source: draft-ietf-httpbis/25/draft-ietf-httpbis-p6-cache-25.xml @ 2656

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prepare publication of -25 drafts on 2013-11-17

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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-25" ipr="pre5378Trust200902" obsoletes="2616">
20
21
22
23<front>
24
25  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): 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="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
43    <organization>Akamai</organization>
44    <address>
45      <email>mnot@mnot.net</email>
46      <uri>http://www.mnot.net/</uri>
47    </address>
48  </author>
49
50  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
51    <organization abbrev="greenbytes">greenbytes GmbH</organization>
52    <address>
53      <postal>
54        <street>Hafenweg 16</street>
55        <city>Muenster</city><region>NW</region><code>48155</code>
56        <country>Germany</country>
57      </postal>
58      <email>julian.reschke@greenbytes.de</email>
59      <uri>http://greenbytes.de/tech/webdav/</uri>
60    </address>
61  </author>
62
63  <date month="November" year="2013" day="17"/>
64  <workgroup>HTTPbis Working Group</workgroup>
65
66<abstract>
67<t>
68   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
69   distributed, collaborative, hypertext information systems. This document
70   defines requirements on HTTP caches and the associated header fields
71   that control cache behavior or indicate cacheable response messages.
72</t>
73</abstract>
74
75<note title="Editorial Note (To be removed by RFC Editor)">
76  <t>
77    Discussion of this draft takes place on the HTTPBIS working group
78    mailing list (ietf-http-wg@w3.org), which is archived at
79    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
80  </t>
81  <t>
82    The current issues list is at
83    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
84    documents (including fancy diffs) can be found at
85    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
86  </t>
87  <t>
88    The changes in this draft are summarized in <xref target="changes.since.24"/>.
89  </t>
90</note>
91
92   </front>
93   <middle>
94
95<section anchor="caching" title="Introduction">
96<t>
97   HTTP is typically used for distributed information systems, where
98   performance can be improved by the use of response caches. This document
99   defines aspects of HTTP/1.1 related to caching and reusing response
100   messages.
101</t>
102
103<iref item="cache"/>
104<t>
105   An HTTP cache is a local store of response messages and the
106   subsystem that controls storage, retrieval, and deletion of messages in it.
107   A cache stores cacheable responses in order to reduce the response time and
108   network bandwidth consumption on future, equivalent requests. Any client or
109   server MAY employ a cache, though a cache cannot be used by a server that
110   is acting as a tunnel.
111</t>
112<iref item="shared cache"/>
113<iref item="private cache"/>
114<t anchor="shared.and.private.caches">
115   A shared cache is a cache that stores responses to be reused
116   by more than one user; shared caches are usually (but not always) deployed
117   as a part of an intermediary. A private cache, in contrast,
118   is dedicated to a single user.
119</t>
120<t>
121   The goal of caching in HTTP/1.1 is to significantly improve performance
122   by reusing a prior response message to satisfy a current request.
123   A stored response is considered "fresh", as defined in
124   <xref target="expiration.model"/>, if the response can be reused without
125   "validation" (checking with the origin server to see if the cached response
126   remains valid for this request).  A fresh response can therefore
127   reduce both latency and network overhead each time it is reused.
128   When a cached response is not fresh, it might still be reusable if it can
129   be freshened by validation (<xref target="validation.model"/>) or if the
130   origin is unavailable (<xref target="serving.stale.responses"/>).
131</t>
132
133<section title="Conformance and Error Handling" anchor="conformance">
134<t>
135   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
136   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
137   document are to be interpreted as described in <xref target="RFC2119"/>.
138</t>
139<t>
140   Conformance criteria and considerations regarding error handling
141   are defined in Section 2.5 of <xref target="Part1"/>.
142</t>
143</section>
144
145<section title="Syntax Notation" anchor="notation">
146<t>
147   This specification uses the Augmented Backus-Naur Form (ABNF) notation
148   of <xref target="RFC5234"/> with the list rule extension defined in
149   Section 7 of <xref target="Part1"/>. <xref target="imported.abnf"/> describes rules imported from
150   other documents. <xref target="collected.abnf"/> shows the collected ABNF
151   with the list rule expanded.
152</t>
153
154<section title="Delta Seconds" anchor="delta-seconds">
155<t>
156   The delta-seconds rule specifies a non-negative integer, representing time
157   in seconds.
158</t>
159<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
160  delta-seconds  = 1*DIGIT
161]]></artwork></figure>
162<t>
163   A recipient parsing a delta-seconds value and converting it to binary form
164   ought to use an arithmetic type of at least 31 bits of non-negative integer
165   range.
166   If a cache receives a delta-seconds value greater than the greatest integer
167   it can represent, or if any of its subsequent calculations overflows,
168   the cache MUST consider the value to be either 2147483648
169   (2^31) or the greatest positive integer it can conveniently
170   represent.
171</t>
172<t><list>
173   <t>
174       Note: The value 2147483648 is here for historical reasons, effectively
175       represents infinity (over 68 years), and does not need to be stored in
176       binary form; an implementation could produce it as a canned string if
177       any overflow occurs, even if the calculations are performed with an
178       arithmetic type incapable of directly representing that number.
179       What matters here is that an overflow be detected and not treated as a
180       negative value in later calculations.
181   </t>
182</list></t>
183</section>
184
185</section>
186</section>
187
188<section anchor="caching.overview" title="Overview of Cache Operation">
189<iref item="cache entry"/>
190<iref item="cache key"/>
191<t>
192   Proper cache operation preserves the semantics of HTTP transfers
193   (<xref target="Part2"/>) while eliminating the transfer of information already held
194   in the cache.  Although caching is an entirely OPTIONAL feature of HTTP,
195   we assume that reusing the cached response is desirable and that such
196   reuse is the default behavior when no requirement or local
197   configuration prevents it.  Therefore, HTTP cache requirements are focused
198   on preventing a cache from either storing a non-reusable response or
199   reusing a stored response inappropriately, rather than mandating that
200   caches always store and reuse particular responses.
201</t>
202<t>
203   Each cache entry consists of a cache key and one or more
204   HTTP responses corresponding to prior requests that used the same key. The
205   most common form of cache entry is a successful result of a retrieval
206   request: i.e., a 200 (OK) response to a GET request, which
207   contains a representation of the resource identified by the request target
208   (Section 4.3.1 of <xref target="Part2"/>). However, it is also possible to cache permanent redirects,
209   negative results (e.g., 404 (Not Found)),
210   incomplete results (e.g., 206 (Partial Content)), and
211   responses to methods other than GET if the method's definition allows such
212   caching and defines something suitable for use as a cache key.
213</t>
214<iref item="cache key"/>
215<t>
216   The primary cache key consists of the request method and
217   target URI.  However, since HTTP caches in common use today are typically
218   limited to caching responses to GET, many caches simply decline
219   other methods and use only the URI as the primary cache key.
220</t>
221<t>
222   If a request target is subject to content negotiation, its cache entry
223   might consist of multiple stored responses, each differentiated by a
224   secondary key for the values of the original request's selecting header
225   fields (<xref target="caching.negotiated.responses"/>).
226</t>
227</section>
228
229<section anchor="response.cacheability" title="Storing Responses in Caches">
230<t>
231   A cache MUST NOT store a response to any request, unless:
232   <list style="symbols">
233      <t>The request method is understood by the cache and defined as being
234      cacheable, and</t>
235      <t>the response status code is understood by the cache, and</t>
236      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
237      header fields, and</t>
238      <t>the "private" cache response directive (see <xref target="cache-response-directive.private"/>) does not appear in the
239      response, if the cache is shared, and</t>
240      <t>the Authorization header field (see
241      Section 4.1 of <xref target="Part7"/>) does not appear in the request, if the cache is
242      shared, unless the response explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
243      <t>the response either:
244         <list style="symbols">
245            <t>contains an <xref target="header.expires" format="none">Expires</xref> header field (see
246            <xref target="header.expires"/>), or</t>
247            <t>contains a max-age response cache directive (see <xref target="cache-response-directive.max-age"/>), or</t>
248            <t>contains a s-maxage response cache directive (see <xref target="cache-response-directive.s-maxage"/>) and the cache is
249            shared, or</t>
250            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
251            or</t>
252            <t>has a status code that is defined as cacheable by default
253            (see <xref target="heuristic.freshness"/>), or</t>
254            <t>contains a public response cache directive (see <xref target="cache-response-directive.public"/>).</t>
255         </list>
256      </t>
257   </list>
258</t>
259<t>
260   Note that any of the requirements listed above can be overridden by a
261   cache-control extension; see <xref target="cache.control.extensions"/>.
262</t>
263<t>
264   In this context, a cache has "understood" a request method or a response
265   status code if it recognizes it and implements all specified
266   caching-related behavior.
267</t>
268<t>
269   Note that, in normal operation, some caches will not store a response that
270   has neither a cache validator nor an explicit expiration time, as such
271   responses are not usually useful to store. However, caches are not
272   prohibited from storing such responses.
273</t>
274
275<section anchor="incomplete.responses" title="Storing Incomplete Responses">
276<t>
277   A response message is considered complete when all of the octets indicated
278   by the message framing (<xref target="Part1"/>) are received prior to the connection
279   being closed. If the request method is GET, the response status code is
280   200 (OK), and the entire response header section has been received, a
281   cache MAY store an incomplete response message body if the cache entry is
282   recorded as incomplete. Likewise, a 206 (Partial Content)
283   response MAY be stored as if it were an incomplete 200
284   (OK) cache entry. However, a cache MUST NOT store incomplete or
285   partial content responses if it does not support the Range
286   and Content-Range header fields or if it does not understand
287   the range units used in those fields.
288</t>
289<t>
290   A cache MAY complete a stored incomplete response by making a subsequent
291   range request (<xref target="Part5"/>) and combining the successful response with the
292   stored entry, as defined in <xref target="combining.responses"/>. A cache
293   MUST NOT use an incomplete response to answer requests unless the
294   response has been made complete or the request is partial and specifies a
295   range that is wholly within the incomplete response. A cache MUST NOT
296   send a partial response to a client without explicitly marking it as such
297   using the 206 (Partial Content) status code.
298</t>
299</section>
300
301
302<section anchor="caching.authenticated.responses" title="Storing Responses to Authenticated Requests">
303<t>
304   A shared cache MUST NOT use a cached response to a request with an
305   Authorization header field (Section 4.1 of <xref target="Part7"/>) to
306   satisfy any subsequent request unless a cache directive that allows such
307   responses to be stored is present in the response.
308</t>
309<t>
310   In this specification, the following <xref target="header.cache-control" format="none">Cache-Control</xref> response
311   directives (<xref target="cache-response-directive"/>) have such an effect:
312   must-revalidate, public, s-maxage.
313</t>
314<t>
315   Note that cached responses that contain the "must-revalidate" and/or
316   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
317   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
318   used to satisfy a subsequent request without revalidating it on the origin
319   server.
320</t>
321</section>
322
323<section anchor="combining.responses" title="Combining Partial Content">
324<t>
325   A response might transfer only a partial representation if the
326   connection closed prematurely or if the request used one or more Range
327   specifiers (<xref target="Part5"/>).  After several such transfers, a cache might have
328   received several ranges of the same representation.  A cache MAY combine
329   these ranges into a single stored response, and reuse that response to
330   satisfy later requests, if they all share the same strong validator and
331   the cache complies with the client requirements in Section 4.3 of <xref target="Part5"/>.
