source: draft-ietf-httpbis/10/draft-ietf-httpbis-p6-cache-10.xml @ 955

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draft 10 (hopefully as submitted by Julian)

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc category="std" docName="draft-ietf-httpbis-p6-cache-10" ipr="pre5378Trust200902" obsoletes="2616">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
23
24  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
25    <organization abbrev="Day Software">Day Software</organization>
26    <address>
27      <postal>
28        <street>23 Corporate Plaza DR, Suite 280</street>
29        <city>Newport Beach</city>
30        <region>CA</region>
31        <code>92660</code>
32        <country>USA</country>
33      </postal>
34      <phone>+1-949-706-5300</phone>
35      <facsimile>+1-949-706-5305</facsimile>
36      <email>fielding@gbiv.com</email>
37      <uri>http://roy.gbiv.com/</uri>
38    </address>
39  </author>
40
41  <author initials="J." surname="Gettys" fullname="Jim Gettys">
42    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
43    <address>
44      <postal>
45        <street>21 Oak Knoll Road</street>
46        <city>Carlisle</city>
47        <region>MA</region>
48        <code>01741</code>
49        <country>USA</country>
50      </postal>
51      <email>jg@freedesktop.org</email>
52      <uri>http://gettys.wordpress.com/</uri>
53    </address>
54  </author>
55
56  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
57    <organization abbrev="HP">Hewlett-Packard Company</organization>
58    <address>
59      <postal>
60        <street>HP Labs, Large Scale Systems Group</street>
61        <street>1501 Page Mill Road, MS 1177</street>
62        <city>Palo Alto</city>
63        <region>CA</region>
64        <code>94304</code>
65        <country>USA</country>
66      </postal>
67      <email>JeffMogul@acm.org</email>
68    </address>
69  </author>
70
71  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
72    <organization abbrev="Microsoft">Microsoft Corporation</organization>
73    <address>
74      <postal>
75        <street>1 Microsoft Way</street>
76        <city>Redmond</city>
77        <region>WA</region>
78        <code>98052</code>
79        <country>USA</country>
80      </postal>
81      <email>henrikn@microsoft.com</email>
82    </address>
83  </author>
84
85  <author fullname="Larry Masinter" initials="L." surname="Masinter">
86    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
87    <address>
88      <postal>
89        <street>345 Park Ave</street>
90        <city>San Jose</city>
91        <region>CA</region>
92        <code>95110</code>
93        <country>USA</country>
94      </postal>
95      <email>LMM@acm.org</email>
96      <uri>http://larry.masinter.net/</uri>
97    </address>
98  </author>
99
100  <author fullname="Paul J. Leach" initials="P." surname="Leach">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108      </postal>
109      <email>paulle@microsoft.com</email>
110    </address>
111  </author>
112
113  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
114    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
115    <address>
116      <postal>
117        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
118        <street>The Stata Center, Building 32</street>
119        <street>32 Vassar Street</street>
120        <city>Cambridge</city>
121        <region>MA</region>
122        <code>02139</code>
123        <country>USA</country>
124      </postal>
125      <email>timbl@w3.org</email>
126      <uri>http://www.w3.org/People/Berners-Lee/</uri>
127    </address>
128  </author>
129
130  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
131    <organization abbrev="W3C">World Wide Web Consortium</organization>
132    <address>
133      <postal>
134        <street>W3C / ERCIM</street>
135        <street>2004, rte des Lucioles</street>
136        <city>Sophia-Antipolis</city>
137        <region>AM</region>
138        <code>06902</code>
139        <country>France</country>
140      </postal>
141      <email>ylafon@w3.org</email>
142      <uri>http://www.raubacapeu.net/people/yves/</uri>
143    </address>
144  </author>
145
146  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
147    <address>
148      <email>mnot@mnot.net</email>
149      <uri>http://www.mnot.net/</uri>
150    </address>
151  </author>
152
153  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
154    <organization abbrev="greenbytes">greenbytes GmbH</organization>
155    <address>
156      <postal>
157        <street>Hafenweg 16</street>
158        <city>Muenster</city><region>NW</region><code>48155</code>
159        <country>Germany</country>
160      </postal>
161      <phone>+49 251 2807760</phone>
162      <facsimile>+49 251 2807761</facsimile>
163      <email>julian.reschke@greenbytes.de</email>
164      <uri>http://greenbytes.de/tech/webdav/</uri>
165    </address>
166  </author>
167
168  <date month="July" year="2010" day="12"/>
169  <workgroup>HTTPbis Working Group</workgroup>
170
171<abstract>
172<t>
173  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
174  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
175  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
176  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
177  fields that control cache behavior or indicate cacheable response messages.
178</t>
179</abstract>
180
181<note title="Editorial Note (To be removed by RFC Editor)">
182  <t>
183    Discussion of this draft should take place on the HTTPBIS working group
184    mailing list (ietf-http-wg@w3.org). The current issues list is
185    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
186    and related documents (including fancy diffs) can be found at
187    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
188  </t>
189  <t>
190    The changes in this draft are summarized in <xref target="changes.since.09"/>.
191  </t>
192</note>
193
194  </front>
195  <middle>
196
197<section anchor="caching" title="Introduction">
198<t>
199  HTTP is typically used for distributed information systems, where performance can be
200  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
201  caching and reusing response messages.
202</t>
203
204<section anchor="intro.purpose" title="Purpose">
205<iref item="cache"/>
206<t>
207  An HTTP cache is a local store of response messages and the subsystem that
208  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
209  in order to reduce the response time and network bandwidth consumption on future,
210  equivalent requests. Any client or server may include a cache, though a cache cannot be
211  used by a server that is acting as a tunnel.
212</t>
213<t>
214  Caching would be useless if it did not significantly improve performance. The goal of
215  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
216  some cases, a stored response can be reused without the need for a network request,
217  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
218  (see <xref target="expiration.model"/>). Even when a new request is required, it is often
219  possible to reuse all or parts of the payload of a prior response to satisfy the request,
220  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
221  purpose (see <xref target="validation.model"/>).
222</t>
223</section>
224
225<section anchor="intro.terminology" title="Terminology">
226<t>
227  This specification uses a number of terms to refer to the roles played by participants
228  in, and objects of, HTTP caching.
229</t>
230<t>
231  <iref item="cacheable"/>
232  <?rfc needLines="4"?>cacheable
233  <list>
234    <t>A response is cacheable if a cache is allowed to store a copy of the response message
235      for use in answering subsequent requests. Even when a response is cacheable, there may
236      be additional constraints on whether a cache can use the cached copy to satisfy a
237      particular request.</t>
238  </list>
239</t>
240<t>
241  <iref item="explicit expiration time"/>
242  <?rfc needLines="4"?>explicit expiration time
243  <list>
244    <t>The time at which the origin server intends that an entity should no longer be
245      returned by a cache without further validation.</t>
246  </list>
247</t>
248<t>
249  <iref item="heuristic expiration time"/>
250  <?rfc needLines="4"?>heuristic expiration time
251  <list>
252    <t>An expiration time assigned by a cache when no explicit expiration time is
253    available.</t>
254  </list>
255</t>
256<t>
257  <iref item="age"/>
258  <?rfc needLines="4"?>age
259  <list>
260    <t>The age of a response is the time since it was sent by, or successfully validated
261      with, the origin server.</t>
262  </list>
263</t>
264<t>
265  <iref item="first-hand"/>
266  <?rfc needLines="4"?>first-hand
267  <list>
268    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
269    0.</t>
270  </list>
271</t>
272<t>
273  <iref item="freshness lifetime"/>
274  <?rfc needLines="4"?>freshness lifetime
275  <list>
276    <t>The length of time between the generation of a response and its expiration time. </t>
277  </list>
278</t>
279<t>
280  <iref item="fresh"/>
281  <?rfc needLines="4"?>fresh
282  <list>
283    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
284  </list>
285</t>
286<t>
287  <iref item="stale"/>
288  <?rfc needLines="4"?>stale
289  <list>
290    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
291  </list>
292</t>
293<t>
294  <iref item="validator"/>
295  <?rfc needLines="4"?>validator
296  <list>
297    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
298      out whether a stored response is an equivalent copy of an entity.</t>
299  </list>
300</t>
301<t anchor="shared.and.non-shared.caches">
302  <iref item="validator"/>
303  <?rfc needLines="4"?>shared cache
304  <list>
305    <t>A cache that is accessible to more than one user. A non-shared cache is
306      dedicated to a single user.</t>
307  </list>
308</t>
309</section>
310
311<section anchor="intro.requirements" title="Requirements">
312<t>
313   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
314   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
315   document are to be interpreted as described in <xref target="RFC2119"/>.
316</t>
317<t>
318   An implementation is not compliant if it fails to satisfy one or more
319   of the "MUST" or "REQUIRED" level requirements for the protocols it
320   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
321   level and all the "SHOULD" level requirements for its protocols is said
322   to be "unconditionally compliant"; one that satisfies all the "MUST"
323   level requirements but not all the "SHOULD" level requirements for its
324   protocols is said to be "conditionally compliant".
325</t>
326</section>
327
328<section title="Syntax Notation" anchor="notation">
329 
330 
331 
332 
333 
334 
335 
336 
337 
338<t>
339  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
340  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
341  <xref target="collected.abnf"/> shows the collected ABNF, with the list
342  rule expanded.
343</t>
344<t>
345  The following core rules are included by
346  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
347  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
348  DIGIT (decimal 0-9), DQUOTE (double quote),
349  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
350  OCTET (any 8-bit sequence of data), SP (space),
351  VCHAR (any visible USASCII character),
352  and WSP (whitespace).
