source: draft-ietf-httpbis/01/p1-messaging.xml @ 875

Last change on this file since 875 was 165, checked in by fielding@…, 12 years ago

set date and version for draft 01

  • Property svn:eol-style set to native
File size: 176.7 KB
Line 
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 "01">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2008">
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="2616" category="std"
50     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
51     xmlns:x='http://purl.org/net/xml2rfc/ext'>
52<front>
53
54  <title abbrev="HTTP/1.1, Part 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;" day="12"/>
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  <t>
219    This draft incorporates those issue resolutions that were either
220    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
221    or which were agreed upon on the mailing list between October 2006 and
222    November 2007 (as published in "draft-lafon-rfc2616bis-03").
223  </t>
224</note>
225</front>
226<middle>
227<section title="Introduction" anchor="introduction">
228<t>
229   The Hypertext Transfer Protocol (HTTP) is an application-level
230   protocol for distributed, collaborative, hypermedia information
231   systems. HTTP has been in use by the World-Wide Web global
232   information initiative since 1990. The first version of HTTP, commonly
233   referred to as HTTP/0.9, was a simple protocol for raw data transfer
234   across the Internet with only a single method and no metadata.
235   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 metadata about the data transferred and
238   modifiers on the request/response semantics. However, HTTP/1.0 did
239   not sufficiently take into consideration the effects of hierarchical
240   proxies, caching, the need for persistent connections, or name-based
241   virtual hosts. In addition, the proliferation of incompletely-implemented
242   applications calling themselves "HTTP/1.0" 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 document is Part 1 of the seven-part specification that defines
248   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
249   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
250   requirements that enable reliable implementations and adding only
251   those new features that will either be safely ignored by an HTTP/1.0
252   recipient or only sent when communicating with a party advertising
253   compliance with HTTP/1.1.
254   Part 1 defines those aspects of HTTP/1.1 related to overall network
255   operation, message framing, interaction with transport protocols, and
256   URI schemes.
257</t>
258<t>
259   This document is currently disorganized in order to minimize the changes
260   between drafts and enable reviewers to see the smaller errata changes.
261   The next draft will reorganize the sections to better reflect the content.
262   In particular, the sections will be organized according to the typical
263   process of deciding when to use HTTP (URI schemes), overall network operation,
264   connection management, message framing, and generic message parsing.
265   The current mess reflects how widely dispersed these topics and associated
266   requirements had become in <xref target="RFC2616"/>.
267</t>
268
269<section title="Purpose" anchor="intro.purpose">
270<t>
271   Practical information systems require more functionality than simple
272   retrieval, including search, front-end update, and annotation. HTTP
273   allows an open-ended set of methods and headers that indicate the
274   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
275   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
276   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
277   method is to be applied. Messages are passed in a format similar to
278   that used by Internet mail <xref target="RFC2822"/> as defined by the Multipurpose
279   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
280</t>
281<t>
282   HTTP is also used as a generic protocol for communication between
283   user agents and proxies/gateways to other Internet systems, including
284   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
285   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
286   access to resources available from diverse applications.
287</t>
288</section>
289
290<section title="Requirements" anchor="intro.requirements">
291<t>
292   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
293   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
294   document are to be interpreted as described in <xref target="RFC2119"/>.
295</t>
296<t>
297   An implementation is not compliant if it fails to satisfy one or more
298   of the &MUST; or &REQUIRED; level requirements for the protocols it
299   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
300   level and all the &SHOULD; level requirements for its protocols is said
301   to be "unconditionally compliant"; one that satisfies all the &MUST;
302   level requirements but not all the &SHOULD; level requirements for its
303   protocols is said to be "conditionally compliant."
304</t>
305</section>
306
307<section title="Terminology" anchor="intro.terminology">
308<t>
309   This specification uses a number of terms to refer to the roles
310   played by participants in, and objects of, the HTTP communication.
311</t>
312<t>
313  <iref item="connection"/>
314  <x:dfn>connection</x:dfn>
315  <list>
316    <t>
317      A transport layer virtual circuit established between two programs
318      for the purpose of communication.
319    </t>
320  </list>
321</t>
322<t>
323  <iref item="message"/>
324  <x:dfn>message</x:dfn>
325  <list>
326    <t>
327      The basic unit of HTTP communication, consisting of a structured
328      sequence of octets matching the syntax defined in <xref target="http.message"/> and
329      transmitted via the connection.
330    </t>
331  </list>
332</t>
333<t>
334  <iref item="request"/>
335  <x:dfn>request</x:dfn>
336  <list>
337    <t>
338      An HTTP request message, as defined in <xref target="request"/>.
339    </t>
340  </list>
341</t>
342<t>
343  <iref item="response"/>
344  <x:dfn>response</x:dfn>
345  <list>
346    <t>
347      An HTTP response message, as defined in <xref target="response"/>.
348    </t>
349  </list>
350</t>
351<t>
352  <iref item="resource"/>
353  <x:dfn>resource</x:dfn>
354  <list>
355    <t>
356      A network data object or service that can be identified by a URI,
357      as defined in <xref target="uri"/>. Resources may be available in multiple
358      representations (e.g. multiple languages, data formats, size, and
359      resolutions) or vary in other ways.
