source: draft-ietf-httpbis/latest/p6-cache.xml @ 2523

Last change on this file since 2523 was 2523, checked in by mnot@…, 7 years ago

Reword security considerations around cache poisioning; see #535.

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 112.9 KB
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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 "December">
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 abnf-extension              "<xref target='Part1' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY conditional-precedence      "<xref target='Part4' x:rel='#precedence' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-etag                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-if-match             "<xref target='Part4' x:rel='#header.if-match' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-if-none-match        "<xref target='Part4' x:rel='#header.if-none-match' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-if-modified-since    "<xref target='Part4' x:rel='#header.if-modified-since' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-if-unmodified-since  "<xref target='Part4' x:rel='#header.if-unmodified-since' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-if-range             "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY header-vary                 "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY GET                         "<xref target='Part2' x:rel='#GET' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-overview             "<xref target='Part2' x:rel='#overview.of.status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY transformations             "<xref target='Part1' x:rel='#message.transformations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY body.length                 "<xref target='Part1' x:rel='#message.body.lenth' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58]>
59<?rfc toc="yes" ?>
60<?rfc symrefs="yes" ?>
61<?rfc sortrefs="yes" ?>
62<?rfc compact="yes"?>
63<?rfc subcompact="no" ?>
64<?rfc linkmailto="no" ?>
65<?rfc editing="no" ?>
66<?rfc comments="yes"?>
67<?rfc inline="yes"?>
68<?rfc rfcedstyle="yes"?>
69<?rfc-ext allow-markup-in-artwork="yes" ?>
70<?rfc-ext include-references-in-index="yes" ?>
71<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
72  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
73<x:link rel="prev" basename="p5-range"/>
74<x:link rel="next" basename="p7-auth"/>
75<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
76<front>
77
78  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
79
80  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
81    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
82    <address>
83      <postal>
84        <street>345 Park Ave</street>
85        <city>San Jose</city>
86        <region>CA</region>
87        <code>95110</code>
88        <country>USA</country>
89      </postal>
90      <email>fielding@gbiv.com</email>
91      <uri>http://roy.gbiv.com/</uri>
92    </address>
93  </author>
94
95  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
96    <organization>Akamai</organization>
97    <address>
98      <email>mnot@mnot.net</email>
99      <uri>http://www.mnot.net/</uri>
100    </address>
101  </author>
102
103  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
104    <organization abbrev="greenbytes">greenbytes GmbH</organization>
105    <address>
106      <postal>
107        <street>Hafenweg 16</street>
108        <city>Muenster</city><region>NW</region><code>48155</code>
109        <country>Germany</country>
110      </postal>
111      <email>julian.reschke@greenbytes.de</email>
112      <uri>http://greenbytes.de/tech/webdav/</uri>
113    </address>
114  </author>
115
116  <date month="&ID-MONTH;" year="&ID-YEAR;" />
117  <workgroup>HTTPbis Working Group</workgroup>
118
119<abstract>
120<t>
121   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
122   distributed, collaborative, hypertext information systems. This document
123   defines HTTP caches and the associated header fields
124   that control cache behavior or indicate cacheable response messages.
125</t>
126</abstract>
127
128<note title="Editorial Note (To be removed by RFC Editor)">
129  <t>
130    Discussion of this draft takes place on the HTTPBIS working group
131    mailing list (ietf-http-wg@w3.org), which is archived at
132    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
133  </t>
134  <t>
135    The current issues list is at
136    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
137    documents (including fancy diffs) can be found at
138    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
139  </t>
140  <t>
141    The changes in this draft are summarized in <xref target="changes.since.25"/>.
142  </t>
143</note>
144
145   </front>
146   <middle>
147
148<section anchor="caching" title="Introduction">
149<t>
150   HTTP is typically used for distributed information systems, where
151   performance can be improved by the use of response caches. This document
152   defines aspects of HTTP/1.1 related to caching and reusing response
153   messages.
154</t>
155
156<iref item="cache" />
157<t>
158   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
159   subsystem that controls storage, retrieval, and deletion of messages in it.
160   A cache stores cacheable responses in order to reduce the response time and
161   network bandwidth consumption on future, equivalent requests. Any client or
162   server &MAY; employ a cache, though a cache cannot be used by a server that
163   is acting as a tunnel.
164</t>
165<iref item="shared cache" />
166<iref item="private cache" />
167<t anchor="shared.and.private.caches">
168   A <x:dfn>shared cache</x:dfn> is a cache that stores responses to be reused
169   by more than one user; shared caches are usually (but not always) deployed
170   as a part of an intermediary. A <x:dfn>private cache</x:dfn>, in contrast,
171   is dedicated to a single user; often, they are deployed as a component of
172   a user agent.
173</t>
174<t>
175   The goal of caching in HTTP/1.1 is to significantly improve performance
176   by reusing a prior response message to satisfy a current request.
177   A stored response is considered "fresh", as defined in
178   <xref target="expiration.model" />, if the response can be reused without
179   "validation" (checking with the origin server to see if the cached response
180   remains valid for this request).  A fresh response can therefore
181   reduce both latency and network overhead each time it is reused.
182   When a cached response is not fresh, it might still be reusable if it can
183   be freshened by validation (<xref target="validation.model" />) or if the
184   origin is unavailable (<xref target="serving.stale.responses" />).
185</t>
186
187<section title="Conformance and Error Handling" anchor="conformance">
188<t>
189   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
190   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
191   document are to be interpreted as described in <xref target="RFC2119"/>.
192</t>
193<t>
194   Conformance criteria and considerations regarding error handling
195   are defined in &conformance;.
196</t>
197</section>
198
199<section title="Syntax Notation" anchor="notation">
200<t>
201   This specification uses the Augmented Backus-Naur Form (ABNF) notation
202   of <xref target="RFC5234"/> with the list rule extension defined in
203   &abnf-extension;. <xref target="imported.abnf"/> describes rules imported from
204   other documents. <xref target="collected.abnf"/> shows the collected ABNF
205   with the list rule expanded.
206</t>
207
208<section title="Delta Seconds" anchor="delta-seconds">
209<t>
210   The delta-seconds rule specifies a non-negative integer, representing time
211   in seconds.
212</t>
213<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
214  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
215</artwork></figure>
216<t>
217   A recipient parsing a delta-seconds value and converting it to binary form
218   ought to use an arithmetic type of at least 31 bits of non-negative integer
219   range.
220   If a cache receives a delta-seconds value greater than the greatest integer
221   it can represent, or if any of its subsequent calculations overflows,
222   the cache &MUST; consider the value to be either 2147483648
223   (2<x:sup>31</x:sup>) or the greatest positive integer it can conveniently
224   represent.
225</t>
226<x:note>
227   <t>
228       &Note; The value 2147483648 is here for historical reasons, effectively
229       represents infinity (over 68 years), and does not need to be stored in
230       binary form; an implementation could produce it as a canned string if
231       any overflow occurs, even if the calculations are performed with an
232       arithmetic type incapable of directly representing that number.
233       What matters here is that an overflow be detected and not treated as a
234       negative value in later calculations.
235   </t>
236</x:note>
237</section>
238
239</section>
240</section>
241
242<section anchor="caching.overview" title="Overview of Cache Operation">
243<iref item="cache entry" />
244<iref item="cache key" />
245<t>
246   Proper cache operation preserves the semantics of HTTP transfers
247   (&semantics;) while eliminating the transfer of information already held
248   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
249   we assume that reusing the cached response is desirable and that such
250   reuse is the default behavior when no requirement or local
251   configuration prevents it.  Therefore, HTTP cache requirements are focused
252   on preventing a cache from either storing a non-reusable response or
253   reusing a stored response inappropriately, rather than mandating that
254   caches always store and reuse particular responses.
255</t>
256<t>
257   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
258   HTTP responses corresponding to prior requests that used the same key. The
259   most common form of cache entry is a successful result of a retrieval
260   request: i.e., a <x:ref>200 (OK)</x:ref> response to a GET request, which
261   contains a representation of the resource identified by the request target
262   (&GET;). However, it is also possible to cache permanent redirects,
263   negative results (e.g., <x:ref>404 (Not Found)</x:ref>),
264   incomplete results (e.g., <x:ref>206 (Partial Content)</x:ref>), and
265   responses to methods other than GET if the method's definition allows such
266   caching and defines something suitable for use as a cache key.
267</t>
268<iref item="cache key" />
269<t>
270   The primary <x:dfn>cache key</x:dfn> consists of the request method and
271   target URI.  However, since HTTP caches in common use today are typically
272   limited to caching responses to GET, many caches simply decline
273   other methods and use only the URI as the primary cache key.
274</t>
275<t>
276   If a request target is subject to content negotiation, its cache entry
277   might consist of multiple stored responses, each differentiated by a
278   secondary key for the values of the original request's selecting header
279   fields (<xref target="caching.negotiated.responses"/>).
280</t>
281</section>
282
283<section anchor="response.cacheability" title="Storing Responses in Caches">
284<t>
285   A cache &MUST-NOT; store a response to any request, unless:
286   <list style="symbols">
287      <t>The request method is understood by the cache and defined as being
288      cacheable, and</t>
289      <t>the response status code is understood by the cache, and</t>
290      <t>the "no-store" cache directive (see <xref
291      target="header.cache-control" />) does not appear in request or response
292      header fields, and</t>
293      <t>the "private" response directive (see <xref
294      target="cache-response-directive.private" />) does not appear in the
295      response, if the cache is shared, and</t>
296      <t>the <x:ref>Authorization</x:ref> header field (see
297      &header-authorization;) does not appear in the request, if the cache is
298      shared, unless the response explicitly allows it (see <xref
299      target="caching.authenticated.responses" />), and</t>
300      <t>the response either:
301         <list style="symbols">
302            <t>contains an <x:ref>Expires</x:ref> header field (see
303            <xref target="header.expires"/>), or</t>
304            <t>contains a max-age response directive (see <xref
305            target="cache-response-directive.max-age" />), or</t>
306            <t>contains a s-maxage response directive (see <xref
307            target="cache-response-directive.s-maxage" />) and the cache is
308            shared, or</t>
309            <t>contains a Cache Control Extension (see <xref
310            target="cache.control.extensions" />) that allows it to be cached,
311            or</t>
312            <t>has a status code that is defined as cacheable by default
313            (see <xref target="heuristic.freshness" />), or</t>
314            <t>contains a public response directive (see <xref
315            target="cache-response-directive.public"/>).</t>
316         </list>
317      </t>
318   </list>
319</t>
320<t>
321   Note that any of the requirements listed above can be overridden by a
322   cache-control extension; see <xref target="cache.control.extensions" />.
323</t>
324<t>
325   In this context, a cache has "understood" a request method or a response
326   status code if it recognizes it and implements all specified
327   caching-related behavior.
328</t>
329<t>
330   Note that, in normal operation, some caches will not store a response that
331   has neither a cache validator nor an explicit expiration time, as such
332   responses are not usually useful to store. However, caches are not
333   prohibited from storing such responses.
334</t>
335
336<section anchor="incomplete.responses" title="Storing Incomplete Responses">
337<t>
338   A response message is considered complete when all of the octets indicated
339   by the message framing (&messaging;) are received prior to the connection
340   being closed. If the request method is GET, the response status code is
341   <x:ref>200 (OK)</x:ref>, and the entire response header section has been received, a
342   cache &MAY; store an incomplete response message body if the cache entry is
343   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
344   response &MAY; be stored as if it were an incomplete <x:ref>200
345   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
346   partial content responses if it does not support the <x:ref>Range</x:ref>
347   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
348   the range units used in those fields.
349</t>
350<t>
351   A cache &MAY; complete a stored incomplete response by making a subsequent
352   range request (&partial;) and combining the successful response with the
353   stored entry, as defined in <xref target="combining.responses"/>. A cache
354   &MUST-NOT; use an incomplete response to answer requests unless the
355   response has been made complete or the request is partial and specifies a
356   range that is wholly within the incomplete response. A cache &MUST-NOT;
357   send a partial response to a client without explicitly marking it as such
358   using the <x:ref>206 (Partial Content)</x:ref> status code.
