source: draft-ietf-httpbis/latest/p6-cache.xml @ 540

Last change on this file since 540 was 540, checked in by julian.reschke@…, 11 years ago

reverted back P6's indentation to match the one used in the other parts; also minor xml2rfc syntax cleanup

  • Property svn:eol-style set to native
File size: 99.9 KB
[29]1<?xml version="1.0" encoding="utf-8"?>
[540]2<?xml-stylesheet type='text/xsl' href='rfc2629.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>">
[29]14  <!ENTITY ID-VERSION "latest">
[460]15  <!ENTITY ID-MONTH "March">
[439]16  <!ENTITY ID-YEAR "2009">
[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=''/>">
24  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x=''/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='' xmlns:x=''/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='' xmlns:x=''/>">
[205]29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x=''/>">
[538]30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
[31]31  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x=''/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x=''/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x=''/>">
36<?rfc toc="yes" ?>
[29]37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
[8]39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
[203]43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
[8]45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
[538]47<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
48  obsoletes="2616" x:maturity-level="draft" xmlns:x="">
49  <front>
[538]51    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
[538]53    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
54      <organization abbrev="Day Software">Day Software</organization>
55      <address>
[8]56      <postal>
[29]57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
[8]59        <region>CA</region>
[29]60        <code>92660</code>
61        <country>USA</country>
[8]62      </postal>
[29]63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email></email>
66      <uri></uri>
[8]67    </address>
[538]68    </author>
[538]70    <author fullname="Jim Gettys" initials="J." surname="Gettys">
71      <organization>One Laptop per Child</organization>
72      <address>
[8]73      <postal>
[29]74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
[8]76        <region>MA</region>
[29]77        <code>01741</code>
78        <country>USA</country>
[8]79      </postal>
[29]80      <email></email>
81      <uri></uri>
[8]82    </address>
[538]83    </author>
85    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
86      <organization abbrev="HP">Hewlett-Packard Company</organization>
87      <address>
[8]88      <postal>
[29]89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
[8]91        <city>Palo Alto</city>
92        <region>CA</region>
[29]93        <code>94304</code>
94        <country>USA</country>
[8]95      </postal>
[29]96      <email></email>
[8]97    </address>
[538]98    </author>
[538]100    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
101      <organization abbrev="Microsoft">Microsoft Corporation</organization>
102      <address>
[8]103      <postal>
[29]104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
[8]109      </postal>
[29]110      <email></email>
[8]111    </address>
[538]112    </author>
[538]114    <author fullname="Larry Masinter" initials="L." surname="Masinter">
115      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116      <address>
[8]117      <postal>
[29]118        <street>345 Park Ave</street>
119        <city>San Jose</city>
[8]120        <region>CA</region>
[29]121        <code>95110</code>
122        <country>USA</country>
[8]123      </postal>
[29]124      <email></email>
125      <uri></uri>
[8]126    </address>
[538]127    </author>
129    <author fullname="Paul J. Leach" initials="P." surname="Leach">
130      <organization abbrev="Microsoft">Microsoft Corporation</organization>
131      <address>
[8]132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email></email>
139    </address>
[538]140    </author>
142    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
143      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144      <address>
[8]145      <postal>
[34]146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
[8]149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
[29]152        <country>USA</country>
[8]153      </postal>
154      <email></email>
[34]155      <uri></uri>
[8]156    </address>
[538]157    </author>
[538]159    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
160      <organization abbrev="W3C">World Wide Web Consortium</organization>
161      <address>
[93]162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email></email>
171      <uri></uri>
172    </address>
[538]173    </author>
[538]175    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
176      <organization abbrev="greenbytes">greenbytes GmbH</organization>
177      <address>
[95]178      <postal>
179        <street>Hafenweg 16</street>
180        <city>Muenster</city><region>NW</region><code>48155</code>
181        <country>Germany</country>
182      </postal>
183      <phone>+49 251 2807760</phone>   
184      <facsimile>+49 251 2807761</facsimile>   
185      <email></email>       
186      <uri></uri>     
187    </address>
[538]188    </author>
[538]190    <date month="&ID-MONTH;" year="&ID-YEAR;" />
191    <workgroup>HTTPbis Working Group</workgroup>
195  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
196  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
197  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
198  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
199  fields that control cache behavior or indicate cacheable response messages.
[540]203<note title="Editorial Note (To be removed by RFC Editor)">
205  Discussion of this draft should take place on the HTTPBIS working group mailing list
206  ( The current issues list is at <eref
207  target="" /> and related documents
208  (including fancy diffs) can be found at <eref target="" />.
211  The changes in this draft are summarized in <xref target="changes.since.05" />.
[538]215  </front>
216  <middle>
[540]218<section anchor="caching" title="Introduction">
220  HTTP is typically used for distributed information systems, where performance can be
221  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
222  caching and reusing response messages.
[540]225<section anchor="intro.purpose" title="Purpose">
226<iref item="cache" />
228  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
229  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
230  in order to reduce the response time and network bandwidth consumption on future,
231  equivalent requests. Any client or server may include a cache, though a cache cannot be
232  used by a server that is acting as a tunnel.
235  Caching would be useless if it did not significantly improve performance. The goal of
236  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
237  some cases, a stored response can be reused without the need for a network request,
238  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
239  (see <xref target="expiration.model" />). Even when a new request is required, it is often
240  possible to reuse all or parts of the payload of a prior response to satisfy the request,
241  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
242  purpose (see <xref target="validation.model" />).
[540]246<section anchor="intro.terminology" title="Terminology">
248  >This specification uses a number of terms to refer to the roles played by participants
249  in, and objects of, HTTP caching.
252  <iref item="cacheable" />
253  <x:dfn>cacheable</x:dfn>
254  <list>
255    <t>A response is cacheable if a cache is allowed to store a copy of the response message
256      for use in answering subsequent requests. Even when a response is cacheable, there may
257      be additional constraints on whether a cache can use the cached copy to satisfy a
258      particular request.</t>
259  </list>
262  <iref item="explicit expiration time" />
263  <x:dfn>explicit expiration time</x:dfn>
264  <list>
265    <t>The time at which the origin server intends that an entity should no longer be
266      returned by a cache without further validation.</t>
267  </list>
270  <iref item="heuristic expiration time" />
271  <x:dfn>heuristic expiration time</x:dfn>
272  <list>
273    <t>An expiration time assigned by a cache when no explicit expiration time is
274    available.</t>
275  </list>
278  <iref item="age" />
279  <x:dfn>age</x:dfn>
280  <list>
281    <t>The age of a response is the time since it was sent by, or successfully validated
282      with, the origin server.</t>
283  </list>
286  <iref item="first-hand" />
287  <x:dfn>first-hand</x:dfn>
288  <list>
289    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
290    0.</t>
291  </list>
294  <iref item="freshness lifetime" />
295  <x:dfn>freshness lifetime</x:dfn>
296  <list>
297    <t>The length of time between the generation of a response and its expiration time. </t>
298  </list>
301  <iref item="fresh" />
302  <x:dfn>fresh</x:dfn>
303  <list>
304    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
305  </list>
308  <iref item="stale" />
309  <x:dfn>stale</x:dfn>
310  <list>
311    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
312  </list>
315  <iref item="validator" />
316  <x:dfn>validator</x:dfn>
317  <list>
318    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
319      out whether a stored response is an equivalent copy of an entity.</t>
320  </list>
322<t anchor="shared.and.non-shared.caches">
323  <iref item="validator" />
324  <x:dfn>shared cache</x:dfn>
325  <list>
326    <t>A cache that is accessible to more than one user. A non-shared cache is
327      dedicated to a single user.</t>
328  </list>
[540]332<section anchor="intro.requirements" title="Requirements">
334  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
335  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
336  described in <xref target="RFC2119" />.
