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'USASCII' -- Obsolete informational reference (is this intentional?): RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) Summary: 0 errors (**), 0 flaws (~~), 7 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTP Working Group R. Fielding, Ed. 3 Internet-Draft Adobe 4 Obsoletes: 7234 (if approved) M. Nottingham, Ed. 5 Intended status: Standards Track Fastly 6 Expires: April 5, 2021 J. Reschke, Ed. 7 greenbytes 8 October 2, 2020 10 HTTP Caching 11 draft-ietf-httpbis-cache-12 13 Abstract 15 The Hypertext Transfer Protocol (HTTP) is a stateless application- 16 level protocol for distributed, collaborative, hypertext information 17 systems. This document defines HTTP caches and the associated header 18 fields that control cache behavior or indicate cacheable response 19 messages. 21 This document obsoletes RFC 7234. 23 Editorial Note 25 This note is to be removed before publishing as an RFC. 27 Discussion of this draft takes place on the HTTP working group 28 mailing list (ietf-http-wg@w3.org), which is archived at 29 . 31 Working Group information can be found at ; 32 source code and issues list for this draft can be found at 33 . 35 The changes in this draft are summarized in Appendix C.13. 37 Status of This Memo 39 This Internet-Draft is submitted in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at https://datatracker.ietf.org/drafts/current/. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 52 This Internet-Draft will expire on April 5, 2021. 54 Copyright Notice 56 Copyright (c) 2020 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 61 license-info) in effect on the date of publication of this document. 62 Please review these documents carefully, as they describe your rights 63 and restrictions with respect to this document. Code Components 64 extracted from this document must include Simplified BSD License text 65 as described in Section 4.e of the Trust Legal Provisions and are 66 provided without warranty as described in the Simplified BSD License. 68 This document may contain material from IETF Documents or IETF 69 Contributions published or made publicly available before November 70 10, 2008. The person(s) controlling the copyright in some of this 71 material may not have granted the IETF Trust the right to allow 72 modifications of such material outside the IETF Standards Process. 73 Without obtaining an adequate license from the person(s) controlling 74 the copyright in such materials, this document may not be modified 75 outside the IETF Standards Process, and derivative works of it may 76 not be created outside the IETF Standards Process, except to format 77 it for publication as an RFC or to translate it into languages other 78 than English. 80 Table of Contents 82 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 83 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5 84 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5 85 1.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 6 86 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 87 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 88 3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8 89 3.2. Storing Incomplete Responses . . . . . . . . . . . . . . 9 90 3.3. Storing Responses to Authenticated Requests . . . . . . . 9 91 3.4. Combining Partial Content . . . . . . . . . . . . . . . . 10 92 4. Constructing Responses from Caches . . . . . . . . . . . . . 10 93 4.1. Calculating Cache Keys with Vary . . . . . . . . . . . . 11 94 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 12 95 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 14 96 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 14 97 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 15 98 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 16 99 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 17 100 4.3.1. Sending a Validation Request . . . . . . . . . . . . 17 101 4.3.2. Handling a Received Validation Request . . . . . . . 18 102 4.3.3. Handling a Validation Response . . . . . . . . . . . 19 103 4.3.4. Freshening Stored Responses upon Validation . . . . . 20 104 4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 21 105 4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . 21 106 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 22 107 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 108 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 23 109 5.2.1. Request Cache-Control Directives . . . . . . . . . . 24 110 5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 24 111 5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 24 112 5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 25 113 5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 25 114 5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 25 115 5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 26 116 5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 26 117 5.2.2. Response Cache-Control Directives . . . . . . . . . . 26 118 5.2.2.1. must-revalidate . . . . . . . . . . . . . . . . . 26 119 5.2.2.2. must-understand . . . . . . . . . . . . . . . . . 27 120 5.2.2.3. no-cache . . . . . . . . . . . . . . . . . . . . 27 121 5.2.2.4. no-store . . . . . . . . . . . . . . . . . . . . 28 122 5.2.2.5. no-transform . . . . . . . . . . . . . . . . . . 28 123 5.2.2.6. public . . . . . . . . . . . . . . . . . . . . . 28 124 5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 28 125 5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 29 126 5.2.2.9. max-age . . . . . . . . . . . . . . . . . . . . . 29 127 5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 30 128 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 30 129 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 31 130 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32 131 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33 132 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 33 133 6. Relationship to Applications . . . . . . . . . . . . . . . . 33 134 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34 135 7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 34 136 7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 34 137 7.3. Caching of Sensitive Information . . . . . . . . . . . . 35 138 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 139 8.1. Field Registration . . . . . . . . . . . . . . . . . . . 35 140 8.2. Cache Directive Registration . . . . . . . . . . . . . . 35 141 8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 35 142 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 143 9.1. Normative References . . . . . . . . . . . . . . . . . . 35 144 9.2. Informative References . . . . . . . . . . . . . . . . . 36 145 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 37 146 Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 37 147 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 38 148 C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 38 149 C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 39 150 C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 39 151 C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 39 152 C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 39 153 C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 40 154 C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 40 155 C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 40 156 C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 41 157 C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 41 158 C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 41 159 C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 41 160 C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 42 161 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42 162 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 164 1. Introduction 166 The Hypertext Transfer Protocol (HTTP) is a stateless application- 167 level request/response protocol that uses extensible semantics and 168 self-descriptive messages for flexible interaction with network-based 169 hypertext information systems. HTTP is defined by a series of 170 documents that collectively form the HTTP/1.1 specification: 172 o "HTTP Semantics" [Semantics] 174 o "HTTP Caching" (this document) 176 o "HTTP/1.1 Messaging" [Messaging] 178 HTTP is typically used for distributed information systems, where the 179 use of response caches can improve performance. This document 180 defines aspects of HTTP related to caching and reusing response 181 messages. 183 An HTTP cache is a local store of response messages and the subsystem 184 that controls storage, retrieval, and deletion of messages in it. A 185 cache stores cacheable responses to reduce the response time and 186 network bandwidth consumption on future equivalent requests. Any 187 client or server MAY use a cache, though a server that is acting as a 188 tunnel cannot. 190 A shared cache is a cache that stores responses for reuse by more 191 than one user; shared caches are usually (but not always) deployed as 192 a part of an intermediary. A private cache, in contrast, is 193 dedicated to a single user; often, they are deployed as a component 194 of a user agent. 196 HTTP caching's goal is significantly improving performance by reusing 197 a prior response message to satisfy a current request. A cache 198 considers a stored response "fresh", as defined in Section 4.2, if it 199 can be reused without "validation" (checking with the origin server 200 to see if the cached response remains valid for this request). A 201 fresh response can therefore reduce both latency and network overhead 202 each time the cache reuses it. When a cached response is not fresh, 203 it might still be reusable if validation can freshen it (Section 4.3) 204 or if the origin is unavailable (Section 4.2.4). 206 This document obsoletes RFC 7234, with the changes being summarized 207 in Appendix B. 209 1.1. Requirements Notation 211 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 212 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 213 "OPTIONAL" in this document are to be interpreted as described in BCP 214 14 [RFC2119] [RFC8174] when, and only when, they appear in all 215 capitals, as shown here. 217 Section 2 of [Semantics] defines conformance criteria and contains 218 considerations regarding error handling. 