360    </t>
361  </list>
362</t>
363<t>
364  <iref item="entity"/>
365  <x:dfn>entity</x:dfn>
366  <list>
367    <t>
368      The information transferred as the payload of a request or
369      response. An entity consists of metainformation in the form of
370      entity-header fields and content in the form of an entity-body, as
371      described in &entity;.
372    </t>
373  </list>
374</t>
375<t>
376  <iref item="representation"/>
377  <x:dfn>representation</x:dfn>
378  <list>
379    <t>
380      An entity included with a response that is subject to content
381      negotiation, as described in &content.negotiation;. There may exist multiple
382      representations associated with a particular response status.
383    </t>
384  </list>
385</t>
386<t>
387  <iref item="content negotiation"/>
388  <x:dfn>content negotiation</x:dfn>
389  <list>
390    <t>
391      The mechanism for selecting the appropriate representation when
392      servicing a request, as described in &content.negotiation;. The
393      representation of entities in any response can be negotiated
394      (including error responses).
395    </t>
396  </list>
397</t>
398<t>
399  <iref item="variant"/>
400  <x:dfn>variant</x:dfn>
401  <list>
402    <t>
403      A resource may have one, or more than one, representation(s)
404      associated with it at any given instant. Each of these
405      representations is termed a `variant'.  Use of the term `variant'
406      does not necessarily imply that the resource is subject to content
407      negotiation.
408    </t>
409  </list>
410</t>
411<t>
412  <iref item="client"/>
413  <x:dfn>client</x:dfn>
414  <list>
415    <t>
416      A program that establishes connections for the purpose of sending
417      requests.
418    </t>
419  </list>
420</t>
421<t>
422  <iref item="user agent"/>
423  <x:dfn>user agent</x:dfn>
424  <list>
425    <t>
426      The client which initiates a request. These are often browsers,
427      editors, spiders (web-traversing robots), or other end user tools.
428    </t>
429  </list>
430</t>
431<t>
432  <iref item="server"/>
433  <x:dfn>server</x:dfn>
434  <list>
435    <t>
436      An application program that accepts connections in order to
437      service requests by sending back responses. Any given program may
438      be capable of being both a client and a server; our use of these
439      terms refers only to the role being performed by the program for a
440      particular connection, rather than to the program's capabilities
441      in general. Likewise, any server may act as an origin server,
442      proxy, gateway, or tunnel, switching behavior based on the nature
443      of each request.
444    </t>
445  </list>
446</t>
447<t>
448  <iref item="origin server"/>
449  <x:dfn>origin server</x:dfn>
450  <list>
451    <t>
452      The server on which a given resource resides or is to be created.
453    </t>
454  </list>
455</t>
456<t>
457  <iref item="proxy"/>
458  <x:dfn>proxy</x:dfn>
459  <list>
460    <t>
461      An intermediary program which acts as both a server and a client
462      for the purpose of making requests on behalf of other clients.
463      Requests are serviced internally or by passing them on, with
464      possible translation, to other servers. A proxy &MUST; implement
465      both the client and server requirements of this specification. A
466      "transparent proxy" is a proxy that does not modify the request or
467      response beyond what is required for proxy authentication and
468      identification. A "non-transparent proxy" is a proxy that modifies
469      the request or response in order to provide some added service to
470      the user agent, such as group annotation services, media type
471      transformation, protocol reduction, or anonymity filtering. Except
472      where either transparent or non-transparent behavior is explicitly
473      stated, the HTTP proxy requirements apply to both types of
474      proxies.
475    </t>
476  </list>
477</t>
478<t>
479  <iref item="gateway"/>
480  <x:dfn>gateway</x:dfn>
481  <list>
482    <t>
483      A server which acts as an intermediary for some other server.
484      Unlike a proxy, a gateway receives requests as if it were the
485      origin server for the requested resource; the requesting client
486      may not be aware that it is communicating with a gateway.
487    </t>
488  </list>
489</t>
490<t>
491  <iref item="tunnel"/>
492  <x:dfn>tunnel</x:dfn>
493  <list>
494    <t>
495      An intermediary program which is acting as a blind relay between
496      two connections. Once active, a tunnel is not considered a party
497      to the HTTP communication, though the tunnel may have been
498      initiated by an HTTP request. The tunnel ceases to exist when both
499      ends of the relayed connections are closed.
500    </t>
501  </list>
502</t>
503<t>
504  <iref item="cache"/>
505  <x:dfn>cache</x:dfn>
506  <list>
507    <t>
508      A program's local store of response messages and the subsystem
509      that controls its message storage, retrieval, and deletion. A
510      cache stores cacheable responses in order to reduce the response
511      time and network bandwidth consumption on future, equivalent
512      requests. Any client or server may include a cache, though a cache
513      cannot be used by a server that is acting as a tunnel.
514    </t>
515  </list>
516</t>
517<t>
518  <iref item="cacheable"/>
519  <x:dfn>cacheable</x:dfn>
520  <list>
521    <t>
522      A response is cacheable if a cache is allowed to store a copy of
523      the response message for use in answering subsequent requests. The
524      rules for determining the cacheability of HTTP responses are
525      defined in &caching;. Even if a resource is cacheable, there may
526      be additional constraints on whether a cache can use the cached
527      copy for a particular request.
