source: draft-ietf-httpbis/07/draft-ietf-httpbis-p6-cache-07.xml @ 2082

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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  When a cache has one or more stored responses for a requested URI, but cannot 
683  serve any of them (e.g., because they are not fresh, or one cannot be selected;
684  see <xref target="caching.negotiated.responses"/>),
685  it can use the conditional request mechanism <xref target="Part4"/> in the forwarded
686  request to give the origin server an opportunity to both select a valid stored
687  response to be used, and to update it. This process is known as "validating"
688  or "revalidating" the stored response.
689</t>
690<t>
691  When sending such a conditional request, the cache SHOULD add an If-Modified-Since
692  header whose value is that of the Last-Modified header from the selected
693  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
694</t>
695<t>
696  Additionally, the cache SHOULD add an If-None-Match header whose value 
697  is that of the ETag header(s) from all responses stored for the requested URI,
698  if present. However, if any of the stored responses contains only partial
699  content, its entity-tag SHOULD NOT be included in the If-None-Match header
700  field unless the request is for a range that would be fully satisfied by
701  that stored response.
702</t>
703<t>
704  A 304 (Not Modified) response status code indicates that the stored
705  response can be updated and reused; see <xref target="combining.headers"/>.
706</t>
707<t>
708  A full response (i.e., one with a response body) indicates that none 
709  of the stored responses nominated in the conditional request is
710  suitable. Instead, the full response is used both to satisfy the
711  request and replace the stored response. <cref>Should there be a requirement here?</cref>
712</t>
713<t>
714  If a cache receives a 5xx response while attempting to validate a response, it MAY
715  either forward this response to the requesting client, or act as if the server failed to
716  respond. In the latter case, it MAY return a previously stored response (see <xref target="serving.stale.responses"/>).
717</t>
718<t>
719  If a cache receives a successful response whose Content-Location field 
720  matches that of an existing stored response for the same Request-URI, 
721  whose entity-tag differs from that of the existing stored response, 
722  and whose Date is more recent than that of the existing response, the 
723  existing response SHOULD NOT be returned in response to future 
724  requests and SHOULD be deleted from the cache. <cref>DISCUSS: Not 
725  sure if this is necessary.</cref>
726</t>
727</section>
728
729<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
730<t>
731  Because unsafe methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing state on the
732  origin server, intervening caches can use them to keep their contents
733  up-to-date.
734</t>
735<t>
736  The following HTTP methods MUST cause a cache to invalidate the Request-URI as well
737  as the URI(s) in the Location and Content-Location headers (if present):
738  <list style="symbols">
739    <t>PUT</t>
740    <t>DELETE</t>
741    <t>POST</t>
742  </list>
743</t>
744<t>
745  An invalidation based on a URI from a Location or Content-Location header MUST NOT
746  be performed if the host part of that URI differs from the host part in the Request-URI.
747  This helps prevent denial of service attacks.
748</t>
749<t>
750  <cref>TODO: "host part" needs to be specified better.</cref>
751</t>
752<t>
753  A cache that passes through requests for methods it does not understand SHOULD
754  invalidate the Request-URI.
755</t>
756<t>
757  Here, "invalidate" means that the cache will either remove all stored responses related
758  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
759  before they can be returned in response to a subsequent request.
760</t>
761<t>
762  Note that this does not guarantee that all appropriate responses are invalidated. For
763  example, the request that caused the change at the origin server might not have gone
764  through the cache where a response is stored.
765</t>
766<t>
767  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
768</t>
769</section>
770
771<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
772<t>
773  When a cache receives a request that can be satisfied by a stored response
774  that has a Vary header field (<xref target="header.vary"/>), it MUST NOT use that
775  response unless all of the selecting request-headers nominated by the Vary header match
776  in both the original request (i.e., that associated with the stored response),
777  and the presented request.
778</t>
779<t>
780  The selecting request-headers from two requests are defined to match if and only if the
781  selecting request-headers in the first request can be transformed to the selecting
782  request-headers in the second request by adding or removing linear white space
783  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
784  combining multiple message-header fields with the same field name following the rules
785  about message headers in Section 4.2 of <xref target="Part1"/>.
786</t>
787<t>
788  If a header field is absent from a request, it can only match another request
789  if it is also absent there.
790</t>
791<t>
792  A Vary header field-value of "*" always fails to match, and subsequent requests to that
793  resource can only be properly interpreted by the origin server.
794</t>
795<t>
796  The stored response with matching selecting request-headers is known as the
797  selected response.
798</t>
799<t>
800  If no selected response is available, the cache MAY forward the presented
801  request to the origin server in a conditional request; see <xref target="validation.model"/>.
802</t>
803</section>
804
805<section anchor="combining.headers" title="Combining Responses">
806<t>
807  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
808  (in this section, the "new" response"), it needs to created an updated response by combining
809  the stored response with the new one, so that the updated response can be used to satisfy the request.
810</t>
811<t>
812  If the new response contains an ETag, it identifies the stored 
813  response to use. <cref>may need language about Content-Location 
814  here</cref><cref>cover case where INM with multiple etags was sent</cref>
815</t>
816<t>
817  If the status code is 206 (partial content), both the stored and new 
818  responses MUST have ETags, and those ETags MUST match using the strong 
819  comparison function (see Section 4 of <xref target="Part4"/>). Otherwise, the 
820  responses MUST NOT be combined.
821</t>
822<t>
823  The stored response headers are used as those of the updated response, except that
824  <list style="symbols">
825    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
826      MUST be deleted from the stored response and the updated response.</t>
827    <t>any stored Warning headers with warn-code 2xx MUST be retained in the stored
828      response and the updated response.</t>
829    <t>any headers provided in the new response MUST replace the corresponding
830      headers from the stored response.</t>
831  </list>
832</t>
833<t>
834  If a header field-name in the new response matches more than one 
835  header in the stored response, all such stored headers MUST be replaced.
836</t>
837<t>
838  The updated response can [[<cref>requirement?</cref>]] be used to replace the 
839  stored response in cache. In the case of a 206 response, the combined 
840  entity-body MAY be stored.
841</t>
842<t>
843  <cref>ISSUE: discuss how to handle HEAD updates</cref>
844</t>
845</section>
846
847</section>
848
849<section anchor="header.fields" title="Header Field Definitions">
850<t>
851  This section defines the syntax and semantics of HTTP/1.1 header fields
852  related to caching.
853</t>
854<t>
855  For entity-header fields, both sender and recipient refer to either the client or the
856  server, depending on who sends and who receives the entity.
