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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPbis Working Group R. Fielding, Ed. 3 Internet-Draft Adobe 4 Obsoletes: 2616 (if approved) J. Gettys 5 Intended status: Standards Track Alcatel-Lucent 6 Expires: May 3, 2012 J. Mogul 7 HP 8 H. Frystyk 9 Microsoft 10 L. Masinter 11 Adobe 12 P. Leach 13 Microsoft 14 T. Berners-Lee 15 W3C/MIT 16 Y. Lafon, Ed. 17 W3C 18 M. Nottingham, Ed. 19 Rackspace 20 J. Reschke, Ed. 21 greenbytes 22 October 31, 2011 24 HTTP/1.1, part 6: Caching 25 draft-ietf-httpbis-p6-cache-17 27 Abstract 29 The Hypertext Transfer Protocol (HTTP) is an application-level 30 protocol for distributed, collaborative, hypertext information 31 systems. HTTP has been in use by the World Wide Web global 32 information initiative since 1990. This document is Part 6 of the 33 seven-part specification that defines the protocol referred to as 34 "HTTP/1.1" and, taken together, obsoletes RFC 2616. 36 Part 6 defines requirements on HTTP caches and the associated header 37 fields that control cache behavior or indicate cacheable response 38 messages. 40 Editorial Note (To be removed by RFC Editor) 42 Discussion of this draft should take place on the HTTPBIS working 43 group mailing list (ietf-http-wg@w3.org), which is archived at 44 . 46 The current issues list is at 47 and related 48 documents (including fancy diffs) can be found at 49 . 51 The changes in this draft are summarized in Appendix C.18. 53 Status of This Memo 55 This Internet-Draft is submitted in full conformance with the 56 provisions of BCP 78 and BCP 79. 58 Internet-Drafts are working documents of the Internet Engineering 59 Task Force (IETF). Note that other groups may also distribute 60 working documents as Internet-Drafts. The list of current Internet- 61 Drafts is at http://datatracker.ietf.org/drafts/current/. 63 Internet-Drafts are draft documents valid for a maximum of six months 64 and may be updated, replaced, or obsoleted by other documents at any 65 time. It is inappropriate to use Internet-Drafts as reference 66 material or to cite them other than as "work in progress." 68 This Internet-Draft will expire on May 3, 2012. 70 Copyright Notice 72 Copyright (c) 2011 IETF Trust and the persons identified as the 73 document authors. All rights reserved. 75 This document is subject to BCP 78 and the IETF Trust's Legal 76 Provisions Relating to IETF Documents 77 (http://trustee.ietf.org/license-info) in effect on the date of 78 publication of this document. Please review these documents 79 carefully, as they describe your rights and restrictions with respect 80 to this document. Code Components extracted from this document must 81 include Simplified BSD License text as described in Section 4.e of 82 the Trust Legal Provisions and are provided without warranty as 83 described in the Simplified BSD License. 85 This document may contain material from IETF Documents or IETF 86 Contributions published or made publicly available before November 87 10, 2008. The person(s) controlling the copyright in some of this 88 material may not have granted the IETF Trust the right to allow 89 modifications of such material outside the IETF Standards Process. 90 Without obtaining an adequate license from the person(s) controlling 91 the copyright in such materials, this document may not be modified 92 outside the IETF Standards Process, and derivative works of it may 93 not be created outside the IETF Standards Process, except to format 94 it for publication as an RFC or to translate it into languages other 95 than English. 97 Table of Contents 99 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 100 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 101 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 102 1.3. Conformance and Error Handling . . . . . . . . . . . . . . 7 103 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7 104 1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 8 105 1.4.2. ABNF Rules defined in other Parts of the 106 Specification . . . . . . . . . . . . . . . . . . . . 8 107 1.5. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 8 108 2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 8 109 2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 9 110 2.2. Constructing Responses from Caches . . . . . . . . . . . . 10 111 2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 11 112 2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 12 113 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 13 114 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 15 115 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 16 116 2.4.1. Freshening Responses . . . . . . . . . . . . . . . . . 17 117 2.5. Request Methods that Invalidate . . . . . . . . . . . . . 18 118 2.6. Shared Caching of Authenticated Responses . . . . . . . . 18 119 2.7. Caching Negotiated Responses . . . . . . . . . . . . . . . 19 120 2.8. Combining Partial Content . . . . . . . . . . . . . . . . 20 121 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 20 122 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 123 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 21 124 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 21 125 3.2.2. Response Cache-Control Directives . . . . . . . . . . 23 126 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26 127 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 128 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28 129 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 130 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29 131 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 32 132 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 133 5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32 134 5.2. Header Field Registration . . . . . . . . . . . . . . . . 33 135 6. Security Considerations . . . . . . . . . . . . . . . . . . . 33 136 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 137 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33 138 8.1. Normative References . . . . . . . . . . . . . . . . . . . 33 139 8.2. Informative References . . . . . . . . . . . . . . . . . . 34 140 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35 141 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 35 142 Appendix C. Change Log (to be removed by RFC Editor before 143 publication) . . . . . . . . . . . . . . . . . . . . 36 144 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 36 145 C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 37 146 C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 37 147 C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 38 148 C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 38 149 C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 38 150 C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 38 151 C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 39 152 C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 39 153 C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 39 154 C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 40 155 C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 40 156 C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 41 157 C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 41 158 C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 41 159 C.16. Since draft-ietf-httpbis-p6-cache-14 . . . . . . . . . . . 41 160 C.17. Since draft-ietf-httpbis-p6-cache-15 . . . . . . . . . . . 42 161 C.18. Since draft-ietf-httpbis-p6-cache-16 . . . . . . . . . . . 42 162 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 164 1. Introduction 166 HTTP is typically used for distributed information systems, where 167 performance can be improved by the use of response caches. This 168 document defines aspects of HTTP/1.1 related to caching and reusing 169 response messages. 171 1.1. Purpose 173 An HTTP cache is a local store of response messages and the subsystem 174 that controls its message storage, retrieval, and deletion. A cache 175 stores cacheable responses in order to reduce the response time and 176 network bandwidth consumption on future, equivalent requests. Any 177 client or server MAY employ a cache, though a cache cannot be used by 178 a server that is acting as a tunnel. 180 The goal of caching in HTTP/1.1 is to significantly improve 181 performance by reusing a prior response message to satisfy a current 182 request. A stored response is considered "fresh", as defined in 183 Section 2.3, if the response can be reused without "validation" 184 (checking with the origin server to see if the cached response 185 remains valid for this request). A fresh cache response can 186 therefore reduce both latency and network transfers each time it is 187 reused. When a cached response is not fresh, it might still be 188 reusable if it can be freshened by validation (Section 2.4) or if the 189 origin is unavailable. 191 1.2. Terminology 193 This specification uses a number of terms to refer to the roles 194 played by participants in, and objects of, HTTP caching. 196 cache 198 A conformant implementation of a HTTP cache. Note that this 199 implies an HTTP/1.1 cache; this specification does not define 200 conformance for HTTP/1.0 caches. 202 shared cache 204 A cache that stores responses to be reused by more than one user; 205 usually (but not always) deployed as part of an intermediary. 207 private cache 209 A cache that is dedicated to a single user. 211 cacheable 213 A response is cacheable if a cache is allowed to store a copy of 214 the response message for use in answering subsequent requests. 215 Even when a response is cacheable, there might be additional 216 constraints on whether a cache can use the stored copy to satisfy 217 a particular request. 