information.

   If an atom:entry element the string
   "http://www.iana.org/assignments/relation/".  The value of "rel"
   describes the meaning of the link, but does not contain an atom:copyright element,
   then impose any behavioral
   requirements on implementations.

   This document defines five initial values for the atom:copyright element Registry of Link
   Relations:

   The value "alternate" signifies that the containing atom:feed element's
   atom:head element, if present, should be considered to apply to IRI in the
   entry.

4.13  The "atom:category" Element

   Category elements contain information about a category to which an
   Atom feed or entry is associated.

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

4.13.1  The "term" Attribute identifies an alternate version of the resource described
   by the containing element.

   The "term" value "related" signifies that the IRI in the value of the href
   attribute is a string which identifies the category a resource related to
   which the entry or resource described by
   the containing element.  For example, the feed belongs.  Category elements MUST have a
   "term" attribute.

4.13.2  The "scheme" Attribute

   The "scheme"  attribute is for a URI site that identifies
   discusses the performance of the search engine at
   "http://search.example.com" might contain, as a categorization
   scheme.  Category elements MAY have child of atom:feed:

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

   An identical link might appear as a "scheme" attribute.

4.13.3  The "label" attribute

   The "label" attribute provides child of any atom:entry whose
   content contains a human-readable label discussion of that may be
   displayed in end-user applications.  Category elements MAY have a
   "label" attribute.

4.14  The "atom:summary" Element same search engine.

   The "atom:summary" element is a Text construct value "self" signifies that conveys a short
   summary, abstract or excerpt the IRI in the value of an entry.

   atomSummary = element atom:summary { atomTextConstruct }

4.15  The "atom:content" Element

   The "atom:content" element either contains or links the href
   attribute identifies a resource equivalent to the content containing element.

   The value "enclosure" signifies that the IRI in the value of the entry.  atom:entry elements MUST contain zero or one atom:content
   elements.

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

   atomInlineXHTMLContent =
      element atom:content {
         atomCommonAttributes, href
   attribute type { "XHTML" | atomMediaType }?,
         (text|anyElement)*
      }

   atomOutOfLineContent =
      element atom:content {
         atomCommonAttributes, identifies a related resource which is potentially large in
   size and may require special handling by consuming software.  For
   Link constructs with rel="enclosure", the length attribute type { "TEXT" | "HTML" | "XHTML" | atomMediaType }?, SHOULD be
   provided.

   The value "via" signifies that the IRI in the value of the href
   attribute src { atomUri },
         empty
      }

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomOutOfLineContent

4.15.1 identifies a resource that is the source of the information
   provided in the containing element.

4.2.9.3  The "type" attribute

   atom:content Attribute

   Link elements MAY have a "type" type attribute, When present, the whose value MAY
   be one of "TEXT", "HTML", or "XHTML".  Failing that, it MUST be conform to
   the syntax of a MIME media type [RFC2045] in which, to use [RFC2045].

   The type attribute's value is an advisory media type; it is a hint
   about the terminology type of Section
   5 the representation that is expected to be returned
   when the value of [RFC2045], the top level href attribute is a discrete type.  If dereferenced.  Note that the
   type attribute is does not provided, software MUST behave as though it were
   present override the actual media type returned with a value of "TEXT".

4.15.2
   the representation.

4.2.9.4  The "src" attribute

   atom:content "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 a "src" an hreflang attribute, whose value MUST be a URI
   reference [RFC3986].  If the "src"
   language tag [RFC3066].

4.2.9.5  The "title" Attribute

   The "title" attribute is present, software MAY
   use the URI to retrieve conveys human-readable information about the content.  If
   link.  The content of the "src" "title" attribute is
   present, atom:content MUST be empty.  That is to say, 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 may
   be retrievable using "src=" URI, or in octets; it may be contained within
   atom:content, but not both.

   If the "src" attribute is present, a hint about the "type" content length of the
   representation returned when the IRI in the href attribute SHOULD be
   provided and MUST be a MIME media type [RFC2045], rather than "TEXT",
   "HTML", or "XHTML".  The value is advisory;
   dereferenced.  Note that is to say, upon
   dereferencing the URI to retrieve the content, if length attribute does not override the server
   providing that
   actual content also provides a media type, length of the
   server-provided media type 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 authoritative.

