idnits 2.17.1 draft-ietf-httpbis-p6-cache-19.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document seems to contain a disclaimer for pre-RFC5378 work, and may have content which was first submitted before 10 November 2008. The disclaimer is necessary when there are original authors that you have been unable to contact, or if some do not wish to grant the BCP78 rights to the IETF Trust. If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (March 12, 2012) is 4428 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p1-messaging-19 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p2-semantics-19 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p4-conditional-19 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p5-range-19 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p7-auth-19 -- Obsolete informational reference (is this intentional?): RFC 1305 (Obsoleted by RFC 5905) -- Obsolete informational reference (is this intentional?): RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) -- Obsolete informational reference (is this intentional?): RFC 5226 (Obsoleted by RFC 8126) Summary: 0 errors (**), 0 flaws (~~), 6 warnings (==), 5 comments (--). 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) Y. Lafon, Ed. 5 Intended status: Standards Track W3C 6 Expires: September 13, 2012 M. Nottingham, Ed. 7 Rackspace 8 J. Reschke, Ed. 9 greenbytes 10 March 12, 2012 12 HTTP/1.1, part 6: Caching 13 draft-ietf-httpbis-p6-cache-19 15 Abstract 17 The Hypertext Transfer Protocol (HTTP) is an application-level 18 protocol for distributed, collaborative, hypertext information 19 systems. HTTP has been in use by the World Wide Web global 20 information initiative since 1990. This document is Part 6 of the 21 seven-part specification that defines the protocol referred to as 22 "HTTP/1.1" and, taken together, obsoletes RFC 2616. 24 Part 6 defines requirements on HTTP caches and the associated header 25 fields that control cache behavior or indicate cacheable response 26 messages. 28 Editorial Note (To be removed by RFC Editor) 30 Discussion of this draft should take place on the HTTPBIS working 31 group mailing list (ietf-http-wg@w3.org), which is archived at 32 . 34 The current issues list is at 35 and related 36 documents (including fancy diffs) can be found at 37 . 39 The changes in this draft are summarized in Appendix C.20. 41 Status of This Memo 43 This Internet-Draft is submitted in full conformance with the 44 provisions of BCP 78 and BCP 79. 46 Internet-Drafts are working documents of the Internet Engineering 47 Task Force (IETF). Note that other groups may also distribute 48 working documents as Internet-Drafts. The list of current Internet- 49 Drafts is at http://datatracker.ietf.org/drafts/current/. 51 Internet-Drafts are draft documents valid for a maximum of six months 52 and may be updated, replaced, or obsoleted by other documents at any 53 time. It is inappropriate to use Internet-Drafts as reference 54 material or to cite them other than as "work in progress." 56 This Internet-Draft will expire on September 13, 2012. 58 Copyright Notice 60 Copyright (c) 2012 IETF Trust and the persons identified as the 61 document authors. All rights reserved. 63 This document is subject to BCP 78 and the IETF Trust's Legal 64 Provisions Relating to IETF Documents 65 (http://trustee.ietf.org/license-info) in effect on the date of 66 publication of this document. Please review these documents 67 carefully, as they describe your rights and restrictions with respect 68 to this document. Code Components extracted from this document must 69 include Simplified BSD License text as described in Section 4.e of 70 the Trust Legal Provisions and are provided without warranty as 71 described in the Simplified BSD License. 73 This document may contain material from IETF Documents or IETF 74 Contributions published or made publicly available before November 75 10, 2008. The person(s) controlling the copyright in some of this 76 material may not have granted the IETF Trust the right to allow 77 modifications of such material outside the IETF Standards Process. 78 Without obtaining an adequate license from the person(s) controlling 79 the copyright in such materials, this document may not be modified 80 outside the IETF Standards Process, and derivative works of it may 81 not be created outside the IETF Standards Process, except to format 82 it for publication as an RFC or to translate it into languages other 83 than English. 85 Table of Contents 87 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 88 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 89 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 90 1.3. Conformance and Error Handling . . . . . . . . . . . . . . 7 91 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7 92 1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 8 93 1.4.2. ABNF Rules defined in other Parts of the 94 Specification . . . . . . . . . . . . . . . . . . . . 8 95 1.5. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 8 96 2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 8 97 2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 9 98 2.2. Constructing Responses from Caches . . . . . . . . . . . . 10 99 2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 12 100 2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 13 101 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 14 102 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 16 103 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 16 104 2.4.1. Freshening Responses with 304 Not Modified . . . . . . 17 105 2.5. Updating Caches with HEAD Responses . . . . . . . . . . . 18 106 2.6. Request Methods that Invalidate . . . . . . . . . . . . . 18 107 2.7. Shared Caching of Authenticated Responses . . . . . . . . 19 108 2.8. Caching Negotiated Responses . . . . . . . . . . . . . . . 19 109 2.9. Combining Partial Content . . . . . . . . . . . . . . . . 20 110 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 21 111 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 112 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 21 113 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 22 114 3.2.2. Response Cache-Control Directives . . . . . . . . . . 24 115 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26 116 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 117 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28 118 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 119 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 30 120 3.6.1. 110 Response is Stale . . . . . . . . . . . . . . . . 31 121 3.6.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 31 122 3.6.3. 112 Disconnected Operation . . . . . . . . . . . . . . 31 123 3.6.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 32 124 3.6.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 32 125 3.6.6. 214 Transformation Applied . . . . . . . . . . . . . . 32 126 3.6.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 32 127 3.6.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 32 128 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 32 129 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 130 5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 33 131 5.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 33 132 5.3. Header Field Registration . . . . . . . . . . . . . . . . 34 133 6. Security Considerations . . . . . . . . . . . . . . . . . . . 34 134 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 135 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35 136 8.1. Normative References . . . . . . . . . . . . . . . . . . . 35 137 8.2. Informative References . . . . . . . . . . . . . . . . . . 35 138 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 36 139 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 36 140 Appendix C. Change Log (to be removed by RFC Editor before 141 publication) . . . . . . . . . . . . . . . . . . . . 37 142 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 38 143 C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 38 144 C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 38 145 C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 39 146 C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 39 147 C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 39 148 C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 39 149 C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 40 150 C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 40 151 C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 40 152 C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 41 153 C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 41 154 C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 42 155 C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 42 156 C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 42 157 C.16. Since draft-ietf-httpbis-p6-cache-14 . . . . . . . . . . . 