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[55]1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?rfc toc="yes" ?>
7<?rfc symrefs="yes" ?>
8<?rfc sortrefs="yes" ?>
9<?rfc compact="yes"?>
10<?rfc subcompact="no" ?>
11<?rfc linkmailto="no" ?>
12<?rfc editing="no" ?>
13<!DOCTYPE rfc
14  PUBLIC "" "rfc2629.dtd">
15<rfc obsoletes="2068, 2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p2-semantics-00">
16<front>
17
18  <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
19
20  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
21    <organization abbrev="Day Software">Day Software</organization>
22    <address>
23      <postal>
24        <street>23 Corporate Plaza DR, Suite 280</street>
25        <city>Newport Beach</city>
26        <region>CA</region>
27        <code>92660</code>
28        <country>USA</country>
29      </postal>
30      <phone>+1-949-706-5300</phone>
31      <facsimile>+1-949-706-5305</facsimile>
32      <email>fielding@gbiv.com</email>
33      <uri>http://roy.gbiv.com/</uri>
34    </address>
35  </author>
36
37  <author initials="J." surname="Gettys" fullname="Jim Gettys">
38    <organization>One Laptop per Child</organization>
39    <address>
40      <postal>
41        <street>21 Oak Knoll Road</street>
42        <city>Carlisle</city>
43        <region>MA</region>
44        <code>01741</code>
45        <country>USA</country>
46      </postal>
47      <email>jg@laptop.org</email>
48      <uri>http://www.laptop.org/</uri>
49    </address>
50  </author>
51 
52  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
53    <organization abbrev="HP">Hewlett-Packard Company</organization>
54    <address>
55      <postal>
56        <street>HP Labs, Large Scale Systems Group</street>
57        <street>1501 Page Mill Road, MS 1177</street>
58        <city>Palo Alto</city>
59        <region>CA</region>
60        <code>94304</code>
61        <country>USA</country>
62      </postal>
63      <email>JeffMogul@acm.org</email>
64    </address>
65  </author>
66
67  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
68    <organization abbrev="Microsoft">Microsoft Corporation</organization>
69    <address>
70      <postal>
71        <street>1 Microsoft Way</street>
72        <city>Redmond</city>
73        <region>WA</region>
74        <code>98052</code>
75        <country>USA</country>
76      </postal>
77      <email>henrikn@microsoft.com</email>
78    </address>
79  </author>
80
81  <author initials="L." surname="Masinter" fullname="Larry Masinter">
82    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>LMM@acm.org</email>
92      <uri>http://larry.masinter.net/</uri>
93    </address>
94  </author>
95 
96  <author initials="P." surname="Leach" fullname="Paul J. Leach">
97    <organization abbrev="Microsoft">Microsoft Corporation</organization>
98    <address>
99      <postal>
100        <street>1 Microsoft Way</street>
101        <city>Redmond</city>
102        <region>WA</region>
103        <code>98052</code>
104      </postal>
105      <email>paulle@microsoft.com</email>
106    </address>
107  </author>
108   
109  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
110    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
111    <address>
112      <postal>
113        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
114        <street>The Stata Center, Building 32</street>
115        <street>32 Vassar Street</street>
116        <city>Cambridge</city>
117        <region>MA</region>
118        <code>02139</code>
119        <country>USA</country>
120      </postal>
121      <email>timbl@w3.org</email>
122      <uri>http://www.w3.org/People/Berners-Lee/</uri>
123    </address>
124  </author>
125
126  <date month="December" year="2007"/>
127
128<abstract>
129<t>
130   The Hypertext Transfer Protocol (HTTP) is an application-level
131   protocol for distributed, collaborative, hypermedia information
132   systems. HTTP has been in use by the World Wide Web global information
133   initiative since 1990. This document is Part 2 of the seven-part specification
134   that defines the protocol referred to as "HTTP/1.1" and, taken together,
135   obsoletes RFC 2616.  Part 2 defines the semantics of HTTP messages
136   as expressed by request methods, request-header fields, response status codes,
137   and response-header fields.
138</t>
139</abstract>
140
141<note title="Editorial Note (To be removed by RFC Editor)">
142  <t>
143    This version of the HTTP specification contains only minimal editorial
144    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
145    and authors' addresses).  All other changes are due to partitioning the
146    original into seven mostly independent parts.  The intent is for readers
147    of future drafts to able to use draft 00 as the basis for comparison
148    when the WG makes later changes to the specification text.  This draft
149    will shortly be followed by draft 01 (containing the first round of changes
150    that have already been agreed to on the mailing list). There is no point in
151    reviewing this draft other than to verify that the partitioning has been
152    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
153    will be the editors after draft 00 is submitted.
154  </t>
155  <t>
156    Discussion of this draft should take place on the HTTPBIS working group
157    mailing list (ietf-http-wg@w3.org). The current issues list is
[63]158    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
[55]159    and related documents (including fancy diffs) can be found at
160    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
161  </t>
162</note>
163</front>
164<middle>
165<section title="Introduction" anchor="introduction">
166<t>
167   This document will define aspects of HTTP related to request and response
168   semantics. Right now it only includes the extracted relevant sections of
169   RFC 2616 with only minor edits.
170</t>
171<t>
172   The HTTP protocol is a request/response protocol. A client sends a
173   request to the server in the form of a request method, URI, and
174   protocol version, followed by a MIME-like message containing request
175   modifiers, client information, and possible body content over a
176   connection with a server. The server responds with a status line,
177   including the message's protocol version and a success or error code,
178   followed by a MIME-like message containing server information, entity
179   metainformation, and possible entity-body content. The relationship
180   between HTTP and MIME is described in Appendix A of <xref target="Part3"/>.
181</t>
182</section>
183
184<section title="Product Tokens" anchor="product.tokens">
185<t>
186   Product tokens are used to allow communicating applications to
187   identify themselves by software name and version. Most fields using
188   product tokens also allow sub-products which form a significant part
189   of the application to be listed, separated by white space. By
190   convention, the products are listed in order of their significance
191   for identifying the application.
192</t>
193<figure><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/><artwork type="abnf2616"><![CDATA[
194    product         = token ["/" product-version]
195    product-version = token
196]]></artwork></figure>
197<t>
198   Examples:
199</t>
200<figure><artwork type="example"><![CDATA[
201    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
202    Server: Apache/0.8.4
203]]></artwork></figure>
204<t>
205   Product tokens SHOULD be short and to the point. They MUST NOT be
206   used for advertising or other non-essential information. Although any
207   token character MAY appear in a product-version, this token SHOULD
208   only be used for a version identifier (i.e., successive versions of
209   the same product SHOULD only differ in the product-version portion of
210   the product value).
211</t>
212</section>
213
214<section title="Method" anchor="method">
215<t>
216   The Method  token indicates the method to be performed on the
217   resource identified by the Request-URI. The method is case-sensitive.
218</t>
219<figure><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/><artwork type="abnf2616"><![CDATA[
220    Method         = "OPTIONS"                ; Section 8.2
221                   | "GET"                    ; Section 8.3
222                   | "HEAD"                   ; Section 8.4
223                   | "POST"                   ; Section 8.5
224                   | "PUT"                    ; Section 8.6
225                   | "DELETE"                 ; Section 8.7
226                   | "TRACE"                  ; Section 8.8
227                   | "CONNECT"                ; Section 8.9
228                   | extension-method
229    extension-method = token
230]]></artwork></figure>
231<t>
232   The list of methods allowed by a resource can be specified in an
233   Allow header field (<xref target="header.allow"/>). The return code of the response
234   always notifies the client whether a method is currently allowed on a
235   resource, since the set of allowed methods can change dynamically. An
236   origin server SHOULD return the status code 405 (Method Not Allowed)
237   if the method is known by the origin server but not allowed for the
238   requested resource, and 501 (Not Implemented) if the method is
239   unrecognized or not implemented by the origin server. The methods GET
240   and HEAD MUST be supported by all general-purpose servers. All other
241   methods are OPTIONAL; however, if the above methods are implemented,
242   they MUST be implemented with the same semantics as those specified
243   in <xref target="method.definitions"/>.
244</t>
245</section>
246
247<section title="Request Header Fields" anchor="request.header.fields">
248<t>
249   The request-header fields allow the client to pass additional
250   information about the request, and about the client itself, to the
251   server. These fields act as request modifiers, with semantics
252   equivalent to the parameters on a programming language method
253   invocation.
254</t>
255<figure><iref primary="true" item="Grammar" subitem="request-header"/><artwork type="abnf2616"><![CDATA[
256    request-header = Accept                   ; [Part3], Section 5.1
257                   | Accept-Charset           ; [Part3], Section 5.2
258                   | Accept-Encoding          ; [Part3], Section 5.3
259                   | Accept-Language          ; [Part3], Section 5.4
260                   | Authorization            ; [Part7], Section 3.1
261                   | Expect                   ; Section 10.2
262                   | From                     ; Section 10.3
263                   | Host                     ; [Part1], Section 8.4
264                   | If-Match                 ; [Part4], Section 6.2
265                   | If-Modified-Since        ; [Part4], Section 6.3
266                   | If-None-Match            ; [Part4], Section 6.4
267                   | If-Range                 ; [Part5], Section 5.3
268                   | If-Unmodified-Since      ; [Part4], Section 6.5
269                   | Max-Forwards             ; Section 10.5
270                   | Proxy-Authorization      ; [Part7], Section 3.3
271                   | Range                    ; [Part5], Section 5.4
272                   | Referer                  ; Section 10.6
273                   | TE                       ; [Part1], Section 8.8
274                   | User-Agent               ; Section 10.9
275]]></artwork></figure>
276<t>
277   Request-header field names can be extended reliably only in
278   combination with a change in the protocol version. However, new or
279   experimental header fields MAY be given the semantics of request-header
280   fields if all parties in the communication recognize them to
281   be request-header fields. Unrecognized header fields are treated as
282   entity-header fields.
283</t>
284</section>
285
286<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
287<t>
288   The Status-Code element is a 3-digit integer result code of the
289   attempt to understand and satisfy the request. These codes are fully
290   defined in <xref target="status.codes"/>. The Reason-Phrase is intended to give a short
291   textual description of the Status-Code. The Status-Code is intended
292   for use by automata and the Reason-Phrase is intended for the human
293   user. The client is not required to examine or display the Reason-Phrase.
294</t>
295<t> 
296   The individual values of the numeric status codes defined for
297   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
298   presented below. The reason phrases listed here are only
299   recommendations -- they MAY be replaced by local equivalents without
300   affecting the protocol.
