source: draft-ietf-httpbis/07/draft-ietf-httpbis-p2-semantics-07.xml @ 609

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HTAB cleanup (not even editorial)

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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-07">
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 day="13" month="July" year="2009"/>
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.06"/>.
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><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>
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><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><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><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><artwork type="abnf2616"><![CDATA[
335  Age           = <Age, defined in [Part6], Section 3.1>
336  Vary          = <Vary, defined in [Part6], Section 3.5>
337]]></artwork></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).
401</t>
402<t>
403  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-methods"/>.
404</t>
405</section>
406</section>
407
408<section title="Request Header Fields" anchor="request.header.fields">
409 
410<t>
411   The request-header fields allow the client to pass additional
412   information about the request, and about the client itself, to the
413   server. These fields act as request modifiers, with semantics
414   equivalent to the parameters on a programming language method
415   invocation.
416</t>
417<figure><iref primary="true" item="Grammar" subitem="request-header"/><artwork type="abnf2616"><![CDATA[
418  request-header = Accept                   ; [Part3], Section 5.1
419                 / Accept-Charset           ; [Part3], Section 5.2
420                 / Accept-Encoding          ; [Part3], Section 5.3
421                 / Accept-Language          ; [Part3], Section 5.4
422                 / Authorization            ; [Part7], Section 3.1
423                 / Expect                   ; Section 9.2
424                 / From                     ; Section 9.3
425                 / Host                     ; [Part1], Section 8.4
426                 / If-Match                 ; [Part4], Section 6.2
427                 / If-Modified-Since        ; [Part4], Section 6.3
428                 / If-None-Match            ; [Part4], Section 6.4
429                 / If-Range                 ; [Part5], Section 5.3
430                 / If-Unmodified-Since      ; [Part4], Section 6.5
431                 / Max-Forwards             ; Section 9.5
432                 / Proxy-Authorization      ; [Part7], Section 3.3
433                 / Range                    ; [Part5], Section 5.4
434                 / Referer                  ; Section 9.6
435                 / TE                       ; [Part1], Section 8.8
436                 / User-Agent               ; Section 9.9
437]]></artwork></figure>
438<t>
439   Request-header field names can be extended reliably only in
440   combination with a change in the protocol version. However, new or
441   experimental header fields MAY be given the semantics of request-header
442   fields if all parties in the communication recognize them to
443   be request-header fields. Unrecognized header fields are treated as
444   entity-header fields.
445</t>
446</section>
447
448<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
449 
450 
451 
452<t>
453   The Status-Code element is a 3-digit integer result code of the
454   attempt to understand and satisfy the request. The status codes listed
455   below are defined in <xref target="status.codes"/>.
456   The Reason-Phrase is intended to give a short
457   textual description of the Status-Code. The Status-Code is intended
458   for use by automata and the Reason-Phrase is intended for the human
459   user. The client is not required to examine or display the Reason-Phrase.
460</t>
461<t> 
462   The individual values of the numeric status codes defined for
463   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
464   presented below. The reason phrases listed here are only
465   recommendations -- they MAY be replaced by local equivalents without
466   affecting the protocol.
467</t>
468<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[
469  Status-Code    =
470         "100"  ; Section 8.1.1: Continue
471       / "101"  ; Section 8.1.2: Switching Protocols
472       / "200"  ; Section 8.2.1: OK
473       / "201"  ; Section 8.2.2: Created
474       / "202"  ; Section 8.2.3: Accepted
475       / "203"  ; Section 8.2.4: Non-Authoritative Information
476       / "204"  ; Section 8.2.5: No Content
477       / "205"  ; Section 8.2.6: Reset Content
478       / "206"  ; Section 8.2.7: Partial Content
479       / "300"  ; Section 8.3.1: Multiple Choices
480       / "301"  ; Section 8.3.2: Moved Permanently
481       / "302"  ; Section 8.3.3: Found
482       / "303"  ; Section 8.3.4: See Other
483       / "304"  ; Section 8.3.5: Not Modified
484       / "305"  ; Section 8.3.6: Use Proxy
485       / "307"  ; Section 8.3.8: Temporary Redirect
486       / "400"  ; Section 8.4.1: Bad Request
487       / "401"  ; Section 8.4.2: Unauthorized
488       / "402"  ; Section 8.4.3: Payment Required
489       / "403"  ; Section 8.4.4: Forbidden
490       / "404"  ; Section 8.4.5: Not Found
491       / "405"  ; Section 8.4.6: Method Not Allowed
492       / "406"  ; Section 8.4.7: Not Acceptable
493       / "407"  ; Section 8.4.8: Proxy Authentication Required
494       / "408"  ; Section 8.4.9: Request Time-out
495       / "409"  ; Section 8.4.10: Conflict
496       / "410"  ; Section 8.4.11: Gone
497       / "411"  ; Section 8.4.12: Length Required
498       / "412"  ; Section 8.4.13: Precondition Failed
499       / "413"  ; Section 8.4.14: Request Entity Too Large
500       / "414"  ; Section 8.4.15: URI Too Long
501       / "415"  ; Section 8.4.16: Unsupported Media Type
502       / "416"  ; Section 8.4.17: Requested range not satisfiable
503       / "417"  ; Section 8.4.18: Expectation Failed
504       / "500"  ; Section 8.5.1: Internal Server Error
505       / "501"  ; Section 8.5.2: Not Implemented
506       / "502"  ; Section 8.5.3: Bad Gateway
507       / "503"  ; Section 8.5.4: Service Unavailable
508       / "504"  ; Section 8.5.5: Gateway Time-out
509       / "505"  ; Section 8.5.6: HTTP Version not supported
510       / extension-code
511
512  extension-code = 3DIGIT
513  Reason-Phrase  = *( WSP / VCHAR / obs-text )
514]]></artwork></figure>
515<t>
516   HTTP status codes are extensible. HTTP applications are not required
517   to understand the meaning of all registered status codes, though such
518   understanding is obviously desirable. However, applications MUST
519   understand the class of any status code, as indicated by the first
520   digit, and treat any unrecognized response as being equivalent to the
521   x00 status code of that class, with the exception that an
522   unrecognized response MUST NOT be cached. For example, if an
523   unrecognized status code of 431 is received by the client, it can
524   safely assume that there was something wrong with its request and
525   treat the response as if it had received a 400 status code. In such
526   cases, user agents SHOULD present to the user the entity returned
527   with the response, since that entity is likely to include human-readable
528   information which will explain the unusual status.
529</t>
530
531<section title="Status Code Registry" anchor="status.code.registry">
532<t>
533  The HTTP Status Code Registry defines the name space for the Status-Code
534  token in the Status line of an HTTP response.
535</t>
536<t>
537  Values to be added to this name space are subject to IETF review
538  (<xref target="RFC5226"/>, Section 4.1).
539</t>
540<t>
541  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-status-codes"/>.
542</t>
543</section>
544
545</section>
546
547<section title="Response Header Fields" anchor="response.header.fields">
548 
549<t>
550   The response-header fields allow the server to pass additional
551   information about the response which cannot be placed in the Status-Line.
552   These header fields give information about the server and about
553   further access to the resource identified by the request-target.
554</t>
555<figure><iref primary="true" item="Grammar" subitem="response-header"/><artwork type="abnf2616"><![CDATA[
556  response-header = Accept-Ranges           ; [Part5], Section 5.1
557                  / Age                     ; [Part6], Section 3.1
558                  / Allow                   ; Section 9.1
559                  / ETag                    ; [Part4], Section 6.1
560                  / Location                ; Section 9.4
561                  / Proxy-Authenticate      ; [Part7], Section 3.2
562                  / Retry-After             ; Section 9.7
563                  / Server                  ; Section 9.8
564                  / Vary                    ; [Part6], Section 3.5
565                  / WWW-Authenticate        ; [Part7], Section 3.4
566]]></artwork></figure>
567<t>
568   Response-header field names can be extended reliably only in
569   combination with a change in the protocol version. However, new or
570   experimental header fields MAY be given the semantics of response-header
571   fields if all parties in the communication recognize them to
572   be response-header fields. Unrecognized header fields are treated as
573   entity-header fields.
574</t>
575</section>
576
577<section title="Entity" anchor="entity">
578<t>
579   Request and Response messages MAY transfer an entity if not otherwise
580   restricted by the request method or response status code. An entity
581   consists of entity-header fields and an entity-body, although some
582   responses will only include the entity-headers. HTTP entity-body and
583   entity-header fields are defined in <xref target="Part3"/>.
584</t>
585<t>
586   An entity-body is only present in a message when a message-body is
587   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
588   from the message-body by decoding any Transfer-Encoding that might
589   have been applied to ensure safe and proper transfer of the message.
590</t>
591</section>
592
593
594<section title="Method Definitions" anchor="method.definitions">
595<t>
596   The set of common methods for HTTP/1.1 is defined below. Although
597   this set can be expanded, additional methods cannot be assumed to
598   share the same semantics for separately extended clients and servers.
599</t>
600
601<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
602
603<section title="Safe Methods" anchor="safe.methods">
604<iref item="Safe Methods" primary="true"/>
605<t>
606   Implementors should be aware that the software represents the user in
607   their interactions over the Internet, and should be careful to allow
608   the user to be aware of any actions they might take which may have an
609   unexpected significance to themselves or others.
610</t>
611<t>
612   In particular, the convention has been established that the GET and
613   HEAD methods SHOULD NOT  have the significance of taking an action
614   other than retrieval. These methods ought to be considered "safe".
615   This allows user agents to represent other methods, such as POST, PUT
616   and DELETE, in a special way, so that the user is made aware of the
617   fact that a possibly unsafe action is being requested.
618</t>
619<t>
620   Naturally, it is not possible to ensure that the server does not
621   generate side-effects as a result of performing a GET request; in
622   fact, some dynamic resources consider that a feature. The important
623   distinction here is that the user did not request the side-effects,
624   so therefore cannot be held accountable for them.
625</t>
626</section>
627
628<section title="Idempotent Methods" anchor="idempotent.methods">
629<iref item="Idempotent Methods" primary="true"/>
630<t>
631   Methods can also have the property of "idempotence" in that (aside
632   from error or expiration issues) the side-effects of N &gt; 0 identical
633   requests is the same as for a single request. The methods GET, HEAD,
634   PUT and DELETE share this property. Also, the methods OPTIONS and
635   TRACE SHOULD NOT  have side effects, and so are inherently idempotent.
636</t>
637<t>
638   However, it is possible that a sequence of several requests is non-idempotent,
639   even if all of the methods executed in that sequence are
640   idempotent. (A sequence is idempotent if a single execution of the
641   entire sequence always yields a result that is not changed by a
642   reexecution of all, or part, of that sequence.) For example, a
643   sequence is non-idempotent if its result depends on a value that is
644   later modified in the same sequence.
645</t>
646<t>
647   A sequence that never has side effects is idempotent, by definition
648   (provided that no concurrent operations are being executed on the
649   same set of resources).
650</t>
651</section>
652</section>
653
654<section title="OPTIONS" anchor="OPTIONS">
655 
656  <iref primary="true" item="OPTIONS method"/>
657  <iref primary="true" item="Methods" subitem="OPTIONS"/>
658<t>
659   The OPTIONS method represents a request for information about the
660   communication options available on the request/response chain
661   identified by the request-target. This method allows the client to
662   determine the options and/or requirements associated with a resource,
663   or the capabilities of a server, without implying a resource action
664   or initiating a resource retrieval.
665</t>
666<t>
667   Responses to this method are not cacheable.
668</t>
669<t>
670   If the OPTIONS request includes an entity-body (as indicated by the
671   presence of Content-Length or Transfer-Encoding), then the media type
672   MUST be indicated by a Content-Type field. Although this
673   specification does not define any use for such a body, future
674   extensions to HTTP might use the OPTIONS body to make more detailed
675   queries on the server.
676</t>
677<t>
678   If the request-target is an asterisk ("*"), the OPTIONS request is
679   intended to apply to the server in general rather than to a specific
680   resource. Since a server's communication options typically depend on
681   the resource, the "*" request is only useful as a "ping" or "no-op"
682   type of method; it does nothing beyond allowing the client to test
683   the capabilities of the server. For example, this can be used to test
684   a proxy for HTTP/1.1 compliance (or lack thereof).
685</t>
686<t>
687   If the request-target is not an asterisk, the OPTIONS request applies
688   only to the options that are available when communicating with that
689   resource.
690</t>
691<t>
692   A 200 response SHOULD include any header fields that indicate
693   optional features implemented by the server and applicable to that
694   resource (e.g., Allow), possibly including extensions not defined by
695   this specification. The response body, if any, SHOULD also include
696   information about the communication options. The format for such a
697   body is not defined by this specification, but might be defined by
698   future extensions to HTTP. Content negotiation MAY be used to select
699   the appropriate response format. If no response body is included, the
700   response MUST include a Content-Length field with a field-value of
701   "0".
702</t>
703<t>
704   The Max-Forwards request-header field MAY be used to target a
705   specific proxy in the request chain. When a proxy receives an OPTIONS
706   request on an absolute-URI for which request forwarding is permitted,
707   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
708   field-value is zero ("0"), the proxy MUST NOT forward the message;
709   instead, the proxy SHOULD respond with its own communication options.
710   If the Max-Forwards field-value is an integer greater than zero, the
711   proxy MUST decrement the field-value when it forwards the request. If
712   no Max-Forwards field is present in the request, then the forwarded
713   request MUST NOT include a Max-Forwards field.
714</t>
715</section>
716
717<section title="GET" anchor="GET">
718 
719  <iref primary="true" item="GET method"/>
720  <iref primary="true" item="Methods" subitem="GET"/>
721<t>
722   The GET method means retrieve whatever information (in the form of an
723   entity) is identified by the request-target. If the request-target refers
724   to a data-producing process, it is the produced data which shall be
725   returned as the entity in the response and not the source text of the
726   process, unless that text happens to be the output of the process.
727</t>
728<t>
729   The semantics of the GET method change to a "conditional GET" if the
730   request message includes an If-Modified-Since, If-Unmodified-Since,
731   If-Match, If-None-Match, or If-Range header field. A conditional GET
732   method requests that the entity be transferred only under the
733   circumstances described by the conditional header field(s). The
734   conditional GET method is intended to reduce unnecessary network
735   usage by allowing cached entities to be refreshed without requiring
736   multiple requests or transferring data already held by the client.
