source: draft-ietf-httpbis/latest/p6-cache.xml @ 1374

Last change on this file since 1374 was 1374, checked in by fielding@…, 8 years ago

Clarify what should happen when a response is incomplete.
Disentangle the requirements surrounding conditional range
requests, strong validators, and recombining partial content
to remove redundant redundancy. Separate handling of 304
responses into a separate section on cache freshening.

Add definitions for "cache entry" and "cache key".
Improve introductions for caching and cache operation.

These changes should all be editorial, hopefully.
Tangentially related to #101 and #304.

  • Property svn:eol-style set to native
File size: 113.1 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "August">
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 uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40]>
41<?rfc toc="yes" ?>
42<?rfc symrefs="yes" ?>
43<?rfc sortrefs="yes" ?>
44<?rfc compact="yes"?>
45<?rfc subcompact="no" ?>
46<?rfc linkmailto="no" ?>
47<?rfc editing="no" ?>
48<?rfc comments="yes"?>
49<?rfc inline="yes"?>
50<?rfc rfcedstyle="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
54  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
55<front>
56
57  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
58
59  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
60    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
61    <address>
62      <postal>
63        <street>345 Park Ave</street>
64        <city>San Jose</city>
65        <region>CA</region>
66        <code>95110</code>
67        <country>USA</country>
68      </postal>
69      <email>fielding@gbiv.com</email>
70      <uri>http://roy.gbiv.com/</uri>
71    </address>
72  </author>
73
74  <author initials="J." surname="Gettys" fullname="Jim Gettys">
75    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
76    <address>
77      <postal>
78        <street>21 Oak Knoll Road</street>
79        <city>Carlisle</city>
80        <region>MA</region>
81        <code>01741</code>
82        <country>USA</country>
83      </postal>
84      <email>jg@freedesktop.org</email>
85      <uri>http://gettys.wordpress.com/</uri>
86    </address>
87  </author>
88
89  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
90    <organization abbrev="HP">Hewlett-Packard Company</organization>
91    <address>
92      <postal>
93        <street>HP Labs, Large Scale Systems Group</street>
94        <street>1501 Page Mill Road, MS 1177</street>
95        <city>Palo Alto</city>
96        <region>CA</region>
97        <code>94304</code>
98        <country>USA</country>
99      </postal>
100      <email>JeffMogul@acm.org</email>
101    </address>
102  </author>
103
104  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
105    <organization abbrev="Microsoft">Microsoft Corporation</organization>
106    <address>
107      <postal>
108        <street>1 Microsoft Way</street>
109        <city>Redmond</city>
110        <region>WA</region>
111        <code>98052</code>
112        <country>USA</country>
113      </postal>
114      <email>henrikn@microsoft.com</email>
115    </address>
116  </author>
117
118  <author fullname="Larry Masinter" initials="L." surname="Masinter">
119    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
120    <address>
121      <postal>
122        <street>345 Park Ave</street>
123        <city>San Jose</city>
124        <region>CA</region>
125        <code>95110</code>
126        <country>USA</country>
127      </postal>
128      <email>LMM@acm.org</email>
129      <uri>http://larry.masinter.net/</uri>
130    </address>
131  </author>
132
133  <author fullname="Paul J. Leach" initials="P." surname="Leach">
134    <organization abbrev="Microsoft">Microsoft Corporation</organization>
135    <address>
136      <postal>
137        <street>1 Microsoft Way</street>
138        <city>Redmond</city>
139        <region>WA</region>
140        <code>98052</code>
141      </postal>
142      <email>paulle@microsoft.com</email>
143    </address>
144  </author>
145
146  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
147    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
148    <address>
149      <postal>
150        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
151        <street>The Stata Center, Building 32</street>
152        <street>32 Vassar Street</street>
153        <city>Cambridge</city>
154        <region>MA</region>
155        <code>02139</code>
156        <country>USA</country>
157      </postal>
158      <email>timbl@w3.org</email>
159      <uri>http://www.w3.org/People/Berners-Lee/</uri>
160    </address>
161  </author>
162
163  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
164    <organization abbrev="W3C">World Wide Web Consortium</organization>
165    <address>
166      <postal>
167        <street>W3C / ERCIM</street>
168        <street>2004, rte des Lucioles</street>
169        <city>Sophia-Antipolis</city>
170        <region>AM</region>
171        <code>06902</code>
172        <country>France</country>
173      </postal>
174      <email>ylafon@w3.org</email>
175      <uri>http://www.raubacapeu.net/people/yves/</uri>
176    </address>
177  </author>
178
179  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
180    <address>
181      <email>mnot@mnot.net</email>
182      <uri>http://www.mnot.net/</uri>
183    </address>
184  </author>
185
186  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
187    <organization abbrev="greenbytes">greenbytes GmbH</organization>
188    <address>
189      <postal>
190        <street>Hafenweg 16</street>
191        <city>Muenster</city><region>NW</region><code>48155</code>
192        <country>Germany</country>
193      </postal>
194      <phone>+49 251 2807760</phone>
195      <facsimile>+49 251 2807761</facsimile>
196      <email>julian.reschke@greenbytes.de</email>
197      <uri>http://greenbytes.de/tech/webdav/</uri>
198    </address>
199  </author>
200
201  <date month="&ID-MONTH;" year="&ID-YEAR;" />
202  <workgroup>HTTPbis Working Group</workgroup>
203
204<abstract>
205<t>
206   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
207   distributed, collaborative, hypertext information systems. HTTP has been in
208   use by the World Wide Web global information initiative since 1990. This
209   document is Part 6 of the seven-part specification that defines the protocol
210   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
211</t>
212<t>
213   Part 6 defines requirements on HTTP caches and the associated header fields
214   that control cache behavior or indicate cacheable response messages.
215</t>
216</abstract>
217
218<note title="Editorial Note (To be removed by RFC Editor)">
219  <t>
220    Discussion of this draft should take place on the HTTPBIS working group
221    mailing list (ietf-http-wg@w3.org), which is archived at
222    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
223  </t>
224  <t>
225    The current issues list is at
226    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
227    documents (including fancy diffs) can be found at
228    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
229  </t>
230  <t>
231    The changes in this draft are summarized in <xref target="changes.since.15"/>.
232  </t>
233</note>
234
235   </front>
236   <middle>
237
238<section anchor="caching" title="Introduction">
239<t>
240   HTTP is typically used for distributed information systems, where
241   performance can be improved by the use of response caches. This document
242   defines aspects of HTTP/1.1 related to caching and reusing response
243   messages.
244</t>
245
246<section anchor="intro.purpose" title="Purpose">
247<iref item="cache" />
248<t>
249   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
250   subsystem that controls its message storage, retrieval, and deletion. A
251   cache stores cacheable responses in order to reduce the response time and
252   network bandwidth consumption on future, equivalent requests. Any client or
253   server &MAY; employ a cache, though a cache cannot be used by a server that
254   is acting as a tunnel.
255</t>
256<t>
257   The goal of caching in HTTP/1.1 is to significantly improve performance
258   by reusing a prior response message to satisfy a current request.
259   A stored response is considered "fresh", as defined in
260   <xref target="expiration.model" />, if the response can be reused without
261   "validation" (checking with the origin server to see if the cached response
262   remains valid for this request).  A fresh cache response can therefore
263   reduce both latency and network transfers each time it is reused.
264   When a cached response is not fresh, it might still be reusable if it can
265   be freshened by validation (<xref target="validation.model" />) or if the
266   origin is unavailable.
267</t>
268</section>
269
270<section anchor="intro.terminology" title="Terminology">
271<t>
272   This specification uses a number of terms to refer to the roles played by
273   participants in, and objects of, HTTP caching.
