source: draft-ietf-httpbis/latest/p2-semantics.xml @ 29

Last change on this file since 29 was 29, checked in by fielding@…, 12 years ago

Partition RFC 2616 into seven (mostly) independent documents.
No semantic changes. Some meaningless crossreferences to prior
editorial decisions have been removed from appendices.

Structural changes minimized to simplify diff versus rfc2616.
This was a lot harder than it looks.

Part 8 (Cookies) is for future specification based on RFC 2965.

  • Property svn:eol-style set to native
File size: 102.0 KB
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[29]1<?xml version="1.0" encoding="utf-8"?>
[8]2<!DOCTYPE rfc [
3  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
4  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
5  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
6  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
7  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
8  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
9  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
10  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
11  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
12  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
[29]13  <!ENTITY ID-VERSION "latest">
14  <!ENTITY messaging                  "[Part 1]">
15  <!ENTITY payload                    "[Part 3]">
16  <!ENTITY conditional                "[Part 4]">
17  <!ENTITY range                      "[Part 5]">
18  <!ENTITY caching                    "[Part 6]">
19  <!ENTITY auth                       "[Part 7]">
20  <!ENTITY content-negotiation        "[Part 3]">
21  <!ENTITY diff2045entity             "[Part 3]">
22  <!ENTITY uri                        "[Part 1]">
23  <!ENTITY http-url                   "[Part 1]">
24  <!ENTITY http-version               "[Part 1]">
25  <!ENTITY use100                     "[Part 1]">
26  <!ENTITY qvalue                     "[Part 3]">
27  <!ENTITY header-accept              "[Part 3]">
28  <!ENTITY header-accept-charset      "[Part 3]">
29  <!ENTITY header-accept-encoding     "[Part 3]">
30  <!ENTITY header-accept-language     "[Part 3]">
31  <!ENTITY header-accept-ranges       "[Part 3]">
32  <!ENTITY header-age                 "[Part 6]">
33  <!ENTITY header-authorization       "[Part 7]">
34  <!ENTITY header-cache-control       "[Part 6]">
35  <!ENTITY header-content-location    "[Part 3]">
36  <!ENTITY header-content-range       "[Part 5]">
37  <!ENTITY header-etag                "[Part 4]">
38  <!ENTITY header-expires             "[Part 6]">
39  <!ENTITY header-host                "[Part 1]">
40  <!ENTITY header-if-match            "[Part 4]">
41  <!ENTITY header-if-modified-since   "[Part 4]">
42  <!ENTITY header-if-none-match       "[Part 4]">
43  <!ENTITY header-if-range            "[Part 5]">
44  <!ENTITY header-if-unmodified-since "[Part 4]">
45  <!ENTITY header-pragma              "[Part 6]">
46  <!ENTITY header-proxy-authenticate  "[Part 7]">
47  <!ENTITY header-proxy-authorization "[Part 7]">
48  <!ENTITY header-range               "[Part 5]">
49  <!ENTITY header-upgrade             "[Part 1]">
50  <!ENTITY header-te                  "[Part 1]">
51  <!ENTITY header-vary                "[Part 6]">
52  <!ENTITY header-via                 "[Part 1]">
53  <!ENTITY header-warning             "[Part 6]">
54  <!ENTITY header-www-authenticate    "[Part 7]">
55  <!ENTITY message-body               "[Part 1]">
[8]56]>
57<?rfc toc="yes" ?>
[29]58<?rfc symrefs="yes" ?>
59<?rfc sortrefs="yes" ?>
[8]60<?rfc compact="yes"?>
61<?rfc subcompact="no" ?>
62<?rfc linkmailto="no" ?>
63<?rfc editing="no" ?>
64<?rfc-ext allow-markup-in-artwork="yes" ?>
65<?rfc-ext include-references-in-index="yes" ?>
[29]66<rfc obsoletes="2068, 2616, 2617" category="std"
67     ipr="full3978" docName="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"
68     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
[8]69<front>
70
[29]71  <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
[8]72
[29]73  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
74    <organization abbrev="Day Software">Day Software</organization>
[8]75    <address>
76      <postal>
[29]77        <street>23 Corporate Plaza DR, Suite 280</street>
78        <city>Newport Beach</city>
[8]79        <region>CA</region>
[29]80        <code>92660</code>
81        <country>USA</country>
[8]82      </postal>
[29]83      <phone>+1-949-706-5300</phone>
84      <facsimile>+1-949-706-5305</facsimile>
85      <email>fielding@gbiv.com</email>
86      <uri>http://roy.gbiv.com/</uri>
[8]87    </address>
88  </author>
89
[29]90  <author initials="J." surname="Gettys" fullname="Jim Gettys">
91    <organization>One Laptop per Child</organization>
[8]92    <address>
93      <postal>
[29]94        <street>21 Oak Knoll Road</street>
95        <city>Carlisle</city>
[8]96        <region>MA</region>
[29]97        <code>01741</code>
98        <country>USA</country>
[8]99      </postal>
[29]100      <email>jg@laptop.org</email>
101      <uri>http://www.laptop.org/</uri>
[8]102    </address>
103  </author>
104 
105  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
[29]106    <organization abbrev="HP">Hewlett-Packard Company</organization>
[8]107    <address>
108      <postal>
[29]109        <street>HP Labs, Large Scale Systems Group</street>
110        <street>1501 Page Mill Road, MS 1177</street>
[8]111        <city>Palo Alto</city>
112        <region>CA</region>
[29]113        <code>94304</code>
114        <country>USA</country>
[8]115      </postal>
[29]116      <email>JeffMogul@acm.org</email>
[8]117    </address>
118  </author>
119
120  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
[29]121    <organization abbrev="Microsoft">Microsoft Corporation</organization>
[8]122    <address>
123      <postal>
[29]124        <street>1 Microsoft Way</street>
125        <city>Redmond</city>
126        <region>WA</region>
127        <code>98052</code>
128        <country>USA</country>
[8]129      </postal>
[29]130      <email>henrikn@microsoft.com</email>
[8]131    </address>
132  </author>
133
134  <author initials="L." surname="Masinter" fullname="Larry Masinter">
[29]135    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
[8]136    <address>
137      <postal>
[29]138        <street>345 Park Ave</street>
139        <city>San Jose</city>
[8]140        <region>CA</region>
[29]141        <code>95110</code>
142        <country>USA</country>
[8]143      </postal>
[29]144      <email>LMM@acm.org</email>
145      <uri>http://larry.masinter.net/</uri>
[8]146    </address>
147  </author>
148 
149  <author initials="P." surname="Leach" fullname="Paul J. Leach">
150    <organization abbrev="Microsoft">Microsoft Corporation</organization>
151    <address>
152      <postal>
153        <street>1 Microsoft Way</street>
154        <city>Redmond</city>
155        <region>WA</region>
156        <code>98052</code>
157      </postal>
158      <email>paulle@microsoft.com</email>
159    </address>
160  </author>
161   
162  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
163    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
164    <address>
165      <postal>
[29]166        <street>MIT Laboratory for Computer Science</street>
[8]167        <street>545 Technology Square</street>
168        <city>Cambridge</city>
169        <region>MA</region>
170        <code>02139</code>
[29]171        <country>USA</country>
[8]172      </postal>
[29]173      <facsimile>+1 (617) 258 8682</facsimile>
[8]174      <email>timbl@w3.org</email>
175    </address>
176  </author>
177
[29]178  <date month="December" year="2007"/>
[8]179
180<abstract>
181<t>
182   The Hypertext Transfer Protocol (HTTP) is an application-level
183   protocol for distributed, collaborative, hypermedia information
[29]184   systems. HTTP has been in use by the World Wide Web global information
185   initiative since 1990. This document is Part 2 of the eight-part specification
186   that defines the protocol referred to as "HTTP/1.1" and, taken together,
187   updates RFC 2616 and RFC 2617.  Part 2 defines the semantics of HTTP messages
188   as expressed by request methods, request-header fields, response status codes,
189   and response-header fields.
[8]190</t>
191</abstract>
192</front>
193<middle>
194<section title="Introduction" anchor="introduction">
195<t>
[29]196   This document will define aspects of HTTP related to request and response
197   semantics. Right now it only includes the extracted relevant sections of
198   RFC 2616 with only minor edits.
[8]199</t>
200<t>
201   The HTTP protocol is a request/response protocol. A client sends a
202   request to the server in the form of a request method, URI, and
203   protocol version, followed by a MIME-like message containing request
204   modifiers, client information, and possible body content over a
205   connection with a server. The server responds with a status line,
206   including the message's protocol version and a success or error code,
207   followed by a MIME-like message containing server information, entity
208   metainformation, and possible entity-body content. The relationship
[29]209   between HTTP and MIME is described in &diff2045entity;.
[8]210</t>
211</section>
212
213<section title="Product Tokens" anchor="product.tokens">
214<t>
215   Product tokens are used to allow communicating applications to
216   identify themselves by software name and version. Most fields using
217   product tokens also allow sub-products which form a significant part
218   of the application to be listed, separated by white space. By
219   convention, the products are listed in order of their significance
220   for identifying the application.
221</t>
222<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
223    product         = token ["/" product-version]
224    product-version = token
225</artwork></figure>
226<t>
227   Examples:
228</t>
229<figure><artwork type="example">
230    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
231    Server: Apache/0.8.4
232</artwork></figure>
233<t>
234   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
235   used for advertising or other non-essential information. Although any
236   token character &MAY; appear in a product-version, this token &SHOULD;
237   only be used for a version identifier (i.e., successive versions of
238   the same product &SHOULD; only differ in the product-version portion of
239   the product value).
240</t>
241</section>
242
243<section title="Method" anchor="method">
244<t>
245   The Method  token indicates the method to be performed on the
246   resource identified by the Request-URI. The method is case-sensitive.
247</t>
248<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
249    Method         = "OPTIONS"                ; <xref target="OPTIONS"/>
250                   | "GET"                    ; <xref target="GET"/>
251                   | "HEAD"                   ; <xref target="HEAD"/>
252                   | "POST"                   ; <xref target="POST"/>
253                   | "PUT"                    ; <xref target="PUT"/>
254                   | "DELETE"                 ; <xref target="DELETE"/>
255                   | "TRACE"                  ; <xref target="TRACE"/>
256                   | "CONNECT"                ; <xref target="CONNECT"/>
257                   | extension-method
258    extension-method = token
259</artwork></figure>
260<t>
261   The list of methods allowed by a resource can be specified in an
262   Allow header field (<xref target="header.allow"/>). The return code of the response
263   always notifies the client whether a method is currently allowed on a
264   resource, since the set of allowed methods can change dynamically. An
265   origin server &SHOULD; return the status code 405 (Method Not Allowed)
266   if the method is known by the origin server but not allowed for the
267   requested resource, and 501 (Not Implemented) if the method is
268   unrecognized or not implemented by the origin server. The methods GET
269   and HEAD &MUST; be supported by all general-purpose servers. All other
270   methods are &OPTIONAL;; however, if the above methods are implemented,
271   they &MUST; be implemented with the same semantics as those specified
272   in <xref target="method.definitions"/>.
273</t>
274</section>
275
276<section title="Request Header Fields" anchor="request.header.fields">
277<t>
278   The request-header fields allow the client to pass additional
279   information about the request, and about the client itself, to the
280   server. These fields act as request modifiers, with semantics
281   equivalent to the parameters on a programming language method
282   invocation.
