Network Working Group M. Nottingham Internet-Draft June 25, 2006 Expires: December 27, 2006 Extensions for Multi-Document Syndicated Feeds draft-nottingham-atompub-feed-history-06 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 December 27, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This specification defines three types of syndicated feeds that enable publication of entries across one or more feed documents. Nottingham Expires December 27, 2006 [Page 1] Internet-Draft Feed History June 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Paged Feeds . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 9. Normative References . . . . . . . . . . . . . . . . . . . . . 14 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15 Appendix B. Reconstructing Archived Feeds . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 Intellectual Property and Copyright Statements . . . . . . . . . . 17 Nottingham Expires December 27, 2006 [Page 2] Internet-Draft Feed History June 2006 1. Introduction Syndicated feeds of information (using such formats as Atom [RFC4287] or RSS 2.0) are often split up into multiple documents to save bandwidth, allow "sliding window" access, or for other purposes. This specification defines three types of feeds that allow the reconstruction of their state from one or more feed documents; "complete" feeds, "paged" feeds and "archived" feeds. These types are complementary; 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. See the feed type definitions below for examples of use cases for each. 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], as scoped to those conformance targets. 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" 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 Nottingham Expires December 27, 2006 [Page 3] Internet-Draft Feed History June 2006 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. Although they refer to Atom normatively, the mechanisms described herein can be used with similar syndication formats, such as the various flavours of RSS. 3. Terminology In this specification, "feed document" refers to an Atom Feed Document, RSS document, or similar syndication instance document. It may contain any number of entries (in RSS, items), and may or may not be a complete representation of the logical feed. "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. 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). 4. Complete Feeds A complete feed is a feed document that contains all of the entries in the logical feed; any entry not actually in the feed document SHOULD NOT be presented as part of that feed. It is sometimes important to distinguish a complete feed, because clients may attempt to keep a history of feed entries seen over time, presenting the aggregate as the feed's contents. This is undesireable in some situations. 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 them as complete feeds, old entries are discarded when the feed 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. For example, Nottingham Expires December 27, 2006 [Page 4] Internet-Draft Feed History June 2006 4.1. Examples Atom-formatted Complete Feed NetTunes Queue The CDs you'll receive next. 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 A Rush of Blood to the Head urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z More jangly guitars from Coldplay... Nottingham Expires December 27, 2006 [Page 5] Internet-Draft Feed History June 2006 RSS 2.0-formatted Complete Feed NetTunes Queue http://nettunes.example.org/ The CDs 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@nettunes.example.org webmaster@nettunes.example.org A Rush of Blood to the Head http://nettunes.example.org/Coldplay/rush More jangly guitars from Coldplay... Tue, 03 Jun 2003 09:39:21 GMT urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 5. Paged Feeds A paged feed is a set of linked feed documents that contain the entries in the 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 the client will be able to reconstruct the logical feed as the server has published it. Some entries may be added to the feed as the pages of the feed are acccessed, without the client becoming aware of them. 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 overwhelm the server, the network, or the client. Nottingham Expires December 27, 2006 [Page 6] Internet-Draft Feed History June 2006 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. 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]. 5.1. Examples 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. Nottingham Expires December 27, 2006 [Page 7] Internet-Draft Feed History June 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 6. Archived Feeds An archived feed is a set of feed documents that can be combined to accurately reconstruct a logical feed. Unlike paged feeds, archived feeds enable clients to do this without losing any entries. This is achieved by publishing a single subscription 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 (often, the feed document that should be subscribed to). Archive documents are feed documents that contain less recent entries in the feed. The set of entries contained in an archive document Nottingham Expires December 27, 2006 [Page 8] Internet-Draft Feed History June 2006 published at a particular URI MUST NOT change over time. Likewise, the URI for a particular archive document MUST NOT change over time, so that clients can recognise it and associate it with the entries contained therein. Typically, a logical feed will make a subscription feed available, and link it to a set of archive documents (also linked together) which contain progressively less recent entries. Clients can then "subscribe" to the feed, polling the subscription document for recent changes. If a client has missed some entries, the archives can be used to synchronise its state by fetching the archive documents it has not yet seen. Note that because archive documents are considered stable, changes to entries in them may not be apparent to all users. 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. The following link relations are used to tie 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 archives available. Archive documents SHOULD have "next-archive" and "current" link relations. 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]. Archive document SHOULD also contain an fh:archive element in their head sections, to indicate that they themselves are archives. For example, Nottingham Expires December 27, 2006 [Page 9] Internet-Draft Feed History June 2006 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. Clients SHOULD warn users when they are not able to reconstruct the complete, 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). 6.1. Examples 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. Nottingham Expires December 27, 2006 [Page 10] Internet-Draft Feed History June 2006 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 December 27, 2006 [Page 11] Internet-Draft Feed History June 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 December 27, 2006 [Page 12] Internet-Draft Feed History June 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 7. 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: Nottingham Expires December 27, 2006 [Page 13] Internet-Draft Feed History June 2006 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 ] 8. Security Considerations Feeds using the mechanisms described here 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. 9. 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 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. Nottingham Expires December 27, 2006 [Page 14] Internet-Draft Feed History June 2006 Appendix A. Acknowledgements The author would like to thank the following people for their contributions, comments and help: Danny Ayers, Thomas Broyer, 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. Reconstructing Archived Feeds One algorithm for reconstructing an archived feed into a complete, logical feed (S), give the subscription document (D) follows. 1. Create an empty list L. 2. Consider the URI of the last archive document successfully stored to local store S as A. 3. Consider the set of entries in document D as E. 4. If the document D has a "prev-archive" link relation value P in its head section, and P is not A, 1. Append P to L. 2. Dereference P and use the resulting feed document as D. 5. Repeat the previous step until no new P is found. 6. Add all of document D's entries to the local store S, replacing any entries with the same identity. 7. Pop the last "prev-archive" link relation from L, dereference its value and use the resulting feed document as D. 8. Repeat the previous two steps until L is empty. 9. Add the entries E to the local store S, replacing any entries with the same identity. In these instructions, the concept of an entry's identity is format- specific; e.g., in Atom, it is conveyed by the atom:id element; in RSS 2, it is indicated by the guid element. Nottingham Expires December 27, 2006 [Page 15] Internet-Draft Feed History June 2006 Author's Address Mark Nottingham Email: mnot@pobox.com URI: http://www.mnot.net/ Nottingham Expires December 27, 2006 [Page 16] Internet-Draft Feed History June 2006 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 (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. Nottingham Expires December 27, 2006 [Page 17]