source: draft-ietf-httpbis/latest/p6-cache.xml @ 1736

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Work-in-progress: hyperlink header field definitions(P7)

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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 <x:ref>200 (OK)</x:ref> response containing a representation
435   of the resource identified by the request target.  However, it is also possible
436   to cache negative results (e.g., <x:ref>404 (Not Found)</x:ref>, incomplete results
437   (e.g., <x:ref>206 (Partial Content)</x:ref>), 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 <x:ref>Authorization</x:ref> 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. If the request is GET, the response
507   status is <x:ref>200 (OK)</x:ref>, 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 <x:ref>206 (Partial Content)</x:ref> response &MAY; be stored as if it were
511   an incomplete <x:ref>200 (OK)</x:ref> 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 <x:ref>206 (Partial Content)</x:ref> 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<t>
675   When there is more than one value present for a given directive (e.g., two
676   Expires header fields, multiple Cache-Control: max-age directives), it is
677   considered invalid. Caches are encouraged to consider responses that have
678   invalid freshness information to be stale.
679</t>
680
681<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
682<t>
683   If no explicit expiration time is present in a stored response that has a
684   status code whose definition allows heuristic freshness to be used
685   (including the following in &status-codes;: <x:ref>200 (OK)</x:ref>, <x:ref>203
686   (Non-Authoritative Information)</x:ref>, <x:ref>206 (Partial Content)</x:ref>,
687   <x:ref>300 (Multiple Choices)</x:ref>, <x:ref>301 (Moved Permanently)</x:ref> and
688   <x:ref>410 (Gone)</x:ref>), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
689   use heuristics to determine freshness for responses with status codes that do
690   not explicitly allow it.
691</t>
692<t>
693   When a heuristic is used to calculate freshness lifetime, a cache
694   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
695   its current_age is more than 24 hours and such a warning is not already
696   present.
697</t>
698<t>
699   Also, if the response has a Last-Modified header field
700   (&header-last-modified;), caches are encouraged to use a heuristic
701   expiration value that is no more than some fraction of the interval since
702   that time. A typical setting of this fraction might be 10%.
703</t>
704<x:note>
705   <t>
706      &Note; RFC 2616 (<xref target="RFC2616" x:fmt=","
707      x:sec="13.9"/>) required that caches do not calculate heuristic
708      freshness for URIs with query components (i.e., those containing '?').
709      In practice, this has not been widely implemented. Therefore, servers
710      are encouraged to send explicit directives (e.g., Cache-Control:
711      no-cache) if they wish to preclude caching.
712   </t>
713</x:note>
714</section>
715</section>
716
717<section anchor="age.calculations" title="Calculating Age">
718<t>
719   HTTP/1.1 uses the Age header field to convey the estimated age of the
720   response message when obtained from a cache. The Age field value is the
721   cache's estimate of the amount of time since the response was generated or
722   validated by the origin server. In essence, the Age value is the sum of the
723   time that the response has been resident in each of the caches along the
724   path from the origin server, plus the amount of time it has been in transit
725   along network paths.
726</t>
727<t>
728   The following data is used for the age calculation:
729</t>
730<t>
731   <x:dfn>age_value</x:dfn>
732   <list>
733      <t>
734         The term "age_value" denotes the value of the Age header field (<xref
735         target="header.age"/>), in a form appropriate for arithmetic
736         operation; or 0, if not available.
737      </t>
738   </list>
739</t>
740<t>
741   <x:dfn>date_value</x:dfn>
742   <list>
743      <t>
744         HTTP/1.1 requires origin servers to send a Date header field, if possible,
745         with every response, giving the time at which the response was
746         generated. The term "date_value" denotes the value of the Date
747         header field, in a form appropriate for arithmetic operations. See
748         &header-date; for the definition of the Date header field, and for
749         requirements regarding responses without it.
750      </t>
751   </list>
752</t>
753<t>
754   <x:dfn>now</x:dfn>
755   <list>
756      <t>
757         The term "now" means "the current value of the clock at the host
758         performing the calculation". A cache &SHOULD; use NTP (<xref
759         target="RFC1305"/>) or some similar protocol to synchronize its
760         clocks to a globally accurate time standard.
761      </t>
762   </list>
763</t>
764<t>
765   <x:dfn>request_time</x:dfn>
766   <list>
767      <t>
768         The current value of the clock at the host at the time the request
769         resulting in the stored response was made.
770      </t>
771   </list>
772</t>
773<t>
774   <x:dfn>response_time</x:dfn>
775   <list>
776      <t>
777         The current value of the clock at the host at the time the response
778         was received.
779      </t>
780   </list>
781</t>
782<t>
783   A response's age can be calculated in two entirely independent ways:
784   <list style="numbers">
785      <t>the "apparent_age": response_time minus date_value, if the local
786      clock is reasonably well synchronized to the origin server's clock. If
787      the result is negative, the result is replaced by zero.</t>
788      <t>the "corrected_age_value", if all of the caches along the response
789      path implement HTTP/1.1. A cache &MUST; interpret this value relative
790      to the time the request was initiated, not the time that the response
791      was received.</t>
792   </list>
793</t>
794<figure>
795<artwork type="code">
796  apparent_age = max(0, response_time - date_value);
797
798  response_delay = response_time - request_time;
799  corrected_age_value = age_value + response_delay; 
800</artwork>
801</figure>
802<figure>
803<preamble>These &SHOULD; be combined as</preamble>
804<artwork type="code">
805  corrected_initial_age = max(apparent_age, corrected_age_value);
806</artwork></figure>
807<t>
808   unless the cache is confident in the value of the Age header (e.g., because
809   there are no HTTP/1.0 hops in the Via header), in which case the
810   corrected_age_value &MAY; be used as the corrected_initial_age.</t>
811<t>
812   The current_age of a stored response can then be calculated by adding the
813   amount of time (in seconds) since the stored response was last validated by
814   the origin server to the corrected_initial_age.
815</t>
816<figure><artwork type="code">
817  resident_time = now - response_time;
818  current_age = corrected_initial_age + resident_time;
819</artwork></figure>
820<t>
821   Additionally, to avoid common problems in date parsing:
822</t>
823<t>
824  <list style="symbols">
825     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
826        which appears to be more than 50 years in the future is in fact
827        in the past (this helps solve the "year 2000" problem).</t>
828
829     <t>Although all date formats are specified to be case-sensitive,
830        recipients &SHOULD; match day, week and timezone names
831        case-insensitively.</t>
832             
833     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
834        Expires date as earlier than the proper value, but &MUST-NOT;
835        internally represent a parsed Expires date as later than the
836        proper value.</t>
837
838     <t>All expiration-related calculations &MUST; be done in GMT. The
839        local time zone &MUST-NOT; influence the calculation or comparison
840        of an age or expiration time.</t>
841
842     <t>If an HTTP header field incorrectly carries a date value with a time
843        zone other than GMT, it &MUST; be converted into GMT using the
844        most conservative possible conversion.</t>
845  </list>
846</t>
847</section>
848
849<section anchor="serving.stale.responses" title="Serving Stale Responses">
850<t>
851   A "stale" response is one that either has explicit expiry information or is
852   allowed to have heuristic expiry calculated, but is not fresh according to
853   the calculations in <xref target="expiration.model" />.
854</t>
855<t>
856   A cache &MUST-NOT; return a stale response if it is prohibited by an
857   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
858   directive, a "must-revalidate" cache-response-directive, or an applicable
859   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
860   target="cache-response-directive"/>).
861</t>
862<t>
863   A cache &MUST-NOT; return stale responses unless it is disconnected
864   (i.e., it cannot contact the origin server or otherwise find a forward
865   path) or doing so is explicitly allowed (e.g., by the max-stale request
866   directive; see <xref target="cache-request-directive" />).
867</t>
868<t>
869   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
870   <xref target="header.warning" />) to stale responses. Likewise, a cache
871   &SHOULD; add the 112 warn-code to stale responses if the cache is
872   disconnected.
873</t>
874<t>
875   If a cache receives a first-hand response (either an entire response, or a
876   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward to the
877   requesting client, and the received response is no longer fresh, the cache
878   can forward it to the requesting client without adding a new Warning (but
879   without removing any existing Warning header fields). A cache shouldn't
880   attempt to validate a response simply because that response became stale in
881   transit.