857</t>
858
859<section anchor="header.age" title="Age">
860  <iref item="Age header" primary="true"/>
861  <iref item="Headers" primary="true" subitem="Age"/>
862 
863 
864 
865<t>
866  The response-header field "Age" conveys the sender's estimate of the amount of time since
867  the response (or its validation) was generated at the origin server. Age values are
868  calculated as specified in <xref target="age.calculations"/>.
869</t>
870<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/><artwork type="abnf2616"><![CDATA[
871  Age   = "Age" ":" OWS Age-v
872  Age-v = delta-seconds
873]]></artwork></figure>
874<t anchor="rule.delta-seconds">
875 
876  Age field-values are non-negative integers, representing time in seconds.
877</t>
878<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
879  delta-seconds  = 1*DIGIT
880]]></artwork></figure>
881<t>
882  If a cache receives a value larger than the largest positive integer it can represent, or
883  if any of its age calculations overflows, it MUST transmit an Age header with a
884  field-value of 2147483648 (2^31). Caches SHOULD use an arithmetic type
885  of at least 31 bits of range.
886</t>
887<t>
888  The presence of an Age header field in a response implies that a response is not
889  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
890  Age header field.
891</t>
892</section>
893
894<section anchor="header.cache-control" title="Cache-Control">
895  <iref item="Cache-Control header" primary="true"/>
896  <iref item="Headers" primary="true" subitem="Cache-Control"/>
897 
898 
899 
900 
901 
902 
903<t>
904  The general-header field "Cache-Control" is used to specify directives that MUST be
905  obeyed by all caches along the request/response chain. The directives specify behavior
906  intended to prevent caches from adversely interfering with the request or response. Cache
907  directives are unidirectional in that the presence of a directive in a request does not
908  imply that the same directive is to be given in the response.
909</t>
910<t><list>
911  <t>
912    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
913    Pragma: no-cache (see <xref target="header.pragma"/>).
914  </t>
915</list></t>
916<t>
917  Cache directives MUST be passed through by a proxy or gateway application,
918  regardless of their significance to that application, since the directives might be
919  applicable to all recipients along the request/response chain. It is not possible to
920  target a directive to a specific cache.
921</t>
922<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[
923  Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
924  Cache-Control-v = 1#cache-directive
925
926  cache-directive = cache-request-directive
927     / cache-response-directive
928
929  cache-extension = token [ "=" ( token / quoted-string ) ]
930]]></artwork></figure>
931
932<section anchor="cache-request-directive" title="Request Cache-Control Directives">
933 
934
935<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
936  cache-request-directive =
937       "no-cache"
938     / "no-store"
939     / "max-age" "=" delta-seconds
940     / "max-stale" [ "=" delta-seconds ]
941     / "min-fresh" "=" delta-seconds
942     / "no-transform"
943     / "only-if-cached"
944     / cache-extension
945]]></artwork></figure>
946
947<t>
948  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
949  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
950  no-cache
951  <list>
952    <t>The no-cache request directive indicates that a stored response MUST NOT be
953      used to satisfy the request without successful validation on the origin server.</t>
954  </list>
955</t>
956<t>
957  <iref item="Cache Directives" primary="true" subitem="no-store"/>
958  <iref item="no-store" primary="true" subitem="Cache Directive"/>
959  no-store
960  <list>
961    <t>The no-store request directive indicates that a cache MUST NOT store any part
962      of either this request or any response to it. This directive applies to both
963      non-shared and shared caches. "MUST NOT store" in this context means that the
964      cache MUST NOT intentionally store the information in non-volatile storage,
965      and MUST make a best-effort attempt to remove the information from volatile
966      storage as promptly as possible after forwarding it.</t>
967    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
968      particular, malicious or compromised caches might not recognize or obey this
969      directive, and communications networks may be vulnerable to eavesdropping.</t>
970  </list>
971</t>
972<t>
973  <iref item="Cache Directives" primary="true" subitem="max-age"/>
974  <iref item="max-age" primary="true" subitem="Cache Directive"/>
975  max-age
976  <list>
977    <t>The max-age request directive indicates that the client is willing to accept a
978      response whose age is no greater than the specified time in seconds. Unless
979      max-stale directive is also included, the client is not willing to accept a stale
980      response.</t>
981  </list>
982</t>
983<t>
984  <iref item="Cache Directives" primary="true" subitem="max-stale"/>
985  <iref item="max-stale" primary="true" subitem="Cache Directive"/>
986  max-stale
987  <list>
988    <t>The max-stale request directive indicates that the client is willing to accept a
989      response that has exceeded its expiration time. If max-stale is assigned a value,
990      then the client is willing to accept a response that has exceeded its expiration
991      time by no more than the specified number of seconds. If no value is assigned to
992      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
993  </list>
994</t>
995<t>
996  <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
997  <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
998  min-fresh
999  <list>
1000    <t>The min-fresh request directive indicates that the client is willing to accept a
1001      response whose freshness lifetime is no less than its current age plus the specified
1002      time in seconds. That is, the client wants a response that will still be fresh for
1003      at least the specified number of seconds.</t>
1004  </list>
1005</t>
1006<t>
1007  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1008  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1009  no-transform
1010  <list>
1011    <t>The no-transform request directive indicates that an intermediate cache or proxy
1012      MUST NOT change the Content-Encoding, Content-Range or Content-Type request
1013      headers, nor the request entity-body.</t>
1014  </list>
1015</t>
1016<t>
1017  <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1018  <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1019  only-if-cached
1020  <list>
1021    <t>The only-if-cached request directive indicates that the client only wishes to
1022      return a stored response. If it receives this directive, a cache SHOULD either
1023      respond using a stored response that is consistent with the other constraints of the
1024      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1025      being operated as a unified system with good internal connectivity, such a request
1026      MAY be forwarded within that group of caches.</t>
1027  </list>
1028</t>
1029</section>
1030
1031<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1032 
1033
1034<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1035  cache-response-directive =
1036       "public"
1037     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1038     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1039     / "no-store"
1040     / "no-transform"
1041     / "must-revalidate"
1042     / "proxy-revalidate"
1043     / "max-age" "=" delta-seconds
1044     / "s-maxage" "=" delta-seconds
1045     / cache-extension
1046]]></artwork></figure>
1047
1048<t>
1049  <iref item="Cache Directives" primary="true" subitem="public"/>
1050  <iref item="public" primary="true" subitem="Cache Directive"/>
1051  public
1052  <list>
1053    <t>The public response directive indicates that the response MAY be cached, even
1054      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1055      (See also Authorization, Section 3.1 of <xref target="Part7"/>, for additional details.) </t>
1056  </list>
1057</t>
1058<t>
1059  <iref item="Cache Directives" primary="true" subitem="private"/>
1060  <iref item="private" primary="true" subitem="Cache Directive"/>
1061  private
1062  <list>
1063    <t>The private response directive indicates that the response message is intended for
1064      a single user and MUST NOT be stored by a shared cache. A private (non-shared)
1065      cache MAY store the response.</t>
1066    <t>If the private response directive specifies one or more field-names, this
1067      requirement is limited to the field-values associated with the listed response
1068      headers. That is, the specified field-names(s) MUST NOT be stored by a shared
1069      cache, whereas the remainder of the response message MAY be.</t>
1070    <t>
1071      Note: This usage of the word private only controls where the response may
1072      be stored, and cannot ensure the privacy of the message content.</t>
1073  </list>
1074</t>
1075<t>
1076  <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1077  <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1078  no-cache
1079  <list>
1080    <t>The no-cache response directive indicates that the response MUST NOT be used to
1081      satisfy a subsequent request without successful validation on the origin server.
