source: draft-ietf-httpbis/latest/p6-cache.xml @ 1697

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bump up document dates and update to latest version of rfc2629.xslt

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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 "July">
16  <!ENTITY ID-YEAR "2012">
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 notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44]>
45<?rfc toc="yes" ?>
46<?rfc symrefs="yes" ?>
47<?rfc sortrefs="yes" ?>
48<?rfc compact="yes"?>
49<?rfc subcompact="no" ?>
50<?rfc linkmailto="no" ?>
51<?rfc editing="no" ?>
52<?rfc comments="yes"?>
53<?rfc inline="yes"?>
54<?rfc rfcedstyle="yes"?>
55<?rfc-ext allow-markup-in-artwork="yes" ?>
56<?rfc-ext include-references-in-index="yes" ?>
57<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
58  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
59<x:link rel="prev" basename="p5-range"/>
60<x:link rel="next" basename="p7-auth"/>
61<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
62<front>
63
64  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
65
66  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
67    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
68    <address>
69      <postal>
70        <street>345 Park Ave</street>
71        <city>San Jose</city>
72        <region>CA</region>
73        <code>95110</code>
74        <country>USA</country>
75      </postal>
76      <email>fielding@gbiv.com</email>
77      <uri>http://roy.gbiv.com/</uri>
78    </address>
79  </author>
80
81  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
82    <organization abbrev="W3C">World Wide Web Consortium</organization>
83    <address>
84      <postal>
85        <street>W3C / ERCIM</street>
86        <street>2004, rte des Lucioles</street>
87        <city>Sophia-Antipolis</city>
88        <region>AM</region>
89        <code>06902</code>
90        <country>France</country>
91      </postal>
92      <email>ylafon@w3.org</email>
93      <uri>http://www.raubacapeu.net/people/yves/</uri>
94    </address>
95  </author>
96
97  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
98    <organization>Rackspace</organization>
99    <address>
100      <email>mnot@mnot.net</email>
101      <uri>http://www.mnot.net/</uri>
102    </address>
103  </author>
104
105  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
106    <organization abbrev="greenbytes">greenbytes GmbH</organization>
107    <address>
108      <postal>
109        <street>Hafenweg 16</street>
110        <city>Muenster</city><region>NW</region><code>48155</code>
111        <country>Germany</country>
112      </postal>
113      <email>julian.reschke@greenbytes.de</email>
114      <uri>http://greenbytes.de/tech/webdav/</uri>
115    </address>
116  </author>
117
118  <date month="&ID-MONTH;" year="&ID-YEAR;" />
119  <workgroup>HTTPbis Working Group</workgroup>
120
121<abstract>
122<t>
123   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
124   distributed, collaborative, hypertext information systems. HTTP has been in
125   use by the World Wide Web global information initiative since 1990. This
126   document is Part 6 of the seven-part specification that defines the protocol
127   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
128</t>
129<t>
130   Part 6 defines requirements on HTTP caches and the associated header fields
131   that control cache behavior or indicate cacheable response messages.
132</t>
133</abstract>
134
135<note title="Editorial Note (To be removed by RFC Editor)">
136  <t>
137    Discussion of this draft ought to take place on the HTTPBIS working group
138    mailing list (ietf-http-wg@w3.org), which is archived at
139    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
140  </t>
141  <t>
142    The current issues list is at
143    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
144    documents (including fancy diffs) can be found at
145    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
146  </t>
147  <t>
148    The changes in this draft are summarized in <xref target="changes.since.19"/>.
149  </t>
150</note>
151
152   </front>
153   <middle>
154
155<section anchor="caching" title="Introduction">
156<t>
157   HTTP is typically used for distributed information systems, where
158   performance can be improved by the use of response caches. This document
159   defines aspects of HTTP/1.1 related to caching and reusing response
160   messages.
161</t>
162
163<section anchor="intro.purpose" title="Purpose">
164<iref item="cache" />
165<t>
166   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
167   subsystem that controls its message storage, retrieval, and deletion. A
168   cache stores cacheable responses in order to reduce the response time and
169   network bandwidth consumption on future, equivalent requests. Any client or
170   server &MAY; employ a cache, though a cache cannot be used by a server that
171   is acting as a tunnel.
172</t>
173<t>
174   The goal of caching in HTTP/1.1 is to significantly improve performance
175   by reusing a prior response message to satisfy a current request.
176   A stored response is considered "fresh", as defined in
177   <xref target="expiration.model" />, if the response can be reused without
178   "validation" (checking with the origin server to see if the cached response
179   remains valid for this request).  A fresh cache response can therefore
180   reduce both latency and network transfers each time it is reused.
181   When a cached response is not fresh, it might still be reusable if it can
182   be freshened by validation (<xref target="validation.model" />) or if the
183   origin is unavailable.
184</t>
185</section>
186
187<section anchor="intro.terminology" title="Terminology">
188<t>
189   This specification uses a number of terms to refer to the roles played by
190   participants in, and objects of, HTTP caching.
191</t>
192<t>
193   <iref item="cache" />
194   <x:dfn>cache</x:dfn>
195   <list>
196      <t>A conformant implementation of a HTTP cache. Note that this implies
197        an HTTP/1.1 cache; this specification does not define conformance
198        for HTTP/1.0 caches.</t>
199   </list>
200</t>
201<t anchor="shared.and.non-shared.caches">
202   <iref item="shared cache" />
203   <x:dfn>shared cache</x:dfn>
204   <list>
205      <t>A cache that stores responses to be reused by more than one user;
206         usually (but not always) deployed as part of an intermediary.</t>
207   </list>
208</t>
209<t>
210   <iref item="private cache" />
211   <x:dfn>private cache</x:dfn>
212   <list>
213      <t>A cache that is dedicated to a single user.</t>
214   </list>
215</t>
216<t>
217   <iref item="cacheable" />
218   <x:dfn>cacheable</x:dfn>
219   <list>
220      <t>A response is cacheable if a cache is allowed to store a copy of the
221      response message for use in answering subsequent requests. Even when a
222      response is cacheable, there might be additional constraints on whether
223      a cache can use the stored copy to satisfy a particular request.</t>
224   </list>
225</t>
226<t>
227   <iref item="explicit expiration time" />
228   <x:dfn>explicit expiration time</x:dfn>
229   <list>
230      <t>The time at which the origin server intends that a representation
231      no longer be returned by a cache without further validation.</t>
232   </list>
233</t>
234<t>
235   <iref item="heuristic expiration time" />
236   <x:dfn>heuristic expiration time</x:dfn>
237   <list>
238      <t>An expiration time assigned by a cache when no explicit expiration
239      time is available.</t>
240   </list>
241</t>
242<t>
243   <iref item="age" />
244   <x:dfn>age</x:dfn>
245   <list>
246      <t>The age of a response is the time since it was sent by, or
247      successfully validated with, the origin server.</t>
248   </list>
249</t>
250<t>
251   <iref item="first-hand" />
252   <x:dfn>first-hand</x:dfn>
253   <list>
254      <t>A response is first-hand if the freshness model is not in use; i.e.,
255      its age is 0.</t>
256   </list>
257</t>
258<t>
259   <iref item="freshness lifetime" />
260   <x:dfn>freshness lifetime</x:dfn>
261   <list>
262      <t>The length of time between the generation of a response and its
263      expiration time.</t>
264   </list>
265</t>
266<t>
267   <iref item="fresh" />
268   <x:dfn>fresh</x:dfn>
269   <list>
270      <t>A response is fresh if its age has not yet exceeded its freshness
271      lifetime.</t>
272   </list>
273</t>
274<t>
275   <iref item="stale" />
276   <x:dfn>stale</x:dfn>
277   <list>
278      <t>A response is stale if its age has passed its freshness lifetime
279      (either explicit or heuristic).</t>
280   </list>
281</t>
282<t>
283   <iref item="validator" />
284   <x:dfn>validator</x:dfn>
285   <list>
286      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
287      is used to find out whether a stored response is an equivalent copy of
288      a representation. See &weak-and-strong;.</t>
289   </list>
290</t>
291<t>
292   <iref item="strong validator" />
293   <iref item="validator" subitem="strong" />
294   <x:dfn>strong validator</x:dfn>
295   <list>
296      <t>A validator that is defined by the origin server such that its
297         current value will change if the representation body changes; i.e.,
298         an entity-tag that is not marked as weak (&entity-tags;) or,
299         if no entity-tag is provided, a Last-Modified value that is strong
300         in the sense defined by &lastmod-comparison;.</t>
301   </list>
302</t>
303</section>
304
305<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
306<t>
307   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
308   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
309   document are to be interpreted as described in <xref target="RFC2119"/>.
310</t>
311<t>
312   This document defines conformance criteria for several roles in HTTP
313   communication, including Senders, Recipients, Clients, Servers, User-Agents,
314   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
315   for definitions of these terms.
316</t>
317<t>
318   An implementation is considered conformant if it complies with all of the
319   requirements associated with its role(s). Note that SHOULD-level requirements
320   are relevant here, unless one of the documented exceptions is applicable.
321</t>
322<t>
323   This document also uses ABNF to define valid protocol elements
324   (<xref target="notation"/>). In addition to the prose requirements placed
325   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
326</t>
327<t>
328   Unless noted otherwise, Recipients &MUST; be able to parse all protocol
329   elements matching the ABNF rules defined for them and &MAY; take steps to
330   recover a usable protocol element from an invalid construct. However, HTTP does not define
331   specific error handling mechanisms, except in cases where it has direct
332   impact on security. This is because different uses of the protocol require
333   different error handling strategies; for example, a Web browser might wish to
334   transparently recover from a response where the Location header field
335   doesn't parse according to the ABNF, whereby in a systems control protocol
336   using HTTP, this type of error recovery could lead to dangerous consequences.
337</t>
338</section>
339
340<section title="Syntax Notation" anchor="notation">
341   <x:anchor-alias value="ALPHA"/>
342   <x:anchor-alias value="CR"/>
343   <x:anchor-alias value="DIGIT"/>
344   <x:anchor-alias value="DQUOTE"/>
345   <x:anchor-alias value="LF"/>
346   <x:anchor-alias value="OCTET"/>
347   <x:anchor-alias value="SP"/>
348   <x:anchor-alias value="VCHAR"/>
349<t>
350   This specification uses the Augmented Backus-Naur Form (ABNF) notation
351   of <xref target="RFC5234"/> with the list rule extension defined in
352   &notation;<xref target="collected.abnf"/> shows the collected ABNF
353   with the list rule expanded.