339  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
340  or &REQUIRED; level requirements for the protocols it implements. An implementation
341  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
342  requirements for its protocols is said to be "unconditionally compliant"; one that
343  satisfies all the &MUST; level requirements but not all the &SHOULD; level
344  requirements for its protocols is said to be "conditionally compliant."
[424]348<section title="Syntax Notation" anchor="notation">
[425]349  <x:anchor-alias value="ALPHA"/>
350  <x:anchor-alias value="CR"/>
351  <x:anchor-alias value="DIGIT"/>
352  <x:anchor-alias value="DQUOTE"/>
353  <x:anchor-alias value="LF"/>
354  <x:anchor-alias value="OCTET"/>
355  <x:anchor-alias value="SP"/>
356  <x:anchor-alias value="VCHAR"/>
357  <x:anchor-alias value="WSP"/>
359  This specification uses the ABNF syntax defined in &notation;.
[425]360  The following core rules are included by
361  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
362  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
363  DIGIT (decimal 0-9), DQUOTE (double quote),
364  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
365  OCTET (any 8-bit sequence of data), SP (space),
366  VCHAR (any visible USASCII character),
367  and WSP (whitespace).
[424]370<section title="Core Rules" anchor="core.rules">
[229]371  <x:anchor-alias value="quoted-string"/>
372  <x:anchor-alias value="token"/>
[364]373  <x:anchor-alias value="OWS"/>
[424]375  The core rules below are defined in &basic-rules;:
377<figure><artwork type="abnf2616">
[229]378  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
379  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
[364]380  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
384<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
[229]385  <x:anchor-alias value="field-name"/>
386  <x:anchor-alias value="HTTP-date"/>
387  <x:anchor-alias value="port"/>
388  <x:anchor-alias value="pseudonym"/>
389  <x:anchor-alias value="uri-host"/>
[205]391  The ABNF rules below are defined in other parts:
[207]393<figure><!--Part1--><artwork type="abnf2616">
[229]394  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
395  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
[374]396  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
[229]397  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
[374]398  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
[540]405<section anchor="caching.overview" title="Cache Operation">
[540]407<section anchor="response.cacheability" title="Response Cacheability">
409  A cache &MUST-NOT; store a response to any request, unless:
410  <list style="symbols">
411    <t>The request method is defined as being cacheable, and</t>
412    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
413       appear in request or response headers, and</t>
414    <t>the "private" cache response directive (see <xref target="header.cache-control" />
415       does not appear in the response, if the cache is shared, and</t>
416    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
417       the cache is shared (unless the "public" directive is present; see <xref
418       target="header.cache-control" />), and</t>           
419    <t>the cache understands partial responses, if the response is partial or incomplete
420       (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
421  </list>
424  Note that in normal operation, most caches will not store a response that has neither a
425  cache validator nor an explicit expiration time, as such responses are not usually
426  useful to store. However, caches are not prohibited from storing such responses.
[540]429<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
431  A cache that receives an incomplete response (for example, with fewer bytes of data
432  than specified in a Content-Length header) can store the response, but &MUST; 
433  treat it as a partial response &partial;. Partial responses
434  can be combined as described in &combining-byte-ranges;; the result might be a
435  full response or might still be partial. A cache &MUST-NOT; return a partial
436  response to a client without explicitly marking it as such using the 206 (Partial
437  Content) status code.
440  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
441  incomplete or partial responses.
[540]448<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
450For a presented request, a cache &MUST-NOT; return a stored response, unless:
451  <list style="symbols">
452    <t>The presented Request-URI and that of the stored response match (see
453      <cref>TBD</cref>), and</t>
454    <t>the request method associated with the stored response allows it to be
455      used for the presented request, and</t>
456    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
457      target="caching.negotiated.responses" />), and</t>
458    <t>the presented request and stored response are free from directives that would prevent
459      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
460      and</t>
461    <t>the stored response is either:
462      <list style="symbols">
463        <t>fresh (see <xref target="expiration.model" />), or</t>
464        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
465        <t>successfully validated (see <xref target="validation.model" />).</t>
466      </list>
467    </t>
468  </list>
471  <cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
474  When a stored response is used to satisfy a request, caches &MUST; include a
475  single Age header field <xref target="header.age" /> in the response with a value equal to the stored response's
476  current_age; see <xref target="age.calculations" />.
477  <cref>DISCUSS: this currently includes successfully validated responses.</cref>
480  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
481  the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
482  corresponding response.
485  Also, note that unsafe requests might invalidate already stored responses; see
486  <xref target="invalidation.after.updates.or.deletions" />.
489  Caches &MUST; use the most recent response (as determined by the Date header) when
490  more than one suitable response is stored. They can also forward a request with
491  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
492  use.
495  <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
[540]499<section anchor="expiration.model" title="Freshness Model">
501  When a response is "fresh" in the cache, it can be used to satisfy subsequent
502  requests without contacting the origin server, thereby improving efficiency.
505  The primary mechanism for determining freshness is for an origin server to provide an
506  explicit expiration time in the future, using either the Expires header (<xref
507  target="header.expires" />) or the max-age response cache directive (<xref
508  target="cache-response-directive" />). Generally, origin servers will assign future
509  explicit expiration times to responses in the belief that the entity is not likely to
510  change in a semantically significant way before the expiration time is reached.
513  If an origin server wishes to force a cache to validate every request, it can
514  assign an explicit expiration time in the past. This means that the response is always
515  stale, so that caches should validate it before using it for subsequent requests.
516  <cref>This wording may cause confusion, because the response may still be served stale.</cref>
519  Since origin servers do not always provide explicit expiration times, HTTP caches may
520  also assign heuristic expiration times when they are not specified, employing algorithms that
521  use other header values (such as the Last-Modified time) to estimate a plausible
522  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
523  impose worst-case constraints on their results.
527The calculation to determine if a response is fresh is:
529<artwork type="code">
[538]530   response_is_fresh = (freshness_lifetime &gt; current_age)
535  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
536  the current_age is defined in <xref target="age.calculations" />.
539  Additionally, clients may need to influence freshness calculation. They can do this using
540  several request cache directives, with the effect of either increasing or loosening
541  constraints on freshness. See <xref target="cache-request-directive" />.
544  <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
545  freshness.</cref>
548  Note that freshness applies only to cache operation; it cannot be used to force a user agent
549  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
550  the difference between caches and history mechanisms.
[540]553<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
555  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
556  response by using the first match of:
557  <list style="symbols">
558    <t>If the cache is shared and the s-maxage response cache directive (<xref
559      target="cache-response-directive" />) is present, use its value, or</t>
560    <t>If the max-age response cache directive (<xref target="cache-response-directive"
561      />) is present, use its value, or</t>
562    <t>If the Expires response header (<xref target="header.expires" />) is present, use
563      its value minus the value of the Date response header, or</t>
564    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
565      may be used; see <xref target="heuristic.freshness" />.</t>
566  </list>
569  Note that this calculation is not vulnerable to clock skew, since all of the
570  information comes from the origin server.
[540]573<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
575  If no explicit expiration time is present in a stored response that has a status code
576  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
577  calculated. Heuristics &MUST-NOT; be used for other response status codes.
580  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
581  attach a Warning header with a 113 warn-code to the response if its current_age is
582  more than 24 hours and such a warning is not already present.