220 1.2. Syntax Notation 222 This specification uses the Augmented Backus-Naur Form (ABNF) 223 notation of [RFC5234], extended with the notation for case- 224 sensitivity in strings defined in [RFC7405]. 226 It also uses a list extension, defined in Section 5.7.1 of 227 [Semantics], that allows for compact definition of comma-separated 228 lists using a '#' operator (similar to how the '*' operator indicates 229 repetition). Appendix A shows the collected grammar with all list 230 operators expanded to standard ABNF notation. 232 The following core rules are included by reference, as defined in 233 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 234 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 235 HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line 236 feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any 237 visible [USASCII] character). 239 [Semantics] defines the following rules: 241 HTTP-date = 242 OWS = 243 field-name = 244 quoted-string = 245 token = 247 1.3. Delta Seconds 249 The delta-seconds rule specifies a non-negative integer, representing 250 time in seconds. 252 delta-seconds = 1*DIGIT 254 A recipient parsing a delta-seconds value and converting it to binary 255 form ought to use an arithmetic type of at least 31 bits of non- 256 negative integer range. If a cache receives a delta-seconds value 257 greater than the greatest integer it can represent, or if any of its 258 subsequent calculations overflows, the cache MUST consider the value 259 to be 2147483648 (2^31) or the greatest positive integer it can 260 conveniently represent. 262 | *Note:* The value 2147483648 is here for historical reasons, 263 | represents infinity (over 68 years), and does not need to be 264 | stored in binary form; an implementation could produce it as a 265 | canned string if any overflow occurs, even if the calculations 266 | are performed with an arithmetic type incapable of directly 267 | representing that number. What matters here is that an 268 | overflow be detected and not treated as a negative value in 269 | later calculations. 271 2. Overview of Cache Operation 273 Proper cache operation preserves the semantics of HTTP transfers 274 ([Semantics]) while reducing the transfer of information already held 275 in the cache. Although caching is an entirely OPTIONAL feature of 276 HTTP, it can be assumed that reusing a cached response is desirable 277 and that such reuse is the default behavior when no requirement or 278 local configuration prevents it. Therefore, HTTP cache requirements 279 are focused on preventing a cache from either storing a non-reusable 280 response or reusing a stored response inappropriately, rather than 281 mandating that caches always store and reuse particular responses. 283 The base cache key comprises the request method and target URI used 284 to retrieve the stored response; the method determines under which 285 circumstances that response can be used to satisfy a request. 286 However, many HTTP caches in common use today only cache GET 287 responses, and therefore only use the URI as the cache key, 288 forwarding other methods. 290 If a request target is subject to content negotiation, the cache 291 might store multiple responses for it. Caches differentiate these 292 responses by incorporating values of the original request's selecting 293 header fields into the cache key as well, as per Section 4.1. 295 Caches might incorporate additional material into the cache key. For 296 example, user agent caches might include the referring site's 297 identity, thereby "double keying" the cache to avoid some privacy 298 risks (see Section 7.2). 300 Most commonly, caches store the successful result of a retrieval 301 request: i.e., a 200 (OK) response to a GET request, which contains a 302 representation of the target resource (Section 8.3.1 of [Semantics]). 303 However, it is also possible to store redirects, negative results 304 (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial 305 Content)), and responses to methods other than GET if the method's 306 definition allows such caching and defines something suitable for use 307 as a cache key. 309 A cache is disconnected when it cannot contact the origin server or 310 otherwise find a forward path for a request. A disconnected cache 311 can serve stale responses in some circumstances (Section 4.2.4). 313 3. Storing Responses in Caches 315 A cache MUST NOT store a response to a request unless: 317 o the request method is understood by the cache; 319 o the response status code is final (see Section 14 of [Semantics]); 321 o if the response status code is 206 or 304, or the "must- 322 understand" cache directive (see Section 5.2) is present: the 323 cache understands the response status code; 325 o the "no-store" cache directive is not present in the response (see 326 Section 5.2); 328 o if the cache is shared: the "private" response directive is either 329 not present or allows a shared cache to store a modified response; 330 see Section 5.2.2.7); 332 o if the cache is shared: the Authorization header field is not 333 present in the request (see Section 10.6.2 of [Semantics]) or a 334 response directive is present that explicitly allows shared 335 caching (see Section 3.3); and, 337 o the response contains at least one of: 339 * a public response directive (see Section 5.2.2.6); 341 * a private response directive, if the cache is not shared (see 342 Section 5.2.2.7); 344 * an Expires header field (see Section 5.3); 346 * a max-age response directive (see Section 5.2.2.9); 348 * if the cache is shared, an s-maxage response directive (see 349 Section 5.2.2.10); 351 * a Cache Control Extension that allows it to be cached (see 352 Section 5.2.3); or, 354 * a status code that is defined as heuristically cacheable (see 355 Section 4.2.2). 357 Note that a cache-control extension can override any of the 358 requirements listed; see Section 5.2.3. 360 In this context, a cache has "understood" a request method or a 361 response status code if it recognizes it and implements all specified 362 caching-related behavior. 364 Note that, in normal operation, some caches will not store a response 365 that has neither a cache validator nor an explicit expiration time, 366 as such responses are not usually useful to store. However, caches 367 are not prohibited from storing such responses. 369 3.1. Storing Header and Trailer Fields 371 Caches MUST include all received header fields - including 372 unrecognised ones - when storing a response; this assures that new 373 HTTP header fields can be successfully deployed. However, the 374 following exceptions are made: 376 o The Connection header field and fields whose names are listed in 377 it are required by Section 13.1 of [Messaging] to be removed 378 before forwarding the message. This MAY be implemented by doing 379 so before storage. 381 o Likewise, some fields' semantics require them to be removed before 382 forwarding the message, and this MAY be implemented by doing so 383 before storage; see Section 13.1 of [Messaging] for some examples. 385 o Header fields that are specific to a client's proxy configuration 386 MUST NOT be stored, unless the cache incorporates the identity of 387 the proxy into the cache key. Effectively, this is limited to 388 Proxy-Authenticate (Section 10.7.1 of [Semantics]), Proxy- 389 Authentication-Info (Section 10.7.3 of [Semantics]), and Proxy- 390 Authorization (Section 10.7.2 of [Semantics]). 392 Caches MAY either store trailer fields separate from header fields, 393 or discard them. Caches MUST NOT combine trailer fields with header 394 fields. 396 3.2. Storing Incomplete Responses 398 If the request method is GET, the response status code is 200 (OK), 399 and the entire response header section has been received, a cache MAY 400 store a response body that is not complete (Section 3.3 of 401 [Semantics]) if the stored response is recorded as being incomplete. 402 Likewise, a 206 (Partial Content) response MAY be stored as if it 403 were an incomplete 200 (OK) response. However, a cache MUST NOT 404 store incomplete or partial-content responses if it does not support 405 the Range and Content-Range header fields or if it does not 406 understand the range units used in those fields. 408 A cache MAY complete a stored incomplete response by making a 409 subsequent range request (Section 13.2 of [Semantics]) and combining 410 the successful response with the stored response, as defined in 411 Section 3.4. A cache MUST NOT use an incomplete response to answer 412 requests unless the response has been made complete, or the request 413 is partial and specifies a range wholly within the incomplete 414 response. A cache MUST NOT send a partial response to a client 415 without explicitly marking it using the 206 (Partial Content) status 416 code. 418 3.3. Storing Responses to Authenticated Requests 420 A shared cache MUST NOT use a cached response to a request with an 421 Authorization header field (Section 10.6.2 of [Semantics]) to satisfy 422 any subsequent request unless the response contains a Cache-Control 423 field with a response directive (Section 5.2.2) that allows it to be 424 stored by a shared cache and the cache conforms to the requirements 425 of that directive for that response. 427 In this specification, the following response directives have such an 428 effect: must-revalidate (Section 5.2.2.1), public (Section 5.2.2.6), 429 and s-maxage (Section 5.2.2.10). 431 3.4. Combining Partial Content 433 A response might transfer only a partial representation if the 434 connection closed prematurely or if the request used one or more 435 Range specifiers (Section 13.2 of [Semantics]). After several such 436 transfers, a cache might have received several ranges of the same 437 representation. A cache MAY combine these ranges into a single 438 stored response, and reuse that response to satisfy later requests, 439 if they all share the same strong validator and the cache complies 440 with the client requirements in Section 14.3.7.3 of [Semantics]. 