source: draft-ietf-httpbis/latest/p6-cache.xml @ 2241

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "May">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY architecture                "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY conformance                 "<xref target='Part1' x:rel='#conformance' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-vary                 "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY GET                         "<xref target='Part2' x:rel='#GET' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY transformations             "<xref target='Part1' x:rel='#message.transformations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48]>
49<?rfc toc="yes" ?>
50<?rfc symrefs="yes" ?>
51<?rfc sortrefs="yes" ?>
52<?rfc compact="yes"?>
53<?rfc subcompact="no" ?>
54<?rfc linkmailto="no" ?>
55<?rfc editing="no" ?>
56<?rfc comments="yes"?>
57<?rfc inline="yes"?>
58<?rfc rfcedstyle="yes"?>
59<?rfc-ext allow-markup-in-artwork="yes" ?>
60<?rfc-ext include-references-in-index="yes" ?>
61<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
62  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
63<x:link rel="prev" basename="p5-range"/>
64<x:link rel="next" basename="p7-auth"/>
65<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
66<front>
67
68  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
69
70  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
71    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
72    <address>
73      <postal>
74        <street>345 Park Ave</street>
75        <city>San Jose</city>
76        <region>CA</region>
77        <code>95110</code>
78        <country>USA</country>
79      </postal>
80      <email>fielding@gbiv.com</email>
81      <uri>http://roy.gbiv.com/</uri>
82    </address>
83  </author>
84
85  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
86    <organization>Akamai</organization>
87    <address>
88      <email>mnot@mnot.net</email>
89      <uri>http://www.mnot.net/</uri>
90    </address>
91  </author>
92
93  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
94    <organization abbrev="greenbytes">greenbytes GmbH</organization>
95    <address>
96      <postal>
97        <street>Hafenweg 16</street>
98        <city>Muenster</city><region>NW</region><code>48155</code>
99        <country>Germany</country>
100      </postal>
101      <email>julian.reschke@greenbytes.de</email>
102      <uri>http://greenbytes.de/tech/webdav/</uri>
103    </address>
104  </author>
105
106  <date month="&ID-MONTH;" year="&ID-YEAR;" />
107  <workgroup>HTTPbis Working Group</workgroup>
108
109<abstract>
110<t>
111   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
112   distributed, collaborative, hypertext information systems. This document
113   defines requirements on HTTP caches and the associated header fields
114   that control cache behavior or indicate cacheable response messages.
115</t>
116</abstract>
117
118<note title="Editorial Note (To be removed by RFC Editor)">
119  <t>
120    Discussion of this draft takes place on the HTTPBIS working group
121    mailing list (ietf-http-wg@w3.org), which is archived at
122    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
123  </t>
124  <t>
125    The current issues list is at
126    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
127    documents (including fancy diffs) can be found at
128    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
129  </t>
130  <t>
131    The changes in this draft are summarized in <xref target="changes.since.22"/>.
132  </t>
133</note>
134
135   </front>
136   <middle>
137
138<section anchor="caching" title="Introduction">
139<t>
140   HTTP is typically used for distributed information systems, where
141   performance can be improved by the use of response caches. This document
142   defines aspects of HTTP/1.1 related to caching and reusing response
143   messages.
144</t>
145
146<iref item="cache" />
147<t>
148   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
149   subsystem that controls storage, retrieval, and deletion of messages in it.
150   A cache stores cacheable responses in order to reduce the response time and
151   network bandwidth consumption on future, equivalent requests. Any client or
152   server &MAY; employ a cache, though a cache cannot be used by a server that
153   is acting as a tunnel.
154</t>
155<iref item="shared cache" />
156<iref item="private cache" />
157<t anchor="shared.and.non-shared.caches">
158   A <x:dfn>shared cache</x:dfn> is a cache that stores responses to be reused
159   by more than one user; shared caches are usually (but not always) deployed
160   as a part of an intermediary. A <x:dfn>private cache</x:dfn>, in contrast,
161   is dedicated to a single user.
162</t>
163<t>
164   The goal of caching in HTTP/1.1 is to significantly improve performance
165   by reusing a prior response message to satisfy a current request.
166   A stored response is considered "fresh", as defined in
167   <xref target="expiration.model" />, if the response can be reused without
168   "validation" (checking with the origin server to see if the cached response
169   remains valid for this request).  A fresh response can therefore
170   reduce both latency and network overhead each time it is reused.
171   When a cached response is not fresh, it might still be reusable if it can
172   be freshened by validation (<xref target="validation.model" />) or if the
173   origin is unavailable (<xref target="serving.stale.responses" />).
174</t>
175
176<section title="Conformance and Error Handling" anchor="conformance">
177<t>
178   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
179   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
180   document are to be interpreted as described in <xref target="RFC2119"/>.
181</t>
182<t>
183   Conformance criteria and considerations regarding error handling
184   are defined in &conformance;.
185</t>
186</section>
187
188<section title="Syntax Notation" anchor="notation">
189<t>
190   This specification uses the Augmented Backus-Naur Form (ABNF) notation
191   of <xref target="RFC5234"/> with the list rule extension defined in
192   &notation;. <xref target="imported.abnf"/> describes rules imported from
193   other documents. <xref target="collected.abnf"/> shows the collected ABNF
194   with the list rule expanded.
195</t>
196
197<section title="Delta Seconds" anchor="delta-seconds">
198<t>
199   The delta-seconds rule specifies a non-negative integer, representing time
200   in seconds.
201</t>
202<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
203  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
204</artwork></figure>
205<t>
206   If an implementation receives a delta-seconds value larger than the largest
207   positive integer it can represent, or if any of its subsequent calculations
208   overflows, it &MUST; consider the value to be 2147483648
209   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
210   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
211   send delta-seconds with a value greater than 2147483648.
212</t>
213</section>
214
215</section>
216</section>
217
218<section anchor="caching.overview" title="Overview of Cache Operation">
219<iref item="cache entry" />
220<iref item="cache key" />
221<t>
222   Proper cache operation preserves the semantics of HTTP transfers
223   (&semantics;) while eliminating the transfer of information already held
224   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
225   we assume that reusing the cached response is desirable and that such
226   reuse is the default behavior when no requirement or local
227   configuration prevents it.  Therefore, HTTP cache requirements are focused
228   on preventing a cache from either storing a non-reusable response or
229   reusing a stored response inappropriately, rather than mandating that
230   caches always store and reuse particular responses.
231</t>
232<t>
233   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
234   HTTP responses corresponding to prior requests that used the same key. The
235   most common form of cache entry is a successful result of a retrieval
236   request: i.e., a <x:ref>200 (OK)</x:ref> response to a GET request, which
237   contains a representation of the resource identified by the request target
238   (&GET;). However, it is also possible to cache permanent redirects,
239   negative results (e.g., <x:ref>404 (Not Found)</x:ref>),
240   incomplete results (e.g., <x:ref>206 (Partial Content)</x:ref>), and
241   responses to methods other than GET if the method's definition allows such
242   caching and defines something suitable for use as a cache key.
243</t>
244<iref item="cache key" />
245<t>
246   The primary <x:dfn>cache key</x:dfn> consists of the request method and
247   target URI.  However, since HTTP caches in common use today are typically
248   limited to caching responses to GET, many implementations simply decline
249   other methods and use only the URI as the primary cache key.
250</t>
251<t>
252   If a request target is subject to content negotiation, its cache entry
253   might consist of multiple stored responses, each differentiated by a
254   secondary key for the values of the original request's selecting header
255   fields (<xref target="caching.negotiated.responses"/>).
256</t>
257</section>
258
259<section anchor="response.cacheability" title="Storing Responses in Caches">
260<t>
261   A cache &MUST-NOT; store a response to any request, unless:
262   <list style="symbols">
263      <t>The request method is understood by the cache and defined as being
264      cacheable, and</t>
265      <t>the response status code is understood by the cache, and</t>
266      <t>the "no-store" cache directive (see <xref
267      target="header.cache-control" />) does not appear in request or response
268      header fields, and</t>
269      <t>the "private" cache response directive (see <xref
270      target="cache-response-directive.private" />) does not appear in the
271      response, if the cache is shared, and</t>
272      <t>the <x:ref>Authorization</x:ref> header field (see
273      &header-authorization;) does not appear in the request, if the cache is
274      shared, unless the response explicitly allows it (see <xref
275      target="caching.authenticated.responses" />), and</t>
276      <t>the response either:
277         <list style="symbols">
278            <t>contains an <x:ref>Expires</x:ref> header field (see
279            <xref target="header.expires"/>), or</t>
280            <t>contains a max-age response cache directive (see <xref
281            target="cache-response-directive.max-age" />), or</t>
282            <t>contains a s-maxage response cache directive (see <xref
283            target="cache-response-directive.s-maxage" />) and the cache is
284            shared, or</t>
285            <t>contains a Cache Control Extension (see <xref
286            target="cache.control.extensions" />) that allows it to be cached,
287            or</t>
288            <t>has a status code that is defined as cacheable
289            (see <xref target="heuristic.freshness" />), or</t>
290            <t>contains a public response cache directive (see <xref
291            target="cache-response-directive.public"/>).</t>
292         </list>
293      </t>
294   </list>
295</t>
296<t>
297   Note that any of the requirements listed above can be overridden by a
298   cache-control extension; see <xref target="cache.control.extensions" />.
299</t>
300<t>
301   In this context, a cache has "understood" a request method or a response
302   status code if it recognizes it and implements all specified
303   caching-related behavior.
304</t>
305<t>
306   Note that, in normal operation, some caches will not store a response that
307   has neither a cache validator nor an explicit expiration time, as such
308   responses are not usually useful to store. However, caches are not
309   prohibited from storing such responses.
310</t>
311
312<section anchor="incomplete.responses" title="Storing Incomplete Responses">
313<t>
314   A response message is considered complete when all of the octets indicated
315   by the message framing (&messaging;) are received prior to the connection
316   being closed. If the request is GET, the response status is <x:ref>200
317   (OK)</x:ref>, and the entire response header block has been received, a
318   cache &MAY; store an incomplete response message body if the cache entry is
319   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
320   response &MAY; be stored as if it were an incomplete <x:ref>200
321   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
322   partial content responses if it does not support the <x:ref>Range</x:ref>
323   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
324   the range units used in those fields.
325</t>
326<t>
327   A cache &MAY; complete a stored incomplete response by making a subsequent
328   range request (&partial;) and combining the successful response with the
329   stored entry, as defined in <xref target="combining.responses"/>. A cache
330   &MUST-NOT; use an incomplete response to answer requests unless the
331   response has been made complete or the request is partial and specifies a
332   range that is wholly within the incomplete response. A cache &MUST-NOT;
333   send a partial response to a client without explicitly marking it as such
334   using the <x:ref>206 (Partial Content)</x:ref> status code.