354</t>
355<t>
356   The following core rules are included by reference, as defined in <xref
357   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
358   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
359   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
360   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
361</t>
362
363<section title="Core Rules" anchor="core.rules">
364   <x:anchor-alias value="quoted-string"/>
365   <x:anchor-alias value="token"/>
366   <x:anchor-alias value="OWS"/>
367<t>
368   The core rules below are defined in <xref target="Part1"/>:
369</t>
370<figure><artwork type="abnf2616">
371  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
372  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
373  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
374</artwork></figure>
375</section>
376
377<section title="ABNF Rules defined in other Parts of the Specification"
378    anchor="abnf.dependencies">
379   <x:anchor-alias value="field-name"/>
380   <x:anchor-alias value="HTTP-date"/>
381   <x:anchor-alias value="port"/>
382   <x:anchor-alias value="pseudonym"/>
383   <x:anchor-alias value="uri-host"/>
384<t>
385   The ABNF rules below are defined in other parts:
386</t>
387<figure><!--Part1--><artwork type="abnf2616">
388  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
389  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
390  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
391  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
392  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
393</artwork></figure>
394</section>
395</section>
396
397<section title="Delta Seconds" anchor="delta-seconds">
398<t>
399   The delta-seconds rule specifies a non-negative integer, representing time
400   in seconds.
401</t>
402<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
403  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
404</artwork></figure>
405<t>
406   If an implementation receives a delta-seconds value larger than the largest
407   positive integer it can represent, or if any of its subsequent calculations
408   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
409   Recipients parsing a delta-seconds value &MUST; use an arithmetic type of
410   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
411   value greater than 2147483648.
412</t>
413</section>
414
415</section>
416
417<section anchor="caching.overview" title="Cache Operation">
418<iref item="cache entry" />
419<iref item="cache key" />
420<t>
421   Proper cache operation preserves the semantics of HTTP transfers
422   (&semantics;) while eliminating the transfer of information already held
423   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
424   we assume that reusing the cached response is desirable and that such
425   reuse is the default behavior when no requirement or locally-desired
426   configuration prevents it.  Therefore, HTTP cache requirements are focused
427   on preventing a cache from either storing a non-reusable response or
428   reusing a stored response inappropriately.
429</t>
430<t>
431   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
432   HTTP responses corresponding to prior requests that used the same key.
433   The most common form of cache entry is a successful result of a retrieval
434   request: i.e., a 200 (OK) response containing a representation of the
435   resource identified by the request target.  However, it is also possible
436   to cache negative results (e.g., 404 not found), incomplete results
437   (e.g., 206 partial content), and responses to safe methods other than
438   GET if the method's definition allows such caching and defines something
439   suitable for use as a cache key.
440</t>
441<t>
442   The default <x:dfn>cache key</x:dfn> consists of the request method and
443   target URI.  However, since HTTP caches in common use today are typically
444   limited to caching responses to GET, most implementations simply decline
445   other methods and use only the URI as the key.
446</t>
447<t>
448   If a request target is subject to content negotiation, its cache entry
449   might consist of multiple stored responses, each differentiated by a
450   secondary key for the values of the original request's selecting header
451   fields (<xref target="caching.negotiated.responses"/>).
452</t>
453
454<section anchor="response.cacheability" title="Response Cacheability">
455<t>
456   A cache &MUST-NOT; store a response to any request, unless:
457   <list style="symbols">
458      <t>The request method is understood by the cache and defined as being
459      cacheable, and</t>
460      <t>the response status code is understood by the cache, and</t>
461      <t>the "no-store" cache directive (see <xref
462      target="header.cache-control" />) does not appear in request or response
463      header fields, and</t>
464      <t>the "private" cache response directive (see <xref
465      target="cache-response-directive" />) does not appear in the response, if
466      the cache is shared, and</t>
467      <t>the "Authorization" header field (see &header-authorization;) does not
468      appear in the request, if the cache is shared, unless the response
469      explicitly allows it (see <xref target="caching.authenticated.responses"
470      />), and</t>
471      <t>the response either:
472         <list style="symbols">
473            <t>contains an Expires header field (see <xref target="header.expires"
474            />), or</t>
475            <t>contains a max-age response cache directive (see <xref
476            target="cache-response-directive" />), or</t>
477            <t>contains a s-maxage response cache directive and the cache is
478            shared, or</t>
479            <t>contains a Cache Control Extension (see <xref
480            target="cache.control.extensions" />) that allows it to be cached,
481            or</t>
482            <t>has a status code that can be served with heuristic freshness
483            (see <xref target="heuristic.freshness" />).</t>
484         </list>
485      </t>
486   </list>
487</t>
488<t>
489   Note that any of the requirements listed above can be overridden by a
490   cache-control extension; see <xref target="cache.control.extensions" />.
491</t>
492<t>
493   In this context, a cache has "understood" a request method or a response
494   status code if it recognizes it and implements any cache-specific
495   behavior.
496</t>
497<t>
498   Note that, in normal operation, most caches will not store a response that
499   has neither a cache validator nor an explicit expiration time, as such
500   responses are not usually useful to store. However, caches are not
501   prohibited from storing such responses.
502</t>
503<t>
504   A response message is considered complete when all of the octets
505   indicated by the message framing (&messaging;) are received
506   prior to the connection being closed.
507   If the request is GET, the response status is 200 (OK), and the entire
508   response header block has been received, a cache &MAY; store an incomplete
509   response message body if the cache entry is recorded as incomplete.
510   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
511   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
512   incomplete or partial content responses if it does not support the Range
513   and Content-Range header fields or if it does not understand the
514   range units used in those fields.
515</t>
516<t>
517   A cache &MAY; complete a stored incomplete response by making a subsequent
518   range request (&partial;) and combining the successful response with the
519   stored entry, as defined in <xref target="combining.responses"/>.
520   A cache &MUST-NOT; use an incomplete response to answer requests
521   unless the response has been made complete or the request is partial and
522   specifies a range that is wholly within the incomplete response.
523   A cache &MUST-NOT; send a partial response to a client without explicitly
524   marking it as such using the 206 (Partial Content) status code.
525</t>
526</section>
527
528
529<section anchor="constructing.responses.from.caches" 
530   title="Constructing Responses from Caches">
531<t>
532   For a presented request, a cache &MUST-NOT; return a stored response,
533   unless:
534   <list style="symbols">
535      <t>The presented effective request URI (&effective-request-uri;) and
536      that of the stored response match, and</t>
537      <t>the request method associated with the stored response allows it to
538      be used for the presented request, and</t>
539      <t>selecting header fields nominated by the stored response (if any)
540      match those presented (see <xref target="caching.negotiated.responses"
541      />), and</t>
542      <t>the presented request does not contain the no-cache pragma (<xref
543      target="header.pragma"/>), nor the no-cache cache directive (<xref
544      target="cache-request-directive"/>), unless the stored response is
545      successfully validated (<xref target="validation.model"/>), and</t>
546      <t>the stored response does not contain the no-cache cache directive
547      (<xref target="cache-response-directive"/>), unless it is successfully
548      validated (<xref target="validation.model"/>), and</t>
549      <t>the stored response is either:
550         <list style="symbols">
551            <t>fresh (see <xref target="expiration.model" />), or</t>
552            <t>allowed to be served stale (see <xref
553            target="serving.stale.responses" />), or</t>
554            <t>successfully validated (see <xref target="validation.model"
555            />).</t>
556         </list>
557      </t>
558  </list>
559</t>
560<t>
561   Note that any of the requirements listed above can be overridden by a
562   cache-control extension; see <xref target="cache.control.extensions" />.
563</t>
564<t>
565   When a stored response is used to satisfy a request without validation,
566   a cache &MUST; include a single Age header field (<xref target="header.age"
567   />) in the response with a value equal to the stored response's
568   current_age; see <xref target="age.calculations" />.
569</t>
570<t>
571   A cache &MUST; write through requests with methods that are unsafe
572   (&safe-methods;) to the origin server; i.e., a cache is not allowed to generate
573   a reply to such a request before having forwarded the request and having
574   received a corresponding response.
575</t>
576<t>
577   Also, note that unsafe requests might invalidate already stored responses;
578   see <xref target="invalidation.after.updates.or.deletions" />.
579</t>
580<t>
581   When more than one suitable response is stored, a cache &MUST; use the
582   most recent response (as determined by the Date header field). It can also
583   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
584   no-cache" to disambiguate which response to use.
585</t>
586<t>
587   A cache that does not have a clock available &MUST-NOT; use stored responses
588   without revalidating them on every use. A cache, especially a shared
589   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
590   synchronize its clock with a reliable external standard.
591</t>
592
593</section>
594
595<section anchor="expiration.model" title="Freshness Model">
596<t>
597   When a response is "fresh" in the cache, it can be used to satisfy
598   subsequent requests without contacting the origin server, thereby improving
599   efficiency.
600</t>
601<t>
602   The primary mechanism for determining freshness is for an origin server to
603   provide an explicit expiration time in the future, using either the Expires
604   header field (<xref target="header.expires" />) or the max-age response cache
605   directive (<xref target="cache-response-directive" />). Generally, origin
606   servers will assign future explicit expiration times to responses in the
607   belief that the representation is not likely to change in a semantically
608   significant way before the expiration time is reached.
609</t>
610<t>
611   If an origin server wishes to force a cache to validate every request, it
612   can assign an explicit expiration time in the past to indicate that the
613   response is already stale. Compliant caches will normally validate the
614   cached response before reusing it for subsequent requests (see <xref
615   target="serving.stale.responses" />).
616</t>
617<t>
618   Since origin servers do not always provide explicit expiration times,
619   a cache &MAY; assign a heuristic expiration time when an explicit time is not
620   specified, employing algorithms that use other header field values (such as the
621   Last-Modified time) to estimate a plausible expiration time. This
622   specification does not provide specific algorithms, but does impose
623   worst-case constraints on their results.
624</t>
625<figure>
626<preamble>
627  The calculation to determine if a response is fresh is:
628</preamble>
629<artwork type="code">
630   response_is_fresh = (freshness_lifetime &gt; current_age)
631</artwork>
632</figure>
633<t>
634   The freshness_lifetime is defined in <xref
635   target="calculating.freshness.lifetime" />; the current_age is defined in
636   <xref target="age.calculations" />.
637</t>
638<t>
639   Additionally, clients can influence freshness calculation &mdash; either
640   constraining it relaxing it &mdash; by using the max-age and min-fresh
641   request cache directives. See <xref target="cache-request-directive" /> 
642   for details.
