source: draft-ietf-httpbis/07/draft-ietf-httpbis-p6-cache-07.xml @ 605

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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='rfc2629.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<!DOCTYPE rfc
17  PUBLIC "" "rfc2629.dtd">
18<rfc category="std" docName="draft-ietf-httpbis-p6-cache-07" ipr="pre5378Trust200902" obsoletes="2616">
19<front>
20
21  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
22
23  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
24    <organization abbrev="Day Software">Day Software</organization>
25    <address>
26      <postal>
27        <street>23 Corporate Plaza DR, Suite 280</street>
28        <city>Newport Beach</city>
29        <region>CA</region>
30        <code>92660</code>
31        <country>USA</country>
32      </postal>
33      <phone>+1-949-706-5300</phone>
34      <facsimile>+1-949-706-5305</facsimile>
35      <email>fielding@gbiv.com</email>
36      <uri>http://roy.gbiv.com/</uri>
37    </address>
38  </author>
39
40  <author fullname="Jim Gettys" initials="J." surname="Gettys">
41    <organization>One Laptop per Child</organization>
42    <address>
43      <postal>
44        <street>21 Oak Knoll Road</street>
45        <city>Carlisle</city>
46        <region>MA</region>
47        <code>01741</code>
48        <country>USA</country>
49      </postal>
50      <email>jg@laptop.org</email>
51      <uri>http://www.laptop.org/</uri>
52    </address>
53  </author>
54
55  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
56    <organization abbrev="HP">Hewlett-Packard Company</organization>
57    <address>
58      <postal>
59        <street>HP Labs, Large Scale Systems Group</street>
60        <street>1501 Page Mill Road, MS 1177</street>
61        <city>Palo Alto</city>
62        <region>CA</region>
63        <code>94304</code>
64        <country>USA</country>
65      </postal>
66      <email>JeffMogul@acm.org</email>
67    </address>
68  </author>
69
70  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
71    <organization abbrev="Microsoft">Microsoft Corporation</organization>
72    <address>
73      <postal>
74        <street>1 Microsoft Way</street>
75        <city>Redmond</city>
76        <region>WA</region>
77        <code>98052</code>
78        <country>USA</country>
79      </postal>
80      <email>henrikn@microsoft.com</email>
81    </address>
82  </author>
83
84  <author fullname="Larry Masinter" initials="L." surname="Masinter">
85    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
86    <address>
87      <postal>
88        <street>345 Park Ave</street>
89        <city>San Jose</city>
90        <region>CA</region>
91        <code>95110</code>
92        <country>USA</country>
93      </postal>
94      <email>LMM@acm.org</email>
95      <uri>http://larry.masinter.net/</uri>
96    </address>
97  </author>
98
99  <author fullname="Paul J. Leach" initials="P." surname="Leach">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107      </postal>
108      <email>paulle@microsoft.com</email>
109    </address>
110  </author>
111
112  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
113    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
114    <address>
115      <postal>
116        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
117        <street>The Stata Center, Building 32</street>
118        <street>32 Vassar Street</street>
119        <city>Cambridge</city>
120        <region>MA</region>
121        <code>02139</code>
122        <country>USA</country>
123      </postal>
124      <email>timbl@w3.org</email>
125      <uri>http://www.w3.org/People/Berners-Lee/</uri>
126    </address>
127  </author>
128
129  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
130    <organization abbrev="W3C">World Wide Web Consortium</organization>
131    <address>
132      <postal>
133        <street>W3C / ERCIM</street>
134        <street>2004, rte des Lucioles</street>
135        <city>Sophia-Antipolis</city>
136        <region>AM</region>
137        <code>06902</code>
138        <country>France</country>
139      </postal>
140      <email>ylafon@w3.org</email>
141      <uri>http://www.raubacapeu.net/people/yves/</uri>
142    </address>
143  </author>
144
145  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
146    <organization/>
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 day="13" month="July" year="2009"/>
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 mailing list
184  (ietf-http-wg@w3.org). The current issues list is at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/> and related documents
185  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/"/>.
186</t>
187<t>
188  The changes in this draft are summarized in <xref target="changes.since.06"/>.
189</t>
190</note>
191
192  </front>
193  <middle>
194
195<section anchor="caching" title="Introduction">
196<t>
197  HTTP is typically used for distributed information systems, where performance can be
198  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
199  caching and reusing response messages.
200</t>
201
202<section anchor="intro.purpose" title="Purpose">
203<iref item="cache"/>
204<t>
205  An HTTP cache is a local store of response messages and the subsystem that
206  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
207  in order to reduce the response time and network bandwidth consumption on future,
208  equivalent requests. Any client or server may include a cache, though a cache cannot be
209  used by a server that is acting as a tunnel.
210</t>
211<t>
212  Caching would be useless if it did not significantly improve performance. The goal of
213  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
214  some cases, a stored response can be reused without the need for a network request,
215  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
216  (see <xref target="expiration.model"/>). Even when a new request is required, it is often
217  possible to reuse all or parts of the payload of a prior response to satisfy the request,
218  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
219  purpose (see <xref target="validation.model"/>).
220</t>
221</section>
222
223<section anchor="intro.terminology" title="Terminology">
224<t>
225  This specification uses a number of terms to refer to the roles played by participants
226  in, and objects of, HTTP caching.
227</t>
228<t>
229  <iref item="cacheable"/>
230  <?rfc needLines="4"?>cacheable
231  <list>
232    <t>A response is cacheable if a cache is allowed to store a copy of the response message
233      for use in answering subsequent requests. Even when a response is cacheable, there may
234      be additional constraints on whether a cache can use the cached copy to satisfy a
235      particular request.</t>
236  </list>
237</t>
238<t>
239  <iref item="explicit expiration time"/>
240  <?rfc needLines="4"?>explicit expiration time
241  <list>
242    <t>The time at which the origin server intends that an entity should no longer be
243      returned by a cache without further validation.</t>
244  </list>
245</t>
246<t>
247  <iref item="heuristic expiration time"/>
248  <?rfc needLines="4"?>heuristic expiration time
249  <list>
250    <t>An expiration time assigned by a cache when no explicit expiration time is
251    available.</t>
252  </list>
253</t>
254<t>
255  <iref item="age"/>
256  <?rfc needLines="4"?>age
257  <list>
258    <t>The age of a response is the time since it was sent by, or successfully validated
259      with, the origin server.</t>
260  </list>
261</t>
262<t>
263  <iref item="first-hand"/>
264  <?rfc needLines="4"?>first-hand
265  <list>
266    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
267    0.</t>
268  </list>
269</t>
270<t>
271  <iref item="freshness lifetime"/>
272  <?rfc needLines="4"?>freshness lifetime
273  <list>
274    <t>The length of time between the generation of a response and its expiration time. </t>
275  </list>
276</t>
277<t>
278  <iref item="fresh"/>
279  <?rfc needLines="4"?>fresh
280  <list>
281    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
282  </list>
283</t>
284<t>
285  <iref item="stale"/>
286  <?rfc needLines="4"?>stale
287  <list>
288    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
289  </list>
290</t>
291<t>
292  <iref item="validator"/>
293  <?rfc needLines="4"?>validator
294  <list>
295    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
296      out whether a stored response is an equivalent copy of an entity.</t>
297  </list>
298</t>
299<t anchor="shared.and.non-shared.caches">
300  <iref item="validator"/>
301  <?rfc needLines="4"?>shared cache
302  <list>
303    <t>A cache that is accessible to more than one user. A non-shared cache is
304      dedicated to a single user.</t>
305  </list>
306</t>
307</section>
308
309<section anchor="intro.requirements" title="Requirements">
310<t>
311  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
312  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
313  described in <xref target="RFC2119"/>.
314</t>
315<t>
316  An implementation is not compliant if it fails to satisfy one or more of the MUST
317  or REQUIRED level requirements for the protocols it implements. An implementation
318  that satisfies all the MUST or REQUIRED level and all the SHOULD level
319  requirements for its protocols is said to be "unconditionally compliant"; one that
320  satisfies all the MUST level requirements but not all the SHOULD level
321  requirements for its protocols is said to be "conditionally compliant."
322</t>
323</section>
324
325<section title="Syntax Notation" anchor="notation">
326 
327 
328 
329 
330 
331 
332 
333 
334 
335<t>
336  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
337  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
338  <xref target="collected.abnf"/> shows the collected ABNF, with the list
339  rule expanded.
340</t>
341<t>
342  The following core rules are included by
343  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
344  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
345  DIGIT (decimal 0-9), DQUOTE (double quote),
346  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
347  OCTET (any 8-bit sequence of data), SP (space),
348  VCHAR (any visible USASCII character),
349  and WSP (whitespace).