335</t>
336</section>
337
338
339<section anchor="caching.authenticated.responses" 
340   title="Storing Responses to Authenticated Requests">
341<t>
342   A shared cache &MUST-NOT; use a cached response to a request with an
343   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
344   satisfy any subsequent request unless a cache directive that allows such
345   responses to be stored is present in the response.
346</t>
347<t>
348   In this specification, the following <x:ref>Cache-Control</x:ref> response
349   directives (<xref target="cache-response-directive"/>) have such an effect:
350   must-revalidate, public, s-maxage.
351</t>
352<t>
353   Note that cached responses that contain the "must-revalidate" and/or
354   "s-maxage" response directives are not allowed to be served stale (<xref
355   target="serving.stale.responses"/>) by shared caches. In particular, a
356   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
357   used to satisfy a subsequent request without revalidating it on the origin
358   server.
359</t>
360</section>
361
362<section anchor="combining.responses" title="Combining Partial Content">
363<t>
364   A response might transfer only a partial representation if the
365   connection closed prematurely or if the request used one or more Range
366   specifiers (&partial;).  After several such transfers, a cache might have
367   received several ranges of the same representation.  A cache &MAY; combine
368   these ranges into a single stored response, and reuse that response to
369   satisfy later requests, if they all share the same strong validator and
370   the cache complies with the client requirements in &combining-byte-ranges;.
371</t>
372<t>
373   When combining the new response with one or more stored responses, a
374   cache &MUST;:
375   <list style="symbols">
376      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
377         with warn-code 1xx (see <xref target="header.warning" />);</t>
378      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
379         with warn-code 2xx; and,</t>
380      <t>use other header fields provided in the new response, aside
381         from <x:ref>Content-Range</x:ref>, to replace all instances of the
382         corresponding header fields in the stored response.</t>
383   </list>
384</t>
385</section>
386
387</section>
388
389
390<section anchor="constructing.responses.from.caches" 
391   title="Constructing Responses from Caches">
392<t>
393   When presented with a request, a cache &MUST-NOT; reuse a stored response,
394   unless:
395   <list style="symbols">
396      <t>The presented effective request URI (&effective-request-uri;) and
397      that of the stored response match, and</t>
398      <t>the request method associated with the stored response allows it to
399      be used for the presented request, and</t>
400      <t>selecting header fields nominated by the stored response (if any)
401      match those presented (see <xref target="caching.negotiated.responses"
402      />), and</t>
403      <t>the presented request does not contain the no-cache pragma (<xref
404      target="header.pragma"/>), nor the no-cache cache directive (<xref
405      target="cache-request-directive"/>), unless the stored response is
406      successfully validated (<xref target="validation.model"/>), and</t>
407      <t>the stored response does not contain the no-cache cache directive
408      (<xref target="cache-response-directive.no-cache"/>), unless it is
409      successfully validated (<xref target="validation.model"/>), and</t>
410      <t>the stored response is either:
411         <list style="symbols">
412            <t>fresh (see <xref target="expiration.model" />), or</t>
413            <t>allowed to be served stale (see <xref
414            target="serving.stale.responses" />), or</t>
415            <t>successfully validated (see <xref target="validation.model"
416            />).</t>
417         </list>
418      </t>
419  </list>
420</t>
421<t>
422   Note that any of the requirements listed above can be overridden by a
423   cache-control extension; see <xref target="cache.control.extensions" />.
424</t>
425<t>
426   When a stored response is used to satisfy a request without validation, a
427   cache &MUST; generate an <x:ref>Age</x:ref> header field (<xref
428   target="header.age"/>), replacing any present in the response with a value
429   equal to the stored response's current_age; see <xref
430   target="age.calculations" />.
431</t>
432<t>
433   A cache &MUST; write through requests with methods that are unsafe
434   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
435   generate a reply to such a request before having forwarded the request and
436   having received a corresponding response.
437</t>
438<t>
439   Also, note that unsafe requests might invalidate already stored responses;
440   see <xref target="invalidation.after.updates.or.deletions" />.
441</t>
442<t>
443   When more than one suitable response is stored, a cache &MUST; use the
444   most recent response (as determined by the <x:ref>Date</x:ref> header
445   field). It can also forward the request with "Cache-Control: max-age=0" or
446   "Cache-Control: no-cache" to disambiguate which response to use.
447</t>
448<t>
449   A cache that does not have a clock available &MUST-NOT; use stored
450   responses without revalidating them upon every use.
451</t>
452
453
454<section anchor="expiration.model" title="Freshness">
455<iref item="fresh" />
456<iref item="stale" />
457<t>
458   A <x:dfn>fresh</x:dfn> response is one whose age has not yet exceeded its
459   freshness lifetime. Conversely, a <x:dfn>stale</x:dfn>
460   response is one where it has.
461</t>
462<iref item="freshness lifetime" />
463<iref item="explicit expiration time" />
464<iref item="heuristic expiration time" />
465<t>
466   A response's <x:dfn>freshness lifetime</x:dfn> is the length of time
467   between its generation by the origin server and its expiration time. An
468   <x:dfn>explicit expiration time</x:dfn> is the time at which the origin
469   server intends that a stored response can no longer be used by a cache
470   without further validation, whereas a <x:dfn>heuristic expiration
471   time</x:dfn> is assigned by a cache when no explicit expiriation time is
472   available.
473</t>
474<iref item="age" />
475<t>
476   A response's <x:dfn>age</x:dfn> is the time that has passed since it was
477   generated by, or successfully validated with, the origin server. 
478</t>
479<t>
480   When a response is "fresh" in the cache, it can be used to satisfy
481   subsequent requests without contacting the origin server, thereby improving
482   efficiency.
483</t>
484<t>
485   The primary mechanism for determining freshness is for an origin server to
486   provide an explicit expiration time in the future, using either the
487   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
488   the max-age response cache directive (<xref
489   target="cache-response-directive.max-age" />). Generally, origin servers
490   will assign future explicit expiration times to responses in the belief
491   that the representation is not likely to change in a semantically
492   significant way 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
499   target="serving.stale.responses" />).
500</t>
501<t>
502   Since origin servers do not always provide explicit expiration times,
503   caches are also allowed to use a heuristic to determine an expiration time
504   under certain circumstances (see <xref target="heuristic.freshness"/>).
505</t>
506<figure>
507<preamble>
508  The calculation to determine if a response is fresh is:
509</preamble>
510<artwork type="code">
511   response_is_fresh = (freshness_lifetime &gt; current_age)
512</artwork>
513</figure>
514<t>
515   freshness_lifetime is defined in <xref
516   target="calculating.freshness.lifetime" />; current_age is defined in
517   <xref target="age.calculations" />.
518</t>
519<t>
520   Clients can send the max-age or min-fresh cache directives in a request to
521   constrain or relax freshness calculations for the corresponding response
522   (<xref target="cache-request-directive" />).
523</t>
524<t>
525   When calculating freshness, to avoid common problems in date parsing:
526</t>
527<t>
528  <list style="symbols">
529     <t>Although all date formats are specified to be case-sensitive,
530        cache recipients &SHOULD; match day, week and timezone names
531        case-insensitively.</t>
532             
533     <t>If a cache recipient's internal implementation of time has less
534        resolution than the value of an HTTP-date, the recipient &MUST;
535        internally represent a parsed <x:ref>Expires</x:ref> date as the
536        nearest time equal to or earlier than the received value.</t>
537
538     <t>Cache recipients &MUST-NOT; allow local time zones to influence the
539        calculation or comparison of an age or expiration time.</t>
540
541     <t>Cache recipients &SHOULD; consider a date with a zone abbreviation
542        other than "GMT" to be invalid for calculating expiration.</t>
543  </list>
544</t>
545<t>
546   Note that freshness applies only to cache operation; it cannot be used to
547   force a user agent to refresh its display or reload a resource. See <xref
548   target="history.lists" /> for an explanation of the difference between
549   caches and history mechanisms.
550</t>
551
552<section anchor="calculating.freshness.lifetime" 
553   title="Calculating Freshness Lifetime">
554<t>
555   A cache can calculate the freshness lifetime (denoted as
556   freshness_lifetime) of a response by using the first match of:
557   <list style="symbols">
558      <t>If the cache is shared and the s-maxage response cache directive
559      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
560      or</t>
561      <t>If the max-age response cache directive (<xref
562      target="cache-response-directive.max-age" />) is present, use its value, or</t>
563      <t>If the <x:ref>Expires</x:ref> response header field
564      (<xref target="header.expires" />) is present, use its value minus the
565      value of the <x:ref>Date</x:ref> response header field, or</t>
566      <t>Otherwise, no explicit expiration time is present in the response. A
567      heuristic freshness lifetime might be applicable; see <xref
568      target="heuristic.freshness" />.</t>
569   </list>
570</t>
571<t>
572   Note that this calculation is not vulnerable to clock skew, since all of
573   the information comes from the origin server.
574</t>
575<t>
576   When there is more than one value present for a given directive (e.g., two
577   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
578   directives), the directive's value is considered invalid. Caches are
579   encouraged to consider responses that have invalid freshness information to
580   be stale.
581</t>
582</section>
583
584<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
585<t>
586   Since origin servers do not always provide explicit expiration times, a
587   cache &MAY; assign a heuristic expiration time when an explicit time is not
588   specified, employing algorithms that use other header field values (such as
589   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
590   time. This specification does not provide specific algorithms, but does
591   impose worst-case constraints on their results.
592</t>
593<t>
594   A cache &MUST-NOT; use heuristics to determine freshness when an explicit
595   expiration time is present in the stored response. Because of the
596   requirements in <xref target="response.cacheability"/>, this means that,
597   effectively, heuristics can only be used on responses without explicit
598   freshness whose status codes are defined as cacheable, and responses
599   without explicit freshness that have been marked as explicitly cacheable
600   (e.g., with a "public" response cache directive).
601</t>
602<t>
603   If the response has a <x:ref>Last-Modified</x:ref> header field
604   (&header-last-modified;), caches are encouraged to use a heuristic
605   expiration value that is no more than some fraction of the interval since
606   that time. A typical setting of this fraction might be 10%.
607</t>
608<t>
609   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
610   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
611   response if its current_age is more than 24 hours and such a warning is not
612   already present.
613</t>
614<x:note>
615   <t>
616      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
617      from calculating heuristic freshness for URIs with query components
618      (i.e., those containing '?'). In practice, this has not been widely
619      implemented. Therefore, origin servers are encouraged to send explicit
620      directives (e.g., Cache-Control: no-cache) if they wish to preclude
621      caching.