737</t>
738<t>
739   The semantics of the GET method change to a "partial GET" if the
740   request message includes a Range header field. A partial GET requests
741   that only part of the entity be transferred, as described in Section 5.4 of <xref target="Part5"/>.
742   The partial GET method is intended to reduce unnecessary
743   network usage by allowing partially-retrieved entities to be
744   completed without transferring data already held by the client.
745</t>
746<t>
747   The response to a GET request is cacheable if and only if it meets
748   the requirements for HTTP caching described in <xref target="Part6"/>.
749</t>
750<t>
751   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
752</t>
753</section>
754
755<section title="HEAD" anchor="HEAD">
756 
757  <iref primary="true" item="HEAD method"/>
758  <iref primary="true" item="Methods" subitem="HEAD"/>
759<t>
760   The HEAD method is identical to GET except that the server MUST NOT
761   return a message-body in the response. The metainformation contained
762   in the HTTP headers in response to a HEAD request SHOULD be identical
763   to the information sent in response to a GET request. This method can
764   be used for obtaining metainformation about the entity implied by the
765   request without transferring the entity-body itself. This method is
766   often used for testing hypertext links for validity, accessibility,
767   and recent modification.
768</t>
769<t>
770   The response to a HEAD request MAY be cacheable in the sense that the
771   information contained in the response MAY be used to update a
772   previously cached entity from that resource. If the new field values
773   indicate that the cached entity differs from the current entity (as
774   would be indicated by a change in Content-Length, Content-MD5, ETag
775   or Last-Modified), then the cache MUST treat the cache entry as
776   stale.
777</t>
778</section>
779
780<section title="POST" anchor="POST">
781  <iref primary="true" item="POST method"/>
782  <iref primary="true" item="Methods" subitem="POST"/>
783<t>
784   The POST method is used to request that the origin server accept the
785   entity enclosed in the request as data to be processed by the resource
786   identified by the request-target in the Request-Line. POST is designed
787   to allow a uniform method to cover the following functions:
788  <list style="symbols">
789    <t>
790      Annotation of existing resources;
791    </t>
792    <t>
793        Posting a message to a bulletin board, newsgroup, mailing list,
794        or similar group of articles;
795    </t>
796    <t>
797        Providing a block of data, such as the result of submitting a
798        form, to a data-handling process;
799    </t>
800    <t>
801        Extending a database through an append operation.
802    </t>
803  </list>
804</t>
805<t>
806   The actual function performed by the POST method is determined by the
807   server and is usually dependent on the request-target.
808</t>
809<t>
810   The action performed by the POST method might not result in a
811   resource that can be identified by a URI. In this case, either 200
812   (OK) or 204 (No Content) is the appropriate response status,
813   depending on whether or not the response includes an entity that
814   describes the result.
815</t>
816<t>
817   If a resource has been created on the origin server, the response
818   SHOULD be 201 (Created) and contain an entity which describes the
819   status of the request and refers to the new resource, and a Location
820   header (see <xref target="header.location"/>).
821</t>
822<t>
823   Responses to this method are not cacheable, unless the response
824   includes appropriate Cache-Control or Expires header fields. However,
825   the 303 (See Other) response can be used to direct the user agent to
826   retrieve a cacheable resource.
827</t>
828</section>
829
830<section title="PUT" anchor="PUT">
831  <iref primary="true" item="PUT method"/>
832  <iref primary="true" item="Methods" subitem="PUT"/>
833<t>
834   The PUT method requests that the enclosed entity be stored at the
835   supplied request-target. If the request-target refers to an already
836   existing resource, the enclosed entity SHOULD be considered as a
837   modified version of the one residing on the origin server. If the
838   request-target does not point to an existing resource, and that URI is
839   capable of being defined as a new resource by the requesting user
840   agent, the origin server can create the resource with that URI. If a
841   new resource is created at the request-target, the origin server MUST
842   inform the user agent
843   via the 201 (Created) response. If an existing resource is modified,
844   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
845   to indicate successful completion of the request. If the resource
846   could not be created or modified with the request-target, an appropriate
847   error response SHOULD be given that reflects the nature of the
848   problem. The recipient of the entity MUST NOT ignore any Content-*
849   headers (headers starting with the prefix 'Content-') that it does
850   not understand or implement
851   and MUST return a 501 (Not Implemented) response in such cases.
852</t>
853<t>
854   If the request passes through a cache and the request-target identifies
855   one or more currently cached entities, those entries SHOULD be
856   treated as stale. Responses to this method are not cacheable.
857</t>
858<t>
859   The fundamental difference between the POST and PUT requests is
860   reflected in the different meaning of the request-target. The URI in a
861   POST request identifies the resource that will handle the enclosed
862   entity. That resource might be a data-accepting process, a gateway to
863   some other protocol, or a separate entity that accepts annotations.
864   In contrast, the URI in a PUT request identifies the entity enclosed
865   with the request -- the user agent knows what URI is intended and the
866   server MUST NOT attempt to apply the request to some other resource.
867   If the server desires that the request be applied to a different URI,
868   it MUST send a 301 (Moved Permanently) response; the user agent MAY
869   then make its own decision regarding whether or not to redirect the
870   request.
871</t>
872<t>
873   A single resource MAY be identified by many different URIs. For
874   example, an article might have a URI for identifying "the current
875   version" which is separate from the URI identifying each particular
876   version. In this case, a PUT request on a general URI might result in
877   several other URIs being defined by the origin server.
878</t>
879<t>
880   HTTP/1.1 does not define how a PUT method affects the state of an
881   origin server.
882</t>
883<t>
884   Unless otherwise specified for a particular entity-header, the
885   entity-headers in the PUT request SHOULD be applied to the resource
886   created or modified by the PUT.
887</t>
888</section>
889
890<section title="DELETE" anchor="DELETE">
891  <iref primary="true" item="DELETE method"/>
892  <iref primary="true" item="Methods" subitem="DELETE"/>
893<t>
894   The DELETE method requests that the origin server delete the resource
895   identified by the request-target. This method MAY be overridden by human
896   intervention (or other means) on the origin server. The client cannot
897   be guaranteed that the operation has been carried out, even if the
898   status code returned from the origin server indicates that the action
899   has been completed successfully. However, the server SHOULD NOT
900   indicate success unless, at the time the response is given, it
901   intends to delete the resource or move it to an inaccessible
902   location.
903</t>
904<t>
905   A successful response SHOULD be 200 (OK) if the response includes an
906   entity describing the status, 202 (Accepted) if the action has not
907   yet been enacted, or 204 (No Content) if the action has been enacted
908   but the response does not include an entity.
909</t>
910<t>
911   If the request passes through a cache and the request-target identifies
912   one or more currently cached entities, those entries SHOULD be
913   treated as stale. Responses to this method are not cacheable.
914</t>
915</section>
916
917<section title="TRACE" anchor="TRACE">
918 
919  <iref primary="true" item="TRACE method"/>
920  <iref primary="true" item="Methods" subitem="TRACE"/>
921<t>
922   The TRACE method is used to invoke a remote, application-layer loop-back
923   of the request message. The final recipient of the request
924   SHOULD reflect the message received back to the client as the
925   entity-body of a 200 (OK) response. The final recipient is either the
926   origin server or the first proxy or gateway to receive a Max-Forwards
927   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
928   MUST NOT include an entity.
929</t>
930<t>
931   TRACE allows the client to see what is being received at the other
932   end of the request chain and use that data for testing or diagnostic
933   information. The value of the Via header field (Section 8.9 of <xref target="Part1"/>) is of
934   particular interest, since it acts as a trace of the request chain.
935   Use of the Max-Forwards header field allows the client to limit the
936   length of the request chain, which is useful for testing a chain of
937   proxies forwarding messages in an infinite loop.
938</t>
939<t>
940   If the request is valid, the response SHOULD contain the entire
941   request message in the entity-body, with a Content-Type of
942   "message/http" (see Section 9.3.1 of <xref target="Part1"/>). Responses to this method
943   MUST NOT be cached.
944</t>
945</section>
946
947<section title="CONNECT" anchor="CONNECT">
948  <iref primary="true" item="CONNECT method"/>
949  <iref primary="true" item="Methods" subitem="CONNECT"/>
950<t>
951   This specification reserves the method name CONNECT for use with a
952   proxy that can dynamically switch to being a tunnel (e.g. SSL
953   tunneling <xref target="RFC2817"/>).
954</t>
955</section>
956</section>
957
958
959<section title="Status Code Definitions" anchor="status.codes">
960<t>
961   Each Status-Code is described below, including any metainformation required
962   in the response.
963</t>
964
965<section title="Informational 1xx" anchor="status.1xx">
966<t>
967   This class of status code indicates a provisional response,
968   consisting only of the Status-Line and optional headers, and is
969   terminated by an empty line. There are no required headers for this
970   class of status code. Since HTTP/1.0 did not define any 1xx status
971   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
972   except under experimental conditions.
973</t>
974<t>
975   A client MUST be prepared to accept one or more 1xx status responses
976   prior to a regular response, even if the client does not expect a 100
977   (Continue) status message. Unexpected 1xx status responses MAY be
978   ignored by a user agent.
979</t>
980<t>
981   Proxies MUST forward 1xx responses, unless the connection between the
982   proxy and its client has been closed, or unless the proxy itself
983   requested the generation of the 1xx response. (For example, if a
984   proxy adds a "Expect: 100-continue" field when it forwards a request,
985   then it need not forward the corresponding 100 (Continue)
986   response(s).)
987</t>
988
989<section title="100 Continue" anchor="status.100">
990  <iref primary="true" item="100 Continue (status code)"/>
991  <iref primary="true" item="Status Codes" subitem="100 Continue"/>
992<t>
993   The client SHOULD continue with its request. This interim response is
994   used to inform the client that the initial part of the request has
995   been received and has not yet been rejected by the server. The client
996   SHOULD continue by sending the remainder of the request or, if the
997   request has already been completed, ignore this response. The server
998   MUST send a final response after the request has been completed. See
999   Section 7.2.3 of <xref target="Part1"/> for detailed discussion of the use and handling of this
1000   status code.
1001</t>
1002</section>
1003
1004<section title="101 Switching Protocols" anchor="status.101">
1005  <iref primary="true" item="101 Switching Protocols (status code)"/>
1006  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols"/>
1007<t>
1008   The server understands and is willing to comply with the client's
1009   request, via the Upgrade message header field (Section 5.4 of <xref target="Part5"/>), for a
1010   change in the application protocol being used on this connection. The
1011   server will switch protocols to those defined by the response's
1012   Upgrade header field immediately after the empty line which
1013   terminates the 101 response.
1014</t>
1015<t>
1016   The protocol SHOULD be switched only when it is advantageous to do
1017   so. For example, switching to a newer version of HTTP is advantageous
1018   over older versions, and switching to a real-time, synchronous
1019   protocol might be advantageous when delivering resources that use
1020   such features.
1021</t>
1022</section>
1023</section>
1024
1025<section title="Successful 2xx" anchor="status.2xx">
1026<t>
1027   This class of status code indicates that the client's request was
1028   successfully received, understood, and accepted.
1029</t>
1030
1031<section title="200 OK" anchor="status.200">
1032  <iref primary="true" item="200 OK (status code)"/>
1033  <iref primary="true" item="Status Codes" subitem="200 OK"/>
1034<t>
1035   The request has succeeded. The information returned with the response
1036   is dependent on the method used in the request, for example:
1037  <list style="hanging">
1038    <t hangText="GET">
1039          an entity corresponding to the requested resource is sent in
1040          the response;
1041    </t>
1042    <t hangText="HEAD">
1043          the entity-header fields corresponding to the requested
1044          resource are sent in the response without any message-body;
1045    </t>
1046    <t hangText="POST">
1047      an entity describing or containing the result of the action;
1048    </t>
1049    <t hangText="TRACE">
1050      an entity containing the request message as received by the
1051      end server.
1052    </t>
1053  </list>
1054</t>
1055</section>
1056
1057<section title="201 Created" anchor="status.201">
1058  <iref primary="true" item="201 Created (status code)"/>
1059  <iref primary="true" item="Status Codes" subitem="201 Created"/>
1060<t>
1061   The request has been fulfilled and resulted in a new resource being
1062   created. The newly created resource can be referenced by the URI(s)
1063   returned in the entity of the response, with the most specific URI
1064   for the resource given by a Location header field. The response
1065   SHOULD include an entity containing a list of resource
1066   characteristics and location(s) from which the user or user agent can
1067   choose the one most appropriate. The entity format is specified by
1068   the media type given in the Content-Type header field. The origin
1069   server MUST create the resource before returning the 201 status code.
1070   If the action cannot be carried out immediately, the server SHOULD
1071   respond with 202 (Accepted) response instead.
1072</t>
1073<t>
1074   A 201 response MAY contain an ETag response header field indicating
1075   the current value of the entity tag for the requested variant just
1076   created, see Section 6.1 of <xref target="Part4"/>.
1077</t>
1078</section>
1079
1080<section title="202 Accepted" anchor="status.202">
1081  <iref primary="true" item="202 Accepted (status code)"/>
1082  <iref primary="true" item="Status Codes" subitem="202 Accepted"/>
1083<t>
1084   The request has been accepted for processing, but the processing has
1085   not been completed.  The request might or might not eventually be
1086   acted upon, as it might be disallowed when processing actually takes
1087   place. There is no facility for re-sending a status code from an
1088   asynchronous operation such as this.
1089</t>
1090<t>
1091   The 202 response is intentionally non-committal. Its purpose is to
1092   allow a server to accept a request for some other process (perhaps a
1093   batch-oriented process that is only run once per day) without
1094   requiring that the user agent's connection to the server persist
1095   until the process is completed. The entity returned with this
1096   response SHOULD include an indication of the request's current status
1097   and either a pointer to a status monitor or some estimate of when the
1098   user can expect the request to be fulfilled.
