source: draft-ietf-httpbis/20/draft-ietf-httpbis-p6-cache-20.xml

Last change on this file was 1809, checked in by julian.reschke@…, 8 years ago

Remove mentions of "seven" parts.

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc category="std" docName="draft-ietf-httpbis-p6-cache-20" ipr="pre5378Trust200902" obsoletes="2616">
20
21
22
23<front>
24
25  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
26
27  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
28    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
29    <address>
30      <postal>
31        <street>345 Park Ave</street>
32        <city>San Jose</city>
33        <region>CA</region>
34        <code>95110</code>
35        <country>USA</country>
36      </postal>
37      <email>fielding@gbiv.com</email>
38      <uri>http://roy.gbiv.com/</uri>
39    </address>
40  </author>
41
42  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
43    <organization abbrev="W3C">World Wide Web Consortium</organization>
44    <address>
45      <postal>
46        <street>W3C / ERCIM</street>
47        <street>2004, rte des Lucioles</street>
48        <city>Sophia-Antipolis</city>
49        <region>AM</region>
50        <code>06902</code>
51        <country>France</country>
52      </postal>
53      <email>ylafon@w3.org</email>
54      <uri>http://www.raubacapeu.net/people/yves/</uri>
55    </address>
56  </author>
57
58  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
59    <organization>Rackspace</organization>
60    <address>
61      <email>mnot@mnot.net</email>
62      <uri>http://www.mnot.net/</uri>
63    </address>
64  </author>
65
66  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
67    <organization abbrev="greenbytes">greenbytes GmbH</organization>
68    <address>
69      <postal>
70        <street>Hafenweg 16</street>
71        <city>Muenster</city><region>NW</region><code>48155</code>
72        <country>Germany</country>
73      </postal>
74      <email>julian.reschke@greenbytes.de</email>
75      <uri>http://greenbytes.de/tech/webdav/</uri>
76    </address>
77  </author>
78
79  <date month="July" year="2012" day="16"/>
80  <workgroup>HTTPbis Working Group</workgroup>
81
82<abstract>
83<t>
84   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
85   distributed, collaborative, hypertext information systems. This document
86   defines requirements on HTTP caches and the associated header fields
87   that control cache behavior or indicate cacheable response messages.
88</t>
89</abstract>
90
91<note title="Editorial Note (To be removed by RFC Editor)">
92  <t>
93    Discussion of this draft takes place on the HTTPBIS working group
94    mailing list (ietf-http-wg@w3.org), which is archived at
95    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
96  </t>
97  <t>
98    The current issues list is at
99    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
100    documents (including fancy diffs) can be found at
101    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
102  </t>
103  <t>
104    The changes in this draft are summarized in <xref target="changes.since.19"/>.
105  </t>
106</note>
107
108   </front>
109   <middle>
110
111<section anchor="caching" title="Introduction">
112<t>
113   HTTP is typically used for distributed information systems, where
114   performance can be improved by the use of response caches. This document
115   defines aspects of HTTP/1.1 related to caching and reusing response
116   messages.
117</t>
118
119<section anchor="intro.purpose" title="Purpose">
120<iref item="cache"/>
121<t>
122   An HTTP cache is a local store of response messages and the
123   subsystem that controls its message storage, retrieval, and deletion. A
124   cache stores cacheable responses in order to reduce the response time and
125   network bandwidth consumption on future, equivalent requests. Any client or
126   server MAY employ a cache, though a cache cannot be used by a server that
127   is acting as a tunnel.
128</t>
129<t>
130   The goal of caching in HTTP/1.1 is to significantly improve performance
131   by reusing a prior response message to satisfy a current request.
132   A stored response is considered "fresh", as defined in
133   <xref target="expiration.model"/>, if the response can be reused without
134   "validation" (checking with the origin server to see if the cached response
135   remains valid for this request).  A fresh cache response can therefore
136   reduce both latency and network transfers each time it is reused.
137   When a cached response is not fresh, it might still be reusable if it can
138   be freshened by validation (<xref target="validation.model"/>) or if the
139   origin is unavailable.
140</t>
141</section>
142
143<section anchor="intro.terminology" title="Terminology">
144<t>
145   This specification uses a number of terms to refer to the roles played by
146   participants in, and objects of, HTTP caching.
147</t>
148<t>
149   <iref item="cache"/>
150   <?rfc needLines="4"?>cache
151   <list>
152      <t>A conformant implementation of a HTTP cache. Note that this implies
153        an HTTP/1.1 cache; this specification does not define conformance
154        for HTTP/1.0 caches.</t>
155   </list>
156</t>
157<t anchor="shared.and.non-shared.caches">
158   <iref item="shared cache"/>
159   <?rfc needLines="4"?>shared cache
160   <list>
161      <t>A cache that stores responses to be reused by more than one user;
162         usually (but not always) deployed as part of an intermediary.</t>
163   </list>
164</t>
165<t>
166   <iref item="private cache"/>
167   <?rfc needLines="4"?>private cache
168   <list>
169      <t>A cache that is dedicated to a single user.</t>
170   </list>
171</t>
172<t>
173   <iref item="cacheable"/>
174   <?rfc needLines="4"?>cacheable
175   <list>
176      <t>A response is cacheable if a cache is allowed to store a copy of the
177      response message for use in answering subsequent requests. Even when a
178      response is cacheable, there might be additional constraints on whether
179      a cache can use the stored copy to satisfy a particular request.</t>
180   </list>
181</t>
182<t>
183   <iref item="explicit expiration time"/>
184   <?rfc needLines="4"?>explicit expiration time
185   <list>
186      <t>The time at which the origin server intends that a representation
187      no longer be returned by a cache without further validation.</t>
188   </list>
189</t>
190<t>
191   <iref item="heuristic expiration time"/>
192   <?rfc needLines="4"?>heuristic expiration time
193   <list>
194      <t>An expiration time assigned by a cache when no explicit expiration
195      time is available.</t>
196   </list>
197</t>
198<t>
199   <iref item="age"/>
200   <?rfc needLines="4"?>age
201   <list>
202      <t>The age of a response is the time since it was sent by, or
203      successfully validated with, the origin server.</t>
204   </list>
205</t>
206<t>
207   <iref item="first-hand"/>
208   <?rfc needLines="4"?>first-hand
209   <list>
210      <t>A response is first-hand if the freshness model is not in use; i.e.,
211      its age is 0.</t>
212   </list>
213</t>
214<t>
215   <iref item="freshness lifetime"/>
216   <?rfc needLines="4"?>freshness lifetime
217   <list>
218      <t>The length of time between the generation of a response and its
219      expiration time.</t>
220   </list>
221</t>
222<t>
223   <iref item="fresh"/>
224   <?rfc needLines="4"?>fresh
225   <list>
226      <t>A response is fresh if its age has not yet exceeded its freshness
227      lifetime.</t>
228   </list>
229</t>
230<t>
231   <iref item="stale"/>
232   <?rfc needLines="4"?>stale
233   <list>
234      <t>A response is stale if its age has passed its freshness lifetime
235      (either explicit or heuristic).</t>
236   </list>
237</t>
238<t>
239   <iref item="validator"/>
240   <?rfc needLines="4"?>validator
241   <list>
242      <t>A protocol element (e.g., an entity-tag or a Last-Modified
243      time) that is used to find out whether a stored response is an equivalent
244      copy of a representation. See Section 2.1 of <xref target="Part4"/>.</t>
245   </list>
246</t>
247<t>
248   <iref item="strong validator"/>
249   <iref item="validator" subitem="strong"/>
250   <?rfc needLines="4"?>strong validator
251   <list>
252      <t>A validator that is defined by the origin server such that its
253         current value will change if the representation body changes; i.e.,
254         an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
255         if no entity-tag is provided, a Last-Modified value
256         that is strong in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
257   </list>
258</t>
259</section>
260
261<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
262<t>
263   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
264   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
265   document are to be interpreted as described in <xref target="RFC2119"/>.
266</t>
267<t>
268   This specification targets conformance criteria according to the role of
269   a participant in HTTP communication.  Hence, HTTP requirements are placed
270   on senders, recipients, clients, servers, user agents, intermediaries,
271   origin servers, proxies, gateways, or caches, depending on what behavior
272   is being constrained by the requirement. See Section 2 of <xref target="Part1"/> for definitions
273   of these terms.
274</t>
275<t>
276   The verb "generate" is used instead of "send" where a requirement
277   differentiates between creating a protocol element and merely forwarding a
278   received element downstream.
279</t>
280<t>
281   An implementation is considered conformant if it complies with all of the
282   requirements associated with the roles it partakes in HTTP. Note that
283   SHOULD-level requirements are relevant here, unless one of the documented
284   exceptions is applicable.
285</t>
286<t>
287   This document also uses ABNF to define valid protocol elements
288   (<xref target="notation"/>).
289   In addition to the prose requirements placed upon them, senders MUST NOT
290   generate protocol elements that do not match the grammar defined by the
291   ABNF rules for those protocol elements that are applicable to the sender's
292   role. If a received protocol element is processed, the recipient MUST be
293   able to parse any value that would match the ABNF rules for that protocol
294   element, excluding only those rules not applicable to the recipient's role.
295</t>
296<t>
297   Unless noted otherwise, a recipient MAY attempt to recover a usable
298   protocol element from an invalid construct.  HTTP does not define
299   specific error handling mechanisms except when they have a direct impact
300   on security, since different applications of the protocol require
301   different error handling strategies.  For example, a Web browser might
302   wish to transparently recover from a response where the
303   Location header field doesn't parse according to the ABNF,
304   whereas a systems control client might consider any form of error recovery
305   to be dangerous.
306</t>
307</section>
308
309<section title="Syntax Notation" anchor="notation">
310<t>
311   This specification uses the Augmented Backus-Naur Form (ABNF) notation
312   of <xref target="RFC5234"/> with the list rule extension defined in
313   Section 1.2 of <xref target="Part1"/>. <xref target="imported.abnf"/> describes rules imported from
314   other documents. <xref target="collected.abnf"/> shows the collected ABNF
315   with the list rule expanded.
316</t>
317
318<section title="Delta Seconds" anchor="delta-seconds">
319<t>
320   The delta-seconds rule specifies a non-negative integer, representing time
321   in seconds.
322</t>
323<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
324  delta-seconds  = 1*DIGIT
325]]></artwork></figure>
326<t>
327   If an implementation receives a delta-seconds value larger than the largest
328   positive integer it can represent, or if any of its subsequent calculations
329   overflows, it MUST consider the value to be 2147483648
330   (2^31). Recipients parsing a delta-seconds value MUST use
331   an arithmetic type of at least 31 bits of range, and senders MUST NOT
332   send delta-seconds with a value greater than 2147483648.