882</t>
883</section>
884</section>
885
886<section anchor="validation.model" title="Validation Model">
887<t>
888   When a cache has one or more stored responses for a requested URI, but
889   cannot serve any of them (e.g., because they are not fresh, or one cannot
890   be selected; see <xref target="caching.negotiated.responses"/>), it can use
891   the conditional request mechanism &conditional; in the forwarded request to
892   give the origin server an opportunity to both select a valid stored
893   response to be used, and to update it. This process is known as
894   "validating" or "revalidating" the stored response.
895</t>
896<t>
897   When sending such a conditional request, a cache adds an If-Modified-Since
898   header field whose value is that of the Last-Modified header field from the
899   selected (see <xref target="caching.negotiated.responses"/>) stored
900   response, if available.
901</t>
902<t>
903   Additionally, a cache can add an If-None-Match header field whose value is
904   that of the ETag header field(s) from all responses stored for the
905   requested URI, if present. However, if any of the stored responses contains
906   only partial content, the cache shouldn't include its entity-tag in the
907   If-None-Match header field unless the request is for a range that would be
908   fully satisfied by that stored response.
909</t>
910
911<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
912
913<t>
914   <list style="symbols">
915      <t>
916         A <x:ref>304 (Not Modified)</x:ref> response status code indicates that the stored
917         response can be updated and reused; see <xref
918         target="freshening.responses"/>.
919      </t>
920      <t>
921         A full response (i.e., one with a response body) indicates that none
922         of the stored responses nominated in the conditional request is
923         suitable. Instead, the cache can use the full response to
924         satisfy the request and &MAY; replace the stored response(s).
925      </t>
926      <t>
927         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
928         response while attempting to validate a response, it can either
929         forward this response to the requesting client, or act as if the
930         server failed to respond. In the latter case, it can return a
931         previously stored response (see <xref target="serving.stale.responses" />).
932      </t>
933   </list>
934</t>
935
936<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
937<t>
938   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and already has one
939   or more stored <x:ref>200 (OK)</x:ref> responses for the same cache key, the cache needs
940   to identify which of the stored responses are updated by this new response
941   and then update the stored response(s) with the new information provided in
942   the <x:ref>304</x:ref> response.
943   <list style="symbols">
944    <t>
945     If the new response contains a strong validator, then that strong
946     validator identifies the selected representation.  All of the stored
947     responses with the same strong validator are selected.
948     If none of the stored responses contain the same strong validator, then
949     this new response corresponds to a new selected representation and
950     &MUST-NOT; update the existing stored responses.
951    </t>
952    <t>
953     If the new response contains a weak validator and that validator
954     corresponds to one of the cache's stored responses, then the most
955     recent of those matching stored responses is selected.
956    </t>
957    <t>
958     If the new response does not include any form of validator, there is
959     only one stored response, and that stored response also lacks a
960     validator, then that stored response is selected.
961    </t>
962   </list>
963</t>
964<t>
965   If a stored response is selected for update, the cache &MUST;:
966   <list style="symbols">
967      <t>delete any Warning header fields in the stored response with
968         warn-code 1xx (see <xref target="header.warning" />);</t>
969      <t>retain any Warning header fields in the stored response with
970         warn-code 2xx; and,</t>
971      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
972         response to replace all instances of the corresponding header
973         fields in the stored response.</t>
974   </list>
975</t>
976</section>
977
978</section>
979
980<section anchor="head.effects" title="Updating Caches with HEAD Responses">
981<t>
982   A response to the HEAD method is identical to what an equivalent request
983   made with a GET would have been, except it lacks a body. This property
984   of HEAD responses is used to both invalidate and update cached GET
985   responses.
986</t>
987<t>
988   If one or more stored GET responses can be selected (as per <xref
989   target="caching.negotiated.responses"/>) for a HEAD request, and the
990   Content-Length, ETag or Last-Modified value of a HEAD response differs from
991   that in a selected GET response, the cache &MUST; consider that selected
992   response to be stale.
993</t>
994<t>
995   If the Content-Length, ETag and Last-Modified values of a HEAD response
996   (when present) are the same as that in a selected GET response (as per
997   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update the
998   remaining headers in the stored response using the following rules:
999   <list style="symbols">
1000      <t>delete any Warning header fields in the stored response with
1001         warn-code 1xx (see <xref target="header.warning" />);</t>
1002      <t>retain any Warning header fields in the stored response with
1003         warn-code 2xx; and,</t>
1004      <t>use other header fields provided in the response to replace
1005         all instances of the corresponding header fields in the stored
1006         response.</t>
1007   </list>
1008</t>
1009
1010</section>
1011
1012<section anchor="invalidation.after.updates.or.deletions" 
1013   title="Request Methods that Invalidate">
1014<t>
1015   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1016   have the potential for changing state on the origin server, intervening
1017   caches can use them to keep their contents up-to-date.
1018</t>
1019<t>
1020   A cache &MUST; invalidate the effective Request URI
1021   (&effective-request-uri;) as well as the URI(s) in the Location
1022   and Content-Location response header fields (if present) when a non-error
1023   response to a request with an unsafe method is received.
1024</t>
1025<t>
1026   However, a cache &MUST-NOT; invalidate a URI from a Location or
1027   Content-Location response header field if the host part of that URI differs
1028   from the host part in the effective request URI (&effective-request-uri;).
1029   This helps prevent denial of service attacks.
1030</t>
1031<t>
1032   A cache &MUST; invalidate the effective request URI
1033   (&effective-request-uri;) when it receives a non-error response
1034   to a request with a method whose safety is unknown.
1035</t>
1036<t>
1037   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref> or
1038   <x:ref>3xx (Redirection)</x:ref> status code.
1039   "Invalidate" means that the cache will either remove all stored
1040   responses related to the effective request URI, or will mark these as
1041   "invalid" and in need of a mandatory validation before they can be returned
1042   in response to a subsequent request.
1043</t>
1044<t>
1045   Note that this does not guarantee that all appropriate responses are
1046   invalidated. For example, the request that caused the change at the origin
1047   server might not have gone through the cache where a response is stored.
1048</t>
1049</section>
1050
1051<section anchor="caching.authenticated.responses" 
1052   title="Shared Caching of Authenticated Responses">
1053<t>
1054   A shared cache &MUST-NOT; use a cached response to a request with an
1055   <x:ref>Authorization</x:ref> header field (&header-authorization;) to satisfy any subsequent
1056   request unless a cache directive that allows such responses to be stored is
1057   present in the response.
1058</t>
1059
1060<t>
1061   In this specification, the following Cache-Control response directives
1062   (<xref target="cache-response-directive"/>) have such an effect:
1063   must-revalidate, public, s-maxage.
1064</t>
1065
1066<t>
1067   Note that cached responses that contain the "must-revalidate" and/or
1068   "s-maxage" response directives are not allowed to be served stale (<xref
1069   target="serving.stale.responses"/>) by shared caches. In particular, a
1070   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1071   used to satisfy a subsequent request without revalidating it on the origin
1072   server.
1073</t>
1074</section>
1075
1076<section anchor="caching.negotiated.responses" 
1077   title="Caching Negotiated Responses">
1078<t>
1079   When a cache receives a request that can be satisfied by a stored response
1080   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1081   use that response unless all of the selecting header fields nominated by
1082   the Vary header field match in both the original request (i.e., that associated
1083   with the stored response), and the presented request.
1084</t>
1085<t>
1086   The selecting header fields from two requests are defined to match if and
1087   only if those in the first request can be transformed to those in the
1088   second request by applying any of the following:
1089   <list style="symbols">
1090      <t>
1091         adding or removing whitespace, where allowed in the header field's syntax
1092      </t>
1093      <t>
1094         combining multiple header fields with the same field name
1095         (see &header-fields;)
1096      </t>
1097      <t>
1098         normalizing both header field values in a way that is known to have
1099         identical semantics, according to the header field's specification (e.g.,
1100         re-ordering field values when order is not significant;
1101         case-normalization, where values are defined to be case-insensitive)
1102      </t>
1103  </list>
1104</t>
1105<t>
1106   If (after any normalization that might take place) a header field is absent
1107   from a request, it can only match another request if it is also absent
1108   there.
1109</t>
1110<t>
1111   A Vary header field-value of "*" always fails to match, and subsequent
1112   requests to that resource can only be properly interpreted by the origin
1113   server.
