source: draft-ietf-httpbis/19/draft-ietf-httpbis-p6-cache-19.xml @ 1592

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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-19" ipr="pre5378Trust200902" obsoletes="2616">
20
21
22
23<front>
24
25  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
26
27  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
28    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
29    <address>
30      <postal>
31        <street>345 Park Ave</street>
32        <city>San Jose</city>
33        <region>CA</region>
34        <code>95110</code>
35        <country>USA</country>
36      </postal>
37      <email>fielding@gbiv.com</email>
38      <uri>http://roy.gbiv.com/</uri>
39    </address>
40  </author>
41
42  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
43    <organization abbrev="W3C">World Wide Web Consortium</organization>
44    <address>
45      <postal>
46        <street>W3C / ERCIM</street>
47        <street>2004, rte des Lucioles</street>
48        <city>Sophia-Antipolis</city>
49        <region>AM</region>
50        <code>06902</code>
51        <country>France</country>
52      </postal>
53      <email>ylafon@w3.org</email>
54      <uri>http://www.raubacapeu.net/people/yves/</uri>
55    </address>
56  </author>
57
58  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
59    <organization>Rackspace</organization>
60    <address>
61      <email>mnot@mnot.net</email>
62      <uri>http://www.mnot.net/</uri>
63    </address>
64  </author>
65
66  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
67    <organization abbrev="greenbytes">greenbytes GmbH</organization>
68    <address>
69      <postal>
70        <street>Hafenweg 16</street>
71        <city>Muenster</city><region>NW</region><code>48155</code>
72        <country>Germany</country>
73      </postal>
74      <phone>+49 251 2807760</phone>
75      <facsimile>+49 251 2807761</facsimile>
76      <email>julian.reschke@greenbytes.de</email>
77      <uri>http://greenbytes.de/tech/webdav/</uri>
78    </address>
79  </author>
80
81  <date month="March" year="2012" day="12"/>
82  <workgroup>HTTPbis Working Group</workgroup>
83
84<abstract>
85<t>
86   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
87   distributed, collaborative, hypertext information systems. HTTP has been in
88   use by the World Wide Web global information initiative since 1990. This
89   document is Part 6 of the seven-part specification that defines the protocol
90   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
91</t>
92<t>
93   Part 6 defines requirements on HTTP caches and the associated header fields
94   that control cache behavior or indicate cacheable response messages.
95</t>
96</abstract>
97
98<note title="Editorial Note (To be removed by RFC Editor)">
99  <t>
100    Discussion of this draft should take place on the HTTPBIS working group
101    mailing list (ietf-http-wg@w3.org), which is archived at
102    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
103  </t>
104  <t>
105    The current issues list is at
106    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
107    documents (including fancy diffs) can be found at
108    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
109  </t>
110  <t>
111    The changes in this draft are summarized in <xref target="changes.since.18"/>.
112  </t>
113</note>
114
115   </front>
116   <middle>
117
118<section anchor="caching" title="Introduction">
119<t>
120   HTTP is typically used for distributed information systems, where
121   performance can be improved by the use of response caches. This document
122   defines aspects of HTTP/1.1 related to caching and reusing response
123   messages.
124</t>
125
126<section anchor="intro.purpose" title="Purpose">
127<iref item="cache"/>
128<t>
129   An HTTP cache is a local store of response messages and the
130   subsystem that controls its message storage, retrieval, and deletion. A
131   cache stores cacheable responses in order to reduce the response time and
132   network bandwidth consumption on future, equivalent requests. Any client or
133   server MAY employ a cache, though a cache cannot be used by a server that
134   is acting as a tunnel.
135</t>
136<t>
137   The goal of caching in HTTP/1.1 is to significantly improve performance
138   by reusing a prior response message to satisfy a current request.
139   A stored response is considered "fresh", as defined in
140   <xref target="expiration.model"/>, if the response can be reused without
141   "validation" (checking with the origin server to see if the cached response
142   remains valid for this request).  A fresh cache response can therefore
143   reduce both latency and network transfers each time it is reused.
144   When a cached response is not fresh, it might still be reusable if it can
145   be freshened by validation (<xref target="validation.model"/>) or if the
146   origin is unavailable.
147</t>
148</section>
149
150<section anchor="intro.terminology" title="Terminology">
151<t>
152   This specification uses a number of terms to refer to the roles played by
153   participants in, and objects of, HTTP caching.
154</t>
155<t>
156   <iref item="cache"/>
157   <?rfc needLines="4"?>cache
158   <list>
159      <t>A conformant implementation of a HTTP cache. Note that this implies
160        an HTTP/1.1 cache; this specification does not define conformance
161        for HTTP/1.0 caches.</t>
162   </list>
163</t>
164<t anchor="shared.and.non-shared.caches">
165   <iref item="shared cache"/>
166   <?rfc needLines="4"?>shared cache
167   <list>
168      <t>A cache that stores responses to be reused by more than one user;
169         usually (but not always) deployed as part of an intermediary.</t>
170   </list>
171</t>
172<t>
173   <iref item="private cache"/>
174   <?rfc needLines="4"?>private cache
175   <list>
176      <t>A cache that is dedicated to a single user.</t>
177   </list>
178</t>
179<t>
180   <iref item="cacheable"/>
181   <?rfc needLines="4"?>cacheable
182   <list>
183      <t>A response is cacheable if a cache is allowed to store a copy of the
184      response message for use in answering subsequent requests. Even when a
185      response is cacheable, there might be additional constraints on whether
186      a cache can use the stored copy to satisfy a particular request.</t>
187   </list>
188</t>
189<t>
190   <iref item="explicit expiration time"/>
191   <?rfc needLines="4"?>explicit expiration time
192   <list>
193      <t>The time at which the origin server intends that a representation
194      no longer be returned by a cache without further validation.</t>
195   </list>
196</t>
197<t>
198   <iref item="heuristic expiration time"/>
199   <?rfc needLines="4"?>heuristic expiration time
200   <list>
201      <t>An expiration time assigned by a cache when no explicit expiration
202      time is available.</t>
203   </list>
204</t>
205<t>
206   <iref item="age"/>
207   <?rfc needLines="4"?>age
208   <list>
209      <t>The age of a response is the time since it was sent by, or
210      successfully validated with, the origin server.</t>
211   </list>
212</t>
213<t>
214   <iref item="first-hand"/>
215   <?rfc needLines="4"?>first-hand
216   <list>
217      <t>A response is first-hand if the freshness model is not in use; i.e.,
218      its age is 0.</t>
219   </list>
220</t>
221<t>
222   <iref item="freshness lifetime"/>
223   <?rfc needLines="4"?>freshness lifetime
224   <list>
225      <t>The length of time between the generation of a response and its
226      expiration time.</t>
227   </list>
228</t>
229<t>
230   <iref item="fresh"/>
231   <?rfc needLines="4"?>fresh
232   <list>
233      <t>A response is fresh if its age has not yet exceeded its freshness
234      lifetime.</t>
235   </list>
236</t>
237<t>
238   <iref item="stale"/>
239   <?rfc needLines="4"?>stale
240   <list>
241      <t>A response is stale if its age has passed its freshness lifetime
242      (either explicit or heuristic).</t>
243   </list>
244</t>
245<t>
246   <iref item="validator"/>
247   <?rfc needLines="4"?>validator
248   <list>
249      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
250      is used to find out whether a stored response is an equivalent copy of
251      a representation. See Section 2.1 of <xref target="Part4"/>.</t>
252   </list>
253</t>
254<t>
255   <iref item="strong validator"/>
256   <iref item="validator" subitem="strong"/>
257   <?rfc needLines="4"?>strong validator
258   <list>
259      <t>A validator that is defined by the origin server such that its
260         current value will change if the representation body changes; i.e.,
261         an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
262         if no entity-tag is provided, a Last-Modified value that is strong
263         in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
264   </list>
265</t>
266</section>
267
268<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
269<t>
270   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
271   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
272   document are to be interpreted as described in <xref target="RFC2119"/>.
273</t>
274<t>
275   This document defines conformance criteria for several roles in HTTP
276   communication, including Senders, Recipients, Clients, Servers, User-Agents,
277   Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of <xref target="Part1"/>
278   for definitions of these terms.
279</t>
280<t>
281   An implementation is considered conformant if it complies with all of the
282   requirements associated with its role(s). Note that SHOULD-level requirements
283   are relevant here, unless one of the documented exceptions is applicable.
284</t>
285<t>
286   This document also uses ABNF to define valid protocol elements
287   (<xref target="notation"/>). In addition to the prose requirements placed
288   upon them, Senders MUST NOT generate protocol elements that are invalid.
289</t>
290<t>
291   Unless noted otherwise, Recipients MAY take steps to recover a usable
292   protocol element from an invalid construct. However, HTTP does not define
293   specific error handling mechanisms, except in cases where it has direct
294   impact on security. This is because different uses of the protocol require
295   different error handling strategies; for example, a Web browser may wish to
296   transparently recover from a response where the Location header field
297   doesn't parse according to the ABNF, whereby in a systems control protocol
298   using HTTP, this type of error recovery could lead to dangerous consequences.
299</t>
300</section>
301
302<section title="Syntax Notation" anchor="notation">
303   
304   
305   
306   
307   
308   
309   
310   
311<t>
312   This specification uses the Augmented Backus-Naur Form (ABNF) notation
313   of <xref target="RFC5234"/> with the list rule extension defined in
314   Section 1.2 of <xref target="Part1"/><xref target="collected.abnf"/> shows the collected ABNF
315   with the list rule expanded.
316</t>
317<t>
318   The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
319   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
320   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
321   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
322</t>
323
324<section title="Core Rules" anchor="core.rules">
325   
326   
327   
328<t>
329   The core rules below are defined in <xref target="Part1"/>:
330</t>
331<figure><artwork type="abnf2616"><![CDATA[
332  OWS           = <OWS, defined in [Part1], Section 3.2.1>
333  quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
334  token         = <token, defined in [Part1], Section 3.2.4>
335]]></artwork></figure>
336</section>
337
338<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
339   
340   
341   
342   
343   
344<t>
345   The ABNF rules below are defined in other parts:
346</t>
347<figure><artwork type="abnf2616"><![CDATA[
348  field-name    = <field-name, defined in [Part1], Section 3.2>
349  HTTP-date     = <HTTP-date, defined in [Part2], Section 8>
350  port          = <port, defined in [Part1], Section 2.7>
351  pseudonym     = <pseudonym, defined in [Part1], Section 6.2>
352  uri-host      = <uri-host, defined in [Part1], Section 2.7>
353]]></artwork></figure>
354</section>
355</section>
356
357<section title="Delta Seconds" anchor="delta-seconds">
358<t>
359   The delta-seconds rule specifies a non-negative integer, representing time
360   in seconds.
