Network Working Group                                 M. Nottingham, Ed.
Internet-Draft
Expires: July 11, 27, 2005                                     R. Sayre, Ed.
                                               Boswijck Memex Consulting
                                                        January 10, 26, 2005

                      The Atom Syndication Format
                      draft-ietf-atompub-format-04
                      draft-ietf-atompub-format-05

Status of this Memo

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   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 . . . . . . . . . . . . . . . . . . .   8   9
     3.1  Text Constructs  . . . . . . . . . . . . . . . . . . . . .   8   9
       3.1.1  "type" Attribute . . . . . . . . . . . . . . . . . . .   8   9
     3.2  Person Constructs  . . . . . . . . . . . . . . . . . . . .   9  10
       3.2.1  The "atom:name" Element  . . . . . . . . . . . . . . . . .   9  10
       3.2.2  The "atom:uri" Element . . . . . . . . . . . . . . . . . .   9  10
       3.2.3  The "atom:email" Element . . . . . . . . . . . . . . . . .   9  11
     3.3  Date Constructs  . . . . . . . . . . . . . . . . . . . . .   9  11
     3.4  Service Constructs . . . . . . . . . . . . . . . . . . . .   9  11
       3.4.1  The "href" Attribute . . . . . . . . . . . . . . . . . . .  10  11
     3.5  Link  Identity Constructs  . . . . . . . . . . . . . . . . . . . . .  10  11
       3.5.1  "rel" Attribute  .  Dereferencing Identity Constructs  . . . . . . . . . .  12
       3.5.2  Comparing Identity Constructs  . . . . . . . .  10
       3.5.2  "type" Attribute . . . .  12
   4.   Element Definitions  . . . . . . . . . . . . . . .  11
       3.5.3  "href" Attribute . . . . .  14
     4.1  The "atom:feed" Element  . . . . . . . . . . . . . .  11
       3.5.4  "hreflang" Attribute . . .  14
       4.1.1  The "version" Attribute  . . . . . . . . . . . . . .  11
       3.5.5  "title" Attribute .  14
     4.2  The "atom:head" Element  . . . . . . . . . . . . . . . . .  11
       3.5.6  "length" Attribute  14
       4.2.1  Usage of "atom:head" within "atom:entry" . . . . . . .  16
     4.3  The "atom:entry" Element . . . . . . . . . . .  11
     3.6  Identity Constructs . . . . . .  16
     4.4  The "atom:title" Element . . . . . . . . . . . . .  12
       3.6.1  Dereferencing Identity Constructs . . . .  18
     4.5  The "atom:id" Element  . . . . . .  12
       3.6.2  Comparing Identity Constructs . . . . . . . . . . . .  12
     3.7  18
     4.6  The Category Construct . "atom:link" Element  . . . . . . . . . . . . . . . . .  13
       3.7.1  18
       4.6.1  The "term" "href" Attribute . . . . . . . . . . . . . . . . .  13
       3.7.2  19
       4.6.2  The "scheme" "rel" Attribute  . . . . . . . . . . . . . . . .  13
       3.7.3  The "label" attribute  . .  19
       4.6.3  The "type" Attribute . . . . . . . . . . . . . .  13
   4.   The "atom:feed" Element . . .  20
       4.6.4  The "hreflang" Attribute . . . . . . . . . . . . . . .  14
     4.1  "version"  20
       4.6.5  The "title" Attribute  . . . . . . . . . . . . . . . . . . .  14
     4.2  20
       4.6.6  The "atom:head" Element  . "length" Attribute . . . . . . . . . . . . . . . .  14
       4.2.1  "atom:title"  20
     4.7  The "atom:updated" Element . . . . . . . . . . . . . . . . .  14
       4.2.2  "atom:link"  20
     4.8  The "atom:published" Element . . . . . . . . . . . . . . . . .  14
       4.2.3  "atom:category"  21
     4.9  The "atom:author" Element  . . . . . . . . . . . . . . .  15
       4.2.4  "atom:introspection" .  21
     4.10   The "atom:contributor" Element . . . . . . . . . . . . .  15
       4.2.5  "atom:post"  21
     4.11   The "atom:host" Element  . . . . . . . . . . . . . . . . .  15
       4.2.6  "atom:author"  21
     4.12   The "atom:copyright" Element . . . . . . . . . . . . . . . .  15
       4.2.7  "atom:contributor"  21
     4.13   The "atom:category" Element  . . . . . . . . . . . . . .  15
       4.2.8  "atom:tagline" Element . .  22
       4.13.1   The "term" Attribute . . . . . . . . . . . . . .  15
       4.2.9  "atom:id" Element . .  22
       4.13.2   The "scheme" Attribute . . . . . . . . . . . . . . .  22
       4.13.3   The "label" attribute  .  16
       4.2.10   "atom:generator" Element . . . . . . . . . . . . . .  16
       4.2.11   "atom:copyright"  22
     4.14   The "atom:summary" Element . . . . . . . . . . . . . .  16
       4.2.12   "atom:info" .  22
     4.15   The "atom:content" Element . . . . . . . . . . . . . . .  23
       4.15.1   The "type" attribute .  16
       4.2.13   "atom:updated" Element . . . . . . . . . . . . . . .  16
   5.  23
       4.15.2   The "atom:entry" Element . . . "src" attribute  . . . . . . . . . . . . . . .  18
     5.1  "atom:title" Element .  23
       4.15.3   Processing Model . . . . . . . . . . . . . . . . . .  18
     5.2  "atom:link"  24
     4.16   The "atom:introspection" Element . . . . . . . . . . . . . . . . . . .  18
     5.3  "atom:category"  25
     4.17   The "atom:post" Element  . . . . . . . . . . . . . . . . .  18
     5.4  25
     4.18   The "atom:edit" Element  . . . . . . . . . . . . . . . . . . .  19
     5.5  "atom:author" Element  . . . . . . . . . . . . . . . . . .  19
     5.6  "atom:contributor" Element . . . . . . . . . . . . . . . .  19
     5.7  "atom:host" Element  . . . . . . . . . . . . . . . . . . .  19
     5.8  "atom:id" Element  . . . . . . . . . . . . . . . . . . . .  19
     5.9  "atom:updated"  25
     4.19   The "atom:tagline" Element . . . . . . . . . . . . . . . . . .  19
     5.10   "atom:published"  25
     4.20   The "atom:generator" Element . . . . . . . . . . . . . . . .  20
     5.11   "atom:summary"  25
     4.21   The "atom:info" Element  . . . . . . . . . . . . . . . . .  20
     5.12   "atom:content" Element . . . . . . . . .  26
   5.   Managing Feed State  . . . . . . . .  20
       5.12.1   "type" attribute . . . . . . . . . . . .  27
   6.   Securing Atom Documents  . . . . . .  20
       5.12.2   "src" attribute . . . . . . . . . . . .  28
     6.1  Digital Signatures . . . . . .  21
       5.12.3   Processing Model . . . . . . . . . . . . . .  28
     6.2  Encryption . . . .  21
     5.13   "atom:copyright" Element . . . . . . . . . . . . . . . .  22
     5.14   "atom:head" Element . . . .  28
   7.   Embedding Atom in Other Formats  . . . . . . . . . . . . . .  22
   6.   Managing Feed State  29
   8.   Extending Atom . . . . . . . . . . . . . . . . . . . .  23
   7.   Securing Atom Documents . . .  30
   9.   IANA Considerations  . . . . . . . . . . . . . . .  24
     7.1  Digital Signatures . . . . .  31
     9.1  Registry of Link Relations . . . . . . . . . . . . . . .  24
     7.2  Encryption .  31
   10.  Security Considerations  . . . . . . . . . . . . . . . . . .  33
   11.  References . . . . .  24
   8.   Embedding Atom in Other Formats . . . . . . . . . . . . . .  25
   9.   Extending Atom . . . . . .  34
   11.1   Normative References . . . . . . . . . . . . . . . . .  26
   10.  IANA Considerations . .  34
   11.2   Informative References . . . . . . . . . . . . . . . . . .  27
     10.1   Registry of Link Relations  35
        Authors' Addresses . . . . . . . . . . . . . . .  27
   11.  Security Considerations . . . . . .  36
   A.   Contributors . . . . . . . . . . . .  29
   12.  References . . . . . . . . . . . .  37
   B.   Collected RELAX NG Compact Schema  . . . . . . . . . . . . .  30
   12.1   Normative References  38
   C.   Revision History . . . . . . . . . . . . . . . . . . .  30
   12.2   Informative References . . .  45
        Intellectual Property and Copyright Statements . . . . . . . . . . . . . . .  31
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  32
   A.   Contributors . . . . . . . . . . . . . . . . . . . . . . . .  33
   B.   Revision History . . . . . . . . . . . . . . . . . . . . . .  34
        Intellectual Property and Copyright Statements . . . . . . .  36  47

