source: draft-ietf-httpbis/05/draft-ietf-httpbis-p2-semantics-05.xml @ 559

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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="full3978" docName="draft-ietf-httpbis-p2-semantics-05">
19<front>
20
21  <title abbrev="HTTP/1.1, Part 2">HTTP/1.1, part 2: Message Semantics</title>
22
23  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
24    <organization abbrev="Day Software">Day Software</organization>
25    <address>
26      <postal>
27        <street>23 Corporate Plaza DR, Suite 280</street>
28        <city>Newport Beach</city>
29        <region>CA</region>
30        <code>92660</code>
31        <country>USA</country>
32      </postal>
33      <phone>+1-949-706-5300</phone>
34      <facsimile>+1-949-706-5305</facsimile>
35      <email>fielding@gbiv.com</email>
36      <uri>http://roy.gbiv.com/</uri>
37    </address>
38  </author>
39
40  <author initials="J." surname="Gettys" fullname="Jim Gettys">
41    <organization>One Laptop per Child</organization>
42    <address>
43      <postal>
44        <street>21 Oak Knoll Road</street>
45        <city>Carlisle</city>
46        <region>MA</region>
47        <code>01741</code>
48        <country>USA</country>
49      </postal>
50      <email>jg@laptop.org</email>
51      <uri>http://www.laptop.org/</uri>
52    </address>
53  </author>
54 
55  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
56    <organization abbrev="HP">Hewlett-Packard Company</organization>
57    <address>
58      <postal>
59        <street>HP Labs, Large Scale Systems Group</street>
60        <street>1501 Page Mill Road, MS 1177</street>
61        <city>Palo Alto</city>
62        <region>CA</region>
63        <code>94304</code>
64        <country>USA</country>
65      </postal>
66      <email>JeffMogul@acm.org</email>
67    </address>
68  </author>
69
70  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
71    <organization abbrev="Microsoft">Microsoft Corporation</organization>
72    <address>
73      <postal>
74        <street>1 Microsoft Way</street>
75        <city>Redmond</city>
76        <region>WA</region>
77        <code>98052</code>
78        <country>USA</country>
79      </postal>
80      <email>henrikn@microsoft.com</email>
81    </address>
82  </author>
83
84  <author initials="L." surname="Masinter" fullname="Larry Masinter">
85    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
86    <address>
87      <postal>
88        <street>345 Park Ave</street>
89        <city>San Jose</city>
90        <region>CA</region>
91        <code>95110</code>
92        <country>USA</country>
93      </postal>
94      <email>LMM@acm.org</email>
95      <uri>http://larry.masinter.net/</uri>
96    </address>
97  </author>
98 
99  <author initials="P." surname="Leach" fullname="Paul J. Leach">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107      </postal>
108      <email>paulle@microsoft.com</email>
109    </address>
110  </author>
111   
112  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
113    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
114    <address>
115      <postal>
116        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
117        <street>The Stata Center, Building 32</street>
118        <street>32 Vassar Street</street>
119        <city>Cambridge</city>
120        <region>MA</region>
121        <code>02139</code>
122        <country>USA</country>
123      </postal>
124      <email>timbl@w3.org</email>
125      <uri>http://www.w3.org/People/Berners-Lee/</uri>
126    </address>
127  </author>
128
129  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
130    <organization abbrev="W3C">World Wide Web Consortium</organization>
131    <address>
132      <postal>
133        <street>W3C / ERCIM</street>
134        <street>2004, rte des Lucioles</street>
135        <city>Sophia-Antipolis</city>
136        <region>AM</region>
137        <code>06902</code>
138        <country>France</country>
139      </postal>
140      <email>ylafon@w3.org</email>
141      <uri>http://www.raubacapeu.net/people/yves/</uri>
142    </address>
143  </author>
144
145  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
146    <organization abbrev="greenbytes">greenbytes GmbH</organization>
147    <address>
148      <postal>
149        <street>Hafenweg 16</street>
150        <city>Muenster</city><region>NW</region><code>48155</code>
151        <country>Germany</country>
152      </postal>
153      <phone>+49 251 2807760</phone>   
154      <facsimile>+49 251 2807761</facsimile>   
155      <email>julian.reschke@greenbytes.de</email>       
156      <uri>http://greenbytes.de/tech/webdav/</uri>     
157    </address>
158  </author>
159
160  <date month="November" year="2008" day="16"/>
161
162<abstract>
163<t>
164   The Hypertext Transfer Protocol (HTTP) is an application-level
165   protocol for distributed, collaborative, hypermedia information
166   systems. HTTP has been in use by the World Wide Web global information
167   initiative since 1990. This document is Part 2 of the seven-part specification
168   that defines the protocol referred to as "HTTP/1.1" and, taken together,
169   obsoletes RFC 2616.  Part 2 defines the semantics of HTTP messages
170   as expressed by request methods, request-header fields, response status codes,
171   and response-header fields.
172</t>
173</abstract>
174
175<note title="Editorial Note (To be removed by RFC Editor)">
176  <t>
177    Discussion of this draft should take place on the HTTPBIS working group
178    mailing list (ietf-http-wg@w3.org). The current issues list is
179    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
180    and related documents (including fancy diffs) can be found at
181    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
182  </t>
183  <t>
184    The changes in this draft are summarized in <xref target="changes.since.04"/>.
185  </t>
186</note>
187</front>
188<middle>
189<section title="Introduction" anchor="introduction">
190<t>
191   This document defines HTTP/1.1 request and response semantics.  Each HTTP
192   message, as defined in <xref target="Part1"/>, is in the form of either a request or
193   a response.  An HTTP server listens on a connection for HTTP requests and
194   responds to each request, in the order received on that connection, with
195   one or more HTTP response messages.  This document defines the commonly
196   agreed upon semantics of the HTTP uniform interface, the intentions defined
197   by each request method, and the various response messages that might be
198   expected as a result of applying that method for the requested resource.
199</t>
200<t>
201   This document is currently disorganized in order to minimize the changes
202   between drafts and enable reviewers to see the smaller errata changes.
203   The next draft will reorganize the sections to better reflect the content.
204   In particular, the sections will be ordered according to the typical
205   processing of an HTTP request message (after message parsing): resource
206   mapping, general header fields, methods, request modifiers, response
207   status, and resource metadata.  The current mess reflects how widely
208   dispersed these topics and associated requirements had become in
209   <xref target="RFC2616"/>.
210</t>
211
212<section title="Requirements" anchor="intro.requirements">
213<t>
214   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216   document are to be interpreted as described in <xref target="RFC2119"/>.
217</t>
218<t>
219   An implementation is not compliant if it fails to satisfy one or more
220   of the MUST or REQUIRED level requirements for the protocols it
221   implements. An implementation that satisfies all the MUST or REQUIRED
222   level and all the SHOULD level requirements for its protocols is said
223   to be "unconditionally compliant"; one that satisfies all the MUST
224   level requirements but not all the SHOULD level requirements for its
225   protocols is said to be "conditionally compliant."
226</t>
227</section>
228</section>
229
230<section title="Notational Conventions and Generic Grammar" anchor="notation">
231 
232 
233 
234 
235 
236 
237<t>
238  This specification uses the ABNF syntax defined in Section 2.1 of <xref target="Part1"/> and
239  the core rules defined in Section 2.2 of <xref target="Part1"/>:
240</t>
241<figure><artwork type="abnf2616"><![CDATA[
242  DIGIT         = <DIGIT, defined in [Part1], Section 2.2>
243]]></artwork></figure>
244<figure><artwork type="abnf2616"><![CDATA[
245  comment       = <comment, defined in [Part1], Section 2.2>
246  quoted-string = <quoted-string, defined in [Part1], Section 2.2>
247  token         = <token, defined in [Part1], Section 2.2>
248  OWS           = <OWS, defined in [Part1], Section 2.2>
249  RWS           = <RWS, defined in [Part1], Section 2.2>
250]]></artwork></figure>
251<t anchor="abnf.dependencies">
252 
253 
254 
255 
256 
257 
258 
259 
260 
261 
262 
263 
264 
265 
266 
267 
268 
269 
270 
271 
272 
273 
274 
275 
276 
277  The ABNF rules below are defined in other parts:
278</t>
279<figure><!--Part1--><artwork type="abnf2616"><![CDATA[
280  absolute-URI  = <absolute-URI, defined in [Part1], Section 3.2>
281  fragment      = <fragment, defined in [Part1], Section 3.2>
282  Host          = <Host, defined in [Part1], Section 3.2>
283  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
284  product       = <product, defined in [Part1], Section 3.5>
285  relativeURI   = <relativeURI, defined in [Part1], Section 3.2>
286  TE            = <TE, defined in [Part1], Section 8.8>
287]]></artwork></figure>
288<figure><!--Part3--><artwork type="abnf2616"><![CDATA[
289  Accept        = <Accept, defined in [Part3], Section 6.1>
290  Accept-Charset =
291             <Accept-Charset, defined in [Part3], Section 6.2>
292  Accept-Encoding =
293             <Accept-Encoding, defined in [Part3], Section 6.3>
294  Accept-Language =
295             <Accept-Language, defined in [Part3], Section 6.4>
296]]></artwork></figure>
297<figure><!--Part4--><artwork type="abnf2616"><![CDATA[
298  ETag          = <ETag, defined in [Part4], Section 7.1>
299  If-Match      = <If-Match, defined in [Part4], Section 7.2>
300  If-Modified-Since =
301             <If-Modified-Since, defined in [Part4], Section 7.3>
302  If-None-Match = <If-None-Match, defined in [Part4], Section 7.4>
303  If-Unmodified-Since =
304             <If-Unmodified-Since, defined in [Part4], Section 7.5>
305]]></artwork></figure>
306<figure><!--Part5--><artwork type="abnf2616"><![CDATA[
307  Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 6.1>
308  If-Range      = <If-Range, defined in [Part5], Section 6.3>
309  Range         = <Range, defined in [Part5], Section 6.4>
310]]></artwork></figure>
311<figure><!--Part6--><artwork type="abnf2616"><![CDATA[
312  Age           = <Age, defined in [Part6], Section 16.1>
313  Vary          = <Vary, defined in [Part6], Section 16.5>
314]]></artwork><!--Part7--></figure>
315<figure><artwork type="abnf2616"><![CDATA[
316  Authorization = <Authorization, defined in [Part7], Section 4.1>
317  Proxy-Authenticate =
318             <Proxy-Authenticate, defined in [Part7], Section 4.2>
319  Proxy-Authorization =
320             <Proxy-Authorization, defined in [Part7], Section 4.3>
321  WWW-Authenticate =
322             <WWW-Authenticate, defined in [Part7], Section 4.4>
323]]></artwork></figure>
324</section>
325
326<section title="Method" anchor="method">
327 
328 
329<t>
330   The Method  token indicates the method to be performed on the
331   resource identified by the Request-URI. The method is case-sensitive.
332</t>
333<figure><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/><artwork type="abnf2616"><![CDATA[
334  Method         = %x4F.50.54.49.4F.4E.53   ; "OPTIONS", Section 8.2
335                 / %x47.45.54               ; "GET", Section 8.3
336                 / %x48.45.41.44            ; "HEAD", Section 8.4
337                 / %x50.4F.53.54            ; "POST", Section 8.5
338                 / %x50.55.54               ; "PUT", Section 8.6
339                 / %x44.45.4C.45.54.45      ; "DELETE", Section 8.7
340                 / %x54.52.41.43.45         ; "TRACE", Section 8.8
341                 / %x43.4F.4E.4E.45.43.54   ; "CONNECT", Section 8.9
342                 / extension-method
343  extension-method = token
344]]></artwork></figure>
345<t>
346   The list of methods allowed by a resource can be specified in an
347   Allow header field (<xref target="header.allow"/>). The return code of the response
348   always notifies the client whether a method is currently allowed on a
349   resource, since the set of allowed methods can change dynamically. An
350   origin server SHOULD return the status code 405 (Method Not Allowed)
351   if the method is known by the origin server but not allowed for the
352   requested resource, and 501 (Not Implemented) if the method is
353   unrecognized or not implemented by the origin server. The methods GET
354   and HEAD MUST be supported by all general-purpose servers. All other
355   methods are OPTIONAL; however, if the above methods are implemented,
356   they MUST be implemented with the same semantics as those specified
357   in <xref target="method.definitions"/>.
358</t>
359
360<section title="Method Registry" anchor="method.registry">
361<t>
362  The HTTP Method Registry defines the name space for the Method token in the
363  Request line of an HTTP request.
364</t>
365<t>
366  Registrations MUST include the following fields:
367  <list style="symbols">
368    <t>Method Name (see <xref target="method"/>)</t>
369    <t>Safe ("yes" or "no", see <xref target="safe.methods"/>)</t>
370    <t>Pointer to specification text</t>
371  </list>
372</t>
373<t>
374  Values to be added to this name space are subject to IETF review
375  (<xref target="RFC5226"/>, Section 4.1).  Any document registering
376  new method names should be traceable through statuses of either 'Obsoletes'
377  or 'Updates' to this document.
378</t>
379<t>
380  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-methods"/>.
381</t>
382</section>
383</section>
384
385<section title="Request Header Fields" anchor="request.header.fields">
386 
387 
388<t>
389   The request-header fields allow the client to pass additional
390   information about the request, and about the client itself, to the
391   server. These fields act as request modifiers, with semantics
392   equivalent to the parameters on a programming language method
393   invocation.
394</t>
395<figure><iref primary="true" item="Grammar" subitem="request-header"/><artwork type="abnf2616"><![CDATA[
396  request-header = Accept                   ; [Part3], Section 6.1
397                 / Accept-Charset           ; [Part3], Section 6.2
398                 / Accept-Encoding          ; [Part3], Section 6.3
399                 / Accept-Language          ; [Part3], Section 6.4
400                 / Authorization            ; [Part7], Section 4.1
401                 / Expect                   ; Section 10.2
402                 / From                     ; Section 10.3
403                 / Host                     ; [Part1], Section 8.4
404                 / If-Match                 ; [Part4], Section 7.2
405                 / If-Modified-Since        ; [Part4], Section 7.3
406                 / If-None-Match            ; [Part4], Section 7.4
407                 / If-Range                 ; [Part5], Section 6.3
408                 / If-Unmodified-Since      ; [Part4], Section 7.5
409                 / Max-Forwards             ; Section 10.5
410                 / Proxy-Authorization      ; [Part7], Section 4.3
411                 / Range                    ; [Part5], Section 6.4
412                 / Referer                  ; Section 10.6
413                 / TE                       ; [Part1], Section 8.8
414                 / User-Agent               ; Section 10.9
415]]></artwork></figure>
416<t>
417   Request-header field names can be extended reliably only in
418   combination with a change in the protocol version. However, new or
419   experimental header fields MAY be given the semantics of request-header
420   fields if all parties in the communication recognize them to
421   be request-header fields. Unrecognized header fields are treated as
422   entity-header fields.