301</t>
302<figure><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/><artwork type="abnf2616"><![CDATA[
303   Status-Code    =
304         "100"  ; Section 9.1.1: Continue
305       | "101"  ; Section 9.1.2: Switching Protocols
306       | "200"  ; Section 9.2.1: OK
307       | "201"  ; Section 9.2.2: Created
308       | "202"  ; Section 9.2.3: Accepted
309       | "203"  ; Section 9.2.4: Non-Authoritative Information
310       | "204"  ; Section 9.2.5: No Content
311       | "205"  ; Section 9.2.6: Reset Content
312       | "206"  ; Section 9.2.7: Partial Content
313       | "300"  ; Section 9.3.1: Multiple Choices
314       | "301"  ; Section 9.3.2: Moved Permanently
315       | "302"  ; Section 9.3.3: Found
316       | "303"  ; Section 9.3.4: See Other
317       | "304"  ; Section 9.3.5: Not Modified
318       | "305"  ; Section 9.3.6: Use Proxy
319       | "307"  ; Section 9.3.8: Temporary Redirect
320       | "400"  ; Section 9.4.1: Bad Request
321       | "401"  ; Section 9.4.2: Unauthorized
322       | "402"  ; Section 9.4.3: Payment Required
323       | "403"  ; Section 9.4.4: Forbidden
324       | "404"  ; Section 9.4.5: Not Found
325       | "405"  ; Section 9.4.6: Method Not Allowed
326       | "406"  ; Section 9.4.7: Not Acceptable
327       | "407"  ; Section 9.4.8: Proxy Authentication Required
328       | "408"  ; Section 9.4.9: Request Time-out
329       | "409"  ; Section 9.4.10: Conflict
330       | "410"  ; Section 9.4.11: Gone
331       | "411"  ; Section 9.4.12: Length Required
332       | "412"  ; Section 9.4.13: Precondition Failed
333       | "413"  ; Section 9.4.14: Request Entity Too Large
334       | "414"  ; Section 9.4.15: Request-URI Too Large
335       | "415"  ; Section 9.4.16: Unsupported Media Type
336       | "416"  ; Section 9.4.17: Requested range not satisfiable
337       | "417"  ; Section 9.4.18: Expectation Failed
338       | "500"  ; Section 9.5.1: Internal Server Error
339       | "501"  ; Section 9.5.2: Not Implemented
340       | "502"  ; Section 9.5.3: Bad Gateway
341       | "503"  ; Section 9.5.4: Service Unavailable
342       | "504"  ; Section 9.5.5: Gateway Time-out
343       | "505"  ; Section 9.5.6: HTTP Version not supported
344       | extension-code
345
346   extension-code = 3DIGIT
347   Reason-Phrase  = *<TEXT, excluding CR, LF>
348]]></artwork></figure>
349<t>
350   HTTP status codes are extensible. HTTP applications are not required
351   to understand the meaning of all registered status codes, though such
352   understanding is obviously desirable. However, applications MUST
353   understand the class of any status code, as indicated by the first
354   digit, and treat any unrecognized response as being equivalent to the
355   x00 status code of that class, with the exception that an
356   unrecognized response MUST NOT be cached. For example, if an
357   unrecognized status code of 431 is received by the client, it can
358   safely assume that there was something wrong with its request and
359   treat the response as if it had received a 400 status code. In such
360   cases, user agents SHOULD present to the user the entity returned
361   with the response, since that entity is likely to include human-readable
362   information which will explain the unusual status.
363</t>
364</section>
365
366<section title="Response Header Fields" anchor="response.header.fields">
367<t>
368   The response-header fields allow the server to pass additional
369   information about the response which cannot be placed in the Status-Line.
370   These header fields give information about the server and about
371   further access to the resource identified by the Request-URI.
372</t>
373<figure><iref primary="true" item="Grammar" subitem="response-header"/><artwork type="abnf2616"><![CDATA[
374    response-header = Accept-Ranges           ; [Part5], Section 5.1
375                    | Age                     ; [Part6], Section 3.1
376                    | ETag                    ; [Part4], Section 6.1
377                    | Location                ; Section 10.4
378                    | Proxy-Authenticate      ; [Part7], Section 3.2
379                    | Retry-After             ; Section 10.7
380                    | Server                  ; Section 10.8
381                    | Vary                    ; [Part6], Section 3.5
382                    | WWW-Authenticate        ; [Part7], Section 3.4
383]]></artwork></figure>
384<t>
385   Response-header field names can be extended reliably only in
386   combination with a change in the protocol version. However, new or
387   experimental header fields MAY be given the semantics of response-header
388   fields if all parties in the communication recognize them to
389   be response-header fields. Unrecognized header fields are treated as
390   entity-header fields.
391</t>
392</section>
393
394<section title="Entity" anchor="entity">
395<t>
396   Request and Response messages MAY transfer an entity if not otherwise
397   restricted by the request method or response status code. An entity
398   consists of entity-header fields and an entity-body, although some
399   responses will only include the entity-headers. HTTP entity-body and
400   entity-header fields are defined in <xref target="Part3"/>.
401</t>
402<t>
403   An entity-body is only present in a message when a message-body is
404   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
405   from the message-body by decoding any Transfer-Encoding that might
406   have been applied to ensure safe and proper transfer of the message.
407</t>
408</section>
409
410
411<section title="Method Definitions" anchor="method.definitions">
412<t>
413   The set of common methods for HTTP/1.1 is defined below. Although
414   this set can be expanded, additional methods cannot be assumed to
415   share the same semantics for separately extended clients and servers.
416
417   The Host request-header field (Section 8.4 of <xref target="Part1"/>) MUST accompany all
418   HTTP/1.1 requests.
419</t>
420
421<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
422
423<section title="Safe Methods" anchor="safe.methods">
424<t>
425   Implementors should be aware that the software represents the user in
426   their interactions over the Internet, and should be careful to allow
427   the user to be aware of any actions they might take which may have an
428   unexpected significance to themselves or others.
429</t>
430<t>
431   In particular, the convention has been established that the GET and
432   HEAD methods SHOULD NOT  have the significance of taking an action
433   other than retrieval. These methods ought to be considered "safe".
434   This allows user agents to represent other methods, such as POST, PUT
435   and DELETE, in a special way, so that the user is made aware of the
436   fact that a possibly unsafe action is being requested.
437</t>
438<t>
439   Naturally, it is not possible to ensure that the server does not
440   generate side-effects as a result of performing a GET request; in
441   fact, some dynamic resources consider that a feature. The important
442   distinction here is that the user did not request the side-effects,
443   so therefore cannot be held accountable for them.
444</t>
445</section>
446
447<section title="Idempotent Methods" anchor="idempotent.methods">
448<t>
449   Methods can also have the property of "idempotence" in that (aside
450   from error or expiration issues) the side-effects of N &gt; 0 identical
451   requests is the same as for a single request. The methods GET, HEAD,
452   PUT and DELETE share this property. Also, the methods OPTIONS and
453   TRACE SHOULD NOT  have side effects, and so are inherently idempotent.
454</t>
455<t>
456   However, it is possible that a sequence of several requests is non-idempotent,
457   even if all of the methods executed in that sequence are
458   idempotent. (A sequence is idempotent if a single execution of the
459   entire sequence always yields a result that is not changed by a
460   reexecution of all, or part, of that sequence.) For example, a
461   sequence is non-idempotent if its result depends on a value that is
462   later modified in the same sequence.
463</t>
464<t>
465   A sequence that never has side effects is idempotent, by definition
466   (provided that no concurrent operations are being executed on the
467   same set of resources).
468</t>
469</section>
470</section>
471
472<section title="OPTIONS" anchor="OPTIONS">
473  <iref primary="true" item="OPTIONS method"/>
474  <iref primary="true" item="Methods" subitem="OPTIONS"/>
475<t>
476   The OPTIONS method represents a request for information about the
477   communication options available on the request/response chain
478   identified by the Request-URI. This method allows the client to
479   determine the options and/or requirements associated with a resource,
480   or the capabilities of a server, without implying a resource action
481   or initiating a resource retrieval.
482</t>
483<t>
484   Responses to this method are not cacheable.
485</t>
486<t>
487   If the OPTIONS request includes an entity-body (as indicated by the
488   presence of Content-Length or Transfer-Encoding), then the media type
489   MUST be indicated by a Content-Type field. Although this
490   specification does not define any use for such a body, future
491   extensions to HTTP might use the OPTIONS body to make more detailed
492   queries on the server. A server that does not support such an
493   extension MAY discard the request body.
494</t>
495<t>
496   If the Request-URI is an asterisk ("*"), the OPTIONS request is
497   intended to apply to the server in general rather than to a specific
498   resource. Since a server's communication options typically depend on
499   the resource, the "*" request is only useful as a "ping" or "no-op"
500   type of method; it does nothing beyond allowing the client to test
501   the capabilities of the server. For example, this can be used to test
502   a proxy for HTTP/1.1 compliance (or lack thereof).
503</t>
504<t>
505   If the Request-URI is not an asterisk, the OPTIONS request applies
506   only to the options that are available when communicating with that
507   resource.
508</t>
509<t>
510   A 200 response SHOULD include any header fields that indicate
511   optional features implemented by the server and applicable to that
512   resource (e.g., Allow), possibly including extensions not defined by
513   this specification. The response body, if any, SHOULD also include
514   information about the communication options. The format for such a
515   body is not defined by this specification, but might be defined by
516   future extensions to HTTP. Content negotiation MAY be used to select
517   the appropriate response format. If no response body is included, the
518   response MUST include a Content-Length field with a field-value of
519   "0".
520</t>
521<t>
522   The Max-Forwards request-header field MAY be used to target a
523   specific proxy in the request chain. When a proxy receives an OPTIONS
524   request on an absoluteURI for which request forwarding is permitted,
525   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
526   field-value is zero ("0"), the proxy MUST NOT forward the message;
527   instead, the proxy SHOULD respond with its own communication options.
528   If the Max-Forwards field-value is an integer greater than zero, the
529   proxy MUST decrement the field-value when it forwards the request. If
530   no Max-Forwards field is present in the request, then the forwarded
531   request MUST NOT include a Max-Forwards field.
532</t>
533</section>
534
535<section title="GET" anchor="GET">
536  <iref primary="true" item="GET method"/>
537  <iref primary="true" item="Methods" subitem="GET"/>
538<t>
539   The GET method means retrieve whatever information (in the form of an
540   entity) is identified by the Request-URI. If the Request-URI refers
541   to a data-producing process, it is the produced data which shall be
542   returned as the entity in the response and not the source text of the
543   process, unless that text happens to be the output of the process.
544</t>
545<t>
546   The semantics of the GET method change to a "conditional GET" if the
547   request message includes an If-Modified-Since, If-Unmodified-Since,
548   If-Match, If-None-Match, or If-Range header field. A conditional GET
549   method requests that the entity be transferred only under the
550   circumstances described by the conditional header field(s). The
551   conditional GET method is intended to reduce unnecessary network
552   usage by allowing cached entities to be refreshed without requiring
553   multiple requests or transferring data already held by the client.
554</t>
555<t>
556   The semantics of the GET method change to a "partial GET" if the
557   request message includes a Range header field. A partial GET requests
558   that only part of the entity be transferred, as described in Section 5.4 of <xref target="Part5"/>.
559   The partial GET method is intended to reduce unnecessary
560   network usage by allowing partially-retrieved entities to be
561   completed without transferring data already held by the client.
562</t>
563<t>
564   The response to a GET request is cacheable if and only if it meets
565   the requirements for HTTP caching described in <xref target="Part6"/>.
566</t>
567<t>
568   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
569</t>
570</section>
571
572<section title="HEAD" anchor="HEAD">
573  <iref primary="true" item="HEAD method"/>
574  <iref primary="true" item="Methods" subitem="HEAD"/>
575<t>
576   The HEAD method is identical to GET except that the server MUST NOT
577   return a message-body in the response. The metainformation contained
578   in the HTTP headers in response to a HEAD request SHOULD be identical
579   to the information sent in response to a GET request. This method can
580   be used for obtaining metainformation about the entity implied by the
581   request without transferring the entity-body itself. This method is
582   often used for testing hypertext links for validity, accessibility,
583   and recent modification.
