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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 (~~), 8 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: February 28, 2021 J. F. Reschke, Ed. 7 greenbytes 8 August 27, 2020 10 HTTP Caching 11 draft-ietf-httpbis-cache-11 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.12. 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 February 28, 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 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42 161 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 163 1. Introduction 165 The Hypertext Transfer Protocol (HTTP) is a stateless application- 166 level request/response protocol that uses extensible semantics and 167 self-descriptive messages for flexible interaction with network-based 168 hypertext information systems. HTTP is defined by a series of 169 documents that collectively form the HTTP/1.1 specification: 171 o "HTTP Semantics" [Semantics] 173 o "HTTP Caching" (this document) 175 o "HTTP/1.1 Messaging" [Messaging] 177 HTTP is typically used for distributed information systems, where the 178 use of response caches can improve performance. This document 179 defines aspects of HTTP related to caching and reusing response 180 messages. 182 An HTTP cache is a local store of response messages and the subsystem 183 that controls storage, retrieval, and deletion of messages in it. A 184 cache stores cacheable responses to reduce the response time and 185 network bandwidth consumption on future equivalent requests. Any 186 client or server MAY use a cache, though a server that is acting as a 187 tunnel cannot. 189 A shared cache is a cache that stores responses for reuse by more 190 than one user; shared caches are usually (but not always) deployed as 191 a part of an intermediary. A private cache, in contrast, is 192 dedicated to a single user; often, they are deployed as a component 193 of a user agent. 195 HTTP caching's goal is significantly improving performance by reusing 196 a prior response message to satisfy a current request. A cache 197 considers a stored response "fresh", as defined in Section 4.2, if it 198 can be reused without "validation" (checking with the origin server 199 to see if the cached response remains valid for this request). A 200 fresh response can therefore reduce both latency and network overhead 201 each time the cache reuses it. When a cached response is not fresh, 202 it might still be reusable if validation can freshen it (Section 4.3) 203 or if the origin is unavailable (Section 4.2.4). 205 This document obsoletes RFC 7234, with the changes being summarized 206 in Appendix B. 208 1.1. Requirements Notation 210 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 211 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 212 "OPTIONAL" in this document are to be interpreted as described in BCP 213 14 [RFC2119] [RFC8174] when, and only when, they appear in all 214 capitals, as shown here. 216 Section 3 of [Semantics] defines conformance criteria and contains 217 considerations regarding error handling. 219 1.2. Syntax Notation 221 This specification uses the Augmented Backus-Naur Form (ABNF) 222 notation of [RFC5234], extended with the notation for case- 223 sensitivity in strings defined in [RFC7405]. 225 It also uses a list extension, defined in Section 5.5 of [Semantics], 226 that allows for compact definition of comma-separated lists using a 227 '#' operator (similar to how the '*' operator indicates repetition). 228 Appendix A shows the collected grammar with all list operators 229 expanded to standard ABNF notation. 231 The following core rules are included by reference, as defined in 232 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 233 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 234 HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line 235 feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any 236 visible [USASCII] character). 238 [Semantics] defines the following rules: 240 HTTP-date = 241 OWS = 242 field-name = 243 quoted-string = 244 token = 246 1.3. Delta Seconds 248 The delta-seconds rule specifies a non-negative integer, representing 249 time in seconds. 251 delta-seconds = 1*DIGIT 253 A recipient parsing a delta-seconds value and converting it to binary 254 form ought to use an arithmetic type of at least 31 bits of non- 255 negative integer range. If a cache receives a delta-seconds value 256 greater than the greatest integer it can represent, or if any of its 257 subsequent calculations overflows, the cache MUST consider the value 258 to be 2147483648 (2^31) or the greatest positive integer it can 259 conveniently represent. 261 | *Note:* The value 2147483648 is here for historical reasons, 262 | represents infinity (over 68 years), and does not need to be 263 | stored in binary form; an implementation could produce it as a 264 | canned string if any overflow occurs, even if the calculations 265 | are performed with an arithmetic type incapable of directly 266 | representing that number. What matters here is that an 267 | overflow be detected and not treated as a negative value in 268 | later calculations. 270 2. Overview of Cache Operation 272 Proper cache operation preserves the semantics of HTTP transfers 273 ([Semantics]) while reducing the transfer of information already held 274 in the cache. Although caching is an entirely OPTIONAL feature of 275 HTTP, it can be assumed that reusing a cached response is desirable 276 and that such reuse is the default behavior when no requirement or 277 local configuration prevents it. Therefore, HTTP cache requirements 278 are focused on preventing a cache from either storing a non-reusable 279 response or reusing a stored response inappropriately, rather than 280 mandating that caches always store and reuse particular responses. 282 The base cache key comprises the request method and target URI used 283 to retrieve the stored response; the method determines under which 284 circumstances that response can be used to satisfy a request. 285 However, many HTTP caches in common use today only cache GET 286 responses, and therefore only use the URI as the cache key, 287 forwarding other methods. 289 If a request target is subject to content negotiation, the cache 290 might store multiple responses for it. Caches differentiate these 291 responses by incorporating values of the original request's selecting 292 header fields into the cache key as well, as per Section 4.1. 294 Caches might incorporate additional material into the cache key. For 295 example, user agent caches might include the referring site's 296 identity, thereby "double keying" the cache to avoid some privacy 297 risks (see Section 7.2). 299 Most commonly, caches store the successful result of a retrieval 300 request: i.e., a 200 (OK) response to a GET request, which contains a 301 representation of the target resource (Section 8.3.1 of [Semantics]). 302 However, it is also possible to store redirects, negative results 303 (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial 304 Content)), and responses to methods other than GET if the method's 305 definition allows such caching and defines something suitable for use 306 as a cache key. 308 A cache is disconnected when it cannot contact the origin server or 309 otherwise find a forward path for a request. A disconnected cache 310 can serve stale responses in some circumstances (Section 4.2.4). 312 3. Storing Responses in Caches 314 A cache MUST NOT store a response to a request unless: 316 o the request method is understood by the cache; 318 o the response status code is final (see Section 10 of [Semantics]); 320 o if the response status code is 206 or 304, or the "must- 321 understand" cache directive (see Section 5.2) is present: the 322 cache understands the response status code; 324 o the "no-store" cache directive is not present in the response (see 325 Section 5.2); 327 o if the cache is shared: the "private" response directive is either 328 not present or allows a shared cache to store a modified response; 329 see Section 5.2.2.7); 331 o if the cache is shared: the Authorization header field is not 332 present in the request (see Section 9.5.3 of [Semantics]) or a 333 response directive is present that explicitly allows shared 334 caching (see Section 3.3); and, 336 o the response contains at least one of: 338 * a public response directive (see Section 5.2.2.6); 340 * a private response directive, if the cache is not shared (see 341 Section 5.2.2.7); 343 * an Expires header field (see Section 5.3); 345 * a max-age response directive (see Section 5.2.2.9); 347 * if the cache is shared, an s-maxage response directive (see 348 Section 5.2.2.10); 350 * a Cache Control Extension that allows it to be cached (see 351 Section 5.2.3); or, 353 * a status code that is defined as heuristically cacheable (see 354 Section 4.2.2). 356 Note that a cache-control extension can override any of the 357 requirements listed; see Section 5.2.3. 359 In this context, a cache has "understood" a request method or a 360 response status code if it recognizes it and implements all specified 361 caching-related behavior. 