Changeset 394
- Timestamp:
- 15/11/08 03:13:58 (14 years ago)
- File:
-
- 1 edited
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draft-ietf-httpbis/latest-roy/p6-cache.xml
r348 r394 20 20 <!ENTITY messaging "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 21 21 <!ENTITY conditional "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 22 <!ENTITY partial "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 22 23 <!ENTITY combining-byte-ranges "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 23 24 <!ENTITY entity-length "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> … … 27 28 <!ENTITY header-date "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 28 29 <!ENTITY header-via "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 30 <!ENTITY header-last-modified "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 29 31 <!ENTITY message-headers "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> 30 32 <!ENTITY message-length "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>"> … … 43 45 <?rfc-ext allow-markup-in-artwork="yes" ?> 44 46 <?rfc-ext include-references-in-index="yes" ?> 45 < rfc obsoletes="2616" category="std" x:maturity-level="draft"46 ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"47 xmlns:x='http://purl.org/net/xml2rfc/ext'>48 <front>49 50 <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>51 52 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">53 <organization abbrev="Day Software">Day Software</organization>54 <address>47 <?oxygen RNGSchema="../../rfc2629xslt/rfc2629-ext.rnc" type="compact"?> 48 <rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="full3978" 49 obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext"> 50 <front> 51 52 <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title> 53 54 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 55 <organization abbrev="Day Software">Day Software</organization> 56 <address> 55 57 <postal> 56 58 <street>23 Corporate Plaza DR, Suite 280</street> … … 65 67 <uri>http://roy.gbiv.com/</uri> 66 68 </address> 67 </author>68 69 <author initials="J." surname="Gettys" fullname="JimGettys">70 <organization>One Laptop per Child</organization>71 <address>69 </author> 70 71 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 72 <organization>One Laptop per Child</organization> 73 <address> 72 74 <postal> 73 75 <street>21 Oak Knoll Road</street> … … 80 82 <uri>http://www.laptop.org/</uri> 81 83 </address> 82 </author>83 84 <author initials="J." surname="Mogul" fullname="Jeffrey C.Mogul">85 <organization abbrev="HP">Hewlett-Packard Company</organization>86 <address>84 </author> 85 86 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 87 <organization abbrev="HP">Hewlett-Packard Company</organization> 88 <address> 87 89 <postal> 88 90 <street>HP Labs, Large Scale Systems Group</street> … … 95 97 <email>JeffMogul@acm.org</email> 96 98 </address> 97 </author>98 99 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">100 <organization abbrev="Microsoft">Microsoft Corporation</organization>101 <address>99 </author> 100 101 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 102 <organization abbrev="Microsoft">Microsoft Corporation</organization> 103 <address> 102 104 <postal> 103 105 <street>1 Microsoft Way</street> … … 109 111 <email>henrikn@microsoft.com</email> 110 112 </address> 111 </author>112 113 <author initials="L." surname="Masinter" fullname="LarryMasinter">114 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>115 <address>113 </author> 114 115 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 116 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 117 <address> 116 118 <postal> 117 119 <street>345 Park Ave</street> … … 124 126 <uri>http://larry.masinter.net/</uri> 125 127 </address> 126 </author>127 128 <author initials="P." surname="Leach" fullname="Paul J.Leach">129 <organization abbrev="Microsoft">Microsoft Corporation</organization>130 <address>128 </author> 129 130 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 131 <organization abbrev="Microsoft">Microsoft Corporation</organization> 132 <address> 131 133 <postal> 132 134 <street>1 Microsoft Way</street> … … 137 139 <email>paulle@microsoft.com</email> 138 140 </address> 139 </author>140 141 <author initials="T." surname="Berners-Lee" fullname="TimBerners-Lee">142 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>143 <address>141 </author> 142 143 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 144 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 145 <address> 144 146 <postal> 145 147 <street>MIT Computer Science and Artificial Intelligence Laboratory</street> … … 154 156 <uri>http://www.w3.org/People/Berners-Lee/</uri> 155 157 </address> 156 </author>157 158 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">159 <organization abbrev="W3C">World Wide Web Consortium</organization>160 <address>158 </author> 159 160 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 161 <organization abbrev="W3C">World Wide Web Consortium</organization> 162 <address> 161 163 <postal> 162 164 <street>W3C / ERCIM</street> … … 170 172 <uri>http://www.raubacapeu.net/people/yves/</uri> 171 173 </address> 172 </author>173 174 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">175 <organization abbrev="greenbytes">greenbytes GmbH</organization>176 <address>174 </author> 175 176 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 177 <organization abbrev="greenbytes">greenbytes GmbH</organization> 178 <address> 177 179 <postal> 178 180 <street>Hafenweg 16</street> … … 185 187 <uri>http://greenbytes.de/tech/webdav/</uri> 186 188 </address> 187 </author> 188 189 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 190 191 <abstract> 192 <t> 193 The Hypertext Transfer Protocol (HTTP) is an application-level 194 protocol for distributed, collaborative, hypermedia information 195 systems. 196 This document is Part 6 of the seven-part specification 197 that defines the protocol referred to as "HTTP/1.1" and, taken together, 198 obsoletes RFC 2616. Part 6 defines requirements on HTTP caches 199 and the associated header fields that control cache behavior or indicate 200 cacheable response messages. 201 </t> 202 </abstract> 203 204 <note title="Editorial Note (To be removed by RFC Editor)"> 205 <t> 206 Discussion of this draft should take place on the HTTPBIS working group 207 mailing list (ietf-http-wg@w3.org). The current issues list is 208 at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/> 209 and related documents (including fancy diffs) can be found at 210 <eref target="http://tools.ietf.org/wg/httpbis/"/>. 211 </t> 212 <t> 213 The changes in this draft are summarized in <xref target="changes.since.04"/>. 214 </t> 215 </note> 216 </front> 217 <middle> 218 <section title="Introduction" anchor="caching"> 219 <t> 220 HTTP is typically used for distributed information systems, where 221 performance can be improved by the use of response caches. This document 222 defines aspects of HTTP/1.1 related to caching and reusing response messages. 223 </t> 224 225 <section title="Purpose" anchor="intro.purpose"> 226 <iref item="cache"/> 227 <t> 228 An HTTP <x:dfn>cache</x:dfn> is a local store of response messages 229 and the subsystem that controls its message storage, retrieval, and 230 deletion. A cache stores cacheable responses in order to reduce the 231 response time and network bandwidth consumption on future, equivalent 232 requests. Any client or server may include a cache, though a cache 233 cannot be used by a server that is acting as a tunnel. 234 </t> 235 <t> 236 Caching would be useless if it did not significantly improve 237 performance. The goal of caching in HTTP/1.1 is to reuse a prior response 238 message to satisfy a current request. In some cases, the existing response 239 can be reused without the need for a network request, reducing latency and 240 network round-trips; we use an "expiration" mechanism for this purpose 241 (see <xref target="expiration.model"/>). Even when a new request is required, 242 it is often possible to reuse all or parts of the payload of a prior response 243 to satisfy the request, thereby reducing network bandwidth usage; we use a 244 "validation" mechanism for this purpose (see <xref target="validation.model"/>). 245 </t> 246 <iref item="semantically transparent"/> 247 <t> 248 A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with 249 respect to a particular response, when its use affects neither the 250 requesting client nor the origin server, except to improve 251 performance. When a cache is semantically transparent, the client 252 receives exactly the same response status and payload 253 that it would have received had its request been handled directly 254 by the origin server. 255 </t> 256 <t> 257 In an ideal world, all interactions with an HTTP cache would be 258 semantically transparent. However, for some resources, semantic 259 transparency is not always necessary and can be effectively traded 260 for the sake of bandwidth scaling, disconnected operation, and 261 high availability. HTTP/1.1 allows origin servers, caches, 262 and clients to explicitly reduce transparency when necessary. 263 However, because non-transparent operation may confuse non-expert 264 users and might be incompatible with certain server applications 265 (such as those for ordering merchandise), the protocol requires that 266 transparency be relaxed 267 <list style="symbols"> 268 <t>only by an explicit protocol-level request when relaxed by 269 client or origin server</t> 270 271 <t>only with an explicit warning to the end user when relaxed by 272 cache or client</t> 273 </list> 274 </t> 275 <t> 276 Therefore, HTTP/1.1 provides these important elements: 277 <list style="numbers"> 278 <t>Protocol features that provide full semantic transparency when 279 this is required by all parties.</t> 280 281 <t>Protocol features that allow an origin server or user agent to 282 explicitly request and control non-transparent operation.</t> 283 284 <t>Protocol features that allow a cache to attach warnings to 285 responses that do not preserve the requested approximation of 286 semantic transparency.</t> 287 </list> 288 </t> 289 <t> 290 A basic principle is that it must be possible for the clients to 291 detect any potential relaxation of semantic transparency. 292 <list><t> 293 <x:h>Note:</x:h> The server, cache, or client implementor might be faced with 294 design decisions not explicitly discussed in this specification. 295 If a decision might affect semantic transparency, the implementor 296 ought to err on the side of maintaining transparency unless a 297 careful and complete analysis shows significant benefits in 298 breaking transparency. 299 </t></list> 300 </t> 301 </section> 302 303 <section title="Terminology" anchor="intro.terminology"> 304 <t> 305 This specification uses a number of terms to refer to the roles 306 played by participants in, and objects of, HTTP caching. 307 </t> 308 <t> 309 <iref item="cacheable"/> 310 <x:dfn>cacheable</x:dfn> 311 <list> 312 <t> 313 A response is cacheable if a cache is allowed to store a copy of 314 the response message for use in answering subsequent requests. 315 Even when a response is cacheable, there may 316 be additional constraints on whether a cache can use the cached 317 copy for a particular request. 318 </t> 319 </list> 320 </t> 321 <t> 322 <iref item="first-hand"/> 323 <x:dfn>first-hand</x:dfn> 324 <list> 325 <t> 326 A response is first-hand if it comes directly and without 327 unnecessary delay from the origin server, perhaps via one or more 328 proxies. A response is also first-hand if its validity has just 329 been checked directly with the origin server. 330 </t> 331 </list> 332 </t> 333 <t> 334 <iref item="explicit expiration time"/> 335 <x:dfn>explicit expiration time</x:dfn> 336 <list> 337 <t> 338 The time at which the origin server intends that an entity should 339 no longer be returned by a cache without further validation. 340 </t> 341 </list> 342 </t> 343 <t> 344 <iref item="heuristic expiration time"/> 345 <x:dfn>heuristic expiration time</x:dfn> 346 <list> 347 <t> 348 An expiration time assigned by a cache when no explicit expiration 349 time is available. 350 </t> 351 </list> 352 </t> 353 <t> 354 <iref item="age"/> 355 <x:dfn>age</x:dfn> 356 <list> 357 <t> 358 The age of a response is the time since it was sent by, or 359 successfully validated with, the origin server. 360 </t> 361 </list> 362 </t> 363 <t> 364 <iref item="freshness lifetime"/> 365 <x:dfn>freshness lifetime</x:dfn> 366 <list> 367 <t> 368 The length of time between the generation of a response and its 369 expiration time. 370 </t> 371 </list> 372 </t> 373 <t> 374 <iref item="fresh"/> 375 <x:dfn>fresh</x:dfn> 376 <list> 377 <t> 378 A response is fresh if its age has not yet exceeded its freshness 379 lifetime. 380 </t> 381 </list> 382 </t> 383 <t> 384 <iref item="stale"/> 385 <x:dfn>stale</x:dfn> 386 <list> 387 <t> 388 A response is stale if its age has passed its freshness lifetime. 389 </t> 390 </list> 391 </t> 392 <t> 393 <iref item="validator"/> 394 <x:dfn>validator</x:dfn> 395 <list> 396 <t> 397 A protocol element (e.g., an entity tag or a Last-Modified time) 398 that is used to find out whether a cache entry is an equivalent 399 copy of an entity. 400 </t> 401 </list> 402 </t> 403 </section> 404 405 <section title="Requirements" anchor="intro.requirements"> 406 <t> 407 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 408 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 409 document are to be interpreted as described in <xref target="RFC2119"/>. 410 </t> 411 <t> 412 An implementation is not compliant if it fails to satisfy one or more 413 of the &MUST; or &REQUIRED; level requirements for the protocols it 414 implements. An implementation that satisfies all the &MUST; or &REQUIRED; 415 level and all the &SHOULD; level requirements for its protocols is said 416 to be "unconditionally compliant"; one that satisfies all the &MUST; 417 level requirements but not all the &SHOULD; level requirements for its 418 protocols is said to be "conditionally compliant." 419 </t> 420 </section> 421 </section> 422 423 424 <section title="Notational Conventions and Generic Grammar" anchor="notation"> 425 <x:anchor-alias value="DIGIT"/> 426 <x:anchor-alias value="DQUOTE"/> 427 <x:anchor-alias value="quoted-string"/> 428 <x:anchor-alias value="SP"/> 429 <x:anchor-alias value="token"/> 430 <t> 431 This specification uses the ABNF syntax defined in ¬ation-abnf; and 432 the core rules defined in &basic-rules;: 433 <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see 434 <eref target="http://ietf.org/wg/httpbis/trac/ticket/36"/>.</cref> 435 </t> 436 <figure><artwork type="abnf2616"> 189 </author> 190 191 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 192 193 <abstract> 194 <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, 195 collaborative, hypermedia information systems. This document is Part 6 of the seven-part 196 specification that defines the protocol referred to as "HTTP/1.1" and, taken together, 197 obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header 198 fields that control cache behavior or indicate cacheable response messages.</t> 199 </abstract> 200 201 <note title="Editorial Note (To be removed by RFC Editor)"> 202 <t>Discussion of this draft should take place on the HTTPBIS working group mailing list 203 (ietf-http-wg@w3.org). The current issues list is at <eref 204 target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents 205 (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t> 206 <t>The changes in this draft are summarized in <xref target="changes.since.04" />.</t> 207 </note> 208 </front> 209 <middle> 210 <section anchor="caching" title="Introduction"> 211 <t>HTTP is typically used for distributed information systems, where performance can be 212 improved by the use of response caches. This document defines aspects of HTTP/1.1 related to 213 caching and reusing response messages.</t> 214 215 <section anchor="intro.purpose" title="Purpose"> 216 <iref item="cache" /> 217 <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that 218 controls its message storage, retrieval, and deletion. A cache stores cacheable responses 219 in order to reduce the response time and network bandwidth consumption on future, 220 equivalent requests. Any client or server may include a cache, though a cache cannot be 221 used by a server that is acting as a tunnel.</t> 222 <t>Caching would be useless if it did not significantly improve performance. The goal of 223 caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In 224 some cases, a stored response can be reused without the need for a network request, 225 reducing latency and network round-trips; a "freshness" mechanism is used for this purpose 226 (see <xref target="expiration.model" />). Even when a new request is required, it is often 227 possible to reuse all or parts of the payload of a prior response to satisfy the request, 228 thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose 229 (see <xref target="validation.model" />).</t> 230 </section> 231 232 <section anchor="intro.terminology" title="Terminology"> 233 <t>This specification uses a number of terms to refer to the roles played by participants 234 in, and objects of, HTTP caching.</t> 235 <t> 236 <iref item="cacheable" /> 237 <x:dfn>cacheable</x:dfn> 238 <list> 239 <t>A response is cacheable if a cache is allowed to store a copy of the response message 240 for use in answering subsequent requests. Even when a response is cacheable, there may 241 be additional constraints on whether a cache can use the cached copy to satisfy a 242 particular request.</t> 243 </list> 244 </t> 245 <t> 246 <iref item="first-hand" /> 247 <x:dfn>first-hand</x:dfn> 248 <list> 249 <t>A response is first-hand if it comes from the origin server, perhaps via one or more 250 proxies, but not from cache. A response is also first-hand if its validity has just 251 been checked directly with the origin server.</t> 252 </list> 253 </t> 254 <t> 255 <iref item="explicit expiration time" /> 256 <x:dfn>explicit expiration time</x:dfn> 257 <list> 258 <t>The time at which the origin server intends that an entity should no longer be 259 returned by a cache without further validation.</t> 260 </list> 261 </t> 262 <t> 263 <iref item="heuristic expiration time" /> 264 <x:dfn>heuristic expiration time</x:dfn> 265 <list> 266 <t>An expiration time assigned by a cache when no explicit expiration time is 267 available.</t> 268 </list> 269 </t> 270 <t> 271 <iref item="age" /> 272 <x:dfn>age</x:dfn> 273 <list> 274 <t>The age of a response is the time since it was sent by, or successfully validated 275 with, the origin server.</t> 276 </list> 277 </t> 278 <t> 279 <iref item="freshness lifetime" /> 280 <x:dfn>freshness lifetime</x:dfn> 281 <list> 282 <t>The length of time between the generation of a response and its expiration time. </t> 283 </list> 284 </t> 285 <t> 286 <iref item="fresh" /> 287 <x:dfn>fresh</x:dfn> 288 <list> 289 <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t> 290 </list> 291 </t> 292 <t> 293 <iref item="stale" /> 294 <x:dfn>stale</x:dfn> 295 <list> 296 <t>A response is stale if its age has passed its freshness lifetime.</t> 297 </list> 298 </t> 299 <t> 300 <iref item="validator" /> 301 <x:dfn>validator</x:dfn> 302 <list> 303 <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find 304 out whether a stored response is an equivalent copy of an entity.</t> 305 </list> 306 </t> 307 <t> 308 <iref item="validator" /> 309 <x:dfn>shared cache</x:dfn> 310 <list> 311 <t>A cache that is accessible to more than one user. A non-shared cache that is 312 dedicated to a single user.</t> 313 </list> 314 </t> 315 </section> 316 317 318 <section anchor="intro.requirements" title="Requirements"> 319 <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD 320 NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as 321 described in <xref target="RFC2119" />.</t> 322 <t>An implementation is not compliant if it fails to satisfy one or more of the &MUST; 323 or &REQUIRED; level requirements for the protocols it implements. An implementation 324 that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level 325 requirements for its protocols is said to be "unconditionally compliant"; one that 326 satisfies all the &MUST; level requirements but not all the &SHOULD; level 327 requirements for its protocols is said to be "conditionally compliant."</t> 328 </section> 329 </section> 330 331 332 <section anchor="notation" title="Notational Conventions and Generic Grammar"> 333 <x:anchor-alias value="DIGIT" /> 334 <x:anchor-alias value="DQUOTE" /> 335 <x:anchor-alias value="quoted-string" /> 336 <x:anchor-alias value="SP" /> 337 <x:anchor-alias value="token" /> 338 <t>This specification uses the ABNF syntax defined in ¬ation-abnf; and the core rules 339 defined in &basic-rules;: <cref anchor="abnf.dep">ABNF syntax and basic rules will be 340 adopted from RFC 5234, see <eref target="http://ietf.org/wg/httpbis/trac/ticket/36" 341 />.</cref> 342 </t> 343 <figure> 344 <artwork type="abnf2616"> 437 345 <x:ref>DIGIT</x:ref> = <DIGIT, defined in &basic-rules;> 438 346 <x:ref>DQUOTE</x:ref> = <DQUOTE, defined in &basic-rules;> 439 347 <x:ref>SP</x:ref> = <SP, defined in &basic-rules;> 440 </artwork></figure> 441 <figure><artwork type="abnf2616"> 348 </artwork> 349 </figure> 350 <figure> 351 <artwork type="abnf2616"> 442 352 <x:ref>quoted-string</x:ref> = <quoted-string, defined in &basic-rules;> 443 353 <x:ref>token</x:ref> = <token, defined in &basic-rules;> 444 </artwork></figure> 445 <t anchor="abnf.dependencies"> 446 <x:anchor-alias value="field-name"/> 447 <x:anchor-alias value="HTTP-date"/> 448 <x:anchor-alias value="port"/> 449 <x:anchor-alias value="pseudonym"/> 450 <x:anchor-alias value="uri-host"/> 451 The ABNF rules below are defined in other parts: 452 </t> 453 <figure><!--Part1--><artwork type="abnf2616"> 354 </artwork> 355 </figure> 356 <t anchor="abnf.dependencies"> 357 <x:anchor-alias value="field-name" /> 358 <x:anchor-alias value="HTTP-date" /> 359 <x:anchor-alias value="port" /> 360 <x:anchor-alias value="pseudonym" /> 361 <x:anchor-alias value="uri-host" /> The ABNF rules below are defined in other parts:</t> 362 <figure> 363 <!--Part1--> 364 <artwork type="abnf2616"> 454 365 <x:ref>field-name</x:ref> = <field-name, defined in &message-headers;> 455 366 <x:ref>HTTP-date</x:ref> = <HTTP-date, defined in &full-date;> … … 457 368 <x:ref>pseudonym</x:ref> = <pseudonym, defined in &header-via;> 458 369 <x:ref>uri-host</x:ref> = <uri-host, defined in &uri;> 459 </artwork></figure> 460 </section> 461 462 <section title="Overview" anchor="caching.overview"> 463 <section title="Cache Correctness" anchor="cache.correctness"> 464 <t> 465 A correct cache &MUST; respond to a request with the most up-to-date 466 response held by the cache that is appropriate to the request (see 467 Sections <xref target="disambiguating.expiration.values" format="counter"/>, 468 <xref target="disambiguating.multiple.responses" format="counter"/>, 469 and <xref target="cache.replacement" format="counter"/>) which meets one of the following 470 conditions: 471 <list style="numbers"> 472 <t>It has been checked for equivalence with what the origin server 473 would have returned by revalidating the response with the 474 origin server (<xref target="validation.model"/>);</t> 475 476 <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case, 477 this means it meets the least restrictive freshness requirement 478 of the client, origin server, and cache (see <xref target="header.cache-control"/>); if 479 the origin server so specifies, it is the freshness requirement 480 of the origin server alone. 481 482 If a stored response is not "fresh enough" by the most 483 restrictive freshness requirement of both the client and the 484 origin server, in carefully considered circumstances the cache 485 &MAY; still return the response with the appropriate Warning 486 header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/> 487 and <xref target="header.warning" format="counter"/>), unless such a response 488 is prohibited (e.g., by a "no-store" cache-directive, or by a 489 "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t> 490 491 <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy), 492 or error (4xx or 5xx) response message.</t> 493 </list> 494 </t> 495 <t> 496 If the cache can not communicate with the origin server, then a 497 correct cache &SHOULD; respond as above if the response can be 498 correctly served from the cache; if not it &MUST; return an error or 499 warning indicating that there was a communication failure. 500 </t> 501 <t> 502 If a cache receives a response (either an entire response, or a 304 503 (Not Modified) response) that it would normally forward to the 504 requesting client, and the received response is no longer fresh, the 505 cache &SHOULD; forward it to the requesting client without adding a new 506 Warning (but without removing any existing Warning headers). A cache 507 &SHOULD-NOT; attempt to revalidate a response simply because that 508 response became stale in transit; this might lead to an infinite 509 loop. A user agent that receives a stale response without a Warning 510 &MAY; display a warning indication to the user. 511 </t> 512 </section> 513 514 <section title="Warnings" anchor="warnings"> 515 <t> 516 Whenever a cache returns a response that is neither first-hand nor 517 "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it 518 &MUST; attach a warning to that effect, using a Warning general-header. 519 The Warning header and the currently defined warnings are described 520 in <xref target="header.warning"/>. 521 </t> 522 </section> 523 524 <section title="Cache-control Mechanisms" anchor="cache-control.mechanisms"> 525 <t> 526 The basic cache mechanisms in HTTP/1.1 (server-specified expiration 527 times and validators) are implicit directives to caches. In some 528 cases, a server or client might need to provide explicit directives 529 to the HTTP caches. We use the Cache-Control header for this purpose. 