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2 Network Working Group J. Snell
3 Internet-Draft September 23, 2005
4 Expires: March 27, 2006
6 Atom Metadata Expiration: Specifying Expiration Timestamps for Atom
7 Feed and Entry metadata
8 draft-snell-atompub-feed-expires-04.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 March 27, 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 . . . . . . . . . . . . . . . . . . . . . 6
53 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
54 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
55 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
56 Intellectual Property and Copyright Statements . . . . . . . . 8
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 ([I-D.ietf-atompub-
63 format]). The mechanism defines two mutually exclusive extension
64 elements that may be used to specify either an exact instant that the
65 information content of an atom:entry expires, or a maximum age from
66 the moment 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:updated element)
74 exceeds the value specified by the 'max-age' extension, or when the
75 moment specified by the 'expires' extension elements passes, the
76 state of the time constrained information content MUST be considered
77 to be 'expired' and no longer valid. It is strongly recommended that
78 implementations either discard 'expired' information content or
79 otherwise warn users that the information content has expired.
81 The mechanism defined herein MUST NOT be used to support the caching
82 of Atom documents and MUST NOT be used to schedule when a client
83 should revisit/refresh local copies of Atom documents. Specifically,
84 the 'expires' and 'max-age' extension elements are relevant only to
85 the informational content within an atom:entry and are not adequate
86 mechanism for determining the freshness of a cached copy of the
87 document in which the elements appear.
89 The presence of an 'expires' or 'max-age' extension within an atom:
90 feed or atom:source element is relevant only to the informational
91 content of the containing element and not to any associated atom:
92 entry elements.
94 Section 6.3 of the Atom Format specification indicates that Atom
95 processors that encounter unknown extensions MUST ignore those
96 extensions without altering their behavior. Because of this
97 requirement, there can be no assumption that a particular software
98 implementation will support the extensions defined herein.
100 2. Notational Conventions
102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
104 document are to be interpreted as described in BCP 14, [RFC2119], as
105 scoped to those conformance targets.
107 In this specification, "entry" refers to an atom:entry element.
109 In this specification, "feed" refers to an Atom Feed Document.
111 In this specification, "head section" refers to the children of a
112 feed's document-wide metadata container; e.g., the child elements of
113 the atom:feed element in an Atom Feed Document.
115 In this specification, the term "extension element" refers to a
116 structured extension element as described in [I-D.ietf-atompub-
117 format].
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".
156 expiresElement = element age:expires { atomDateConstruct }
158 o atom:entry elements MAY contain exactly one 'expires' extension
159 element.
160 o atom:feed elements MAY contain exactly one 'expires' extension
161 element.
162 o atom:source elements MAY contain exactly one 'expires' extension
163 element.
164 o An atom:entry that contains an 'expires' extension element MUST
165 NOT contain a 'max-age' extension element.
166 o An atom:feed that contains an 'expires' extension element MUST NOT
167 contain a 'max-age' extension element.
168 o An atom:source that contains an 'expires' extension element MUST
169 NOT contain a 'max-age' extension element.
170 o The value of the 'expires' extension element SHOULD NOT specify a
171 date and time earlier than the moment specified by the atom:
172 updated element.
174 For example
175
177 http://www.example.com/myfeed
178 My Example Feed
179 2005-07-28T12:00:00Z
180
181 James
182
183 tag:entries.com,2005:1
184 An entry with an expiration timestamp
185
186 2005-07-28T12:00:00Z
187 2005-12-12T12:00:00Z
188 An entry with an expiration
189
190
192 4. The 'max-age' extension element
194 The 'max-age' extension element is used to indicate a maximum age of
195 the information content in miliseconds from the moment specified by
196 the atom:updated element after which the information content MUST be
197 considered to have "expires". The value is specified as a non-
198 negative integer conforming to the nonNegativeIntegerConstruct.
199 maxageElement = element age:max-age { nonNegativeIntegerConstruct }
201 o atom:entry elements MAY contain exactly one 'max-age' extension
202 element.
204 o atom:feed elements MAY contain exactly one 'max-age' extension
205 element.
206 o atom:source elements MAY contain exactly one 'max-age' extension
207 element.
208 o An atom:entry that contains a 'max-age' extension element MUST NOT
209 contain an 'expires' extension element.
210 o An atom:feed that contains a 'max-age' extension element MUST NOT
211 contain an 'expires' extension element.
212 o An atom:source that contains a 'max-age' extension element MUST
213 NOT contain an 'expires' extension element.
215 For example
216
218 http://www.example.com/myfeed
219 My Example Feed
220 2005-07-28T12:00:00Z
221
222 James
223
224 tag:entries.com,2005:1
225 An entry with a maximum age of 20 seconds
226
227 2005-07-28T12:00:00Z
228 20000
229 expire after a mere 20 seconds
230
231
233 5. Security Considerations
235 It is possible for malicious intermediaries to alter the expiration
236 metadata in an Atom document. This risk can be mitigated by
237 digitally signing the elements in which the 'expires' or 'max-age'
238 extension elements appear. It is strongly recommended that
239 implementations discard expired information content or otherwise warn
240 users that the information content has expired.
242 6. IANA Considerations
244 There are no IANA considerations introduced by this specification.
246 7. References
248 [I-D.ietf-atompub-format]
249 Sayre, R. and M. Nottingham, "The Atom Syndication
250 Format", draft-ietf-atompub-format-11 (work in progress),
251 August 2005.
253 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
254 Requirement Levels", BCP 14, RFC 2119, March 1997.
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 Author's Address
272 James M Snell
274 Phone:
275 Email: jasnell@gmail.com
276 URI: http://snellspace.com
278 Appendix A. Acknowledgements
280 The author gratefully acknowledges the feedback from Eric Scheid,
281 Walter Underwood, Henry Story, Tim Bray and the other members of Atom
282 Publishing Format and Protocol working group during the development
283 of this specification.
285 Intellectual Property Statement
287 The IETF takes no position regarding the validity or scope of any
288 Intellectual Property Rights or other rights that might be claimed to
289 pertain to the implementation or use of the technology described in
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297 assurances of licenses to be made available, or the result of an
298 attempt made to obtain a general license or permission for the use of
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300 specification can be obtained from the IETF on-line IPR repository at
301 http://www.ietf.org/ipr.
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304 copyrights, patents or patent applications, or other proprietary
305 rights that may cover technology that may be required to implement
306 this standard. Please address the information to the IETF at
307 ietf-ipr@ietf.org.
309 Disclaimer of Validity
311 This document and the information contained herein are provided on an
312 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
313 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
314 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
315 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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317 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
319 Copyright Statement
321 Copyright (C) The Internet Society (2005). This document is subject
322 to the rights, licenses and restrictions contained in BCP 78, and
323 except as set forth therein, the authors retain all their rights.
325 Acknowledgment
327 Funding for the RFC Editor function is currently provided by the
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