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Network Working Group                                      M. NottinghamRequest for Comments: 5005                                September 2007Category: Standards TrackFeed Paging and ArchivingStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This specification defines three types of syndicated Web feeds that   enable publication of entries across one or more feed documents.   This includes "paged" feeds for piecemeal access, "archived" feeds   that allow reconstruction of the feed's contents, and feeds that are   explicitly "complete".Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Notational Conventions . . . . . . . . . . . . . . . . . .21.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .32.  Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . .33.  Paged Feeds  . . . . . . . . . . . . . . . . . . . . . . . . .44.  Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . .64.1.  Publishing Archived Feeds  . . . . . . . . . . . . . . . .84.2.  Consuming Archived Feeds . . . . . . . . . . . . . . . . .85.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .96.  Security Considerations  . . . . . . . . . . . . . . . . . . .97.  References . . . . . . . . . . . . . . . . . . . . . . . . . .107.1.  Normative References . . . . . . . . . . . . . . . . . . .107.2.  Informative References . . . . . . . . . . . . . . . . . .10Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .12Appendix B.  Use in RSS 2.0  . . . . . . . . . . . . . . . . . . .12Nottingham                  Standards Track                     [Page 1]

RFC 5005               Feed Paging and Archiving          September 20071.  Introduction   Syndicated Web feeds (using formats such as Atom [1]) are often split   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 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 the entries 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; seeAppendix 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 inRFC 2119 [2].   This specification uses XML Namespaces [3] 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"Nottingham                  Standards Track                     [Page 2]

RFC 5005               Feed Paging and Archiving          September 20071.2.  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   representation of the logical feed.   A "logical feed" is the complete set of entries associated with a   feed (as contrasted with a feed document, which may contain a subset   of entries).   "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 [4].  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 [1] 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 inSection5.1 of [5].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 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   feed 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                  Standards Track                     [Page 3]

RFC 5005               Feed Paging and Archiving          September 2007   Example: Atom-formatted Complete Feed   <?xml version="1.0" encoding="utf-8"?>   <feed xmlns="http://www.w3.org/2005/Atom"    xmlns:fh="http://purl.org/syndication/history/1.0">    <title>NetMovies Queue</title>    <subtitle>The DVDs you'll receive next.</subtitle>    <link href="http://example.org/"/>    <fh:complete/>    <link rel="self"     href="http://netmovies.example.org/jdoe/queue/index.atom"/>    <updated>2003-12-13T18:30:02Z</updated>    <author>      <name>John Doe</name>    </author>    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>    <entry>      <title>Casablanca</title>      <link href="http://netmovies.example.org/movies/Casablanca"/>      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>      <updated>2003-12-13T18:30:02Z</updated>      <summary>Here's looking at you, kid...</summary>    </entry>   </feed>   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 each document's 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.Nottingham                  Standards Track                     [Page 4]

RFC 5005               Feed Paging and Archiving          September 2007   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.   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   <?xml version="1.0" encoding="utf-8"?>   <feed xmlns="http://www.w3.org/2005/Atom">    <title>Example Feed</title>    <link href="http://example.org/"/>    <link rel="self" href="http://example.org/index.atom"/>    <link rel="next" href="http://example.org/index.atom?page=2"/>    <updated>2003-12-13T18:30:02Z</updated>    <author>      <name>John Doe</name>    </author>    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>    <entry>      <title>Atom-Powered Robots Run Amok</title>      <link href="http://example.org/2003/12/13/atom03"/>      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>      <updated>2003-12-13T18:30:02Z</updated>      <summary>Some text.</summary>    </entry>   </feed>   This specification does not address duplicate entries in paged feeds.Nottingham                  Standards Track                     [Page 5]

RFC 5005               Feed Paging and Archiving          September 20074.  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   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.  Archive documents SHOULD also have a "next-archive" link   relation, unless there are no following archives available.   Archive documents SHOULD indicate their associated subscription   documents using the "current" link relation.   Archive documents 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                  Standards Track                     [Page 6]