528    </t>
529  </list>
530</t>
531<t>
532  <iref item="upstream"/>
533  <iref item="downstream"/>
534  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
535  <list>
536    <t>
537      Upstream and downstream describe the flow of a message: all
538      messages flow from upstream to downstream.
539    </t>
540  </list>
541</t>
542<t>
543  <iref item="inbound"/>
544  <iref item="outbound"/>
545  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
546  <list>
547    <t>
548      Inbound and outbound refer to the request and response paths for
549      messages: "inbound" means "traveling toward the origin server",
550      and "outbound" means "traveling toward the user agent"
551    </t>
552  </list>
553</t>
554</section>
555
556<section title="Overall Operation" anchor="intro.overall.operation">
557<t>
558   The HTTP protocol is a request/response protocol. A client sends a
559   request to the server in the form of a request method, URI, and
560   protocol version, followed by a MIME-like message containing request
561   modifiers, client information, and possible body content over a
562   connection with a server. The server responds with a status line,
563   including the message's protocol version and a success or error code,
564   followed by a MIME-like message containing server information, entity
565   metainformation, and possible entity-body content. The relationship
566   between HTTP and MIME is described in &diff2045entity;.
567</t>
568<t>
569   Most HTTP communication is initiated by a user agent and consists of
570   a request to be applied to a resource on some origin server. In the
571   simplest case, this may be accomplished via a single connection (v)
572   between the user agent (UA) and the origin server (O).
573</t>
574<figure><artwork type="drawing">
575       request chain ------------------------&gt;
576    UA -------------------v------------------- O
577       &lt;----------------------- response chain
578</artwork></figure>
579<t>
580   A more complicated situation occurs when one or more intermediaries
581   are present in the request/response chain. There are three common
582   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
583   forwarding agent, receiving requests for a URI in its absolute form,
584   rewriting all or part of the message, and forwarding the reformatted
585   request toward the server identified by the URI. A gateway is a
586   receiving agent, acting as a layer above some other server(s) and, if
587   necessary, translating the requests to the underlying server's
588   protocol. A tunnel acts as a relay point between two connections
589   without changing the messages; tunnels are used when the
590   communication needs to pass through an intermediary (such as a
591   firewall) even when the intermediary cannot understand the contents
592   of the messages.
593</t>
594<figure><artwork type="drawing">
595       request chain --------------------------------------&gt;
596    UA -----v----- A -----v----- B -----v----- C -----v----- O
597       &lt;------------------------------------- response chain
598</artwork></figure>
599<t>
600   The figure above shows three intermediaries (A, B, and C) between the
601   user agent and origin server. A request or response message that
602   travels the whole chain will pass through four separate connections.
603   This distinction is important because some HTTP communication options
604   may apply only to the connection with the nearest, non-tunnel
605   neighbor, only to the end-points of the chain, or to all connections
606   along the chain. Although the diagram is linear, each participant may
607   be engaged in multiple, simultaneous communications. For example, B
608   may be receiving requests from many clients other than A, and/or
609   forwarding requests to servers other than C, at the same time that it
610   is handling A's request.
611</t>
612<t>
613   Any party to the communication which is not acting as a tunnel may
614   employ an internal cache for handling requests. The effect of a cache
615   is that the request/response chain is shortened if one of the
616   participants along the chain has a cached response applicable to that
617   request. The following illustrates the resulting chain if B has a
618   cached copy of an earlier response from O (via C) for a request which
619   has not been cached by UA or A.
620</t>
621<figure><artwork type="drawing">
622          request chain ----------&gt;
623       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
624          &lt;--------- response chain
625</artwork></figure>
626<t>
627   Not all responses are usefully cacheable, and some requests may
628   contain modifiers which place special requirements on cache behavior.
629   HTTP requirements for cache behavior and cacheable responses are
630   defined in &caching;.
631</t>
632<t>
633   In fact, there are a wide variety of architectures and configurations
634   of caches and proxies currently being experimented with or deployed
635   across the World Wide Web. These systems include national hierarchies
636   of proxy caches to save transoceanic bandwidth, systems that
637   broadcast or multicast cache entries, organizations that distribute
638   subsets of cached data via CD-ROM, and so on. HTTP systems are used
639   in corporate intranets over high-bandwidth links, and for access via
640   PDAs with low-power radio links and intermittent connectivity. The
641   goal of HTTP/1.1 is to support the wide diversity of configurations
642   already deployed while introducing protocol constructs that meet the
643   needs of those who build web applications that require high
644   reliability and, failing that, at least reliable indications of
645   failure.
646</t>
647<t>
648   HTTP communication usually takes place over TCP/IP connections. The
649   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
650   not preclude HTTP from being implemented on top of any other protocol
651   on the Internet, or on other networks. HTTP only presumes a reliable
652   transport; any protocol that provides such guarantees can be used;
653   the mapping of the HTTP/1.1 request and response structures onto the
654   transport data units of the protocol in question is outside the scope
655   of this specification.
656</t>
657<t>
658   In HTTP/1.0, most implementations used a new connection for each
659   request/response exchange. In HTTP/1.1, a connection may be used for
660   one or more request/response exchanges, although connections may be
661   closed for a variety of reasons (see <xref target="persistent.connections"/>).