530 </t> 531 <t> 532 The Cache-Control header allows a client or server to transmit a 533 variety of directives in either requests or responses. These 534 directives typically override the default caching algorithms. As a 535 general rule, if there is any apparent conflict between header 536 values, the most restrictive interpretation is applied (that is, the 537 one that is most likely to preserve semantic transparency). However, 538 in some cases, cache-control directives are explicitly specified as 539 weakening the approximation of semantic transparency (for example, 540 "max-stale" or "public"). 541 </t> 542 <t> 543 The cache-control directives are described in detail in <xref target="header.cache-control"/>. 544 </t> 545 </section> 546 547 <section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings"> 548 <t> 549 Many user agents make it possible for users to override the basic 550 caching mechanisms. For example, the user agent might allow the user 551 to specify that cached entities (even explicitly stale ones) are 552 never validated. Or the user agent might habitually add "Cache-Control: 553 max-stale=3600" to every request. The user agent &SHOULD-NOT; 554 default to either non-transparent behavior, or behavior that results 555 in abnormally ineffective caching, but &MAY; be explicitly configured 556 to do so by an explicit action of the user. 557 </t> 558 <t> 559 If the user has overridden the basic caching mechanisms, the user 560 agent &SHOULD; explicitly indicate to the user whenever this results in 561 the display of information that might not meet the server's 562 transparency requirements (in particular, if the displayed entity is 563 known to be stale). Since the protocol normally allows the user agent 564 to determine if responses are stale or not, this indication need only 565 be displayed when this actually happens. The indication need not be a 566 dialog box; it could be an icon (for example, a picture of a rotting 567 fish) or some other indicator. 568 </t> 569 <t> 570 If the user has overridden the caching mechanisms in a way that would 571 abnormally reduce the effectiveness of caches, the user agent &SHOULD; 572 continually indicate this state to the user (for example, by a 573 display of a picture of currency in flames) so that the user does not 574 inadvertently consume excess resources or suffer from excessive 575 latency. 576 </t> 577 </section> 578 579 <section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings"> 580 <t> 581 In some cases, the operator of a cache &MAY; choose to configure it to 582 return stale responses even when not requested by clients. This 583 decision ought not be made lightly, but may be necessary for reasons 584 of availability or performance, especially when the cache is poorly 585 connected to the origin server. Whenever a cache returns a stale 586 response, it &MUST; mark it as such (using a Warning header) enabling 587 the client software to alert the user that there might be a potential 588 problem. 589 </t> 590 <t> 591 It also allows the user agent to take steps to obtain a first-hand or 592 fresh response. For this reason, a cache &SHOULD-NOT; return a stale 593 response if the client explicitly requests a first-hand or fresh one, 594 unless it is impossible to comply for technical or policy reasons. 595 </t> 596 </section> 597 598 <section title="Client-controlled Behavior" anchor="client-controlled.behavior"> 599 <t> 600 While the origin server (and to a lesser extent, intermediate caches, 601 by their contribution to the age of a response) are the primary 602 source of expiration information, in some cases the client might need 603 to control a cache's decision about whether to return a cached 604 response without validating it. Clients do this using several 605 directives of the Cache-Control header. 606 </t> 607 <t> 608 A client's request &MAY; specify the maximum age it is willing to 609 accept of an unvalidated response; specifying a value of zero forces 610 the cache(s) to revalidate all responses. A client &MAY; also specify 611 the minimum time remaining before a response expires. Both of these 612 options increase constraints on the behavior of caches, and so cannot 613 further relax the cache's approximation of semantic transparency. 614 </t> 615 <t> 616 A client &MAY; also specify that it will accept stale responses, up to 617 some maximum amount of staleness. This loosens the constraints on the 618 caches, and so might violate the origin server's specified 619 constraints on semantic transparency, but might be necessary to 620 support disconnected operation, or high availability in the face of 621 poor connectivity. 622 </t> 623 </section> 624 </section> 625 626 <section title="Expiration Model" anchor="expiration.model"> 627 628 <section title="Server-Specified Expiration" anchor="server-specified.expiration"> 629 <t> 630 HTTP caching works best when caches can entirely avoid making 631 requests to the origin server. The primary mechanism for avoiding 632 requests is for an origin server to provide an explicit expiration 633 time in the future, indicating that a response &MAY; be used to satisfy 634 subsequent requests. In other words, a cache can return a fresh 635 response without first contacting the server. 636 </t> 637 <t> 638 Our expectation is that servers will assign future explicit 639 expiration times to responses in the belief that the entity is not 640 likely to change, in a semantically significant way, before the 641 expiration time is reached. This normally preserves semantic 642 transparency, as long as the server's expiration times are carefully 643 chosen. 644 </t> 645 <t> 646 The expiration mechanism applies only to responses taken from a cache 647 and not to first-hand responses forwarded immediately to the 648 requesting client. 649 </t> 650 <t> 651 If an origin server wishes to force a semantically transparent cache 652 to validate every request, it &MAY; assign an explicit expiration time 653 in the past. This means that the response is always stale, and so the 654 cache &SHOULD; validate it before using it for subsequent requests. See 655 <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation. 656 </t> 657 <t> 658 If an origin server wishes to force any HTTP/1.1 cache, no matter how 659 it is configured, to validate every request, it &SHOULD; use the "must-revalidate" 660 cache-control directive (see <xref target="header.cache-control"/>). 661 </t> 662 <t> 663 Servers specify explicit expiration times using either the Expires 664 header, or the max-age directive of the Cache-Control header. 665 </t> 666 <t> 667 An expiration time cannot be used to force a user agent to refresh 668 its display or reload a resource; its semantics apply only to caching 669 mechanisms, and such mechanisms need only check a resource's 670 expiration status when a new request for that resource is initiated. 671 See <xref target="history.lists"/> for an explanation of the difference between caches 672 and history mechanisms. 673 </t> 674 </section> 675 676 <section title="Heuristic Expiration" anchor="heuristic.expiration"> 677 <t> 678 Since origin servers do not always provide explicit expiration times, 679 HTTP caches typically assign heuristic expiration times, employing 680 algorithms that use other header values (such as the Last-Modified 681 time) to estimate a plausible expiration time. The HTTP/1.1 682 specification does not provide specific algorithms, but does impose 683 worst-case constraints on their results. Since heuristic expiration 684 times might compromise semantic transparency, they ought to be used 685 cautiously, and we encourage origin servers to provide explicit 686 expiration times as much as possible. 687 </t> 688 </section> 689 690 <section title="Age Calculations" anchor="age.calculations"> 691 <t> 692 In order to know if a cached entry is fresh, a cache needs to know if 693 its age exceeds its freshness lifetime. We discuss how to calculate 694 the latter in <xref target="expiration.calculations"/>; this section describes how to calculate 695 the age of a response or cache entry. 696 </t> 697 <t> 698 In this discussion, we use the term "now" to mean "the current value 699 of the clock at the host performing the calculation." Hosts that use 700 HTTP, but especially hosts running origin servers and caches, &SHOULD; 701 use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to 702 a globally accurate time standard. 703 </t> 704 <t> 705 HTTP/1.1 requires origin servers to send a Date header, if possible, 706 with every response, giving the time at which the response was 707 generated (see &header-date;). We use the term "date_value" to denote 708 the value of the Date header, in a form appropriate for arithmetic 709 operations. 710 </t> 711 <t> 712 HTTP/1.1 uses the Age response-header to convey the estimated age of 713 the response message when obtained from a cache. The Age field value 714 is the cache's estimate of the amount of time since the response was 715 generated or revalidated by the origin server. 716 </t> 717 <t> 718 In essence, the Age value is the sum of the time that the response 719 has been resident in each of the caches along the path from the 720 origin server, plus the amount of time it has been in transit along 721 network paths. 722 </t> 723 <t> 724 We use the term "age_value" to denote the value of the Age header, in 725 a form appropriate for arithmetic operations. 726 </t> 727 <t> 728 A response's age can be calculated in two entirely independent ways: 729 <list style="numbers"> 730 <t>now minus date_value, if the local clock is reasonably well 731 synchronized to the origin server's clock. If the result is 732 negative, the result is replaced by zero.</t> 733 734 <t>age_value, if all of the caches along the response path 735 implement HTTP/1.1.</t> 736 </list> 737 </t> 738 <t> 739 Given that we have two independent ways to compute the age of a 740 response when it is received, we can combine these as 741 </t> 742 <figure><artwork type="code"> 370 </artwork> 371 </figure> 372 </section> 373 374 <section anchor="caching.overview" title="Cache Operation"> 375 376 <section anchor="cache.correctness" title="Cache Correctness"> 377 <iref item="cache.correctness" /> 378 <t>When a cache is "<x:dfn>correct</x:dfn>", the client receives exactly the same response 379 status and payload that it would have received had its request been handled directly by 380 the origin server.</t> 381 <t>Ideally, all interactions with an HTTP cache would be correct. However, for some 382 resources, complete correctness is not always necessary and can be effectively traded for 383 the sake of bandwidth scaling, disconnected operation, and high availability. HTTP/1.1 384 allows origin servers, caches, and clients to explicitly reduce correctness when 385 necessary.</t> 386 <t>However, because incorrect operation may confuse users and might be incompatible with 387 server applications (such as those for ordering merchandise), caches MUST NOT relax 388 correctness unless: <list style="symbols"> 389 <t>the client or origin server permits it with an explicit protocol-level element; see 390 <xref target="header.cache-control" />, or</t> 391 <t>the cache provides an explicit warning to the end user; see <xref 392 target="header.warning" />.</t> 393 </list> 394 </t> 395 <t> 396 <cref>REVIEW: previous semantic transparency text didn't make a lot of sense; replacing 397 with "correctness"</cref> 398 </t> 399 <t> 400 <cref>TODO: align with intermediary semantic transparency in p1</cref> 401 </t> 402 <t> 403 <cref>REVIEW: removed Explicit User Agent Warnings section</cref> 404 </t> 405 </section> 406 407 <section anchor="response.cacheability" title="Response Cacheability"> 408 <t>A cache MAY store a response to any request, provided that: <list style="symbols"> 409 <t>the "no-store" cache-control directive (see <xref target="header.cache-control" />) 410 does not appear in request or response headers.</t> 411 <t>the cache understands partial responses, if the response is partial or incomplete 412 (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t> 413 </list> 414 </t> 415 <t>Note that in normal operation, most caches will not store a response that has neither a 416 cache validator nor an explicitly expiration time, as such responses are not usually 417 useful to store. However, caches are not prohibited from storing such responses.</t> 418 </section> 419 420 <section anchor="constructing.responses.from.caches" 421 title="Constructing Responses from Caches"> 422 <t>For a given request, a non-shared cache &MAY; return a stored response, provided 423 that: <list style="symbols"> 424 <t>the presented request-URI and that of the stored response match (see 425 <cref>TBD</cref>), and</t> 426 <t>selecting headers nominated by the stored response (if any) match (see <xref 427 target="caching.negotiated.responses" />), and</t> 428 <t>the stored response is either fresh (see <xref target="expiration.model" />) or 429 allowed to be served stale (see <xref target="serving.stale.responses" />), and</t> 430 <t>the presented request and stored response are free from directives that would prevent 431 it (see <xref target="header.cache-control" /> and <xref target="header.pragma" 432 />).</t> 433 </list> 434 </t> 435 <t> 436 <cref>ISSUE: This doesn't specify whether the request method is part of the cache key.</cref> 437 </t> 438 <t>A shared cache &MAY; return a stored response, provided that: <list style="symbols"> 439 <t>the criteria for non-shared caches above are met (including directives for shared 440 caches; see <xref target="header.cache-control" />), and</t> 441 <t>the stored response was not associated with an authenticated request (see 442 &header-authorization;), unless explicitly allowed (see <xref 443 target="header.cache-control" />).</t> 444 </list> 445 </t> 446 <t>All request methods other than GET and HEAD &MUST; be written through the cache to 447 the origin server. Note that such requests might invalidate already stored responses; see 448 <xref target="invalidation.after.updates.or.deletions" />.</t> 449 <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when 450 more than one applicable response is stored. They &MAY; also send a request with 451 "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to 452 use.</t> 453 <t>In the process of determining whether a stored response is fresh or not, a cache 454 &MAY; validate that response (see <xref target="validation.model" />).</t> 455 </section> 456 457 458 459 460 <section anchor="expiration.model" title="Freshness Model"> 461 462 <t>HTTP caching works best when caches can entirely avoid making requests to the origin 463 server. When a response is "fresh" in the cache, it can be used to satisfy subsequent 464 requests without contacting the origin server. This is also referred to as "expiration."</t> 465 <t>Expiration applies only to responses taken from a cache and not to first-hand responses. 466 It cannot be used to force a user agent to refresh its display or reload a resource; its 467 semantics apply only to caches. See <xref target="history.lists" /> for an explanation of 468 the difference between caches and history mechanisms.</t> 469 <t>The primary mechanism for avoiding requests is for an origin server to provide an 470 explicit expiration time in the future, using either the Expires header <xref 471 target="header.expires" /> or the max-age directive of the Cache-Control header <xref 472 target="header.cache-control" />. Generally, origin servers will assign future explicit 473 expiration times to responses in the belief that the entity is not likely to change in a 474 semantically significant way before the expiration time is reached. This normally 475 preserves cache correctness, as long as the server's expiration times are carefully 476 chosen.</t> 477 <t>If an origin server wishes to force a cache to validate every request, it &MAY; 478 assign an explicit expiration time in the past. This means that the response is always 479 stale, and so the cache &SHOULD; validate it before using it for subsequent requests.</t> 480 <t>Since origin servers do not always provide explicit expiration times, HTTP caches may 481 assign heuristic expiration times when they are not specified, employing algorithms that 482 use other header values (such as the Last-Modified time) to estimate a plausible 483 expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does 484 impose worst-case constraints on their results.</t> 485 <t>Additionally, in some cases the client might need to influence freshness calculation. 486 Clients can do this using several directives of the Cache-Control header, with the effect 487 of either increasing or loosening constraints on freshness.</t> 488 489 <t>The calculation to determine if a response has expired is:</t> 490 <figure> 491 <artwork type="code"> 492 response_is_fresh = (freshness_lifetime > current_age) 493 </artwork> 494 </figure> 495 496 <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />; 497 the current_age is defined in <xref target="age.calculations" />.</t> 498 499 <t> 500 <cref>TODO: incorporate client-specified freshness controls.</cref> 501 </t> 502 <t> 503 <cref>TODO: incorporate s-maxage</cref> 504 </t> 505 506 507 <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime"> 508 <t>"expires_value" denotes the value of the Expires header <xref target="header.expires" 509 />. "max_age_value" denotes the number of seconds carried by the "max-age" directive of 510 the Cache-Control header in a response (see <xref target="header.cache-control" />).</t> 511 <t>The max-age directive takes priority over Expires, so if max-age is present in a 512 response, the calculation:</t> 513 <figure> 514 <artwork type="code"> 515 freshness_lifetime = max_age_value 516 </artwork> 517 </figure> 518 <t>Otherwise, if Expires is present in the response, the calculation is:</t> 519 <figure> 520 <artwork type="code"> 521 freshness_lifetime = expires_value - date_value 522 </artwork> 523 </figure> 524 <t>Note that the calculations above are not vulnerable to clock skew, since all of the 525 information comes from the origin server.</t> 526 </section> 527 528 <section anchor="heuristic.freshness" title="Heuristic Freshness"> 529 <t>If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage appears in the 530 response, and the response does not include other restrictions on caching, the cache 531 &MAY; compute a freshness_lifetime using a heuristic, if the stored response's 532 status code is one of 200, 203, 206, 300, 301 or 410. Heuristics &MUST-NOT; be used 533 for other response status codes. When a heuristic is used to calculate 534 freshness_lifetime, the cache &MUST; attach a Warning header with a 113 warn-code to the response if its 535 current_age is more than 24 hours and such a warning is not already present.</t> 536 <t>Also, if the response has a Last-Modified header &header-last-modified;, the 537 heuristic expiration value &SHOULD; be no more than some fraction of the interval 538 since that time. A typical setting of this fraction might be 10%.</t> 539 <t> 540 <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref> 541 </t> 542 </section> 543 544 <section anchor="age.calculations" title="Calculating Age"> 545 <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response 546 message when obtained from a cache. The Age field value is the cache's estimate of the 547 amount of time since the response was generated or validated by the origin server. In 548 essence, the Age value is the sum of the time that the response has been resident in 549 each of the caches along the path from the origin server, plus the amount of time it has 550 been in transit along network paths.</t> 551 <t>When a response is generated from a stored response, the cache &MUST; include a single 552 Age header field in the response with a value equal to the stored response's current_age, 553 calculated using the algorithm described in this section.</t> 554 <t>The term "age_value" denotes the value of the Age header, in a form appropriate for 555 arithmetic operations.</t> 556 <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every 557 response, giving the time at which the response was generated (see &header-date;). 558 The term "date_value" denotes the value of the Date header, in a form appropriate for 559 arithmetic operations.</t> 560 <t>The term "now" means "the current value of the clock at the host performing the 561 calculation." Hosts that use HTTP, but especially hosts running origin servers and 562 caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to 563 synchronize their clocks to a globally accurate time standard.</t> 564 <t>A response's age can be calculated in two entirely independent ways: <list 565 style="numbers"> 566 <t>now minus date_value, if the local clock is reasonably well synchronized to the 567 origin server's clock. If the result is negative, the result is replaced by zero.</t> 568 569 <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t> 570 </list> 571 </t> 572 <t>These are combined as</t> 573 <figure> 574 <artwork type="code"> 743 575 corrected_received_age = max(now - date_value, age_value) 744 </artwork></figure> 745 <t> 746 and as long as we have either nearly synchronized clocks or all-HTTP/1.1 747 paths, one gets a reliable (conservative) result. 748 </t> 749 <t> 750 Because of network-imposed delays, some significant interval might 751 pass between the time that a server generates a response and the time 752 it is received at the next outbound cache or client. If uncorrected, 753 this delay could result in improperly low ages. 754 </t> 755 <t> 756 Because the request that resulted in the returned Age value must have 757 been initiated prior to that Age value's generation, we can correct 758 for delays imposed by the network by recording the time at which the 759 request was initiated. Then, when an Age value is received, it &MUST; 760 be interpreted relative to the time the request was initiated, not 761 the time that the response was received. This algorithm results in 762 conservative behavior no matter how much delay is experienced. So, we 763 compute: 764 </t> 765 <figure><artwork type="code"> 576 </artwork> 577 </figure> 578 <t>When an Age value is 579 received, it &MUST; be interpreted relative to the time the request was initiated, 580 not the time that the response was received.</t> 581 <figure> 582 <artwork type="code"> 766 583 corrected_initial_age = corrected_received_age 767 584 + (now - request_time) 768 </artwork> </figure>769 <t>770 where "request_time" is the time (according to the local clock) when771 the request that elicited this response was sent.772 </t> 773 <t> 774 Summary of age calculation algorithm, when a cache receives a775 response:776 </t>777 <figure><artwork type="code">585 </artwork> 586 </figure> 587 <t>where "request_time" is the time (according to the local clock) when the request that 588 elicited this response was sent.</t> 589 <t>The current_age of a stored response can then be calculated by adding the amount of time 590 (in seconds) since the stored response was last validated by the origin server to the 591 corrected_initial_age.</t> 592 <t>In summary:</t> 593 <figure> 594 <artwork type="code"> 778 595 /* 779 596 * age_value … … 784 601 * request_time 785 602 * is the (local) time when the cache made the request 786 * that resulted in this cached response603 * that resulted in this stored response 787 604 * response_time 788 605 * is the (local) time when the cache received the … … 798 615 resident_time = now - response_time; 799 616 current_age = corrected_initial_age + resident_time; 800 </artwork></figure> 801 <t> 802 The current_age of a cache entry is calculated by adding the amount 803 of time (in seconds) since the cache entry was last validated by the 804 origin server to the corrected_initial_age. When a response is 805 generated from a cache entry, the cache &MUST; include a single Age 806 header field in the response with a value equal to the cache entry's 807 current_age. 808 </t> 809 <t> 810 The presence of an Age header field in a response implies that a 811 response is not first-hand. However, the converse is not true, since 812 the lack of an Age header field in a response does not imply that the 813 response is first-hand unless all caches along the request path are 814 compliant with HTTP/1.1 (i.e., older HTTP caches did not implement 815 the Age header field). 816 </t> 817 </section> 818 819 <section title="Expiration Calculations" anchor="expiration.calculations"> 820 <t> 821 In order to decide whether a response is fresh or stale, we need to 822 compare its freshness lifetime to its age. The age is calculated as 823 described in <xref target="age.calculations"/>; this section describes how to calculate 824 the freshness lifetime, and to determine if a response has expired. 825 In the discussion below, the values can be represented in any form 826 appropriate for arithmetic operations. 827 </t> 828 <t> 829 We use the term "expires_value" to denote the value of the Expires 830 header. We use the term "max_age_value" to denote an appropriate 831 value of the number of seconds carried by the "max-age" directive of 832 the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>). 833 </t> 834 <t> 835 The max-age directive takes priority over Expires, so if max-age is 836 present in a response, the calculation is simply: 837 </t> 838 <figure><artwork type="code"> 839 freshness_lifetime = max_age_value 840 </artwork></figure> 841 <t> 842 Otherwise, if Expires is present in the response, the calculation is: 843 </t> 844 <figure><artwork type="code"> 845 freshness_lifetime = expires_value - date_value 846 </artwork></figure> 847 <t> 848 Note that neither of these calculations is vulnerable to clock skew, 849 since all of the information comes from the origin server. 850 </t> 851 <t> 852 If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage 853 (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response 854 does not include other restrictions on caching, the cache &MAY; compute 855 a freshness lifetime using a heuristic. The cache &MUST; attach Warning 856 113 to any response whose age is more than 24 hours if such warning 857 has not already been added. 858 </t> 859 <t> 860 Also, if the response does have a Last-Modified time, the heuristic 861 expiration value &SHOULD; be no more than some fraction of the interval 862 since that time. A typical setting of this fraction might be 10%. 