Network Working Group M. Nottingham, Ed. Internet-Draft Expires: July 11, 2005 R. Sayre, Ed. Boswijck Memex Consulting January 10, 2005 The Atom Syndication Format draft-ietf-atompub-format-04 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 3668. 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 http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 11, 2005. 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 11, 2005 [Page 1] Internet-Draft Atom Format January 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Editorial Notes . . . . . . . . . . . . . . . . . . . . . 4 1.2 Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Conformance . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Notational Conventions . . . . . . . . . . . . . . . . . . 5 2. Atom Documents . . . . . . . . . . . . . . . . . . . . . . . 7 3. Common Atom Constructs . . . . . . . . . . . . . . . . . . . 8 3.1 Text Constructs . . . . . . . . . . . . . . . . . . . . . 8 3.1.1 "type" Attribute . . . . . . . . . . . . . . . . . . . 8 3.2 Person Constructs . . . . . . . . . . . . . . . . . . . . 9 3.2.1 "atom:name" Element . . . . . . . . . . . . . . . . . 9 3.2.2 "atom:uri" Element . . . . . . . . . . . . . . . . . . 9 3.2.3 "atom:email" Element . . . . . . . . . . . . . . . . . 9 3.3 Date Constructs . . . . . . . . . . . . . . . . . . . . . 9 3.4 Service Constructs . . . . . . . . . . . . . . . . . . . . 9 3.4.1 "href" Attribute . . . . . . . . . . . . . . . . . . . 10 3.5 Link Constructs . . . . . . . . . . . . . . . . . . . . . 10 3.5.1 "rel" Attribute . . . . . . . . . . . . . . . . . . . 10 3.5.2 "type" Attribute . . . . . . . . . . . . . . . . . . . 11 3.5.3 "href" Attribute . . . . . . . . . . . . . . . . . . . 11 3.5.4 "hreflang" Attribute . . . . . . . . . . . . . . . . . 11 3.5.5 "title" Attribute . . . . . . . . . . . . . . . . . . 11 3.5.6 "length" Attribute . . . . . . . . . . . . . . . . . . 11 3.6 Identity Constructs . . . . . . . . . . . . . . . . . . . 12 3.6.1 Dereferencing Identity Constructs . . . . . . . . . . 12 3.6.2 Comparing Identity Constructs . . . . . . . . . . . . 12 3.7 The Category Construct . . . . . . . . . . . . . . . . . . 13 3.7.1 The "term" Attribute . . . . . . . . . . . . . . . . . 13 3.7.2 The "scheme" Attribute . . . . . . . . . . . . . . . . 13 3.7.3 The "label" attribute . . . . . . . . . . . . . . . . 13 4. The "atom:feed" Element . . . . . . . . . . . . . . . . . . 14 4.1 "version" Attribute . . . . . . . . . . . . . . . . . . . 14 4.2 The "atom:head" Element . . . . . . . . . . . . . . . . . 14 4.2.1 "atom:title" Element . . . . . . . . . . . . . . . . . 14 4.2.2 "atom:link" Element . . . . . . . . . . . . . . . . . 14 4.2.3 "atom:category" Element . . . . . . . . . . . . . . . 15 4.2.4 "atom:introspection" Element . . . . . . . . . . . . . 15 4.2.5 "atom:post" Element . . . . . . . . . . . . . . . . . 15 4.2.6 "atom:author" Element . . . . . . . . . . . . . . . . 15 4.2.7 "atom:contributor" Element . . . . . . . . . . . . . . 15 4.2.8 "atom:tagline" Element . . . . . . . . . . . . . . . . 15 4.2.9 "atom:id" Element . . . . . . . . . . . . . . . . . . 16 4.2.10 "atom:generator" Element . . . . . . . . . . . . . . 16 4.2.11 "atom:copyright" Element . . . . . . . . . . . . . . 16 4.2.12 "atom:info" Element . . . . . . . . . . . . . . . . 16 4.2.13 "atom:updated" Element . . . . . . . . . . . . . . . 16 Nottingham & Sayre Expires July 11, 2005 [Page 2] Internet-Draft Atom Format January 2005 5. The "atom:entry" Element . . . . . . . . . . . . . . . . . . 18 5.1 "atom:title" Element . . . . . . . . . . . . . . . . . . . 18 5.2 "atom:link" Element . . . . . . . . . . . . . . . . . . . 18 5.3 "atom:category" Element . . . . . . . . . . . . . . . . . 18 5.4 "atom:edit" Element . . . . . . . . . . . . . . . . . . . 19 5.5 "atom:author" Element . . . . . . . . . . . . . . . . . . 19 5.6 "atom:contributor" Element . . . . . . . . . . . . . . . . 19 5.7 "atom:host" Element . . . . . . . . . . . . . . . . . . . 19 5.8 "atom:id" Element . . . . . . . . . . . . . . . . . . . . 19 5.9 "atom:updated" Element . . . . . . . . . . . . . . . . . . 19 5.10 "atom:published" Element . . . . . . . . . . . . . . . . 20 5.11 "atom:summary" Element . . . . . . . . . . . . . . . . . 20 5.12 "atom:content" Element . . . . . . . . . . . . . . . . . 20 5.12.1 "type" attribute . . . . . . . . . . . . . . . . . . 20 5.12.2 "src" attribute . . . . . . . . . . . . . . . . . . 21 5.12.3 Processing Model . . . . . . . . . . . . . . . . . . 21 5.13 "atom:copyright" Element . . . . . . . . . . . . . . . . 22 5.14 "atom:head" Element . . . . . . . . . . . . . . . . . . 22 6. Managing Feed State . . . . . . . . . . . . . . . . . . . . 23 7. Securing Atom Documents . . . . . . . . . . . . . . . . . . 