662</t>
663</section>
664</section>
665
666<section title="Notational Conventions and Generic Grammar" anchor="notation">
667
668<section title="Augmented BNF" anchor="notation.abnf">
669<t>
670   All of the mechanisms specified in this document are described in
671   both prose and an augmented Backus-Naur Form (BNF) similar to that
672   used by <xref target="RFC822ABNF"/>. Implementors will need to be familiar with the
673   notation in order to understand this specification. The augmented BNF
674   includes the following constructs:
675</t>
676<t>
677   name = definition
678  <list>
679    <t>
680      The name of a rule is simply the name itself (without any
681      enclosing "&lt;" and "&gt;") and is separated from its definition by the
682      equal "=" character. White space is only significant in that
683      indentation of continuation lines is used to indicate a rule
684      definition that spans more than one line. Certain basic rules are
685      in uppercase, such as SP, LWS, HTAB, CRLF, DIGIT, ALPHA, etc. Angle
686      brackets are used within definitions whenever their presence will
687      facilitate discerning the use of rule names.
688    </t>
689  </list>
690</t>
691<t>
692   "literal"
693  <list>
694    <t>
695      Quotation marks surround literal text. Unless stated otherwise,
696      the text is case-insensitive.
697    </t>
698  </list>
699</t>
700<t>
701   rule1 | rule2
702  <list>
703    <t>
704      Elements separated by a bar ("|") are alternatives, e.g., "yes |
705      no" will accept yes or no.
706    </t>
707  </list>
708</t>
709<t>
710   (rule1 rule2)
711  <list>
712    <t>
713      Elements enclosed in parentheses are treated as a single element.
714      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
715      foo elem" and "elem bar elem".
716    </t>
717  </list>
718</t>
719<t>
720   *rule
721  <list>
722    <t>
723      The character "*" preceding an element indicates repetition. The
724      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
725      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
726      that "*(element)" allows any number, including zero; "1*element"
727      requires at least one; and "1*2element" allows one or two.
728    </t>
729  </list>
730</t>
731<t>
732   [rule]
733  <list>
734    <t>
735      Square brackets enclose optional elements; "[foo bar]" is
736      equivalent to "*1(foo bar)".
737    </t>
738  </list>
739</t>
740<t>
741   N rule
742  <list>
743    <t>
744      Specific repetition: "&lt;n&gt;(element)" is equivalent to
745      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
746      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
747      alphabetic characters.
748    </t>
749  </list>
750</t>
751<t>
752   #rule
753  <list>
754    <t>
755      A construct "#" is defined, similar to "*", for defining lists of
756      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
757      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
758      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
759      form of lists very easy; a rule such as
760    </t>
761    <t>
762         ( *LWS element *( *LWS "," *LWS element ))
763    </t>
764    <t>
765      can be shown as
766    </t>
767    <t>
768         1#element
769    </t>
770    <t>
771      Wherever this construct is used, null elements are allowed, but do
772      not contribute to the count of elements present. That is,
773      "(element), , (element) " is permitted, but counts as only two
774      elements. Therefore, where at least one element is required, at
775      least one non-null element &MUST; be present. Default values are 0
776      and infinity so that "#element" allows any number, including zero;
777      "1#element" requires at least one; and "1#2element" allows one or
778      two.
779    </t>
780  </list>
781</t>
782<t>
783   ; comment
784  <list>
785    <t>
786      A semi-colon, set off some distance to the right of rule text,
787      starts a comment that continues to the end of line. This is a
788      simple way of including useful notes in parallel with the
789      specifications.
790    </t>
791  </list>
792</t>
793<t>
794   implied *LWS
795  <list>
796    <t>
797      The grammar described by this specification is word-based. Except
798      where noted otherwise, linear white space (LWS) can be included
799      between any two adjacent words (token or quoted-string), and
800      between adjacent words and separators, without changing the
801      interpretation of a field. At least one delimiter (LWS and/or
802      separators) &MUST; exist between any two tokens (for the definition
803      of "token" below), since they would otherwise be interpreted as a
804      single token.
805    </t>
806  </list>
807</t>
808</section>
809
810<section title="Basic Rules" anchor="basic.rules">
811<x:anchor-alias value="OCTET"/>
812<x:anchor-alias value="CHAR"/>
813<x:anchor-alias value="UPALPHA"/>
814<x:anchor-alias value="LOALPHA"/>
815<x:anchor-alias value="ALPHA"/>
816<x:anchor-alias value="DIGIT"/>
817<x:anchor-alias value="CTL"/>
818<x:anchor-alias value="CR"/>
819<x:anchor-alias value="LF"/>
820<x:anchor-alias value="SP"/>
821<x:anchor-alias value="HTAB"/>
822<x:anchor-alias value="CRLF"/>
823<x:anchor-alias value="LWS"/>
824<x:anchor-alias value="TEXT"/>
825<x:anchor-alias value="HEX"/>
826<x:anchor-alias value="token"/>
827<x:anchor-alias value="separators"/>
828<x:anchor-alias value="comment"/>
829<x:anchor-alias value="ctext"/>
830<x:anchor-alias value="quoted-string"/>
831<x:anchor-alias value="qdtext"/>
832<x:anchor-alias value="quoted-pair"/>
833<t>
834   The following rules are used throughout this specification to
835   describe basic parsing constructs. The US-ASCII coded character set
836   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
837</t>
838<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="HTAB"/><iref primary="true" item="Grammar" subitem="DQUOTE"/>
839  OCTET          = &lt;any 8-bit sequence of data&gt;
840  CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
841  UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
842  LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
843  ALPHA          = UPALPHA | LOALPHA
844  DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
845  CTL            = &lt;any US-ASCII control character
846                   (octets 0 - 31) and DEL (127)&gt;
847  CR             = &lt;US-ASCII CR, carriage return (13)&gt;
848  LF             = &lt;US-ASCII LF, linefeed (10)&gt;
849  SP             = &lt;US-ASCII SP, space (32)&gt;
850  HTAB           = &lt;US-ASCII HT, horizontal-tab (9)&gt;
851  DQUOTE         = &lt;US-ASCII double-quote mark (34)&gt;
852</artwork></figure>
853<t>
854   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
855   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
856   tolerant applications). The end-of-line marker within an entity-body
857   is defined by its associated media type, as described in &media-types;.
