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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 "September">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY field-rules                 "<xref target='Part1' x:rel='#field.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc rfcedstyle="yes"?>
52<?rfc-ext allow-markup-in-artwork="yes" ?>
53<?rfc-ext include-references-in-index="yes" ?>
54<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
55  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
56<front>
57
58  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
59
60  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
61    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
62    <address>
63      <postal>
64        <street>345 Park Ave</street>
65        <city>San Jose</city>
66        <region>CA</region>
67        <code>95110</code>
68        <country>USA</country>
69      </postal>
70      <email>fielding@gbiv.com</email>
71      <uri>http://roy.gbiv.com/</uri>
72    </address>
73  </author>
74
75  <author initials="J." surname="Gettys" fullname="Jim Gettys">
76    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
77    <address>
78      <postal>
79        <street>21 Oak Knoll Road</street>
80        <city>Carlisle</city>
81        <region>MA</region>
82        <code>01741</code>
83        <country>USA</country>
84      </postal>
85      <email>jg@freedesktop.org</email>
86      <uri>http://gettys.wordpress.com/</uri>
87    </address>
88  </author>
89
90  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
91    <organization abbrev="HP">Hewlett-Packard Company</organization>
92    <address>
93      <postal>
94        <street>HP Labs, Large Scale Systems Group</street>
95        <street>1501 Page Mill Road, MS 1177</street>
96        <city>Palo Alto</city>
97        <region>CA</region>
98        <code>94304</code>
99        <country>USA</country>
100      </postal>
101      <email>JeffMogul@acm.org</email>
102    </address>
103  </author>
104
105  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
106    <organization abbrev="Microsoft">Microsoft Corporation</organization>
107    <address>
108      <postal>
109        <street>1 Microsoft Way</street>
110        <city>Redmond</city>
111        <region>WA</region>
112        <code>98052</code>
113        <country>USA</country>
114      </postal>
115      <email>henrikn@microsoft.com</email>
116    </address>
117  </author>
118
119  <author fullname="Larry Masinter" initials="L." surname="Masinter">
120    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
121    <address>
122      <postal>
123        <street>345 Park Ave</street>
124        <city>San Jose</city>
125        <region>CA</region>
126        <code>95110</code>
127        <country>USA</country>
128      </postal>
129      <email>LMM@acm.org</email>
130      <uri>http://larry.masinter.net/</uri>
131    </address>
132  </author>
133
134  <author fullname="Paul J. Leach" initials="P." surname="Leach">
135    <organization abbrev="Microsoft">Microsoft Corporation</organization>
136    <address>
137      <postal>
138        <street>1 Microsoft Way</street>
139        <city>Redmond</city>
140        <region>WA</region>
141        <code>98052</code>
142      </postal>
143      <email>paulle@microsoft.com</email>
144    </address>
145  </author>
146
147  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
148    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
149    <address>
150      <postal>
151        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
152        <street>The Stata Center, Building 32</street>
153        <street>32 Vassar Street</street>
154        <city>Cambridge</city>
155        <region>MA</region>
156        <code>02139</code>
157        <country>USA</country>
158      </postal>
159      <email>timbl@w3.org</email>
160      <uri>http://www.w3.org/People/Berners-Lee/</uri>
161    </address>
162  </author>
163
164  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
165    <organization abbrev="W3C">World Wide Web Consortium</organization>
166    <address>
167      <postal>
168        <street>W3C / ERCIM</street>
169        <street>2004, rte des Lucioles</street>
170        <city>Sophia-Antipolis</city>
171        <region>AM</region>
172        <code>06902</code>
173        <country>France</country>
174      </postal>
175      <email>ylafon@w3.org</email>
176      <uri>http://www.raubacapeu.net/people/yves/</uri>
177    </address>
178  </author>
179
180  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
181    <organization>Rackspace</organization>
182    <address>
183      <email>mnot@mnot.net</email>
184      <uri>http://www.mnot.net/</uri>
185    </address>
186  </author>
187
188  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
189    <organization abbrev="greenbytes">greenbytes GmbH</organization>
190    <address>
191      <postal>
192        <street>Hafenweg 16</street>
193        <city>Muenster</city><region>NW</region><code>48155</code>
194        <country>Germany</country>
195      </postal>
196      <phone>+49 251 2807760</phone>
197      <facsimile>+49 251 2807761</facsimile>
198      <email>julian.reschke@greenbytes.de</email>
199      <uri>http://greenbytes.de/tech/webdav/</uri>
200    </address>
201  </author>
202
203  <date month="&ID-MONTH;" year="&ID-YEAR;" />
204  <workgroup>HTTPbis Working Group</workgroup>
205
206<abstract>
207<t>
208   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
209   distributed, collaborative, hypertext information systems. HTTP has been in
210   use by the World Wide Web global information initiative since 1990. This
211   document is Part 6 of the seven-part specification that defines the protocol
212   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
213</t>
214<t>
215   Part 6 defines requirements on HTTP caches and the associated header fields
216   that control cache behavior or indicate cacheable response messages.
217</t>
218</abstract>
219
220<note title="Editorial Note (To be removed by RFC Editor)">
221  <t>
222    Discussion of this draft should take place on the HTTPBIS working group
223    mailing list (ietf-http-wg@w3.org), which is archived at
224    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
225  </t>
226  <t>
227    The current issues list is at
228    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
229    documents (including fancy diffs) can be found at
230    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
231  </t>
232  <t>
233    The changes in this draft are summarized in <xref target="changes.since.16"/>.
234  </t>
235</note>
236
237   </front>
238   <middle>
239
240<section anchor="caching" title="Introduction">
241<t>
242   HTTP is typically used for distributed information systems, where
243   performance can be improved by the use of response caches. This document
244   defines aspects of HTTP/1.1 related to caching and reusing response
245   messages.
246</t>
247
248<section anchor="intro.purpose" title="Purpose">
249<iref item="cache" />
250<t>
251   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
252   subsystem that controls its message storage, retrieval, and deletion. A
253   cache stores cacheable responses in order to reduce the response time and
254   network bandwidth consumption on future, equivalent requests. Any client or
255   server &MAY; employ a cache, though a cache cannot be used by a server that
256   is acting as a tunnel.
257</t>
258<t>
259   The goal of caching in HTTP/1.1 is to significantly improve performance
260   by reusing a prior response message to satisfy a current request.
261   A stored response is considered "fresh", as defined in
262   <xref target="expiration.model" />, if the response can be reused without
263   "validation" (checking with the origin server to see if the cached response
264   remains valid for this request).  A fresh cache response can therefore
265   reduce both latency and network transfers each time it is reused.
266   When a cached response is not fresh, it might still be reusable if it can
267   be freshened by validation (<xref target="validation.model" />) or if the
268   origin is unavailable.
269</t>
270</section>
271
272<section anchor="intro.terminology" title="Terminology">
273<t>
274   This specification uses a number of terms to refer to the roles played by
275   participants in, and objects of, HTTP caching.
276</t>
277<t>
278   <iref item="cache" />
279   <x:dfn>cache</x:dfn>
280   <list>
281      <t>A conformant implementation of a HTTP cache. Note that this implies
282        an HTTP/1.1 cache; this specification does not define conformance
283        for HTTP/1.0 caches.</t>
284   </list>
285</t>
286<t anchor="shared.and.non-shared.caches">
287   <iref item="shared cache" />
288   <x:dfn>shared cache</x:dfn>
289   <list>
290      <t>A cache that stores responses to be reused by more than one user;
291         usually (but not always) deployed as part of an intermediary.</t>
292   </list>
293</t>
294<t>
295   <iref item="private cache" />
296   <x:dfn>private cache</x:dfn>
297   <list>
298      <t>A cache that is dedicated to a single user.</t>
299   </list>
300</t>
301<t>
302   <iref item="cacheable" />
303   <x:dfn>cacheable</x:dfn>
304   <list>
305      <t>A response is cacheable if a cache is allowed to store a copy of the
306      response message for use in answering subsequent requests. Even when a
307      response is cacheable, there might be additional constraints on whether
308      a cache can use the stored copy to satisfy a particular request.</t>
309   </list>
310</t>
311<t>
312   <iref item="explicit expiration time" />
313   <x:dfn>explicit expiration time</x:dfn>
314   <list>
315      <t>The time at which the origin server intends that a representation
316      no longer be returned by a cache without further validation.</t>
317   </list>
318</t>
319<t>
320   <iref item="heuristic expiration time" />
321   <x:dfn>heuristic expiration time</x:dfn>
322   <list>
323      <t>An expiration time assigned by a cache when no explicit expiration
324      time is available.</t>
325   </list>
326</t>
327<t>
328   <iref item="age" />
329   <x:dfn>age</x:dfn>
330   <list>
331      <t>The age of a response is the time since it was sent by, or
332      successfully validated with, the origin server.</t>
333   </list>
334</t>
335<t>
336   <iref item="first-hand" />
337   <x:dfn>first-hand</x:dfn>
338   <list>
339      <t>A response is first-hand if the freshness model is not in use; i.e.,
340      its age is 0.</t>
341   </list>
342</t>
343<t>
344   <iref item="freshness lifetime" />
345   <x:dfn>freshness lifetime</x:dfn>
346   <list>
347      <t>The length of time between the generation of a response and its
348      expiration time.</t>
349   </list>
350</t>
351<t>
352   <iref item="fresh" />
353   <x:dfn>fresh</x:dfn>
354   <list>
355      <t>A response is fresh if its age has not yet exceeded its freshness
356      lifetime.</t>
357   </list>
358</t>
359<t>
360   <iref item="stale" />
361   <x:dfn>stale</x:dfn>
362   <list>
363      <t>A response is stale if its age has passed its freshness lifetime
364      (either explicit or heuristic).</t>
365   </list>
366</t>
367<t>
368   <iref item="validator" />
369   <x:dfn>validator</x:dfn>
370   <list>
371      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
372      is used to find out whether a stored response is an equivalent copy of
373      a representation. See &weak-and-strong;.</t>
374   </list>
375</t>
376<t>
377   <iref item="strong validator" />
378   <iref item="validator" subitem="strong" />
379   <x:dfn>strong validator</x:dfn>
380   <list>
381      <t>A validator that is defined by the origin server such that its
382         current value will change if the representation body changes; i.e.,
383         an entity-tag that is not marked as weak (&entity-tags;) or,
384         if no entity-tag is provided, a Last-Modified value that is strong
385         in the sense defined by &lastmod-comparison;.</t>
386   </list>
387</t>
388</section>
389
390<section anchor="intro.requirements" title="Requirements">
391<t>
392   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
393   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
394   document are to be interpreted as described in <xref target="RFC2119"/>.
395</t>
396<t>
397   An implementation is not compliant if it fails to satisfy one or more of
398   the "MUST" or "REQUIRED" level requirements for the protocols it
399   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
400   level and all the "SHOULD" level requirements for its protocols is said to
401   be "unconditionally compliant"; one that satisfies all the "MUST" level
402   requirements but not all the "SHOULD" level requirements for its protocols
403   is said to be "conditionally compliant".