333</t>
334</section>
335
336</section>
337</section>
338
339<section anchor="caching.overview" title="Overview of Cache Operation">
340<iref item="cache entry"/>
341<iref item="cache key"/>
342<t>
343   Proper cache operation preserves the semantics of HTTP transfers
344   (<xref target="Part2"/>) while eliminating the transfer of information already held
345   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
346   we assume that reusing the cached response is desirable and that such
347   reuse is the default behavior when no requirement or locally-desired
348   configuration prevents it.  Therefore, HTTP cache requirements are focused
349   on preventing a cache from either storing a non-reusable response or
350   reusing a stored response inappropriately.
351</t>
352<t>
353   Each cache entry consists of a cache key and one or more
354   HTTP responses corresponding to prior requests that used the same key. The
355   most common form of cache entry is a successful result of a retrieval
356   request: i.e., a 200 (OK) response containing a
357   representation of the resource identified by the request target. However,
358   it is also possible to cache negative results (e.g., 404 (Not
359   Found), incomplete results (e.g., 206 (Partial
360   Content)), and responses to methods other than GET if the method's
361   definition allows such caching and defines something suitable for use as a
362   cache key.
363</t>
364<t>
365   The default cache key consists of the request method and
366   target URI.  However, since HTTP caches in common use today are typically
367   limited to caching responses to GET, many implementations simply decline
368   other methods and use only the URI as the key.
369</t>
370<t>
371   If a request target is subject to content negotiation, its cache entry
372   might consist of multiple stored responses, each differentiated by a
373   secondary key for the values of the original request's selecting header
374   fields (<xref target="caching.negotiated.responses"/>).
375</t>
376</section>
377
378<section anchor="response.cacheability" title="Storing Responses in Caches">
379<t>
380   A cache MUST NOT store a response to any request, unless:
381   <list style="symbols">
382      <t>The request method is understood by the cache and defined as being
383      cacheable, and</t>
384      <t>the response status code is understood by the cache, and</t>
385      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
386      header fields, and</t>
387      <t>the "private" cache response directive (see <xref target="cache-response-directive.private"/>) does not appear in the
388      response, if the cache is shared, and</t>
389      <t>the Authorization header field (see
390      Section 4.1 of <xref target="Part7"/>) does not appear in the request, if the cache is
391      shared, unless the response explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
392      <t>the response either:
393         <list style="symbols">
394            <t>contains an <xref target="header.expires" format="none">Expires</xref> header field (see
395            <xref target="header.expires"/>), or</t>
396            <t>contains a max-age response cache directive (see <xref target="cache-response-directive.max-age"/>), or</t>
397            <t>contains a s-maxage response cache directive and the cache is
398            shared, or</t>
399            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
400            or</t>
401            <t>has a status code that can be served with heuristic freshness
402            (see <xref target="heuristic.freshness"/>).</t>
403         </list>
404      </t>
405   </list>
406</t>
407<t>
408   Note that any of the requirements listed above can be overridden by a
409   cache-control extension; see <xref target="cache.control.extensions"/>.
410</t>
411<t>
412   In this context, a cache has "understood" a request method or a response
413   status code if it recognizes it and implements any cache-specific
414   behavior.
415</t>
416<t>
417   Note that, in normal operation, many caches will not store a response that
418   has neither a cache validator nor an explicit expiration time, as such
419   responses are not usually useful to store. However, caches are not
420   prohibited from storing such responses.
421</t>
422
423<section anchor="incomplete.responses" title="Storing Incomplete Responses">
424<t>
425   A response message is considered complete when all of the octets indicated
426   by the message framing (<xref target="Part1"/>) are received prior to the connection
427   being closed. If the request is GET, the response status is 200
428   (OK), and the entire response header block has been received, a
429   cache MAY store an incomplete response message body if the cache entry is
430   recorded as incomplete. Likewise, a 206 (Partial Content)
431   response MAY be stored as if it were an incomplete 200
432   (OK) cache entry. However, a cache MUST NOT store incomplete or
433   partial content responses if it does not support the Range
434   and Content-Range header fields or if it does not understand
435   the range units used in those fields.
436</t>
437<t>
438   A cache MAY complete a stored incomplete response by making a subsequent
439   range request (<xref target="Part5"/>) and combining the successful response with the
440   stored entry, as defined in <xref target="combining.responses"/>. A cache
441   MUST NOT use an incomplete response to answer requests unless the
442   response has been made complete or the request is partial and specifies a
443   range that is wholly within the incomplete response. A cache MUST NOT
444   send a partial response to a client without explicitly marking it as such
445   using the 206 (Partial Content) status code.
446</t>
447</section>
448
449
450<section anchor="caching.authenticated.responses" title="Storing Responses to Authenticated Requests">
451<t>
452   A shared cache MUST NOT use a cached response to a request with an
453   Authorization header field (Section 4.1 of <xref target="Part7"/>) to
454   satisfy any subsequent request unless a cache directive that allows such
455   responses to be stored is present in the response.
456</t>
457<t>
458   In this specification, the following <xref target="header.cache-control" format="none">Cache-Control</xref> response
459   directives (<xref target="cache-response-directive"/>) have such an effect:
460   must-revalidate, public, s-maxage.
461</t>
462<t>
463   Note that cached responses that contain the "must-revalidate" and/or
464   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
465   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
466   used to satisfy a subsequent request without revalidating it on the origin
467   server.
468</t>
469</section>
470</section>
471
472
473<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
474<t>
475   For a presented request, a cache MUST NOT return a stored response,
476   unless:
477   <list style="symbols">
478      <t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
479      that of the stored response match, and</t>
480      <t>the request method associated with the stored response allows it to
481      be used for the presented request, and</t>
482      <t>selecting header fields nominated by the stored response (if any)
483      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
484      <t>the presented request does not contain the no-cache pragma (<xref target="header.pragma"/>), nor the no-cache cache directive (<xref target="cache-request-directive"/>), unless the stored response is
485      successfully validated (<xref target="validation.model"/>), and</t>
486      <t>the stored response does not contain the no-cache cache directive
487      (<xref target="cache-response-directive.no-cache"/>), unless it is
488      successfully validated (<xref target="validation.model"/>), and</t>
489      <t>the stored response is either:
490         <list style="symbols">
491            <t>fresh (see <xref target="expiration.model"/>), or</t>
492            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
493            <t>successfully validated (see <xref target="validation.model"/>).</t>
494         </list>
495      </t>
496  </list>
497</t>
498<t>
499   Note that any of the requirements listed above can be overridden by a
500   cache-control extension; see <xref target="cache.control.extensions"/>.
501</t>
502<t>
503   When a stored response is used to satisfy a request without validation,
504   a cache MUST include a single <xref target="header.age" format="none">Age</xref> header field
505   (<xref target="header.age"/>) in the response with a value equal to the
506   stored response's current_age; see <xref target="age.calculations"/>.
507</t>
508<t>
509   A cache MUST write through requests with methods that are unsafe
510   (Section 2.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache is not allowed to
511   generate a reply to such a request before having forwarded the request and
512   having received a corresponding response.
513</t>
514<t>
515   Also, note that unsafe requests might invalidate already stored responses;
516   see <xref target="invalidation.after.updates.or.deletions"/>.
517</t>
518<t>
519   When more than one suitable response is stored, a cache MUST use the
520   most recent response (as determined by the Date header
521   field). It can also forward a request with "Cache-Control: max-age=0" or
522   "Cache-Control: no-cache" to disambiguate which response to use.
523</t>
524<t>
525   A cache that does not have a clock available MUST NOT use stored
526   responses without revalidating them on every use. A cache, especially a
527   shared cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its clock with a reliable external
528   standard.
529</t>
530
531
532<section anchor="expiration.model" title="Freshness Model">
533<t>
534   When a response is "fresh" in the cache, it can be used to satisfy
535   subsequent requests without contacting the origin server, thereby improving
536   efficiency.
537</t>
538<t>
539   The primary mechanism for determining freshness is for an origin server to
540   provide an explicit expiration time in the future, using either the
541   <xref target="header.expires" format="none">Expires</xref> header field (<xref target="header.expires"/>) or
542   the max-age response cache directive (<xref target="cache-response-directive.max-age"/>). Generally, origin servers will
543   assign future explicit expiration times to responses in the belief that the
544   representation is not likely to change in a semantically significant way
545   before the expiration time is reached.
546</t>
547<t>
548   If an origin server wishes to force a cache to validate every request, it
549   can assign an explicit expiration time in the past to indicate that the
550   response is already stale. Compliant caches will normally validate the
551   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
552</t>
553<t>
554   Since origin servers do not always provide explicit expiration times, a
555   cache MAY assign a heuristic expiration time when an explicit time is not
556   specified, employing algorithms that use other header field values (such as
557   the Last-Modified time) to estimate a plausible expiration
558   time. This specification does not provide specific algorithms, but does
559   impose worst-case constraints on their results.
560</t>
561<figure>
562<preamble>
563  The calculation to determine if a response is fresh is:
564</preamble>
565<artwork type="code"><![CDATA[
566   response_is_fresh = (freshness_lifetime > current_age)
567]]></artwork>
568</figure>
569<t>
570   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
571   <xref target="age.calculations"/>.
572</t>
573<t>
574   Additionally, clients can influence freshness calculation — either
575   constraining it relaxing it — by using the max-age and min-fresh
576   request cache directives. See <xref target="cache-request-directive"/>
577   for details.
578</t>
579<t>
580   Note that freshness applies only to cache operation; it cannot be used to
581   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
582   caches and history mechanisms.
583</t>
584
585<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
586<t>
587   A cache can calculate the freshness lifetime (denoted as
588   freshness_lifetime) of a response by using the first match of:
589   <list style="symbols">
590      <t>If the cache is shared and the s-maxage response cache directive
591      (<xref target="cache-response-directive.s-maxage"/>) is present, use its value,
592      or</t>
593      <t>If the max-age response cache directive (<xref target="cache-response-directive.max-age"/>) is present, use its value, or</t>
594      <t>If the <xref target="header.expires" format="none">Expires</xref> response header field
595      (<xref target="header.expires"/>) is present, use its value minus the
596      value of the Date response header field, or</t>
597      <t>Otherwise, no explicit expiration time is present in the response. A
598      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
599   </list>
600</t>
601<t>
602   Note that this calculation is not vulnerable to clock skew, since all of
603   the information comes from the origin server.
604</t>
605<t>
606   When there is more than one value present for a given directive (e.g., two
607   <xref target="header.expires" format="none">Expires</xref> header fields, multiple Cache-Control: max-age
608   directives), it is considered invalid. Caches are encouraged to consider
609   responses that have invalid freshness information to be stale.