643</t>
644<t>
645   Note that freshness applies only to cache operation; it cannot be used to
646   force a user agent to refresh its display or reload a resource. See <xref
647   target="history.lists" /> for an explanation of the difference between
648   caches and history mechanisms.
649</t>
650
651<section anchor="calculating.freshness.lifetime" 
652   title="Calculating Freshness Lifetime">
653<t>
654   A cache can calculate the freshness lifetime (denoted as
655   freshness_lifetime) of a response by using the first match of:
656   <list style="symbols">
657      <t>If the cache is shared and the s-maxage response cache directive
658      (<xref target="cache-response-directive" />) is present, use its value,
659      or</t>
660      <t>If the max-age response cache directive (<xref
661      target="cache-response-directive" />) is present, use its value, or</t>
662      <t>If the Expires response header field (<xref target="header.expires" />) is
663      present, use its value minus the value of the Date response header field,
664      or</t>
665      <t>Otherwise, no explicit expiration time is present in the response. A
666      heuristic freshness lifetime might be applicable; see <xref
667      target="heuristic.freshness" />.</t>
668   </list>
669</t>
670<t>
671   Note that this calculation is not vulnerable to clock skew, since all of
672   the information comes from the origin server.
673</t>
674
675<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
676<t>
677   If no explicit expiration time is present in a stored response that has a
678   status code whose definition allows heuristic freshness to be used
679   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
680   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
681   use heuristics to determine freshness for responses with status codes that do
682   not explicitly allow it.
683</t>
684<t>
685   When a heuristic is used to calculate freshness lifetime, a cache
686   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
687   its current_age is more than 24 hours and such a warning is not already
688   present.
689</t>
690<t>
691   Also, if the response has a Last-Modified header field
692   (&header-last-modified;), caches are encouraged to use a heuristic
693   expiration value that is no more than some fraction of the interval since
694   that time. A typical setting of this fraction might be 10%.
695</t>
696<x:note>
697   <t>
698      &Note; RFC 2616 (<xref target="RFC2616" x:fmt=","
699      x:sec="13.9"/>) required that caches do not calculate heuristic
700      freshness for URIs with query components (i.e., those containing '?').
701      In practice, this has not been widely implemented. Therefore, servers
702      are encouraged to send explicit directives (e.g., Cache-Control:
703      no-cache) if they wish to preclude caching.
704   </t>
705</x:note>
706</section>
707</section>
708
709<section anchor="age.calculations" title="Calculating Age">
710<t>
711   HTTP/1.1 uses the Age header field to convey the estimated age of the
712   response message when obtained from a cache. The Age field value is the
713   cache's estimate of the amount of time since the response was generated or
714   validated by the origin server. In essence, the Age value is the sum of the
715   time that the response has been resident in each of the caches along the
716   path from the origin server, plus the amount of time it has been in transit
717   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 Age header field (<xref
727         target="header.age"/>), in a form appropriate for arithmetic
728         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         HTTP/1.1 requires origin servers to send a Date header field, if possible,
737         with every response, giving the time at which the response was
738         generated. The term "date_value" denotes the value of the Date
739         header field, in a form appropriate for arithmetic operations. See
740         &header-date; for the definition of the Date header field, and for
741         requirements regarding responses without it.
742      </t>
743   </list>
744</t>
745<t>
746   <x:dfn>now</x:dfn>
747   <list>
748      <t>
749         The term "now" means "the current value of the clock at the host
750         performing the calculation". A cache &SHOULD; use NTP (<xref
751         target="RFC1305"/>) or some similar protocol to synchronize its
752         clocks to a globally accurate time standard.
753      </t>
754   </list>
755</t>
756<t>
757   <x:dfn>request_time</x:dfn>
758   <list>
759      <t>
760         The current value of the clock at the host at the time the request
761         resulting in the stored response was made.
762      </t>
763   </list>
764</t>
765<t>
766   <x:dfn>response_time</x:dfn>
767   <list>
768      <t>
769         The current value of the clock at the host at the time the response
770         was received.
771      </t>
772   </list>
773</t>
774<t>
775   A response's age can be calculated in two entirely independent ways:
776   <list style="numbers">
777      <t>the "apparent_age": response_time minus date_value, if the local
778      clock is reasonably well synchronized to the origin server's clock. If
779      the result is negative, the result is replaced by zero.</t>
780      <t>the "corrected_age_value", if all of the caches along the response
781      path implement HTTP/1.1. A cache &MUST; interpret this value relative
782      to the time the request was initiated, not the time that the response
783      was received.</t>
784   </list>
785</t>
786<figure>
787<artwork type="code">
788  apparent_age = max(0, response_time - date_value);
789
790  response_delay = response_time - request_time;
791  corrected_age_value = age_value + response_delay; 
792</artwork>
793</figure>
794<figure>
795<preamble>These &SHOULD; be combined as</preamble>
796<artwork type="code">
797  corrected_initial_age = max(apparent_age, corrected_age_value);
798</artwork></figure>
799<t>
800   unless the cache is confident in the value of the Age header (e.g., because
801   there are no HTTP/1.0 hops in the Via header), in which case the
802   corrected_age_value &MAY; be used as the corrected_initial_age.</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<t>
813   Additionally, to avoid common problems in date parsing:
814</t>
815<t>
816  <list style="symbols">
817     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
818        which appears to be more than 50 years in the future is in fact
819        in the past (this helps solve the "year 2000" problem).</t>
820
821     <t>Although all date formats are specified to be case-sensitive,
822        recipients &SHOULD; match day, week and timezone names
823        case-insensitively.</t>
824             
825     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
826        Expires date as earlier than the proper value, but &MUST-NOT;
827        internally represent a parsed Expires date as later than the
828        proper value.</t>
829
830     <t>All expiration-related calculations &MUST; be done in GMT. The
831        local time zone &MUST-NOT; influence the calculation or comparison
832        of an age or expiration time.</t>
833
834     <t>If an HTTP header field incorrectly carries a date value with a time
835        zone other than GMT, it &MUST; be converted into GMT using the
836        most conservative possible conversion.</t>
837  </list>
838</t>
839</section>
840
841<section anchor="serving.stale.responses" title="Serving Stale Responses">
842<t>
843   A "stale" response is one that either has explicit expiry information or is
844   allowed to have heuristic expiry calculated, but is not fresh according to
845   the calculations in <xref target="expiration.model" />.
846</t>
847<t>
848   A cache &MUST-NOT; return a stale response if it is prohibited by an
849   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
850   directive, a "must-revalidate" cache-response-directive, or an applicable
851   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
852   target="cache-response-directive"/>).
853</t>
854<t>
855   A cache &MUST-NOT; return stale responses unless it is disconnected
856   (i.e., it cannot contact the origin server or otherwise find a forward
857   path) or doing so is explicitly allowed (e.g., by the max-stale request
858   directive; see <xref target="cache-request-directive" />).
859</t>
860<t>
861   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
862   <xref target="header.warning" />) to stale responses. Likewise, a cache
863   &SHOULD; add the 112 warn-code to stale responses if the cache is
864   disconnected.
865</t>
866<t>
867   If a cache receives a first-hand response (either an entire response, or a
868   304 (Not Modified) response) that it would normally forward to the
869   requesting client, and the received response is no longer fresh, the cache
870   can forward it to the requesting client without adding a new Warning (but
871   without removing any existing Warning header fields). A cache shouldn't
872   attempt to validate a response simply because that response became stale in
873   transit.
874</t>
875</section>
876</section>
877
878<section anchor="validation.model" title="Validation Model">
879<t>
880   When a cache has one or more stored responses for a requested URI, but
881   cannot serve any of them (e.g., because they are not fresh, or one cannot
882   be selected; see <xref target="caching.negotiated.responses"/>), it can use
883   the conditional request mechanism &conditional; in the forwarded request to
884   give the origin server an opportunity to both select a valid stored
885   response to be used, and to update it. This process is known as
886   "validating" or "revalidating" the stored response.
887</t>
888<t>
889   When sending such a conditional request, a cache adds an If-Modified-Since
890   header field whose value is that of the Last-Modified header field from the
891   selected (see <xref target="caching.negotiated.responses"/>) stored
892   response, if available.
893</t>
894<t>
895   Additionally, a cache can add an If-None-Match header field whose value is
896   that of the ETag header field(s) from all responses stored for the
897   requested URI, if present. However, if any of the stored responses contains
898   only partial content, the cache shouldn't include its entity-tag in the
899   If-None-Match header field unless the request is for a range that would be
900   fully satisfied by that stored response.
901</t>
902
903<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
904
905<t>
906   <list style="symbols">
907      <t>
908         A 304 (Not Modified) response status code indicates that the stored
909         response can be updated and reused; see <xref
910         target="freshening.responses"/>.
911      </t>
912      <t>
913         A full response (i.e., one with a response body) indicates that none
914         of the stored responses nominated in the conditional request is
915         suitable. Instead, the cache can use the full response to
916         satisfy the request and &MAY; replace the stored response(s).
917      </t>
918      <t>
919         However, if a cache receives a 5xx response while attempting to
920         validate a response, it can either forward this response to the
921         requesting client, or act as if the server failed to respond. In the
922         latter case, it can return a previously stored response (see <xref
923         target="serving.stale.responses" />).
924      </t>
925   </list>
926</t>
927
928<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
929<t>
930   When a cache receives a 304 (Not Modified) response and already has one
931   or more stored 200 (OK) responses for the same cache key, the cache needs
932   to identify which of the stored responses are updated by this new response
933   and then update the stored response(s) with the new information provided in
934   the 304 response.
935   <list style="symbols">
936    <t>
937     If the new response contains a strong validator, then that strong
938     validator identifies the selected representation.  All of the stored
939     responses with the same strong validator are selected.
940     If none of the stored responses contain the same strong validator, then
941     this new response corresponds to a new selected representation and
942     &MUST-NOT; update the existing stored responses.
943    </t>
944    <t>
945     If the new response contains a weak validator and that validator
946     corresponds to one of the cache's stored responses, then the most
947     recent of those matching stored responses is selected.
948    </t>
949    <t>
950     If the new response does not include any form of validator, there is
951     only one stored response, and that stored response also lacks a
952     validator, then that stored response is selected.
953    </t>
954   </list>
955</t>
956<t>
957   If a stored response is selected for update, the cache &MUST;:
958   <list style="symbols">
959      <t>delete any Warning header fields in the stored response with
960         warn-code 1xx (see <xref target="header.warning" />);</t>
961      <t>retain any Warning header fields in the stored response with
962         warn-code 2xx; and,</t>
963      <t>use other header fields provided in the 304 response to replace
964         all instances of the corresponding header fields in the stored
965         response.</t>
966   </list>
967</t>
968</section>
969
970</section>
971
972<section anchor="head.effects" title="Updating Caches with HEAD Responses">
973<t>
974   A response to the HEAD method is identical to what an equivalent request
975   made with a GET would have been, except it lacks a body. This property
976   of HEAD responses is used to both invalidate and update cached GET
977   responses.
