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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-23" ipr="pre5378Trust200902" obsoletes="2616">
20
21
22
23<front>
24
25  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): 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="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
43    <organization>Akamai</organization>
44    <address>
45      <email>mnot@mnot.net</email>
46      <uri>http://www.mnot.net/</uri>
47    </address>
48  </author>
49
50  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
51    <organization abbrev="greenbytes">greenbytes GmbH</organization>
52    <address>
53      <postal>
54        <street>Hafenweg 16</street>
55        <city>Muenster</city><region>NW</region><code>48155</code>
56        <country>Germany</country>
57      </postal>
58      <email>julian.reschke@greenbytes.de</email>
59      <uri>http://greenbytes.de/tech/webdav/</uri>
60    </address>
61  </author>
62
63  <date month="July" year="2013" day="15"/>
64  <workgroup>HTTPbis Working Group</workgroup>
65
66<abstract>
67<t>
68   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
69   distributed, collaborative, hypertext information systems. This document
70   defines requirements on HTTP caches and the associated header fields
71   that control cache behavior or indicate cacheable response messages.
72</t>
73</abstract>
74
75<note title="Editorial Note (To be removed by RFC Editor)">
76  <t>
77    Discussion of this draft takes place on the HTTPBIS working group
78    mailing list (ietf-http-wg@w3.org), which is archived at
79    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
80  </t>
81  <t>
82    The current issues list is at
83    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
84    documents (including fancy diffs) can be found at
85    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
86  </t>
87  <t>
88    The changes in this draft are summarized in <xref target="changes.since.22"/>.
89  </t>
90</note>
91
92   </front>
93   <middle>
94
95<section anchor="caching" title="Introduction">
96<t>
97   HTTP is typically used for distributed information systems, where
98   performance can be improved by the use of response caches. This document
99   defines aspects of HTTP/1.1 related to caching and reusing response
100   messages.
101</t>
102
103<iref item="cache"/>
104<t>
105   An HTTP cache is a local store of response messages and the
106   subsystem that controls storage, retrieval, and deletion of messages in it.
107   A cache stores cacheable responses in order to reduce the response time and
108   network bandwidth consumption on future, equivalent requests. Any client or
109   server MAY employ a cache, though a cache cannot be used by a server that
110   is acting as a tunnel.
111</t>
112<iref item="shared cache"/>
113<iref item="private cache"/>
114<t anchor="shared.and.non-shared.caches">
115   A shared cache is a cache that stores responses to be reused
116   by more than one user; shared caches are usually (but not always) deployed
117   as a part of an intermediary. A private cache, in contrast,
118   is dedicated to a single user.
119</t>
120<t>
121   The goal of caching in HTTP/1.1 is to significantly improve performance
122   by reusing a prior response message to satisfy a current request.
123   A stored response is considered "fresh", as defined in
124   <xref target="expiration.model"/>, if the response can be reused without
125   "validation" (checking with the origin server to see if the cached response
126   remains valid for this request).  A fresh response can therefore
127   reduce both latency and network overhead each time it is reused.
128   When a cached response is not fresh, it might still be reusable if it can
129   be freshened by validation (<xref target="validation.model"/>) or if the
130   origin is unavailable (<xref target="serving.stale.responses"/>).
131</t>
132
133<section title="Conformance and Error Handling" anchor="conformance">
134<t>
135   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
136   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
137   document are to be interpreted as described in <xref target="RFC2119"/>.
138</t>
139<t>
140   Conformance criteria and considerations regarding error handling
141   are defined in Section 2.5 of <xref target="Part1"/>.
142</t>
143</section>
144
145<section title="Syntax Notation" anchor="notation">
146<t>
147   This specification uses the Augmented Backus-Naur Form (ABNF) notation
148   of <xref target="RFC5234"/> with the list rule extension defined in
149   Section 1.2 of <xref target="Part1"/>. <xref target="imported.abnf"/> describes rules imported from
150   other documents. <xref target="collected.abnf"/> shows the collected ABNF
151   with the list rule expanded.
152</t>
153
154<section title="Delta Seconds" anchor="delta-seconds">
155<t>
156   The delta-seconds rule specifies a non-negative integer, representing time
157   in seconds.
158</t>
159<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
160  delta-seconds  = 1*DIGIT
161]]></artwork></figure>
162<t>
163   If a cache receives a delta-seconds value larger than the largest
164   positive integer it can represent, or if any of its subsequent calculations
165   overflows, it MUST consider the value to be 2147483648
166   (2^31). Recipients parsing a delta-seconds value MUST use
167   an arithmetic type of at least 31 bits of range, and senders MUST NOT
168   generate delta-seconds with a value greater than 2147483648.
169</t>
170</section>
171
172</section>
173</section>
174
175<section anchor="caching.overview" title="Overview of Cache Operation">
176<iref item="cache entry"/>
177<iref item="cache key"/>
178<t>
179   Proper cache operation preserves the semantics of HTTP transfers
180   (<xref target="Part2"/>) while eliminating the transfer of information already held
181   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
182   we assume that reusing the cached response is desirable and that such
183   reuse is the default behavior when no requirement or local
184   configuration prevents it.  Therefore, HTTP cache requirements are focused
185   on preventing a cache from either storing a non-reusable response or
186   reusing a stored response inappropriately, rather than mandating that
187   caches always store and reuse particular responses.
188</t>
189<t>
190   Each cache entry consists of a cache key and one or more
191   HTTP responses corresponding to prior requests that used the same key. The
192   most common form of cache entry is a successful result of a retrieval
193   request: i.e., a 200 (OK) response to a GET request, which
194   contains a representation of the resource identified by the request target
195   (Section 4.3.1 of <xref target="Part2"/>). However, it is also possible to cache permanent redirects,
196   negative results (e.g., 404 (Not Found)),
197   incomplete results (e.g., 206 (Partial Content)), and
198   responses to methods other than GET if the method's definition allows such
199   caching and defines something suitable for use as a cache key.
200</t>
201<iref item="cache key"/>
202<t>
203   The primary cache key consists of the request method and
204   target URI.  However, since HTTP caches in common use today are typically
205   limited to caching responses to GET, many caches simply decline
206   other methods and use only the URI as the primary cache key.
207</t>
208<t>
209   If a request target is subject to content negotiation, its cache entry
210   might consist of multiple stored responses, each differentiated by a
211   secondary key for the values of the original request's selecting header
212   fields (<xref target="caching.negotiated.responses"/>).
213</t>
214</section>
215
216<section anchor="response.cacheability" title="Storing Responses in Caches">
217<t>
218   A cache MUST NOT store a response to any request, unless:
219   <list style="symbols">
220      <t>The request method is understood by the cache and defined as being
221      cacheable, and</t>
222      <t>the response status code is understood by the cache, and</t>
223      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
224      header fields, and</t>
225      <t>the "private" cache response directive (see <xref target="cache-response-directive.private"/>) does not appear in the
226      response, if the cache is shared, and</t>
227      <t>the Authorization header field (see
228      Section 4.1 of <xref target="Part7"/>) does not appear in the request, if the cache is
229      shared, unless the response explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
230      <t>the response either:
231         <list style="symbols">
232            <t>contains an <xref target="header.expires" format="none">Expires</xref> header field (see
233            <xref target="header.expires"/>), or</t>
234            <t>contains a max-age response cache directive (see <xref target="cache-response-directive.max-age"/>), or</t>
235            <t>contains a s-maxage response cache directive (see <xref target="cache-response-directive.s-maxage"/>) and the cache is
236            shared, or</t>
237            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
238            or</t>
239            <t>has a status code that is defined as cacheable
240            (see <xref target="heuristic.freshness"/>), or</t>
241            <t>contains a public response cache directive (see <xref target="cache-response-directive.public"/>).</t>
242         </list>
243      </t>
244   </list>
245</t>
246<t>
247   Note that any of the requirements listed above can be overridden by a
248   cache-control extension; see <xref target="cache.control.extensions"/>.
249</t>
250<t>
251   In this context, a cache has "understood" a request method or a response
252   status code if it recognizes it and implements all specified
253   caching-related behavior.
254</t>
255<t>
256   Note that, in normal operation, some caches will not store a response that
257   has neither a cache validator nor an explicit expiration time, as such
258   responses are not usually useful to store. However, caches are not
259   prohibited from storing such responses.
260</t>
261
262<section anchor="incomplete.responses" title="Storing Incomplete Responses">
263<t>
264   A response message is considered complete when all of the octets indicated
265   by the message framing (<xref target="Part1"/>) are received prior to the connection
266   being closed. If the request is GET, the response status is 200
267   (OK), and the entire response header block has been received, a
268   cache MAY store an incomplete response message body if the cache entry is
269   recorded as incomplete. Likewise, a 206 (Partial Content)
270   response MAY be stored as if it were an incomplete 200
271   (OK) cache entry. However, a cache MUST NOT store incomplete or
272   partial content responses if it does not support the Range
273   and Content-Range header fields or if it does not understand
274   the range units used in those fields.
275</t>
276<t>
277   A cache MAY complete a stored incomplete response by making a subsequent
278   range request (<xref target="Part5"/>) and combining the successful response with the
279   stored entry, as defined in <xref target="combining.responses"/>. A cache
280   MUST NOT use an incomplete response to answer requests unless the
281   response has been made complete or the request is partial and specifies a
282   range that is wholly within the incomplete response. A cache MUST NOT
283   send a partial response to a client without explicitly marking it as such
284   using the 206 (Partial Content) status code.
285</t>
286</section>
287
288
289<section anchor="caching.authenticated.responses" title="Storing Responses to Authenticated Requests">
290<t>
291   A shared cache MUST NOT use a cached response to a request with an
292   Authorization header field (Section 4.1 of <xref target="Part7"/>) to
293   satisfy any subsequent request unless a cache directive that allows such
294   responses to be stored is present in the response.
295</t>
296<t>
297   In this specification, the following <xref target="header.cache-control" format="none">Cache-Control</xref> response
298   directives (<xref target="cache-response-directive"/>) have such an effect:
299   must-revalidate, public, s-maxage.
300</t>
301<t>
302   Note that cached responses that contain the "must-revalidate" and/or
303   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
304   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
305   used to satisfy a subsequent request without revalidating it on the origin
306   server.
307</t>
308</section>
309
310<section anchor="combining.responses" title="Combining Partial Content">
311<t>
312   A response might transfer only a partial representation if the
313   connection closed prematurely or if the request used one or more Range
314   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
315   received several ranges of the same representation.  A cache MAY combine
316   these ranges into a single stored response, and reuse that response to
317   satisfy later requests, if they all share the same strong validator and
318   the cache complies with the client requirements in Section 4.3 of <xref target="Part5"/>.
