Network Working Group                                           J. Snell
Internet-Draft                                           January 4, 2008
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: July 7, 2008

                   Atom Syndication Format Tombstones

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This specification adds mechanisms to the Atom Syndication Format
   which Atom Feed publishers can use to explicitly indicate that
   specific Atom entries have been removed from an Atom feed.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   3.  The at:deleted-entry element  . . . . . . . . . . . . . . . . . 3
   4.  The Trash Link Relation . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
     6.1.  The Link Relation Registration "Trash"  . . . . . . . . . . 6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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1.  Introduction

   This specification adds a new element to the Atom Syndication Format
   [RFC4287] that can be used to explicitly indicate that specific Atom
   entries have been removed from an Atom Feed Document.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, [RFC2119]

   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;

    "at": ""

3.  The at:deleted-entry element

   The at:deleted-entry element MAY appear as a child of atom:feed to
   represent an Atom Entry that has been removed from the feed.

     deletedEntry = element at:deleted-entry {
       attribute ref { atomUri },
       attribute when { atomDateConstruct }?,
       ( element at:by { atomPersonConstruct}?,
       & element at:comment {atomTextConstruct}?,
       & extensionElement* )

   The at:deleted-entry element MUST contain a ref attribute whose value
   specifies the atom:id of the entry that has been removed.

   The at:deleted-entry element MAY contain a when element whose value
   is an [RFC3339] "date-time" specifying the instant the entry was
   deleted.  An uppercase "T" character MUST be used to separate date
   and time, and an uppercase "Z" character MUST be present in the
   absence of a numeric time zone offset

   The at:deleted-entry element MAY contain one at:by element used to
   identify the entity that removed the entry from the feed.  The at:by
   element is an Atom Person Construct as defined by Section 3.2 of

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   The at:deleted-entry element MAY contain one at:comment element whose
   value provides additional information about the deletion operation.
   The atom:comment element is an Atom Text Construct as defined by
   Section 3.1 of [RFC4287].

   Atom Feed Documents MAY contain any number of at:deleted-entry
   elements, but MUST NOT contain more than one with the same ref
   attribute value.

     <feed xmlns=""
        <!-- Minimal deleted-entry -->
          ref=",2005:/entries/1" />

        <!-- Extended deleted-entry -->
            <name>John Doe</name>
          <at:comment>Removed comment spam</at:comment>

   An Atom feed MAY contain atom:entry elements and at:deleted-entry
   elements sharing the same atom:id value.  Atom processors MUST ignore
   any at:deleted-entry elements sharing an atom:id value with an atom:
   entry whose atom:updated element specifies a date and time more
   recent than the at:deleted-entry element's when value.  If the at:
   deleted-entry element does not contain a when element, the at:
   deleted-entry sharing the same atom:id as an entry MUST be ignored.

4.  The Trash Link Relation

   This specification registers a new "trash" link relation to the Atom
   Registry of Link Relations defined in Section 7.1 of [RFC4287].  When
   appearing on atom:link elements within either the atom:feed or atom:
   source elements, The "trash" rel attribute value signifies that the
   IRI in the value of href attribute identifies a resource listing
   entries that have been deleted from the Feed document.  If the linked
   resource is an Atom Feed Document, each of the entries in that feed
   will contain copies of the atom:entry elements removed from the feed

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   containing the trash links.

   For example,

    <feed xmlns="">
      <link rel="trash" type="application/atom+xml" href="trash.xml" />
      <link rel="trash" type="application/xhtml+xml" href="trash.xml" />

   A single linked trash resource may list entries deleted from multiple
   feed documents.  In such cases, each entry in the trash resource
   SHOULD reference the feed from which the entry was removed.  For
   instance, if the trash resource is an Atom Feed Document, each entry
   would contain an atom:source element identifying the feed the entry
   was deleted from.

   Linked "trash" resources are orthogonal to the at:deleted-entry
   element and are intended to provide servers with a way of allowing
   clients to recover deleted entries.  For instance, the entries in a
   feed may be managed by two separate authors, one of whom may delete
   an entry that the other author wishes to later recover.  If a trash
   feed is provided, the second author can look for a copy of the
   deleted entry there.

   While it will be common for servers to provide only a single trash
   resource per feed, or to have multiple feeds share a single trash
   resource, it is possible for a single feed to contain multiple trash
   links.  The relationship between multiple trash links is undefined.
   Individual entries deleted from the feed containing the linked trash
   resources MAY appear in any, all or none of the linked trash
   resources.  It is up to the server implementation to decide whether
   to provide a trash resource and whether or not to add deleted entries
   to those resources.

5.  Security Considerations

   As specified in [RFC4287], Atom processors should be aware of the
   potential for spoofing attacks where an attacker publishes atom:entry
   or atom:deleted-entry elements using the same atom:id values as
   entries from other Atom feeds.  An attacker may attempt to trick an
   application into believing that a given entry has either been removed
   from or added to a feed.  To mitigate this issue, Atom processors are
   advised to ignore atom:deleted-entry elements referencing entries
   that have not previously appeared within the containing Feed document
   and should take steps to verify the origin of the Atom feed before

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   considering the entries to be removed.

6.  IANA Considerations

6.1.  The Link Relation Registration "Trash"

   Attribute Value:  trash
   Description:  An IRI of a resource listing deleted resources.  When
      appearing within an atom:feed or atom:source elements, signifies
      that the IRI in the value of the href attribute identifies a
      resource listing entries that have been deleted from the Feed
   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.
   Security considerations:  Automated agents should take care when this
      relation crosses administrative domains (e.g., the URI has a
      different authority than the current document).

7.  Acknowledgements

   The author gratefully acknowledges the feedback from the members of
   the Atom Publishing Format and Protocol working group during the
   development of this specification.

8.  Normative References

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

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

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

              Hollander, D., Layman, A., and T. Bray, "Namespaces in
              XML", World Wide Web Consortium FirstEdition REC-xml-
              names-19990114, January 1999,

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Author's Address

   James M Snell


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