858</t>
859<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
860  CRLF           = CR LF
861</artwork></figure>
862<t>
863   HTTP/1.1 header field values can be folded onto multiple lines if the
864   continuation line begins with a space or horizontal tab. All linear
865   white space, including folding, has the same semantics as SP. A
866   recipient &MAY; replace any linear white space with a single SP before
867   interpreting the field value or forwarding the message downstream.
868</t>
869<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
870  LWS            = [CRLF] 1*( SP | HTAB )
871</artwork></figure>
872<t>
873   The TEXT rule is only used for descriptive field contents and values
874   that are not intended to be interpreted by the message parser. Words
875   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
876   <xref target="ISO-8859-1"/> only when encoded according to the rules of
877   <xref target="RFC2047"/>.
878</t>
879<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
880  TEXT           = &lt;any OCTET except CTLs,
881                   but including LWS&gt;
882</artwork></figure>
883<t>
884   A CRLF is allowed in the definition of TEXT only as part of a header
885   field continuation. It is expected that the folding LWS will be
886   replaced with a single SP before interpretation of the TEXT value.
887</t>
888<t>
889   Hexadecimal numeric characters are used in several protocol elements.
890</t>
891<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
892  HEX            = "A" | "B" | "C" | "D" | "E" | "F"
893                 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
894</artwork></figure>
895<t>
896   Many HTTP/1.1 header field values consist of words separated by LWS
897   or special characters. These special characters &MUST; be in a quoted
898   string to be used within a parameter value (as defined in
899   <xref target="transfer.codings"/>).
900</t>
901<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
902  token          = 1*&lt;any CHAR except CTLs or separators&gt;
903  separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
904                 | "," | ";" | ":" | "\" | DQUOTE
905                 | "/" | "[" | "]" | "?" | "="
906                 | "{" | "}" | SP | HTAB
907</artwork></figure>
908<t>
909   Comments can be included in some HTTP header fields by surrounding
910   the comment text with parentheses. Comments are only allowed in
911   fields containing "comment" as part of their field value definition.
912   In all other fields, parentheses are considered part of the field
913   value.
914</t>
915<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
916  comment        = "(" *( ctext | quoted-pair | comment ) ")"
917  ctext          = &lt;any TEXT excluding "(" and ")"&gt;
918</artwork></figure>
919<t>
920   A string of text is parsed as a single word if it is quoted using
921   double-quote marks.
922</t>
923<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
924  quoted-string  = ( DQUOTE *(qdtext | quoted-pair ) DQUOTE )
925  qdtext         = &lt;any TEXT excluding DQUOTE and "\">
926</artwork></figure>
927<t>
928   The backslash character ("\") &MAY; be used as a single-character
929   quoting mechanism only within quoted-string and comment constructs.
930</t>
931<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
932  quoted-pair    = "\" CHAR
933</artwork></figure>
934</section>
935</section>
936
937<section title="Protocol Parameters" anchor="protocol.parameters">
938
939<section title="HTTP Version" anchor="http.version">
940<t>
941   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
942   of the protocol. The protocol versioning policy is intended to allow
943   the sender to indicate the format of a message and its capacity for
944   understanding further HTTP communication, rather than the features
945   obtained via that communication. No change is made to the version
946   number for the addition of message components which do not affect
947   communication behavior or which only add to extensible field values.
948   The &lt;minor&gt; number is incremented when the changes made to the
949   protocol add features which do not change the general message parsing
950   algorithm, but which may add to the message semantics and imply
951   additional capabilities of the sender. The &lt;major&gt; number is
952   incremented when the format of a message within the protocol is
953   changed. See <xref target="RFC2145"/> for a fuller explanation.
954</t>
955<t>
956   The version of an HTTP message is indicated by an HTTP-Version field
957   in the first line of the message. HTTP-Version is case-sensitive.
958</t>
959<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
960  HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
961</artwork></figure>
962<t>
963   Note that the major and minor numbers &MUST; be treated as separate
964   integers and that each &MAY; be incremented higher than a single digit.
965   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
966   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
967   &MUST-NOT; be sent.
968</t>
969<t>
970   An application that sends a request or response message that includes
971   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
972   with this specification. Applications that are at least conditionally
973   compliant with this specification &SHOULD; use an HTTP-Version of
974   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
975   not compatible with HTTP/1.0. For more details on when to send
976   specific HTTP-Version values, see <xref target="RFC2145"/>.