   If a Date construct indicating an
   instant in time associated with an event early in the value life cycle of type begins
   the entry.  Typically, atom:published will be associated with "text/" the
   initial creation or ends with "+xml", first availability of the
   content SHOULD be local; that resource.

   atomPublished = element atom:published { atomDateConstruct }

4.2.11  The "atom:source-feed" Element

   If an atom:entry is to say, no "src" attribute should be
   provided.

4.15.3  Processing Model

   Software MUST apply copied from one feed into another feed, then the following rules in
   source atom:feed's metadata (all child elements of atom:feed other
   than the order below to
   ascertain atom:entry elements) MAY be preserved within the rules governing copied
   entry by adding an atom:source-feed child element, if it is not
   already present in the content entry, and including some or all of "atom:content".

   1.  If the value is "TEXT", source
   feed's metadata elements as the content of atom:content MUST NOT
       contain child elements. atom:source-feed element's children.
   Such text is intended to metadata SHOULD 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 preserved if the value source atom:feed contains
   any of "type" is "HTML", the content of atom:content
       MUST NOT contain child elements, elements atom:author, atom:contributor,
   atom:copyright, or atom:category and SHOULD be suitable those child elements are not
   present in the source atom:entry.

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

4.2.12  The "atom:subtitle" Element

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

   atomSubtitle = element atom:subtitle { atomTextConstruct }

4.2.13  The "atom:summary" Element

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

   atomSummary = element atom:summary { atomTextConstruct }

4.2.14  The HTML markup must be
       escaped; for example, "<br>" as "&lt;br>". "atom:title" Element

   The HTML markup
       SHOULD be such "atom:title" element is a Text construct that it could validly appear directly within conveys a
   human-readable title for an
       HTML <DIV> element.  Receiving software which displays 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
       content SHOULD use most
   recent instant in time when an entry or feed was modified in a way
   the markup to aid publisher considers significant.  Therefore, not all
   modifications necessarily result in displaying it.
   3.  If a changed atom:updated value.

   atomUpdated = element atom:updated { atomDateConstruct }

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

5.  Securing Atom Documents

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

5.1  Digital Signatures

   The root of an Atom document (i.e., atom:feed in displaying it.  Escaped markup is
       interpreted an Atom Feed
   Document, atom:entry in an Atom Entry Document) MAY have an Enveloped
   Signature, as a text representation of markup, described by XML-Signature and Syntax Processing
   [W3C.REC-xmldsig-core-20020212].

   Processors MUST NOT be
       interpreted as markup itself.
   4.  If the value of "type" ends with "+xml" or "/xml", the content of
       atom:content may include child elements, and SHOULD be suitable
       for handling by software that knows the indicated media type.  If
       the "src" attribute is not provided, this would normally mean
       that the "atom:content" element would contain reject an Atom Document containing such a single child
       element which would serve as the root element
   signature because they are not capable of verifying it; they MUST
   continue processing and MAY inform the XML document user of their failure to
   validate the indicated type.
   5.  If signature.

   In other words, the value presence of "type" begins an element with "text/" the content namespace IRI
   "http://www.w3.org/2000/09/xmldsig#" and a local name of
       atom:content MUST NOT contain "Signature"
   as a child elements.

   6.  For all other values of "type", the content document element must not cause an Atom Processor
   to fail merely because of atom:content its presence.

   Other elements in an Atom Document MUST NOT be signed unless their
   definitions explicitly specify such a valid Base64 encoding [RFC3548], which when decoded SHOULD
       be suitable for handling by software that knows the indicated
       media type.  In this case, the characters capability.

5.2  Encryption

   The root of an Atom Document (i.e., atom:feed in the Base64 encoding
       may be preceded and followed an Atom Feed
   Document, atom:entry in an Atom Entry Document) MAY be encrypted,
   using the atom:content element mechanisms described by
       whitespace, XML Encryption Syntax and lines are separated by a single newline (U+000A)
       character.

4.16  The "atom:introspection" Element

   The "atom:introspection" element is a Service construct that conveys
   the URI of
   Processing [W3C.REC-xmlenc-core-20021210].