219 explicit expiration time 221 The time at which the origin server intends that a representation 222 no longer be returned by a cache without further validation. 224 heuristic expiration time 226 An expiration time assigned by a cache when no explicit expiration 227 time is available. 229 age 231 The age of a response is the time since it was sent by, or 232 successfully validated with, the origin server. 234 first-hand 236 A response is first-hand if the freshness model is not in use; 237 i.e., its age is 0. 239 freshness lifetime 241 The length of time between the generation of a response and its 242 expiration time. 244 fresh 246 A response is fresh if its age has not yet exceeded its freshness 247 lifetime. 249 stale 251 A response is stale if its age has passed its freshness lifetime 252 (either explicit or heuristic). 254 validator 256 A protocol element (e.g., an entity-tag or a Last-Modified time) 257 that is used to find out whether a stored response is an 258 equivalent copy of a representation. See Section 2.1 of [Part4]. 260 strong validator 262 A validator that is defined by the origin server such that its 263 current value will change if the representation body changes; 264 i.e., an entity-tag that is not marked as weak (Section 2.3 of 265 [Part4]) or, if no entity-tag is provided, a Last-Modified value 266 that is strong in the sense defined by Section 2.2.2 of [Part4]. 268 1.3. Conformance and Error Handling 270 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 271 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 272 document are to be interpreted as described in [RFC2119]. 274 This document defines conformance criteria for several roles in HTTP 275 communication, including Senders, Recipients, Clients, Servers, User- 276 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See 277 Section 2 of [Part1] for definitions of these terms. 279 An implementation is considered conformant if it complies with all of 280 the requirements associated with its role(s). Note that SHOULD-level 281 requirements are relevant here, unless one of the documented 282 exceptions is applicable. 284 This document also uses ABNF to define valid protocol elements 285 (Section 1.4). In addition to the prose requirements placed upon 286 them, Senders MUST NOT generate protocol elements that are invalid. 288 Unless noted otherwise, Recipients MAY take steps to recover a usable 289 protocol element from an invalid construct. However, HTTP does not 290 define specific error handling mechanisms, except in cases where it 291 has direct impact on security. This is because different uses of the 292 protocol require different error handling strategies; for example, a 293 Web browser may wish to transparently recover from a response where 294 the Location header field doesn't parse according to the ABNF, 295 whereby in a systems control protocol using HTTP, this type of error 296 recovery could lead to dangerous consequences. 298 1.4. Syntax Notation 300 This specification uses the ABNF syntax defined in Section 1.2 of 301 [Part1] (which extends the syntax defined in [RFC5234] with a list 302 rule). Appendix B shows the collected ABNF, with the list rule 303 expanded. 305 The following core rules are included by reference, as defined in 306 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 307 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 308 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit 309 sequence of data), SP (space), and VCHAR (any visible US-ASCII 310 character). 312 1.4.1. Core Rules 314 The core rules below are defined in [Part1]: 316 OWS = 317 quoted-string = 318 token = 320 1.4.2. ABNF Rules defined in other Parts of the Specification 322 The ABNF rules below are defined in other parts: 324 field-name = 325 HTTP-date = 326 port = 327 pseudonym = 328 uri-host = 330 1.5. Delta Seconds 332 The delta-seconds rule specifies a non-negative integer, representing 333 time in seconds. 335 delta-seconds = 1*DIGIT 337 If an implementation receives a delta-seconds value larger than the 338 largest positive integer it can represent, or if any of its 339 subsequent calculations overflows, it MUST consider the value to be 340 2147483648 (2^31). Recipients parsing a delta-seconds value MUST use 341 an arithmetic type of at least 31 bits of range, and senders MUST NOT 342 send delta-seconds with a value greater than 2147483648. 344 2. Cache Operation 346 Proper cache operation preserves the semantics of HTTP transfers 347 ([Part2]) while eliminating the transfer of information already held 348 in the cache. Although caching is an entirely OPTIONAL feature of 349 HTTP, we assume that reusing the cached response is desirable and 350 that such reuse is the default behavior when no requirement or 351 locally-desired configuration prevents it. Therefore, HTTP cache 352 requirements are focused on preventing a cache from either storing a 353 non-reusable response or reusing a stored response inappropriately. 355 Each cache entry consists of a cache key and one or more HTTP 356 responses corresponding to prior requests that used the same key. 357 The most common form of cache entry is a successful result of a 358 retrieval request: i.e., a 200 (OK) response containing a 359 representation of the resource identified by the request target. 360 However, it is also possible to cache negative results (e.g., 404 not 361 found), incomplete results (e.g., 206 partial content), and responses 362 to safe methods other than GET if the method's definition allows such 363 caching and defines something suitable for use as a cache key. 365 The default cache key consists of the request method and target URI. 366 However, since HTTP caches in common use today are typically limited 367 to caching responses to GET, most implementations simply decline 368 other methods and use only the URI as the key. 370 If a request target is subject to content negotiation, its cache 371 entry might consist of multiple stored responses, each differentiated 372 by a secondary key for the values of the original request's selecting 373 header fields (Section 2.7). 375 2.1. Response Cacheability 377 A cache MUST NOT store a response to any request, unless: 379 o The request method is understood by the cache and defined as being 380 cacheable, and 382 o the response status code is understood by the cache, and 384 o the "no-store" cache directive (see Section 3.2) does not appear 385 in request or response header fields, and 387 o the "private" cache response directive (see Section 3.2.2 does not 388 appear in the response, if the cache is shared, and 390 o the "Authorization" header field (see Section 4.1 of [Part7]) does 391 not appear in the request, if the cache is shared, unless the 392 response explicitly allows it (see Section 2.6), and 394 o the response either: 396 * contains an Expires header field (see Section 3.3), or 398 * contains a max-age response cache directive (see 399 Section 3.2.2), or 401 * contains a s-maxage response cache directive and the cache is 402 shared, or 404 * contains a Cache Control Extension (see Section 3.2.3) that 405 allows it to be cached, or 407 * has a status code that can be served with heuristic freshness 408 (see Section 2.3.1.1). 410 Note that any of the requirements listed above can be overridden by a 411 cache-control extension; see Section 3.2.3. 413 In this context, a cache has "understood" a request method or a 414 response status code if it recognizes it and implements any cache- 415 specific behavior. 417 Note that, in normal operation, most caches will not store a response 418 that has neither a cache validator nor an explicit expiration time, 419 as such responses are not usually useful to store. However, caches 420 are not prohibited from storing such responses. 422 A response message is considered complete when all of the octets 423 indicated by the message framing ([Part1]) are received prior to the 424 connection being closed. If the request is GET, the response status 425 is 200 (OK), and the entire response header block has been received, 426 a cache MAY store an incomplete response message-body if the cache 427 entry is recorded as incomplete. Likewise, a 206 (Partial Content) 428 response MAY be stored as if it were an incomplete 200 (OK) cache 429 entry. However, a cache MUST NOT store incomplete or partial content 430 responses if it does not support the Range and Content-Range header 431 fields or if it does not understand the range units used in those 432 fields. 434 A cache MAY complete a stored incomplete response by making a 435 subsequent range request ([Part5]) and combining the successful 436 response with the stored entry, as defined in Section 2.8. A cache 437 MUST NOT use an incomplete response to answer requests unless the 438 response has been made complete or the request is partial and 439 specifies a range that is wholly within the incomplete response. A 440 cache MUST NOT send a partial response to a client without explicitly 441 marking it as such using the 206 (Partial Content) status code. 443 2.2. Constructing Responses from Caches 445 For a presented request, a cache MUST NOT return a stored response, 446 unless: 448 o The presented effective request URI (Section 4.3 of [Part1]) and 449 that of the stored response match, and 451 o the request method associated with the stored response allows it 452 to be used for the presented request, and 454 o selecting header fields nominated by the stored response (if any) 455 match those presented (see Section 2.7), and 457 o the presented request and stored response are free from directives 458 that would prevent its use (see Section 3.2 and Section 3.4), and 460 o the stored response is either: 462 * fresh (see Section 2.3), or 464 * allowed to be served stale (see Section 2.3.3), or 466 * successfully validated (see Section 2.4). 