361</t>
362<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
363  delta-seconds  = 1*DIGIT
364]]></artwork></figure>
365<t>
366   If an implementation receives a delta-seconds value larger than the largest
367   positive integer it can represent, or if any of its subsequent calculations
368   overflows, it MUST consider the value to be 2147483648 (2^31).
369   Recipients parsing a delta-seconds value MUST use an arithmetic type of
370   at least 31 bits of range, and senders MUST NOT send delta-seconds with a
371   value greater than 2147483648.
372</t>
373</section>
374
375</section>
376
377<section anchor="caching.overview" title="Cache Operation">
378<iref item="cache entry"/>
379<iref item="cache key"/>
380<t>
381   Proper cache operation preserves the semantics of HTTP transfers
382   (<xref target="Part2"/>) while eliminating the transfer of information already held
383   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
384   we assume that reusing the cached response is desirable and that such
385   reuse is the default behavior when no requirement or locally-desired
386   configuration prevents it.  Therefore, HTTP cache requirements are focused
387   on preventing a cache from either storing a non-reusable response or
388   reusing a stored response inappropriately.
389</t>
390<t>
391   Each cache entry consists of a cache key and one or more
392   HTTP responses corresponding to prior requests that used the same key.
393   The most common form of cache entry is a successful result of a retrieval
394   request: i.e., a 200 (OK) response containing a representation of the
395   resource identified by the request target.  However, it is also possible
396   to cache negative results (e.g., 404 not found), incomplete results
397   (e.g., 206 partial content), and responses to safe methods other than
398   GET if the method's definition allows such caching and defines something
399   suitable for use as a cache key.
400</t>
401<t>
402   The default cache key consists of the request method and
403   target URI.  However, since HTTP caches in common use today are typically
404   limited to caching responses to GET, most implementations simply decline
405   other methods and use only the URI as the key.
406</t>
407<t>
408   If a request target is subject to content negotiation, its cache entry
409   might consist of multiple stored responses, each differentiated by a
410   secondary key for the values of the original request's selecting header
411   fields (<xref target="caching.negotiated.responses"/>).
412</t>
413
414<section anchor="response.cacheability" title="Response Cacheability">
415<t>
416   A cache MUST NOT store a response to any request, unless:
417   <list style="symbols">
418      <t>The request method is understood by the cache and defined as being
419      cacheable, and</t>
420      <t>the response status code is understood by the cache, and</t>
421      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
422      header fields, and</t>
423      <t>the "private" cache response directive (see <xref target="cache-response-directive"/>) does not appear in the response, if
424      the cache is shared, and</t>
425      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
426      appear in the request, if the cache is shared, unless the response
427      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
428      <t>the response either:
429         <list style="symbols">
430            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
431            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
432            <t>contains a s-maxage response cache directive and the cache is
433            shared, or</t>
434            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
435            or</t>
436            <t>has a status code that can be served with heuristic freshness
437            (see <xref target="heuristic.freshness"/>).</t>
438         </list>
439      </t>
440   </list>
441</t>
442<t>
443   Note that any of the requirements listed above can be overridden by a
444   cache-control extension; see <xref target="cache.control.extensions"/>.
445</t>
446<t>
447   In this context, a cache has "understood" a request method or a response
448   status code if it recognizes it and implements any cache-specific
449   behavior.
450</t>
451<t>
452   Note that, in normal operation, most caches will not store a response that
453   has neither a cache validator nor an explicit expiration time, as such
454   responses are not usually useful to store. However, caches are not
455   prohibited from storing such responses.
456</t>
457<t>
458   A response message is considered complete when all of the octets
459   indicated by the message framing (<xref target="Part1"/>) are received
460   prior to the connection being closed.
461   If the request is GET, the response status is 200 (OK), and the entire
462   response header block has been received, a cache MAY store an incomplete
463   response message body if the cache entry is recorded as incomplete.
464   Likewise, a 206 (Partial Content) response MAY be stored as if it were
465   an incomplete 200 (OK) cache entry.  However, a cache MUST NOT store
466   incomplete or partial content responses if it does not support the Range
467   and Content-Range header fields or if it does not understand the
468   range units used in those fields.
469</t>
470<t>
471   A cache MAY complete a stored incomplete response by making a subsequent
472   range request (<xref target="Part5"/>) and combining the successful response with the
473   stored entry, as defined in <xref target="combining.responses"/>.
474   A cache MUST NOT use an incomplete response to answer requests
475   unless the response has been made complete or the request is partial and
476   specifies a range that is wholly within the incomplete response.
477   A cache MUST NOT send a partial response to a client without explicitly
478   marking it as such using the 206 (Partial Content) status code.
479</t>
480</section>
481
482
483<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
484<t>
485   For a presented request, a cache MUST NOT return a stored response,
486   unless:
487   <list style="symbols">
488      <t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
489      that of the stored response match, and</t>
490      <t>the request method associated with the stored response allows it to
491      be used for the presented request, and</t>
492      <t>selecting header fields nominated by the stored response (if any)
493      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
494      <t>the presented request does not contain the no-cache pragma (<xref target="header.pragma"/>), nor the no-cache cache directive (<xref target="cache-request-directive"/>), unless the stored response is
495      successfully validated (<xref target="validation.model"/>), and</t>
496      <t>the stored response does not contain the no-cache cache directive
497      (<xref target="cache-response-directive"/>), unless it is successfully
498      validated (<xref target="validation.model"/>), and</t>
499      <t>the stored response is either:
500         <list style="symbols">
501            <t>fresh (see <xref target="expiration.model"/>), or</t>
502            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
503            <t>successfully validated (see <xref target="validation.model"/>).</t>
504         </list>
505      </t>
506  </list>
507</t>
508<t>
509   Note that any of the requirements listed above can be overridden by a
510   cache-control extension; see <xref target="cache.control.extensions"/>.
511</t>
512<t>
513   When a stored response is used to satisfy a request without validation,
514   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
515   current_age; see <xref target="age.calculations"/>.
516</t>
517<t>
518   A cache MUST write through requests with methods that are unsafe
519   (Section 6.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
520   a reply to such a request before having forwarded the request and having
521   received a corresponding response.
522</t>
523<t>
524   Also, note that unsafe requests might invalidate already stored responses;
525   see <xref target="invalidation.after.updates.or.deletions"/>.
526</t>
527<t>
528   When more than one suitable response is stored, a cache MUST use the
529   most recent response (as determined by the Date header field). It can also
530   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
531   no-cache" to disambiguate which response to use.
532</t>
533<t>
534   A cache that does not have a clock available MUST NOT use stored responses
535   without revalidating them on every use. A cache, especially a shared
536   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
537   synchronize its clock with a reliable external standard.
538</t>
539
540</section>
541
542<section anchor="expiration.model" title="Freshness Model">
543<t>
544   When a response is "fresh" in the cache, it can be used to satisfy
545   subsequent requests without contacting the origin server, thereby improving
546   efficiency.
547</t>
548<t>
549   The primary mechanism for determining freshness is for an origin server to
550   provide an explicit expiration time in the future, using either the Expires
551   header field (<xref target="header.expires"/>) or the max-age response cache
552   directive (<xref target="cache-response-directive"/>). Generally, origin
553   servers will assign future explicit expiration times to responses in the
554   belief that the representation is not likely to change in a semantically
555   significant way before the expiration time is reached.
556</t>
557<t>
558   If an origin server wishes to force a cache to validate every request, it
559   can assign an explicit expiration time in the past to indicate that the
560   response is already stale. Compliant caches will normally validate the
561   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
562</t>
563<t>
564   Since origin servers do not always provide explicit expiration times,
565   a cache MAY assign a heuristic expiration time when an explicit time is not
566   specified, employing algorithms that use other header field values (such as the
567   Last-Modified time) to estimate a plausible expiration time. This
568   specification does not provide specific algorithms, but does impose
569   worst-case constraints on their results.
570</t>
571<figure>
572<preamble>
573  The calculation to determine if a response is fresh is:
574</preamble>
575<artwork type="code"><![CDATA[
576   response_is_fresh = (freshness_lifetime > current_age)
577]]></artwork>
578</figure>
579<t>
580   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
581   <xref target="age.calculations"/>.
582</t>
583<t>
584   Additionally, clients can influence freshness calculation -- either
585   constraining it relaxing it -- by using the max-age and min-fresh
586   request cache directives. See <xref target="cache-request-directive"/> 
587   for details.
588</t>
589<t>
590   Note that freshness applies only to cache operation; it cannot be used to
591   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
592   caches and history mechanisms.
593</t>
594
595<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
596<t>
597   A cache can calculate the freshness lifetime (denoted as
598   freshness_lifetime) of a response by using the first match of:
599   <list style="symbols">
600      <t>If the cache is shared and the s-maxage response cache directive
601      (<xref target="cache-response-directive"/>) is present, use its value,
602      or</t>
603      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
604      <t>If the Expires response header field (<xref target="header.expires"/>) is
605      present, use its value minus the value of the Date response header field,
606      or</t>
607      <t>Otherwise, no explicit expiration time is present in the response. A
608      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
609   </list>
610</t>
611<t>
612   Note that this calculation is not vulnerable to clock skew, since all of
613   the information comes from the origin server.
614</t>
615
616<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
617<t>
618   If no explicit expiration time is present in a stored response that has a
619   status code whose definition allows heuristic freshness to be used
620   (including the following in Section 7 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
621   410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
622   use heuristics to determine freshness for responses with status codes that do
623   not explicitly allow it.
624</t>
625<t>
626   When a heuristic is used to calculate freshness lifetime, a cache
627   SHOULD attach a Warning header field with a 113 warn-code to the response if
628   its current_age is more than 24 hours and such a warning is not already
629   present.
630</t>
631<t>
632   Also, if the response has a Last-Modified header field
633   (Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
634   expiration value that is no more than some fraction of the interval since
635   that time. A typical setting of this fraction might be 10%.
636</t>
637<t><list>
638   <t>
639      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
640      freshness for URIs with query components (i.e., those containing '?').
641      In practice, this has not been widely implemented. Therefore, servers
642      are encouraged to send explicit directives (e.g., Cache-Control:
643      no-cache) if they wish to preclude caching.
644   </t>
645</list></t>
646</section>
647</section>
648
649<section anchor="age.calculations" title="Calculating Age">
650<t>
651   HTTP/1.1 uses the Age header field to convey the estimated age of the
652   response message when obtained from a cache. The Age field value is the
653   cache's estimate of the amount of time since the response was generated or
654   validated by the origin server. In essence, the Age value is the sum of the
655   time that the response has been resident in each of the caches along the
656   path from the origin server, plus the amount of time it has been in transit
657   along network paths.
658</t>
659<t>
660   The following data is used for the age calculation:
661</t>
662<t>
663   <?rfc needLines="4"?>age_value
664   <list>
665      <t>
666         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
667         operation; or 0, if not available.
