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 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 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. Snell Expires February 17, 2006 [Page 1]
Internet-Draft Feed Thread August 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 February 17, 2006 [Page 2]
Internet-Draft Feed Thread August 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 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- Snell Expires February 17, 2006 [Page 3]
Internet-Draft Feed Thread August 2005 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 Snell Expires February 17, 2006 [Page 4]
Internet-Draft Feed Thread August 2005 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 Snell Expires February 17, 2006 [Page 5]
Internet-Draft Feed Thread August 2005 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. Snell Expires February 17, 2006 [Page 6]
Internet-Draft Feed Thread August 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 and the other members of Atom Publishing Format and Protocol working group during the development of this specification. Snell Expires February 17, 2006 [Page 7]
Internet-Draft Feed Thread August 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 February 17, 2006 [Page 8]