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Network Working Group                                           J. Snell
Internet-Draft                                           August 16, 2005
Expires: February 17, 2006


 Atom Entry Expiration: Specifying Expiration Timestamps for Atom Entry
                                metadata
                draft-snell-atompub-feed-expires-00.txt

Status of this Memo

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   This Internet-Draft will expire on February 17, 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  . . . . . . . . . . . . . . . . . . . . . 6
   7.   References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
        Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
   A.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
        Intellectual Property and Copyright Statements . . . . . . . . 8







































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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 ([I-D.ietf-atompub-
   format]).  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 entry contains an 'expires' or 'max-age' extension element,
   the information content of that entry 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:updated element) 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' entries or otherwise warn users that the
   information content of the entry has expired.

   The mechanism defined herein MUST NOT be used mechanism to support
   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 relevant to the Atom Feed and Entry Documents in which they
   happen to appear.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

   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 [I-D.ietf-atompub-



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   format].

   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;
     "age": "http://purl.org/syndication/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
   and is used to indicate a precise moment in time (inclusive) when the
   information content of an entry expires.
     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  If an atom:entry element does not contain an 'expires' extension
      element, the 'expires' extension element of the contained atom:
      source is considered to apply
   o  In an Atom Feed Document, if an atom:entry element or a contained
      atom:source element do not contain an 'expires' extension element,
      the 'expires' extension element in the feed head section are



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      considered to apply.
   o  An atom:entry 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  An atom:feed that contains an 'expires' extension element MUST NOT
      contain a 'max-age' extension element.

   For example
     <feed xmlns="http://www.w3.org/2005/Atom"
       xmlns:age="http://purl.org/syndication/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" />
         <age:expires>2005-12-12T12:00:00Z</age:expires>
         <summary>An entry with an expiration</summary>
       </entry>
     </feed>

   While the 'expires' element MAY appear as a child of the atom:feed
   and atom:source elements, it's effect is constrained to the
   information content of atom:entry elements.

4.  The 'max-age' extension element

   The 'max-age' extension element is used to indicate the maximum age
   of the information content in miliseconds from the moment specified
   by the atom:updated element.  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  If an atom:entry element does not contain an 'max-age' extension
      element, the 'max-age' extension element of the contained atom:
      source is considered to apply





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   o  In an Atom Feed Document, if an atom:entry element or a contained
      atom:source element do not contain an 'max-age' extension element,
      the 'max-age' extension element in the feed head section are
      considered to apply.
   o  An atom:entry that contains an 'max-age' extension element MUST
      NOT contain a 'expires' extension element.
   o  An atom:source that contains an 'max-age' extension element MUST
      NOT contain a 'expires' extension element.
   o  An atom:feed that contains an 'max-age' extension element MUST NOT
      contain a 'expires' extension element.

   For example
     <feed xmlns="http://www.w3.org/2005/Atom"
            xmlns:age="http://purl.org/syndication/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" />
         <age:max-age>20000</age:max-age>
         <summary>expire after a mere 20 seconds</summary>
       </entry>
     </feed>

   While the 'max-age' element MAY appear as a child of the atom:feed
   and atom:source elements, it's effect is constrained to the
   information content of atom:entry elements.

5.  Security Considerations

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

6.  IANA Considerations

   There are no IANA considerations introduced by this specification.







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7.  References

   [I-D.ietf-atompub-format]
              Sayre, R. and M. Nottingham, "The Atom Syndication
              Format", draft-ietf-atompub-format-11 (work in progress),
              August 2005.

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

   [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.


Author's Address

   James M Snell


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

Appendix A.  Acknowledgements

   The author gratefully acknowledges the feedback from Eric Scheid,
   Walter Underwood, Henry Story and the other members of Atom
   Publishing Format and Protocol working group during the development
   of this specification.











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