source: draft-ietf-httpbis/22/draft-ietf-httpbis-p6-cache-22.xml @ 2190

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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-22" 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="February" year="2013" day="23"/>
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.21"/>.
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<section anchor="intro.purpose" title="Purpose">
104<iref item="cache"/>
105<t>
106   An HTTP cache is a local store of response messages and the
107   subsystem that controls its message storage, retrieval, and deletion. A
108   cache stores cacheable responses in order to reduce the response time and
109   network bandwidth consumption on future, equivalent requests. Any client or
110   server MAY employ a cache, though a cache cannot be used by a server that
111   is acting as a tunnel.
112</t>
113<t>
114   The goal of caching in HTTP/1.1 is to significantly improve performance
115   by reusing a prior response message to satisfy a current request.
116   A stored response is considered "fresh", as defined in
117   <xref target="expiration.model"/>, if the response can be reused without
118   "validation" (checking with the origin server to see if the cached response
119   remains valid for this request).  A fresh cache response can therefore
120   reduce both latency and network transfers each time it is reused.
121   When a cached response is not fresh, it might still be reusable if it can
122   be freshened by validation (<xref target="validation.model"/>) or if the
123   origin is unavailable.
124</t>
125</section>
126
127<section anchor="intro.terminology" title="Terminology">
128<t>
129   This specification uses a number of terms to refer to the roles played by
130   participants in, and objects of, HTTP caching.
131</t>
132<t>
133   <iref item="cache"/>
134   <?rfc needLines="4"?>cache
135   <list>
136      <t>A conformant implementation of an HTTP cache. Note that this implies
137        an HTTP/1.1 cache; this specification does not define conformance
138        for HTTP/1.0 caches.</t>
139   </list>
140</t>
141<t anchor="shared.and.non-shared.caches">
142   <iref item="shared cache"/>
143   <?rfc needLines="4"?>shared cache
144   <list>
145      <t>A cache that stores responses to be reused by more than one user;
146         usually (but not always) deployed as part of an intermediary.</t>
147   </list>
148</t>
149<t>
150   <iref item="private cache"/>
151   <?rfc needLines="4"?>private cache
152   <list>
153      <t>A cache that is dedicated to a single user.</t>
154   </list>
155</t>
156<t>
157   <iref item="cacheable"/>
158   <?rfc needLines="4"?>cacheable
159   <list>
160      <t>A response is cacheable if a cache is allowed to store a copy of the
161      response message for use in answering subsequent requests. Even when a
162      response is cacheable, there might be additional constraints on whether
163      a cache can use the stored copy to satisfy a particular request.</t>
164   </list>
165</t>
166<t>
167   <iref item="explicit expiration time"/>
168   <?rfc needLines="4"?>explicit expiration time
169   <list>
170      <t>The time at which the origin server intends that a stored response
171      no longer be used by a cache without further validation.</t>
172   </list>
173</t>
174<t>
175   <iref item="heuristic expiration time"/>
176   <?rfc needLines="4"?>heuristic expiration time
177   <list>
178      <t>An expiration time assigned by a cache when no explicit expiration
179      time is available.</t>
180   </list>
181</t>
182<t>
183   <iref item="age"/>
184   <?rfc needLines="4"?>age
185   <list>
186      <t>The age of a response is the time since it was sent by, or
187      successfully validated with, the origin server.</t>
188   </list>
189</t>
190<t>
191   <iref item="first-hand"/>
192   <?rfc needLines="4"?>first-hand
193   <list>
194      <t>A response is first-hand if the freshness model is not in use; i.e.,
195      its age is 0.</t>
196   </list>
197</t>
198<t>
199   <iref item="freshness lifetime"/>
200   <?rfc needLines="4"?>freshness lifetime
201   <list>
202      <t>The length of time between the generation of a response and its
203      expiration time.</t>
204   </list>
205</t>
206<t>
207   <iref item="fresh"/>
208   <?rfc needLines="4"?>fresh
209   <list>
210      <t>A response is fresh if its age has not yet exceeded its freshness
211      lifetime.</t>
212   </list>
213</t>
214<t>
215   <iref item="stale"/>
216   <?rfc needLines="4"?>stale
217   <list>
218      <t>A response is stale if its age has passed its freshness lifetime
219      (either explicit or heuristic).</t>
220   </list>
221</t>
222<t>
223   <iref item="validator"/>
224   <?rfc needLines="4"?>validator
225   <list>
226      <t>A protocol element (e.g., an entity-tag or a Last-Modified
227      time) that is used to find out whether a stored response is an equivalent
228      copy of a representation. See Section 2.1 of <xref target="Part4"/>.</t>
229   </list>
230</t>
231<t>
232   <iref item="strong validator"/>
233   <iref item="validator" subitem="strong"/>
234   <?rfc needLines="4"?>strong validator
235   <list>
236      <t>A validator that is defined by the origin server such that its
237         current value will change if the representation data changes; i.e.,
238         an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
239         if no entity-tag is provided, a Last-Modified value
240         that is strong in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
241   </list>
242</t>
243</section>
244
245<section title="Conformance and Error Handling" anchor="conformance">
246<t>
247   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
248   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
249   document are to be interpreted as described in <xref target="RFC2119"/>.
250</t>
251<t>
252   Conformance criteria and considerations regarding error handling
253   are defined in Section 2.5 of <xref target="Part1"/>.
254</t>
255</section>
256
257<section title="Syntax Notation" anchor="notation">
258<t>
259   This specification uses the Augmented Backus-Naur Form (ABNF) notation
260   of <xref target="RFC5234"/> with the list rule extension defined in
261   Section 1.2 of <xref target="Part1"/>. <xref target="imported.abnf"/> describes rules imported from
262   other documents. <xref target="collected.abnf"/> shows the collected ABNF
263   with the list rule expanded.
264</t>
265
266<section title="Delta Seconds" anchor="delta-seconds">
267<t>
268   The delta-seconds rule specifies a non-negative integer, representing time
269   in seconds.
270</t>
271<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
272  delta-seconds  = 1*DIGIT
273]]></artwork></figure>
274<t>
275   If an implementation receives a delta-seconds value larger than the largest
276   positive integer it can represent, or if any of its subsequent calculations
277   overflows, it MUST consider the value to be 2147483648
278   (2^31). Recipients parsing a delta-seconds value MUST use
279   an arithmetic type of at least 31 bits of range, and senders MUST NOT
280   send delta-seconds with a value greater than 2147483648.
281</t>
282</section>
283
284</section>
285</section>
286
287<section anchor="caching.overview" title="Overview of Cache Operation">
288<iref item="cache entry"/>
289<iref item="cache key"/>
290<t>
291   Proper cache operation preserves the semantics of HTTP transfers
292   (<xref target="Part2"/>) while eliminating the transfer of information already held
293   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
294   we assume that reusing the cached response is desirable and that such
295   reuse is the default behavior when no requirement or locally-desired
296   configuration prevents it.  Therefore, HTTP cache requirements are focused
297   on preventing a cache from either storing a non-reusable response or
298   reusing a stored response inappropriately.
299</t>
300<t>
301   Each cache entry consists of a cache key and one or more
302   HTTP responses corresponding to prior requests that used the same key. The
303   most common form of cache entry is a successful result of a retrieval
304   request: i.e., a 200 (OK) response to a GET request, which
305   contains a representation of the resource identified by the request target
306   (Section 4.3.1 of <xref target="Part2"/>). However, it is also possible to cache permanent redirects,
307   negative results (e.g., 404 (Not Found)),
308   incomplete results (e.g., 206 (Partial Content)), and
309   responses to methods other than GET if the method's definition allows such
310   caching and defines something suitable for use as a cache key.
311</t>
312<t>
313   The default cache key consists of the request method and
314   target URI.  However, since HTTP caches in common use today are typically
315   limited to caching responses to GET, many implementations simply decline
316   other methods and use only the URI as the key.
317</t>
318<t>
319   If a request target is subject to content negotiation, its cache entry
320   might consist of multiple stored responses, each differentiated by a
321   secondary key for the values of the original request's selecting header
322   fields (<xref target="caching.negotiated.responses"/>).
323</t>
324</section>
325
326<section anchor="response.cacheability" title="Storing Responses in Caches">
327<t>
328   A cache MUST NOT store a response to any request, unless:
329   <list style="symbols">
330      <t>The request method is understood by the cache and defined as being
331      cacheable, and</t>
332      <t>the response status code is understood by the cache, and</t>
333      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
334      header fields, and</t>
335      <t>the "private" cache response directive (see <xref target="cache-response-directive.private"/>) does not appear in the
336      response, if the cache is shared, and</t>
337      <t>the Authorization header field (see
338      Section 4.1 of <xref target="Part7"/>) does not appear in the request, if the cache is
339      shared, unless the response explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
340      <t>the response either:
341         <list style="symbols">
342            <t>contains an <xref target="header.expires" format="none">Expires</xref> header field (see
343            <xref target="header.expires"/>), or</t>
344            <t>contains a max-age response cache directive (see <xref target="cache-response-directive.max-age"/>), or</t>
345            <t>contains a s-maxage response cache directive and the cache is
346            shared, or</t>
347            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
348            or</t>
349            <t>has a status code that is defined as cacheable
350            (see <xref target="heuristic.freshness"/>), or</t>
351            <t>contains a public response cache directive (see <xref target="cache-response-directive.public"/>).</t>
352         </list>
353      </t>
354   </list>
355</t>
356<t>
357   Note that any of the requirements listed above can be overridden by a
358   cache-control extension; see <xref target="cache.control.extensions"/>.
359</t>
360<t>
361   In this context, a cache has "understood" a request method or a response
362   status code if it recognizes it and implements any cache-specific
363   behavior.
364</t>
365<t>
366   Note that, in normal operation, many caches will not store a response that
367   has neither a cache validator nor an explicit expiration time, as such
368   responses are not usually useful to store. However, caches are not
369   prohibited from storing such responses.
370</t>
371
372<section anchor="incomplete.responses" title="Storing Incomplete Responses">
373<t>
374   A response message is considered complete when all of the octets indicated
375   by the message framing (<xref target="Part1"/>) are received prior to the connection
376   being closed. If the request is GET, the response status is 200
377   (OK), and the entire response header block has been received, a
378   cache MAY store an incomplete response message body if the cache entry is
379   recorded as incomplete. Likewise, a 206 (Partial Content)
380   response MAY be stored as if it were an incomplete 200
381   (OK) cache entry. However, a cache MUST NOT store incomplete or
382   partial content responses if it does not support the Range
383   and Content-Range header fields or if it does not understand
384   the range units used in those fields.
