path: root/doc/rfc/rfc4287.txt

Network Working Group                                 M. Nottingham, Ed.
Request for Comments: 4287                                 R. Sayre, Ed.
Category: Standards Track                                  December 2005


                      The Atom Syndication Format

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

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

Table of Contents

   1. Introduction ....................................................3
      1.1. Examples ...................................................3
      1.2. Namespace and Version ......................................5
      1.3. Notational Conventions .....................................5
   2. Atom Documents ..................................................6
   3. Common Atom Constructs ..........................................7
      3.1. Text Constructs ............................................7
           3.1.1. The "type" Attribute ................................8
      3.2. Person Constructs .........................................10
           3.2.1. The "atom:name" Element ............................10
           3.2.2. The "atom:uri" Element .............................10
           3.2.3. The "atom:email" Element ...........................10
      3.3. Date Constructs ...........................................10
   4. Atom Element Definitions .......................................11
      4.1. Container Elements ........................................11
           4.1.1. The "atom:feed" Element ............................11
           4.1.2. The "atom:entry" Element ...........................13
           4.1.3. The "atom:content" Element .........................14
      4.2. Metadata Elements .........................................17
           4.2.1. The "atom:author" Element ..........................17
           4.2.2. The "atom:category" Element ........................18
           4.2.3. The "atom:contributor" Element .....................18



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           4.2.4. The "atom:generator" Element .......................18
           4.2.5. The "atom:icon" Element ............................19
           4.2.6. The "atom:id" Element ..............................19
           4.2.7. The "atom:link" Element ............................21
           4.2.8. The "atom:logo" Element ............................23
           4.2.9. The "atom:published" Element .......................23
           4.2.10. The "atom:rights" Element .........................24
           4.2.11. The "atom:source" Element .........................24
           4.2.12. The "atom:subtitle" Element .......................25
           4.2.13. The "atom:summary" Element ........................25
           4.2.14. The "atom:title" Element ..........................25
           4.2.15. The "atom:updated" Element ........................25
   5. Securing Atom Documents ........................................26
      5.1. Digital Signatures ........................................26
      5.2. Encryption ................................................27
      5.3. Signing and Encrypting ....................................28
   6. Extending Atom .................................................28
      6.1. Extensions from Non-Atom Vocabularies .....................28
      6.2. Extensions to the Atom Vocabulary .........................28
      6.3. Processing Foreign Markup .................................28
      6.4. Extension Elements ........................................29
           6.4.1. Simple Extension Elements ..........................29
           6.4.2. Structured Extension Elements ......................29
   7. IANA Considerations ............................................30
      7.1. Registry of Link Relations ................................31
   8. Security Considerations ........................................31
      8.1. HTML and XHTML Content ....................................31
      8.2. URIs ......................................................31
      8.3. IRIs ......................................................31
      8.4. Spoofing ..................................................31
      8.5. Encryption and Signing ....................................32
   9. References .....................................................32
      9.1. Normative References ......................................32
      9.2. Informative References ....................................34
   Appendix A. Contributors ..........................................35
   Appendix B. RELAX NG Compact Schema ...............................35















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1.  Introduction

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

   The primary use case that Atom addresses is the syndication of Web
   content such as weblogs and news headlines to Web sites as well as
   directly to user agents.

1.1.  Examples

   A brief, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom">

     <title>Example Feed</title>
     <link href="http://example.org/"/>
     <updated>2003-12-13T18:30:02Z</updated>
     <author>
       <name>John Doe</name>
     </author>
     <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

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

   </feed>
















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   A more extensive, single-entry Atom Feed Document:

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom">
     <title type="text">dive into mark</title>
     <subtitle type="html">
       A &lt;em&gt;lot&lt;/em&gt; of effort
       went into making this effortless
     </subtitle>
     <updated>2005-07-31T12:29:29Z</updated>
     <id>tag:example.org,2003:3</id>
     <link rel="alternate" type="text/html"
      hreflang="en" href="http://example.org/"/>
     <link rel="self" type="application/atom+xml"
      href="http://example.org/feed.atom"/>
     <rights>Copyright (c) 2003, Mark Pilgrim</rights>
     <generator uri="http://www.example.com/" version="1.0">
       Example Toolkit
     </generator>
     <entry>
       <title>Atom draft-07 snapshot</title>
       <link rel="alternate" type="text/html"
        href="http://example.org/2005/04/02/atom"/>
       <link rel="enclosure" type="audio/mpeg" length="1337"
        href="http://example.org/audio/ph34r_my_podcast.mp3"/>
       <id>tag:example.org,2003:3.2397</id>
       <updated>2005-07-31T12:29:29Z</updated>
       <published>2003-12-13T08:29:29-04:00</published>
       <author>
         <name>Mark Pilgrim</name>
         <uri>http://example.org/</uri>
         <email>f8dy@example.com</email>
       </author>
       <contributor>
         <name>Sam Ruby</name>
       </contributor>
       <contributor>
         <name>Joe Gregorio</name>
       </contributor>
       <content type="xhtml" xml:lang="en"
        xml:base="http://diveintomark.org/">
         <div xmlns="http://www.w3.org/1999/xhtml">
           <p><i>[Update: The Atom draft is finished.]</i></p>
         </div>
       </content>
     </entry>
   </feed>