1082      This allows an origin server to prevent caching even by caches that have been
1083      configured to return stale responses.</t>
1084    <t>If the no-cache response directive specifies one or more field-names, this
1085      requirement is limited to the field-values associated with the listed response
1086      headers. That is, the specified field-name(s) MUST NOT be sent in the response
1087      to a subsequent request without successful validation on the origin server. This
1088      allows an origin server to prevent the re-use of certain header fields in a
1089      response, while still allowing caching of the rest of the response.</t>
1090    <t>
1091      Note: Most HTTP/1.0 caches will not recognize or obey this directive.
1092    </t>
1093  </list>
1094</t>
1095
1096<t>
1097  <iref item="Cache Directives" primary="true" subitem="no-store"/>
1098  <iref item="no-store" primary="true" subitem="Cache Directive"/>
1099  no-store
1100  <list>
1101    <t>The no-store response directive indicates that a cache MUST NOT store any
1102      part of either the immediate request or response. This directive applies to both
1103      non-shared and shared caches. "MUST NOT store" in this context means that the
1104      cache MUST NOT intentionally store the information in non-volatile storage,
1105      and MUST make a best-effort attempt to remove the information from volatile
1106      storage as promptly as possible after forwarding it.</t>
1107    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1108      particular, malicious or compromised caches might not recognize or obey this
1109      directive, and communications networks may be vulnerable to eavesdropping.</t>
1110  </list>
1111</t>
1112<t>
1113  <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1114  <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1115  must-revalidate
1116  <list>
1117    <t>The must-revalidate response directive indicates that once it has become stale, the response MUST NOT be
1118     used to satisfy subsequent requests without successful validation on the origin server.</t>
1119    <t>The must-revalidate directive is necessary to support reliable operation for
1120      certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey
1121      the must-revalidate directive; in particular, if the cache cannot reach the origin
1122      server for any reason, it MUST generate a 504 (Gateway Timeout) response.</t>
1123    <t>Servers SHOULD send the must-revalidate directive if and only if failure to
1124      validate a request on the entity could result in incorrect operation, such as a
1125      silently unexecuted financial transaction.</t>
1126  </list>
1127</t>
1128<t>
1129  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1130  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1131  proxy-revalidate
1132  <list>
1133    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1134      response directive, except that it does not apply to non-shared caches.</t>
1135  </list>
1136</t>
1137<t>
1138  <iref item="Cache Directives" primary="true" subitem="max-age"/>
1139  <iref item="max-age" primary="true" subitem="Cache Directive"/>
1140  max-age
1141  <list>
1142    <t>The max-age response directive indicates that response is to be considered stale
1143      after its age is greater than the specified number of seconds.</t>
1144  </list>
1145</t>
1146<t>
1147  <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1148  <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1149  s-maxage
1150  <list>
1151    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1152      specified by this directive overrides the maximum age specified by either the
1153      max-age directive or the Expires header. The s-maxage directive also implies the
1154      semantics of the proxy-revalidate response directive.</t>
1155  </list>
1156</t>
1157<t>
1158  <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1159  <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1160  no-transform
1161  <list>
1162    <t>The no-transform response directive indicates that an intermediate cache or proxy
1163      MUST NOT change the Content-Encoding, Content-Range or Content-Type response
1164      headers, nor the response entity-body.</t>
1165  </list>
1166</t>
1167
1168</section>
1169
1170<section anchor="cache.control.extensions" title="Cache Control Extensions">
1171<t>
1172  The Cache-Control header field can be extended through the use of one or more
1173  cache-extension tokens, each with an optional value. Informational extensions (those
1174  that do not require a change in cache behavior) can be added without changing the
1175  semantics of other directives. Behavioral extensions are designed to work by acting as
1176  modifiers to the existing base of cache directives. Both the new directive and the
1177  standard directive are supplied, such that applications that do not understand the new
1178  directive will default to the behavior specified by the standard directive, and those
1179  that understand the new directive will recognize it as modifying the requirements
1180  associated with the standard directive. In this way, extensions to the cache-control
1181  directives can be made without requiring changes to the base protocol.
1182</t>
1183<t>
1184  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1185  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1186  all directives that it does not understand.
1187</t>
1188<t>
1189  For example, consider a hypothetical new response directive called "community" that
1190  acts as a modifier to the private directive. We define this new directive to mean that,
1191  in addition to any non-shared cache, any cache that is shared only by members of the
1192  community named within its value may cache the response. An origin server wishing to
1193  allow the UCI community to use an otherwise private response in their shared cache(s)
1194  could do so by including
1195</t>
1196<figure><artwork type="example"><![CDATA[
1197  Cache-Control: private, community="UCI"
1198]]></artwork></figure>
1199<t>
1200  A cache seeing this header field will act correctly even if the cache does not
1201  understand the community cache-extension, since it will also see and understand the
1202  private directive and thus default to the safe behavior.
1203</t>
1204<t>
1205  Unrecognized cache directives MUST be ignored; it is assumed that any cache
1206  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1207  directives (or the response's default cacheability) such that the cache behavior will
1208  remain minimally correct even if the cache does not understand the extension(s).
1209</t>
1210</section>
1211
1212</section>
1213
1214<section anchor="header.expires" title="Expires">
1215  <iref item="Expires header" primary="true"/>
1216  <iref item="Headers" primary="true" subitem="Expires"/>
1217 
1218 
1219<t>
1220  The entity-header field "Expires" gives the date/time after which the response is
1221  considered stale. See <xref target="expiration.model"/> for further discussion of the
1222  freshness model.
1223</t>
1224<t>
1225  The presence of an Expires field does not imply that the original resource will change or
1226  cease to exist at, before, or after that time.
1227</t>
1228<t>
1229  The field-value is an absolute date and time as defined by HTTP-date in Section 3.2 of <xref target="Part1"/>;
1230  it MUST be sent in rfc1123-date format.
