Internet-Draft. Boswijck Memex Consulting January 26, The Atom Syndication Format draft-ietf-atompub-format-05. Status of this Memo

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1 Network Working Group Internet-Draft Expires: July 27, 2005 M. Nottingham, Ed. R. Sayre, Ed. Boswijck Memex Consulting January 26, 2005 The Atom Syndication Format draft-ietf-atompub-format-05 Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at The list of Internet-Draft Shadow Directories can be accessed at This Internet-Draft will expire on July 27, Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract This document specifies Atom, an XML-based Web content and metadata syndication format. Nottingham & Sayre Expires July 27, 2005 [Page 1]

2 Table of Contents 1. Introduction Editorial Notes Example Conformance Notational Conventions Atom Documents Common Atom Constructs Text Constructs "type" Attribute Person Constructs The "atom:name" Element The "atom:uri" Element The "atom: " Element Date Constructs Service Constructs The "href" Attribute Identity Constructs Dereferencing Identity Constructs Comparing Identity Constructs Element Definitions The "atom:feed" Element The "version" Attribute The "atom:head" Element Usage of "atom:head" within "atom:entry" The "atom:entry" Element The "atom:title" Element The "atom:id" Element The "atom:link" Element The "href" Attribute The "rel" Attribute The "type" Attribute The "hreflang" Attribute The "title" Attribute The "length" Attribute The "atom:updated" Element The "atom:published" Element The "atom:author" Element The "atom:contributor" Element The "atom:host" Element The "atom:copyright" Element The "atom:category" Element The "term" Attribute The "scheme" Attribute The "label" attribute The "atom:summary" Element The "atom:content" Element Nottingham & Sayre Expires July 27, 2005 [Page 2]

3 The "type" attribute The "src" attribute Processing Model The "atom:introspection" Element The "atom:post" Element The "atom:edit" Element The "atom:tagline" Element The "atom:generator" Element The "atom:info" Element Managing Feed State Securing Atom Documents Digital Signatures Encryption Embedding Atom in Other Formats Extending Atom IANA Considerations Registry of Link Relations Security Considerations References Normative References Informative References Authors Addresses A. Contributors B. Collected RELAX NG Compact Schema C. Revision History Intellectual Property and Copyright Statements Nottingham & Sayre Expires July 27, 2005 [Page 3]

4 1. Introduction Atom is an XML-based document format which describes lists of related information known as "feeds". Feeds are composed of a number of items, known as "entries", each with an extensible set of attached metadata. For example, each entry has a title. The primary use case that Atom addresses is the syndication of Web content such as Weblogs and news headlines to Web sites as well as directly to user agents. However, nothing precludes it from being used for other purposes and kinds of content. Details of communication protocols between software agents using Atom can be found in the Atom Protocol specification [Atom-protocol]. [[ more motivation / design principles ]] 1.1 Editorial Notes The Atom format is a work-in-progress, and this draft is both incomplete and likely to change rapidly. As a result, THE FORMAT DESCRIBED BY THIS DRAFT SHOULD NOT BE DEPLOYED, either in production systems or in any non-experimental fashion on the Internet. Discussion of this draft happens in two fora; The mailing list [1] The Atom Wiki Web site [2] Active development takes place on the mailing list, while the Wiki is used for issue tracking and new proposals. This document is an early draft and known to be incomplete. Topics marked [[like this]] indicate where additional text is likely to be added. Nottingham & Sayre Expires July 27, 2005 [Page 4]

5 1.2 Example A minimal, single-entry Atom Feed Document: <?xml version="1.0" encoding="utf-8"?> <feed version="draft-ietf-atompub-format-05:do not deploy" xmlns=" <head> <title>example Feed</title> <link href=" <updated> t18:30:02z</updated> <author> <name>john Doe</name> </author> </head> <entry> <title>atom-powered Robots Run Amok</title> <link href=" <id>vemmi://example.org/2003/32397</id> <updated> t18:30:02z</updated> </entry> </feed> 1.3 Conformance [[ talk about atom documents and atom consumers, and how requirements are placed on them ]] 1.4 Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119]. This specification uses XML Namespaces [W3C.REC-xml-names ] to uniquely identify XML elements and attribute names. It uses the following namespace prefixes for the indicated namespace URIs; "atom": Note that the choice of any namespace prefix is arbitrary and not semantically significant. Atom is specified using terms from the XML Infoset [W3C.REC-xml-infoset ]. However, this specification uses a shorthand for two common terms; the phrase "Information Item" is omitted when naming Element Information Items and Attribute Nottingham & Sayre Expires July 27, 2005 [Page 5]

