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Network Working Group                                      M. Nottingham
Internet-Draft                                         February 26,                                             June 25, 2006
Expires: August 30, December 27, 2006


             Feed History: Enabling Incremental Syndication
                draft-nottingham-atompub-feed-history-05


             Extensions for Multi-Document Syndicated Feeds
                draft-nottingham-atompub-feed-history-06

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   This Internet-Draft will expire on August 30, December 27, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification describes a technique for feed publishers to give
   hints about the completeness defines three types of a feed, and a means syndicated feeds that
   enable publication of retrieving
   "missed" entries from a incremental feed. across one or more feed documents.










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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . .  3
   3.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   4.  Publishing Incremental  Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Publishing Non-Incremental  Paged Feeds  . . . . . . . . . . . . . . .  5 . . . . . . . . . .  6
     5.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Reconstructing Feed State  Archived Feeds . . . . . . . . . . . . . . . . . .  5
   7. . . . . . .  8
     6.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . .  7 . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10 14
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 11 14
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11 15
   Appendix B.  Reconstructing Archived Feeds . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 13 17

































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1.  Introduction

   Syndication documents (e.g., those in formats

   Syndicated feeds of information (using such formats as Atom [RFC4287]
   and
   or RSS 2.0 [1]) usually only contain the last several entries in a
   larger channel (or "feed") of information.  Often, consuming software
   keeps copies 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 all entries feeds that have been previously seen,
   effectively keeping a history of allow the feed's contents.

   However, not all feeds benefit
   reconstruction of their state from this practice; in some, previous
   entries one or more feed documents;
   "complete" feeds, "paged" feeds and "archived" feeds.

   These types are not relevant to complementary; each has different properties and
   trade-offs:

   o  Complete feeds contain the current contents entire set of the feed.  For
   example, it's not desirable to keep history in this manner with a
   "top ten" feed; showing old entries would imply that the previous
   number in one is now number eleven, document,
      and so forth.

   Feeds that encourage this practice have a different problem.  If
   consuming software does not poll often enough, some entries may can be
   missed, causing them useful when it isn't desirable to be silently omitted.  For some applications,
   this is a serious error on its own.  Even in non-critical
   applications, this phenomenon "remember"
      previously-seen entries.
   o  Paged feeds split the logical feed's entries among multiple
      temporary documents.  This can cause publishers to make Feed
   Documents contain more be useful when entries than reasonably necessary, just to
   assure that consumers have an amply large window in which to
   reconstruct the feed's state.

   This specification describes a technique that allows feed publishers
   to give hints as to
      are not long-lived or stable, and the completeness client needs to access an
      arbitrary portion of a feed document, them, usually in close succession.
   o  Archived feeds split them among multiple permanent documents, and a means
   of retrieving "missed"
      can be useful when entries from a partial, or incremental, feed
   by linking archives of the feed together.

   Although are long-lived and it refers is important for
      clients to Atom normatively, the mechanism described
   herein can be used with similar syndication formats, such as see every one.

   See the
   various flavours feed type definitions below for examples of RSS. use cases for
   each.


2.  Terminology

   In  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification, "feed document" refers to an Atom Feed
   Document, RSS
   document or similar syndication format instance
   document.  It may contain any number of entries (in RSS, items), and
   may or may not are to be a complete representation of the logical feed.

   "Subscription document" refers interpreted as described in BCP 14 [RFC2119], as
   scoped to a feed document that always
   contains those conformance targets.

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML element names.  It uses the most recent entries available in following
   namespace prefix for the feed (often, the
   feed document that should be subscribed to).

   "Archive document" refers to a feed document that is archived; i.e.,
   the set of entries inside it does not change over time.  Entries
   within an archive MAY themselves change, however.




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   "Incremental feed" refers to a set of associated feed documents
   (namely, one subscription document and any number of archive
   documents) that can be combined to form a single, logical feed.

   Finally, "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.


3.  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 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 [2]
   for more information about specific values.


4.  Publishing Incremental Feeds

   Publishers who wish

   Although they refer to make a feed available incrementally should:

   1.  Periodically remove the least recent entries from the
       subscription document, moving them into archive documents.
   2.  In Atom normatively, the subscription document's head section, add a "previous"
       link relation, containing a URI reference to mechanisms described
   herein can be used with similar syndication formats, such as the most recent
       archive documents.
   various flavours of RSS.


3.  Terminology

   In archive documents' head sections, add a "previous" link
       relation, containing a URI reference this specification, "feed document" refers to the next most recent
       archive (if any).





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   4.  In archive documents' head sections, add a "current" link
       relation, containing the subscription document's URI reference.

   Publishers are not required to make all archive documents available;
   they may refuse to serve (e.g., with HTTP status code 403),
   Document, RSS document, or be
   unable to serve (e.g., with HTTP status code 404) an archive similar syndication instance document.

   Publishers  It
   may also duplicate contain any number of entries between the subscription
   document (in RSS, items), and the most current archive document; i.e., they may
   archive current entries.