283</t>
284<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-header"/>
[29]285    request-header = Accept                   ; &header-accept;
286                   | Accept-Charset           ; &header-accept-charset;
287                   | Accept-Encoding          ; &header-accept-encoding;
288                   | Accept-Language          ; &header-accept-language;
289                   | Authorization            ; &header-authorization;
[8]290                   | Expect                   ; <xref target="header.expect"/>
291                   | From                     ; <xref target="header.from"/>
[29]292                   | Host                     ; &header-host;
293                   | If-Match                 ; &header-if-match;
294                   | If-Modified-Since        ; &header-if-modified-since;
295                   | If-None-Match            ; &header-if-none-match;
296                   | If-Range                 ; &header-if-range;
297                   | If-Unmodified-Since      ; &header-if-unmodified-since;
[8]298                   | Max-Forwards             ; <xref target="header.max-forwards"/>
[29]299                   | Proxy-Authorization      ; &header-proxy-authorization;
300                   | Range                    ; &header-range;
[8]301                   | Referer                  ; <xref target="header.referer"/>
[29]302                   | TE                       ; &header-te;
[8]303                   | User-Agent               ; <xref target="header.user-agent"/>
304</artwork></figure>
305<t>
306   Request-header field names can be extended reliably only in
307   combination with a change in the protocol version. However, new or
308   experimental header fields &MAY; be given the semantics of request-header
309   fields if all parties in the communication recognize them to
310   be request-header fields. Unrecognized header fields are treated as
311   entity-header fields.
312</t>
313</section>
314
315<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
316<t>
317   The Status-Code element is a 3-digit integer result code of the
318   attempt to understand and satisfy the request. These codes are fully
319   defined in <xref target="status.codes"/>. The Reason-Phrase is intended to give a short
320   textual description of the Status-Code. The Status-Code is intended
321   for use by automata and the Reason-Phrase is intended for the human
322   user. The client is not required to examine or display the Reason-Phrase.
323</t>
324<t> 
325   The individual values of the numeric status codes defined for
326   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
327   presented below. The reason phrases listed here are only
328   recommendations -- they &MAY; be replaced by local equivalents without
329   affecting the protocol.
330</t>
331<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/>
332   Status-Code    =
333         "100"  ; <xref target="status.100"/>: Continue
334       | "101"  ; <xref target="status.101"/>: Switching Protocols
335       | "200"  ; <xref target="status.200"/>: OK
336       | "201"  ; <xref target="status.201"/>: Created
337       | "202"  ; <xref target="status.202"/>: Accepted
338       | "203"  ; <xref target="status.203"/>: Non-Authoritative Information
339       | "204"  ; <xref target="status.204"/>: No Content
340       | "205"  ; <xref target="status.205"/>: Reset Content
341       | "206"  ; <xref target="status.206"/>: Partial Content
342       | "300"  ; <xref target="status.300"/>: Multiple Choices
343       | "301"  ; <xref target="status.301"/>: Moved Permanently
344       | "302"  ; <xref target="status.302"/>: Found
345       | "303"  ; <xref target="status.303"/>: See Other
346       | "304"  ; <xref target="status.304"/>: Not Modified
347       | "305"  ; <xref target="status.305"/>: Use Proxy
348       | "307"  ; <xref target="status.307"/>: Temporary Redirect
349       | "400"  ; <xref target="status.400"/>: Bad Request
350       | "401"  ; <xref target="status.401"/>: Unauthorized
351       | "402"  ; <xref target="status.402"/>: Payment Required
352       | "403"  ; <xref target="status.403"/>: Forbidden
353       | "404"  ; <xref target="status.404"/>: Not Found
354       | "405"  ; <xref target="status.405"/>: Method Not Allowed
355       | "406"  ; <xref target="status.406"/>: Not Acceptable
356       | "407"  ; <xref target="status.407"/>: Proxy Authentication Required
357       | "408"  ; <xref target="status.408"/>: Request Time-out
358       | "409"  ; <xref target="status.409"/>: Conflict
359       | "410"  ; <xref target="status.410"/>: Gone
360       | "411"  ; <xref target="status.411"/>: Length Required
361       | "412"  ; <xref target="status.412"/>: Precondition Failed
362       | "413"  ; <xref target="status.413"/>: Request Entity Too Large
363       | "414"  ; <xref target="status.414"/>: Request-URI Too Large
364       | "415"  ; <xref target="status.415"/>: Unsupported Media Type
365       | "416"  ; <xref target="status.416"/>: Requested range not satisfiable
366       | "417"  ; <xref target="status.417"/>: Expectation Failed
367       | "500"  ; <xref target="status.500"/>: Internal Server Error
368       | "501"  ; <xref target="status.501"/>: Not Implemented
369       | "502"  ; <xref target="status.502"/>: Bad Gateway
370       | "503"  ; <xref target="status.503"/>: Service Unavailable
371       | "504"  ; <xref target="status.504"/>: Gateway Time-out
372       | "505"  ; <xref target="status.505"/>: HTTP Version not supported
373       | extension-code
374
375   extension-code = 3DIGIT
376   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
377</artwork></figure>
378<t>
379   HTTP status codes are extensible. HTTP applications are not required
380   to understand the meaning of all registered status codes, though such
381   understanding is obviously desirable. However, applications &MUST;
382   understand the class of any status code, as indicated by the first
383   digit, and treat any unrecognized response as being equivalent to the
384   x00 status code of that class, with the exception that an
385   unrecognized response &MUST-NOT; be cached. For example, if an
386   unrecognized status code of 431 is received by the client, it can
387   safely assume that there was something wrong with its request and
388   treat the response as if it had received a 400 status code. In such
389   cases, user agents &SHOULD; present to the user the entity returned
390   with the response, since that entity is likely to include human-readable
391   information which will explain the unusual status.
392</t>
393</section>
394
395<section title="Response Header Fields" anchor="response.header.fields">
396<t>
397   The response-header fields allow the server to pass additional
398   information about the response which cannot be placed in the Status-Line.
399   These header fields give information about the server and about
400   further access to the resource identified by the Request-URI.
401</t>
402<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="response-header"/>
[29]403    response-header = Accept-Ranges           ; &header-accept-ranges;
404                    | Age                     ; &header-age;
405                    | ETag                    ; &header-etag;
[8]406                    | Location                ; <xref target="header.location"/>
[29]407                    | Proxy-Authenticate      ; &header-proxy-authenticate;
[8]408                    | Retry-After             ; <xref target="header.retry-after"/>
409                    | Server                  ; <xref target="header.server"/>
[29]410                    | Vary                    ; &header-vary;
411                    | WWW-Authenticate        ; &header-www-authenticate;
[8]412</artwork></figure>
413<t>
414   Response-header field names can be extended reliably only in
415   combination with a change in the protocol version. However, new or
416   experimental header fields &MAY; be given the semantics of response-header
417   fields if all parties in the communication recognize them to
418   be response-header fields. Unrecognized header fields are treated as
419   entity-header fields.
420</t>
421</section>
422
423<section title="Entity" anchor="entity">
424<t>
425   Request and Response messages &MAY; transfer an entity if not otherwise
426   restricted by the request method or response status code. An entity
427   consists of entity-header fields and an entity-body, although some
[29]428   responses will only include the entity-headers. HTTP entity-body and
429   entity-header fields are defined in &payload;.
[8]430</t>
431<t>
432   An entity-body is only present in a message when a message-body is
[29]433   present, as described in &message-body;. The entity-body is obtained
[8]434   from the message-body by decoding any Transfer-Encoding that might
435   have been applied to ensure safe and proper transfer of the message.
436</t>
437</section>
438
439
440<section title="Method Definitions" anchor="method.definitions">
441<t>
442   The set of common methods for HTTP/1.1 is defined below. Although
443   this set can be expanded, additional methods cannot be assumed to
444   share the same semantics for separately extended clients and servers.
445
[29]446   The Host request-header field (&header-host;) &MUST; accompany all
[8]447   HTTP/1.1 requests.
448</t>
449
450<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
451
452<section title="Safe Methods" anchor="safe.methods">
453<t>
454   Implementors should be aware that the software represents the user in
455   their interactions over the Internet, and should be careful to allow
456   the user to be aware of any actions they might take which may have an
457   unexpected significance to themselves or others.
458</t>
459<t>
460   In particular, the convention has been established that the GET and
461   HEAD methods &SHOULD-NOT;  have the significance of taking an action
462   other than retrieval. These methods ought to be considered "safe".
463   This allows user agents to represent other methods, such as POST, PUT
464   and DELETE, in a special way, so that the user is made aware of the
465   fact that a possibly unsafe action is being requested.
466</t>
467<t>
468   Naturally, it is not possible to ensure that the server does not
469   generate side-effects as a result of performing a GET request; in
470   fact, some dynamic resources consider that a feature. The important
471   distinction here is that the user did not request the side-effects,
472   so therefore cannot be held accountable for them.
473</t>
474</section>
475
476<section title="Idempotent Methods" anchor="idempotent.methods">
477<t>
478   Methods can also have the property of "idempotence" in that (aside
479   from error or expiration issues) the side-effects of N &gt; 0 identical
480   requests is the same as for a single request. The methods GET, HEAD,
481   PUT and DELETE share this property. Also, the methods OPTIONS and
482   TRACE &SHOULD-NOT;  have side effects, and so are inherently idempotent.
483</t>
484<t>
485   However, it is possible that a sequence of several requests is non-idempotent,
486   even if all of the methods executed in that sequence are
487   idempotent. (A sequence is idempotent if a single execution of the
488   entire sequence always yields a result that is not changed by a
489   reexecution of all, or part, of that sequence.) For example, a
490   sequence is non-idempotent if its result depends on a value that is
491   later modified in the same sequence.
492</t>
493<t>
494   A sequence that never has side effects is idempotent, by definition
495   (provided that no concurrent operations are being executed on the
496   same set of resources).
497</t>
498</section>
499</section>
500
501<section title="OPTIONS" anchor="OPTIONS">
502  <iref primary="true" item="OPTIONS method" x:for-anchor=""/>
503  <iref primary="true" item="Methods" subitem="OPTIONS" x:for-anchor=""/>
504<t>
505   The OPTIONS method represents a request for information about the
506   communication options available on the request/response chain
507   identified by the Request-URI. This method allows the client to
508   determine the options and/or requirements associated with a resource,
509   or the capabilities of a server, without implying a resource action
510   or initiating a resource retrieval.
511</t>
512<t>
513   Responses to this method are not cacheable.
514</t>
515<t>
516   If the OPTIONS request includes an entity-body (as indicated by the
517   presence of Content-Length or Transfer-Encoding), then the media type
518   &MUST; be indicated by a Content-Type field. Although this
519   specification does not define any use for such a body, future
520   extensions to HTTP might use the OPTIONS body to make more detailed
521   queries on the server. A server that does not support such an
522   extension &MAY; discard the request body.
523</t>
524<t>
525   If the Request-URI is an asterisk ("*"), the OPTIONS request is
526   intended to apply to the server in general rather than to a specific
527   resource. Since a server's communication options typically depend on
528   the resource, the "*" request is only useful as a "ping" or "no-op"
529   type of method; it does nothing beyond allowing the client to test
530   the capabilities of the server. For example, this can be used to test
531   a proxy for HTTP/1.1 compliance (or lack thereof).
532</t>
533<t>
534   If the Request-URI is not an asterisk, the OPTIONS request applies
535   only to the options that are available when communicating with that
536   resource.
537</t>
538<t>
539   A 200 response &SHOULD; include any header fields that indicate
540   optional features implemented by the server and applicable to that
541   resource (e.g., Allow), possibly including extensions not defined by
542   this specification. The response body, if any, &SHOULD; also include
543   information about the communication options. The format for such a
544   body is not defined by this specification, but might be defined by
545   future extensions to HTTP. Content negotiation &MAY; be used to select
546   the appropriate response format. If no response body is included, the
547   response &MUST; include a Content-Length field with a field-value of
548   "0".
549</t>
550<t>
551   The Max-Forwards request-header field &MAY; be used to target a
552   specific proxy in the request chain. When a proxy receives an OPTIONS
553   request on an absoluteURI for which request forwarding is permitted,
554   the proxy &MUST; check for a Max-Forwards field. If the Max-Forwards
555   field-value is zero ("0"), the proxy &MUST-NOT; forward the message;
556   instead, the proxy &SHOULD; respond with its own communication options.
557   If the Max-Forwards field-value is an integer greater than zero, the
558   proxy &MUST; decrement the field-value when it forwards the request. If
559   no Max-Forwards field is present in the request, then the forwarded
560   request &MUST-NOT; include a Max-Forwards field.