6 Information Items. Therefore, when this specification uses the term "element," it is referring to an Element Information Item in Infoset terms. Likewise, when it uses the term "attribute," it is referring to an Attribute Information Item. Some sections of this specification are illustrated with fragments of a non-normative RELAX NG Compact schema [RELAX-NG]. However, the text of this specification provides the definition of conformance. A collected schema appears in an informative appendix. Nottingham & Sayre Expires July 27, 2005 [Page 6]

7 2. Atom Documents This specification describes two kinds of Atom Documents; Atom Feed Documents and Atom Entry Documents. An Atom Feed Document is a representation of an Atom feed, including metadata about the feed, and some or all of the entries associated with it. Its document element is atom:feed. An Atom Entry Document represents exactly one Atom Entry, outside of the context of an Atom Feed. Its document element is atom:entry. namespace atom = " start = atomfeed atomentry Both kinds of Atom documents are specified in terms of the XML Information Set, serialised as XML 1.0 [W3C.REC-xml ] and identified with the "application/atom+xml" media type. Atom Documents MUST be well-formed XML. Atom constrains the appearance and content of elements and attributes; unless otherwise stated, Atom Documents MAY contain other Information Items as appropriate. In particular, Comment Information Items and Processing Instruction Information Items SHOULD be ignored in the normal processing of an Atom Document. Any element in an Atom Document MAY have an xml:base attribute. XML Base [W3C.REC-xmlbase ] processing MUST be applied to any relative reference [RFC3986] present in an Atom Document. This includes such elements and attributes as specified by Atom itself, as well as those specified by extensions to Atom. Any element in an Atom Document MAY have an xml:lang attribute, whose content indicates the natural language of the element s content. Requirements regarding the content and interpretation of xml:lang are specified in XML 1.0 [W3C.REC-xml ] Section atomcommonattributes = attribute xml:base { atomuri }?, attribute xml:lang { atomlanguagetag }? [[ discussion of URI escaping and i18n, IRI ]] [[ discussion of white space ]] Atom is extensible. See the section titled Extending Atom later in Nottingham & Sayre Expires July 27, 2005 [Page 7]

8 this document for a full description of how Atom Documents can be extended. Nottingham & Sayre Expires July 27, 2005 [Page 8]

9 3. Common Atom Constructs Many of Atom s elements share a few common structures. This section defines a few such structures and their requirements for convenient reference by the appropriate element definitions. When an element is identified as being a particular kind of construct, it inherits the corresponding requirements from that construct s definition in this section. 3.1 Text Constructs A Text construct contains human readable text, usually in fairly small quantities. atomplaintextconstruct = attribute type { "TEXT" "HTML" }?, text atomxhtmltextconstruct = attribute type { "XHTML" }, (text anyelement)* atomtextconstruct = atomplaintextconstruct atomxhtmltextconstruct "type" Attribute Text constructs MAY have a "type" attribute. When present, the value MUST be one of "TEXT", "HTML" or "XHTML". If the "type" attribute is not provided, software MUST behave as though it were present with a value of "TEXT". Note that MIME media types [RFC2045] are not acceptable values for the "type" attribute. If the value is "TEXT", the content of the Text construct MUST NOT contain child elements. Such text is intended to be presented to humans in a readable fashion. Thus, software MAY display it using normal text rendering techniques such as proportional fonts, white-space collapsing, and justification. If the value of "type" is "HTML", the content of the Text construct MUST NOT contain child elements, and SHOULD be suitable for handling by software that knows HTML. The HTML markup must be escaped; for example, "<br>" as "<br>". The HTML markup SHOULD be such that it Nottingham & Sayre Expires July 27, 2005 [Page 9]

10 could validly appear directly within an HTML <DIV> element. Receiving software which displays the content MAY use the markup to aid in displaying it. [[example atom entry w/ escaped markup]] If the value of "type" is "XHTML", the content of the Text construct MAY contain child elements. The content SHOULD be XHTML text and markup that could validly appear directly within an xhtml:div element. Receiving software which displays the content MAY use the markup to aid in displaying it. Escaped markup is interpreted as a text representation of markup, and MUST NOT be interpreted as markup itself. 3.2 Person Constructs A Person construct is an element that describes a person, corporation, or similar entity. Person constructs MAY be extended by namespace-qualified element children. This specification assigns no significance to the order of appearance of the child elements in a Person construct. atompersonconstruct = (element atom:name { text } & element atom:uri { atomuri }? & element atom: { atom address }?) The "atom:name" Element The "atom:name" element s content conveys a human-readable name for the person. Person constructs MUST contain exactly one "atom:name" element The "atom:uri" Element The "atom:uri" element s content conveys a URI associated with the person. Person constructs MAY contain an atom:uri element, but MUST NOT contain more than one. The content of atom:uri in a Person construct MUST be a URI reference [RFC3986]. xml:base [W3C.REC-xmlbase ] processing MUST be applied to the atom:uri element s content. Nottingham & Sayre Expires July 27, 2005 [Page 10]