350</t>
351
352<section title="Core Rules" anchor="core.rules">
353 
354 
355 
356<t>
357  The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
358</t>
359<figure><artwork type="abnf2616"><![CDATA[
360  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
361  token         = <token, defined in [Part1], Section 1.2.2>
362  OWS           = <OWS, defined in [Part1], Section 1.2.2>
363]]></artwork></figure>
364</section>
365
366<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
367 
368 
369 
370 
371 
372<t>
373  The ABNF rules below are defined in other parts:
374</t>
375<figure><artwork type="abnf2616"><![CDATA[
376  field-name    = <field-name, defined in [Part1], Section 4.2>
377  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.2>
378  port          = <port, defined in [Part1], Section 2.1>
379  pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
380  uri-host      = <uri-host, defined in [Part1], Section 2.1>
381]]></artwork></figure>
382</section>
383
384</section>
385</section>
386
387<section anchor="caching.overview" title="Cache Operation">
388
389<section anchor="response.cacheability" title="Response Cacheability">
390<t>
391  A cache MUST NOT store a response to any request, unless:
392  <list style="symbols">
393    <t>The request method is defined as being cacheable, and</t>
394    <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not
395       appear in request or response headers, and</t>
396    <t>the "private" cache response directive (see <xref target="header.cache-control"/>
397       does not appear in the response, if the cache is shared, and</t>
398    <t>the "Authorization" header (see Section 3.1 of <xref target="Part7"/>) does not appear in the request, if
399       the cache is shared (unless the "public" directive is present; see <xref target="header.cache-control"/>), and</t>           
400    <t>the cache understands partial responses, if the response is partial or incomplete
401       (see <xref target="errors.or.incomplete.response.cache.behavior"/>).</t>
402  </list>
403</t>
404<t>
405  Note that in normal operation, most caches will not store a response that has neither a
406  cache validator nor an explicit expiration time, as such responses are not usually
407  useful to store. However, caches are not prohibited from storing such responses.
408</t>
409
410<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
411<t>
412  A cache that receives an incomplete response (for example, with fewer bytes of data
413  than specified in a Content-Length header) can store the response, but MUST
414  treat it as a partial response <xref target="Part5"/>. Partial responses
415  can be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
416  full response or might still be partial. A cache MUST NOT return a partial
417  response to a client without explicitly marking it as such using the 206 (Partial
418  Content) status code.
419</t>
420<t>
421  A cache that does not support the Range and Content-Range headers MUST NOT store
422  incomplete or partial responses.
423</t>
424</section>
425
426</section>
427
428
429<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
430<t>
431  For a presented request, a cache MUST NOT return a stored response, unless:
432  <list style="symbols">
433    <t>The presented Request-URI and that of the stored response match
434      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
435      1 doesn't define Request-URI anymore; the new term request-target does not
436      work for this.</cref>), and</t>
437    <t>the request method associated with the stored response allows it to be
438      used for the presented request, and</t>
439    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
440    <t>the presented request and stored response are free from directives that would prevent
441      its use (see <xref target="header.cache-control"/> and <xref target="header.pragma"/>),
442      and</t>
443    <t>the stored response is either:
444      <list style="symbols">
445        <t>fresh (see <xref target="expiration.model"/>), or</t>
446        <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
447        <t>successfully validated (see <xref target="validation.model"/>).</t>
448      </list>
449    </t>
450  </list>
451</t>
452<t>
453  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
454</t>
455<t>
456  When a stored response is used to satisfy a request, caches MUST include a
457  single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
458  current_age; see <xref target="age.calculations"/>.
459  <cref>DISCUSS: this currently includes successfully validated responses.</cref>
460</t>
461<t>
462  Requests with methods that are unsafe (Section 7.1.1 of <xref target="Part2"/>) MUST be written through the cache to
463  the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
464  corresponding response.
465</t>
466<t>
467  Also, note that unsafe requests might invalidate already stored responses; see
468  <xref target="invalidation.after.updates.or.deletions"/>.
469</t>
470<t>
471  Caches MUST use the most recent response (as determined by the Date header) when
472  more than one suitable response is stored. They can also forward a request with
473  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
474  use.
475</t>
476<t>
477  <cref anchor="TODO-header-properties">end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
478</t>
479</section>
480
481<section anchor="expiration.model" title="Freshness Model">
482<t>
483  When a response is "fresh" in the cache, it can be used to satisfy subsequent
484  requests without contacting the origin server, thereby improving efficiency.
485</t>
486<t>
487  The primary mechanism for determining freshness is for an origin server to provide an
488  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
489  explicit expiration times to responses in the belief that the entity is not likely to
490  change in a semantically significant way before the expiration time is reached.
491</t>
492<t>
493  If an origin server wishes to force a cache to validate every request, it can
494  assign an explicit expiration time in the past. This means that the response is always
495  stale, so that caches should validate it before using it for subsequent requests.
496  <cref>This wording may cause confusion, because the response may still be served stale.</cref>
497</t>
498<t>
499  Since origin servers do not always provide explicit expiration times, HTTP caches may
500  also assign heuristic expiration times when they are not specified, employing algorithms that
501  use other header values (such as the Last-Modified time) to estimate a plausible
502  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
503  impose worst-case constraints on their results.
504</t>
505<figure>
506<preamble>
507  The calculation to determine if a response is fresh is:
508</preamble>
509<artwork type="code"><![CDATA[
510   response_is_fresh = (freshness_lifetime > current_age)
511]]></artwork>
512</figure>
513
514<t>
515  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>;
516  the current_age is defined in <xref target="age.calculations"/>.
517</t>
518<t>
519  Additionally, clients may need to influence freshness calculation. They can do this using
520  several request cache directives, with the effect of either increasing or loosening
521  constraints on freshness. See <xref target="cache-request-directive"/>.
522</t>
523<t>
524  <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
525  freshness.</cref>
526</t>
527<t>
528  Note that freshness applies only to cache operation; it cannot be used to force a user agent
529  to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of
530  the difference between caches and history mechanisms.
531</t>
532
533<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
534<t>
535  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
536  response by using the first match of:
537  <list style="symbols">
538    <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>
539    <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
540    <t>If the Expires response header (<xref target="header.expires"/>) is present, use
541      its value minus the value of the Date response header, or</t>
542    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
543      may be used; see <xref target="heuristic.freshness"/>.</t>
544  </list>
545</t>
546<t>
547  Note that this calculation is not vulnerable to clock skew, since all of the
548  information comes from the origin server.
549</t>
550
551<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
552<t>
553  If no explicit expiration time is present in a stored response that has a status code
554  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
555  calculated. Heuristics MUST NOT be used for other response status codes.
556</t>
557<t>
558  When a heuristic is used to calculate freshness lifetime, the cache SHOULD
559  attach a Warning header with a 113 warn-code to the response if its current_age is
560  more than 24 hours and such a warning is not already present.
561</t>
562<t>
563  Also, if the response has a Last-Modified header (Section 6.6 of <xref target="Part4"/>), the
564  heuristic expiration value SHOULD be no more than some fraction of the interval
565  since that time. A typical setting of this fraction might be 10%.
566</t>
567<t>
568  <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
569</t>
570</section>
571</section>
572
573<section anchor="age.calculations" title="Calculating Age">
574<t>
575  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
576  message when obtained from a cache. The Age field value is the cache's estimate of the
577  amount of time since the response was generated or validated by the origin server. In
578  essence, the Age value is the sum of the time that the response has been resident in
579  each of the caches along the path from the origin server, plus the amount of time it has
580  been in transit along network paths.
581</t>
582<t>
583  The term "age_value" denotes the value of the Age header, in a form appropriate for
584  arithmetic operations.
585</t>
586<t>
587  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
588  response, giving the time at which the response was generated (see Section 8.3 of <xref target="Part1"/>).
589  The term "date_value" denotes the value of the Date header, in a form appropriate for
590  arithmetic operations.
591</t>
592<t>
593  The term "now" means "the current value of the clock at the host performing the
594  calculation." Hosts that use HTTP, but especially hosts running origin servers and
595  caches, SHOULD use NTP <xref target="RFC1305"/> or some similar protocol to
596  synchronize their clocks to a globally accurate time standard.
597</t>
598<t>
599  A response's age can be calculated in two entirely independent ways:
600  <list style="numbers">
601    <t>now minus date_value, if the local clock is reasonably well synchronized to the
602      origin server's clock. If the result is negative, the result is replaced by zero.</t>
603    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
604  </list>
605</t>
606<figure>
607<preamble>These are combined as</preamble>
608<artwork type="code"><![CDATA[
609    corrected_received_age = max(now - date_value, age_value)
610]]></artwork></figure>
611<t>
612  When an Age value is received, it MUST be interpreted relative to the time the
613  request was initiated, not the time that the response was received.
614</t>
615<figure><artwork type="code"><![CDATA[
616   corrected_initial_age = corrected_received_age
617                         + (now - request_time)
618]]></artwork></figure>
619<t>
620  where "request_time" is the time (according to the local clock) when the request that
621  elicited this response was sent.
622</t>
623<t>
624  The current_age of a stored response can then be calculated by adding the amount of
625  time (in seconds) since the stored response was last validated by the origin server to
626  the corrected_initial_age.