978</t>
979<t>
980   If one or more stored GET responses can be selected (as per <xref
981   target="caching.negotiated.responses"/>) for a HEAD request, and the
982   Content-Length, ETag or Last-Modified value of a HEAD response differs from
983   that in a selected GET response, the cache &MUST; consider that selected
984   response to be stale.
985</t>
986<t>
987   If the Content-Length, ETag and Last-Modified values of a HEAD response
988   (when present) are the same as that in a selected GET response (as per
989   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update the
990   remaining headers in the stored response using the following rules:
991   <list style="symbols">
992      <t>delete any Warning header fields in the stored response with
993         warn-code 1xx (see <xref target="header.warning" />);</t>
994      <t>retain any Warning header fields in the stored response with
995         warn-code 2xx; and,</t>
996      <t>use other header fields provided in the response to replace
997         all instances of the corresponding header fields in the stored
998         response.</t>
999   </list>
1000</t>
1001
1002</section>
1003
1004<section anchor="invalidation.after.updates.or.deletions" 
1005   title="Request Methods that Invalidate">
1006<t>
1007   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1008   have the potential for changing state on the origin server, intervening
1009   caches can use them to keep their contents up-to-date.
1010</t>
1011<t>
1012   A cache &MUST; invalidate the effective Request URI
1013   (&effective-request-uri;) as well as the URI(s) in the Location
1014   and Content-Location response header fields (if present) when a non-error
1015   response to a request with an unsafe method is received.
1016</t>
1017<t>
1018   However, a cache &MUST-NOT; invalidate a URI from a Location or
1019   Content-Location response header field if the host part of that URI differs
1020   from the host part in the effective request URI (&effective-request-uri;).
1021   This helps prevent denial of service attacks.
1022</t>
1023<t>
1024   A cache &MUST; invalidate the effective request URI
1025   (&effective-request-uri;) when it receives a non-error response
1026   to a request with a method whose safety is unknown.
1027</t>
1028<t>
1029   Here, a "non-error response" is one with a 2xx or 3xx status code.
1030   "Invalidate" means that the cache will either remove all stored
1031   responses related to the effective request URI, or will mark these as
1032   "invalid" and in need of a mandatory validation before they can be returned
1033   in response to a subsequent request.
1034</t>
1035<t>
1036   Note that this does not guarantee that all appropriate responses are
1037   invalidated. For example, the request that caused the change at the origin
1038   server might not have gone through the cache where a response is stored.
1039</t>
1040</section>
1041
1042<section anchor="caching.authenticated.responses" 
1043   title="Shared Caching of Authenticated Responses">
1044
1045<t>
1046   A shared cache &MUST-NOT; use a cached response to a request with an
1047   Authorization header field (&header-authorization;) to satisfy any subsequent
1048   request unless a cache directive that allows such responses to be stored is
1049   present in the response.
1050</t>
1051
1052<t>
1053   In this specification, the following Cache-Control response directives
1054   (<xref target="cache-response-directive"/>) have such an effect:
1055   must-revalidate, public, s-maxage.
1056</t>
1057
1058<t>
1059   Note that cached responses that contain the "must-revalidate" and/or
1060   "s-maxage" response directives are not allowed to be served stale (<xref
1061   target="serving.stale.responses"/>) by shared caches. In particular, a
1062   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1063   used to satisfy a subsequent request without revalidating it on the origin
1064   server.
1065</t>
1066</section>
1067
1068<section anchor="caching.negotiated.responses" 
1069   title="Caching Negotiated Responses">
1070<t>
1071   When a cache receives a request that can be satisfied by a stored response
1072   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1073   use that response unless all of the selecting header fields nominated by
1074   the Vary header field match in both the original request (i.e., that associated
1075   with the stored response), and the presented request.
1076</t>
1077<t>
1078   The selecting header fields from two requests are defined to match if and
1079   only if those in the first request can be transformed to those in the
1080   second request by applying any of the following:
1081   <list style="symbols">
1082      <t>
1083         adding or removing whitespace, where allowed in the header field's syntax
1084      </t>
1085      <t>
1086         combining multiple header fields with the same field name
1087         (see &header-fields;)
1088      </t>
1089      <t>
1090         normalizing both header field values in a way that is known to have
1091         identical semantics, according to the header field's specification (e.g.,
1092         re-ordering field values when order is not significant;
1093         case-normalization, where values are defined to be case-insensitive)
1094      </t>
1095  </list>
1096</t>
1097<t>
1098   If (after any normalization that might take place) a header field is absent
1099   from a request, it can only match another request if it is also absent
1100   there.
1101</t>
1102<t>
1103   A Vary header field-value of "*" always fails to match, and subsequent
1104   requests to that resource can only be properly interpreted by the origin
1105   server.
1106</t>
1107<t>
1108   The stored response with matching selecting header fields is known as the
1109   selected response.
1110</t>
1111<t>
1112   If multiple selected responses are available, the most recent response
1113   (as determined by the Date header field) is used; see <xref 
1114   target="constructing.responses.from.caches"/>.
1115</t>
1116<t>
1117   If no selected response is available, the cache can forward the presented
1118   request to the origin server in a conditional request; see <xref
1119   target="validation.model"/>.
1120</t>
1121</section>
1122
1123<section anchor="combining.responses" title="Combining Partial Content">
1124<t>
1125   A response might transfer only a partial representation if the
1126   connection closed prematurely or if the request used one or more Range
1127   specifiers (&partial;).  After several such transfers, a cache might have
1128   received several ranges of the same representation.  A cache &MAY; combine
1129   these ranges into a single stored response, and reuse that response to
1130   satisfy later requests, if they all share the same strong validator and
1131   the cache complies with the client requirements in &combining-byte-ranges;.
1132</t>
1133<t>
1134   When combining the new response with one or more stored responses, a
1135   cache &MUST;:
1136   <list style="symbols">
1137      <t>delete any Warning header fields in the stored response with
1138         warn-code 1xx (see <xref target="header.warning" />);</t>
1139      <t>retain any Warning header fields in the stored response with
1140         warn-code 2xx; and,</t>
1141      <t>use other header fields provided in the new response, aside
1142         from Content-Range, to replace all instances of the corresponding
1143         header fields in the stored response.</t>
1144   </list>
1145</t>
1146</section>
1147
1148
1149</section>
1150
1151<section anchor="header.field.definitions" title="Header Field Definitions">
1152<t>
1153   This section defines the syntax and semantics of HTTP/1.1 header fields
1154   related to caching.
1155</t>
1156
1157<section anchor="header.age" title="Age">
1158   <iref item="Age header field" primary="true" x:for-anchor="" />
1159   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1160   <x:anchor-alias value="Age"/>
1161   <x:anchor-alias value="age-value"/>
1162<t>
1163   The "Age" header field conveys the sender's estimate of the amount
1164   of time since the response was generated or successfully validated at the
1165   origin server. Age values are calculated as specified in <xref
1166   target="age.calculations" />.
1167</t>
1168<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1169  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1170</artwork></figure>
1171<t>
1172  Age field-values are non-negative integers, representing time in seconds
1173  (see <xref target="delta-seconds"/>).
1174</t>
1175<t>
1176   The presence of an Age header field in a response implies that a response
1177   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1178   might not implement the Age header field.
1179</t>
1180</section>
1181
1182<section anchor="header.cache-control" title="Cache-Control">
1183   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1184   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1185      x:for-anchor="" />
1186   <x:anchor-alias value="Cache-Control"/>
1187   <x:anchor-alias value="cache-directive"/>
1188<t>
1189   The "Cache-Control" header field is used to specify directives for
1190   caches along the request/response chain. Such cache directives are
1191   unidirectional in that the presence of a directive in a request does not
1192   imply that the same directive is to be given in the response.
1193</t>
1194<t>
1195   A cache &MUST; obey the requirements of the Cache-Control
1196   directives defined in this section. See <xref
1197   target="cache.control.extensions"/> for information about how Cache-Control
1198   directives defined elsewhere are handled.
1199</t>
1200<x:note>
1201   <t>
1202       &Note; HTTP/1.0 caches might not implement Cache-Control and
1203       might only implement Pragma: no-cache (see <xref target="header.pragma"
1204       />).
1205   </t>
1206</x:note>
1207<t>
1208   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1209   through in forwarded messages, regardless of their
1210   significance to that application, since the directives might be applicable
1211   to all recipients along the request/response chain. It is not possible to
1212   target a directive to a specific cache.
1213</t>
1214<t>
1215   Cache directives are identified by a token, to be compared case-insensitively,
1216   and have an optional argument, that can use both token and quoted-string
1217   syntax. For the directives defined below that define arguments, recipients
1218   ought to accept both forms, even if one is documented to be preferred. For
1219   any directive not defined by this specification, recipients &MUST; accept
1220   both forms.
1221</t>
1222<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1223  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1224
1225  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1226</artwork></figure>
1227<t>
1228   For the cache directives defined below, no argument is defined (nor allowed)
1229   otherwise stated otherwise.
1230</t>
1231
1232<section anchor="cache-request-directive" 
1233   title="Request Cache-Control Directives">
1234
1235<section title="no-cache" anchor="cache-request-directive.no-cache">
1236   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1237   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1238<t>
1239   The no-cache request directive indicates that a cache &MUST-NOT; 
1240   use a stored response to satisfy the request without successful
1241   validation on the origin server.
1242</t>
1243</section>
1244 
1245<section title="no-store" anchor="cache-request-directive.no-store">
1246   <iref item="Cache Directives" primary="true" subitem="no-store" />
1247   <iref item="no-store" primary="true" subitem="Cache Directive" />
1248<t>
1249   The no-store request directive indicates that a cache &MUST-NOT;
1250   store any part of either this request or any response to it. This
1251   directive applies to both private and shared caches. "&MUST-NOT;
1252   store" in this context means that the cache &MUST-NOT; intentionally
1253   store the information in non-volatile storage, and &MUST; make a
1254   best-effort attempt to remove the information from volatile storage as
1255   promptly as possible after forwarding it.
1256</t>
1257<t>
1258   This directive is NOT a reliable or sufficient mechanism for ensuring
1259   privacy. In particular, malicious or compromised caches might not
1260   recognize or obey this directive, and communications networks might be
1261   vulnerable to eavesdropping.
