source: draft-ietf-httpbis/09/p6-cache.xml @ 802

Last change on this file since 802 was 772, checked in by julian.reschke@…, 10 years ago

Prepare publication of -09 drafts on March 08

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[29]1<?xml version="1.0" encoding="utf-8"?>
[768]2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
[8]3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
[772]14  <!ENTITY ID-VERSION "09">
[764]15  <!ENTITY ID-MONTH "March">
[741]16  <!ENTITY ID-YEAR "2010">
[424]17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x=''/>">
[205]18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
[538]19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x=''/>">
[31]20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x=''/>">
[183]21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x=''/>">
[538]22  <!ENTITY partial                     "<xref target='Part5' xmlns:x=''/>">
[31]23  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x=''/>">
[580]24  <!ENTITY full-date                   "<xref target='Part1' x:rel='' xmlns:x=''/>">
[31]25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='' xmlns:x=''/>">
[205]28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x=''/>">
[538]29  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
[697]30  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x=''/>">
[31]31  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x=''/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
[607]33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x=''/>">
35<?rfc toc="yes" ?>
[29]36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
[8]38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
[203]42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
[8]44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
[538]46<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
47  obsoletes="2616" x:maturity-level="draft" xmlns:x="">
[547]50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
[547]52  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
[8]55      <postal>
[29]56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
[8]58        <region>CA</region>
[29]59        <code>92660</code>
60        <country>USA</country>
[8]61      </postal>
[29]62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email></email>
65      <uri></uri>
[8]66    </address>
[547]67  </author>
[547]69  <author fullname="Jim Gettys" initials="J." surname="Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
[8]72      <postal>
[29]73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
[8]75        <region>MA</region>
[29]76        <code>01741</code>
77        <country>USA</country>
[8]78      </postal>
[29]79      <email></email>
80      <uri></uri>
[8]81    </address>
[547]82  </author>
[547]84  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
[8]87      <postal>
[29]88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
[8]90        <city>Palo Alto</city>
91        <region>CA</region>
[29]92        <code>94304</code>
93        <country>USA</country>
[8]94      </postal>
[29]95      <email></email>
[8]96    </address>
[547]97  </author>
[547]99  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
[8]102      <postal>
[29]103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
[8]108      </postal>
[29]109      <email></email>
[8]110    </address>
[547]111  </author>
[547]113  <author fullname="Larry Masinter" initials="L." surname="Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
[8]116      <postal>
[29]117        <street>345 Park Ave</street>
118        <city>San Jose</city>
[8]119        <region>CA</region>
[29]120        <code>95110</code>
121        <country>USA</country>
[8]122      </postal>
[29]123      <email></email>
124      <uri></uri>
[8]125    </address>
[547]126  </author>
[547]128  <author fullname="Paul J. Leach" initials="P." surname="Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
[8]131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email></email>
138    </address>
[547]139  </author>
[547]141  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
[8]144      <postal>
[34]145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
[8]148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
[29]151        <country>USA</country>
[8]152      </postal>
153      <email></email>
[34]154      <uri></uri>
[8]155    </address>
[547]156  </author>
[547]158  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
[93]161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email></email>
170      <uri></uri>
171    </address>
[547]172  </author>
[601]174  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
[607]175    <address>
176      <email></email>
177      <uri></uri>
[601]178    </address>
179  </author>
[547]181  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
182    <organization abbrev="greenbytes">greenbytes GmbH</organization>
183    <address>
[95]184      <postal>
185        <street>Hafenweg 16</street>
186        <city>Muenster</city><region>NW</region><code>48155</code>
187        <country>Germany</country>
188      </postal>
[607]189      <phone>+49 251 2807760</phone>
190      <facsimile>+49 251 2807761</facsimile>
191      <email></email>
192      <uri></uri>
[95]193    </address>
[547]194  </author>
[772]196  <date month="&ID-MONTH;" year="&ID-YEAR;" day="8"/>
[547]197  <workgroup>HTTPbis Working Group</workgroup>
201  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
202  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
203  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
204  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
205  fields that control cache behavior or indicate cacheable response messages.
[540]209<note title="Editorial Note (To be removed by RFC Editor)">
211  Discussion of this draft should take place on the HTTPBIS working group mailing list
212  ( The current issues list is at <eref
213  target="" /> and related documents
214  (including fancy diffs) can be found at <eref target="" />.
[720]217  The changes in this draft are summarized in <xref target="changes.since.08" />.
[538]221  </front>
222  <middle>
[540]224<section anchor="caching" title="Introduction">
226  HTTP is typically used for distributed information systems, where performance can be
227  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
228  caching and reusing response messages.
[540]231<section anchor="intro.purpose" title="Purpose">
232<iref item="cache" />
234  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
235  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
236  in order to reduce the response time and network bandwidth consumption on future,
237  equivalent requests. Any client or server may include a cache, though a cache cannot be
238  used by a server that is acting as a tunnel.
241  Caching would be useless if it did not significantly improve performance. The goal of
242  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
243  some cases, a stored response can be reused without the need for a network request,
244  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
245  (see <xref target="expiration.model" />). Even when a new request is required, it is often
246  possible to reuse all or parts of the payload of a prior response to satisfy the request,
247  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
248  purpose (see <xref target="validation.model" />).
[540]252<section anchor="intro.terminology" title="Terminology">
254  This specification uses a number of terms to refer to the roles played by participants
[540]255  in, and objects of, HTTP caching.
258  <iref item="cacheable" />
259  <x:dfn>cacheable</x:dfn>
260  <list>
261    <t>A response is cacheable if a cache is allowed to store a copy of the response message
262      for use in answering subsequent requests. Even when a response is cacheable, there may
263      be additional constraints on whether a cache can use the cached copy to satisfy a
264      particular request.</t>
265  </list>
268  <iref item="explicit expiration time" />
269  <x:dfn>explicit expiration time</x:dfn>
270  <list>
271    <t>The time at which the origin server intends that an entity should no longer be
272      returned by a cache without further validation.</t>
273  </list>
276  <iref item="heuristic expiration time" />
277  <x:dfn>heuristic expiration time</x:dfn>
278  <list>
279    <t>An expiration time assigned by a cache when no explicit expiration time is
280    available.</t>
281  </list>
284  <iref item="age" />
285  <x:dfn>age</x:dfn>
286  <list>
287    <t>The age of a response is the time since it was sent by, or successfully validated
288      with, the origin server.</t>
289  </list>
292  <iref item="first-hand" />
293  <x:dfn>first-hand</x:dfn>
294  <list>
295    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
296    0.</t>
297  </list>
300  <iref item="freshness lifetime" />
301  <x:dfn>freshness lifetime</x:dfn>
302  <list>
303    <t>The length of time between the generation of a response and its expiration time. </t>
304  </list>
307  <iref item="fresh" />
308  <x:dfn>fresh</x:dfn>
309  <list>
310    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
311  </list>
314  <iref item="stale" />
315  <x:dfn>stale</x:dfn>
316  <list>
317    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
318  </list>
321  <iref item="validator" />
322  <x:dfn>validator</x:dfn>
323  <list>
324    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
325      out whether a stored response is an equivalent copy of an entity.</t>
326  </list>
328<t anchor="shared.and.non-shared.caches">
329  <iref item="validator" />
330  <x:dfn>shared cache</x:dfn>
331  <list>
332    <t>A cache that is accessible to more than one user. A non-shared cache is
333      dedicated to a single user.</t>
334  </list>
[540]338<section anchor="intro.requirements" title="Requirements">
340  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
341  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
342  described in <xref target="RFC2119" />.