353</t>
354
355<section title="Core Rules" anchor="core.rules">
356 
357 
358 
359<t>
360  The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
361</t>
362<figure><artwork type="abnf2616"><![CDATA[
363  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
364  token         = <token, defined in [Part1], Section 1.2.2>
365  OWS           = <OWS, defined in [Part1], Section 1.2.2>
366]]></artwork></figure>
367</section>
368
369<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
370 
371 
372 
373 
374 
375<t>
376  The ABNF rules below are defined in other parts:
377</t>
378<figure><artwork type="abnf2616"><![CDATA[
379  field-name    = <field-name, defined in [Part1], Section 3.2>
380  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
381  port          = <port, defined in [Part1], Section 2.6>
382  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
383  uri-host      = <uri-host, defined in [Part1], Section 2.6>
384]]></artwork></figure>
385</section>
386
387</section>
388</section>
389
390<section anchor="caching.overview" title="Cache Operation">
391
392<section anchor="response.cacheability" title="Response Cacheability">
393<t>
394  A cache MUST NOT store a response to any request, unless:
395  <list style="symbols">
396    <t>The request method is understood by the cache and defined as being cacheable, and</t>
397    <t>the response status code is understood by the cache, and</t>
398    <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not
399       appear in request or response headers, and</t>
400    <t>the "private" cache response directive (see <xref target="cache-response-directive"/>
401       does not appear in the response, if the cache is shared, and</t>
402    <t>the "Authorization" header (see Section 3.1 of <xref target="Part7"/>) does not appear in the request, if
403       the cache is shared, unless the response explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
404    <t>the response either:
405      <list style="symbols">
406        <t>contains an Expires header (see <xref target="header.expires"/>), or</t>
407        <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
408        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
409        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached, or</t>
410        <t>has a status code that can be served with heuristic freshness (see <xref target="heuristic.freshness"/>).</t>
411      </list>
412    </t>
413  </list>
414</t>
415<t>
416  In this context, a cache has "understood" a request method or a response status
417  code if it recognises it and implements any cache-specific behaviour. In
418  particular, 206 Partial Content responses cannot be cached by an
419  implementation that does not handle partial content
420  (see <xref target="errors.or.incomplete.response.cache.behavior"/>).
421</t>
422<t>
423  Note that in normal operation, most caches will not store a response that has neither a
424  cache validator nor an explicit expiration time, as such responses are not usually
425  useful to store. However, caches are not prohibited from storing such responses.
426</t>
427
428<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
429<t>
430  A cache that receives an incomplete response (for example, with fewer bytes of data
431  than specified in a Content-Length header) can store the response, but MUST
432  treat it as a partial response <xref target="Part5"/>. Partial responses
433  can be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
434  full response or might still be partial. A cache MUST NOT return a partial
435  response to a client without explicitly marking it as such using the 206 (Partial
436  Content) status code.
437</t>
438<t>
439  A cache that does not support the Range and Content-Range headers MUST NOT store
440  incomplete or partial responses.
441</t>
442</section>
443
444</section>
445
446
447<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
448<t>
449  For a presented request, a cache MUST NOT return a stored response, unless:
450  <list style="symbols">
451    <t>The presented Effective Request URI (Section 4.3 of <xref target="Part1"/>) and that of the stored response match, and</t>
452    <t>the request method associated with the stored response allows it to be
453      used for the presented request, and</t>
454    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
455    <t>the presented request and stored response are free from directives that would prevent
456      its use (see <xref target="header.cache-control"/> and <xref target="header.pragma"/>),
457      and</t>
458    <t>the stored response is either:
459      <list style="symbols">
460        <t>fresh (see <xref target="expiration.model"/>), or</t>
461        <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
462        <t>successfully validated (see <xref target="validation.model"/>).</t>
463      </list>
464    </t>
465  </list>
466</t>
467<t>
468  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
469</t>
470<t>
471  When a stored response is used to satisfy a request, caches MUST include a
472  single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
473  current_age; see <xref target="age.calculations"/>.
474  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
475</t>
476<t>
477  Requests with methods that are unsafe (Section 7.1.1 of <xref target="Part2"/>) MUST be written through the cache to
478  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
479  corresponding response.
480</t>
481<t>
482  Also, note that unsafe requests might invalidate already stored responses; see
483  <xref target="invalidation.after.updates.or.deletions"/>.
484</t>
485<t>
486  Caches MUST use the most recent response (as determined by the Date header) when
487  more than one suitable response is stored. They can also forward a request with
488  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
489  use.
490</t>
491</section>
492
493<section anchor="expiration.model" title="Freshness Model">
494<t>
495  When a response is "fresh" in the cache, it can be used to satisfy subsequent
496  requests without contacting the origin server, thereby improving efficiency.
497</t>
498<t>
499  The primary mechanism for determining freshness is for an origin server to provide an
500  explicit expiration time in the future, using either the Expires header (<xref target="header.expires"/>) or the max-age response cache directive (<xref target="cache-response-directive"/>). Generally, origin servers will assign future
501  explicit expiration times to responses in the belief that the entity is not likely to
502  change in a semantically significant way before the expiration time is reached.
503</t>
504<t>
505  If an origin server wishes to force a cache to validate every request, it can
506  assign an explicit expiration time in the past. This means that the response is always
507  stale, so that caches should validate it before using it for subsequent requests.
508  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
509</t>
510<t>
511  Since origin servers do not always provide explicit expiration times, HTTP caches may
512  also assign heuristic expiration times when they are not specified, employing algorithms that
513  use other header values (such as the Last-Modified time) to estimate a plausible
514  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
515  impose worst-case constraints on their results.
516</t>
517<figure>
518<preamble>
519  The calculation to determine if a response is fresh is:
520</preamble>
521<artwork type="code"><![CDATA[
522   response_is_fresh = (freshness_lifetime > current_age)
523]]></artwork>
524</figure>
525<t>
526  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>;
527  the current_age is defined in <xref target="age.calculations"/>.
528</t>
529<t>
530  Additionally, clients may need to influence freshness calculation. They can do this using
531  several request cache directives, with the effect of either increasing or loosening
532  constraints on freshness. See <xref target="cache-request-directive"/>.
533</t>
534<t>
535  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
536  freshness.</cref>
537</t>
538<t>
539  Note that freshness applies only to cache operation; it cannot be used to force a user agent
540  to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of
541  the difference between caches and history mechanisms.
542</t>
543
544<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
545<t>
546  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
547  response by using the first match of:
548  <list style="symbols">
549    <t>If the cache is shared and the s-maxage response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
550    <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
551    <t>If the Expires response header (<xref target="header.expires"/>) is present, use
552      its value minus the value of the Date response header, or</t>
553    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
554      freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
555  </list>
556</t>
557<t>
558  Note that this calculation is not vulnerable to clock skew, since all of the
559  information comes from the origin server.
560</t>
561
562<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
563<t>
564  If no explicit expiration time is present in a stored response that has a status code
565  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
566  calculated. Heuristics MUST NOT be used for other response status codes.
567</t>
568<t>
569  When a heuristic is used to calculate freshness lifetime, the cache SHOULD
570  attach a Warning header with a 113 warn-code to the response if its current_age is
571  more than 24 hours and such a warning is not already present.
572</t>
573<t>
574  Also, if the response has a Last-Modified header (Section 6.6 of <xref target="Part4"/>), the
575  heuristic expiration value SHOULD be no more than some fraction of the interval
576  since that time. A typical setting of this fraction might be 10%.
577</t>
578<t><list>
579  <t>
580    Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9)
581    required that caches do not calculate heuristic freshness for URLs with
582    query components (i.e., those containing '?'). In practice, this has not
583    been widely implemented. Therefore, servers are encouraged to send explicit
584    directives (e.g., Cache-Control: no-cache) if they wish to preclude
585    caching.
586  </t>
587</list></t>
588</section>
589</section>
590
591<section anchor="age.calculations" title="Calculating Age">
592<t>
593  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
594  message when obtained from a cache. The Age field value is the cache's estimate of the
595  amount of time since the response was generated or validated by the origin server. In
596  essence, the Age value is the sum of the time that the response has been resident in
597  each of the caches along the path from the origin server, plus the amount of time it has
598  been in transit along network paths.
599</t>
600<t>
601  The following data is used for the age calculation:
602</t>
603<t>
604  <?rfc needLines="4"?>age_value
605  <list>
606    <t>
607      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
608      in a form appropriate for arithmetic operation; or 0, if not available.
609    </t>
610  </list>
611</t>
612<t>
613  <?rfc needLines="4"?>date_value
614  <list>
615    <t>
616      HTTP/1.1 requires origin servers to send a Date header, if possible,
617      with every response, giving the time at which the response was generated.
618      The term "date_value" denotes the value of the Date header, in a form
619      appropriate for arithmetic operations. See Section 9.3 of <xref target="Part1"/> for the definition
620      of the Date header, and for requirements regarding responses without a
621      Date response header.
622    </t>
623  </list>
624</t>
625<t>
626  <?rfc needLines="4"?>now
627  <list>
628    <t>
629      The term "now" means "the current value of the clock at the host
630      performing the calculation". Hosts that use HTTP, but especially hosts
631      running origin servers and caches, SHOULD use NTP
632      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
633      clocks to a globally accurate time standard.
634    </t>
635  </list>
636</t>
637<t>
638  <?rfc needLines="4"?>request_time
639  <list>
640    <t>
641      The current value of the clock at the host at the time the request
642      resulting in the stored response was made.