1262</t>
1263<t>
1264   Note that if a request containing this directive is satisfied from a
1265   cache, the no-store request directive does not apply to the already
1266   stored response.
1267</t>
1268</section>
1269
1270<section title="max-age" anchor="cache-request-directive.max-age">
1271   <iref item="Cache Directives" primary="true" subitem="max-age" />
1272   <iref item="max-age" primary="true" subitem="Cache Directive" />
1273<t>
1274   Argument syntax:
1275   <list>
1276      <t>
1277        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1278      </t>
1279   </list>
1280</t>
1281<t>
1282   The max-age request directive indicates that the client is unwilling to
1283   accept a response whose age is greater than the specified number of
1284   seconds. Unless the max-stale request directive is also present, the
1285   client is not willing to accept a stale response.
1286</t>
1287<t>
1288   &Note; This directive uses the token form of the argument syntax;
1289   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1290   quoted-string form.
1291</t>
1292</section>
1293
1294<section title="max-stale" anchor="cache-request-directive.max-stale">
1295   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1296   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1297<t>
1298   Argument syntax:
1299   <list>
1300      <t>
1301        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1302      </t>
1303   </list>
1304</t>
1305<t>
1306   The max-stale request directive indicates that the client is willing
1307   to accept a response that has exceeded its expiration time. If max-stale
1308   is assigned a value, then the client is willing to accept a response
1309   that has exceeded its expiration time by no more than the specified
1310   number of seconds. If no value is assigned to max-stale, then the client
1311   is willing to accept a stale response of any age.
1312</t>
1313<t>
1314   &Note; This directive uses the token form of the argument syntax;
1315   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1316   quoted-string form.
1317</t>
1318</section>
1319
1320<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1321   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1322   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1323<t>
1324   Argument syntax:
1325   <list>
1326      <t>
1327        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1328      </t>
1329   </list>
1330</t>
1331<t>
1332   The min-fresh request directive indicates that the client is willing
1333   to accept a response whose freshness lifetime is no less than its
1334   current age plus the specified time in seconds. That is, the client
1335   wants a response that will still be fresh for at least the specified
1336   number of seconds.
1337</t>
1338<t>
1339   &Note; This directive uses the token form of the argument syntax;
1340   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1341   quoted-string form.
1342</t>
1343</section>
1344
1345<section title="no-transform" anchor="cache-request-directive.no-transform">
1346   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1347   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1348<t>
1349   The no-transform request directive indicates that an intermediary
1350   (whether or not it implements a cache) &MUST-NOT; change the
1351   Content-Encoding, Content-Range or Content-Type request header fields,
1352   nor the request representation.
1353</t>
1354</section>
1355
1356<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1357   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1358   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1359<t>
1360   The only-if-cached request directive indicates that the client only
1361   wishes to obtain a stored response. If it receives 3ive, a
1362   cache &SHOULD; either respond using a stored response that is consistent
1363   with the other constraints of the request, or respond with a 504
1364   (Gateway Timeout) status code. If a group of caches is being operated as
1365   a unified system with good internal connectivity, a member cache &MAY;
1366   forward such a request within that group of caches.
1367</t>
1368</section>
1369</section>
1370
1371<section anchor="cache-response-directive" 
1372   title="Response Cache-Control Directives">
1373   <x:anchor-alias value="cache-response-directive" />
1374
1375<section title="public" anchor="cache-response-directive.only-if-cached">
1376   <iref item="Cache Directives" primary="true" subitem="public" />
1377   <iref item="public" primary="true" subitem="Cache Directive" />
1378<t>
1379   The public response directive indicates that a response whose
1380   associated request contains an 'Authentication' header &MAY; be
1381   stored (see <xref target="caching.authenticated.responses" />).
1382</t>
1383</section>
1384
1385<section title="private" anchor="cache-response-directive.private">
1386   <iref item="Cache Directives" primary="true" subitem="private" />
1387   <iref item="private" primary="true" subitem="Cache Directive" />
1388<t>
1389   Argument syntax:
1390   <list>
1391      <t>
1392        #<x:ref>field-name</x:ref>
1393      </t>
1394   </list>
1395</t>
1396<t>
1397   The private response directive indicates that the response message is
1398   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1399   private cache &MAY; store the response.
1400</t>
1401<t>
1402   If the private response directive specifies one or more field-names,
1403   this requirement is limited to the field-values associated with the
1404   listed response header fields. That is, a shared cache &MUST-NOT; store
1405   the specified field-names(s), whereas it &MAY; store the remainder of the
1406   response message.
1407</t>
1408<t>
1409   The field-names given are not limited to the set of standard header
1410   fields defined by this specification. Field names are case-insensitive.
1411</t>
1412<t>
1413   &Note; This usage of the word "private" only controls
1414   where the response can be stored; it cannot ensure the privacy of the
1415   message content. Also, private response directives with field-names are
1416   often handled by implementations as if an unqualified private directive
1417   was received; i.e., the special handling for the qualified form is not
1418   widely implemented.
1419</t>
1420<t>
1421   &Note; This directive uses the quoted-string form of the argument
1422   syntax. Senders &SHOULD-NOT; use the token form (even if quoting appears not
1423   to be needed for single-entry lists).
1424</t>
1425</section>
1426
1427<section title="no-cache" anchor="cache-response-directive.no-cache">
1428   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1429   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1430<t>
1431   Argument syntax:
1432   <list>
1433      <t>
1434        #<x:ref>field-name</x:ref>
1435      </t>
1436   </list>
1437</t>
1438<t>
1439   The no-cache response directive indicates that the response &MUST-NOT;
1440   be used to satisfy a subsequent request without successful validation on
1441   the origin server. This allows an origin server to prevent a cache from
1442   using it to satisfy a request without contacting it, even by caches that
1443   have been configured to return stale responses.
1444</t>
1445<t>
1446   If the no-cache response directive specifies one or more field-names,
1447   then a cache &MAY; use the response to satisfy a subsequent request,
1448   subject to any other restrictions on caching. However, any header fields
1449   in the response that have the field-name(s) listed &MUST-NOT; be sent
1450   in the response to a subsequent request without successful revalidation
1451   with the origin server. This allows an origin server to prevent the
1452   re-use of certain header fields in a response, while still allowing
1453   caching of the rest of the response.
1454</t> 
1455<t>
1456   The field-names given are not limited to the set of standard header
1457   fields defined by this specification. Field names are case-insensitive.
1458</t>
1459<t>
1460   &Note; Most HTTP/1.0 caches will not recognize or obey
1461   this directive. Also, no-cache response directives with field-names are
1462   often handled by implementations as if an unqualified no-cache directive
1463   was received; i.e., the special handling for the qualified form is not
1464   widely implemented.
1465</t>
1466<t>
1467   &Note; This directive uses the quoted-string form of the argument
1468   syntax. Senders &SHOULD-NOT; use the token form (even if quoting appears not
1469   to be needed for single-entry lists).
1470</t>
1471</section>
1472
1473<section title="no-store" anchor="cache-response-directive.no-store">
1474   <iref item="Cache Directives" primary="true" subitem="no-store" />
1475   <iref item="no-store" primary="true" subitem="Cache Directive" />
1476<t>
1477   The no-store response directive indicates that a cache &MUST-NOT;
1478   store any part of either the immediate request or response. This
1479   directive applies to both private and shared caches. "&MUST-NOT;
1480   store" in this context means that the cache &MUST-NOT; intentionally
1481   store the information in non-volatile storage, and &MUST; make a
1482   best-effort attempt to remove the information from volatile storage as
1483   promptly as possible after forwarding it.
1484</t>
1485<t>
1486   This directive is NOT a reliable or sufficient mechanism for ensuring
1487   privacy. In particular, malicious or compromised caches might not
1488   recognize or obey this directive, and communications networks might be
1489   vulnerable to eavesdropping.
1490</t>
1491</section>
1492
1493<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1494   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1495   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1496<t>
1497   The must-revalidate response directive indicates that once it has
1498   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1499   requests without successful validation on the origin server.
1500</t>
1501<t>
1502   The must-revalidate directive is necessary to support reliable
1503   operation for certain protocol features. In all circumstances a
1504   cache &MUST; obey the must-revalidate directive; in particular,
1505   if a cache cannot reach the origin server for any reason, it &MUST;
1506   generate a 504 (Gateway Timeout) response.
1507</t>
1508<t>
1509   The must-revalidate directive ought to be used by servers if and only
1510   if failure to validate a request on the representation could result in
1511   incorrect operation, such as a silently unexecuted financial
1512   transaction.
1513</t>
1514</section>
1515
1516<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1517   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1518   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1519<t>
1520   The proxy-revalidate response directive has the same meaning as the
1521   must-revalidate response directive, except that it does not apply to
1522   private caches.
1523</t>
1524</section>
1525
1526<section title="max-age" anchor="cache-response-directive.max-age">
1527   <iref item="Cache Directives" primary="true" subitem="max-age" />
1528   <iref item="max-age" primary="true" subitem="Cache Directive" />
1529<t>
1530   Argument syntax:
1531   <list>
1532      <t>
1533        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1534      </t>
1535   </list>
1536</t>
1537<t>
1538   The max-age response directive indicates that the response is to be
1539   considered stale after its age is greater than the specified number of
1540   seconds.
1541</t>
1542<t>
1543   &Note; This directive uses the token form of the argument syntax;
1544   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1545   quoted-string form.
1546</t>
1547</section>     
1548
1549<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1550   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1551   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1552<t>
1553   Argument syntax:
1554   <list>
1555      <t>
1556        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1557      </t>
1558   </list>
1559</t>
1560<t>
1561   The s-maxage response directive indicates that, in shared caches, the
1562   maximum age specified by this directive overrides the maximum age
1563   specified by either the max-age directive or the Expires header field. The
1564   s-maxage directive also implies the semantics of the proxy-revalidate
1565   response directive.
1566</t>
1567<t>
1568   &Note; This directive uses the token form of the argument syntax;
1569   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1570   quoted-string form.
1571</t>
1572</section>
1573
1574<section title="no-transform" anchor="cache-response-directive.no-transform">
1575   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1576   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1577<t>
1578   The no-transform response directive indicates that an intermediary
1579   (regardless of whether it implements a cache) &MUST-NOT; change the
1580   Content-Encoding, Content-Range or Content-Type response header fields,
1581   nor the response representation.