359</t>
360</section>
361
362
363<section anchor="caching.authenticated.responses"
364   title="Storing Responses to Authenticated Requests">
365<t>
366   A shared cache &MUST-NOT; use a cached response to a request with an
367   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
368   satisfy any subsequent request unless a cache directive that allows such
369   responses to be stored is present in the response.
370</t>
371<t>
372   In this specification, the following <x:ref>Cache-Control</x:ref> response
373   directives (<xref target="cache-response-directive"/>) have such an effect:
374   must-revalidate, public, s-maxage.
375</t>
376<t>
377   Note that cached responses that contain the "must-revalidate" and/or
378   "s-maxage" response directives are not allowed to be served stale (<xref
379   target="serving.stale.responses"/>) by shared caches. In particular, a
380   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
381   used to satisfy a subsequent request without revalidating it on the origin
382   server.
383</t>
384</section>
385
386<section anchor="combining.responses" title="Combining Partial Content">
387<t>
388   A response might transfer only a partial representation if the
389   connection closed prematurely or if the request used one or more Range
390   specifiers (&partial;).  After several such transfers, a cache might have
391   received several ranges of the same representation.  A cache &MAY; combine
392   these ranges into a single stored response, and reuse that response to
393   satisfy later requests, if they all share the same strong validator and
394   the cache complies with the client requirements in &combining-byte-ranges;.
395</t>
396<t>
397   When combining the new response with one or more stored responses, a
398   cache &MUST;:
399   <list style="symbols">
400      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
401         with warn-code 1xx (see <xref target="header.warning" />);</t>
402      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
403         with warn-code 2xx; and,</t>
404      <t>use other header fields provided in the new response, aside
405         from <x:ref>Content-Range</x:ref>, to replace all instances of the
406         corresponding header fields in the stored response.</t>
407   </list>
408</t>
409</section>
410
411</section>
412
413
414<section anchor="constructing.responses.from.caches"
415   title="Constructing Responses from Caches">
416<t>
417   When presented with a request, a cache &MUST-NOT; reuse a stored response,
418   unless:
419   <list style="symbols">
420      <t>The presented effective request URI (&effective-request-uri;) and
421      that of the stored response match, and</t>
422      <t>the request method associated with the stored response allows it to
423      be used for the presented request, and</t>
424      <t>selecting header fields nominated by the stored response (if any)
425      match those presented (see <xref target="caching.negotiated.responses"
426      />), and</t>
427      <t>the presented request does not contain the no-cache pragma (<xref
428      target="header.pragma"/>), nor the no-cache cache directive (<xref
429      target="cache-request-directive"/>), unless the stored response is
430      successfully validated (<xref target="validation.model"/>), and</t>
431      <t>the stored response does not contain the no-cache cache directive
432      (<xref target="cache-response-directive.no-cache"/>), unless it is
433      successfully validated (<xref target="validation.model"/>), and</t>
434      <t>the stored response is either:
435         <list style="symbols">
436            <t>fresh (see <xref target="expiration.model" />), or</t>
437            <t>allowed to be served stale (see <xref
438            target="serving.stale.responses" />), or</t>
439            <t>successfully validated (see <xref target="validation.model"
440            />).</t>
441         </list>
442      </t>
443  </list>
444</t>
445<t>
446   Note that any of the requirements listed above can be overridden by a
447   cache-control extension; see <xref target="cache.control.extensions" />.
448</t>
449<t>
450   When a stored response is used to satisfy a request without validation, a
451   cache &MUST; generate an <x:ref>Age</x:ref> header field (<xref
452   target="header.age"/>), replacing any present in the response with a value
453   equal to the stored response's current_age; see <xref
454   target="age.calculations" />.
455</t>
456<t>
457   A cache &MUST; write through requests with methods that are unsafe
458   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
459   generate a reply to such a request before having forwarded the request and
460   having received a corresponding response.
461</t>
462<t>
463   Also, note that unsafe requests might invalidate already stored responses;
464   see <xref target="invalidation" />.
465</t>
466<t>
467   When more than one suitable response is stored, a cache &MUST; use the
468   most recent response (as determined by the <x:ref>Date</x:ref> header
469   field). It can also forward the request with "Cache-Control: max-age=0" or
470   "Cache-Control: no-cache" to disambiguate which response to use.
471</t>
472<t>
473   A cache that does not have a clock available &MUST-NOT; use stored
474   responses without revalidating them upon every use.
475</t>
476
477<section anchor="caching.negotiated.responses"
478   title="Calculating Secondary Keys with Vary">
479<t>
480   When a cache receives a request that can be satisfied by a stored response
481   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
482   it &MUST-NOT; use that response unless all of the selecting header fields
483   nominated by the Vary header field match in both the original request
484   (i.e., that associated with the stored response), and the presented
485   request.
486</t>
487<t>
488   The selecting header fields from two requests are defined to match if and
489   only if those in the first request can be transformed to those in the
490   second request by applying any of the following:
491   <list style="symbols">
492      <t>
493         adding or removing whitespace, where allowed in the header field's
494         syntax
495      </t>
496      <t>
497         combining multiple header fields with the same field name
498         (see &header-fields;)
499      </t>
500      <t>
501         normalizing both header field values in a way that is known to have
502         identical semantics, according to the header field's specification
503         (e.g., re-ordering field values when order is not significant;
504         case-normalization, where values are defined to be case-insensitive)
505      </t>
506  </list>
507</t>
508<t>
509   If (after any normalization that might take place) a header field is absent
510   from a request, it can only match another request if it is also absent
511   there.
512</t>
513<t>
514   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match.
515</t>
516<t>
517   The stored response with matching selecting header fields is known as the
518   selected response.
519</t>
520<t>
521   If multiple selected responses are available (potentially including
522   responses without a Vary header field), the cache will need to choose one to use.
523   When a selecting header field has a known mechanism for doing so (e.g., qvalues on
524   <x:ref>Accept</x:ref> and similar request header fields), that mechanism &MAY; be
525   used to select preferred responses; of the remainder, the most recent
526   response (as determined by the <x:ref>Date</x:ref> header field) is used, as
527   per <xref target="constructing.responses.from.caches"/>.
528</t>
529<t>
530   If no selected response is available, the cache cannot satisfy the
531   presented request. Typically, it is forwarded to the origin server
532   in a (possibly conditional; see <xref target="validation.model"/>) request.
533</t>
534</section>
535
536<section anchor="expiration.model" title="Freshness">
537<iref item="fresh" />
538<iref item="stale" />
539<t>
540   A <x:dfn>fresh</x:dfn> response is one whose age has not yet exceeded its
541   freshness lifetime. Conversely, a <x:dfn>stale</x:dfn>
542   response is one where it has.
543</t>
544<iref item="freshness lifetime" />
545<iref item="explicit expiration time" />
546<iref item="heuristic expiration time" />
547<t>
548   A response's <x:dfn>freshness lifetime</x:dfn> is the length of time
549   between its generation by the origin server and its expiration time. An
550   <x:dfn>explicit expiration time</x:dfn> is the time at which the origin
551   server intends that a stored response can no longer be used by a cache
552   without further validation, whereas a <x:dfn>heuristic expiration
553   time</x:dfn> is assigned by a cache when no explicit expiration time is
554   available.
555</t>
556<iref item="age" />
557<t>
558   A response's <x:dfn>age</x:dfn> is the time that has passed since it was
559   generated by, or successfully validated with, the origin server. 
560</t>
561<t>
562   When a response is "fresh" in the cache, it can be used to satisfy
563   subsequent requests without contacting the origin server, thereby improving
564   efficiency.
565</t>
566<t>
567   The primary mechanism for determining freshness is for an origin server to
568   provide an explicit expiration time in the future, using either the
569   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
570   the max-age response directive (<xref
571   target="cache-response-directive.max-age" />). Generally, origin servers
572   will assign future explicit expiration times to responses in the belief
573   that the representation is not likely to change in a semantically
574   significant way before the expiration time is reached.
575</t>
576<t>
577   If an origin server wishes to force a cache to validate every request, it
578   can assign an explicit expiration time in the past to indicate that the
579   response is already stale. Compliant caches will normally validate a stale
580   cached response before reusing it for subsequent requests (see <xref
581   target="serving.stale.responses" />).
582</t>
583<t>
584   Since origin servers do not always provide explicit expiration times,
585   caches are also allowed to use a heuristic to determine an expiration time
586   under certain circumstances (see <xref target="heuristic.freshness"/>).
587</t>
588<figure>
589<preamble>
590  The calculation to determine if a response is fresh is:
591</preamble>
592<artwork type="code">
593   response_is_fresh = (freshness_lifetime &gt; current_age)
594</artwork>
595</figure>
596<t>
597   freshness_lifetime is defined in <xref
598   target="calculating.freshness.lifetime" />; current_age is defined in
599   <xref target="age.calculations" />.
600</t>
601<t>
602   Clients can send the max-age or min-fresh cache directives in a request to
603   constrain or relax freshness calculations for the corresponding response
604   (<xref target="cache-request-directive" />).
605</t>
606<t>
607   When calculating freshness, to avoid common problems in date parsing:
608</t>
609<t>
610  <list style="symbols">
611     <t>Although all date formats are specified to be case-sensitive,
612        a cache recipient &SHOULD; match day, week, and timezone names
613        case-insensitively.</t>
614             
615     <t>If a cache recipient's internal implementation of time has less
616        resolution than the value of an HTTP-date, the recipient &MUST;
617        internally represent a parsed <x:ref>Expires</x:ref> date as the
618        nearest time equal to or earlier than the received value.</t>
619
620     <t>A cache recipient &MUST-NOT; allow local time zones to influence the
621        calculation or comparison of an age or expiration time.</t>
622
623     <t>A cache recipient &SHOULD; consider a date with a zone abbreviation
624        other than GMT or UTC to be invalid for calculating expiration.</t>
625  </list>
626</t>
627<t>
628   Note that freshness applies only to cache operation; it cannot be used to
629   force a user agent to refresh its display or reload a resource. See <xref
630   target="history.lists" /> for an explanation of the difference between
631   caches and history mechanisms.
632</t>
633
634<section anchor="calculating.freshness.lifetime"
635   title="Calculating Freshness Lifetime">
636<t>
637   A cache can calculate the freshness lifetime (denoted as
638   freshness_lifetime) of a response by using the first match of:
639   <list style="symbols">
640      <t>If the cache is shared and the s-maxage response directive
641      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
642      or</t>
643      <t>If the max-age response directive (<xref
644      target="cache-response-directive.max-age" />) is present, use its value, or</t>
645      <t>If the <x:ref>Expires</x:ref> response header field
646      (<xref target="header.expires" />) is present, use its value minus the
647      value of the <x:ref>Date</x:ref> response header field, or</t>
648      <t>Otherwise, no explicit expiration time is present in the response. A
649      heuristic freshness lifetime might be applicable; see <xref
650      target="heuristic.freshness" />.</t>
651   </list>
652</t>
653<t>
654   Note that this calculation is not vulnerable to clock skew, since all of
655   the information comes from the origin server.
656</t>
657<t>
658   When there is more than one value present for a given directive (e.g., two
659   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
660   directives), the directive's value is considered invalid. Caches are
661   encouraged to consider responses that have invalid freshness information to
662   be stale.
663</t>
664</section>
665
666<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
667<t>
668   Since origin servers do not always provide explicit expiration times, a
669   cache &MAY; assign a heuristic expiration time when an explicit time is not
670   specified, employing algorithms that use other header field values (such as
671   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
672   time. This specification does not provide specific algorithms, but does
673   impose worst-case constraints on their results.
674</t>
675<t>
676   A cache &MUST-NOT; use heuristics to determine freshness when an explicit
677   expiration time is present in the stored response. Because of the
678   requirements in <xref target="response.cacheability"/>, this means that,
679   effectively, heuristics can only be used on responses without explicit
680   freshness whose status codes are defined as cacheable by default (see
681   &status-overview;), and those responses without explicit freshness that have
682   been marked as explicitly cacheable (e.g., with a "public" response
683   directive).