668      </t>
669   </list>
670</t>
671<t>
672   <?rfc needLines="4"?>date_value
673   <list>
674      <t>
675         HTTP/1.1 requires origin servers to send a Date header field, if possible,
676         with every response, giving the time at which the response was
677         generated. The term "date_value" denotes the value of the Date
678         header field, in a form appropriate for arithmetic operations. See
679         Section 10.2 of <xref target="Part2"/> for the definition of the Date header field, and for
680         requirements regarding responses without it.
681      </t>
682   </list>
683</t>
684<t>
685   <?rfc needLines="4"?>now
686   <list>
687      <t>
688         The term "now" means "the current value of the clock at the host
689         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
690         clocks to a globally accurate time standard.
691      </t>
692   </list>
693</t>
694<t>
695   <?rfc needLines="4"?>request_time
696   <list>
697      <t>
698         The current value of the clock at the host at the time the request
699         resulting in the stored response was made.
700      </t>
701   </list>
702</t>
703<t>
704   <?rfc needLines="4"?>response_time
705   <list>
706      <t>
707         The current value of the clock at the host at the time the response
708         was received.
709      </t>
710   </list>
711</t>
712<t>
713   A response's age can be calculated in two entirely independent ways:
714   <list style="numbers">
715      <t>the "apparent_age": response_time minus date_value, if the local
716      clock is reasonably well synchronized to the origin server's clock. If
717      the result is negative, the result is replaced by zero.</t>
718      <t>the "corrected_age_value", if all of the caches along the response
719      path implement HTTP/1.1. A cache MUST interpret this value relative
720      to the time the request was initiated, not the time that the response
721      was received.</t>
722   </list>
723</t>
724<figure>
725<artwork type="code"><![CDATA[
726  apparent_age = max(0, response_time - date_value);
727
728  response_delay = response_time - request_time;
729  corrected_age_value = age_value + response_delay; 
730]]></artwork>
731</figure>
732<figure>
733<preamble>These SHOULD be combined as</preamble>
734<artwork type="code"><![CDATA[
735  corrected_initial_age = max(apparent_age, corrected_age_value);
736]]></artwork></figure>
737<t>
738   unless the cache is confident in the value of the Age header (e.g., because
739   there are no HTTP/1.0 hops in the Via header), in which case the
740   corrected_age_value MAY be used as the corrected_initial_age.</t>
741<t>
742   The current_age of a stored response can then be calculated by adding the
743   amount of time (in seconds) since the stored response was last validated by
744   the origin server to the corrected_initial_age.
745</t>
746<figure><artwork type="code"><![CDATA[
747  resident_time = now - response_time;
748  current_age = corrected_initial_age + resident_time;
749]]></artwork></figure>
750<t>
751   Additionally, to avoid common problems in date parsing:
752</t>
753<t>
754  <list style="symbols">
755     <t>HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
756        which appears to be more than 50 years in the future is in fact
757        in the past (this helps solve the "year 2000" problem).</t>
758
759     <t>Although all date formats are specified to be case-sensitive,
760        recipients SHOULD match day, week and timezone names
761        case-insensitively.</t>
762             
763     <t>An HTTP/1.1 implementation MAY internally represent a parsed
764        Expires date as earlier than the proper value, but MUST NOT
765        internally represent a parsed Expires date as later than the
766        proper value.</t>
767
768     <t>All expiration-related calculations MUST be done in GMT. The
769        local time zone MUST NOT influence the calculation or comparison
770        of an age or expiration time.</t>
771
772     <t>If an HTTP header field incorrectly carries a date value with a time
773        zone other than GMT, it MUST be converted into GMT using the
774        most conservative possible conversion.</t>
775  </list>
776</t>
777</section>
778
779<section anchor="serving.stale.responses" title="Serving Stale Responses">
780<t>
781   A "stale" response is one that either has explicit expiry information or is
782   allowed to have heuristic expiry calculated, but is not fresh according to
783   the calculations in <xref target="expiration.model"/>.
784</t>
785<t>
786   A cache MUST NOT return a stale response if it is prohibited by an
787   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
788   directive, a "must-revalidate" cache-response-directive, or an applicable
789   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
790</t>
791<t>
792   A cache MUST NOT return stale responses unless it is disconnected
793   (i.e., it cannot contact the origin server or otherwise find a forward
794   path) or doing so is explicitly allowed (e.g., by the max-stale request
795   directive; see <xref target="cache-request-directive"/>).
796</t>
797<t>
798   A cache SHOULD append a Warning header field with the 110 warn-code (see
799   <xref target="header.warning"/>) to stale responses. Likewise, a cache
800   SHOULD add the 112 warn-code to stale responses if the cache is
801   disconnected.
802</t>
803<t>
804   If a cache receives a first-hand response (either an entire response, or a
805   304 (Not Modified) response) that it would normally forward to the
806   requesting client, and the received response is no longer fresh, the cache
807   can forward it to the requesting client without adding a new Warning (but
808   without removing any existing Warning header fields). A cache shouldn't
809   attempt to validate a response simply because that response became stale in
810   transit.
811</t>
812</section>
813</section>
814
815<section anchor="validation.model" title="Validation Model">
816<t>
817   When a cache has one or more stored responses for a requested URI, but
818   cannot serve any of them (e.g., because they are not fresh, or one cannot
819   be selected; see <xref target="caching.negotiated.responses"/>), it can use
820   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
821   give the origin server an opportunity to both select a valid stored
822   response to be used, and to update it. This process is known as
823   "validating" or "revalidating" the stored response.
824</t>
825<t>
826   When sending such a conditional request, a cache adds an If-Modified-Since
827   header field whose value is that of the Last-Modified header field from the
828   selected (see <xref target="caching.negotiated.responses"/>) stored
829   response, if available.
830</t>
831<t>
832   Additionally, a cache can add an If-None-Match header field whose value is
833   that of the ETag header field(s) from all responses stored for the
834   requested URI, if present. However, if any of the stored responses contains
835   only partial content, the cache shouldn't include its entity-tag in the
836   If-None-Match header field unless the request is for a range that would be
837   fully satisfied by that stored response.
838</t>
839
840<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
841
842<t>
843   <list style="symbols">
844      <t>
845         A 304 (Not Modified) response status code indicates that the stored
846         response can be updated and reused; see <xref target="freshening.responses"/>.
847      </t>
848      <t>
849         A full response (i.e., one with a response body) indicates that none
850         of the stored responses nominated in the conditional request is
851         suitable. Instead, the cache can use the full response to
852         satisfy the request and MAY replace the stored response(s).
853      </t>
854      <t>
855         However, if a cache receives a 5xx response while attempting to
856         validate a response, it can either forward this response to the
857         requesting client, or act as if the server failed to respond. In the
858         latter case, it can return a previously stored response (see <xref target="serving.stale.responses"/>).
859      </t>
860   </list>
861</t>
862
863<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
864<t>
865   When a cache receives a 304 (Not Modified) response and already has one
866   or more stored 200 (OK) responses for the same cache key, the cache needs
867   to identify which of the stored responses are updated by this new response
868   and then update the stored response(s) with the new information provided in
869   the 304 response.
870   <list style="symbols">
871    <t>
872     If the new response contains a strong validator, then that strong
873     validator identifies the selected representation.  All of the stored
874     responses with the same strong validator are selected.
875     If none of the stored responses contain the same strong validator, then
876     this new response corresponds to a new selected representation and
877     MUST NOT update the existing stored responses.
878    </t>
879    <t>
880     If the new response contains a weak validator and that validator
881     corresponds to one of the cache's stored responses, then the most
882     recent of those matching stored responses is selected.
883    </t>
884    <t>
885     If the new response does not include any form of validator, there is
886     only one stored response, and that stored response also lacks a
887     validator, then that stored response is selected.
888    </t>
889   </list>
890</t>
891<t>
892   If a stored response is selected for update, the cache MUST:
893   <list style="symbols">
894      <t>delete any Warning header fields in the stored response with
895         warn-code 1xx (see <xref target="header.warning"/>);</t>
896      <t>retain any Warning header fields in the stored response with
897         warn-code 2xx; and,</t>
898      <t>use other header fields provided in the 304 response to replace
899         all instances of the corresponding header fields in the stored
900         response.</t>
901   </list>
902</t>
903</section>
904
905</section>
906
907<section anchor="head.effects" title="Updating Caches with HEAD Responses">
908<t>
909   A response to the HEAD method is identical to what an equivalent request
910   made with a GET would have been, except it lacks a body. This property
911   of HEAD responses is used to both invalidate and update cached GET
912   responses.
913</t>
914<t>
915   If one or more stored GET responses can be selected (as per <xref target="caching.negotiated.responses"/>) for a HEAD request, and the
916   Content-Length, ETag or Last-Modified value of a HEAD response differs from
917   that in a selected GET response, the cache MUST consider that selected
918   response to be stale.
919</t>
920<t>
921   If the Content-Length, ETag and Last-Modified values of a HEAD response
922   (when present) are the same as that in a selected GET response (as per
923   <xref target="caching.negotiated.responses"/>), the cache SHOULD update the
924   remaining headers in the stored response using the following rules:
925   <list style="symbols">
926      <t>delete any Warning header fields in the stored response with
927         warn-code 1xx (see <xref target="header.warning"/>);</t>
928      <t>retain any Warning header fields in the stored response with
929         warn-code 2xx; and,</t>
930      <t>use other header fields provided in the response to replace
931         all instances of the corresponding header fields in the stored
932         response.</t>
933   </list>
934</t>
935
936</section>
937
938<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
939<t>
940   Because unsafe request methods (Section 6.1.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
941   have the potential for changing state on the origin server, intervening
942   caches can use them to keep their contents up-to-date.
943</t>
944<t>
945   A cache MUST invalidate the effective Request URI
946   (Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the Location
947   and Content-Location response header fields (if present) when a non-error
948   response to a request with an unsafe method is received.
949</t>
950<t>
951   However, a cache MUST NOT invalidate a URI from a Location or
952   Content-Location response header field if the host part of that URI differs
953   from the host part in the effective request URI (Section 5.5 of <xref target="Part1"/>).
954   This helps prevent denial of service attacks.
955</t>
956<t>
957   A cache MUST invalidate the effective request URI
958   (Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
959   to a request with a method whose safety is unknown.
960</t>
961<t>
962   Here, a "non-error response" is one with a 2xx or 3xx status code.
963   "Invalidate" means that the cache will either remove all stored
964   responses related to the effective request URI, or will mark these as
965   "invalid" and in need of a mandatory validation before they can be returned
966   in response to a subsequent request.
967</t>
968<t>
969   Note that this does not guarantee that all appropriate responses are
970   invalidated. For example, the request that caused the change at the origin
971   server might not have gone through the cache where a response is stored.
