source: draft-ietf-httpbis/06/draft-ietf-httpbis-p2-semantics-06.xml @ 784

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<!DOCTYPE rfc
17  PUBLIC "" "rfc2629.dtd">
18<rfc obsoletes="2616" updates="2817" category="std" ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p2-semantics-06">
19<front>
20
21  <title abbrev="HTTP/1.1, Part 2">HTTP/1.1, part 2: Message Semantics</title>
22
23  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
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 initials="J." surname="Gettys" fullname="Jim 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 initials="J." surname="Mogul" fullname="Jeffrey C. 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 initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
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 initials="L." surname="Masinter" fullname="Larry 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 initials="P." surname="Leach" fullname="Paul J. 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 initials="T." surname="Berners-Lee" fullname="Tim 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 initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
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 initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
146    <organization abbrev="greenbytes">greenbytes GmbH</organization>
147    <address>
148      <postal>
149        <street>Hafenweg 16</street>
150        <city>Muenster</city><region>NW</region><code>48155</code>
151        <country>Germany</country>
152      </postal>
153      <phone>+49 251 2807760</phone>   
154      <facsimile>+49 251 2807761</facsimile>   
155      <email>julian.reschke@greenbytes.de</email>       
156      <uri>http://greenbytes.de/tech/webdav/</uri>     
157    </address>
158  </author>
159
160  <date month="March" year="2009" day="9"/>
161  <workgroup>HTTPbis Working Group</workgroup>
162
163<abstract>
164<t>
165   The Hypertext Transfer Protocol (HTTP) is an application-level
166   protocol for distributed, collaborative, hypermedia information
167   systems. HTTP has been in use by the World Wide Web global information
168   initiative since 1990. This document is Part 2 of the seven-part specification
169   that defines the protocol referred to as "HTTP/1.1" and, taken together,
170   obsoletes RFC 2616.  Part 2 defines the semantics of HTTP messages
171   as expressed by request methods, request-header fields, response status codes,
172   and response-header fields.
173</t>
174</abstract>
175
176<note title="Editorial Note (To be removed by RFC Editor)">
177  <t>
178    Discussion of this draft should take place on the HTTPBIS working group
179    mailing list (ietf-http-wg@w3.org). The current issues list is
180    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
181    and related documents (including fancy diffs) can be found at
182    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
183  </t>
184  <t>
185    The changes in this draft are summarized in <xref target="changes.since.05"/>.
186  </t>
187</note>
188</front>
189<middle>
190<section title="Introduction" anchor="introduction">
191<t>
192   This document defines HTTP/1.1 request and response semantics.  Each HTTP
193   message, as defined in <xref target="Part1"/>, is in the form of either a request or
194   a response.  An HTTP server listens on a connection for HTTP requests and
195   responds to each request, in the order received on that connection, with
196   one or more HTTP response messages.  This document defines the commonly
197   agreed upon semantics of the HTTP uniform interface, the intentions defined
198   by each request method, and the various response messages that might be
199   expected as a result of applying that method for the requested resource.
200</t>
201<t>
202   This document is currently disorganized in order to minimize the changes
203   between drafts and enable reviewers to see the smaller errata changes.
204   The next draft will reorganize the sections to better reflect the content.
205   In particular, the sections will be ordered according to the typical
206   processing of an HTTP request message (after message parsing): resource
207   mapping, general header fields, methods, request modifiers, response
208   status, and resource metadata.  The current mess reflects how widely
209   dispersed these topics and associated requirements had become in
210   <xref target="RFC2616"/>.
211</t>
212
213<section title="Requirements" anchor="intro.requirements">
214<t>
215   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
216   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
217   document are to be interpreted as described in <xref target="RFC2119"/>.
218</t>
219<t>
220   An implementation is not compliant if it fails to satisfy one or more
221   of the MUST or REQUIRED level requirements for the protocols it
222   implements. An implementation that satisfies all the MUST or REQUIRED
223   level and all the SHOULD level requirements for its protocols is said
224   to be "unconditionally compliant"; one that satisfies all the MUST
225   level requirements but not all the SHOULD level requirements for its
226   protocols is said to be "conditionally compliant."
227</t>
228</section>
229
230<section title="Syntax Notation" anchor="notation">
231 
232 
233 
234 
235 
236<t>
237  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
238  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
239  <xref target="collected.abnf"/> shows the collected ABNF, with the list
240  rule expanded.
241</t>
242<t>
243  The following core rules are included by
244  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
245  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
246  DIGIT (decimal 0-9), DQUOTE (double quote),
247  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
248  OCTET (any 8-bit sequence of data), SP (space),
249  VCHAR (any visible USASCII character),
250  and WSP (whitespace).
251</t>
252
253<section title="Core Rules" anchor="core.rules">
254 
255 
256 
257 
258 
259 
260<t>
261  The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
262</t>
263<figure><artwork type="abnf2616"><![CDATA[
264  comment       = <comment, defined in [Part1], Section 1.2.2>
265  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
266  token         = <token, defined in [Part1], Section 1.2.2>
267  OWS           = <OWS, defined in [Part1], Section 1.2.2>
268  RWS           = <RWS, defined in [Part1], Section 1.2.2>
269  obs-text      = <obs-text, defined in [Part1], Section 1.2.2>
270]]></artwork></figure>
271</section>
272
273<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
274 
275 
276 
277 
278 
279 
280 
281 
282 
283 
284 
285 
286 
287 
288 
289 
290 
291 
292 
293 
294 
295 
296 
297 
298 
299<t>
300  The ABNF rules below are defined in other parts:
301</t>
302<figure><!--Part1--><artwork type="abnf2616"><![CDATA[
303  absolute-URI  = <absolute-URI, defined in [Part1], Section 2.1>
304  fragment      = <fragment, defined in [Part1], Section 2.1>
305  Host          = <Host, defined in [Part1], Section 2.1>
306  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.2.1>
307  partial-URI   = <partial-URI, defined in [Part1], Section 2.1>
308  product       = <product, defined in [Part1], Section 3.4>
309  TE            = <TE, defined in [Part1], Section 8.8>
310]]></artwork></figure>
311<figure><!--Part3--><artwork type="abnf2616"><![CDATA[
312  Accept        = <Accept, defined in [Part3], Section 5.1>
313  Accept-Charset =
314             <Accept-Charset, defined in [Part3], Section 5.2>
315  Accept-Encoding =
316             <Accept-Encoding, defined in [Part3], Section 5.3>
317  Accept-Language =
318             <Accept-Language, defined in [Part3], Section 5.4>
319]]></artwork></figure>
320<figure><!--Part4--><artwork type="abnf2616"><![CDATA[
321  ETag          = <ETag, defined in [Part4], Section 6.1>
322  If-Match      = <If-Match, defined in [Part4], Section 6.2>
323  If-Modified-Since =
324             <If-Modified-Since, defined in [Part4], Section 6.3>
325  If-None-Match = <If-None-Match, defined in [Part4], Section 6.4>
326  If-Unmodified-Since =
327             <If-Unmodified-Since, defined in [Part4], Section 6.5>
328]]></artwork></figure>
329<figure><!--Part5--><artwork type="abnf2616"><![CDATA[
330  Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 5.1>
331  If-Range      = <If-Range, defined in [Part5], Section 5.3>
332  Range         = <Range, defined in [Part5], Section 5.4>
333]]></artwork></figure>
334<figure><!--Part6--><artwork type="abnf2616"><![CDATA[
335  Age           = <Age, defined in [Part6], Section 3.1>
336  Vary          = <Vary, defined in [Part6], Section 3.5>
337]]></artwork><!--Part7--></figure>
338<figure><artwork type="abnf2616"><![CDATA[
339  Authorization = <Authorization, defined in [Part7], Section 3.1>
340  Proxy-Authenticate =
341             <Proxy-Authenticate, defined in [Part7], Section 3.2>
342  Proxy-Authorization =
343             <Proxy-Authorization, defined in [Part7], Section 3.3>
344  WWW-Authenticate =
345             <WWW-Authenticate, defined in [Part7], Section 3.4>
346]]></artwork></figure>
347</section>
348</section>
349</section>
350
351<section title="Method" anchor="method">
352 
353 
354<t>
355   The Method  token indicates the method to be performed on the
356   resource identified by the request-target. The method is case-sensitive.
357</t>
358<figure><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/><artwork type="abnf2616"><![CDATA[
359  Method         = %x4F.50.54.49.4F.4E.53   ; "OPTIONS", Section 7.2
360                 / %x47.45.54               ; "GET", Section 7.3
361                 / %x48.45.41.44            ; "HEAD", Section 7.4
362                 / %x50.4F.53.54            ; "POST", Section 7.5
363                 / %x50.55.54               ; "PUT", Section 7.6
364                 / %x44.45.4C.45.54.45      ; "DELETE", Section 7.7
365                 / %x54.52.41.43.45         ; "TRACE", Section 7.8
366                 / %x43.4F.4E.4E.45.43.54   ; "CONNECT", Section 7.9
367                 / extension-method
368  extension-method = token
369]]></artwork></figure>
370<t>
371   The list of methods allowed by a resource can be specified in an
372   Allow header field (<xref target="header.allow"/>). The return code of the response
373   always notifies the client whether a method is currently allowed on a
374   resource, since the set of allowed methods can change dynamically. An
375   origin server SHOULD return the status code 405 (Method Not Allowed)
376   if the method is known by the origin server but not allowed for the
377   requested resource, and 501 (Not Implemented) if the method is
378   unrecognized or not implemented by the origin server. The methods GET
379   and HEAD MUST be supported by all general-purpose servers. All other
380   methods are OPTIONAL; however, if the above methods are implemented,
381   they MUST be implemented with the same semantics as those specified
382   in <xref target="method.definitions"/>.
383</t>
384
385<section title="Method Registry" anchor="method.registry">
386<t>
387  The HTTP Method Registry defines the name space for the Method token in the
388  Request line of an HTTP request.
389</t>
390<t>
391  Registrations MUST include the following fields:
392  <list style="symbols">
393    <t>Method Name (see <xref target="method"/>)</t>
394    <t>Safe ("yes" or "no", see <xref target="safe.methods"/>)</t>
395    <t>Pointer to specification text</t>
396  </list>
397</t>
398<t>
399  Values to be added to this name space are subject to IETF review
400  (<xref target="RFC5226"/>, Section 4.1).  Any document registering
401  new method names should be traceable through statuses of either 'Obsoletes'
402  or 'Updates' to this document.
403</t>
404<t>
405  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-methods"/>.
406</t>
407</section>
408</section>
409
410<section title="Request Header Fields" anchor="request.header.fields">
411 
412<t>
413   The request-header fields allow the client to pass additional
414   information about the request, and about the client itself, to the
415   server. These fields act as request modifiers, with semantics
416   equivalent to the parameters on a programming language method
417   invocation.
418</t>
419<figure><iref primary="true" item="Grammar" subitem="request-header"/><artwork type="abnf2616"><![CDATA[
420  request-header = Accept                   ; [Part3], Section 5.1
421                 / Accept-Charset           ; [Part3], Section 5.2
422                 / Accept-Encoding          ; [Part3], Section 5.3
423                 / Accept-Language          ; [Part3], Section 5.4
424                 / Authorization            ; [Part7], Section 3.1
425                 / Expect                   ; Section 9.2
426                 / From                     ; Section 9.3
427                 / Host                     ; [Part1], Section 8.4
428                 / If-Match                 ; [Part4], Section 6.2
429                 / If-Modified-Since        ; [Part4], Section 6.3
430                 / If-None-Match            ; [Part4], Section 6.4
431                 / If-Range                 ; [Part5], Section 5.3
432                 / If-Unmodified-Since      ; [Part4], Section 6.5
433                 / Max-Forwards             ; Section 9.5
434                 / Proxy-Authorization      ; [Part7], Section 3.3
435                 / Range                    ; [Part5], Section 5.4
436                 / Referer                  ; Section 9.6
437                 / TE                       ; [Part1], Section 8.8
438                 / User-Agent               ; Section 9.9
439]]></artwork></figure>
440<t>
441   Request-header field names can be extended reliably only in
442   combination with a change in the protocol version. However, new or
443   experimental header fields MAY be given the semantics of request-header
444   fields if all parties in the communication recognize them to
445   be request-header fields. Unrecognized header fields are treated as
446   entity-header fields.
447</t>
448</section>
449
450<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
451 
452 
453 
454<t>
455   The Status-Code element is a 3-digit integer result code of the
456   attempt to understand and satisfy the request. The status codes listed
457   below are defined in <xref target="status.codes"/>.
458   The Reason-Phrase is intended to give a short
459   textual description of the Status-Code. The Status-Code is intended
460   for use by automata and the Reason-Phrase is intended for the human
461   user. The client is not required to examine or display the Reason-Phrase.
462</t>
463<t> 
464   The individual values of the numeric status codes defined for
465   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
466   presented below. The reason phrases listed here are only
467   recommendations -- they MAY be replaced by local equivalents without
468   affecting the protocol.
469</t>
470<figure><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/><artwork type="abnf2616"><![CDATA[
471  Status-Code    =
472         "100"  ; Section 8.1.1: Continue
473       / "101"  ; Section 8.1.2: Switching Protocols
474       / "200"  ; Section 8.2.1: OK
475       / "201"  ; Section 8.2.2: Created
476       / "202"  ; Section 8.2.3: Accepted
477       / "203"  ; Section 8.2.4: Non-Authoritative Information
478       / "204"  ; Section 8.2.5: No Content
479       / "205"  ; Section 8.2.6: Reset Content
480       / "206"  ; Section 8.2.7: Partial Content
481       / "300"  ; Section 8.3.1: Multiple Choices
482       / "301"  ; Section 8.3.2: Moved Permanently
483       / "302"  ; Section 8.3.3: Found
484       / "303"  ; Section 8.3.4: See Other
485       / "304"  ; Section 8.3.5: Not Modified
486       / "305"  ; Section 8.3.6: Use Proxy
487       / "307"  ; Section 8.3.8: Temporary Redirect
488       / "400"  ; Section 8.4.1: Bad Request
489       / "401"  ; Section 8.4.2: Unauthorized
490       / "402"  ; Section 8.4.3: Payment Required
491       / "403"  ; Section 8.4.4: Forbidden
492       / "404"  ; Section 8.4.5: Not Found
493       / "405"  ; Section 8.4.6: Method Not Allowed
494       / "406"  ; Section 8.4.7: Not Acceptable
495       / "407"  ; Section 8.4.8: Proxy Authentication Required
496       / "408"  ; Section 8.4.9: Request Time-out
497       / "409"  ; Section 8.4.10: Conflict
498       / "410"  ; Section 8.4.11: Gone
499       / "411"  ; Section 8.4.12: Length Required
500       / "412"  ; Section 8.4.13: Precondition Failed
501       / "413"  ; Section 8.4.14: Request Entity Too Large
502       / "414"  ; Section 8.4.15: URI Too Long
503       / "415"  ; Section 8.4.16: Unsupported Media Type
504       / "416"  ; Section 8.4.17: Requested range not satisfiable
505       / "417"  ; Section 8.4.18: Expectation Failed
506       / "500"  ; Section 8.5.1: Internal Server Error
507       / "501"  ; Section 8.5.2: Not Implemented
508       / "502"  ; Section 8.5.3: Bad Gateway
509       / "503"  ; Section 8.5.4: Service Unavailable
510       / "504"  ; Section 8.5.5: Gateway Time-out
511       / "505"  ; Section 8.5.6: HTTP Version not supported
512       / extension-code
513
514  extension-code = 3DIGIT
515  Reason-Phrase  = *( WSP / VCHAR / obs-text )
516]]></artwork></figure>
517<t>
518   HTTP status codes are extensible. HTTP applications are not required
519   to understand the meaning of all registered status codes, though such
520   understanding is obviously desirable. However, applications MUST
521   understand the class of any status code, as indicated by the first
522   digit, and treat any unrecognized response as being equivalent to the
523   x00 status code of that class, with the exception that an
524   unrecognized response MUST NOT be cached. For example, if an
525   unrecognized status code of 431 is received by the client, it can
526   safely assume that there was something wrong with its request and
527   treat the response as if it had received a 400 status code. In such
528   cases, user agents SHOULD present to the user the entity returned
529   with the response, since that entity is likely to include human-readable
530   information which will explain the unusual status.