363 Note that, in normal operation, some caches will not store a response 364 that has neither a cache validator nor an explicit expiration time, 365 as such responses are not usually useful to store. However, caches 366 are not prohibited from storing such responses. 368 3.1. Storing Header and Trailer Fields 370 Caches MUST include all received header fields - including 371 unrecognised ones - when storing a response; this assures that new 372 HTTP header fields can be successfully deployed. However, the 373 following exceptions are made: 375 o The Connection header field and fields whose names are listed in 376 it are required by Section 13.1 of [Messaging] to be removed 377 before forwarding the message. This MAY be implemented by doing 378 so before storage. 380 o Likewise, some fields' semantics require them to be removed before 381 forwarding the message, and this MAY be implemented by doing so 382 before storage; see Section 13.1 of [Messaging] for some examples. 384 o Header fields that are specific to a client's proxy configuration 385 MUST NOT be stored, unless the cache incorporates the identity of 386 the proxy into the cache key. Effectively, this is limited to 387 Proxy-Authenticate (Section 11.3.2 of [Semantics]), Proxy- 388 Authentication-Info (Section 11.3.4 of [Semantics]), and Proxy- 389 Authorization (Section 9.5.4 of [Semantics]). 391 Caches MAY either store trailer fields separate from header fields, 392 or discard them. Caches MUST NOT combine trailer fields with header 393 fields. 395 3.2. Storing Incomplete Responses 397 If the request method is GET, the response status code is 200 (OK), 398 and the entire response header section has been received, a cache MAY 399 store a response body that is not complete (Section 2.1 of 400 [Semantics]) if the stored response is recorded as being incomplete. 401 Likewise, a 206 (Partial Content) response MAY be stored as if it 402 were an incomplete 200 (OK) response. However, a cache MUST NOT 403 store incomplete or partial-content responses if it does not support 404 the Range and Content-Range header fields or if it does not 405 understand the range units used in those fields. 407 A cache MAY complete a stored incomplete response by making a 408 subsequent range request (Section 9.3 of [Semantics]) and combining 409 the successful response with the stored response, as defined in 410 Section 3.4. A cache MUST NOT use an incomplete response to answer 411 requests unless the response has been made complete, or the request 412 is partial and specifies a range wholly within the incomplete 413 response. A cache MUST NOT send a partial response to a client 414 without explicitly marking it using the 206 (Partial Content) status 415 code. 417 3.3. Storing Responses to Authenticated Requests 419 A shared cache MUST NOT use a cached response to a request with an 420 Authorization header field (Section 9.5.3 of [Semantics]) to satisfy 421 any subsequent request unless the response contains a Cache-Control 422 field with a response directive (Section 5.2.2) that allows it to be 423 stored by a shared cache and the cache conforms to the requirements 424 of that directive for that response. 426 In this specification, the following response directives have such an 427 effect: must-revalidate (Section 5.2.2.1), public (Section 5.2.2.6), 428 and s-maxage (Section 5.2.2.10). 430 3.4. Combining Partial Content 432 A response might transfer only a partial representation if the 433 connection closed prematurely or if the request used one or more 434 Range specifiers (Section 9.3 of [Semantics]). After several such 435 transfers, a cache might have received several ranges of the same 436 representation. A cache MAY combine these ranges into a single 437 stored response, and reuse that response to satisfy later requests, 438 if they all share the same strong validator and the cache complies 439 with the client requirements in Section 10.3.7.3 of [Semantics]. 441 When combining the new response with one or more stored responses, a 442 cache MUST use the header fields provided in the new response, aside 443 from Content-Range, to replace all instances of the corresponding 444 header fields in the stored response. 446 4. Constructing Responses from Caches 448 When presented with a request, a cache MUST NOT reuse a stored 449 response, unless: 451 o The presented target URI (Section 6.1 of [Semantics]) and that of 452 the stored response match, and 454 o the request method associated with the stored response allows it 455 to be used for the presented request, and 457 o selecting header fields nominated by the stored response (if any) 458 match those presented (see Section 4.1), and 460 o the stored response does not contain the no-cache cache directive 461 (Section 5.2.2.3), unless it is successfully validated 462 (Section 4.3), and 464 o the stored response is either: 466 * fresh (see Section 4.2), or 468 * allowed to be served stale (see Section 4.2.4), or 470 * successfully validated (see Section 4.3). 472 Note that a cache-control extension can override any of the 473 requirements listed; see Section 5.2.3. 475 When a stored response is used to satisfy a request without 476 validation, a cache MUST generate an Age header field (Section 5.1), 477 replacing any present in the response with a value equal to the 478 stored response's current_age; see Section 4.2.3. 480 A cache MUST write through requests with methods that are unsafe 481 (Section 8.2.1 of [Semantics]) to the origin server; i.e., a cache is 482 not allowed to generate a reply to such a request before having 483 forwarded the request and having received a corresponding response. 485 Also, note that unsafe requests might invalidate already-stored 486 responses; see Section 4.4. 488 When more than one suitable response is stored, a cache MUST use the 489 most recent one (as determined by the Date header field). It can 490 also forward the request with "Cache-Control: max-age=0" or "Cache- 491 Control: no-cache" to disambiguate which response to use. 493 A cache that does not have a clock available MUST NOT use stored 494 responses without revalidating them upon every use. 496 4.1. Calculating Cache Keys with Vary 498 When a cache receives a request that can be satisfied by a stored 499 response that has a Vary header field (Section 11.1.4 of 500 [Semantics]), it MUST NOT use that response unless all the selecting 501 header fields nominated by the Vary header field match in both the 502 original request (i.e., that associated with the stored response), 503 and the presented request. 505 The selecting header fields from two requests are defined to match if 506 and only if those in the first request can be transformed to those in 507 the second request by applying any of: 509 o adding or removing whitespace, where allowed in the header field's 510 syntax 512 o combining multiple header fields with the same field name (see 513 Section 5.4 of [Semantics]) 515 o normalizing both header field values in a way that is known to 516 have identical semantics, according to the header field's 517 specification (e.g., reordering field values when order is not 518 significant; case-normalization, where values are defined to be 519 case-insensitive) 521 If (after any normalization that might take place) a header field is 522 absent from a request, it can only match another request if it is 523 also absent there. 525 A Vary header field value containing a member "*" always fails to 526 match. 528 The stored response with matching selecting header fields is known as 529 the selected response. 531 If multiple selected responses are available (potentially including 532 responses without a Vary header field), the cache will need to choose 533 one to use. When a selecting header field has a known mechanism for 534 doing so (e.g., qvalues on Accept and similar request header fields), 535 that mechanism MAY be used to select preferred responses; of the 536 remainder, the most recent response (as determined by the Date header 537 field) is used, as per Section 4. 539 Note that in practice, some resources might send the Vary header 540 field on responses inconsistently. When a cache has multiple 541 responses for a target URI, and one or more omits the Vary header 542 field, it SHOULD use the most recent non-empty value available to 543 select an appropriate response for the request. 545 If no selected response is available, the cache cannot satisfy the 546 presented request. Typically, it is forwarded to the origin server 547 in a (possibly conditional; see Section 4.3) request. 549 4.2. Freshness 551 A fresh response is one whose age has not yet exceeded its freshness 552 lifetime. Conversely, a stale response is one where it has. 554 A response's freshness lifetime is the length of time between its 555 generation by the origin server and its expiration time. An explicit 556 expiration time is the time at which the origin server intends that a 557 stored response can no longer be used by a cache without further 558 validation, whereas a heuristic expiration time is assigned by a 559 cache when no explicit expiration time is available. 561 A response's age is the time that has passed since it was generated 562 by, or successfully validated with, the origin server. 