42 158 C.17. Since draft-ietf-httpbis-p6-cache-15 . . . . . . . . . . . 43 159 C.18. Since draft-ietf-httpbis-p6-cache-16 . . . . . . . . . . . 43 160 C.19. Since draft-ietf-httpbis-p6-cache-17 . . . . . . . . . . . 43 161 C.20. Since draft-ietf-httpbis-p6-cache-18 . . . . . . . . . . . 43 162 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 164 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 Augmented Backus-Naur Form (ABNF) 301 notation of [RFC5234] with the list rule extension defined in Section 302 1.2 of [Part1]. Appendix B shows the collected ABNF with the list 303 rule 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 2.8). 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 388 not 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 2.7), 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 2.9. 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 5.5 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 2.8), and 457 o the presented request does not contain the no-cache pragma 458 (Section 3.4), nor the no-cache cache directive (Section 3.2.1), 459 unless the stored response is successfully validated 460 (Section 2.4), and 462 o the stored response does not contain the no-cache cache directive 463 (Section 3.2.2), unless it is successfully validated 464 (Section 2.4), and 466 o the stored response is either: 468 * fresh (see Section 2.3), or 470 * allowed to be served stale (see Section 2.3.3), or 472 * successfully validated (see Section 2.4). 474 Note that any of the requirements listed above can be overridden by a 475 cache-control extension; see Section 3.2.3. 477 When a stored response is used to satisfy a request without 478 validation, a cache MUST include a single Age header field 479 (Section 3.1) in the response with a value equal to the stored 480 response's current_age; see Section 2.3.2. 482 A cache MUST write through requests with methods that are unsafe 483 (Section 6.1.1 of [Part2]) to the origin server; i.e., a cache must 484 not generate a reply to such a request before having forwarded the 485 request and having received a corresponding response. 487 Also, note that unsafe requests might invalidate already stored 488 responses; see Section 2.6. 490 When more than one suitable response is stored, a cache MUST use the 491 most recent response (as determined by the Date header field). It 492 can also forward a request with "Cache-Control: max-age=0" or "Cache- 493 Control: no-cache" to disambiguate which response to use. 495 A cache that does not have a clock available MUST NOT use stored 496 responses without revalidating them on every use. A cache, 497 especially a shared cache, SHOULD use a mechanism, such as NTP 498 [RFC1305], to synchronize its clock with a reliable external 499 standard. 501 2.3. Freshness Model 503 When a response is "fresh" in the cache, it can be used to satisfy 504 subsequent requests without contacting the origin server, thereby 505 improving efficiency. 507 The primary mechanism for determining freshness is for an origin 508 server to provide an explicit expiration time in the future, using 509 either the Expires header field (Section 3.3) or the max-age response 510 cache directive (Section 3.2.2). Generally, origin servers will 511 assign future explicit expiration times to responses in the belief 512 that the representation is not likely to change in a semantically 513 significant way before the expiration time is reached. 515 If an origin server wishes to force a cache to validate every 516 request, it can assign an explicit expiration time in the past to 517 indicate that the response is already stale. Compliant caches will 518 normally validate the cached response before reusing it for 519 subsequent requests (see Section 2.3.3). 521 Since origin servers do not always provide explicit expiration times, 522 a cache MAY assign a heuristic expiration time when an explicit time 523 is not specified, employing algorithms that use other header field 524 values (such as the Last-Modified time) to estimate a plausible 525 expiration time. This specification does not provide specific 526 algorithms, but does impose worst-case constraints on their results. 528 The calculation to determine if a response is fresh is: 530 response_is_fresh = (freshness_lifetime > current_age) 532 The freshness_lifetime is defined in Section 2.3.1; the current_age 533 is defined in Section 2.3.2. 535 Additionally, clients can influence freshness calculation -- either 536 constraining it relaxing it -- by using the max-age and min-fresh 537 request cache directives. See Section 3.2.1 for details. 539 Note that freshness applies only to cache operation; it cannot be 540 used to force a user agent to refresh its display or reload a 541 resource. See Section 4 for an explanation of the difference between 542 caches and history mechanisms. 544 2.3.1. Calculating Freshness Lifetime 546 A cache can calculate the freshness lifetime (denoted as 547 freshness_lifetime) of a response by using the first match of: 549 o If the cache is shared and the s-maxage response cache directive 550 (Section 3.2.2) is present, use its value, or 552 o If the max-age response cache directive (Section 3.2.2) is 553 present, use its value, or 555 o If the Expires response header field (Section 3.3) is present, use 556 its value minus the value of the Date response header field, or 558 o Otherwise, no explicit expiration time is present in the response. 559 A heuristic freshness lifetime might be applicable; see 560 Section 2.3.1.1. 562 Note that this calculation is not vulnerable to clock skew, since all 563 of the information comes from the origin server. 565 2.3.1.1. Calculating Heuristic Freshness 567 If no explicit expiration time is present in a stored response that 568 has a status code whose definition allows heuristic freshness to be 569 used (including the following in Section 7 of [Part2]: 200, 203, 206, 570 300, 301 and 410), a cache MAY calculate a heuristic expiration time. 571 A cache MUST NOT use heuristics to determine freshness for responses 572 with status codes that do not explicitly allow it. 574 When a heuristic is used to calculate freshness lifetime, a cache 575 SHOULD attach a Warning header field with a 113 warn-code to the 576 response if its current_age is more than 24 hours and such a warning 577 is not already present. 579 Also, if the response has a Last-Modified header field (Section 2.2 580 of [Part4]), caches are encouraged to use a heuristic expiration 581 value that is no more than some fraction of the interval since that 582 time. A typical setting of this fraction might be 10%. 584 Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do 585 not calculate heuristic freshness for URIs with query components 586 (i.e., those containing '?'). In practice, this has not been 587 widely implemented. Therefore, servers are encouraged to send 588 explicit directives (e.g., Cache-Control: no-cache) if they wish 589 to preclude caching. 591 2.3.2. Calculating Age 593 HTTP/1.1 uses the Age header field to convey the estimated age of the 594 response message when obtained from a cache. The Age field value is 595 the cache's estimate of the amount of time since the response was 596 generated or validated by the origin server. In essence, the Age 597 value is the sum of the time that the response has been resident in 598 each of the caches along the path from the origin server, plus the 599 amount of time it has been in transit along network paths. 601 The following data is used for the age calculation: 603 age_value 605 The term "age_value" denotes the value of the Age header field 606 (Section 3.1), in a form appropriate for arithmetic operation; or 607 0, if not available. 609 date_value 611 HTTP/1.1 requires origin servers to send a Date header field, if 612 possible, with every response, giving the time at which the 613 response was generated. The term "date_value" denotes the value 614 of the Date header field, in a form appropriate for arithmetic 615 operations. See Section 10.2 of [Part2] for the definition of the 616 Date header field, and for requirements regarding responses 617 without it. 619 now 621 The term "now" means "the current value of the clock at the host 622 performing the calculation". A cache SHOULD use NTP ([RFC1305]) 623 or some similar protocol to synchronize its clocks to a globally 624 accurate time standard. 626 request_time 628 The current value of the clock at the host at the time the request 629 resulting in the stored response was made. 631 response_time 633 The current value of the clock at the host at the time the 634 response was received. 