622   </t>
623</x:note>
624</section>
625
626<section anchor="age.calculations" title="Calculating Age">
627<t>
628   The <x:ref>Age</x:ref> header field is used to convey an estimated
629   age of the response message when obtained from a cache. The Age field value
630   is the cache's estimate of the number of seconds since the response was
631   generated or validated by the origin server. In essence, the Age value is
632   the sum of the time that the response has been resident in each of the
633   caches along the path from the origin server, plus the amount of time it
634   has been in transit along network paths.
635</t>
636<t>
637   The following data is used for the age calculation:
638</t>
639<t>
640   <x:dfn>age_value</x:dfn>
641   <list>
642      <t>
643         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
644         header field (<xref target="header.age"/>), in a form appropriate for
645         arithmetic operation; or 0, if not available.
646      </t>
647   </list>
648</t>
649<t>
650   <x:dfn>date_value</x:dfn>
651   <list>
652      <t>
653         The term "date_value" denotes the value of
654         the Date header field, in a form appropriate for arithmetic
655         operations. See &header-date; for the definition of the Date header
656         field, and for requirements regarding responses without it.
657      </t>
658   </list>
659</t>
660<t>
661   <x:dfn>now</x:dfn>
662   <list>
663      <t>
664         The term "now" means "the current value of the clock at the host
665         performing the calculation". A host ought to use NTP (<xref
666         target="RFC1305"/>) or some similar protocol to synchronize its
667         clocks to Coordinated Universal Time.
668      </t>
669   </list>
670</t>
671<t>
672   <x:dfn>request_time</x:dfn>
673   <list>
674      <t>
675         The current value of the clock at the host at the time the request
676         resulting in the stored response was made.
677      </t>
678   </list>
679</t>
680<t>
681   <x:dfn>response_time</x:dfn>
682   <list>
683      <t>
684         The current value of the clock at the host at the time the response
685         was received.
686      </t>
687   </list>
688</t>
689<t>
690   A response's age can be calculated in two entirely independent ways:
691   <list style="numbers">
692      <t>the "apparent_age": response_time minus date_value, if the local
693      clock is reasonably well synchronized to the origin server's clock. If
694      the result is negative, the result is replaced by zero.</t>
695      <t>the "corrected_age_value", if all of the caches along the response
696      path implement HTTP/1.1. A cache &MUST; interpret this value relative
697      to the time the request was initiated, not the time that the response
698      was received.</t>
699   </list>
700</t>
701<figure>
702<artwork type="code">
703  apparent_age = max(0, response_time - date_value);
704
705  response_delay = response_time - request_time;
706  corrected_age_value = age_value + response_delay; 
707</artwork>
708</figure>
709<figure>
710<preamble>These &SHOULD; be combined as</preamble>
711<artwork type="code">
712  corrected_initial_age = max(apparent_age, corrected_age_value);
713</artwork></figure>
714<t>
715   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
716   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
717   header field), in which case the corrected_age_value &MAY; be used as the
718   corrected_initial_age.
719</t>
720<t>
721   The current_age of a stored response can then be calculated by adding the
722   amount of time (in seconds) since the stored response was last validated by
723   the origin server to the corrected_initial_age.
724</t>
725<figure><artwork type="code">
726  resident_time = now - response_time;
727  current_age = corrected_initial_age + resident_time;
728</artwork></figure>
729</section>
730
731<section anchor="serving.stale.responses" title="Serving Stale Responses">
732<t>
733   A "stale" response is one that either has explicit expiry information or is
734   allowed to have heuristic expiry calculated, but is not fresh according to
735   the calculations in <xref target="expiration.model" />.
736</t>
737<t>
738   A cache &MUST-NOT; send a stale response if it is prohibited by an
739   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
740   directive, a "must-revalidate" cache-response-directive, or an applicable
741   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
742   target="cache-response-directive"/>).
743</t>
744<t>
745   A cache &MUST-NOT; send stale responses unless it is disconnected
746   (i.e., it cannot contact the origin server or otherwise find a forward
747   path) or doing so is explicitly allowed (e.g., by the max-stale request
748   directive; see <xref target="cache-request-directive" />).
749</t>
750<t>
751   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
752   warn-code (see <xref target="header.warning"/>) to stale responses.
753   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
754   cache is disconnected.
755</t>
756<iref item="first-hand" />
757<t>
758   Note that if a cache receives a <x:dfn>first-hand</x:dfn> response (one
759   where the freshness model is not in use; i.e., its age is 0, whether it is
760   an entire response, or a <x:ref>304 (Not Modified)</x:ref> response) that
761   it would normally forward to the requesting client, and the received
762   response is no longer fresh, the cache can forward it to the requesting
763   client without adding a new <x:ref>Warning</x:ref> (but without removing
764   any existing Warning header fields). A cache shouldn't attempt to validate
765   a response simply because that response became stale in transit.
766</t>
767</section>
768</section>
769
770<section anchor="validation.model" title="Validation">
771<t>
772   When a cache has one or more stored responses for a requested URI, but
773   cannot serve any of them (e.g., because they are not fresh, or one cannot
774   be selected; see <xref target="caching.negotiated.responses"/>), it can use
775   the conditional request mechanism &conditional; in the forwarded request to
776   give the origin server an opportunity to both select a valid stored
777   response to be used, and to update it. This process is known as
778   "validating" or "revalidating" the stored response.
779</t>
780<iref item="validator" />
781<t>
782   When sending such a conditional request, a cache adds a
783   <x:dfn>validator</x:dfn> (or more than one), that is used to find
784   out whether a stored response is an equivalent copy of a current
785   representation of the resource.
786</t>
787<t>
788   One such validator is the <x:ref>If-Modified-Since</x:ref> header field,
789   whose value is that of the <x:ref>Last-Modified</x:ref> header field from
790   the selected (see <xref target="caching.negotiated.responses"/>) stored
791   response, if available.
792</t>
793<t>
794   Another is the <x:ref>If-None-Match</x:ref> header field,
795   whose value is that of the <x:ref>ETag</x:ref> header field(s) from
796   relevant responses stored for the primary cache key, if present. However,
797   if any of the stored responses contains only partial content, the cache
798   shouldn't include its entity-tag in the If-None-Match header field unless
799   the request is for a range that would be fully satisfied by that stored
800   response.
801</t>
802
803<t>Cache handling of a response to a conditional request is dependent upon its
804status code:</t>
805
806<t>
807   <list style="symbols">
808      <t>
809         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
810         that the stored response can be updated and reused; see <xref
811         target="freshening.responses"/>.
812      </t>
813      <t>
814         A full response (i.e., one with a payload body) indicates that none
815         of the stored responses nominated in the conditional request is
816         suitable. Instead, the cache can use the full response to
817         satisfy the request and &MAY; replace the stored response(s).
818      </t>
819      <t>
820         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
821         response while attempting to validate a response, it can either
822         forward this response to the requesting client, or act as if the
823         server failed to respond. In the latter case, it can send a
824         previously stored response (see <xref
825         target="serving.stale.responses" />).
826      </t>
827   </list>
828</t>
829
830<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
831<t>
832   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
833   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
834   same cache key, the cache needs to identify which of the stored responses
835   are updated by this new response and then update the stored response(s)
836   with the new information provided in the <x:ref>304</x:ref> response.
837</t>
838<iref item="strong validator" />
839<t>
840   The stored response to update is identified by using the first match (if
841   any) of:
842   <list style="symbols">
843    <t>
844     If the new response contains a <x:dfn>strong validator</x:dfn> (see
845     &weak-and-strong;), then that strong validator identifies the selected
846     representation for update. All of the stored responses with the same
847     strong validator are selected. If none of the stored responses contain
848     the same strong validator, then the new response &MUST-NOT; be used to
849     update any stored responses.
850    </t>
851    <t>
852     If the new response contains a weak validator and that validator
853     corresponds to one of the cache's stored responses, then the most
854     recent of those matching stored responses is selected for update.
855    </t>
856    <t>
857     If the new response does not include any form of validator (such as in
858     the case where a client generates an If-Modified-Since request from a
859     source other than the Last-Modified response header field), and there is
860     only one stored response, and that stored response also lacks a
861     validator, then that stored response is selected for update.
862    </t>
863   </list>
864</t>
865<t>
866   If a stored response is selected for update, the cache &MUST;:
867   <list style="symbols">
868      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
869         with warn-code 1xx (see <xref target="header.warning" />);</t>
870      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
871         with warn-code 2xx; and,</t>
872      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
873         response to replace all instances of the corresponding header
874         fields in the stored response.</t>
875   </list>
876</t>
877</section>
878
879</section>
880
881<section anchor="caching.negotiated.responses" 
882   title="Calculating Secondary Keys with Vary">
883<t>
884   When a cache receives a request that can be satisfied by a stored response
885   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
886   it &MUST-NOT; use that response unless all of the selecting header fields
887   nominated by the Vary header field match in both the original request
888   (i.e., that associated with the stored response), and the presented
889   request.
890</t>
891<t>
892   The selecting header fields from two requests are defined to match if and
893   only if those in the first request can be transformed to those in the
894   second request by applying any of the following:
895   <list style="symbols">
896      <t>
897         adding or removing whitespace, where allowed in the header field's
898         syntax
899      </t>
900      <t>
901         combining multiple header fields with the same field name
902         (see &header-fields;)
903      </t>
904      <t>
905         normalizing both header field values in a way that is known to have
906         identical semantics, according to the header field's specification
907         (e.g., re-ordering field values when order is not significant;
908         case-normalization, where values are defined to be case-insensitive)
909      </t>
910  </list>
911</t>
912<t>
913   If (after any normalization that might take place) a header field is absent
914   from a request, it can only match another request if it is also absent
915   there.
916</t>
917<t>
918   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
919   subsequent requests to that resource can only be properly interpreted by the
920   origin server.
921</t>
922<t>
923   The stored response with matching selecting header fields is known as the
924   selected response.
925</t>
926<t>
927   If multiple selected responses are available, the most recent response
928   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref 
929   target="constructing.responses.from.caches"/>.
930</t>
931<t>
932   If no selected response is available, the cache cannot satisfy the
933   presented request. Typically, it is forwarded to the origin server
934   in a (possibly conditional; see <xref target="validation.model"/>) request.
935</t>
936</section>
937
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
950   target="caching.negotiated.responses"/>) for a HEAD request, and the
951   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
952   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
953   selected GET response, the cache &MUST; consider that selected response to
954   be stale.
955</t>
956<t>
957   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
958   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
959   the same as that in a selected GET response (as per
960   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
961   the remaining header fields in the stored response using the following
962   rules:
963   <list style="symbols">
964      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
965         with warn-code 1xx (see <xref target="header.warning" />);</t>
966      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
967         with warn-code 2xx; and,</t>
968      <t>use other header fields provided in the response to replace
969         all instances of the corresponding header fields in the stored
970         response.</t>
971   </list>
972</t>
973
974</section>
975
976
977<section anchor="invalidation.after.updates.or.deletions" 
978   title="Request Methods that Invalidate">
979<t>
980   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
981   have the potential for changing state on the origin server, intervening
982   caches can use them to keep their contents up-to-date.