561</t>
562</section>
563
564<section title="GET" anchor="GET">
565  <iref primary="true" item="GET method" x:for-anchor=""/>
566  <iref primary="true" item="Methods" subitem="GET" x:for-anchor=""/>
567<t>
568   The GET method means retrieve whatever information (in the form of an
569   entity) is identified by the Request-URI. If the Request-URI refers
570   to a data-producing process, it is the produced data which shall be
571   returned as the entity in the response and not the source text of the
572   process, unless that text happens to be the output of the process.
573</t>
574<t>
575   The semantics of the GET method change to a "conditional GET" if the
576   request message includes an If-Modified-Since, If-Unmodified-Since,
577   If-Match, If-None-Match, or If-Range header field. A conditional GET
578   method requests that the entity be transferred only under the
579   circumstances described by the conditional header field(s). The
580   conditional GET method is intended to reduce unnecessary network
581   usage by allowing cached entities to be refreshed without requiring
582   multiple requests or transferring data already held by the client.
583</t>
584<t>
585   The semantics of the GET method change to a "partial GET" if the
586   request message includes a Range header field. A partial GET requests
[29]587   that only part of the entity be transferred, as described in &header-range;.
[8]588   The partial GET method is intended to reduce unnecessary
589   network usage by allowing partially-retrieved entities to be
590   completed without transferring data already held by the client.
591</t>
592<t>
593   The response to a GET request is cacheable if and only if it meets
[29]594   the requirements for HTTP caching described in &caching;.
[8]595</t>
596<t>
597   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
598</t>
599</section>
600
601<section title="HEAD" anchor="HEAD">
602  <iref primary="true" item="HEAD method" x:for-anchor=""/>
603  <iref primary="true" item="Methods" subitem="HEAD" x:for-anchor=""/>
604<t>
605   The HEAD method is identical to GET except that the server &MUST-NOT;
606   return a message-body in the response. The metainformation contained
607   in the HTTP headers in response to a HEAD request &SHOULD; be identical
608   to the information sent in response to a GET request. This method can
609   be used for obtaining metainformation about the entity implied by the
610   request without transferring the entity-body itself. This method is
611   often used for testing hypertext links for validity, accessibility,
612   and recent modification.
613</t>
614<t>
615   The response to a HEAD request &MAY; be cacheable in the sense that the
616   information contained in the response &MAY; be used to update a
617   previously cached entity from that resource. If the new field values
618   indicate that the cached entity differs from the current entity (as
619   would be indicated by a change in Content-Length, Content-MD5, ETag
620   or Last-Modified), then the cache &MUST; treat the cache entry as
621   stale.
622</t>
623</section>
624
625<section title="POST" anchor="POST">
626  <iref primary="true" item="POST method" x:for-anchor=""/>
627  <iref primary="true" item="Methods" subitem="POST" x:for-anchor=""/>
628<t>
629   The POST method is used to request that the origin server accept the
630   entity enclosed in the request as a new subordinate of the resource
631   identified by the Request-URI in the Request-Line. POST is designed
632   to allow a uniform method to cover the following functions:
633  <list style="symbols">
634    <t>
635      Annotation of existing resources;
636    </t>
637    <t>
638        Posting a message to a bulletin board, newsgroup, mailing list,
639        or similar group of articles;
640    </t>
641    <t>
642        Providing a block of data, such as the result of submitting a
643        form, to a data-handling process;
644    </t>
645    <t>
646        Extending a database through an append operation.
647    </t>
648  </list>
649</t>
650<t>
651   The actual function performed by the POST method is determined by the
652   server and is usually dependent on the Request-URI. The posted entity
653   is subordinate to that URI in the same way that a file is subordinate
654   to a directory containing it, a news article is subordinate to a
655   newsgroup to which it is posted, or a record is subordinate to a
656   database.
657</t>
658<t>
659   The action performed by the POST method might not result in a
660   resource that can be identified by a URI. In this case, either 200
661   (OK) or 204 (No Content) is the appropriate response status,
662   depending on whether or not the response includes an entity that
663   describes the result.
664</t>
665<t>
666   If a resource has been created on the origin server, the response
667   &SHOULD; be 201 (Created) and contain an entity which describes the
668   status of the request and refers to the new resource, and a Location
669   header (see <xref target="header.location"/>).
670</t>
671<t>
672   Responses to this method are not cacheable, unless the response
673   includes appropriate Cache-Control or Expires header fields. However,
674   the 303 (See Other) response can be used to direct the user agent to
675   retrieve a cacheable resource.
676</t>
677<t>
678   POST requests &MUST; obey the message transmission requirements set out
[29]679   in [message.transmission.requirements].
[8]680</t>
681<t>
682   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
683</t>
684</section>
685
686<section title="PUT" anchor="PUT">
687  <iref primary="true" item="PUT method" x:for-anchor=""/>
688  <iref primary="true" item="Methods" subitem="PUT" x:for-anchor=""/>
689<t>
690   The PUT method requests that the enclosed entity be stored under the
691   supplied Request-URI. If the Request-URI refers to an already
692   existing resource, the enclosed entity &SHOULD; be considered as a
693   modified version of the one residing on the origin server. If the
694   Request-URI does not point to an existing resource, and that URI is
695   capable of being defined as a new resource by the requesting user
696   agent, the origin server can create the resource with that URI. If a
697   new resource is created, the origin server &MUST; inform the user agent
698   via the 201 (Created) response. If an existing resource is modified,
699   either the 200 (OK) or 204 (No Content) response codes &SHOULD; be sent
700   to indicate successful completion of the request. If the resource
701   could not be created or modified with the Request-URI, an appropriate
702   error response &SHOULD; be given that reflects the nature of the
703   problem. The recipient of the entity &MUST-NOT; ignore any Content-*
704   (e.g. Content-Range) headers that it does not understand or implement
705   and &MUST; return a 501 (Not Implemented) response in such cases.
706</t>
707<t>
708   If the request passes through a cache and the Request-URI identifies
709   one or more currently cached entities, those entries &SHOULD; be
710   treated as stale. Responses to this method are not cacheable.
711</t>
712<t>
713   The fundamental difference between the POST and PUT requests is
714   reflected in the different meaning of the Request-URI. The URI in a
715   POST request identifies the resource that will handle the enclosed
716   entity. That resource might be a data-accepting process, a gateway to
717   some other protocol, or a separate entity that accepts annotations.
718   In contrast, the URI in a PUT request identifies the entity enclosed
719   with the request -- the user agent knows what URI is intended and the
720   server &MUST-NOT; attempt to apply the request to some other resource.
721   If the server desires that the request be applied to a different URI,
722   it &MUST; send a 301 (Moved Permanently) response; the user agent &MAY;
723   then make its own decision regarding whether or not to redirect the
724   request.
725</t>
726<t>
727   A single resource &MAY; be identified by many different URIs. For
728   example, an article might have a URI for identifying "the current
729   version" which is separate from the URI identifying each particular
730   version. In this case, a PUT request on a general URI might result in
731   several other URIs being defined by the origin server.
732</t>
733<t>
734   HTTP/1.1 does not define how a PUT method affects the state of an
735   origin server.
736</t>
737<t>
738   PUT requests &MUST; obey the message transmission requirements set out
[29]739   in [message.transmission.requirements].
[8]740</t>
741<t>
742   Unless otherwise specified for a particular entity-header, the
743   entity-headers in the PUT request &SHOULD; be applied to the resource
744   created or modified by the PUT.
745</t>
746</section>
747
748<section title="DELETE" anchor="DELETE">
749  <iref primary="true" item="DELETE method" x:for-anchor=""/>
750  <iref primary="true" item="Methods" subitem="DELETE" x:for-anchor=""/>
751<t>
752   The DELETE method requests that the origin server delete the resource
753   identified by the Request-URI. This method &MAY; be overridden by human
754   intervention (or other means) on the origin server. The client cannot
755   be guaranteed that the operation has been carried out, even if the
756   status code returned from the origin server indicates that the action
757   has been completed successfully. However, the server &SHOULD-NOT; 
758   indicate success unless, at the time the response is given, it
759   intends to delete the resource or move it to an inaccessible
760   location.
761</t>
762<t>
763   A successful response &SHOULD; be 200 (OK) if the response includes an
764   entity describing the status, 202 (Accepted) if the action has not
765   yet been enacted, or 204 (No Content) if the action has been enacted
766   but the response does not include an entity.
767</t>
768<t>
769   If the request passes through a cache and the Request-URI identifies
770   one or more currently cached entities, those entries &SHOULD; be
771   treated as stale. Responses to this method are not cacheable.
772</t>
773</section>
774
775<section title="TRACE" anchor="TRACE">
776  <iref primary="true" item="TRACE method" x:for-anchor=""/>
777  <iref primary="true" item="Methods" subitem="TRACE" x:for-anchor=""/>
778<t>
779   The TRACE method is used to invoke a remote, application-layer loop-back
780   of the request message. The final recipient of the request
781   &SHOULD; reflect the message received back to the client as the
782   entity-body of a 200 (OK) response. The final recipient is either the
783   origin server or the first proxy or gateway to receive a Max-Forwards
784   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
785   &MUST-NOT; include an entity.
786</t>
787<t>
788   TRACE allows the client to see what is being received at the other
789   end of the request chain and use that data for testing or diagnostic
[29]790   information. The value of the Via header field (&header-via;) is of
[8]791   particular interest, since it acts as a trace of the request chain.
792   Use of the Max-Forwards header field allows the client to limit the
793   length of the request chain, which is useful for testing a chain of
794   proxies forwarding messages in an infinite loop.
795</t>
796<t>
797   If the request is valid, the response &SHOULD; contain the entire
798   request message in the entity-body, with a Content-Type of
799   "message/http". Responses to this method &MUST-NOT; be cached.
800</t>
801</section>
802
803<section title="CONNECT" anchor="CONNECT">
804  <iref primary="true" item="CONNECT method" x:for-anchor=""/>
805  <iref primary="true" item="Methods" subitem="CONNECT" x:for-anchor=""/>
806<t>
807   This specification reserves the method name CONNECT for use with a
808   proxy that can dynamically switch to being a tunnel (e.g. SSL
809   tunneling <xref target="Luo1998"/>).
810</t>
811</section>
812</section>
813
814
815<section title="Status Code Definitions" anchor="status.codes">
816<t>
817   Each Status-Code is described below, including a description of which
818   method(s) it can follow and any metainformation required in the
819   response.
820</t>
821
822<section title="Informational 1xx" anchor="status.1xx">
823<t>
824   This class of status code indicates a provisional response,
825   consisting only of the Status-Line and optional headers, and is
826   terminated by an empty line. There are no required headers for this
827   class of status code. Since HTTP/1.0 did not define any 1xx status
828   codes, servers &MUST-NOT; send a 1xx response to an HTTP/1.0 client
829   except under experimental conditions.
830</t>
831<t>
832   A client &MUST; be prepared to accept one or more 1xx status responses
833   prior to a regular response, even if the client does not expect a 100
834   (Continue) status message. Unexpected 1xx status responses &MAY; be
835   ignored by a user agent.
836</t>
837<t>
838   Proxies &MUST; forward 1xx responses, unless the connection between the
839   proxy and its client has been closed, or unless the proxy itself
840   requested the generation of the 1xx response. (For example, if a
841   proxy adds a "Expect: 100-continue" field when it forwards a request,
842   then it need not forward the corresponding 100 (Continue)
843   response(s).)
844</t>
845
846<section title="100 Continue" anchor="status.100">
847  <iref primary="true" item="100 Continue (status code)" x:for-anchor=""/>
848  <iref primary="true" item="Status Codes" subitem="100 Continue" x:for-anchor=""/>
849<t>
850   The client &SHOULD; continue with its request. This interim response is
851   used to inform the client that the initial part of the request has
852   been received and has not yet been rejected by the server. The client
853   &SHOULD; continue by sending the remainder of the request or, if the
854   request has already been completed, ignore this response. The server
855   &MUST; send a final response after the request has been completed. See
[29]856   &use100; for detailed discussion of the use and handling of this
[8]857   status code.