972</t>
973</section>
974
975<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
976
977<t>
978   A shared cache MUST NOT use a cached response to a request with an
979   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
980   request unless a cache directive that allows such responses to be stored is
981   present in the response.
982</t>
983
984<t>
985   In this specification, the following Cache-Control response directives
986   (<xref target="cache-response-directive"/>) have such an effect:
987   must-revalidate, public, s-maxage.
988</t>
989
990<t>
991   Note that cached responses that contain the "must-revalidate" and/or
992   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
993   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
994   used to satisfy a subsequent request without revalidating it on the origin
995   server.
996</t>
997</section>
998
999<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
1000<t>
1001   When a cache receives a request that can be satisfied by a stored response
1002   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
1003   use that response unless all of the selecting header fields nominated by
1004   the Vary header field match in both the original request (i.e., that associated
1005   with the stored response), and the presented request.
1006</t>
1007<t>
1008   The selecting header fields from two requests are defined to match if and
1009   only if those in the first request can be transformed to those in the
1010   second request by applying any of the following:
1011   <list style="symbols">
1012      <t>
1013         adding or removing whitespace, where allowed in the header field's syntax
1014      </t>
1015      <t>
1016         combining multiple header fields with the same field name
1017         (see Section 3.2 of <xref target="Part1"/>)
1018      </t>
1019      <t>
1020         normalizing both header field values in a way that is known to have
1021         identical semantics, according to the header field's specification (e.g.,
1022         re-ordering field values when order is not significant;
1023         case-normalization, where values are defined to be case-insensitive)
1024      </t>
1025  </list>
1026</t>
1027<t>
1028   If (after any normalization that might take place) a header field is absent
1029   from a request, it can only match another request if it is also absent
1030   there.
1031</t>
1032<t>
1033   A Vary header field-value of "*" always fails to match, and subsequent
1034   requests to that resource can only be properly interpreted by the origin
1035   server.
1036</t>
1037<t>
1038   The stored response with matching selecting header fields is known as the
1039   selected response.
1040</t>
1041<t>
1042   If multiple selected responses are available, the most recent response
1043   (as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
1044</t>
1045<t>
1046   If no selected response is available, the cache can forward the presented
1047   request to the origin server in a conditional request; see <xref target="validation.model"/>.
1048</t>
1049</section>
1050
1051<section anchor="combining.responses" title="Combining Partial Content">
1052<t>
1053   A response might transfer only a partial representation if the
1054   connection closed prematurely or if the request used one or more Range
1055   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
1056   received several ranges of the same representation.  A cache MAY combine
1057   these ranges into a single stored response, and reuse that response to
1058   satisfy later requests, if they all share the same strong validator and
1059   the cache complies with the client requirements in Section 4.2 of <xref target="Part5"/>.
1060</t>
1061<t>
1062   When combining the new response with one or more stored responses, a
1063   cache MUST:
1064   <list style="symbols">
1065      <t>delete any Warning header fields in the stored response with
1066         warn-code 1xx (see <xref target="header.warning"/>);</t>
1067      <t>retain any Warning header fields in the stored response with
1068         warn-code 2xx; and,</t>
1069      <t>use other header fields provided in the new response, aside
1070         from Content-Range, to replace all instances of the corresponding
1071         header fields in the stored response.</t>
1072   </list>
1073</t>
1074</section>
1075
1076
1077</section>
1078
1079<section anchor="header.field.definitions" title="Header Field Definitions">
1080<t>
1081   This section defines the syntax and semantics of HTTP/1.1 header fields
1082   related to caching.
1083</t>
1084
1085<section anchor="header.age" title="Age">
1086   <iref item="Age header field" primary="true"/>
1087   <iref item="Header Fields" primary="true" subitem="Age"/>
1088   
1089   
1090<t>
1091   The "Age" header field conveys the sender's estimate of the amount
1092   of time since the response was generated or successfully validated at the
1093   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1094</t>
1095<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1096  Age = delta-seconds
1097]]></artwork></figure>
1098<t>
1099  Age field-values are non-negative integers, representing time in seconds
1100  (see <xref target="delta-seconds"/>).
1101</t>
1102<t>
1103   The presence of an Age header field in a response implies that a response
1104   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1105   might not implement the Age header field.
1106</t>
1107</section>
1108
1109<section anchor="header.cache-control" title="Cache-Control">
1110   <iref item="Cache-Control header field" primary="true"/>
1111   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1112   
1113   
1114   
1115   
1116   
1117<t>
1118   The "Cache-Control" header field is used to specify directives for
1119   caches along the request/response chain. Such cache directives are
1120   unidirectional in that the presence of a directive in a request does not
1121   imply that the same directive is to be given in the response.
1122</t>
1123<t>
1124   A cache MUST obey the requirements of the Cache-Control
1125   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1126   directives defined elsewhere are handled.
1127</t>
1128<t><list>
1129   <t>
1130       Note: HTTP/1.0 caches might not implement Cache-Control and
1131       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1132   </t>
1133</list></t>
1134<t>
1135   A proxy, whether or not it implements a cache, MUST pass cache directives
1136   through in forwarded messages, regardless of their
1137   significance to that application, since the directives might be applicable
1138   to all recipients along the request/response chain. It is not possible to
1139   target a directive to a specific cache.
1140</t>
1141<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
1142<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1143  Cache-Control   = 1#cache-directive
1144
1145  cache-directive = cache-request-directive
1146     / cache-response-directive
1147
1148  cache-extension = token [ "=" ( token / quoted-string ) ]
1149]]></artwork></figure>
1150
1151<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1152   
1153
1154<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1155  cache-request-directive =
1156       "no-cache"
1157     / "no-store"
1158     / "max-age" "=" delta-seconds
1159     / "max-stale" [ "=" delta-seconds ]
1160     / "min-fresh" "=" delta-seconds
1161     / "no-transform"
1162     / "only-if-cached"
1163     / cache-extension
1164]]></artwork></figure>
1165
1166<t>
1167   <?rfc needLines="4"?>no-cache
1168   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1169   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1170   <list>
1171      <t>The no-cache request directive indicates that a cache MUST NOT
1172      use a stored response to satisfy the request without successful
1173      validation on the origin server.</t> 
1174   </list>
1175</t>
1176<t>
1177   <?rfc needLines="4"?>no-store
1178   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1179   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1180   <list>
1181      <t>The no-store request directive indicates that a cache MUST NOT
1182      store any part of either this request or any response to it. This
1183      directive applies to both private and shared caches. "MUST NOT
1184      store" in this context means that the cache MUST NOT intentionally
1185      store the information in non-volatile storage, and MUST make a
1186      best-effort attempt to remove the information from volatile storage as
1187      promptly as possible after forwarding it.</t>
1188      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1189      privacy. In particular, malicious or compromised caches might not
1190      recognize or obey this directive, and communications networks might be
1191      vulnerable to eavesdropping.</t>
1192      <t>Note that if a request containing this directive is satisfied from a
1193      cache, the no-store request directive does not apply to the already
1194      stored response.</t>
1195   </list>
1196</t>
1197<t>
1198   <?rfc needLines="4"?>max-age
1199   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1200   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1201   <list>
1202      <t>The max-age request directive indicates that the client is unwilling to
1203      accept a response whose age is greater than the specified number of
1204      seconds. Unless the max-stale request directive is also present, the
1205      client is not willing to accept a stale response.</t>
1206   </list>
1207</t>
1208<t>
1209   <?rfc needLines="4"?>max-stale
1210   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1211   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1212   <list>
1213      <t>The max-stale request directive indicates that the client is willing
1214      to accept a response that has exceeded its expiration time. If max-stale
1215      is assigned a value, then the client is willing to accept a response
1216      that has exceeded its expiration time by no more than the specified
1217      number of seconds. If no value is assigned to max-stale, then the client
1218      is willing to accept a stale response of any age.</t>
1219   </list>
1220</t>
1221<t>
1222   <?rfc needLines="4"?>min-fresh
1223   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1224   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1225   <list>
1226      <t>The min-fresh request directive indicates that the client is willing
1227      to accept a response whose freshness lifetime is no less than its
1228      current age plus the specified time in seconds. That is, the client
1229      wants a response that will still be fresh for at least the specified
1230      number of seconds.</t>
1231   </list>
1232</t>
1233<t>
1234   <?rfc needLines="4"?>no-transform
1235   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1236   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1237   <list>
1238      <t>The no-transform request directive indicates that an intermediary
1239        (whether or not it implements a cache) MUST NOT change the
1240        Content-Encoding, Content-Range or Content-Type request header fields,
1241        nor the request representation.</t>
1242   </list>
1243</t>
1244<t>
1245   <?rfc needLines="4"?>only-if-cached
1246   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1247   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1248   <list>
1249      <t>The only-if-cached request directive indicates that the client only
1250      wishes to obtain a stored response. If it receives this directive, a
1251      cache SHOULD either respond using a stored response that is consistent
1252      with the other constraints of the request, or respond with a 504
1253      (Gateway Timeout) status code. If a group of caches is being operated as
1254      a unified system with good internal connectivity, a member cache MAY
1255      forward such a request within that group of caches.</t>
1256   </list>
1257</t>
1258</section>
1259
1260<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1261   
1262
1263<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1264  cache-response-directive =
1265       "public"
1266     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1267     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1268     / "no-store"
1269     / "no-transform"
1270     / "must-revalidate"
1271     / "proxy-revalidate"
1272     / "max-age" "=" delta-seconds
1273     / "s-maxage" "=" delta-seconds
1274     / cache-extension
1275]]></artwork></figure>
1276
1277<t>
1278   <?rfc needLines="4"?>public
1279   <iref item="Cache Directives" primary="true" subitem="public"/>
1280   <iref item="public" primary="true" subitem="Cache Directive"/>
1281   <list>
1282      <t>The public response directive indicates that a response whose
1283        associated request contains an 'Authentication' header MAY be
1284        stored (see <xref target="caching.authenticated.responses"/>).</t>
1285  </list>
1286</t>
1287<t>
1288   <?rfc needLines="4"?>private
1289   <iref item="Cache Directives" primary="true" subitem="private"/>
1290   <iref item="private" primary="true" subitem="Cache Directive"/>
1291   <list>
1292      <t>The private response directive indicates that the response message is
1293      intended for a single user and MUST NOT be stored by a shared cache. A
1294      private cache MAY store the response.</t>
1295      <t>If the private response directive specifies one or more field-names,
1296      this requirement is limited to the field-values associated with the
1297      listed response header fields. That is, a shared cache MUST NOT store
1298      the specified field-names(s), whereas it MAY store the remainder of the
1299      response message.</t>