404</t>
405</section>
406
407<section title="Syntax Notation" anchor="notation">
408   <x:anchor-alias value="ALPHA"/>
409   <x:anchor-alias value="CR"/>
410   <x:anchor-alias value="DIGIT"/>
411   <x:anchor-alias value="DQUOTE"/>
412   <x:anchor-alias value="LF"/>
413   <x:anchor-alias value="OCTET"/>
414   <x:anchor-alias value="SP"/>
415   <x:anchor-alias value="VCHAR"/>
416<t>
417   This specification uses the ABNF syntax defined in &notation; (which
418   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
419   <xref target="collected.abnf"/> shows the collected ABNF, with the list
420   rule expanded.
421</t>
422<t>
423   The following core rules are included by reference, as defined in <xref
424   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
425   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
426   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
427   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
428</t>
429
430<section title="Core Rules" anchor="core.rules">
431   <x:anchor-alias value="quoted-string"/>
432   <x:anchor-alias value="token"/>
433   <x:anchor-alias value="OWS"/>
434<t>
435   The core rules below are defined in <xref target="Part1"/>:
436</t>
437<figure><artwork type="abnf2616">
438  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
439  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
440  <x:ref>token</x:ref>         = &lt;token, defined in &field-rules;&gt;
441</artwork></figure>
442</section>
443
444<section title="ABNF Rules defined in other Parts of the Specification"
445    anchor="abnf.dependencies">
446   <x:anchor-alias value="field-name"/>
447   <x:anchor-alias value="HTTP-date"/>
448   <x:anchor-alias value="port"/>
449   <x:anchor-alias value="pseudonym"/>
450   <x:anchor-alias value="uri-host"/>
451<t>
452   The ABNF rules below are defined in other parts:
453</t>
454<figure><!--Part1--><artwork type="abnf2616">
455  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
456  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
457  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
458  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
459  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
460</artwork></figure>
461</section>
462</section>
463
464<section title="Delta Seconds" anchor="delta-seconds">
465<t>
466   The delta-seconds rule specifies a non-negative integer, representing time
467   in seconds.
468</t>
469<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
470  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
471</artwork></figure>
472<t>
473   If an implementation receives a delta-seconds value larger than the largest
474   positive integer it can represent, or if any of its subsequent calculations
475   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
476   Recipients parsing a delta-seconds value &MUST; use an arithmetic type of
477   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
478   value greater than 2147483648.
479</t>
480</section>
481
482</section>
483
484<section anchor="caching.overview" title="Cache Operation">
485<iref item="cache entry" />
486<iref item="cache key" />
487<t>
488   Proper cache operation preserves the semantics of HTTP transfers
489   (&semantics;) while eliminating the transfer of information already held
490   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
491   we assume that reusing the cached response is desirable and that such
492   reuse is the default behavior when no requirement or locally-desired
493   configuration prevents it.  Therefore, HTTP cache requirements are focused
494   on preventing a cache from either storing a non-reusable response or
495   reusing a stored response inappropriately.
496</t>
497<t>
498   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
499   HTTP responses corresponding to prior requests that used the same key.
500   The most common form of cache entry is a successful result of a retrieval
501   request: i.e., a 200 (OK) response containing a representation of the
502   resource identified by the request target.  However, it is also possible
503   to cache negative results (e.g., 404 not found), incomplete results
504   (e.g., 206 partial content), and responses to safe methods other than
505   GET if the method's definition allows such caching and defines something
506   suitable for use as a cache key.
507</t>
508<t>
509   The default <x:dfn>cache key</x:dfn> consists of the request method and
510   target URI.  However, since HTTP caches in common use today are typically
511   limited to caching responses to GET, most implementations simply decline
512   other methods and use only the URI as the key.
513</t>
514<t>
515   If a request target is subject to content negotiation, its cache entry
516   might consist of multiple stored responses, each differentiated by a
517   secondary key for the values of the original request's selecting header
518   fields (<xref target="caching.negotiated.responses"/>).
519</t>
520
521<section anchor="response.cacheability" title="Response Cacheability">
522<t>
523   A cache &MUST-NOT; store a response to any request, unless:
524   <list style="symbols">
525      <t>The request method is understood by the cache and defined as being
526      cacheable, and</t>
527      <t>the response status code is understood by the cache, and</t>
528      <t>the "no-store" cache directive (see <xref
529      target="header.cache-control" />) does not appear in request or response
530      header fields, and</t>
531      <t>the "private" cache response directive (see <xref
532      target="cache-response-directive" /> does not appear in the response, if
533      the cache is shared, and</t>
534      <t>the "Authorization" header field (see &header-authorization;) does not
535      appear in the request, if the cache is shared, unless the response
536      explicitly allows it (see <xref target="caching.authenticated.responses"
537      />), and</t>
538      <t>the response either:
539         <list style="symbols">
540            <t>contains an Expires header field (see <xref target="header.expires"
541            />), or</t>
542            <t>contains a max-age response cache directive (see <xref
543            target="cache-response-directive" />), or</t>
544            <t>contains a s-maxage response cache directive and the cache is
545            shared, or</t>
546            <t>contains a Cache Control Extension (see <xref
547            target="cache.control.extensions" />) that allows it to be cached,
548            or</t>
549            <t>has a status code that can be served with heuristic freshness
550            (see <xref target="heuristic.freshness" />).</t>
551         </list>
552      </t>
553   </list>
554</t>
555<t>
556   Note that any of the requirements listed above can be overridden by a
557   cache-control extension; see <xref target="cache.control.extensions" />.
558</t>
559<t>
560   In this context, a cache has "understood" a request method or a response
561   status code if it recognizes it and implements any cache-specific
562   behavior.
563</t>
564<t>
565   Note that, in normal operation, most caches will not store a response that
566   has neither a cache validator nor an explicit expiration time, as such
567   responses are not usually useful to store. However, caches are not
568   prohibited from storing such responses.
569</t>
570<t>
571   A response message is considered complete when all of the octets
572   indicated by the message framing (&messaging;) are received
573   prior to the connection being closed.
574   If the request is GET, the response status is 200 (OK), and the entire
575   response header block has been received, a cache &MAY; store an incomplete
576   response message-body if the cache entry is recorded as incomplete.
577   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
578   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
579   incomplete or partial content responses if it does not support the Range
580   and Content-Range header fields or if it does not understand the
581   range units used in those fields.
582</t>
583<t>
584   A cache &MAY; complete a stored incomplete response by making a subsequent
585   range request (&partial;) and combining the successful response with the
586   stored entry, as defined in <xref target="combining.responses"/>.
587   A cache &MUST-NOT; use an incomplete response to answer requests
588   unless the response has been made complete or the request is partial and
589   specifies a range that is wholly within the incomplete response.
590   A cache &MUST-NOT; send a partial response to a client without explicitly
591   marking it as such using the 206 (Partial Content) status code.
592</t>
593</section>
594
595
596<section anchor="constructing.responses.from.caches" 
597   title="Constructing Responses from Caches">
598<t>
599   For a presented request, a cache &MUST-NOT; return a stored response,
600   unless:
601   <list style="symbols">
602      <t>The presented effective request URI (&effective-request-uri;) and
603      that of the stored response match, and</t>
604      <t>the request method associated with the stored response allows it to
605      be used for the presented request, and</t>
606      <t>selecting header fields nominated by the stored response (if any)
607      match those presented (see <xref target="caching.negotiated.responses"
608      />), and</t>
609      <t>the presented request and stored response are free from directives
610      that would prevent its use (see <xref target="header.cache-control" />
611      and <xref target="header.pragma"/>), and</t>
612      <t>the stored response is either:
613         <list style="symbols">
614            <t>fresh (see <xref target="expiration.model" />), or</t>
615            <t>allowed to be served stale (see <xref
616            target="serving.stale.responses" />), or</t>
617            <t>successfully validated (see <xref target="validation.model"
618            />).</t>
619         </list>
620      </t>
621  </list>
622</t>
623<t>
624   Note that any of the requirements listed above can be overridden by a
625   cache-control extension; see <xref target="cache.control.extensions" />.
626</t>
627<t>
628   When a stored response is used to satisfy a request without validation,
629   a cache &MUST; include a single Age header field (<xref target="header.age"
630   />) in the response with a value equal to the stored response's
631   current_age; see <xref target="age.calculations" />.
632</t>
633<t>
634   A cache &MUST; write through requests with methods that are unsafe
635   (&safe-methods;) to the origin server; i.e., a cache must not generate
636   a reply to such a request before having forwarded the request and having
637   received a corresponding response.
638</t>
639<t>
640   Also, note that unsafe requests might invalidate already stored responses;
641   see <xref target="invalidation.after.updates.or.deletions" />.
642</t>
643<t>
644   When more than one suitable response is stored, a cache &MUST; use the
645   most recent response (as determined by the Date header field). It can also
646   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
647   no-cache" to disambiguate which response to use.
648</t>
649<t>
650   A cache that does not have a clock available &MUST-NOT; use stored responses
651   without revalidating them on every use. A cache, especially a shared
652   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
653   synchronize its clock with a reliable external standard.
654</t>
655
656</section>
657
658<section anchor="expiration.model" title="Freshness Model">
659<t>
660   When a response is "fresh" in the cache, it can be used to satisfy
661   subsequent requests without contacting the origin server, thereby improving
662   efficiency.
663</t>
664<t>
665   The primary mechanism for determining freshness is for an origin server to
666   provide an explicit expiration time in the future, using either the Expires
667   header field (<xref target="header.expires" />) or the max-age response cache
668   directive (<xref target="cache-response-directive" />). Generally, origin
669   servers will assign future explicit expiration times to responses in the
670   belief that the representation is not likely to change in a semantically
671   significant way before the expiration time is reached.
672</t>
673<t>
674   If an origin server wishes to force a cache to validate every request, it
675   can assign an explicit expiration time in the past to indicate that the
676   response is already stale. Compliant caches will normally validate the
677   cached response before reusing it for subsequent requests (see <xref
678   target="serving.stale.responses" />).
679</t>
680<t>
681   Since origin servers do not always provide explicit expiration times,
682   a cache &MAY; assign a heuristic expiration time when an explicit time is not
683   specified, employing algorithms that use other header field values (such as the
684   Last-Modified time) to estimate a plausible expiration time. This
685   specification does not provide specific algorithms, but does impose
686   worst-case constraints on their results.
687</t>
688<figure>
689<preamble>
690  The calculation to determine if a response is fresh is:
691</preamble>
692<artwork type="code">
693   response_is_fresh = (freshness_lifetime &gt; current_age)
694</artwork>
695</figure>
696<t>
697   The freshness_lifetime is defined in <xref
698   target="calculating.freshness.lifetime" />; the current_age is defined in
699   <xref target="age.calculations" />.
700</t>
701<t>
702   Additionally, clients might need to influence freshness calculation. They
703   can do this using several request cache directives, with the effect of
704   either increasing or loosening constraints on freshness. See <xref
705   target="cache-request-directive" />.
706</t>
707<t>
708   Note that freshness applies only to cache operation; it cannot be used to
709   force a user agent to refresh its display or reload a resource. See <xref
710   target="history.lists" /> for an explanation of the difference between
711   caches and history mechanisms.