274</t>
275<t>
276   <iref item="cache" />
277   <x:dfn>cache</x:dfn>
278   <list>
279      <t>A conformant implementation of a HTTP cache. Note that this implies
280        an HTTP/1.1 cache; this specification does not define conformance
281        for HTTP/1.0 caches.</t>
282   </list>
283</t>
284<t anchor="shared.and.non-shared.caches">
285   <iref item="shared cache" />
286   <x:dfn>shared cache</x:dfn>
287   <list>
288      <t>A cache that stores responses to be reused by more than one user;
289         usually (but not always) deployed as part of an intermediary.</t>
290   </list>
291</t>
292<t>
293   <iref item="private cache" />
294   <x:dfn>private cache</x:dfn>
295   <list>
296      <t>A cache that is dedicated to a single user.</t>
297   </list>
298</t>
299<t>
300   <iref item="cacheable" />
301   <x:dfn>cacheable</x:dfn>
302   <list>
303      <t>A response is cacheable if a cache is allowed to store a copy of the
304      response message for use in answering subsequent requests. Even when a
305      response is cacheable, there might be additional constraints on whether
306      a cache can use the stored copy to satisfy a particular request.</t>
307   </list>
308</t>
309<t>
310   <iref item="explicit expiration time" />
311   <x:dfn>explicit expiration time</x:dfn>
312   <list>
313      <t>The time at which the origin server intends that a representation
314      no longer be returned by a cache without further validation.</t>
315   </list>
316</t>
317<t>
318   <iref item="heuristic expiration time" />
319   <x:dfn>heuristic expiration time</x:dfn>
320   <list>
321      <t>An expiration time assigned by a cache when no explicit expiration
322      time is available.</t>
323   </list>
324</t>
325<t>
326   <iref item="age" />
327   <x:dfn>age</x:dfn>
328   <list>
329      <t>The age of a response is the time since it was sent by, or
330      successfully validated with, the origin server.</t>
331   </list>
332</t>
333<t>
334   <iref item="first-hand" />
335   <x:dfn>first-hand</x:dfn>
336   <list>
337      <t>A response is first-hand if the freshness model is not in use; i.e.,
338      its age is 0.</t>
339   </list>
340</t>
341<t>
342   <iref item="freshness lifetime" />
343   <x:dfn>freshness lifetime</x:dfn>
344   <list>
345      <t>The length of time between the generation of a response and its
346      expiration time.</t>
347   </list>
348</t>
349<t>
350   <iref item="fresh" />
351   <x:dfn>fresh</x:dfn>
352   <list>
353      <t>A response is fresh if its age has not yet exceeded its freshness
354      lifetime.</t>
355   </list>
356</t>
357<t>
358   <iref item="stale" />
359   <x:dfn>stale</x:dfn>
360   <list>
361      <t>A response is stale if its age has passed its freshness lifetime
362      (either explicit or heuristic).</t>
363   </list>
364</t>
365<t>
366   <iref item="validator" />
367   <x:dfn>validator</x:dfn>
368   <list>
369      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
370      is used to find out whether a stored response is an equivalent copy of
371      a representation. See &weak-and-strong;.</t>
372   </list>
373</t>
374<t>
375   <iref item="strong validator" />
376   <iref item="validator, strong" />
377   <x:dfn>strong validator</x:dfn>
378   <list>
379      <t>A validator that is defined by the origin server such that its
380         current value will change if the representation body changes; i.e.,
381         an entity-tag that is not marked as weak (&entity-tags;) or,
382         if no entity-tag is provided, a Last-Modified value that is strong
383         in the sense defined by &lastmod-comparison;.</t>
384   </list>
385</t>
386</section>
387
388<section anchor="intro.requirements" title="Requirements">
389<t>
390   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
391   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
392   document are to be interpreted as described in <xref target="RFC2119"/>.
393</t>
394<t>
395   An implementation is not compliant if it fails to satisfy one or more of
396   the "MUST" or "REQUIRED" level requirements for the protocols it
397   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
398   level and all the "SHOULD" level requirements for its protocols is said to
399   be "unconditionally compliant"; one that satisfies all the "MUST" level
400   requirements but not all the "SHOULD" level requirements for its protocols
401   is said to be "conditionally compliant".
402</t>
403</section>
404
405<section title="Syntax Notation" anchor="notation">
406   <x:anchor-alias value="ALPHA"/>
407   <x:anchor-alias value="CR"/>
408   <x:anchor-alias value="DIGIT"/>
409   <x:anchor-alias value="DQUOTE"/>
410   <x:anchor-alias value="LF"/>
411   <x:anchor-alias value="OCTET"/>
412   <x:anchor-alias value="SP"/>
413   <x:anchor-alias value="VCHAR"/>
414   <x:anchor-alias value="WSP"/>
415<t>
416   This specification uses the ABNF syntax defined in &notation; (which
417   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
418   <xref target="collected.abnf"/> shows the collected ABNF, with the list
419   rule expanded.
420</t>
421<t>
422   The following core rules are included by reference, as defined in <xref
423   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
424   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
425   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
426   sequence of data), SP (space), VCHAR (any visible USASCII character), and
427   WSP (whitespace).
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 &basic-rules;:
436</t>
437<figure><artwork type="abnf2616">
438  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
439  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
440  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-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 &full-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 &SHOULD; 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 (&header-last-modified;),
755   a cache &SHOULD-NOT; use a heuristic expiration value that is more than some
756   fraction of the interval since that time. A typical setting of this fraction
757   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</section>
872
873<section anchor="serving.stale.responses" title="Serving Stale Responses">
874<t>
875   A "stale" response is one that either has explicit expiry information or is
876   allowed to have heuristic expiry calculated, but is not fresh according to
877   the calculations in <xref target="expiration.model" />.
878</t>
879<t>
880   A cache &MUST-NOT; return a stale response if it is prohibited by an
881   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
882   directive, a "must-revalidate" cache-response-directive, or an applicable
883   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
884   target="cache-response-directive"/>).
885</t>
886<t>
887   A cache &SHOULD-NOT; return stale responses unless it is disconnected
888   (i.e., it cannot contact the origin server or otherwise find a forward
889   path) or doing so is explicitly allowed (e.g., by the max-stale request
890   directive; see <xref target="cache-request-directive" />).
891</t>
892<t>
893   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
894   <xref target="header.warning" />) to stale responses. Likewise, a cache
895   &SHOULD; add the 112 warn-code to stale responses if the cache is
896   disconnected.
897</t>
898<t>
899   If a cache receives a first-hand response (either an entire response, or a
900   304 (Not Modified) response) that it would normally forward to the
901   requesting client, and the received response is no longer fresh, the cache
902   &SHOULD; forward it to the requesting client without adding a new Warning
903   (but without removing any existing Warning header fields). A cache &SHOULD-NOT;
904   attempt to validate a response simply because that response became stale in
905   transit.
906</t>
907</section>
908</section>
909
910<section anchor="validation.model" title="Validation Model">
911<t>
912   When a cache has one or more stored responses for a requested URI, but
913   cannot serve any of them (e.g., because they are not fresh, or one cannot
914   be selected; see <xref target="caching.negotiated.responses"/>), it can use
915   the conditional request mechanism &conditional; in the forwarded request to
916   give the origin server an opportunity to both select a valid stored
917   response to be used, and to update it. This process is known as
918   "validating" or "revalidating" the stored response.
919</t>
920<t>
921   When sending such a conditional request, a cache &SHOULD; add an
922   If-Modified-Since header field whose value is that of the Last-Modified header
923   field from the selected (see <xref target="caching.negotiated.responses"/>)
924   stored response, if available.
925</t>
926<t>
927   Additionally, a cache &SHOULD; add an If-None-Match header field whose value is
928   that of the ETag header field(s) from all responses stored for the requested URI,
929   if present. However, if any of the stored responses contains only partial
930   content, the cache &SHOULD-NOT; include its entity-tag in the If-None-Match
931   header field unless the request is for a range that would be fully
932   satisfied by that stored response.
933</t>
934<t>
935   A 304 (Not Modified) response status code indicates that the stored
936   response can be updated and reused; see <xref target="freshening.responses"/>.
937</t>
938<t>
939   A full response (i.e., one with a response body) indicates that none of the
940   stored responses nominated in the conditional request is suitable. Instead,
941   a cache &SHOULD; use the full response to satisfy the request and &MAY; 
942   replace the stored response(s).
943</t>
944<t>
945   If a cache receives a 5xx response while attempting to validate a response,
946   it &MAY; either forward this response to the requesting client, or act as
947   if the server failed to respond. In the latter case, it &MAY; return a
948   previously stored response (see <xref target="serving.stale.responses" />).
949</t>
950</section>
951
952<section anchor="invalidation.after.updates.or.deletions" 
953   title="Request Methods that Invalidate">
954<t>
955   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
956   have the potential for changing state on the origin server, intervening
957   caches can use them to keep their contents up-to-date.
958</t>
959<t>
960   A cache &MUST; invalidate the effective Request URI
961   (&effective-request-uri;) as well as the URI(s) in the Location
962   and Content-Location header fields (if present) when a non-error
963   response to a request with an unsafe method is received.
964</t>
965<t>
966   However, a cache &MUST-NOT; invalidate a URI from a
967   Location or Content-Location header field if the host part of that URI
968   differs from the host part in the effective request URI
969   (&effective-request-uri;). This helps prevent denial of service attacks.
970</t>
971<t>
972   A cache &SHOULD; invalidate the effective request URI
973   (&effective-request-uri;) when it receives a non-error response
974   to a request with a method whose safety is unknown.
975</t>
976<t>
977   Here, a "non-error response" is one with a 2xx or 3xx status code.
978   "Invalidate" means that the cache will either remove all stored
979   responses related to the effective request URI, or will mark these as
980   "invalid" and in need of a mandatory validation before they can be returned
981   in response to a subsequent request.
982</t>
983<t>
984   Note that this does not guarantee that all appropriate responses are
985   invalidated. For example, the request that caused the change at the origin
986   server might not have gone through the cache where a response is stored.
987</t>
988</section>
989
990<section anchor="caching.authenticated.responses" 
991   title="Shared Caching of Authenticated Responses">
992
993<t>
994   A shared cache &MUST-NOT; use a cached response to a request with an
995   Authorization header field (&header-authorization;) to satisfy any subsequent
996   request unless a cache directive that allows such responses to be stored is
997   present in the response.
998</t>
999
1000<t>
1001   In this specification, the following Cache-Control response directives
1002   (<xref target="cache-response-directive"/>) have such an effect:
1003   must-revalidate, public, s-maxage.