423</t>
424</section>
425
426<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
427 
428 
429<t>
430   The Status-Code element is a 3-digit integer result code of the
431   attempt to understand and satisfy the request. The status codes listed
432   below are defined in <xref target="status.codes"/>.
433   The Reason-Phrase is intended to give a short
434   textual description of the Status-Code. The Status-Code is intended
435   for use by automata and the Reason-Phrase is intended for the human
436   user. The client is not required to examine or display the Reason-Phrase.
437</t>
438<t> 
439   The individual values of the numeric status codes defined for
440   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
441   presented below. The reason phrases listed here are only
442   recommendations -- they MAY be replaced by local equivalents without
443   affecting the protocol.
444</t>
445<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[
446  Status-Code    =
447         "100"  ; Section 9.1.1: Continue
448       / "101"  ; Section 9.1.2: Switching Protocols
449       / "200"  ; Section 9.2.1: OK
450       / "201"  ; Section 9.2.2: Created
451       / "202"  ; Section 9.2.3: Accepted
452       / "203"  ; Section 9.2.4: Non-Authoritative Information
453       / "204"  ; Section 9.2.5: No Content
454       / "205"  ; Section 9.2.6: Reset Content
455       / "206"  ; Section 9.2.7: Partial Content
456       / "300"  ; Section 9.3.1: Multiple Choices
457       / "301"  ; Section 9.3.2: Moved Permanently
458       / "302"  ; Section 9.3.3: Found
459       / "303"  ; Section 9.3.4: See Other
460       / "304"  ; Section 9.3.5: Not Modified
461       / "305"  ; Section 9.3.6: Use Proxy
462       / "307"  ; Section 9.3.8: Temporary Redirect
463       / "400"  ; Section 9.4.1: Bad Request
464       / "401"  ; Section 9.4.2: Unauthorized
465       / "402"  ; Section 9.4.3: Payment Required
466       / "403"  ; Section 9.4.4: Forbidden
467       / "404"  ; Section 9.4.5: Not Found
468       / "405"  ; Section 9.4.6: Method Not Allowed
469       / "406"  ; Section 9.4.7: Not Acceptable
470       / "407"  ; Section 9.4.8: Proxy Authentication Required
471       / "408"  ; Section 9.4.9: Request Time-out
472       / "409"  ; Section 9.4.10: Conflict
473       / "410"  ; Section 9.4.11: Gone
474       / "411"  ; Section 9.4.12: Length Required
475       / "412"  ; Section 9.4.13: Precondition Failed
476       / "413"  ; Section 9.4.14: Request Entity Too Large
477       / "414"  ; Section 9.4.15: Request-URI Too Large
478       / "415"  ; Section 9.4.16: Unsupported Media Type
479       / "416"  ; Section 9.4.17: Requested range not satisfiable
480       / "417"  ; Section 9.4.18: Expectation Failed
481       / "500"  ; Section 9.5.1: Internal Server Error
482       / "501"  ; Section 9.5.2: Not Implemented
483       / "502"  ; Section 9.5.3: Bad Gateway
484       / "503"  ; Section 9.5.4: Service Unavailable
485       / "504"  ; Section 9.5.5: Gateway Time-out
486       / "505"  ; Section 9.5.6: HTTP Version not supported
487       / extension-code
488
489  extension-code = 3DIGIT
490  Reason-Phrase  = *<TEXT, excluding CR, LF>
491]]></artwork></figure>
492<t>
493   HTTP status codes are extensible. HTTP applications are not required
494   to understand the meaning of all registered status codes, though such
495   understanding is obviously desirable. However, applications MUST
496   understand the class of any status code, as indicated by the first
497   digit, and treat any unrecognized response as being equivalent to the
498   x00 status code of that class, with the exception that an
499   unrecognized response MUST NOT be cached. For example, if an
500   unrecognized status code of 431 is received by the client, it can
501   safely assume that there was something wrong with its request and
502   treat the response as if it had received a 400 status code. In such
503   cases, user agents SHOULD present to the user the entity returned
504   with the response, since that entity is likely to include human-readable
505   information which will explain the unusual status.
506</t>
507
508<section title="Status Code Registry" anchor="status.code.registry">
509<t>
510  The HTTP Status Code Registry defines the name space for the Status-Code
511  token in the Status line of an HTTP response.
512</t>
513<t>
514  Values to be added to this name space are subject to IETF review
515  (<xref target="RFC5226"/>, Section 4.1).  Any document registering
516  new status codes should be traceable through statuses of either 'Obsoletes'
517  or 'Updates' to this document.
518</t>
519<t>
520  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-status-codes"/>.
521</t>
522</section>
523
524</section>
525
526<section title="Response Header Fields" anchor="response.header.fields">
527 
528<t>
529   The response-header fields allow the server to pass additional
530   information about the response which cannot be placed in the Status-Line.
531   These header fields give information about the server and about
532   further access to the resource identified by the Request-URI.
533</t>
534<figure><iref primary="true" item="Grammar" subitem="response-header"/><artwork type="abnf2616"><![CDATA[
535  response-header = Accept-Ranges           ; [Part5], Section 6.1
536                  / Age                     ; [Part6], Section 16.1
537                  / Allow                   ; Section 10.1
538                  / ETag                    ; [Part4], Section 7.1
539                  / Location                ; Section 10.4
540                  / Proxy-Authenticate      ; [Part7], Section 4.2
541                  / Retry-After             ; Section 10.7
542                  / Server                  ; Section 10.8
543                  / Vary                    ; [Part6], Section 16.5
544                  / WWW-Authenticate        ; [Part7], Section 4.4
545]]></artwork></figure>
546<t>
547   Response-header field names can be extended reliably only in
548   combination with a change in the protocol version. However, new or
549   experimental header fields MAY be given the semantics of response-header
550   fields if all parties in the communication recognize them to
551   be response-header fields. Unrecognized header fields are treated as
552   entity-header fields.
553</t>
554</section>
555
556<section title="Entity" anchor="entity">
557<t>
558   Request and Response messages MAY transfer an entity if not otherwise
559   restricted by the request method or response status code. An entity
560   consists of entity-header fields and an entity-body, although some
561   responses will only include the entity-headers. HTTP entity-body and
562   entity-header fields are defined in <xref target="Part3"/>.
563</t>
564<t>
565   An entity-body is only present in a message when a message-body is
566   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
567   from the message-body by decoding any Transfer-Encoding that might
568   have been applied to ensure safe and proper transfer of the message.
569</t>
570</section>
571
572
573<section title="Method Definitions" anchor="method.definitions">
574<t>
575   The set of common methods for HTTP/1.1 is defined below. Although
576   this set can be expanded, additional methods cannot be assumed to
577   share the same semantics for separately extended clients and servers.
578</t>
579
580<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
581
582<section title="Safe Methods" anchor="safe.methods">
583<iref item="Safe Methods" primary="true"/>
584<t>
585   Implementors should be aware that the software represents the user in
586   their interactions over the Internet, and should be careful to allow
587   the user to be aware of any actions they might take which may have an
588   unexpected significance to themselves or others.
589</t>
590<t>
591   In particular, the convention has been established that the GET and
592   HEAD methods SHOULD NOT  have the significance of taking an action
593   other than retrieval. These methods ought to be considered "safe".
594   This allows user agents to represent other methods, such as POST, PUT
595   and DELETE, in a special way, so that the user is made aware of the
596   fact that a possibly unsafe action is being requested.
597</t>
598<t>
599   Naturally, it is not possible to ensure that the server does not
600   generate side-effects as a result of performing a GET request; in
601   fact, some dynamic resources consider that a feature. The important
602   distinction here is that the user did not request the side-effects,
603   so therefore cannot be held accountable for them.
604</t>
605</section>
606
607<section title="Idempotent Methods" anchor="idempotent.methods">
608<iref item="Idempotent Methods" primary="true"/>
609<t>
610   Methods can also have the property of "idempotence" in that (aside
611   from error or expiration issues) the side-effects of N &gt; 0 identical
612   requests is the same as for a single request. The methods GET, HEAD,
613   PUT and DELETE share this property. Also, the methods OPTIONS and
614   TRACE SHOULD NOT  have side effects, and so are inherently idempotent.
615</t>
616<t>
617   However, it is possible that a sequence of several requests is non-idempotent,
618   even if all of the methods executed in that sequence are
619   idempotent. (A sequence is idempotent if a single execution of the
620   entire sequence always yields a result that is not changed by a
621   reexecution of all, or part, of that sequence.) For example, a
622   sequence is non-idempotent if its result depends on a value that is
623   later modified in the same sequence.
624</t>
625<t>
626   A sequence that never has side effects is idempotent, by definition
627   (provided that no concurrent operations are being executed on the
628   same set of resources).
629</t>
630</section>
631</section>
632
633<section title="OPTIONS" anchor="OPTIONS">
634 
635  <iref primary="true" item="OPTIONS method"/>
636  <iref primary="true" item="Methods" subitem="OPTIONS"/>
637<t>
638   The OPTIONS method represents a request for information about the
639   communication options available on the request/response chain
640   identified by the Request-URI. This method allows the client to
641   determine the options and/or requirements associated with a resource,
642   or the capabilities of a server, without implying a resource action
643   or initiating a resource retrieval.
644</t>
645<t>
646   Responses to this method are not cacheable.
647</t>
648<t>
649   If the OPTIONS request includes an entity-body (as indicated by the
650   presence of Content-Length or Transfer-Encoding), then the media type
651   MUST be indicated by a Content-Type field. Although this
652   specification does not define any use for such a body, future
653   extensions to HTTP might use the OPTIONS body to make more detailed
654   queries on the server.
655</t>
656<t>
657   If the Request-URI is an asterisk ("*"), the OPTIONS request is
658   intended to apply to the server in general rather than to a specific
659   resource. Since a server's communication options typically depend on
660   the resource, the "*" request is only useful as a "ping" or "no-op"
661   type of method; it does nothing beyond allowing the client to test
662   the capabilities of the server. For example, this can be used to test
663   a proxy for HTTP/1.1 compliance (or lack thereof).
664</t>
665<t>
666   If the Request-URI is not an asterisk, the OPTIONS request applies
667   only to the options that are available when communicating with that
668   resource.
669</t>
670<t>
671   A 200 response SHOULD include any header fields that indicate
672   optional features implemented by the server and applicable to that
673   resource (e.g., Allow), possibly including extensions not defined by
674   this specification. The response body, if any, SHOULD also include
675   information about the communication options. The format for such a
676   body is not defined by this specification, but might be defined by
677   future extensions to HTTP. Content negotiation MAY be used to select
678   the appropriate response format. If no response body is included, the
679   response MUST include a Content-Length field with a field-value of
680   "0".
681</t>
682<t>
683   The Max-Forwards request-header field MAY be used to target a
684   specific proxy in the request chain. When a proxy receives an OPTIONS
685   request on an absolute-URI for which request forwarding is permitted,
686   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
687   field-value is zero ("0"), the proxy MUST NOT forward the message;
688   instead, the proxy SHOULD respond with its own communication options.
689   If the Max-Forwards field-value is an integer greater than zero, the
690   proxy MUST decrement the field-value when it forwards the request. If
691   no Max-Forwards field is present in the request, then the forwarded
692   request MUST NOT include a Max-Forwards field.
693</t>
694</section>
695
696<section title="GET" anchor="GET">
697 
698  <iref primary="true" item="GET method"/>
699  <iref primary="true" item="Methods" subitem="GET"/>
700<t>
701   The GET method means retrieve whatever information (in the form of an
702   entity) is identified by the Request-URI. If the Request-URI refers
703   to a data-producing process, it is the produced data which shall be
704   returned as the entity in the response and not the source text of the
705   process, unless that text happens to be the output of the process.
706</t>
707<t>
708   The semantics of the GET method change to a "conditional GET" if the
709   request message includes an If-Modified-Since, If-Unmodified-Since,
710   If-Match, If-None-Match, or If-Range header field. A conditional GET
711   method requests that the entity be transferred only under the
712   circumstances described by the conditional header field(s). The
713   conditional GET method is intended to reduce unnecessary network
714   usage by allowing cached entities to be refreshed without requiring
715   multiple requests or transferring data already held by the client.
716</t>
717<t>
718   The semantics of the GET method change to a "partial GET" if the
719   request message includes a Range header field. A partial GET requests
720   that only part of the entity be transferred, as described in Section 6.4 of <xref target="Part5"/>.
721   The partial GET method is intended to reduce unnecessary
722   network usage by allowing partially-retrieved entities to be
723   completed without transferring data already held by the client.
724</t>
725<t>
726   The response to a GET request is cacheable if and only if it meets
727   the requirements for HTTP caching described in <xref target="Part6"/>.
728</t>
729<t>
730   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
731</t>
732</section>
733
734<section title="HEAD" anchor="HEAD">
735 
736  <iref primary="true" item="HEAD method"/>
737  <iref primary="true" item="Methods" subitem="HEAD"/>
738<t>
739   The HEAD method is identical to GET except that the server MUST NOT
740   return a message-body in the response. The metainformation contained
741   in the HTTP headers in response to a HEAD request SHOULD be identical
742   to the information sent in response to a GET request. This method can
743   be used for obtaining metainformation about the entity implied by the
744   request without transferring the entity-body itself. This method is
745   often used for testing hypertext links for validity, accessibility,
746   and recent modification.
747</t>
748<t>
749   The response to a HEAD request MAY be cacheable in the sense that the
750   information contained in the response MAY be used to update a
751   previously cached entity from that resource. If the new field values
752   indicate that the cached entity differs from the current entity (as
753   would be indicated by a change in Content-Length, Content-MD5, ETag
754   or Last-Modified), then the cache MUST treat the cache entry as
755   stale.
756</t>
757</section>
758
759<section title="POST" anchor="POST">
760  <iref primary="true" item="POST method"/>
761  <iref primary="true" item="Methods" subitem="POST"/>
762<t>
763   The POST method is used to request that the origin server accept the
764   entity enclosed in the request as data to be processed by the resource
765   identified by the Request-URI in the Request-Line. POST is designed
766   to allow a uniform method to cover the following functions:
767  <list style="symbols">
768    <t>
769      Annotation of existing resources;
770    </t>
771    <t>
772        Posting a message to a bulletin board, newsgroup, mailing list,
773        or similar group of articles;
774    </t>
775    <t>
776        Providing a block of data, such as the result of submitting a
777        form, to a data-handling process;
778    </t>
779    <t>
780        Extending a database through an append operation.
781    </t>
782  </list>
783</t>
784<t>
785   The actual function performed by the POST method is determined by the
786   server and is usually dependent on the Request-URI.