1114</t>
1115<t>
1116   The stored response with matching selecting header fields is known as the
1117   selected response.
1118</t>
1119<t>
1120   If multiple selected responses are available, the most recent response
1121   (as determined by the Date header field) is used; see <xref 
1122   target="constructing.responses.from.caches"/>.
1123</t>
1124<t>
1125   If no selected response is available, the cache can forward the presented
1126   request to the origin server in a conditional request; see <xref
1127   target="validation.model"/>.
1128</t>
1129</section>
1130
1131<section anchor="combining.responses" title="Combining Partial Content">
1132<t>
1133   A response might transfer only a partial representation if the
1134   connection closed prematurely or if the request used one or more Range
1135   specifiers (&partial;).  After several such transfers, a cache might have
1136   received several ranges of the same representation.  A cache &MAY; combine
1137   these ranges into a single stored response, and reuse that response to
1138   satisfy later requests, if they all share the same strong validator and
1139   the cache complies with the client requirements in &combining-byte-ranges;.
1140</t>
1141<t>
1142   When combining the new response with one or more stored responses, a
1143   cache &MUST;:
1144   <list style="symbols">
1145      <t>delete any Warning header fields in the stored response with
1146         warn-code 1xx (see <xref target="header.warning" />);</t>
1147      <t>retain any Warning header fields in the stored response with
1148         warn-code 2xx; and,</t>
1149      <t>use other header fields provided in the new response, aside
1150         from Content-Range, to replace all instances of the corresponding
1151         header fields in the stored response.</t>
1152   </list>
1153</t>
1154</section>
1155
1156
1157</section>
1158
1159<section anchor="header.field.definitions" title="Header Field Definitions">
1160<t>
1161   This section defines the syntax and semantics of HTTP/1.1 header fields
1162   related to caching.
1163</t>
1164
1165<section anchor="header.age" title="Age">
1166   <iref item="Age header field" primary="true" x:for-anchor="" />
1167   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1168   <x:anchor-alias value="Age"/>
1169   <x:anchor-alias value="age-value"/>
1170<t>
1171   The "Age" header field conveys the sender's estimate of the amount
1172   of time since the response was generated or successfully validated at the
1173   origin server. Age values are calculated as specified in <xref
1174   target="age.calculations" />.
1175</t>
1176<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1177  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1178</artwork></figure>
1179<t>
1180  Age field-values are non-negative integers, representing time in seconds
1181  (see <xref target="delta-seconds"/>).
1182</t>
1183<t>
1184   The presence of an Age header field in a response implies that a response
1185   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1186   might not implement the Age header field.
1187</t>
1188</section>
1189
1190<section anchor="header.cache-control" title="Cache-Control">
1191   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1192   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1193      x:for-anchor="" />
1194   <x:anchor-alias value="Cache-Control"/>
1195   <x:anchor-alias value="cache-directive"/>
1196<t>
1197   The "Cache-Control" header field is used to specify directives for
1198   caches along the request/response chain. Such cache directives are
1199   unidirectional in that the presence of a directive in a request does not
1200   imply that the same directive is to be given in the response.
1201</t>
1202<t>
1203   A cache &MUST; obey the requirements of the Cache-Control
1204   directives defined in this section. See <xref
1205   target="cache.control.extensions"/> for information about how Cache-Control
1206   directives defined elsewhere are handled.
1207</t>
1208<x:note>
1209   <t>
1210       &Note; HTTP/1.0 caches might not implement Cache-Control and
1211       might only implement Pragma: no-cache (see <xref target="header.pragma"
1212       />).
1213   </t>
1214</x:note>
1215<t>
1216   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1217   through in forwarded messages, regardless of their
1218   significance to that application, since the directives might be applicable
1219   to all recipients along the request/response chain. It is not possible to
1220   target a directive to a specific cache.
1221</t>
1222<t>
1223   Cache directives are identified by a token, to be compared case-insensitively,
1224   and have an optional argument, that can use both token and quoted-string
1225   syntax. For the directives defined below that define arguments, recipients
1226   ought to accept both forms, even if one is documented to be preferred. For
1227   any directive not defined by this specification, recipients &MUST; accept
1228   both forms.
1229</t>
1230<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1231  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1232
1233  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1234</artwork></figure>
1235<t>
1236   For the cache directives defined below, no argument is defined (nor allowed)
1237   otherwise stated otherwise.
1238</t>
1239
1240<section anchor="cache-request-directive" 
1241   title="Request Cache-Control Directives">
1242
1243<section title="no-cache" anchor="cache-request-directive.no-cache">
1244   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1245   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1246<t>
1247   The no-cache request directive indicates that a cache &MUST-NOT; 
1248   use a stored response to satisfy the request without successful
1249   validation on the origin server.
1250</t>
1251</section>
1252 
1253<section title="no-store" anchor="cache-request-directive.no-store">
1254   <iref item="Cache Directives" primary="true" subitem="no-store" />
1255   <iref item="no-store" primary="true" subitem="Cache Directive" />
1256<t>
1257   The no-store request directive indicates that a cache &MUST-NOT;
1258   store any part of either this request or any response to it. This
1259   directive applies to both private and shared caches. "&MUST-NOT;
1260   store" in this context means that the cache &MUST-NOT; intentionally
1261   store the information in non-volatile storage, and &MUST; make a
1262   best-effort attempt to remove the information from volatile storage as
1263   promptly as possible after forwarding it.
1264</t>
1265<t>
1266   This directive is NOT a reliable or sufficient mechanism for ensuring
1267   privacy. In particular, malicious or compromised caches might not
1268   recognize or obey this directive, and communications networks might be
1269   vulnerable to eavesdropping.
1270</t>
1271<t>
1272   Note that if a request containing this directive is satisfied from a
1273   cache, the no-store request directive does not apply to the already
1274   stored response.
1275</t>
1276</section>
1277
1278<section title="max-age" anchor="cache-request-directive.max-age">
1279   <iref item="Cache Directives" primary="true" subitem="max-age" />
1280   <iref item="max-age" primary="true" subitem="Cache Directive" />
1281<t>
1282   Argument syntax:
1283   <list>
1284      <t>
1285        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1286      </t>
1287   </list>
1288</t>
1289<t>
1290   The max-age request directive indicates that the client is unwilling to
1291   accept a response whose age is greater than the specified number of
1292   seconds. Unless the max-stale request directive is also present, the
1293   client is not willing to accept a stale response.
1294</t>
1295<t>
1296   &Note; This directive uses the token form of the argument syntax;
1297   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1298   quoted-string form.
1299</t>
1300</section>
1301
1302<section title="max-stale" anchor="cache-request-directive.max-stale">
1303   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1304   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1305<t>
1306   Argument syntax:
1307   <list>
1308      <t>
1309        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1310      </t>
1311   </list>
1312</t>
1313<t>
1314   The max-stale request directive indicates that the client is willing
1315   to accept a response that has exceeded its expiration time. If max-stale
1316   is assigned a value, then the client is willing to accept a response
1317   that has exceeded its expiration time by no more than the specified
1318   number of seconds. If no value is assigned to max-stale, then the client
1319   is willing to accept a stale response of any age.
1320</t>
1321<t>
1322   &Note; This directive uses the token form of the argument syntax;
1323   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1324   quoted-string form.
1325</t>
1326</section>
1327
1328<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1329   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1330   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1331<t>
1332   Argument syntax:
1333   <list>
1334      <t>
1335        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1336      </t>
1337   </list>
1338</t>
1339<t>
1340   The min-fresh request directive indicates that the client is willing
1341   to accept a response whose freshness lifetime is no less than its
1342   current age plus the specified time in seconds. That is, the client
1343   wants a response that will still be fresh for at least the specified
1344   number of seconds.
1345</t>
1346<t>
1347   &Note; This directive uses the token form of the argument syntax;
1348   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1349   quoted-string form.
1350</t>
1351</section>
1352
1353<section title="no-transform" anchor="cache-request-directive.no-transform">
1354   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1355   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1356<t>
1357   The no-transform request directive indicates that an intermediary
1358   (whether or not it implements a cache) &MUST-NOT; change the
1359   Content-Encoding, Content-Range or Content-Type request header fields,
1360   nor the request representation.