11 3.2.3 The "atom: " Element The "atom: " element s content conveys an address associated with the persons. Person constructs MAY contain an atom: element, but MUST NOT contain more than one. Its content MUST be an address [RFC2822]. 3.3 Date Constructs A Date construct is an element whose content MUST conform to the date-time BNF rule in [RFC3339]. atomdateconstruct = xsd:datetime 3.4 Service Constructs A Service construct is an empty element that conveys the URI of an Atom Publishing Protocol [Atom-protocol] service associated with an entry or feed. atomserviceconstruct = attribute href { atomuri } The "href" Attribute The "href" attribute contains the a URI of the service. atom:service elements MUST have a "href" attribute, whose value MUST be a URI reference [RFC3986]. xml:base processing MUST be applied to the "href" attribute. 3.5 Identity Constructs An Identity construct is an element whose content conveys a permanent, universally unique identifier for the construct s parent. Its content MUST be a URI, as defined by [RFC3986]. Note that the definition of "URI" excludes relative references. When an Atom document is relocated, migrated, syndicated, republished, exported or imported, the content of its Identity construct MUST NOT change. Put another way, an Identity construct Nottingham & Sayre Expires July 27, 2005 [Page 11]

12 pertains to all instantiations of a particular Atom entry or feed; revisions retain the same content in their Identity constructs. atomidentityconstruct = (atomuri) Dereferencing Identity Constructs The content of an Identity construct MAY be dereferencable (e.g. an HTTP URI). However, processors MUST NOT assume it to be dereferencable. The content of an Identity construct MUST be created in a way that assures uniqueness, and it is suggested that the Identity construct be stored along with the associated resource. Because of the risk of confusion between URIs that would be equivalent if dereferenced, the following normalization strategy is strongly encouraged when generating Identity constructs: o Provide the scheme in lowercase characters. 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 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 portions of the URI are UTF-8 encoded NFC form Unicode strings Comparing Identity Constructs Instances of Identity constructs can be compared to determine whether an entry or feed is the same as one seen before. Processors MUST compare Identity constructs on a character-by-character basis in a case-sensitive fashion. As a result, two URIs that resolve to the same resource but are not character-for-character identical will be considered different for the purposes of Identifier comparison. Nottingham & Sayre Expires July 27, 2005 [Page 12]

13 For example, " " " and " will all be considered different identifiers, despite their differences in case. Likewise, " bob", " and " will all be considered different identifiers, because URI %-escaping is significant for the purposes of comparison. Nottingham & Sayre Expires July 27, 2005 [Page 13]

14 4. Element Definitions 4.1 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 first element child MUST be atom:head, which MAY be followed zero or more atom:entry child elements. atomfeed = element atom:feed { atomversionattribute, (atomhead & atomentry* & anyelement*) } The "version" Attribute atom:feed elements MUST have a "version" attribute whose content indicates the version of the Atom specification that the feed conforms to. The content of this attribute is unstructured text. The version identifier for this specification is "draft-ietf-atompub-format-05: do not deploy". atomversionattribute = attribute version {"draft-ietf-atompub-format-05 : do not deploy"} 4.2 The "atom:head" Element The atom:head element acts as a container for metadata about the feed itself. The atom:head element MAY contain any namespace-qualified [W3C.REC-xml-names ] elements as children. This specification assigns no significance to the order of appearance of the child elements of atom:head. Nottingham & Sayre Expires July 27, 2005 [Page 14]

15 atomhead = element atom:head { (atomtitle & atomupdated & atomlink+ & atomid? & atomauthor? & atomcontributor* & atomtagline? & atomgenerator? & atominfo? & atomcopyright? & atomcategory* & atomintrospection? & atompost? & anyelement*) } The following child elements are defined by this specification (note that the presence of some of these elements is required): o atom:head elements MUST contain exactly one atom:title element. o atom:head elements MUST contain at least one atom:link element with a rel attribute value of "alternate". o atom:head elements MUST NOT contain more than one atom:link element with a rel attribute value of "alternate" that has the same type attribute value. If a feed s atom:link element with type="alternate" resolves to an HTML document, then that document SHOULD have a autodiscovery link element [Atom-autodiscovery] that reflects back to the feed. atom:head elements MAY contain additional atom:link elements beyond those described above. o atom:head elements MUST contain exactly one atom:author element, UNLESS all of the atom:feed element s child atom:entry elements contain an atom:author element. atom:head elements MUST NOT contain more than one atom:author element. [[inheritance]] o atom:head elements MUST NOT contain more than one an atom:contributor element. o atom:head elements MUST NOT contain more than one an atom:copyright element. o atom:head elements MUST NOT contain more than one an atom:id element. o atom:head elements MUST contain exactly one atom:updated element. o atom:head elements MAY contain any number of atom:category elements. o atom:head elements MUST NOT contain more than one atom:post element. Nottingham & Sayre Expires July 27, 2005 [Page 15]

