source: draft-ietf-httpbis/latest/p1-messaging.xml @ 115

Last change on this file since 115 was 115, checked in by julian.reschke@…, 12 years ago

Add templates for Changes sections throughout, remove intro specific to draft -00, enhance a few more inter-document references, cleanup some leftovers of the symref switchover.

  • Property svn:eol-style set to native
File size: 170.1 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "December">
16  <!ENTITY ID-YEAR "2007">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39]>
40<?rfc toc="yes" ?>
41<?rfc symrefs="yes" ?>
42<?rfc sortrefs="yes" ?>
43<?rfc compact="yes"?>
44<?rfc subcompact="no" ?>
45<?rfc linkmailto="no" ?>
46<?rfc editing="no" ?>
47<?rfc-ext allow-markup-in-artwork="yes" ?>
48<?rfc-ext include-references-in-index="yes" ?>
49<rfc obsoletes="2068, 2616" category="std"
50     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
51     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
52<front>
53
54  <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
55
56  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
57    <organization abbrev="Day Software">Day Software</organization>
58    <address>
59      <postal>
60        <street>23 Corporate Plaza DR, Suite 280</street>
61        <city>Newport Beach</city>
62        <region>CA</region>
63        <code>92660</code>
64        <country>USA</country>
65      </postal>
66      <phone>+1-949-706-5300</phone>
67      <facsimile>+1-949-706-5305</facsimile>
68      <email>fielding@gbiv.com</email>
69      <uri>http://roy.gbiv.com/</uri>
70    </address>
71  </author>
72
73  <author initials="J." surname="Gettys" fullname="Jim Gettys">
74    <organization>One Laptop per Child</organization>
75    <address>
76      <postal>
77        <street>21 Oak Knoll Road</street>
78        <city>Carlisle</city>
79        <region>MA</region>
80        <code>01741</code>
81        <country>USA</country>
82      </postal>
83      <email>jg@laptop.org</email>
84      <uri>http://www.laptop.org/</uri>
85    </address>
86  </author>
87 
88  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
89    <organization abbrev="HP">Hewlett-Packard Company</organization>
90    <address>
91      <postal>
92        <street>HP Labs, Large Scale Systems Group</street>
93        <street>1501 Page Mill Road, MS 1177</street>
94        <city>Palo Alto</city>
95        <region>CA</region>
96        <code>94304</code>
97        <country>USA</country>
98      </postal>
99      <email>JeffMogul@acm.org</email>
100    </address>
101  </author>
102
103  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
104    <organization abbrev="Microsoft">Microsoft Corporation</organization>
105    <address>
106      <postal>
107        <street>1 Microsoft Way</street>
108        <city>Redmond</city>
109        <region>WA</region>
110        <code>98052</code>
111        <country>USA</country>
112      </postal>
113      <email>henrikn@microsoft.com</email>
114    </address>
115  </author>
116
117  <author initials="L." surname="Masinter" fullname="Larry Masinter">
118    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
119    <address>
120      <postal>
121        <street>345 Park Ave</street>
122        <city>San Jose</city>
123        <region>CA</region>
124        <code>95110</code>
125        <country>USA</country>
126      </postal>
127      <email>LMM@acm.org</email>
128      <uri>http://larry.masinter.net/</uri>
129    </address>
130  </author>
131 
132  <author initials="P." surname="Leach" fullname="Paul J. Leach">
133    <organization abbrev="Microsoft">Microsoft Corporation</organization>
134    <address>
135      <postal>
136        <street>1 Microsoft Way</street>
137        <city>Redmond</city>
138        <region>WA</region>
139        <code>98052</code>
140      </postal>
141      <email>paulle@microsoft.com</email>
142    </address>
143  </author>
144   
145  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
146    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
147    <address>
148      <postal>
149        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
150        <street>The Stata Center, Building 32</street>
151        <street>32 Vassar Street</street>
152        <city>Cambridge</city>
153        <region>MA</region>
154        <code>02139</code>
155        <country>USA</country>
156      </postal>
157      <email>timbl@w3.org</email>
158      <uri>http://www.w3.org/People/Berners-Lee/</uri>
159    </address>
160  </author>
161
162  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
163    <organization abbrev="W3C">World Wide Web Consortium</organization>
164    <address>
165      <postal>
166        <street>W3C / ERCIM</street>
167        <street>2004, rte des Lucioles</street>
168        <city>Sophia-Antipolis</city>
169        <region>AM</region>
170        <code>06902</code>
171        <country>France</country>
172      </postal>
173      <email>ylafon@w3.org</email>
174      <uri>http://www.raubacapeu.net/people/yves/</uri>
175    </address>
176  </author>
177
178  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
179    <organization abbrev="greenbytes">greenbytes GmbH</organization>
180    <address>
181      <postal>
182        <street>Hafenweg 16</street>
183        <city>Muenster</city><region>NW</region><code>48155</code>
184        <country>Germany</country>
185      </postal>
186      <phone>+49 251 2807760</phone>   
187      <facsimile>+49 251 2807761</facsimile>   
188      <email>julian.reschke@greenbytes.de</email>       
189      <uri>http://greenbytes.de/tech/webdav/</uri>     
190    </address>
191  </author>
192
193  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
194
195<abstract>
196<t>
197   The Hypertext Transfer Protocol (HTTP) is an application-level
198   protocol for distributed, collaborative, hypermedia information
199   systems. HTTP has been in use by the World Wide Web global information
200   initiative since 1990. This document is Part 1 of the seven-part specification
201   that defines the protocol referred to as "HTTP/1.1" and, taken together,
202   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
203   its associated terminology, defines the "http" and "https" Uniform
204   Resource Identifier (URI) schemes, defines the generic message syntax
205   and parsing requirements for HTTP message frames, and describes
206   general security concerns for implementations.
207</t>
208</abstract>
209
210<note title="Editorial Note (To be removed by RFC Editor)">
211  <t>
212    Discussion of this draft should take place on the HTTPBIS working group
213    mailing list (ietf-http-wg@w3.org). The current issues list is
214    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
215    and related documents (including fancy diffs) can be found at
216    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
217  </t>
218</note>
219</front>
220<middle>
221<section title="Introduction" anchor="introduction">
222<t>
223   This document will define aspects of HTTP related to overall network
224   operation, message framing, interaction with transport protocols, and
225   URI schemes. Right now it only includes the extracted relevant sections
226   of <xref target="RFC2616"/>.
227</t>
228<section title="Purpose" anchor="intro.purpose">
229<t>
230   The Hypertext Transfer Protocol (HTTP) is an application-level
231   protocol for distributed, collaborative, hypermedia information
232   systems. HTTP has been in use by the World-Wide Web global
233   information initiative since 1990. The first version of HTTP,
234   referred to as HTTP/0.9, was a simple protocol for raw data transfer
235   across the Internet. HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
236   the protocol by allowing messages to be in the format of MIME-like
237   messages, containing metainformation about the data transferred and
238   modifiers on the request/response semantics. However, HTTP/1.0 does
239   not sufficiently take into consideration the effects of hierarchical
240   proxies, caching, the need for persistent connections, or virtual
241   hosts. In addition, the proliferation of incompletely-implemented
242   applications calling themselves "HTTP/1.0" has necessitated a
243   protocol version change in order for two communicating applications
244   to determine each other's true capabilities.
245</t>
246<t>
247   This specification defines the protocol referred to as "HTTP/1.1".
248   This protocol includes more stringent requirements than HTTP/1.0 in
249   order to ensure reliable implementation of its features.
250</t>
251<t>
252   Practical information systems require more functionality than simple
253   retrieval, including search, front-end update, and annotation. HTTP
254   allows an open-ended set of methods and headers that indicate the
255   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
256   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
257   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
258   method is to be applied. Messages are passed in a format similar to
259   that used by Internet mail <xref target="RFC822"/> as defined by the Multipurpose
260   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
261</t>
262<t>
263   HTTP is also used as a generic protocol for communication between
264   user agents and proxies/gateways to other Internet systems, including
265   those supported by the SMTP <xref target="RFC821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
266   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
267   access to resources available from diverse applications.
268</t>
269</section>
270
271<section title="Requirements" anchor="intro.requirements">
272<t>
273   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
274   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
275   document are to be interpreted as described in <xref target="RFC2119"/>.
276</t>
277<t>
278   An implementation is not compliant if it fails to satisfy one or more
279   of the &MUST; or &REQUIRED; level requirements for the protocols it
280   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
281   level and all the &SHOULD; level requirements for its protocols is said
282   to be "unconditionally compliant"; one that satisfies all the &MUST;
283   level requirements but not all the &SHOULD; level requirements for its
284   protocols is said to be "conditionally compliant."
285</t>
286</section>
287
288<section title="Terminology" anchor="intro.terminology">
289<t>
290   This specification uses a number of terms to refer to the roles
291   played by participants in, and objects of, the HTTP communication.
292</t>
293<t>
294  <iref item="connection"/>
295  <x:dfn>connection</x:dfn>
296  <list>
297    <t>
298      A transport layer virtual circuit established between two programs
299      for the purpose of communication.
300    </t>
301  </list>
302</t>
303<t>
304  <iref item="message"/>
305  <x:dfn>message</x:dfn>
306  <list>
307    <t>
308      The basic unit of HTTP communication, consisting of a structured
309      sequence of octets matching the syntax defined in <xref target="http.message"/> and
310      transmitted via the connection.
311    </t>
312  </list>
313</t>
314<t>
315  <iref item="request"/>
316  <x:dfn>request</x:dfn>
317  <list>
318    <t>
319      An HTTP request message, as defined in <xref target="request"/>.
320    </t>
321  </list>
322</t>
323<t>
324  <iref item="response"/>
325  <x:dfn>response</x:dfn>
326  <list>
327    <t>
328      An HTTP response message, as defined in <xref target="response"/>.
329    </t>
330  </list>
331</t>
332<t>
333  <iref item="resource"/>
334  <x:dfn>resource</x:dfn>
335  <list>
336    <t>
337      A network data object or service that can be identified by a URI,
338      as defined in <xref target="uri"/>. Resources may be available in multiple
339      representations (e.g. multiple languages, data formats, size, and
340      resolutions) or vary in other ways.
341    </t>
342  </list>
343</t>
344<t>
345  <iref item="entity"/>
346  <x:dfn>entity</x:dfn>
347  <list>
348    <t>
349      The information transferred as the payload of a request or
350      response. An entity consists of metainformation in the form of
351      entity-header fields and content in the form of an entity-body, as
352      described in &entity;.
353    </t>
354  </list>
355</t>
356<t>
357  <iref item="representation"/>
358  <x:dfn>representation</x:dfn>
359  <list>
360    <t>
361      An entity included with a response that is subject to content
362      negotiation, as described in &content.negotiation;. There may exist multiple
363      representations associated with a particular response status.
364    </t>
365  </list>
366</t>
367<t>
368  <iref item="content negotiation"/>
369  <x:dfn>content negotiation</x:dfn>
370  <list>
371    <t>
372      The mechanism for selecting the appropriate representation when
373      servicing a request, as described in &content.negotiation;. The
374      representation of entities in any response can be negotiated
375      (including error responses).
376    </t>
377  </list>
378</t>
379<t>
380  <iref item="variant"/>
381  <x:dfn>variant</x:dfn>
382  <list>
383    <t>
384      A resource may have one, or more than one, representation(s)
385      associated with it at any given instant. Each of these
386      representations is termed a `variant'.  Use of the term `variant'
387      does not necessarily imply that the resource is subject to content
388      negotiation.
389    </t>
390  </list>
391</t>
392<t>
393  <iref item="client"/>
394  <x:dfn>client</x:dfn>
395  <list>
396    <t>
397      A program that establishes connections for the purpose of sending
398      requests.
399    </t>
400  </list>
401</t>
402<t>
403  <iref item="user agent"/>
404  <x:dfn>user agent</x:dfn>
405  <list>
406    <t>
407      The client which initiates a request. These are often browsers,
408      editors, spiders (web-traversing robots), or other end user tools.
409    </t>
410  </list>
411</t>
412<t>
413  <iref item="server"/>
414  <x:dfn>server</x:dfn>
415  <list>
416    <t>
417      An application program that accepts connections in order to
418      service requests by sending back responses. Any given program may
419      be capable of being both a client and a server; our use of these
420      terms refers only to the role being performed by the program for a
421      particular connection, rather than to the program's capabilities
422      in general. Likewise, any server may act as an origin server,
423      proxy, gateway, or tunnel, switching behavior based on the nature
424      of each request.
425    </t>
426  </list>
427</t>
428<t>
429  <iref item="origin server"/>
430  <x:dfn>origin server</x:dfn>
431  <list>
432    <t>
433      The server on which a given resource resides or is to be created.
434    </t>
435  </list>
436</t>
437<t>
438  <iref item="proxy"/>
439  <x:dfn>proxy</x:dfn>
440  <list>
441    <t>
442      An intermediary program which acts as both a server and a client
443      for the purpose of making requests on behalf of other clients.
444      Requests are serviced internally or by passing them on, with
445      possible translation, to other servers. A proxy &MUST; implement
446      both the client and server requirements of this specification. A
447      "transparent proxy" is a proxy that does not modify the request or
448      response beyond what is required for proxy authentication and
449      identification. A "non-transparent proxy" is a proxy that modifies
450      the request or response in order to provide some added service to
451      the user agent, such as group annotation services, media type
452      transformation, protocol reduction, or anonymity filtering. Except
453      where either transparent or non-transparent behavior is explicitly
454      stated, the HTTP proxy requirements apply to both types of
455      proxies.
456    </t>
457  </list>
458</t>
459<t>
460  <iref item="gateway"/>
461  <x:dfn>gateway</x:dfn>
462  <list>
463    <t>
464      A server which acts as an intermediary for some other server.
465      Unlike a proxy, a gateway receives requests as if it were the
466      origin server for the requested resource; the requesting client
467      may not be aware that it is communicating with a gateway.
468    </t>
469  </list>
470</t>
471<t>
472  <iref item="tunnel"/>
473  <x:dfn>tunnel</x:dfn>
474  <list>
475    <t>
476      An intermediary program which is acting as a blind relay between
477      two connections. Once active, a tunnel is not considered a party
478      to the HTTP communication, though the tunnel may have been
479      initiated by an HTTP request. The tunnel ceases to exist when both
480      ends of the relayed connections are closed.
481    </t>
482  </list>
483</t>
484<t>
485  <iref item="cache"/>
486  <x:dfn>cache</x:dfn>
487  <list>
488    <t>
489      A program's local store of response messages and the subsystem
490      that controls its message storage, retrieval, and deletion. A
491      cache stores cacheable responses in order to reduce the response
492      time and network bandwidth consumption on future, equivalent
493      requests. Any client or server may include a cache, though a cache
494      cannot be used by a server that is acting as a tunnel.
495    </t>
496  </list>
497</t>
498<t>
499  <iref item="cacheable"/>
500  <x:dfn>cacheable</x:dfn>
501  <list>
502    <t>
503      A response is cacheable if a cache is allowed to store a copy of
504      the response message for use in answering subsequent requests. The
505      rules for determining the cacheability of HTTP responses are
506      defined in &caching;. Even if a resource is cacheable, there may
507      be additional constraints on whether a cache can use the cached
508      copy for a particular request.
509    </t>
510  </list>
511</t>
512<t>
513  <iref item="upstream"/>
514  <iref item="downstream"/>
515  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
516  <list>
517    <t>
518      Upstream and downstream describe the flow of a message: all
519      messages flow from upstream to downstream.
520    </t>
521  </list>
522</t>
523<t>
524  <iref item="inbound"/>
525  <iref item="outbound"/>
526  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
527  <list>
528    <t>
529      Inbound and outbound refer to the request and response paths for
530      messages: "inbound" means "traveling toward the origin server",
531      and "outbound" means "traveling toward the user agent"
532    </t>
533  </list>
534</t>
535</section>
536
537<section title="Overall Operation" anchor="intro.overall.operation">
538<t>
539   The HTTP protocol is a request/response protocol. A client sends a
540   request to the server in the form of a request method, URI, and
541   protocol version, followed by a MIME-like message containing request
542   modifiers, client information, and possible body content over a
543   connection with a server. The server responds with a status line,
544   including the message's protocol version and a success or error code,
545   followed by a MIME-like message containing server information, entity
546   metainformation, and possible entity-body content. The relationship
547   between HTTP and MIME is described in &diff2045entity;.
548</t>
549<t>
550   Most HTTP communication is initiated by a user agent and consists of
551   a request to be applied to a resource on some origin server. In the
552   simplest case, this may be accomplished via a single connection (v)
553   between the user agent (UA) and the origin server (O).
554</t>
555<figure><artwork type="drawing">
556       request chain ------------------------&gt;
557    UA -------------------v------------------- O
558       &lt;----------------------- response chain
559</artwork></figure>
560<t>
561   A more complicated situation occurs when one or more intermediaries
562   are present in the request/response chain. There are three common
563   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
564   forwarding agent, receiving requests for a URI in its absolute form,
565   rewriting all or part of the message, and forwarding the reformatted
566   request toward the server identified by the URI. A gateway is a
567   receiving agent, acting as a layer above some other server(s) and, if
568   necessary, translating the requests to the underlying server's
569   protocol. A tunnel acts as a relay point between two connections
570   without changing the messages; tunnels are used when the
571   communication needs to pass through an intermediary (such as a
572   firewall) even when the intermediary cannot understand the contents
573   of the messages.