585  Also, if the response has a Last-Modified header (&header-last-modified;), the
586  heuristic expiration value &SHOULD; be no more than some fraction of the interval
587  since that time. A typical setting of this fraction might be 10%.
590  <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
[540]595<section anchor="age.calculations" title="Calculating Age">
597  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
598  message when obtained from a cache. The Age field value is the cache's estimate of the
599  amount of time since the response was generated or validated by the origin server. In
600  essence, the Age value is the sum of the time that the response has been resident in
601  each of the caches along the path from the origin server, plus the amount of time it has
602  been in transit along network paths.
605  The term "age_value" denotes the value of the Age header, in a form appropriate for
606  arithmetic operations.
609  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
610  response, giving the time at which the response was generated (see &header-date;).
611  The term "date_value" denotes the value of the Date header, in a form appropriate for
612  arithmetic operations.
615  The term "now" means "the current value of the clock at the host performing the
616  calculation." Hosts that use HTTP, but especially hosts running origin servers and
617  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
618  synchronize their clocks to a globally accurate time standard.
620<t>A response's age can be calculated in two entirely independent ways:
621  <list style="numbers">
622    <t>now minus date_value, if the local clock is reasonably well synchronized to the
623      origin server's clock. If the result is negative, the result is replaced by zero.</t>
624    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
625  </list>
628<preamble>These are combined as</preamble>
629<artwork type="code">
[8]630    corrected_received_age = max(now - date_value, age_value)
633  When an Age value is received, it &MUST; be interpreted relative to the time the
634  request was initiated, not the time that the response was received.
636<figure><artwork type="code">
[8]637   corrected_initial_age = corrected_received_age
638                         + (now - request_time)
641  where "request_time" is the time (according to the local clock) when the request that
642  elicited this response was sent.
645  The current_age of a stored response can then be calculated by adding the amount of
646  time (in seconds) since the stored response was last validated by the origin server to
647  the corrected_initial_age.
650  In summary:
652<figure><artwork type="code">
653  age_value     - Age header field-value received with the response
654  date_value    - Date header field-value received with the response
655  request_time  - local time when the cache made the request
656                 resulting in the stored response
657  response_time - local time when the cache received the response
658  now           - current local time
660  apparent_age = max(0, response_time - date_value);
661  corrected_received_age = max(apparent_age, age_value);
662  response_delay = response_time - request_time;
663  corrected_initial_age = corrected_received_age + response_delay;
664  resident_time = now - response_time;
665  current_age   = corrected_initial_age + resident_time;
[540]669<section anchor="serving.stale.responses" title="Serving Stale Responses">
671  A "stale" response is one that either has explicit expiry information, or is allowed to
672  have heuristic expiry calculated, but is not fresh according to the calculations in
673  <xref target="expiration.model" />.
676  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
677  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
678  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
679  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
682  Caches &SHOULD-NOT; return stale responses unless they are
683  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
684  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
687  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
688  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
689  the cache is disconnected.
692  If a cache receives a first-hand response (either an entire response, or a 304 (Not
693  Modified) response) that it would normally forward to the requesting client, and the
694  received response is no longer fresh, the cache &SHOULD; forward it to the
695  requesting client without adding a new Warning (but without removing any existing
696  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
697  that response became stale in transit.
[540]702<section anchor="validation.model" title="Validation Model">
704  Checking with the origin server to see if a stale or otherwise unusable cached response
705  can be reused is called "validating" or "revalidating." Doing so potentially avoids
706  the overhead of retransmitting the response body when the stored response is valid.
709  HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
710  response includes one or more validators, such as the field values of an ETag or
711  Last-Modified header field, then a validating request &SHOULD; be made conditional
712  to those field values.
715  A 304 (Not Modified) response status code indicates that the stored
716  response can be updated and reused; see <xref target="combining.headers"/>.
719  If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
720  request and replace the stored response. <cref>Should there be a requirement here?</cref>
723  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
724  either forward this response to the requesting client, or act as if the server failed to
725  respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
726  111 warn-code; see <xref target="header.warning"/>) unless the
727  stored response includes the "must-revalidate" cache directive (see <xref
728  target="serving.stale.responses" />).
[540]732<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
734  Because unsafe methods (&safe-methods;) have the potential for changing state on the
735  origin server, intervening caches can use them to keep their contents
736  up-to-date.
739  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
740  as the Location and Content-Location headers (if present):
741  <list style="symbols">
742    <t>PUT</t>
743    <t>DELETE</t>
744    <t>POST</t>
745  </list>
748  An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
749  be performed if the host part of that URI differs from the host part in the Request-URI.
750  This helps prevent denial of service attacks.
753  <cref>TODO: "host part" needs to be specified better.</cref>
756  A cache that passes through requests for methods it does not understand &SHOULD;
757  invalidate the Request-URI.
760  Here, "invalidate" means that the cache will either remove all stored responses related
761  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
762  before they can be returned in response to a subsequent request.
765  Note that this does not guarantee that all appropriate responses are invalidated. For
766  example, the request that caused the change at the origin server might not have gone
767  through the cache where a response is stored.
770  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
[540]774<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
776  Use of server-driven content negotiation (&server-driven-negotiation;) alters
777  the conditions under which a cache can use the response for subsequent
778  requests.
781  When a cache receives a request that can be satisfied by a stored response
782  that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
783  all of the selecting request-headers in the presented request match the corresponding
784  stored request-headers from the original request.
787  The selecting request-headers from two requests are defined to match if and only if the
788  selecting request-headers in the first request can be transformed to the selecting
789  request-headers in the second request by adding or removing linear white space
790  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
791  combining multiple message-header fields with the same field name following the rules
792  about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref>
795  A Vary header field-value of "*" always fails to match, and subsequent requests to that
796  resource can only be properly interpreted by the origin server.
799  If no stored response matches, the cache &MAY; forward the presented request to the origin
800  server in a conditional request, and &SHOULD; include all ETags stored with
801  potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
802  includes an entity tag or Content-Location that indicates the entity to be used, that
803  cached response &MUST; be used to satisfy the presented request, and &SHOULD;
804  be used to update the corresponding stored response; see <xref target="combining.headers"/>.
807  If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
808  be included in the If-None-Match header field unless the request is for a range that would
809  be fully satisfied by that stored response.
812  If a cache receives a successful response whose Content-Location field matches that of an
813  existing stored response for the same Request-URI, whose entity-tag differs from that of
814  the existing stored response, and whose Date is more recent than that of the existing
815  response, the existing response &SHOULD-NOT; be returned in response to future
816  requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref>
[540]820<section anchor="combining.headers" title="Combining Responses">
822  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
823  it needs to update the stored response with the new one, so that the updated response can
824  be sent to the client.
827  If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
828  the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
829  Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
830  the stored response with the updated entity-body received in the response and use the
831  result as the updated entity-body (see &combining-byte-ranges;).
834  The stored response headers are used for the updated response, except that
835  <list style="symbols">
836    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
837      &MUST; be deleted from the stored response and the forwarded response.</t>
838    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
839      response and the forwarded response.</t>
840    <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
841      headers from the stored response.</t>
842  </list>
845  A cache &MUST; also replace any stored headers with corresponding headers received in the
846  incoming response, except for Warning headers as described immediately above. If a header
847  field-name in the incoming response matches more than one header in the stored response,
848  all such old headers &MUST; be replaced. It &MAY; store the combined
849  entity-body.