   Note that because consumers are not required to re-fetch archived
   feeds that they've previously stored, changes to entries in those
   documents or may not
   be apparent a complete representation of the logical feed.

   "Head section" refers to all users.  Therefore, if the children of a
   publisher requires 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 change to be visible to 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 users (e.g.,
   correcting factual errors), they should consider publishing of the
   revised entry entries
   in the subscription feed, logical feed; any entry not actually in addition to (or instead
   of) the appropriate archive feed.  Conversely, unimportant changes
   (e.g., spelling corrections) might feed document
   SHOULD NOT be only effected in archive feeds.


5.  Publishing Non-Incremental Feeds

   Some consumers 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, even
   without explicit hints that enable it.  Because this entries seen over time,
   presenting the aggregate as the feed's contents.  This is undesirable
   for
   undesireable in some feeds, this section defines an explicit means of indicating
   that history should not be kept. situations.

   For example, 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.

   The fh:complete element, when present  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 subscription feed document it occurs in is a complete
   representation of the logical feed's entries.

   For example,

     <fh:complete/>


6.  Reconstructing



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   When presented with a feed document, a consumer MAY reconstruct History                     June 2006


4.1.  Examples

   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>NetTunes Queue</title>
    <description>The CDs you'll receive next.</description>
    <link href="http://example.org/"/>
    <fh:complete/>
    <link rel="self"
     href="http://nettunes.example.org/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>A Rush of Blood to the
   feed's state in a local store S by following these steps: Head</title>
      <link href="http://nettunes.example.org/Coldplay/rush"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
      <updated>2003-12-13T18:30:02Z</updated>
      <summary>More jangly guitars from Coldplay...</summary>
    </entry>
   </feed>

























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   1.  If


   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>NetTunes Queue</title>
     <link>http://nettunes.example.org/</link>
     <description>The CDs you'll receive next.</description>
     <language>en-us</language>
     <pubDate>Tue, 10 Jun 2003 04:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 10 Jun 2003 09:41:01 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@nettunes.example.org</managingEditor>
     <webMaster>webmaster@nettunes.example.org</webMaster>
     <fh:complete/>
     <item>
      <title>A Rush of Blood to the Head</title>
      <link>http://nettunes.example.org/Coldplay/rush</link>
      <description>More jangly guitars from Coldplay...
      </description>
      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
      <guid>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>
     </item>
    </channel>
   </rss>


5.  Paged Feeds

   A paged feed document's head section contains is a "current" link
       relation value C, dereference C and use the resulting set of linked feed
       document as D.
   2.  If documents that contain the fh:complete element is present in D's head section:
       1.  Remove all
   entries present in local store S.
       2.  Add all of the document D's entries to local store S.
       3.  Stop processing.
   3.  Otherwise:
       1.  Create an empty list L.
       2.  Consider logical feed, without any guarantees about the URI
   stability of the last archive document successfully
           stored documents' contents.

   Paged feeds are lossy; that is, it is not possible to local store S as A.
       3.  Consider guarantee that
   the entries in document D client will be able to reconstruct the logical feed as E.
       4.  If the document D server
   has a "previous" link relation value P in
           its head section, and P is not A,
           1.  Append P published it.  Some entries may be added 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 pages
   of document D's entries to the local store S,
           replacing any entries with the same identity.
       7.  Pop the last "previous" link relation from L, dereference its
           value and use the resulting feed document as D.
       8.  Repeat are acccessed, without the previous two steps until L is empty.
       9.  Add client becoming aware of them.

   Paged feeds can be useful when the number of entries E to is very large,
   infinite, or indeterminate.  Clients can "page" through the local store S, replacing any 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 same identity.

   In server, the instructions above, network, or the concept 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 an entry's identity is
   format-specific; e.g., documents.
   o  "last" - A URI that refers to the furthest following document in Atom, it is conveyed by a
      series of documents.
   o  "previous" - A URI that refers to the atom:id
   element; immediately preceding
      document in RSS 2, it is indicated by a series of documents.
   o  "next" - A URI that refers to the guid element.

   Consumers immediately following document
      in a series of documents.

   Paged feed documents MUST have at least one of these link relations
   present, and SHOULD warn users 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

   <?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>









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   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.nasa.gov/</link>
     <description>Liftoff to Space Exploration.</description>
     <language>en-us</language>
     <pubDate>Tue, 10 Jun 2003 04:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 10 Jun 2003 09:41:01 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@example.com</managingEditor>
     <webMaster>webmaster@example.com</webMaster>
     <atom:link rel="next"
      href="http://liftof.nasa.gov/index.rss?page=2"/>
     <item>
      <title>Star City</title>
      <link>http://liftoff.nasa.gov/2003/06/news-starcity</link>
      <description>How do not have Americans get ready to work with Russians
      aboard the complete feed,
   or an error International Space Station? They take a crash course
      in culture, language and protocol at Russia's <a
      href="http://howe.iki.rssi.ru/GCTC/gctc_e.htm"&gt;Star
      City</a>.</description>
      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
      <guid>http://liftoff.nasa.gov/2003/06/03.html#item573</guid>
     </item>
    </channel>
   </rss>


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 encountered a feed document that always contains the
   most recently added or changed entries available in the reconstruction process (e.g., by
   alerting logical feed
   (often, the user 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 is unavailable, or
   displaying pseudo-entries that inform



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   published at a particular URI MUST NOT change over time.