319</t>
320<t>
321   When combining the new response with one or more stored responses, a
322   cache MUST:
323   <list style="symbols">
324      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
325         with warn-code 1xx (see <xref target="header.warning"/>);</t>
326      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
327         with warn-code 2xx; and,</t>
328      <t>use other header fields provided in the new response, aside
329         from Content-Range, to replace all instances of the
330         corresponding header fields in the stored response.</t>
331   </list>
332</t>
333</section>
334
335</section>
336
337
338<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
339<t>
340   When presented with a request, a cache MUST NOT reuse a stored response,
341   unless:
342   <list style="symbols">
343      <t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
344      that of the stored response match, and</t>
345      <t>the request method associated with the stored response allows it to
346      be used for the presented request, and</t>
347      <t>selecting header fields nominated by the stored response (if any)
348      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
349      <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
350      successfully validated (<xref target="validation.model"/>), and</t>
351      <t>the stored response does not contain the no-cache cache directive
352      (<xref target="cache-response-directive.no-cache"/>), unless it is
353      successfully validated (<xref target="validation.model"/>), and</t>
354      <t>the stored response is either:
355         <list style="symbols">
356            <t>fresh (see <xref target="expiration.model"/>), or</t>
357            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
358            <t>successfully validated (see <xref target="validation.model"/>).</t>
359         </list>
360      </t>
361  </list>
362</t>
363<t>
364   Note that any of the requirements listed above can be overridden by a
365   cache-control extension; see <xref target="cache.control.extensions"/>.
366</t>
367<t>
368   When a stored response is used to satisfy a request without validation, a
369   cache MUST generate an <xref target="header.age" format="none">Age</xref> header field (<xref target="header.age"/>), replacing any present in the response with a value
370   equal to the stored response's current_age; see <xref target="age.calculations"/>.
371</t>
372<t>
373   A cache MUST write through requests with methods that are unsafe
374   (Section 4.2.1 of <xref target="Part2"/>) to the origin server; i.e., a cache is not allowed to
375   generate a reply to such a request before having forwarded the request and
376   having received a corresponding response.
377</t>
378<t>
379   Also, note that unsafe requests might invalidate already stored responses;
380   see <xref target="invalidation.after.updates.or.deletions"/>.
381</t>
382<t>
383   When more than one suitable response is stored, a cache MUST use the
384   most recent response (as determined by the Date header
385   field). It can also forward the request with "Cache-Control: max-age=0" or
386   "Cache-Control: no-cache" to disambiguate which response to use.
387</t>
388<t>
389   A cache that does not have a clock available MUST NOT use stored
390   responses without revalidating them upon every use.
391</t>
392
393
394<section anchor="expiration.model" title="Freshness">
395<iref item="fresh"/>
396<iref item="stale"/>
397<t>
398   A fresh response is one whose age has not yet exceeded its
399   freshness lifetime. Conversely, a stale
400   response is one where it has.
401</t>
402<iref item="freshness lifetime"/>
403<iref item="explicit expiration time"/>
404<iref item="heuristic expiration time"/>
405<t>
406   A response's freshness lifetime is the length of time
407   between its generation by the origin server and its expiration time. An
408   explicit expiration time is the time at which the origin
409   server intends that a stored response can no longer be used by a cache
410   without further validation, whereas a heuristic expiration
411   time is assigned by a cache when no explicit expiriation time is
412   available.
413</t>
414<iref item="age"/>
415<t>
416   A response's age is the time that has passed since it was
417   generated by, or successfully validated with, the origin server. 
418</t>
419<t>
420   When a response is "fresh" in the cache, it can be used to satisfy
421   subsequent requests without contacting the origin server, thereby improving
422   efficiency.
423</t>
424<t>
425   The primary mechanism for determining freshness is for an origin server to
426   provide an explicit expiration time in the future, using either the
427   <xref target="header.expires" format="none">Expires</xref> header field (<xref target="header.expires"/>) or
428   the max-age response cache directive (<xref target="cache-response-directive.max-age"/>). Generally, origin servers
429   will assign future explicit expiration times to responses in the belief
430   that the representation is not likely to change in a semantically
431   significant way before the expiration time is reached.
432</t>
433<t>
434   If an origin server wishes to force a cache to validate every request, it
435   can assign an explicit expiration time in the past to indicate that the
436   response is already stale. Compliant caches will normally validate a stale
437   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
438</t>
439<t>
440   Since origin servers do not always provide explicit expiration times,
441   caches are also allowed to use a heuristic to determine an expiration time
442   under certain circumstances (see <xref target="heuristic.freshness"/>).
443</t>
444<figure>
445<preamble>
446  The calculation to determine if a response is fresh is:
447</preamble>
448<artwork type="code"><![CDATA[
449   response_is_fresh = (freshness_lifetime > current_age)
450]]></artwork>
451</figure>
452<t>
453   freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; current_age is defined in
454   <xref target="age.calculations"/>.
455</t>
456<t>
457   Clients can send the max-age or min-fresh cache directives in a request to
458   constrain or relax freshness calculations for the corresponding response
459   (<xref target="cache-request-directive"/>).
460</t>
461<t>
462   When calculating freshness, to avoid common problems in date parsing:
463</t>
464<t>
465  <list style="symbols">
466     <t>Although all date formats are specified to be case-sensitive,
467        cache recipients SHOULD match day, week and timezone names
468        case-insensitively.</t>
469             
470     <t>If a cache recipient's internal implementation of time has less
471        resolution than the value of an HTTP-date, the recipient MUST
472        internally represent a parsed <xref target="header.expires" format="none">Expires</xref> date as the
473        nearest time equal to or earlier than the received value.</t>
474
475     <t>Cache recipients MUST NOT allow local time zones to influence the
476        calculation or comparison of an age or expiration time.</t>
477
478     <t>Cache recipients SHOULD consider a date with a zone abbreviation
479        other than "GMT" to be invalid for calculating expiration.</t>
480  </list>
481</t>
482<t>
483   Note that freshness applies only to cache operation; it cannot be used to
484   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
485   caches and history mechanisms.
486</t>
487
488<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
489<t>
490   A cache can calculate the freshness lifetime (denoted as
491   freshness_lifetime) of a response by using the first match of:
492   <list style="symbols">
493      <t>If the cache is shared and the s-maxage response cache directive
494      (<xref target="cache-response-directive.s-maxage"/>) is present, use its value,
495      or</t>
496      <t>If the max-age response cache directive (<xref target="cache-response-directive.max-age"/>) is present, use its value, or</t>
497      <t>If the <xref target="header.expires" format="none">Expires</xref> response header field
498      (<xref target="header.expires"/>) is present, use its value minus the
499      value of the Date response header field, or</t>
500      <t>Otherwise, no explicit expiration time is present in the response. A
501      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
502   </list>
503</t>
504<t>
505   Note that this calculation is not vulnerable to clock skew, since all of
506   the information comes from the origin server.
507</t>
508<t>
509   When there is more than one value present for a given directive (e.g., two
510   <xref target="header.expires" format="none">Expires</xref> header fields, multiple Cache-Control: max-age
511   directives), the directive's value is considered invalid. Caches are
512   encouraged to consider responses that have invalid freshness information to
513   be stale.
514</t>
515</section>
516
517<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
518<t>
519   Since origin servers do not always provide explicit expiration times, a
520   cache MAY assign a heuristic expiration time when an explicit time is not
521   specified, employing algorithms that use other header field values (such as
522   the Last-Modified time) to estimate a plausible expiration
523   time. This specification does not provide specific algorithms, but does
524   impose worst-case constraints on their results.
525</t>
526<t>
527   A cache MUST NOT use heuristics to determine freshness when an explicit
528   expiration time is present in the stored response. Because of the
529   requirements in <xref target="response.cacheability"/>, this means that,
530   effectively, heuristics can only be used on responses without explicit
531   freshness whose status codes are defined as cacheable, and responses
532   without explicit freshness that have been marked as explicitly cacheable
533   (e.g., with a "public" response cache directive).
534</t>
535<t>
536   If the response has a Last-Modified header field
537   (Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
538   expiration value that is no more than some fraction of the interval since
539   that time. A typical setting of this fraction might be 10%.
540</t>
541<t>
542   When a heuristic is used to calculate freshness lifetime, a cache SHOULD
543   attach a <xref target="header.warning" format="none">Warning</xref> header field with a 113 warn-code to the
544   response if its current_age is more than 24 hours and such a warning is not
545   already present.
546</t>
547<t><list>
548   <t>
549      Note: Section 13.9 of <xref target="RFC2616"/> prohibited caches
550      from calculating heuristic freshness for URIs with query components
551      (i.e., those containing '?'). In practice, this has not been widely
552      implemented. Therefore, origin servers are encouraged to send explicit
553      directives (e.g., Cache-Control: no-cache) if they wish to preclude
554      caching.
555   </t>
556</list></t>
557</section>
558
559<section anchor="age.calculations" title="Calculating Age">
560<t>
561   The <xref target="header.age" format="none">Age</xref> header field is used to convey an estimated
562   age of the response message when obtained from a cache. The Age field value
563   is the cache's estimate of the number of seconds since the response was
564   generated or validated by the origin server. In essence, the Age value is
565   the sum of the time that the response has been resident in each of the
566   caches along the path from the origin server, plus the amount of time it
567   has been in transit along network paths.
568</t>
569<t>
570   The following data is used for the age calculation:
571</t>
572<t>
573   <?rfc needLines="4"?>age_value
574   <list>
575      <t>
576         The term "age_value" denotes the value of the <xref target="header.age" format="none">Age</xref>
577         header field (<xref target="header.age"/>), in a form appropriate for
578         arithmetic operation; or 0, if not available.
579      </t>
580   </list>
581</t>
582<t>
583   <?rfc needLines="4"?>date_value
584   <list>
585      <t>
586         The term "date_value" denotes the value of
587         the Date header field, in a form appropriate for arithmetic
588         operations. See Section 7.1.1.2 of <xref target="Part2"/> for the definition of the Date header
589         field, and for requirements regarding responses without it.
590      </t>
591   </list>
592</t>
593<t>
594   <?rfc needLines="4"?>now
595   <list>
596      <t>
597         The term "now" means "the current value of the clock at the host
598         performing the calculation". A host ought to use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
599         clocks to Coordinated Universal Time.
600      </t>
601   </list>
602</t>
603<t>
604   <?rfc needLines="4"?>request_time
605   <list>
606      <t>
607         The current value of the clock at the host at the time the request
608         resulting in the stored response was made.
609      </t>
610   </list>
611</t>
612<t>
613   <?rfc needLines="4"?>response_time
614   <list>
615      <t>
616         The current value of the clock at the host at the time the response
617         was received.