332</t>
333<t>
334   When combining the new response with one or more stored responses, a
335   cache MUST:
336   <list style="symbols">
337      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
338         with warn-code 1xx (see <xref target="header.warning"/>);</t>
339      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
340         with warn-code 2xx; and,</t>
341      <t>use other header fields provided in the new response, aside
342         from Content-Range, to replace all instances of the
343         corresponding header fields in the stored response.</t>
344   </list>
345</t>
346</section>
347
348</section>
349
350
351<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
352<t>
353   When presented with a request, a cache MUST NOT reuse a stored response,
354   unless:
355   <list style="symbols">
356      <t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
357      that of the stored response match, and</t>
358      <t>the request method associated with the stored response allows it to
359      be used for the presented request, and</t>
360      <t>selecting header fields nominated by the stored response (if any)
361      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
362      <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
363      successfully validated (<xref target="validation.model"/>), and</t>
364      <t>the stored response does not contain the no-cache cache directive
365      (<xref target="cache-response-directive.no-cache"/>), unless it is
366      successfully validated (<xref target="validation.model"/>), and</t>
367      <t>the stored response is either:
368         <list style="symbols">
369            <t>fresh (see <xref target="expiration.model"/>), or</t>
370            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
371            <t>successfully validated (see <xref target="validation.model"/>).</t>
372         </list>
373      </t>
374  </list>
375</t>
376<t>
377   Note that any of the requirements listed above can be overridden by a
378   cache-control extension; see <xref target="cache.control.extensions"/>.
379</t>
380<t>
381   When a stored response is used to satisfy a request without validation, a
382   cache MUST generate an <xref target="header.age" format="none">Age</xref> header field (<xref target="header.age"/>), replacing any present in the response with a value
383   equal to the stored response's current_age; see <xref target="age.calculations"/>.
384</t>
385<t>
386   A cache MUST write through requests with methods that are unsafe
387   (Section 4.2.1 of <xref target="Part2"/>) to the origin server; i.e., a cache is not allowed to
388   generate a reply to such a request before having forwarded the request and
389   having received a corresponding response.
390</t>
391<t>
392   Also, note that unsafe requests might invalidate already stored responses;
393   see <xref target="invalidation"/>.
394</t>
395<t>
396   When more than one suitable response is stored, a cache MUST use the
397   most recent response (as determined by the Date header
398   field). It can also forward the request with "Cache-Control: max-age=0" or
399   "Cache-Control: no-cache" to disambiguate which response to use.
400</t>
401<t>
402   A cache that does not have a clock available MUST NOT use stored
403   responses without revalidating them upon every use.
404</t>
405
406<section anchor="caching.negotiated.responses" title="Calculating Secondary Keys with Vary">
407<t>
408   When a cache receives a request that can be satisfied by a stored response
409   that has a Vary header field (Section 7.1.4 of <xref target="Part2"/>),
410   it MUST NOT use that response unless all of the selecting header fields
411   nominated by the Vary header field match in both the original request
412   (i.e., that associated with the stored response), and the presented
413   request.
414</t>
415<t>
416   The selecting header fields from two requests are defined to match if and
417   only if those in the first request can be transformed to those in the
418   second request by applying any of the following:
419   <list style="symbols">
420      <t>
421         adding or removing whitespace, where allowed in the header field's
422         syntax
423      </t>
424      <t>
425         combining multiple header fields with the same field name
426         (see Section 3.2 of <xref target="Part1"/>)
427      </t>
428      <t>
429         normalizing both header field values in a way that is known to have
430         identical semantics, according to the header field's specification
431         (e.g., re-ordering field values when order is not significant;
432         case-normalization, where values are defined to be case-insensitive)
433      </t>
434  </list>
435</t>
436<t>
437   If (after any normalization that might take place) a header field is absent
438   from a request, it can only match another request if it is also absent
439   there.
440</t>
441<t>
442   A Vary header field-value of "*" always fails to match.
443</t>
444<t>
445   The stored response with matching selecting header fields is known as the
446   selected response.
447</t>
448<t>
449   If multiple selected responses are available (potentially including
450   responses without a Vary header field), the cache will need to choose one to use.
451   When a selecting header field has a known mechanism for doing so (e.g., qvalues on
452   Accept and similar request header fields), that mechanism MAY be
453   used to select preferred responses; of the remainder, the most recent
454   response (as determined by the Date header field) is used, as
455   per <xref target="constructing.responses.from.caches"/>.
456</t>
457<t>
458   If no selected response is available, the cache cannot satisfy the
459   presented request. Typically, it is forwarded to the origin server
460   in a (possibly conditional; see <xref target="validation.model"/>) request.
461</t>
462</section>
463
464<section anchor="expiration.model" title="Freshness">
465<iref item="fresh"/>
466<iref item="stale"/>
467<t>
468   A fresh response is one whose age has not yet exceeded its
469   freshness lifetime. Conversely, a stale
470   response is one where it has.
471</t>
472<iref item="freshness lifetime"/>
473<iref item="explicit expiration time"/>
474<iref item="heuristic expiration time"/>
475<t>
476   A response's freshness lifetime is the length of time
477   between its generation by the origin server and its expiration time. An
478   explicit expiration time is the time at which the origin
479   server intends that a stored response can no longer be used by a cache
480   without further validation, whereas a heuristic expiration
481   time is assigned by a cache when no explicit expiration time is
482   available.
483</t>
484<iref item="age"/>
485<t>
486   A response's age is the time that has passed since it was
487   generated by, or successfully validated with, the origin server. 
488</t>
489<t>
490   When a response is "fresh" in the cache, it can be used to satisfy
491   subsequent requests without contacting the origin server, thereby improving
492   efficiency.
493</t>
494<t>
495   The primary mechanism for determining freshness is for an origin server to
496   provide an explicit expiration time in the future, using either the
497   <xref target="header.expires" format="none">Expires</xref> header field (<xref target="header.expires"/>) or
498   the max-age response cache directive (<xref target="cache-response-directive.max-age"/>). Generally, origin servers
499   will assign future explicit expiration times to responses in the belief
500   that the representation is not likely to change in a semantically
501   significant way before the expiration time is reached.
502</t>
503<t>
504   If an origin server wishes to force a cache to validate every request, it
505   can assign an explicit expiration time in the past to indicate that the
506   response is already stale. Compliant caches will normally validate a stale
507   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
508</t>
509<t>
510   Since origin servers do not always provide explicit expiration times,
511   caches are also allowed to use a heuristic to determine an expiration time
512   under certain circumstances (see <xref target="heuristic.freshness"/>).
513</t>
514<figure>
515<preamble>
516  The calculation to determine if a response is fresh is:
517</preamble>
518<artwork type="code"><![CDATA[
519   response_is_fresh = (freshness_lifetime > current_age)
520]]></artwork>
521</figure>
522<t>
523   freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; current_age is defined in
524   <xref target="age.calculations"/>.
525</t>
526<t>
527   Clients can send the max-age or min-fresh cache directives in a request to
528   constrain or relax freshness calculations for the corresponding response
529   (<xref target="cache-request-directive"/>).
530</t>
531<t>
532   When calculating freshness, to avoid common problems in date parsing:
533</t>
534<t>
535  <list style="symbols">
536     <t>Although all date formats are specified to be case-sensitive,
537        a cache recipient SHOULD match day, week, and timezone names
538        case-insensitively.</t>
539             
540     <t>If a cache recipient's internal implementation of time has less
541        resolution than the value of an HTTP-date, the recipient MUST
542        internally represent a parsed <xref target="header.expires" format="none">Expires</xref> date as the
543        nearest time equal to or earlier than the received value.</t>
544
545     <t>A cache recipient MUST NOT allow local time zones to influence the
546        calculation or comparison of an age or expiration time.</t>
547
548     <t>A cache recipient SHOULD consider a date with a zone abbreviation
549        other than GMT or UTC to be invalid for calculating expiration.</t>
550  </list>
551</t>
552<t>
553   Note that freshness applies only to cache operation; it cannot be used to
554   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
555   caches and history mechanisms.
556</t>
557
558<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
559<t>
560   A cache can calculate the freshness lifetime (denoted as
561   freshness_lifetime) of a response by using the first match of:
562   <list style="symbols">
563      <t>If the cache is shared and the s-maxage response cache directive
564      (<xref target="cache-response-directive.s-maxage"/>) is present, use its value,
565      or</t>
566      <t>If the max-age response cache directive (<xref target="cache-response-directive.max-age"/>) is present, use its value, or</t>
567      <t>If the <xref target="header.expires" format="none">Expires</xref> response header field
568      (<xref target="header.expires"/>) is present, use its value minus the
569      value of the Date response header field, or</t>
570      <t>Otherwise, no explicit expiration time is present in the response. A
571      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
572   </list>
573</t>
574<t>
575   Note that this calculation is not vulnerable to clock skew, since all of
576   the information comes from the origin server.
577</t>
578<t>
579   When there is more than one value present for a given directive (e.g., two
580   <xref target="header.expires" format="none">Expires</xref> header fields, multiple Cache-Control: max-age
581   directives), the directive's value is considered invalid. Caches are
582   encouraged to consider responses that have invalid freshness information to
583   be stale.
584</t>
585</section>
586
587<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
588<t>
589   Since origin servers do not always provide explicit expiration times, a
590   cache MAY assign a heuristic expiration time when an explicit time is not
591   specified, employing algorithms that use other header field values (such as
592   the Last-Modified time) to estimate a plausible expiration
593   time. This specification does not provide specific algorithms, but does
594   impose worst-case constraints on their results.
595</t>
596<t>
597   A cache MUST NOT use heuristics to determine freshness when an explicit
598   expiration time is present in the stored response. Because of the
599   requirements in <xref target="response.cacheability"/>, this means that,
600   effectively, heuristics can only be used on responses without explicit
601   freshness whose status codes are defined as cacheable by default (see
602   Section 6.1 of <xref target="Part2"/>), and those responses without explicit freshness that have
603   been marked as explicitly cacheable (e.g., with a "public" response cache
604   directive).
605</t>
606<t>
607   If the response has a Last-Modified header field
608   (Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
609   expiration value that is no more than some fraction of the interval since
610   that time. A typical setting of this fraction might be 10%.
611</t>
612<t>
613   When a heuristic is used to calculate freshness lifetime, a cache SHOULD
614   generate a <xref target="header.warning" format="none">Warning</xref> header field with a 113 warn-code (see
615   <xref target="warn.113"/>) in the response if its current_age is more than
616   24 hours and such a warning is not already present.
617</t>
618<t><list>
619   <t>
620      Note: Section 13.9 of <xref target="RFC2616"/> prohibited caches
621      from calculating heuristic freshness for URIs with query components
622      (i.e., those containing '?'). In practice, this has not been widely
623      implemented. Therefore, origin servers are encouraged to send explicit
624      directives (e.g., Cache-Control: no-cache) if they wish to preclude
625      caching.