24 7.1 Digital Signatures . . . . . . . . . . . . . . . . . . . . 24 7.2 Encryption . . . . . . . . . . . . . . . . . . . . . . . . 24 8. Embedding Atom in Other Formats . . . . . . . . . . . . . . 25 9. Extending Atom . . . . . . . . . . . . . . . . . . . . . . . 26 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27 10.1 Registry of Link Relations . . . . . . . . . . . . . . . 27 11. Security Considerations . . . . . . . . . . . . . . . . . . 29 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 12.1 Normative References . . . . . . . . . . . . . . . . . . . 30 12.2 Informative References . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 32 A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 33 B. Revision History . . . . . . . . . . . . . . . . . . . . . . 34 Intellectual Property and Copyright Statements . . . . . . . 36 Nottingham & Sayre Expires July 11, 2005 [Page 3] Internet-Draft Atom Format January 2005 1. Introduction Atom is an XML-based document format intended to allow 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 11, 2005 [Page 4] Internet-Draft Atom Format January 2005 1.2 Example A minimal, single-entry Atom Feed Document: Example Feed 2003-12-13T18:30:02Z John Doe Atom-Powered Robots Run Amok vemmi://example.org/2003/32397 2003-12-13T18:30:02Z 1.3 Conformance [[ talk about atom documents and atom consumers, and how requirements are placed on them ]] 1.4 Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119]. This specification uses XML Namespaces [W3C.REC-xml-names-19990114] to uniquely identify XML elements and attribute names. It uses the following namespace prefixes for the indicated namespace URIs; "atom": http://purl.org/atom/ns#draft-ietf-atompub-format-04 Note that the choice of any namespace prefix is arbitrary and not semantically significant. Atom is specified using terms from the XML Infoset [W3C.REC-xml-infoset-20011024]. However, this specification uses a Nottingham & Sayre Expires July 11, 2005 [Page 5] Internet-Draft Atom Format January 2005 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. Nottingham & Sayre Expires July 11, 2005 [Page 6] Internet-Draft Atom Format January 2005 2. Atom Documents This specification describes two kinds of Atom Documents; Atom Feed Documents and Atom Entry Documents. An Atom Feed Document is a representation of an Atom feed, including metadata about the feed, and some or all of the entries associated with it. Its 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. Both kinds of Atom documents are specified in terms of the XML Information Set, serialised as XML 1.0 [W3C.REC-xml-20040204] and identified with the "application/atom+xml" media type. Atom Documents MUST be well-formed XML. [[ Validity? ]] Atom constrains the appearance and content of elements and attributes; unless otherwise stated, Atom Documents MAY contain other Information Items as appropriate. In particular, Comment Information Items and Processing Instruction Information Items SHOULD be ignored in the normal processing of an Atom Document. Any element in an Atom Document MAY have an xml:base attribute. XML Base [W3C.REC-xmlbase-20010627] processing MUST be applied to any relative reference [RFC2396bis] present in an Atom Document. This includes such elements and attributes as specified by Atom itself, as well as those specified by extensions to Atom. Any element in an Atom Document MAY have an xml:lang attribute, whose content indicates the default natural language of the element's content. Requirements regarding the content and interpretation of xml:lang are specified in XML 1.0 [W3C.REC-xml-20040204] Section 2.12. [[ discussion of URI escaping and i18n, IRI ]] [[ discussion of white space ]] Atom is extensible. See the section titled 'Extending Atom' later in this document for a full description of how Atom Documents can be extended. Nottingham & Sayre Expires July 11, 2005 [Page 7] Internet-Draft Atom Format January 2005 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. 3.1.1 "type" Attribute Text constructs MAY have a "type" attribute. When present, the value MUST be one of "TEXT", "HTML" or "XHTML". If the "type" attribute is not provided, software MUST behave as though it were present with a value of "TEXT". Note that MIME media types [RFC2045] are not acceptable values for the "type" attribute. If the value is "TEXT", the content of the Text construct MUST NOT contain child elements. Such text is intended to be presented to humans in a readable fashion. Thus, software MAY display it using normal text rendering techniques such as proportional fonts, white-space collapsing, and justification. If the value of "type" is "HTML", the content of the Text construct MUST NOT contain child elements, and SHOULD be suitable for handling by software that knows HTML. The HTML markup must be escaped; for example, "