531</t>
532
533<section title="Status Code Registry" anchor="status.code.registry">
534<t>
535  The HTTP Status Code Registry defines the name space for the Status-Code
536  token in the Status line of an HTTP response.
537</t>
538<t>
539  Values to be added to this name space are subject to IETF review
540  (<xref target="RFC5226"/>, Section 4.1).  Any document registering
541  new status codes should be traceable through statuses of either 'Obsoletes'
542  or 'Updates' to this document.
543</t>
544<t>
545  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-status-codes"/>.
546</t>
547</section>
548
549</section>
550
551<section title="Response Header Fields" anchor="response.header.fields">
552 
553<t>
554   The response-header fields allow the server to pass additional
555   information about the response which cannot be placed in the Status-Line.
556   These header fields give information about the server and about
557   further access to the resource identified by the request-target.
558</t>
559<figure><iref primary="true" item="Grammar" subitem="response-header"/><artwork type="abnf2616"><![CDATA[
560  response-header = Accept-Ranges           ; [Part5], Section 5.1
561                  / Age                     ; [Part6], Section 3.1
562                  / Allow                   ; Section 9.1
563                  / ETag                    ; [Part4], Section 6.1
564                  / Location                ; Section 9.4
565                  / Proxy-Authenticate      ; [Part7], Section 3.2
566                  / Retry-After             ; Section 9.7
567                  / Server                  ; Section 9.8
568                  / Vary                    ; [Part6], Section 3.5
569                  / WWW-Authenticate        ; [Part7], Section 3.4
570]]></artwork></figure>
571<t>
572   Response-header field names can be extended reliably only in
573   combination with a change in the protocol version. However, new or
574   experimental header fields MAY be given the semantics of response-header
575   fields if all parties in the communication recognize them to
576   be response-header fields. Unrecognized header fields are treated as
577   entity-header fields.
578</t>
579</section>
580
581<section title="Entity" anchor="entity">
582<t>
583   Request and Response messages MAY transfer an entity if not otherwise
584   restricted by the request method or response status code. An entity
585   consists of entity-header fields and an entity-body, although some
586   responses will only include the entity-headers. HTTP entity-body and
587   entity-header fields are defined in <xref target="Part3"/>.
588</t>
589<t>
590   An entity-body is only present in a message when a message-body is
591   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
592   from the message-body by decoding any Transfer-Encoding that might
593   have been applied to ensure safe and proper transfer of the message.
594</t>
595</section>
596
597
598<section title="Method Definitions" anchor="method.definitions">
599<t>
600   The set of common methods for HTTP/1.1 is defined below. Although
601   this set can be expanded, additional methods cannot be assumed to
602   share the same semantics for separately extended clients and servers.
603</t>
604
605<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
606
607<section title="Safe Methods" anchor="safe.methods">
608<iref item="Safe Methods" primary="true"/>
609<t>
610   Implementors should be aware that the software represents the user in
611   their interactions over the Internet, and should be careful to allow
612   the user to be aware of any actions they might take which may have an
613   unexpected significance to themselves or others.
614</t>
615<t>
616   In particular, the convention has been established that the GET and
617   HEAD methods SHOULD NOT  have the significance of taking an action
618   other than retrieval. These methods ought to be considered "safe".
619   This allows user agents to represent other methods, such as POST, PUT
620   and DELETE, in a special way, so that the user is made aware of the
621   fact that a possibly unsafe action is being requested.
622</t>
623<t>
624   Naturally, it is not possible to ensure that the server does not
625   generate side-effects as a result of performing a GET request; in
626   fact, some dynamic resources consider that a feature. The important
627   distinction here is that the user did not request the side-effects,
628   so therefore cannot be held accountable for them.
629</t>
630</section>
631
632<section title="Idempotent Methods" anchor="idempotent.methods">
633<iref item="Idempotent Methods" primary="true"/>
634<t>
635   Methods can also have the property of "idempotence" in that (aside
636   from error or expiration issues) the side-effects of N &gt; 0 identical
637   requests is the same as for a single request. The methods GET, HEAD,
638   PUT and DELETE share this property. Also, the methods OPTIONS and
639   TRACE SHOULD NOT  have side effects, and so are inherently idempotent.
640</t>
641<t>
642   However, it is possible that a sequence of several requests is non-idempotent,
643   even if all of the methods executed in that sequence are
644   idempotent. (A sequence is idempotent if a single execution of the
645   entire sequence always yields a result that is not changed by a
646   reexecution of all, or part, of that sequence.) For example, a
647   sequence is non-idempotent if its result depends on a value that is
648   later modified in the same sequence.
649</t>
650<t>
651   A sequence that never has side effects is idempotent, by definition
652   (provided that no concurrent operations are being executed on the
653   same set of resources).
654</t>
655</section>
656</section>
657
658<section title="OPTIONS" anchor="OPTIONS">
659 
660  <iref primary="true" item="OPTIONS method"/>
661  <iref primary="true" item="Methods" subitem="OPTIONS"/>
662<t>
663   The OPTIONS method represents a request for information about the
664   communication options available on the request/response chain
665   identified by the request-target. This method allows the client to
666   determine the options and/or requirements associated with a resource,
667   or the capabilities of a server, without implying a resource action
668   or initiating a resource retrieval.
669</t>
670<t>
671   Responses to this method are not cacheable.
672</t>
673<t>
674   If the OPTIONS request includes an entity-body (as indicated by the
675   presence of Content-Length or Transfer-Encoding), then the media type
676   MUST be indicated by a Content-Type field. Although this
677   specification does not define any use for such a body, future
678   extensions to HTTP might use the OPTIONS body to make more detailed
679   queries on the server.
680</t>
681<t>
682   If the request-target is an asterisk ("*"), the OPTIONS request is
683   intended to apply to the server in general rather than to a specific
684   resource. Since a server's communication options typically depend on
685   the resource, the "*" request is only useful as a "ping" or "no-op"
686   type of method; it does nothing beyond allowing the client to test
687   the capabilities of the server. For example, this can be used to test
688   a proxy for HTTP/1.1 compliance (or lack thereof).
689</t>
690<t>
691   If the request-target is not an asterisk, the OPTIONS request applies
692   only to the options that are available when communicating with that
693   resource.
694</t>
695<t>
696   A 200 response SHOULD include any header fields that indicate
697   optional features implemented by the server and applicable to that
698   resource (e.g., Allow), possibly including extensions not defined by
699   this specification. The response body, if any, SHOULD also include
700   information about the communication options. The format for such a
701   body is not defined by this specification, but might be defined by
702   future extensions to HTTP. Content negotiation MAY be used to select
703   the appropriate response format. If no response body is included, the
704   response MUST include a Content-Length field with a field-value of
705   "0".
706</t>
707<t>
708   The Max-Forwards request-header field MAY be used to target a
709   specific proxy in the request chain. When a proxy receives an OPTIONS
710   request on an absolute-URI for which request forwarding is permitted,
711   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
712   field-value is zero ("0"), the proxy MUST NOT forward the message;
713   instead, the proxy SHOULD respond with its own communication options.
714   If the Max-Forwards field-value is an integer greater than zero, the
715   proxy MUST decrement the field-value when it forwards the request. If
716   no Max-Forwards field is present in the request, then the forwarded
717   request MUST NOT include a Max-Forwards field.
718</t>
719</section>
720
721<section title="GET" anchor="GET">
722 
723  <iref primary="true" item="GET method"/>
724  <iref primary="true" item="Methods" subitem="GET"/>
725<t>
726   The GET method means retrieve whatever information (in the form of an
727   entity) is identified by the request-target. If the request-target refers
728   to a data-producing process, it is the produced data which shall be
729   returned as the entity in the response and not the source text of the
730   process, unless that text happens to be the output of the process.
731</t>
732<t>
733   The semantics of the GET method change to a "conditional GET" if the
734   request message includes an If-Modified-Since, If-Unmodified-Since,
735   If-Match, If-None-Match, or If-Range header field. A conditional GET
736   method requests that the entity be transferred only under the
737   circumstances described by the conditional header field(s). The
738   conditional GET method is intended to reduce unnecessary network
739   usage by allowing cached entities to be refreshed without requiring
740   multiple requests or transferring data already held by the client.
741</t>
742<t>
743   The semantics of the GET method change to a "partial GET" if the
744   request message includes a Range header field. A partial GET requests
745   that only part of the entity be transferred, as described in Section 5.4 of <xref target="Part5"/>.
746   The partial GET method is intended to reduce unnecessary
747   network usage by allowing partially-retrieved entities to be
748   completed without transferring data already held by the client.
749</t>
750<t>
751   The response to a GET request is cacheable if and only if it meets
752   the requirements for HTTP caching described in <xref target="Part6"/>.
753</t>
754<t>
755   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
756</t>
757</section>
758
759<section title="HEAD" anchor="HEAD">
760 
761  <iref primary="true" item="HEAD method"/>
762  <iref primary="true" item="Methods" subitem="HEAD"/>
763<t>
764   The HEAD method is identical to GET except that the server MUST NOT
765   return a message-body in the response. The metainformation contained
766   in the HTTP headers in response to a HEAD request SHOULD be identical
767   to the information sent in response to a GET request. This method can
768   be used for obtaining metainformation about the entity implied by the
769   request without transferring the entity-body itself. This method is
770   often used for testing hypertext links for validity, accessibility,
771   and recent modification.
772</t>
773<t>
774   The response to a HEAD request MAY be cacheable in the sense that the
775   information contained in the response MAY be used to update a
776   previously cached entity from that resource. If the new field values
777   indicate that the cached entity differs from the current entity (as
778   would be indicated by a change in Content-Length, Content-MD5, ETag
779   or Last-Modified), then the cache MUST treat the cache entry as
780   stale.
781</t>
782</section>
783
784<section title="POST" anchor="POST">
785  <iref primary="true" item="POST method"/>
786  <iref primary="true" item="Methods" subitem="POST"/>
787<t>
788   The POST method is used to request that the origin server accept the
789   entity enclosed in the request as data to be processed by the resource
790   identified by the request-target in the Request-Line. POST is designed
791   to allow a uniform method to cover the following functions:
792  <list style="symbols">
793    <t>
794      Annotation of existing resources;
795    </t>
796    <t>
797        Posting a message to a bulletin board, newsgroup, mailing list,
798        or similar group of articles;
799    </t>
800    <t>
801        Providing a block of data, such as the result of submitting a
802        form, to a data-handling process;
803    </t>
804    <t>
805        Extending a database through an append operation.
806    </t>
807  </list>
808</t>
809<t>
810   The actual function performed by the POST method is determined by the
811   server and is usually dependent on the request-target.
812</t>
813<t>
814   The action performed by the POST method might not result in a
815   resource that can be identified by a URI. In this case, either 200
816   (OK) or 204 (No Content) is the appropriate response status,
817   depending on whether or not the response includes an entity that
818   describes the result.
819</t>
820<t>
821   If a resource has been created on the origin server, the response
822   SHOULD be 201 (Created) and contain an entity which describes the
823   status of the request and refers to the new resource, and a Location
824   header (see <xref target="header.location"/>).
825</t>
826<t>
827   Responses to this method are not cacheable, unless the response
828   includes appropriate Cache-Control or Expires header fields. However,
829   the 303 (See Other) response can be used to direct the user agent to
830   retrieve a cacheable resource.
831</t>
832</section>
833
834<section title="PUT" anchor="PUT">
835  <iref primary="true" item="PUT method"/>
836  <iref primary="true" item="Methods" subitem="PUT"/>
837<t>
838   The PUT method requests that the enclosed entity be stored at the
839   supplied request-target. If the request-target refers to an already
840   existing resource, the enclosed entity SHOULD be considered as a
841   modified version of the one residing on the origin server. If the
842   request-target does not point to an existing resource, and that URI is
843   capable of being defined as a new resource by the requesting user
844   agent, the origin server can create the resource with that URI. If a
845   new resource is created at the request-target, the origin server MUST
846         inform the user agent
847   via the 201 (Created) response. If an existing resource is modified,
848   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
849   to indicate successful completion of the request. If the resource
850   could not be created or modified with the request-target, an appropriate
851   error response SHOULD be given that reflects the nature of the
852   problem. The recipient of the entity MUST NOT ignore any Content-*
853   headers (headers starting with the prefix 'Content-') that it does
854   not understand or implement
855   and MUST return a 501 (Not Implemented) response in such cases.
856</t>
857<t>
858   If the request passes through a cache and the request-target identifies
859   one or more currently cached entities, those entries SHOULD be
860   treated as stale. Responses to this method are not cacheable.
861</t>
862<t>
863   The fundamental difference between the POST and PUT requests is
864   reflected in the different meaning of the request-target. The URI in a
865   POST request identifies the resource that will handle the enclosed
866   entity. That resource might be a data-accepting process, a gateway to
867   some other protocol, or a separate entity that accepts annotations.
868   In contrast, the URI in a PUT request identifies the entity enclosed
869   with the request -- the user agent knows what URI is intended and the
870   server MUST NOT attempt to apply the request to some other resource.
871   If the server desires that the request be applied to a different URI,
872   it MUST send a 301 (Moved Permanently) response; the user agent MAY
873   then make its own decision regarding whether or not to redirect the
874   request.
875</t>
876<t>
877   A single resource MAY be identified by many different URIs. For
878   example, an article might have a URI for identifying "the current
879   version" which is separate from the URI identifying each particular
880   version. In this case, a PUT request on a general URI might result in
881   several other URIs being defined by the origin server.
882</t>
883<t>
884   HTTP/1.1 does not define how a PUT method affects the state of an
885   origin server.
886</t>
887<t>
888   Unless otherwise specified for a particular entity-header, the
889   entity-headers in the PUT request SHOULD be applied to the resource
890   created or modified by the PUT.
891</t>
892</section>
893
894<section title="DELETE" anchor="DELETE">
895  <iref primary="true" item="DELETE method"/>
896  <iref primary="true" item="Methods" subitem="DELETE"/>
897<t>
898   The DELETE method requests that the origin server delete the resource
899   identified by the request-target. This method MAY be overridden by human
900   intervention (or other means) on the origin server. The client cannot
901   be guaranteed that the operation has been carried out, even if the
902   status code returned from the origin server indicates that the action
903   has been completed successfully. However, the server SHOULD NOT
904   indicate success unless, at the time the response is given, it
905   intends to delete the resource or move it to an inaccessible
906   location.