787</t>
788<t>
789   The action performed by the POST method might not result in a
790   resource that can be identified by a URI. In this case, either 200
791   (OK) or 204 (No Content) is the appropriate response status,
792   depending on whether or not the response includes an entity that
793   describes the result.
794</t>
795<t>
796   If a resource has been created on the origin server, the response
797   SHOULD be 201 (Created) and contain an entity which describes the
798   status of the request and refers to the new resource, and a Location
799   header (see <xref target="header.location"/>).
800</t>
801<t>
802   Responses to this method are not cacheable, unless the response
803   includes appropriate Cache-Control or Expires header fields. However,
804   the 303 (See Other) response can be used to direct the user agent to
805   retrieve a cacheable resource.
806</t>
807</section>
808
809<section title="PUT" anchor="PUT">
810  <iref primary="true" item="PUT method"/>
811  <iref primary="true" item="Methods" subitem="PUT"/>
812<t>
813   The PUT method requests that the enclosed entity be stored at the
814   supplied Request-URI. If the Request-URI refers to an already
815   existing resource, the enclosed entity SHOULD be considered as a
816   modified version of the one residing on the origin server. If the
817   Request-URI does not point to an existing resource, and that URI is
818   capable of being defined as a new resource by the requesting user
819   agent, the origin server can create the resource with that URI. If a
820   new resource is created at the Request-URI, the origin server MUST
821         inform the user agent
822   via the 201 (Created) response. If an existing resource is modified,
823   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
824   to indicate successful completion of the request. If the resource
825   could not be created or modified with the Request-URI, an appropriate
826   error response SHOULD be given that reflects the nature of the
827   problem. The recipient of the entity MUST NOT ignore any Content-*
828   headers (headers starting with the prefix 'Content-') that it does
829   not understand or implement
830   and MUST return a 501 (Not Implemented) response in such cases.
831</t>
832<t>
833   If the request passes through a cache and the Request-URI identifies
834   one or more currently cached entities, those entries SHOULD be
835   treated as stale. Responses to this method are not cacheable.
836</t>
837<t>
838   The fundamental difference between the POST and PUT requests is
839   reflected in the different meaning of the Request-URI. The URI in a
840   POST request identifies the resource that will handle the enclosed
841   entity. That resource might be a data-accepting process, a gateway to
842   some other protocol, or a separate entity that accepts annotations.
843   In contrast, the URI in a PUT request identifies the entity enclosed
844   with the request -- the user agent knows what URI is intended and the
845   server MUST NOT attempt to apply the request to some other resource.
846   If the server desires that the request be applied to a different URI,
847   it MUST send a 301 (Moved Permanently) response; the user agent MAY
848   then make its own decision regarding whether or not to redirect the
849   request.
850</t>
851<t>
852   A single resource MAY be identified by many different URIs. For
853   example, an article might have a URI for identifying "the current
854   version" which is separate from the URI identifying each particular
855   version. In this case, a PUT request on a general URI might result in
856   several other URIs being defined by the origin server.
857</t>
858<t>
859   HTTP/1.1 does not define how a PUT method affects the state of an
860   origin server.
861</t>
862<t>
863   Unless otherwise specified for a particular entity-header, the
864   entity-headers in the PUT request SHOULD be applied to the resource
865   created or modified by the PUT.
866</t>
867</section>
868
869<section title="DELETE" anchor="DELETE">
870  <iref primary="true" item="DELETE method"/>
871  <iref primary="true" item="Methods" subitem="DELETE"/>
872<t>
873   The DELETE method requests that the origin server delete the resource
874   identified by the Request-URI. This method MAY be overridden by human
875   intervention (or other means) on the origin server. The client cannot
876   be guaranteed that the operation has been carried out, even if the
877   status code returned from the origin server indicates that the action
878   has been completed successfully. However, the server SHOULD NOT
879   indicate success unless, at the time the response is given, it
880   intends to delete the resource or move it to an inaccessible
881   location.
882</t>
883<t>
884   A successful response SHOULD be 200 (OK) if the response includes an
885   entity describing the status, 202 (Accepted) if the action has not
886   yet been enacted, or 204 (No Content) if the action has been enacted
887   but the response does not include an entity.
888</t>
889<t>
890   If the request passes through a cache and the Request-URI identifies
891   one or more currently cached entities, those entries SHOULD be
892   treated as stale. Responses to this method are not cacheable.
893</t>
894</section>
895
896<section title="TRACE" anchor="TRACE">
897 
898  <iref primary="true" item="TRACE method"/>
899  <iref primary="true" item="Methods" subitem="TRACE"/>
900<t>
901   The TRACE method is used to invoke a remote, application-layer loop-back
902   of the request message. The final recipient of the request
903   SHOULD reflect the message received back to the client as the
904   entity-body of a 200 (OK) response. The final recipient is either the
905   origin server or the first proxy or gateway to receive a Max-Forwards
906   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
907   MUST NOT include an entity.
908</t>
909<t>
910   TRACE allows the client to see what is being received at the other
911   end of the request chain and use that data for testing or diagnostic
912   information. The value of the Via header field (Section 8.9 of <xref target="Part1"/>) is of
913   particular interest, since it acts as a trace of the request chain.
914   Use of the Max-Forwards header field allows the client to limit the
915   length of the request chain, which is useful for testing a chain of
916   proxies forwarding messages in an infinite loop.
917</t>
918<t>
919   If the request is valid, the response SHOULD contain the entire
920   request message in the entity-body, with a Content-Type of
921   "message/http" (see Section 9.3.1 of <xref target="Part1"/>). Responses to this method
922   MUST NOT be cached.
923</t>
924</section>
925
926<section title="CONNECT" anchor="CONNECT">
927  <iref primary="true" item="CONNECT method"/>
928  <iref primary="true" item="Methods" subitem="CONNECT"/>
929<t>
930   This specification reserves the method name CONNECT for use with a
931   proxy that can dynamically switch to being a tunnel (e.g. SSL
932   tunneling <xref target="RFC2817"/>).
933</t>
934</section>
935</section>
936
937
938<section title="Status Code Definitions" anchor="status.codes">
939<t>
940   Each Status-Code is described below, including a description of which
941   method(s) it can follow and any metainformation required in the
942   response.
943</t>
944
945<section title="Informational 1xx" anchor="status.1xx">
946<t>
947   This class of status code indicates a provisional response,
948   consisting only of the Status-Line and optional headers, and is
949   terminated by an empty line. There are no required headers for this
950   class of status code. Since HTTP/1.0 did not define any 1xx status
951   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
952   except under experimental conditions.
953</t>
954<t>
955   A client MUST be prepared to accept one or more 1xx status responses
956   prior to a regular response, even if the client does not expect a 100
957   (Continue) status message. Unexpected 1xx status responses MAY be
958   ignored by a user agent.
959</t>
960<t>
961   Proxies MUST forward 1xx responses, unless the connection between the
962   proxy and its client has been closed, or unless the proxy itself
963   requested the generation of the 1xx response. (For example, if a
964   proxy adds a "Expect: 100-continue" field when it forwards a request,
965   then it need not forward the corresponding 100 (Continue)
966   response(s).)
967</t>
968
969<section title="100 Continue" anchor="status.100">
970  <iref primary="true" item="100 Continue (status code)"/>
971  <iref primary="true" item="Status Codes" subitem="100 Continue"/>
972<t>
973   The client SHOULD continue with its request. This interim response is
974   used to inform the client that the initial part of the request has
975   been received and has not yet been rejected by the server. The client
976   SHOULD continue by sending the remainder of the request or, if the
977   request has already been completed, ignore this response. The server
978   MUST send a final response after the request has been completed. See
979   Section 7.2.3 of <xref target="Part1"/> for detailed discussion of the use and handling of this
980   status code.
981</t>
982</section>
983
984<section title="101 Switching Protocols" anchor="status.101">
985  <iref primary="true" item="101 Switching Protocols (status code)"/>
986  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols"/>
987<t>
988   The server understands and is willing to comply with the client's
989   request, via the Upgrade message header field (Section 6.4 of <xref target="Part5"/>), for a
990   change in the application protocol being used on this connection. The
991   server will switch protocols to those defined by the response's
992   Upgrade header field immediately after the empty line which
993   terminates the 101 response.
994</t>
995<t>
996   The protocol SHOULD be switched only when it is advantageous to do
997   so. For example, switching to a newer version of HTTP is advantageous
998   over older versions, and switching to a real-time, synchronous
999   protocol might be advantageous when delivering resources that use
1000   such features.
1001</t>
1002</section>
1003</section>
1004
1005<section title="Successful 2xx" anchor="status.2xx">
1006<t>
1007   This class of status code indicates that the client's request was
1008   successfully received, understood, and accepted.
1009</t>
1010
1011<section title="200 OK" anchor="status.200">
1012  <iref primary="true" item="200 OK (status code)"/>
1013  <iref primary="true" item="Status Codes" subitem="200 OK"/>
1014<t>
1015   The request has succeeded. The information returned with the response
1016   is dependent on the method used in the request, for example:
1017  <list style="hanging">
1018    <t hangText="GET">
1019          an entity corresponding to the requested resource is sent in
1020          the response;
1021    </t>
1022    <t hangText="HEAD">
1023          the entity-header fields corresponding to the requested
1024          resource are sent in the response without any message-body;
1025    </t>
1026    <t hangText="POST">
1027      an entity describing or containing the result of the action;
1028    </t>
1029    <t hangText="TRACE">
1030      an entity containing the request message as received by the
1031      end server.
1032    </t>
1033  </list>
1034</t>
1035</section>
1036
1037<section title="201 Created" anchor="status.201">
1038  <iref primary="true" item="201 Created (status code)"/>
1039  <iref primary="true" item="Status Codes" subitem="201 Created"/>
1040<t>
1041   The request has been fulfilled and resulted in a new resource being
1042   created. The newly created resource can be referenced by the URI(s)
1043   returned in the entity of the response, with the most specific URI
1044   for the resource given by a Location header field. The response
1045   SHOULD include an entity containing a list of resource
1046   characteristics and location(s) from which the user or user agent can
1047   choose the one most appropriate. The entity format is specified by
1048   the media type given in the Content-Type header field. The origin
1049   server MUST create the resource before returning the 201 status code.
1050   If the action cannot be carried out immediately, the server SHOULD
1051   respond with 202 (Accepted) response instead.
1052</t>
1053<t>
1054   A 201 response MAY contain an ETag response header field indicating
1055   the current value of the entity tag for the requested variant just
1056   created, see Section 7.1 of <xref target="Part4"/>.
1057</t>
1058</section>
1059
1060<section title="202 Accepted" anchor="status.202">
1061  <iref primary="true" item="202 Accepted (status code)"/>
1062  <iref primary="true" item="Status Codes" subitem="202 Accepted"/>
1063<t>
1064   The request has been accepted for processing, but the processing has
1065   not been completed.  The request might or might not eventually be
1066   acted upon, as it might be disallowed when processing actually takes
1067   place. There is no facility for re-sending a status code from an
1068   asynchronous operation such as this.
1069</t>
1070<t>
1071   The 202 response is intentionally non-committal. Its purpose is to
1072   allow a server to accept a request for some other process (perhaps a
1073   batch-oriented process that is only run once per day) without
1074   requiring that the user agent's connection to the server persist
1075   until the process is completed. The entity returned with this
1076   response SHOULD include an indication of the request's current status
1077   and either a pointer to a status monitor or some estimate of when the
1078   user can expect the request to be fulfilled.
1079</t>
1080</section>
1081
1082<section title="203 Non-Authoritative Information" anchor="status.203">
1083  <iref primary="true" item="203 Non-Authoritative Information (status code)"/>
1084  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information"/>
1085<t>
1086   The returned metainformation in the entity-header is not the
1087   definitive set as available from the origin server, but is gathered
1088   from a local or a third-party copy. The set presented MAY be a subset
1089   or superset of the original version. For example, including local
1090   annotation information about the resource might result in a superset
1091   of the metainformation known by the origin server. Use of this
1092   response code is not required and is only appropriate when the
1093   response would otherwise be 200 (OK).
1094</t>
1095</section>
1096
1097<section title="204 No Content" anchor="status.204">
1098  <iref primary="true" item="204 No Content (status code)"/>
1099  <iref primary="true" item="Status Codes" subitem="204 No Content"/>
1100<t>
1101   The server has fulfilled the request but does not need to return an
1102   entity-body, and might want to return updated metainformation. The
1103   response MAY include new or updated metainformation in the form of
1104   entity-headers, which if present SHOULD be associated with the
1105   requested variant.
1106</t>
1107<t>
1108   If the client is a user agent, it SHOULD NOT  change its document view
1109   from that which caused the request to be sent. This response is
1110   primarily intended to allow input for actions to take place without
1111   causing a change to the user agent's active document view, although
1112   any new or updated metainformation SHOULD be applied to the document
1113   currently in the user agent's active view.
1114</t>
1115<t>
1116   The 204 response MUST NOT include a message-body, and thus is always
1117   terminated by the first empty line after the header fields.
1118</t>
1119</section>
1120
1121<section title="205 Reset Content" anchor="status.205">
1122  <iref primary="true" item="205 Reset Content (status code)"/>
1123  <iref primary="true" item="Status Codes" subitem="205 Reset Content"/>
1124<t>
1125   The server has fulfilled the request and the user agent SHOULD reset
1126   the document view which caused the request to be sent. This response
1127   is primarily intended to allow input for actions to take place via
1128   user input, followed by a clearing of the form in which the input is
1129   given so that the user can easily initiate another input action. The
1130   response MUST NOT include an entity.
1131</t>
1132</section>
1133
1134<section title="206 Partial Content" anchor="status.206">
1135  <iref primary="true" item="206 Partial Content (status code)"/>
1136  <iref primary="true" item="Status Codes" subitem="206 Partial Content"/>
1137<t>
1138   The server has fulfilled the partial GET request for the resource
1139   and the enclosed entity is a partial representation as defined in <xref target="Part5"/>.
1140</t>
1141</section>
1142</section>
1143
1144<section title="Redirection 3xx" anchor="status.3xx">
1145<t>
1146   This class of status code indicates that further action needs to be
1147   taken by the user agent in order to fulfill the request.  The action
1148   required MAY be carried out by the user agent without interaction
1149   with the user if and only if the method used in the second request is
1150   GET or HEAD. A client SHOULD detect infinite redirection loops, since
1151   such loops generate network traffic for each redirection.
1152  <list><t>
1153      Note: previous versions of this specification recommended a
1154      maximum of five redirections. Content developers should be aware
1155      that there might be clients that implement such a fixed
1156      limitation.