610</t>
611</section>
612
613<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
614<t>
615   If no explicit expiration time is present in a stored response that has a
616   status code whose definition allows heuristic freshness to be used
617   (including the following in Section 4 of <xref target="Part2"/>: 200 (OK),
618   203 (Non-Authoritative Information), 206 (Partial
619   Content), 300 (Multiple Choices), 301 (Moved
620   Permanently) and 410 (Gone)), a cache MAY
621   calculate a heuristic expiration time. A cache MUST NOT use heuristics to
622   determine freshness for responses with status codes that do not explicitly
623   allow it.
624</t>
625<t>
626   When a heuristic is used to calculate freshness lifetime, a cache SHOULD
627   attach a <xref target="header.warning" format="none">Warning</xref> header field with a 113 warn-code to the
628   response if its current_age is more than 24 hours and such a warning is not
629   already present.
630</t>
631<t>
632   Also, if the response has a Last-Modified header field
633   (Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
634   expiration value that is no more than some fraction of the interval since
635   that time. A typical setting of this fraction might be 10%.
636</t>
637<t><list>
638   <t>
639      Note: Section 13.9 of <xref target="RFC2616"/> prohibited caches
640      from calculating heuristic freshness for URIs with query components
641      (i.e., those containing '?'). In practice, this has not been widely
642      implemented. Therefore, servers are encouraged to send explicit
643      directives (e.g., Cache-Control: no-cache) if they wish to preclude
644      caching.
645   </t>
646</list></t>
647</section>
648
649<section anchor="age.calculations" title="Calculating Age">
650<t>
651   HTTP/1.1 uses the <xref target="header.age" format="none">Age</xref> header field to convey the estimated
652   age of the response message when obtained from a cache. The Age field value
653   is the cache's estimate of the amount of time since the response was
654   generated or validated by the origin server. In essence, the Age value is
655   the sum of the time that the response has been resident in each of the
656   caches along the path from the origin server, plus the amount of time it
657   has been in transit along network paths.
658</t>
659<t>
660   The following data is used for the age calculation:
661</t>
662<t>
663   <?rfc needLines="4"?>age_value
664   <list>
665      <t>
666         The term "age_value" denotes the value of the <xref target="header.age" format="none">Age</xref>
667         header field (<xref target="header.age"/>), in a form appropriate for
668         arithmetic operation; or 0, if not available.
669      </t>
670   </list>
671</t>
672<t>
673   <?rfc needLines="4"?>date_value
674   <list>
675      <t>
676         HTTP/1.1 requires origin servers to send a Date header
677         field, if possible, with every response, giving the time at which the
678         response was generated. The term "date_value" denotes the value of
679         the Date header field, in a form appropriate for arithmetic
680         operations. See Section 9.10 of <xref target="Part2"/> for the definition of the Date header
681         field, and for requirements regarding responses without it.
682      </t>
683   </list>
684</t>
685<t>
686   <?rfc needLines="4"?>now
687   <list>
688      <t>
689         The term "now" means "the current value of the clock at the host
690         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
691         clocks to a globally accurate time standard.
692      </t>
693   </list>
694</t>
695<t>
696   <?rfc needLines="4"?>request_time
697   <list>
698      <t>
699         The current value of the clock at the host at the time the request
700         resulting in the stored response was made.
701      </t>
702   </list>
703</t>
704<t>
705   <?rfc needLines="4"?>response_time
706   <list>
707      <t>
708         The current value of the clock at the host at the time the response
709         was received.
710      </t>
711   </list>
712</t>
713<t>
714   A response's age can be calculated in two entirely independent ways:
715   <list style="numbers">
716      <t>the "apparent_age": response_time minus date_value, if the local
717      clock is reasonably well synchronized to the origin server's clock. If
718      the result is negative, the result is replaced by zero.</t>
719      <t>the "corrected_age_value", if all of the caches along the response
720      path implement HTTP/1.1. A cache MUST interpret this value relative
721      to the time the request was initiated, not the time that the response
722      was received.</t>
723   </list>
724</t>
725<figure>
726<artwork type="code"><![CDATA[
727  apparent_age = max(0, response_time - date_value);
728
729  response_delay = response_time - request_time;
730  corrected_age_value = age_value + response_delay; 
731]]></artwork>
732</figure>
733<figure>
734<preamble>These SHOULD be combined as</preamble>
735<artwork type="code"><![CDATA[
736  corrected_initial_age = max(apparent_age, corrected_age_value);
737]]></artwork></figure>
738<t>
739   unless the cache is confident in the value of the <xref target="header.age" format="none">Age</xref> header
740   field (e.g., because there are no HTTP/1.0 hops in the Via
741   header field), in which case the corrected_age_value MAY be used as the
742   corrected_initial_age.</t>
743<t>
744   The current_age of a stored response can then be calculated by adding the
745   amount of time (in seconds) since the stored response was last validated by
746   the origin server to the corrected_initial_age.
747</t>
748<figure><artwork type="code"><![CDATA[
749  resident_time = now - response_time;
750  current_age = corrected_initial_age + resident_time;
751]]></artwork></figure>
752<t>
753   Additionally, to avoid common problems in date parsing:
754</t>
755<t>
756  <list style="symbols">
757     <t>HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
758        which appears to be more than 50 years in the future is in fact
759        in the past (this helps solve the "year 2000" problem).</t>
760
761     <t>Although all date formats are specified to be case-sensitive,
762        recipients SHOULD match day, week and timezone names
763        case-insensitively.</t>
764             
765     <t>An HTTP/1.1 implementation MAY internally represent a parsed
766        <xref target="header.expires" format="none">Expires</xref> date as earlier than the proper value, but
767        MUST NOT internally represent a parsed Expires date as later than the
768        proper value.</t>
769
770     <t>All expiration-related calculations MUST be done in GMT. The
771        local time zone MUST NOT influence the calculation or comparison
772        of an age or expiration time.</t>
773
774     <t>If an HTTP header field incorrectly carries a date value with a time
775        zone other than GMT, it MUST be converted into GMT using the
776        most conservative possible conversion.</t>
777  </list>
778</t>
779</section>
780
781<section anchor="serving.stale.responses" title="Serving Stale Responses">
782<t>
783   A "stale" response is one that either has explicit expiry information or is
784   allowed to have heuristic expiry calculated, but is not fresh according to
785   the calculations in <xref target="expiration.model"/>.
786</t>
787<t>
788   A cache MUST NOT return a stale response if it is prohibited by an
789   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
790   directive, a "must-revalidate" cache-response-directive, or an applicable
791   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
792</t>
793<t>
794   A cache MUST NOT return stale responses unless it is disconnected
795   (i.e., it cannot contact the origin server or otherwise find a forward
796   path) or doing so is explicitly allowed (e.g., by the max-stale request
797   directive; see <xref target="cache-request-directive"/>).
798</t>
799<t>
800   A cache SHOULD append a <xref target="header.warning" format="none">Warning</xref> header field with the 110
801   warn-code (see <xref target="header.warning"/>) to stale responses.
802   Likewise, a cache SHOULD add the 112 warn-code to stale responses if the
803   cache is disconnected.
804</t>
805<t>
806   If a cache receives a first-hand response (either an entire response, or a
807   304 (Not Modified) response) that it would normally forward
808   to the requesting client, and the received response is no longer fresh, the
809   cache can forward it to the requesting client without adding a new
810   <xref target="header.warning" format="none">Warning</xref> (but without removing any existing Warning header
811   fields). A cache shouldn't attempt to validate a response simply because
812   that response became stale in transit.
813</t>
814</section>
815</section>
816
817<section anchor="validation.model" title="Validation Model">
818<t>
819   When a cache has one or more stored responses for a requested URI, but
820   cannot serve any of them (e.g., because they are not fresh, or one cannot
821   be selected; see <xref target="caching.negotiated.responses"/>), it can use
822   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
823   give the origin server an opportunity to both select a valid stored
824   response to be used, and to update it. This process is known as
825   "validating" or "revalidating" the stored response.
826</t>
827<t>
828   When sending such a conditional request, a cache adds an
829   If-Modified-Since header field whose value is that of the
830   Last-Modified header field from the selected
831   (see <xref target="caching.negotiated.responses"/>) stored response, if
832   available.
833</t>
834<t>
835   Additionally, a cache can add an If-None-Match header field
836   whose value is that of the ETag header field(s) from all
837   responses stored for the requested URI, if present. However, if any of the
838   stored responses contains only partial content, the cache shouldn't
839   include its entity-tag in the If-None-Match header field unless the request
840   is for a range that would be fully satisfied by that stored response.
841</t>
842
843<t>Cache handling of a response to a conditional request is dependent upon its
844status code:</t>
845
846<t>
847   <list style="symbols">
848      <t>
849         A 304 (Not Modified) response status code indicates
850         that the stored response can be updated and reused; see <xref target="freshening.responses"/>.
851      </t>
852      <t>
853         A full response (i.e., one with a response body) indicates that none
854         of the stored responses nominated in the conditional request is
855         suitable. Instead, the cache can use the full response to
856         satisfy the request and MAY replace the stored response(s).
857      </t>
858      <t>
859         However, if a cache receives a 5xx (Server Error)
860         response while attempting to validate a response, it can either
861         forward this response to the requesting client, or act as if the
862         server failed to respond. In the latter case, it can return a
863         previously stored response (see <xref target="serving.stale.responses"/>).
864      </t>
865   </list>
866</t>
867
868<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
869<t>
870   When a cache receives a 304 (Not Modified) response and
871   already has one or more stored 200 (OK) responses for the
872   same cache key, the cache needs to identify which of the stored responses
873   are updated by this new response and then update the stored response(s)
874   with the new information provided in the 304 response.
875   <list style="symbols">
876    <t>
877     If the new response contains a strong validator, then that strong
878     validator identifies the selected representation.  All of the stored
879     responses with the same strong validator are selected.
880     If none of the stored responses contain the same strong validator, then
881     this new response corresponds to a new selected representation and
882     MUST NOT update the existing stored responses.
883    </t>
884    <t>
885     If the new response contains a weak validator and that validator
886     corresponds to one of the cache's stored responses, then the most
887     recent of those matching stored responses is selected.
888    </t>
889    <t>
890     If the new response does not include any form of validator, there is
891     only one stored response, and that stored response also lacks a
892     validator, then that stored response is selected.