643    </t>
644  </list>
645</t>
646<t>
647  <?rfc needLines="4"?>response_time
648  <list>
649    <t>
650      The current value of the clock at the host at the time the response was
651      received.
652    </t>
653  </list>
654</t>
655<t>
656  A response's age can be calculated in two entirely independent ways:
657  <list style="numbers">
658    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
659      origin server's clock. If the result is negative, the result is replaced by zero.</t>
660    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
661      note this value MUST be interpreted relative to the time the
662      request was initiated, not the time that the response was received.</t>
663  </list>
664</t>
665<figure>
666<artwork type="code"><![CDATA[
667  apparent_age = max(0, response_time - date_value);
668
669  response_delay = response_time - request_time;
670  corrected_age_value = age_value + response_delay; 
671]]></artwork>
672</figure>
673<figure>
674<preamble>These are combined as</preamble>
675<artwork type="code"><![CDATA[
676  corrected_initial_age = max(apparent_age, corrected_age_value);
677]]></artwork></figure>
678<t>
679  The current_age of a stored response can then be calculated by adding the amount of
680  time (in seconds) since the stored response was last validated by the origin server to
681  the corrected_initial_age.
682</t>
683<figure><artwork type="code"><![CDATA[
684  resident_time = now - response_time;
685  current_age = corrected_initial_age + resident_time;
686]]></artwork></figure>
687</section>
688
689<section anchor="serving.stale.responses" title="Serving Stale Responses">
690<t>
691  A "stale" response is one that either has explicit expiry information or is allowed to
692  have heuristic expiry calculated, but is not fresh according to the calculations in
693  <xref target="expiration.model"/>.
694</t>
695<t>
696  Caches MUST NOT return a stale response if it is prohibited by an explicit
697  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
698  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
699  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
700</t>
701<t>
702  Caches SHOULD NOT return stale responses unless they are
703  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
704  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive"/>).
705</t>
706<t>
707  Stale responses SHOULD have a Warning header with the 110 warn-code (see <xref target="header.warning"/>). Likewise, the 112 warn-code SHOULD be sent on stale responses if
708  the cache is disconnected.
709</t>
710<t>
711  If a cache receives a first-hand response (either an entire response, or a 304 (Not
712  Modified) response) that it would normally forward to the requesting client, and the
713  received response is no longer fresh, the cache SHOULD forward it to the
714  requesting client without adding a new Warning (but without removing any existing
715  Warning headers). A cache SHOULD NOT attempt to validate a response simply because
716  that response became stale in transit.
717</t>
718</section>
719</section>
720
721<section anchor="validation.model" title="Validation Model">
722<t>
723  When a cache has one or more stored responses for a requested URI, but cannot 
724  serve any of them (e.g., because they are not fresh, or one cannot be selected;
725  see <xref target="caching.negotiated.responses"/>),
726  it can use the conditional request mechanism <xref target="Part4"/> in the forwarded
727  request to give the origin server an opportunity to both select a valid stored
728  response to be used, and to update it. This process is known as "validating"
729  or "revalidating" the stored response.
730</t>
731<t>
732  When sending such a conditional request, the cache SHOULD add an If-Modified-Since
733  header whose value is that of the Last-Modified header from the selected
734  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
735</t>
736<t>
737  Additionally, the cache SHOULD add an If-None-Match header whose value 
738  is that of the ETag header(s) from all responses stored for the requested URI,
739  if present. However, if any of the stored responses contains only partial
740  content, its entity-tag SHOULD NOT be included in the If-None-Match header
741  field unless the request is for a range that would be fully satisfied by
742  that stored response.
743</t>
744<t>
745  A 304 (Not Modified) response status code indicates that the stored
746  response can be updated and reused; see <xref target="combining.headers"/>.
747</t>
748<t>
749  A full response (i.e., one with a response body) indicates that none 
750  of the stored responses nominated in the conditional request is
751  suitable. Instead, the full response is used both to satisfy the
752  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
753</t>
754<t>
755  If a cache receives a 5xx response while attempting to validate a response, it MAY
756  either forward this response to the requesting client, or act as if the server failed to
757  respond. In the latter case, it MAY return a previously stored response (see <xref target="serving.stale.responses"/>).
758</t>
759</section>
760
761<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
762<t>
763  Because unsafe methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing state on the
764  origin server, intervening caches can use them to keep their contents
765  up-to-date.
766</t>
767<t>
768  The following HTTP methods MUST cause a cache to invalidate the Effective Request URI (Section 4.3 of <xref target="Part1"/>) as well
769  as the URI(s) in the Location and Content-Location headers (if present):
770  <list style="symbols">
771    <t>PUT</t>
772    <t>DELETE</t>
773    <t>POST</t>
774  </list>
775</t>
776<t>
777  An invalidation based on a URI from a Location or Content-Location header MUST NOT
778  be performed if the host part of that URI differs from the host part in the Effective Request URI (Section 4.3 of <xref target="Part1"/>).
779  This helps prevent denial of service attacks.
780</t>
781<t>
782  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
783</t>
784<t>
785  A cache that passes through requests for methods it does not understand SHOULD
786  invalidate the Effective Request URI (Section 4.3 of <xref target="Part1"/>).
787</t>
788<t>
789  Here, "invalidate" means that the cache will either remove all stored responses related
790  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
791  before they can be returned in response to a subsequent request.
792</t>
793<t>
794  Note that this does not guarantee that all appropriate responses are invalidated. For
795  example, the request that caused the change at the origin server might not have gone
796  through the cache where a response is stored.
797</t>
798<t>
799  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
800</t>
801</section>
802
803<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
804
805<t>Shared caches MUST NOT use a cached response to a request with an Authorization header (Section 3.1 of <xref target="Part7"/>) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
806
807<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
808
809<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
810</section>
811
812<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
813<t>
814  When a cache receives a request that can be satisfied by a stored response
815  that has a Vary header field (<xref target="header.vary"/>), it MUST NOT use that
816  response unless all of the selecting request-headers nominated by the Vary header match
817  in both the original request (i.e., that associated with the stored response),
818  and the presented request.
819</t>
820<t>
821  The selecting request-headers from two requests are defined to match
822  if and only if those in the first request can be transformed to those in the
823  second request by applying any of the following:
824  <list style="symbols">
825    <t>
826      adding or removing whitespace, where allowed in the header's syntax
827    </t>
828    <t>
829      combining multiple message-header fields with the same field name (see
830      Section 3.2 of <xref target="Part1"/>)
831    </t>
832    <t>
833      normalizing both header values in a way that is known to have identical
834      semantics, according to the header's specification (e.g., re-ordering field values
835      when order is not significant; case-normalization, where values are defined to be
836      case-insensitive)   
837    </t>
838  </list>
839</t>
840<t>
841  If (after any normalisation that may take place) a header field is absent
842  from a request, it can only match another request if it is also absent there.
843</t>
844<t>
845  A Vary header field-value of "*" always fails to match, and subsequent requests to that
846  resource can only be properly interpreted by the origin server.
847</t>
848<t>
849  The stored response with matching selecting request-headers is known as the
850  selected response.
851</t>
852<t>
853  If no selected response is available, the cache MAY forward the presented
854  request to the origin server in a conditional request; see <xref target="validation.model"/>.
855</t>
856</section>
857
858<section anchor="combining.headers" title="Combining Responses">
859<t>
860  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
861  (in this section, the "new" response"), it needs to created an updated response by combining
862  the stored response with the new one, so that the updated response can be used to satisfy the request.
863</t>
864<t>
865  If the new response contains an ETag, it identifies the stored 
866  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
867  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
868</t>
869<t>
870  If the status code is 206 (partial content), both the stored and new 
871  responses MUST have validators, and those validators MUST match using the strong 
872  comparison function (see Section 4 of <xref target="Part4"/>). Otherwise, the 
873  responses MUST NOT be combined.
874</t>
875<t>
876  The stored response headers are used as those of the updated response, except that
877  <list style="symbols">
878    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
879      MUST be deleted from the stored response and the updated response.</t>
880    <t>any stored Warning headers with warn-code 2xx MUST be retained in the stored
881      response and the updated response.</t>
882    <t>any headers provided in the new response MUST replace the corresponding
883      headers from the stored response.</t>
884  </list>
885</t>
886<t>
887  If a header field-name in the new response matches more than one 
888  header in the stored response, all such stored headers MUST be replaced.
889</t>
890<t>
891  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
892  stored response in cache. In the case of a 206 response, the combined 
893  entity-body MAY be stored.
894</t>
895<t>
896  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
897</t>
898</section>
899
900</section>
901
902<section anchor="header.fields" title="Header Field Definitions">
903<t>
904  This section defines the syntax and semantics of HTTP/1.1 header fields
905  related to caching.
906</t>
907<t>
908  For entity-header fields, both sender and recipient refer to either the client or the
909  server, depending on who sends and who receives the entity.
910</t>
911
912<section anchor="header.age" title="Age">
913  <iref item="Age header" primary="true"/>
914  <iref item="Headers" primary="true" subitem="Age"/>
915 
916 
917 
918<t>
919  The "Age" response-header field conveys the sender's estimate of the amount
920  of time since the response was generated or successfully validated at the
921  origin server. Age values are calculated as specified in
922  <xref target="age.calculations"/>.
923</t>
924<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/><artwork type="abnf2616"><![CDATA[
925  Age   = "Age" ":" OWS Age-v
926  Age-v = delta-seconds
927]]></artwork></figure>
928<t anchor="rule.delta-seconds">
929 
930  Age field-values are non-negative integers, representing time in seconds.
931</t>
932<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
933  delta-seconds  = 1*DIGIT
934]]></artwork></figure>
935<t>
936  If a cache receives a value larger than the largest positive integer it can represent, or
937  if any of its age calculations overflows, it MUST transmit an Age header with a
938  field-value of 2147483648 (2^31). Caches SHOULD use an arithmetic type
939  of at least 31 bits of range.