584</t>
585<t>
586   The response to a HEAD request MAY be cacheable in the sense that the
587   information contained in the response MAY be used to update a
588   previously cached entity from that resource. If the new field values
589   indicate that the cached entity differs from the current entity (as
590   would be indicated by a change in Content-Length, Content-MD5, ETag
591   or Last-Modified), then the cache MUST treat the cache entry as
592   stale.
593</t>
594</section>
595
596<section title="POST" anchor="POST">
597  <iref primary="true" item="POST method"/>
598  <iref primary="true" item="Methods" subitem="POST"/>
599<t>
600   The POST method is used to request that the origin server accept the
601   entity enclosed in the request as a new subordinate of the resource
602   identified by the Request-URI in the Request-Line. POST is designed
603   to allow a uniform method to cover the following functions:
604  <list style="symbols">
605    <t>
606      Annotation of existing resources;
607    </t>
608    <t>
609        Posting a message to a bulletin board, newsgroup, mailing list,
610        or similar group of articles;
611    </t>
612    <t>
613        Providing a block of data, such as the result of submitting a
614        form, to a data-handling process;
615    </t>
616    <t>
617        Extending a database through an append operation.
618    </t>
619  </list>
620</t>
621<t>
622   The actual function performed by the POST method is determined by the
623   server and is usually dependent on the Request-URI. The posted entity
624   is subordinate to that URI in the same way that a file is subordinate
625   to a directory containing it, a news article is subordinate to a
626   newsgroup to which it is posted, or a record is subordinate to a
627   database.
628</t>
629<t>
630   The action performed by the POST method might not result in a
631   resource that can be identified by a URI. In this case, either 200
632   (OK) or 204 (No Content) is the appropriate response status,
633   depending on whether or not the response includes an entity that
634   describes the result.
635</t>
636<t>
637   If a resource has been created on the origin server, the response
638   SHOULD be 201 (Created) and contain an entity which describes the
639   status of the request and refers to the new resource, and a Location
640   header (see <xref target="header.location"/>).
641</t>
642<t>
643   Responses to this method are not cacheable, unless the response
644   includes appropriate Cache-Control or Expires header fields. However,
645   the 303 (See Other) response can be used to direct the user agent to
646   retrieve a cacheable resource.
647</t>
648<t>
649   POST requests MUST obey the message transmission requirements set out
650   in Section 7.2 of <xref target="Part1"/>.
651</t>
652<t>
653   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
654</t>
655</section>
656
657<section title="PUT" anchor="PUT">
658  <iref primary="true" item="PUT method"/>
659  <iref primary="true" item="Methods" subitem="PUT"/>
660<t>
661   The PUT method requests that the enclosed entity be stored under the
662   supplied Request-URI. If the Request-URI refers to an already
663   existing resource, the enclosed entity SHOULD be considered as a
664   modified version of the one residing on the origin server. If the
665   Request-URI does not point to an existing resource, and that URI is
666   capable of being defined as a new resource by the requesting user
667   agent, the origin server can create the resource with that URI. If a
668   new resource is created, the origin server MUST inform the user agent
669   via the 201 (Created) response. If an existing resource is modified,
670   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
671   to indicate successful completion of the request. If the resource
672   could not be created or modified with the Request-URI, an appropriate
673   error response SHOULD be given that reflects the nature of the
674   problem. The recipient of the entity MUST NOT ignore any Content-*
675   (e.g. Content-Range) headers that it does not understand or implement
676   and MUST return a 501 (Not Implemented) response in such cases.
677</t>
678<t>
679   If the request passes through a cache and the Request-URI identifies
680   one or more currently cached entities, those entries SHOULD be
681   treated as stale. Responses to this method are not cacheable.
682</t>
683<t>
684   The fundamental difference between the POST and PUT requests is
685   reflected in the different meaning of the Request-URI. The URI in a
686   POST request identifies the resource that will handle the enclosed
687   entity. That resource might be a data-accepting process, a gateway to
688   some other protocol, or a separate entity that accepts annotations.
689   In contrast, the URI in a PUT request identifies the entity enclosed
690   with the request -- the user agent knows what URI is intended and the
691   server MUST NOT attempt to apply the request to some other resource.
692   If the server desires that the request be applied to a different URI,
693   it MUST send a 301 (Moved Permanently) response; the user agent MAY
694   then make its own decision regarding whether or not to redirect the
695   request.
696</t>
697<t>
698   A single resource MAY be identified by many different URIs. For
699   example, an article might have a URI for identifying "the current
700   version" which is separate from the URI identifying each particular
701   version. In this case, a PUT request on a general URI might result in
702   several other URIs being defined by the origin server.
703</t>
704<t>
705   HTTP/1.1 does not define how a PUT method affects the state of an
706   origin server.
707</t>
708<t>
709   PUT requests MUST obey the message transmission requirements set out
710   in Section 7.2 of <xref target="Part1"/>.
711</t>
712<t>
713   Unless otherwise specified for a particular entity-header, the
714   entity-headers in the PUT request SHOULD be applied to the resource
715   created or modified by the PUT.
716</t>
717</section>
718
719<section title="DELETE" anchor="DELETE">
720  <iref primary="true" item="DELETE method"/>
721  <iref primary="true" item="Methods" subitem="DELETE"/>
722<t>
723   The DELETE method requests that the origin server delete the resource
724   identified by the Request-URI. This method MAY be overridden by human
725   intervention (or other means) on the origin server. The client cannot
726   be guaranteed that the operation has been carried out, even if the
727   status code returned from the origin server indicates that the action
728   has been completed successfully. However, the server SHOULD NOT
729   indicate success unless, at the time the response is given, it
730   intends to delete the resource or move it to an inaccessible
731   location.
732</t>
733<t>
734   A successful response SHOULD be 200 (OK) if the response includes an
735   entity describing the status, 202 (Accepted) if the action has not
736   yet been enacted, or 204 (No Content) if the action has been enacted
737   but the response does not include an entity.
738</t>
739<t>
740   If the request passes through a cache and the Request-URI identifies
741   one or more currently cached entities, those entries SHOULD be
742   treated as stale. Responses to this method are not cacheable.
743</t>
744</section>
745
746<section title="TRACE" anchor="TRACE">
747  <iref primary="true" item="TRACE method"/>
748  <iref primary="true" item="Methods" subitem="TRACE"/>
749<t>
750   The TRACE method is used to invoke a remote, application-layer loop-back
751   of the request message. The final recipient of the request
752   SHOULD reflect the message received back to the client as the
753   entity-body of a 200 (OK) response. The final recipient is either the
754   origin server or the first proxy or gateway to receive a Max-Forwards
755   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
756   MUST NOT include an entity.
757</t>
758<t>
759   TRACE allows the client to see what is being received at the other
760   end of the request chain and use that data for testing or diagnostic
761   information. The value of the Via header field (Section 8.9 of <xref target="Part1"/>) is of
762   particular interest, since it acts as a trace of the request chain.
763   Use of the Max-Forwards header field allows the client to limit the
764   length of the request chain, which is useful for testing a chain of
765   proxies forwarding messages in an infinite loop.
766</t>
767<t>
768   If the request is valid, the response SHOULD contain the entire
769   request message in the entity-body, with a Content-Type of
770   "message/http". Responses to this method MUST NOT be cached.
771</t>
772</section>
773
774<section title="CONNECT" anchor="CONNECT">
775  <iref primary="true" item="CONNECT method"/>
776  <iref primary="true" item="Methods" subitem="CONNECT"/>
777<t>
778   This specification reserves the method name CONNECT for use with a
779   proxy that can dynamically switch to being a tunnel (e.g. SSL
780   tunneling <xref target="Luo1998"/>).
781</t>
782</section>
783</section>
784
785
786<section title="Status Code Definitions" anchor="status.codes">
787<t>
788   Each Status-Code is described below, including a description of which
789   method(s) it can follow and any metainformation required in the
790   response.
791</t>
792
793<section title="Informational 1xx" anchor="status.1xx">
794<t>
795   This class of status code indicates a provisional response,
796   consisting only of the Status-Line and optional headers, and is
797   terminated by an empty line. There are no required headers for this
798   class of status code. Since HTTP/1.0 did not define any 1xx status
799   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
800   except under experimental conditions.
801</t>
802<t>
803   A client MUST be prepared to accept one or more 1xx status responses
804   prior to a regular response, even if the client does not expect a 100
805   (Continue) status message. Unexpected 1xx status responses MAY be
806   ignored by a user agent.
807</t>
808<t>
809   Proxies MUST forward 1xx responses, unless the connection between the
810   proxy and its client has been closed, or unless the proxy itself
811   requested the generation of the 1xx response. (For example, if a
812   proxy adds a "Expect: 100-continue" field when it forwards a request,
813   then it need not forward the corresponding 100 (Continue)
814   response(s).)
815</t>
816
817<section title="100 Continue" anchor="status.100">
818  <iref primary="true" item="100 Continue (status code)"/>
819  <iref primary="true" item="Status Codes" subitem="100 Continue"/>
820<t>
821   The client SHOULD continue with its request. This interim response is
822   used to inform the client that the initial part of the request has
823   been received and has not yet been rejected by the server. The client
824   SHOULD continue by sending the remainder of the request or, if the
825   request has already been completed, ignore this response. The server
826   MUST send a final response after the request has been completed. See
827   Section 7.2.3 of <xref target="Part1"/> for detailed discussion of the use and handling of this
828   status code.
829</t>
830</section>
831
832<section title="101 Switching Protocols" anchor="status.101">
833  <iref primary="true" item="101 Switching Protocols (status code)"/>
834  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols"/>
835<t>
836   The server understands and is willing to comply with the client's
837   request, via the Upgrade message header field (Section 5.4 of <xref target="Part5"/>), for a
838   change in the application protocol being used on this connection. The
839   server will switch protocols to those defined by the response's
840   Upgrade header field immediately after the empty line which
841   terminates the 101 response.
842</t>
843<t>
844   The protocol SHOULD be switched only when it is advantageous to do
845   so. For example, switching to a newer version of HTTP is advantageous
846   over older versions, and switching to a real-time, synchronous
847   protocol might be advantageous when delivering resources that use
848   such features.
849</t>
850</section>
851</section>
852
853<section title="Successful 2xx" anchor="status.2xx">
854<t>
855   This class of status code indicates that the client's request was
856   successfully received, understood, and accepted.
857</t>
858
859<section title="200 OK" anchor="status.200">
860  <iref primary="true" item="200 OK (status code)"/>
861  <iref primary="true" item="Status Codes" subitem="200 OK"/>
862<t>
863   The request has succeeded. The information returned with the response
864   is dependent on the method used in the request, for example:
865  <list style="hanging">
866    <t hangText="GET">
867          an entity corresponding to the requested resource is sent in
868          the response;
869    </t>
870    <t hangText="HEAD">
871          the entity-header fields corresponding to the requested
872          resource are sent in the response without any message-body;
873    </t>
874    <t hangText="POST">
875      an entity describing or containing the result of the action;
876    </t>
877    <t hangText="TRACE">
878      an entity containing the request message as received by the
879      end server.
880    </t>
881  </list>
882</t>
883</section>
884
885<section title="201 Created" anchor="status.201">
886  <iref primary="true" item="201 Created (status code)"/>
887  <iref primary="true" item="Status Codes" subitem="201 Created"/>
888<t>
889   The request has been fulfilled and resulted in a new resource being
890   created. The newly created resource can be referenced by the URI(s)
891   returned in the entity of the response, with the most specific URI
892   for the resource given by a Location header field. The response
893   SHOULD include an entity containing a list of resource
894   characteristics and location(s) from which the user or user agent can
895   choose the one most appropriate. The entity format is specified by
896   the media type given in the Content-Type header field. The origin
897   server MUST create the resource before returning the 201 status code.
