source: draft-ietf-httpbis/16/draft-ietf-httpbis-p6-cache-16.xml @ 1831

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fix mime types

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc category="std" docName="draft-ietf-httpbis-p6-cache-16" ipr="pre5378Trust200902" obsoletes="2616">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
23
24  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
25    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
26    <address>
27      <postal>
28        <street>345 Park Ave</street>
29        <city>San Jose</city>
30        <region>CA</region>
31        <code>95110</code>
32        <country>USA</country>
33      </postal>
34      <email>fielding@gbiv.com</email>
35      <uri>http://roy.gbiv.com/</uri>
36    </address>
37  </author>
38
39  <author initials="J." surname="Gettys" fullname="Jim Gettys">
40    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
41    <address>
42      <postal>
43        <street>21 Oak Knoll Road</street>
44        <city>Carlisle</city>
45        <region>MA</region>
46        <code>01741</code>
47        <country>USA</country>
48      </postal>
49      <email>jg@freedesktop.org</email>
50      <uri>http://gettys.wordpress.com/</uri>
51    </address>
52  </author>
53
54  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
55    <organization abbrev="HP">Hewlett-Packard Company</organization>
56    <address>
57      <postal>
58        <street>HP Labs, Large Scale Systems Group</street>
59        <street>1501 Page Mill Road, MS 1177</street>
60        <city>Palo Alto</city>
61        <region>CA</region>
62        <code>94304</code>
63        <country>USA</country>
64      </postal>
65      <email>JeffMogul@acm.org</email>
66    </address>
67  </author>
68
69  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
70    <organization abbrev="Microsoft">Microsoft Corporation</organization>
71    <address>
72      <postal>
73        <street>1 Microsoft Way</street>
74        <city>Redmond</city>
75        <region>WA</region>
76        <code>98052</code>
77        <country>USA</country>
78      </postal>
79      <email>henrikn@microsoft.com</email>
80    </address>
81  </author>
82
83  <author fullname="Larry Masinter" initials="L." surname="Masinter">
84    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
85    <address>
86      <postal>
87        <street>345 Park Ave</street>
88        <city>San Jose</city>
89        <region>CA</region>
90        <code>95110</code>
91        <country>USA</country>
92      </postal>
93      <email>LMM@acm.org</email>
94      <uri>http://larry.masinter.net/</uri>
95    </address>
96  </author>
97
98  <author fullname="Paul J. Leach" initials="P." surname="Leach">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106      </postal>
107      <email>paulle@microsoft.com</email>
108    </address>
109  </author>
110
111  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
112    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
113    <address>
114      <postal>
115        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
116        <street>The Stata Center, Building 32</street>
117        <street>32 Vassar Street</street>
118        <city>Cambridge</city>
119        <region>MA</region>
120        <code>02139</code>
121        <country>USA</country>
122      </postal>
123      <email>timbl@w3.org</email>
124      <uri>http://www.w3.org/People/Berners-Lee/</uri>
125    </address>
126  </author>
127
128  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
129    <organization abbrev="W3C">World Wide Web Consortium</organization>
130    <address>
131      <postal>
132        <street>W3C / ERCIM</street>
133        <street>2004, rte des Lucioles</street>
134        <city>Sophia-Antipolis</city>
135        <region>AM</region>
136        <code>06902</code>
137        <country>France</country>
138      </postal>
139      <email>ylafon@w3.org</email>
140      <uri>http://www.raubacapeu.net/people/yves/</uri>
141    </address>
142  </author>
143
144  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
145    <address>
146      <email>mnot@mnot.net</email>
147      <uri>http://www.mnot.net/</uri>
148    </address>
149  </author>
150
151  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
152    <organization abbrev="greenbytes">greenbytes GmbH</organization>
153    <address>
154      <postal>
155        <street>Hafenweg 16</street>
156        <city>Muenster</city><region>NW</region><code>48155</code>
157        <country>Germany</country>
158      </postal>
159      <phone>+49 251 2807760</phone>
160      <facsimile>+49 251 2807761</facsimile>
161      <email>julian.reschke@greenbytes.de</email>
162      <uri>http://greenbytes.de/tech/webdav/</uri>
163    </address>
164  </author>
165
166  <date month="August" year="2011" day="24"/>
167  <workgroup>HTTPbis Working Group</workgroup>
168
169<abstract>
170<t>
171   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
172   distributed, collaborative, hypertext information systems. HTTP has been in
173   use by the World Wide Web global information initiative since 1990. This
174   document is Part 6 of the seven-part specification that defines the protocol
175   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
176</t>
177<t>
178   Part 6 defines requirements on HTTP caches and the associated header fields
179   that control cache behavior or indicate cacheable response messages.
180</t>
181</abstract>
182
183<note title="Editorial Note (To be removed by RFC Editor)">
184  <t>
185    Discussion of this draft should take place on the HTTPBIS working group
186    mailing list (ietf-http-wg@w3.org), which is archived at
187    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
188  </t>
189  <t>
190    The current issues list is at
191    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
192    documents (including fancy diffs) can be found at
193    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
194  </t>
195  <t>
196    The changes in this draft are summarized in <xref target="changes.since.15"/>.
197  </t>
198</note>
199
200   </front>
201   <middle>
202
203<section anchor="caching" title="Introduction">
204<t>
205   HTTP is typically used for distributed information systems, where
206   performance can be improved by the use of response caches. This document
207   defines aspects of HTTP/1.1 related to caching and reusing response
208   messages.
209</t>
210
211<section anchor="intro.purpose" title="Purpose">
212<iref item="cache"/>
213<t>
214   An HTTP cache is a local store of response messages and the
215   subsystem that controls its message storage, retrieval, and deletion. A
216   cache stores cacheable responses in order to reduce the response time and
217   network bandwidth consumption on future, equivalent requests. Any client or
218   server MAY employ a cache, though a cache cannot be used by a server that
219   is acting as a tunnel.
220</t>
221<t>
222   The goal of caching in HTTP/1.1 is to significantly improve performance
223   by reusing a prior response message to satisfy a current request.
224   A stored response is considered "fresh", as defined in
225   <xref target="expiration.model"/>, if the response can be reused without
226   "validation" (checking with the origin server to see if the cached response
227   remains valid for this request).  A fresh cache response can therefore
228   reduce both latency and network transfers each time it is reused.
229   When a cached response is not fresh, it might still be reusable if it can
230   be freshened by validation (<xref target="validation.model"/>) or if the
231   origin is unavailable.
232</t>
233</section>
234
235<section anchor="intro.terminology" title="Terminology">
236<t>
237   This specification uses a number of terms to refer to the roles played by
238   participants in, and objects of, HTTP caching.
239</t>
240<t>
241   <iref item="cache"/>
242   <?rfc needLines="4"?>cache
243   <list>
244      <t>A conformant implementation of a HTTP cache. Note that this implies
245        an HTTP/1.1 cache; this specification does not define conformance
246        for HTTP/1.0 caches.</t>
247   </list>
248</t>
249<t anchor="shared.and.non-shared.caches">
250   <iref item="shared cache"/>
251   <?rfc needLines="4"?>shared cache
252   <list>
253      <t>A cache that stores responses to be reused by more than one user;
254         usually (but not always) deployed as part of an intermediary.</t>
255   </list>
256</t>
257<t>
258   <iref item="private cache"/>
259   <?rfc needLines="4"?>private cache
260   <list>
261      <t>A cache that is dedicated to a single user.</t>
262   </list>
263</t>
264<t>
265   <iref item="cacheable"/>
266   <?rfc needLines="4"?>cacheable
267   <list>
268      <t>A response is cacheable if a cache is allowed to store a copy of the
269      response message for use in answering subsequent requests. Even when a
270      response is cacheable, there might be additional constraints on whether
271      a cache can use the stored copy to satisfy a particular request.</t>
272   </list>
273</t>
274<t>
275   <iref item="explicit expiration time"/>
276   <?rfc needLines="4"?>explicit expiration time
277   <list>
278      <t>The time at which the origin server intends that a representation
279      no longer be returned by a cache without further validation.</t>
280   </list>
281</t>
282<t>
283   <iref item="heuristic expiration time"/>
284   <?rfc needLines="4"?>heuristic expiration time
285   <list>
286      <t>An expiration time assigned by a cache when no explicit expiration
287      time is available.</t>
288   </list>
289</t>
290<t>
291   <iref item="age"/>
292   <?rfc needLines="4"?>age
293   <list>
294      <t>The age of a response is the time since it was sent by, or
295      successfully validated with, the origin server.</t>
296   </list>
297</t>
298<t>
299   <iref item="first-hand"/>
300   <?rfc needLines="4"?>first-hand
301   <list>
302      <t>A response is first-hand if the freshness model is not in use; i.e.,
303      its age is 0.</t>
304   </list>
305</t>
306<t>
307   <iref item="freshness lifetime"/>
308   <?rfc needLines="4"?>freshness lifetime
309   <list>
310      <t>The length of time between the generation of a response and its
311      expiration time.</t>
312   </list>
313</t>
314<t>
315   <iref item="fresh"/>
316   <?rfc needLines="4"?>fresh
317   <list>
318      <t>A response is fresh if its age has not yet exceeded its freshness
319      lifetime.</t>
320   </list>
321</t>
322<t>
323   <iref item="stale"/>
324   <?rfc needLines="4"?>stale
325   <list>
326      <t>A response is stale if its age has passed its freshness lifetime
327      (either explicit or heuristic).</t>
328   </list>
329</t>
330<t>
331   <iref item="validator"/>
332   <?rfc needLines="4"?>validator
333   <list>
334      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
335      is used to find out whether a stored response is an equivalent copy of
336      a representation. See Section 2.1 of <xref target="Part4"/>.</t>
337   </list>
338</t>
339<t>
340   <iref item="strong validator"/>
341   <iref item="validator" subitem="strong"/>
342   <?rfc needLines="4"?>strong validator
343   <list>
344      <t>A validator that is defined by the origin server such that its
345         current value will change if the representation body changes; i.e.,
346         an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
347         if no entity-tag is provided, a Last-Modified value that is strong
348         in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
349   </list>
350</t>
351</section>
352
353<section anchor="intro.requirements" title="Requirements">
354<t>
355   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
356   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
357   document are to be interpreted as described in <xref target="RFC2119"/>.
358</t>
359<t>
360   An implementation is not compliant if it fails to satisfy one or more of
361   the "MUST" or "REQUIRED" level requirements for the protocols it
362   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
363   level and all the "SHOULD" level requirements for its protocols is said to
364   be "unconditionally compliant"; one that satisfies all the "MUST" level
365   requirements but not all the "SHOULD" level requirements for its protocols
366   is said to be "conditionally compliant".