1300      <t> Note: This usage of the word "private" only controls
1301      where the response can be stored; it cannot ensure the privacy of the
1302      message content. Also, private response directives with field-names are
1303      often handled by implementations as if an unqualified private directive
1304      was received; i.e., the special handling for the qualified form is not
1305      widely implemented.</t>
1306   </list>
1307</t>
1308<t>
1309   <?rfc needLines="4"?>no-cache
1310   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1311   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1312   <list>
1313      <t>The no-cache response directive indicates that the response MUST NOT
1314      be used to satisfy a subsequent request without successful validation on
1315      the origin server. This allows an origin server to prevent a cache from
1316      using it to satisfy a request without contacting it, even by caches that
1317      have been configured to return stale responses.</t>
1318      <t>If the no-cache response directive specifies one or more field-names,
1319      then a cache MAY use the response to satisfy a subsequent request,
1320      subject to any other restrictions on caching. However, any header fields
1321      in the response that have the field-name(s) listed MUST NOT be sent
1322      in the response to a subsequent request without successful revalidation
1323      with the origin server. This allows an origin server to prevent the
1324      re-use of certain header fields in a response, while still allowing
1325      caching of the rest of the response.</t> 
1326      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1327      this directive. Also, no-cache response directives with field-names are
1328      often handled by implementations as if an unqualified no-cache directive
1329      was received; i.e., the special handling for the qualified form is not
1330      widely implemented.</t>
1331   </list>
1332</t>
1333<t>
1334   <?rfc needLines="4"?>no-store
1335   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1336   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1337   <list>
1338      <t>The no-store response directive indicates that a cache MUST NOT
1339      store any part of either the immediate request or response. This
1340      directive applies to both private and shared caches. "MUST NOT
1341      store" in this context means that the cache MUST NOT intentionally
1342      store the information in non-volatile storage, and MUST make a
1343      best-effort attempt to remove the information from volatile storage as
1344      promptly as possible after forwarding it.</t>
1345      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1346      privacy. In particular, malicious or compromised caches might not
1347      recognize or obey this directive, and communications networks might be
1348      vulnerable to eavesdropping.</t>
1349   </list>
1350</t>
1351<t>
1352   <?rfc needLines="4"?>must-revalidate
1353   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1354   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1355   <list>
1356      <t>The must-revalidate response directive indicates that once it has
1357      become stale, a cache MUST NOT use the response to satisfy subsequent
1358      requests without successful validation on the origin server.</t>
1359      <t>The must-revalidate directive is necessary to support reliable
1360      operation for certain protocol features. In all circumstances a
1361      cache MUST obey the must-revalidate directive; in particular,
1362      if a cache cannot reach the origin server for any reason, it MUST
1363      generate a 504 (Gateway Timeout) response.</t>
1364      <t>The must-revalidate directive ought to be used by servers if and only
1365      if failure to validate a request on the representation could result in
1366      incorrect operation, such as a silently unexecuted financial
1367      transaction.</t>
1368   </list>
1369</t>
1370<t>
1371   <?rfc needLines="4"?>proxy-revalidate
1372   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1373   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1374   <list>
1375      <t>The proxy-revalidate response directive has the same meaning as the
1376      must-revalidate response directive, except that it does not apply to
1377      private caches.</t>
1378   </list>
1379</t>
1380<t>
1381   <?rfc needLines="4"?>max-age
1382   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1383   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1384   <list>
1385      <t>The max-age response directive indicates that the response is to be
1386      considered stale after its age is greater than the specified number of
1387      seconds.</t>
1388   </list>
1389</t>
1390<t>
1391   <?rfc needLines="4"?>s-maxage
1392   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1393   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1394   <list>
1395      <t>The s-maxage response directive indicates that, in shared caches, the
1396      maximum age specified by this directive overrides the maximum age
1397      specified by either the max-age directive or the Expires header field. The
1398      s-maxage directive also implies the semantics of the proxy-revalidate
1399      response directive.</t>
1400   </list>
1401</t>
1402<t>
1403   <?rfc needLines="4"?>no-transform
1404   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1405   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1406   <list>
1407      <t>The no-transform response directive indicates that an intermediary
1408      (regardless of whether it implements a cache) MUST NOT change the
1409      Content-Encoding, Content-Range or Content-Type response header fields,
1410      nor the response representation.</t>
1411   </list>
1412</t>
1413
1414</section>
1415
1416<section anchor="cache.control.extensions" title="Cache Control Extensions">
1417<t>
1418   The Cache-Control header field can be extended through the use of one or
1419   more cache-extension tokens, each with an optional value. Informational
1420   extensions (those that do not require a change in cache behavior) can be
1421   added without changing the semantics of other directives. Behavioral
1422   extensions are designed to work by acting as modifiers to the existing base
1423   of cache directives. Both the new directive and the standard directive are
1424   supplied, such that applications that do not understand the new directive
1425   will default to the behavior specified by the standard directive, and those
1426   that understand the new directive will recognize it as modifying the
1427   requirements associated with the standard directive. In this way,
1428   extensions to the cache-control directives can be made without requiring
1429   changes to the base protocol.
1430</t>
1431<t>
1432   This extension mechanism depends on an HTTP cache obeying all of the
1433   cache-control directives defined for its native HTTP-version, obeying
1434   certain extensions, and ignoring all directives that it does not
1435   understand.
1436</t>
1437<t>
1438   For example, consider a hypothetical new response directive called
1439   "community" that acts as a modifier to the private directive. We define
1440   this new directive to mean that, in addition to any private cache, any
1441   cache that is shared only by members of the community named within its
1442   value may cache the response. An origin server wishing to allow the UCI
1443   community to use an otherwise private response in their shared cache(s)
1444   could do so by including
1445</t>
1446<figure><artwork type="example"><![CDATA[
1447  Cache-Control: private, community="UCI"
1448]]></artwork></figure>
1449<t>
1450   A cache seeing this header field will act correctly even if the cache does
1451   not understand the community cache-extension, since it will also see and
1452   understand the private directive and thus default to the safe behavior.
1453</t>
1454<t>
1455   A cache MUST ignore unrecognized cache directives; it is assumed that any
1456   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1457   combined with standard directives (or the response's default cacheability)
1458   such that the cache behavior will remain minimally correct even if the
1459   cache does not understand the extension(s).
1460</t>
1461<t>
1462   The HTTP Cache Directive Registry defines the name space for the cache
1463   directives.
1464</t>
1465<t>
1466   A registration MUST include the following fields:
1467   <list style="symbols">
1468      <t>Cache Directive Name</t>
1469      <t>Pointer to specification text</t>
1470   </list>
1471</t>
1472<t>
1473   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1474</t>
1475<t>
1476   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1477</t>
1478</section>
1479
1480</section>
1481
1482<section anchor="header.expires" title="Expires">
1483   <iref item="Expires header field" primary="true"/>
1484   <iref item="Header Fields" primary="true" subitem="Expires"/>
1485   
1486<t>
1487   The "Expires" header field gives the date/time after which the
1488   response is considered stale. See <xref target="expiration.model"/> for
1489   further discussion of the freshness model.
1490</t>
1491<t>
1492   The presence of an Expires field does not imply that the original resource
1493   will change or cease to exist at, before, or after that time.
1494</t>
1495<t>
1496   The field-value is an absolute date and time as defined by HTTP-date in
1497   Section 8 of <xref target="Part2"/>; a sender MUST use the rfc1123-date format.
1498</t>
1499<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1500  Expires = HTTP-date
1501]]></artwork></figure>
1502<figure>
1503  <preamble>For example</preamble>
1504<artwork type="example"><![CDATA[
1505  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1506]]></artwork></figure>
1507<t>
1508   A cache MUST treat other invalid date formats,
1509   especially including the value "0", as in the past (i.e., "already
1510   expired").
1511</t>
1512<t><list>
1513   <t>
1514       Note: If a response includes a Cache-Control field with the
1515       max-age directive (see <xref target="cache-response-directive"/>),
1516       that directive overrides the Expires field. Likewise, the s-maxage
1517       directive overrides Expires in shared caches.
1518   </t>
1519</list></t>
1520<t>
1521   Historically, HTTP required the Expires field-value to be no more than a
1522   year in the future. While longer freshness lifetimes are no longer
1523   prohibited, extremely large values have been demonstrated to cause
1524   problems (e.g., clock overflows due to use of 32-bit integers for
1525   time values), and most caches will evict a response far sooner than
1526   that. Therefore, senders ought not produce them.
1527</t>
1528<t>
1529   An origin server without a clock MUST NOT assign Expires
1530   values to a response unless these values were associated
1531   with the resource by a system or user with a reliable clock. It MAY
1532   assign an Expires value that is known, at or before server
1533   configuration time, to be in the past (this allows "pre-expiration"
1534   of responses without storing separate Expires values for each
1535   resource).
1536</t>
1537</section>
1538
1539<section anchor="header.pragma" title="Pragma">
1540   <iref item="Pragma header field" primary="true"/>
1541   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1542   
1543   
1544   
1545<t>
1546   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1547   caches, so that clients can specify a "no-cache" request that they will
1548   understand (as Cache-Control was not defined until HTTP/1.1). When the
1549   Cache-Control header is also present and understood in a request, Pragma is
1550   ignored.
1551</t>
1552<t>
1553   In HTTP/1.0, Pragma was defined as an extensible field for
1554   implementation-specified directives for recipients. This specification
1555   deprecates such extensions to improve interoperability.
1556</t>
1557<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[
1558  Pragma           = 1#pragma-directive
1559  pragma-directive = "no-cache" / extension-pragma
1560  extension-pragma = token [ "=" ( token / quoted-string ) ]
1561]]></artwork></figure>
1562<t>
1563   When the Cache-Control header is not present in a request, the no-cache
1564   request pragma-directive MUST have the same effect on caches as if
1565   "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
1566</t>
1567<t>
1568   When sending a no-cache request, a client ought to include both the pragma
1569   and cache-control directives, unless Cache-Control: no-cache is
1570   purposefully omitted to target other Cache-Control response directives at
1571   HTTP/1.1 caches. For example:
1572</t>
1573<figure>
1574<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1575  GET / HTTP/1.1
1576  Host: www.example.com
1577  Cache-Control: max-age=30
1578  Pragma: no-cache
1579 
1580  ]]></artwork>
1581</figure>
1582<t>
1583   will constrain HTTP/1.1 caches to serve a response no older than 30
1584   seconds, while precluding implementations that do not understand
1585   Cache-Control from serving a cached response.
1586</t>
1587<t><list>
1588   <t>
1589      Note: Because the meaning of "Pragma: no-cache" in responses is not
1590      specified, it does not provide a reliable replacement for
1591      "Cache-Control: no-cache" in them.
1592   </t>
1593</list></t>
1594</section>
1595
1596<section anchor="header.vary" title="Vary">
1597   <iref item="Vary header field" primary="true"/>
1598   <iref item="Header Fields" primary="true" subitem="Vary"/>
1599   
1600<t>
1601   The "Vary" header field conveys the set of header fields
1602   that were used to select the representation.