" as "<br>". The HTML markup SHOULD be such that it could validly appear directly within an HTML
element. Receiving software which displays the content MAY use the markup to aid in displaying it. 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. Nottingham & Sayre Expires July 11, 2005 [Page 8] Internet-Draft Atom Format January 2005 3.2 Person Constructs A Person construct is an element that describes a person, corporation, or similar entity. Person constructs MAY be extended by namespace-qualified element children. This specification assigns no significance to the order of appearance of the child elements of a Person construct. 3.2.1 "atom:name" Element The "atom:name" element's content conveys a human-readable name for the person. Person constructs MUST contain exactly one "atom:name" element. 3.2.2 "atom:uri" Element The "atom:uri" element's content conveys a URI associated with the person. Person constructs MAY contain an atom:uri element, but MUST NOT contain more than one. The content of atom:uri in a Person construct MUST be a URI reference [RFC2396bis]. xml:base [W3C.REC-xmlbase-20010627] processing MUST be applied to the atom:uri element's content. 3.2.3 "atom:email" Element The "atom:email" element's content conveys an e-mail address associated with the persons. Person constructs MAY contain an atom:email element, but MUST NOT contain more than one. Its content MUST be an e-mail address [RFC2822]. 3.3 Date Constructs A Date construct is an element whose content MUST conform to the date-time BNF rule in [RFC3339]. 3.4 Service Constructs A Service construct is an empty element that conveys the URI of an Atom Publishing Protocol [Atom-protocol] service associated with an entry or feed. The type of service is identified by the element name. A Service construct has the following attribute: Nottingham & Sayre Expires July 11, 2005 [Page 9] Internet-Draft Atom Format January 2005 3.4.1 "href" Attribute The "href" attribute contains the a URI pointing to the endpoint of the service named by the name attribute. atom:service elements MUST have a "href" attribute, whose value MUST be a URI reference [RFC2396bis]. xml:base processing MUST be applied to the "href" attribute. 3.5 Link Constructs A Link construct is an empty element that describes a connection from an Atom Document to another Web resource. 3.5.1 "rel" Attribute Link constructs MAY have an optional "rel" attribute that indicates the link relation type. If the "rel" attribute is not present, the link construct 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 [RFC2396bis]. Note that use of a relative reference to the "rel" value URI 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 10, and thus the URI 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 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 to which the resource described by the containing atom:feed or atom:entry element is related. For example, the feed for a site which discusses the performance of the search engine at "http://search.example.com" might contain, as a child of atom:head: Nottingham & Sayre Expires July 11, 2005 [Page 10] Internet-Draft Atom Format January 2005 An identical link might appear as a child of any atom:entry whose content contains a discussion of that same search engine. 3.5.2 "type" Attribute Link constructs 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 MAY be used as a hint to determine 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. 3.5.3 "href" Attribute The "href" attribute contains the link's URI. Link constructs MUST have a href attribute, whose value MUST be a URI reference [RFC2396bis]. xml:base [W3C.REC-xmlbase-20010627] processing MUST be applied to the href attribute's content. 3.5.4 "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 constructs MAY have an hreflang attribute, whose value MUST be a language tag [RFC3066]. 3.5.5 "title" Attribute The "title" attribute conveys human-readable information about the link. Link constructs MAY have a title attribute. 3.5.6 "length" Attribute The "length" attribute indicates an advisory length of the linked content in octets; it MAY be used as a hint to determine 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 constructs MAY have a length attribute. Nottingham & Sayre Expires July 11, 2005 [Page 11] Internet-Draft Atom Format January 2005 3.6 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 [RFC2396bis]. 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 pertains to all instantiations of a particular Atom entry or feed; revisions retain the same content in their Identity constructs. 3.6.1 Dereferencing Identity Constructs The content of an Identity construct MAY be dereferencable (e.g. an HTTP URI). However, processors MUST NOT assume it 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. 