1099</t>
1100</section>
1101
1102<section title="203 Non-Authoritative Information" anchor="status.203">
1103  <iref primary="true" item="203 Non-Authoritative Information (status code)"/>
1104  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information"/>
1105<t>
1106   The returned metainformation in the entity-header is not the
1107   definitive set as available from the origin server, but is gathered
1108   from a local or a third-party copy. The set presented MAY be a subset
1109   or superset of the original version. For example, including local
1110   annotation information about the resource might result in a superset
1111   of the metainformation known by the origin server. Use of this
1112   response code is not required and is only appropriate when the
1113   response would otherwise be 200 (OK).
1114</t>
1115</section>
1116
1117<section title="204 No Content" anchor="status.204">
1118  <iref primary="true" item="204 No Content (status code)"/>
1119  <iref primary="true" item="Status Codes" subitem="204 No Content"/>
1120<t>
1121   The server has fulfilled the request but does not need to return an
1122   entity-body, and might want to return updated metainformation. The
1123   response MAY include new or updated metainformation in the form of
1124   entity-headers, which if present SHOULD be associated with the
1125   requested variant.
1126</t>
1127<t>
1128   If the client is a user agent, it SHOULD NOT  change its document view
1129   from that which caused the request to be sent. This response is
1130   primarily intended to allow input for actions to take place without
1131   causing a change to the user agent's active document view, although
1132   any new or updated metainformation SHOULD be applied to the document
1133   currently in the user agent's active view.
1134</t>
1135<t>
1136   The 204 response MUST NOT include a message-body, and thus is always
1137   terminated by the first empty line after the header fields.
1138</t>
1139</section>
1140
1141<section title="205 Reset Content" anchor="status.205">
1142  <iref primary="true" item="205 Reset Content (status code)"/>
1143  <iref primary="true" item="Status Codes" subitem="205 Reset Content"/>
1144<t>
1145   The server has fulfilled the request and the user agent SHOULD reset
1146   the document view which caused the request to be sent. This response
1147   is primarily intended to allow input for actions to take place via
1148   user input, followed by a clearing of the form in which the input is
1149   given so that the user can easily initiate another input action. The
1150   response MUST NOT include an entity.
1151</t>
1152</section>
1153
1154<section title="206 Partial Content" anchor="status.206">
1155  <iref primary="true" item="206 Partial Content (status code)"/>
1156  <iref primary="true" item="Status Codes" subitem="206 Partial Content"/>
1157<t>
1158   The server has fulfilled the partial GET request for the resource
1159   and the enclosed entity is a partial representation as defined in <xref target="Part5"/>.
1160</t>
1161</section>
1162</section>
1163
1164<section title="Redirection 3xx" anchor="status.3xx">
1165<t>
1166   This class of status code indicates that further action needs to be
1167   taken by the user agent in order to fulfill the request.  The action
1168   required MAY be carried out by the user agent without interaction
1169   with the user if and only if the method used in the second request is
1170   GET or HEAD. A client SHOULD detect infinite redirection loops, since
1171   such loops generate network traffic for each redirection.
1172</t>
1173<t><list>
1174  <t>
1175    Note: previous versions of this specification recommended a
1176    maximum of five redirections. Content developers should be aware
1177    that there might be clients that implement such a fixed
1178    limitation.
1179  </t>
1180</list></t>
1181
1182<section title="300 Multiple Choices" anchor="status.300">
1183  <iref primary="true" item="300 Multiple Choices (status code)"/>
1184  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices"/>
1185<t>
1186   The requested resource corresponds to any one of a set of
1187   representations, each with its own specific location, and agent-driven
1188   negotiation information (Section 4 of <xref target="Part3"/>) is being provided so that
1189   the user (or user agent) can select a preferred representation and
1190   redirect its request to that location.
1191</t>
1192<t>
1193   Unless it was a HEAD request, the response SHOULD include an entity
1194   containing a list of resource characteristics and location(s) from
1195   which the user or user agent can choose the one most appropriate. The
1196   entity format is specified by the media type given in the Content-Type
1197   header field. Depending upon the format and the capabilities of
1198   the user agent, selection of the most appropriate choice MAY be
1199   performed automatically. However, this specification does not define
1200   any standard for such automatic selection.
1201</t>
1202<t>
1203   If the server has a preferred choice of representation, it SHOULD
1204   include the specific URI for that representation in the Location
1205   field; user agents MAY use the Location field value for automatic
1206   redirection. This response is cacheable unless indicated otherwise.
1207</t>
1208</section>
1209
1210<section title="301 Moved Permanently" anchor="status.301">
1211  <iref primary="true" item="301 Moved Permanently (status code)"/>
1212  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently"/>
1213<t>
1214   The requested resource has been assigned a new permanent URI and any
1215   future references to this resource SHOULD use one of the returned
1216   URIs.  Clients with link editing capabilities ought to automatically
1217   re-link references to the request-target to one or more of the new
1218   references returned by the server, where possible. This response is
1219   cacheable unless indicated otherwise.
1220</t>
1221<t>
1222   The new permanent URI SHOULD be given by the Location field in the
1223   response. Unless the request method was HEAD, the entity of the
1224   response SHOULD contain a short hypertext note with a hyperlink to
1225   the new URI(s).
1226</t>
1227<t>
1228   If the 301 status code is received in response to a request method
1229   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1230   then the request MAY be automatically redirected by the user agent without
1231   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1232   request unless it can be confirmed by the user, since this might
1233   change the conditions under which the request was issued.
1234</t>
1235<t><list>
1236  <t>
1237    Note: When automatically redirecting a POST request after
1238    receiving a 301 status code, some existing HTTP/1.0 user agents
1239    will erroneously change it into a GET request.
1240  </t>
1241</list></t>
1242</section>
1243
1244<section title="302 Found" anchor="status.302">
1245  <iref primary="true" item="302 Found (status code)"/>
1246  <iref primary="true" item="Status Codes" subitem="302 Found"/>
1247<t>
1248   The requested resource resides temporarily under a different URI.
1249   Since the redirection might be altered on occasion, the client SHOULD
1250   continue to use the request-target for future requests.  This response
1251   is only cacheable if indicated by a Cache-Control or Expires header
1252   field.
1253</t>
1254<t>
1255   The temporary URI SHOULD be given by the Location field in the
1256   response. Unless the request method was HEAD, the entity of the
1257   response SHOULD contain a short hypertext note with a hyperlink to
1258   the new URI(s).
1259</t>
1260<t>
1261   If the 302 status code is received in response to a request method
1262   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1263   then the request MAY be automatically redirected by the user agent without
1264   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1265   request unless it can be confirmed by the user, since this might
1266   change the conditions under which the request was issued.
1267</t>
1268<t><list>
1269  <t>
1270    Note: <xref target="RFC1945"/> and <xref target="RFC2068"/> specify that the client is not allowed
1271    to change the method on the redirected request.  However, most
1272    existing user agent implementations treat 302 as if it were a 303
1273    response, performing a GET on the Location field-value regardless
1274    of the original request method. The status codes 303 and 307 have
1275    been added for servers that wish to make unambiguously clear which
1276    kind of reaction is expected of the client.
1277  </t>
1278</list></t>
1279</section>
1280
1281<section title="303 See Other" anchor="status.303">
1282  <iref primary="true" item="303 See Other (status code)"/>
1283  <iref primary="true" item="Status Codes" subitem="303 See Other"/>
1284<t>
1285   The server directs the user agent to a different resource, indicated
1286   by a URI in the Location header field, that provides an indirect
1287   response to the original request.  The user agent MAY perform a GET
1288   request on the URI in the Location field in order to obtain a
1289   representation corresponding to the response, be redirected again,
1290   or end with an error status.  The Location URI is not a substitute
1291   reference for the originally requested resource.
1292</t>
1293<t>
1294   The 303 status is generally applicable to any HTTP method.  It is
1295   primarily used to allow the output of a POST action to redirect
1296   the user agent to a selected resource, since doing so provides the
1297   information corresponding to the POST response in a form that
1298   can be separately identified, bookmarked, and cached independent
1299   of the original request.
1300</t>
1301<t>
1302   A 303 response to a GET request indicates that the requested
1303   resource does not have a representation of its own that can be
1304   transferred by the server over HTTP.  The Location URI indicates a
1305   resource that is descriptive of the requested resource such that
1306   the follow-on representation may be useful without implying that
1307   it adequately represents the previously requested resource.
1308   Note that answers to the questions of what can be represented, what
1309   representations are adequate, and what might be a useful description
1310   are outside the scope of HTTP and thus entirely determined by the
1311   URI owner(s).
1312</t>
1313<t>
1314   A 303 response SHOULD NOT be cached unless it is indicated as
1315   cacheable by Cache-Control or Expires header fields.  Except for
1316   responses to a HEAD request, the entity of a 303 response SHOULD
1317   contain a short hypertext note with a hyperlink to the Location URI.
1318</t>
1319</section>
1320
1321<section title="304 Not Modified" anchor="status.304">
1322  <iref primary="true" item="304 Not Modified (status code)"/>
1323  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1324<t>
1325   The response to the request has not been modified since the conditions
1326   indicated by the client's conditional GET request, as defined in <xref target="Part4"/>.
1327</t>
1328</section>
1329
1330<section title="305 Use Proxy" anchor="status.305">
1331  <iref primary="true" item="305 Use Proxy (status code)"/>
1332  <iref primary="true" item="Status Codes" subitem="305 Use Proxy"/>
1333<t>
1334   The 305 status was defined in a previous version of this specification
1335   (see <xref target="changes.from.rfc.2616"/>), and is now deprecated.
1336</t>
1337</section>
1338
1339<section title="306 (Unused)" anchor="status.306">
1340  <iref primary="true" item="306 (Unused) (status code)"/>
1341  <iref primary="true" item="Status Codes" subitem="306 (Unused)"/>
1342<t>
1343   The 306 status code was used in a previous version of the
1344   specification, is no longer used, and the code is reserved.
1345</t>
1346</section>
1347
1348<section title="307 Temporary Redirect" anchor="status.307">
1349  <iref primary="true" item="307 Temporary Redirect (status code)"/>
1350  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect"/>
1351<t>
1352   The requested resource resides temporarily under a different URI.
1353   Since the redirection MAY be altered on occasion, the client SHOULD
1354   continue to use the request-target for future requests.  This response
1355   is only cacheable if indicated by a Cache-Control or Expires header
1356   field.
1357</t>
1358<t>
1359   The temporary URI SHOULD be given by the Location field in the
1360   response. Unless the request method was HEAD, the entity of the
1361   response SHOULD contain a short hypertext note with a hyperlink to
1362   the new URI(s) , since many pre-HTTP/1.1 user agents do not
1363   understand the 307 status. Therefore, the note SHOULD contain the
1364   information necessary for a user to repeat the original request on
1365   the new URI.
1366</t>
1367<t>
1368   If the 307 status code is received in response to a request method
1369   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1370   then the request MAY be automatically redirected by the user agent without
1371   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1372   request unless it can be confirmed by the user, since this might
1373   change the conditions under which the request was issued.
1374</t>
1375</section>
1376</section>
1377
1378<section title="Client Error 4xx" anchor="status.4xx">
1379<t>
1380   The 4xx class of status code is intended for cases in which the
1381   client seems to have erred. Except when responding to a HEAD request,
1382   the server SHOULD include an entity containing an explanation of the
1383   error situation, and whether it is a temporary or permanent
1384   condition. These status codes are applicable to any request method.
1385   User agents SHOULD display any included entity to the user.
1386</t>
1387<t>
1388   If the client is sending data, a server implementation using TCP
1389   SHOULD be careful to ensure that the client acknowledges receipt of
1390   the packet(s) containing the response, before the server closes the
1391   input connection. If the client continues sending data to the server
1392   after the close, the server's TCP stack will send a reset packet to
1393   the client, which may erase the client's unacknowledged input buffers
1394   before they can be read and interpreted by the HTTP application.
1395</t>
1396
1397<section title="400 Bad Request" anchor="status.400">
1398  <iref primary="true" item="400 Bad Request (status code)"/>
1399  <iref primary="true" item="Status Codes" subitem="400 Bad Request"/>
1400<t>
1401   The request could not be understood by the server due to malformed
1402   syntax. The client SHOULD NOT  repeat the request without
1403   modifications.
1404</t>
1405</section>
1406
1407<section title="401 Unauthorized" anchor="status.401">
1408  <iref primary="true" item="401 Unauthorized (status code)"/>
1409  <iref primary="true" item="Status Codes" subitem="401 Unauthorized"/>
1410<t>
1411   The request requires user authentication (see <xref target="Part7"/>).
1412</t>
1413</section>
1414
1415<section title="402 Payment Required" anchor="status.402">
1416  <iref primary="true" item="402 Payment Required (status code)"/>
1417  <iref primary="true" item="Status Codes" subitem="402 Payment Required"/>
1418<t>
1419   This code is reserved for future use.
1420</t>
1421</section>
1422
1423<section title="403 Forbidden" anchor="status.403">
1424  <iref primary="true" item="403 Forbidden (status code)"/>
1425  <iref primary="true" item="Status Codes" subitem="403 Forbidden"/>
1426<t>
1427   The server understood the request, but is refusing to fulfill it.
1428   Authorization will not help and the request SHOULD NOT  be repeated.
1429   If the request method was not HEAD and the server wishes to make
1430   public why the request has not been fulfilled, it SHOULD describe the
1431   reason for the refusal in the entity.  If the server does not wish to
1432   make this information available to the client, the status code 404
1433   (Not Found) can be used instead.
1434</t>
1435</section>
1436
1437<section title="404 Not Found" anchor="status.404">
1438  <iref primary="true" item="404 Not Found (status code)"/>
1439  <iref primary="true" item="Status Codes" subitem="404 Not Found"/>
1440<t>
1441   The server has not found anything matching the request-target. No
1442   indication is given of whether the condition is temporary or
1443   permanent. The 410 (Gone) status code SHOULD be used if the server
1444   knows, through some internally configurable mechanism, that an old
1445   resource is permanently unavailable and has no forwarding address.
1446   This status code is commonly used when the server does not wish to
1447   reveal exactly why the request has been refused, or when no other
1448   response is applicable.
1449</t>
1450</section>
1451
1452<section title="405 Method Not Allowed" anchor="status.405">
1453  <iref primary="true" item="405 Method Not Allowed (status code)"/>
1454  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed"/>
1455<t>
1456   The method specified in the Request-Line is not allowed for the
1457   resource identified by the request-target. The response MUST include an
1458   Allow header containing a list of valid methods for the requested
1459   resource.