385</t>
386<t>
387   A cache MAY complete a stored incomplete response by making a subsequent
388   range request (<xref target="Part5"/>) and combining the successful response with the
389   stored entry, as defined in <xref target="combining.responses"/>. A cache
390   MUST NOT use an incomplete response to answer requests unless the
391   response has been made complete or the request is partial and specifies a
392   range that is wholly within the incomplete response. A cache MUST NOT
393   send a partial response to a client without explicitly marking it as such
394   using the 206 (Partial Content) status code.
395</t>
396</section>
397
398
399<section anchor="caching.authenticated.responses" title="Storing Responses to Authenticated Requests">
400<t>
401   A shared cache MUST NOT use a cached response to a request with an
402   Authorization header field (Section 4.1 of <xref target="Part7"/>) to
403   satisfy any subsequent request unless a cache directive that allows such
404   responses to be stored is present in the response.
405</t>
406<t>
407   In this specification, the following <xref target="header.cache-control" format="none">Cache-Control</xref> response
408   directives (<xref target="cache-response-directive"/>) have such an effect:
409   must-revalidate, public, s-maxage.
410</t>
411<t>
412   Note that cached responses that contain the "must-revalidate" and/or
413   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
414   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
415   used to satisfy a subsequent request without revalidating it on the origin
416   server.
417</t>
418</section>
419</section>
420
421
422<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
423<t>
424   For a presented request, a cache MUST NOT send a stored response,
425   unless:
426   <list style="symbols">
427      <t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
428      that of the stored response match, and</t>
429      <t>the request method associated with the stored response allows it to
430      be used for the presented request, and</t>
431      <t>selecting header fields nominated by the stored response (if any)
432      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
433      <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
434      successfully validated (<xref target="validation.model"/>), and</t>
435      <t>the stored response does not contain the no-cache cache directive
436      (<xref target="cache-response-directive.no-cache"/>), unless it is
437      successfully validated (<xref target="validation.model"/>), and</t>
438      <t>the stored response is either:
439         <list style="symbols">
440            <t>fresh (see <xref target="expiration.model"/>), or</t>
441            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
442            <t>successfully validated (see <xref target="validation.model"/>).</t>
443         </list>
444      </t>
445  </list>
446</t>
447<t>
448   Note that any of the requirements listed above can be overridden by a
449   cache-control extension; see <xref target="cache.control.extensions"/>.
450</t>
451<t>
452   When a stored response is used to satisfy a request without validation,
453   a cache MUST send a single <xref target="header.age" format="none">Age</xref> header field
454   (<xref target="header.age"/>) in the response with a value equal to the
455   stored response's current_age; see <xref target="age.calculations"/>.
456</t>
457<t>
458   A cache MUST write through requests with methods that are unsafe
459   (Section 4.2.1 of <xref target="Part2"/>) to the origin server; i.e., a cache is not allowed to
460   generate a reply to such a request before having forwarded the request and
461   having received a corresponding response.
462</t>
463<t>
464   Also, note that unsafe requests might invalidate already stored responses;
465   see <xref target="invalidation.after.updates.or.deletions"/>.
466</t>
467<t>
468   When more than one suitable response is stored, a cache MUST use the
469   most recent response (as determined by the Date header
470   field). It can also forward a request with "Cache-Control: max-age=0" or
471   "Cache-Control: no-cache" to disambiguate which response to use.
472</t>
473<t>
474   A cache that does not have a clock available MUST NOT use stored
475   responses without revalidating them on every use.
476</t>
477
478
479<section anchor="expiration.model" title="Freshness Model">
480<t>
481   When a response is "fresh" in the cache, it can be used to satisfy
482   subsequent requests without contacting the origin server, thereby improving
483   efficiency.
484</t>
485<t>
486   The primary mechanism for determining freshness is for an origin server to
487   provide an explicit expiration time in the future, using either the
488   <xref target="header.expires" format="none">Expires</xref> header field (<xref target="header.expires"/>) or
489   the max-age response cache directive (<xref target="cache-response-directive.max-age"/>). Generally, origin servers will
490   assign future explicit expiration times to responses in the belief that the
491   representation is not likely to change in a semantically significant way
492   before the expiration time is reached.
493</t>
494<t>
495   If an origin server wishes to force a cache to validate every request, it
496   can assign an explicit expiration time in the past to indicate that the
497   response is already stale. Compliant caches will normally validate a stale
498   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
499</t>
500<t>
501   Since origin servers do not always provide explicit expiration times,
502   caches are also allowed to use a heuristic to determine an expiration time
503   under certain circumstances (see <xref target="heuristic.freshness"/>).
504</t>
505<figure>
506<preamble>
507  The calculation to determine if a response is fresh is:
508</preamble>
509<artwork type="code"><![CDATA[
510   response_is_fresh = (freshness_lifetime > current_age)
511]]></artwork>
512</figure>
513<t>
514   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
515   <xref target="age.calculations"/>.
516</t>
517<t>
518   Clients can send the max-age or min-fresh cache directives in a request to
519   constrain or relax freshness calculations for the corresponding response
520   (<xref target="cache-request-directive"/>).
521</t>
522<t>
523   Note that freshness applies only to cache operation; it cannot be used to
524   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
525   caches and history mechanisms.
526</t>
527
528<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
529<t>
530   A cache can calculate the freshness lifetime (denoted as
531   freshness_lifetime) of a response by using the first match of:
532   <list style="symbols">
533      <t>If the cache is shared and the s-maxage response cache directive
534      (<xref target="cache-response-directive.s-maxage"/>) is present, use its value,
535      or</t>
536      <t>If the max-age response cache directive (<xref target="cache-response-directive.max-age"/>) is present, use its value, or</t>
537      <t>If the <xref target="header.expires" format="none">Expires</xref> response header field
538      (<xref target="header.expires"/>) is present, use its value minus the
539      value of the Date response header field, or</t>
540      <t>Otherwise, no explicit expiration time is present in the response. A
541      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
542   </list>
543</t>
544<t>
545   Note that this calculation is not vulnerable to clock skew, since all of
546   the information comes from the origin server.
547</t>
548<t>
549   When there is more than one value present for a given directive (e.g., two
550   <xref target="header.expires" format="none">Expires</xref> header fields, multiple Cache-Control: max-age
551   directives), it is considered invalid. Caches are encouraged to consider
552   responses that have invalid freshness information to be stale.
553</t>
554</section>
555
556<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
557<t>
558   Since origin servers do not always provide explicit expiration times, a
559   cache MAY assign a heuristic expiration time when an explicit time is not
560   specified, employing algorithms that use other header field values (such as
561   the Last-Modified time) to estimate a plausible expiration
562   time. This specification does not provide specific algorithms, but does
563   impose worst-case constraints on their results.
564</t>
565<t>
566   A cache MUST NOT use heuristics to determine freshness when an explicit
567   expiration time is present in the stored response. Because of the
568   requirements in <xref target="response.cacheability"/>, this means that,
569   effectively, heuristics can only be used on responses without explicit
570   freshness whose status codes are defined as cacheable, and responses
571   without explicit freshness that have been marked as explicitly cacheable
572   (e.g., with a "public" response cache directive).
573</t>
574<t>
575   If the response has a Last-Modified header field
576   (Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
577   expiration value that is no more than some fraction of the interval since
578   that time. A typical setting of this fraction might be 10%.
579</t>
580<t>
581   When a heuristic is used to calculate freshness lifetime, a cache SHOULD
582   attach a <xref target="header.warning" format="none">Warning</xref> header field with a 113 warn-code to the
583   response if its current_age is more than 24 hours and such a warning is not
584   already present.
585</t>
586<t><list>
587   <t>
588      Note: Section 13.9 of <xref target="RFC2616"/> prohibited caches
589      from calculating heuristic freshness for URIs with query components
590      (i.e., those containing '?'). In practice, this has not been widely
591      implemented. Therefore, servers are encouraged to send explicit
592      directives (e.g., Cache-Control: no-cache) if they wish to preclude
593      caching.
594   </t>
595</list></t>
596</section>
597
598<section anchor="age.calculations" title="Calculating Age">
599<t>
600   The <xref target="header.age" format="none">Age</xref> header field is used to convey an estimated
601   age of the response message when obtained from a cache. The Age field value
602   is the cache's estimate of the number of seconds since the response was
603   generated or validated by the origin server. In essence, the Age value is
604   the sum of the time that the response has been resident in each of the
605   caches along the path from the origin server, plus the amount of time it
606   has been in transit along network paths.
607</t>
608<t>
609   The following data is used for the age calculation:
610</t>
611<t>
612   <?rfc needLines="4"?>age_value
613   <list>
614      <t>
615         The term "age_value" denotes the value of the <xref target="header.age" format="none">Age</xref>
616         header field (<xref target="header.age"/>), in a form appropriate for
617         arithmetic operation; or 0, if not available.
618      </t>
619   </list>
620</t>
621<t>
622   <?rfc needLines="4"?>date_value
623   <list>
624      <t>
625         The term "date_value" denotes the value of
626         the Date header field, in a form appropriate for arithmetic
627         operations. See Section 7.1.1.2 of <xref target="Part2"/> for the definition of the Date header
628         field, and for requirements regarding responses without it.
629      </t>
630   </list>
631</t>
632<t>
633   <?rfc needLines="4"?>now
634   <list>
635      <t>
636         The term "now" means "the current value of the clock at the host
637         performing the calculation". A host ought to use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
638         clocks to Coordinated Universal Time.
639      </t>
640   </list>
641</t>
642<t>
643   <?rfc needLines="4"?>request_time
644   <list>
645      <t>
646         The current value of the clock at the host at the time the request
647         resulting in the stored response was made.
648      </t>
649   </list>
650</t>
651<t>
652   <?rfc needLines="4"?>response_time
653   <list>
654      <t>
655         The current value of the clock at the host at the time the response
656         was received.