977</t>
978<t>
979   The HTTP version of an application is the highest HTTP version for
980   which the application is at least conditionally compliant.
981</t>
982<t>
983   Proxy and gateway applications need to be careful when forwarding
984   messages in protocol versions different from that of the application.
985   Since the protocol version indicates the protocol capability of the
986   sender, a proxy/gateway &MUST-NOT; send a message with a version
987   indicator which is greater than its actual version. If a higher
988   version request is received, the proxy/gateway &MUST; either downgrade
989   the request version, or respond with an error, or switch to tunnel
990   behavior.
991</t>
992<t>
993   Due to interoperability problems with HTTP/1.0 proxies discovered
994   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
995   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
996   they support. The proxy/gateway's response to that request &MUST; be in
997   the same major version as the request.
998</t>
999<t>
1000  <list>
1001    <t>
1002      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1003      of header fields required or forbidden by the versions involved.
1004    </t>
1005  </list>
1006</t>
1007</section>
1008
1009<section title="Uniform Resource Identifiers" anchor="uri">
1010<t>
1011   URIs have been known by many names: WWW addresses, Universal Document
1012   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
1013   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
1014   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
1015   simply formatted strings which identify--via name, location, or any
1016   other characteristic--a resource.
1017</t>
1018
1019<section title="General Syntax" anchor="general.syntax">
1020<t>
1021   URIs in HTTP can be represented in absolute form or relative to some
1022   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1023   forms are differentiated by the fact that absolute URIs always begin
1024   with a scheme name followed by a colon. For definitive information on
1025   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1026   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1027   and <xref target="RFC1808"/>). This specification adopts the
1028   definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
1029   "host", "abs_path", "rel_path", "query", and "authority" from that
1030   specification.
1031</t>
1032<t>
1033   The HTTP protocol does not place any a priori limit on the length of
1034   a URI. Servers &MUST; be able to handle the URI of any resource they
1035   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1036   provide GET-based forms that could generate such URIs. A server
1037   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1038   than the server can handle (see &status-414;).
1039</t>
1040<t>
1041  <list>
1042    <t>
1043      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1044      above 255 bytes, because some older client or proxy
1045      implementations might not properly support these lengths.
1046    </t>
1047  </list>
1048</t>
1049</section>
1050
1051<section title="http URL" anchor="http.url">
1052<t>
1053   The "http" scheme is used to locate network resources via the HTTP
1054   protocol. This section defines the scheme-specific syntax and
1055   semantics for http URLs.
1056</t>
1057<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http_URL"/>
1058  http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
1059</artwork></figure>
1060<t>
1061   If the port is empty or not given, port 80 is assumed. The semantics
1062   are that the identified resource is located at the server listening
1063   for TCP connections on that port of that host, and the Request-URI
1064   for the resource is abs_path (<xref target="request-uri"/>). The use of IP addresses
1065   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1066   the abs_path is not present in the URL, it &MUST; be given as "/" when
1067   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1068
1069   receives a host name which is not a fully qualified domain name, it
1070   &MAY; add its domain to the host name it received. If a proxy receives
1071   a fully qualified domain name, the proxy &MUST-NOT; change the host
1072   name.
1073</t>
1074</section>
1075
1076<section title="URI Comparison" anchor="uri.comparison">
1077<t>
1078   When comparing two URIs to decide if they match or not, a client
1079   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1080   URIs, with these exceptions:
1081  <list style="symbols">
1082    <t>A port that is empty or not given is equivalent to the default
1083        port for that URI-reference;</t>
1084    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1085    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1086    <t>An empty abs_path is equivalent to an abs_path of "/".</t>
1087  </list>
1088</t>
1089<t>
1090   Characters other than those in the "reserved" set (see
1091   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1092</t>
1093<t>
1094   For example, the following three URIs are equivalent:
1095</t>
1096<figure><artwork type="example">
1097   http://example.com:80/~smith/home.html
1098   http://EXAMPLE.com/%7Esmith/home.html
1099   http://EXAMPLE.com:/%7esmith/home.html
1100</artwork></figure>
1101</section>
1102</section>
1103
1104<section title="Date/Time Formats" anchor="date.time.formats">
1105<section title="Full Date" anchor="full.date">
1106<t>
1107   HTTP applications have historically allowed three different formats
1108   for the representation of date/time stamps:
1109</t>
1110<figure><artwork type="example">
1111   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1112   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1113   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1114</artwork></figure>
1115<t>
1116   The first format is preferred as an Internet standard and represents
1117   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1118   <xref target="RFC822"/>). The other formats are described here only for
1119   compatibility with obsolete implementations.
1120   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1121   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1122   only generate the RFC 1123 format for representing HTTP-date values
1123   in header fields. See <xref target="tolerant.applications"/> for further information.
1124</t>
1125<t><list><t>
1126      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1127      accepting date values that may have been sent by non-HTTP
1128      applications, as is sometimes the case when retrieving or posting
1129      messages via proxies/gateways to SMTP or NNTP.
1130</t></list></t>
1131<t>
1132   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1133   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1134   equal to UTC (Coordinated Universal Time). This is indicated in the
1135   first two formats by the inclusion of "GMT" as the three-letter
1136   abbreviation for time zone, and &MUST; be assumed when reading the
1137   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1138   additional LWS beyond that specifically included as SP in the
1139   grammar.