852  <cref>ISSUE: discuss how to handle HEAD updates</cref>
[540]858<section anchor="header.fields" title="Header Field Definitions">
859<t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
860<t>For entity-header fields, both sender and recipient refer to either the client or the
861server, depending on who sends and who receives the entity.</t>
[540]863<section anchor="header.age" title="Age">
864  <iref item="Age header" primary="true" x:for-anchor="" />
865  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
866  <x:anchor-alias value="Age"/>
867  <x:anchor-alias value="Age-v"/>
868  <x:anchor-alias value="age-value"/>
870  The response-header field "Age" conveys the sender's estimate of the amount of time since
871  the response (or its validation) was generated at the origin server. Age values are
872  calculated as specified in <xref target="age.calculations" />.
[364]874<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
[366]875  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
[364]876  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
[540]878<t anchor="">
879  <x:anchor-alias value="delta-seconds" />
880  Age field-values are non-negative decimal integers, representing time in seconds.
882<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
[229]883  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
886  If a cache receives a value larger than the largest positive integer it can represent, or
887  if any of its age calculations overflows, it &MUST; transmit an Age header with a
888  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
889  of at least 31 bits of range.
892  The presence of an Age header field in a response implies that a response is not
893  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
894  Age header field.
[540]898<section anchor="header.cache-control" title="Cache-Control">
899  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
900  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
901  <x:anchor-alias value="Cache-Control"/>
902  <x:anchor-alias value="Cache-Control-v"/>
903  <x:anchor-alias value="cache-directive"/>
904  <x:anchor-alias value="cache-extension"/>
905  <x:anchor-alias value="cache-request-directive"/>
906  <x:anchor-alias value="cache-response-directive"/>
908  The general-header field "Cache-Control" is used to specify directives that &MUST; be
909  obeyed by all caches along the request/response chain. The directives specify behavior
910  intended to prevent caches from adversely interfering with the request or response. Cache
911  directives are unidirectional in that the presence of a directive in a request does not
912  imply that the same directive is to be given in the response.
915  <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
916  Pragma: no-cache (see <xref target="header.pragma" />).</t>
919  Cache directives &MUST; be passed through by a proxy or gateway application,
920  regardless of their significance to that application, since the directives might be
921  applicable to all recipients along the request/response chain. It is not possible to
922  target a directive to a specific cache.
[538]924<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]925  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
[364]926  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
[229]928  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
[334]929     / <x:ref>cache-response-directive</x:ref>
[334]931  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[540]934<section anchor="cache-request-directive" title="Request Cache-Control Directives">
935  <x:anchor-alias value="cache-request-directive" />
[540]937<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
[538]938  <x:ref>cache-request-directive</x:ref> =
939       "no-cache"
940     / "no-store"
941     / "max-age" "=" <x:ref>delta-seconds</x:ref>
942     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
943     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
944     / "no-transform"
945     / "only-if-cached"
946     / <x:ref>cache-extension</x:ref>
950  <iref item="Cache Directives" primary="true" subitem="no-cache" />
951  <iref item="no-cache" primary="true" subitem="Cache Directive" />
952  no-cache
953  <list>
954    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
955      used to satisfy the request without successful validation on the origin server.</t>
956  </list>
959  <iref item="Cache Directives" primary="true" subitem="no-store" />
960  <iref item="no-store" primary="true" subitem="Cache Directive" />
961  no-store
962  <list>
963    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
964      of either this request or any response to it. This directive applies to both
965      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
966      cache &MUST-NOT; intentionally store the information in non-volatile storage,
967      and &MUST; make a best-effort attempt to remove the information from volatile
968      storage as promptly as possible after forwarding it.</t>
969    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
970      particular, malicious or compromised caches might not recognize or obey this
971      directive, and communications networks may be vulnerable to eavesdropping.</t>
972  </list>
975  <iref item="Cache Directives" primary="true" subitem="max-age" />
976  <iref item="max-age" primary="true" subitem="Cache Directive" />
977  max-age
978  <list>
979    <t>The max-age request directive indicates that the client is willing to accept a
980      response whose age is no greater than the specified time in seconds. Unless
981      max-stale directive is also included, the client is not willing to accept a stale
982      response.</t>
983  </list>
986  <iref item="Cache Directives" primary="true" subitem="max-stale" />
987  <iref item="max-stale" primary="true" subitem="Cache Directive" />
988  max-stale
989  <list>
990    <t>The max-stale request directive indicates that the client is willing to accept a
991      response that has exceeded its expiration time. If max-stale is assigned a value,
992      then the client is willing to accept a response that has exceeded its expiration
993      time by no more than the specified number of seconds. If no value is assigned to
994      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
995  </list>
998  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
999  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1000  min-fresh
1001  <list>
1002    <t>The min-fresh request directive indicates that the client is willing to accept a
1003      response whose freshness lifetime is no less than its current age plus the specified
1004      time in seconds. That is, the client wants a response that will still be fresh for
1005      at least the specified number of seconds.</t>
1006  </list>
1009  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1010  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1011  no-transform
1012  <list>
1013    <t>The no-transform request directive indicates that an intermediate cache or proxy
1014      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1015      headers, nor the request entity-body.</t>
1016  </list>
1019  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1020  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1021  only-if-cached
1022  <list>
1023    <t>The only-if-cached request directive indicates that the client only wishes to
1024      return a stored response. If it receives this directive, a cache &SHOULD; either
1025      respond using a stored response that is consistent with the other constraints of the
1026      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1027      being operated as a unified system with good internal connectivity, such a request
1028      &MAY; be forwarded within that group of caches.</t>
1029  </list>
[540]1033<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1034  <x:anchor-alias value="cache-response-directive" />
[540]1036<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
[538]1037  <x:ref>cache-response-directive</x:ref> =
1038       "public"
1039     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1040     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1041     / "no-store"
1042     / "no-transform"
1043     / "must-revalidate"
1044     / "proxy-revalidate"
1045     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1046     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1047     / <x:ref>cache-extension</x:ref>
1051  <iref item="Cache Directives" primary="true" subitem="public" />
1052  <iref item="public" primary="true" subitem="Cache Directive" />
1053  public
1054  <list>
1055    <t>The public response directive indicates that the response &MAY; be cached, even
1056      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1057      (See also Authorization, &header-authorization;, for additional details.) </t>
1058  </list>
1061  <iref item="Cache Directives" primary="true" subitem="private" />
1062  <iref item="private" primary="true" subitem="Cache Directive" />
1063  private
1064  <list>
1065    <t>The private response directive indicates that the response message is intended for
1066      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1067      cache &MAY; store the response.</t>
1068    <t>If the private response directive specifies one or more field-names, this
1069      requirement is limited to the field-values associated with the listed response
1070      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1071      cache, whereas the remainder of the response message &MAY; be.</t>
1072    <t>
1073      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1074      be stored, and cannot ensure the privacy of the message content.</t>
1075  </list>
1078  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1079  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1080  no-cache
1081  <list>
1082    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1083      satisfy a subsequent request without successful validation on the origin server.
