Atom Syndication Format Tombstones
draft-snell-atompub-tombstones-04.txt

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Abstract

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.

Table of Contents

1.  Introduction
2.  Notational Conventions
3.  The at: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;

 "at": "http://purl.org/atompub/tombstones/1.0"

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 {
    atomCommonAttributes,
    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] (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 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 [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

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] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

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="http://www.w3.org/2005/Atom"
        xmlns:at="http://purl.org/atompub/tombstones/1.0">
     ...
     <!-- Minimal deleted-entry -->
     <at:deleted-entry
       ref="tag:example.org,2005:/entries/1" />

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

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] (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. 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 containing the trash links.

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>

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] (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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