468 Note that any of the requirements listed above can be overridden by a 469 cache-control extension; see Section 3.2.3. 471 When a stored response is used to satisfy a request without 472 validation, a cache MUST include a single Age header field 473 (Section 3.1) in the response with a value equal to the stored 474 response's current_age; see Section 2.3.2. 476 A cache MUST write through requests with methods that are unsafe 477 (Section 6.1.1 of [Part2]) to the origin server; i.e., a cache must 478 not generate a reply to such a request before having forwarded the 479 request and having received a corresponding response. 481 Also, note that unsafe requests might invalidate already stored 482 responses; see Section 2.5. 484 When more than one suitable response is stored, a cache MUST use the 485 most recent response (as determined by the Date header field). It 486 can also forward a request with "Cache-Control: max-age=0" or "Cache- 487 Control: no-cache" to disambiguate which response to use. 489 A cache that does not have a clock available MUST NOT use stored 490 responses without revalidating them on every use. A cache, 491 especially a shared cache, SHOULD use a mechanism, such as NTP 492 [RFC1305], to synchronize its clock with a reliable external 493 standard. 495 2.3. Freshness Model 497 When a response is "fresh" in the cache, it can be used to satisfy 498 subsequent requests without contacting the origin server, thereby 499 improving efficiency. 501 The primary mechanism for determining freshness is for an origin 502 server to provide an explicit expiration time in the future, using 503 either the Expires header field (Section 3.3) or the max-age response 504 cache directive (Section 3.2.2). Generally, origin servers will 505 assign future explicit expiration times to responses in the belief 506 that the representation is not likely to change in a semantically 507 significant way before the expiration time is reached. 509 If an origin server wishes to force a cache to validate every 510 request, it can assign an explicit expiration time in the past to 511 indicate that the response is already stale. Compliant caches will 512 normally validate the cached response before reusing it for 513 subsequent requests (see Section 2.3.3). 515 Since origin servers do not always provide explicit expiration times, 516 a cache MAY assign a heuristic expiration time when an explicit time 517 is not specified, employing algorithms that use other header field 518 values (such as the Last-Modified time) to estimate a plausible 519 expiration time. This specification does not provide specific 520 algorithms, but does impose worst-case constraints on their results. 522 The calculation to determine if a response is fresh is: 524 response_is_fresh = (freshness_lifetime > current_age) 526 The freshness_lifetime is defined in Section 2.3.1; the current_age 527 is defined in Section 2.3.2. 529 Additionally, clients might need to influence freshness calculation. 530 They can do this using several request cache directives, with the 531 effect of either increasing or loosening constraints on freshness. 532 See Section 3.2.1. 534 Note that freshness applies only to cache operation; it cannot be 535 used to force a user agent to refresh its display or reload a 536 resource. See Section 4 for an explanation of the difference between 537 caches and history mechanisms. 539 2.3.1. Calculating Freshness Lifetime 541 A cache can calculate the freshness lifetime (denoted as 542 freshness_lifetime) of a response by using the first match of: 544 o If the cache is shared and the s-maxage response cache directive 545 (Section 3.2.2) is present, use its value, or 547 o If the max-age response cache directive (Section 3.2.2) is 548 present, use its value, or 550 o If the Expires response header field (Section 3.3) is present, use 551 its value minus the value of the Date response header field, or 553 o Otherwise, no explicit expiration time is present in the response. 554 A heuristic freshness lifetime might be applicable; see 555 Section 2.3.1.1. 557 Note that this calculation is not vulnerable to clock skew, since all 558 of the information comes from the origin server. 560 2.3.1.1. Calculating Heuristic Freshness 562 If no explicit expiration time is present in a stored response that 563 has a status code whose definition allows heuristic freshness to be 564 used (including the following in Section 7 of [Part2]: 200, 203, 206, 565 300, 301 and 410), a cache MAY calculate a heuristic expiration time. 566 A cache MUST NOT use heuristics to determine freshness for responses 567 with status codes that do not explicitly allow it. 569 When a heuristic is used to calculate freshness lifetime, a cache 570 SHOULD attach a Warning header field with a 113 warn-code to the 571 response if its current_age is more than 24 hours and such a warning 572 is not already present. 574 Also, if the response has a Last-Modified header field (Section 2.2 575 of [Part4]), caches are encouraged to use a heuristic expiration 576 value that is no more than some fraction of the interval since that 577 time. A typical setting of this fraction might be 10%. 579 Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do 580 not calculate heuristic freshness for URIs with query components 581 (i.e., those containing '?'). In practice, this has not been 582 widely implemented. Therefore, servers are encouraged to send 583 explicit directives (e.g., Cache-Control: no-cache) if they wish 584 to preclude caching. 586 2.3.2. Calculating Age 588 HTTP/1.1 uses the Age header field to convey the estimated age of the 589 response message when obtained from a cache. The Age field value is 590 the cache's estimate of the amount of time since the response was 591 generated or validated by the origin server. In essence, the Age 592 value is the sum of the time that the response has been resident in 593 each of the caches along the path from the origin server, plus the 594 amount of time it has been in transit along network paths. 596 The following data is used for the age calculation: 598 age_value 600 The term "age_value" denotes the value of the Age header field 601 (Section 3.1), in a form appropriate for arithmetic operation; or 602 0, if not available. 604 date_value 606 HTTP/1.1 requires origin servers to send a Date header field, if 607 possible, with every response, giving the time at which the 608 response was generated. The term "date_value" denotes the value 609 of the Date header field, in a form appropriate for arithmetic 610 operations. See Section 9.2 of [Part2] for the definition of the 611 Date header field, and for requirements regarding responses 612 without it. 614 now 616 The term "now" means "the current value of the clock at the host 617 performing the calculation". A cache SHOULD use NTP ([RFC1305]) 618 or some similar protocol to synchronize its clocks to a globally 619 accurate time standard. 621 request_time 623 The current value of the clock at the host at the time the request 624 resulting in the stored response was made. 626 response_time 628 The current value of the clock at the host at the time the 629 response was received. 631 A response's age can be calculated in two entirely independent ways: 633 1. the "apparent_age": response_time minus date_value, if the local 634 clock is reasonably well synchronized to the origin server's 635 clock. If the result is negative, the result is replaced by 636 zero. 638 2. the "corrected_age_value", if all of the caches along the 639 response path implement HTTP/1.1. A cache MUST interpret this 640 value relative to the time the request was initiated, not the 641 time that the response was received. 643 apparent_age = max(0, response_time - date_value); 645 response_delay = response_time - request_time; 646 corrected_age_value = age_value + response_delay; 648 These are combined as 650 corrected_initial_age = max(apparent_age, corrected_age_value); 652 The current_age of a stored response can then be calculated by adding 653 the amount of time (in seconds) since the stored response was last 654 validated by the origin server to the corrected_initial_age. 656 resident_time = now - response_time; 657 current_age = corrected_initial_age + resident_time; 659 Additionally, to avoid common problems in date parsing: 661 o HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date 662 which appears to be more than 50 years in the future is in fact in 663 the past (this helps solve the "year 2000" problem). 665 o Although all date formats are specified to be case-sensitive, 666 recipients SHOULD match day, week and timezone names case- 667 insensitively. 669 o An HTTP/1.1 implementation MAY internally represent a parsed 670 Expires date as earlier than the proper value, but MUST NOT 671 internally represent a parsed Expires date as later than the 672 proper value. 674 o All expiration-related calculations MUST be done in GMT. The 675 local time zone MUST NOT influence the calculation or comparison 676 of an age or expiration time. 678 o If an HTTP header field incorrectly carries a date value with a 679 time zone other than GMT, it MUST be converted into GMT using the 680 most conservative possible conversion. 682 2.3.3. Serving Stale Responses 684 A "stale" response is one that either has explicit expiry information 685 or is allowed to have heuristic expiry calculated, but is not fresh 686 according to the calculations in Section 2.3. 688 A cache MUST NOT return a stale response if it is prohibited by an 689 explicit in-protocol directive (e.g., by a "no-store" or "no-cache" 690 cache directive, a "must-revalidate" cache-response-directive, or an 691 applicable "s-maxage" or "proxy-revalidate" cache-response-directive; 692 see Section 3.2.2). 