1460</t>
1461</section>
1462
1463<section title="406 Not Acceptable" anchor="status.406">
1464  <iref primary="true" item="406 Not Acceptable (status code)"/>
1465  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable"/>
1466<t>
1467   The resource identified by the request is only capable of generating
1468   response entities which have content characteristics not acceptable
1469   according to the accept headers sent in the request.
1470</t>
1471<t>
1472   Unless it was a HEAD request, the response SHOULD include an entity
1473   containing a list of available entity characteristics and location(s)
1474   from which the user or user agent can choose the one most
1475   appropriate. The entity format is specified by the media type given
1476   in the Content-Type header field. Depending upon the format and the
1477   capabilities of the user agent, selection of the most appropriate
1478   choice MAY be performed automatically. However, this specification
1479   does not define any standard for such automatic selection.
1480</t>
1481<t><list>
1482  <t>
1483    Note: HTTP/1.1 servers are allowed to return responses which are
1484    not acceptable according to the accept headers sent in the
1485    request. In some cases, this may even be preferable to sending a
1486    406 response. User agents are encouraged to inspect the headers of
1487    an incoming response to determine if it is acceptable.
1488  </t>
1489</list></t>
1490<t>
1491   If the response could be unacceptable, a user agent SHOULD
1492   temporarily stop receipt of more data and query the user for a
1493   decision on further actions.
1494</t>
1495</section>
1496
1497<section title="407 Proxy Authentication Required" anchor="status.407">
1498  <iref primary="true" item="407 Proxy Authentication Required (status code)"/>
1499  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required"/>
1500<t>
1501   This code is similar to 401 (Unauthorized), but indicates that the
1502   client must first authenticate itself with the proxy (see <xref target="Part7"/>).
1503</t>
1504</section>
1505
1506<section title="408 Request Timeout" anchor="status.408">
1507  <iref primary="true" item="408 Request Timeout (status code)"/>
1508  <iref primary="true" item="Status Codes" subitem="408 Request Timeout"/>
1509<t>
1510   The client did not produce a request within the time that the server
1511   was prepared to wait. The client MAY repeat the request without
1512   modifications at any later time.
1513</t>
1514</section>
1515
1516<section title="409 Conflict" anchor="status.409">
1517  <iref primary="true" item="409 Conflict (status code)"/>
1518  <iref primary="true" item="Status Codes" subitem="409 Conflict"/>
1519<t>
1520   The request could not be completed due to a conflict with the current
1521   state of the resource. This code is only allowed in situations where
1522   it is expected that the user might be able to resolve the conflict
1523   and resubmit the request. The response body SHOULD include enough
1524   information for the user to recognize the source of the conflict.
1525   Ideally, the response entity would include enough information for the
1526   user or user agent to fix the problem; however, that might not be
1527   possible and is not required.
1528</t>
1529<t>
1530   Conflicts are most likely to occur in response to a PUT request. For
1531   example, if versioning were being used and the entity being PUT
1532   included changes to a resource which conflict with those made by an
1533   earlier (third-party) request, the server might use the 409 response
1534   to indicate that it can't complete the request. In this case, the
1535   response entity would likely contain a list of the differences
1536   between the two versions in a format defined by the response
1537   Content-Type.
1538</t>
1539</section>
1540
1541<section title="410 Gone" anchor="status.410">
1542  <iref primary="true" item="410 Gone (status code)"/>
1543  <iref primary="true" item="Status Codes" subitem="410 Gone"/>
1544<t>
1545   The requested resource is no longer available at the server and no
1546   forwarding address is known. This condition is expected to be
1547   considered permanent. Clients with link editing capabilities SHOULD
1548   delete references to the request-target after user approval. If the
1549   server does not know, or has no facility to determine, whether or not
1550   the condition is permanent, the status code 404 (Not Found) SHOULD be
1551   used instead. This response is cacheable unless indicated otherwise.
1552</t>
1553<t>
1554   The 410 response is primarily intended to assist the task of web
1555   maintenance by notifying the recipient that the resource is
1556   intentionally unavailable and that the server owners desire that
1557   remote links to that resource be removed. Such an event is common for
1558   limited-time, promotional services and for resources belonging to
1559   individuals no longer working at the server's site. It is not
1560   necessary to mark all permanently unavailable resources as "gone" or
1561   to keep the mark for any length of time -- that is left to the
1562   discretion of the server owner.
1563</t>
1564</section>
1565
1566<section title="411 Length Required" anchor="status.411">
1567  <iref primary="true" item="411 Length Required (status code)"/>
1568  <iref primary="true" item="Status Codes" subitem="411 Length Required"/>
1569<t>
1570   The server refuses to accept the request without a defined Content-Length.
1571   The client MAY repeat the request if it adds a valid
1572   Content-Length header field containing the length of the message-body
1573   in the request message.
1574</t>
1575</section>
1576
1577<section title="412 Precondition Failed" anchor="status.412">
1578  <iref primary="true" item="412 Precondition Failed (status code)"/>
1579  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1580<t>
1581   The precondition given in one or more of the request-header fields
1582   evaluated to false when it was tested on the server, as defined in
1583   <xref target="Part4"/>.
1584</t>
1585</section>
1586
1587<section title="413 Request Entity Too Large" anchor="status.413">
1588  <iref primary="true" item="413 Request Entity Too Large (status code)"/>
1589  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large"/>
1590<t>
1591   The server is refusing to process a request because the request
1592   entity is larger than the server is willing or able to process. The
1593   server MAY close the connection to prevent the client from continuing
1594   the request.
1595</t>
1596<t>
1597   If the condition is temporary, the server SHOULD include a Retry-After
1598   header field to indicate that it is temporary and after what
1599   time the client MAY try again.
1600</t>
1601</section>
1602
1603<section title="414 URI Too Long" anchor="status.414">
1604  <iref primary="true" item="414 URI Too Long (status code)"/>
1605  <iref primary="true" item="Status Codes" subitem="414 URI Too Long"/>
1606<t>
1607   The server is refusing to service the request because the request-target
1608   is longer than the server is willing to interpret. This rare
1609   condition is only likely to occur when a client has improperly
1610   converted a POST request to a GET request with long query
1611   information, when the client has descended into a URI "black hole" of
1612   redirection (e.g., a redirected URI prefix that points to a suffix of
1613   itself), or when the server is under attack by a client attempting to
1614   exploit security holes present in some servers using fixed-length
1615   buffers for reading or manipulating the request-target.
1616</t>
1617</section>
1618
1619<section title="415 Unsupported Media Type" anchor="status.415">
1620  <iref primary="true" item="415 Unsupported Media Type (status code)"/>
1621  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type"/>
1622<t>
1623   The server is refusing to service the request because the entity of
1624   the request is in a format not supported by the requested resource
1625   for the requested method.
1626</t>
1627</section>
1628
1629<section title="416 Requested Range Not Satisfiable" anchor="status.416">
1630  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)"/>
1631  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable"/>
1632<t>
1633   The request included a Range request-header field (Section 5.4 of <xref target="Part5"/>) and none of
1634   the range-specifier values in this field overlap the current extent
1635   of the selected resource.
1636</t>
1637</section>
1638
1639<section title="417 Expectation Failed" anchor="status.417">
1640  <iref primary="true" item="417 Expectation Failed (status code)"/>
1641  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed"/>
1642<t>
1643   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
1644   could not be met by this server, or, if the server is a proxy,
1645   the server has unambiguous evidence that the request could not be met
1646   by the next-hop server.
1647</t>
1648</section>
1649</section>
1650
1651<section title="Server Error 5xx" anchor="status.5xx">
1652<t>
1653   Response status codes beginning with the digit "5" indicate cases in
1654   which the server is aware that it has erred or is incapable of
1655   performing the request. Except when responding to a HEAD request, the
1656   server SHOULD include an entity containing an explanation of the
1657   error situation, and whether it is a temporary or permanent
1658   condition. User agents SHOULD display any included entity to the
1659   user. These response codes are applicable to any request method.
1660</t>
1661
1662<section title="500 Internal Server Error" anchor="status.500">
1663  <iref primary="true" item="500 Internal Server Error (status code)"/>
1664  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error"/>
1665<t>
1666   The server encountered an unexpected condition which prevented it
1667   from fulfilling the request.
1668</t>
1669</section>
1670
1671<section title="501 Not Implemented" anchor="status.501">
1672  <iref primary="true" item="501 Not Implemented (status code)"/>
1673  <iref primary="true" item="Status Codes" subitem="501 Not Implemented"/>
1674<t>
1675   The server does not support the functionality required to fulfill the
1676   request. This is the appropriate response when the server does not
1677   recognize the request method and is not capable of supporting it for
1678   any resource.
1679</t>
1680</section>
1681
1682<section title="502 Bad Gateway" anchor="status.502">
1683  <iref primary="true" item="502 Bad Gateway (status code)"/>
1684  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway"/>
1685<t>
1686   The server, while acting as a gateway or proxy, received an invalid
1687   response from the upstream server it accessed in attempting to
1688   fulfill the request.
1689</t>
1690</section>
1691
1692<section title="503 Service Unavailable" anchor="status.503">
1693  <iref primary="true" item="503 Service Unavailable (status code)"/>
1694  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable"/>
1695<t>
1696   The server is currently unable to handle the request due to a
1697   temporary overloading or maintenance of the server. The implication
1698   is that this is a temporary condition which will be alleviated after
1699   some delay. If known, the length of the delay MAY be indicated in a
1700   Retry-After header. If no Retry-After is given, the client SHOULD
1701   handle the response as it would for a 500 response.
1702</t>
1703<t><list>
1704  <t>
1705    Note: The existence of the 503 status code does not imply that a
1706    server must use it when becoming overloaded. Some servers may wish
1707    to simply refuse the connection.
1708  </t>
1709</list></t>
1710</section>
1711
1712<section title="504 Gateway Timeout" anchor="status.504">
1713  <iref primary="true" item="504 Gateway Timeout (status code)"/>
1714  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout"/>
1715<t>
1716   The server, while acting as a gateway or proxy, did not receive a
1717   timely response from the upstream server specified by the URI (e.g.
1718   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
1719   to access in attempting to complete the request.
1720</t>
1721<t><list>
1722  <t>
1723    Note: Note to implementors: some deployed proxies are known to
1724    return 400 or 500 when DNS lookups time out.
1725  </t>
1726</list></t>
1727</section>
1728
1729<section title="505 HTTP Version Not Supported" anchor="status.505">
1730  <iref primary="true" item="505 HTTP Version Not Supported (status code)"/>
1731  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported"/>
1732<t>
1733   The server does not support, or refuses to support, the protocol
1734   version that was used in the request message. The server is
1735   indicating that it is unable or unwilling to complete the request
1736   using the same major version as the client, as described in Section 3.1 of <xref target="Part1"/>,
1737   other than with this error message. The response SHOULD contain
1738   an entity describing why that version is not supported and what other
1739   protocols are supported by that server.
1740</t>
1741
1742</section>
1743</section>
1744</section>
1745
1746
1747<section title="Header Field Definitions" anchor="header.fields">
1748<t>
1749   This section defines the syntax and semantics of HTTP/1.1 header fields
1750   related to request and response semantics.
1751</t>
1752<t>
1753   For entity-header fields, both sender and recipient refer to either the
1754   client or the server, depending on who sends and who receives the entity.
1755</t>
1756
1757<section title="Allow" anchor="header.allow">
1758  <iref primary="true" item="Allow header"/>
1759  <iref primary="true" item="Headers" subitem="Allow"/>
1760 
1761 
1762<t>
1763      The response-header field "Allow" lists the set of methods advertised as
1764      supported by the resource identified by the request-target. The purpose of
1765      this field is strictly to inform the recipient of valid methods
1766      associated with the resource. An Allow header field MUST be
1767      present in a 405 (Method Not Allowed) response.
1768</t>
1769<figure><iref primary="true" item="Grammar" subitem="Allow"/><iref primary="true" item="Grammar" subitem="Allow-v"/><artwork type="abnf2616"><![CDATA[
1770  Allow   = "Allow" ":" OWS Allow-v
1771  Allow-v = #Method
1772]]></artwork></figure>
1773<t>
1774      Example of use:
1775</t>
1776<figure><artwork type="example"><![CDATA[
1777  Allow: GET, HEAD, PUT
1778]]></artwork></figure>
1779<t>
1780      The actual set of allowed methods is defined
1781      by the origin server at the time of each request.
1782</t>
1783<t>
1784      A proxy MUST NOT modify the Allow header field even if it does not
1785      understand all the methods specified, since the user agent might
1786      have other means of communicating with the origin server.
1787</t>
1788</section>
1789
1790<section title="Expect" anchor="header.expect">
1791  <iref primary="true" item="Expect header"/>
1792  <iref primary="true" item="Headers" subitem="Expect"/>
1793 
1794 
1795 
1796 
1797 
1798<t>
1799   The request-header field "Expect" is used to indicate that particular
1800   server behaviors are required by the client.
1801</t>
1802<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[
1803  Expect       = "Expect" ":" OWS Expect-v
1804  Expect-v     = 1#expectation
1805 
1806  expectation  = "100-continue" / expectation-extension
1807  expectation-extension = token [ "=" ( token / quoted-string )
1808                           *expect-params ]
1809  expect-params = ";" token [ "=" ( token / quoted-string ) ]
1810]]></artwork></figure>
1811<t>
1812   A server that does not understand or is unable to comply with any of
1813   the expectation values in the Expect field of a request MUST respond
1814   with appropriate error status. The server MUST respond with a 417
1815   (Expectation Failed) status if any of the expectations cannot be met
1816   or, if there are other problems with the request, some other 4xx
1817   status.
1818</t>
1819<t>
1820   This header field is defined with extensible syntax to allow for
1821   future extensions. If a server receives a request containing an
1822   Expect field that includes an expectation-extension that it does not
1823   support, it MUST respond with a 417 (Expectation Failed) status.
1824</t>
1825<t>
1826   Comparison of expectation values is case-insensitive for unquoted
1827   tokens (including the 100-continue token), and is case-sensitive for
1828   quoted-string expectation-extensions.