627</t>
628<t>
629  In summary:
630</t>
631<figure><artwork type="code"><![CDATA[
632  age_value     - Age header field-value received with the response
633  date_value    - Date header field-value received with the response
634  request_time  - local time when the cache made the request
635                 resulting in the stored response
636  response_time - local time when the cache received the response
637  now           - current local time
638 
639  apparent_age = max(0, response_time - date_value);
640  corrected_received_age = max(apparent_age, age_value);
641  response_delay = response_time - request_time;
642  corrected_initial_age = corrected_received_age + response_delay;
643  resident_time = now - response_time;
644  current_age   = corrected_initial_age + resident_time;
645]]></artwork></figure>
646</section>
647
648<section anchor="serving.stale.responses" title="Serving Stale Responses">
649<t>
650  A "stale" response is one that either has explicit expiry information, or is allowed to
651  have heuristic expiry calculated, but is not fresh according to the calculations in
652  <xref target="expiration.model"/>.
653</t>
654<t>
655  Caches MUST NOT return a stale response if it is prohibited by an explicit
656  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
657  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
658  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
659</t>
660<t>
661  Caches SHOULD NOT return stale responses unless they are
662  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
663  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive"/>).
664</t>
665<t>
666  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
667  the cache is disconnected.
668</t>
669<t>
670  If a cache receives a first-hand response (either an entire response, or a 304 (Not
671  Modified) response) that it would normally forward to the requesting client, and the
672  received response is no longer fresh, the cache SHOULD forward it to the
673  requesting client without adding a new Warning (but without removing any existing
674  Warning headers). A cache SHOULD NOT attempt to validate a response simply because
675  that response became stale in transit.
676</t>
677</section>
678</section>
679
680<section anchor="validation.model" title="Validation Model">
681<t>
682  Checking with the origin server to see if a stale or otherwise unusable cached response
683  can be reused is called "validating" or "revalidating." Doing so potentially avoids
684  the overhead of retransmitting the response body when the stored response is valid.
685</t>
686<t>
687  HTTP's conditional request mechanism <xref target="Part4"/> is used for this purpose. When a stored
688  response includes one or more validators, such as the field values of an ETag or
689  Last-Modified header field, then a validating request SHOULD be made conditional
690  to those field values.
691</t>
692<t>
693  A 304 (Not Modified) response status code indicates that the stored
694  response can be updated and reused; see <xref target="combining.headers"/>.
695</t>
696<t>
697  If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
698  request and replace the stored response. <cref>Should there be a requirement here?</cref>
699</t>
700<t>
701  If a cache receives a 5xx response while attempting to validate a response, it MAY
702  either forward this response to the requesting client, or act as if the server failed to
703  respond. In the latter case, it MAY return a previously stored response (which SHOULD include the
704  111 warn-code; see <xref target="header.warning"/>) unless the
705  stored response includes the "must-revalidate" cache directive (see <xref target="serving.stale.responses"/>).
706</t>
707</section>
708
709<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
710<t>
711  Because unsafe methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing state on the
712  origin server, intervening caches can use them to keep their contents
713  up-to-date.
714</t>
715<t>
716  The following HTTP methods MUST cause a cache to invalidate the Request-URI as well
717  as the URI(s) in the Location and Content-Location headers (if present):
718  <list style="symbols">
719    <t>PUT</t>
720    <t>DELETE</t>
721    <t>POST</t>
722  </list>
723</t>
724<t>
725  An invalidation based on a URI from a Location or Content-Location header MUST NOT
726  be performed if the host part of that URI differs from the host part in the Request-URI.
727  This helps prevent denial of service attacks.
728</t>
729<t>
730  <cref>TODO: "host part" needs to be specified better.</cref>
731</t>
732<t>
733  A cache that passes through requests for methods it does not understand SHOULD
734  invalidate the Request-URI.
735</t>
736<t>
737  Here, "invalidate" means that the cache will either remove all stored responses related
738  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
739  before they can be returned in response to a subsequent request.
740</t>
741<t>
742  Note that this does not guarantee that all appropriate responses are invalidated. For
743  example, the request that caused the change at the origin server might not have gone
744  through the cache where a response is stored.
745</t>
746<t>
747  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
748</t>
749</section>
750
751<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
752<t>
753  Use of server-driven content negotiation (Section 4.1 of <xref target="Part3"/>) alters
754  the conditions under which a cache can use the response for subsequent
755  requests.
756</t>
757<t>
758  When a cache receives a request that can be satisfied by a stored response
759  that includes a Vary header field (<xref target="header.vary"/>), it MUST NOT use that response unless
760  all of the selecting request-headers in the presented request match the corresponding
761  stored request-headers from the original request.
762</t>
763<t>
764  The selecting request-headers from two requests are defined to match if and only if the
765  selecting request-headers in the first request can be transformed to the selecting
766  request-headers in the second request by adding or removing linear white space
767  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
768  combining multiple message-header fields with the same field name following the rules
769  about message headers in Section 4.2 of <xref target="Part1"/>. <cref anchor="DISCUSS-header-specific-canonicalization">
770  Should the matching requirement be relaxed so that it would be ok to use a cached response
771  if the selecting request headers match after header-specific canonicalization?
772  (see &lt;http://trac.tools.ietf.org/wg/httpbis/trac/ticket/147&gt;)
773  </cref>
774</t>
775<t>
776  A Vary header field-value of "*" always fails to match, and subsequent requests to that
777  resource can only be properly interpreted by the origin server.
778</t>
779<t>
780  If no stored response matches, the cache MAY forward the presented request to the origin
781  server in a conditional request, and SHOULD include all ETags stored with
782  potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
783  includes an entity tag or Content-Location that indicates the entity to be used, that
784  cached response MUST be used to satisfy the presented request, and SHOULD
785  be used to update the corresponding stored response; see <xref target="combining.headers"/>.
786</t>
787<t>
788  If any of the stored responses contains only partial content, its entity-tag SHOULD NOT
789  be included in the If-None-Match header field unless the request is for a range that would
790  be fully satisfied by that stored response.
791</t>
792<t>
793  If a cache receives a successful response whose Content-Location field matches that of an
794  existing stored response for the same Request-URI, whose entity-tag differs from that of
795  the existing stored response, and whose Date is more recent than that of the existing
796  response, the existing response SHOULD NOT be returned in response to future
797  requests and SHOULD be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref>
798</t>
799</section>
800
801<section anchor="combining.headers" title="Combining Responses">
802<t>
803  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
804  it needs to update the stored response with the new one, so that the updated response can
805  be sent to the client.
806</t>
807<t>
808  If the status code is 304 (Not Modified), the cache SHOULD use the stored entity-body as
809  the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
810  Last-Modified headers match exactly, the cache MAY combine the stored entity-body in
811  the stored response with the updated entity-body received in the response and use the
812  result as the updated entity-body (see Section 4 of <xref target="Part5"/>).
813</t>
814<t>
815  The stored response headers are used for the updated response, except that
816  <list style="symbols">
817    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
818      MUST be deleted from the stored response and the forwarded response.</t>
819    <t>any stored Warning headers with warn-code 2xx MUST be retained in the stored
820      response and the forwarded response.</t>
821    <t>any headers provided in the 304 or 206 response MUST replace the corresponding
822      headers from the stored response.</t>
823  </list>
824</t>
825<t>
826  A cache MUST also replace any stored headers with corresponding headers received in the
827  incoming response, except for Warning headers as described immediately above. If a header
828  field-name in the incoming response matches more than one header in the stored response,
829  all such old headers MUST be replaced. It MAY store the combined
830  entity-body.
831</t>
832<t>
833  <cref>ISSUE: discuss how to handle HEAD updates</cref>
834</t>
835</section>
836
837</section>
838
839<section anchor="header.fields" title="Header Field Definitions">
840<t>
841  This section defines the syntax and semantics of HTTP/1.1 header fields
842  related to caching.
843</t>
844<t>
845  For entity-header fields, both sender and recipient refer to either the client or the
846  server, depending on who sends and who receives the entity.
847</t>
848
849<section anchor="header.age" title="Age">
850  <iref item="Age header" primary="true"/>
851  <iref item="Headers" primary="true" subitem="Age"/>
852 
853 
854 
855<t>
856  The response-header field "Age" conveys the sender's estimate of the amount of time since
857  the response (or its validation) was generated at the origin server. Age values are
858  calculated as specified in <xref target="age.calculations"/>.
859</t>
860<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/><artwork type="abnf2616"><![CDATA[
861  Age   = "Age" ":" OWS Age-v
862  Age-v = delta-seconds
863]]></artwork></figure>
864<t anchor="rule.delta-seconds">
865 
866  Age field-values are non-negative integers, representing time in seconds.
867</t>
868<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
869  delta-seconds  = 1*DIGIT
870]]></artwork></figure>
871<t>
872  If a cache receives a value larger than the largest positive integer it can represent, or
873  if any of its age calculations overflows, it MUST transmit an Age header with a
874  field-value of 2147483648 (2^31). Caches SHOULD use an arithmetic type
875  of at least 31 bits of range.