1084      This allows an origin server to prevent caching even by caches that have been
1085      configured to return stale responses.</t>
1086    <t>If the no-cache response directive specifies one or more field-names, this
1087      requirement is limited to the field-values assosicated with the listed response
1088      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1089      to a subsequent request without successful validation on the origin server. This
1090      allows an origin server to prevent the re-use of certain header fields in a
1091      response, while still allowing caching of the rest of the response.</t>
1092    <t>
1093      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1094    </t>
1095  </list>
1099  <iref item="Cache Directives" primary="true" subitem="no-store" />
1100  <iref item="no-store" primary="true" subitem="Cache Directive" />
1101  no-store
1102  <list>
1103    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1104      part of either the immediate request or response. This directive applies to both
1105      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1106      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1107      and &MUST; make a best-effort attempt to remove the information from volatile
1108      storage as promptly as possible after forwarding it.</t>
1109    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1110      particular, malicious or compromised caches might not recognize or obey this
1111      directive, and communications networks may be vulnerable to eavesdropping.</t>
1112  </list>
1115  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1116  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1117  must-revalidate
1118  <list>
1119    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1120     used to satisfy subsequent requests without successful validation on the origin server.</t>
1121    <t>The must-revalidate directive is necessary to support reliable operation for
1122      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1123      the must-revalidate directive; in particular, if the cache cannot reach the origin
1124      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1125    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1126      validate a request on the entity could result in incorrect operation, such as a
1127      silently unexecuted financial transaction.</t>
1128  </list>
1131  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1132  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1133  proxy-revalidate
1134  <list>
1135    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1136      response directive, except that it does not apply to non-shared caches.</t>
1137  </list>
1140  <iref item="Cache Directives" primary="true" subitem="max-age" />
1141  <iref item="max-age" primary="true" subitem="Cache Directive" />
1142  max-age
1143  <list>
1144    <t>The max-age response directive indicates that response is to be considered stale
1145      after its age is greater than the specified number of seconds.</t>
1146  </list>
1149  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1150  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1151  s-maxage
1152  <list>
1153    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1154      specified by this directive overrides the maximum age specified by either the
1155      max-age directive or the Expires header. The s-maxage directive also implies the
1156      semantics of the proxy-revalidate response directive.</t>
1157  </list>
1160  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1161  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1162  no-transform
1163  <list>
1164    <t>The no-transform response directive indicates that an intermediate cache or proxy
1165      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1166      headers, nor the response entity-body.</t>
1167  </list>
[540]1172<section anchor="cache.control.extensions" title="Cache Control Extensions">
1174  The Cache-Control header field can be extended through the use of one or more
1175  cache-extension tokens, each with an optional value. Informational extensions (those
1176  that do not require a change in cache behavior) can be added without changing the
1177  semantics of other directives. Behavioral extensions are designed to work by acting as
1178  modifiers to the existing base of cache directives. Both the new directive and the
1179  standard directive are supplied, such that applications that do not understand the new
1180  directive will default to the behavior specified by the standard directive, and those
1181  that understand the new directive will recognize it as modifying the requirements
1182  associated with the standard directive. In this way, extensions to the cache-control
1183  directives can be made without requiring changes to the base protocol.
1186  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1187  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1188  all directives that it does not understand.
1191  For example, consider a hypothetical new response directive called "community" that
1192  acts as a modifier to the private directive. We define this new directive to mean that,
1193  in addition to any non-shared cache, any cache that is shared only by members of the
1194  community named within its value may cache the response. An origin server wishing to
1195  allow the UCI community to use an otherwise private response in their shared cache(s)
1196  could do so by including
1198<figure><artwork type="example">
[458]1199  Cache-Control: private, community="UCI"
1202  A cache seeing this header field will act correctly even if the cache does not
1203  understand the community cache-extension, since it will also see and understand the
1204  private directive and thus default to the safe behavior.
1207  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1208  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1209  directives (or the response's default cacheability) such that the cache behavior will
1210  remain minimally correct even if the cache does not understand the extension(s).
[540]1216<section anchor="header.expires" title="Expires">
1217  <iref item="Expires header" primary="true" x:for-anchor="" />
1218  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1219  <x:anchor-alias value="Expires"/>
1220  <x:anchor-alias value="Expires-v"/>
1222  The entity-header field "Expires" gives the date/time after which the response is
1223  considered stale. See <xref target="expiration.model" /> for further discussion of the
1224  freshness model.
1227  The presence of an Expires field does not imply that the original resource will change or
1228  cease to exist at, before, or after that time.
1231  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1232  it &MUST; be sent in rfc1123-date format.
[364]1234<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
[366]1235  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
[364]1236  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1239  <preamble>For example</preamble>
1240<artwork type="example">
[364]1241  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1244    <t>
1245      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1246      directive (see <xref target="cache-response-directive" />), that directive overrides
1247      the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.</t>
1250  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1253  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1254  including the value "0", as in the past (i.e., "already expired").
[540]1258<section anchor="header.pragma" title="Pragma">
1259  <iref item="Pragma header" primary="true" x:for-anchor="" />
1260  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1261  <x:anchor-alias value="extension-pragma"/>
1262  <x:anchor-alias value="Pragma"/>
1263  <x:anchor-alias value="Pragma-v"/>
1264  <x:anchor-alias value="pragma-directive"/>
1266  >The general-header field "Pragma" is used to include implementation-specific directives
1267  that might apply to any recipient along the request/response chain. All pragma directives
1268  specify optional behavior from the viewpoint of the protocol; however, some systems
1269  &MAY; require that behavior be consistent with the directives.
[364]1271<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]1272  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
[364]1273  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
[334]1274  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1275  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1278  When the no-cache directive is present in a request message, an application &SHOULD;
1279  forward the request toward the origin server even if it has a cached copy of what is being
1280  requested. This pragma directive has the same semantics as the no-cache response directive
1281  (see <xref target="cache-response-directive" />) and is defined here for backward
1282  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1283  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1284  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1287  <t>
1288    <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1289    is not actually specified, it does not provide a reliable replacement for
1290    "Cache-Control: no-cache" in a response.</t>
1293  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
[540]1297<section anchor="header.vary" title="Vary">
1298  <iref item="Vary header" primary="true" x:for-anchor="" />
1299  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1300  <x:anchor-alias value="Vary"/>
1301  <x:anchor-alias value="Vary-v"/>
1303  The "Vary" response-header field's value indicates the set of request-header fields that
1304  determines, while the response is fresh, whether a cache is permitted to use the
1305  response to reply to a subsequent request without validation; see <xref 
1306  target="caching.negotiated.responses" />.
1309  In uncacheable or stale responses, the Vary field value advises the user agent about
1310  the criteria that were used to select the representation.
[364]1312<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
[366]1313  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
[364]1314  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1317  The set of header fields named by the Vary field value is known as the selecting
1318  request-headers.
1321  Servers &SHOULD; include a Vary header field with any cacheable response that is
1322  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1323  requests on that resource and informs the user agent about the presence of negotiation on
1324  that resource. A server &MAY; include a Vary header field with a non-cacheable
1325  response that is subject to server-driven negotiation, since this might provide the user
1326  agent with useful information about the dimensions over which the response varies at the
1327  time of the response.
1330  A Vary field value of "*" signals that unspecified parameters not limited to the
1331  request-headers (e.g., the network address of the client), play a role in the selection of
1332  the response representation; therefore, a cache cannot determine whether this response is
1333  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1334  it may only be generated by an origin server.
1337  The field-names given are not limited to the set of standard request-header fields
1338  defined by this specification. Field names are case-insensitive.
[540]1342<section anchor="header.warning" title="Warning">
1343  <iref item="Warning header" primary="true" x:for-anchor="" />
1344  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1345  <x:anchor-alias value="Warning"/>
1346  <x:anchor-alias value="Warning-v"/>
1347  <x:anchor-alias value="warning-value"/>
1348  <x:anchor-alias value="warn-agent"/>
1349  <x:anchor-alias value="warn-code"/>
1350  <x:anchor-alias value="warn-date"/>
1351  <x:anchor-alias value="warn-text"/>
1353  The general-header field "Warning" is used to carry additional information about the status
1354  or transformation of a message that might not be reflected in the message. This
1355  information is typically used to warn about possible incorrectness introduced by caching
1356  operations or transformations applied to the entity body of the message.