1004</t>
1005
1006<t>
1007   Note that cached responses that contain the "must-revalidate" and/or
1008   "s-maxage" response directives are not allowed to be served stale (<xref
1009   target="serving.stale.responses"/>) by shared caches. In particular, a
1010   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1011   used to satisfy a subsequent request without revalidating it on the origin
1012   server.
1013</t>
1014</section>
1015
1016<section anchor="caching.negotiated.responses" 
1017   title="Caching Negotiated Responses">
1018<t>
1019   When a cache receives a request that can be satisfied by a stored response
1020   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1021   use that response unless all of the selecting header fields nominated by
1022   the Vary header field match in both the original request (i.e., that associated
1023   with the stored response), and the presented request.
1024</t>
1025<t>
1026   The selecting header fields from two requests are defined to match if and
1027   only if those in the first request can be transformed to those in the
1028   second request by applying any of the following:
1029   <list style="symbols">
1030      <t>
1031         adding or removing whitespace, where allowed in the header field's syntax
1032      </t>
1033      <t>
1034         combining multiple header fields with the same field name
1035         (see &header-fields;)
1036      </t>
1037      <t>
1038         normalizing both header field values in a way that is known to have
1039         identical semantics, according to the header field's specification (e.g.,
1040         re-ordering field values when order is not significant;
1041         case-normalization, where values are defined to be case-insensitive)
1042      </t>
1043  </list>
1044</t>
1045<t>
1046   If (after any normalization that might take place) a header field is absent
1047   from a request, it can only match another request if it is also absent
1048   there.
1049</t>
1050<t>
1051   A Vary header field-value of "*" always fails to match, and subsequent
1052   requests to that resource can only be properly interpreted by the origin
1053   server.
1054</t>
1055<t>
1056   The stored response with matching selecting header fields is known as the
1057   selected response.
1058</t>
1059<t>
1060   If multiple selected responses are available, the most recent response
1061   (as determined by the Date header field) is used; see <xref 
1062   target="constructing.responses.from.caches"/>.
1063</t>
1064<t>
1065   If no selected response is available, the cache &MAY; forward the presented
1066   request to the origin server in a conditional request; see <xref
1067   target="validation.model"/>.
1068</t>
1069</section>
1070
1071<section anchor="combining.responses" title="Combining Partial Content">
1072<t>
1073   A response might transfer only a partial representation if the
1074   connection closed prematurely or if the request used one or more Range
1075   specifiers (&partial;).  After several such transfers, a cache might have
1076   received several ranges of the same representation.  A cache &MAY; combine
1077   these ranges into a single stored response, and reuse that response to
1078   satisfy later requests, if they all share the same strong validator and
1079   the cache complies with the client requirements in &combining-byte-ranges;.
1080</t>
1081<t>
1082   When combining the new response with one or more stored responses, a
1083   cache &MUST;:
1084   <list style="symbols">
1085      <t>delete any Warning header fields in the stored response with
1086         warn-code 1xx (see <xref target="header.warning" />);</t>
1087      <t>retain any Warning header fields in the stored response with
1088         warn-code 2xx; and,</t>
1089      <t>use other header fields provided in the new response, aside
1090         from Content-Range, to replace all instances of the corresponding
1091         header fields in the stored response.</t>
1092   </list>
1093</t>
1094</section>
1095
1096<section anchor="freshening.responses" title="Freshening Responses">
1097<t>
1098   When a cache receives a 304 (Not Modified) response and already has one
1099   or more stored 200 (OK) responses for the same cache key, the cache needs
1100   to identify which of the stored responses are updated by this new response
1101   and then update the stored response(s) with the new information provided in
1102   the 304 response.
1103   <list style="symbols">
1104    <t>
1105     If the new response contains a strong validator, then that strong
1106     validator identifies the selected representation.  All of the stored
1107     responses with the same strong validator are selected.
1108     If none of the stored responses contain the same strong validator, then
1109     this new response corresponds to a new selected representation and
1110     &MUST-NOT; update the existing stored responses.
1111    </t>
1112    <t>
1113     If the new response contains a weak validator and that validator
1114     corresponds to one of the cache's stored responses, then the most
1115     recent of those matching stored responses is selected.
1116    </t>
1117    <t>
1118     If the new response does not include any form of validator, there is
1119     only one stored response, and that stored response also lacks a
1120     validator, then that stored response is selected.
1121    </t>
1122   </list>
1123</t>
1124<t>
1125   If a stored response is selected for update, the cache &MUST;:
1126   <list style="symbols">
1127      <t>delete any Warning header fields in the stored response with
1128         warn-code 1xx (see <xref target="header.warning" />);</t>
1129      <t>retain any Warning header fields in the stored response with
1130         warn-code 2xx; and,</t>
1131      <t>use other header fields provided in the 304 response to replace
1132         all instances of the corresponding header fields in the stored
1133         response.</t>
1134   </list>
1135</t>
1136</section>
1137
1138</section>
1139
1140<section anchor="header.fields" title="Header Field Definitions">
1141<t>
1142   This section defines the syntax and semantics of HTTP/1.1 header fields
1143   related to caching.
1144</t>
1145
1146<section anchor="header.age" title="Age">
1147   <iref item="Age header field" primary="true" x:for-anchor="" />
1148   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1149   <x:anchor-alias value="Age"/>
1150   <x:anchor-alias value="age-value"/>
1151<t>
1152   The "Age" header field conveys the sender's estimate of the amount
1153   of time since the response was generated or successfully validated at the
1154   origin server. Age values are calculated as specified in <xref
1155   target="age.calculations" />.
1156</t>
1157<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1158  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1159</artwork></figure>
1160<t>
1161  Age field-values are non-negative integers, representing time in seconds
1162  (see <xref target="delta-seconds"/>).
1163</t>
1164<t>
1165   The presence of an Age header field in a response implies that a response
1166   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1167   might not implement the Age header field.
1168</t>
1169</section>
1170
1171<section anchor="header.cache-control" title="Cache-Control">
1172   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1173   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1174      x:for-anchor="" />
1175   <x:anchor-alias value="Cache-Control"/>
1176   <x:anchor-alias value="cache-directive"/>
1177   <x:anchor-alias value="cache-extension"/>
1178   <x:anchor-alias value="cache-request-directive"/>
1179   <x:anchor-alias value="cache-response-directive"/>
1180<t>
1181   The "Cache-Control" header field is used to specify directives for
1182   caches along the request/response chain. Such cache directives are
1183   unidirectional in that the presence of a directive in a request does not
1184   imply that the same directive is to be given in the response.
1185</t>
1186<t>
1187   A cache &MUST; obey the requirements of the Cache-Control
1188   directives defined in this section. See <xref
1189   target="cache.control.extensions"/> for information about how Cache-Control
1190   directives defined elsewhere are handled.
1191</t>
1192<x:note>
1193   <t>
1194       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1195       might only implement Pragma: no-cache (see <xref target="header.pragma"
1196       />).
1197   </t>
1198</x:note>
1199<t>
1200   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1201   through in forwarded messages, regardless of their
1202   significance to that application, since the directives might be applicable
1203   to all recipients along the request/response chain. It is not possible to
1204   target a directive to a specific cache.