1361</t>
1362</section>
1363
1364<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1365   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1366   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1367<t>
1368   The only-if-cached request directive indicates that the client only
1369   wishes to obtain a stored response. If it receives 3ive, a
1370   cache &SHOULD; either respond using a stored response that is consistent
1371   with the other constraints of the request, or respond with a <x:ref>504
1372   (Gateway Timeout)</x:ref> status code. If a group of caches is being operated as
1373   a unified system with good internal connectivity, a member cache &MAY;
1374   forward such a request within that group of caches.
1375</t>
1376</section>
1377</section>
1378
1379<section anchor="cache-response-directive" 
1380   title="Response Cache-Control Directives">
1381   <x:anchor-alias value="cache-response-directive" />
1382
1383<section title="public" anchor="cache-response-directive.only-if-cached">
1384   <iref item="Cache Directives" primary="true" subitem="public" />
1385   <iref item="public" primary="true" subitem="Cache Directive" />
1386<t>
1387   The public response directive indicates that a response whose
1388   associated request contains an 'Authentication' header &MAY; be
1389   stored (see <xref target="caching.authenticated.responses" />).
1390</t>
1391</section>
1392
1393<section title="private" anchor="cache-response-directive.private">
1394   <iref item="Cache Directives" primary="true" subitem="private" />
1395   <iref item="private" primary="true" subitem="Cache Directive" />
1396<t>
1397   Argument syntax:
1398   <list>
1399      <t>
1400        #<x:ref>field-name</x:ref>
1401      </t>
1402   </list>
1403</t>
1404<t>
1405   The private response directive indicates that the response message is
1406   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1407   private cache &MAY; store the response.
1408</t>
1409<t>
1410   If the private response directive specifies one or more field-names,
1411   this requirement is limited to the field-values associated with the
1412   listed response header fields. That is, a shared cache &MUST-NOT; store
1413   the specified field-names(s), whereas it &MAY; store the remainder of the
1414   response message.
1415</t>
1416<t>
1417   The field-names given are not limited to the set of standard header
1418   fields defined by this specification. Field names are case-insensitive.
1419</t>
1420<t>
1421   &Note; This usage of the word "private" only controls
1422   where the response can be stored; it cannot ensure the privacy of the
1423   message content. Also, private response directives with field-names are
1424   often handled by implementations as if an unqualified private directive
1425   was received; i.e., the special handling for the qualified form is not
1426   widely implemented.
1427</t>
1428<t>
1429   &Note; This directive uses the quoted-string form of the argument
1430   syntax. Senders &SHOULD-NOT; use the token form (even if quoting appears not
1431   to be needed for single-entry lists).
1432</t>
1433</section>
1434
1435<section title="no-cache" anchor="cache-response-directive.no-cache">
1436   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1437   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1438<t>
1439   Argument syntax:
1440   <list>
1441      <t>
1442        #<x:ref>field-name</x:ref>
1443      </t>
1444   </list>
1445</t>
1446<t>
1447   The no-cache response directive indicates that the response &MUST-NOT;
1448   be used to satisfy a subsequent request without successful validation on
1449   the origin server. This allows an origin server to prevent a cache from
1450   using it to satisfy a request without contacting it, even by caches that
1451   have been configured to return stale responses.
1452</t>
1453<t>
1454   If the no-cache response directive specifies one or more field-names,
1455   then a cache &MAY; use the response to satisfy a subsequent request,
1456   subject to any other restrictions on caching. However, any header fields
1457   in the response that have the field-name(s) listed &MUST-NOT; be sent
1458   in the response to a subsequent request without successful revalidation
1459   with the origin server. This allows an origin server to prevent the
1460   re-use of certain header fields in a response, while still allowing
1461   caching of the rest of the response.
1462</t> 
1463<t>
1464   The field-names given are not limited to the set of standard header
1465   fields defined by this specification. Field names are case-insensitive.
1466</t>
1467<t>
1468   &Note; Most HTTP/1.0 caches will not recognize or obey
1469   this directive. Also, no-cache response directives with field-names are
1470   often handled by implementations as if an unqualified no-cache directive
1471   was received; i.e., the special handling for the qualified form is not
1472   widely implemented.
1473</t>
1474<t>
1475   &Note; This directive uses the quoted-string form of the argument
1476   syntax. Senders &SHOULD-NOT; use the token form (even if quoting appears not
1477   to be needed for single-entry lists).
1478</t>
1479</section>
1480
1481<section title="no-store" anchor="cache-response-directive.no-store">
1482   <iref item="Cache Directives" primary="true" subitem="no-store" />
1483   <iref item="no-store" primary="true" subitem="Cache Directive" />
1484<t>
1485   The no-store response directive indicates that a cache &MUST-NOT;
1486   store any part of either the immediate request or response. This
1487   directive applies to both private and shared caches. "&MUST-NOT;
1488   store" in this context means that the cache &MUST-NOT; intentionally
1489   store the information in non-volatile storage, and &MUST; make a
1490   best-effort attempt to remove the information from volatile storage as
1491   promptly as possible after forwarding it.
1492</t>
1493<t>
1494   This directive is NOT a reliable or sufficient mechanism for ensuring
1495   privacy. In particular, malicious or compromised caches might not
1496   recognize or obey this directive, and communications networks might be
1497   vulnerable to eavesdropping.
1498</t>
1499</section>
1500
1501<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1502   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1503   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1504<t>
1505   The must-revalidate response directive indicates that once it has
1506   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1507   requests without successful validation on the origin server.
1508</t>
1509<t>
1510   The must-revalidate directive is necessary to support reliable
1511   operation for certain protocol features. In all circumstances a
1512   cache &MUST; obey the must-revalidate directive; in particular,
1513   if a cache cannot reach the origin server for any reason, it &MUST;
1514   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1515</t>
1516<t>
1517   The must-revalidate directive ought to be used by servers if and only
1518   if failure to validate a request on the representation could result in
1519   incorrect operation, such as a silently unexecuted financial
1520   transaction.
1521</t>
1522</section>
1523
1524<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1525   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1526   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1527<t>
1528   The proxy-revalidate response directive has the same meaning as the
1529   must-revalidate response directive, except that it does not apply to
1530   private caches.
1531</t>
1532</section>
1533
1534<section title="max-age" anchor="cache-response-directive.max-age">
1535   <iref item="Cache Directives" primary="true" subitem="max-age" />
1536   <iref item="max-age" primary="true" subitem="Cache Directive" />
1537<t>
1538   Argument syntax:
1539   <list>
1540      <t>
1541        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1542      </t>
1543   </list>
1544</t>
1545<t>
1546   The max-age response directive indicates that the response is to be
1547   considered stale after its age is greater than the specified number of
1548   seconds.
1549</t>
1550<t>
1551   &Note; This directive uses the token form of the argument syntax;
1552   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1553   quoted-string form.
1554</t>
1555</section>     
1556
1557<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1558   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1559   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1560<t>
1561   Argument syntax:
1562   <list>
1563      <t>
1564        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1565      </t>
1566   </list>
1567</t>
1568<t>
1569   The s-maxage response directive indicates that, in shared caches, the
1570   maximum age specified by this directive overrides the maximum age
1571   specified by either the max-age directive or the Expires header field. The
1572   s-maxage directive also implies the semantics of the proxy-revalidate
1573   response directive.
1574</t>
1575<t>
1576   &Note; This directive uses the token form of the argument syntax;
1577   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1578   quoted-string form.
1579</t>
1580</section>
1581
1582<section title="no-transform" anchor="cache-response-directive.no-transform">
1583   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1584   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1585<t>
1586   The no-transform response directive indicates that an intermediary
1587   (regardless of whether it implements a cache) &MUST-NOT; change the
1588   Content-Encoding, Content-Range or Content-Type response header fields,
1589   nor the response representation.
1590</t>
1591</section>
1592
1593</section>
1594
1595<section anchor="cache.control.extensions" title="Cache Control Extensions">
1596<t>
1597   The Cache-Control header field can be extended through the use of one or
1598   more cache-extension tokens, each with an optional value. Informational
1599   extensions (those that do not require a change in cache behavior) can be
1600   added without changing the semantics of other directives. Behavioral
1601   extensions are designed to work by acting as modifiers to the existing base
1602   of cache directives. Both the new directive and the standard directive are
1603   supplied, such that applications that do not understand the new directive
1604   will default to the behavior specified by the standard directive, and those
1605   that understand the new directive will recognize it as modifying the
1606   requirements associated with the standard directive. In this way,
1607   extensions to the cache-control directives can be made without requiring
1608   changes to the base protocol.