893    </t>
894   </list>
895</t>
896<t>
897   If a stored response is selected for update, the cache MUST:
898   <list style="symbols">
899      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
900         with warn-code 1xx (see <xref target="header.warning"/>);</t>
901      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
902         with warn-code 2xx; and,</t>
903      <t>use other header fields provided in the 304 (Not Modified)
904         response to replace all instances of the corresponding header
905         fields in the stored response.</t>
906   </list>
907</t>
908</section>
909
910</section>
911
912<section anchor="caching.negotiated.responses" title="Using Negotiated Responses">
913<t>
914   When a cache receives a request that can be satisfied by a stored response
915   that has a <xref target="header.vary" format="none">Vary</xref> header field (<xref target="header.vary"/>),
916   it MUST NOT use that response unless all of the selecting header fields
917   nominated by the Vary header field match in both the original request
918   (i.e., that associated with the stored response), and the presented
919   request.
920</t>
921<t>
922   The selecting header fields from two requests are defined to match if and
923   only if those in the first request can be transformed to those in the
924   second request by applying any of the following:
925   <list style="symbols">
926      <t>
927         adding or removing whitespace, where allowed in the header field's
928         syntax
929      </t>
930      <t>
931         combining multiple header fields with the same field name
932         (see Section 3.2 of <xref target="Part1"/>)
933      </t>
934      <t>
935         normalizing both header field values in a way that is known to have
936         identical semantics, according to the header field's specification
937         (e.g., re-ordering field values when order is not significant;
938         case-normalization, where values are defined to be case-insensitive)
939      </t>
940  </list>
941</t>
942<t>
943   If (after any normalization that might take place) a header field is absent
944   from a request, it can only match another request if it is also absent
945   there.
946</t>
947<t>
948   A <xref target="header.vary" format="none">Vary</xref> header field-value of "*" always fails to match, and
949   subsequent requests to that resource can only be properly interpreted by the
950   origin server.
951</t>
952<t>
953   The stored response with matching selecting header fields is known as the
954   selected response.
955</t>
956<t>
957   If multiple selected responses are available, the most recent response
958   (as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
959</t>
960<t>
961   If no selected response is available, the cache can forward the presented
962   request to the origin server in a conditional request; see <xref target="validation.model"/>.
963</t>
964</section>
965
966
967<section anchor="combining.responses" title="Combining Partial Content">
968<t>
969   A response might transfer only a partial representation if the
970   connection closed prematurely or if the request used one or more Range
971   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
972   received several ranges of the same representation.  A cache MAY combine
973   these ranges into a single stored response, and reuse that response to
974   satisfy later requests, if they all share the same strong validator and
975   the cache complies with the client requirements in Section 4.2 of <xref target="Part5"/>.
976</t>
977<t>
978   When combining the new response with one or more stored responses, a
979   cache MUST:
980   <list style="symbols">
981      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
982         with warn-code 1xx (see <xref target="header.warning"/>);</t>
983      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
984         with warn-code 2xx; and,</t>
985      <t>use other header fields provided in the new response, aside
986         from Content-Range, to replace all instances of the
987         corresponding header fields in the stored response.</t>
988   </list>
989</t>
990</section>
991</section>
992
993
994<section anchor="head.effects" title="Updating Caches with HEAD Responses">
995<t>
996   A response to the HEAD method is identical to what an equivalent request
997   made with a GET would have been, except it lacks a body. This property
998   of HEAD responses is used to both invalidate and update cached GET
999   responses.
1000</t>
1001<t>
1002   If one or more stored GET responses can be selected (as per <xref target="caching.negotiated.responses"/>) for a HEAD request, and the
1003   Content-Length, ETag or
1004   Last-Modified value of a HEAD response differs from that in a
1005   selected GET response, the cache MUST consider that selected response to
1006   be stale.
1007</t>
1008<t>
1009   If the Content-Length, ETag and
1010   Last-Modified values of a HEAD response (when present) are
1011   the same as that in a selected GET response (as per
1012   <xref target="caching.negotiated.responses"/>), the cache SHOULD update
1013   the remaining header fields in the stored response using the following
1014   rules:
1015   <list style="symbols">
1016      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
1017         with warn-code 1xx (see <xref target="header.warning"/>);</t>
1018      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
1019         with warn-code 2xx; and,</t>
1020      <t>use other header fields provided in the response to replace
1021         all instances of the corresponding header fields in the stored
1022         response.</t>
1023   </list>
1024</t>
1025
1026</section>
1027
1028
1029<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
1030<t>
1031   Because unsafe request methods (Section 2.1.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
1032   have the potential for changing state on the origin server, intervening
1033   caches can use them to keep their contents up-to-date.
1034</t>
1035<t>
1036   A cache MUST invalidate the effective Request URI
1037   (Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the
1038   Location and Content-Location response header
1039   fields (if present) when a non-error response to a request with an unsafe
1040   method is received.
1041</t>
1042<t>
1043   However, a cache MUST NOT invalidate a URI from a Location
1044   or Content-Location response header field if the host part of
1045   that URI differs from the host part in the effective request URI
1046   (Section 5.5 of <xref target="Part1"/>). This helps prevent denial of service attacks.
1047</t>
1048<t>
1049   A cache MUST invalidate the effective request URI
1050   (Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
1051   to a request with a method whose safety is unknown.
1052</t>
1053<t>
1054   Here, a "non-error response" is one with a 2xx (Successful)
1055   or 3xx (Redirection) status code. "Invalidate" means that
1056   the cache will either remove all stored responses related to the effective
1057   request URI, or will mark these as "invalid" and in need of a mandatory
1058   validation before they can be returned in response to a subsequent request.
1059</t>
1060<t>
1061   Note that this does not guarantee that all appropriate responses are
1062   invalidated. For example, the request that caused the change at the origin
1063   server might not have gone through the cache where a response is stored.
1064</t>
1065</section>
1066
1067
1068
1069
1070<section anchor="header.field.definitions" title="Header Field Definitions">
1071<t>
1072   This section defines the syntax and semantics of HTTP/1.1 header fields
1073   related to caching.
1074</t>
1075
1076<section anchor="header.age" title="Age">
1077   <iref item="Age header field" primary="true"/>
1078   <iref item="Header Fields" primary="true" subitem="Age"/>
1079   
1080   
1081<t>
1082   The "Age" header field conveys the sender's estimate of the amount
1083   of time since the response was generated or successfully validated at the
1084   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1085</t>
1086<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1087  Age = delta-seconds
1088]]></artwork></figure>
1089<t>
1090  Age field-values are non-negative integers, representing time in seconds
1091  (see <xref target="delta-seconds"/>).
1092</t>
1093<t>
1094   The presence of an Age header field in a response implies that a response
1095   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1096   might not implement the Age header field.
1097</t>
1098</section>
1099
1100<section anchor="header.cache-control" title="Cache-Control">
1101   <iref item="Cache-Control header field" primary="true"/>
1102   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1103   
1104   
1105<t>
1106   The "Cache-Control" header field is used to specify directives for
1107   caches along the request/response chain. Such cache directives are
1108   unidirectional in that the presence of a directive in a request does not
1109   imply that the same directive is to be given in the response.
1110</t>
1111<t>
1112   A cache MUST obey the requirements of the Cache-Control
1113   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1114   directives defined elsewhere are handled.
1115</t>
1116<t><list>
1117   <t>
1118       Note: HTTP/1.0 caches might not implement Cache-Control and
1119       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1120   </t>
1121</list></t>
1122<t>
1123   A proxy, whether or not it implements a cache, MUST pass cache directives
1124   through in forwarded messages, regardless of their
1125   significance to that application, since the directives might be applicable
1126   to all recipients along the request/response chain. It is not possible to
1127   target a directive to a specific cache.
1128</t>
1129<t>
1130   Cache directives are identified by a token, to be compared case-insensitively,
1131   and have an optional argument, that can use both token and quoted-string
1132   syntax. For the directives defined below that define arguments, recipients
1133   ought to accept both forms, even if one is documented to be preferred. For
1134   any directive not defined by this specification, recipients MUST accept
1135   both forms.
1136</t>
1137<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><artwork type="abnf2616"><![CDATA[
1138  Cache-Control   = 1#cache-directive
1139
1140  cache-directive = token [ "=" ( token / quoted-string ) ]
1141]]></artwork></figure>
1142<t>
1143   For the cache directives defined below, no argument is defined (nor allowed)
1144   otherwise stated otherwise.
1145</t>
1146
1147<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1148
1149<section title="no-cache" anchor="cache-request-directive.no-cache">
1150   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1151   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1152<t>
1153   The "no-cache" request directive indicates that a cache MUST NOT
1154   use a stored response to satisfy the request without successful
1155   validation on the origin server.
1156</t>
1157</section>
1158 
1159<section title="no-store" anchor="cache-request-directive.no-store">
1160   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1161   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1162<t>
1163   The "no-store" request directive indicates that a cache MUST NOT
1164   store any part of either this request or any response to it. This
1165   directive applies to both private and shared caches. "MUST NOT
1166   store" in this context means that the cache MUST NOT intentionally
1167   store the information in non-volatile storage, and MUST make a
1168   best-effort attempt to remove the information from volatile storage as
1169   promptly as possible after forwarding it.
1170</t>
1171<t>
1172   This directive is NOT a reliable or sufficient mechanism for ensuring
1173   privacy. In particular, malicious or compromised caches might not
1174   recognize or obey this directive, and communications networks might be
1175   vulnerable to eavesdropping.
1176</t>
1177<t>
1178   Note that if a request containing this directive is satisfied from a
1179   cache, the no-store request directive does not apply to the already
1180   stored response.
1181</t>
1182</section>
1183
1184<section title="max-age" anchor="cache-request-directive.max-age">
1185   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1186   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1187<t>
1188   Argument syntax:
1189   <list>
1190      <t>
1191        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1192      </t>
1193   </list>
1194</t>
1195<t>
1196   The "max-age" request directive indicates that the client is unwilling to
1197   accept a response whose age is greater than the specified number of
1198   seconds. Unless the max-stale request directive is also present, the
1199   client is not willing to accept a stale response.
1200</t>
1201<t>
1202   Note: This directive uses the token form of the argument syntax;
1203   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1204   quoted-string form.
1205</t>
1206</section>
1207
1208<section title="max-stale" anchor="cache-request-directive.max-stale">
1209   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1210   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1211<t>
1212   Argument syntax:
1213   <list>
1214      <t>
1215        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1216      </t>
1217   </list>
1218</t>
1219<t>
1220   The "max-stale" request directive indicates that the client is willing
1221   to accept a response that has exceeded its expiration time. If max-stale
1222   is assigned a value, then the client is willing to accept a response
1223   that has exceeded its expiration time by no more than the specified
1224   number of seconds. If no value is assigned to max-stale, then the client
1225   is willing to accept a stale response of any age.
1226</t>
1227<t>
1228   Note: This directive uses the token form of the argument syntax;
1229   e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use the
1230   quoted-string form.