1359  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1360  warning, rather than an error status code, distinguish these responses from true failures.
1363  Warning headers can in general be applied to any message, however some warn-codes are
1364  specific to caches and can only be applied to response messages.
[364]1366<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]1367  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
[364]1368  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
[229]1370  <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>
1371                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
[229]1373  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
[334]1374  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
[135]1375                  ; the name or pseudonym of the server adding
1376                  ; the Warning header, for use in debugging
[229]1377  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1378  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1381  Multiple warnings can be attached to a response (either by the origin server or by
1382  a cache), including multiple warnings with the same code number. For example, a server
1383  might provide the same warning with texts in both English and Basque.
1386  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1387  possible, in the order that they appear in the response. If it is not possible to inform
1388  the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1389  <list style="symbols">
1390    <t>Warnings that appear early in the response take priority over those appearing later
1391      in the response.</t>
1392    <t>Warnings in the user's preferred character set take priority over warnings in other
1393      character sets but with identical warn-codes and warn-agents.</t>
1394  </list>
1397  Systems that generate multiple Warning headers &SHOULD; order them with this user
1398  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1399  Warning headers.
1402  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1403  Warning is required to be deleted from a stored response after validation:
1404  <list style="symbols">
1405    <t>1xx Warnings that describe the freshness or validation status of the response, and so
1406      &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1407      except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1408    <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1409      not rectified by a validation (for example, a lossy compression of the entity bodies)
1410      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1411      returned, in which case they &MUST; be.</t>
1412  </list>
1415  The warn-text &SHOULD; be in a natural language and character set that is most likely
1416  to be intelligible to the human user receiving the response. This decision can be based on
1417  any available knowledge, such as the location of the cache or user, the Accept-Language
1418  field in a request, the Content-Language field in a response, etc. The default language is
1419  English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).
1422  If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1423  warn-text using the method described in <xref target="RFC2047" />.
1426  If an implementation sends a message with one or more Warning headers to a receiver whose
1427  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1428  warn-date that matches the Date header in the message.
1431  If an implementation receives a message with a warning-value that includes a warn-date,
1432  and that warn-date is different from the Date value in the response, then that
1433  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1434  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1435  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1436  well.
1439  The following warn-codes are defined by this specification, each with a recommended
1440  warn-text in English, and a description of its meaning.
1443  110 Response is stale
1444  <list>
1445    <t>&SHOULD; be included whenever the returned response is stale.</t>
1446  </list>
1449  111 Revalidation failed
1450  <list>
1451    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1452      validate the response failed, due to an inability to reach the server.</t>
1453  </list>
1456  112 Disconnected operation
1457  <list>
1458    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1459      the network for a period of time.</t>
1460  </list>
1463  113 Heuristic expiration
1464  <list>
1465    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1466      greater than 24 hours and the response's age is greater than 24 hours.</t>
1467  </list>
1470  199 Miscellaneous warning
1471  <list>
1472    <t>The warning text can include arbitrary information to be presented to a human
1473      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1474      action, besides presenting the warning to the user.</t>
1475  </list>
1478  214 Transformation applied
1479  <list>
1480    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1481      transformation changing the content-coding (as specified in the Content-Encoding
1482      header) or media-type (as specified in the Content-Type header) of the response, or
1483      the entity-body of the response, unless this Warning code already appears in the
1484      response.</t>
1485  </list>
1488  299 Miscellaneous persistent warning
1489  <list>
1490    <t>The warning text can include arbitrary information to be presented to a human
1491      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1492      action.</t>
1493  </list>
[540]1499<section anchor="history.lists" title="History Lists">
1501  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1502  can be used to redisplay an entity retrieved earlier in a session.
1505  History mechanisms and caches are different. In particular history mechanisms
1506  &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1507  history mechanism is meant to show exactly what the user saw at the time when the resource
1508  was retrieved.
1509  </t>
1511  By default, an expiration time does not apply to history mechanisms. If the entity is still
1512  in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1513  unless the user has specifically configured the agent to refresh expired history documents.
1516  This is not to be construed to prohibit the history mechanism from telling the user that a
1517  view might be stale.
1518  <list>
1519    <t>
1520      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1521      stale resources, this will tend to force service authors to avoid using HTTP expiration
1522      controls and cache controls when they would otherwise like to. Service authors may
1523      consider it important that users not be presented with error messages or warning
1524      messages when they use navigation controls (such as BACK) to view previously fetched
1525      resources. Even though sometimes such resources ought not be cached, or ought to expire
1526      quickly, user interface considerations may force service authors to resort to other
1527      means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1528      of improperly functioning history mechanisms.</t>
1529  </list>
[540]1534<section anchor="IANA.considerations" title="IANA Considerations">
[540]1536<section anchor="message.header.registration" title="Message Header Registration">
1538  The Message Header Registry located at <eref
1539  target="" />
1540  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
[540]1543<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1544<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1545  <ttcol>Header Field Name</ttcol>
1546  <ttcol>Protocol</ttcol>
1547  <ttcol>Status</ttcol>
1548  <ttcol>Reference</ttcol>
[540]1550  <c>Age</c>
1551  <c>http</c>
1552  <c>standard</c>
1553  <c>
1554    <xref target="header.age" />
1555  </c>
[540]1557  <c>Cache-Control</c>
1558  <c>http</c>
1559  <c>standard</c>
1560  <c>
1561    <xref target="header.cache-control" />
1562  </c>
[540]1564  <c>Expires</c>
1565  <c>http</c>
1566  <c>standard</c>
1567  <c>
1568    <xref target="header.expires" />
1569  </c>
[540]1571  <c>Pragma</c>
1572  <c>http</c>
1573  <c>standard</c>
1574  <c>
1575    <xref target="header.pragma" />
1576  </c>
[540]1578  <c>Vary</c>
1579  <c>http</c>
1580  <c>standard</c>
1581  <c>
1582    <xref target="header.vary" />
1583  </c>
[540]1585  <c>Warning</c>
1586  <c>http</c>
1587  <c>standard</c>
1588  <c>
1589    <xref target="header.warning" />
1590  </c>
1594  The change controller is: "IETF ( - Internet Engineering Task Force".
[540]1600<section anchor="security.considerations" title="Security Considerations">
1602  Caches expose additional potential vulnerabilities, since the contents of the cache
1603  represent an attractive target for malicious exploitation. Because cache contents persist
1604  after an HTTP request is complete, an attack on the cache can reveal information long after
1605  a user believes that the information has been removed from the network. Therefore, cache
1606  contents should be protected as sensitive information.
[540]1610<section anchor="ack" title="Acknowledgments">
1612  Much of the content and presentation of the caching design is due to suggestions and
1613  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1614  and Larry Masinter.