907</t>
908<t>
909   A successful response SHOULD be 200 (OK) if the response includes an
910   entity describing the status, 202 (Accepted) if the action has not
911   yet been enacted, or 204 (No Content) if the action has been enacted
912   but the response does not include an entity.
913</t>
914<t>
915   If the request passes through a cache and the request-target identifies
916   one or more currently cached entities, those entries SHOULD be
917   treated as stale. Responses to this method are not cacheable.
918</t>
919</section>
920
921<section title="TRACE" anchor="TRACE">
922 
923  <iref primary="true" item="TRACE method"/>
924  <iref primary="true" item="Methods" subitem="TRACE"/>
925<t>
926   The TRACE method is used to invoke a remote, application-layer loop-back
927   of the request message. The final recipient of the request
928   SHOULD reflect the message received back to the client as the
929   entity-body of a 200 (OK) response. The final recipient is either the
930   origin server or the first proxy or gateway to receive a Max-Forwards
931   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
932   MUST NOT include an entity.
933</t>
934<t>
935   TRACE allows the client to see what is being received at the other
936   end of the request chain and use that data for testing or diagnostic
937   information. The value of the Via header field (Section 8.9 of <xref target="Part1"/>) is of
938   particular interest, since it acts as a trace of the request chain.
939   Use of the Max-Forwards header field allows the client to limit the
940   length of the request chain, which is useful for testing a chain of
941   proxies forwarding messages in an infinite loop.
942</t>
943<t>
944   If the request is valid, the response SHOULD contain the entire
945   request message in the entity-body, with a Content-Type of
946   "message/http" (see Section 9.3.1 of <xref target="Part1"/>). Responses to this method
947   MUST NOT be cached.
948</t>
949</section>
950
951<section title="CONNECT" anchor="CONNECT">
952  <iref primary="true" item="CONNECT method"/>
953  <iref primary="true" item="Methods" subitem="CONNECT"/>
954<t>
955   This specification reserves the method name CONNECT for use with a
956   proxy that can dynamically switch to being a tunnel (e.g. SSL
957   tunneling <xref target="RFC2817"/>).
958</t>
959</section>
960</section>
961
962
963<section title="Status Code Definitions" anchor="status.codes">
964<t>
965   Each Status-Code is described below, including a description of which
966   method(s) it can follow and any metainformation required in the
967   response.
968</t>
969
970<section title="Informational 1xx" anchor="status.1xx">
971<t>
972   This class of status code indicates a provisional response,
973   consisting only of the Status-Line and optional headers, and is
974   terminated by an empty line. There are no required headers for this
975   class of status code. Since HTTP/1.0 did not define any 1xx status
976   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
977   except under experimental conditions.
978</t>
979<t>
980   A client MUST be prepared to accept one or more 1xx status responses
981   prior to a regular response, even if the client does not expect a 100
982   (Continue) status message. Unexpected 1xx status responses MAY be
983   ignored by a user agent.
984</t>
985<t>
986   Proxies MUST forward 1xx responses, unless the connection between the
987   proxy and its client has been closed, or unless the proxy itself
988   requested the generation of the 1xx response. (For example, if a
989   proxy adds a "Expect: 100-continue" field when it forwards a request,
990   then it need not forward the corresponding 100 (Continue)
991   response(s).)
992</t>
993
994<section title="100 Continue" anchor="status.100">
995  <iref primary="true" item="100 Continue (status code)"/>
996  <iref primary="true" item="Status Codes" subitem="100 Continue"/>
997<t>
998   The client SHOULD continue with its request. This interim response is
999   used to inform the client that the initial part of the request has
1000   been received and has not yet been rejected by the server. The client
1001   SHOULD continue by sending the remainder of the request or, if the
1002   request has already been completed, ignore this response. The server
1003   MUST send a final response after the request has been completed. See
1004   Section 7.2.3 of <xref target="Part1"/> for detailed discussion of the use and handling of this
1005   status code.
1006</t>
1007</section>
1008
1009<section title="101 Switching Protocols" anchor="status.101">
1010  <iref primary="true" item="101 Switching Protocols (status code)"/>
1011  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols"/>
1012<t>
1013   The server understands and is willing to comply with the client's
1014   request, via the Upgrade message header field (Section 5.4 of <xref target="Part5"/>), for a
1015   change in the application protocol being used on this connection. The
1016   server will switch protocols to those defined by the response's
1017   Upgrade header field immediately after the empty line which
1018   terminates the 101 response.
1019</t>
1020<t>
1021   The protocol SHOULD be switched only when it is advantageous to do
1022   so. For example, switching to a newer version of HTTP is advantageous
1023   over older versions, and switching to a real-time, synchronous
1024   protocol might be advantageous when delivering resources that use
1025   such features.
1026</t>
1027</section>
1028</section>
1029
1030<section title="Successful 2xx" anchor="status.2xx">
1031<t>
1032   This class of status code indicates that the client's request was
1033   successfully received, understood, and accepted.
1034</t>
1035
1036<section title="200 OK" anchor="status.200">
1037  <iref primary="true" item="200 OK (status code)"/>
1038  <iref primary="true" item="Status Codes" subitem="200 OK"/>
1039<t>
1040   The request has succeeded. The information returned with the response
1041   is dependent on the method used in the request, for example:
1042  <list style="hanging">
1043    <t hangText="GET">
1044          an entity corresponding to the requested resource is sent in
1045          the response;
1046    </t>
1047    <t hangText="HEAD">
1048          the entity-header fields corresponding to the requested
1049          resource are sent in the response without any message-body;
1050    </t>
1051    <t hangText="POST">
1052      an entity describing or containing the result of the action;
1053    </t>
1054    <t hangText="TRACE">
1055      an entity containing the request message as received by the
1056      end server.
1057    </t>
1058  </list>
1059</t>
1060</section>
1061
1062<section title="201 Created" anchor="status.201">
1063  <iref primary="true" item="201 Created (status code)"/>
1064  <iref primary="true" item="Status Codes" subitem="201 Created"/>
1065<t>
1066   The request has been fulfilled and resulted in a new resource being
1067   created. The newly created resource can be referenced by the URI(s)
1068   returned in the entity of the response, with the most specific URI
1069   for the resource given by a Location header field. The response
1070   SHOULD include an entity containing a list of resource
1071   characteristics and location(s) from which the user or user agent can
1072   choose the one most appropriate. The entity format is specified by
1073   the media type given in the Content-Type header field. The origin
1074   server MUST create the resource before returning the 201 status code.
1075   If the action cannot be carried out immediately, the server SHOULD
1076   respond with 202 (Accepted) response instead.
1077</t>
1078<t>
1079   A 201 response MAY contain an ETag response header field indicating
1080   the current value of the entity tag for the requested variant just
1081   created, see Section 6.1 of <xref target="Part4"/>.
1082</t>
1083</section>
1084
1085<section title="202 Accepted" anchor="status.202">
1086  <iref primary="true" item="202 Accepted (status code)"/>
1087  <iref primary="true" item="Status Codes" subitem="202 Accepted"/>
1088<t>
1089   The request has been accepted for processing, but the processing has
1090   not been completed.  The request might or might not eventually be
1091   acted upon, as it might be disallowed when processing actually takes
1092   place. There is no facility for re-sending a status code from an
1093   asynchronous operation such as this.
1094</t>
1095<t>
1096   The 202 response is intentionally non-committal. Its purpose is to
1097   allow a server to accept a request for some other process (perhaps a
1098   batch-oriented process that is only run once per day) without
1099   requiring that the user agent's connection to the server persist
1100   until the process is completed. The entity returned with this
1101   response SHOULD include an indication of the request's current status
1102   and either a pointer to a status monitor or some estimate of when the
1103   user can expect the request to be fulfilled.
1104</t>
1105</section>
1106
1107<section title="203 Non-Authoritative Information" anchor="status.203">
1108  <iref primary="true" item="203 Non-Authoritative Information (status code)"/>
1109  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information"/>
1110<t>
1111   The returned metainformation in the entity-header is not the
1112   definitive set as available from the origin server, but is gathered
1113   from a local or a third-party copy. The set presented MAY be a subset
1114   or superset of the original version. For example, including local
1115   annotation information about the resource might result in a superset
1116   of the metainformation known by the origin server. Use of this
1117   response code is not required and is only appropriate when the
1118   response would otherwise be 200 (OK).
1119</t>
1120</section>
1121
1122<section title="204 No Content" anchor="status.204">
1123  <iref primary="true" item="204 No Content (status code)"/>
1124  <iref primary="true" item="Status Codes" subitem="204 No Content"/>
1125<t>
1126   The server has fulfilled the request but does not need to return an
1127   entity-body, and might want to return updated metainformation. The
1128   response MAY include new or updated metainformation in the form of
1129   entity-headers, which if present SHOULD be associated with the
1130   requested variant.
1131</t>
1132<t>
1133   If the client is a user agent, it SHOULD NOT  change its document view
1134   from that which caused the request to be sent. This response is
1135   primarily intended to allow input for actions to take place without
1136   causing a change to the user agent's active document view, although
1137   any new or updated metainformation SHOULD be applied to the document
1138   currently in the user agent's active view.
1139</t>
1140<t>
1141   The 204 response MUST NOT include a message-body, and thus is always
1142   terminated by the first empty line after the header fields.
1143</t>
1144</section>
1145
1146<section title="205 Reset Content" anchor="status.205">
1147  <iref primary="true" item="205 Reset Content (status code)"/>
1148  <iref primary="true" item="Status Codes" subitem="205 Reset Content"/>
1149<t>
1150   The server has fulfilled the request and the user agent SHOULD reset
1151   the document view which caused the request to be sent. This response
1152   is primarily intended to allow input for actions to take place via
1153   user input, followed by a clearing of the form in which the input is
1154   given so that the user can easily initiate another input action. The
1155   response MUST NOT include an entity.
1156</t>
1157</section>
1158
1159<section title="206 Partial Content" anchor="status.206">
1160  <iref primary="true" item="206 Partial Content (status code)"/>
1161  <iref primary="true" item="Status Codes" subitem="206 Partial Content"/>
1162<t>
1163   The server has fulfilled the partial GET request for the resource
1164   and the enclosed entity is a partial representation as defined in <xref target="Part5"/>.
1165</t>
1166</section>
1167</section>
1168
1169<section title="Redirection 3xx" anchor="status.3xx">
1170<t>
1171   This class of status code indicates that further action needs to be
1172   taken by the user agent in order to fulfill the request.  The action
1173   required MAY be carried out by the user agent without interaction
1174   with the user if and only if the method used in the second request is
1175   GET or HEAD. A client SHOULD detect infinite redirection loops, since
1176   such loops generate network traffic for each redirection.
1177  <list><t>
1178      Note: previous versions of this specification recommended a
1179      maximum of five redirections. Content developers should be aware
1180      that there might be clients that implement such a fixed
1181      limitation.
1182  </t></list>
1183</t>
1184
1185<section title="300 Multiple Choices" anchor="status.300">
1186  <iref primary="true" item="300 Multiple Choices (status code)"/>
1187  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices"/>
1188<t>
1189   The requested resource corresponds to any one of a set of
1190   representations, each with its own specific location, and agent-driven
1191   negotiation information (Section 4 of <xref target="Part3"/>) is being provided so that
1192   the user (or user agent) can select a preferred representation and
1193   redirect its request to that location.
1194</t>
1195<t>
1196   Unless it was a HEAD request, the response SHOULD include an entity
1197   containing a list of resource characteristics and location(s) from
1198   which the user or user agent can choose the one most appropriate. The
1199   entity format is specified by the media type given in the Content-Type
1200   header field. Depending upon the format and the capabilities of
1201   the user agent, selection of the most appropriate choice MAY be
1202   performed automatically. However, this specification does not define
1203   any standard for such automatic selection.
1204</t>
1205<t>
1206   If the server has a preferred choice of representation, it SHOULD
1207   include the specific URI for that representation in the Location
1208   field; user agents MAY use the Location field value for automatic
1209   redirection. This response is cacheable unless indicated otherwise.
1210</t>
1211</section>
1212
1213<section title="301 Moved Permanently" anchor="status.301">
1214  <iref primary="true" item="301 Moved Permanently (status code)"/>
1215  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently"/>
1216<t>
1217   The requested resource has been assigned a new permanent URI and any
1218   future references to this resource SHOULD use one of the returned
1219   URIs.  Clients with link editing capabilities ought to automatically
1220   re-link references to the request-target to one or more of the new
1221   references returned by the server, where possible. This response is
1222   cacheable unless indicated otherwise.
1223</t>
1224<t>
1225   The new permanent URI SHOULD be given by the Location field in the
1226   response. Unless the request method was HEAD, the entity of the
1227   response SHOULD contain a short hypertext note with a hyperlink to
1228   the new URI(s).
1229</t>
1230<t>
1231   If the 301 status code is received in response to a request method
1232   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1233   then the request MAY be automatically redirected by the user agent without
1234   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1235   request unless it can be confirmed by the user, since this might
1236   change the conditions under which the request was issued.
1237  <list><t>
1238      Note: When automatically redirecting a POST request after
1239      receiving a 301 status code, some existing HTTP/1.0 user agents
1240      will erroneously change it into a GET request.
1241  </t></list>
1242</t>
1243</section>
1244
1245<section title="302 Found" anchor="status.302">
1246  <iref primary="true" item="302 Found (status code)"/>
1247  <iref primary="true" item="Status Codes" subitem="302 Found"/>
1248<t>
1249   The requested resource resides temporarily under a different URI.
1250   Since the redirection might be altered on occasion, the client SHOULD
1251   continue to use the request-target for future requests.  This response
1252   is only cacheable if indicated by a Cache-Control or Expires header
1253   field.
1254</t>
1255<t>
1256   The temporary URI SHOULD be given by the Location field in the
1257   response. Unless the request method was HEAD, the entity of the
1258   response SHOULD contain a short hypertext note with a hyperlink to
1259   the new URI(s).
1260</t>
1261<t>
1262   If the 302 status code is received in response to a request method
1263   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1264   then the request MAY be automatically redirected by the user agent without
1265   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1266   request unless it can be confirmed by the user, since this might
1267   change the conditions under which the request was issued.
1268  <list><t>
1269      Note: <xref target="RFC1945"/> and <xref target="RFC2068"/> specify that the client is not allowed
1270      to change the method on the redirected request.  However, most
1271      existing user agent implementations treat 302 as if it were a 303
1272      response, performing a GET on the Location field-value regardless
1273      of the original request method. The status codes 303 and 307 have
1274      been added for servers that wish to make unambiguously clear which
1275      kind of reaction is expected of the client.
1276  </t></list>
1277</t>
1278</section>
1279
1280<section title="303 See Other" anchor="status.303">
1281  <iref primary="true" item="303 See Other (status code)"/>
1282  <iref primary="true" item="Status Codes" subitem="303 See Other"/>
1283<t>
1284   The server directs the user agent to a different resource, indicated
1285   by a URI in the Location header field, that provides an indirect
1286   response to the original request.  The user agent MAY perform a GET
1287   request on the URI in the Location field in order to obtain a
1288   representation corresponding to the response, be redirected again,
1289   or end with an error status.  The Location URI is not a substitute
1290   reference for the originally requested resource.
1291</t>
1292<t>
1293   The 303 status is generally applicable to any HTTP method.  It is
1294   primarily used to allow the output of a POST action to redirect
1295   the user agent to a selected resource, since doing so provides the
1296   information corresponding to the POST response in a form that
1297   can be separately identified, bookmarked, and cached independent
1298   of the original request.