1231</t>
1232<figure><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/><artwork type="abnf2616"><![CDATA[
1233  Expires   = "Expires" ":" OWS Expires-v
1234  Expires-v = HTTP-date
1235]]></artwork></figure>
1236<figure>
1237  <preamble>For example</preamble>
1238<artwork type="example"><![CDATA[
1239  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1240]]></artwork></figure>
1241<t><list>
1242  <t>
1243    Note: if a response includes a Cache-Control field with the max-age
1244    directive (see <xref target="cache-response-directive"/>), that directive overrides
1245    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1246  </t>
1247</list></t>
1248<t>
1249  HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.
1250</t>
1251<t>
1252  HTTP/1.1 clients and caches MUST treat other invalid date formats, especially
1253  including the value "0", as in the past (i.e., "already expired").
1254</t>
1255</section>
1256
1257<section anchor="header.pragma" title="Pragma">
1258  <iref item="Pragma header" primary="true"/>
1259  <iref item="Headers" primary="true" subitem="Pragma"/>
1260 
1261 
1262 
1263 
1264<t>
1265  The general-header field "Pragma" is used to include implementation-specific directives
1266  that might apply to any recipient along the request/response chain. All pragma directives
1267  specify optional behavior from the viewpoint of the protocol; however, some systems
1268  MAY require that behavior be consistent with the directives.
1269</t>
1270<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[
1271  Pragma            = "Pragma" ":" OWS Pragma-v
1272  Pragma-v          = 1#pragma-directive
1273  pragma-directive  = "no-cache" / extension-pragma
1274  extension-pragma  = token [ "=" ( token / quoted-string ) ]
1275]]></artwork></figure>
1276<t>
1277  When the no-cache directive is present in a request message, an application SHOULD
1278  forward the request toward the origin server even if it has a cached copy of what is being
1279  requested. This pragma directive has the same semantics as the no-cache response directive
1280  (see <xref target="cache-response-directive"/>) and is defined here for backward
1281  compatibility with HTTP/1.0. Clients SHOULD include both header fields when a
1282  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1283  SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1284</t>
1285<t><list>
1286  <t>
1287    Note: because the meaning of "Pragma: no-cache" as a response-header field
1288    is not actually specified, it does not provide a reliable replacement for
1289    "Cache-Control: no-cache" in a response.
1290  </t>
1291</list></t>
1292<t>
1293  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1294</t>
1295</section>
1296
1297<section anchor="header.vary" title="Vary">
1298  <iref item="Vary header" primary="true"/>
1299  <iref item="Headers" primary="true" subitem="Vary"/>
1300 
1301 
1302<t>
1303  The "Vary" response-header field's value indicates the set of request-header fields that
1304  determines, while the response is fresh, whether a cache is permitted to use the
1305  response to reply to a subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1306</t>
1307<t>
1308  In uncacheable or stale responses, the Vary field value advises the user agent about
1309  the criteria that were used to select the representation.
1310</t>
1311<figure><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/><artwork type="abnf2616"><![CDATA[
1312  Vary   = "Vary" ":" OWS Vary-v
1313  Vary-v = "*" / 1#field-name
1314]]></artwork></figure>
1315<t>
1316  The set of header fields named by the Vary field value is known as the selecting
1317  request-headers.
1318</t>
1319<t>
1320  Servers SHOULD include a Vary header field with any cacheable response that is
1321  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1322  requests on that resource and informs the user agent about the presence of negotiation on
1323  that resource. A server MAY include a Vary header field with a non-cacheable
1324  response that is subject to server-driven negotiation, since this might provide the user
1325  agent with useful information about the dimensions over which the response varies at the
1326  time of the response.
1327</t>
1328<t>
1329  A Vary field value of "*" signals that unspecified parameters not limited to the
1330  request-headers (e.g., the network address of the client), play a role in the selection of
1331  the response representation; therefore, a cache cannot determine whether this response is
1332  appropriate. The "*" value MUST NOT be generated by a proxy server;
1333  it may only be generated by an origin server.
1334</t>
1335<t>
1336  The field-names given are not limited to the set of standard request-header fields
1337  defined by this specification. Field names are case-insensitive.
1338</t>
1339</section>
1340
1341<section anchor="header.warning" title="Warning">
1342  <iref item="Warning header" primary="true"/>
1343  <iref item="Headers" primary="true" subitem="Warning"/>
1344 
1345 
1346 
1347 
1348 
1349 
1350 
1351<t>
1352  The general-header field "Warning" is used to carry additional information about the status
1353  or transformation of a message that might not be reflected in the message. This
1354  information is typically used to warn about possible incorrectness introduced by caching
1355  operations or transformations applied to the entity body of the message.
1356</t>
1357<t>
1358  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1359  warning, rather than an error status code, distinguish these responses from true failures.
1360</t>
1361<t>
1362  Warning headers can in general be applied to any message, however some warn-codes are
1363  specific to caches and can only be applied to response messages.
1364</t>
1365<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[
1366  Warning    = "Warning" ":" OWS Warning-v
1367  Warning-v  = 1#warning-value
1368 
1369  warning-value = warn-code SP warn-agent SP warn-text
1370                                        [SP warn-date]
1371 
1372  warn-code  = 3DIGIT
1373  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1374                  ; the name or pseudonym of the server adding
1375                  ; the Warning header, for use in debugging
1376  warn-text  = quoted-string
1377  warn-date  = DQUOTE HTTP-date DQUOTE
1378]]></artwork></figure>
1379<t>
1380  Multiple warnings can be attached to a response (either by the origin server or by
1381  a cache), including multiple warnings with the same code number. For example, a server
1382  might provide the same warning with texts in both English and Basque.
1383</t>
1384<t>
1385  When this occurs, the user agent SHOULD inform the user of as many of them as
1386  possible, in the order that they appear in the response. If it is not possible to inform
1387  the user of all of the warnings, the user agent SHOULD follow these heuristics:
1388  <list style="symbols">
1389    <t>Warnings that appear early in the response take priority over those appearing later
1390      in the response.</t>
1391    <t>Warnings in the user's preferred character set take priority over warnings in other
1392      character sets but with identical warn-codes and warn-agents.</t>
1393  </list>
1394</t>
1395<t>
1396  Systems that generate multiple Warning headers SHOULD order them with this user
1397  agent behavior in mind. New Warning headers SHOULD be added after any existing
1398  Warning headers.
1399</t>
1400<t>
1401  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1402  Warning is required to be deleted from a stored response after validation:
1403  <list style="symbols">
1404    <t>1xx Warnings that describe the freshness or validation status of the response, and so
1405      MUST be deleted by caches after validation. They MUST NOT be generated by a cache
1406      except when validating a cached entry, and MUST NOT be generated by clients.</t>
1407    <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1408      not rectified by a validation (for example, a lossy compression of the entity bodies)
1409      and MUST NOT be deleted by caches after validation, unless a full response is
1410      returned, in which case they MUST be.</t>
1411  </list>
1412</t>
1413<t>
1414  The warn-text SHOULD be in a natural language and character set that is most likely
1415  to be intelligible to the human user receiving the response. This decision can be based on
1416  any available knowledge, such as the location of the cache or user, the Accept-Language
1417  field in a request, the Content-Language field in a response, etc. The default language is
1418  English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
1419</t>
1420<t>
1421  If a character set other than ISO-8859-1 is used, it MUST be encoded in the
1422  warn-text using the method described in <xref target="RFC2047"/>.
