The Atom Syndication Format

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

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

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

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.

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>

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 <em>lot</em> 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>

The XML Namespaces URI 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.

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 , above. Note that the choice of namespace prefix is arbitrary and not semantically significant. Atom is specified using terms from the XML Infoset . 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 . However, the text of this specification provides the definition of conformance. A complete schema appears in . 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, , as scoped to those conformance targets.

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 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 . Every URI 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 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 . Any element defined by this specification &MAY; have an xml:base attribute . When xml:base is used in an Atom Document, it serves the function described in , 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, .

atomCommonAttributes = attribute xml:base { atomUri }?, attribute xml:lang { atomLanguageTag }?, undefinedAttribute* Atom is an extensible format. See 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.

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.

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

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 , MIME media types &MUST-NOT; be used as values for the "type" attribute on Text constructs.

Example atom:title with text content:

... <title type="text"> Less: < </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.

Example atom:title with HTML content:

... <title type="html"> Less: <em> &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 . Any markup within &MUST; be escaped; for example, "<br>" as "<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.

Example atom:title with XHTML content:

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

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 ).

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.

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 .

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 .

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

atomDateConstruct = atomCommonAttributes, xsd:dateTime Such date values happen to be compatible with the following specifications: , , and . 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.

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. The following child elements are defined by this specification (note that the presence of some of these elements is required): 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. atom:feed elements &MAY; contain any number of atom:category elements. atom:feed elements &MAY; contain any number of atom:contributor elements. atom:feed elements &MUST-NOT; contain more than one atom:generator element. atom:feed elements &MUST-NOT; contain more than one atom:icon element. atom:feed elements &MUST-NOT; contain more than one atom:logo element. atom:feed elements &MUST; contain exactly one atom:id element. 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. 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. atom:feed elements &MAY; contain additional atom:link elements beyond those described above. atom:feed elements &MUST-NOT; contain more than one atom:rights element. atom:feed elements &MUST-NOT; contain more than one atom:subtitle element. atom:feed elements &MUST; contain exactly one atom:title element. 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.

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 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.

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): 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. atom:entry elements &MAY; contain any number of atom:category elements. atom:entry elements &MUST-NOT; contain more than one atom:content element. atom:entry elements &MAY; contain any number of atom:contributor elements. atom:entry elements &MUST; contain exactly one atom:id element. atom:entry elements that contain no child atom:content element &MUST; contain at least one atom:link element with a rel attribute value of "alternate". atom: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. atom:entry elements &MAY; contain additional atom:link elements beyond those described above. atom:entry elements &MUST-NOT; contain more than one atom:published element. atom:entry elements &MUST-NOT; contain more than one atom:rights element. atom:entry elements &MUST-NOT; contain more than one atom:source element. 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 , but is not an XML media type , does not begin with "text/", and does not end with "/xml" or "+xml". atom:entry elements &MUST-NOT; contain more than one atom:summary element. atom:entry elements &MUST; contain exactly one atom:title element. atom:entry elements &MUST; contain exactly one atom:updated element.

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

atomInlineTextContent = element atom:content { atomCommonAttributes, attribute type { "text" | "html" }?, (text)* } atomInlineXHTMLContent = element atom:content { atomCommonAttributes, attribute type { "xhtml" }, xhtmlDiv } atomInlineOtherContent = element atom:content { atomCommonAttributes, attribute type { atomMediaType }?, (text|anyElement)* } atomOutOfLineContent = element atom:content { atomCommonAttributes, attribute type { atomMediaType }?, attribute src { atomUri }, empty } atomContent = atomInlineTextContent | atomInlineXHTMLContent | atomInlineOtherContent | atomOutOfLineContent

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 ). 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".

atom:content &MAY; have a "src" attribute, whose value &MUST; be an IRI reference . 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 , 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.

Atom Documents &MUST; conform to the following rules. Atom Processors &MUST; interpret atom:content according to the first applicable rule. 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. 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 . The HTML markup &MUST; be escaped; for example, "<br>" as "<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. If the value of "type" is "xhtml", the content of atom:content &MUST; be a single XHTML div element 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. If the value of "type" is an XML media type 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. If the value of "type" begins with "text/" (case insensitive), the content of atom:content &MUST-NOT; contain child elements. For all other values of "type", the content of atom:content &MUST; be a valid Base64 encoding, as described in . 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.

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> ...

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.

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 }

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

The "scheme" attribute is an IRI that identifies a categorization scheme. Category elements &MAY; have a "scheme" 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 "&" and "<" represent their corresponding characters ("&" and "<", respectively), not markup. Category elements &MAY; have a "label" attribute.

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 }

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

atomGenerator = element atom:generator { atomCommonAttributes, attribute uri { atomUri }?, attribute version { text }?, text } The content of this element, when present, &MUST; be a string that is a human-readable name for the generating agent. Entities such as "&" and "<" represent their corresponding characters ("&" and "<" respectively), not markup. The atom:generator element &MAY; have a "uri" attribute whose value &MUST; be an IRI reference . 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.

The "atom:icon" element's content is an IRI reference 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.