858</t>
859</section>
860
861<section title="101 Switching Protocols" anchor="status.101">
862  <iref primary="true" item="101 Switching Protocols (status code)" x:for-anchor=""/>
863  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols" x:for-anchor=""/>
864<t>
865   The server understands and is willing to comply with the client's
[29]866   request, via the Upgrade message header field (&header-upgrade;), for a
[8]867   change in the application protocol being used on this connection. The
868   server will switch protocols to those defined by the response's
869   Upgrade header field immediately after the empty line which
870   terminates the 101 response.
871</t>
872<t>
873   The protocol &SHOULD; be switched only when it is advantageous to do
874   so. For example, switching to a newer version of HTTP is advantageous
875   over older versions, and switching to a real-time, synchronous
876   protocol might be advantageous when delivering resources that use
877   such features.
878</t>
879</section>
880</section>
881
882<section title="Successful 2xx" anchor="status.2xx">
883<t>
884   This class of status code indicates that the client's request was
885   successfully received, understood, and accepted.
886</t>
887
888<section title="200 OK" anchor="status.200">
889  <iref primary="true" item="200 OK (status code)" x:for-anchor=""/>
890  <iref primary="true" item="Status Codes" subitem="200 OK" x:for-anchor=""/>
891<t>
892   The request has succeeded. The information returned with the response
893   is dependent on the method used in the request, for example:
894  <list style="hanging">
895    <t hangText="GET">
896          an entity corresponding to the requested resource is sent in
897          the response;
898    </t>
899    <t hangText="HEAD">
900          the entity-header fields corresponding to the requested
901          resource are sent in the response without any message-body;
902    </t>
903    <t hangText="POST">
904      an entity describing or containing the result of the action;
905    </t>
906    <t hangText="TRACE">
907      an entity containing the request message as received by the
908      end server.
909    </t>
910  </list>
911</t>
912</section>
913
914<section title="201 Created" anchor="status.201">
915  <iref primary="true" item="201 Created (status code)" x:for-anchor=""/>
916  <iref primary="true" item="Status Codes" subitem="201 Created" x:for-anchor=""/>
917<t>
918   The request has been fulfilled and resulted in a new resource being
919   created. The newly created resource can be referenced by the URI(s)
920   returned in the entity of the response, with the most specific URI
921   for the resource given by a Location header field. The response
922   &SHOULD; include an entity containing a list of resource
923   characteristics and location(s) from which the user or user agent can
924   choose the one most appropriate. The entity format is specified by
925   the media type given in the Content-Type header field. The origin
926   server &MUST; create the resource before returning the 201 status code.
927   If the action cannot be carried out immediately, the server &SHOULD;
928   respond with 202 (Accepted) response instead.
929</t>
930<t>
931   A 201 response &MAY; contain an ETag response header field indicating
932   the current value of the entity tag for the requested variant just
[29]933   created, see &header-etag;.
[8]934</t>
935</section>
936
937<section title="202 Accepted" anchor="status.202">
938  <iref primary="true" item="202 Accepted (status code)" x:for-anchor=""/>
939  <iref primary="true" item="Status Codes" subitem="202 Accepted" x:for-anchor=""/>
940<t>
941   The request has been accepted for processing, but the processing has
942   not been completed.  The request might or might not eventually be
943   acted upon, as it might be disallowed when processing actually takes
944   place. There is no facility for re-sending a status code from an
945   asynchronous operation such as this.
946</t>
947<t>
948   The 202 response is intentionally non-committal. Its purpose is to
949   allow a server to accept a request for some other process (perhaps a
950   batch-oriented process that is only run once per day) without
951   requiring that the user agent's connection to the server persist
952   until the process is completed. The entity returned with this
953   response &SHOULD; include an indication of the request's current status
954   and either a pointer to a status monitor or some estimate of when the
955   user can expect the request to be fulfilled.
956</t>
957</section>
958
959<section title="203 Non-Authoritative Information" anchor="status.203">
960  <iref primary="true" item="203 Non-Authoritative Information (status code)" x:for-anchor=""/>
961  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information" x:for-anchor=""/>
962<t>
963   The returned metainformation in the entity-header is not the
964   definitive set as available from the origin server, but is gathered
965   from a local or a third-party copy. The set presented &MAY; be a subset
966   or superset of the original version. For example, including local
967   annotation information about the resource might result in a superset
968   of the metainformation known by the origin server. Use of this
969   response code is not required and is only appropriate when the
970   response would otherwise be 200 (OK).
971</t>
972</section>
973
974<section title="204 No Content" anchor="status.204">
975  <iref primary="true" item="204 No Content (status code)" x:for-anchor=""/>
976  <iref primary="true" item="Status Codes" subitem="204 No Content" x:for-anchor=""/>
977<t>
978   The server has fulfilled the request but does not need to return an
979   entity-body, and might want to return updated metainformation. The
980   response &MAY; include new or updated metainformation in the form of
981   entity-headers, which if present &SHOULD; be associated with the
982   requested variant.
983</t>
984<t>
985   If the client is a user agent, it &SHOULD-NOT;  change its document view
986   from that which caused the request to be sent. This response is
987   primarily intended to allow input for actions to take place without
988   causing a change to the user agent's active document view, although
989   any new or updated metainformation &SHOULD; be applied to the document
990   currently in the user agent's active view.
991</t>
992<t>
993   The 204 response &MUST-NOT; include a message-body, and thus is always
994   terminated by the first empty line after the header fields.
995</t>
996</section>
997
998<section title="205 Reset Content" anchor="status.205">
999  <iref primary="true" item="205 Reset Content (status code)" x:for-anchor=""/>
1000  <iref primary="true" item="Status Codes" subitem="205 Reset Content" x:for-anchor=""/>
1001<t>
1002   The server has fulfilled the request and the user agent &SHOULD; reset
1003   the document view which caused the request to be sent. This response
1004   is primarily intended to allow input for actions to take place via
1005   user input, followed by a clearing of the form in which the input is
1006   given so that the user can easily initiate another input action. The
1007   response &MUST-NOT; include an entity.
1008</t>
1009</section>
1010
1011<section title="206 Partial Content" anchor="status.206">
1012  <iref primary="true" item="206 Partial Content (status code)" x:for-anchor=""/>
1013  <iref primary="true" item="Status Codes" subitem="206 Partial Content" x:for-anchor=""/>
1014<t>
[29]1015   The server has fulfilled the partial GET request for the resource
1016   and the enclosed entity is a partial representation as defined in &range;.
[8]1017</t>
1018</section>
1019</section>
1020
1021<section title="Redirection 3xx" anchor="status.3xx">
1022<t>
1023   This class of status code indicates that further action needs to be
1024   taken by the user agent in order to fulfill the request.  The action
1025   required &MAY; be carried out by the user agent without interaction
1026   with the user if and only if the method used in the second request is
1027   GET or HEAD. A client &SHOULD; detect infinite redirection loops, since
1028   such loops generate network traffic for each redirection.
1029  <list><t>
1030      <x:h>Note:</x:h> previous versions of this specification recommended a
1031      maximum of five redirections. Content developers should be aware
1032      that there might be clients that implement such a fixed
1033      limitation.
1034  </t></list>
1035</t>
1036
1037<section title="300 Multiple Choices" anchor="status.300">
1038  <iref primary="true" item="300 Multiple Choices (status code)" x:for-anchor=""/>
1039  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices" x:for-anchor=""/>
1040<t>
1041   The requested resource corresponds to any one of a set of
1042   representations, each with its own specific location, and agent-driven
[29]1043   negotiation information (&content-negotiation;) is being provided so that
[8]1044   the user (or user agent) can select a preferred representation and
1045   redirect its request to that location.
1046</t>
1047<t>
1048   Unless it was a HEAD request, the response &SHOULD; include an entity
1049   containing a list of resource characteristics and location(s) from
1050   which the user or user agent can choose the one most appropriate. The
1051   entity format is specified by the media type given in the Content-Type
1052   header field. Depending upon the format and the capabilities of
1053   the user agent, selection of the most appropriate choice &MAY; be
1054   performed automatically. However, this specification does not define
1055   any standard for such automatic selection.
1056</t>
1057<t>
1058   If the server has a preferred choice of representation, it &SHOULD;
1059   include the specific URI for that representation in the Location
1060   field; user agents &MAY; use the Location field value for automatic
1061   redirection. This response is cacheable unless indicated otherwise.
1062</t>
1063</section>
1064
1065<section title="301 Moved Permanently" anchor="status.301">
1066  <iref primary="true" item="301 Moved Permanently (status code)" x:for-anchor=""/>
1067  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently" x:for-anchor=""/>
1068<t>
1069   The requested resource has been assigned a new permanent URI and any
1070   future references to this resource &SHOULD; use one of the returned
1071   URIs.  Clients with link editing capabilities ought to automatically
1072   re-link references to the Request-URI to one or more of the new
1073   references returned by the server, where possible. This response is
1074   cacheable unless indicated otherwise.
1075</t>
1076<t>
1077   The new permanent URI &SHOULD; be given by the Location field in the
1078   response. Unless the request method was HEAD, the entity of the
1079   response &SHOULD; contain a short hypertext note with a hyperlink to
1080   the new URI(s).
1081</t>
1082<t>
1083   If the 301 status code is received in response to a request other
1084   than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
1085   request unless it can be confirmed by the user, since this might
1086   change the conditions under which the request was issued.
1087  <list><t>
1088      <x:h>Note:</x:h> When automatically redirecting a POST request after
1089      receiving a 301 status code, some existing HTTP/1.0 user agents
1090      will erroneously change it into a GET request.
1091  </t></list>
1092</t>
1093</section>
1094
1095<section title="302 Found" anchor="status.302">
1096  <iref primary="true" item="302 Found (status code)" x:for-anchor=""/>
1097  <iref primary="true" item="Status Codes" subitem="302 Found" x:for-anchor=""/>
1098<t>
1099   The requested resource resides temporarily under a different URI.
1100   Since the redirection might be altered on occasion, the client &SHOULD;
1101   continue to use the Request-URI for future requests.  This response
1102   is only cacheable if indicated by a Cache-Control or Expires header
1103   field.
1104</t>
1105<t>
1106   The temporary URI &SHOULD; be given by the Location field in the
1107   response. Unless the request method was HEAD, the entity of the
1108   response &SHOULD; contain a short hypertext note with a hyperlink to
1109   the new URI(s).
1110</t>
1111<t>
1112   If the 302 status code is received in response to a request other
1113   than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
1114   request unless it can be confirmed by the user, since this might
1115   change the conditions under which the request was issued.
1116  <list><t>
1117      <x:h>Note:</x:h> RFC 1945 and RFC 2068 specify that the client is not allowed
1118      to change the method on the redirected request.  However, most
1119      existing user agent implementations treat 302 as if it were a 303
1120      response, performing a GET on the Location field-value regardless
1121      of the original request method. The status codes 303 and 307 have
1122      been added for servers that wish to make unambiguously clear which
1123      kind of reaction is expected of the client.
1124  </t></list>
1125</t>
1126</section>
1127
1128<section title="303 See Other" anchor="status.303">
1129  <iref primary="true" item="303 See Other (status code)" x:for-anchor=""/>
1130  <iref primary="true" item="Status Codes" subitem="303 See Other" x:for-anchor=""/>
1131<t>
1132   The response to the request can be found under a different URI and
1133   &SHOULD; be retrieved using a GET method on that resource. This method
1134   exists primarily to allow the output of a POST-activated script to
1135   redirect the user agent to a selected resource. The new URI is not a
1136   substitute reference for the originally requested resource. The 303
1137   response &MUST-NOT; be cached, but the response to the second
1138   (redirected) request might be cacheable.
