Network Working Group Internet-Draft January 25, 2006 Expires: July 29, Feed Rank draft-snell-atompub-feed-index-05.txt. Status of this Memo

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1 Network Working Group J. Snell Internet-Draft January 25, 2006 Expires: July 29, 2006 Status of this Memo Feed Rank draft-snell-atompub-feed-index-05.txt By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 29, Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document defines a mechanism for numerically ranking entries within a syndication feed. Snell Expires July 29, 2006 [Page 1]

2 Table of Contents 1. Introduction Notational Conventions Ranking Domains and Schemes Ranking Domain Scope Ranking Domain Identifiers Ranking Entries Processing Rankings Well-known Domains and the Default Ranking Scheme Security Considerations IANA Considerations References Author s Address Intellectual Property and Copyright Statements Snell Expires July 29, 2006 [Page 2]

3 1. Introduction In syndication document formats such as Atom [RFC4287] and RSS, the order of entries as presented in the list is typically insignificant. This presents a challenge when the list of entries is intended to represent an ordered or ranked set. This document specifies a mechanism that allows feed publishers to establish numeric rankings for entries within a feed to be used as a means of organizing and sorting those entries. Although this document refers to Atom normatively, the mechanism described herein can be used with similar syndication formats, such as the various flavors of RSS. 2. 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]. In this specification, "entry" refers to an atom:entry element or similar construct from other syndication formats (e.g., RSS) that are contained within a feed. In this specification, "feed" refers to an Atom Feed Document or similar syndication format (e.g., RSS) that contains a collection of entries. In this specification, "head section" refers to the children of a feed document s document-wide metadata container; e.g., the child elements of the atom:feed element in an Atom Feed Document or the children of the RSS channel element. This specification uses XML Namespaces [W3C.REC-xml-names ] to uniquely identify XML element names. It uses the following namespace prefix for the indicated namespace URI; "r": " This specification uses terms from the XML Infoset [W3C.REC-xmlinfoset ]. However, this specification uses a shorthand; the phrase "Information Item" is omitted when naming Element Information Items. Therefore, when this specification uses the term "element," it is referring to an Element Information Item in Infoset terms. 3. Ranking Domains and Schemes A Ranking Domain is a uniquely identifiable logical collection of entries containing numeric ranking values conforming to a Ranking Snell Expires July 29, 2006 [Page 3]

4 Scheme. Ranking Schemes identify the specific rules for how the numeric ranking values associated with a given Ranking Domain are to be interpreted. Each Ranking Scheme is associated with exactly one Ranking Domain. Ranking Schemes are defined using the r:ranking-scheme element. rankingscheme = element r:ranking-scheme { atomcommonattributes, attribute domain { IRI }?, attribute label { text }?, attribute significance { ascending descending }?, attribute precision { nonnegativeinteger }?, attribute minimum { decimal }?, attribute maximum { decimal }? } o The domain attribute identifies the Ranking Domain for the Ranking Scheme. o The label attribute specifies a language-sensitive, human readable label for the Ranking Scheme. o The significance attribute indicates how implementations are to interpret the significance of an entries numeric ranking value. A value of descending indicates that the significance of the rank descends as the numeric ranking value increases. A value of ascending indicates that the significance of the rank increases as the numeric ranking value increases. If not specified, the significance is considered to be ascending. o The precision attribute specifies the level of precision to be applied to the value of the numeric ranking value. The value is expressed as a non-negative integer. If not specified the value is considered to be indeterminate (e.g., the number 2.0 is not distinct from the number 2.00). Ranking schemes that are based on fractional numeric ranking values SHOULD specify a precision. o The minumum attribute specifies the lowest possible numeric ranking value (inclusive). If not specified, the minimum value is considered to be 0. o The maximum attribute specifies the highest possible numeric ranking value (inclusive). If not specified, no maximum value is considered to apply. Feeds MAY contain any number of r:ranking-scheme elements. A Feed MUST NOT contain more than one r:ranking-scheme element with the same Ranking Domain. Snell Expires July 29, 2006 [Page 4]