1582</t>
1583</section>
1584
1585</section>
1586
1587<section anchor="cache.control.extensions" title="Cache Control Extensions">
1588<t>
1589   The Cache-Control header field can be extended through the use of one or
1590   more cache-extension tokens, each with an optional value. Informational
1591   extensions (those that do not require a change in cache behavior) can be
1592   added without changing the semantics of other directives. Behavioral
1593   extensions are designed to work by acting as modifiers to the existing base
1594   of cache directives. Both the new directive and the standard directive are
1595   supplied, such that applications that do not understand the new directive
1596   will default to the behavior specified by the standard directive, and those
1597   that understand the new directive will recognize it as modifying the
1598   requirements associated with the standard directive. In this way,
1599   extensions to the cache-control directives can be made without requiring
1600   changes to the base protocol.
1601</t>
1602<t>
1603   This extension mechanism depends on an HTTP cache obeying all of the
1604   cache-control directives defined for its native HTTP-version, obeying
1605   certain extensions, and ignoring all directives that it does not
1606   understand.
1607</t>
1608<t>
1609   For example, consider a hypothetical new response directive called
1610   "community" that acts as a modifier to the private directive. We define
1611   this new directive to mean that, in addition to any private cache, any
1612   cache that is shared only by members of the community named within its
1613   value may cache the response. An origin server wishing to allow the UCI
1614   community to use an otherwise private response in their shared cache(s)
1615   could do so by including
1616</t>
1617<figure><artwork type="example">
1618  Cache-Control: private, community="UCI"
1619</artwork></figure>
1620<t>
1621   A cache seeing this header field will act correctly even if the cache does
1622   not understand the community cache-extension, since it will also see and
1623   understand the private directive and thus default to the safe behavior.
1624</t>
1625<t>
1626   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1627   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1628   combined with standard directives (or the response's default cacheability)
1629   such that the cache behavior will remain minimally correct even if the
1630   cache does not understand the extension(s).
1631</t>
1632<t>
1633   The HTTP Cache Directive Registry defines the name space for the cache
1634   directives.
1635</t>
1636<t>
1637   A registration &MUST; include the following fields:
1638   <list style="symbols">
1639      <t>Cache Directive Name</t>
1640      <t>Pointer to specification text</t>
1641   </list>
1642</t>
1643<t>
1644   Values to be added to this name space require IETF Review (see <xref
1645   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1646</t>
1647<t>
1648   The registry itself is maintained at <eref
1649   target="http://www.iana.org/assignments/http-cache-directives"/>.
1650</t>
1651</section>
1652
1653</section>
1654
1655<section anchor="header.expires" title="Expires">
1656   <iref item="Expires header field" primary="true" x:for-anchor="" />
1657   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1658   <x:anchor-alias value="Expires"/>
1659<t>
1660   The "Expires" header field gives the date/time after which the
1661   response is considered stale. See <xref target="expiration.model" /> for
1662   further discussion of the freshness model.
1663</t>
1664<t>
1665   The presence of an Expires field does not imply that the original resource
1666   will change or cease to exist at, before, or after that time.
1667</t>
1668<t>
1669   The field-value is an absolute date and time as defined by HTTP-date in
1670   &http-date;; a sender &MUST; use the rfc1123-date format.
1671</t>
1672<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1673  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1674</artwork></figure>
1675<figure>
1676  <preamble>For example</preamble>
1677<artwork type="example">
1678  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1679</artwork></figure>
1680<t>
1681   A cache &MUST; treat other invalid date formats,
1682   especially including the value "0", as in the past (i.e., "already
1683   expired").
1684</t>
1685<x:note>
1686   <t>
1687       &Note; If a response includes a Cache-Control field with the
1688       max-age directive (see <xref target="cache-response-directive" />),
1689       that directive overrides the Expires field. Likewise, the s-maxage
1690       directive overrides Expires in shared caches.
1691   </t>
1692</x:note>
1693<t>
1694   Historically, HTTP required the Expires field-value to be no more than a
1695   year in the future. While longer freshness lifetimes are no longer
1696   prohibited, extremely large values have been demonstrated to cause
1697   problems (e.g., clock overflows due to use of 32-bit integers for
1698   time values), and most caches will evict a response far sooner than
1699   that. Therefore, senders ought not produce them.
1700</t>
1701<t>
1702   An origin server without a clock &MUST-NOT; assign Expires
1703   values to a response unless these values were associated
1704   with the resource by a system or user with a reliable clock. It &MAY;
1705   assign an Expires value that is known, at or before server
1706   configuration time, to be in the past (this allows "pre-expiration"
1707   of responses without storing separate Expires values for each
1708   resource).
1709</t>
1710</section>
1711
1712<section anchor="header.pragma" title="Pragma">
1713   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1714   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1715   <x:anchor-alias value="extension-pragma"/>
1716   <x:anchor-alias value="Pragma"/>
1717   <x:anchor-alias value="pragma-directive"/>
1718<t>
1719   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1720   caches, so that clients can specify a "no-cache" request that they will
1721   understand (as Cache-Control was not defined until HTTP/1.1). When the
1722   Cache-Control header is also present and understood in a request, Pragma is
1723   ignored.
1724</t>
1725<t>
1726   In HTTP/1.0, Pragma was defined as an extensible field for
1727   implementation-specified directives for recipients. This specification
1728   deprecates such extensions to improve interoperability.
1729</t>
1730<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"/>
1731  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1732  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1733  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1734</artwork></figure>
1735<t>
1736   When the Cache-Control header is not present in a request, the no-cache
1737   request pragma-directive &MUST; have the same effect on caches as if
1738   "Cache-Control: no-cache" were present (see <xref
1739   target="cache-request-directive" />).
1740</t>
1741<t>
1742   When sending a no-cache request, a client ought to include both the pragma
1743   and cache-control directives, unless Cache-Control: no-cache is
1744   purposefully omitted to target other Cache-Control response directives at
1745   HTTP/1.1 caches. For example:
1746</t>
1747<figure>
1748<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1749GET / HTTP/1.1
1750Host: www.example.com
1751Cache-Control: max-age=30
1752Pragma: no-cache
1753
1754</artwork>
1755</figure>
1756<t>
1757   will constrain HTTP/1.1 caches to serve a response no older than 30
1758   seconds, while precluding implementations that do not understand
1759   Cache-Control from serving a cached response.
1760</t>
1761<x:note>
1762   <t>
1763      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1764      specified, it does not provide a reliable replacement for
1765      "Cache-Control: no-cache" in them.
1766   </t>
1767</x:note>
1768</section>
1769
1770<section anchor="header.vary" title="Vary">
1771   <iref item="Vary header field" primary="true" x:for-anchor="" />
1772   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1773   <x:anchor-alias value="Vary"/>
1774<t>
1775   The "Vary" header field conveys the set of header fields
1776   that were used to select the representation.
1777</t>
1778<t>
1779   Caches use this information, in part, to determine whether a stored
1780   response can be used to satisfy a given request; see <xref
1781   target="caching.negotiated.responses" />. determines, while the response is
1782   fresh, whether a cache is permitted to use the response to reply to a
1783   subsequent request without validation; see <xref
1784   target="caching.negotiated.responses" />.
1785</t>
1786<t>
1787   In uncacheable or stale responses, the Vary field value advises the user
1788   agent about the criteria that were used to select the representation.
1789</t>
1790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1791  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1792</artwork></figure>
1793<t>
1794   The set of header fields named by the Vary field value is known as the
1795   selecting header fields.
1796</t>
1797<t>
1798   A server &SHOULD; include a Vary header field with any cacheable response
1799   that is subject to server-driven negotiation. Doing so allows a cache to
1800   properly interpret future requests on that resource and informs the user
1801   agent about the presence of negotiation on that resource. A server &MAY;
1802   include a Vary header field with a non-cacheable response that is subject
1803   to server-driven negotiation, since this might provide the user agent with
1804   useful information about the dimensions over which the response varies at
1805   the time of the response.
1806</t>
1807<t>
1808   A Vary field value of "*" signals that unspecified parameters not limited
1809   to the header fields (e.g., the network address of the client), play a
1810   role in the selection of the response representation; therefore, a cache
1811   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1812   generate the "*" value.
1813</t>
1814<t>
1815   The field-names given are not limited to the set of standard header
1816   fields defined by this specification. Field names are case-insensitive.
1817</t>
1818</section>
1819
1820<section anchor="header.warning" title="Warning">
1821   <iref item="Warning header field" primary="true" x:for-anchor="" />
1822   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1823   <x:anchor-alias value="Warning"/>
1824   <x:anchor-alias value="warning-value"/>
1825   <x:anchor-alias value="warn-agent"/>
1826   <x:anchor-alias value="warn-code"/>
1827   <x:anchor-alias value="warn-date"/>
1828   <x:anchor-alias value="warn-text"/>
1829<t>
1830   The "Warning" header field is used to carry additional information
1831   about the status or transformation of a message that might not be reflected
1832   in the message. This information is typically used to warn about possible
1833   incorrectness introduced by caching operations or transformations applied
1834   to the payload of the message.
1835</t>
1836<t>
1837   Warnings can be used for other purposes, both cache-related and otherwise.
1838   The use of a warning, rather than an error status code, distinguishes these
1839   responses from true failures.
1840</t>
1841<t>
1842   Warning header fields can in general be applied to any message, however some
1843   warn-codes are specific to caches and can only be applied to response
1844   messages.
1845</t>
1846<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"/>
1847  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1848 
1849  <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>
1850                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1851 
1852  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1853  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1854                  ; the name or pseudonym of the server adding
1855                  ; the Warning header field, for use in debugging
1856  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1857  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1858</artwork></figure>
1859<t>
1860   Multiple warnings can be attached to a response (either by the origin
1861   server or by a cache), including multiple warnings with the same code
1862   number, only differing in warn-text.
1863</t>
1864<t>
1865   When this occurs, the user agent &SHOULD; inform the user of as many of
1866   them as possible, in the order that they appear in the response.
1867</t>
1868<t>
1869   Systems that generate multiple Warning header fields are encouraged to
1870   order them with this user agent behavior in mind. New Warning header fields
1871   are added after any existing Warning headers fields.
1872</t>
1873<t>
1874   Warnings are assigned three digit warn-codes. The first digit indicates
1875   whether the Warning is required to be deleted from a stored response after
1876   validation:
1877   <list style="symbols">
1878      <t>1xx Warnings describe the freshness or validation status of the
1879      response, and so &MUST; be deleted by a cache after validation. They can
1880      only be generated by a cache when validating a cached entry, and
1881      &MUST-NOT; be generated in any other situation.</t>
1882      <t>2xx Warnings describe some aspect of the representation that is not
1883      rectified by a validation (for example, a lossy compression of the
1884      representation) and &MUST-NOT; be deleted by a cache after validation,
1885      unless a full response is returned, in which case they &MUST; be.</t>
1886   </list>
1887</t>
1888<t>
1889   If an implementation sends a message with one or more Warning header fields to a
1890   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1891   in each warning-value a warn-date that matches the Date header field in the
1892   message.