564 When a response is "fresh" in the cache, it can be used to satisfy 565 subsequent requests without contacting the origin server, thereby 566 improving efficiency. 568 The primary mechanism for determining freshness is for an origin 569 server to provide an explicit expiration time in the future, using 570 either the Expires header field (Section 5.3) or the max-age response 571 directive (Section 5.2.2.9). Generally, origin servers will assign 572 future explicit expiration times to responses in the belief that the 573 representation is not likely to change in a semantically significant 574 way before the expiration time is reached. 576 If an origin server wishes to force a cache to validate every 577 request, it can assign an explicit expiration time in the past to 578 indicate that the response is already stale. Compliant caches will 579 normally validate a stale cached response before reusing it for 580 subsequent requests (see Section 4.2.4). 582 Since origin servers do not always provide explicit expiration times, 583 caches are also allowed to use a heuristic to determine an expiration 584 time under certain circumstances (see Section 4.2.2). 586 The calculation to determine if a response is fresh is: 588 response_is_fresh = (freshness_lifetime > current_age) 590 freshness_lifetime is defined in Section 4.2.1; current_age is 591 defined in Section 4.2.3. 593 Clients can send the max-age or min-fresh request directives 594 (Section 5.2.1) to constrain or relax freshness calculations for the 595 corresponding response. However, caches are not required to honor 596 them. 598 When calculating freshness, to avoid common problems in date parsing: 600 o Although all date formats are specified to be case-sensitive, a 601 cache recipient SHOULD match day, week, and time-zone names case- 602 insensitively. 604 o If a cache recipient's internal implementation of time has less 605 resolution than the value of an HTTP-date, the recipient MUST 606 internally represent a parsed Expires date as the nearest time 607 equal to or earlier than the received value. 609 o A cache recipient MUST NOT allow local time zones to influence the 610 calculation or comparison of an age or expiration time. 612 o A cache recipient SHOULD consider a date with a zone abbreviation 613 other than GMT or UTC to be invalid for calculating expiration. 615 Note that freshness applies only to cache operation; it cannot be 616 used to force a user agent to refresh its display or reload a 617 resource. See Section 6 for an explanation of the difference between 618 caches and history mechanisms. 620 4.2.1. Calculating Freshness Lifetime 622 A cache can calculate the freshness lifetime (denoted as 623 freshness_lifetime) of a response by using the first match of: 625 o If the cache is shared and the s-maxage response directive 626 (Section 5.2.2.10) is present, use its value, or 628 o If the max-age response directive (Section 5.2.2.9) is present, 629 use its value, or 631 o If the Expires response header field (Section 5.3) is present, use 632 its value minus the value of the Date response header field, or 634 o Otherwise, no explicit expiration time is present in the response. 635 A heuristic freshness lifetime might be applicable; see 636 Section 4.2.2. 638 Note that this calculation is not vulnerable to clock skew, since all 639 of the information comes from the origin server. 641 When there is more than one value present for a given directive 642 (e.g., two Expires header fields, multiple Cache-Control: max-age 643 directives), the directive's value is considered invalid. Caches are 644 encouraged to consider responses that have invalid freshness 645 information to be stale. 647 4.2.2. Calculating Heuristic Freshness 649 Since origin servers do not always provide explicit expiration times, 650 a cache MAY assign a heuristic expiration time when an explicit time 651 is not specified, employing algorithms that use other header field 652 values (such as the Last-Modified time) to estimate a plausible 653 expiration time. This specification does not provide specific 654 algorithms, but does impose worst-case constraints on their results. 656 A cache MUST NOT use heuristics to determine freshness when an 657 explicit expiration time is present in the stored response. Because 658 of the requirements in Section 3, this means that heuristics can only 659 be used on responses without explicit freshness whose status codes 660 are defined as "heuristically cacheable" (e.g., see Section 10.1 of 661 [Semantics]), and those responses without explicit freshness that 662 have been marked as explicitly cacheable (e.g., with a "public" 663 response directive). 665 Note that in previous specifications heuristically cacheable response 666 status codes were called "cacheable by default." 668 If the response has a Last-Modified header field (Section 11.2.2 of 669 [Semantics]), caches are encouraged to use a heuristic expiration 670 value that is no more than some fraction of the interval since that 671 time. A typical setting of this fraction might be 10%. 673 | *Note:* Section 13.9 of [RFC2616] prohibited caches from 674 | calculating heuristic freshness for URIs with query components 675 | (i.e., those containing '?'). In practice, this has not been 676 | widely implemented. Therefore, origin servers are encouraged 677 | to send explicit directives (e.g., Cache-Control: no-cache) if 678 | they wish to prevent caching. 680 4.2.3. Calculating Age 682 The Age header field is used to convey an estimated age of the 683 response message when obtained from a cache. The Age field value is 684 the cache's estimate of the number of seconds since the origin server 685 generated or validated the response. The Age value is therefore the 686 sum of the time that the response has been resident in each of the 687 caches along the path from the origin server, plus the time it has 688 been in transit along network paths. 690 Age calculation uses the following data: 692 age_value The term "age_value" denotes the value of the Age header 693 field (Section 5.1), in a form appropriate for arithmetic 694 operation; or 0, if not available. 696 date_value The term "date_value" denotes the value of the Date 697 header field, in a form appropriate for arithmetic operations. 698 See Section 11.1.1 of [Semantics] for the definition of the Date 699 header field, and for requirements regarding responses without it. 701 now The term "now" means "the current value of the clock at the host 702 performing the calculation". A host ought to use NTP ([RFC5905]) 703 or some similar protocol to synchronize its clocks to Coordinated 704 Universal Time. 706 request_time The current value of the clock at the host at the time 707 the request resulting in the stored response was made. 709 response_time The current value of the clock at the host at the time 710 the response was received. 712 A response's age can be calculated in two entirely independent ways: 714 1. the "apparent_age": response_time minus date_value, if the local 715 clock is reasonably well synchronized to the origin server's 716 clock. If the result is negative, the result is replaced by 717 zero. 719 2. the "corrected_age_value", if all of the caches along the 720 response path implement HTTP/1.1 or greater. A cache MUST 721 interpret this value relative to the time the request was 722 initiated, not the time that the response was received. 724 apparent_age = max(0, response_time - date_value); 726 response_delay = response_time - request_time; 727 corrected_age_value = age_value + response_delay; 729 These are combined as 731 corrected_initial_age = max(apparent_age, corrected_age_value); 733 unless the cache is confident in the value of the Age header field 734 (e.g., because there are no HTTP/1.0 hops in the Via header field), 735 in which case the corrected_age_value MAY be used as the 736 corrected_initial_age. 738 The current_age of a stored response can then be calculated by adding 739 the time (in seconds) since the stored response was last validated by 740 the origin server to the corrected_initial_age. 742 resident_time = now - response_time; 743 current_age = corrected_initial_age + resident_time; 745 4.2.4. Serving Stale Responses 747 A "stale" response is one that either has explicit expiry information 748 or is allowed to have heuristic expiry calculated, but is not fresh 749 according to the calculations in Section 4.2. 751 A cache MUST NOT generate a stale response if it is prohibited by an 752 explicit in-protocol directive (e.g., by a "no-store" or "no-cache" 753 cache directive, a "must-revalidate" cache-response-directive, or an 754 applicable "s-maxage" or "proxy-revalidate" cache-response-directive; 755 see Section 5.2.2). 757 A cache MUST NOT generate a stale response unless it is disconnected 758 or doing so is explicitly permitted by the client or origin server 759 (e.g., by the max-stale request directive in Section 5.2.1, by 760 extension directives such as those defined in [RFC5861], or by 761 configuration in accordance with an out-of-band contract). 