712</t>
713
714<section anchor="calculating.freshness.lifetime" 
715   title="Calculating Freshness Lifetime">
716<t>
717   A cache can calculate the freshness lifetime (denoted as
718   freshness_lifetime) of a response by using the first match of:
719   <list style="symbols">
720      <t>If the cache is shared and the s-maxage response cache directive
721      (<xref target="cache-response-directive" />) is present, use its value,
722      or</t>
723      <t>If the max-age response cache directive (<xref
724      target="cache-response-directive" />) is present, use its value, or</t>
725      <t>If the Expires response header field (<xref target="header.expires" />) is
726      present, use its value minus the value of the Date response header field,
727      or</t>
728      <t>Otherwise, no explicit expiration time is present in the response. A
729      heuristic freshness lifetime might be applicable; see <xref
730      target="heuristic.freshness" />.</t>
731   </list>
732</t>
733<t>
734   Note that this calculation is not vulnerable to clock skew, since all of
735   the information comes from the origin server.
736</t>
737
738<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
739<t>
740   If no explicit expiration time is present in a stored response that has a
741   status code whose definition allows heuristic freshness to be used
742   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
743   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
744   use heuristics to determine freshness for responses with status codes that do
745   not explicitly allow it.
746</t>
747<t>
748   When a heuristic is used to calculate freshness lifetime, a cache
749   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
750   its current_age is more than 24 hours and such a warning is not already
751   present.
752</t>
753<t>
754   Also, if the response has a Last-Modified header field
755   (&header-last-modified;), caches are encouraged to use a heuristic
756   expiration value that is no more than some fraction of the interval since
757   that time. A typical setting of this fraction might be 10%.
758</t>
759<x:note>
760   <t>
761      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
762      x:sec="13.9"/>) required that caches do not calculate heuristic
763      freshness for URIs with query components (i.e., those containing '?').
764      In practice, this has not been widely implemented. Therefore, servers
765      are encouraged to send explicit directives (e.g., Cache-Control:
766      no-cache) if they wish to preclude caching.
767   </t>
768</x:note>
769</section>
770</section>
771
772<section anchor="age.calculations" title="Calculating Age">
773<t>
774   HTTP/1.1 uses the Age header field to convey the estimated age of the
775   response message when obtained from a cache. The Age field value is the
776   cache's estimate of the amount of time since the response was generated or
777   validated by the origin server. In essence, the Age value is the sum of the
778   time that the response has been resident in each of the caches along the
779   path from the origin server, plus the amount of time it has been in transit
780   along network paths.
781</t>
782<t>
783   The following data is used for the age calculation:
784</t>
785<t>
786   <x:dfn>age_value</x:dfn>
787   <list>
788      <t>
789         The term "age_value" denotes the value of the Age header field (<xref
790         target="header.age"/>), in a form appropriate for arithmetic
791         operation; or 0, if not available.
792      </t>
793   </list>
794</t>
795<t>
796   <x:dfn>date_value</x:dfn>
797   <list>
798      <t>
799         HTTP/1.1 requires origin servers to send a Date header field, if possible,
800         with every response, giving the time at which the response was
801         generated. The term "date_value" denotes the value of the Date
802         header field, in a form appropriate for arithmetic operations. See
803         &header-date; for the definition of the Date header field, and for
804         requirements regarding responses without it.
805      </t>
806   </list>
807</t>
808<t>
809   <x:dfn>now</x:dfn>
810   <list>
811      <t>
812         The term "now" means "the current value of the clock at the host
813         performing the calculation". A cache &SHOULD; use NTP (<xref
814         target="RFC1305"/>) or some similar protocol to synchronize its
815         clocks to a globally accurate time standard.
816      </t>
817   </list>
818</t>
819<t>
820   <x:dfn>request_time</x:dfn>
821   <list>
822      <t>
823         The current value of the clock at the host at the time the request
824         resulting in the stored response was made.
825      </t>
826   </list>
827</t>
828<t>
829   <x:dfn>response_time</x:dfn>
830   <list>
831      <t>
832         The current value of the clock at the host at the time the response
833         was received.
834      </t>
835   </list>
836</t>
837<t>
838   A response's age can be calculated in two entirely independent ways:
839   <list style="numbers">
840      <t>the "apparent_age": response_time minus date_value, if the local
841      clock is reasonably well synchronized to the origin server's clock. If
842      the result is negative, the result is replaced by zero.</t>
843      <t>the "corrected_age_value", if all of the caches along the response
844      path implement HTTP/1.1. A cache &MUST; interpret this value relative
845      to the time the request was initiated, not the time that the response
846      was received.</t>
847   </list>
848</t>
849<figure>
850<artwork type="code">
851  apparent_age = max(0, response_time - date_value);
852
853  response_delay = response_time - request_time;
854  corrected_age_value = age_value + response_delay; 
855</artwork>
856</figure>
857<figure>
858<preamble>These are combined as</preamble>
859<artwork type="code">
860  corrected_initial_age = max(apparent_age, corrected_age_value);
861</artwork></figure>
862<t>
863   The current_age of a stored response can then be calculated by adding the
864   amount of time (in seconds) since the stored response was last validated by
865   the origin server to the corrected_initial_age.
866</t>
867<figure><artwork type="code">
868  resident_time = now - response_time;
869  current_age = corrected_initial_age + resident_time;
870</artwork></figure>
871<t>
872   Additionally, to avoid common problems in date parsing:
873</t>
874<t>
875  <list style="symbols">
876     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
877        which appears to be more than 50 years in the future is in fact
878        in the past (this helps solve the "year 2000" problem).</t>
879
880     <t>Although all date formats are specified to be case-sensitive,
881        recipients &SHOULD; match day, week and timezone names
882        case-insensitively.</t>
883             
884     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
885        Expires date as earlier than the proper value, but &MUST-NOT;
886        internally represent a parsed Expires date as later than the
887        proper value.</t>
888
889     <t>All expiration-related calculations &MUST; be done in GMT. The
890        local time zone &MUST-NOT; influence the calculation or comparison
891        of an age or expiration time.</t>
892
893     <t>If an HTTP header field incorrectly carries a date value with a time
894        zone other than GMT, it &MUST; be converted into GMT using the
895        most conservative possible conversion.</t>
896  </list>
897</t>
898</section>
899
900<section anchor="serving.stale.responses" title="Serving Stale Responses">
901<t>
902   A "stale" response is one that either has explicit expiry information or is
903   allowed to have heuristic expiry calculated, but is not fresh according to
904   the calculations in <xref target="expiration.model" />.
905</t>
906<t>
907   A cache &MUST-NOT; return a stale response if it is prohibited by an
908   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
909   directive, a "must-revalidate" cache-response-directive, or an applicable
910   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
911   target="cache-response-directive"/>).
912</t>
913<t>
914   A cache &MUST-NOT; return stale responses unless it is disconnected
915   (i.e., it cannot contact the origin server or otherwise find a forward
916   path) or doing so is explicitly allowed (e.g., by the max-stale request
917   directive; see <xref target="cache-request-directive" />).
918</t>
919<t>
920   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
921   <xref target="header.warning" />) to stale responses. Likewise, a cache
922   &SHOULD; add the 112 warn-code to stale responses if the cache is
923   disconnected.
924</t>
925<t>
926   If a cache receives a first-hand response (either an entire response, or a
927   304 (Not Modified) response) that it would normally forward to the
928   requesting client, and the received response is no longer fresh, the cache
929   can forward it to the requesting client without adding a new Warning (but
930   without removing any existing Warning header fields). A cache shouldn't
931   attempt to validate a response simply because that response became stale in
932   transit.
933</t>
934</section>
935</section>
936
937<section anchor="validation.model" title="Validation Model">
938<t>
939   When a cache has one or more stored responses for a requested URI, but
940   cannot serve any of them (e.g., because they are not fresh, or one cannot
941   be selected; see <xref target="caching.negotiated.responses"/>), it can use
942   the conditional request mechanism &conditional; in the forwarded request to
943   give the origin server an opportunity to both select a valid stored
944   response to be used, and to update it. This process is known as
945   "validating" or "revalidating" the stored response.
946</t>
947<t>
948   When sending such a conditional request, a cache adds an If-Modified-Since
949   header field whose value is that of the Last-Modified header field from the
950   selected (see <xref target="caching.negotiated.responses"/>) stored
951   response, if available.
952</t>
953<t>
954   Additionally, a cache can add an If-None-Match header field whose value is
955   that of the ETag header field(s) from all responses stored for the
956   requested URI, if present. However, if any of the stored responses contains
957   only partial content, the cache shouldn't include its entity-tag in the
958   If-None-Match header field unless the request is for a range that would be
959   fully satisfied by that stored response.
960</t>
961
962<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
963
964<t>
965   <list style="symbols">
966      <t>
967         A 304 (Not Modified) response status code indicates that the stored
968         response can be updated and reused; see <xref
969         target="freshening.responses"/>.
970      </t>
971      <t>
972         A full response (i.e., one with a response body) indicates that none
973         of the stored responses nominated in the conditional request is
974         suitable. Instead, the cache can use the full response to
975         satisfy the request and &MAY; replace the stored response(s).
976      </t>
977      <t>
978         However, if a cache receives a 5xx response while attempting to
979         validate a response, it can either forward this response to the
980         requesting client, or act as if the server failed to respond. In the
981         latter case, it can return a previously stored response (see <xref
982         target="serving.stale.responses" />).
983      </t>
984   </list>
985</t>
986
987<section anchor="freshening.responses" title="Freshening Responses">
988<t>
989   When a cache receives a 304 (Not Modified) response and already has one
990   or more stored 200 (OK) responses for the same cache key, the cache needs
991   to identify which of the stored responses are updated by this new response
992   and then update the stored response(s) with the new information provided in
993   the 304 response.
994   <list style="symbols">
995    <t>
996     If the new response contains a strong validator, then that strong
997     validator identifies the selected representation.  All of the stored
998     responses with the same strong validator are selected.
999     If none of the stored responses contain the same strong validator, then
1000     this new response corresponds to a new selected representation and
1001     &MUST-NOT; update the existing stored responses.
1002    </t>
1003    <t>
1004     If the new response contains a weak validator and that validator
1005     corresponds to one of the cache's stored responses, then the most
1006     recent of those matching stored responses is selected.
1007    </t>
1008    <t>
1009     If the new response does not include any form of validator, there is
1010     only one stored response, and that stored response also lacks a
1011     validator, then that stored response is selected.
1012    </t>
1013   </list>
1014</t>
1015<t>
1016   If a stored response is selected for update, the cache &MUST;:
1017   <list style="symbols">
1018      <t>delete any Warning header fields in the stored response with
1019         warn-code 1xx (see <xref target="header.warning" />);</t>
1020      <t>retain any Warning header fields in the stored response with
1021         warn-code 2xx; and,</t>
1022      <t>use other header fields provided in the 304 response to replace
1023         all instances of the corresponding header fields in the stored
1024         response.</t>
1025   </list>
1026</t>
1027</section>
1028
1029</section>
1030
1031<section anchor="invalidation.after.updates.or.deletions" 
1032   title="Request Methods that Invalidate">
1033<t>
1034   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1035   have the potential for changing state on the origin server, intervening
1036   caches can use them to keep their contents up-to-date.