618      </t>
619   </list>
620</t>
621<t>
622   A response's age can be calculated in two entirely independent ways:
623   <list style="numbers">
624      <t>the "apparent_age": response_time minus date_value, if the local
625      clock is reasonably well synchronized to the origin server's clock. If
626      the result is negative, the result is replaced by zero.</t>
627      <t>the "corrected_age_value", if all of the caches along the response
628      path implement HTTP/1.1. A cache MUST interpret this value relative
629      to the time the request was initiated, not the time that the response
630      was received.</t>
631   </list>
632</t>
633<figure>
634<artwork type="code"><![CDATA[
635  apparent_age = max(0, response_time - date_value);
636
637  response_delay = response_time - request_time;
638  corrected_age_value = age_value + response_delay; 
639]]></artwork>
640</figure>
641<figure>
642<preamble>These are combined as</preamble>
643<artwork type="code"><![CDATA[
644  corrected_initial_age = max(apparent_age, corrected_age_value);
645]]></artwork></figure>
646<t>
647   unless the cache is confident in the value of the <xref target="header.age" format="none">Age</xref> header
648   field (e.g., because there are no HTTP/1.0 hops in the Via
649   header field), in which case the corrected_age_value MAY be used as the
650   corrected_initial_age.
651</t>
652<t>
653   The current_age of a stored response can then be calculated by adding the
654   amount of time (in seconds) since the stored response was last validated by
655   the origin server to the corrected_initial_age.
656</t>
657<figure><artwork type="code"><![CDATA[
658  resident_time = now - response_time;
659  current_age = corrected_initial_age + resident_time;
660]]></artwork></figure>
661</section>
662
663<section anchor="serving.stale.responses" title="Serving Stale Responses">
664<t>
665   A "stale" response is one that either has explicit expiry information or is
666   allowed to have heuristic expiry calculated, but is not fresh according to
667   the calculations in <xref target="expiration.model"/>.
668</t>
669<t>
670   A cache MUST NOT generate a stale response if it is prohibited by an
671   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
672   directive, a "must-revalidate" cache-response-directive, or an applicable
673   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
674</t>
675<t>
676   A cache MUST NOT send stale responses unless it is disconnected
677   (i.e., it cannot contact the origin server or otherwise find a forward
678   path) or doing so is explicitly allowed (e.g., by the max-stale request
679   directive; see <xref target="cache-request-directive"/>).
680</t>
681<t>
682   A cache SHOULD append a <xref target="header.warning" format="none">Warning</xref> header field with the 110
683   warn-code (see <xref target="header.warning"/>) to stale responses.
684   Likewise, a cache SHOULD add the 112 warn-code to stale responses if the
685   cache is disconnected.
686</t>
687<iref item="first-hand"/>
688<t>
689   Note that if a cache receives a first-hand response (one
690   where the freshness model is not in use; i.e., its age is 0, whether it is
691   an entire response, or a 304 (Not Modified) response) that
692   it would normally forward to the requesting client, and the received
693   response is no longer fresh, the cache MAY forward it to the requesting
694   client without adding a new <xref target="header.warning" format="none">Warning</xref> (but without removing
695   any existing Warning header fields). A cache ought not attempt to validate
696   a response simply because that response became stale in transit.
697</t>
698</section>
699</section>
700
701<section anchor="validation.model" title="Validation">
702<t>
703   When a cache has one or more stored responses for a requested URI, but
704   cannot serve any of them (e.g., because they are not fresh, or one cannot
705   be selected; see <xref target="caching.negotiated.responses"/>), it can use
706   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
707   give the origin server an opportunity to both select a valid stored
708   response to be used, and to update it. This process is known as
709   "validating" or "revalidating" the stored response.
710</t>
711<iref item="validator"/>
712<t>
713   When sending such a conditional request, a cache adds a
714   validator (or more than one), that is used to find
715   out whether a stored response is an equivalent copy of a current
716   representation of the resource.
717</t>
718<t>
719   One such validator is the If-Modified-Since header field,
720   whose value is that of the Last-Modified header field from
721   the selected (see <xref target="caching.negotiated.responses"/>) stored
722   response, if available.
723</t>
724<t>
725   Another is the If-None-Match header field,
726   whose value is that of the ETag header field(s) from
727   relevant responses stored for the primary cache key, if present. However,
728   if any of the stored responses contains only partial content, the cache
729   ought not include its entity-tag in the If-None-Match header field unless
730   the request is for a range that would be fully satisfied by that stored
731   response.
732</t>
733
734<t>Cache handling of a response to a conditional request is dependent upon its
735status code:</t>
736
737<t>
738   <list style="symbols">
739      <t>
740         A 304 (Not Modified) response status code indicates
741         that the stored response can be updated and reused; see <xref target="freshening.responses"/>.
742      </t>
743      <t>
744         A full response (i.e., one with a payload body) indicates that none
745         of the stored responses nominated in the conditional request is
746         suitable. Instead, the cache can use the full response to
747         satisfy the request and MAY replace the stored response(s).
748      </t>
749      <t>
750         However, if a cache receives a 5xx (Server Error)
751         response while attempting to validate a response, it can either
752         forward this response to the requesting client, or act as if the
753         server failed to respond. In the latter case, it can send a
754         previously stored response (see <xref target="serving.stale.responses"/>).
755      </t>
756   </list>
757</t>
758
759<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
760<t>
761   When a cache receives a 304 (Not Modified) response and
762   already has one or more stored 200 (OK) responses for the
763   same cache key, the cache needs to identify which of the stored responses
764   are updated by this new response and then update the stored response(s)
765   with the new information provided in the 304 response.
766</t>
767<iref item="strong validator"/>
768<t>
769   The stored response to update is identified by using the first match (if
770   any) of:
771   <list style="symbols">
772    <t>
773     If the new response contains a strong validator (see
774     Section 2.1 of <xref target="Part4"/>), then that strong validator identifies the selected
775     representation for update. All of the stored responses with the same
776     strong validator are selected. If none of the stored responses contain the
777     same strong validator, then the cache MUST NOT use the new response to
778     update any stored responses.
779    </t>
780    <t>
781     If the new response contains a weak validator and that validator
782     corresponds to one of the cache's stored responses, then the most
783     recent of those matching stored responses is selected for update.
784    </t>
785    <t>
786     If the new response does not include any form of validator (such as in
787     the case where a client generates an If-Modified-Since request from a
788     source other than the Last-Modified response header field), and there is
789     only one stored response, and that stored response also lacks a
790     validator, then that stored response is selected for update.
791    </t>
792   </list>
793</t>
794<t>
795   If a stored response is selected for update, the cache MUST:
796   <list style="symbols">
797      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
798         with warn-code 1xx (see <xref target="header.warning"/>);</t>
799      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
800         with warn-code 2xx; and,</t>
801      <t>use other header fields provided in the 304 (Not Modified)
802         response to replace all instances of the corresponding header
803         fields in the stored response.</t>
804   </list>
805</t>
806</section>
807
808</section>
809
810<section anchor="caching.negotiated.responses" title="Calculating Secondary Keys with Vary">
811<t>
812   When a cache receives a request that can be satisfied by a stored response
813   that has a Vary header field (Section 7.1.4 of <xref target="Part2"/>),
814   it MUST NOT use that response unless all of the selecting header fields
815   nominated by the Vary header field match in both the original request
816   (i.e., that associated with the stored response), and the presented
817   request.
818</t>
819<t>
820   The selecting header fields from two requests are defined to match if and
821   only if those in the first request can be transformed to those in the
822   second request by applying any of the following:
823   <list style="symbols">
824      <t>
825         adding or removing whitespace, where allowed in the header field's
826         syntax
827      </t>
828      <t>
829         combining multiple header fields with the same field name
830         (see Section 3.2 of <xref target="Part1"/>)
831      </t>
832      <t>
833         normalizing both header field values in a way that is known to have
834         identical semantics, according to the header field's specification
835         (e.g., re-ordering field values when order is not significant;
836         case-normalization, where values are defined to be case-insensitive)
837      </t>
838  </list>
839</t>
840<t>
841   If (after any normalization that might take place) a header field is absent
842   from a request, it can only match another request if it is also absent
843   there.
844</t>
845<t>
846   A Vary header field-value of "*" always fails to match.
847</t>
848<t>
849   The stored response with matching selecting header fields is known as the
850   selected response.
851</t>
852<t>
853   If multiple selected responses are available (potentially including
854   responses without a Vary header field), the cache will need to choose one to use.
855   When a selecting header field has a known mechanism for doing so (e.g., qvalues on
856   Accept and similar request header fields), that mechanism MAY be
857   used to select preferred responses; of the remainder, the most recent
858   response (as determined by the Date header field) is used, as
859   per <xref target="constructing.responses.from.caches"/>.
860</t>
861<t>
862   If no selected response is available, the cache cannot satisfy the
863   presented request. Typically, it is forwarded to the origin server
864   in a (possibly conditional; see <xref target="validation.model"/>) request.
865</t>
866</section>
867
868</section>
869
870
871<section anchor="head.effects" title="Updating Caches with HEAD Responses">
872<t>
873   A response to the HEAD method is identical to what an equivalent request
874   made with a GET would have been, except it lacks a body. This property
875   of HEAD responses is used to both invalidate and update cached GET
876   responses.
877</t>
878<t>
879   If one or more stored GET responses can be selected (as per <xref target="caching.negotiated.responses"/>) for a HEAD request, and the
880   Content-Length, ETag or
881   Last-Modified value of a HEAD response differs from that in a
882   selected GET response, the cache MUST consider that selected response to
883   be stale.
884</t>
885<t>
886   If the Content-Length, ETag and
887   Last-Modified values of a HEAD response (when present) are
888   the same as that in a selected GET response (as per
889   <xref target="caching.negotiated.responses"/>), the cache SHOULD update
890   the remaining header fields in the stored response using the following
891   rules:
892   <list style="symbols">
893      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
894         with warn-code 1xx (see <xref target="header.warning"/>);</t>
895      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
896         with warn-code 2xx; and,</t>
897      <t>use other header fields provided in the response to replace
898         all instances of the corresponding header fields in the stored
899         response.</t>
900   </list>
901</t>
902
903</section>
904
905
906<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
907<t>
908   Because unsafe request methods (Section 4.2.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
909   have the potential for changing state on the origin server, intervening
910   caches can use them to keep their contents up-to-date.
911</t>
912<t>
913   A cache MUST invalidate the effective Request URI
914   (Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the
915   Location and Content-Location response header
916   fields (if present) when a non-error response to a request with an unsafe
917   method is received.