16 o atom:head elements MUST NOT contain more than one atom:introspection element. o atom:head elements MUST NOT contain more than one atom:tagline element. o atom:head elements MUST NOT contain more than one atom:generator element. o atom:head elements MUST NOT contain more than one atom:info element Usage of "atom:head" within "atom:entry" In this case, the atom:head element acts as a container for metadata about the feed within which the entry was created. If an atom:entry is copied into one feed from another feed, then the atom:head element of the source feed SHOULD be inserted into the atom:entry unless the entry, as copied, already contains an atom:head element. If the atom:entry already contains an atom:head, then that atom:head SHOULD be copied without modification. [[... example... ]] 4.3 The "atom:entry" Element The "atom:entry" element represents an individual entry. This element can appear as a child of the atom:feed element, or it can appear as the document (i.e., top-level) element of a standalone Atom Entry Document. When appearing in an Atom Entry Document, atom:entry elements MUST have a "version" attribute whose content indicates the version of the Atom specification that the entry conforms to. The version identifier for this specification is "draft-ietf-atompub-format-05: do not deploy". The atom:entry element MAY contain any namespace-qualified [W3C.REC-xml-names ] elements as children. This specification assigns no significance to the order of appearance of the child elements of atom:entry. Nottingham & Sayre Expires July 27, 2005 [Page 16]

17 atomentry = element atom:entry { atomversionattribute?, (atomtitle & atomid & atomlink* & atomupdated & atompublished? & atomauthor? & atomcontributor* & atomhost? & atomcopyright? & atomcategory* & atomedit? & atomsummary? & atomcontent? & atomhead? & anyelement*) } The following child elements are defined by this specification (note that it requires the presence of some of these elements): o atom:entry elements MUST have exactly one "atom:title" element. 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". atom:entry elements MUST NOT contain more than one atom:link element with a rel attribute value of "alternate" that has the same type attribute value. atom:entry elements MAY contain additional atom:link elements beyond those described above. o atom:entry elements MUST contain exactly one atom:updated element. o atom:entry elements MUST NOT contain more than one atom:published element. o atom:entry elements MUST contain exactly one atom:author element, unless, in an Atom Feed Document, the atom:head element contains an atom:author element itself. atom:entry elements MUST NOT contain more than one atom:author element. o atom:entry elements MUST NOT contain more than one atom:contributor element. o atom:entry elements MUST NOT contain more than one atom:host element. o atom:entry elements MUST NOT contain more than one atom:copyright element. o atom:entry elements MAY contain any number of atom:category elements. Nottingham & Sayre Expires July 27, 2005 [Page 17]

18 o atom:entry elements MUST contain an atom:summary element in any of the following cases: * the atom:entry element contains no atom:content element. * the atom:entry contains an atom:content which has a "src" attribute (and is thus empty). * the atom:entry contains content which is encoded in Base64; i.e. the "type" attribute of atom:content is a MIME media type [RFC2045] and does not begin with "text/" nor end with "+xml". o atom:entry elements MUST NOT contain more than one atom:summary element. o atom:entry elements MUST NOT contain more than one atom:edit element. o atom:entry elements MUST NOT contain more than one atom:head element. 4.4 The "atom:title" Element The "atom:title" element is a Text construct that conveys a human-readable title for an entry or feed. atomtitle = element atom:title { atomtextconstruct } 4.5 The "atom:id" Element The "atom:id" element is an Identity construct that conveys a permanent, universally unique identifier for an entry or feed. atomid = element atom:id { atomidentityconstruct } 4.6 The "atom:link" Element The atom:link element is an empty element that defines a reference from an Atom Document to a Web resource. atomlink = element atom:link { attribute href { atomuri }, attribute rel { atomncname atomuri }?, attribute type { atommediatype }?, attribute hreflang { atomlanguagetag }?, attribute title { text }?, attribute length { text }?, empty Nottingham & Sayre Expires July 27, 2005 [Page 18]

19 } The "href" Attribute The "href" attribute contains the link s URI. Link constructs MUST have a href attribute, whose value MUST be a URI reference [RFC3986]. xml:base [W3C.REC-xmlbase ] processing MUST be applied to the href attribute s content The "rel" Attribute Link elements MAY have an "rel" attribute that indicates the link relation type. If the "rel" attribute is not present, the link element MUST be interpreted as if the link relation type is "alternate". rel_attribute = segment-nz-nc / URI The value of "rel" MUST be either a name, which is non-empty and does not contain any colon (":") characters, or a URI [RFC3986]. Note that use of a relative reference is not allowed. If a name is given, implementations MUST consider the link relation type to be equivalent to the same name registered within the IANA Registry of Link Relations Section 9, and thus the URI that would be obtained by appending the value of the rel attribute to the string " The value of "rel" describes the meaning of the link, but does not impose any behavioral requirements on implementations. This document defines two initial values for the Registry of Link Relations: The value "alternate" signifies that the URI 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 URI in the value of the href attribute identifies a resource related to the resource described by the containing atom:feed or atom:entry element. For example, the feed for a site which discusses the performance of the search engine at " might contain, as a child of atom:head: <link rel="related" href=" Nottingham & Sayre Expires July 27, 2005 [Page 19]