763 4.3. Validation 765 When a cache has one or more stored responses for a requested URI, 766 but cannot serve any of them (e.g., because they are not fresh, or 767 one cannot be selected; see Section 4.1), it can use the conditional 768 request mechanism Section 9.2 of [Semantics] in the forwarded request 769 to give the next inbound server an opportunity to select a valid 770 stored response to use, updating the stored metadata in the process, 771 or to replace the stored response(s) with a new response. This 772 process is known as "validating" or "revalidating" the stored 773 response. 775 4.3.1. Sending a Validation Request 777 When generating a conditional request for validation, a cache starts 778 with either a request it is attempting to satisfy, or - if it is 779 initiating the request independently - it synthesises a request using 780 a stored response by copying the method, target URI, and request 781 header fields identified by the Vary header field Section 4.1. 783 It then updates that request with one or more precondition header 784 fields. These contain validator metadata sourced from stored 785 response(s) that have the same cache key. 787 The precondition header fields are then compared by recipients to 788 determine whether any stored response is equivalent to a current 789 representation of the resource. 791 One such validator is the timestamp given in a Last-Modified header 792 field (Section 11.2.2 of [Semantics]), which can be used in an If- 793 Modified-Since header field for response validation, or in an If- 794 Unmodified-Since or If-Range header field for representation 795 selection (i.e., the client is referring specifically to a previously 796 obtained representation with that timestamp). 798 Another validator is the entity-tag given in an ETag field 799 (Section 11.2.3 of [Semantics]). One or more entity-tags, indicating 800 one or more stored responses, can be used in an If-None-Match header 801 field for response validation, or in an If-Match or If-Range header 802 field for representation selection (i.e., the client is referring 803 specifically to one or more previously obtained representations with 804 the listed entity-tags). 806 4.3.2. Handling a Received Validation Request 808 Each client in the request chain may have its own cache, so it is 809 common for a cache at an intermediary to receive conditional requests 810 from other (outbound) caches. Likewise, some user agents make use of 811 conditional requests to limit data transfers to recently modified 812 representations or to complete the transfer of a partially retrieved 813 representation. 815 If a cache receives a request that can be satisfied by reusing one of 816 its stored 200 (OK) or 206 (Partial Content) responses, the cache 817 SHOULD evaluate any applicable conditional header field preconditions 818 received in that request with respect to the corresponding validators 819 contained within the selected response. A cache MUST NOT evaluate 820 conditional header fields that only apply to an origin server, occur 821 in a request with semantics that cannot be satisfied with a cached 822 response, or occur in a request with a target resource for which it 823 has no stored responses; such preconditions are likely intended for 824 some other (inbound) server. 826 The proper evaluation of conditional requests by a cache depends on 827 the received precondition header fields and their precedence, as 828 defined in Section 9.2.2 of [Semantics]. The If-Match and If- 829 Unmodified-Since conditional header fields are not applicable to a 830 cache. 832 A request containing an If-None-Match header field (Section 9.2.4 of 833 [Semantics]) indicates that the client wants to validate one or more 834 of its own stored responses in comparison to whichever stored 835 response is selected by the cache. If the field value is "*", or if 836 the field value is a list of entity-tags and at least one of them 837 matches the entity-tag of the selected stored response, a cache 838 recipient SHOULD generate a 304 (Not Modified) response (using the 839 metadata of the selected stored response) instead of sending that 840 stored response. 842 When a cache decides to revalidate its own stored responses for a 843 request that contains an If-None-Match list of entity-tags, the cache 844 MAY combine the received list with a list of entity-tags from its own 845 stored set of responses (fresh or stale) and send the union of the 846 two lists as a replacement If-None-Match header field value in the 847 forwarded request. If a stored response contains only partial 848 content, the cache MUST NOT include its entity-tag in the union 849 unless the request is for a range that would be fully satisfied by 850 that partial stored response. If the response to the forwarded 851 request is 304 (Not Modified) and has an ETag field value with an 852 entity-tag that is not in the client's list, the cache MUST generate 853 a 200 (OK) response for the client by reusing its corresponding 854 stored response, as updated by the 304 response metadata 855 (Section 4.3.4). 857 If an If-None-Match header field is not present, a request containing 858 an If-Modified-Since header field (Section 9.2.5 of [Semantics]) 859 indicates that the client wants to validate one or more of its own 860 stored responses by modification date. A cache recipient SHOULD 861 generate a 304 (Not Modified) response (using the metadata of the 862 selected stored response) if one of the following cases is true: 1) 863 the selected stored response has a Last-Modified field value that is 864 earlier than or equal to the conditional timestamp; 2) no Last- 865 Modified field is present in the selected stored response, but it has 866 a Date field value that is earlier than or equal to the conditional 867 timestamp; or, 3) neither Last-Modified nor Date is present in the 868 selected stored response, but the cache recorded it as having been 869 received at a time earlier than or equal to the conditional 870 timestamp. 872 A cache that implements partial responses to range requests, as 873 defined in Section 9.3 of [Semantics], also needs to evaluate a 874 received If-Range header field (Section 9.2.7 of [Semantics]) 875 regarding its selected stored response. 877 4.3.3. Handling a Validation Response 879 Cache handling of a response to a conditional request depends upon 880 its status code: 882 o A 304 (Not Modified) response status code indicates that the 883 stored response can be updated and reused; see Section 4.3.4. 885 o A full response (i.e., one with a payload body) indicates that 886 none of the stored responses nominated in the conditional request 887 is suitable. Instead, the cache MUST use the full response to 888 satisfy the request and MAY replace the stored response(s). 890 o However, if a cache receives a 5xx (Server Error) response while 891 attempting to validate a response, it can either forward this 892 response to the requesting client, or act as if the server failed 893 to respond. In the latter case, the cache MAY send a previously 894 stored response (see Section 4.2.4). 896 4.3.4. Freshening Stored Responses upon Validation 898 When a cache receives a 304 (Not Modified) response and already has 899 one or more stored 200 (OK) responses for the applicable cache key, 900 the cache needs to identify which (if any) are to be updated by the 901 new information provided, and then do so. 903 The stored response(s) to update are identified by using the first 904 match (if any) of: 906 o If the new response contains a strong validator (see 907 Section 11.2.1 of [Semantics]), then that strong validator 908 identifies the selected representation for update. All the stored 909 responses with the same strong validator are identified for 910 update. If none of the stored responses contain the same strong 911 validator, then the cache MUST NOT use the new response to update 912 any stored responses. 914 o If the new response contains a weak validator and that validator 915 corresponds to one of the cache's stored responses, then the most 916 recent of those matching stored responses is identified for 917 update. 919 o If the new response does not include any form of validator (such 920 as where a client generates an If-Modified-Since request from a 921 source other than the Last-Modified response header field), and 922 there is only one stored response, and that stored response also 923 lacks a validator, then that stored response is identified for 924 update. 926 For each stored response identified for update, the cache MUST use 927 the header fields provided in the 304 (Not Modified) response to 928 replace all instances of the corresponding header fields in the 929 stored response, with the following exceptions: 931 o The exceptions to header field storage in Section 3.1 also apply 932 to header field updates. 934 o Caches MUST NOT update the following header fields: Content- 935 Encoding, Content-Length, Content-MD5 (Section 14.15 of 936 [RFC2616]), Content-Range, ETag. 938 4.3.5. Freshening Responses with HEAD 940 A response to the HEAD method is identical to what an equivalent 941 request made with a GET would have been, except it lacks a body. 942 This property of HEAD responses can be used to invalidate or update a 943 cached GET response if the more efficient conditional GET request 944 mechanism is not available (due to no validators being present in the 945 stored response) or if transmission of the representation body is not 946 desired even if it has changed. 