863 </t> 864 <t> 865 The calculation to determine if a response has expired is quite 866 simple: 867 </t> 868 <figure><artwork type="code"> 869 response_is_fresh = (freshness_lifetime > current_age) 870 </artwork></figure> 871 </section> 872 873 <section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values"> 874 <t> 875 Because expiration values are assigned optimistically, it is possible 876 for two caches to contain fresh values for the same resource that are 877 different. 878 </t> 879 <t> 880 If a client performing a retrieval receives a non-first-hand response 881 for a request that was already fresh in its own cache, and the Date 882 header in its existing cache entry is newer than the Date on the new 883 response, then the client &MAY; ignore the response. If so, it &MAY; 884 retry the request with a "Cache-Control: max-age=0" directive (see 885 <xref target="header.cache-control"/>), to force a check with the origin server. 886 </t> 887 <t> 888 If a cache has two fresh responses for the same representation with 889 different validators, it &MUST; use the one with the more recent Date 890 header. This situation might arise because the cache is pooling 891 responses from other caches, or because a client has asked for a 892 reload or a revalidation of an apparently fresh cache entry. 893 </t> 894 </section> 895 896 <section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses"> 897 <t> 898 Because a client might be receiving responses via multiple paths, so 899 that some responses flow through one set of caches and other 900 responses flow through a different set of caches, a client might 901 receive responses in an order different from that in which the origin 902 server sent them. We would like the client to use the most recently 903 generated response, even if older responses are still apparently 904 fresh. 905 </t> 906 <t> 907 Neither the entity tag nor the expiration value can impose an 908 ordering on responses, since it is possible that a later response 909 intentionally carries an earlier expiration time. The Date values are 910 ordered to a granularity of one second. 911 </t> 912 <t> 913 When a client tries to revalidate a cache entry, and the response it 914 receives contains a Date header that appears to be older than the one 915 for the existing entry, then the client &SHOULD; repeat the request 916 unconditionally, and include 917 </t> 918 <figure><artwork type="example"> 919 Cache-Control: max-age=0 920 </artwork></figure> 921 <t> 922 to force any intermediate caches to validate their copies directly 923 with the origin server, or 924 </t> 925 <figure><artwork type="example"> 926 Cache-Control: no-cache 927 </artwork></figure> 928 <t> 929 to force any intermediate caches to obtain a new copy from the origin 930 server. 931 </t> 932 <t> 933 If the Date values are equal, then the client &MAY; use either response 934 (or &MAY;, if it is being extremely prudent, request a new response). 935 Servers &MUST-NOT; depend on clients being able to choose 936 deterministically between responses generated during the same second, 937 if their expiration times overlap. 938 </t> 939 </section> 940 </section> 941 942 <section title="Validation Model" anchor="validation.model"> 943 <t> 944 When a cache has a stale entry that it would like to use as a 945 response to a client's request, it first has to check with the origin 946 server (or possibly an intermediate cache with a fresh response) to 947 see if its cached entry is still usable. We call this "validating" 948 the cache entry. 949 </t> 950 <t> 951 HTTP's conditional request mechanism, defined in &conditional;, is 952 used to avoid retransmitting the response payload when the cached entry 953 is valid. When a cached response includes one or more "cache validators," 954 such as the field values of an ETag or Last-Modified header field, then 955 a validating GET request &SHOULD; be made conditional to those field values. 956 The server checks the conditional request's validator against the current 957 state of the requested resource and, if they match, the server responds 958 with a 304 (Not Modified) status code to indicate that the cached response 959 can be refreshed and reused without retransmitting the response payload. 960 If the validator does not match the current state of the requested 961 resource, then the server returns a full response, including payload, 962 so that the request can be satisfied and the cache entry supplanted 963 without the need for an additional network round-trip. 964 </t> 965 </section> 966 967 <section title="Response Cacheability" anchor="response.cacheability"> 968 <t> 969 Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>) 970 directive, a caching system &MAY; always store a successful response 971 (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation 972 if it is fresh, and &MAY; return it after successful validation. If 973 there is neither a cache validator nor an explicit expiration time 974 associated with a response, we do not expect it to be cached, but 975 certain caches &MAY; violate this expectation (for example, when little 976 or no network connectivity is available). A client can usually detect 977 that such a response was taken from a cache by comparing the Date 978 header to the current time. 979 <list><t> 980 <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation 981 without providing any Warning. 982 </t></list> 983 </t> 984 <t> 985 However, in some cases it might be inappropriate for a cache to 986 retain an entity, or to return it in response to a subsequent 987 request. This might be because absolute semantic transparency is 988 deemed necessary by the service author, or because of security or 989 privacy considerations. Certain cache-control directives are 990 therefore provided so that the server can indicate that certain 991 resource entities, or portions thereof, are not to be cached 992 regardless of other considerations. 993 </t> 994 <t> 995 Note that &header-authorization; normally prevents a shared cache from saving 996 and returning a response to a previous request if that request 997 included an Authorization header. 998 </t> 999 <t> 1000 A response received with a status code of 200, 203, 206, 300, 301 or 1001 410 &MAY; be stored by a cache and used in reply to a subsequent 1002 request, subject to the expiration mechanism, unless a cache-control 1003 directive prohibits caching. However, a cache that does not support 1004 the Range and Content-Range headers &MUST-NOT; cache 206 (Partial 1005 Content) responses. 1006 </t> 1007 <t> 1008 A response received with any other status code (e.g. status codes 302 1009 and 307) &MUST-NOT; be returned in a reply to a subsequent request 1010 unless there are cache-control directives or another header(s) that 1011 explicitly allow it. For example, these include the following: an 1012 Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage", "must-revalidate", 1013 "proxy-revalidate", "public" or "private" cache-control 1014 directive (<xref target="header.cache-control"/>). 1015 </t> 1016 </section> 1017 1018 <section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches"> 1019 <t> 1020 The purpose of an HTTP cache is to store information received in 1021 response to requests for use in responding to future requests. In 1022 many cases, a cache simply returns the appropriate parts of a 1023 response to the requester. However, if the cache holds a cache entry 1024 based on a previous response, it might have to combine parts of a new 1025 response with what is held in the cache entry. 1026 </t> 1027 1028 <section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers"> 1029 <t> 1030 For the purpose of defining the behavior of caches and non-caching 1031 proxies, we divide HTTP headers into two categories: 1032 <list style="symbols"> 1033 <t>End-to-end headers, which are transmitted to the ultimate 1034 recipient of a request or response. End-to-end headers in 1035 responses &MUST; be stored as part of a cache entry and &MUST; be 1036 transmitted in any response formed from a cache entry.</t> 1037 1038 <t>Hop-by-hop headers, which are meaningful only for a single 1039 transport-level connection, and are not stored by caches or 1040 forwarded by proxies.</t> 1041 </list> 1042 </t> 1043 <t> 1044 The following HTTP/1.1 headers are hop-by-hop headers: 1045 <list style="symbols"> 1046 <t>Connection</t> 1047 <t>Keep-Alive</t> 1048 <t>Proxy-Authenticate</t> 1049 <t>Proxy-Authorization</t> 1050 <t>TE</t> 1051 <t>Trailer</t> 1052 <t>Transfer-Encoding</t> 1053 <t>Upgrade</t> 1054 </list> 1055 </t> 1056 <t> 1057 All other headers defined by HTTP/1.1 are end-to-end headers. 1058 </t> 1059 <t> 1060 Other hop-by-hop headers &MUST; be listed in a Connection header 1061 (&header-connection;). 1062 </t> 1063 </section> 1064 1065 <section title="Non-modifiable Headers" anchor="non-modifiable.headers"> 1066 <t> 1067 Some features of HTTP/1.1, such as Digest 1068 Authentication, depend on the value of certain end-to-end headers. A 1069 transparent proxy &SHOULD-NOT; modify an end-to-end header unless the 1070 definition of that header requires or specifically allows that. 1071 </t> 1072 <t> 1073 A transparent proxy &MUST-NOT; modify any of the following fields in a 1074 request or response, and it &MUST-NOT; add any of these fields if not 1075 already present: 1076 <list style="symbols"> 1077 <t>Content-Location</t> 1078 <t>Content-MD5</t> 1079 <t>ETag</t> 1080 <t>Last-Modified</t> 1081 </list> 1082 </t> 1083 <t> 1084 A transparent proxy &MUST-NOT; modify any of the following fields in a 1085 response: 1086 <list style="symbols"> 1087 <t>Expires</t> 1088 </list> 1089 </t> 1090 <t> 1091 but it &MAY; add any of these fields if not already present. If an 1092 Expires header is added, it &MUST; be given a field-value identical to 1093 that of the Date header in that response. 1094 </t> 1095 <t> 1096 A proxy &MUST-NOT; modify or add any of the following fields in a 1097 message that contains the no-transform cache-control directive, or in 1098 any request: 1099 <list style="symbols"> 1100 <t>Content-Encoding</t> 1101 <t>Content-Range</t> 1102 <t>Content-Type</t> 1103 </list> 1104 </t> 1105 <t> 1106 A non-transparent proxy &MAY; modify or add these fields to a message 1107 that does not include no-transform, but if it does so, it &MUST; add a 1108 Warning 214 (Transformation applied) if one does not already appear 1109 in the message (see <xref target="header.warning"/>). 1110 <list><t> 1111 Warning: unnecessary modification of end-to-end headers might 1112 cause authentication failures if stronger authentication 1113 mechanisms are introduced in later versions of HTTP. Such 1114 authentication mechanisms &MAY; rely on the values of header fields 1115 not listed here. 1116 </t></list> 1117 </t> 1118 <t> 1119 The Content-Length field of a request or response is added or deleted 1120 according to the rules in &message-length;. A transparent proxy &MUST; 1121 preserve the entity-length (&entity-length;) of the entity-body, 1122 although it &MAY; change the transfer-length (&message-length;). 1123 </t> 1124 </section> 1125 1126 <section title="Combining Headers" anchor="combining.headers"> 1127 <t> 1128 When a cache makes a validating request to a server, and the server 1129 provides a 304 (Not Modified) response or a 206 (Partial Content) 1130 response, the cache then constructs a response to send to the 1131 requesting client. 1132 </t> 1133 <t> 1134 If the status code is 304 (Not Modified), the cache uses the entity-body 1135 stored in the cache entry as the entity-body of this outgoing 1136 response. If the status code is 206 (Partial Content) and the ETag or 1137 Last-Modified headers match exactly, the cache &MAY; combine the 1138 contents stored in the cache entry with the new contents received in 1139 the response and use the result as the entity-body of this outgoing 1140 response, (see &combining-byte-ranges;). 1141 </t> 1142 <t> 1143 The end-to-end headers stored in the cache entry are used for the 1144 constructed response, except that 1145 <list style="symbols"> 1146 <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>) 1147 &MUST; be deleted from the cache entry and the forwarded response.</t> 1148 <t>any stored Warning headers with warn-code 2xx &MUST; be retained 1149 in the cache entry and the forwarded response.</t> 1150 <t>any end-to-end headers provided in the 304 or 206 response &MUST; 1151 replace the corresponding headers from the cache entry.</t> 1152 </list> 1153 </t> 1154 <t> 1155 Unless the cache decides to remove the cache entry, it &MUST; also 1156 replace the end-to-end headers stored with the cache entry with 1157 corresponding headers received in the incoming response, except for 1158 Warning headers as described immediately above. If a header field-name 1159 in the incoming response matches more than one header in the 1160 cache entry, all such old headers &MUST; be replaced. 1161 </t> 1162 <t> 1163 In other words, the set of end-to-end headers received in the 1164 incoming response overrides all corresponding end-to-end headers 1165 stored with the cache entry (except for stored Warning headers with 1166 warn-code 1xx, which are deleted even if not overridden). 1167 <list><t> 1168 <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not 1169 Modified) or a 206 (Partial Content) response to update any header 1170 associated with a previous response for the same entity or sub-ranges 1171 thereof, although it might not always be meaningful or 1172 correct to do so. This rule does not allow an origin server to use 1173 a 304 (Not Modified) or a 206 (Partial Content) response to 1174 entirely delete a header that it had provided with a previous 1175 response. 1176 </t></list> 1177 </t> 1178 </section> 1179 1180 </section> 1181 1182 <section title="Caching Negotiated Responses" anchor="caching.negotiated.responses"> 1183 <t> 1184 Use of server-driven content negotiation (&server-driven-negotiation;), as indicated 1185 by the presence of a Vary header field in a response, alters the 1186 conditions and procedure by which a cache can use the response for 1187 subsequent requests. See <xref target="header.vary"/> for use of the Vary header 1188 field by servers. 1189 </t> 1190 <t> 1191 A server &SHOULD; use the Vary header field to inform a cache of what 1192 request-header fields were used to select among multiple 1193 representations of a cacheable response subject to server-driven 1194 negotiation. The set of header fields named by the Vary field value 1195 is known as the "selecting" request-headers. 1196 </t> 1197 <t> 1198 When the cache receives a subsequent request whose Request-URI 1199 specifies one or more cache entries including a Vary header field, 1200 the cache &MUST-NOT; use such a cache entry to construct a response to 1201 the new request unless all of the selecting request-headers present 1202 in the new request match the corresponding stored request-headers in 1203 the original request. 1204 </t> 1205 <t> 1206 The selecting request-headers from two requests are defined to match 1207 if and only if the selecting request-headers in the first request can 1208 be transformed to the selecting request-headers in the second request 1209 by adding or removing linear white space (LWS) at places where this 1210 is allowed by the corresponding BNF, and/or combining multiple 1211 message-header fields with the same field name following the rules 1212 about message headers in &message-headers;. 1213 </t> 1214 <t> 1215 A Vary header field-value of "*" always fails to match and subsequent 1216 requests on that resource can only be properly interpreted by the 1217 origin server. 1218 </t> 1219 <t> 1220 If the selecting request header fields for the cached entry do not 1221 match the selecting request header fields of the new request, then 1222 the cache &MUST-NOT; use a cached entry to satisfy the request unless 1223 it first relays the new request to the origin server in a conditional 1224 request and the server responds with 304 (Not Modified), including an 1225 entity tag or Content-Location that indicates the entity to be used. 1226 </t> 1227 <t> 1228 If an entity tag was assigned to a cached representation, the 1229 forwarded request &SHOULD; be conditional and include the entity tags 1230 in an If-None-Match header field from all its cache entries for the 1231 resource. This conveys to the server the set of entities currently 1232 held by the cache, so that if any one of these entities matches the 1233 requested entity, the server can use the ETag header field in its 304 1234 (Not Modified) response to tell the cache which entry is appropriate. 1235 If the entity-tag of the new response matches that of an existing 1236 entry, the new response &SHOULD; be used to update the header fields of 1237 the existing entry, and the result &MUST; be returned to the client. 1238 </t> 1239 <t> 1240 If any of the existing cache entries contains only partial content 1241 for the associated entity, its entity-tag &SHOULD-NOT; be included in 1242 the If-None-Match header field unless the request is for a range that 1243 would be fully satisfied by that entry. 1244 </t> 1245 <t> 1246 If a cache receives a successful response whose Content-Location 1247 field matches that of an existing cache entry for the same Request-URI, 1248 whose entity-tag differs from that of the existing entry, and 1249 whose Date is more recent than that of the existing entry, the 1250 existing entry &SHOULD-NOT; be returned in response to future requests 1251 and &SHOULD; be deleted from the cache. 1252 </t> 1253 </section> 1254 1255 <section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches"> 1256 <t> 1257 For reasons of security and privacy, it is necessary to make a 1258 distinction between "shared" and "non-shared" caches. A non-shared 1259 cache is one that is accessible only to a single user. Accessibility 1260 in this case &SHOULD; be enforced by appropriate security mechanisms. 1261 All other caches are considered to be "shared." Other sections of 1262 this specification place certain constraints on the operation of 1263 shared caches in order to prevent loss of privacy or failure of 1264 access controls. 1265 </t> 1266 </section> 1267 1268 <section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior"> 1269 <t> 1270 A cache that receives an incomplete response (for example, with fewer 1271 bytes of data than specified in a Content-Length header) &MAY; store 1272 the response. However, the cache &MUST; treat this as a partial 1273 response. Partial responses &MAY; be combined as described in &combining-byte-ranges;; 1274 the result might be a full response or might still be 1275 partial. A cache &MUST-NOT; return a partial response to a client 1276 without explicitly marking it as such, using the 206 (Partial 1277 Content) status code. A cache &MUST-NOT; return a partial response 1278 using a status code of 200 (OK). 1279 </t> 1280 <t> 1281 If a cache receives a 5xx response while attempting to revalidate an 1282 entry, it &MAY; either forward this response to the requesting client, 1283 or act as if the server failed to respond. In the latter case, it &MAY; 1284 return a previously received response unless the cached entry 1285 includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>). 1286 </t> 1287 </section> 1288 1289 <section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head"> 1290 <t> 1291 Unless the origin server explicitly prohibits the caching of their 1292 responses, the application of GET and HEAD methods to any resources 1293 &SHOULD-NOT; have side effects that would lead to erroneous behavior if 1294 these responses are taken from a cache. They &MAY; still have side 1295 effects, but a cache is not required to consider such side effects in 1296 its caching decisions. Caches are always expected to observe an 1297 origin server's explicit restrictions on caching. 1298 </t> 1299 <t> 1300 We note one exception to this rule: since some applications have 1301 traditionally used GET and HEAD requests with URLs containing a query part 1302 to perform operations with significant side 1303 effects, caches &MUST-NOT; treat responses to such URIs as fresh unless 1304 the server provides an explicit expiration time. This specifically 1305 means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 1306 be taken from a cache. See &safe-methods; for related information. 1307 </t> 1308 </section> 1309 1310 <section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions"> 1311 <t> 1312 The effect of certain methods performed on a resource at the origin 1313 server might cause one or more existing cache entries to become non-transparently 1314 invalid. That is, although they might continue to be 1315 "fresh," they do not accurately reflect what the origin server would 1316 return for a new request on that resource. 1317 </t> 1318 <t> 1319 There is no way for HTTP to guarantee that all such 1320 cache entries are marked invalid. For example, the request that 1321 caused the change at the origin server might not have gone through 1322 the proxy where a cache entry is stored. However, several rules help 1323 reduce the likelihood of erroneous behavior. 1324 </t> 1325 <t> 1326 In this section, the phrase "invalidate an entity" means that the 1327 cache will either remove all instances of that entity from its 1328 storage, or will mark these as "invalid" and in need of a mandatory 1329 revalidation before they can be returned in response to a subsequent 1330 request. 1331 </t> 1332 <t> 1333 Some HTTP methods &MUST; cause a cache to invalidate an entity. This is 1334 either the entity referred to by the Request-URI, or by the Location 1335 or Content-Location headers (if present). These methods are: 1336 <list style="symbols"> 1337 <t>PUT</t> 1338 <t>DELETE</t> 1339 <t>POST</t> 1340 </list> 1341 </t> 1342 <t> 1343 An invalidation based 1344 on the URI in a Location or Content-Location header &MUST-NOT; be 1345 performed if the host part of that URI differs from the host part 1346 in the Request-URI. This helps prevent denial of service attacks. 1347 </t> 1348 <t> 1349 A cache that passes through requests for methods it does not 1350 understand &SHOULD; invalidate any entities referred to by the 1351 Request-URI. 1352 </t> 1353 </section> 1354 1355 <section title="Write-Through Mandatory" anchor="write-through.mandatory"> 1356 <t> 1357 All methods that might be expected to cause modifications to the 1358 origin server's resources &MUST; be written through to the origin 1359 server. This currently includes all methods except for GET and HEAD. 1360 A cache &MUST-NOT; reply to such a request from a client before having 1361 transmitted the request to the inbound server, and having received a 1362 corresponding response from the inbound server. This does not prevent 1363 a proxy cache from sending a 100 (Continue) response before the 1364 inbound server has sent its final reply. 1365 </t> 1366 <t> 1367 The alternative (known as "write-back" or "copy-back" caching) is not 1368 allowed in HTTP/1.1, due to the difficulty of providing consistent 1369 updates and the problems arising from server, cache, or network 1370 failure prior to write-back. 1371 </t> 1372 </section> 1373 1374 <section title="Cache Replacement" anchor="cache.replacement"> 1375 <t> 1376 If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>, 1377 <xref target="disambiguating.expiration.values" format="counter"/>, 1378 <xref target="disambiguating.multiple.responses" format="counter"/> 1379 and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>) 1380 response is received from a resource while any existing responses for 1381 the same resource are cached, the cache &SHOULD; use the new response 1382 to reply to the current request. It &MAY; insert it into cache storage 1383 and &MAY;, if it meets all other requirements, use it to respond to any 1384 future requests that would previously have caused the old response to 1385 be returned. If it inserts the new response into cache storage the 1386 rules in <xref target="combining.headers"/> apply. 1387 <list><t> 1388 <x:h>Note:</x:h> a new response that has an older Date header value than 1389 existing cached responses is not cacheable. 1390 </t></list> 1391 </t> 1392 </section> 1393 1394 <section title="History Lists" anchor="history.lists"> 1395 <t> 1396 User agents often have history mechanisms, such as "Back" buttons and 1397 history lists, which can be used to redisplay an entity retrieved 1398 earlier in a session. 1399 </t> 1400 <t> 1401 History mechanisms and caches are different. In particular history 1402 mechanisms &SHOULD-NOT; try to show a semantically transparent view of 1403 the current state of a resource. Rather, a history mechanism is meant 1404 to show exactly what the user saw at the time when the resource was 1405 retrieved. 1406 </t> 1407 <t> 1408 By default, an expiration time does not apply to history mechanisms. 1409 If the entity is still in storage, a history mechanism &SHOULD; display 1410 it even if the entity has expired, unless the user has specifically 1411 configured the agent to refresh expired history documents. 1412 </t> 1413 <t> 1414 This is not to be construed to prohibit the history mechanism from 1415 telling the user that a view might be stale. 1416 <list><t> 1417 <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from 1418 viewing stale resources, this will tend to force service authors 1419 to avoid using HTTP expiration controls and cache controls when 1420 they would otherwise like to. Service authors may consider it 1421 important that users not be presented with error messages or 1422 warning messages when they use navigation controls (such as BACK) 1423 to view previously fetched resources. Even though sometimes such 1424 resources ought not be cached, or ought to expire quickly, user 1425 interface considerations may force service authors to resort to 1426 other means of preventing caching (e.g. "once-only" URLs) in order 1427 not to suffer the effects of improperly functioning history 1428 mechanisms. 1429 </t></list> 1430 </t> 1431 </section> 1432 1433 <section title="Header Field Definitions" anchor="header.fields"> 1434 <t> 1435 This section defines the syntax and semantics of HTTP/1.1 header fields 1436 related to caching. 1437 </t> 1438 <t> 1439 For entity-header fields, both sender and recipient refer to either the 1440 client or the server, depending on who sends and who receives the entity. 