1157  </t></list>
1158</t>
1159
1160<section title="300 Multiple Choices" anchor="status.300">
1161  <iref primary="true" item="300 Multiple Choices (status code)"/>
1162  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices"/>
1163<t>
1164   The requested resource corresponds to any one of a set of
1165   representations, each with its own specific location, and agent-driven
1166   negotiation information (Section 5 of <xref target="Part3"/>) is being provided so that
1167   the user (or user agent) can select a preferred representation and
1168   redirect its request to that location.
1169</t>
1170<t>
1171   Unless it was a HEAD request, the response SHOULD include an entity
1172   containing a list of resource characteristics and location(s) from
1173   which the user or user agent can choose the one most appropriate. The
1174   entity format is specified by the media type given in the Content-Type
1175   header field. Depending upon the format and the capabilities of
1176   the user agent, selection of the most appropriate choice MAY be
1177   performed automatically. However, this specification does not define
1178   any standard for such automatic selection.
1179</t>
1180<t>
1181   If the server has a preferred choice of representation, it SHOULD
1182   include the specific URI for that representation in the Location
1183   field; user agents MAY use the Location field value for automatic
1184   redirection. This response is cacheable unless indicated otherwise.
1185</t>
1186</section>
1187
1188<section title="301 Moved Permanently" anchor="status.301">
1189  <iref primary="true" item="301 Moved Permanently (status code)"/>
1190  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently"/>
1191<t>
1192   The requested resource has been assigned a new permanent URI and any
1193   future references to this resource SHOULD use one of the returned
1194   URIs.  Clients with link editing capabilities ought to automatically
1195   re-link references to the Request-URI to one or more of the new
1196   references returned by the server, where possible. This response is
1197   cacheable unless indicated otherwise.
1198</t>
1199<t>
1200   The new permanent URI SHOULD be given by the Location field in the
1201   response. Unless the request method was HEAD, the entity of the
1202   response SHOULD contain a short hypertext note with a hyperlink to
1203   the new URI(s).
1204</t>
1205<t>
1206   If the 301 status code is received in response to a request method
1207   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1208   then the request MAY be automatically redirected by the user agent without
1209   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1210   request unless it can be confirmed by the user, since this might
1211   change the conditions under which the request was issued.
1212  <list><t>
1213      Note: When automatically redirecting a POST request after
1214      receiving a 301 status code, some existing HTTP/1.0 user agents
1215      will erroneously change it into a GET request.
1216  </t></list>
1217</t>
1218</section>
1219
1220<section title="302 Found" anchor="status.302">
1221  <iref primary="true" item="302 Found (status code)"/>
1222  <iref primary="true" item="Status Codes" subitem="302 Found"/>
1223<t>
1224   The requested resource resides temporarily under a different URI.
1225   Since the redirection might be altered on occasion, the client SHOULD
1226   continue to use the Request-URI for future requests.  This response
1227   is only cacheable if indicated by a Cache-Control or Expires header
1228   field.
1229</t>
1230<t>
1231   The temporary URI SHOULD be given by the Location field in the
1232   response. Unless the request method was HEAD, the entity of the
1233   response SHOULD contain a short hypertext note with a hyperlink to
1234   the new URI(s).
1235</t>
1236<t>
1237   If the 302 status code is received in response to a request method
1238   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1239   then the request MAY be automatically redirected by the user agent without
1240   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1241   request unless it can be confirmed by the user, since this might
1242   change the conditions under which the request was issued.
1243  <list><t>
1244      Note: <xref target="RFC1945"/> and <xref target="RFC2068"/> specify that the client is not allowed
1245      to change the method on the redirected request.  However, most
1246      existing user agent implementations treat 302 as if it were a 303
1247      response, performing a GET on the Location field-value regardless
1248      of the original request method. The status codes 303 and 307 have
1249      been added for servers that wish to make unambiguously clear which
1250      kind of reaction is expected of the client.
1251  </t></list>
1252</t>
1253</section>
1254
1255<section title="303 See Other" anchor="status.303">
1256  <iref primary="true" item="303 See Other (status code)"/>
1257  <iref primary="true" item="Status Codes" subitem="303 See Other"/>
1258<t>
1259   The server directs the user agent to a different resource, indicated
1260   by a URI in the Location header field, that provides an indirect
1261   response to the original request.  The user agent MAY perform a GET
1262   request on the URI in the Location field in order to obtain a
1263   representation corresponding to the response, be redirected again,
1264   or end with an error status.  The Location URI is not a substitute
1265   reference for the originally requested resource.
1266</t>
1267<t>
1268   The 303 status is generally applicable to any HTTP method.  It is
1269   primarily used to allow the output of a POST action to redirect
1270   the user agent to a selected resource, since doing so provides the
1271   information corresponding to the POST response in a form that
1272   can be separately identified, bookmarked, and cached independent
1273   of the original request.
1274</t>
1275<t>
1276   A 303 response to a GET request indicates that the requested
1277   resource does not have a representation of its own that can be
1278   transferred by the server over HTTP.  The Location URI indicates a
1279   resource that is descriptive of the requested resource such that
1280   the follow-on representation may be useful without implying that
1281   it adequately represents the previously requested resource.
1282   Note that answers to the questions of what can be represented, what
1283   representations are adequate, and what might be a useful description
1284   are outside the scope of HTTP and thus entirely determined by the
1285   resource owner(s).
1286</t>
1287<t>
1288   A 303 response SHOULD NOT be cached unless it is indicated as
1289   cacheable by Cache-Control or Expires header fields.  Except for
1290   responses to a HEAD request, the entity of a 303 response SHOULD
1291   contain a short hypertext note with a hyperlink to the Location URI.
1292</t>
1293</section>
1294
1295<section title="304 Not Modified" anchor="status.304">
1296  <iref primary="true" item="304 Not Modified (status code)"/>
1297  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1298<t>
1299   The response to the request has not been modified since the conditions
1300   indicated by the client's conditional GET request, as defined in <xref target="Part4"/>.
1301</t>
1302</section>
1303
1304<section title="305 Use Proxy" anchor="status.305">
1305  <iref primary="true" item="305 Use Proxy (status code)"/>
1306  <iref primary="true" item="Status Codes" subitem="305 Use Proxy"/>
1307<t>
1308   The 305 status was defined in a previous version of this specification
1309   (see <xref target="changes.from.rfc.2616"/>), and is now deprecated.
1310</t>
1311</section>
1312
1313<section title="306 (Unused)" anchor="status.306">
1314  <iref primary="true" item="306 (Unused) (status code)"/>
1315  <iref primary="true" item="Status Codes" subitem="306 (Unused)"/>
1316<t>
1317   The 306 status code was used in a previous version of the
1318   specification, is no longer used, and the code is reserved.
1319</t>
1320</section>
1321
1322<section title="307 Temporary Redirect" anchor="status.307">
1323  <iref primary="true" item="307 Temporary Redirect (status code)"/>
1324  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect"/>
1325<t>
1326   The requested resource resides temporarily under a different URI.
1327   Since the redirection MAY be altered on occasion, the client SHOULD
1328   continue to use the Request-URI for future requests.  This response
1329   is only cacheable if indicated by a Cache-Control or Expires header
1330   field.
1331</t>
1332<t>
1333   The temporary URI SHOULD be given by the Location field in the
1334   response. Unless the request method was HEAD, the entity of the
1335   response SHOULD contain a short hypertext note with a hyperlink to
1336   the new URI(s) , since many pre-HTTP/1.1 user agents do not
1337   understand the 307 status. Therefore, the note SHOULD contain the
1338   information necessary for a user to repeat the original request on
1339   the new URI.
1340</t>
1341<t>
1342   If the 307 status code is received in response to a request method
1343   that is known to be "safe", as defined in <xref target="safe.methods"/>,
1344   then the request MAY be automatically redirected by the user agent without
1345   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
1346   request unless it can be confirmed by the user, since this might
1347   change the conditions under which the request was issued.
1348</t>
1349</section>
1350</section>
1351
1352<section title="Client Error 4xx" anchor="status.4xx">
1353<t>
1354   The 4xx class of status code is intended for cases in which the
1355   client seems to have erred. Except when responding to a HEAD request,
1356   the server SHOULD include an entity containing an explanation of the
1357   error situation, and whether it is a temporary or permanent
1358   condition. These status codes are applicable to any request method.
1359   User agents SHOULD display any included entity to the user.
1360</t>
1361<t>
1362   If the client is sending data, a server implementation using TCP
1363   SHOULD be careful to ensure that the client acknowledges receipt of
1364   the packet(s) containing the response, before the server closes the
1365   input connection. If the client continues sending data to the server
1366   after the close, the server's TCP stack will send a reset packet to
1367   the client, which may erase the client's unacknowledged input buffers
1368   before they can be read and interpreted by the HTTP application.
1369</t>
1370
1371<section title="400 Bad Request" anchor="status.400">
1372  <iref primary="true" item="400 Bad Request (status code)"/>
1373  <iref primary="true" item="Status Codes" subitem="400 Bad Request"/>
1374<t>
1375   The request could not be understood by the server due to malformed
1376   syntax. The client SHOULD NOT  repeat the request without
1377   modifications.
1378</t>
1379</section>
1380
1381<section title="401 Unauthorized" anchor="status.401">
1382  <iref primary="true" item="401 Unauthorized (status code)"/>
1383  <iref primary="true" item="Status Codes" subitem="401 Unauthorized"/>
1384<t>
1385   The request requires user authentication (see <xref target="Part7"/>).
1386</t>
1387</section>
1388
1389<section title="402 Payment Required" anchor="status.402">
1390  <iref primary="true" item="402 Payment Required (status code)"/>
1391  <iref primary="true" item="Status Codes" subitem="402 Payment Required"/>
1392<t>
1393   This code is reserved for future use.
1394</t>
1395</section>
1396
1397<section title="403 Forbidden" anchor="status.403">
1398  <iref primary="true" item="403 Forbidden (status code)"/>
1399  <iref primary="true" item="Status Codes" subitem="403 Forbidden"/>
1400<t>
1401   The server understood the request, but is refusing to fulfill it.
1402   Authorization will not help and the request SHOULD NOT  be repeated.
1403   If the request method was not HEAD and the server wishes to make
1404   public why the request has not been fulfilled, it SHOULD describe the
1405   reason for the refusal in the entity.  If the server does not wish to
1406   make this information available to the client, the status code 404
1407   (Not Found) can be used instead.
1408</t>
1409</section>
1410
1411<section title="404 Not Found" anchor="status.404">
1412  <iref primary="true" item="404 Not Found (status code)"/>
1413  <iref primary="true" item="Status Codes" subitem="404 Not Found"/>
1414<t>
1415   The server has not found anything matching the Request-URI. No
1416   indication is given of whether the condition is temporary or
1417   permanent. The 410 (Gone) status code SHOULD be used if the server
1418   knows, through some internally configurable mechanism, that an old
1419   resource is permanently unavailable and has no forwarding address.
1420   This status code is commonly used when the server does not wish to
1421   reveal exactly why the request has been refused, or when no other
1422   response is applicable.
1423</t>
1424</section>
1425
1426<section title="405 Method Not Allowed" anchor="status.405">
1427  <iref primary="true" item="405 Method Not Allowed (status code)"/>
1428  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed"/>
1429<t>
1430   The method specified in the Request-Line is not allowed for the
1431   resource identified by the Request-URI. The response MUST include an
1432   Allow header containing a list of valid methods for the requested
1433   resource.
1434</t>
1435</section>
1436
1437<section title="406 Not Acceptable" anchor="status.406">
1438  <iref primary="true" item="406 Not Acceptable (status code)"/>
1439  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable"/>
1440<t>
1441   The resource identified by the request is only capable of generating
1442   response entities which have content characteristics not acceptable
1443   according to the accept headers sent in the request.
1444</t>
1445<t>
1446   Unless it was a HEAD request, the response SHOULD include an entity
1447   containing a list of available entity characteristics and location(s)
1448   from which the user or user agent can choose the one most
1449   appropriate. The entity format is specified by the media type given
1450   in the Content-Type header field. Depending upon the format and the
1451   capabilities of the user agent, selection of the most appropriate
1452   choice MAY be performed automatically. However, this specification
1453   does not define any standard for such automatic selection.
1454  <list><t>
1455      Note: HTTP/1.1 servers are allowed to return responses which are
1456      not acceptable according to the accept headers sent in the
1457      request. In some cases, this may even be preferable to sending a
1458      406 response. User agents are encouraged to inspect the headers of
1459      an incoming response to determine if it is acceptable.
1460  </t></list>
1461</t>
1462<t>
1463   If the response could be unacceptable, a user agent SHOULD
1464   temporarily stop receipt of more data and query the user for a
1465   decision on further actions.
1466</t>
1467</section>
1468
1469<section title="407 Proxy Authentication Required" anchor="status.407">
1470  <iref primary="true" item="407 Proxy Authentication Required (status code)"/>
1471  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required"/>
1472<t>
1473   This code is similar to 401 (Unauthorized), but indicates that the
1474   client must first authenticate itself with the proxy (see <xref target="Part7"/>).
1475</t>
1476</section>
1477
1478<section title="408 Request Timeout" anchor="status.408">
1479  <iref primary="true" item="408 Request Timeout (status code)"/>
1480  <iref primary="true" item="Status Codes" subitem="408 Request Timeout"/>
1481<t>
1482   The client did not produce a request within the time that the server
1483   was prepared to wait. The client MAY repeat the request without
1484   modifications at any later time.
1485</t>
1486</section>
1487
1488<section title="409 Conflict" anchor="status.409">
1489  <iref primary="true" item="409 Conflict (status code)"/>
1490  <iref primary="true" item="Status Codes" subitem="409 Conflict"/>
1491<t>
1492   The request could not be completed due to a conflict with the current
1493   state of the resource. This code is only allowed in situations where
1494   it is expected that the user might be able to resolve the conflict
1495   and resubmit the request. The response body SHOULD include enough
1496   information for the user to recognize the source of the conflict.
1497   Ideally, the response entity would include enough information for the
1498   user or user agent to fix the problem; however, that might not be
1499   possible and is not required.
1500</t>
1501<t>
1502   Conflicts are most likely to occur in response to a PUT request. For
1503   example, if versioning were being used and the entity being PUT
1504   included changes to a resource which conflict with those made by an
1505   earlier (third-party) request, the server might use the 409 response
1506   to indicate that it can't complete the request. In this case, the
1507   response entity would likely contain a list of the differences
1508   between the two versions in a format defined by the response
1509   Content-Type.
1510</t>
1511</section>
1512
1513<section title="410 Gone" anchor="status.410">
1514  <iref primary="true" item="410 Gone (status code)"/>
1515  <iref primary="true" item="Status Codes" subitem="410 Gone"/>
1516<t>
1517   The requested resource is no longer available at the server and no
1518   forwarding address is known. This condition is expected to be
1519   considered permanent. Clients with link editing capabilities SHOULD
1520   delete references to the Request-URI after user approval. If the
1521   server does not know, or has no facility to determine, whether or not
1522   the condition is permanent, the status code 404 (Not Found) SHOULD be
1523   used instead. This response is cacheable unless indicated otherwise.