1037</t>
1038<t>
1039   A cache &MUST; invalidate the effective Request URI
1040   (&effective-request-uri;) as well as the URI(s) in the Location
1041   and Content-Location header fields (if present) when a non-error
1042   response to a request with an unsafe method is received.
1043</t>
1044<t>
1045   However, a cache &MUST-NOT; invalidate a URI from a
1046   Location or Content-Location header field if the host part of that URI
1047   differs from the host part in the effective request URI
1048   (&effective-request-uri;). This helps prevent denial of service attacks.
1049</t>
1050<t>
1051   A cache &MUST; invalidate the effective request URI
1052   (&effective-request-uri;) when it receives a non-error response
1053   to a request with a method whose safety is unknown.
1054</t>
1055<t>
1056   Here, a "non-error response" is one with a 2xx or 3xx status code.
1057   "Invalidate" means that the cache will either remove all stored
1058   responses related to the effective request URI, or will mark these as
1059   "invalid" and in need of a mandatory validation before they can be returned
1060   in response to a subsequent request.
1061</t>
1062<t>
1063   Note that this does not guarantee that all appropriate responses are
1064   invalidated. For example, the request that caused the change at the origin
1065   server might not have gone through the cache where a response is stored.
1066</t>
1067</section>
1068
1069<section anchor="caching.authenticated.responses" 
1070   title="Shared Caching of Authenticated Responses">
1071
1072<t>
1073   A shared cache &MUST-NOT; use a cached response to a request with an
1074   Authorization header field (&header-authorization;) to satisfy any subsequent
1075   request unless a cache directive that allows such responses to be stored is
1076   present in the response.
1077</t>
1078
1079<t>
1080   In this specification, the following Cache-Control response directives
1081   (<xref target="cache-response-directive"/>) have such an effect:
1082   must-revalidate, public, s-maxage.
1083</t>
1084
1085<t>
1086   Note that cached responses that contain the "must-revalidate" and/or
1087   "s-maxage" response directives are not allowed to be served stale (<xref
1088   target="serving.stale.responses"/>) by shared caches. In particular, a
1089   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1090   used to satisfy a subsequent request without revalidating it on the origin
1091   server.
1092</t>
1093</section>
1094
1095<section anchor="caching.negotiated.responses" 
1096   title="Caching Negotiated Responses">
1097<t>
1098   When a cache receives a request that can be satisfied by a stored response
1099   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1100   use that response unless all of the selecting header fields nominated by
1101   the Vary header field match in both the original request (i.e., that associated
1102   with the stored response), and the presented request.
1103</t>
1104<t>
1105   The selecting header fields from two requests are defined to match if and
1106   only if those in the first request can be transformed to those in the
1107   second request by applying any of the following:
1108   <list style="symbols">
1109      <t>
1110         adding or removing whitespace, where allowed in the header field's syntax
1111      </t>
1112      <t>
1113         combining multiple header fields with the same field name
1114         (see &header-fields;)
1115      </t>
1116      <t>
1117         normalizing both header field values in a way that is known to have
1118         identical semantics, according to the header field's specification (e.g.,
1119         re-ordering field values when order is not significant;
1120         case-normalization, where values are defined to be case-insensitive)
1121      </t>
1122  </list>
1123</t>
1124<t>
1125   If (after any normalization that might take place) a header field is absent
1126   from a request, it can only match another request if it is also absent
1127   there.
1128</t>
1129<t>
1130   A Vary header field-value of "*" always fails to match, and subsequent
1131   requests to that resource can only be properly interpreted by the origin
1132   server.
1133</t>
1134<t>
1135   The stored response with matching selecting header fields is known as the
1136   selected response.
1137</t>
1138<t>
1139   If multiple selected responses are available, the most recent response
1140   (as determined by the Date header field) is used; see <xref 
1141   target="constructing.responses.from.caches"/>.
1142</t>
1143<t>
1144   If no selected response is available, the cache can forward the presented
1145   request to the origin server in a conditional request; see <xref
1146   target="validation.model"/>.
1147</t>
1148</section>
1149
1150<section anchor="combining.responses" title="Combining Partial Content">
1151<t>
1152   A response might transfer only a partial representation if the
1153   connection closed prematurely or if the request used one or more Range
1154   specifiers (&partial;).  After several such transfers, a cache might have
1155   received several ranges of the same representation.  A cache &MAY; combine
1156   these ranges into a single stored response, and reuse that response to
1157   satisfy later requests, if they all share the same strong validator and
1158   the cache complies with the client requirements in &combining-byte-ranges;.
1159</t>
1160<t>
1161   When combining the new response with one or more stored responses, a
1162   cache &MUST;:
1163   <list style="symbols">
1164      <t>delete any Warning header fields in the stored response with
1165         warn-code 1xx (see <xref target="header.warning" />);</t>
1166      <t>retain any Warning header fields in the stored response with
1167         warn-code 2xx; and,</t>
1168      <t>use other header fields provided in the new response, aside
1169         from Content-Range, to replace all instances of the corresponding
1170         header fields in the stored response.</t>
1171   </list>
1172</t>
1173</section>
1174
1175
1176</section>
1177
1178<section anchor="header.field.definitions" title="Header Field Definitions">
1179<t>
1180   This section defines the syntax and semantics of HTTP/1.1 header fields
1181   related to caching.
1182</t>
1183
1184<section anchor="header.age" title="Age">
1185   <iref item="Age header field" primary="true" x:for-anchor="" />
1186   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1187   <x:anchor-alias value="Age"/>
1188   <x:anchor-alias value="age-value"/>
1189<t>
1190   The "Age" header field conveys the sender's estimate of the amount
1191   of time since the response was generated or successfully validated at the
1192   origin server. Age values are calculated as specified in <xref
1193   target="age.calculations" />.
1194</t>
1195<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1196  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1197</artwork></figure>
1198<t>
1199  Age field-values are non-negative integers, representing time in seconds
1200  (see <xref target="delta-seconds"/>).
1201</t>
1202<t>
1203   The presence of an Age header field in a response implies that a response
1204   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1205   might not implement the Age header field.
1206</t>
1207</section>
1208
1209<section anchor="header.cache-control" title="Cache-Control">
1210   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1211   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1212      x:for-anchor="" />
1213   <x:anchor-alias value="Cache-Control"/>
1214   <x:anchor-alias value="cache-directive"/>
1215   <x:anchor-alias value="cache-extension"/>
1216   <x:anchor-alias value="cache-request-directive"/>
1217   <x:anchor-alias value="cache-response-directive"/>
1218<t>
1219   The "Cache-Control" header field is used to specify directives for
1220   caches along the request/response chain. Such cache directives are
1221   unidirectional in that the presence of a directive in a request does not
1222   imply that the same directive is to be given in the response.
1223</t>
1224<t>
1225   A cache &MUST; obey the requirements of the Cache-Control
1226   directives defined in this section. See <xref
1227   target="cache.control.extensions"/> for information about how Cache-Control
1228   directives defined elsewhere are handled.
1229</t>
1230<x:note>
1231   <t>
1232       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1233       might only implement Pragma: no-cache (see <xref target="header.pragma"
1234       />).
1235   </t>
1236</x:note>
1237<t>
1238   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1239   through in forwarded messages, regardless of their
1240   significance to that application, since the directives might be applicable
1241   to all recipients along the request/response chain. It is not possible to
1242   target a directive to a specific cache.
1243</t>
1244<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1245  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1246
1247  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1248     / <x:ref>cache-response-directive</x:ref>
1249
1250  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1251</artwork></figure>
1252
1253<section anchor="cache-request-directive" 
1254   title="Request Cache-Control Directives">
1255   <x:anchor-alias value="cache-request-directive" />
1256
1257<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1258   subitem="cache-request-directive" />
1259  <x:ref>cache-request-directive</x:ref> =
1260       "no-cache"
1261     / "no-store"
1262     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1263     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1264     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1265     / "no-transform"
1266     / "only-if-cached"
1267     / <x:ref>cache-extension</x:ref>
1268</artwork></figure>
1269
1270<t>
1271   <x:dfn>no-cache</x:dfn>
1272   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1273   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1274   <list>
1275      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1276      use a stored response to satisfy the request without successful
1277      validation on the origin server.</t> 
1278   </list>
1279</t>
1280<t>
1281   <x:dfn>no-store</x:dfn>
1282   <iref item="Cache Directives" primary="true" subitem="no-store" />
1283   <iref item="no-store" primary="true" subitem="Cache Directive" />
1284   <list>
1285      <t>The no-store request directive indicates that a cache &MUST-NOT;
1286      store any part of either this request or any response to it. This
1287      directive applies to both private and shared caches. "&MUST-NOT;
1288      store" in this context means that the cache &MUST-NOT; intentionally
1289      store the information in non-volatile storage, and &MUST; make a
1290      best-effort attempt to remove the information from volatile storage as
1291      promptly as possible after forwarding it.</t>
1292      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1293      privacy. In particular, malicious or compromised caches might not
1294      recognize or obey this directive, and communications networks might be
1295      vulnerable to eavesdropping.</t>
1296      <t>Note that if a request containing this directive is satisfied from a
1297      cache, the no-store request directive does not apply to the already
1298      stored response.</t>
1299   </list>
1300</t>
1301<t>
1302   <x:dfn>max-age</x:dfn>
1303   <iref item="Cache Directives" primary="true" subitem="max-age" />
1304   <iref item="max-age" primary="true" subitem="Cache Directive" />
1305   <list>
1306      <t>The max-age request directive indicates that the client is unwilling to
1307      accept a response whose age is greater than the specified number of
1308      seconds. Unless the max-stale request directive is also present, the
1309      client is not willing to accept a stale response.</t>
1310   </list>
1311</t>
1312<t>
1313   <x:dfn>max-stale</x:dfn>
1314   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1315   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1316   <list>
1317      <t>The max-stale request directive indicates that the client is willing
1318      to accept a response that has exceeded its expiration time. If max-stale
1319      is assigned a value, then the client is willing to accept a response
1320      that has exceeded its expiration time by no more than the specified
1321      number of seconds. If no value is assigned to max-stale, then the client
1322      is willing to accept a stale response of any age.</t>
1323   </list>
1324</t>
1325<t>
1326   <x:dfn>min-fresh</x:dfn>
1327   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1328   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1329   <list>
1330      <t>The min-fresh request directive indicates that the client is willing
1331      to accept a response whose freshness lifetime is no less than its
1332      current age plus the specified time in seconds. That is, the client
1333      wants a response that will still be fresh for at least the specified
1334      number of seconds.</t>
1335   </list>
1336</t>
1337<t>
1338   <x:dfn>no-transform</x:dfn>
1339   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1340   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1341   <list>
1342      <t>The no-transform request directive indicates that an intermediary
1343        (whether or not it implements a cache) &MUST-NOT; change the
1344        Content-Encoding, Content-Range or Content-Type request header fields,
1345        nor the request representation.</t>
1346   </list>
1347</t>
1348<t>
1349   <x:dfn>only-if-cached</x:dfn>
1350   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1351   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1352   <list>
1353      <t>The only-if-cached request directive indicates that the client only
1354      wishes to obtain a stored response. If it receives this directive, a
1355      cache &SHOULD; either respond using a stored response that is consistent
1356      with the other constraints of the request, or respond with a 504
1357      (Gateway Timeout) status code. If a group of caches is being operated as