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1.2.  Namespace and Version

   The XML Namespaces URI [W3C.REC-xml-names-19990114] for the XML data
   format described in this specification is:

   http://www.w3.org/2005/Atom

   For convenience, this data format may be referred to as "Atom 1.0".
   This specification uses "Atom" internally.

1.3.  Notational Conventions

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

   This specification uses the namespace prefix "atom:" for the
   Namespace URI identified in Section 1.2, above.  Note that the choice
   of namespace prefix is arbitrary and not semantically significant.

   Atom is specified using terms from the XML Infoset
   [W3C.REC-xml-infoset-20040204].  However, this specification uses a
   shorthand for two common terms: the phrase "Information Item" is
   omitted when naming Element Information Items and Attribute
   Information Items.  Therefore, when this specification uses the term
   "element," it is referring to an Element Information Item in Infoset
   terms.  Likewise, when it uses the term "attribute," it is referring
   to an Attribute Information Item.

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

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













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2.  Atom Documents

   This specification describes two kinds of Atom Documents: Atom Feed
   Documents and Atom Entry Documents.

   An Atom Feed Document is a representation of an Atom feed, including
   metadata about the feed, and some or all of the entries associated
   with it.  Its root is the atom:feed element.

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

   namespace atom = "http://www.w3.org/2005/Atom"
   start = atomFeed | atomEntry

   Both kinds of Atom Documents are specified in terms of the XML
   Information Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and
   identified with the "application/atom+xml" media type.  Atom
   Documents MUST be well-formed XML.  This specification does not
   define a DTD for Atom Documents, and hence does not require them to
   be valid (in the sense used by XML).

   Atom allows the use of IRIs [RFC3987].  Every URI [RFC3986] is also
   an IRI, so a URI may be used wherever below an IRI is named.  There
   are two special considerations: (1) when an IRI that is not also a
   URI is given for dereferencing, it MUST be mapped to a URI using the
   steps in Section 3.1 of [RFC3987] and (2) when an IRI is serving as
   an atom:id value, it MUST NOT be so mapped, so that the comparison
   works as described in Section 4.2.6.1.

   Any element defined by this specification MAY have an xml:base
   attribute [W3C.REC-xmlbase-20010627].  When xml:base is used in an
   Atom Document, it serves the function described in section 5.1.1 of
   [RFC3986], establishing the base URI (or IRI) for resolving any
   relative references found within the effective scope of the xml:base
   attribute.

   Any element defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its descendents.  The language context is only
   significant for elements and attributes declared to be "Language-
   Sensitive" by this specification.  Requirements regarding the content
   and interpretation of xml:lang are specified in XML 1.0
   [W3C.REC-xml-20040204], Section 2.12.







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

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

   Atom Processors MAY keep state 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.

3.  Common Atom Constructs

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

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

   Note that there MUST NOT be any white space in a Date construct or in
   any IRI.  Some XML-emitting implementations erroneously insert white
   space around values by default, and such implementations will emit
   invalid Atom Documents.

3.1.  Text Constructs

   A Text construct contains human-readable text, usually in small
   quantities.  The content of Text constructs is Language-Sensitive.

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

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

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct





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3.1.1.  The "type" Attribute

   Text constructs MAY have a "type" attribute.  When present, the value
   MUST be one of "text", "html", or "xhtml".  If the "type" attribute
   is not provided, Atom Processors MUST behave as though it were
   present with a value of "text".  Unlike the atom:content element
   defined in Section 4.1.3, MIME media types [MIMEREG] MUST NOT be used
   as values for the "type" attribute on Text constructs.

3.1.1.1.  Text

   Example atom:title with text content:

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

   If the value is "text", the content of the Text construct MUST NOT
   contain child elements.  Such text is intended to be presented to
   humans in a readable fashion.  Thus, Atom Processors MAY collapse
   white space (including line breaks) and display the text using
   typographic techniques such as justification and proportional fonts.