636 A response's age can be calculated in two entirely independent ways: 638 1. the "apparent_age": response_time minus date_value, if the local 639 clock is reasonably well synchronized to the origin server's 640 clock. If the result is negative, the result is replaced by 641 zero. 643 2. the "corrected_age_value", if all of the caches along the 644 response path implement HTTP/1.1. A cache MUST interpret this 645 value relative to the time the request was initiated, not the 646 time that the response was received. 648 apparent_age = max(0, response_time - date_value); 650 response_delay = response_time - request_time; 651 corrected_age_value = age_value + response_delay; 653 These SHOULD be combined as 655 corrected_initial_age = max(apparent_age, corrected_age_value); 657 unless the cache is confident in the value of the Age header (e.g., 658 because there are no HTTP/1.0 hops in the Via header), in which case 659 the corrected_age_value MAY be used as the corrected_initial_age. 661 The current_age of a stored response can then be calculated by adding 662 the amount of time (in seconds) since the stored response was last 663 validated by the origin server to the corrected_initial_age. 665 resident_time = now - response_time; 666 current_age = corrected_initial_age + resident_time; 668 Additionally, to avoid common problems in date parsing: 670 o HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date 671 which appears to be more than 50 years in the future is in fact in 672 the past (this helps solve the "year 2000" problem). 674 o Although all date formats are specified to be case-sensitive, 675 recipients SHOULD match day, week and timezone names case- 676 insensitively. 678 o An HTTP/1.1 implementation MAY internally represent a parsed 679 Expires date as earlier than the proper value, but MUST NOT 680 internally represent a parsed Expires date as later than the 681 proper value. 683 o All expiration-related calculations MUST be done in GMT. The 684 local time zone MUST NOT influence the calculation or comparison 685 of an age or expiration time. 687 o If an HTTP header field incorrectly carries a date value with a 688 time zone other than GMT, it MUST be converted into GMT using the 689 most conservative possible conversion. 691 2.3.3. Serving Stale Responses 693 A "stale" response is one that either has explicit expiry information 694 or is allowed to have heuristic expiry calculated, but is not fresh 695 according to the calculations in Section 2.3. 697 A cache MUST NOT return a stale response if it is prohibited by an 698 explicit in-protocol directive (e.g., by a "no-store" or "no-cache" 699 cache directive, a "must-revalidate" cache-response-directive, or an 700 applicable "s-maxage" or "proxy-revalidate" cache-response-directive; 701 see Section 3.2.2). 703 A cache MUST NOT return stale responses unless it is disconnected 704 (i.e., it cannot contact the origin server or otherwise find a 705 forward path) or doing so is explicitly allowed (e.g., by the max- 706 stale request directive; see Section 3.2.1). 708 A cache SHOULD append a Warning header field with the 110 warn-code 709 (see Section 3.6) to stale responses. Likewise, a cache SHOULD add 710 the 112 warn-code to stale responses if the cache is disconnected. 712 If a cache receives a first-hand response (either an entire response, 713 or a 304 (Not Modified) response) that it would normally forward to 714 the requesting client, and the received response is no longer fresh, 715 the cache can forward it to the requesting client without adding a 716 new Warning (but without removing any existing Warning header 717 fields). A cache shouldn't attempt to validate a response simply 718 because that response became stale in transit. 720 2.4. Validation Model 722 When a cache has one or more stored responses for a requested URI, 723 but cannot serve any of them (e.g., because they are not fresh, or 724 one cannot be selected; see Section 2.8), it can use the conditional 725 request mechanism [Part4] in the forwarded request to give the origin 726 server an opportunity to both select a valid stored response to be 727 used, and to update it. This process is known as "validating" or 728 "revalidating" the stored response. 730 When sending such a conditional request, a cache adds an If-Modified- 731 Since header field whose value is that of the Last-Modified header 732 field from the selected (see Section 2.8) stored response, if 733 available. 735 Additionally, a cache can add an If-None-Match header field whose 736 value is that of the ETag header field(s) from all responses stored 737 for the requested URI, if present. However, if any of the stored 738 responses contains only partial content, the cache shouldn't include 739 its entity-tag in the If-None-Match header field unless the request 740 is for a range that would be fully satisfied by that stored response. 742 Cache handling of a response to a conditional request is dependent 743 upon its status code: 745 o A 304 (Not Modified) response status code indicates that the 746 stored response can be updated and reused; see Section 2.4.1. 748 o A full response (i.e., one with a response body) indicates that 749 none of the stored responses nominated in the conditional request 750 is suitable. Instead, the cache can use the full response to 751 satisfy the request and MAY replace the stored response(s). 753 o However, if a cache receives a 5xx response while attempting to 754 validate a response, it can either forward this response to the 755 requesting client, or act as if the server failed to respond. In 756 the latter case, it can return a previously stored response (see 757 Section 2.3.3). 759 2.4.1. Freshening Responses with 304 Not Modified 761 When a cache receives a 304 (Not Modified) response and already has 762 one or more stored 200 (OK) responses for the same cache key, the 763 cache needs to identify which of the stored responses are updated by 764 this new response and then update the stored response(s) with the new 765 information provided in the 304 response. 767 o If the new response contains a strong validator, then that strong 768 validator identifies the selected representation. All of the 769 stored responses with the same strong validator are selected. If 770 none of the stored responses contain the same strong validator, 771 then this new response corresponds to a new selected 772 representation and MUST NOT update the existing stored responses. 774 o If the new response contains a weak validator and that validator 775 corresponds to one of the cache's stored responses, then the most 776 recent of those matching stored responses is selected. 778 o If the new response does not include any form of validator, there 779 is only one stored response, and that stored response also lacks a 780 validator, then that stored response is selected. 782 If a stored response is selected for update, the cache MUST: 784 o delete any Warning header fields in the stored response with warn- 785 code 1xx (see Section 3.6); 787 o retain any Warning header fields in the stored response with warn- 788 code 2xx; and, 790 o use other header fields provided in the 304 response to replace 791 all instances of the corresponding header fields in the stored 792 response. 794 2.5. Updating Caches with HEAD Responses 796 A response to the HEAD method is identical to what an equivalent 797 request made with a GET would have been, except it lacks a body. 798 This property of HEAD responses is used to both invalidate and update 799 cached GET responses. 801 If one or more stored GET responses can be selected (as per 802 Section 2.8) for a HEAD request, and the Content-Length, ETag or 803 Last-Modified value of a HEAD response differs from that in a 804 selected GET response, the cache MUST consider that selected response 805 to be stale. 807 If the Content-Length, ETag and Last-Modified values of a HEAD 808 response (when present) are the same as that in a selected GET 809 response (as per Section 2.8), the cache SHOULD update the remaining 810 headers in the stored response using the following rules: 812 o delete any Warning header fields in the stored response with warn- 813 code 1xx (see Section 3.6); 815 o retain any Warning header fields in the stored response with warn- 816 code 2xx; and, 818 o use other header fields provided in the response to replace all 819 instances of the corresponding header fields in the stored 820 response. 822 2.6. Request Methods that Invalidate 824 Because unsafe request methods (Section 6.1.1 of [Part2]) such as 825 PUT, POST or DELETE have the potential for changing state on the 826 origin server, intervening caches can use them to keep their contents 827 up-to-date. 829 A cache MUST invalidate the effective Request URI (Section 5.5 of 831 [Part1]) as well as the URI(s) in the Location and Content-Location 832 response header fields (if present) when a non-error response to a 833 request with an unsafe method is received. 835 However, a cache MUST NOT invalidate a URI from a Location or 836 Content-Location response header field if the host part of that URI 837 differs from the host part in the effective request URI (Section 5.5 838 of [Part1]). This helps prevent denial of service attacks. 