876</t>
877<t>
878  The presence of an Age header field in a response implies that a response is not
879  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
880  Age header field.
881</t>
882</section>
883
884<section anchor="header.cache-control" title="Cache-Control">
885  <iref item="Cache-Control header" primary="true"/>
886  <iref item="Headers" primary="true" subitem="Cache-Control"/>
887 
888 
889 
890 
891 
892 
893<t>
894  The general-header field "Cache-Control" is used to specify directives that MUST be
895  obeyed by all caches along the request/response chain. The directives specify behavior
896  intended to prevent caches from adversely interfering with the request or response. Cache
897  directives are unidirectional in that the presence of a directive in a request does not
898  imply that the same directive is to be given in the response.
899</t>
900<t><list>
901  <t>
902    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
903    Pragma: no-cache (see <xref target="header.pragma"/>).
904  </t>
905</list></t>
906<t>
907  Cache directives MUST be passed through by a proxy or gateway application,
908  regardless of their significance to that application, since the directives might be
909  applicable to all recipients along the request/response chain. It is not possible to
910  target a directive to a specific cache.
911</t>
912<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[
913  Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
914  Cache-Control-v = 1#cache-directive
915
916  cache-directive = cache-request-directive
917     / cache-response-directive
918
919  cache-extension = token [ "=" ( token / quoted-string ) ]
920]]></artwork></figure>
921
922<section anchor="cache-request-directive" title="Request Cache-Control Directives">
923 
924
925<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
926  cache-request-directive =
927       "no-cache"
928     / "no-store"
929     / "max-age" "=" delta-seconds
930     / "max-stale" [ "=" delta-seconds ]
931     / "min-fresh" "=" delta-seconds
932     / "no-transform"
933     / "only-if-cached"
934     / cache-extension
935]]></artwork></figure>
936
937<t>
938  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
939  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
940  no-cache
941  <list>
942    <t>The no-cache request directive indicates that a stored response MUST NOT be
943      used to satisfy the request without successful validation on the origin server.</t>
944  </list>
945</t>
946<t>
947  <iref item="Cache Directives" primary="true" subitem="no-store"/>
948  <iref item="no-store" primary="true" subitem="Cache Directive"/>
949  no-store
950  <list>
951    <t>The no-store request directive indicates that a cache MUST NOT store any part
952      of either this request or any response to it. This directive applies to both
953      non-shared and shared caches. "MUST NOT store" in this context means that the
954      cache MUST NOT intentionally store the information in non-volatile storage,
955      and MUST make a best-effort attempt to remove the information from volatile
956      storage as promptly as possible after forwarding it.</t>
957    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
958      particular, malicious or compromised caches might not recognize or obey this
959      directive, and communications networks may be vulnerable to eavesdropping.</t>
960  </list>
961</t>
962<t>
963  <iref item="Cache Directives" primary="true" subitem="max-age"/>
964  <iref item="max-age" primary="true" subitem="Cache Directive"/>
965  max-age
966  <list>
967    <t>The max-age request directive indicates that the client is willing to accept a
968      response whose age is no greater than the specified time in seconds. Unless
969      max-stale directive is also included, the client is not willing to accept a stale
970      response.</t>
971  </list>
972</t>
973<t>
974  <iref item="Cache Directives" primary="true" subitem="max-stale"/>
975  <iref item="max-stale" primary="true" subitem="Cache Directive"/>
976  max-stale
977  <list>
978    <t>The max-stale request directive indicates that the client is willing to accept a
979      response that has exceeded its expiration time. If max-stale is assigned a value,
980      then the client is willing to accept a response that has exceeded its expiration
981      time by no more than the specified number of seconds. If no value is assigned to
982      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
983  </list>
984</t>
985<t>
986  <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
987  <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
988  min-fresh
989  <list>
990    <t>The min-fresh request directive indicates that the client is willing to accept a
991      response whose freshness lifetime is no less than its current age plus the specified
992      time in seconds. That is, the client wants a response that will still be fresh for
993      at least the specified number of seconds.</t>
994  </list>
995</t>
996<t>
997  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
998  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
999  no-transform
1000  <list>
1001    <t>The no-transform request directive indicates that an intermediate cache or proxy
1002      MUST NOT change the Content-Encoding, Content-Range or Content-Type request
1003      headers, nor the request entity-body.</t>
1004  </list>
1005</t>
1006<t>
1007  <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1008  <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1009  only-if-cached
1010  <list>
1011    <t>The only-if-cached request directive indicates that the client only wishes to
1012      return a stored response. If it receives this directive, a cache SHOULD either
1013      respond using a stored response that is consistent with the other constraints of the
1014      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1015      being operated as a unified system with good internal connectivity, such a request
1016      MAY be forwarded within that group of caches.</t>
1017  </list>
1018</t>
1019</section>
1020
1021<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1022 
1023
1024<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1025  cache-response-directive =
1026       "public"
1027     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1028     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1029     / "no-store"
1030     / "no-transform"
1031     / "must-revalidate"
1032     / "proxy-revalidate"
1033     / "max-age" "=" delta-seconds
1034     / "s-maxage" "=" delta-seconds
1035     / cache-extension
1036]]></artwork></figure>
1037
1038<t>
1039  <iref item="Cache Directives" primary="true" subitem="public"/>
1040  <iref item="public" primary="true" subitem="Cache Directive"/>
1041  public
1042  <list>
1043    <t>The public response directive indicates that the response MAY be cached, even
1044      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1045      (See also Authorization, Section 3.1 of <xref target="Part7"/>, for additional details.) </t>
1046  </list>
1047</t>
1048<t>
1049  <iref item="Cache Directives" primary="true" subitem="private"/>
1050  <iref item="private" primary="true" subitem="Cache Directive"/>
1051  private
1052  <list>
1053    <t>The private response directive indicates that the response message is intended for
1054      a single user and MUST NOT be stored by a shared cache. A private (non-shared)
1055      cache MAY store the response.</t>
1056    <t>If the private response directive specifies one or more field-names, this
1057      requirement is limited to the field-values associated with the listed response
1058      headers. That is, the specified field-names(s) MUST NOT be stored by a shared
1059      cache, whereas the remainder of the response message MAY be.</t>
1060    <t>
1061      Note: This usage of the word private only controls where the response may
1062      be stored, and cannot ensure the privacy of the message content.</t>
1063  </list>
1064</t>
1065<t>
1066  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1067  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1068  no-cache
1069  <list>
1070    <t>The no-cache response directive indicates that the response MUST NOT be used to
1071      satisfy a subsequent request without successful validation on the origin server.
1072      This allows an origin server to prevent caching even by caches that have been
1073      configured to return stale responses.</t>
1074    <t>If the no-cache response directive specifies one or more field-names, this
1075      requirement is limited to the field-values associated with the listed response
1076      headers. That is, the specified field-name(s) MUST NOT be sent in the response
1077      to a subsequent request without successful validation on the origin server. This
1078      allows an origin server to prevent the re-use of certain header fields in a
1079      response, while still allowing caching of the rest of the response.</t>
1080    <t>
1081      Note: Most HTTP/1.0 caches will not recognize or obey this directive.
1082    </t>
1083  </list>
1084</t>
1085
1086<t>
1087  <iref item="Cache Directives" primary="true" subitem="no-store"/>
1088  <iref item="no-store" primary="true" subitem="Cache Directive"/>
1089  no-store
1090  <list>
1091    <t>The no-store response directive indicates that a cache MUST NOT store any
1092      part of either the immediate request or response. This directive applies to both
1093      non-shared and shared caches. "MUST NOT store" in this context means that the
1094      cache MUST NOT intentionally store the information in non-volatile storage,
1095      and MUST make a best-effort attempt to remove the information from volatile
1096      storage as promptly as possible after forwarding it.</t>
1097    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1098      particular, malicious or compromised caches might not recognize or obey this
1099      directive, and communications networks may be vulnerable to eavesdropping.</t>
1100  </list>
1101</t>
1102<t>
1103  <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1104  <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1105  must-revalidate
1106  <list>
1107    <t>The must-revalidate response directive indicates that once it has become stale, the response MUST NOT be
1108     used to satisfy subsequent requests without successful validation on the origin server.</t>
1109    <t>The must-revalidate directive is necessary to support reliable operation for
1110      certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey
1111      the must-revalidate directive; in particular, if the cache cannot reach the origin
1112      server for any reason, it MUST generate a 504 (Gateway Timeout) response.</t>
1113    <t>Servers SHOULD send the must-revalidate directive if and only if failure to
1114      validate a request on the entity could result in incorrect operation, such as a
1115      silently unexecuted financial transaction.</t>
1116  </list>
1117</t>
1118<t>
1119  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1120  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1121  proxy-revalidate
1122  <list>
1123    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1124      response directive, except that it does not apply to non-shared caches.</t>
1125  </list>
1126</t>
1127<t>
1128  <iref item="Cache Directives" primary="true" subitem="max-age"/>
1129  <iref item="max-age" primary="true" subitem="Cache Directive"/>
1130  max-age
1131  <list>
1132    <t>The max-age response directive indicates that response is to be considered stale
1133      after its age is greater than the specified number of seconds.</t>
1134  </list>
1135</t>
1136<t>
1137  <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1138  <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1139  s-maxage
1140  <list>
1141    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1142      specified by this directive overrides the maximum age specified by either the
1143      max-age directive or the Expires header. The s-maxage directive also implies the
1144      semantics of the proxy-revalidate response directive.</t>
1145  </list>
1146</t>
1147<t>
1148  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1149  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1150  no-transform
1151  <list>
1152    <t>The no-transform response directive indicates that an intermediate cache or proxy
1153      MUST NOT change the Content-Encoding, Content-Range or Content-Type response
1154      headers, nor the response entity-body.</t>
1155  </list>
1156</t>
1157
1158</section>
1159
1160<section anchor="cache.control.extensions" title="Cache Control Extensions">
1161<t>
1162  The Cache-Control header field can be extended through the use of one or more
1163  cache-extension tokens, each with an optional value. Informational extensions (those
1164  that do not require a change in cache behavior) can be added without changing the
1165  semantics of other directives. Behavioral extensions are designed to work by acting as
1166  modifiers to the existing base of cache directives. Both the new directive and the
1167  standard directive are supplied, such that applications that do not understand the new
1168  directive will default to the behavior specified by the standard directive, and those
1169  that understand the new directive will recognize it as modifying the requirements
1170  associated with the standard directive. In this way, extensions to the cache-control
1171  directives can be made without requiring changes to the base protocol.