1441 </t> 1442 1443 <section title="Age" anchor="header.age"> 1444 <iref primary="true" item="Age header" x:for-anchor=""/> 1445 <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/> 1446 <x:anchor-alias value="Age"/> 1447 <x:anchor-alias value="age-value"/> 1448 <t> 1449 The Age response-header field conveys the sender's estimate of the 1450 amount of time since the response (or its revalidation) was 1451 generated at the origin server. A cached response is "fresh" if 1452 its age does not exceed its freshness lifetime. Age values are 1453 calculated as specified in <xref target="age.calculations"/>. 1454 </t> 1455 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/> 617 </artwork> 618 </figure> 619 </section> 620 621 622 <section anchor="serving.stale.responses" title="Serving Stale Responses"> 623 <t>Caches &MAY; return a stale response, unless such a response is otherwise prohibited 624 (e.g., by a "no-store" or "no-cache" cache-request-directive, or a "must-revalidate" 625 cache-response-directive; see <xref target="header.cache-control" />). Such stale responses 626 MUST have a Warning header with the 110 warn-code (see <xref format="counter" 627 target="header.warning" />)</t> 628 <t>A stale response &MUST; have freshness information available, either explicitly or 629 heuristically. See <xref target="calculating.freshness.lifetime" /></t> 630 <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not 631 Modified) response) that it would normally forward to the requesting client, and the 632 received response is no longer fresh, the cache &SHOULD; forward it to the requesting 633 client without adding a new Warning (but without removing any existing Warning headers). A 634 cache &SHOULD-NOT; attempt to validate a response simply because that response 635 became stale in transit.</t> 636 </section> 637 638 </section> 639 640 641 <section anchor="validation.model" title="Validation Model"> 642 <t>When a cache has a stale response that it would like to use, it should first check with 643 the origin server (or possibly an intermediate cache with a fresh response) to see if it 644 is still usable. This is called "validating" or "revalidating" the stored response.</t> 645 <t>HTTP's conditional request mechanism, defined in &conditional;, is used to avoid 646 retransmitting the response payload when the stored response is valid. When a stored response 647 includes one or more "cache validators," such as the field values of an ETag or 648 Last-Modified header field, then a validating GET request &SHOULD; be made conditional 649 to those field values. The server checks the conditional request's validator against the 650 current state of the requested resource and, if they match, the server responds with a 304 651 (Not Modified) status code to indicate that the stored response can be refreshed and 652 reused without retransmitting the response payload. If the validator does not match the 653 current state of the requested resource, then the server returns a full response, 654 including payload, so that the request can be satisfied and the stored response supplanted 655 without the need for an additional network round-trip.</t> 656 <t>See <xref target="combining.headers" /> for information about combining cached headers 657 with those in a 304 response.</t> 658 <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY; 659 either forward this response to the requesting client, or act as if the server failed to 660 respond. In the latter case, it &MAY; return a previously received response unless the 661 stored response includes the "must-revalidate" cache-control directive (see <xref 662 target="header.cache-control" />).</t> 663 <t> 664 <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in 665 p1</cref> 666 </t> 667 </section> 668 669 670 <section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate"> 671 <t>Because unsafe methods &safe-methods; have the potential for changing state on the 672 origin server, intervening caches have the opportunity to use them to keep their contents 673 up-to-date.</t> 674 <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well 675 as the Location and Content-Location headers (if present): <list style="symbols"> 676 <t>PUT</t> 677 <t>DELETE</t> 678 <t>POST</t> 679 </list> 680 </t> 681 <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT; 682 be performed if the host part of that URI differs from the host part in the Request-URI. 683 This helps prevent denial of service attacks.</t> 684 <t> 685 <cref>TODO: "host part" needs to be specified better.</cref> 686 </t> 687 <t>A cache that passes through requests for methods it does not understand &SHOULD; 688 invalidate the Request-URI.</t> 689 <t>Here, "invalidate" means that the cache will either remove all stored responses related 690 to the Request-URI, or will mark these as "invalid" and in need of a mandatory 691 validation before they can be returned in response to a subsequent request.</t> 692 <t>Note that this does not guarantee that all appropriate responses are invalidated. For 693 example, the request that caused the change at the origin server might not have gone 694 through the cache where a response is stored.</t> 695 <t> 696 <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref> 697 </t> 698 </section> 699 700 701 <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses"> 702 <t>Use of server-driven content negotiation (&server-driven-negotiation;), as indicated 703 by the presence of a Vary header field <xref target="header.vary" /> in a response, alters 704 the conditions and procedure by which a cache can use the response for subsequent 705 requests.</t> 706 <t>When the cache receives a subsequent request which may be satisfied by a stored responses 707 that include a Vary header field, it &MUST-NOT; use it to satisfy the request unless 708 all of the selecting request-headers present in the new request match the corresponding 709 stored request-headers from the original request.</t> 710 <t>The selecting request-headers from two requests are defined to match if and only if the 711 selecting request-headers in the first request can be transformed to the selecting 712 request-headers in the second request by adding or removing linear white space 713 <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or 714 combining multiple message-header fields with the same field name following the rules 715 about message headers in &message-headers;.</t> 716 <t>A Vary header field-value of "*" always fails to match, and subsequent requests on that 717 resource can only be properly interpreted by the origin server.</t> 718 <t>If the selecting request header fields for the stored response do not match the selecting 719 request header fields of the new request, then the cache &MUST-NOT; use the stored 720 response to satisfy the request unless it first relays the new request to the origin 721 server in a conditional request and the server responds with 304 (Not Modified), including 722 an entity tag or Content-Location that indicates the entity to be used.</t> 723 <t>If one or more applicable stored response has an entity tag, the forwarded request 724 &SHOULD; be conditional and include all of these entity tags in an If-None-Match 725 header field. This conveys to the server the set of entities currently stored by the 726 cache, so that if any one of these entities matches the requested entity, the server can 727 use the ETag header field in its 304 (Not Modified) response to tell the cache which 728 stored response is appropriate. If the entity-tag of the new response matches that of an 729 existing stored resopnse, the new response &SHOULD; be used to update its header 730 fields, and the result &MUST; be returned to the client.</t> 731 <t>If any of the existing stored responses contains only partial content for the associated 732 entity, its entity-tag &SHOULD-NOT; be included in the If-None-Match header field 733 unless the request is for a range that would be fully satisfied by that stored response.</t> 734 <t>If a cache receives a successful response whose Content-Location field matches that of an 735 existing stored response for the same Request-URI, whose entity-tag differs from that of the 736 existing stored response, and whose Date is more recent than that of the existing response, 737 the existing response &SHOULD-NOT; be returned in response to future requests and 738 &SHOULD; be deleted from the cache.</t> 739 <t> 740 <cref>TODO: this is still really messed up.</cref> 741 </t> 742 </section> 743 744 745 <section anchor="errors.or.incomplete.response.cache.behavior" 746 title="Caching Incomplete Responses"> 747 <t>A cache that receives an incomplete response (for example, with fewer bytes of data than 748 specified in a Content-Length header) &MAY; store the response. However, the cache 749 &MUST; treat this as a partial response &partial;. Partial responses &MAY; be 750 combined as described in &combining-byte-ranges;; the result might be a full response 751 or might still be partial. A cache &MUST-NOT; return a partial response to a client 752 without explicitly marking it as such using the 206 (Partial Content) status code.</t> 753 <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; cache 754 incomplete or partial responses.</t> 755 </section> 756 757 758 <section anchor="combining.headers" title="Combining Responses"> 759 <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response, 760 it needs to update the stored response with the new one, so that the updated response can 761 be sent to the client.</t> 762 <t>If the status code is 304 (Not Modified), the cache SHOULD use the stored entity-body as 763 the updated entity-body. If the status code is 206 (Partial Content) and the ETag or 764 Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in 765 the stored response with the updated entity-body received in the response and use the result 766 as the updated entity-body (see &combining-byte-ranges;).</t> 767 <t>The stored response headers are used for the updated response, except that <list 768 style="symbols"> 769 <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />) 770 &MUST; be deleted from the stored response and the forwarded response.</t> 771 <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored 772 response and the forwarded response.</t> 773 <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding 774 headers from the stored response.</t> 775 </list> 776 </t> 777 <t>A cache &MUST; also replace stored headers with corresponding headers received in the 778 incoming response, except for Warning headers as described immediately above. If a header 779 field-name in the incoming response matches more than one header in the stored response, all 780 such old headers &MUST; be replaced. it &MAY; store the combined entity-body.</t> 781 </section> 782 783 </section> 784 785 786 787 788 <section anchor="header.fields" title="Header Field Definitions"> 789 <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t> 790 <t>For entity-header fields, both sender and recipient refer to either the client or the 791 server, depending on who sends and who receives the entity.</t> 792 793 <section anchor="header.age" title="Age"> 794 <iref item="Age header" primary="true" x:for-anchor="" /> 795 <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" /> 796 <x:anchor-alias value="Age" /> 797 <x:anchor-alias value="age-value" /> 798 <t>The Age response-header field conveys the sender's estimate of the amount of time since 799 the response (or its validation) was generated at the origin server. Age values are 800 calculated as specified in <xref target="age.calculations" />.</t> 801 <figure> 802 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Age" /><iref item="Grammar" primary="true" subitem="age-value" /> 1456 803 <x:ref>Age</x:ref> = "Age" ":" <x:ref>age-value</x:ref> 1457 804 <x:ref>age-value</x:ref> = <x:ref>delta-seconds</x:ref> 1458 </artwork> </figure>1459 <t anchor="rule.delta-seconds">1460 <x:anchor-alias value="delta-seconds"/>1461 Age values are non-negative decimal integers, representing time in1462 seconds.1463 </t>1464 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>805 </artwork> 806 </figure> 807 <t anchor="rule.delta-seconds"> 808 <x:anchor-alias value="delta-seconds" /> Age values are non-negative decimal integers, 809 representing time in seconds.</t> 810 <figure> 811 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" /> 1465 812 <x:ref>delta-seconds</x:ref> = 1*<x:ref>DIGIT</x:ref> 1466 </artwork></figure> 1467 <t> 1468 If a cache receives a value larger than the largest positive 1469 integer it can represent, or if any of its age calculations 1470 overflows, it &MUST; transmit an Age header with a value of 1471 2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST; 1472 include an Age header field in every response generated from its 1473 own cache. Caches &SHOULD; use an arithmetic type of at least 31 1474 bits of range. 1475 </t> 1476 </section> 1477 1478 <section title="Cache-Control" anchor="header.cache-control"> 1479 <iref primary="true" item="Cache-Control header" x:for-anchor=""/> 1480 <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/> 1481 <x:anchor-alias value="Cache-Control"/> 1482 <x:anchor-alias value="cache-directive"/> 1483 <x:anchor-alias value="cache-extension"/> 1484 <x:anchor-alias value="cache-request-directive"/> 1485 <x:anchor-alias value="cache-response-directive"/> 1486 <t> 1487 The Cache-Control general-header field is used to specify directives 1488 that &MUST; be obeyed by all caching mechanisms along the 1489 request/response chain. The directives specify behavior intended to 1490 prevent caches from adversely interfering with the request or 1491 response. These directives typically override the default caching 1492 algorithms. Cache directives are unidirectional in that the presence 1493 of a directive in a request does not imply that the same directive is 1494 to be given in the response. 1495 <list><t> 1496 Note that HTTP/1.0 caches might not implement Cache-Control and 1497 might only implement Pragma: no-cache (see <xref target="header.pragma"/>). 1498 </t></list> 1499 </t> 1500 <t> 1501 Cache directives &MUST; be passed through by a proxy or gateway 1502 application, regardless of their significance to that application, 1503 since the directives might be applicable to all recipients along the 1504 request/response chain. It is not possible to specify a cache-directive 1505 for a specific cache. 1506 </t> 1507 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/> 813 </artwork> 814 </figure> 815 <t>If a cache receives a value larger than the largest positive integer it can represent, or 816 if any of its age calculations overflows, it &MUST; transmit an Age header with a 817 value of 2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache 818 &MUST; include an Age header field in every response generated from its own cache. 819 Caches &SHOULD; use an arithmetic type of at least 31 bits of range.</t> 820 <t>The presence of an Age header field in a response implies that a response is not 821 first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the 822 Age header field.</t> 823 </section> 824 825 <section anchor="header.cache-control" title="Cache-Control"> 826 <iref item="Cache-Control header" primary="true" x:for-anchor="" /> 827 <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" /> 828 <x:anchor-alias value="Cache-Control" /> 829 <x:anchor-alias value="cache-directive" /> 830 <x:anchor-alias value="cache-extension" /> 831 <t>The Cache-Control general-header field is used to specify directives that &MUST; be 832 obeyed by all caches along the request/response chain. The directives specify behavior 833 intended to prevent caches from adversely interfering with the request or response. Cache 834 directives are unidirectional in that the presence of a directive in a request does not 835 imply that the same directive is to be given in the response. <list> 836 <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement 837 Pragma: no-cache (see <xref target="header.pragma" />).</t> 838 </list> 839 </t> 840 <t>Cache directives &MUST; be passed through by a proxy or gateway application, 841 regardless of their significance to that application, since the directives might be 842 applicable to all recipients along the request/response chain. It is not possible to 843 target a cache-directive to a specific cache.</t> 844 <figure> 845 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Cache-Control" /><iref item="Grammar" primary="true" subitem="cache-directive" /><iref item="Grammar" primary="true" subitem="cache-extension" /> 1508 846 <x:ref>Cache-Control</x:ref> = "Cache-Control" ":" 1#<x:ref>cache-directive</x:ref> 1509 847 … … 1511 849 / <x:ref>cache-response-directive</x:ref> 1512 850 851 <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ] 852 </artwork> 853 </figure> 854 855 856 <section anchor="cache-request-directive" title="Request Cache-Control Directives"> 857 <x:anchor-alias value="cache-request-directive" /> 858 859 <figure> 860 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" /> 1513 861 <x:ref>cache-request-directive</x:ref> = 1514 "no-cache" ; <xref target="what.is.cacheable"/> 1515 / "no-store" ; <xref target="what.may.be.stored.by.caches"/> 1516 / "max-age" "=" <x:ref>delta-seconds</x:ref> ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/> 1517 / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ] ; <xref target="modifications.of.the.basic.expiration.mechanism"/> 1518 / "min-fresh" "=" <x:ref>delta-seconds</x:ref> ; <xref target="modifications.of.the.basic.expiration.mechanism"/> 1519 / "no-transform" ; <xref target="no-transform.directive"/> 1520 / "only-if-cached" ; <xref target="cache.revalidation.and.reload.controls"/> 1521 / <x:ref>cache-extension</x:ref> ; <xref target="cache.control.extensions"/> 1522 862 "no-cache" 863 / "no-store" 864 / "max-age" "=" <x:ref>delta-seconds</x:ref> 865 / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ] 866 / "min-fresh" "=" <x:ref>delta-seconds</x:ref> 867 / "no-transform" 868 / "only-if-cached" 869 / <x:ref>cache-extension</x:ref> 870 </artwork> 871 </figure> 872 873 874 <t> 875 <iref item="Cache Directives" primary="true" subitem="no-cache" /> 876 <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list> 877 <t>The no-cache request directive indicates that a stored response &MUST-NOT; be 878 used to satisfy the request without successful validation on the origin server. 879 </t> 880 </list> 881 </t> 882 883 <t> 884 <iref item="Cache Directives" primary="true" subitem="no-store" /> 885 <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list> 886 <t>The no-store request directive indicates that a cache &MUST-NOT; store any part 887 of either this request or any response to it. This directive applies to both 888 non-shared and shared caches. "&MUST-NOT; store" in this context means that the 889 cache &MUST-NOT; intentionally store the information in non-volatile storage, 890 and &MUST; make a best-effort attempt to remove the information from volatile 891 storage as promptly as possible after forwarding it.</t> 892 <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In 893 particular, malicious or compromised caches might not recognize or obey this 894 directive, and communications networks may be vulnerable to eavesdropping.</t> 895 </list> 896 </t> 897 898 <t> 899 <iref item="Cache Directives" primary="true" subitem="max-age" /> 900 <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list> 901 <t>The no-cache request directive indicates that the client is willing to accept a 902 response whose age is no greater than the specified time in seconds. Unless 903 max-stale directive is also included, the client is not willing to accept a stale 904 response.</t> 905 </list> 906 </t> 907 <t> 908 <iref item="Cache Directives" primary="true" subitem="max-stale" /> 909 <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list> 910 <t>The max-stale request directive indicates that the client is willing to accept a 911 response that has exceeded its expiration time. If max-stale is assigned a value, 912 then the client is willing to accept a response that has exceeded its expiration 913 time by no more than the specified number of seconds. If no value is assigned to 914 max-stale, then the client is willing to accept a stale response of any age.</t> 915 </list> 916 </t> 917 <t> 918 <iref item="Cache Directives" primary="true" subitem="min-fresh" /> 919 <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list> 920 <t>The min-fresh request directive indicates that the client is willing to accept a 921 response whose freshness lifetime is no less than its current age plus the specified 922 time in seconds. That is, the client wants a response that will still be fresh for 923 at least the specified number of seconds.</t> 924 </list> 925 </t> 926 <t> 927 <iref item="Cache Directives" primary="true" subitem="no-transform" /> 928 <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list> 929 <t>The no-transform request directive indicates that an intermediate cache or proxy 930 &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request 931 headers, nor the request entity-body.</t> 932 </list> 933 </t> 934 935 <t> 936 <iref item="Cache Directives" primary="true" subitem="only-if-cached" /> 937 <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list> 938 <t>The only-if-cached request directive indicates that the client only wishes to 939 return a stored response. If it receives this directive, a cache &SHOULD; either 940 respond using a stored response that is consistent with the other constraints of the 941 request, or respond with a 504 (Gateway Timeout) status. If a group of caches is 942 being operated as a unified system with good internal connectivity, such a request 943 &MAY; be forwarded within that group of caches.</t> 944 </list> 945 </t> 946 </section> 947 948 <section anchor="cache-response-directive" title="Response Cache-Control Directives"> 949 <x:anchor-alias value="cache-response-directive" /> 950 951 <figure> 952 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" /> 1523 953 <x:ref>cache-response-directive</x:ref> = 1524 "public" ; <xref target="what.is.cacheable"/> 1525 / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/> 1526 / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/> 1527 / "no-store" ; <xref target="what.may.be.stored.by.caches"/> 1528 / "no-transform" ; <xref target="no-transform.directive"/> 1529 / "must-revalidate" ; <xref target="cache.revalidation.and.reload.controls"/> 1530 / "proxy-revalidate" ; <xref target="cache.revalidation.and.reload.controls"/> 1531 / "max-age" "=" <x:ref>delta-seconds</x:ref> ; <xref target="modifications.of.the.basic.expiration.mechanism"/> 1532 / "s-maxage" "=" <x:ref>delta-seconds</x:ref> ; <xref target="modifications.of.the.basic.expiration.mechanism"/> 1533 / <x:ref>cache-extension</x:ref> ; <xref target="cache.control.extensions"/> 1534 1535 <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ] 1536 </artwork></figure> 1537 <t> 1538 When a directive appears without any 1#field-name parameter, the 1539 directive applies to the entire request or response. When such a 1540 directive appears with a 1#field-name parameter, it applies only to 1541 the named field or fields, and not to the rest of the request or 1542 response. This mechanism supports extensibility; implementations of 1543 future versions of HTTP might apply these directives to 1544 header fields not defined in HTTP/1.1. 1545 </t> 1546 <t> 1547 The cache-control directives can be broken down into these general 1548 categories: 1549 <list style="symbols"> 1550 <t>Restrictions on what are cacheable; these may only be imposed by 1551 the origin server.</t> 1552 1553 <t>Restrictions on what may be stored by a cache; these may be 1554 imposed by either the origin server or the user agent.</t> 1555 1556 <t>Modifications of the basic expiration mechanism; these may be 1557 imposed by either the origin server or the user agent.</t> 1558 1559 <t>Controls over cache revalidation and reload; these may only be 1560 imposed by a user agent.</t> 1561 1562 <t>Control over transformation of entities.</t> 1563 1564 <t>Extensions to the caching system.</t> 1565 </list> 1566 </t> 1567 1568 <section title="What is Cacheable" anchor="what.is.cacheable"> 1569 <t> 1570 By default, a response is cacheable if the requirements of the 1571 request method, request header fields, and the response status 1572 indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults 1573 for cacheability. The following Cache-Control response directives 1574 allow an origin server to override the default cacheability of a 1575 response: 1576 </t> 1577 <t> 1578 <iref item="Cache Directives" subitem="public" primary="true"/> 1579 <iref item="public" subitem="Cache Directive" primary="true"/> 1580 public 1581 <list><t> 1582 Indicates that the response &MAY; be cached by any cache, even if it 1583 would normally be non-cacheable or cacheable only within a non-shared 1584 cache. (See also Authorization, &header-authorization;, for 1585 additional details.) 1586 </t></list> 1587 </t> 1588 <t> 1589 <iref item="Cache Directives" subitem="private" primary="true"/> 1590 <iref item="private" subitem="Cache Directive" primary="true"/> 1591 private 1592 <list><t> 1593 Indicates that all or part of the response message is intended for 1594 a single user and &MUST-NOT; be cached by a shared cache. This 1595 allows an origin server to state that the specified parts of the 1596 response are intended for only one user and are not a valid 1597 response for requests by other users. A private (non-shared) cache 1598 &MAY; cache the response. 1599 </t><t> 1600 <x:h>Note:</x:h> This usage of the word private only controls where the 1601 response may be cached, and cannot ensure the privacy of the 1602 message content. 1603 </t></list> 1604 </t> 1605 <t> 1606 <iref item="Cache Directives" subitem="no-cache" primary="true"/> 1607 <iref item="no-cache" subitem="Cache Directive" primary="true"/> 1608 no-cache 1609 <list><t> 1610 If the no-cache directive does not specify a field-name, then a 1611 cache &MUST-NOT; use the response to satisfy a subsequent request 1612 without successful revalidation with the origin server. This 1613 allows an origin server to prevent caching even by caches that 1614 have been configured to return stale responses to client requests. 