20 An identical link might appear as a child of any atom:entry whose content contains a discussion of that same search engine The "type" Attribute Link elements MAY have a type attribute, whose value MUST conform to the syntax of a MIME media type [RFC2045]. The type attribute s value is an advisory media type; it is a hint about the type of the representation which 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 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] The "title" Attribute The "title" attribute conveys human-readable information about the link. Link elements MAY have a title attribute 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 URI in the href attribute is 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. 4.7 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 producer considers significant. Therefore, not all modifications necessarily result in a changed atom:updated value. Publishers MAY change the value of this element over time. Processors MAY present entries sorted using this value. Processors MAY choose not to present entries until the instant in time specified Nottingham & Sayre Expires July 27, 2005 [Page 20]

21 in the atom:updated element has passed. atomupdated = element atom:updated { atomdateconstruct } 4.8 The "atom:published" Element The "atom:published" element is a Date construct indicating an instant in time associated with an event early in the life cycle of the entry. Typically, atom:published will be associated with the initial creation or first availability of the resource. Processors MAY present entries sorted using this value. Processors MAY choose not to present entries until the instant in time specified in the atom:published element has passed. 4.9 The "atom:author" Element The "atom:author" element is a Person construct that indicates the default author of the an entry or feed atomauthor = element atom:author { atompersonconstruct } 4.10 The "atom:contributor" Element The "atom:contributor" element is a Person construct that indicates a person or other entity who contributes to the entry. atomcontributor = element atom:contributor { atompersonconstruct } 4.11 The "atom:host" Element The "atom:host" element s content conveys a domain name or network address associated with an entry s origin. Its content MUST be a domain name [RFC1035], a dotted-decimal IPv4 address [RFC0791], or a colon-delimited IPv6 address [RFC2460] The "atom:copyright" Element The "atom:copyright" element is a Text construct that conveys a human-readable copyright statement for an entry or feed. The atom:copyright element SHOULD NOT be used to convey Nottingham & Sayre Expires July 27, 2005 [Page 21]

22 machine-readable licensing information. If an atom:entry element does not contain an atom:copyright element, then the atom:copyright element of the containing atom:feed element s atom:head element, if present, should be considered to apply to the entry The "atom:category" Element Category elements contain information about a category to which an Atom feed or entry is associated. atomcategory = element atom:category { attribute term { text }, attribute scheme { atomuri }?, attribute label { text }?, empty } The "term" Attribute The "term" attribute is a string which identifies the category to which the entry or feed belongs. Category elements MUST have a "term" attribute The "scheme" Attribute The "scheme" attribute is a URI that identifies a categorization scheme. Category elements MAY have a "scheme" attribute The "label" attribute The "label" attribute provides a human-readable label that may be displayed in end-user applications. Category elements MAY have a "label" attribute 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 } Nottingham & Sayre Expires July 27, 2005 [Page 22]

23 4.15 The "atom:content" Element The "atom:content" element either contains or links to the content of the entry. atom:entry elements MUST contain zero or one atom:content elements. atominlinetextcontent = element atom:content { attribute type { "TEXT" "HTML" atommediatype }?, (text)* } atominlinexhtmlcontent = element atom:content { attribute type { "XHTML" atommediatype }?, (text anyelement)* } atomoutoflinecontent = element atom:content { attribute type { "TEXT" "HTML" "XHTML" atommediatype }?, attribute src { atomuri }, empty } atomcontent = atominlinetextcontent atominlinexhtmlcontent atomoutoflinecontent The "type" attribute atom:content MAY have a "type" attribute, When present, the value MAY be one of "TEXT", "HTML", or "XHTML". Failing that, it MUST be a MIME media type [RFC2045] in which, to use the terminology of Section 5 of [RFC2045], the top level is a discrete type. If the type attribute is not provided, software MUST behave as though it were present with a value of "TEXT" The "src" attribute atom:content MAY have a "src" attribute, whose value MUST be a URI reference [RFC3986]. If the "src" attribute is present, software MAY use the URI to retrieve the content. If the "src" attribute is Nottingham & Sayre Expires July 27, 2005 [Page 23]