1172</t>
1173<t>
1174  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1175  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1176  all directives that it does not understand.
1177</t>
1178<t>
1179  For example, consider a hypothetical new response directive called "community" that
1180  acts as a modifier to the private directive. We define this new directive to mean that,
1181  in addition to any non-shared cache, any cache that is shared only by members of the
1182  community named within its value may cache the response. An origin server wishing to
1183  allow the UCI community to use an otherwise private response in their shared cache(s)
1184  could do so by including
1185</t>
1186<figure><artwork type="example"><![CDATA[
1187  Cache-Control: private, community="UCI"
1188]]></artwork></figure>
1189<t>
1190  A cache seeing this header field will act correctly even if the cache does not
1191  understand the community cache-extension, since it will also see and understand the
1192  private directive and thus default to the safe behavior.
1193</t>
1194<t>
1195  Unrecognized cache directives MUST be ignored; it is assumed that any cache
1196  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1197  directives (or the response's default cacheability) such that the cache behavior will
1198  remain minimally correct even if the cache does not understand the extension(s).
1199</t>
1200</section>
1201
1202</section>
1203
1204<section anchor="header.expires" title="Expires">
1205  <iref item="Expires header" primary="true"/>
1206  <iref item="Headers" primary="true" subitem="Expires"/>
1207 
1208 
1209<t>
1210  The entity-header field "Expires" gives the date/time after which the response is
1211  considered stale. See <xref target="expiration.model"/> for further discussion of the
1212  freshness model.
1213</t>
1214<t>
1215  The presence of an Expires field does not imply that the original resource will change or
1216  cease to exist at, before, or after that time.
1217</t>
1218<t>
1219  The field-value is an absolute date and time as defined by HTTP-date in Section 3.2 of <xref target="Part1"/>;
1220  it MUST be sent in rfc1123-date format.
1221</t>
1222<figure><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/><artwork type="abnf2616"><![CDATA[
1223  Expires   = "Expires" ":" OWS Expires-v
1224  Expires-v = HTTP-date
1225]]></artwork></figure>
1226<figure>
1227  <preamble>For example</preamble>
1228<artwork type="example"><![CDATA[
1229  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1230]]></artwork></figure>
1231<t><list>
1232  <t>
1233    Note: if a response includes a Cache-Control field with the max-age
1234    directive (see <xref target="cache-response-directive"/>), that directive overrides
1235    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1236  </t>
1237</list></t>
1238<t>
1239  HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.
1240</t>
1241<t>
1242  HTTP/1.1 clients and caches MUST treat other invalid date formats, especially
1243  including the value "0", as in the past (i.e., "already expired").
1244</t>
1245</section>
1246
1247<section anchor="header.pragma" title="Pragma">
1248  <iref item="Pragma header" primary="true"/>
1249  <iref item="Headers" primary="true" subitem="Pragma"/>
1250 
1251 
1252 
1253 
1254<t>
1255  The general-header field "Pragma" is used to include implementation-specific directives
1256  that might apply to any recipient along the request/response chain. All pragma directives
1257  specify optional behavior from the viewpoint of the protocol; however, some systems
1258  MAY require that behavior be consistent with the directives.
1259</t>
1260<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[
1261  Pragma            = "Pragma" ":" OWS Pragma-v
1262  Pragma-v          = 1#pragma-directive
1263  pragma-directive  = "no-cache" / extension-pragma
1264  extension-pragma  = token [ "=" ( token / quoted-string ) ]
1265]]></artwork></figure>
1266<t>
1267  When the no-cache directive is present in a request message, an application SHOULD
1268  forward the request toward the origin server even if it has a cached copy of what is being
1269  requested. This pragma directive has the same semantics as the no-cache response directive
1270  (see <xref target="cache-response-directive"/>) and is defined here for backward
1271  compatibility with HTTP/1.0. Clients SHOULD include both header fields when a
1272  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1273  SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1274</t>
1275<t><list>
1276  <t>
1277    Note: because the meaning of "Pragma: no-cache" as a response-header field
1278    is not actually specified, it does not provide a reliable replacement for
1279    "Cache-Control: no-cache" in a response.
1280  </t>
1281</list></t>
1282<t>
1283  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1284</t>
1285</section>
1286
1287<section anchor="header.vary" title="Vary">
1288  <iref item="Vary header" primary="true"/>
1289  <iref item="Headers" primary="true" subitem="Vary"/>
1290 
1291 
1292<t>
1293  The "Vary" response-header field's value indicates the set of request-header fields that
1294  determines, while the response is fresh, whether a cache is permitted to use the
1295  response to reply to a subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1296</t>
1297<t>
1298  In uncacheable or stale responses, the Vary field value advises the user agent about
1299  the criteria that were used to select the representation.
1300</t>
1301<figure><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/><artwork type="abnf2616"><![CDATA[
1302  Vary   = "Vary" ":" OWS Vary-v
1303  Vary-v = "*" / 1#field-name
1304]]></artwork></figure>
1305<t>
1306  The set of header fields named by the Vary field value is known as the selecting
1307  request-headers.
1308</t>
1309<t>
1310  Servers SHOULD include a Vary header field with any cacheable response that is
1311  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1312  requests on that resource and informs the user agent about the presence of negotiation on
1313  that resource. A server MAY include a Vary header field with a non-cacheable
1314  response that is subject to server-driven negotiation, since this might provide the user
1315  agent with useful information about the dimensions over which the response varies at the
1316  time of the response.
1317</t>
1318<t>
1319  A Vary field value of "*" signals that unspecified parameters not limited to the
1320  request-headers (e.g., the network address of the client), play a role in the selection of
1321  the response representation; therefore, a cache cannot determine whether this response is
1322  appropriate. The "*" value MUST NOT be generated by a proxy server;
1323  it may only be generated by an origin server.
1324</t>
1325<t>
1326  The field-names given are not limited to the set of standard request-header fields
1327  defined by this specification. Field names are case-insensitive.
1328</t>
1329</section>
1330
1331<section anchor="header.warning" title="Warning">
1332  <iref item="Warning header" primary="true"/>
1333  <iref item="Headers" primary="true" subitem="Warning"/>
1334 
1335 
1336 
1337 
1338 
1339 
1340 
1341<t>
1342  The general-header field "Warning" is used to carry additional information about the status
1343  or transformation of a message that might not be reflected in the message. This
1344  information is typically used to warn about possible incorrectness introduced by caching
1345  operations or transformations applied to the entity body of the message.
1346</t>
1347<t>
1348  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1349  warning, rather than an error status code, distinguish these responses from true failures.
1350</t>
1351<t>
1352  Warning headers can in general be applied to any message, however some warn-codes are
1353  specific to caches and can only be applied to response messages.
1354</t>
1355<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[
1356  Warning    = "Warning" ":" OWS Warning-v
1357  Warning-v  = 1#warning-value
1358 
1359  warning-value = warn-code SP warn-agent SP warn-text
1360                                        [SP warn-date]
1361 
1362  warn-code  = 3DIGIT
1363  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1364                  ; the name or pseudonym of the server adding
1365                  ; the Warning header, for use in debugging
1366  warn-text  = quoted-string
1367  warn-date  = DQUOTE HTTP-date DQUOTE
1368]]></artwork></figure>
1369<t>
1370  Multiple warnings can be attached to a response (either by the origin server or by
1371  a cache), including multiple warnings with the same code number. For example, a server
1372  might provide the same warning with texts in both English and Basque.