345  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
346  or &REQUIRED; level requirements for the protocols it implements. An implementation
347  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
348  requirements for its protocols is said to be "unconditionally compliant"; one that
349  satisfies all the &MUST; level requirements but not all the &SHOULD; level
350  requirements for its protocols is said to be "conditionally compliant."
[424]354<section title="Syntax Notation" anchor="notation">
[425]355  <x:anchor-alias value="ALPHA"/>
356  <x:anchor-alias value="CR"/>
357  <x:anchor-alias value="DIGIT"/>
358  <x:anchor-alias value="DQUOTE"/>
359  <x:anchor-alias value="LF"/>
360  <x:anchor-alias value="OCTET"/>
361  <x:anchor-alias value="SP"/>
362  <x:anchor-alias value="VCHAR"/>
363  <x:anchor-alias value="WSP"/>
[543]365  This specification uses the ABNF syntax defined in &notation; (which
366  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
367  <xref target="collected.abnf"/> shows the collected ABNF, with the list
368  rule expanded.
[425]371  The following core rules are included by
372  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
373  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
374  DIGIT (decimal 0-9), DQUOTE (double quote),
375  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
376  OCTET (any 8-bit sequence of data), SP (space),
377  VCHAR (any visible USASCII character),
378  and WSP (whitespace).
[424]381<section title="Core Rules" anchor="core.rules">
[229]382  <x:anchor-alias value="quoted-string"/>
383  <x:anchor-alias value="token"/>
[364]384  <x:anchor-alias value="OWS"/>
[424]386  The core rules below are defined in &basic-rules;:
388<figure><artwork type="abnf2616">
[229]389  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
390  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
[364]391  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
395<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
[229]396  <x:anchor-alias value="field-name"/>
397  <x:anchor-alias value="HTTP-date"/>
398  <x:anchor-alias value="port"/>
399  <x:anchor-alias value="pseudonym"/>
400  <x:anchor-alias value="uri-host"/>
[205]402  The ABNF rules below are defined in other parts:
[207]404<figure><!--Part1--><artwork type="abnf2616">
[647]405  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
[229]406  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
[374]407  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
[229]408  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
[374]409  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
[540]416<section anchor="caching.overview" title="Cache Operation">
[540]418<section anchor="response.cacheability" title="Response Cacheability">
420  A cache &MUST-NOT; store a response to any request, unless:
421  <list style="symbols">
[721]422    <t>The request method is understood by the cache and defined as being cacheable, and</t>
[737]423    <t>the response status code is understood by the cache, and</t>
[540]424    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
425       appear in request or response headers, and</t>
[737]426    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
[540]427       does not appear in the response, if the cache is shared, and</t>
428    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
429       the cache is shared (unless the "public" directive is present; see <xref
[737]430       target="header.cache-control" />), and</t>
431    <t>the response either:
432      <list style="symbols">
433        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
434        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
435        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
436        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
437        <t>has a status code that can be served with heuristic freshness (see <xref
438           target="heuristic.freshness" />).</t>
439      </list>
440    </t>
[540]441  </list>
[737]444  In this context, a cache has "understood" a request method or a response status
445  code if it recognises it and implements any cache-specific behaviour. In
446  particular, 206 Partial Content responses cannot be cached by an
447  implementation that does not handle partial content
448  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
[540]451  Note that in normal operation, most caches will not store a response that has neither a
452  cache validator nor an explicit expiration time, as such responses are not usually
453  useful to store. However, caches are not prohibited from storing such responses.
[540]456<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
458  A cache that receives an incomplete response (for example, with fewer bytes of data
459  than specified in a Content-Length header) can store the response, but &MUST;
460  treat it as a partial response &partial;. Partial responses
461  can be combined as described in &combining-byte-ranges;; the result might be a
462  full response or might still be partial. A cache &MUST-NOT; return a partial
463  response to a client without explicitly marking it as such using the 206 (Partial
464  Content) status code.
467  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
468  incomplete or partial responses.
[540]475<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
[575]477  For a presented request, a cache &MUST-NOT; return a stored response, unless:
[540]478  <list style="symbols">
[561]479    <t>The presented Request-URI and that of the stored response match
480      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
481      1 doesn't define Request-URI anymore; the new term request-target does not
[694]482      work for this. (see <eref target="" />)</cref>), and</t>
[540]483    <t>the request method associated with the stored response allows it to be
484      used for the presented request, and</t>
485    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
486      target="caching.negotiated.responses" />), and</t>
487    <t>the presented request and stored response are free from directives that would prevent
488      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
489      and</t>
490    <t>the stored response is either:
491      <list style="symbols">
492        <t>fresh (see <xref target="expiration.model" />), or</t>
493        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
494        <t>successfully validated (see <xref target="validation.model" />).</t>
495      </list>
496    </t>
497  </list>
[562]500  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
503  When a stored response is used to satisfy a request, caches &MUST; include a
[594]504  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
[540]505  current_age; see <xref target="age.calculations" />.
[767]506  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
509  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
[732]510  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
[540]511  corresponding response.
514  Also, note that unsafe requests might invalidate already stored responses; see
515  <xref target="invalidation.after.updates.or.deletions" />.
518  Caches &MUST; use the most recent response (as determined by the Date header) when
519  more than one suitable response is stored. They can also forward a request with
520  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
521  use.
[562]524  <cref anchor="TODO-header-properties">end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
[540]528<section anchor="expiration.model" title="Freshness Model">
530  When a response is "fresh" in the cache, it can be used to satisfy subsequent
531  requests without contacting the origin server, thereby improving efficiency.
534  The primary mechanism for determining freshness is for an origin server to provide an
535  explicit expiration time in the future, using either the Expires header (<xref
536  target="header.expires" />) or the max-age response cache directive (<xref
537  target="cache-response-directive" />). Generally, origin servers will assign future
538  explicit expiration times to responses in the belief that the entity is not likely to
539  change in a semantically significant way before the expiration time is reached.
542  If an origin server wishes to force a cache to validate every request, it can
543  assign an explicit expiration time in the past. This means that the response is always
544  stale, so that caches should validate it before using it for subsequent requests.
[767]545  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
548  Since origin servers do not always provide explicit expiration times, HTTP caches may
549  also assign heuristic expiration times when they are not specified, employing algorithms that
550  use other header values (such as the Last-Modified time) to estimate a plausible
551  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
552  impose worst-case constraints on their results.
[575]556  The calculation to determine if a response is fresh is:
558<artwork type="code">
[538]559   response_is_fresh = (freshness_lifetime &gt; current_age)
564  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
565  the current_age is defined in <xref target="age.calculations" />.
568  Additionally, clients may need to influence freshness calculation. They can do this using
569  several request cache directives, with the effect of either increasing or loosening
570  constraints on freshness. See <xref target="cache-request-directive" />.
[767]573  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
[540]574  freshness.</cref>
577  Note that freshness applies only to cache operation; it cannot be used to force a user agent
578  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
579  the difference between caches and history mechanisms.
[540]582<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
584  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
585  response by using the first match of:
586  <list style="symbols">
587    <t>If the cache is shared and the s-maxage response cache directive (<xref
588      target="cache-response-directive" />) is present, use its value, or</t>
589    <t>If the max-age response cache directive (<xref target="cache-response-directive"
590      />) is present, use its value, or</t>
591    <t>If the Expires response header (<xref target="header.expires" />) is present, use
592      its value minus the value of the Date response header, or</t>
593    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
594      may be used; see <xref target="heuristic.freshness" />.</t>
595  </list>
598  Note that this calculation is not vulnerable to clock skew, since all of the
599  information comes from the origin server.
[540]602<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
604  If no explicit expiration time is present in a stored response that has a status code
605  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
606  calculated. Heuristics &MUST-NOT; be used for other response status codes.