1139</t>
1140<t>
1141   The different URI &SHOULD; be given by the Location field in the
1142   response. Unless the request method was HEAD, the entity of the
1143   response &SHOULD; contain a short hypertext note with a hyperlink to
1144   the new URI(s).
1145  <list><t>
1146      <x:h>Note:</x:h> Many pre-HTTP/1.1 user agents do not understand the 303
1147      status. When interoperability with such clients is a concern, the
1148      302 status code may be used instead, since most user agents react
1149      to a 302 response as described here for 303.
1150  </t></list>
1151</t>
1152</section>
1153
1154<section title="304 Not Modified" anchor="status.304">
1155  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1156  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1157<t>
1158   If the client has performed a conditional GET request and access is
1159   allowed, but the document has not been modified, the server &SHOULD;
1160   respond with this status code. The 304 response &MUST-NOT; contain a
1161   message-body, and thus is always terminated by the first empty line
1162   after the header fields.
1163</t>
1164<t>
1165   The response &MUST; include the following header fields:
1166  <list style="symbols">
[29]1167    <t>Date, unless its omission is required by [clockless.origin.server.operation]</t>
[8]1168  </list>
1169</t>
1170<t>
1171   If a clockless origin server obeys these rules, and proxies and
1172   clients add their own Date to any response received without one (as
1173   already specified by [RFC 2068], section <xref target="RFC2068" x:sec="14.19" x:fmt="number"/>), caches will operate
1174   correctly.
1175  <list style="symbols">
1176    <t>ETag and/or Content-Location, if the header would have been sent
1177        in a 200 response to the same request</t>
1178    <t>Expires, Cache-Control, and/or Vary, if the field-value might
1179        differ from that sent in any previous response for the same
1180        variant</t>
1181  </list>
1182</t>
1183<t>
[29]1184   If the conditional GET used a strong cache validator (see &caching;),
[8]1185   the response &SHOULD-NOT;  include other entity-headers.
1186   Otherwise (i.e., the conditional GET used a weak validator), the
1187   response &MUST-NOT; include other entity-headers; this prevents
1188   inconsistencies between cached entity-bodies and updated headers.
1189</t>
1190<t>
1191   If a 304 response indicates an entity not currently cached, then the
1192   cache &MUST; disregard the response and repeat the request without the
1193   conditional.
1194</t>
1195<t>
1196   If a cache uses a received 304 response to update a cache entry, the
1197   cache &MUST; update the entry to reflect any new field values given in
1198   the response.
1199</t>
1200</section>
1201
1202<section title="305 Use Proxy" anchor="status.305">
1203  <iref primary="true" item="305 Use Proxy (status code)" x:for-anchor=""/>
1204  <iref primary="true" item="Status Codes" subitem="305 Use Proxy" x:for-anchor=""/>
1205<t>
1206   The requested resource &MUST; be accessed through the proxy given by
1207   the Location field. The Location field gives the URI of the proxy.
1208   The recipient is expected to repeat this single request via the
1209   proxy. 305 responses &MUST; only be generated by origin servers.
1210  <list><t>
1211      <x:h>Note:</x:h> RFC 2068 was not clear that 305 was intended to redirect a
1212      single request, and to be generated by origin servers only.  Not
1213      observing these limitations has significant security consequences.
1214  </t></list>
1215</t>
1216</section>
1217
1218<section title="306 (Unused)" anchor="status.306">
1219  <iref primary="true" item="306 (Unused) (status code)" x:for-anchor=""/>
1220  <iref primary="true" item="Status Codes" subitem="306 (Unused)" x:for-anchor=""/>
1221<t>
1222   The 306 status code was used in a previous version of the
1223   specification, is no longer used, and the code is reserved.
1224</t>
1225</section>
1226
1227<section title="307 Temporary Redirect" anchor="status.307">
1228  <iref primary="true" item="307 Temporary Redirect (status code)" x:for-anchor=""/>
1229  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect" x:for-anchor=""/>
1230<t>
1231   The requested resource resides temporarily under a different URI.
1232   Since the redirection &MAY; be altered on occasion, the client &SHOULD;
1233   continue to use the Request-URI for future requests.  This response
1234   is only cacheable if indicated by a Cache-Control or Expires header
1235   field.
1236</t>
1237<t>
1238   The temporary URI &SHOULD; be given by the Location field in the
1239   response. Unless the request method was HEAD, the entity of the
1240   response &SHOULD; contain a short hypertext note with a hyperlink to
1241   the new URI(s) , since many pre-HTTP/1.1 user agents do not
1242   understand the 307 status. Therefore, the note &SHOULD; contain the
1243   information necessary for a user to repeat the original request on
1244   the new URI.
1245</t>
1246<t>
1247   If the 307 status code is received in response to a request other
1248   than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
1249   request unless it can be confirmed by the user, since this might
1250   change the conditions under which the request was issued.
1251</t>
1252</section>
1253</section>
1254
1255<section title="Client Error 4xx" anchor="status.4xx">
1256<t>
1257   The 4xx class of status code is intended for cases in which the
1258   client seems to have erred. Except when responding to a HEAD request,
1259   the server &SHOULD; include an entity containing an explanation of the
1260   error situation, and whether it is a temporary or permanent
1261   condition. These status codes are applicable to any request method.
1262   User agents &SHOULD; display any included entity to the user.
1263</t>
1264<t>
1265   If the client is sending data, a server implementation using TCP
1266   &SHOULD; be careful to ensure that the client acknowledges receipt of
1267   the packet(s) containing the response, before the server closes the
1268   input connection. If the client continues sending data to the server
1269   after the close, the server's TCP stack will send a reset packet to
1270   the client, which may erase the client's unacknowledged input buffers
1271   before they can be read and interpreted by the HTTP application.
1272</t>
1273
1274<section title="400 Bad Request" anchor="status.400">
1275  <iref primary="true" item="400 Bad Request (status code)" x:for-anchor=""/>
1276  <iref primary="true" item="Status Codes" subitem="400 Bad Request" x:for-anchor=""/>
1277<t>
1278   The request could not be understood by the server due to malformed
1279   syntax. The client &SHOULD-NOT;  repeat the request without
1280   modifications.
1281</t>
1282</section>
1283
1284<section title="401 Unauthorized" anchor="status.401">
1285  <iref primary="true" item="401 Unauthorized (status code)" x:for-anchor=""/>
1286  <iref primary="true" item="Status Codes" subitem="401 Unauthorized" x:for-anchor=""/>
1287<t>
1288   The request requires user authentication. The response &MUST; include a
[29]1289   WWW-Authenticate header field (&header-www-authenticate;) containing a challenge
[8]1290   applicable to the requested resource. The client &MAY; repeat the
[29]1291   request with a suitable Authorization header field (&header-authorization;). If
[8]1292   the request already included Authorization credentials, then the 401
1293   response indicates that authorization has been refused for those
1294   credentials. If the 401 response contains the same challenge as the
1295   prior response, and the user agent has already attempted
1296   authentication at least once, then the user &SHOULD; be presented the
1297   entity that was given in the response, since that entity might
1298   include relevant diagnostic information. HTTP access authentication
1299   is explained in "HTTP Authentication: Basic and Digest Access
1300   Authentication" <xref target="RFC2617"/>.
1301</t>
1302</section>
1303
1304<section title="402 Payment Required" anchor="status.402">
1305  <iref primary="true" item="402 Payment Required (status code)" x:for-anchor=""/>
1306  <iref primary="true" item="Status Codes" subitem="402 Payment Required" x:for-anchor=""/>
1307<t>
1308   This code is reserved for future use.
1309</t>
1310</section>
1311
1312<section title="403 Forbidden" anchor="status.403">
1313  <iref primary="true" item="403 Forbidden (status code)" x:for-anchor=""/>
1314  <iref primary="true" item="Status Codes" subitem="403 Forbidden" x:for-anchor=""/>
1315<t>
1316   The server understood the request, but is refusing to fulfill it.
1317   Authorization will not help and the request &SHOULD-NOT;  be repeated.
1318   If the request method was not HEAD and the server wishes to make
1319   public why the request has not been fulfilled, it &SHOULD; describe the
1320   reason for the refusal in the entity.  If the server does not wish to
1321   make this information available to the client, the status code 404
1322   (Not Found) can be used instead.
1323</t>
1324</section>
1325
1326<section title="404 Not Found" anchor="status.404">
1327  <iref primary="true" item="404 Not Found (status code)" x:for-anchor=""/>
1328  <iref primary="true" item="Status Codes" subitem="404 Not Found" x:for-anchor=""/>
1329<t>
1330   The server has not found anything matching the Request-URI. No
1331   indication is given of whether the condition is temporary or
1332   permanent. The 410 (Gone) status code &SHOULD; be used if the server
1333   knows, through some internally configurable mechanism, that an old
1334   resource is permanently unavailable and has no forwarding address.
1335   This status code is commonly used when the server does not wish to
1336   reveal exactly why the request has been refused, or when no other
1337   response is applicable.
1338</t>
1339</section>
1340
1341<section title="405 Method Not Allowed" anchor="status.405">
1342  <iref primary="true" item="405 Method Not Allowed (status code)" x:for-anchor=""/>
1343  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed" x:for-anchor=""/>
1344<t>
1345   The method specified in the Request-Line is not allowed for the
1346   resource identified by the Request-URI. The response &MUST; include an
1347   Allow header containing a list of valid methods for the requested
1348   resource.
1349</t>
1350</section>
1351
1352<section title="406 Not Acceptable" anchor="status.406">
1353  <iref primary="true" item="406 Not Acceptable (status code)" x:for-anchor=""/>
1354  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable" x:for-anchor=""/>
1355<t>
1356   The resource identified by the request is only capable of generating
1357   response entities which have content characteristics not acceptable
1358   according to the accept headers sent in the request.
1359</t>
1360<t>
1361   Unless it was a HEAD request, the response &SHOULD; include an entity
1362   containing a list of available entity characteristics and location(s)
1363   from which the user or user agent can choose the one most
1364   appropriate. The entity format is specified by the media type given
1365   in the Content-Type header field. Depending upon the format and the
1366   capabilities of the user agent, selection of the most appropriate
1367   choice &MAY; be performed automatically. However, this specification
1368   does not define any standard for such automatic selection.
1369  <list><t>
1370      <x:h>Note:</x:h> HTTP/1.1 servers are allowed to return responses which are
1371      not acceptable according to the accept headers sent in the
1372      request. In some cases, this may even be preferable to sending a
1373      406 response. User agents are encouraged to inspect the headers of
1374      an incoming response to determine if it is acceptable.
1375  </t></list>
1376</t>
1377<t>
1378   If the response could be unacceptable, a user agent &SHOULD;
1379   temporarily stop receipt of more data and query the user for a
1380   decision on further actions.
1381</t>
1382</section>
1383
1384<section title="407 Proxy Authentication Required" anchor="status.407">
1385  <iref primary="true" item="407 Proxy Authentication Required (status code)" x:for-anchor=""/>
1386  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required" x:for-anchor=""/>
1387<t>
1388   This code is similar to 401 (Unauthorized), but indicates that the
1389   client must first authenticate itself with the proxy. The proxy &MUST;
[29]1390   return a Proxy-Authenticate header field (&header-proxy-authenticate;) containing a
[8]1391   challenge applicable to the proxy for the requested resource. The
1392   client &MAY; repeat the request with a suitable Proxy-Authorization
[29]1393   header field (&header-proxy-authorization;). HTTP access authentication is explained
[8]1394   in "HTTP Authentication: Basic and Digest Access Authentication"
1395   <xref target="RFC2617"/>.
1396</t>
1397</section>
1398
1399<section title="408 Request Timeout" anchor="status.408">
1400  <iref primary="true" item="408 Request Timeout (status code)" x:for-anchor=""/>
1401  <iref primary="true" item="Status Codes" subitem="408 Request Timeout" x:for-anchor=""/>
1402<t>
1403   The client did not produce a request within the time that the server
1404   was prepared to wait. The client &MAY; repeat the request without
1405   modifications at any later time.