694 A cache MUST NOT return stale responses unless it is disconnected 695 (i.e., it cannot contact the origin server or otherwise find a 696 forward path) or doing so is explicitly allowed (e.g., by the max- 697 stale request directive; see Section 3.2.1). 699 A cache SHOULD append a Warning header field with the 110 warn-code 700 (see Section 3.6) to stale responses. Likewise, a cache SHOULD add 701 the 112 warn-code to stale responses if the cache is disconnected. 703 If a cache receives a first-hand response (either an entire response, 704 or a 304 (Not Modified) response) that it would normally forward to 705 the requesting client, and the received response is no longer fresh, 706 the cache can forward it to the requesting client without adding a 707 new Warning (but without removing any existing Warning header 708 fields). A cache shouldn't attempt to validate a response simply 709 because that response became stale in transit. 711 2.4. Validation Model 713 When a cache has one or more stored responses for a requested URI, 714 but cannot serve any of them (e.g., because they are not fresh, or 715 one cannot be selected; see Section 2.7), it can use the conditional 716 request mechanism [Part4] in the forwarded request to give the origin 717 server an opportunity to both select a valid stored response to be 718 used, and to update it. This process is known as "validating" or 719 "revalidating" the stored response. 721 When sending such a conditional request, a cache adds an If-Modified- 722 Since header field whose value is that of the Last-Modified header 723 field from the selected (see Section 2.7) stored response, if 724 available. 726 Additionally, a cache can add an If-None-Match header field whose 727 value is that of the ETag header field(s) from all responses stored 728 for the requested URI, if present. However, if any of the stored 729 responses contains only partial content, the cache shouldn't include 730 its entity-tag in the If-None-Match header field unless the request 731 is for a range that would be fully satisfied by that stored response. 733 Cache handling of a response to a conditional request is dependent 734 upon its status code: 736 o A 304 (Not Modified) response status code indicates that the 737 stored response can be updated and reused; see Section 2.4.1. 739 o A full response (i.e., one with a response body) indicates that 740 none of the stored responses nominated in the conditional request 741 is suitable. Instead, the cache can use the full response to 742 satisfy the request and MAY replace the stored response(s). 744 o However, if a cache receives a 5xx response while attempting to 745 validate a response, it can either forward this response to the 746 requesting client, or act as if the server failed to respond. In 747 the latter case, it can return a previously stored response (see 748 Section 2.3.3). 750 2.4.1. Freshening Responses 752 When a cache receives a 304 (Not Modified) response and already has 753 one or more stored 200 (OK) responses for the same cache key, the 754 cache needs to identify which of the stored responses are updated by 755 this new response and then update the stored response(s) with the new 756 information provided in the 304 response. 758 o If the new response contains a strong validator, then that strong 759 validator identifies the selected representation. All of the 760 stored responses with the same strong validator are selected. If 761 none of the stored responses contain the same strong validator, 762 then this new response corresponds to a new selected 763 representation and MUST NOT update the existing stored responses. 765 o If the new response contains a weak validator and that validator 766 corresponds to one of the cache's stored responses, then the most 767 recent of those matching stored responses is selected. 769 o If the new response does not include any form of validator, there 770 is only one stored response, and that stored response also lacks a 771 validator, then that stored response is selected. 773 If a stored response is selected for update, the cache MUST: 775 o delete any Warning header fields in the stored response with warn- 776 code 1xx (see Section 3.6); 778 o retain any Warning header fields in the stored response with warn- 779 code 2xx; and, 781 o use other header fields provided in the 304 response to replace 782 all instances of the corresponding header fields in the stored 783 response. 785 2.5. Request Methods that Invalidate 787 Because unsafe request methods (Section 6.1.1 of [Part2]) such as 788 PUT, POST or DELETE have the potential for changing state on the 789 origin server, intervening caches can use them to keep their contents 790 up-to-date. 792 A cache MUST invalidate the effective Request URI (Section 4.3 of 793 [Part1]) as well as the URI(s) in the Location and Content-Location 794 header fields (if present) when a non-error response to a request 795 with an unsafe method is received. 797 However, a cache MUST NOT invalidate a URI from a Location or 798 Content-Location header field if the host part of that URI differs 799 from the host part in the effective request URI (Section 4.3 of 800 [Part1]). This helps prevent denial of service attacks. 802 A cache MUST invalidate the effective request URI (Section 4.3 of 803 [Part1]) when it receives a non-error response to a request with a 804 method whose safety is unknown. 806 Here, a "non-error response" is one with a 2xx or 3xx status code. 807 "Invalidate" means that the cache will either remove all stored 808 responses related to the effective request URI, or will mark these as 809 "invalid" and in need of a mandatory validation before they can be 810 returned in response to a subsequent request. 812 Note that this does not guarantee that all appropriate responses are 813 invalidated. For example, the request that caused the change at the 814 origin server might not have gone through the cache where a response 815 is stored. 817 2.6. Shared Caching of Authenticated Responses 819 A shared cache MUST NOT use a cached response to a request with an 820 Authorization header field (Section 4.1 of [Part7]) to satisfy any 821 subsequent request unless a cache directive that allows such 822 responses to be stored is present in the response. 824 In this specification, the following Cache-Control response 825 directives (Section 3.2.2) have such an effect: must-revalidate, 826 public, s-maxage. 828 Note that cached responses that contain the "must-revalidate" and/or 829 "s-maxage" response directives are not allowed to be served stale 830 (Section 2.3.3) by shared caches. In particular, a response with 831 either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to 832 satisfy a subsequent request without revalidating it on the origin 833 server. 835 2.7. Caching Negotiated Responses 837 When a cache receives a request that can be satisfied by a stored 838 response that has a Vary header field (Section 3.5), it MUST NOT use 839 that response unless all of the selecting header fields nominated by 840 the Vary header field match in both the original request (i.e., that 841 associated with the stored response), and the presented request. 843 The selecting header fields from two requests are defined to match if 844 and only if those in the first request can be transformed to those in 845 the second request by applying any of the following: 847 o adding or removing whitespace, where allowed in the header field's 848 syntax 850 o combining multiple header fields with the same field name (see 851 Section 3.2 of [Part1]) 853 o normalizing both header field values in a way that is known to 854 have identical semantics, according to the header field's 855 specification (e.g., re-ordering field values when order is not 856 significant; case-normalization, where values are defined to be 857 case-insensitive) 859 If (after any normalization that might take place) a header field is 860 absent from a request, it can only match another request if it is 861 also absent there. 863 A Vary header field-value of "*" always fails to match, and 864 subsequent requests to that resource can only be properly interpreted 865 by the origin server. 867 The stored response with matching selecting header fields is known as 868 the selected response. 870 If multiple selected responses are available, the most recent 871 response (as determined by the Date header field) is used; see 872 Section 2.2. 874 If no selected response is available, the cache can forward the 875 presented request to the origin server in a conditional request; see 876 Section 2.4. 878 2.8. Combining Partial Content 880 A response might transfer only a partial representation if the 881 connection closed prematurely or if the request used one or more 882 Range specifiers ([Part5]). After several such transfers, a cache 883 might have received several ranges of the same representation. A 884 cache MAY combine these ranges into a single stored response, and 885 reuse that response to satisfy later requests, if they all share the 886 same strong validator and the cache complies with the client 887 requirements in Section 4 of [Part5]. 889 When combining the new response with one or more stored responses, a 890 cache MUST: 892 o delete any Warning header fields in the stored response with warn- 893 code 1xx (see Section 3.6); 895 o retain any Warning header fields in the stored response with warn- 896 code 2xx; and, 898 o use other header fields provided in the new response, aside from 899 Content-Range, to replace all instances of the corresponding 900 header fields in the stored response. 