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 introspection file associated with a feed.

   atomIntrospection =
             element atom:introspection { atomServiceConstruct }

4.17  The "atom:post" Element

   The "atom:post" element is a Service construct Atom
   document.  Note that conveys the URI
   used to add entries to a feed.

   atomPost = element atom:post { atomServiceConstruct }

4.18  The "atom:edit" Element

   The "atom:edit" atom:content element is a Service construct that conveys the URI
   used designed to retrieve and edit support
   the source representation inclusion of an entry.

   atomEdit = element atom:edit { atomServiceConstruct }

4.19  The "atom:tagline" Element

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

   atomTagline = element atom:tagline { atomTextConstruct }

4.20  The "atom:generator" Element

   The "atom:generator" element's content identifies arbitrary foreign markup.

6.2  Extensions To the software agent
   used to generate a feed, for debugging Atom Vocabulary

   Future versions of this specification may add new elements and other purposes.

   atomGenerator = element atom:generator {
      atomCommonAttributes,
      attribute url { atomUri }?,
      attribute
   attributes to the Atom markup vocabulary.  Software written to
   conform to this version { text }?,
      text
   }

   The content 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 element, when present, MUST
   discussion, unrecognized markup from the Atom vocabulary will be a string that is a
   human-readable name for
   considered "foreign "markup".

   Unlike markup from other vocabularies, foreign markup from the generating agent.

   The atom:generator element Atom
   vocabulary MAY have a "uri" attribute whose value
   MUST be a URI reference [RFC3986].  When dereferenced, that URI
   SHOULD produce appear at any location in an Atom document.

6.3  Software Processing of Foreign Markup

   Software processing an Atom Document which encounters foreign markup
   in a representation location that is relevant legal according to this specification MUST NOT
   stop processing or signal an error.  It may be the case that agent.

   The atom:generator element MAY have a "version" attribute that
   indicates the version of
   software is able to process the generating agent.  When present, its
   value foreign markup correctly and does so.
   Otherwise, such markup is unstructured text.

4.21  The "atom:info" Element

   The "atom:info" element termed "unknown foreign markup".

   When unknown foreign markup is encountered as a Text construct that conveys a
   human-readable explanation child of the feed format itself.  The atom:info
   element SHOULD NOT be considered meaningful by processors; it is atom:entry,
   atom:feed, or a
   convenience Person Construct, software MAY [[anchor67: Changed to publishers.

   atomInfo = element atom:info { atomTextConstruct }

5.  Managing Feed State

   [[ talk about what it means
   MAY from SHOULD.  Republishers might want to keep copy it.  --R.  Sayre]]
   bypass the markup and any textual content and MUST NOT change its
   behavior as a view result of a feed ]]

6.  Securing Atom Documents

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

6.1  Digital Signatures

   The document element 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 an foreign elements as though the surrounding markup
   were not present.

6.4  Extension Elements

   Atom document (i.e., atom:feed allows foreign markup anywhere in an Atom
   Feed Document, document.  Child
   elements of atom:entry in an Atom Entry Document) MAY have an
   Enveloped Signature, as and atom:feed 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 XML-Signature and Syntax
   Processing [W3C.REC-xmldsig-core-20020212].

   Processors this specification.

6.4.1  Simple Extension Elements

   A Simple Extension element MUST NOT reject an Atom document containing such have any attributes or child
   elements.  The element MAY contain either character data, or be
   empty.

   The element can be interpreted as a
   signature because they are not capable simple property (or name/value
   pair) of verifying it; they MUST
   continue processing and MAY inform the user parent element that encloses it.  The pair consisting of their failure to
   validate the signature.

   In other words,
   the presence namespace-URI of an element with the namespace URI
   "http://www.w3.org/2000/09/xmldsig#" element and a the local name of "Signature" the element
   can be interpreted as a child the name of the property.  The character data
   content of the document element must not cause a processor to fail
   merely because can be interpreted as the value of its presence.

   Other elements in the
   property.  If the element is empty, then the property value can be
   interpreted as an Atom document empty string.

6.4.2  Structured Extension Elements

   The root element of a Structured Extension element MUST NOT 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 signed unless their
   definitions explicitly specify such a capability.