684</t>
685<t>
686   If the response has a <x:ref>Last-Modified</x:ref> header field
687   (&header-last-modified;), caches are encouraged to use a heuristic
688   expiration value that is no more than some fraction of the interval since
689   that time. A typical setting of this fraction might be 10%.
690</t>
691<t>
692   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
693   generate a <x:ref>Warning</x:ref> header field with a 113 warn-code (see
694   <xref target="warn.113"/>) in the response if its current_age is more than
695   24 hours and such a warning is not already present.
696</t>
697<x:note>
698   <t>
699      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
700      from calculating heuristic freshness for URIs with query components
701      (i.e., those containing '?'). In practice, this has not been widely
702      implemented. Therefore, origin servers are encouraged to send explicit
703      directives (e.g., Cache-Control: no-cache) if they wish to preclude
704      caching.
705   </t>
706</x:note>
707</section>
708
709<section anchor="age.calculations" title="Calculating Age">
710<t>
711   The <x:ref>Age</x:ref> header field is used to convey an estimated
712   age of the response message when obtained from a cache. The Age field value
713   is the cache's estimate of the number of seconds since the response was
714   generated or validated by the origin server. In essence, the Age value is
715   the sum of the time that the response has been resident in each of the
716   caches along the path from the origin server, plus the amount of time it
717   has been in transit along network paths.
718</t>
719<t>
720   The following data is used for the age calculation:
721</t>
722<t>
723   <x:dfn>age_value</x:dfn>
724   <list>
725      <t>
726         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
727         header field (<xref target="header.age"/>), in a form appropriate for
728         arithmetic operation; or 0, if not available.
729      </t>
730   </list>
731</t>
732<t>
733   <x:dfn>date_value</x:dfn>
734   <list>
735      <t>
736         The term "date_value" denotes the value of
737         the Date header field, in a form appropriate for arithmetic
738         operations. See &header-date; for the definition of the Date header
739         field, and for requirements regarding responses without it.
740      </t>
741   </list>
742</t>
743<t>
744   <x:dfn>now</x:dfn>
745   <list>
746      <t>
747         The term "now" means "the current value of the clock at the host
748         performing the calculation". A host ought to use NTP (<xref
749         target="RFC5905"/>) or some similar protocol to synchronize its
750         clocks to Coordinated Universal Time.
751      </t>
752   </list>
753</t>
754<t>
755   <x:dfn>request_time</x:dfn>
756   <list>
757      <t>
758         The current value of the clock at the host at the time the request
759         resulting in the stored response was made.
760      </t>
761   </list>
762</t>
763<t>
764   <x:dfn>response_time</x:dfn>
765   <list>
766      <t>
767         The current value of the clock at the host at the time the response
768         was received.
769      </t>
770   </list>
771</t>
772<t>
773   A response's age can be calculated in two entirely independent ways:
774   <list style="numbers">
775      <t>the "apparent_age": response_time minus date_value, if the local
776      clock is reasonably well synchronized to the origin server's clock. If
777      the result is negative, the result is replaced by zero.</t>
778      <t>the "corrected_age_value", if all of the caches along the response
779      path implement HTTP/1.1. A cache &MUST; interpret this value relative
780      to the time the request was initiated, not the time that the response
781      was received.</t>
782   </list>
783</t>
784<figure>
785<artwork type="code">
786  apparent_age = max(0, response_time - date_value);
787
788  response_delay = response_time - request_time;
789  corrected_age_value = age_value + response_delay; 
790</artwork>
791</figure>
792<figure>
793<preamble>These are combined as</preamble>
794<artwork type="code">
795  corrected_initial_age = max(apparent_age, corrected_age_value);
796</artwork></figure>
797<t>
798   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
799   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
800   header field), in which case the corrected_age_value &MAY; be used as the
801   corrected_initial_age.
802</t>
803<t>
804   The current_age of a stored response can then be calculated by adding the
805   amount of time (in seconds) since the stored response was last validated by
806   the origin server to the corrected_initial_age.
807</t>
808<figure><artwork type="code">
809  resident_time = now - response_time;
810  current_age = corrected_initial_age + resident_time;
811</artwork></figure>
812</section>
813
814<section anchor="serving.stale.responses" title="Serving Stale Responses">
815<t>
816   A "stale" response is one that either has explicit expiry information or is
817   allowed to have heuristic expiry calculated, but is not fresh according to
818   the calculations in <xref target="expiration.model" />.
819</t>
820<t>
821   A cache &MUST-NOT; generate a stale response if it is prohibited by an
822   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
823   directive, a "must-revalidate" cache-response-directive, or an applicable
824   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
825   target="cache-response-directive"/>).
826</t>
827<t>
828   A cache &MUST-NOT; send stale responses unless it is disconnected
829   (i.e., it cannot contact the origin server or otherwise find a forward
830   path) or doing so is explicitly allowed (e.g., by the max-stale request
831   directive; see <xref target="cache-request-directive" />).
832</t>
833<t>
834   A cache &SHOULD; generate a <x:ref>Warning</x:ref> header field with the
835   110 warn-code (see <xref target="warn.110"/>) in stale responses.
836   Likewise, a cache &SHOULD; generate a 112 warn-code (see
837   <xref target="warn.112"/>) in stale responses if the cache is disconnected.
838</t>
839<t>
840   A cache &SHOULD-NOT; generate a new <x:ref>Warning</x:ref> header field
841   when forwarding a response that does not have an <x:ref>Age</x:ref> header
842   field, even if the response is already stale.  A cache need not validate
843   a response that merely became stale in transit.
844</t>
845</section>
846</section>
847
848<section anchor="validation.model" title="Validation">
849<t>
850   When a cache has one or more stored responses for a requested URI, but
851   cannot serve any of them (e.g., because they are not fresh, or one cannot
852   be selected; see <xref target="caching.negotiated.responses"/>), it can use
853   the conditional request mechanism &conditional; in the forwarded request to
854   give the next inbound server an opportunity to select a valid stored
855   response to use, updating the stored metadata in the process, or to replace
856   the stored response(s) with a new response. This process is known as
857   "validating" or "revalidating" the stored response.
858</t>
859
860<section anchor="validation.sent" title="Sending a Validation Request"><iref item="validator" />
861<t>
862   When sending a conditional request for cache validation, a cache sends one
863   or more precondition header fields containing <x:dfn>validator</x:dfn>
864   metadata from its stored response(s), which is then compared by recipients
865   to determine whether a stored response is equivalent to a current
866   representation of the resource.
867</t>
868<t>
869   One such validator is the timestamp given in a <x:ref>Last-Modified</x:ref>
870   header field (&header-last-modified;), which can be used in an
871   <x:ref>If-Modified-Since</x:ref> header field for response validation, or
872   in an <x:ref>If-Unmodified-Since</x:ref> or <x:ref>If-Range</x:ref> header
873   field for representation selection (i.e., the client is referring
874   specifically to a previously obtained representation with that timestamp).
875</t>
876<t>
877   Another validator is the entity-tag given in an <x:ref>ETag</x:ref> header
878   field (&header-etag;). One or more entity-tags, indicating one or more
879   stored responses, can be used in an <x:ref>If-None-Match</x:ref> header
880   field for response validation, or in an <x:ref>If-Match</x:ref> or
881   <x:ref>If-Range</x:ref> header field for representation selection (i.e.,
882   the client is referring specifically to one or more previously obtained
883   representations with the listed entity-tags).
884</t>
885</section>
886
887<section anchor="validation.received" title="Handling a Received Validation Request">
888<t>
889   Each client in the request chain may have its own cache, so it is common
890   for a cache at an intermediary to receive conditional requests from other
891   (outbound) caches. Likewise, some user agents make use of conditional
892   requests to limit data transfers to recently modified representations or to
893   complete the transfer of a partially retrieved representation.
894</t>
895<t>
896   If a cache receives a request that can be satisfied by reusing one of
897   its stored <x:ref>200 (OK)</x:ref> or <x:ref>206 (Partial Content)</x:ref>
898   responses, the cache &SHOULD; evaluate any applicable conditional header
899   field preconditions received in that request with respect to the
900   corresponding validators contained within the selected response.
901   A cache &MUST-NOT; evaluate conditional header fields that are only
902   applicable to an origin server, found in a request with semantics that
903   cannot be satisfied with a cached response, or applied to a target resource
904   for which it has no stored responses; such preconditions are likely
905   intended for some other (inbound) server.
906</t>
907<t>
908   The proper evaluation of conditional requests by a cache depends on the
909   received precondition header fields and their precedence, as defined in
910   &conditional-precedence;. The <x:ref>If-Match</x:ref> and
911   <x:ref>If-Unmodified-Since</x:ref> conditional header fields are not
912   applicable to a cache.
913</t>
914<t>
915   A request containing an <x:ref>If-None-Match</x:ref> header field
916   (&header-if-none-match;) indicates that the client wants to validate one
917   or more of its own stored responses in comparison to whichever stored
918   response is selected by the cache.
919   If the field-value is "*", or if the field-value is a list of entity-tags
920   and at least one of them match the entity-tag of the selected stored
921   response, a cache recipient &SHOULD; generate a
922   <x:ref>304 (Not Modified)</x:ref> response (using the metadata of the
923   selected stored response) instead of sending that stored response.
924</t>
925<t>
926   When a cache decides to revalidate its own stored responses for a
927   request that contains an <x:ref>If-None-Match</x:ref> list of entity-tags,
928   the cache &MAY; combine the received list with a list of entity-tags
929   from its own stored set of responses (fresh or stale) and send the union of
930   the two lists as a replacement <x:ref>If-None-Match</x:ref> header
931   field value in the forwarded request.
932   If a stored response contains only partial content, the
933   cache &MUST-NOT; include its entity-tag in the union unless the request is
934   for a range that would be fully satisfied by that partial stored response.
935   If the response to the forwarded request is
936   <x:ref>304 (Not Modified)</x:ref> and has an ETag header field value with
937   an entity-tag that is not in the client's list, the cache &MUST;
938   generate a <x:ref>200 (OK)</x:ref> response for the client by reusing its
939   corresponding stored response, as updated by the 304 response metadata
940   (<xref target="freshening.responses"/>).
941</t>
942<t>
943   If an <x:ref>If-None-Match</x:ref> header field is not present, a request
944   containing an <x:ref>If-Modified-Since</x:ref> header field
945   (&header-if-modified-since;) indicates that the client wants to validate
946   one or more of its own stored responses by modification date.
947   A cache recipient &SHOULD; generate a <x:ref>304 (Not Modified)</x:ref>
948   response (using the metadata of the selected stored response) if one of the
949   following cases is true:
950   1) the selected stored response has a <x:ref>Last-Modified</x:ref>
951   field-value that is earlier than or equal to the conditional timestamp;
952   2) no <x:ref>Last-Modified</x:ref> field is present in the selected stored
953   response, but it has a <x:ref>Date</x:ref> field-value that is earlier than
954   or equal to the conditional timestamp; or,
955   3) neither <x:ref>Last-Modified</x:ref> nor <x:ref>Date</x:ref> is present
956   in the selected stored response, but the cache recorded it as having been
957   received at a time earlier than or equal to the conditional timestamp.
958</t>
959<t>
960   A cache that implements partial responses to range requests, as defined in
961   &partial;, also needs to evaluate a received <x:ref>If-Range</x:ref> header
962   field (&header-if-range;) with respect to its selected stored response.
963</t>
964</section>
965
966<section anchor="validation.response" title="Handling a Validation Response">
967<t>
968   Cache handling of a response to a conditional request is dependent upon its
969   status code:
970</t>
971<t>
972   <list style="symbols">
973      <t>
974         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
975         that the stored response can be updated and reused; see <xref
976         target="freshening.responses"/>.
977      </t>
978      <t>
979         A full response (i.e., one with a payload body) indicates that none
980         of the stored responses nominated in the conditional request is
981         suitable. Instead, the cache &MUST; use the full response to
982         satisfy the request and &MAY; replace the stored response(s).
