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2 Network Working Group J. Snell
3 Internet-Draft December 6, 2005
4 Expires: June 9, 2006
6 Atom Metadata Expiration: Specifying Expiration Timestamps for Atom
7 Feed and Entry metadata
8 draft-snell-atompub-feed-expires-06.txt
10 Status of this Memo
12 By submitting this Internet-Draft, each author represents that any
13 applicable patent or other IPR claims of which he or she is aware
14 have been or will be disclosed, and any of which he or she becomes
15 aware will be disclosed, in accordance with Section 6 of BCP 79.
17 Internet-Drafts are working documents of the Internet Engineering
18 Task Force (IETF), its areas, and its working groups. Note that
19 other groups may also distribute working documents as Internet-
20 Drafts.
22 Internet-Drafts are draft documents valid for a maximum of six months
23 and may be updated, replaced, or obsoleted by other documents at any
24 time. It is inappropriate to use Internet-Drafts as reference
25 material or to cite them other than as "work in progress."
27 The list of current Internet-Drafts can be accessed at
28 http://www.ietf.org/ietf/1id-abstracts.txt.
30 The list of Internet-Draft Shadow Directories can be accessed at
31 http://www.ietf.org/shadow.html.
33 This Internet-Draft will expire on June 9, 2006.
35 Copyright Notice
37 Copyright (C) The Internet Society (2005).
39 Abstract
41 This memo presents a mechanism that allows feed publishers to express
42 maximum age and expiration properties for information content within
43 an Atom entry.
45 Table of Contents
47 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
48 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
49 3. The 'expires' extension element . . . . . . . . . . . . . . . . 4
50 4. The 'max-age' extension element . . . . . . . . . . . . . . . . 5
51 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
52 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
53 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
54 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
55 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
56 Intellectual Property and Copyright Statements . . . . . . . . . . 9
58 1. Introduction
60 This document specifies a mechanism that allows the expression of
61 expiration timestamps and maximum age properties for information
62 content within the Atom Syndication Format ([RFC4287]). The
63 mechanism defines two mutually exclusive extension elements that may
64 be used to specify either an exact instant that the information
65 content of an atom:entry expires, or a maximum age from the moment
66 specified by an entries atom:updated element.
68 When an atom:feed, atom:entry or atom:source contains an 'expires' or
69 'max-age' extension element, the information content of the contained
70 element is considered to be 'time constrained'. Time constrainted
71 information content is considered to be either 'active' or 'expired'.
72 The default state is 'active'. When the age (calculated in
73 miliseconds from the moment specified by the atom:published or atom:
74 updated elements) exceeds the value specified by the 'max-age'
75 extension, or when the moment specified by the 'expires' extension
76 elements passes, the state of the time constrained information
77 content MUST be considered to be 'expired' and no longer valid. It
78 is strongly recommended that implementations either discard 'expired'
79 information content or otherwise warn users that the information
80 content has expired.
82 The mechanism defined herein MUST NOT be used to support the caching
83 of Atom documents and MUST NOT be used to schedule when a client
84 should revisit/refresh local copies of Atom documents. Specifically,
85 the 'expires' and 'max-age' extension elements are relevant only to
86 the informational content within an atom:entry and are not adequate
87 mechanism for determining the freshness of a cached copy of the
88 document in which the elements appear.
90 The presence of an 'expires' or 'max-age' extension within an atom:
91 feed or atom:source element is relevant only to the informational
92 content of the containing element and not to any associated atom:
93 entry elements.
95 Section 6.3 of the Atom Format specification indicates that Atom
96 processors that encounter unknown extensions MUST ignore those
97 extensions without altering their behavior. Because of this
98 requirement, there can be no assumption that a particular software
99 implementation will support the extensions defined herein.
101 2. Notational Conventions
103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
105 document are to be interpreted as described in BCP 14, [RFC2119], as
106 scoped to those conformance targets.
108 In this specification, "entry" refers to an atom:entry element.
110 In this specification, "feed" refers to an Atom Feed Document.
112 In this specification, "head section" refers to the children of a
113 feed's document-wide metadata container; e.g., the child elements of
114 the atom:feed element in an Atom Feed Document.
116 In this specification, the term "extension element" refers to a
117 structured extension element as described in [RFC4287].
119 In this specification, the term "information content" refers to the
120 collection of metadata relevant to an atom:entry element.
122 This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
123 to uniquely identify XML element names. It uses the following
124 namespace prefix for the indicated namespace URI;
126 {Ed. Note: this namespace MUST be changed to a proper IETF namespace
127 scheme prior to publication}
128 "age": "http://purl.org/atompub/age/1.0"
130 This specification uses terms from the XML Infoset [W3C.REC-xml-
131 infoset-20040204]. However, this specification uses a shorthand; the
132 phrase "Information Item" is omitted when naming Element Information
133 Items. Therefore, when this specification uses the term "element,"
134 it is referring to an Element Information Item in Infoset terms.
136 This specification defines the notion of a Non-Negative Integer
137 Construct whose value MUST conform to the canonical representation of
138 the "nonNegativeInteger" construction defined by [W3C.REC-xmlschema-
139 2-20041028]. The value of the Non-Negative Integer Construct MUST
140 NOT contain any leading or trailing whitespace.
141 nonNegativeIntegerConstruct =
142 atomCommonAttributes,
143 xsd:nonNegativeInteger
145 Example Non-Negative Integer Constructs
146 0
147 1000000
149 3. The 'expires' extension element
151 The 'expires' extension element conforms to the Atom Date Construct
152 and is used to indicate a precise moment in time after which the
153 information content of the containing element MUST be considered to
154 have "expired".