983</t>
984<t>
985   A cache &MUST; invalidate the effective Request URI
986   (&effective-request-uri;) as well as the URI(s) in the
987   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
988   fields (if present) when a non-error response to a request with an unsafe
989   method is received.
990</t>
991<t>
992   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
993   or <x:ref>Content-Location</x:ref> response header field if the host part of
994   that URI differs from the host part in the effective request URI
995   (&effective-request-uri;). This helps prevent denial of service attacks.
996</t>
997<t>
998   A cache &MUST; invalidate the effective request URI
999   (&effective-request-uri;) when it receives a non-error response
1000   to a request with a method whose safety is unknown.
1001</t>
1002<t>
1003   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1004   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1005   the cache will either remove all stored responses related to the effective
1006   request URI, or will mark these as "invalid" and in need of a mandatory
1007   validation before they can be sent in response to a subsequent request.
1008</t>
1009<t>
1010   Note that this does not guarantee that all appropriate responses are
1011   invalidated. For example, a state-changing request might invalidate
1012   responses in the caches it travels through, but relevant responses still
1013   might be stored in other caches that it has not.</t>
1014</section>
1015
1016
1017
1018
1019<section anchor="header.field.definitions" title="Header Field Definitions">
1020<t>
1021   This section defines the syntax and semantics of HTTP/1.1 header fields
1022   related to caching.
1023</t>
1024
1025<section anchor="header.age" title="Age">
1026   <iref item="Age header field" primary="true" x:for-anchor="" />
1027   <x:anchor-alias value="Age"/>
1028   <x:anchor-alias value="age-value"/>
1029<t>
1030   The "Age" header field conveys the sender's estimate of the amount
1031   of time since the response was generated or successfully validated at the
1032   origin server. Age values are calculated as specified in <xref
1033   target="age.calculations" />.
1034</t>
1035<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1036  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1037</artwork></figure>
1038<t>
1039  Age field-values are non-negative integers, representing time in seconds
1040  (see <xref target="delta-seconds"/>).
1041</t>
1042<t>
1043   The presence of an Age header field in a response implies that a response
1044   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1045   might not implement the Age header field.
1046</t>
1047</section>
1048
1049<section anchor="header.cache-control" title="Cache-Control">
1050   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1051   <x:anchor-alias value="Cache-Control"/>
1052   <x:anchor-alias value="cache-directive"/>
1053<t>
1054   The "Cache-Control" header field is used to specify directives for
1055   caches along the request/response chain. Such cache directives are
1056   unidirectional in that the presence of a directive in a request does not
1057   imply that the same directive is to be given in the response.
1058</t>
1059<t>
1060   A cache &MUST; obey the requirements of the Cache-Control
1061   directives defined in this section. See <xref
1062   target="cache.control.extensions"/> for information about how Cache-Control
1063   directives defined elsewhere are handled.
1064</t>
1065<x:note>
1066   <t>
1067       &Note; Some HTTP/1.0 caches might not implement Cache-Control.
1068   </t>
1069</x:note>
1070<t>
1071   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1072   through in forwarded messages, regardless of their
1073   significance to that application, since the directives might be applicable
1074   to all recipients along the request/response chain. It is not possible to
1075   target a directive to a specific cache.
1076</t>
1077<t>
1078   Cache directives are identified by a token, to be compared case-insensitively,
1079   and have an optional argument, that can use both token and quoted-string
1080   syntax. For the directives defined below that define arguments, recipients
1081   ought to accept both forms, even if one is documented to be preferred. For
1082   any directive not defined by this specification, recipients &MUST; accept
1083   both forms.
1084</t>
1085<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1086  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1087
1088  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1089</artwork></figure>
1090<t>
1091   For the cache directives defined below, no argument is defined (nor allowed)
1092   unless stated otherwise.
1093</t>
1094
1095<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1096
1097<section title="max-age" anchor="cache-request-directive.max-age">
1098   <iref item="max-age (cache directive)" primary="true" />
1099<t>
1100   Argument syntax:
1101   <list>
1102      <t>
1103        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1104      </t>
1105   </list>
1106</t>
1107<t>
1108   The "max-age" request directive indicates that the client is unwilling to
1109   accept a response whose age is greater than the specified number of
1110   seconds. Unless the max-stale request directive is also present, the
1111   client is not willing to accept a stale response.
1112</t>
1113<t>
1114   &Note; This directive uses the token form of the argument syntax;
1115   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1116   quoted-string form.
1117</t>
1118</section>
1119
1120<section title="max-stale" anchor="cache-request-directive.max-stale">
1121   <iref item="max-stale (cache directive)" primary="true" />
1122<t>
1123   Argument syntax:
1124   <list>
1125      <t>
1126        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1127      </t>
1128   </list>
1129</t>
1130<t>
1131   The "max-stale" request directive indicates that the client is willing
1132   to accept a response that has exceeded its freshness lifetime. If max-stale
1133   is assigned a value, then the client is willing to accept a response
1134   that has exceeded its freshness lifetime by no more than the specified
1135   number of seconds. If no value is assigned to max-stale, then the client
1136   is willing to accept a stale response of any age.
1137</t>
1138<t>
1139   &Note; This directive uses the token form of the argument syntax;
1140   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1141   quoted-string form.
1142</t>
1143</section>
1144
1145<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1146   <iref item="min-fresh (cache directive)" primary="true" />
1147<t>
1148   Argument syntax:
1149   <list>
1150      <t>
1151        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1152      </t>
1153   </list>
1154</t>
1155<t>
1156   The "min-fresh" request directive indicates that the client is willing
1157   to accept a response whose freshness lifetime is no less than its
1158   current age plus the specified time in seconds. That is, the client
1159   wants a response that will still be fresh for at least the specified
1160   number of seconds.
1161</t>
1162<t>
1163   &Note; This directive uses the token form of the argument syntax;
1164   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1165   quoted-string form.
1166</t>
1167</section>
1168
1169<section title="no-cache" anchor="cache-request-directive.no-cache">
1170   <iref item="no-cache (cache directive)" primary="true" />
1171<t>
1172   The "no-cache" request directive indicates that a cache &MUST-NOT; 
1173   use a stored response to satisfy the request without successful
1174   validation on the origin server.
1175</t>
1176</section>
1177 
1178<section title="no-store" anchor="cache-request-directive.no-store">
1179   <iref item="no-store (cache directive)" primary="true" />
1180<t>
1181   The "no-store" request directive indicates that a cache &MUST-NOT;
1182   store any part of either this request or any response to it. This
1183   directive applies to both private and shared caches. "&MUST-NOT;
1184   store" in this context means that the cache &MUST-NOT; intentionally
1185   store the information in non-volatile storage, and &MUST; make a
1186   best-effort attempt to remove the information from volatile storage as
1187   promptly as possible after forwarding it.
1188</t>
1189<t>
1190   This directive is NOT a reliable or sufficient mechanism for ensuring
1191   privacy. In particular, malicious or compromised caches might not
1192   recognize or obey this directive, and communications networks might be
1193   vulnerable to eavesdropping.
1194</t>
1195<t>
1196   Note that if a request containing this directive is satisfied from a
1197   cache, the no-store request directive does not apply to the already
1198   stored response.
1199</t>
1200</section>
1201
1202<section title="no-transform" anchor="cache-request-directive.no-transform">
1203   <iref item="no-transform (cache directive)" primary="true" />
1204<t>
1205   The "no-transform" request directive indicates that an intermediary
1206   (whether or not it implements a cache) &MUST-NOT; transform the payload,
1207   as defined in &transformations;.
1208</t>
1209</section>
1210
1211<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1212   <iref item="only-if-cached (cache directive)" primary="true" />
1213<t>
1214   The "only-if-cached" request directive indicates that the client only wishes
1215   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1216   either respond using a stored response that is consistent with the other
1217   constraints of the request, or respond with a <x:ref>504 (Gateway
1218   Timeout)</x:ref> status code. If a group of caches is being operated as a
1219   unified system with good internal connectivity, a member cache &MAY;
1220   forward such a request within that group of caches.
1221</t>
1222</section>
1223</section>
1224
1225<section anchor="cache-response-directive" 
1226   title="Response Cache-Control Directives">
1227   <x:anchor-alias value="cache-response-directive" />
1228
1229<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1230   <iref item="must-revalidate (cache directive)" primary="true" />
1231<t>
1232   The "must-revalidate" response directive indicates that once it has
1233   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1234   requests without successful validation on the origin server.
1235</t>
1236<t>
1237   The must-revalidate directive is necessary to support reliable
1238   operation for certain protocol features. In all circumstances a
1239   cache &MUST; obey the must-revalidate directive; in particular,
1240   if a cache cannot reach the origin server for any reason, it &MUST;
1241   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1242</t>
1243<t>
1244   The must-revalidate directive ought to be used by servers if and only
1245   if failure to validate a request on the representation could result in
1246   incorrect operation, such as a silently unexecuted financial
1247   transaction.
1248</t>
1249</section>
1250
1251<section title="no-cache" anchor="cache-response-directive.no-cache">
1252   <iref item="no-cache (cache directive)" primary="true" />
1253<t>
1254   Argument syntax:
1255   <list>
1256      <t>
1257        #<x:ref>field-name</x:ref>
1258      </t>
1259   </list>
1260</t>
1261<t>
1262   The "no-cache" response directive indicates that the response &MUST-NOT;
1263   be used to satisfy a subsequent request without successful validation on
1264   the origin server. This allows an origin server to prevent a cache from
1265   using it to satisfy a request without contacting it, even by caches that
1266   have been configured to send stale responses.
1267</t>
1268<t>
1269   If the no-cache response directive specifies one or more field-names,
1270   then a cache &MAY; use the response to satisfy a subsequent request,
1271   subject to any other restrictions on caching. However, any header fields
1272   in the response that have the field-name(s) listed &MUST-NOT; be sent
1273   in the response to a subsequent request without successful revalidation
1274   with the origin server. This allows an origin server to prevent the
1275   re-use of certain header fields in a response, while still allowing
1276   caching of the rest of the response.
1277</t> 
1278<t>
1279   The field-names given are not limited to the set of header
1280   fields defined by this specification. Field names are case-insensitive.
1281</t>
1282<t>
1283   &Note; Although it has been back-ported to many implementations, some
1284   HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
1285   response directives with field-names are often handled by implementations
1286   as if an unqualified no-cache directive was received; i.e., the special
1287   handling for the qualified form is not widely implemented.
1288</t>
1289<t>
1290   &Note; This directive uses the quoted-string form of the argument syntax.
1291   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1292   needed for single-entry lists).