1.  Introduction

   Atom is an XML-based document format intended to allow which describes lists of related information,
   information known as "feeds".  Feeds are composed of a number of
   items, known as "entries", each with an extensible set of attached
   metadata.  For example, each entry has a title.

   The primary use case that Atom addresses is the syndication of Web
   content such as Weblogs and news headlines to Web sites as well as
   directly to user agents.  However, nothing precludes it from being
   used for other purposes and kinds of content.

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

   [[ more motivation / design principles ]]

1.1  Editorial Notes

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

   Discussion of this draft happens in two fora;

      The mailing list [1]
      The Atom Wiki Web site [2]

   Active development takes place on the mailing list, while the Wiki is
   used for issue tracking and new proposals.

   This document is an early draft and known to be incomplete.  Topics
   marked [[like this]] indicate where additional text is likely to be
   added.

1.2  Example

   A minimal, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="UTF-8"?>
   <feed version="draft-ietf-atompub-format-04
   : do version="draft-ietf-atompub-format-05:do not deploy"
    xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-04
   ">
   xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-05">
     <head>
       <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>
       <updated>2003-12-13T18:30:02Z</updated>
     </entry>
   </feed>

1.3  Conformance

   [[ talk about atom documents and atom consumers, and how requirements
   are placed on them ]]

1.4  Notational Conventions

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

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML elements and attribute names.  It uses the
   following namespace prefixes for the indicated namespace URIs;

   "atom":  http://purl.org/atom/ns#draft-ietf-atompub-format-04  http://purl.org/atom/ns#draft-ietf-atompub-format-05

   Note that the choice of any namespace prefix is arbitrary and not
   semantically significant.

   Atom is specified using terms from the XML Infoset
   [W3C.REC-xml-infoset-20011024].  However, this specification uses a
   shorthand for two common terms; the phrase "Information Item" is
   omitted when naming Element Information Items and Attribute
   Information Items.

   Therefore, when this specification uses the term "element," it is
   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 is atom:feed.

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

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

   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.

   [[ Validity? ]]

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

   Any element in an Atom Document MAY have an xml:base attribute.  XML
   Base [W3C.REC-xmlbase-20010627] processing MUST be applied to any
   relative reference [RFC2396bis] [RFC3986] present in an Atom Document.  This
   includes such elements and attributes as specified by Atom itself, as
   well as those specified by extensions to Atom.

   Any element in an Atom Document MAY have an xml:lang attribute, whose
   content indicates the default natural language of the element's content.
   Requirements regarding the content and interpretation of xml:lang are
   specified in XML 1.0 [W3C.REC-xml-20040204] Section 2.12.

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

   [[ discussion of URI escaping and i18n, IRI ]]

   [[ discussion of white space ]]

   Atom is extensible.  See the section titled 'Extending Atom' later in
   this document for a full description of how Atom Documents can be
   extended.

3.  Common Atom Constructs

   Many of Atom's elements share a few common structures.  This section
   defines a few such structures and their requirements, 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.

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

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

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct

3.1.1  "type" Attribute

   Text constructs MAY have a "type" attribute.  When present, the value
   MUST be one of "TEXT", "HTML" or "XHTML".  If the "type" attribute is
   not provided, software MUST behave as though it were present with a
   value of "TEXT".