1231</t>
1232</section>
1233
1234<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1235   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1236   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1237<t>
1238   Argument syntax:
1239   <list>
1240      <t>
1241        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1242      </t>
1243   </list>
1244</t>
1245<t>
1246   The "min-fresh" request directive indicates that the client is willing
1247   to accept a response whose freshness lifetime is no less than its
1248   current age plus the specified time in seconds. That is, the client
1249   wants a response that will still be fresh for at least the specified
1250   number of seconds.
1251</t>
1252<t>
1253   Note: This directive uses the token form of the argument syntax;
1254   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the
1255   quoted-string form.
1256</t>
1257</section>
1258
1259<section title="no-transform" anchor="cache-request-directive.no-transform">
1260   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1261   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1262<t>
1263   The "no-transform" request directive indicates that an intermediary
1264   (whether or not it implements a cache) MUST NOT change the
1265   Content-Encoding, Content-Range or
1266   Content-Type request header fields, nor the request
1267   representation.
1268</t>
1269</section>
1270
1271<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1272   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1273   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1274<t>
1275   The "only-if-cached" request directive indicates that the client only wishes
1276   to obtain a stored response. If it receives this directive, a cache SHOULD
1277   either respond using a stored response that is consistent with the other
1278   constraints of the request, or respond with a 504 (Gateway
1279   Timeout) status code. If a group of caches is being operated as a
1280   unified system with good internal connectivity, a member cache MAY
1281   forward such a request within that group of caches.
1282</t>
1283</section>
1284</section>
1285
1286<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1287   
1288
1289<section title="public" anchor="cache-response-directive.only-if-cached">
1290   <iref item="Cache Directives" primary="true" subitem="public"/>
1291   <iref item="public" primary="true" subitem="Cache Directive"/>
1292<t>
1293   The "public" response directive indicates that a response whose
1294   associated request contains an 'Authentication' header MAY be
1295   stored (see <xref target="caching.authenticated.responses"/>).
1296</t>
1297</section>
1298
1299<section title="private" anchor="cache-response-directive.private">
1300   <iref item="Cache Directives" primary="true" subitem="private"/>
1301   <iref item="private" primary="true" subitem="Cache Directive"/>
1302<t>
1303   Argument syntax:
1304   <list>
1305      <t>
1306        #<xref target="imported.abnf" format="none">field-name</xref>
1307      </t>
1308   </list>
1309</t>
1310<t>
1311   The "private" response directive indicates that the response message is
1312   intended for a single user and MUST NOT be stored by a shared cache. A
1313   private cache MAY store the response.
1314</t>
1315<t>
1316   If the private response directive specifies one or more field-names,
1317   this requirement is limited to the field-values associated with the
1318   listed response header fields. That is, a shared cache MUST NOT store
1319   the specified field-names(s), whereas it MAY store the remainder of the
1320   response message.
1321</t>
1322<t>
1323   The field-names given are not limited to the set of standard header
1324   fields defined by this specification. Field names are case-insensitive.
1325</t>
1326<t>
1327   Note: This usage of the word "private" only controls
1328   where the response can be stored; it cannot ensure the privacy of the
1329   message content. Also, private response directives with field-names are
1330   often handled by implementations as if an unqualified private directive
1331   was received; i.e., the special handling for the qualified form is not
1332   widely implemented.
1333</t>
1334<t>
1335   Note: This directive uses the quoted-string form of the argument syntax.
1336   Senders SHOULD NOT use the token form (even if quoting appears not to be
1337   needed for single-entry lists).
1338</t>
1339</section>
1340
1341<section title="no-cache" anchor="cache-response-directive.no-cache">
1342   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1343   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1344<t>
1345   Argument syntax:
1346   <list>
1347      <t>
1348        #<xref target="imported.abnf" format="none">field-name</xref>
1349      </t>
1350   </list>
1351</t>
1352<t>
1353   The "no-cache" response directive indicates that the response MUST NOT
1354   be used to satisfy a subsequent request without successful validation on
1355   the origin server. This allows an origin server to prevent a cache from
1356   using it to satisfy a request without contacting it, even by caches that
1357   have been configured to return stale responses.
1358</t>
1359<t>
1360   If the no-cache response directive specifies one or more field-names,
1361   then a cache MAY use the response to satisfy a subsequent request,
1362   subject to any other restrictions on caching. However, any header fields
1363   in the response that have the field-name(s) listed MUST NOT be sent
1364   in the response to a subsequent request without successful revalidation
1365   with the origin server. This allows an origin server to prevent the
1366   re-use of certain header fields in a response, while still allowing
1367   caching of the rest of the response.
1368</t>
1369<t>
1370   The field-names given are not limited to the set of standard header
1371   fields defined by this specification. Field names are case-insensitive.
1372</t>
1373<t>
1374   Note: Many HTTP/1.0 caches will not recognize or obey
1375   this directive. Also, no-cache response directives with field-names are
1376   often handled by implementations as if an unqualified no-cache directive
1377   was received; i.e., the special handling for the qualified form is not
1378   widely implemented.
1379</t>
1380<t>
1381   Note: This directive uses the quoted-string form of the argument syntax.
1382   Senders SHOULD NOT use the token form (even if quoting appears not to be
1383   needed for single-entry lists).
1384</t>
1385</section>
1386
1387<section title="no-store" anchor="cache-response-directive.no-store">
1388   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1389   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1390<t>
1391   The "no-store" response directive indicates that a cache MUST NOT
1392   store any part of either the immediate request or response. This
1393   directive applies to both private and shared caches. "MUST NOT
1394   store" in this context means that the cache MUST NOT intentionally
1395   store the information in non-volatile storage, and MUST make a
1396   best-effort attempt to remove the information from volatile storage as
1397   promptly as possible after forwarding it.
1398</t>
1399<t>
1400   This directive is NOT a reliable or sufficient mechanism for ensuring
1401   privacy. In particular, malicious or compromised caches might not
1402   recognize or obey this directive, and communications networks might be
1403   vulnerable to eavesdropping.
1404</t>
1405</section>
1406
1407<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1408   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1409   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1410<t>
1411   The "must-revalidate" response directive indicates that once it has
1412   become stale, a cache MUST NOT use the response to satisfy subsequent
1413   requests without successful validation on the origin server.
1414</t>
1415<t>
1416   The must-revalidate directive is necessary to support reliable
1417   operation for certain protocol features. In all circumstances a
1418   cache MUST obey the must-revalidate directive; in particular,
1419   if a cache cannot reach the origin server for any reason, it MUST
1420   generate a 504 (Gateway Timeout) response.
1421</t>
1422<t>
1423   The must-revalidate directive ought to be used by servers if and only
1424   if failure to validate a request on the representation could result in
1425   incorrect operation, such as a silently unexecuted financial
1426   transaction.
1427</t>
1428</section>
1429
1430<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1431   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1432   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1433<t>
1434   The "proxy-revalidate" response directive has the same meaning as the
1435   must-revalidate response directive, except that it does not apply to
1436   private caches.
1437</t>
1438</section>
1439
1440<section title="max-age" anchor="cache-response-directive.max-age">
1441   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1442   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1443<t>
1444   Argument syntax:
1445   <list>
1446      <t>
1447        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1448      </t>
1449   </list>
1450</t>
1451<t>
1452   The "max-age" response directive indicates that the response is to be
1453   considered stale after its age is greater than the specified number of
1454   seconds.
1455</t>
1456<t>
1457   Note: This directive uses the token form of the argument syntax;
1458   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1459   quoted-string form.
1460</t>
1461</section>     
1462
1463<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1464   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1465   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1466<t>
1467   Argument syntax:
1468   <list>
1469      <t>
1470        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1471      </t>
1472   </list>
1473</t>
1474<t>
1475   The "s-maxage" response directive indicates that, in shared caches, the
1476   maximum age specified by this directive overrides the maximum age
1477   specified by either the max-age directive or the <xref target="header.expires" format="none">Expires</xref>
1478   header field. The s-maxage directive also implies the semantics of the
1479   proxy-revalidate response directive.
1480</t>
1481<t>
1482   Note: This directive uses the token form of the argument syntax;
1483   e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
1484   quoted-string form.
1485</t>
1486</section>
1487
1488<section title="no-transform" anchor="cache-response-directive.no-transform">
1489   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1490   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1491<t>
1492   The "no-transform" response directive indicates that an intermediary
1493   (regardless of whether it implements a cache) MUST NOT change the
1494   Content-Encoding, Content-Range or
1495   Content-Type response header fields, nor the response
1496   representation.
1497</t>
1498</section>
1499
1500</section>
1501
1502<section anchor="cache.control.extensions" title="Cache Control Extensions">
1503<t>
1504   The Cache-Control header field can be extended through the use of one or
1505   more cache-extension tokens, each with an optional value. Informational
1506   extensions (those that do not require a change in cache behavior) can be
1507   added without changing the semantics of other directives. Behavioral
1508   extensions are designed to work by acting as modifiers to the existing base
1509   of cache directives. Both the new directive and the standard directive are
1510   supplied, such that applications that do not understand the new directive
1511   will default to the behavior specified by the standard directive, and those
1512   that understand the new directive will recognize it as modifying the
1513   requirements associated with the standard directive. In this way,
1514   extensions to the cache-control directives can be made without requiring
1515   changes to the base protocol.
1516</t>
1517<t>
1518   This extension mechanism depends on an HTTP cache obeying all of the
1519   cache-control directives defined for its native HTTP-version, obeying
1520   certain extensions, and ignoring all directives that it does not
1521   understand.
1522</t>
1523<t>
1524   For example, consider a hypothetical new response directive called
1525   "community" that acts as a modifier to the private directive. We define
1526   this new directive to mean that, in addition to any private cache, any
1527   cache that is shared only by members of the community named within its
1528   value is allowed to cache the response. An origin server wishing to allow
1529   the UCI community to use an otherwise private response in their shared
1530   cache(s) could do so by including
1531</t>
1532<figure><artwork type="example"><![CDATA[
1533  Cache-Control: private, community="UCI"
1534]]></artwork></figure>
1535<t>
1536   A cache seeing this header field will act correctly even if the cache does
1537   not understand the community cache-extension, since it will also see and
1538   understand the private directive and thus default to the safe behavior.
1539</t>
1540<t>
1541   A cache MUST ignore unrecognized cache directives; it is assumed that any
1542   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1543   combined with standard directives (or the response's default cacheability)
1544   such that the cache behavior will remain minimally correct even if the
1545   cache does not understand the extension(s).