1406</t>
1407</section>
1408
1409<section title="409 Conflict" anchor="status.409">
1410  <iref primary="true" item="409 Conflict (status code)" x:for-anchor=""/>
1411  <iref primary="true" item="Status Codes" subitem="409 Conflict" x:for-anchor=""/>
1412<t>
1413   The request could not be completed due to a conflict with the current
1414   state of the resource. This code is only allowed in situations where
1415   it is expected that the user might be able to resolve the conflict
1416   and resubmit the request. The response body &SHOULD; include enough
1417   information for the user to recognize the source of the conflict.
1418   Ideally, the response entity would include enough information for the
1419   user or user agent to fix the problem; however, that might not be
1420   possible and is not required.
1421</t>
1422<t>
1423   Conflicts are most likely to occur in response to a PUT request. For
1424   example, if versioning were being used and the entity being PUT
1425   included changes to a resource which conflict with those made by an
1426   earlier (third-party) request, the server might use the 409 response
1427   to indicate that it can't complete the request. In this case, the
1428   response entity would likely contain a list of the differences
1429   between the two versions in a format defined by the response
1430   Content-Type.
1431</t>
1432</section>
1433
1434<section title="410 Gone" anchor="status.410">
1435  <iref primary="true" item="410 Gone (status code)" x:for-anchor=""/>
1436  <iref primary="true" item="Status Codes" subitem="410 Gone" x:for-anchor=""/>
1437<t>
1438   The requested resource is no longer available at the server and no
1439   forwarding address is known. This condition is expected to be
1440   considered permanent. Clients with link editing capabilities &SHOULD;
1441   delete references to the Request-URI after user approval. If the
1442   server does not know, or has no facility to determine, whether or not
1443   the condition is permanent, the status code 404 (Not Found) &SHOULD; be
1444   used instead. This response is cacheable unless indicated otherwise.
1445</t>
1446<t>
1447   The 410 response is primarily intended to assist the task of web
1448   maintenance by notifying the recipient that the resource is
1449   intentionally unavailable and that the server owners desire that
1450   remote links to that resource be removed. Such an event is common for
1451   limited-time, promotional services and for resources belonging to
1452   individuals no longer working at the server's site. It is not
1453   necessary to mark all permanently unavailable resources as "gone" or
1454   to keep the mark for any length of time -- that is left to the
1455   discretion of the server owner.
1456</t>
1457</section>
1458
1459<section title="411 Length Required" anchor="status.411">
1460  <iref primary="true" item="411 Length Required (status code)" x:for-anchor=""/>
1461  <iref primary="true" item="Status Codes" subitem="411 Length Required" x:for-anchor=""/>
1462<t>
1463   The server refuses to accept the request without a defined Content-Length.
1464   The client &MAY; repeat the request if it adds a valid
1465   Content-Length header field containing the length of the message-body
1466   in the request message.
1467</t>
1468</section>
1469
1470<section title="412 Precondition Failed" anchor="status.412">
1471  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1472  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1473<t>
1474   The precondition given in one or more of the request-header fields
1475   evaluated to false when it was tested on the server. This response
1476   code allows the client to place preconditions on the current resource
1477   metainformation (header field data) and thus prevent the requested
1478   method from being applied to a resource other than the one intended.
1479</t>
1480</section>
1481
1482<section title="413 Request Entity Too Large" anchor="status.413">
1483  <iref primary="true" item="413 Request Entity Too Large (status code)" x:for-anchor=""/>
1484  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large" x:for-anchor=""/>
1485<t>
1486   The server is refusing to process a request because the request
1487   entity is larger than the server is willing or able to process. The
1488   server &MAY; close the connection to prevent the client from continuing
1489   the request.
1490</t>
1491<t>
1492   If the condition is temporary, the server &SHOULD; include a Retry-After
1493   header field to indicate that it is temporary and after what
1494   time the client &MAY; try again.
1495</t>
1496</section>
1497
1498<section title="414 Request-URI Too Long" anchor="status.414">
1499  <iref primary="true" item="414 Request-URI Too Long (status code)" x:for-anchor=""/>
1500  <iref primary="true" item="Status Codes" subitem="414 Request-URI Too Long" x:for-anchor=""/>
1501<t>
1502   The server is refusing to service the request because the Request-URI
1503   is longer than the server is willing to interpret. This rare
1504   condition is only likely to occur when a client has improperly
1505   converted a POST request to a GET request with long query
1506   information, when the client has descended into a URI "black hole" of
1507   redirection (e.g., a redirected URI prefix that points to a suffix of
1508   itself), or when the server is under attack by a client attempting to
1509   exploit security holes present in some servers using fixed-length
1510   buffers for reading or manipulating the Request-URI.
1511</t>
1512</section>
1513
1514<section title="415 Unsupported Media Type" anchor="status.415">
1515  <iref primary="true" item="415 Unsupported Media Type (status code)" x:for-anchor=""/>
1516  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type" x:for-anchor=""/>
1517<t>
1518   The server is refusing to service the request because the entity of
1519   the request is in a format not supported by the requested resource
1520   for the requested method.
1521</t>
1522</section>
1523
1524<section title="416 Requested Range Not Satisfiable" anchor="status.416">
1525  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)" x:for-anchor=""/>
1526  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable" x:for-anchor=""/>
1527<t>
[29]1528   The request included a Range request-header field (&header-range;) and none of
[8]1529   the range-specifier values in this field overlap the current extent
[29]1530   of the selected resource.
[8]1531</t>
1532</section>
1533
1534<section title="417 Expectation Failed" anchor="status.417">
1535  <iref primary="true" item="417 Expectation Failed (status code)" x:for-anchor=""/>
1536  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed" x:for-anchor=""/>
1537<t>
1538   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
1539   could not be met by this server, or, if the server is a proxy,
1540   the server has unambiguous evidence that the request could not be met
1541   by the next-hop server.
1542</t>
1543</section>
1544</section>
1545
1546<section title="Server Error 5xx" anchor="status.5xx">
1547<t>
1548   Response status codes beginning with the digit "5" indicate cases in
1549   which the server is aware that it has erred or is incapable of
1550   performing the request. Except when responding to a HEAD request, the
1551   server &SHOULD; include an entity containing an explanation of the
1552   error situation, and whether it is a temporary or permanent
1553   condition. User agents &SHOULD; display any included entity to the
1554   user. These response codes are applicable to any request method.
1555</t>
1556
1557<section title="500 Internal Server Error" anchor="status.500">
1558  <iref primary="true" item="500 Internal Server Error (status code)" x:for-anchor=""/>
1559  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error" x:for-anchor=""/>
1560<t>
1561   The server encountered an unexpected condition which prevented it
1562   from fulfilling the request.
1563</t>
1564</section>
1565
1566<section title="501 Not Implemented" anchor="status.501">
1567  <iref primary="true" item="501 Not Implemented (status code)" x:for-anchor=""/>
1568  <iref primary="true" item="Status Codes" subitem="501 Not Implemented" x:for-anchor=""/>
1569<t>
1570   The server does not support the functionality required to fulfill the
1571   request. This is the appropriate response when the server does not
1572   recognize the request method and is not capable of supporting it for
1573   any resource.
1574</t>
1575</section>
1576
1577<section title="502 Bad Gateway" anchor="status.502">
1578  <iref primary="true" item="502 Bad Gateway (status code)" x:for-anchor=""/>
1579  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway" x:for-anchor=""/>
1580<t>
1581   The server, while acting as a gateway or proxy, received an invalid
1582   response from the upstream server it accessed in attempting to
1583   fulfill the request.
1584</t>
1585</section>
1586
1587<section title="503 Service Unavailable" anchor="status.503">
1588  <iref primary="true" item="503 Service Unavailable (status code)" x:for-anchor=""/>
1589  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable" x:for-anchor=""/>
1590<t>
1591   The server is currently unable to handle the request due to a
1592   temporary overloading or maintenance of the server. The implication
1593   is that this is a temporary condition which will be alleviated after
1594   some delay. If known, the length of the delay &MAY; be indicated in a
1595   Retry-After header. If no Retry-After is given, the client &SHOULD;
1596   handle the response as it would for a 500 response.
1597  <list><t>
1598      <x:h>Note:</x:h> The existence of the 503 status code does not imply that a
1599      server must use it when becoming overloaded. Some servers may wish
1600      to simply refuse the connection.
1601  </t></list>
1602</t>
1603</section>
1604
1605<section title="504 Gateway Timeout" anchor="status.504">
1606  <iref primary="true" item="504 Gateway Timeout (status code)" x:for-anchor=""/>
1607  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout" x:for-anchor=""/>
1608<t>
1609   The server, while acting as a gateway or proxy, did not receive a
1610   timely response from the upstream server specified by the URI (e.g.
1611   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
1612   to access in attempting to complete the request.
1613  <list><t>
1614      <x:h>Note:</x:h> Note to implementors: some deployed proxies are known to
1615      return 400 or 500 when DNS lookups time out.
1616  </t></list>
1617</t>
1618</section>
1619
1620<section title="505 HTTP Version Not Supported" anchor="status.505">
1621  <iref primary="true" item="505 HTTP Version Not Supported (status code)" x:for-anchor=""/>
1622  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported" x:for-anchor=""/>
1623<t>
1624   The server does not support, or refuses to support, the HTTP protocol
1625   version that was used in the request message. The server is
1626   indicating that it is unable or unwilling to complete the request
[29]1627   using the same major version as the client, as described in &http-version;,
[8]1628   other than with this error message. The response &SHOULD; contain
1629   an entity describing why that version is not supported and what other
1630   protocols are supported by that server.
1631</t>
1632
1633</section>
1634</section>
1635</section>
1636
1637
1638<section title="Header Field Definitions" anchor="header.fields">
1639<t>
1640   This section defines the syntax and semantics of all standard
1641   HTTP/1.1 header fields. For entity-header fields, both sender and
1642   recipient refer to either the client or the server, depending on who
1643   sends and who receives the entity.
1644</t>
1645
1646<section title="Allow" anchor="header.allow">
1647  <iref primary="true" item="Allow header" x:for-anchor=""/>
1648  <iref primary="true" item="Headers" subitem="Allow" x:for-anchor=""/>
1649<t>
1650      The Allow entity-header field lists the set of methods supported
1651      by the resource identified by the Request-URI. The purpose of this
1652      field is strictly to inform the recipient of valid methods
1653      associated with the resource. An Allow header field &MUST; be
1654      present in a 405 (Method Not Allowed) response.
1655</t>
1656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Allow"/>
1657       Allow   = "Allow" ":" #Method
1658</artwork></figure>
1659<t>
1660      Example of use:
1661</t>
1662<figure><artwork type="example">
1663       Allow: GET, HEAD, PUT
1664</artwork></figure>
1665<t>
1666      This field cannot prevent a client from trying other methods.
1667      However, the indications given by the Allow header field value
1668      &SHOULD; be followed. The actual set of allowed methods is defined
1669      by the origin server at the time of each request.
1670</t>
1671<t>
1672      The Allow header field &MAY; be provided with a PUT request to
1673      recommend the methods to be supported by the new or modified
1674      resource. The server is not required to support these methods and
1675      &SHOULD; include an Allow header in the response giving the actual
1676      supported methods.
1677</t>
1678<t>
1679      A proxy &MUST-NOT; modify the Allow header field even if it does not
1680      understand all the methods specified, since the user agent might
1681      have other means of communicating with the origin server.
1682</t>
1683</section>
1684
1685<section title="Expect" anchor="header.expect">
1686  <iref primary="true" item="Expect header" x:for-anchor=""/>
1687  <iref primary="true" item="Headers" subitem="Expect" x:for-anchor=""/>
1688<t>
1689   The Expect request-header field is used to indicate that particular
1690   server behaviors are required by the client.