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

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

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

   [[example atom entry w/ escaped markup]]

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

3.2  Person Constructs

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

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

   This specification assigns no significance to the order of appearance
   of the child elements of 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.  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 associated with the
   person.  Person constructs MAY contain an atom:uri element, but MUST
   NOT contain more than one.  The content of atom:uri in a Person
   construct MUST be a URI reference [RFC2396bis]. [RFC3986].

   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 constructs MAY contain an
   atom:email element, but MUST NOT contain more than one.  Its content
   MUST be an e-mail address [RFC2822].

3.3  Date Constructs

   A Date construct is an element whose content MUST conform to the
   date-time BNF rule in [RFC3339].

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime

3.4  Service Constructs

   A Service construct is an empty element that conveys the URI of an
   Atom Publishing Protocol [Atom-protocol] service associated with an
   entry or feed.  The type of service is identified by the element
   name.

   A Service construct has the following attribute:

   atomServiceConstruct =
      atomCommonAttributes,
      attribute href { atomUri }

3.4.1  The "href" Attribute

   The "href" attribute contains the a URI pointing to the endpoint of the service named by the name attribute. service.  atom:service
   elements MUST have a "href" attribute, whose value MUST be a URI
   reference
   [RFC2396bis]. [RFC3986].

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

3.5  Link  Identity Constructs

   A Link

   An Identity construct is an empty element that describes whose content conveys a connection from
   an Atom Document to another Web resource.

3.5.1  "rel" Attribute

   Link constructs MAY have an optional "rel" attribute that indicates
   the link relation type.  If the "rel" attribute is not present,
   permanent, universally unique identifier for the
   link construct construct's parent.
   Its content MUST be interpreted a URI, as if defined by [RFC3986].  Note that the link relation type is
   "alternate".

   rel_attribute = segment-nz-nc / URI

   The value
   definition of "rel" MUST be either a name, which "URI" excludes relative references.

   When an Atom document is non-empty and does
   not contain any colon (":") characters, relocated, migrated, syndicated,
   republished, exported or a URI [RFC2396bis].  Note
   that use of a relative reference to imported, the "rel" value URI is not
   allowed.  If a name is given, implementations content of its Identity
   construct MUST consider the link
   relation type to be equivalent NOT change.  Put another way, an Identity construct
   pertains to all instantiations of a particular Atom entry or feed;
   revisions retain the same name registered within the
   IANA Registry content in their Identity constructs.

   atomIdentityConstruct =
      atomCommonAttributes,
      (atomUri)

3.5.1  Dereferencing Identity Constructs

   The content of Link Relations Section 10, and thus the URI that
   would an Identity construct MAY be obtained by appending the value of the rel attribute dereferencable (e.g.  an
   HTTP URI).  However, processors MUST NOT assume it to the
   string "http://www.iana.org/assignments/relation/". be
   dereferencable.

   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 content of Link
   Relations:

   The value "alternate" signifies an Identity construct MUST be created in a way that
   assures uniqueness, and it is suggested that the URI in Identity construct
   be stored along with the value associated resource.

   Because of the href
   attribute identifies an alternate version risk of the resource described
   by the containing element.

   The value "related" signifies confusion between URIs that would be
   equivalent if dereferenced, the URI following normalization strategy is
   strongly encouraged when generating Identity constructs:

   o  Provide the scheme in lowercase characters.
   o  Provide the value of the href
   attribute identifies a resource to which the resource described by
   the containing atom:feed or atom:entry element host, if any, in lowercase characters.
   o  Only perform percent-encoding where it is related. essential.
   o  Use uppercase A-through-F characters when percent-encoding.
   o  Prevent dot-segments appearing in paths.
   o  For
   example, the feed for schemes that define a site which discusses the performance of default authority, use an empty
      authority if 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 default is desired.
   o  For schemes that define an empty path to be equivalent to a child path
      of any atom:entry whose
   content contains "/", use "/".
   o  For schemes that define a discussion of port, use an empty port if the default
      is desired.
   o  Preserve empty fragment identifiers and queries.
   o  Ensure that same search engine. all portions of the URI are UTF-8 encoded NFC form
      Unicode strings.

3.5.2  "type" Attribute

   Link  Comparing Identity Constructs

   Instances of Identity constructs MAY have a type attribute, whose value MUST conform can be compared to the syntax of a MIME media type [RFC2045].

   The type attribute's value is determine whether
   an advisory media type; it MAY be used entry or feed is the same as one seen before.  Processors MUST
   compare Identity constructs on a hint character-by-character basis in a
   case-sensitive fashion.

   As a result, two URIs that resolve to determine the type of the representation which is
   expected to same resource but are not
   character-for-character identical will be returned when considered different for
   the value purposes of the href attribute is
   dereferenced.  Note that the type attribute does not override the
   actual media type returned with the representation.

3.5.3  "href" Attribute

   The "href" attribute contains the link's URI.  Link constructs MUST
   have a href attribute, whose value MUST 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 a URI reference
   [RFC2396bis].

   xml:base [W3C.REC-xmlbase-20010627] processing MUST 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 applied to considered different identifiers, because URI %-escaping
   is significant for the
   href attribute's content.

3.5.4  "hreflang" Attribute purposes of comparison.

4.  Element Definitions

4.1  The "hreflang" attribute's content describes "atom:feed" Element

   The "atom:feed" element is the language document (i.e., top-level) element of the
   resource pointed to by the href attribute.  When used together with
   the rel="alternate", it implies
   an Atom Feed Document, acting as a translated version of container for metadata and data
   associated with the entry.
   Link constructs MAY have an hreflang attribute, whose value feed.  Its first element child MUST be a
   language tag [RFC3066].

3.5.5  "title" Attribute

   The "title" attribute conveys human-readable information about the
   link.  Link constructs atom:head,
   which MAY be followed zero or more atom:entry child elements.

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

4.1.1  The "version" Attribute

   atom:feed elements MUST have a title attribute.

3.5.6  "length" Attribute

   The "length" "version" attribute whose content
   indicates an advisory length the version of the linked
   content in octets; it MAY be used as a hint to determine Atom specification that the feed
   conforms to.  The content
   length of the representation returned when the URI in the href this attribute is dereferenced.  Note that the length unstructured text.