155 expiresElement = element age:expires { atomDateConstruct }
157 o atom:entry elements MAY contain exactly one 'expires' extension
158 element.
159 o atom:feed elements MAY contain exactly one 'expires' extension
160 element.
161 o atom:source elements MAY contain exactly one 'expires' extension
162 element.
163 o An atom:entry that contains an 'expires' extension element MUST
164 NOT contain a 'max-age' extension element.
165 o An atom:feed that contains an 'expires' extension element MUST NOT
166 contain a 'max-age' extension element.
167 o An atom:source that contains an 'expires' extension element MUST
168 NOT contain a 'max-age' extension element.
169 o The value of the 'expires' extension element MUST NOT specify a
170 date and time earlier than the moment specified by the atom:
171 updated element or the atom:published element (if present). If an
172 'expires' element does specify a date and time earlier than either
173 the atom:updated or atom:published elements, the 'expires' element
174 MUST be ignored.
176 For example
177
179 http://www.example.com/myfeed
180 My Example Feed
181 2005-07-28T12:00:00Z
182
183 James
184
185 tag:entries.com,2005:1
186 An entry with an expiration timestamp
187
188 2005-07-28T12:00:00Z
189 2005-12-12T12:00:00Z
190 An entry with an expiration
191
192
194 4. The 'max-age' extension element
196 The 'max-age' extension element is used to indicate a maximum age of
197 the information content in miliseconds from the moment specified by
198 the atom:published element (if present) or the atom:updated element
199 after which the information content MUST be considered to have
200 "expires". The value is specified as a non-negative integer
201 conforming to the nonNegativeIntegerConstruct.
202 maxageElement = element age:max-age { nonNegativeIntegerConstruct }
204 o atom:entry elements MAY contain exactly one 'max-age' extension
205 element.
206 o atom:feed elements MAY contain exactly one 'max-age' extension
207 element.
208 o atom:source elements MAY contain exactly one 'max-age' extension
209 element.
210 o An atom:entry that contains a 'max-age' extension element MUST NOT
211 contain an 'expires' extension element.
212 o An atom:feed that contains a 'max-age' extension element MUST NOT
213 contain an 'expires' extension element.
214 o An atom:source that contains a 'max-age' extension element MUST
215 NOT contain an 'expires' extension element.
217 For example
218
220 http://www.example.com/myfeed
221 My Example Feed
222 2005-07-28T12:00:00Z
223
224 James
225
226 tag:entries.com,2005:1
227 An entry with a maximum age of 20 seconds
228
229 2005-07-28T12:00:00Z
230 20000
231 expire after a mere 20 seconds
232
233
235 5. Security Considerations
237 It is possible for malicious intermediaries to alter the expiration
238 metadata in an Atom document. This risk can be mitigated by
239 digitally signing the elements in which the 'expires' or 'max-age'
240 extension elements appear. It is strongly recommended that
241 implementations discard expired information content or otherwise warn
242 users that the information content has expired.
244 6. IANA Considerations
246 There are no IANA considerations introduced by this specification.
248 7. References
250 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
251 Requirement Levels", BCP 14, RFC 2119, March 1997.
253 [RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
254 Format", RFC 4287, December 2005.
256 [W3C.REC-xml-infoset-20040204]
257 Tobin, R. and J. Cowan, "XML Information Set (Second
258 Edition)", W3C REC REC-xml-infoset-20040204,
259 February 2004.
261 [W3C.REC-xml-names-19990114]
262 Hollander, D., Bray, T., and A. Layman, "Namespaces in
263 XML", W3C REC REC-xml-names-19990114, January 1999.
265 [W3C.REC-xmlschema-2-20041028]
266 Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
267 Second Edition", W3C REC REC-xmlschema-2-20041028,
268 October 2004.
270 Appendix A. Acknowledgements
272 The author gratefully acknowledges the feedback from Eric Scheid,
273 Walter Underwood, Henry Story, Tim Bray, Mark Nottingham, Robert
274 Sayrer, James Holderness, Phil Ringnalda, Antone Roundy and the other
275 members of Atom Publishing Format and Protocol working group during
276 the development of this specification.
278 Author's Address
280 James M Snell
282 Phone:
283 Email: jasnell@gmail.com
284 URI: http://snellspace.com
286 Intellectual Property Statement
288 The IETF takes no position regarding the validity or scope of any
289 Intellectual Property Rights or other rights that might be claimed to
290 pertain to the implementation or use of the technology described in
291 this document or the extent to which any license under such rights
292 might or might not be available; nor does it represent that it has
293 made any independent effort to identify any such rights. Information
294 on the procedures with respect to rights in RFC documents can be
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297 Copies of IPR disclosures made to the IETF Secretariat and any
298 assurances of licenses to be made available, or the result of an
299 attempt made to obtain a general license or permission for the use of
300 such proprietary rights by implementers or users of this
301 specification can be obtained from the IETF on-line IPR repository at
302 http://www.ietf.org/ipr.
304 The IETF invites any interested party to bring to its attention any
305 copyrights, patents or patent applications, or other proprietary
306 rights that may cover technology that may be required to implement
307 this standard. Please address the information to the IETF at
308 ietf-ipr@ietf.org.
310 Disclaimer of Validity
312 This document and the information contained herein are provided on an
313 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
314 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
315 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
316 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
317 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
318 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
320 Copyright Statement
322 Copyright (C) The Internet Society (2005). This document is subject
323 to the rights, licenses and restrictions contained in BCP 78, and
324 except as set forth therein, the authors retain all their rights.
326 Acknowledgment
328 Funding for the RFC Editor function is currently provided by the
329 Internet Society.