1358      a unified system with good internal connectivity, a member cache &MAY;
1359      forward such a request within that group of caches.</t>
1360   </list>
1361</t>
1362</section>
1363
1364<section anchor="cache-response-directive" 
1365   title="Response Cache-Control Directives">
1366   <x:anchor-alias value="cache-response-directive" />
1367
1368<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1369   subitem="cache-response-directive" />
1370  <x:ref>cache-response-directive</x:ref> =
1371       "public"
1372     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1373     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1374     / "no-store"
1375     / "no-transform"
1376     / "must-revalidate"
1377     / "proxy-revalidate"
1378     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1379     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1380     / <x:ref>cache-extension</x:ref>
1381</artwork></figure>
1382
1383<t>
1384   <x:dfn>public</x:dfn>
1385   <iref item="Cache Directives" primary="true" subitem="public" />
1386   <iref item="public" primary="true" subitem="Cache Directive" />
1387   <list>
1388      <t>The public response directive indicates that a response whose
1389        associated request contains an 'Authentication' header &MAY; be
1390        stored (see <xref target="caching.authenticated.responses" />).</t>
1391  </list>
1392</t>
1393<t>
1394   <x:dfn>private</x:dfn>
1395   <iref item="Cache Directives" primary="true" subitem="private" />
1396   <iref item="private" primary="true" subitem="Cache Directive" />
1397   <list>
1398      <t>The private response directive indicates that the response message is
1399      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1400      private cache &MAY; store the response.</t>
1401      <t>If the private response directive specifies one or more field-names,
1402      this requirement is limited to the field-values associated with the
1403      listed response header fields. That is, a shared cache &MUST-NOT; store
1404      the specified field-names(s), whereas it &MAY; store the remainder of the
1405      response message.</t>
1406      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1407      the response can be stored; it cannot ensure the privacy of the message
1408      content. Also, private response directives with field-names are often
1409      handled by implementations as if an unqualified private directive was
1410      received; i.e., the special handling for the qualified form is not
1411      widely implemented.</t>
1412   </list>
1413</t>
1414<t>
1415   <x:dfn>no-cache</x:dfn>
1416   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1417   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1418   <list>
1419      <t>The no-cache response directive indicates that the response MUST NOT
1420      be used to satisfy a subsequent request without successful validation on
1421      the origin server. This allows an origin server to prevent a cache from
1422      using it to satisfy a request without contacting it, even by caches that
1423      have been configured to return stale responses.</t>
1424      <t>If the no-cache response directive specifies one or more field-names,
1425      this requirement is limited to the field-values associated with the
1426      listed response header fields. That is, a cache &MUST-NOT; send the
1427      specified field-name(s) in the response to a subsequent request without successful
1428      validation on the origin server. This allows an origin server to prevent
1429      the re-use of certain header fields in a response, while still allowing
1430      caching of the rest of the response.</t>
1431      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1432      this directive. Also, no-cache response directives with field-names are
1433      often handled by implementations as if an unqualified no-cache directive
1434      was received; i.e., the special handling for the qualified form is not
1435      widely implemented. </t>
1436   </list>
1437</t>
1438<t>
1439   <x:dfn>no-store</x:dfn>
1440   <iref item="Cache Directives" primary="true" subitem="no-store" />
1441   <iref item="no-store" primary="true" subitem="Cache Directive" />
1442   <list>
1443      <t>The no-store response directive indicates that a cache &MUST-NOT;
1444      store any part of either the immediate request or response. This
1445      directive applies to both private and shared caches. "&MUST-NOT;
1446      store" in this context means that the cache &MUST-NOT; intentionally
1447      store the information in non-volatile storage, and &MUST; make a
1448      best-effort attempt to remove the information from volatile storage as
1449      promptly as possible after forwarding it.</t>
1450      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1451      privacy. In particular, malicious or compromised caches might not
1452      recognize or obey this directive, and communications networks might be
1453      vulnerable to eavesdropping.</t>
1454   </list>
1455</t>
1456<t>
1457   <x:dfn>must-revalidate</x:dfn>
1458   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1459   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1460   <list>
1461      <t>The must-revalidate response directive indicates that once it has
1462      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1463      requests without successful validation on the origin server.</t>
1464      <t>The must-revalidate directive is necessary to support reliable
1465      operation for certain protocol features. In all circumstances a
1466      cache &MUST; obey the must-revalidate directive; in particular,
1467      if a cache cannot reach the origin server for any reason, it &MUST;
1468      generate a 504 (Gateway Timeout) response.</t>
1469      <t>The must-revalidate directive ought to be used by servers if and only
1470      if failure to validate a request on the representation could result in
1471      incorrect operation, such as a silently unexecuted financial
1472      transaction.</t>
1473   </list>
1474</t>
1475<t>
1476   <x:dfn>proxy-revalidate</x:dfn>
1477   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1478   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1479   <list>
1480      <t>The proxy-revalidate response directive has the same meaning as the
1481      must-revalidate response directive, except that it does not apply to
1482      private caches.</t>
1483   </list>
1484</t>
1485<t>
1486   <x:dfn>max-age</x:dfn>
1487   <iref item="Cache Directives" primary="true" subitem="max-age" />
1488   <iref item="max-age" primary="true" subitem="Cache Directive" />
1489   <list>
1490      <t>The max-age response directive indicates that the response is to be
1491      considered stale after its age is greater than the specified number of
1492      seconds.</t>
1493   </list>
1494</t>
1495<t>
1496   <x:dfn>s-maxage</x:dfn>
1497   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1498   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1499   <list>
1500      <t>The s-maxage response directive indicates that, in shared caches, the
1501      maximum age specified by this directive overrides the maximum age
1502      specified by either the max-age directive or the Expires header field. The
1503      s-maxage directive also implies the semantics of the proxy-revalidate
1504      response directive.</t>
1505   </list>
1506</t>
1507<t>
1508   <x:dfn>no-transform</x:dfn>
1509   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1510   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1511   <list>
1512      <t>The no-transform response directive indicates that an intermediary
1513      (regardless of whether it implements a cache) &MUST-NOT; change the
1514      Content-Encoding, Content-Range or Content-Type response header fields,
1515      nor the response representation.</t>
1516   </list>
1517</t>
1518
1519</section>
1520
1521<section anchor="cache.control.extensions" title="Cache Control Extensions">
1522<t>
1523   The Cache-Control header field can be extended through the use of one or
1524   more cache-extension tokens, each with an optional value. Informational
1525   extensions (those that do not require a change in cache behavior) can be
1526   added without changing the semantics of other directives. Behavioral
1527   extensions are designed to work by acting as modifiers to the existing base
1528   of cache directives. Both the new directive and the standard directive are
1529   supplied, such that applications that do not understand the new directive
1530   will default to the behavior specified by the standard directive, and those
1531   that understand the new directive will recognize it as modifying the
1532   requirements associated with the standard directive. In this way,
1533   extensions to the cache-control directives can be made without requiring
1534   changes to the base protocol.
1535</t>
1536<t>
1537   This extension mechanism depends on an HTTP cache obeying all of the
1538   cache-control directives defined for its native HTTP-version, obeying
1539   certain extensions, and ignoring all directives that it does not
1540   understand.
1541</t>
1542<t>
1543   For example, consider a hypothetical new response directive called
1544   "community" that acts as a modifier to the private directive. We define
1545   this new directive to mean that, in addition to any private cache, any
1546   cache that is shared only by members of the community named within its
1547   value may cache the response. An origin server wishing to allow the UCI
1548   community to use an otherwise private response in their shared cache(s)
1549   could do so by including
1550</t>
1551<figure><artwork type="example">
1552  Cache-Control: private, community="UCI"
1553</artwork></figure>
1554<t>
1555   A cache seeing this header field will act correctly even if the cache does
1556   not understand the community cache-extension, since it will also see and
1557   understand the private directive and thus default to the safe behavior.
1558</t>
1559<t>
1560   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1561   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1562   combined with standard directives (or the response's default cacheability)
1563   such that the cache behavior will remain minimally correct even if the
1564   cache does not understand the extension(s).
1565</t>
1566<t>
1567   The HTTP Cache Directive Registry defines the name space for the cache
1568   directives.
1569</t>
1570<t>
1571   A registration &MUST; include the following fields:
1572   <list style="symbols">
1573      <t>Cache Directive Name</t>
1574      <t>Pointer to specification text</t>
1575   </list>
1576</t>
1577<t>
1578   Values to be added to this name space are subject to IETF review (<xref
1579   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1580</t>
1581<t>
1582   The registry itself is maintained at <eref
1583   target="http://www.iana.org/assignments/http-cache-directives"/>.
1584</t>
1585</section>
1586
1587</section>
1588
1589<section anchor="header.expires" title="Expires">
1590   <iref item="Expires header field" primary="true" x:for-anchor="" />
1591   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1592   <x:anchor-alias value="Expires"/>
1593<t>
1594   The "Expires" header field gives the date/time after which the
1595   response is considered stale. See <xref target="expiration.model" /> for
1596   further discussion of the freshness model.
1597</t>
1598<t>
1599   The presence of an Expires field does not imply that the original resource
1600   will change or cease to exist at, before, or after that time.
1601</t>
1602<t>
1603   The field-value is an absolute date and time as defined by HTTP-date in
1604   &http-date;; a sender &MUST; use the rfc1123-date format.
1605</t>
1606<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1607  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1608</artwork></figure>
1609<figure>
1610  <preamble>For example</preamble>
1611<artwork type="example">
1612  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1613</artwork></figure>
1614<t>
1615   A cache &MUST; treat other invalid date formats,
1616   especially including the value "0", as in the past (i.e., "already
1617   expired").
1618</t>
1619<x:note>
1620   <t>
1621       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1622       max-age directive (see <xref target="cache-response-directive" />),
1623       that directive overrides the Expires field. Likewise, the s-maxage
1624       directive overrides Expires in shared caches.
1625   </t>
1626</x:note>
1627<t>
1628   Historically, HTTP required the Expires field-value to be no more than a
1629   year in the future. While longer freshness lifetimes are no longer
1630   prohibited, extremely large values have been demonstrated to cause
1631   problems (e.g., clock overflows due to use of 32-bit integers for
1632   time values), and most caches will evict a response far sooner than
1633   that. Therefore, senders ought not produce them.
1634</t>
1635<t>
1636   An origin server without a clock &MUST-NOT; assign Expires
1637   values to a response unless these values were associated
1638   with the resource by a system or user with a reliable clock. It &MAY;
1639   assign an Expires value that is known, at or before server
1640   configuration time, to be in the past (this allows "pre-expiration"
1641   of responses without storing separate Expires values for each
1642   resource).
1643</t>
1644</section>
1645
1646<section anchor="header.pragma" title="Pragma">
1647   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1648   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1649   <x:anchor-alias value="extension-pragma"/>
1650   <x:anchor-alias value="Pragma"/>
1651   <x:anchor-alias value="pragma-directive"/>
1652<t>
1653   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1654   caches, so that clients can specify a "no-cache" request that they will
1655   understand (as Cache-Control was not defined until HTTP/1.1). When the
1656   Cache-Control header is also present and understood in a request, Pragma is
1657   ignored.