3.1.1.2.  HTML

   Example atom:title with HTML content:

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

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









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3.1.1.3.  XHTML

   Example atom:title with XHTML content:

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

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


   Examples of valid XHTML content:

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

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

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




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3.2.  Person Constructs

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

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

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

3.2.1.  The "atom:name" Element

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

3.2.2.  The "atom:uri" Element

   The "atom:uri" element's content conveys an IRI associated with the
   person.  Person constructs MAY contain an atom:uri element, but MUST
   NOT contain more than one.  The content of atom:uri in a Person
   construct MUST be an IRI reference [RFC3987].

3.2.3.  The "atom:email" Element

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

3.3.  Date Constructs

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

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime




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

   Example Date constructs:

   <updated>2003-12-13T18:30:02Z</updated>
   <updated>2003-12-13T18:30:02.25Z</updated>
   <updated>2003-12-13T18:30:02+01:00</updated>
   <updated>2003-12-13T18:30:02.25+01:00</updated>

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

4.  Atom Element Definitions

4.1.  Container Elements

4.1.1.  The "atom:feed" Element

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

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

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



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

   o  atom:feed elements MUST contain one or more atom:author elements,
      unless all of the atom:feed element's child atom:entry elements
      contain at least one atom:author element.
   o  atom:feed elements MAY contain any number of atom:category
      elements.
   o  atom:feed elements MAY contain any number of atom:contributor
      elements.
   o  atom:feed elements MUST NOT contain more than one atom:generator
      element.
   o  atom:feed elements MUST NOT contain more than one atom:icon
      element.
   o  atom:feed elements MUST NOT contain more than one atom:logo
      element.
   o  atom:feed elements MUST contain exactly one atom:id element.
   o  atom:feed elements SHOULD contain one atom:link element with a rel
      attribute value of "self".  This is the preferred URI for
      retrieving Atom Feed Documents representing this Atom feed.
   o  atom:feed elements MUST NOT contain more than one atom:link
      element with a rel attribute value of "alternate" that has the
      same combination of type and hreflang attribute values.
   o  atom:feed elements MAY contain additional atom:link elements
      beyond those described above.
   o  atom:feed elements MUST NOT contain more than one atom:rights
      element.
   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.

   If multiple atom:entry elements with the same atom:id value appear in
   an Atom Feed Document, they represent the same entry.  Their
   atom:updated timestamps SHOULD be different.  If an Atom Feed
   Document contains multiple entries with the same atom:id, Atom
   Processors MAY choose to display all of them or some subset of them.
   One typical behavior would be to display only the entry with the
   latest atom:updated timestamp.

4.1.1.1.  Providing Textual Content

   Experience teaches that feeds that contain textual content are in
   general more useful than those that do not.  Some applications (one
   example is full-text indexers) require a minimum amount of text or
   (X)HTML to function reliably and predictably.  Feed producers should
   be aware of these issues.  It is advisable that each atom:entry
   element contain a non-empty atom:title element, a non-empty



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   atom:content element when that element is present, and a non-empty
   atom:summary element when the entry contains no atom:content element.
   However, the absence of atom:summary is not an error, and Atom
   Processors MUST NOT fail to function correctly as a consequence of
   such an absence.

4.1.2.  The "atom:entry" Element

   The "atom:entry" element represents an individual entry, acting as a
   container for metadata and data associated with the entry.  This
   element can appear as a child of the atom:feed element, or it can
   appear as the document (i.e., top-level) element of a stand-alone
   Atom Entry Document.

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

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

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

   o  atom:entry elements MUST contain one or more atom:author elements,
      unless the atom:entry contains an atom:source element that
      contains an atom:author element or, in an Atom Feed Document, the
      atom:feed element contains an atom:author element itself.
   o  atom:entry elements MAY contain any number of atom:category
      elements.
   o  atom:entry elements MUST NOT contain more than one atom:content
      element.
   o  atom:entry elements MAY contain any number of atom:contributor
      elements.



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   o  atom:entry elements MUST contain exactly one atom:id element.
   o  atom:entry elements that contain no child atom:content element
      MUST contain at least one atom:link element with a rel attribute
      value of "alternate".
   o  atom:entry elements MUST NOT contain more than one atom:link
      element with a rel attribute value of "alternate" that has the
      same combination of type and hreflang attribute values.
   o  atom:entry elements MAY contain additional atom:link elements
      beyond those described above.
   o  atom:entry elements MUST NOT contain more than one atom:published
      element.
   o  atom:entry elements MUST NOT contain more than one atom:rights
      element.
   o  atom:entry elements MUST NOT contain more than one atom:source
      element.
   o  atom:entry elements MUST contain an atom:summary element in either
      of the following cases:
      *  the atom:entry contains an atom:content that has a "src"
         attribute (and is thus empty).
      *  the atom:entry contains content that is encoded in Base64;
         i.e., the "type" attribute of atom:content is a MIME media type
         [MIMEREG], but is not an XML media type [RFC3023], does not
         begin with "text/", and does not end with "/xml" or "+xml".
   o  atom:entry elements MUST NOT contain more than one atom:summary
      element.
   o  atom:entry elements MUST contain exactly one atom:title element.
   o  atom:entry elements MUST contain exactly one atom:updated element.