918</t>
919<t>
920   However, a cache MUST NOT invalidate a URI from a Location
921   or Content-Location response header field if the host part of
922   that URI differs from the host part in the effective request URI
923   (Section 5.5 of <xref target="Part1"/>). This helps prevent denial of service attacks.
924</t>
925<t>
926   A cache MUST invalidate the effective request URI
927   (Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
928   to a request with a method whose safety is unknown.
929</t>
930<t>
931   Here, a "non-error response" is one with a 2xx (Successful)
932   or 3xx (Redirection) status code. "Invalidate" means that
933   the cache will either remove all stored responses related to the effective
934   request URI, or will mark these as "invalid" and in need of a mandatory
935   validation before they can be sent in response to a subsequent request.
936</t>
937<t>
938   Note that this does not guarantee that all appropriate responses are
939   invalidated. For example, a state-changing request might invalidate
940   responses in the caches it travels through, but relevant responses still
941   might be stored in other caches that it has not.</t>
942</section>
943
944
945
946
947<section anchor="header.field.definitions" title="Header Field Definitions">
948<t>
949   This section defines the syntax and semantics of HTTP/1.1 header fields
950   related to caching.
951</t>
952
953<section anchor="header.age" title="Age">
954   <iref item="Age header field" primary="true"/>
955   
956   
957<t>
958   The "Age" header field conveys the sender's estimate of the amount
959   of time since the response was generated or successfully validated at the
960   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
961</t>
962<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
963  Age = delta-seconds
964]]></artwork></figure>
965<t>
966  Age field-values are non-negative integers, representing time in seconds
967  (see <xref target="delta-seconds"/>).
968</t>
969<t>
970   The presence of an Age header field in a response implies that a response
971   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
972   might not implement the Age header field.
973</t>
974</section>
975
976<section anchor="header.cache-control" title="Cache-Control">
977   <iref item="Cache-Control header field" primary="true"/>
978   
979   
980<t>
981   The "Cache-Control" header field is used to specify directives for
982   caches along the request/response chain. Such cache directives are
983   unidirectional in that the presence of a directive in a request does not
984   imply that the same directive is to be given in the response.
985</t>
986<t>
987   A cache MUST obey the requirements of the Cache-Control
988   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
989   directives defined elsewhere are handled.
990</t>
991<t><list>
992   <t>
993       Note: Some HTTP/1.0 caches might not implement Cache-Control.
994   </t>
995</list></t>
996<t>
997   A proxy, whether or not it implements a cache, MUST pass cache directives
998   through in forwarded messages, regardless of their
999   significance to that application, since the directives might be applicable
1000   to all recipients along the request/response chain. It is not possible to
1001   target a directive to a specific cache.
1002</t>
1003<t>
1004   Cache directives are identified by a token, to be compared case-insensitively,
1005   and have an optional argument, that can use both token and quoted-string
1006   syntax. For the directives defined below that define arguments, recipients
1007   ought to accept both forms, even if one is documented to be preferred. For
1008   any directive not defined by this specification, recipients MUST accept
1009   both forms.
1010</t>
1011<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><artwork type="abnf2616"><![CDATA[
1012  Cache-Control   = 1#cache-directive
1013
1014  cache-directive = token [ "=" ( token / quoted-string ) ]
1015]]></artwork></figure>
1016<t>
1017   For the cache directives defined below, no argument is defined (nor allowed)
1018   unless stated otherwise.
1019</t>
1020
1021<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1022
1023<section title="max-age" anchor="cache-request-directive.max-age">
1024   <iref item="max-age (cache directive)" primary="true"/>
1025<t>
1026   Argument syntax:
1027   <list>
1028      <t>
1029        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1030      </t>
1031   </list>
1032</t>
1033<t>
1034   The "max-age" request directive indicates that the client is unwilling to
1035   accept a response whose age is greater than the specified number of
1036   seconds. Unless the max-stale request directive is also present, the
1037   client is not willing to accept a stale response.
1038</t>
1039<t>
1040   Note: This directive uses the token form of the argument syntax;
1041   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1042   quoted-string form.
1043</t>
1044</section>
1045
1046<section title="max-stale" anchor="cache-request-directive.max-stale">
1047   <iref item="max-stale (cache directive)" primary="true"/>
1048<t>
1049   Argument syntax:
1050   <list>
1051      <t>
1052        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1053      </t>
1054   </list>
1055</t>
1056<t>
1057   The "max-stale" request directive indicates that the client is willing
1058   to accept a response that has exceeded its freshness lifetime. If max-stale
1059   is assigned a value, then the client is willing to accept a response
1060   that has exceeded its freshness lifetime by no more than the specified
1061   number of seconds. If no value is assigned to max-stale, then the client
1062   is willing to accept a stale response of any age.
1063</t>
1064<t>
1065   Note: This directive uses the token form of the argument syntax;
1066   e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use the
1067   quoted-string form.
1068</t>
1069</section>
1070
1071<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1072   <iref item="min-fresh (cache directive)" primary="true"/>
1073<t>
1074   Argument syntax:
1075   <list>
1076      <t>
1077        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1078      </t>
1079   </list>
1080</t>
1081<t>
1082   The "min-fresh" request directive indicates that the client is willing
1083   to accept a response whose freshness lifetime is no less than its
1084   current age plus the specified time in seconds. That is, the client
1085   wants a response that will still be fresh for at least the specified
1086   number of seconds.
1087</t>
1088<t>
1089   Note: This directive uses the token form of the argument syntax;
1090   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the
1091   quoted-string form.
1092</t>
1093</section>
1094
1095<section title="no-cache" anchor="cache-request-directive.no-cache">
1096   <iref item="no-cache (cache directive)" primary="true"/>
1097<t>
1098   The "no-cache" request directive indicates that a cache MUST NOT
1099   use a stored response to satisfy the request without successful
1100   validation on the origin server.
1101</t>
1102</section>
1103 
1104<section title="no-store" anchor="cache-request-directive.no-store">
1105   <iref item="no-store (cache directive)" primary="true"/>
1106<t>
1107   The "no-store" request directive indicates that a cache MUST NOT
1108   store any part of either this request or any response to it. This
1109   directive applies to both private and shared caches. "MUST NOT
1110   store" in this context means that the cache MUST NOT intentionally
1111   store the information in non-volatile storage, and MUST make a
1112   best-effort attempt to remove the information from volatile storage as
1113   promptly as possible after forwarding it.
1114</t>
1115<t>
1116   This directive is NOT a reliable or sufficient mechanism for ensuring
1117   privacy. In particular, malicious or compromised caches might not
1118   recognize or obey this directive, and communications networks might be
1119   vulnerable to eavesdropping.
1120</t>
1121<t>
1122   Note that if a request containing this directive is satisfied from a
1123   cache, the no-store request directive does not apply to the already
1124   stored response.
1125</t>
1126</section>
1127
1128<section title="no-transform" anchor="cache-request-directive.no-transform">
1129   <iref item="no-transform (cache directive)" primary="true"/>
1130<t>
1131   The "no-transform" request directive indicates that an intermediary
1132   (whether or not it implements a cache) MUST NOT transform the payload,
1133   as defined in Section 5.7.2 of <xref target="Part1"/>.
1134</t>
1135</section>
1136
1137<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1138   <iref item="only-if-cached (cache directive)" primary="true"/>
1139<t>
1140   The "only-if-cached" request directive indicates that the client only wishes
1141   to obtain a stored response. If it receives this directive, a cache SHOULD
1142   either respond using a stored response that is consistent with the other
1143   constraints of the request, or respond with a 504 (Gateway
1144   Timeout) status code. If a group of caches is being operated as a
1145   unified system with good internal connectivity, a member cache MAY
1146   forward such a request within that group of caches.
1147</t>
1148</section>
1149</section>
1150
1151<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1152   
1153
1154<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1155   <iref item="must-revalidate (cache directive)" primary="true"/>
1156<t>
1157   The "must-revalidate" response directive indicates that once it has
1158   become stale, a cache MUST NOT use the response to satisfy subsequent
1159   requests without successful validation on the origin server.
1160</t>
1161<t>
1162   The must-revalidate directive is necessary to support reliable
1163   operation for certain protocol features. In all circumstances a
1164   cache MUST obey the must-revalidate directive; in particular,
1165   if a cache cannot reach the origin server for any reason, it MUST
1166   generate a 504 (Gateway Timeout) response.
1167</t>
1168<t>
1169   The must-revalidate directive ought to be used by servers if and only
1170   if failure to validate a request on the representation could result in
1171   incorrect operation, such as a silently unexecuted financial
1172   transaction.
1173</t>
1174</section>
1175
1176<section title="no-cache" anchor="cache-response-directive.no-cache">
1177   <iref item="no-cache (cache directive)" primary="true"/>
1178<t>
1179   Argument syntax:
1180   <list>
1181      <t>
1182        #<xref target="imported.abnf" format="none">field-name</xref>
1183      </t>
1184   </list>
1185</t>
1186<t>
1187   The "no-cache" response directive indicates that the response MUST NOT
1188   be used to satisfy a subsequent request without successful validation on
1189   the origin server. This allows an origin server to prevent a cache from
1190   using it to satisfy a request without contacting it, even by caches that
1191   have been configured to send stale responses.
1192</t>
1193<t>
1194   If the no-cache response directive specifies one or more field-names,
1195   then a cache MAY use the response to satisfy a subsequent request,
1196   subject to any other restrictions on caching. However, any header fields
1197   in the response that have the field-name(s) listed MUST NOT be sent
1198   in the response to a subsequent request without successful revalidation
1199   with the origin server. This allows an origin server to prevent the
1200   re-use of certain header fields in a response, while still allowing
1201   caching of the rest of the response.
1202</t>
1203<t>
1204   The field-names given are not limited to the set of header
1205   fields defined by this specification. Field names are case-insensitive.
1206</t>
1207<t>
1208   Note: Although it has been back-ported to many implementations, some
1209   HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
1210   response directives with field-names are often handled by caches
1211   as if an unqualified no-cache directive was received; i.e., the special
1212   handling for the qualified form is not widely implemented.
1213</t>
1214<t>
1215   Note: This directive uses the quoted-string form of the argument syntax.
1216   Senders SHOULD NOT use the token form (even if quoting appears not to be
1217   needed for single-entry lists).
1218</t>
1219</section>
1220
1221<section title="no-store" anchor="cache-response-directive.no-store">
1222   <iref item="no-store (cache directive)" primary="true"/>
1223<t>
1224   The "no-store" response directive indicates that a cache MUST NOT
1225   store any part of either the immediate request or response. This
1226   directive applies to both private and shared caches. "MUST NOT
1227   store" in this context means that the cache MUST NOT intentionally
1228   store the information in non-volatile storage, and MUST make a
1229   best-effort attempt to remove the information from volatile storage as
1230   promptly as possible after forwarding it.