626   </t>
627</list></t>
628</section>
629
630<section anchor="age.calculations" title="Calculating Age">
631<t>
632   The <xref target="header.age" format="none">Age</xref> header field is used to convey an estimated
633   age of the response message when obtained from a cache. The Age field value
634   is the cache's estimate of the number of seconds since the response was
635   generated or validated by the origin server. In essence, the Age value is
636   the sum of the time that the response has been resident in each of the
637   caches along the path from the origin server, plus the amount of time it
638   has been in transit along network paths.
639</t>
640<t>
641   The following data is used for the age calculation:
642</t>
643<t>
644   <?rfc needLines="4"?>age_value
645   <list>
646      <t>
647         The term "age_value" denotes the value of the <xref target="header.age" format="none">Age</xref>
648         header field (<xref target="header.age"/>), in a form appropriate for
649         arithmetic operation; or 0, if not available.
650      </t>
651   </list>
652</t>
653<t>
654   <?rfc needLines="4"?>date_value
655   <list>
656      <t>
657         The term "date_value" denotes the value of
658         the Date header field, in a form appropriate for arithmetic
659         operations. See Section 7.1.1.2 of <xref target="Part2"/> for the definition of the Date header
660         field, and for requirements regarding responses without it.
661      </t>
662   </list>
663</t>
664<t>
665   <?rfc needLines="4"?>now
666   <list>
667      <t>
668         The term "now" means "the current value of the clock at the host
669         performing the calculation". A host ought to use NTP (<xref target="RFC5905"/>) or some similar protocol to synchronize its
670         clocks to Coordinated Universal Time.
671      </t>
672   </list>
673</t>
674<t>
675   <?rfc needLines="4"?>request_time
676   <list>
677      <t>
678         The current value of the clock at the host at the time the request
679         resulting in the stored response was made.
680      </t>
681   </list>
682</t>
683<t>
684   <?rfc needLines="4"?>response_time
685   <list>
686      <t>
687         The current value of the clock at the host at the time the response
688         was received.
689      </t>
690   </list>
691</t>
692<t>
693   A response's age can be calculated in two entirely independent ways:
694   <list style="numbers">
695      <t>the "apparent_age": response_time minus date_value, if the local
696      clock is reasonably well synchronized to the origin server's clock. If
697      the result is negative, the result is replaced by zero.</t>
698      <t>the "corrected_age_value", if all of the caches along the response
699      path implement HTTP/1.1. A cache MUST interpret this value relative
700      to the time the request was initiated, not the time that the response
701      was received.</t>
702   </list>
703</t>
704<figure>
705<artwork type="code"><![CDATA[
706  apparent_age = max(0, response_time - date_value);
707
708  response_delay = response_time - request_time;
709  corrected_age_value = age_value + response_delay; 
710]]></artwork>
711</figure>
712<figure>
713<preamble>These are combined as</preamble>
714<artwork type="code"><![CDATA[
715  corrected_initial_age = max(apparent_age, corrected_age_value);
716]]></artwork></figure>
717<t>
718   unless the cache is confident in the value of the <xref target="header.age" format="none">Age</xref> header
719   field (e.g., because there are no HTTP/1.0 hops in the Via
720   header field), in which case the corrected_age_value MAY be used as the
721   corrected_initial_age.
722</t>
723<t>
724   The current_age of a stored response can then be calculated by adding the
725   amount of time (in seconds) since the stored response was last validated by
726   the origin server to the corrected_initial_age.
727</t>
728<figure><artwork type="code"><![CDATA[
729  resident_time = now - response_time;
730  current_age = corrected_initial_age + resident_time;
731]]></artwork></figure>
732</section>
733
734<section anchor="serving.stale.responses" title="Serving Stale Responses">
735<t>
736   A "stale" response is one that either has explicit expiry information or is
737   allowed to have heuristic expiry calculated, but is not fresh according to
738   the calculations in <xref target="expiration.model"/>.
739</t>
740<t>
741   A cache MUST NOT generate a stale response if it is prohibited by an
742   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
743   directive, a "must-revalidate" cache-response-directive, or an applicable
744   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
745</t>
746<t>
747   A cache MUST NOT send stale responses unless it is disconnected
748   (i.e., it cannot contact the origin server or otherwise find a forward
749   path) or doing so is explicitly allowed (e.g., by the max-stale request
750   directive; see <xref target="cache-request-directive"/>).
751</t>
752<t>
753   A cache SHOULD generate a <xref target="header.warning" format="none">Warning</xref> header field with the
754   110 warn-code (see <xref target="warn.110"/>) in stale responses.
755   Likewise, a cache SHOULD generate a 112 warn-code (see
756   <xref target="warn.112"/>) in stale responses if the cache is disconnected.
757</t>
758<t>
759   A cache SHOULD NOT generate a new <xref target="header.warning" format="none">Warning</xref> header field
760   when forwarding a response that does not have an <xref target="header.age" format="none">Age</xref> header
761   field, even if the response is already stale.  A cache need not validate
762   a response that merely became stale in transit.
763</t>
764</section>
765</section>
766
767<section anchor="validation.model" title="Validation">
768<t>
769   When a cache has one or more stored responses for a requested URI, but
770   cannot serve any of them (e.g., because they are not fresh, or one cannot
771   be selected; see <xref target="caching.negotiated.responses"/>), it can use
772   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
773   give the next inbound server an opportunity to select a valid stored
774   response to use, updating the stored metadata in the process, or to replace
775   the stored response(s) with a new response. This process is known as
776   "validating" or "revalidating" the stored response.
777</t>
778
779<section anchor="validation.sent" title="Sending a Validation Request"><iref item="validator"/>
780<t>
781   When sending a conditional request for cache validation, a cache sends one
782   or more precondition header fields containing validator
783   metadata from its stored response(s), which is then compared by recipients
784   to determine whether a stored response is equivalent to a current
785   representation of the resource.
786</t>
787<t>
788   One such validator is the timestamp given in a Last-Modified
789   header field (Section 2.2 of <xref target="Part4"/>), which can be used in an
790   If-Modified-Since header field for response validation, or
791   in an If-Unmodified-Since or If-Range header
792   field for representation selection (i.e., the client is referring
793   specifically to a previously obtained representation with that timestamp).
794</t>
795<t>
796   Another validator is the entity-tag given in an ETag header
797   field (Section 2.3 of <xref target="Part4"/>). One or more entity-tags, indicating one or more
798   stored responses, can be used in an If-None-Match header
799   field for response validation, or in an If-Match or
800   If-Range header field for representation selection (i.e.,
801   the client is referring specifically to one or more previously obtained
802   representations with the listed entity-tags).
803</t>
804</section>
805
806<section anchor="validation.received" title="Handling a Received Validation Request">
807<t>
808   Each client in the request chain may have its own cache, so it is common
809   for a cache at an intermediary to receive conditional requests from other
810   (outbound) caches. Likewise, some user agents make use of conditional
811   requests to limit data transfers to recently modified representations or to
812   complete the transfer of a partially retrieved representation.
813</t>
814<t>
815   If a cache receives a request that can be satisfied by reusing one of
816   its stored 200 (OK) or 206 (Partial Content)
817   responses, the cache SHOULD evaluate any applicable conditional header
818   field preconditions received in that request with respect to the
819   corresponding validators contained within the selected response.
820   A cache MUST NOT evaluate conditional header fields that are only
821   applicable to an origin server, found in a request with semantics that
822   cannot be satisfied with a cached response, or applied to a target resource
823   for which it has no stored responses; such preconditions are likely
824   intended for some other (inbound) server.
825</t>
826<t>
827   The proper evaluation of conditional requests by a cache depends on the
828   received precondition header fields and their precedence, as defined in
829   Section 6 of <xref target="Part4"/>. The If-Match and
830   If-Unmodified-Since conditional header fields are not
831   applicable to a cache.
832</t>
833<t>
834   A request containing an If-None-Match header field
835   (Section 3.2 of <xref target="Part4"/>) indicates that the client wants to validate one
836   or more of its own stored responses in comparison to whichever stored
837   response is selected by the cache.
838   If the field-value is "*", or if the field-value is a list of entity-tags
839   and at least one of them match the entity-tag of the selected stored
840   response, a cache recipient SHOULD generate a
841   304 (Not Modified) response (using the metadata of the
842   selected stored response) instead of sending that stored response.
843</t>
844<t>
845   When a cache decides to revalidate its own stored responses for a
846   request that contains an If-None-Match list of entity-tags,
847   the cache MAY combine the received list with a list of entity-tags
848   from its own stored set of responses (fresh or stale) and send the union of
849   the two lists as a replacement If-None-Match header
850   field value in the forwarded request.
851   If a stored response contains only partial content, the
852   cache MUST NOT include its entity-tag in the union unless the request is
853   for a range that would be fully satisfied by that partial stored response.
854   If the response to the forwarded request is
855   304 (Not Modified) and has an ETag header field value with
856   an entity-tag that is not in the client's list, the cache MUST
857   generate a 200 (OK) response for the client by reusing its
858   corresponding stored response, as updated by the 304 response metadata
859   (<xref target="freshening.responses"/>).
860</t>
861<t>
862   If an If-None-Match header field is not present, a request
863   containing an If-Modified-Since header field
864   (Section 3.3 of <xref target="Part4"/>) indicates that the client wants to validate
865   one or more of its own stored responses by modification date.
866   A cache recipient SHOULD generate a 304 (Not Modified)
867   response (using the metadata of the selected stored response) if one of the
868   following cases is true:
869   1) the selected stored response has a Last-Modified
870   field-value that is earlier than or equal to the conditional timestamp;
871   2) no Last-Modified field is present in the selected stored
872   response, but it has a Date field-value that is earlier than
873   or equal to the conditional timestamp; or,
874   3) neither Last-Modified nor Date is present
875   in the selected stored response, but the cache recorded it as having been
876   received at a time earlier than or equal to the conditional timestamp.
877</t>
878<t>
879   A cache that implements partial responses to range requests, as defined in
880   <xref target="Part5"/>, also needs to evaluate a received If-Range header
881   field (Section 3.2 of <xref target="Part5"/>) with respect to its selected stored response.
882</t>
883</section>
884
885<section anchor="validation.response" title="Handling a Validation Response">
886<t>
887   Cache handling of a response to a conditional request is dependent upon its
888   status code:
889</t>
890<t>
891   <list style="symbols">
892      <t>
893         A 304 (Not Modified) response status code indicates
894         that the stored response can be updated and reused; see <xref target="freshening.responses"/>.
895      </t>
896      <t>
897         A full response (i.e., one with a payload body) indicates that none
898         of the stored responses nominated in the conditional request is
899         suitable. Instead, the cache MUST use the full response to
900         satisfy the request and MAY replace the stored response(s).
901      </t>
902      <t>
903         However, if a cache receives a 5xx (Server Error)
904         response while attempting to validate a response, it can either
905         forward this response to the requesting client, or act as if the
906         server failed to respond. In the latter case, the cache MAY send a
907         previously stored response (see <xref target="serving.stale.responses"/>).
908      </t>
909   </list>
910</t>
911</section>
912
913<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
914<t>
915   When a cache receives a 304 (Not Modified) response and
916   already has one or more stored 200 (OK) responses for the
917   same cache key, the cache needs to identify which of the stored responses
918   are updated by this new response and then update the stored response(s)
919   with the new information provided in the 304 response.