609  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
610  attach a Warning header with a 113 warn-code to the response if its current_age is
611  more than 24 hours and such a warning is not already present.
614  Also, if the response has a Last-Modified header (&header-last-modified;), the
615  heuristic expiration value &SHOULD; be no more than some fraction of the interval
616  since that time. A typical setting of this fraction might be 10%.
[767]619  <cref anchor="REVIEW-query-string-heuristics">took away HTTP/1.0 query string heuristic uncacheability.</cref>
[540]624<section anchor="age.calculations" title="Calculating Age">
626  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
627  message when obtained from a cache. The Age field value is the cache's estimate of the
628  amount of time since the response was generated or validated by the origin server. In
629  essence, the Age value is the sum of the time that the response has been resident in
630  each of the caches along the path from the origin server, plus the amount of time it has
631  been in transit along network paths.
634  The term "age_value" denotes the value of the Age header, in a form appropriate for
635  arithmetic operations.
638  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
639  response, giving the time at which the response was generated (see &header-date;).
640  The term "date_value" denotes the value of the Date header, in a form appropriate for
641  arithmetic operations.
644  The term "now" means "the current value of the clock at the host performing the
645  calculation." Hosts that use HTTP, but especially hosts running origin servers and
646  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
647  synchronize their clocks to a globally accurate time standard.
650  A response's age can be calculated in two entirely independent ways:
[540]651  <list style="numbers">
652    <t>now minus date_value, if the local clock is reasonably well synchronized to the
653      origin server's clock. If the result is negative, the result is replaced by zero.</t>
654    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
655  </list>
658<preamble>These are combined as</preamble>
659<artwork type="code">
[8]660    corrected_received_age = max(now - date_value, age_value)
663  When an Age value is received, it &MUST; be interpreted relative to the time the
664  request was initiated, not the time that the response was received.
666<figure><artwork type="code">
[8]667   corrected_initial_age = corrected_received_age
668                         + (now - request_time)
671  where "request_time" is the time (according to the local clock) when the request that
672  elicited this response was sent.
675  The current_age of a stored response can then be calculated by adding the amount of
676  time (in seconds) since the stored response was last validated by the origin server to
677  the corrected_initial_age.
680  In summary:
682<figure><artwork type="code">
683  age_value     - Age header field-value received with the response
684  date_value    - Date header field-value received with the response
685  request_time  - local time when the cache made the request
686                 resulting in the stored response
687  response_time - local time when the cache received the response
688  now           - current local time
690  apparent_age = max(0, response_time - date_value);
691  corrected_received_age = max(apparent_age, age_value);
692  response_delay = response_time - request_time;
693  corrected_initial_age = corrected_received_age + response_delay;
694  resident_time = now - response_time;
695  current_age   = corrected_initial_age + resident_time;
[540]699<section anchor="serving.stale.responses" title="Serving Stale Responses">
701  A "stale" response is one that either has explicit expiry information, or is allowed to
702  have heuristic expiry calculated, but is not fresh according to the calculations in
703  <xref target="expiration.model" />.
706  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
707  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
708  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
709  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
712  Caches &SHOULD-NOT; return stale responses unless they are
713  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
714  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
717  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
718  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
719  the cache is disconnected.
722  If a cache receives a first-hand response (either an entire response, or a 304 (Not
723  Modified) response) that it would normally forward to the requesting client, and the
724  received response is no longer fresh, the cache &SHOULD; forward it to the
725  requesting client without adding a new Warning (but without removing any existing
726  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
727  that response became stale in transit.
[540]732<section anchor="validation.model" title="Validation Model">
[607]734  When a cache has one or more stored responses for a requested URI, but cannot 
735  serve any of them (e.g., because they are not fresh, or one cannot be selected;
736  see <xref target="caching.negotiated.responses"/>),
737  it can use the conditional request mechanism &conditional; in the forwarded
738  request to give the origin server an opportunity to both select a valid stored
739  response to be used, and to update it. This process is known as "validating"
740  or "revalidating" the stored response.
[607]743  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
744  header whose value is that of the Last-Modified header from the selected
745  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
[607]748  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
749  is that of the ETag header(s) from all responses stored for the requested URI,
750  if present. However, if any of the stored responses contains only partial
751  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
752  field unless the request is for a range that would be fully satisfied by
753  that stored response.
[540]756  A 304 (Not Modified) response status code indicates that the stored
757  response can be updated and reused; see <xref target="combining.headers"/>.
[607]760  A full response (i.e., one with a response body) indicates that none 
761  of the stored responses nominated in the conditional request is
762  suitable. Instead, the full response is used both to satisfy the
[767]763  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
766  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
767  either forward this response to the requesting client, or act as if the server failed to
[607]768  respond. In the latter case, it &MAY; return a previously stored response (see <xref
[540]769  target="serving.stale.responses" />).
[540]773<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
775  Because unsafe methods (&safe-methods;) have the potential for changing state on the
776  origin server, intervening caches can use them to keep their contents
777  up-to-date.
780  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
[589]781  as the URI(s) in the Location and Content-Location headers (if present):
[540]782  <list style="symbols">
783    <t>PUT</t>
784    <t>DELETE</t>
785    <t>POST</t>
786  </list>
[589]789  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
[540]790  be performed if the host part of that URI differs from the host part in the Request-URI.
791  This helps prevent denial of service attacks.
[767]794  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
797  A cache that passes through requests for methods it does not understand &SHOULD;
798  invalidate the Request-URI.
801  Here, "invalidate" means that the cache will either remove all stored responses related
802  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
803  before they can be returned in response to a subsequent request.
806  Note that this does not guarantee that all appropriate responses are invalidated. For
807  example, the request that caused the change at the origin server might not have gone
808  through the cache where a response is stored.
[767]811  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
[540]815<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
817  When a cache receives a request that can be satisfied by a stored response
[607]818  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
819  response unless all of the selecting request-headers nominated by the Vary header match
820  in both the original request (i.e., that associated with the stored response),
821  and the presented request.
[771]824  The selecting request-headers from two requests are defined to match
825  if and only if those in the first request can be transformed to those in the
826  second request by applying any of the following:
827  <list style="symbols">
828    <t>
829      adding or removing whitespace, where allowed in the header's syntax
830    </t>
831    <t>
832      combining multiple message-header fields with the same field name (see
833      &header-fields;)
834    </t>
835    <t>
836      normalizing both header values in a way that is known to have identical
837      semantics, according to the header's specification (e.g., re-ordering field values
838      when order is not significant; case-normalization, where values are defined to be
839      case-insensitive)   
840    </t>
841  </list>
[771]844  If (after any normalisation that may take place) a header field is absent
845  from a request, it can only match another request if it is also absent there.
[540]848  A Vary header field-value of "*" always fails to match, and subsequent requests to that
849  resource can only be properly interpreted by the origin server.
[607]852  The stored response with matching selecting request-headers is known as the
853  selected response.
[607]856  If no selected response is available, the cache &MAY; forward the presented
857  request to the origin server in a conditional request; see <xref target="validation.model"/>.
[540]861<section anchor="combining.headers" title="Combining Responses">
[607]863  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
864  (in this section, the "new" response"), it needs to created an updated response by combining
865  the stored response with the new one, so that the updated response can be used to satisfy the request.
[607]868  If the new response contains an ETag, it identifies the stored 
[767]869  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
870  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
[607]873  If the status code is 206 (partial content), both the stored and new 
[615]874  responses &MUST; have validators, and those validators &MUST; match using the strong 
[607]875  comparison function (see &weak-and-strong;). Otherwise, the 
876  responses &MUST-NOT; be combined.