983      </t>
984      <t>
985         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
986         response while attempting to validate a response, it can either
987         forward this response to the requesting client, or act as if the
988         server failed to respond. In the latter case, the cache &MAY; send a
989         previously stored response (see <xref
990         target="serving.stale.responses" />).
991      </t>
992   </list>
993</t>
994</section>
995
996<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
997<t>
998   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
999   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
1000   same cache key, the cache needs to identify which of the stored responses
1001   are updated by this new response and then update the stored response(s)
1002   with the new information provided in the <x:ref>304</x:ref> response.
1003</t>
1004<iref item="strong validator" />
1005<t>
1006   The stored response to update is identified by using the first match (if
1007   any) of:
1008   <list style="symbols">
1009    <t>
1010     If the new response contains a <x:dfn>strong validator</x:dfn> (see
1011     &weak-and-strong;), then that strong validator identifies the selected
1012     representation for update. All of the stored responses with the same
1013     strong validator are selected. If none of the stored responses contain the
1014     same strong validator, then the cache &MUST-NOT; use the new response to
1015     update any stored responses.
1016    </t>
1017    <t>
1018     If the new response contains a weak validator and that validator
1019     corresponds to one of the cache's stored responses, then the most
1020     recent of those matching stored responses is selected for update.
1021    </t>
1022    <t>
1023     If the new response does not include any form of validator (such as in
1024     the case where a client generates an If-Modified-Since request from a
1025     source other than the Last-Modified response header field), and there is
1026     only one stored response, and that stored response also lacks a
1027     validator, then that stored response is selected for update.
1028    </t>
1029   </list>
1030</t>
1031<t>
1032   If a stored response is selected for update, the cache &MUST;:
1033   <list style="symbols">
1034      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1035         with warn-code 1xx (see <xref target="header.warning" />);</t>
1036      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1037         with warn-code 2xx; and,</t>
1038      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
1039         response to replace all instances of the corresponding header
1040         fields in the stored response.</t>
1041   </list>
1042</t>
1043</section>
1044
1045<section anchor="head.effects" title="Freshening Responses via HEAD">
1046<t>
1047   A response to the HEAD method is identical to what an equivalent request
1048   made with a GET would have been, except it lacks a body. This property
1049   of HEAD responses can be used to invalidate or update a cached GET
1050   response if the more efficient conditional GET request mechanism is not
1051   available (due to no validators being present in the stored response) or
1052   if transmission of the representation body is not desired even if it has
1053   changed.
1054</t>
1055<t>
1056   When a cache makes an inbound HEAD request for a given request target and
1057   receives a <x:ref>200 (OK)</x:ref> response, the cache &SHOULD; update or
1058   invalidate each of its stored GET responses that could have been selected
1059   for that request (see <xref target="caching.negotiated.responses"/>).
1060</t>
1061<t>
1062   For each of the stored responses that could have been selected, if the
1063   stored response and HEAD response have matching values for any received
1064   validator fields (<x:ref>ETag</x:ref> and <x:ref>Last-Modified</x:ref>)
1065   and, if the HEAD response has a <x:ref>Content-Length</x:ref> header field,
1066   the value of <x:ref>Content-Length</x:ref> matches that of the stored
1067   response, the cache &SHOULD; update the stored response as described below;
1068   otherwise, the cache &SHOULD; consider the stored response to be stale.
1069</t>
1070<t>
1071   If a cache updates a stored response with the metadata provided in a HEAD
1072   response, the cache &MUST;:
1073   <list style="symbols">
1074      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1075         with warn-code 1xx (see <xref target="header.warning" />);</t>
1076      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1077         with warn-code 2xx; and,</t>
1078      <t>use other header fields provided in the HEAD response to replace all
1079         instances of the corresponding header fields in the stored response
1080         and append new header fields to the stored response's header section
1081         unless otherwise restricted by the <x:ref>Cache-Control</x:ref>
1082         header field.</t>
1083   </list>
1084</t>
1085</section>
1086</section>
1087
1088
1089<section anchor="invalidation" title="Invalidation">
1090<t>
1091   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1092   have the potential for changing state on the origin server, intervening
1093   caches can use them to keep their contents up-to-date.
1094</t>
1095<t>
1096   A cache &MUST; invalidate the effective Request URI
1097   (&effective-request-uri;) as well as the URI(s) in the
1098   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1099   fields (if present) when a non-error status code is received in response to
1100   an unsafe request method.
1101</t>
1102<t>
1103   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1104   or <x:ref>Content-Location</x:ref> response header field if the host part of
1105   that URI differs from the host part in the effective request URI
1106   (&effective-request-uri;). This helps prevent denial of service attacks.
1107</t>
1108<t>
1109   A cache &MUST; invalidate the effective request URI
1110   (&effective-request-uri;) when it receives a non-error response
1111   to a request with a method whose safety is unknown.
1112</t>
1113<t>
1114   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1115   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1116   the cache will either remove all stored responses related to the effective
1117   request URI, or will mark these as "invalid" and in need of a mandatory
1118   validation before they can be sent in response to a subsequent request.
1119</t>
1120<t>
1121   Note that this does not guarantee that all appropriate responses are
1122   invalidated. For example, a state-changing request might invalidate
1123   responses in the caches it travels through, but relevant responses still
1124   might be stored in other caches that it has not.</t>
1125</section>
1126</section>
1127
1128
1129
1130<section anchor="header.field.definitions" title="Header Field Definitions">
1131<t>
1132   This section defines the syntax and semantics of HTTP/1.1 header fields
1133   related to caching.
1134</t>
1135
1136<section anchor="header.age" title="Age">
1137   <iref item="Age header field" primary="true" x:for-anchor="" />
1138   <x:anchor-alias value="Age"/>
1139   <x:anchor-alias value="age-value"/>
1140<t>
1141   The "Age" header field conveys the sender's estimate of the amount
1142   of time since the response was generated or successfully validated at the
1143   origin server. Age values are calculated as specified in <xref
1144   target="age.calculations" />.
1145</t>
1146<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1147  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1148</artwork></figure>
1149<t>
1150   The Age field-value is a non-negative integer, representing time in seconds
1151   (see <xref target="delta-seconds"/>).
1152</t>
1153<t>
1154   The presence of an Age header field implies that the response was not
1155   generated or validated by the origin server for this request. However,
1156   lack of an Age header field does not imply the origin was contacted, since
1157   the response might have been received from an HTTP/1.0 cache that does not
1158   implement Age.
1159</t>
1160</section>
1161
1162<section anchor="header.cache-control" title="Cache-Control">
1163   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1164   <x:anchor-alias value="Cache-Control"/>
1165   <x:anchor-alias value="cache-directive"/>
1166<t>
1167   The "Cache-Control" header field is used to specify directives for
1168   caches along the request/response chain. Such cache directives are
1169   unidirectional in that the presence of a directive in a request does not
1170   imply that the same directive is to be given in the response.
1171</t>
1172<t>
1173   A cache &MUST; obey the requirements of the Cache-Control
1174   directives defined in this section. See <xref
1175   target="cache.control.extensions"/> for information about how Cache-Control
1176   directives defined elsewhere are handled.
1177</t>
1178<x:note>
1179   <t>
1180       &Note; Some HTTP/1.0 caches might not implement Cache-Control.
1181   </t>
1182</x:note>
1183<t>
1184   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1185   through in forwarded messages, regardless of their
1186   significance to that application, since the directives might be applicable
1187   to all recipients along the request/response chain. It is not possible to
1188   target a directive to a specific cache.
1189</t>
1190<t>
1191   Cache directives are identified by a token, to be compared case-insensitively,
1192   and have an optional argument, that can use both token and quoted-string
1193   syntax. For the directives defined below that define arguments, recipients
1194   ought to accept both forms, even if one is documented to be preferred. For
1195   any directive not defined by this specification, a recipient &MUST; accept
1196   both forms.
1197</t>
1198<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1199  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1200
1201  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1202</artwork></figure>
1203<t>
1204   For the cache directives defined below, no argument is defined (nor allowed)
1205   unless stated otherwise.
1206</t>
1207
1208<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1209
1210<section title="max-age" anchor="cache-request-directive.max-age">
1211   <iref item="max-age (cache directive)" primary="true" />
1212<t>
1213   Argument syntax:
1214   <list>
1215      <t>
1216        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1217      </t>
1218   </list>
1219</t>
1220<t>
1221   The "max-age" request directive indicates that the client is unwilling
1222   to accept a response whose age is greater than the specified number of
1223   seconds. Unless the max-stale request directive is also present, the client
1224   is not willing to accept a stale response.
1225</t>
1226<t>
1227   This directive uses the token form of the argument syntax;
1228   e.g., 'max-age=5', not 'max-age="5"'. A sender &SHOULD-NOT; generate the
1229   quoted-string form.
1230</t>
1231</section>
1232
1233<section title="max-stale" anchor="cache-request-directive.max-stale">
1234   <iref item="max-stale (cache directive)" primary="true" />
1235<t>
1236   Argument syntax:
1237   <list>
1238      <t>
1239        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1240      </t>
1241   </list>
1242</t>
1243<t>
1244   The "max-stale" request directive indicates that the client is willing
1245   to accept a response that has exceeded its freshness lifetime. If max-stale
1246   is assigned a value, then the client is willing to accept a response
1247   that has exceeded its freshness lifetime by no more than the specified
1248   number of seconds. If no value is assigned to max-stale, then the client
1249   is willing to accept a stale response of any age.
1250</t>
1251<t>
1252   This directive uses the token form of the argument syntax;
1253   e.g., 'max-stale=10', not 'max-stale="10"'. A sender &SHOULD-NOT; generate
1254   the quoted-string form.
1255</t>
1256</section>
1257
1258<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1259   <iref item="min-fresh (cache directive)" primary="true" />
1260<t>
1261   Argument syntax:
1262   <list>
1263      <t>
1264        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1265      </t>
1266   </list>
1267</t>
1268<t>
1269   The "min-fresh" request directive indicates that the client is willing
1270   to accept a response whose freshness lifetime is no less than its
1271   current age plus the specified time in seconds. That is, the client
1272   wants a response that will still be fresh for at least the specified
1273   number of seconds.
1274</t>
1275<t>
1276   This directive uses the token form of the argument syntax;
1277   e.g., 'min-fresh=20', not 'min-fresh="20"'. A sender &SHOULD-NOT; generate
1278   the quoted-string form.
1279</t>
1280</section>
1281
1282<section title="no-cache" anchor="cache-request-directive.no-cache">
1283   <iref item="no-cache (cache directive)" primary="true" />
1284<t>
1285   The "no-cache" request directive indicates that a cache &MUST-NOT;
1286   use a stored response to satisfy the request without successful
1287   validation on the origin server.
1288</t>
1289</section>
1290 
1291<section title="no-store" anchor="cache-request-directive.no-store">
1292   <iref item="no-store (cache directive)" primary="true" />
1293<t>
1294   The "no-store" request directive indicates that a cache &MUST-NOT;
1295   store any part of either this request or any response to it. This
1296   directive applies to both private and shared caches. "&MUST-NOT;
1297   store" in this context means that the cache &MUST-NOT; intentionally
1298   store the information in non-volatile storage, and &MUST; make a
1299   best-effort attempt to remove the information from volatile storage as
1300   promptly as possible after forwarding it.
1301</t>
1302<t>
1303   This directive is NOT a reliable or sufficient mechanism for ensuring
1304   privacy. In particular, malicious or compromised caches might not
1305   recognize or obey this directive, and communications networks might be
1306   vulnerable to eavesdropping.
1307</t>
1308<t>
1309   Note that if a request containing this directive is satisfied from a
1310   cache, the no-store request directive does not apply to the already
1311   stored response.
1312</t>
1313</section>
1314
1315<section title="no-transform" anchor="cache-request-directive.no-transform">
1316   <iref item="no-transform (cache directive)" primary="true" />
1317<t>
1318   The "no-transform" request directive indicates that an intermediary
1319   (whether or not it implements a cache) &MUST-NOT; transform the payload,
1320   as defined in &transformations;.