948 When a cache makes an inbound HEAD request for a target URI and 949 receives a 200 (OK) response, the cache SHOULD update or invalidate 950 each of its stored GET responses that could have been selected for 951 that request (see Section 4.1). 953 For each of the stored responses that could have been selected, if 954 the stored response and HEAD response have matching values for any 955 received validator fields (ETag and Last-Modified) and, if the HEAD 956 response has a Content-Length header field, the value of Content- 957 Length matches that of the stored response, the cache SHOULD update 958 the stored response as described below; otherwise, the cache SHOULD 959 consider the stored response to be stale. 961 If a cache updates a stored response with the metadata provided in a 962 HEAD response, the cache MUST use the header fields provided in the 963 HEAD response to replace all instances of the corresponding header 964 fields in the stored response (subject to the exceptions in 965 Section 4.3.4) and append new header fields to the stored response's 966 header section unless otherwise restricted by the Cache-Control 967 header field. 969 4.4. Invalidation 971 Because unsafe request methods (Section 8.2.1 of [Semantics]) such as 972 PUT, POST or DELETE have the potential for changing state on the 973 origin server, intervening caches are required to invalidate stored 974 responses to keep their contents up to date. Invalidate means that 975 the cache will either remove all stored responses whose target URI 976 matches the given URI, or will mark them as "invalid" and in need of 977 a mandatory validation before they can be sent in response to a 978 subsequent request. 980 Note that this does not guarantee that all appropriate responses are 981 invalidated globally; a state-changing request would only invalidate 982 responses in the caches it travels through. 984 A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) 985 and the URI(s) in the Location and Content-Location response header 986 fields (if present) when a non-error status code is received in 987 response to an unsafe request method. 989 However, a cache MUST NOT invalidate a URI from a Location or 990 Content-Location response header field if the host part of that URI 991 differs from the host part in the target URI (Section 6.1 of 992 [Semantics]). This helps prevent denial-of-service attacks. 994 A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) 995 when it receives a non-error response to a request with a method 996 whose safety is unknown. 998 Here, a "non-error response" is one with a 2xx (Successful) or 3xx 999 (Redirection) status code. 1001 5. Field Definitions 1003 This section defines the syntax and semantics of HTTP fields related 1004 to caching. 1006 --------------- ----------- ------ 1007 Field Name Status Ref. 1008 --------------- ----------- ------ 1009 Age standard 5.1 1010 Cache-Control standard 5.2 1011 Expires standard 5.3 1012 Pragma standard 5.4 1013 Warning obsoleted 5.5 1014 --------------- ----------- ------ 1016 Table 1 1018 5.1. Age 1020 The "Age" header field conveys the sender's estimate of the time 1021 since the response was generated or successfully validated at the 1022 origin server. Age values are calculated as specified in 1023 Section 4.2.3. 1025 Age = delta-seconds 1027 The Age field value is a non-negative integer, representing time in 1028 seconds (see Section 1.3). A cache SHOULD consider a response to be 1029 stale if an Age field is present and its value is invalid (i.e., 1030 contains a list or something other than a non-negative integer). 1032 The presence of an Age header field implies that the response was not 1033 generated or validated by the origin server for this request. 1034 However, lack of an Age header field does not imply the origin was 1035 contacted, since the response might have been received from an 1036 HTTP/1.0 cache that does not implement Age. 1038 5.2. Cache-Control 1040 The "Cache-Control" header field is used to list directives for 1041 caches along the request/response chain. Such cache directives are 1042 unidirectional in that the presence of a directive in a request does 1043 not imply that the same directive is present in the response, or to 1044 be repeated in it. 1046 See Section 5.2.3 for information about how Cache-Control directives 1047 defined elsewhere are handled. 1049 | *Note:* Some HTTP/1.0 caches might not implement Cache-Control. 1051 A proxy, whether or not it implements a cache, MUST pass cache 1052 directives through in forwarded messages, regardless of their 1053 significance to that application, since the directives might apply to 1054 all recipients along the request/response chain. It is not possible 1055 to target a directive to a specific cache. 1057 Cache directives are identified by a token, to be compared case- 1058 insensitively, and have an optional argument that can use both token 1059 and quoted-string syntax. For the directives defined below that 1060 define arguments, recipients ought to accept both forms, even if a 1061 specific form is required for generation. 1063 Cache-Control = #cache-directive 1065 cache-directive = token [ "=" ( token / quoted-string ) ] 1067 For the cache directives defined below, no argument is defined (nor 1068 allowed) unless stated otherwise. 1070 ------------------ ---------------------------------- 1071 Cache Directive Reference 1072 ------------------ ---------------------------------- 1073 max-age Section 5.2.1.1, Section 5.2.2.9 1074 max-stale Section 5.2.1.2 1075 min-fresh Section 5.2.1.3 1076 must-revalidate Section 5.2.2.1 1077 must-understand Section 5.2.2.2 1078 no-cache Section 5.2.1.4, Section 5.2.2.3 1079 no-store Section 5.2.1.5, Section 5.2.2.4 1080 no-transform Section 5.2.1.6, Section 5.2.2.5 1081 only-if-cached Section 5.2.1.7 1082 private Section 5.2.2.7 1083 proxy-revalidate Section 5.2.2.8 1084 public Section 5.2.2.6 1085 s-maxage Section 5.2.2.10 1086 ------------------ ---------------------------------- 1088 Table 2 1090 5.2.1. Request Cache-Control Directives 1092 This section defines cache request directives. They are advisory; 1093 caches MAY implement them, but are not required to. 1095 5.2.1.1. max-age 1097 Argument syntax: 1099 delta-seconds (see Section 1.3) 1101 The "max-age" request directive indicates that the client prefers a 1102 response whose age is less than or equal to the specified number of 1103 seconds. Unless the max-stale request directive is also present, the 1104 client does not wish to receive a stale response. 1106 This directive uses the token form of the argument syntax: e.g., 1107 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the 1108 quoted-string form. 1110 5.2.1.2. max-stale 1112 Argument syntax: 1114 delta-seconds (see Section 1.3) 1116 The "max-stale" request directive indicates that the client will 1117 accept a response that has exceeded its freshness lifetime. If a 1118 value is present, then the client is willing to accept a response 1119 that has exceeded its freshness lifetime by no more than the 1120 specified number of seconds. If no value is assigned to max-stale, 1121 then the client will accept a stale response of any age. 1123 This directive uses the token form of the argument syntax: e.g., 1124 'max-stale=10' not 'max-stale="10"'. A sender MUST NOT generate the 1125 quoted-string form. 1127 5.2.1.3. min-fresh 1129 Argument syntax: 1131 delta-seconds (see Section 1.3) 1133 The "min-fresh" request directive indicates that the client prefers a 1134 response whose freshness lifetime is no less than its current age 1135 plus the specified time in seconds. That is, the client wants a 1136 response that will still be fresh for at least the specified number 1137 of seconds. 1139 This directive uses the token form of the argument syntax: e.g., 1140 'min-fresh=20' not 'min-fresh="20"'. A sender MUST NOT generate the 1141 quoted-string form. 1143 5.2.1.4. no-cache 1145 The "no-cache" request directive indicates that the client prefers 1146 stored response not be used to satisfy the request without successful 1147 validation on the origin server. 1149 5.2.1.5. no-store 1151 The "no-store" request directive indicates that a cache MUST NOT 1152 store any part of either this request or any response to it. This 1153 directive applies to both private and shared caches. "MUST NOT 1154 store" in this context means that the cache MUST NOT intentionally 1155 store the information in non-volatile storage, and MUST make a best- 1156 effort attempt to remove the information from volatile storage as 1157 promptly as possible after forwarding it. 1159 This directive is NOT a reliable or sufficient mechanism for ensuring 1160 privacy. In particular, malicious or compromised caches might not 1161 recognize or obey this directive, and communications networks might 1162 be vulnerable to eavesdropping. 1164 Note that if a request containing this directive is satisfied from a 1165 cache, the no-store request directive does not apply to the already 1166 stored response. 