1603</t>
1604<t>
1605   Caches use this information, in part, to determine whether a stored
1606   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1607   fresh, whether a cache is permitted to use the response to reply to a
1608   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1609</t>
1610<t>
1611   In uncacheable or stale responses, the Vary field value advises the user
1612   agent about the criteria that were used to select the representation.
1613</t>
1614<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
1615  Vary = "*" / 1#field-name
1616]]></artwork></figure>
1617<t>
1618   The set of header fields named by the Vary field value is known as the
1619   selecting header fields.
1620</t>
1621<t>
1622   A server SHOULD include a Vary header field with any cacheable response
1623   that is subject to server-driven negotiation. Doing so allows a cache to
1624   properly interpret future requests on that resource and informs the user
1625   agent about the presence of negotiation on that resource. A server MAY
1626   include a Vary header field with a non-cacheable response that is subject
1627   to server-driven negotiation, since this might provide the user agent with
1628   useful information about the dimensions over which the response varies at
1629   the time of the response.
1630</t>
1631<t>
1632   A Vary field value of "*" signals that unspecified parameters not limited
1633   to the header fields (e.g., the network address of the client), play a
1634   role in the selection of the response representation; therefore, a cache
1635   cannot determine whether this response is appropriate. A proxy MUST NOT
1636   generate the "*" value.
1637</t>
1638<t>
1639   The field-names given are not limited to the set of standard header
1640   fields defined by this specification. Field names are case-insensitive.
1641</t>
1642</section>
1643
1644<section anchor="header.warning" title="Warning">
1645   <iref item="Warning header field" primary="true"/>
1646   <iref item="Header Fields" primary="true" subitem="Warning"/>
1647   
1648   
1649   
1650   
1651   
1652   
1653<t>
1654   The "Warning" header field is used to carry additional information
1655   about the status or transformation of a message that might not be reflected
1656   in the message. This information is typically used to warn about possible
1657   incorrectness introduced by caching operations or transformations applied
1658   to the payload of the message.
1659</t>
1660<t>
1661   Warnings can be used for other purposes, both cache-related and otherwise.
1662   The use of a warning, rather than an error status code, distinguishes these
1663   responses from true failures.
1664</t>
1665<t>
1666   Warning header fields can in general be applied to any message, however some
1667   warn-codes are specific to caches and can only be applied to response
1668   messages.
1669</t>
1670<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[
1671  Warning       = 1#warning-value
1672 
1673  warning-value = warn-code SP warn-agent SP warn-text
1674                                        [SP warn-date]
1675 
1676  warn-code  = 3DIGIT
1677  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1678                  ; the name or pseudonym of the server adding
1679                  ; the Warning header field, for use in debugging
1680  warn-text  = quoted-string
1681  warn-date  = DQUOTE HTTP-date DQUOTE
1682]]></artwork></figure>
1683<t>
1684   Multiple warnings can be attached to a response (either by the origin
1685   server or by a cache), including multiple warnings with the same code
1686   number, only differing in warn-text.
1687</t>
1688<t>
1689   When this occurs, the user agent SHOULD inform the user of as many of
1690   them as possible, in the order that they appear in the response.
1691</t>
1692<t>
1693   Systems that generate multiple Warning header fields are encouraged to
1694   order them with this user agent behavior in mind. New Warning header fields
1695   are added after any existing Warning headers fields.
1696</t>
1697<t>
1698   Warnings are assigned three digit warn-codes. The first digit indicates
1699   whether the Warning is required to be deleted from a stored response after
1700   validation:
1701   <list style="symbols">
1702      <t>1xx Warnings describe the freshness or validation status of the
1703      response, and so MUST be deleted by a cache after validation. They can
1704      only be generated by a cache when validating a cached entry, and
1705      MUST NOT be generated in any other situation.</t>
1706      <t>2xx Warnings describe some aspect of the representation that is not
1707      rectified by a validation (for example, a lossy compression of the
1708      representation) and MUST NOT be deleted by a cache after validation,
1709      unless a full response is returned, in which case they MUST be.</t>
1710   </list>
1711</t>
1712<t>
1713   If an implementation sends a message with one or more Warning header fields to a
1714   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1715   in each warning-value a warn-date that matches the Date header field in the
1716   message.
1717</t>
1718<t>
1719   If a system receives a message with a warning-value that includes
1720   a warn-date, and that warn-date is different from the Date value in the
1721   response, then that warning-value MUST be deleted from the message before
1722   storing, forwarding, or using it. (preventing the consequences of naive
1723   caching of Warning header fields.) If all of the warning-values are deleted
1724   for this reason, the Warning header field MUST be deleted as well.
1725</t>
1726<t>
1727   The following warn-codes are defined by this specification, each with a
1728   recommended warn-text in English, and a description of its meaning.
1729</t>
1730
1731<section title="110 Response is Stale" anchor="warn.110">
1732  <iref primary="true" item="110 Response is Stale (warn code)"/>
1733  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale"/>
1734<t>
1735   A cache SHOULD include this whenever the returned response is stale.
1736</t>
1737</section>
1738
1739<section title="111 Revalidation Failed" anchor="warn.111">
1740  <iref primary="true" item="111 Revalidation Failed (warn code)"/>
1741  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed"/>
1742<t>
1743   A cache SHOULD include this when returning a stale response because an
1744   attempt to validate the response failed, due to an inability to reach
1745   the server.
1746</t>
1747</section>
1748
1749<section title="112 Disconnected Operation" anchor="warn.112">
1750  <iref primary="true" item="112 Disconnected Operation (warn code)"/>
1751  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation"/>
1752<t>
1753   A cache SHOULD include this if it is intentionally disconnected from
1754   the rest of the network for a period of time.
1755</t>
1756</section>
1757
1758<section title="113 Heuristic Expiration" anchor="warn.113">
1759  <iref primary="true" item="113 Heuristic Expiration (warn code)"/>
1760  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration"/>
1761<t>
1762   A cache SHOULD include this if it heuristically chose a freshness
1763   lifetime greater than 24 hours and the response's age is greater than 24
1764   hours.
1765</t>
1766</section>
1767
1768<section title="199 Miscellaneous Warning" anchor="warn.199">
1769  <iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
1770  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning"/>
1771<t>
1772   The warning text can include arbitrary information to be presented to
1773   a human user, or logged. A system receiving this warning MUST NOT take
1774   any automated action, besides presenting the warning to the user.
1775</t>
1776</section>
1777
1778<section title="214 Transformation Applied" anchor="warn.214">
1779  <iref primary="true" item="214 Transformation Applied (warn code)"/>
1780  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied"/>
1781<t>
1782   MUST be added by a proxy if it applies any transformation to the
1783   representation, such as changing the content-coding, media-type, or
1784   modifying the representation data, unless this Warning code already appears
1785   in the response.
1786</t>
1787</section>
1788
1789<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1790  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)"/>
1791  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning"/>
1792<t>
1793   The warning text can include arbitrary information to be presented to
1794   a human user, or logged. A system receiving this warning MUST NOT take
1795   any automated action.
1796</t>
1797</section>
1798
1799<section title="Warn Code Extensions" anchor="warn.code.extensions">
1800<t>
1801   The HTTP Warn Code Registry defines the name space for warn codes.
1802</t>
1803<t>
1804   A registration MUST include the following fields:
1805   <list style="symbols">
1806      <t>Warn Code (3 digits)</t>
1807      <t>Short Description</t>
1808      <t>Pointer to specification text</t>
1809   </list>
1810</t>
1811<t>
1812   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1813</t>
1814<t>
1815   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
1816</t>
1817</section>
1818</section>
1819</section>
1820
1821<section anchor="history.lists" title="History Lists">
1822<t>
1823   User agents often have history mechanisms, such as "Back" buttons and
1824   history lists, that can be used to redisplay a representation retrieved
1825   earlier in a session.
1826</t>
1827<t>
1828   The freshness model (<xref target="expiration.model"/>) does not
1829   necessarily apply to history mechanisms. I.e., a history mechanism can
1830   display a previous representation even if it has expired.
1831</t>
1832<t>
1833   This does not prohibit the history mechanism from telling the user that a
1834   view might be stale, or from honoring cache directives (e.g.,
1835   Cache-Control: no-store).
1836</t>
1837</section>
1838
1839
1840<section anchor="IANA.considerations" title="IANA Considerations">
1841
1842<section title="Cache Directive Registry" anchor="cache.directive.registration">
1843<t>
1844   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1845</t>
1846<t>
1847   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1848   populated with the registrations below:
1849</t>
1850
1851<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1852<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1853   <ttcol>Cache Directive</ttcol>
1854   <ttcol>Reference</ttcol>
1855
1856   <c>max-age</c>
1857   <c>
1858      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1859   </c>
1860   <c>max-stale</c>
1861   <c>
1862      <xref target="cache-request-directive"/>
1863   </c>
1864   <c>min-fresh</c>
1865   <c>
1866      <xref target="cache-request-directive"/>
1867   </c>
1868   <c>must-revalidate</c>
1869   <c>
1870      <xref target="cache-response-directive"/>
1871   </c>
1872   <c>no-cache</c>
1873   <c>
1874      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1875   </c>
1876   <c>no-store</c>
1877   <c>
1878      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1879   </c>
1880   <c>no-transform</c>
1881   <c>
1882      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1883   </c>
1884   <c>only-if-cached</c>
1885   <c>
1886      <xref target="cache-request-directive"/>
1887   </c>
1888   <c>private</c>
1889   <c>
1890      <xref target="cache-response-directive"/>
1891   </c>
1892   <c>proxy-revalidate</c>
1893   <c>
1894      <xref target="cache-response-directive"/>
1895   </c>
1896   <c>public</c>
1897   <c>
1898      <xref target="cache-response-directive"/>
1899   </c>
1900   <c>s-maxage</c>
1901   <c>
1902      <xref target="cache-response-directive"/>
1903   </c>
1904   <c>stale-if-error</c>
1905   <c>
1906      <xref target="RFC5861"/>, Section 4
1907   </c>
1908   <c>stale-while-revalidate</c>
1909   <c>
1910      <xref target="RFC5861"/>, Section 3
1911   </c>
1912</texttable>
1913<!--(END)-->
1914
1915</section>
1916
1917<section title="Warn Code Registry" anchor="warn.code.registration">
1918<t>
1919   The registration procedure for HTTP Warn Codes is defined by <xref target="warn.code.extensions"/> of this document.