1423</t>
1424<t>
1425  If an implementation sends a message with one or more Warning headers to a receiver whose
1426  version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a
1427  warn-date that matches the Date header in the message.
1428</t>
1429<t>
1430  If an implementation receives a message with a warning-value that includes a warn-date,
1431  and that warn-date is different from the Date value in the response, then that
1432  warning-value MUST be deleted from the message before storing, forwarding, or using
1433  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1434  warning-values are deleted for this reason, the Warning header MUST be deleted as
1435  well.
1436</t>
1437<t>
1438  The following warn-codes are defined by this specification, each with a recommended
1439  warn-text in English, and a description of its meaning.
1440</t>
1441<t>
1442<?rfc needLines="4"?>
1443  110 Response is stale
1444  <list>
1445    <t>SHOULD be included whenever the returned response is stale.</t>
1446  </list>
1447</t>
1448<t>
1449<?rfc needLines="4"?>
1450  111 Revalidation failed
1451  <list>
1452    <t>SHOULD be included if a cache returns a stale response because an attempt to
1453      validate the response failed, due to an inability to reach the server.</t>
1454  </list>
1455</t>
1456<t>
1457<?rfc needLines="4"?>
1458  112 Disconnected operation
1459  <list>
1460    <t>SHOULD be included if the cache is intentionally disconnected from the rest of
1461      the network for a period of time.</t>
1462  </list>
1463</t>
1464<t>
1465<?rfc needLines="4"?>
1466  113 Heuristic expiration
1467  <list>
1468    <t>SHOULD be included if the cache heuristically chose a freshness lifetime
1469      greater than 24 hours and the response's age is greater than 24 hours.</t>
1470  </list>
1471</t>
1472<t>
1473<?rfc needLines="4"?>
1474  199 Miscellaneous warning
1475  <list>
1476    <t>The warning text can include arbitrary information to be presented to a human
1477      user, or logged. A system receiving this warning MUST NOT take any automated
1478      action, besides presenting the warning to the user.</t>
1479  </list>
1480</t>
1481<t>
1482<?rfc needLines="4"?>
1483  214 Transformation applied
1484  <list>
1485    <t>MUST be added by an intermediate cache or proxy if it applies any
1486      transformation changing the content-coding (as specified in the Content-Encoding
1487      header) or media-type (as specified in the Content-Type header) of the response, or
1488      the entity-body of the response, unless this Warning code already appears in the
1489      response.</t>
1490  </list>
1491</t>
1492<t>
1493<?rfc needLines="4"?>
1494  299 Miscellaneous persistent warning
1495  <list>
1496    <t>The warning text can include arbitrary information to be presented to a human
1497      user, or logged. A system receiving this warning MUST NOT take any automated
1498      action.</t>
1499  </list>
1500</t>
1501</section>
1502
1503</section>
1504
1505<section anchor="history.lists" title="History Lists">
1506<t>
1507  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1508  can be used to redisplay an entity retrieved earlier in a session.
1509</t>
1510<t>
1511  History mechanisms and caches are different. In particular history mechanisms
1512  SHOULD NOT try to show a correct view of the current state of a resource. Rather, a
1513  history mechanism is meant to show exactly what the user saw at the time when the resource
1514  was retrieved.
1515</t>
1516<t>
1517  By default, an expiration time does not apply to history mechanisms. If the entity is still
1518  in storage, a history mechanism SHOULD display it even if the entity has expired,
1519  unless the user has specifically configured the agent to refresh expired history documents.
1520</t>
1521<t>
1522  This is not to be construed to prohibit the history mechanism from telling the user that a
1523  view might be stale.
1524</t>
1525<t><list>
1526  <t>
1527    Note: if history list mechanisms unnecessarily prevent users from viewing
1528    stale resources, this will tend to force service authors to avoid using HTTP expiration
1529    controls and cache controls when they would otherwise like to. Service authors may
1530    consider it important that users not be presented with error messages or warning
1531    messages when they use navigation controls (such as BACK) to view previously fetched
1532    resources. Even though sometimes such resources ought not be cached, or ought to expire
1533    quickly, user interface considerations may force service authors to resort to other
1534    means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1535    of improperly functioning history mechanisms.
1536  </t>
1537</list></t>
1538</section>
1539
1540
1541<section anchor="IANA.considerations" title="IANA Considerations">
1542
1543<section anchor="message.header.registration" title="Message Header Registration">
1544<t>
1545  The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1546  should be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1547</t>
1548
1549<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1550<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1551  <ttcol>Header Field Name</ttcol>
1552  <ttcol>Protocol</ttcol>
1553  <ttcol>Status</ttcol>
1554  <ttcol>Reference</ttcol>
1555
1556  <c>Age</c>
1557  <c>http</c>
1558  <c>standard</c>
1559  <c>
1560    <xref target="header.age"/>
1561  </c>
1562
1563  <c>Cache-Control</c>
1564  <c>http</c>
1565  <c>standard</c>
1566  <c>
1567    <xref target="header.cache-control"/>
1568  </c>
1569
1570  <c>Expires</c>
1571  <c>http</c>
1572  <c>standard</c>
1573  <c>
1574    <xref target="header.expires"/>
1575  </c>
1576
1577  <c>Pragma</c>
1578  <c>http</c>
1579  <c>standard</c>
1580  <c>
1581    <xref target="header.pragma"/>
1582  </c>
1583
1584  <c>Vary</c>
1585  <c>http</c>
1586  <c>standard</c>
1587  <c>
1588    <xref target="header.vary"/>
1589  </c>
1590
1591  <c>Warning</c>
1592  <c>http</c>
1593  <c>standard</c>
1594  <c>
1595    <xref target="header.warning"/>
1596  </c>
1597</texttable>
1598<!--(END)-->
1599<t>
1600  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1601</t>
1602</section>
1603
1604</section>
1605
1606<section anchor="security.considerations" title="Security Considerations">
1607<t>
1608  Caches expose additional potential vulnerabilities, since the contents of the cache
1609  represent an attractive target for malicious exploitation. Because cache contents persist
1610  after an HTTP request is complete, an attack on the cache can reveal information long after
1611  a user believes that the information has been removed from the network. Therefore, cache
1612  contents should be protected as sensitive information.