940</t>
941<t>
942  The presence of an Age header field in a response implies that a response is not
943  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
944  Age header field.
945</t>
946</section>
947
948<section anchor="header.cache-control" title="Cache-Control">
949  <iref item="Cache-Control header" primary="true"/>
950  <iref item="Headers" primary="true" subitem="Cache-Control"/>
951 
952 
953 
954 
955 
956 
957<t>
958  The "Cache-Control" general-header field is used to specify directives for
959  caches along the request/response chain. Such cache directives are
960  unidirectional in that the presence of a directive in a request does not
961  imply that the same directive is to be given in the response.
962</t>
963<t>
964  HTTP/1.1 caches MUST obey the requirements of the Cache-Control directives
965  defined in this section. See <xref target="cache.control.extensions"/> for
966  information about how Cache-Control directives defined elsewhere are handled.
967</t>
968<t><list>
969  <t>
970    Note: HTTP/1.0 caches might not implement Cache-Control and
971    might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
972  </t>
973</list></t>
974<t>
975  Cache directives MUST be passed through by a proxy or gateway application,
976  regardless of their significance to that application, since the directives might be
977  applicable to all recipients along the request/response chain. It is not possible to
978  target a directive to a specific cache.
979</t>
980<figure><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"/><artwork type="abnf2616"><![CDATA[
981  Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
982  Cache-Control-v = 1#cache-directive
983
984  cache-directive = cache-request-directive
985     / cache-response-directive
986
987  cache-extension = token [ "=" ( token / quoted-string ) ]
988]]></artwork></figure>
989
990<section anchor="cache-request-directive" title="Request Cache-Control Directives">
991 
992
993<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
994  cache-request-directive =
995       "no-cache"
996     / "no-store"
997     / "max-age" "=" delta-seconds
998     / "max-stale" [ "=" delta-seconds ]
999     / "min-fresh" "=" delta-seconds
1000     / "no-transform"
1001     / "only-if-cached"
1002     / cache-extension
1003]]></artwork></figure>
1004
1005<t>
1006  <?rfc needLines="4"?>no-cache
1007  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1008  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1009  <list>
1010    <t>The no-cache request directive indicates that a stored response MUST NOT be
1011      used to satisfy the request without successful validation on the origin server.</t>
1012  </list>
1013</t>
1014<t>
1015  <?rfc needLines="4"?>no-store
1016  <iref item="Cache Directives" primary="true" subitem="no-store"/>
1017  <iref item="no-store" primary="true" subitem="Cache Directive"/>
1018  <list>
1019    <t>The no-store request directive indicates that a cache MUST NOT store any part
1020      of either this request or any response to it. This directive applies to both
1021      non-shared and shared caches. "MUST NOT store" in this context means that the
1022      cache MUST NOT intentionally store the information in non-volatile storage,
1023      and MUST make a best-effort attempt to remove the information from volatile
1024      storage as promptly as possible after forwarding it.</t>
1025    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1026      particular, malicious or compromised caches might not recognize or obey this
1027      directive, and communications networks may be vulnerable to eavesdropping.</t>
1028  </list>
1029</t>
1030<t>
1031  <?rfc needLines="4"?>max-age
1032  <iref item="Cache Directives" primary="true" subitem="max-age"/>
1033  <iref item="max-age" primary="true" subitem="Cache Directive"/>
1034  <list>
1035    <t>The max-age request directive indicates that the client is willing to accept a
1036      response whose age is no greater than the specified time in seconds. Unless
1037      the max-stale request directive is also present, the client is not willing to accept a stale
1038      response.</t>
1039  </list>
1040</t>
1041<t>
1042  <?rfc needLines="4"?>max-stale
1043  <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1044  <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1045  <list>
1046    <t>The max-stale request directive indicates that the client is willing to accept a
1047      response that has exceeded its expiration time. If max-stale is assigned a value,
1048      then the client is willing to accept a response that has exceeded its expiration
1049      time by no more than the specified number of seconds. If no value is assigned to
1050      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1051  </list>
1052</t>
1053<t>
1054  <?rfc needLines="4"?>min-fresh
1055  <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1056  <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1057  <list>
1058    <t>The min-fresh request directive indicates that the client is willing to accept a
1059      response whose freshness lifetime is no less than its current age plus the specified
1060      time in seconds. That is, the client wants a response that will still be fresh for
1061      at least the specified number of seconds.</t>
1062  </list>
1063</t>
1064<t>
1065  <?rfc needLines="4"?>no-transform
1066  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1067  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1068  <list>
1069    <t>The no-transform request directive indicates that an intermediate cache or proxy
1070      MUST NOT change the Content-Encoding, Content-Range or Content-Type request
1071      headers, nor the request entity-body.</t>
1072  </list>
1073</t>
1074<t>
1075  <?rfc needLines="4"?>only-if-cached
1076  <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1077  <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1078  <list>
1079    <t>The only-if-cached request directive indicates that the client only wishes to
1080      return a stored response. If it receives this directive, a cache SHOULD either
1081      respond using a stored response that is consistent with the other constraints of the
1082      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1083      being operated as a unified system with good internal connectivity, such a request
1084      MAY be forwarded within that group of caches.</t>
1085  </list>
1086</t>
1087</section>
1088
1089<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1090 
1091
1092<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1093  cache-response-directive =
1094       "public"
1095     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1096     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1097     / "no-store"
1098     / "no-transform"
1099     / "must-revalidate"
1100     / "proxy-revalidate"
1101     / "max-age" "=" delta-seconds
1102     / "s-maxage" "=" delta-seconds
1103     / cache-extension
1104]]></artwork></figure>
1105
1106<t>
1107  <?rfc needLines="4"?>public
1108  <iref item="Cache Directives" primary="true" subitem="public"/>
1109  <iref item="public" primary="true" subitem="Cache Directive"/>
1110  <list>
1111    <t>The public response directive indicates that the response MAY be cached, even
1112      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1113      (See also Authorization, Section 3.1 of <xref target="Part7"/>, for additional details.) </t>
1114  </list>
1115</t>
1116<t>
1117  <?rfc needLines="4"?>private
1118  <iref item="Cache Directives" primary="true" subitem="private"/>
1119  <iref item="private" primary="true" subitem="Cache Directive"/>
1120  <list>
1121    <t>The private response directive indicates that the response message is intended for
1122      a single user and MUST NOT be stored by a shared cache. A private (non-shared)
1123      cache MAY store the response.</t>
1124    <t>If the private response directive specifies one or more field-names, this
1125      requirement is limited to the field-values associated with the listed response
1126      headers. That is, the specified field-names(s) MUST NOT be stored by a shared
1127      cache, whereas the remainder of the response message MAY be.</t>
1128    <t>
1129      Note: This usage of the word private only controls where the response may
1130      be stored, and cannot ensure the privacy of the message content.
1131      Also, private response directives with field-names are often handled by
1132      implementations as if an unqualified private directive was received; i.e.,
1133      the special handling for the qualified form is not widely implemented.</t>
1134  </list>
1135</t>
1136<t>
1137  <?rfc needLines="4"?>no-cache
1138  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1139  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1140  <list>
1141    <t>The no-cache response directive indicates that the response MUST NOT be used to
1142      satisfy a subsequent request without successful validation on the origin server.
1143      This allows an origin server to prevent caching even by caches that have been
1144      configured to return stale responses.</t>
1145    <t>If the no-cache response directive specifies one or more field-names, this
1146      requirement is limited to the field-values associated with the listed response
1147      headers. That is, the specified field-name(s) MUST NOT be sent in the response
1148      to a subsequent request without successful validation on the origin server. This
1149      allows an origin server to prevent the re-use of certain header fields in a
1150      response, while still allowing caching of the rest of the response.</t>
1151    <t>
1152      Note: Most HTTP/1.0 caches will not recognize or obey this directive.
1153      Also, no-cache response directives with field-names are often handled by
1154      implementations as if an unqualified no-cache directive was received; i.e.,
1155      the special handling for the qualified form is not widely implemented.