879  The stored response headers are used as those of the updated response, except that
[540]880  <list style="symbols">
881    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
[607]882      &MUST; be deleted from the stored response and the updated response.</t>
[540]883    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
[607]884      response and the updated response.</t>
885    <t>any headers provided in the new response &MUST; replace the corresponding
[540]886      headers from the stored response.</t>
887  </list>
[607]890  If a header field-name in the new response matches more than one 
891  header in the stored response, all such stored headers &MUST; be replaced.
[767]894  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
[607]895  stored response in cache. In the case of a 206 response, the combined 
896  entity-body &MAY; be stored.
[767]899  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
[540]905<section anchor="header.fields" title="Header Field Definitions">
907  This section defines the syntax and semantics of HTTP/1.1 header fields
908  related to caching.
911  For entity-header fields, both sender and recipient refer to either the client or the
912  server, depending on who sends and who receives the entity.
[540]915<section anchor="header.age" title="Age">
916  <iref item="Age header" primary="true" x:for-anchor="" />
917  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
918  <x:anchor-alias value="Age"/>
919  <x:anchor-alias value="Age-v"/>
920  <x:anchor-alias value="age-value"/>
[698]922  The "Age" response-header field conveys the sender's estimate of the amount
923  of time since the response was generated or successfully validated at the
924  origin server. Age values are calculated as specified in
925  <xref target="age.calculations" />.
[364]927<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
[366]928  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
[364]929  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
[540]931<t anchor="">
932  <x:anchor-alias value="delta-seconds" />
[576]933  Age field-values are non-negative integers, representing time in seconds.
935<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
[229]936  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
939  If a cache receives a value larger than the largest positive integer it can represent, or
940  if any of its age calculations overflows, it &MUST; transmit an Age header with a
941  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
942  of at least 31 bits of range.
945  The presence of an Age header field in a response implies that a response is not
946  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
947  Age header field.
[540]951<section anchor="header.cache-control" title="Cache-Control">
952  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
953  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
954  <x:anchor-alias value="Cache-Control"/>
955  <x:anchor-alias value="Cache-Control-v"/>
956  <x:anchor-alias value="cache-directive"/>
957  <x:anchor-alias value="cache-extension"/>
958  <x:anchor-alias value="cache-request-directive"/>
959  <x:anchor-alias value="cache-response-directive"/>
[698]961  The "Cache-Control" general-header field is used to specify directives that
962  &MUST; be obeyed by all caches along the request/response chain. Such cache
963  directives are unidirectional in that the presence of a directive in a
964  request does not imply that the same directive is to be given in the
965  response.
[575]968  <t>
969    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
970    Pragma: no-cache (see <xref target="header.pragma" />).
971  </t>
974  Cache directives &MUST; be passed through by a proxy or gateway application,
975  regardless of their significance to that application, since the directives might be
976  applicable to all recipients along the request/response chain. It is not possible to
977  target a directive to a specific cache.
[538]979<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
[366]980  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
[364]981  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
[229]983  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
[334]984     / <x:ref>cache-response-directive</x:ref>
[334]986  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[540]989<section anchor="cache-request-directive" title="Request Cache-Control Directives">
990  <x:anchor-alias value="cache-request-directive" />
[540]992<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
[538]993  <x:ref>cache-request-directive</x:ref> =
994       "no-cache"
995     / "no-store"
996     / "max-age" "=" <x:ref>delta-seconds</x:ref>
997     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
998     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
999     / "no-transform"
1000     / "only-if-cached"
1001     / <x:ref>cache-extension</x:ref>
1005  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1006  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1007  no-cache
1008  <list>
1009    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1010      used to satisfy the request without successful validation on the origin server.</t>
1011  </list>
1014  <iref item="Cache Directives" primary="true" subitem="no-store" />
1015  <iref item="no-store" primary="true" subitem="Cache Directive" />
1016  no-store
1017  <list>
1018    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1019      of either this request or any response to it. This directive applies to both
1020      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1021      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1022      and &MUST; make a best-effort attempt to remove the information from volatile
1023      storage as promptly as possible after forwarding it.</t>
1024    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1025      particular, malicious or compromised caches might not recognize or obey this
1026      directive, and communications networks may be vulnerable to eavesdropping.</t>
1027  </list>
1030  <iref item="Cache Directives" primary="true" subitem="max-age" />
1031  <iref item="max-age" primary="true" subitem="Cache Directive" />
1032  max-age
1033  <list>
1034    <t>The max-age request directive indicates that the client is willing to accept a
1035      response whose age is no greater than the specified time in seconds. Unless
1036      max-stale directive is also included, the client is not willing to accept a stale
1037      response.</t>
1038  </list>
1041  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1042  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1043  max-stale
1044  <list>
1045    <t>The max-stale request directive indicates that the client is willing to accept a
1046      response that has exceeded its expiration time. If max-stale is assigned a value,
1047      then the client is willing to accept a response that has exceeded its expiration
1048      time by no more than the specified number of seconds. If no value is assigned to
[767]1049      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
[540]1050  </list>
1053  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1054  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1055  min-fresh
1056  <list>
1057    <t>The min-fresh request directive indicates that the client is willing to accept a
1058      response whose freshness lifetime is no less than its current age plus the specified
1059      time in seconds. That is, the client wants a response that will still be fresh for
1060      at least the specified number of seconds.</t>
1061  </list>
1064  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1065  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1066  no-transform
1067  <list>
1068    <t>The no-transform request directive indicates that an intermediate cache or proxy
1069      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1070      headers, nor the request entity-body.</t>
1071  </list>
1074  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1075  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1076  only-if-cached
1077  <list>
1078    <t>The only-if-cached request directive indicates that the client only wishes to
1079      return a stored response. If it receives this directive, a cache &SHOULD; either
1080      respond using a stored response that is consistent with the other constraints of the
1081      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1082      being operated as a unified system with good internal connectivity, such a request
1083      &MAY; be forwarded within that group of caches.</t>
1084  </list>
[540]1088<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1089  <x:anchor-alias value="cache-response-directive" />
[540]1091<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
[538]1092  <x:ref>cache-response-directive</x:ref> =
1093       "public"
1094     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1095     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1096     / "no-store"
1097     / "no-transform"
1098     / "must-revalidate"
1099     / "proxy-revalidate"
1100     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1101     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1102     / <x:ref>cache-extension</x:ref>
1106  <iref item="Cache Directives" primary="true" subitem="public" />
1107  <iref item="public" primary="true" subitem="Cache Directive" />
1108  public
1109  <list>
1110    <t>The public response directive indicates that the response &MAY; be cached, even
1111      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1112      (See also Authorization, &header-authorization;, for additional details.) </t>
1113  </list>
1116  <iref item="Cache Directives" primary="true" subitem="private" />
1117  <iref item="private" primary="true" subitem="Cache Directive" />
1118  private
1119  <list>
1120    <t>The private response directive indicates that the response message is intended for
1121      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1122      cache &MAY; store the response.</t>
1123    <t>If the private response directive specifies one or more field-names, this
1124      requirement is limited to the field-values associated with the listed response
1125      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1126      cache, whereas the remainder of the response message &MAY; be.</t>
1127    <t>
1128      <x:h>Note:</x:h> This usage of the word private only controls where the response may
[710]1129      be stored, and cannot ensure the privacy of the message content.
1130      Also, private response directives with field-names are often handled by
[732]1131      implementations as if an unqualified private directive was received; i.e.,
[710]1132      the special handling for the qualified form is not widely implemented.
[540]1134  </list>
1137  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1138  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1139  no-cache
1140  <list>
1141    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1142      satisfy a subsequent request without successful validation on the origin server.