1373</t>
1374<t>
1375  When this occurs, the user agent SHOULD inform the user of as many of them as
1376  possible, in the order that they appear in the response. If it is not possible to inform
1377  the user of all of the warnings, the user agent SHOULD follow these heuristics:
1378  <list style="symbols">
1379    <t>Warnings that appear early in the response take priority over those appearing later
1380      in the response.</t>
1381    <t>Warnings in the user's preferred character set take priority over warnings in other
1382      character sets but with identical warn-codes and warn-agents.</t>
1383  </list>
1384</t>
1385<t>
1386  Systems that generate multiple Warning headers SHOULD order them with this user
1387  agent behavior in mind. New Warning headers SHOULD be added after any existing
1388  Warning headers.
1389</t>
1390<t>
1391  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1392  Warning is required to be deleted from a stored response after validation:
1393  <list style="symbols">
1394    <t>1xx Warnings that describe the freshness or validation status of the response, and so
1395      MUST be deleted by caches after validation. They MUST NOT be generated by a cache
1396      except when validating a cached entry, and MUST NOT be generated by clients.</t>
1397    <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1398      not rectified by a validation (for example, a lossy compression of the entity bodies)
1399      and MUST NOT be deleted by caches after validation, unless a full response is
1400      returned, in which case they MUST be.</t>
1401  </list>
1402</t>
1403<t>
1404  The warn-text SHOULD be in a natural language and character set that is most likely
1405  to be intelligible to the human user receiving the response. This decision can be based on
1406  any available knowledge, such as the location of the cache or user, the Accept-Language
1407  field in a request, the Content-Language field in a response, etc. The default language is
1408  English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
1409</t>
1410<t>
1411  If a character set other than ISO-8859-1 is used, it MUST be encoded in the
1412  warn-text using the method described in <xref target="RFC2047"/>.
1413</t>
1414<t>
1415  If an implementation sends a message with one or more Warning headers to a receiver whose
1416  version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a
1417  warn-date that matches the Date header in the message.
1418</t>
1419<t>
1420  If an implementation receives a message with a warning-value that includes a warn-date,
1421  and that warn-date is different from the Date value in the response, then that
1422  warning-value MUST be deleted from the message before storing, forwarding, or using
1423  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1424  warning-values are deleted for this reason, the Warning header MUST be deleted as
1425  well.
1426</t>
1427<t>
1428  The following warn-codes are defined by this specification, each with a recommended
1429  warn-text in English, and a description of its meaning.
1430</t>
1431<t>
1432<?rfc needLines="4"?>
1433  110 Response is stale
1434  <list>
1435    <t>SHOULD be included whenever the returned response is stale.</t>
1436  </list>
1437</t>
1438<t>
1439<?rfc needLines="4"?>
1440  111 Revalidation failed
1441  <list>
1442    <t>SHOULD be included if a cache returns a stale response because an attempt to
1443      validate the response failed, due to an inability to reach the server.</t>
1444  </list>
1445</t>
1446<t>
1447<?rfc needLines="4"?>
1448  112 Disconnected operation
1449  <list>
1450    <t>SHOULD be included if the cache is intentionally disconnected from the rest of
1451      the network for a period of time.</t>
1452  </list>
1453</t>
1454<t>
1455<?rfc needLines="4"?>
1456  113 Heuristic expiration
1457  <list>
1458    <t>SHOULD be included if the cache heuristically chose a freshness lifetime
1459      greater than 24 hours and the response's age is greater than 24 hours.</t>
1460  </list>
1461</t>
1462<t>
1463<?rfc needLines="4"?>
1464  199 Miscellaneous warning
1465  <list>
1466    <t>The warning text can include arbitrary information to be presented to a human
1467      user, or logged. A system receiving this warning MUST NOT take any automated
1468      action, besides presenting the warning to the user.</t>
1469  </list>
1470</t>
1471<t>
1472<?rfc needLines="4"?>
1473  214 Transformation applied
1474  <list>
1475    <t>MUST be added by an intermediate cache or proxy if it applies any
1476      transformation changing the content-coding (as specified in the Content-Encoding
1477      header) or media-type (as specified in the Content-Type header) of the response, or
1478      the entity-body of the response, unless this Warning code already appears in the
1479      response.</t>
1480  </list>
1481</t>
1482<t>
1483<?rfc needLines="4"?>
1484  299 Miscellaneous persistent warning
1485  <list>
1486    <t>The warning text can include arbitrary information to be presented to a human
1487      user, or logged. A system receiving this warning MUST NOT take any automated
1488      action.</t>
1489  </list>
1490</t>
1491</section>
1492
1493</section>
1494
1495<section anchor="history.lists" title="History Lists">
1496<t>
1497  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1498  can be used to redisplay an entity retrieved earlier in a session.
1499</t>
1500<t>
1501  History mechanisms and caches are different. In particular history mechanisms
1502  SHOULD NOT try to show a correct view of the current state of a resource. Rather, a
1503  history mechanism is meant to show exactly what the user saw at the time when the resource
1504  was retrieved.
1505</t>
1506<t>
1507  By default, an expiration time does not apply to history mechanisms. If the entity is still
1508  in storage, a history mechanism SHOULD display it even if the entity has expired,
1509  unless the user has specifically configured the agent to refresh expired history documents.
1510</t>
1511<t>
1512  This is not to be construed to prohibit the history mechanism from telling the user that a
1513  view might be stale.
1514</t>
1515<t><list>
1516  <t>
1517    Note: if history list mechanisms unnecessarily prevent users from viewing
1518    stale resources, this will tend to force service authors to avoid using HTTP expiration
1519    controls and cache controls when they would otherwise like to. Service authors may
1520    consider it important that users not be presented with error messages or warning
1521    messages when they use navigation controls (such as BACK) to view previously fetched
1522    resources. Even though sometimes such resources ought not be cached, or ought to expire
1523    quickly, user interface considerations may force service authors to resort to other
1524    means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1525    of improperly functioning history mechanisms.
1526  </t>
1527</list></t>
1528</section>
1529
1530
1531<section anchor="IANA.considerations" title="IANA Considerations">
1532
1533<section anchor="message.header.registration" title="Message Header Registration">
1534<t>
1535  The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1536  should be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1537</t>
1538
1539<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1540<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1541  <ttcol>Header Field Name</ttcol>
1542  <ttcol>Protocol</ttcol>
1543  <ttcol>Status</ttcol>
1544  <ttcol>Reference</ttcol>
1545
1546  <c>Age</c>
1547  <c>http</c>
1548  <c>standard</c>
1549  <c>
1550    <xref target="header.age"/>
1551  </c>
1552
1553  <c>Cache-Control</c>
1554  <c>http</c>
1555  <c>standard</c>
1556  <c>
1557    <xref target="header.cache-control"/>
1558  </c>
1559
1560  <c>Expires</c>
1561  <c>http</c>
1562  <c>standard</c>
1563  <c>
1564    <xref target="header.expires"/>
1565  </c>
1566
1567  <c>Pragma</c>
1568  <c>http</c>
1569  <c>standard</c>
1570  <c>
1571    <xref target="header.pragma"/>
1572  </c>
1573
1574  <c>Vary</c>
1575  <c>http</c>
1576  <c>standard</c>
1577  <c>
1578    <xref target="header.vary"/>
1579  </c>
1580
1581  <c>Warning</c>
1582  <c>http</c>
1583  <c>standard</c>
1584  <c>
1585    <xref target="header.warning"/>
1586  </c>
1587</texttable>
1588<!--(END)-->
1589<t>
1590  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1591</t>
1592</section>
1593
1594</section>
1595
1596<section anchor="security.considerations" title="Security Considerations">
1597<t>
1598  Caches expose additional potential vulnerabilities, since the contents of the cache
1599  represent an attractive target for malicious exploitation. Because cache contents persist
1600  after an HTTP request is complete, an attack on the cache can reveal information long after
1601  a user believes that the information has been removed from the network. Therefore, cache
1602  contents should be protected as sensitive information.
1603</t>
1604</section>
1605
1606<section anchor="ack" title="Acknowledgments">
1607<t>
1608  Much of the content and presentation of the caching design is due to suggestions and
1609  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1610  and Larry Masinter.