1293</t>
1294</section>
1295
1296<section title="no-store" anchor="cache-response-directive.no-store">
1297   <iref item="no-store (cache directive)" primary="true" />
1298<t>
1299   The "no-store" response directive indicates that a cache &MUST-NOT;
1300   store any part of either the immediate request or response. This
1301   directive applies to both private and shared caches. "&MUST-NOT;
1302   store" in this context means that the cache &MUST-NOT; intentionally
1303   store the information in non-volatile storage, and &MUST; make a
1304   best-effort attempt to remove the information from volatile storage as
1305   promptly as possible after forwarding it.
1306</t>
1307<t>
1308   This directive is NOT a reliable or sufficient mechanism for ensuring
1309   privacy. In particular, malicious or compromised caches might not
1310   recognize or obey this directive, and communications networks might be
1311   vulnerable to eavesdropping.
1312</t>
1313</section>
1314
1315<section title="no-transform" anchor="cache-response-directive.no-transform">
1316   <iref item="no-transform (cache directive)" primary="true" />
1317<t>
1318   The "no-transform" response directive indicates that an intermediary
1319   (regardless of whether it implements a cache) &MUST-NOT; transform the
1320   payload, as defined in &transformations;.
1321</t>
1322</section>
1323
1324<section title="public" anchor="cache-response-directive.public">
1325   <iref item="public (cache directive)" primary="true" />
1326<t>
1327   The "public" response directive indicates that any cache &MAY; store the
1328   response, even if the response would normally be non-cacheable or cacheable
1329   only within a non-shared cache. (See <xref
1330   target="caching.authenticated.responses"/> for additional details related
1331   to the use of public in response to a request containing
1332   <x:ref>Authorization</x:ref>, and <xref target="response.cacheability"/>
1333   for details of how public affects responses that would normally not be
1334   stored, due to their status codes not being defined as cacheable.)
1335</t>
1336</section>
1337
1338<section title="private" anchor="cache-response-directive.private">
1339   <iref item="private (cache directive)" primary="true" />
1340<t>
1341   Argument syntax:
1342   <list>
1343      <t>
1344        #<x:ref>field-name</x:ref>
1345      </t>
1346   </list>
1347</t>
1348<t>
1349   The "private" response directive indicates that the response message is
1350   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1351   private cache &MAY; store the response and reuse it for later requests,
1352   even if the response would normally be non-cacheable.
1353</t>
1354<t>
1355   If the private response directive specifies one or more field-names,
1356   this requirement is limited to the field-values associated with the
1357   listed response header fields. That is, a shared cache &MUST-NOT; store
1358   the specified field-names(s), whereas it &MAY; store the remainder of the
1359   response message.
1360</t>
1361<t>
1362   The field-names given are not limited to the set of header
1363   fields defined by this specification. Field names are case-insensitive.
1364</t>
1365<t>
1366   &Note; This usage of the word "private" only controls
1367   where the response can be stored; it cannot ensure the privacy of the
1368   message content. Also, private response directives with field-names are
1369   often handled by implementations as if an unqualified private directive
1370   was received; i.e., the special handling for the qualified form is not
1371   widely implemented.
1372</t>
1373<t>
1374   &Note; This directive uses the quoted-string form of the argument syntax.
1375   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1376   needed for single-entry lists).
1377</t>
1378</section>
1379
1380<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1381   <iref item="proxy-revalidate (cache directive)" primary="true" />
1382<t>
1383   The "proxy-revalidate" response directive has the same meaning as the
1384   must-revalidate response directive, except that it does not apply to
1385   private caches.
1386</t>
1387</section>
1388
1389<section title="max-age" anchor="cache-response-directive.max-age">
1390   <iref item="max-age (cache directive)" primary="true" />
1391<t>
1392   Argument syntax:
1393   <list>
1394      <t>
1395        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1396      </t>
1397   </list>
1398</t>
1399<t>
1400   The "max-age" response directive indicates that the response is to be
1401   considered stale after its age is greater than the specified number of
1402   seconds.
1403</t>
1404<t>
1405   &Note; This directive uses the token form of the argument syntax;
1406   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1407   quoted-string form.
1408</t>
1409</section>     
1410
1411<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1412   <iref item="s-maxage (cache directive)" primary="true" />
1413<t>
1414   Argument syntax:
1415   <list>
1416      <t>
1417        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1418      </t>
1419   </list>
1420</t>
1421<t>
1422   The "s-maxage" response directive indicates that, in shared caches, the
1423   maximum age specified by this directive overrides the maximum age
1424   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1425   header field. The s-maxage directive also implies the semantics of the
1426   proxy-revalidate response directive.
1427</t>
1428<t>
1429   &Note; This directive uses the token form of the argument syntax;
1430   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1431   quoted-string form.
1432</t>
1433</section>
1434
1435</section>
1436
1437<section anchor="cache.control.extensions" title="Cache Control Extensions">
1438<t>
1439   The Cache-Control header field can be extended through the use of one or
1440   more cache-extension tokens, each with an optional value.
1441</t>
1442<t>
1443   Informational extensions (those that do not require a change in cache
1444   behavior) can be added without changing the semantics of other directives.
1445   Behavioral extensions are designed to work by acting as modifiers to the
1446   existing base of cache directives.
1447</t>
1448<t>   
1449   Both the new directive and the standard directive are supplied, such that
1450   applications that do not understand the new directive will default to the
1451   behavior specified by the standard directive, and those that understand the
1452   new directive will recognize it as modifying the requirements associated
1453   with the standard directive. In this way, extensions to the cache-control
1454   directives can be made without requiring changes to the base protocol.
1455</t>
1456<t>
1457   This extension mechanism depends on an HTTP cache obeying all of the
1458   cache-control directives defined for its native HTTP-version, obeying
1459   certain extensions, and ignoring all directives that it does not
1460   understand.
1461</t>
1462<t>
1463   For example, consider a hypothetical new response directive called
1464   "community" that acts as a modifier to the private directive. We define
1465   this new directive to mean that, in addition to any private cache, any
1466   cache that is shared only by members of the community named within its
1467   value is allowed to cache the response. An origin server wishing to allow
1468   the UCI community to use an otherwise private response in their shared
1469   cache(s) could do so by including
1470</t>
1471<figure><artwork type="example">
1472  Cache-Control: private, community="UCI"
1473</artwork></figure>
1474<t>
1475   A cache seeing this header field will act correctly even if the cache does
1476   not understand the community cache-extension, since it will also see and
1477   understand the private directive and thus default to the safe behavior.
1478</t>
1479<t>
1480   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1481   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1482   combined with standard directives (or the response's default cacheability)
1483   such that the cache behavior will remain minimally correct even if the
1484   cache does not understand the extension(s).
1485</t>
1486<t>
1487   New extension directives ought to consider defining:
1488</t>
1489<t>
1490   <list style="symbols">
1491      <t>What it means for a directive to be specified multiple times,</t>
1492      <t>When the directive does not take an argument, what it means when an
1493      argument is present,</t>
1494      <t>When the directive requires an argument, what it means when it is
1495      missing,</t>
1496      <t>Whether the directive is specific to requests, responses, or able
1497        to be used in either.</t>
1498   </list>
1499</t>
1500<t>
1501   The HTTP Cache Directive Registry defines the name space for the cache
1502   directives.
1503</t>
1504<t>
1505   A registration &MUST; include the following fields:
1506   <list style="symbols">
1507      <t>Cache Directive Name</t>
1508      <t>Pointer to specification text</t>
1509   </list>
1510</t>
1511<t>
1512   Values to be added to this name space require IETF Review (see <xref
1513   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1514</t>
1515<t>
1516   The registry itself is maintained at <eref
1517   target="http://www.iana.org/assignments/http-cache-directives"/>.
1518</t>
1519</section>
1520
1521</section>
1522
1523<section anchor="header.expires" title="Expires">
1524   <iref item="Expires header field" primary="true" x:for-anchor="" />
1525   <x:anchor-alias value="Expires"/>
1526<t>
1527   The "Expires" header field gives the date/time after which the
1528   response is considered stale. See <xref target="expiration.model" /> for
1529   further discussion of the freshness model.
1530</t>
1531<t>
1532   The presence of an Expires field does not imply that the original resource
1533   will change or cease to exist at, before, or after that time.
1534</t>
1535<t>
1536   The Expires value is an HTTP-date timestamp, as defined in &http-date;.
1537</t>
1538<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1539  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1540</artwork></figure>
1541<figure>
1542  <preamble>For example</preamble>
1543<artwork type="example">
1544  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1545</artwork></figure>
1546<t>
1547   A cache recipient &MUST; interpret invalid date formats, especially the
1548   value "0", as representing a time in the past (i.e., "already expired").
1549</t>
1550<t>
1551   If a response includes a <x:ref>Cache-Control</x:ref> field with
1552   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1553   a recipient &MUST; ignore the Expires field.
1554   Likewise, if a response includes the s-maxage directive
1555   (<xref target="cache-response-directive.s-maxage" />), a shared cache
1556   recipient &MUST; ignore the Expires field. In both these cases, the value
1557   in Expires is only intended for recipients that have not yet implemented
1558   the Cache-Control field.
1559</t>
1560<t>
1561   An origin server without a clock &MUST-NOT; generate an Expires field
1562   unless its value represents a fixed time in the past (always expired)
1563   or its value has been associated with the resource by a system or user
1564   with a reliable clock.
1565</t>
1566<t>
1567   Historically, HTTP required the Expires field-value to be no more than a
1568   year in the future. While longer freshness lifetimes are no longer
1569   prohibited, extremely large values have been demonstrated to cause
1570   problems (e.g., clock overflows due to use of 32-bit integers for
1571   time values), and many caches will evict a response far sooner than
1572   that.
1573</t>
1574</section>
1575
1576<section anchor="header.pragma" title="Pragma">
1577   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1578   <x:anchor-alias value="extension-pragma"/>
1579   <x:anchor-alias value="Pragma"/>
1580   <x:anchor-alias value="pragma-directive"/>
1581<t>
1582   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1583   caches, so that clients can specify a "no-cache" request that they will
1584   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1585   When the Cache-Control header field is also present and understood in a
1586   request, Pragma is ignored.
1587</t>
1588<t>
1589   In HTTP/1.0, Pragma was defined as an extensible field for
1590   implementation-specified directives for recipients. This specification
1591   deprecates such extensions to improve interoperability.
1592</t>
1593<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1594  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1595  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1596  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1597</artwork></figure>
1598<t>
1599   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1600   request, the no-cache request pragma-directive &MUST; have the same effect
1601   on caches as if "Cache-Control: no-cache" were present (see <xref
1602   target="cache-request-directive" />).