902 3. Header Field Definitions 904 This section defines the syntax and semantics of HTTP/1.1 header 905 fields related to caching. 907 3.1. Age 909 The "Age" header field conveys the sender's estimate of the amount of 910 time since the response was generated or successfully validated at 911 the origin server. Age values are calculated as specified in 912 Section 2.3.2. 914 Age = delta-seconds 916 Age field-values are non-negative integers, representing time in 917 seconds (see Section 1.5). 919 The presence of an Age header field in a response implies that a 920 response is not first-hand. However, the converse is not true, since 921 HTTP/1.0 caches might not implement the Age header field. 923 3.2. Cache-Control 925 The "Cache-Control" header field is used to specify directives for 926 caches along the request/response chain. Such cache directives are 927 unidirectional in that the presence of a directive in a request does 928 not imply that the same directive is to be given in the response. 930 A cache MUST obey the requirements of the Cache-Control directives 931 defined in this section. See Section 3.2.3 for information about how 932 Cache-Control directives defined elsewhere are handled. 934 Note: HTTP/1.0 caches might not implement Cache-Control and might 935 only implement Pragma: no-cache (see Section 3.4). 937 A proxy, whether or not it implements a cache, MUST pass cache 938 directives through in forwarded messages, regardless of their 939 significance to that application, since the directives might be 940 applicable to all recipients along the request/response chain. It is 941 not possible to target a directive to a specific cache. 943 Cache directives are identified by a token, to be compared case- 944 insensitively, and have an optional argument. 946 Cache-Control = 1#cache-directive 948 cache-directive = cache-request-directive 949 / cache-response-directive 951 cache-extension = token [ "=" ( token / quoted-string ) ] 953 3.2.1. Request Cache-Control Directives 955 cache-request-directive = 956 "no-cache" 957 / "no-store" 958 / "max-age" "=" delta-seconds 959 / "max-stale" [ "=" delta-seconds ] 960 / "min-fresh" "=" delta-seconds 961 / "no-transform" 962 / "only-if-cached" 963 / cache-extension 965 no-cache 967 The no-cache request directive indicates that a cache MUST NOT use 968 a stored response to satisfy the request without successful 969 validation on the origin server. 971 no-store 973 The no-store request directive indicates that a cache MUST NOT 974 store any part of either this request or any response to it. This 975 directive applies to both private and shared caches. "MUST NOT 976 store" in this context means that the cache MUST NOT intentionally 977 store the information in non-volatile storage, and MUST make a 978 best-effort attempt to remove the information from volatile 979 storage as promptly as possible after forwarding it. 981 This directive is NOT a reliable or sufficient mechanism for 982 ensuring privacy. In particular, malicious or compromised caches 983 might not recognize or obey this directive, and communications 984 networks might be vulnerable to eavesdropping. 986 Note that if a request containing this directive is satisfied from 987 a cache, the no-store request directive does not apply to the 988 already stored response. 990 max-age 992 The max-age request directive indicates that the client is 993 unwilling to accept a response whose age is greater than the 994 specified number of seconds. Unless the max-stale request 995 directive is also present, the client is not willing to accept a 996 stale response. 998 max-stale 1000 The max-stale request directive indicates that the client is 1001 willing to accept a response that has exceeded its expiration 1002 time. If max-stale is assigned a value, then the client is 1003 willing to accept a response that has exceeded its expiration time 1004 by no more than the specified number of seconds. If no value is 1005 assigned to max-stale, then the client is willing to accept a 1006 stale response of any age. 1008 min-fresh 1010 The min-fresh request directive indicates that the client is 1011 willing to accept a response whose freshness lifetime is no less 1012 than its current age plus the specified time in seconds. That is, 1013 the client wants a response that will still be fresh for at least 1014 the specified number of seconds. 1016 no-transform 1018 The no-transform request directive indicates that an intermediary 1019 (whether or not it implements a cache) MUST NOT change the 1020 Content-Encoding, Content-Range or Content-Type request header 1021 fields, nor the request representation. 1023 only-if-cached 1025 The only-if-cached request directive indicates that the client 1026 only wishes to obtain a stored response. If it receives this 1027 directive, a cache SHOULD either respond using a stored response 1028 that is consistent with the other constraints of the request, or 1029 respond with a 504 (Gateway Timeout) status code. If a group of 1030 caches is being operated as a unified system with good internal 1031 connectivity, a member cache MAY forward such a request within 1032 that group of caches. 1034 3.2.2. Response Cache-Control Directives 1036 cache-response-directive = 1037 "public" 1038 / "private" [ "=" DQUOTE 1#field-name DQUOTE ] 1039 / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] 1040 / "no-store" 1041 / "no-transform" 1042 / "must-revalidate" 1043 / "proxy-revalidate" 1044 / "max-age" "=" delta-seconds 1045 / "s-maxage" "=" delta-seconds 1046 / cache-extension 1048 public 1050 The public response directive indicates that a response whose 1051 associated request contains an 'Authentication' header MAY be 1052 stored (see Section 2.6). 1054 private 1056 The private response directive indicates that the response message 1057 is intended for a single user and MUST NOT be stored by a shared 1058 cache. A private cache MAY store the response. 1060 If the private response directive specifies one or more field- 1061 names, this requirement is limited to the field-values associated 1062 with the listed response header fields. That is, a shared cache 1063 MUST NOT store the specified field-names(s), whereas it MAY store 1064 the remainder of the response message. 1066 Note: This usage of the word private only controls where the 1067 response can be stored; it cannot ensure the privacy of the 1068 message content. Also, private response directives with field- 1069 names are often handled by implementations as if an unqualified 1070 private directive was received; i.e., the special handling for the 1071 qualified form is not widely implemented. 1073 no-cache 1075 The no-cache response directive indicates that the response MUST 1076 NOT be used to satisfy a subsequent request without successful 1077 validation on the origin server. This allows an origin server to 1078 prevent a cache from using it to satisfy a request without 1079 contacting it, even by caches that have been configured to return 1080 stale responses. 1082 If the no-cache response directive specifies one or more field- 1083 names, this requirement is limited to the field-values associated 1084 with the listed response header fields. That is, a cache MUST NOT 1085 send the specified field-name(s) in the response to a subsequent 1086 request without successful validation on the origin server. This 1087 allows an origin server to prevent the re-use of certain header 1088 fields in a response, while still allowing caching of the rest of 1089 the response. 1091 Note: Most HTTP/1.0 caches will not recognize or obey this 1092 directive. Also, no-cache response directives with field-names 1093 are often handled by implementations as if an unqualified no-cache 1094 directive was received; i.e., the special handling for the 1095 qualified form is not widely implemented. 1097 no-store 1099 The no-store response directive indicates that a cache MUST NOT 1100 store any part of either the immediate request or response. This 1101 directive applies to both private and shared caches. "MUST NOT 1102 store" in this context means that the cache MUST NOT intentionally 1103 store the information in non-volatile storage, and MUST make a 1104 best-effort attempt to remove the information from volatile 1105 storage as promptly as possible after forwarding it. 1107 This directive is NOT a reliable or sufficient mechanism for 1108 ensuring privacy. In particular, malicious or compromised caches 1109 might not recognize or obey this directive, and communications 1110 networks might be vulnerable to eavesdropping. 1112 must-revalidate 1114 The must-revalidate response directive indicates that once it has 1115 become stale, a cache MUST NOT use the response to satisfy 1116 subsequent requests without successful validation on the origin 1117 server. 1119 The must-revalidate directive is necessary to support reliable 1120 operation for certain protocol features. In all circumstances a 1121 cache MUST obey the must-revalidate directive; in particular, if a 1122 cache cannot reach the origin server for any reason, it MUST 1123 generate a 504 (Gateway Timeout) response. 1125 The must-revalidate directive ought to be used by servers if and 1126 only if failure to validate a request on the representation could 1127 result in incorrect operation, such as a silently unexecuted 1128 financial transaction. 1130 proxy-revalidate 1132 The proxy-revalidate response directive has the same meaning as 1133 the must-revalidate response directive, except that it does not 1134 apply to private caches. 1136 max-age 1138 The max-age response directive indicates that the response is to 1139 be considered stale after its age is greater than the specified 1140 number of seconds. 