   The version identifier for this specification is
   "draft-ietf-atompub-format-05: do not deploy".

   atomVersionAttribute =
    attribute does version {"draft-ietf-atompub-format-05 : do not
   override the actual content length of the representation as reported
   by the underlying protocol.

   Link constructs MAY have a length attribute.

3.6  Identity Constructs

   An Identity construct is an deploy"}

4.2  The "atom:head" Element

   The atom:head element whose content conveys acts as a
   permanent, universally unique identifier container for metadata about the construct's parent.
   Its content MUST be a URI, 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?
          & atomContributor*
          & atomTagline?
          & atomGenerator?
          & atomInfo?
          & atomCopyright?
          & atomCategory*
          & atomIntrospection?
          & atomPost?
          & anyElement*)
      }

   The following child elements are defined by [RFC2396bis].  Note this specification (note
   that the
   definition presence of "URI" excludes relative references.

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

   o  atom:head elements MUST contain exactly one atom:title element.
   o  atom:head elements MUST contain at least one atom:link element
      with a rel attribute value of its Identity
   construct "alternate".
   o  atom:head elements MUST NOT change.  Put another way, an Identity construct
   pertains to all instantiations of contain more than one atom:link
      element with a particular Atom entry or feed;
   revisions retain rel attribute value of "alternate" that has the
      same content in their Identity constructs.

3.6.1  Dereferencing Identity Constructs

   The content of an Identity construct MAY be dereferencable (e.g.  an
   HTTP URI).  However, processors MUST NOT assume it type attribute value.  If a feed's atom:link element with
      type="alternate" resolves to be
   dereferencable.

   The content of an Identity construct MUST be created in a way HTML document, then that
   assures uniqueness, and it is suggested document
      SHOULD have a autodiscovery link element [Atom-autodiscovery] that
      reflects back to the Identity construct
   be stored along with the associated resource.

   Because of the risk of confusion between URIs that would be
   equivalent if dereferenced, the following normalization strategy is
   strongly encouraged when generating Identity constructs:

   o  Provide the scheme in lowercase characters. feed.  atom:head elements MAY contain
      additional atom:link elements beyond those described above.
   o  Provide  atom:head elements MUST contain exactly one atom:author element,
      UNLESS all of the host, if any, in lowercase characters.
   o  Only perform percent-encoding where it is essential.
   o  Use uppercase A-through-F characters when percent-encoding.
   o  Prevent dot-segments appearing in paths. atom:feed element's child atom:entry elements
      contain an atom:author element.  atom:head elements MUST NOT
      contain more than one atom:author element.  [[inheritance]]
   o  For schemes that define a default authority, use  atom:head elements MUST NOT contain more than one an empty
      authority if the default is desired.
      atom:contributor element.
   o  For schemes that define  atom:head elements MUST NOT contain more than one an empty path to be equivalent to a path
      of "/", use "/".
      atom:copyright element.
   o  For schemes that define a port, use  atom:head elements MUST NOT contain more than one an empty port if the default
      is desired. atom:id
      element.
   o  Preserve empty fragment identifiers and queries.  atom:head elements MUST contain exactly one atom:updated element.
   o  Ensure that all portions of the URI are UTF-8 encoded NFC form
      Unicode strings.

3.6.2  Comparing Identity Constructs

   Instances  atom:head elements MAY contain any number of Identity constructs can be compared to determine whether
   an entry or feed is the same as atom:category
      elements.
   o  atom:head elements MUST NOT contain more than one seen before.  Processors atom:post
      element.

   o  atom:head elements MUST
   compare Identity constructs on a character-by-character basis in a
   case-sensitive fashion.

   As a result, two URIs that resolve to NOT contain more than one
      atom:introspection element.
   o  atom:head elements MUST NOT contain more than one atom:tagline
      element.
   o  atom:head elements MUST NOT contain more than one atom:generator
      element.
   o  atom:head elements MUST NOT contain more than one atom:info
      element.

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

   In this case, the same resource but are not
   character-for-character identical will be considered different atom:head element acts as a container for metadata
   about 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 feed within which the entry was created.

   If an atom:entry is significant for copied into one feed from another feed, then the purposes
   atom:head element of comparison.

3.7  The Category Construct

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

3.7.1  The "term" Attribute

   The "term" attribute will SHOULD be a string which identifies inserted into the category
   within
   atom:entry unless the categorization scheme to which entry, as copied, already contains an atom:head
   element.  If the entry or feed belongs.
   Category constructs MUST have a "term" attribute.

3.7.2  The "scheme" Attribute

   The is a URI that identifies a categorization scheme.  Category
   constructs MAY have a "scheme" attribute.

3.7.3  The "label" attribute

   The "label" attribute provides a human-readable label atom:entry already contains an atom:head, then that may
   atom:head SHOULD be
   displayed in end-user applications.  Category constructs MAY have a
   "label" attribute.

4. copied without modification.

   [[ ...  example ...  ]]

4.3  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, acting individual entry.  This
   element can appear as a container for metadata and data
   associated with the feed.  Its first element 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.

   When appearing in an Atom Entry Document, atom:entry child elements.

4.1  "version" Attribute

   atom:feed elements MUST
   have a "version" attribute whose content indicates the version of the
   Atom specification that the feed entry conforms to.

   The content of this attribute is unstructured text.

   The version identifier for this specification is
   "draft-ietf-atompub-format-04 :
   "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 atom:entry element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  This
   specification assigns no significance to the order of appearance of
   the child elements of atom:head. 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
   that it requires the presence of some of these elements):

   o  atom:entry elements is required):

4.2.1  "atom:title" Element

   The MUST have exactly one "atom:title" element is a Text construct that conveys a
   human-readable title for the feed.  atom:head element.
   o  atom:entry elements MUST contain exactly one atom:title atom:id element.