1205</t>
1206<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1207  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1208
1209  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1210     / <x:ref>cache-response-directive</x:ref>
1211
1212  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1213</artwork></figure>
1214
1215<section anchor="cache-request-directive" 
1216   title="Request Cache-Control Directives">
1217   <x:anchor-alias value="cache-request-directive" />
1218
1219<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1220   subitem="cache-request-directive" />
1221  <x:ref>cache-request-directive</x:ref> =
1222       "no-cache"
1223     / "no-store"
1224     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1225     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1226     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1227     / "no-transform"
1228     / "only-if-cached"
1229     / <x:ref>cache-extension</x:ref>
1230</artwork></figure>
1231
1232<t>
1233   <x:dfn>no-cache</x:dfn>
1234   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1235   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1236   <list>
1237      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1238      use a stored response to satisfy the request without successful
1239      validation on the origin server.</t> 
1240   </list>
1241</t>
1242<t>
1243   <x:dfn>no-store</x:dfn>
1244   <iref item="Cache Directives" primary="true" subitem="no-store" />
1245   <iref item="no-store" primary="true" subitem="Cache Directive" />
1246   <list>
1247      <t>The no-store request directive indicates that a cache &MUST-NOT;
1248      store any part of either this request or any response to it. This
1249      directive applies to both private and shared caches. "&MUST-NOT;
1250      store" in this context means that the cache &MUST-NOT; intentionally
1251      store the information in non-volatile storage, and &MUST; make a
1252      best-effort attempt to remove the information from volatile storage as
1253      promptly as possible after forwarding it.</t>
1254      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1255      privacy. In particular, malicious or compromised caches might not
1256      recognize or obey this directive, and communications networks might be
1257      vulnerable to eavesdropping.</t>
1258      <t>Note that if a request containing this directive is satisfied from a
1259      cache, the no-store request directive does not apply to the already
1260      stored response.</t>
1261   </list>
1262</t>
1263<t>
1264   <x:dfn>max-age</x:dfn>
1265   <iref item="Cache Directives" primary="true" subitem="max-age" />
1266   <iref item="max-age" primary="true" subitem="Cache Directive" />
1267   <list>
1268      <t>The max-age request directive indicates that the client is unwilling to
1269      accept a response whose age is greater than the specified number of
1270      seconds. Unless the max-stale request directive is also present, the
1271      client is not willing to accept a stale response.</t>
1272   </list>
1273</t>
1274<t>
1275   <x:dfn>max-stale</x:dfn>
1276   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1277   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1278   <list>
1279      <t>The max-stale request directive indicates that the client is willing
1280      to accept a response that has exceeded its expiration time. If max-stale
1281      is assigned a value, then the client is willing to accept a response
1282      that has exceeded its expiration time by no more than the specified
1283      number of seconds. If no value is assigned to max-stale, then the client
1284      is willing to accept a stale response of any age.</t>
1285   </list>
1286</t>
1287<t>
1288   <x:dfn>min-fresh</x:dfn>
1289   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1290   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1291   <list>
1292      <t>The min-fresh request directive indicates that the client is willing
1293      to accept a response whose freshness lifetime is no less than its
1294      current age plus the specified time in seconds. That is, the client
1295      wants a response that will still be fresh for at least the specified
1296      number of seconds.</t>
1297   </list>
1298</t>
1299<t>
1300   <x:dfn>no-transform</x:dfn>
1301   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1302   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1303   <list>
1304      <t>The no-transform request directive indicates that an intermediary
1305        (whether or not it implements a cache) &MUST-NOT; change the
1306        Content-Encoding, Content-Range or Content-Type request header fields,
1307        nor the request representation.</t>
1308   </list>
1309</t>
1310<t>
1311   <x:dfn>only-if-cached</x:dfn>
1312   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1313   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1314   <list>
1315      <t>The only-if-cached request directive indicates that the client only
1316      wishes to obtain a stored response. If it receives this directive, a
1317      cache &SHOULD; either respond using a stored response that is consistent
1318      with the other constraints of the request, or respond with a 504
1319      (Gateway Timeout) status code. If a group of caches is being operated as
1320      a unified system with good internal connectivity, a member cache &MAY;
1321      forward such a request within that group of caches.</t>
1322   </list>
1323</t>
1324</section>
1325
1326<section anchor="cache-response-directive" 
1327   title="Response Cache-Control Directives">
1328   <x:anchor-alias value="cache-response-directive" />
1329
1330<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1331   subitem="cache-response-directive" />
1332  <x:ref>cache-response-directive</x:ref> =
1333       "public"
1334     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1335     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1336     / "no-store"
1337     / "no-transform"
1338     / "must-revalidate"
1339     / "proxy-revalidate"
1340     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1341     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1342     / <x:ref>cache-extension</x:ref>
1343</artwork></figure>
1344
1345<t>
1346   <x:dfn>public</x:dfn>
1347   <iref item="Cache Directives" primary="true" subitem="public" />
1348   <iref item="public" primary="true" subitem="Cache Directive" />
1349   <list>
1350      <t>The public response directive indicates that a response whose
1351        associated request contains an 'Authentication' header &MAY; be
1352        stored (see <xref target="caching.authenticated.responses" />).</t>
1353  </list>
1354</t>
1355<t>
1356   <x:dfn>private</x:dfn>
1357   <iref item="Cache Directives" primary="true" subitem="private" />
1358   <iref item="private" primary="true" subitem="Cache Directive" />
1359   <list>
1360      <t>The private response directive indicates that the response message is
1361      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1362      private cache &MAY; store the response.</t>
1363      <t>If the private response directive specifies one or more field-names,
1364      this requirement is limited to the field-values associated with the
1365      listed response header fields. That is, a shared cache &MUST-NOT; store
1366      the specified field-names(s), whereas it &MAY; store the remainder of the
1367      response message.</t>
1368      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1369      the response can be stored; it cannot ensure the privacy of the message
1370      content. Also, private response directives with field-names are often
1371      handled by implementations as if an unqualified private directive was
1372      received; i.e., the special handling for the qualified form is not
1373      widely implemented.</t>
1374   </list>
1375</t>
1376<t>
1377   <x:dfn>no-cache</x:dfn>
1378   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1379   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1380   <list>
1381      <t>The no-cache response directive indicates that the response MUST NOT
1382      be used to satisfy a subsequent request without successful validation on
1383      the origin server. This allows an origin server to prevent a cache from
1384      using it to satisfy a request without contacting it, even by caches that
1385      have been configured to return stale responses.</t>
1386      <t>If the no-cache response directive specifies one or more field-names,
1387      this requirement is limited to the field-values associated with the
1388      listed response header fields. That is, a cache &MUST-NOT; send the
1389      specified field-name(s) in the response to a subsequent request without successful
1390      validation on the origin server. This allows an origin server to prevent
1391      the re-use of certain header fields in a response, while still allowing
1392      caching of the rest of the response.</t>
1393      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1394      this directive. Also, no-cache response directives with field-names are
1395      often handled by implementations as if an unqualified no-cache directive
1396      was received; i.e., the special handling for the qualified form is not
1397      widely implemented. </t>
1398   </list>
1399</t>
1400<t>
1401   <x:dfn>no-store</x:dfn>
1402   <iref item="Cache Directives" primary="true" subitem="no-store" />
1403   <iref item="no-store" primary="true" subitem="Cache Directive" />
1404   <list>
1405      <t>The no-store response directive indicates that a cache &MUST-NOT;
1406      store any part of either the immediate request or response. This
1407      directive applies to both private and shared caches. "&MUST-NOT;
1408      store" in this context means that the cache &MUST-NOT; intentionally
1409      store the information in non-volatile storage, and &MUST; make a
1410      best-effort attempt to remove the information from volatile storage as
1411      promptly as possible after forwarding it.</t>
1412      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1413      privacy. In particular, malicious or compromised caches might not
1414      recognize or obey this directive, and communications networks might be
1415      vulnerable to eavesdropping.</t>
1416   </list>
1417</t>
1418<t>
1419   <x:dfn>must-revalidate</x:dfn>
1420   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1421   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1422   <list>
1423      <t>The must-revalidate response directive indicates that once it has
1424      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1425      requests without successful validation on the origin server.</t>
1426      <t>The must-revalidate directive is necessary to support reliable
1427      operation for certain protocol features. In all circumstances a
1428      cache &MUST; obey the must-revalidate directive; in particular,
1429      if a cache cannot reach the origin server for any reason, it &MUST;
1430      generate a 504 (Gateway Timeout) response.</t>
1431      <t>A server &SHOULD; send the must-revalidate directive if and only if
1432      failure to validate a request on the representation could result in
1433      incorrect operation, such as a silently unexecuted financial
1434      transaction.</t>
1435   </list>
1436</t>
1437<t>
1438   <x:dfn>proxy-revalidate</x:dfn>
1439   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1440   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1441   <list>
1442      <t>The proxy-revalidate response directive has the same meaning as the
1443      must-revalidate response directive, except that it does not apply to
1444      private caches.</t>
1445   </list>
1446</t>
1447<t>
1448   <x:dfn>max-age</x:dfn>
1449   <iref item="Cache Directives" primary="true" subitem="max-age" />
1450   <iref item="max-age" primary="true" subitem="Cache Directive" />
1451   <list>
1452      <t>The max-age response directive indicates that the response is to be
1453      considered stale after its age is greater than the specified number of
1454      seconds.</t>
1455   </list>
1456</t>
1457<t>
1458   <x:dfn>s-maxage</x:dfn>
1459   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1460   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1461   <list>
1462      <t>The s-maxage response directive indicates that, in shared caches, the
1463      maximum age specified by this directive overrides the maximum age
1464      specified by either the max-age directive or the Expires header field. The
1465      s-maxage directive also implies the semantics of the proxy-revalidate
1466      response directive.</t>
1467   </list>
1468</t>
1469<t>
1470   <x:dfn>no-transform</x:dfn>
1471   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1472   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1473   <list>
1474      <t>The no-transform response directive indicates that an intermediary
1475      (regardless of whether it implements a cache) &MUST-NOT; change the
1476      Content-Encoding, Content-Range or Content-Type response header fields,
1477      nor the response representation.</t>
1478   </list>
1479</t>
1480
1481</section>
1482
1483<section anchor="cache.control.extensions" title="Cache Control Extensions">
1484<t>
1485   The Cache-Control header field can be extended through the use of one or
1486   more cache-extension tokens, each with an optional value. Informational
1487   extensions (those that do not require a change in cache behavior) can be
1488   added without changing the semantics of other directives. Behavioral
1489   extensions are designed to work by acting as modifiers to the existing base
1490   of cache directives. Both the new directive and the standard directive are
1491   supplied, such that applications that do not understand the new directive
1492   will default to the behavior specified by the standard directive, and those
1493   that understand the new directive will recognize it as modifying the
1494   requirements associated with the standard directive. In this way,
1495   extensions to the cache-control directives can be made without requiring
1496   changes to the base protocol.
1497</t>
1498<t>
1499   This extension mechanism depends on an HTTP cache obeying all of the
1500   cache-control directives defined for its native HTTP-version, obeying
1501   certain extensions, and ignoring all directives that it does not
1502   understand.
