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