1168 5.2.1.6. no-transform 1170 The "no-transform" request directive indicates that the client is 1171 asking for intermediaries to avoid transforming the payload, as 1172 defined in Section 6.6.2 of [Semantics]. 1174 5.2.1.7. only-if-cached 1176 The "only-if-cached" request directive indicates that the client only 1177 wishes to obtain a stored response. Caches that honor this request 1178 directive SHOULD, upon receiving it, either respond using a stored 1179 response consistent with the other constraints of the request, or 1180 respond with a 504 (Gateway Timeout) status code. 1182 5.2.2. Response Cache-Control Directives 1184 This section defines cache response directives. A cache MUST obey 1185 the Cache-Control directives defined in this section. 1187 5.2.2.1. must-revalidate 1189 The "must-revalidate" response directive indicates that once the 1190 response has become stale, a cache MUST NOT reuse that response to 1191 satisfy another request until it has been successfully validated by 1192 the origin, as defined by Section 4.3. 1194 The must-revalidate directive is necessary to support reliable 1195 operation for certain protocol features. In all circumstances a 1196 cache MUST obey the must-revalidate directive; in particular, if a 1197 cache is disconnected, the cache MUST generate a 504 (Gateway 1198 Timeout) response rather than reuse the stale response. 1200 The must-revalidate directive ought to be used by servers if and only 1201 if failure to validate a request on the representation could cause 1202 incorrect operation, such as a silently unexecuted financial 1203 transaction. 1205 The must-revalidate directive also permits a shared cache to reuse a 1206 response to a request containing an Authorization header field, 1207 subject to the above requirement on revalidation (Section 3.3). 1209 5.2.2.2. must-understand 1211 The "must-understand" response directive limits caching of the 1212 response to a cache that understands and conforms to the requirements 1213 for that response's status code. A cache MUST NOT store a response 1214 containing the must-understand directive if the cache does not 1215 understand the response status code. 1217 5.2.2.3. no-cache 1219 Argument syntax: 1221 #field-name 1223 The "no-cache" response directive, in its unqualified form (without 1224 an argument), indicates that the response MUST NOT be used to satisfy 1225 any other request without forwarding it for validation and receiving 1226 a successful response; see Section 4.3. 1228 This allows an origin server to prevent a cache from using the 1229 response to satisfy a request without contacting it, even by caches 1230 that have been configured to send stale responses. 1232 The qualified form of no-cache response directive, with an argument 1233 that lists one or more field names, indicates that a cache MAY use 1234 the response to satisfy a subsequent request, subject to any other 1235 restrictions on caching, if the listed header fields are excluded 1236 from the subsequent response or the subsequent response has been 1237 successfully revalidated with the origin server (updating or removing 1238 those fields). This allows an origin server to prevent the re-use of 1239 certain header fields in a response, while still allowing caching of 1240 the rest of the response. 1242 The field names given are not limited to the set of header fields 1243 defined by this specification. Field names are case-insensitive. 1245 This directive uses the quoted-string form of the argument syntax. A 1246 sender SHOULD NOT generate the token form (even if quoting appears 1247 not to be needed for single-entry lists). 1249 *Note:* Although it has been back-ported to many implementations, 1250 some HTTP/1.0 caches will not recognize or obey this directive. 1251 Also, the qualified form of the directive is often handled by caches 1252 as if an unqualified no-cache directive was received; i.e., the 1253 special handling for the qualified form is not widely implemented. 1255 5.2.2.4. no-store 1257 The "no-store" response directive indicates that a cache MUST NOT 1258 store any part of either the immediate request or response, and MUST 1259 NOT use the response to satisfy any other request. 1261 This directive applies to both private and shared caches. "MUST NOT 1262 store" in this context means that the cache MUST NOT intentionally 1263 store the information in non-volatile storage, and MUST make a best- 1264 effort attempt to remove the information from volatile storage as 1265 promptly as possible after forwarding it. 1267 This directive is NOT a reliable or sufficient mechanism for ensuring 1268 privacy. In particular, malicious or compromised caches might not 1269 recognize or obey this directive, and communications networks might 1270 be vulnerable to eavesdropping. 1272 5.2.2.5. no-transform 1274 The "no-transform" response directive indicates that an intermediary 1275 (regardless of whether it implements a cache) MUST NOT transform the 1276 payload, as defined in Section 6.6.2 of [Semantics]. 1278 5.2.2.6. public 1280 The "public" response directive indicates that a cache MAY store the 1281 response even if it would otherwise be prohibited, subject to the 1282 constraints defined in Section 3. In other words, public explicitly 1283 marks the response as cacheable. For example, public permits a 1284 shared cache to reuse a response to a request containing an 1285 Authorization header field (Section 3.3). 1287 Note that it is unnecessary to add the public directive to a response 1288 that is already cacheable according to Section 3. 1290 If a response with the public directive has no explicit freshness 1291 information, it is heuristically cacheable (Section 4.2.2). 1293 5.2.2.7. private 1295 Argument syntax: 1297 #field-name 1299 The unqualified "private" response directive indicates that a shared 1300 cache MUST NOT store the response (i.e., the response is intended for 1301 a single user). It also indicates that a private cache MAY store the 1302 response, subject the constraints defined in Section 3, even if the 1303 response would not otherwise be heuristically cacheable by a private 1304 cache. 1306 If a qualified private response directive is present, with an 1307 argument that lists one or more field names, then only the listed 1308 fields are limited to a single user: a shared cache MUST NOT store 1309 the listed fields if they are present in the original response, but 1310 MAY store the remainder of the response message without those fields, 1311 subject the constraints defined in Section 3. 1313 The field names given are not limited to the set of header fields 1314 defined by this specification. Field names are case-insensitive. 1316 This directive uses the quoted-string form of the argument syntax. A 1317 sender SHOULD NOT generate the token form (even if quoting appears 1318 not to be needed for single-entry lists). 1320 *Note:* This usage of the word "private" only controls where the 1321 response can be stored; it cannot ensure the privacy of the message 1322 content. Also, the qualified form of the directive is often handled 1323 by caches as if an unqualified private directive was received; i.e., 1324 the special handling for the qualified form is not widely 1325 implemented. 1327 5.2.2.8. proxy-revalidate 1329 The "proxy-revalidate" response directive indicates that once the 1330 response has become stale, a shared cache MUST NOT reuse that 1331 response to satisfy another request until it has been successfully 1332 validated by the origin, as defined by Section 4.3. This is 1333 analogous to must-revalidate (Section 5.2.2.1), except that proxy- 1334 revalidate does not apply to private caches. 1336 Note that "proxy-revalidate" on its own does not imply that a 1337 response is cacheable. For example, it might be combined with the 1338 public directive (Section 5.2.2.6), allowing the response to be 1339 cached while requiring only a shared cache to revalidate when stale. 1341 5.2.2.9. max-age 1343 Argument syntax: 1345 delta-seconds (see Section 1.3) 1347 The "max-age" response directive indicates that the response is to be 1348 considered stale after its age is greater than the specified number 1349 of seconds. 1351 This directive uses the token form of the argument syntax: e.g., 1352 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the 1353 quoted-string form. 1355 5.2.2.10. s-maxage 1357 Argument syntax: 1359 delta-seconds (see Section 1.3) 1361 The "s-maxage" response directive indicates that, for a shared cache, 1362 the maximum age specified by this directive overrides the maximum age 1363 specified by either the max-age directive or the Expires header 1364 field. 1366 The s-maxage directive incorporates the proxy-revalidate 1367 (Section 5.2.2.8) response directive's semantics for a shared cache. 1368 A shared cache MUST NOT reuse a stale response with s-maxage to 1369 satisfy another request until it has been successfully validated by 1370 the origin, as defined by Section 4.3. This directive also permits a 1371 shared cache to reuse a response to a request containing an 1372 Authorization header field, subject to the above requirements on 1373 maximum age and revalidation (Section 3.3). 