1299</t>
1300<t>
1301   A 303 response to a GET request indicates that the requested
1302   resource does not have a representation of its own that can be
1303   transferred by the server over HTTP.  The Location URI indicates a
1304   resource that is descriptive of the requested resource such that
1305   the follow-on representation may be useful without implying that
1306   it adequately represents the previously requested resource.
1307   Note that answers to the questions of what can be represented, what
1308   representations are adequate, and what might be a useful description
1309   are outside the scope of HTTP and thus entirely determined by the
1310   resource owner(s).
1311</t>
1312<t>
1313   A 303 response SHOULD NOT be cached unless it is indicated as
1314   cacheable by Cache-Control or Expires header fields.  Except for
1315   responses to a HEAD request, the entity of a 303 response SHOULD
1316   contain a short hypertext note with a hyperlink to the Location URI.
1317</t>
1318</section>
1319
1320<section title="304 Not Modified" anchor="status.304">
1321  <iref primary="true" item="304 Not Modified (status code)"/>
1322  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1323<t>
1324   The response to the request has not been modified since the conditions
1325   indicated by the client's conditional GET request, as defined in <xref target="Part4"/>.
1326</t>
1327</section>
1328
1329<section title="305 Use Proxy" anchor="status.305">
1330  <iref primary="true" item="305 Use Proxy (status code)"/>
1331  <iref primary="true" item="Status Codes" subitem="305 Use Proxy"/>
1332<t>
1333   The 305 status was defined in a previous version of this specification
1334   (see <xref target="changes.from.rfc.2616"/>), and is now deprecated.
1335</t>
1336</section>
1337
1338<section title="306 (Unused)" anchor="status.306">
1339  <iref primary="true" item="306 (Unused) (status code)"/>
1340  <iref primary="true" item="Status Codes" subitem="306 (Unused)"/>
1341<t>
1342   The 306 status code was used in a previous version of the
1343   specification, is no longer used, and the code is reserved.
1344</t>
1345</section>
1346
1347<section title="307 Temporary Redirect" anchor="status.307">
1348  <iref primary="true" item="307 Temporary Redirect (status code)"/>
1349  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect"/>
1350<t>
1351   The requested resource resides temporarily under a different URI.
1352   Since the redirection MAY be altered on occasion, the client SHOULD
1353   continue to use the request-target for future requests.  This response
1354   is only cacheable if indicated by a Cache-Control or Expires header
1355   field.
1356</t>
1357<t>
1358   The temporary URI SHOULD be given by the Location field in the
1359   response. Unless the request method was HEAD, the entity of the
1360   response SHOULD contain a short hypertext note with a hyperlink to
1361   the new URI(s) , since many pre-HTTP/1.1 user agents do not
1362   understand the 307 status. Therefore, the note SHOULD contain the
1363   information necessary for a user to repeat the original request on
1364   the new URI.
1365</t>
1366<t>
1367   If the 307 status code is received in response to a request method
1368   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1369   then the request MAY be automatically redirected by the user agent without
1370   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1371   request unless it can be confirmed by the user, since this might
1372   change the conditions under which the request was issued.
1373</t>
1374</section>
1375</section>
1376
1377<section title="Client Error 4xx" anchor="status.4xx">
1378<t>
1379   The 4xx class of status code is intended for cases in which the
1380   client seems to have erred. Except when responding to a HEAD request,
1381   the server SHOULD include an entity containing an explanation of the
1382   error situation, and whether it is a temporary or permanent
1383   condition. These status codes are applicable to any request method.
1384   User agents SHOULD display any included entity to the user.
1385</t>
1386<t>
1387   If the client is sending data, a server implementation using TCP
1388   SHOULD be careful to ensure that the client acknowledges receipt of
1389   the packet(s) containing the response, before the server closes the
1390   input connection. If the client continues sending data to the server
1391   after the close, the server's TCP stack will send a reset packet to
1392   the client, which may erase the client's unacknowledged input buffers
1393   before they can be read and interpreted by the HTTP application.
1394</t>
1395
1396<section title="400 Bad Request" anchor="status.400">
1397  <iref primary="true" item="400 Bad Request (status code)"/>
1398  <iref primary="true" item="Status Codes" subitem="400 Bad Request"/>
1399<t>
1400   The request could not be understood by the server due to malformed
1401   syntax. The client SHOULD NOT  repeat the request without
1402   modifications.
1403</t>
1404</section>
1405
1406<section title="401 Unauthorized" anchor="status.401">
1407  <iref primary="true" item="401 Unauthorized (status code)"/>
1408  <iref primary="true" item="Status Codes" subitem="401 Unauthorized"/>
1409<t>
1410   The request requires user authentication (see <xref target="Part7"/>).
1411</t>
1412</section>
1413
1414<section title="402 Payment Required" anchor="status.402">
1415  <iref primary="true" item="402 Payment Required (status code)"/>
1416  <iref primary="true" item="Status Codes" subitem="402 Payment Required"/>
1417<t>
1418   This code is reserved for future use.
1419</t>
1420</section>
1421
1422<section title="403 Forbidden" anchor="status.403">
1423  <iref primary="true" item="403 Forbidden (status code)"/>
1424  <iref primary="true" item="Status Codes" subitem="403 Forbidden"/>
1425<t>
1426   The server understood the request, but is refusing to fulfill it.
1427   Authorization will not help and the request SHOULD NOT  be repeated.
1428   If the request method was not HEAD and the server wishes to make
1429   public why the request has not been fulfilled, it SHOULD describe the
1430   reason for the refusal in the entity.  If the server does not wish to
1431   make this information available to the client, the status code 404
1432   (Not Found) can be used instead.
1433</t>
1434</section>
1435
1436<section title="404 Not Found" anchor="status.404">
1437  <iref primary="true" item="404 Not Found (status code)"/>
1438  <iref primary="true" item="Status Codes" subitem="404 Not Found"/>
1439<t>
1440   The server has not found anything matching the request-target. No
1441   indication is given of whether the condition is temporary or
1442   permanent. The 410 (Gone) status code SHOULD be used if the server
1443   knows, through some internally configurable mechanism, that an old
1444   resource is permanently unavailable and has no forwarding address.
1445   This status code is commonly used when the server does not wish to
1446   reveal exactly why the request has been refused, or when no other
1447   response is applicable.
1448</t>
1449</section>
1450
1451<section title="405 Method Not Allowed" anchor="status.405">
1452  <iref primary="true" item="405 Method Not Allowed (status code)"/>
1453  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed"/>
1454<t>
1455   The method specified in the Request-Line is not allowed for the
1456   resource identified by the request-target. The response MUST include an
1457   Allow header containing a list of valid methods for the requested
1458   resource.
1459</t>
1460</section>
1461
1462<section title="406 Not Acceptable" anchor="status.406">
1463  <iref primary="true" item="406 Not Acceptable (status code)"/>
1464  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable"/>
1465<t>
1466   The resource identified by the request is only capable of generating
1467   response entities which have content characteristics not acceptable
1468   according to the accept headers sent in the request.
1469</t>
1470<t>
1471   Unless it was a HEAD request, the response SHOULD include an entity
1472   containing a list of available entity characteristics and location(s)
1473   from which the user or user agent can choose the one most
1474   appropriate. The entity format is specified by the media type given
1475   in the Content-Type header field. Depending upon the format and the
1476   capabilities of the user agent, selection of the most appropriate
1477   choice MAY be performed automatically. However, this specification
1478   does not define any standard for such automatic selection.
1479  <list><t>
1480      Note: HTTP/1.1 servers are allowed to return responses which are
1481      not acceptable according to the accept headers sent in the
1482      request. In some cases, this may even be preferable to sending a
1483      406 response. User agents are encouraged to inspect the headers of
1484      an incoming response to determine if it is acceptable.
1485  </t></list>
1486</t>
1487<t>
1488   If the response could be unacceptable, a user agent SHOULD
1489   temporarily stop receipt of more data and query the user for a
1490   decision on further actions.
1491</t>
1492</section>
1493
1494<section title="407 Proxy Authentication Required" anchor="status.407">
1495  <iref primary="true" item="407 Proxy Authentication Required (status code)"/>
1496  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required"/>
1497<t>
1498   This code is similar to 401 (Unauthorized), but indicates that the
1499   client must first authenticate itself with the proxy (see <xref target="Part7"/>).
1500</t>
1501</section>
1502
1503<section title="408 Request Timeout" anchor="status.408">
1504  <iref primary="true" item="408 Request Timeout (status code)"/>
1505  <iref primary="true" item="Status Codes" subitem="408 Request Timeout"/>
1506<t>
1507   The client did not produce a request within the time that the server
1508   was prepared to wait. The client MAY repeat the request without
1509   modifications at any later time.
1510</t>
1511</section>
1512
1513<section title="409 Conflict" anchor="status.409">
1514  <iref primary="true" item="409 Conflict (status code)"/>
1515  <iref primary="true" item="Status Codes" subitem="409 Conflict"/>
1516<t>
1517   The request could not be completed due to a conflict with the current
1518   state of the resource. This code is only allowed in situations where
1519   it is expected that the user might be able to resolve the conflict
1520   and resubmit the request. The response body SHOULD include enough
1521   information for the user to recognize the source of the conflict.
1522   Ideally, the response entity would include enough information for the
1523   user or user agent to fix the problem; however, that might not be
1524   possible and is not required.
1525</t>
1526<t>
1527   Conflicts are most likely to occur in response to a PUT request. For
1528   example, if versioning were being used and the entity being PUT
1529   included changes to a resource which conflict with those made by an
1530   earlier (third-party) request, the server might use the 409 response
1531   to indicate that it can't complete the request. In this case, the
1532   response entity would likely contain a list of the differences
1533   between the two versions in a format defined by the response
1534   Content-Type.
1535</t>
1536</section>
1537
1538<section title="410 Gone" anchor="status.410">
1539  <iref primary="true" item="410 Gone (status code)"/>
1540  <iref primary="true" item="Status Codes" subitem="410 Gone"/>
1541<t>
1542   The requested resource is no longer available at the server and no
1543   forwarding address is known. This condition is expected to be
1544   considered permanent. Clients with link editing capabilities SHOULD
1545   delete references to the request-target after user approval. If the
1546   server does not know, or has no facility to determine, whether or not
1547   the condition is permanent, the status code 404 (Not Found) SHOULD be
1548   used instead. This response is cacheable unless indicated otherwise.
1549</t>
1550<t>
1551   The 410 response is primarily intended to assist the task of web
1552   maintenance by notifying the recipient that the resource is
1553   intentionally unavailable and that the server owners desire that
1554   remote links to that resource be removed. Such an event is common for
1555   limited-time, promotional services and for resources belonging to
1556   individuals no longer working at the server's site. It is not
1557   necessary to mark all permanently unavailable resources as "gone" or
1558   to keep the mark for any length of time -- that is left to the
1559   discretion of the server owner.
1560</t>
1561</section>
1562
1563<section title="411 Length Required" anchor="status.411">
1564  <iref primary="true" item="411 Length Required (status code)"/>
1565  <iref primary="true" item="Status Codes" subitem="411 Length Required"/>
1566<t>
1567   The server refuses to accept the request without a defined Content-Length.
1568   The client MAY repeat the request if it adds a valid
1569   Content-Length header field containing the length of the message-body
1570   in the request message.
1571</t>
1572</section>
1573
1574<section title="412 Precondition Failed" anchor="status.412">
1575  <iref primary="true" item="412 Precondition Failed (status code)"/>
1576  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1577<t>
1578   The precondition given in one or more of the request-header fields
1579   evaluated to false when it was tested on the server, as defined in
1580   <xref target="Part4"/>.
1581</t>
1582</section>
1583
1584<section title="413 Request Entity Too Large" anchor="status.413">
1585  <iref primary="true" item="413 Request Entity Too Large (status code)"/>
1586  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large"/>
1587<t>
1588   The server is refusing to process a request because the request
1589   entity is larger than the server is willing or able to process. The
1590   server MAY close the connection to prevent the client from continuing
1591   the request.
1592</t>
1593<t>
1594   If the condition is temporary, the server SHOULD include a Retry-After
1595   header field to indicate that it is temporary and after what
1596   time the client MAY try again.
1597</t>
1598</section>
1599
1600<section title="414 URI Too Long" anchor="status.414">
1601  <iref primary="true" item="414 URI Too Long (status code)"/>
1602  <iref primary="true" item="Status Codes" subitem="414 URI Too Long"/>
1603<t>
1604   The server is refusing to service the request because the request-target
1605   is longer than the server is willing to interpret. This rare
1606   condition is only likely to occur when a client has improperly
1607   converted a POST request to a GET request with long query
1608   information, when the client has descended into a URI "black hole" of
1609   redirection (e.g., a redirected URI prefix that points to a suffix of
1610   itself), or when the server is under attack by a client attempting to
1611   exploit security holes present in some servers using fixed-length
1612   buffers for reading or manipulating the request-target.
1613</t>
1614</section>
1615
1616<section title="415 Unsupported Media Type" anchor="status.415">
1617  <iref primary="true" item="415 Unsupported Media Type (status code)"/>
1618  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type"/>
1619<t>
1620   The server is refusing to service the request because the entity of
1621   the request is in a format not supported by the requested resource
1622   for the requested method.
1623</t>
1624</section>
1625
1626<section title="416 Requested Range Not Satisfiable" anchor="status.416">
1627  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)"/>
1628  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable"/>
1629<t>
1630   The request included a Range request-header field (Section 5.4 of <xref target="Part5"/>) and none of
1631   the range-specifier values in this field overlap the current extent
1632   of the selected resource.
1633</t>
1634</section>
1635
1636<section title="417 Expectation Failed" anchor="status.417">
1637  <iref primary="true" item="417 Expectation Failed (status code)"/>
1638  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed"/>
1639<t>
1640   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
1641   could not be met by this server, or, if the server is a proxy,
1642   the server has unambiguous evidence that the request could not be met
1643   by the next-hop server.
1644</t>
1645</section>
1646</section>
1647
1648<section title="Server Error 5xx" anchor="status.5xx">
1649<t>
1650   Response status codes beginning with the digit "5" indicate cases in
1651   which the server is aware that it has erred or is incapable of
1652   performing the request. Except when responding to a HEAD request, the
1653   server SHOULD include an entity containing an explanation of the
1654   error situation, and whether it is a temporary or permanent
1655   condition. User agents SHOULD display any included entity to the
1656   user. These response codes are applicable to any request method.
1657</t>
1658
1659<section title="500 Internal Server Error" anchor="status.500">
1660  <iref primary="true" item="500 Internal Server Error (status code)"/>
1661  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error"/>
1662<t>
1663   The server encountered an unexpected condition which prevented it
1664   from fulfilling the request.
1665</t>
1666</section>
1667
1668<section title="501 Not Implemented" anchor="status.501">
1669  <iref primary="true" item="501 Not Implemented (status code)"/>
1670  <iref primary="true" item="Status Codes" subitem="501 Not Implemented"/>
1671<t>
1672   The server does not support the functionality required to fulfill the
1673   request. This is the appropriate response when the server does not
1674   recognize the request method and is not capable of supporting it for
1675   any resource.
