Network Working Group M. Nottingham Internet-Draft November 22, 2006 Intended status: Standards Track Expires: May 26, 2007 Feed Paging and Archiving draft-nottingham-atompub-feed-history-08 Status of This Memo 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 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 May 26, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This specification defines three types of syndicated Web feeds that enable publication of entries across one or more feed documents. Nottingham Expires May 26, 2007 [Page 1] Internet-Draft Feed Paging and Archiving November 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Paged Feeds . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Publishing Archived Feeds . . . . . . . . . . . . . . . . 9 4.2. Consuming Archived Feeds . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11 Appendix B. Use in RSS 2.0 . . . . . . . . . . . . . . . . . . . 11 Nottingham Expires May 26, 2007 [Page 2] Internet-Draft Feed Paging and Archiving November 2006 1. Introduction Syndicated Web feeds (using such formats as Atom [RFC4287]) are often split up into multiple documents to save bandwidth, allow "sliding window" access, or for other purposes. This specification formalizes two types of feeds that can span one or more feed documents; "paged" feeds and "archived" feeds. Additionally, it defines "complete" feeds to cover the case when a single feed document explicitly represents all of the feed's entries. Each has different properties and trade-offs: o Complete feeds contain the entire set of entries in one document, and can be useful when it isn't desirable to "remember" previously-seen entries. o Paged feeds split the logical feed's entries among multiple temporary documents. This can be useful when entries in the feed are not long-lived or stable, and the client needs to access an arbitrary portion of them, usually in close succession. o Archived feeds split them among multiple permanent documents, and can be useful when entries are long-lived and it is important for clients to see every one. The semantics of a feed that combines these types is undefined by this specification. Although they refer to Atom normatively, the mechanisms described herein can be used with similar syndication formats; see Appendix B for one such use. 1.1. 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 element names. It uses the following namespace prefix for the indicated namespace URI; "fh": "http://purl.org/syndication/history/1.0" 1.2. Terminology In this specification, "feed document" refers to an Atom Feed Document or similar syndication instance document. It may contain any number of entries, and may or may not be a complete Nottingham Expires May 26, 2007 [Page 3] Internet-Draft Feed Paging and Archiving November 2006 representation of the logical feed. A "logical feed" is the set of entries associated with a particular feed (as contrasted with a feed document, which may contain a subset of them). "Head section" refers to a document's feed-wide metadata container; e.g., the child elements of the atom:feed element in an Atom Feed Document. This specification uses terms from the XML Infoset [W3C.REC-xml-infoset-20040204]. 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. This specification also uses Atom link relations to identify different types of links; see the Atom specification [RFC4287] for information about their syntax, and the IANA link relation registry for more information about specific values. Note that URI references in link relation values may be relative, and when they are used they must be absolutised, as described in Section 5.1 of [RFC3986]. 2. Complete Feeds A complete feed is a feed document that contains all of the entries of a logical feed; any entry not actually in the feed document SHOULD NOT be considered to be part of that feed. For example, a feed that represents a ranking that varies over time (such as "Top Twenty Records" or "Most Popular Items") should not have newer entries displayed alongside older ones. By marking this type feeds as complete, old entries are discarded when it is refreshed. The fh:complete element, when present in a feed's head section, indicates that the feed document it occurs in is a complete representation of the logical feed's entries. It is an empty element; this specification does not define any content for it. Nottingham Expires May 26, 2007 [Page 4] Internet-Draft Feed Paging and Archiving November 2006 Example: Atom-formatted Complete Feed NetMovies Queue The DVDs you'll receive next. 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Casablanca urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z Here's looking at you, kid... This specification does not address duplicate entries in complete feeds. 3. Paged Feeds A paged feed is a set of linked feed documents that together contain the entries of a logical feed, without any guarantees about the stability of the documents' contents. Paged feeds are lossy; that is, it is not possible to guarantee that clients will be able to reconstruct the contents of the logical feed at a particular time. Entries may be added or changed as the pages of the feed are accessed, without the client becoming aware of them. Therefore, clients SHOULD NOT present paged feeds as coherent or complete, or make assumptions to that effect. Paged feeds can be useful when the number of entries is very large, infinite, or indeterminate. Clients can "page" through the feed, only accessing a subset of the feed's entries as necessary. For example, a search engine might make query results available as a paged feed, so that queries with very large result sets do not Nottingham