4.2.2  "atom:link" Element

   The "atom:link" element is a Link construct that conveys a URI
   associated with the feed.  The nature of the relationship is
   determined by the construct's rel attribute.

   atom:head
   o  atom:entry elements that contain no child atom:content element
      MUST contain at least one atom:link element with a rel attribute
      value of "alternate".

   atom:head  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  atom:entry
      elements MAY contain additional atom:link elements beyond those
      described above.

4.2.3  "atom:category" Element

   A Category Construct identifying a category with which the feed is
   associated.  atom:head elements MAY contain any number of
   atom:category elements.

4.2.4  "atom:introspection" Element

   The "atom:introspection" element is a Service construct that conveys
   the URI of an introspection file associated with the feed.  atom:head
   elements MUST NOT contain more than one atom:introspection element.

4.2.5  "atom:post" Element

   The "atom:post" element is a Service construct that conveys the URI
   used to add entries to the feed.  atom:head
   o  atom:entry elements MUST NOT contain
   more than one atom:post element.

4.2.6  "atom:author" Element

   The "atom:author" element is a Person construct that indicates the
   default author of the feed.  atom:head contain exactly one atom:updated element.
   o  atom:entry elements MUST NOT contain more than one atom:published
      element.
   o  atom:entry elements MUST contain exactly one atom:author element, UNLESS all of
      unless, in an Atom Feed Document, the atom:feed element's child
   atom:entry elements contain atom:head element contains
      an atom:author element.  atom:head element itself.  atom:entry elements MUST NOT
      contain more than one atom:author element.

   [[explain inheritance]]

4.2.7  "atom:contributor" Element

   The "atom:contributor" element is a Person construct that indicates a
   person or other entity who contributes to the feed.  atom:head
   o  atom:entry elements MAY MUST NOT contain one or more than one
      atom:contributor elements.

4.2.8  "atom:tagline" Element

   The "atom:tagline" element is a Text construct that conveys a
   human-readable description or tagline for the feed.  atom:head element.
   o  atom:entry elements MAY contain an atom:tagline element, but MUST NOT contain more than one.

4.2.9  "atom:id" Element

   The "atom:id" element is an Identity construct that conveys a
   permanent, universally unique identifier for a feed.  atom:head one atom:host
      element.
   o  atom:entry elements MAY contain an atom:id element, but MUST NOT contain more than one.

4.2.10  "atom:generator" Element

   The "atom:generator" element's content identifies the software agent
   used to generate the feed, for debugging and other purposes.
   atom:head one atom:copyright
      element.
   o  atom:entry elements MAY contain an atom:generator element, but any number of atom:category
      elements.

   o  atom:entry elements MUST
   NOT contain more than one.

   The content an atom:summary element in any of this element, when present, MUST be a string that is a
   human-readable name for
      the generating agent.

   The atom:generator element MAY have a "uri" attribute whose value
   MUST be a URI reference [RFC2396bis].  When dereferenced, that URI
   SHOULD produce a representation that is relevant to that agent.

   The atom:generator following cases:
      *  the atom:entry element MAY have contains no atom:content element.
      *  the atom:entry contains an atom:content which has a "version" "src"
         attribute that
   indicates the version of (and is thus empty).
      *  the generating agent.  When present, its
   value atom:entry contains content which is unstructured text.

4.2.11  "atom:copyright" Element

   The "atom:copyright" element encoded in Base64;
         i.e.  the "type" attribute of atom:content is Text construct that conveys a
   human-readable copyright statement for the feed.  atom:head MIME media type
         [RFC2045] and does not begin with "text/" nor end with "+xml".
   o  atom:entry elements
   MAY MUST NOT contain an atom:copyright element, but more than one atom:summary
      element.
   o  atom:entry elements MUST NOT contain more than
   one.

   The atom:copyright element SHOULD one atom:edit
      element.
   o  atom:entry elements MUST NOT be used to convey
   machine-readable licensing information.

4.2.12  "atom:info" contain more than one atom:head
      element.

4.4  The "atom:title" Element

   The "atom:info" "atom:title" element is a Text construct that conveys a
   human-readable explanation of the feed format itself.  atom:head
   elements MAY contain title for an atom:info element, but MUST NOT contain more
   than one.

   The atom:info entry or feed.

   atomTitle = element SHOULD NOT considered meaningful by processors;
   it is a convenience to publishers in certain situations.

4.2.13  "atom:updated" atom:title { atomTextConstruct }

4.5  The "atom:id" Element

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

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

5.
   permanent, universally unique identifier for an entry or feed.

   atomId = element atom:id { atomIdentityConstruct }

4.6  The "atom:entry" "atom:link" Element

   The "atom:entry" atom:link element represents is an individual entry.  This empty element can appear as that defines a child of the atom:feed element, or it can
   appear as reference
   from an Atom Document 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
   }

4.6.1  The "href" Attribute

   The "href" attribute contains the document (i.e., top-level) element of a standalone Atom
   Entry Document.

   When appearing in an Atom Entry Document, atom:entry elements link's URI.  Link constructs MUST
   have a "version" attribute href attribute, whose content 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 version of link
   relation type.  If the
   Atom specification that "rel" attribute is not present, the entry conforms to. link
   element MUST be interpreted as if the link relation type is
   "alternate".

   rel_attribute = segment-nz-nc / URI

   The version identifier for this specification value of "rel" MUST be either a name, which is
   "draft-ietf-atompub-format-04 : do non-empty and does
   not deploy".

   The atom:entry element MAY contain any namespace-qualified
   [W3C.REC-xml-names-19990114] elements as children.  This
   specification assigns no significance 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 order of appearance same name registered within the IANA Registry of Link
   Relations Section 9, and thus the child elements URI that would be obtained by
   appending the value of atom:entry. the rel attribute to the string
   "http://www.iana.org/assignments/relation/".  The following child elements are defined by this specification (note
   that it requires value of "rel"
   describes the presence meaning of some the link, but does not impose any behavioral
   requirements on implementations.

   This document defines two initial values for the Registry of these elements):

5.1  "atom:title" Element Link
   Relations:

   The "atom:title" element is a Text construct value "alternate" signifies that conveys a
   human-readable title for the entry.  atom:entry elements MUST have
   exactly one "atom:title" URI in the value of the href
   attribute identifies an alternate version of the resource described
   by the containing element.