657      </t>
658   </list>
659</t>
660<t>
661   A response's age can be calculated in two entirely independent ways:
662   <list style="numbers">
663      <t>the "apparent_age": response_time minus date_value, if the local
664      clock is reasonably well synchronized to the origin server's clock. If
665      the result is negative, the result is replaced by zero.</t>
666      <t>the "corrected_age_value", if all of the caches along the response
667      path implement HTTP/1.1. A cache MUST interpret this value relative
668      to the time the request was initiated, not the time that the response
669      was received.</t>
670   </list>
671</t>
672<figure>
673<artwork type="code"><![CDATA[
674  apparent_age = max(0, response_time - date_value);
675
676  response_delay = response_time - request_time;
677  corrected_age_value = age_value + response_delay; 
678]]></artwork>
679</figure>
680<figure>
681<preamble>These SHOULD be combined as</preamble>
682<artwork type="code"><![CDATA[
683  corrected_initial_age = max(apparent_age, corrected_age_value);
684]]></artwork></figure>
685<t>
686   unless the cache is confident in the value of the <xref target="header.age" format="none">Age</xref> header
687   field (e.g., because there are no HTTP/1.0 hops in the Via
688   header field), in which case the corrected_age_value MAY be used as the
689   corrected_initial_age.</t>
690<t>
691   The current_age of a stored response can then be calculated by adding the
692   amount of time (in seconds) since the stored response was last validated by
693   the origin server to the corrected_initial_age.
694</t>
695<figure><artwork type="code"><![CDATA[
696  resident_time = now - response_time;
697  current_age = corrected_initial_age + resident_time;
698]]></artwork></figure>
699<t>
700   Additionally, to avoid common problems in date parsing:
701</t>
702<t>
703  <list style="symbols">
704     <t>Although all date formats are specified to be case-sensitive,
705        cache recipients SHOULD match day, week and timezone names
706        case-insensitively.</t>
707             
708     <t>If a cache recipient's internal implementation of time has less
709        resolution than the value of an HTTP-date, the recipient MUST
710        internally represent a parsed <xref target="header.expires" format="none">Expires</xref> date as the
711        nearest time equal to or earlier than the received value.</t>
712
713     <t>Cache recipients MUST NOT allow local time zones to influence the
714        calculation or comparison of an age or expiration time.</t>
715
716     <t>Cache recipients SHOULD consider a date with a zone abbreviation
717        other than "GMT" to be invalid for calculating expiration.</t>
718  </list>
719</t>
720</section>
721
722<section anchor="serving.stale.responses" title="Serving Stale Responses">
723<t>
724   A "stale" response is one that either has explicit expiry information or is
725   allowed to have heuristic expiry calculated, but is not fresh according to
726   the calculations in <xref target="expiration.model"/>.
727</t>
728<t>
729   A cache MUST NOT send a stale response if it is prohibited by an
730   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
731   directive, a "must-revalidate" cache-response-directive, or an applicable
732   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
733</t>
734<t>
735   A cache MUST NOT send stale responses unless it is disconnected
736   (i.e., it cannot contact the origin server or otherwise find a forward
737   path) or doing so is explicitly allowed (e.g., by the max-stale request
738   directive; see <xref target="cache-request-directive"/>).
739</t>
740<t>
741   A cache SHOULD append a <xref target="header.warning" format="none">Warning</xref> header field with the 110
742   warn-code (see <xref target="header.warning"/>) to stale responses.
743   Likewise, a cache SHOULD add the 112 warn-code to stale responses if the
744   cache is disconnected.
745</t>
746<t>
747   If a cache receives a first-hand response (either an entire response, or a
748   304 (Not Modified) response) that it would normally forward
749   to the requesting client, and the received response is no longer fresh, the
750   cache can forward it to the requesting client without adding a new
751   <xref target="header.warning" format="none">Warning</xref> (but without removing any existing Warning header
752   fields). A cache shouldn't attempt to validate a response simply because
753   that response became stale in transit.
754</t>
755</section>
756</section>
757
758<section anchor="validation.model" title="Validation Model">
759<t>
760   When a cache has one or more stored responses for a requested URI, but
761   cannot serve any of them (e.g., because they are not fresh, or one cannot
762   be selected; see <xref target="caching.negotiated.responses"/>), it can use
763   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
764   give the origin server an opportunity to both select a valid stored
765   response to be used, and to update it. This process is known as
766   "validating" or "revalidating" the stored response.
767</t>
768<t>
769   When sending such a conditional request, a cache adds an
770   If-Modified-Since header field whose value is that of the
771   Last-Modified header field from the selected
772   (see <xref target="caching.negotiated.responses"/>) stored response, if
773   available.
774</t>
775<t>
776   Additionally, a cache can add an If-None-Match header field
777   whose value is that of the ETag header field(s) from all
778   responses stored for the requested URI, if present. However, if any of the
779   stored responses contains only partial content, the cache shouldn't
780   include its entity-tag in the If-None-Match header field unless the request
781   is for a range that would be fully satisfied by that stored response.
782</t>
783
784<t>Cache handling of a response to a conditional request is dependent upon its
785status code:</t>
786
787<t>
788   <list style="symbols">
789      <t>
790         A 304 (Not Modified) response status code indicates
791         that the stored response can be updated and reused; see <xref target="freshening.responses"/>.
792      </t>
793      <t>
794         A full response (i.e., one with a payload body) indicates that none
795         of the stored responses nominated in the conditional request is
796         suitable. Instead, the cache can use the full response to
797         satisfy the request and MAY replace the stored response(s).
798      </t>
799      <t>
800         However, if a cache receives a 5xx (Server Error)
801         response while attempting to validate a response, it can either
802         forward this response to the requesting client, or act as if the
803         server failed to respond. In the latter case, it can send a
804         previously stored response (see <xref target="serving.stale.responses"/>).
805      </t>
806   </list>
807</t>
808
809<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
810<t>
811   When a cache receives a 304 (Not Modified) response and
812   already has one or more stored 200 (OK) responses for the
813   same cache key, the cache needs to identify which of the stored responses
814   are updated by this new response and then update the stored response(s)
815   with the new information provided in the 304 response.
816</t>
817<t>
818   The stored response to update is identified by using the first match (if
819   any) of:
820   <list style="symbols">
821    <t>
822     If the new response contains a strong validator, then that strong
823     validator identifies the selected representation.  All of the stored
824     responses with the same strong validator are selected.
825     If none of the stored responses contain the same strong validator,
826     then the new response MUST NOT be used to update any stored responses.
827    </t>
828    <t>
829     If the new response contains a weak validator and that validator
830     corresponds to one of the cache's stored responses, then the most
831     recent of those matching stored responses is selected.
832    </t>
833    <t>
834     If the new response does not include any form of validator (such as in
835     the case where a client generates an If-Modified-Since request from a
836     source other than the Last-Modified response header field), and there is
837     only one stored response, and that stored response also lacks a
838     validator, then that stored response is selected.
839    </t>
840   </list>
841</t>
842<t>
843   If a stored response is selected for update, the cache MUST:
844   <list style="symbols">
845      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
846         with warn-code 1xx (see <xref target="header.warning"/>);</t>
847      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
848         with warn-code 2xx; and,</t>
849      <t>use other header fields provided in the 304 (Not Modified)
850         response to replace all instances of the corresponding header
851         fields in the stored response.</t>
852   </list>
853</t>
854</section>
855
856</section>
857
858<section anchor="caching.negotiated.responses" title="Using Negotiated Responses">
859<t>
860   When a cache receives a request that can be satisfied by a stored response
861   that has a Vary header field (Section 7.1.4 of <xref target="Part2"/>),
862   it MUST NOT use that response unless all of the selecting header fields
863   nominated by the Vary header field match in both the original request
864   (i.e., that associated with the stored response), and the presented
865   request.
866</t>
867<t>
868   The selecting header fields from two requests are defined to match if and
869   only if those in the first request can be transformed to those in the
870   second request by applying any of the following:
871   <list style="symbols">
872      <t>
873         adding or removing whitespace, where allowed in the header field's
874         syntax
875      </t>
876      <t>
877         combining multiple header fields with the same field name
878         (see Section 3.2 of <xref target="Part1"/>)
879      </t>
880      <t>
881         normalizing both header field values in a way that is known to have
882         identical semantics, according to the header field's specification
883         (e.g., re-ordering field values when order is not significant;
884         case-normalization, where values are defined to be case-insensitive)
885      </t>
886  </list>
887</t>
888<t>
889   If (after any normalization that might take place) a header field is absent
890   from a request, it can only match another request if it is also absent
891   there.
892</t>
893<t>
894   A Vary header field-value of "*" always fails to match, and
895   subsequent requests to that resource can only be properly interpreted by the
896   origin server.
897</t>
898<t>
899   The stored response with matching selecting header fields is known as the
900   selected response.
901</t>
902<t>
903   If multiple selected responses are available, the most recent response
904   (as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
905</t>
906<t>
907   If no selected response is available, the cache cannot satisfy the
908   presented request. Typically, it is forwarded to the origin server
909   in a (possibly conditional; see <xref target="validation.model"/>) request.
910</t>
911</section>
912
913
914<section anchor="combining.responses" title="Combining Partial Content">
915<t>
916   A response might transfer only a partial representation if the
917   connection closed prematurely or if the request used one or more Range
918   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
919   received several ranges of the same representation.  A cache MAY combine
920   these ranges into a single stored response, and reuse that response to
921   satisfy later requests, if they all share the same strong validator and
922   the cache complies with the client requirements in Section 4.3 of <xref target="Part5"/>.
923</t>
924<t>
925   When combining the new response with one or more stored responses, a
926   cache MUST:
927   <list style="symbols">
928      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
929         with warn-code 1xx (see <xref target="header.warning"/>);</t>
930      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
931         with warn-code 2xx; and,</t>
932      <t>use other header fields provided in the new response, aside
933         from Content-Range, to replace all instances of the
934         corresponding header fields in the stored response.</t>
935   </list>
936</t>
937</section>
938</section>
939
940
941<section anchor="head.effects" title="Updating Caches with HEAD Responses">
942<t>
943   A response to the HEAD method is identical to what an equivalent request
944   made with a GET would have been, except it lacks a body. This property
945   of HEAD responses is used to both invalidate and update cached GET
946   responses.
947</t>
948<t>
949   If one or more stored GET responses can be selected (as per <xref target="caching.negotiated.responses"/>) for a HEAD request, and the
950   Content-Length, ETag or
951   Last-Modified value of a HEAD response differs from that in a
952   selected GET response, the cache MUST consider that selected response to
953   be stale.