1609</t>
1610<t>
1611   This extension mechanism depends on an HTTP cache obeying all of the
1612   cache-control directives defined for its native HTTP-version, obeying
1613   certain extensions, and ignoring all directives that it does not
1614   understand.
1615</t>
1616<t>
1617   For example, consider a hypothetical new response directive called
1618   "community" that acts as a modifier to the private directive. We define
1619   this new directive to mean that, in addition to any private cache, any
1620   cache that is shared only by members of the community named within its
1621   value is allowed to cache the response. An origin server wishing to allow
1622   the UCI community to use an otherwise private response in their shared
1623   cache(s) could do so by including
1624</t>
1625<figure><artwork type="example">
1626  Cache-Control: private, community="UCI"
1627</artwork></figure>
1628<t>
1629   A cache seeing this header field will act correctly even if the cache does
1630   not understand the community cache-extension, since it will also see and
1631   understand the private directive and thus default to the safe behavior.
1632</t>
1633<t>
1634   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1635   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1636   combined with standard directives (or the response's default cacheability)
1637   such that the cache behavior will remain minimally correct even if the
1638   cache does not understand the extension(s).
1639</t>
1640<t>
1641   New extension directives ought to consider defining:
1642</t>
1643<t>
1644   <list style="symbols">
1645      <t>What it means for a directive to be specified multiple times,</t>
1646      <t>When the directive does not take an argument, what it means when an
1647      argument is present,</t>
1648      <t>When the directive requires an argument, what it means when it is
1649      missing.</t>
1650   </list>
1651</t>
1652<t>
1653   The HTTP Cache Directive Registry defines the name space for the cache
1654   directives.
1655</t>
1656<t>
1657   A registration &MUST; include the following fields:
1658   <list style="symbols">
1659      <t>Cache Directive Name</t>
1660      <t>Pointer to specification text</t>
1661   </list>
1662</t>
1663<t>
1664   Values to be added to this name space require IETF Review (see <xref
1665   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1666</t>
1667<t>
1668   The registry itself is maintained at <eref
1669   target="http://www.iana.org/assignments/http-cache-directives"/>.
1670</t>
1671</section>
1672
1673</section>
1674
1675<section anchor="header.expires" title="Expires">
1676   <iref item="Expires header field" primary="true" x:for-anchor="" />
1677   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1678   <x:anchor-alias value="Expires"/>
1679<t>
1680   The "Expires" header field gives the date/time after which the
1681   response is considered stale. See <xref target="expiration.model" /> for
1682   further discussion of the freshness model.
1683</t>
1684<t>
1685   The presence of an Expires field does not imply that the original resource
1686   will change or cease to exist at, before, or after that time.
1687</t>
1688<t>
1689   The field-value is an absolute date and time as defined by HTTP-date in
1690   &http-date;; a sender &MUST; use the rfc1123-date format.
1691</t>
1692<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1693  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1694</artwork></figure>
1695<figure>
1696  <preamble>For example</preamble>
1697<artwork type="example">
1698  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1699</artwork></figure>
1700<t>
1701   A cache &MUST; treat other invalid date formats,
1702   especially including the value "0", as in the past (i.e., "already
1703   expired").
1704</t>
1705<x:note>
1706   <t>
1707       &Note; If a response includes a Cache-Control field with the
1708       max-age directive (see <xref target="cache-response-directive" />),
1709       that directive overrides the Expires field. Likewise, the s-maxage
1710       directive overrides Expires in shared caches.
1711   </t>
1712</x:note>
1713<t>
1714   Historically, HTTP required the Expires field-value to be no more than a
1715   year in the future. While longer freshness lifetimes are no longer
1716   prohibited, extremely large values have been demonstrated to cause
1717   problems (e.g., clock overflows due to use of 32-bit integers for
1718   time values), and most caches will evict a response far sooner than
1719   that. Therefore, senders ought not produce them.
1720</t>
1721<t>
1722   An origin server without a clock &MUST-NOT; assign Expires
1723   values to a response unless these values were associated
1724   with the resource by a system or user with a reliable clock. It &MAY;
1725   assign an Expires value that is known, at or before server
1726   configuration time, to be in the past (this allows "pre-expiration"
1727   of responses without storing separate Expires values for each
1728   resource).
1729</t>
1730</section>
1731
1732<section anchor="header.pragma" title="Pragma">
1733   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1734   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1735   <x:anchor-alias value="extension-pragma"/>
1736   <x:anchor-alias value="Pragma"/>
1737   <x:anchor-alias value="pragma-directive"/>
1738<t>
1739   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1740   caches, so that clients can specify a "no-cache" request that they will
1741   understand (as Cache-Control was not defined until HTTP/1.1). When the
1742   Cache-Control header is also present and understood in a request, Pragma is
1743   ignored.
1744</t>
1745<t>
1746   In HTTP/1.0, Pragma was defined as an extensible field for
1747   implementation-specified directives for recipients. This specification
1748   deprecates such extensions to improve interoperability.
1749</t>
1750<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"/>
1751  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1752  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1753  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1754</artwork></figure>
1755<t>
1756   When the Cache-Control header is not present in a request, the no-cache
1757   request pragma-directive &MUST; have the same effect on caches as if
1758   "Cache-Control: no-cache" were present (see <xref
1759   target="cache-request-directive" />).
1760</t>
1761<t>
1762   When sending a no-cache request, a client ought to include both the pragma
1763   and cache-control directives, unless Cache-Control: no-cache is
1764   purposefully omitted to target other Cache-Control response directives at
1765   HTTP/1.1 caches. For example:
1766</t>
1767<figure>
1768<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1769GET / HTTP/1.1
1770Host: www.example.com
1771Cache-Control: max-age=30
1772Pragma: no-cache
1773
1774</artwork>
1775</figure>
1776<t>
1777   will constrain HTTP/1.1 caches to serve a response no older than 30
1778   seconds, while precluding implementations that do not understand
1779   Cache-Control from serving a cached response.
1780</t>
1781<x:note>
1782   <t>
1783      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1784      specified, it does not provide a reliable replacement for
1785      "Cache-Control: no-cache" in them.
1786   </t>
1787</x:note>
1788</section>
1789
1790<section anchor="header.vary" title="Vary">
1791   <iref item="Vary header field" primary="true" x:for-anchor="" />
1792   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1793   <x:anchor-alias value="Vary"/>
1794<t>
1795   The "Vary" header field conveys the set of header fields
1796   that were used to select the representation.
1797</t>
1798<t>
1799   Caches use this information, in part, to determine whether a stored
1800   response can be used to satisfy a given request; see <xref
1801   target="caching.negotiated.responses" />. determines, while the response is
1802   fresh, whether a cache is permitted to use the response to reply to a
1803   subsequent request without validation; see <xref
1804   target="caching.negotiated.responses" />.
1805</t>
1806<t>
1807   In uncacheable or stale responses, the Vary field value advises the user
1808   agent about the criteria that were used to select the representation.
1809</t>
1810<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1811  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1812</artwork></figure>
1813<t>
1814   The set of header fields named by the Vary field value is known as the
1815   selecting header fields.
1816</t>
1817<t>
1818   A server &SHOULD; include a Vary header field with any cacheable response
1819   that is subject to server-driven negotiation. Doing so allows a cache to
1820   properly interpret future requests on that resource and informs the user
1821   agent about the presence of negotiation on that resource. A server &MAY;
1822   include a Vary header field with a non-cacheable response that is subject
1823   to server-driven negotiation, since this might provide the user agent with
1824   useful information about the dimensions over which the response varies at
1825   the time of the response.
1826</t>
1827<t>
1828   A Vary field value of "*" signals that unspecified parameters not limited
1829   to the header fields (e.g., the network address of the client), play a
1830   role in the selection of the response representation; therefore, a cache
1831   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1832   generate the "*" value.
1833</t>
1834<t>
1835   The field-names given are not limited to the set of standard header
1836   fields defined by this specification. Field names are case-insensitive.