1546</t>
1547<t>
1548   New extension directives ought to consider defining:
1549</t>
1550<t>
1551   <list style="symbols">
1552      <t>What it means for a directive to be specified multiple times,</t>
1553      <t>When the directive does not take an argument, what it means when an
1554      argument is present,</t>
1555      <t>When the directive requires an argument, what it means when it is
1556      missing.</t>
1557   </list>
1558</t>
1559<t>
1560   The HTTP Cache Directive Registry defines the name space for the cache
1561   directives.
1562</t>
1563<t>
1564   A registration MUST include the following fields:
1565   <list style="symbols">
1566      <t>Cache Directive Name</t>
1567      <t>Pointer to specification text</t>
1568   </list>
1569</t>
1570<t>
1571   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1572</t>
1573<t>
1574   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1575</t>
1576</section>
1577
1578</section>
1579
1580<section anchor="header.expires" title="Expires">
1581   <iref item="Expires header field" primary="true"/>
1582   <iref item="Header Fields" primary="true" subitem="Expires"/>
1583   
1584<t>
1585   The "Expires" header field gives the date/time after which the
1586   response is considered stale. See <xref target="expiration.model"/> for
1587   further discussion of the freshness model.
1588</t>
1589<t>
1590   The presence of an Expires field does not imply that the original resource
1591   will change or cease to exist at, before, or after that time.
1592</t>
1593<t>
1594   The field-value is an absolute date and time as defined by HTTP-date in
1595   Section 5.1 of <xref target="Part2"/>; a sender MUST use the rfc1123-date format.
1596</t>
1597<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1598  Expires = HTTP-date
1599]]></artwork></figure>
1600<figure>
1601  <preamble>For example</preamble>
1602<artwork type="example"><![CDATA[
1603  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1604]]></artwork></figure>
1605<t>
1606   A cache MUST treat other invalid date formats,
1607   especially including the value "0", as in the past (i.e., "already
1608   expired").
1609</t>
1610<t><list>
1611   <t>
1612       Note: If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
1613       the max-age directive (see <xref target="cache-response-directive.max-age"/>),
1614       that directive overrides the Expires field. Likewise, the s-maxage
1615       directive (<xref target="cache-response-directive.s-maxage"/>) overrides
1616       the <xref target="header.expires" format="none">Expires</xref> header fieldin shared caches.
1617   </t>
1618</list></t>
1619<t>
1620   Historically, HTTP required the Expires field-value to be no more than a
1621   year in the future. While longer freshness lifetimes are no longer
1622   prohibited, extremely large values have been demonstrated to cause
1623   problems (e.g., clock overflows due to use of 32-bit integers for
1624   time values), and many caches will evict a response far sooner than
1625   that. Therefore, senders ought not produce them.
1626</t>
1627<t>
1628   An origin server without a clock MUST NOT assign Expires
1629   values to a response unless these values were associated
1630   with the resource by a system or user with a reliable clock. It MAY
1631   assign an Expires value that is known, at or before server
1632   configuration time, to be in the past (this allows "pre-expiration"
1633   of responses without storing separate Expires values for each
1634   resource).
1635</t>
1636</section>
1637
1638<section anchor="header.pragma" title="Pragma">
1639   <iref item="Pragma header field" primary="true"/>
1640   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1641   
1642   
1643   
1644<t>
1645   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1646   caches, so that clients can specify a "no-cache" request that they will
1647   understand (as <xref target="header.cache-control" format="none">Cache-Control</xref> was not defined until HTTP/1.1).
1648   When the Cache-Control header field is also present and understood in a
1649   request, Pragma is ignored.
1650</t>
1651<t>
1652   In HTTP/1.0, Pragma was defined as an extensible field for
1653   implementation-specified directives for recipients. This specification
1654   deprecates such extensions to improve interoperability.
1655</t>
1656<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
1657  Pragma           = 1#pragma-directive
1658  pragma-directive = "no-cache" / extension-pragma
1659  extension-pragma = token [ "=" ( token / quoted-string ) ]
1660]]></artwork></figure>
1661<t>
1662   When the <xref target="header.cache-control" format="none">Cache-Control</xref> header field is not present in a
1663   request, the no-cache request pragma-directive MUST have the same effect
1664   on caches as if "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
1665</t>
1666<t>
1667   When sending a no-cache request, a client ought to include both the pragma
1668   and cache-control directives, unless Cache-Control: no-cache is
1669   purposefully omitted to target other <xref target="header.cache-control" format="none">Cache-Control</xref> response
1670   directives at HTTP/1.1 caches. For example:
1671</t>
1672<figure>
1673<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1674  GET / HTTP/1.1
1675  Host: www.example.com
1676  Cache-Control: max-age=30
1677  Pragma: no-cache
1678 
1679  ]]></artwork>
1680</figure>
1681<t>
1682   will constrain HTTP/1.1 caches to serve a response no older than 30
1683   seconds, while precluding implementations that do not understand
1684   <xref target="header.cache-control" format="none">Cache-Control</xref> from serving a cached response.
1685</t>
1686<t><list>
1687   <t>
1688      Note: Because the meaning of "Pragma: no-cache" in responses is not
1689      specified, it does not provide a reliable replacement for
1690      "Cache-Control: no-cache" in them.
1691   </t>
1692</list></t>
1693</section>
1694
1695<section anchor="header.vary" title="Vary">
1696   <iref item="Vary header field" primary="true"/>
1697   <iref item="Header Fields" primary="true" subitem="Vary"/>
1698   
1699<t>
1700   The "Vary" header field conveys the set of header fields
1701   that were used to select the representation.
1702</t>
1703<t>
1704   Caches use this information, in part, to determine whether a stored
1705   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>.
1706</t>
1707<t>
1708   In uncacheable or stale responses, the Vary field value advises the user
1709   agent about the criteria that were used to select the representation.
1710</t>
1711<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
1712  Vary = "*" / 1#field-name
1713]]></artwork></figure>
1714<t>
1715   The set of header fields named by the Vary field value is known as the
1716   selecting header fields.
1717</t>
1718<t>
1719   A server SHOULD include a Vary header field with any cacheable response
1720   that is subject to server-driven negotiation. Doing so allows a cache to
1721   properly interpret future requests on that resource and informs the user
1722   agent about the presence of negotiation on that resource. A server MAY
1723   include a Vary header field with a non-cacheable response that is subject
1724   to server-driven negotiation, since this might provide the user agent with
1725   useful information about the dimensions over which the response varies at
1726   the time of the response.
1727</t>
1728<t>
1729   A Vary field value of "*" signals that unspecified parameters not limited
1730   to the header fields (e.g., the network address of the client), play a
1731   role in the selection of the response representation; therefore, a cache
1732   cannot determine whether this response is appropriate. A proxy MUST NOT
1733   generate the "*" value.
1734</t>
1735<t>
1736   The field-names given are not limited to the set of standard header
1737   fields defined by this specification. Field names are case-insensitive.
1738</t>
1739</section>
1740
1741<section anchor="header.warning" title="Warning">
1742   <iref item="Warning header field" primary="true"/>
1743   <iref item="Header Fields" primary="true" subitem="Warning"/>
1744   
1745   
1746   
1747   
1748   
1749   
1750<t>
1751   The "Warning" header field is used to carry additional information
1752   about the status or transformation of a message that might not be reflected
1753   in the message. This information is typically used to warn about possible
1754   incorrectness introduced by caching operations or transformations applied
1755   to the payload of the message.
1756</t>
1757<t>
1758   Warnings can be used for other purposes, both cache-related and otherwise.
1759   The use of a warning, rather than an error status code, distinguishes these
1760   responses from true failures.
1761</t>
1762<t>
1763   Warning header fields can in general be applied to any message, however some
1764   warn-codes are specific to caches and can only be applied to response
1765   messages.
1766</t>
1767<figure><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/><artwork type="abnf2616"><![CDATA[
1768  Warning       = 1#warning-value
1769 
1770  warning-value = warn-code SP warn-agent SP warn-text
1771                                        [SP warn-date]
1772 
1773  warn-code  = 3DIGIT
1774  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1775                  ; the name or pseudonym of the server adding
1776                  ; the Warning header field, for use in debugging
1777  warn-text  = quoted-string
1778  warn-date  = DQUOTE HTTP-date DQUOTE
1779]]></artwork></figure>
1780<t>
1781   Multiple warnings can be attached to a response (either by the origin
1782   server or by a cache), including multiple warnings with the same code
1783   number, only differing in warn-text.
1784</t>
1785<t>
1786   When this occurs, the user agent SHOULD inform the user of as many of
1787   them as possible, in the order that they appear in the response.
1788</t>
1789<t>
1790   Systems that generate multiple Warning header fields are encouraged to
1791   order them with this user agent behavior in mind. New Warning header fields
1792   are added after any existing Warning header fields.
1793</t>
1794<t>
1795   Warnings are assigned three digit warn-codes. The first digit indicates
1796   whether the Warning is required to be deleted from a stored response after
1797   validation:
1798   <list style="symbols">
1799      <t>1xx Warnings describe the freshness or validation status of the
1800      response, and so MUST be deleted by a cache after validation. They can
1801      only be generated by a cache when validating a cached entry, and
1802      MUST NOT be generated in any other situation.</t>
1803      <t>2xx Warnings describe some aspect of the representation that is not
1804      rectified by a validation (for example, a lossy compression of the
1805      representation) and MUST NOT be deleted by a cache after validation,
1806      unless a full response is returned, in which case they MUST be.</t>
1807   </list>
1808</t>
1809<t>
1810   If an implementation sends a message with one or more Warning header fields
1811   to a receiver whose version is HTTP/1.0 or lower, then the sender MUST
1812   include in each warning-value a warn-date that matches the
1813   Date header field in the message.
1814</t>
1815<t>
1816   If a system receives a message with a warning-value that includes a
1817   warn-date, and that warn-date is different from the Date
1818   value in the response, then that warning-value MUST be deleted from the
1819   message before storing, forwarding, or using it. (preventing the
1820   consequences of naive caching of Warning header fields.) If all of the
1821   warning-values are deleted for this reason, the Warning header field MUST
1822   be deleted as well.
1823</t>
1824<t>
1825   The following warn-codes are defined by this specification, each with a
1826   recommended warn-text in English, and a description of its meaning.
1827</t>
1828
1829<section title="110 Response is Stale" anchor="warn.110">
1830  <iref primary="true" item="110 Response is Stale (warn code)"/>
1831  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale"/>
1832<t>
1833   A cache SHOULD include this whenever the returned response is stale.
1834</t>
1835</section>
1836
1837<section title="111 Revalidation Failed" anchor="warn.111">
1838  <iref primary="true" item="111 Revalidation Failed (warn code)"/>
1839  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed"/>
1840<t>
1841   A cache SHOULD include this when returning a stale response because an
1842   attempt to validate the response failed, due to an inability to reach
1843   the server.