1621<references title="Normative References">
[540]1623  <reference anchor="ISO-8859-1">
1624    <front>
1625      <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1626        1: Latin alphabet No. 1 </title>
1627      <author>
1628        <organization>International Organization for Standardization</organization>
1629      </author>
1630      <date year="1998" />
1631    </front>
1632    <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1633  </reference>
[540]1635  <reference anchor="Part1">
1636    <front>
1637      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1638      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1639        <organization abbrev="Day Software">Day Software</organization>
1640        <address><email></email></address>
1641      </author>
1642      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1643        <organization>One Laptop per Child</organization>
1644        <address><email></email></address>
1645      </author>
1646      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1647        <organization abbrev="HP">Hewlett-Packard Company</organization>
1648        <address><email></email></address>
1649      </author>
1650      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1651        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1652        <address><email></email></address>
1653      </author>
1654      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1655        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1656        <address><email></email></address>
1657      </author>
1658      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1659        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1660        <address><email></email></address>
1661      </author>
1662      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1663        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1664        <address><email></email></address>
1665      </author>
1666      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1667        <organization abbrev="W3C">World Wide Web Consortium</organization>
1668        <address><email></email></address>
1669      </author>
1670      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1671        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1672        <address><email></email></address>
1673      </author>
1674      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1675    </front>
1676    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1677    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1678  </reference>
[540]1680  <reference anchor="Part2">
1681    <front>
1682      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1683      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1684        <organization abbrev="Day Software">Day Software</organization>
1685        <address><email></email></address>
1686      </author>
1687      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1688        <organization>One Laptop per Child</organization>
1689        <address><email></email></address>
1690      </author>
1691      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1692        <organization abbrev="HP">Hewlett-Packard Company</organization>
1693        <address><email></email></address>
1694      </author>
1695      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1696        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1697        <address><email></email></address>
1698      </author>
1699      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1700        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1701        <address><email></email></address>
1702      </author>
1703      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1704        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1705        <address><email></email></address>
1706      </author>
1707      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1708        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1709        <address><email></email></address>
1710      </author>
1711      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1712        <organization abbrev="W3C">World Wide Web Consortium</organization>
1713        <address><email></email></address>
1714      </author>
1715      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1716        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1717        <address><email></email></address>
1718      </author>
1719      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1720    </front>
1721    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1722    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1723  </reference>
[540]1725  <reference anchor="Part3">
1726    <front>
1727      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1728      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1729        <organization abbrev="Day Software">Day Software</organization>
1730        <address><email></email></address>
1731      </author>
1732      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1733        <organization>One Laptop per Child</organization>
1734        <address><email></email></address>
1735      </author>
1736      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1737        <organization abbrev="HP">Hewlett-Packard Company</organization>
1738        <address><email></email></address>
1739      </author>
1740      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1741        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1742        <address><email></email></address>
1743      </author>
1744      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1745        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1746        <address><email></email></address>
1747      </author>
1748      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1749        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1750        <address><email></email></address>
1751      </author>
1752      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1753        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1754        <address><email></email></address>
1755      </author>
1756      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1757        <organization abbrev="W3C">World Wide Web Consortium</organization>
1758        <address><email></email></address>
1759      </author>
1760      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1761        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1762        <address><email></email></address>
1763      </author>
1764      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1765    </front>
1766    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1767    <x:source basename="p3-payload" href="p3-payload.xml" />
1768  </reference>
[540]1770  <reference anchor="Part4">
1771    <front>
1772      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1773      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1774        <organization abbrev="Day Software">Day Software</organization>
1775        <address><email></email></address>
1776      </author>
1777      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1778        <organization>One Laptop per Child</organization>
1779        <address><email></email></address>
1780      </author>
1781      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1782        <organization abbrev="HP">Hewlett-Packard Company</organization>
1783        <address><email></email></address>
1784      </author>
1785      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1786        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1787        <address><email></email></address>
1788      </author>
1789      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1790        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1791        <address><email></email></address>
1792      </author>
1793      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1794        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1795        <address><email></email></address>
1796      </author>
1797      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1798        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1799        <address><email></email></address>
1800      </author>
1801      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1802        <organization abbrev="W3C">World Wide Web Consortium</organization>
1803        <address><email></email></address>
1804      </author>
1805      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1806        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1807        <address><email></email></address>
1808      </author>
1809      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1810    </front>
1811    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1812    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1813  </reference>
[540]1815  <reference anchor="Part5">
1816    <front>
1817      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1818      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1819        <organization abbrev="Day Software">Day Software</organization>
1820        <address><email></email></address>
1821      </author>
1822      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1823        <organization>One Laptop per Child</organization>
1824        <address><email></email></address>
1825      </author>
1826      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1827        <organization abbrev="HP">Hewlett-Packard Company</organization>
1828        <address><email></email></address>
1829      </author>
1830      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1831        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1832        <address><email></email></address>
1833      </author>
1834      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1835        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1836        <address><email></email></address>
1837      </author>
1838      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1839        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1840        <address><email></email></address>
1841      </author>
1842      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1843        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1844        <address><email></email></address>
1845      </author>
1846      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1847        <organization abbrev="W3C">World Wide Web Consortium</organization>
1848        <address><email></email></address>
1849      </author>
1850      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1851        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1852        <address><email></email></address>
1853      </author>
1854      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1855    </front>
1856    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1857    <x:source basename="p5-range" href="p5-range.xml" />
1858  </reference>
[540]1860  <reference anchor="Part7">
1861    <front>
1862      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1863      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1864        <organization abbrev="Day Software">Day Software</organization>
1865        <address><email></email></address>
1866      </author>
1867      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1868        <organization>One Laptop per Child</organization>
1869        <address><email></email></address>
1870      </author>
1871      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1872        <organization abbrev="HP">Hewlett-Packard Company</organization>
1873        <address><email></email></address>
1874      </author>
1875      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1876        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1877        <address><email></email></address>
1878      </author>
1879      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1880        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1881        <address><email></email></address>
1882      </author>
1883      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1884        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1885        <address><email></email></address>
1886      </author>
1887      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1888        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1889        <address><email></email></address>
1890      </author>
1891      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1892        <organization abbrev="W3C">World Wide Web Consortium</organization>
1893        <address><email></email></address>
1894      </author>
1895      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1896        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1897        <address><email></email></address>