1613</t>
1614</section>
1615
1616<section anchor="ack" title="Acknowledgments">
1617<t>
1618  Much of the content and presentation of the caching design is due to suggestions and
1619  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1620  and Larry Masinter.
1621</t>
1622</section>
1623
1624</middle>
1625
1626<back>
1627<references title="Normative References">
1628
1629  <reference anchor="ISO-8859-1">
1630    <front>
1631      <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1632        1: Latin alphabet No. 1 </title>
1633      <author>
1634        <organization>International Organization for Standardization</organization>
1635      </author>
1636      <date year="1998"/>
1637    </front>
1638    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1639  </reference>
1640
1641  <reference anchor="Part1">
1642    <front>
1643      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1644      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1645        <organization abbrev="Day Software">Day Software</organization>
1646        <address><email>fielding@gbiv.com</email></address>
1647      </author>
1648      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1649        <organization>One Laptop per Child</organization>
1650        <address><email>jg@laptop.org</email></address>
1651      </author>
1652      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1653        <organization abbrev="HP">Hewlett-Packard Company</organization>
1654        <address><email>JeffMogul@acm.org</email></address>
1655      </author>
1656      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1657        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1658        <address><email>henrikn@microsoft.com</email></address>
1659      </author>
1660      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1661        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1662        <address><email>LMM@acm.org</email></address>
1663      </author>
1664      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1665        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1666        <address><email>paulle@microsoft.com</email></address>
1667      </author>
1668      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1669        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1670        <address><email>timbl@w3.org</email></address>
1671      </author>
1672      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1673        <organization abbrev="W3C">World Wide Web Consortium</organization>
1674        <address><email>ylafon@w3.org</email></address>
1675      </author>
1676      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1677        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1678        <address><email>julian.reschke@greenbytes.de</email></address>
1679      </author>
1680      <date month="July" year="2009"/>
1681    </front>
1682    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-07"/>
1683   
1684  </reference>
1685
1686  <reference anchor="Part2">
1687    <front>
1688      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1689      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1690        <organization abbrev="Day Software">Day Software</organization>
1691        <address><email>fielding@gbiv.com</email></address>
1692      </author>
1693      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1694        <organization>One Laptop per Child</organization>
1695        <address><email>jg@laptop.org</email></address>
1696      </author>
1697      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1698        <organization abbrev="HP">Hewlett-Packard Company</organization>
1699        <address><email>JeffMogul@acm.org</email></address>
1700      </author>
1701      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1702        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1703        <address><email>henrikn@microsoft.com</email></address>
1704      </author>
1705      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1706        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1707        <address><email>LMM@acm.org</email></address>
1708      </author>
1709      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1710        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1711        <address><email>paulle@microsoft.com</email></address>
1712      </author>
1713      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1714        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1715        <address><email>timbl@w3.org</email></address>
1716      </author>
1717      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1718        <organization abbrev="W3C">World Wide Web Consortium</organization>
1719        <address><email>ylafon@w3.org</email></address>
1720      </author>
1721      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1722        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1723        <address><email>julian.reschke@greenbytes.de</email></address>
1724      </author>
1725      <date month="July" year="2009"/>
1726    </front>
1727    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-07"/>
1728   
1729  </reference>
1730
1731  <reference anchor="Part3">
1732    <front>
1733      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1734      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1735        <organization abbrev="Day Software">Day Software</organization>
1736        <address><email>fielding@gbiv.com</email></address>
1737      </author>
1738      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1739        <organization>One Laptop per Child</organization>
1740        <address><email>jg@laptop.org</email></address>
1741      </author>
1742      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1743        <organization abbrev="HP">Hewlett-Packard Company</organization>
1744        <address><email>JeffMogul@acm.org</email></address>
1745      </author>
1746      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1747        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1748        <address><email>henrikn@microsoft.com</email></address>
1749      </author>
1750      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1751        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1752        <address><email>LMM@acm.org</email></address>
1753      </author>
1754      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1755        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1756        <address><email>paulle@microsoft.com</email></address>
1757      </author>
1758      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1759        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1760        <address><email>timbl@w3.org</email></address>
1761      </author>
1762      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1763        <organization abbrev="W3C">World Wide Web Consortium</organization>
1764        <address><email>ylafon@w3.org</email></address>
1765      </author>
1766      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1767        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1768        <address><email>julian.reschke@greenbytes.de</email></address>
1769      </author>
1770      <date month="July" year="2009"/>
1771    </front>
1772    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-07"/>
1773   
1774  </reference>
1775
1776  <reference anchor="Part4">
1777    <front>
1778      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1779      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1780        <organization abbrev="Day Software">Day Software</organization>
1781        <address><email>fielding@gbiv.com</email></address>
1782      </author>
1783      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1784        <organization>One Laptop per Child</organization>
1785        <address><email>jg@laptop.org</email></address>
1786      </author>
1787      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1788        <organization abbrev="HP">Hewlett-Packard Company</organization>
1789        <address><email>JeffMogul@acm.org</email></address>
1790      </author>
1791      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1792        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1793        <address><email>henrikn@microsoft.com</email></address>
1794      </author>
1795      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1796        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1797        <address><email>LMM@acm.org</email></address>
1798      </author>
1799      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1800        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1801        <address><email>paulle@microsoft.com</email></address>
1802      </author>
1803      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1804        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1805        <address><email>timbl@w3.org</email></address>
1806      </author>
1807      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1808        <organization abbrev="W3C">World Wide Web Consortium</organization>
1809        <address><email>ylafon@w3.org</email></address>
1810      </author>
1811      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1812        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1813        <address><email>julian.reschke@greenbytes.de</email></address>
1814      </author>
1815      <date month="July" year="2009"/>
1816    </front>
1817    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-07"/>
1818   
1819  </reference>
1820
1821  <reference anchor="Part5">
1822    <front>
1823      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1824      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1825        <organization abbrev="Day Software">Day Software</organization>