1615 </t><t> 1616 If the no-cache directive does specify one or more field-names, 1617 then a cache &MAY; use the response to satisfy a subsequent request, 1618 subject to any other restrictions on caching. However, the 1619 specified field-name(s) &MUST-NOT; be sent in the response to a 1620 subsequent request without successful revalidation with the origin 1621 server. This allows an origin server to prevent the re-use of 1622 certain header fields in a response, while still allowing caching 1623 of the rest of the response. 1624 <list><t> 1625 <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this 1626 directive. 1627 </t></list> 1628 </t></list> 1629 </t> 1630 </section> 1631 1632 <section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches"> 1633 <t> 1634 <iref item="Cache Directives" subitem="no-store" primary="true"/> 1635 <iref item="no-store" subitem="Cache Directive" primary="true"/> 1636 no-store 1637 <list><t> 1638 The purpose of the no-store directive is to prevent the 1639 inadvertent release or retention of sensitive information (for 1640 example, on backup tapes). The no-store directive applies to the 1641 entire message, and &MAY; be sent either in a response or in a 1642 request. If sent in a request, a cache &MUST-NOT; store any part of 1643 either this request or any response to it. If sent in a response, 1644 a cache &MUST-NOT; store any part of either this response or the 1645 request that elicited it. This directive applies to both non-shared 1646 and shared caches. "&MUST-NOT; store" in this context means 1647 that the cache &MUST-NOT; intentionally store the information in 1648 non-volatile storage, and &MUST; make a best-effort attempt to 1649 remove the information from volatile storage as promptly as 1650 possible after forwarding it. 1651 </t><t> 1652 Even when this directive is associated with a response, users 1653 might explicitly store such a response outside of the caching 1654 system (e.g., with a "Save As" dialog). History buffers &MAY; store 1655 such responses as part of their normal operation. 1656 </t><t> 1657 The purpose of this directive is to meet the stated requirements 1658 of certain users and service authors who are concerned about 1659 accidental releases of information via unanticipated accesses to 1660 cache data structures. While the use of this directive might 1661 improve privacy in some cases, we caution that it is NOT in any 1662 way a reliable or sufficient mechanism for ensuring privacy. In 1663 particular, malicious or compromised caches might not recognize or 1664 obey this directive, and communications networks might be 1665 vulnerable to eavesdropping. 1666 </t></list> 1667 </t> 1668 </section> 1669 1670 <section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism"> 1671 <t> 1672 The expiration time of an entity &MAY; be specified by the origin 1673 server using the Expires header (see <xref target="header.expires"/>). Alternatively, 1674 it &MAY; be specified using the max-age directive in a response. When 1675 the max-age cache-control directive is present in a cached response, 1676 the response is stale if its current age is greater than the age 1677 value given (in seconds) at the time of a new request for that 1678 resource. The max-age directive on a response implies that the 1679 response is cacheable (i.e., "public") unless some other, more 1680 restrictive cache directive is also present. 1681 </t> 1682 <t> 1683 If a response includes both an Expires header and a max-age 1684 directive, the max-age directive overrides the Expires header, even 1685 if the Expires header is more restrictive. This rule allows an origin 1686 server to provide, for a given response, a longer expiration time to 1687 an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be 1688 useful if certain HTTP/1.0 caches improperly calculate ages or 1689 expiration times, perhaps due to desynchronized clocks. 1690 </t> 1691 <t> 1692 Many HTTP/1.0 cache implementations will treat an Expires value that 1693 is less than or equal to the response Date value as being equivalent 1694 to the Cache-Control response directive "no-cache". If an HTTP/1.1 1695 cache receives such a response, and the response does not include a 1696 Cache-Control header field, it &SHOULD; consider the response to be 1697 non-cacheable in order to retain compatibility with HTTP/1.0 servers. 1698 <list><t> 1699 <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP 1700 cache control feature, such as the "private" directive, on a 1701 network including older caches that do not understand that 1702 feature. The origin server will need to combine the new feature 1703 with an Expires field whose value is less than or equal to the 1704 Date value. This will prevent older caches from improperly 1705 caching the response. 1706 </t></list> 1707 </t> 1708 <t> 1709 <iref item="Cache Directives" subitem="s-maxage" primary="true"/> 1710 <iref item="s-maxage" subitem="Cache Directive" primary="true"/> 1711 s-maxage 1712 <list><t> 1713 If a response includes an s-maxage directive, then for a shared 1714 cache (but not for a private cache), the maximum age specified by 1715 this directive overrides the maximum age specified by either the 1716 max-age directive or the Expires header. The s-maxage directive 1717 also implies the semantics of the proxy-revalidate directive (see 1718 <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the 1719 entry after it becomes stale to respond to a subsequent request 1720 without first revalidating it with the origin server. The s-maxage 1721 directive is always ignored by a private cache. 1722 </t></list> 1723 </t> 1724 <t> 1725 Note that most older caches, not compliant with this specification, 1726 do not implement any cache-control directives. An origin server 1727 wishing to use a cache-control directive that restricts, but does not 1728 prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the 1729 requirement that the max-age directive overrides the Expires header, 1730 and the fact that pre-HTTP/1.1-compliant caches do not observe the 1731 max-age directive. 1732 </t> 1733 <t> 1734 Other directives allow a user agent to modify the basic expiration 1735 mechanism. These directives &MAY; be specified on a request: 1736 </t> 1737 <t> 1738 <iref item="Cache Directives" subitem="max-age" primary="true"/> 1739 <iref item="max-age" subitem="Cache Directive" primary="true"/> 1740 max-age 1741 <list><t> 1742 Indicates that the client is willing to accept a response whose 1743 age is no greater than the specified time in seconds. Unless max-stale 1744 directive is also included, the client is not willing to 1745 accept a stale response. 1746 </t></list> 1747 </t> 1748 <t> 1749 <iref item="Cache Directives" subitem="min-fresh" primary="true"/> 1750 <iref item="min-fresh" subitem="Cache Directive" primary="true"/> 1751 min-fresh 1752 <list><t> 1753 Indicates that the client is willing to accept a response whose 1754 freshness lifetime is no less than its current age plus the 1755 specified time in seconds. That is, the client wants a response 1756 that will still be fresh for at least the specified number of 1757 seconds. 1758 </t></list> 1759 </t> 1760 <t> 1761 <iref item="Cache Directives" subitem="max-stale" primary="true"/> 1762 <iref item="max-stale" subitem="Cache Directive" primary="true"/> 1763 max-stale 1764 <list><t> 1765 Indicates that the client is willing to accept a response that has 1766 exceeded its expiration time. If max-stale is assigned a value, 1767 then the client is willing to accept a response that has exceeded 1768 its expiration time by no more than the specified number of 1769 seconds. If no value is assigned to max-stale, then the client is 1770 willing to accept a stale response of any age. 1771 </t></list> 1772 </t> 1773 <t> 1774 If a cache returns a stale response, either because of a max-stale 1775 directive on a request, or because the cache is configured to 1776 override the expiration time of a response, the cache &MUST; attach a 1777 Warning header to the stale response, using Warning 110 (Response is 1778 stale). 1779 </t> 1780 <t> 1781 A cache &MAY; be configured to return stale responses without 1782 validation, but only if this does not conflict with any "MUST"-level 1783 requirements concerning cache validation (e.g., a "must-revalidate" 1784 cache-control directive). 1785 </t> 1786 <t> 1787 If both the new request and the cached entry include "max-age" 1788 directives, then the lesser of the two values is used for determining 1789 the freshness of the cached entry for that request. 1790 </t> 1791 </section> 1792 1793 <section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls"> 1794 <t> 1795 Sometimes a user agent might want or need to insist that a cache 1796 revalidate its cache entry with the origin server (and not just with 1797 the next cache along the path to the origin server), or to reload its 1798 cache entry from the origin server. End-to-end revalidation might be 1799 necessary if either the cache or the origin server has overestimated 1800 the expiration time of the cached response. End-to-end reload may be 1801 necessary if the cache entry has become corrupted for some reason. 1802 </t> 1803 <t> 1804 End-to-end revalidation may be requested either when the client does 1805 not have its own local cached copy, in which case we call it 1806 "unspecified end-to-end revalidation", or when the client does have a 1807 local cached copy, in which case we call it "specific end-to-end 1808 revalidation." 1809 </t> 1810 <t> 1811 The client can specify these three kinds of action using Cache-Control 1812 request directives: 1813 </t> 1814 <t> 1815 End-to-end reload 1816 <list><t> 1817 The request includes a "no-cache" cache-control directive or, for 1818 compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field 1819 names &MUST-NOT; be included with the no-cache directive in a 1820 request. The server &MUST-NOT; use a cached copy when responding to 1821 such a request. 1822 </t></list> 1823 </t> 1824 <t> 1825 Specific end-to-end revalidation 1826 <list><t> 1827 The request includes a "max-age=0" cache-control directive, which 1828 forces each cache along the path to the origin server to 1829 revalidate its own entry, if any, with the next cache or server. 1830 The initial request includes a cache-validating conditional with 1831 the client's current validator. 1832 </t></list> 1833 </t> 1834 <t> 1835 Unspecified end-to-end revalidation 1836 <list><t> 1837 The request includes "max-age=0" cache-control directive, which 1838 forces each cache along the path to the origin server to 1839 revalidate its own entry, if any, with the next cache or server. 1840 The initial request does not include a cache-validating 1841 conditional; the first cache along the path (if any) that holds a 1842 cache entry for this resource includes a cache-validating 1843 conditional with its current validator. 1844 </t></list> 1845 </t> 1846 <t> 1847 <iref item="Cache Directives" subitem="max-age" primary="true"/> 1848 <iref item="max-age" subitem="Cache Directive" primary="true"/> 1849 max-age 1850 <list><t> 1851 When an intermediate cache is forced, by means of a max-age=0 1852 directive, to revalidate its own cache entry, and the client has 1853 supplied its own validator in the request, the supplied validator 1854 might differ from the validator currently stored with the cache 1855 entry. In this case, the cache &MAY; use either validator in making 1856 its own request without affecting semantic transparency. 1857 </t><t> 1858 However, the choice of validator might affect performance. The 1859 best approach is for the intermediate cache to use its own 1860 validator when making its request. If the server replies with 304 1861 (Not Modified), then the cache can return its now validated copy 1862 to the client with a 200 (OK) response. If the server replies with 1863 a new entity and cache validator, however, the intermediate cache 1864 can compare the returned validator with the one provided in the 1865 client's request, using the strong comparison function. If the 1866 client's validator is equal to the origin server's, then the 1867 intermediate cache simply returns 304 (Not Modified). Otherwise, 1868 it returns the new entity with a 200 (OK) response. 1869 </t><t> 1870 If a request includes the no-cache directive, it &SHOULD-NOT; 1871 include min-fresh, max-stale, or max-age. 1872 </t></list> 1873 </t> 1874 <t> 1875 <iref item="Cache Directives" subitem="only-if-cached" primary="true"/> 1876 <iref item="only-if-cached" subitem="Cache Directive" primary="true"/> 1877 only-if-cached 1878 <list><t> 1879 In some cases, such as times of extremely poor network 1880 connectivity, a client may want a cache to return only those 1881 responses that it currently has stored, and not to reload or 1882 revalidate with the origin server. To do this, the client may 1883 include the only-if-cached directive in a request. If it receives 1884 this directive, a cache &SHOULD; either respond using a cached entry 1885 that is consistent with the other constraints of the request, or 1886 respond with a 504 (Gateway Timeout) status. However, if a group 1887 of caches is being operated as a unified system with good internal 1888 connectivity, such a request &MAY; be forwarded within that group of 1889 caches. 1890 </t></list> 1891 </t> 1892 <t> 1893 <iref item="Cache Directives" subitem="must-revalidate" primary="true"/> 1894 <iref item="must-revalidate" subitem="Cache Directive" primary="true"/> 1895 must-revalidate 1896 <list><t> 1897 Because a cache &MAY; be configured to ignore a server's specified 1898 expiration time, and because a client request &MAY; include a max-stale 1899 directive (which has a similar effect), the protocol also 1900 includes a mechanism for the origin server to require revalidation 1901 of a cache entry on any subsequent use. When the must-revalidate 1902 directive is present in a response received by a cache, that cache 1903 &MUST-NOT; use the entry after it becomes stale to respond to a 1904 subsequent request without first revalidating it with the origin 1905 server. (I.e., the cache &MUST; do an end-to-end revalidation every 1906 time, if, based solely on the origin server's Expires or max-age 1907 value, the cached response is stale.) 1908 </t><t> 1909 The must-revalidate directive is necessary to support reliable 1910 operation for certain protocol features. In all circumstances an 1911 HTTP/1.1 cache &MUST; obey the must-revalidate directive; in 1912 particular, if the cache cannot reach the origin server for any 1913 reason, it &MUST; generate a 504 (Gateway Timeout) response. 1914 </t><t> 1915 Servers &SHOULD; send the must-revalidate directive if and only if 1916 failure to revalidate a request on the entity could result in 1917 incorrect operation, such as a silently unexecuted financial 1918 transaction. Recipients &MUST-NOT; take any automated action that 1919 violates this directive, and &MUST-NOT; automatically provide an 1920 unvalidated copy of the entity if revalidation fails. 1921 </t><t> 1922 Although this is not recommended, user agents operating under 1923 severe connectivity constraints &MAY; violate this directive but, if 1924 so, &MUST; explicitly warn the user that an unvalidated response has 1925 been provided. The warning &MUST; be provided on each unvalidated 1926 access, and &SHOULD; require explicit user confirmation. 1927 </t></list> 1928 </t> 1929 <t> 1930 <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/> 1931 <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/> 1932 proxy-revalidate 1933 <list><t> 1934 The proxy-revalidate directive has the same meaning as the must-revalidate 1935 directive, except that it does not apply to non-shared 1936 user agent caches. It can be used on a response to an 1937 authenticated request to permit the user's cache to store and 1938 later return the response without needing to revalidate it (since 1939 it has already been authenticated once by that user), while still 1940 requiring proxies that service many users to revalidate each time 1941 (in order to make sure that each user has been authenticated). 1942 Note that such authenticated responses also need the public cache 1943 control directive in order to allow them to be cached at all. 1944 </t></list> 1945 </t> 1946 </section> 1947 1948 <section title="No-Transform Directive" anchor="no-transform.directive"> 1949 <t> 1950 <iref item="Cache Directives" subitem="no-transform" primary="true"/> 1951 <iref item="no-transform" subitem="Cache Directive" primary="true"/> 1952 no-transform 1953 <list><t> 1954 Implementors of intermediate caches (proxies) have found it useful 1955 to convert the media type of certain entity bodies. A non-transparent 1956 proxy might, for example, convert between image 1957 formats in order to save cache space or to reduce the amount of 1958 traffic on a slow link. 1959 </t><t> 1960 Serious operational problems occur, however, when these 1961 transformations are applied to entity bodies intended for certain 1962 kinds of applications. For example, applications for medical 1963 imaging, scientific data analysis and those using end-to-end 1964 authentication, all depend on receiving an entity body that is bit 1965 for bit identical to the original entity-body. 1966 </t><t> 1967 Therefore, if a message includes the no-transform directive, an 1968 intermediate cache or proxy &MUST-NOT; change those headers that are 1969 listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform 1970 directive. This implies that the cache or proxy &MUST-NOT; change 1971 any aspect of the entity-body that is specified by these headers, 1972 including the value of the entity-body itself. 1973 </t></list> 1974 </t> 1975 </section> 1976 1977 <section title="Cache Control Extensions" anchor="cache.control.extensions"> 1978 <t> 1979 The Cache-Control header field can be extended through the use of one 1980 or more cache-extension tokens, each with an optional assigned value. 1981 Informational extensions (those which do not require a change in 1982 cache behavior) &MAY; be added without changing the semantics of other 1983 directives. Behavioral extensions are designed to work by acting as 1984 modifiers to the existing base of cache directives. Both the new 1985 directive and the standard directive are supplied, such that 1986 applications which do not understand the new directive will default 1987 to the behavior specified by the standard directive, and those that 1988 understand the new directive will recognize it as modifying the 1989 requirements associated with the standard directive. In this way, 1990 extensions to the cache-control directives can be made without 1991 requiring changes to the base protocol. 1992 </t> 1993 <t> 1994 This extension mechanism depends on an HTTP cache obeying all of the 1995 cache-control directives defined for its native HTTP-version, obeying 1996 certain extensions, and ignoring all directives that it does not 1997 understand. 1998 </t> 1999 <t> 2000 For example, consider a hypothetical new response directive called 2001 community which acts as a modifier to the private directive. We 2002 define this new directive to mean that, in addition to any non-shared 2003 cache, any cache which is shared only by members of the community 2004 named within its value may cache the response. An origin server 2005 wishing to allow the UCI community to use an otherwise private 2006 response in their shared cache(s) could do so by including 2007 </t> 2008 <figure><artwork type="example"> 954 "public" 955 / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] 956 / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] 957 / "no-store" 958 / "no-transform" 959 / "must-revalidate" 960 / "proxy-revalidate" 961 / "max-age" "=" <x:ref>delta-seconds</x:ref> 962 / "s-maxage" "=" <x:ref>delta-seconds</x:ref> 963 / <x:ref>cache-extension</x:ref> 964 </artwork> 965 </figure> 966 967 <t> 968 <iref item="Cache Directives" primary="true" subitem="public" /> 969 <iref item="public" primary="true" subitem="Cache Directive" /> public <list> 970 <t>The public response directive indicates that the response &MAY; be cached, even 971 if it would normally be non-cacheable or cacheable only within a non-shared cache. 972 (See also Authorization, &header-authorization;, for additional details.) </t> 973 </list> 974 </t> 975 976 <t> 977 <iref item="Cache Directives" primary="true" subitem="private" /> 978 <iref item="private" primary="true" subitem="Cache Directive" /> private <list> 979 <t>The private response directive indicates that the response message is intended for 980 a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared) 981 cache &MAY; store the response.</t> 982 <t>If the private response directive specifies one or more field-names, this 983 requirement is limited to the field-values associated with the listed response 984 headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared 985 cache, whereas the remainder of the response message &MAY; be.</t> 986 <t> 987 <cref>ISSUE: What does this really mean? Is this a good idea?</cref> 988 </t> 989 <t> 990 <x:h>Note:</x:h> This usage of the word private only controls where the response may 991 be stored, and cannot ensure the privacy of the message content.</t> 992 </list> 993 </t> 994 995 <t> 996 <iref item="Cache Directives" primary="true" subitem="no-cache" /> 997 <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list> 998 <t>The no-cache response directive indicates that a response &MUST-NOT; be used to 999 satisfy a subseqent request without successful validation on the origin server. 1000 This allows an origin server to prevent caching even by caches that have been 1001 configured to return stale responses.</t> 1002 <t>If the no-cache response directive specifies one or more field-names, this 1003 requirement is limited to the field-values assosicated with the listed response 1004 headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response 1005 to a subsequent request without successful validation on the origin server. This 1006 allows an origin server to prevent the re-use of certain header fields in a 1007 response, while still allowing caching of the rest of the response.</t> 1008 <t> 1009 <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive. 1010 </t> 1011 </list> 1012 </t> 1013 1014 <t> 1015 <iref item="Cache Directives" primary="true" subitem="no-store" /> 1016 <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list> 1017 <t>The no-store response directive indicates that a cache &MUST-NOT; store any 1018 part of either the immediate request or response. This directive applies to both 1019 non-shared and shared caches. "&MUST-NOT; store" in this context means that the 1020 cache &MUST-NOT; intentionally store the information in non-volatile storage, 1021 and &MUST; make a best-effort attempt to remove the information from volatile 1022 storage as promptly as possible after forwarding it.</t> 1023 <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In 1024 particular, malicious or compromised caches might not recognize or obey this 1025 directive, and communications networks may be vulnerable to eavesdropping.</t> 1026 </list> 1027 </t> 1028 1029 <t> 1030 <iref item="Cache Directives" primary="true" subitem="no-transform" /> 1031 <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list> 1032 <t>The no-transform response directive indicates that an intermediate cache or proxy 1033 &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response 1034 headers, nor the response entity-body.</t> 1035 </list> 1036 </t> 1037 1038 <t> 1039 <iref item="Cache Directives" primary="true" subitem="must-revalidate" /> 1040 <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list> 1041 <t>The must-revalidate response-directive indicates that validation is required before 1042 the response is used by a cache to satisfy any request.</t> 1043 <t>When the present, caches &MUST-NOT; use a stored after it becomes stale to respond to a subsequent 1044 request without first validating it with the origin server.</t> 1045 <t>The must-revalidate directive is necessary to support reliable operation for 1046 certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey 1047 the must-revalidate directive; in particular, if the cache cannot reach the origin 1048 server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t> 1049 <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to 1050 validate a request on the entity could result in incorrect operation, such as a 1051 silently unexecuted financial transaction. Recipients &MUST-NOT; take any 1052 automated action that violates this directive, and &MUST-NOT; automatically 1053 provide an unvalidated copy of the entity if validation fails.</t> 1054 <t>Although this is not recommended, user agents operating under severe connectivity 1055 constraints &MAY; violate this directive but, if so, &MUST; explicitly warn 1056 the user that an unvalidated response has been provided. The warning &MUST; be 1057 provided on each unvalidated access, and &SHOULD; require explicit user 1058 confirmation.</t> 1059 <t><cref>TODO: last two paragraphs seem nonsensical.</cref></t> 1060 </list> 1061 </t> 1062 1063 <t> 1064 <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" /> 1065 <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" /> 1066 proxy-revalidate <list> 1067 <t>The proxy-revalidate directive has the same meaning as the must-revalidate 1068 directive, except that it does not apply to non-shared caches.</t> 1069 </list> 1070 </t> 1071 <t> 1072 <iref item="Cache Directives" primary="true" subitem="s-maxage" /> 1073 <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list> 1074 <t>The s-maxage response directive indicates that, in shared caches, the maximum age 1075 specified by this directive overrides the maximum age specified by either the 1076 max-age directive or the Expires header. The s-maxage directive also implies the 1077 semantics of the proxy-revalidate response directive.