1893</t>
1894<t>
1895   If a system receives a message with a warning-value that includes
1896   a warn-date, and that warn-date is different from the Date value in the
1897   response, then that warning-value &MUST; be deleted from the message before
1898   storing, forwarding, or using it. (preventing the consequences of naive
1899   caching of Warning header fields.) If all of the warning-values are deleted
1900   for this reason, the Warning header field &MUST; be deleted as well.
1901</t>
1902<t>
1903   The following warn-codes are defined by this specification, each with a
1904   recommended warn-text in English, and a description of its meaning.
1905</t>
1906
1907<section title="110 Response is Stale" anchor="warn.110">
1908  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1909  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale" x:for-anchor=""/>
1910<t>
1911   A cache &SHOULD; include this whenever the returned response is stale.
1912</t>
1913</section>
1914
1915<section title="111 Revalidation Failed" anchor="warn.111">
1916  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1917  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed" x:for-anchor=""/>
1918<t>
1919   A cache &SHOULD; include this when returning a stale response because an
1920   attempt to validate the response failed, due to an inability to reach
1921   the server.
1922</t>
1923</section>
1924
1925<section title="112 Disconnected Operation" anchor="warn.112">
1926  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1927  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation" x:for-anchor=""/>
1928<t>
1929   A cache &SHOULD; include this if it is intentionally disconnected from
1930   the rest of the network for a period of time.
1931</t>
1932</section>
1933
1934<section title="113 Heuristic Expiration" anchor="warn.113">
1935  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1936  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration" x:for-anchor=""/>
1937<t>
1938   A cache &SHOULD; include this if it heuristically chose a freshness
1939   lifetime greater than 24 hours and the response's age is greater than 24
1940   hours.
1941</t>
1942</section>
1943
1944<section title="199 Miscellaneous Warning" anchor="warn.199">
1945  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1946  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning" x:for-anchor=""/>
1947<t>
1948   The warning text can include arbitrary information to be presented to
1949   a human user, or logged. A system receiving this warning &MUST-NOT; take
1950   any automated action, besides presenting the warning to the user.
1951</t>
1952</section>
1953
1954<section title="214 Transformation Applied" anchor="warn.214">
1955  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1956  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied" x:for-anchor=""/>
1957<t>
1958   &MUST; be added by a proxy if it applies any transformation to the
1959   representation, such as changing the content-coding, media-type, or
1960   modifying the representation data, unless this Warning code already appears
1961   in the response.
1962</t>
1963</section>
1964
1965<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1966  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1967  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning" x:for-anchor=""/>
1968<t>
1969   The warning text can include arbitrary information to be presented to
1970   a human user, or logged. A system receiving this warning &MUST-NOT; take
1971   any automated action.
1972</t>
1973</section>
1974
1975<section title="Warn Code Extensions" anchor="warn.code.extensions">
1976<t>
1977   The HTTP Warn Code Registry defines the name space for warn codes.
1978</t>
1979<t>
1980   A registration &MUST; include the following fields:
1981   <list style="symbols">
1982      <t>Warn Code (3 digits)</t>
1983      <t>Short Description</t>
1984      <t>Pointer to specification text</t>
1985   </list>
1986</t>
1987<t>
1988   Values to be added to this name space require IETF Review (see <xref
1989   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1990</t>
1991<t>
1992   The registry itself is maintained at <eref
1993   target="http://www.iana.org/assignments/http-warn-codes"/>.
1994</t>
1995</section>
1996</section>
1997</section>
1998
1999<section anchor="history.lists" title="History Lists">
2000<t>
2001   User agents often have history mechanisms, such as "Back" buttons and
2002   history lists, that can be used to redisplay a representation retrieved
2003   earlier in a session.
2004</t>
2005<t>
2006   The freshness model (<xref target="expiration.model"/>) does not
2007   necessarily apply to history mechanisms. I.e., a history mechanism can
2008   display a previous representation even if it has expired.
2009</t>
2010<t>
2011   This does not prohibit the history mechanism from telling the user that a
2012   view might be stale, or from honoring cache directives (e.g.,
2013   Cache-Control: no-store).
2014</t>
2015</section>
2016
2017
2018<section anchor="IANA.considerations" title="IANA Considerations">
2019
2020<section title="Cache Directive Registry" 
2021   anchor="cache.directive.registration">
2022<t>
2023   The registration procedure for HTTP Cache Directives is defined by <xref
2024   target="cache.control.extensions"/> of this document.
2025</t>
2026<t>
2027   The HTTP Cache Directive Registry shall be created at <eref
2028   target="http://www.iana.org/assignments/http-cache-directives"/> and be
2029   populated with the registrations below:
2030</t>
2031<?BEGININC p6-cache.cache-directives ?>
2032<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
2033<texttable xmlns:my="#my" align="left" suppress-title="true"
2034           anchor="iana.cache.directive.registration.table">
2035   <ttcol>Cache Directive</ttcol>
2036   <ttcol>Reference</ttcol>
2037
2038   <c>max-age</c>
2039   <c>
2040      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
2041   </c>
2042   <c>max-stale</c>
2043   <c>
2044      <xref target="cache-request-directive.max-stale"/>
2045   </c>
2046   <c>min-fresh</c>
2047   <c>
2048      <xref target="cache-request-directive.min-fresh"/>
2049   </c>
2050   <c>must-revalidate</c>
2051   <c>
2052      <xref target="cache-response-directive.must-revalidate"/>
2053   </c>
2054   <c>no-cache</c>
2055   <c>
2056      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
2057   </c>
2058   <c>no-store</c>
2059   <c>
2060      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
2061   </c>
2062   <c>no-transform</c>
2063   <c>
2064      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
2065   </c>
2066   <c>only-if-cached</c>
2067   <c>
2068      <xref target="cache-request-directive.only-if-cached"/>
2069   </c>
2070   <c>private</c>
2071   <c>
2072      <xref target="cache-response-directive.private"/>
2073   </c>
2074   <c>proxy-revalidate</c>
2075   <c>
2076      <xref target="cache-response-directive.proxy-revalidate"/>
2077   </c>
2078   <c>public</c>
2079   <c>
2080      <xref target="cache-response-directive.only-if-cached"/>
2081   </c>
2082   <c>s-maxage</c>
2083   <c>
2084      <xref target="cache-response-directive.s-maxage"/>
2085   </c>
2086   <c>stale-if-error</c>
2087   <c>
2088      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2089   </c>
2090   <c>stale-while-revalidate</c>
2091   <c>
2092      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2093   </c>
2094</texttable>
2095<!--(END)-->
2096<?ENDINC p6-cache.cache-directives ?>
2097</section>
2098
2099<section title="Warn Code Registry" 
2100   anchor="warn.code.registration">
2101<t>
2102   The registration procedure for HTTP Warn Codes is defined by <xref
2103   target="warn.code.extensions"/> of this document.
2104</t>
2105<t>
2106   The HTTP Warn Code Registry shall be created at <eref
2107   target="http://www.iana.org/assignments/http-cache-directives"/> and be
2108   populated with the registrations below:
2109</t>
2110<?BEGININC p6-cache.iana-warn-codes ?>
2111<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2112<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
2113   <ttcol>Warn Code</ttcol>
2114   <ttcol>Short Description</ttcol>
2115   <ttcol>Reference</ttcol>
2116   <c>110</c>
2117   <c>Response is Stale</c>
2118   <c>
2119      <xref target="warn.110"/>
2120   </c>
2121   <c>111</c>
2122   <c>Revalidation Failed</c>
2123   <c>
2124      <xref target="warn.111"/>
2125   </c>
2126   <c>112</c>
2127   <c>Disconnected Operation</c>
2128   <c>
2129      <xref target="warn.112"/>
2130   </c>
2131   <c>113</c>
2132   <c>Heuristic Expiration</c>
2133   <c>
2134      <xref target="warn.113"/>
2135   </c>
2136   <c>199</c>
2137   <c>Miscellaneous Warning</c>
2138   <c>
2139      <xref target="warn.199"/>
2140   </c>
2141   <c>214</c>
2142   <c>Transformation Applied</c>
2143   <c>
2144      <xref target="warn.214"/>
2145   </c>
2146   <c>299</c>
2147   <c>Miscellaneous Persistent Warning</c>
2148   <c>
2149      <xref target="warn.299"/>
2150   </c>
2151</texttable>
2152<!--(END)-->
2153<?ENDINC p6-cache.iana-warn-codes ?>
2154</section>
2155
2156<section title="Header Field Registration" anchor="header.field.registration">
2157<t>
2158  The Message Header Field Registry located at <eref 
2159  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2160  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2161</t>
2162<?BEGININC p6-cache.iana-headers ?>
2163<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2164<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2165   <ttcol>Header Field Name</ttcol>
2166   <ttcol>Protocol</ttcol>
2167   <ttcol>Status</ttcol>
2168   <ttcol>Reference</ttcol>
2169
2170   <c>Age</c>
2171   <c>http</c>
2172   <c>standard</c>
2173   <c>
2174      <xref target="header.age"/>
2175   </c>
2176   <c>Cache-Control</c>
2177   <c>http</c>
2178   <c>standard</c>
2179   <c>
2180      <xref target="header.cache-control"/>
2181   </c>
2182   <c>Expires</c>
2183   <c>http</c>
2184   <c>standard</c>
2185   <c>
2186      <xref target="header.expires"/>
2187   </c>
2188   <c>Pragma</c>
2189   <c>http</c>
2190   <c>standard</c>
2191   <c>
2192      <xref target="header.pragma"/>
2193   </c>
2194   <c>Vary</c>
2195   <c>http</c>
2196   <c>standard</c>
2197   <c>
2198      <xref target="header.vary"/>
2199   </c>
2200   <c>Warning</c>
2201   <c>http</c>
2202   <c>standard</c>
2203   <c>
2204      <xref target="header.warning"/>
2205   </c>
2206</texttable>
2207<!--(END)-->
2208<?ENDINC p6-cache.iana-headers ?>
2209<t>
2210   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2211   Force".
2212</t>
2213</section>
2214</section>
2215
2216<section anchor="security.considerations" title="Security Considerations">
2217<t>
2218   Caches expose additional potential vulnerabilities, since the contents of
2219   the cache represent an attractive target for malicious exploitation.