1231</t>
1232<t>
1233   This directive is NOT a reliable or sufficient mechanism for ensuring
1234   privacy. In particular, malicious or compromised caches might not
1235   recognize or obey this directive, and communications networks might be
1236   vulnerable to eavesdropping.
1237</t>
1238</section>
1239
1240<section title="no-transform" anchor="cache-response-directive.no-transform">
1241   <iref item="no-transform (cache directive)" primary="true"/>
1242<t>
1243   The "no-transform" response directive indicates that an intermediary
1244   (regardless of whether it implements a cache) MUST NOT transform the
1245   payload, as defined in Section 5.7.2 of <xref target="Part1"/>.
1246</t>
1247</section>
1248
1249<section title="public" anchor="cache-response-directive.public">
1250   <iref item="public (cache directive)" primary="true"/>
1251<t>
1252   The "public" response directive indicates that any cache MAY store the
1253   response, even if the response would normally be non-cacheable or cacheable
1254   only within a non-shared cache. (See <xref target="caching.authenticated.responses"/> for additional details related
1255   to the use of public in response to a request containing
1256   Authorization, and <xref target="response.cacheability"/>
1257   for details of how public affects responses that would normally not be
1258   stored, due to their status codes not being defined as cacheable.)
1259</t>
1260</section>
1261
1262<section title="private" anchor="cache-response-directive.private">
1263   <iref item="private (cache directive)" primary="true"/>
1264<t>
1265   Argument syntax:
1266   <list>
1267      <t>
1268        #<xref target="imported.abnf" format="none">field-name</xref>
1269      </t>
1270   </list>
1271</t>
1272<t>
1273   The "private" response directive indicates that the response message is
1274   intended for a single user and MUST NOT be stored by a shared cache. A
1275   private cache MAY store the response and reuse it for later requests,
1276   even if the response would normally be non-cacheable.
1277</t>
1278<t>
1279   If the private response directive specifies one or more field-names,
1280   this requirement is limited to the field-values associated with the
1281   listed response header fields. That is, a shared cache MUST NOT store
1282   the specified field-names(s), whereas it MAY store the remainder of the
1283   response message.
1284</t>
1285<t>
1286   The field-names given are not limited to the set of header
1287   fields defined by this specification. Field names are case-insensitive.
1288</t>
1289<t>
1290   Note: This usage of the word "private" only controls
1291   where the response can be stored; it cannot ensure the privacy of the
1292   message content. Also, private response directives with field-names are
1293   often handled by caches as if an unqualified private directive
1294   was received; i.e., the special handling for the qualified form is not
1295   widely implemented.
1296</t>
1297<t>
1298   Note: This directive uses the quoted-string form of the argument syntax.
1299   Senders SHOULD NOT use the token form (even if quoting appears not to be
1300   needed for single-entry lists).
1301</t>
1302</section>
1303
1304<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1305   <iref item="proxy-revalidate (cache directive)" primary="true"/>
1306<t>
1307   The "proxy-revalidate" response directive has the same meaning as the
1308   must-revalidate response directive, except that it does not apply to
1309   private caches.
1310</t>
1311</section>
1312
1313<section title="max-age" anchor="cache-response-directive.max-age">
1314   <iref item="max-age (cache directive)" primary="true"/>
1315<t>
1316   Argument syntax:
1317   <list>
1318      <t>
1319        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1320      </t>
1321   </list>
1322</t>
1323<t>
1324   The "max-age" response directive indicates that the response is to be
1325   considered stale after its age is greater than the specified number of
1326   seconds.
1327</t>
1328<t>
1329   Note: This directive uses the token form of the argument syntax;
1330   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1331   quoted-string form.
1332</t>
1333</section>     
1334
1335<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1336   <iref item="s-maxage (cache directive)" primary="true"/>
1337<t>
1338   Argument syntax:
1339   <list>
1340      <t>
1341        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1342      </t>
1343   </list>
1344</t>
1345<t>
1346   The "s-maxage" response directive indicates that, in shared caches, the
1347   maximum age specified by this directive overrides the maximum age
1348   specified by either the max-age directive or the <xref target="header.expires" format="none">Expires</xref>
1349   header field. The s-maxage directive also implies the semantics of the
1350   proxy-revalidate response directive.
1351</t>
1352<t>
1353   Note: This directive uses the token form of the argument syntax;
1354   e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
1355   quoted-string form.
1356</t>
1357</section>
1358
1359</section>
1360
1361<section anchor="cache.control.extensions" title="Cache Control Extensions">
1362<t>
1363   The Cache-Control header field can be extended through the use of one or
1364   more cache-extension tokens, each with an optional value.
1365</t>
1366<t>
1367   Informational extensions (those that do not require a change in cache
1368   behavior) can be added without changing the semantics of other directives.
1369   Behavioral extensions are designed to work by acting as modifiers to the
1370   existing base of cache directives.
1371</t>
1372<t>  
1373   Both the new directive and the standard directive are supplied, such that
1374   applications that do not understand the new directive will default to the
1375   behavior specified by the standard directive, and those that understand the
1376   new directive will recognize it as modifying the requirements associated
1377   with the standard directive. In this way, extensions to the cache-control
1378   directives can be made without requiring changes to the base protocol.
1379</t>
1380<t>
1381   This extension mechanism depends on an HTTP cache obeying all of the
1382   cache-control directives defined for its native HTTP-version, obeying
1383   certain extensions, and ignoring all directives that it does not
1384   understand.
1385</t>
1386<t>
1387   For example, consider a hypothetical new response directive called
1388   "community" that acts as a modifier to the private directive. We define
1389   this new directive to mean that, in addition to any private cache, any
1390   cache that is shared only by members of the community named within its
1391   value is allowed to cache the response. An origin server wishing to allow
1392   the UCI community to use an otherwise private response in their shared
1393   cache(s) could do so by including
1394</t>
1395<figure><artwork type="example"><![CDATA[
1396  Cache-Control: private, community="UCI"
1397]]></artwork></figure>
1398<t>
1399   A cache seeing this header field will act correctly even if the cache does
1400   not understand the community cache-extension, since it will also see and
1401   understand the private directive and thus default to the safe behavior.
1402</t>
1403<t>
1404   A cache MUST ignore unrecognized cache directives; it is assumed that any
1405   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1406   combined with standard directives (or the response's default cacheability)
1407   such that the cache behavior will remain minimally correct even if the
1408   cache does not understand the extension(s).
1409</t>
1410</section>
1411
1412</section>
1413
1414<section anchor="header.expires" title="Expires">
1415   <iref item="Expires header field" primary="true"/>
1416   
1417<t>
1418   The "Expires" header field gives the date/time after which the
1419   response is considered stale. See <xref target="expiration.model"/> for
1420   further discussion of the freshness model.
1421</t>
1422<t>
1423   The presence of an Expires field does not imply that the original resource
1424   will change or cease to exist at, before, or after that time.
1425</t>
1426<t>
1427   The Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of <xref target="Part2"/>.
1428</t>
1429<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1430  Expires = HTTP-date
1431]]></artwork></figure>
1432<figure>
1433  <preamble>For example</preamble>
1434<artwork type="example"><![CDATA[
1435  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1436]]></artwork></figure>
1437<t>
1438   A cache recipient MUST interpret invalid date formats, especially the
1439   value "0", as representing a time in the past (i.e., "already expired").
1440</t>
1441<t>
1442   If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
1443   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1444   a recipient MUST ignore the Expires field.
1445   Likewise, if a response includes the s-maxage directive
1446   (<xref target="cache-response-directive.s-maxage"/>), a shared cache
1447   recipient MUST ignore the Expires field. In both these cases, the value
1448   in Expires is only intended for recipients that have not yet implemented
1449   the Cache-Control field.
1450</t>
1451<t>
1452   An origin server without a clock MUST NOT generate an Expires field
1453   unless its value represents a fixed time in the past (always expired)
1454   or its value has been associated with the resource by a system or user
1455   with a reliable clock.
1456</t>
1457<t>
1458   Historically, HTTP required the Expires field-value to be no more than a
1459   year in the future. While longer freshness lifetimes are no longer
1460   prohibited, extremely large values have been demonstrated to cause
1461   problems (e.g., clock overflows due to use of 32-bit integers for
1462   time values), and many caches will evict a response far sooner than
1463   that.
1464</t>
1465</section>
1466
1467<section anchor="header.pragma" title="Pragma">
1468   <iref item="Pragma header field" primary="true"/>
1469   
1470   
1471   
1472<t>
1473   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1474   caches, so that clients can specify a "no-cache" request that they will
1475   understand (as <xref target="header.cache-control" format="none">Cache-Control</xref> was not defined until HTTP/1.1).
1476   When the Cache-Control header field is also present and understood in a
1477   request, Pragma is ignored.
1478</t>
1479<t>
1480   In HTTP/1.0, Pragma was defined as an extensible field for
1481   implementation-specified directives for recipients. This specification
1482   deprecates such extensions to improve interoperability.
1483</t>
1484<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[
1485  Pragma           = 1#pragma-directive
1486  pragma-directive = "no-cache" / extension-pragma
1487  extension-pragma = token [ "=" ( token / quoted-string ) ]
1488]]></artwork></figure>
1489<t>
1490   When the <xref target="header.cache-control" format="none">Cache-Control</xref> header field is not present in a
1491   request, caches MUST consider the no-cache request pragma-directive as
1492   having the same effect as if "Cache-Control: no-cache" were present (see
1493   <xref target="cache-request-directive"/>).
1494</t>
1495<t>
1496   When sending a no-cache request, a client ought to include both the pragma
1497   and cache-control directives, unless Cache-Control: no-cache is
1498   purposefully omitted to target other <xref target="header.cache-control" format="none">Cache-Control</xref> response
1499   directives at HTTP/1.1 caches. For example:
1500</t>
1501<figure>
1502<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1503  GET / HTTP/1.1
1504  Host: www.example.com
1505  Cache-Control: max-age=30
1506  Pragma: no-cache
1507 
1508  ]]></artwork>
1509</figure>
1510<t>
1511   will constrain HTTP/1.1 caches to serve a response no older than 30
1512   seconds, while precluding implementations that do not understand
1513   <xref target="header.cache-control" format="none">Cache-Control</xref> from serving a cached response.
1514</t>
1515<t><list>
1516   <t>
1517      Note: Because the meaning of "Pragma: no-cache" in responses is not
1518      specified, it does not provide a reliable replacement for
1519      "Cache-Control: no-cache" in them.