1143      This allows an origin server to prevent caching even by caches that have been
1144      configured to return stale responses.</t>
1145    <t>If the no-cache response directive specifies one or more field-names, this
[555]1146      requirement is limited to the field-values associated with the listed response
[540]1147      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1148      to a subsequent request without successful validation on the origin server. This
1149      allows an origin server to prevent the re-use of certain header fields in a
1150      response, while still allowing caching of the rest of the response.</t>
1151    <t>
1152      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
[710]1153      Also, no-cache response directives with field-names are often handled by
[736]1154      implementations as if an unqualified no-cache directive was received; i.e.,
[710]1155      the special handling for the qualified form is not widely implemented.
[540]1156    </t>
1157  </list>
1161  <iref item="Cache Directives" primary="true" subitem="no-store" />
1162  <iref item="no-store" primary="true" subitem="Cache Directive" />
1163  no-store
1164  <list>
1165    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1166      part of either the immediate request or response. This directive applies to both
1167      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1168      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1169      and &MUST; make a best-effort attempt to remove the information from volatile
1170      storage as promptly as possible after forwarding it.</t>
1171    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1172      particular, malicious or compromised caches might not recognize or obey this
1173      directive, and communications networks may be vulnerable to eavesdropping.</t>
1174  </list>
1177  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1178  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1179  must-revalidate
1180  <list>
1181    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1182     used to satisfy subsequent requests without successful validation on the origin server.</t>
1183    <t>The must-revalidate directive is necessary to support reliable operation for
1184      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1185      the must-revalidate directive; in particular, if the cache cannot reach the origin
1186      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1187    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1188      validate a request on the entity could result in incorrect operation, such as a
1189      silently unexecuted financial transaction.</t>
1190  </list>
1193  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1194  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1195  proxy-revalidate
1196  <list>
1197    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1198      response directive, except that it does not apply to non-shared caches.</t>
1199  </list>
1202  <iref item="Cache Directives" primary="true" subitem="max-age" />
1203  <iref item="max-age" primary="true" subitem="Cache Directive" />
1204  max-age
1205  <list>
1206    <t>The max-age response directive indicates that response is to be considered stale
1207      after its age is greater than the specified number of seconds.</t>
1208  </list>
1211  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1212  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1213  s-maxage
1214  <list>
1215    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1216      specified by this directive overrides the maximum age specified by either the
1217      max-age directive or the Expires header. The s-maxage directive also implies the
1218      semantics of the proxy-revalidate response directive.</t>
1219  </list>
1222  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1223  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1224  no-transform
1225  <list>
1226    <t>The no-transform response directive indicates that an intermediate cache or proxy
1227      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1228      headers, nor the response entity-body.</t>
1229  </list>
[540]1234<section anchor="cache.control.extensions" title="Cache Control Extensions">
1236  The Cache-Control header field can be extended through the use of one or more
1237  cache-extension tokens, each with an optional value. Informational extensions (those
1238  that do not require a change in cache behavior) can be added without changing the
1239  semantics of other directives. Behavioral extensions are designed to work by acting as
1240  modifiers to the existing base of cache directives. Both the new directive and the
1241  standard directive are supplied, such that applications that do not understand the new
1242  directive will default to the behavior specified by the standard directive, and those
1243  that understand the new directive will recognize it as modifying the requirements
1244  associated with the standard directive. In this way, extensions to the cache-control
1245  directives can be made without requiring changes to the base protocol.
1248  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1249  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1250  all directives that it does not understand.
1253  For example, consider a hypothetical new response directive called "community" that
1254  acts as a modifier to the private directive. We define this new directive to mean that,
1255  in addition to any non-shared cache, any cache that is shared only by members of the
1256  community named within its value may cache the response. An origin server wishing to
1257  allow the UCI community to use an otherwise private response in their shared cache(s)
1258  could do so by including
1260<figure><artwork type="example">
[458]1261  Cache-Control: private, community="UCI"
1264  A cache seeing this header field will act correctly even if the cache does not
1265  understand the community cache-extension, since it will also see and understand the
1266  private directive and thus default to the safe behavior.
1269  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1270  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1271  directives (or the response's default cacheability) such that the cache behavior will
1272  remain minimally correct even if the cache does not understand the extension(s).
[540]1278<section anchor="header.expires" title="Expires">
1279  <iref item="Expires header" primary="true" x:for-anchor="" />
1280  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1281  <x:anchor-alias value="Expires"/>
1282  <x:anchor-alias value="Expires-v"/>
[697]1284  The "Expires" entity-header field gives the date/time after which the response is
[540]1285  considered stale. See <xref target="expiration.model" /> for further discussion of the
1286  freshness model.
1289  The presence of an Expires field does not imply that the original resource will change or
1290  cease to exist at, before, or after that time.
1293  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1294  it &MUST; be sent in rfc1123-date format.
[364]1296<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
[366]1297  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
[364]1298  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1301  <preamble>For example</preamble>
1302<artwork type="example">
[364]1303  Expires: Thu, 01 Dec 1994 16:00:00 GMT
[575]1306  <t>
[756]1307    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
[575]1308    directive (see <xref target="cache-response-directive" />), that directive overrides
1309    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1310  </t>
1313  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1316  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1317  including the value "0", as in the past (i.e., "already expired").
[540]1321<section anchor="header.pragma" title="Pragma">
1322  <iref item="Pragma header" primary="true" x:for-anchor="" />
1323  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1324  <x:anchor-alias value="extension-pragma"/>
1325  <x:anchor-alias value="Pragma"/>
1326  <x:anchor-alias value="Pragma-v"/>
1327  <x:anchor-alias value="pragma-directive"/>
[697]1329  The "Pragma" general-header field is used to include implementation-specific directives
[540]1330  that might apply to any recipient along the request/response chain. All pragma directives
1331  specify optional behavior from the viewpoint of the protocol; however, some systems
1332  &MAY; require that behavior be consistent with the directives.
[364]1334<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
[366]1335  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
[364]1336  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
[334]1337  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1338  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1341  When the no-cache directive is present in a request message, an application &SHOULD;
1342  forward the request toward the origin server even if it has a cached copy of what is being
1343  requested. This pragma directive has the same semantics as the no-cache response directive
1344  (see <xref target="cache-response-directive" />) and is defined here for backward
1345  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1346  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1347  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1350  <t>
[756]1351    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
[540]1352    is not actually specified, it does not provide a reliable replacement for
[575]1353    "Cache-Control: no-cache" in a response.
1354  </t>
1357  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
[540]1361<section anchor="header.vary" title="Vary">
1362  <iref item="Vary header" primary="true" x:for-anchor="" />
1363  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1364  <x:anchor-alias value="Vary"/>
1365  <x:anchor-alias value="Vary-v"/>
[698]1367  The "Vary" response-header field conveys the set of request-header fields
1368  that were used to select the representation.
1371  Caches use this information, in part, to determine whether a stored response
[732]1372  can be used to satisfy a given request; see
[698]1373  <xref target="caching.negotiated.responses" />.
[540]1374  determines, while the response is fresh, whether a cache is permitted to use the
1375  response to reply to a subsequent request without validation; see <xref
1376  target="caching.negotiated.responses" />.
1379  In uncacheable or stale responses, the Vary field value advises the user agent about
1380  the criteria that were used to select the representation.
[364]1382<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
[366]1383  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
[364]1384  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1387  The set of header fields named by the Vary field value is known as the selecting
1388  request-headers.
1391  Servers &SHOULD; include a Vary header field with any cacheable response that is
1392  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1393  requests on that resource and informs the user agent about the presence of negotiation on
1394  that resource. A server &MAY; include a Vary header field with a non-cacheable
1395  response that is subject to server-driven negotiation, since this might provide the user
1396  agent with useful information about the dimensions over which the response varies at the
1397  time of the response.