4.1.3.  The "atom:content" Element

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

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

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





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

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

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomInlineOtherContent
    | atomOutOfLineContent

4.1.3.1.  The "type" Attribute

   On the atom:content element, the value of the "type" attribute MAY be
   one of "text", "html", or "xhtml".  Failing that, it MUST conform to
   the syntax of a MIME media type, but MUST NOT be a composite type
   (see Section 4.2.6 of [MIMEREG]).  If neither the type attribute nor
   the src attribute is provided, Atom Processors MUST behave as though
   the type attribute were present with a value of "text".

4.1.3.2.  The "src" Attribute

   atom:content MAY have a "src" attribute, whose value MUST be an IRI
   reference [RFC3987].  If the "src" attribute is present, atom:content
   MUST be empty.  Atom Processors MAY use the IRI to retrieve the
   content and MAY choose to ignore remote content or to present it in a
   different manner than local content.

   If the "src" attribute is present, the "type" attribute SHOULD be
   provided and MUST be a MIME media type [MIMEREG], rather than "text",
   "html", or "xhtml".  The value is advisory; that is to say, when the
   corresponding URI (mapped from an IRI, if necessary) is dereferenced,
   if the server providing that content also provides a media type, the
   server-provided media type is authoritative.








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4.1.3.3.  Processing Model

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

   1.  If the value of "type" is "text", the content of atom:content
       MUST NOT contain child elements.  Such text is intended to be
       presented to humans in a readable fashion.  Thus, Atom Processors
       MAY collapse white space (including line breaks), and display the
       text using typographic techniques such as justification and
       proportional fonts.

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

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

   4.  If the value of "type" is an XML media type [RFC3023] or ends
       with "+xml" or "/xml" (case insensitive), the content of
       atom:content MAY include child elements and SHOULD be suitable
       for handling as the indicated media type.  If the "src" attribute
       is not provided, this would normally mean that the "atom:content"
       element would contain a single child element that would serve as
       the root element of the XML document of the indicated type.

   5.  If the value of "type" begins with "text/" (case insensitive),
       the content of atom:content MUST NOT contain child elements.

   6.  For all other values of "type", the content of atom:content MUST
       be a valid Base64 encoding, as described in [RFC3548], section 3.
       When decoded, it SHOULD be suitable for handling as the indicated
       media type.  In this case, the characters in the Base64 encoding
       MAY be preceded and followed in the atom:content element by white
       space, and lines are separated by a single newline (U+000A)
       character.





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4.1.3.4.  Examples

   XHTML inline:

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

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

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

4.2.  Metadata Elements

4.2.1.  The "atom:author" Element

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

   atomAuthor = element atom:author { atomPersonConstruct }

   If an atom:entry element does not contain atom:author elements, then
   the atom:author elements of the contained atom:source element are
   considered to apply.  In an Atom Feed Document, the atom:author
   elements of the containing atom:feed element are considered to apply
   to the entry if there are no atom:author elements in the locations
   described above.







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4.2.2.  The "atom:category" Element

   The "atom:category" element conveys information about a category
   associated with an entry or feed.  This specification assigns no
   meaning to the content (if any) of this element.

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

4.2.2.1.  The "term" Attribute

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

4.2.2.2.  The "scheme" Attribute

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

4.2.2.3.  The "label" Attribute

   The "label" attribute provides a human-readable label for display in
   end-user applications.  The content of the "label" attribute is
   Language-Sensitive.  Entities such as "&amp;" and "&lt;" represent
   their corresponding characters ("&" and "<", respectively), not
   markup.  Category elements MAY have a "label" attribute.

4.2.3.  The "atom:contributor" Element

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

   atomContributor = element atom:contributor { atomPersonConstruct }

4.2.4.  The "atom:generator" Element

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






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

   The content of this element, when present, MUST be a string that is a
   human-readable name for the generating agent.  Entities such as
   "&amp;" and "&lt;" represent their corresponding characters ("&" and
   "<" respectively), not markup.

   The atom:generator element MAY have a "uri" attribute whose value
   MUST be an IRI reference [RFC3987].  When dereferenced, the resulting
   URI (mapped from an IRI, if necessary) SHOULD produce a
   representation that is relevant to that agent.

   The atom:generator element MAY have a "version" attribute that
   indicates the version of the generating agent.

