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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 295 found in BCP 78 and BCP 79. 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.