442 When combining the new response with one or more stored responses, a 443 cache MUST use the header fields provided in the new response, aside 444 from Content-Range, to replace all instances of the corresponding 445 header fields in the stored response. 447 4. Constructing Responses from Caches 449 When presented with a request, a cache MUST NOT reuse a stored 450 response, unless: 452 o The presented target URI (Section 6.1 of [Semantics]) and that of 453 the stored response match, and 455 o the request method associated with the stored response allows it 456 to be used for the presented request, and 458 o selecting header fields nominated by the stored response (if any) 459 match those presented (see Section 4.1), and 461 o the stored response does not contain the no-cache cache directive 462 (Section 5.2.2.3), unless it is successfully validated 463 (Section 4.3), and 465 o the stored response is either: 467 * fresh (see Section 4.2), or 469 * allowed to be served stale (see Section 4.2.4), or 471 * successfully validated (see Section 4.3). 473 Note that a cache-control extension can override any of the 474 requirements listed; see Section 5.2.3. 476 When a stored response is used to satisfy a request without 477 validation, a cache MUST generate an Age header field (Section 5.1), 478 replacing any present in the response with a value equal to the 479 stored response's current_age; see Section 4.2.3. 481 A cache MUST write through requests with methods that are unsafe 482 (Section 8.2.1 of [Semantics]) to the origin server; i.e., a cache is 483 not allowed to generate a reply to such a request before having 484 forwarded the request and having received a corresponding response. 486 Also, note that unsafe requests might invalidate already-stored 487 responses; see Section 4.4. 489 When more than one suitable response is stored, a cache MUST use the 490 most recent one (as determined by the Date header field). It can 491 also forward the request with "Cache-Control: max-age=0" or "Cache- 492 Control: no-cache" to disambiguate which response to use. 494 A cache that does not have a clock available MUST NOT use stored 495 responses without revalidating them upon every use. 497 4.1. Calculating Cache Keys with Vary 499 When a cache receives a request that can be satisfied by a stored 500 response that has a Vary header field (Section 11.2.1 of 501 [Semantics]), it MUST NOT use that response unless all the selecting 502 header fields nominated by the Vary header field match in both the 503 original request (i.e., that associated with the stored response), 504 and the presented request. 506 The selecting header fields from two requests are defined to match if 507 and only if those in the first request can be transformed to those in 508 the second request by applying any of: 510 o adding or removing whitespace, where allowed in the header field's 511 syntax 513 o combining multiple header fields with the same field name (see 514 Section 5.4.4 of [Semantics]) 516 o normalizing both header field values in a way that is known to 517 have identical semantics, according to the header field's 518 specification (e.g., reordering field values when order is not 519 significant; case-normalization, where values are defined to be 520 case-insensitive) 522 If (after any normalization that might take place) a header field is 523 absent from a request, it can only match another request if it is 524 also absent there. 526 A Vary header field value containing a member "*" always fails to 527 match. 529 The stored response with matching selecting header fields is known as 530 the selected response. 532 If multiple selected responses are available (potentially including 533 responses without a Vary header field), the cache will need to choose 534 one to use. When a selecting header field has a known mechanism for 535 doing so (e.g., qvalues on Accept and similar request header fields), 536 that mechanism MAY be used to select preferred responses; of the 537 remainder, the most recent response (as determined by the Date header 538 field) is used, as per Section 4. 540 Note that in practice, some resources might send the Vary header 541 field on responses inconsistently. When a cache has multiple 542 responses for a target URI, and one or more omits the Vary header 543 field, it SHOULD use the most recent non-empty value available to 544 select an appropriate response for the request. 546 If no selected response is available, the cache cannot satisfy the 547 presented request. Typically, it is forwarded to the origin server 548 in a (possibly conditional; see Section 4.3) request. 550 4.2. Freshness 552 A fresh response is one whose age has not yet exceeded its freshness 553 lifetime. Conversely, a stale response is one where it has. 555 A response's freshness lifetime is the length of time between its 556 generation by the origin server and its expiration time. An explicit 557 expiration time is the time at which the origin server intends that a 558 stored response can no longer be used by a cache without further 559 validation, whereas a heuristic expiration time is assigned by a 560 cache when no explicit expiration time is available. 562 A response's age is the time that has passed since it was generated 563 by, or successfully validated with, the origin server. 565 When a response is "fresh" in the cache, it can be used to satisfy 566 subsequent requests without contacting the origin server, thereby 567 improving efficiency. 569 The primary mechanism for determining freshness is for an origin 570 server to provide an explicit expiration time in the future, using 571 either the Expires header field (Section 5.3) or the max-age response 572 directive (Section 5.2.2.9). Generally, origin servers will assign 573 future explicit expiration times to responses in the belief that the 574 representation is not likely to change in a semantically significant 575 way before the expiration time is reached. 577 If an origin server wishes to force a cache to validate every 578 request, it can assign an explicit expiration time in the past to 579 indicate that the response is already stale. Compliant caches will 580 normally validate a stale cached response before reusing it for 581 subsequent requests (see Section 4.2.4). 583 Since origin servers do not always provide explicit expiration times, 584 caches are also allowed to use a heuristic to determine an expiration 585 time under certain circumstances (see Section 4.2.2). 587 The calculation to determine if a response is fresh is: 589 response_is_fresh = (freshness_lifetime > current_age) 591 freshness_lifetime is defined in Section 4.2.1; current_age is 592 defined in Section 4.2.3. 594 Clients can send the max-age or min-fresh request directives 595 (Section 5.2.1) to constrain or relax freshness calculations for the 596 corresponding response. However, caches are not required to honor 597 them. 599 When calculating freshness, to avoid common problems in date parsing: 601 o Although all date formats are specified to be case-sensitive, a 602 cache recipient SHOULD match day, week, and time-zone names case- 603 insensitively. 605 o If a cache recipient's internal implementation of time has less 606 resolution than the value of an HTTP-date, the recipient MUST 607 internally represent a parsed Expires date as the nearest time 608 equal to or earlier than the received value. 610 o A cache recipient MUST NOT allow local time zones to influence the 611 calculation or comparison of an age or expiration time. 613 o A cache recipient SHOULD consider a date with a zone abbreviation 614 other than GMT or UTC to be invalid for calculating expiration. 616 Note that freshness applies only to cache operation; it cannot be 617 used to force a user agent to refresh its display or reload a 618 resource. See Section 6 for an explanation of the difference between 619 caches and history mechanisms. 621 4.2.1. Calculating Freshness Lifetime 623 A cache can calculate the freshness lifetime (denoted as 624 freshness_lifetime) of a response by using the first match of: 626 o If the cache is shared and the s-maxage response directive 627 (Section 5.2.2.10) is present, use its value, or 629 o If the max-age response directive (Section 5.2.2.9) is present, 630 use its value, or 632 o If the Expires response header field (Section 5.3) is present, use 633 its value minus the value of the Date response header field, or 635 o Otherwise, no explicit expiration time is present in the response. 636 A heuristic freshness lifetime might be applicable; see 637 Section 4.2.2. 639 Note that this calculation is not vulnerable to clock skew, since all 640 of the information comes from the origin server. 642 When there is more than one value present for a given directive 643 (e.g., two Expires header fields, multiple Cache-Control: max-age 644 directives), the directive's value is considered invalid. Caches are 645 encouraged to consider responses that have invalid freshness 646 information to be stale. 648 4.2.2. Calculating Heuristic Freshness 650 Since origin servers do not always provide explicit expiration times, 651 a cache MAY assign a heuristic expiration time when an explicit time 652 is not specified, employing algorithms that use other header field 653 values (such as the Last-Modified time) to estimate a plausible 654 expiration time. This specification does not provide specific 655 algorithms, but does impose worst-case constraints on their results. 657 A cache MUST NOT use heuristics to determine freshness when an 658 explicit expiration time is present in the stored response. Because 659 of the requirements in Section 3, this means that heuristics can only 660 be used on responses without explicit freshness whose status codes 661 are defined as "heuristically cacheable" (e.g., see Section 14.1 of 662 [Semantics]), and those responses without explicit freshness that 663 have been marked as explicitly cacheable (e.g., with a "public" 664 response directive). 666 Note that in previous specifications heuristically cacheable response 667 status codes were called "cacheable by default." 