1658</t>
1659<t>
1660   In HTTP/1.0, Pragma was defined as an extensible field for
1661   implementation-specified directives for recipients. This specification
1662   deprecates such extensions to improve interoperability.
1663</t>
1664<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"/>
1665  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1666  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1667  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1668</artwork></figure>
1669<t>
1670   When the Cache-Control header is not present in a request, the no-cache
1671   request pragma-directive &MUST; have the same effect on caches as if
1672   "Cache-Control: no-cache" were present (see <xref
1673   target="cache-request-directive" />).
1674</t>
1675<t>
1676   When sending a no-cache request, a client ought to include both the pragma
1677   and cache-control directives, unless Cache-Control: no-cache is
1678   purposefully omitted to target other Cache-Control response directives at
1679   HTTP/1.1 caches. For example:
1680</t>
1681<figure>
1682<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1683GET / HTTP/1.1
1684Host: www.example.com
1685Cache-Control: max-age=30
1686Pragma: no-cache
1687
1688</artwork>
1689</figure>
1690<t>
1691   will constrain HTTP/1.1 caches to serve a response no older than 30
1692   seconds, while precluding implementations that do not understand
1693   Cache-Control from serving a cached response.
1694</t>
1695<x:note>
1696   <t>
1697      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1698      specified, it does not provide a reliable replacement for
1699      "Cache-Control: no-cache" in them.
1700   </t>
1701</x:note>
1702</section>
1703
1704<section anchor="header.vary" title="Vary">
1705   <iref item="Vary header field" primary="true" x:for-anchor="" />
1706   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1707   <x:anchor-alias value="Vary"/>
1708<t>
1709   The "Vary" header field conveys the set of header fields
1710   that were used to select the representation.
1711</t>
1712<t>
1713   Caches use this information, in part, to determine whether a stored
1714   response can be used to satisfy a given request; see <xref
1715   target="caching.negotiated.responses" />. determines, while the response is
1716   fresh, whether a cache is permitted to use the response to reply to a
1717   subsequent request without validation; see <xref
1718   target="caching.negotiated.responses" />.
1719</t>
1720<t>
1721   In uncacheable or stale responses, the Vary field value advises the user
1722   agent about the criteria that were used to select the representation.
1723</t>
1724<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1725  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1726</artwork></figure>
1727<t>
1728   The set of header fields named by the Vary field value is known as the
1729   selecting header fields.
1730</t>
1731<t>
1732   A server &SHOULD; include a Vary header field with any cacheable response
1733   that is subject to server-driven negotiation. Doing so allows a cache to
1734   properly interpret future requests on that resource and informs the user
1735   agent about the presence of negotiation on that resource. A server &MAY;
1736   include a Vary header field with a non-cacheable response that is subject
1737   to server-driven negotiation, since this might provide the user agent with
1738   useful information about the dimensions over which the response varies at
1739   the time of the response.
1740</t>
1741<t>
1742   A Vary field value of "*" signals that unspecified parameters not limited
1743   to the header fields (e.g., the network address of the client), play a
1744   role in the selection of the response representation; therefore, a cache
1745   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1746   generate the "*" value.
1747</t>
1748<t>
1749   The field-names given are not limited to the set of standard header
1750   fields defined by this specification. Field names are case-insensitive.
1751</t>
1752</section>
1753
1754<section anchor="header.warning" title="Warning">
1755   <iref item="Warning header field" primary="true" x:for-anchor="" />
1756   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1757   <x:anchor-alias value="Warning"/>
1758   <x:anchor-alias value="warning-value"/>
1759   <x:anchor-alias value="warn-agent"/>
1760   <x:anchor-alias value="warn-code"/>
1761   <x:anchor-alias value="warn-date"/>
1762   <x:anchor-alias value="warn-text"/>
1763<t>
1764   The "Warning" header field is used to carry additional information
1765   about the status or transformation of a message that might not be reflected
1766   in the message. This information is typically used to warn about possible
1767   incorrectness introduced by caching operations or transformations applied
1768   to the payload of the message.
1769</t>
1770<t>
1771   Warnings can be used for other purposes, both cache-related and otherwise.
1772   The use of a warning, rather than an error status code, distinguishes these
1773   responses from true failures.
1774</t>
1775<t>
1776   Warning header fields can in general be applied to any message, however some
1777   warn-codes are specific to caches and can only be applied to response
1778   messages.
1779</t>
1780<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"/>
1781  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1782 
1783  <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>
1784                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1785 
1786  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1787  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1788                  ; the name or pseudonym of the server adding
1789                  ; the Warning header field, for use in debugging
1790  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1791  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1792</artwork></figure>
1793<t>
1794   Multiple warnings can be attached to a response (either by the origin
1795   server or by a cache), including multiple warnings with the same code
1796   number, only differing in warn-text.
1797</t>
1798<t>
1799   When this occurs, the user agent &SHOULD; inform the user of as many of
1800   them as possible, in the order that they appear in the response.
1801</t>
1802<t>
1803   Systems that generate multiple Warning header fields are encouraged to
1804   order them with this user agent behavior in mind. New Warning header fields
1805   are added after any existing Warning headers fields.
1806</t>
1807<t>
1808   Warnings are assigned three digit warn-codes. The first digit indicates
1809   whether the Warning is required to be deleted from a stored response after
1810   validation:
1811   <list style="symbols">
1812      <t>1xx Warnings describe the freshness or validation status of the
1813      response, and so &MUST; be deleted by a cache after validation. They can
1814      only be generated by a cache when validating a cached entry, and
1815      &MUST-NOT; be generated in any other situation.</t>
1816      <t>2xx Warnings describe some aspect of the representation that is not
1817      rectified by a validation (for example, a lossy compression of the
1818      representation) and &MUST-NOT; be deleted by a cache after validation,
1819      unless a full response is returned, in which case they &MUST; be.</t>
1820   </list>
1821</t>
1822<t>
1823   If an implementation sends a message with one or more Warning header fields to a
1824   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1825   in each warning-value a warn-date that matches the Date header field in the
1826   message.
1827</t>
1828<t>
1829   If a system receives a message with a warning-value that includes
1830   a warn-date, and that warn-date is different from the Date value in the
1831   response, then that warning-value &MUST; be deleted from the message before
1832   storing, forwarding, or using it. (preventing the consequences of naive
1833   caching of Warning header fields.) If all of the warning-values are deleted
1834   for this reason, the Warning header field &MUST; be deleted as well.
1835</t>
1836<t>
1837   The following warn-codes are defined by this specification, each with a
1838   recommended warn-text in English, and a description of its meaning.
1839</t>
1840<t><?rfc needLines="4"?>
1841   110 Response is stale
1842   <list>
1843      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1844   </list>
1845</t>
1846<t><?rfc needLines="4"?>
1847   111 Revalidation failed
1848   <list>
1849      <t>A cache &SHOULD; include this when returning a stale response because an
1850      attempt to validate the response failed, due to an inability to reach
1851      the server.</t>
1852   </list>
1853</t>
1854<t><?rfc needLines="4"?>
1855   112 Disconnected operation
1856   <list>
1857      <t>A cache &SHOULD; include this if it is intentionally disconnected from
1858      the rest of the network for a period of time.</t>
1859   </list>
1860</t>
1861<t><?rfc needLines="4"?>
1862   113 Heuristic expiration
1863   <list>
1864      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1865      lifetime greater than 24 hours and the response's age is greater than 24
1866      hours.</t>
1867   </list>
1868</t>
1869<t><?rfc needLines="4"?>
1870   199 Miscellaneous warning
1871   <list>
1872      <t>The warning text can include arbitrary information to be presented to
1873      a human user, or logged. A system receiving this warning &MUST-NOT; take
1874      any automated action, besides presenting the warning to the user.</t>
1875   </list>
1876</t>
1877<t><?rfc needLines="4"?>
1878   214 Transformation applied
1879   <list>
1880      <t>&MUST; be added by a proxy if it applies any
1881      transformation to the representation, such as changing the
1882      content-coding, media-type, or modifying the representation data, unless
1883      this Warning code already appears in the response.</t>
1884   </list>
1885</t>
1886<t><?rfc needLines="4"?>
1887   299 Miscellaneous persistent warning
1888   <list>
1889      <t>The warning text can include arbitrary information to be presented to
1890      a human user, or logged. A system receiving this warning &MUST-NOT; take
1891      any automated action.</t>
1892   </list>
1893</t>
1894</section>
1895
1896</section>
1897
1898<section anchor="history.lists" title="History Lists">
1899<t>
1900   User agents often have history mechanisms, such as "Back" buttons and
1901   history lists, that can be used to redisplay a representation retrieved
1902   earlier in a session.
1903</t>
1904<t>
1905   The freshness model (<xref target="expiration.model"/>) does not
1906   necessarily apply to history mechanisms. I.e., a history mechanism can
1907   display a previous representation even if it has expired.
1908</t>
1909<t>
1910   This does not prohibit the history mechanism from telling the user that a
1911   view might be stale, or from honoring cache directives (e.g.,
1912   Cache-Control: no-store).
1913</t>
1914</section>
1915
1916
1917<section anchor="IANA.considerations" title="IANA Considerations">
1918
1919<section title="Cache Directive Registry" 
1920   anchor="cache.directive.registration">
1921<t>
1922   The registration procedure for HTTP Cache Directives is defined by <xref
1923   target="cache.control.extensions"/> of this document.