1520   </t>
1521</list></t>
1522</section>
1523
1524<section anchor="header.warning" title="Warning">
1525   <iref item="Warning header field" primary="true"/>
1526   
1527   
1528   
1529   
1530   
1531   
1532<t>
1533   The "Warning" header field is used to carry additional information
1534   about the status or transformation of a message that might not be reflected
1535   in the message. This information is typically used to warn about possible
1536   incorrectness introduced by caching operations or transformations applied
1537   to the payload of the message.
1538</t>
1539<t>
1540   Warnings can be used for other purposes, both cache-related and otherwise.
1541   The use of a warning, rather than an error status code, distinguishes these
1542   responses from true failures.
1543</t>
1544<t>
1545   Warning header fields can in general be applied to any message, however some
1546   warn-codes are specific to caches and can only be applied to response
1547   messages.
1548</t>
1549<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[
1550  Warning       = 1#warning-value
1551 
1552  warning-value = warn-code SP warn-agent SP warn-text
1553                                        [SP warn-date]
1554 
1555  warn-code  = 3DIGIT
1556  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1557                  ; the name or pseudonym of the server adding
1558                  ; the Warning header field, for use in debugging
1559  warn-text  = quoted-string
1560  warn-date  = DQUOTE HTTP-date DQUOTE
1561]]></artwork></figure>
1562<t>
1563   Multiple warnings can be attached to a response (either by the origin
1564   server or by a cache), including multiple warnings with the same code
1565   number, only differing in warn-text.
1566</t>
1567<t>
1568   When this occurs, the user agent SHOULD inform the user of as many of
1569   them as possible, in the order that they appear in the response.
1570</t>
1571<t>
1572   Systems that generate multiple Warning header fields are encouraged to
1573   order them with this user agent behavior in mind. New Warning header fields
1574   are added after any existing Warning header fields.
1575</t>
1576<t>
1577   Warnings are assigned three digit warn-codes. The first digit indicates
1578   whether the Warning is required to be deleted from a stored response after
1579   validation:
1580   <list style="symbols">
1581      <t>1xx Warnings describe the freshness or validation status of the
1582      response, and so MUST be deleted by a cache after validation. They can
1583      only be generated by a cache when validating a cached entry, and
1584      MUST NOT be generated in any other situation.</t>
1585      <t>2xx Warnings describe some aspect of the representation that is not
1586      rectified by a validation (for example, a lossy compression of the
1587      representation) and MUST NOT be deleted by a cache after validation,
1588      unless a full response is sent, in which case they MUST be.</t>
1589   </list>
1590</t>
1591<t>
1592   If an implementation sends a message with one or more Warning header fields
1593   to a receiver whose version is HTTP/1.0 or lower, then the sender MUST
1594   include in each warning-value a warn-date that matches the
1595   Date header field in the message.
1596</t>
1597<t>
1598   If a system receives a message with a warning-value that includes a
1599   warn-date, and that warn-date is different from the Date
1600   value in the response, then that warning-value MUST be deleted from the
1601   message before storing, forwarding, or using it. (preventing the
1602   consequences of naive caching of Warning header fields.) If all of the
1603   warning-values are deleted for this reason, the Warning header field MUST
1604   be deleted as well.
1605</t>
1606<t>
1607   The following warn-codes are defined by this specification, each with a
1608   recommended warn-text in English, and a description of its meaning.
1609</t>
1610
1611<section title="110 Response is Stale" anchor="warn.110">
1612  <iref primary="true" item="110 Response is Stale (warn code)"/>
1613<t>
1614   A cache SHOULD generate this whenever the sent response is stale.
1615</t>
1616</section>
1617
1618<section title="111 Revalidation Failed" anchor="warn.111">
1619  <iref primary="true" item="111 Revalidation Failed (warn code)"/>
1620<t>
1621   A cache SHOULD generate this when sending a stale response because an
1622   attempt to validate the response failed, due to an inability to reach
1623   the server.
1624</t>
1625</section>
1626
1627<section title="112 Disconnected Operation" anchor="warn.112">
1628  <iref primary="true" item="112 Disconnected Operation (warn code)"/>
1629<t>
1630   A cache SHOULD generate this if it is intentionally disconnected from
1631   the rest of the network for a period of time.
1632</t>
1633</section>
1634
1635<section title="113 Heuristic Expiration" anchor="warn.113">
1636  <iref primary="true" item="113 Heuristic Expiration (warn code)"/>
1637<t>
1638   A cache SHOULD generate this if it heuristically chose a freshness
1639   lifetime greater than 24 hours and the response's age is greater than 24
1640   hours.
1641</t>
1642</section>
1643
1644<section title="199 Miscellaneous Warning" anchor="warn.199">
1645  <iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
1646<t>
1647   The warning text can include arbitrary information to be presented to
1648   a human user, or logged. A system receiving this warning MUST NOT take
1649   any automated action, besides presenting the warning to the user.
1650</t>
1651</section>
1652
1653<section title="214 Transformation Applied" anchor="warn.214">
1654  <iref primary="true" item="214 Transformation Applied (warn code)"/>
1655<t>
1656   MUST be added by a proxy if it applies any transformation to the
1657   representation, such as changing the content-coding, media-type, or
1658   modifying the representation data, unless this Warning code already appears
1659   in the response.
1660</t>
1661</section>
1662
1663<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1664  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)"/>
1665<t>
1666   The warning text can include arbitrary information to be presented to
1667   a human user, or logged. A system receiving this warning MUST NOT take
1668   any automated action.
1669</t>
1670</section>
1671
1672<section title="Warn Code Extensions" anchor="warn.code.extensions">
1673<t>
1674   Extension warn codes can be defined; see <xref target="warn.code.registry.procedure"/>
1675   for details.
1676</t>
1677</section>
1678</section>
1679</section>
1680
1681<section anchor="history.lists" title="History Lists">
1682<t>
1683   User agents often have history mechanisms, such as "Back" buttons and
1684   history lists, that can be used to redisplay a representation retrieved
1685   earlier in a session.
1686</t>
1687<t>
1688   The freshness model (<xref target="expiration.model"/>) does not
1689   necessarily apply to history mechanisms. I.e., a history mechanism can
1690   display a previous representation even if it has expired.
1691</t>
1692<t>
1693   This does not prohibit the history mechanism from telling the user that a
1694   view might be stale, or from honoring cache directives (e.g.,
1695   Cache-Control: no-store).
1696</t>
1697</section>
1698
1699
1700<section title="IANA Considerations" anchor="iana.considerations">
1701
1702<section title="Cache Directive Registry" anchor="cache.directive.registry">
1703<t>
1704   The HTTP Cache Directive Registry defines the name space for the
1705   cache directives. It will be created and maintained at
1706   <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1707</t>
1708
1709<section title="Procedure" anchor="cache.directive.registry.procedure">
1710<t>
1711   A registration MUST include the following fields:
1712   <list style="symbols">
1713      <t>Cache Directive Name</t>
1714      <t>Pointer to specification text</t>
1715   </list>
1716</t>
1717<t>
1718   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1719</t>
1720</section>
1721
1722<section title="Considerations for New Cache Control Directives" anchor="cache.directive.considerations">
1723<t>
1724   New extension directives ought to consider defining:
1725</t>
1726<t>
1727   <list style="symbols">
1728      <t>What it means for a directive to be specified multiple times,</t>
1729      <t>When the directive does not take an argument, what it means when an
1730      argument is present,</t>
1731      <t>When the directive requires an argument, what it means when it is
1732      missing,</t>
1733      <t>Whether the directive is specific to requests, responses, or able
1734        to be used in either.</t>
1735   </list>
1736</t>
1737<t>
1738  See also <xref target="cache.control.extensions"/>.
1739</t>
1740</section>
1741
1742<section title="Registrations" anchor="cache.directive.registration">
1743<t>
1744  The HTTP Cache Directive Registry shall be populated with the registrations below:
1745</t>
1746
1747<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1748<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1749   <ttcol>Cache Directive</ttcol>