920</t>
921<iref item="strong validator"/>
922<t>
923   The stored response to update is identified by using the first match (if
924   any) of:
925   <list style="symbols">
926    <t>
927     If the new response contains a strong validator (see
928     Section 2.1 of <xref target="Part4"/>), then that strong validator identifies the selected
929     representation for update. All of the stored responses with the same
930     strong validator are selected. If none of the stored responses contain the
931     same strong validator, then the cache MUST NOT use the new response to
932     update any stored responses.
933    </t>
934    <t>
935     If the new response contains a weak validator and that validator
936     corresponds to one of the cache's stored responses, then the most
937     recent of those matching stored responses is selected for update.
938    </t>
939    <t>
940     If the new response does not include any form of validator (such as in
941     the case where a client generates an If-Modified-Since request from a
942     source other than the Last-Modified response header field), and there is
943     only one stored response, and that stored response also lacks a
944     validator, then that stored response is selected for update.
945    </t>
946   </list>
947</t>
948<t>
949   If a stored response is selected for update, the cache MUST:
950   <list style="symbols">
951      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
952         with warn-code 1xx (see <xref target="header.warning"/>);</t>
953      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
954         with warn-code 2xx; and,</t>
955      <t>use other header fields provided in the 304 (Not Modified)
956         response to replace all instances of the corresponding header
957         fields in the stored response.</t>
958   </list>
959</t>
960</section>
961
962<section anchor="head.effects" title="Freshening Responses via HEAD">
963<t>
964   A response to the HEAD method is identical to what an equivalent request
965   made with a GET would have been, except it lacks a body. This property
966   of HEAD responses can be used to invalidate or update a cached GET
967   response if the more efficient conditional GET request mechanism is not
968   available (due to no validators being present in the stored response) or
969   if transmission of the representation body is not desired even if it has
970   changed.
971</t>
972<t>
973   When a cache makes an inbound HEAD request for a given request target and
974   receives a 200 (OK) response, the cache SHOULD update or
975   invalidate each of its stored GET responses that could have been selected
976   for that request (see <xref target="caching.negotiated.responses"/>).
977</t>
978<t>
979   For each of the stored responses that could have been selected, if the
980   stored response and HEAD response have matching values for any received
981   validator fields (ETag and Last-Modified)
982   and, if the HEAD response has a Content-Length header field,
983   the value of Content-Length matches that of the stored
984   response, the cache SHOULD update the stored response a described below;
985   otherwise, the cache SHOULD consider the stored response to be stale.
986</t>
987<t>
988   If a cache updates a stored response with the metadata provided in a HEAD
989   response, the cache MUST:
990   <list style="symbols">
991      <t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
992         with warn-code 1xx (see <xref target="header.warning"/>);</t>
993      <t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
994         with warn-code 2xx; and,</t>
995      <t>use other header fields provided in the HEAD response to replace all
996         instances of the corresponding header fields in the stored response
997         and append new header fields to the stored response's header section
998         unless otherwise restricted by the <xref target="header.cache-control" format="none">Cache-Control</xref>
999         header field.</t>
1000   </list>
1001</t>
1002</section>
1003</section>
1004
1005
1006<section anchor="invalidation" title="Invalidation">
1007<t>
1008   Because unsafe request methods (Section 4.2.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
1009   have the potential for changing state on the origin server, intervening
1010   caches can use them to keep their contents up-to-date.
1011</t>
1012<t>
1013   A cache MUST invalidate the effective Request URI
1014   (Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the
1015   Location and Content-Location response header
1016   fields (if present) when a non-error status code is received in response to
1017   an unsafe request method.
1018</t>
1019<t>
1020   However, a cache MUST NOT invalidate a URI from a Location
1021   or Content-Location response header field if the host part of
1022   that URI differs from the host part in the effective request URI
1023   (Section 5.5 of <xref target="Part1"/>). This helps prevent denial of service attacks.
1024</t>
1025<t>
1026   A cache MUST invalidate the effective request URI
1027   (Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
1028   to a request with a method whose safety is unknown.
1029</t>
1030<t>
1031   Here, a "non-error response" is one with a 2xx (Successful)
1032   or 3xx (Redirection) status code. "Invalidate" means that
1033   the cache will either remove all stored responses related to the effective
1034   request URI, or will mark these as "invalid" and in need of a mandatory
1035   validation before they can be sent in response to a subsequent request.
1036</t>
1037<t>
1038   Note that this does not guarantee that all appropriate responses are
1039   invalidated. For example, a state-changing request might invalidate
1040   responses in the caches it travels through, but relevant responses still
1041   might be stored in other caches that it has not.</t>
1042</section>
1043</section>
1044
1045
1046
1047<section anchor="header.field.definitions" title="Header Field Definitions">
1048<t>
1049   This section defines the syntax and semantics of HTTP/1.1 header fields
1050   related to caching.
1051</t>
1052
1053<section anchor="header.age" title="Age">
1054   <iref item="Age header field" primary="true"/>
1055   
1056   
1057<t>
1058   The "Age" header field conveys the sender's estimate of the amount
1059   of time since the response was generated or successfully validated at the
1060   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1061</t>
1062<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1063  Age = delta-seconds
1064]]></artwork></figure>
1065<t>
1066   The Age field-value is a non-negative integer, representing time in seconds
1067   (see <xref target="delta-seconds"/>).
1068</t>
1069<t>
1070   The presence of an Age header field implies that the response was not
1071   generated or validated by the origin server for this request. However,
1072   lack of an Age header field does not imply the origin was contacted, since
1073   the response might have been received from an HTTP/1.0 cache that does not
1074   implement Age.
1075</t>
1076</section>
1077
1078<section anchor="header.cache-control" title="Cache-Control">
1079   <iref item="Cache-Control header field" primary="true"/>
1080   
1081   
1082<t>
1083   The "Cache-Control" header field is used to specify directives for
1084   caches along the request/response chain. Such cache directives are
1085   unidirectional in that the presence of a directive in a request does not
1086   imply that the same directive is to be given in the response.
1087</t>
1088<t>
1089   A cache MUST obey the requirements of the Cache-Control
1090   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1091   directives defined elsewhere are handled.
1092</t>
1093<t><list>
1094   <t>
1095       Note: Some HTTP/1.0 caches might not implement Cache-Control.
1096   </t>
1097</list></t>
1098<t>
1099   A proxy, whether or not it implements a cache, MUST pass cache directives
1100   through in forwarded messages, regardless of their
1101   significance to that application, since the directives might be applicable
1102   to all recipients along the request/response chain. It is not possible to
1103   target a directive to a specific cache.
1104</t>
1105<t>
1106   Cache directives are identified by a token, to be compared case-insensitively,
1107   and have an optional argument, that can use both token and quoted-string
1108   syntax. For the directives defined below that define arguments, recipients
1109   ought to accept both forms, even if one is documented to be preferred. For
1110   any directive not defined by this specification, a recipient MUST accept
1111   both forms.
1112</t>
1113<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><artwork type="abnf2616"><![CDATA[
1114  Cache-Control   = 1#cache-directive
1115
1116  cache-directive = token [ "=" ( token / quoted-string ) ]
1117]]></artwork></figure>
1118<t>
1119   For the cache directives defined below, no argument is defined (nor allowed)
1120   unless stated otherwise.
1121</t>
1122
1123<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1124
1125<section title="max-age" anchor="cache-request-directive.max-age">
1126   <iref item="max-age (cache directive)" primary="true"/>
1127<t>
1128   Argument syntax:
1129   <list>
1130      <t>
1131        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1132      </t>
1133   </list>
1134</t>
1135<t>
1136   The "max-age" request directive indicates that the client is unwilling to
1137   accept a response whose age is greater than the specified number of
1138   seconds. Unless the max-stale request directive is also present, the
1139   client is not willing to accept a stale response.
1140</t>
1141<t>
1142   This directive uses the token form of the argument syntax;
1143   e.g., 'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate the
1144   quoted-string form.
1145</t>
1146</section>
1147
1148<section title="max-stale" anchor="cache-request-directive.max-stale">
1149   <iref item="max-stale (cache directive)" primary="true"/>
1150<t>
1151   Argument syntax:
1152   <list>
1153      <t>
1154        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1155      </t>
1156   </list>
1157</t>
1158<t>
1159   The "max-stale" request directive indicates that the client is willing
1160   to accept a response that has exceeded its freshness lifetime. If max-stale
1161   is assigned a value, then the client is willing to accept a response
1162   that has exceeded its freshness lifetime by no more than the specified
1163   number of seconds. If no value is assigned to max-stale, then the client
1164   is willing to accept a stale response of any age.
1165</t>
1166<t>
1167   This directive uses the token form of the argument syntax;
1168   e.g., 'max-stale=10', not 'max-stale="10"'. A sender SHOULD NOT generate
1169   the quoted-string form.
1170</t>
1171</section>
1172
1173<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1174   <iref item="min-fresh (cache directive)" primary="true"/>
1175<t>
1176   Argument syntax:
1177   <list>
1178      <t>
1179        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1180      </t>
1181   </list>
1182</t>
1183<t>
1184   The "min-fresh" request directive indicates that the client is willing
1185   to accept a response whose freshness lifetime is no less than its
1186   current age plus the specified time in seconds. That is, the client
1187   wants a response that will still be fresh for at least the specified
1188   number of seconds.
1189</t>
1190<t>
1191   This directive uses the token form of the argument syntax;
1192   e.g., 'min-fresh=20', not 'min-fresh="20"'. A sender SHOULD NOT generate
1193   the quoted-string form.
1194</t>
1195</section>
1196
1197<section title="no-cache" anchor="cache-request-directive.no-cache">
1198   <iref item="no-cache (cache directive)" primary="true"/>
1199<t>
1200   The "no-cache" request directive indicates that a cache MUST NOT
1201   use a stored response to satisfy the request without successful
1202   validation on the origin server.
1203</t>
1204</section>
1205 
1206<section title="no-store" anchor="cache-request-directive.no-store">
1207   <iref item="no-store (cache directive)" primary="true"/>
1208<t>
1209   The "no-store" request directive indicates that a cache MUST NOT
1210   store any part of either this request or any response to it. This
1211   directive applies to both private and shared caches. "MUST NOT
1212   store" in this context means that the cache MUST NOT intentionally
1213   store the information in non-volatile storage, and MUST make a
1214   best-effort attempt to remove the information from volatile storage as
1215   promptly as possible after forwarding it.
1216</t>
1217<t>
1218   This directive is NOT a reliable or sufficient mechanism for ensuring
1219   privacy. In particular, malicious or compromised caches might not
1220   recognize or obey this directive, and communications networks might be
1221   vulnerable to eavesdropping.
1222</t>
1223<t>
1224   Note that if a request containing this directive is satisfied from a
1225   cache, the no-store request directive does not apply to the already
1226   stored response.
1227</t>
1228</section>
1229
1230<section title="no-transform" anchor="cache-request-directive.no-transform">
1231   <iref item="no-transform (cache directive)" primary="true"/>
1232<t>
1233   The "no-transform" request directive indicates that an intermediary
1234   (whether or not it implements a cache) MUST NOT transform the payload,
1235   as defined in Section 5.7.2 of <xref target="Part1"/>.