1924</t>
1925<t>
1926   The HTTP Cache Directive Registry shall be created at <eref
1927   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1928   populated with the registrations below:
1929</t>
1930<?BEGININC p6-cache.cache-directives ?>
1931<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1932<texttable xmlns:my="#my" align="left" suppress-title="true"
1933           anchor="iana.cache.directive.registration.table">
1934   <ttcol>Cache Directive</ttcol>
1935   <ttcol>Reference</ttcol>
1936
1937   <c>max-age</c>
1938   <c>
1939      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1940   </c>
1941   <c>max-stale</c>
1942   <c>
1943      <xref target="cache-request-directive"/>
1944   </c>
1945   <c>min-fresh</c>
1946   <c>
1947      <xref target="cache-request-directive"/>
1948   </c>
1949   <c>must-revalidate</c>
1950   <c>
1951      <xref target="cache-response-directive"/>
1952   </c>
1953   <c>no-cache</c>
1954   <c>
1955      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1956   </c>
1957   <c>no-store</c>
1958   <c>
1959      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1960   </c>
1961   <c>no-transform</c>
1962   <c>
1963      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1964   </c>
1965   <c>only-if-cached</c>
1966   <c>
1967      <xref target="cache-request-directive"/>
1968   </c>
1969   <c>private</c>
1970   <c>
1971      <xref target="cache-response-directive"/>
1972   </c>
1973   <c>proxy-revalidate</c>
1974   <c>
1975      <xref target="cache-response-directive"/>
1976   </c>
1977   <c>public</c>
1978   <c>
1979      <xref target="cache-response-directive"/>
1980   </c>
1981   <c>s-maxage</c>
1982   <c>
1983      <xref target="cache-response-directive"/>
1984   </c>
1985   <c>stale-if-error</c>
1986   <c>
1987      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1988   </c>
1989   <c>stale-while-revalidate</c>
1990   <c>
1991      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1992   </c>
1993</texttable>
1994<!--(END)-->
1995<?ENDINC p6-cache.cache-directives ?>
1996</section>
1997
1998<section title="Header Field Registration" anchor="header.field.registration">
1999<t>
2000  The Message Header Field Registry located at <eref 
2001  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2002  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2003</t>
2004<?BEGININC p6-cache.iana-headers ?>
2005<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2006<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2007   <ttcol>Header Field Name</ttcol>
2008   <ttcol>Protocol</ttcol>
2009   <ttcol>Status</ttcol>
2010   <ttcol>Reference</ttcol>
2011
2012   <c>Age</c>
2013   <c>http</c>
2014   <c>standard</c>
2015   <c>
2016      <xref target="header.age"/>
2017   </c>
2018   <c>Cache-Control</c>
2019   <c>http</c>
2020   <c>standard</c>
2021   <c>
2022      <xref target="header.cache-control"/>
2023   </c>
2024   <c>Expires</c>
2025   <c>http</c>
2026   <c>standard</c>
2027   <c>
2028      <xref target="header.expires"/>
2029   </c>
2030   <c>Pragma</c>
2031   <c>http</c>
2032   <c>standard</c>
2033   <c>
2034      <xref target="header.pragma"/>
2035   </c>
2036   <c>Vary</c>
2037   <c>http</c>
2038   <c>standard</c>
2039   <c>
2040      <xref target="header.vary"/>
2041   </c>
2042   <c>Warning</c>
2043   <c>http</c>
2044   <c>standard</c>
2045   <c>
2046      <xref target="header.warning"/>
2047   </c>
2048</texttable>
2049<!--(END)-->
2050<?ENDINC p6-cache.iana-headers ?>
2051<t>
2052   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2053   Force".
2054</t>
2055</section>
2056
2057</section>
2058
2059<section anchor="security.considerations" title="Security Considerations">
2060<t>
2061   Caches expose additional potential vulnerabilities, since the contents of
2062   the cache represent an attractive target for malicious exploitation.
2063   Because cache contents persist after an HTTP request is complete, an attack
2064   on the cache can reveal information long after a user believes that the
2065   information has been removed from the network. Therefore, cache contents
2066   need to be protected as sensitive information.
2067</t>
2068</section>
2069
2070<section title="Acknowledgments" anchor="acks">
2071<t>
2072  See &acks;.
2073</t>
2074</section>
2075
2076</middle>
2077
2078<back>
2079<references title="Normative References">
2080
2081  <reference anchor="Part1">
2082    <front>
2083      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2084      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2085        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2086        <address><email>fielding@gbiv.com</email></address>
2087      </author>
2088      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2089        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2090        <address><email>jg@freedesktop.org</email></address>
2091      </author>
2092      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2093        <organization abbrev="HP">Hewlett-Packard Company</organization>
2094        <address><email>JeffMogul@acm.org</email></address>
2095      </author>
2096      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2097        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2098        <address><email>henrikn@microsoft.com</email></address>
2099      </author>
2100      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2101        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2102        <address><email>LMM@acm.org</email></address>
2103      </author>
2104      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2105        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2106        <address><email>paulle@microsoft.com</email></address>
2107      </author>
2108      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2109        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2110        <address><email>timbl@w3.org</email></address>
2111      </author>
2112      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2113        <organization abbrev="W3C">World Wide Web Consortium</organization>
2114        <address><email>ylafon@w3.org</email></address>
2115      </author>
2116      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2117        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2118        <address><email>julian.reschke@greenbytes.de</email></address>
2119      </author>
2120      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2121    </front>
2122    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2123    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2124  </reference>
2125
2126  <reference anchor="Part2">
2127    <front>
2128      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2129      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2130        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2131        <address><email>fielding@gbiv.com</email></address>
2132      </author>
2133      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2134        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2135        <address><email>jg@freedesktop.org</email></address>
2136      </author>
2137      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2138        <organization abbrev="HP">Hewlett-Packard Company</organization>
2139        <address><email>JeffMogul@acm.org</email></address>
2140      </author>
2141      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2142        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2143        <address><email>henrikn@microsoft.com</email></address>
2144      </author>
2145      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2146        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2147        <address><email>LMM@acm.org</email></address>
2148      </author>
2149      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2150        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2151        <address><email>paulle@microsoft.com</email></address>
2152      </author>
2153      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2154        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2155        <address><email>timbl@w3.org</email></address>
2156      </author>
2157      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2158        <organization abbrev="W3C">World Wide Web Consortium</organization>
2159        <address><email>ylafon@w3.org</email></address>
2160      </author>
2161      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2162        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2163        <address><email>julian.reschke@greenbytes.de</email></address>
2164      </author>
2165      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2166    </front>
2167    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2168    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2169  </reference>
2170
2171  <reference anchor="Part4">
2172    <front>
2173      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2174      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2175        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2176        <address><email>fielding@gbiv.com</email></address>
2177      </author>
2178      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2179        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2180        <address><email>jg@freedesktop.org</email></address>
2181      </author>
2182      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2183        <organization abbrev="HP">Hewlett-Packard Company</organization>
2184        <address><email>JeffMogul@acm.org</email></address>
2185      </author>
2186      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2187        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2188        <address><email>henrikn@microsoft.com</email></address>
2189      </author>
2190      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2191        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2192        <address><email>LMM@acm.org</email></address>
2193      </author>
2194      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2195        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2196        <address><email>paulle@microsoft.com</email></address>
2197      </author>
2198      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2199        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2200        <address><email>timbl@w3.org</email></address>
2201      </author>
2202      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2203        <organization abbrev="W3C">World Wide Web Consortium</organization>
2204        <address><email>ylafon@w3.org</email></address>
2205      </author>
2206      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2207        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2208        <address><email>julian.reschke@greenbytes.de</email></address>
2209      </author>
2210      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2211    </front>
2212    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2213    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2214  </reference>
2215
2216  <reference anchor="Part5">
2217    <front>
2218      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2219      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2220        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2221        <address><email>fielding@gbiv.com</email></address>
2222      </author>
2223      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2224        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2225        <address><email>jg@freedesktop.org</email></address>
2226      </author>
2227      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2228        <organization abbrev="HP">Hewlett-Packard Company</organization>
2229        <address><email>JeffMogul@acm.org</email></address>
2230      </author>
2231      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2232        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2233        <address><email>henrikn@microsoft.com</email></address>
2234      </author>
2235      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2236        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2237        <address><email>LMM@acm.org</email></address>
2238      </author>
2239      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2240        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2241        <address><email>paulle@microsoft.com</email></address>
2242      </author>
2243      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2244        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2245        <address><email>timbl@w3.org</email></address>
2246      </author>
2247      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2248        <organization abbrev="W3C">World Wide Web Consortium</organization>
2249        <address><email>ylafon@w3.org</email></address>
2250      </author>
2251      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2252        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2253        <address><email>julian.reschke@greenbytes.de</email></address>
2254      </author>
2255      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2256    </front>
2257    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2258    <x:source basename="p5-range" href="p5-range.xml" />
2259  </reference>
2260
2261  <reference anchor="Part7">
2262    <front>
2263      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2264      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2265        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2266        <address><email>fielding@gbiv.com</email></address>
2267      </author>
2268      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2269        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2270        <address><email>jg@freedesktop.org</email></address>
2271      </author>
2272      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2273        <organization abbrev="HP">Hewlett-Packard Company</organization>
2274        <address><email>JeffMogul@acm.org</email></address>
2275      </author>
2276      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2277        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2278        <address><email>henrikn@microsoft.com</email></address>
2279      </author>
2280      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2281        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2282        <address><email>LMM@acm.org</email></address>
2283      </author>
2284      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2285        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2286        <address><email>paulle@microsoft.com</email></address>
2287      </author>
2288      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2289        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2290        <address><email>timbl@w3.org</email></address>
2291      </author>
2292      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2293        <organization abbrev="W3C">World Wide Web Consortium</organization>
2294        <address><email>ylafon@w3.org</email></address>
2295      </author>
2296      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2297        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2298        <address><email>julian.reschke@greenbytes.de</email></address>
2299      </author>
2300      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2301    </front>
2302    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2303    <x:source basename="p7-auth" href="p7-auth.xml" />
2304  </reference>
2305
2306  <reference anchor="RFC2119">
2307    <front>
2308      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2309      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2310        <organization>Harvard University</organization>
2311        <address><email>sob@harvard.edu</email></address>
2312      </author>
2313      <date month="March" year="1997" />
2314    </front>
2315    <seriesInfo name="BCP" value="14" />
2316    <seriesInfo name="RFC" value="2119" />
2317  </reference>
2318
2319  <reference anchor="RFC5234">
2320    <front>
2321      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2322      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2323        <organization>Brandenburg InternetWorking</organization>
2324        <address>
2325          <email>dcrocker@bbiw.net</email>
2326        </address> 
2327      </author>
2328      <author initials="P." surname="Overell" fullname="Paul Overell">
2329        <organization>THUS plc.</organization>
2330        <address>
2331          <email>paul.overell@thus.net</email>
2332        </address>
2333      </author>
2334      <date month="January" year="2008"/>
2335    </front>
2336    <seriesInfo name="STD" value="68"/>
2337    <seriesInfo name="RFC" value="5234"/>
2338  </reference>
2339 
2340</references>
2341
2342<references title="Informative References">
2343
2344  <reference anchor="RFC1305">
2345    <front>
2346      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2347      <author fullname="David L. Mills" initials="D." surname="Mills">
2348        <organization>University of Delaware, Electrical Engineering Department</organization>
2349        <address><email>mills@udel.edu</email></address>
2350      </author>
2351      <date month="March" year="1992" />
2352    </front>
2353    <seriesInfo name="RFC" value="1305" />
2354  </reference>
2355
2356  <reference anchor="RFC2616">
2357    <front>
2358      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2359      <author fullname="R. Fielding" initials="R." surname="Fielding">
2360        <organization>University of California, Irvine</organization>
2361        <address><email>fielding@ics.uci.edu</email></address>
2362      </author>
2363      <author fullname="J. Gettys" initials="J." surname="Gettys">
2364        <organization>W3C</organization>
2365        <address><email>jg@w3.org</email></address>
2366      </author>
2367      <author fullname="J. Mogul" initials="J." surname="Mogul">
2368        <organization>Compaq Computer Corporation</organization>
2369        <address><email>mogul@wrl.dec.com</email></address>
2370      </author>
2371      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2372        <organization>MIT Laboratory for Computer Science</organization>
2373        <address><email>frystyk@w3.org</email></address>
2374      </author>
2375      <author fullname="L. Masinter" initials="L." surname="Masinter">
2376        <organization>Xerox Corporation</organization>
2377        <address><email>masinter@parc.xerox.com</email></address>
2378      </author>
2379      <author fullname="P. Leach" initials="P." surname="Leach">
2380        <organization>Microsoft Corporation</organization>
2381        <address><email>paulle@microsoft.com</email></address>
2382      </author>
2383      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2384        <organization>W3C</organization>
2385        <address><email>timbl@w3.org</email></address>
2386      </author>
2387      <date month="June" year="1999" />
2388    </front>
2389    <seriesInfo name="RFC" value="2616" />
2390  </reference>
2391
2392  <reference anchor="RFC3864">
2393    <front>
2394      <title>Registration Procedures for Message Header Fields</title>
2395      <author fullname="G. Klyne" initials="G." surname="Klyne">
2396        <organization>Nine by Nine</organization>
2397        <address><email>GK-IETF@ninebynine.org</email></address>
2398      </author>
2399      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2400        <organization>BEA Systems</organization>
2401        <address><email>mnot@pobox.com</email></address>
2402      </author>
2403      <author fullname="J. Mogul" initials="J." surname="Mogul">
2404        <organization>HP Labs</organization>
2405        <address><email>JeffMogul@acm.org</email></address>
2406      </author>
2407      <date month="September" year="2004" />
2408    </front>
2409    <seriesInfo name="BCP" value="90" />
2410    <seriesInfo name="RFC" value="3864" />
2411  </reference>
2412
2413  <reference anchor='RFC5226'>
2414    <front>
2415      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2416      <author initials='T.' surname='Narten' fullname='T. Narten'>
2417        <organization>IBM</organization>
2418        <address><email>narten@us.ibm.com</email></address>
2419      </author>
2420      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2421        <organization>Google</organization>
2422        <address><email>Harald@Alvestrand.no</email></address>
2423      </author>
2424      <date year='2008' month='May' />
2425    </front>
2426    <seriesInfo name='BCP' value='26' />
2427    <seriesInfo name='RFC' value='5226' />
2428  </reference>
2429
2430  <reference anchor='RFC5861'>
2431    <front>
2432      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2433      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2434        <organization>Yahoo! Inc.</organization>