1140</t>
1141<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"/>
1142  HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1143  rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1144  rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1145  asctime-date = wkday SP date3 SP time SP 4DIGIT
1146  date1        = 2DIGIT SP month SP 4DIGIT
1147                 ; day month year (e.g., 02 Jun 1982)
1148  date2        = 2DIGIT "-" month "-" 2DIGIT
1149                 ; day-month-year (e.g., 02-Jun-82)
1150  date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1151                 ; month day (e.g., Jun  2)
1152  time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1153                 ; 00:00:00 - 23:59:59
1154  wkday        = "Mon" | "Tue" | "Wed"
1155               | "Thu" | "Fri" | "Sat" | "Sun"
1156  weekday      = "Monday" | "Tuesday" | "Wednesday"
1157               | "Thursday" | "Friday" | "Saturday" | "Sunday"
1158  month        = "Jan" | "Feb" | "Mar" | "Apr"
1159               | "May" | "Jun" | "Jul" | "Aug"
1160               | "Sep" | "Oct" | "Nov" | "Dec"
1161</artwork></figure>
1162<t>
1163      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1164      to their usage within the protocol stream. Clients and servers are
1165      not required to use these formats for user presentation, request
1166      logging, etc.
1167</t>
1168</section>
1169</section>
1170
1171<section title="Transfer Codings" anchor="transfer.codings">
1172<t>
1173   Transfer-coding values are used to indicate an encoding
1174   transformation that has been, can be, or may need to be applied to an
1175   entity-body in order to ensure "safe transport" through the network.
1176   This differs from a content coding in that the transfer-coding is a
1177   property of the message, not of the original entity.
1178</t>
1179<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1180  transfer-coding         = "chunked" | transfer-extension
1181  transfer-extension      = token *( ";" parameter )
1182</artwork></figure>
1183<t>
1184   Parameters are in  the form of attribute/value pairs.
1185</t>
1186<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"/>
1187  parameter               = attribute "=" value
1188  attribute               = token
1189  value                   = token | quoted-string
1190</artwork></figure>
1191<t>
1192   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1193   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1194   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1195</t>
1196<t>
1197   Whenever a transfer-coding is applied to a message-body, the set of
1198   transfer-codings &MUST; include "chunked", unless the message is
1199   terminated by closing the connection. When the "chunked" transfer-coding
1200   is used, it &MUST; be the last transfer-coding applied to the
1201   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1202   than once to a message-body. These rules allow the recipient to
1203   determine the transfer-length of the message (<xref target="message.length"/>).
1204</t>
1205<t>
1206   Transfer-codings are analogous to the Content-Transfer-Encoding
1207   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1208   binary data over a 7-bit transport service. However, safe transport
1209   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1210   the only unsafe characteristic of message-bodies is the difficulty in
1211   determining the exact body length (<xref target="message.length"/>), or the desire to
1212   encrypt data over a shared transport.
1213</t>
1214<t>
1215   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1216   transfer-coding value tokens. Initially, the registry contains the
1217   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1218   "gzip", "compress", and "deflate" (&content-codings;).
1219</t>
1220<t>
1221   New transfer-coding value tokens &SHOULD; be registered in the same way
1222   as new content-coding value tokens (&content-codings;).
1223</t>
1224<t>
1225   A server which receives an entity-body with a transfer-coding it does
1226   not understand &SHOULD; return 501 (Not Implemented), and close the
1227   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1228   client.
1229</t>
1230
1231<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1232<t>
1233   The chunked encoding modifies the body of a message in order to
1234   transfer it as a series of chunks, each with its own size indicator,
1235   followed by an &OPTIONAL; trailer containing entity-header fields. This
1236   allows dynamically produced content to be transferred along with the
1237   information necessary for the recipient to verify that it has
1238   received the full message.
1239</t>
1240<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"/>
1241  Chunked-Body   = *chunk
1242                   last-chunk
1243                   trailer
1244                   CRLF
1245 
1246  chunk          = chunk-size [ chunk-extension ] CRLF
1247                   chunk-data CRLF
1248  chunk-size     = 1*HEX
1249  last-chunk     = 1*("0") [ chunk-extension ] CRLF
1250 
1251  chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1252  chunk-ext-name = token
1253  chunk-ext-val  = token | quoted-string
1254  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
1255  trailer        = *(entity-header CRLF)
1256</artwork></figure>
1257<t>
1258   The chunk-size field is a string of hex digits indicating the size of
1259   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1260   zero, followed by the trailer, which is terminated by an empty line.
1261</t>
1262<t>
1263   The trailer allows the sender to include additional HTTP header
1264   fields at the end of the message. The Trailer header field can be
1265   used to indicate which header fields are included in a trailer (see
1266   <xref target="header.trailer"/>).
1267</t>
1268<t>
1269   A server using chunked transfer-coding in a response &MUST-NOT; use the
1270   trailer for any header fields unless at least one of the following is
1271   true:
1272  <list style="numbers">
1273    <t>the request included a TE header field that indicates "trailers" is
1274     acceptable in the transfer-coding of the  response, as described in
1275     <xref target="header.te"/>; or,</t>
1276
1277    <t>the server is the origin server for the response, the trailer
1278     fields consist entirely of optional metadata, and the recipient
1279     could use the message (in a manner acceptable to the origin server)
1280     without receiving this metadata.  In other words, the origin server
1281     is willing to accept the possibility that the trailer fields might
1282     be silently discarded along the path to the client.</t>
1283  </list>
1284</t>
1285<t>
1286   This requirement prevents an interoperability failure when the
1287   message is being received by an HTTP/1.1 (or later) proxy and
1288   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1289   compliance with the protocol would have necessitated a possibly
1290   infinite buffer on the proxy.