1236</t>
1237</section>
1238
1239<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1240   <iref item="only-if-cached (cache directive)" primary="true"/>
1241<t>
1242   The "only-if-cached" request directive indicates that the client only wishes
1243   to obtain a stored response. If it receives this directive, a cache SHOULD
1244   either respond using a stored response that is consistent with the other
1245   constraints of the request, or respond with a 504 (Gateway
1246   Timeout) status code. If a group of caches is being operated as a
1247   unified system with good internal connectivity, a member cache MAY
1248   forward such a request within that group of caches.
1249</t>
1250</section>
1251</section>
1252
1253<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1254   
1255
1256<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1257   <iref item="must-revalidate (cache directive)" primary="true"/>
1258<t>
1259   The "must-revalidate" response directive indicates that once it has
1260   become stale, a cache MUST NOT use the response to satisfy subsequent
1261   requests without successful validation on the origin server.
1262</t>
1263<t>
1264   The must-revalidate directive is necessary to support reliable
1265   operation for certain protocol features. In all circumstances a
1266   cache MUST obey the must-revalidate directive; in particular,
1267   if a cache cannot reach the origin server for any reason, it MUST
1268   generate a 504 (Gateway Timeout) response.
1269</t>
1270<t>
1271   The must-revalidate directive ought to be used by servers if and only
1272   if failure to validate a request on the representation could result in
1273   incorrect operation, such as a silently unexecuted financial
1274   transaction.
1275</t>
1276</section>
1277
1278<section title="no-cache" anchor="cache-response-directive.no-cache">
1279   <iref item="no-cache (cache directive)" primary="true"/>
1280<t>
1281   Argument syntax:
1282   <list>
1283      <t>
1284        #<xref target="imported.abnf" format="none">field-name</xref>
1285      </t>
1286   </list>
1287</t>
1288<t>
1289   The "no-cache" response directive indicates that the response MUST NOT
1290   be used to satisfy a subsequent request without successful validation on
1291   the origin server. This allows an origin server to prevent a cache from
1292   using it to satisfy a request without contacting it, even by caches that
1293   have been configured to send stale responses.
1294</t>
1295<t>
1296   If the no-cache response directive specifies one or more field-names,
1297   then a cache MAY use the response to satisfy a subsequent request,
1298   subject to any other restrictions on caching. However, any header fields
1299   in the response that have the field-name(s) listed MUST NOT be sent
1300   in the response to a subsequent request without successful revalidation
1301   with the origin server. This allows an origin server to prevent the
1302   re-use of certain header fields in a response, while still allowing
1303   caching of the rest of the response.
1304</t>
1305<t>
1306   The field-names given are not limited to the set of header
1307   fields defined by this specification. Field names are case-insensitive.
1308</t>
1309<t>
1310   This directive uses the quoted-string form of the argument syntax.
1311   A sender SHOULD NOT generate the token form (even if quoting appears not
1312   to be needed for single-entry lists).
1313</t>
1314<t>
1315   Note: Although it has been back-ported to many implementations, some
1316   HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
1317   response directives with field-names are often handled by caches
1318   as if an unqualified no-cache directive was received; i.e., the special
1319   handling for the qualified form is not widely implemented.
1320</t>
1321</section>
1322
1323<section title="no-store" anchor="cache-response-directive.no-store">
1324   <iref item="no-store (cache directive)" primary="true"/>
1325<t>
1326   The "no-store" response directive indicates that a cache MUST NOT
1327   store any part of either the immediate request or response. This
1328   directive applies to both private and shared caches. "MUST NOT
1329   store" in this context means that the cache MUST NOT intentionally
1330   store the information in non-volatile storage, and MUST make a
1331   best-effort attempt to remove the information from volatile storage as
1332   promptly as possible after forwarding it.
1333</t>
1334<t>
1335   This directive is NOT a reliable or sufficient mechanism for ensuring
1336   privacy. In particular, malicious or compromised caches might not
1337   recognize or obey this directive, and communications networks might be
1338   vulnerable to eavesdropping.
1339</t>
1340</section>
1341
1342<section title="no-transform" anchor="cache-response-directive.no-transform">
1343   <iref item="no-transform (cache directive)" primary="true"/>
1344<t>
1345   The "no-transform" response directive indicates that an intermediary
1346   (regardless of whether it implements a cache) MUST NOT transform the
1347   payload, as defined in Section 5.7.2 of <xref target="Part1"/>.
1348</t>
1349</section>
1350
1351<section title="public" anchor="cache-response-directive.public">
1352   <iref item="public (cache directive)" primary="true"/>
1353<t>
1354   The "public" response directive indicates that any cache MAY store the
1355   response, even if the response would normally be non-cacheable or cacheable
1356   only within a private cache. (See <xref target="caching.authenticated.responses"/> for additional details related to
1357   the use of public in response to a request containing
1358   Authorization, and <xref target="response.cacheability"/> for
1359   details of how public affects responses that would normally not be stored,
1360   due to their status codes not being defined as cacheable by default; see
1361   <xref target="heuristic.freshness"/>.)
1362</t>
1363</section>
1364
1365<section title="private" anchor="cache-response-directive.private">
1366   <iref item="private (cache directive)" primary="true"/>
1367<t>
1368   Argument syntax:
1369   <list>
1370      <t>
1371        #<xref target="imported.abnf" format="none">field-name</xref>
1372      </t>
1373   </list>
1374</t>
1375<t>
1376   The "private" response directive indicates that the response message is
1377   intended for a single user and MUST NOT be stored by a shared cache. A
1378   private cache MAY store the response and reuse it for later requests,
1379   even if the response would normally be non-cacheable.
1380</t>
1381<t>
1382   If the private response directive specifies one or more field-names,
1383   this requirement is limited to the field-values associated with the
1384   listed response header fields. That is, a shared cache MUST NOT store
1385   the specified field-names(s), whereas it MAY store the remainder of the
1386   response message.
1387</t>
1388<t>
1389   The field-names given are not limited to the set of header
1390   fields defined by this specification. Field names are case-insensitive.
1391</t>
1392<t>
1393   This directive uses the quoted-string form of the argument syntax.
1394   A sender SHOULD NOT generate the token form (even if quoting appears not
1395   to be needed for single-entry lists).
1396</t>
1397<t>
1398   Note: This usage of the word "private" only controls
1399   where the response can be stored; it cannot ensure the privacy of the
1400   message content. Also, private response directives with field-names are
1401   often handled by caches as if an unqualified private directive
1402   was received; i.e., the special handling for the qualified form is not
1403   widely implemented.
1404</t>
1405</section>
1406
1407<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1408   <iref item="proxy-revalidate (cache directive)" primary="true"/>
1409<t>
1410   The "proxy-revalidate" response directive has the same meaning as the
1411   must-revalidate response directive, except that it does not apply to
1412   private caches.
1413</t>
1414</section>
1415
1416<section title="max-age" anchor="cache-response-directive.max-age">
1417   <iref item="max-age (cache directive)" primary="true"/>
1418<t>
1419   Argument syntax:
1420   <list>
1421      <t>
1422        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1423      </t>
1424   </list>
1425</t>
1426<t>
1427   The "max-age" response directive indicates that the response is to be
1428   considered stale after its age is greater than the specified number of
1429   seconds.
1430</t>
1431<t>
1432   This directive uses the token form of the argument syntax;
1433   e.g., 'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate the
1434   quoted-string form.
1435</t>
1436</section>     
1437
1438<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1439   <iref item="s-maxage (cache directive)" primary="true"/>
1440<t>
1441   Argument syntax:
1442   <list>
1443      <t>
1444        <xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
1445      </t>
1446   </list>
1447</t>
1448<t>
1449   The "s-maxage" response directive indicates that, in shared caches, the
1450   maximum age specified by this directive overrides the maximum age
1451   specified by either the max-age directive or the <xref target="header.expires" format="none">Expires</xref>
1452   header field. The s-maxage directive also implies the semantics of the
1453   proxy-revalidate response directive.
1454</t>
1455<t>
1456   This directive uses the token form of the argument syntax;
1457   e.g., 's-maxage=10', not 's-maxage="10"'. A sender SHOULD NOT generate
1458   the quoted-string form.
1459</t>
1460</section>
1461
1462</section>
1463
1464<section anchor="cache.control.extensions" title="Cache Control Extensions">
1465<t>
1466   The Cache-Control header field can be extended through the use of one or
1467   more cache-extension tokens, each with an optional value.
1468</t>
1469<t>
1470   Informational extensions (those that do not require a change in cache
1471   behavior) can be added without changing the semantics of other directives.
1472   Behavioral extensions are designed to work by acting as modifiers to the
1473   existing base of cache directives.
1474</t>
1475<t>  
1476   Both the new directive and the standard directive are supplied, such that
1477   applications that do not understand the new directive will default to the
1478   behavior specified by the standard directive, and those that understand the
1479   new directive will recognize it as modifying the requirements associated
1480   with the standard directive. In this way, extensions to the cache-control
1481   directives can be made without requiring changes to the base protocol.
1482</t>
1483<t>
1484   This extension mechanism depends on an HTTP cache obeying all of the
1485   cache-control directives defined for its native HTTP-version, obeying
1486   certain extensions, and ignoring all directives that it does not
1487   understand.
1488</t>
1489<t>
1490   For example, consider a hypothetical new response directive called
1491   "community" that acts as a modifier to the private directive. We define
1492   this new directive to mean that, in addition to any private cache, any
1493   cache that is shared only by members of the community named within its
1494   value is allowed to cache the response. An origin server wishing to allow
1495   the UCI community to use an otherwise private response in their shared
1496   cache(s) could do so by including
1497</t>
1498<figure><artwork type="example"><![CDATA[
1499  Cache-Control: private, community="UCI"
1500]]></artwork></figure>
1501<t>
1502   A cache seeing this header field will act correctly even if the cache does
1503   not understand the community cache-extension, since it will also see and
1504   understand the private directive and thus default to the safe behavior.
1505</t>
1506<t>
1507   A cache MUST ignore unrecognized cache directives; it is assumed that any
1508   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1509   combined with standard directives (or the response's default cacheability)
1510   such that the cache behavior will remain minimally correct even if the
1511   cache does not understand the extension(s).
1512</t>
1513</section>
1514
1515</section>
1516
1517<section anchor="header.expires" title="Expires">
1518   <iref item="Expires header field" primary="true"/>
1519   
1520<t>
1521   The "Expires" header field gives the date/time after which the
1522   response is considered stale. See <xref target="expiration.model"/> for
1523   further discussion of the freshness model.
1524</t>
1525<t>
1526   The presence of an Expires field does not imply that the original resource
1527   will change or cease to exist at, before, or after that time.
1528</t>
1529<t>
1530   The Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of <xref target="Part2"/>.
1531</t>
1532<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1533  Expires = HTTP-date
1534]]></artwork></figure>
1535<figure>
1536  <preamble>For example</preamble>
1537<artwork type="example"><![CDATA[
1538  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1539]]></artwork></figure>
1540<t>
1541   A cache recipient MUST interpret invalid date formats, especially the
1542   value "0", as representing a time in the past (i.e., "already expired").
1543</t>
1544<t>
1545   If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
1546   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1547   a recipient MUST ignore the Expires field.
1548   Likewise, if a response includes the s-maxage directive
1549   (<xref target="cache-response-directive.s-maxage"/>), a shared cache
1550   recipient MUST ignore the Expires field. In both these cases, the value
1551   in Expires is only intended for recipients that have not yet implemented
1552   the Cache-Control field.