367</t>
368</section>
369
370<section title="Syntax Notation" anchor="notation">
371   
372   
373   
374   
375   
376   
377   
378   
379   
380<t>
381   This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
382   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
383   <xref target="collected.abnf"/> shows the collected ABNF, with the list
384   rule expanded.
385</t>
386<t>
387   The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
388   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
389   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
390   sequence of data), SP (space), VCHAR (any visible USASCII character), and
391   WSP (whitespace).
392</t>
393
394<section title="Core Rules" anchor="core.rules">
395   
396   
397   
398<t>
399   The core rules below are defined in <xref target="Part1"/>:
400</t>
401<figure><artwork type="abnf2616"><![CDATA[
402  OWS           = <OWS, defined in [Part1], Section 1.2.2>
403  quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
404  token         = <token, defined in [Part1], Section 3.2.3>
405]]></artwork></figure>
406</section>
407
408<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
409   
410   
411   
412   
413   
414<t>
415   The ABNF rules below are defined in other parts:
416</t>
417<figure><artwork type="abnf2616"><![CDATA[
418  field-name    = <field-name, defined in [Part1], Section 3.2>
419  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
420  port          = <port, defined in [Part1], Section 2.7>
421  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
422  uri-host      = <uri-host, defined in [Part1], Section 2.7>
423]]></artwork></figure>
424</section>
425</section>
426
427<section title="Delta Seconds" anchor="delta-seconds">
428<t>
429   The delta-seconds rule specifies a non-negative integer, representing time
430   in seconds.
431</t>
432<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
433  delta-seconds  = 1*DIGIT
434]]></artwork></figure>
435<t>
436   If an implementation receives a delta-seconds value larger than the largest
437   positive integer it can represent, or if any of its subsequent calculations
438   overflows, it MUST consider the value to be 2147483648 (2^31).
439   Recipients parsing a delta-seconds value SHOULD use an arithmetic type of
440   at least 31 bits of range, and senders MUST NOT send delta-seconds with a
441   value greater than 2147483648.
442</t>
443</section>
444
445</section>
446
447<section anchor="caching.overview" title="Cache Operation">
448<iref item="cache entry"/>
449<iref item="cache key"/>
450<t>
451   Proper cache operation preserves the semantics of HTTP transfers
452   (<xref target="Part2"/>) while eliminating the transfer of information already held
453   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
454   we assume that reusing the cached response is desirable and that such
455   reuse is the default behavior when no requirement or locally-desired
456   configuration prevents it.  Therefore, HTTP cache requirements are focused
457   on preventing a cache from either storing a non-reusable response or
458   reusing a stored response inappropriately.
459</t>
460<t>
461   Each cache entry consists of a cache key and one or more
462   HTTP responses corresponding to prior requests that used the same key.
463   The most common form of cache entry is a successful result of a retrieval
464   request: i.e., a 200 (OK) response containing a representation of the
465   resource identified by the request target.  However, it is also possible
466   to cache negative results (e.g., 404 not found), incomplete results
467   (e.g., 206 partial content), and responses to safe methods other than
468   GET if the method's definition allows such caching and defines something
469   suitable for use as a cache key.
470</t>
471<t>
472   The default cache key consists of the request method and
473   target URI.  However, since HTTP caches in common use today are typically
474   limited to caching responses to GET, most implementations simply decline
475   other methods and use only the URI as the key.
476</t>
477<t>
478   If a request target is subject to content negotiation, its cache entry
479   might consist of multiple stored responses, each differentiated by a
480   secondary key for the values of the original request's selecting header
481   fields (<xref target="caching.negotiated.responses"/>).
482</t>
483
484<section anchor="response.cacheability" title="Response Cacheability">
485<t>
486   A cache MUST NOT store a response to any request, unless:
487   <list style="symbols">
488      <t>The request method is understood by the cache and defined as being
489      cacheable, and</t>
490      <t>the response status code is understood by the cache, and</t>
491      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
492      header fields, and</t>
493      <t>the "private" cache response directive (see <xref target="cache-response-directive"/> does not appear in the response, if
494      the cache is shared, and</t>
495      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
496      appear in the request, if the cache is shared, unless the response
497      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
498      <t>the response either:
499         <list style="symbols">
500            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
501            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
502            <t>contains a s-maxage response cache directive and the cache is
503            shared, or</t>
504            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
505            or</t>
506            <t>has a status code that can be served with heuristic freshness
507            (see <xref target="heuristic.freshness"/>).</t>
508         </list>
509      </t>
510   </list>
511</t>
512<t>
513   Note that any of the requirements listed above can be overridden by a
514   cache-control extension; see <xref target="cache.control.extensions"/>.
515</t>
516<t>
517   In this context, a cache has "understood" a request method or a response
518   status code if it recognizes it and implements any cache-specific
519   behavior.
520</t>
521<t>
522   Note that, in normal operation, most caches will not store a response that
523   has neither a cache validator nor an explicit expiration time, as such
524   responses are not usually useful to store. However, caches are not
525   prohibited from storing such responses.
526</t>
527<t>
528   A response message is considered complete when all of the octets
529   indicated by the message framing (<xref target="Part1"/>) are received
530   prior to the connection being closed.
531   If the request is GET, the response status is 200 (OK), and the entire
532   response header block has been received, a cache MAY store an incomplete
533   response message-body if the cache entry is recorded as incomplete.
534   Likewise, a 206 (Partial Content) response MAY be stored as if it were
535   an incomplete 200 (OK) cache entry.  However, a cache MUST NOT store
536   incomplete or partial content responses if it does not support the Range
537   and Content-Range header fields or if it does not understand the
538   range units used in those fields.
539</t>
540<t>
541   A cache MAY complete a stored incomplete response by making a subsequent
542   range request (<xref target="Part5"/>) and combining the successful response with the
543   stored entry, as defined in <xref target="combining.responses"/>.
544   A cache MUST NOT use an incomplete response to answer requests
545   unless the response has been made complete or the request is partial and
546   specifies a range that is wholly within the incomplete response.
547   A cache MUST NOT send a partial response to a client without explicitly
548   marking it as such using the 206 (Partial Content) status code.
549</t>
550</section>
551
552
553<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
554<t>
555   For a presented request, a cache MUST NOT return a stored response,
556   unless:
557   <list style="symbols">
558      <t>The presented effective request URI (Section 4.3 of <xref target="Part1"/>) and
559      that of the stored response match, and</t>
560      <t>the request method associated with the stored response allows it to
561      be used for the presented request, and</t>
562      <t>selecting header fields nominated by the stored response (if any)
563      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
564      <t>the presented request and stored response are free from directives
565      that would prevent its use (see <xref target="header.cache-control"/>
566      and <xref target="header.pragma"/>), and</t>
567      <t>the stored response is either:
568         <list style="symbols">
569            <t>fresh (see <xref target="expiration.model"/>), or</t>
570            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
571            <t>successfully validated (see <xref target="validation.model"/>).</t>
572         </list>
573      </t>
574  </list>
575</t>
576<t>
577   Note that any of the requirements listed above can be overridden by a
578   cache-control extension; see <xref target="cache.control.extensions"/>.
579</t>
580<t>
581   When a stored response is used to satisfy a request without validation,
582   a cache MUST include a single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
583   current_age; see <xref target="age.calculations"/>.
584</t>
585<t>
586   A cache MUST write through requests with methods that are unsafe
587   (Section 7.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
588   a reply to such a request before having forwarded the request and having
589   received a corresponding response.
590</t>
591<t>
592   Also, note that unsafe requests might invalidate already stored responses;
593   see <xref target="invalidation.after.updates.or.deletions"/>.
594</t>
595<t>
596   When more than one suitable response is stored, a cache MUST use the
597   most recent response (as determined by the Date header field). It can also
598   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
599   no-cache" to disambiguate which response to use.
600</t>
601<t>
602   A cache that does not have a clock available MUST NOT use stored responses
603   without revalidating them on every use. A cache, especially a shared
604   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
605   synchronize its clock with a reliable external standard.
606</t>
607
608</section>
609
610<section anchor="expiration.model" title="Freshness Model">
611<t>
612   When a response is "fresh" in the cache, it can be used to satisfy
613   subsequent requests without contacting the origin server, thereby improving
614   efficiency.
615</t>
616<t>
617   The primary mechanism for determining freshness is for an origin server to
618   provide an explicit expiration time in the future, using either the Expires
619   header field (<xref target="header.expires"/>) or the max-age response cache
620   directive (<xref target="cache-response-directive"/>). Generally, origin
621   servers will assign future explicit expiration times to responses in the
622   belief that the representation is not likely to change in a semantically
623   significant way before the expiration time is reached.
624</t>
625<t>
626   If an origin server wishes to force a cache to validate every request, it
627   can assign an explicit expiration time in the past to indicate that the
628   response is already stale. Compliant caches will normally validate the
629   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
630</t>
631<t>
632   Since origin servers do not always provide explicit expiration times,
633   a cache MAY assign a heuristic expiration time when an explicit time is not
634   specified, employing algorithms that use other header field values (such as the
635   Last-Modified time) to estimate a plausible expiration time. This
636   specification does not provide specific algorithms, but does impose
637   worst-case constraints on their results.
638</t>
639<figure>
640<preamble>
641  The calculation to determine if a response is fresh is:
642</preamble>
643<artwork type="code"><![CDATA[
644   response_is_fresh = (freshness_lifetime > current_age)
645]]></artwork>
646</figure>
647<t>
648   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
649   <xref target="age.calculations"/>.
650</t>
651<t>
652   Additionally, clients might need to influence freshness calculation. They
653   can do this using several request cache directives, with the effect of
654   either increasing or loosening constraints on freshness. See <xref target="cache-request-directive"/>.
655</t>
656<t>
657   Note that freshness applies only to cache operation; it cannot be used to
658   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
659   caches and history mechanisms.
660</t>
661
662<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
663<t>
664   A cache can calculate the freshness lifetime (denoted as
665   freshness_lifetime) of a response by using the first match of:
666   <list style="symbols">
667      <t>If the cache is shared and the s-maxage response cache directive
668      (<xref target="cache-response-directive"/>) is present, use its value,
669      or</t>
670      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
671      <t>If the Expires response header field (<xref target="header.expires"/>) is
672      present, use its value minus the value of the Date response header field,
673      or</t>
674      <t>Otherwise, no explicit expiration time is present in the response. A
675      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
676   </list>
677</t>
678<t>
679   Note that this calculation is not vulnerable to clock skew, since all of
680   the information comes from the origin server.
681</t>
682
683<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
684<t>
685   If no explicit expiration time is present in a stored response that has a
686   status code whose definition allows heuristic freshness to be used
687   (including the following in Section 8 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
688   410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
689   use heuristics to determine freshness for responses with status codes that do
690   not explicitly allow it.