1837</t>
1838</section>
1839
1840<section anchor="header.warning" title="Warning">
1841   <iref item="Warning header field" primary="true" x:for-anchor="" />
1842   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1843   <x:anchor-alias value="Warning"/>
1844   <x:anchor-alias value="warning-value"/>
1845   <x:anchor-alias value="warn-agent"/>
1846   <x:anchor-alias value="warn-code"/>
1847   <x:anchor-alias value="warn-date"/>
1848   <x:anchor-alias value="warn-text"/>
1849<t>
1850   The "Warning" header field is used to carry additional information
1851   about the status or transformation of a message that might not be reflected
1852   in the message. This information is typically used to warn about possible
1853   incorrectness introduced by caching operations or transformations applied
1854   to the payload of the message.
1855</t>
1856<t>
1857   Warnings can be used for other purposes, both cache-related and otherwise.
1858   The use of a warning, rather than an error status code, distinguishes these
1859   responses from true failures.
1860</t>
1861<t>
1862   Warning header fields can in general be applied to any message, however some
1863   warn-codes are specific to caches and can only be applied to response
1864   messages.
1865</t>
1866<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"/>
1867  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1868 
1869  <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>
1870                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1871 
1872  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1873  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1874                  ; the name or pseudonym of the server adding
1875                  ; the Warning header field, for use in debugging
1876  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1877  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1878</artwork></figure>
1879<t>
1880   Multiple warnings can be attached to a response (either by the origin
1881   server or by a cache), including multiple warnings with the same code
1882   number, only differing in warn-text.
1883</t>
1884<t>
1885   When this occurs, the user agent &SHOULD; inform the user of as many of
1886   them as possible, in the order that they appear in the response.
1887</t>
1888<t>
1889   Systems that generate multiple Warning header fields are encouraged to
1890   order them with this user agent behavior in mind. New Warning header fields
1891   are added after any existing Warning headers fields.
1892</t>
1893<t>
1894   Warnings are assigned three digit warn-codes. The first digit indicates
1895   whether the Warning is required to be deleted from a stored response after
1896   validation:
1897   <list style="symbols">
1898      <t>1xx Warnings describe the freshness or validation status of the
1899      response, and so &MUST; be deleted by a cache after validation. They can
1900      only be generated by a cache when validating a cached entry, and
1901      &MUST-NOT; be generated in any other situation.</t>
1902      <t>2xx Warnings describe some aspect of the representation that is not
1903      rectified by a validation (for example, a lossy compression of the
1904      representation) and &MUST-NOT; be deleted by a cache after validation,
1905      unless a full response is returned, in which case they &MUST; be.</t>
1906   </list>
1907</t>
1908<t>
1909   If an implementation sends a message with one or more Warning header fields to a
1910   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1911   in each warning-value a warn-date that matches the Date header field in the
1912   message.
1913</t>
1914<t>
1915   If a system receives a message with a warning-value that includes
1916   a warn-date, and that warn-date is different from the Date value in the
1917   response, then that warning-value &MUST; be deleted from the message before
1918   storing, forwarding, or using it. (preventing the consequences of naive
1919   caching of Warning header fields.) If all of the warning-values are deleted
1920   for this reason, the Warning header field &MUST; be deleted as well.
1921</t>
1922<t>
1923   The following warn-codes are defined by this specification, each with a
1924   recommended warn-text in English, and a description of its meaning.
1925</t>
1926
1927<section title="110 Response is Stale" anchor="warn.110">
1928  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1929  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale" x:for-anchor=""/>
1930<t>
1931   A cache &SHOULD; include this whenever the returned response is stale.
1932</t>
1933</section>
1934
1935<section title="111 Revalidation Failed" anchor="warn.111">
1936  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1937  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed" x:for-anchor=""/>
1938<t>
1939   A cache &SHOULD; include this when returning a stale response because an
1940   attempt to validate the response failed, due to an inability to reach
1941   the server.
1942</t>
1943</section>
1944
1945<section title="112 Disconnected Operation" anchor="warn.112">
1946  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1947  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation" x:for-anchor=""/>
1948<t>
1949   A cache &SHOULD; include this if it is intentionally disconnected from
1950   the rest of the network for a period of time.
1951</t>
1952</section>
1953
1954<section title="113 Heuristic Expiration" anchor="warn.113">
1955  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1956  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration" x:for-anchor=""/>
1957<t>
1958   A cache &SHOULD; include this if it heuristically chose a freshness
1959   lifetime greater than 24 hours and the response's age is greater than 24
1960   hours.
1961</t>
1962</section>
1963
1964<section title="199 Miscellaneous Warning" anchor="warn.199">
1965  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1966  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning" x:for-anchor=""/>
1967<t>
1968   The warning text can include arbitrary information to be presented to
1969   a human user, or logged. A system receiving this warning &MUST-NOT; take
1970   any automated action, besides presenting the warning to the user.
1971</t>
1972</section>
1973
1974<section title="214 Transformation Applied" anchor="warn.214">
1975  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1976  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied" x:for-anchor=""/>
1977<t>
1978   &MUST; be added by a proxy if it applies any transformation to the
1979   representation, such as changing the content-coding, media-type, or
1980   modifying the representation data, unless this Warning code already appears
1981   in the response.
1982</t>
1983</section>
1984
1985<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1986  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1987  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning" x:for-anchor=""/>
1988<t>
1989   The warning text can include arbitrary information to be presented to
1990   a human user, or logged. A system receiving this warning &MUST-NOT; take
1991   any automated action.
1992</t>
1993</section>
1994
1995<section title="Warn Code Extensions" anchor="warn.code.extensions">
1996<t>
1997   The HTTP Warn Code Registry defines the name space for warn codes.
1998</t>
1999<t>
2000   A registration &MUST; include the following fields:
2001   <list style="symbols">
2002      <t>Warn Code (3 digits)</t>
2003      <t>Short Description</t>
2004      <t>Pointer to specification text</t>
2005   </list>
2006</t>
2007<t>
2008   Values to be added to this name space require IETF Review (see <xref
2009   target="RFC5226" x:fmt="," x:sec="4.1"/>).
2010</t>
2011<t>
2012   The registry itself is maintained at <eref
2013   target="http://www.iana.org/assignments/http-warn-codes"/>.
2014</t>
2015</section>
2016</section>
2017</section>
2018
2019<section anchor="history.lists" title="History Lists">
2020<t>
2021   User agents often have history mechanisms, such as "Back" buttons and
2022   history lists, that can be used to redisplay a representation retrieved
2023   earlier in a session.
2024</t>
2025<t>
2026   The freshness model (<xref target="expiration.model"/>) does not
2027   necessarily apply to history mechanisms. I.e., a history mechanism can
2028   display a previous representation even if it has expired.
2029</t>
2030<t>
2031   This does not prohibit the history mechanism from telling the user that a
2032   view might be stale, or from honoring cache directives (e.g.,
2033   Cache-Control: no-store).
2034</t>
2035</section>
2036
2037
2038<section anchor="IANA.considerations" title="IANA Considerations">
2039
2040<section title="Cache Directive Registry" 
2041   anchor="cache.directive.registration">
2042<t>
2043   The registration procedure for HTTP Cache Directives is defined by <xref
2044   target="cache.control.extensions"/> of this document.
2045</t>
2046<t>
2047   The HTTP Cache Directive Registry shall be created at <eref
2048   target="http://www.iana.org/assignments/http-cache-directives"/> and be
2049   populated with the registrations below:
2050</t>
2051<?BEGININC p6-cache.cache-directives ?>
2052<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
2053<texttable xmlns:my="#my" align="left" suppress-title="true"
2054           anchor="iana.cache.directive.registration.table">
2055   <ttcol>Cache Directive</ttcol>
2056   <ttcol>Reference</ttcol>
2057
2058   <c>max-age</c>
2059   <c>
2060      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
2061   </c>
2062   <c>max-stale</c>
2063   <c>
2064      <xref target="cache-request-directive.max-stale"/>
2065   </c>
2066   <c>min-fresh</c>
2067   <c>
2068      <xref target="cache-request-directive.min-fresh"/>
2069   </c>
2070   <c>must-revalidate</c>
2071   <c>
2072      <xref target="cache-response-directive.must-revalidate"/>
2073   </c>
2074   <c>no-cache</c>
2075   <c>
2076      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
2077   </c>
2078   <c>no-store</c>
2079   <c>
2080      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
2081   </c>
2082   <c>no-transform</c>
2083   <c>
2084      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
2085   </c>
2086   <c>only-if-cached</c>
2087   <c>
2088      <xref target="cache-request-directive.only-if-cached"/>
2089   </c>
2090   <c>private</c>
2091   <c>
2092      <xref target="cache-response-directive.private"/>
2093   </c>
2094   <c>proxy-revalidate</c>
2095   <c>
2096      <xref target="cache-response-directive.proxy-revalidate"/>
2097   </c>
2098   <c>public</c>
2099   <c>
2100      <xref target="cache-response-directive.only-if-cached"/>
2101   </c>
2102   <c>s-maxage</c>
2103   <c>
2104      <xref target="cache-response-directive.s-maxage"/>
2105   </c>
2106   <c>stale-if-error</c>
2107   <c>
2108      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2109   </c>
2110   <c>stale-while-revalidate</c>
2111   <c>
2112      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2113   </c>
2114</texttable>
2115<!--(END)-->
2116<?ENDINC p6-cache.cache-directives ?>
2117</section>
2118
2119<section title="Warn Code Registry" 
2120   anchor="warn.code.registration">
2121<t>
2122   The registration procedure for HTTP Warn Codes is defined by <xref
2123   target="warn.code.extensions"/> of this document.