3.6.2 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. Nottingham & Sayre Expires July 11, 2005 [Page 12] Internet-Draft Atom Format January 2005 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. For example, "http://www.example.org/thing", "http://www.example.org/Thing", "http://www.EXAMPLE.org/thing" and "HTTP://www.example.org/thing" will all be considered different identifiers, despite their differences in case. Likewise, "http://www.example.com/~bob", "http://www.example.com/%7ebob" and "http://www.example.com/%7Ebob" will all be considered different identifiers, because URI %-escaping is significant for the purposes of comparison. 3.7 The Category Construct Category constructs contain information about a category to which an Atom feed or entry is associated. 3.7.1 The "term" Attribute The "term" attribute will be a string which identifies the category within the categorization scheme to which the entry or feed belongs. Category constructs MUST have a "term" attribute. 3.7.2 The "scheme" Attribute The is a URI that identifies a categorization scheme. Category constructs MAY have a "scheme" attribute. 3.7.3 The "label" attribute The "label" attribute provides a human-readable label that may be displayed in end-user applications. Category constructs MAY have a "label" attribute. Nottingham & Sayre Expires July 11, 2005 [Page 13] Internet-Draft Atom Format January 2005 4. 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. 4.1 "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-04 : 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-19990114] elements as children. This specification assigns no significance to the order of appearance of the child elements of atom:head. The following child elements are defined by this specification (note that the presence of some of these elements is required): 4.2.1 "atom:title" Element The "atom:title" element is a Text construct that conveys a human-readable title for the feed. atom:head elements MUST contain exactly one atom:title element. 4.2.2 "atom:link" Element The "atom:link" element is a Link construct that conveys a URI associated with the feed. The nature of the relationship is determined by the construct's rel attribute. atom:head elements MUST contain at least one atom:link element with a rel attribute value of "alternate". 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. Nottingham & Sayre Expires July 11, 2005 [Page 14] Internet-Draft Atom Format January 2005 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. 4.2.3 "atom:category" Element A Category Construct identifying a category with which the feed is associated. atom:head elements MAY contain any number of atom:category elements. 4.2.4 "atom:introspection" Element The "atom:introspection" element is a Service construct that conveys the URI of an introspection file associated with the feed. atom:head elements MUST NOT contain more than one atom:introspection element. 4.2.5 "atom:post" Element The "atom:post" element is a Service construct that conveys the URI used to add entries to the feed. atom:head elements MUST NOT contain more than one atom:post element. 4.2.6 "atom:author" Element The "atom:author" element is a Person construct that indicates the default author of the feed. atom:head 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. [[explain inheritance]] 4.2.7 "atom:contributor" Element The "atom:contributor" element is a Person construct that indicates a person or other entity who contributes to the feed. atom:head elements MAY contain one or more atom:contributor elements. 4.2.8 "atom:tagline" Element The "atom:tagline" element is a Text construct that conveys a human-readable description or tagline for the feed. atom:head elements MAY contain an atom:tagline element, but MUST NOT contain more than one. Nottingham & Sayre Expires July 11, 2005 [Page 15] Internet-Draft Atom Format January 2005 4.2.9 "atom:id" Element The "atom:id" element is an Identity construct that conveys a permanent, universally unique identifier for a feed. atom:head elements MAY contain an atom:id element, but MUST NOT contain more than one. 4.2.10 "atom:generator" Element The "atom:generator" element's content identifies the software agent used to generate the feed, for debugging and other purposes. atom:head elements MAY contain an atom:generator element, but MUST NOT contain more than one. 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 [RFC2396bis]. 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. 4.2.11 "atom:copyright" Element The "atom:copyright" element is Text construct that conveys a human-readable copyright statement for the feed. atom:head elements MAY contain an atom:copyright element, but MUST NOT contain more than one. The atom:copyright element SHOULD NOT be used to convey machine-readable licensing information. 