1321</t>
1322</section>
1323
1324<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1325   <iref item="only-if-cached (cache directive)" primary="true" />
1326<t>
1327   The "only-if-cached" request directive indicates that the client only wishes
1328   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1329   either respond using a stored response that is consistent with the other
1330   constraints of the request, or respond with a <x:ref>504 (Gateway
1331   Timeout)</x:ref> status code. If a group of caches is being operated as a
1332   unified system with good internal connectivity, a member cache &MAY;
1333   forward such a request within that group of caches.
1334</t>
1335</section>
1336</section>
1337
1338<section anchor="cache-response-directive"
1339   title="Response Cache-Control Directives">
1340   <x:anchor-alias value="cache-response-directive" />
1341
1342<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1343   <iref item="must-revalidate (cache directive)" primary="true" />
1344<t>
1345   The "must-revalidate" response directive indicates that once it has
1346   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1347   requests without successful validation on the origin server.
1348</t>
1349<t>
1350   The must-revalidate directive is necessary to support reliable
1351   operation for certain protocol features. In all circumstances a
1352   cache &MUST; obey the must-revalidate directive; in particular,
1353   if a cache cannot reach the origin server for any reason, it &MUST;
1354   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1355</t>
1356<t>
1357   The must-revalidate directive ought to be used by servers if and only
1358   if failure to validate a request on the representation could result in
1359   incorrect operation, such as a silently unexecuted financial
1360   transaction.
1361</t>
1362</section>
1363
1364<section title="no-cache" anchor="cache-response-directive.no-cache">
1365   <iref item="no-cache (cache directive)" primary="true" />
1366<t>
1367   Argument syntax:
1368   <list>
1369      <t>
1370        #<x:ref>field-name</x:ref>
1371      </t>
1372   </list>
1373</t>
1374<t>
1375   The "no-cache" response directive indicates that the response &MUST-NOT;
1376   be used to satisfy a subsequent request without successful validation on
1377   the origin server. This allows an origin server to prevent a cache from
1378   using it to satisfy a request without contacting it, even by caches that
1379   have been configured to send stale responses.
1380</t>
1381<t>
1382   If the no-cache response directive specifies one or more field-names,
1383   then a cache &MAY; use the response to satisfy a subsequent request,
1384   subject to any other restrictions on caching. However, any header fields
1385   in the response that have the field-name(s) listed &MUST-NOT; be sent
1386   in the response to a subsequent request without successful revalidation
1387   with the origin server. This allows an origin server to prevent the
1388   re-use of certain header fields in a response, while still allowing
1389   caching of the rest of the response.
1390</t>
1391<t>
1392   The field-names given are not limited to the set of header
1393   fields defined by this specification. Field names are case-insensitive.
1394</t>
1395<t>
1396   This directive uses the quoted-string form of the argument syntax.
1397   A sender &SHOULD-NOT; generate the token form (even if quoting appears not
1398   to be needed for single-entry lists).
1399</t>
1400<t>
1401   &Note; Although it has been back-ported to many implementations, some
1402   HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
1403   response directives with field-names are often handled by caches
1404   as if an unqualified no-cache directive was received; i.e., the special
1405   handling for the qualified form is not widely implemented.
1406</t>
1407</section>
1408
1409<section title="no-store" anchor="cache-response-directive.no-store">
1410   <iref item="no-store (cache directive)" primary="true" />
1411<t>
1412   The "no-store" response directive indicates that a cache &MUST-NOT;
1413   store any part of either the immediate request or response. This
1414   directive applies to both private and shared caches. "&MUST-NOT;
1415   store" in this context means that the cache &MUST-NOT; intentionally
1416   store the information in non-volatile storage, and &MUST; make a
1417   best-effort attempt to remove the information from volatile storage as
1418   promptly as possible after forwarding it.
1419</t>
1420<t>
1421   This directive is NOT a reliable or sufficient mechanism for ensuring
1422   privacy. In particular, malicious or compromised caches might not
1423   recognize or obey this directive, and communications networks might be
1424   vulnerable to eavesdropping.
1425</t>
1426</section>
1427
1428<section title="no-transform" anchor="cache-response-directive.no-transform">
1429   <iref item="no-transform (cache directive)" primary="true" />
1430<t>
1431   The "no-transform" response directive indicates that an intermediary
1432   (regardless of whether it implements a cache) &MUST-NOT; transform the
1433   payload, as defined in &transformations;.
1434</t>
1435</section>
1436
1437<section title="public" anchor="cache-response-directive.public">
1438   <iref item="public (cache directive)" primary="true" />
1439<t>
1440   The "public" response directive indicates that any cache &MAY; store the
1441   response, even if the response would normally be non-cacheable or cacheable
1442   only within a private cache. (See <xref
1443   target="caching.authenticated.responses"/> for additional details related to
1444   the use of public in response to a request containing
1445   <x:ref>Authorization</x:ref>, and <xref target="response.cacheability"/> for
1446   details of how public affects responses that would normally not be stored,
1447   due to their status codes not being defined as cacheable by default; see
1448   <xref target="heuristic.freshness"/>.)
1449</t>
1450</section>
1451
1452<section title="private" anchor="cache-response-directive.private">
1453   <iref item="private (cache directive)" primary="true" />
1454<t>
1455   Argument syntax:
1456   <list>
1457      <t>
1458        #<x:ref>field-name</x:ref>
1459      </t>
1460   </list>
1461</t>
1462<t>
1463   The "private" response directive indicates that the response message is
1464   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1465   private cache &MAY; store the response and reuse it for later requests,
1466   even if the response would normally be non-cacheable.
1467</t>
1468<t>
1469   If the private response directive specifies one or more field-names,
1470   this requirement is limited to the field-values associated with the
1471   listed response header fields. That is, a shared cache &MUST-NOT; store
1472   the specified field-names(s), whereas it &MAY; store the remainder of the
1473   response message.
1474</t>
1475<t>
1476   The field-names given are not limited to the set of header
1477   fields defined by this specification. Field names are case-insensitive.
1478</t>
1479<t>
1480   This directive uses the quoted-string form of the argument syntax.
1481   A sender &SHOULD-NOT; generate the token form (even if quoting appears not
1482   to be needed for single-entry lists).
1483</t>
1484<t>
1485   &Note; This usage of the word "private" only controls
1486   where the response can be stored; it cannot ensure the privacy of the
1487   message content. Also, private response directives with field-names are
1488   often handled by caches as if an unqualified private directive
1489   was received; i.e., the special handling for the qualified form is not
1490   widely implemented.
1491</t>
1492</section>
1493
1494<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1495   <iref item="proxy-revalidate (cache directive)" primary="true" />
1496<t>
1497   The "proxy-revalidate" response directive has the same meaning as the
1498   must-revalidate response directive, except that it does not apply to
1499   private caches.
1500</t>
1501</section>
1502
1503<section title="max-age" anchor="cache-response-directive.max-age">
1504   <iref item="max-age (cache directive)" primary="true" />
1505<t>
1506   Argument syntax:
1507   <list>
1508      <t>
1509        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1510      </t>
1511   </list>
1512</t>
1513<t>
1514   The "max-age" response directive indicates that the response is to be
1515   considered stale after its age is greater than the specified number of
1516   seconds.
1517</t>
1518<t>
1519   This directive uses the token form of the argument syntax;
1520   e.g., 'max-age=5', not 'max-age="5"'. A sender &SHOULD-NOT; generate the
1521   quoted-string form.
1522</t>
1523</section>     
1524
1525<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1526   <iref item="s-maxage (cache directive)" primary="true" />
1527<t>
1528   Argument syntax:
1529   <list>
1530      <t>
1531        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1532      </t>
1533   </list>
1534</t>
1535<t>
1536   The "s-maxage" response directive indicates that, in shared caches, the
1537   maximum age specified by this directive overrides the maximum age
1538   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1539   header field. The s-maxage directive also implies the semantics of the
1540   proxy-revalidate response directive.
1541</t>
1542<t>
1543   This directive uses the token form of the argument syntax;
1544   e.g., 's-maxage=10', not 's-maxage="10"'. A sender &SHOULD-NOT; generate
1545   the quoted-string form.
1546</t>
1547</section>
1548
1549</section>
1550
1551<section anchor="cache.control.extensions" title="Cache Control Extensions">
1552<t>
1553   The Cache-Control header field can be extended through the use of one or
1554   more cache-extension tokens, each with an optional value.
1555</t>
1556<t>
1557   Informational extensions (those that do not require a change in cache
1558   behavior) can be added without changing the semantics of other directives.
1559   Behavioral extensions are designed to work by acting as modifiers to the
1560   existing base of cache directives.
1561</t>
1562<t>  
1563   Both the new directive and the standard directive are supplied, such that
1564   applications that do not understand the new directive will default to the
1565   behavior specified by the standard directive, and those that understand the
1566   new directive will recognize it as modifying the requirements associated
1567   with the standard directive. In this way, extensions to the cache-control
1568   directives can be made without requiring changes to the base protocol.
1569</t>
1570<t>
1571   This extension mechanism depends on an HTTP cache obeying all of the
1572   cache-control directives defined for its native HTTP-version, obeying
1573   certain extensions, and ignoring all directives that it does not
1574   understand.
1575</t>
1576<t>
1577   For example, consider a hypothetical new response directive called
1578   "community" that acts as a modifier to the private directive. We define
1579   this new directive to mean that, in addition to any private cache, any
1580   cache that is shared only by members of the community named within its
1581   value is allowed to cache the response. An origin server wishing to allow
1582   the UCI community to use an otherwise private response in their shared
1583   cache(s) could do so by including
1584</t>
1585<figure><artwork type="example">
1586  Cache-Control: private, community="UCI"
1587</artwork></figure>
1588<t>
1589   A cache seeing this header field will act correctly even if the cache does
1590   not understand the community cache-extension, since it will also see and
1591   understand the private directive and thus default to the safe behavior.
1592</t>
1593<t>
1594   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1595   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1596   combined with standard directives (or the response's default cacheability)
1597   such that the cache behavior will remain minimally correct even if the
1598   cache does not understand the extension(s).
1599</t>
1600</section>
1601
1602</section>
1603
1604<section anchor="header.expires" title="Expires">
1605   <iref item="Expires header field" primary="true" x:for-anchor="" />
1606   <x:anchor-alias value="Expires"/>
1607<t>
1608   The "Expires" header field gives the date/time after which the
1609   response is considered stale. See <xref target="expiration.model" /> for
1610   further discussion of the freshness model.
1611</t>
1612<t>
1613   The presence of an Expires field does not imply that the original resource
1614   will change or cease to exist at, before, or after that time.
1615</t>
1616<t>
1617   The Expires value is an HTTP-date timestamp, as defined in &http-date;.
1618</t>
1619<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1620  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1621</artwork></figure>
1622<figure>
1623  <preamble>For example</preamble>
1624<artwork type="example">
1625  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1626</artwork></figure>
1627<t>
1628   A cache recipient &MUST; interpret invalid date formats, especially the
1629   value "0", as representing a time in the past (i.e., "already expired").
1630</t>
1631<t>
1632   If a response includes a <x:ref>Cache-Control</x:ref> field with
1633   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1634   a recipient &MUST; ignore the Expires field.
1635   Likewise, if a response includes the s-maxage directive
1636   (<xref target="cache-response-directive.s-maxage" />), a shared cache
1637   recipient &MUST; ignore the Expires field. In both these cases, the value
1638   in Expires is only intended for recipients that have not yet implemented
1639   the Cache-Control field.
1640</t>
1641<t>
1642   An origin server without a clock &MUST-NOT; generate an Expires field
1643   unless its value represents a fixed time in the past (always expired)
1644   or its value has been associated with the resource by a system or user
1645   with a reliable clock.
1646</t>
1647<t>
1648   Historically, HTTP required the Expires field-value to be no more than a
1649   year in the future. While longer freshness lifetimes are no longer
1650   prohibited, extremely large values have been demonstrated to cause
1651   problems (e.g., clock overflows due to use of 32-bit integers for
1652   time values), and many caches will evict a response far sooner than
1653   that.
