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rfc:rfc4287

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

Nottingham & Sayre Standards Track [Page 1] RFC 4287 Atom Format December 2005

         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

Nottingham & Sayre Standards Track [Page 2] RFC 4287 Atom Format December 2005

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>

Nottingham & Sayre Standards Track [Page 3] RFC 4287 Atom Format December 2005

 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>

Nottingham & Sayre Standards Track [Page 4] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 5] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 6] RFC 4287 Atom Format December 2005

 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

Nottingham & Sayre Standards Track [Page 7] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 8] RFC 4287 Atom Format December 2005

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>
 ...

Nottingham & Sayre Standards Track [Page 9] RFC 4287 Atom Format December 2005

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

Nottingham & Sayre Standards Track [Page 10] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 11] RFC 4287 Atom Format December 2005

 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

Nottingham & Sayre Standards Track [Page 12] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 13] RFC 4287 Atom Format December 2005

 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
    }

Nottingham & Sayre Standards Track [Page 14] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 15] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 16] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 17] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 18] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 19] RFC 4287 Atom Format December 2005

 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:

Nottingham & Sayre Standards Track [Page 20] RFC 4287 Atom Format December 2005

    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

Nottingham & Sayre Standards Track [Page 21] RFC 4287 Atom Format December 2005

 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].

Nottingham & Sayre Standards Track [Page 22] RFC 4287 Atom Format December 2005

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 }

Nottingham & Sayre Standards Track [Page 23] RFC 4287 Atom Format December 2005

 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*)
    }

Nottingham & Sayre Standards Track [Page 24] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 25] RFC 4287 Atom Format December 2005

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

Nottingham & Sayre Standards Track [Page 26] RFC 4287 Atom Format December 2005

 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).

Nottingham & Sayre Standards Track [Page 27] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 28] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 29] RFC 4287 Atom Format December 2005

 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

Nottingham & Sayre Standards Track [Page 30] RFC 4287 Atom Format December 2005

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

Nottingham & Sayre Standards Track [Page 31] RFC 4287 Atom Format December 2005

 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.

Nottingham & Sayre Standards Track [Page 32] RFC 4287 Atom Format December 2005

 [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>.

Nottingham & Sayre Standards Track [Page 33] RFC 4287 Atom Format December 2005

 [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>.

Nottingham & Sayre Standards Track [Page 34] RFC 4287 Atom Format December 2005

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

Nottingham & Sayre Standards Track [Page 35] RFC 4287 Atom Format December 2005

 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

Nottingham & Sayre Standards Track [Page 36] RFC 4287 Atom Format December 2005

        & 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

Nottingham & Sayre Standards Track [Page 37] RFC 4287 Atom Format December 2005

        & 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 {

Nottingham & Sayre Standards Track [Page 38] RFC 4287 Atom Format December 2005

       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 }?,

Nottingham & Sayre Standards Track [Page 39] RFC 4287 Atom Format December 2005

       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

Nottingham & Sayre Standards Track [Page 40] RFC 4287 Atom Format December 2005

 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)*

Nottingham & Sayre Standards Track [Page 41] RFC 4287 Atom Format December 2005

 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

Nottingham & Sayre Standards Track [Page 42] RFC 4287 Atom Format December 2005

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.

Nottingham & Sayre Standards Track [Page 43]

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