5 <feed xmlns=" xmlns:r=" <r:ranking-scheme domain="tag:example.org,2006:movie_reviews" xml:lang="en-us" label="five Star Reviews" precision="0" significance="ascending" minimum="0" maximum="5" />... </feed> 3.1. Ranking Domain Scope Ranking Domain s provide a logical mechanism used to associate a numerically ranked set of resources. Ranking Domains are identified by IRI s. Ranking Domains fall into one of three scopes: o Feed Scope (known as the "Feed Ranking Domain") o Document Scope (known as the "Document Ranking Domain") o Domain Scope Ranking Schemes and numeric ranking values that do not specify a domain attribute are automatically associated with the Feed Ranking Domain. The IRI identity of the Feed Ranking Domain is the same as the feed s unique identifier (e.g., atom:id in atom:feed elements). A Ranking-Scheme associated with the Feed Ranking Domain <r:ranking-scheme xml:lang="en-us" label="five Star Reviews" significance="ascending" precision="0" min-value="0" max-value="5" /> The set of entries contained within the Feed Ranking Domain is limited to the set of entries contained within the Feed. Ranking Schemes and numeric rankings that specify a domain equal or equivalent to the Feed Document s Base URI are associated with the Document Ranking Domain. The IRI identity of the Document Ranking Domain is the normalized form of the containing Feed Document s Base URI. Snell Expires July 29, 2006 [Page 5]

6 A Ranking-Scheme associated with the Document Ranking Domain <r:ranking-scheme domain="" xml:lang="en-us" label="five Star Reviews" significance="ascending" precision="0" min-value="0" max-value="5" /> The set of entries contained within the Document Ranking Domain is limited to the set entries contained within the Document. Ranking Schemes and numeric ranking values that specify any IRI value other than the Base URI of the containing document are associated with a Domain Scope. A Ranking-Scheme associated with a Domain Scope <r:ranking-scheme domain="tag:example.com,2006:movie_reviews" xml:lang="en-us" label="five Star Reviews" significance="ascending" precision="0" min-value="0" max-value="5" /> Domain Scoped SHOULD be considered to be open sets consistings of entries from any number of feeds Ranking Domain Identifiers The IRI s identifying Ranking Domains are subject to the same construction and comparison rules as the atom:id element. 4. Ranking Entries Entries within a feed MAY contain zero or more r:rank elements specifing a numeric ranking value within a given Ranking Domain. An entry MUST NOT contain more than one r:rank element per Ranking Domain. rankingvalue = element r:rank { attribute domain {IRI}?, (decimal} } The domain attribute identifies the Ranking Domain. Snell Expires July 29, 2006 [Page 6]

7 The value of the r:rank is a decimal value conforming to the XML Schema decimal data type [W3C.REC-xmlschema ]. The value MUST NOT contain any leading or trailing whitespace. <r:rank domain="tag:example.com,2006:movie_reviews">3.5</r:rank> 5. Processing Rankings Processing a Ranking Domain involves the following steps: Identify the Ranking Domain Resolve the applicable Ranking Scheme for the identified Ranking Domain Identify the available set of entries containing numeric ranking values within the identified Ranking Domain. The members of this set MAY span multiple feeds. Remove from the set all entries whose rankings fall outside the minimum and maximum values set by the domain s Ranking Scheme. Sort the remaining set of ranked entries according to the significance and precision of the numeric ranking as specified by the domain s Ranking Scheme. 6. Well-known Domains and the Default Ranking Scheme Feeds MAY contain ranked entries within domains that have no corresponding r:ranking-scheme element. Software implementations MAY attempt to match such domains to well-known Ranking Domains and Schemes. For instance, an online search engine may choose to define a ranking scheme that is reflective of the relevance of a given result to a search query; rather than require that a r:ranking-scheme be included in every feed where the Ranking Scheme may be used, the search engine may separately publish its Ranking Scheme and associated Ranking Domain. If multiple Ranking Schemes are resolved for a given Ranking Domain, implementations MAY select any of the resolved schemes to apply while ignoring the remainder. If a Ranking Scheme for a given domain cannot be resolved (e.g., no r:ranking-scheme with a matching domain attribute can be found and the domain is not well-known), the Ranking Domain SHOULD be associated with the Default Ranking Scheme defined below. o label = Default Ranking o significance = ascending Snell Expires July 29, 2006 [Page 7]

8 o precision = unspecified o minumum = 0 o maximum = unspecified 7. Security Considerations There are no security considerations introduced by this specification. 8. IANA Considerations There are no IANA considerations introduced by this specification. 9. References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March [RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, December [W3C.REC-xml-infoset ] Tobin, R. and J. Cowan, "XML Information Set (Second Edition)", W3C REC REC-xml-infoset , February [W3C.REC-xml-names ] Hollander, D., Bray, T., and A. Layman, "Namespaces in XML", W3C REC REC-xml-names , January [W3C.REC-xmlschema ] Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes Second Edition", W3C REC REC-xmlschema , October Snell Expires July 29, 2006 [Page 8]

9 Author s Address James M Snell Phone: jasnell@gmail.com URI: Snell Expires July 29, 2006 [Page 9]

10 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 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 (2006). 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. Snell Expires July 29, 2006 [Page 10]

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