1750   <ttcol>Reference</ttcol>
1751
1752   <c>max-age</c>
1753   <c>
1754      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1755   </c>
1756   <c>max-stale</c>
1757   <c>
1758      <xref target="cache-request-directive.max-stale"/>
1759   </c>
1760   <c>min-fresh</c>
1761   <c>
1762      <xref target="cache-request-directive.min-fresh"/>
1763   </c>
1764   <c>must-revalidate</c>
1765   <c>
1766      <xref target="cache-response-directive.must-revalidate"/>
1767   </c>
1768   <c>no-cache</c>
1769   <c>
1770      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1771   </c>
1772   <c>no-store</c>
1773   <c>
1774      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1775   </c>
1776   <c>no-transform</c>
1777   <c>
1778      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1779   </c>
1780   <c>only-if-cached</c>
1781   <c>
1782      <xref target="cache-request-directive.only-if-cached"/>
1783   </c>
1784   <c>private</c>
1785   <c>
1786      <xref target="cache-response-directive.private"/>
1787   </c>
1788   <c>proxy-revalidate</c>
1789   <c>
1790      <xref target="cache-response-directive.proxy-revalidate"/>
1791   </c>
1792   <c>public</c>
1793   <c>
1794      <xref target="cache-response-directive.public"/>
1795   </c>
1796   <c>s-maxage</c>
1797   <c>
1798      <xref target="cache-response-directive.s-maxage"/>
1799   </c>
1800   <c>stale-if-error</c>
1801   <c>
1802      <xref target="RFC5861"/>, Section 4
1803   </c>
1804   <c>stale-while-revalidate</c>
1805   <c>
1806      <xref target="RFC5861"/>, Section 3
1807   </c>
1808</texttable>
1809<!--(END)-->
1810
1811</section>
1812</section>
1813
1814<section title="Warn Code Registry" anchor="warn.code.registry">
1815<t>
1816   The HTTP Warn Code Registry defines the name space for warn codes.
1817   It will be created and maintained at
1818   <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
1819</t>
1820
1821<section title="Procedure" anchor="warn.code.registry.procedure">
1822<t>
1823   A registration MUST include the following fields:
1824   <list style="symbols">
1825      <t>Warn Code (3 digits)</t>
1826      <t>Short Description</t>
1827      <t>Pointer to specification text</t>
1828   </list>
1829</t>
1830<t>
1831   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1832</t>
1833</section>
1834
1835<section title="Registrations" anchor="warn.code.registration">
1836<t>
1837  The HTTP Warn Code Registry shall be populated with the registrations below:
1838</t>
1839
1840<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1841<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1842   <ttcol>Warn Code</ttcol>
1843   <ttcol>Short Description</ttcol>
1844   <ttcol>Reference</ttcol>
1845   <c>110</c>
1846   <c>Response is Stale</c>
1847   <c>
1848      <xref target="warn.110"/>
1849   </c>
1850   <c>111</c>
1851   <c>Revalidation Failed</c>
1852   <c>
1853      <xref target="warn.111"/>
1854   </c>
1855   <c>112</c>
1856   <c>Disconnected Operation</c>
1857   <c>
1858      <xref target="warn.112"/>
1859   </c>
1860   <c>113</c>
1861   <c>Heuristic Expiration</c>
1862   <c>
1863      <xref target="warn.113"/>
1864   </c>
1865   <c>199</c>
1866   <c>Miscellaneous Warning</c>
1867   <c>
1868      <xref target="warn.199"/>
1869   </c>
1870   <c>214</c>
1871   <c>Transformation Applied</c>
1872   <c>
1873      <xref target="warn.214"/>
1874   </c>
1875   <c>299</c>
1876   <c>Miscellaneous Persistent Warning</c>
1877   <c>
1878      <xref target="warn.299"/>
1879   </c>
1880</texttable>
1881<!--(END)-->
1882
1883</section>
1884</section>
1885
1886<section title="Header Field Registration" anchor="header.field.registration">
1887<t>
1888   HTTP header fields are registered within the Message Header Field Registry
1889   maintained at
1890   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
1891</t>
1892<t>
1893   This document defines the following HTTP header fields, so their
1894   associated registry entries shall be updated according to the permanent
1895   registrations below (see <xref target="BCP90"/>):
1896</t>
1897
1898<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1899<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1900   <ttcol>Header Field Name</ttcol>
1901   <ttcol>Protocol</ttcol>
1902   <ttcol>Status</ttcol>
1903   <ttcol>Reference</ttcol>
1904
1905   <c>Age</c>
1906   <c>http</c>
1907   <c>standard</c>
1908   <c>
1909      <xref target="header.age"/>
1910   </c>
1911   <c>Cache-Control</c>
1912   <c>http</c>
1913   <c>standard</c>
1914   <c>
1915      <xref target="header.cache-control"/>
1916   </c>
1917   <c>Expires</c>
1918   <c>http</c>
1919   <c>standard</c>
1920   <c>
1921      <xref target="header.expires"/>
1922   </c>
1923   <c>Pragma</c>
1924   <c>http</c>
1925   <c>standard</c>
1926   <c>
1927      <xref target="header.pragma"/>
1928   </c>
1929   <c>Warning</c>
1930   <c>http</c>
1931   <c>standard</c>
1932   <c>
1933      <xref target="header.warning"/>
1934   </c>
1935</texttable>
1936<!--(END)-->
1937
1938<t>
1939   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1940   Force".
1941</t>
1942</section>
1943</section>
1944
1945<section anchor="security.considerations" title="Security Considerations">
1946<t>
1947   This section is meant to inform developers, information providers, and
1948   users of known security concerns specific to HTTP/1.1 caching.
1949   More general security considerations are addressed in HTTP messaging
1950   <xref target="Part1"/> and semantics <xref target="Part2"/>.
1951</t>
1952<t>
1953   Caches expose additional potential vulnerabilities, since the contents of
1954   the cache represent an attractive target for malicious exploitation.
1955   Because cache contents persist after an HTTP request is complete, an attack
1956   on the cache can reveal information long after a user believes that the
1957   information has been removed from the network. Therefore, cache contents
1958   need to be protected as sensitive information.
1959</t>
1960<t>
1961   Furthermore, the very use of a cache can bring about privacy concerns. For
1962   example, if two users share a cache, and the first one browses to a site,
1963   the second may be able to detect that the other has been to that site,
1964   because the resources from it load more quickly, thanks to the cache.
1965</t>
1966<t>
1967   Implementation flaws might allow attackers to insert content into a cache
1968   ("cache poisoning"), leading to compromise of clients that trust that
1969   content. Because of their nature, these attacks are difficult to mitigate.
1970</t>
1971<t>
1972   Likewise, implementation flaws (as well as misunderstanding of cache
1973   operation) might lead to caching of sensitive information (e.g.,
1974   authentication credentials) that is thought to be private, exposing it to
1975   unauthorized parties.
1976</t>
1977<t>
1978   Note that the Set-Cookie response header field <xref target="RFC6265"/> does not
1979   inhibit caching; a cacheable response with a Set-Cookie header field can be (and
1980   often is) used to satisfy subsequent requests to caches. Servers who wish
1981   to control caching of these responses are encouraged to emit appropriate
1982   Cache-Control response header fields.
1983</t>
1984
1985</section>
1986
1987<section title="Acknowledgments" anchor="acks">
1988<t>
1989  See Section 9 of <xref target="Part1"/>.
1990</t>
1991</section>
1992
1993</middle>
1994
1995<back>
1996<references title="Normative References">
1997
1998  <reference anchor="Part1">
1999    <front>
2000      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2001      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2002        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2003        <address><email>fielding@gbiv.com</email></address>
2004      </author>
2005      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2006        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2007        <address><email>julian.reschke@greenbytes.de</email></address>
2008      </author>
2009      <date month="July" year="2013"/>
2010    </front>
2011    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-23"/>
2012   
2013  </reference>
2014
2015  <reference anchor="Part2">
2016    <front>
2017      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2018      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2019        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2020        <address><email>fielding@gbiv.com</email></address>
2021      </author>
2022      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2023        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2024        <address><email>julian.reschke@greenbytes.de</email></address>
2025      </author>
2026      <date month="July" year="2013"/>
2027    </front>
2028    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-23"/>
2029   
2030  </reference>
2031
2032  <reference anchor="Part4">
2033    <front>
2034      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2035      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2036        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2037        <address><email>fielding@gbiv.com</email></address>
2038      </author>
2039      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2040        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2041        <address><email>julian.reschke@greenbytes.de</email></address>
2042      </author>
2043      <date month="July" year="2013"/>
2044    </front>
2045    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-23"/>
2046   
2047  </reference>
2048
2049  <reference anchor="Part5">
2050    <front>
2051      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2052      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2053        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2054        <address><email>fielding@gbiv.com</email></address>
2055      </author>
2056      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2057        <organization abbrev="W3C">World Wide Web Consortium</organization>
2058        <address><email>ylafon@w3.org</email></address>
2059      </author>
2060      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2061        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2062        <address><email>julian.reschke@greenbytes.de</email></address>
2063      </author>
2064      <date month="July" year="2013"/>
2065    </front>
2066    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-23"/>
2067   
2068  </reference>
2069
2070  <reference anchor="Part7">
2071    <front>
2072      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2073      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2074        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2075        <address><email>fielding@gbiv.com</email></address>
2076      </author>
2077      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2078        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2079        <address><email>julian.reschke@greenbytes.de</email></address>
2080      </author>
2081      <date month="July" year="2013"/>
2082    </front>
2083    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-23"/>
2084   
2085  </reference>
2086
2087  <reference anchor="RFC2119">
2088    <front>
2089      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2090      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2091        <organization>Harvard University</organization>
2092        <address><email>sob@harvard.edu</email></address>
2093      </author>
2094      <date month="March" year="1997"/>
2095    </front>
2096    <seriesInfo name="BCP" value="14"/>
2097    <seriesInfo name="RFC" value="2119"/>
2098  </reference>
2099
2100  <reference anchor="RFC5234">
2101    <front>
2102      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2103      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2104        <organization>Brandenburg InternetWorking</organization>
2105        <address>
2106          <email>dcrocker@bbiw.net</email>
2107        </address> 
2108      </author>
2109      <author initials="P." surname="Overell" fullname="Paul Overell">
2110        <organization>THUS plc.</organization>
2111        <address>
2112          <email>paul.overell@thus.net</email>
2113        </address>
2114      </author>
2115      <date month="January" year="2008"/>
2116    </front>
2117    <seriesInfo name="STD" value="68"/>
2118    <seriesInfo name="RFC" value="5234"/>
2119  </reference>
2120 
2121</references>
2122
2123<references title="Informative References">
2124
2125  <reference anchor="RFC1305">
2126    <front>
2127      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2128      <author fullname="David L. Mills" initials="D." surname="Mills">
2129        <organization>University of Delaware, Electrical Engineering Department</organization>
2130        <address><email>mills@udel.edu</email></address>
2131      </author>
2132      <date month="March" year="1992"/>
2133    </front>
2134    <seriesInfo name="RFC" value="1305"/>
2135  </reference>
2136
2137  <reference anchor="RFC2616">
2138    <front>
2139      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2140      <author fullname="R. Fielding" initials="R." surname="Fielding">
2141        <organization>University of California, Irvine</organization>
2142        <address><email>fielding@ics.uci.edu</email></address>
2143      </author>
2144      <author fullname="J. Gettys" initials="J." surname="Gettys">
2145        <organization>W3C</organization>
2146        <address><email>jg@w3.org</email></address>
2147      </author>
2148      <author fullname="J. Mogul" initials="J." surname="Mogul">
2149        <organization>Compaq Computer Corporation</organization>
2150        <address><email>mogul@wrl.dec.com</email></address>
2151      </author>
2152      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2153        <organization>MIT Laboratory for Computer Science</organization>
2154        <address><email>frystyk@w3.org</email></address>
2155      </author>
2156      <author fullname="L. Masinter" initials="L." surname="Masinter">
2157        <organization>Xerox Corporation</organization>
2158        <address><email>masinter@parc.xerox.com</email></address>
2159      </author>
2160      <author fullname="P. Leach" initials="P." surname="Leach">
2161        <organization>Microsoft Corporation</organization>
2162        <address><email>paulle@microsoft.com</email></address>
2163      </author>
2164      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2165        <organization>W3C</organization>
2166        <address><email>timbl@w3.org</email></address>
2167      </author>
2168      <date month="June" year="1999"/>
2169    </front>
2170    <seriesInfo name="RFC" value="2616"/>
2171  </reference>
2172
2173  <reference anchor="BCP90">
2174    <front>
2175      <title>Registration Procedures for Message Header Fields</title>
2176      <author fullname="G. Klyne" initials="G." surname="Klyne">
2177        <organization>Nine by Nine</organization>
2178        <address><email>GK-IETF@ninebynine.org</email></address>
2179      </author>
2180      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2181        <organization>BEA Systems</organization>
2182        <address><email>mnot@pobox.com</email></address>
2183      </author>
2184      <author fullname="J. Mogul" initials="J." surname="Mogul">
2185        <organization>HP Labs</organization>
2186        <address><email>JeffMogul@acm.org</email></address>
2187      </author>
2188      <date month="September" year="2004"/>
2189    </front>
2190    <seriesInfo name="BCP" value="90"/>
2191    <seriesInfo name="RFC" value="3864"/>
2192  </reference>
2193
2194  <reference anchor="RFC5226">
2195    <front>
2196      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2197      <author initials="T." surname="Narten" fullname="T. Narten">
2198        <organization>IBM</organization>
2199        <address><email>narten@us.ibm.com</email></address>
2200      </author>
2201      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2202        <organization>Google</organization>
2203        <address><email>Harald@Alvestrand.no</email></address>
2204      </author>
2205      <date year="2008" month="May"/>
2206    </front>
2207    <seriesInfo name="BCP" value="26"/>
2208    <seriesInfo name="RFC" value="5226"/>
2209  </reference>
2210
2211  <reference anchor="RFC5861">
2212    <front>
2213      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2214      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2215        <organization>Yahoo! Inc.</organization>