1829</t>
1830<t>
1831   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
1832   return a 417 (Expectation Failed) status if it receives a request
1833   with an expectation that it cannot meet. However, the Expect
1834   request-header itself is end-to-end; it MUST be forwarded if the
1835   request is forwarded.
1836</t>
1837<t>
1838   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
1839   Expect header.
1840</t>
1841<t>
1842   See Section 7.2.3 of <xref target="Part1"/> for the use of the 100 (Continue) status.
1843</t>
1844</section>
1845
1846<section title="From" anchor="header.from">
1847  <iref primary="true" item="From header"/>
1848  <iref primary="true" item="Headers" subitem="From"/>
1849 
1850 
1851 
1852<t>
1853   The request-header field "From", if given, SHOULD contain an Internet
1854   e-mail address for the human user who controls the requesting user
1855   agent. The address SHOULD be machine-usable, as defined by "mailbox"
1856   in Section 3.4 of <xref target="RFC5322"/>:
1857</t>
1858<figure><iref primary="true" item="Grammar" subitem="From"/><iref primary="true" item="Grammar" subitem="From-v"/><artwork type="abnf2616"><![CDATA[
1859  From    = "From" ":" OWS From-v
1860  From-v  = mailbox
1861 
1862  mailbox = <mailbox, defined in [RFC5322], Section 3.4>
1863]]></artwork></figure>
1864<t>
1865   An example is:
1866</t>
1867<figure><artwork type="example"><![CDATA[
1868  From: webmaster@example.org
1869]]></artwork></figure>
1870<t>
1871   This header field MAY be used for logging purposes and as a means for
1872   identifying the source of invalid or unwanted requests. It SHOULD NOT
1873   be used as an insecure form of access protection. The interpretation
1874   of this field is that the request is being performed on behalf of the
1875   person given, who accepts responsibility for the method performed. In
1876   particular, robot agents SHOULD include this header so that the
1877   person responsible for running the robot can be contacted if problems
1878   occur on the receiving end.
1879</t>
1880<t>
1881   The Internet e-mail address in this field MAY be separate from the
1882   Internet host which issued the request. For example, when a request
1883   is passed through a proxy the original issuer's address SHOULD be
1884   used.
1885</t>
1886<t>
1887   The client SHOULD NOT  send the From header field without the user's
1888   approval, as it might conflict with the user's privacy interests or
1889   their site's security policy. It is strongly recommended that the
1890   user be able to disable, enable, and modify the value of this field
1891   at any time prior to a request.
1892</t>
1893</section>
1894
1895<section title="Location" anchor="header.location">
1896  <iref primary="true" item="Location header"/>
1897  <iref primary="true" item="Headers" subitem="Location"/>
1898 
1899 
1900<t>
1901   The response-header field "Location" is used for the identification of a
1902   new resource or to redirect the recipient to a location other than the
1903   request-target for completion of the request.  For 201 (Created)
1904   responses, the Location is that of the new resource which was created
1905   by the request. For 3xx responses, the location SHOULD indicate the
1906   server's preferred URI for automatic redirection to the resource. The
1907   field value consists of a single absolute URI.
1908</t>
1909<figure><iref primary="true" item="Grammar" subitem="Location"/><iref primary="true" item="Grammar" subitem="Location-v"/><artwork type="abnf2616"><![CDATA[
1910  Location       = "Location" ":" OWS Location-v
1911  Location-v     = absolute-URI [ "#" fragment ]
1912]]></artwork></figure>
1913<t>
1914   An example is:
1915</t>
1916<figure><artwork type="example"><![CDATA[
1917  Location: http://www.example.org/pub/WWW/People.html
1918]]></artwork></figure>
1919<t><list>
1920  <t>
1921    Note: The Content-Location header field (Section 5.7 of <xref target="Part3"/>) differs
1922    from Location in that the Content-Location identifies the original
1923    location of the entity enclosed in the response. It is therefore
1924    possible for a response to contain header fields for both Location
1925    and Content-Location.
1926  </t>
1927</list></t>
1928<t>
1929   There are circumstances in which a fragment identifier in a Location URL would not be appropriate:
1930   <list style="symbols">
1931      <t>With a 201 Created response, because in this usage the Location header specifies the URL for the entire created resource.</t>
1932      <t>With a 300 Multiple Choices, since the choice decision is intended to be made on resource characteristics and not fragment characteristics.</t>
1933      <t>With 305 Use Proxy.</t>
1934   </list>
1935</t>
1936</section>
1937
1938<section title="Max-Forwards" anchor="header.max-forwards">
1939  <iref primary="true" item="Max-Forwards header"/>
1940  <iref primary="true" item="Headers" subitem="Max-Forwards"/>
1941 
1942 
1943<t>
1944   The request-header "Max-Forwards" field provides a mechanism with the
1945   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
1946   number of proxies or gateways that can forward the request to the
1947   next inbound server. This can be useful when the client is attempting
1948   to trace a request chain which appears to be failing or looping in
1949   mid-chain.
1950</t>
1951<figure><iref primary="true" item="Grammar" subitem="Max-Forwards"/><iref primary="true" item="Grammar" subitem="Max-Forwards-v"/><artwork type="abnf2616"><![CDATA[
1952  Max-Forwards   = "Max-Forwards" ":" OWS Max-Forwards-v
1953  Max-Forwards-v = 1*DIGIT
1954]]></artwork></figure>
1955<t>
1956   The Max-Forwards value is a decimal integer indicating the remaining
1957   number of times this request message may be forwarded.
1958</t>
1959<t>
1960   Each proxy or gateway recipient of a TRACE or OPTIONS request
1961   containing a Max-Forwards header field MUST check and update its
1962   value prior to forwarding the request. If the received value is zero
1963   (0), the recipient MUST NOT forward the request; instead, it MUST
1964   respond as the final recipient. If the received Max-Forwards value is
1965   greater than zero, then the forwarded message MUST contain an updated
1966   Max-Forwards field with a value decremented by one (1).
1967</t>
1968<t>
1969   The Max-Forwards header field MAY be ignored for all other methods
1970   defined by this specification and for any extension methods for which
1971   it is not explicitly referred to as part of that method definition.
1972</t>
1973</section>
1974
1975<section title="Referer" anchor="header.referer">
1976  <iref primary="true" item="Referer header"/>
1977  <iref primary="true" item="Headers" subitem="Referer"/>
1978 
1979 
1980<t>
1981   The request-header field "Referer" [sic] allows the client to specify, for
1982   the server's benefit, the address (URI) of the resource from which the
1983   request-target was obtained (the "referrer", although the header field is
1984   misspelled.).
1985</t>
1986<t>
1987   The Referer header allows servers to generate lists of back-links to
1988   resources for interest, logging, optimized caching, etc. It also allows
1989   obsolete or mistyped links to be traced for maintenance. Some servers use
1990   Referer as a means of controlling where they allow links from (so-called
1991   "deep linking"), but it should be noted that legitimate requests are not
1992   required to contain a Referer header field.
1993</t>
1994<t>
1995   If the request-target was obtained from a source that does not have its own
1996   URI (e.g., input from the user keyboard), the Referer field MUST either be
1997   sent with the value "about:blank", or not be sent at all. Note that this
1998   requirement does not apply to sources with non-HTTP URIs (e.g., FTP).
1999</t>
2000<figure><iref primary="true" item="Grammar" subitem="Referer"/><iref primary="true" item="Grammar" subitem="Referer-v"/><artwork type="abnf2616"><![CDATA[
2001  Referer        = "Referer" ":" OWS Referer-v
2002  Referer-v      = absolute-URI / partial-URI
2003]]></artwork></figure>
2004<t>
2005   Example:
2006</t>
2007<figure><artwork type="example"><![CDATA[
2008  Referer: http://www.example.org/hypertext/Overview.html
2009]]></artwork></figure>
2010<t>
2011   If the field value is a relative URI, it SHOULD be interpreted
2012   relative to the request-target. The URI MUST NOT include a fragment. See
2013   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
2014</t>
2015</section>
2016
2017<section title="Retry-After" anchor="header.retry-after">
2018  <iref primary="true" item="Retry-After header"/>
2019  <iref primary="true" item="Headers" subitem="Retry-After"/>
2020 
2021 
2022<t>
2023   The response-header "Retry-After" field can be used with a 503 (Service
2024   Unavailable) response to indicate how long the service is expected to
2025   be unavailable to the requesting client. This field MAY also be used
2026   with any 3xx (Redirection) response to indicate the minimum time the
2027   user-agent is asked wait before issuing the redirected request. The
2028   value of this field can be either an HTTP-date or an integer number
2029   of seconds (in decimal) after the time of the response.
2030</t>
2031<figure><iref primary="true" item="Grammar" subitem="Retry-After"/><iref primary="true" item="Grammar" subitem="Retry-After-v"/><artwork type="abnf2616"><![CDATA[
2032  Retry-After   = "Retry-After" ":" OWS Retry-After-v
2033  Retry-After-v = HTTP-date / delta-seconds
2034]]></artwork></figure>
2035<t anchor="rule.delta-seconds">
2036 
2037   Time spans are non-negative decimal integers, representing time in
2038   seconds.
2039</t>
2040<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
2041  delta-seconds  = 1*DIGIT
2042]]></artwork></figure>
2043<t>
2044   Two examples of its use are
2045</t>
2046<figure><artwork type="example"><![CDATA[
2047  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
2048  Retry-After: 120
2049]]></artwork></figure>
2050<t>
2051   In the latter example, the delay is 2 minutes.
2052</t>
2053</section>
2054
2055<section title="Server" anchor="header.server">
2056  <iref primary="true" item="Server header"/>
2057  <iref primary="true" item="Headers" subitem="Server"/>
2058 
2059 
2060<t>
2061   The response-header field "Server" contains information about the
2062   software used by the origin server to handle the request. The field
2063   can contain multiple product tokens (Section 3.4 of <xref target="Part1"/>) and comments
2064   identifying the server and any significant subproducts. The product
2065   tokens are listed in order of their significance for identifying the
2066   application.
2067</t>
2068<figure><iref primary="true" item="Grammar" subitem="Server"/><iref primary="true" item="Grammar" subitem="Server-v"/><artwork type="abnf2616"><![CDATA[
2069  Server         = "Server" ":" OWS Server-v
2070  Server-v       = product
2071                   *( RWS ( product / comment ) )
2072]]></artwork></figure>
2073<t>
2074   Example:
2075</t>
2076<figure><artwork type="example"><![CDATA[
2077  Server: CERN/3.0 libwww/2.17
2078]]></artwork></figure>
2079<t>
2080   If the response is being forwarded through a proxy, the proxy
2081   application MUST NOT modify the Server response-header. Instead, it
2082   MUST include a Via field (as described in Section 8.9 of <xref target="Part1"/>).
2083</t>
2084<t><list>
2085  <t>
2086    Note: Revealing the specific software version of the server might
2087    allow the server machine to become more vulnerable to attacks
2088    against software that is known to contain security holes. Server
2089    implementors are encouraged to make this field a configurable
2090    option.
2091  </t>
2092</list></t>
2093</section>
2094
2095<section title="User-Agent" anchor="header.user-agent">
2096  <iref primary="true" item="User-Agent header"/>
2097  <iref primary="true" item="Headers" subitem="User-Agent"/>
2098 
2099 
2100<t>
2101   The request-header field "User-Agent" contains information about the
2102   user agent originating the request. This is for statistical purposes,
2103   the tracing of protocol violations, and automated recognition of user
2104   agents for the sake of tailoring responses to avoid particular user
2105   agent limitations. User agents SHOULD include this field with
2106   requests. The field can contain multiple product tokens (Section 3.4 of <xref target="Part1"/>)
2107   and comments identifying the agent and any subproducts which form a
2108   significant part of the user agent. By convention, the product tokens
2109   are listed in order of their significance for identifying the
2110   application.
2111</t>
2112<figure><iref primary="true" item="Grammar" subitem="User-Agent"/><iref primary="true" item="Grammar" subitem="User-Agent-v"/><artwork type="abnf2616"><![CDATA[
2113  User-Agent     = "User-Agent" ":" OWS User-Agent-v
2114  User-Agent-v   = product
2115                   *( RWS ( product / comment ) )
2116]]></artwork></figure>
2117<t>
2118   Example:
2119</t>
2120<figure><artwork type="example"><![CDATA[
2121  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
2122]]></artwork></figure>
2123</section>
2124
2125</section>
2126
2127<section title="IANA Considerations" anchor="IANA.considerations">
2128
2129<section title="Method Registry" anchor="method.registration">
2130<t>
2131  The registration procedure for HTTP Methods is defined by
2132  <xref target="method.registry"/> of this document.
2133</t>
2134<t>
2135   The HTTP Method Registry located at <eref target="http://www.iana.org/assignments/http-methods"/>
2136   should be populated with the registrations below:
2137</t>
2138<!--AUTOGENERATED FROM extract-method-defs.xslt, do not edit manually-->
2139<texttable align="left" suppress-title="true" anchor="iana.method.registration.table">
2140   <ttcol>Method</ttcol>
2141   <ttcol>Safe</ttcol>
2142   <ttcol>Reference</ttcol>
2143   <c>CONNECT</c>
2144   <c>no</c>
2145   <c>
2146      <xref target="CONNECT"/>
2147   </c>
2148   <c>DELETE</c>
2149   <c>no</c>
2150   <c>
2151      <xref target="DELETE"/>
2152   </c>
2153   <c>GET</c>
2154   <c>yes</c>
2155   <c>
2156      <xref target="GET"/>
2157   </c>
2158   <c>HEAD</c>
2159   <c>yes</c>
2160   <c>
2161      <xref target="HEAD"/>
2162   </c>
2163   <c>OPTIONS</c>
2164   <c>yes</c>
2165   <c>
2166      <xref target="OPTIONS"/>
2167   </c>
2168   <c>POST</c>
2169   <c>no</c>
2170   <c>
2171      <xref target="POST"/>
2172   </c>
2173   <c>PUT</c>
2174   <c>no</c>
2175   <c>
2176      <xref target="PUT"/>
2177   </c>
2178   <c>TRACE</c>
2179   <c>yes</c>
2180   <c>
2181      <xref target="TRACE"/>
2182   </c>
2183</texttable>
2184<!--(END)-->
2185</section>
2186
2187<section title="Status Code Registry" anchor="status.code.registration">
2188<t>
2189   The registration procedure for HTTP Status Codes -- previously defined
2190   in Section 7.1 of <xref target="RFC2817"/> -- is now defined
2191   by <xref target="status.code.registry"/> of this document.
