    Network Working Group                                      M. Nottingham
    Internet-Draft                                             June 25, September 10, 2006
    Intended status: Standards Track
    Expires: December 27, 2006

                 Extensions for Multi-Document Syndicated Feeds
                    draft-nottingham-atompub-feed-history-06
    March 14, 2007

                           Feed Paging and Archiving
                    draft-nottingham-atompub-feed-history-07

    Status of this This Memo

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       This Internet-Draft will expire on December 27, 2006.
    March 14, 2007.

    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.

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    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.   1.1.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
       3.   1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
       4.  3
       2.  Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . .  4
         4.1. 2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
       5. 3.  Paged Feeds  . . . . . . . . . . . . . . . . . . . . . . . . .  6
         5.1. 3.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  7
       6. 4.  Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . .  8
         6.1. 4.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
       7. 5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
       8. 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
       9. 7.  Normative References . . . . . . . . . . . . . . . . . . . . . 14
       Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
       Appendix B.  Reconstructing Archived Feeds . . . . . . . . . . . . 15
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
       Intellectual Property and Copyright Statements . . . . . . . . . . 17

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    1.  Introduction

       Syndicated Web feeds of information (using such formats as Atom [RFC4287]
       [RFC4287] or RSS RSS
       2.0) are often split up into multiple documents to save
       bandwidth, save bandwidth,
       allow "sliding window" access, or for other purposes.

       This specification defines three formalizes two types of feeds that allow the
       reconstruction of their state from can span one or or
       more feed documents;
       "complete" feeds, 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.

       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 Although they refer to Atom normatively, the mechanisms described
       herein can be used with similar syndication formats, such as the
       various flavors of use cases for
       each.

    2. RSS.

    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], 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

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       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. 1.2.  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

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    Paging and Archiving         September 2006

       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.    "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.

       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.

    2.  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 considered to be 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,

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    4.1. 2.1.  Examples

       Atom-formatted Complete Feed

        The CDs DVDs you'll receive next.

        2003-12-13T18:30:02Z

          John Doe

        urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6

          <del class="diff modified">A Rush of Blood to the Head</del><ins class="diff modified">Casablanca</ins>

          urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a
          2003-12-13T18:30:02Z
          More jangly guitars from Coldplay...Here's looking at you, kid...

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       RSS 2.0-formatted Complete Feed

         <del class="diff modified">NetTunes </del><ins class="diff modified">NetMovies </ins>Queue
         http://nettunes.example.org/http://netmovies.example.org/
         The CDs 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@nettunes.example.orgeditor@netmovies.example.org
         webmaster@nettunes.example.orgwebmaster@netmovies.example.org

          <del class="diff modified">A Rush of Blood to the Head</del><ins class="diff modified">Casablanca</ins>
          http://nettunes.example.org/Coldplay/rushhttp://netmovies.example.org/movies/Casablanca
          More jangly guitars from Coldplay...
    Here's looking at you, kid...

          Tue, 03 Jun 2003 09:39:21 GMT
          urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a

    5. 3.  Paged Feeds

       A paged feed is a set of linked feed documents that contain the
       together 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 contents of the logical
       feed as the server
       has published it. at a particular time.  Some entries may be added to the feed as the pages
       or changed as
       the pages of the feed are acccessed, accessed, without the client becoming aware 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.

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       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. 3.1.  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.

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       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

          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

    6. 4.  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

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       published at a particular URI MUST SHOULD NOT change over time.

       Likewise, time.  Likewise,

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       the URI for a particular archive document MUST SHOULD NOT change
       over time, so change over
       time.

       These stability requirements allow clients to safely assume that if
       they have retrieved the archive document at a particular URI once, it
       will not meaningfully change in the future.  As a result, if an
       archive document's contents are changed, clients can recognise it and associate it with the
       entries contained therein.
    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.

       Typically, a logical feed will make a subscription feed available,
       and will link it to a set of archive archive
       documents (also linked together)
       together) which contain progressively less 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 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 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,

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       For example,

       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. 4.1.  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.

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       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.

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       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

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       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. 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:

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          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. 6.  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. 7.  Normative References

       [RFC2119]                       Bradner, S., "Key words for use in in
                                       RFCs to Indicate
                  Indicate Requirement Levels", Levels",
                                       BCP 14, RFC 2119, March 1997.

       [RFC3986]                       Berners-Lee, T., Fielding, R., and L. L.
                                       Masinter, "Uniform
                  Resource "Uniform Resource
                                       Identifier (URI): Generic Syntax", Syntax",
                                       STD 66,
                  66, RFC 3986, January 2005.

       [RFC4287]                       Nottingham, M. and R. Sayre, "The "The
                                       Atom Syndication
                  Syndication Format", RFC 4287, 4287,
                                       December 2005.

       [W3C.REC-xml-infoset-20040204]
                 [W3C.REC-xml-infoset-20040204]  Cowan, J. and R. Tobin, "XML "XML
                                       Information Set (Second
                  Edition)", (Second Edition)",
                                       W3C REC REC-xml-infoset-20040204,
                                       February 2004.

       [W3C.REC-xml-names-19990114]
               [W3C.REC-xml-names-19990114]    Bray, T., Hollander, D., and A. Layman, "Namespaces in
                  XML", W3C REC REC-xml-names-19990114, January 1999.
    A.

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                                       Layman, "Namespaces in XML", W3C
                                       REC REC-xml-names-19990114,
                                       January 1999.

    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.

    Author's Address

       Mark Nottingham

       EMail: mnot@pobox.com
       URI:   http://www.mnot.net/

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    Author's Address

       Mark Nottingham

       Email: mnot@pobox.com
       URI:   http://www.mnot.net/

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    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.

       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.

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    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
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       Copies of IPR disclosures made to the IETF Secretariat and any
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       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
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    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
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    Acknowledgment
    Acknowledgement

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    the IETF
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