24 present, atom:content MUST be empty. That is to say, the content may be retrievable using "src=" URI, or it may be contained within atom:content, but not both. If the "src" attribute is present, the "type" attribute SHOULD be provided and MUST be a MIME media type [RFC2045], rather than "TEXT", "HTML", or "XHTML". The value is advisory; that is to say, upon dereferencing the URI to retrieve the content, if the server providing that content also provides a media type, the server-provided media type is authoritative. If the value of type begins with "text/" or ends with "+xml", the content SHOULD be local; that is to say, no "src" attribute should be provided Processing Model Software MUST apply the following rules in the order below to ascertain the rules governing the content of "atom:content". 1. If the value is "TEXT", the content of atom:content MUST NOT contain child elements. Such text is intended to be presented to humans in a readable fashion. Thus, software MAY display it using normal text rendering techniques such as proportional fonts, white-space collapsing, and justification. 2. If the value of "type" is "HTML", the content of atom:content MUST NOT contain child elements, and SHOULD be suitable for handling by software that knows HTML. The HTML markup must be escaped; for example, "<br>" as "<br>". The HTML markup SHOULD be such that it could validly appear directly within an HTML <DIV> element. Receiving software which displays the content SHOULD use the markup to aid in displaying it. 3. If the value of "type" is "XHTML", the content of atom:content MAY contain child elements. The content SHOULD be XHTML text and markup that could validly appear directly within an xhtml:div element. Receiving software which displays the content SHOULD use the markup to aid in displaying it. Escaped markup is interpreted as a text representation of markup, and MUST NOT be interpreted as markup itself. 4. If the value of "type" ends with "+xml" or "/xml", the content of atom:content may include child elements, and SHOULD be suitable for handling by software that knows the indicated media type. If the "src" attribute is not provided, this would normally mean that the "atom:content" element would contain a single child element which would serve as the root element of the XML document of the indicated type. 5. If the value of "type" begins with "text/" the content of atom:content MUST NOT contain child elements. Nottingham & Sayre Expires July 27, 2005 [Page 24]

25 6. For all other values of "type", the content of atom:content MUST be a valid Base64 encoding [RFC3548], which when decoded SHOULD be suitable for handling by software that knows the indicated media type. In this case, the characters in the Base64 encoding may be preceded and followed in the atom:content element by whitespace, and lines are separated by a single newline (U+000A) character The "atom:introspection" Element The "atom:introspection" element is a Service construct that conveys the URI of an introspection file associated with a feed. atomintrospection = element atom:introspection { atomserviceconstruct } 4.17 The "atom:post" Element The "atom:post" element is a Service construct that conveys the URI used to add entries to a feed. atompost = element atom:post { atomserviceconstruct } 4.18 The "atom:edit" Element The "atom:edit" element is a Service construct that conveys the URI used to retrieve and edit the source representation of an entry. atomedit = element atom:edit { atomserviceconstruct } 4.19 The "atom:tagline" Element The "atom:tagline" element is a Text construct that conveys a human-readable description or tagline for the feed. atomtagline = element atom:tagline { atomtextconstruct } 4.20 The "atom:generator" Element The "atom:generator" element s content identifies the software agent Nottingham & Sayre Expires July 27, 2005 [Page 25]

26 used to generate a feed, for debugging and other purposes. atomgenerator = element atom:generator { attribute url { 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. The atom:generator element MAY have a "uri" attribute whose value MUST be a URI reference [RFC3986]. When dereferenced, that URI 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. When present, its value is unstructured text The "atom:info" Element The "atom:info" element is a Text construct that conveys a human-readable explanation of the feed format itself. The atom:info element SHOULD NOT be considered meaningful by processors; it is a convenience to publishers. atominfo = element atom:info { atomtextconstruct } Nottingham & Sayre Expires July 27, 2005 [Page 26]

27 5. Managing Feed State [[ talk about what it means to keep a view of a feed ]] Nottingham & Sayre Expires July 27, 2005 [Page 27]

28 6. Securing Atom Documents Because Atom is an XML-based format, existing XML security mechanisms can be used to secure its content. 6.1 Digital Signatures The document element of an Atom document (i.e., atom:feed in an Atom Feed Document, atom:entry in an Atom Entry Document) MAY have an Enveloped Signature, as described by XML-Signature and Syntax Processing [W3C.REC-xmldsig-core ]. 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 " and a local name of "Signature" as a child of the document element must not cause a processor to fail merely because of its presence. Other elements in an Atom document MUST NOT be signed unless their definitions explicitly specify such a capability. 6.2 Encryption The document element of an Atom document (i.e., atom:feed in an Atom Feed Document, atom:entry in an Atom Entry Document) MAY be encrypted, using the mechanisms described by XML Encryption Syntax and Processing [W3C.REC-xmlenc-core ]. Nottingham & Sayre Expires July 27, 2005 [Page 28]

29 7. Embedding Atom in Other Formats [[... ]] Nottingham & Sayre Expires July 27, 2005 [Page 29]