1156    </t>
1157  </list>
1158</t>
1159<t>
1160  <?rfc needLines="4"?>no-store
1161  <iref item="Cache Directives" primary="true" subitem="no-store"/>
1162  <iref item="no-store" primary="true" subitem="Cache Directive"/>
1163  <list>
1164    <t>The no-store response directive indicates that a cache MUST NOT store any
1165      part of either the immediate request or response. This directive applies to both
1166      non-shared and shared caches. "MUST NOT store" in this context means that the
1167      cache MUST NOT intentionally store the information in non-volatile storage,
1168      and MUST make a best-effort attempt to remove the information from volatile
1169      storage as promptly as possible after forwarding it.</t>
1170    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1171      particular, malicious or compromised caches might not recognize or obey this
1172      directive, and communications networks may be vulnerable to eavesdropping.</t>
1173  </list>
1174</t>
1175<t>
1176  <?rfc needLines="4"?>must-revalidate
1177  <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1178  <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1179  <list>
1180    <t>The must-revalidate response directive indicates that once it has become stale, the response MUST NOT be
1181     used to satisfy subsequent requests without successful validation on the origin server.</t>
1182    <t>The must-revalidate directive is necessary to support reliable operation for
1183      certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey
1184      the must-revalidate directive; in particular, if the cache cannot reach the origin
1185      server for any reason, it MUST generate a 504 (Gateway Timeout) response.</t>
1186    <t>Servers SHOULD send the must-revalidate directive if and only if failure to
1187      validate a request on the entity could result in incorrect operation, such as a
1188      silently unexecuted financial transaction.</t>
1189  </list>
1190</t>
1191<t>
1192  <?rfc needLines="4"?>proxy-revalidate
1193  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1194  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1195  <list>
1196    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1197      response directive, except that it does not apply to non-shared caches.</t>
1198  </list>
1199</t>
1200<t>
1201  <?rfc needLines="4"?>max-age
1202  <iref item="Cache Directives" primary="true" subitem="max-age"/>
1203  <iref item="max-age" primary="true" subitem="Cache Directive"/>
1204  <list>
1205    <t>The max-age response directive indicates that response is to be considered stale
1206      after its age is greater than the specified number of seconds.</t>
1207  </list>
1208</t>
1209<t>
1210  <?rfc needLines="4"?>s-maxage
1211  <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1212  <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1213  <list>
1214    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1215      specified by this directive overrides the maximum age specified by either the
1216      max-age directive or the Expires header. The s-maxage directive also implies the
1217      semantics of the proxy-revalidate response directive.</t>
1218  </list>
1219</t>
1220<t>
1221  <?rfc needLines="4"?>no-transform
1222  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1223  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1224  <list>
1225    <t>The no-transform response directive indicates that an intermediate cache or proxy
1226      MUST NOT change the Content-Encoding, Content-Range or Content-Type response
1227      headers, nor the response entity-body.</t>
1228  </list>
1229</t>
1230
1231</section>
1232
1233<section anchor="cache.control.extensions" title="Cache Control Extensions">
1234<t>
1235  The Cache-Control header field can be extended through the use of one or more
1236  cache-extension tokens, each with an optional value. Informational extensions (those
1237  that do not require a change in cache behavior) can be added without changing the
1238  semantics of other directives. Behavioral extensions are designed to work by acting as
1239  modifiers to the existing base of cache directives. Both the new directive and the
1240  standard directive are supplied, such that applications that do not understand the new
1241  directive will default to the behavior specified by the standard directive, and those
1242  that understand the new directive will recognize it as modifying the requirements
1243  associated with the standard directive. In this way, extensions to the cache-control
1244  directives can be made without requiring changes to the base protocol.
1245</t>
1246<t>
1247  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1248  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1249  all directives that it does not understand.
1250</t>
1251<t>
1252  For example, consider a hypothetical new response directive called "community" that
1253  acts as a modifier to the private directive. We define this new directive to mean that,
1254  in addition to any non-shared cache, any cache that is shared only by members of the
1255  community named within its value may cache the response. An origin server wishing to
1256  allow the UCI community to use an otherwise private response in their shared cache(s)
1257  could do so by including
1258</t>
1259<figure><artwork type="example"><![CDATA[
1260  Cache-Control: private, community="UCI"
1261]]></artwork></figure>
1262<t>
1263  A cache seeing this header field will act correctly even if the cache does not
1264  understand the community cache-extension, since it will also see and understand the
1265  private directive and thus default to the safe behavior.
1266</t>
1267<t>
1268  Unrecognized cache directives MUST be ignored; it is assumed that any cache
1269  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1270  directives (or the response's default cacheability) such that the cache behavior will
1271  remain minimally correct even if the cache does not understand the extension(s).
1272</t>
1273<t>
1274  The HTTP Cache Directive Registry defines the name space for the cache
1275  directives.
1276</t>
1277<t>
1278  Registrations MUST include the following fields:
1279  <list style="symbols">
1280    <t>Cache Directive Name</t>
1281    <t>Pointer to specification text</t>
1282  </list>
1283</t>
1284<t>
1285  Values to be added to this name space are subject to IETF review
1286  (<xref target="RFC5226"/>, Section 4.1).
1287</t>
1288<t>
1289  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1290</t>
1291</section>
1292
1293</section>
1294
1295<section anchor="header.expires" title="Expires">
1296  <iref item="Expires header" primary="true"/>
1297  <iref item="Headers" primary="true" subitem="Expires"/>
1298 
1299 
1300<t>
1301  The "Expires" entity-header field gives the date/time after which the response is
1302  considered stale. See <xref target="expiration.model"/> for further discussion of the
1303  freshness model.
1304</t>
1305<t>
1306  The presence of an Expires field does not imply that the original resource will change or
1307  cease to exist at, before, or after that time.
1308</t>
1309<t>
1310  The field-value is an absolute date and time as defined by HTTP-date in Section 6.1 of <xref target="Part1"/>;
1311  it MUST be sent in rfc1123-date format.
1312</t>
1313<figure><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/><artwork type="abnf2616"><![CDATA[
1314  Expires   = "Expires" ":" OWS Expires-v
1315  Expires-v = HTTP-date
1316]]></artwork></figure>
1317<figure>
1318  <preamble>For example</preamble>
1319<artwork type="example"><![CDATA[
1320  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1321]]></artwork></figure>
1322<t><list>
1323  <t>
1324    Note: If a response includes a Cache-Control field with the max-age
1325    directive (see <xref target="cache-response-directive"/>), that directive overrides
1326    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1327  </t>
1328</list></t>
1329<t>
1330  HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.
1331</t>
1332<t>
1333  HTTP/1.1 clients and caches MUST treat other invalid date formats, especially
1334  including the value "0", as in the past (i.e., "already expired").
1335</t>
1336</section>
1337
1338<section anchor="header.pragma" title="Pragma">
1339  <iref item="Pragma header" primary="true"/>
1340  <iref item="Headers" primary="true" subitem="Pragma"/>
1341 
1342 
1343 
1344 
1345<t>
1346  The "Pragma" general-header field is used to include implementation-specific directives
1347  that might apply to any recipient along the request/response chain. All pragma directives
1348  specify optional behavior from the viewpoint of the protocol; however, some systems
1349  MAY require that behavior be consistent with the directives.
1350</t>
1351<figure><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"/><artwork type="abnf2616"><![CDATA[
1352  Pragma            = "Pragma" ":" OWS Pragma-v
1353  Pragma-v          = 1#pragma-directive
1354  pragma-directive  = "no-cache" / extension-pragma
1355  extension-pragma  = token [ "=" ( token / quoted-string ) ]
1356]]></artwork></figure>
1357<t>
1358  When the no-cache directive is present in a request message, an application SHOULD
1359  forward the request toward the origin server even if it has a cached copy of what is being
1360  requested. This pragma directive has the same semantics as the no-cache response directive
1361  (see <xref target="cache-response-directive"/>) and is defined here for backward
1362  compatibility with HTTP/1.0. Clients SHOULD include both header fields when a
1363  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1364  SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1365</t>
1366<t><list>
1367  <t>
1368    Note: Because the meaning of "Pragma: no-cache" as a response-header field
1369    is not actually specified, it does not provide a reliable replacement for
1370    "Cache-Control: no-cache" in a response.
1371  </t>
1372</list></t>
1373<t>
1374  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1375</t>
1376</section>
1377
1378<section anchor="header.vary" title="Vary">
1379  <iref item="Vary header" primary="true"/>
1380  <iref item="Headers" primary="true" subitem="Vary"/>
1381 
1382 
1383<t>
1384  The "Vary" response-header field conveys the set of request-header fields
1385  that were used to select the representation.
1386</t>
1387<t>
1388  Caches use this information, in part, to determine whether a stored response
1389  can be used to satisfy a given request; see
1390  <xref target="caching.negotiated.responses"/>.
1391  determines, while the response is fresh, whether a cache is permitted to use the
1392  response to reply to a subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1393</t>
1394<t>
1395  In uncacheable or stale responses, the Vary field value advises the user agent about
1396  the criteria that were used to select the representation.
1397</t>
1398<figure><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/><artwork type="abnf2616"><![CDATA[
1399  Vary   = "Vary" ":" OWS Vary-v
1400  Vary-v = "*" / 1#field-name
1401]]></artwork></figure>
1402<t>
1403  The set of header fields named by the Vary field value is known as the selecting
1404  request-headers.
1405</t>
1406<t>
1407  Servers SHOULD include a Vary header field with any cacheable response that is
1408  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1409  requests on that resource and informs the user agent about the presence of negotiation on
1410  that resource. A server MAY include a Vary header field with a non-cacheable
1411  response that is subject to server-driven negotiation, since this might provide the user
1412  agent with useful information about the dimensions over which the response varies at the
1413  time of the response.
1414</t>
1415<t>
1416  A Vary field value of "*" signals that unspecified parameters not limited to the
1417  request-headers (e.g., the network address of the client), play a role in the selection of
1418  the response representation; therefore, a cache cannot determine whether this response is
1419  appropriate. The "*" value MUST NOT be generated by a proxy server;
1420  it may only be generated by an origin server.
1421</t>
1422<t>
1423  The field-names given are not limited to the set of standard request-header fields
1424  defined by this specification. Field names are case-insensitive.
1425</t>
1426</section>
1427
1428<section anchor="header.warning" title="Warning">
1429  <iref item="Warning header" primary="true"/>
1430  <iref item="Headers" primary="true" subitem="Warning"/>
1431 
1432 
1433 
1434 
1435 
1436 
1437 
1438<t>
1439  The "Warning" general-header field is used to carry additional information about the status
1440  or transformation of a message that might not be reflected in the message. This
1441  information is typically used to warn about possible incorrectness introduced by caching
1442  operations or transformations applied to the entity body of the message.
1443</t>
1444<t>
1445  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1446  warning, rather than an error status code, distinguishes these responses from true failures.