2216        <address><email>mnot@yahoo-inc.com</email></address>
2217      </author>
2218      <date month="April" year="2010"/>
2219    </front>
2220    <seriesInfo name="RFC" value="5861"/>
2221  </reference>
2222
2223  <reference anchor="RFC6265">
2224    <front>
2225      <title>HTTP State Management Mechanism</title>
2226      <author initials="A." surname="Barth" fullname="Adam Barth">
2227        <organization abbrev="U.C. Berkeley">
2228          University of California, Berkeley
2229        </organization>
2230        <address><email>abarth@eecs.berkeley.edu</email></address>
2231      </author>
2232      <date year="2011" month="April"/>
2233    </front>
2234    <seriesInfo name="RFC" value="6265"/>
2235  </reference>
2236
2237</references>
2238
2239<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2240<t>
2241  Caching-related text has been substantially rewritten for clarity.
2242</t>
2243<t>
2244  The algorithm for calculating age is now less conservative.
2245  (<xref target="age.calculations"/>)
2246</t>
2247<t>
2248  Caches are now required to handle dates with timezones as if they're
2249  invalid, because it's not possible to accurately guess.
2250  (<xref target="age.calculations"/>)
2251</t>
2252<t>
2253  The Content-Location response header field is no longer used
2254  to determine the appropriate response to use when validating.
2255  (<xref target="validation.model"/>)
2256</t>
2257<t>
2258  The algorithm for selecting a cached negotiated response to use has been
2259  clarified in several ways. In particular, it now explicitly allows
2260  header-specific canonicalization when processing selecting header fields.
2261  (<xref target="caching.negotiated.responses"/>)
2262</t>
2263<t>
2264  Requirements regarding denial of service attack avoidance when performing
2265  invalidation have been clarified.
2266  (<xref target="invalidation.after.updates.or.deletions"/>)
2267</t>
2268<t>
2269  Cache invalidation only occurs when a successful response is received.
2270  (<xref target="invalidation.after.updates.or.deletions"/>)
2271</t>
2272<t>
2273  The conditions under which an authenticated response can be cached have been
2274  clarified.
2275  (<xref target="caching.authenticated.responses"/>)
2276</t>
2277<t>
2278  The one-year limit on Expires header field values has been removed; instead,
2279  the reasoning for using a sensible value is given.
2280  (<xref target="header.expires"/>)
2281</t>
2282<t>
2283  The Pragma header field is now only defined for backwards compatibility;
2284  future pragmas are deprecated.
2285  (<xref target="header.pragma"/>)
2286</t>
2287<t>
2288  Cache directives are explicitly defined to be case-insensitive.
2289  (<xref target="header.cache-control"/>)
2290</t>
2291<t>
2292  Handling of multiple instances of cache directives when only one is
2293  expected is now defined.
2294  (<xref target="header.cache-control"/>)
2295</t>
2296<t>
2297  The qualified forms of the private and no-cache cache directives are noted
2298  to not be widely implemented; e.g., "private=foo" is interpreted by many
2299  caches as simply "private". Additionally, the meaning of the qualified
2300  form of no-cache has been clarified.
2301  (<xref target="cache-response-directive"/>)
2302</t>
2303<t>
2304  The "no-store" cache request directive doesn't apply to responses; i.e.,
2305  a cache can satisfy a request with no-store on it, and does not invalidate
2306  it.
2307  (<xref target="cache-request-directive.no-store"/>)
2308</t>
2309<t>
2310  The "no-cache" response cache directive's meaning has been clarified.
2311  (<xref target="cache-response-directive.no-cache"/>)
2312</t>
2313<t>
2314  New status codes can now define that caches are allowed to use heuristic
2315  freshness with them.
2316  (<xref target="heuristic.freshness"/>)
2317</t>
2318<t>
2319  Caches are now allow to calculate heuristic freshness for URLs with query
2320  components.
2321  (<xref target="heuristic.freshness"/>)
2322</t>
2323<t>
2324  Some requirements regarding production of the <xref target="header.warning" format="none">Warning</xref>
2325  header fields have been relaxed, as it is not widely implemented.
2326  Furthermore, the <xref target="header.warning" format="none">Warning</xref> header field no longer uses RFC 2047 encoding,
2327  nor allows multiple languages, as these aspects were not implemented.
2328  (<xref target="header.warning"/>)
2329</t>
2330<t>
2331  This specification introduces the Cache Directive and Warn Code Registries,
2332  and defines considerations for new cache directives.
2333  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2334</t>
2335</section>
2336
2337<section title="Imported ABNF" anchor="imported.abnf">
2338   
2339   
2340   
2341   
2342   
2343   
2344   
2345   
2346   
2347   
2348   
2349   
2350   
2351   
2352   
2353   
2354<t>
2355   The following core rules are included by reference, as defined in Appendix B.1 of <xref target="RFC5234"/>: ALPHA (letters), CR (carriage
2356   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2357   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2358   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2359</t>
2360<t>
2361   The rules below are defined in <xref target="Part1"/>:
2362</t>
2363<figure><artwork type="abnf2616"><![CDATA[
2364  OWS           = <OWS, defined in [Part1], Section 3.2.3>
2365  field-name    = <field-name, defined in [Part1], Section 3.2>
2366  quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2367  token         = <token, defined in [Part1], Section 3.2.6>
2368
2369  port          = <port, defined in [Part1], Section 2.7>
2370  pseudonym     = <pseudonym, defined in [Part1], Section 5.7.1>
2371  uri-host      = <uri-host, defined in [Part1], Section 2.7>
2372]]></artwork></figure>
2373<t>
2374   The rules below are defined in other parts:
2375</t>
2376<figure><artwork type="abnf2616"><![CDATA[
2377  HTTP-date     = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2378]]></artwork></figure>
2379</section>
2380
2381
2382<section title="Collected ABNF" anchor="collected.abnf">
2383<t>
2384  In the collected ABNF below, list rules are expanded as per Section 1.2 of <xref target="Part1"/>.
2385</t><figure>
2386<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2387Age = delta-seconds
2388
2389Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2390 cache-directive ] )
2391
2392Expires = HTTP-date
2393
2394HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2395
2396OWS = <OWS, defined in [Part1], Section 3.2.3>
2397
2398Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2399 pragma-directive ] )
2400
2401Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2402 )
2403
2404cache-directive = token [ "=" ( token / quoted-string ) ]
2405
2406delta-seconds = 1*DIGIT
2407
2408extension-pragma = token [ "=" ( token / quoted-string ) ]
2409
2410field-name = <field-name, defined in [Part1], Section 3.2>
2411
2412port = <port, defined in [Part1], Section 2.7>
2413pragma-directive = "no-cache" / extension-pragma
2414pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
2415
2416quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2417
2418token = <token, defined in [Part1], Section 3.2.6>
2419
2420uri-host = <uri-host, defined in [Part1], Section 2.7>
2421
2422warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2423warn-code = 3DIGIT
2424warn-date = DQUOTE HTTP-date DQUOTE
2425warn-text = quoted-string
2426warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2427 ]
2428]]></artwork>
2429</figure>
2430</section>
2431
2432
2433<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2434<t>
2435  Changes up to the first Working Group Last Call draft are summarized
2436  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2437</t>
2438
2439<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2440<t>
2441  Closed issues:
2442  <list style="symbols">
2443    <t>
2444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2445      "untangle Cache-Control ABNF"
2446    </t>
2447    <t>
2448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2449      "Multiple values in Cache-Control header fields"
2450    </t>
2451    <t>
2452      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2453      "Case sensitivity of header fields in CC values"
2454    </t>
2455    <t>
2456      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2457      "Spurious 'MAYs'"
2458    </t>
2459    <t>
2460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2461      "enhance considerations for new cache control directives"
2462    </t>
2463    <t>
2464      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2465      "ABNF requirements for recipients"
2466    </t>
2467    <t>
2468      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2469      "note introduction of new IANA registries as normative changes"
2470    </t>
2471    <t>
2472      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2473      "broken prose in description of 'Vary'"
2474    </t>
2475  </list>
2476</t>
2477</section>
2478
2479<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2480<t>
2481  Closed issues:
2482  <list style="symbols">
2483    <t>
2484      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2485      "'Most Conservative'"
2486    </t>
2487  </list>
2488</t>
2489<t>
2490  Other changes:
2491  <list style="symbols">
2492    <t>
2493      Conformance criteria and considerations regarding error handling are
2494      now defined in Part 1.
2495    </t>
2496    <t>
2497      Move definition of "Vary" header field into Part 2.
2498    </t>
2499    <t>
2500      Add security considerations with respect to cache poisoning and
2501      the "Set-Cookie" header field.
2502    </t>
2503  </list>
2504</t>
2505</section>
2506
2507<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2508<t>
2509  Closed issues:
2510  <list style="symbols">
2511    <t>
2512      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2513      "Allowing heuristic caching for new status codes"
2514    </t>
2515    <t>
2516      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
2517      "304 without validator"
2518    </t>
2519    <t>
2520      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2521      "No-Transform"
2522    </t>
2523    <t>
2524      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
2525      "Revert prior change to the meaning of the public cache response
2526       directive.
2527    </t>
2528  </list>
2529</t>
2530</section>
2531
2532<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
2533<t>
2534  Closed issues:
2535  <list style="symbols">
2536    <t>
2537      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
2538      "explain list expansion in ABNF appendices"
2539    </t>
2540    <t>
2541      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/453"/>:
2542      "Returning the freshest response"
2543    </t>
2544    <t>
2545      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/464"/>:
2546      "placement of extension point considerations"
2547    </t>
2548    <t>
2549      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/469"/>:
2550      "Editorial notes for p6"
2551    </t>
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/471"/>:
2554      "Vary and future requests"
2555    </t>
2556  </list>
2557</t>
2558</section>
2559</section>
2560  </back>
2561</rfc>
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