5.2  "atom:link" Element

   The "atom:link" element is a Link construct value "related" signifies that conveys a the URI
   associated with in the entry.  The nature value of the relationship as well as href
   attribute identifies a resource related to the link itself is determined resource described by
   the element's content. containing atom:feed or atom:entry elements that contain no child atom:content element MUST
   contain element.  For example, the
   feed for a site which discusses the performance of the search engine
   at least one atom:link element with "http://search.example.com" might contain, as a rel attribute value child of
   "alternate".
   atom:head:

   <link rel="related" href="http://search.example.com/">
   An identical link might appear as a child of any atom:entry elements MUST NOT contain more than one atom:link element
   with whose
   content contains a rel attribute value discussion of "alternate" that has the same type
   attribute value.

   atom:entry search engine.

4.6.3  The "type" Attribute

   Link elements MAY contain additional atom:link elements beyond
   those described above.

5.3  "atom:category" Element

   A Category Construct identifying have a category with which type attribute, whose value MUST conform to
   the entry is
   associated.  atom:entry elements MAY contain any number syntax of
   atom:category elements.

5.4  "atom:edit" Element a MIME media type [RFC2045].

   The "atom:edit" element type attribute's value is an advisory media type; it is a Service construct hint
   about the type of the representation which is expected to be returned
   when the value of the href attribute is dereferenced.  Note that conveys the URI
   type attribute does not override the actual media type returned with
   the representation.

4.6.4  The "hreflang" Attribute

   The "hreflang" attribute's content describes the language of the
   resource pointed to by the href attribute.  When used to retrieve and edit together with
   the source representation rel="alternate", it implies a translated version of the entry.
   atom:entry
   Link elements MAY have an hreflang attribute, whose value MUST NOT contain more than one atom:edit element.

5.5  "atom:author" Element be a
   language tag [RFC3066].

4.6.5  The "atom:author" element is "title" Attribute

   The "title" attribute conveys human-readable information about the
   link.  Link elements MAY have a Person construct that title attribute.

4.6.6  The "length" Attribute

   The "length" attribute indicates an advisory length of the
   default author linked
   content in octets; it is a hint about the content length of the entry.  atom:entry elements MUST contain
   exactly one atom:author element, unless,
   representation returned when the URI in an Atom Feed Document, the atom:head element contains an atom:author element itself.
   atom:entry elements MUST NOT contain more than one atom:author
   element.

5.6  "atom:contributor" Element

   The "atom:contributor" element href attribute is a Person construct
   dereferenced.  Note that indicates a
   person or other entity who contributes to the entry.  atom:entry
   elements MAY contain one or more atom:contributor elements.

5.7  "atom:host" Element

   The "atom:host" element's length attribute does not override the
   actual content conveys a domain name or network
   address associated with length of the entry's origin.  atom:entry representation as reported by the
   underlying protocol.

   Link elements MAY
   contain a single atom:host element.  Its content MUST be a domain
   name [RFC1035], a dotted-decimal IPv4 address [RFC0791], or have a
   colon-delimited IPv6 address [RFC2460].

5.8  "atom:id" Element length attribute.

4.7  The "atom:id" element is an Identity construct that conveys a
   permanent, universally unique identifier for an entry.  atom:entry
   elements MUST contain exactly one atom:id element.

5.9 "atom:updated" Element

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

   atom:entry elements MUST contain exactly one a changed atom:updated element. value.

   Publishers MAY change the value of this element over time.
   Processors MAY present entries sorted using this value.  Processors
   MAY choose not to present entries until the instant in time specified
   in the atom:updated element has passed.

5.10

   atomUpdated = element atom:updated { atomDateConstruct }

4.8  The "atom:published" Element

   The "atom:published" element is a Date construct indicating an
   instant in time associated with an event early in the life cycle of
   the entry.  Typically, atom:published will be associated with the
   initial creation or first availability of the resource.

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

4.9  The "atom:author" Element

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

   atomAuthor = element atom:author { atomPersonConstruct }

4.10  The "atom:contributor" Element

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

   atomContributor = element atom:contributor { atomPersonConstruct }

4.11  The "atom:host" Element

   The "atom:host" element's content conveys a domain name or network
   address associated with an entry's origin.  Its content MUST be a
   domain name [RFC1035], a dotted-decimal IPv4 address [RFC0791], or a
   colon-delimited IPv6 address [RFC2460].

4.12  The "atom:copyright" Element

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

   The atom:copyright element SHOULD NOT be used to convey
   machine-readable licensing information.

   If an atom:entry element does not contain an atom:copyright element,
   then the atom:copyright element of the containing atom:feed element's
   atom:head element, if present, should be considered to apply to the
   entry.

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 }?,
      attribute label { text }?,
      empty
   }

4.13.1  The "term" Attribute

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

4.13.2  The "scheme" Attribute

   The "scheme"  attribute is a URI  that identifies a categorization
   scheme.  Category elements MAY contain an atom:published element, but MUST
   NOT contain more than one.

   Processors MAY present entries sorted using this value.  Processors
   MAY choose not to present entries until the instant in time specified have a "scheme" attribute.

4.13.3  The "label" attribute

   The "label" attribute provides a human-readable label that may be
   displayed in the atom:published element has passed.

5.11 end-user applications.  Category elements MAY have a
   "label" attribute.

4.14  The "atom:summary" Element

   The "atom:summary" element is a Text construct that conveys a short
   summary, abstract or excerpt of the entry.  atom:entry elements MAY
   contain an atom:summary element, but MUST NOT contain more than one.

   atom:entry elements MUST contain an atom:summary element in any of
   the following cases:

   o  the atom:entry entry.

   atomSummary = element contains no atom:content element.
   o  the atom:entry contains an atom:content which has a "src"
      attribute (and is thus empty).
   o  the atom:entry contains content which is encoded in Base64; i.e.
      the "type" attribute of atom:content is a MIME media type
      [RFC2045] and does not begin with "text/" nor end with "+xml".

5.12 atom:summary { atomTextConstruct }

4.15  The "atom:content" Element

   The "atom:content" element either contains or links to the content of
   the entry.  atom:entry elements MUST contain zero or one atom:content
   elements.