1654</t>
1655</section>
1656
1657<section anchor="header.pragma" title="Pragma">
1658   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1659   <x:anchor-alias value="extension-pragma"/>
1660   <x:anchor-alias value="Pragma"/>
1661   <x:anchor-alias value="pragma-directive"/>
1662<t>
1663   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1664   caches, so that clients can specify a "no-cache" request that they will
1665   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1666   When the Cache-Control header field is also present and understood in a
1667   request, Pragma is ignored.
1668</t>
1669<t>
1670   In HTTP/1.0, Pragma was defined as an extensible field for
1671   implementation-specified directives for recipients. This specification
1672   deprecates such extensions to improve interoperability.
1673</t>
1674<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"/>
1675  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1676  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1677  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1678</artwork></figure>
1679<t>
1680   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1681   request, caches &MUST; consider the no-cache request pragma-directive as
1682   having the same effect as if "Cache-Control: no-cache" were present (see
1683   <xref target="cache-request-directive" />).
1684</t>
1685<t>
1686   When sending a no-cache request, a client ought to include both the pragma
1687   and cache-control directives, unless Cache-Control: no-cache is
1688   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1689   directives at HTTP/1.1 caches. For example:
1690</t>
1691<figure>
1692<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1693GET / HTTP/1.1
1694Host: www.example.com
1695Cache-Control: max-age=30
1696Pragma: no-cache
1697
1698</artwork>
1699</figure>
1700<t>
1701   will constrain HTTP/1.1 caches to serve a response no older than 30
1702   seconds, while precluding implementations that do not understand
1703   <x:ref>Cache-Control</x:ref> from serving a cached response.
1704</t>
1705<x:note>
1706   <t>
1707      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1708      specified, it does not provide a reliable replacement for
1709      "Cache-Control: no-cache" in them.
1710   </t>
1711</x:note>
1712</section>
1713
1714<section anchor="header.warning" title="Warning">
1715   <iref item="Warning header field" primary="true" x:for-anchor="" />
1716   <x:anchor-alias value="Warning"/>
1717   <x:anchor-alias value="warning-value"/>
1718   <x:anchor-alias value="warn-agent"/>
1719   <x:anchor-alias value="warn-code"/>
1720   <x:anchor-alias value="warn-date"/>
1721   <x:anchor-alias value="warn-text"/>
1722<t>
1723   The "Warning" header field is used to carry additional information
1724   about the status or transformation of a message that might not be reflected
1725   in the status code. This information is typically used to warn about possible
1726   incorrectness introduced by caching operations or transformations applied
1727   to the payload of the message.
1728</t>
1729<t>
1730   Warnings can be used for other purposes, both cache-related and otherwise.
1731   The use of a warning, rather than an error status code, distinguishes these
1732   responses from true failures.
1733</t>
1734<t>
1735   Warning header fields can in general be applied to any message, however some
1736   warn-codes are specific to caches and can only be applied to response
1737   messages.
1738</t>
1739<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"/>
1740  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1741 
1742  <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>
1743                                        [ <x:ref>SP</x:ref> <x:ref>warn-date</x:ref> ]
1744 
1745  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1746  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1747                  ; the name or pseudonym of the server adding
1748                  ; the Warning header field, for use in debugging
1749                  ; a single "-" is recommended when agent unknown
1750  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1751  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1752</artwork></figure>
1753<t>
1754   Multiple warnings can be generated in a response (either by the origin
1755   server or by a cache), including multiple warnings with the same warn-code
1756   number that only differ in warn-text.
1757</t>
1758<t>
1759   A user agent that receives one or more Warning header fields &SHOULD;
1760   inform the user of as many of them as possible, in the order that they
1761   appear in the response. Senders that generate multiple Warning header
1762   fields are encouraged to order them with this user agent behavior in mind.
1763   A sender that generates new Warning header fields &MUST; append them after
1764   any existing Warning header fields.
1765</t>
1766<t>
1767   Warnings are assigned three digit warn-codes. The first digit indicates
1768   whether the Warning is required to be deleted from a stored response after
1769   validation:
1770   <list style="symbols">
1771      <t>1xx warn-codes describe the freshness or validation status of the
1772      response, and so &MUST; be deleted by a cache after validation. They can
1773      only be generated by a cache when validating a cached entry, and
1774      &MUST-NOT; be generated in any other situation.</t>
1775      <t>2xx warn-codes describe some aspect of the representation that is not
1776      rectified by a validation (for example, a lossy compression of the
1777      representation) and &MUST-NOT; be deleted by a cache after validation,
1778      unless a full response is sent, in which case they &MUST; be.</t>
1779   </list>
1780</t>
1781<t>
1782   If a sender generates one or more 1xx warn-codes in a message to be
1783   sent to a recipient known to implement only HTTP/1.0, the sender &MUST;
1784   include in each corresponding warning-value a warn-date that matches the
1785   <x:ref>Date</x:ref> header field in the message. For example:
1786</t>
1787<figure><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1788HTTP/1.1 200 OK
1789Date: Sat, 25 Aug 2012 23:34:45 GMT
1790Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
1791
1792</artwork></figure>
1793<t>
1794  Warnings have accompanying warn-text that describes the error, e.g., for
1795  logging. It is advisory only, and its content does not affect interpretation
1796  of the warn-code.
1797</t>
1798<t>
1799   If a recipient that uses, evaluates, or displays Warning header fields
1800   receives a warn-date that is different from the <x:ref>Date</x:ref> value
1801   in the same message, the recipient &MUST; exclude the warning-value
1802   containing that warn-date before storing, forwarding, or using the message.
1803   This allows recipients to exclude warning-values that were improperly
1804   retained after a cache validation.
1805   If all of the warning-values are excluded, the recipient &MUST; exclude
1806   the Warning header field as well.
1807</t>
1808<t>
1809   The following warn-codes are defined by this specification, each with a
1810   recommended warn-text in English, and a description of its meaning.
1811   The procedure for defining additional warn codes is described in
1812   <xref target="warn.code.registry.procedure"/>.
1813</t>
1814
1815<section title='Warning: 110 - "Response is Stale"' anchor="warn.110">
1816  <iref primary="true" item="110 (warn-code)" x:for-anchor=""/>
1817  <iref primary="true" item="Response is Stale (warn-text)" x:for-anchor=""/>
1818<t>
1819   A cache &SHOULD; generate this whenever the sent response is stale.
1820</t>
1821</section>
1822
1823<section title='Warning: 111 - "Revalidation Failed"' anchor="warn.111">
1824  <iref primary="true" item="111 (warn-code)" x:for-anchor=""/>
1825  <iref primary="true" item="Revalidation Failed (warn-text)" x:for-anchor=""/>
1826<t>
1827   A cache &SHOULD; generate this when sending a stale response because an
1828   attempt to validate the response failed, due to an inability to reach
1829   the server.
1830</t>
1831</section>
1832
1833<section title='Warning: 112 - "Disconnected Operation"' anchor="warn.112">
1834  <iref primary="true" item="112 (warn-code)" x:for-anchor=""/>
1835  <iref primary="true" item="Disconnected Operation (warn-text)" x:for-anchor=""/>
1836<t>
1837   A cache &SHOULD; generate this if it is intentionally disconnected from
1838   the rest of the network for a period of time.
1839</t>
1840</section>
1841
1842<section title='Warning: 113 - "Heuristic Expiration"' anchor="warn.113">
1843  <iref primary="true" item="113 (warn-code)" x:for-anchor=""/>
1844  <iref primary="true" item="Heuristic Expiration (warn-text)" x:for-anchor=""/>
1845<t>
1846   A cache &SHOULD; generate this if it heuristically chose a freshness
1847   lifetime greater than 24 hours and the response's age is greater than 24
1848   hours.
1849</t>
1850</section>
1851
1852<section title='Warning: 199 - "Miscellaneous Warning"' anchor="warn.199">
1853  <iref primary="true" item="199 (warn-code)" x:for-anchor=""/>
1854  <iref primary="true" item="Miscellaneous Warning (warn-text)" x:for-anchor=""/>
1855<t>
1856   The warning text can include arbitrary information to be presented to
1857   a human user, or logged. A system receiving this warning &MUST-NOT; take
1858   any automated action, besides presenting the warning to the user.
1859</t>
1860</section>
1861
1862<section title='Warning: 214 - "Transformation Applied"' anchor="warn.214">
1863  <iref primary="true" item="214 (warn-code)" x:for-anchor=""/>
1864  <iref primary="true" item="Transformation Applied (warn-text)" x:for-anchor=""/>
1865<t>
1866   &MUST; be added by a proxy if it applies any transformation to the
1867   representation, such as changing the content-coding, media-type, or
1868   modifying the representation data, unless this Warning code already appears
1869   in the response.
1870</t>
1871</section>
1872
1873<section title='Warning: 299 - "Miscellaneous Persistent Warning"' anchor="warn.299">
1874  <iref primary="true" item="299 (warn-code)" x:for-anchor=""/>
1875  <iref primary="true" item="Miscellaneous Persistent Warning (warn-text)" x:for-anchor=""/>
1876<t>
1877   The warning text can include arbitrary information to be presented to
1878   a human user, or logged. A system receiving this warning &MUST-NOT; take
1879   any automated action.
1880</t>
1881</section>
1882</section>
1883</section>
1884
1885<section anchor="history.lists" title="History Lists">
1886<t>
1887   User agents often have history mechanisms, such as "Back" buttons and
1888   history lists, that can be used to redisplay a representation retrieved
1889   earlier in a session.
1890</t>
1891<t>
1892   The freshness model (<xref target="expiration.model"/>) does not
1893   necessarily apply to history mechanisms. I.e., a history mechanism can
1894   display a previous representation even if it has expired.
1895</t>
1896<t>
1897   This does not prohibit the history mechanism from telling the user that a
1898   view might be stale, or from honoring cache directives (e.g.,
1899   Cache-Control: no-store).
1900</t>
1901</section>
1902
1903
1904<section title="IANA Considerations" anchor="iana.considerations">
1905
1906<section title="Cache Directive Registry" anchor="cache.directive.registry">
1907<t>
1908   The HTTP Cache Directive Registry defines the name space for the
1909   cache directives. It will be created and maintained at (the suggested URI)
1910   <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1911</t>
1912
1913<section title="Procedure" anchor="cache.directive.registry.procedure">
1914<t>
1915   A registration &MUST; include the following fields:
1916   <list style="symbols">
1917      <t>Cache Directive Name</t>
1918      <t>Pointer to specification text</t>
1919   </list>
1920</t>
1921<t>
1922   Values to be added to this name space require IETF Review (see <xref
1923   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1924</t>
1925</section>
1926
1927<section title="Considerations for New Cache Control Directives" anchor="cache.directive.considerations">