4.2.5.  The "atom:icon" Element

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

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

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

4.2.6.  The "atom:id" Element

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

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

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



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

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

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

   o  Provide the scheme in lowercase characters.
   o  Provide the host, if any, in lowercase characters.
   o  Only perform percent-encoding where it is essential.
   o  Use uppercase A through F characters when percent-encoding.
   o  Prevent dot-segments from appearing in paths.
   o  For schemes that define a default authority, use an empty
      authority if the default is desired.
   o  For schemes that define an empty path to be equivalent to a path
      of "/", use "/".
   o  For schemes that define a port, use an empty port if the default
      is desired.
   o  Preserve empty fragment identifiers and queries.
   o  Ensure that all components of the IRI are appropriately character
      normalized, e.g., by using NFC or NFKC.

4.2.6.1.  Comparing atom:id

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

   As a result, two IRIs that resolve to the same resource but are not
   character-for-character identical will be considered different for
   the purposes of identifier comparison.

   For example, these are four distinct identifiers, despite the fact
   that they differ only in case:





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      http://www.example.org/thing
      http://www.example.org/Thing
      http://www.EXAMPLE.org/thing
      HTTP://www.example.org/thing

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

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

4.2.7.  The "atom:link" Element

   The "atom:link" element defines a reference from an entry or feed to
   a Web resource.  This specification assigns no meaning to the content
   (if any) of this element.

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

4.2.7.1.  The "href" Attribute

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

4.2.7.2.  The "rel" Attribute

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

   The value of "rel" MUST be a string that is non-empty and matches
   either the "isegment-nz-nc" or the "IRI" production in [RFC3987].
   Note that use of a relative reference other than a simple name is not
   allowed.  If a name is given, implementations MUST consider the link
   relation type equivalent to the same name registered within the IANA



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   Registry of Link Relations (Section 7), and thus to the IRI that
   would be obtained by appending the value of the rel attribute to the
   string "http://www.iana.org/assignments/relation/".  The value of
   "rel" describes the meaning of the link, but does not impose any
   behavioral requirements on Atom Processors.

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

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

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

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

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

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

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

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

4.2.7.3.  The "type" Attribute

   On the link element, the "type" attribute's value is an advisory
   media type: it is a hint about the type of the representation that is
   expected to be returned when the value of the href attribute is
   dereferenced.  Note that the type attribute does not override the
   actual media type returned with the representation.  Link elements
   MAY have a type attribute, whose value MUST conform to the syntax of
   a MIME media type [MIMEREG].



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4.2.7.4.  The "hreflang" Attribute

   The "hreflang" attribute's content describes the language of the
   resource pointed to by the href attribute.  When used together with
   the rel="alternate", it implies a translated version of the entry.
   Link elements MAY have an hreflang attribute, whose value MUST be a
   language tag [RFC3066].

4.2.7.5.  The "title" Attribute

   The "title" attribute conveys human-readable information about the
   link.  The content of the "title" attribute is Language-Sensitive.
   Entities such as "&amp;" and "&lt;" represent their corresponding
   characters ("&" and "<", respectively), not markup.  Link elements
   MAY have a title attribute.

4.2.7.6.  The "length" Attribute

   The "length" attribute indicates an advisory length of the linked
   content in octets; it is a hint about the content length of the
   representation returned when the IRI in the href attribute is mapped
   to a URI and dereferenced.  Note that the length attribute does not
   override the actual content length of the representation as reported
   by the underlying protocol.  Link elements MAY have a length
   attribute.

4.2.8.  The "atom:logo" Element

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

   atomLogo = element atom:logo {
      atomCommonAttributes,
      (atomUri)
   }

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

4.2.9.  The "atom:published" Element

   The "atom:published" element is a Date construct indicating an
   instant in time associated with an event early in the life cycle of
   the entry.

   atomPublished = element atom:published { atomDateConstruct }





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

4.2.10.  The "atom:rights" Element

   The "atom:rights" element is a Text construct that conveys
   information about rights held in and over an entry or feed.

   atomRights = element atom:rights { atomTextConstruct }

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

   If an atom:entry element does not contain an atom:rights element,
   then the atom:rights element of the containing atom:feed element, if
   present, is considered to apply to the entry.

4.2.11.  The "atom:source" Element

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

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




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   The atom:source element is designed to allow the aggregation of
   entries from different feeds while retaining information about an
   entry's source feed.  For this reason, Atom Processors that are
   performing such aggregation SHOULD include at least the required
   feed-level Metadata elements (atom:id, atom:title, and atom:updated)
   in the atom:source element.

4.2.12.  The "atom:subtitle" Element

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

   atomSubtitle = element atom:subtitle { atomTextConstruct }

4.2.13.  The "atom:summary" Element

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

   atomSummary = element atom:summary { atomTextConstruct }

   It is not advisable for the atom:summary element to duplicate
   atom:title or atom:content because Atom Processors might assume there
   is a useful summary when there is none.