1844</t>
1845</section>
1846
1847<section title="112 Disconnected Operation" anchor="warn.112">
1848  <iref primary="true" item="112 Disconnected Operation (warn code)"/>
1849  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation"/>
1850<t>
1851   A cache SHOULD include this if it is intentionally disconnected from
1852   the rest of the network for a period of time.
1853</t>
1854</section>
1855
1856<section title="113 Heuristic Expiration" anchor="warn.113">
1857  <iref primary="true" item="113 Heuristic Expiration (warn code)"/>
1858  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration"/>
1859<t>
1860   A cache SHOULD include this if it heuristically chose a freshness
1861   lifetime greater than 24 hours and the response's age is greater than 24
1862   hours.
1863</t>
1864</section>
1865
1866<section title="199 Miscellaneous Warning" anchor="warn.199">
1867  <iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
1868  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning"/>
1869<t>
1870   The warning text can include arbitrary information to be presented to
1871   a human user, or logged. A system receiving this warning MUST NOT take
1872   any automated action, besides presenting the warning to the user.
1873</t>
1874</section>
1875
1876<section title="214 Transformation Applied" anchor="warn.214">
1877  <iref primary="true" item="214 Transformation Applied (warn code)"/>
1878  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied"/>
1879<t>
1880   MUST be added by a proxy if it applies any transformation to the
1881   representation, such as changing the content-coding, media-type, or
1882   modifying the representation data, unless this Warning code already appears
1883   in the response.
1884</t>
1885</section>
1886
1887<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1888  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)"/>
1889  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning"/>
1890<t>
1891   The warning text can include arbitrary information to be presented to
1892   a human user, or logged. A system receiving this warning MUST NOT take
1893   any automated action.
1894</t>
1895</section>
1896
1897<section title="Warn Code Extensions" anchor="warn.code.extensions">
1898<t>
1899   The HTTP Warn Code Registry defines the name space for warn codes.
1900</t>
1901<t>
1902   A registration MUST include the following fields:
1903   <list style="symbols">
1904      <t>Warn Code (3 digits)</t>
1905      <t>Short Description</t>
1906      <t>Pointer to specification text</t>
1907   </list>
1908</t>
1909<t>
1910   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1911</t>
1912<t>
1913   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
1914</t>
1915</section>
1916</section>
1917</section>
1918
1919<section anchor="history.lists" title="History Lists">
1920<t>
1921   User agents often have history mechanisms, such as "Back" buttons and
1922   history lists, that can be used to redisplay a representation retrieved
1923   earlier in a session.
1924</t>
1925<t>
1926   The freshness model (<xref target="expiration.model"/>) does not
1927   necessarily apply to history mechanisms. I.e., a history mechanism can
1928   display a previous representation even if it has expired.
1929</t>
1930<t>
1931   This does not prohibit the history mechanism from telling the user that a
1932   view might be stale, or from honoring cache directives (e.g.,
1933   Cache-Control: no-store).
1934</t>
1935</section>
1936
1937
1938<section anchor="IANA.considerations" title="IANA Considerations">
1939
1940<section title="Cache Directive Registry" anchor="cache.directive.registration">
1941<t>
1942   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1943</t>
1944<t>
1945   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1946   populated with the registrations below:
1947</t>
1948
1949<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1950<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1951   <ttcol>Cache Directive</ttcol>
1952   <ttcol>Reference</ttcol>
1953
1954   <c>max-age</c>
1955   <c>
1956      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1957   </c>
1958   <c>max-stale</c>
1959   <c>
1960      <xref target="cache-request-directive.max-stale"/>
1961   </c>
1962   <c>min-fresh</c>
1963   <c>
1964      <xref target="cache-request-directive.min-fresh"/>
1965   </c>
1966   <c>must-revalidate</c>
1967   <c>
1968      <xref target="cache-response-directive.must-revalidate"/>
1969   </c>
1970   <c>no-cache</c>
1971   <c>
1972      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1973   </c>
1974   <c>no-store</c>
1975   <c>
1976      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1977   </c>
1978   <c>no-transform</c>
1979   <c>
1980      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1981   </c>
1982   <c>only-if-cached</c>
1983   <c>
1984      <xref target="cache-request-directive.only-if-cached"/>
1985   </c>
1986   <c>private</c>
1987   <c>
1988      <xref target="cache-response-directive.private"/>
1989   </c>
1990   <c>proxy-revalidate</c>
1991   <c>
1992      <xref target="cache-response-directive.proxy-revalidate"/>
1993   </c>
1994   <c>public</c>
1995   <c>
1996      <xref target="cache-response-directive.only-if-cached"/>
1997   </c>
1998   <c>s-maxage</c>
1999   <c>
2000      <xref target="cache-response-directive.s-maxage"/>
2001   </c>
2002   <c>stale-if-error</c>
2003   <c>
2004      <xref target="RFC5861"/>, Section 4
2005   </c>
2006   <c>stale-while-revalidate</c>
2007   <c>
2008      <xref target="RFC5861"/>, Section 3
2009   </c>
2010</texttable>
2011<!--(END)-->
2012
2013</section>
2014
2015<section title="Warn Code Registry" anchor="warn.code.registration">
2016<t>
2017   The registration procedure for HTTP Warn Codes is defined by <xref target="warn.code.extensions"/> of this document.
2018</t>
2019<t>
2020   The HTTP Warn Code Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
2021   populated with the registrations below:
2022</t>
2023
2024<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2025<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
2026   <ttcol>Warn Code</ttcol>
2027   <ttcol>Short Description</ttcol>
2028   <ttcol>Reference</ttcol>
2029   <c>110</c>
2030   <c>Response is Stale</c>
2031   <c>
2032      <xref target="warn.110"/>
2033   </c>
2034   <c>111</c>
2035   <c>Revalidation Failed</c>
2036   <c>
2037      <xref target="warn.111"/>
2038   </c>
2039   <c>112</c>
2040   <c>Disconnected Operation</c>
2041   <c>
2042      <xref target="warn.112"/>
2043   </c>
2044   <c>113</c>
2045   <c>Heuristic Expiration</c>
2046   <c>
2047      <xref target="warn.113"/>
2048   </c>
2049   <c>199</c>
2050   <c>Miscellaneous Warning</c>
2051   <c>
2052      <xref target="warn.199"/>
2053   </c>
2054   <c>214</c>
2055   <c>Transformation Applied</c>
2056   <c>
2057      <xref target="warn.214"/>
2058   </c>
2059   <c>299</c>
2060   <c>Miscellaneous Persistent Warning</c>
2061   <c>
2062      <xref target="warn.299"/>
2063   </c>
2064</texttable>
2065<!--(END)-->
2066
2067</section>
2068
2069<section title="Header Field Registration" anchor="header.field.registration">
2070<t>
2071  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
2072  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
2073</t>
2074
2075<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2076<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2077   <ttcol>Header Field Name</ttcol>
2078   <ttcol>Protocol</ttcol>
2079   <ttcol>Status</ttcol>
2080   <ttcol>Reference</ttcol>
2081
2082   <c>Age</c>
2083   <c>http</c>
2084   <c>standard</c>
2085   <c>
2086      <xref target="header.age"/>
2087   </c>
2088   <c>Cache-Control</c>
2089   <c>http</c>
2090   <c>standard</c>
2091   <c>
2092      <xref target="header.cache-control"/>
2093   </c>
2094   <c>Expires</c>
2095   <c>http</c>
2096   <c>standard</c>
2097   <c>
2098      <xref target="header.expires"/>
2099   </c>
2100   <c>Pragma</c>
2101   <c>http</c>
2102   <c>standard</c>
2103   <c>
2104      <xref target="header.pragma"/>
2105   </c>
2106   <c>Vary</c>
2107   <c>http</c>
2108   <c>standard</c>
2109   <c>
2110      <xref target="header.vary"/>
2111   </c>
2112   <c>Warning</c>
2113   <c>http</c>
2114   <c>standard</c>
2115   <c>
2116      <xref target="header.warning"/>
2117   </c>
2118</texttable>
2119<!--(END)-->
2120
2121<t>
2122   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2123   Force".
2124</t>
2125</section>
2126</section>
2127
2128<section anchor="security.considerations" title="Security Considerations">
2129<t>
2130   Caches expose additional potential vulnerabilities, since the contents of
2131   the cache represent an attractive target for malicious exploitation.
2132   Because cache contents persist after an HTTP request is complete, an attack
2133   on the cache can reveal information long after a user believes that the
2134   information has been removed from the network. Therefore, cache contents
2135   need to be protected as sensitive information.