1503</t>
1504<t>
1505   For example, consider a hypothetical new response directive called
1506   "community" that acts as a modifier to the private directive. We define
1507   this new directive to mean that, in addition to any private cache, any
1508   cache that is shared only by members of the community named within its
1509   value may cache the response. An origin server wishing to allow the UCI
1510   community to use an otherwise private response in their shared cache(s)
1511   could do so by including
1512</t>
1513<figure><artwork type="example">
1514  Cache-Control: private, community="UCI"
1515</artwork></figure>
1516<t>
1517   A cache seeing this header field will act correctly even if the cache does
1518   not understand the community cache-extension, since it will also see and
1519   understand the private directive and thus default to the safe behavior.
1520</t>
1521<t>
1522   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1523   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1524   combined with standard directives (or the response's default cacheability)
1525   such that the cache behavior will remain minimally correct even if the
1526   cache does not understand the extension(s).
1527</t>
1528<t>
1529   The HTTP Cache Directive Registry defines the name space for the cache
1530   directives.
1531</t>
1532<t>
1533   A registration &MUST; include the following fields:
1534   <list style="symbols">
1535      <t>Cache Directive Name</t>
1536      <t>Pointer to specification text</t>
1537   </list>
1538</t>
1539<t>
1540   Values to be added to this name space are subject to IETF review (<xref
1541   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1542</t>
1543<t>
1544   The registry itself is maintained at <eref
1545   target="http://www.iana.org/assignments/http-cache-directives"/>.
1546</t>
1547</section>
1548
1549</section>
1550
1551<section anchor="header.expires" title="Expires">
1552   <iref item="Expires header field" primary="true" x:for-anchor="" />
1553   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1554   <x:anchor-alias value="Expires"/>
1555<t>
1556   The "Expires" header field gives the date/time after which the
1557   response is considered stale. See <xref target="expiration.model" /> for
1558   further discussion of the freshness model.
1559</t>
1560<t>
1561   The presence of an Expires field does not imply that the original resource
1562   will change or cease to exist at, before, or after that time.
1563</t>
1564<t>
1565   The field-value is an absolute date and time as defined by HTTP-date in
1566   &full-date;; a sender &MUST; use the rfc1123-date format.
1567</t>
1568<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1569  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1570</artwork></figure>
1571<figure>
1572  <preamble>For example</preamble>
1573<artwork type="example">
1574  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1575</artwork></figure>
1576<x:note>
1577   <t>
1578       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1579       max-age directive (see <xref target="cache-response-directive" />),
1580       that directive overrides the Expires field. Likewise, the s-maxage
1581       directive overrides Expires in shared caches.
1582   </t>
1583</x:note>
1584<t>
1585   A server &SHOULD-NOT; send Expires dates more than one year in the
1586   future.
1587</t>
1588<t>
1589   A cache &MUST; treat other invalid date formats,
1590   especially including the value "0", as in the past (i.e., "already
1591   expired").
1592</t>
1593</section>
1594
1595<section anchor="header.pragma" title="Pragma">
1596   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1597   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1598   <x:anchor-alias value="extension-pragma"/>
1599   <x:anchor-alias value="Pragma"/>
1600   <x:anchor-alias value="pragma-directive"/>
1601<t>
1602   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1603   caches, so that clients can specify a "no-cache" request that they will
1604   understand (as Cache-Control was not defined until HTTP/1.1). When the
1605   Cache-Control header is also present and understood in a request, Pragma is
1606   ignored.
1607</t>
1608<t>
1609   In HTTP/1.0, Pragma was defined as an extensible field for
1610   implementation-specified directives for recipients. This specification
1611   deprecates such extensions to improve interoperability.
1612</t>
1613<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"/>
1614  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1615  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1616  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1617</artwork></figure>
1618<t>
1619   When the Cache-Control header is not present in a request, the no-cache
1620   request pragma-directive &MUST; have the same effect on caches as if
1621   "Cache-Control: no-cache" were present (see <xref
1622   target="cache-request-directive" />).
1623</t>
1624<t>
1625   When sending a no-cache request, a client &SHOULD; include both pragma and
1626   cache-control directives unless Cache-Control: no-cache is purposefully
1627   omitted to target other Cache-Control response directives at HTTP/1.1
1628   caches. For example:
1629</t>
1630<figure>
1631<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1632GET / HTTP/1.1
1633Host: www.example.com
1634Cache-Control: max-age=30
1635Pragma: no-cache
1636
1637</artwork>
1638</figure>
1639<t>
1640   will constrain HTTP/1.1 caches to serve a response no older than 30
1641   seconds, while precluding implementations that do not understand
1642   Cache-Control from serving a cached response.
1643</t>
1644<x:note>
1645   <t>
1646      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1647      specified, it does not provide a reliable replacement for
1648      "Cache-Control: no-cache" in them.
1649   </t>
1650</x:note>
1651</section>
1652
1653<section anchor="header.vary" title="Vary">
1654   <iref item="Vary header field" primary="true" x:for-anchor="" />
1655   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1656   <x:anchor-alias value="Vary"/>
1657<t>
1658   The "Vary" header field conveys the set of header fields
1659   that were used to select the representation.
1660</t>
1661<t>
1662   Caches use this information, in part, to determine whether a stored
1663   response can be used to satisfy a given request; see <xref
1664   target="caching.negotiated.responses" />. determines, while the response is
1665   fresh, whether a cache is permitted to use the response to reply to a
1666   subsequent request without validation; see <xref
1667   target="caching.negotiated.responses" />.
1668</t>
1669<t>
1670   In uncacheable or stale responses, the Vary field value advises the user
1671   agent about the criteria that were used to select the representation.
1672</t>
1673<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1674  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1675</artwork></figure>
1676<t>
1677   The set of header fields named by the Vary field value is known as the
1678   selecting header fields.
1679</t>
1680<t>
1681   A server &SHOULD; include a Vary header field with any cacheable response
1682   that is subject to server-driven negotiation. Doing so allows a cache to
1683   properly interpret future requests on that resource and informs the user
1684   agent about the presence of negotiation on that resource. A server &MAY;
1685   include a Vary header field with a non-cacheable response that is subject
1686   to server-driven negotiation, since this might provide the user agent with
1687   useful information about the dimensions over which the response varies at
1688   the time of the response.
1689</t>
1690<t>
1691   A Vary field value of "*" signals that unspecified parameters not limited
1692   to the header fields (e.g., the network address of the client), play a
1693   role in the selection of the response representation; therefore, a cache
1694   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1695   generate the "*" value.
1696</t>
1697<t>
1698   The field-names given are not limited to the set of standard header
1699   fields defined by this specification. Field names are case-insensitive.
1700</t>
1701</section>
1702
1703<section anchor="header.warning" title="Warning">
1704   <iref item="Warning header field" primary="true" x:for-anchor="" />
1705   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1706   <x:anchor-alias value="Warning"/>
1707   <x:anchor-alias value="warning-value"/>
1708   <x:anchor-alias value="warn-agent"/>
1709   <x:anchor-alias value="warn-code"/>
1710   <x:anchor-alias value="warn-date"/>
1711   <x:anchor-alias value="warn-text"/>
1712<t>
1713   The "Warning" header field is used to carry additional information
1714   about the status or transformation of a message that might not be reflected
1715   in the message. This information is typically used to warn about possible
1716   incorrectness introduced by caching operations or transformations applied
1717   to the payload of the message.
1718</t>
1719<t>
1720   Warnings can be used for other purposes, both cache-related and otherwise.
1721   The use of a warning, rather than an error status code, distinguishes these
1722   responses from true failures.
1723</t>
1724<t>
1725   Warning header fields can in general be applied to any message, however some
1726   warn-codes are specific to caches and can only be applied to response
1727   messages.
1728</t>
1729<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"/>
1730  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1731 
1732  <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>
1733                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1734 
1735  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1736  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1737                  ; the name or pseudonym of the server adding
1738                  ; the Warning header field, for use in debugging
1739  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1740  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1741</artwork></figure>
1742<t>
1743   Multiple warnings can be attached to a response (either by the origin
1744   server or by a cache), including multiple warnings with the same code
1745   number, only differing in warn-text.
1746</t>
1747<t>
1748   When this occurs, the user agent &SHOULD; inform the user of as many of
1749   them as possible, in the order that they appear in the response.
1750</t>
1751<t>
1752   Systems that generate multiple Warning header fields &SHOULD; order them with
1753   this user agent behavior in mind. New Warning header fields &SHOULD; be added
1754   after any existing Warning headers fields.
1755</t>
1756<t>
1757   Warnings are assigned three digit warn-codes. The first digit indicates
1758   whether the Warning is required to be deleted from a stored response after
1759   validation:
1760   <list style="symbols">
1761      <t>1xx Warnings describe the freshness or validation status of the
1762      response, and so &MUST; be deleted by a cache after validation. They can
1763      only be generated by a cache when validating a cached entry, and
1764      &MUST-NOT; be generated in any other situation.</t>
1765      <t>2xx Warnings describe some aspect of the representation that is not
1766      rectified by a validation (for example, a lossy compression of the
1767      representation) and &MUST-NOT; be deleted by a cache after validation,
1768      unless a full response is returned, in which case they &MUST; be.</t>
1769   </list>
1770</t>
1771<t>
1772   If an implementation sends a message with one or more Warning header fields to a
1773   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1774   in each warning-value a warn-date that matches the Date header field in the
1775   message.