1524</t>
1525<t>
1526   The 410 response is primarily intended to assist the task of web
1527   maintenance by notifying the recipient that the resource is
1528   intentionally unavailable and that the server owners desire that
1529   remote links to that resource be removed. Such an event is common for
1530   limited-time, promotional services and for resources belonging to
1531   individuals no longer working at the server's site. It is not
1532   necessary to mark all permanently unavailable resources as "gone" or
1533   to keep the mark for any length of time -- that is left to the
1534   discretion of the server owner.
1535</t>
1536</section>
1537
1538<section title="411 Length Required" anchor="status.411">
1539  <iref primary="true" item="411 Length Required (status code)"/>
1540  <iref primary="true" item="Status Codes" subitem="411 Length Required"/>
1541<t>
1542   The server refuses to accept the request without a defined Content-Length.
1543   The client MAY repeat the request if it adds a valid
1544   Content-Length header field containing the length of the message-body
1545   in the request message.
1546</t>
1547</section>
1548
1549<section title="412 Precondition Failed" anchor="status.412">
1550  <iref primary="true" item="412 Precondition Failed (status code)"/>
1551  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1552<t>
1553   The precondition given in one or more of the request-header fields
1554   evaluated to false when it was tested on the server, as defined in
1555   <xref target="Part4"/>.
1556</t>
1557</section>
1558
1559<section title="413 Request Entity Too Large" anchor="status.413">
1560  <iref primary="true" item="413 Request Entity Too Large (status code)"/>
1561  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large"/>
1562<t>
1563   The server is refusing to process a request because the request
1564   entity is larger than the server is willing or able to process. The
1565   server MAY close the connection to prevent the client from continuing
1566   the request.
1567</t>
1568<t>
1569   If the condition is temporary, the server SHOULD include a Retry-After
1570   header field to indicate that it is temporary and after what
1571   time the client MAY try again.
1572</t>
1573</section>
1574
1575<section title="414 Request-URI Too Long" anchor="status.414">
1576  <iref primary="true" item="414 Request-URI Too Long (status code)"/>
1577  <iref primary="true" item="Status Codes" subitem="414 Request-URI Too Long"/>
1578<t>
1579   The server is refusing to service the request because the Request-URI
1580   is longer than the server is willing to interpret. This rare
1581   condition is only likely to occur when a client has improperly
1582   converted a POST request to a GET request with long query
1583   information, when the client has descended into a URI "black hole" of
1584   redirection (e.g., a redirected URI prefix that points to a suffix of
1585   itself), or when the server is under attack by a client attempting to
1586   exploit security holes present in some servers using fixed-length
1587   buffers for reading or manipulating the Request-URI.
1588</t>
1589</section>
1590
1591<section title="415 Unsupported Media Type" anchor="status.415">
1592  <iref primary="true" item="415 Unsupported Media Type (status code)"/>
1593  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type"/>
1594<t>
1595   The server is refusing to service the request because the entity of
1596   the request is in a format not supported by the requested resource
1597   for the requested method.
1598</t>
1599</section>
1600
1601<section title="416 Requested Range Not Satisfiable" anchor="status.416">
1602  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)"/>
1603  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable"/>
1604<t>
1605   The request included a Range request-header field (Section 6.4 of <xref target="Part5"/>) and none of
1606   the range-specifier values in this field overlap the current extent
1607   of the selected resource.
1608</t>
1609</section>
1610
1611<section title="417 Expectation Failed" anchor="status.417">
1612  <iref primary="true" item="417 Expectation Failed (status code)"/>
1613  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed"/>
1614<t>
1615   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
1616   could not be met by this server, or, if the server is a proxy,
1617   the server has unambiguous evidence that the request could not be met
1618   by the next-hop server.
1619</t>
1620</section>
1621</section>
1622
1623<section title="Server Error 5xx" anchor="status.5xx">
1624<t>
1625   Response status codes beginning with the digit "5" indicate cases in
1626   which the server is aware that it has erred or is incapable of
1627   performing the request. Except when responding to a HEAD request, the
1628   server SHOULD include an entity containing an explanation of the
1629   error situation, and whether it is a temporary or permanent
1630   condition. User agents SHOULD display any included entity to the
1631   user. These response codes are applicable to any request method.
1632</t>
1633
1634<section title="500 Internal Server Error" anchor="status.500">
1635  <iref primary="true" item="500 Internal Server Error (status code)"/>
1636  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error"/>
1637<t>
1638   The server encountered an unexpected condition which prevented it
1639   from fulfilling the request.
1640</t>
1641</section>
1642
1643<section title="501 Not Implemented" anchor="status.501">
1644  <iref primary="true" item="501 Not Implemented (status code)"/>
1645  <iref primary="true" item="Status Codes" subitem="501 Not Implemented"/>
1646<t>
1647   The server does not support the functionality required to fulfill the
1648   request. This is the appropriate response when the server does not
1649   recognize the request method and is not capable of supporting it for
1650   any resource.
1651</t>
1652</section>
1653
1654<section title="502 Bad Gateway" anchor="status.502">
1655  <iref primary="true" item="502 Bad Gateway (status code)"/>
1656  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway"/>
1657<t>
1658   The server, while acting as a gateway or proxy, received an invalid
1659   response from the upstream server it accessed in attempting to
1660   fulfill the request.
1661</t>
1662</section>
1663
1664<section title="503 Service Unavailable" anchor="status.503">
1665  <iref primary="true" item="503 Service Unavailable (status code)"/>
1666  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable"/>
1667<t>
1668   The server is currently unable to handle the request due to a
1669   temporary overloading or maintenance of the server. The implication
1670   is that this is a temporary condition which will be alleviated after
1671   some delay. If known, the length of the delay MAY be indicated in a
1672   Retry-After header. If no Retry-After is given, the client SHOULD
1673   handle the response as it would for a 500 response.
1674  <list><t>
1675      Note: The existence of the 503 status code does not imply that a
1676      server must use it when becoming overloaded. Some servers may wish
1677      to simply refuse the connection.
1678  </t></list>
1679</t>
1680</section>
1681
1682<section title="504 Gateway Timeout" anchor="status.504">
1683  <iref primary="true" item="504 Gateway Timeout (status code)"/>
1684  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout"/>
1685<t>
1686   The server, while acting as a gateway or proxy, did not receive a
1687   timely response from the upstream server specified by the URI (e.g.
1688   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
1689   to access in attempting to complete the request.
1690  <list><t>
1691      Note: Note to implementors: some deployed proxies are known to
1692      return 400 or 500 when DNS lookups time out.
1693  </t></list>
1694</t>
1695</section>
1696
1697<section title="505 HTTP Version Not Supported" anchor="status.505">
1698  <iref primary="true" item="505 HTTP Version Not Supported (status code)"/>
1699  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported"/>
1700<t>
1701   The server does not support, or refuses to support, the protocol
1702   version that was used in the request message. The server is
1703   indicating that it is unable or unwilling to complete the request
1704   using the same major version as the client, as described in Section 3.1 of <xref target="Part1"/>,
1705   other than with this error message. The response SHOULD contain
1706   an entity describing why that version is not supported and what other
1707   protocols are supported by that server.
1708</t>
1709
1710</section>
1711</section>
1712</section>
1713
1714
1715<section title="Header Field Definitions" anchor="header.fields">
1716<t>
1717   This section defines the syntax and semantics of HTTP/1.1 header fields
1718   related to request and response semantics.
1719</t>
1720<t>
1721   For entity-header fields, both sender and recipient refer to either the
1722   client or the server, depending on who sends and who receives the entity.
1723</t>
1724
1725<section title="Allow" anchor="header.allow">
1726  <iref primary="true" item="Allow header"/>
1727  <iref primary="true" item="Headers" subitem="Allow"/>
1728 
1729 
1730<t>
1731      The response-header field "Allow" lists the set of methods advertised as
1732      supported by the resource identified by the Request-URI. The purpose of
1733      this field is strictly to inform the recipient of valid methods
1734      associated with the resource. An Allow header field MUST be
1735      present in a 405 (Method Not Allowed) response.
1736</t>
1737<figure><iref primary="true" item="Grammar" subitem="Allow"/><iref primary="true" item="Grammar" subitem="Allow-v"/><artwork type="abnf2616"><![CDATA[
1738  Allow   = "Allow" ":" OWS Allow-v
1739  Allow-v = #Method
1740]]></artwork></figure>
1741<t>
1742      Example of use:
1743</t>
1744<figure><artwork type="example"><![CDATA[
1745  Allow: GET, HEAD, PUT
1746]]></artwork></figure>
1747<t>
1748      The actual set of allowed methods is defined
1749      by the origin server at the time of each request.
1750</t>
1751<t>
1752      A proxy MUST NOT modify the Allow header field even if it does not
1753      understand all the methods specified, since the user agent might
1754      have other means of communicating with the origin server.
1755</t>
1756</section>
1757
1758<section title="Expect" anchor="header.expect">
1759  <iref primary="true" item="Expect header"/>
1760  <iref primary="true" item="Headers" subitem="Expect"/>
1761 
1762 
1763 
1764 
1765 
1766<t>
1767   The request-header field "Expect" is used to indicate that particular
1768   server behaviors are required by the client.
1769</t>
1770<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[
1771  Expect       = "Expect" ":" OWS Expect-v
1772  Expect-v     = 1#expectation
1773 
1774  expectation  = "100-continue" / expectation-extension
1775  expectation-extension = token [ "=" ( token / quoted-string )
1776                           *expect-params ]
1777  expect-params = ";" token [ "=" ( token / quoted-string ) ]
1778]]></artwork></figure>
1779<t>
1780   A server that does not understand or is unable to comply with any of
1781   the expectation values in the Expect field of a request MUST respond
1782   with appropriate error status. The server MUST respond with a 417
1783   (Expectation Failed) status if any of the expectations cannot be met
1784   or, if there are other problems with the request, some other 4xx
1785   status.
1786</t>
1787<t>
1788   This header field is defined with extensible syntax to allow for
1789   future extensions. If a server receives a request containing an
1790   Expect field that includes an expectation-extension that it does not
1791   support, it MUST respond with a 417 (Expectation Failed) status.
1792</t>
1793<t>
1794   Comparison of expectation values is case-insensitive for unquoted
1795   tokens (including the 100-continue token), and is case-sensitive for
1796   quoted-string expectation-extensions.
1797</t>
1798<t>
1799   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
1800   return a 417 (Expectation Failed) status if it receives a request
1801   with an expectation that it cannot meet. However, the Expect
1802   request-header itself is end-to-end; it MUST be forwarded if the
1803   request is forwarded.
1804</t>
1805<t>
1806   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
1807   Expect header.
1808</t>
1809<t>
1810   See Section 7.2.3 of <xref target="Part1"/> for the use of the 100 (Continue) status.
1811</t>
1812</section>
1813
1814<section title="From" anchor="header.from">
1815  <iref primary="true" item="From header"/>
1816  <iref primary="true" item="Headers" subitem="From"/>
1817 
1818 
1819 
1820<t>
1821   The request-header field "From", if given, SHOULD contain an Internet
1822   e-mail address for the human user who controls the requesting user
1823   agent. The address SHOULD be machine-usable, as defined by "mailbox"
1824   in Section 3.4 of <xref target="RFC5322"/>:
1825</t>
1826<figure><iref primary="true" item="Grammar" subitem="From"/><iref primary="true" item="Grammar" subitem="From-v"/><artwork type="abnf2616"><![CDATA[
1827  From    = "From" ":" OWS From-v
1828  From-v  = mailbox
1829 
1830  mailbox = <mailbox, defined in [RFC5322], Section 3.4>
1831]]></artwork></figure>
1832<t>
1833   An example is:
1834</t>
1835<figure><artwork type="example"><![CDATA[
1836  From: webmaster@example.org
1837]]></artwork></figure>
1838<t>
1839   This header field MAY be used for logging purposes and as a means for
1840   identifying the source of invalid or unwanted requests. It SHOULD NOT
1841   be used as an insecure form of access protection. The interpretation
1842   of this field is that the request is being performed on behalf of the
1843   person given, who accepts responsibility for the method performed. In
1844   particular, robot agents SHOULD include this header so that the
1845   person responsible for running the robot can be contacted if problems
1846   occur on the receiving end.
1847</t>
1848<t>
1849   The Internet e-mail address in this field MAY be separate from the
1850   Internet host which issued the request. For example, when a request
1851   is passed through a proxy the original issuer's address SHOULD be
1852   used.
1853</t>
1854<t>
1855   The client SHOULD NOT  send the From header field without the user's
1856   approval, as it might conflict with the user's privacy interests or
1857   their site's security policy. It is strongly recommended that the
1858   user be able to disable, enable, and modify the value of this field
1859   at any time prior to a request.
1860</t>
1861</section>
1862
1863<section title="Location" anchor="header.location">
1864  <iref primary="true" item="Location header"/>
1865  <iref primary="true" item="Headers" subitem="Location"/>
1866 
1867 
1868<t>
1869   The response-header field "Location" is used for the identification of a
1870   new resource or to redirect the recipient to a location other than the
1871   Request-URI for completion of the request.  For 201 (Created)
1872   responses, the Location is that of the new resource which was created
1873   by the request. For 3xx responses, the location SHOULD indicate the
1874   server's preferred URI for automatic redirection to the resource. The
1875   field value consists of a single absolute URI.
1876</t>
1877<figure><iref primary="true" item="Grammar" subitem="Location"/><iref primary="true" item="Grammar" subitem="Location-v"/><artwork type="abnf2616"><![CDATA[
1878  Location       = "Location" ":" OWS Location-v
1879  Location-v     = absolute-URI [ "#" fragment ]
1880]]></artwork></figure>
1881<t>
1882   An example is:
1883</t>
1884<figure><artwork type="example"><![CDATA[
1885  Location: http://www.example.org/pub/WWW/People.html
1886]]></artwork></figure>
1887<t>
1888  <list><t>
1889      Note: The Content-Location header field (Section 6.7 of <xref target="Part3"/>) differs
1890      from Location in that the Content-Location identifies the original
1891      location of the entity enclosed in the response. It is therefore
1892      possible for a response to contain header fields for both Location
1893      and Content-Location.