840 A cache MUST invalidate the effective request URI (Section 5.5 of 841 [Part1]) when it receives a non-error response to a request with a 842 method whose safety is unknown. 844 Here, a "non-error response" is one with a 2xx or 3xx status code. 845 "Invalidate" means that the cache will either remove all stored 846 responses related to the effective request URI, or will mark these as 847 "invalid" and in need of a mandatory validation before they can be 848 returned in response to a subsequent request. 850 Note that this does not guarantee that all appropriate responses are 851 invalidated. For example, the request that caused the change at the 852 origin server might not have gone through the cache where a response 853 is stored. 855 2.7. Shared Caching of Authenticated Responses 857 A shared cache MUST NOT use a cached response to a request with an 858 Authorization header field (Section 4.1 of [Part7]) to satisfy any 859 subsequent request unless a cache directive that allows such 860 responses to be stored is present in the response. 862 In this specification, the following Cache-Control response 863 directives (Section 3.2.2) have such an effect: must-revalidate, 864 public, s-maxage. 866 Note that cached responses that contain the "must-revalidate" and/or 867 "s-maxage" response directives are not allowed to be served stale 868 (Section 2.3.3) by shared caches. In particular, a response with 869 either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to 870 satisfy a subsequent request without revalidating it on the origin 871 server. 873 2.8. Caching Negotiated Responses 875 When a cache receives a request that can be satisfied by a stored 876 response that has a Vary header field (Section 3.5), it MUST NOT use 877 that response unless all of the selecting header fields nominated by 878 the Vary header field match in both the original request (i.e., that 879 associated with the stored response), and the presented request. 881 The selecting header fields from two requests are defined to match if 882 and only if those in the first request can be transformed to those in 883 the second request by applying any of the following: 885 o adding or removing whitespace, where allowed in the header field's 886 syntax 888 o combining multiple header fields with the same field name (see 889 Section 3.2 of [Part1]) 891 o normalizing both header field values in a way that is known to 892 have identical semantics, according to the header field's 893 specification (e.g., re-ordering field values when order is not 894 significant; case-normalization, where values are defined to be 895 case-insensitive) 897 If (after any normalization that might take place) a header field is 898 absent from a request, it can only match another request if it is 899 also absent there. 901 A Vary header field-value of "*" always fails to match, and 902 subsequent requests to that resource can only be properly interpreted 903 by the origin server. 905 The stored response with matching selecting header fields is known as 906 the selected response. 908 If multiple selected responses are available, the most recent 909 response (as determined by the Date header field) is used; see 910 Section 2.2. 912 If no selected response is available, the cache can forward the 913 presented request to the origin server in a conditional request; see 914 Section 2.4. 916 2.9. Combining Partial Content 918 A response might transfer only a partial representation if the 919 connection closed prematurely or if the request used one or more 920 Range specifiers ([Part5]). After several such transfers, a cache 921 might have received several ranges of the same representation. A 922 cache MAY combine these ranges into a single stored response, and 923 reuse that response to satisfy later requests, if they all share the 924 same strong validator and the cache complies with the client 925 requirements in Section 4.2 of [Part5]. 927 When combining the new response with one or more stored responses, a 928 cache MUST: 930 o delete any Warning header fields in the stored response with warn- 931 code 1xx (see Section 3.6); 933 o retain any Warning header fields in the stored response with warn- 934 code 2xx; and, 936 o use other header fields provided in the new response, aside from 937 Content-Range, to replace all instances of the corresponding 938 header fields in the stored response. 940 3. Header Field Definitions 942 This section defines the syntax and semantics of HTTP/1.1 header 943 fields related to caching. 945 3.1. Age 947 The "Age" header field conveys the sender's estimate of the amount of 948 time since the response was generated or successfully validated at 949 the origin server. Age values are calculated as specified in 950 Section 2.3.2. 952 Age = delta-seconds 954 Age field-values are non-negative integers, representing time in 955 seconds (see Section 1.5). 957 The presence of an Age header field in a response implies that a 958 response is not first-hand. However, the converse is not true, since 959 HTTP/1.0 caches might not implement the Age header field. 961 3.2. Cache-Control 963 The "Cache-Control" header field is used to specify directives for 964 caches along the request/response chain. Such cache directives are 965 unidirectional in that the presence of a directive in a request does 966 not imply that the same directive is to be given in the response. 968 A cache MUST obey the requirements of the Cache-Control directives 969 defined in this section. See Section 3.2.3 for information about how 970 Cache-Control directives defined elsewhere are handled. 972 Note: HTTP/1.0 caches might not implement Cache-Control and might 973 only implement Pragma: no-cache (see Section 3.4). 975 A proxy, whether or not it implements a cache, MUST pass cache 976 directives through in forwarded messages, regardless of their 977 significance to that application, since the directives might be 978 applicable to all recipients along the request/response chain. It is 979 not possible to target a directive to a specific cache. 981 Cache directives are identified by a token, to be compared case- 982 insensitively, and have an optional argument. 984 Cache-Control = 1#cache-directive 986 cache-directive = cache-request-directive 987 / cache-response-directive 989 cache-extension = token [ "=" ( token / quoted-string ) ] 991 3.2.1. Request Cache-Control Directives 993 cache-request-directive = 994 "no-cache" 995 / "no-store" 996 / "max-age" "=" delta-seconds 997 / "max-stale" [ "=" delta-seconds ] 998 / "min-fresh" "=" delta-seconds 999 / "no-transform" 1000 / "only-if-cached" 1001 / cache-extension 1003 no-cache 1005 The no-cache request directive indicates that a cache MUST NOT use 1006 a stored response to satisfy the request without successful 1007 validation on the origin server. 1009 no-store 1011 The no-store request directive indicates that a cache MUST NOT 1012 store any part of either this request or any response to it. This 1013 directive applies to both private and shared caches. "MUST NOT 1014 store" in this context means that the cache MUST NOT intentionally 1015 store the information in non-volatile storage, and MUST make a 1016 best-effort attempt to remove the information from volatile 1017 storage as promptly as possible after forwarding it. 1019 This directive is NOT a reliable or sufficient mechanism for 1020 ensuring privacy. In particular, malicious or compromised caches 1021 might not recognize or obey this directive, and communications 1022 networks might be vulnerable to eavesdropping. 1024 Note that if a request containing this directive is satisfied from 1025 a cache, the no-store request directive does not apply to the 1026 already stored response. 1028 max-age 1030 The max-age request directive indicates that the client is 1031 unwilling to accept a response whose age is greater than the 1032 specified number of seconds. Unless the max-stale request 1033 directive is also present, the client is not willing to accept a 1034 stale response. 1036 max-stale 1038 The max-stale request directive indicates that the client is 1039 willing to accept a response that has exceeded its expiration 1040 time. If max-stale is assigned a value, then the client is 1041 willing to accept a response that has exceeded its expiration time 1042 by no more than the specified number of seconds. If no value is 1043 assigned to max-stale, then the client is willing to accept a 1044 stale response of any age. 1046 min-fresh 1048 The min-fresh request directive indicates that the client is 1049 willing to accept a response whose freshness lifetime is no less 1050 than its current age plus the specified time in seconds. That is, 1051 the client wants a response that will still be fresh for at least 1052 the specified number of seconds. 