1400  A Vary field value of "*" signals that unspecified parameters not limited to the
1401  request-headers (e.g., the network address of the client), play a role in the selection of
1402  the response representation; therefore, a cache cannot determine whether this response is
1403  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1404  it may only be generated by an origin server.
1407  The field-names given are not limited to the set of standard request-header fields
1408  defined by this specification. Field names are case-insensitive.
[540]1412<section anchor="header.warning" title="Warning">
1413  <iref item="Warning header" primary="true" x:for-anchor="" />
1414  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1415  <x:anchor-alias value="Warning"/>
1416  <x:anchor-alias value="Warning-v"/>
1417  <x:anchor-alias value="warning-value"/>
1418  <x:anchor-alias value="warn-agent"/>
1419  <x:anchor-alias value="warn-code"/>
1420  <x:anchor-alias value="warn-date"/>
1421  <x:anchor-alias value="warn-text"/>
[697]1423  The "Warning" general-header field is used to carry additional information about the status
[540]1424  or transformation of a message that might not be reflected in the message. This
1425  information is typically used to warn about possible incorrectness introduced by caching
1426  operations or transformations applied to the entity body of the message.
1429  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
[732]1430  warning, rather than an error status code, distinguishes these responses from true failures.
1433  Warning headers can in general be applied to any message, however some warn-codes are
1434  specific to caches and can only be applied to response messages.
[364]1436<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><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"/>
[366]1437  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
[364]1438  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
[229]1440  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1441                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
[229]1443  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
[334]1444  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
[135]1445                  ; the name or pseudonym of the server adding
1446                  ; the Warning header, for use in debugging
[229]1447  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1448  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1451  Multiple warnings can be attached to a response (either by the origin server or by
[669]1452  a cache), including multiple warnings with the same code number, only differing
1453  in warn-text.
1456  When this occurs, the user agent &SHOULD; inform the user of as many of them as
[669]1457  possible, in the order that they appear in the response.
1460  Systems that generate multiple Warning headers &SHOULD; order them with this user
1461  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1462  Warning headers.
1465  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1466  Warning is required to be deleted from a stored response after validation:
1467  <list style="symbols">
[709]1468    <t>1xx Warnings describe the freshness or validation status of the response, and so
1469      &MUST; be deleted by caches after validation. They can only be generated by a cache
1470      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1471    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
[540]1472      not rectified by a validation (for example, a lossy compression of the entity bodies)
1473      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1474      returned, in which case they &MUST; be.</t>
1475  </list>
1478  If an implementation sends a message with one or more Warning headers to a receiver whose
1479  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1480  warn-date that matches the Date header in the message.
1483  If an implementation receives a message with a warning-value that includes a warn-date,
1484  and that warn-date is different from the Date value in the response, then that
1485  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1486  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1487  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1488  well.
1491  The following warn-codes are defined by this specification, each with a recommended
1492  warn-text in English, and a description of its meaning.
[541]1494<t><?rfc needLines="4"?>
[540]1495  110 Response is stale
1496  <list>
1497    <t>&SHOULD; be included whenever the returned response is stale.</t>
1498  </list>
[541]1500<t><?rfc needLines="4"?>
[540]1501  111 Revalidation failed
1502  <list>
1503    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1504      validate the response failed, due to an inability to reach the server.</t>
1505  </list>
[541]1507<t><?rfc needLines="4"?>
[540]1508  112 Disconnected operation
1509  <list>
1510    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1511      the network for a period of time.</t>
1512  </list>
[541]1514<t><?rfc needLines="4"?>
[540]1515  113 Heuristic expiration
1516  <list>
1517    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1518      greater than 24 hours and the response's age is greater than 24 hours.</t>
1519  </list>
[541]1521<t><?rfc needLines="4"?>
[540]1522  199 Miscellaneous warning
1523  <list>
1524    <t>The warning text can include arbitrary information to be presented to a human
1525      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1526      action, besides presenting the warning to the user.</t>
1527  </list>
[541]1529<t><?rfc needLines="4"?>
[540]1530  214 Transformation applied
1531  <list>
1532    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1533      transformation changing the content-coding (as specified in the Content-Encoding
1534      header) or media-type (as specified in the Content-Type header) of the response, or
1535      the entity-body of the response, unless this Warning code already appears in the
1536      response.</t>
1537  </list>
[541]1539<t><?rfc needLines="4"?>
[540]1540  299 Miscellaneous persistent warning
1541  <list>
1542    <t>The warning text can include arbitrary information to be presented to a human
1543      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1544      action.</t>
1545  </list>
[540]1551<section anchor="history.lists" title="History Lists">
1553  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1554  can be used to redisplay an entity retrieved earlier in a session.
[766]1557  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1558  a history mechanism can display a previous representation even if it has expired.
1560  <t>
[766]1561  This does not prohibit the history mechanism from telling the user that a
1562  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
[575]1563  </t>
[540]1567<section anchor="IANA.considerations" title="IANA Considerations">
[540]1569<section anchor="message.header.registration" title="Message Header Registration">
1571  The Message Header Registry located at <eref
1572  target="" />
1573  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
[680]1575<?BEGININC p6-cache.iana-headers ?>
[540]1576<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[680]1577<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1578   <ttcol>Header Field Name</ttcol>
1579   <ttcol>Protocol</ttcol>
1580   <ttcol>Status</ttcol>
1581   <ttcol>Reference</ttcol>
[680]1583   <c>Age</c>
1584   <c>http</c>
1585   <c>standard</c>
1586   <c>
1587      <xref target="header.age"/>
1588   </c>
1589   <c>Cache-Control</c>
1590   <c>http</c>
1591   <c>standard</c>
1592   <c>
1593      <xref target="header.cache-control"/>
1594   </c>
1595   <c>Expires</c>
1596   <c>http</c>
1597   <c>standard</c>
1598   <c>
1599      <xref target="header.expires"/>
1600   </c>
1601   <c>Pragma</c>
1602   <c>http</c>
1603   <c>standard</c>
1604   <c>
1605      <xref target="header.pragma"/>
1606   </c>
1607   <c>Vary</c>
1608   <c>http</c>
1609   <c>standard</c>
1610   <c>
1611      <xref target="header.vary"/>
1612   </c>
1613   <c>Warning</c>
1614   <c>http</c>
1615   <c>standard</c>
1616   <c>
1617      <xref target="header.warning"/>
1618   </c>
[680]1621<?ENDINC p6-cache.iana-headers ?>
1623  The change controller is: "IETF ( - Internet Engineering Task Force".
[540]1629<section anchor="security.considerations" title="Security Considerations">
1631  Caches expose additional potential vulnerabilities, since the contents of the cache
1632  represent an attractive target for malicious exploitation. Because cache contents persist
1633  after an HTTP request is complete, an attack on the cache can reveal information long after
1634  a user believes that the information has been removed from the network. Therefore, cache
1635  contents should be protected as sensitive information.
[540]1639<section anchor="ack" title="Acknowledgments">
1641  Much of the content and presentation of the caching design is due to suggestions and
1642  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1643  and Larry Masinter.