1375 This directive uses the token form of the argument syntax: e.g., 1376 's-maxage=10' not 's-maxage="10"'. A sender MUST NOT generate the 1377 quoted-string form. 1379 5.2.3. Cache Control Extensions 1381 The Cache-Control header field can be extended through the use of one 1382 or more cache-extension tokens, each with an optional value. A cache 1383 MUST ignore unrecognized cache directives. 1385 Informational extensions (those that do not require a change in cache 1386 behavior) can be added without changing the semantics of other 1387 directives. 1389 Behavioral extensions are designed to work by acting as modifiers to 1390 the existing base of cache directives. Both the new directive and 1391 the old directive are supplied, such that applications that do not 1392 understand the new directive will default to the behavior specified 1393 by the old directive, and those that understand the new directive 1394 will recognize it as modifying the requirements associated with the 1395 old directive. In this way, extensions to the existing cache-control 1396 directives can be made without breaking deployed caches. 1398 For example, consider a hypothetical new response directive called 1399 "community" that acts as a modifier to the private directive: in 1400 addition to private caches, any cache that is shared only by members 1401 of the named community is allowed to cache the response. An origin 1402 server wishing to allow the UCI community to use an otherwise private 1403 response in their shared cache(s) could do so by including 1405 Cache-Control: private, community="UCI" 1407 A cache that recognizes such a community cache-extension could 1408 broaden its behavior in accordance with that extension. A cache that 1409 does not recognize the community cache-extension would ignore it and 1410 adhere to the private directive. 1412 New extension directives ought to consider defining: 1414 o What it means for a directive to be specified multiple times, 1416 o When the directive does not take an argument, what it means when 1417 an argument is present, 1419 o When the directive requires an argument, what it means when it is 1420 missing, 1422 o Whether the directive is specific to requests, responses, or able 1423 to be used in either. 1425 5.2.4. Cache Directive Registry 1427 The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" 1428 defines the namespace for the cache directives. It has been created 1429 and is now maintained at . 1432 A registration MUST include the following fields: 1434 o Cache Directive Name 1436 o Pointer to specification text 1437 Values to be added to this namespace require IETF Review (see 1438 [RFC8126], Section 4.8). 1440 5.3. Expires 1442 The "Expires" header field gives the date/time after which the 1443 response is considered stale. See Section 4.2 for further discussion 1444 of the freshness model. 1446 The presence of an Expires field does not imply that the original 1447 resource will change or cease to exist at, before, or after that 1448 time. 1450 The Expires value is an HTTP-date timestamp, as defined in 1451 Section 5.4.1.5 of [Semantics]. 1453 Expires = HTTP-date 1455 For example 1457 Expires: Thu, 01 Dec 1994 16:00:00 GMT 1459 A cache recipient MUST interpret invalid date formats, especially the 1460 value "0", as representing a time in the past (i.e., "already 1461 expired"). 1463 If a response includes a Cache-Control field with the max-age 1464 directive (Section 5.2.2.9), a recipient MUST ignore the Expires 1465 field. Likewise, if a response includes the s-maxage directive 1466 (Section 5.2.2.10), a shared cache recipient MUST ignore the Expires 1467 field. In both these cases, the value in Expires is only intended 1468 for recipients that have not yet implemented the Cache-Control field. 1470 An origin server without a clock MUST NOT generate an Expires field 1471 unless its value represents a fixed time in the past (always expired) 1472 or its value has been associated with the resource by a system or 1473 user with a reliable clock. 1475 Historically, HTTP required the Expires field value to be no more 1476 than a year in the future. While longer freshness lifetimes are no 1477 longer prohibited, extremely large values have been demonstrated to 1478 cause problems (e.g., clock overflows due to use of 32-bit integers 1479 for time values), and many caches will evict a response far sooner 1480 than that. 1482 5.4. Pragma 1484 The "Pragma" header field was defined for HTTP/1.0 caches, so that 1485 clients could specify a "no-cache" request (as Cache-Control was not 1486 defined until HTTP/1.1). 1488 However, support for Cache-Control is now widespread. As a result, 1489 this specification deprecates Pragma. 1491 | *Note:* Because the meaning of "Pragma: no-cache" in responses 1492 | was never specified, it does not provide a reliable replacement 1493 | for "Cache-Control: no-cache" in them. 1495 5.5. Warning 1497 The "Warning" header field was used to carry additional information 1498 about the status or transformation of a message that might not be 1499 reflected in the status code. This specification obsoletes it, as it 1500 is not widely generated or surfaced to users. The information it 1501 carried can be gleaned from examining other header fields, such as 1502 Age. 1504 6. Relationship to Applications 1506 Applications using HTTP often specify additional forms of caching. 1507 For example, Web browsers often have history mechanisms such as 1508 "Back" buttons that can be used to redisplay a representation 1509 retrieved earlier in a session. 1511 Likewise, some Web browsers implement caching of images and other 1512 assets within a page view; they may or may not honor HTTP caching 1513 semantics. 1515 The requirements in this specification do not necessarily apply to 1516 how applications use data after it is retrieved from a HTTP cache. 1517 That is, a history mechanism can display a previous representation 1518 even if it has expired, and an application can use cached data in 1519 other ways beyond its freshness lifetime. 1521 This does not prohibit the application from taking HTTP caching into 1522 account; for example, a history mechanism might tell the user that a 1523 view is stale, or it might honor cache directives (e.g., Cache- 1524 Control: no-store). 1526 7. Security Considerations 1528 This section is meant to inform developers, information providers, 1529 and users of known security concerns specific to HTTP caching. More 1530 general security considerations are addressed in HTTP messaging 1531 [Messaging] and semantics [Semantics]. 1533 Caches expose additional potential vulnerabilities, since the 1534 contents of the cache represent an attractive target for malicious 1535 exploitation. Because cache contents persist after an HTTP request 1536 is complete, an attack on the cache can reveal information long after 1537 a user believes that the information has been removed from the 1538 network. Therefore, cache contents need to be protected as sensitive 1539 information. 1541 7.1. Cache Poisoning 1543 Various attacks might be amplified by being stored in a shared cache. 1544 Such "cache poisoning" attacks use the cache to distribute a 1545 malicious payload to many clients, and are especially effective when 1546 an attacker can use implementation flaws, elevated privileges, or 1547 other techniques to insert such a response into a cache. 1549 One common attack vector for cache poisoning is to exploit 1550 differences in message parsing on proxies and in user agents; see 1551 Section 6.3 of [Messaging] for the relevant requirements regarding 1552 HTTP/1.1. 1554 7.2. Timing Attacks 1556 Because one of the primary uses of a cache is to optimise 1557 performance, its use can "leak" information about what resources have 1558 been previously requested. 1560 For example, if a user visits a site and their browser caches some of 1561 its responses, and then navigates to a second site, that site can 1562 attempt to load responses it knows exists on the first site. If they 1563 load quickly, it can be assumed that the user has visited that site, 1564 or even a specific page on it. 1566 Such "timing attacks" can be mitigated by adding more information to 1567 the cache key, such as the identity of the referring site (to prevent 1568 the attack described above). This is sometimes called "double 1569 keying." 1571 7.3. Caching of Sensitive Information 1573 Implementation and deployment flaws (as well as misunderstanding of 1574 cache operation) might lead to caching of sensitive information 1575 (e.g., authentication credentials) that is thought to be private, 1576 exposing it to unauthorized parties. 1578 Note that the Set-Cookie response header field [RFC6265] does not 1579 inhibit caching; a cacheable response with a Set-Cookie header field 1580 can be (and often is) used to satisfy subsequent requests to caches. 1581 Servers who wish to control caching of these responses are encouraged 1582 to emit appropriate Cache-Control response header fields. 1584 8. IANA Considerations 1586 The change controller for the following registrations is: "IETF 1587 (iesg@ietf.org) - Internet Engineering Task Force". 