1553</t>
1554<t>
1555   An origin server without a clock MUST NOT generate an Expires field
1556   unless its value represents a fixed time in the past (always expired)
1557   or its value has been associated with the resource by a system or user
1558   with a reliable clock.
1559</t>
1560<t>
1561   Historically, HTTP required the Expires field-value to be no more than a
1562   year in the future. While longer freshness lifetimes are no longer
1563   prohibited, extremely large values have been demonstrated to cause
1564   problems (e.g., clock overflows due to use of 32-bit integers for
1565   time values), and many caches will evict a response far sooner than
1566   that.
1567</t>
1568</section>
1569
1570<section anchor="header.pragma" title="Pragma">
1571   <iref item="Pragma header field" primary="true"/>
1572   
1573   
1574   
1575<t>
1576   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1577   caches, so that clients can specify a "no-cache" request that they will
1578   understand (as <xref target="header.cache-control" format="none">Cache-Control</xref> was not defined until HTTP/1.1).
1579   When the Cache-Control header field is also present and understood in a
1580   request, Pragma is ignored.
1581</t>
1582<t>
1583   In HTTP/1.0, Pragma was defined as an extensible field for
1584   implementation-specified directives for recipients. This specification
1585   deprecates such extensions to improve interoperability.
1586</t>
1587<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[
1588  Pragma           = 1#pragma-directive
1589  pragma-directive = "no-cache" / extension-pragma
1590  extension-pragma = token [ "=" ( token / quoted-string ) ]
1591]]></artwork></figure>
1592<t>
1593   When the <xref target="header.cache-control" format="none">Cache-Control</xref> header field is not present in a
1594   request, caches MUST consider the no-cache request pragma-directive as
1595   having the same effect as if "Cache-Control: no-cache" were present (see
1596   <xref target="cache-request-directive"/>).
1597</t>
1598<t>
1599   When sending a no-cache request, a client ought to include both the pragma
1600   and cache-control directives, unless Cache-Control: no-cache is
1601   purposefully omitted to target other <xref target="header.cache-control" format="none">Cache-Control</xref> response
1602   directives at HTTP/1.1 caches. For example:
1603</t>
1604<figure>
1605<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1606  GET / HTTP/1.1
1607  Host: www.example.com
1608  Cache-Control: max-age=30
1609  Pragma: no-cache
1610 
1611  ]]></artwork>
1612</figure>
1613<t>
1614   will constrain HTTP/1.1 caches to serve a response no older than 30
1615   seconds, while precluding implementations that do not understand
1616   <xref target="header.cache-control" format="none">Cache-Control</xref> from serving a cached response.
1617</t>
1618<t><list>
1619   <t>
1620      Note: Because the meaning of "Pragma: no-cache" in responses is not
1621      specified, it does not provide a reliable replacement for
1622      "Cache-Control: no-cache" in them.
1623   </t>
1624</list></t>
1625</section>
1626
1627<section anchor="header.warning" title="Warning">
1628   <iref item="Warning header field" primary="true"/>
1629   
1630   
1631   
1632   
1633   
1634   
1635<t>
1636   The "Warning" header field is used to carry additional information
1637   about the status or transformation of a message that might not be reflected
1638   in the status code. This information is typically used to warn about possible
1639   incorrectness introduced by caching operations or transformations applied
1640   to the payload of the message.
1641</t>
1642<t>
1643   Warnings can be used for other purposes, both cache-related and otherwise.
1644   The use of a warning, rather than an error status code, distinguishes these
1645   responses from true failures.
1646</t>
1647<t>
1648   Warning header fields can in general be applied to any message, however some
1649   warn-codes are specific to caches and can only be applied to response
1650   messages.
1651</t>
1652<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[
1653  Warning       = 1#warning-value
1654 
1655  warning-value = warn-code SP warn-agent SP warn-text
1656                                        [ SP warn-date ]
1657 
1658  warn-code  = 3DIGIT
1659  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1660                  ; the name or pseudonym of the server adding
1661                  ; the Warning header field, for use in debugging
1662                  ; a single "-" is recommended when agent unknown
1663  warn-text  = quoted-string
1664  warn-date  = DQUOTE HTTP-date DQUOTE
1665]]></artwork></figure>
1666<t>
1667   Multiple warnings can be generated in a response (either by the origin
1668   server or by a cache), including multiple warnings with the same warn-code
1669   number that only differ in warn-text.
1670</t>
1671<t>
1672   A user agent that receives one or more Warning header fields SHOULD
1673   inform the user of as many of them as possible, in the order that they
1674   appear in the response. Senders that generate multiple Warning header
1675   fields are encouraged to order them with this user agent behavior in mind.
1676   A sender that generates new Warning header fields MUST append them after
1677   any existing Warning header fields.
1678</t>
1679<t>
1680   Warnings are assigned three digit warn-codes. The first digit indicates
1681   whether the Warning is required to be deleted from a stored response after
1682   validation:
1683   <list style="symbols">
1684      <t>1xx warn-codes describe the freshness or validation status of the
1685      response, and so MUST be deleted by a cache after validation. They can
1686      only be generated by a cache when validating a cached entry, and
1687      MUST NOT be generated in any other situation.</t>
1688      <t>2xx warn-codes describe some aspect of the representation that is not
1689      rectified by a validation (for example, a lossy compression of the
1690      representation) and MUST NOT be deleted by a cache after validation,
1691      unless a full response is sent, in which case they MUST be.</t>
1692   </list>
1693</t>
1694<t>
1695   If a sender generates one or more 1xx warn-codes in a message to be
1696   sent to a recipient known to implement only HTTP/1.0, the sender MUST
1697   include in each corresponding warning-value a warn-date that matches the
1698   Date header field in the message. For example:
1699</t>
1700<figure><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1701  HTTP/1.1 200 OK
1702  Date: Sat, 25 Aug 2012 23:34:45 GMT
1703  Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
1704 
1705  ]]></artwork></figure>
1706<t>
1707   If a recipient that uses, evaluates, or displays Warning header fields
1708   receives a warn-date that is different from the Date value
1709   in the same message, the recipient MUST exclude the warning-value
1710   containing that warn-date before storing, forwarding, or using the message.
1711   This allows recipients to exclude warning-values that were improperly
1712   retained after a cache validation.
1713   If all of the warning-values are excluded, the recipient MUST exclude
1714   the Warning header field as well.
1715</t>
1716<t>
1717   The following warn-codes are defined by this specification, each with a
1718   recommended warn-text in English, and a description of its meaning.
1719   The procedure for defining additional warn codes is described in
1720   <xref target="warn.code.registry.procedure"/>.
1721</t>
1722
1723<section title="Warning: 110 - &#34;Response is Stale&#34;" anchor="warn.110">
1724  <iref primary="true" item="110 (warn-code)"/>
1725  <iref primary="true" item="Response is Stale (warn-text)"/>
1726<t>
1727   A cache SHOULD generate this whenever the sent response is stale.
1728</t>
1729</section>
1730
1731<section title="Warning: 111 - &#34;Revalidation Failed&#34;" anchor="warn.111">
1732  <iref primary="true" item="111 (warn-code)"/>
1733  <iref primary="true" item="Revalidation Failed (warn-text)"/>
1734<t>
1735   A cache SHOULD generate this when sending a stale response because an
1736   attempt to validate the response failed, due to an inability to reach
1737   the server.
1738</t>
1739</section>
1740
1741<section title="Warning: 112 - &#34;Disconnected Operation&#34;" anchor="warn.112">
1742  <iref primary="true" item="112 (warn-code)"/>
1743  <iref primary="true" item="Disconnected Operation (warn-text)"/>
1744<t>
1745   A cache SHOULD generate this if it is intentionally disconnected from
1746   the rest of the network for a period of time.
1747</t>
1748</section>
1749
1750<section title="Warning: 113 - &#34;Heuristic Expiration&#34;" anchor="warn.113">
1751  <iref primary="true" item="113 (warn-code)"/>
1752  <iref primary="true" item="Heuristic Expiration (warn-text)"/>
1753<t>
1754   A cache SHOULD generate this if it heuristically chose a freshness
1755   lifetime greater than 24 hours and the response's age is greater than 24
1756   hours.
1757</t>
1758</section>
1759
1760<section title="Warning: 199 - &#34;Miscellaneous Warning&#34;" anchor="warn.199">
1761  <iref primary="true" item="199 (warn-code)"/>
1762  <iref primary="true" item="Miscellaneous Warning (warn-text)"/>
1763<t>
1764   The warning text can include arbitrary information to be presented to
1765   a human user, or logged. A system receiving this warning MUST NOT take
1766   any automated action, besides presenting the warning to the user.
1767</t>
1768</section>
1769
1770<section title="Warning: 214 - &#34;Transformation Applied&#34;" anchor="warn.214">
1771  <iref primary="true" item="214 (warn-code)"/>
1772  <iref primary="true" item="Transformation Applied (warn-text)"/>
1773<t>
1774   MUST be added by a proxy if it applies any transformation to the
1775   representation, such as changing the content-coding, media-type, or
1776   modifying the representation data, unless this Warning code already appears
1777   in the response.
1778</t>
1779</section>
1780
1781<section title="Warning: 299 - &#34;Miscellaneous Persistent Warning&#34;" anchor="warn.299">
1782  <iref primary="true" item="299 (warn-code)"/>
1783  <iref primary="true" item="Miscellaneous Persistent Warning (warn-text)"/>
1784<t>
1785   The warning text can include arbitrary information to be presented to
1786   a human user, or logged. A system receiving this warning MUST NOT take
1787   any automated action.
1788</t>
1789</section>
1790</section>
1791</section>
1792
1793<section anchor="history.lists" title="History Lists">
1794<t>
1795   User agents often have history mechanisms, such as "Back" buttons and
1796   history lists, that can be used to redisplay a representation retrieved
1797   earlier in a session.
1798</t>
1799<t>
1800   The freshness model (<xref target="expiration.model"/>) does not
1801   necessarily apply to history mechanisms. I.e., a history mechanism can
1802   display a previous representation even if it has expired.