1650<references title="Normative References">
[540]1652  <reference anchor="Part1">
1653    <front>
1654      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1655      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1656        <organization abbrev="Day Software">Day Software</organization>
1657        <address><email></email></address>
1658      </author>
1659      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1660        <organization>One Laptop per Child</organization>
1661        <address><email></email></address>
1662      </author>
1663      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1664        <organization abbrev="HP">Hewlett-Packard Company</organization>
1665        <address><email></email></address>
1666      </author>
1667      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1668        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1669        <address><email></email></address>
1670      </author>
1671      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1672        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1673        <address><email></email></address>
1674      </author>
1675      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1676        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1677        <address><email></email></address>
1678      </author>
1679      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1680        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1681        <address><email></email></address>
1682      </author>
1683      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1684        <organization abbrev="W3C">World Wide Web Consortium</organization>
1685        <address><email></email></address>
1686      </author>
1687      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1688        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1689        <address><email></email></address>
1690      </author>
1691      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1692    </front>
1693    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1694    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1695  </reference>
[540]1697  <reference anchor="Part2">
1698    <front>
1699      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1700      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1701        <organization abbrev="Day Software">Day Software</organization>
1702        <address><email></email></address>
1703      </author>
1704      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1705        <organization>One Laptop per Child</organization>
1706        <address><email></email></address>
1707      </author>
1708      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1709        <organization abbrev="HP">Hewlett-Packard Company</organization>
1710        <address><email></email></address>
1711      </author>
1712      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1713        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1714        <address><email></email></address>
1715      </author>
1716      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1717        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1718        <address><email></email></address>
1719      </author>
1720      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1721        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1722        <address><email></email></address>
1723      </author>
1724      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1725        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1726        <address><email></email></address>
1727      </author>
1728      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1729        <organization abbrev="W3C">World Wide Web Consortium</organization>
1730        <address><email></email></address>
1731      </author>
1732      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1733        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1734        <address><email></email></address>
1735      </author>
1736      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1737    </front>
1738    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1739    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1740  </reference>
[540]1742  <reference anchor="Part4">
1743    <front>
1744      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1745      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1746        <organization abbrev="Day Software">Day Software</organization>
1747        <address><email></email></address>
1748      </author>
1749      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1750        <organization>One Laptop per Child</organization>
1751        <address><email></email></address>
1752      </author>
1753      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1754        <organization abbrev="HP">Hewlett-Packard Company</organization>
1755        <address><email></email></address>
1756      </author>
1757      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1758        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1759        <address><email></email></address>
1760      </author>
1761      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1762        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1763        <address><email></email></address>
1764      </author>
1765      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1766        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1767        <address><email></email></address>
1768      </author>
1769      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1770        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1771        <address><email></email></address>
1772      </author>
1773      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1774        <organization abbrev="W3C">World Wide Web Consortium</organization>
1775        <address><email></email></address>
1776      </author>
1777      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1778        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1779        <address><email></email></address>
1780      </author>
1781      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1782    </front>
1783    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1784    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1785  </reference>
[540]1787  <reference anchor="Part5">
1788    <front>
1789      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1790      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1791        <organization abbrev="Day Software">Day Software</organization>
1792        <address><email></email></address>
1793      </author>
1794      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1795        <organization>One Laptop per Child</organization>
1796        <address><email></email></address>
1797      </author>
1798      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1799        <organization abbrev="HP">Hewlett-Packard Company</organization>
1800        <address><email></email></address>
1801      </author>
1802      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1803        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1804        <address><email></email></address>
1805      </author>
1806      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1807        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1808        <address><email></email></address>
1809      </author>
1810      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1811        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1812        <address><email></email></address>
1813      </author>
1814      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1815        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1816        <address><email></email></address>
1817      </author>
1818      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1819        <organization abbrev="W3C">World Wide Web Consortium</organization>
1820        <address><email></email></address>
1821      </author>
1822      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1823        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1824        <address><email></email></address>
1825      </author>
1826      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1827    </front>
1828    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1829    <x:source basename="p5-range" href="p5-range.xml" />
1830  </reference>
[540]1832  <reference anchor="Part7">
1833    <front>
1834      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1835      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1836        <organization abbrev="Day Software">Day Software</organization>
1837        <address><email></email></address>
1838      </author>
1839      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1840        <organization>One Laptop per Child</organization>
1841        <address><email></email></address>
1842      </author>
1843      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1844        <organization abbrev="HP">Hewlett-Packard Company</organization>
1845        <address><email></email></address>
1846      </author>
1847      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1848        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1849        <address><email></email></address>
1850      </author>
1851      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1852        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1853        <address><email></email></address>
1854      </author>
1855      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1856        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1857        <address><email></email></address>
1858      </author>
1859      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1860        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1861        <address><email></email></address>
1862      </author>
1863      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1864        <organization abbrev="W3C">World Wide Web Consortium</organization>
1865        <address><email></email></address>
1866      </author>
1867      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1868        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1869        <address><email></email></address>
1870      </author>
1871      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1872    </front>
1873    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1874    <x:source basename="p7-auth" href="p7-auth.xml" />
1875  </reference>
[540]1877  <reference anchor="RFC2119">
1878    <front>
1879      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1880      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1881        <organization>Harvard University</organization>
1882        <address><email></email></address>
1883      </author>
1884      <date month="March" year="1997" />
1885    </front>
1886    <seriesInfo name="BCP" value="14" />
1887    <seriesInfo name="RFC" value="2119" />
1888  </reference>
[540]1890  <reference anchor="RFC5234">
1891    <front>
1892      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1893      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1894        <organization>Brandenburg InternetWorking</organization>
1895        <address>
[728]1896          <email></email>
1897        </address> 
[540]1898      </author>
1899      <author initials="P." surname="Overell" fullname="Paul Overell">
1900        <organization>THUS plc.</organization>
1901        <address>
[728]1902          <email></email>
1903        </address>
[540]1904      </author>
1905      <date month="January" year="2008"/>
1906    </front>
1907    <seriesInfo name="STD" value="68"/>
1908    <seriesInfo name="RFC" value="5234"/>
1909  </reference>
[540]1913<references title="Informative References">
[540]1915  <reference anchor="RFC1305">
1916    <front>
1917      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1918      <author fullname="David L. Mills" initials="D." surname="Mills">
1919        <organization>University of Delaware, Electrical Engineering Department</organization>
1920        <address><email></email></address>
1921      </author>
1922      <date month="March" year="1992" />
1923    </front>
1924    <seriesInfo name="RFC" value="1305" />
1925  </reference>
[540]1927  <reference anchor="RFC2616">
1928    <front>
1929      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1930      <author fullname="R. Fielding" initials="R." surname="Fielding">
1931        <organization>University of California, Irvine</organization>
1932        <address><email></email></address>
1933      </author>
1934      <author fullname="J. Gettys" initials="J." surname="Gettys">
1935        <organization>W3C</organization>
1936        <address><email></email></address>
1937      </author>
1938      <author fullname="J. Mogul" initials="J." surname="Mogul">
1939        <organization>Compaq Computer Corporation</organization>
1940        <address><email></email></address>
1941      </author>
1942      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1943        <organization>MIT Laboratory for Computer Science</organization>
1944        <address><email></email></address>
1945      </author>
1946      <author fullname="L. Masinter" initials="L." surname="Masinter">
1947        <organization>Xerox Corporation</organization>
1948        <address><email></email></address>
1949      </author>
1950      <author fullname="P. Leach" initials="P." surname="Leach">
1951        <organization>Microsoft Corporation</organization>
1952        <address><email></email></address>
1953      </author>
1954      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1955        <organization>W3C</organization>
1956        <address><email></email></address>
1957      </author>
1958      <date month="June" year="1999" />
1959    </front>
1960    <seriesInfo name="RFC" value="2616" />
1961  </reference>
1963  <reference anchor="RFC3864">
1964    <front>
1965      <title>Registration Procedures for Message Header Fields</title>
1966      <author fullname="G. Klyne" initials="G." surname="Klyne">
1967        <organization>Nine by Nine</organization>
1968        <address><email></email></address>
1969      </author>
1970      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1971        <organization>BEA Systems</organization>
1972        <address><email></email></address>
1973      </author>
1974      <author fullname="J. Mogul" initials="J." surname="Mogul">
1975        <organization>HP Labs</organization>
1976        <address><email></email></address>
1977      </author>
1978      <date month="September" year="2004" />
1979    </front>
1980    <seriesInfo name="BCP" value="90" />
1981    <seriesInfo name="RFC" value="3864" />
1982  </reference>
1986<section anchor="compatibility" title="Compatibility with Previous Versions">
1988<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1990  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1991  this missing case.