4.2.12 "atom:info" Element The "atom:info" element is a Text construct that conveys a human-readable explanation of the feed format itself. atom:head elements MAY contain an atom:info element, but MUST NOT contain more than one. The atom:info element SHOULD NOT considered meaningful by processors; it is a convenience to publishers in certain situations. 4.2.13 "atom:updated" Element The "atom:updated" element is a Date construct indicating the most Nottingham & Sayre Expires July 11, 2005 [Page 16] Internet-Draft Atom Format January 2005 recent instant in time when the feed was modified in a way the producer considers significant. Therefore, not all modifications necessarily result in a changed atom:updated value. atom:head elements MUST contain exactly one atom:updated element. Nottingham & Sayre Expires July 11, 2005 [Page 17] Internet-Draft Atom Format January 2005 5. 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-04 : do not deploy". The atom:entry element MAY contain any namespace-qualified [W3C.REC-xml-names-19990114] elements as children. This specification assigns no significance to the order of appearance of the child elements of atom:entry. The following child elements are defined by this specification (note that it requires the presence of some of these elements): 5.1 "atom:title" Element The "atom:title" element is a Text construct that conveys a human-readable title for the entry. atom:entry elements MUST have exactly one "atom:title" element. 5.2 "atom:link" Element The "atom:link" element is a Link construct that conveys a URI associated with the entry. The nature of the relationship as well as the link itself is determined by the element's content. 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. 5.3 "atom:category" Element A Category Construct identifying a category with which the entry is Nottingham & Sayre Expires July 11, 2005 [Page 18] Internet-Draft Atom Format January 2005 associated. atom:entry elements MAY contain any number of atom:category elements. 5.4 "atom:edit" Element The "atom:edit" element is a Service construct that conveys the URI used to retrieve and edit the source representation of the entry. atom:entry elements MUST NOT contain more than one atom:edit element. 5.5 "atom:author" Element The "atom:author" element is a Person construct that indicates the default author of the entry. 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. 5.6 "atom:contributor" Element The "atom:contributor" element is a Person construct that indicates a person or other entity who contributes to the entry. atom:entry elements MAY contain one or more atom:contributor elements. 5.7 "atom:host" Element The "atom:host" element's content conveys a domain name or network address associated with the entry's origin. atom:entry elements MAY contain a single atom:host element. Its content MUST be a domain name [RFC1035], a dotted-decimal IPv4 address [RFC0791], or a colon-delimited IPv6 address [RFC2460]. 5.8 "atom:id" Element The "atom:id" element is an Identity construct that conveys a permanent, universally unique identifier for an entry. atom:entry elements MUST contain exactly one atom:id element. 5.9 "atom:updated" Element The "atom:updated" element is a Date construct indicating the most recent instant in time when the entry was modified in a way the producer considers significant. Therefore, not all modifications necessarily result in a changed atom:updated value. atom:entry elements MUST contain exactly one atom:updated element. Publishers MAY change the value of this element over time. Nottingham & Sayre Expires July 11, 2005 [Page 19] Internet-Draft Atom Format January 2005 Processors MAY present entries sorted using this value. Processors MAY choose not to present entries until the instant in time specified in the atom:updated element has passed. 5.10 "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. atom:entry elements MAY contain an atom:published element, but MUST NOT contain more than one. Processors MAY present entries sorted using this value. Processors MAY choose not to present entries until the instant in time specified in the atom:published element has passed. 5.11 "atom:summary" Element The "atom:summary" element is a Text construct that conveys a short summary, abstract or excerpt of the entry. atom:entry elements MAY contain an atom:summary element, but MUST NOT contain more than one. atom:entry elements MUST contain an atom:summary element in any of the following cases: o the atom:entry element contains no atom:content element. o the atom:entry contains an atom:content which has a "src" attribute (and is thus empty). o the atom:entry contains content which is encoded in Base64; i.e. the "type" attribute of atom:content is a MIME media type [RFC2045] and does not begin with "text/" nor end with "+xml". 