898   If the action cannot be carried out immediately, the server SHOULD
899   respond with 202 (Accepted) response instead.
900</t>
901<t>
902   A 201 response MAY contain an ETag response header field indicating
903   the current value of the entity tag for the requested variant just
904   created, see Section 6.1 of <xref target="Part4"/>.
905</t>
906</section>
907
908<section title="202 Accepted" anchor="status.202">
909  <iref primary="true" item="202 Accepted (status code)"/>
910  <iref primary="true" item="Status Codes" subitem="202 Accepted"/>
911<t>
912   The request has been accepted for processing, but the processing has
913   not been completed.  The request might or might not eventually be
914   acted upon, as it might be disallowed when processing actually takes
915   place. There is no facility for re-sending a status code from an
916   asynchronous operation such as this.
917</t>
918<t>
919   The 202 response is intentionally non-committal. Its purpose is to
920   allow a server to accept a request for some other process (perhaps a
921   batch-oriented process that is only run once per day) without
922   requiring that the user agent's connection to the server persist
923   until the process is completed. The entity returned with this
924   response SHOULD include an indication of the request's current status
925   and either a pointer to a status monitor or some estimate of when the
926   user can expect the request to be fulfilled.
927</t>
928</section>
929
930<section title="203 Non-Authoritative Information" anchor="status.203">
931  <iref primary="true" item="203 Non-Authoritative Information (status code)"/>
932  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information"/>
933<t>
934   The returned metainformation in the entity-header is not the
935   definitive set as available from the origin server, but is gathered
936   from a local or a third-party copy. The set presented MAY be a subset
937   or superset of the original version. For example, including local
938   annotation information about the resource might result in a superset
939   of the metainformation known by the origin server. Use of this
940   response code is not required and is only appropriate when the
941   response would otherwise be 200 (OK).
942</t>
943</section>
944
945<section title="204 No Content" anchor="status.204">
946  <iref primary="true" item="204 No Content (status code)"/>
947  <iref primary="true" item="Status Codes" subitem="204 No Content"/>
948<t>
949   The server has fulfilled the request but does not need to return an
950   entity-body, and might want to return updated metainformation. The
951   response MAY include new or updated metainformation in the form of
952   entity-headers, which if present SHOULD be associated with the
953   requested variant.
954</t>
955<t>
956   If the client is a user agent, it SHOULD NOT  change its document view
957   from that which caused the request to be sent. This response is
958   primarily intended to allow input for actions to take place without
959   causing a change to the user agent's active document view, although
960   any new or updated metainformation SHOULD be applied to the document
961   currently in the user agent's active view.
962</t>
963<t>
964   The 204 response MUST NOT include a message-body, and thus is always
965   terminated by the first empty line after the header fields.
966</t>
967</section>
968
969<section title="205 Reset Content" anchor="status.205">
970  <iref primary="true" item="205 Reset Content (status code)"/>
971  <iref primary="true" item="Status Codes" subitem="205 Reset Content"/>
972<t>
973   The server has fulfilled the request and the user agent SHOULD reset
974   the document view which caused the request to be sent. This response
975   is primarily intended to allow input for actions to take place via
976   user input, followed by a clearing of the form in which the input is
977   given so that the user can easily initiate another input action. The
978   response MUST NOT include an entity.
979</t>
980</section>
981
982<section title="206 Partial Content" anchor="status.206">
983  <iref primary="true" item="206 Partial Content (status code)"/>
984  <iref primary="true" item="Status Codes" subitem="206 Partial Content"/>
985<t>
986   The server has fulfilled the partial GET request for the resource
987   and the enclosed entity is a partial representation as defined in <xref target="Part5"/>.
988</t>
989</section>
990</section>
991
992<section title="Redirection 3xx" anchor="status.3xx">
993<t>
994   This class of status code indicates that further action needs to be
995   taken by the user agent in order to fulfill the request.  The action
996   required MAY be carried out by the user agent without interaction
997   with the user if and only if the method used in the second request is
998   GET or HEAD. A client SHOULD detect infinite redirection loops, since
999   such loops generate network traffic for each redirection.
1000  <list><t>
1001      Note: previous versions of this specification recommended a
1002      maximum of five redirections. Content developers should be aware
1003      that there might be clients that implement such a fixed
1004      limitation.
1005  </t></list>
1006</t>
1007
1008<section title="300 Multiple Choices" anchor="status.300">
1009  <iref primary="true" item="300 Multiple Choices (status code)"/>
1010  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices"/>
1011<t>
1012   The requested resource corresponds to any one of a set of
1013   representations, each with its own specific location, and agent-driven
1014   negotiation information (Section 4 of <xref target="Part3"/>) is being provided so that
1015   the user (or user agent) can select a preferred representation and
1016   redirect its request to that location.
1017</t>
1018<t>
1019   Unless it was a HEAD request, the response SHOULD include an entity
1020   containing a list of resource characteristics and location(s) from
1021   which the user or user agent can choose the one most appropriate. The
1022   entity format is specified by the media type given in the Content-Type
1023   header field. Depending upon the format and the capabilities of
1024   the user agent, selection of the most appropriate choice MAY be
1025   performed automatically. However, this specification does not define
1026   any standard for such automatic selection.
1027</t>
1028<t>
1029   If the server has a preferred choice of representation, it SHOULD
1030   include the specific URI for that representation in the Location
1031   field; user agents MAY use the Location field value for automatic
1032   redirection. This response is cacheable unless indicated otherwise.
1033</t>
1034</section>
1035
1036<section title="301 Moved Permanently" anchor="status.301">
1037  <iref primary="true" item="301 Moved Permanently (status code)"/>
1038  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently"/>
1039<t>
1040   The requested resource has been assigned a new permanent URI and any
1041   future references to this resource SHOULD use one of the returned
1042   URIs.  Clients with link editing capabilities ought to automatically
1043   re-link references to the Request-URI to one or more of the new
1044   references returned by the server, where possible. This response is
1045   cacheable unless indicated otherwise.
1046</t>
1047<t>
1048   The new permanent URI SHOULD be given by the Location field in the
1049   response. Unless the request method was HEAD, the entity of the
1050   response SHOULD contain a short hypertext note with a hyperlink to
1051   the new URI(s).
1052</t>
1053<t>
1054   If the 301 status code is received in response to a request other
1055   than GET or HEAD, the user agent MUST NOT automatically redirect the
1056   request unless it can be confirmed by the user, since this might
1057   change the conditions under which the request was issued.
1058  <list><t>
1059      Note: When automatically redirecting a POST request after
1060      receiving a 301 status code, some existing HTTP/1.0 user agents
1061      will erroneously change it into a GET request.
1062  </t></list>
1063</t>
1064</section>
1065
1066<section title="302 Found" anchor="status.302">
1067  <iref primary="true" item="302 Found (status code)"/>
1068  <iref primary="true" item="Status Codes" subitem="302 Found"/>
1069<t>
1070   The requested resource resides temporarily under a different URI.
1071   Since the redirection might be altered on occasion, the client SHOULD
1072   continue to use the Request-URI for future requests.  This response
1073   is only cacheable if indicated by a Cache-Control or Expires header
1074   field.
1075</t>
1076<t>
1077   The temporary 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 302 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      Note: RFC 1945 and RFC 2068 specify that the client is not allowed
1089      to change the method on the redirected request.  However, most
1090      existing user agent implementations treat 302 as if it were a 303
1091      response, performing a GET on the Location field-value regardless
1092      of the original request method. The status codes 303 and 307 have
1093      been added for servers that wish to make unambiguously clear which
1094      kind of reaction is expected of the client.
1095  </t></list>
1096</t>
1097</section>
1098
1099<section title="303 See Other" anchor="status.303">
1100  <iref primary="true" item="303 See Other (status code)"/>
1101  <iref primary="true" item="Status Codes" subitem="303 See Other"/>
1102<t>
1103   The response to the request can be found under a different URI and
1104   SHOULD be retrieved using a GET method on that resource. This method
1105   exists primarily to allow the output of a POST-activated script to
1106   redirect the user agent to a selected resource. The new URI is not a
1107   substitute reference for the originally requested resource. The 303
1108   response MUST NOT be cached, but the response to the second
1109   (redirected) request might be cacheable.
1110</t>
1111<t>
1112   The different URI SHOULD be given by the Location field in the
1113   response. Unless the request method was HEAD, the entity of the
1114   response SHOULD contain a short hypertext note with a hyperlink to
1115   the new URI(s).
1116  <list><t>
1117      Note: Many pre-HTTP/1.1 user agents do not understand the 303
1118      status. When interoperability with such clients is a concern, the
1119      302 status code may be used instead, since most user agents react
1120      to a 302 response as described here for 303.
1121  </t></list>
1122</t>
1123</section>
1124
1125<section title="304 Not Modified" anchor="status.304">
1126  <iref primary="true" item="304 Not Modified (status code)"/>
1127  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1128<t>
1129   The response to the request has not been modified since the conditions
1130   indicated by the client's conditional GET request, as defined in <xref target="Part4"/>.
1131</t>
1132</section>
1133
1134<section title="305 Use Proxy" anchor="status.305">
1135  <iref primary="true" item="305 Use Proxy (status code)"/>
1136  <iref primary="true" item="Status Codes" subitem="305 Use Proxy"/>
1137<t>
1138   The requested resource MUST be accessed through the proxy given by
1139   the Location field. The Location field gives the URI of the proxy.
1140   The recipient is expected to repeat this single request via the
1141   proxy. 305 responses MUST only be generated by origin servers.
1142  <list><t>
1143      Note: RFC 2068 was not clear that 305 was intended to redirect a
1144      single request, and to be generated by origin servers only.  Not
1145      observing these limitations has significant security consequences.
1146  </t></list>
1147</t>
1148</section>
1149
1150<section title="306 (Unused)" anchor="status.306">
1151  <iref primary="true" item="306 (Unused) (status code)"/>
1152  <iref primary="true" item="Status Codes" subitem="306 (Unused)"/>
1153<t>
1154   The 306 status code was used in a previous version of the
1155   specification, is no longer used, and the code is reserved.
1156</t>
1157</section>
1158
1159<section title="307 Temporary Redirect" anchor="status.307">
1160  <iref primary="true" item="307 Temporary Redirect (status code)"/>
1161  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect"/>
1162<t>
1163   The requested resource resides temporarily under a different URI.
1164   Since the redirection MAY be altered on occasion, the client SHOULD
1165   continue to use the Request-URI for future requests.  This response
1166   is only cacheable if indicated by a Cache-Control or Expires header
1167   field.
1168</t>
1169<t>
1170   The temporary URI SHOULD be given by the Location field in the
1171   response. Unless the request method was HEAD, the entity of the
1172   response SHOULD contain a short hypertext note with a hyperlink to
1173   the new URI(s) , since many pre-HTTP/1.1 user agents do not
1174   understand the 307 status. Therefore, the note SHOULD contain the
1175   information necessary for a user to repeat the original request on
1176   the new URI.
1177</t>
1178<t>
1179   If the 307 status code is received in response to a request other
1180   than GET or HEAD, the user agent MUST NOT automatically redirect the
1181   request unless it can be confirmed by the user, since this might
1182   change the conditions under which the request was issued.