1611</t>
1612</section>
1613
1614</middle>
1615
1616<back>
1617<references title="Normative References">
1618
1619  <reference anchor="ISO-8859-1">
1620    <front>
1621      <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1622        1: Latin alphabet No. 1 </title>
1623      <author>
1624        <organization>International Organization for Standardization</organization>
1625      </author>
1626      <date year="1998"/>
1627    </front>
1628    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1629  </reference>
1630
1631  <reference anchor="Part1">
1632    <front>
1633      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1634      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1635        <organization abbrev="Day Software">Day Software</organization>
1636        <address><email>fielding@gbiv.com</email></address>
1637      </author>
1638      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1639        <organization>One Laptop per Child</organization>
1640        <address><email>jg@laptop.org</email></address>
1641      </author>
1642      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1643        <organization abbrev="HP">Hewlett-Packard Company</organization>
1644        <address><email>JeffMogul@acm.org</email></address>
1645      </author>
1646      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1647        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1648        <address><email>henrikn@microsoft.com</email></address>
1649      </author>
1650      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1651        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1652        <address><email>LMM@acm.org</email></address>
1653      </author>
1654      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1655        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1656        <address><email>paulle@microsoft.com</email></address>
1657      </author>
1658      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1659        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1660        <address><email>timbl@w3.org</email></address>
1661      </author>
1662      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1663        <organization abbrev="W3C">World Wide Web Consortium</organization>
1664        <address><email>ylafon@w3.org</email></address>
1665      </author>
1666      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1667        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1668        <address><email>julian.reschke@greenbytes.de</email></address>
1669      </author>
1670      <date month="July" year="2009"/>
1671    </front>
1672    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-07"/>
1673   
1674  </reference>
1675
1676  <reference anchor="Part2">
1677    <front>
1678      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1679      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1680        <organization abbrev="Day Software">Day Software</organization>
1681        <address><email>fielding@gbiv.com</email></address>
1682      </author>
1683      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1684        <organization>One Laptop per Child</organization>
1685        <address><email>jg@laptop.org</email></address>
1686      </author>
1687      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1688        <organization abbrev="HP">Hewlett-Packard Company</organization>
1689        <address><email>JeffMogul@acm.org</email></address>
1690      </author>
1691      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1692        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1693        <address><email>henrikn@microsoft.com</email></address>
1694      </author>
1695      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1696        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1697        <address><email>LMM@acm.org</email></address>
1698      </author>
1699      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1700        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1701        <address><email>paulle@microsoft.com</email></address>
1702      </author>
1703      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1704        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1705        <address><email>timbl@w3.org</email></address>
1706      </author>
1707      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1708        <organization abbrev="W3C">World Wide Web Consortium</organization>
1709        <address><email>ylafon@w3.org</email></address>
1710      </author>
1711      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1712        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1713        <address><email>julian.reschke@greenbytes.de</email></address>
1714      </author>
1715      <date month="July" year="2009"/>
1716    </front>
1717    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-07"/>
1718   
1719  </reference>
1720
1721  <reference anchor="Part3">
1722    <front>
1723      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1724      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1725        <organization abbrev="Day Software">Day Software</organization>
1726        <address><email>fielding@gbiv.com</email></address>
1727      </author>
1728      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1729        <organization>One Laptop per Child</organization>
1730        <address><email>jg@laptop.org</email></address>
1731      </author>
1732      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1733        <organization abbrev="HP">Hewlett-Packard Company</organization>
1734        <address><email>JeffMogul@acm.org</email></address>
1735      </author>
1736      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1737        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1738        <address><email>henrikn@microsoft.com</email></address>
1739      </author>
1740      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1741        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1742        <address><email>LMM@acm.org</email></address>
1743      </author>
1744      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1745        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1746        <address><email>paulle@microsoft.com</email></address>
1747      </author>
1748      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1749        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1750        <address><email>timbl@w3.org</email></address>
1751      </author>
1752      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1753        <organization abbrev="W3C">World Wide Web Consortium</organization>
1754        <address><email>ylafon@w3.org</email></address>
1755      </author>
1756      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1757        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1758        <address><email>julian.reschke@greenbytes.de</email></address>
1759      </author>
1760      <date month="July" year="2009"/>
1761    </front>
1762    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-07"/>
1763   
1764  </reference>
1765
1766  <reference anchor="Part4">
1767    <front>
1768      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1769      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1770        <organization abbrev="Day Software">Day Software</organization>
1771        <address><email>fielding@gbiv.com</email></address>
1772      </author>
1773      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1774        <organization>One Laptop per Child</organization>
1775        <address><email>jg@laptop.org</email></address>
1776      </author>
1777      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1778        <organization abbrev="HP">Hewlett-Packard Company</organization>
1779        <address><email>JeffMogul@acm.org</email></address>
1780      </author>
1781      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1782        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1783        <address><email>henrikn@microsoft.com</email></address>
1784      </author>
1785      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1786        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1787        <address><email>LMM@acm.org</email></address>
1788      </author>
1789      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1790        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1791        <address><email>paulle@microsoft.com</email></address>
1792      </author>
1793      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1794        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1795        <address><email>timbl@w3.org</email></address>
1796      </author>
1797      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1798        <organization abbrev="W3C">World Wide Web Consortium</organization>
1799        <address><email>ylafon@w3.org</email></address>
1800      </author>
1801      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1802        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1803        <address><email>julian.reschke@greenbytes.de</email></address>
1804      </author>
1805      <date month="July" year="2009"/>
1806    </front>
1807    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-07"/>
1808   
1809  </reference>
1810
1811  <reference anchor="Part5">
1812    <front>
1813      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1814      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1815        <organization abbrev="Day Software">Day Software</organization>
1816        <address><email>fielding@gbiv.com</email></address>
1817      </author>
1818      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1819        <organization>One Laptop per Child</organization>
1820        <address><email>jg@laptop.org</email></address>
1821      </author>
1822      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1823        <organization abbrev="HP">Hewlett-Packard Company</organization>
1824        <address><email>JeffMogul@acm.org</email></address>
1825      </author>
1826      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1827        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1828        <address><email>henrikn@microsoft.com</email></address>
1829      </author>
1830      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1831        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1832        <address><email>LMM@acm.org</email></address>
1833      </author>
1834      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1835        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1836        <address><email>paulle@microsoft.com</email></address>
1837      </author>
1838      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1839        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1840        <address><email>timbl@w3.org</email></address>
1841      </author>
1842      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1843        <organization abbrev="W3C">World Wide Web Consortium</organization>
1844        <address><email>ylafon@w3.org</email></address>
1845      </author>
1846      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1847        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1848        <address><email>julian.reschke@greenbytes.de</email></address>
1849      </author>
1850      <date month="July" year="2009"/>
1851    </front>
1852    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-07"/>
1853   
1854  </reference>
1855
1856  <reference anchor="Part7">
1857    <front>
1858      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1859      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1860        <organization abbrev="Day Software">Day Software</organization>
1861        <address><email>fielding@gbiv.com</email></address>
1862      </author>
1863      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1864        <organization>One Laptop per Child</organization>
1865        <address><email>jg@laptop.org</email></address>
1866      </author>
1867      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1868        <organization abbrev="HP">Hewlett-Packard Company</organization>
1869        <address><email>JeffMogul@acm.org</email></address>
1870      </author>
1871      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1872        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1873        <address><email>henrikn@microsoft.com</email></address>
1874      </author>
1875      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1876        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1877        <address><email>LMM@acm.org</email></address>
1878      </author>
1879      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1880        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1881        <address><email>paulle@microsoft.com</email></address>
1882      </author>
1883      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1884        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1885        <address><email>timbl@w3.org</email></address>
1886      </author>
1887      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1888        <organization abbrev="W3C">World Wide Web Consortium</organization>
1889        <address><email>ylafon@w3.org</email></address>
1890      </author>
1891      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1892        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1893        <address><email>julian.reschke@greenbytes.de</email></address>
1894      </author>
1895      <date month="July" year="2009"/>
1896    </front>
1897    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-07"/>
1898   
1899  </reference>
1900
1901  <reference anchor="RFC2047">
1902    <front>