691</t>
692<t>
693   When a heuristic is used to calculate freshness lifetime, a cache
694   SHOULD attach a Warning header field with a 113 warn-code to the response if
695   its current_age is more than 24 hours and such a warning is not already
696   present.
697</t>
698<t>
699   Also, if the response has a Last-Modified header field (Section 2.2 of <xref target="Part4"/>),
700   a cache SHOULD NOT use a heuristic expiration value that is more than some
701   fraction of the interval since that time. A typical setting of this fraction
702   might be 10%.
703</t>
704<t><list>
705   <t>
706      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
707      freshness for URIs with query components (i.e., those containing '?').
708      In practice, this has not been widely implemented. Therefore, servers
709      are encouraged to send explicit directives (e.g., Cache-Control:
710      no-cache) if they wish to preclude caching.
711   </t>
712</list></t>
713</section>
714</section>
715
716<section anchor="age.calculations" title="Calculating Age">
717<t>
718   HTTP/1.1 uses the Age header field to convey the estimated age of the
719   response message when obtained from a cache. The Age field value is the
720   cache's estimate of the amount of time since the response was generated or
721   validated by the origin server. In essence, the Age value is the sum of the
722   time that the response has been resident in each of the caches along the
723   path from the origin server, plus the amount of time it has been in transit
724   along network paths.
725</t>
726<t>
727   The following data is used for the age calculation:
728</t>
729<t>
730   <?rfc needLines="4"?>age_value
731   <list>
732      <t>
733         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
734         operation; or 0, if not available.
735      </t>
736   </list>
737</t>
738<t>
739   <?rfc needLines="4"?>date_value
740   <list>
741      <t>
742         HTTP/1.1 requires origin servers to send a Date header field, if possible,
743         with every response, giving the time at which the response was
744         generated. The term "date_value" denotes the value of the Date
745         header field, in a form appropriate for arithmetic operations. See
746         Section 9.3 of <xref target="Part1"/> for the definition of the Date header field, and for
747         requirements regarding responses without it.
748      </t>
749   </list>
750</t>
751<t>
752   <?rfc needLines="4"?>now
753   <list>
754      <t>
755         The term "now" means "the current value of the clock at the host
756         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
757         clocks to a globally accurate time standard.
758      </t>
759   </list>
760</t>
761<t>
762   <?rfc needLines="4"?>request_time
763   <list>
764      <t>
765         The current value of the clock at the host at the time the request
766         resulting in the stored response was made.
767      </t>
768   </list>
769</t>
770<t>
771   <?rfc needLines="4"?>response_time
772   <list>
773      <t>
774         The current value of the clock at the host at the time the response
775         was received.
776      </t>
777   </list>
778</t>
779<t>
780   A response's age can be calculated in two entirely independent ways:
781   <list style="numbers">
782      <t>the "apparent_age": response_time minus date_value, if the local
783      clock is reasonably well synchronized to the origin server's clock. If
784      the result is negative, the result is replaced by zero.</t>
785      <t>the "corrected_age_value", if all of the caches along the response
786      path implement HTTP/1.1. A cache MUST interpret this value relative
787      to the time the request was initiated, not the time that the response
788      was received.</t>
789   </list>
790</t>
791<figure>
792<artwork type="code"><![CDATA[
793  apparent_age = max(0, response_time - date_value);
794
795  response_delay = response_time - request_time;
796  corrected_age_value = age_value + response_delay; 
797]]></artwork>
798</figure>
799<figure>
800<preamble>These are combined as</preamble>
801<artwork type="code"><![CDATA[
802  corrected_initial_age = max(apparent_age, corrected_age_value);
803]]></artwork></figure>
804<t>
805   The current_age of a stored response can then be calculated by adding the
806   amount of time (in seconds) since the stored response was last validated by
807   the origin server to the corrected_initial_age.
808</t>
809<figure><artwork type="code"><![CDATA[
810  resident_time = now - response_time;
811  current_age = corrected_initial_age + resident_time;
812]]></artwork></figure>
813<t>
814   Additional rules for requirements on parsing and encoding of dates
815   and other potential problems with date encodings include:
816</t>
817<t>
818  <list style="symbols">
819     <t>HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
820        which appears to be more than 50 years in the future is in fact
821        in the past (this helps solve the "year 2000" problem).</t>
822
823     <t>Although all date formats are specified to be case-sensitive,
824        recipients SHOULD match day, week and timezone names
825        case-insensitively.</t>
826             
827     <t>An HTTP/1.1 implementation MAY internally represent a parsed
828        Expires date as earlier than the proper value, but MUST NOT
829        internally represent a parsed Expires date as later than the
830        proper value.</t>
831
832     <t>All expiration-related calculations MUST be done in GMT. The
833        local time zone MUST NOT influence the calculation or comparison
834        of an age or expiration time.</t>
835
836     <t>If an HTTP header field incorrectly carries a date value with a time
837        zone other than GMT, it MUST be converted into GMT using the
838        most conservative possible conversion.</t>
839  </list>
840</t>
841</section>
842
843<section anchor="serving.stale.responses" title="Serving Stale Responses">
844<t>
845   A "stale" response is one that either has explicit expiry information or is
846   allowed to have heuristic expiry calculated, but is not fresh according to
847   the calculations in <xref target="expiration.model"/>.
848</t>
849<t>
850   A cache MUST NOT return a stale response if it is prohibited by an
851   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
852   directive, a "must-revalidate" cache-response-directive, or an applicable
853   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
854</t>
855<t>
856   A cache SHOULD NOT return stale responses unless it is disconnected
857   (i.e., it cannot contact the origin server or otherwise find a forward
858   path) or doing so is explicitly allowed (e.g., by the max-stale request
859   directive; see <xref target="cache-request-directive"/>).
860</t>
861<t>
862   A cache SHOULD append a Warning header field with the 110 warn-code (see
863   <xref target="header.warning"/>) to stale responses. Likewise, a cache
864   SHOULD add the 112 warn-code to stale responses if the cache is
865   disconnected.
866</t>
867<t>
868   If a cache receives a first-hand response (either an entire response, or a
869   304 (Not Modified) response) that it would normally forward to the
870   requesting client, and the received response is no longer fresh, the cache
871   SHOULD forward it to the requesting client without adding a new Warning
872   (but without removing any existing Warning header fields). A cache SHOULD NOT
873   attempt to validate a response simply because that response became stale in
874   transit.
875</t>
876</section>
877</section>
878
879<section anchor="validation.model" title="Validation Model">
880<t>
881   When a cache has one or more stored responses for a requested URI, but
882   cannot serve any of them (e.g., because they are not fresh, or one cannot
883   be selected; see <xref target="caching.negotiated.responses"/>), it can use
884   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
885   give the origin server an opportunity to both select a valid stored
886   response to be used, and to update it. This process is known as
887   "validating" or "revalidating" the stored response.
888</t>
889<t>
890   When sending such a conditional request, a cache SHOULD add an
891   If-Modified-Since header field whose value is that of the Last-Modified header
892   field from the selected (see <xref target="caching.negotiated.responses"/>)
893   stored response, if available.
894</t>
895<t>
896   Additionally, a cache SHOULD add an If-None-Match header field whose value is
897   that of the ETag header field(s) from all responses stored for the requested URI,
898   if present. However, if any of the stored responses contains only partial
899   content, the cache SHOULD NOT include its entity-tag in the If-None-Match
900   header field unless the request is for a range that would be fully
901   satisfied by that stored response.
902</t>
903<t>
904   A 304 (Not Modified) response status code indicates that the stored
905   response can be updated and reused; see <xref target="freshening.responses"/>.
906</t>
907<t>
908   A full response (i.e., one with a response body) indicates that none of the
909   stored responses nominated in the conditional request is suitable. Instead,
910   a cache SHOULD use the full response to satisfy the request and MAY
911   replace the stored response(s).
912</t>
913<t>
914   If a cache receives a 5xx response while attempting to validate a response,
915   it MAY either forward this response to the requesting client, or act as
916   if the server failed to respond. In the latter case, it MAY return a
917   previously stored response (see <xref target="serving.stale.responses"/>).
918</t>
919</section>
920
921<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
922<t>
923   Because unsafe request methods (Section 7.1.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
924   have the potential for changing state on the origin server, intervening
925   caches can use them to keep their contents up-to-date.
926</t>
927<t>
928   A cache MUST invalidate the effective Request URI
929   (Section 4.3 of <xref target="Part1"/>) as well as the URI(s) in the Location
930   and Content-Location header fields (if present) when a non-error
931   response to a request with an unsafe method is received.
932</t>
933<t>
934   However, a cache MUST NOT invalidate a URI from a
935   Location or Content-Location header field if the host part of that URI
936   differs from the host part in the effective request URI
937   (Section 4.3 of <xref target="Part1"/>). This helps prevent denial of service attacks.
938</t>
939<t>
940   A cache SHOULD invalidate the effective request URI
941   (Section 4.3 of <xref target="Part1"/>) when it receives a non-error response
942   to a request with a method whose safety is unknown.
943</t>
944<t>
945   Here, a "non-error response" is one with a 2xx or 3xx status code.
946   "Invalidate" means that the cache will either remove all stored
947   responses related to the effective request URI, or will mark these as
948   "invalid" and in need of a mandatory validation before they can be returned
949   in response to a subsequent request.
950</t>
951<t>
952   Note that this does not guarantee that all appropriate responses are
953   invalidated. For example, the request that caused the change at the origin
954   server might not have gone through the cache where a response is stored.
955</t>
956</section>
957
958<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
959
960<t>
961   A shared cache MUST NOT use a cached response to a request with an
962   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
963   request unless a cache directive that allows such responses to be stored is
964   present in the response.
965</t>
966
967<t>
968   In this specification, the following Cache-Control response directives
969   (<xref target="cache-response-directive"/>) have such an effect:
970   must-revalidate, public, s-maxage.
971</t>
972
973<t>
974   Note that cached responses that contain the "must-revalidate" and/or
975   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
976   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
977   used to satisfy a subsequent request without revalidating it on the origin
978   server.
979</t>
980</section>
981
982<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
983<t>
984   When a cache receives a request that can be satisfied by a stored response
985   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
986   use that response unless all of the selecting header fields nominated by
987   the Vary header field match in both the original request (i.e., that associated
988   with the stored response), and the presented request.