1603</t>
1604<t>
1605   When sending a no-cache request, a client ought to include both the pragma
1606   and cache-control directives, unless Cache-Control: no-cache is
1607   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1608   directives at HTTP/1.1 caches. For example:
1609</t>
1610<figure>
1611<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1612GET / HTTP/1.1
1613Host: www.example.com
1614Cache-Control: max-age=30
1615Pragma: no-cache
1616
1617</artwork>
1618</figure>
1619<t>
1620   will constrain HTTP/1.1 caches to serve a response no older than 30
1621   seconds, while precluding implementations that do not understand
1622   <x:ref>Cache-Control</x:ref> from serving a cached response.
1623</t>
1624<x:note>
1625   <t>
1626      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1627      specified, it does not provide a reliable replacement for
1628      "Cache-Control: no-cache" in them.
1629   </t>
1630</x:note>
1631</section>
1632
1633<section anchor="header.warning" title="Warning">
1634   <iref item="Warning header field" primary="true" x:for-anchor="" />
1635   <x:anchor-alias value="Warning"/>
1636   <x:anchor-alias value="warning-value"/>
1637   <x:anchor-alias value="warn-agent"/>
1638   <x:anchor-alias value="warn-code"/>
1639   <x:anchor-alias value="warn-date"/>
1640   <x:anchor-alias value="warn-text"/>
1641<t>
1642   The "Warning" header field is used to carry additional information
1643   about the status or transformation of a message that might not be reflected
1644   in the message. This information is typically used to warn about possible
1645   incorrectness introduced by caching operations or transformations applied
1646   to the payload of the message.
1647</t>
1648<t>
1649   Warnings can be used for other purposes, both cache-related and otherwise.
1650   The use of a warning, rather than an error status code, distinguishes these
1651   responses from true failures.
1652</t>
1653<t>
1654   Warning header fields can in general be applied to any message, however some
1655   warn-codes are specific to caches and can only be applied to response
1656   messages.
1657</t>
1658<figure><artwork type="abnf2616"><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"/>
1659  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1660 
1661  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1662                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1663 
1664  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1665  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1666                  ; the name or pseudonym of the server adding
1667                  ; the Warning header field, for use in debugging
1668  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1669  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1670</artwork></figure>
1671<t>
1672   Multiple warnings can be attached to a response (either by the origin
1673   server or by a cache), including multiple warnings with the same code
1674   number, only differing in warn-text.
1675</t>
1676<t>
1677   When this occurs, the user agent &SHOULD; inform the user of as many of
1678   them as possible, in the order that they appear in the response.
1679</t>
1680<t>
1681   Systems that generate multiple Warning header fields are encouraged to
1682   order them with this user agent behavior in mind. New Warning header fields
1683   are added after any existing Warning header fields.
1684</t>
1685<t>
1686   Warnings are assigned three digit warn-codes. The first digit indicates
1687   whether the Warning is required to be deleted from a stored response after
1688   validation:
1689   <list style="symbols">
1690      <t>1xx Warnings describe the freshness or validation status of the
1691      response, and so &MUST; be deleted by a cache after validation. They can
1692      only be generated by a cache when validating a cached entry, and
1693      &MUST-NOT; be generated in any other situation.</t>
1694      <t>2xx Warnings describe some aspect of the representation that is not
1695      rectified by a validation (for example, a lossy compression of the
1696      representation) and &MUST-NOT; be deleted by a cache after validation,
1697      unless a full response is sent, in which case they &MUST; be.</t>
1698   </list>
1699</t>
1700<t>
1701   If an implementation sends a message with one or more Warning header fields
1702   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1703   include in each warning-value a warn-date that matches the
1704   <x:ref>Date</x:ref> header field in the message.
1705</t>
1706<t>
1707   If a system receives a message with a warning-value that includes a
1708   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
1709   value in the response, then that warning-value &MUST; be deleted from the
1710   message before storing, forwarding, or using it. (preventing the
1711   consequences of naive caching of Warning header fields.) If all of the
1712   warning-values are deleted for this reason, the Warning header field &MUST;
1713   be deleted as well.
1714</t>
1715<t>
1716   The following warn-codes are defined by this specification, each with a
1717   recommended warn-text in English, and a description of its meaning.
1718</t>
1719
1720<section title="110 Response is Stale" anchor="warn.110">
1721  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1722<t>
1723   A cache &SHOULD; generate this whenever the sent response is stale.
1724</t>
1725</section>
1726
1727<section title="111 Revalidation Failed" anchor="warn.111">
1728  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1729<t>
1730   A cache &SHOULD; generate this when sending a stale response because an
1731   attempt to validate the response failed, due to an inability to reach
1732   the server.
1733</t>
1734</section>
1735
1736<section title="112 Disconnected Operation" anchor="warn.112">
1737  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1738<t>
1739   A cache &SHOULD; generate this if it is intentionally disconnected from
1740   the rest of the network for a period of time.
1741</t>
1742</section>
1743
1744<section title="113 Heuristic Expiration" anchor="warn.113">
1745  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1746<t>
1747   A cache &SHOULD; generate this if it heuristically chose a freshness
1748   lifetime greater than 24 hours and the response's age is greater than 24
1749   hours.
1750</t>
1751</section>
1752
1753<section title="199 Miscellaneous Warning" anchor="warn.199">
1754  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1755<t>
1756   The warning text can include arbitrary information to be presented to
1757   a human user, or logged. A system receiving this warning &MUST-NOT; take
1758   any automated action, besides presenting the warning to the user.
1759</t>
1760</section>
1761
1762<section title="214 Transformation Applied" anchor="warn.214">
1763  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1764<t>
1765   &MUST; be added by a proxy if it applies any transformation to the
1766   representation, such as changing the content-coding, media-type, or
1767   modifying the representation data, unless this Warning code already appears
1768   in the response.
1769</t>
1770</section>
1771
1772<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1773  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1774<t>
1775   The warning text can include arbitrary information to be presented to
1776   a human user, or logged. A system receiving this warning &MUST-NOT; take
1777   any automated action.
1778</t>
1779</section>
1780
1781<section title="Warn Code Extensions" anchor="warn.code.extensions">
1782<t>
1783   The HTTP Warn Code Registry defines the name space for warn codes.
1784</t>
1785<t>
1786   A registration &MUST; include the following fields:
1787   <list style="symbols">
1788      <t>Warn Code (3 digits)</t>
1789      <t>Short Description</t>
1790      <t>Pointer to specification text</t>
1791   </list>
1792</t>
1793<t>
1794   Values to be added to this name space require IETF Review (see <xref
1795   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1796</t>
1797<t>
1798   The registry itself is maintained at <eref
1799   target="http://www.iana.org/assignments/http-warn-codes"/>.
1800</t>
1801</section>
1802</section>
1803</section>
1804
1805<section anchor="history.lists" title="History Lists">
1806<t>
1807   User agents often have history mechanisms, such as "Back" buttons and
1808   history lists, that can be used to redisplay a representation retrieved
1809   earlier in a session.
1810</t>
1811<t>
1812   The freshness model (<xref target="expiration.model"/>) does not
1813   necessarily apply to history mechanisms. I.e., a history mechanism can
1814   display a previous representation even if it has expired.
1815</t>
1816<t>
1817   This does not prohibit the history mechanism from telling the user that a
1818   view might be stale, or from honoring cache directives (e.g.,
1819   Cache-Control: no-store).
1820</t>
1821</section>
1822
1823
1824<section anchor="IANA.considerations" title="IANA Considerations">
1825
1826<section title="Cache Directive Registry" 
1827   anchor="cache.directive.registration">
1828<t>
1829   The registration procedure for HTTP Cache Directives is defined by <xref
1830   target="cache.control.extensions"/> of this document.
1831</t>
1832<t>
1833   The HTTP Cache Directive Registry shall be created at <eref
1834   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1835   populated with the registrations below:
1836</t>
1837<?BEGININC p6-cache.cache-directives ?>
1838<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1839<texttable xmlns:my="#my" align="left" suppress-title="true"
1840           anchor="iana.cache.directive.registration.table">
1841   <ttcol>Cache Directive</ttcol>
1842   <ttcol>Reference</ttcol>
1843
1844   <c>max-age</c>
1845   <c>
1846      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1847   </c>
1848   <c>max-stale</c>
1849   <c>
1850      <xref target="cache-request-directive.max-stale"/>
1851   </c>
1852   <c>min-fresh</c>
1853   <c>
1854      <xref target="cache-request-directive.min-fresh"/>
1855   </c>
1856   <c>must-revalidate</c>
1857   <c>
1858      <xref target="cache-response-directive.must-revalidate"/>
1859   </c>
1860   <c>no-cache</c>
1861   <c>
1862      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1863   </c>
1864   <c>no-store</c>
1865   <c>
1866      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1867   </c>
1868   <c>no-transform</c>
1869   <c>
1870      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1871   </c>
1872   <c>only-if-cached</c>
1873   <c>
1874      <xref target="cache-request-directive.only-if-cached"/>
1875   </c>
1876   <c>private</c>
1877   <c>
1878      <xref target="cache-response-directive.private"/>
1879   </c>
1880   <c>proxy-revalidate</c>
1881   <c>
1882      <xref target="cache-response-directive.proxy-revalidate"/>
1883   </c>
1884   <c>public</c>
1885   <c>
1886      <xref target="cache-response-directive.public"/>
1887   </c>
1888   <c>s-maxage</c>
1889   <c>
1890      <xref target="cache-response-directive.s-maxage"/>
1891   </c>
1892   <c>stale-if-error</c>
1893   <c>
1894      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1895   </c>
1896   <c>stale-while-revalidate</c>
1897   <c>
1898      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1899   </c>
1900</texttable>
1901<!--(END)-->
1902<?ENDINC p6-cache.cache-directives ?>
1903</section>
1904
1905<section title="Warn Code Registry" 
1906   anchor="warn.code.registration">
1907<t>
1908   The registration procedure for HTTP Warn Codes is defined by <xref
1909   target="warn.code.extensions"/> of this document.
