Network Working Group J. Snell
Internet-Draft September 2005
Expires: March 5, 2006
Atom Metadata Expiration: Specifying Expiration Timestamps for Atom
Feed and Entry metadata
draft-snell-atompub-feed-expires-05.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
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 5, 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.
Snell Expires March 5, 2006 [Page 1]
Internet-Draft Feed Thread September 2005
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
Snell Expires March 5, 2006 [Page 2]
Internet-Draft Feed Thread September 2005
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 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
document are to be interpreted as described in BCP 14, [RFC2119], as
Snell Expires March 5, 2006 [Page 3]
Internet-Draft Feed Thread September 2005
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-
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;
{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
01000000
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 after which the
Snell Expires March 5, 2006 [Page 4]
Internet-Draft Feed Thread September 2005
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
http://www.example.com/myfeedMy Example Feed2005-07-28T12:00:00ZJamestag:entries.com,2005:1
An entry with an expiration timestamp2005-07-28T12:00:00Z2005-12-12T12:00:00ZAn entry with an expiration
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
after which the information content MUST be considered to have
"expires". The value is specified as a non-negative integer
Snell Expires March 5, 2006 [Page 5]
Internet-Draft Feed Thread September 2005
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
http://www.example.com/myfeedMy Example Feed2005-07-28T12:00:00ZJamestag:entries.com,2005:1
An entry with a maximum age of 20 seconds2005-07-28T12:00:00Z20000expire after a mere 20 seconds
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.
6. IANA Considerations
There are no IANA considerations introduced by this specification.
Snell Expires March 5, 2006 [Page 6]
Internet-Draft Feed Thread September 2005
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, 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.
Snell Expires March 5, 2006 [Page 7]
Internet-Draft Feed Thread September 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Snell Expires March 5, 2006 [Page 8]