1183</t>
1184</section>
1185</section>
1186
1187<section title="Client Error 4xx" anchor="status.4xx">
1188<t>
1189   The 4xx class of status code is intended for cases in which the
1190   client seems to have erred. Except when responding to a HEAD request,
1191   the server SHOULD include an entity containing an explanation of the
1192   error situation, and whether it is a temporary or permanent
1193   condition. These status codes are applicable to any request method.
1194   User agents SHOULD display any included entity to the user.
1195</t>
1196<t>
1197   If the client is sending data, a server implementation using TCP
1198   SHOULD be careful to ensure that the client acknowledges receipt of
1199   the packet(s) containing the response, before the server closes the
1200   input connection. If the client continues sending data to the server
1201   after the close, the server's TCP stack will send a reset packet to
1202   the client, which may erase the client's unacknowledged input buffers
1203   before they can be read and interpreted by the HTTP application.
1204</t>
1205
1206<section title="400 Bad Request" anchor="status.400">
1207  <iref primary="true" item="400 Bad Request (status code)"/>
1208  <iref primary="true" item="Status Codes" subitem="400 Bad Request"/>
1209<t>
1210   The request could not be understood by the server due to malformed
1211   syntax. The client SHOULD NOT  repeat the request without
1212   modifications.
1213</t>
1214</section>
1215
1216<section title="401 Unauthorized" anchor="status.401">
1217  <iref primary="true" item="401 Unauthorized (status code)"/>
1218  <iref primary="true" item="Status Codes" subitem="401 Unauthorized"/>
1219<t>
1220   The request requires user authentication (see <xref target="Part7"/>).
1221</t>
1222</section>
1223
1224<section title="402 Payment Required" anchor="status.402">
1225  <iref primary="true" item="402 Payment Required (status code)"/>
1226  <iref primary="true" item="Status Codes" subitem="402 Payment Required"/>
1227<t>
1228   This code is reserved for future use.
1229</t>
1230</section>
1231
1232<section title="403 Forbidden" anchor="status.403">
1233  <iref primary="true" item="403 Forbidden (status code)"/>
1234  <iref primary="true" item="Status Codes" subitem="403 Forbidden"/>
1235<t>
1236   The server understood the request, but is refusing to fulfill it.
1237   Authorization will not help and the request SHOULD NOT  be repeated.
1238   If the request method was not HEAD and the server wishes to make
1239   public why the request has not been fulfilled, it SHOULD describe the
1240   reason for the refusal in the entity.  If the server does not wish to
1241   make this information available to the client, the status code 404
1242   (Not Found) can be used instead.
1243</t>
1244</section>
1245
1246<section title="404 Not Found" anchor="status.404">
1247  <iref primary="true" item="404 Not Found (status code)"/>
1248  <iref primary="true" item="Status Codes" subitem="404 Not Found"/>
1249<t>
1250   The server has not found anything matching the Request-URI. No
1251   indication is given of whether the condition is temporary or
1252   permanent. The 410 (Gone) status code SHOULD be used if the server
1253   knows, through some internally configurable mechanism, that an old
1254   resource is permanently unavailable and has no forwarding address.
1255   This status code is commonly used when the server does not wish to
1256   reveal exactly why the request has been refused, or when no other
1257   response is applicable.
1258</t>
1259</section>
1260
1261<section title="405 Method Not Allowed" anchor="status.405">
1262  <iref primary="true" item="405 Method Not Allowed (status code)"/>
1263  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed"/>
1264<t>
1265   The method specified in the Request-Line is not allowed for the
1266   resource identified by the Request-URI. The response MUST include an
1267   Allow header containing a list of valid methods for the requested
1268   resource.
1269</t>
1270</section>
1271
1272<section title="406 Not Acceptable" anchor="status.406">
1273  <iref primary="true" item="406 Not Acceptable (status code)"/>
1274  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable"/>
1275<t>
1276   The resource identified by the request is only capable of generating
1277   response entities which have content characteristics not acceptable
1278   according to the accept headers sent in the request.
1279</t>
1280<t>
1281   Unless it was a HEAD request, the response SHOULD include an entity
1282   containing a list of available entity characteristics and location(s)
1283   from which the user or user agent can choose the one most
1284   appropriate. The entity format is specified by the media type given
1285   in the Content-Type header field. Depending upon the format and the
1286   capabilities of the user agent, selection of the most appropriate
1287   choice MAY be performed automatically. However, this specification
1288   does not define any standard for such automatic selection.
1289  <list><t>
1290      Note: HTTP/1.1 servers are allowed to return responses which are
1291      not acceptable according to the accept headers sent in the
1292      request. In some cases, this may even be preferable to sending a
1293      406 response. User agents are encouraged to inspect the headers of
1294      an incoming response to determine if it is acceptable.
1295  </t></list>
1296</t>
1297<t>
1298   If the response could be unacceptable, a user agent SHOULD
1299   temporarily stop receipt of more data and query the user for a
1300   decision on further actions.
1301</t>
1302</section>
1303
1304<section title="407 Proxy Authentication Required" anchor="status.407">
1305  <iref primary="true" item="407 Proxy Authentication Required (status code)"/>
1306  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required"/>
1307<t>
1308   This code is similar to 401 (Unauthorized), but indicates that the
1309   client must first authenticate itself with the proxy (see <xref target="Part7"/>).
1310</t>
1311</section>
1312
1313<section title="408 Request Timeout" anchor="status.408">
1314  <iref primary="true" item="408 Request Timeout (status code)"/>
1315  <iref primary="true" item="Status Codes" subitem="408 Request Timeout"/>
1316<t>
1317   The client did not produce a request within the time that the server
1318   was prepared to wait. The client MAY repeat the request without
1319   modifications at any later time.
1320</t>
1321</section>
1322
1323<section title="409 Conflict" anchor="status.409">
1324  <iref primary="true" item="409 Conflict (status code)"/>
1325  <iref primary="true" item="Status Codes" subitem="409 Conflict"/>
1326<t>
1327   The request could not be completed due to a conflict with the current
1328   state of the resource. This code is only allowed in situations where
1329   it is expected that the user might be able to resolve the conflict
1330   and resubmit the request. The response body SHOULD include enough
1331   information for the user to recognize the source of the conflict.
1332   Ideally, the response entity would include enough information for the
1333   user or user agent to fix the problem; however, that might not be
1334   possible and is not required.
1335</t>
1336<t>
1337   Conflicts are most likely to occur in response to a PUT request. For
1338   example, if versioning were being used and the entity being PUT
1339   included changes to a resource which conflict with those made by an
1340   earlier (third-party) request, the server might use the 409 response
1341   to indicate that it can't complete the request. In this case, the
1342   response entity would likely contain a list of the differences
1343   between the two versions in a format defined by the response
1344   Content-Type.
1345</t>
1346</section>
1347
1348<section title="410 Gone" anchor="status.410">
1349  <iref primary="true" item="410 Gone (status code)"/>
1350  <iref primary="true" item="Status Codes" subitem="410 Gone"/>
1351<t>
1352   The requested resource is no longer available at the server and no
1353   forwarding address is known. This condition is expected to be
1354   considered permanent. Clients with link editing capabilities SHOULD
1355   delete references to the Request-URI after user approval. If the
1356   server does not know, or has no facility to determine, whether or not
1357   the condition is permanent, the status code 404 (Not Found) SHOULD be
1358   used instead. This response is cacheable unless indicated otherwise.
1359</t>
1360<t>
1361   The 410 response is primarily intended to assist the task of web
1362   maintenance by notifying the recipient that the resource is
1363   intentionally unavailable and that the server owners desire that
1364   remote links to that resource be removed. Such an event is common for
1365   limited-time, promotional services and for resources belonging to
1366   individuals no longer working at the server's site. It is not
1367   necessary to mark all permanently unavailable resources as "gone" or
1368   to keep the mark for any length of time -- that is left to the
1369   discretion of the server owner.
1370</t>
1371</section>
1372
1373<section title="411 Length Required" anchor="status.411">
1374  <iref primary="true" item="411 Length Required (status code)"/>
1375  <iref primary="true" item="Status Codes" subitem="411 Length Required"/>
1376<t>
1377   The server refuses to accept the request without a defined Content-Length.
1378   The client MAY repeat the request if it adds a valid
1379   Content-Length header field containing the length of the message-body
1380   in the request message.
1381</t>
1382</section>
1383
1384<section title="412 Precondition Failed" anchor="status.412">
1385  <iref primary="true" item="412 Precondition Failed (status code)"/>
1386  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1387<t>
1388   The precondition given in one or more of the request-header fields
1389   evaluated to false when it was tested on the server, as defined in
1390   <xref target="Part4"/>.
1391</t>
1392</section>
1393
1394<section title="413 Request Entity Too Large" anchor="status.413">
1395  <iref primary="true" item="413 Request Entity Too Large (status code)"/>
1396  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large"/>
1397<t>
1398   The server is refusing to process a request because the request
1399   entity is larger than the server is willing or able to process. The
1400   server MAY close the connection to prevent the client from continuing
1401   the request.
1402</t>
1403<t>
1404   If the condition is temporary, the server SHOULD include a Retry-After
1405   header field to indicate that it is temporary and after what
1406   time the client MAY try again.
1407</t>
1408</section>
1409
1410<section title="414 Request-URI Too Long" anchor="status.414">
1411  <iref primary="true" item="414 Request-URI Too Long (status code)"/>
1412  <iref primary="true" item="Status Codes" subitem="414 Request-URI Too Long"/>
1413<t>
1414   The server is refusing to service the request because the Request-URI
1415   is longer than the server is willing to interpret. This rare
1416   condition is only likely to occur when a client has improperly
1417   converted a POST request to a GET request with long query
1418   information, when the client has descended into a URI "black hole" of
1419   redirection (e.g., a redirected URI prefix that points to a suffix of
1420   itself), or when the server is under attack by a client attempting to
1421   exploit security holes present in some servers using fixed-length
1422   buffers for reading or manipulating the Request-URI.
1423</t>
1424</section>
1425
1426<section title="415 Unsupported Media Type" anchor="status.415">
1427  <iref primary="true" item="415 Unsupported Media Type (status code)"/>
1428  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type"/>
1429<t>
1430   The server is refusing to service the request because the entity of
1431   the request is in a format not supported by the requested resource
1432   for the requested method.
1433</t>
1434</section>
1435
1436<section title="416 Requested Range Not Satisfiable" anchor="status.416">
1437  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)"/>
1438  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable"/>
1439<t>
1440   The request included a Range request-header field (Section 5.4 of <xref target="Part5"/>) and none of
1441   the range-specifier values in this field overlap the current extent
1442   of the selected resource.
1443</t>
1444</section>
1445
1446<section title="417 Expectation Failed" anchor="status.417">
1447  <iref primary="true" item="417 Expectation Failed (status code)"/>
1448  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed"/>
1449<t>
1450   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
1451   could not be met by this server, or, if the server is a proxy,
1452   the server has unambiguous evidence that the request could not be met
1453   by the next-hop server.
1454</t>
1455</section>
1456</section>
1457
1458<section title="Server Error 5xx" anchor="status.5xx">
1459<t>
1460   Response status codes beginning with the digit "5" indicate cases in
1461   which the server is aware that it has erred or is incapable of
1462   performing the request. Except when responding to a HEAD request, the
1463   server SHOULD include an entity containing an explanation of the
1464   error situation, and whether it is a temporary or permanent
1465   condition. User agents SHOULD display any included entity to the
1466   user. These response codes are applicable to any request method.