1903      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1904        Part Three: Message Header Extensions for Non-ASCII Text</title>
1905      <author fullname="Keith Moore" initials="K." surname="Moore">
1906        <organization>University of Tennessee</organization>
1907        <address><email>moore@cs.utk.edu</email></address>
1908      </author>
1909      <date month="November" year="1996"/>
1910    </front>
1911    <seriesInfo name="RFC" value="2047"/>
1912  </reference>
1913
1914  <reference anchor="RFC2119">
1915    <front>
1916      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1917      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1918        <organization>Harvard University</organization>
1919        <address><email>sob@harvard.edu</email></address>
1920      </author>
1921      <date month="March" year="1997"/>
1922    </front>
1923    <seriesInfo name="BCP" value="14"/>
1924    <seriesInfo name="RFC" value="2119"/>
1925  </reference>
1926
1927  <reference anchor="RFC5234">
1928    <front>
1929      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1930      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1931        <organization>Brandenburg InternetWorking</organization>
1932        <address>
1933        <postal>
1934        <street>675 Spruce Dr.</street>
1935        <city>Sunnyvale</city>
1936        <region>CA</region>
1937        <code>94086</code>
1938        <country>US</country></postal>
1939        <phone>+1.408.246.8253</phone>
1940        <email>dcrocker@bbiw.net</email></address> 
1941      </author>
1942      <author initials="P." surname="Overell" fullname="Paul Overell">
1943        <organization>THUS plc.</organization>
1944        <address>
1945        <postal>
1946        <street>1/2 Berkeley Square</street>
1947        <street>99 Berkely Street</street>
1948        <city>Glasgow</city>
1949        <code>G3 7HR</code>
1950        <country>UK</country></postal>
1951        <email>paul.overell@thus.net</email></address>
1952      </author>
1953      <date month="January" year="2008"/>
1954    </front>
1955    <seriesInfo name="STD" value="68"/>
1956    <seriesInfo name="RFC" value="5234"/>
1957  </reference>
1958 
1959</references>
1960
1961<references title="Informative References">
1962
1963  <reference anchor="RFC1305">
1964    <front>
1965      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1966      <author fullname="David L. Mills" initials="D." surname="Mills">
1967        <organization>University of Delaware, Electrical Engineering Department</organization>
1968        <address><email>mills@udel.edu</email></address>
1969      </author>
1970      <date month="March" year="1992"/>
1971    </front>
1972    <seriesInfo name="RFC" value="1305"/>
1973  </reference>
1974
1975  <reference anchor="RFC2616">
1976    <front>
1977      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1978      <author fullname="R. Fielding" initials="R." surname="Fielding">
1979        <organization>University of California, Irvine</organization>
1980        <address><email>fielding@ics.uci.edu</email></address>
1981      </author>
1982      <author fullname="J. Gettys" initials="J." surname="Gettys">
1983        <organization>W3C</organization>
1984        <address><email>jg@w3.org</email></address>
1985      </author>
1986      <author fullname="J. Mogul" initials="J." surname="Mogul">
1987        <organization>Compaq Computer Corporation</organization>
1988        <address><email>mogul@wrl.dec.com</email></address>
1989      </author>
1990      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1991        <organization>MIT Laboratory for Computer Science</organization>
1992        <address><email>frystyk@w3.org</email></address>
1993      </author>
1994      <author fullname="L. Masinter" initials="L." surname="Masinter">
1995        <organization>Xerox Corporation</organization>
1996        <address><email>masinter@parc.xerox.com</email></address>
1997      </author>
1998      <author fullname="P. Leach" initials="P." surname="Leach">
1999        <organization>Microsoft Corporation</organization>
2000        <address><email>paulle@microsoft.com</email></address>
2001      </author>
2002      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2003        <organization>W3C</organization>
2004        <address><email>timbl@w3.org</email></address>
2005      </author>
2006      <date month="June" year="1999"/>
2007    </front>
2008    <seriesInfo name="RFC" value="2616"/>
2009  </reference>
2010
2011  <reference anchor="RFC3864">
2012    <front>
2013      <title>Registration Procedures for Message Header Fields</title>
2014      <author fullname="G. Klyne" initials="G." surname="Klyne">
2015        <organization>Nine by Nine</organization>
2016        <address><email>GK-IETF@ninebynine.org</email></address>
2017      </author>
2018      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2019        <organization>BEA Systems</organization>
2020        <address><email>mnot@pobox.com</email></address>
2021      </author>
2022      <author fullname="J. Mogul" initials="J." surname="Mogul">
2023        <organization>HP Labs</organization>
2024        <address><email>JeffMogul@acm.org</email></address>
2025      </author>
2026      <date month="September" year="2004"/>
2027    </front>
2028    <seriesInfo name="BCP" value="90"/>
2029    <seriesInfo name="RFC" value="3864"/>
2030  </reference>
2031
2032</references>
2033
2034<section anchor="compatibility" title="Compatibility with Previous Versions">
2035
2036<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2037<t>
2038  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2039  this missing case.
2040  (Sections <xref format="counter" target="response.cacheability"/>, <xref format="counter" target="header.cache-control"/>).
2041</t>
2042<t>
2043  Transfer-coding and message lengths all interact in ways that required fixing exactly
2044  when chunked encoding is used (to allow for transfer encoding that may not be self
2045  delimiting); it was important to straighten out exactly how message lengths are computed.
2046  (see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2047  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2048</t>
2049<t>
2050  Proxies should be able to add Content-Length when appropriate.
2051  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2052</t>
2053<t>Range request responses would become very verbose if all meta-data were always returned;
2054  by allowing the server to only send needed headers in a 206 response, this problem can be
2055  avoided.
2056  (<xref target="combining.headers"/>)
2057</t>
2058<t>
2059  The Cache-Control: max-age directive was not properly defined for responses.
2060  (<xref target="cache-response-directive"/>)
2061</t>
2062<t>
2063  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"/>,
2064  and <xref format="counter" target="header.warning"/>) Warning also needed to be a general
2065  header, as PUT or other methods may have need for it in requests.
2066</t>
2067</section>
2068
2069<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2070<t>
2071  Clarify denial of service attack avoidance requirement.
2072  (<xref target="invalidation.after.updates.or.deletions"/>)
2073</t>
2074</section>
2075
2076    </section>
2077
2078<section title="Collected ABNF" anchor="collected.abnf">
2079<figure>
2080<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2081Age = "Age:" OWS Age-v
2082Age-v = delta-seconds
2083
2084Cache-Control = "Cache-Control:" OWS Cache-Control-v
2085Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
2086 cache-directive ] )
2087
2088Expires = "Expires:" OWS Expires-v
2089Expires-v = HTTP-date
2090
2091HTTP-date = <HTTP-date, defined in [Part1], Section 3.2>
2092
2093OWS = <OWS, defined in [Part1], Section 1.2.2>
2094
2095Pragma = "Pragma:" OWS Pragma-v
2096Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2097 pragma-directive ] )
2098
2099Vary = "Vary:" OWS Vary-v
2100Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2101 ] ) )
2102
2103Warning = "Warning:" OWS Warning-v
2104Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2105 ] )
2106
2107cache-directive = cache-request-directive / cache-response-directive
2108cache-extension = token [ "=" ( token / quoted-string ) ]
2109cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2110 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2111 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2112 cache-extension
2113cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2114 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2115 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2116 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2117 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2118 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2119
2120delta-seconds = 1*DIGIT
2121
2122extension-pragma = token [ "=" ( token / quoted-string ) ]
2123
2124field-name = <field-name, defined in [Part1], Section 4.2>
2125
2126port = <port, defined in [Part1], Section 2.1>
2127pragma-directive = "no-cache" / extension-pragma
2128pseudonym = <pseudonym, defined in [Part1], Section 8.9>
2129
2130quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2131
2132token = <token, defined in [Part1], Section 1.2.2>
2133
2134uri-host = <uri-host, defined in [Part1], Section 2.1>
2135
2136warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2137warn-code = 3DIGIT
2138warn-date = DQUOTE HTTP-date DQUOTE
2139warn-text = quoted-string
2140warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2141 ]
2142
2143
2144]]></artwork>
2145</figure>
2146<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2147; Age defined but not used
2148; Cache-Control defined but not used
2149; Expires defined but not used
2150; Pragma defined but not used
2151; Vary defined but not used
2152; Warning defined but not used
2153]]></artwork></figure></section>
2154
2155<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2156
2157<section title="Since RFC2616">
2158  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2159</section>
2160
2161<section title="Since draft-ietf-httpbis-p6-cache-00">
2162<t>
2163  Closed issues:
2164  <list style="symbols">
2165    <t>
2166      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2167    <t>
2168      <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>
2169    <t>
2170      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2171    <t>
2172      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2173    <t>
2174      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2175    <t>
2176      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2177    <t>
2178      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2179    <t>
2180      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2181    <t>
2182      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2183  </list>
2184</t>
2185<t>
2186  Other changes:
2187  <list style="symbols">
2188    <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>
2189  </list>
2190</t>
2191</section>
2192
2193<section title="Since draft-ietf-httpbis-p6-cache-01">
2194<t>
2195  Closed issues:
2196  <list style="symbols">
2197    <t>
2198      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2199  </list>
2200</t>
2201<t>
2202  Other changes:
2203  <list style="symbols">
2204    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2205      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2206    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2207      specification.</t>
2208  </list>
2209</t>
2210</section>
2211
2212<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2213<t>
2214  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2215  <list style="symbols">
2216    <t>Reference RFC 3984, and update header registrations for headers defined in this
2217      document.</t>
2218  </list>
2219</t>
2220</section>
2221
2222<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2223<t>
2224  Closed issues:
2225  <list style="symbols">
2226    <t>
2227      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2228  </list>
2229</t>
2230</section>
2231
2232<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2233<t>
2234  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2235  <list style="symbols"> 
2236    <t>
2237      Use "/" instead of "|" for alternatives.
2238    </t>
2239    <t>
2240      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2241      whitespace ("OWS") and required whitespace ("RWS").
2242    </t>
2243    <t>
2244      Rewrite ABNFs to spell out whitespace rules, factor out
2245      header value format definitions.
2246    </t>
2247  </list>
2248</t>
2249</section>
2250
2251<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2252<t>
2253  This is a total rewrite of this part of the specification.
2254</t>
2255<t>
2256  Affected issues:
2257  <list style="symbols">
2258    <t>
2259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2260    <t>
2261      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2262    <t>
2263      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2264    <t>
2265      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2266  </list>
2267</t>
2268<t>
2269  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2270  <list style="symbols"> 
2271    <t>
2272      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2273    </t>
2274  </list>
2275</t>
2276</section>
2277
2278<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2279<t>
2280  Closed issues:
2281  <list style="symbols"> 
2282    <t>
2283      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2284      "base for numeric protocol elements"
2285    </t>
2286  </list>
2287</t>
2288</section>
2289
2290</section>
2291  </back>
2292</rfc>
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