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Network Working Group                                           J. Snell
Internet-Draft                                          December 6, 2005
Expires: June 9, 2006


  Atom Metadata Expiration: Specifying Expiration  Timestamps for Atom
                        Feed and Entry metadata
                draft-snell-atompub-feed-expires-06.txt

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
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   This Internet-Draft will expire on June 9, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo presents a mechanism that allows feed publishers to express
   maximum age and expiration properties for information content within
   an Atom entry.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   3.  The 'expires' extension element . . . . . . . . . . . . . . . . 4
   4.  The 'max-age' extension element . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9







































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1.  Introduction

   This document specifies a mechanism that allows the expression of
   expiration timestamps and maximum age properties for information
   content within the Atom Syndication Format ([RFC4287]).  The
   mechanism defines two mutually exclusive extension elements that may
   be used to specify either an exact instant that the information
   content of an atom:entry expires, or a maximum age from the moment
   specified by an entries atom:updated element.

   When an atom:feed, atom:entry or atom:source contains an 'expires' or
   'max-age' extension element, the information content of the contained
   element is considered to be 'time constrained'.  Time constrainted
   information content is considered to be either 'active' or 'expired'.
   The default state is 'active'.  When the age (calculated in
   miliseconds from the moment specified by the atom:published or atom:
   updated elements) exceeds the value specified by the 'max-age'
   extension, or when the moment specified by the 'expires' extension
   elements passes, the state of the time constrained information
   content MUST be considered to be 'expired' and no longer valid.  It
   is strongly recommended that implementations either discard 'expired'
   information content or otherwise warn users that the information
   content has expired.

   The mechanism defined herein MUST NOT be used to support the caching
   of Atom documents and MUST NOT be used to schedule when a client
   should revisit/refresh local copies of Atom documents.  Specifically,
   the 'expires' and 'max-age' extension elements are relevant only to
   the informational content within an atom:entry and are not adequate
   mechanism for determining the freshness of a cached copy of the
   document in which the elements appear.

   The presence of an 'expires' or 'max-age' extension within an atom:
   feed or atom:source element is relevant only to the informational
   content of the containing element and not to any associated atom:
   entry elements.

   Section 6.3 of the Atom Format specification indicates that Atom
   processors that encounter unknown extensions MUST ignore those
   extensions without altering their behavior.  Because of this
   requirement, there can be no assumption that a particular software
   implementation will support the extensions defined herein.


2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

   In this specification, "entry" refers to an atom:entry element.

   In this specification, "feed" refers to an Atom Feed Document.

   In this specification, "head section" refers to the children of a
   feed's document-wide metadata container; e.g., the child elements of
   the atom:feed element in an Atom Feed Document.

   In this specification, the term "extension element" refers to a
   structured extension element as described in [RFC4287].

   In this specification, the term "information content" refers to the
   collection of metadata relevant to an atom:entry element.

   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;

   {Ed.  Note: this namespace MUST be changed to a proper IETF namespace
   scheme prior to publication}
     "age": "http://purl.org/atompub/age/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 defines the notion of a Non-Negative Integer
   Construct whose value MUST conform to the canonical representation of
   the "nonNegativeInteger" construction defined by [W3C.REC-xmlschema-
   2-20041028].  The value of the Non-Negative Integer Construct MUST
   NOT contain any leading or trailing whitespace.
     nonNegativeIntegerConstruct =
         atomCommonAttributes,
         xsd:nonNegativeInteger

   Example Non-Negative Integer Constructs
     <age:max-age>0</age:max-age>
     <age:max-age>1000000</age:max-age>