2124</t>
2125<t>
2126   The HTTP Warn Code Registry shall be created at <eref
2127   target="http://www.iana.org/assignments/http-cache-directives"/> and be
2128   populated with the registrations below:
2129</t>
2130<?BEGININC p6-cache.iana-warn-codes ?>
2131<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2132<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
2133   <ttcol>Warn Code</ttcol>
2134   <ttcol>Short Description</ttcol>
2135   <ttcol>Reference</ttcol>
2136   <c>110</c>
2137   <c>Response is Stale</c>
2138   <c>
2139      <xref target="warn.110"/>
2140   </c>
2141   <c>111</c>
2142   <c>Revalidation Failed</c>
2143   <c>
2144      <xref target="warn.111"/>
2145   </c>
2146   <c>112</c>
2147   <c>Disconnected Operation</c>
2148   <c>
2149      <xref target="warn.112"/>
2150   </c>
2151   <c>113</c>
2152   <c>Heuristic Expiration</c>
2153   <c>
2154      <xref target="warn.113"/>
2155   </c>
2156   <c>199</c>
2157   <c>Miscellaneous Warning</c>
2158   <c>
2159      <xref target="warn.199"/>
2160   </c>
2161   <c>214</c>
2162   <c>Transformation Applied</c>
2163   <c>
2164      <xref target="warn.214"/>
2165   </c>
2166   <c>299</c>
2167   <c>Miscellaneous Persistent Warning</c>
2168   <c>
2169      <xref target="warn.299"/>
2170   </c>
2171</texttable>
2172<!--(END)-->
2173<?ENDINC p6-cache.iana-warn-codes ?>
2174</section>
2175
2176<section title="Header Field Registration" anchor="header.field.registration">
2177<t>
2178  The Message Header Field Registry located at <eref 
2179  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2180  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2181</t>
2182<?BEGININC p6-cache.iana-headers ?>
2183<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2184<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2185   <ttcol>Header Field Name</ttcol>
2186   <ttcol>Protocol</ttcol>
2187   <ttcol>Status</ttcol>
2188   <ttcol>Reference</ttcol>
2189
2190   <c>Age</c>
2191   <c>http</c>
2192   <c>standard</c>
2193   <c>
2194      <xref target="header.age"/>
2195   </c>
2196   <c>Cache-Control</c>
2197   <c>http</c>
2198   <c>standard</c>
2199   <c>
2200      <xref target="header.cache-control"/>
2201   </c>
2202   <c>Expires</c>
2203   <c>http</c>
2204   <c>standard</c>
2205   <c>
2206      <xref target="header.expires"/>
2207   </c>
2208   <c>Pragma</c>
2209   <c>http</c>
2210   <c>standard</c>
2211   <c>
2212      <xref target="header.pragma"/>
2213   </c>
2214   <c>Vary</c>
2215   <c>http</c>
2216   <c>standard</c>
2217   <c>
2218      <xref target="header.vary"/>
2219   </c>
2220   <c>Warning</c>
2221   <c>http</c>
2222   <c>standard</c>
2223   <c>
2224      <xref target="header.warning"/>
2225   </c>
2226</texttable>
2227<!--(END)-->
2228<?ENDINC p6-cache.iana-headers ?>
2229<t>
2230   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2231   Force".
2232</t>
2233</section>
2234</section>
2235
2236<section anchor="security.considerations" title="Security Considerations">
2237<t>
2238   Caches expose additional potential vulnerabilities, since the contents of
2239   the cache represent an attractive target for malicious exploitation.
2240   Because cache contents persist after an HTTP request is complete, an attack
2241   on the cache can reveal information long after a user believes that the
2242   information has been removed from the network. Therefore, cache contents
2243   need to be protected as sensitive information.
2244</t>
2245</section>
2246
2247<section title="Acknowledgments" anchor="acks">
2248<t>
2249  See &acks;.
2250</t>
2251</section>
2252
2253</middle>
2254
2255<back>
2256<references title="Normative References">
2257
2258  <reference anchor="Part1">
2259    <front>
2260      <title>HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2261      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2262        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2263        <address><email>fielding@gbiv.com</email></address>
2264      </author>
2265      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2266        <organization abbrev="W3C">World Wide Web Consortium</organization>
2267        <address><email>ylafon@w3.org</email></address>
2268      </author>
2269      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2270        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2271        <address><email>julian.reschke@greenbytes.de</email></address>
2272      </author>
2273      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2274    </front>
2275    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2276    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2277  </reference>
2278
2279  <reference anchor="Part2">
2280    <front>
2281      <title>HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation</title>
2282      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2283        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2284        <address><email>fielding@gbiv.com</email></address>
2285      </author>
2286      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2287        <organization abbrev="W3C">World Wide Web Consortium</organization>
2288        <address><email>ylafon@w3.org</email></address>
2289      </author>
2290      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2291        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2292        <address><email>julian.reschke@greenbytes.de</email></address>
2293      </author>
2294      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2295    </front>
2296    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2297    <x:source basename="p2-semantics" href="p2-semantics.xml">
2298      <x:defines>2xx (Successful)</x:defines>
2299      <x:defines>200 (OK)</x:defines>
2300      <x:defines>203 (Non-Authoritative Information)</x:defines>
2301      <x:defines>3xx (Redirection)</x:defines>
2302      <x:defines>300 (Multiple Choices)</x:defines>
2303      <x:defines>301 (Moved Permanently)</x:defines>
2304      <x:defines>404 (Not Found)</x:defines>
2305      <x:defines>410 (Gone)</x:defines>
2306      <x:defines>5xx (Server Error)</x:defines>
2307      <x:defines>504 (Gateway Timeout)</x:defines>
2308    </x:source>
2309  </reference>
2310
2311  <reference anchor="Part4">
2312    <front>
2313      <title>HTTP/1.1, part 4: Conditional Requests</title>
2314      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2315        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2316        <address><email>fielding@gbiv.com</email></address>
2317      </author>
2318      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2319        <organization abbrev="W3C">World Wide Web Consortium</organization>
2320        <address><email>ylafon@w3.org</email></address>
2321      </author>
2322      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2323        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2324        <address><email>julian.reschke@greenbytes.de</email></address>
2325      </author>
2326      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2327    </front>
2328    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2329    <x:source basename="p4-conditional" href="p4-conditional.xml">
2330      <x:defines>304</x:defines>
2331      <x:defines>304 (Not Modified)</x:defines>
2332    </x:source>
2333  </reference>
2334
2335  <reference anchor="Part5">
2336    <front>
2337      <title>HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2338      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2339        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2340        <address><email>fielding@gbiv.com</email></address>
2341      </author>
2342      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2343        <organization abbrev="W3C">World Wide Web Consortium</organization>
2344        <address><email>ylafon@w3.org</email></address>
2345      </author>
2346      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2347        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2348        <address><email>julian.reschke@greenbytes.de</email></address>
2349      </author>
2350      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2351    </front>
2352    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2353    <x:source basename="p5-range" href="p5-range.xml">
2354      <x:defines>206 (Partial Content)</x:defines>
2355    </x:source>
2356  </reference>
2357
2358  <reference anchor="Part7">
2359    <front>
2360      <title>HTTP/1.1, part 7: Authentication</title>
2361      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2362        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2363        <address><email>fielding@gbiv.com</email></address>
2364      </author>
2365      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2366        <organization abbrev="W3C">World Wide Web Consortium</organization>
2367        <address><email>ylafon@w3.org</email></address>
2368      </author>
2369      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2370        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2371        <address><email>julian.reschke@greenbytes.de</email></address>
2372      </author>
2373      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2374    </front>
2375    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2376    <x:source basename="p7-auth" href="p7-auth.xml">
2377      <x:defines>Authorization</x:defines>
2378    </x:source>
2379  </reference>
2380
2381  <reference anchor="RFC2119">
2382    <front>
2383      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2384      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2385        <organization>Harvard University</organization>