954</t>
955<t>
956   If the Content-Length, ETag and
957   Last-Modified values of a HEAD response (when present) are
958   the same as that in a selected GET response (as per
959   <xref target="caching.negotiated.responses"/>), the cache SHOULD update
960   the remaining header fields in the stored response using the following
961   rules:
962   <list style="symbols">
963      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
964         with warn-code 1xx (see <xref target="header.warning"/>);</t>
965      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
966         with warn-code 2xx; and,</t>
967      <t>use other header fields provided in the response to replace
968         all instances of the corresponding header fields in the stored
969         response.</t>
970   </list>
971</t>
972
973</section>
974
975
976<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
977<t>
978   Because unsafe request methods (Section 4.2.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
979   have the potential for changing state on the origin server, intervening
980   caches can use them to keep their contents up-to-date.
981</t>
982<t>
983   A cache MUST invalidate the effective Request URI
984   (Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the
985   Location and Content-Location response header
986   fields (if present) when a non-error response to a request with an unsafe
987   method is received.
988</t>
989<t>
990   However, a cache MUST NOT invalidate a URI from a Location
991   or Content-Location response header field if the host part of
992   that URI differs from the host part in the effective request URI
993   (Section 5.5 of <xref target="Part1"/>). This helps prevent denial of service attacks.
994</t>
995<t>
996   A cache MUST invalidate the effective request URI
997   (Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
998   to a request with a method whose safety is unknown.
999</t>
1000<t>
1001   Here, a "non-error response" is one with a 2xx (Successful)
1002   or 3xx (Redirection) status code. "Invalidate" means that
1003   the cache will either remove all stored responses related to the effective
1004   request URI, or will mark these as "invalid" and in need of a mandatory
1005   validation before they can be sent in response to a subsequent request.
1006</t>
1007<t>
1008   Note that this does not guarantee that all appropriate responses are
1009   invalidated. For example, the request that caused the change at the origin
1010   server might not have gone through the cache where a response is stored.
1011</t>
1012</section>
1013
1014
1015
1016
1017<section anchor="header.field.definitions" title="Header Field Definitions">
1018<t>
1019   This section defines the syntax and semantics of HTTP/1.1 header fields
1020   related to caching.
1021</t>
1022
1023<section anchor="header.age" title="Age">
1024   <iref item="Age header field" primary="true"/>
1025   
1026   
1027<t>
1028   The "Age" header field conveys the sender's estimate of the amount
1029   of time since the response was generated or successfully validated at the
1030   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1031</t>
1032<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1033  Age = delta-seconds
1034]]></artwork></figure>
1035<t>
1036  Age field-values are non-negative integers, representing time in seconds
1037  (see <xref target="delta-seconds"/>).
1038</t>
1039<t>
1040   The presence of an Age header field in a response implies that a response
1041   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1042   might not implement the Age header field.
1043</t>
1044</section>
1045
1046<section anchor="header.cache-control" title="Cache-Control">
1047   <iref item="Cache-Control header field" primary="true"/>
1048   
1049   
1050<t>
1051   The "Cache-Control" header field is used to specify directives for
1052   caches along the request/response chain. Such cache directives are
1053   unidirectional in that the presence of a directive in a request does not
1054   imply that the same directive is to be given in the response.
1055</t>
1056<t>
1057   A cache MUST obey the requirements of the Cache-Control
1058   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1059   directives defined elsewhere are handled.
1060</t>
1061<t><list>
1062   <t>
1063       Note: HTTP/1.0 caches might not implement Cache-Control and
1064       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1065   </t>
1066</list></t>
1067<t>
1068   A proxy, whether or not it implements a cache, MUST pass cache directives
1069   through in forwarded messages, regardless of their
1070   significance to that application, since the directives might be applicable
1071   to all recipients along the request/response chain. It is not possible to
1072   target a directive to a specific cache.
1073</t>
1074<t>
1075   Cache directives are identified by a token, to be compared case-insensitively,
1076   and have an optional argument, that can use both token and quoted-string
1077   syntax. For the directives defined below that define arguments, recipients
1078   ought to accept both forms, even if one is documented to be preferred. For
1079   any directive not defined by this specification, recipients MUST accept
1080   both forms.
1081</t>
1082<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><artwork type="abnf2616"><![CDATA[
1083  Cache-Control   = 1#cache-directive
1084
1085  cache-directive = token [ "=" ( token / quoted-string ) ]
1086]]></artwork></figure>
1087<t>
1088   For the cache directives defined below, no argument is defined (nor allowed)
1089   otherwise stated otherwise.
1090</t>
1091
1092<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1093
1094<section title="no-cache" anchor="cache-request-directive.no-cache">
1095   <iref item="no-cache (cache directive)" primary="true"/>
1096<t>
1097   The "no-cache" request directive indicates that a cache MUST NOT
1098   use a stored response to satisfy the request without successful
1099   validation on the origin server.
1100</t>
1101</section>
1102 
1103<section title="no-store" anchor="cache-request-directive.no-store">
1104   <iref item="no-store (cache directive)" primary="true"/>
1105<t>
1106   The "no-store" request directive indicates that a cache MUST NOT
1107   store any part of either this request or any response to it. This
1108   directive applies to both private and shared caches. "MUST NOT
1109   store" in this context means that the cache MUST NOT intentionally
1110   store the information in non-volatile storage, and MUST make a
1111   best-effort attempt to remove the information from volatile storage as
1112   promptly as possible after forwarding it.
1113</t>
1114<t>
1115   This directive is NOT a reliable or sufficient mechanism for ensuring
1116   privacy. In particular, malicious or compromised caches might not
1117   recognize or obey this directive, and communications networks might be
1118   vulnerable to eavesdropping.
1119</t>
1120<t>
1121   Note that if a request containing this directive is satisfied from a
1122   cache, the no-store request directive does not apply to the already
1123   stored response.
1124</t>
1125</section>
1126
1127<section title="max-age" anchor="cache-request-directive.max-age">
1128   <iref item="max-age (cache directive)" primary="true"/>
1129<t>
1130   Argument syntax:
1131   <list>
1132      <t>
1133        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1134      </t>
1135   </list>
1136</t>
1137<t>
1138   The "max-age" request directive indicates that the client is unwilling to
1139   accept a response whose age is greater than the specified number of
1140   seconds. Unless the max-stale request directive is also present, the
1141   client is not willing to accept a stale response.
1142</t>
1143<t>
1144   Note: This directive uses the token form of the argument syntax;
1145   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1146   quoted-string form.
1147</t>
1148</section>
1149
1150<section title="max-stale" anchor="cache-request-directive.max-stale">
1151   <iref item="max-stale (cache directive)" primary="true"/>
1152<t>
1153   Argument syntax:
1154   <list>
1155      <t>
1156        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1157      </t>
1158   </list>
1159</t>
1160<t>
1161   The "max-stale" request directive indicates that the client is willing
1162   to accept a response that has exceeded its expiration time. If max-stale
1163   is assigned a value, then the client is willing to accept a response
1164   that has exceeded its expiration time by no more than the specified
1165   number of seconds. If no value is assigned to max-stale, then the client
1166   is willing to accept a stale response of any age.
1167</t>
1168<t>
1169   Note: This directive uses the token form of the argument syntax;
1170   e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use the
1171   quoted-string form.
1172</t>
1173</section>
1174
1175<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1176   <iref item="min-fresh (cache directive)" primary="true"/>
1177<t>
1178   Argument syntax:
1179   <list>
1180      <t>
1181        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1182      </t>
1183   </list>
1184</t>
1185<t>
1186   The "min-fresh" request directive indicates that the client is willing
1187   to accept a response whose freshness lifetime is no less than its
1188   current age plus the specified time in seconds. That is, the client
1189   wants a response that will still be fresh for at least the specified
1190   number of seconds.
1191</t>
1192<t>
1193   Note: This directive uses the token form of the argument syntax;
1194   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the
1195   quoted-string form.
1196</t>
1197</section>
1198
1199<section title="no-transform" anchor="cache-request-directive.no-transform">
1200   <iref item="no-transform (cache directive)" primary="true"/>
1201<t>
1202   The "no-transform" request directive indicates that an intermediary
1203   (whether or not it implements a cache) MUST NOT transform the payload,
1204   as defined in Section 5.7.2 of <xref target="Part1"/>.
1205</t>
1206</section>
1207
1208<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1209   <iref item="only-if-cached (cache directive)" primary="true"/>
1210<t>
1211   The "only-if-cached" request directive indicates that the client only wishes
1212   to obtain a stored response. If it receives this directive, a cache SHOULD
1213   either respond using a stored response that is consistent with the other
1214   constraints of the request, or respond with a 504 (Gateway
1215   Timeout) status code. If a group of caches is being operated as a
1216   unified system with good internal connectivity, a member cache MAY
1217   forward such a request within that group of caches.
1218</t>
1219</section>
1220</section>
1221
1222<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1223   
1224
1225<section title="public" anchor="cache-response-directive.public">
1226   <iref item="public (cache directive)" primary="true"/>
1227<t>
1228   The "public" response directive indicates that any cache MAY store the
1229   response, even if the response would normally be non-cacheable or cacheable
1230   only within a non-shared cache. (See <xref target="caching.authenticated.responses"/> for additional details related
1231   to the use of public in response to a request containing
1232   Authorization, and <xref target="response.cacheability"/>
1233   for details of how public affects responses that would normally not be
1234   stored, due to their status codes not being defined as cacheable.)
1235</t>
1236</section>
1237
1238<section title="private" anchor="cache-response-directive.private">
1239   <iref item="private (cache directive)" primary="true"/>
1240<t>
1241   Argument syntax:
1242   <list>
1243      <t>
1244        #<xref target="imported.abnf" format="none">field-name</xref>
1245      </t>
1246   </list>
1247</t>
1248<t>
1249   The "private" response directive indicates that the response message is
1250   intended for a single user and MUST NOT be stored by a shared cache. A
1251   private cache MAY store the response and reuse it for later requests,
1252   even if the response would normally be non-cacheable.
1253</t>
1254<t>
1255   If the private response directive specifies one or more field-names,
1256   this requirement is limited to the field-values associated with the
1257   listed response header fields. That is, a shared cache MUST NOT store
1258   the specified field-names(s), whereas it MAY store the remainder of the
1259   response message.
1260</t>
1261<t>
1262   The field-names given are not limited to the set of standard header
1263   fields defined by this specification. Field names are case-insensitive.
1264</t>
1265<t>
1266   Note: This usage of the word "private" only controls
1267   where the response can be stored; it cannot ensure the privacy of the
1268   message content. Also, private response directives with field-names are
1269   often handled by implementations as if an unqualified private directive
1270   was received; i.e., the special handling for the qualified form is not
1271   widely implemented.
1272</t>
1273<t>
1274   Note: This directive uses the quoted-string form of the argument syntax.
1275   Senders SHOULD NOT use the token form (even if quoting appears not to be
1276   needed for single-entry lists).