669 If the response has a Last-Modified header field (Section 7.9.2 of 670 [Semantics]), caches are encouraged to use a heuristic expiration 671 value that is no more than some fraction of the interval since that 672 time. A typical setting of this fraction might be 10%. 674 | *Note:* Section 13.9 of [RFC2616] prohibited caches from 675 | calculating heuristic freshness for URIs with query components 676 | (i.e., those containing '?'). In practice, this has not been 677 | widely implemented. Therefore, origin servers are encouraged 678 | to send explicit directives (e.g., Cache-Control: no-cache) if 679 | they wish to prevent caching. 681 4.2.3. Calculating Age 683 The Age header field is used to convey an estimated age of the 684 response message when obtained from a cache. The Age field value is 685 the cache's estimate of the number of seconds since the origin server 686 generated or validated the response. The Age value is therefore the 687 sum of the time that the response has been resident in each of the 688 caches along the path from the origin server, plus the time it has 689 been in transit along network paths. 691 Age calculation uses the following data: 693 age_value The term "age_value" denotes the value of the Age header 694 field (Section 5.1), in a form appropriate for arithmetic 695 operation; or 0, if not available. 697 date_value The term "date_value" denotes the value of the Date 698 header field, in a form appropriate for arithmetic operations. 699 See Section 9.2.2 of [Semantics] for the definition of the Date 700 header field, and for requirements regarding responses without it. 702 now The term "now" means "the current value of the clock at the host 703 performing the calculation". A host ought to use NTP ([RFC5905]) 704 or some similar protocol to synchronize its clocks to Coordinated 705 Universal Time. 707 request_time The current value of the clock at the host at the time 708 the request resulting in the stored response was made. 710 response_time The current value of the clock at the host at the time 711 the response was received. 713 A response's age can be calculated in two entirely independent ways: 715 1. the "apparent_age": response_time minus date_value, if the local 716 clock is reasonably well synchronized to the origin server's 717 clock. If the result is negative, the result is replaced by 718 zero. 720 2. the "corrected_age_value", if all of the caches along the 721 response path implement HTTP/1.1 or greater. A cache MUST 722 interpret this value relative to the time the request was 723 initiated, not the time that the response was received. 725 apparent_age = max(0, response_time - date_value); 727 response_delay = response_time - request_time; 728 corrected_age_value = age_value + response_delay; 730 These are combined as 732 corrected_initial_age = max(apparent_age, corrected_age_value); 734 unless the cache is confident in the value of the Age header field 735 (e.g., because there are no HTTP/1.0 hops in the Via header field), 736 in which case the corrected_age_value MAY be used as the 737 corrected_initial_age. 739 The current_age of a stored response can then be calculated by adding 740 the time (in seconds) since the stored response was last validated by 741 the origin server to the corrected_initial_age. 743 resident_time = now - response_time; 744 current_age = corrected_initial_age + resident_time; 746 4.2.4. Serving Stale Responses 748 A "stale" response is one that either has explicit expiry information 749 or is allowed to have heuristic expiry calculated, but is not fresh 750 according to the calculations in Section 4.2. 752 A cache MUST NOT generate a stale response if it is prohibited by an 753 explicit in-protocol directive (e.g., by a "no-store" or "no-cache" 754 cache directive, a "must-revalidate" cache-response-directive, or an 755 applicable "s-maxage" or "proxy-revalidate" cache-response-directive; 756 see Section 5.2.2). 758 A cache MUST NOT generate a stale response unless it is disconnected 759 or doing so is explicitly permitted by the client or origin server 760 (e.g., by the max-stale request directive in Section 5.2.1, by 761 extension directives such as those defined in [RFC5861], or by 762 configuration in accordance with an out-of-band contract). 764 4.3. Validation 766 When a cache has one or more stored responses for a requested URI, 767 but cannot serve any of them (e.g., because they are not fresh, or 768 one cannot be selected; see Section 4.1), it can use the conditional 769 request mechanism Section 12.1 of [Semantics] in the forwarded 770 request to give the next inbound server an opportunity to select a 771 valid stored response to use, updating the stored metadata in the 772 process, or to replace the stored response(s) with a new response. 773 This process is known as "validating" or "revalidating" the stored 774 response. 776 4.3.1. Sending a Validation Request 778 When generating a conditional request for validation, a cache starts 779 with either a request it is attempting to satisfy, or - if it is 780 initiating the request independently - it synthesises a request using 781 a stored response by copying the method, target URI, and request 782 header fields identified by the Vary header field Section 4.1. 784 It then updates that request with one or more precondition header 785 fields. These contain validator metadata sourced from stored 786 response(s) that have the same cache key. 788 The precondition header fields are then compared by recipients to 789 determine whether any stored response is equivalent to a current 790 representation of the resource. 792 One such validator is the timestamp given in a Last-Modified header 793 field (Section 7.9.2 of [Semantics]), which can be used in an If- 794 Modified-Since header field for response validation, or in an If- 795 Unmodified-Since or If-Range header field for representation 796 selection (i.e., the client is referring specifically to a previously 797 obtained representation with that timestamp). 799 Another validator is the entity-tag given in an ETag field 800 (Section 7.9.3 of [Semantics]). One or more entity-tags, indicating 801 one or more stored responses, can be used in an If-None-Match header 802 field for response validation, or in an If-Match or If-Range header 803 field for representation selection (i.e., the client is referring 804 specifically to one or more previously obtained representations with 805 the listed entity-tags). 807 4.3.2. Handling a Received Validation Request 809 Each client in the request chain may have its own cache, so it is 810 common for a cache at an intermediary to receive conditional requests 811 from other (outbound) caches. Likewise, some user agents make use of 812 conditional requests to limit data transfers to recently modified 813 representations or to complete the transfer of a partially retrieved 814 representation. 816 If a cache receives a request that can be satisfied by reusing one of 817 its stored 200 (OK) or 206 (Partial Content) responses, the cache 818 SHOULD evaluate any applicable conditional header field preconditions 819 received in that request with respect to the corresponding validators 820 contained within the selected response. A cache MUST NOT evaluate 821 conditional header fields that only apply to an origin server, occur 822 in a request with semantics that cannot be satisfied with a cached 823 response, or occur in a request with a target resource for which it 824 has no stored responses; such preconditions are likely intended for 825 some other (inbound) server. 827 The proper evaluation of conditional requests by a cache depends on 828 the received precondition header fields and their precedence, as 829 defined in Section 12.3 of [Semantics]. The If-Match and If- 830 Unmodified-Since conditional header fields are not applicable to a 831 cache. 833 A request containing an If-None-Match header field (Section 12.1.2 of 834 [Semantics]) indicates that the client wants to validate one or more 835 of its own stored responses in comparison to whichever stored 836 response is selected by the cache. If the field value is "*", or if 837 the field value is a list of entity-tags and at least one of them 838 matches the entity-tag of the selected stored response, a cache 839 recipient SHOULD generate a 304 (Not Modified) response (using the 840 metadata of the selected stored response) instead of sending that 841 stored response. 843 When a cache decides to revalidate its own stored responses for a 844 request that contains an If-None-Match list of entity-tags, the cache 845 MAY combine the received list with a list of entity-tags from its own 846 stored set of responses (fresh or stale) and send the union of the 847 two lists as a replacement If-None-Match header field value in the 848 forwarded request. If a stored response contains only partial 849 content, the cache MUST NOT include its entity-tag in the union 850 unless the request is for a range that would be fully satisfied by 851 that partial stored response. If the response to the forwarded 852 request is 304 (Not Modified) and has an ETag field value with an 853 entity-tag that is not in the client's list, the cache MUST generate 854 a 200 (OK) response for the client by reusing its corresponding 855 stored response, as updated by the 304 response metadata 856 (Section 4.3.4). 858 If an If-None-Match header field is not present, a request containing 859 an If-Modified-Since header field (Section 12.1.3 of [Semantics]) 860 indicates that the client wants to validate one or more of its own 861 stored responses by modification date. A cache recipient SHOULD 862 generate a 304 (Not Modified) response (using the metadata of the 863 selected stored response) if one of the following cases is true: 1) 864 the selected stored response has a Last-Modified field value that is 865 earlier than or equal to the conditional timestamp; 2) no Last- 866 Modified field is present in the selected stored response, but it has 867 a Date field value that is earlier than or equal to the conditional 868 timestamp; or, 3) neither Last-Modified nor Date is present in the 869 selected stored response, but the cache recorded it as having been 870 received at a time earlier than or equal to the conditional 871 timestamp. 