1928<t>
1929   New extension directives ought to consider defining:
1930</t>
1931<t>
1932   <list style="symbols">
1933      <t>What it means for a directive to be specified multiple times,</t>
1934      <t>When the directive does not take an argument, what it means when an
1935      argument is present,</t>
1936      <t>When the directive requires an argument, what it means when it is
1937      missing,</t>
1938      <t>Whether the directive is specific to requests, responses, or able
1939        to be used in either.</t>
1940   </list>
1941</t>
1942<t>
1943  See also <xref target="cache.control.extensions"/>.
1944</t>
1945</section>
1946
1947<section title="Registrations" anchor="cache.directive.registration">
1948<t>
1949  The HTTP Cache Directive Registry shall be populated with the registrations below:
1950</t>
1951<?BEGININC p6-cache.cache-directives ?>
1952<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1953<texttable xmlns:my="#my" align="left" suppress-title="true"
1954           anchor="iana.cache.directive.registration.table">
1955   <ttcol>Cache Directive</ttcol>
1956   <ttcol>Reference</ttcol>
1957
1958   <c>max-age</c>
1959   <c>
1960      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1961   </c>
1962   <c>max-stale</c>
1963   <c>
1964      <xref target="cache-request-directive.max-stale"/>
1965   </c>
1966   <c>min-fresh</c>
1967   <c>
1968      <xref target="cache-request-directive.min-fresh"/>
1969   </c>
1970   <c>must-revalidate</c>
1971   <c>
1972      <xref target="cache-response-directive.must-revalidate"/>
1973   </c>
1974   <c>no-cache</c>
1975   <c>
1976      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1977   </c>
1978   <c>no-store</c>
1979   <c>
1980      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1981   </c>
1982   <c>no-transform</c>
1983   <c>
1984      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1985   </c>
1986   <c>only-if-cached</c>
1987   <c>
1988      <xref target="cache-request-directive.only-if-cached"/>
1989   </c>
1990   <c>private</c>
1991   <c>
1992      <xref target="cache-response-directive.private"/>
1993   </c>
1994   <c>proxy-revalidate</c>
1995   <c>
1996      <xref target="cache-response-directive.proxy-revalidate"/>
1997   </c>
1998   <c>public</c>
1999   <c>
2000      <xref target="cache-response-directive.public"/>
2001   </c>
2002   <c>s-maxage</c>
2003   <c>
2004      <xref target="cache-response-directive.s-maxage"/>
2005   </c>
2006   <c>stale-if-error</c>
2007   <c>
2008      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2009   </c>
2010   <c>stale-while-revalidate</c>
2011   <c>
2012      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2013   </c>
2014</texttable>
2015<!--(END)-->
2016<?ENDINC p6-cache.cache-directives ?>
2017</section>
2018</section>
2019
2020<section title="Warn Code Registry" anchor="warn.code.registry">
2021<t>
2022   The HTTP Warn Code Registry defines the name space for warn codes.
2023   It will be created and maintained at (the suggested URI)
2024   <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
2025</t>
2026
2027<section title="Procedure" anchor="warn.code.registry.procedure">
2028<t>
2029   A registration &MUST; include the following fields:
2030   <list style="symbols">
2031      <t>Warn Code (3 digits)</t>
2032      <t>Short Description</t>
2033      <t>Pointer to specification text</t>
2034   </list>
2035</t>
2036<t>
2037   Values to be added to this name space require IETF Review (see <xref
2038   target="RFC5226" x:fmt="," x:sec="4.1"/>).
2039</t>
2040</section>
2041
2042<section title="Registrations" anchor="warn.code.registration">
2043<t>
2044  The HTTP Warn Code Registry shall be populated with the registrations below:
2045</t>
2046<?BEGININC p6-cache.iana-warn-codes ?>
2047<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2048<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
2049   <ttcol>Warn Code</ttcol>
2050   <ttcol>Short Description</ttcol>
2051   <ttcol>Reference</ttcol>
2052   <c>110</c>
2053   <c>Response is Stale</c>
2054   <c>
2055      <xref target="warn.110"/>
2056   </c>
2057   <c>111</c>
2058   <c>Revalidation Failed</c>
2059   <c>
2060      <xref target="warn.111"/>
2061   </c>
2062   <c>112</c>
2063   <c>Disconnected Operation</c>
2064   <c>
2065      <xref target="warn.112"/>
2066   </c>
2067   <c>113</c>
2068   <c>Heuristic Expiration</c>
2069   <c>
2070      <xref target="warn.113"/>
2071   </c>
2072   <c>199</c>
2073   <c>Miscellaneous Warning</c>
2074   <c>
2075      <xref target="warn.199"/>
2076   </c>
2077   <c>214</c>
2078   <c>Transformation Applied</c>
2079   <c>
2080      <xref target="warn.214"/>
2081   </c>
2082   <c>299</c>
2083   <c>Miscellaneous Persistent Warning</c>
2084   <c>
2085      <xref target="warn.299"/>
2086   </c>
2087</texttable>
2088<!--(END)-->
2089<?ENDINC p6-cache.iana-warn-codes ?>
2090</section>
2091</section>
2092
2093<section title="Header Field Registration" anchor="header.field.registration">
2094<t>
2095   HTTP header fields are registered within the Message Header Field Registry
2096   maintained at
2097   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
2098</t>
2099<t>
2100   This document defines the following HTTP header fields, so their
2101   associated registry entries shall be updated according to the permanent
2102   registrations below (see <xref target="BCP90"/>):
2103</t>
2104<?BEGININC p6-cache.iana-headers ?>
2105<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2106<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2107   <ttcol>Header Field Name</ttcol>
2108   <ttcol>Protocol</ttcol>
2109   <ttcol>Status</ttcol>
2110   <ttcol>Reference</ttcol>
2111
2112   <c>Age</c>
2113   <c>http</c>
2114   <c>standard</c>
2115   <c>
2116      <xref target="header.age"/>
2117   </c>
2118   <c>Cache-Control</c>
2119   <c>http</c>
2120   <c>standard</c>
2121   <c>
2122      <xref target="header.cache-control"/>
2123   </c>
2124   <c>Expires</c>
2125   <c>http</c>
2126   <c>standard</c>
2127   <c>
2128      <xref target="header.expires"/>
2129   </c>
2130   <c>Pragma</c>
2131   <c>http</c>
2132   <c>standard</c>
2133   <c>
2134      <xref target="header.pragma"/>
2135   </c>
2136   <c>Warning</c>
2137   <c>http</c>
2138   <c>standard</c>
2139   <c>
2140      <xref target="header.warning"/>
2141   </c>
2142</texttable>
2143<!--(END)-->
2144<?ENDINC p6-cache.iana-headers ?>
2145<t>
2146   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2147   Force".
2148</t>
2149</section>
2150</section>
2151
2152<section anchor="security.considerations" title="Security Considerations">
2153<t>
2154   This section is meant to inform developers, information providers, and
2155   users of known security concerns specific to HTTP/1.1 caching.
2156   More general security considerations are addressed in HTTP messaging
2157   &messaging; and semantics &semantics;.
2158</t>
2159<t>
2160   Caches expose additional potential vulnerabilities, since the contents of
2161   the cache represent an attractive target for malicious exploitation.
2162   Because cache contents persist after an HTTP request is complete, an attack
2163   on the cache can reveal information long after a user believes that the
2164   information has been removed from the network. Therefore, cache contents
2165   need to be protected as sensitive information.
2166</t>
2167<t>
2168   In particular, various attacks might be amplified by being stored in a
2169   shared cache; such "cache poisoning" attacks use the cache to distribute a
2170   malicious payload to many clients, and are especially effective when an
2171   attacker can use implmentation flaws, elevated priviledges or other
2172   techniques to insert such a response into a cache. One common attack vector
2173   for cache poisoning is to exploit differences in message parsing on proxies
2174   and in user agents; see &body.length; for the relevant requirements.
2175</t>
2176<t>
2177   Likewise, implementation flaws (as well as misunderstanding of cache
2178   operation) might lead to caching of sensitive information (e.g.,
2179   authentication credentials) that is thought to be private, exposing it to
2180   unauthorized parties.
2181</t>
2182<t>
2183   Furthermore, the very use of a cache can bring about privacy concerns. For
2184   example, if two users share a cache, and the first one browses to a site,
2185   the second may be able to detect that the other has been to that site,
2186   because the resources from it load more quickly, thanks to the cache.
2187</t>
2188<t>
2189   Note that the Set-Cookie response header field <xref target="RFC6265"/> does not
2190   inhibit caching; a cacheable response with a Set-Cookie header field can be (and
2191   often is) used to satisfy subsequent requests to caches. Servers who wish
2192   to control caching of these responses are encouraged to emit appropriate
2193   Cache-Control response header fields.
2194</t>
2195
2196</section>
2197
2198<section title="Acknowledgments" anchor="acks">
2199<t>
2200  See &acks;.
2201</t>
2202</section>
2203
2204</middle>
2205
2206<back>
2207<references title="Normative References">
2208
2209  <reference anchor="Part1">
2210    <front>
2211      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2212      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2213        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2214        <address><email>fielding@gbiv.com</email></address>
2215      </author>
2216      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2217        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2218        <address><email>julian.reschke@greenbytes.de</email></address>
2219      </author>
2220      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2221    </front>
2222    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2223    <x:source basename="p1-messaging" href="p1-messaging.xml">
2224      <x:defines>Content-Length</x:defines>
2225      <x:defines>Via</x:defines>
2226    </x:source>
2227  </reference>
2228
2229  <reference anchor="Part2">
2230    <front>
2231      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2232      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2233        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2234        <address><email>fielding@gbiv.com</email></address>
2235      </author>
2236      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2237        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2238        <address><email>julian.reschke@greenbytes.de</email></address>
2239      </author>
2240      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2241    </front>
2242    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2243    <x:source basename="p2-semantics" href="p2-semantics.xml">
2244      <x:defines>2xx (Successful)</x:defines>
2245      <x:defines>200 (OK)</x:defines>
2246      <x:defines>203 (Non-Authoritative Information)</x:defines>
2247      <x:defines>3xx (Redirection)</x:defines>
2248      <x:defines>300 (Multiple Choices)</x:defines>
2249      <x:defines>301 (Moved Permanently)</x:defines>
2250      <x:defines>404 (Not Found)</x:defines>
2251      <x:defines>410 (Gone)</x:defines>
2252      <x:defines>5xx (Server Error)</x:defines>
2253      <x:defines>504 (Gateway Timeout)</x:defines>
2254      <x:defines>Accept</x:defines>
2255      <x:defines>Content-Encoding</x:defines>
2256      <x:defines>Content-Location</x:defines>
2257      <x:defines>Content-Type</x:defines>
2258      <x:defines>Date</x:defines>
2259      <x:defines>Location</x:defines>
2260      <x:defines>Vary</x:defines>
2261    </x:source>
2262  </reference>
2263
2264  <reference anchor="Part4">
2265    <front>
2266      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2267      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2268        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2269        <address><email>fielding@gbiv.com</email></address>
2270      </author>
2271      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2272        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2273        <address><email>julian.reschke@greenbytes.de</email></address>
2274      </author>
2275      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2276    </front>
2277    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2278    <x:source basename="p4-conditional" href="p4-conditional.xml">
2279      <x:defines>304</x:defines>
2280      <x:defines>304 (Not Modified)</x:defines>
2281      <x:defines>412 (Precondition Failed)</x:defines>
2282      <x:defines>ETag</x:defines>
2283      <x:defines>If-Modified-Since</x:defines>
2284      <x:defines>If-Unmodified-Since</x:defines>
2285      <x:defines>If-Match</x:defines>
2286      <x:defines>If-None-Match</x:defines>
2287      <x:defines>Last-Modified</x:defines>
2288    </x:source>
2289  </reference>
2290
2291  <reference anchor="Part5">
2292    <front>
2293      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2294      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2295        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2296        <address><email>fielding@gbiv.com</email></address>
2297      </author>
2298      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2299        <organization abbrev="W3C">World Wide Web Consortium</organization>
2300        <address><email>ylafon@w3.org</email></address>
2301      </author>
2302      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2303        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2304        <address><email>julian.reschke@greenbytes.de</email></address>
2305      </author>
2306      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2307    </front>
2308    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2309    <x:source basename="p5-range" href="p5-range.xml">
2310      <x:defines>206 (Partial Content)</x:defines>
2311      <x:defines>Content-Range</x:defines>
2312      <x:defines>If-Range</x:defines>
2313      <x:defines>Range</x:defines>
2314    </x:source>
2315  </reference>
2316
2317  <reference anchor="Part7">
2318    <front>
2319      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2320      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2321        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2322        <address><email>fielding@gbiv.com</email></address>
2323      </author>
2324      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2325        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2326        <address><email>julian.reschke@greenbytes.de</email></address>
2327      </author>
2328      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2329    </front>