   Likewise, the user URI for a particular archive document MUST NOT change
   over time, so that some clients can recognise it and associate it with the
   entries may
   be missing).

   Consumers MAY cache archive documents and/or use contained therein.

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

   Clients can then "subscribe" to the feed, as long as polling the result is subscription
   document for recent changes.  If a client has missed some entries,
   the same as that
   achieved archives can be used to synchronise its state by following these steps.

   In particular, consumers MAY use fetching the "first" and "next" link
   relations, if available,
   archive documents it has not yet seen.

   Note that because archive documents are considered stable, changes to traverse the feed
   entries in a different manner.
   However, such consumers MUST NOT require their presense them may not be apparent to
   reconstruct all users.  Therefore, if a feed's state.

   When comparing URIs
   publisher requires a change to determine if an archive has been successfully
   stored, be visible to all users (e.g.,
   correcting factual errors), they should consider publishing the value of
   revised entry in the "self" link relation, if present, SHOULD 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 as to tie archived feeds together:

   o  "prev-archive" - A URI that refers to the immediately preceding
      archive document.
   o  "next-archive" - A URI of that refers to the immediately following
      archive document.
   o  "current" - A URI that, when dereferenced, returns a feed document it occurs in.




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      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].

   This specification does not bestow any semantics on the ordering of
   entries

   Archive document SHOULD also contain an fh:archive element in a feed; the only purpose of introducing ordering between their
   head sections, to indicate that they themselves are archives.

   For example,

     <fh:archive/>



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   Publishers are not required to make all archive documents is available;
   they may refuse to allow 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 to (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 reconstructed.


7. missing).

6.1.  Examples

   Non-Incremental Atom Feed

   Atom-formatted Subscription 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"> xmlns="http://www.w3.org/2005/Atom">
    <title>Example Feed</title>
    <link href="http://example.org/"/>
    <fh:complete/>
    <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>

















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   Incremental Atom Feed: 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="previous" 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>

   Incremental Atom Feed:


   Atom-formatted Archive Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom">
    <title>Example Feed</title>
    <link rel="current" href="http://example.org/index.atom"/>
    <link rel="self" href="http://example.org/2003/11/index.atom"/>
    <link rel="previous" 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>





























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   Incremental


   RSS 2.0 Feed: 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.nasa.gov/</link>
     <description>Liftoff to Space Exploration.</description>
     <language>en-us</language>
     <pubDate>Tue, 10 Jun 2003 04:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 10 Jun 2003 09:41:01 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@example.com</managingEditor>
     <webMaster>webmaster@example.com</webMaster>
     <atom:link rel="previous" rel="prev-archive"
      href="http://liftoff.nasa.gov/2003/05/index.rss"/>

     <item>
      <title>Star City</title>
      <link>http://liftoff.nasa.gov/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 <a
      href="http://howe.iki.rssi.ru/GCTC/gctc_e.htm"&gt;Star
      City</a&gt;.</description>
      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
      <guid>http://liftoff.nasa.gov/2003/06/03.html#item573</guid>
     </item>
    </channel>
   </rss>




















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   Incremental


   RSS 2.0 Feed: 2.0-formatted Archive 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.nasa.gov/</link>
     <description>Liftoff to Space Exploration.</description>
     <language>en-us</language>
     <pubDate>Tue, 30 May 2003 08:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 30 May 2003 10:31:52 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@example.com</managingEditor>
     <webMaster>webmaster@example.com</webMaster>
     <atom:link rel="current"
      href="http://liftoff.nasa.gov/index.rss"/>
     <atom:link rel="previous" rel="prev-archive"
      href="http://liftoff.nasa.gov/2003/04/index.rss"/>

     <item>
      <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.nasa.gov/2003/05/30.html#item572</guid>
     </item>
     <item>
      <title>The Engine That Does More</title>
      <link>http://liftoff.nasa.gov/2003/05/news-VASIMR.asp</link>
      <description>Before man travels to Mars, NASA hopes to
      design new engines that will let us fly through 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.nasa.gov/2003/05/27.html#item571</guid>
     </item>
    </channel>
   </rss>


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:




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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 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.nasa.gov/2003/05/27.html#item571</guid>
     </item>
    </channel>
   </rss> 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 consumer client to initiate excessive (or even an unending
   sequence of) network requests, causing denial of service (either to
   the consumer, client, the target server, and/or intervening networks).
   Consumers  Clients
   can mitigate this risk by requiring user intervention after a certain
   number of requests, or by limiting requests either



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   hard limit, or with heuristics.

   Consumers

   Clients should be mindful of resource limits when storing feed
   documents; to
   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.

   [1]  <http://blogs.law.harvard.edu/tech/rss>

   [2]  <http://www.iana.org/assignments/link-relations>



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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.
















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Author's Address

   Mark Nottingham

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













































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