2136</t>
2137</section>
2138
2139<section title="Acknowledgments" anchor="acks">
2140<t>
2141  See Section 9 of <xref target="Part1"/>.
2142</t>
2143</section>
2144
2145</middle>
2146
2147<back>
2148<references title="Normative References">
2149
2150  <reference anchor="Part1">
2151    <front>
2152      <title>HTTP/1.1, part 1: Message Routing and Syntax"</title>
2153      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2154        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2155        <address><email>fielding@gbiv.com</email></address>
2156      </author>
2157      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2158        <organization abbrev="W3C">World Wide Web Consortium</organization>
2159        <address><email>ylafon@w3.org</email></address>
2160      </author>
2161      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2162        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2163        <address><email>julian.reschke@greenbytes.de</email></address>
2164      </author>
2165      <date month="July" year="2012"/>
2166    </front>
2167    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-20"/>
2168   
2169  </reference>
2170
2171  <reference anchor="Part2">
2172    <front>
2173      <title>HTTP/1.1, part 2: Semantics and Payloads</title>
2174      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2175        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2176        <address><email>fielding@gbiv.com</email></address>
2177      </author>
2178      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2179        <organization abbrev="W3C">World Wide Web Consortium</organization>
2180        <address><email>ylafon@w3.org</email></address>
2181      </author>
2182      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2183        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2184        <address><email>julian.reschke@greenbytes.de</email></address>
2185      </author>
2186      <date month="July" year="2012"/>
2187    </front>
2188    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-20"/>
2189   
2190  </reference>
2191
2192  <reference anchor="Part4">
2193    <front>
2194      <title>HTTP/1.1, part 4: Conditional Requests</title>
2195      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2196        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2197        <address><email>fielding@gbiv.com</email></address>
2198      </author>
2199      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2200        <organization abbrev="W3C">World Wide Web Consortium</organization>
2201        <address><email>ylafon@w3.org</email></address>
2202      </author>
2203      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2204        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2205        <address><email>julian.reschke@greenbytes.de</email></address>
2206      </author>
2207      <date month="July" year="2012"/>
2208    </front>
2209    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-20"/>
2210   
2211  </reference>
2212
2213  <reference anchor="Part5">
2214    <front>
2215      <title>HTTP/1.1, part 5: Range Requests</title>
2216      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2217        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2218        <address><email>fielding@gbiv.com</email></address>
2219      </author>
2220      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2221        <organization abbrev="W3C">World Wide Web Consortium</organization>
2222        <address><email>ylafon@w3.org</email></address>
2223      </author>
2224      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2225        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2226        <address><email>julian.reschke@greenbytes.de</email></address>
2227      </author>
2228      <date month="July" year="2012"/>
2229    </front>
2230    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-20"/>
2231   
2232  </reference>
2233
2234  <reference anchor="Part7">
2235    <front>
2236      <title>HTTP/1.1, part 7: Authentication</title>
2237      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2238        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2239        <address><email>fielding@gbiv.com</email></address>
2240      </author>
2241      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2242        <organization abbrev="W3C">World Wide Web Consortium</organization>
2243        <address><email>ylafon@w3.org</email></address>
2244      </author>
2245      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2246        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2247        <address><email>julian.reschke@greenbytes.de</email></address>
2248      </author>
2249      <date month="July" year="2012"/>
2250    </front>
2251    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-20"/>
2252   
2253  </reference>
2254
2255  <reference anchor="RFC2119">
2256    <front>
2257      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2258      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2259        <organization>Harvard University</organization>
2260        <address><email>sob@harvard.edu</email></address>
2261      </author>
2262      <date month="March" year="1997"/>
2263    </front>
2264    <seriesInfo name="BCP" value="14"/>
2265    <seriesInfo name="RFC" value="2119"/>
2266  </reference>
2267
2268  <reference anchor="RFC5234">
2269    <front>
2270      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2271      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2272        <organization>Brandenburg InternetWorking</organization>
2273        <address>
2274          <email>dcrocker@bbiw.net</email>
2275        </address> 
2276      </author>
2277      <author initials="P." surname="Overell" fullname="Paul Overell">
2278        <organization>THUS plc.</organization>
2279        <address>
2280          <email>paul.overell@thus.net</email>
2281        </address>
2282      </author>
2283      <date month="January" year="2008"/>
2284    </front>
2285    <seriesInfo name="STD" value="68"/>
2286    <seriesInfo name="RFC" value="5234"/>
2287  </reference>
2288 
2289</references>
2290
2291<references title="Informative References">
2292
2293  <reference anchor="RFC1305">
2294    <front>
2295      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2296      <author fullname="David L. Mills" initials="D." surname="Mills">
2297        <organization>University of Delaware, Electrical Engineering Department</organization>
2298        <address><email>mills@udel.edu</email></address>
2299      </author>
2300      <date month="March" year="1992"/>
2301    </front>
2302    <seriesInfo name="RFC" value="1305"/>
2303  </reference>
2304
2305  <reference anchor="RFC2616">
2306    <front>
2307      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2308      <author fullname="R. Fielding" initials="R." surname="Fielding">
2309        <organization>University of California, Irvine</organization>
2310        <address><email>fielding@ics.uci.edu</email></address>
2311      </author>
2312      <author fullname="J. Gettys" initials="J." surname="Gettys">
2313        <organization>W3C</organization>
2314        <address><email>jg@w3.org</email></address>
2315      </author>
2316      <author fullname="J. Mogul" initials="J." surname="Mogul">
2317        <organization>Compaq Computer Corporation</organization>
2318        <address><email>mogul@wrl.dec.com</email></address>
2319      </author>
2320      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2321        <organization>MIT Laboratory for Computer Science</organization>
2322        <address><email>frystyk@w3.org</email></address>
2323      </author>
2324      <author fullname="L. Masinter" initials="L." surname="Masinter">
2325        <organization>Xerox Corporation</organization>
2326        <address><email>masinter@parc.xerox.com</email></address>
2327      </author>
2328      <author fullname="P. Leach" initials="P." surname="Leach">
2329        <organization>Microsoft Corporation</organization>
2330        <address><email>paulle@microsoft.com</email></address>
2331      </author>
2332      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2333        <organization>W3C</organization>
2334        <address><email>timbl@w3.org</email></address>
2335      </author>
2336      <date month="June" year="1999"/>
2337    </front>
2338    <seriesInfo name="RFC" value="2616"/>
2339  </reference>
2340
2341  <reference anchor="RFC3864">
2342    <front>
2343      <title>Registration Procedures for Message Header Fields</title>
2344      <author fullname="G. Klyne" initials="G." surname="Klyne">
2345        <organization>Nine by Nine</organization>
2346        <address><email>GK-IETF@ninebynine.org</email></address>
2347      </author>
2348      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2349        <organization>BEA Systems</organization>
2350        <address><email>mnot@pobox.com</email></address>
2351      </author>
2352      <author fullname="J. Mogul" initials="J." surname="Mogul">
2353        <organization>HP Labs</organization>
2354        <address><email>JeffMogul@acm.org</email></address>
2355      </author>
2356      <date month="September" year="2004"/>
2357    </front>
2358    <seriesInfo name="BCP" value="90"/>
2359    <seriesInfo name="RFC" value="3864"/>
2360  </reference>
2361
2362  <reference anchor="RFC5226">
2363    <front>
2364      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2365      <author initials="T." surname="Narten" fullname="T. Narten">
2366        <organization>IBM</organization>
2367        <address><email>narten@us.ibm.com</email></address>
2368      </author>
2369      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2370        <organization>Google</organization>
2371        <address><email>Harald@Alvestrand.no</email></address>
2372      </author>
2373      <date year="2008" month="May"/>
2374    </front>
2375    <seriesInfo name="BCP" value="26"/>
2376    <seriesInfo name="RFC" value="5226"/>
2377  </reference>
2378
2379  <reference anchor="RFC5861">
2380    <front>
2381      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2382      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2383        <organization>Yahoo! Inc.</organization>
2384        <address><email>mnot@yahoo-inc.com</email></address>
2385      </author>
2386      <date month="April" year="2010"/>
2387    </front>
2388    <seriesInfo name="RFC" value="5861"/>
2389  </reference>
2390
2391</references>
2392
2393<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2394<t>
2395  Make the specified age calculation algorithm less conservative.
2396  (<xref target="age.calculations"/>)
2397</t>
2398<t>
2399  Remove requirement to consider Content-Location in successful
2400  responses in order to determine the appropriate response to use.
2401  (<xref target="validation.model"/>)
2402</t>
2403<t>
2404  Clarify denial of service attack avoidance requirement.
2405  (<xref target="invalidation.after.updates.or.deletions"/>)
2406</t>
2407<t>
2408  Change ABNF productions for header fields to only define the field value.
2409  (<xref target="header.field.definitions"/>)
2410</t>
2411<t>
2412  Do not mention RFC 2047 encoding and multiple languages in <xref target="header.warning" format="none">Warning</xref>
2413  header fields anymore, as these aspects never were implemented.
2414  (<xref target="header.warning"/>)
2415</t>
2416<t>
2417  Introduce Cache Directive and Warn Code Registries.
2418  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2419</t>
2420</section>
2421
2422<section title="Imported ABNF" anchor="imported.abnf">
2423   
2424   
2425   
2426   
2427   
2428   
2429   
2430   
2431   
2432   
2433   
2434   
2435   
2436   
2437   
2438   
2439<t>
2440   The following core rules are included by reference, as defined in Appendix B.1 of <xref target="RFC5234"/>: ALPHA (letters), CR (carriage
2441   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2442   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2443   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2444</t>
2445<t>
2446   The rules below are defined in <xref target="Part1"/>:
2447</t>
2448<figure><artwork type="abnf2616"><![CDATA[
2449  OWS           = <OWS, defined in [Part1], Section 3.2.1>
2450  field-name    = <field-name, defined in [Part1], Section 3.2>
2451  quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
2452  token         = <token, defined in [Part1], Section 3.2.4>
2453
2454  port          = <port, defined in [Part1], Section 2.8>
2455  pseudonym     = <pseudonym, defined in [Part1], Section 6.2>
2456  uri-host      = <uri-host, defined in [Part1], Section 2.8>
2457]]></artwork></figure>
2458<t>
2459   The rules below are defined in other parts:
2460</t>
2461<figure><artwork type="abnf2616"><![CDATA[
2462  HTTP-date     = <HTTP-date, defined in [Part2], Section 5.1>
2463]]></artwork></figure>
2464</section>
2465
2466
2467<section title="Collected ABNF" anchor="collected.abnf">
2468<figure>
2469<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2470Age = delta-seconds
2471
2472Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2473 cache-directive ] )
2474
2475Expires = HTTP-date
2476
2477HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
2478
2479OWS = <OWS, defined in [Part1], Section 3.2.1>
2480
2481Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2482 pragma-directive ] )
2483
2484Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2485 ) )
2486
2487Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2488 )
2489
2490cache-directive = token [ "=" ( token / quoted-string ) ]
2491
2492delta-seconds = 1*DIGIT
2493
2494extension-pragma = token [ "=" ( token / quoted-string ) ]
2495
2496field-name = <field-name, defined in [Part1], Section 3.2>
2497
2498port = <port, defined in [Part1], Section 2.8>
2499pragma-directive = "no-cache" / extension-pragma
2500pseudonym = <pseudonym, defined in [Part1], Section 6.2>
2501
2502quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
2503
2504token = <token, defined in [Part1], Section 3.2.4>
2505
2506uri-host = <uri-host, defined in [Part1], Section 2.8>
2507
2508warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2509warn-code = 3DIGIT
2510warn-date = DQUOTE HTTP-date DQUOTE
2511warn-text = quoted-string
2512warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2513 ]
2514]]></artwork>
2515</figure>
2516</section>
2517
2518
2519<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2520<t>
2521  Changes up to the first Working Group Last Call draft are summarized
2522  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2523</t>
2524
2525<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2526<t>
2527  Closed issues:
2528  <list style="symbols">
2529    <t>
2530      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2531      "untangle Cache-Control ABNF"
2532    </t>
2533    <t>
2534      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2535      "Multiple values in Cache-Control header fields"
2536    </t>
2537    <t>
2538      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2539      "Case sensitivity of header fields in CC values"
2540    </t>
2541    <t>
2542      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2543      "Spurious 'MAYs'"
2544    </t>
2545    <t>
2546      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2547      "enhance considerations for new cache control directives"
2548    </t>
2549    <t>
2550      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2551      "ABNF requirements for recipients"
2552    </t>
2553    <t>
2554      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2555      "note introduction of new IANA registries as normative changes"
2556    </t>
2557  </list>
2558</t>
2559</section>
2560
2561</section>
2562  </back>
2563</rfc>
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