1776</t>
1777<t>
1778   If a system receives a message with a warning-value that includes
1779   a warn-date, and that warn-date is different from the Date value in the
1780   response, then that warning-value &MUST; be deleted from the message before
1781   storing, forwarding, or using it. (preventing the consequences of naive
1782   caching of Warning header fields.) If all of the warning-values are deleted
1783   for this reason, the Warning header field &MUST; be deleted as well.
1784</t>
1785<t>
1786   The following warn-codes are defined by this specification, each with a
1787   recommended warn-text in English, and a description of its meaning.
1788</t>
1789<t><?rfc needLines="4"?>
1790   110 Response is stale
1791   <list>
1792      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1793   </list>
1794</t>
1795<t><?rfc needLines="4"?>
1796   111 Revalidation failed
1797   <list>
1798      <t>A cache &SHOULD; include this when returning a stale response because an
1799      attempt to validate the response failed, due to an inability to reach
1800      the server.</t>
1801   </list>
1802</t>
1803<t><?rfc needLines="4"?>
1804   112 Disconnected operation
1805   <list>
1806      <t>A cache &SHOULD; b include this if it is intentionally disconnected from
1807      the rest of the network for a period of time.</t>
1808   </list>
1809</t>
1810<t><?rfc needLines="4"?>
1811   113 Heuristic expiration
1812   <list>
1813      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1814      lifetime greater than 24 hours and the response's age is greater than 24
1815      hours.</t>
1816   </list>
1817</t>
1818<t><?rfc needLines="4"?>
1819   199 Miscellaneous warning
1820   <list>
1821      <t>The warning text can include arbitrary information to be presented to
1822      a human user, or logged. A system receiving this warning &MUST-NOT; take
1823      any automated action, besides presenting the warning to the user.</t>
1824   </list>
1825</t>
1826<t><?rfc needLines="4"?>
1827   214 Transformation applied
1828   <list>
1829      <t>&MUST; be added by a proxy if it applies any
1830      transformation to the representation, such as changing the
1831      content-coding, media-type, or modifying the representation data, unless
1832      this Warning code already appears in the response.</t>
1833   </list>
1834</t>
1835<t><?rfc needLines="4"?>
1836   299 Miscellaneous persistent warning
1837   <list>
1838      <t>The warning text can include arbitrary information to be presented to
1839      a human user, or logged. A system receiving this warning &MUST-NOT; take
1840      any automated action.</t>
1841   </list>
1842</t>
1843</section>
1844
1845</section>
1846
1847<section anchor="history.lists" title="History Lists">
1848<t>
1849   User agents often have history mechanisms, such as "Back" buttons and
1850   history lists, that can be used to redisplay a representation retrieved
1851   earlier in a session.
1852</t>
1853<t>
1854   The freshness model (<xref target="expiration.model"/>) does not
1855   necessarily apply to history mechanisms. I.e., a history mechanism can
1856   display a previous representation even if it has expired.
1857</t>
1858<t>
1859   This does not prohibit the history mechanism from telling the user that a
1860   view might be stale, or from honoring cache directives (e.g.,
1861   Cache-Control: no-store).
1862</t>
1863</section>
1864
1865
1866<section anchor="IANA.considerations" title="IANA Considerations">
1867
1868<section title="Cache Directive Registry" 
1869   anchor="cache.directive.registration">
1870<t>
1871   The registration procedure for HTTP Cache Directives is defined by <xref
1872   target="cache.control.extensions"/> of this document.
1873</t>
1874<t>
1875   The HTTP Cache Directive Registry shall be created at <eref
1876   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1877   populated with the registrations below:
1878</t>
1879<?BEGININC p6-cache.cache-directives ?>
1880<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1881<texttable xmlns:my="#my" align="left" suppress-title="true"
1882           anchor="iana.cache.directive.registration.table">
1883   <ttcol>Cache Directive</ttcol>
1884   <ttcol>Reference</ttcol>
1885
1886   <c>max-age</c>
1887   <c>
1888      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1889   </c>
1890   <c>max-stale</c>
1891   <c>
1892      <xref target="cache-request-directive"/>
1893   </c>
1894   <c>min-fresh</c>
1895   <c>
1896      <xref target="cache-request-directive"/>
1897   </c>
1898   <c>must-revalidate</c>
1899   <c>
1900      <xref target="cache-response-directive"/>
1901   </c>
1902   <c>no-cache</c>
1903   <c>
1904      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1905   </c>
1906   <c>no-store</c>
1907   <c>
1908      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1909   </c>
1910   <c>no-transform</c>
1911   <c>
1912      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1913   </c>
1914   <c>only-if-cached</c>
1915   <c>
1916      <xref target="cache-request-directive"/>
1917   </c>
1918   <c>private</c>
1919   <c>
1920      <xref target="cache-response-directive"/>
1921   </c>
1922   <c>proxy-revalidate</c>
1923   <c>
1924      <xref target="cache-response-directive"/>
1925   </c>
1926   <c>public</c>
1927   <c>
1928      <xref target="cache-response-directive"/>
1929   </c>
1930   <c>s-maxage</c>
1931   <c>
1932      <xref target="cache-response-directive"/>
1933   </c>
1934   <c>stale-if-error</c>
1935   <c>
1936      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1937   </c>
1938   <c>stale-while-revalidate</c>
1939   <c>
1940      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1941   </c>
1942</texttable>
1943<!--(END)-->
1944<?ENDINC p6-cache.cache-directives ?>
1945</section>
1946
1947<section title="Header Field Registration" anchor="header.field.registration">
1948<t>
1949  The Message Header Field Registry located at <eref 
1950  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1951  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1952</t>
1953<?BEGININC p6-cache.iana-headers ?>
1954<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1955<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1956   <ttcol>Header Field Name</ttcol>
1957   <ttcol>Protocol</ttcol>
1958   <ttcol>Status</ttcol>
1959   <ttcol>Reference</ttcol>
1960
1961   <c>Age</c>
1962   <c>http</c>
1963   <c>standard</c>
1964   <c>
1965      <xref target="header.age"/>
1966   </c>
1967   <c>Cache-Control</c>
1968   <c>http</c>
1969   <c>standard</c>
1970   <c>
1971      <xref target="header.cache-control"/>
1972   </c>
1973   <c>Expires</c>
1974   <c>http</c>
1975   <c>standard</c>
1976   <c>
1977      <xref target="header.expires"/>
1978   </c>
1979   <c>Pragma</c>
1980   <c>http</c>
1981   <c>standard</c>
1982   <c>
1983      <xref target="header.pragma"/>
1984   </c>
1985   <c>Vary</c>
1986   <c>http</c>
1987   <c>standard</c>
1988   <c>
1989      <xref target="header.vary"/>
1990   </c>
1991   <c>Warning</c>
1992   <c>http</c>
1993   <c>standard</c>
1994   <c>
1995      <xref target="header.warning"/>
1996   </c>
1997</texttable>
1998<!--(END)-->
1999<?ENDINC p6-cache.iana-headers ?>
2000<t>
2001   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2002   Force".
2003</t>
2004</section>
2005
2006</section>
2007
2008<section anchor="security.considerations" title="Security Considerations">
2009<t>
2010   Caches expose additional potential vulnerabilities, since the contents of
2011   the cache represent an attractive target for malicious exploitation.
2012   Because cache contents persist after an HTTP request is complete, an attack
2013   on the cache can reveal information long after a user believes that the
2014   information has been removed from the network. Therefore, cache contents
2015   need to be protected as sensitive information.