1920</t>
1921<t>
1922   The HTTP Warn Code Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1923   populated with the registrations below:
1924</t>
1925
1926<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1927<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1928   <ttcol>Warn Code</ttcol>
1929   <ttcol>Short Description</ttcol>
1930   <ttcol>Reference</ttcol>
1931   <c>110</c>
1932   <c>Response is Stale</c>
1933   <c>
1934      <xref target="warn.110"/>
1935   </c>
1936   <c>111</c>
1937   <c>Revalidation Failed</c>
1938   <c>
1939      <xref target="warn.111"/>
1940   </c>
1941   <c>112</c>
1942   <c>Disconnected Operation</c>
1943   <c>
1944      <xref target="warn.112"/>
1945   </c>
1946   <c>113</c>
1947   <c>Heuristic Expiration</c>
1948   <c>
1949      <xref target="warn.113"/>
1950   </c>
1951   <c>199</c>
1952   <c>Miscellaneous Warning</c>
1953   <c>
1954      <xref target="warn.199"/>
1955   </c>
1956   <c>214</c>
1957   <c>Transformation Applied</c>
1958   <c>
1959      <xref target="warn.214"/>
1960   </c>
1961   <c>299</c>
1962   <c>Miscellaneous Persistent Warning</c>
1963   <c>
1964      <xref target="warn.299"/>
1965   </c>
1966</texttable>
1967<!--(END)-->
1968
1969</section>
1970
1971<section title="Header Field Registration" anchor="header.field.registration">
1972<t>
1973  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1974  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1975</t>
1976
1977<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1978<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1979   <ttcol>Header Field Name</ttcol>
1980   <ttcol>Protocol</ttcol>
1981   <ttcol>Status</ttcol>
1982   <ttcol>Reference</ttcol>
1983
1984   <c>Age</c>
1985   <c>http</c>
1986   <c>standard</c>
1987   <c>
1988      <xref target="header.age"/>
1989   </c>
1990   <c>Cache-Control</c>
1991   <c>http</c>
1992   <c>standard</c>
1993   <c>
1994      <xref target="header.cache-control"/>
1995   </c>
1996   <c>Expires</c>
1997   <c>http</c>
1998   <c>standard</c>
1999   <c>
2000      <xref target="header.expires"/>
2001   </c>
2002   <c>Pragma</c>
2003   <c>http</c>
2004   <c>standard</c>
2005   <c>
2006      <xref target="header.pragma"/>
2007   </c>
2008   <c>Vary</c>
2009   <c>http</c>
2010   <c>standard</c>
2011   <c>
2012      <xref target="header.vary"/>
2013   </c>
2014   <c>Warning</c>
2015   <c>http</c>
2016   <c>standard</c>
2017   <c>
2018      <xref target="header.warning"/>
2019   </c>
2020</texttable>
2021<!--(END)-->
2022
2023<t>
2024   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2025   Force".
2026</t>
2027</section>
2028</section>
2029
2030<section anchor="security.considerations" title="Security Considerations">
2031<t>
2032   Caches expose additional potential vulnerabilities, since the contents of
2033   the cache represent an attractive target for malicious exploitation.
2034   Because cache contents persist after an HTTP request is complete, an attack
2035   on the cache can reveal information long after a user believes that the
2036   information has been removed from the network. Therefore, cache contents
2037   need to be protected as sensitive information.
2038</t>
2039</section>
2040
2041<section title="Acknowledgments" anchor="acks">
2042<t>
2043  See Section 9 of <xref target="Part1"/>.
2044</t>
2045</section>
2046
2047</middle>
2048
2049<back>
2050<references title="Normative References">
2051
2052  <reference anchor="Part1">
2053    <front>
2054      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2055      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2056        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2057        <address><email>fielding@gbiv.com</email></address>
2058      </author>
2059      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2060        <organization abbrev="W3C">World Wide Web Consortium</organization>
2061        <address><email>ylafon@w3.org</email></address>
2062      </author>
2063      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2064        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2065        <address><email>julian.reschke@greenbytes.de</email></address>
2066      </author>
2067      <date month="March" year="2012"/>
2068    </front>
2069    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-19"/>
2070   
2071  </reference>
2072
2073  <reference anchor="Part2">
2074    <front>
2075      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2076      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2077        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2078        <address><email>fielding@gbiv.com</email></address>
2079      </author>
2080      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2081        <organization abbrev="W3C">World Wide Web Consortium</organization>
2082        <address><email>ylafon@w3.org</email></address>
2083      </author>
2084      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2085        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2086        <address><email>julian.reschke@greenbytes.de</email></address>
2087      </author>
2088      <date month="March" year="2012"/>
2089    </front>
2090    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-19"/>
2091   
2092  </reference>
2093
2094  <reference anchor="Part4">
2095    <front>
2096      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2097      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2098        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2099        <address><email>fielding@gbiv.com</email></address>
2100      </author>
2101      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2102        <organization abbrev="W3C">World Wide Web Consortium</organization>
2103        <address><email>ylafon@w3.org</email></address>
2104      </author>
2105      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2106        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2107        <address><email>julian.reschke@greenbytes.de</email></address>
2108      </author>
2109      <date month="March" year="2012"/>
2110    </front>
2111    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-19"/>
2112   
2113  </reference>
2114
2115  <reference anchor="Part5">
2116    <front>
2117      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2118      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2119        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2120        <address><email>fielding@gbiv.com</email></address>
2121      </author>
2122      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2123        <organization abbrev="W3C">World Wide Web Consortium</organization>
2124        <address><email>ylafon@w3.org</email></address>
2125      </author>
2126      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2127        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2128        <address><email>julian.reschke@greenbytes.de</email></address>
2129      </author>
2130      <date month="March" year="2012"/>
2131    </front>
2132    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-19"/>
2133   
2134  </reference>
2135
2136  <reference anchor="Part7">
2137    <front>
2138      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2139      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2140        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2141        <address><email>fielding@gbiv.com</email></address>
2142      </author>
2143      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2144        <organization abbrev="W3C">World Wide Web Consortium</organization>
2145        <address><email>ylafon@w3.org</email></address>
2146      </author>
2147      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2148        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2149        <address><email>julian.reschke@greenbytes.de</email></address>
2150      </author>
2151      <date month="March" year="2012"/>
2152    </front>
2153    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-19"/>
2154   
2155  </reference>
2156
2157  <reference anchor="RFC2119">
2158    <front>
2159      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2160      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2161        <organization>Harvard University</organization>
2162        <address><email>sob@harvard.edu</email></address>
2163      </author>
2164      <date month="March" year="1997"/>
2165    </front>
2166    <seriesInfo name="BCP" value="14"/>
2167    <seriesInfo name="RFC" value="2119"/>
2168  </reference>
2169
2170  <reference anchor="RFC5234">
2171    <front>
2172      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2173      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2174        <organization>Brandenburg InternetWorking</organization>
2175        <address>
2176          <email>dcrocker@bbiw.net</email>
2177        </address> 
2178      </author>
2179      <author initials="P." surname="Overell" fullname="Paul Overell">
2180        <organization>THUS plc.</organization>
2181        <address>
2182          <email>paul.overell@thus.net</email>
2183        </address>
2184      </author>
2185      <date month="January" year="2008"/>
2186    </front>
2187    <seriesInfo name="STD" value="68"/>
2188    <seriesInfo name="RFC" value="5234"/>
2189  </reference>
2190 
2191</references>
2192
2193<references title="Informative References">
2194
2195  <reference anchor="RFC1305">
2196    <front>
2197      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2198      <author fullname="David L. Mills" initials="D." surname="Mills">
2199        <organization>University of Delaware, Electrical Engineering Department</organization>
2200        <address><email>mills@udel.edu</email></address>
2201      </author>
2202      <date month="March" year="1992"/>
2203    </front>
2204    <seriesInfo name="RFC" value="1305"/>
2205  </reference>
2206
2207  <reference anchor="RFC2616">
2208    <front>
2209      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2210      <author fullname="R. Fielding" initials="R." surname="Fielding">
2211        <organization>University of California, Irvine</organization>
2212        <address><email>fielding@ics.uci.edu</email></address>
2213      </author>
2214      <author fullname="J. Gettys" initials="J." surname="Gettys">
2215        <organization>W3C</organization>
2216        <address><email>jg@w3.org</email></address>
2217      </author>
2218      <author fullname="J. Mogul" initials="J." surname="Mogul">
2219        <organization>Compaq Computer Corporation</organization>
2220        <address><email>mogul@wrl.dec.com</email></address>
2221      </author>
2222      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2223        <organization>MIT Laboratory for Computer Science</organization>
2224        <address><email>frystyk@w3.org</email></address>
2225      </author>
2226      <author fullname="L. Masinter" initials="L." surname="Masinter">
2227        <organization>Xerox Corporation</organization>
2228        <address><email>masinter@parc.xerox.com</email></address>
2229      </author>
2230      <author fullname="P. Leach" initials="P." surname="Leach">
2231        <organization>Microsoft Corporation</organization>
2232        <address><email>paulle@microsoft.com</email></address>
2233      </author>
2234      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2235        <organization>W3C</organization>
2236        <address><email>timbl@w3.org</email></address>
2237      </author>
2238      <date month="June" year="1999"/>
2239    </front>
2240    <seriesInfo name="RFC" value="2616"/>
2241  </reference>
2242
2243  <reference anchor="RFC3864">
2244    <front>
2245      <title>Registration Procedures for Message Header Fields</title>
2246      <author fullname="G. Klyne" initials="G." surname="Klyne">
2247        <organization>Nine by Nine</organization>
2248        <address><email>GK-IETF@ninebynine.org</email></address>
2249      </author>
2250      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2251        <organization>BEA Systems</organization>
2252        <address><email>mnot@pobox.com</email></address>
2253      </author>
2254      <author fullname="J. Mogul" initials="J." surname="Mogul">
2255        <organization>HP Labs</organization>
2256        <address><email>JeffMogul@acm.org</email></address>
2257      </author>
2258      <date month="September" year="2004"/>
2259    </front>
2260    <seriesInfo name="BCP" value="90"/>
2261    <seriesInfo name="RFC" value="3864"/>
2262  </reference>
2263
2264  <reference anchor="RFC5226">
2265    <front>
2266      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2267      <author initials="T." surname="Narten" fullname="T. Narten">
2268        <organization>IBM</organization>
2269        <address><email>narten@us.ibm.com</email></address>
2270      </author>
2271      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2272        <organization>Google</organization>
2273        <address><email>Harald@Alvestrand.no</email></address>
2274      </author>
2275      <date year="2008" month="May"/>
2276    </front>
2277    <seriesInfo name="BCP" value="26"/>
2278    <seriesInfo name="RFC" value="5226"/>
2279  </reference>
2280
2281  <reference anchor="RFC5861">
2282    <front>
2283      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2284      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2285        <organization>Yahoo! Inc.</organization>
2286        <address><email>mnot@yahoo-inc.com</email></address>
2287      </author>
2288      <date month="April" year="2010"/>
2289    </front>
2290    <seriesInfo name="RFC" value="5861"/>
2291  </reference>
2292
2293</references>
2294
2295<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2296<t>
2297  Make the specified age calculation algorithm less conservative.
2298  (<xref target="age.calculations"/>)
2299</t>
2300<t>
2301  Remove requirement to consider Content-Location in successful responses
2302  in order to determine the appropriate response to use.