1054 no-transform 1056 The no-transform request directive indicates that an intermediary 1057 (whether or not it implements a cache) MUST NOT change the 1058 Content-Encoding, Content-Range or Content-Type request header 1059 fields, nor the request representation. 1061 only-if-cached 1063 The only-if-cached request directive indicates that the client 1064 only wishes to obtain a stored response. If it receives this 1065 directive, a cache SHOULD either respond using a stored response 1066 that is consistent with the other constraints of the request, or 1067 respond with a 504 (Gateway Timeout) status code. If a group of 1068 caches is being operated as a unified system with good internal 1069 connectivity, a member cache MAY forward such a request within 1070 that group of caches. 1072 3.2.2. Response Cache-Control Directives 1074 cache-response-directive = 1075 "public" 1076 / "private" [ "=" DQUOTE 1#field-name DQUOTE ] 1077 / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] 1078 / "no-store" 1079 / "no-transform" 1080 / "must-revalidate" 1081 / "proxy-revalidate" 1082 / "max-age" "=" delta-seconds 1083 / "s-maxage" "=" delta-seconds 1084 / cache-extension 1086 public 1088 The public response directive indicates that a response whose 1089 associated request contains an 'Authentication' header MAY be 1090 stored (see Section 2.7). 1092 private 1094 The private response directive indicates that the response message 1095 is intended for a single user and MUST NOT be stored by a shared 1096 cache. A private cache MAY store the response. 1098 If the private response directive specifies one or more field- 1099 names, this requirement is limited to the field-values associated 1100 with the listed response header fields. That is, a shared cache 1101 MUST NOT store the specified field-names(s), whereas it MAY store 1102 the remainder of the response message. 1104 Note: This usage of the word "private" only controls where the 1105 response can be stored; it cannot ensure the privacy of the 1106 message content. Also, private response directives with field- 1107 names are often handled by implementations as if an unqualified 1108 private directive was received; i.e., the special handling for the 1109 qualified form is not widely implemented. 1111 no-cache 1113 The no-cache response directive indicates that the response MUST 1114 NOT be used to satisfy a subsequent request without successful 1115 validation on the origin server. This allows an origin server to 1116 prevent a cache from using it to satisfy a request without 1117 contacting it, even by caches that have been configured to return 1118 stale responses. 1120 If the no-cache response directive specifies one or more field- 1121 names, then a cache MAY use the response to satisfy a subsequent 1122 request, subject to any other restrictions on caching. However, 1123 any header fields in the response that have the field-name(s) 1124 listed MUST NOT be sent in the response to a subsequent request 1125 without successful revalidation with the origin server. This 1126 allows an origin server to prevent the re-use of certain header 1127 fields in a response, while still allowing caching of the rest of 1128 the response. 1130 Note: Most HTTP/1.0 caches will not recognize or obey this 1131 directive. Also, no-cache response directives with field-names 1132 are often handled by implementations as if an unqualified no-cache 1133 directive was received; i.e., the special handling for the 1134 qualified form is not widely implemented. 1136 no-store 1138 The no-store response directive indicates that a cache MUST NOT 1139 store any part of either the immediate request or response. This 1140 directive applies to both private and shared caches. "MUST NOT 1141 store" in this context means that the cache MUST NOT intentionally 1142 store the information in non-volatile storage, and MUST make a 1143 best-effort attempt to remove the information from volatile 1144 storage as promptly as possible after forwarding it. 1146 This directive is NOT a reliable or sufficient mechanism for 1147 ensuring privacy. In particular, malicious or compromised caches 1148 might not recognize or obey this directive, and communications 1149 networks might be vulnerable to eavesdropping. 1151 must-revalidate 1153 The must-revalidate response directive indicates that once it has 1154 become stale, a cache MUST NOT use the response to satisfy 1155 subsequent requests without successful validation on the origin 1156 server. 1158 The must-revalidate directive is necessary to support reliable 1159 operation for certain protocol features. In all circumstances a 1160 cache MUST obey the must-revalidate directive; in particular, if a 1161 cache cannot reach the origin server for any reason, it MUST 1162 generate a 504 (Gateway Timeout) response. 1164 The must-revalidate directive ought to be used by servers if and 1165 only if failure to validate a request on the representation could 1166 result in incorrect operation, such as a silently unexecuted 1167 financial transaction. 1169 proxy-revalidate 1171 The proxy-revalidate response directive has the same meaning as 1172 the must-revalidate response directive, except that it does not 1173 apply to private caches. 1175 max-age 1177 The max-age response directive indicates that the response is to 1178 be considered stale after its age is greater than the specified 1179 number of seconds. 1181 s-maxage 1183 The s-maxage response directive indicates that, in shared caches, 1184 the maximum age specified by this directive overrides the maximum 1185 age specified by either the max-age directive or the Expires 1186 header field. The s-maxage directive also implies the semantics 1187 of the proxy-revalidate response directive. 1189 no-transform 1191 The no-transform response directive indicates that an intermediary 1192 (regardless of whether it implements a cache) MUST NOT change the 1193 Content-Encoding, Content-Range or Content-Type response header 1194 fields, nor the response representation. 1196 3.2.3. Cache Control Extensions 1198 The Cache-Control header field can be extended through the use of one 1199 or more cache-extension tokens, each with an optional value. 1200 Informational extensions (those that do not require a change in cache 1201 behavior) can be added without changing the semantics of other 1202 directives. Behavioral extensions are designed to work by acting as 1203 modifiers to the existing base of cache directives. Both the new 1204 directive and the standard directive are supplied, such that 1205 applications that do not understand the new directive will default to 1206 the behavior specified by the standard directive, and those that 1207 understand the new directive will recognize it as modifying the 1208 requirements associated with the standard directive. In this way, 1209 extensions to the cache-control directives can be made without 1210 requiring changes to the base protocol. 1212 This extension mechanism depends on an HTTP cache obeying all of the 1213 cache-control directives defined for its native HTTP-version, obeying 1214 certain extensions, and ignoring all directives that it does not 1215 understand. 1217 For example, consider a hypothetical new response directive called 1218 "community" that acts as a modifier to the private directive. We 1219 define this new directive to mean that, in addition to any private 1220 cache, any cache that is shared only by members of the community 1221 named within its value may cache the response. An origin server 1222 wishing to allow the UCI community to use an otherwise private 1223 response in their shared cache(s) could do so by including 1225 Cache-Control: private, community="UCI" 1227 A cache seeing this header field will act correctly even if the cache 1228 does not understand the community cache-extension, since it will also 1229 see and understand the private directive and thus default to the safe 1230 behavior. 1232 A cache MUST ignore unrecognized cache directives; it is assumed that 1233 any cache directive likely to be unrecognized by an HTTP/1.1 cache 1234 will be combined with standard directives (or the response's default 1235 cacheability) such that the cache behavior will remain minimally 1236 correct even if the cache does not understand the extension(s). 1238 The HTTP Cache Directive Registry defines the name space for the 1239 cache directives. 1241 A registration MUST include the following fields: 1243 o Cache Directive Name 1245 o Pointer to specification text 1247 Values to be added to this name space require IETF Review (see 1248 [RFC5226], Section 4.1). 1250 The registry itself is maintained at 1251 . 1253 3.3. Expires 1255 The "Expires" header field gives the date/time after which the 1256 response is considered stale. See Section 2.3 for further discussion 1257 of the freshness model. 