574</t>
575<figure><artwork type="drawing">
576       request chain --------------------------------------&gt;
577    UA -----v----- A -----v----- B -----v----- C -----v----- O
578       &lt;------------------------------------- response chain
579</artwork></figure>
580<t>
581   The figure above shows three intermediaries (A, B, and C) between the
582   user agent and origin server. A request or response message that
583   travels the whole chain will pass through four separate connections.
584   This distinction is important because some HTTP communication options
585   may apply only to the connection with the nearest, non-tunnel
586   neighbor, only to the end-points of the chain, or to all connections
587   along the chain. Although the diagram is linear, each participant may
588   be engaged in multiple, simultaneous communications. For example, B
589   may be receiving requests from many clients other than A, and/or
590   forwarding requests to servers other than C, at the same time that it
591   is handling A's request.
592</t>
593<t>
594   Any party to the communication which is not acting as a tunnel may
595   employ an internal cache for handling requests. The effect of a cache
596   is that the request/response chain is shortened if one of the
597   participants along the chain has a cached response applicable to that
598   request. The following illustrates the resulting chain if B has a
599   cached copy of an earlier response from O (via C) for a request which
600   has not been cached by UA or A.
601</t>
602<figure><artwork type="drawing">
603          request chain ----------&gt;
604       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
605          &lt;--------- response chain
606</artwork></figure>
607<t>
608   Not all responses are usefully cacheable, and some requests may
609   contain modifiers which place special requirements on cache behavior.
610   HTTP requirements for cache behavior and cacheable responses are
611   defined in &caching;.
612</t>
613<t>
614   In fact, there are a wide variety of architectures and configurations
615   of caches and proxies currently being experimented with or deployed
616   across the World Wide Web. These systems include national hierarchies
617   of proxy caches to save transoceanic bandwidth, systems that
618   broadcast or multicast cache entries, organizations that distribute
619   subsets of cached data via CD-ROM, and so on. HTTP systems are used
620   in corporate intranets over high-bandwidth links, and for access via
621   PDAs with low-power radio links and intermittent connectivity. The
622   goal of HTTP/1.1 is to support the wide diversity of configurations
623   already deployed while introducing protocol constructs that meet the
624   needs of those who build web applications that require high
625   reliability and, failing that, at least reliable indications of
626   failure.
627</t>
628<t>
629   HTTP communication usually takes place over TCP/IP connections. The
630   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
631   not preclude HTTP from being implemented on top of any other protocol
632   on the Internet, or on other networks. HTTP only presumes a reliable
633   transport; any protocol that provides such guarantees can be used;
634   the mapping of the HTTP/1.1 request and response structures onto the
635   transport data units of the protocol in question is outside the scope
636   of this specification.
637</t>
638<t>
639   In HTTP/1.0, most implementations used a new connection for each
640   request/response exchange. In HTTP/1.1, a connection may be used for
641   one or more request/response exchanges, although connections may be
642   closed for a variety of reasons (see <xref target="persistent.connections"/>).
643</t>
644</section>
645</section>
646
647<section title="Notational Conventions and Generic Grammar" anchor="notation">
648
649<section title="Augmented BNF" anchor="notation.abnf">
650<t>
651   All of the mechanisms specified in this document are described in
652   both prose and an augmented Backus-Naur Form (BNF) similar to that
653   used by <xref target="RFC822"/>. Implementors will need to be familiar with the
654   notation in order to understand this specification. The augmented BNF
655   includes the following constructs:
656</t>
657<t>
658   name = definition
659  <list>
660    <t>
661      The name of a rule is simply the name itself (without any
662      enclosing "&lt;" and "&gt;") and is separated from its definition by the
663      equal "=" character. White space is only significant in that
664      indentation of continuation lines is used to indicate a rule
665      definition that spans more than one line. Certain basic rules are
666      in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
667      brackets are used within definitions whenever their presence will
668      facilitate discerning the use of rule names.
669    </t>
670  </list>
671</t>
672<t>
673   "literal"
674  <list>
675    <t>
676      Quotation marks surround literal text. Unless stated otherwise,
677      the text is case-insensitive.
678    </t>
679  </list>
680</t>
681<t>
682   rule1 | rule2
683  <list>
684    <t>
685      Elements separated by a bar ("|") are alternatives, e.g., "yes |
686      no" will accept yes or no.
687    </t>
688  </list>
689</t>
690<t>
691   (rule1 rule2)
692  <list>
693    <t>
694      Elements enclosed in parentheses are treated as a single element.
695      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
696      foo elem" and "elem bar elem".
697    </t>
698  </list>
699</t>
700<t>
701   *rule
702  <list>
703    <t>
704      The character "*" preceding an element indicates repetition. The
705      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
706      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
707      that "*(element)" allows any number, including zero; "1*element"
708      requires at least one; and "1*2element" allows one or two.
709    </t>
710  </list>
711</t>
712<t>
713   [rule]
714  <list>
715    <t>
716      Square brackets enclose optional elements; "[foo bar]" is
717      equivalent to "*1(foo bar)".
718    </t>
719  </list>
720</t>
721<t>
722   N rule
723  <list>
724    <t>
725      Specific repetition: "&lt;n&gt;(element)" is equivalent to
726      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
727      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
728      alphabetic characters.
729    </t>
730  </list>
731</t>
732<t>
733   #rule
734  <list>
735    <t>
736      A construct "#" is defined, similar to "*", for defining lists of
737      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
738      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
739      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
740      form of lists very easy; a rule such as
741    </t>
742    <t>
743         ( *LWS element *( *LWS "," *LWS element ))
744    </t>
745    <t>
746      can be shown as
747    </t>
748    <t>
749         1#element
750    </t>
751    <t>
752      Wherever this construct is used, null elements are allowed, but do
753      not contribute to the count of elements present. That is,
754      "(element), , (element) " is permitted, but counts as only two
755      elements. Therefore, where at least one element is required, at
756      least one non-null element &MUST; be present. Default values are 0
757      and infinity so that "#element" allows any number, including zero;
758      "1#element" requires at least one; and "1#2element" allows one or
759      two.
760    </t>
761  </list>
762</t>
763<t>
764   ; comment
765  <list>
766    <t>
767      A semi-colon, set off some distance to the right of rule text,
768      starts a comment that continues to the end of line. This is a
769      simple way of including useful notes in parallel with the
770      specifications.
771    </t>
772  </list>
773</t>
774<t>
775   implied *LWS
776  <list>
777    <t>
778      The grammar described by this specification is word-based. Except
779      where noted otherwise, linear white space (LWS) can be included
780      between any two adjacent words (token or quoted-string), and
781      between adjacent words and separators, without changing the
782      interpretation of a field. At least one delimiter (LWS and/or
783      separators) &MUST; exist between any two tokens (for the definition
784      of "token" below), since they would otherwise be interpreted as a
785      single token.
786    </t>
787  </list>
788</t>
789</section>
790
791<section title="Basic Rules" anchor="basic.rules">
792<x:anchor-alias value="OCTET"/>
793<x:anchor-alias value="CHAR"/>
794<x:anchor-alias value="UPALPHA"/>
795<x:anchor-alias value="LOALPHA"/>
796<x:anchor-alias value="ALPHA"/>
797<x:anchor-alias value="DIGIT"/>
798<x:anchor-alias value="CTL"/>
799<x:anchor-alias value="CR"/>
800<x:anchor-alias value="LF"/>
801<x:anchor-alias value="SP"/>
802<x:anchor-alias value="HT"/>
803<x:anchor-alias value="CRLF"/>
804<x:anchor-alias value="LWS"/>
805<x:anchor-alias value="TEXT"/>
806<x:anchor-alias value="HEX"/>
807<x:anchor-alias value="token"/>
808<x:anchor-alias value="separators"/>
809<x:anchor-alias value="comment"/>
810<x:anchor-alias value="ctext"/>
811<x:anchor-alias value="quoted-string"/>
812<x:anchor-alias value="qdtext"/>
813<x:anchor-alias value="quoted-pair"/>
814<t>
815   The following rules are used throughout this specification to
816   describe basic parsing constructs. The US-ASCII coded character set
817   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
818</t>
819<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><iref primary="true" item="Grammar" subitem="UPALPHA"/><iref primary="true" item="Grammar" subitem="LOALPHA"/><iref primary="true" item="Grammar" subitem="ALPHA"/><iref primary="true" item="Grammar" subitem="DIGIT"/><iref primary="true" item="Grammar" subitem="CTL"/><iref primary="true" item="Grammar" subitem="CR"/><iref primary="true" item="Grammar" subitem="LF"/><iref primary="true" item="Grammar" subitem="SP"/><iref primary="true" item="Grammar" subitem="HT"/>
820    OCTET          = &lt;any 8-bit sequence of data&gt;
821    CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
822    UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
823    LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
824    ALPHA          = UPALPHA | LOALPHA
825    DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
826    CTL            = &lt;any US-ASCII control character
827                     (octets 0 - 31) and DEL (127)&gt;
828    CR             = &lt;US-ASCII CR, carriage return (13)&gt;
829    LF             = &lt;US-ASCII LF, linefeed (10)&gt;
830    SP             = &lt;US-ASCII SP, space (32)&gt;
831    HT             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
832    &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
833</artwork></figure>
834<t>
835   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
836   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
837   tolerant applications). The end-of-line marker within an entity-body
838   is defined by its associated media type, as described in &media-types;.
839</t>
840<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
841    CRLF           = CR LF
842</artwork></figure>
843<t>
844   HTTP/1.1 header field values can be folded onto multiple lines if the
845   continuation line begins with a space or horizontal tab. All linear
846   white space, including folding, has the same semantics as SP. A
847   recipient &MAY; replace any linear white space with a single SP before
848   interpreting the field value or forwarding the message downstream.
849</t>
850<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
851    LWS            = [CRLF] 1*( SP | HT )
852</artwork></figure>
853<t>
854   The TEXT rule is only used for descriptive field contents and values
855   that are not intended to be interpreted by the message parser. Words
856   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
857   <xref target="ISO-8859"/> only when encoded according to the rules of
858   <xref target="RFC2047"/>.
859</t>
860<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
861    TEXT           = &lt;any OCTET except CTLs,
862                     but including LWS&gt;
863</artwork></figure>
864<t>
865   A CRLF is allowed in the definition of TEXT only as part of a header
866   field continuation. It is expected that the folding LWS will be
867   replaced with a single SP before interpretation of the TEXT value.
868</t>
869<t>
870   Hexadecimal numeric characters are used in several protocol elements.
871</t>
872<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
873    HEX            = "A" | "B" | "C" | "D" | "E" | "F"
874                   | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
875</artwork></figure>
876<t>
877   Many HTTP/1.1 header field values consist of words separated by LWS
878   or special characters. These special characters &MUST; be in a quoted
879   string to be used within a parameter value (as defined in
880   <xref target="transfer.codings"/>).
881</t>
882<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
883    token          = 1*&lt;any CHAR except CTLs or separators&gt;
884    separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
885                   | "," | ";" | ":" | "\" | &lt;"&gt;
886                   | "/" | "[" | "]" | "?" | "="
887                   | "{" | "}" | SP | HT
888</artwork></figure>
889<t>
890   Comments can be included in some HTTP header fields by surrounding
891   the comment text with parentheses. Comments are only allowed in
892   fields containing "comment" as part of their field value definition.
893   In all other fields, parentheses are considered part of the field
894   value.
895</t>
896<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
897    comment        = "(" *( ctext | quoted-pair | comment ) ")"
898    ctext          = &lt;any TEXT excluding "(" and ")"&gt;
899</artwork></figure>
900<t>
901   A string of text is parsed as a single word if it is quoted using
902   double-quote marks.
903</t>
904<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
905    quoted-string  = ( &lt;"&gt; *(qdtext | quoted-pair ) &lt;"&gt; )
906    qdtext         = &lt;any TEXT except &lt;"&gt;&gt;
907</artwork></figure>
908<t>
909   The backslash character ("\") &MAY; be used as a single-character
910   quoting mechanism only within quoted-string and comment constructs.
911</t>
912<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
913    quoted-pair    = "\" CHAR
914</artwork></figure>
915</section>
916</section>
917
918<section title="Protocol Parameters" anchor="protocol.parameters">
919
920<section title="HTTP Version" anchor="http.version">
921<t>
922   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
923   of the protocol. The protocol versioning policy is intended to allow
924   the sender to indicate the format of a message and its capacity for
925   understanding further HTTP communication, rather than the features
926   obtained via that communication. No change is made to the version
927   number for the addition of message components which do not affect
928   communication behavior or which only add to extensible field values.
929   The &lt;minor&gt; number is incremented when the changes made to the
930   protocol add features which do not change the general message parsing
931   algorithm, but which may add to the message semantics and imply
932   additional capabilities of the sender. The &lt;major&gt; number is
933   incremented when the format of a message within the protocol is
934   changed. See <xref target="RFC2145"/> for a fuller explanation.
935</t>
936<t>
937   The version of an HTTP message is indicated by an HTTP-Version field
938   in the first line of the message. HTTP-Version is case-sensitive.
939</t>
940<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
941       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
942</artwork></figure>
943<t>
944   Note that the major and minor numbers &MUST; be treated as separate
945   integers and that each &MAY; be incremented higher than a single digit.
946   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
947   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
948   &MUST-NOT; be sent.
949</t>
950<t>
951   An application that sends a request or response message that includes
952   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
953   with this specification. Applications that are at least conditionally
954   compliant with this specification &SHOULD; use an HTTP-Version of
955   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
956   not compatible with HTTP/1.0. For more details on when to send
957   specific HTTP-Version values, see <xref target="RFC2145"/>.
958</t>
959<t>
960   The HTTP version of an application is the highest HTTP version for
961   which the application is at least conditionally compliant.
962</t>
963<t>
964   Proxy and gateway applications need to be careful when forwarding
965   messages in protocol versions different from that of the application.
966   Since the protocol version indicates the protocol capability of the
967   sender, a proxy/gateway &MUST-NOT; send a message with a version
968   indicator which is greater than its actual version. If a higher
969   version request is received, the proxy/gateway &MUST; either downgrade
970   the request version, or respond with an error, or switch to tunnel
971   behavior.
972</t>
973<t>
974   Due to interoperability problems with HTTP/1.0 proxies discovered
975   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
976   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
977   they support. The proxy/gateway's response to that request &MUST; be in
978   the same major version as the request.
979</t>
980<t>
981  <list>
982    <t>
983      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
984      of header fields required or forbidden by the versions involved.
985    </t>
986  </list>
987</t>
988</section>
989
990<section title="Uniform Resource Identifiers" anchor="uri">
991<t>
992   URIs have been known by many names: WWW addresses, Universal Document
993   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
994   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
995   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
996   simply formatted strings which identify--via name, location, or any
997   other characteristic--a resource.
998</t>
999
1000<section title="General Syntax" anchor="general.syntax">
1001<t>
1002   URIs in HTTP can be represented in absolute form or relative to some
1003   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1004   forms are differentiated by the fact that absolute URIs always begin
1005   with a scheme name followed by a colon. For definitive information on
1006   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1007   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1008   and <xref target="RFC1808"/>). This specification adopts the
1009   definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
1010   "host","abs_path", "rel_path", and "authority" from that
1011   specification.
1012</t>
1013<t>
1014   The HTTP protocol does not place any a priori limit on the length of
1015   a URI. Servers &MUST; be able to handle the URI of any resource they
1016   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1017   provide GET-based forms that could generate such URIs. A server
1018   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1019   than the server can handle (see &status-414;).
1020</t>
1021<t>
1022  <list>
1023    <t>
1024      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1025      above 255 bytes, because some older client or proxy
1026      implementations might not properly support these lengths.
1027    </t>
1028  </list>
1029</t>
1030</section>
1031
1032<section title="http URL" anchor="http.url">
1033<t>
1034   The "http" scheme is used to locate network resources via the HTTP
1035   protocol. This section defines the scheme-specific syntax and
1036   semantics for http URLs.
1037</t>
1038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http_URL"/>
1039http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
1040</artwork></figure>
1041<t>
1042   If the port is empty or not given, port 80 is assumed. The semantics
1043   are that the identified resource is located at the server listening
1044   for TCP connections on that port of that host, and the Request-URI
1045   for the resource is abs_path (<xref target="request-uri"/>). The use of IP addresses
1046   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1047   the abs_path is not present in the URL, it &MUST; be given as "/" when
1048   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1049
1050   receives a host name which is not a fully qualified domain name, it
1051   &MAY; add its domain to the host name it received. If a proxy receives
1052   a fully qualified domain name, the proxy &MUST-NOT; change the host
1053   name.