1277</t>
1278</section>
1279
1280<section title="no-cache" anchor="cache-response-directive.no-cache">
1281   <iref item="no-cache (cache directive)" primary="true"/>
1282<t>
1283   Argument syntax:
1284   <list>
1285      <t>
1286        #<xref target="imported.abnf" format="none">field-name</xref>
1287      </t>
1288   </list>
1289</t>
1290<t>
1291   The "no-cache" response directive indicates that the response MUST NOT
1292   be used to satisfy a subsequent request without successful validation on
1293   the origin server. This allows an origin server to prevent a cache from
1294   using it to satisfy a request without contacting it, even by caches that
1295   have been configured to send stale responses.
1296</t>
1297<t>
1298   If the no-cache response directive specifies one or more field-names,
1299   then a cache MAY use the response to satisfy a subsequent request,
1300   subject to any other restrictions on caching. However, any header fields
1301   in the response that have the field-name(s) listed MUST NOT be sent
1302   in the response to a subsequent request without successful revalidation
1303   with the origin server. This allows an origin server to prevent the
1304   re-use of certain header fields in a response, while still allowing
1305   caching of the rest of the response.
1306</t>
1307<t>
1308   The field-names given are not limited to the set of standard header
1309   fields defined by this specification. Field names are case-insensitive.
1310</t>
1311<t>
1312   Note: Many HTTP/1.0 caches will not recognize or obey
1313   this directive. Also, no-cache response directives with field-names are
1314   often handled by implementations as if an unqualified no-cache directive
1315   was received; i.e., the special handling for the qualified form is not
1316   widely implemented.
1317</t>
1318<t>
1319   Note: This directive uses the quoted-string form of the argument syntax.
1320   Senders SHOULD NOT use the token form (even if quoting appears not to be
1321   needed for single-entry lists).
1322</t>
1323</section>
1324
1325<section title="no-store" anchor="cache-response-directive.no-store">
1326   <iref item="no-store (cache directive)" primary="true"/>
1327<t>
1328   The "no-store" response directive indicates that a cache MUST NOT
1329   store any part of either the immediate request or response. This
1330   directive applies to both private and shared caches. "MUST NOT
1331   store" in this context means that the cache MUST NOT intentionally
1332   store the information in non-volatile storage, and MUST make a
1333   best-effort attempt to remove the information from volatile storage as
1334   promptly as possible after forwarding it.
1335</t>
1336<t>
1337   This directive is NOT a reliable or sufficient mechanism for ensuring
1338   privacy. In particular, malicious or compromised caches might not
1339   recognize or obey this directive, and communications networks might be
1340   vulnerable to eavesdropping.
1341</t>
1342</section>
1343
1344<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1345   <iref item="must-revalidate (cache directive)" primary="true"/>
1346<t>
1347   The "must-revalidate" response directive indicates that once it has
1348   become stale, a cache MUST NOT use the response to satisfy subsequent
1349   requests without successful validation on the origin server.
1350</t>
1351<t>
1352   The must-revalidate directive is necessary to support reliable
1353   operation for certain protocol features. In all circumstances a
1354   cache MUST obey the must-revalidate directive; in particular,
1355   if a cache cannot reach the origin server for any reason, it MUST
1356   generate a 504 (Gateway Timeout) response.
1357</t>
1358<t>
1359   The must-revalidate directive ought to be used by servers if and only
1360   if failure to validate a request on the representation could result in
1361   incorrect operation, such as a silently unexecuted financial
1362   transaction.
1363</t>
1364</section>
1365
1366<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1367   <iref item="proxy-revalidate (cache directive)" primary="true"/>
1368<t>
1369   The "proxy-revalidate" response directive has the same meaning as the
1370   must-revalidate response directive, except that it does not apply to
1371   private caches.
1372</t>
1373</section>
1374
1375<section title="max-age" anchor="cache-response-directive.max-age">
1376   <iref item="max-age (cache directive)" primary="true"/>
1377<t>
1378   Argument syntax:
1379   <list>
1380      <t>
1381        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1382      </t>
1383   </list>
1384</t>
1385<t>
1386   The "max-age" response directive indicates that the response is to be
1387   considered stale after its age is greater than the specified number of
1388   seconds.
1389</t>
1390<t>
1391   Note: This directive uses the token form of the argument syntax;
1392   e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
1393   quoted-string form.
1394</t>
1395</section>     
1396
1397<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1398   <iref item="s-maxage (cache directive)" primary="true"/>
1399<t>
1400   Argument syntax:
1401   <list>
1402      <t>
1403        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1404      </t>
1405   </list>
1406</t>
1407<t>
1408   The "s-maxage" response directive indicates that, in shared caches, the
1409   maximum age specified by this directive overrides the maximum age
1410   specified by either the max-age directive or the <xref target="header.expires" format="none">Expires</xref>
1411   header field. The s-maxage directive also implies the semantics of the
1412   proxy-revalidate response directive.
1413</t>
1414<t>
1415   Note: This directive uses the token form of the argument syntax;
1416   e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
1417   quoted-string form.
1418</t>
1419</section>
1420
1421<section title="no-transform" anchor="cache-response-directive.no-transform">
1422   <iref item="no-transform (cache directive)" primary="true"/>
1423<t>
1424   The "no-transform" response directive indicates that an intermediary
1425   (regardless of whether it implements a cache) MUST NOT transform the
1426   payload, as defined in Section 5.7.2 of <xref target="Part1"/>.
1427</t>
1428</section>
1429
1430</section>
1431
1432<section anchor="cache.control.extensions" title="Cache Control Extensions">
1433<t>
1434   The Cache-Control header field can be extended through the use of one or
1435   more cache-extension tokens, each with an optional value. Informational
1436   extensions (those that do not require a change in cache behavior) can be
1437   added without changing the semantics of other directives. Behavioral
1438   extensions are designed to work by acting as modifiers to the existing base
1439   of cache directives. Both the new directive and the standard directive are
1440   supplied, such that applications that do not understand the new directive
1441   will default to the behavior specified by the standard directive, and those
1442   that understand the new directive will recognize it as modifying the
1443   requirements associated with the standard directive. In this way,
1444   extensions to the cache-control directives can be made without requiring
1445   changes to the base protocol.
1446</t>
1447<t>
1448   This extension mechanism depends on an HTTP cache obeying all of the
1449   cache-control directives defined for its native HTTP-version, obeying
1450   certain extensions, and ignoring all directives that it does not
1451   understand.
1452</t>
1453<t>
1454   For example, consider a hypothetical new response directive called
1455   "community" that acts as a modifier to the private directive. We define
1456   this new directive to mean that, in addition to any private cache, any
1457   cache that is shared only by members of the community named within its
1458   value is allowed to cache the response. An origin server wishing to allow
1459   the UCI community to use an otherwise private response in their shared
1460   cache(s) could do so by including
1461</t>
1462<figure><artwork type="example"><![CDATA[
1463  Cache-Control: private, community="UCI"
1464]]></artwork></figure>
1465<t>
1466   A cache seeing this header field will act correctly even if the cache does
1467   not understand the community cache-extension, since it will also see and
1468   understand the private directive and thus default to the safe behavior.
1469</t>
1470<t>
1471   A cache MUST ignore unrecognized cache directives; it is assumed that any
1472   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1473   combined with standard directives (or the response's default cacheability)
1474   such that the cache behavior will remain minimally correct even if the
1475   cache does not understand the extension(s).
1476</t>
1477<t>
1478   New extension directives ought to consider defining:
1479</t>
1480<t>
1481   <list style="symbols">
1482      <t>What it means for a directive to be specified multiple times,</t>
1483      <t>When the directive does not take an argument, what it means when an
1484      argument is present,</t>
1485      <t>When the directive requires an argument, what it means when it is
1486      missing.</t>
1487   </list>
1488</t>
1489<t>
1490   The HTTP Cache Directive Registry defines the name space for the cache
1491   directives.
1492</t>
1493<t>
1494   A registration MUST include the following fields:
1495   <list style="symbols">
1496      <t>Cache Directive Name</t>
1497      <t>Pointer to specification text</t>
1498   </list>
1499</t>
1500<t>
1501   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1502</t>
1503<t>
1504   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1505</t>
1506</section>
1507
1508</section>
1509
1510<section anchor="header.expires" title="Expires">
1511   <iref item="Expires header field" primary="true"/>
1512   
1513<t>
1514   The "Expires" header field gives the date/time after which the
1515   response is considered stale. See <xref target="expiration.model"/> for
1516   further discussion of the freshness model.
1517</t>
1518<t>
1519   The presence of an Expires field does not imply that the original resource
1520   will change or cease to exist at, before, or after that time.
1521</t>
1522<t>
1523   The Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of <xref target="Part2"/>.
1524</t>
1525<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1526  Expires = HTTP-date
1527]]></artwork></figure>
1528<figure>
1529  <preamble>For example</preamble>
1530<artwork type="example"><![CDATA[
1531  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1532]]></artwork></figure>
1533<t>
1534   A cache recipient MUST interpret invalid date formats, especially the
1535   value "0", as representing a time in the past (i.e., "already expired").
1536</t>
1537<t>
1538   If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
1539   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1540   a recipient MUST ignore the Expires field.
1541   Likewise, if a response includes the s-maxage directive
1542   (<xref target="cache-response-directive.s-maxage"/>), a shared cache
1543   recipient MUST ignore the Expires field. In both these cases, the value
1544   in Expires is only intended for recipients that have not yet implemented
1545   the Cache-Control field.
1546</t>
1547<t>
1548   An origin server without a clock MUST NOT generate an Expires field
1549   unless its value represents a fixed time in the past (always expired)
1550   or its value has been associated with the resource by a system or user
1551   with a reliable clock.
1552</t>
1553<t>
1554   Historically, HTTP required the Expires field-value to be no more than a
1555   year in the future. While longer freshness lifetimes are no longer
1556   prohibited, extremely large values have been demonstrated to cause
1557   problems (e.g., clock overflows due to use of 32-bit integers for
1558   time values), and many caches will evict a response far sooner than
1559   that.
1560</t>
1561</section>
1562
1563<section anchor="header.pragma" title="Pragma">
1564   <iref item="Pragma header field" primary="true"/>
1565   
1566   
1567   
1568<t>
1569   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1570   caches, so that clients can specify a "no-cache" request that they will
1571   understand (as <xref target="header.cache-control" format="none">Cache-Control</xref> was not defined until HTTP/1.1).