1676</t>
1677</section>
1678
1679<section title="502 Bad Gateway" anchor="status.502">
1680  <iref primary="true" item="502 Bad Gateway (status code)"/>
1681  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway"/>
1682<t>
1683   The server, while acting as a gateway or proxy, received an invalid
1684   response from the upstream server it accessed in attempting to
1685   fulfill the request.
1686</t>
1687</section>
1688
1689<section title="503 Service Unavailable" anchor="status.503">
1690  <iref primary="true" item="503 Service Unavailable (status code)"/>
1691  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable"/>
1692<t>
1693   The server is currently unable to handle the request due to a
1694   temporary overloading or maintenance of the server. The implication
1695   is that this is a temporary condition which will be alleviated after
1696   some delay. If known, the length of the delay MAY be indicated in a
1697   Retry-After header. If no Retry-After is given, the client SHOULD
1698   handle the response as it would for a 500 response.
1699  <list><t>
1700      Note: The existence of the 503 status code does not imply that a
1701      server must use it when becoming overloaded. Some servers may wish
1702      to simply refuse the connection.
1703  </t></list>
1704</t>
1705</section>
1706
1707<section title="504 Gateway Timeout" anchor="status.504">
1708  <iref primary="true" item="504 Gateway Timeout (status code)"/>
1709  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout"/>
1710<t>
1711   The server, while acting as a gateway or proxy, did not receive a
1712   timely response from the upstream server specified by the URI (e.g.
1713   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
1714   to access in attempting to complete the request.
1715  <list><t>
1716      Note: Note to implementors: some deployed proxies are known to
1717      return 400 or 500 when DNS lookups time out.
1718  </t></list>
1719</t>
1720</section>
1721
1722<section title="505 HTTP Version Not Supported" anchor="status.505">
1723  <iref primary="true" item="505 HTTP Version Not Supported (status code)"/>
1724  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported"/>
1725<t>
1726   The server does not support, or refuses to support, the protocol
1727   version that was used in the request message. The server is
1728   indicating that it is unable or unwilling to complete the request
1729   using the same major version as the client, as described in Section 3.1 of <xref target="Part1"/>,
1730   other than with this error message. The response SHOULD contain
1731   an entity describing why that version is not supported and what other
1732   protocols are supported by that server.
1733</t>
1734
1735</section>
1736</section>
1737</section>
1738
1739
1740<section title="Header Field Definitions" anchor="header.fields">
1741<t>
1742   This section defines the syntax and semantics of HTTP/1.1 header fields
1743   related to request and response semantics.
1744</t>
1745<t>
1746   For entity-header fields, both sender and recipient refer to either the
1747   client or the server, depending on who sends and who receives the entity.
1748</t>
1749
1750<section title="Allow" anchor="header.allow">
1751  <iref primary="true" item="Allow header"/>
1752  <iref primary="true" item="Headers" subitem="Allow"/>
1753 
1754 
1755<t>
1756      The response-header field "Allow" lists the set of methods advertised as
1757      supported by the resource identified by the request-target. The purpose of
1758      this field is strictly to inform the recipient of valid methods
1759      associated with the resource. An Allow header field MUST be
1760      present in a 405 (Method Not Allowed) response.
1761</t>
1762<figure><iref primary="true" item="Grammar" subitem="Allow"/><iref primary="true" item="Grammar" subitem="Allow-v"/><artwork type="abnf2616"><![CDATA[
1763  Allow   = "Allow" ":" OWS Allow-v
1764  Allow-v = #Method
1765]]></artwork></figure>
1766<t>
1767      Example of use:
1768</t>
1769<figure><artwork type="example"><![CDATA[
1770  Allow: GET, HEAD, PUT
1771]]></artwork></figure>
1772<t>
1773      The actual set of allowed methods is defined
1774      by the origin server at the time of each request.
1775</t>
1776<t>
1777      A proxy MUST NOT modify the Allow header field even if it does not
1778      understand all the methods specified, since the user agent might
1779      have other means of communicating with the origin server.
1780</t>
1781</section>
1782
1783<section title="Expect" anchor="header.expect">
1784  <iref primary="true" item="Expect header"/>
1785  <iref primary="true" item="Headers" subitem="Expect"/>
1786 
1787 
1788 
1789 
1790 
1791<t>
1792   The request-header field "Expect" is used to indicate that particular
1793   server behaviors are required by the client.
1794</t>
1795<figure><iref primary="true" item="Grammar" subitem="Expect"/><iref primary="true" item="Grammar" subitem="Expect-v"/><iref primary="true" item="Grammar" subitem="expectation"/><iref primary="true" item="Grammar" subitem="expectation-extension"/><iref primary="true" item="Grammar" subitem="expect-params"/><artwork type="abnf2616"><![CDATA[
1796  Expect       = "Expect" ":" OWS Expect-v
1797  Expect-v     = 1#expectation
1798 
1799  expectation  = "100-continue" / expectation-extension
1800  expectation-extension = token [ "=" ( token / quoted-string )
1801                           *expect-params ]
1802  expect-params = ";" token [ "=" ( token / quoted-string ) ]
1803]]></artwork></figure>
1804<t>
1805   A server that does not understand or is unable to comply with any of
1806   the expectation values in the Expect field of a request MUST respond
1807   with appropriate error status. The server MUST respond with a 417
1808   (Expectation Failed) status if any of the expectations cannot be met
1809   or, if there are other problems with the request, some other 4xx
1810   status.
1811</t>
1812<t>
1813   This header field is defined with extensible syntax to allow for
1814   future extensions. If a server receives a request containing an
1815   Expect field that includes an expectation-extension that it does not
1816   support, it MUST respond with a 417 (Expectation Failed) status.
1817</t>
1818<t>
1819   Comparison of expectation values is case-insensitive for unquoted
1820   tokens (including the 100-continue token), and is case-sensitive for
1821   quoted-string expectation-extensions.
1822</t>
1823<t>
1824   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
1825   return a 417 (Expectation Failed) status if it receives a request
1826   with an expectation that it cannot meet. However, the Expect
1827   request-header itself is end-to-end; it MUST be forwarded if the
1828   request is forwarded.
1829</t>
1830<t>
1831   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
1832   Expect header.
1833</t>
1834<t>
1835   See Section 7.2.3 of <xref target="Part1"/> for the use of the 100 (Continue) status.
1836</t>
1837</section>
1838
1839<section title="From" anchor="header.from">
1840  <iref primary="true" item="From header"/>
1841  <iref primary="true" item="Headers" subitem="From"/>
1842 
1843 
1844 
1845<t>
1846   The request-header field "From", if given, SHOULD contain an Internet
1847   e-mail address for the human user who controls the requesting user
1848   agent. The address SHOULD be machine-usable, as defined by "mailbox"
1849   in Section 3.4 of <xref target="RFC5322"/>:
1850</t>
1851<figure><iref primary="true" item="Grammar" subitem="From"/><iref primary="true" item="Grammar" subitem="From-v"/><artwork type="abnf2616"><![CDATA[
1852  From    = "From" ":" OWS From-v
1853  From-v  = mailbox
1854 
1855  mailbox = <mailbox, defined in [RFC5322], Section 3.4>
1856]]></artwork></figure>
1857<t>
1858   An example is:
1859</t>
1860<figure><artwork type="example"><![CDATA[
1861  From: webmaster@example.org
1862]]></artwork></figure>
1863<t>
1864   This header field MAY be used for logging purposes and as a means for
1865   identifying the source of invalid or unwanted requests. It SHOULD NOT
1866   be used as an insecure form of access protection. The interpretation
1867   of this field is that the request is being performed on behalf of the
1868   person given, who accepts responsibility for the method performed. In
1869   particular, robot agents SHOULD include this header so that the
1870   person responsible for running the robot can be contacted if problems
1871   occur on the receiving end.
1872</t>
1873<t>
1874   The Internet e-mail address in this field MAY be separate from the
1875   Internet host which issued the request. For example, when a request
1876   is passed through a proxy the original issuer's address SHOULD be
1877   used.
1878</t>
1879<t>
1880   The client SHOULD NOT  send the From header field without the user's
1881   approval, as it might conflict with the user's privacy interests or
1882   their site's security policy. It is strongly recommended that the
1883   user be able to disable, enable, and modify the value of this field
1884   at any time prior to a request.
1885</t>
1886</section>
1887
1888<section title="Location" anchor="header.location">
1889  <iref primary="true" item="Location header"/>
1890  <iref primary="true" item="Headers" subitem="Location"/>
1891 
1892 
1893<t>
1894   The response-header field "Location" is used for the identification of a
1895   new resource or to redirect the recipient to a location other than the
1896   request-target for completion of the request.  For 201 (Created)
1897   responses, the Location is that of the new resource which was created
1898   by the request. For 3xx responses, the location SHOULD indicate the
1899   server's preferred URI for automatic redirection to the resource. The
1900   field value consists of a single absolute URI.
1901</t>
1902<figure><iref primary="true" item="Grammar" subitem="Location"/><iref primary="true" item="Grammar" subitem="Location-v"/><artwork type="abnf2616"><![CDATA[
1903  Location       = "Location" ":" OWS Location-v
1904  Location-v     = absolute-URI [ "#" fragment ]
1905]]></artwork></figure>
1906<t>
1907   An example is:
1908</t>
1909<figure><artwork type="example"><![CDATA[
1910  Location: http://www.example.org/pub/WWW/People.html
1911]]></artwork></figure>
1912<t>
1913  <list><t>
1914      Note: The Content-Location header field (Section 5.7 of <xref target="Part3"/>) differs
1915      from Location in that the Content-Location identifies the original
1916      location of the entity enclosed in the response. It is therefore
1917      possible for a response to contain header fields for both Location
1918      and Content-Location.
1919  </t></list>
1920</t>
1921<t>
1922   There are circumstances in which a fragment identifier in a Location URL would not be appropriate:
1923   <list style="symbols">
1924      <t>With a 201 Created response, because in this usage the Location header specifies the URL for the entire created resource.</t>
1925      <t>With a 300 Multiple Choices, since the choice decision is intended to be made on resource characteristics and not fragment characteristics.</t>
1926      <t>With 305 Use Proxy.</t>
1927   </list>
1928</t>
1929</section>
1930
1931<section title="Max-Forwards" anchor="header.max-forwards">
1932  <iref primary="true" item="Max-Forwards header"/>
1933  <iref primary="true" item="Headers" subitem="Max-Forwards"/>
1934 
1935 
1936<t>
1937   The request-header "Max-Forwards" field provides a mechanism with the
1938   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
1939   number of proxies or gateways that can forward the request to the
1940   next inbound server. This can be useful when the client is attempting
1941   to trace a request chain which appears to be failing or looping in
1942   mid-chain.
1943</t>
1944<figure><iref primary="true" item="Grammar" subitem="Max-Forwards"/><iref primary="true" item="Grammar" subitem="Max-Forwards-v"/><artwork type="abnf2616"><![CDATA[
1945  Max-Forwards   = "Max-Forwards" ":" OWS Max-Forwards-v
1946  Max-Forwards-v = 1*DIGIT
1947]]></artwork></figure>
1948<t>
1949   The Max-Forwards value is a decimal integer indicating the remaining
1950   number of times this request message may be forwarded.
1951</t>
1952<t>
1953   Each proxy or gateway recipient of a TRACE or OPTIONS request
1954   containing a Max-Forwards header field MUST check and update its
1955   value prior to forwarding the request. If the received value is zero
1956   (0), the recipient MUST NOT forward the request; instead, it MUST
1957   respond as the final recipient. If the received Max-Forwards value is
1958   greater than zero, then the forwarded message MUST contain an updated
1959   Max-Forwards field with a value decremented by one (1).
1960</t>
1961<t>
1962   The Max-Forwards header field MAY be ignored for all other methods
1963   defined by this specification and for any extension methods for which
1964   it is not explicitly referred to as part of that method definition.
1965</t>
1966</section>
1967
1968<section title="Referer" anchor="header.referer">
1969  <iref primary="true" item="Referer header"/>
1970  <iref primary="true" item="Headers" subitem="Referer"/>
1971 
1972 
1973<t>
1974   The request-header field "Referer" [sic] allows the client to specify,
1975   for the server's benefit, the address (URI) of the resource from
1976   which the request-target was obtained (the "referrer", although the
1977   header field is misspelled.) The Referer request-header allows a
1978   server to generate lists of back-links to resources for interest,
1979   logging, optimized caching, etc. It also allows obsolete or mistyped
1980   links to be traced for maintenance. The Referer field MUST NOT be
1981   sent if the request-target was obtained from a source that does not have
1982   its own URI, such as input from the user keyboard.
1983</t>
1984<figure><iref primary="true" item="Grammar" subitem="Referer"/><iref primary="true" item="Grammar" subitem="Referer-v"/><artwork type="abnf2616"><![CDATA[
1985  Referer        = "Referer" ":" OWS Referer-v
1986  Referer-v      = absolute-URI / partial-URI
1987]]></artwork></figure>
1988<t>
1989   Example:
1990</t>
1991<figure><artwork type="example"><![CDATA[
1992  Referer: http://www.example.org/hypertext/Overview.html
1993]]></artwork></figure>
1994<t>
1995   If the field value is a relative URI, it SHOULD be interpreted
1996   relative to the request-target. The URI MUST NOT include a fragment. See
1997   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
1998</t>
1999</section>
2000
2001<section title="Retry-After" anchor="header.retry-after">
2002  <iref primary="true" item="Retry-After header"/>
2003  <iref primary="true" item="Headers" subitem="Retry-After"/>
2004 
2005 
2006<t>
2007   The response-header "Retry-After" field can be used with a 503 (Service
2008   Unavailable) response to indicate how long the service is expected to
2009   be unavailable to the requesting client. This field MAY also be used
2010   with any 3xx (Redirection) response to indicate the minimum time the
2011   user-agent is asked wait before issuing the redirected request. The
2012   value of this field can be either an HTTP-date or an integer number
2013   of seconds (in decimal) after the time of the response.
2014</t>
2015<figure><iref primary="true" item="Grammar" subitem="Retry-After"/><iref primary="true" item="Grammar" subitem="Retry-After-v"/><artwork type="abnf2616"><![CDATA[
2016  Retry-After   = "Retry-After" ":" OWS Retry-After-v
2017  Retry-After-v = HTTP-date / delta-seconds
2018]]></artwork></figure>
2019<t anchor="rule.delta-seconds">
2020 
2021   Time spans are non-negative decimal integers, representing time in
2022   seconds.
2023</t>
2024<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
2025  delta-seconds  = 1*DIGIT
2026]]></artwork></figure>
2027<t>
2028   Two examples of its use are
2029</t>
2030<figure><artwork type="example"><![CDATA[
2031  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
2032  Retry-After: 120
2033]]></artwork></figure>
2034<t>
2035   In the latter example, the delay is 2 minutes.
2036</t>
2037</section>
2038
2039<section title="Server" anchor="header.server">
2040  <iref primary="true" item="Server header"/>
2041  <iref primary="true" item="Headers" subitem="Server"/>
2042 
2043 
2044<t>
2045   The response-header field "Server" contains information about the
2046   software used by the origin server to handle the request. The field
2047   can contain multiple product tokens (Section 3.4 of <xref target="Part1"/>) and comments
2048   identifying the server and any significant subproducts. The product
2049   tokens are listed in order of their significance for identifying the
2050   application.