2016</t>
2017</section>
2018
2019<section title="Acknowledgments" anchor="acks">
2020<t>
2021  See &acks;.
2022</t>
2023</section>
2024
2025</middle>
2026
2027<back>
2028<references title="Normative References">
2029
2030  <reference anchor="Part1">
2031    <front>
2032      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2033      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2034        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2035        <address><email>fielding@gbiv.com</email></address>
2036      </author>
2037      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2038        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2039        <address><email>jg@freedesktop.org</email></address>
2040      </author>
2041      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2042        <organization abbrev="HP">Hewlett-Packard Company</organization>
2043        <address><email>JeffMogul@acm.org</email></address>
2044      </author>
2045      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2046        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2047        <address><email>henrikn@microsoft.com</email></address>
2048      </author>
2049      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2050        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2051        <address><email>LMM@acm.org</email></address>
2052      </author>
2053      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2054        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2055        <address><email>paulle@microsoft.com</email></address>
2056      </author>
2057      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2058        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2059        <address><email>timbl@w3.org</email></address>
2060      </author>
2061      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2062        <organization abbrev="W3C">World Wide Web Consortium</organization>
2063        <address><email>ylafon@w3.org</email></address>
2064      </author>
2065      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2066        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2067        <address><email>julian.reschke@greenbytes.de</email></address>
2068      </author>
2069      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2070    </front>
2071    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2072    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2073  </reference>
2074
2075  <reference anchor="Part2">
2076    <front>
2077      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2078      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2079        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2080        <address><email>fielding@gbiv.com</email></address>
2081      </author>
2082      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2083        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2084        <address><email>jg@freedesktop.org</email></address>
2085      </author>
2086      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2087        <organization abbrev="HP">Hewlett-Packard Company</organization>
2088        <address><email>JeffMogul@acm.org</email></address>
2089      </author>
2090      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2091        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2092        <address><email>henrikn@microsoft.com</email></address>
2093      </author>
2094      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2095        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2096        <address><email>LMM@acm.org</email></address>
2097      </author>
2098      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2099        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2100        <address><email>paulle@microsoft.com</email></address>
2101      </author>
2102      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2103        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2104        <address><email>timbl@w3.org</email></address>
2105      </author>
2106      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2107        <organization abbrev="W3C">World Wide Web Consortium</organization>
2108        <address><email>ylafon@w3.org</email></address>
2109      </author>
2110      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2111        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2112        <address><email>julian.reschke@greenbytes.de</email></address>
2113      </author>
2114      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2115    </front>
2116    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2117    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2118  </reference>
2119
2120  <reference anchor="Part4">
2121    <front>
2122      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2123      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2124        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2125        <address><email>fielding@gbiv.com</email></address>
2126      </author>
2127      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2128        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2129        <address><email>jg@freedesktop.org</email></address>
2130      </author>
2131      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2132        <organization abbrev="HP">Hewlett-Packard Company</organization>
2133        <address><email>JeffMogul@acm.org</email></address>
2134      </author>
2135      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2136        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2137        <address><email>henrikn@microsoft.com</email></address>
2138      </author>
2139      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2140        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2141        <address><email>LMM@acm.org</email></address>
2142      </author>
2143      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2144        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2145        <address><email>paulle@microsoft.com</email></address>
2146      </author>
2147      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2148        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2149        <address><email>timbl@w3.org</email></address>
2150      </author>
2151      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2152        <organization abbrev="W3C">World Wide Web Consortium</organization>
2153        <address><email>ylafon@w3.org</email></address>
2154      </author>
2155      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2156        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2157        <address><email>julian.reschke@greenbytes.de</email></address>
2158      </author>
2159      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2160    </front>
2161    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2162    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2163  </reference>
2164
2165  <reference anchor="Part5">
2166    <front>
2167      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2168      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2169        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2170        <address><email>fielding@gbiv.com</email></address>
2171      </author>
2172      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2173        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2174        <address><email>jg@freedesktop.org</email></address>
2175      </author>
2176      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2177        <organization abbrev="HP">Hewlett-Packard Company</organization>
2178        <address><email>JeffMogul@acm.org</email></address>
2179      </author>
2180      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2181        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2182        <address><email>henrikn@microsoft.com</email></address>
2183      </author>
2184      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2185        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2186        <address><email>LMM@acm.org</email></address>
2187      </author>
2188      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2189        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2190        <address><email>paulle@microsoft.com</email></address>
2191      </author>
2192      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2193        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2194        <address><email>timbl@w3.org</email></address>
2195      </author>
2196      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2197        <organization abbrev="W3C">World Wide Web Consortium</organization>
2198        <address><email>ylafon@w3.org</email></address>
2199      </author>
2200      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2201        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2202        <address><email>julian.reschke@greenbytes.de</email></address>
2203      </author>
2204      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2205    </front>
2206    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2207    <x:source basename="p5-range" href="p5-range.xml" />
2208  </reference>
2209
2210  <reference anchor="Part7">
2211    <front>
2212      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2213      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2214        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2215        <address><email>fielding@gbiv.com</email></address>
2216      </author>
2217      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2218        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2219        <address><email>jg@freedesktop.org</email></address>
2220      </author>
2221      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2222        <organization abbrev="HP">Hewlett-Packard Company</organization>
2223        <address><email>JeffMogul@acm.org</email></address>
2224      </author>
2225      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2226        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2227        <address><email>henrikn@microsoft.com</email></address>
2228      </author>
2229      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2230        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2231        <address><email>LMM@acm.org</email></address>
2232      </author>
2233      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2234        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2235        <address><email>paulle@microsoft.com</email></address>
2236      </author>
2237      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2238        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2239        <address><email>timbl@w3.org</email></address>
2240      </author>
2241      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2242        <organization abbrev="W3C">World Wide Web Consortium</organization>
2243        <address><email>ylafon@w3.org</email></address>
2244      </author>
2245      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2246        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2247        <address><email>julian.reschke@greenbytes.de</email></address>
2248      </author>
2249      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2250    </front>
2251    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2252    <x:source basename="p7-auth" href="p7-auth.xml" />
2253  </reference>
2254
2255  <reference anchor="RFC2119">
2256    <front>
2257      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2258      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2259        <organization>Harvard University</organization>
2260        <address><email>sob@harvard.edu</email></address>
2261      </author>
2262      <date month="March" year="1997" />
2263    </front>
2264    <seriesInfo name="BCP" value="14" />
2265    <seriesInfo name="RFC" value="2119" />
2266  </reference>
2267
2268  <reference anchor="RFC5234">
2269    <front>
2270      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2271      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2272        <organization>Brandenburg InternetWorking</organization>
2273        <address>
2274          <email>dcrocker@bbiw.net</email>
2275        </address> 
2276      </author>
2277      <author initials="P." surname="Overell" fullname="Paul Overell">
2278        <organization>THUS plc.</organization>
2279        <address>
2280          <email>paul.overell@thus.net</email>
2281        </address>
2282      </author>
2283      <date month="January" year="2008"/>
2284    </front>
2285    <seriesInfo name="STD" value="68"/>
2286    <seriesInfo name="RFC" value="5234"/>
2287  </reference>
2288 
2289</references>
2290
2291<references title="Informative References">
2292
2293  <reference anchor="RFC1305">
2294    <front>
2295      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2296      <author fullname="David L. Mills" initials="D." surname="Mills">
2297        <organization>University of Delaware, Electrical Engineering Department</organization>
2298        <address><email>mills@udel.edu</email></address>
2299      </author>
2300      <date month="March" year="1992" />
2301    </front>
2302    <seriesInfo name="RFC" value="1305" />
2303  </reference>
2304
2305  <reference anchor="RFC2616">
2306    <front>
2307      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2308      <author fullname="R. Fielding" initials="R." surname="Fielding">
2309        <organization>University of California, Irvine</organization>
2310        <address><email>fielding@ics.uci.edu</email></address>
2311      </author>
2312      <author fullname="J. Gettys" initials="J." surname="Gettys">
2313        <organization>W3C</organization>
2314        <address><email>jg@w3.org</email></address>
2315      </author>
2316      <author fullname="J. Mogul" initials="J." surname="Mogul">
2317        <organization>Compaq Computer Corporation</organization>
2318        <address><email>mogul@wrl.dec.com</email></address>
2319      </author>
2320      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2321        <organization>MIT Laboratory for Computer Science</organization>
2322        <address><email>frystyk@w3.org</email></address>
2323      </author>
2324      <author fullname="L. Masinter" initials="L." surname="Masinter">
2325        <organization>Xerox Corporation</organization>
2326        <address><email>masinter@parc.xerox.com</email></address>
2327      </author>
2328      <author fullname="P. Leach" initials="P." surname="Leach">
2329        <organization>Microsoft Corporation</organization>
2330        <address><email>paulle@microsoft.com</email></address>
2331      </author>
2332      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2333        <organization>W3C</organization>
2334        <address><email>timbl@w3.org</email></address>
2335      </author>
2336      <date month="June" year="1999" />
2337    </front>
2338    <seriesInfo name="RFC" value="2616" />
2339  </reference>
2340
2341  <reference anchor="RFC3864">
2342    <front>
2343      <title>Registration Procedures for Message Header Fields</title>
2344      <author fullname="G. Klyne" initials="G." surname="Klyne">
2345        <organization>Nine by Nine</organization>
2346        <address><email>GK-IETF@ninebynine.org</email></address>
2347      </author>
2348      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2349        <organization>BEA Systems</organization>
2350        <address><email>mnot@pobox.com</email></address>
2351      </author>
2352      <author fullname="J. Mogul" initials="J." surname="Mogul">
2353        <organization>HP Labs</organization>
2354        <address><email>JeffMogul@acm.org</email></address>
2355      </author>
2356      <date month="September" year="2004" />
2357    </front>
2358    <seriesInfo name="BCP" value="90" />
2359    <seriesInfo name="RFC" value="3864" />
2360  </reference>
2361
2362  <reference anchor='RFC5226'>
2363    <front>
2364      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2365      <author initials='T.' surname='Narten' fullname='T. Narten'>
2366        <organization>IBM</organization>
2367        <address><email>narten@us.ibm.com</email></address>
2368      </author>
2369      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2370        <organization>Google</organization>
2371        <address><email>Harald@Alvestrand.no</email></address>
2372      </author>
2373      <date year='2008' month='May' />
2374    </front>
2375    <seriesInfo name='BCP' value='26' />
2376    <seriesInfo name='RFC' value='5226' />
2377  </reference>
2378
2379  <reference anchor='RFC5861'>
2380    <front>
2381      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2382      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2383        <organization>Yahoo! Inc.</organization>
2384        <address><email>mnot@yahoo-inc.com</email></address>
2385      </author>
2386      <date month="April" year="2010"/>
2387    </front>
2388    <seriesInfo name='RFC' value='5861' />
2389  </reference>
2390
2391</references>
2392
2393<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2394<t>
2395  Make the specified age calculation algorithm less conservative.
2396  (<xref target="age.calculations"/>)
2397</t>
2398<t>
2399  Remove requirement to consider Content-Location in successful responses
2400  in order to determine the appropriate response to use.