1572   When the Cache-Control header field is also present and understood in a
1573   request, Pragma is ignored.
1574</t>
1575<t>
1576   In HTTP/1.0, Pragma was defined as an extensible field for
1577   implementation-specified directives for recipients. This specification
1578   deprecates such extensions to improve interoperability.
1579</t>
1580<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[
1581  Pragma           = 1#pragma-directive
1582  pragma-directive = "no-cache" / extension-pragma
1583  extension-pragma = token [ "=" ( token / quoted-string ) ]
1584]]></artwork></figure>
1585<t>
1586   When the <xref target="header.cache-control" format="none">Cache-Control</xref> header field is not present in a
1587   request, the no-cache request pragma-directive MUST have the same effect
1588   on caches as if "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
1589</t>
1590<t>
1591   When sending a no-cache request, a client ought to include both the pragma
1592   and cache-control directives, unless Cache-Control: no-cache is
1593   purposefully omitted to target other <xref target="header.cache-control" format="none">Cache-Control</xref> response
1594   directives at HTTP/1.1 caches. For example:
1595</t>
1596<figure>
1597<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1598  GET / HTTP/1.1
1599  Host: www.example.com
1600  Cache-Control: max-age=30
1601  Pragma: no-cache
1602 
1603  ]]></artwork>
1604</figure>
1605<t>
1606   will constrain HTTP/1.1 caches to serve a response no older than 30
1607   seconds, while precluding implementations that do not understand
1608   <xref target="header.cache-control" format="none">Cache-Control</xref> from serving a cached response.
1609</t>
1610<t><list>
1611   <t>
1612      Note: Because the meaning of "Pragma: no-cache" in responses is not
1613      specified, it does not provide a reliable replacement for
1614      "Cache-Control: no-cache" in them.
1615   </t>
1616</list></t>
1617</section>
1618
1619<section anchor="header.warning" title="Warning">
1620   <iref item="Warning header field" primary="true"/>
1621   
1622   
1623   
1624   
1625   
1626   
1627<t>
1628   The "Warning" header field is used to carry additional information
1629   about the status or transformation of a message that might not be reflected
1630   in the message. This information is typically used to warn about possible
1631   incorrectness introduced by caching operations or transformations applied
1632   to the payload of the message.
1633</t>
1634<t>
1635   Warnings can be used for other purposes, both cache-related and otherwise.
1636   The use of a warning, rather than an error status code, distinguishes these
1637   responses from true failures.
1638</t>
1639<t>
1640   Warning header fields can in general be applied to any message, however some
1641   warn-codes are specific to caches and can only be applied to response
1642   messages.
1643</t>
1644<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[
1645  Warning       = 1#warning-value
1646 
1647  warning-value = warn-code SP warn-agent SP warn-text
1648                                        [SP warn-date]
1649 
1650  warn-code  = 3DIGIT
1651  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1652                  ; the name or pseudonym of the server adding
1653                  ; the Warning header field, for use in debugging
1654  warn-text  = quoted-string
1655  warn-date  = DQUOTE HTTP-date DQUOTE
1656]]></artwork></figure>
1657<t>
1658   Multiple warnings can be attached to a response (either by the origin
1659   server or by a cache), including multiple warnings with the same code
1660   number, only differing in warn-text.
1661</t>
1662<t>
1663   When this occurs, the user agent SHOULD inform the user of as many of
1664   them as possible, in the order that they appear in the response.
1665</t>
1666<t>
1667   Systems that generate multiple Warning header fields are encouraged to
1668   order them with this user agent behavior in mind. New Warning header fields
1669   are added after any existing Warning header fields.
1670</t>
1671<t>
1672   Warnings are assigned three digit warn-codes. The first digit indicates
1673   whether the Warning is required to be deleted from a stored response after
1674   validation:
1675   <list style="symbols">
1676      <t>1xx Warnings describe the freshness or validation status of the
1677      response, and so MUST be deleted by a cache after validation. They can
1678      only be generated by a cache when validating a cached entry, and
1679      MUST NOT be generated in any other situation.</t>
1680      <t>2xx Warnings describe some aspect of the representation that is not
1681      rectified by a validation (for example, a lossy compression of the
1682      representation) and MUST NOT be deleted by a cache after validation,
1683      unless a full response is sent, in which case they MUST be.</t>
1684   </list>
1685</t>
1686<t>
1687   If an implementation sends a message with one or more Warning header fields
1688   to a receiver whose version is HTTP/1.0 or lower, then the sender MUST
1689   include in each warning-value a warn-date that matches the
1690   Date header field in the message.
1691</t>
1692<t>
1693   If a system receives a message with a warning-value that includes a
1694   warn-date, and that warn-date is different from the Date
1695   value in the response, then that warning-value MUST be deleted from the
1696   message before storing, forwarding, or using it. (preventing the
1697   consequences of naive caching of Warning header fields.) If all of the
1698   warning-values are deleted for this reason, the Warning header field MUST
1699   be deleted as well.
1700</t>
1701<t>
1702   The following warn-codes are defined by this specification, each with a
1703   recommended warn-text in English, and a description of its meaning.
1704</t>
1705
1706<section title="110 Response is Stale" anchor="warn.110">
1707  <iref primary="true" item="110 Response is Stale (warn code)"/>
1708<t>
1709   A cache SHOULD generate this whenever the sent response is stale.
1710</t>
1711</section>
1712
1713<section title="111 Revalidation Failed" anchor="warn.111">
1714  <iref primary="true" item="111 Revalidation Failed (warn code)"/>
1715<t>
1716   A cache SHOULD generate this when sending a stale response because an
1717   attempt to validate the response failed, due to an inability to reach
1718   the server.
1719</t>
1720</section>
1721
1722<section title="112 Disconnected Operation" anchor="warn.112">
1723  <iref primary="true" item="112 Disconnected Operation (warn code)"/>
1724<t>
1725   A cache SHOULD generate this if it is intentionally disconnected from
1726   the rest of the network for a period of time.
1727</t>
1728</section>
1729
1730<section title="113 Heuristic Expiration" anchor="warn.113">
1731  <iref primary="true" item="113 Heuristic Expiration (warn code)"/>
1732<t>
1733   A cache SHOULD generate this if it heuristically chose a freshness
1734   lifetime greater than 24 hours and the response's age is greater than 24
1735   hours.
1736</t>
1737</section>
1738
1739<section title="199 Miscellaneous Warning" anchor="warn.199">
1740  <iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
1741<t>
1742   The warning text can include arbitrary information to be presented to
1743   a human user, or logged. A system receiving this warning MUST NOT take
1744   any automated action, besides presenting the warning to the user.
1745</t>
1746</section>
1747
1748<section title="214 Transformation Applied" anchor="warn.214">
1749  <iref primary="true" item="214 Transformation Applied (warn code)"/>
1750<t>
1751   MUST be added by a proxy if it applies any transformation to the
1752   representation, such as changing the content-coding, media-type, or
1753   modifying the representation data, unless this Warning code already appears
1754   in the response.
1755</t>
1756</section>
1757
1758<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1759  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)"/>
1760<t>
1761   The warning text can include arbitrary information to be presented to
1762   a human user, or logged. A system receiving this warning MUST NOT take
1763   any automated action.
1764</t>
1765</section>
1766
1767<section title="Warn Code Extensions" anchor="warn.code.extensions">
1768<t>
1769   The HTTP Warn Code Registry defines the name space for warn codes.
1770</t>
1771<t>
1772   A registration MUST include the following fields:
1773   <list style="symbols">
1774      <t>Warn Code (3 digits)</t>
1775      <t>Short Description</t>
1776      <t>Pointer to specification text</t>
1777   </list>
1778</t>
1779<t>
1780   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1781</t>
1782<t>
1783   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
1784</t>
1785</section>
1786</section>
1787</section>
1788
1789<section anchor="history.lists" title="History Lists">
1790<t>
1791   User agents often have history mechanisms, such as "Back" buttons and
1792   history lists, that can be used to redisplay a representation retrieved
1793   earlier in a session.
1794</t>
1795<t>
1796   The freshness model (<xref target="expiration.model"/>) does not
1797   necessarily apply to history mechanisms. I.e., a history mechanism can
1798   display a previous representation even if it has expired.
1799</t>
1800<t>
1801   This does not prohibit the history mechanism from telling the user that a
1802   view might be stale, or from honoring cache directives (e.g.,
1803   Cache-Control: no-store).
1804</t>
1805</section>
1806
1807
1808<section anchor="IANA.considerations" title="IANA Considerations">
1809
1810<section title="Cache Directive Registry" anchor="cache.directive.registration">
1811<t>
1812   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1813</t>
1814<t>
1815   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1816   populated with the registrations below:
1817</t>
1818
1819<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1820<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1821   <ttcol>Cache Directive</ttcol>
1822   <ttcol>Reference</ttcol>
1823
1824   <c>max-age</c>
1825   <c>
1826      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1827   </c>
1828   <c>max-stale</c>
1829   <c>
1830      <xref target="cache-request-directive.max-stale"/>
1831   </c>
1832   <c>min-fresh</c>
1833   <c>
1834      <xref target="cache-request-directive.min-fresh"/>
1835   </c>
1836   <c>must-revalidate</c>
1837   <c>
1838      <xref target="cache-response-directive.must-revalidate"/>
1839   </c>
1840   <c>no-cache</c>
1841   <c>
1842      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1843   </c>
1844   <c>no-store</c>
1845   <c>
1846      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1847   </c>
1848   <c>no-transform</c>
1849   <c>
1850      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1851   </c>
1852   <c>only-if-cached</c>
1853   <c>
1854      <xref target="cache-request-directive.only-if-cached"/>
1855   </c>
1856   <c>private</c>
1857   <c>
1858      <xref target="cache-response-directive.private"/>
1859   </c>
1860   <c>proxy-revalidate</c>
1861   <c>
1862      <xref target="cache-response-directive.proxy-revalidate"/>
1863   </c>
1864   <c>public</c>
1865   <c>
1866      <xref target="cache-response-directive.public"/>
1867   </c>
1868   <c>s-maxage</c>
1869   <c>
1870      <xref target="cache-response-directive.s-maxage"/>
1871   </c>
1872   <c>stale-if-error</c>
1873   <c>
1874      <xref target="RFC5861"/>, Section 4
1875   </c>
1876   <c>stale-while-revalidate</c>
1877   <c>
1878      <xref target="RFC5861"/>, Section 3
1879   </c>
1880</texttable>
1881<!--(END)-->
1882
1883</section>
1884
1885<section title="Warn Code Registry" anchor="warn.code.registration">
1886<t>
1887   The registration procedure for HTTP Warn Codes is defined by <xref target="warn.code.extensions"/> of this document.