2051</t>
2052<figure><iref primary="true" item="Grammar" subitem="Server"/><iref primary="true" item="Grammar" subitem="Server-v"/><artwork type="abnf2616"><![CDATA[
2053  Server         = "Server" ":" OWS Server-v
2054  Server-v       = product
2055                   *( RWS ( product / comment ) )
2056]]></artwork></figure>
2057<t>
2058   Example:
2059</t>
2060<figure><artwork type="example"><![CDATA[
2061  Server: CERN/3.0 libwww/2.17
2062]]></artwork></figure>
2063<t>
2064   If the response is being forwarded through a proxy, the proxy
2065   application MUST NOT modify the Server response-header. Instead, it
2066   MUST include a Via field (as described in Section 8.9 of <xref target="Part1"/>).
2067  <list><t>
2068      Note: Revealing the specific software version of the server might
2069      allow the server machine to become more vulnerable to attacks
2070      against software that is known to contain security holes. Server
2071      implementors are encouraged to make this field a configurable
2072      option.
2073  </t></list>
2074</t>
2075</section>
2076
2077<section title="User-Agent" anchor="header.user-agent">
2078  <iref primary="true" item="User-Agent header"/>
2079  <iref primary="true" item="Headers" subitem="User-Agent"/>
2080 
2081 
2082<t>
2083   The request-header field "User-Agent" contains information about the
2084   user agent originating the request. This is for statistical purposes,
2085   the tracing of protocol violations, and automated recognition of user
2086   agents for the sake of tailoring responses to avoid particular user
2087   agent limitations. User agents SHOULD include this field with
2088   requests. The field can contain multiple product tokens (Section 3.4 of <xref target="Part1"/>)
2089   and comments identifying the agent and any subproducts which form a
2090   significant part of the user agent. By convention, the product tokens
2091   are listed in order of their significance for identifying the
2092   application.
2093</t>
2094<figure><iref primary="true" item="Grammar" subitem="User-Agent"/><iref primary="true" item="Grammar" subitem="User-Agent-v"/><artwork type="abnf2616"><![CDATA[
2095  User-Agent     = "User-Agent" ":" OWS User-Agent-v
2096  User-Agent-v   = product
2097                   *( RWS ( product / comment ) )
2098]]></artwork></figure>
2099<t>
2100   Example:
2101</t>
2102<figure><artwork type="example"><![CDATA[
2103  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
2104]]></artwork></figure>
2105</section>
2106
2107</section>
2108
2109<section title="IANA Considerations" anchor="IANA.considerations">
2110
2111<section title="Method Registry" anchor="method.registration">
2112<t>
2113  The registration procedure for HTTP Methods is defined by
2114  <xref target="method.registry"/> of this document.
2115</t>
2116<t>
2117   The HTTP Method Registry located at <eref target="http://www.iana.org/assignments/http-methods"/>
2118   should be populated with the registrations below:
2119</t>
2120<!--AUTOGENERATED FROM extract-method-defs.xslt, do not edit manually-->
2121<texttable align="left" suppress-title="true" anchor="iana.method.registration.table">
2122   <ttcol>Method</ttcol>
2123   <ttcol>Safe</ttcol>
2124   <ttcol>Reference</ttcol>
2125   <c>CONNECT</c>
2126   <c>no</c>
2127   <c>
2128      <xref target="CONNECT"/>
2129   </c>
2130   <c>DELETE</c>
2131   <c>no</c>
2132   <c>
2133      <xref target="DELETE"/>
2134   </c>
2135   <c>GET</c>
2136   <c>yes</c>
2137   <c>
2138      <xref target="GET"/>
2139   </c>
2140   <c>HEAD</c>
2141   <c>yes</c>
2142   <c>
2143      <xref target="HEAD"/>
2144   </c>
2145   <c>OPTIONS</c>
2146   <c>yes</c>
2147   <c>
2148      <xref target="OPTIONS"/>
2149   </c>
2150   <c>POST</c>
2151   <c>no</c>
2152   <c>
2153      <xref target="POST"/>
2154   </c>
2155   <c>PUT</c>
2156   <c>no</c>
2157   <c>
2158      <xref target="PUT"/>
2159   </c>
2160   <c>TRACE</c>
2161   <c>yes</c>
2162   <c>
2163      <xref target="TRACE"/>
2164   </c>
2165</texttable>
2166<!--(END)-->
2167</section>
2168
2169<section title="Status Code Registry" anchor="status.code.registration">
2170<t>
2171   The registration procedure for HTTP Status Codes -- previously defined
2172   in Section 7.1 of <xref target="RFC2817"/> -- is now defined
2173   by <xref target="status.code.registry"/> of this document.
2174</t>
2175<t>
2176   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
2177   should be updated with the registrations below:
2178</t>
2179<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
2180<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
2181   <ttcol>Value</ttcol>
2182   <ttcol>Description</ttcol>
2183   <ttcol>Reference</ttcol>
2184   <c>100</c>
2185   <c>Continue</c>
2186   <c>
2187      <xref target="status.100"/>
2188   </c>
2189   <c>101</c>
2190   <c>Switching Protocols</c>
2191   <c>
2192      <xref target="status.101"/>
2193   </c>
2194   <c>200</c>
2195   <c>OK</c>
2196   <c>
2197      <xref target="status.200"/>
2198   </c>
2199   <c>201</c>
2200   <c>Created</c>
2201   <c>
2202      <xref target="status.201"/>
2203   </c>
2204   <c>202</c>
2205   <c>Accepted</c>
2206   <c>
2207      <xref target="status.202"/>
2208   </c>
2209   <c>203</c>
2210   <c>Non-Authoritative Information</c>
2211   <c>
2212      <xref target="status.203"/>
2213   </c>
2214   <c>204</c>
2215   <c>No Content</c>
2216   <c>
2217      <xref target="status.204"/>
2218   </c>
2219   <c>205</c>
2220   <c>Reset Content</c>
2221   <c>
2222      <xref target="status.205"/>
2223   </c>
2224   <c>206</c>
2225   <c>Partial Content</c>
2226   <c>
2227      <xref target="status.206"/>
2228   </c>
2229   <c>300</c>
2230   <c>Multiple Choices</c>
2231   <c>
2232      <xref target="status.300"/>
2233   </c>
2234   <c>301</c>
2235   <c>Moved Permanently</c>
2236   <c>
2237      <xref target="status.301"/>
2238   </c>
2239   <c>302</c>
2240   <c>Found</c>
2241   <c>
2242      <xref target="status.302"/>
2243   </c>
2244   <c>303</c>
2245   <c>See Other</c>
2246   <c>
2247      <xref target="status.303"/>
2248   </c>
2249   <c>304</c>
2250   <c>Not Modified</c>
2251   <c>
2252      <xref target="status.304"/>
2253   </c>
2254   <c>305</c>
2255   <c>Use Proxy</c>
2256   <c>
2257      <xref target="status.305"/>
2258   </c>
2259   <c>306</c>
2260   <c>(Unused)</c>
2261   <c>
2262      <xref target="status.306"/>
2263   </c>
2264   <c>307</c>
2265   <c>Temporary Redirect</c>
2266   <c>
2267      <xref target="status.307"/>
2268   </c>
2269   <c>400</c>
2270   <c>Bad Request</c>
2271   <c>
2272      <xref target="status.400"/>
2273   </c>
2274   <c>401</c>
2275   <c>Unauthorized</c>
2276   <c>
2277      <xref target="status.401"/>
2278   </c>
2279   <c>402</c>
2280   <c>Payment Required</c>
2281   <c>
2282      <xref target="status.402"/>
2283   </c>
2284   <c>403</c>
2285   <c>Forbidden</c>
2286   <c>
2287      <xref target="status.403"/>
2288   </c>
2289   <c>404</c>
2290   <c>Not Found</c>
2291   <c>
2292      <xref target="status.404"/>
2293   </c>
2294   <c>405</c>
2295   <c>Method Not Allowed</c>
2296   <c>
2297      <xref target="status.405"/>
2298   </c>
2299   <c>406</c>
2300   <c>Not Acceptable</c>
2301   <c>
2302      <xref target="status.406"/>
2303   </c>
2304   <c>407</c>
2305   <c>Proxy Authentication Required</c>
2306   <c>
2307      <xref target="status.407"/>
2308   </c>
2309   <c>408</c>
2310   <c>Request Timeout</c>
2311   <c>
2312      <xref target="status.408"/>
2313   </c>
2314   <c>409</c>
2315   <c>Conflict</c>
2316   <c>
2317      <xref target="status.409"/>
2318   </c>
2319   <c>410</c>
2320   <c>Gone</c>
2321   <c>
2322      <xref target="status.410"/>
2323   </c>
2324   <c>411</c>
2325   <c>Length Required</c>
2326   <c>
2327      <xref target="status.411"/>
2328   </c>
2329   <c>412</c>
2330   <c>Precondition Failed</c>
2331   <c>
2332      <xref target="status.412"/>
2333   </c>
2334   <c>413</c>
2335   <c>Request Entity Too Large</c>
2336   <c>
2337      <xref target="status.413"/>
2338   </c>
2339   <c>414</c>
2340   <c>URI Too Long</c>
2341   <c>
2342      <xref target="status.414"/>
2343   </c>
2344   <c>415</c>
2345   <c>Unsupported Media Type</c>
2346   <c>
2347      <xref target="status.415"/>
2348   </c>
2349   <c>416</c>
2350   <c>Requested Range Not Satisfiable</c>
2351   <c>
2352      <xref target="status.416"/>
2353   </c>
2354   <c>417</c>
2355   <c>Expectation Failed</c>
2356   <c>
2357      <xref target="status.417"/>
2358   </c>
2359   <c>500</c>
2360   <c>Internal Server Error</c>
2361   <c>
2362      <xref target="status.500"/>
2363   </c>
2364   <c>501</c>
2365   <c>Not Implemented</c>
2366   <c>
2367      <xref target="status.501"/>
2368   </c>
2369   <c>502</c>
2370   <c>Bad Gateway</c>
2371   <c>
2372      <xref target="status.502"/>
2373   </c>
2374   <c>503</c>
2375   <c>Service Unavailable</c>
2376   <c>
2377      <xref target="status.503"/>
2378   </c>
2379   <c>504</c>
2380   <c>Gateway Timeout</c>
2381   <c>
2382      <xref target="status.504"/>
2383   </c>
2384   <c>505</c>
2385   <c>HTTP Version Not Supported</c>
2386   <c>
2387      <xref target="status.505"/>
2388   </c>
2389</texttable>
2390<!--(END)-->
2391</section>
2392<section title="Message Header Registration" anchor="message.header.registration">
2393<t>
2394   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2395   with the permanent registrations below (see <xref target="RFC3864"/>):
2396</t>
2397<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2398<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2399   <ttcol>Header Field Name</ttcol>
2400   <ttcol>Protocol</ttcol>
2401   <ttcol>Status</ttcol>
2402   <ttcol>Reference</ttcol>
2403   <c>Allow</c>
2404   <c>http</c>
2405   <c>standard</c>
2406   <c>
2407      <xref target="header.allow"/>
2408   </c>
2409   <c>Expect</c>
2410   <c>http</c>
2411   <c>standard</c>
2412   <c>
2413      <xref target="header.expect"/>
2414   </c>
2415   <c>From</c>
2416   <c>http</c>
2417   <c>standard</c>
2418   <c>
2419      <xref target="header.from"/>
2420   </c>
2421   <c>Location</c>
2422   <c>http</c>
2423   <c>standard</c>
2424   <c>
2425      <xref target="header.location"/>
2426   </c>
2427   <c>Max-Forwards</c>
2428   <c>http</c>
2429   <c>standard</c>
2430   <c>
2431      <xref target="header.max-forwards"/>
2432   </c>
2433   <c>Referer</c>
2434   <c>http</c>
2435   <c>standard</c>
2436   <c>
2437      <xref target="header.referer"/>
2438   </c>
2439   <c>Retry-After</c>
2440   <c>http</c>
2441   <c>standard</c>
2442   <c>
2443      <xref target="header.retry-after"/>
2444   </c>
2445   <c>Server</c>
2446   <c>http</c>
2447   <c>standard</c>
2448   <c>
2449      <xref target="header.server"/>
2450   </c>
2451   <c>User-Agent</c>
2452   <c>http</c>
2453   <c>standard</c>
2454   <c>
2455      <xref target="header.user-agent"/>
2456   </c>
2457</texttable>
2458<!--(END)-->
2459<t>
2460   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2461</t>
2462</section>
2463</section>
2464
2465<section title="Security Considerations" anchor="security.considerations">
2466<t>
2467   This section is meant to inform application developers, information
2468   providers, and users of the security limitations in HTTP/1.1 as
2469   described by this document. The discussion does not include
2470   definitive solutions to the problems revealed, though it does make
2471   some suggestions for reducing security risks.
2472</t>
2473
2474<section title="Transfer of Sensitive Information" anchor="security.sensitive">
2475<t>
2476   Like any generic data transfer protocol, HTTP cannot regulate the
2477   content of the data that is transferred, nor is there any a priori
2478   method of determining the sensitivity of any particular piece of
2479   information within the context of any given request. Therefore,
2480   applications SHOULD supply as much control over this information as
2481   possible to the provider of that information. Four header fields are
2482   worth special mention in this context: Server, Via, Referer and From.
2483</t>
2484<t>
2485   Revealing the specific software version of the server might allow the
2486   server machine to become more vulnerable to attacks against software
2487   that is known to contain security holes. Implementors SHOULD make the
2488   Server header field a configurable option.
2489</t>
2490<t>
2491   Proxies which serve as a portal through a network firewall SHOULD
2492   take special precautions regarding the transfer of header information
2493   that identifies the hosts behind the firewall. In particular, they
2494   SHOULD remove, or replace with sanitized versions, any Via fields
2495   generated behind the firewall.
2496</t>
2497<t>
2498   The Referer header allows reading patterns to be studied and reverse
2499   links drawn. Although it can be very useful, its power can be abused
2500   if user details are not separated from the information contained in
2501   the Referer. Even when the personal information has been removed, the
2502   Referer header might indicate a private document's URI whose
2503   publication would be inappropriate.
2504</t>
2505<t>
2506   The information sent in the From field might conflict with the user's
2507   privacy interests or their site's security policy, and hence it
2508   SHOULD NOT  be transmitted without the user being able to disable,
2509   enable, and modify the contents of the field. The user MUST be able
2510   to set the contents of this field within a user preference or
2511   application defaults configuration.
2512</t>
2513<t>
2514   We suggest, though do not require, that a convenient toggle interface
2515   be provided for the user to enable or disable the sending of From and
2516   Referer information.
2517</t>
2518<t>
2519   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
2520   fields can sometimes be used to determine that a specific client or
2521   server have a particular security hole which might be exploited.
2522   Unfortunately, this same information is often used for other valuable
2523   purposes for which HTTP currently has no better mechanism.
2524</t>
2525</section>
2526
2527<section title="Encoding Sensitive Information in URIs" anchor="encoding.sensitive.information.in.uris">
2528<t>
2529   Because the source of a link might be private information or might
2530   reveal an otherwise private information source, it is strongly
2531   recommended that the user be able to select whether or not the
2532   Referer field is sent. For example, a browser client could have a
2533   toggle switch for browsing openly/anonymously, which would
2534   respectively enable/disable the sending of Referer and From
2535   information.
2536</t>
2537<t>
2538   Clients SHOULD NOT include a Referer header field in a (non-secure)
2539   HTTP request if the referring page was transferred with a secure
2540   protocol.