RFC 5005               Feed Paging and Archiving          September 2007   Example: Atom-formatted Subscription Document   <?xml version="1.0" encoding="utf-8"?>   <feed xmlns="http://www.w3.org/2005/Atom">    <title>Example Feed</title>    <link href="http://example.org/"/>    <link rel="self" href="http://example.org/index.atom"/>    <link rel="prev-archive"     href="http://example.org/2003/11/index.atom"/>    <updated>2003-12-13T18:30:02Z</updated>    <author>      <name>John Doe</name>    </author>    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>    <entry>      <title>Atom-Powered Robots Run Amok</title>      <link href="http://example.org/2003/12/13/atom03"/>      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>      <updated>2003-12-13T18:30:02Z</updated>      <summary>Some text.</summary>    </entry>   </feed>   Example: Atom-formatted Archive Document   <?xml version="1.0" encoding="utf-8"?>   <feed xmlns="http://www.w3.org/2005/Atom"    xmlns:fh="http://purl.org/syndication/history/1.0">    <title>Example Feed</title>    <link rel="current" href="http://example.org/index.atom"/>    <link rel="self" href="http://example.org/2003/11/index.atom"/>    <fh:archive/>    <link rel="prev-archive"     href="http://example.org/2003/10/index.atom"/>    <updated>2003-11-24T12:00:00Z</updated>    <author>      <name>John Doe</name>    </author>    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>    <entry>      <title>Atom-Powered Robots Scheduled To Run Amok</title>      <link href="http://example.org/2003/11/24/robots_coming"/>      <id>urn:uuid:cdef5c6d5-gff8-4ebb-assa-80dwe44efkjo</id>      <updated>2003-11-24T12:00:00Z</updated>      <summary>Some text from an old, different entry.</summary>    </entry>   </feed>Nottingham                  Standards Track                     [Page 7]

RFC 5005               Feed Paging and Archiving          September 2007   In this example, the feed archives are split into monthly chunks, and   the subscription document points to the most recent complete archive   "http://example.org/2003/11/index.atom" using the "prev-archive"   relation.  That document, in turn points to the previous archive   "http://example.org/2003/10/index.atom", and so on.  Note that the   "2003/11" archive does not have a "next-archive" relation, because it   is the most recent complete archive; although another archive   ("2003/12") may be under construction, it would be an error to link   to it before completion.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   the changes.   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 document, in   addition to (or instead of) the appropriate archive document.   Conversely, unimportant changes (e.g., spelling corrections) might be   only effected in archive documents.   Publishers SHOULD construct their feed documents in such a way as to   make duplicate removal unambiguous (seeSection 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 a 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 no time-stamps areNottingham                  Standards Track                     [Page 8]

RFC 5005               Feed Paging and Archiving          September 2007   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   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   This specification defines the following new relations that have been   added to the Link Relations registry:      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 [RFC5005]      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 [RFC5005]   Additionally, the "previous," "next", and "current" link relations   should be updated to refer to this document.6.  Security Considerations   Feeds using this mechanism have the same security considerations as   Atom [1].  Encryption and authentication security services can be   obtained by encrypting and/or signing the feed, as described in [1],   and may also be obtained through channel-based mechanisms (e.g., TLS   [6], HTTP authentication [7]) and/or transport (e.g., IPsec [8]).   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 aNottingham                  Standards Track                     [Page 9]

RFC 5005               Feed Paging and Archiving          September 2007   hard limit, or with heuristics.  Servers can mitigate this risk by   denying requests that they consider abusive (e.g., by closing the   connection or generating an error).   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 to inform the user when the reconstructed feed is not complete.   This specification does not define what it means when a logical   feed's component feed documents have different security mechanisms   applied.7.  References7.1.  Normative References   [1]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication        Format",RFC 4287, December 2005.   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [3]  Bray, T., Hollander, D., and A. Layman, "Namespaces in XML",        World Wide Web Consortium First Edition REC-xml-names-19990114,        January 1999,        <http://www.w3.org/TR/1999/REC-xml-names-19990114>.   [4]  Tobin, R. and J. Cowan, "XML Information Set (Second Edition)",        World Wide Web Consortium Recommendation REC-xml-infoset-        20040204, February 2004,        <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.   [5]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform        Resource Identifier (URI): Generic Syntax", STD 66,RFC 3986,        January 2005.7.2.  Informative References   [6]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)        Protocol Version 1.1",RFC 4346, April 2006.   [7]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,        Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:        Basic and Digest Access Authentication",RFC 2617, June 1999.Nottingham                  Standards Track                    [Page 10]