1826        <address><email>fielding@gbiv.com</email></address>
1827      </author>
1828      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1829        <organization>One Laptop per Child</organization>
1830        <address><email>jg@laptop.org</email></address>
1831      </author>
1832      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1833        <organization abbrev="HP">Hewlett-Packard Company</organization>
1834        <address><email>JeffMogul@acm.org</email></address>
1835      </author>
1836      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1837        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1838        <address><email>henrikn@microsoft.com</email></address>
1839      </author>
1840      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1841        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1842        <address><email>LMM@acm.org</email></address>
1843      </author>
1844      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1845        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1846        <address><email>paulle@microsoft.com</email></address>
1847      </author>
1848      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1849        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1850        <address><email>timbl@w3.org</email></address>
1851      </author>
1852      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1853        <organization abbrev="W3C">World Wide Web Consortium</organization>
1854        <address><email>ylafon@w3.org</email></address>
1855      </author>
1856      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1857        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1858        <address><email>julian.reschke@greenbytes.de</email></address>
1859      </author>
1860      <date month="July" year="2009"/>
1861    </front>
1862    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-07"/>
1863   
1864  </reference>
1865
1866  <reference anchor="Part7">
1867    <front>
1868      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1869      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1870        <organization abbrev="Day Software">Day Software</organization>
1871        <address><email>fielding@gbiv.com</email></address>
1872      </author>
1873      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1874        <organization>One Laptop per Child</organization>
1875        <address><email>jg@laptop.org</email></address>
1876      </author>
1877      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1878        <organization abbrev="HP">Hewlett-Packard Company</organization>
1879        <address><email>JeffMogul@acm.org</email></address>
1880      </author>
1881      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1882        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1883        <address><email>henrikn@microsoft.com</email></address>
1884      </author>
1885      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1886        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1887        <address><email>LMM@acm.org</email></address>
1888      </author>
1889      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1890        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1891        <address><email>paulle@microsoft.com</email></address>
1892      </author>
1893      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1894        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1895        <address><email>timbl@w3.org</email></address>
1896      </author>
1897      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1898        <organization abbrev="W3C">World Wide Web Consortium</organization>
1899        <address><email>ylafon@w3.org</email></address>
1900      </author>
1901      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1902        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1903        <address><email>julian.reschke@greenbytes.de</email></address>
1904      </author>
1905      <date month="July" year="2009"/>
1906    </front>
1907    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-07"/>
1908   
1909  </reference>
1910
1911  <reference anchor="RFC2047">
1912    <front>
1913      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1914        Part Three: Message Header Extensions for Non-ASCII Text</title>
1915      <author fullname="Keith Moore" initials="K." surname="Moore">
1916        <organization>University of Tennessee</organization>
1917        <address><email>moore@cs.utk.edu</email></address>
1918      </author>
1919      <date month="November" year="1996"/>
1920    </front>
1921    <seriesInfo name="RFC" value="2047"/>
1922  </reference>
1923
1924  <reference anchor="RFC2119">
1925    <front>
1926      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1927      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1928        <organization>Harvard University</organization>
1929        <address><email>sob@harvard.edu</email></address>
1930      </author>
1931      <date month="March" year="1997"/>
1932    </front>
1933    <seriesInfo name="BCP" value="14"/>
1934    <seriesInfo name="RFC" value="2119"/>
1935  </reference>
1936
1937  <reference anchor="RFC5234">
1938    <front>
1939      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1940      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1941        <organization>Brandenburg InternetWorking</organization>
1942        <address>
1943        <postal>
1944        <street>675 Spruce Dr.</street>
1945        <city>Sunnyvale</city>
1946        <region>CA</region>
1947        <code>94086</code>
1948        <country>US</country></postal>
1949        <phone>+1.408.246.8253</phone>
1950        <email>dcrocker@bbiw.net</email></address> 
1951      </author>
1952      <author initials="P." surname="Overell" fullname="Paul Overell">
1953        <organization>THUS plc.</organization>
1954        <address>
1955        <postal>
1956        <street>1/2 Berkeley Square</street>
1957        <street>99 Berkely Street</street>
1958        <city>Glasgow</city>
1959        <code>G3 7HR</code>
1960        <country>UK</country></postal>
1961        <email>paul.overell@thus.net</email></address>
1962      </author>
1963      <date month="January" year="2008"/>
1964    </front>
1965    <seriesInfo name="STD" value="68"/>
1966    <seriesInfo name="RFC" value="5234"/>
1967  </reference>
1968 
1969</references>
1970
1971<references title="Informative References">
1972
1973  <reference anchor="RFC1305">
1974    <front>
1975      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1976      <author fullname="David L. Mills" initials="D." surname="Mills">
1977        <organization>University of Delaware, Electrical Engineering Department</organization>
1978        <address><email>mills@udel.edu</email></address>
1979      </author>
1980      <date month="March" year="1992"/>
1981    </front>
1982    <seriesInfo name="RFC" value="1305"/>
1983  </reference>
1984
1985  <reference anchor="RFC2616">
1986    <front>
1987      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1988      <author fullname="R. Fielding" initials="R." surname="Fielding">
1989        <organization>University of California, Irvine</organization>
1990        <address><email>fielding@ics.uci.edu</email></address>
1991      </author>
1992      <author fullname="J. Gettys" initials="J." surname="Gettys">
1993        <organization>W3C</organization>
1994        <address><email>jg@w3.org</email></address>
1995      </author>
1996      <author fullname="J. Mogul" initials="J." surname="Mogul">
1997        <organization>Compaq Computer Corporation</organization>
1998        <address><email>mogul@wrl.dec.com</email></address>
1999      </author>
2000      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2001        <organization>MIT Laboratory for Computer Science</organization>
2002        <address><email>frystyk@w3.org</email></address>
2003      </author>
2004      <author fullname="L. Masinter" initials="L." surname="Masinter">
2005        <organization>Xerox Corporation</organization>
2006        <address><email>masinter@parc.xerox.com</email></address>
2007      </author>
2008      <author fullname="P. Leach" initials="P." surname="Leach">
2009        <organization>Microsoft Corporation</organization>
2010        <address><email>paulle@microsoft.com</email></address>
2011      </author>
2012      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2013        <organization>W3C</organization>
2014        <address><email>timbl@w3.org</email></address>
2015      </author>
2016      <date month="June" year="1999"/>
2017    </front>
2018    <seriesInfo name="RFC" value="2616"/>
2019  </reference>
2020
2021  <reference anchor="RFC3864">
2022    <front>
2023      <title>Registration Procedures for Message Header Fields</title>
2024      <author fullname="G. Klyne" initials="G." surname="Klyne">
2025        <organization>Nine by Nine</organization>
2026        <address><email>GK-IETF@ninebynine.org</email></address>
2027      </author>
2028      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2029        <organization>BEA Systems</organization>
2030        <address><email>mnot@pobox.com</email></address>
2031      </author>
2032      <author fullname="J. Mogul" initials="J." surname="Mogul">
2033        <organization>HP Labs</organization>
2034        <address><email>JeffMogul@acm.org</email></address>
2035      </author>
2036      <date month="September" year="2004"/>
2037    </front>
2038    <seriesInfo name="BCP" value="90"/>
2039    <seriesInfo name="RFC" value="3864"/>
2040  </reference>
2041
2042</references>
2043
2044<section anchor="compatibility" title="Compatibility with Previous Versions">
2045
2046<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2047<t>
2048  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2049  this missing case.
