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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Snell 3 Internet-Draft September 2005 4 Expires: March 5, 2006 6 Atom Metadata Expiration: Specifying Expiration Timestamps for Atom 7 Feed and Entry metadata 8 draft-snell-atompub-feed-expires-05.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 5, 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 7 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: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 [I-D.ietf-atompub- 118 format]. 120 In this specification, the term "information content" refers to the 121 collection of metadata relevant to an atom:entry element. 123 This specification uses XML Namespaces [W3C.REC-xml-names-19990114] 124 to uniquely identify XML element names. It uses the following 125 namespace prefix for the indicated namespace URI; 127 {Ed. Note: this namespace MUST be changed to a proper IETF namespace 128 scheme prior to publication} 129 "age": "http://purl.org/atompub/age/1.0" 131 This specification uses terms from the XML Infoset [W3C.REC-xml- 132 infoset-20040204]. However, this specification uses a shorthand; the 133 phrase "Information Item" is omitted when naming Element Information 134 Items. Therefore, when this specification uses the term "element," 135 it is referring to an Element Information Item in Infoset terms. 137 This specification defines the notion of a Non-Negative Integer 138 Construct whose value MUST conform to the canonical representation of 139 the "nonNegativeInteger" construction defined by [W3C.REC-xmlschema- 140 2-20041028]. The value of the Non-Negative Integer Construct MUST 141 NOT contain any leading or trailing whitespace. 142 nonNegativeIntegerConstruct = 143 atomCommonAttributes, 144 xsd:nonNegativeInteger 146 Example Non-Negative Integer Constructs 147 0 148 1000000 150 3. The 'expires' extension element 152 The 'expires' extension element conforms to the Atom Date Construct 153 and is used to indicate a precise moment in time after which the 154 information content of the containing element MUST be considered to 155 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 MUST NOT specify a 171 date and time earlier than the moment specified by the atom: 172 updated element or the atom:published element (if present). If an 173 'expires' element does specify a date and time earlier than either 174 the atom:updated or atom:published elements, the 'expires' element 175 MUST be ignored. 177 For example 178 180 http://www.example.com/myfeed 181 My Example Feed 182 2005-07-28T12:00:00Z 183 184 James 185 186 tag:entries.com,2005:1 187 An entry with an expiration timestamp 188 189 2005-07-28T12:00:00Z 190 2005-12-12T12:00:00Z 191 An entry with an expiration 192 193 195 4. The 'max-age' extension element 197 The 'max-age' extension element is used to indicate a maximum age of 198 the information content in miliseconds from the moment specified by 199 the atom:published element (if present) or the atom:updated element 200 after which the information content MUST be considered to have 201 "expires". The value is specified as a non-negative integer 202 conforming to the nonNegativeIntegerConstruct. 203 maxageElement = element age:max-age { nonNegativeIntegerConstruct } 205 o atom:entry elements MAY contain exactly one 'max-age' extension 206 element. 207 o atom:feed elements MAY contain exactly one 'max-age' extension 208 element. 209 o atom:source elements MAY contain exactly one 'max-age' extension 210 element. 211 o An atom:entry that contains a 'max-age' extension element MUST NOT 212 contain an 'expires' extension element. 213 o An atom:feed that contains a 'max-age' extension element MUST NOT 214 contain an 'expires' extension element. 215 o An atom:source that contains a 'max-age' extension element MUST 216 NOT contain an 'expires' extension element. 218 For example 219 221 http://www.example.com/myfeed 222 My Example Feed 223 2005-07-28T12:00:00Z 224 225 James 226 227 tag:entries.com,2005:1 228 An entry with a maximum age of 20 seconds 229 230 2005-07-28T12:00:00Z 231 20000 232 expire after a mere 20 seconds 233 234 236 5. Security Considerations 238 It is possible for malicious intermediaries to alter the expiration 239 metadata in an Atom document. This risk can be mitigated by 240 digitally signing the elements in which the 'expires' or 'max-age' 241 extension elements appear. It is strongly recommended that 242 implementations discard expired information content or otherwise warn 243 users that the information content has expired. 245 6. IANA Considerations 247 There are no IANA considerations introduced by this specification. 249 7. References 251 [I-D.ietf-atompub-format] 252 Sayre, R. and M. Nottingham, "The Atom Syndication 253 Format", draft-ietf-atompub-format-11 (work in progress), 254 August 2005. 256 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 257 Requirement Levels", BCP 14, RFC 2119, March 1997. 259 [W3C.REC-xml-infoset-20040204] 260 Tobin, R. and J. Cowan, "XML Information Set (Second 261 Edition)", W3C REC REC-xml-infoset-20040204, 262 February 2004. 264 [W3C.REC-xml-names-19990114] 265 Hollander, D., Bray, T., and A. Layman, "Namespaces in 266 XML", W3C REC REC-xml-names-19990114, January 1999. 268 [W3C.REC-xmlschema-2-20041028] 269 Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes 270 Second Edition", W3C REC REC-xmlschema-2-20041028, 271 October 2004. 273 Author's Address 275 James M Snell 277 Phone: 278 Email: jasnell@gmail.com 279 URI: http://snellspace.com 281 Appendix A. Acknowledgements 283 The author gratefully acknowledges the feedback from Eric Scheid, 284 Walter Underwood, Henry Story, Tim Bray, Mark Nottingham, Robert 285 Sayrer, James Holderness, Phil Ringnalda, Antone Roundy and the other 286 members of Atom Publishing Format and Protocol working group during 287 the development of this specification. 289 Intellectual Property Statement 291 The IETF takes no position regarding the validity or scope of any 292 Intellectual Property Rights or other rights that might be claimed to 293 pertain to the implementation or use of the technology described in 294 this document or the extent to which any license under such rights 295 might or might not be available; nor does it represent that it has 296 made any independent effort to identify any such rights. 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Please address the information to the IETF at 311 ietf-ipr@ietf.org. 313 Disclaimer of Validity 315 This document and the information contained herein are provided on an 316 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 317 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 318 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 319 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 320 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 321 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 323 Copyright Statement 325 Copyright (C) The Internet Society (2005). This document is subject 326 to the rights, licenses and restrictions contained in BCP 78, and 327 except as set forth therein, the authors retain all their rights. 329 Acknowledgment 331 Funding for the RFC Editor function is currently provided by the 332 Internet Society.