</t> 1078 </list> 1079 </t> 1080 1081 </section> 1082 1083 <section anchor="cache.control.extensions" title="Cache Control Extensions"> 1084 <t>The Cache-Control header field can be extended through the use of one or more 1085 cache-extension tokens, each with an optional value. Informational extensions (those 1086 which do not require a change in cache behavior) &MAY; be added without changing the 1087 semantics of other directives. Behavioral extensions are designed to work by acting as 1088 modifiers to the existing base of cache directives. Both the new directive and the 1089 standard directive are supplied, such that applications which do not understand the new 1090 directive will default to the behavior specified by the standard directive, and those 1091 that understand the new directive will recognize it as modifying the requirements 1092 associated with the standard directive. In this way, extensions to the cache-control 1093 directives can be made without requiring changes to the base protocol.</t> 1094 <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control 1095 directives defined for its native HTTP-version, obeying certain extensions, and ignoring 1096 all directives that it does not understand.</t> 1097 <t>For example, consider a hypothetical new response directive called "community" which 1098 acts as a modifier to the private directive. We define this new directive to mean that, 1099 in addition to any non-shared cache, any cache which is shared only by members of the 1100 community named within its value may cache the response. An origin server wishing to 1101 allow the UCI community to use an otherwise private response in their shared cache(s) 1102 could do so by including</t> 1103 <figure> 1104 <artwork type="example"> 2009 1105 Cache-Control: private, community="UCI" 2010 </artwork></figure> 2011 <t> 2012 A cache seeing this header field will act correctly even if the cache 2013 does not understand the community cache-extension, since it will also 2014 see and understand the private directive and thus default to the safe 2015 behavior. 2016 </t> 2017 <t> 2018 Unrecognized cache-directives &MUST; be ignored; it is assumed that any 2019 cache-directive likely to be unrecognized by an HTTP/1.1 cache will 2020 be combined with standard directives (or the response's default 2021 cacheability) such that the cache behavior will remain minimally 2022 correct even if the cache does not understand the extension(s). 2023 </t> 2024 </section> 2025 </section> 2026 2027 <section title="Expires" anchor="header.expires"> 2028 <iref primary="true" item="Expires header" x:for-anchor=""/> 2029 <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/> 2030 <x:anchor-alias value="Expires"/> 2031 <t> 2032 The Expires entity-header field gives the date/time after which the 2033 response is considered stale. A stale cache entry may not normally be 2034 returned by a cache (either a proxy cache or a user agent cache) 2035 unless it is first validated with the origin server (or with an 2036 intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/> 2037 for further discussion of the expiration model. 2038 </t> 2039 <t> 2040 The presence of an Expires field does not imply that the original 2041 resource will change or cease to exist at, before, or after that 2042 time. 2043 </t> 2044 <t> 2045 The format is an absolute date and time as defined by HTTP-date in 2046 &full-date;; it &MUST; be sent in rfc1123-date format. 2047 </t> 2048 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/> 1106 </artwork> 1107 </figure> 1108 <t>A cache seeing this header field will act correctly even if the cache does not 1109 understand the community cache-extension, since it will also see and understand the 1110 private directive and thus default to the safe behavior.</t> 1111 <t>Unrecognized cache-directives &MUST; be ignored; it is assumed that any 1112 cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with 1113 standard directives (or the response's default cacheability) such that the cache 1114 behavior will remain minimally correct even if the cache does not understand the 1115 extension(s).</t> 1116 </section> 1117 1118 </section> 1119 1120 <section anchor="header.expires" title="Expires"> 1121 <iref item="Expires header" primary="true" x:for-anchor="" /> 1122 <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" /> 1123 <x:anchor-alias value="Expires" /> 1124 <t>The Expires entity-header field gives the date/time after which the response is 1125 considered stale. See <xref target="expiration.model" /> for further discussion of the 1126 expiration model.</t> 1127 <t>The presence of an Expires field does not imply that the original resource will change or 1128 cease to exist at, before, or after that time.</t> 1129 <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;; 1130 it &MUST; be sent in rfc1123-date format.</t> 1131 <figure> 1132 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Expires" /> 2049 1133 <x:ref>Expires</x:ref> = "Expires" ":" <x:ref>HTTP-date</x:ref> 2050 </artwork> </figure>2051 <t>2052 An example of its use is2053 </t>2054 <figure><artwork type="example">1134 </artwork> 1135 </figure> 1136 <t>For example</t> 1137 <figure> 1138 <artwork type="example"> 2055 1139 Expires: Thu, 01 Dec 1994 16:00:00 GMT 2056 </artwork></figure> 2057 <t> 2058 <list><t> 2059 <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age 2060 directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the 2061 Expires field. 2062 </t></list> 2063 </t> 2064 <t> 2065 HTTP/1.1 clients and caches &MUST; treat other invalid date formats, 2066 especially including the value "0", as in the past (i.e., "already 2067 expired"). 2068 </t> 2069 <t> 2070 To mark a response as "already expired," an origin server sends an 2071 Expires date that is equal to the Date header value. (See the rules 2072 for expiration calculations in <xref target="expiration.calculations"/>.) 2073 </t> 2074 <t> 2075 To mark a response as "never expires," an origin server sends an 2076 Expires date approximately one year from the time the response is 2077 sent. HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one 2078 year in the future. 2079 </t> 2080 <t> 2081 The presence of an Expires header field with a date value of some 2082 time in the future on a response that otherwise would by default be 2083 non-cacheable indicates that the response is cacheable, unless 2084 indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>). 2085 </t> 2086 </section> 2087 2088 <section title="Pragma" anchor="header.pragma"> 2089 <iref primary="true" item="Pragma header" x:for-anchor=""/> 2090 <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/> 2091 <x:anchor-alias value="extension-pragma"/> 2092 <x:anchor-alias value="Pragma"/> 2093 <x:anchor-alias value="pragma-directive"/> 2094 <t> 2095 The Pragma general-header field is used to include implementation-specific 2096 directives that might apply to any recipient along the 2097 request/response chain. All pragma directives specify optional 2098 behavior from the viewpoint of the protocol; however, some systems 2099 &MAY; require that behavior be consistent with the directives. 2100 </t> 2101 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/> 1140 </artwork> 1141 </figure> 1142 <t> 1143 <list> 1144 <t> 1145 <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age 1146 directive (see <xref target="header.cache-control" />), that directive overrides the 1147 Expires field.</t> 1148 </list> 1149 </t> 1150 <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t> 1151 <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially 1152 including the value "0", as in the past (i.e., "already expired").</t> 1153 </section> 1154 1155 <section anchor="header.pragma" title="Pragma"> 1156 <iref item="Pragma header" primary="true" x:for-anchor="" /> 1157 <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" /> 1158 <x:anchor-alias value="extension-pragma" /> 1159 <x:anchor-alias value="Pragma" /> 1160 <x:anchor-alias value="pragma-directive" /> 1161 <t>The Pragma general-header field is used to include implementation-specific directives 1162 that might apply to any recipient along the request/response chain. All pragma directives 1163 specify optional behavior from the viewpoint of the protocol; however, some systems 1164 &MAY; require that behavior be consistent with the directives.</t> 1165 <figure> 1166 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Pragma" /><iref item="Grammar" primary="true" subitem="pragma-directive" /><iref item="Grammar" primary="true" subitem="extension-pragma" /> 2102 1167 <x:ref>Pragma</x:ref> = "Pragma" ":" 1#<x:ref>pragma-directive</x:ref> 2103 1168 <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref> 2104 1169 <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ] 2105 </artwork></figure> 2106 <t> 2107 When the no-cache directive is present in a request message, an 2108 application &SHOULD; forward the request toward the origin server even 2109 if it has a cached copy of what is being requested. This pragma 2110 directive has the same semantics as the no-cache cache-directive (see 2111 <xref target="header.cache-control"/>) and is defined here for backward compatibility with 2112 HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache 2113 request is sent to a server not known to be HTTP/1.1 compliant. 2114 </t> 2115 <t> 2116 Pragma directives &MUST; be passed through by a proxy or gateway 2117 application, regardless of their significance to that application, 2118 since the directives might be applicable to all recipients along the 2119 request/response chain. It is not possible to specify a pragma for a 2120 specific recipient; however, any pragma directive not relevant to a 2121 recipient &SHOULD; be ignored by that recipient. 2122 </t> 2123 <t> 2124 HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had 2125 sent "Cache-Control: no-cache". No new Pragma directives will be 2126 defined in HTTP. 2127 <list><t> 2128 <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a 2129 response-header field is not actually specified, it does not provide a 2130 reliable replacement for "Cache-Control: no-cache" in a response. 2131 </t></list> 2132 </t> 2133 </section> 2134 2135 <section title="Vary" anchor="header.vary"> 2136 <iref primary="true" item="Vary header" x:for-anchor=""/> 2137 <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/> 2138 <x:anchor-alias value="Vary"/> 2139 <t> 2140 The Vary response-header field's value indicates the set of request-header 2141 fields that fully determines, while the response is fresh, whether a cache 2142 is permitted to use the response to reply to a subsequent request 2143 without revalidation. For uncacheable or stale responses, the Vary 2144 field value advises the user agent about the criteria that were used 2145 to select the representation. A Vary field value of "*" implies that 2146 a cache cannot determine from the request headers of a subsequent 2147 request whether this response is the appropriate representation. See 2148 <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches. 2149 </t> 2150 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/> 1170 </artwork> 1171 </figure> 1172 <t>When the no-cache directive is present in a request message, an application &SHOULD; 1173 forward the request toward the origin server even if it has a cached copy of what is being 1174 requested. This pragma directive has the same semantics as the no-cache cache-directive 1175 (see <xref target="header.cache-control" />) and is defined here for backward 1176 compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a 1177 no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches 1178 &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t> 1179 <t> 1180 <list> 1181 <t> 1182 <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field 1183 is not actually specified, it does not provide a reliable replacement for 1184 "Cache-Control: no-cache" in a response.</t> 1185 </list> 1186 </t> 1187 <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t> 1188 </section> 1189 1190 <section anchor="header.vary" title="Vary"> 1191 <iref item="Vary header" primary="true" x:for-anchor="" /> 1192 <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" /> 1193 <x:anchor-alias value="Vary" /> 1194 <t>The Vary response-header field's value indicates the set of request-header fields that 1195 fully determines, while the response is fresh, whether a cache is permitted to use the 1196 response to reply to a subsequent request without validation. For uncacheable or stale 1197 responses, the Vary field value advises the user agent about the criteria that were used 1198 to select the representation. A Vary field value of "*" implies that a cache cannot 1199 determine from the request headers of a subsequent request whether this response is the 1200 appropriate representation. See <xref target="caching.negotiated.responses" /> for use of 1201 the Vary header field by caches.</t> 1202 <figure> 1203 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Vary" /> 2151 1204 <x:ref>Vary</x:ref> = "Vary" ":" ( "*" / 1#<x:ref>field-name</x:ref> ) 2152 </artwork></figure> 2153 <t> 2154 An HTTP/1.1 server &SHOULD; include a Vary header field with any 2155 cacheable response that is subject to server-driven negotiation. 2156 Doing so allows a cache to properly interpret future requests on that 2157 resource and informs the user agent about the presence of negotiation 2158 on that resource. A server &MAY; include a Vary header field with a 2159 non-cacheable response that is subject to server-driven negotiation, 2160 since this might provide the user agent with useful information about 2161 the dimensions over which the response varies at the time of the 2162 response. 2163 </t> 2164 <t> 2165 A Vary field value consisting of a list of field-names signals that 2166 the representation selected for the response is based on a selection 2167 algorithm which considers ONLY the listed request-header field values 2168 in selecting the most appropriate representation. A cache &MAY; assume 2169 that the same selection will be made for future requests with the 2170 same values for the listed field names, for the duration of time for 2171 which the response is fresh. 2172 </t> 2173 <t> 2174 The field-names given are not limited to the set of standard 2175 request-header fields defined by this specification. Field names are 2176 case-insensitive. 2177 </t> 2178 <t> 2179 A Vary field value of "*" signals that unspecified parameters not 2180 limited to the request-headers (e.g., the network address of the 2181 client), play a role in the selection of the response representation. 2182 The "*" value &MUST-NOT; be generated by a proxy server; it may only be 2183 generated by an origin server. 2184 </t> 2185 </section> 2186 2187 <section title="Warning" anchor="header.warning"> 2188 <iref primary="true" item="Warning header" x:for-anchor=""/> 2189 <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/> 2190 <x:anchor-alias value="Warning"/> 2191 <x:anchor-alias value="warning-value"/> 2192 <x:anchor-alias value="warn-agent"/> 2193 <x:anchor-alias value="warn-code"/> 2194 <x:anchor-alias value="warn-date"/> 2195 <x:anchor-alias value="warn-text"/> 2196 <t> 2197 The Warning general-header field is used to carry additional 2198 information about the status or transformation of a message which 2199 might not be reflected in the message. This information is typically 2200 used to warn about a possible lack of semantic transparency from 2201 caching operations or transformations applied to the entity body of 2202 the message. 2203 </t> 2204 <t> 2205 Warnings MAY be used for other purposes, both cache-related and 2206 otherwise. The use of a warning, rather than an error status code, 2207 distinguish these responses from true failures. 2208 </t> 2209 2210 <t> 2211 Warning headers can in general be applied to any message, however 2212 some warn-codes are specific to caches and can only be 2213 applied to response messages. 2214 </t> 2215 2216 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/> 1205 </artwork> 1206 </figure> 1207 <t>The set of header fields named by the Vary field value is known as the "selecting" 1208 request-headers.</t> 1209 <t>Servers &SHOULD; include a Vary header field with any cacheable response 1210 that is subject to server-driven negotiation. Doing so allows a cache to properly 1211 interpret future requests on that resource and informs the user agent about the presence 1212 of negotiation on that resource. A server &MAY; include a Vary header field with a 1213 non-cacheable response that is subject to server-driven negotiation, since this might 1214 provide the user agent with useful information about the dimensions over which the 1215 response varies at the time of the response.</t> 1216 <t>A Vary field value of "*" signals that unspecified parameters not limited to the 1217 request-headers (e.g., the network address of the client), play a role in the selection of 1218 the response representation. The "*" value &MUST-NOT; be generated by a proxy server; 1219 it may only be generated by an origin server.</t> 1220 <t>The field-names given are not limited to the set of standard request-header fields 1221 defined by this specification. Field names are case-insensitive.</t> 1222 <t> 1223 <cref>ISSUE: Does 'server' here imply that non-origin servers can generate vary? note use of 'proxy'</cref></t> 1224 </section> 1225 1226 <section anchor="header.warning" title="Warning"> 1227 <iref item="Warning header" primary="true" x:for-anchor="" /> 1228 <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" /> 1229 <x:anchor-alias value="Warning" /> 1230 <x:anchor-alias value="warning-value" /> 1231 <x:anchor-alias value="warn-agent" /> 1232 <x:anchor-alias value="warn-code" /> 1233 <x:anchor-alias value="warn-date" /> 1234 <x:anchor-alias value="warn-text" /> 1235 <t>The Warning general-header field is used to carry additional information about the status 1236 or transformation of a message which might not be reflected in the message. This 1237 information is typically used to warn about possible incorrectness introduced by caching 1238 operations or transformations applied to the entity body of the message.</t> 1239 <t>Warnings MAY be used for other purposes, both cache-related and otherwise. The use of a 1240 warning, rather than an error status code, distinguish these responses from true failures.</t> 1241 1242 <t>Warning headers can in general be applied to any message, however some warn-codes are 1243 specific to caches and can only be applied to response messages.</t> 1244 1245 <figure> 1246 <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Warning" /><iref item="Grammar" primary="true" subitem="warning-value" /><iref item="Grammar" primary="true" subitem="warn-code" /><iref item="Grammar" primary="true" subitem="warn-agent" /><iref item="Grammar" primary="true" subitem="warn-text" /><iref item="Grammar" primary="true" subitem="warn-date" /> 2217 1247 <x:ref>Warning</x:ref> = "Warning" ":" 1#<x:ref>warning-value</x:ref> 2218 1248 … … 2226 1256 <x:ref>warn-text</x:ref> = <x:ref>quoted-string</x:ref> 2227 1257 <x:ref>warn-date</x:ref> = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref> 2228 </artwork></figure> 2229 2230 <t> 2231 Multiple warnings &MAY; be attached to a response (either by the origin 2232 server or by a cache), including multiple warnings with the same code 2233 number. For example, a server might provide the same warning with 2234 texts in both English and Basque. 2235 </t> 2236 <t> 2237 When this occurs, the user agent ought to inform the user of as many 2238 of them as possible, in the order that they appear in the response. 2239 If it is not possible to inform the user of all of the warnings, 2240 the user agent SHOULD follow these heuristics: 2241 <list style="symbols"> 2242 <t>Warnings that appear early in the response take priority over 2243 those appearing later in the response.</t> 2244 2245 <t>Warnings in the user's preferred character set take priority 2246 over warnings in other character sets but with identical warn-codes 2247 and warn-agents.</t> 2248 </list> 2249 </t> 2250 <t> 2251 Systems that generate multiple Warning headers &SHOULD; order them with 2252 this user agent behavior in mind. New Warning headers &SHOULD; be added 2253 after any existing Warning headers. 2254 </t> 2255 <t> 2256 Warnings are assigned three digit warn-codes. The first digit 2257 indicates whether the Warning is required to be deleted from a 2258 stored cache entry after validation: 2259 <list style="symbols"> 2260 <t>1xx Warnings that describe the freshness or revalidation status of 2261 the response, and so MUST be deleted by caches after validation.</t> 2262 <t> 2xx Warnings that describe some aspect of the entity body or entity 2263 headers that is not rectified by a revalidation (for example, a 2264 lossy compression of the entity bodies) and which MUST NOT be 2265 deleted by caches after validation, unless a full response is 2266 returned, in which case they MUST be.</t> 2267 </list> 2268 </t> 2269 <t> 2270 The warn-text &SHOULD; be in a natural language and character set that 2271 is most likely to be intelligible to the human user receiving the 2272 response. This decision &MAY; be based on any available knowledge, such 2273 as the location of the cache or user, the Accept-Language field in a 2274 request, the Content-Language field in a response, etc. The default 2275 language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>). 2276 </t> 2277 <t> 2278 If a character set other than ISO-8859-1 is used, it &MUST; be encoded 2279 in the warn-text using the method described in <xref target="RFC2047"/>. 2280 </t> 2281 <t> 2282 If an implementation sends a message with one or more Warning headers 2283 to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST; 2284 include in each warning-value a warn-date that matches the Date header 2285 in the message. 2286 </t> 2287 <t> 2288 If an implementation receives a message with a warning-value that 2289 includes a warn-date, and that warn-date is different from the Date 2290 value in the response, then that warning-value &MUST; be deleted from 2291 the message before storing, forwarding, or using it. (This prevents 2292 bad consequences of naive caching of Warning header fields.) If all 2293 of the warning-values are deleted for this reason, the Warning header 2294 &MUST; be deleted as well. 2295 </t> 2296 <t> 2297 The following warn-codes are defined by this specification, each with a 2298 recommended warn-text in English, and a description of its meaning. 2299 </t> 2300 <t> 2301 110 Response is stale 2302 <list><t> 2303 &MUST; be included whenever the returned response is stale. 2304 </t></list> 2305 </t> 2306 <t> 2307 111 Revalidation failed 2308 <list><t> 2309 &MUST; be included if a cache returns a stale response because an 2310 attempt to revalidate the response failed, due to an inability to 2311 reach the server. 2312 </t></list> 2313 </t> 2314 <t> 2315 112 Disconnected operation 2316 <list><t> 2317 &SHOULD; be included if the cache is intentionally disconnected from 2318 the rest of the network for a period of time. 2319 </t></list> 2320 </t> 2321 <t> 2322 113 Heuristic expiration 2323 <list><t> 2324 &MUST; be included if the cache heuristically chose a freshness 2325 lifetime greater than 24 hours and the response's age is greater 2326 than 24 hours. 2327 </t></list> 2328 </t> 2329 <t> 2330 199 Miscellaneous warning 2331 <list><t> 2332 The warning text &MAY; include arbitrary information to be presented 2333 to a human user, or logged. A system receiving this warning &MUST-NOT; 2334 take any automated action, besides presenting the warning to 2335 the user. 2336 </t></list> 2337 </t> 2338 <t> 2339 214 Transformation applied 2340 <list><t> 2341 &MUST; be added by an intermediate cache or proxy if it applies any 2342 transformation changing the content-coding (as specified in the 2343 Content-Encoding header) or media-type (as specified in the 2344 Content-Type header) of the response, or the entity-body of the 2345 response, unless this Warning code already appears in the response. 2346 </t></list> 2347 </t> 2348 <t> 2349 299 Miscellaneous persistent warning 2350 <list><t> 2351 The warning text &MAY; include arbitrary information to be presented 2352 to a human user, or logged. A system receiving this warning &MUST-NOT; 2353 take any automated action. 2354 </t></list> 2355 </t> 2356 </section> 2357 2358 </section> 2359 2360 <section title="IANA Considerations" anchor="IANA.considerations"> 2361 <section title="Message Header Registration" anchor="message.header.registration"> 2362 <t> 2363 The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated 2364 with the permanent registrations below (see <xref target="RFC3864"/>): 2365 </t> 2366 <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually--> 2367 <texttable align="left" suppress-title="true" anchor="iana.header.registration.table"> 2368 <ttcol>Header Field Name</ttcol> 2369 <ttcol>Protocol</ttcol> 2370 <ttcol>Status</ttcol> 2371 <ttcol>Reference</ttcol> 2372 2373 <c>Age</c> 2374 <c>http</c> 2375 <c>standard</c> 2376 <c> 2377 <xref target="header.age"/> 2378 </c> 2379 <c>Cache-Control</c> 2380 <c>http</c> 2381 <c>standard</c> 2382 <c> 2383 <xref target="header.cache-control"/> 2384 </c> 2385 <c>Expires</c> 2386 <c>http</c> 2387 <c>standard</c> 2388 <c> 2389 <xref target="header.expires"/> 2390 </c> 2391 <c>Pragma</c> 2392 <c>http</c> 2393 <c>standard</c> 2394 <c> 2395 <xref target="header.pragma"/> 2396 </c> 2397 <c>Vary</c> 2398 <c>http</c> 2399 <c>standard</c> 2400 <c> 2401 <xref target="header.vary"/> 2402 </c> 2403 <c>Warning</c> 2404 <c>http</c> 2405 <c>standard</c> 2406 <c> 2407 <xref target="header.warning"/> 2408 </c> 2409 </texttable> 2410 <!--(END)--> 2411 <t> 2412 The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force". 2413 </t> 2414 </section> 2415 </section> 2416 2417 <section title="Security Considerations" anchor="security.considerations"> 2418 <t> 2419 Caching proxies provide additional potential vulnerabilities, since 2420 the contents of the cache represent an attractive target for 2421 malicious exploitation. Because cache contents persist after an HTTP 2422 request is complete, an attack on the cache can reveal information 2423 long after a user believes that the information has been removed from 2424 the network. Therefore, cache contents should be protected as 2425 sensitive information. 