5.12 "atom:content" Element The "atom:content" element either contains or links to the content of the entry. atom:entry elements MUST contain zero or one atom:content elements. 5.12.1 "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". Nottingham & Sayre Expires July 11, 2005 [Page 20] Internet-Draft Atom Format January 2005 5.12.2 "src" attribute atom:content MAY have a "src" attribute, whose value MUST be a URI reference [RFC2396bis]. If the "src" attribute is present, software MAY use the URI to retrieve the content. If the "src" attribute is present, atom:content MUST be empty. That is to say, the content may be retrievable using "src=" URI, or it may be contained within atom:content, but not both. If the "src" attribute is present, the "type" attribute SHOULD be provided and MUST be a MIME media type [RFC2045], rather than "TEXT", "HTML", or "XHTML". The value is advisory; that is to say, upon dereferencing the URI to retrieve the content, if the server providing that content also provides a media type, the server-provided media type is authoritative. If the value of type begins with "text/" or ends with "+xml", the content SHOULD be local; that is to say, no "src" attribute should be provided. 5.12.3 Processing Model Software MUST apply the following rules in succession in the order below to ascertain the rules governing the content of "atom:content". 1. If the value is "TEXT", the content of atom:content MUST NOT contain child elements. Such text is intended to be presented to humans in a readable fashion. Thus, software MAY display it using normal text rendering techniques such as proportional fonts, white-space collapsing, and justification. 2. If the value of "type" is "HTML", the content of atom:content MUST NOT contain child elements, and SHOULD be suitable for handling by software that knows HTML. The HTML markup must be escaped; for example, "
" as "<br>". The HTML markup SHOULD be such that it could validly appear directly within an HTML
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 Nottingham & Sayre Expires July 11, 2005 [Page 21] Internet-Draft Atom Format January 2005 that the "atom:content" element would contain a single child element which would serve as the root element of the XML document of the indicated type. 5. If the value of "type" begins with "text/" the content of atom:content MUST NOT contain child elements. 6. For all other values of "type", the content of atom:content MUST be a valid Base64 encoding [RFC3548], which when decoded SHOULD be suitable for handling by software that knows the indicated media type. In this case, the characters in the Base64 encoding may be preceded and followed in the atom:content element by whitespace, and lines are separated by a single newline (U+000A) character, as required by XML. 5.13 "atom:copyright" Element The "atom:copyright" element is a Text construct that conveys a human-readable copyright statement for the entry. atom:entry elements MAY contain an atom:copyright element, but MUST NOT contain more than one. The atom:copyright element SHOULD NOT be used to convey machine-readable licensing information. If an atom:entry element does not contain an atom:copyright element, then the atom:copyright element of the containing atom:feed element's atom:head element, if present, should be considered to apply to the entry. 5.14 "atom:head" Element The atom:head element acts as a container for metadata about the feed within which the entry was created. atom:entry elements MAY contain at most one atom:head element. If the atom:head element is present, it SHOULD be the first child element of atom:entry. 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 ... ]] Nottingham & Sayre Expires July 11, 2005 [Page 22] Internet-Draft Atom Format January 2005 6. Managing Feed State [[ talk about what it means to keep a view of a feed ]] Nottingham & Sayre Expires July 11, 2005 [Page 23] Internet-Draft Atom Format January 2005 7. Securing Atom Documents Because Atom is an XML-based format, existing XML security mechanisms can be used to secure its content. Note that while these mechanisms are available to secure Atom documents, they should not be used indiscriminately. 7.1 Digital Signatures The document element of an Atom document (i.e., atom:feed in an Atom Feed Document, atom:entry in an Atom Entry Document) MAY have an Enveloped Signature, as described by XML-Signature and Syntax Processing [W3C.REC-xmldsig-core-20020212]. Other XML signature mechanisms MUST NOT be used on the document element of an Atom document. Processors MUST NOT reject an Atom document containing such a signature because they are not capable of verifying it; they MUST continue processing and MAY inform the user of their failure to validate the signature. In other words, the presence of an element with the namespace URI "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature" as a child of the document element must not cause a processor to fail merely because of its presence. Other elements in an Atom document MUST NOT be signed unless their definitions explicitly specify such a capability. 