1054</t>
1055</section>
1056
1057<section title="URI Comparison" anchor="uri.comparison">
1058<t>
1059   When comparing two URIs to decide if they match or not, a client
1060   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1061   URIs, with these exceptions:
1062  <list style="symbols">
1063    <t>A port that is empty or not given is equivalent to the default
1064        port for that URI-reference;</t>
1065    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1066    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1067    <t>An empty abs_path is equivalent to an abs_path of "/".</t>
1068  </list>
1069</t>
1070<t>
1071   Characters other than those in the "reserved" set (see
1072   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1073</t>
1074<t>
1075   For example, the following three URIs are equivalent:
1076</t>
1077<figure><artwork type="example">
1078   http://example.com:80/~smith/home.html
1079   http://EXAMPLE.com/%7Esmith/home.html
1080   http://EXAMPLE.com:/%7esmith/home.html
1081</artwork></figure>
1082</section>
1083</section>
1084
1085<section title="Date/Time Formats" anchor="date.time.formats">
1086<section title="Full Date" anchor="full.date">
1087<t>
1088   HTTP applications have historically allowed three different formats
1089   for the representation of date/time stamps:
1090</t>
1091<figure><artwork type="example">
1092   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1093   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1094   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1095</artwork></figure>
1096<t>
1097   The first format is preferred as an Internet standard and represents
1098   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1099   <xref target="RFC822"/>). The other formats are described here only for
1100   compatibility with obsolete implementations.
1101   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1102   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1103   only generate the RFC 1123 format for representing HTTP-date values
1104   in header fields. See <xref target="tolerant.applications"/> for further information.
1105</t>
1106<t><list><t>
1107      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1108      accepting date values that may have been sent by non-HTTP
1109      applications, as is sometimes the case when retrieving or posting
1110      messages via proxies/gateways to SMTP or NNTP.
1111</t></list></t>
1112<t>
1113   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1114   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1115   equal to UTC (Coordinated Universal Time). This is indicated in the
1116   first two formats by the inclusion of "GMT" as the three-letter
1117   abbreviation for time zone, and &MUST; be assumed when reading the
1118   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1119   additional LWS beyond that specifically included as SP in the
1120   grammar.
1121</t>
1122<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-date"/><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="rfc850-date"/><iref primary="true" item="Grammar" subitem="asctime-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/><iref primary="true" item="Grammar" subitem="time"/><iref primary="true" item="Grammar" subitem="wkday"/><iref primary="true" item="Grammar" subitem="weekday"/><iref primary="true" item="Grammar" subitem="month"/>
1123    HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1124    rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1125    rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1126    asctime-date = wkday SP date3 SP time SP 4DIGIT
1127    date1        = 2DIGIT SP month SP 4DIGIT
1128                   ; day month year (e.g., 02 Jun 1982)
1129    date2        = 2DIGIT "-" month "-" 2DIGIT
1130                   ; day-month-year (e.g., 02-Jun-82)
1131    date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1132                   ; month day (e.g., Jun  2)
1133    time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1134                   ; 00:00:00 - 23:59:59
1135    wkday        = "Mon" | "Tue" | "Wed"
1136                 | "Thu" | "Fri" | "Sat" | "Sun"
1137    weekday      = "Monday" | "Tuesday" | "Wednesday"
1138                 | "Thursday" | "Friday" | "Saturday" | "Sunday"
1139    month        = "Jan" | "Feb" | "Mar" | "Apr"
1140                 | "May" | "Jun" | "Jul" | "Aug"
1141                 | "Sep" | "Oct" | "Nov" | "Dec"
1142</artwork></figure>
1143<t>
1144      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1145      to their usage within the protocol stream. Clients and servers are
1146      not required to use these formats for user presentation, request
1147      logging, etc.
1148</t>
1149</section>
1150</section>
1151
1152<section title="Transfer Codings" anchor="transfer.codings">
1153<t>
1154   Transfer-coding values are used to indicate an encoding
1155   transformation that has been, can be, or may need to be applied to an
1156   entity-body in order to ensure "safe transport" through the network.
1157   This differs from a content coding in that the transfer-coding is a
1158   property of the message, not of the original entity.
1159</t>
1160<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1161    transfer-coding         = "chunked" | transfer-extension
1162    transfer-extension      = token *( ";" parameter )
1163</artwork></figure>
1164<t>
1165   Parameters are in  the form of attribute/value pairs.
1166</t>
1167<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/>
1168    parameter               = attribute "=" value
1169    attribute               = token
1170    value                   = token | quoted-string
1171</artwork></figure>
1172<t>
1173   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1174   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1175   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1176</t>
1177<t>
1178   Whenever a transfer-coding is applied to a message-body, the set of
1179   transfer-codings &MUST; include "chunked", unless the message is
1180   terminated by closing the connection. When the "chunked" transfer-coding
1181   is used, it &MUST; be the last transfer-coding applied to the
1182   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1183   than once to a message-body. These rules allow the recipient to
1184   determine the transfer-length of the message (<xref target="message.length"/>).
1185</t>
1186<t>
1187   Transfer-codings are analogous to the Content-Transfer-Encoding
1188   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1189   binary data over a 7-bit transport service. However, safe transport
1190   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1191   the only unsafe characteristic of message-bodies is the difficulty in
1192   determining the exact body length (<xref target="message.length"/>), or the desire to
1193   encrypt data over a shared transport.
1194</t>
1195<t>
1196   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1197   transfer-coding value tokens. Initially, the registry contains the
1198   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1199   "gzip", "compress", and "deflate" (&content-codings;).
1200</t>
1201<t>
1202   New transfer-coding value tokens &SHOULD; be registered in the same way
1203   as new content-coding value tokens (&content-codings;).
1204</t>
1205<t>
1206   A server which receives an entity-body with a transfer-coding it does
1207   not understand &SHOULD; return 501 (Unimplemented), and close the
1208   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1209   client.
1210</t>
1211
1212<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1213<t>
1214   The chunked encoding modifies the body of a message in order to
1215   transfer it as a series of chunks, each with its own size indicator,
1216   followed by an &OPTIONAL; trailer containing entity-header fields. This
1217   allows dynamically produced content to be transferred along with the
1218   information necessary for the recipient to verify that it has
1219   received the full message.
1220</t>
1221<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Chunked-Body"/><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="true" item="Grammar" subitem="chunk-extension"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer"/>
1222    Chunked-Body   = *chunk
1223                     last-chunk
1224                     trailer
1225                     CRLF
1226
1227    chunk          = chunk-size [ chunk-extension ] CRLF
1228                     chunk-data CRLF
1229    chunk-size     = 1*HEX
1230    last-chunk     = 1*("0") [ chunk-extension ] CRLF
1231
1232    chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1233    chunk-ext-name = token
1234    chunk-ext-val  = token | quoted-string
1235    chunk-data     = chunk-size(OCTET)
1236    trailer        = *(entity-header CRLF)
1237</artwork></figure>
1238<t>
1239   The chunk-size field is a string of hex digits indicating the size of
1240   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1241   zero, followed by the trailer, which is terminated by an empty line.
1242</t>
1243<t>
1244   The trailer allows the sender to include additional HTTP header
1245   fields at the end of the message. The Trailer header field can be
1246   used to indicate which header fields are included in a trailer (see
1247   <xref target="header.trailer"/>).
1248</t>
1249<t>
1250   A server using chunked transfer-coding in a response &MUST-NOT; use the
1251   trailer for any header fields unless at least one of the following is
1252   true:
1253  <list style="numbers">
1254    <t>the request included a TE header field that indicates "trailers" is
1255     acceptable in the transfer-coding of the  response, as described in
1256     <xref target="header.te"/>; or,</t>
1257
1258    <t>the server is the origin server for the response, the trailer
1259     fields consist entirely of optional metadata, and the recipient
1260     could use the message (in a manner acceptable to the origin server)
1261     without receiving this metadata.  In other words, the origin server
1262     is willing to accept the possibility that the trailer fields might
1263     be silently discarded along the path to the client.</t>
1264  </list>
1265</t>
1266<t>
1267   This requirement prevents an interoperability failure when the
1268   message is being received by an HTTP/1.1 (or later) proxy and
1269   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1270   compliance with the protocol would have necessitated a possibly
1271   infinite buffer on the proxy.
1272</t>
1273<t>
1274   A process for decoding the "chunked" transfer-coding
1275   can be represented in pseudo-code as:
1276</t>
1277<figure><artwork type="code">
1278    length := 0
1279    read chunk-size, chunk-extension (if any) and CRLF
1280    while (chunk-size &gt; 0) {
1281       read chunk-data and CRLF
1282       append chunk-data to entity-body
1283       length := length + chunk-size
1284       read chunk-size and CRLF
1285    }
1286    read entity-header
1287    while (entity-header not empty) {
1288       append entity-header to existing header fields
1289       read entity-header
1290    }
1291    Content-Length := length
1292    Remove "chunked" from Transfer-Encoding
1293</artwork></figure>
1294<t>
1295   All HTTP/1.1 applications &MUST; be able to receive and decode the
1296   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1297   they do not understand.
1298</t>
1299</section>
1300</section>
1301
1302</section>
1303
1304<section title="HTTP Message" anchor="http.message">
1305
1306<section title="Message Types" anchor="message.types">
1307<t>
1308   HTTP messages consist of requests from client to server and responses
1309   from server to client.
1310</t>
1311<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1312    HTTP-message   = Request | Response     ; HTTP/1.1 messages
1313</artwork></figure>
1314<t>
1315   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1316   message format of <xref target="RFC822"/> for transferring entities (the payload
1317   of the message). Both types of message consist of a start-line, zero
1318   or more header fields (also known as "headers"), an empty line (i.e.,
1319   a line with nothing preceding the CRLF) indicating the end of the
1320   header fields, and possibly a message-body.
1321</t>
1322<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1323     generic-message = start-line
1324                       *(message-header CRLF)
1325                       CRLF
1326                       [ message-body ]
1327     start-line      = Request-Line | Status-Line
1328</artwork></figure>
1329<t>
1330   In the interest of robustness, servers &SHOULD; ignore any empty
1331   line(s) received where a Request-Line is expected. In other words, if
1332   the server is reading the protocol stream at the beginning of a
1333   message and receives a CRLF first, it should ignore the CRLF.
1334</t>
1335<t>
1336   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1337   after a POST request. To restate what is explicitly forbidden by the
1338   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1339   extra CRLF.
1340</t>
1341</section>
1342
1343<section title="Message Headers" anchor="message.headers">
1344<t>
1345   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1346   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1347   entity-header (&entity-header-fields;) fields, follow the same generic format as
1348   that given in <xref target="RFC822" x:fmt="of" x:sec="3.1"/>. Each header field consists
1349   of a name followed by a colon (":") and the field value. Field names
1350   are case-insensitive. The field value &MAY; be preceded by any amount
1351   of LWS, though a single SP is preferred. Header fields can be
1352   extended over multiple lines by preceding each extra line with at
1353   least one SP or HT. Applications ought to follow "common form", where
1354   one is known or indicated, when generating HTTP constructs, since
1355   there might exist some implementations that fail to accept anything
1356   beyond the common forms.
1357</t>
1358<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-header"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/>
1359    message-header = field-name ":" [ field-value ]
1360    field-name     = token
1361    field-value    = *( field-content | LWS )
1362    field-content  = &lt;the OCTETs making up the field-value
1363                     and consisting of either *TEXT or combinations
1364                     of token, separators, and quoted-string&gt;
1365</artwork></figure>
1366<t>
1367   The field-content does not include any leading or trailing LWS:
1368   linear white space occurring before the first non-whitespace
1369   character of the field-value or after the last non-whitespace
1370   character of the field-value. Such leading or trailing LWS &MAY; be
1371   removed without changing the semantics of the field value. Any LWS
1372   that occurs between field-content &MAY; be replaced with a single SP
1373   before interpreting the field value or forwarding the message
1374   downstream.
1375</t>
1376<t>
1377   The order in which header fields with differing field names are
1378   received is not significant. However, it is "good practice" to send
1379   general-header fields first, followed by request-header or response-header
1380   fields, and ending with the entity-header fields.
1381</t>
1382<t>
1383   Multiple message-header fields with the same field-name &MAY; be
1384   present in a message if and only if the entire field-value for that
1385   header field is defined as a comma-separated list [i.e., #(values)].
1386   It &MUST; be possible to combine the multiple header fields into one
1387   "field-name: field-value" pair, without changing the semantics of the
1388   message, by appending each subsequent field-value to the first, each
1389   separated by a comma. The order in which header fields with the same
1390   field-name are received is therefore significant to the
1391   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1392   change the order of these field values when a message is forwarded.
1393</t>
1394</section>
1395
1396<section title="Message Body" anchor="message.body">
1397<t>
1398   The message-body (if any) of an HTTP message is used to carry the
1399   entity-body associated with the request or response. The message-body
1400   differs from the entity-body only when a transfer-coding has been
1401   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1402</t>
1403<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1404    message-body = entity-body
1405                 | &lt;entity-body encoded as per Transfer-Encoding&gt;
1406</artwork></figure>
1407<t>
1408   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1409   applied by an application to ensure safe and proper transfer of the
1410   message. Transfer-Encoding is a property of the message, not of the
1411   entity, and thus &MAY; be added or removed by any application along the
1412   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1413   when certain transfer-codings may be used.)
1414</t>
1415<t>
1416   The rules for when a message-body is allowed in a message differ for
1417   requests and responses.
1418</t>
1419<t>
1420   The presence of a message-body in a request is signaled by the
1421   inclusion of a Content-Length or Transfer-Encoding header field in
1422   the request's message-headers. A message-body &MUST-NOT; be included in
1423   a request if the specification of the request method (&method;)
1424   does not allow sending an entity-body in requests. A server &SHOULD;
1425   read and forward a message-body on any request; if the request method
1426   does not include defined semantics for an entity-body, then the
1427   message-body &SHOULD; be ignored when handling the request.
1428</t>
1429<t>
1430   For response messages, whether or not a message-body is included with
1431   a message is dependent on both the request method and the response
1432   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1433   &MUST-NOT; include a message-body, even though the presence of entity-header
1434   fields might lead one to believe they do. All 1xx
1435   (informational), 204 (no content), and 304 (not modified) responses
1436   &MUST-NOT; include a message-body. All other responses do include a
1437   message-body, although it &MAY; be of zero length.
1438</t>
1439</section>
1440
1441<section title="Message Length" anchor="message.length">
1442<t>
1443   The transfer-length of a message is the length of the message-body as
1444   it appears in the message; that is, after any transfer-codings have
1445   been applied. When a message-body is included with a message, the
1446   transfer-length of that body is determined by one of the following
1447   (in order of precedence):
1448</t>
1449<t>
1450  <list style="numbers">
1451    <x:lt><t>
1452     Any response message which "&MUST-NOT;" include a message-body (such
1453     as the 1xx, 204, and 304 responses and any response to a HEAD
1454     request) is always terminated by the first empty line after the
1455     header fields, regardless of the entity-header fields present in
1456     the message.
1457    </t></x:lt>
1458    <x:lt><t>
1459     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1460     is present, then the transfer-length is
1461     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1462     unless the message is terminated by closing the connection.
1463    </t></x:lt>
1464    <x:lt><t>
1465     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1466     decimal value in OCTETs represents both the entity-length and the
1467     transfer-length. The Content-Length header field &MUST-NOT; be sent
1468     if these two lengths are different (i.e., if a Transfer-Encoding
1469     header field is present). If a message is received with both a
1470     Transfer-Encoding header field and a Content-Length header field,
1471     the latter &MUST; be ignored.
1472    </t></x:lt>
1473    <x:lt><t>
1474     If the message uses the media type "multipart/byteranges", and the
1475     transfer-length is not otherwise specified, then this self-delimiting
1476     media type defines the transfer-length. This media type
1477     &MUST-NOT; be used unless the sender knows that the recipient can parse
1478     it; the presence in a request of a Range header with multiple byte-range
1479     specifiers from a 1.1 client implies that the client can parse
1480     multipart/byteranges responses.
1481    <list style="empty"><t>
1482       A range header might be forwarded by a 1.0 proxy that does not
1483       understand multipart/byteranges; in this case the server &MUST;
1484       delimit the message using methods defined in items 1, 3 or 5 of
1485       this section.
1486    </t></list>
1487    </t></x:lt>
1488    <x:lt><t>
1489     By the server closing the connection. (Closing the connection
1490     cannot be used to indicate the end of a request body, since that
1491     would leave no possibility for the server to send back a response.)
1492    </t></x:lt>
1493  </list>
1494</t>
1495<t>
1496   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1497   containing a message-body &MUST; include a valid Content-Length header
1498   field unless the server is known to be HTTP/1.1 compliant. If a
1499   request contains a message-body and a Content-Length is not given,
1500   the server &SHOULD; respond with 400 (bad request) if it cannot
1501   determine the length of the message, or with 411 (length required) if
1502   it wishes to insist on receiving a valid Content-Length.
1503</t>
1504<t>
1505   All HTTP/1.1 applications that receive entities &MUST; accept the
1506   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1507   to be used for messages when the message length cannot be determined
1508   in advance.