2050  (Sections <xref format="counter" target="response.cacheability"/>, <xref format="counter" target="header.cache-control"/>).
2051</t>
2052<t>
2053  Transfer-coding and message lengths all interact in ways that required fixing exactly
2054  when chunked encoding is used (to allow for transfer encoding that may not be self
2055  delimiting); it was important to straighten out exactly how message lengths are computed.
2056  (see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2057  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2058</t>
2059<t>
2060  Proxies should be able to add Content-Length when appropriate.
2061  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2062</t>
2063<t>Range request responses would become very verbose if all meta-data were always returned;
2064  by allowing the server to only send needed headers in a 206 response, this problem can be
2065  avoided.
2066  (<xref target="combining.headers"/>)
2067</t>
2068<t>
2069  The Cache-Control: max-age directive was not properly defined for responses.
2070  (<xref target="cache-response-directive"/>)
2071</t>
2072<t>
2073  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"/>,
2074  and <xref format="counter" target="header.warning"/>) Warning also needed to be a general
2075  header, as PUT or other methods may have need for it in requests.
2076</t>
2077</section>
2078
2079<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2080<t>
2081  Clarify denial of service attack avoidance requirement.
2082  (<xref target="invalidation.after.updates.or.deletions"/>)
2083</t>
2084</section>
2085
2086    </section>
2087
2088<section title="Collected ABNF" anchor="collected.abnf">
2089<figure>
2090<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2091Age = "Age:" OWS Age-v
2092Age-v = delta-seconds
2093
2094Cache-Control = "Cache-Control:" OWS Cache-Control-v
2095Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
2096 cache-directive ] )
2097
2098Expires = "Expires:" OWS Expires-v
2099Expires-v = HTTP-date
2100
2101HTTP-date = <HTTP-date, defined in [Part1], Section 3.2>
2102
2103OWS = <OWS, defined in [Part1], Section 1.2.2>
2104
2105Pragma = "Pragma:" OWS Pragma-v
2106Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2107 pragma-directive ] )
2108
2109Vary = "Vary:" OWS Vary-v
2110Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2111 ] ) )
2112
2113Warning = "Warning:" OWS Warning-v
2114Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2115 ] )
2116
2117cache-directive = cache-request-directive / cache-response-directive
2118cache-extension = token [ "=" ( token / quoted-string ) ]
2119cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2120 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2121 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2122 cache-extension
2123cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2124 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2125 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2126 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2127 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2128 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2129
2130delta-seconds = 1*DIGIT
2131
2132extension-pragma = token [ "=" ( token / quoted-string ) ]
2133
2134field-name = <field-name, defined in [Part1], Section 4.2>
2135
2136port = <port, defined in [Part1], Section 2.1>
2137pragma-directive = "no-cache" / extension-pragma
2138pseudonym = <pseudonym, defined in [Part1], Section 8.9>
2139
2140quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2141
2142token = <token, defined in [Part1], Section 1.2.2>
2143
2144uri-host = <uri-host, defined in [Part1], Section 2.1>
2145
2146warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2147warn-code = 3DIGIT
2148warn-date = DQUOTE HTTP-date DQUOTE
2149warn-text = quoted-string
2150warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2151 ]
2152
2153
2154]]></artwork>
2155</figure>
2156<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2157; Age defined but not used
2158; Cache-Control defined but not used
2159; Expires defined but not used
2160; Pragma defined but not used
2161; Vary defined but not used
2162; Warning defined but not used
2163]]></artwork></figure></section>
2164
2165<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2166
2167<section title="Since RFC2616">
2168  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2169</section>
2170
2171<section title="Since draft-ietf-httpbis-p6-cache-00">
2172<t>
2173  Closed issues:
2174  <list style="symbols">
2175    <t>
2176      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2177    <t>
2178      <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>
2179    <t>
2180      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2181    <t>
2182      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2183    <t>
2184      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2185    <t>
2186      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2187    <t>
2188      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2189    <t>
2190      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2191    <t>
2192      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2193  </list>
2194</t>
2195<t>
2196  Other changes:
2197  <list style="symbols">
2198    <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>
2199  </list>
2200</t>
2201</section>
2202
2203<section title="Since draft-ietf-httpbis-p6-cache-01">
2204<t>
2205  Closed issues:
2206  <list style="symbols">
2207    <t>
2208      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2209  </list>
2210</t>
2211<t>
2212  Other changes:
2213  <list style="symbols">
2214    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2215      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2216    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2217      specification.</t>
2218  </list>
2219</t>
2220</section>
2221
2222<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2223<t>
2224  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2225  <list style="symbols">
2226    <t>Reference RFC 3984, and update header registrations for headers defined in this
2227      document.</t>
2228  </list>
2229</t>
2230</section>
2231
2232<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2233<t>
2234  Closed issues:
2235  <list style="symbols">
2236    <t>
2237      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2238  </list>
2239</t>
2240</section>
2241
2242<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2243<t>
2244  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2245  <list style="symbols"> 
2246    <t>
2247      Use "/" instead of "|" for alternatives.
2248    </t>
2249    <t>
2250      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2251      whitespace ("OWS") and required whitespace ("RWS").
2252    </t>
2253    <t>
2254      Rewrite ABNFs to spell out whitespace rules, factor out
2255      header value format definitions.
2256    </t>
2257  </list>
2258</t>
2259</section>
2260
2261<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2262<t>
2263  This is a total rewrite of this part of the specification.
2264</t>
2265<t>
2266  Affected issues:
2267  <list style="symbols">
2268    <t>
2269      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2270    <t>
2271      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2272    <t>
2273      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2274    <t>
2275      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2276  </list>
2277</t>
2278<t>
2279  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2280  <list style="symbols"> 
2281    <t>
2282      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2283    </t>
2284  </list>
2285</t>
2286</section>
2287
2288<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2289<t>
2290  Closed issues:
2291  <list style="symbols"> 
2292    <t>
2293      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2294      "base for numeric protocol elements"
2295    </t>
2296  </list>
2297</t>
2298<t>
2299  Affected issues:
2300  <list style="symbols">
2301    <t>
2302      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2303      Vary and non-existant headers
2304    </t>
2305  </list>
2306</t>
2307</section>
2308
2309</section>
2310  </back>
2311</rfc>
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