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 . Note that the definition of "IRI" excludes relative references. Though the IRI might use a dereferencable scheme, Atom Processors &MUST-NOT; assume it can be dereferenced. When an Atom Document is relocated, migrated, syndicated, republished, exported, or imported, the content of its atom:id element &MUST-NOT; change. Put another way, an atom:id element pertains to all instantiations of a particular Atom entry or feed; revisions retain the same content in their atom:id elements. It is suggested that the atom:id element be stored along with the associated resource. The content of an atom:id element &MUST; be created in a way that assures uniqueness. Because of the risk of confusion between IRIs that would be equivalent if they were mapped to URIs and dereferenced, the following normalization strategy &SHOULD; be applied when generating atom:id elements: Provide the scheme in lowercase characters. Provide the host, if any, in lowercase characters. Only perform percent-encoding where it is essential. Use uppercase A through F characters when percent-encoding. Prevent dot-segments from appearing in paths. For schemes that define a default authority, use an empty authority if the default is desired. For schemes that define an empty path to be equivalent to a path of "/", use "/". For schemes that define a port, use an empty port if the default is desired. Preserve empty fragment identifiers and queries. Ensure that all components of the IRI are appropriately character normalized, e.g., by using NFC or NFKC.

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: 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

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 }

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

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 . 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 Registry of Link Relations (), 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: 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. 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:

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 .

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 .

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

The "atom:logo" element's content is an IRI reference 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).

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

atomPublished = element atom:published { atomDateConstruct} Typically, atom:published will be associated with the initial creation or first availability of the resource.

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.

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*) } 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.

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

atomSubtitle = element atom:subtitle { atomTextConstruct }

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.

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

atomTitle = element atom:title { atomTextConstruct }

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.

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.

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. 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. requires support for Canonical XML . 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 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 . 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. 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.

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. 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 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).

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.

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.

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".

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.

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.

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.

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

structuredExtensionElement = element * - atom:* { (attribute * { text }+, (text|anyElement)*) | (attribute * { text }*, (text?, anyElement+, (text|anyElement)*)) } The structure of a Structured Extension element, including the order of its child elements, could be significant. This specification does not provide an interpretation of a Structured Extension element. The syntax of the XML contained in the element (and an interpretation of how the element relates to its containing element) is defined by the specification of the Atom extension.

An Atom Document, when serialized as XML 1.0, can be identified with the following media type: application atom+xml None. This parameter has semantics identical to the charset parameter of the "application/xml" media type as specified in . Identical to those of "application/xml" as described in . As defined in this specification. In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in . There are no known interoperability issues. This specification. No known applications currently use this media type. Additional information: As specified for "application/xml" in . .atom As specified for "application/xml" in . As specified in . TEXT Mark Nottingham <mnot@pobox.com> COMMON IESG

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

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 and 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.

Atom Processors handle URIs. See .

Atom Processors handle IRIs. See .

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 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.

Atom Documents can be encrypted and signed using and , 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.

Media Type Specifications and Registration Procedures Key words for use in RFCs to Indicate Requirement Levels Harvard University

1350 Mass. Ave. Cambridge MA 02138 - +1 617 495 3864 sob@harvard.edu

General keyword In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. Authors who follow these guidelines should incorporate this phrase near the beginning of their document: 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 RFC 2119. Note that the force of these words is modified by the requirement level of the document in which they are used. Internet Message Format This document specifies a syntax for text messages that are sent betweencomputer users, within the framework of "electronic mail" messages.[STANDARDS TRACK] The 'text/html' Media Type XML Media Types This document standardizes five new media types -- text/xml, application/xml, text/xml-external-parsed-entity, application/xml-external-parsed-entity, and application/xml-dtd -- for use in exchanging network entities that are related to the Extensible Markup Language (XML). This document also standardizes a convention (using the suffix '+xml') for naming media types outside of these five types when those media types represent XML MIME (Multipurpose Internet Mail Extensions) entities. Tags for the Identification of Languages This document describes a language tag for use in cases where it is desired to indicate the language used in an information object, how to register values for use in this language tag, and a construct for matching such language tags. Date and Time on the Internet: Timestamps This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar. The Base16, Base32, and Base64 Data Encodings This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, and use of different encoding alphabets. Uniform Resource Identifier (URI): Generic Syntax Internationalized Resource Identifiers (IRIs) XML Information Set (Second Edition) XML Base Namespaces in XML Extensible Markup Language (XML) 1.0 (Third Edition) XML-Signature Syntax and Processing XML Encryption Syntax and Processing HTML 4.01 Specification Modularization of XHTML™ Canonical XML Version 1.0 Exclusive XML Canonicalization Version 1.0 Guidelines for Writing an IANA Considerations Section in RFCs IBM Corporation

3039 Cornwallis Ave. PO Box 12195 - BRQA/502 Research Triangle Park NC 27709-2195 919-254-7798 narten@raleigh.ibm.com

Maxware

Pirsenteret N-7005 Trondheim Norway +47 73 54 57 97 Harald@Alvestrand.no

General Internet Assigned Numbers Authority IANA Many protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication algorithm for IPSec). To insure that such quantities have consistent values and interpretations in different implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA). In order for the IANA to manage a given name space prudently, it needs guidelines describing the conditions under which new values can be assigned. If the IANA is expected to play a role in the management of a name space, the IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a name space and provides guidelines to document authors on the specific text that must be included in documents that place demands on the IANA. Data elements and interchange formats - Information interchange - Representation of dates and times International Organization for Standardization XML Schema Part 2: Datatypes Second Edition Date and Time Formats RELAX NG Compact Syntax OASIS Technical Committee: RELAX NG

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.

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 and .

# -*- 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 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 & 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 & 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 { 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 }?, 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 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)* 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