1691</t>
1692<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expect"/><iref primary="true" item="Grammar" subitem="expectation"/><iref primary="true" item="Grammar" subitem="expectation-extension"/><iref primary="true" item="Grammar" subitem="expect-params"/>
1693   Expect       =  "Expect" ":" 1#expectation
1694
1695   expectation  =  "100-continue" | expectation-extension
1696   expectation-extension =  token [ "=" ( token | quoted-string )
1697                            *expect-params ]
1698   expect-params =  ";" token [ "=" ( token | quoted-string ) ]
1699</artwork></figure>
1700<t>
1701   A server that does not understand or is unable to comply with any of
1702   the expectation values in the Expect field of a request &MUST; respond
1703   with appropriate error status. The server &MUST; respond with a 417
1704   (Expectation Failed) status if any of the expectations cannot be met
1705   or, if there are other problems with the request, some other 4xx
1706   status.
1707</t>
1708<t>
1709   This header field is defined with extensible syntax to allow for
1710   future extensions. If a server receives a request containing an
1711   Expect field that includes an expectation-extension that it does not
1712   support, it &MUST; respond with a 417 (Expectation Failed) status.
1713</t>
1714<t>
1715   Comparison of expectation values is case-insensitive for unquoted
1716   tokens (including the 100-continue token), and is case-sensitive for
1717   quoted-string expectation-extensions.
1718</t>
1719<t>
1720   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy &MUST;
1721   return a 417 (Expectation Failed) status if it receives a request
1722   with an expectation that it cannot meet. However, the Expect
1723   request-header itself is end-to-end; it &MUST; be forwarded if the
1724   request is forwarded.
1725</t>
1726<t>
1727   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
1728   Expect header.
1729</t>
1730<t>
[29]1731   See &use100; for the use of the 100 (continue) status.
[8]1732</t>
1733</section>
1734
1735<section title="From" anchor="header.from">
1736  <iref primary="true" item="From header" x:for-anchor=""/>
1737  <iref primary="true" item="Headers" subitem="From" x:for-anchor=""/>
1738<t>
1739   The From request-header field, if given, &SHOULD; contain an Internet
1740   e-mail address for the human user who controls the requesting user
1741   agent. The address &SHOULD; be machine-usable, as defined by "mailbox"
1742   in RFC 822 <xref target="RFC822"/> as updated by RFC 1123 <xref target="RFC1123"/>:
1743</t>
1744<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="From"/>
1745    From   = "From" ":" mailbox
1746</artwork></figure>
1747<t>
1748   An example is:
1749</t>
1750<figure><artwork type="example">
1751    From: webmaster@w3.org
1752</artwork></figure>
1753<t>
1754   This header field &MAY; be used for logging purposes and as a means for
1755   identifying the source of invalid or unwanted requests. It &SHOULD-NOT; 
1756   be used as an insecure form of access protection. The interpretation
1757   of this field is that the request is being performed on behalf of the
1758   person given, who accepts responsibility for the method performed. In
1759   particular, robot agents &SHOULD; include this header so that the
1760   person responsible for running the robot can be contacted if problems
1761   occur on the receiving end.
1762</t>
1763<t>
1764   The Internet e-mail address in this field &MAY; be separate from the
1765   Internet host which issued the request. For example, when a request
1766   is passed through a proxy the original issuer's address &SHOULD; be
1767   used.
1768</t>
1769<t>
1770   The client &SHOULD-NOT;  send the From header field without the user's
1771   approval, as it might conflict with the user's privacy interests or
1772   their site's security policy. It is strongly recommended that the
1773   user be able to disable, enable, and modify the value of this field
1774   at any time prior to a request.
1775</t>
1776</section>
1777
1778<section title="Location" anchor="header.location">
1779  <iref primary="true" item="Location header" x:for-anchor=""/>
1780  <iref primary="true" item="Headers" subitem="Location" x:for-anchor=""/>
1781<t>
1782   The Location response-header field is used to redirect the recipient
1783   to a location other than the Request-URI for completion of the
1784   request or identification of a new resource. For 201 (Created)
1785   responses, the Location is that of the new resource which was created
1786   by the request. For 3xx responses, the location &SHOULD; indicate the
1787   server's preferred URI for automatic redirection to the resource. The
1788   field value consists of a single absolute URI.
1789</t>
1790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Location"/>
1791    Location       = "Location" ":" absoluteURI
1792</artwork></figure>
1793<t>
1794   An example is:
1795</t>
1796<figure><artwork type="example">
1797    Location: http://www.w3.org/pub/WWW/People.html
1798</artwork></figure>
1799<t>
1800  <list><t>
[29]1801      <x:h>Note:</x:h> The Content-Location header field (&header-content-location;) differs
[8]1802      from Location in that the Content-Location identifies the original
1803      location of the entity enclosed in the request. It is therefore
1804      possible for a response to contain header fields for both Location
[29]1805      and Content-Location.
[8]1806  </t></list>
1807</t>
1808</section>
1809
1810<section title="Max-Forwards" anchor="header.max-forwards">
1811  <iref primary="true" item="Max-Forwards header" x:for-anchor=""/>
1812  <iref primary="true" item="Headers" subitem="Max-Forwards" x:for-anchor=""/>
1813<t>
1814   The Max-Forwards request-header field provides a mechanism with the
1815   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
1816   number of proxies or gateways that can forward the request to the
1817   next inbound server. This can be useful when the client is attempting
1818   to trace a request chain which appears to be failing or looping in
1819   mid-chain.
1820</t>
1821<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Max-Forwards"/>
1822    Max-Forwards   = "Max-Forwards" ":" 1*DIGIT
1823</artwork></figure>
1824<t>
1825   The Max-Forwards value is a decimal integer indicating the remaining
1826   number of times this request message may be forwarded.
1827</t>
1828<t>
1829   Each proxy or gateway recipient of a TRACE or OPTIONS request
1830   containing a Max-Forwards header field &MUST; check and update its
1831   value prior to forwarding the request. If the received value is zero
1832   (0), the recipient &MUST-NOT; forward the request; instead, it &MUST;
1833   respond as the final recipient. If the received Max-Forwards value is
1834   greater than zero, then the forwarded message &MUST; contain an updated
1835   Max-Forwards field with a value decremented by one (1).
1836</t>
1837<t>
1838   The Max-Forwards header field &MAY; be ignored for all other methods
1839   defined by this specification and for any extension methods for which
1840   it is not explicitly referred to as part of that method definition.
1841</t>
1842</section>
1843
1844<section title="Referer" anchor="header.referer">
1845  <iref primary="true" item="Referer header" x:for-anchor=""/>
1846  <iref primary="true" item="Headers" subitem="Referer" x:for-anchor=""/>
1847<t>
1848   The Referer[sic] request-header field allows the client to specify,
1849   for the server's benefit, the address (URI) of the resource from
1850   which the Request-URI was obtained (the "referrer", although the
1851   header field is misspelled.) The Referer request-header allows a
1852   server to generate lists of back-links to resources for interest,
1853   logging, optimized caching, etc. It also allows obsolete or mistyped
1854   links to be traced for maintenance. The Referer field &MUST-NOT; be
1855   sent if the Request-URI was obtained from a source that does not have
1856   its own URI, such as input from the user keyboard.
1857</t>
1858<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Referer"/>
1859    Referer        = "Referer" ":" ( absoluteURI | relativeURI )
1860</artwork></figure>
1861<t>
1862   Example:
1863</t>
1864<figure><artwork type="example">
1865    Referer: http://www.w3.org/hypertext/DataSources/Overview.html
1866</artwork></figure>
1867<t>
1868   If the field value is a relative URI, it &SHOULD; be interpreted
1869   relative to the Request-URI. The URI &MUST-NOT; include a fragment. See
1870   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
1871</t>
1872</section>
1873
1874<section title="Retry-After" anchor="header.retry-after">
1875  <iref primary="true" item="Retry-After header" x:for-anchor=""/>
1876  <iref primary="true" item="Headers" subitem="Retry-After" x:for-anchor=""/>
1877<t>
1878   The Retry-After response-header field can be used with a 503 (Service
1879   Unavailable) response to indicate how long the service is expected to
1880   be unavailable to the requesting client. This field &MAY; also be used
1881   with any 3xx (Redirection) response to indicate the minimum time the
1882   user-agent is asked wait before issuing the redirected request. The
1883   value of this field can be either an HTTP-date or an integer number
1884   of seconds (in decimal) after the time of the response.
1885</t>
1886<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Retry-After"/>
1887    Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )
1888</artwork></figure>
1889<t>
1890   Two examples of its use are
1891</t>
1892<figure><artwork type="example">
1893    Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
1894    Retry-After: 120
1895</artwork></figure>
1896<t>
1897   In the latter example, the delay is 2 minutes.
1898</t>
1899</section>
1900
1901<section title="Server" anchor="header.server">
1902  <iref primary="true" item="Server header" x:for-anchor=""/>
1903  <iref primary="true" item="Headers" subitem="Server" x:for-anchor=""/>
1904<t>
1905   The Server response-header field contains information about the
1906   software used by the origin server to handle the request. The field
1907   can contain multiple product tokens (<xref target="product.tokens"/>) and comments
1908   identifying the server and any significant subproducts. The product
1909   tokens are listed in order of their significance for identifying the
1910   application.
1911</t>
1912<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Server"/>
1913    Server         = "Server" ":" 1*( product | comment )
1914</artwork></figure>
1915<t>
1916   Example:
1917</t>
1918<figure><artwork type="example">
1919    Server: CERN/3.0 libwww/2.17
1920</artwork></figure>
1921<t>
1922   If the response is being forwarded through a proxy, the proxy
1923   application &MUST-NOT; modify the Server response-header. Instead, it
[29]1924   &SHOULD; include a Via field (as described in &header-via;).
[8]1925  <list><t>
1926      <x:h>Note:</x:h> Revealing the specific software version of the server might
1927      allow the server machine to become more vulnerable to attacks
1928      against software that is known to contain security holes. Server
1929      implementors are encouraged to make this field a configurable
1930      option.
1931  </t></list>
1932</t>
1933</section>
1934
1935<section title="User-Agent" anchor="header.user-agent">
1936  <iref primary="true" item="User-Agent header" x:for-anchor=""/>
1937  <iref primary="true" item="Headers" subitem="User-Agent" x:for-anchor=""/>
1938<t>
1939   The User-Agent request-header field contains information about the
1940   user agent originating the request. This is for statistical purposes,
1941   the tracing of protocol violations, and automated recognition of user
1942   agents for the sake of tailoring responses to avoid particular user
1943   agent limitations. User agents &SHOULD; include this field with
1944   requests. The field can contain multiple product tokens (<xref target="product.tokens"/>)
1945   and comments identifying the agent and any subproducts which form a
1946   significant part of the user agent. By convention, the product tokens
1947   are listed in order of their significance for identifying the
1948   application.
1949</t>
1950<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="User-Agent"/>
1951    User-Agent     = "User-Agent" ":" 1*( product | comment )
1952</artwork></figure>
1953<t>
1954   Example:
1955</t>
1956<figure><artwork type="example">
1957    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1958</artwork></figure>
1959</section>
1960
1961</section>
1962
[29]1963<section title="IANA Considerations" anchor="IANA.considerations">
[8]1964<t>
[29]1965   TBD.
[8]1966</t>
1967</section>
1968
1969<section title="Security Considerations" anchor="security.considerations">
1970<t>
1971   This section is meant to inform application developers, information
1972   providers, and users of the security limitations in HTTP/1.1 as
1973   described by this document. The discussion does not include
1974   definitive solutions to the problems revealed, though it does make
1975   some suggestions for reducing security risks.
1976</t>
1977
1978<section title="Transfer of Sensitive Information" anchor="security.sensitive">
1979<t>
1980   Like any generic data transfer protocol, HTTP cannot regulate the
1981   content of the data that is transferred, nor is there any a priori
1982   method of determining the sensitivity of any particular piece of
1983   information within the context of any given request. Therefore,
1984   applications &SHOULD; supply as much control over this information as
1985   possible to the provider of that information. Four header fields are
1986   worth special mention in this context: Server, Via, Referer and From.
