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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 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 290 this document or the extent to which any license under such rights 291 might or might not be available; nor does it represent that it has 292 made any independent effort to identify any such rights. 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