1447</t>
1448<t>
1449  Warning headers can in general be applied to any message, however some warn-codes are
1450  specific to caches and can only be applied to response messages.
1451</t>
1452<figure><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"/><artwork type="abnf2616"><![CDATA[
1453  Warning    = "Warning" ":" OWS Warning-v
1454  Warning-v  = 1#warning-value
1455 
1456  warning-value = warn-code SP warn-agent SP warn-text
1457                                        [SP warn-date]
1458 
1459  warn-code  = 3DIGIT
1460  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1461                  ; the name or pseudonym of the server adding
1462                  ; the Warning header, for use in debugging
1463  warn-text  = quoted-string
1464  warn-date  = DQUOTE HTTP-date DQUOTE
1465]]></artwork></figure>
1466<t>
1467  Multiple warnings can be attached to a response (either by the origin server or by
1468  a cache), including multiple warnings with the same code number, only differing
1469  in warn-text.
1470</t>
1471<t>
1472  When this occurs, the user agent SHOULD inform the user of as many of them as
1473  possible, in the order that they appear in the response.
1474</t>
1475<t>
1476  Systems that generate multiple Warning headers SHOULD order them with this user
1477  agent behavior in mind. New Warning headers SHOULD be added after any existing
1478  Warning headers.
1479</t>
1480<t>
1481  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1482  Warning is required to be deleted from a stored response after validation:
1483  <list style="symbols">
1484    <t>1xx Warnings describe the freshness or validation status of the response, and so
1485      MUST be deleted by caches after validation. They can only be generated by a cache
1486      when validating a cached entry, and MUST NOT be generated in any other situation.</t>
1487    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1488      not rectified by a validation (for example, a lossy compression of the entity bodies)
1489      and MUST NOT be deleted by caches after validation, unless a full response is
1490      returned, in which case they MUST be.</t>
1491  </list>
1492</t>
1493<t>
1494  If an implementation sends a message with one or more Warning headers to a receiver whose
1495  version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a
1496  warn-date that matches the Date header in the message.
1497</t>
1498<t>
1499  If an implementation receives a message with a warning-value that includes a warn-date,
1500  and that warn-date is different from the Date value in the response, then that
1501  warning-value MUST be deleted from the message before storing, forwarding, or using
1502  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1503  warning-values are deleted for this reason, the Warning header MUST be deleted as
1504  well.
1505</t>
1506<t>
1507  The following warn-codes are defined by this specification, each with a recommended
1508  warn-text in English, and a description of its meaning.
1509</t>
1510<t>
1511<?rfc needLines="4"?>
1512  110 Response is stale
1513  <list>
1514    <t>SHOULD be included whenever the returned response is stale.</t>
1515  </list>
1516</t>
1517<t>
1518<?rfc needLines="4"?>
1519  111 Revalidation failed
1520  <list>
1521    <t>SHOULD be included if a cache returns a stale response because an attempt to
1522      validate the response failed, due to an inability to reach the server.</t>
1523  </list>
1524</t>
1525<t>
1526<?rfc needLines="4"?>
1527  112 Disconnected operation
1528  <list>
1529    <t>SHOULD be included if the cache is intentionally disconnected from the rest of
1530      the network for a period of time.</t>
1531  </list>
1532</t>
1533<t>
1534<?rfc needLines="4"?>
1535  113 Heuristic expiration
1536  <list>
1537    <t>SHOULD be included if the cache heuristically chose a freshness lifetime
1538      greater than 24 hours and the response's age is greater than 24 hours.</t>
1539  </list>
1540</t>
1541<t>
1542<?rfc needLines="4"?>
1543  199 Miscellaneous warning
1544  <list>
1545    <t>The warning text can include arbitrary information to be presented to a human
1546      user, or logged. A system receiving this warning MUST NOT take any automated
1547      action, besides presenting the warning to the user.</t>
1548  </list>
1549</t>
1550<t>
1551<?rfc needLines="4"?>
1552  214 Transformation applied
1553  <list>
1554    <t>MUST be added by an intermediate cache or proxy if it applies any
1555      transformation changing the content-coding (as specified in the Content-Encoding
1556      header) or media-type (as specified in the Content-Type header) of the response, or
1557      the entity-body of the response, unless this Warning code already appears in the
1558      response.</t>
1559  </list>
1560</t>
1561<t>
1562<?rfc needLines="4"?>
1563  299 Miscellaneous persistent warning
1564  <list>
1565    <t>The warning text can include arbitrary information to be presented to a human
1566      user, or logged. A system receiving this warning MUST NOT take any automated
1567      action.</t>
1568  </list>
1569</t>
1570</section>
1571
1572</section>
1573
1574<section anchor="history.lists" title="History Lists">
1575<t>
1576  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1577  can be used to redisplay an entity retrieved earlier in a session.
1578</t>
1579<t>
1580  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1581  a history mechanism can display a previous representation even if it has expired.
1582</t>
1583  <t>
1584  This does not prohibit the history mechanism from telling the user that a
1585  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1586  </t>
1587</section>
1588
1589
1590<section anchor="IANA.considerations" title="IANA Considerations">
1591
1592<section title="Cache Directive Registry" anchor="cache.directive.registration">
1593<t>
1594  The registration procedure for HTTP Cache Directives is defined by
1595  <xref target="cache.control.extensions"/> of this document.
1596</t>
1597<t>
1598   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1599   and be populated with the registrations below:
1600</t>
1601
1602<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1603<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1604   <ttcol>Cache Directive</ttcol>
1605   <ttcol>Reference</ttcol>
1606
1607   <c>max-age</c>
1608   <c>
1609      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1610   </c>
1611   <c>max-stale</c>
1612   <c>
1613      <xref target="cache-request-directive"/>
1614   </c>
1615   <c>min-fresh</c>
1616   <c>
1617      <xref target="cache-request-directive"/>
1618   </c>
1619   <c>must-revalidate</c>
1620   <c>
1621      <xref target="cache-response-directive"/>
1622   </c>
1623   <c>no-cache</c>
1624   <c>
1625      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1626   </c>
1627   <c>no-store</c>
1628   <c>
1629      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1630   </c>
1631   <c>no-transform</c>
1632   <c>
1633      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1634   </c>
1635   <c>only-if-cached</c>
1636   <c>
1637      <xref target="cache-request-directive"/>
1638   </c>
1639   <c>private</c>
1640   <c>
1641      <xref target="cache-response-directive"/>
1642   </c>
1643   <c>proxy-revalidate</c>
1644   <c>
1645      <xref target="cache-response-directive"/>
1646   </c>
1647   <c>public</c>
1648   <c>
1649      <xref target="cache-response-directive"/>
1650   </c>
1651   <c>s-maxage</c>
1652   <c>
1653      <xref target="cache-response-directive"/>
1654   </c>
1655   <c>stale-if-error</c>
1656   <c>
1657      <xref target="RFC5861"/>, Section 4
1658   </c>
1659   <c>stale-while-revalidate</c>
1660   <c>
1661      <xref target="RFC5861"/>, Section 3
1662   </c>
1663</texttable>
1664<!--(END)-->
1665
1666</section>
1667
1668<section anchor="message.header.registration" title="Message Header Registration">
1669<t>
1670  The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1671  should be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1672</t>
1673
1674<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1675<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1676   <ttcol>Header Field Name</ttcol>
1677   <ttcol>Protocol</ttcol>
1678   <ttcol>Status</ttcol>
1679   <ttcol>Reference</ttcol>
1680
1681   <c>Age</c>
1682   <c>http</c>
1683   <c>standard</c>
1684   <c>
1685      <xref target="header.age"/>
1686   </c>
1687   <c>Cache-Control</c>
1688   <c>http</c>
1689   <c>standard</c>
1690   <c>
1691      <xref target="header.cache-control"/>
1692   </c>
1693   <c>Expires</c>
1694   <c>http</c>
1695   <c>standard</c>
1696   <c>
1697      <xref target="header.expires"/>
1698   </c>
1699   <c>Pragma</c>
1700   <c>http</c>
1701   <c>standard</c>
1702   <c>
1703      <xref target="header.pragma"/>
1704   </c>
1705   <c>Vary</c>
1706   <c>http</c>
1707   <c>standard</c>
1708   <c>
1709      <xref target="header.vary"/>
1710   </c>
1711   <c>Warning</c>
1712   <c>http</c>
1713   <c>standard</c>
1714   <c>
1715      <xref target="header.warning"/>
1716   </c>
1717</texttable>
1718<!--(END)-->
1719
1720<t>
1721  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1722</t>
1723</section>
1724
1725</section>
1726
1727<section anchor="security.considerations" title="Security Considerations">
1728<t>
1729  Caches expose additional potential vulnerabilities, since the contents of the cache
1730  represent an attractive target for malicious exploitation. Because cache contents persist
1731  after an HTTP request is complete, an attack on the cache can reveal information long after
1732  a user believes that the information has been removed from the network. Therefore, cache
1733  contents should be protected as sensitive information.
1734</t>
1735</section>
1736
1737<section anchor="ack" title="Acknowledgments">
1738<t>
1739  Much of the content and presentation of the caching design is due to suggestions and
1740  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1741  and Larry Masinter.