1467</t>
1468
1469<section title="500 Internal Server Error" anchor="status.500">
1470  <iref primary="true" item="500 Internal Server Error (status code)"/>
1471  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error"/>
1472<t>
1473   The server encountered an unexpected condition which prevented it
1474   from fulfilling the request.
1475</t>
1476</section>
1477
1478<section title="501 Not Implemented" anchor="status.501">
1479  <iref primary="true" item="501 Not Implemented (status code)"/>
1480  <iref primary="true" item="Status Codes" subitem="501 Not Implemented"/>
1481<t>
1482   The server does not support the functionality required to fulfill the
1483   request. This is the appropriate response when the server does not
1484   recognize the request method and is not capable of supporting it for
1485   any resource.
1486</t>
1487</section>
1488
1489<section title="502 Bad Gateway" anchor="status.502">
1490  <iref primary="true" item="502 Bad Gateway (status code)"/>
1491  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway"/>
1492<t>
1493   The server, while acting as a gateway or proxy, received an invalid
1494   response from the upstream server it accessed in attempting to
1495   fulfill the request.
1496</t>
1497</section>
1498
1499<section title="503 Service Unavailable" anchor="status.503">
1500  <iref primary="true" item="503 Service Unavailable (status code)"/>
1501  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable"/>
1502<t>
1503   The server is currently unable to handle the request due to a
1504   temporary overloading or maintenance of the server. The implication
1505   is that this is a temporary condition which will be alleviated after
1506   some delay. If known, the length of the delay MAY be indicated in a
1507   Retry-After header. If no Retry-After is given, the client SHOULD
1508   handle the response as it would for a 500 response.
1509  <list><t>
1510      Note: The existence of the 503 status code does not imply that a
1511      server must use it when becoming overloaded. Some servers may wish
1512      to simply refuse the connection.
1513  </t></list>
1514</t>
1515</section>
1516
1517<section title="504 Gateway Timeout" anchor="status.504">
1518  <iref primary="true" item="504 Gateway Timeout (status code)"/>
1519  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout"/>
1520<t>
1521   The server, while acting as a gateway or proxy, did not receive a
1522   timely response from the upstream server specified by the URI (e.g.
1523   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
1524   to access in attempting to complete the request.
1525  <list><t>
1526      Note: Note to implementors: some deployed proxies are known to
1527      return 400 or 500 when DNS lookups time out.
1528  </t></list>
1529</t>
1530</section>
1531
1532<section title="505 HTTP Version Not Supported" anchor="status.505">
1533  <iref primary="true" item="505 HTTP Version Not Supported (status code)"/>
1534  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported"/>
1535<t>
1536   The server does not support, or refuses to support, the HTTP protocol
1537   version that was used in the request message. The server is
1538   indicating that it is unable or unwilling to complete the request
1539   using the same major version as the client, as described in Section 3.1 of <xref target="Part1"/>,
1540   other than with this error message. The response SHOULD contain
1541   an entity describing why that version is not supported and what other
1542   protocols are supported by that server.
1543</t>
1544
1545</section>
1546</section>
1547</section>
1548
1549
1550<section title="Header Field Definitions" anchor="header.fields">
1551<t>
1552   This section defines the syntax and semantics of all standard
1553   HTTP/1.1 header fields. For entity-header fields, both sender and
1554   recipient refer to either the client or the server, depending on who
1555   sends and who receives the entity.
1556</t>
1557
1558<section title="Allow" anchor="header.allow">
1559  <iref primary="true" item="Allow header"/>
1560  <iref primary="true" item="Headers" subitem="Allow"/>
1561<t>
1562      The Allow entity-header field lists the set of methods supported
1563      by the resource identified by the Request-URI. The purpose of this
1564      field is strictly to inform the recipient of valid methods
1565      associated with the resource. An Allow header field MUST be
1566      present in a 405 (Method Not Allowed) response.
1567</t>
1568<figure><iref primary="true" item="Grammar" subitem="Allow"/><artwork type="abnf2616"><![CDATA[
1569       Allow   = "Allow" ":" #Method
1570]]></artwork></figure>
1571<t>
1572      Example of use:
1573</t>
1574<figure><artwork type="example"><![CDATA[
1575       Allow: GET, HEAD, PUT
1576]]></artwork></figure>
1577<t>
1578      This field cannot prevent a client from trying other methods.
1579      However, the indications given by the Allow header field value
1580      SHOULD be followed. The actual set of allowed methods is defined
1581      by the origin server at the time of each request.
1582</t>
1583<t>
1584      The Allow header field MAY be provided with a PUT request to
1585      recommend the methods to be supported by the new or modified
1586      resource. The server is not required to support these methods and
1587      SHOULD include an Allow header in the response giving the actual
1588      supported methods.
1589</t>
1590<t>
1591      A proxy MUST NOT modify the Allow header field even if it does not
1592      understand all the methods specified, since the user agent might
1593      have other means of communicating with the origin server.
1594</t>
1595</section>
1596
1597<section title="Expect" anchor="header.expect">
1598  <iref primary="true" item="Expect header"/>
1599  <iref primary="true" item="Headers" subitem="Expect"/>
1600<t>
1601   The Expect request-header field is used to indicate that particular
1602   server behaviors are required by the client.
1603</t>
1604<figure><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"/><artwork type="abnf2616"><![CDATA[
1605   Expect       =  "Expect" ":" 1#expectation
1606
1607   expectation  =  "100-continue" | expectation-extension
1608   expectation-extension =  token [ "=" ( token | quoted-string )
1609                            *expect-params ]
1610   expect-params =  ";" token [ "=" ( token | quoted-string ) ]
1611]]></artwork></figure>
1612<t>
1613   A server that does not understand or is unable to comply with any of
1614   the expectation values in the Expect field of a request MUST respond
1615   with appropriate error status. The server MUST respond with a 417
1616   (Expectation Failed) status if any of the expectations cannot be met
1617   or, if there are other problems with the request, some other 4xx
1618   status.
1619</t>
1620<t>
1621   This header field is defined with extensible syntax to allow for
1622   future extensions. If a server receives a request containing an
1623   Expect field that includes an expectation-extension that it does not
1624   support, it MUST respond with a 417 (Expectation Failed) status.
1625</t>
1626<t>
1627   Comparison of expectation values is case-insensitive for unquoted
1628   tokens (including the 100-continue token), and is case-sensitive for
1629   quoted-string expectation-extensions.
1630</t>
1631<t>
1632   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
1633   return a 417 (Expectation Failed) status if it receives a request
1634   with an expectation that it cannot meet. However, the Expect
1635   request-header itself is end-to-end; it MUST be forwarded if the
1636   request is forwarded.
1637</t>
1638<t>
1639   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
1640   Expect header.
1641</t>
1642<t>
1643   See Section 7.2.3 of <xref target="Part1"/> for the use of the 100 (continue) status.
1644</t>
1645</section>
1646
1647<section title="From" anchor="header.from">
1648  <iref primary="true" item="From header"/>
1649  <iref primary="true" item="Headers" subitem="From"/>
1650<t>
1651   The From request-header field, if given, SHOULD contain an Internet
1652   e-mail address for the human user who controls the requesting user
1653   agent. The address SHOULD be machine-usable, as defined by "mailbox"
1654   in RFC 822 <xref target="RFC822"/> as updated by RFC 1123 <xref target="RFC1123"/>:
1655</t>
1656<figure><iref primary="true" item="Grammar" subitem="From"/><artwork type="abnf2616"><![CDATA[
1657    From   = "From" ":" mailbox
1658]]></artwork></figure>
1659<t>
1660   An example is:
1661</t>
1662<figure><artwork type="example"><![CDATA[
1663    From: webmaster@w3.org
1664]]></artwork></figure>
1665<t>
1666   This header field MAY be used for logging purposes and as a means for
1667   identifying the source of invalid or unwanted requests. It SHOULD NOT
1668   be used as an insecure form of access protection. The interpretation
1669   of this field is that the request is being performed on behalf of the
1670   person given, who accepts responsibility for the method performed. In
1671   particular, robot agents SHOULD include this header so that the
1672   person responsible for running the robot can be contacted if problems
1673   occur on the receiving end.
1674</t>
1675<t>
1676   The Internet e-mail address in this field MAY be separate from the
1677   Internet host which issued the request. For example, when a request
1678   is passed through a proxy the original issuer's address SHOULD be
1679   used.
1680</t>
1681<t>
1682   The client SHOULD NOT  send the From header field without the user's
1683   approval, as it might conflict with the user's privacy interests or
1684   their site's security policy. It is strongly recommended that the
1685   user be able to disable, enable, and modify the value of this field
1686   at any time prior to a request.
1687</t>
1688</section>
1689
1690<section title="Location" anchor="header.location">
1691  <iref primary="true" item="Location header"/>
1692  <iref primary="true" item="Headers" subitem="Location"/>
1693<t>
1694   The Location response-header field is used to redirect the recipient
1695   to a location other than the Request-URI for completion of the
1696   request or identification of a new resource. For 201 (Created)
1697   responses, the Location is that of the new resource which was created
1698   by the request. For 3xx responses, the location SHOULD indicate the
1699   server's preferred URI for automatic redirection to the resource. The
1700   field value consists of a single absolute URI.
1701</t>
1702<figure><iref primary="true" item="Grammar" subitem="Location"/><artwork type="abnf2616"><![CDATA[
1703    Location       = "Location" ":" absoluteURI
1704]]></artwork></figure>
1705<t>
1706   An example is:
1707</t>
1708<figure><artwork type="example"><![CDATA[
1709    Location: http://www.w3.org/pub/WWW/People.html
1710]]></artwork></figure>
1711<t>
1712  <list><t>
1713      Note: The Content-Location header field (Section 5.7 of <xref target="Part3"/>) differs
1714      from Location in that the Content-Location identifies the original
1715      location of the entity enclosed in the request. It is therefore
1716      possible for a response to contain header fields for both Location
1717      and Content-Location.
1718  </t></list>
1719</t>
1720</section>
1721
1722<section title="Max-Forwards" anchor="header.max-forwards">
1723  <iref primary="true" item="Max-Forwards header"/>
1724  <iref primary="true" item="Headers" subitem="Max-Forwards"/>
1725<t>
1726   The Max-Forwards request-header field provides a mechanism with the
1727   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
1728   number of proxies or gateways that can forward the request to the
1729   next inbound server. This can be useful when the client is attempting
1730   to trace a request chain which appears to be failing or looping in
1731   mid-chain.
1732</t>
1733<figure><iref primary="true" item="Grammar" subitem="Max-Forwards"/><artwork type="abnf2616"><![CDATA[
1734    Max-Forwards   = "Max-Forwards" ":" 1*DIGIT
1735]]></artwork></figure>
1736<t>
1737   The Max-Forwards value is a decimal integer indicating the remaining
1738   number of times this request message may be forwarded.
1739</t>
1740<t>
1741   Each proxy or gateway recipient of a TRACE or OPTIONS request
1742   containing a Max-Forwards header field MUST check and update its
1743   value prior to forwarding the request. If the received value is zero
1744   (0), the recipient MUST NOT forward the request; instead, it MUST
1745   respond as the final recipient. If the received Max-Forwards value is
1746   greater than zero, then the forwarded message MUST contain an updated
1747   Max-Forwards field with a value decremented by one (1).
1748</t>
1749<t>
1750   The Max-Forwards header field MAY be ignored for all other methods
1751   defined by this specification and for any extension methods for which
1752   it is not explicitly referred to as part of that method definition.