1910</t>
1911<t>
1912   The HTTP Warn Code Registry shall be created at <eref
1913   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1914   populated with the registrations below:
1915</t>
1916<?BEGININC p6-cache.iana-warn-codes ?>
1917<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1918<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1919   <ttcol>Warn Code</ttcol>
1920   <ttcol>Short Description</ttcol>
1921   <ttcol>Reference</ttcol>
1922   <c>110</c>
1923   <c>Response is Stale</c>
1924   <c>
1925      <xref target="warn.110"/>
1926   </c>
1927   <c>111</c>
1928   <c>Revalidation Failed</c>
1929   <c>
1930      <xref target="warn.111"/>
1931   </c>
1932   <c>112</c>
1933   <c>Disconnected Operation</c>
1934   <c>
1935      <xref target="warn.112"/>
1936   </c>
1937   <c>113</c>
1938   <c>Heuristic Expiration</c>
1939   <c>
1940      <xref target="warn.113"/>
1941   </c>
1942   <c>199</c>
1943   <c>Miscellaneous Warning</c>
1944   <c>
1945      <xref target="warn.199"/>
1946   </c>
1947   <c>214</c>
1948   <c>Transformation Applied</c>
1949   <c>
1950      <xref target="warn.214"/>
1951   </c>
1952   <c>299</c>
1953   <c>Miscellaneous Persistent Warning</c>
1954   <c>
1955      <xref target="warn.299"/>
1956   </c>
1957</texttable>
1958<!--(END)-->
1959<?ENDINC p6-cache.iana-warn-codes ?>
1960</section>
1961
1962<section title="Header Field Registration" anchor="header.field.registration">
1963<t>
1964  The Message Header Field Registry located at <eref 
1965  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1966  shall be updated with the permanent registrations below (see <xref target="BCP90" />):
1967</t>
1968<?BEGININC p6-cache.iana-headers ?>
1969<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1970<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1971   <ttcol>Header Field Name</ttcol>
1972   <ttcol>Protocol</ttcol>
1973   <ttcol>Status</ttcol>
1974   <ttcol>Reference</ttcol>
1975
1976   <c>Age</c>
1977   <c>http</c>
1978   <c>standard</c>
1979   <c>
1980      <xref target="header.age"/>
1981   </c>
1982   <c>Cache-Control</c>
1983   <c>http</c>
1984   <c>standard</c>
1985   <c>
1986      <xref target="header.cache-control"/>
1987   </c>
1988   <c>Expires</c>
1989   <c>http</c>
1990   <c>standard</c>
1991   <c>
1992      <xref target="header.expires"/>
1993   </c>
1994   <c>Pragma</c>
1995   <c>http</c>
1996   <c>standard</c>
1997   <c>
1998      <xref target="header.pragma"/>
1999   </c>
2000   <c>Warning</c>
2001   <c>http</c>
2002   <c>standard</c>
2003   <c>
2004      <xref target="header.warning"/>
2005   </c>
2006</texttable>
2007<!--(END)-->
2008<?ENDINC p6-cache.iana-headers ?>
2009<t>
2010   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2011   Force".
2012</t>
2013</section>
2014</section>
2015
2016<section anchor="security.considerations" title="Security Considerations">
2017<t>
2018   This section is meant to inform developers, information providers, and
2019   users of known security concerns specific to HTTP/1.1 caching.
2020   More general security considerations are addressed in HTTP messaging
2021   &messaging; and semantics &semantics;.
2022</t>
2023<t>
2024   Caches expose additional potential vulnerabilities, since the contents of
2025   the cache represent an attractive target for malicious exploitation.
2026   Because cache contents persist after an HTTP request is complete, an attack
2027   on the cache can reveal information long after a user believes that the
2028   information has been removed from the network. Therefore, cache contents
2029   need to be protected as sensitive information.
2030</t>
2031<t>
2032   Furthermore, the very use of a cache can bring about privacy concerns. For
2033   example, if two users share a cache, and the first one browses to a site,
2034   the second may be able to detect that the other has been to that site,
2035   because the resources from it load more quickly, thanks to the cache.
2036</t>
2037<t>
2038   Implementation flaws might allow attackers to insert content into a cache
2039   ("cache poisoning"), leading to compromise of clients that trust that
2040   content. Because of their nature, these attacks are difficult to mitigate.
2041</t>
2042<t>
2043   Likewise, implementation flaws (as well as misunderstanding of cache
2044   operation) might lead to caching of sensitive information (e.g.,
2045   authentication credentials) that is thought to be private, exposing it to
2046   unauthorized parties.
2047</t>
2048<t>
2049   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2050   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2051   often is) used to satisfy subsequent requests to caches. Servers who wish
2052   to control caching of these responses are encouraged to emit appropriate
2053   Cache-Control response headers.
2054</t>
2055
2056</section>
2057
2058<section title="Acknowledgments" anchor="acks">
2059<t>
2060  See &acks;.
2061</t>
2062</section>
2063
2064</middle>
2065
2066<back>
2067<references title="Normative References">
2068
2069  <reference anchor="Part1">
2070    <front>
2071      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2072      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2073        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2074        <address><email>fielding@gbiv.com</email></address>
2075      </author>
2076      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2077        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2078        <address><email>julian.reschke@greenbytes.de</email></address>
2079      </author>
2080      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2081    </front>
2082    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2083    <x:source basename="p1-messaging" href="p1-messaging.xml">
2084      <x:defines>Content-Length</x:defines>
2085      <x:defines>Via</x:defines>
2086    </x:source>
2087  </reference>
2088
2089  <reference anchor="Part2">
2090    <front>
2091      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2092      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2093        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2094        <address><email>fielding@gbiv.com</email></address>
2095      </author>
2096      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2097        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2098        <address><email>julian.reschke@greenbytes.de</email></address>
2099      </author>
2100      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2101    </front>
2102    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2103    <x:source basename="p2-semantics" href="p2-semantics.xml">
2104      <x:defines>2xx (Successful)</x:defines>
2105      <x:defines>200 (OK)</x:defines>
2106      <x:defines>203 (Non-Authoritative Information)</x:defines>
2107      <x:defines>3xx (Redirection)</x:defines>
2108      <x:defines>300 (Multiple Choices)</x:defines>
2109      <x:defines>301 (Moved Permanently)</x:defines>
2110      <x:defines>404 (Not Found)</x:defines>
2111      <x:defines>410 (Gone)</x:defines>
2112      <x:defines>5xx (Server Error)</x:defines>
2113      <x:defines>504 (Gateway Timeout)</x:defines>
2114      <x:defines>Content-Encoding</x:defines>
2115      <x:defines>Content-Location</x:defines>
2116      <x:defines>Content-Type</x:defines>
2117      <x:defines>Date</x:defines>
2118      <x:defines>Location</x:defines>
2119      <x:defines>Vary</x:defines>
2120    </x:source>
2121  </reference>
2122
2123  <reference anchor="Part4">
2124    <front>
2125      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2126      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2127        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2128        <address><email>fielding@gbiv.com</email></address>
2129      </author>
2130      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2131        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2132        <address><email>julian.reschke@greenbytes.de</email></address>
2133      </author>
2134      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2135    </front>
2136    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2137    <x:source basename="p4-conditional" href="p4-conditional.xml">
2138      <x:defines>304</x:defines>
2139      <x:defines>304 (Not Modified)</x:defines>
2140      <x:defines>ETag</x:defines>
2141      <x:defines>If-Modified-Since</x:defines>
2142      <x:defines>If-None-Match</x:defines>
2143      <x:defines>Last-Modified</x:defines>
2144    </x:source>
2145  </reference>
2146
2147  <reference anchor="Part5">
2148    <front>
2149      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2150      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2151        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2152        <address><email>fielding@gbiv.com</email></address>
2153      </author>
2154      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2155        <organization abbrev="W3C">World Wide Web Consortium</organization>
2156        <address><email>ylafon@w3.org</email></address>
2157      </author>
2158      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2159        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2160        <address><email>julian.reschke@greenbytes.de</email></address>
2161      </author>
2162      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2163    </front>
2164    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2165    <x:source basename="p5-range" href="p5-range.xml">
2166      <x:defines>206 (Partial Content)</x:defines>
2167      <x:defines>Content-Range</x:defines>
2168      <x:defines>Range</x:defines>
2169    </x:source>
2170  </reference>
2171
2172  <reference anchor="Part7">
2173    <front>
2174      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2175      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2176        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2177        <address><email>fielding@gbiv.com</email></address>
2178      </author>
2179      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2180        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2181        <address><email>julian.reschke@greenbytes.de</email></address>
2182      </author>
2183      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2184    </front>
2185    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2186    <x:source basename="p7-auth" href="p7-auth.xml">
2187      <x:defines>Authorization</x:defines>
2188    </x:source>
2189  </reference>
2190
2191  <reference anchor="RFC2119">
2192    <front>
2193      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2194      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2195        <organization>Harvard University</organization>
2196        <address><email>sob@harvard.edu</email></address>
2197      </author>
2198      <date month="March" year="1997" />
2199    </front>
2200    <seriesInfo name="BCP" value="14" />
2201    <seriesInfo name="RFC" value="2119" />
2202  </reference>
2203
2204  <reference anchor="RFC5234">
2205    <front>
2206      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2207      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2208        <organization>Brandenburg InternetWorking</organization>
2209        <address>
2210          <email>dcrocker@bbiw.net</email>
2211        </address> 
2212      </author>
2213      <author initials="P." surname="Overell" fullname="Paul Overell">
2214        <organization>THUS plc.</organization>
2215        <address>
2216          <email>paul.overell@thus.net</email>
2217        </address>
2218      </author>
2219      <date month="January" year="2008"/>
2220    </front>
2221    <seriesInfo name="STD" value="68"/>
2222    <seriesInfo name="RFC" value="5234"/>
2223  </reference>
2224 
2225</references>
2226
2227<references title="Informative References">
2228
2229  <reference anchor="RFC1305">
2230    <front>
2231      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2232      <author fullname="David L. Mills" initials="D." surname="Mills">
2233        <organization>University of Delaware, Electrical Engineering Department</organization>
2234        <address><email>mills@udel.edu</email></address>
2235      </author>
2236      <date month="March" year="1992" />
2237    </front>
2238    <seriesInfo name="RFC" value="1305" />
2239  </reference>
2240
2241  <reference anchor="RFC2616">
2242    <front>
2243      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2244      <author fullname="R. Fielding" initials="R." surname="Fielding">
2245        <organization>University of California, Irvine</organization>
2246        <address><email>fielding@ics.uci.edu</email></address>
2247      </author>
2248      <author fullname="J. Gettys" initials="J." surname="Gettys">
2249        <organization>W3C</organization>
2250        <address><email>jg@w3.org</email></address>
2251      </author>
2252      <author fullname="J. Mogul" initials="J." surname="Mogul">
2253        <organization>Compaq Computer Corporation</organization>
2254        <address><email>mogul@wrl.dec.com</email></address>
2255      </author>
2256      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2257        <organization>MIT Laboratory for Computer Science</organization>
2258        <address><email>frystyk@w3.org</email></address>
2259      </author>
2260      <author fullname="L. Masinter" initials="L." surname="Masinter">
2261        <organization>Xerox Corporation</organization>
2262        <address><email>masinter@parc.xerox.com</email></address>
2263      </author>
2264      <author fullname="P. Leach" initials="P." surname="Leach">
2265        <organization>Microsoft Corporation</organization>
2266        <address><email>paulle@microsoft.com</email></address>
2267      </author>
2268      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2269        <organization>W3C</organization>
2270        <address><email>timbl@w3.org</email></address>
2271      </author>
2272      <date month="June" year="1999" />
2273    </front>
2274    <seriesInfo name="RFC" value="2616" />
2275  </reference>
2276
2277  <reference anchor="BCP90">
2278    <front>
2279      <title>Registration Procedures for Message Header Fields</title>
2280      <author fullname="G. Klyne" initials="G." surname="Klyne">
2281        <organization>Nine by Nine</organization>
2282        <address><email>GK-IETF@ninebynine.org</email></address>
2283      </author>
2284      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2285        <organization>BEA Systems</organization>
2286        <address><email>mnot@pobox.com</email></address>
2287      </author>
2288      <author fullname="J. Mogul" initials="J." surname="Mogul">
2289        <organization>HP Labs</organization>
2290        <address><email>JeffMogul@acm.org</email></address>
2291      </author>
2292      <date month="September" year="2004" />
2293    </front>
2294    <seriesInfo name="BCP" value="90" />
2295    <seriesInfo name="RFC" value="3864" />
2296  </reference>
2297
2298  <reference anchor='RFC5226'>
2299    <front>
2300      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2301      <author initials='T.' surname='Narten' fullname='T. Narten'>
2302        <organization>IBM</organization>
2303        <address><email>narten@us.ibm.com</email></address>
2304      </author>
2305      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2306        <organization>Google</organization>
2307        <address><email>Harald@Alvestrand.no</email></address>
2308      </author>
2309      <date year='2008' month='May' />
2310    </front>
2311    <seriesInfo name='BCP' value='26' />
2312    <seriesInfo name='RFC' value='5226' />
2313  </reference>
2314
2315  <reference anchor='RFC5861'>
2316    <front>
2317      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2318      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2319        <organization>Yahoo! Inc.</organization>