1509</t>
1510<t>
1511   Messages &MUST-NOT; include both a Content-Length header field and a
1512   transfer-coding. If the message does include a
1513   transfer-coding, the Content-Length &MUST; be ignored.
1514</t>
1515<t>
1516   When a Content-Length is given in a message where a message-body is
1517   allowed, its field value &MUST; exactly match the number of OCTETs in
1518   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1519   invalid length is received and detected.
1520</t>
1521</section>
1522
1523<section title="General Header Fields" anchor="general.header.fields">
1524<t>
1525   There are a few header fields which have general applicability for
1526   both request and response messages, but which do not apply to the
1527   entity being transferred. These header fields apply only to the
1528   message being transmitted.
1529</t>
1530<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1531    general-header = Cache-Control            ; &header-cache-control;
1532                   | Connection               ; <xref target="header.connection"/>
1533                   | Date                     ; <xref target="header.date"/>
1534                   | Pragma                   ; &header-pragma;
1535                   | Trailer                  ; <xref target="header.trailer"/>
1536                   | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1537                   | Upgrade                  ; <xref target="header.upgrade"/>
1538                   | Via                      ; <xref target="header.via"/>
1539                   | Warning                  ; &header-warning;
1540</artwork></figure>
1541<t>
1542   General-header field names can be extended reliably only in
1543   combination with a change in the protocol version. However, new or
1544   experimental header fields may be given the semantics of general
1545   header fields if all parties in the communication recognize them to
1546   be general-header fields. Unrecognized header fields are treated as
1547   entity-header fields.
1548</t>
1549</section>
1550</section>
1551
1552<section title="Request" anchor="request">
1553<t>
1554   A request message from a client to a server includes, within the
1555   first line of that message, the method to be applied to the resource,
1556   the identifier of the resource, and the protocol version in use.
1557</t>
1558<!--                 Host                      ; should be moved here eventually -->
1559<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1560     Request       = Request-Line              ; <xref target="request-line"/>
1561                     *(( general-header        ; <xref target="general.header.fields"/>
1562                      | request-header         ; &request-header-fields;
1563                      | entity-header ) CRLF)  ; &entity-header-fields;
1564                     CRLF
1565                     [ message-body ]          ; <xref target="message.body"/>
1566</artwork></figure>
1567
1568<section title="Request-Line" anchor="request-line">
1569<t>
1570   The Request-Line begins with a method token, followed by the
1571   Request-URI and the protocol version, and ending with CRLF. The
1572   elements are separated by SP characters. No CR or LF is allowed
1573   except in the final CRLF sequence.
1574</t>
1575<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1576     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1577</artwork></figure>
1578
1579<section title="Method" anchor="method">
1580<t>
1581   The Method  token indicates the method to be performed on the
1582   resource identified by the Request-URI. The method is case-sensitive.
1583</t>
1584<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1585    Method         = token
1586</artwork></figure>
1587</section>
1588
1589<section title="Request-URI" anchor="request-uri">
1590<t>
1591   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1592   identifies the resource upon which to apply the request.
1593</t>
1594<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1595    Request-URI    = "*"
1596                   | absoluteURI
1597                   | ( abs_path [ "?" query ] )
1598                   | authority
1599</artwork></figure>
1600<t>
1601   The four options for Request-URI are dependent on the nature of the
1602   request. The asterisk "*" means that the request does not apply to a
1603   particular resource, but to the server itself, and is only allowed
1604   when the method used does not necessarily apply to a resource. One
1605   example would be
1606</t>
1607<figure><artwork type="example">
1608    OPTIONS * HTTP/1.1
1609</artwork></figure>
1610<t>
1611   The absoluteURI form is &REQUIRED; when the request is being made to a
1612   proxy. The proxy is requested to forward the request or service it
1613   from a valid cache, and return the response. Note that the proxy &MAY;
1614   forward the request on to another proxy or directly to the server
1615   specified by the absoluteURI. In order to avoid request loops, a
1616   proxy &MUST; be able to recognize all of its server names, including
1617   any aliases, local variations, and the numeric IP address. An example
1618   Request-Line would be:
1619</t>
1620<figure><artwork type="example">
1621    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1622</artwork></figure>
1623<t>
1624   To allow for transition to absoluteURIs in all requests in future
1625   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1626   form in requests, even though HTTP/1.1 clients will only generate
1627   them in requests to proxies.
1628</t>
1629<t>
1630   The authority form is only used by the CONNECT method (&CONNECT;).
1631</t>
1632<t>
1633   The most common form of Request-URI is that used to identify a
1634   resource on an origin server or gateway. In this case the absolute
1635   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, abs_path) as
1636   the Request-URI, and the network location of the URI (authority) &MUST;
1637   be transmitted in a Host header field. For example, a client wishing
1638   to retrieve the resource above directly from the origin server would
1639   create a TCP connection to port 80 of the host "www.example.org" and send
1640   the lines:
1641</t>
1642<figure><artwork type="example">
1643    GET /pub/WWW/TheProject.html HTTP/1.1
1644    Host: www.example.org
1645</artwork></figure>
1646<t>
1647   followed by the remainder of the Request. Note that the absolute path
1648   cannot be empty; if none is present in the original URI, it &MUST; be
1649   given as "/" (the server root).
1650</t>
1651<t>
1652   The Request-URI is transmitted in the format specified in
1653   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1654   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1655   properly interpret the request. Servers &SHOULD; respond to invalid
1656   Request-URIs with an appropriate status code.
1657</t>
1658<t>
1659   A transparent proxy &MUST-NOT; rewrite the "abs_path" part of the
1660   received Request-URI when forwarding it to the next inbound server,
1661   except as noted above to replace a null abs_path with "/".
1662</t>
1663<t>
1664  <list><t>
1665      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1666      meaning of the request when the origin server is improperly using
1667      a non-reserved URI character for a reserved purpose.  Implementors
1668      should be aware that some pre-HTTP/1.1 proxies have been known to
1669      rewrite the Request-URI.
1670  </t></list>
1671</t>
1672</section>
1673</section>
1674
1675<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1676<t>
1677   The exact resource identified by an Internet request is determined by
1678   examining both the Request-URI and the Host header field.
1679</t>
1680<t>
1681   An origin server that does not allow resources to differ by the
1682   requested host &MAY; ignore the Host header field value when
1683   determining the resource identified by an HTTP/1.1 request. (But see
1684   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1685   for other requirements on Host support in HTTP/1.1.)
1686</t>
1687<t>
1688   An origin server that does differentiate resources based on the host
1689   requested (sometimes referred to as virtual hosts or vanity host
1690   names) &MUST; use the following rules for determining the requested
1691   resource on an HTTP/1.1 request:
1692  <list style="numbers">
1693    <t>If Request-URI is an absoluteURI, the host is part of the
1694     Request-URI. Any Host header field value in the request &MUST; be
1695     ignored.</t>
1696    <t>If the Request-URI is not an absoluteURI, and the request includes
1697     a Host header field, the host is determined by the Host header
1698     field value.</t>
1699    <t>If the host as determined by rule 1 or 2 is not a valid host on
1700     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1701  </list>
1702</t>
1703<t>
1704   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1705   attempt to use heuristics (e.g., examination of the URI path for
1706   something unique to a particular host) in order to determine what
1707   exact resource is being requested.
1708</t>
1709</section>
1710
1711</section>
1712
1713
1714<section title="Response" anchor="response">
1715<t>
1716   After receiving and interpreting a request message, a server responds
1717   with an HTTP response message.
1718</t>
1719<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1720    Response      = Status-Line               ; <xref target="status-line"/>
1721                    *(( general-header        ; <xref target="general.header.fields"/>
1722                     | response-header        ; &response-header-fields;
1723                     | entity-header ) CRLF)  ; &entity-header-fields;
1724                    CRLF
1725                    [ message-body ]          ; <xref target="message.body"/>
1726</artwork></figure>
1727
1728<section title="Status-Line" anchor="status-line">
1729<t>
1730   The first line of a Response message is the Status-Line, consisting
1731   of the protocol version followed by a numeric status code and its
1732   associated textual phrase, with each element separated by SP
1733   characters. No CR or LF is allowed except in the final CRLF sequence.
1734</t>
1735<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1736    Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1737</artwork></figure>
1738
1739<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1740<t>
1741   The Status-Code element is a 3-digit integer result code of the
1742   attempt to understand and satisfy the request. These codes are fully
1743   defined in &status-codes;. The Reason-Phrase is intended to give a short
1744   textual description of the Status-Code. The Status-Code is intended
1745   for use by automata and the Reason-Phrase is intended for the human
1746   user. The client is not required to examine or display the Reason-Phrase.
1747</t>
1748<t>
1749   The first digit of the Status-Code defines the class of response. The
1750   last two digits do not have any categorization role. There are 5
1751   values for the first digit:
1752  <list style="symbols">
1753    <t>
1754      1xx: Informational - Request received, continuing process
1755    </t>
1756    <t>
1757      2xx: Success - The action was successfully received,
1758        understood, and accepted
1759    </t>
1760    <t>
1761      3xx: Redirection - Further action must be taken in order to
1762        complete the request
1763    </t>
1764    <t>
1765      4xx: Client Error - The request contains bad syntax or cannot
1766        be fulfilled
1767    </t>
1768    <t>
1769      5xx: Server Error - The server failed to fulfill an apparently
1770        valid request
1771    </t>
1772  </list>
1773</t>
1774<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/>
1775   Status-Code    = 3DIGIT
1776   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1777</artwork></figure>
1778</section>
1779</section>
1780
1781</section>
1782
1783
1784<section title="Connections" anchor="connections">
1785
1786<section title="Persistent Connections" anchor="persistent.connections">
1787
1788<section title="Purpose" anchor="persistent.purpose">
1789<t>
1790   Prior to persistent connections, a separate TCP connection was
1791   established to fetch each URL, increasing the load on HTTP servers
1792   and causing congestion on the Internet. The use of inline images and
1793   other associated data often require a client to make multiple
1794   requests of the same server in a short amount of time. Analysis of
1795   these performance problems and results from a prototype
1796   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1797   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1798   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1799   T/TCP <xref target="Tou1998"/>.
1800</t>
1801<t>
1802   Persistent HTTP connections have a number of advantages:
1803  <list style="symbols">
1804      <t>
1805        By opening and closing fewer TCP connections, CPU time is saved
1806        in routers and hosts (clients, servers, proxies, gateways,
1807        tunnels, or caches), and memory used for TCP protocol control
1808        blocks can be saved in hosts.
1809      </t>
1810      <t>
1811        HTTP requests and responses can be pipelined on a connection.
1812        Pipelining allows a client to make multiple requests without
1813        waiting for each response, allowing a single TCP connection to
1814        be used much more efficiently, with much lower elapsed time.
1815      </t>
1816      <t>
1817        Network congestion is reduced by reducing the number of packets
1818        caused by TCP opens, and by allowing TCP sufficient time to
1819        determine the congestion state of the network.
1820      </t>
1821      <t>
1822        Latency on subsequent requests is reduced since there is no time
1823        spent in TCP's connection opening handshake.
1824      </t>
1825      <t>
1826        HTTP can evolve more gracefully, since errors can be reported
1827        without the penalty of closing the TCP connection. Clients using
1828        future versions of HTTP might optimistically try a new feature,
1829        but if communicating with an older server, retry with old
1830        semantics after an error is reported.
1831      </t>
1832    </list>
1833</t>
1834<t>
1835   HTTP implementations &SHOULD; implement persistent connections.
1836</t>
1837</section>
1838
1839<section title="Overall Operation" anchor="persistent.overall">
1840<t>
1841   A significant difference between HTTP/1.1 and earlier versions of
1842   HTTP is that persistent connections are the default behavior of any
1843   HTTP connection. That is, unless otherwise indicated, the client
1844   &SHOULD; assume that the server will maintain a persistent connection,
1845   even after error responses from the server.
1846</t>
1847<t>
1848   Persistent connections provide a mechanism by which a client and a
1849   server can signal the close of a TCP connection. This signaling takes
1850   place using the Connection header field (<xref target="header.connection"/>). Once a close
1851   has been signaled, the client &MUST-NOT; send any more requests on that
1852   connection.
1853</t>
1854
1855<section title="Negotiation" anchor="persistent.negotiation">
1856<t>
1857   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1858   maintain a persistent connection unless a Connection header including
1859   the connection-token "close" was sent in the request. If the server
1860   chooses to close the connection immediately after sending the
1861   response, it &SHOULD; send a Connection header including the
1862   connection-token close.
1863</t>
1864<t>
1865   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1866   decide to keep it open based on whether the response from a server
1867   contains a Connection header with the connection-token close. In case
1868   the client does not want to maintain a connection for more than that
1869   request, it &SHOULD; send a Connection header including the
1870   connection-token close.
1871</t>
1872<t>
1873   If either the client or the server sends the close token in the
1874   Connection header, that request becomes the last one for the
1875   connection.
1876</t>
1877<t>
1878   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1879   maintained for HTTP versions less than 1.1 unless it is explicitly
1880   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1881   compatibility with HTTP/1.0 clients.
1882</t>
1883<t>
1884   In order to remain persistent, all messages on the connection &MUST;
1885   have a self-defined message length (i.e., one not defined by closure
1886   of the connection), as described in <xref target="message.length"/>.
1887</t>
1888</section>
1889
1890<section title="Pipelining" anchor="pipelining">
1891<t>
1892   A client that supports persistent connections &MAY; "pipeline" its
1893   requests (i.e., send multiple requests without waiting for each
1894   response). A server &MUST; send its responses to those requests in the
1895   same order that the requests were received.
1896</t>
1897<t>
1898   Clients which assume persistent connections and pipeline immediately
1899   after connection establishment &SHOULD; be prepared to retry their
1900   connection if the first pipelined attempt fails. If a client does
1901   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1902   persistent. Clients &MUST; also be prepared to resend their requests if
1903   the server closes the connection before sending all of the
1904   corresponding responses.
1905</t>
1906<t>
1907   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1908   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1909   premature termination of the transport connection could lead to
1910   indeterminate results. A client wishing to send a non-idempotent
1911   request &SHOULD; wait to send that request until it has received the
1912   response status for the previous request.
1913</t>
1914</section>
1915</section>
1916
1917<section title="Proxy Servers" anchor="persistent.proxy">
1918<t>
1919   It is especially important that proxies correctly implement the
1920   properties of the Connection header field as specified in <xref target="header.connection"/>.
1921</t>
1922<t>
1923   The proxy server &MUST; signal persistent connections separately with
1924   its clients and the origin servers (or other proxy servers) that it
1925   connects to. Each persistent connection applies to only one transport
1926   link.
1927</t>
1928<t>
1929   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1930   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
1931   discussion of the problems with the Keep-Alive header implemented by
1932   many HTTP/1.0 clients).
1933</t>
1934</section>
1935
1936<section title="Practical Considerations" anchor="persistent.practical">
1937<t>
1938   Servers will usually have some time-out value beyond which they will
1939   no longer maintain an inactive connection. Proxy servers might make
1940   this a higher value since it is likely that the client will be making
1941   more connections through the same server. The use of persistent
1942   connections places no requirements on the length (or existence) of
1943   this time-out for either the client or the server.
1944</t>
1945<t>
1946   When a client or server wishes to time-out it &SHOULD; issue a graceful
1947   close on the transport connection. Clients and servers &SHOULD; both
1948   constantly watch for the other side of the transport close, and
1949   respond to it as appropriate. If a client or server does not detect
1950   the other side's close promptly it could cause unnecessary resource
1951   drain on the network.
1952</t>
1953<t>
1954   A client, server, or proxy &MAY; close the transport connection at any
1955   time. For example, a client might have started to send a new request
1956   at the same time that the server has decided to close the "idle"
1957   connection. From the server's point of view, the connection is being
1958   closed while it was idle, but from the client's point of view, a
1959   request is in progress.
1960</t>
1961<t>
1962   This means that clients, servers, and proxies &MUST; be able to recover
1963   from asynchronous close events. Client software &SHOULD; reopen the
1964   transport connection and retransmit the aborted sequence of requests
1965   without user interaction so long as the request sequence is
1966   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1967   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1968   human operator the choice of retrying the request(s). Confirmation by
1969   user-agent software with semantic understanding of the application
1970   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
1971   be repeated if the second sequence of requests fails.
1972</t>
1973<t>
1974   Servers &SHOULD; always respond to at least one request per connection,
1975   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1976   middle of transmitting a response, unless a network or client failure
1977   is suspected.
1978</t>
1979<t>
1980   Clients that use persistent connections &SHOULD; limit the number of
1981   simultaneous connections that they maintain to a given server. A
1982   single-user client &SHOULD-NOT; maintain more than 2 connections with
1983   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1984   another server or proxy, where N is the number of simultaneously
1985   active users. These guidelines are intended to improve HTTP response
1986   times and avoid congestion.
1987</t>
1988</section>
1989</section>
1990
1991<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1992
1993<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1994<t>
1995   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1996   flow control mechanisms to resolve temporary overloads, rather than
1997   terminating connections with the expectation that clients will retry.