2426 </t> 2427 </section> 2428 2429 <section title="Acknowledgments" anchor="ack"> 2430 <t> 2431 Much of the content and presentation of the caching design is due to 2432 suggestions and comments from individuals including: Shel Kaphan, 2433 Paul Leach, Koen Holtman, David Morris, and Larry Masinter. 2434 </t> 2435 </section> 2436 </middle> 2437 <back> 2438 2439 <references title="Normative References"> 2440 2441 <reference anchor="ISO-8859-1"> 2442 <front> 2443 <title> 2444 Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1 2445 </title> 2446 <author> 2447 <organization>International Organization for Standardization</organization> 2448 </author> 2449 <date year="1998"/> 2450 </front> 2451 <seriesInfo name="ISO/IEC" value="8859-1:1998"/> 2452 </reference> 2453 2454 <reference anchor="Part1"> 2455 <front> 2456 <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title> 2457 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2458 <organization abbrev="Day Software">Day Software</organization> 2459 <address><email>fielding@gbiv.com</email></address> 2460 </author> 2461 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2462 <organization>One Laptop per Child</organization> 2463 <address><email>jg@laptop.org</email></address> 2464 </author> 2465 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2466 <organization abbrev="HP">Hewlett-Packard Company</organization> 2467 <address><email>JeffMogul@acm.org</email></address> 2468 </author> 2469 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2470 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2471 <address><email>henrikn@microsoft.com</email></address> 2472 </author> 2473 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2474 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2475 <address><email>LMM@acm.org</email></address> 2476 </author> 2477 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2478 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2479 <address><email>paulle@microsoft.com</email></address> 2480 </author> 2481 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2482 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2483 <address><email>timbl@w3.org</email></address> 2484 </author> 2485 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2486 <organization abbrev="W3C">World Wide Web Consortium</organization> 2487 <address><email>ylafon@w3.org</email></address> 2488 </author> 2489 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2490 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2491 <address><email>julian.reschke@greenbytes.de</email></address> 2492 </author> 2493 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2494 </front> 2495 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/> 2496 <x:source href="p1-messaging.xml" basename="p1-messaging"/> 2497 </reference> 2498 2499 <reference anchor="Part2"> 2500 <front> 2501 <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title> 2502 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2503 <organization abbrev="Day Software">Day Software</organization> 2504 <address><email>fielding@gbiv.com</email></address> 2505 </author> 2506 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2507 <organization>One Laptop per Child</organization> 2508 <address><email>jg@laptop.org</email></address> 2509 </author> 2510 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2511 <organization abbrev="HP">Hewlett-Packard Company</organization> 2512 <address><email>JeffMogul@acm.org</email></address> 2513 </author> 2514 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2515 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2516 <address><email>henrikn@microsoft.com</email></address> 2517 </author> 2518 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2519 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2520 <address><email>LMM@acm.org</email></address> 2521 </author> 2522 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2523 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2524 <address><email>paulle@microsoft.com</email></address> 2525 </author> 2526 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2527 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2528 <address><email>timbl@w3.org</email></address> 2529 </author> 2530 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2531 <organization abbrev="W3C">World Wide Web Consortium</organization> 2532 <address><email>ylafon@w3.org</email></address> 2533 </author> 2534 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2535 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2536 <address><email>julian.reschke@greenbytes.de</email></address> 2537 </author> 2538 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2539 </front> 2540 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/> 2541 <x:source href="p2-semantics.xml" basename="p2-semantics"/> 2542 </reference> 2543 2544 <reference anchor="Part3"> 2545 <front> 2546 <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title> 2547 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2548 <organization abbrev="Day Software">Day Software</organization> 2549 <address><email>fielding@gbiv.com</email></address> 2550 </author> 2551 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2552 <organization>One Laptop per Child</organization> 2553 <address><email>jg@laptop.org</email></address> 2554 </author> 2555 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2556 <organization abbrev="HP">Hewlett-Packard Company</organization> 2557 <address><email>JeffMogul@acm.org</email></address> 2558 </author> 2559 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2560 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2561 <address><email>henrikn@microsoft.com</email></address> 2562 </author> 2563 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2564 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2565 <address><email>LMM@acm.org</email></address> 2566 </author> 2567 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2568 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2569 <address><email>paulle@microsoft.com</email></address> 2570 </author> 2571 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2572 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2573 <address><email>timbl@w3.org</email></address> 2574 </author> 2575 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2576 <organization abbrev="W3C">World Wide Web Consortium</organization> 2577 <address><email>ylafon@w3.org</email></address> 2578 </author> 2579 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2580 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2581 <address><email>julian.reschke@greenbytes.de</email></address> 2582 </author> 2583 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2584 </front> 2585 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/> 2586 <x:source href="p3-payload.xml" basename="p3-payload"/> 2587 </reference> 2588 2589 <reference anchor="Part4"> 2590 <front> 2591 <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title> 2592 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2593 <organization abbrev="Day Software">Day Software</organization> 2594 <address><email>fielding@gbiv.com</email></address> 2595 </author> 2596 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2597 <organization>One Laptop per Child</organization> 2598 <address><email>jg@laptop.org</email></address> 2599 </author> 2600 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2601 <organization abbrev="HP">Hewlett-Packard Company</organization> 2602 <address><email>JeffMogul@acm.org</email></address> 2603 </author> 2604 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2605 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2606 <address><email>henrikn@microsoft.com</email></address> 2607 </author> 2608 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2609 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2610 <address><email>LMM@acm.org</email></address> 2611 </author> 2612 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2613 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2614 <address><email>paulle@microsoft.com</email></address> 2615 </author> 2616 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2617 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2618 <address><email>timbl@w3.org</email></address> 2619 </author> 2620 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2621 <organization abbrev="W3C">World Wide Web Consortium</organization> 2622 <address><email>ylafon@w3.org</email></address> 2623 </author> 2624 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2625 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2626 <address><email>julian.reschke@greenbytes.de</email></address> 2627 </author> 2628 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2629 </front> 2630 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/> 2631 <x:source href="p4-conditional.xml" basename="p4-conditional"/> 2632 </reference> 2633 2634 <reference anchor="Part5"> 2635 <front> 2636 <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title> 2637 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2638 <organization abbrev="Day Software">Day Software</organization> 2639 <address><email>fielding@gbiv.com</email></address> 2640 </author> 2641 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2642 <organization>One Laptop per Child</organization> 2643 <address><email>jg@laptop.org</email></address> 2644 </author> 2645 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2646 <organization abbrev="HP">Hewlett-Packard Company</organization> 2647 <address><email>JeffMogul@acm.org</email></address> 2648 </author> 2649 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2650 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2651 <address><email>henrikn@microsoft.com</email></address> 2652 </author> 2653 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2654 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2655 <address><email>LMM@acm.org</email></address> 2656 </author> 2657 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2658 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2659 <address><email>paulle@microsoft.com</email></address> 2660 </author> 2661 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2662 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2663 <address><email>timbl@w3.org</email></address> 2664 </author> 2665 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2666 <organization abbrev="W3C">World Wide Web Consortium</organization> 2667 <address><email>ylafon@w3.org</email></address> 2668 </author> 2669 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2670 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2671 <address><email>julian.reschke@greenbytes.de</email></address> 2672 </author> 2673 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2674 </front> 2675 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/> 2676 <x:source href="p5-range.xml" basename="p5-range"/> 2677 </reference> 2678 2679 <reference anchor="Part7"> 2680 <front> 2681 <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title> 2682 <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"> 2683 <organization abbrev="Day Software">Day Software</organization> 2684 <address><email>fielding@gbiv.com</email></address> 2685 </author> 2686 <author initials="J." surname="Gettys" fullname="Jim Gettys"> 2687 <organization>One Laptop per Child</organization> 2688 <address><email>jg@laptop.org</email></address> 2689 </author> 2690 <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul"> 2691 <organization abbrev="HP">Hewlett-Packard Company</organization> 2692 <address><email>JeffMogul@acm.org</email></address> 2693 </author> 2694 <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"> 2695 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2696 <address><email>henrikn@microsoft.com</email></address> 2697 </author> 2698 <author initials="L." surname="Masinter" fullname="Larry Masinter"> 2699 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 2700 <address><email>LMM@acm.org</email></address> 2701 </author> 2702 <author initials="P." surname="Leach" fullname="Paul J. Leach"> 2703 <organization abbrev="Microsoft">Microsoft Corporation</organization> 2704 <address><email>paulle@microsoft.com</email></address> 2705 </author> 2706 <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee"> 2707 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 2708 <address><email>timbl@w3.org</email></address> 2709 </author> 2710 <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor"> 2711 <organization abbrev="W3C">World Wide Web Consortium</organization> 2712 <address><email>ylafon@w3.org</email></address> 2713 </author> 2714 <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"> 2715 <organization abbrev="greenbytes">greenbytes GmbH</organization> 2716 <address><email>julian.reschke@greenbytes.de</email></address> 2717 </author> 2718 <date month="&ID-MONTH;" year="&ID-YEAR;"/> 2719 </front> 2720 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/> 2721 <x:source href="p7-auth.xml" basename="p7-auth"/> 2722 </reference> 2723 2724 <reference anchor="RFC2047"> 2725 <front> 2726 <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title> 2727 <author initials="K." surname="Moore" fullname="Keith Moore"> 2728 <organization>University of Tennessee</organization> 2729 <address><email>moore@cs.utk.edu</email></address> 2730 </author> 2731 <date month="November" year="1996"/> 2732 </front> 2733 <seriesInfo name="RFC" value="2047"/> 2734 </reference> 2735 2736 <reference anchor="RFC2119"> 2737 <front> 2738 <title>Key words for use in RFCs to Indicate Requirement Levels</title> 2739 <author initials="S." surname="Bradner" fullname="Scott Bradner"> 2740 <organization>Harvard University</organization> 2741 <address><email>sob@harvard.edu</email></address> 2742 </author> 2743 <date month="March" year="1997"/> 2744 </front> 2745 <seriesInfo name="BCP" value="14"/> 2746 <seriesInfo name="RFC" value="2119"/> 2747 </reference> 2748 2749 </references> 2750 2751 <references title="Informative References"> 2752 2753 <reference anchor="RFC1305"> 2754 <front> 2755 <title>Network Time Protocol (Version 3) Specification, Implementation</title> 2756 <author initials="D." surname="Mills" fullname="David L. Mills"> 2757 <organization>University of Delaware, Electrical Engineering Department</organization> 2758 <address><email>mills@udel.edu</email></address> 2759 </author> 2760 <date month="March" year="1992"/> 2761 </front> 2762 <seriesInfo name="RFC" value="1305"/> 2763 </reference> 2764 2765 <reference anchor="RFC2616"> 2766 <front> 2767 <title>Hypertext Transfer Protocol -- HTTP/1.1</title> 2768 <author initials="R." surname="Fielding" fullname="R. Fielding"> 2769 <organization>University of California, Irvine</organization> 2770 <address><email>fielding@ics.uci.edu</email></address> 2771 </author> 2772 <author initials="J." surname="Gettys" fullname="J. Gettys"> 2773 <organization>W3C</organization> 2774 <address><email>jg@w3.org</email></address> 2775 </author> 2776 <author initials="J." surname="Mogul" fullname="J. Mogul"> 2777 <organization>Compaq Computer Corporation</organization> 2778 <address><email>mogul@wrl.dec.com</email></address> 2779 </author> 2780 <author initials="H." surname="Frystyk" fullname="H. Frystyk"> 2781 <organization>MIT Laboratory for Computer Science</organization> 2782 <address><email>frystyk@w3.org</email></address> 2783 </author> 2784 <author initials="L." surname="Masinter" fullname="L. Masinter"> 2785 <organization>Xerox Corporation</organization> 2786 <address><email>masinter@parc.xerox.com</email></address> 2787 </author> 2788 <author initials="P." surname="Leach" fullname="P. Leach"> 2789 <organization>Microsoft Corporation</organization> 2790 <address><email>paulle@microsoft.com</email></address> 2791 </author> 2792 <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee"> 2793 <organization>W3C</organization> 2794 <address><email>timbl@w3.org</email></address> 2795 </author> 2796 <date month="June" year="1999"/> 2797 </front> 2798 <seriesInfo name="RFC" value="2616"/> 2799 </reference> 2800 2801 <reference anchor='RFC3864'> 2802 <front> 2803 <title>Registration Procedures for Message Header Fields</title> 2804 <author initials='G.' surname='Klyne' fullname='G. Klyne'> 2805 <organization>Nine by Nine</organization> 2806 <address><email>GK-IETF@ninebynine.org</email></address> 2807 </author> 2808 <author initials='M.' surname='Nottingham' fullname='M. Nottingham'> 2809 <organization>BEA Systems</organization> 2810 <address><email>mnot@pobox.com</email></address> 2811 </author> 2812 <author initials='J.' surname='Mogul' fullname='J. Mogul'> 2813 <organization>HP Labs</organization> 2814 <address><email>JeffMogul@acm.org</email></address> 2815 </author> 2816 <date year='2004' month='September' /> 2817 </front> 2818 <seriesInfo name='BCP' value='90' /> 2819 <seriesInfo name='RFC' value='3864' /> 2820 </reference> 2821 2822 </references> 2823 2824 <section title="Compatibility with Previous Versions" anchor="compatibility"> 2825 2826 <section title="Changes from RFC 2068" anchor="changes.from.rfc.2068"> 2827 <t> 2828 A case was missed in the Cache-Control model of HTTP/1.1; s-maxage 2829 was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>, 2830 <xref target="header.cache-control" format="counter"/>, 2831 <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>) 2832 </t> 2833 <t> 2834 Transfer-coding and message lengths all interact in ways that 2835 required fixing exactly when chunked encoding is used (to allow for 2836 transfer encoding that may not be self delimiting); it was important 2837 to straighten out exactly how message lengths are computed. 2838 (<xref target="non-modifiable.headers"/>, 2839 see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>) 2840 </t> 2841 <t> 2842 Proxies should be able to add Content-Length when appropriate. 2843 (<xref target="non-modifiable.headers"/>) 2844 </t> 2845 <t> 2846 Range request responses would become very verbose if all meta-data 2847 were always returned; by allowing the server to only send needed 2848 headers in a 206 response, this problem can be avoided. 2849 (<xref target="combining.headers"/>) 2850 </t> 2851 <t> 2852 The Cache-Control: max-age directive was not properly defined for 2853 responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>) 2854 </t> 2855 <t> 2856 Warnings could be cached incorrectly, or not updated appropriately. 2857 (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>, 2858 <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>, 2859 and <xref target="header.warning" format="counter"/>) Warning 2860 also needed to be a general header, as PUT or other methods may have 2861 need for it in requests. 2862 </t> 2863 </section> 2864 2865 <section title="Changes from RFC 2616" anchor="changes.from.rfc.2616"> 2866 <t> 2867 Clarify denial of service attack avoidance requirement. 2868 (<xref target="invalidation.after.updates.or.deletions"/>) 2869 </t> 2870 </section> 2871 2872 </section> 2873 2874 <section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log"> 2875 2876 <section title="Since RFC2616"> 2877 <t> 2878 Extracted relevant partitions from <xref target="RFC2616"/>. 2879 </t> 2880 </section> 2881 2882 <section title="Since draft-ietf-httpbis-p6-cache-00"> 2883 <t> 2884 Closed issues: 2885 <list style="symbols"> 2886 <t> 2887 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: 2888 "Trailer" 2889 (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>) 2890 </t> 2891 <t> 2892 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>: 2893 "Invalidation after Update or Delete" 2894 (<eref target="http://purl.org/NET/http-errata#invalidupd"/>) 2895 </t> 2896 <t> 2897 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: 2898 "Normative and Informative references" 2899 </t> 2900 <t> 2901 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: 2902 "Date reference typo" 2903 </t> 2904 <t> 2905 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: 2906 "Connection header text" 2907 </t> 2908 <t> 2909 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: 2910 "Informative references" 2911 </t> 2912 <t> 2913 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: 2914 "ISO-8859-1 Reference" 2915 </t> 2916 <t> 2917 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: 2918 "Normative up-to-date references" 2919 </t> 2920 <t> 2921 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: 2922 "typo in 13.2.2" 2923 </t> 2924 </list> 2925 </t> 2926 <t> 2927 Other changes: 2928 <list style="symbols"> 2929 <t> 2930 Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>) 2931 </t> 2932 </list> 2933 </t> 2934 </section> 2935 2936 <section title="Since draft-ietf-httpbis-p6-cache-01"> 2937 <t> 2938 Closed issues: 2939 <list style="symbols"> 2940 <t> 2941 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: 2942 "rel_path not used" 2943 </t> 2944 </list> 2945 </t> 2946 <t> 2947 Other changes: 2948 <list style="symbols"> 2949 <t> 2950 Get rid of duplicate BNF rule names ("host" -> "uri-host") 2951 (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>) 2952 </t> 2953 <t> 2954 Add explicit references to BNF syntax and rules imported from other parts of the specification. 2955 </t> 2956 </list> 2957 </t> 2958 </section> 2959 2960 <section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02"> 2961 <t> 2962 Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>): 2963 <list style="symbols"> 2964 <t> 2965 Reference RFC 3984, and update header registrations for headers defined 2966 in this document. 2967 </t> 2968 </list> 2969 </t> 2970 </section> 2971 2972 <section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03"> 2973 <t> 2974 Closed issues: 2975 <list style="symbols"> 2976 <t> 2977 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: 2978 "Vary header classification" 2979 </t> 2980 </list> 2981 </t> 2982 </section> 2983 2984 <section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04"> 2985 <t> 2986 Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>): 2987 <list style="symbols"> 2988 <t> 2989 Use "/" instead of "|" for alternatives. 2990 </t> 2991 </list> 2992 </t> 2993 </section> 2994 2995 </section> 2996 2997 </back> 1258 </artwork> 1259 </figure> 1260 1261 <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by 1262 a cache), including multiple warnings with the same code number. For example, a server 1263 might provide the same warning with texts in both English and Basque.</t> 1264 <t>When this occurs, the user agent ought to inform the user of as many of them as possible, 1265 in the order that they appear in the response. If it is not possible to inform the user of 1266 all of the warnings, the user agent SHOULD follow these heuristics: <list style="symbols"> 1267 <t>Warnings that appear early in the response take priority over those appearing later 1268 in the response.</t> 1269 1270 <t>Warnings in the user's preferred character set take priority over warnings in other 1271 character sets but with identical warn-codes and warn-agents.</t> 1272 </list> 1273 </t> 1274 <t>Systems that generate multiple Warning headers &SHOULD; order them with this user 1275 agent behavior in mind. New Warning headers &SHOULD; be added after any existing 1276 Warning headers.</t> 1277 <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the 1278 Warning is required to be deleted from a stored response after validation: <list 1279 style="symbols"> 1280 <t>1xx Warnings that describe the freshness or validation status of the response, and 1281 so MUST be deleted by caches after validation. They MUST NOT be generated by a cache except when 1282 validating a cached entry, and MUST NOT be generated by clients.</t> 1283 <t>2xx Warnings that describe some aspect of the entity body or entity headers that is 1284 not rectified by a validation (for example, a lossy compression of the entity 1285 bodies) and which MUST NOT be deleted by caches after validation, unless a full 1286 response is returned, in which case they MUST be.</t> 1287 </list> 1288 </t> 1289 <t>The warn-text &SHOULD; be in a natural language and character set that is most likely 1290 to be intelligible to the human user receiving the response. This decision &MAY; be 1291 based on any available knowledge, such as the location of the cache or user, the 1292 Accept-Language field in a request, the Content-Language field in a response, etc. The 1293 default language is English and the default character set is ISO-8859-1 (<xref 1294 target="ISO-8859-1" />).</t> 1295 <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the 1296 warn-text using the method described in <xref target="RFC2047" />.</t> 1297 <t>If an implementation sends a message with one or more Warning headers to a receiver whose 1298 version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a 1299 warn-date that matches the Date header in the message.</t> 1300 <t>If an implementation receives a message with a warning-value that includes a warn-date, 1301 and that warn-date is different from the Date value in the response, then that 1302 warning-value &MUST; be deleted from the message before storing, forwarding, or using 1303 it. (This prevents bad consequences of naive caching of Warning header fields.) If all of 1304 the warning-values are deleted for this reason, the Warning header &MUST; be deleted 1305 as well.</t> 1306 <t>The following warn-codes are defined by this specification, each with a recommended 1307 warn-text in English, and a description of its meaning.</t> 1308 <t>110 Response is stale <list> 1309 <t>&MUST; be included whenever the returned response is stale.</t> 1310 </list> 1311 </t> 1312 <t>111 Revalidation failed <list> 1313 <t>&MUST; be included if a cache returns a stale response because an attempt to 1314 validate the response failed, due to an inability to reach the server.</t> 1315 </list> 1316 </t> 1317 <t>112 Disconnected operation <list> 1318 <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of 1319 the network for a period of time.</t> 1320 </list> 1321 </t> 1322 <t>113 Heuristic expiration <list> 1323 <t>&MUST; be included if the cache heuristically chose a freshness lifetime greater 1324 than 24 hours and the response's age is greater than 24 hours.