5.12.1

   atomInlineTextContent =
      element 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

4.15.1  The "type" attribute

   atom:content MAY have a "type" attribute, When present, the value MAY
   be one of "TEXT", "HTML", or "XHTML".  Failing that, it MUST be a
   MIME media type [RFC2045] in which, to use the terminology of Section
   5 of [RFC2045], the top level is a discrete type.  If the type
   attribute is not provided, software MUST behave as though it were
   present with a value of "TEXT".

5.12.2

4.15.2  The "src" attribute

   atom:content MAY have a "src" attribute, whose value MUST be a URI
   reference [RFC2396bis]. [RFC3986].  If the "src" attribute is present, software MAY
   use the URI to retrieve the content.  If the "src" attribute is
   present, atom:content MUST be empty.  That is to say, the content may
   be retrievable using "src=" URI, or it may be contained within
   atom:content, but not both.

   If the "src" attribute is present, the "type" attribute SHOULD be
   provided and MUST be a MIME media type [RFC2045], rather than "TEXT",
   "HTML", or "XHTML".  The value is advisory; that is to say, upon
   dereferencing the URI to retrieve the content, if the server
   providing that content also provides a media type, the
   server-provided media type is authoritative.

   If the value of type begins with "text/" or ends with "+xml", the
   content SHOULD be local; that is to say, no "src" attribute should be
   provided.

5.12.3

4.15.3  Processing Model

   Software MUST apply the following rules in succession in the order below to
   ascertain the rules governing the content of "atom:content".

   1.  If the value is "TEXT", the content of atom:content MUST NOT
       contain child elements.  Such text is intended to be presented to
       humans in a readable fashion.  Thus, software MAY display it
       using normal text rendering techniques such as proportional
       fonts, white-space collapsing, and justification.
   2.  If the value of "type" is "HTML", the content of atom:content
       MUST NOT contain child elements, and SHOULD be suitable for
       handling by software that knows HTML.  The HTML markup must be
       escaped; for example, "<br>" as "&lt;br>".  The HTML markup
       SHOULD be such that it could validly appear directly within an
       HTML <DIV> element.  Receiving software which displays the
       content SHOULD use the markup to aid in displaying it.
   3.  If the value of "type" is "XHTML", the content of atom:content
       MAY contain child elements.  The content SHOULD be XHTML text and
       markup that could validly appear directly within an xhtml:div
       element.  Receiving software which displays the content SHOULD
       use the markup to aid in displaying it.  Escaped markup is
       interpreted as a text representation of markup, and MUST NOT be
       interpreted as markup itself.
   4.  If the value of "type" ends with "+xml" or "/xml", the content of
       atom:content may include child elements, and SHOULD be suitable
       for handling by software that knows the indicated media type.  If
       the "src" attribute is not provided, this would normally mean
       that the "atom:content" element would contain a single child
       element which would serve as the root element of the XML document
       of the indicated type.
   5.  If the value of "type" begins with "text/" the content of
       atom:content MUST NOT contain child elements.

   6.  For all other values of "type", the content of atom:content MUST
       be a valid Base64 encoding [RFC3548], which when decoded SHOULD
       be suitable for handling by software that knows the indicated
       media type.  In this case, the characters in the Base64 encoding
       may be preceded Base64 encoding
       may be preceded and followed in the atom:content element by
       whitespace, and lines are separated by a single newline (U+000A)
       character.

4.16  The "atom:introspection" Element

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

   atomEdit = element by
       whitespace, and lines are separated by a single newline (U+000A)
       character, as required by XML.

5.13  "atom:copyright" atom:edit { atomServiceConstruct }

4.19  The "atom:tagline" Element

   The "atom:copyright" "atom:tagline" element is a Text construct that conveys a
   human-readable copyright statement description or tagline for the entry.  atom:entry
   elements MAY contain an atom:copyright element, but MUST NOT contain
   more than one.

   The atom:copyright feed.

   atomTagline = element SHOULD NOT be atom:tagline { atomTextConstruct }

4.20  The "atom:generator" Element

   The "atom:generator" element's content identifies the software agent
   used to convey
   machine-readable licensing information.

   If an atom:entry element does not contain an atom:copyright element,
   then the atom:copyright generate a feed, for debugging and other purposes.

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

   The content of the containing atom:feed element's
   atom:head this element, if when present, should MUST be considered to apply to the
   entry.

5.14  "atom:head" Element

   The atom:head element acts as a container string that is a
   human-readable name for metadata about the feed
   within which the entry was created.

   atom:entry elements MAY contain at most one atom:head element.  If the atom:head generating agent.

   The atom:generator element is present, it SHOULD MAY have a "uri" attribute whose value
   MUST be the first child
   element of atom:entry.

   If an atom:entry a URI reference [RFC3986].  When dereferenced, that URI
   SHOULD produce a representation that is copied into one feed from another feed, then the
   atom:head relevant to that agent.

   The atom:generator element of the source feed SHOULD be inserted into the
   atom:entry unless MAY have a "version" attribute that
   indicates the entry, as copied, already contains an atom:head
   element.  If version of the atom:entry already contains an atom:head, then generating agent.  When present, its
   value is unstructured text.

4.21  The "atom:info" Element

   The "atom:info" element is a Text construct that
   atom:head conveys a
   human-readable explanation of the feed format itself.  The atom:info
   element SHOULD NOT be copied without modification.

   [[ ...  example ...  ]]

6. considered meaningful by processors; it is a
   convenience to publishers.

   atomInfo = element atom:info { atomTextConstruct }

5.  Managing Feed State

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

7.

6.  Securing Atom Documents

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

   Note that while these mechanisms are available to secure Atom
   documents, they should not be used indiscriminately.

7.1

6.1  Digital Signatures

   The document element of an Atom document (i.e., atom:feed in an Atom
   Feed Document, atom:entry in an Atom Entry Document) MAY have an
   Enveloped Signature, as described by XML-Signature and Syntax
   Processing [W3C.REC-xmldsig-core-20020212].  Other XML signature
   mechanisms MUST NOT be used on the document element of an Atom
   document.