1894  </t></list>
1895</t>
1896<t>
1897   There are circumstances in which a fragment identifier in a Location URL would not be appropriate:
1898   <list style="symbols">
1899      <t>With a 201 Created response, because in this usage the Location header specifies the URL for the entire created resource.</t>
1900      <t>With a 300 Multiple Choices, since the choice decision is intended to be made on resource characteristics and not fragment characteristics.</t>
1901      <t>With 305 Use Proxy.</t>
1902   </list>
1903</t>
1904</section>
1905
1906<section title="Max-Forwards" anchor="header.max-forwards">
1907  <iref primary="true" item="Max-Forwards header"/>
1908  <iref primary="true" item="Headers" subitem="Max-Forwards"/>
1909 
1910 
1911<t>
1912   The request-header "Max-Forwards" field provides a mechanism with the
1913   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
1914   number of proxies or gateways that can forward the request to the
1915   next inbound server. This can be useful when the client is attempting
1916   to trace a request chain which appears to be failing or looping in
1917   mid-chain.
1918</t>
1919<figure><iref primary="true" item="Grammar" subitem="Max-Forwards"/><iref primary="true" item="Grammar" subitem="Max-Forwards-v"/><artwork type="abnf2616"><![CDATA[
1920  Max-Forwards   = "Max-Forwards" ":" OWS Max-Forwards-v
1921  Max-Forwards-v = 1*DIGIT
1922]]></artwork></figure>
1923<t>
1924   The Max-Forwards value is a decimal integer indicating the remaining
1925   number of times this request message may be forwarded.
1926</t>
1927<t>
1928   Each proxy or gateway recipient of a TRACE or OPTIONS request
1929   containing a Max-Forwards header field MUST check and update its
1930   value prior to forwarding the request. If the received value is zero
1931   (0), the recipient MUST NOT forward the request; instead, it MUST
1932   respond as the final recipient. If the received Max-Forwards value is
1933   greater than zero, then the forwarded message MUST contain an updated
1934   Max-Forwards field with a value decremented by one (1).
1935</t>
1936<t>
1937   The Max-Forwards header field MAY be ignored for all other methods
1938   defined by this specification and for any extension methods for which
1939   it is not explicitly referred to as part of that method definition.
1940</t>
1941</section>
1942
1943<section title="Referer" anchor="header.referer">
1944  <iref primary="true" item="Referer header"/>
1945  <iref primary="true" item="Headers" subitem="Referer"/>
1946 
1947 
1948<t>
1949   The request-header field "Referer" [sic] allows the client to specify,
1950   for the server's benefit, the address (URI) of the resource from
1951   which the Request-URI was obtained (the "referrer", although the
1952   header field is misspelled.) The Referer request-header allows a
1953   server to generate lists of back-links to resources for interest,
1954   logging, optimized caching, etc. It also allows obsolete or mistyped
1955   links to be traced for maintenance. The Referer field MUST NOT be
1956   sent if the Request-URI was obtained from a source that does not have
1957   its own URI, such as input from the user keyboard.
1958</t>
1959<figure><iref primary="true" item="Grammar" subitem="Referer"/><iref primary="true" item="Grammar" subitem="Referer-v"/><artwork type="abnf2616"><![CDATA[
1960  Referer        = "Referer" ":" OWS Referer-v
1961  Referer-v      = absolute-URI / relativeURI
1962]]></artwork></figure>
1963<t>
1964   Example:
1965</t>
1966<figure><artwork type="example"><![CDATA[
1967  Referer: http://www.example.org/hypertext/Overview.html
1968]]></artwork></figure>
1969<t>
1970   If the field value is a relative URI, it SHOULD be interpreted
1971   relative to the Request-URI. The URI MUST NOT include a fragment. See
1972   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
1973</t>
1974</section>
1975
1976<section title="Retry-After" anchor="header.retry-after">
1977  <iref primary="true" item="Retry-After header"/>
1978  <iref primary="true" item="Headers" subitem="Retry-After"/>
1979 
1980 
1981<t>
1982   The response-header "Retry-After" field can be used with a 503 (Service
1983   Unavailable) response to indicate how long the service is expected to
1984   be unavailable to the requesting client. This field MAY also be used
1985   with any 3xx (Redirection) response to indicate the minimum time the
1986   user-agent is asked wait before issuing the redirected request. The
1987   value of this field can be either an HTTP-date or an integer number
1988   of seconds (in decimal) after the time of the response.
1989</t>
1990<figure><iref primary="true" item="Grammar" subitem="Retry-After"/><iref primary="true" item="Grammar" subitem="Retry-After-v"/><artwork type="abnf2616"><![CDATA[
1991  Retry-After   = "Retry-After" ":" OWS Retry-After-v
1992  Retry-After-v = HTTP-date / delta-seconds
1993]]></artwork></figure>
1994<t anchor="rule.delta-seconds">
1995 
1996   Time spans are non-negative decimal integers, representing time in
1997   seconds.
1998</t>
1999<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
2000  delta-seconds  = 1*DIGIT
2001]]></artwork></figure>
2002<t>
2003   Two examples of its use are
2004</t>
2005<figure><artwork type="example"><![CDATA[
2006  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
2007  Retry-After: 120
2008]]></artwork></figure>
2009<t>
2010   In the latter example, the delay is 2 minutes.
2011</t>
2012</section>
2013
2014<section title="Server" anchor="header.server">
2015  <iref primary="true" item="Server header"/>
2016  <iref primary="true" item="Headers" subitem="Server"/>
2017 
2018 
2019<t>
2020   The response-header field "Server" contains information about the
2021   software used by the origin server to handle the request. The field
2022   can contain multiple product tokens (Section 3.5 of <xref target="Part1"/>) and comments
2023   identifying the server and any significant subproducts. The product
2024   tokens are listed in order of their significance for identifying the
2025   application.
2026</t>
2027<figure><iref primary="true" item="Grammar" subitem="Server"/><iref primary="true" item="Grammar" subitem="Server-v"/><artwork type="abnf2616"><![CDATA[
2028  Server         = "Server" ":" OWS Server-v
2029  Server-v       = product
2030                   *( RWS ( product / comment ) )
2031]]></artwork></figure>
2032<t>
2033   Example:
2034</t>
2035<figure><artwork type="example"><![CDATA[
2036  Server: CERN/3.0 libwww/2.17
2037]]></artwork></figure>
2038<t>
2039   If the response is being forwarded through a proxy, the proxy
2040   application MUST NOT modify the Server response-header. Instead, it
2041   MUST include a Via field (as described in Section 8.9 of <xref target="Part1"/>).
2042  <list><t>
2043      Note: Revealing the specific software version of the server might
2044      allow the server machine to become more vulnerable to attacks
2045      against software that is known to contain security holes. Server
2046      implementors are encouraged to make this field a configurable
2047      option.
2048  </t></list>
2049</t>
2050</section>
2051
2052<section title="User-Agent" anchor="header.user-agent">
2053  <iref primary="true" item="User-Agent header"/>
2054  <iref primary="true" item="Headers" subitem="User-Agent"/>
2055 
2056 
2057<t>
2058   The request-header field "User-Agent" contains information about the
2059   user agent originating the request. This is for statistical purposes,
2060   the tracing of protocol violations, and automated recognition of user
2061   agents for the sake of tailoring responses to avoid particular user
2062   agent limitations. User agents SHOULD include this field with
2063   requests. The field can contain multiple product tokens (Section 3.5 of <xref target="Part1"/>)
2064   and comments identifying the agent and any subproducts which form a
2065   significant part of the user agent. By convention, the product tokens
2066   are listed in order of their significance for identifying the
2067   application.
2068</t>
2069<figure><iref primary="true" item="Grammar" subitem="User-Agent"/><iref primary="true" item="Grammar" subitem="User-Agent-v"/><artwork type="abnf2616"><![CDATA[
2070  User-Agent     = "User-Agent" ":" OWS User-Agent-v
2071  User-Agent-v   = product
2072                   *( RWS ( product / comment ) )
2073]]></artwork></figure>
2074<t>
2075   Example:
2076</t>
2077<figure><artwork type="example"><![CDATA[
2078  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
2079]]></artwork></figure>
2080</section>
2081
2082</section>
2083
2084<section title="IANA Considerations" anchor="IANA.considerations">
2085
2086<section title="Method Registry" anchor="method.registration">
2087<t>
2088  The registration procedure for HTTP Methods is defined by
2089  <xref target="method.registry"/> of this document.
2090</t>
2091<t>
2092   The HTTP Method Registry located at <eref target="http://www.iana.org/assignments/http-methods"/>
2093   should be populated with the registrations below:
2094</t>
2095<!--AUTOGENERATED FROM extract-method-defs.xslt, do not edit manually-->
2096<texttable align="left" suppress-title="true" anchor="iana.method.registration.table">
2097   <ttcol>Method</ttcol>
2098   <ttcol>Safe</ttcol>
2099   <ttcol>Reference</ttcol>
2100   <c>CONNECT</c>
2101   <c>no</c>
2102   <c>
2103      <xref target="CONNECT"/>
2104   </c>
2105   <c>DELETE</c>
2106   <c>no</c>
2107   <c>
2108      <xref target="DELETE"/>
2109   </c>
2110   <c>GET</c>
2111   <c>yes</c>
2112   <c>
2113      <xref target="GET"/>
2114   </c>
2115   <c>HEAD</c>
2116   <c>yes</c>
2117   <c>
2118      <xref target="HEAD"/>
2119   </c>
2120   <c>OPTIONS</c>
2121   <c>yes</c>
2122   <c>
2123      <xref target="OPTIONS"/>
2124   </c>
2125   <c>POST</c>
2126   <c>no</c>
2127   <c>
2128      <xref target="POST"/>
2129   </c>
2130   <c>PUT</c>
2131   <c>no</c>
2132   <c>
2133      <xref target="PUT"/>
2134   </c>
2135   <c>TRACE</c>
2136   <c>yes</c>
2137   <c>
2138      <xref target="TRACE"/>
2139   </c>
2140</texttable>
2141<!--(END)-->
2142</section>
2143
2144<section title="Status Code Registry" anchor="status.code.registration">
2145<t>
2146   The registration procedure for HTTP Status Codes -- previously defined
2147   in Section 7.1 of <xref target="RFC2817"/> -- is now defined
2148   by <xref target="status.code.registry"/> of this document.
2149</t>
2150<t>
2151   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
2152   should be updated with the registrations below:
2153</t>
2154<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
2155<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
2156   <ttcol>Value</ttcol>
2157   <ttcol>Description</ttcol>
2158   <ttcol>Reference</ttcol>
2159   <c>100</c>
2160   <c>Continue</c>
2161   <c>
2162      <xref target="status.100"/>
2163   </c>
2164   <c>101</c>
2165   <c>Switching Protocols</c>
2166   <c>
2167      <xref target="status.101"/>
2168   </c>
2169   <c>200</c>
2170   <c>OK</c>
2171   <c>
2172      <xref target="status.200"/>
2173   </c>
2174   <c>201</c>
2175   <c>Created</c>
2176   <c>
2177      <xref target="status.201"/>
2178   </c>
2179   <c>202</c>
2180   <c>Accepted</c>
2181   <c>
2182      <xref target="status.202"/>
2183   </c>
2184   <c>203</c>
2185   <c>Non-Authoritative Information</c>
2186   <c>
2187      <xref target="status.203"/>
2188   </c>
2189   <c>204</c>
2190   <c>No Content</c>
2191   <c>
2192      <xref target="status.204"/>
2193   </c>
2194   <c>205</c>
2195   <c>Reset Content</c>
2196   <c>
2197      <xref target="status.205"/>
2198   </c>
2199   <c>206</c>
2200   <c>Partial Content</c>
2201   <c>
2202      <xref target="status.206"/>
2203   </c>
2204   <c>300</c>
2205   <c>Multiple Choices</c>
2206   <c>
2207      <xref target="status.300"/>
2208   </c>
2209   <c>301</c>
2210   <c>Moved Permanently</c>
2211   <c>
2212      <xref target="status.301"/>
2213   </c>
2214   <c>302</c>
2215   <c>Found</c>
2216   <c>
2217      <xref target="status.302"/>
2218   </c>
2219   <c>303</c>
2220   <c>See Other</c>
2221   <c>
2222      <xref target="status.303"/>
2223   </c>
2224   <c>304</c>
2225   <c>Not Modified</c>
2226   <c>
2227      <xref target="status.304"/>
2228   </c>
2229   <c>305</c>
2230   <c>Use Proxy</c>
2231   <c>
2232      <xref target="status.305"/>
2233   </c>
2234   <c>306</c>
2235   <c>(Unused)</c>
2236   <c>
2237      <xref target="status.306"/>
2238   </c>
2239   <c>307</c>
2240   <c>Temporary Redirect</c>
2241   <c>
2242      <xref target="status.307"/>
2243   </c>
2244   <c>400</c>
2245   <c>Bad Request</c>
2246   <c>
2247      <xref target="status.400"/>
2248   </c>
2249   <c>401</c>
2250   <c>Unauthorized</c>
2251   <c>
2252      <xref target="status.401"/>
2253   </c>
2254   <c>402</c>
2255   <c>Payment Required</c>
2256   <c>
2257      <xref target="status.402"/>
2258   </c>
2259   <c>403</c>
2260   <c>Forbidden</c>
2261   <c>
2262      <xref target="status.403"/>
2263   </c>
2264   <c>404</c>
2265   <c>Not Found</c>
2266   <c>
2267      <xref target="status.404"/>
2268   </c>
2269   <c>405</c>
2270   <c>Method Not Allowed</c>
2271   <c>
2272      <xref target="status.405"/>
2273   </c>
2274   <c>406</c>
2275   <c>Not Acceptable</c>
2276   <c>
2277      <xref target="status.406"/>
2278   </c>
2279   <c>407</c>
2280   <c>Proxy Authentication Required</c>
2281   <c>
2282      <xref target="status.407"/>
2283   </c>
2284   <c>408</c>
2285   <c>Request Timeout</c>
2286   <c>
2287      <xref target="status.408"/>
2288   </c>
2289   <c>409</c>
2290   <c>Conflict</c>
2291   <c>
2292      <xref target="status.409"/>
2293   </c>
2294   <c>410</c>
2295   <c>Gone</c>
2296   <c>
2297      <xref target="status.410"/>
2298   </c>
2299   <c>411</c>
2300   <c>Length Required</c>
2301   <c>
2302      <xref target="status.411"/>
2303   </c>
2304   <c>412</c>
2305   <c>Precondition Failed</c>
2306   <c>
2307      <xref target="status.412"/>
2308   </c>
2309   <c>413</c>
2310   <c>Request Entity Too Large</c>
2311   <c>
2312      <xref target="status.413"/>
2313   </c>
2314   <c>414</c>
2315   <c>Request-URI Too Long</c>
2316   <c>
2317      <xref target="status.414"/>
2318   </c>
2319   <c>415</c>
2320   <c>Unsupported Media Type</c>
2321   <c>
2322      <xref target="status.415"/>
2323   </c>
2324   <c>416</c>
2325   <c>Requested Range Not Satisfiable</c>
2326   <c>
2327      <xref target="status.416"/>
2328   </c>
2329   <c>417</c>
2330   <c>Expectation Failed</c>
2331   <c>
2332      <xref target="status.417"/>
2333   </c>
2334   <c>500</c>
2335   <c>Internal Server Error</c>
2336   <c>
2337      <xref target="status.500"/>
2338   </c>
2339   <c>501</c>
2340   <c>Not Implemented</c>
2341   <c>
2342      <xref target="status.501"/>
2343   </c>
2344   <c>502</c>
2345   <c>Bad Gateway</c>
2346   <c>
2347      <xref target="status.502"/>
2348   </c>
2349   <c>503</c>
2350   <c>Service Unavailable</c>
2351   <c>
2352      <xref target="status.503"/>
2353   </c>
2354   <c>504</c>
2355   <c>Gateway Timeout</c>
2356   <c>
2357      <xref target="status.504"/>
2358   </c>
2359   <c>505</c>
2360   <c>HTTP Version Not Supported</c>
2361   <c>
2362      <xref target="status.505"/>
2363   </c>
2364</texttable>
2365<!--(END)-->
2366</section>
2367<section title="Message Header Registration" anchor="message.header.registration">
2368<t>
2369   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2370   with the permanent registrations below (see <xref target="RFC3864"/>):
2371</t>
2372<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2373<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2374   <ttcol>Header Field Name</ttcol>
2375   <ttcol>Protocol</ttcol>
2376   <ttcol>Status</ttcol>
2377   <ttcol>Reference</ttcol>
2378   <c>Allow</c>
2379   <c>http</c>
2380   <c>standard</c>
2381   <c>
2382      <xref target="header.allow"/>
2383   </c>
2384   <c>Expect</c>
2385   <c>http</c>
2386   <c>standard</c>
2387   <c>
2388      <xref target="header.expect"/>
2389   </c>
2390   <c>From</c>
2391   <c>http</c>
2392   <c>standard</c>
2393   <c>
2394      <xref target="header.from"/>
2395   </c>
2396   <c>Location</c>
2397   <c>http</c>
2398   <c>standard</c>
2399   <c>
2400      <xref target="header.location"/>
2401   </c>
2402   <c>Max-Forwards</c>
2403   <c>http</c>
2404   <c>standard</c>
2405   <c>
2406      <xref target="header.max-forwards"/>
2407   </c>
2408   <c>Referer</c>
2409   <c>http</c>
2410   <c>standard</c>
2411   <c>
2412      <xref target="header.referer"/>
2413   </c>
2414   <c>Retry-After</c>
2415   <c>http</c>
2416   <c>standard</c>
2417   <c>
2418      <xref target="header.retry-after"/>
2419   </c>
2420   <c>Server</c>
2421   <c>http</c>
2422   <c>standard</c>
2423   <c>
2424      <xref target="header.server"/>
2425   </c>
2426   <c>User-Agent</c>
2427   <c>http</c>
2428   <c>standard</c>
2429   <c>
2430      <xref target="header.user-agent"/>
2431   </c>
2432</texttable>
2433<!--(END)-->
2434<t>
2435   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2436</t>
2437</section>
2438</section>
2439
2440<section title="Security Considerations" anchor="security.considerations">
2441<t>
2442   This section is meant to inform application developers, information
2443   providers, and users of the security limitations in HTTP/1.1 as
2444   described by this document. The discussion does not include
2445   definitive solutions to the problems revealed, though it does make
2446   some suggestions for reducing security risks.