7.2 Encryption The document element of an Atom document (i.e., atom:feed in an Atom Feed Document, atom:entry in an Atom Entry Document) MAY be encrypted, using the mechanisms described by XML Encryption Syntax and Processing [W3C.REC-xmlenc-core-20021210]. Other XML encryption mechanisms MUST NOT be used on the document element of an Atom document. Nottingham & Sayre Expires July 11, 2005 [Page 24] Internet-Draft Atom Format January 2005 8. Embedding Atom in Other Formats [[ ... ]] Nottingham & Sayre Expires July 11, 2005 [Page 25] Internet-Draft Atom Format January 2005 9. Extending Atom [[ ... ]] Nottingham & Sayre Expires July 11, 2005 [Page 26] Internet-Draft Atom Format January 2005 10. IANA Considerations An Atom Document, when serialized as XML 1.0, can be identified with the following media type: MIME media type name: application MIME subtype name: atom+xml Mandatory parameters: None. Optional parameters: "charset": This parameter has identical semantics to the charset parameter of the "application/xml" media type as specified in RFC 3023 [RFC3023]. [RFC3023]. Encoding considerations: Identical to those of "application/xml" as described in RFC 3023 [RFC3023], section 3.2. Security considerations: As defined in this specification. [[update upon publication]] In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in RFC 3023 [RFC3023], section 10. Interoperability considerations: There are no known interoperability issues. Published specification: This specification. [[update upon publication]] Applications which use this media type: No known applications currently use this media type. Additional information: Magic number(s): As specified for "application/xml" in RFC 3023 [RFC3023], section 3.2. File extension: .atom Fragment identifiers: As specified for "application/xml" in RFC 3023 [RFC3023], section 5. Base URI: As specified in RFC 3023 [RFC3023], section 6. Macintosh File Type code: TEXT Person and email address to contact for further information: Mark Nottingham Intended usage: COMMON Author/Change controller: This specification's author(s). [[update upon publication]] 10.1 Registry of Link Relations This registry is maintained by IANA and initially contains the two values: "alternate" and "related". New assignments are subject to IESG Approval, as outlined in [RFC2434]. Requests should be made by email to IANA, which will then forward the request to the IESG requesting approval. The request should contain discussion of at Nottingham & Sayre Expires July 11, 2005 [Page 27] Internet-Draft Atom Format January 2005 least the following five topics: o A value for the "rel" attribute that conforms to the syntax rule given in Section 3.5.1 o Common name for link type. o Description of link type semantics. o Expected display characteristics. o Security considerations. Nottingham & Sayre Expires July 11, 2005 [Page 28] Internet-Draft Atom Format January 2005 11. Security Considerations Atom document can be encrypted and signed using [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212], respectively, and is subject to the security considerations implied by their use. Nottingham & Sayre Expires July 11, 2005 [Page 29] Internet-Draft Atom Format January 2005 12. References 12.1 Normative References [Atom-autodiscovery] Pilgrim, M., "Atom Feed Autodiscovery", work-in-progress, August 2004. [Atom-protocol] Gregorio, J. and R. Sayre, "The Atom Publishing Protocol", work-in-progress, July 2004. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2396bis] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", awaiting RFC number, December 2004. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001. [RFC3066] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001. [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002. [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. Nottingham & Sayre Expires July 11, 2005 [Page 30] Internet-Draft Atom Format January 2005 [W3C.NOTE-datetime-19980827] Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C NOTE NOTE-datetime-19980827, August 1998. [W3C.REC-xml-20040204] Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T. and E. Maler, "Extensible Markup Language (XML) 1.0 (Third Edition)", W3C REC REC-xml-20040204, February 2004. [W3C.REC-xml-infoset-20011024] Tobin, R. and J. Cowan, "XML Information Set", W3C FirstEdition REC-xml-infoset-20011024, October 2001. [W3C.REC-xml-names-19990114] Hollander, D., Bray, T. and A. Layman, "Namespaces in XML", W3C REC REC-xml-names-19990114, January 1999. [W3C.REC-xmlbase-20010627] Marsh, J., "XML Base", W3C REC REC-xmlbase-20010627, June 2001. [W3C.REC-xmldsig-core-20020212] Solo, D., Reagle, J. and D. Eastlake, "XML-Signature Syntax and Processing", W3C REC REC-xmldsig-core-20020212, February 2002. [W3C.REC-xmlenc-core-20021210] Reagle, J. and D. Eastlake, "XML Encryption Syntax and Processing", W3C REC REC-xmlenc-core-20021210, December 2002. 