   Processors MUST NOT reject an Atom document containing such a
   signature because they are not capable of verifying it; they MUST
   continue processing and MAY inform the user of their failure to
   validate the signature.

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

   Other elements in an Atom document MUST NOT be signed unless their
   definitions explicitly specify such a capability.

7.2

6.2  Encryption

   The document element of an Atom document (i.e., atom:feed in an Atom
   Feed Document, atom:entry in an Atom Entry Document) MAY be
   encrypted, using the mechanisms described by XML Encryption Syntax
   and Processing [W3C.REC-xmlenc-core-20021210].  Other XML encryption
   mechanisms MUST NOT be used on the document element of an Atom
   document.

8.

7.  Embedding Atom in Other Formats

   [[ ...  ]]

9.

8.  Extending Atom

   [[ ...  ]]

10.

9.  IANA Considerations

   An Atom Document, when serialized as XML 1.0, can be identified with
   the following media type:

   MIME media type name: application
   MIME subtype name: atom+xml
   Mandatory parameters: None.
   Optional parameters:
      "charset": This parameter has identical semantics to the charset
         parameter of the "application/xml" media type as specified in
         RFC 3023 [RFC3023].  [RFC3023].
   Encoding considerations: Identical to those of "application/xml" as
      described in RFC 3023 [RFC3023], section 3.2.
   Security considerations: As defined in this specification.  [[update
      upon publication]]
      In addition, as this media type uses the "+xml" convention, it
      shares the same security considerations as described in RFC 3023
      [RFC3023], section 10.
   Interoperability considerations: There are no known interoperability
      issues.
   Published specification: This specification.  [[update upon
      publication]]
   Applications which use this media type: No known applications
      currently use this media type.

   Additional information:

   Magic number(s): As specified for "application/xml" in RFC 3023
      [RFC3023], section 3.2.
   File extension: .atom
   Fragment identifiers: As specified for "application/xml" in RFC 3023
      [RFC3023], section 5.
   Base URI: As specified in RFC 3023 [RFC3023], section 6.
   Macintosh File Type code: TEXT
   Person and email address to contact for further information: Mark
      Nottingham <mnot@pobox.com>
   Intended usage: COMMON
   Author/Change controller: This specification's author(s).  [[update
      upon publication]]

10.1

9.1  Registry of Link Relations

   This registry is maintained by IANA and initially contains the two
   values: "alternate" and "related".  New assignments are subject to
   IESG Approval, as outlined in [RFC2434].  Requests should be made by
   email to IANA, which will then forward the request to the IESG
   requesting approval.  The request should contain discussion of at
   least the following five topics:

   o  A value for the "rel" attribute that conforms to the syntax rule
      given in Section 3.5.1 4.6.2
   o  Common name for link type.
   o  Description of link type semantics.
   o  Expected display characteristics.
   o  Security considerations.

11.

10.  Security Considerations

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

12.

11.  References

12.1

11.1  Normative References

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

   [Atom-protocol]
              Gregorio, J. and R. Sayre, "The Atom Publishing Protocol",
              work-in-progress, July 2004.

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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

   [RFC2396bis]
              Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax",  awaiting RFC
              number, December 2004.

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

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

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

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

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

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

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

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

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

   [W3C.REC-xml-infoset-20011024]
              Tobin, R. and J. Cowan, "XML Information Set", W3C
              FirstEdition REC-xml-infoset-20011024, October 2001.

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

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

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

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

12.2

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

   Robert Sayre (editor)
   Boswijck Memex Consulting

   EMail: rfsayre@boswijck.com
   URI:   http://boswijck.com

Appendix A.  Contributors

   The following people contributed to preliminary drafts of this
   document: Tim Bray, Mark Pilgrim, and Sam Ruby.  Norman Walsh
   provided the Relax NG schema.  The content and concepts within are a
   product of the Atom community and 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"
   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)

   # 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"
            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"
            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 {
         atomCommonAttributes,
         (atomTitle
          & atomUpdated
          & atomLink+
          & atomId?
          & atomAuthor?
          & atomContributor*
          & atomTagline?
          & atomGenerator?
          & atomInfo?
          & atomCopyright?
          & atomCategory*
          & atomIntrospection?
          & atomPost?
          & anyElement*)
      }

   # 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 Atom
   Publishing Format and Protocol Working Group. parent atom:feed does not."
            ]
         ]
      ]
      element atom:entry {
         atomCommonAttributes,
         atomVersionAttribute?,
         (atomTitle
          & atomId
          & atomLink*
          & atomUpdated
          & atomPublished?
          & atomAuthor?
          & atomContributor*
          & atomHost?
          & atomCopyright?
          & atomCategory*
          & atomEdit?
          & atomSummary?
          & atomContent?
          & atomHead?
          & anyElement*)
      }

   # atom:title

   atomTitle = element atom:title { atomTextConstruct }

   # atom:id

   atomId = element atom:id { atomIdentityConstruct }

   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
   }

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

   atomUpdated = element atom:updated { atomDateConstruct }

   # atom:published
   atomPublished = element atom:published { atomDateConstruct }

   # atom:author

   atomAuthor = element atom:author { atomPersonConstruct }

   # atom:contributor

   atomContributor = element atom:contributor { atomPersonConstruct }

   # atom:host

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

   # atom:copyright

   atomCopyright = element atom:copyright { atomTextConstruct }

   # atom:category

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

   # atom:summary

   atomSummary = element atom:summary { atomTextConstruct }

   # atom:content

   atomInlineTextContent =
      element 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

   # atom:introspection

   atomIntrospection =
             element atom:introspection { atomServiceConstruct }

   # atom:post

   atomPost = element atom:post { atomServiceConstruct }

   # atom:edit

   atomEdit = element atom:edit { atomServiceConstruct }

   # atom:tagline

   atomTagline = element atom:tagline { atomTextConstruct }

   # atom:generator

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

   # atom:info

   atomInfo = element atom:info { atomTextConstruct }
   # 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 B. C.  Revision History

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

   -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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Disclaimer of Validity

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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