1998   The latter technique can exacerbate network congestion.
1999</t>
2000</section>
2001
2002<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2003<t>
2004   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2005   the network connection for an error status while it is transmitting
2006   the request. If the client sees an error status, it &SHOULD;
2007   immediately cease transmitting the body. If the body is being sent
2008   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2009   empty trailer &MAY; be used to prematurely mark the end of the message.
2010   If the body was preceded by a Content-Length header, the client &MUST;
2011   close the connection.
2012</t>
2013</section>
2014
2015<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2016<t>
2017   The purpose of the 100 (Continue) status (see &status-100;) is to
2018   allow a client that is sending a request message with a request body
2019   to determine if the origin server is willing to accept the request
2020   (based on the request headers) before the client sends the request
2021   body. In some cases, it might either be inappropriate or highly
2022   inefficient for the client to send the body if the server will reject
2023   the message without looking at the body.
2024</t>
2025<t>
2026   Requirements for HTTP/1.1 clients:
2027  <list style="symbols">
2028    <t>
2029        If a client will wait for a 100 (Continue) response before
2030        sending the request body, it &MUST; send an Expect request-header
2031        field (&header-expect;) with the "100-continue" expectation.
2032    </t>
2033    <t>
2034        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2035        with the "100-continue" expectation if it does not intend
2036        to send a request body.
2037    </t>
2038  </list>
2039</t>
2040<t>
2041   Because of the presence of older implementations, the protocol allows
2042   ambiguous situations in which a client may send "Expect: 100-continue"
2043   without receiving either a 417 (Expectation Failed) status
2044   or a 100 (Continue) status. Therefore, when a client sends this
2045   header field to an origin server (possibly via a proxy) from which it
2046   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2047   for an indefinite period before sending the request body.
2048</t>
2049<t>
2050   Requirements for HTTP/1.1 origin servers:
2051  <list style="symbols">
2052    <t> Upon receiving a request which includes an Expect request-header
2053        field with the "100-continue" expectation, an origin server &MUST;
2054        either respond with 100 (Continue) status and continue to read
2055        from the input stream, or respond with a final status code. The
2056        origin server &MUST-NOT; wait for the request body before sending
2057        the 100 (Continue) response. If it responds with a final status
2058        code, it &MAY; close the transport connection or it &MAY; continue
2059        to read and discard the rest of the request.  It &MUST-NOT;
2060        perform the requested method if it returns a final status code.
2061    </t>
2062    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2063        the request message does not include an Expect request-header
2064        field with the "100-continue" expectation, and &MUST-NOT; send a
2065        100 (Continue) response if such a request comes from an HTTP/1.0
2066        (or earlier) client. There is an exception to this rule: for
2067        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2068        status in response to an HTTP/1.1 PUT or POST request that does
2069        not include an Expect request-header field with the "100-continue"
2070        expectation. This exception, the purpose of which is
2071        to minimize any client processing delays associated with an
2072        undeclared wait for 100 (Continue) status, applies only to
2073        HTTP/1.1 requests, and not to requests with any other HTTP-version
2074        value.
2075    </t>
2076    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2077        already received some or all of the request body for the
2078        corresponding request.
2079    </t>
2080    <t> An origin server that sends a 100 (Continue) response &MUST;
2081    ultimately send a final status code, once the request body is
2082        received and processed, unless it terminates the transport
2083        connection prematurely.
2084    </t>
2085    <t> If an origin server receives a request that does not include an
2086        Expect request-header field with the "100-continue" expectation,
2087        the request includes a request body, and the server responds
2088        with a final status code before reading the entire request body
2089        from the transport connection, then the server &SHOULD-NOT;  close
2090        the transport connection until it has read the entire request,
2091        or until the client closes the connection. Otherwise, the client
2092        might not reliably receive the response message. However, this
2093        requirement is not be construed as preventing a server from
2094        defending itself against denial-of-service attacks, or from
2095        badly broken client implementations.
2096      </t>
2097    </list>
2098</t>
2099<t>
2100   Requirements for HTTP/1.1 proxies:
2101  <list style="symbols">
2102    <t> If a proxy receives a request that includes an Expect request-header
2103        field with the "100-continue" expectation, and the proxy
2104        either knows that the next-hop server complies with HTTP/1.1 or
2105        higher, or does not know the HTTP version of the next-hop
2106        server, it &MUST; forward the request, including the Expect header
2107        field.
2108    </t>
2109    <t> If the proxy knows that the version of the next-hop server is
2110        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2111        respond with a 417 (Expectation Failed) status.
2112    </t>
2113    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2114        numbers received from recently-referenced next-hop servers.
2115    </t>
2116    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2117        request message was received from an HTTP/1.0 (or earlier)
2118        client and did not include an Expect request-header field with
2119        the "100-continue" expectation. This requirement overrides the
2120        general rule for forwarding of 1xx responses (see &status-1xx;).
2121    </t>
2122  </list>
2123</t>
2124</section>
2125
2126<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2127<t>
2128   If an HTTP/1.1 client sends a request which includes a request body,
2129   but which does not include an Expect request-header field with the
2130   "100-continue" expectation, and if the client is not directly
2131   connected to an HTTP/1.1 origin server, and if the client sees the
2132   connection close before receiving any status from the server, the
2133   client &SHOULD; retry the request.  If the client does retry this
2134   request, it &MAY; use the following "binary exponential backoff"
2135   algorithm to be assured of obtaining a reliable response:
2136  <list style="numbers">
2137    <t>
2138      Initiate a new connection to the server
2139    </t>
2140    <t>
2141      Transmit the request-headers
2142    </t>
2143    <t>
2144      Initialize a variable R to the estimated round-trip time to the
2145         server (e.g., based on the time it took to establish the
2146         connection), or to a constant value of 5 seconds if the round-trip
2147         time is not available.
2148    </t>
2149    <t>
2150       Compute T = R * (2**N), where N is the number of previous
2151         retries of this request.
2152    </t>
2153    <t>
2154       Wait either for an error response from the server, or for T
2155         seconds (whichever comes first)
2156    </t>
2157    <t>
2158       If no error response is received, after T seconds transmit the
2159         body of the request.
2160    </t>
2161    <t>
2162       If client sees that the connection is closed prematurely,
2163         repeat from step 1 until the request is accepted, an error
2164         response is received, or the user becomes impatient and
2165         terminates the retry process.
2166    </t>
2167  </list>
2168</t>
2169<t>
2170   If at any point an error status is received, the client
2171  <list style="symbols">
2172      <t>&SHOULD-NOT;  continue and</t>
2173
2174      <t>&SHOULD; close the connection if it has not completed sending the
2175        request message.</t>
2176    </list>
2177</t>
2178</section>
2179</section>
2180</section>
2181
2182
2183<section title="Header Field Definitions" anchor="header.fields">
2184<t>
2185   This section defines the syntax and semantics of all standard
2186   HTTP/1.1 header fields. For entity-header fields, both sender and
2187   recipient refer to either the client or the server, depending on who
2188   sends and who receives the entity.
2189</t>
2190
2191<section title="Connection" anchor="header.connection">
2192  <iref primary="true" item="Connection header" x:for-anchor=""/>
2193  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2194<t>
2195   The Connection general-header field allows the sender to specify
2196   options that are desired for that particular connection and &MUST-NOT;
2197   be communicated by proxies over further connections.
2198</t>
2199<t>
2200   The Connection header has the following grammar:
2201</t>
2202<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2203    Connection = "Connection" ":" 1#(connection-token)
2204    connection-token  = token
2205</artwork></figure>
2206<t>
2207   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2208   message is forwarded and, for each connection-token in this field,
2209   remove any header field(s) from the message with the same name as the
2210   connection-token. Connection options are signaled by the presence of
2211   a connection-token in the Connection header field, not by any
2212   corresponding additional header field(s), since the additional header
2213   field may not be sent if there are no parameters associated with that
2214   connection option.
2215</t>
2216<t>
2217   Message headers listed in the Connection header &MUST-NOT; include
2218   end-to-end headers, such as Cache-Control.
2219</t>
2220<t>
2221   HTTP/1.1 defines the "close" connection option for the sender to
2222   signal that the connection will be closed after completion of the
2223   response. For example,
2224</t>
2225<figure><artwork type="example">
2226    Connection: close
2227</artwork></figure>
2228<t>
2229   in either the request or the response header fields indicates that
2230   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2231   after the current request/response is complete.
2232</t>
2233<t>
2234   An HTTP/1.1 client that does not support persistent connections &MUST;
2235   include the "close" connection option in every request message.
2236</t>
2237<t>
2238   An HTTP/1.1 server that does not support persistent connections &MUST;
2239   include the "close" connection option in every response message that
2240   does not have a 1xx (informational) status code.
2241</t>
2242<t>
2243   A system receiving an HTTP/1.0 (or lower-version) message that
2244   includes a Connection header &MUST;, for each connection-token in this
2245   field, remove and ignore any header field(s) from the message with
2246   the same name as the connection-token. This protects against mistaken
2247   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2248</t>
2249</section>
2250
2251<section title="Content-Length" anchor="header.content-length">
2252  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2253  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2254<t>
2255   The Content-Length entity-header field indicates the size of the
2256   entity-body, in decimal number of OCTETs, sent to the recipient or,
2257   in the case of the HEAD method, the size of the entity-body that
2258   would have been sent had the request been a GET.
2259</t>
2260<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2261    Content-Length    = "Content-Length" ":" 1*DIGIT
2262</artwork></figure>
2263<t>
2264   An example is
2265</t>
2266<figure><artwork type="example">
2267    Content-Length: 3495
2268</artwork></figure>
2269<t>
2270   Applications &SHOULD; use this field to indicate the transfer-length of
2271   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2272</t>
2273<t>
2274   Any Content-Length greater than or equal to zero is a valid value.
2275   <xref target="message.length"/> describes how to determine the length of a message-body
2276   if a Content-Length is not given.
2277</t>
2278<t>
2279   Note that the meaning of this field is significantly different from
2280   the corresponding definition in MIME, where it is an optional field
2281   used within the "message/external-body" content-type. In HTTP, it
2282   &SHOULD; be sent whenever the message's length can be determined prior
2283   to being transferred, unless this is prohibited by the rules in
2284   <xref target="message.length"/>.
2285</t>
2286</section>
2287
2288<section title="Date" anchor="header.date">
2289  <iref primary="true" item="Date header" x:for-anchor=""/>
2290  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2291<t>
2292   The Date general-header field represents the date and time at which
2293   the message was originated, having the same semantics as orig-date in
2294   <xref target="RFC822"/>. The field value is an HTTP-date, as described in <xref target="full.date"/>;
2295   it &MUST; be sent in rfc1123-date format.
2296</t>
2297<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2298    Date  = "Date" ":" HTTP-date
2299</artwork></figure>
2300<t>
2301   An example is
2302</t>
2303<figure><artwork type="example">
2304    Date: Tue, 15 Nov 1994 08:12:31 GMT
2305</artwork></figure>
2306<t>
2307   Origin servers &MUST; include a Date header field in all responses,
2308   except in these cases:
2309  <list style="numbers">
2310      <t>If the response status code is 100 (Continue) or 101 (Switching
2311         Protocols), the response &MAY; include a Date header field, at
2312         the server's option.</t>
2313
2314      <t>If the response status code conveys a server error, e.g. 500
2315         (Internal Server Error) or 503 (Service Unavailable), and it is
2316         inconvenient or impossible to generate a valid Date.</t>
2317
2318      <t>If the server does not have a clock that can provide a
2319         reasonable approximation of the current time, its responses
2320         &MUST-NOT; include a Date header field. In this case, the rules
2321         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2322  </list>
2323</t>
2324<t>
2325   A received message that does not have a Date header field &MUST; be
2326   assigned one by the recipient if the message will be cached by that
2327   recipient or gatewayed via a protocol which requires a Date. An HTTP
2328   implementation without a clock &MUST-NOT; cache responses without
2329   revalidating them on every use. An HTTP cache, especially a shared
2330   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2331   clock with a reliable external standard.
2332</t>
2333<t>
2334   Clients &SHOULD; only send a Date header field in messages that include
2335   an entity-body, as in the case of the PUT and POST requests, and even
2336   then it is optional. A client without a clock &MUST-NOT; send a Date
2337   header field in a request.
2338</t>
2339<t>
2340   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2341   time subsequent to the generation of the message. It &SHOULD; represent
2342   the best available approximation of the date and time of message
2343   generation, unless the implementation has no means of generating a
2344   reasonably accurate date and time. In theory, the date ought to
2345   represent the moment just before the entity is generated. In
2346   practice, the date can be generated at any time during the message
2347   origination without affecting its semantic value.
2348</t>
2349
2350<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2351<t>
2352   Some origin server implementations might not have a clock available.
2353   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2354   values to a response, unless these values were associated
2355   with the resource by a system or user with a reliable clock. It &MAY;
2356   assign an Expires value that is known, at or before server
2357   configuration time, to be in the past (this allows "pre-expiration"
2358   of responses without storing separate Expires values for each
2359   resource).
2360</t>
2361</section>
2362</section>
2363
2364<section title="Host" anchor="header.host">
2365  <iref primary="true" item="Host header" x:for-anchor=""/>
2366  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2367<t>
2368   The Host request-header field specifies the Internet host and port
2369   number of the resource being requested, as obtained from the original
2370   URI given by the user or referring resource (generally an HTTP URL,
2371   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2372   the naming authority of the origin server or gateway given by the
2373   original URL. This allows the origin server or gateway to
2374   differentiate between internally-ambiguous URLs, such as the root "/"
2375   URL of a server for multiple host names on a single IP address.
2376</t>
2377<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2378    Host = "Host" ":" host [ ":" port ] ; <xref target="http.url"/>
2379</artwork></figure>
2380<t>
2381   A "host" without any trailing port information implies the default
2382   port for the service requested (e.g., "80" for an HTTP URL). For
2383   example, a request on the origin server for
2384   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2385</t>
2386<figure><artwork type="example">
2387    GET /pub/WWW/ HTTP/1.1
2388    Host: www.example.org
2389</artwork></figure>
2390<t>
2391   A client &MUST; include a Host header field in all HTTP/1.1 request
2392   messages . If the requested URI does not include an Internet host
2393   name for the service being requested, then the Host header field &MUST;
2394   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2395   request message it forwards does contain an appropriate Host header
2396   field that identifies the service being requested by the proxy. All
2397   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2398   status code to any HTTP/1.1 request message which lacks a Host header
2399   field.
2400</t>
2401<t>
2402   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2403   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2404   for other requirements relating to Host.
2405</t>
2406</section>
2407
2408<section title="TE" anchor="header.te">
2409  <iref primary="true" item="TE header" x:for-anchor=""/>
2410  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2411<t>
2412   The TE request-header field indicates what extension transfer-codings
2413   it is willing to accept in the response and whether or not it is
2414   willing to accept trailer fields in a chunked transfer-coding. Its
2415   value may consist of the keyword "trailers" and/or a comma-separated
2416   list of extension transfer-coding names with optional accept
2417   parameters (as described in <xref target="transfer.codings"/>).
2418</t>
2419<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2420    TE        = "TE" ":" #( t-codings )
2421    t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2422</artwork></figure>
2423<t>
2424   The presence of the keyword "trailers" indicates that the client is
2425   willing to accept trailer fields in a chunked transfer-coding, as
2426   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2427   transfer-coding values even though it does not itself represent a
2428   transfer-coding.
2429</t>
2430<t>
2431   Examples of its use are:
2432</t>
2433<figure><artwork type="example">
2434    TE: deflate
2435    TE:
2436    TE: trailers, deflate;q=0.5
2437</artwork></figure>
2438<t>
2439   The TE header field only applies to the immediate connection.
2440   Therefore, the keyword &MUST; be supplied within a Connection header
2441   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2442</t>
2443<t>
2444   A server tests whether a transfer-coding is acceptable, according to
2445   a TE field, using these rules:
2446  <list style="numbers">
2447    <x:lt>
2448      <t>The "chunked" transfer-coding is always acceptable. If the
2449         keyword "trailers" is listed, the client indicates that it is
2450         willing to accept trailer fields in the chunked response on
2451         behalf of itself and any downstream clients. The implication is
2452         that, if given, the client is stating that either all
2453         downstream clients are willing to accept trailer fields in the
2454         forwarded response, or that it will attempt to buffer the
2455         response on behalf of downstream recipients.
2456      </t><t>
2457         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2458         chunked response such that a client can be assured of buffering
2459         the entire response.</t>
2460    </x:lt>
2461    <x:lt>
2462      <t>If the transfer-coding being tested is one of the transfer-codings
2463         listed in the TE field, then it is acceptable unless it
2464         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2465         qvalue of 0 means "not acceptable.")</t>
2466    </x:lt>
2467    <x:lt>
2468      <t>If multiple transfer-codings are acceptable, then the
2469         acceptable transfer-coding with the highest non-zero qvalue is
2470         preferred.  The "chunked" transfer-coding always has a qvalue
2471         of 1.</t>
2472    </x:lt>
2473  </list>
2474</t>
2475<t>
2476   If the TE field-value is empty or if no TE field is present, the only
2477   transfer-coding  is "chunked". A message with no transfer-coding is
2478   always acceptable.