Expires May 26, 2007 [Page 5] Internet-Draft Feed Paging and Archiving November 2006 overwhelm the server, the network, or the client. The feed documents in a paged feed are tied together with the following link relations: o "first" - A URI that refers to the furthest preceding document in a series of documents. o "last" - A URI that refers to the furthest following document in a series of documents. o "previous" - A URI that refers to the immediately preceding document in a series of documents. o "next" - A URI that refers to the immediately following document in a series of documents. Paged feed documents MUST have at least one of these link relations present, and should contain as many as practical and applicable. Example: Atom-formatted Paged Feed Example Feed 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Run Amok urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z Some text. This specification does not address duplicate entries in paged feeds. 4. Archived Feeds An archived feed is a set of feed documents that can be combined to accurately reconstruct the entries of a logical feed. Unlike paged feeds, archived feeds enable clients to do this without losing entries. This is achieved by publishing a single subscription Nottingham Expires May 26, 2007 [Page 6] Internet-Draft Feed Paging and Archiving November 2006 document and (potentially) many archive documents. A subscription document is a feed document that always contains the most recently added or changed entries available in the logical feed. Archive documents are feed documents that contain less recent entries in the feed. The set of entries contained in an archive document published at a particular URI SHOULD NOT change over time. Likewise, the URI for a particular archive document SHOULD NOT change over time. The following link relations are used to tie subscription and archived feeds together: o "prev-archive" - A URI that refers to the immediately preceding archive document. o "next-archive" - A URI that refers to the immediately following archive document. o "current" - A URI that, when dereferenced, returns a feed document containing the most recent entries in the feed. Subscription documents and archive documents MUST have a "prev- archive" link relation, unless there are no preceding archives available. Additionally, archive documents SHOULD have "next- archive" and "current" link relations. Archive document SHOULD also contain an fh:archive element in their head sections to indicate that they are archives. fh:archive is an empty element; this specification does not define any content for it. Nottingham Expires May 26, 2007 [Page 7] Internet-Draft Feed Paging and Archiving November 2006 Example: Atom-formatted Subscription Document Example Feed 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Run Amok urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z Some text. Example: Atom-formatted Archive Document Example Feed 2003-11-24T12:00:00Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Scheduled To Run Amok urn:uuid:cdef5c6d5-gff8-4ebb-assa-80dwe44efkjo 2003-11-24T12:00:00Z Some text from an old, different entry. Nottingham Expires May 26, 2007 [Page 8] Internet-Draft Feed Paging and Archiving November 2006 4.1. Publishing Archived Feeds The requirement that archive documents be stable allows clients to safely assume that if they have retrieved one in the past, it will not meaningfully change in the future. As a result, if an archive document's contents are changed, some clients may not become aware of it. Therefore, if a publisher requires a change to be visible to all users (e.g., correcting factual errors), they should consider publishing the revised entry in the subscription feed, in addition to (or instead of) the appropriate archive feed. Conversely, unimportant changes (e.g., spelling corrections) might be only effected in archive feeds. Publishers SHOULD construct their feed documents in such a way as to make duplicate removal unambiguous (see Section 4.2). Publishers are not required to make all archive documents available; they may refuse to serve (e.g., with HTTP status code 403 or 410), or be unable to serve (e.g., with HTTP status code 404) an archive document. 4.2. Consuming Archived Feeds Typically, clients will "subscribe" to an archived feed by polling the subscription document for recent changes. If URI contained in the prev-archive link relation has not been processed in the past, the client can "catch up" with any missed entries by dereferencing it and adding the contained entries to the logical feed. This process should be repeated recursively until the client encounters a prev- archive link relation that has been processed, the end of the archive is indicated by a missing prev-archive link relation, or an error is encountered. If duplicate entries are found, clients SHOULD consider only the most recently updated entry to be part of the logical feed. If duplicate entries have the same update time-stamp, or none is available, the entry sourced from the most recently updated feed document SHOULD replace all other duplicates of that entry. In Atom-formatted archived feeds, two entries are duplicates if they have the same atom:id element. The update time of an entry is determined by its atom:updated element, and likewise the update time of a feed document is determined by its feed-level atom:updated element. Clients SHOULD warn users when they are not able to reconstruct the Nottingham Expires May 26, 2007 [Page 9] Internet-Draft Feed Paging and Archiving November 2006 entire logical feed (e.g., by alerting the user that an archive document is unavailable, or displaying pseudo-entries that inform the user that some entries may be missing). 