2401  (<xref target="validation.model" />)
2402</t>
2403<t>
2404  Clarify denial of service attack avoidance requirement.
2405  (<xref target="invalidation.after.updates.or.deletions" />)
2406</t>
2407<t>
2408  Change ABNF productions for header fields to only define the field value.
2409  (<xref target="header.fields"/>)
2410</t>
2411<t>
2412  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2413  anymore, as these aspects never were implemented.
2414  (<xref target="header.warning" />)
2415</t>
2416</section>
2417
2418<?BEGININC p6-cache.abnf-appendix ?>
2419<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2420<figure>
2421<artwork type="abnf" name="p6-cache.parsed-abnf">
2422<x:ref>Age</x:ref> = delta-seconds
2423
2424<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2425 cache-directive ] )
2426
2427<x:ref>Expires</x:ref> = HTTP-date
2428
2429<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2430
2431<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2432
2433<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2434 pragma-directive ] )
2435
2436<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2437 ) )
2438
2439<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2440 )
2441
2442<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2443<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2444<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2445 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2446 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2447 cache-extension
2448<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2449 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2450 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2451 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2452 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2453 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2454
2455<x:ref>delta-seconds</x:ref> = 1*DIGIT
2456
2457<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2458
2459<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2460
2461<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2462<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2463<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2464
2465<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2466
2467<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2468
2469<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2470
2471<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2472<x:ref>warn-code</x:ref> = 3DIGIT
2473<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2474<x:ref>warn-text</x:ref> = quoted-string
2475<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2476 ]
2477</artwork>
2478</figure>
2479<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2480; Age defined but not used
2481; Cache-Control defined but not used
2482; Expires defined but not used
2483; Pragma defined but not used
2484; Vary defined but not used
2485; Warning defined but not used
2486</artwork></figure></section>
2487<?ENDINC p6-cache.abnf-appendix ?>
2488
2489<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2490
2491<section title="Since RFC 2616">
2492  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2493</section>
2494
2495<section title="Since draft-ietf-httpbis-p6-cache-00">
2496<t>
2497  Closed issues:
2498  <list style="symbols">
2499    <t>
2500      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2501    <t>
2502      <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>
2503    <t>
2504      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2505    <t>
2506      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2507    <t>
2508      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2509    <t>
2510      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2511    <t>
2512      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2513    <t>
2514      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2515    <t>
2516      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2517  </list>
2518</t>
2519<t>
2520  Other changes:
2521  <list style="symbols">
2522    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2523        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2524  </list>
2525</t>
2526</section>
2527
2528<section title="Since draft-ietf-httpbis-p6-cache-01">
2529<t>
2530  Closed issues:
2531  <list style="symbols">
2532    <t>
2533      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2534  </list>
2535</t>
2536<t>
2537  Other changes:
2538  <list style="symbols">
2539    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2540      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2541    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2542      specification.</t>
2543  </list>
2544</t>
2545</section>
2546
2547<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2548<t>
2549  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2550  <list style="symbols">
2551    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2552      document.</t>
2553  </list>
2554</t>
2555</section>
2556
2557<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2558<t>
2559  Closed issues:
2560  <list style="symbols">
2561    <t>
2562      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2563  </list>
2564</t>
2565</section>
2566
2567<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2568<t>
2569  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2570  <list style="symbols"> 
2571    <t>
2572      Use "/" instead of "|" for alternatives.
2573    </t>
2574    <t>
2575      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2576      whitespace ("OWS") and required whitespace ("RWS").
2577    </t>
2578    <t>
2579      Rewrite ABNFs to spell out whitespace rules, factor out
2580      header field value format definitions.
2581    </t>
2582  </list>
2583</t>
2584</section>
2585
2586<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2587<t>
2588  This is a total rewrite of this part of the specification.
2589</t>
2590<t>
2591  Affected issues:
2592  <list style="symbols">
2593    <t>
2594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2595    <t>
2596      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2597    <t>
2598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2599    <t>
2600      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2601  </list>
2602</t>
2603<t>
2604  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2605  <list style="symbols"> 
2606    <t>
2607      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2608    </t>
2609  </list>
2610</t>
2611</section>
2612
2613<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2614<t>
2615  Closed issues:
2616  <list style="symbols"> 
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2619      "base for numeric protocol elements"
2620    </t>
2621  </list>
2622</t>
2623<t>
2624  Affected issues:
2625  <list style="symbols">
2626    <t>
2627      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2628      "Vary and non-existant headers"
2629    </t>
2630  </list>
2631</t>
2632</section>
2633
2634<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2635<t>
2636  Closed issues:
2637  <list style="symbols"> 
2638    <t>
2639      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2640      "Definition of 1xx Warn-Codes"
2641    </t>
2642    <t>
2643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2644      "Content-Location on 304 responses"
2645    </t>
2646    <t>
2647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2648      "private and no-cache CC directives with headers"
2649    </t>
2650    <t>
2651      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2652      "RFC2047 and warn-text"
2653    </t>
2654  </list>
2655</t>
2656</section>
2657
2658<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2659<t>
2660  Closed issues:
2661  <list style="symbols"> 
2662    <t>
2663      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2664      "serving negotiated responses from cache: header-specific canonicalization"
2665    </t>
2666    <t>
2667      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2668      "Effect of CC directives on history lists"
2669    </t>
2670    <t>
2671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2672      "Cache Extensions can override no-store, etc."
2673    </t>
2674  </list>
2675</t>
2676<t>
2677  Affected issues:
2678  <list style="symbols">
2679    <t>
2680      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2681      Status codes and caching
2682    </t>
2683  </list>
2684</t>
2685<t>
2686  Partly resolved issues:
2687  <list style="symbols"> 
2688    <t>
2689      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2690      "Placement of 13.5.1 and 13.5.2"
2691    </t>
2692  </list>
2693</t>
2694</section>
2695
2696<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2697<t>
2698  Closed issues:
2699  <list style="symbols"> 
2700    <t>
2701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2702      "Age calculation"
2703    </t>
2704    <t>
2705      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2706      "Clarify differences between / requirements for request and response CC directives"
2707    </t>
2708    <t>
2709      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2710      "Caching authenticated responses"
2711    </t>
2712    <t>
2713      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2714      "IANA registry for cache-control directives"
2715    </t>
2716    <t>
2717      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2718      "Heuristic caching of URLs with query components"
2719    </t>
2720  </list>
2721</t>
2722<t>
2723  Partly resolved issues:
2724  <list style="symbols"> 
2725    <t>
2726      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2727      "Term for the requested resource's URI"
2728    </t>
2729  </list>
2730</t>
2731</section>
2732
2733<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2734<t>
2735  Closed issues:
2736  <list style="symbols"> 
2737    <t>
2738      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2739      "Clarify entity / representation / variant terminology"
2740    </t>
2741    <t>
2742      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2743      "consider removing the 'changes from 2068' sections"
2744    </t>
2745    <t>
2746      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2747      "Allowing heuristic caching for new status codes"
2748    </t>
2749    <t>
2750      Clean up TODOs and prose in "Combining Responses."
2751    </t>
2752  </list>
2753</t>
2754</section>
2755
2756<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2757<t>
2758  Closed issues:
2759  <list style="symbols"> 
2760    <t>
2761      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2762      "Text about clock requirement for caches belongs in p6"
2763    </t>
2764  </list>
2765</t>
2766</section>
2767
2768<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2769<t>
2770  Closed issues:
2771  <list style="symbols"> 
2772    <t>
2773      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2774      "Header Classification"
2775    </t>
2776    <t>
2777      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2778      "Clarify 'public'"
2779    </t>
2780  </list>
2781</t>
2782</section>
2783
2784<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2785<t>
2786  Closed issues:
2787  <list style="symbols">
2788    <t>
2789      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2790      "untangle ABNFs for header fields"
2791    </t>
2792  </list>
2793</t>
2794</section>
2795
2796<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2797<t>
2798  Closed issues:
2799  <list style="symbols">
2800    <t>
2801      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2802      "Mismatch Vary"
2803    </t>
2804    <t>
2805      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2806      "Cache Invalidation only happens upon successful responses"
2807    </t>
2808    <t>
2809      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2810      "Recommend minimum sizes for protocol elements"
2811    </t>
2812    <t>
2813      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2814      "Proxies don't 'understand' methods"
2815    </t>
2816    <t>
2817      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2818      "Cache Extensions can override no-store, etc."
2819    </t>
2820    <t>
2821      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2822      "Pragma"
2823    </t>
2824  </list>
2825</t>
2826</section>
2827
2828<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2829<t>
2830  None yet.
2831</t>
2832</section>
2833
2834</section>
2835  </back>
2836</rfc>
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