2447</t>
2448
2449<section title="Transfer of Sensitive Information" anchor="security.sensitive">
2450<t>
2451   Like any generic data transfer protocol, HTTP cannot regulate the
2452   content of the data that is transferred, nor is there any a priori
2453   method of determining the sensitivity of any particular piece of
2454   information within the context of any given request. Therefore,
2455   applications SHOULD supply as much control over this information as
2456   possible to the provider of that information. Four header fields are
2457   worth special mention in this context: Server, Via, Referer and From.
2458</t>
2459<t>
2460   Revealing the specific software version of the server might allow the
2461   server machine to become more vulnerable to attacks against software
2462   that is known to contain security holes. Implementors SHOULD make the
2463   Server header field a configurable option.
2464</t>
2465<t>
2466   Proxies which serve as a portal through a network firewall SHOULD
2467   take special precautions regarding the transfer of header information
2468   that identifies the hosts behind the firewall. In particular, they
2469   SHOULD remove, or replace with sanitized versions, any Via fields
2470   generated behind the firewall.
2471</t>
2472<t>
2473   The Referer header allows reading patterns to be studied and reverse
2474   links drawn. Although it can be very useful, its power can be abused
2475   if user details are not separated from the information contained in
2476   the Referer. Even when the personal information has been removed, the
2477   Referer header might indicate a private document's URI whose
2478   publication would be inappropriate.
2479</t>
2480<t>
2481   The information sent in the From field might conflict with the user's
2482   privacy interests or their site's security policy, and hence it
2483   SHOULD NOT  be transmitted without the user being able to disable,
2484   enable, and modify the contents of the field. The user MUST be able
2485   to set the contents of this field within a user preference or
2486   application defaults configuration.
2487</t>
2488<t>
2489   We suggest, though do not require, that a convenient toggle interface
2490   be provided for the user to enable or disable the sending of From and
2491   Referer information.
2492</t>
2493<t>
2494   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
2495   fields can sometimes be used to determine that a specific client or
2496   server have a particular security hole which might be exploited.
2497   Unfortunately, this same information is often used for other valuable
2498   purposes for which HTTP currently has no better mechanism.
2499</t>
2500</section>
2501
2502<section title="Encoding Sensitive Information in URIs" anchor="encoding.sensitive.information.in.uris">
2503<t>
2504   Because the source of a link might be private information or might
2505   reveal an otherwise private information source, it is strongly
2506   recommended that the user be able to select whether or not the
2507   Referer field is sent. For example, a browser client could have a
2508   toggle switch for browsing openly/anonymously, which would
2509   respectively enable/disable the sending of Referer and From
2510   information.
2511</t>
2512<t>
2513   Clients SHOULD NOT include a Referer header field in a (non-secure)
2514   HTTP request if the referring page was transferred with a secure
2515   protocol.
2516</t>
2517<t>
2518   Authors of services should not use
2519   GET-based forms for the submission of sensitive data because that
2520   data will be encoded in the Request-URI. Many existing
2521   servers, proxies, and user agents log or display the Request-URI in
2522   places where it might be visible to third parties. Such services can
2523   use POST-based form submission instead.
2524</t>
2525</section>
2526
2527<section title="Location Headers and Spoofing" anchor="location.spoofing">
2528<t>
2529   If a single server supports multiple organizations that do not trust
2530   one another, then it MUST check the values of Location and Content-Location
2531   headers in responses that are generated under control of
2532   said organizations to make sure that they do not attempt to
2533   invalidate resources over which they have no authority.
2534</t>
2535</section>
2536
2537</section>
2538
2539<section title="Acknowledgments" anchor="ack">
2540</section>
2541</middle>
2542<back>
2543
2544<references title="Normative References">
2545
2546<reference anchor="Part1">
2547  <front>
2548    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2549    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2550      <organization abbrev="Day Software">Day Software</organization>
2551      <address><email>fielding@gbiv.com</email></address>
2552    </author>
2553    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2554      <organization>One Laptop per Child</organization>
2555      <address><email>jg@laptop.org</email></address>
2556    </author>
2557    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2558      <organization abbrev="HP">Hewlett-Packard Company</organization>
2559      <address><email>JeffMogul@acm.org</email></address>
2560    </author>
2561    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2562      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2563      <address><email>henrikn@microsoft.com</email></address>
2564    </author>
2565    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2566      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2567      <address><email>LMM@acm.org</email></address>
2568    </author>
2569    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2570      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2571      <address><email>paulle@microsoft.com</email></address>
2572    </author>
2573    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2574      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2575      <address><email>timbl@w3.org</email></address>
2576    </author>
2577    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2578      <organization abbrev="W3C">World Wide Web Consortium</organization>
2579      <address><email>ylafon@w3.org</email></address>
2580    </author>
2581    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2582      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2583      <address><email>julian.reschke@greenbytes.de</email></address>
2584    </author>
2585    <date month="November" year="2008"/>
2586  </front>
2587  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-05"/>
2588 
2589</reference>
2590
2591<reference anchor="Part3">
2592  <front>
2593    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2594    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2595      <organization abbrev="Day Software">Day Software</organization>
2596      <address><email>fielding@gbiv.com</email></address>
2597    </author>
2598    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2599      <organization>One Laptop per Child</organization>
2600      <address><email>jg@laptop.org</email></address>
2601    </author>
2602    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2603      <organization abbrev="HP">Hewlett-Packard Company</organization>
2604      <address><email>JeffMogul@acm.org</email></address>
2605    </author>
2606    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2607      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2608      <address><email>henrikn@microsoft.com</email></address>
2609    </author>
2610    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2611      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2612      <address><email>LMM@acm.org</email></address>
2613    </author>
2614    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2615      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2616      <address><email>paulle@microsoft.com</email></address>
2617    </author>
2618    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2619      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2620      <address><email>timbl@w3.org</email></address>
2621    </author>
2622    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2623      <organization abbrev="W3C">World Wide Web Consortium</organization>
2624      <address><email>ylafon@w3.org</email></address>
2625    </author>
2626    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2627      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2628      <address><email>julian.reschke@greenbytes.de</email></address>
2629    </author>
2630    <date month="November" year="2008"/>
2631  </front>
2632  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-05"/>
2633 
2634</reference>
2635
2636<reference anchor="Part4">
2637  <front>
2638    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2639    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2640      <organization abbrev="Day Software">Day Software</organization>
2641      <address><email>fielding@gbiv.com</email></address>
2642    </author>
2643    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2644      <organization>One Laptop per Child</organization>
2645      <address><email>jg@laptop.org</email></address>
2646    </author>
2647    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2648      <organization abbrev="HP">Hewlett-Packard Company</organization>
2649      <address><email>JeffMogul@acm.org</email></address>
2650    </author>
2651    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2652      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2653      <address><email>henrikn@microsoft.com</email></address>
2654    </author>
2655    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2656      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2657      <address><email>LMM@acm.org</email></address>
2658    </author>
2659    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2660      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2661      <address><email>paulle@microsoft.com</email></address>
2662    </author>
2663    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2664      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2665      <address><email>timbl@w3.org</email></address>
2666    </author>
2667    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2668      <organization abbrev="W3C">World Wide Web Consortium</organization>
2669      <address><email>ylafon@w3.org</email></address>
2670    </author>
2671    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2672      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2673      <address><email>julian.reschke@greenbytes.de</email></address>
2674    </author>
2675    <date month="November" year="2008"/>
2676  </front>
2677  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-05"/>
2678 
2679</reference>
2680
2681<reference anchor="Part5">
2682  <front>
2683    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2684    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2685      <organization abbrev="Day Software">Day Software</organization>
2686      <address><email>fielding@gbiv.com</email></address>
2687    </author>
2688    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2689      <organization>One Laptop per Child</organization>
2690      <address><email>jg@laptop.org</email></address>
2691    </author>
2692    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2693      <organization abbrev="HP">Hewlett-Packard Company</organization>
2694      <address><email>JeffMogul@acm.org</email></address>
2695    </author>
2696    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2697      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2698      <address><email>henrikn@microsoft.com</email></address>
2699    </author>
2700    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2701      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2702      <address><email>LMM@acm.org</email></address>
2703    </author>
2704    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2705      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2706      <address><email>paulle@microsoft.com</email></address>
2707    </author>
2708    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2709      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2710      <address><email>timbl@w3.org</email></address>
2711    </author>
2712    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2713      <organization abbrev="W3C">World Wide Web Consortium</organization>
2714      <address><email>ylafon@w3.org</email></address>
2715    </author>
2716    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2717      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2718      <address><email>julian.reschke@greenbytes.de</email></address>
2719    </author>
2720    <date month="November" year="2008"/>
2721  </front>
2722  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-05"/>
2723 
2724</reference>
2725
2726<reference anchor="Part6">
2727  <front>
2728    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
2729    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2730      <organization abbrev="Day Software">Day Software</organization>
2731      <address><email>fielding@gbiv.com</email></address>
2732    </author>
2733    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2734      <organization>One Laptop per Child</organization>
2735      <address><email>jg@laptop.org</email></address>
2736    </author>
2737    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2738      <organization abbrev="HP">Hewlett-Packard Company</organization>
2739      <address><email>JeffMogul@acm.org</email></address>
2740    </author>
2741    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2742      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2743      <address><email>henrikn@microsoft.com</email></address>
2744    </author>
2745    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2746      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2747      <address><email>LMM@acm.org</email></address>
2748    </author>
2749    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2750      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2751      <address><email>paulle@microsoft.com</email></address>
2752    </author>
2753    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2754      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2755      <address><email>timbl@w3.org</email></address>
2756    </author>
2757    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2758      <organization abbrev="W3C">World Wide Web Consortium</organization>
2759      <address><email>ylafon@w3.org</email></address>
2760    </author>
2761    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2762      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2763      <address><email>julian.reschke@greenbytes.de</email></address>
2764    </author>
2765    <date month="November" year="2008"/>
2766  </front>
2767  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-05"/>
2768 
2769</reference>
2770
2771<reference anchor="Part7">
2772  <front>
2773    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2774    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2775      <organization abbrev="Day Software">Day Software</organization>
2776      <address><email>fielding@gbiv.com</email></address>
2777    </author>
2778    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2779      <organization>One Laptop per Child</organization>
2780      <address><email>jg@laptop.org</email></address>
2781    </author>
2782    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2783      <organization abbrev="HP">Hewlett-Packard Company</organization>
2784      <address><email>JeffMogul@acm.org</email></address>
2785    </author>
2786    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2787      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2788      <address><email>henrikn@microsoft.com</email></address>
2789    </author>
2790    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2791      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2792      <address><email>LMM@acm.org</email></address>
2793    </author>
2794    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2795      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2796      <address><email>paulle@microsoft.com</email></address>
2797    </author>
2798    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2799      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2800      <address><email>timbl@w3.org</email></address>
2801    </author>
2802    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2803      <organization abbrev="W3C">World Wide Web Consortium</organization>
2804      <address><email>ylafon@w3.org</email></address>
2805    </author>
2806    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2807      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2808      <address><email>julian.reschke@greenbytes.de</email></address>
2809    </author>
2810    <date month="November" year="2008"/>
2811  </front>
2812  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-05"/>
2813 
2814</reference>
2815
2816<reference anchor="RFC2119">
2817  <front>
2818    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2819    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2820      <organization>Harvard University</organization>