2541</t>
2542<t>
2543   Authors of services should not use
2544   GET-based forms for the submission of sensitive data because that
2545   data will be encoded in the Request-target. Many existing
2546   servers, proxies, and user agents log or display the Request-target in
2547   places where it might be visible to third parties. Such services can
2548   use POST-based form submission instead.
2549</t>
2550</section>
2551
2552<section title="Location Headers and Spoofing" anchor="location.spoofing">
2553<t>
2554   If a single server supports multiple organizations that do not trust
2555   one another, then it MUST check the values of Location and Content-Location
2556   headers in responses that are generated under control of
2557   said organizations to make sure that they do not attempt to
2558   invalidate resources over which they have no authority.
2559</t>
2560</section>
2561
2562</section>
2563
2564<section title="Acknowledgments" anchor="ack">
2565</section>
2566</middle>
2567<back>
2568
2569<references title="Normative References">
2570
2571<reference anchor="Part1">
2572  <front>
2573    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2574    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2575      <organization abbrev="Day Software">Day Software</organization>
2576      <address><email>fielding@gbiv.com</email></address>
2577    </author>
2578    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2579      <organization>One Laptop per Child</organization>
2580      <address><email>jg@laptop.org</email></address>
2581    </author>
2582    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2583      <organization abbrev="HP">Hewlett-Packard Company</organization>
2584      <address><email>JeffMogul@acm.org</email></address>
2585    </author>
2586    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2587      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2588      <address><email>henrikn@microsoft.com</email></address>
2589    </author>
2590    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2591      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2592      <address><email>LMM@acm.org</email></address>
2593    </author>
2594    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2595      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2596      <address><email>paulle@microsoft.com</email></address>
2597    </author>
2598    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2599      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2600      <address><email>timbl@w3.org</email></address>
2601    </author>
2602    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2603      <organization abbrev="W3C">World Wide Web Consortium</organization>
2604      <address><email>ylafon@w3.org</email></address>
2605    </author>
2606    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2607      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2608      <address><email>julian.reschke@greenbytes.de</email></address>
2609    </author>
2610    <date month="March" year="2009"/>
2611  </front>
2612  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-06"/>
2613 
2614</reference>
2615
2616<reference anchor="Part3">
2617  <front>
2618    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2619    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2620      <organization abbrev="Day Software">Day Software</organization>
2621      <address><email>fielding@gbiv.com</email></address>
2622    </author>
2623    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2624      <organization>One Laptop per Child</organization>
2625      <address><email>jg@laptop.org</email></address>
2626    </author>
2627    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2628      <organization abbrev="HP">Hewlett-Packard Company</organization>
2629      <address><email>JeffMogul@acm.org</email></address>
2630    </author>
2631    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2632      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2633      <address><email>henrikn@microsoft.com</email></address>
2634    </author>
2635    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2636      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2637      <address><email>LMM@acm.org</email></address>
2638    </author>
2639    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2640      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2641      <address><email>paulle@microsoft.com</email></address>
2642    </author>
2643    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2644      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2645      <address><email>timbl@w3.org</email></address>
2646    </author>
2647    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2648      <organization abbrev="W3C">World Wide Web Consortium</organization>
2649      <address><email>ylafon@w3.org</email></address>
2650    </author>
2651    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2652      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2653      <address><email>julian.reschke@greenbytes.de</email></address>
2654    </author>
2655    <date month="March" year="2009"/>
2656  </front>
2657  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-06"/>
2658 
2659</reference>
2660
2661<reference anchor="Part4">
2662  <front>
2663    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2664    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2665      <organization abbrev="Day Software">Day Software</organization>
2666      <address><email>fielding@gbiv.com</email></address>
2667    </author>
2668    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2669      <organization>One Laptop per Child</organization>
2670      <address><email>jg@laptop.org</email></address>
2671    </author>
2672    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2673      <organization abbrev="HP">Hewlett-Packard Company</organization>
2674      <address><email>JeffMogul@acm.org</email></address>
2675    </author>
2676    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2677      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2678      <address><email>henrikn@microsoft.com</email></address>
2679    </author>
2680    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2681      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2682      <address><email>LMM@acm.org</email></address>
2683    </author>
2684    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2685      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2686      <address><email>paulle@microsoft.com</email></address>
2687    </author>
2688    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2689      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2690      <address><email>timbl@w3.org</email></address>
2691    </author>
2692    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2693      <organization abbrev="W3C">World Wide Web Consortium</organization>
2694      <address><email>ylafon@w3.org</email></address>
2695    </author>
2696    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2697      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2698      <address><email>julian.reschke@greenbytes.de</email></address>
2699    </author>
2700    <date month="March" year="2009"/>
2701  </front>
2702  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-06"/>
2703 
2704</reference>
2705
2706<reference anchor="Part5">
2707  <front>
2708    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2709    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2710      <organization abbrev="Day Software">Day Software</organization>
2711      <address><email>fielding@gbiv.com</email></address>
2712    </author>
2713    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2714      <organization>One Laptop per Child</organization>
2715      <address><email>jg@laptop.org</email></address>
2716    </author>
2717    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2718      <organization abbrev="HP">Hewlett-Packard Company</organization>
2719      <address><email>JeffMogul@acm.org</email></address>
2720    </author>
2721    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2722      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2723      <address><email>henrikn@microsoft.com</email></address>
2724    </author>
2725    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2726      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2727      <address><email>LMM@acm.org</email></address>
2728    </author>
2729    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2730      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2731      <address><email>paulle@microsoft.com</email></address>
2732    </author>
2733    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2734      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2735      <address><email>timbl@w3.org</email></address>
2736    </author>
2737    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2738      <organization abbrev="W3C">World Wide Web Consortium</organization>
2739      <address><email>ylafon@w3.org</email></address>
2740    </author>
2741    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2742      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2743      <address><email>julian.reschke@greenbytes.de</email></address>
2744    </author>
2745    <date month="March" year="2009"/>
2746  </front>
2747  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-06"/>
2748 
2749</reference>
2750
2751<reference anchor="Part6">
2752  <front>
2753    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
2754    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2755      <organization abbrev="Day Software">Day Software</organization>
2756      <address><email>fielding@gbiv.com</email></address>
2757    </author>
2758    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2759      <organization>One Laptop per Child</organization>
2760      <address><email>jg@laptop.org</email></address>
2761    </author>
2762    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2763      <organization abbrev="HP">Hewlett-Packard Company</organization>
2764      <address><email>JeffMogul@acm.org</email></address>
2765    </author>
2766    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2767      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2768      <address><email>henrikn@microsoft.com</email></address>
2769    </author>
2770    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2771      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2772      <address><email>LMM@acm.org</email></address>
2773    </author>
2774    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2775      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2776      <address><email>paulle@microsoft.com</email></address>
2777    </author>
2778    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2779      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2780      <address><email>timbl@w3.org</email></address>
2781    </author>
2782    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2783      <organization abbrev="W3C">World Wide Web Consortium</organization>
2784      <address><email>ylafon@w3.org</email></address>
2785    </author>
2786    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2787      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2788      <address><email>julian.reschke@greenbytes.de</email></address>
2789    </author>
2790    <date month="March" year="2009"/>
2791  </front>
2792  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-06"/>
2793 
2794</reference>
2795
2796<reference anchor="Part7">
2797  <front>
2798    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2799    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2800      <organization abbrev="Day Software">Day Software</organization>
2801      <address><email>fielding@gbiv.com</email></address>
2802    </author>
2803    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2804      <organization>One Laptop per Child</organization>
2805      <address><email>jg@laptop.org</email></address>
2806    </author>
2807    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2808      <organization abbrev="HP">Hewlett-Packard Company</organization>
2809      <address><email>JeffMogul@acm.org</email></address>
2810    </author>
2811    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2812      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2813      <address><email>henrikn@microsoft.com</email></address>
2814    </author>
2815    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2816      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2817      <address><email>LMM@acm.org</email></address>
2818    </author>
2819    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2820      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2821      <address><email>paulle@microsoft.com</email></address>
2822    </author>
2823    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2824      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2825      <address><email>timbl@w3.org</email></address>
2826    </author>
2827    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2828      <organization abbrev="W3C">World Wide Web Consortium</organization>
2829      <address><email>ylafon@w3.org</email></address>
2830    </author>
2831    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2832      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2833      <address><email>julian.reschke@greenbytes.de</email></address>
2834    </author>
2835    <date month="March" year="2009"/>
2836  </front>
2837  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-06"/>
2838 
2839</reference>
2840
2841<reference anchor="RFC2119">
2842  <front>
2843    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2844    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2845      <organization>Harvard University</organization>
2846      <address><email>sob@harvard.edu</email></address>
2847    </author>
2848    <date month="March" year="1997"/>
2849  </front>
2850  <seriesInfo name="BCP" value="14"/>
2851  <seriesInfo name="RFC" value="2119"/>
2852</reference>
2853
2854<reference anchor="RFC5234">
2855  <front>
2856    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2857    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2858      <organization>Brandenburg InternetWorking</organization>
2859      <address>
2860      <postal>
2861      <street>675 Spruce Dr.</street>
2862      <city>Sunnyvale</city>
2863      <region>CA</region>
2864      <code>94086</code>
2865      <country>US</country></postal>
2866      <phone>+1.408.246.8253</phone>
2867      <email>dcrocker@bbiw.net</email></address> 
2868    </author>
2869    <author initials="P." surname="Overell" fullname="Paul Overell">
2870      <organization>THUS plc.</organization>
2871      <address>
2872      <postal>
2873      <street>1/2 Berkeley Square</street>
2874      <street>99 Berkely Street</street>
2875      <city>Glasgow</city>
2876      <code>G3 7HR</code>
2877      <country>UK</country></postal>
2878      <email>paul.overell@thus.net</email></address>
2879    </author>
2880    <date month="January" year="2008"/>
2881  </front>
2882  <seriesInfo name="STD" value="68"/>
2883  <seriesInfo name="RFC" value="5234"/>
2884</reference>
2885
2886</references>
2887
2888<references title="Informative References">
2889
2890<reference anchor="RFC1945">
2891  <front>
2892    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
2893    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2894      <organization>MIT, Laboratory for Computer Science</organization>
2895      <address><email>timbl@w3.org</email></address>
2896    </author>
2897    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2898      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2899      <address><email>fielding@ics.uci.edu</email></address>
2900    </author>
2901    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2902      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
2903      <address><email>frystyk@w3.org</email></address>
2904    </author>
2905    <date month="May" year="1996"/>
2906  </front>
2907  <seriesInfo name="RFC" value="1945"/>
2908</reference>
2909
2910<reference anchor="RFC2068">
2911  <front>
2912    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2913    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2914      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2915      <address><email>fielding@ics.uci.edu</email></address>
2916    </author>
2917    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2918      <organization>MIT Laboratory for Computer Science</organization>
2919      <address><email>jg@w3.org</email></address>
2920    </author>
2921    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2922      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2923      <address><email>mogul@wrl.dec.com</email></address>
2924    </author>
2925    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2926      <organization>MIT Laboratory for Computer Science</organization>
2927      <address><email>frystyk@w3.org</email></address>
2928    </author>
2929    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2930      <organization>MIT Laboratory for Computer Science</organization>
2931      <address><email>timbl@w3.org</email></address>
2932    </author>
2933    <date month="January" year="1997"/>
2934  </front>
2935  <seriesInfo name="RFC" value="2068"/>
2936</reference>
2937
2938<reference anchor="RFC2616">
2939  <front>
2940    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2941    <author initials="R." surname="Fielding" fullname="R. Fielding">
2942      <organization>University of California, Irvine</organization>
2943      <address><email>fielding@ics.uci.edu</email></address>
2944    </author>
2945    <author initials="J." surname="Gettys" fullname="J. Gettys">
2946      <organization>W3C</organization>
2947      <address><email>jg@w3.org</email></address>
2948    </author>
2949    <author initials="J." surname="Mogul" fullname="J. Mogul">
2950      <organization>Compaq Computer Corporation</organization>
2951      <address><email>mogul@wrl.dec.com</email></address>
2952    </author>
2953    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2954      <organization>MIT Laboratory for Computer Science</organization>
2955      <address><email>frystyk@w3.org</email></address>
2956    </author>
2957    <author initials="L." surname="Masinter" fullname="L. Masinter">
2958      <organization>Xerox Corporation</organization>
2959      <address><email>masinter@parc.xerox.com</email></address>
2960    </author>
2961    <author initials="P." surname="Leach" fullname="P. Leach">
2962      <organization>Microsoft Corporation</organization>
2963      <address><email>paulle@microsoft.com</email></address>
2964    </author>
2965    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2966      <organization>W3C</organization>
2967      <address><email>timbl@w3.org</email></address>
2968    </author>
2969    <date month="June" year="1999"/>
2970  </front>
2971  <seriesInfo name="RFC" value="2616"/>
2972</reference>
2973
2974<reference anchor="RFC2817">
2975  <front>
2976    <title>Upgrading to TLS Within HTTP/1.1</title>
2977    <author initials="R." surname="Khare" fullname="R. Khare">
2978      <organization>4K Associates / UC Irvine</organization>
2979      <address><email>rohit@4K-associates.com</email></address>
2980    </author>
2981    <author initials="S." surname="Lawrence" fullname="S. Lawrence">
2982      <organization>Agranat Systems, Inc.</organization>
2983      <address><email>lawrence@agranat.com</email></address>
2984    </author>
2985    <date year="2000" month="May"/>
2986  </front>
2987  <seriesInfo name="RFC" value="2817"/>
2988</reference>
2989
2990<reference anchor="RFC3864">
2991  <front>
2992    <title>Registration Procedures for Message Header Fields</title>
2993    <author initials="G." surname="Klyne" fullname="G. Klyne">
2994      <organization>Nine by Nine</organization>
2995      <address><email>GK-IETF@ninebynine.org</email></address>
2996    </author>
2997    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
2998      <organization>BEA Systems</organization>
2999      <address><email>mnot@pobox.com</email></address>
3000    </author>
3001    <author initials="J." surname="Mogul" fullname="J. Mogul">
3002      <organization>HP Labs</organization>
3003      <address><email>JeffMogul@acm.org</email></address>
3004    </author>
3005    <date year="2004" month="September"/>
3006  </front>
3007  <seriesInfo name="BCP" value="90"/>
3008  <seriesInfo name="RFC" value="3864"/>
3009</reference>
3010
3011<reference anchor="RFC5226">
3012  <front>
3013    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
3014    <author initials="T." surname="Narten" fullname="T. Narten">
3015      <organization>IBM</organization>
3016      <address><email>narten@us.ibm.com</email></address>
3017    </author>
3018    <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
3019      <organization>Google</organization>
3020      <address><email>Harald@Alvestrand.no</email></address>
3021    </author>
3022    <date year="2008" month="May"/>
3023  </front>
3024  <seriesInfo name="BCP" value="26"/>
3025  <seriesInfo name="RFC" value="5226"/>
3026</reference>
3027
3028<reference anchor="RFC5322">
3029  <front>
3030    <title>Internet Message Format</title>
3031    <author initials="P." surname="Resnick" fullname="P. Resnick">
3032      <organization>Qualcomm Incorporated</organization>
3033    </author>
3034    <date year="2008" month="October"/>
3035  </front> 
3036  <seriesInfo name="RFC" value="5322"/>
3037</reference>
3038
3039</references>
3040
3041<section title="Compatibility with Previous Versions" anchor="compatibility">
3042<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
3043<t>
3044   Clarified which error code should be used for inbound server failures
3045   (e.g. DNS failures). (<xref target="status.504"/>).