1142 s-maxage 1144 The s-maxage response directive indicates that, in shared caches, 1145 the maximum age specified by this directive overrides the maximum 1146 age specified by either the max-age directive or the Expires 1147 header field. The s-maxage directive also implies the semantics 1148 of the proxy-revalidate response directive. 1150 no-transform 1152 The no-transform response directive indicates that an intermediary 1153 (regardless of whether it implements a cache) MUST NOT change the 1154 Content-Encoding, Content-Range or Content-Type response header 1155 fields, nor the response representation. 1157 3.2.3. Cache Control Extensions 1159 The Cache-Control header field can be extended through the use of one 1160 or more cache-extension tokens, each with an optional value. 1161 Informational extensions (those that do not require a change in cache 1162 behavior) can be added without changing the semantics of other 1163 directives. Behavioral extensions are designed to work by acting as 1164 modifiers to the existing base of cache directives. Both the new 1165 directive and the standard directive are supplied, such that 1166 applications that do not understand the new directive will default to 1167 the behavior specified by the standard directive, and those that 1168 understand the new directive will recognize it as modifying the 1169 requirements associated with the standard directive. In this way, 1170 extensions to the cache-control directives can be made without 1171 requiring changes to the base protocol. 1173 This extension mechanism depends on an HTTP cache obeying all of the 1174 cache-control directives defined for its native HTTP-version, obeying 1175 certain extensions, and ignoring all directives that it does not 1176 understand. 1178 For example, consider a hypothetical new response directive called 1179 "community" that acts as a modifier to the private directive. We 1180 define this new directive to mean that, in addition to any private 1181 cache, any cache that is shared only by members of the community 1182 named within its value may cache the response. An origin server 1183 wishing to allow the UCI community to use an otherwise private 1184 response in their shared cache(s) could do so by including 1186 Cache-Control: private, community="UCI" 1188 A cache seeing this header field will act correctly even if the cache 1189 does not understand the community cache-extension, since it will also 1190 see and understand the private directive and thus default to the safe 1191 behavior. 1193 A cache MUST ignore unrecognized cache directives; it is assumed that 1194 any cache directive likely to be unrecognized by an HTTP/1.1 cache 1195 will be combined with standard directives (or the response's default 1196 cacheability) such that the cache behavior will remain minimally 1197 correct even if the cache does not understand the extension(s). 1199 The HTTP Cache Directive Registry defines the name space for the 1200 cache directives. 1202 A registration MUST include the following fields: 1204 o Cache Directive Name 1206 o Pointer to specification text 1208 Values to be added to this name space are subject to IETF review 1209 ([RFC5226], Section 4.1). 1211 The registry itself is maintained at 1212 . 1214 3.3. Expires 1216 The "Expires" header field gives the date/time after which the 1217 response is considered stale. See Section 2.3 for further discussion 1218 of the freshness model. 1220 The presence of an Expires field does not imply that the original 1221 resource will change or cease to exist at, before, or after that 1222 time. 1224 The field-value is an absolute date and time as defined by HTTP-date 1225 in Section 8 of [Part2]; a sender MUST use the rfc1123-date format. 1227 Expires = HTTP-date 1229 For example 1231 Expires: Thu, 01 Dec 1994 16:00:00 GMT 1233 A cache MUST treat other invalid date formats, especially including 1234 the value "0", as in the past (i.e., "already expired"). 1236 Note: If a response includes a Cache-Control field with the max- 1237 age directive (see Section 3.2.2), that directive overrides the 1238 Expires field. Likewise, the s-maxage directive overrides Expires 1239 in shared caches. 1241 Historically, HTTP required the Expires field-value to be no more 1242 than a year in the future. While longer freshness lifetimes are no 1243 longer prohibited, extremely large values have been demonstrated to 1244 cause problems (e.g., clock overflows due to use of 32-bit integers 1245 for time values), and most caches will evict a response far sooner 1246 than that. Therefore, senders ought not produce them. 1248 An origin server without a clock MUST NOT assign Expires values to a 1249 response unless these values were associated with the resource by a 1250 system or user with a reliable clock. It MAY assign an Expires value 1251 that is known, at or before server configuration time, to be in the 1252 past (this allows "pre-expiration" of responses without storing 1253 separate Expires values for each resource). 1255 3.4. Pragma 1257 The "Pragma" header field allows backwards compatibility with 1258 HTTP/1.0 caches, so that clients can specify a "no-cache" request 1259 that they will understand (as Cache-Control was not defined until 1260 HTTP/1.1). When the Cache-Control header is also present and 1261 understood in a request, Pragma is ignored. 1263 In HTTP/1.0, Pragma was defined as an extensible field for 1264 implementation-specified directives for recipients. This 1265 specification deprecates such extensions to improve interoperability. 1267 Pragma = 1#pragma-directive 1268 pragma-directive = "no-cache" / extension-pragma 1269 extension-pragma = token [ "=" ( token / quoted-string ) ] 1271 When the Cache-Control header is not present in a request, the no- 1272 cache request pragma-directive MUST have the same effect on caches as 1273 if "Cache-Control: no-cache" were present (see Section 3.2.1). 1275 When sending a no-cache request, a client ought to include both the 1276 pragma and cache-control directives, unless Cache-Control: no-cache 1277 is purposefully omitted to target other Cache-Control response 1278 directives at HTTP/1.1 caches. For example: 1280 GET / HTTP/1.1 1281 Host: www.example.com 1282 Cache-Control: max-age=30 1283 Pragma: no-cache 1285 will constrain HTTP/1.1 caches to serve a response no older than 30 1286 seconds, while precluding implementations that do not understand 1287 Cache-Control from serving a cached response. 1289 Note: Because the meaning of "Pragma: no-cache" in responses is 1290 not specified, it does not provide a reliable replacement for 1291 "Cache-Control: no-cache" in them. 1293 3.5. Vary 1295 The "Vary" header field conveys the set of header fields that were 1296 used to select the representation. 1298 Caches use this information, in part, to determine whether a stored 1299 response can be used to satisfy a given request; see Section 2.7. 1300 determines, while the response is fresh, whether a cache is permitted 1301 to use the response to reply to a subsequent request without 1302 validation; see Section 2.7. 1304 In uncacheable or stale responses, the Vary field value advises the 1305 user agent about the criteria that were used to select the 1306 representation. 1308 Vary = "*" / 1#field-name 1310 The set of header fields named by the Vary field value is known as 1311 the selecting header fields. 1313 A server SHOULD include a Vary header field with any cacheable 1314 response that is subject to server-driven negotiation. Doing so 1315 allows a cache to properly interpret future requests on that resource 1316 and informs the user agent about the presence of negotiation on that 1317 resource. A server MAY include a Vary header field with a non- 1318 cacheable response that is subject to server-driven negotiation, 1319 since this might provide the user agent with useful information about 1320 the dimensions over which the response varies at the time of the 1321 response. 1323 A Vary field value of "*" signals that unspecified parameters not 1324 limited to the header fields (e.g., the network address of the 1325 client), play a role in the selection of the response representation; 1326 therefore, a cache cannot determine whether this response is 1327 appropriate. A proxy MUST NOT generate the "*" value. 1329 The field-names given are not limited to the set of standard header 1330 fields defined by this specification. Field names are case- 1331 insensitive. 1333 3.6. Warning 1335 The "Warning" header field is used to carry additional information 1336 about the status or transformation of a message that might not be 1337 reflected in the message. This information is typically used to warn 1338 about possible incorrectness introduced by caching operations or 1339 transformations applied to the payload of the message. 1341 Warnings can be used for other purposes, both cache-related and 1342 otherwise. The use of a warning, rather than an error status code, 1343 distinguishes these responses from true failures. 1345 Warning header fields can in general be applied to any message, 1346 however some warn-codes are specific to caches and can only be 1347 applied to response messages. 1349 Warning = 1#warning-value 1351 warning-value = warn-code SP warn-agent SP warn-text 1352 [SP warn-date] 1354 warn-code = 3DIGIT 1355 warn-agent = ( uri-host [ ":" port ] ) / pseudonym 1356 ; the name or pseudonym of the server adding 1357 ; the Warning header field, for use in debugging 1358 warn-text = quoted-string 1359 warn-date = DQUOTE HTTP-date DQUOTE 1361 Multiple warnings can be attached to a response (either by the origin 1362 server or by a cache), including multiple warnings with the same code 1363 number, only differing in warn-text. 1365 When this occurs, the user agent SHOULD inform the user of as many of 1366 them as possible, in the order that they appear in the response. 