2821      <address><email>sob@harvard.edu</email></address>
2822    </author>
2823    <date month="March" year="1997"/>
2824  </front>
2825  <seriesInfo name="BCP" value="14"/>
2826  <seriesInfo name="RFC" value="2119"/>
2827</reference>
2828
2829</references>
2830
2831<references title="Informative References">
2832
2833<reference anchor="RFC1945">
2834  <front>
2835    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
2836    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2837      <organization>MIT, Laboratory for Computer Science</organization>
2838      <address><email>timbl@w3.org</email></address>
2839    </author>
2840    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2841      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2842      <address><email>fielding@ics.uci.edu</email></address>
2843    </author>
2844    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2845      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
2846      <address><email>frystyk@w3.org</email></address>
2847    </author>
2848    <date month="May" year="1996"/>
2849  </front>
2850  <seriesInfo name="RFC" value="1945"/>
2851</reference>
2852
2853<reference anchor="RFC2068">
2854  <front>
2855    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2856    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2857      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2858      <address><email>fielding@ics.uci.edu</email></address>
2859    </author>
2860    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2861      <organization>MIT Laboratory for Computer Science</organization>
2862      <address><email>jg@w3.org</email></address>
2863    </author>
2864    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2865      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2866      <address><email>mogul@wrl.dec.com</email></address>
2867    </author>
2868    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2869      <organization>MIT Laboratory for Computer Science</organization>
2870      <address><email>frystyk@w3.org</email></address>
2871    </author>
2872    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2873      <organization>MIT Laboratory for Computer Science</organization>
2874      <address><email>timbl@w3.org</email></address>
2875    </author>
2876    <date month="January" year="1997"/>
2877  </front>
2878  <seriesInfo name="RFC" value="2068"/>
2879</reference>
2880
2881<reference anchor="RFC2616">
2882  <front>
2883    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2884    <author initials="R." surname="Fielding" fullname="R. Fielding">
2885      <organization>University of California, Irvine</organization>
2886      <address><email>fielding@ics.uci.edu</email></address>
2887    </author>
2888    <author initials="J." surname="Gettys" fullname="J. Gettys">
2889      <organization>W3C</organization>
2890      <address><email>jg@w3.org</email></address>
2891    </author>
2892    <author initials="J." surname="Mogul" fullname="J. Mogul">
2893      <organization>Compaq Computer Corporation</organization>
2894      <address><email>mogul@wrl.dec.com</email></address>
2895    </author>
2896    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2897      <organization>MIT Laboratory for Computer Science</organization>
2898      <address><email>frystyk@w3.org</email></address>
2899    </author>
2900    <author initials="L." surname="Masinter" fullname="L. Masinter">
2901      <organization>Xerox Corporation</organization>
2902      <address><email>masinter@parc.xerox.com</email></address>
2903    </author>
2904    <author initials="P." surname="Leach" fullname="P. Leach">
2905      <organization>Microsoft Corporation</organization>
2906      <address><email>paulle@microsoft.com</email></address>
2907    </author>
2908    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2909      <organization>W3C</organization>
2910      <address><email>timbl@w3.org</email></address>
2911    </author>
2912    <date month="June" year="1999"/>
2913  </front>
2914  <seriesInfo name="RFC" value="2616"/>
2915</reference>
2916
2917<reference anchor="RFC2817">
2918  <front>
2919    <title>Upgrading to TLS Within HTTP/1.1</title>
2920    <author initials="R." surname="Khare" fullname="R. Khare">
2921      <organization>4K Associates / UC Irvine</organization>
2922      <address><email>rohit@4K-associates.com</email></address>
2923    </author>
2924    <author initials="S." surname="Lawrence" fullname="S. Lawrence">
2925      <organization>Agranat Systems, Inc.</organization>
2926      <address><email>lawrence@agranat.com</email></address>
2927    </author>
2928    <date year="2000" month="May"/>
2929  </front>
2930  <seriesInfo name="RFC" value="2817"/>
2931</reference>
2932
2933<reference anchor="RFC3864">
2934  <front>
2935    <title>Registration Procedures for Message Header Fields</title>
2936    <author initials="G." surname="Klyne" fullname="G. Klyne">
2937      <organization>Nine by Nine</organization>
2938      <address><email>GK-IETF@ninebynine.org</email></address>
2939    </author>
2940    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
2941      <organization>BEA Systems</organization>
2942      <address><email>mnot@pobox.com</email></address>
2943    </author>
2944    <author initials="J." surname="Mogul" fullname="J. Mogul">
2945      <organization>HP Labs</organization>
2946      <address><email>JeffMogul@acm.org</email></address>
2947    </author>
2948    <date year="2004" month="September"/>
2949  </front>
2950  <seriesInfo name="BCP" value="90"/>
2951  <seriesInfo name="RFC" value="3864"/>
2952</reference>
2953
2954<reference anchor="RFC5226">
2955  <front>
2956    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2957    <author initials="T." surname="Narten" fullname="T. Narten">
2958      <organization>IBM</organization>
2959      <address><email>narten@us.ibm.com</email></address>
2960    </author>
2961    <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2962      <organization>Google</organization>
2963      <address><email>Harald@Alvestrand.no</email></address>
2964    </author>
2965    <date year="2008" month="May"/>
2966  </front>
2967  <seriesInfo name="BCP" value="26"/>
2968  <seriesInfo name="RFC" value="5226"/>
2969</reference>
2970
2971<reference anchor="RFC5322">
2972  <front>
2973    <title>Internet Message Format</title>
2974    <author initials="P." surname="Resnick" fullname="P. Resnick">
2975      <organization>Qualcomm Incorporated</organization>
2976    </author>
2977    <date year="2008" month="October"/>
2978  </front> 
2979  <seriesInfo name="RFC" value="5322"/>
2980</reference>
2981
2982</references>
2983
2984<section title="Compatibility with Previous Versions" anchor="compatibility">
2985<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2986<t>
2987   Clarified which error code should be used for inbound server failures
2988   (e.g. DNS failures). (<xref target="status.504"/>).
2989</t>
2990<t>
2991   201 (Created) had a race that required an Etag be sent when a resource is
2992   first created. (<xref target="status.201"/>).
2993</t>
2994<t>
2995   Rewrite of message transmission requirements to make it much harder
2996   for implementors to get it wrong, as the consequences of errors here
2997   can have significant impact on the Internet, and to deal with the
2998   following problems:
2999  <list style="numbers">
3000      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
3001         this was incorrectly placing a requirement on the behavior of
3002         an implementation of a future version of HTTP/1.x</t>
3003
3004      <t>Made it clear that user-agents should retry requests, not
3005         "clients" in general.</t>
3006
3007      <t>Converted requirements for clients to ignore unexpected 100
3008         (Continue) responses, and for proxies to forward 100 responses,
3009         into a general requirement for 1xx responses.</t>
3010
3011      <t>Modified some TCP-specific language, to make it clearer that
3012         non-TCP transports are possible for HTTP.</t>
3013
3014      <t>Require that the origin server MUST NOT wait for the request
3015         body before it sends a required 100 (Continue) response.</t>
3016
3017      <t>Allow, rather than require, a server to omit 100 (Continue) if
3018         it has already seen some of the request body.</t>
3019
3020      <t>Allow servers to defend against denial-of-service attacks and
3021         broken clients.</t>
3022  </list>
3023</t>
3024<t>
3025   This change adds the Expect header and 417 status code.
3026</t>
3027<t>
3028   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
3029   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
3030</t>
3031<t>
3032   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
3033   implemented in previous versions of this specification. See Section 19.6.1 of <xref target="RFC2068"/>.
3034</t>
3035</section>
3036
3037<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
3038<t>
3039  This document takes over the Status Code Registry, previously defined
3040  in Section 7.1 of <xref target="RFC2817"/>.
3041  (<xref target="status.code.registry"/>)
3042</t>
3043<t>
3044  Clarify definition of POST.
3045  (<xref target="POST"/>)
3046</t>
3047<t>
3048  Failed to consider that there are
3049  many other request methods that are safe to automatically redirect,
3050  and further that the user agent is able to make that determination
3051  based on the request method semantics.
3052  (Sections <xref format="counter" target="status.301"/>,
3053  <xref format="counter" target="status.302"/> and
3054  <xref format="counter" target="status.307"/>)
3055</t>
3056<t>
3057  Deprecate 305 Use Proxy status code, because user agents did not implement it.
3058  It used to indicate that the requested resource must be accessed through the
3059  proxy given by the Location field. The Location field gave the URI of the
3060  proxy. The recipient was expected to repeat this single request via the proxy.
3061  (<xref target="status.305"/>)
3062</t>
3063<t>
3064  Reclassify Allow header as response header, removing the option to
3065  specify it in a PUT request.
3066  Relax the server requirement on the contents of the Allow header and
3067  remove requirement on clients to always trust the header value.
3068  (<xref target="header.allow"/>)
3069</t>
3070<t>
3071  Correct syntax of Location header to allow fragment,
3072  as referred symbol wasn't what was expected, and add some
3073  clarifications as to when it would not be appropriate.
3074  (<xref target="header.location"/>)
3075</t>
3076<t>
3077  In the description of the Server header, the Via field
3078  was described as a SHOULD. The requirement was and is stated
3079  correctly in the description of the Via header in Section 8.9 of <xref target="Part1"/>.
3080  (<xref target="header.server"/>)
3081</t>
3082</section>
3083
3084</section>
3085
3086<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
3087
3088<section title="Since RFC2616">
3089<t>
3090  Extracted relevant partitions from <xref target="RFC2616"/>.
3091</t>
3092</section>
3093
3094<section title="Since draft-ietf-httpbis-p2-semantics-00">
3095<t>
3096  Closed issues:
3097  <list style="symbols"> 
3098    <t>
3099      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/5"/>:
3100      "Via is a MUST"
3101      (<eref target="http://purl.org/NET/http-errata#via-must"/>)
3102    </t>
3103    <t>
3104      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/6"/>:
3105      "Fragments allowed in Location"
3106      (<eref target="http://purl.org/NET/http-errata#location-fragments"/>)
3107    </t>
3108    <t>
3109      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/10"/>:
3110      "Safe Methods vs Redirection"
3111      (<eref target="http://purl.org/NET/http-errata#saferedirect"/>)
3112    </t>
3113    <t>
3114      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/17"/>:
3115      "Revise description of the POST method"
3116      (<eref target="http://purl.org/NET/http-errata#post"/>)
3117    </t>
3118    <t>
3119      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
3120      "Normative and Informative references"
3121    </t>
3122    <t>
3123      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
3124      "RFC2606 Compliance"
3125    </t>
3126    <t>
3127      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
3128      "Informative references"
3129    </t>
3130    <t>
3131      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/84"/>:
3132      "Redundant cross-references"
3133    </t>
3134  </list>
3135</t>
3136<t>
3137  Other changes:
3138  <list style="symbols"> 
3139    <t>
3140      Move definitions of 304 and 412 condition codes to <xref target="Part4"/>
3141    </t>
3142  </list>
3143</t>
3144</section>
3145
3146<section title="Since draft-ietf-httpbis-p2-semantics-01">
3147<t>
3148  Closed issues:
3149  <list style="symbols"> 
3150    <t>
3151      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/21"/>:
3152      "PUT side effects"
3153    </t>
3154    <t>
3155      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/91"/>:
3156      "Duplicate Host header requirements"
3157    </t>
3158  </list>
3159</t>
3160<t>
3161  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3162  <list style="symbols"> 
3163    <t>
3164      Move "Product Tokens" section (back) into Part 1, as "token" is used
3165      in the definition of the Upgrade header.
3166    </t>
3167    <t>
3168      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3169    </t>
3170    <t>
3171      Copy definition of delta-seconds from Part6 instead of referencing it.
3172    </t>
3173  </list>
3174</t>
3175</section>
3176
3177<section title="Since draft-ietf-httpbis-p2-semantics-02" anchor="changes.since.02">
3178<t>
3179  Closed issues:
3180  <list style="symbols"> 
3181    <t>
3182      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/24"/>:
3183      "Requiring Allow in 405 responses"
3184    </t>
3185    <t>
3186      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/59"/>:
3187      "Status Code Registry"
3188    </t>
3189    <t>
3190      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/61"/>:
3191      "Redirection vs. Location"
3192    </t>
3193    <t>
3194      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/70"/>:
3195      "Cacheability of 303 response"
3196    </t>
3197    <t>
3198      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/76"/>:
3199      "305 Use Proxy"
3200    </t>
3201    <t>
3202      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
3203      "Classification for Allow header"
3204    </t>
3205    <t>
3206      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/112"/>:
3207      "PUT - 'store under' vs 'store at'"
3208    </t>
3209  </list>
3210</t>
3211<t>
3212  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
3213  <list style="symbols"> 
3214    <t>
3215      Reference RFC 3984, and update header registrations for headers defined
3216      in this document.
3217    </t>
3218  </list>
3219</t>
3220<t>
3221  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3222  <list style="symbols"> 
3223    <t>
3224      Replace string literals when the string really is case-sensitive (method).
3225    </t>
3226  </list>
3227</t>
3228</section>
3229
3230<section title="Since draft-ietf-httpbis-p2-semantics-03" anchor="changes.since.03">
3231<t>
3232  Closed issues:
3233  <list style="symbols"> 
3234    <t>
3235      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/98"/>:
3236      "OPTIONS request bodies"
3237    </t>
3238    <t>
3239      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/119"/>:
3240      "Description of CONNECT should refer to RFC2817"
3241    </t>
3242    <t>
3243      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/125"/>:
3244      "Location Content-Location reference request/response mixup"
3245    </t>
3246  </list>
3247</t>
3248<t>
3249  Ongoing work on Method Registry (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/72"/>):
3250  <list style="symbols"> 
3251    <t>
3252      Added initial proposal for registration process, plus initial
3253      content (non-HTTP/1.1 methods to be added by a separate specification).
3254    </t>
3255  </list>
3256</t>
3257</section>
3258
3259<section title="Since draft-ietf-httpbis-p2-semantics-04" anchor="changes.since.04">
3260<t>
3261  Closed issues:
3262  <list style="symbols"> 
3263    <t>
3264      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/103"/>:
3265      "Content-*"
3266    </t>
3267    <t>
3268      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
3269      "RFC 2822 is updated by RFC 5322"
3270    </t>
3271  </list>
3272</t>
3273<t>
3274  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3275  <list style="symbols"> 
3276    <t>
3277      Use "/" instead of "|" for alternatives.
3278    </t>
3279    <t>
3280      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3281      whitespace ("OWS") and required whitespace ("RWS").
3282    </t>
3283    <t>
3284      Rewrite ABNFs to spell out whitespace rules, factor out
3285      header value format definitions.
3286    </t>
3287  </list>
3288</t>
3289</section>
3290
3291</section>
3292
3293</back>
3294</rfc>
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