2330    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2331    <x:source basename="p7-auth" href="p7-auth.xml">
2332      <x:defines>Authorization</x:defines>
2333    </x:source>
2334  </reference>
2335
2336  <reference anchor="RFC2119">
2337    <front>
2338      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2339      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2340        <organization>Harvard University</organization>
2341        <address><email>sob@harvard.edu</email></address>
2342      </author>
2343      <date month="March" year="1997" />
2344    </front>
2345    <seriesInfo name="BCP" value="14" />
2346    <seriesInfo name="RFC" value="2119" />
2347  </reference>
2348
2349  <reference anchor="RFC5234">
2350    <front>
2351      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2352      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2353        <organization>Brandenburg InternetWorking</organization>
2354        <address>
2355          <email>dcrocker@bbiw.net</email>
2356        </address> 
2357      </author>
2358      <author initials="P." surname="Overell" fullname="Paul Overell">
2359        <organization>THUS plc.</organization>
2360        <address>
2361          <email>paul.overell@thus.net</email>
2362        </address>
2363      </author>
2364      <date month="January" year="2008"/>
2365    </front>
2366    <seriesInfo name="STD" value="68"/>
2367    <seriesInfo name="RFC" value="5234"/>
2368  </reference>
2369 
2370</references>
2371
2372<references title="Informative References">
2373
2374  <reference anchor="RFC2616">
2375    <front>
2376      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2377      <author fullname="R. Fielding" initials="R." surname="Fielding">
2378        <organization>University of California, Irvine</organization>
2379        <address><email>fielding@ics.uci.edu</email></address>
2380      </author>
2381      <author fullname="J. Gettys" initials="J." surname="Gettys">
2382        <organization>W3C</organization>
2383        <address><email>jg@w3.org</email></address>
2384      </author>
2385      <author fullname="J. Mogul" initials="J." surname="Mogul">
2386        <organization>Compaq Computer Corporation</organization>
2387        <address><email>mogul@wrl.dec.com</email></address>
2388      </author>
2389      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2390        <organization>MIT Laboratory for Computer Science</organization>
2391        <address><email>frystyk@w3.org</email></address>
2392      </author>
2393      <author fullname="L. Masinter" initials="L." surname="Masinter">
2394        <organization>Xerox Corporation</organization>
2395        <address><email>masinter@parc.xerox.com</email></address>
2396      </author>
2397      <author fullname="P. Leach" initials="P." surname="Leach">
2398        <organization>Microsoft Corporation</organization>
2399        <address><email>paulle@microsoft.com</email></address>
2400      </author>
2401      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2402        <organization>W3C</organization>
2403        <address><email>timbl@w3.org</email></address>
2404      </author>
2405      <date month="June" year="1999" />
2406    </front>
2407    <seriesInfo name="RFC" value="2616" />
2408  </reference>
2409
2410  <reference anchor="BCP90">
2411    <front>
2412      <title>Registration Procedures for Message Header Fields</title>
2413      <author fullname="G. Klyne" initials="G." surname="Klyne">
2414        <organization>Nine by Nine</organization>
2415        <address><email>GK-IETF@ninebynine.org</email></address>
2416      </author>
2417      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2418        <organization>BEA Systems</organization>
2419        <address><email>mnot@pobox.com</email></address>
2420      </author>
2421      <author fullname="J. Mogul" initials="J." surname="Mogul">
2422        <organization>HP Labs</organization>
2423        <address><email>JeffMogul@acm.org</email></address>
2424      </author>
2425      <date month="September" year="2004" />
2426    </front>
2427    <seriesInfo name="BCP" value="90" />
2428    <seriesInfo name="RFC" value="3864" />
2429  </reference>
2430
2431  <reference anchor='RFC5226'>
2432    <front>
2433      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2434      <author initials='T.' surname='Narten' fullname='T. Narten'>
2435        <organization>IBM</organization>
2436        <address><email>narten@us.ibm.com</email></address>
2437      </author>
2438      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2439        <organization>Google</organization>
2440        <address><email>Harald@Alvestrand.no</email></address>
2441      </author>
2442      <date year='2008' month='May' />
2443    </front>
2444    <seriesInfo name='BCP' value='26' />
2445    <seriesInfo name='RFC' value='5226' />
2446  </reference>
2447
2448  <reference anchor='RFC5861'>
2449    <front>
2450      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2451      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2452        <organization>Yahoo! Inc.</organization>
2453        <address><email>mnot@yahoo-inc.com</email></address>
2454      </author>
2455      <date month="April" year="2010"/>
2456    </front>
2457    <seriesInfo name='RFC' value='5861' />
2458  </reference>
2459
2460  <reference anchor='RFC5905'>
2461    <front>
2462      <title>Network Time Protocol Version 4: Protocol and Algorithms Specification</title>
2463      <author initials='D.' surname='Mills' fullname='David L. Mills'/>
2464      <author initials='J.' surname='Martin' fullname='Jim Martin' role="editor"/>
2465      <author initials='J.' surname='Burbank' fullname='Jack Burbank'/>
2466      <author initials='W.' surname='Kasch' fullname='William Kasch'/>
2467      <date year='2010' month='June' />
2468    </front>
2469    <seriesInfo name='RFC' value='5905' />
2470  </reference>
2471
2472  <reference anchor="RFC6265">
2473    <front>
2474      <title>HTTP State Management Mechanism</title>
2475      <author initials="A." surname="Barth" fullname="Adam Barth">
2476        <organization abbrev="U.C. Berkeley">
2477          University of California, Berkeley
2478        </organization>
2479        <address><email>abarth@eecs.berkeley.edu</email></address>
2480      </author>
2481      <date year="2011" month="April" />
2482    </front>
2483    <seriesInfo name="RFC" value="6265"/>
2484  </reference>
2485
2486</references>
2487
2488<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2489<t>
2490  The specification has been substantially rewritten for clarity.
2491</t>
2492<t>
2493  The conditions under which an authenticated response can be cached have been
2494  clarified.
2495  (<xref target="caching.authenticated.responses" />)
2496</t>
2497<t>
2498  New status codes can now define that caches are allowed to use heuristic
2499  freshness with them.
2500  Caches are now allowed to calculate heuristic freshness for URIs with query
2501  components.
2502  (<xref target="heuristic.freshness" />)
2503</t>
2504<t>
2505  The algorithm for calculating age is now less conservative.
2506  Caches are now required to handle dates with timezones as if they're
2507  invalid, because it's not possible to accurately guess.
2508  (<xref target="age.calculations"/>)
2509</t>
2510<t>
2511  The <x:ref>Content-Location</x:ref> response header field is no longer used
2512  to determine the appropriate response to use when validating.
2513  (<xref target="validation.model" />)
2514</t>
2515<t>
2516  The algorithm for selecting a cached negotiated response to use has been
2517  clarified in several ways. In particular, it now explicitly allows
2518  header-specific canonicalization when processing selecting header fields.
2519  (<xref target="caching.negotiated.responses" />)
2520</t>
2521<t>
2522  Requirements regarding denial of service attack avoidance when performing
2523  invalidation have been clarified.
2524  (<xref target="invalidation" />)
2525</t>
2526<t>
2527  Cache invalidation only occurs when a successful response is received.
2528  (<xref target="invalidation" />)
2529</t>
2530<t>
2531  Cache directives are explicitly defined to be case-insensitive.
2532  Handling of multiple instances of cache directives when only one is
2533  expected is now defined.
2534  (<xref target="header.cache-control" />)
2535</t>
2536<t>
2537  The "no-store" request directive doesn't apply to responses; i.e.,
2538  a cache can satisfy a request with no-store on it, and does not invalidate
2539  it.
2540  (<xref target="cache-request-directive.no-store" />)
2541</t>
2542<t>
2543  The qualified forms of the private and no-cache cache directives are noted
2544  to not be widely implemented; e.g., "private=foo" is interpreted by many
2545  caches as simply "private". Additionally, the meaning of the qualified
2546  form of no-cache has been clarified.
2547  (<xref target="cache-response-directive" />)
2548</t>
2549<t>
2550  The "no-cache" response directive's meaning has been clarified.
2551  (<xref target="cache-response-directive.no-cache" />)
2552</t>
2553<t>
2554  The one-year limit on <x:ref>Expires</x:ref> header field values has been removed; instead,
2555  the reasoning for using a sensible value is given.
2556  (<xref target="header.expires" />)
2557</t>
2558<t>
2559  The <x:ref>Pragma</x:ref> header field is now only defined for backwards compatibility;
2560  future pragmas are deprecated.
2561  (<xref target="header.pragma" />)
2562</t>
2563<t>
2564  Some requirements regarding production and processing of the <x:ref>Warning</x:ref>
2565  header fields have been relaxed, as it is not widely implemented.
2566  Furthermore, the <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2567  nor allows multiple languages, as these aspects were not implemented.
2568  (<xref target="header.warning" />)
2569</t>
2570<t>
2571  This specification introduces the Cache Directive and Warn Code Registries,
2572  and defines considerations for new cache directives.
2573  (<xref target="cache.directive.registry"/> and <xref target="warn.code.registry"/>)
2574</t>
2575</section>
2576
2577<section title="Imported ABNF" anchor="imported.abnf">
2578   <x:anchor-alias value="ALPHA"/>
2579   <x:anchor-alias value="CR"/>
2580   <x:anchor-alias value="DIGIT"/>
2581   <x:anchor-alias value="DQUOTE"/>
2582   <x:anchor-alias value="LF"/>
2583   <x:anchor-alias value="OCTET"/>
2584   <x:anchor-alias value="SP"/>
2585   <x:anchor-alias value="VCHAR"/>
2586   <x:anchor-alias value="quoted-string"/>
2587   <x:anchor-alias value="token"/>
2588   <x:anchor-alias value="OWS"/>
2589   <x:anchor-alias value="field-name"/>
2590   <x:anchor-alias value="HTTP-date"/>
2591   <x:anchor-alias value="port"/>
2592   <x:anchor-alias value="pseudonym"/>
2593   <x:anchor-alias value="uri-host"/>
2594<t>
2595   The following core rules are included by reference, as defined in <xref
2596   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2597   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2598   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2599   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2600</t>
2601<t>
2602   The rules below are defined in <xref target="Part1"/>:
2603</t>
2604<figure><artwork type="abnf2616">
2605  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2606  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2607  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2608  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2609
2610  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2611  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
2612  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2613</artwork></figure>
2614<t>
2615   The rules below are defined in other parts:
2616</t>
2617<figure><artwork type="abnf2616">
2618  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2619</artwork></figure>
2620</section>
2621
2622<?BEGININC p6-cache.abnf-appendix ?>
2623<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2624<t>
2625  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
2626</t><figure>
2627<artwork type="abnf" name="p6-cache.parsed-abnf">
2628<x:ref>Age</x:ref> = delta-seconds
2629
2630<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2631 cache-directive ] )
2632
2633<x:ref>Expires</x:ref> = HTTP-date
2634
2635<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
2636
2637<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
2638
2639<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2640 pragma-directive ] )
2641
2642<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2643 )
2644
2645<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2646
2647<x:ref>delta-seconds</x:ref> = 1*DIGIT
2648
2649<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2650
2651<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2652
2653<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2654<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2655<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
2656
2657<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
2658
2659<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.6&gt;
2660
2661<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2662
2663<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2664<x:ref>warn-code</x:ref> = 3DIGIT
2665<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2666<x:ref>warn-text</x:ref> = quoted-string
2667<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2668 ]
2669</artwork>
2670</figure>
2671</section>
2672<?ENDINC p6-cache.abnf-appendix ?>
2673
2674<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2675<t>
2676  Changes up to the IETF Last Call draft are summarized
2677  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-24#appendix-D"/>.
2678</t>
2679
2680<section title="Since draft-ietf-httpbis-p6-cache-24" anchor="changes.since.24">
2681<t>
2682  Closed issues:
2683  <list style="symbols">
2684    <t>
2685      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/499"/>:
2686      "RFC 1305 ref needs to be updated to 5905"
2687    </t>
2688    <t>
2689      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/500"/>:
2690      "dangling reference to cacheable status codes"
2691    </t>
2692    <t>
2693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/512"/>:
2694      "APPSDIR review of draft-ietf-httpbis-p6-cache-24"
2695    </t>
2696  </list>
2697</t>
2698</section>
2699
2700<section title="Since draft-ietf-httpbis-p6-cache-25" anchor="changes.since.25">
2701<t>
2702  None yet.
2703</t>
2704</section>
2705</section>
2706  </back>
2707</rfc>
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