5. IANA Considerations The "previous", "next" and "current" link relations have been previously registered, and no IANA action regarding them is required. This specification defines the following link relations: o Attribute Value: prev-archive o Description: A URI that refers to the immediately preceding archive document. o Expected display characteristics: none o Security considerations: See [ this document ] o Attribute Value: next-archive o Description: A URI that refers to the immediately following archive document. o Expected display characteristics: none o Security considerations: See [ this document ] 6. Security Considerations Feeds using these mechanisms could be crafted in such a way as to cause a client to initiate excessive (or even an unending sequence of) network requests, causing denial of service (either to the client, the target server, and/or intervening networks). Clients can mitigate this risk by requiring user intervention after a certain number of requests, or by limiting requests either according to a hard limit, or with heuristics. Clients should be mindful of resource limits when storing feed documents. To reiterate, they are not required to always store or reconstruct the feed when conforming to this specification; they only need inform the user when the reconstructed feed is not complete. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Nottingham Expires May 26, 2007 [Page 10] Internet-Draft Feed Paging and Archiving November 2006 Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, December 2005. [W3C.REC-xml-infoset-20040204] Cowan, J. and R. Tobin, "XML Information Set (Second Edition)", W3C REC REC-xml-infoset-20040204, February 2004. [W3C.REC-xml-names-19990114] Bray, T., Hollander, D., and A. Layman, "Namespaces in XML", W3C REC REC-xml-names-19990114, January 1999. 7.2. Informative References [RSS2] Winer, D., "RSS 2.0 Specification", 2005, . Appendix A. Acknowledgements The author would like to thank the following people for their contributions, comments and help: Danny Ayers, Thomas Broyer, Lisa Dusseault, Stefan Eissing, David Hall, Bill de Hora, Aristotle Pagaltzis, John Panzer, Dave Pawson, Garrett Rooney, Robert Sayre, James Snell, Henry Story. Any errors herein remain the author's, not theirs. Appendix B. Use in RSS 2.0 As previously noted, while this specification's extensions are described in terms of the Atom feed format, they are also useful in similar formats. This informative appendix demonstrates how they can be used in an RSS 2.0-formatted [RSS2] feed. In RSS 2.0-formatted feeds, two entries are duplicates if they have the same guid element. The update time of an entry is not defined by RSS 2.0, but the feed-level update time can be determined by the pubDate element. Nottingham Expires May 26, 2007 [Page 11] Internet-Draft Feed Paging and Archiving November 2006 RSS 2.0-formatted Complete Feed NetMovies Queue http://netmovies.example.org/ The DVDs you'll receive next. en-us Tue, 10 Jun 2003 04:00:00 GMT Tue, 10 Jun 2003 09:41:01 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@netmovies.example.org webmaster@netmovies.example.org Casablanca http://netmovies.example.org/movies/Casablanca Here's looking at you, kid... Tue, 03 Jun 2003 09:39:21 GMT urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a Nottingham Expires May 26, 2007 [Page 12] Internet-Draft Feed Paging and Archiving November 2006 RSS 2.0-formatted Paged Feed Liftoff News http://liftoff.nasa.gov/ Liftoff to Space Exploration. en-us Tue, 10 Jun 2003 04:00:00 GMT Tue, 10 Jun 2003 09:41:01 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@example.com webmaster@example.com Nottingham Expires May 26, 2007 [Page 13] Internet-Draft Feed Paging and Archiving November 2006 RSS 2.0-formatted Subscription Document Liftoff News http://liftoff.nasa.gov/ Liftoff to Space Exploration. en-us Tue, 10 Jun 2003 04:00:00 GMT Tue, 10 Jun 2003 09:41:01 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@example.com webmaster@example.com Star City http://liftoff.nasa.gov/2003/06/news-starcity How do Americans get ready to work with Russians aboard the International Space Station? They take a crash course in culture, language and protocol at Russia's Tue, 03 Jun 2003 09:39:21 GMT http://liftoff.nasa.gov/2003/06/03.html#item573 Nottingham Expires May 26, 2007 [Page 14] Internet-Draft Feed Paging and Archiving November 2006 RSS 2.0-formatted Archive Document Liftoff News http://liftoff.nasa.gov/ Liftoff to Space Exploration. en-us Tue, 30 May 2003 08:00:00 GMT Tue, 30 May 2003 10:31:52 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@example.com webmaster@example.com Sky watchers in Europe, Asia, and parts of Alaska and Canada will experience a partial eclipse of the Sun on Saturday, May 31st. Fri, 30 May 2003 11:06:42 GMT http://liftoff.nasa.gov/2003/05/30.html#item572 The Engine That Does More http://liftoff.nasa.gov/2003/05/news-VASIMR.asp Before man travels to Mars, NASA hopes to design new engines that will let us fly through the Solar System more quickly. The proposed VASIMR engine would do that. Tue, 27 May 2003 08:37:32 GMT http://liftoff.nasa.gov/2003/05/27.html#item571 Nottingham Expires May 26, 2007 [Page 15] Internet-Draft Feed Paging and Archiving November 2006 Author's Address Mark Nottingham EMail: mnot@pobox.com URI: http://www.mnot.net/ Nottingham Expires May 26, 2007 [Page 16] Internet-Draft Feed Paging and Archiving November 2006 Full 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. 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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Nottingham Expires May 26, 2007 [Page 17]