2386        <address><email>sob@harvard.edu</email></address>
2387      </author>
2388      <date month="March" year="1997" />
2389    </front>
2390    <seriesInfo name="BCP" value="14" />
2391    <seriesInfo name="RFC" value="2119" />
2392  </reference>
2393
2394  <reference anchor="RFC5234">
2395    <front>
2396      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2397      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2398        <organization>Brandenburg InternetWorking</organization>
2399        <address>
2400          <email>dcrocker@bbiw.net</email>
2401        </address> 
2402      </author>
2403      <author initials="P." surname="Overell" fullname="Paul Overell">
2404        <organization>THUS plc.</organization>
2405        <address>
2406          <email>paul.overell@thus.net</email>
2407        </address>
2408      </author>
2409      <date month="January" year="2008"/>
2410    </front>
2411    <seriesInfo name="STD" value="68"/>
2412    <seriesInfo name="RFC" value="5234"/>
2413  </reference>
2414 
2415</references>
2416
2417<references title="Informative References">
2418
2419  <reference anchor="RFC1305">
2420    <front>
2421      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2422      <author fullname="David L. Mills" initials="D." surname="Mills">
2423        <organization>University of Delaware, Electrical Engineering Department</organization>
2424        <address><email>mills@udel.edu</email></address>
2425      </author>
2426      <date month="March" year="1992" />
2427    </front>
2428    <seriesInfo name="RFC" value="1305" />
2429  </reference>
2430
2431  <reference anchor="RFC2616">
2432    <front>
2433      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2434      <author fullname="R. Fielding" initials="R." surname="Fielding">
2435        <organization>University of California, Irvine</organization>
2436        <address><email>fielding@ics.uci.edu</email></address>
2437      </author>
2438      <author fullname="J. Gettys" initials="J." surname="Gettys">
2439        <organization>W3C</organization>
2440        <address><email>jg@w3.org</email></address>
2441      </author>
2442      <author fullname="J. Mogul" initials="J." surname="Mogul">
2443        <organization>Compaq Computer Corporation</organization>
2444        <address><email>mogul@wrl.dec.com</email></address>
2445      </author>
2446      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2447        <organization>MIT Laboratory for Computer Science</organization>
2448        <address><email>frystyk@w3.org</email></address>
2449      </author>
2450      <author fullname="L. Masinter" initials="L." surname="Masinter">
2451        <organization>Xerox Corporation</organization>
2452        <address><email>masinter@parc.xerox.com</email></address>
2453      </author>
2454      <author fullname="P. Leach" initials="P." surname="Leach">
2455        <organization>Microsoft Corporation</organization>
2456        <address><email>paulle@microsoft.com</email></address>
2457      </author>
2458      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2459        <organization>W3C</organization>
2460        <address><email>timbl@w3.org</email></address>
2461      </author>
2462      <date month="June" year="1999" />
2463    </front>
2464    <seriesInfo name="RFC" value="2616" />
2465  </reference>
2466
2467  <reference anchor="RFC3864">
2468    <front>
2469      <title>Registration Procedures for Message Header Fields</title>
2470      <author fullname="G. Klyne" initials="G." surname="Klyne">
2471        <organization>Nine by Nine</organization>
2472        <address><email>GK-IETF@ninebynine.org</email></address>
2473      </author>
2474      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2475        <organization>BEA Systems</organization>
2476        <address><email>mnot@pobox.com</email></address>
2477      </author>
2478      <author fullname="J. Mogul" initials="J." surname="Mogul">
2479        <organization>HP Labs</organization>
2480        <address><email>JeffMogul@acm.org</email></address>
2481      </author>
2482      <date month="September" year="2004" />
2483    </front>
2484    <seriesInfo name="BCP" value="90" />
2485    <seriesInfo name="RFC" value="3864" />
2486  </reference>
2487
2488  <reference anchor='RFC5226'>
2489    <front>
2490      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2491      <author initials='T.' surname='Narten' fullname='T. Narten'>
2492        <organization>IBM</organization>
2493        <address><email>narten@us.ibm.com</email></address>
2494      </author>
2495      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2496        <organization>Google</organization>
2497        <address><email>Harald@Alvestrand.no</email></address>
2498      </author>
2499      <date year='2008' month='May' />
2500    </front>
2501    <seriesInfo name='BCP' value='26' />
2502    <seriesInfo name='RFC' value='5226' />
2503  </reference>
2504
2505  <reference anchor='RFC5861'>
2506    <front>
2507      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2508      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2509        <organization>Yahoo! Inc.</organization>
2510        <address><email>mnot@yahoo-inc.com</email></address>
2511      </author>
2512      <date month="April" year="2010"/>
2513    </front>
2514    <seriesInfo name='RFC' value='5861' />
2515  </reference>
2516
2517</references>
2518
2519<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2520<t>
2521  Make the specified age calculation algorithm less conservative.
2522  (<xref target="age.calculations"/>)
2523</t>
2524<t>
2525  Remove requirement to consider Content-Location in successful responses
2526  in order to determine the appropriate response to use.
2527  (<xref target="validation.model" />)
2528</t>
2529<t>
2530  Clarify denial of service attack avoidance requirement.
2531  (<xref target="invalidation.after.updates.or.deletions" />)
2532</t>
2533<t>
2534  Change ABNF productions for header fields to only define the field value.
2535  (<xref target="header.field.definitions"/>)
2536</t>
2537<t>
2538  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2539  anymore, as these aspects never were implemented.
2540  (<xref target="header.warning" />)
2541</t>
2542</section>
2543
2544<?BEGININC p6-cache.abnf-appendix ?>
2545<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2546<figure>
2547<artwork type="abnf" name="p6-cache.parsed-abnf">
2548<x:ref>Age</x:ref> = delta-seconds
2549
2550<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2551 cache-directive ] )
2552
2553<x:ref>Expires</x:ref> = HTTP-date
2554
2555<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
2556
2557<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
2558
2559<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2560 pragma-directive ] )
2561
2562<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2563 ) )
2564
2565<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2566 )
2567
2568<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2569
2570<x:ref>delta-seconds</x:ref> = 1*DIGIT
2571
2572<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2573
2574<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2575
2576<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2577<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2578<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 6.2&gt;
2579
2580<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
2581
2582<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.4&gt;
2583
2584<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2585
2586<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2587<x:ref>warn-code</x:ref> = 3DIGIT
2588<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2589<x:ref>warn-text</x:ref> = quoted-string
2590<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2591 ]
2592</artwork>
2593</figure>
2594<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2595; Age defined but not used
2596; Cache-Control defined but not used
2597; Expires defined but not used
2598; Pragma defined but not used
2599; Vary defined but not used
2600; Warning defined but not used
2601</artwork></figure></section>
2602<?ENDINC p6-cache.abnf-appendix ?>
2603
2604<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2605<t>
2606  Changes up to the first Working Group Last Call draft are summarized
2607  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2608</t>
2609
2610<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2611<t>
2612  Closed issues:
2613  <list style="symbols">
2614    <t>
2615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2616      "untangle Cache-Control ABNF"
2617    </t>
2618    <t>
2619      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2620      "Multiple values in Cache-Control headers"
2621    </t>
2622    <t>
2623      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2624      "Case sensitivity of header fields in CC values"
2625    </t>
2626    <t>
2627      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2628      "Spurious 'MAY's"
2629    </t>
2630    <t>
2631      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2632      "enhance considerations for new cache control directives"
2633    </t>
2634    <t>
2635      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2636      "ABNF requirements for recipients"
2637    </t>
2638  </list>
2639</t>
2640</section>
2641
2642</section>
2643  </back>
2644</rfc>
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