30 8. Extending Atom [[... ]] Nottingham & Sayre Expires July 27, 2005 [Page 30]

31 9. IANA Considerations An Atom Document, when serialized as XML 1.0, can be identified with the following media type: MIME media type name: application MIME subtype name: atom+xml Mandatory parameters: None. Optional parameters: "charset": This parameter has identical semantics to the charset parameter of the "application/xml" media type as specified in RFC 3023 [RFC3023]. [RFC3023]. Encoding considerations: Identical to those of "application/xml" as described in RFC 3023 [RFC3023], section 3.2. Security considerations: As defined in this specification. [[update upon publication]] In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in RFC 3023 [RFC3023], section 10. Interoperability considerations: There are no known interoperability issues. Published specification: This specification. [[update upon publication]] Applications which use this media type: No known applications currently use this media type. Additional information: Magic number(s): As specified for "application/xml" in RFC 3023 [RFC3023], section 3.2. File extension:.atom Fragment identifiers: As specified for "application/xml" in RFC 3023 [RFC3023], section 5. Base URI: As specified in RFC 3023 [RFC3023], section 6. Macintosh File Type code: TEXT Person and address to contact for further information: Mark Nottingham Intended usage: COMMON Author/Change controller: This specification s author(s). [[update upon publication]] 9.1 Registry of Link Relations This registry is maintained by IANA and initially contains the two values: "alternate" and "related". New assignments are subject to IESG Approval, as outlined in [RFC2434]. Requests should be made by to IANA, which will then forward the request to the IESG requesting approval. The request should contain discussion of at Nottingham & Sayre Expires July 27, 2005 [Page 31]

32 least the following five topics: o A value for the "rel" attribute that conforms to the syntax rule given in Section o Common name for link type. o Description of link type semantics. o Expected display characteristics. o Security considerations. Nottingham & Sayre Expires July 27, 2005 [Page 32]

33 10. Security Considerations Atom document can be encrypted and signed using [W3C.REC-xmlenc-core ] and [W3C.REC-xmldsig-core ], respectively, and is subject to the security considerations implied by their use. Nottingham & Sayre Expires July 27, 2005 [Page 33]

34 11. References 11.1 Normative References [Atom-autodiscovery] Pilgrim, M., "Atom Feed Autodiscovery", work-in-progress, August [Atom-protocol] Gregorio, J. and R. Sayre, "The Atom Publishing Protocol", work-in-progress, July [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April [RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January [RFC3066] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July [RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January [W3C.NOTE-datetime ] Nottingham & Sayre Expires July 27, 2005 [Page 34]

35 Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C NOTE NOTE-datetime , August [W3C.REC-xml ] Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T. and E. Maler, "Extensible Markup Language (XML) 1.0 (Third Edition)", W3C REC REC-xml , February [W3C.REC-xml-infoset ] Tobin, R. and J. Cowan, "XML Information Set", W3C FirstEdition REC-xml-infoset , October [W3C.REC-xml-names ] Hollander, D., Bray, T. and A. Layman, "Namespaces in XML", W3C REC REC-xml-names , January [W3C.REC-xmlbase ] Marsh, J., "XML Base", W3C REC REC-xmlbase , June [W3C.REC-xmldsig-core ] Solo, D., Reagle, J. and D. Eastlake, "XML-Signature Syntax and Processing", W3C REC REC-xmldsig-core , February [W3C.REC-xmlenc-core ] Reagle, J. and D. Eastlake, "XML Encryption Syntax and Processing", W3C REC REC-xmlenc-core , December Informative References [RELAX-NG] OASIS Technical Committee: RELAX NG, "RELAX NG Specification", December [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October Nottingham & Sayre Expires July 27, 2005 [Page 35]

36 URIs [1] < [2] < Authors Addresses Mark Nottingham (editor) URI: Robert Sayre (editor) Boswijck Memex Consulting URI: Nottingham & Sayre Expires July 27, 2005 [Page 36]

37 Appendix A. Contributors The following people contributed to preliminary drafts of this document: Tim Bray, Mark Pilgrim, and Sam Ruby. Norman Walsh provided the Relax NG schema. The content and concepts within are a product of the Atom community and the Atom Publishing Format and Protocol Working Group. Nottingham & Sayre Expires July 27, 2005 [Page 37]

38 Appendix B. Collected RELAX NG Compact Schema This appendix is informative. # -*- Relax NG -*- namespace local = "" namespace atom = " namespace s = " start = atomfeed atomentry # Attribute definitions atomcommonattributes = attribute xml:base { atomuri }?, attribute xml:lang { atomlanguagetag }? atomversionattribute = attribute version {"draft-ietf-atompub-format-05 : do not deploy"} # Common Atom Constructs atomplaintextconstruct = attribute type { "TEXT" "HTML" }?, text atomxhtmltextconstruct = attribute type { "XHTML" }, (text anyelement)* atomtextconstruct = atomplaintextconstruct atomxhtmltextconstruct atompersonconstruct = (element atom:name { text } & element atom:uri { atomuri }? & element atom: { atom address }?) atomdateconstruct = xsd:datetime atomserviceconstruct = Nottingham & Sayre Expires July 27, 2005 [Page 38]