RFC 5005               Feed Paging and Archiving          September 2007   [8]  Kent, S. and K. Seo, "Security Architecture for the Internet        Protocol",RFC 4301, December 2005.   [9]  Winer, D., "RSS 2.0 Specification", 2005,        <http://www.rssboard.org/rss-specification>.Nottingham                  Standards Track                    [Page 11]

RFC 5005               Feed Paging and Archiving          September 2007Appendix 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, Vidya Narayanan,   Aristotle Pagaltzis, John Panzer, Dave Pawson, Garrett Rooney, Robert   Sayre, James Snell, Henry Story, and Franklin Tse.   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 [9] 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   lastBuildDate element, if present.   RSS 2.0-formatted Complete Feed   <?xml version="1.0"?>   <rss version="2.0"    xmlns:fh="http://purl.org/syndication/history/1.0">    <channel>     <title>NetMovies Queue</title>     <link>http://netmovies.example.org/</link>     <description>The DVDs you'll receive next.</description>     <fh:complete/>     <item>      <title>Casablanca</title>      <link>http://netmovies.example.org/movies/Casablanca</link>      <description>Here's looking at you, kid...      </description>      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>      <guid isPermaLink="false"      >urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>     </item>    </channel>   </rss>Nottingham                  Standards Track                    [Page 12]

RFC 5005               Feed Paging and Archiving          September 2007   RSS 2.0-formatted Paged Feed   <?xml version="1.0"?>   <rss version="2.0"    xmlns:atom="http://www.w3.org/2005/Atom">    <channel>     <title>Liftoff News</title>     <link>http://liftoff.example.net/</link>     <description>Liftoff to Space Exploration.</description>     <atom:link rel="next"      href="http://liftof.example.net/index.rss?page=2"/>     <item>      <title>Star City</title>      <link>http://liftoff.example.net/2003/06/news-starcity</link>      <description>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 Star City.      </description>      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>      <guid>http://liftoff.example.net/2003/06/03/starcity</guid>     </item>    </channel>   </rss>   RSS 2.0-formatted Subscription Document   <?xml version="1.0"?>   <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">    <channel>     <title>Liftoff News</title>     <link>http://liftoff.example.net/</link>     <description>Liftoff to Space Exploration.</description>     <atom:link rel="prev-archive"      href="http://liftoff.example.net/2003/05/index.rss"/>     <item>      <title>Star City</title>      <link>http://liftoff.example.net/2003/06/news-starcity</link>      <description>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 Star City.      </description>      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>      <guid>http://liftoff.example.net/2003/06/03/starcity</guid>     </item>    </channel>   </rss>Nottingham                  Standards Track                    [Page 13]

RFC 5005               Feed Paging and Archiving          September 2007   RSS 2.0-formatted Archive Document   <?xml version="1.0"?>   <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom"    xmlns:fh="http://purl.org/syndication/history/1.0">    <channel>     <title>Liftoff News</title>     <link>http://liftoff.example.net/</link>     <description>Liftoff to Space Exploration.</description>     <lastBuildDate>Fri, 30 May 2003 11:06:42 GMT</lastBuildDate>     <fh:archive/>     <atom:link rel="current"      href="http://liftoff.example.net/index.rss"/>     <atom:link rel="prev-archive"      href="http://liftoff.example.net/2003/04/index.rss"/>     <item>      <title>Upcoming Eclipse</title>      <link>http://liftoff.example.net/2003/05/30/eclipse</link>      <description>Sky watchers in Europe, Asia, and parts of      Alaska and Canada will experience a partial eclipse of the Sun      on Saturday, May 31st.</description>      <pubDate>Fri, 30 May 2003 11:06:42 GMT</pubDate>      <guid>http://liftoff.example.net/2003/05/30/eclipse</guid>     </item>     <item>      <title>The Engine That Does More</title>      <link>http://liftoff.example.net/2003/05/27/vasmir</link>      <description>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.</description>      <pubDate>Tue, 27 May 2003 08:37:32 GMT</pubDate>      <guid>http://liftoff.example.net/2003/05/27/vasmir</guid>     </item>    </channel>   </rss>Author's Address   Mark Nottingham   EMail: mnot@pobox.com   URI:http://www.mnot.net/Nottingham                  Standards Track                    [Page 14]

RFC 5005               Feed Paging and Archiving          September 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Nottingham                  Standards Track                    [Page 15]