4.2.14.  The "atom:title" Element

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

   atomTitle = element atom:title { atomTextConstruct }

4.2.15.  The "atom:updated" Element

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

   atomUpdated = element atom:updated { atomDateConstruct }

   Publishers MAY change the value of this element over time.









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5.  Securing Atom Documents

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

   Producers of feeds and/or entries, and intermediaries who aggregate
   feeds and/or entries, may have sound reasons for signing and/or
   encrypting otherwise-unprotected content.  For example, a merchant
   might digitally sign a message that contains a discount coupon for
   its products.  A bank that uses Atom to deliver customer statements
   is very likely to want to sign and encrypt those messages to protect
   their customers' financial information and to assure the customer of
   their authenticity.  Intermediaries may want to encrypt aggregated
   feeds so that a passive observer cannot tell what topics the
   recipient is interested in.  Of course, many other examples exist as
   well.

   The algorithm requirements in this section pertain to the Atom
   Processor.  They require that a recipient, at a minimum, be able to
   handle messages that use the specified cryptographic algorithms.
   These requirements do not limit the algorithms that the sender can
   choose.

5.1.  Digital Signatures

   The root of an Atom Document (i.e., atom:feed in an Atom Feed
   Document, atom:entry in an Atom Entry Document) or any atom:entry
   element MAY have an Enveloped Signature, as described by XML-
   Signature and Syntax Processing [W3C.REC-xmldsig-core-20020212].

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

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

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

   Section 6.5.1 of [W3C.REC-xmldsig-core-20020212] requires support for
   Canonical XML [W3C.REC-xml-c14n-20010315].  However, many
   implementers do not use it because signed XML documents enclosed in
   other XML documents have their signatures broken.  Thus, Atom
   Processors that verify signed Atom Documents MUST be able to



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   canonicalize with the exclusive XML canonicalization method
   identified by the URI "http://www.w3.org/2001/10/xml-exc-c14n#", as
   specified in Exclusive XML Canonicalization
   [W3C.REC-xml-exc-c14n-20020718].

   Intermediaries such as aggregators may need to add an atom:source
   element to an entry that does not contain its own atom:source
   element.  If such an entry is signed, the addition will break the
   signature.  Thus, a publisher of individually-signed entries should
   strongly consider adding an atom:source element to those entries
   before signing them.  Implementers should also be aware of the issues
   concerning the use of markup in the "xml:" namespace as it interacts
   with canonicalization.

   Section 4.4.2 of [W3C.REC-xmldsig-core-20020212] requires support for
   DSA signatures and recommends support for RSA signatures.  However,
   because of the much greater popularity in the market of RSA versus
   DSA, Atom Processors that verify signed Atom Documents MUST be able
   to verify RSA signatures, but do not need be able to verify DSA
   signatures.  Due to security issues that can arise if the keying
   material for message authentication code (MAC) authentication is not
   handled properly, Atom Documents SHOULD NOT use MACs for signatures.

5.2.  Encryption

   The root of an Atom Document (i.e., atom:feed in an Atom Feed
   Document, atom:entry in an Atom Entry Document) MAY be encrypted,
   using the mechanisms described by XML Encryption Syntax and
   Processing [W3C.REC-xmlenc-core-20021210].

   Section 5.1 of [W3C.REC-xmlenc-core-20021210] requires support of
   TripleDES, AES-128, and AES-256.  Atom Processors that decrypt Atom
   Documents MUST be able to decrypt with AES-128 in Cipher Block
   Chaining (CBC) mode.

   Encryption based on [W3C.REC-xmlenc-core-20021210] does not ensure
   integrity of the original document.  There are known cryptographic
   attacks where someone who cannot decrypt a message can still change
   bits in a way where part or all the decrypted message makes sense but
   has a different meaning.  Thus, Atom Processors that decrypt Atom
   Documents SHOULD check the integrity of the decrypted document by
   verifying the hash in the signature (if any) in the document, or by
   verifying a hash of the document within the document (if any).








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5.3.  Signing and Encrypting

   When an Atom Document is to be both signed and encrypted, it is
   generally a good idea to first sign the document, then encrypt the
   signed document.  This provides integrity to the base document while
   encrypting all the information, including the identity of the entity
   that signed the document.  Note that, if MACs are used for
   authentication, the order MUST be that the document is signed and
   then encrypted, and not the other way around.