1803</t>
1804<t>
1805   This does not prohibit the history mechanism from telling the user that a
1806   view might be stale, or from honoring cache directives (e.g.,
1807   Cache-Control: no-store).
1808</t>
1809</section>
1810
1811
1812<section title="IANA Considerations" anchor="iana.considerations">
1813
1814<section title="Cache Directive Registry" anchor="cache.directive.registry">
1815<t>
1816   The HTTP Cache Directive Registry defines the name space for the
1817   cache directives. It will be created and maintained at (the suggested URI)
1818   <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1819</t>
1820
1821<section title="Procedure" anchor="cache.directive.registry.procedure">
1822<t>
1823   A registration MUST include the following fields:
1824   <list style="symbols">
1825      <t>Cache Directive Name</t>
1826      <t>Pointer to specification text</t>
1827   </list>
1828</t>
1829<t>
1830   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1831</t>
1832</section>
1833
1834<section title="Considerations for New Cache Control Directives" anchor="cache.directive.considerations">
1835<t>
1836   New extension directives ought to consider defining:
1837</t>
1838<t>
1839   <list style="symbols">
1840      <t>What it means for a directive to be specified multiple times,</t>
1841      <t>When the directive does not take an argument, what it means when an
1842      argument is present,</t>
1843      <t>When the directive requires an argument, what it means when it is
1844      missing,</t>
1845      <t>Whether the directive is specific to requests, responses, or able
1846        to be used in either.</t>
1847   </list>
1848</t>
1849<t>
1850  See also <xref target="cache.control.extensions"/>.
1851</t>
1852</section>
1853
1854<section title="Registrations" anchor="cache.directive.registration">
1855<t>
1856  The HTTP Cache Directive Registry shall be populated with the registrations below:
1857</t>
1858
1859<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1860<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1861   <ttcol>Cache Directive</ttcol>
1862   <ttcol>Reference</ttcol>
1863
1864   <c>max-age</c>
1865   <c>
1866      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1867   </c>
1868   <c>max-stale</c>
1869   <c>
1870      <xref target="cache-request-directive.max-stale"/>
1871   </c>
1872   <c>min-fresh</c>
1873   <c>
1874      <xref target="cache-request-directive.min-fresh"/>
1875   </c>
1876   <c>must-revalidate</c>
1877   <c>
1878      <xref target="cache-response-directive.must-revalidate"/>
1879   </c>
1880   <c>no-cache</c>
1881   <c>
1882      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1883   </c>
1884   <c>no-store</c>
1885   <c>
1886      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1887   </c>
1888   <c>no-transform</c>
1889   <c>
1890      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1891   </c>
1892   <c>only-if-cached</c>
1893   <c>
1894      <xref target="cache-request-directive.only-if-cached"/>
1895   </c>
1896   <c>private</c>
1897   <c>
1898      <xref target="cache-response-directive.private"/>
1899   </c>
1900   <c>proxy-revalidate</c>
1901   <c>
1902      <xref target="cache-response-directive.proxy-revalidate"/>
1903   </c>
1904   <c>public</c>
1905   <c>
1906      <xref target="cache-response-directive.public"/>
1907   </c>
1908   <c>s-maxage</c>
1909   <c>
1910      <xref target="cache-response-directive.s-maxage"/>
1911   </c>
1912   <c>stale-if-error</c>
1913   <c>
1914      <xref target="RFC5861"/>, Section 4
1915   </c>
1916   <c>stale-while-revalidate</c>
1917   <c>
1918      <xref target="RFC5861"/>, Section 3
1919   </c>
1920</texttable>
1921<!--(END)-->
1922
1923</section>
1924</section>
1925
1926<section title="Warn Code Registry" anchor="warn.code.registry">
1927<t>
1928   The HTTP Warn Code Registry defines the name space for warn codes.
1929   It will be created and maintained at (the suggested URI)
1930   <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
1931</t>
1932
1933<section title="Procedure" anchor="warn.code.registry.procedure">
1934<t>
1935   A registration MUST include the following fields:
1936   <list style="symbols">
1937      <t>Warn Code (3 digits)</t>
1938      <t>Short Description</t>
1939      <t>Pointer to specification text</t>
1940   </list>
1941</t>
1942<t>
1943   Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
1944</t>
1945</section>
1946
1947<section title="Registrations" anchor="warn.code.registration">
1948<t>
1949  The HTTP Warn Code Registry shall be populated with the registrations below:
1950</t>
1951
1952<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1953<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1954   <ttcol>Warn Code</ttcol>
1955   <ttcol>Short Description</ttcol>
1956   <ttcol>Reference</ttcol>
1957   <c>110</c>
1958   <c>Response is Stale</c>
1959   <c>
1960      <xref target="warn.110"/>
1961   </c>
1962   <c>111</c>
1963   <c>Revalidation Failed</c>
1964   <c>
1965      <xref target="warn.111"/>
1966   </c>
1967   <c>112</c>
1968   <c>Disconnected Operation</c>
1969   <c>
1970      <xref target="warn.112"/>
1971   </c>
1972   <c>113</c>
1973   <c>Heuristic Expiration</c>
1974   <c>
1975      <xref target="warn.113"/>
1976   </c>
1977   <c>199</c>
1978   <c>Miscellaneous Warning</c>
1979   <c>
1980      <xref target="warn.199"/>
1981   </c>
1982   <c>214</c>
1983   <c>Transformation Applied</c>
1984   <c>
1985      <xref target="warn.214"/>
1986   </c>
1987   <c>299</c>
1988   <c>Miscellaneous Persistent Warning</c>
1989   <c>
1990      <xref target="warn.299"/>
1991   </c>
1992</texttable>
1993<!--(END)-->
1994
1995</section>
1996</section>
1997
1998<section title="Header Field Registration" anchor="header.field.registration">
1999<t>
2000   HTTP header fields are registered within the Message Header Field Registry
2001   maintained at
2002   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
2003</t>
2004<t>
2005   This document defines the following HTTP header fields, so their
2006   associated registry entries shall be updated according to the permanent
2007   registrations below (see <xref target="BCP90"/>):
2008</t>
2009
2010<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2011<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2012   <ttcol>Header Field Name</ttcol>
2013   <ttcol>Protocol</ttcol>
2014   <ttcol>Status</ttcol>
2015   <ttcol>Reference</ttcol>
2016
2017   <c>Age</c>
2018   <c>http</c>
2019   <c>standard</c>
2020   <c>
2021      <xref target="header.age"/>
2022   </c>
2023   <c>Cache-Control</c>
2024   <c>http</c>
2025   <c>standard</c>
2026   <c>
2027      <xref target="header.cache-control"/>
2028   </c>
2029   <c>Expires</c>
2030   <c>http</c>
2031   <c>standard</c>
2032   <c>
2033      <xref target="header.expires"/>
2034   </c>
2035   <c>Pragma</c>
2036   <c>http</c>
2037   <c>standard</c>
2038   <c>
2039      <xref target="header.pragma"/>
2040   </c>
2041   <c>Warning</c>
2042   <c>http</c>
2043   <c>standard</c>
2044   <c>
2045      <xref target="header.warning"/>
2046   </c>
2047</texttable>
2048<!--(END)-->
2049
2050<t>
2051   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2052   Force".
2053</t>
2054</section>
2055</section>
2056
2057<section anchor="security.considerations" title="Security Considerations">
2058<t>
2059   This section is meant to inform developers, information providers, and
2060   users of known security concerns specific to HTTP/1.1 caching.
2061   More general security considerations are addressed in HTTP messaging
2062   <xref target="Part1"/> and semantics <xref target="Part2"/>.
2063</t>
2064<t>
2065   Caches expose additional potential vulnerabilities, since the contents of
2066   the cache represent an attractive target for malicious exploitation.
2067   Because cache contents persist after an HTTP request is complete, an attack
2068   on the cache can reveal information long after a user believes that the
2069   information has been removed from the network. Therefore, cache contents
2070   need to be protected as sensitive information.
2071</t>
2072<t>
2073   Furthermore, the very use of a cache can bring about privacy concerns. For
2074   example, if two users share a cache, and the first one browses to a site,
2075   the second may be able to detect that the other has been to that site,
2076   because the resources from it load more quickly, thanks to the cache.
2077</t>
2078<t>
2079   Implementation flaws might allow attackers to insert content into a cache
2080   ("cache poisoning"), leading to compromise of clients that trust that
2081   content. Because of their nature, these attacks are difficult to mitigate.
2082</t>
2083<t>
2084   Likewise, implementation flaws (as well as misunderstanding of cache
2085   operation) might lead to caching of sensitive information (e.g.,
2086   authentication credentials) that is thought to be private, exposing it to
2087   unauthorized parties.
2088</t>
2089<t>
2090   Note that the Set-Cookie response header field <xref target="RFC6265"/> does not
2091   inhibit caching; a cacheable response with a Set-Cookie header field can be (and
2092   often is) used to satisfy subsequent requests to caches. Servers who wish
2093   to control caching of these responses are encouraged to emit appropriate
2094   Cache-Control response header fields.
2095</t>
2096
2097</section>
2098
2099<section title="Acknowledgments" anchor="acks">
2100<t>
2101  See Section 10 of <xref target="Part1"/>.
2102</t>
2103</section>
2104
2105</middle>
2106
2107<back>
2108<references title="Normative References">
2109
2110  <reference anchor="Part1">
2111    <front>
2112      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2113      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2114        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2115        <address><email>fielding@gbiv.com</email></address>
2116      </author>
2117      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2118        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2119        <address><email>julian.reschke@greenbytes.de</email></address>
2120      </author>
2121      <date month="November" year="2013"/>
2122    </front>
2123    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-25"/>
2124   
2125  </reference>
2126
2127  <reference anchor="Part2">
2128    <front>
2129      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2130      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2131        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2132        <address><email>fielding@gbiv.com</email></address>
2133      </author>
2134      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2135        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2136        <address><email>julian.reschke@greenbytes.de</email></address>
2137      </author>
2138      <date month="November" year="2013"/>
2139    </front>
2140    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-25"/>
2141   
2142  </reference>
2143
2144  <reference anchor="Part4">
2145    <front>
2146      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2147      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2148        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2149        <address><email>fielding@gbiv.com</email></address>
2150      </author>
2151      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2152        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2153        <address><email>julian.reschke@greenbytes.de</email></address>
2154      </author>
2155      <date month="November" year="2013"/>
2156    </front>
2157    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-25"/>
2158   
2159  </reference>
2160
2161  <reference anchor="Part5">
2162    <front>
2163      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2164      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2165        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2166        <address><email>fielding@gbiv.com</email></address>
2167      </author>
2168      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2169        <organization abbrev="W3C">World Wide Web Consortium</organization>
2170        <address><email>ylafon@w3.org</email></address>
2171      </author>
2172      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2173        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2174        <address><email>julian.reschke@greenbytes.de</email></address>
2175      </author>
2176      <date month="November" year="2013"/>
2177    </front>
2178    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-25"/>
2179   
2180  </reference>
2181
2182  <reference anchor="Part7">
2183    <front>
2184      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2185      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2186        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2187        <address><email>fielding@gbiv.com</email></address>
2188      </author>
2189      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2190        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2191        <address><email>julian.reschke@greenbytes.de</email></address>
2192      </author>
2193      <date month="November" year="2013"/>
2194    </front>
2195    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-25"/>
2196   
2197  </reference>
2198
2199  <reference anchor="RFC2119">
2200    <front>
2201      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2202      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2203        <organization>Harvard University</organization>
2204        <address><email>sob@harvard.edu</email></address>