2220   Because cache contents persist after an HTTP request is complete, an attack
2221   on the cache can reveal information long after a user believes that the
2222   information has been removed from the network. Therefore, cache contents
2223   need to be protected as sensitive information.
2224</t>
2225</section>
2226
2227<section title="Acknowledgments" anchor="acks">
2228<t>
2229  See &acks;.
2230</t>
2231</section>
2232
2233</middle>
2234
2235<back>
2236<references title="Normative References">
2237
2238  <reference anchor="Part1">
2239    <front>
2240      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2241      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2242        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2243        <address><email>fielding@gbiv.com</email></address>
2244      </author>
2245      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2246        <organization abbrev="W3C">World Wide Web Consortium</organization>
2247        <address><email>ylafon@w3.org</email></address>
2248      </author>
2249      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2250        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2251        <address><email>julian.reschke@greenbytes.de</email></address>
2252      </author>
2253      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2254    </front>
2255    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2256    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2257  </reference>
2258
2259  <reference anchor="Part2">
2260    <front>
2261      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2262      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2263        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2264        <address><email>fielding@gbiv.com</email></address>
2265      </author>
2266      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2267        <organization abbrev="W3C">World Wide Web Consortium</organization>
2268        <address><email>ylafon@w3.org</email></address>
2269      </author>
2270      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2271        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2272        <address><email>julian.reschke@greenbytes.de</email></address>
2273      </author>
2274      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2275    </front>
2276    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2277    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2278  </reference>
2279
2280  <reference anchor="Part4">
2281    <front>
2282      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2283      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2284        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2285        <address><email>fielding@gbiv.com</email></address>
2286      </author>
2287      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2288        <organization abbrev="W3C">World Wide Web Consortium</organization>
2289        <address><email>ylafon@w3.org</email></address>
2290      </author>
2291      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2292        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2293        <address><email>julian.reschke@greenbytes.de</email></address>
2294      </author>
2295      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2296    </front>
2297    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2298    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2299  </reference>
2300
2301  <reference anchor="Part5">
2302    <front>
2303      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2304      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2305        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2306        <address><email>fielding@gbiv.com</email></address>
2307      </author>
2308      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2309        <organization abbrev="W3C">World Wide Web Consortium</organization>
2310        <address><email>ylafon@w3.org</email></address>
2311      </author>
2312      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2313        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2314        <address><email>julian.reschke@greenbytes.de</email></address>
2315      </author>
2316      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2317    </front>
2318    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2319    <x:source basename="p5-range" href="p5-range.xml" />
2320  </reference>
2321
2322  <reference anchor="Part7">
2323    <front>
2324      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2325      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2326        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2327        <address><email>fielding@gbiv.com</email></address>
2328      </author>
2329      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2330        <organization abbrev="W3C">World Wide Web Consortium</organization>
2331        <address><email>ylafon@w3.org</email></address>
2332      </author>
2333      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2334        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2335        <address><email>julian.reschke@greenbytes.de</email></address>
2336      </author>
2337      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2338    </front>
2339    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2340    <x:source basename="p7-auth" href="p7-auth.xml" />
2341  </reference>
2342
2343  <reference anchor="RFC2119">
2344    <front>
2345      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2346      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2347        <organization>Harvard University</organization>
2348        <address><email>sob@harvard.edu</email></address>
2349      </author>
2350      <date month="March" year="1997" />
2351    </front>
2352    <seriesInfo name="BCP" value="14" />
2353    <seriesInfo name="RFC" value="2119" />
2354  </reference>
2355
2356  <reference anchor="RFC5234">
2357    <front>
2358      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2359      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2360        <organization>Brandenburg InternetWorking</organization>
2361        <address>
2362          <email>dcrocker@bbiw.net</email>
2363        </address> 
2364      </author>
2365      <author initials="P." surname="Overell" fullname="Paul Overell">
2366        <organization>THUS plc.</organization>
2367        <address>
2368          <email>paul.overell@thus.net</email>
2369        </address>
2370      </author>
2371      <date month="January" year="2008"/>
2372    </front>
2373    <seriesInfo name="STD" value="68"/>
2374    <seriesInfo name="RFC" value="5234"/>
2375  </reference>
2376 
2377</references>
2378
2379<references title="Informative References">
2380
2381  <reference anchor="RFC1305">
2382    <front>
2383      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2384      <author fullname="David L. Mills" initials="D." surname="Mills">
2385        <organization>University of Delaware, Electrical Engineering Department</organization>
2386        <address><email>mills@udel.edu</email></address>
2387      </author>
2388      <date month="March" year="1992" />
2389    </front>
2390    <seriesInfo name="RFC" value="1305" />
2391  </reference>
2392
2393  <reference anchor="RFC2616">
2394    <front>
2395      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2396      <author fullname="R. Fielding" initials="R." surname="Fielding">
2397        <organization>University of California, Irvine</organization>
2398        <address><email>fielding@ics.uci.edu</email></address>
2399      </author>
2400      <author fullname="J. Gettys" initials="J." surname="Gettys">
2401        <organization>W3C</organization>
2402        <address><email>jg@w3.org</email></address>
2403      </author>
2404      <author fullname="J. Mogul" initials="J." surname="Mogul">
2405        <organization>Compaq Computer Corporation</organization>
2406        <address><email>mogul@wrl.dec.com</email></address>
2407      </author>
2408      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2409        <organization>MIT Laboratory for Computer Science</organization>
2410        <address><email>frystyk@w3.org</email></address>
2411      </author>
2412      <author fullname="L. Masinter" initials="L." surname="Masinter">
2413        <organization>Xerox Corporation</organization>
2414        <address><email>masinter@parc.xerox.com</email></address>
2415      </author>
2416      <author fullname="P. Leach" initials="P." surname="Leach">
2417        <organization>Microsoft Corporation</organization>
2418        <address><email>paulle@microsoft.com</email></address>
2419      </author>
2420      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2421        <organization>W3C</organization>
2422        <address><email>timbl@w3.org</email></address>
2423      </author>
2424      <date month="June" year="1999" />
2425    </front>
2426    <seriesInfo name="RFC" value="2616" />
2427  </reference>
2428
2429  <reference anchor="RFC3864">
2430    <front>
2431      <title>Registration Procedures for Message Header Fields</title>
2432      <author fullname="G. Klyne" initials="G." surname="Klyne">
2433        <organization>Nine by Nine</organization>
2434        <address><email>GK-IETF@ninebynine.org</email></address>
2435      </author>
2436      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2437        <organization>BEA Systems</organization>
2438        <address><email>mnot@pobox.com</email></address>
2439      </author>
2440      <author fullname="J. Mogul" initials="J." surname="Mogul">
2441        <organization>HP Labs</organization>
2442        <address><email>JeffMogul@acm.org</email></address>
2443      </author>
2444      <date month="September" year="2004" />
2445    </front>
2446    <seriesInfo name="BCP" value="90" />
2447    <seriesInfo name="RFC" value="3864" />
2448  </reference>
2449
2450  <reference anchor='RFC5226'>
2451    <front>
2452      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2453      <author initials='T.' surname='Narten' fullname='T. Narten'>
2454        <organization>IBM</organization>
2455        <address><email>narten@us.ibm.com</email></address>
2456      </author>
2457      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2458        <organization>Google</organization>
2459        <address><email>Harald@Alvestrand.no</email></address>
2460      </author>
2461      <date year='2008' month='May' />
2462    </front>
2463    <seriesInfo name='BCP' value='26' />
2464    <seriesInfo name='RFC' value='5226' />
2465  </reference>
2466
2467  <reference anchor='RFC5861'>
2468    <front>
2469      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2470      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2471        <organization>Yahoo! Inc.</organization>
2472        <address><email>mnot@yahoo-inc.com</email></address>
2473      </author>
2474      <date month="April" year="2010"/>
2475    </front>
2476    <seriesInfo name='RFC' value='5861' />
2477  </reference>
2478
2479</references>
2480
2481<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2482<t>
2483  Make the specified age calculation algorithm less conservative.
2484  (<xref target="age.calculations"/>)
2485</t>
2486<t>
2487  Remove requirement to consider Content-Location in successful responses
2488  in order to determine the appropriate response to use.
2489  (<xref target="validation.model" />)
2490</t>
2491<t>
2492  Clarify denial of service attack avoidance requirement.
2493  (<xref target="invalidation.after.updates.or.deletions" />)
2494</t>
2495<t>
2496  Change ABNF productions for header fields to only define the field value.
2497  (<xref target="header.field.definitions"/>)
2498</t>
2499<t>
2500  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2501  anymore, as these aspects never were implemented.
2502  (<xref target="header.warning" />)
2503</t>
2504</section>
2505
2506<?BEGININC p6-cache.abnf-appendix ?>
2507<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2508<figure>
2509<artwork type="abnf" name="p6-cache.parsed-abnf">
2510<x:ref>Age</x:ref> = delta-seconds
2511
2512<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2513 cache-directive ] )
2514
2515<x:ref>Expires</x:ref> = HTTP-date
2516
2517<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
2518
2519<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
2520
2521<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2522 pragma-directive ] )
2523
2524<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2525 ) )
2526
2527<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2528 )
2529
2530<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2531
2532<x:ref>delta-seconds</x:ref> = 1*DIGIT
2533
2534<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2535
2536<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2537
2538<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2539<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2540<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 6.2&gt;
2541
2542<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
2543
2544<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.4&gt;
2545
2546<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2547
2548<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2549<x:ref>warn-code</x:ref> = 3DIGIT
2550<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2551<x:ref>warn-text</x:ref> = quoted-string
2552<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2553 ]
2554</artwork>
2555</figure>
2556<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2557; Age defined but not used
2558; Cache-Control defined but not used
2559; Expires defined but not used
2560; Pragma defined but not used
2561; Vary defined but not used
2562; Warning defined but not used
2563</artwork></figure></section>
2564<?ENDINC p6-cache.abnf-appendix ?>
2565
2566<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2567<t>
2568  Changes up to the first Working Group Last Call draft are summarized
2569  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2570</t>
2571
2572<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2573<t>
2574  Closed issues:
2575  <list style="symbols">
2576    <t>
2577      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2578      "untangle Cache-Control ABNF"
2579    </t>
2580    <t>
2581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2582      "Case sensitivity of header fields in CC values"
2583    </t>
2584    <t>
2585      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2586      "Spurious 'MAY's"
2587    </t>
2588    <t>
2589      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2590      "ABNF requirements for recipients"
2591    </t>
2592  </list>
2593</t>
2594</section>
2595
2596</section>
2597  </back>
2598</rfc>
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