1753</t>
1754</section>
1755
1756<section title="Referer" anchor="header.referer">
1757  <iref primary="true" item="Referer header"/>
1758  <iref primary="true" item="Headers" subitem="Referer"/>
1759<t>
1760   The Referer[sic] request-header field allows the client to specify,
1761   for the server's benefit, the address (URI) of the resource from
1762   which the Request-URI was obtained (the "referrer", although the
1763   header field is misspelled.) The Referer request-header allows a
1764   server to generate lists of back-links to resources for interest,
1765   logging, optimized caching, etc. It also allows obsolete or mistyped
1766   links to be traced for maintenance. The Referer field MUST NOT be
1767   sent if the Request-URI was obtained from a source that does not have
1768   its own URI, such as input from the user keyboard.
1769</t>
1770<figure><iref primary="true" item="Grammar" subitem="Referer"/><artwork type="abnf2616"><![CDATA[
1771    Referer        = "Referer" ":" ( absoluteURI | relativeURI )
1772]]></artwork></figure>
1773<t>
1774   Example:
1775</t>
1776<figure><artwork type="example"><![CDATA[
1777    Referer: http://www.w3.org/hypertext/DataSources/Overview.html
1778]]></artwork></figure>
1779<t>
1780   If the field value is a relative URI, it SHOULD be interpreted
1781   relative to the Request-URI. The URI MUST NOT include a fragment. See
1782   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
1783</t>
1784</section>
1785
1786<section title="Retry-After" anchor="header.retry-after">
1787  <iref primary="true" item="Retry-After header"/>
1788  <iref primary="true" item="Headers" subitem="Retry-After"/>
1789<t>
1790   The Retry-After response-header field can be used with a 503 (Service
1791   Unavailable) response to indicate how long the service is expected to
1792   be unavailable to the requesting client. This field MAY also be used
1793   with any 3xx (Redirection) response to indicate the minimum time the
1794   user-agent is asked wait before issuing the redirected request. The
1795   value of this field can be either an HTTP-date or an integer number
1796   of seconds (in decimal) after the time of the response.
1797</t>
1798<figure><iref primary="true" item="Grammar" subitem="Retry-After"/><artwork type="abnf2616"><![CDATA[
1799    Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )
1800]]></artwork></figure>
1801<t>
1802   Two examples of its use are
1803</t>
1804<figure><artwork type="example"><![CDATA[
1805    Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
1806    Retry-After: 120
1807]]></artwork></figure>
1808<t>
1809   In the latter example, the delay is 2 minutes.
1810</t>
1811</section>
1812
1813<section title="Server" anchor="header.server">
1814  <iref primary="true" item="Server header"/>
1815  <iref primary="true" item="Headers" subitem="Server"/>
1816<t>
1817   The Server response-header field contains information about the
1818   software used by the origin server to handle the request. The field
1819   can contain multiple product tokens (<xref target="product.tokens"/>) and comments
1820   identifying the server and any significant subproducts. The product
1821   tokens are listed in order of their significance for identifying the
1822   application.
1823</t>
1824<figure><iref primary="true" item="Grammar" subitem="Server"/><artwork type="abnf2616"><![CDATA[
1825    Server         = "Server" ":" 1*( product | comment )
1826]]></artwork></figure>
1827<t>
1828   Example:
1829</t>
1830<figure><artwork type="example"><![CDATA[
1831    Server: CERN/3.0 libwww/2.17
1832]]></artwork></figure>
1833<t>
1834   If the response is being forwarded through a proxy, the proxy
1835   application MUST NOT modify the Server response-header. Instead, it
1836   SHOULD include a Via field (as described in Section 8.9 of <xref target="Part1"/>).
1837  <list><t>
1838      Note: Revealing the specific software version of the server might
1839      allow the server machine to become more vulnerable to attacks
1840      against software that is known to contain security holes. Server
1841      implementors are encouraged to make this field a configurable
1842      option.
1843  </t></list>
1844</t>
1845</section>
1846
1847<section title="User-Agent" anchor="header.user-agent">
1848  <iref primary="true" item="User-Agent header"/>
1849  <iref primary="true" item="Headers" subitem="User-Agent"/>
1850<t>
1851   The User-Agent request-header field contains information about the
1852   user agent originating the request. This is for statistical purposes,
1853   the tracing of protocol violations, and automated recognition of user
1854   agents for the sake of tailoring responses to avoid particular user
1855   agent limitations. User agents SHOULD include this field with
1856   requests. The field can contain multiple product tokens (<xref target="product.tokens"/>)
1857   and comments identifying the agent and any subproducts which form a
1858   significant part of the user agent. By convention, the product tokens
1859   are listed in order of their significance for identifying the
1860   application.
1861</t>
1862<figure><iref primary="true" item="Grammar" subitem="User-Agent"/><artwork type="abnf2616"><![CDATA[
1863    User-Agent     = "User-Agent" ":" 1*( product | comment )
1864]]></artwork></figure>
1865<t>
1866   Example:
1867</t>
1868<figure><artwork type="example"><![CDATA[
1869    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1870]]></artwork></figure>
1871</section>
1872
1873</section>
1874
1875<section title="IANA Considerations" anchor="IANA.considerations">
1876<t>
1877   TBD.
1878</t>
1879</section>
1880
1881<section title="Security Considerations" anchor="security.considerations">
1882<t>
1883   This section is meant to inform application developers, information
1884   providers, and users of the security limitations in HTTP/1.1 as
1885   described by this document. The discussion does not include
1886   definitive solutions to the problems revealed, though it does make
1887   some suggestions for reducing security risks.
1888</t>
1889
1890<section title="Transfer of Sensitive Information" anchor="security.sensitive">
1891<t>
1892   Like any generic data transfer protocol, HTTP cannot regulate the
1893   content of the data that is transferred, nor is there any a priori
1894   method of determining the sensitivity of any particular piece of
1895   information within the context of any given request. Therefore,
1896   applications SHOULD supply as much control over this information as
1897   possible to the provider of that information. Four header fields are
1898   worth special mention in this context: Server, Via, Referer and From.
1899</t>
1900<t>
1901   Revealing the specific software version of the server might allow the
1902   server machine to become more vulnerable to attacks against software
1903   that is known to contain security holes. Implementors SHOULD make the
1904   Server header field a configurable option.
1905</t>
1906<t>
1907   Proxies which serve as a portal through a network firewall SHOULD
1908   take special precautions regarding the transfer of header information
1909   that identifies the hosts behind the firewall. In particular, they
1910   SHOULD remove, or replace with sanitized versions, any Via fields
1911   generated behind the firewall.
1912</t>
1913<t>
1914   The Referer header allows reading patterns to be studied and reverse
1915   links drawn. Although it can be very useful, its power can be abused
1916   if user details are not separated from the information contained in
1917   the Referer. Even when the personal information has been removed, the
1918   Referer header might indicate a private document's URI whose
1919   publication would be inappropriate.
1920</t>
1921<t>
1922   The information sent in the From field might conflict with the user's
1923   privacy interests or their site's security policy, and hence it
1924   SHOULD NOT  be transmitted without the user being able to disable,
1925   enable, and modify the contents of the field. The user MUST be able
1926   to set the contents of this field within a user preference or
1927   application defaults configuration.
1928</t>
1929<t>
1930   We suggest, though do not require, that a convenient toggle interface
1931   be provided for the user to enable or disable the sending of From and
1932   Referer information.
1933</t>
1934<t>
1935   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
1936   fields can sometimes be used to determine that a specific client or
1937   server have a particular security hole which might be exploited.
1938   Unfortunately, this same information is often used for other valuable
1939   purposes for which HTTP currently has no better mechanism.
1940</t>
1941</section>
1942
1943<section title="Encoding Sensitive Information in URI's" anchor="encoding.sensitive.information.in.uris">
1944<t>
1945   Because the source of a link might be private information or might
1946   reveal an otherwise private information source, it is strongly
1947   recommended that the user be able to select whether or not the
1948   Referer field is sent. For example, a browser client could have a
1949   toggle switch for browsing openly/anonymously, which would
1950   respectively enable/disable the sending of Referer and From
1951   information.
1952</t>
1953<t>
1954   Clients SHOULD NOT  include a Referer header field in a (non-secure)
1955   HTTP request if the referring page was transferred with a secure
1956   protocol.
1957</t>
1958<t>
1959   Authors of services which use the HTTP protocol SHOULD NOT  use GET
1960   based forms for the submission of sensitive data, because this will
1961   cause this data to be encoded in the Request-URI. Many existing
1962   servers, proxies, and user agents will log the request URI in some
1963   place where it might be visible to third parties. Servers can use
1964   POST-based form submission instead
1965</t>
1966</section>
1967
1968<section title="Location Headers and Spoofing" anchor="location.spoofing">
1969<t>
1970   If a single server supports multiple organizations that do not trust
1971   one another, then it MUST check the values of Location and Content-Location
1972   headers in responses that are generated under control of
1973   said organizations to make sure that they do not attempt to
1974   invalidate resources over which they have no authority.