2320        <address><email>mnot@yahoo-inc.com</email></address>
2321      </author>
2322      <date month="April" year="2010"/>
2323    </front>
2324    <seriesInfo name='RFC' value='5861' />
2325  </reference>
2326
2327  <reference anchor="RFC6265">
2328    <front>
2329      <title>HTTP State Management Mechanism</title>
2330      <author initials="A." surname="Barth" fullname="Adam Barth">
2331        <organization abbrev="U.C. Berkeley">
2332          University of California, Berkeley
2333        </organization>
2334        <address><email>abarth@eecs.berkeley.edu</email></address>
2335      </author>
2336      <date year="2011" month="April" />
2337    </front>
2338    <seriesInfo name="RFC" value="6265"/>
2339  </reference>
2340
2341</references>
2342
2343<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2344<t>
2345  Caching-related text has been substantially rewritten for clarity.
2346</t>
2347<t>
2348  The algorithm for calculating age is now less conservative.
2349  (<xref target="age.calculations"/>)
2350</t>
2351<t>
2352  Caches are now required to handle dates with timezones as if they're
2353  invalid, because it's not possible to accurately guess.
2354  (<xref target="age.calculations"/>)
2355</t>
2356<t>
2357  The <x:ref>Content-Location</x:ref> response header field is no longer used
2358  to determine the appropriate response to use when validating.
2359  (<xref target="validation.model" />)
2360</t>
2361<t>
2362  The algorithm for selecting a cached negotiated response to use has been
2363  clarified in several ways. In particular, it now explicitly allows
2364  header-specific canonicalization when processing selecting header fields.
2365  (<xref target="caching.negotiated.responses" />)
2366</t>
2367<t>
2368  Requirements regarding denial of service attack avoidance when performing
2369  invalidation have been clarified.
2370  (<xref target="invalidation.after.updates.or.deletions" />)
2371</t>
2372<t>
2373  Cache invalidation only occurs when a successful response is received.
2374  (<xref target="invalidation.after.updates.or.deletions" />)
2375</t>
2376<t>
2377  The conditions under which an authenticated response can be cached have been
2378  clarified.
2379  (<xref target="caching.authenticated.responses" />)
2380</t>
2381<t>
2382  The one-year limit on Expires header field values has been removed; instead,
2383  the reasoning for using a sensible value is given.
2384  (<xref target="header.expires" />)
2385</t>
2386<t>
2387  The Pragma header field is now only defined for backwards compatibility;
2388  future pragmas are deprecated.
2389  (<xref target="header.pragma" />)
2390</t>
2391<t>
2392  Cache directives are explicitly defined to be case-insensitive.
2393  (<xref target="header.cache-control" />)
2394</t>
2395<t>
2396  Handling of multiple instances of cache directives when only one is
2397  expected is now defined.
2398  (<xref target="header.cache-control" />)
2399</t>
2400<t>
2401  The qualified forms of the private and no-cache cache directives are noted
2402  to not be widely implemented; e.g., "private=foo" is interpreted by many
2403  caches as simply "private". Additionally, the meaning of the qualified
2404  form of no-cache has been clarified.
2405  (<xref target="cache-response-directive" />)
2406</t>
2407<t>
2408  The "no-store" cache request directive doesn't apply to responses; i.e.,
2409  a cache can satisfy a request with no-store on it, and does not invalidate
2410  it.
2411  (<xref target="cache-request-directive.no-store" />)
2412</t>
2413<t>
2414  The "no-cache" response cache directive's meaning has been clarified.
2415  (<xref target="cache-response-directive.no-cache" />)
2416</t>
2417<t>
2418  New status codes can now define that caches are allowed to use heuristic
2419  freshness with them.
2420  (<xref target="heuristic.freshness" />)
2421</t>
2422<t>
2423  Caches are now allow to calculate heuristic freshness for URLs with query
2424  components.
2425  (<xref target="heuristic.freshness" />)
2426</t>
2427<t>
2428  Some requirements regarding production of the <x:ref>Warning</x:ref> 
2429  header have been relaxed, as it is not widely implemented.
2430  (<xref target="header.warning" />)
2431</t>
2432<t>
2433  The <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2434  nor allows multiple languages, as these aspects were not implemented.
2435  (<xref target="header.warning" />)
2436</t>
2437<t>
2438  This specification introduces the Cache Directive and Warn Code Registries,
2439  and defines considerations for new cache directives.
2440  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2441</t>
2442</section>
2443
2444<section title="Imported ABNF" anchor="imported.abnf">
2445   <x:anchor-alias value="ALPHA"/>
2446   <x:anchor-alias value="CR"/>
2447   <x:anchor-alias value="DIGIT"/>
2448   <x:anchor-alias value="DQUOTE"/>
2449   <x:anchor-alias value="LF"/>
2450   <x:anchor-alias value="OCTET"/>
2451   <x:anchor-alias value="SP"/>
2452   <x:anchor-alias value="VCHAR"/>
2453   <x:anchor-alias value="quoted-string"/>
2454   <x:anchor-alias value="token"/>
2455   <x:anchor-alias value="OWS"/>
2456   <x:anchor-alias value="field-name"/>
2457   <x:anchor-alias value="HTTP-date"/>
2458   <x:anchor-alias value="port"/>
2459   <x:anchor-alias value="pseudonym"/>
2460   <x:anchor-alias value="uri-host"/>
2461<t>
2462   The following core rules are included by reference, as defined in <xref
2463   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2464   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2465   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2466   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2467</t>
2468<t>
2469   The rules below are defined in <xref target="Part1"/>:
2470</t>
2471<figure><artwork type="abnf2616">
2472  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2473  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2474  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2475  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2476
2477  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2478  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
2479  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2480</artwork></figure>
2481<t>
2482   The rules below are defined in other parts:
2483</t>
2484<figure><artwork type="abnf2616">
2485  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2486</artwork></figure>
2487</section>
2488
2489<?BEGININC p6-cache.abnf-appendix ?>
2490<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2491<t>
2492  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
2493</t><figure>
2494<artwork type="abnf" name="p6-cache.parsed-abnf">
2495<x:ref>Age</x:ref> = delta-seconds
2496
2497<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2498 cache-directive ] )
2499
2500<x:ref>Expires</x:ref> = HTTP-date
2501
2502<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
2503
2504<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
2505
2506<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2507 pragma-directive ] )
2508
2509<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2510 )
2511
2512<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2513
2514<x:ref>delta-seconds</x:ref> = 1*DIGIT
2515
2516<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2517
2518<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2519
2520<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2521<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2522<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
2523
2524<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
2525
2526<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.6&gt;
2527
2528<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2529
2530<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2531<x:ref>warn-code</x:ref> = 3DIGIT
2532<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2533<x:ref>warn-text</x:ref> = quoted-string
2534<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2535 ]
2536</artwork>
2537</figure>
2538</section>
2539<?ENDINC p6-cache.abnf-appendix ?>
2540
2541<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2542<t>
2543  Changes up to the first Working Group Last Call draft are summarized
2544  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2545</t>
2546
2547<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2548<t>
2549  Closed issues:
2550  <list style="symbols">
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2553      "untangle Cache-Control ABNF"
2554    </t>
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2557      "Multiple values in Cache-Control header fields"
2558    </t>
2559    <t>
2560      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2561      "Case sensitivity of header fields in CC values"
2562    </t>
2563    <t>
2564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2565      "Spurious 'MAYs'"
2566    </t>
2567    <t>
2568      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2569      "enhance considerations for new cache control directives"
2570    </t>
2571    <t>
2572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2573      "ABNF requirements for recipients"
2574    </t>
2575    <t>
2576      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2577      "note introduction of new IANA registries as normative changes"
2578    </t>
2579    <t>
2580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2581      "broken prose in description of 'Vary'"
2582    </t>
2583  </list>
2584</t>
2585</section>
2586
2587<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2588<t>
2589  Closed issues:
2590  <list style="symbols">
2591    <t>
2592      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2593      "'Most Conservative'"
2594    </t>
2595  </list>
2596</t>
2597<t>
2598  Other changes:
2599  <list style="symbols">
2600    <t>
2601      Conformance criteria and considerations regarding error handling are
2602      now defined in Part 1.
2603    </t>
2604    <t>
2605      Move definition of "Vary" header field into Part 2.
2606    </t>
2607    <t>
2608      Add security considerations with respect to cache poisoning and
2609      the "Set-Cookie" header field.
2610    </t>
2611  </list>
2612</t>
2613</section>
2614
2615<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2616<t>
2617  Closed issues:
2618  <list style="symbols">
2619    <t>
2620      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2621      "Allowing heuristic caching for new status codes"
2622    </t>
2623    <t>
2624      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
2625      "304 without validator"
2626    </t>
2627    <t>
2628      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2629      "No-Transform"
2630    </t>
2631    <t>
2632      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
2633      "Revert prior change to the meaning of the public cache response
2634       directive.
2635    </t>
2636  </list>
2637</t>
2638</section>
2639
2640<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
2641<t>
2642  Closed issues:
2643  <list style="symbols">
2644    <t>
2645      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
2646      "explain list expansion in ABNF appendices"
2647    </t>
2648  </list>
2649</t>
2650</section>
2651</section>
2652  </back>
2653</rfc>
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