2303  (<xref target="validation.model"/>)
2304</t>
2305<t>
2306  Clarify denial of service attack avoidance requirement.
2307  (<xref target="invalidation.after.updates.or.deletions"/>)
2308</t>
2309<t>
2310  Change ABNF productions for header fields to only define the field value.
2311  (<xref target="header.field.definitions"/>)
2312</t>
2313<t>
2314  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2315  anymore, as these aspects never were implemented.
2316  (<xref target="header.warning"/>)
2317</t>
2318</section>
2319
2320
2321<section title="Collected ABNF" anchor="collected.abnf">
2322<figure>
2323<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2324Age = delta-seconds
2325
2326Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2327 cache-directive ] )
2328
2329Expires = HTTP-date
2330
2331HTTP-date = <HTTP-date, defined in [Part2], Section 8>
2332
2333OWS = <OWS, defined in [Part1], Section 3.2.1>
2334
2335Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2336 pragma-directive ] )
2337
2338Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2339 ) )
2340
2341Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2342 )
2343
2344cache-directive = cache-request-directive / cache-response-directive
2345cache-extension = token [ "=" ( token / quoted-string ) ]
2346cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2347 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2348 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2349 cache-extension
2350cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2351 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2352 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2353 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2354 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2355 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2356
2357delta-seconds = 1*DIGIT
2358
2359extension-pragma = token [ "=" ( token / quoted-string ) ]
2360
2361field-name = <field-name, defined in [Part1], Section 3.2>
2362
2363port = <port, defined in [Part1], Section 2.7>
2364pragma-directive = "no-cache" / extension-pragma
2365pseudonym = <pseudonym, defined in [Part1], Section 6.2>
2366
2367quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
2368
2369token = <token, defined in [Part1], Section 3.2.4>
2370
2371uri-host = <uri-host, defined in [Part1], Section 2.7>
2372
2373warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2374warn-code = 3DIGIT
2375warn-date = DQUOTE HTTP-date DQUOTE
2376warn-text = quoted-string
2377warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2378 ]
2379]]></artwork>
2380</figure>
2381<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2382; Age defined but not used
2383; Cache-Control defined but not used
2384; Expires defined but not used
2385; Pragma defined but not used
2386; Vary defined but not used
2387; Warning defined but not used
2388]]></artwork></figure></section>
2389
2390
2391<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2392
2393<section title="Since RFC 2616">
2394  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2395</section>
2396
2397<section title="Since draft-ietf-httpbis-p6-cache-00">
2398<t>
2399  Closed issues:
2400  <list style="symbols">
2401    <t>
2402      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2403    <t>
2404      <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>
2405    <t>
2406      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2407    <t>
2408      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2409    <t>
2410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2411    <t>
2412      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2413    <t>
2414      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2415    <t>
2416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2417    <t>
2418      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2419  </list>
2420</t>
2421<t>
2422  Other changes:
2423  <list style="symbols">
2424    <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>
2425  </list>
2426</t>
2427</section>
2428
2429<section title="Since draft-ietf-httpbis-p6-cache-01">
2430<t>
2431  Closed issues:
2432  <list style="symbols">
2433    <t>
2434      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2435  </list>
2436</t>
2437<t>
2438  Other changes:
2439  <list style="symbols">
2440    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2441      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2442    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2443      specification.</t>
2444  </list>
2445</t>
2446</section>
2447
2448<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2449<t>
2450  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2451  <list style="symbols">
2452    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2453      document.</t>
2454  </list>
2455</t>
2456</section>
2457
2458<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2459<t>
2460  Closed issues:
2461  <list style="symbols">
2462    <t>
2463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2464  </list>
2465</t>
2466</section>
2467
2468<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2469<t>
2470  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2471  <list style="symbols"> 
2472    <t>
2473      Use "/" instead of "|" for alternatives.
2474    </t>
2475    <t>
2476      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2477      whitespace ("OWS") and required whitespace ("RWS").
2478    </t>
2479    <t>
2480      Rewrite ABNFs to spell out whitespace rules, factor out
2481      header field value format definitions.
2482    </t>
2483  </list>
2484</t>
2485</section>
2486
2487<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2488<t>
2489  This is a total rewrite of this part of the specification.
2490</t>
2491<t>
2492  Affected issues:
2493  <list style="symbols">
2494    <t>
2495      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2496    <t>
2497      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2498    <t>
2499      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2500    <t>
2501      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2502  </list>
2503</t>
2504<t>
2505  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2506  <list style="symbols"> 
2507    <t>
2508      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2509    </t>
2510  </list>
2511</t>
2512</section>
2513
2514<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2515<t>
2516  Closed issues:
2517  <list style="symbols"> 
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2520      "base for numeric protocol elements"
2521    </t>
2522  </list>
2523</t>
2524<t>
2525  Affected issues:
2526  <list style="symbols">
2527    <t>
2528      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2529      "Vary and non-existant headers"
2530    </t>
2531  </list>
2532</t>
2533</section>
2534
2535<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2536<t>
2537  Closed issues:
2538  <list style="symbols"> 
2539    <t>
2540      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2541      "Definition of 1xx Warn-Codes"
2542    </t>
2543    <t>
2544      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2545      "Content-Location on 304 responses"
2546    </t>
2547    <t>
2548      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2549      "private and no-cache CC directives with headers"
2550    </t>
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2553      "RFC2047 and warn-text"
2554    </t>
2555  </list>
2556</t>
2557</section>
2558
2559<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2560<t>
2561  Closed issues:
2562  <list style="symbols"> 
2563    <t>
2564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2565      "serving negotiated responses from cache: header-specific canonicalization"
2566    </t>
2567    <t>
2568      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2569      "Effect of CC directives on history lists"
2570    </t>
2571    <t>
2572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2573      "Cache Extensions can override no-store, etc."
2574    </t>
2575  </list>
2576</t>
2577<t>
2578  Affected issues:
2579  <list style="symbols">
2580    <t>
2581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2582      Status codes and caching
2583    </t>
2584  </list>
2585</t>
2586<t>
2587  Partly resolved issues:
2588  <list style="symbols"> 
2589    <t>
2590      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2591      "Placement of 13.5.1 and 13.5.2"
2592    </t>
2593  </list>
2594</t>
2595</section>
2596
2597<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2598<t>
2599  Closed issues:
2600  <list style="symbols"> 
2601    <t>
2602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2603      "Age calculation"
2604    </t>
2605    <t>
2606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2607      "Clarify differences between / requirements for request and response CC directives"
2608    </t>
2609    <t>
2610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2611      "Caching authenticated responses"
2612    </t>
2613    <t>
2614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2615      "IANA registry for cache-control directives"
2616    </t>
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2619      "Heuristic caching of URLs with query components"
2620    </t>
2621  </list>
2622</t>
2623<t>
2624  Partly resolved issues:
2625  <list style="symbols"> 
2626    <t>
2627      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2628      "Term for the requested resource's URI"
2629    </t>
2630  </list>
2631</t>
2632</section>
2633
2634<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2635<t>
2636  Closed issues:
2637  <list style="symbols"> 
2638    <t>
2639      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2640      "Clarify entity / representation / variant terminology"
2641    </t>
2642    <t>
2643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2644      "consider removing the 'changes from 2068' sections"
2645    </t>
2646    <t>
2647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2648      "Allowing heuristic caching for new status codes"
2649    </t>
2650    <t>
2651      Clean up TODOs and prose in "Combining Responses."
2652    </t>
2653  </list>
2654</t>
2655</section>
2656
2657<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2658<t>
2659  Closed issues:
2660  <list style="symbols"> 
2661    <t>
2662      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2663      "Text about clock requirement for caches belongs in p6"
2664    </t>
2665  </list>
2666</t>
2667</section>
2668
2669<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2670<t>
2671  Closed issues:
2672  <list style="symbols"> 
2673    <t>
2674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2675      "Header Classification"
2676    </t>
2677    <t>
2678      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2679      "Clarify 'public'"
2680    </t>
2681  </list>
2682</t>
2683</section>
2684
2685<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2686<t>
2687  Closed issues:
2688  <list style="symbols">
2689    <t>
2690      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2691      "untangle ABNFs for header fields"
2692    </t>
2693  </list>
2694</t>
2695</section>
2696
2697<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2698<t>
2699  Closed issues:
2700  <list style="symbols">
2701    <t>
2702      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2703      "Mismatch Vary"
2704    </t>
2705    <t>
2706      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2707      "Cache Invalidation only happens upon successful responses"
2708    </t>
2709    <t>
2710      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2711      "Recommend minimum sizes for protocol elements"
2712    </t>
2713    <t>
2714      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2715      "Proxies don't 'understand' methods"
2716    </t>
2717    <t>
2718      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2719      "Cache Extensions can override no-store, etc."
2720    </t>
2721    <t>
2722      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2723      "Pragma"
2724    </t>
2725  </list>
2726</t>
2727</section>
2728
2729<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2730<t>
2731  Closed issues:
2732  <list style="symbols">
2733    <t>
2734      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2735      "Motivate one-year limit for Expires"
2736    </t>
2737  </list>
2738</t>
2739</section>
2740
2741<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
2742<t>
2743  Closed issues:
2744  <list style="symbols">
2745    <t>
2746      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
2747      "Document HTTP's error-handling philosophy"
2748    </t>
2749    <t>
2750      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/317"/>:
2751      "Cache-Control directive case sensitivity"
2752    </t>
2753  </list>
2754</t>
2755</section>
2756
2757<section title="Since draft-ietf-httpbis-p6-cache-17" anchor="changes.since.17">
2758<t>
2759  Closed issues:
2760  <list style="symbols">
2761    <t>
2762      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/293"/>:
2763      "Interaction of request and response Cache-Control"
2764    </t>
2765    <t>
2766      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/212"/>:
2767      "Refining age for 1.1 proxy chains"
2768    </t>
2769    <t>
2770      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/274"/>:
2771      "warn-code registry"
2772    </t>
2773  </list>
2774</t>
2775</section>
2776
2777<section title="Since draft-ietf-httpbis-p6-cache-18" anchor="changes.since.18">
2778<t>
2779  Closed issues:
2780  <list style="symbols">
2781    <t>
2782      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/227"/>:
2783      "Combining HEAD responses"
2784    </t>
2785    <t>
2786      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/337"/>:
2787      "Field names in cache-control header arguments"
2788    </t>
2789  </list>
2790</t>
2791</section>
2792
2793</section>
2794  </back>
2795</rfc>
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