1589 8.1. Field Registration 1591 Please update the "Hypertext Transfer Protocol (HTTP) Field Name 1592 Registry" at with the 1593 field names listed in the two tables of Section 5. 1595 8.2. Cache Directive Registration 1597 Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive 1598 Registry" at 1599 with the registration procedure of Section 5.2.4 and the cache 1600 directive names summarized in the table of Section 5.2. 1602 8.3. Warn Code Registry 1604 Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn 1605 Codes" registry at 1606 to the effect that Warning is obsoleted. 1608 9. References 1610 9.1. Normative References 1612 [Messaging] 1613 Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, 1614 Ed., "HTTP/1.1 Messaging", Work in Progress, Internet- 1615 Draft, draft-ietf-httpbis-messaging-11, August 27, 2020, 1616 . 1619 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1620 Requirement Levels", BCP 14, RFC 2119, 1621 DOI 10.17487/RFC2119, March 1997, 1622 . 1624 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1625 Resource Identifier (URI): Generic Syntax", STD 66, 1626 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1627 . 1629 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1630 Specifications: ABNF", STD 68, RFC 5234, 1631 DOI 10.17487/RFC5234, January 2008, 1632 . 1634 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 1635 RFC 7405, DOI 10.17487/RFC7405, December 2014, 1636 . 1638 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1639 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1640 May 2017, . 1642 [Semantics] 1643 Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, 1644 Ed., "HTTP Semantics", Work in Progress, Internet-Draft, 1645 draft-ietf-httpbis-semantics-11, August 27, 2020, 1646 . 1649 [USASCII] American National Standards Institute, "Coded Character 1650 Set -- 7-bit American Standard Code for Information 1651 Interchange", ANSI X3.4, 1986. 1653 9.2. Informative References 1655 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1656 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1657 Transfer Protocol -- HTTP/1.1", RFC 2616, 1658 DOI 10.17487/RFC2616, June 1999, 1659 . 1661 [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale 1662 Content", RFC 5861, DOI 10.17487/RFC5861, April 2010, 1663 . 1665 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 1666 "Network Time Protocol Version 4: Protocol and Algorithms 1667 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 1668 . 1670 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1671 DOI 10.17487/RFC6265, April 2011, 1672 . 1674 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, 1675 Ed., "Hypertext Transfer Protocol (HTTP): Caching", 1676 RFC 7234, DOI 10.17487/RFC7234, June 2014, 1677 . 1679 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1680 Writing an IANA Considerations Section in RFCs", BCP 26, 1681 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1682 . 1684 Appendix A. Collected ABNF 1686 In the collected ABNF below, list rules are expanded as per 1687 Section 5.5.1 of [Semantics]. 1689 Age = delta-seconds 1691 Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ] 1693 Expires = HTTP-date 1695 HTTP-date = 1697 OWS = 1699 cache-directive = token [ "=" ( token / quoted-string ) ] 1701 delta-seconds = 1*DIGIT 1703 field-name = 1705 quoted-string = 1707 token = 1709 Appendix B. Changes from RFC 7234 1711 Handling invalid and multiple Age header field values has been 1712 clarified. (Section 5.1) 1713 Some cache directives defined by this specification now have stronger 1714 prohibitions against generating the quoted form of their values, 1715 since this has been found to create interoperability problems. 1716 Consumers of extension cache directives are no longer required to 1717 accept both token and quoted-string forms, but they still need to 1718 parse them properly for unknown extensions. (Section 5.2) 1720 The "public" and "private" cache directives were clarified, so that 1721 they do not make responses reusable under any condition. 1722 (Section 5.2.2) 1724 The "must-understand" cache directive was introduced; caches are no 1725 longer required to understand the semantics of new response status 1726 codes unless it is present. (Section 5.2.2.2) 1728 The Warning response header was obsoleted. Much of the information 1729 supported by Warning could be gleaned by examining the response, and 1730 the remaining warn-codes - although potentially useful - were 1731 entirely advisory. In practice, Warning was not added by caches or 1732 intermediaries. (Section 5.5) 1734 Appendix C. Change Log 1736 This section is to be removed before publishing as an RFC. 1738 C.1. Between RFC7234 and draft 00 1740 The changes were purely editorial: 1742 o Change boilerplate and abstract to indicate the "draft" status, 1743 and update references to ancestor specifications. 1745 o Remove version "1.1" from document title, indicating that this 1746 specification applies to all HTTP versions. 1748 o Adjust historical notes. 1750 o Update links to sibling specifications. 1752 o Replace sections listing changes from RFC 2616 by new empty 1753 sections referring to RFC 723x. 1755 o Remove acknowledgements specific to RFC 723x. 1757 o Move "Acknowledgements" to the very end and make them unnumbered. 1759 C.2. Since draft-ietf-httpbis-cache-00 1761 The changes are purely editorial: 1763 o Moved all extensibility tips, registration procedures, and 1764 registry tables from the IANA considerations to normative 1765 sections, reducing the IANA considerations to just instructions 1766 that will be removed prior to publication as an RFC. 1768 C.3. Since draft-ietf-httpbis-cache-01 1770 o Cite RFC 8126 instead of RFC 5226 () 1773 o In Section 5.4, misleading statement about the relation between 1774 Pragma and Cache-Control (, ) 1777 C.4. Since draft-ietf-httpbis-cache-02 1779 o In Section 3, explain that only final responses are cacheable 1780 () 1782 o In Section 5.2.2, clarify what responses various directives apply 1783 to () 1785 o In Section 4.3.1, clarify the source of validators in conditional 1786 requests () 1788 o Revise Section 6 to apply to more than just History Lists 1789 () 1791 o In Section 5.5, deprecated "Warning" header field 1792 () 1794 o In Section 3.3, remove a spurious note 1795 () 1797 C.5. Since draft-ietf-httpbis-cache-03 1799 o In Section 2, define what a disconnected cache is 1800 () 1802 o In Section 4, clarify language around how to select a response 1803 when more than one matches () 1806 o in Section 4.2.4, mention stale-while-revalidate and stale-if- 1807 error () 1809 o Remove requirements around cache request directives 1810 () 1812 o Deprecate Pragma () 1815 o In Section 3.3 and Section 5.2.2, note effect of some directives 1816 on authenticated requests () 1819 C.6. Since draft-ietf-httpbis-cache-04 1821 o In Section 5.2, remove the registrations for stale-if-error and 1822 stale-while-revalidate which happened in RFC 7234 1823 () 1825 C.7. Since draft-ietf-httpbis-cache-05 1827 o In Section 3.2, clarify how weakly framed content is considered 1828 for purposes of completeness () 1831 o Throughout, describe Vary and cache key operations more clearly 1832 () 1834 o In Section 3, remove concept of "cacheable methods" in favor of 1835 prose (, 1836 ) 1838 o Refactored Section 7, and added a section on timing attacks 1839 () 1841 o Changed "cacheable by default" to "heuristically cacheable" 1842 throughout () 1844 C.8. Since draft-ietf-httpbis-cache-06 1846 o In Section 3 and Section 5.2.2.2, change response cacheability to 1847 only require understanding the response status code if the must- 1848 understand cache directive is present () 1851 o Change requirements for handling different forms of cache 1852 directives in Section 5.2 () 1855 o Fix typo in Section 5.2.2.10 () 1858 o In Section 5.2.2.6 and Section 5.2.2.7, clarify "private" and 1859 "public" so that they do not override all other cache directives 1860 () 1862 o In Section 3, distinguish between private with and without 1863 qualifying headers () 1866 o In Section 4.1, clarify that any "*" as a member of Vary will 1867 disable caching () 1869 o In Section 1.1, reference RFC 8174 as well 1870 () 1872 C.9. Since draft-ietf-httpbis-cache-07 1874 o Throughout, replace "effective request URI", "request-target" and 1875 similar with "target URI" () 1878 o In Section 5.2.2.6 and Section 5.2.2.7, make it clear that these 1879 directives do not ignore other requirements for caching 1880 () 1882 o In Section 3.2, move definition of "complete" into semantics 1883 () 1885 C.10. Since draft-ietf-httpbis-cache-08 1887 o Appendix A now uses the sender variant of the "#" list expansion 1888 () 1890 C.11. Since draft-ietf-httpbis-cache-09 1892 o In Section 5.1, discuss handling of invalid and multiple Age 1893 header field values () 1896 o Switch to xml2rfc v3 mode for draft generation 1897 () 1899 C.12. Since draft-ietf-httpbis-cache-10 1901 o In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists 1902 () 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 F. 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/