39 attribute href { atomuri } atomidentityconstruct = (atomuri) # atom:feed # TODO: Test for multiple atom:link/@rel= alternate with # the The following tests are simple to do, # but my validator is giving me trouble. # TODO: Debug and add them back # Test for at least one atom:link/@rel= alternate # Test for atom:author or all atom:entry have atom:author # atom:feed atomfeed = element atom:feed { atomversionattribute, (atomhead & atomentry* & anyelement*) } # atom:head atomhead = [ s:rule [ context = "atom:head" s:assert [ test = "atom:link[@rel= alternate ]" "An atom:feed must have at least one link element with a rel attribute of alternate." ] ] s:rule [ context = "atom:head" s:assert [ test = "atom:author or not(../atom:entry[count(atom:author) = 0])" "An atom:feed must have an atom:author unless all of its atom:entry children have an atom:author." ] ] ] element atom:head { Nottingham & Sayre Expires July 27, 2005 [Page 39]

40 } (atomtitle & atomupdated & atomlink+ & atomid? & atomauthor? & atomcontributor* & atomtagline? & atomgenerator? & atominfo? & atomcopyright? & atomcategory* & atomintrospection? & atompost? & anyelement*) # atom:entry # TODO: Test for multiple alternate # with the # TODO: Test for required atom:summary atomentry = [ s:rule [ context = "/atom:entry" s:assert [ test = "@version" "The version attribute is required on standalone atom:entry elements." ] ] s:rule [ context = "atom:entry" s:assert [ test = "atom:link[@rel= alternate ]" "An atom:entry must have at least one link element with a rel attribute of alternate." ] ] s:rule [ context = "atom:entry" s:assert [ test = "atom:author or../atom:author" "An atom:entry must have an atom:author if the parent atom:feed does not." ] ] Nottingham & Sayre Expires July 27, 2005 [Page 40]

41 ] element atom:entry { atomversionattribute?, (atomtitle & atomid & atomlink* & atomupdated & atompublished? & atomauthor? & atomcontributor* & atomhost? & atomcopyright? & atomcategory* & atomedit? & atomsummary? & atomcontent? & atomhead? & anyelement*) } # atom:title atomtitle = element atom:title { atomtextconstruct } # atom:id atomid = element atom:id { atomidentityconstruct } atomlink = element atom:link { attribute href { atomuri }, attribute rel { atomncname atomuri }?, attribute type { atommediatype }?, attribute hreflang { atomlanguagetag }?, attribute title { text }?, attribute length { text }?, empty } # atom:updated # TODO: Test for a timezone that SHOULD be UTC atomupdated = element atom:updated { atomdateconstruct } # atom:published Nottingham & Sayre Expires July 27, 2005 [Page 41]

42 atompublished = element atom:published { atomdateconstruct } # atom:author atomauthor = element atom:author { atompersonconstruct } # atom:contributor atomcontributor = element atom:contributor { atompersonconstruct } # atom:host # TODO: tighten this up atomhost = element atom:host { text } # atom:copyright atomcopyright = element atom:copyright { atomtextconstruct } # atom:category atomcategory = element atom:category { attribute term { text }, attribute scheme { atomuri }?, attribute label { text }?, empty } # atom:summary atomsummary = element atom:summary { atomtextconstruct } # atom:content atominlinetextcontent = element atom:content { attribute type { "TEXT" "HTML" atommediatype }?, (text)* } atominlinexhtmlcontent = element atom:content { attribute type { "XHTML" atommediatype }?, (text anyelement)* } Nottingham & Sayre Expires July 27, 2005 [Page 42]

43 atomoutoflinecontent = element atom:content { attribute type { "TEXT" "HTML" "XHTML" atommediatype }?, attribute src { atomuri }, empty } atomcontent = atominlinetextcontent atominlinexhtmlcontent atomoutoflinecontent # atom:introspection atomintrospection = element atom:introspection { atomserviceconstruct } # atom:post atompost = element atom:post { atomserviceconstruct } # atom:edit atomedit = element atom:edit { atomserviceconstruct } # atom:tagline atomtagline = element atom:tagline { atomtextconstruct } # atom:generator atomgenerator = element atom:generator { attribute url { atomuri }?, attribute version { text }?, text } # atom:info atominfo = element atom:info { atomtextconstruct } Nottingham & Sayre Expires July 27, 2005 [Page 43]