989</t>
990<t>
991   The selecting header fields from two requests are defined to match if and
992   only if those in the first request can be transformed to those in the
993   second request by applying any of the following:
994   <list style="symbols">
995      <t>
996         adding or removing whitespace, where allowed in the header field's syntax
997      </t>
998      <t>
999         combining multiple header fields with the same field name
1000         (see Section 3.2 of <xref target="Part1"/>)
1001      </t>
1002      <t>
1003         normalizing both header field values in a way that is known to have
1004         identical semantics, according to the header field's specification (e.g.,
1005         re-ordering field values when order is not significant;
1006         case-normalization, where values are defined to be case-insensitive)
1007      </t>
1008  </list>
1009</t>
1010<t>
1011   If (after any normalization that might take place) a header field is absent
1012   from a request, it can only match another request if it is also absent
1013   there.
1014</t>
1015<t>
1016   A Vary header field-value of "*" always fails to match, and subsequent
1017   requests to that resource can only be properly interpreted by the origin
1018   server.
1019</t>
1020<t>
1021   The stored response with matching selecting header fields is known as the
1022   selected response.
1023</t>
1024<t>
1025   If multiple selected responses are available, the most recent response
1026   (as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
1027</t>
1028<t>
1029   If no selected response is available, the cache MAY forward the presented
1030   request to the origin server in a conditional request; see <xref target="validation.model"/>.
1031</t>
1032</section>
1033
1034<section anchor="combining.responses" title="Combining Partial Content">
1035<t>
1036   A response might transfer only a partial representation if the
1037   connection closed prematurely or if the request used one or more Range
1038   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
1039   received several ranges of the same representation.  A cache MAY combine
1040   these ranges into a single stored response, and reuse that response to
1041   satisfy later requests, if they all share the same strong validator and
1042   the cache complies with the client requirements in Section 4 of <xref target="Part5"/>.
1043</t>
1044<t>
1045   When combining the new response with one or more stored responses, a
1046   cache MUST:
1047   <list style="symbols">
1048      <t>delete any Warning header fields in the stored response with
1049         warn-code 1xx (see <xref target="header.warning"/>);</t>
1050      <t>retain any Warning header fields in the stored response with
1051         warn-code 2xx; and,</t>
1052      <t>use other header fields provided in the new response, aside
1053         from Content-Range, to replace all instances of the corresponding
1054         header fields in the stored response.</t>
1055   </list>
1056</t>
1057</section>
1058
1059<section anchor="freshening.responses" title="Freshening Responses">
1060<t>
1061   When a cache receives a 304 (Not Modified) response and already has one
1062   or more stored 200 (OK) responses for the same cache key, the cache needs
1063   to identify which of the stored responses are updated by this new response
1064   and then update the stored response(s) with the new information provided in
1065   the 304 response.
1066   <list style="symbols">
1067    <t>
1068     If the new response contains a strong validator, then that strong
1069     validator identifies the selected representation.  All of the stored
1070     responses with the same strong validator are selected.
1071     If none of the stored responses contain the same strong validator, then
1072     this new response corresponds to a new selected representation and
1073     MUST NOT update the existing stored responses.
1074    </t>
1075    <t>
1076     If the new response contains a weak validator and that validator
1077     corresponds to one of the cache's stored responses, then the most
1078     recent of those matching stored responses is selected.
1079    </t>
1080    <t>
1081     If the new response does not include any form of validator, there is
1082     only one stored response, and that stored response also lacks a
1083     validator, then that stored response is selected.
1084    </t>
1085   </list>
1086</t>
1087<t>
1088   If a stored response is selected for update, the cache MUST:
1089   <list style="symbols">
1090      <t>delete any Warning header fields in the stored response with
1091         warn-code 1xx (see <xref target="header.warning"/>);</t>
1092      <t>retain any Warning header fields in the stored response with
1093         warn-code 2xx; and,</t>
1094      <t>use other header fields provided in the 304 response to replace
1095         all instances of the corresponding header fields in the stored
1096         response.</t>
1097   </list>
1098</t>
1099</section>
1100
1101</section>
1102
1103<section anchor="header.fields" title="Header Field Definitions">
1104<t>
1105   This section defines the syntax and semantics of HTTP/1.1 header fields
1106   related to caching.
1107</t>
1108
1109<section anchor="header.age" title="Age">
1110   <iref item="Age header field" primary="true"/>
1111   <iref item="Header Fields" primary="true" subitem="Age"/>
1112   
1113   
1114<t>
1115   The "Age" header field conveys the sender's estimate of the amount
1116   of time since the response was generated or successfully validated at the
1117   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1118</t>
1119<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1120  Age = delta-seconds
1121]]></artwork></figure>
1122<t>
1123  Age field-values are non-negative integers, representing time in seconds
1124  (see <xref target="delta-seconds"/>).
1125</t>
1126<t>
1127   The presence of an Age header field in a response implies that a response
1128   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1129   might not implement the Age header field.
1130</t>
1131</section>
1132
1133<section anchor="header.cache-control" title="Cache-Control">
1134   <iref item="Cache-Control header field" primary="true"/>
1135   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1136   
1137   
1138   
1139   
1140   
1141<t>
1142   The "Cache-Control" header field is used to specify directives for
1143   caches along the request/response chain. Such cache directives are
1144   unidirectional in that the presence of a directive in a request does not
1145   imply that the same directive is to be given in the response.
1146</t>
1147<t>
1148   A cache MUST obey the requirements of the Cache-Control
1149   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1150   directives defined elsewhere are handled.
1151</t>
1152<t><list>
1153   <t>
1154       Note: HTTP/1.0 caches might not implement Cache-Control and
1155       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1156   </t>
1157</list></t>
1158<t>
1159   A proxy, whether or not it implements a cache, MUST pass cache directives
1160   through in forwarded messages, regardless of their
1161   significance to that application, since the directives might be applicable
1162   to all recipients along the request/response chain. It is not possible to
1163   target a directive to a specific cache.
1164</t>
1165<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1166  Cache-Control   = 1#cache-directive
1167
1168  cache-directive = cache-request-directive
1169     / cache-response-directive
1170
1171  cache-extension = token [ "=" ( token / quoted-string ) ]
1172]]></artwork></figure>
1173
1174<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1175   
1176
1177<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1178  cache-request-directive =
1179       "no-cache"
1180     / "no-store"
1181     / "max-age" "=" delta-seconds
1182     / "max-stale" [ "=" delta-seconds ]
1183     / "min-fresh" "=" delta-seconds
1184     / "no-transform"
1185     / "only-if-cached"
1186     / cache-extension
1187]]></artwork></figure>
1188
1189<t>
1190   <?rfc needLines="4"?>no-cache
1191   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1192   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1193   <list>
1194      <t>The no-cache request directive indicates that a cache MUST NOT
1195      use a stored response to satisfy the request without successful
1196      validation on the origin server.</t>
1197   </list>
1198</t>
1199<t>
1200   <?rfc needLines="4"?>no-store
1201   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1202   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1203   <list>
1204      <t>The no-store request directive indicates that a cache MUST NOT
1205      store any part of either this request or any response to it. This
1206      directive applies to both private and shared caches. "MUST NOT
1207      store" in this context means that the cache MUST NOT intentionally
1208      store the information in non-volatile storage, and MUST make a
1209      best-effort attempt to remove the information from volatile storage as
1210      promptly as possible after forwarding it.</t>
1211      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1212      privacy. In particular, malicious or compromised caches might not
1213      recognize or obey this directive, and communications networks might be
1214      vulnerable to eavesdropping.</t>
1215      <t>Note that if a request containing this directive is satisfied from a
1216      cache, the no-store request directive does not apply to the already
1217      stored response.</t>
1218   </list>
1219</t>
1220<t>
1221   <?rfc needLines="4"?>max-age
1222   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1223   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1224   <list>
1225      <t>The max-age request directive indicates that the client is unwilling to
1226      accept a response whose age is greater than the specified number of
1227      seconds. Unless the max-stale request directive is also present, the
1228      client is not willing to accept a stale response.</t>
1229   </list>
1230</t>
1231<t>
1232   <?rfc needLines="4"?>max-stale
1233   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1234   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1235   <list>
1236      <t>The max-stale request directive indicates that the client is willing
1237      to accept a response that has exceeded its expiration time. If max-stale
1238      is assigned a value, then the client is willing to accept a response
1239      that has exceeded its expiration time by no more than the specified
1240      number of seconds. If no value is assigned to max-stale, then the client
1241      is willing to accept a stale response of any age.</t>
1242   </list>
1243</t>
1244<t>
1245   <?rfc needLines="4"?>min-fresh
1246   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1247   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1248   <list>
1249      <t>The min-fresh request directive indicates that the client is willing
1250      to accept a response whose freshness lifetime is no less than its
1251      current age plus the specified time in seconds. That is, the client
1252      wants a response that will still be fresh for at least the specified
1253      number of seconds.</t>
1254   </list>
1255</t>
1256<t>
1257   <?rfc needLines="4"?>no-transform
1258   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1259   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1260   <list>
1261      <t>The no-transform request directive indicates that an intermediary
1262        (whether or not it implements a cache) MUST NOT change the
1263        Content-Encoding, Content-Range or Content-Type request header fields,
1264        nor the request representation.</t>
1265   </list>
1266</t>
1267<t>
1268   <?rfc needLines="4"?>only-if-cached
1269   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1270   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1271   <list>
1272      <t>The only-if-cached request directive indicates that the client only
1273      wishes to obtain a stored response. If it receives this directive, a
1274      cache SHOULD either respond using a stored response that is consistent
1275      with the other constraints of the request, or respond with a 504
1276      (Gateway Timeout) status code. If a group of caches is being operated as
1277      a unified system with good internal connectivity, a member cache MAY
1278      forward such a request within that group of caches.</t>
1279   </list>
1280</t>
1281</section>
1282
1283<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1284   
1285
1286<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1287  cache-response-directive =
1288       "public"
1289     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1290     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1291     / "no-store"
1292     / "no-transform"
1293     / "must-revalidate"
1294     / "proxy-revalidate"
1295     / "max-age" "=" delta-seconds
1296     / "s-maxage" "=" delta-seconds
1297     / cache-extension
1298]]></artwork></figure>
1299
1300<t>
1301   <?rfc needLines="4"?>public
1302   <iref item="Cache Directives" primary="true" subitem="public"/>
1303   <iref item="public" primary="true" subitem="Cache Directive"/>
1304   <list>
1305      <t>The public response directive indicates that a response whose
1306        associated request contains an 'Authentication' header MAY be
1307        stored (see <xref target="caching.authenticated.responses"/>).</t>
1308  </list>
1309</t>
1310<t>
1311   <?rfc needLines="4"?>private