2192</t>
2193<t>
2194   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
2195   should be updated with the registrations below:
2196</t>
2197<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
2198<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
2199   <ttcol>Value</ttcol>
2200   <ttcol>Description</ttcol>
2201   <ttcol>Reference</ttcol>
2202   <c>100</c>
2203   <c>Continue</c>
2204   <c>
2205      <xref target="status.100"/>
2206   </c>
2207   <c>101</c>
2208   <c>Switching Protocols</c>
2209   <c>
2210      <xref target="status.101"/>
2211   </c>
2212   <c>200</c>
2213   <c>OK</c>
2214   <c>
2215      <xref target="status.200"/>
2216   </c>
2217   <c>201</c>
2218   <c>Created</c>
2219   <c>
2220      <xref target="status.201"/>
2221   </c>
2222   <c>202</c>
2223   <c>Accepted</c>
2224   <c>
2225      <xref target="status.202"/>
2226   </c>
2227   <c>203</c>
2228   <c>Non-Authoritative Information</c>
2229   <c>
2230      <xref target="status.203"/>
2231   </c>
2232   <c>204</c>
2233   <c>No Content</c>
2234   <c>
2235      <xref target="status.204"/>
2236   </c>
2237   <c>205</c>
2238   <c>Reset Content</c>
2239   <c>
2240      <xref target="status.205"/>
2241   </c>
2242   <c>206</c>
2243   <c>Partial Content</c>
2244   <c>
2245      <xref target="status.206"/>
2246   </c>
2247   <c>300</c>
2248   <c>Multiple Choices</c>
2249   <c>
2250      <xref target="status.300"/>
2251   </c>
2252   <c>301</c>
2253   <c>Moved Permanently</c>
2254   <c>
2255      <xref target="status.301"/>
2256   </c>
2257   <c>302</c>
2258   <c>Found</c>
2259   <c>
2260      <xref target="status.302"/>
2261   </c>
2262   <c>303</c>
2263   <c>See Other</c>
2264   <c>
2265      <xref target="status.303"/>
2266   </c>
2267   <c>304</c>
2268   <c>Not Modified</c>
2269   <c>
2270      <xref target="status.304"/>
2271   </c>
2272   <c>305</c>
2273   <c>Use Proxy</c>
2274   <c>
2275      <xref target="status.305"/>
2276   </c>
2277   <c>306</c>
2278   <c>(Unused)</c>
2279   <c>
2280      <xref target="status.306"/>
2281   </c>
2282   <c>307</c>
2283   <c>Temporary Redirect</c>
2284   <c>
2285      <xref target="status.307"/>
2286   </c>
2287   <c>400</c>
2288   <c>Bad Request</c>
2289   <c>
2290      <xref target="status.400"/>
2291   </c>
2292   <c>401</c>
2293   <c>Unauthorized</c>
2294   <c>
2295      <xref target="status.401"/>
2296   </c>
2297   <c>402</c>
2298   <c>Payment Required</c>
2299   <c>
2300      <xref target="status.402"/>
2301   </c>
2302   <c>403</c>
2303   <c>Forbidden</c>
2304   <c>
2305      <xref target="status.403"/>
2306   </c>
2307   <c>404</c>
2308   <c>Not Found</c>
2309   <c>
2310      <xref target="status.404"/>
2311   </c>
2312   <c>405</c>
2313   <c>Method Not Allowed</c>
2314   <c>
2315      <xref target="status.405"/>
2316   </c>
2317   <c>406</c>
2318   <c>Not Acceptable</c>
2319   <c>
2320      <xref target="status.406"/>
2321   </c>
2322   <c>407</c>
2323   <c>Proxy Authentication Required</c>
2324   <c>
2325      <xref target="status.407"/>
2326   </c>
2327   <c>408</c>
2328   <c>Request Timeout</c>
2329   <c>
2330      <xref target="status.408"/>
2331   </c>
2332   <c>409</c>
2333   <c>Conflict</c>
2334   <c>
2335      <xref target="status.409"/>
2336   </c>
2337   <c>410</c>
2338   <c>Gone</c>
2339   <c>
2340      <xref target="status.410"/>
2341   </c>
2342   <c>411</c>
2343   <c>Length Required</c>
2344   <c>
2345      <xref target="status.411"/>
2346   </c>
2347   <c>412</c>
2348   <c>Precondition Failed</c>
2349   <c>
2350      <xref target="status.412"/>
2351   </c>
2352   <c>413</c>
2353   <c>Request Entity Too Large</c>
2354   <c>
2355      <xref target="status.413"/>
2356   </c>
2357   <c>414</c>
2358   <c>URI Too Long</c>
2359   <c>
2360      <xref target="status.414"/>
2361   </c>
2362   <c>415</c>
2363   <c>Unsupported Media Type</c>
2364   <c>
2365      <xref target="status.415"/>
2366   </c>
2367   <c>416</c>
2368   <c>Requested Range Not Satisfiable</c>
2369   <c>
2370      <xref target="status.416"/>
2371   </c>
2372   <c>417</c>
2373   <c>Expectation Failed</c>
2374   <c>
2375      <xref target="status.417"/>
2376   </c>
2377   <c>500</c>
2378   <c>Internal Server Error</c>
2379   <c>
2380      <xref target="status.500"/>
2381   </c>
2382   <c>501</c>
2383   <c>Not Implemented</c>
2384   <c>
2385      <xref target="status.501"/>
2386   </c>
2387   <c>502</c>
2388   <c>Bad Gateway</c>
2389   <c>
2390      <xref target="status.502"/>
2391   </c>
2392   <c>503</c>
2393   <c>Service Unavailable</c>
2394   <c>
2395      <xref target="status.503"/>
2396   </c>
2397   <c>504</c>
2398   <c>Gateway Timeout</c>
2399   <c>
2400      <xref target="status.504"/>
2401   </c>
2402   <c>505</c>
2403   <c>HTTP Version Not Supported</c>
2404   <c>
2405      <xref target="status.505"/>
2406   </c>
2407</texttable>
2408<!--(END)-->
2409</section>
2410<section title="Message Header Registration" anchor="message.header.registration">
2411<t>
2412   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2413   with the permanent registrations below (see <xref target="RFC3864"/>):
2414</t>
2415<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2416<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2417   <ttcol>Header Field Name</ttcol>
2418   <ttcol>Protocol</ttcol>
2419   <ttcol>Status</ttcol>
2420   <ttcol>Reference</ttcol>
2421   <c>Allow</c>
2422   <c>http</c>
2423   <c>standard</c>
2424   <c>
2425      <xref target="header.allow"/>
2426   </c>
2427   <c>Expect</c>
2428   <c>http</c>
2429   <c>standard</c>
2430   <c>
2431      <xref target="header.expect"/>
2432   </c>
2433   <c>From</c>
2434   <c>http</c>
2435   <c>standard</c>
2436   <c>
2437      <xref target="header.from"/>
2438   </c>
2439   <c>Location</c>
2440   <c>http</c>
2441   <c>standard</c>
2442   <c>
2443      <xref target="header.location"/>
2444   </c>
2445   <c>Max-Forwards</c>
2446   <c>http</c>
2447   <c>standard</c>
2448   <c>
2449      <xref target="header.max-forwards"/>
2450   </c>
2451   <c>Referer</c>
2452   <c>http</c>
2453   <c>standard</c>
2454   <c>
2455      <xref target="header.referer"/>
2456   </c>
2457   <c>Retry-After</c>
2458   <c>http</c>
2459   <c>standard</c>
2460   <c>
2461      <xref target="header.retry-after"/>
2462   </c>
2463   <c>Server</c>
2464   <c>http</c>
2465   <c>standard</c>
2466   <c>
2467      <xref target="header.server"/>
2468   </c>
2469   <c>User-Agent</c>
2470   <c>http</c>
2471   <c>standard</c>
2472   <c>
2473      <xref target="header.user-agent"/>
2474   </c>
2475</texttable>
2476<!--(END)-->
2477<t>
2478   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2479</t>
2480</section>
2481</section>
2482
2483<section title="Security Considerations" anchor="security.considerations">
2484<t>
2485   This section is meant to inform application developers, information
2486   providers, and users of the security limitations in HTTP/1.1 as
2487   described by this document. The discussion does not include
2488   definitive solutions to the problems revealed, though it does make
2489   some suggestions for reducing security risks.
2490</t>
2491
2492<section title="Transfer of Sensitive Information" anchor="security.sensitive">
2493<t>
2494   Like any generic data transfer protocol, HTTP cannot regulate the
2495   content of the data that is transferred, nor is there any a priori
2496   method of determining the sensitivity of any particular piece of
2497   information within the context of any given request. Therefore,
2498   applications SHOULD supply as much control over this information as
2499   possible to the provider of that information. Four header fields are
2500   worth special mention in this context: Server, Via, Referer and From.
2501</t>
2502<t>
2503   Revealing the specific software version of the server might allow the
2504   server machine to become more vulnerable to attacks against software
2505   that is known to contain security holes. Implementors SHOULD make the
2506   Server header field a configurable option.
2507</t>
2508<t>
2509   Proxies which serve as a portal through a network firewall SHOULD
2510   take special precautions regarding the transfer of header information
2511   that identifies the hosts behind the firewall. In particular, they
2512   SHOULD remove, or replace with sanitized versions, any Via fields
2513   generated behind the firewall.
2514</t>
2515<t>
2516   The Referer header allows reading patterns to be studied and reverse
2517   links drawn. Although it can be very useful, its power can be abused
2518   if user details are not separated from the information contained in
2519   the Referer. Even when the personal information has been removed, the
2520   Referer header might indicate a private document's URI whose
2521   publication would be inappropriate.
2522</t>
2523<t>
2524   The information sent in the From field might conflict with the user's
2525   privacy interests or their site's security policy, and hence it
2526   SHOULD NOT  be transmitted without the user being able to disable,
2527   enable, and modify the contents of the field. The user MUST be able
2528   to set the contents of this field within a user preference or
2529   application defaults configuration.
2530</t>
2531<t>
2532   We suggest, though do not require, that a convenient toggle interface
2533   be provided for the user to enable or disable the sending of From and
2534   Referer information.
2535</t>
2536<t>
2537   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
2538   fields can sometimes be used to determine that a specific client or
2539   server have a particular security hole which might be exploited.
2540   Unfortunately, this same information is often used for other valuable
2541   purposes for which HTTP currently has no better mechanism.
2542</t>
2543</section>
2544
2545<section title="Encoding Sensitive Information in URIs" anchor="encoding.sensitive.information.in.uris">
2546<t>
2547   Because the source of a link might be private information or might
2548   reveal an otherwise private information source, it is strongly
2549   recommended that the user be able to select whether or not the
2550   Referer field is sent. For example, a browser client could have a
2551   toggle switch for browsing openly/anonymously, which would
2552   respectively enable/disable the sending of Referer and From
2553   information.
2554</t>
2555<t>
2556   Clients SHOULD NOT include a Referer header field in a (non-secure)
2557   HTTP request if the referring page was transferred with a secure
2558   protocol.
2559</t>
2560<t>
2561   Authors of services should not use
2562   GET-based forms for the submission of sensitive data because that
2563   data will be encoded in the Request-target. Many existing
2564   servers, proxies, and user agents log or display the Request-target in
2565   places where it might be visible to third parties. Such services can
2566   use POST-based form submission instead.
2567</t>
2568</section>
2569
2570<section title="Location Headers and Spoofing" anchor="location.spoofing">
2571<t>
2572   If a single server supports multiple organizations that do not trust
2573   one another, then it MUST check the values of Location and Content-Location
2574   headers in responses that are generated under control of
2575   said organizations to make sure that they do not attempt to
2576   invalidate resources over which they have no authority.