6.2  Encryption empty.

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

   This specification does not provide an Atom document (i.e., atom:feed in an Atom
   Feed Document, atom:entry interpretation of a Structured
   Extension element.  The syntax of the XML contained in the element,
   and an Atom Entry Document) MAY be
   encrypted, using interpretation of how the mechanisms described element relates to its containing
   element is defined by XML Encryption Syntax
   and Processing [W3C.REC-xmlenc-core-20021210].

7.  Embedding Atom in Other Formats

   [[ ...  ]]

8.  Extending the specification of the Atom

   [[ ...  ]]

9. extension.

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
         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
      [[anchor71: 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  [[anchor72: update upon
      publication]]
   Applications which that 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
      [[anchor73: update upon publication]]

9.1

7.1  Registry of Link Relations

   This registry is maintained by IANA and initially contains the two five
   values: "alternate" "alternate", "related", "self", "enclosure", and "related". "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 contain
   discussion of at least the following five topics:

   o  A value for the "rel" attribute that conforms to the syntax rule
      given in Section 4.6.2 4.2.9.2
   o  Common name for link type.
   o  Description of link type semantics.
   o  Expected display characteristics.
   o  Security considerations.

10.

8.  Security Considerations

8.1  HTML and XHTML Content

   Text Constructs and atom:content allow the delivery of HTML and XHTML
   to receiving software, which may process it.  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 RFC 2854 and HTML 4.01 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 may 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 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.

11.

9.  References

11.1

9.1  Normative References

   [Atom-autodiscovery]
              Pilgrim, M., "Atom Feed Autodiscovery",  work-in-progress,
              August 2004.

   [Atom-protocol]
              Gregorio, J.

   [ISO.8601.1988]
              International Organization for Standardization, "Data
              elements and R. Sayre, "The Atom Publishing Protocol",
              work-in-progress, July 2004.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC1035]  Mockapetris, P., "Domain names interchange formats - implementation Information interchange
              - Representation of dates and
              specification", STD 13, RFC 1035, November 1987. times", ISO Standard 8601,
              June 1988.

   [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.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC3023]  Murata, M., St. Laurent, S. and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

   [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.NOTE-datetime-19980827]
              Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C
              NOTE NOTE-datetime-19980827, August 1998.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A. and I. Jacobs, "HTML 4.01
              Specification", W3C REC REC-html401-19991224, December
              1999.

   [W3C.REC-xhtml-basic-20001219]
              Baker, M., Ishikawa, M., Matsui, S., Stark, P., Wugofski,
              T. and T. Yamakami, "XHTML Basic", W3C
              REC REC-xhtml-basic-20001219, December 2000.

   [W3C.REC-xml-20040204]
              Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T. and
              E. Maler, "Extensible Markup Language (XML) 1.0 (Third
              Edition)", W3C REC REC-xml-20040204, February 2004.

   [W3C.REC-xml-infoset-20011024]
              Tobin, R. and J.

   [W3C.REC-xml-infoset-20040204]
              Cowan, J. and R. Tobin, "XML Information Set", Set (Second
              Edition)", W3C
              FirstEdition REC-xml-infoset-20011024, October 2001. REC REC-xml-infoset-20040204, February
              2004.

   [W3C.REC-xml-names-19990114]
              Hollander, D., Bray, T. and A. Layman, "Namespaces in
              XML", W3C REC REC-xml-names-19990114, January 1999.

   [W3C.REC-xmlbase-20010627]
              Marsh, J., "XML Base", W3C REC REC-xmlbase-20010627, June
              2001.

   [W3C.REC-xmldsig-core-20020212]
              Solo, D., Reagle, J. and D. Eastlake, "XML-Signature
              Syntax and Processing", W3C REC REC-xmldsig-core-20020212,
              February 2002.

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

11.2

   [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.

9.2  Informative References

   [RELAX-NG]
              OASIS Technical Committee: RELAX NG, "RELAX NG
              Specification", December 2001.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

URIs

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

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

Authors' Addresses

   Mark Nottingham (editor)

   EMail:

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

   Robert Sayre (editor)
   Boswijck Memex Consulting

   EMail:

   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.  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.

Appendix B.  Collected RELAX NG Compact Schema

   This appendix is informative.