2479</t>
2480</section>
2481
2482<section title="Trailer" anchor="header.trailer">
2483  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2484  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2485<t>
2486   The Trailer general field value indicates that the given set of
2487   header fields is present in the trailer of a message encoded with
2488   chunked transfer-coding.
2489</t>
2490<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2491    Trailer  = "Trailer" ":" 1#field-name
2492</artwork></figure>
2493<t>
2494   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2495   message using chunked transfer-coding with a non-empty trailer. Doing
2496   so allows the recipient to know which header fields to expect in the
2497   trailer.
2498</t>
2499<t>
2500   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2501   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2502   trailer fields in a "chunked" transfer-coding.
2503</t>
2504<t>
2505   Message header fields listed in the Trailer header field &MUST-NOT;
2506   include the following header fields:
2507  <list style="symbols">
2508    <t>Transfer-Encoding</t>
2509    <t>Content-Length</t>
2510    <t>Trailer</t>
2511  </list>
2512</t>
2513</section>
2514
2515<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2516  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2517  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2518<t>
2519   The Transfer-Encoding general-header field indicates what (if any)
2520   type of transformation has been applied to the message body in order
2521   to safely transfer it between the sender and the recipient. This
2522   differs from the content-coding in that the transfer-coding is a
2523   property of the message, not of the entity.
2524</t>
2525<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2526  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2527</artwork></figure>
2528<t>
2529   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2530</t>
2531<figure><artwork type="example">
2532  Transfer-Encoding: chunked
2533</artwork></figure>
2534<t>
2535   If multiple encodings have been applied to an entity, the transfer-codings
2536   &MUST; be listed in the order in which they were applied.
2537   Additional information about the encoding parameters &MAY; be provided
2538   by other entity-header fields not defined by this specification.
2539</t>
2540<t>
2541   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2542   header.
2543</t>
2544</section>
2545
2546<section title="Upgrade" anchor="header.upgrade">
2547  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2548  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2549<t>
2550   The Upgrade general-header allows the client to specify what
2551   additional communication protocols it supports and would like to use
2552   if the server finds it appropriate to switch protocols. The server
2553   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2554   response to indicate which protocol(s) are being switched.
2555</t>
2556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2557    Upgrade        = "Upgrade" ":" 1#product
2558</artwork></figure>
2559<t>
2560   For example,
2561</t>
2562<figure><artwork type="example">
2563    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2564</artwork></figure>
2565<t>
2566   The Upgrade header field is intended to provide a simple mechanism
2567   for transition from HTTP/1.1 to some other, incompatible protocol. It
2568   does so by allowing the client to advertise its desire to use another
2569   protocol, such as a later version of HTTP with a higher major version
2570   number, even though the current request has been made using HTTP/1.1.
2571   This eases the difficult transition between incompatible protocols by
2572   allowing the client to initiate a request in the more commonly
2573   supported protocol while indicating to the server that it would like
2574   to use a "better" protocol if available (where "better" is determined
2575   by the server, possibly according to the nature of the method and/or
2576   resource being requested).
2577</t>
2578<t>
2579   The Upgrade header field only applies to switching application-layer
2580   protocols upon the existing transport-layer connection. Upgrade
2581   cannot be used to insist on a protocol change; its acceptance and use
2582   by the server is optional. The capabilities and nature of the
2583   application-layer communication after the protocol change is entirely
2584   dependent upon the new protocol chosen, although the first action
2585   after changing the protocol &MUST; be a response to the initial HTTP
2586   request containing the Upgrade header field.
2587</t>
2588<t>
2589   The Upgrade header field only applies to the immediate connection.
2590   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2591   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2592   HTTP/1.1 message.
2593</t>
2594<t>
2595   The Upgrade header field cannot be used to indicate a switch to a
2596   protocol on a different connection. For that purpose, it is more
2597   appropriate to use a 301, 302, 303, or 305 redirection response.
2598</t>
2599<t>
2600   This specification only defines the protocol name "HTTP" for use by
2601   the family of Hypertext Transfer Protocols, as defined by the HTTP
2602   version rules of <xref target="http.version"/> and future updates to this
2603   specification. Any token can be used as a protocol name; however, it
2604   will only be useful if both the client and server associate the name
2605   with the same protocol.
2606</t>
2607</section>
2608
2609<section title="Via" anchor="header.via">
2610  <iref primary="true" item="Via header" x:for-anchor=""/>
2611  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2612<t>
2613   The Via general-header field &MUST; be used by gateways and proxies to
2614   indicate the intermediate protocols and recipients between the user
2615   agent and the server on requests, and between the origin server and
2616   the client on responses. It is analogous to the "Received" field of
2617   <xref target="RFC822"/> and is intended to be used for tracking message forwards,
2618   avoiding request loops, and identifying the protocol capabilities of
2619   all senders along the request/response chain.
2620</t>
2621<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Via"/><iref primary="true" item="Grammar" subitem="received-protocol"/><iref primary="true" item="Grammar" subitem="protocol-name"/><iref primary="true" item="Grammar" subitem="protocol-version"/><iref primary="true" item="Grammar" subitem="received-by"/><iref primary="true" item="Grammar" subitem="pseudonym"/>
2622   Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2623   received-protocol = [ protocol-name "/" ] protocol-version
2624   protocol-name     = token
2625   protocol-version  = token
2626   received-by       = ( host [ ":" port ] ) | pseudonym
2627   pseudonym         = token
2628</artwork></figure>
2629<t>
2630   The received-protocol indicates the protocol version of the message
2631   received by the server or client along each segment of the
2632   request/response chain. The received-protocol version is appended to
2633   the Via field value when the message is forwarded so that information
2634   about the protocol capabilities of upstream applications remains
2635   visible to all recipients.
2636</t>
2637<t>
2638   The protocol-name is optional if and only if it would be "HTTP". The
2639   received-by field is normally the host and optional port number of a
2640   recipient server or client that subsequently forwarded the message.
2641   However, if the real host is considered to be sensitive information,
2642   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2643   be assumed to be the default port of the received-protocol.
2644</t>
2645<t>
2646   Multiple Via field values represents each proxy or gateway that has
2647   forwarded the message. Each recipient &MUST; append its information
2648   such that the end result is ordered according to the sequence of
2649   forwarding applications.
2650</t>
2651<t>
2652   Comments &MAY; be used in the Via header field to identify the software
2653   of the recipient proxy or gateway, analogous to the User-Agent and
2654   Server header fields. However, all comments in the Via field are
2655   optional and &MAY; be removed by any recipient prior to forwarding the
2656   message.
2657</t>
2658<t>
2659   For example, a request message could be sent from an HTTP/1.0 user
2660   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2661   forward the request to a public proxy at p.example.net, which completes
2662   the request by forwarding it to the origin server at www.example.com.
2663   The request received by www.example.com would then have the following
2664   Via header field:
2665</t>
2666<figure><artwork type="example">
2667    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2668</artwork></figure>
2669<t>
2670   Proxies and gateways used as a portal through a network firewall
2671   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2672   the firewall region. This information &SHOULD; only be propagated if
2673   explicitly enabled. If not enabled, the received-by host of any host
2674   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2675   for that host.
2676</t>
2677<t>
2678   For organizations that have strong privacy requirements for hiding
2679   internal structures, a proxy &MAY; combine an ordered subsequence of
2680   Via header field entries with identical received-protocol values into
2681   a single such entry. For example,
2682</t>
2683<figure><artwork type="example">
2684    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2685</artwork></figure>
2686<t>
2687        could be collapsed to
2688</t>
2689<figure><artwork type="example">
2690    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2691</artwork></figure>
2692<t>
2693   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2694   under the same organizational control and the hosts have already been
2695   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2696   have different received-protocol values.
2697</t>
2698</section>
2699
2700</section>
2701
2702<section title="IANA Considerations" anchor="IANA.considerations">
2703<t>
2704   TBD.
2705</t>
2706</section>
2707
2708<section title="Security Considerations" anchor="security.considerations">
2709<t>
2710   This section is meant to inform application developers, information
2711   providers, and users of the security limitations in HTTP/1.1 as
2712   described by this document. The discussion does not include
2713   definitive solutions to the problems revealed, though it does make
2714   some suggestions for reducing security risks.
2715</t>
2716
2717<section title="Personal Information" anchor="personal.information">
2718<t>
2719   HTTP clients are often privy to large amounts of personal information
2720   (e.g. the user's name, location, mail address, passwords, encryption
2721   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2722   leakage of this information via the HTTP protocol to other sources.
2723   We very strongly recommend that a convenient interface be provided
2724   for the user to control dissemination of such information, and that
2725   designers and implementors be particularly careful in this area.
2726   History shows that errors in this area often create serious security
2727   and/or privacy problems and generate highly adverse publicity for the
2728   implementor's company.
2729</t>
2730</section>
2731
2732<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2733<t>
2734   A server is in the position to save personal data about a user's
2735   requests which might identify their reading patterns or subjects of
2736   interest. This information is clearly confidential in nature and its
2737   handling can be constrained by law in certain countries. People using
2738   the HTTP protocol to provide data are responsible for ensuring that
2739   such material is not distributed without the permission of any
2740   individuals that are identifiable by the published results.
2741</t>
2742</section>
2743
2744<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2745<t>
2746   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2747   the documents returned by HTTP requests to be only those that were
2748   intended by the server administrators. If an HTTP server translates
2749   HTTP URIs directly into file system calls, the server &MUST; take
2750   special care not to serve files that were not intended to be
2751   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2752   other operating systems use ".." as a path component to indicate a
2753   directory level above the current one. On such a system, an HTTP
2754   server &MUST; disallow any such construct in the Request-URI if it
2755   would otherwise allow access to a resource outside those intended to
2756   be accessible via the HTTP server. Similarly, files intended for
2757   reference only internally to the server (such as access control
2758   files, configuration files, and script code) &MUST; be protected from
2759   inappropriate retrieval, since they might contain sensitive
2760   information. Experience has shown that minor bugs in such HTTP server
2761   implementations have turned into security risks.
2762</t>
2763</section>
2764
2765<section title="DNS Spoofing" anchor="dns.spoofing">
2766<t>
2767   Clients using HTTP rely heavily on the Domain Name Service, and are
2768   thus generally prone to security attacks based on the deliberate
2769   mis-association of IP addresses and DNS names. Clients need to be
2770   cautious in assuming the continuing validity of an IP number/DNS name
2771   association.
2772</t>
2773<t>
2774   In particular, HTTP clients &SHOULD; rely on their name resolver for
2775   confirmation of an IP number/DNS name association, rather than
2776   caching the result of previous host name lookups. Many platforms
2777   already can cache host name lookups locally when appropriate, and
2778   they &SHOULD; be configured to do so. It is proper for these lookups to
2779   be cached, however, only when the TTL (Time To Live) information
2780   reported by the name server makes it likely that the cached
2781   information will remain useful.
2782</t>
2783<t>
2784   If HTTP clients cache the results of host name lookups in order to
2785   achieve a performance improvement, they &MUST; observe the TTL
2786   information reported by DNS.
2787</t>
2788<t>
2789   If HTTP clients do not observe this rule, they could be spoofed when
2790   a previously-accessed server's IP address changes. As network
2791   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2792   possibility of this form of attack will grow. Observing this
2793   requirement thus reduces this potential security vulnerability.
2794</t>
2795<t>
2796   This requirement also improves the load-balancing behavior of clients
2797   for replicated servers using the same DNS name and reduces the
2798   likelihood of a user's experiencing failure in accessing sites which
2799   use that strategy.
2800</t>
2801</section>
2802
2803<section title="Proxies and Caching" anchor="attack.proxies">
2804<t>
2805   By their very nature, HTTP proxies are men-in-the-middle, and
2806   represent an opportunity for man-in-the-middle attacks. Compromise of
2807   the systems on which the proxies run can result in serious security
2808   and privacy problems. Proxies have access to security-related
2809   information, personal information about individual users and
2810   organizations, and proprietary information belonging to users and
2811   content providers. A compromised proxy, or a proxy implemented or
2812   configured without regard to security and privacy considerations,
2813   might be used in the commission of a wide range of potential attacks.
2814</t>
2815<t>
2816   Proxy operators should protect the systems on which proxies run as
2817   they would protect any system that contains or transports sensitive
2818   information. In particular, log information gathered at proxies often
2819   contains highly sensitive personal information, and/or information
2820   about organizations. Log information should be carefully guarded, and
2821   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2822</t>
2823<t>
2824   Proxy implementors should consider the privacy and security
2825   implications of their design and coding decisions, and of the
2826   configuration options they provide to proxy operators (especially the
2827   default configuration).
2828</t>
2829<t>
2830   Users of a proxy need to be aware that they are no trustworthier than
2831   the people who run the proxy; HTTP itself cannot solve this problem.
2832</t>
2833<t>
2834   The judicious use of cryptography, when appropriate, may suffice to
2835   protect against a broad range of security and privacy attacks. Such
2836   cryptography is beyond the scope of the HTTP/1.1 specification.
2837</t>
2838</section>
2839
2840<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2841<t>
2842   They exist. They are hard to defend against. Research continues.
2843   Beware.
2844</t>
2845</section>
2846</section>
2847
2848<section title="Acknowledgments" anchor="ack">
2849<t>
2850   This specification makes heavy use of the augmented BNF and generic
2851   constructs defined by David H. Crocker for <xref target="RFC822"/>. Similarly, it
2852   reuses many of the definitions provided by Nathaniel Borenstein and
2853   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2854   specification will help reduce past confusion over the relationship
2855   between HTTP and Internet mail message formats.
2856</t>
2857<t>
2858   The HTTP protocol has evolved considerably over the years. It has
2859   benefited from a large and active developer community--the many
2860   people who have participated on the www-talk mailing list--and it is
2861   that community which has been most responsible for the success of
2862   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2863   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2864   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2865   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2866   VanHeyningen deserve special recognition for their efforts in
2867   defining early aspects of the protocol.
2868</t>
2869<t>
2870   This document has benefited greatly from the comments of all those
2871   participating in the HTTP-WG. In addition to those already mentioned,
2872   the following individuals have contributed to this specification:
2873</t>
2874<t>
2875   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2876   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2877   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2878   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2879   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2880   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2881   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2882   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2883   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2884   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2885   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2886   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2887   Josh Cohen.
2888</t>
2889<t>
2890   Thanks to the "cave men" of Palo Alto. You know who you are.
2891</t>
2892<t>
2893   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2894   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2895   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2896   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2897   Larry Masinter for their help. And thanks go particularly to Jeff
2898   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2899</t>
2900<t>
2901   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
2902   Frystyk implemented RFC 2068 early, and we wish to thank them for the
2903   discovery of many of the problems that this document attempts to
2904   rectify.