1259 The presence of an Expires field does not imply that the original 1260 resource will change or cease to exist at, before, or after that 1261 time. 1263 The field-value is an absolute date and time as defined by HTTP-date 1264 in Section 8 of [Part2]; a sender MUST use the rfc1123-date format. 1266 Expires = HTTP-date 1268 For example 1270 Expires: Thu, 01 Dec 1994 16:00:00 GMT 1272 A cache MUST treat other invalid date formats, especially including 1273 the value "0", as in the past (i.e., "already expired"). 1275 Note: If a response includes a Cache-Control field with the max- 1276 age directive (see Section 3.2.2), that directive overrides the 1277 Expires field. Likewise, the s-maxage directive overrides Expires 1278 in shared caches. 1280 Historically, HTTP required the Expires field-value to be no more 1281 than a year in the future. While longer freshness lifetimes are no 1282 longer prohibited, extremely large values have been demonstrated to 1283 cause problems (e.g., clock overflows due to use of 32-bit integers 1284 for time values), and most caches will evict a response far sooner 1285 than that. Therefore, senders ought not produce them. 1287 An origin server without a clock MUST NOT assign Expires values to a 1288 response unless these values were associated with the resource by a 1289 system or user with a reliable clock. It MAY assign an Expires value 1290 that is known, at or before server configuration time, to be in the 1291 past (this allows "pre-expiration" of responses without storing 1292 separate Expires values for each resource). 1294 3.4. Pragma 1296 The "Pragma" header field allows backwards compatibility with 1297 HTTP/1.0 caches, so that clients can specify a "no-cache" request 1298 that they will understand (as Cache-Control was not defined until 1299 HTTP/1.1). When the Cache-Control header is also present and 1300 understood in a request, Pragma is ignored. 1302 In HTTP/1.0, Pragma was defined as an extensible field for 1303 implementation-specified directives for recipients. This 1304 specification deprecates such extensions to improve interoperability. 1306 Pragma = 1#pragma-directive 1307 pragma-directive = "no-cache" / extension-pragma 1308 extension-pragma = token [ "=" ( token / quoted-string ) ] 1310 When the Cache-Control header is not present in a request, the no- 1311 cache request pragma-directive MUST have the same effect on caches as 1312 if "Cache-Control: no-cache" were present (see Section 3.2.1). 1314 When sending a no-cache request, a client ought to include both the 1315 pragma and cache-control directives, unless Cache-Control: no-cache 1316 is purposefully omitted to target other Cache-Control response 1317 directives at HTTP/1.1 caches. For example: 1319 GET / HTTP/1.1 1320 Host: www.example.com 1321 Cache-Control: max-age=30 1322 Pragma: no-cache 1324 will constrain HTTP/1.1 caches to serve a response no older than 30 1325 seconds, while precluding implementations that do not understand 1326 Cache-Control from serving a cached response. 1328 Note: Because the meaning of "Pragma: no-cache" in responses is 1329 not specified, it does not provide a reliable replacement for 1330 "Cache-Control: no-cache" in them. 1332 3.5. Vary 1334 The "Vary" header field conveys the set of header fields that were 1335 used to select the representation. 1337 Caches use this information, in part, to determine whether a stored 1338 response can be used to satisfy a given request; see Section 2.8. 1339 determines, while the response is fresh, whether a cache is permitted 1340 to use the response to reply to a subsequent request without 1341 validation; see Section 2.8. 1343 In uncacheable or stale responses, the Vary field value advises the 1344 user agent about the criteria that were used to select the 1345 representation. 1347 Vary = "*" / 1#field-name 1349 The set of header fields named by the Vary field value is known as 1350 the selecting header fields. 1352 A server SHOULD include a Vary header field with any cacheable 1353 response that is subject to server-driven negotiation. Doing so 1354 allows a cache to properly interpret future requests on that resource 1355 and informs the user agent about the presence of negotiation on that 1356 resource. A server MAY include a Vary header field with a non- 1357 cacheable response that is subject to server-driven negotiation, 1358 since this might provide the user agent with useful information about 1359 the dimensions over which the response varies at the time of the 1360 response. 1362 A Vary field value of "*" signals that unspecified parameters not 1363 limited to the header fields (e.g., the network address of the 1364 client), play a role in the selection of the response representation; 1365 therefore, a cache cannot determine whether this response is 1366 appropriate. A proxy MUST NOT generate the "*" value. 1368 The field-names given are not limited to the set of standard header 1369 fields defined by this specification. Field names are case- 1370 insensitive. 1372 3.6. Warning 1374 The "Warning" header field is used to carry additional information 1375 about the status or transformation of a message that might not be 1376 reflected in the message. This information is typically used to warn 1377 about possible incorrectness introduced by caching operations or 1378 transformations applied to the payload of the message. 1380 Warnings can be used for other purposes, both cache-related and 1381 otherwise. The use of a warning, rather than an error status code, 1382 distinguishes these responses from true failures. 1384 Warning header fields can in general be applied to any message, 1385 however some warn-codes are specific to caches and can only be 1386 applied to response messages. 1388 Warning = 1#warning-value 1390 warning-value = warn-code SP warn-agent SP warn-text 1391 [SP warn-date] 1393 warn-code = 3DIGIT 1394 warn-agent = ( uri-host [ ":" port ] ) / pseudonym 1395 ; the name or pseudonym of the server adding 1396 ; the Warning header field, for use in debugging 1397 warn-text = quoted-string 1398 warn-date = DQUOTE HTTP-date DQUOTE 1400 Multiple warnings can be attached to a response (either by the origin 1401 server or by a cache), including multiple warnings with the same code 1402 number, only differing in warn-text. 1404 When this occurs, the user agent SHOULD inform the user of as many of 1405 them as possible, in the order that they appear in the response. 1407 Systems that generate multiple Warning header fields are encouraged 1408 to order them with this user agent behavior in mind. New Warning 1409 header fields are added after any existing Warning headers fields. 1411 Warnings are assigned three digit warn-codes. The first digit 1412 indicates whether the Warning is required to be deleted from a stored 1413 response after validation: 1415 o 1xx Warnings describe the freshness or validation status of the 1416 response, and so MUST be deleted by a cache after validation. 1417 They can only be generated by a cache when validating a cached 1418 entry, and MUST NOT be generated in any other situation. 1420 o 2xx Warnings describe some aspect of the representation that is 1421 not rectified by a validation (for example, a lossy compression of 1422 the representation) and MUST NOT be deleted by a cache after 1423 validation, unless a full response is returned, in which case they 1424 MUST be. 1426 If an implementation sends a message with one or more Warning header 1427 fields to a receiver whose version is HTTP/1.0 or lower, then the 1428 sender MUST include in each warning-value a warn-date that matches 1429 the Date header field in the message. 1431 If a system receives a message with a warning-value that includes a 1432 warn-date, and that warn-date is different from the Date value in the 1433 response, then that warning-value MUST be deleted from the message 1434 before storing, forwarding, or using it. (preventing the consequences 1435 of naive caching of Warning header fields.) If all of the warning- 1436 values are deleted for this reason, the Warning header field MUST be 1437 deleted as well. 1439 The following warn-codes are defined by this specification, each with 1440 a recommended warn-text in English, and a description of its meaning. 1442 3.6.1. 110 Response is Stale 1444 A cache SHOULD include this whenever the returned response is stale. 1446 3.6.2. 111 Revalidation Failed 1448 A cache SHOULD include this when returning a stale response because 1449 an attempt to validate the response failed, due to an inability to 1450 reach the server. 1452 3.6.3. 112 Disconnected Operation 1454 A cache SHOULD include this if it is intentionally disconnected from 1455 the rest of the network for a period of time. 1457 3.6.4. 113 Heuristic Expiration 1459 A cache SHOULD include this if it heuristically chose a freshness 1460 lifetime greater than 24 hours and the response's age is greater than 1461 24 hours. 1463 3.6.5. 