   # -*- Relax NG -*-

   namespace local = ""

   namespace
     atom =
      "http://purl.org/atom/ns#draft-ietf-atompub-format-04" "http://purl.org/atom/ns#draft-ietf-atompub-format-06"

   namespace s = "http://www.ascc.net/xml/schematron"

   start = atomFeed | atomEntry

   #

   ## Attribute definitions

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

   atomVersionAttribute =
    attribute version {"draft-ietf-atompub-format-05 : do not deploy"}

   #

   ## Common Atom Constructs

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

   atomXHTMLTextConstruct =
      atomCommonAttributes,
      attribute type { "XHTML" },
      (text|anyElement)*

   atomTextConstruct =
      atomPlainTextConstruct | atomXHTMLTextConstruct

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

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime

   atomServiceConstruct =
      atomCommonAttributes,
      attribute href { atomUri }

   atomIdentityConstruct =
      atomCommonAttributes,
      (atomUri)
   ## Container elements

   # atom:feed
   # TODO: Test for multiple atom:link/@rel='alternate' with
   # the same @type The following tests are simple to do,
   # but my validator is giving me trouble.
   # TODO: Debug and add them back
   # Test for at least one atom:link/@rel='alternate'
   # Test for atom:author or all atom:entry have atom:author

   # atom:feed

   atomFeed =
      element atom:feed {
         atomCommonAttributes,
         atomVersionAttribute,
         (atomHead
          & atomEntry*
          & anyElement*)
      }

   # atom:head

   atomHead =
      [
         s:rule [
            context = "atom:head" "atom:feed"
   	 s:assert [
               test = "atom:link[@rel='alternate']"
   	    "An atom:feed must have at least one link element
   	    with a rel attribute of 'alternate'."
            ]
         ]
         s:rule [
            context = "atom:head" "atom:feed"
            s:assert [
      test = "atom:author or not(../atom:entry[count(atom:author) = 0])"
      "An atom:feed must have an atom:author unless all of
      its atom:entry children have an atom:author."
            ]
         ]
      ]

      element atom:head atom:feed {
         atomCommonAttributes,
         (atomTitle
          & atomUpdated
          & atomLink+
          & atomId?
         (atomAuthor?
          & atomAuthor? atomCategory*
          & atomContributor*
          & atomTagline? atomCopyright?
          & atomGenerator?
          & atomInfo? atomIcon?
          & atomCopyright? atomId?
          & atomCategory* atomImage?
          & atomLink+
          & atomIntrospection? atomSubtitle?
          & atomPost? atomTitle
          & anyElement*) atomUpdated
          & anyElement* ),
         atomEntry*
      }

   # atom:entry
   # TODO: Test for multiple atom:link @rel='alternate'
   # with the same @type
   # TODO: Test for required atom:summary

   atomEntry =
      [
         s:rule [
            context = "/atom:entry"
            s:assert [
               test = "@version"
               "The version attribute is required on standalone
                atom:entry elements."
            ]
         ]
         s:rule [
            context = "atom:entry"
   	 s:assert [
               test = "atom:link[@rel='alternate']"
               "An atom:entry must have at least one link element
                with a rel attribute of 'alternate'."
            ]
         ]
         s:rule [
            context = "atom:entry"
            s:assert [
               test = "atom:author or ../atom:author"
   	    "An atom:entry must have an atom:author
   	    if the parent atom:feed does not."
            ]
         ]

      ]

      element atom:entry {
         atomCommonAttributes,
         atomVersionAttribute?,
         (atomTitle
          & atomId
          & atomLink*
         (atomAuthor?
          & atomUpdated
          & atomPublished? atomCategory*
          & atomAuthor? atomContent?
          & atomContributor*
          & atomHost?
          & atomCopyright?
          & atomCategory* atomId
          & atomLink*
          & atomPublished?
          & atomEdit? atomSourceFeed?
          & atomSummary?
          & atomContent? atomTitle
          & atomHead? atomUpdated &
          anyElement*)
      }
   # atom:title

   atomTitle atom:content

   atomInlineTextContent =
      element atom:title atom:content { atomTextConstruct }