2905</t>
2906</section>
2907
2908</middle>
2909<back>
2910<references>
2911
2912<reference anchor="Part2">
2913   <front>
2914      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2915      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2916         <organization abbrev="Day Software">Day Software</organization>
2917         <address><email>fielding@gbiv.com</email></address>
2918      </author>
2919      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2920         <organization>One Laptop per Child</organization>
2921         <address><email>jg@laptop.org</email></address>
2922      </author>
2923      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2924         <organization abbrev="HP">Hewlett-Packard Company</organization>
2925         <address><email>JeffMogul@acm.org</email></address>
2926      </author>
2927      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2928         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2929         <address><email>henrikn@microsoft.com</email></address>
2930      </author>
2931      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2932         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2933         <address><email>LMM@acm.org</email></address>
2934      </author>
2935      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2936         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2937         <address><email>paulle@microsoft.com</email></address>
2938      </author>
2939      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2940         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2941         <address><email>timbl@w3.org</email></address>
2942      </author>
2943      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2944         <organization abbrev="W3C">World Wide Web Consortium</organization>
2945         <address><email>ylafon@w3.org</email></address>
2946      </author>
2947      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2948         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2949         <address><email>julian.reschke@greenbytes.de</email></address>
2950      </author>
2951      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2952   </front>
2953   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2954   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2955</reference>
2956
2957<reference anchor="Part3">
2958   <front>
2959      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2960      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2961         <organization abbrev="Day Software">Day Software</organization>
2962         <address><email>fielding@gbiv.com</email></address>
2963      </author>
2964      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2965         <organization>One Laptop per Child</organization>
2966         <address><email>jg@laptop.org</email></address>
2967      </author>
2968      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2969         <organization abbrev="HP">Hewlett-Packard Company</organization>
2970         <address><email>JeffMogul@acm.org</email></address>
2971      </author>
2972      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2973         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2974         <address><email>henrikn@microsoft.com</email></address>
2975      </author>
2976      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2977         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2978         <address><email>LMM@acm.org</email></address>
2979      </author>
2980      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2981         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2982         <address><email>paulle@microsoft.com</email></address>
2983      </author>
2984      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2985         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2986         <address><email>timbl@w3.org</email></address>
2987      </author>
2988      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2989         <organization abbrev="W3C">World Wide Web Consortium</organization>
2990         <address><email>ylafon@w3.org</email></address>
2991      </author>
2992      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2993         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2994         <address><email>julian.reschke@greenbytes.de</email></address>
2995      </author>
2996      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2997   </front>
2998   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2999   <x:source href="p3-payload.xml" basename="p3-payload"/>
3000</reference>
3001
3002<reference anchor="Part6">
3003   <front>
3004      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3005      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3006         <organization abbrev="Day Software">Day Software</organization>
3007         <address><email>fielding@gbiv.com</email></address>
3008      </author>
3009      <author initials="J." surname="Gettys" fullname="Jim Gettys">
3010         <organization>One Laptop per Child</organization>
3011         <address><email>jg@laptop.org</email></address>
3012      </author>
3013      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3014         <organization abbrev="HP">Hewlett-Packard Company</organization>
3015         <address><email>JeffMogul@acm.org</email></address>
3016      </author>
3017      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3018         <organization abbrev="Microsoft">Microsoft Corporation</organization>
3019         <address><email>henrikn@microsoft.com</email></address>
3020      </author>
3021      <author initials="L." surname="Masinter" fullname="Larry Masinter">
3022         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3023         <address><email>LMM@acm.org</email></address>
3024      </author>
3025      <author initials="P." surname="Leach" fullname="Paul J. Leach">
3026         <organization abbrev="Microsoft">Microsoft Corporation</organization>
3027         <address><email>paulle@microsoft.com</email></address>
3028      </author>
3029      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3030         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3031         <address><email>timbl@w3.org</email></address>
3032      </author>
3033      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3034         <organization abbrev="W3C">World Wide Web Consortium</organization>
3035         <address><email>ylafon@w3.org</email></address>
3036      </author>
3037      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3038         <organization abbrev="greenbytes">greenbytes GmbH</organization>
3039         <address><email>julian.reschke@greenbytes.de</email></address>
3040      </author>
3041      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3042   </front>
3043   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3044   <x:source href="p6-cache.xml" basename="p6-cache"/>
3045</reference>
3046
3047<reference anchor="RFC2616">
3048   <front>
3049      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3050      <author initials="R." surname="Fielding" fullname="R. Fielding">
3051         <organization>University of California, Irvine</organization>
3052         <address><email>fielding@ics.uci.edu</email></address>
3053      </author>
3054      <author initials="J." surname="Gettys" fullname="J. Gettys">
3055         <organization>W3C</organization>
3056         <address><email>jg@w3.org</email></address>
3057      </author>
3058      <author initials="J." surname="Mogul" fullname="J. Mogul">
3059         <organization>Compaq Computer Corporation</organization>
3060         <address><email>mogul@wrl.dec.com</email></address>
3061      </author>
3062      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3063         <organization>MIT Laboratory for Computer Science</organization>
3064         <address><email>frystyk@w3.org</email></address>
3065      </author>
3066      <author initials="L." surname="Masinter" fullname="L. Masinter">
3067         <organization>Xerox Corporation</organization>
3068         <address><email>masinter@parc.xerox.com</email></address>
3069      </author>
3070      <author initials="P." surname="Leach" fullname="P. Leach">
3071         <organization>Microsoft Corporation</organization>
3072         <address><email>paulle@microsoft.com</email></address>
3073      </author>
3074      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3075         <organization>W3C</organization>
3076         <address><email>timbl@w3.org</email></address>
3077      </author>
3078      <date month="June" year="1999"/>
3079   </front>
3080   <seriesInfo name="RFC" value="2616"/>
3081</reference>
3082
3083<reference anchor="RFC1436">
3084<front>
3085<title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3086<author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3087<organization>University of Minnesota, Computer and Information Services</organization>
3088<address>
3089<postal>
3090<street>100 Union Street SE</street>
3091<street>Room 152 Shepherd Labs</street>
3092<city>Minneapolis</city>
3093<region>MN</region>
3094<code>55455</code>
3095<country>US</country></postal>
3096<phone>+1 612 625 1300</phone>
3097<email>fxa@boombox.micro.umn.edu</email></address></author>
3098<author initials="M." surname="McCahill" fullname="Mark McCahill">
3099<organization>University of Minnesota, Computer and Information Services</organization>
3100<address>
3101<postal>
3102<street>100 Union Street SE</street>
3103<street>Room 152 Shepherd Labs</street>
3104<city>Minneapolis</city>
3105<region>MN</region>
3106<code>55455</code>
3107<country>US</country></postal>
3108<phone>+1 612 625 1300</phone>
3109<email>mpm@boombox.micro.umn.edu</email></address></author>
3110<author initials="P." surname="Lindner" fullname="Paul Lindner">
3111<organization>University of Minnesota, Computer and Information Services</organization>
3112<address>
3113<postal>
3114<street>100 Union Street SE</street>
3115<street>Room 152 Shepherd Labs</street>
3116<city>Minneapolis</city>
3117<region>MN</region>
3118<code>55455</code>
3119<country>US</country></postal>
3120<phone>+1 612 625 1300</phone>
3121<email>lindner@boombox.micro.umn.edu</email></address></author>
3122<author initials="D." surname="Johnson" fullname="David Johnson">
3123<organization>University of Minnesota, Computer and Information Services</organization>
3124<address>
3125<postal>
3126<street>100 Union Street SE</street>
3127<street>Room 152 Shepherd Labs</street>
3128<city>Minneapolis</city>
3129<region>MN</region>
3130<code>55455</code>
3131<country>US</country></postal>
3132<phone>+1 612 625 1300</phone>
3133<email>dmj@boombox.micro.umn.edu</email></address></author>
3134<author initials="D." surname="Torrey" fullname="Daniel Torrey">
3135<organization>University of Minnesota, Computer and Information Services</organization>
3136<address>
3137<postal>
3138<street>100 Union Street SE</street>
3139<street>Room 152 Shepherd Labs</street>
3140<city>Minneapolis</city>
3141<region>MN</region>
3142<code>55455</code>
3143<country>US</country></postal>
3144<phone>+1 612 625 1300</phone>
3145<email>daniel@boombox.micro.umn.edu</email></address></author>
3146<author initials="B." surname="Alberti" fullname="Bob Alberti">
3147<organization>University of Minnesota, Computer and Information Services</organization>
3148<address>
3149<postal>
3150<street>100 Union Street SE</street>
3151<street>Room 152 Shepherd Labs</street>
3152<city>Minneapolis</city>
3153<region>MN</region>
3154<code>55455</code>
3155<country>US</country></postal>
3156<phone>+1 612 625 1300</phone>
3157<email>alberti@boombox.micro.umn.edu</email></address></author>
3158<date month="March" year="1993"/>
3159</front>
3160<seriesInfo name="RFC" value="1436"/>
3161</reference>
3162
3163<reference anchor="RFC1630">
3164<front>
3165<title abbrev="URIs in WWW">Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web</title>
3166<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3167<organization>CERN, World-Wide Web project</organization>
3168<address>
3169<postal>
3170<street>1211 Geneva 23</street>
3171<city/>
3172<region/>
3173<code/>
3174<country>CH</country></postal>
3175<phone>+41 22 7673755</phone>
3176<facsimile>+41 22 7677155</facsimile>
3177<email>timbl@info.cern.ch</email></address></author>
3178<date month="June" year="1994"/></front>
3179<seriesInfo name="RFC" value="1630"/>
3180</reference>
3181
3182<reference anchor="RFC1738">
3183  <front>
3184    <title>Uniform Resource Locators (URL)</title>
3185    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3186      <organization>CERN, World-Wide Web project</organization>
3187      <address><email>timbl@info.cern.ch</email></address>
3188    </author>
3189    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3190      <organization>Xerox PARC</organization>
3191      <address><email>masinter@parc.xerox.com</email></address>
3192    </author>
3193    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3194      <organization>University of Minnesota, Computer and Information Services</organization>
3195      <address><email>mpm@boombox.micro.umn.edu</email></address>
3196    </author>
3197    <date month="December" year="1994"/>
3198  </front>
3199  <seriesInfo name="RFC" value="1738"/>
3200</reference>
3201
3202<reference anchor="RFC1945">
3203  <front>
3204    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3205    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3206      <organization>MIT, Laboratory for Computer Science</organization>
3207      <address><email>timbl@w3.org</email></address>
3208    </author>
3209    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3210      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3211      <address><email>fielding@ics.uci.edu</email></address>
3212    </author>
3213    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3214      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3215      <address><email>frystyk@w3.org</email></address>
3216    </author>
3217    <date month="May" year="1996"/>
3218  </front>
3219  <seriesInfo name="RFC" value="1945"/>
3220</reference>
3221
3222<reference anchor="RFC2045">
3223  <front>
3224    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3225    <author initials="N." surname="Freed" fullname="Ned Freed">
3226      <organization>Innosoft International, Inc.</organization>
3227      <address><email>ned@innosoft.com</email></address>
3228    </author>
3229    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3230      <organization>First Virtual Holdings</organization>
3231      <address><email>nsb@nsb.fv.com</email></address>
3232    </author>
3233    <date month="November" year="1996"/>
3234  </front>
3235  <seriesInfo name="RFC" value="2045"/>
3236</reference>
3237
3238<reference anchor="RFC1123">
3239  <front>
3240    <title>Requirements for Internet Hosts - Application and Support</title>
3241    <author initials="R." surname="Braden" fullname="Robert Braden">
3242      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3243      <address><email>Braden@ISI.EDU</email></address>
3244    </author>
3245    <date month="October" year="1989"/>
3246  </front>
3247  <seriesInfo name="STD" value="3"/>
3248  <seriesInfo name="RFC" value="1123"/>
3249</reference>
3250
3251<reference anchor="RFC822">
3252  <front>
3253    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3254    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3255      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3256      <address><email>DCrocker@UDel-Relay</email></address>
3257    </author>
3258    <date month="August" day="13" year="1982"/>
3259  </front>
3260  <seriesInfo name="STD" value="11"/>
3261  <seriesInfo name="RFC" value="822"/>
3262</reference>
3263
3264<reference anchor="WAIS">
3265<front>
3266<title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3267<author initials="F." surname="Davis" fullname="F. Davis">
3268<organization>Thinking Machines Corporation</organization>
3269</author>
3270<author initials="B." surname="Kahle" fullname="B. Kahle">
3271<organization>Thinking Machines Corporation</organization>
3272</author>
3273<author initials="H." surname="Morris" fullname="H. Morris">
3274<organization>Thinking Machines Corporation</organization>
3275</author>
3276<author initials="J." surname="Salem" fullname="J. Salem">
3277<organization>Thinking Machines Corporation</organization>
3278</author>
3279<author initials="T." surname="Shen" fullname="T. Shen">
3280<organization>Thinking Machines Corporation</organization>
3281</author>
3282<author initials="R." surname="Wang" fullname="R. Wang">
3283<organization>Thinking Machines Corporation</organization>
3284</author>
3285<author initials="J." surname="Sui" fullname="J. Sui">
3286<organization>Thinking Machines Corporation</organization>
3287</author>
3288<author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3289<organization>Thinking Machines Corporation</organization>
3290</author>
3291<date month="April" year="1990"/></front>
3292<seriesInfo name="Thinking Machines Corporation" value=""/>
3293</reference>
3294
3295<reference anchor="RFC1808">
3296  <front>
3297    <title>Relative Uniform Resource Locators</title>
3298    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3299      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3300      <address><email>fielding@ics.uci.edu</email></address>
3301    </author>
3302    <date month="June" year="1995"/>
3303  </front>
3304  <seriesInfo name="RFC" value="1808"/>
3305</reference>
3306
3307<reference anchor='RFC3977'>
3308   <front>
3309      <title>Network News Transfer Protocol (NNTP)</title>
3310      <author initials='C.' surname='Feather' fullname='C. Feather'>
3311         <organization>THUS plc</organization>
3312         <address>
3313            <email>clive@demon.net</email>
3314         </address>
3315      </author>
3316      <date year='2006' month='October' />
3317   </front>
3318</reference>
3319
3320<reference anchor="RFC2047">
3321  <front>
3322    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3323    <author initials="K." surname="Moore" fullname="Keith Moore">
3324      <organization>University of Tennessee</organization>
3325      <address><email>moore@cs.utk.edu</email></address>
3326    </author>
3327    <date month="November" year="1996"/>
3328  </front>
3329  <seriesInfo name="RFC" value="2047"/>
3330</reference>
3331
3332<reference anchor="RFC821">
3333<front>
3334<title>Simple Mail Transfer Protocol</title>
3335<author initials="J.B." surname="Postel" fullname="Jonathan B. Postel">
3336<organization>University of Southern California (USC)/Information Sciences Institute</organization>
3337<address>
3338<postal>
3339<street>4676 Admiralty Way</street>
3340<city>Marina del Rey</city>
3341<region>CA</region>
3342<code>90291</code>
3343<country>US</country></postal>
3344<phone>+1 213 822 1511</phone></address></author>
3345<date month="August" year="1982"/></front>
3346<seriesInfo name="STD" value="10"/>
3347<seriesInfo name="RFC" value="821"/>
3348</reference>
3349
3350<reference anchor="RFC4288">
3351  <front>
3352    <title>Media Type Specifications and Registration Procedures</title>
3353    <author initials="N." surname="Freed" fullname="N. Freed">
3354      <organization>Sun Microsystems</organization>
3355      <address>
3356        <email>ned.freed@mrochek.com</email>
3357      </address>
3358    </author>
3359    <author initials="J." surname="Klensin" fullname="J. Klensin">
3360      <organization/>
3361      <address>
3362        <email>klensin+ietf@jck.com</email>
3363      </address>
3364    </author>
3365    <date year="2005" month="December"/>
3366  </front>
3367  <seriesInfo name="BCP" value="13"/>
3368  <seriesInfo name="RFC" value="4288"/>
3369</reference>
3370
3371<reference anchor="RFC959">
3372<front>
3373<title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3374<author initials="J." surname="Postel" fullname="J. Postel">
3375<organization>Information Sciences Institute (ISI)</organization></author>
3376<author initials="J." surname="Reynolds" fullname="J. Reynolds">
3377<organization/></author>
3378<date month="October" year="1985"/></front>
3379<seriesInfo name="STD" value="9"/>
3380<seriesInfo name="RFC" value="959"/>
3381</reference>
3382
3383<reference anchor="RFC1737">
3384<front>
3385<title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3386<author initials="L." surname="Masinter" fullname="Larry Masinter">
3387<organization>Xerox Palo Alto Research Center</organization>
3388<address>
3389<postal>
3390<street>3333 Coyote Hill Road</street>
3391<city>Palo Alto</city>
3392<region>CA</region>
3393<code>94304</code>
3394<country>US</country></postal>
3395<phone>+1 415 812 4365</phone>
3396<facsimile>+1 415 812 4333</facsimile>
3397<email>masinter@parc.xerox.com</email></address></author>
3398<author initials="K." surname="Sollins" fullname="Karen Sollins">
3399<organization>MIT Laboratory for Computer Science</organization>
3400<address>
3401<postal>
3402<street>545 Technology Square</street>
3403<city>Cambridge</city>
3404<region>MA</region>
3405<code>02139</code>
3406<country>US</country></postal>
3407<phone>+1 617 253 2673</phone>
3408<email>sollins@lcs.mit.edu</email></address></author>
3409<date month="December" year="1994"/></front>
3410<seriesInfo name="RFC" value="1737"/>
3411</reference>
3412
3413<reference anchor="USASCII">
3414  <front>
3415    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3416    <author>
3417      <organization>American National Standards Institute</organization>
3418    </author>
3419    <date year="1986"/>
3420  </front>
3421  <seriesInfo name="ANSI" value="X3.4"/>
3422</reference>
3423
3424<reference anchor="ISO-8859">
3425<front>
3426<title>Information technology - 8-bit single byte coded graphic - character sets</title>
3427<author>
3428<organization>International Organization for Standardization</organization>
3429</author>
3430<date year="1987-1990"/>
3431</front>
3432<annotation>
3433  Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
3434  Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
3435  Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
3436  Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
3437  Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
3438  Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
3439  Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
3440  Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