1742</t>
1743</section>
1744
1745</middle>
1746
1747<back>
1748<references title="Normative References">
1749
1750  <reference anchor="Part1">
1751    <front>
1752      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1753      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1754        <organization abbrev="Day Software">Day Software</organization>
1755        <address><email>fielding@gbiv.com</email></address>
1756      </author>
1757      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1758        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1759        <address><email>jg@freedesktop.org</email></address>
1760      </author>
1761      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1762        <organization abbrev="HP">Hewlett-Packard Company</organization>
1763        <address><email>JeffMogul@acm.org</email></address>
1764      </author>
1765      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1766        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1767        <address><email>henrikn@microsoft.com</email></address>
1768      </author>
1769      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1770        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1771        <address><email>LMM@acm.org</email></address>
1772      </author>
1773      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1774        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1775        <address><email>paulle@microsoft.com</email></address>
1776      </author>
1777      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1778        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1779        <address><email>timbl@w3.org</email></address>
1780      </author>
1781      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1782        <organization abbrev="W3C">World Wide Web Consortium</organization>
1783        <address><email>ylafon@w3.org</email></address>
1784      </author>
1785      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1786        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1787        <address><email>julian.reschke@greenbytes.de</email></address>
1788      </author>
1789      <date month="July" year="2010"/>
1790    </front>
1791    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-10"/>
1792   
1793  </reference>
1794
1795  <reference anchor="Part2">
1796    <front>
1797      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1798      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1799        <organization abbrev="Day Software">Day Software</organization>
1800        <address><email>fielding@gbiv.com</email></address>
1801      </author>
1802      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1803        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1804        <address><email>jg@freedesktop.org</email></address>
1805      </author>
1806      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1807        <organization abbrev="HP">Hewlett-Packard Company</organization>
1808        <address><email>JeffMogul@acm.org</email></address>
1809      </author>
1810      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1811        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1812        <address><email>henrikn@microsoft.com</email></address>
1813      </author>
1814      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1815        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1816        <address><email>LMM@acm.org</email></address>
1817      </author>
1818      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1819        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1820        <address><email>paulle@microsoft.com</email></address>
1821      </author>
1822      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1823        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1824        <address><email>timbl@w3.org</email></address>
1825      </author>
1826      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1827        <organization abbrev="W3C">World Wide Web Consortium</organization>
1828        <address><email>ylafon@w3.org</email></address>
1829      </author>
1830      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1831        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1832        <address><email>julian.reschke@greenbytes.de</email></address>
1833      </author>
1834      <date month="July" year="2010"/>
1835    </front>
1836    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-10"/>
1837   
1838  </reference>
1839
1840  <reference anchor="Part4">
1841    <front>
1842      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1843      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1844        <organization abbrev="Day Software">Day Software</organization>
1845        <address><email>fielding@gbiv.com</email></address>
1846      </author>
1847      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1848        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1849        <address><email>jg@freedesktop.org</email></address>
1850      </author>
1851      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1852        <organization abbrev="HP">Hewlett-Packard Company</organization>
1853        <address><email>JeffMogul@acm.org</email></address>
1854      </author>
1855      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1856        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1857        <address><email>henrikn@microsoft.com</email></address>
1858      </author>
1859      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1860        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1861        <address><email>LMM@acm.org</email></address>
1862      </author>
1863      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1864        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1865        <address><email>paulle@microsoft.com</email></address>
1866      </author>
1867      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1868        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1869        <address><email>timbl@w3.org</email></address>
1870      </author>
1871      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1872        <organization abbrev="W3C">World Wide Web Consortium</organization>
1873        <address><email>ylafon@w3.org</email></address>
1874      </author>
1875      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1876        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1877        <address><email>julian.reschke@greenbytes.de</email></address>
1878      </author>
1879      <date month="July" year="2010"/>
1880    </front>
1881    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-10"/>
1882   
1883  </reference>
1884
1885  <reference anchor="Part5">
1886    <front>
1887      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1888      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1889        <organization abbrev="Day Software">Day Software</organization>
1890        <address><email>fielding@gbiv.com</email></address>
1891      </author>
1892      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1893        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1894        <address><email>jg@freedesktop.org</email></address>
1895      </author>
1896      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1897        <organization abbrev="HP">Hewlett-Packard Company</organization>
1898        <address><email>JeffMogul@acm.org</email></address>
1899      </author>
1900      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1901        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1902        <address><email>henrikn@microsoft.com</email></address>
1903      </author>
1904      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1905        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1906        <address><email>LMM@acm.org</email></address>
1907      </author>
1908      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1909        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1910        <address><email>paulle@microsoft.com</email></address>
1911      </author>
1912      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1913        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1914        <address><email>timbl@w3.org</email></address>
1915      </author>
1916      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1917        <organization abbrev="W3C">World Wide Web Consortium</organization>
1918        <address><email>ylafon@w3.org</email></address>
1919      </author>
1920      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1921        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1922        <address><email>julian.reschke@greenbytes.de</email></address>
1923      </author>
1924      <date month="July" year="2010"/>
1925    </front>
1926    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-10"/>
1927   
1928  </reference>
1929
1930  <reference anchor="Part7">
1931    <front>
1932      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1933      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1934        <organization abbrev="Day Software">Day Software</organization>
1935        <address><email>fielding@gbiv.com</email></address>
1936      </author>
1937      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1938        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1939        <address><email>jg@freedesktop.org</email></address>
1940      </author>
1941      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1942        <organization abbrev="HP">Hewlett-Packard Company</organization>
1943        <address><email>JeffMogul@acm.org</email></address>
1944      </author>
1945      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1946        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1947        <address><email>henrikn@microsoft.com</email></address>
1948      </author>
1949      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1950        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1951        <address><email>LMM@acm.org</email></address>
1952      </author>
1953      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1954        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1955        <address><email>paulle@microsoft.com</email></address>
1956      </author>
1957      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1958        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1959        <address><email>timbl@w3.org</email></address>
1960      </author>
1961      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1962        <organization abbrev="W3C">World Wide Web Consortium</organization>
1963        <address><email>ylafon@w3.org</email></address>
1964      </author>
1965      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1966        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1967        <address><email>julian.reschke@greenbytes.de</email></address>
1968      </author>
1969      <date month="July" year="2010"/>
1970    </front>
1971    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-10"/>
1972   
1973  </reference>
1974
1975  <reference anchor="RFC2119">
1976    <front>
1977      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1978      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1979        <organization>Harvard University</organization>
1980        <address><email>sob@harvard.edu</email></address>
1981      </author>
1982      <date month="March" year="1997"/>
1983    </front>
1984    <seriesInfo name="BCP" value="14"/>
1985    <seriesInfo name="RFC" value="2119"/>
1986  </reference>
1987
1988  <reference anchor="RFC5234">
1989    <front>
1990      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1991      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1992        <organization>Brandenburg InternetWorking</organization>
1993        <address>
1994          <email>dcrocker@bbiw.net</email>
1995        </address> 
1996      </author>
1997      <author initials="P." surname="Overell" fullname="Paul Overell">
1998        <organization>THUS plc.</organization>
1999        <address>
2000          <email>paul.overell@thus.net</email>
2001        </address>
2002      </author>
2003      <date month="January" year="2008"/>
2004    </front>
2005    <seriesInfo name="STD" value="68"/>
2006    <seriesInfo name="RFC" value="5234"/>
2007  </reference>
2008 
2009</references>
2010
2011<references title="Informative References">
2012
2013  <reference anchor="RFC1305">
2014    <front>
2015      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2016      <author fullname="David L. Mills" initials="D." surname="Mills">
2017        <organization>University of Delaware, Electrical Engineering Department</organization>
2018        <address><email>mills@udel.edu</email></address>
2019      </author>
2020      <date month="March" year="1992"/>
2021    </front>
2022    <seriesInfo name="RFC" value="1305"/>
2023  </reference>
2024
2025  <reference anchor="RFC2616">
2026    <front>
2027      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2028      <author fullname="R. Fielding" initials="R." surname="Fielding">
2029        <organization>University of California, Irvine</organization>
2030        <address><email>fielding@ics.uci.edu</email></address>
2031      </author>
2032      <author fullname="J. Gettys" initials="J." surname="Gettys">
2033        <organization>W3C</organization>
2034        <address><email>jg@w3.org</email></address>
2035      </author>
2036      <author fullname="J. Mogul" initials="J." surname="Mogul">
2037        <organization>Compaq Computer Corporation</organization>
2038        <address><email>mogul@wrl.dec.com</email></address>
2039      </author>
2040      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2041        <organization>MIT Laboratory for Computer Science</organization>
2042        <address><email>frystyk@w3.org</email></address>
2043      </author>
2044      <author fullname="L. Masinter" initials="L." surname="Masinter">
2045        <organization>Xerox Corporation</organization>
2046        <address><email>masinter@parc.xerox.com</email></address>
2047      </author>
2048      <author fullname="P. Leach" initials="P." surname="Leach">
2049        <organization>Microsoft Corporation</organization>
2050        <address><email>paulle@microsoft.com</email></address>
2051      </author>
2052      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2053        <organization>W3C</organization>
2054        <address><email>timbl@w3.org</email></address>
2055      </author>
2056      <date month="June" year="1999"/>
2057    </front>
2058    <seriesInfo name="RFC" value="2616"/>
2059  </reference>
2060
2061  <reference anchor="RFC3864">
2062    <front>
2063      <title>Registration Procedures for Message Header Fields</title>
2064      <author fullname="G. Klyne" initials="G." surname="Klyne">
2065        <organization>Nine by Nine</organization>
2066        <address><email>GK-IETF@ninebynine.org</email></address>
2067      </author>
2068      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2069        <organization>BEA Systems</organization>
2070        <address><email>mnot@pobox.com</email></address>
2071      </author>
2072      <author fullname="J. Mogul" initials="J." surname="Mogul">
2073        <organization>HP Labs</organization>
2074        <address><email>JeffMogul@acm.org</email></address>
2075      </author>
2076      <date month="September" year="2004"/>
2077    </front>
2078    <seriesInfo name="BCP" value="90"/>
2079    <seriesInfo name="RFC" value="3864"/>
2080  </reference>
2081
2082  <reference anchor="RFC5226">
2083    <front>
2084      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2085      <author initials="T." surname="Narten" fullname="T. Narten">
2086        <organization>IBM</organization>
2087        <address><email>narten@us.ibm.com</email></address>
2088      </author>
2089      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2090        <organization>Google</organization>
2091        <address><email>Harald@Alvestrand.no</email></address>
2092      </author>
2093      <date year="2008" month="May"/>
2094    </front>
2095    <seriesInfo name="BCP" value="26"/>
2096    <seriesInfo name="RFC" value="5226"/>
2097  </reference>
2098
2099  <reference anchor="RFC5861">
2100    <front>
2101      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2102      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2103        <organization>Yahoo! Inc.</organization>
2104        <address><email>mnot@yahoo-inc.com</email></address>
2105      </author>
2106      <date month="April" year="2010"/>
2107    </front>
2108    <seriesInfo name="RFC" value="5861"/>
2109  </reference>
2110
2111</references>
2112
2113<section anchor="compatibility" title="Compatibility with Previous Versions">
2114
2115<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2116<t>
2117  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2118  this missing case.