2435        <address><email>mnot@yahoo-inc.com</email></address>
2436      </author>
2437      <date month="April" year="2010"/>
2438    </front>
2439    <seriesInfo name='RFC' value='5861' />
2440  </reference>
2441
2442</references>
2443
2444<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2445<t>
2446  Make the specified age calculation algorithm less conservative.
2447  (<xref target="age.calculations"/>)
2448</t>
2449<t>
2450  Remove requirement to consider Content-Location in successful responses
2451  in order to determine the appropriate response to use.
2452  (<xref target="validation.model" />)
2453</t>
2454<t>
2455  Clarify denial of service attack avoidance requirement.
2456  (<xref target="invalidation.after.updates.or.deletions" />)
2457</t>
2458<t>
2459  Change ABNF productions for header fields to only define the field value.
2460  (<xref target="header.field.definitions"/>)
2461</t>
2462<t>
2463  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2464  anymore, as these aspects never were implemented.
2465  (<xref target="header.warning" />)
2466</t>
2467</section>
2468
2469<?BEGININC p6-cache.abnf-appendix ?>
2470<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2471<figure>
2472<artwork type="abnf" name="p6-cache.parsed-abnf">
2473<x:ref>Age</x:ref> = delta-seconds
2474
2475<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2476 cache-directive ] )
2477
2478<x:ref>Expires</x:ref> = HTTP-date
2479
2480<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2481
2482<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2483
2484<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2485 pragma-directive ] )
2486
2487<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2488 ) )
2489
2490<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2491 )
2492
2493<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2494<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2495<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2496 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2497 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2498 cache-extension
2499<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2500 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2501 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2502 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2503 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2504 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2505
2506<x:ref>delta-seconds</x:ref> = 1*DIGIT
2507
2508<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2509
2510<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2511
2512<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2513<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2514<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
2515
2516<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
2517
2518<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
2519
2520<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2521
2522<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2523<x:ref>warn-code</x:ref> = 3DIGIT
2524<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2525<x:ref>warn-text</x:ref> = quoted-string
2526<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2527 ]
2528</artwork>
2529</figure>
2530<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2531; Age defined but not used
2532; Cache-Control defined but not used
2533; Expires defined but not used
2534; Pragma defined but not used
2535; Vary defined but not used
2536; Warning defined but not used
2537</artwork></figure></section>
2538<?ENDINC p6-cache.abnf-appendix ?>
2539
2540<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2541
2542<section title="Since RFC 2616">
2543  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2544</section>
2545
2546<section title="Since draft-ietf-httpbis-p6-cache-00">
2547<t>
2548  Closed issues:
2549  <list style="symbols">
2550    <t>
2551      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
2554    <t>
2555      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2556    <t>
2557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2558    <t>
2559      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2560    <t>
2561      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2562    <t>
2563      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2564    <t>
2565      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2566    <t>
2567      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2568  </list>
2569</t>
2570<t>
2571  Other changes:
2572  <list style="symbols">
2573    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2574        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2575  </list>
2576</t>
2577</section>
2578
2579<section title="Since draft-ietf-httpbis-p6-cache-01">
2580<t>
2581  Closed issues:
2582  <list style="symbols">
2583    <t>
2584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2585  </list>
2586</t>
2587<t>
2588  Other changes:
2589  <list style="symbols">
2590    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2591      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2592    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2593      specification.</t>
2594  </list>
2595</t>
2596</section>
2597
2598<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2599<t>
2600  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2601  <list style="symbols">
2602    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2603      document.</t>
2604  </list>
2605</t>
2606</section>
2607
2608<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2609<t>
2610  Closed issues:
2611  <list style="symbols">
2612    <t>
2613      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2614  </list>
2615</t>
2616</section>
2617
2618<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2619<t>
2620  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2621  <list style="symbols"> 
2622    <t>
2623      Use "/" instead of "|" for alternatives.
2624    </t>
2625    <t>
2626      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2627      whitespace ("OWS") and required whitespace ("RWS").
2628    </t>
2629    <t>
2630      Rewrite ABNFs to spell out whitespace rules, factor out
2631      header field value format definitions.
2632    </t>
2633  </list>
2634</t>
2635</section>
2636
2637<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2638<t>
2639  This is a total rewrite of this part of the specification.
2640</t>
2641<t>
2642  Affected issues:
2643  <list style="symbols">
2644    <t>
2645      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2646    <t>
2647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2648    <t>
2649      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2650    <t>
2651      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2652  </list>
2653</t>
2654<t>
2655  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2656  <list style="symbols"> 
2657    <t>
2658      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2659    </t>
2660  </list>
2661</t>
2662</section>
2663
2664<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2665<t>
2666  Closed issues:
2667  <list style="symbols"> 
2668    <t>
2669      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2670      "base for numeric protocol elements"
2671    </t>
2672  </list>
2673</t>
2674<t>
2675  Affected issues:
2676  <list style="symbols">
2677    <t>
2678      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2679      "Vary and non-existant headers"
2680    </t>
2681  </list>
2682</t>
2683</section>
2684
2685<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2686<t>
2687  Closed issues:
2688  <list style="symbols"> 
2689    <t>
2690      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2691      "Definition of 1xx Warn-Codes"
2692    </t>
2693    <t>
2694      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2695      "Content-Location on 304 responses"
2696    </t>
2697    <t>
2698      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2699      "private and no-cache CC directives with headers"
2700    </t>
2701    <t>
2702      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2703      "RFC2047 and warn-text"
2704    </t>
2705  </list>
2706</t>
2707</section>
2708
2709<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2710<t>
2711  Closed issues:
2712  <list style="symbols"> 
2713    <t>
2714      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2715      "serving negotiated responses from cache: header-specific canonicalization"
2716    </t>
2717    <t>
2718      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2719      "Effect of CC directives on history lists"
2720    </t>
2721    <t>
2722      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2723      "Cache Extensions can override no-store, etc."
2724    </t>
2725  </list>
2726</t>
2727<t>
2728  Affected issues:
2729  <list style="symbols">
2730    <t>
2731      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2732      Status codes and caching
2733    </t>
2734  </list>
2735</t>
2736<t>
2737  Partly resolved issues:
2738  <list style="symbols"> 
2739    <t>
2740      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2741      "Placement of 13.5.1 and 13.5.2"
2742    </t>
2743  </list>
2744</t>
2745</section>
2746
2747<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2748<t>
2749  Closed issues:
2750  <list style="symbols"> 
2751    <t>
2752      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2753      "Age calculation"
2754    </t>
2755    <t>
2756      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2757      "Clarify differences between / requirements for request and response CC directives"
2758    </t>
2759    <t>
2760      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2761      "Caching authenticated responses"
2762    </t>
2763    <t>
2764      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2765      "IANA registry for cache-control directives"
2766    </t>
2767    <t>
2768      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2769      "Heuristic caching of URLs with query components"
2770    </t>
2771  </list>
2772</t>
2773<t>
2774  Partly resolved issues:
2775  <list style="symbols"> 
2776    <t>
2777      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2778      "Term for the requested resource's URI"
2779    </t>
2780  </list>
2781</t>
2782</section>
2783
2784<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2785<t>
2786  Closed issues:
2787  <list style="symbols"> 
2788    <t>
2789      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2790      "Clarify entity / representation / variant terminology"
2791    </t>
2792    <t>
2793      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2794      "consider removing the 'changes from 2068' sections"
2795    </t>
2796    <t>
2797      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2798      "Allowing heuristic caching for new status codes"
2799    </t>
2800    <t>
2801      Clean up TODOs and prose in "Combining Responses."
2802    </t>
2803  </list>
2804</t>
2805</section>
2806
2807<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2808<t>
2809  Closed issues:
2810  <list style="symbols"> 
2811    <t>
2812      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2813      "Text about clock requirement for caches belongs in p6"
2814    </t>
2815  </list>
2816</t>
2817</section>
2818
2819<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2820<t>
2821  Closed issues:
2822  <list style="symbols"> 
2823    <t>
2824      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2825      "Header Classification"
2826    </t>
2827    <t>
2828      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2829      "Clarify 'public'"
2830    </t>
2831  </list>
2832</t>
2833</section>
2834
2835<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2836<t>
2837  Closed issues:
2838  <list style="symbols">
2839    <t>
2840      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2841      "untangle ABNFs for header fields"
2842    </t>
2843  </list>
2844</t>
2845</section>
2846
2847<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2848<t>
2849  Closed issues:
2850  <list style="symbols">
2851    <t>
2852      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2853      "Mismatch Vary"
2854    </t>
2855    <t>
2856      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2857      "Cache Invalidation only happens upon successful responses"
2858    </t>
2859    <t>
2860      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2861      "Recommend minimum sizes for protocol elements"
2862    </t>
2863    <t>
2864      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2865      "Proxies don't 'understand' methods"
2866    </t>
2867    <t>
2868      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2869      "Cache Extensions can override no-store, etc."
2870    </t>
2871    <t>
2872      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2873      "Pragma"
2874    </t>
2875  </list>
2876</t>
2877</section>
2878
2879<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2880<t>
2881  Closed issues:
2882  <list style="symbols">
2883    <t>
2884      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2885      "Motivate one-year limit for Expires"
2886    </t>
2887  </list>
2888</t>
2889</section>
2890
2891<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
2892<t>
2893  None yet.
2894</t>
2895</section>
2896
2897</section>
2898  </back>
2899</rfc>
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