2205      </author>
2206      <date month="March" year="1997"/>
2207    </front>
2208    <seriesInfo name="BCP" value="14"/>
2209    <seriesInfo name="RFC" value="2119"/>
2210  </reference>
2211
2212  <reference anchor="RFC5234">
2213    <front>
2214      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2215      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2216        <organization>Brandenburg InternetWorking</organization>
2217        <address>
2218          <email>dcrocker@bbiw.net</email>
2219        </address> 
2220      </author>
2221      <author initials="P." surname="Overell" fullname="Paul Overell">
2222        <organization>THUS plc.</organization>
2223        <address>
2224          <email>paul.overell@thus.net</email>
2225        </address>
2226      </author>
2227      <date month="January" year="2008"/>
2228    </front>
2229    <seriesInfo name="STD" value="68"/>
2230    <seriesInfo name="RFC" value="5234"/>
2231  </reference>
2232 
2233</references>
2234
2235<references title="Informative References">
2236
2237  <reference anchor="RFC2616">
2238    <front>
2239      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2240      <author fullname="R. Fielding" initials="R." surname="Fielding">
2241        <organization>University of California, Irvine</organization>
2242        <address><email>fielding@ics.uci.edu</email></address>
2243      </author>
2244      <author fullname="J. Gettys" initials="J." surname="Gettys">
2245        <organization>W3C</organization>
2246        <address><email>jg@w3.org</email></address>
2247      </author>
2248      <author fullname="J. Mogul" initials="J." surname="Mogul">
2249        <organization>Compaq Computer Corporation</organization>
2250        <address><email>mogul@wrl.dec.com</email></address>
2251      </author>
2252      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2253        <organization>MIT Laboratory for Computer Science</organization>
2254        <address><email>frystyk@w3.org</email></address>
2255      </author>
2256      <author fullname="L. Masinter" initials="L." surname="Masinter">
2257        <organization>Xerox Corporation</organization>
2258        <address><email>masinter@parc.xerox.com</email></address>
2259      </author>
2260      <author fullname="P. Leach" initials="P." surname="Leach">
2261        <organization>Microsoft Corporation</organization>
2262        <address><email>paulle@microsoft.com</email></address>
2263      </author>
2264      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2265        <organization>W3C</organization>
2266        <address><email>timbl@w3.org</email></address>
2267      </author>
2268      <date month="June" year="1999"/>
2269    </front>
2270    <seriesInfo name="RFC" value="2616"/>
2271  </reference>
2272
2273  <reference anchor="BCP90">
2274    <front>
2275      <title>Registration Procedures for Message Header Fields</title>
2276      <author fullname="G. Klyne" initials="G." surname="Klyne">
2277        <organization>Nine by Nine</organization>
2278        <address><email>GK-IETF@ninebynine.org</email></address>
2279      </author>
2280      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2281        <organization>BEA Systems</organization>
2282        <address><email>mnot@pobox.com</email></address>
2283      </author>
2284      <author fullname="J. Mogul" initials="J." surname="Mogul">
2285        <organization>HP Labs</organization>
2286        <address><email>JeffMogul@acm.org</email></address>
2287      </author>
2288      <date month="September" year="2004"/>
2289    </front>
2290    <seriesInfo name="BCP" value="90"/>
2291    <seriesInfo name="RFC" value="3864"/>
2292  </reference>
2293
2294  <reference anchor="RFC5226">
2295    <front>
2296      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2297      <author initials="T." surname="Narten" fullname="T. Narten">
2298        <organization>IBM</organization>
2299        <address><email>narten@us.ibm.com</email></address>
2300      </author>
2301      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2302        <organization>Google</organization>
2303        <address><email>Harald@Alvestrand.no</email></address>
2304      </author>
2305      <date year="2008" month="May"/>
2306    </front>
2307    <seriesInfo name="BCP" value="26"/>
2308    <seriesInfo name="RFC" value="5226"/>
2309  </reference>
2310
2311  <reference anchor="RFC5861">
2312    <front>
2313      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2314      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2315        <organization>Yahoo! Inc.</organization>
2316        <address><email>mnot@yahoo-inc.com</email></address>
2317      </author>
2318      <date month="April" year="2010"/>
2319    </front>
2320    <seriesInfo name="RFC" value="5861"/>
2321  </reference>
2322
2323  <reference anchor="RFC5905">
2324    <front>
2325      <title>Network Time Protocol Version 4: Protocol and Algorithms Specification</title>
2326      <author initials="D." surname="Mills" fullname="David L. Mills"/>
2327      <author initials="J." surname="Martin" fullname="Jim Martin" role="editor"/>
2328      <author initials="J." surname="Burbank" fullname="Jack Burbank"/>
2329      <author initials="W." surname="Kasch" fullname="William Kasch"/>
2330      <date year="2010" month="June"/>
2331    </front>
2332    <seriesInfo name="RFC" value="5905"/>
2333  </reference>
2334
2335  <reference anchor="RFC6265">
2336    <front>
2337      <title>HTTP State Management Mechanism</title>
2338      <author initials="A." surname="Barth" fullname="Adam Barth">
2339        <organization abbrev="U.C. Berkeley">
2340          University of California, Berkeley
2341        </organization>
2342        <address><email>abarth@eecs.berkeley.edu</email></address>
2343      </author>
2344      <date year="2011" month="April"/>
2345    </front>
2346    <seriesInfo name="RFC" value="6265"/>
2347  </reference>
2348
2349</references>
2350
2351<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2352<t>
2353  The specification has been substantially rewritten for clarity.
2354</t>
2355<t>
2356  The conditions under which an authenticated response can be cached have been
2357  clarified.
2358  (<xref target="caching.authenticated.responses"/>)
2359</t>
2360<t>
2361  New status codes can now define that caches are allowed to use heuristic
2362  freshness with them.
2363  Caches are now allowed to calculate heuristic freshness for URIs with query
2364  components.
2365  (<xref target="heuristic.freshness"/>)
2366</t>
2367<t>
2368  The algorithm for calculating age is now less conservative.
2369  Caches are now required to handle dates with timezones as if they're
2370  invalid, because it's not possible to accurately guess.
2371  (<xref target="age.calculations"/>)
2372</t>
2373<t>
2374  The Content-Location response header field is no longer used
2375  to determine the appropriate response to use when validating.
2376  (<xref target="validation.model"/>)
2377</t>
2378<t>
2379  The algorithm for selecting a cached negotiated response to use has been
2380  clarified in several ways. In particular, it now explicitly allows
2381  header-specific canonicalization when processing selecting header fields.
2382  (<xref target="caching.negotiated.responses"/>)
2383</t>
2384<t>
2385  Requirements regarding denial of service attack avoidance when performing
2386  invalidation have been clarified.
2387  (<xref target="invalidation"/>)
2388</t>
2389<t>
2390  Cache invalidation only occurs when a successful response is received.
2391  (<xref target="invalidation"/>)
2392</t>
2393<t>
2394  Cache directives are explicitly defined to be case-insensitive.
2395  Handling of multiple instances of cache directives when only one is
2396  expected is now defined.
2397  (<xref target="header.cache-control"/>)
2398</t>
2399<t>
2400  The "no-store" cache request directive doesn't apply to responses; i.e.,
2401  a cache can satisfy a request with no-store on it, and does not invalidate
2402  it.
2403  (<xref target="cache-request-directive.no-store"/>)
2404</t>
2405<t>
2406  The qualified forms of the private and no-cache cache directives are noted
2407  to not be widely implemented; e.g., "private=foo" is interpreted by many
2408  caches as simply "private". Additionally, the meaning of the qualified
2409  form of no-cache has been clarified.
2410  (<xref target="cache-response-directive"/>)
2411</t>
2412<t>
2413  The "no-cache" response cache directive's meaning has been clarified.
2414  (<xref target="cache-response-directive.no-cache"/>)
2415</t>
2416<t>
2417  The one-year limit on <xref target="header.expires" format="none">Expires</xref> header field values has been removed; instead,
2418  the reasoning for using a sensible value is given.
2419  (<xref target="header.expires"/>)
2420</t>
2421<t>
2422  The <xref target="header.pragma" format="none">Pragma</xref> header field is now only defined for backwards compatibility;
2423  future pragmas are deprecated.
2424  (<xref target="header.pragma"/>)
2425</t>
2426<t>
2427  Some requirements regarding production and processing of the <xref target="header.warning" format="none">Warning</xref>
2428  header fields have been relaxed, as it is not widely implemented.
2429  Furthermore, the <xref target="header.warning" format="none">Warning</xref> header field no longer uses RFC 2047 encoding,
2430  nor allows multiple languages, as these aspects were not implemented.
2431  (<xref target="header.warning"/>)
2432</t>
2433<t>
2434  This specification introduces the Cache Directive and Warn Code Registries,
2435  and defines considerations for new cache directives.
2436  (<xref target="cache.directive.registry"/> and <xref target="warn.code.registry"/>)
2437</t>
2438</section>
2439
2440<section title="Imported ABNF" anchor="imported.abnf">
2441   
2442   
2443   
2444   
2445   
2446   
2447   
2448   
2449   
2450   
2451   
2452   
2453   
2454   
2455   
2456   
2457<t>
2458   The following core rules are included by reference, as defined in Appendix B.1 of <xref target="RFC5234"/>: ALPHA (letters), CR (carriage
2459   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2460   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2461   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2462</t>
2463<t>
2464   The rules below are defined in <xref target="Part1"/>:
2465</t>
2466<figure><artwork type="abnf2616"><![CDATA[
2467  OWS           = <OWS, defined in [Part1], Section 3.2.3>
2468  field-name    = <field-name, defined in [Part1], Section 3.2>
2469  quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2470  token         = <token, defined in [Part1], Section 3.2.6>
2471
2472  port          = <port, defined in [Part1], Section 2.7>
2473  pseudonym     = <pseudonym, defined in [Part1], Section 5.7.1>
2474  uri-host      = <uri-host, defined in [Part1], Section 2.7>
2475]]></artwork></figure>
2476<t>
2477   The rules below are defined in other parts:
2478</t>
2479<figure><artwork type="abnf2616"><![CDATA[
2480  HTTP-date     = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2481]]></artwork></figure>
2482</section>
2483
2484
2485<section title="Collected ABNF" anchor="collected.abnf">
2486<t>
2487  In the collected ABNF below, list rules are expanded as per Section 1.2 of <xref target="Part1"/>.
2488</t><figure>
2489<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2490Age = delta-seconds
2491
2492Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2493 cache-directive ] )
2494
2495Expires = HTTP-date
2496
2497HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
2498
2499OWS = <OWS, defined in [Part1], Section 3.2.3>
2500
2501Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2502 pragma-directive ] )
2503
2504Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2505 )
2506
2507cache-directive = token [ "=" ( token / quoted-string ) ]
2508
2509delta-seconds = 1*DIGIT
2510
2511extension-pragma = token [ "=" ( token / quoted-string ) ]
2512
2513field-name = <field-name, defined in [Part1], Section 3.2>
2514
2515port = <port, defined in [Part1], Section 2.7>
2516pragma-directive = "no-cache" / extension-pragma
2517pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
2518
2519quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
2520
2521token = <token, defined in [Part1], Section 3.2.6>
2522
2523uri-host = <uri-host, defined in [Part1], Section 2.7>
2524
2525warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2526warn-code = 3DIGIT
2527warn-date = DQUOTE HTTP-date DQUOTE
2528warn-text = quoted-string
2529warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2530 ]
2531]]></artwork>
2532</figure>
2533</section>
2534
2535
2536<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2537<t>
2538  Changes up to the IETF Last Call draft are summarized
2539  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-24#appendix-D"/>.
2540</t>
2541
2542<section title="Since draft-ietf-httpbis-p6-cache-24" anchor="changes.since.24">
2543<t>
2544  Closed issues:
2545  <list style="symbols">
2546    <t>
2547      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/499"/>:
2548      "RFC 1305 ref needs to be updated to 5905"
2549    </t>
2550    <t>
2551      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/500"/>:
2552      "dangling reference to cacheable status codes"
2553    </t>
2554    <t>
2555      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/512"/>:
2556      "APPSDIR review of draft-ietf-httpbis-p6-cache-24"
2557    </t>
2558  </list>
2559</t>
2560</section>
2561</section>
2562  </back>
2563</rfc>
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