2119  (Sections <xref format="counter" target="response.cacheability"/>, <xref format="counter" target="header.cache-control"/>).
2120</t>
2121<t>
2122  Range request responses would become very verbose if all meta-data were always returned;
2123  by allowing the server to only send needed headers in a 206 response, this problem can be
2124  avoided.
2125  (<xref target="combining.headers"/>)
2126</t>
2127<t>
2128  The Cache-Control: max-age directive was not properly defined for responses.
2129  (<xref target="cache-response-directive"/>)
2130</t>
2131<t>
2132  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref format="counter" target="expiration.model"/>, <xref format="counter" target="combining.headers"/>, <xref format="counter" target="header.cache-control"/>,
2133  and <xref format="counter" target="header.warning"/>) Warning also needed to be a general
2134  header, as PUT or other methods may have need for it in requests.
2135</t>
2136</section>
2137
2138<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2139<t>
2140  Make the specified age calculation algorithm less conservative.
2141  (<xref target="age.calculations"/>)
2142</t>
2143<t>
2144  Remove requirement to consider Content-Location in successful responses
2145  in order to determine the appropriate response to use.
2146  (<xref target="validation.model"/>)
2147</t>
2148<t>
2149  Clarify denial of service attack avoidance requirement.
2150  (<xref target="invalidation.after.updates.or.deletions"/>)
2151</t>
2152<t>
2153  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2154  anymore, as these aspects never were implemented.
2155  (<xref target="header.warning"/>)
2156</t>
2157</section>
2158
2159    </section>
2160
2161
2162<section title="Collected ABNF" anchor="collected.abnf">
2163<figure>
2164<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2165Age = "Age:" OWS Age-v
2166Age-v = delta-seconds
2167
2168Cache-Control = "Cache-Control:" OWS Cache-Control-v
2169Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
2170 cache-directive ] )
2171
2172Expires = "Expires:" OWS Expires-v
2173Expires-v = HTTP-date
2174
2175HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2176
2177OWS = <OWS, defined in [Part1], Section 1.2.2>
2178
2179Pragma = "Pragma:" OWS Pragma-v
2180Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2181 pragma-directive ] )
2182
2183Vary = "Vary:" OWS Vary-v
2184Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2185 ] ) )
2186
2187Warning = "Warning:" OWS Warning-v
2188Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2189 ] )
2190
2191cache-directive = cache-request-directive / cache-response-directive
2192cache-extension = token [ "=" ( token / quoted-string ) ]
2193cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2194 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2195 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2196 cache-extension
2197cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2198 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2199 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2200 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2201 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2202 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2203
2204delta-seconds = 1*DIGIT
2205
2206extension-pragma = token [ "=" ( token / quoted-string ) ]
2207
2208field-name = <field-name, defined in [Part1], Section 3.2>
2209
2210port = <port, defined in [Part1], Section 2.6>
2211pragma-directive = "no-cache" / extension-pragma
2212pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2213
2214quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2215
2216token = <token, defined in [Part1], Section 1.2.2>
2217
2218uri-host = <uri-host, defined in [Part1], Section 2.6>
2219
2220warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2221warn-code = 3DIGIT
2222warn-date = DQUOTE HTTP-date DQUOTE
2223warn-text = quoted-string
2224warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2225 ]
2226]]></artwork>
2227</figure>
2228<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2229; Age defined but not used
2230; Cache-Control defined but not used
2231; Expires defined but not used
2232; Pragma defined but not used
2233; Vary defined but not used
2234; Warning defined but not used
2235]]></artwork></figure></section>
2236
2237
2238<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2239
2240<section title="Since RFC2616">
2241  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2242</section>
2243
2244<section title="Since draft-ietf-httpbis-p6-cache-00">
2245<t>
2246  Closed issues:
2247  <list style="symbols">
2248    <t>
2249      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2250    <t>
2251      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)</t>
2252    <t>
2253      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2254    <t>
2255      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2256    <t>
2257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2258    <t>
2259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2260    <t>
2261      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2262    <t>
2263      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2264    <t>
2265      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2266  </list>
2267</t>
2268<t>
2269  Other changes:
2270  <list style="symbols">
2271    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2272  </list>
2273</t>
2274</section>
2275
2276<section title="Since draft-ietf-httpbis-p6-cache-01">
2277<t>
2278  Closed issues:
2279  <list style="symbols">
2280    <t>
2281      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2282  </list>
2283</t>
2284<t>
2285  Other changes:
2286  <list style="symbols">
2287    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2288      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2289    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2290      specification.</t>
2291  </list>
2292</t>
2293</section>
2294
2295<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2296<t>
2297  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2298  <list style="symbols">
2299    <t>Reference RFC 3984, and update header registrations for headers defined in this
2300      document.</t>
2301  </list>
2302</t>
2303</section>
2304
2305<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2306<t>
2307  Closed issues:
2308  <list style="symbols">
2309    <t>
2310      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2311  </list>
2312</t>
2313</section>
2314
2315<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2316<t>
2317  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2318  <list style="symbols"> 
2319    <t>
2320      Use "/" instead of "|" for alternatives.
2321    </t>
2322    <t>
2323      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2324      whitespace ("OWS") and required whitespace ("RWS").
2325    </t>
2326    <t>
2327      Rewrite ABNFs to spell out whitespace rules, factor out
2328      header value format definitions.
2329    </t>
2330  </list>
2331</t>
2332</section>
2333
2334<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2335<t>
2336  This is a total rewrite of this part of the specification.
2337</t>
2338<t>
2339  Affected issues:
2340  <list style="symbols">
2341    <t>
2342      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2343    <t>
2344      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2345    <t>
2346      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2347    <t>
2348      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2349  </list>
2350</t>
2351<t>
2352  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2353  <list style="symbols"> 
2354    <t>
2355      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2356    </t>
2357  </list>
2358</t>
2359</section>
2360
2361<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2362<t>
2363  Closed issues:
2364  <list style="symbols"> 
2365    <t>
2366      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2367      "base for numeric protocol elements"
2368    </t>
2369  </list>
2370</t>
2371<t>
2372  Affected issues:
2373  <list style="symbols">
2374    <t>
2375      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2376      Vary and non-existant headers
2377    </t>
2378  </list>
2379</t>
2380</section>
2381
2382<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2383<t>
2384  Closed issues:
2385  <list style="symbols"> 
2386    <t>
2387      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2388      "Definition of 1xx Warn-Codes"
2389    </t>
2390    <t>
2391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2392      "Content-Location on 304 responses"
2393    </t>
2394    <t>
2395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2396      "private and no-cache CC directives with headers"
2397    </t>
2398    <t>
2399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2400      "RFC2047 and warn-text"
2401    </t>
2402  </list>
2403</t>
2404</section>
2405
2406<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2407<t>
2408  Closed issues:
2409  <list style="symbols"> 
2410    <t>
2411      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2412      "serving negotiated responses from cache: header-specific canonicalization"
2413    </t>
2414    <t>
2415      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2416      "Effect of CC directives on history lists"
2417    </t>
2418  </list>
2419</t>
2420<t>
2421  Affected issues:
2422  <list style="symbols">
2423    <t>
2424      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2425      Status codes and caching
2426    </t>
2427  </list>
2428</t>
2429<t>
2430  Partly resolved issues:
2431  <list style="symbols"> 
2432    <t>
2433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2434      "Placement of 13.5.1 and 13.5.2"
2435    </t>
2436  </list>
2437</t>
2438</section>
2439
2440<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2441<t>
2442  Closed issues:
2443  <list style="symbols"> 
2444    <t>
2445      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2446      "Age calculation"
2447    </t>
2448    <t>
2449      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2450      "Clarify differences between / requirements for request and response CC directives"
2451    </t>
2452        <t>
2453                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2454                "Caching authenticated responses"
2455        </t>
2456    <t>
2457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2458      "IANA registry for cache-control directives"
2459    </t>
2460    <t>
2461      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2462      "Heuristic caching of URLs with query components"
2463    </t>
2464  </list>
2465</t>
2466<t>
2467  Partly resolved issues:
2468  <list style="symbols"> 
2469    <t>
2470      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2471      "Term for the requested resource's URI"
2472    </t>
2473  </list>
2474</t>
2475</section>
2476
2477</section>
2478  </back>
2479</rfc>
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