6.  Extending Atom

6.1.  Extensions from Non-Atom Vocabularies

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

6.2.  Extensions to the Atom Vocabulary

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

6.3.  Processing Foreign Markup

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

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

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




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6.4.  Extension Elements

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

6.4.1.  Simple Extension Elements

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

   simpleExtensionElement =
      element * - atom:* {
         text
      }

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

6.4.2.  Structured Extension Elements

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

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

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






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   This specification does not provide an interpretation of a Structured
   Extension element.  The syntax of the XML contained in the element
   (and an interpretation of how the element relates to its containing
   element) is defined by the specification of the Atom 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 semantics identical to the charset
         parameter of the "application/xml" media type as specified in
         [RFC3023].
   Encoding considerations:  Identical to those of "application/xml" as
      described in [RFC3023], Section 3.2.
   Security considerations:  As defined in this specification.
      In addition, as this media type uses the "+xml" convention, it
      shares the same security considerations as described in [RFC3023],
      Section 10.
   Interoperability considerations:  There are no known interoperability
      issues.
   Published specification:  This specification.
   Applications that use this media type:  No known applications
      currently use this media type.

   Additional information:

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









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7.1.  Registry of Link Relations

   This registry is maintained by IANA and initially contains five
   values: "alternate", "related", "self", "enclosure", and "via".  New
   assignments are subject to IESG Approval, as outlined in [RFC2434].
   Requests should be made by email to IANA, which will then forward the
   request to the IESG, requesting approval.  The request should use the
   following template:

   o  Attribute Value: (A value for the "rel" attribute that conforms to
      the syntax rule given in Section 4.2.7.2)
   o  Description:
   o  Expected display characteristics:
   o  Security considerations:

8.  Security Considerations

8.1.  HTML and XHTML Content

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

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

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

8.2.  URIs

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

8.3.  IRIs

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

8.4.  Spoofing

   Atom Processors should be aware of the potential for spoofing attacks
   where the attacker publishes an atom:entry with the atom:id value of
   an entry from another feed, perhaps with a falsified atom:source



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   element duplicating the atom:id of the other feed.  For example, an
   Atom Processor could suppress display of duplicate entries by
   displaying only one entry from a set of entries with identical
   atom:id values.  In that situation, the Atom Processor might also
   take steps to determine whether the entries originated from the same
   publisher before considering them duplicates.

8.5.  Encryption and Signing

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

   Digital signatures provide authentication, message integrity, and
   non-repudiation with proof of origin.  Encryption provides data
   confidentiality.

9.  References

9.1.  Normative References

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

   [MIMEREG]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

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

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

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

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

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

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




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

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

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

   [W3C.REC-xml-20040204]
              Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T.,
              and E. Maler, "Extensible Markup Language (XML) 1.0 (Third
              Edition)", W3C REC REC-xml-20040204, February 2004,
              <http://www.w3.org/TR/2004/REC-xml-20040204>.

   [W3C.REC-xml-c14n-20010315]
              Boyer, J., "Canonical XML Version 1.0", W3C REC REC-xml-
              c14n-20010315, March 2001,
              <http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.

   [W3C.REC-xml-exc-c14n-20020718]
              Eastlake, D., Boyer, J., and J. Reagle, "Exclusive XML
              Canonicalization Version 1.0", W3C REC REC-xml-exc-c14n-
              20020718, July 2002,
              <http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718>.

   [W3C.REC-xml-infoset-20040204]
              Cowan, J. and R. Tobin, "XML Information Set (Second
              Edition)", W3C REC REC-xml-infoset-20040204,
              February 2004,
              <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.

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

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

   [W3C.REC-xmldsig-core-20020212]
              Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
              Syntax and Processing", W3C REC REC-xmldsig-core-20020212,
              February 2002,
              <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212>.



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

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

9.2.  Informative References

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

   [RELAX-NG] Clark, J., "RELAX NG Compact Syntax", December 2001,
              <http://www.oasis-open.org/committees/relax-ng/
              compact-20021121.html>.

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

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

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













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Appendix A.  Contributors

   The following people contributed to preliminary versions 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 Atompub Working Group.

   The Atompub Working Group has dozens of very active contributors who
   proposed ideas and wording for this document, including:

   Danny Ayers, James Aylett, Roger Benningfield, Arve Bersvendsen, Tim
   Bray, Dan Brickley, Thomas Broyer, Robin Cover, Bill de hOra, Martin
   Duerst, Roy Fielding, Joe Gregorio, Bjoern Hoehrmann, Paul Hoffman,
   Anne van Kesteren, Brett Lindsley, Dare Obasanjo, David Orchard,
   Aristotle Pagaltzis, John Panzer, Graham Parks, Dave Pawson, Mark
   Pilgrim, David Powell, Julian Reschke, Phil Ringnalda, Antone Roundy,
   Sam Ruby, Eric Scheid, Brent Simmons, Henri Sivonen, Ray Slakinski,
   James Snell, Henry Story, Asbjorn Ulsberg, Walter Underwood, Norman
   Walsh, Dave Winer, and Bob Wyman.

Appendix B.  RELAX NG Compact Schema

   This appendix is informative.