3441  Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
3442</annotation>
3443</reference>
3444
3445<reference anchor="RFC1900">
3446  <front>
3447    <title>Renumbering Needs Work</title>
3448    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3449      <organization>CERN, Computing and Networks Division</organization>
3450      <address><email>brian@dxcoms.cern.ch</email></address>
3451    </author>
3452    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3453      <organization>cisco Systems</organization>
3454      <address><email>yakov@cisco.com</email></address>
3455    </author>
3456    <date month="February" year="1996"/>
3457  </front>
3458  <seriesInfo name="RFC" value="1900"/>
3459</reference>
3460
3461<reference anchor="Pad1995">
3462<front>
3463<title>Improving HTTP Latency</title>
3464<author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan"><organization/></author>
3465<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul"><organization/></author>
3466<date year="1995" month="Dec"/>
3467</front>
3468<seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3469<annotation>
3470  Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3471  which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3472</annotation>
3473</reference>
3474
3475<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3476<front>
3477<title>Analysis of HTTP Performance</title>
3478<author initials="J." surname="Touch" fullname="Joe Touch">
3479  <organization>USC/Information Sciences Institute</organization>
3480  <address><email>touch@isi.edu</email></address>
3481</author>
3482<author initials="J." surname="Heidemann" fullname="John Heidemann">
3483  <organization>USC/Information Sciences Institute</organization>
3484  <address><email>johnh@isi.edu</email></address>
3485</author>
3486<author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3487  <organization>USC/Information Sciences Institute</organization>
3488  <address><email>katia@isi.edu</email></address>
3489</author>
3490<date year="1998" month="Aug"/>
3491</front>
3492<seriesInfo name="ISI Research Report" value="ISI/RR-98-463 (original report dated Aug.1996)"/>
3493</reference>
3494
3495<reference anchor="RFC1305">
3496<front>
3497<title>Network Time Protocol (Version 3) Specification, Implementation</title>
3498<author initials="D." surname="Mills" fullname="David L. Mills">
3499<organization>University of Delaware, Electrical Engineering Department</organization>
3500<address>
3501<postal>
3502<street/>
3503<city>Newark</city>
3504<region>DE</region>
3505<code>19716</code>
3506<country>US</country></postal>
3507<phone>+1 302 451 8247</phone>
3508<email>mills@udel.edu</email></address></author>
3509<date month="March" year="1992"/>
3510<abstract>
3511<t>This document describes the Network Time Protocol (NTP), specifies its normal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons.</t></abstract></front>
3512<seriesInfo name="RFC" value="1305"/>
3513</reference>
3514
3515<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3516<front>
3517<title>Analysis of HTTP Performance Problems</title>
3518<author initials="S." surname="Spero" fullname="Simon E. Spero">
3519  <organization/>
3520</author>
3521<date/>
3522</front>
3523</reference>
3524
3525<reference anchor="RFC2068">
3526  <front>
3527    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3528    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3529      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3530      <address><email>fielding@ics.uci.edu</email></address>
3531    </author>
3532    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3533      <organization>MIT Laboratory for Computer Science</organization>
3534      <address><email>jg@w3.org</email></address>
3535    </author>
3536    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3537      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3538      <address><email>mogul@wrl.dec.com</email></address>
3539    </author>
3540    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3541      <organization>MIT Laboratory for Computer Science</organization>
3542      <address><email>frystyk@w3.org</email></address>
3543    </author>
3544    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3545      <organization>MIT Laboratory for Computer Science</organization>
3546      <address><email>timbl@w3.org</email></address>
3547    </author>
3548    <date month="January" year="1997"/>
3549  </front>
3550  <seriesInfo name="RFC" value="2068"/>
3551</reference>
3552
3553<reference anchor="RFC2119">
3554  <front>
3555    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3556    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3557      <organization>Harvard University</organization>
3558      <address><email>sob@harvard.edu</email></address>
3559    </author>
3560    <date month="March" year="1997"/>
3561  </front>
3562  <seriesInfo name="BCP" value="14"/>
3563  <seriesInfo name="RFC" value="2119"/>
3564</reference>
3565
3566<reference anchor="RFC2145">
3567  <front>
3568    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3569    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3570      <organization>Western Research Laboratory</organization>
3571      <address><email>mogul@wrl.dec.com</email></address>
3572    </author>
3573    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3574      <organization>Department of Information and Computer Science</organization>
3575      <address><email>fielding@ics.uci.edu</email></address>
3576    </author>
3577    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3578      <organization>MIT Laboratory for Computer Science</organization>
3579      <address><email>jg@w3.org</email></address>
3580    </author>
3581    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3582      <organization>W3 Consortium</organization>
3583      <address><email>frystyk@w3.org</email></address>
3584    </author>
3585    <date month="May" year="1997"/>
3586  </front>
3587  <seriesInfo name="RFC" value="2145"/>
3588</reference>
3589
3590<reference anchor="Nie1997">
3591<front>
3592<title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3593<author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3594  <organization/>
3595</author>
3596<author initials="J." surname="Gettys" fullname="J. Gettys">
3597  <organization/>
3598</author>
3599<author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3600  <organization/>
3601</author>
3602<author initials="H." surname="Lie" fullname="H. Lie">
3603  <organization/>
3604</author>
3605<author initials="C." surname="Lilley" fullname="C. Lilley">
3606  <organization/>
3607</author>
3608<date year="1997" month="Sep"/>
3609</front>
3610<seriesInfo name="Proceedings of ACM SIGCOMM '97, Cannes France" value=""/>
3611</reference>
3612
3613<reference anchor="RFC2396">
3614  <front>
3615    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3616    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3617      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3618      <address><email>timbl@w3.org</email></address>
3619    </author>
3620    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3621      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3622      <address><email>fielding@ics.uci.edu</email></address>
3623    </author>
3624    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3625      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3626      <address><email>masinter@parc.xerox.com</email></address>
3627    </author>
3628    <date month="August" year="1998"/>
3629  </front>
3630  <seriesInfo name="RFC" value="2396"/>
3631</reference>
3632
3633<reference anchor="RFC2324">
3634<front>
3635<title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3636<author initials="L." surname="Masinter" fullname="Larry Masinter">
3637<organization>Xerox Palo Alto Research Center</organization>
3638<address>
3639<postal>
3640<street>3333 Coyote Hill Road</street>
3641<city>Palo Alto</city>
3642<region>CA</region>
3643<code>94304</code></postal>
3644<email>masinter@parc.xerox.com</email></address></author>
3645<date month="April" day="1" year="1998"/>
3646<area>General</area>
3647<keyword>control protocol</keyword>
3648<keyword>coffee</keyword>
3649</front>
3650<seriesInfo name="RFC" value="2324"/>
3651</reference>
3652
3653</references>
3654
3655<section title="Internet Media Type message/http and application/http" anchor="internet.media.type.http">
3656<iref item="Media Type" subitem="message/http" primary="true"/>
3657<iref item="message/http Media Type" primary="true"/>
3658<iref item="Media Type" subitem="application/http" primary="true"/>
3659<iref item="application/http Media Type" primary="true"/>
3660<t>
3661   In addition to defining the HTTP/1.1 protocol, this document serves
3662   as the specification for the Internet media type "message/http" and
3663   "application/http". The message/http type can be used to enclose a
3664   single HTTP request or response message, provided that it obeys the
3665   MIME restrictions for all "message" types regarding line length and
3666   encodings. The application/http type can be used to enclose a
3667   pipeline of one or more HTTP request or response messages (not
3668   intermixed). The following is to be registered with IANA <xref target="RFC4288"/>.
3669</t>
3670<t>
3671  <list style="hanging" x:indent="12em">
3672    <t hangText="Media Type name:">
3673      message
3674    </t>
3675    <t hangText="Media subtype name:">
3676      http
3677    </t>
3678    <t hangText="Required parameters:">
3679      none
3680    </t>
3681    <t hangText="Optional parameters:">
3682      version, msgtype
3683      <list style="hanging">
3684        <t hangText="version:">
3685          The HTTP-Version number of the enclosed message
3686          (e.g., "1.1"). If not present, the version can be
3687          determined from the first line of the body.
3688        </t>
3689        <t hangText="msgtype:">
3690          The message type -- "request" or "response". If not
3691          present, the type can be determined from the first
3692          line of the body.
3693        </t>
3694      </list>
3695    </t>
3696    <t hangText="Encoding considerations:">
3697      only "7bit", "8bit", or "binary" are permitted
3698    </t>
3699    <t hangText="Security considerations:">
3700      none
3701    </t>
3702  </list>
3703</t>
3704<t>
3705  <list style="hanging" x:indent="12em">
3706    <t hangText="Media Type name:">
3707      application
3708    </t>
3709    <t hangText="Media subtype name:">
3710      http
3711    </t>
3712    <t hangText="Required parameters:">
3713      none
3714    </t>
3715    <t hangText="Optional parameters:">
3716      version, msgtype
3717      <list style="hanging">
3718        <t hangText="version:">
3719          The HTTP-Version number of the enclosed messages
3720          (e.g., "1.1"). If not present, the version can be
3721          determined from the first line of the body.
3722        </t>
3723        <t hangText="msgtype:">
3724          The message type -- "request" or "response". If not
3725          present, the type can be determined from the first
3726          line of the body.
3727        </t>
3728      </list>
3729    </t>
3730    <t hangText="Encoding considerations:">
3731      HTTP messages enclosed by this type
3732      are in "binary" format; use of an appropriate
3733      Content-Transfer-Encoding is required when
3734      transmitted via E-mail.
3735    </t>
3736    <t hangText="Security considerations:">
3737      none
3738    </t>
3739  </list>
3740</t>
3741</section>
3742
3743<section title="Tolerant Applications" anchor="tolerant.applications">
3744<t>
3745   Although this document specifies the requirements for the generation
3746   of HTTP/1.1 messages, not all applications will be correct in their
3747   implementation. We therefore recommend that operational applications
3748   be tolerant of deviations whenever those deviations can be
3749   interpreted unambiguously.
3750</t>
3751<t>
3752   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3753   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3754   accept any amount of SP or HT characters between fields, even though
3755   only a single SP is required.
3756</t>
3757<t>
3758   The line terminator for message-header fields is the sequence CRLF.
3759   However, we recommend that applications, when parsing such headers,
3760   recognize a single LF as a line terminator and ignore the leading CR.
3761</t>
3762<t>
3763   The character set of an entity-body &SHOULD; be labeled as the lowest
3764   common denominator of the character codes used within that body, with
3765   the exception that not labeling the entity is preferred over labeling
3766   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3767</t>
3768<t>
3769   Additional rules for requirements on parsing and encoding of dates
3770   and other potential problems with date encodings include:
3771</t>
3772<t>
3773  <list style="symbols">
3774     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3775        which appears to be more than 50 years in the future is in fact
3776        in the past (this helps solve the "year 2000" problem).</t>
3777
3778     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3779        Expires date as earlier than the proper value, but &MUST-NOT;
3780        internally represent a parsed Expires date as later than the
3781        proper value.</t>
3782
3783     <t>All expiration-related calculations &MUST; be done in GMT. The
3784        local time zone &MUST-NOT; influence the calculation or comparison
3785        of an age or expiration time.</t>
3786
3787     <t>If an HTTP header incorrectly carries a date value with a time
3788        zone other than GMT, it &MUST; be converted into GMT using the
3789        most conservative possible conversion.</t>
3790  </list>
3791</t>
3792</section>
3793
3794<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3795<t>
3796   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3797   simplify the process of date comparison. Proxies and gateways from
3798   other protocols &SHOULD; ensure that any Date header field present in a
3799   message conforms to one of the HTTP/1.1 formats and rewrite the date
3800   if necessary.
3801</t>
3802</section>
3803
3804<section title="Compatibility with Previous Versions" anchor="compatibility">
3805<t>
3806   It is beyond the scope of a protocol specification to mandate
3807   compliance with previous versions. HTTP/1.1 was deliberately
3808   designed, however, to make supporting previous versions easy. It is
3809   worth noting that, at the time of composing this specification
3810   (1996), we would expect commercial HTTP/1.1 servers to:
3811  <list style="symbols">
3812     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3813        1.1 requests;</t>
3814
3815     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3816        1.1;</t>
3817
3818     <t>respond appropriately with a message in the same major version
3819        used by the client.</t>
3820  </list>
3821</t>
3822<t>
3823   And we would expect HTTP/1.1 clients to:
3824  <list style="symbols">
3825     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3826        responses;</t>
3827
3828     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3829        1.1.</t>
3830  </list>
3831</t>
3832<t>
3833   For most implementations of HTTP/1.0, each connection is established
3834   by the client prior to the request and closed by the server after
3835   sending the response. Some implementations implement the Keep-Alive
3836   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3837</t>
3838
3839<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3840<t>
3841   This section summarizes major differences between versions HTTP/1.0
3842   and HTTP/1.1.
3843</t>
3844
3845<section title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
3846<t>
3847   The requirements that clients and servers support the Host request-header,
3848   report an error if the Host request-header (<xref target="header.host"/>) is
3849   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3850   are among the most important changes defined by this
3851   specification.
3852</t>
3853<t>
3854   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
3855   addresses and servers; there was no other established mechanism for
3856   distinguishing the intended server of a request than the IP address
3857   to which that request was directed. The changes outlined above will
3858   allow the Internet, once older HTTP clients are no longer common, to
3859   support multiple Web sites from a single IP address, greatly
3860   simplifying large operational Web servers, where allocation of many
3861   IP addresses to a single host has created serious problems. The
3862   Internet will also be able to recover the IP addresses that have been
3863   allocated for the sole purpose of allowing special-purpose domain
3864   names to be used in root-level HTTP URLs. Given the rate of growth of
3865   the Web, and the number of servers already deployed, it is extremely
3866   important that all implementations of HTTP (including updates to
3867   existing HTTP/1.0 applications) correctly implement these
3868   requirements:
3869  <list style="symbols">
3870     <t>Both clients and servers &MUST; support the Host request-header.</t>
3871
3872     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
3873
3874     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
3875        request does not include a Host request-header.</t>
3876
3877     <t>Servers &MUST; accept absolute URIs.</t>
3878  </list>
3879</t>
3880</section>
3881</section>
3882
3883<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
3884<t>
3885   Some clients and servers might wish to be compatible with some
3886   previous implementations of persistent connections in HTTP/1.0
3887   clients and servers. Persistent connections in HTTP/1.0 are
3888   explicitly negotiated as they are not the default behavior. HTTP/1.0
3889   experimental implementations of persistent connections are faulty,
3890   and the new facilities in HTTP/1.1 are designed to rectify these
3891   problems. The problem was that some existing 1.0 clients may be
3892   sending Keep-Alive to a proxy server that doesn't understand
3893   Connection, which would then erroneously forward it to the next
3894   inbound server, which would establish the Keep-Alive connection and
3895   result in a hung HTTP/1.0 proxy waiting for the close on the
3896   response. The result is that HTTP/1.0 clients must be prevented from
3897   using Keep-Alive when talking to proxies.
3898</t>
3899<t>
3900   However, talking to proxies is the most important use of persistent
3901   connections, so that prohibition is clearly unacceptable. Therefore,
3902   we need some other mechanism for indicating a persistent connection
3903   is desired, which is safe to use even when talking to an old proxy
3904   that ignores Connection. Persistent connections are the default for
3905   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
3906   declaring non-persistence. See <xref target="header.connection"/>.
3907</t>
3908<t>
3909   The original HTTP/1.0 form of persistent connections (the Connection:
3910   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
3911</t>
3912</section>
3913
3914<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
3915<t>
3916   This specification has been carefully audited to correct and
3917   disambiguate key word usage; RFC 2068 had many problems in respect to
3918   the conventions laid out in <xref target="RFC2119"/>.
3919</t>
3920<t>
3921   Transfer-coding and message lengths all interact in ways that
3922   required fixing exactly when chunked encoding is used (to allow for
3923   transfer encoding that may not be self delimiting); it was important
3924   to straighten out exactly how message lengths are computed.
3925</t>
3926<t>
3927   The use and interpretation of HTTP version numbers has been clarified
3928   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
3929   version they support to deal with problems discovered in HTTP/1.0
3930   implementations (<xref target="http.version"/>)
3931</t>
3932<t>
3933   Proxies should be able to add Content-Length when appropriate.
3934</t>
3935<t>
3936   Transfer-coding had significant problems, particularly with
3937   interactions with chunked encoding. The solution is that transfer-codings
3938   become as full fledged as content-codings. This involves
3939   adding an IANA registry for transfer-codings (separate from content
3940   codings), a new header field (TE) and enabling trailer headers in the
3941   future. Transfer encoding is a major performance benefit, so it was
3942   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
3943   interoperability problem that could have occurred due to interactions
3944   between authentication trailers, chunked encoding and HTTP/1.0
3945   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
3946   and <xref target="header.te" format="counter"/>)
3947</t>
3948</section>
3949
3950<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
3951<t>
3952  Clarify that HTTP-Version is case sensitive.
3953  (<xref target="http.version"/>)
3954</t>
3955<t>
3956  Remove reference to non-existant identity transfer-coding value tokens.
3957  (Sections <xref format="counter" target="transfer.codings"/> and
3958  <xref format="counter" target="message.length"/>)
3959</t>
3960<t>
3961  Clarification that the chunk length does not include
3962  the count of the octets in the chunk header and trailer.
3963  (<xref target="chunked.transfer.encoding"/>)
3964</t>
3965<t>
3966  Fix BNF to add query, as the abs_path production in
3967  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
3968  (<xref target="request-uri"/>)
3969</t>
3970<t>
3971  Clarify exactly when close connection options must be sent.
3972  (<xref target="header.connection"/>)
3973</t>
3974</section>
3975</section>
3976
3977<section title="Change Log (to be removed by RFC Editor before publication)">
3978
3979<section title="Since RFC2616">
3980<t>
3981  Extracted relevant partitions from <xref target="RFC2616"/>.
3982</t>
3983</section>
3984
3985<section title="Since draft-ietf-httpbis-p1-messaging-00">
3986<t>
3987  TBD.
3988</t>
3989</section>
3990
3991</section>
3992
3993</back>
3994</rfc>
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