1368 Systems that generate multiple Warning header fields are encouraged 1369 to order them with this user agent behavior in mind. New Warning 1370 header fields are added after any existing Warning headers fields. 1372 Warnings are assigned three digit warn-codes. The first digit 1373 indicates whether the Warning is required to be deleted from a stored 1374 response after validation: 1376 o 1xx Warnings describe the freshness or validation status of the 1377 response, and so MUST be deleted by a cache after validation. 1378 They can only be generated by a cache when validating a cached 1379 entry, and MUST NOT be generated in any other situation. 1381 o 2xx Warnings describe some aspect of the representation that is 1382 not rectified by a validation (for example, a lossy compression of 1383 the representation) and MUST NOT be deleted by a cache after 1384 validation, unless a full response is returned, in which case they 1385 MUST be. 1387 If an implementation sends a message with one or more Warning header 1388 fields to a receiver whose version is HTTP/1.0 or lower, then the 1389 sender MUST include in each warning-value a warn-date that matches 1390 the Date header field in the message. 1392 If a system receives a message with a warning-value that includes a 1393 warn-date, and that warn-date is different from the Date value in the 1394 response, then that warning-value MUST be deleted from the message 1395 before storing, forwarding, or using it. (preventing the consequences 1396 of naive caching of Warning header fields.) If all of the warning- 1397 values are deleted for this reason, the Warning header field MUST be 1398 deleted as well. 1400 The following warn-codes are defined by this specification, each with 1401 a recommended warn-text in English, and a description of its meaning. 1403 110 Response is stale 1405 A cache SHOULD include this whenever the returned response is 1406 stale. 1408 111 Revalidation failed 1410 A cache SHOULD include this when returning a stale response 1411 because an attempt to validate the response failed, due to an 1412 inability to reach the server. 1414 112 Disconnected operation 1416 A cache SHOULD include this if it is intentionally disconnected 1417 from the rest of the network for a period of time. 1419 113 Heuristic expiration 1421 A cache SHOULD include this if it heuristically chose a freshness 1422 lifetime greater than 24 hours and the response's age is greater 1423 than 24 hours. 1425 199 Miscellaneous warning 1427 The warning text can include arbitrary information to be presented 1428 to a human user, or logged. A system receiving this warning MUST 1429 NOT take any automated action, besides presenting the warning to 1430 the user. 1432 214 Transformation applied 1434 MUST be added by a proxy if it applies any transformation to the 1435 representation, such as changing the content-coding, media-type, 1436 or modifying the representation data, unless this Warning code 1437 already appears in the response. 1439 299 Miscellaneous persistent warning 1441 The warning text can include arbitrary information to be presented 1442 to a human user, or logged. A system receiving this warning MUST 1443 NOT take any automated action. 1445 4. History Lists 1447 User agents often have history mechanisms, such as "Back" buttons and 1448 history lists, that can be used to redisplay a representation 1449 retrieved earlier in a session. 1451 The freshness model (Section 2.3) does not necessarily apply to 1452 history mechanisms. I.e., a history mechanism can display a previous 1453 representation even if it has expired. 1455 This does not prohibit the history mechanism from telling the user 1456 that a view might be stale, or from honoring cache directives (e.g., 1457 Cache-Control: no-store). 1459 5. IANA Considerations 1461 5.1. Cache Directive Registry 1463 The registration procedure for HTTP Cache Directives is defined by 1464 Section 3.2.3 of this document. 1466 The HTTP Cache Directive Registry shall be created at 1467 and be 1468 populated with the registrations below: 1470 +------------------------+------------------------------+ 1471 | Cache Directive | Reference | 1472 +------------------------+------------------------------+ 1473 | max-age | Section 3.2.1, Section 3.2.2 | 1474 | max-stale | Section 3.2.1 | 1475 | min-fresh | Section 3.2.1 | 1476 | must-revalidate | Section 3.2.2 | 1477 | no-cache | Section 3.2.1, Section 3.2.2 | 1478 | no-store | Section 3.2.1, Section 3.2.2 | 1479 | no-transform | Section 3.2.1, Section 3.2.2 | 1480 | only-if-cached | Section 3.2.1 | 1481 | private | Section 3.2.2 | 1482 | proxy-revalidate | Section 3.2.2 | 1483 | public | Section 3.2.2 | 1484 | s-maxage | Section 3.2.2 | 1485 | stale-if-error | [RFC5861], Section 4 | 1486 | stale-while-revalidate | [RFC5861], Section 3 | 1487 +------------------------+------------------------------+ 1489 5.2. Header Field Registration 1491 The Message Header Field Registry located at shall be 1493 updated with the permanent registrations below (see [RFC3864]): 1495 +-------------------+----------+----------+-------------+ 1496 | Header Field Name | Protocol | Status | Reference | 1497 +-------------------+----------+----------+-------------+ 1498 | Age | http | standard | Section 3.1 | 1499 | Cache-Control | http | standard | Section 3.2 | 1500 | Expires | http | standard | Section 3.3 | 1501 | Pragma | http | standard | Section 3.4 | 1502 | Vary | http | standard | Section 3.5 | 1503 | Warning | http | standard | Section 3.6 | 1504 +-------------------+----------+----------+-------------+ 1506 The change controller is: "IETF (iesg@ietf.org) - Internet 1507 Engineering Task Force". 1509 6. Security Considerations 1511 Caches expose additional potential vulnerabilities, since the 1512 contents of the cache represent an attractive target for malicious 1513 exploitation. Because cache contents persist after an HTTP request 1514 is complete, an attack on the cache can reveal information long after 1515 a user believes that the information has been removed from the 1516 network. Therefore, cache contents need to be protected as sensitive 1517 information. 1519 7. Acknowledgments 1521 See Section 11 of [Part1]. 1523 8. References 1525 8.1. Normative References 1527 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1528 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1529 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, 1530 and Message Parsing", draft-ietf-httpbis-p1-messaging-17 1531 (work in progress), October 2011. 1533 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1534 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1535 and J. Reschke, Ed., "HTTP/1.1, part 2: Message 1536 Semantics", draft-ietf-httpbis-p2-semantics-17 (work in 1537 progress), October 2011. 1539 [Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1540 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1541 and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional 1542 Requests", draft-ietf-httpbis-p4-conditional-17 (work in 1543 progress), October 2011. 1545 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1546 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1547 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and 1548 Partial Responses", draft-ietf-httpbis-p5-range-17 (work 1549 in progress), October 2011. 1551 [Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1552 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1553 and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication", 1554 draft-ietf-httpbis-p7-auth-17 (work in progress), 1555 October 2011. 1557 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1558 Requirement Levels", BCP 14, RFC 2119, March 1997. 1560 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1561 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1563 8.2. Informative References 1565 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 1566 Specification, Implementation", RFC 1305, March 1992. 1568 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1569 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1570 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1572 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1573 Procedures for Message Header Fields", BCP 90, RFC 3864, 1574 September 2004. 1576 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1577 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1578 May 2008. 1580 [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale 1581 Content", RFC 5861, April 2010. 1583 Appendix A. Changes from RFC 2616 1585 Make the specified age calculation algorithm less conservative. 1586 (Section 2.3.2) 1588 Remove requirement to consider Content-Location in successful 1589 responses in order to determine the appropriate response to use. 1590 (Section 2.4) 1592 Clarify denial of service attack avoidance requirement. 1593 (Section 2.5) 1595 Change ABNF productions for header fields to only define the field 1596 value. (Section 3) 1598 Do not mention RFC 2047 encoding and multiple languages in Warning 1599 header fields anymore, as these aspects never were implemented. 