2577</t>
2578</section>
2579
2580</section>
2581
2582<section title="Acknowledgments" anchor="ack">
2583</section>
2584</middle>
2585<back>
2586
2587<references title="Normative References">
2588
2589<reference anchor="Part1">
2590  <front>
2591    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2592    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2593      <organization abbrev="Day Software">Day Software</organization>
2594      <address><email>fielding@gbiv.com</email></address>
2595    </author>
2596    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2597      <organization>One Laptop per Child</organization>
2598      <address><email>jg@laptop.org</email></address>
2599    </author>
2600    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2601      <organization abbrev="HP">Hewlett-Packard Company</organization>
2602      <address><email>JeffMogul@acm.org</email></address>
2603    </author>
2604    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2605      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2606      <address><email>henrikn@microsoft.com</email></address>
2607    </author>
2608    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2609      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2610      <address><email>LMM@acm.org</email></address>
2611    </author>
2612    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2613      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2614      <address><email>paulle@microsoft.com</email></address>
2615    </author>
2616    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2617      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2618      <address><email>timbl@w3.org</email></address>
2619    </author>
2620    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2621      <organization abbrev="W3C">World Wide Web Consortium</organization>
2622      <address><email>ylafon@w3.org</email></address>
2623    </author>
2624    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2625      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2626      <address><email>julian.reschke@greenbytes.de</email></address>
2627    </author>
2628    <date month="July" year="2009"/>
2629  </front>
2630  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-07"/>
2631 
2632</reference>
2633
2634<reference anchor="Part3">
2635  <front>
2636    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2637    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2638      <organization abbrev="Day Software">Day Software</organization>
2639      <address><email>fielding@gbiv.com</email></address>
2640    </author>
2641    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2642      <organization>One Laptop per Child</organization>
2643      <address><email>jg@laptop.org</email></address>
2644    </author>
2645    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2646      <organization abbrev="HP">Hewlett-Packard Company</organization>
2647      <address><email>JeffMogul@acm.org</email></address>
2648    </author>
2649    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2650      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2651      <address><email>henrikn@microsoft.com</email></address>
2652    </author>
2653    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2654      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2655      <address><email>LMM@acm.org</email></address>
2656    </author>
2657    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2658      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2659      <address><email>paulle@microsoft.com</email></address>
2660    </author>
2661    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2662      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2663      <address><email>timbl@w3.org</email></address>
2664    </author>
2665    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2666      <organization abbrev="W3C">World Wide Web Consortium</organization>
2667      <address><email>ylafon@w3.org</email></address>
2668    </author>
2669    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2670      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2671      <address><email>julian.reschke@greenbytes.de</email></address>
2672    </author>
2673    <date month="July" year="2009"/>
2674  </front>
2675  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-07"/>
2676 
2677</reference>
2678
2679<reference anchor="Part4">
2680  <front>
2681    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2682    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2683      <organization abbrev="Day Software">Day Software</organization>
2684      <address><email>fielding@gbiv.com</email></address>
2685    </author>
2686    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2687      <organization>One Laptop per Child</organization>
2688      <address><email>jg@laptop.org</email></address>
2689    </author>
2690    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2691      <organization abbrev="HP">Hewlett-Packard Company</organization>
2692      <address><email>JeffMogul@acm.org</email></address>
2693    </author>
2694    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2695      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2696      <address><email>henrikn@microsoft.com</email></address>
2697    </author>
2698    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2699      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2700      <address><email>LMM@acm.org</email></address>
2701    </author>
2702    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2703      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2704      <address><email>paulle@microsoft.com</email></address>
2705    </author>
2706    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2707      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2708      <address><email>timbl@w3.org</email></address>
2709    </author>
2710    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2711      <organization abbrev="W3C">World Wide Web Consortium</organization>
2712      <address><email>ylafon@w3.org</email></address>
2713    </author>
2714    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2715      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2716      <address><email>julian.reschke@greenbytes.de</email></address>
2717    </author>
2718    <date month="July" year="2009"/>
2719  </front>
2720  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-07"/>
2721 
2722</reference>
2723
2724<reference anchor="Part5">
2725  <front>
2726    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2727    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2728      <organization abbrev="Day Software">Day Software</organization>
2729      <address><email>fielding@gbiv.com</email></address>
2730    </author>
2731    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2732      <organization>One Laptop per Child</organization>
2733      <address><email>jg@laptop.org</email></address>
2734    </author>
2735    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2736      <organization abbrev="HP">Hewlett-Packard Company</organization>
2737      <address><email>JeffMogul@acm.org</email></address>
2738    </author>
2739    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2740      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2741      <address><email>henrikn@microsoft.com</email></address>
2742    </author>
2743    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2744      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2745      <address><email>LMM@acm.org</email></address>
2746    </author>
2747    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2748      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2749      <address><email>paulle@microsoft.com</email></address>
2750    </author>
2751    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2752      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2753      <address><email>timbl@w3.org</email></address>
2754    </author>
2755    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2756      <organization abbrev="W3C">World Wide Web Consortium</organization>
2757      <address><email>ylafon@w3.org</email></address>
2758    </author>
2759    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2760      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2761      <address><email>julian.reschke@greenbytes.de</email></address>
2762    </author>
2763    <date month="July" year="2009"/>
2764  </front>
2765  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-07"/>
2766 
2767</reference>
2768
2769<reference anchor="Part6">
2770  <front>
2771    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
2772    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2773      <organization abbrev="Day Software">Day Software</organization>
2774      <address><email>fielding@gbiv.com</email></address>
2775    </author>
2776    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2777      <organization>One Laptop per Child</organization>
2778      <address><email>jg@laptop.org</email></address>
2779    </author>
2780    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2781      <organization abbrev="HP">Hewlett-Packard Company</organization>
2782      <address><email>JeffMogul@acm.org</email></address>
2783    </author>
2784    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2785      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2786      <address><email>henrikn@microsoft.com</email></address>
2787    </author>
2788    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2789      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2790      <address><email>LMM@acm.org</email></address>
2791    </author>
2792    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2793      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2794      <address><email>paulle@microsoft.com</email></address>
2795    </author>
2796    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2797      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2798      <address><email>timbl@w3.org</email></address>
2799    </author>
2800    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2801      <organization abbrev="W3C">World Wide Web Consortium</organization>
2802      <address><email>ylafon@w3.org</email></address>
2803    </author>
2804    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
2805      <organization/>
2806      <address><email>mnot@mnot.net</email></address>
2807    </author>
2808    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2809      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2810      <address><email>julian.reschke@greenbytes.de</email></address>
2811    </author>
2812    <date month="July" year="2009"/>
2813  </front>
2814  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-07"/>
2815 
2816</reference>
2817
2818<reference anchor="Part7">
2819  <front>
2820    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2821    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2822      <organization abbrev="Day Software">Day Software</organization>
2823      <address><email>fielding@gbiv.com</email></address>
2824    </author>
2825    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2826      <organization>One Laptop per Child</organization>
2827      <address><email>jg@laptop.org</email></address>
2828    </author>
2829    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2830      <organization abbrev="HP">Hewlett-Packard Company</organization>
2831      <address><email>JeffMogul@acm.org</email></address>
2832    </author>
2833    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2834      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2835      <address><email>henrikn@microsoft.com</email></address>
2836    </author>
2837    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2838      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2839      <address><email>LMM@acm.org</email></address>
2840    </author>
2841    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2842      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2843      <address><email>paulle@microsoft.com</email></address>
2844    </author>
2845    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2846      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2847      <address><email>timbl@w3.org</email></address>
2848    </author>
2849    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2850      <organization abbrev="W3C">World Wide Web Consortium</organization>
2851      <address><email>ylafon@w3.org</email></address>
2852    </author>
2853    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2854      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2855      <address><email>julian.reschke@greenbytes.de</email></address>
2856    </author>
2857    <date month="July" year="2009"/>
2858  </front>
2859  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-07"/>
2860 
2861</reference>
2862
2863<reference anchor="RFC2119">
2864  <front>
2865    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2866    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2867      <organization>Harvard University</organization>
2868      <address><email>sob@harvard.edu</email></address>
2869    </author>
2870    <date month="March" year="1997"/>
2871  </front>
2872  <seriesInfo name="BCP" value="14"/>
2873  <seriesInfo name="RFC" value="2119"/>
2874</reference>
2875
2876<reference anchor="RFC5234">
2877  <front>
2878    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2879    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2880      <organization>Brandenburg InternetWorking</organization>
2881      <address>
2882      <postal>
2883      <street>675 Spruce Dr.</street>
2884      <city>Sunnyvale</city>
2885      <region>CA</region>
2886      <code>94086</code>
2887      <country>US</country></postal>
2888      <phone>+1.408.246.8253</phone>
2889      <email>dcrocker@bbiw.net</email></address> 
2890    </author>
2891    <author initials="P." surname="Overell" fullname="Paul Overell">
2892      <organization>THUS plc.</organization>
2893      <address>
2894      <postal>
2895      <street>1/2 Berkeley Square</street>
2896      <street>99 Berkely Street</street>
2897      <city>Glasgow</city>
2898      <code>G3 7HR</code>
2899      <country>UK</country></postal>
2900      <email>paul.overell@thus.net</email></address>
2901    </author>
2902    <date month="January" year="2008"/>
2903  </front>
2904  <seriesInfo name="STD" value="68"/>
2905  <seriesInfo name="RFC" value="5234"/>
2906</reference>
2907
2908</references>
2909
2910<references title="Informative References">
2911
2912<reference anchor="RFC1945">
2913  <front>
2914    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
2915    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2916      <organization>MIT, Laboratory for Computer Science</organization>
2917      <address><email>timbl@w3.org</email></address>
2918    </author>
2919    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2920      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2921      <address><email>fielding@ics.uci.edu</email></address>
2922    </author>
2923    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2924      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
2925      <address><email>frystyk@w3.org</email></address>
2926    </author>
2927    <date month="May" year="1996"/>
2928  </front>
2929  <seriesInfo name="RFC" value="1945"/>
2930</reference>
2931
2932<reference anchor="RFC2068">
2933  <front>
2934    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2935    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2936      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2937      <address><email>fielding@ics.uci.edu</email></address>
2938    </author>
2939    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2940      <organization>MIT Laboratory for Computer Science</organization>
2941      <address><email>jg@w3.org</email></address>
2942    </author>
2943    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2944      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2945      <address><email>mogul@wrl.dec.com</email></address>
2946    </author>
2947    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2948      <organization>MIT Laboratory for Computer Science</organization>
2949      <address><email>frystyk@w3.org</email></address>
2950    </author>
2951    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2952      <organization>MIT Laboratory for Computer Science</organization>
2953      <address><email>timbl@w3.org</email></address>
2954    </author>
2955    <date month="January" year="1997"/>
2956  </front>
2957  <seriesInfo name="RFC" value="2068"/>
2958</reference>
2959
2960<reference anchor="RFC2616">
2961  <front>
2962    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2963    <author initials="R." surname="Fielding" fullname="R. Fielding">
2964      <organization>University of California, Irvine</organization>
2965      <address><email>fielding@ics.uci.edu</email></address>
2966    </author>
2967    <author initials="J." surname="Gettys" fullname="J. Gettys">
2968      <organization>W3C</organization>
2969      <address><email>jg@w3.org</email></address>
2970    </author>
2971    <author initials="J." surname="Mogul" fullname="J. Mogul">
2972      <organization>Compaq Computer Corporation</organization>
2973      <address><email>mogul@wrl.dec.com</email></address>
2974    </author>
2975    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2976      <organization>MIT Laboratory for Computer Science</organization>
2977      <address><email>frystyk@w3.org</email></address>
2978    </author>
2979    <author initials="L." surname="Masinter" fullname="L. Masinter">
2980      <organization>Xerox Corporation</organization>
2981      <address><email>masinter@parc.xerox.com</email></address>
2982    </author>
2983    <author initials="P." surname="Leach" fullname="P. Leach">
2984      <organization>Microsoft Corporation</organization>
2985      <address><email>paulle@microsoft.com</email></address>
2986    </author>
2987    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2988      <organization>W3C</organization>
2989      <address><email>timbl@w3.org</email></address>
2990    </author>
2991    <date month="June" year="1999"/>
2992  </front>
2993  <seriesInfo name="RFC" value="2616"/>
2994</reference>
2995
2996<reference anchor="RFC2817">
2997  <front>
2998    <title>Upgrading to TLS Within HTTP/1.1</title>
2999    <author initials="R." surname="Khare" fullname="R. Khare">
3000      <organization>4K Associates / UC Irvine</organization>
3001      <address><email>rohit@4K-associates.com</email></address>
3002    </author>
3003    <author initials="S." surname="Lawrence" fullname="S. Lawrence">
3004      <organization>Agranat Systems, Inc.</organization>
3005      <address><email>lawrence@agranat.com</email></address>
3006    </author>
3007    <date year="2000" month="May"/>
3008  </front>
3009  <seriesInfo name="RFC" value="2817"/>
3010</reference>
3011
3012<reference anchor="RFC3864">
3013  <front>
3014    <title>Registration Procedures for Message Header Fields</title>
3015    <author initials="G." surname="Klyne" fullname="G. Klyne">
3016      <organization>Nine by Nine</organization>
3017      <address><email>GK-IETF@ninebynine.org</email></address>
3018    </author>
3019    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
3020      <organization>BEA Systems</organization>
3021      <address><email>mnot@pobox.com</email></address>
3022    </author>
3023    <author initials="J." surname="Mogul" fullname="J. Mogul">
3024      <organization>HP Labs</organization>
3025      <address><email>JeffMogul@acm.org</email></address>
3026    </author>
3027    <date year="2004" month="September"/>
3028  </front>
3029  <seriesInfo name="BCP" value="90"/>
3030  <seriesInfo name="RFC" value="3864"/>
3031</reference>
3032
3033<reference anchor="RFC5226">
3034  <front>
3035    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
3036    <author initials="T." surname="Narten" fullname="T. Narten">
3037      <organization>IBM</organization>
3038      <address><email>narten@us.ibm.com</email></address>
3039    </author>
3040    <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
3041      <organization>Google</organization>
3042      <address><email>Harald@Alvestrand.no</email></address>
3043    </author>
3044    <date year="2008" month="May"/>
3045  </front>
3046  <seriesInfo name="BCP" value="26"/>
3047  <seriesInfo name="RFC" value="5226"/>
3048</reference>
3049
3050<reference anchor="RFC5322">
3051  <front>
3052    <title>Internet Message Format</title>
3053    <author initials="P." surname="Resnick" fullname="P. Resnick">
3054      <organization>Qualcomm Incorporated</organization>
3055    </author>
3056    <date year="2008" month="October"/>
3057  </front> 
3058  <seriesInfo name="RFC" value="5322"/>
3059</reference>
3060
3061</references>
3062
3063<section title="Compatibility with Previous Versions" anchor="compatibility">
3064<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
3065<t>
3066   Clarified which error code should be used for inbound server failures
3067   (e.g. DNS failures). (<xref target="status.504"/>).
3068</t>
3069<t>
3070   201 (Created) had a race that required an Etag be sent when a resource is
3071   first created. (<xref target="status.201"/>).
3072</t>
3073<t>
3074   Rewrite of message transmission requirements to make it much harder
3075   for implementors to get it wrong, as the consequences of errors here
3076   can have significant impact on the Internet, and to deal with the
3077   following problems:
3078  <list style="numbers">
3079      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
3080         this was incorrectly placing a requirement on the behavior of
3081         an implementation of a future version of HTTP/1.x</t>
3082
3083      <t>Made it clear that user-agents should retry requests, not
3084         "clients" in general.</t>
3085
3086      <t>Converted requirements for clients to ignore unexpected 100
3087         (Continue) responses, and for proxies to forward 100 responses,
3088         into a general requirement for 1xx responses.</t>
3089
3090      <t>Modified some TCP-specific language, to make it clearer that
3091         non-TCP transports are possible for HTTP.</t>
3092
3093      <t>Require that the origin server MUST NOT wait for the request
3094         body before it sends a required 100 (Continue) response.</t>
3095
3096      <t>Allow, rather than require, a server to omit 100 (Continue) if
3097         it has already seen some of the request body.</t>
3098
3099      <t>Allow servers to defend against denial-of-service attacks and
3100         broken clients.</t>
3101  </list>
3102</t>
3103<t>
3104   This change adds the Expect header and 417 status code.
3105</t>
3106<t>
3107   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
3108   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
3109</t>
3110<t>
3111   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
3112   implemented in previous versions of this specification. See Section 19.6.1 of <xref target="RFC2068"/>.