873 A cache that implements partial responses to range requests, as 874 defined in Section 13.2 of [Semantics], also needs to evaluate a 875 received If-Range header field (Section 12.1.5 of [Semantics]) 876 regarding its selected stored response. 878 4.3.3. Handling a Validation Response 880 Cache handling of a response to a conditional request depends upon 881 its status code: 883 o A 304 (Not Modified) response status code indicates that the 884 stored response can be updated and reused; see Section 4.3.4. 886 o A full response (i.e., one with a payload body) indicates that 887 none of the stored responses nominated in the conditional request 888 is suitable. Instead, the cache MUST use the full response to 889 satisfy the request and MAY replace the stored response(s). 891 o However, if a cache receives a 5xx (Server Error) response while 892 attempting to validate a response, it can either forward this 893 response to the requesting client, or act as if the server failed 894 to respond. In the latter case, the cache MAY send a previously 895 stored response (see Section 4.2.4). 897 4.3.4. Freshening Stored Responses upon Validation 899 When a cache receives a 304 (Not Modified) response and already has 900 one or more stored 200 (OK) responses for the applicable cache key, 901 the cache needs to identify which (if any) are to be updated by the 902 new information provided, and then do so. 904 The stored response(s) to update are identified by using the first 905 match (if any) of: 907 o If the new response contains a strong validator (see Section 7.9.1 908 of [Semantics]), then that strong validator identifies the 909 selected representation for update. All the stored responses with 910 the same strong validator are identified for update. If none of 911 the stored responses contain the same strong validator, then the 912 cache MUST NOT use the new response to update any stored 913 responses. 915 o If the new response contains a weak validator and that validator 916 corresponds to one of the cache's stored responses, then the most 917 recent of those matching stored responses is identified for 918 update. 920 o If the new response does not include any form of validator (such 921 as where a client generates an If-Modified-Since request from a 922 source other than the Last-Modified response header field), and 923 there is only one stored response, and that stored response also 924 lacks a validator, then that stored response is identified for 925 update. 927 For each stored response identified for update, the cache MUST use 928 the header fields provided in the 304 (Not Modified) response to 929 replace all instances of the corresponding header fields in the 930 stored response, with the following exceptions: 932 o The exceptions to header field storage in Section 3.1 also apply 933 to header field updates. 935 o Caches MUST NOT update the following header fields: Content- 936 Encoding, Content-Length, Content-MD5 (Section 14.15 of 937 [RFC2616]), Content-Range, ETag. 939 4.3.5. Freshening Responses with HEAD 941 A response to the HEAD method is identical to what an equivalent 942 request made with a GET would have been, except it lacks a body. 943 This property of HEAD responses can be used to invalidate or update a 944 cached GET response if the more efficient conditional GET request 945 mechanism is not available (due to no validators being present in the 946 stored response) or if transmission of the representation body is not 947 desired even if it has changed. 949 When a cache makes an inbound HEAD request for a target URI and 950 receives a 200 (OK) response, the cache SHOULD update or invalidate 951 each of its stored GET responses that could have been selected for 952 that request (see Section 4.1). 954 For each of the stored responses that could have been selected, if 955 the stored response and HEAD response have matching values for any 956 received validator fields (ETag and Last-Modified) and, if the HEAD 957 response has a Content-Length header field, the value of Content- 958 Length matches that of the stored response, the cache SHOULD update 959 the stored response as described below; otherwise, the cache SHOULD 960 consider the stored response to be stale. 962 If a cache updates a stored response with the metadata provided in a 963 HEAD response, the cache MUST use the header fields provided in the 964 HEAD response to replace all instances of the corresponding header 965 fields in the stored response (subject to the exceptions in 966 Section 4.3.4) and append new header fields to the stored response's 967 header section unless otherwise restricted by the Cache-Control 968 header field. 970 4.4. Invalidation 972 Because unsafe request methods (Section 8.2.1 of [Semantics]) such as 973 PUT, POST or DELETE have the potential for changing state on the 974 origin server, intervening caches are required to invalidate stored 975 responses to keep their contents up to date. Invalidate means that 976 the cache will either remove all stored responses whose target URI 977 matches the given URI, or will mark them as "invalid" and in need of 978 a mandatory validation before they can be sent in response to a 979 subsequent request. 981 Note that this does not guarantee that all appropriate responses are 982 invalidated globally; a state-changing request would only invalidate 983 responses in the caches it travels through. 985 A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) 986 and the URI(s) in the Location and Content-Location response header 987 fields (if present) when a non-error status code is received in 988 response to an unsafe request method. 990 However, a cache MUST NOT invalidate a URI from a Location or 991 Content-Location response header field if the host part of that URI 992 differs from the host part in the target URI (Section 6.1 of 993 [Semantics]). This helps prevent denial-of-service attacks. 995 A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) 996 when it receives a non-error response to a request with a method 997 whose safety is unknown. 999 Here, a "non-error response" is one with a 2xx (Successful) or 3xx 1000 (Redirection) status code. 1002 5. Field Definitions 1004 This section defines the syntax and semantics of HTTP fields related 1005 to caching. 1007 --------------- ----------- ------ 1008 Field Name Status Ref. 1009 --------------- ----------- ------ 1010 Age standard 5.1 1011 Cache-Control standard 5.2 1012 Expires standard 5.3 1013 Pragma standard 5.4 1014 Warning obsoleted 5.5 1015 --------------- ----------- ------ 1017 Table 1 1019 5.1. Age 1021 The "Age" header field conveys the sender's estimate of the time 1022 since the response was generated or successfully validated at the 1023 origin server. Age values are calculated as specified in 1024 Section 4.2.3. 1026 Age = delta-seconds 1028 The Age field value is a non-negative integer, representing time in 1029 seconds (see Section 1.3). A cache SHOULD consider a response to be 1030 stale if an Age field is present and its value is invalid (i.e., 1031 contains a list or something other than a non-negative integer). 1033 The presence of an Age header field implies that the response was not 1034 generated or validated by the origin server for this request. 1035 However, lack of an Age header field does not imply the origin was 1036 contacted, since the response might have been received from an 1037 HTTP/1.0 cache that does not implement Age. 1039 5.2. Cache-Control 1041 The "Cache-Control" header field is used to list directives for 1042 caches along the request/response chain. Such cache directives are 1043 unidirectional in that the presence of a directive in a request does 1044 not imply that the same directive is present in the response, or to 1045 be repeated in it. 1047 See Section 5.2.3 for information about how Cache-Control directives 1048 defined elsewhere are handled. 1050 | *Note:* Some HTTP/1.0 caches might not implement Cache-Control. 1052 A proxy, whether or not it implements a cache, MUST pass cache 1053 directives through in forwarded messages, regardless of their 1054 significance to that application, since the directives might apply to 1055 all recipients along the request/response chain. It is not possible 1056 to target a directive to a specific cache. 1058 Cache directives are identified by a token, to be compared case- 1059 insensitively, and have an optional argument that can use both token 1060 and quoted-string syntax. For the directives defined below that 1061 define arguments, recipients ought to accept both forms, even if a 1062 specific form is required for generation. 1064 Cache-Control = #cache-directive 1066 cache-directive = token [ "=" ( token / quoted-string ) ] 1068 For the cache directives defined below, no argument is defined (nor 1069 allowed) unless stated otherwise. 1071 ------------------ ---------------------------------- 1072 Cache Directive Reference 1073 ------------------ ---------------------------------- 1074 max-age Section 5.2.1.1, Section 5.2.2.9 1075 max-stale Section 5.2.1.2 1076 min-fresh Section 5.2.1.3 1077 must-revalidate Section 5.2.2.1 1078 must-understand Section 5.2.2.2 1079 no-cache Section 5.2.1.4, Section 5.2.2.3 1080 no-store Section 5.2.1.5, Section 5.2.2.4 1081 no-transform Section 5.2.1.6, Section 5.2.2.5 1082 only-if-cached Section 5.2.1.7 1083 private Section 5.2.2.7 1084 proxy-revalidate Section 5.2.2.8 1085 public Section 5.2.2.6 1086 s-maxage Section 5.2.2.10 1087 ------------------ ---------------------------------- 1089 Table 2 1091 5.2.1. Request Cache-Control Directives 1093 This section defines cache request directives. They are advisory; 1094 caches MAY implement them, but are not required to. 1096 5.2.1.1. max-age 1098 Argument syntax: 1100 delta-seconds (see Section 1.3) 1102 The "max-age" request directive indicates that the client prefers a 1103 response whose age is less than or equal to the specified number of 1104 seconds. Unless the max-stale request directive is also present, the 1105 client does not wish to receive a stale response. 