1975</t>
1976</section>
1977
1978</section>
1979
1980<section title="Acknowledgments" anchor="ack">
1981<t>
1982   Based on an XML translation of RFC 2616 by Julian Reschke.
1983</t>
1984</section>
1985</middle>
1986<back>
1987<references>
1988
1989<reference anchor="Part1">
1990   <front>
1991      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1992      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1993         <organization abbrev="Day Software">Day Software</organization>
1994         <address><email>fielding@gbiv.com</email></address>
1995      </author>
1996      <author initials="J." surname="Gettys" fullname="Jim Gettys">
1997         <organization>One Laptop per Child</organization>
1998         <address><email>jg@laptop.org</email></address>
1999      </author>
2000      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2001         <organization abbrev="HP">Hewlett-Packard Company</organization>
2002         <address><email>JeffMogul@acm.org</email></address>
2003      </author>
2004      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2005         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2006         <address><email>henrikn@microsoft.com</email></address>
2007      </author>
2008      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2009         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2010         <address><email>LMM@acm.org</email></address>
2011      </author>
2012      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2013         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2014         <address><email>paulle@microsoft.com</email></address>
2015      </author>
2016      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2017         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2018         <address><email>timbl@w3.org</email></address>
2019      </author>
2020      <date month="December" year="2007"/>
2021   </front>
[61]2022   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-00"/>
[55]2023   
2024</reference>
2025
2026<reference anchor="Part3">
2027   <front>
2028      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2029      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2030         <organization abbrev="Day Software">Day Software</organization>
2031         <address><email>fielding@gbiv.com</email></address>
2032      </author>
2033      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2034         <organization>One Laptop per Child</organization>
2035         <address><email>jg@laptop.org</email></address>
2036      </author>
2037      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2038         <organization abbrev="HP">Hewlett-Packard Company</organization>
2039         <address><email>JeffMogul@acm.org</email></address>
2040      </author>
2041      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2042         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2043         <address><email>henrikn@microsoft.com</email></address>
2044      </author>
2045      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2046         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2047         <address><email>LMM@acm.org</email></address>
2048      </author>
2049      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2050         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2051         <address><email>paulle@microsoft.com</email></address>
2052      </author>
2053      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2054         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2055         <address><email>timbl@w3.org</email></address>
2056      </author>
2057      <date month="December" year="2007"/>
2058   </front>
[61]2059   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-00"/>
[55]2060   
2061</reference>
2062
2063<reference anchor="Part4">
2064   <front>
2065      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2066      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2067         <organization abbrev="Day Software">Day Software</organization>
2068         <address><email>fielding@gbiv.com</email></address>
2069      </author>
2070      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2071         <organization>One Laptop per Child</organization>
2072         <address><email>jg@laptop.org</email></address>
2073      </author>
2074      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2075         <organization abbrev="HP">Hewlett-Packard Company</organization>
2076         <address><email>JeffMogul@acm.org</email></address>
2077      </author>
2078      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2079         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2080         <address><email>henrikn@microsoft.com</email></address>
2081      </author>
2082      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2083         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2084         <address><email>LMM@acm.org</email></address>
2085      </author>
2086      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2087         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2088         <address><email>paulle@microsoft.com</email></address>
2089      </author>
2090      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2091         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2092         <address><email>timbl@w3.org</email></address>
2093      </author>
2094      <date month="December" year="2007"/>
2095   </front>
[61]2096   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-00"/>
[55]2097   
2098</reference>
2099
2100<reference anchor="Part5">
2101   <front>
2102      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2103      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2104         <organization abbrev="Day Software">Day Software</organization>
2105         <address><email>fielding@gbiv.com</email></address>
2106      </author>
2107      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2108         <organization>One Laptop per Child</organization>
2109         <address><email>jg@laptop.org</email></address>
2110      </author>
2111      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2112         <organization abbrev="HP">Hewlett-Packard Company</organization>
2113         <address><email>JeffMogul@acm.org</email></address>
2114      </author>
2115      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2116         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2117         <address><email>henrikn@microsoft.com</email></address>
2118      </author>
2119      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2120         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2121         <address><email>LMM@acm.org</email></address>
2122      </author>
2123      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2124         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2125         <address><email>paulle@microsoft.com</email></address>
2126      </author>
2127      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2128         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2129         <address><email>timbl@w3.org</email></address>
2130      </author>
2131      <date month="December" year="2007"/>
2132   </front>
[61]2133   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-00"/>
[55]2134   
2135</reference>
2136
2137<reference anchor="Part6">
2138   <front>
2139      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
2140      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2141         <organization abbrev="Day Software">Day Software</organization>
2142         <address><email>fielding@gbiv.com</email></address>
2143      </author>
2144      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2145         <organization>One Laptop per Child</organization>
2146         <address><email>jg@laptop.org</email></address>
2147      </author>
2148      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2149         <organization abbrev="HP">Hewlett-Packard Company</organization>
2150         <address><email>JeffMogul@acm.org</email></address>
2151      </author>
2152      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2153         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2154         <address><email>henrikn@microsoft.com</email></address>
2155      </author>
2156      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2157         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2158         <address><email>LMM@acm.org</email></address>
2159      </author>
2160      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2161         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2162         <address><email>paulle@microsoft.com</email></address>
2163      </author>
2164      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2165         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2166         <address><email>timbl@w3.org</email></address>
2167      </author>
2168      <date month="December" year="2007"/>
2169   </front>
[61]2170   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-00"/>
[55]2171   
2172</reference>
2173
2174<reference anchor="Part7">
2175   <front>
2176      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2177      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2178         <organization abbrev="Day Software">Day Software</organization>
2179         <address><email>fielding@gbiv.com</email></address>
2180      </author>
2181      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2182         <organization>One Laptop per Child</organization>
2183         <address><email>jg@laptop.org</email></address>
2184      </author>
2185      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2186         <organization abbrev="HP">Hewlett-Packard Company</organization>
2187         <address><email>JeffMogul@acm.org</email></address>
2188      </author>
2189      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2190         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2191         <address><email>henrikn@microsoft.com</email></address>
2192      </author>
2193      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2194         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2195         <address><email>LMM@acm.org</email></address>
2196      </author>
2197      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2198         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2199         <address><email>paulle@microsoft.com</email></address>
2200      </author>
2201      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2202         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2203         <address><email>timbl@w3.org</email></address>
2204      </author>
2205      <date month="December" year="2007"/>
2206   </front>
[61]2207   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-00"/>
[55]2208   
2209</reference>
2210
2211<reference anchor="RFC2616">
2212   <front>
2213      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2214      <author initials="R." surname="Fielding" fullname="R. Fielding">
2215         <organization>University of California, Irvine</organization>
2216         <address><email>fielding@ics.uci.edu</email></address>
2217      </author>
2218      <author initials="J." surname="Gettys" fullname="J. Gettys">
2219         <organization>W3C</organization>
2220         <address><email>jg@w3.org</email></address>
2221      </author>
2222      <author initials="J." surname="Mogul" fullname="J. Mogul">
2223         <organization>Compaq Computer Corporation</organization>
2224         <address><email>mogul@wrl.dec.com</email></address>
2225      </author>
2226      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2227         <organization>MIT Laboratory for Computer Science</organization>
2228         <address><email>frystyk@w3.org</email></address>
2229      </author>
2230      <author initials="L." surname="Masinter" fullname="L. Masinter">
2231         <organization>Xerox Corporation</organization>
2232         <address><email>masinter@parc.xerox.com</email></address>
2233      </author>
2234      <author initials="P." surname="Leach" fullname="P. Leach">
2235         <organization>Microsoft Corporation</organization>
2236         <address><email>paulle@microsoft.com</email></address>
2237      </author>
2238      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2239         <organization>W3C</organization>
2240         <address><email>timbl@w3.org</email></address>
2241      </author>
2242      <date month="June" year="1999"/>
2243   </front>
2244   <seriesInfo name="RFC" value="2616"/>
2245</reference>
2246
2247<reference anchor="RFC1123">
2248<front>
2249<title>Requirements for Internet Hosts - Application and Support</title>
2250<author initials="R." surname="Braden" fullname="Robert Braden">
2251<organization>University of Southern California (USC), Information Sciences Institute</organization>
2252<address>
2253<postal>
2254<street>4676 Admiralty Way</street>
2255<city>Marina del Rey</city>
2256<region>CA</region>
2257<code>90292-6695</code>
2258<country>US</country></postal>
2259<phone>+1 213 822 1511</phone>
2260<email>Braden@ISI.EDU</email></address></author>
2261<date month="October" year="1989"/></front>
2262<seriesInfo name="STD" value="3"/>
2263<seriesInfo name="RFC" value="1123"/>
2264</reference>
2265
2266<reference anchor="RFC822">
2267<front>
2268<title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
2269<author initials="D.H." surname="Crocker" fullname="David H. Crocker">
2270<organization>University of Delaware, Dept. of Electrical Engineering</organization>
2271<address>
2272<postal>
2273<street/>
2274<city>Newark</city>
2275<region>DE</region>
2276<code>19711</code>
2277<country>US</country></postal>
2278<email>DCrocker@UDel-Relay</email></address></author>
2279<date month="August" day="13" year="1982"/></front>
2280<seriesInfo name="STD" value="11"/>
2281<seriesInfo name="RFC" value="822"/>
2282</reference>
2283
2284<reference anchor="RFC2068">
2285<front>
2286<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2287<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2288<organization>University of California, Irvine, Department of Information and Computer Science</organization>
2289<address>
2290<postal>
2291<street/>
2292<city>Irvine</city>
2293<region>CA</region>
2294<code>92717-3425</code>
2295<country>US</country></postal>
2296<facsimile>+1 714 824 4056</facsimile>
2297<email>fielding@ics.uci.edu</email></address></author>
2298<author initials="J." surname="Gettys" fullname="Jim Gettys">
2299<organization>MIT Laboratory for Computer Science</organization>
2300<address>
2301<postal>
2302<street>545 Technology Square</street>
2303<city>Cambridge</city>
2304<region>MA</region>
2305<code>02139</code>
2306<country>US</country></postal>
2307<facsimile>+1 617 258 8682</facsimile>
2308<email>jg@w3.org</email></address></author>
2309<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2310<organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2311<address>
2312<postal>
2313<street>250 University Avenue</street>
2314<city>Palo Alto</city>
2315<region>CA</region>
2316<code>94301</code>
2317<country>US</country></postal>
2318<email>mogul@wrl.dec.com</email></address></author>
2319<author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2320<organization>MIT Laboratory for Computer Science</organization>
2321<address>
2322<postal>
2323<street>545 Technology Square</street>
2324<city>Cambridge</city>
2325<region>MA</region>
2326<code>02139</code>
2327<country>US</country></postal>
2328<facsimile>+1 617 258 8682</facsimile>
2329<email>frystyk@w3.org</email></address></author>
2330<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2331<organization>MIT Laboratory for Computer Science</organization>
2332<address>
2333<postal>
2334<street>545 Technology Square</street>
2335<city>Cambridge</city>
2336<region>MA</region>
2337<code>02139</code>
2338<country>US</country></postal>
2339<facsimile>+1 617 258 8682</facsimile>
2340<email>timbl@w3.org</email></address></author>
2341<date month="January" year="1997"/>
2342<abstract>
2343<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>
2344<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>
2345<seriesInfo name="RFC" value="2068"/>
2346</reference>
2347
2348<reference anchor="Luo1998">
2349<front>
2350<title>Tunneling TCP based protocols through Web proxy servers</title>
2351<author initials="A." surname="Luotonen" fullname="A. Luotonen">
2352  <organization/>
2353</author>
2354<date/>
2355</front>
2356<seriesInfo name="" value="Work in Progress"/>
2357</reference>
2358
2359</references>
2360
2361<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2362<t>
2363   Clarified which error code should be used for inbound server failures
2364   (e.g. DNS failures). (<xref target="status.504"/>).
2365</t>
2366<t>
2367   CREATE had a race that required an Etag be sent when a resource is
2368   first created. (<xref target="status.201"/>).
2369</t>
2370<t>
2371   Rewrite of message transmission requirements to make it much harder
2372   for implementors to get it wrong, as the consequences of errors here
2373   can have significant impact on the Internet, and to deal with the
2374   following problems:
2375  <list style="numbers">
2376      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
2377         this was incorrectly placing a requirement on the behavior of
2378         an implementation of a future version of HTTP/1.x</t>
2379
2380      <t>Made it clear that user-agents should retry requests, not
2381         "clients" in general.</t>
2382
2383      <t>Converted requirements for clients to ignore unexpected 100
2384         (Continue) responses, and for proxies to forward 100 responses,
2385         into a general requirement for 1xx responses.</t>
2386
2387      <t>Modified some TCP-specific language, to make it clearer that
2388         non-TCP transports are possible for HTTP.</t>
2389
2390      <t>Require that the origin server MUST NOT wait for the request
2391         body before it sends a required 100 (Continue) response.</t>
2392
2393      <t>Allow, rather than require, a server to omit 100 (Continue) if
2394         it has already seen some of the request body.</t>
2395
2396      <t>Allow servers to defend against denial-of-service attacks and
2397         broken clients.</t>
2398  </list>
2399</t>
2400<t>
2401   This change adds the Expect header and 417 status code.
2402</t>
2403<t>
2404   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
2405   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
2406</t>
2407<t>
2408   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
2409   implemented in previous versions of this specification. See RFC 2068
2410   <xref target="RFC2068"/>.
2411</t>
2412</section>
2413</back>
2414</rfc>
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