3046</t>
3047<t>
3048   201 (Created) had a race that required an Etag be sent when a resource is
3049   first created. (<xref target="status.201"/>).
3050</t>
3051<t>
3052   Rewrite of message transmission requirements to make it much harder
3053   for implementors to get it wrong, as the consequences of errors here
3054   can have significant impact on the Internet, and to deal with the
3055   following problems:
3056  <list style="numbers">
3057      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
3058         this was incorrectly placing a requirement on the behavior of
3059         an implementation of a future version of HTTP/1.x</t>
3060
3061      <t>Made it clear that user-agents should retry requests, not
3062         "clients" in general.</t>
3063
3064      <t>Converted requirements for clients to ignore unexpected 100
3065         (Continue) responses, and for proxies to forward 100 responses,
3066         into a general requirement for 1xx responses.</t>
3067
3068      <t>Modified some TCP-specific language, to make it clearer that
3069         non-TCP transports are possible for HTTP.</t>
3070
3071      <t>Require that the origin server MUST NOT wait for the request
3072         body before it sends a required 100 (Continue) response.</t>
3073
3074      <t>Allow, rather than require, a server to omit 100 (Continue) if
3075         it has already seen some of the request body.</t>
3076
3077      <t>Allow servers to defend against denial-of-service attacks and
3078         broken clients.</t>
3079  </list>
3080</t>
3081<t>
3082   This change adds the Expect header and 417 status code.
3083</t>
3084<t>
3085   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
3086   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
3087</t>
3088<t>
3089   The PATCH<iref item="PATCH method" primary="true"/><iref item="Methods" subitem="PATCH" primary="true"/>, LINK<iref item="LINK method" primary="true"/><iref item="Methods" subitem="LINK" primary="true"/>, UNLINK<iref item="UNLINK method" primary="true"/><iref item="Methods" subitem="UNLINK" primary="true"/> methods were defined but not commonly
3090   implemented in previous versions of this specification. See Section 19.6.1 of <xref target="RFC2068"/>.
3091</t>
3092</section>
3093
3094<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
3095<t>
3096  This document takes over the Status Code Registry, previously defined
3097  in Section 7.1 of <xref target="RFC2817"/>.
3098  (<xref target="status.code.registry"/>)
3099</t>
3100<t>
3101  Clarify definition of POST.
3102  (<xref target="POST"/>)
3103</t>
3104<t>
3105  Failed to consider that there are
3106  many other request methods that are safe to automatically redirect,
3107  and further that the user agent is able to make that determination
3108  based on the request method semantics.
3109  (Sections <xref format="counter" target="status.301"/>,
3110  <xref format="counter" target="status.302"/> and
3111  <xref format="counter" target="status.307"/>)
3112</t>
3113<t>
3114  Deprecate 305 Use Proxy status code, because user agents did not implement it.
3115  It used to indicate that the requested resource must be accessed through the
3116  proxy given by the Location field. The Location field gave the URI of the
3117  proxy. The recipient was expected to repeat this single request via the proxy.
3118  (<xref target="status.305"/>)
3119</t>
3120<t>
3121  Reclassify Allow header as response header, removing the option to
3122  specify it in a PUT request.
3123  Relax the server requirement on the contents of the Allow header and
3124  remove requirement on clients to always trust the header value.
3125  (<xref target="header.allow"/>)
3126</t>
3127<t>
3128  Correct syntax of Location header to allow fragment,
3129  as referred symbol wasn't what was expected, and add some
3130  clarifications as to when it would not be appropriate.
3131  (<xref target="header.location"/>)
3132</t>
3133<t>
3134  In the description of the Server header, the Via field
3135  was described as a SHOULD. The requirement was and is stated
3136  correctly in the description of the Via header in Section 8.9 of <xref target="Part1"/>.
3137  (<xref target="header.server"/>)
3138</t>
3139</section>
3140
3141</section>
3142
3143<section title="Collected ABNF" anchor="collected.abnf">
3144<figure>
3145<artwork type="abnf" name="p2-semantics.parsed-abnf"><![CDATA[
3146Accept = <Accept, defined in [Part3], Section 5.1>
3147Accept-Charset = <Accept-Charset, defined in [Part3], Section 5.2>
3148Accept-Encoding = <Accept-Encoding, defined in [Part3], Section 5.3>
3149Accept-Language = <Accept-Language, defined in [Part3], Section 5.4>
3150Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 5.1>
3151Age = <Age, defined in [Part6], Section 3.1>
3152Allow = "Allow:" OWS Allow-v
3153Allow-v = [ ( "," / Method ) *( OWS "," [ OWS Method ] ) ]
3154Authorization = <Authorization, defined in [Part7], Section 3.1>
3155
3156ETag = <ETag, defined in [Part4], Section 6.1>
3157Expect = "Expect:" OWS Expect-v
3158Expect-v = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )
3159
3160From = "From:" OWS From-v
3161From-v = mailbox
3162
3163HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
3164Host = <Host, defined in [Part1], Section 2.1>
3165
3166If-Match = <If-Match, defined in [Part4], Section 6.2>
3167If-Modified-Since =
3168 <If-Modified-Since, defined in [Part4], Section 6.3>
3169If-None-Match = <If-None-Match, defined in [Part4], Section 6.4>
3170If-Range = <If-Range, defined in [Part5], Section 5.3>
3171If-Unmodified-Since =
3172 <If-Unmodified-Since, defined in [Part4], Section 6.5>
3173
3174Location = "Location:" OWS Location-v
3175Location-v = absolute-URI [ "#" fragment ]
3176
3177Max-Forwards = "Max-Forwards:" OWS Max-Forwards-v
3178Max-Forwards-v = 1*DIGIT
3179Method = %x4F.50.54.49.4F.4E.53 / %x47.45.54 / %x48.45.41.44 /
3180 %x50.4F.54 / %x50.55.54 / %x44.45.4C.45.54.45 / %x54.52.41.43.45 /
3181 %x43.4E.4E.45.43.54 / extension-method
3182
3183OWS = <OWS, defined in [Part1], Section 1.2.2>
3184
3185Proxy-Authenticate =
3186 <Proxy-Authenticate, defined in [Part7], Section 3.2>
3187Proxy-Authorization =
3188 <Proxy-Authorization, defined in [Part7], Section 3.3>
3189
3190RWS = <RWS, defined in [Part1], Section 1.2.2>
3191Range = <Range, defined in [Part5], Section 5.4>
3192Reason-Phrase = *( WSP / VCHAR / obs-text )
3193Referer = "Referer:" OWS Referer-v
3194Referer-v = absolute-URI / partial-URI
3195Retry-After = "Retry-After:" OWS Retry-After-v
3196Retry-After-v = HTTP-date / delta-seconds
3197
3198Server = "Server:" OWS Server-v
3199Server-v = product *( RWS ( product / comment ) )
3200Status-Code = "100" / "101" / "200" / "201" / "202" / "203" / "204" /
3201 "205" / "206" / "300" / "301" / "302" / "303" / "304" / "305" /
3202 "307" / "400" / "401" / "402" / "403" / "404" / "405" / "406" /
3203 "407" / "408" / "409" / "410" / "411" / "412" / "413" / "414" /
3204 "415" / "416" / "417" / "500" / "501" / "502" / "503" / "504" /
3205 "505" / extension-code
3206
3207TE = <TE, defined in [Part1], Section 8.8>
3208
3209User-Agent = "User-Agent:" OWS User-Agent-v
3210User-Agent-v = product *( RWS ( product / comment ) )
3211
3212Vary = <Vary, defined in [Part6], Section 3.5>
3213
3214WWW-Authenticate =
3215 <WWW-Authenticate, defined in [Part7], Section 3.4>
3216
3217absolute-URI = <absolute-URI, defined in [Part1], Section 2.1>
3218
3219comment = <comment, defined in [Part1], Section 1.2.2>
3220
3221delta-seconds = 1*DIGIT
3222
3223expect-params = ";" token [ "=" ( token / quoted-string ) ]
3224expectation = "100-continue" / expectation-extension
3225expectation-extension = token [ "=" ( token / quoted-string )
3226 *expect-params ]
3227extension-code = 3DIGIT
3228extension-method = token
3229
3230fragment = <fragment, defined in [Part1], Section 2.1>
3231
3232mailbox = <mailbox, defined in [RFC5322], Section 3.4>
3233
3234obs-text = <obs-text, defined in [Part1], Section 1.2.2>
3235
3236partial-URI = <partial-URI, defined in [Part1], Section 2.1>
3237product = <product, defined in [Part1], Section 3.4>
3238
3239quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
3240
3241request-header = Accept / Accept-Charset / Accept-Encoding /
3242 Accept-Language / Authorization / Expect / From / Host / If-Match /
3243 If-Modified-Since / If-None-Match / If-Range / If-Unmodified-Since /
3244 Max-Forwards / Proxy-Authorization / Range / Referer / TE /
3245 User-Agent
3246response-header = Accept-Ranges / Age / Allow / ETag / Location /
3247 Proxy-Authenticate / Retry-After / Server / Vary / WWW-Authenticate
3248
3249token = <token, defined in [Part1], Section 1.2.2>
3250
3251
3252]]></artwork>
3253</figure>
3254<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
3255; Reason-Phrase defined but not used
3256; Status-Code defined but not used
3257; request-header defined but not used
3258; response-header defined but not used
3259]]></artwork></figure></section>
3260
3261<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
3262
3263<section title="Since RFC2616">
3264<t>
3265  Extracted relevant partitions from <xref target="RFC2616"/>.
3266</t>
3267</section>
3268
3269<section title="Since draft-ietf-httpbis-p2-semantics-00">
3270<t>
3271  Closed issues:
3272  <list style="symbols"> 
3273    <t>
3274      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/5"/>:
3275      "Via is a MUST"
3276      (<eref target="http://purl.org/NET/http-errata#via-must"/>)
3277    </t>
3278    <t>
3279      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/6"/>:
3280      "Fragments allowed in Location"
3281      (<eref target="http://purl.org/NET/http-errata#location-fragments"/>)
3282    </t>
3283    <t>
3284      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/10"/>:
3285      "Safe Methods vs Redirection"
3286      (<eref target="http://purl.org/NET/http-errata#saferedirect"/>)
3287    </t>
3288    <t>
3289      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/17"/>:
3290      "Revise description of the POST method"
3291      (<eref target="http://purl.org/NET/http-errata#post"/>)
3292    </t>
3293    <t>
3294      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
3295      "Normative and Informative references"
3296    </t>
3297    <t>
3298      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
3299      "RFC2606 Compliance"
3300    </t>
3301    <t>
3302      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
3303      "Informative references"
3304    </t>
3305    <t>
3306      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/84"/>:
3307      "Redundant cross-references"
3308    </t>
3309  </list>
3310</t>
3311<t>
3312  Other changes:
3313  <list style="symbols"> 
3314    <t>
3315      Move definitions of 304 and 412 condition codes to <xref target="Part4"/>
3316    </t>
3317  </list>
3318</t>
3319</section>
3320
3321<section title="Since draft-ietf-httpbis-p2-semantics-01">
3322<t>
3323  Closed issues:
3324  <list style="symbols"> 
3325    <t>
3326      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/21"/>:
3327      "PUT side effects"
3328    </t>
3329    <t>
3330      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/91"/>:
3331      "Duplicate Host header requirements"
3332    </t>
3333  </list>
3334</t>
3335<t>
3336  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3337  <list style="symbols"> 
3338    <t>
3339      Move "Product Tokens" section (back) into Part 1, as "token" is used
3340      in the definition of the Upgrade header.
3341    </t>
3342    <t>
3343      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3344    </t>
3345    <t>
3346      Copy definition of delta-seconds from Part6 instead of referencing it.
3347    </t>
3348  </list>
3349</t>
3350</section>
3351
3352<section title="Since draft-ietf-httpbis-p2-semantics-02" anchor="changes.since.02">
3353<t>
3354  Closed issues:
3355  <list style="symbols"> 
3356    <t>
3357      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/24"/>:
3358      "Requiring Allow in 405 responses"
3359    </t>
3360    <t>
3361      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/59"/>:
3362      "Status Code Registry"
3363    </t>
3364    <t>
3365      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/61"/>:
3366      "Redirection vs. Location"
3367    </t>
3368    <t>
3369      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/70"/>:
3370      "Cacheability of 303 response"
3371    </t>
3372    <t>
3373      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/76"/>:
3374      "305 Use Proxy"
3375    </t>
3376    <t>
3377      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
3378      "Classification for Allow header"
3379    </t>
3380    <t>
3381      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/112"/>:
3382      "PUT - 'store under' vs 'store at'"
3383    </t>
3384  </list>
3385</t>
3386<t>
3387  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
3388  <list style="symbols"> 
3389    <t>
3390      Reference RFC 3984, and update header registrations for headers defined
3391      in this document.
3392    </t>
3393  </list>
3394</t>
3395<t>
3396  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3397  <list style="symbols"> 
3398    <t>
3399      Replace string literals when the string really is case-sensitive (method).
3400    </t>
3401  </list>
3402</t>
3403</section>
3404
3405<section title="Since draft-ietf-httpbis-p2-semantics-03" anchor="changes.since.03">
3406<t>
3407  Closed issues:
3408  <list style="symbols"> 
3409    <t>
3410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/98"/>:
3411      "OPTIONS request bodies"
3412    </t>
3413    <t>
3414      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/119"/>:
3415      "Description of CONNECT should refer to RFC2817"
3416    </t>
3417    <t>
3418      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/125"/>:
3419      "Location Content-Location reference request/response mixup"
3420    </t>
3421  </list>
3422</t>
3423<t>
3424  Ongoing work on Method Registry (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/72"/>):
3425  <list style="symbols"> 
3426    <t>
3427      Added initial proposal for registration process, plus initial
3428      content (non-HTTP/1.1 methods to be added by a separate specification).
3429    </t>
3430  </list>
3431</t>
3432</section>
3433
3434<section title="Since draft-ietf-httpbis-p2-semantics-04" anchor="changes.since.04">
3435<t>
3436  Closed issues:
3437  <list style="symbols"> 
3438    <t>
3439      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/103"/>:
3440      "Content-*"
3441    </t>
3442    <t>
3443      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
3444      "RFC 2822 is updated by RFC 5322"
3445    </t>
3446  </list>
3447</t>
3448<t>
3449  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3450  <list style="symbols"> 
3451    <t>
3452      Use "/" instead of "|" for alternatives.
3453    </t>
3454    <t>
3455      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3456      whitespace ("OWS") and required whitespace ("RWS").
3457    </t>
3458    <t>
3459      Rewrite ABNFs to spell out whitespace rules, factor out
3460      header value format definitions.
3461    </t>
3462  </list>
3463</t>
3464</section>
3465
3466<section title="Since draft-ietf-httpbis-p2-semantics-05" anchor="changes.since.05">
3467<t>
3468  Closed issues:
3469  <list style="symbols"> 
3470    <t>
3471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/94"/>:
3472      "Reason-Phrase BNF"
3473    </t>
3474  </list>
3475</t>
3476<t>
3477  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3478  <list style="symbols"> 
3479    <t>
3480      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
3481    </t>
3482  </list>
3483</t>
3484</section>
3485
3486</section>
3487
3488</back>
3489</rfc>
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