1107 This directive uses the token form of the argument syntax: e.g., 1108 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the 1109 quoted-string form. 1111 5.2.1.2. max-stale 1113 Argument syntax: 1115 delta-seconds (see Section 1.3) 1117 The "max-stale" request directive indicates that the client will 1118 accept a response that has exceeded its freshness lifetime. If a 1119 value is present, then the client is willing to accept a response 1120 that has exceeded its freshness lifetime by no more than the 1121 specified number of seconds. If no value is assigned to max-stale, 1122 then the client will accept a stale response of any age. 1124 This directive uses the token form of the argument syntax: e.g., 1125 'max-stale=10' not 'max-stale="10"'. A sender MUST NOT generate the 1126 quoted-string form. 1128 5.2.1.3. min-fresh 1130 Argument syntax: 1132 delta-seconds (see Section 1.3) 1134 The "min-fresh" request directive indicates that the client prefers a 1135 response whose freshness lifetime is no less than its current age 1136 plus the specified time in seconds. That is, the client wants a 1137 response that will still be fresh for at least the specified number 1138 of seconds. 1140 This directive uses the token form of the argument syntax: e.g., 1141 'min-fresh=20' not 'min-fresh="20"'. A sender MUST NOT generate the 1142 quoted-string form. 1144 5.2.1.4. no-cache 1146 The "no-cache" request directive indicates that the client prefers 1147 stored response not be used to satisfy the request without successful 1148 validation on the origin server. 1150 5.2.1.5. no-store 1152 The "no-store" request directive indicates that a cache MUST NOT 1153 store any part of either this request or any response to it. This 1154 directive applies to both private and shared caches. "MUST NOT 1155 store" in this context means that the cache MUST NOT intentionally 1156 store the information in non-volatile storage, and MUST make a best- 1157 effort attempt to remove the information from volatile storage as 1158 promptly as possible after forwarding it. 1160 This directive is NOT a reliable or sufficient mechanism for ensuring 1161 privacy. In particular, malicious or compromised caches might not 1162 recognize or obey this directive, and communications networks might 1163 be vulnerable to eavesdropping. 1165 Note that if a request containing this directive is satisfied from a 1166 cache, the no-store request directive does not apply to the already 1167 stored response. 1169 5.2.1.6. no-transform 1171 The "no-transform" request directive indicates that the client is 1172 asking for intermediaries to avoid transforming the payload, as 1173 defined in Section 6.5 of [Semantics]. 1175 5.2.1.7. only-if-cached 1177 The "only-if-cached" request directive indicates that the client only 1178 wishes to obtain a stored response. Caches that honor this request 1179 directive SHOULD, upon receiving it, either respond using a stored 1180 response consistent with the other constraints of the request, or 1181 respond with a 504 (Gateway Timeout) status code. 1183 5.2.2. Response Cache-Control Directives 1185 This section defines cache response directives. A cache MUST obey 1186 the Cache-Control directives defined in this section. 1188 5.2.2.1. must-revalidate 1190 The "must-revalidate" response directive indicates that once the 1191 response has become stale, a cache MUST NOT reuse that response to 1192 satisfy another request until it has been successfully validated by 1193 the origin, as defined by Section 4.3. 1195 The must-revalidate directive is necessary to support reliable 1196 operation for certain protocol features. In all circumstances a 1197 cache MUST obey the must-revalidate directive; in particular, if a 1198 cache is disconnected, the cache MUST generate a 504 (Gateway 1199 Timeout) response rather than reuse the stale response. 1201 The must-revalidate directive ought to be used by servers if and only 1202 if failure to validate a request on the representation could cause 1203 incorrect operation, such as a silently unexecuted financial 1204 transaction. 1206 The must-revalidate directive also permits a shared cache to reuse a 1207 response to a request containing an Authorization header field, 1208 subject to the above requirement on revalidation (Section 3.3). 1210 5.2.2.2. must-understand 1212 The "must-understand" response directive limits caching of the 1213 response to a cache that understands and conforms to the requirements 1214 for that response's status code. A cache MUST NOT store a response 1215 containing the must-understand directive if the cache does not 1216 understand the response status code. 1218 5.2.2.3. no-cache 1220 Argument syntax: 1222 #field-name 1224 The "no-cache" response directive, in its unqualified form (without 1225 an argument), indicates that the response MUST NOT be used to satisfy 1226 any other request without forwarding it for validation and receiving 1227 a successful response; see Section 4.3. 1229 This allows an origin server to prevent a cache from using the 1230 response to satisfy a request without contacting it, even by caches 1231 that have been configured to send stale responses. 1233 The qualified form of no-cache response directive, with an argument 1234 that lists one or more field names, indicates that a cache MAY use 1235 the response to satisfy a subsequent request, subject to any other 1236 restrictions on caching, if the listed header fields are excluded 1237 from the subsequent response or the subsequent response has been 1238 successfully revalidated with the origin server (updating or removing 1239 those fields). This allows an origin server to prevent the re-use of 1240 certain header fields in a response, while still allowing caching of 1241 the rest of the response. 1243 The field names given are not limited to the set of header fields 1244 defined by this specification. Field names are case-insensitive. 1246 This directive uses the quoted-string form of the argument syntax. A 1247 sender SHOULD NOT generate the token form (even if quoting appears 1248 not to be needed for single-entry lists). 1250 *Note:* Although it has been back-ported to many implementations, 1251 some HTTP/1.0 caches will not recognize or obey this directive. 1252 Also, the qualified form of the directive is often handled by caches 1253 as if an unqualified no-cache directive was received; i.e., the 1254 special handling for the qualified form is not widely implemented. 1256 5.2.2.4. no-store 1258 The "no-store" response directive indicates that a cache MUST NOT 1259 store any part of either the immediate request or response, and MUST 1260 NOT use the response to satisfy any other request. 1262 This directive applies to both private and shared caches. "MUST NOT 1263 store" in this context means that the cache MUST NOT intentionally 1264 store the information in non-volatile storage, and MUST make a best- 1265 effort attempt to remove the information from volatile storage as 1266 promptly as possible after forwarding it. 1268 This directive is NOT a reliable or sufficient mechanism for ensuring 1269 privacy. In particular, malicious or compromised caches might not 1270 recognize or obey this directive, and communications networks might 1271 be vulnerable to eavesdropping. 1273 5.2.2.5. no-transform 1275 The "no-transform" response directive indicates that an intermediary 1276 (regardless of whether it implements a cache) MUST NOT transform the 1277 payload, as defined in Section 6.5 of [Semantics]. 1279 5.2.2.6. public 1281 The "public" response directive indicates that a cache MAY store the 1282 response even if it would otherwise be prohibited, subject to the 1283 constraints defined in Section 3. In other words, public explicitly 1284 marks the response as cacheable. For example, public permits a 1285 shared cache to reuse a response to a request containing an 1286 Authorization header field (Section 3.3). 1288 Note that it is unnecessary to add the public directive to a response 1289 that is already cacheable according to Section 3. 1291 If a response with the public directive has no explicit freshness 1292 information, it is heuristically cacheable (Section 4.2.2). 1294 5.2.2.7. private 1296 Argument syntax: 1298 #field-name 1300 The unqualified "private" response directive indicates that a shared 1301 cache MUST NOT store the response (i.e., the response is intended for 1302 a single user). It also indicates that a private cache MAY store the 1303 response, subject the constraints defined in Section 3, even if the 1304 response would not otherwise be heuristically cacheable by a private 1305 cache. 1307 If a qualified private response directive is present, with an 1308 argument that lists one or more field names, then only the listed 1309 fields are limited to a single user: a shared cache MUST NOT store 1310 the listed fields if they are present in the original response, but 1311 MAY store the remainder of the response message without those fields, 1312 subject the constraints defined in Section 3. 1314 The field names given are not limited to the set of header fields 1315 defined by this specification. Field names are case-insensitive. 1317 This directive uses the quoted-string form of the argument syntax. A 1318 sender SHOULD NOT generate the token form (even if quoting appears 1319 not to be needed for single-entry lists). 1321 *Note:* This usage of the word "private" only controls where the 1322 response can be stored; it cannot ensure the privacy of the message 1323 content. Also, the qualified form of the directive is often handled 1324 by caches as if an unqualified private directive was received; i.e., 1325 the special handling for the qualified form is not widely 1326 implemented. 1328 5.2.2.8. proxy-revalidate 1330 The "proxy-revalidate" response directive indicates that once the 1331 response has become stale, a shared cache MUST NOT reuse that 1332 response to satisfy another request until it has been successfully 1333 validated by the origin, as defined by Section 4.3. This is 1334 analogous to must-revalidate (Section 5.2.2.1), except that proxy- 1335 revalidate does not apply to private caches. 