1888</t>
1889<t>
1890   The HTTP Warn Code Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1891   populated with the registrations below:
1892</t>
1893
1894<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1895<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1896   <ttcol>Warn Code</ttcol>
1897   <ttcol>Short Description</ttcol>
1898   <ttcol>Reference</ttcol>
1899   <c>110</c>
1900   <c>Response is Stale</c>
1901   <c>
1902      <xref target="warn.110"/>
1903   </c>
1904   <c>111</c>
1905   <c>Revalidation Failed</c>
1906   <c>
1907      <xref target="warn.111"/>
1908   </c>
1909   <c>112</c>
1910   <c>Disconnected Operation</c>
1911   <c>
1912      <xref target="warn.112"/>
1913   </c>
1914   <c>113</c>
1915   <c>Heuristic Expiration</c>
1916   <c>
1917      <xref target="warn.113"/>
1918   </c>
1919   <c>199</c>
1920   <c>Miscellaneous Warning</c>
1921   <c>
1922      <xref target="warn.199"/>
1923   </c>
1924   <c>214</c>
1925   <c>Transformation Applied</c>
1926   <c>
1927      <xref target="warn.214"/>
1928   </c>
1929   <c>299</c>
1930   <c>Miscellaneous Persistent Warning</c>
1931   <c>
1932      <xref target="warn.299"/>
1933   </c>
1934</texttable>
1935<!--(END)-->
1936
1937</section>
1938
1939<section title="Header Field Registration" anchor="header.field.registration">
1940<t>
1941  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1942  shall be updated with the permanent registrations below (see <xref target="BCP90"/>):
1943</t>
1944
1945<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1946<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1947   <ttcol>Header Field Name</ttcol>
1948   <ttcol>Protocol</ttcol>
1949   <ttcol>Status</ttcol>
1950   <ttcol>Reference</ttcol>
1951
1952   <c>Age</c>
1953   <c>http</c>
1954   <c>standard</c>
1955   <c>
1956      <xref target="header.age"/>
1957   </c>
1958   <c>Cache-Control</c>
1959   <c>http</c>
1960   <c>standard</c>
1961   <c>
1962      <xref target="header.cache-control"/>
1963   </c>
1964   <c>Expires</c>
1965   <c>http</c>
1966   <c>standard</c>
1967   <c>
1968      <xref target="header.expires"/>
1969   </c>
1970   <c>Pragma</c>
1971   <c>http</c>
1972   <c>standard</c>
1973   <c>
1974      <xref target="header.pragma"/>
1975   </c>
1976   <c>Warning</c>
1977   <c>http</c>
1978   <c>standard</c>
1979   <c>
1980      <xref target="header.warning"/>
1981   </c>
1982</texttable>
1983<!--(END)-->
1984
1985<t>
1986   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1987   Force".
1988</t>
1989</section>
1990</section>
1991
1992<section anchor="security.considerations" title="Security Considerations">
1993<t>
1994   This section is meant to inform developers, information providers, and
1995   users of known security concerns specific to HTTP/1.1 caching.
1996   More general security considerations are addressed in HTTP messaging
1997   <xref target="Part1"/> and semantics <xref target="Part2"/>.
1998</t>
1999<t>
2000   Caches expose additional potential vulnerabilities, since the contents of
2001   the cache represent an attractive target for malicious exploitation.
2002   Because cache contents persist after an HTTP request is complete, an attack
2003   on the cache can reveal information long after a user believes that the
2004   information has been removed from the network. Therefore, cache contents
2005   need to be protected as sensitive information.
2006</t>
2007<t>
2008   Furthermore, the very use of a cache can bring about privacy concerns. For
2009   example, if two users share a cache, and the first one browses to a site,
2010   the second may be able to detect that the other has been to that site,
2011   because the resources from it load more quickly, thanks to the cache.
2012</t>
2013<t>
2014   Implementation flaws might allow attackers to insert content into a cache
2015   ("cache poisoning"), leading to compromise of clients that trust that
2016   content. Because of their nature, these attacks are difficult to mitigate.
2017</t>
2018<t>
2019   Likewise, implementation flaws (as well as misunderstanding of cache
2020   operation) might lead to caching of sensitive information (e.g.,
2021   authentication credentials) that is thought to be private, exposing it to
2022   unauthorized parties.
2023</t>
2024<t>
2025   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2026   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2027   often is) used to satisfy subsequent requests to caches. Servers who wish
2028   to control caching of these responses are encouraged to emit appropriate
2029   Cache-Control response headers.
2030</t>
2031
2032</section>
2033
2034<section title="Acknowledgments" anchor="acks">
2035<t>
2036  See Section 9 of <xref target="Part1"/>.
2037</t>
2038</section>
2039
2040</middle>
2041
2042<back>
2043<references title="Normative References">
2044
2045  <reference anchor="Part1">
2046    <front>
2047      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2048      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2049        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2050        <address><email>fielding@gbiv.com</email></address>
2051      </author>
2052      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2053        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2054        <address><email>julian.reschke@greenbytes.de</email></address>
2055      </author>
2056      <date month="February" year="2013"/>
2057    </front>
2058    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-22"/>
2059   
2060  </reference>
2061
2062  <reference anchor="Part2">
2063    <front>
2064      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2065      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2066        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2067        <address><email>fielding@gbiv.com</email></address>
2068      </author>
2069      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2070        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2071        <address><email>julian.reschke@greenbytes.de</email></address>
2072      </author>
2073      <date month="February" year="2013"/>
2074    </front>
2075    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-22"/>
2076   
2077  </reference>
2078
2079  <reference anchor="Part4">
2080    <front>
2081      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2082      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2083        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2084        <address><email>fielding@gbiv.com</email></address>
2085      </author>
2086      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2087        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2088        <address><email>julian.reschke@greenbytes.de</email></address>
2089      </author>
2090      <date month="February" year="2013"/>
2091    </front>
2092    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-22"/>
2093   
2094  </reference>
2095
2096  <reference anchor="Part5">
2097    <front>
2098      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2099      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2100        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2101        <address><email>fielding@gbiv.com</email></address>
2102      </author>
2103      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2104        <organization abbrev="W3C">World Wide Web Consortium</organization>
2105        <address><email>ylafon@w3.org</email></address>
2106      </author>
2107      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2108        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2109        <address><email>julian.reschke@greenbytes.de</email></address>
2110      </author>
2111      <date month="February" year="2013"/>
2112    </front>
2113    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-22"/>
2114   
2115  </reference>
2116
2117  <reference anchor="Part7">
2118    <front>
2119      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2120      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2121        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2122        <address><email>fielding@gbiv.com</email></address>
2123      </author>
2124      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2125        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2126        <address><email>julian.reschke@greenbytes.de</email></address>
2127      </author>
2128      <date month="February" year="2013"/>
2129    </front>
2130    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-22"/>
2131   
2132  </reference>
2133
2134  <reference anchor="RFC2119">
2135    <front>
2136      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2137      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2138        <organization>Harvard University</organization>
2139        <address><email>sob@harvard.edu</email></address>
2140      </author>
2141      <date month="March" year="1997"/>
2142    </front>
2143    <seriesInfo name="BCP" value="14"/>
2144    <seriesInfo name="RFC" value="2119"/>
2145  </reference>
2146
2147  <reference anchor="RFC5234">
2148    <front>
2149      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2150      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2151        <organization>Brandenburg InternetWorking</organization>
2152        <address>
2153          <email>dcrocker@bbiw.net</email>
2154        </address> 
2155      </author>
2156      <author initials="P." surname="Overell" fullname="Paul Overell">
2157        <organization>THUS plc.</organization>
2158        <address>
2159          <email>paul.overell@thus.net</email>
2160        </address>
2161      </author>
2162      <date month="January" year="2008"/>
2163    </front>
2164    <seriesInfo name="STD" value="68"/>
2165    <seriesInfo name="RFC" value="5234"/>
2166  </reference>
2167 
2168</references>
2169
2170<references title="Informative References">
2171
2172  <reference anchor="RFC1305">
2173    <front>
2174      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2175      <author fullname="David L. Mills" initials="D." surname="Mills">
2176        <organization>University of Delaware, Electrical Engineering Department</organization>
2177        <address><email>mills@udel.edu</email></address>
2178      </author>
2179      <date month="March" year="1992"/>
2180    </front>
2181    <seriesInfo name="RFC" value="1305"/>
2182  </reference>
2183
2184  <reference anchor="RFC2616">
2185    <front>
2186      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2187      <author fullname="R. Fielding" initials="R." surname="Fielding">
2188        <organization>University of California, Irvine</organization>
2189        <address><email>fielding@ics.uci.edu</email></address>
2190      </author>
2191      <author fullname="J. Gettys" initials="J." surname="Gettys">
2192        <organization>W3C</organization>
2193        <address><email>jg@w3.org</email></address>
2194      </author>
2195      <author fullname="J. Mogul" initials="J." surname="Mogul">
2196        <organization>Compaq Computer Corporation</organization>
2197        <address><email>mogul@wrl.dec.com</email></address>
2198      </author>
2199      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2200        <organization>MIT Laboratory for Computer Science</organization>
2201        <address><email>frystyk@w3.org</email></address>
2202      </author>
2203      <author fullname="L. Masinter" initials="L." surname="Masinter">
2204        <organization>Xerox Corporation</organization>
2205        <address><email>masinter@parc.xerox.com</email></address>
2206      </author>
2207      <author fullname="P. Leach" initials="P." surname="Leach">
2208        <organization>Microsoft Corporation</organization>
2209        <address><email>paulle@microsoft.com</email></address>
2210      </author>
2211      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2212        <organization>W3C</organization>
2213        <address><email>timbl@w3.org</email></address>
2214      </author>
2215      <date month="June" year="1999"/>
2216    </front>
2217    <seriesInfo name="RFC" value="2616"/>
2218  </reference>
2219
2220  <reference anchor="BCP90">
2221    <front>
2222      <title>Registration Procedures for Message Header Fields</title>
2223      <author fullname="G. Klyne" initials="G." surname="Klyne">
2224        <organization>Nine by Nine</organization>
2225        <address><email>GK-IETF@ninebynine.org</email></address>
2226      </author>
2227      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2228        <organization>BEA Systems</organization>
2229        <address><email>mnot@pobox.com</email></address>
2230      </author>
2231      <author fullname="J. Mogul" initials="J." surname="Mogul">
2232        <organization>HP Labs</organization>
2233        <address><email>JeffMogul@acm.org</email></address>
2234      </author>
2235      <date month="September" year="2004"/>
2236    </front>
2237    <seriesInfo name="BCP" value="90"/>
2238    <seriesInfo name="RFC" value="3864"/>
2239  </reference>
2240
2241  <reference anchor="RFC5226">
2242    <front>
2243      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2244      <author initials="T." surname="Narten" fullname="T. Narten">
2245        <organization>IBM</organization>
2246        <address><email>narten@us.ibm.com</email></address>
2247      </author>
2248      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2249        <organization>Google</organization>
2250        <address><email>Harald@Alvestrand.no</email></address>
2251      </author>
2252      <date year="2008" month="May"/>
2253    </front>
2254    <seriesInfo name="BCP" value="26"/>
2255    <seriesInfo name="RFC" value="5226"/>
2256  </reference>
2257
2258  <reference anchor="RFC5861">
2259    <front>
2260      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2261      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2262        <organization>Yahoo! Inc.</organization>