1992  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
[768]1995  Range request responses would become very verbose if all meta-data were always returned;
[540]1996  by allowing the server to only send needed headers in a 206 response, this problem can be
1997  avoided.
1998  (<xref target="combining.headers" />)
2001  The Cache-Control: max-age directive was not properly defined for responses.
2002  (<xref target="cache-response-directive"/>)
2005  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2006    format="counter" target="expiration.model" />, <xref format="counter"
2007    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2008  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2009  header, as PUT or other methods may have need for it in requests.
2013<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
[691]2015  Remove requirement to consider Content-Location in successful responses
2016  in order to determine the appropriate response to use.
2017  (<xref target="validation.model" />)
[540]2020  Clarify denial of service attack avoidance requirement.
2021  (<xref target="invalidation.after.updates.or.deletions" />)
2024  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2025  anymore, as these aspects never were implemented.
2026  (<xref target="header.warning" />)
[538]2030    </section>
[680]2032<?BEGININC p6-cache.abnf-appendix ?>
[427]2033<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
2035<artwork type="abnf" name="p6-cache.parsed-abnf">
2036<x:ref>Age</x:ref> = "Age:" OWS Age-v
2037<x:ref>Age-v</x:ref> = delta-seconds
2039<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2040<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
[425]2041 cache-directive ] )
[427]2043<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2044<x:ref>Expires-v</x:ref> = HTTP-date
[678]2046<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2048<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2050<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2051<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
[425]2052 pragma-directive ] )
[427]2054<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2055<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
[425]2056 ] ) )
[427]2058<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2059<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
[425]2060 ] )
[427]2062<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2063<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2064<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
[425]2065 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2066 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2067 cache-extension
[427]2068<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
[425]2069 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2070 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2071 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2072 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2073 ) / ( "s-maxage=" delta-seconds ) / cache-extension
[427]2075<x:ref>delta-seconds</x:ref> = 1*DIGIT
2077<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
[678]2079<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
[648]2081<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
[427]2082<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
[678]2083<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2085<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2087<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
[648]2089<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2091<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2092<x:ref>warn-code</x:ref> = 3DIGIT
2093<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2094<x:ref>warn-text</x:ref> = quoted-string
2095<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
[425]2096 ]
[532]2099<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2100; Age defined but not used
[425]2101; Cache-Control defined but not used
2102; Expires defined but not used
2103; Pragma defined but not used
2104; Vary defined but not used
2105; Warning defined but not used
[680]2107<?ENDINC p6-cache.abnf-appendix ?>
[540]2109<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
[540]2111<section title="Since RFC2616">
2112  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
[540]2115<section title="Since draft-ietf-httpbis-p6-cache-00">
2117  Closed issues:
2118  <list style="symbols">
2119    <t>
[542]2120      <eref target="" />: "Trailer" (<eref target="" />)</t>
[540]2121    <t>
2122      <eref target="" />: "Invalidation after Update or Delete" (<eref target="" />)</t>
2123    <t>
2124      <eref target="" />: "Normative and Informative references"</t>
2125    <t>
2126      <eref target="" />: "Date reference typo"</t>
2127    <t>
2128      <eref target="" />: "Connection header text"</t>
2129    <t>
2130      <eref target="" />: "Informative references"</t>
2131    <t>
2132      <eref target="" />: "ISO-8859-1 Reference"</t>
2133    <t>
2134      <eref target="" />: "Normative up-to-date references"</t>
2135    <t>
2136      <eref target="" />: "typo in 13.2.2"</t>
2137  </list>
2140  Other changes:
2141  <list style="symbols">
2142    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2143        target="" />)</t>
2144  </list>
[540]2148<section title="Since draft-ietf-httpbis-p6-cache-01">
2150  Closed issues:
2151  <list style="symbols">
2152    <t>
2153      <eref target="" />: "rel_path not used"</t>
2154  </list>
2157  Other changes:
2158  <list style="symbols">
2159    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2160      on <eref target="" />)</t>
2161    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2162      specification.</t>
2163  </list>
[540]2167<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2169  Ongoing work on IANA Message Header Registration (<eref target="" />):
2170  <list style="symbols">
2171    <t>Reference RFC 3984, and update header registrations for headers defined in this
2172      document.</t>
2173  </list>
[540]2177<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2179  Closed issues:
2180  <list style="symbols">
2181    <t>
2182      <eref target="" />: "Vary header classification"</t>
2183  </list>
[540]2187<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2189  Ongoing work on ABNF conversion (<eref target=""/>):
2190  <list style="symbols">
2191    <t>
2192      Use "/" instead of "|" for alternatives.
2193    </t>
2194    <t>
2195      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2196      whitespace ("OWS") and required whitespace ("RWS").
2197    </t>
2198    <t>
2199      Rewrite ABNFs to spell out whitespace rules, factor out
2200      header value format definitions.
2201    </t>
2202  </list>
[540]2206<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2208  This is a total rewrite of this part of the specification.
2211  Affected issues:
2212  <list style="symbols">
2213    <t>
2214      <eref target="" />: "Definition of 1xx Warn-Codes"</t>
2215    <t>
2216      <eref target="" />: "Placement of 13.5.1 and 13.5.2"</t>
2217    <t>
2218      <eref target="" />: "The role of Warning and Semantic Transparency in Caching"</t>
2219    <t>
2220      <eref target="" />: "Methods and Caching"</t>
2221  </list>
[543]2224  In addition: Final work on ABNF conversion (<eref target=""/>):
[542]2225  <list style="symbols">
2226    <t>
2227      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2228    </t>
2229  </list>
[547]2233<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
[576]2235  Closed issues:
2236  <list style="symbols">
2237    <t>
2238      <eref target=""/>:
2239      "base for numeric protocol elements"
2240    </t>
2241  </list>
2244  Affected issues:
2245  <list style="symbols">
2246    <t>
2247      <eref target=""/>:
2248      Vary and non-existant headers
2249    </t>
2250  </list>
[604]2254<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
[669]2256  Closed issues:
2257  <list style="symbols">
2258    <t>
[711]2259      <eref target="" />:
2260      "Definition of 1xx Warn-Codes"
2261    </t>
2262    <t>
[691]2263      <eref target=""/>:
2264      "Content-Location on 304 responses"
2265    </t>
2266    <t>
[711]2267      <eref target="" />:
2268      "private and no-cache CC directives with headers"
2269    </t>
2270    <t>
[669]2271      <eref target=""/>:
2272      "RFC2047 and warn-text"
2273    </t>
2274  </list>
[720]2278<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
[766]2280  Closed issues:
2281  <list style="symbols">
2282    <t>
[771]2283      <eref target="" />:
2284      "serving negotiated responses from cache: header-specific canonicalization"
2285    </t>
2286    <t>
[766]2287      <eref target="" />:
2288      "Effect of CC directives on history lists"
2289    </t>
2290  </list>
[722]2293  Affected issues:
2294  <list style="symbols">
2295    <t>
2296      <eref target=""/>:
2297      Status codes and caching
2298    </t>
2299  </list>
2302  Partly resolved issues:
2303  <list style="symbols">
2304    <t>
2305      <eref target=""/>:
2306      "Placement of 13.5.1 and 13.5.2"
2307    </t>
2308  </list>
[538]2313  </back>
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