1337 Note that "proxy-revalidate" on its own does not imply that a 1338 response is cacheable. For example, it might be combined with the 1339 public directive (Section 5.2.2.6), allowing the response to be 1340 cached while requiring only a shared cache to revalidate when stale. 1342 5.2.2.9. max-age 1344 Argument syntax: 1346 delta-seconds (see Section 1.3) 1348 The "max-age" response directive indicates that the response is to be 1349 considered stale after its age is greater than the specified number 1350 of seconds. 1352 This directive uses the token form of the argument syntax: e.g., 1353 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the 1354 quoted-string form. 1356 5.2.2.10. s-maxage 1358 Argument syntax: 1360 delta-seconds (see Section 1.3) 1362 The "s-maxage" response directive indicates that, for a shared cache, 1363 the maximum age specified by this directive overrides the maximum age 1364 specified by either the max-age directive or the Expires header 1365 field. 1367 The s-maxage directive incorporates the proxy-revalidate 1368 (Section 5.2.2.8) response directive's semantics for a shared cache. 1369 A shared cache MUST NOT reuse a stale response with s-maxage to 1370 satisfy another request until it has been successfully validated by 1371 the origin, as defined by Section 4.3. This directive also permits a 1372 shared cache to reuse a response to a request containing an 1373 Authorization header field, subject to the above requirements on 1374 maximum age and revalidation (Section 3.3). 1376 This directive uses the token form of the argument syntax: e.g., 1377 's-maxage=10' not 's-maxage="10"'. A sender MUST NOT generate the 1378 quoted-string form. 1380 5.2.3. Cache Control Extensions 1382 The Cache-Control header field can be extended through the use of one 1383 or more cache-extension tokens, each with an optional value. A cache 1384 MUST ignore unrecognized cache directives. 1386 Informational extensions (those that do not require a change in cache 1387 behavior) can be added without changing the semantics of other 1388 directives. 1390 Behavioral extensions are designed to work by acting as modifiers to 1391 the existing base of cache directives. Both the new directive and 1392 the old directive are supplied, such that applications that do not 1393 understand the new directive will default to the behavior specified 1394 by the old directive, and those that understand the new directive 1395 will recognize it as modifying the requirements associated with the 1396 old directive. In this way, extensions to the existing cache-control 1397 directives can be made without breaking deployed caches. 1399 For example, consider a hypothetical new response directive called 1400 "community" that acts as a modifier to the private directive: in 1401 addition to private caches, any cache that is shared only by members 1402 of the named community is allowed to cache the response. An origin 1403 server wishing to allow the UCI community to use an otherwise private 1404 response in their shared cache(s) could do so by including 1406 Cache-Control: private, community="UCI" 1408 A cache that recognizes such a community cache-extension could 1409 broaden its behavior in accordance with that extension. A cache that 1410 does not recognize the community cache-extension would ignore it and 1411 adhere to the private directive. 1413 New extension directives ought to consider defining: 1415 o What it means for a directive to be specified multiple times, 1417 o When the directive does not take an argument, what it means when 1418 an argument is present, 1420 o When the directive requires an argument, what it means when it is 1421 missing, 1423 o Whether the directive is specific to requests, responses, or able 1424 to be used in either. 1426 5.2.4. Cache Directive Registry 1428 The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" 1429 defines the namespace for the cache directives. It has been created 1430 and is now maintained at . 1433 A registration MUST include the following fields: 1435 o Cache Directive Name 1437 o Pointer to specification text 1438 Values to be added to this namespace require IETF Review (see 1439 [RFC8126], Section 4.8). 1441 5.3. Expires 1443 The "Expires" header field gives the date/time after which the 1444 response is considered stale. See Section 4.2 for further discussion 1445 of the freshness model. 1447 The presence of an Expires field does not imply that the original 1448 resource will change or cease to exist at, before, or after that 1449 time. 1451 The Expires value is an HTTP-date timestamp, as defined in 1452 Section 5.7.7 of [Semantics]. 1454 Expires = HTTP-date 1456 For example 1458 Expires: Thu, 01 Dec 1994 16:00:00 GMT 1460 A cache recipient MUST interpret invalid date formats, especially the 1461 value "0", as representing a time in the past (i.e., "already 1462 expired"). 1464 If a response includes a Cache-Control field with the max-age 1465 directive (Section 5.2.2.9), a recipient MUST ignore the Expires 1466 field. Likewise, if a response includes the s-maxage directive 1467 (Section 5.2.2.10), a shared cache recipient MUST ignore the Expires 1468 field. In both these cases, the value in Expires is only intended 1469 for recipients that have not yet implemented the Cache-Control field. 1471 An origin server without a clock MUST NOT generate an Expires field 1472 unless its value represents a fixed time in the past (always expired) 1473 or its value has been associated with the resource by a system or 1474 user with a reliable clock. 1476 Historically, HTTP required the Expires field value to be no more 1477 than a year in the future. While longer freshness lifetimes are no 1478 longer prohibited, extremely large values have been demonstrated to 1479 cause problems (e.g., clock overflows due to use of 32-bit integers 1480 for time values), and many caches will evict a response far sooner 1481 than that. 1483 5.4. Pragma 1485 The "Pragma" header field was defined for HTTP/1.0 caches, so that 1486 clients could specify a "no-cache" request (as Cache-Control was not 1487 defined until HTTP/1.1). 1489 However, support for Cache-Control is now widespread. As a result, 1490 this specification deprecates Pragma. 1492 | *Note:* Because the meaning of "Pragma: no-cache" in responses 1493 | was never specified, it does not provide a reliable replacement 1494 | for "Cache-Control: no-cache" in them. 1496 5.5. Warning 1498 The "Warning" header field was used to carry additional information 1499 about the status or transformation of a message that might not be 1500 reflected in the status code. This specification obsoletes it, as it 1501 is not widely generated or surfaced to users. The information it 1502 carried can be gleaned from examining other header fields, such as 1503 Age. 1505 6. Relationship to Applications 1507 Applications using HTTP often specify additional forms of caching. 1508 For example, Web browsers often have history mechanisms such as 1509 "Back" buttons that can be used to redisplay a representation 1510 retrieved earlier in a session. 1512 Likewise, some Web browsers implement caching of images and other 1513 assets within a page view; they may or may not honor HTTP caching 1514 semantics. 1516 The requirements in this specification do not necessarily apply to 1517 how applications use data after it is retrieved from a HTTP cache. 1518 That is, a history mechanism can display a previous representation 1519 even if it has expired, and an application can use cached data in 1520 other ways beyond its freshness lifetime. 1522 This does not prohibit the application from taking HTTP caching into 1523 account; for example, a history mechanism might tell the user that a 1524 view is stale, or it might honor cache directives (e.g., Cache- 1525 Control: no-store). 1527 7. Security Considerations 1529 This section is meant to inform developers, information providers, 1530 and users of known security concerns specific to HTTP caching. More 1531 general security considerations are addressed in HTTP messaging 1532 [Messaging] and semantics [Semantics]. 1534 Caches expose additional potential vulnerabilities, since the 1535 contents of the cache represent an attractive target for malicious 1536 exploitation. Because cache contents persist after an HTTP request 1537 is complete, an attack on the cache can reveal information long after 1538 a user believes that the information has been removed from the 1539 network. Therefore, cache contents need to be protected as sensitive 1540 information. 1542 7.1. Cache Poisoning 1544 Various attacks might be amplified by being stored in a shared cache. 1545 Such "cache poisoning" attacks use the cache to distribute a 1546 malicious payload to many clients, and are especially effective when 1547 an attacker can use implementation flaws, elevated privileges, or 1548 other techniques to insert such a response into a cache. 1550 One common attack vector for cache poisoning is to exploit 1551 differences in message parsing on proxies and in user agents; see 1552 Section 6.3 of [Messaging] for the relevant requirements regarding 1553 HTTP/1.1. 1555 7.2. Timing Attacks 1557 Because one of the primary uses of a cache is to optimise 1558 performance, its use can "leak" information about what resources have 1559 been previously requested. 1561 For example, if a user visits a site and their browser caches some of 1562 its responses, and then navigates to a second site, that site can 1563 attempt to load responses it knows exists on the first site. If they 1564 load quickly, it can be assumed that the user has visited that site, 1565 or even a specific page on it. 1567 Such "timing attacks" can be mitigated by adding more information to 1568 the cache key, such as the identity of the referring site (to prevent 1569 the attack described above). This is sometimes called "double 1570 keying." 