1898      </author>
1899      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1900    </front>
1901    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1902    <x:source basename="p7-auth" href="p7-auth.xml" />
1903  </reference>
[540]1905  <reference anchor="RFC2047">
1906    <front>
1907      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1908        Part Three: Message Header Extensions for Non-ASCII Text</title>
1909      <author fullname="Keith Moore" initials="K." surname="Moore">
1910        <organization>University of Tennessee</organization>
1911        <address><email></email></address>
1912      </author>
1913      <date month="November" year="1996" />
1914    </front>
1915    <seriesInfo name="RFC" value="2047" />
1916  </reference>
[540]1918  <reference anchor="RFC2119">
1919    <front>
1920      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1921      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1922        <organization>Harvard University</organization>
1923        <address><email></email></address>
1924      </author>
1925      <date month="March" year="1997" />
1926    </front>
1927    <seriesInfo name="BCP" value="14" />
1928    <seriesInfo name="RFC" value="2119" />
1929  </reference>
[540]1931  <reference anchor="RFC5234">
1932    <front>
1933      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1934      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1935        <organization>Brandenburg InternetWorking</organization>
1936        <address>
1937        <postal>
1938        <street>675 Spruce Dr.</street>
1939        <city>Sunnyvale</city>
1940        <region>CA</region>
1941        <code>94086</code>
1942        <country>US</country></postal>
1943        <phone>+1.408.246.8253</phone>
1944        <email></email></address> 
1945      </author>
1946      <author initials="P." surname="Overell" fullname="Paul Overell">
1947        <organization>THUS plc.</organization>
1948        <address>
1949        <postal>
1950        <street>1/2 Berkeley Square</street>
1951        <street>99 Berkely Street</street>
1952        <city>Glasgow</city>
1953        <code>G3 7HR</code>
1954        <country>UK</country></postal>
1955        <email></email></address>
1956      </author>
1957      <date month="January" year="2008"/>
1958    </front>
1959    <seriesInfo name="STD" value="68"/>
1960    <seriesInfo name="RFC" value="5234"/>
1961  </reference>
[540]1965<references title="Informative References">
[540]1967  <reference anchor="RFC1305">
1968    <front>
1969      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1970      <author fullname="David L. Mills" initials="D." surname="Mills">
1971        <organization>University of Delaware, Electrical Engineering Department</organization>
1972        <address><email></email></address>
1973      </author>
1974      <date month="March" year="1992" />
1975    </front>
1976    <seriesInfo name="RFC" value="1305" />
1977  </reference>
[540]1979  <reference anchor="RFC2616">
1980    <front>
1981      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1982      <author fullname="R. Fielding" initials="R." surname="Fielding">
1983        <organization>University of California, Irvine</organization>
1984        <address><email></email></address>
1985      </author>
1986      <author fullname="J. Gettys" initials="J." surname="Gettys">
1987        <organization>W3C</organization>
1988        <address><email></email></address>
1989      </author>
1990      <author fullname="J. Mogul" initials="J." surname="Mogul">
1991        <organization>Compaq Computer Corporation</organization>
1992        <address><email></email></address>
1993      </author>
1994      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1995        <organization>MIT Laboratory for Computer Science</organization>
1996        <address><email></email></address>
1997      </author>
1998      <author fullname="L. Masinter" initials="L." surname="Masinter">
1999        <organization>Xerox Corporation</organization>
2000        <address><email></email></address>
2001      </author>
2002      <author fullname="P. Leach" initials="P." surname="Leach">
2003        <organization>Microsoft Corporation</organization>
2004        <address><email></email></address>
2005      </author>
2006      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2007        <organization>W3C</organization>
2008        <address><email></email></address>
2009      </author>
2010      <date month="June" year="1999" />
2011    </front>
2012    <seriesInfo name="RFC" value="2616" />
2013  </reference>
2015  <reference anchor="RFC3864">
2016    <front>
2017      <title>Registration Procedures for Message Header Fields</title>
2018      <author fullname="G. Klyne" initials="G." surname="Klyne">
2019        <organization>Nine by Nine</organization>
2020        <address><email></email></address>
2021      </author>
2022      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2023        <organization>BEA Systems</organization>
2024        <address><email></email></address>
2025      </author>
2026      <author fullname="J. Mogul" initials="J." surname="Mogul">
2027        <organization>HP Labs</organization>
2028        <address><email></email></address>
2029      </author>
2030      <date month="September" year="2004" />
2031    </front>
2032    <seriesInfo name="BCP" value="90" />
2033    <seriesInfo name="RFC" value="3864" />
2034  </reference>
2038<section anchor="compatibility" title="Compatibility with Previous Versions">
2040<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2042  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2043  this missing case.
2044  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2047  Transfer-coding and message lengths all interact in ways that required fixing exactly
2048  when chunked encoding is used (to allow for transfer encoding that may not be self
2049  delimiting); it was important to straighten out exactly how message lengths are computed.
2050  (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
2051  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2054  Proxies should be able to add Content-Length when appropriate.
2055  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2058  >Range request responses would become very verbose if all meta-data were always returned;
2059  by allowing the server to only send needed headers in a 206 response, this problem can be
2060  avoided.
2061  (<xref target="combining.headers" />)
2064  The Cache-Control: max-age directive was not properly defined for responses.
2065  (<xref target="cache-response-directive"/>)
2068  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2069    format="counter" target="expiration.model" />, <xref format="counter"
2070    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2071  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2072  header, as PUT or other methods may have need for it in requests.
2076<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2078  Clarify denial of service attack avoidance requirement.
2079  (<xref target="invalidation.after.updates.or.deletions" />)
[538]2083    </section>
[427]2085<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
2087<artwork type="abnf" name="p6-cache.parsed-abnf">
2088<x:ref>Age</x:ref> = "Age:" OWS Age-v
2089<x:ref>Age-v</x:ref> = delta-seconds
2091<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2092<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
[425]2093 cache-directive ] )
[427]2095<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2096<x:ref>Expires-v</x:ref> = HTTP-date
2098<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
2100<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2102<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2103<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
[425]2104 pragma-directive ] )
[427]2106<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2107<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
[425]2108 ] ) )
[427]2110<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2111<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
[425]2112 ] )
[427]2114<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2115<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2116<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
[425]2117 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2118 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2119 cache-extension
[427]2120<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
[425]2121 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2122 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2123 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2124 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2125 ) / ( "s-maxage=" delta-seconds ) / cache-extension
[427]2127<x:ref>delta-seconds</x:ref> = 1*DIGIT
2129<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2131<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
2133<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
2134<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2135<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
2137<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2139<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2141<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
2143<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2144<x:ref>warn-code</x:ref> = 3DIGIT
2145<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2146<x:ref>warn-text</x:ref> = quoted-string
2147<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
[425]2148 ]
[532]2153<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2154; Age defined but not used
[425]2155; Cache-Control defined but not used
2156; Expires defined but not used
2157; Pragma defined but not used
2158; Vary defined but not used
2159; Warning defined but not used
[540]2162<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
[540]2164<section title="Since RFC2616">
2165  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
[540]2168<section title="Since draft-ietf-httpbis-p6-cache-00">
2170  Closed issues:
2171  <list style="symbols">
2172    <t>
2173      <eref target="" />: "Trailer"
2174        (<eref target="" />)</t>
2175    <t>
2176      <eref target="" />: "Invalidation after Update or Delete" (<eref target="" />)</t>
2177    <t>
2178      <eref target="" />: "Normative and Informative references"</t>
2179    <t>
2180      <eref target="" />: "Date reference typo"</t>
2181    <t>
2182      <eref target="" />: "Connection header text"</t>
2183    <t>
2184      <eref target="" />: "Informative references"</t>
2185    <t>
2186      <eref target="" />: "ISO-8859-1 Reference"</t>
2187    <t>
2188      <eref target="" />: "Normative up-to-date references"</t>
2189    <t>
2190      <eref target="" />: "typo in 13.2.2"</t>
2191  </list>
2194  Other changes:
2195  <list style="symbols">
2196    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2197        target="" />)</t>
2198  </list>
[540]2202<section title="Since draft-ietf-httpbis-p6-cache-01">
2204  Closed issues:
2205  <list style="symbols">
2206    <t>
2207      <eref target="" />: "rel_path not used"</t>
2208  </list>
2211  Other changes:
2212  <list style="symbols">
2213    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2214      on <eref target="" />)</t>
2215    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2216      specification.</t>
2217  </list>
[540]2221<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2223  Ongoing work on IANA Message Header Registration (<eref target="" />):
2224  <list style="symbols">
2225    <t>Reference RFC 3984, and update header registrations for headers defined in this
2226      document.</t>
2227  </list>
[540]2231<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2233  Closed issues:
2234  <list style="symbols">
2235    <t>
2236      <eref target="" />: "Vary header classification"</t>
2237  </list>
[540]2241<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2243  Ongoing work on ABNF conversion (<eref target=""/>):
2244  <list style="symbols"> 
2245    <t>
2246      Use "/" instead of "|" for alternatives.
2247    </t>
2248    <t>
2249      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2250      whitespace ("OWS") and required whitespace ("RWS").
2251    </t>
2252    <t>
2253      Rewrite ABNFs to spell out whitespace rules, factor out
2254      header value format definitions.
2255    </t>
2256  </list>
[540]2260<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2261  <t>
2262    This is a total rewrite of this part of the specification.
2263  </t>
2264  <t>
2265    In addition: Ongoing work on ABNF conversion (<eref target=""/>):
2266    <list style="symbols"> 
2267      <t>
2268        Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2269      </t>
2270    </list>
2271  </t>
[538]2275  </back>
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