3113</t>
3114</section>
3115
3116<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
3117<t>
3118  This document takes over the Status Code Registry, previously defined
3119  in Section 7.1 of <xref target="RFC2817"/>.
3120  (<xref target="status.code.registry"/>)
3121</t>
3122<t>
3123  Clarify definition of POST.
3124  (<xref target="POST"/>)
3125</t>
3126<t>
3127  Failed to consider that there are
3128  many other request methods that are safe to automatically redirect,
3129  and further that the user agent is able to make that determination
3130  based on the request method semantics.
3131  (Sections <xref format="counter" target="status.301"/>,
3132  <xref format="counter" target="status.302"/> and
3133  <xref format="counter" target="status.307"/>)
3134</t>
3135<t>
3136  Deprecate 305 Use Proxy status code, because user agents did not implement it.
3137  It used to indicate that the requested resource must be accessed through the
3138  proxy given by the Location field. The Location field gave the URI of the
3139  proxy. The recipient was expected to repeat this single request via the proxy.
3140  (<xref target="status.305"/>)
3141</t>
3142<t>
3143  Reclassify Allow header as response header, removing the option to
3144  specify it in a PUT request.
3145  Relax the server requirement on the contents of the Allow header and
3146  remove requirement on clients to always trust the header value.
3147  (<xref target="header.allow"/>)
3148</t>
3149<t>
3150  Correct syntax of Location header to allow fragment,
3151  as referred symbol wasn't what was expected, and add some
3152  clarifications as to when it would not be appropriate.
3153  (<xref target="header.location"/>)
3154</t>
3155<t>
3156  Allow Referer value of "about:blank" as alternative to not specifying it.
3157  (<xref target="header.referer"/>)
3158</t>
3159<t>
3160  In the description of the Server header, the Via field
3161  was described as a SHOULD. The requirement was and is stated
3162  correctly in the description of the Via header in Section 8.9 of <xref target="Part1"/>.
3163  (<xref target="header.server"/>)
3164</t>
3165</section>
3166
3167</section>
3168
3169<section title="Collected ABNF" anchor="collected.abnf">
3170<figure>
3171<artwork type="abnf" name="p2-semantics.parsed-abnf"><![CDATA[
3172Accept = <Accept, defined in [Part3], Section 5.1>
3173Accept-Charset = <Accept-Charset, defined in [Part3], Section 5.2>
3174Accept-Encoding = <Accept-Encoding, defined in [Part3], Section 5.3>
3175Accept-Language = <Accept-Language, defined in [Part3], Section 5.4>
3176Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 5.1>
3177Age = <Age, defined in [Part6], Section 3.1>
3178Allow = "Allow:" OWS Allow-v
3179Allow-v = [ ( "," / Method ) *( OWS "," [ OWS Method ] ) ]
3180Authorization = <Authorization, defined in [Part7], Section 3.1>
3181
3182ETag = <ETag, defined in [Part4], Section 6.1>
3183Expect = "Expect:" OWS Expect-v
3184Expect-v = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )
3185
3186From = "From:" OWS From-v
3187From-v = mailbox
3188
3189HTTP-date = <HTTP-date, defined in [Part1], Section 3.2>
3190Host = <Host, defined in [Part1], Section 2.1>
3191
3192If-Match = <If-Match, defined in [Part4], Section 6.2>
3193If-Modified-Since =
3194 <If-Modified-Since, defined in [Part4], Section 6.3>
3195If-None-Match = <If-None-Match, defined in [Part4], Section 6.4>
3196If-Range = <If-Range, defined in [Part5], Section 5.3>
3197If-Unmodified-Since =
3198 <If-Unmodified-Since, defined in [Part4], Section 6.5>
3199
3200Location = "Location:" OWS Location-v
3201Location-v = absolute-URI [ "#" fragment ]
3202
3203Max-Forwards = "Max-Forwards:" OWS Max-Forwards-v
3204Max-Forwards-v = 1*DIGIT
3205Method = %x4F.50.54.49.4F.4E.53 ; OPTIONS
3206 / %x47.45.54 ; GET
3207 / %x48.45.41.44 ; HEAD
3208 / %x50.4F.53.54 ; POST
3209 / %x50.55.54 ; PUT
3210 / %x44.45.4C.45.54.45 ; DELETE
3211 / %x54.52.41.43.45 ; TRACE
3212 / %x43.4F.4E.4E.45.43.54 ; CONNECT
3213 / extension-method
3214
3215OWS = <OWS, defined in [Part1], Section 1.2.2>
3216
3217Proxy-Authenticate =
3218 <Proxy-Authenticate, defined in [Part7], Section 3.2>
3219Proxy-Authorization =
3220 <Proxy-Authorization, defined in [Part7], Section 3.3>
3221
3222RWS = <RWS, defined in [Part1], Section 1.2.2>
3223Range = <Range, defined in [Part5], Section 5.4>
3224Reason-Phrase = *( WSP / VCHAR / obs-text )
3225Referer = "Referer:" OWS Referer-v
3226Referer-v = absolute-URI / partial-URI
3227Retry-After = "Retry-After:" OWS Retry-After-v
3228Retry-After-v = HTTP-date / delta-seconds
3229
3230Server = "Server:" OWS Server-v
3231Server-v = product *( RWS ( product / comment ) )
3232Status-Code = "100" / "101" / "200" / "201" / "202" / "203" / "204" /
3233 "205" / "206" / "300" / "301" / "302" / "303" / "304" / "305" /
3234 "307" / "400" / "401" / "402" / "403" / "404" / "405" / "406" /
3235 "407" / "408" / "409" / "410" / "411" / "412" / "413" / "414" /
3236 "415" / "416" / "417" / "500" / "501" / "502" / "503" / "504" /
3237 "505" / extension-code
3238
3239TE = <TE, defined in [Part1], Section 8.8>
3240
3241User-Agent = "User-Agent:" OWS User-Agent-v
3242User-Agent-v = product *( RWS ( product / comment ) )
3243
3244Vary = <Vary, defined in [Part6], Section 3.5>
3245
3246WWW-Authenticate =
3247 <WWW-Authenticate, defined in [Part7], Section 3.4>
3248
3249absolute-URI = <absolute-URI, defined in [Part1], Section 2.1>
3250
3251comment = <comment, defined in [Part1], Section 1.2.2>
3252
3253delta-seconds = 1*DIGIT
3254
3255expect-params = ";" token [ "=" ( token / quoted-string ) ]
3256expectation = "100-continue" / expectation-extension
3257expectation-extension = token [ "=" ( token / quoted-string )
3258 *expect-params ]
3259extension-code = 3DIGIT
3260extension-method = token
3261
3262fragment = <fragment, defined in [Part1], Section 2.1>
3263
3264mailbox = <mailbox, defined in [RFC5322], Section 3.4>
3265
3266obs-text = <obs-text, defined in [Part1], Section 1.2.2>
3267
3268partial-URI = <partial-URI, defined in [Part1], Section 2.1>
3269product = <product, defined in [Part1], Section 3.4>
3270
3271quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
3272
3273request-header = Accept / Accept-Charset / Accept-Encoding /
3274 Accept-Language / Authorization / Expect / From / Host / If-Match /
3275 If-Modified-Since / If-None-Match / If-Range / If-Unmodified-Since /
3276 Max-Forwards / Proxy-Authorization / Range / Referer / TE /
3277 User-Agent
3278response-header = Accept-Ranges / Age / Allow / ETag / Location /
3279 Proxy-Authenticate / Retry-After / Server / Vary / WWW-Authenticate
3280
3281token = <token, defined in [Part1], Section 1.2.2>
3282]]></artwork>
3283</figure>
3284<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
3285; Reason-Phrase defined but not used
3286; Status-Code defined but not used
3287; request-header defined but not used
3288; response-header defined but not used
3289]]></artwork></figure></section>
3290
3291<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
3292
3293<section title="Since RFC2616">
3294<t>
3295  Extracted relevant partitions from <xref target="RFC2616"/>.
3296</t>
3297</section>
3298
3299<section title="Since draft-ietf-httpbis-p2-semantics-00">
3300<t>
3301  Closed issues:
3302  <list style="symbols"> 
3303    <t>
3304      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/5"/>:
3305      "Via is a MUST"
3306      (<eref target="http://purl.org/NET/http-errata#via-must"/>)
3307    </t>
3308    <t>
3309      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/6"/>:
3310      "Fragments allowed in Location"
3311      (<eref target="http://purl.org/NET/http-errata#location-fragments"/>)
3312    </t>
3313    <t>
3314      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/10"/>:
3315      "Safe Methods vs Redirection"
3316      (<eref target="http://purl.org/NET/http-errata#saferedirect"/>)
3317    </t>
3318    <t>
3319      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/17"/>:
3320      "Revise description of the POST method"
3321      (<eref target="http://purl.org/NET/http-errata#post"/>)
3322    </t>
3323    <t>
3324      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
3325      "Normative and Informative references"
3326    </t>
3327    <t>
3328      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
3329      "RFC2606 Compliance"
3330    </t>
3331    <t>
3332      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
3333      "Informative references"
3334    </t>
3335    <t>
3336      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/84"/>:
3337      "Redundant cross-references"
3338    </t>
3339  </list>
3340</t>
3341<t>
3342  Other changes:
3343  <list style="symbols"> 
3344    <t>
3345      Move definitions of 304 and 412 condition codes to <xref target="Part4"/>
3346    </t>
3347  </list>
3348</t>
3349</section>
3350
3351<section title="Since draft-ietf-httpbis-p2-semantics-01">
3352<t>
3353  Closed issues:
3354  <list style="symbols"> 
3355    <t>
3356      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/21"/>:
3357      "PUT side effects"
3358    </t>
3359    <t>
3360      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/91"/>:
3361      "Duplicate Host header requirements"
3362    </t>
3363  </list>
3364</t>
3365<t>
3366  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3367  <list style="symbols"> 
3368    <t>
3369      Move "Product Tokens" section (back) into Part 1, as "token" is used
3370      in the definition of the Upgrade header.
3371    </t>
3372    <t>
3373      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3374    </t>
3375    <t>
3376      Copy definition of delta-seconds from Part6 instead of referencing it.
3377    </t>
3378  </list>
3379</t>
3380</section>
3381
3382<section title="Since draft-ietf-httpbis-p2-semantics-02" anchor="changes.since.02">
3383<t>
3384  Closed issues:
3385  <list style="symbols"> 
3386    <t>
3387      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/24"/>:
3388      "Requiring Allow in 405 responses"
3389    </t>
3390    <t>
3391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/59"/>:
3392      "Status Code Registry"
3393    </t>
3394    <t>
3395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/61"/>:
3396      "Redirection vs. Location"
3397    </t>
3398    <t>
3399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/70"/>:
3400      "Cacheability of 303 response"
3401    </t>
3402    <t>
3403      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/76"/>:
3404      "305 Use Proxy"
3405    </t>
3406    <t>
3407      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
3408      "Classification for Allow header"
3409    </t>
3410    <t>
3411      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/112"/>:
3412      "PUT - 'store under' vs 'store at'"
3413    </t>
3414  </list>
3415</t>
3416<t>
3417  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
3418  <list style="symbols"> 
3419    <t>
3420      Reference RFC 3984, and update header registrations for headers defined
3421      in this document.
3422    </t>
3423  </list>
3424</t>
3425<t>
3426  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3427  <list style="symbols"> 
3428    <t>
3429      Replace string literals when the string really is case-sensitive (method).
3430    </t>
3431  </list>
3432</t>
3433</section>
3434
3435<section title="Since draft-ietf-httpbis-p2-semantics-03" anchor="changes.since.03">
3436<t>
3437  Closed issues:
3438  <list style="symbols"> 
3439    <t>
3440      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/98"/>:
3441      "OPTIONS request bodies"
3442    </t>
3443    <t>
3444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/119"/>:
3445      "Description of CONNECT should refer to RFC2817"
3446    </t>
3447    <t>
3448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/125"/>:
3449      "Location Content-Location reference request/response mixup"
3450    </t>
3451  </list>
3452</t>
3453<t>
3454  Ongoing work on Method Registry (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/72"/>):
3455  <list style="symbols"> 
3456    <t>
3457      Added initial proposal for registration process, plus initial
3458      content (non-HTTP/1.1 methods to be added by a separate specification).
3459    </t>
3460  </list>
3461</t>
3462</section>
3463
3464<section title="Since draft-ietf-httpbis-p2-semantics-04" anchor="changes.since.04">
3465<t>
3466  Closed issues:
3467  <list style="symbols"> 
3468    <t>
3469      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/103"/>:
3470      "Content-*"
3471    </t>
3472    <t>
3473      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
3474      "RFC 2822 is updated by RFC 5322"
3475    </t>
3476  </list>
3477</t>
3478<t>
3479  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3480  <list style="symbols"> 
3481    <t>
3482      Use "/" instead of "|" for alternatives.
3483    </t>
3484    <t>
3485      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3486      whitespace ("OWS") and required whitespace ("RWS").
3487    </t>
3488    <t>
3489      Rewrite ABNFs to spell out whitespace rules, factor out
3490      header value format definitions.
3491    </t>
3492  </list>
3493</t>
3494</section>
3495
3496<section title="Since draft-ietf-httpbis-p2-semantics-05" anchor="changes.since.05">
3497<t>
3498  Closed issues:
3499  <list style="symbols"> 
3500    <t>
3501      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/94"/>:
3502      "Reason-Phrase BNF"
3503    </t>
3504  </list>
3505</t>
3506<t>
3507  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3508  <list style="symbols"> 
3509    <t>
3510      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
3511    </t>
3512  </list>
3513</t>
3514</section>
3515
3516<section title="Since draft-ietf-httpbis-p2-semantics-06" anchor="changes.since.06">
3517<t>
3518  Closed issues:
3519  <list style="symbols"> 
3520    <t>
3521      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/144"/>:
3522      "Clarify when Referer is sent"
3523    </t>
3524    <t>
3525      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/164"/>:
3526      "status codes vs methods"
3527    </t>
3528    <t>
3529      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/170"/>:
3530      "Do not require "updates" relation for specs that register status codes or method names"
3531    </t>
3532  </list>
3533</t>
3534</section>
3535
3536</section>
3537
3538</back>
3539</rfc>
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