3.  The 'expires' extension element

   The 'expires' extension element conforms to the Atom Date Construct



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   and is used to indicate a precise moment in time after which the
   information content of the containing element MUST be considered to
   have "expired".
     expiresElement = element age:expires { atomDateConstruct }

   o  atom:entry elements MAY contain exactly one 'expires' extension
      element.
   o  atom:feed elements MAY contain exactly one 'expires' extension
      element.
   o  atom:source elements MAY contain exactly one 'expires' extension
      element.
   o  An atom:entry that contains an 'expires' extension element MUST
      NOT contain a 'max-age' extension element.
   o  An atom:feed that contains an 'expires' extension element MUST NOT
      contain a 'max-age' extension element.
   o  An atom:source that contains an 'expires' extension element MUST
      NOT contain a 'max-age' extension element.
   o  The value of the 'expires' extension element MUST NOT specify a
      date and time earlier than the moment specified by the atom:
      updated element or the atom:published element (if present).  If an
      'expires' element does specify a date and time earlier than either
      the atom:updated or atom:published elements, the 'expires' element
      MUST be ignored.

   For example
     <feed xmlns="http://www.w3.org/2005/Atom"
       xmlns:age="http://purl.org/atompub/age/1.0">
       <id>http://www.example.com/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.com/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:entries.com,2005:1</td>
         <title>An entry with an expiration timestamp</title>
         <link href="http://www.example.com/entries/1/1" />
         <updated>2005-07-28T12:00:00Z</updated>
         <age:expires>2005-12-12T12:00:00Z</age:expires>
         <summary>An entry with an expiration</summary>
       </entry>
     </feed>


4.  The 'max-age' extension element

   The 'max-age' extension element is used to indicate a maximum age of
   the information content in miliseconds from the moment specified by
   the atom:published element (if present) or the atom:updated element



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   after which the information content MUST be considered to have
   "expires".  The value is specified as a non-negative integer
   conforming to the nonNegativeIntegerConstruct.
     maxageElement = element age:max-age { nonNegativeIntegerConstruct }

   o  atom:entry elements MAY contain exactly one 'max-age' extension
      element.
   o  atom:feed elements MAY contain exactly one 'max-age' extension
      element.
   o  atom:source elements MAY contain exactly one 'max-age' extension
      element.
   o  An atom:entry that contains a 'max-age' extension element MUST NOT
      contain an 'expires' extension element.
   o  An atom:feed that contains a 'max-age' extension element MUST NOT
      contain an 'expires' extension element.
   o  An atom:source that contains a 'max-age' extension element MUST
      NOT contain an 'expires' extension element.

   For example
     <feed xmlns="http://www.w3.org/2005/Atom"
            xmlns:age="http://purl.org/atompub/age/1.0">
       <id>http://www.example.com/myfeed</id>
       <title>My Example Feed</title>
       <updated>2005-07-28T12:00:00Z</updated>
       <link href="http://www.example.com/myfeed" />
       <author><name>James</name></author>
       <entry>
         <id>tag:entries.com,2005:1</td>
         <title>An entry with a maximum age of 20 seconds</title>
         <link href="http://www.example.com/entries/1/1" />
         <updated>2005-07-28T12:00:00Z</updated>
         <age:max-age>20000</age:max-age>
         <summary>expire after a mere 20 seconds</summary>
       </entry>
     </feed>


5.  Security Considerations

   It is possible for malicious intermediaries to alter the expiration
   metadata in an Atom document.  This risk can be mitigated by
   digitally signing the elements in which the 'expires' or 'max-age'
   extension elements appear.  It is strongly recommended that
   implementations discard expired information content or otherwise warn
   users that the information content has expired.






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6.  IANA Considerations

   There are no IANA considerations introduced by this specification.

7.  References

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

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

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

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

   [W3C.REC-xmlschema-2-20041028]
              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", W3C REC REC-xmlschema-2-20041028,
              October 2004.


Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from Eric Scheid,
   Walter Underwood, Henry Story, Tim Bray, Mark Nottingham, Robert
   Sayrer, James Holderness, Phil Ringnalda, Antone Roundy and the other
   members of Atom Publishing Format and Protocol working group during
   the development of this specification.

















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Author's Address

   James M Snell


   Phone:
   Email: jasnell@gmail.com
   URI:   http://snellspace.com











































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   Internet Society.




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