2263        <address><email>mnot@yahoo-inc.com</email></address>
2264      </author>
2265      <date month="April" year="2010"/>
2266    </front>
2267    <seriesInfo name="RFC" value="5861"/>
2268  </reference>
2269
2270  <reference anchor="RFC6265">
2271    <front>
2272      <title>HTTP State Management Mechanism</title>
2273      <author initials="A." surname="Barth" fullname="Adam Barth">
2274        <organization abbrev="U.C. Berkeley">
2275          University of California, Berkeley
2276        </organization>
2277        <address><email>abarth@eecs.berkeley.edu</email></address>
2278      </author>
2279      <date year="2011" month="April"/>
2280    </front>
2281    <seriesInfo name="RFC" value="6265"/>
2282  </reference>
2283
2284</references>
2285
2286<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2287<t>
2288  Caching-related text has been substantially rewritten for clarity.
2289</t>
2290<t>
2291  The algorithm for calculating age is now less conservative.
2292  (<xref target="age.calculations"/>)
2293</t>
2294<t>
2295  Caches are now required to handle dates with timezones as if they're
2296  invalid, because it's not possible to accurately guess.
2297  (<xref target="age.calculations"/>)
2298</t>
2299<t>
2300  The Content-Location response header field is no longer used
2301  to determine the appropriate response to use when validating.
2302  (<xref target="validation.model"/>)
2303</t>
2304<t>
2305  The algorithm for selecting a cached negotiated response to use has been
2306  clarified in several ways. In particular, it now explicitly allows
2307  header-specific canonicalization when processing selecting header fields.
2308  (<xref target="caching.negotiated.responses"/>)
2309</t>
2310<t>
2311  Requirements regarding denial of service attack avoidance when performing
2312  invalidation have been clarified.
2313  (<xref target="invalidation.after.updates.or.deletions"/>)
2314</t>
2315<t>
2316  Cache invalidation only occurs when a successful response is received.
2317  (<xref target="invalidation.after.updates.or.deletions"/>)
2318</t>
2319<t>
2320  The conditions under which an authenticated response can be cached have been
2321  clarified.
2322  (<xref target="caching.authenticated.responses"/>)
2323</t>
2324<t>
2325  The one-year limit on Expires header field values has been removed; instead,
2326  the reasoning for using a sensible value is given.
2327  (<xref target="header.expires"/>)
2328</t>
2329<t>
2330  The Pragma header field is now only defined for backwards compatibility;
2331  future pragmas are deprecated.
2332  (<xref target="header.pragma"/>)
2333</t>
2334<t>
2335  Cache directives are explicitly defined to be case-insensitive.
2336  (<xref target="header.cache-control"/>)
2337</t>
2338<t>
2339  Handling of multiple instances of cache directives when only one is
2340  expected is now defined.
2341  (<xref target="header.cache-control"/>)
2342</t>
2343<t>
2344  The qualified forms of the private and no-cache cache directives are noted
2345  to not be widely implemented; e.g., "private=foo" is interpreted by many
2346  caches as simply "private". Additionally, the meaning of the qualified
2347  form of no-cache has been clarified.
2348  (<xref target="cache-response-directive"/>)
2349</t>
2350<t>
2351  The "no-store" cache request directive doesn't apply to responses; i.e.,
2352  a cache can satisfy a request with no-store on it, and does not invalidate
2353  it.
2354  (<xref target="cache-request-directive.no-store"/>)
2355</t>
2356<t>
2357  The "no-cache" response cache directive's meaning has been clarified.
2358  (<xref target="cache-response-directive.no-cache"/>)
2359</t>
2360<t>
2361  New status codes can now define that caches are allowed to use heuristic
2362  freshness with them.
2363  (<xref target="heuristic.freshness"/>)
2364</t>
2365<t>
2366  Caches are now allow to calculate heuristic freshness for URLs with query
2367  components.
2368  (<xref target="heuristic.freshness"/>)
2369</t>
2370<t>
2371  Some requirements regarding production of the <xref target="header.warning" format="none">Warning</xref>
2372  header have been relaxed, as it is not widely implemented.
2373  (<xref target="header.warning"/>)
2374</t>
2375<t>
2376  The <xref target="header.warning" format="none">Warning</xref> header field no longer uses RFC 2047 encoding,
2377  nor allows multiple languages, as these aspects were not implemented.
2378  (<xref target="header.warning"/>)
2379</t>
2380<t>
2381  This specification introduces the Cache Directive and Warn Code Registries,
2382  and defines considerations for new cache directives.
2383  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2384</t>
2385</section>
2386
2387<section title="Imported ABNF" anchor="imported.abnf">
2388   
2389   
2390   
2391   
2392   
2393   
2394   
2395   
2396   
2397   
2398   
2399   
2400   
2401   
2402   
2403   
2404<t>
2405   The following core rules are included by reference, as defined in Appendix B.1 of <xref target="RFC5234"/>: ALPHA (letters), CR (carriage
2406   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2407   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2408   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2409</t>
2410<t>
2411   The rules below are defined in <xref target="Part1"/>:
2412</t>
2413<figure><artwork type="abnf2616"><![CDATA[
2414  OWS           = <OWS, defined in [Part1], Section 3.2.3>
2415  field-name    = <field-name, defined in [Part1], Section 3.2>
2416  quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2417  token         = <token, defined in [Part1], Section 3.2.6>
2418
2419  port          = <port, defined in [Part1], Section 2.7>
2420  pseudonym     = <pseudonym, defined in [Part1], Section 5.7.1>
2421  uri-host      = <uri-host, defined in [Part1], Section 2.7>
2422]]></artwork></figure>
2423<t>
2424   The rules below are defined in other parts:
2425</t>
2426<figure><artwork type="abnf2616"><![CDATA[
2427  HTTP-date     = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2428]]></artwork></figure>
2429</section>
2430
2431
2432<section title="Collected ABNF" anchor="collected.abnf">
2433<figure>
2434<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2435Age = delta-seconds
2436
2437Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2438 cache-directive ] )
2439
2440Expires = HTTP-date
2441
2442HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2443
2444OWS = <OWS, defined in [Part1], Section 3.2.3>
2445
2446Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2447 pragma-directive ] )
2448
2449Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2450 )
2451
2452cache-directive = token [ "=" ( token / quoted-string ) ]
2453
2454delta-seconds = 1*DIGIT
2455
2456extension-pragma = token [ "=" ( token / quoted-string ) ]
2457
2458field-name = <field-name, defined in [Part1], Section 3.2>
2459
2460port = <port, defined in [Part1], Section 2.7>
2461pragma-directive = "no-cache" / extension-pragma
2462pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
2463
2464quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2465
2466token = <token, defined in [Part1], Section 3.2.6>
2467
2468uri-host = <uri-host, defined in [Part1], Section 2.7>
2469
2470warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2471warn-code = 3DIGIT
2472warn-date = DQUOTE HTTP-date DQUOTE
2473warn-text = quoted-string
2474warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2475 ]
2476]]></artwork>
2477</figure>
2478</section>
2479
2480
2481<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2482<t>
2483  Changes up to the first Working Group Last Call draft are summarized
2484  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2485</t>
2486
2487<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2488<t>
2489  Closed issues:
2490  <list style="symbols">
2491    <t>
2492      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2493      "untangle Cache-Control ABNF"
2494    </t>
2495    <t>
2496      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2497      "Multiple values in Cache-Control header fields"
2498    </t>
2499    <t>
2500      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2501      "Case sensitivity of header fields in CC values"
2502    </t>
2503    <t>
2504      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2505      "Spurious 'MAYs'"
2506    </t>
2507    <t>
2508      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2509      "enhance considerations for new cache control directives"
2510    </t>
2511    <t>
2512      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2513      "ABNF requirements for recipients"
2514    </t>
2515    <t>
2516      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2517      "note introduction of new IANA registries as normative changes"
2518    </t>
2519    <t>
2520      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2521      "broken prose in description of 'Vary'"
2522    </t>
2523  </list>
2524</t>
2525</section>
2526
2527<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2528<t>
2529  Closed issues:
2530  <list style="symbols">
2531    <t>
2532      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2533      "'Most Conservative'"
2534    </t>
2535  </list>
2536</t>
2537<t>
2538  Other changes:
2539  <list style="symbols">
2540    <t>
2541      Conformance criteria and considerations regarding error handling are
2542      now defined in Part 1.
2543    </t>
2544    <t>
2545      Move definition of "Vary" header field into Part 2.
2546    </t>
2547    <t>
2548      Add security considerations with respect to cache poisoning and
2549      the "Set-Cookie" header field.
2550    </t>
2551  </list>
2552</t>
2553</section>
2554
2555<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2556<t>
2557  Closed issues:
2558  <list style="symbols">
2559    <t>
2560      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2561      "Allowing heuristic caching for new status codes"
2562    </t>
2563    <t>
2564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
2565      "304 without validator"
2566    </t>
2567    <t>
2568      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2569      "No-Transform"
2570    </t>
2571    <t>
2572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
2573      "Revert prior change to the meaning of the public cache response
2574       directive.
2575    </t>
2576  </list>
2577</t>
2578</section>
2579
2580<!--<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
2581<t>
2582  None yet.
2583</t>
2584</section>-->
2585</section>
2586  </back>
2587</rfc>
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