12.2 Informative References [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. Nottingham & Sayre Expires July 11, 2005 [Page 31] Internet-Draft Atom Format January 2005 URIs [1] [2] Authors' Addresses Mark Nottingham (editor) EMail: mnot@pobox.com URI: http://www.mnot.net/ Robert Sayre (editor) Boswijck Memex Consulting EMail: rfsayre@boswijck.com URI: http://boswijck.com Nottingham & Sayre Expires July 11, 2005 [Page 32] Internet-Draft Atom Format January 2005 Appendix A. Contributors The following people contributed to preliminary drafts of this document: Tim Bray, Mark Pilgrim, and Sam Ruby. 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 11, 2005 [Page 33] Internet-Draft Atom Format January 2005 Appendix B. Revision History [[ this section should be removed before final publication. ]] -04: Update URI terms for 2396bis. Add Category construct (PaceCategoryRevised). Insert paranoid XHTML interpretation guidelines. Adjust atom:copyright, per chairs' instruction. Add atom:host as child element of atom:entry, per chairs' direction (PacePersonConstruct). Add link/content co-constraint (PaceContentOrLink). Remove atom:origin as a side effect of adding atom:head to atom:entry (PaceHeadInEntry). Add optional length attribute to atom:link (PaceLinkRelated). Add Link registry to Link Construct, IANA Considerations placeholder (PaceFieldingLinks). Change definition of atom:updated (PaceUpdatedDefinition). -03: Move definition of Link @rel to format spec, restrict acceptable values (PaceMoveLinkElement, PaceLinkAttrDefaults). Add Service Construct, head/post, head/introspection, entry/edit (PaceServiceElement). Add Text Construct, entry/content (PaceReformedContent3). Add entry/published (PaceDatePublished). Adjust definition of Identity Construct per chairs' direction to "fix it." Add Sayre to editors. -02: Removed entry/modified, entry/issued, entry/created; added entry/updated (PaceDateUpdated). Changed date construct from W3C date-time to RFC3339 (PaceDateUpdated). Feed links to HTML pages should be reflected back (PaceLinkReflection). Added Identity construct (PaceIdConstruct). Changed feed/id and entry/id to be Identity constructs (PaceIdConstruct). Changed entry/origin's content so that it's the same as the feed's id, rather than its link/@rel="alternate" (PaceIdConstruct). Added "Securing Atom Documents" (PaceDigitalSignatures). -01: Constrained omission of "Information Item" to just elements and attributes. Clarified xml:lang inheritence. Removed entry- and feed-specific langauge about xml:lang (covered by general discussion of xml:lang) Changed xml:lang to reference XML for normative requirements. Changed "... MUST be a string" to "... is unstructued text." Nottingham & Sayre Expires July 11, 2005 [Page 34] Internet-Draft Atom Format January 2005 Remomved langauge about DOCTYPEs, PIs, Comments, Entities. Changed atom:url to atom:uri, @url to @uri Introduced atom:head Introduced "Atom Feed Document" and "Atom Entry Document". Removed requirement for all elements and attributes to be namespace-qualified; now children of selective elements Added extensibility to Person constructs. Removed requirement for media types to be registered (non-registered media types are legal) Added atom:origin (PaceEntryOrigin) Added requirement for entry/id to be present and a URI (PaceEntryIdRequired). Clarified approach to Comments, PIs and well-formedness, as per RFC3470. Referenced escaping algorithm in XML. Assorted editorial nits and cleanup, refactoring -00: Initial IETF Internet-Draft submission. Added optional version attribute to entry (PaceEntryElementNeedsVersionAttribute). Added hreflang attribute (PaceHrefLang). Clarified inheritence of copyright element (PaceItemCopyright). Added xml:lang to entries (PaceItemLang). Tweaked Infoset-related language (PaceNoInfoSet). Clarified lack of structure in version attribute (PaceVersionAsText). Changed approach to XML Base (PaceXmlBaseEverywhere). Added XML Base processing to atom:id (PaceXmlBaseId). Various editorial cleanup and adjustments for IETF publication. Nottingham & Sayre Expires July 11, 2005 [Page 35] Internet-Draft Atom Format January 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Nottingham & Sayre Expires July 11, 2005 [Page 36]