1987</t>
1988<t>
1989   Revealing the specific software version of the server might allow the
1990   server machine to become more vulnerable to attacks against software
1991   that is known to contain security holes. Implementors &SHOULD; make the
1992   Server header field a configurable option.
1993</t>
1994<t>
1995   Proxies which serve as a portal through a network firewall &SHOULD;
1996   take special precautions regarding the transfer of header information
1997   that identifies the hosts behind the firewall. In particular, they
1998   &SHOULD; remove, or replace with sanitized versions, any Via fields
1999   generated behind the firewall.
2000</t>
2001<t>
2002   The Referer header allows reading patterns to be studied and reverse
2003   links drawn. Although it can be very useful, its power can be abused
2004   if user details are not separated from the information contained in
2005   the Referer. Even when the personal information has been removed, the
2006   Referer header might indicate a private document's URI whose
2007   publication would be inappropriate.
2008</t>
2009<t>
2010   The information sent in the From field might conflict with the user's
2011   privacy interests or their site's security policy, and hence it
2012   &SHOULD-NOT;  be transmitted without the user being able to disable,
2013   enable, and modify the contents of the field. The user &MUST; be able
2014   to set the contents of this field within a user preference or
2015   application defaults configuration.
2016</t>
2017<t>
2018   We suggest, though do not require, that a convenient toggle interface
2019   be provided for the user to enable or disable the sending of From and
2020   Referer information.
2021</t>
2022<t>
2023   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
2024   fields can sometimes be used to determine that a specific client or
2025   server have a particular security hole which might be exploited.
2026   Unfortunately, this same information is often used for other valuable
2027   purposes for which HTTP currently has no better mechanism.
2028</t>
2029</section>
2030
2031<section title="Encoding Sensitive Information in URI's" anchor="encoding.sensitive.information.in.uris">
2032<t>
2033   Because the source of a link might be private information or might
2034   reveal an otherwise private information source, it is strongly
2035   recommended that the user be able to select whether or not the
2036   Referer field is sent. For example, a browser client could have a
2037   toggle switch for browsing openly/anonymously, which would
2038   respectively enable/disable the sending of Referer and From
2039   information.
2040</t>
2041<t>
2042   Clients &SHOULD-NOT;  include a Referer header field in a (non-secure)
2043   HTTP request if the referring page was transferred with a secure
2044   protocol.
2045</t>
2046<t>
2047   Authors of services which use the HTTP protocol &SHOULD-NOT;  use GET
2048   based forms for the submission of sensitive data, because this will
2049   cause this data to be encoded in the Request-URI. Many existing
2050   servers, proxies, and user agents will log the request URI in some
2051   place where it might be visible to third parties. Servers can use
2052   POST-based form submission instead
2053</t>
2054</section>
2055
2056<section title="Location Headers and Spoofing" anchor="location.spoofing">
2057<t>
2058   If a single server supports multiple organizations that do not trust
2059   one another, then it &MUST; check the values of Location and Content-Location
2060   headers in responses that are generated under control of
2061   said organizations to make sure that they do not attempt to
2062   invalidate resources over which they have no authority.
2063</t>
2064</section>
2065
2066</section>
2067
2068<section title="Acknowledgments" anchor="ack">
2069<t>
[29]2070   Based on an XML translation of RFC 2616 by Julian Reschke.
[8]2071</t>
2072</section>
2073</middle>
2074<back>
2075<references>
2076
2077<reference anchor="RFC1123">
2078<front>
2079<title>Requirements for Internet Hosts - Application and Support</title>
2080<author initials="R." surname="Braden" fullname="Robert Braden">
2081<organization>University of Southern California (USC), Information Sciences Institute</organization>
2082<address>
2083<postal>
2084<street>4676 Admiralty Way</street>
2085<city>Marina del Rey</city>
2086<region>CA</region>
2087<code>90292-6695</code>
2088<country>US</country></postal>
2089<phone>+1 213 822 1511</phone>
2090<email>Braden@ISI.EDU</email></address></author>
2091<date month="October" year="1989"/></front>
2092<seriesInfo name="STD" value="3"/>
2093<seriesInfo name="RFC" value="1123"/>
2094</reference>
2095
2096<reference anchor="RFC822">
2097<front>
2098<title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
2099<author initials="D.H." surname="Crocker" fullname="David H. Crocker">
2100<organization>University of Delaware, Dept. of Electrical Engineering</organization>
2101<address>
2102<postal>
2103<street/>
2104<city>Newark</city>
2105<region>DE</region>
2106<code>19711</code>
2107<country>US</country></postal>
2108<email>DCrocker@UDel-Relay</email></address></author>
2109<date month="August" day="13" year="1982"/></front>
2110<seriesInfo name="STD" value="11"/>
2111<seriesInfo name="RFC" value="822"/>
2112</reference>
2113
2114<reference anchor="RFC2068">
2115<front>
2116<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2117<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2118<organization>University of California, Irvine, Department of Information and Computer Science</organization>
2119<address>
2120<postal>
2121<street/>
2122<city>Irvine</city>
2123<region>CA</region>
2124<code>92717-3425</code>
2125<country>US</country></postal>
2126<facsimile>+1 714 824 4056</facsimile>
2127<email>fielding@ics.uci.edu</email></address></author>
2128<author initials="J." surname="Gettys" fullname="Jim Gettys">
2129<organization>MIT Laboratory for Computer Science</organization>
2130<address>
2131<postal>
2132<street>545 Technology Square</street>
2133<city>Cambridge</city>
2134<region>MA</region>
2135<code>02139</code>
2136<country>US</country></postal>
2137<facsimile>+1 617 258 8682</facsimile>
2138<email>jg@w3.org</email></address></author>
2139<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2140<organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2141<address>
2142<postal>
2143<street>250 University Avenue</street>
2144<city>Palo Alto</city>
2145<region>CA</region>
2146<code>94301</code>
2147<country>US</country></postal>
2148<email>mogul@wrl.dec.com</email></address></author>
2149<author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2150<organization>MIT Laboratory for Computer Science</organization>
2151<address>
2152<postal>
2153<street>545 Technology Square</street>
2154<city>Cambridge</city>
2155<region>MA</region>
2156<code>02139</code>
2157<country>US</country></postal>
2158<facsimile>+1 617 258 8682</facsimile>
2159<email>frystyk@w3.org</email></address></author>
2160<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2161<organization>MIT Laboratory for Computer Science</organization>
2162<address>
2163<postal>
2164<street>545 Technology Square</street>
2165<city>Cambridge</city>
2166<region>MA</region>
2167<code>02139</code>
2168<country>US</country></postal>
2169<facsimile>+1 617 258 8682</facsimile>
2170<email>timbl@w3.org</email></address></author>
2171<date month="January" year="1997"/>
2172<abstract>
2173<t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.</t>
2174<t>HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1".</t></abstract></front>
2175<seriesInfo name="RFC" value="2068"/>
2176</reference>
2177
2178<reference anchor="RFC2617">
2179<front>
2180<title abbrev="HTTP Authentication">HTTP Authentication: Basic and Digest Access Authentication</title>
2181<author initials="J." surname="Franks" fullname="John Franks">
2182<organization>Northwestern University, Department of Mathematics</organization>
2183<address>
2184<postal>
2185<street/>
2186<city>Evanston</city>
2187<region>IL</region>
2188<code>60208-2730</code>
2189<country>US</country></postal>
2190<email>john@math.nwu.edu</email></address></author>
2191<author initials="P.M." surname="Hallam-Baker" fullname="Phillip M. Hallam-Baker">
2192<organization>Verisign Inc.</organization>
2193<address>
2194<postal>
2195<street>301 Edgewater Place</street>
2196<street>Suite 210</street>
2197<city>Wakefield</city>
2198<region>MA</region>
2199<code>01880</code>
2200<country>US</country></postal>
2201<email>pbaker@verisign.com</email></address></author>
2202<author initials="J.L." surname="Hostetler" fullname="Jeffery L. Hostetler">
2203<organization>AbiSource, Inc.</organization>
2204<address>
2205<postal>
2206<street>6 Dunlap Court</street>
2207<city>Savoy</city>
2208<region>IL</region>
2209<code>61874</code>
2210<country>US</country></postal>
2211<email>jeff@AbiSource.com</email></address></author>
2212<author initials="S.D." surname="Lawrence" fullname="Scott D. Lawrence">
2213<organization>Agranat Systems, Inc.</organization>
2214<address>
2215<postal>
2216<street>5 Clocktower Place</street>
2217<street>Suite 400</street>
2218<city>Maynard</city>
2219<region>MA</region>
2220<code>01754</code>
2221<country>US</country></postal>
2222<email>lawrence@agranat.com</email></address></author>
2223<author initials="P.J." surname="Leach" fullname="Paul J. Leach">
2224<organization>Microsoft Corporation</organization>
2225<address>
2226<postal>
2227<street>1 Microsoft Way</street>
2228<city>Redmond</city>
2229<region>WA</region>
2230<code>98052</code>
2231<country>US</country></postal>
2232<email>paulle@microsoft.com</email></address></author>
2233<author initials="A." surname="Luotonen" fullname="Ari Luotonen">
2234<organization>Netscape Communications Corporation</organization>
2235<address>
2236<postal>
2237<street>501 East Middlefield Road</street>
2238<city>Mountain View</city>
2239<region>CA</region>
2240<code>94043</code>
2241<country>US</country></postal></address></author>
2242<author initials="L." surname="Stewart" fullname="Lawrence C. Stewart">
2243<organization>Open Market, Inc.</organization>
2244<address>
2245<postal>
2246<street>215 First Street</street>
2247<city>Cambridge</city>
2248<region>MA</region>
2249<code>02142</code>
2250<country>US</country></postal>
2251<email>stewart@OpenMarket.com</email></address></author>
2252<date month="June" year="1999"/>
2253</front>
2254<seriesInfo name="RFC" value="2617"/>
2255</reference>
2256
2257<reference anchor="Luo1998">
2258<front>
2259<title>Tunneling TCP based protocols through Web proxy servers</title>
2260<author initials="A." surname="Luotonen" fullname="A. Luotonen">
2261  <organization/>
2262</author>
2263<date/>
2264</front>
2265<seriesInfo name="" value="Work in Progress"/>
2266</reference>
2267
2268</references>
2269
2270<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2271<t>
2272   Clarified which error code should be used for inbound server failures
2273   (e.g. DNS failures). (<xref target="status.504"/>).
2274</t>
2275<t>
2276   CREATE had a race that required an Etag be sent when a resource is
2277   first created. (<xref target="status.201"/>).
2278</t>
2279<t>
2280   Rewrite of message transmission requirements to make it much harder
2281   for implementors to get it wrong, as the consequences of errors here
2282   can have significant impact on the Internet, and to deal with the
2283   following problems:
2284  <list style="numbers">
2285      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
2286         this was incorrectly placing a requirement on the behavior of
2287         an implementation of a future version of HTTP/1.x</t>
2288
2289      <t>Made it clear that user-agents should retry requests, not
2290         "clients" in general.</t>
2291
2292      <t>Converted requirements for clients to ignore unexpected 100
2293         (Continue) responses, and for proxies to forward 100 responses,
2294         into a general requirement for 1xx responses.</t>
2295
2296      <t>Modified some TCP-specific language, to make it clearer that
2297         non-TCP transports are possible for HTTP.</t>
2298
2299      <t>Require that the origin server &MUST-NOT; wait for the request
2300         body before it sends a required 100 (Continue) response.</t>
2301
2302      <t>Allow, rather than require, a server to omit 100 (Continue) if
2303         it has already seen some of the request body.</t>
2304
2305      <t>Allow servers to defend against denial-of-service attacks and
2306         broken clients.</t>
2307  </list>
2308</t>
2309<t>
[29]2310   This change adds the Expect header and 417 status code.
[8]2311</t>
2312<t>
2313   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
2314   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
2315</t>
2316<t>
2317   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
2318   implemented in previous versions of this specification. See RFC 2068
2319   <xref target="RFC2068"/>.
2320</t>
2321</section>
2322</back>
2323</rfc>
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