</t> 1325 </list> 1326 </t> 1327 <t>199 Miscellaneous warning <list> 1328 <t>The warning text &MAY; include arbitrary information to be presented to a human 1329 user, or logged. A system receiving this warning &MUST-NOT; take any automated 1330 action, besides presenting the warning to the user.</t> 1331 </list> 1332 </t> 1333 <t>214 Transformation applied <list> 1334 <t>&MUST; be added by an intermediate cache or proxy if it applies any 1335 transformation changing the content-coding (as specified in the Content-Encoding 1336 header) or media-type (as specified in the Content-Type header) of the response, or 1337 the entity-body of the response, unless this Warning code already appears in the 1338 response.</t> 1339 </list> 1340 </t> 1341 <t>299 Miscellaneous persistent warning <list> 1342 <t>The warning text &MAY; include arbitrary information to be presented to a human 1343 user, or logged. A system receiving this warning &MUST-NOT; take any automated 1344 action.</t> 1345 </list> 1346 </t> 1347 </section> 1348 1349 </section> 1350 1351 1352 <section anchor="history.lists" title="History Lists"> 1353 <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which 1354 can be used to redisplay an entity retrieved earlier in a session.</t> 1355 <t>History mechanisms and caches are different. In particular history mechanisms 1356 &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a 1357 history mechanism is meant to show exactly what the user saw at the time when the resource 1358 was retrieved.</t> 1359 <t>By default, an expiration time does not apply to history mechanisms. If the entity is still 1360 in storage, a history mechanism &SHOULD; display it even if the entity has expired, 1361 unless the user has specifically configured the agent to refresh expired history documents.</t> 1362 <t>This is not to be construed to prohibit the history mechanism from telling the user that a 1363 view might be stale. <list> 1364 <t> 1365 <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing 1366 stale resources, this will tend to force service authors to avoid using HTTP expiration 1367 controls and cache controls when they would otherwise like to. Service authors may 1368 consider it important that users not be presented with error messages or warning 1369 messages when they use navigation controls (such as BACK) to view previously fetched 1370 resources. Even though sometimes such resources ought not be cached, or ought to expire 1371 quickly, user interface considerations may force service authors to resort to other 1372 means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects 1373 of improperly functioning history mechanisms.</t> 1374 </list> 1375 </t> 1376 </section> 1377 1378 1379 <section anchor="IANA.considerations" title="IANA Considerations"> 1380 <section anchor="message.header.registration" title="Message Header Registration"> 1381 <t>The Message Header Registry located at <eref 1382 target="http://www.iana.org/assignments/message-headers/message-header-index.html" /> 1383 should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t> 1384 <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually--> 1385 <texttable align="left" anchor="iana.header.registration.table" suppress-title="true"> 1386 <ttcol>Header Field Name</ttcol> 1387 <ttcol>Protocol</ttcol> 1388 <ttcol>Status</ttcol> 1389 <ttcol>Reference</ttcol> 1390 1391 <c>Age</c> 1392 <c>http</c> 1393 <c>standard</c> 1394 <c> 1395 <xref target="header.age" /> 1396 </c> 1397 <c>Cache-Control</c> 1398 <c>http</c> 1399 <c>standard</c> 1400 <c> 1401 <xref target="header.cache-control" /> 1402 </c> 1403 <c>Expires</c> 1404 <c>http</c> 1405 <c>standard</c> 1406 <c> 1407 <xref target="header.expires" /> 1408 </c> 1409 <c>Pragma</c> 1410 <c>http</c> 1411 <c>standard</c> 1412 <c> 1413 <xref target="header.pragma" /> 1414 </c> 1415 <c>Vary</c> 1416 <c>http</c> 1417 <c>standard</c> 1418 <c> 1419 <xref target="header.vary" /> 1420 </c> 1421 <c>Warning</c> 1422 <c>http</c> 1423 <c>standard</c> 1424 <c> 1425 <xref target="header.warning" /> 1426 </c> 1427 </texttable> 1428 <!--(END)--> 1429 <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t> 1430 </section> 1431 </section> 1432 1433 <section anchor="security.considerations" title="Security Considerations"> 1434 <t>Caches expose additional potential vulnerabilities, since the contents of the 1435 cache represent an attractive target for malicious exploitation. Because cache contents 1436 persist after an HTTP request is complete, an attack on the cache can reveal information 1437 long after a user believes that the information has been removed from the network. 1438 Therefore, cache contents should be protected as sensitive information.</t> 1439 </section> 1440 1441 <section anchor="ack" title="Acknowledgments"> 1442 <t>Much of the content and presentation of the caching design is due to suggestions and 1443 comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, 1444 and Larry Masinter.</t> 1445 </section> 1446 </middle> 1447 1448 <back> 1449 <references title="Normative References"> 1450 1451 <reference anchor="ISO-8859-1"> 1452 <front> 1453 <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part 1454 1: Latin alphabet No. 1 </title> 1455 <author> 1456 <organization>International Organization for Standardization</organization> 1457 </author> 1458 <date year="1998" /> 1459 </front> 1460 <seriesInfo name="ISO/IEC" value="8859-1:1998" /> 1461 </reference> 1462 1463 <reference anchor="Part1"> 1464 <front> 1465 <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title> 1466 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1467 <organization abbrev="Day Software">Day Software</organization> 1468 <address><email>fielding@gbiv.com</email></address> 1469 </author> 1470 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1471 <organization>One Laptop per Child</organization> 1472 <address><email>jg@laptop.org</email></address> 1473 </author> 1474 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1475 <organization abbrev="HP">Hewlett-Packard Company</organization> 1476 <address><email>JeffMogul@acm.org</email></address> 1477 </author> 1478 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1479 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1480 <address><email>henrikn@microsoft.com</email></address> 1481 </author> 1482 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1483 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1484 <address><email>LMM@acm.org</email></address> 1485 </author> 1486 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1487 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1488 <address><email>paulle@microsoft.com</email></address> 1489 </author> 1490 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1491 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1492 <address><email>timbl@w3.org</email></address> 1493 </author> 1494 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1495 <organization abbrev="W3C">World Wide Web Consortium</organization> 1496 <address><email>ylafon@w3.org</email></address> 1497 </author> 1498 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1499 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1500 <address><email>julian.reschke@greenbytes.de</email></address> 1501 </author> 1502 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1503 </front> 1504 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" /> 1505 <x:source basename="p1-messaging" href="p1-messaging.xml" /> 1506 </reference> 1507 1508 <reference anchor="Part2"> 1509 <front> 1510 <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title> 1511 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1512 <organization abbrev="Day Software">Day Software</organization> 1513 <address><email>fielding@gbiv.com</email></address> 1514 </author> 1515 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1516 <organization>One Laptop per Child</organization> 1517 <address><email>jg@laptop.org</email></address> 1518 </author> 1519 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1520 <organization abbrev="HP">Hewlett-Packard Company</organization> 1521 <address><email>JeffMogul@acm.org</email></address> 1522 </author> 1523 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1524 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1525 <address><email>henrikn@microsoft.com</email></address> 1526 </author> 1527 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1528 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1529 <address><email>LMM@acm.org</email></address> 1530 </author> 1531 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1532 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1533 <address><email>paulle@microsoft.com</email></address> 1534 </author> 1535 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1536 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1537 <address><email>timbl@w3.org</email></address> 1538 </author> 1539 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1540 <organization abbrev="W3C">World Wide Web Consortium</organization> 1541 <address><email>ylafon@w3.org</email></address> 1542 </author> 1543 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1544 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1545 <address><email>julian.reschke@greenbytes.de</email></address> 1546 </author> 1547 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1548 </front> 1549 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" /> 1550 <x:source basename="p2-semantics" href="p2-semantics.xml" /> 1551 </reference> 1552 1553 <reference anchor="Part3"> 1554 <front> 1555 <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title> 1556 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1557 <organization abbrev="Day Software">Day Software</organization> 1558 <address><email>fielding@gbiv.com</email></address> 1559 </author> 1560 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1561 <organization>One Laptop per Child</organization> 1562 <address><email>jg@laptop.org</email></address> 1563 </author> 1564 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1565 <organization abbrev="HP">Hewlett-Packard Company</organization> 1566 <address><email>JeffMogul@acm.org</email></address> 1567 </author> 1568 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1569 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1570 <address><email>henrikn@microsoft.com</email></address> 1571 </author> 1572 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1573 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1574 <address><email>LMM@acm.org</email></address> 1575 </author> 1576 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1577 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1578 <address><email>paulle@microsoft.com</email></address> 1579 </author> 1580 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1581 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1582 <address><email>timbl@w3.org</email></address> 1583 </author> 1584 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1585 <organization abbrev="W3C">World Wide Web Consortium</organization> 1586 <address><email>ylafon@w3.org</email></address> 1587 </author> 1588 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1589 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1590 <address><email>julian.reschke@greenbytes.de</email></address> 1591 </author> 1592 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1593 </front> 1594 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" /> 1595 <x:source basename="p3-payload" href="p3-payload.xml" /> 1596 </reference> 1597 1598 <reference anchor="Part4"> 1599 <front> 1600 <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title> 1601 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1602 <organization abbrev="Day Software">Day Software</organization> 1603 <address><email>fielding@gbiv.com</email></address> 1604 </author> 1605 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1606 <organization>One Laptop per Child</organization> 1607 <address><email>jg@laptop.org</email></address> 1608 </author> 1609 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1610 <organization abbrev="HP">Hewlett-Packard Company</organization> 1611 <address><email>JeffMogul@acm.org</email></address> 1612 </author> 1613 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1614 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1615 <address><email>henrikn@microsoft.com</email></address> 1616 </author> 1617 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1618 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1619 <address><email>LMM@acm.org</email></address> 1620 </author> 1621 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1622 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1623 <address><email>paulle@microsoft.com</email></address> 1624 </author> 1625 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1626 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1627 <address><email>timbl@w3.org</email></address> 1628 </author> 1629 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1630 <organization abbrev="W3C">World Wide Web Consortium</organization> 1631 <address><email>ylafon@w3.org</email></address> 1632 </author> 1633 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1634 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1635 <address><email>julian.reschke@greenbytes.de</email></address> 1636 </author> 1637 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1638 </front> 1639 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" /> 1640 <x:source basename="p4-conditional" href="p4-conditional.xml" /> 1641 </reference> 1642 1643 <reference anchor="Part5"> 1644 <front> 1645 <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title> 1646 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1647 <organization abbrev="Day Software">Day Software</organization> 1648 <address><email>fielding@gbiv.com</email></address> 1649 </author> 1650 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1651 <organization>One Laptop per Child</organization> 1652 <address><email>jg@laptop.org</email></address> 1653 </author> 1654 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1655 <organization abbrev="HP">Hewlett-Packard Company</organization> 1656 <address><email>JeffMogul@acm.org</email></address> 1657 </author> 1658 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1659 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1660 <address><email>henrikn@microsoft.com</email></address> 1661 </author> 1662 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1663 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1664 <address><email>LMM@acm.org</email></address> 1665 </author> 1666 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1667 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1668 <address><email>paulle@microsoft.com</email></address> 1669 </author> 1670 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1671 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1672 <address><email>timbl@w3.org</email></address> 1673 </author> 1674 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1675 <organization abbrev="W3C">World Wide Web Consortium</organization> 1676 <address><email>ylafon@w3.org</email></address> 1677 </author> 1678 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1679 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1680 <address><email>julian.reschke@greenbytes.de</email></address> 1681 </author> 1682 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1683 </front> 1684 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" /> 1685 <x:source basename="p5-range" href="p5-range.xml" /> 1686 </reference> 1687 1688 <reference anchor="Part7"> 1689 <front> 1690 <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title> 1691 <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding"> 1692 <organization abbrev="Day Software">Day Software</organization> 1693 <address><email>fielding@gbiv.com</email></address> 1694 </author> 1695 <author fullname="Jim Gettys" initials="J." surname="Gettys"> 1696 <organization>One Laptop per Child</organization> 1697 <address><email>jg@laptop.org</email></address> 1698 </author> 1699 <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul"> 1700 <organization abbrev="HP">Hewlett-Packard Company</organization> 1701 <address><email>JeffMogul@acm.org</email></address> 1702 </author> 1703 <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk"> 1704 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1705 <address><email>henrikn@microsoft.com</email></address> 1706 </author> 1707 <author fullname="Larry Masinter" initials="L." surname="Masinter"> 1708 <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization> 1709 <address><email>LMM@acm.org</email></address> 1710 </author> 1711 <author fullname="Paul J. Leach" initials="P." surname="Leach"> 1712 <organization abbrev="Microsoft">Microsoft Corporation</organization> 1713 <address><email>paulle@microsoft.com</email></address> 1714 </author> 1715 <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee"> 1716 <organization abbrev="W3C/MIT">World Wide Web Consortium</organization> 1717 <address><email>timbl@w3.org</email></address> 1718 </author> 1719 <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon"> 1720 <organization abbrev="W3C">World Wide Web Consortium</organization> 1721 <address><email>ylafon@w3.org</email></address> 1722 </author> 1723 <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke"> 1724 <organization abbrev="greenbytes">greenbytes GmbH</organization> 1725 <address><email>julian.reschke@greenbytes.de</email></address> 1726 </author> 1727 <date month="&ID-MONTH;" year="&ID-YEAR;" /> 1728 </front> 1729 <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" /> 1730 <x:source basename="p7-auth" href="p7-auth.xml" /> 1731 </reference> 1732 1733 <reference anchor="RFC2047"> 1734 <front> 1735 <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) 1736 Part Three: Message Header Extensions for Non-ASCII Text</title> 1737 <author fullname="Keith Moore" initials="K." surname="Moore"> 1738 <organization>University of Tennessee</organization> 1739 <address><email>moore@cs.utk.edu</email></address> 1740 </author> 1741 <date month="November" year="1996" /> 1742 </front> 1743 <seriesInfo name="RFC" value="2047" /> 1744 </reference> 1745 1746 <reference anchor="RFC2119"> 1747 <front> 1748 <title>Key words for use in RFCs to Indicate Requirement Levels</title> 1749 <author fullname="Scott Bradner" initials="S." surname="Bradner"> 1750 <organization>Harvard University</organization> 1751 <address><email>sob@harvard.edu</email></address> 1752 </author> 1753 <date month="March" year="1997" /> 1754 </front> 1755 <seriesInfo name="BCP" value="14" /> 1756 <seriesInfo name="RFC" value="2119" /> 1757 </reference> 1758 1759 </references> 1760 1761 <references title="Informative References"> 1762 1763 <reference anchor="RFC1305"> 1764 <front> 1765 <title>Network Time Protocol (Version 3) Specification, Implementation</title> 1766 <author fullname="David L. Mills" initials="D." surname="Mills"> 1767 <organization>University of Delaware, Electrical Engineering Department</organization> 1768 <address><email>mills@udel.edu</email></address> 1769 </author> 1770 <date month="March" year="1992" /> 1771 </front> 1772 <seriesInfo name="RFC" value="1305" /> 1773 </reference> 1774 1775 <reference anchor="RFC2616"> 1776 <front> 1777 <title>Hypertext Transfer Protocol -- HTTP/1.1</title> 1778 <author fullname="R. Fielding" initials="R." surname="Fielding"> 1779 <organization>University of California, Irvine</organization> 1780 <address><email>fielding@ics.uci.edu</email></address> 1781 </author> 1782 <author fullname="J. Gettys" initials="J." surname="Gettys"> 1783 <organization>W3C</organization> 1784 <address><email>jg@w3.org</email></address> 1785 </author> 1786 <author fullname="J. Mogul" initials="J." surname="Mogul"> 1787 <organization>Compaq Computer Corporation</organization> 1788 <address><email>mogul@wrl.dec.com</email></address> 1789 </author> 1790 <author fullname="H. Frystyk" initials="H." surname="Frystyk"> 1791 <organization>MIT Laboratory for Computer Science</organization> 1792 <address><email>frystyk@w3.org</email></address> 1793 </author> 1794 <author fullname="L. Masinter" initials="L." surname="Masinter"> 1795 <organization>Xerox Corporation</organization> 1796 <address><email>masinter@parc.xerox.com</email></address> 1797 </author> 1798 <author fullname="P. Leach" initials="P." surname="Leach"> 1799 <organization>Microsoft Corporation</organization> 1800 <address><email>paulle@microsoft.com</email></address> 1801 </author> 1802 <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee"> 1803 <organization>W3C</organization> 1804 <address><email>timbl@w3.org</email></address> 1805 </author> 1806 <date month="June" year="1999" /> 1807 </front> 1808 <seriesInfo name="RFC" value="2616" /> 1809 </reference> 1810 1811 <reference anchor="RFC3864"> 1812 <front> 1813 <title>Registration Procedures for Message Header Fields</title> 1814 <author fullname="G. Klyne" initials="G." surname="Klyne"> 1815 <organization>Nine by Nine</organization> 1816 <address><email>GK-IETF@ninebynine.org</email></address> 1817 </author> 1818 <author fullname="M. Nottingham" initials="M." surname="Nottingham"> 1819 <organization>BEA Systems</organization> 1820 <address><email>mnot@pobox.com</email></address> 1821 </author> 1822 <author fullname="J. Mogul" initials="J." surname="Mogul"> 1823 <organization>HP Labs</organization> 1824 <address><email>JeffMogul@acm.org</email></address> 1825 </author> 1826 <date month="September" year="2004" /> 1827 </front> 1828 <seriesInfo name="BCP" value="90" /> 1829 <seriesInfo name="RFC" value="3864" /> 1830 </reference> 1831 1832 </references> 1833 1834 <section anchor="compatibility" title="Compatibility with Previous Versions"> 1835 1836 <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068"> 1837 <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add 1838 this missing case. (Sections <xref format="counter" target="response.cacheability" />, 1839 <xref format="counter" target="header.cache-control" />.</t> 1840 <t>Transfer-coding and message lengths all interact in ways that required fixing exactly 1841 when chunked encoding is used (to allow for transfer encoding that may not be self 1842 delimiting); it was important to straighten out exactly how message lengths are computed. 1843 (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t> 1844 <t>Proxies should be able to add Content-Length when appropriate.</t> 1845 <t>Range request responses would become very verbose if all meta-data were always returned; 1846 by allowing the server to only send needed headers in a 206 response, this problem can be 1847 avoided. (<xref target="combining.headers" />)</t> 1848 <t>The Cache-Control: max-age directive was not properly defined for responses.</t> 1849 <t>Warnings could be cached incorrectly, or not updated appropriately. <xref 1850 format="counter" target="expiration.model" />, <xref format="counter" 1851 target="combining.headers" />, <xref format="counter" target="header.cache-control" />, 1852 and <xref format="counter" target="header.warning" />) Warning also needed to be a general 1853 header, as PUT or other methods may have need for it in requests.</t> 1854 </section> 1855 1856 <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616"> 1857 <t>Clarify denial of service attack avoidance requirement. (<xref 1858 target="invalidation.after.updates.or.deletions" />)</t> 1859 </section> 1860 1861 </section> 1862 1863 <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)"> 1864 1865 <section title="Since RFC2616"> 1866 <t>Extracted relevant partitions from <xref target="RFC2616" />.</t> 1867 </section> 1868 1869 <section title="Since draft-ietf-httpbis-p6-cache-00"> 1870 <t>Closed issues: <list style="symbols"> 1871 <t> 1872 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" 1873 (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t> 1874 <t> 1875 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation 1876 after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t> 1877 <t> 1878 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and 1879 Informative references"</t> 1880 <t> 1881 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date 1882 reference typo"</t> 1883 <t> 1884 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection 1885 header text"</t> 1886 <t> 1887 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative 1888 references"</t> 1889 <t> 1890 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 1891 Reference"</t> 1892 <t> 1893 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative 1894 up-to-date references"</t> 1895 <t> 1896 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 1897 13.2.2"</t> 1898 </list> 1899 </t> 1900 <t>Other changes: <list style="symbols"> 1901 <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref 1902 target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t> 1903 </list> 1904 </t> 1905 </section> 1906 1907 <section title="Since draft-ietf-httpbis-p6-cache-01"> 1908 <t>Closed issues: <list style="symbols"> 1909 <t> 1910 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not 1911 used"</t> 1912 </list> 1913 </t> 1914 <t>Other changes: <list style="symbols"> 1915 <t>Get rid of duplicate BNF rule names ("host" -> "uri-host") (work in progress 1916 on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t> 1917 <t>Add explicit references to BNF syntax and rules imported from other parts of the 1918 specification.</t> 1919 </list> 1920 </t> 1921 </section> 1922 1923 <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02"> 1924 <t>Ongoing work on IANA Message Header Registration (<eref 1925 target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols"> 1926 <t>Reference RFC 3984, and update header registrations for headers defined in this 1927 document.</t> 1928 </list> 1929 </t> 1930 </section> 1931 1932 <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03"> 1933 <t>Closed issues: <list style="symbols"> 1934 <t> 1935 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header 1936 classification"</t> 1937 </list> 1938 </t> 1939 </section> 1940 1941 <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04"> </section> 1942 1943 </section> 1944 </back> 2998 1945 </rfc>
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