1312   <iref item="Cache Directives" primary="true" subitem="private"/>
1313   <iref item="private" primary="true" subitem="Cache Directive"/>
1314   <list>
1315      <t>The private response directive indicates that the response message is
1316      intended for a single user and MUST NOT be stored by a shared cache. A
1317      private cache MAY store the response.</t>
1318      <t>If the private response directive specifies one or more field-names,
1319      this requirement is limited to the field-values associated with the
1320      listed response header fields. That is, a shared cache MUST NOT store
1321      the specified field-names(s), whereas it MAY store the remainder of the
1322      response message.</t>
1323      <t> Note: This usage of the word private only controls where
1324      the response can be stored; it cannot ensure the privacy of the message
1325      content. Also, private response directives with field-names are often
1326      handled by implementations as if an unqualified private directive was
1327      received; i.e., the special handling for the qualified form is not
1328      widely implemented.</t>
1329   </list>
1330</t>
1331<t>
1332   <?rfc needLines="4"?>no-cache
1333   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1334   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1335   <list>
1336      <t>The no-cache response directive indicates that the response MUST NOT
1337      be used to satisfy a subsequent request without successful validation on
1338      the origin server. This allows an origin server to prevent a cache from
1339      using it to satisfy a request without contacting it, even by caches that
1340      have been configured to return stale responses.</t>
1341      <t>If the no-cache response directive specifies one or more field-names,
1342      this requirement is limited to the field-values associated with the
1343      listed response header fields. That is, a cache MUST NOT send the
1344      specified field-name(s) in the response to a subsequent request without successful
1345      validation on the origin server. This allows an origin server to prevent
1346      the re-use of certain header fields in a response, while still allowing
1347      caching of the rest of the response.</t>
1348      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1349      this directive. Also, no-cache response directives with field-names are
1350      often handled by implementations as if an unqualified no-cache directive
1351      was received; i.e., the special handling for the qualified form is not
1352      widely implemented. </t>
1353   </list>
1354</t>
1355<t>
1356   <?rfc needLines="4"?>no-store
1357   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1358   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1359   <list>
1360      <t>The no-store response directive indicates that a cache MUST NOT
1361      store any part of either the immediate request or response. This
1362      directive applies to both private and shared caches. "MUST NOT
1363      store" in this context means that the cache MUST NOT intentionally
1364      store the information in non-volatile storage, and MUST make a
1365      best-effort attempt to remove the information from volatile storage as
1366      promptly as possible after forwarding it.</t>
1367      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1368      privacy. In particular, malicious or compromised caches might not
1369      recognize or obey this directive, and communications networks might be
1370      vulnerable to eavesdropping.</t>
1371   </list>
1372</t>
1373<t>
1374   <?rfc needLines="4"?>must-revalidate
1375   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1376   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1377   <list>
1378      <t>The must-revalidate response directive indicates that once it has
1379      become stale, a cache MUST NOT use the response to satisfy subsequent
1380      requests without successful validation on the origin server.</t>
1381      <t>The must-revalidate directive is necessary to support reliable
1382      operation for certain protocol features. In all circumstances a
1383      cache MUST obey the must-revalidate directive; in particular,
1384      if a cache cannot reach the origin server for any reason, it MUST
1385      generate a 504 (Gateway Timeout) response.</t>
1386      <t>A server SHOULD send the must-revalidate directive if and only if
1387      failure to validate a request on the representation could result in
1388      incorrect operation, such as a silently unexecuted financial
1389      transaction.</t>
1390   </list>
1391</t>
1392<t>
1393   <?rfc needLines="4"?>proxy-revalidate
1394   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1395   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1396   <list>
1397      <t>The proxy-revalidate response directive has the same meaning as the
1398      must-revalidate response directive, except that it does not apply to
1399      private caches.</t>
1400   </list>
1401</t>
1402<t>
1403   <?rfc needLines="4"?>max-age
1404   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1405   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1406   <list>
1407      <t>The max-age response directive indicates that the response is to be
1408      considered stale after its age is greater than the specified number of
1409      seconds.</t>
1410   </list>
1411</t>
1412<t>
1413   <?rfc needLines="4"?>s-maxage
1414   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1415   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1416   <list>
1417      <t>The s-maxage response directive indicates that, in shared caches, the
1418      maximum age specified by this directive overrides the maximum age
1419      specified by either the max-age directive or the Expires header field. The
1420      s-maxage directive also implies the semantics of the proxy-revalidate
1421      response directive.</t>
1422   </list>
1423</t>
1424<t>
1425   <?rfc needLines="4"?>no-transform
1426   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1427   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1428   <list>
1429      <t>The no-transform response directive indicates that an intermediary
1430      (regardless of whether it implements a cache) MUST NOT change the
1431      Content-Encoding, Content-Range or Content-Type response header fields,
1432      nor the response representation.</t>
1433   </list>
1434</t>
1435
1436</section>
1437
1438<section anchor="cache.control.extensions" title="Cache Control Extensions">
1439<t>
1440   The Cache-Control header field can be extended through the use of one or
1441   more cache-extension tokens, each with an optional value. Informational
1442   extensions (those that do not require a change in cache behavior) can be
1443   added without changing the semantics of other directives. Behavioral
1444   extensions are designed to work by acting as modifiers to the existing base
1445   of cache directives. Both the new directive and the standard directive are
1446   supplied, such that applications that do not understand the new directive
1447   will default to the behavior specified by the standard directive, and those
1448   that understand the new directive will recognize it as modifying the
1449   requirements associated with the standard directive. In this way,
1450   extensions to the cache-control directives can be made without requiring
1451   changes to the base protocol.
1452</t>
1453<t>
1454   This extension mechanism depends on an HTTP cache obeying all of the
1455   cache-control directives defined for its native HTTP-version, obeying
1456   certain extensions, and ignoring all directives that it does not
1457   understand.
1458</t>
1459<t>
1460   For example, consider a hypothetical new response directive called
1461   "community" that acts as a modifier to the private directive. We define
1462   this new directive to mean that, in addition to any private cache, any
1463   cache that is shared only by members of the community named within its
1464   value may cache the response. An origin server wishing to allow the UCI
1465   community to use an otherwise private response in their shared cache(s)
1466   could do so by including
1467</t>
1468<figure><artwork type="example"><![CDATA[
1469  Cache-Control: private, community="UCI"
1470]]></artwork></figure>
1471<t>
1472   A cache seeing this header field will act correctly even if the cache does
1473   not understand the community cache-extension, since it will also see and
1474   understand the private directive and thus default to the safe behavior.
1475</t>
1476<t>
1477   A cache MUST ignore unrecognized cache directives; it is assumed that any
1478   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1479   combined with standard directives (or the response's default cacheability)
1480   such that the cache behavior will remain minimally correct even if the
1481   cache does not understand the extension(s).
1482</t>
1483<t>
1484   The HTTP Cache Directive Registry defines the name space for the cache
1485   directives.
1486</t>
1487<t>
1488   A registration MUST include the following fields:
1489   <list style="symbols">
1490      <t>Cache Directive Name</t>
1491      <t>Pointer to specification text</t>
1492   </list>
1493</t>
1494<t>
1495   Values to be added to this name space are subject to IETF review (<xref target="RFC5226"/>, Section 4.1).
1496</t>
1497<t>
1498   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1499</t>
1500</section>
1501
1502</section>
1503
1504<section anchor="header.expires" title="Expires">
1505   <iref item="Expires header field" primary="true"/>
1506   <iref item="Header Fields" primary="true" subitem="Expires"/>
1507   
1508<t>
1509   The "Expires" header field gives the date/time after which the
1510   response is considered stale. See <xref target="expiration.model"/> for
1511   further discussion of the freshness model.
1512</t>
1513<t>
1514   The presence of an Expires field does not imply that the original resource
1515   will change or cease to exist at, before, or after that time.
1516</t>
1517<t>
1518   The field-value is an absolute date and time as defined by HTTP-date in
1519   Section 6.1 of <xref target="Part1"/>; a sender MUST use the rfc1123-date format.
1520</t>
1521<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1522  Expires = HTTP-date
1523]]></artwork></figure>
1524<figure>
1525  <preamble>For example</preamble>
1526<artwork type="example"><![CDATA[
1527  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1528]]></artwork></figure>
1529<t>
1530   A cache MUST treat other invalid date formats,
1531   especially including the value "0", as in the past (i.e., "already
1532   expired").
1533</t>
1534<t><list>
1535   <t>
1536       Note: If a response includes a Cache-Control field with the
1537       max-age directive (see <xref target="cache-response-directive"/>),
1538       that directive overrides the Expires field. Likewise, the s-maxage
1539       directive overrides Expires in shared caches.
1540   </t>
1541</list></t>
1542<t>
1543   Historically, HTTP required the Expires field-value to be no more than a
1544   year in the future. While longer freshness lifetimes are no longer
1545   prohibited, extremely large values have been demonstrated to cause
1546   problems (e.g., clock overflows due to use of 32-bit integers for
1547   time values), and most caches will evict a response far sooner than
1548   that. Therefore, senders ought not produce them.
1549</t>
1550</section>
1551
1552<section anchor="header.pragma" title="Pragma">
1553   <iref item="Pragma header field" primary="true"/>
1554   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1555   
1556   
1557   
1558<t>
1559   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1560   caches, so that clients can specify a "no-cache" request that they will
1561   understand (as Cache-Control was not defined until HTTP/1.1). When the
1562   Cache-Control header is also present and understood in a request, Pragma is
1563   ignored.
1564</t>
1565<t>
1566   In HTTP/1.0, Pragma was defined as an extensible field for
1567   implementation-specified directives for recipients. This specification
1568   deprecates such extensions to improve interoperability.
1569</t>
1570<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
1571  Pragma           = 1#pragma-directive
1572  pragma-directive = "no-cache" / extension-pragma
1573  extension-pragma = token [ "=" ( token / quoted-string ) ]
1574]]></artwork></figure>
1575<t>
1576   When the Cache-Control header is not present in a request, the no-cache
1577   request pragma-directive MUST have the same effect on caches as if
1578   "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
1579</t>
1580<t>
1581   When sending a no-cache request, a client SHOULD include both pragma and
1582   cache-control directives unless Cache-Control: no-cache is purposefully
1583   omitted to target other Cache-Control response directives at HTTP/1.1
1584   caches. For example:
1585</t>
1586<figure>
1587<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1588  GET / HTTP/1.1
1589  Host: www.example.com
1590  Cache-Control: max-age=30
1591  Pragma: no-cache
1592 
1593  ]]></artwork>
1594</figure>
1595<t>
1596   will constrain HTTP/1.1 caches to serve a response no older than 30
1597   seconds, while precluding implementations that do not understand
1598   Cache-Control from serving a cached response.
1599</t>
1600<t><list>
1601   <t>
1602      Note: Because the meaning of "Pragma: no-cache" in responses is not
1603      specified, it does not provide a reliable replacement for
1604      "Cache-Control: no-cache" in them.