   # atom:id

   atomId = element atom:id
         atomCommonAttributes,
         attribute type { atomIdentityConstruct "text | "html" | atomMediaType }?,
         (text)*
      }

   atomLink

   atomInlineXHTMLContent =
      element atom:link atom:content {
         atomCommonAttributes,
         attribute href { atomUri },
      attribute rel { atomNCName | atomUri }?,
      attribute type { "xhtml" | atomMediaType }?,
      attribute hreflang
         (text|anyElement)*
      }

   atomOutOfLineContent =
      element atom:content { atomLanguageTag }?,
         atomCommonAttributes,
         attribute title type { text "text" | "html" | "xhtml" | atomMediaType }?,
         attribute length src { text }?, atomUri },
         empty
      }

   # atom:updated
   # TODO: Test for a timezone that SHOULD be UTC

   atomUpdated = element atom:updated { atomDateConstruct }

   # atom:published
   atomPublished

   atomContent = element atom:published { atomDateConstruct } atomInlineTextContent
    | atomInlineXHTMLContent
    | atomOutOfLineContent

   ## Metadata Elements

   # atom:author

   atomAuthor = element atom:author { atomPersonConstruct }

   # atom:contributor

   atomContributor atom:category

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

   # atom:host

   # TODO: tighten this up
   atomHost atom:contributor
   atomContributor = element atom:host atom:contributor { text atomPersonConstruct }

   # atom:copyright

   atomCopyright = element atom:copyright { atomTextConstruct }

   # atom:category

   atomCategory atom:generator

   atomGenerator = element atom:category atom:generator {
      atomCommonAttributes,
      attribute term { text },
      attribute scheme uri { atomUri }?,
      attribute label version { text }?,
      empty
      text
   }

   # atom:icon

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

   # atom:id

   atomId = element atom:id {  }

   # atom:summary

   atomSummary atom:image

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

   # atom:content

   atomInlineTextContent atom:link

   atomLink = element atom:content atom:link {
      atomCommonAttributes,
      attribute type href { "TEXT" | "HTML" atomUri },
      attribute rel { atomNCName | atomMediaType atomUri }?,
         (text)*
      }

   atomInlineXHTMLContent =
      element atom:content {
         atomCommonAttributes,
      attribute type { "XHTML" | atomMediaType }?,
         (text|anyElement)*
      }
   atomOutOfLineContent =
      element atom:content
      attribute hreflang {
         atomCommonAttributes, atomLanguageTag }?,
      attribute type title { "TEXT" | "HTML" | "XHTML" | atomMediaType text }?,
      attribute src length { atomUri }, text }?,
      empty
   }

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomOutOfLineContent

   # atom:introspection

   atomIntrospection atom:published

   atomPublished = element atom:introspection atom:published { atomServiceConstruct atomDateConstruct }

   # atom:post

   atomPost atom:source-feed

   atomSourceFeed =
      element atom:post atom:source-feed { atomServiceConstruct
         atomCommonAttributes,
         (  atomTitle
         & atomUpdated
         & atomLink+
         & atomIcon
         & atomId?
         & atomImage?
         & atomSubtitle?
         & atomAuthor?
         & atomContributor*
         & atomCopyright?
         & atomCategory*
         & atomGenerator?
         & anyElement* )
      }

   # atom:edit

   atomEdit atom:subtitle

   atomSubtitle = element atom:edit atom:subtitle { atomServiceConstruct atomTextConstruct }

   # atom:tagline

   atomTagline atom:summary

   atomSummary = element atom:tagline atom:summary { atomTextConstruct }

   # atom:generator

   atomGenerator atom:title

   atomTitle = element atom:generator {
      atomCommonAttributes,
      attribute url { atomUri }?,
      attribute version atom:title { text }?,
      text atomTextConstruct }

   # atom:info

   atomInfo atom:updated
   # TODO: Test for a timezone that SHOULD be UTC

   atomUpdated = element atom:info atom:updated { atomTextConstruct atomDateConstruct }

   # Low-level simple types

   # TODO: can anything more specific be said about these types?

   atomNCName = xsd:string { minLength = "1" pattern = "[^:]*" }
   atomMediaType = text
   atomLanguageTag = text
   atomUri = text
   atomEmailAddress = text

   # Extensibility

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

   anyForeignAttribute =
      attribute * - (atom:* | local:* | xml:*) { text }

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

   # EOF

Appendix C.  Revision History

   [[ this  Change Log

   [[anchor85: This section should be removed before final publication.  ]]
   publication.]]

   -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.
      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).
      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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