1600 (Section 3.6) 1602 Appendix B. Collected ABNF 1604 Age = delta-seconds 1606 Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS 1607 cache-directive ] ) 1609 Expires = HTTP-date 1611 HTTP-date = 1613 OWS = 1615 Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS 1616 pragma-directive ] ) 1618 Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ] 1619 ) ) 1621 Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] 1622 ) 1624 cache-directive = cache-request-directive / cache-response-directive 1625 cache-extension = token [ "=" ( token / quoted-string ) ] 1626 cache-request-directive = "no-cache" / "no-store" / ( "max-age=" 1627 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / ( 1628 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" / 1629 cache-extension 1631 cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( "," 1632 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / ( 1633 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS 1634 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" / 1635 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds 1636 ) / ( "s-maxage=" delta-seconds ) / cache-extension 1638 delta-seconds = 1*DIGIT 1640 extension-pragma = token [ "=" ( token / quoted-string ) ] 1642 field-name = 1644 port = 1645 pragma-directive = "no-cache" / extension-pragma 1646 pseudonym = 1648 quoted-string = 1650 token = 1652 uri-host = 1654 warn-agent = ( uri-host [ ":" port ] ) / pseudonym 1655 warn-code = 3DIGIT 1656 warn-date = DQUOTE HTTP-date DQUOTE 1657 warn-text = quoted-string 1658 warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date 1659 ] 1661 ABNF diagnostics: 1663 ; Age defined but not used 1664 ; Cache-Control defined but not used 1665 ; Expires defined but not used 1666 ; Pragma defined but not used 1667 ; Vary defined but not used 1668 ; Warning defined but not used 1670 Appendix C. Change Log (to be removed by RFC Editor before publication) 1672 C.1. Since RFC 2616 1674 Extracted relevant partitions from [RFC2616]. 1676 C.2. Since draft-ietf-httpbis-p6-cache-00 1678 Closed issues: 1680 o : "Trailer" 1681 () 1683 o : "Invalidation 1684 after Update or Delete" 1685 () 1687 o : "Normative and 1688 Informative references" 1690 o : "Date reference 1691 typo" 1693 o : "Connection 1694 header text" 1696 o : "Informative 1697 references" 1699 o : "ISO-8859-1 1700 Reference" 1702 o : "Normative up- 1703 to-date references" 1705 o : "typo in 1706 13.2.2" 1708 Other changes: 1710 o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress 1711 on ) 1713 C.3. Since draft-ietf-httpbis-p6-cache-01 1715 Closed issues: 1717 o : "rel_path not 1718 used" 1720 Other changes: 1722 o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work 1723 in progress on ) 1725 o Add explicit references to BNF syntax and rules imported from 1726 other parts of the specification. 1728 C.4. Since draft-ietf-httpbis-p6-cache-02 1730 Ongoing work on IANA Message Header Field Registration 1731 (): 1733 o Reference RFC 3984, and update header field registrations for 1734 header fields defined in this document. 1736 C.5. Since draft-ietf-httpbis-p6-cache-03 1738 Closed issues: 1740 o : "Vary header 1741 classification" 1743 C.6. Since draft-ietf-httpbis-p6-cache-04 1745 Ongoing work on ABNF conversion 1746 (): 1748 o Use "/" instead of "|" for alternatives. 1750 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional 1751 whitespace ("OWS") and required whitespace ("RWS"). 1753 o Rewrite ABNFs to spell out whitespace rules, factor out header 1754 field value format definitions. 1756 C.7. Since draft-ietf-httpbis-p6-cache-05 1758 This is a total rewrite of this part of the specification. 1760 Affected issues: 1762 o : "Definition of 1763 1xx Warn-Codes" 1765 o : "Placement of 1766 13.5.1 and 13.5.2" 1768 o : "The role of 1769 Warning and Semantic Transparency in Caching" 1771 o : "Methods and 1772 Caching" 1774 In addition: Final work on ABNF conversion 1775 (): 1777 o Add appendix containing collected and expanded ABNF, reorganize 1778 ABNF introduction. 1780 C.8. Since draft-ietf-httpbis-p6-cache-06 1782 Closed issues: 1784 o : "base for 1785 numeric protocol elements" 1787 Affected issues: 1789 o : "Vary and non- 1790 existant headers" 1792 C.9. Since draft-ietf-httpbis-p6-cache-07 1794 Closed issues: 1796 o : "Definition of 1797 1xx Warn-Codes" 1799 o : "Content- 1800 Location on 304 responses" 1802 o : "private and 1803 no-cache CC directives with headers" 1805 o : "RFC2047 and 1806 warn-text" 1808 C.10. Since draft-ietf-httpbis-p6-cache-08 1810 Closed issues: 1812 o : "serving 1813 negotiated responses from cache: header-specific canonicalization" 1815 o : "Effect of CC 1816 directives on history lists" 1818 o : "Cache 1819 Extensions can override no-store, etc." 1821 Affected issues: 1823 o : Status codes 1824 and caching 1826 Partly resolved issues: 1828 o : "Placement of 1829 13.5.1 and 13.5.2" 1831 C.11. Since draft-ietf-httpbis-p6-cache-09 1833 Closed issues: 1835 o : "Age 1836 calculation" 1838 o : "Clarify 1839 differences between / requirements for request and response CC 1840 directives" 1842 o : "Caching 1843 authenticated responses" 1845 o : "IANA registry 1846 for cache-control directives" 1848 o : "Heuristic 1849 caching of URLs with query components" 1851 Partly resolved issues: 1853 o : "Term for the 1854 requested resource's URI" 1856 C.12. Since draft-ietf-httpbis-p6-cache-10 1858 Closed issues: 1860 o : "Clarify 1861 entity / representation / variant terminology" 1863 o : "consider 1864 removing the 'changes from 2068' sections" 1866 o : "Allowing 1867 heuristic caching for new status codes" 1869 o Clean up TODOs and prose in "Combining Responses." 1871 C.13. Since draft-ietf-httpbis-p6-cache-11 1873 Closed issues: 1875 o : "Text about 1876 clock requirement for caches belongs in p6" 1878 C.14. Since draft-ietf-httpbis-p6-cache-12 1880 Closed issues: 1882 o : "Header 1883 Classification" 1885 o : "Clarify 1886 'public'" 1888 C.15. Since draft-ietf-httpbis-p6-cache-13 1890 Closed issues: 1892 o : "untangle 1893 ABNFs for header fields" 1895 C.16. Since draft-ietf-httpbis-p6-cache-14 1897 Closed issues: 1899 o : "Mismatch Vary" 1901 o : "Cache 1902 Invalidation only happens upon successful responses" 1904 o : "Recommend 1905 minimum sizes for protocol elements" 1907 o : "Proxies don't 1908 'understand' methods" 1910 o : "Cache 1911 Extensions can override no-store, etc." 1913 o : "Pragma" 1915 C.17. Since draft-ietf-httpbis-p6-cache-15 1917 Closed issues: 1919 o : "Motivate one- 1920 year limit for Expires" 1922 C.18. Since draft-ietf-httpbis-p6-cache-16 1924 Closed issues: 1926 o : "Document 1927 HTTP's error-handling philosophy" 1929 o : "Cache-Control 1930 directive case sensitivity" 1932 Index 1934 A 1935 age 6 1936 Age header field 20 1938 C 1939 cache 5 1940 Cache Directives 1941 max-age 22, 25 1942 max-stale 22 1943 min-fresh 22 1944 must-revalidate 25 1945 no-cache 21, 24 1946 no-store 22, 24 1947 no-transform 23, 25 1948 only-if-cached 23 1949 private 23 1950 proxy-revalidate 25 1951 public 23 1952 s-maxage 25 1953 cache entry 8 1954 cache key 8 1955 Cache-Control header field 21 1956 cacheable 5 1958 E 1959 Expires header field 27 1960 explicit expiration time 6 1962 F 1963 first-hand 6 1964 fresh 6 1965 freshness lifetime 6 1967 G 1968 Grammar 1969 Age 20 1970 Cache-Control 21 1971 cache-extension 21 1972 cache-request-directive 21 1973 cache-response-directive 23 1974 delta-seconds 8 1975 Expires 27 1976 extension-pragma 28 1977 Pragma 28 1978 pragma-directive 28 1979 Vary 29 1980 warn-agent 30 1981 warn-code 30 1982 warn-date 30 1983 warn-text 30 1984 Warning 30 1985 warning-value 30 1987 H 1988 Header Fields 1989 Age 20 1990 Cache-Control 21 1991 Expires 27 1992 Pragma 28 1993 Vary 28 1994 Warning 29 1995 heuristic expiration time 6 1997 M 1998 max-age 1999 Cache Directive 22, 25 2000 max-stale 2001 Cache Directive 22 2002 min-fresh 2003 Cache Directive 22 2004 must-revalidate 2005 Cache Directive 25 2007 N 2008 no-cache 2009 Cache Directive 21, 24 2010 no-store 2011 Cache Directive 22, 24 2012 no-transform 2013 Cache Directive 23, 25 2015 O 2016 only-if-cached 2017 Cache Directive 23 2019 P 2020 Pragma header field 28 2021 private 2022 Cache Directive 23 2023 private cache 5 2024 proxy-revalidate 2025 Cache Directive 25 2026 public 2027 Cache Directive 23 2029 S 2030 s-maxage 2031 Cache Directive 25 2032 shared cache 5 2033 stale 6 2034 strong validator 7 2036 V 2037 validator 6 2038 strong 7 2039 Vary header field 28 2041 W 2042 Warning header field 29 2044 Authors' Addresses 2046 Roy T. Fielding (editor) 2047 Adobe Systems Incorporated 2048 345 Park Ave 2049 San Jose, CA 95110 2050 USA 2052 EMail: fielding@gbiv.com 2053 URI: http://roy.gbiv.com/ 2054 Jim Gettys 2055 Alcatel-Lucent Bell Labs 2056 21 Oak Knoll Road 2057 Carlisle, MA 01741 2058 USA 2060 EMail: jg@freedesktop.org 2061 URI: http://gettys.wordpress.com/ 2063 Jeffrey C. Mogul 2064 Hewlett-Packard Company 2065 HP Labs, Large Scale Systems Group 2066 1501 Page Mill Road, MS 1177 2067 Palo Alto, CA 94304 2068 USA 2070 EMail: JeffMogul@acm.org 2072 Henrik Frystyk Nielsen 2073 Microsoft Corporation 2074 1 Microsoft Way 2075 Redmond, WA 98052 2076 USA 2078 EMail: henrikn@microsoft.com 2080 Larry Masinter 2081 Adobe Systems Incorporated 2082 345 Park Ave 2083 San Jose, CA 95110 2084 USA 2086 EMail: LMM@acm.org 2087 URI: http://larry.masinter.net/ 2089 Paul J. Leach 2090 Microsoft Corporation 2091 1 Microsoft Way 2092 Redmond, WA 98052 2094 EMail: paulle@microsoft.com 2095 Tim Berners-Lee 2096 World Wide Web Consortium 2097 MIT Computer Science and Artificial Intelligence Laboratory 2098 The Stata Center, Building 32 2099 32 Vassar Street 2100 Cambridge, MA 02139 2101 USA 2103 EMail: timbl@w3.org 2104 URI: http://www.w3.org/People/Berners-Lee/ 2106 Yves Lafon (editor) 2107 World Wide Web Consortium 2108 W3C / ERCIM 2109 2004, rte des Lucioles 2110 Sophia-Antipolis, AM 06902 2111 France 2113 EMail: ylafon@w3.org 2114 URI: http://www.raubacapeu.net/people/yves/ 2116 Mark Nottingham (editor) 2117 Rackspace 2119 EMail: mnot@mnot.net 2120 URI: http://www.mnot.net/ 2122 Julian F. Reschke (editor) 2123 greenbytes GmbH 2124 Hafenweg 16 2125 Muenster, NW 48155 2126 Germany 2128 Phone: +49 251 2807760 2129 Fax: +49 251 2807761 2130 EMail: julian.reschke@greenbytes.de 2131 URI: http://greenbytes.de/tech/webdav/