199 Miscellaneous Warning 1465 The warning text can include arbitrary information to be presented to 1466 a human user, or logged. A system receiving this warning MUST NOT 1467 take any automated action, besides presenting the warning to the 1468 user. 1470 3.6.6. 214 Transformation Applied 1472 MUST be added by a proxy if it applies any transformation to the 1473 representation, such as changing the content-coding, media-type, or 1474 modifying the representation data, unless this Warning code already 1475 appears in the response. 1477 3.6.7. 299 Miscellaneous Persistent Warning 1479 The warning text can include arbitrary information to be presented to 1480 a human user, or logged. A system receiving this warning MUST NOT 1481 take any automated action. 1483 3.6.8. Warn Code Extensions 1485 The HTTP Warn Code Registry defines the name space for warn codes. 1487 A registration MUST include the following fields: 1489 o Warn Code (3 digits) 1491 o Short Description 1493 o Pointer to specification text 1495 Values to be added to this name space require IETF Review (see 1496 [RFC5226], Section 4.1). 1498 The registry itself is maintained at 1499 . 1501 4. History Lists 1503 User agents often have history mechanisms, such as "Back" buttons and 1504 history lists, that can be used to redisplay a representation 1505 retrieved earlier in a session. 1507 The freshness model (Section 2.3) does not necessarily apply to 1508 history mechanisms. I.e., a history mechanism can display a previous 1509 representation even if it has expired. 1511 This does not prohibit the history mechanism from telling the user 1512 that a view might be stale, or from honoring cache directives (e.g., 1513 Cache-Control: no-store). 1515 5. IANA Considerations 1517 5.1. Cache Directive Registry 1519 The registration procedure for HTTP Cache Directives is defined by 1520 Section 3.2.3 of this document. 1522 The HTTP Cache Directive Registry shall be created at 1523 and be 1524 populated with the registrations below: 1526 +------------------------+------------------------------+ 1527 | Cache Directive | Reference | 1528 +------------------------+------------------------------+ 1529 | max-age | Section 3.2.1, Section 3.2.2 | 1530 | max-stale | Section 3.2.1 | 1531 | min-fresh | Section 3.2.1 | 1532 | must-revalidate | Section 3.2.2 | 1533 | no-cache | Section 3.2.1, Section 3.2.2 | 1534 | no-store | Section 3.2.1, Section 3.2.2 | 1535 | no-transform | Section 3.2.1, Section 3.2.2 | 1536 | only-if-cached | Section 3.2.1 | 1537 | private | Section 3.2.2 | 1538 | proxy-revalidate | Section 3.2.2 | 1539 | public | Section 3.2.2 | 1540 | s-maxage | Section 3.2.2 | 1541 | stale-if-error | [RFC5861], Section 4 | 1542 | stale-while-revalidate | [RFC5861], Section 3 | 1543 +------------------------+------------------------------+ 1545 5.2. Warn Code Registry 1547 The registration procedure for HTTP Warn Codes is defined by 1548 Section 3.6.8 of this document. 1550 The HTTP Warn Code Registry shall be created at 1551 and be 1552 populated with the registrations below: 1554 +-----------+----------------------------------+---------------+ 1555 | Warn Code | Short Description | Reference | 1556 +-----------+----------------------------------+---------------+ 1557 | 110 | Response is Stale | Section 3.6.1 | 1558 | 111 | Revalidation Failed | Section 3.6.2 | 1559 | 112 | Disconnected Operation | Section 3.6.3 | 1560 | 113 | Heuristic Expiration | Section 3.6.4 | 1561 | 199 | Miscellaneous Warning | Section 3.6.5 | 1562 | 214 | Transformation Applied | Section 3.6.6 | 1563 | 299 | Miscellaneous Persistent Warning | Section 3.6.7 | 1564 +-----------+----------------------------------+---------------+ 1566 5.3. Header Field Registration 1568 The Message Header Field Registry located at shall be 1570 updated with the permanent registrations below (see [RFC3864]): 1572 +-------------------+----------+----------+-------------+ 1573 | Header Field Name | Protocol | Status | Reference | 1574 +-------------------+----------+----------+-------------+ 1575 | Age | http | standard | Section 3.1 | 1576 | Cache-Control | http | standard | Section 3.2 | 1577 | Expires | http | standard | Section 3.3 | 1578 | Pragma | http | standard | Section 3.4 | 1579 | Vary | http | standard | Section 3.5 | 1580 | Warning | http | standard | Section 3.6 | 1581 +-------------------+----------+----------+-------------+ 1583 The change controller is: "IETF (iesg@ietf.org) - Internet 1584 Engineering Task Force". 1586 6. Security Considerations 1588 Caches expose additional potential vulnerabilities, since the 1589 contents of the cache represent an attractive target for malicious 1590 exploitation. Because cache contents persist after an HTTP request 1591 is complete, an attack on the cache can reveal information long after 1592 a user believes that the information has been removed from the 1593 network. Therefore, cache contents need to be protected as sensitive 1594 information. 1596 7. Acknowledgments 1598 See Section 9 of [Part1]. 1600 8. References 1601 8.1. Normative References 1603 [Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1604 "HTTP/1.1, part 1: URIs, Connections, and Message 1605 Parsing", draft-ietf-httpbis-p1-messaging-19 (work in 1606 progress), March 2012. 1608 [Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1609 "HTTP/1.1, part 2: Message Semantics", 1610 draft-ietf-httpbis-p2-semantics-19 (work in progress), 1611 March 2012. 1613 [Part4] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1614 "HTTP/1.1, part 4: Conditional Requests", 1615 draft-ietf-httpbis-p4-conditional-19 (work in progress), 1616 March 2012. 1618 [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1619 "HTTP/1.1, part 5: Range Requests and Partial Responses", 1620 draft-ietf-httpbis-p5-range-19 (work in progress), 1621 March 2012. 1623 [Part7] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1624 "HTTP/1.1, part 7: Authentication", 1625 draft-ietf-httpbis-p7-auth-19 (work in progress), 1626 March 2012. 1628 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1629 Requirement Levels", BCP 14, RFC 2119, March 1997. 1631 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1632 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1634 8.2. Informative References 1636 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 1637 Specification, Implementation", RFC 1305, March 1992. 1639 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1640 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1641 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1643 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1644 Procedures for Message Header Fields", BCP 90, RFC 3864, 1645 September 2004. 1647 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1648 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1649 May 2008. 1651 [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale 1652 Content", RFC 5861, April 2010. 1654 Appendix A. Changes from RFC 2616 1656 Make the specified age calculation algorithm less conservative. 1657 (Section 2.3.2) 1659 Remove requirement to consider Content-Location in successful 1660 responses in order to determine the appropriate response to use. 1661 (Section 2.4) 1663 Clarify denial of service attack avoidance requirement. 1664 (Section 2.6) 1666 Change ABNF productions for header fields to only define the field 1667 value. (Section 3) 1669 Do not mention RFC 2047 encoding and multiple languages in Warning 1670 header fields anymore, as these aspects never were implemented. 1671 (Section 3.6) 1673 Appendix B. Collected ABNF 1675 Age = delta-seconds 1677 Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS 1678 cache-directive ] ) 1680 Expires = HTTP-date 1682 HTTP-date = 1684 OWS = 1686 Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS 1687 pragma-directive ] ) 1689 Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ] 1690 ) ) 1692 Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] 1693 ) 1695 cache-directive = cache-request-directive / cache-response-directive 1696 cache-extension = token [ "=" ( token / quoted-string ) ] 1697 cache-request-directive = "no-cache" / "no-store" / ( "max-age=" 1698 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / ( 1699 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" / 1700 cache-extension 1701 cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( "," 1702 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / ( 1703 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS 1704 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" / 1705 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds 1706 ) / ( "s-maxage=" delta-seconds ) / cache-extension 1708 delta-seconds = 1*DIGIT 1710 extension-pragma = token [ "=" ( token / quoted-string ) ] 1712 field-name = 1714 port = 1715 pragma-directive = "no-cache" / extension-pragma 1716 pseudonym = 1718 quoted-string = 1720 token = 1722 uri-host = 1724 warn-agent = ( uri-host [ ":" port ] ) / pseudonym 1725 warn-code = 3DIGIT 1726 warn-date = DQUOTE HTTP-date DQUOTE 1727 warn-text = quoted-string 1728 warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date 1729 ] 1731 ABNF diagnostics: 1733 ; Age defined but not used 1734 ; Cache-Control defined but not used 1735 ; Expires defined but not used 1736 ; Pragma defined but not used 1737 ; Vary defined but not used 1738 ; Warning defined but not used 1740 Appendix C. Change Log (to be removed by RFC Editor before publication) 1741 C.1. Since RFC 2616 1743 Extracted relevant partitions from [RFC2616]. 