1605   </t>
1606</list></t>
1607</section>
1608
1609<section anchor="header.vary" title="Vary">
1610   <iref item="Vary header field" primary="true"/>
1611   <iref item="Header Fields" primary="true" subitem="Vary"/>
1612   
1613<t>
1614   The "Vary" header field conveys the set of header fields
1615   that were used to select the representation.
1616</t>
1617<t>
1618   Caches use this information, in part, to determine whether a stored
1619   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1620   fresh, whether a cache is permitted to use the response to reply to a
1621   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1622</t>
1623<t>
1624   In uncacheable or stale responses, the Vary field value advises the user
1625   agent about the criteria that were used to select the representation.
1626</t>
1627<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
1628  Vary = "*" / 1#field-name
1629]]></artwork></figure>
1630<t>
1631   The set of header fields named by the Vary field value is known as the
1632   selecting header fields.
1633</t>
1634<t>
1635   A server SHOULD include a Vary header field with any cacheable response
1636   that is subject to server-driven negotiation. Doing so allows a cache to
1637   properly interpret future requests on that resource and informs the user
1638   agent about the presence of negotiation on that resource. A server MAY
1639   include a Vary header field with a non-cacheable response that is subject
1640   to server-driven negotiation, since this might provide the user agent with
1641   useful information about the dimensions over which the response varies at
1642   the time of the response.
1643</t>
1644<t>
1645   A Vary field value of "*" signals that unspecified parameters not limited
1646   to the header fields (e.g., the network address of the client), play a
1647   role in the selection of the response representation; therefore, a cache
1648   cannot determine whether this response is appropriate. A proxy MUST NOT
1649   generate the "*" value.
1650</t>
1651<t>
1652   The field-names given are not limited to the set of standard header
1653   fields defined by this specification. Field names are case-insensitive.
1654</t>
1655</section>
1656
1657<section anchor="header.warning" title="Warning">
1658   <iref item="Warning header field" primary="true"/>
1659   <iref item="Header Fields" primary="true" subitem="Warning"/>
1660   
1661   
1662   
1663   
1664   
1665   
1666<t>
1667   The "Warning" header field is used to carry additional information
1668   about the status or transformation of a message that might not be reflected
1669   in the message. This information is typically used to warn about possible
1670   incorrectness introduced by caching operations or transformations applied
1671   to the payload of the message.
1672</t>
1673<t>
1674   Warnings can be used for other purposes, both cache-related and otherwise.
1675   The use of a warning, rather than an error status code, distinguishes these
1676   responses from true failures.
1677</t>
1678<t>
1679   Warning header fields can in general be applied to any message, however some
1680   warn-codes are specific to caches and can only be applied to response
1681   messages.
1682</t>
1683<figure><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"/><artwork type="abnf2616"><![CDATA[
1684  Warning       = 1#warning-value
1685 
1686  warning-value = warn-code SP warn-agent SP warn-text
1687                                        [SP warn-date]
1688 
1689  warn-code  = 3DIGIT
1690  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1691                  ; the name or pseudonym of the server adding
1692                  ; the Warning header field, for use in debugging
1693  warn-text  = quoted-string
1694  warn-date  = DQUOTE HTTP-date DQUOTE
1695]]></artwork></figure>
1696<t>
1697   Multiple warnings can be attached to a response (either by the origin
1698   server or by a cache), including multiple warnings with the same code
1699   number, only differing in warn-text.
1700</t>
1701<t>
1702   When this occurs, the user agent SHOULD inform the user of as many of
1703   them as possible, in the order that they appear in the response.
1704</t>
1705<t>
1706   Systems that generate multiple Warning header fields SHOULD order them with
1707   this user agent behavior in mind. New Warning header fields SHOULD be added
1708   after any existing Warning headers fields.
1709</t>
1710<t>
1711   Warnings are assigned three digit warn-codes. The first digit indicates
1712   whether the Warning is required to be deleted from a stored response after
1713   validation:
1714   <list style="symbols">
1715      <t>1xx Warnings describe the freshness or validation status of the
1716      response, and so MUST be deleted by a cache after validation. They can
1717      only be generated by a cache when validating a cached entry, and
1718      MUST NOT be generated in any other situation.</t>
1719      <t>2xx Warnings describe some aspect of the representation that is not
1720      rectified by a validation (for example, a lossy compression of the
1721      representation) and MUST NOT be deleted by a cache after validation,
1722      unless a full response is returned, in which case they MUST be.</t>
1723   </list>
1724</t>
1725<t>
1726   If an implementation sends a message with one or more Warning header fields to a
1727   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1728   in each warning-value a warn-date that matches the Date header field in the
1729   message.
1730</t>
1731<t>
1732   If a system receives a message with a warning-value that includes
1733   a warn-date, and that warn-date is different from the Date value in the
1734   response, then that warning-value MUST be deleted from the message before
1735   storing, forwarding, or using it. (preventing the consequences of naive
1736   caching of Warning header fields.) If all of the warning-values are deleted
1737   for this reason, the Warning header field MUST be deleted as well.
1738</t>
1739<t>
1740   The following warn-codes are defined by this specification, each with a
1741   recommended warn-text in English, and a description of its meaning.
1742</t>
1743<t>
1744<?rfc needLines="4"?>
1745   110 Response is stale
1746   <list>
1747      <t>A cache SHOULD include this whenever the returned response is stale.</t>
1748   </list>
1749</t>
1750<t>
1751<?rfc needLines="4"?>
1752   111 Revalidation failed
1753   <list>
1754      <t>A cache SHOULD include this when returning a stale response because an
1755      attempt to validate the response failed, due to an inability to reach
1756      the server.</t>
1757   </list>
1758</t>
1759<t>
1760<?rfc needLines="4"?>
1761   112 Disconnected operation
1762   <list>
1763      <t>A cache SHOULD b include this if it is intentionally disconnected from
1764      the rest of the network for a period of time.</t>
1765   </list>
1766</t>
1767<t>
1768<?rfc needLines="4"?>
1769   113 Heuristic expiration
1770   <list>
1771      <t>A cache SHOULD include this if it heuristically chose a freshness
1772      lifetime greater than 24 hours and the response's age is greater than 24
1773      hours.</t>
1774   </list>
1775</t>
1776<t>
1777<?rfc needLines="4"?>
1778   199 Miscellaneous warning
1779   <list>
1780      <t>The warning text can include arbitrary information to be presented to
1781      a human user, or logged. A system receiving this warning MUST NOT take
1782      any automated action, besides presenting the warning to the user.</t>
1783   </list>
1784</t>
1785<t>
1786<?rfc needLines="4"?>
1787   214 Transformation applied
1788   <list>
1789      <t>MUST be added by a proxy if it applies any
1790      transformation to the representation, such as changing the
1791      content-coding, media-type, or modifying the representation data, unless
1792      this Warning code already appears in the response.</t>
1793   </list>
1794</t>
1795<t>
1796<?rfc needLines="4"?>
1797   299 Miscellaneous persistent warning
1798   <list>
1799      <t>The warning text can include arbitrary information to be presented to
1800      a human user, or logged. A system receiving this warning MUST NOT take
1801      any automated action.</t>
1802   </list>
1803</t>
1804</section>
1805
1806</section>
1807
1808<section anchor="history.lists" title="History Lists">
1809<t>
1810   User agents often have history mechanisms, such as "Back" buttons and
1811   history lists, that can be used to redisplay a representation retrieved
1812   earlier in a session.
1813</t>
1814<t>
1815   The freshness model (<xref target="expiration.model"/>) does not
1816   necessarily apply to history mechanisms. I.e., a history mechanism can
1817   display a previous representation even if it has expired.
1818</t>
1819<t>
1820   This does not prohibit the history mechanism from telling the user that a
1821   view might be stale, or from honoring cache directives (e.g.,
1822   Cache-Control: no-store).
1823</t>
1824</section>
1825
1826
1827<section anchor="IANA.considerations" title="IANA Considerations">
1828
1829<section title="Cache Directive Registry" anchor="cache.directive.registration">
1830<t>
1831   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1832</t>
1833<t>
1834   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1835   populated with the registrations below:
1836</t>
1837
1838<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1839<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1840   <ttcol>Cache Directive</ttcol>
1841   <ttcol>Reference</ttcol>
1842
1843   <c>max-age</c>
1844   <c>
1845      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1846   </c>
1847   <c>max-stale</c>
1848   <c>
1849      <xref target="cache-request-directive"/>
1850   </c>
1851   <c>min-fresh</c>
1852   <c>
1853      <xref target="cache-request-directive"/>
1854   </c>
1855   <c>must-revalidate</c>
1856   <c>
1857      <xref target="cache-response-directive"/>
1858   </c>
1859   <c>no-cache</c>
1860   <c>
1861      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1862   </c>
1863   <c>no-store</c>
1864   <c>
1865      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1866   </c>
1867   <c>no-transform</c>
1868   <c>
1869      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1870   </c>
1871   <c>only-if-cached</c>
1872   <c>
1873      <xref target="cache-request-directive"/>
1874   </c>
1875   <c>private</c>
1876   <c>
1877      <xref target="cache-response-directive"/>
1878   </c>
1879   <c>proxy-revalidate</c>
1880   <c>
1881      <xref target="cache-response-directive"/>
1882   </c>
1883   <c>public</c>
1884   <c>
1885      <xref target="cache-response-directive"/>
1886   </c>
1887   <c>s-maxage</c>
1888   <c>
1889      <xref target="cache-response-directive"/>
1890   </c>
1891   <c>stale-if-error</c>
1892   <c>
1893      <xref target="RFC5861"/>, Section 4
1894   </c>
1895   <c>stale-while-revalidate</c>
1896   <c>
1897      <xref target="RFC5861"/>, Section 3
1898   </c>
1899</texttable>
1900<!--(END)-->
1901
1902</section>
1903
1904<section title="Header Field Registration" anchor="header.field.registration">
1905<t>
1906  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1907  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1908</t>
1909
1910<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1911<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1912   <ttcol>Header Field Name</ttcol>
1913   <ttcol>Protocol</ttcol>
1914   <ttcol>Status</ttcol>
1915   <ttcol>Reference</ttcol>
1916
1917   <c>Age</c>
1918   <c>http</c>
1919   <c>standard</c>
1920   <c>
1921      <xref target="header.age"/>
1922   </c>
1923   <c>Cache-Control</c>
1924   <c>http</c>
1925   <c>standard</c>
1926   <c>
1927      <xref target="header.cache-control"/>
1928   </c>
1929   <c>Expires</c>
1930   <c>http</c>
1931   <c>standard</c>
1932   <c>
1933      <xref target="header.expires"/>
1934   </c>
1935   <c>Pragma</c>
1936   <c>http</c>
1937   <c>standard</c>
1938   <c>
1939      <xref target="header.pragma"/>
1940   </c>
1941   <c>Vary</c>
1942   <c>http</c>
1943   <c>standard</c>
1944   <c>
1945      <xref target="header.vary"/>
1946   </c>
1947   <c>Warning</c>
1948   <c>http</c>
1949   <c>standard</c>
1950   <c>
1951      <xref target="header.warning"/>
1952   </c>
1953</texttable>
1954<!--(END)-->
1955
1956<t>
1957   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1958   Force".