   The Relax NG schema explicitly excludes elements in the Atom
   namespace that are not defined in this revision of the specification.
   Requirements for Atom Processors encountering such markup are given
   in Sections 6.2 and 6.3.

   # -*- rnc -*-
   # RELAX NG Compact Syntax Grammar for the
   # Atom Format Specification Version 11

   namespace atom = "http://www.w3.org/2005/Atom"
   namespace xhtml = "http://www.w3.org/1999/xhtml"
   namespace s = "http://www.ascc.net/xml/schematron"
   namespace local = ""

   start = atomFeed | atomEntry

   # Common attributes

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

   # Text Constructs



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

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

   atomTextConstruct = atomPlainTextConstruct | atomXHTMLTextConstruct

   # Person Construct

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

   # Date Construct

   atomDateConstruct =
      atomCommonAttributes,
      xsd:dateTime

   # atom:feed

   atomFeed =
      [
         s:rule [
            context = "atom:feed"
            s:assert [
               test = "atom:author or not(atom:entry[not(atom:author)])"
               "An atom:feed must have an atom:author unless all "
               ~ "of its atom:entry children have an atom:author."
            ]
         ]
      ]
      element atom:feed {
         atomCommonAttributes,
         (atomAuthor*
          & atomCategory*
          & atomContributor*
          & atomGenerator?
          & atomIcon?
          & atomId



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          & atomLink*
          & atomLogo?
          & atomRights?
          & atomSubtitle?
          & atomTitle
          & atomUpdated
          & extensionElement*),
         atomEntry*
      }

   # atom:entry

   atomEntry =
      [
         s:rule [
            context = "atom:entry"
            s:assert [
               test = "atom:link[@rel='alternate'] "
               ~ "or atom:link[not(@rel)] "
               ~ "or atom:content"
               "An atom:entry must have at least one atom:link element "
               ~ "with a rel attribute of 'alternate' "
               ~ "or an atom:content."
            ]
         ]
         s:rule [
            context = "atom:entry"
            s:assert [
               test = "atom:author or "
               ~ "../atom:author or atom:source/atom:author"
               "An atom:entry must have an atom:author "
               ~ "if its feed does not."
            ]
         ]
      ]
      element atom:entry {
         atomCommonAttributes,
         (atomAuthor*
          & atomCategory*
          & atomContent?
          & atomContributor*
          & atomId
          & atomLink*
          & atomPublished?
          & atomRights?
          & atomSource?
          & atomSummary?
          & atomTitle



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          & atomUpdated
          & extensionElement*)
      }

   # atom:content

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

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

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

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

   atomContent = atomInlineTextContent
    | atomInlineXHTMLContent
    | atomInlineOtherContent
    | atomOutOfLineContent

   # atom:author

   atomAuthor = element atom:author { atomPersonConstruct }

   # atom:category

   atomCategory =
      element atom:category {



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         atomCommonAttributes,
         attribute term { text },
         attribute scheme { atomUri }?,
         attribute label { text }?,
         undefinedContent
      }

   # atom:contributor

   atomContributor = element atom:contributor { atomPersonConstruct }

   # atom:generator

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

   # atom:icon

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

   # atom:id

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

   # atom:logo

   atomLogo = element atom:logo {
      atomCommonAttributes,
      (atomUri)
   }

   # atom:link

   atomLink =
      element atom:link {
         atomCommonAttributes,
         attribute href { atomUri },
         attribute rel { atomNCName | atomUri }?,



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         attribute type { atomMediaType }?,
         attribute hreflang { atomLanguageTag }?,
         attribute title { text }?,
         attribute length { text }?,
         undefinedContent
      }

   # atom:published

   atomPublished = element atom:published { atomDateConstruct }

   # atom:rights

   atomRights = element atom:rights { atomTextConstruct }

   # atom:source

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

   # atom:subtitle

   atomSubtitle = element atom:subtitle { atomTextConstruct }

   # atom:summary

   atomSummary = element atom:summary { atomTextConstruct }

   # atom:title

   atomTitle = element atom:title { atomTextConstruct }

   # atom:updated



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   atomUpdated = element atom:updated { atomDateConstruct }

   # Low-level simple types

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

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

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

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

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

   # Simple Extension

   simpleExtensionElement =
      element * - atom:* {
         text
      }

   # Structured Extension

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

   # Other Extensibility

   extensionElement =
      simpleExtensionElement | structuredExtensionElement

   undefinedAttribute =
     attribute * - (xml:base | xml:lang | local:*) { text }

   undefinedContent = (text|anyForeignElement)*




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

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

   # XHTML

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

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

   # EOF

Authors' Addresses

   Mark Nottingham (editor)

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


   Robert Sayre (editor)

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









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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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