1745 C.2. Since draft-ietf-httpbis-p6-cache-00 1747 Closed issues: 1749 o : "Trailer" 1750 () 1752 o : "Invalidation 1753 after Update or Delete" 1754 () 1756 o : "Normative and 1757 Informative references" 1759 o : "Date reference 1760 typo" 1762 o : "Connection 1763 header text" 1765 o : "Informative 1766 references" 1768 o : "ISO-8859-1 1769 Reference" 1771 o : "Normative up- 1772 to-date references" 1774 o : "typo in 1775 13.2.2" 1777 Other changes: 1779 o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress 1780 on ) 1782 C.3. Since draft-ietf-httpbis-p6-cache-01 1784 Closed issues: 1786 o : "rel_path not 1787 used" 1789 Other changes: 1791 o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work 1792 in progress on ) 1794 o Add explicit references to BNF syntax and rules imported from 1795 other parts of the specification. 1797 C.4. Since draft-ietf-httpbis-p6-cache-02 1799 Ongoing work on IANA Message Header Field Registration 1800 (): 1802 o Reference RFC 3984, and update header field registrations for 1803 header fields defined in this document. 1805 C.5. Since draft-ietf-httpbis-p6-cache-03 1807 Closed issues: 1809 o : "Vary header 1810 classification" 1812 C.6. Since draft-ietf-httpbis-p6-cache-04 1814 Ongoing work on ABNF conversion 1815 (): 1817 o Use "/" instead of "|" for alternatives. 1819 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional 1820 whitespace ("OWS") and required whitespace ("RWS"). 1822 o Rewrite ABNFs to spell out whitespace rules, factor out header 1823 field value format definitions. 1825 C.7. Since draft-ietf-httpbis-p6-cache-05 1827 This is a total rewrite of this part of the specification. 1829 Affected issues: 1831 o : "Definition of 1832 1xx Warn-Codes" 1834 o : "Placement of 1835 13.5.1 and 13.5.2" 1837 o : "The role of 1838 Warning and Semantic Transparency in Caching" 1840 o : "Methods and 1841 Caching" 1843 In addition: Final work on ABNF conversion 1844 (): 1846 o Add appendix containing collected and expanded ABNF, reorganize 1847 ABNF introduction. 1849 C.8. Since draft-ietf-httpbis-p6-cache-06 1851 Closed issues: 1853 o : "base for 1854 numeric protocol elements" 1856 Affected issues: 1858 o : "Vary and non- 1859 existant headers" 1861 C.9. Since draft-ietf-httpbis-p6-cache-07 1863 Closed issues: 1865 o : "Definition of 1866 1xx Warn-Codes" 1868 o : "Content- 1869 Location on 304 responses" 1871 o : "private and 1872 no-cache CC directives with headers" 1874 o : "RFC2047 and 1875 warn-text" 1877 C.10. Since draft-ietf-httpbis-p6-cache-08 1879 Closed issues: 1881 o : "serving 1882 negotiated responses from cache: header-specific canonicalization" 1884 o : "Effect of CC 1885 directives on history lists" 1887 o : "Cache 1888 Extensions can override no-store, etc." 1890 Affected issues: 1892 o : Status codes 1893 and caching 1895 Partly resolved issues: 1897 o : "Placement of 1898 13.5.1 and 13.5.2" 1900 C.11. Since draft-ietf-httpbis-p6-cache-09 1902 Closed issues: 1904 o : "Age 1905 calculation" 1907 o : "Clarify 1908 differences between / requirements for request and response CC 1909 directives" 1911 o : "Caching 1912 authenticated responses" 1914 o : "IANA registry 1915 for cache-control directives" 1917 o : "Heuristic 1918 caching of URLs with query components" 1920 Partly resolved issues: 1922 o : "Term for the 1923 requested resource's URI" 1925 C.12. Since draft-ietf-httpbis-p6-cache-10 1927 Closed issues: 1929 o : "Clarify 1930 entity / representation / variant terminology" 1932 o : "consider 1933 removing the 'changes from 2068' sections" 1935 o : "Allowing 1936 heuristic caching for new status codes" 1938 o Clean up TODOs and prose in "Combining Responses." 1940 C.13. Since draft-ietf-httpbis-p6-cache-11 1942 Closed issues: 1944 o : "Text about 1945 clock requirement for caches belongs in p6" 1947 C.14. Since draft-ietf-httpbis-p6-cache-12 1949 Closed issues: 1951 o : "Header 1952 Classification" 1954 o : "Clarify 1955 'public'" 1957 C.15. Since draft-ietf-httpbis-p6-cache-13 1959 Closed issues: 1961 o : "untangle 1962 ABNFs for header fields" 1964 C.16. Since draft-ietf-httpbis-p6-cache-14 1966 Closed issues: 1968 o : "Mismatch Vary" 1970 o : "Cache 1971 Invalidation only happens upon successful responses" 1973 o : "Recommend 1974 minimum sizes for protocol elements" 1976 o : "Proxies don't 1977 'understand' methods" 1979 o : "Cache 1980 Extensions can override no-store, etc." 1982 o : "Pragma" 1984 C.17. Since draft-ietf-httpbis-p6-cache-15 1986 Closed issues: 1988 o : "Motivate one- 1989 year limit for Expires" 1991 C.18. Since draft-ietf-httpbis-p6-cache-16 1993 Closed issues: 1995 o : "Document 1996 HTTP's error-handling philosophy" 1998 o : "Cache-Control 1999 directive case sensitivity" 2001 C.19. Since draft-ietf-httpbis-p6-cache-17 2003 Closed issues: 2005 o : "Interaction 2006 of request and response Cache-Control" 2008 o : "Refining age 2009 for 1.1 proxy chains" 2011 o : "warn-code 2012 registry" 2014 C.20. Since draft-ietf-httpbis-p6-cache-18 2016 Closed issues: 2018 o : "Combining 2019 HEAD responses" 2021 o : "Field names 2022 in cache-control header arguments" 2024 Index 2026 1 2027 110 Response is Stale (warn code) 31 2028 111 Revalidation Failed (warn code) 31 2029 112 Disconnected Operation (warn code) 31 2030 113 Heuristic Expiration (warn code) 32 2031 199 Miscellaneous Warning (warn code) 32 2033 2 2034 214 Transformation Applied (warn code) 32 2035 299 Miscellaneous Persistent Warning (warn code) 32 2037 A 2038 age 6 2039 Age header field 21 2041 C 2042 cache 5 2043 Cache Directives 2044 max-age 23, 26 2045 max-stale 23 2046 min-fresh 23 2047 must-revalidate 25 2048 no-cache 22, 24 2049 no-store 22, 25 2050 no-transform 23, 26 2051 only-if-cached 23 2052 private 24 2053 proxy-revalidate 26 2054 public 24 2055 s-maxage 26 2056 cache entry 8 2057 cache key 8 2058 Cache-Control header field 21 2059 cacheable 5 2061 E 2062 Expires header field 27 2063 explicit expiration time 6 2065 F 2066 first-hand 6 2067 fresh 6 2068 freshness lifetime 6 2070 G 2071 Grammar 2072 Age 21 2073 Cache-Control 22 2074 cache-extension 22 2075 cache-request-directive 22 2076 cache-response-directive 24 2077 delta-seconds 8 2078 Expires 28 2079 extension-pragma 28 2080 Pragma 28 2081 pragma-directive 28 2082 Vary 29 2083 warn-agent 30 2084 warn-code 30 2085 warn-date 30 2086 warn-text 30 2087 Warning 30 2088 warning-value 30 2090 H 2091 Header Fields 2092 Age 21 2093 Cache-Control 21 2094 Expires 27 2095 Pragma 28 2096 Vary 29 2097 Warning 30 2098 heuristic expiration time 6 2100 M 2101 max-age 2102 Cache Directive 23, 26 2103 max-stale 2104 Cache Directive 23 2105 min-fresh 2106 Cache Directive 23 2107 must-revalidate 2108 Cache Directive 25 2110 N 2111 no-cache 2112 Cache Directive 22, 24 2113 no-store 2114 Cache Directive 22, 25 2115 no-transform 2116 Cache Directive 23, 26 2118 O 2119 only-if-cached 2120 Cache Directive 23 2122 P 2123 Pragma header field 28 2124 private 2125 Cache Directive 24 2126 private cache 5 2127 proxy-revalidate 2128 Cache Directive 26 2129 public 2130 Cache Directive 24 2132 S 2133 s-maxage 2134 Cache Directive 26 2135 shared cache 5 2136 stale 6 2137 strong validator 7 2139 V 2140 validator 6 2141 strong 7 2142 Vary header field 29 2144 W 2145 Warn Codes 2146 110 Response is Stale 31 2147 111 Revalidation Failed 31 2148 112 Disconnected Operation 31 2149 113 Heuristic Expiration 32 2150 199 Miscellaneous Warning 32 2151 214 Transformation Applied 32 2152 299 Miscellaneous Persistent Warning 32 2153 Warning header field 30 2155 Authors' Addresses 2157 Roy T. Fielding (editor) 2158 Adobe Systems Incorporated 2159 345 Park Ave 2160 San Jose, CA 95110 2161 USA 2163 EMail: fielding@gbiv.com 2164 URI: http://roy.gbiv.com/ 2165 Yves Lafon (editor) 2166 World Wide Web Consortium 2167 W3C / ERCIM 2168 2004, rte des Lucioles 2169 Sophia-Antipolis, AM 06902 2170 France 2172 EMail: ylafon@w3.org 2173 URI: http://www.raubacapeu.net/people/yves/ 2175 Mark Nottingham (editor) 2176 Rackspace 2178 EMail: mnot@mnot.net 2179 URI: http://www.mnot.net/ 2181 Julian F. Reschke (editor) 2182 greenbytes GmbH 2183 Hafenweg 16 2184 Muenster, NW 48155 2185 Germany 2187 Phone: +49 251 2807760 2188 Fax: +49 251 2807761 2189 EMail: julian.reschke@greenbytes.de 2190 URI: http://greenbytes.de/tech/webdav/