1572 7.3. Caching of Sensitive Information 1574 Implementation and deployment flaws (as well as misunderstanding of 1575 cache operation) might lead to caching of sensitive information 1576 (e.g., authentication credentials) that is thought to be private, 1577 exposing it to unauthorized parties. 1579 Note that the Set-Cookie response header field [RFC6265] does not 1580 inhibit caching; a cacheable response with a Set-Cookie header field 1581 can be (and often is) used to satisfy subsequent requests to caches. 1582 Servers who wish to control caching of these responses are encouraged 1583 to emit appropriate Cache-Control response header fields. 1585 8. IANA Considerations 1587 The change controller for the following registrations is: "IETF 1588 (iesg@ietf.org) - Internet Engineering Task Force". 1590 8.1. Field Registration 1592 Please update the "Hypertext Transfer Protocol (HTTP) Field Name 1593 Registry" at with the 1594 field names listed in the two tables of Section 5. 1596 8.2. Cache Directive Registration 1598 Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive 1599 Registry" at 1600 with the registration procedure of Section 5.2.4 and the cache 1601 directive names summarized in the table of Section 5.2. 1603 8.3. Warn Code Registry 1605 Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn 1606 Codes" registry at 1607 to the effect that Warning is obsoleted. 1609 9. References 1611 9.1. Normative References 1613 [Messaging] 1614 Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1615 Ed., "HTTP/1.1 Messaging", Work in Progress, Internet- 1616 Draft, draft-ietf-httpbis-messaging-12, October 2, 2020, 1617 . 1620 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1621 Requirement Levels", BCP 14, RFC 2119, 1622 DOI 10.17487/RFC2119, March 1997, 1623 . 1625 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1626 Specifications: ABNF", STD 68, RFC 5234, 1627 DOI 10.17487/RFC5234, January 2008, 1628 . 1630 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 1631 RFC 7405, DOI 10.17487/RFC7405, December 2014, 1632 . 1634 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1635 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1636 May 2017, . 1638 [Semantics] 1639 Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1640 Ed., "HTTP Semantics", Work in Progress, Internet-Draft, 1641 draft-ietf-httpbis-semantics-12, October 2, 2020, 1642 . 1645 [USASCII] American National Standards Institute, "Coded Character 1646 Set -- 7-bit American Standard Code for Information 1647 Interchange", ANSI X3.4, 1986. 1649 9.2. Informative References 1651 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1652 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1653 Transfer Protocol -- HTTP/1.1", RFC 2616, 1654 DOI 10.17487/RFC2616, June 1999, 1655 . 1657 [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale 1658 Content", RFC 5861, DOI 10.17487/RFC5861, April 2010, 1659 . 1661 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 1662 "Network Time Protocol Version 4: Protocol and Algorithms 1663 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 1664 . 1666 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1667 DOI 10.17487/RFC6265, April 2011, 1668 . 1670 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, 1671 Ed., "Hypertext Transfer Protocol (HTTP): Caching", 1672 RFC 7234, DOI 10.17487/RFC7234, June 2014, 1673 . 1675 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1676 Writing an IANA Considerations Section in RFCs", BCP 26, 1677 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1678 . 1680 Appendix A. Collected ABNF 1682 In the collected ABNF below, list rules are expanded as per 1683 Section 5.7.1.1 of [Semantics]. 1685 Age = delta-seconds 1687 Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ] 1689 Expires = HTTP-date 1691 HTTP-date = 1693 OWS = 1695 cache-directive = token [ "=" ( token / quoted-string ) ] 1697 delta-seconds = 1*DIGIT 1699 field-name = 1701 quoted-string = 1703 token = 1705 Appendix B. Changes from RFC 7234 1707 Handling invalid and multiple Age header field values has been 1708 clarified. (Section 5.1) 1709 Some cache directives defined by this specification now have stronger 1710 prohibitions against generating the quoted form of their values, 1711 since this has been found to create interoperability problems. 1712 Consumers of extension cache directives are no longer required to 1713 accept both token and quoted-string forms, but they still need to 1714 parse them properly for unknown extensions. (Section 5.2) 1716 The "public" and "private" cache directives were clarified, so that 1717 they do not make responses reusable under any condition. 1718 (Section 5.2.2) 1720 The "must-understand" cache directive was introduced; caches are no 1721 longer required to understand the semantics of new response status 1722 codes unless it is present. (Section 5.2.2.2) 1724 The Warning response header was obsoleted. Much of the information 1725 supported by Warning could be gleaned by examining the response, and 1726 the remaining warn-codes - although potentially useful - were 1727 entirely advisory. In practice, Warning was not added by caches or 1728 intermediaries. (Section 5.5) 1730 Appendix C. Change Log 1732 This section is to be removed before publishing as an RFC. 1734 C.1. Between RFC7234 and draft 00 1736 The changes were purely editorial: 1738 o Change boilerplate and abstract to indicate the "draft" status, 1739 and update references to ancestor specifications. 1741 o Remove version "1.1" from document title, indicating that this 1742 specification applies to all HTTP versions. 1744 o Adjust historical notes. 1746 o Update links to sibling specifications. 1748 o Replace sections listing changes from RFC 2616 by new empty 1749 sections referring to RFC 723x. 1751 o Remove acknowledgements specific to RFC 723x. 1753 o Move "Acknowledgements" to the very end and make them unnumbered. 1755 C.2. Since draft-ietf-httpbis-cache-00 1757 The changes are purely editorial: 1759 o Moved all extensibility tips, registration procedures, and 1760 registry tables from the IANA considerations to normative 1761 sections, reducing the IANA considerations to just instructions 1762 that will be removed prior to publication as an RFC. 1764 C.3. Since draft-ietf-httpbis-cache-01 1766 o Cite RFC 8126 instead of RFC 5226 () 1769 o In Section 5.4, misleading statement about the relation between 1770 Pragma and Cache-Control (, ) 1773 C.4. Since draft-ietf-httpbis-cache-02 1775 o In Section 3, explain that only final responses are cacheable 1776 () 1778 o In Section 5.2.2, clarify what responses various directives apply 1779 to () 1781 o In Section 4.3.1, clarify the source of validators in conditional 1782 requests () 1784 o Revise Section 6 to apply to more than just History Lists 1785 () 1787 o In Section 5.5, deprecated "Warning" header field 1788 () 1790 o In Section 3.3, remove a spurious note 1791 () 1793 C.5. Since draft-ietf-httpbis-cache-03 1795 o In Section 2, define what a disconnected cache is 1796 () 1798 o In Section 4, clarify language around how to select a response 1799 when more than one matches () 1802 o in Section 4.2.4, mention stale-while-revalidate and stale-if- 1803 error () 1805 o Remove requirements around cache request directives 1806 () 1808 o Deprecate Pragma () 1811 o In Section 3.3 and Section 5.2.2, note effect of some directives 1812 on authenticated requests () 1815 C.6. Since draft-ietf-httpbis-cache-04 1817 o In Section 5.2, remove the registrations for stale-if-error and 1818 stale-while-revalidate which happened in RFC 7234 1819 () 1821 C.7. Since draft-ietf-httpbis-cache-05 1823 o In Section 3.2, clarify how weakly framed content is considered 1824 for purposes of completeness () 1827 o Throughout, describe Vary and cache key operations more clearly 1828 () 1830 o In Section 3, remove concept of "cacheable methods" in favor of 1831 prose (, 1832 ) 1834 o Refactored Section 7, and added a section on timing attacks 1835 () 1837 o Changed "cacheable by default" to "heuristically cacheable" 1838 throughout () 1840 C.8. Since draft-ietf-httpbis-cache-06 1842 o In Section 3 and Section 5.2.2.2, change response cacheability to 1843 only require understanding the response status code if the must- 1844 understand cache directive is present () 1847 o Change requirements for handling different forms of cache 1848 directives in Section 5.2 () 1851 o Fix typo in Section 5.2.2.10 () 1854 o In Section 5.2.2.6 and Section 5.2.2.7, clarify "private" and 1855 "public" so that they do not override all other cache directives 1856 () 1858 o In Section 3, distinguish between private with and without 1859 qualifying headers () 1862 o In Section 4.1, clarify that any "*" as a member of Vary will 1863 disable caching () 1865 o In Section 1.1, reference RFC 8174 as well 1866 () 1868 C.9. Since draft-ietf-httpbis-cache-07 1870 o Throughout, replace "effective request URI", "request-target" and 1871 similar with "target URI" () 1874 o In Section 5.2.2.6 and Section 5.2.2.7, make it clear that these 1875 directives do not ignore other requirements for caching 1876 () 1878 o In Section 3.2, move definition of "complete" into semantics 1879 () 1881 C.10. Since draft-ietf-httpbis-cache-08 1883 o Appendix A now uses the sender variant of the "#" list expansion 1884 () 1886 C.11. Since draft-ietf-httpbis-cache-09 1888 o In Section 5.1, discuss handling of invalid and multiple Age 1889 header field values () 1892 o Switch to xml2rfc v3 mode for draft generation 1893 () 1895 C.12. Since draft-ietf-httpbis-cache-10 1897 o In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists 1898 () 1900 C.13. Since draft-ietf-httpbis-cache-11 1902 o None. 1904 Acknowledgments 1906 See Appendix "Acknowledgments" of [Semantics]. 1908 Authors' Addresses 1910 Roy T. Fielding (editor) 1911 Adobe 1912 345 Park Ave 1913 San Jose, CA 95110 1914 United States of America 1916 Email: fielding@gbiv.com 1917 URI: https://roy.gbiv.com/ 1919 Mark Nottingham (editor) 1920 Fastly 1921 Prahran VIC 1922 Australia 1924 Email: mnot@mnot.net 1925 URI: https://www.mnot.net/ 1927 Julian Reschke (editor) 1928 greenbytes GmbH 1929 Hafenweg 16 1930 48155 Münster 1931 Germany 1933 Email: julian.reschke@greenbytes.de 1934 URI: https://greenbytes.de/tech/webdav/