1959</t>
1960</section>
1961
1962</section>
1963
1964<section anchor="security.considerations" title="Security Considerations">
1965<t>
1966   Caches expose additional potential vulnerabilities, since the contents of
1967   the cache represent an attractive target for malicious exploitation.
1968   Because cache contents persist after an HTTP request is complete, an attack
1969   on the cache can reveal information long after a user believes that the
1970   information has been removed from the network. Therefore, cache contents
1971   need to be protected as sensitive information.
1972</t>
1973</section>
1974
1975<section title="Acknowledgments" anchor="acks">
1976<t>
1977  See Section 12 of <xref target="Part1"/>.
1978</t>
1979</section>
1980
1981</middle>
1982
1983<back>
1984<references title="Normative References">
1985
1986  <reference anchor="Part1">
1987    <front>
1988      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1989      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1990        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1991        <address><email>fielding@gbiv.com</email></address>
1992      </author>
1993      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1994        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1995        <address><email>jg@freedesktop.org</email></address>
1996      </author>
1997      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1998        <organization abbrev="HP">Hewlett-Packard Company</organization>
1999        <address><email>JeffMogul@acm.org</email></address>
2000      </author>
2001      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2002        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2003        <address><email>henrikn@microsoft.com</email></address>
2004      </author>
2005      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2006        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2007        <address><email>LMM@acm.org</email></address>
2008      </author>
2009      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2010        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2011        <address><email>paulle@microsoft.com</email></address>
2012      </author>
2013      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2014        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2015        <address><email>timbl@w3.org</email></address>
2016      </author>
2017      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2018        <organization abbrev="W3C">World Wide Web Consortium</organization>
2019        <address><email>ylafon@w3.org</email></address>
2020      </author>
2021      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2022        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2023        <address><email>julian.reschke@greenbytes.de</email></address>
2024      </author>
2025      <date month="August" year="2011"/>
2026    </front>
2027    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-16"/>
2028   
2029  </reference>
2030
2031  <reference anchor="Part2">
2032    <front>
2033      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2034      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2035        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2036        <address><email>fielding@gbiv.com</email></address>
2037      </author>
2038      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2039        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2040        <address><email>jg@freedesktop.org</email></address>
2041      </author>
2042      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2043        <organization abbrev="HP">Hewlett-Packard Company</organization>
2044        <address><email>JeffMogul@acm.org</email></address>
2045      </author>
2046      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2047        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2048        <address><email>henrikn@microsoft.com</email></address>
2049      </author>
2050      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2051        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2052        <address><email>LMM@acm.org</email></address>
2053      </author>
2054      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2055        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2056        <address><email>paulle@microsoft.com</email></address>
2057      </author>
2058      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2059        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2060        <address><email>timbl@w3.org</email></address>
2061      </author>
2062      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2063        <organization abbrev="W3C">World Wide Web Consortium</organization>
2064        <address><email>ylafon@w3.org</email></address>
2065      </author>
2066      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2067        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2068        <address><email>julian.reschke@greenbytes.de</email></address>
2069      </author>
2070      <date month="August" year="2011"/>
2071    </front>
2072    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-16"/>
2073   
2074  </reference>
2075
2076  <reference anchor="Part4">
2077    <front>
2078      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2079      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2080        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2081        <address><email>fielding@gbiv.com</email></address>
2082      </author>
2083      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2084        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2085        <address><email>jg@freedesktop.org</email></address>
2086      </author>
2087      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2088        <organization abbrev="HP">Hewlett-Packard Company</organization>
2089        <address><email>JeffMogul@acm.org</email></address>
2090      </author>
2091      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2092        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2093        <address><email>henrikn@microsoft.com</email></address>
2094      </author>
2095      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2096        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2097        <address><email>LMM@acm.org</email></address>
2098      </author>
2099      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2100        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2101        <address><email>paulle@microsoft.com</email></address>
2102      </author>
2103      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2104        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2105        <address><email>timbl@w3.org</email></address>
2106      </author>
2107      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2108        <organization abbrev="W3C">World Wide Web Consortium</organization>
2109        <address><email>ylafon@w3.org</email></address>
2110      </author>
2111      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2112        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2113        <address><email>julian.reschke@greenbytes.de</email></address>
2114      </author>
2115      <date month="August" year="2011"/>
2116    </front>
2117    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-16"/>
2118   
2119  </reference>
2120
2121  <reference anchor="Part5">
2122    <front>
2123      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2124      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2125        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2126        <address><email>fielding@gbiv.com</email></address>
2127      </author>
2128      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2129        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2130        <address><email>jg@freedesktop.org</email></address>
2131      </author>
2132      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2133        <organization abbrev="HP">Hewlett-Packard Company</organization>
2134        <address><email>JeffMogul@acm.org</email></address>
2135      </author>
2136      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2137        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2138        <address><email>henrikn@microsoft.com</email></address>
2139      </author>
2140      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2141        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2142        <address><email>LMM@acm.org</email></address>
2143      </author>
2144      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2145        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2146        <address><email>paulle@microsoft.com</email></address>
2147      </author>
2148      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2149        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2150        <address><email>timbl@w3.org</email></address>
2151      </author>
2152      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2153        <organization abbrev="W3C">World Wide Web Consortium</organization>
2154        <address><email>ylafon@w3.org</email></address>
2155      </author>
2156      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2157        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2158        <address><email>julian.reschke@greenbytes.de</email></address>
2159      </author>
2160      <date month="August" year="2011"/>
2161    </front>
2162    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-16"/>
2163   
2164  </reference>
2165
2166  <reference anchor="Part7">
2167    <front>
2168      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2169      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2170        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2171        <address><email>fielding@gbiv.com</email></address>
2172      </author>
2173      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2174        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2175        <address><email>jg@freedesktop.org</email></address>
2176      </author>
2177      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2178        <organization abbrev="HP">Hewlett-Packard Company</organization>
2179        <address><email>JeffMogul@acm.org</email></address>
2180      </author>
2181      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2182        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2183        <address><email>henrikn@microsoft.com</email></address>
2184      </author>
2185      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2186        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2187        <address><email>LMM@acm.org</email></address>
2188      </author>
2189      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2190        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2191        <address><email>paulle@microsoft.com</email></address>
2192      </author>
2193      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2194        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2195        <address><email>timbl@w3.org</email></address>
2196      </author>
2197      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2198        <organization abbrev="W3C">World Wide Web Consortium</organization>
2199        <address><email>ylafon@w3.org</email></address>
2200      </author>
2201      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2202        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2203        <address><email>julian.reschke@greenbytes.de</email></address>
2204      </author>
2205      <date month="August" year="2011"/>
2206    </front>
2207    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-16"/>
2208   
2209  </reference>
2210
2211  <reference anchor="RFC2119">
2212    <front>
2213      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2214      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2215        <organization>Harvard University</organization>
2216        <address><email>sob@harvard.edu</email></address>
2217      </author>
2218      <date month="March" year="1997"/>
2219    </front>
2220    <seriesInfo name="BCP" value="14"/>
2221    <seriesInfo name="RFC" value="2119"/>
2222  </reference>
2223
2224  <reference anchor="RFC5234">
2225    <front>
2226      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2227      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2228        <organization>Brandenburg InternetWorking</organization>
2229        <address>
2230          <email>dcrocker@bbiw.net</email>
2231        </address> 
2232      </author>
2233      <author initials="P." surname="Overell" fullname="Paul Overell">
2234        <organization>THUS plc.</organization>
2235        <address>
2236          <email>paul.overell@thus.net</email>
2237        </address>
2238      </author>
2239      <date month="January" year="2008"/>
2240    </front>
2241    <seriesInfo name="STD" value="68"/>
2242    <seriesInfo name="RFC" value="5234"/>
2243  </reference>
2244 
2245</references>
2246
2247<references title="Informative References">
2248
2249  <reference anchor="RFC1305">
2250    <front>
2251      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2252      <author fullname="David L. Mills" initials="D." surname="Mills">
2253        <organization>University of Delaware, Electrical Engineering Department</organization>
2254        <address><email>mills@udel.edu</email></address>
2255      </author>
2256      <date month="March" year="1992"/>
2257    </front>
2258    <seriesInfo name="RFC" value="1305"/>
2259  </reference>
2260
2261  <reference anchor="RFC2616">
2262    <front>
2263      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2264      <author fullname="R. Fielding" initials="R." surname="Fielding">
2265        <organization>University of California, Irvine</organization>
2266        <address><email>fielding@ics.uci.edu</email></address>
2267      </author>
2268      <author fullname="J. Gettys" initials="J." surname="Gettys">
2269        <organization>W3C</organization>
2270        <address><email>jg@w3.org</email></address>
2271      </author>
2272      <author fullname="J. Mogul" initials="J." surname="Mogul">
2273        <organization>Compaq Computer Corporation</organization>
2274        <address><email>mogul@wrl.dec.com</email></address>
2275      </author>
2276      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2277        <organization>MIT Laboratory for Computer Science</organization>
2278        <address><email>frystyk@w3.org</email></address>
2279      </author>
2280      <author fullname="L. Masinter" initials="L." surname="Masinter">
2281        <organization>Xerox Corporation</organization>
2282        <address><email>masinter@parc.xerox.com</email></address>
2283      </author>
2284      <author fullname="P. Leach" initials="P." surname="Leach">
2285        <organization>Microsoft Corporation</organization>
2286        <address><email>paulle@microsoft.com</email></address>
2287      </author>
2288      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2289        <organization>W3C</organization>
2290        <address><email>timbl@w3.org</email></address>
2291      </author>
2292      <date month="June" year="1999"/>
2293    </front>
2294    <seriesInfo name="RFC" value="2616"/>
2295  </reference>
2296
2297  <reference anchor="RFC3864">
2298    <front>
2299      <title>Registration Procedures for Message Header Fields</title>
2300      <author fullname="G. Klyne" initials="G." surname="Klyne">
2301        <organization>Nine by Nine</organization>
2302        <address><email>GK-IETF@ninebynine.org</email></address>
2303      </author>
2304      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2305        <organization>BEA Systems</organization>
2306        <address><email>mnot@pobox.com</email></address>
2307      </author>
2308      <author fullname="J. Mogul" initials="J." surname="Mogul">
2309        <organization>HP Labs</organization>
2310        <address><email>JeffMogul@acm.org</email></address>
2311      </author>
2312      <date month="September" year="2004"/>
2313    </front>
2314    <seriesInfo name="BCP" value="90"/>
2315    <seriesInfo name="RFC" value="3864"/>
2316  </reference>
2317
2318  <reference anchor="RFC5226">
2319    <front>
2320      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2321      <author initials="T." surname="Narten" fullname="T. Narten">
2322        <organization>IBM</organization>
2323        <address><email>narten@us.ibm.com</email></address>
2324      </author>
2325      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2326        <organization>Google</organization>
2327        <address><email>Harald@Alvestrand.no</email></address>
2328      </author>
2329      <date year="2008" month="May"/>
2330    </front>
2331    <seriesInfo name="BCP" value="26"/>
2332    <seriesInfo name="RFC" value="5226"/>
2333  </reference>
2334
2335  <reference anchor="RFC5861">
2336    <front>
2337      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2338      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2339        <organization>Yahoo! Inc.</organization>
