Atom Syndication Format Tombstones
draft-snell-atompub-tombstones-03.txt

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Abstract

This specification defines mechanisms by which Atom Feed publishers can explicitly indicate that specific Atom Entries have been removed from an Atom feed.

Table of Contents

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

1.  Introduction

This specification adds a new element to the Atom Syndication Format [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.) 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)

This specification uses XML Namespaces [W3C.REC‑xml‑names‑19990114] (Hollander, D., Layman, A., and T. Bray, “Namespaces in XML,” January 1999.) to uniquely identify XML element names. It uses the following namespace prefix for the indicated namespace URI;

 "atom": "http://www.w3.org/2005/Atom"

3.  The atom:deleted-entry element

The atom: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 atom:deleted-entry {
    atomCommonAttributes,
    ( & atomID,
      & element atom:when { atomDateConstruct }?,
      & element atom:by { atomPersonConstruct}?,
      & element atom:comment {atomTextConstruct}?,
      & extensionElement* )
  }

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

The atom:deleted-entry element MAY contain one atom:when element whose value is an [RFC3339] (Klyne, G., Ed. and C. Newman, “Date and Time on the Internet: Timestamps,” July 2002.) "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 atom:deleted-entry element MAY contain one atom:by element used to identify the entity that removed the entry from the feed. The atom:by element is an Atom Person Construct as defined by Section 3.2 of [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

The atom:deleted-entry element MAY contain one atom: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] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

Atom Feed Documents MAY contain any number of atom:deleted-entry elements.

  <feed xmlns="http://www.w3.org/2005/Atom">
     ...
     <!-- Minimal deleted-entry -->
     <deleted-entry>
       <id>tag:example.org,2005:/entries/1</id>
     </deleted-entry>

     <!-- Extended deleted-entry -->
     <deleted-entry>
       <id>tag:example.org,2005:/entries/2</id>
       <when>2005-11-29T12:11:12Z</when>
       <by>
         <name>John Doe</name>
         <email>jdoe@example.org</email>
       </by>
       <comment>Removed comment spam</comment>
     </deleted-entry>
     ...
  </feed>

An Atom feed MAY contain atom:entry elements and atom:deleted-entry elements sharing the same atom:id value. Atom processors MUST ignore any atom: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 atom:deleted-entry element's atom:when value. If the atom:deleted-entry element does not contain an atom:when element, the atom:deleted-entry sharing the same atom:id as an entry MUST be ignored.

4.  The Trash Link Relation

When an entry is deleted from one feed, a server can choose to move that entry into another Feed document whose member entries consist entirely of deleted entries. Such "trash" feeds are similar in nature to desktop operating system trash folders that are used to collect, and potentially recover items that have been deleted.

This specification registers a new "trash" link relation to the Atom Registry of Link Relations defined in Section 7.1 of [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). 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.

For example,

  <feed xmlns="http://www.w3.org/2005/Atom">
    ...
    <link rel="trash" type="application/atom+xml" href="trash.xml" />
    <link rel="trash" type="application/xhtml+xml" href="trash.xml" />
    ...
  </feed>

5.  Security Considerations

As specified in [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.), 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 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 document.

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

Author's Address

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