2340        <address><email>mnot@yahoo-inc.com</email></address>
2341      </author>
2342      <date month="April" year="2010"/>
2343    </front>
2344    <seriesInfo name="RFC" value="5861"/>
2345  </reference>
2346
2347</references>
2348
2349<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2350<t>
2351  Make the specified age calculation algorithm less conservative.
2352  (<xref target="age.calculations"/>)
2353</t>
2354<t>
2355  Remove requirement to consider Content-Location in successful responses
2356  in order to determine the appropriate response to use.
2357  (<xref target="validation.model"/>)
2358</t>
2359<t>
2360  Clarify denial of service attack avoidance requirement.
2361  (<xref target="invalidation.after.updates.or.deletions"/>)
2362</t>
2363<t>
2364  Change ABNF productions for header fields to only define the field value.
2365  (<xref target="header.fields"/>)
2366</t>
2367<t>
2368  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2369  anymore, as these aspects never were implemented.
2370  (<xref target="header.warning"/>)
2371</t>
2372</section>
2373
2374
2375<section title="Collected ABNF" anchor="collected.abnf">
2376<figure>
2377<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2378Age = delta-seconds
2379
2380Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2381 cache-directive ] )
2382
2383Expires = HTTP-date
2384
2385HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2386
2387OWS = <OWS, defined in [Part1], Section 1.2.2>
2388
2389Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2390 pragma-directive ] )
2391
2392Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2393 ) )
2394
2395Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2396 )
2397
2398cache-directive = cache-request-directive / cache-response-directive
2399cache-extension = token [ "=" ( token / quoted-string ) ]
2400cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2401 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2402 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2403 cache-extension
2404cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2405 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2406 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2407 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2408 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2409 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2410
2411delta-seconds = 1*DIGIT
2412
2413extension-pragma = token [ "=" ( token / quoted-string ) ]
2414
2415field-name = <field-name, defined in [Part1], Section 3.2>
2416
2417port = <port, defined in [Part1], Section 2.7>
2418pragma-directive = "no-cache" / extension-pragma
2419pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2420
2421quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
2422
2423token = <token, defined in [Part1], Section 3.2.3>
2424
2425uri-host = <uri-host, defined in [Part1], Section 2.7>
2426
2427warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2428warn-code = 3DIGIT
2429warn-date = DQUOTE HTTP-date DQUOTE
2430warn-text = quoted-string
2431warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2432 ]
2433]]></artwork>
2434</figure>
2435<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2436; Age defined but not used
2437; Cache-Control defined but not used
2438; Expires defined but not used
2439; Pragma defined but not used
2440; Vary defined but not used
2441; Warning defined but not used
2442]]></artwork></figure></section>
2443
2444
2445<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2446
2447<section title="Since RFC 2616">
2448  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2449</section>
2450
2451<section title="Since draft-ietf-httpbis-p6-cache-00">
2452<t>
2453  Closed issues:
2454  <list style="symbols">
2455    <t>
2456      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2457    <t>
2458      <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>
2459    <t>
2460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2461    <t>
2462      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2463    <t>
2464      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2465    <t>
2466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2467    <t>
2468      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2469    <t>
2470      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2471    <t>
2472      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2473  </list>
2474</t>
2475<t>
2476  Other changes:
2477  <list style="symbols">
2478    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2479  </list>
2480</t>
2481</section>
2482
2483<section title="Since draft-ietf-httpbis-p6-cache-01">
2484<t>
2485  Closed issues:
2486  <list style="symbols">
2487    <t>
2488      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2489  </list>
2490</t>
2491<t>
2492  Other changes:
2493  <list style="symbols">
2494    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2495      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2496    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2497      specification.</t>
2498  </list>
2499</t>
2500</section>
2501
2502<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2503<t>
2504  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2505  <list style="symbols">
2506    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2507      document.</t>
2508  </list>
2509</t>
2510</section>
2511
2512<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2513<t>
2514  Closed issues:
2515  <list style="symbols">
2516    <t>
2517      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2518  </list>
2519</t>
2520</section>
2521
2522<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2523<t>
2524  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2525  <list style="symbols">
2526    <t>
2527      Use "/" instead of "|" for alternatives.
2528    </t>
2529    <t>
2530      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2531      whitespace ("OWS") and required whitespace ("RWS").
2532    </t>
2533    <t>
2534      Rewrite ABNFs to spell out whitespace rules, factor out
2535      header field value format definitions.
2536    </t>
2537  </list>
2538</t>
2539</section>
2540
2541<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2542<t>
2543  This is a total rewrite of this part of the specification.
2544</t>
2545<t>
2546  Affected issues:
2547  <list style="symbols">
2548    <t>
2549      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2550    <t>
2551      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2554    <t>
2555      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2556  </list>
2557</t>
2558<t>
2559  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2560  <list style="symbols">
2561    <t>
2562      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2563    </t>
2564  </list>
2565</t>
2566</section>
2567
2568<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2569<t>
2570  Closed issues:
2571  <list style="symbols">
2572    <t>
2573      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2574      "base for numeric protocol elements"
2575    </t>
2576  </list>
2577</t>
2578<t>
2579  Affected issues:
2580  <list style="symbols">
2581    <t>
2582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2583      "Vary and non-existant headers"
2584    </t>
2585  </list>
2586</t>
2587</section>
2588
2589<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2590<t>
2591  Closed issues:
2592  <list style="symbols">
2593    <t>
2594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2595      "Definition of 1xx Warn-Codes"
2596    </t>
2597    <t>
2598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2599      "Content-Location on 304 responses"
2600    </t>
2601    <t>
2602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2603      "private and no-cache CC directives with headers"
2604    </t>
2605    <t>
2606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2607      "RFC2047 and warn-text"
2608    </t>
2609  </list>
2610</t>
2611</section>
2612
2613<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2614<t>
2615  Closed issues:
2616  <list style="symbols">
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2619      "serving negotiated responses from cache: header-specific canonicalization"
2620    </t>
2621    <t>
2622      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2623      "Effect of CC directives on history lists"
2624    </t>
2625    <t>
2626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2627      "Cache Extensions can override no-store, etc."
2628    </t>
2629  </list>
2630</t>
2631<t>
2632  Affected issues:
2633  <list style="symbols">
2634    <t>
2635      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2636      Status codes and caching
2637    </t>
2638  </list>
2639</t>
2640<t>
2641  Partly resolved issues:
2642  <list style="symbols">
2643    <t>
2644      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2645      "Placement of 13.5.1 and 13.5.2"
2646    </t>
2647  </list>
2648</t>
2649</section>
2650
2651<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2652<t>
2653  Closed issues:
2654  <list style="symbols">
2655    <t>
2656      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2657      "Age calculation"
2658    </t>
2659    <t>
2660      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2661      "Clarify differences between / requirements for request and response CC directives"
2662    </t>
2663    <t>
2664      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2665      "Caching authenticated responses"
2666    </t>
2667    <t>
2668      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2669      "IANA registry for cache-control directives"
2670    </t>
2671    <t>
2672      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2673      "Heuristic caching of URLs with query components"
2674    </t>
2675  </list>
2676</t>
2677<t>
2678  Partly resolved issues:
2679  <list style="symbols">
2680    <t>
2681      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2682      "Term for the requested resource's URI"
2683    </t>
2684  </list>
2685</t>
2686</section>
2687
2688<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2689<t>
2690  Closed issues:
2691  <list style="symbols">
2692    <t>
2693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2694      "Clarify entity / representation / variant terminology"
2695    </t>
2696    <t>
2697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2698      "consider removing the 'changes from 2068' sections"
2699    </t>
2700    <t>
2701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2702      "Allowing heuristic caching for new status codes"
2703    </t>
2704    <t>
2705      Clean up TODOs and prose in "Combining Responses."
2706    </t>
2707  </list>
2708</t>
2709</section>
2710
2711<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2712<t>
2713  Closed issues:
2714  <list style="symbols">
2715    <t>
2716      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2717      "Text about clock requirement for caches belongs in p6"
2718    </t>
2719  </list>
2720</t>
2721</section>
2722
2723<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2724<t>
2725  Closed issues:
2726  <list style="symbols">
2727    <t>
2728      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2729      "Header Classification"
2730    </t>
2731    <t>
2732      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2733      "Clarify 'public'"
2734    </t>
2735  </list>
2736</t>
2737</section>
2738
2739<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2740<t>
2741  Closed issues:
2742  <list style="symbols">
2743    <t>
2744      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2745      "untangle ABNFs for header fields"
2746    </t>
2747  </list>
2748</t>
2749</section>
2750
2751<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2752<t>
2753  Closed issues:
2754  <list style="symbols">
2755    <t>
2756      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2757      "Mismatch Vary"
2758    </t>
2759    <t>
2760      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2761      "Cache Invalidation only happens upon successful responses"
2762    </t>
2763    <t>
2764      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2765      "Recommend minimum sizes for protocol elements"
2766    </t>
2767    <t>
2768      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2769      "Proxies don't 'understand' methods"
2770    </t>
2771    <t>
2772      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2773      "Cache Extensions can override no-store, etc."
2774    </t>
2775    <t>
2776      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2777      "Pragma"
2778    </t>
2779  </list>
2780</t>
2781</section>
2782
2783<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2784<t>
2785  Closed issues:
2786  <list style="symbols">
2787    <t>
2788      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2789      "Motivate one-year limit for Expires"
2790    </t>
2791  </list>
2792</t>
2793</section>
2794
2795</section>
2796  </back>
2797</rfc>
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