1291</t>
1292<t>
1293   A process for decoding the "chunked" transfer-coding
1294   can be represented in pseudo-code as:
1295</t>
1296<figure><artwork type="code">
1297    length := 0
1298    read chunk-size, chunk-extension (if any) and CRLF
1299    while (chunk-size &gt; 0) {
1300       read chunk-data and CRLF
1301       append chunk-data to entity-body
1302       length := length + chunk-size
1303       read chunk-size and CRLF
1304    }
1305    read entity-header
1306    while (entity-header not empty) {
1307       append entity-header to existing header fields
1308       read entity-header
1309    }
1310    Content-Length := length
1311    Remove "chunked" from Transfer-Encoding
1312</artwork></figure>
1313<t>
1314   All HTTP/1.1 applications &MUST; be able to receive and decode the
1315   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1316   they do not understand.
1317</t>
1318</section>
1319</section>
1320
1321</section>
1322
1323<section title="HTTP Message" anchor="http.message">
1324
1325<section title="Message Types" anchor="message.types">
1326<t>
1327   HTTP messages consist of requests from client to server and responses
1328   from server to client.
1329</t>
1330<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1331  HTTP-message   = Request | Response     ; HTTP/1.1 messages
1332</artwork></figure>
1333<t>
1334   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1335   message format of <xref target="RFC2822"/> for transferring entities (the payload
1336   of the message). Both types of message consist of a start-line, zero
1337   or more header fields (also known as "headers"), an empty line (i.e.,
1338   a line with nothing preceding the CRLF) indicating the end of the
1339   header fields, and possibly a message-body.
1340</t>
1341<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1342  generic-message = start-line
1343                    *(message-header CRLF)
1344                    CRLF
1345                    [ message-body ]
1346  start-line      = Request-Line | Status-Line
1347</artwork></figure>
1348<t>
1349   In the interest of robustness, servers &SHOULD; ignore any empty
1350   line(s) received where a Request-Line is expected. In other words, if
1351   the server is reading the protocol stream at the beginning of a
1352   message and receives a CRLF first, it should ignore the CRLF.
1353</t>
1354<t>
1355   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1356   after a POST request. To restate what is explicitly forbidden by the
1357   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1358   extra CRLF.
1359</t>
1360</section>
1361
1362<section title="Message Headers" anchor="message.headers">
1363<t>
1364   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1365   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1366   entity-header (&entity-header-fields;) fields, follow the same generic format as
1367   that given in <xref target="RFC2822" x:fmt="of" x:sec="2.1"/>. Each header field consists
1368   of a name followed by a colon (":") and the field value. Field names
1369   are case-insensitive. The field value &MAY; be preceded by any amount
1370   of LWS, though a single SP is preferred. Header fields can be
1371   extended over multiple lines by preceding each extra line with at
1372   least one SP or HTAB. Applications ought to follow "common form", where
1373   one is known or indicated, when generating HTTP constructs, since
1374   there might exist some implementations that fail to accept anything
1375   beyond the common forms.
1376</t>
1377<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"/>
1378  message-header = field-name ":" [ field-value ]
1379  field-name     = token
1380  field-value    = *( field-content | LWS )
1381  field-content  = &lt;the OCTETs making up the field-value
1382                   and consisting of either *TEXT or combinations
1383                   of token, separators, and quoted-string&gt;
1384</artwork></figure>
1385<t>
1386   The field-content does not include any leading or trailing LWS:
1387   linear white space occurring before the first non-whitespace
1388   character of the field-value or after the last non-whitespace
1389   character of the field-value. Such leading or trailing LWS &MAY; be
1390   removed without changing the semantics of the field value. Any LWS
1391   that occurs between field-content &MAY; be replaced with a single SP
1392   before interpreting the field value or forwarding the message
1393   downstream.
1394</t>
1395<t>
1396   The order in which header fields with differing field names are
1397   received is not significant. However, it is "good practice" to send
1398   general-header fields first, followed by request-header or response-header
1399   fields, and ending with the entity-header fields.
1400</t>
1401<t>
1402   Multiple message-header fields with the same field-name &MAY; be
1403   present in a message if and only if the entire field-value for that
1404   header field is defined as a comma-separated list [i.e., #(values)].
1405   It &MUST; be possible to combine the multiple header fields into one
1406   "field-name: field-value" pair, without changing the semantics of the
1407   message, by appending each subsequent field-value to the first, each
1408   separated by a comma. The order in which header fields with the same
1409   field-name are received is therefore significant to the
1410   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1411   change the order of these field values when a message is forwarded.
1412</t>
1413</section>
1414
1415<section title="Message Body" anchor="message.body">
1416<t>
1417   The message-body (if any) of an HTTP message is used to carry the
1418   entity-body associated with the request or response. The message-body
1419   differs from the entity-body only when a transfer-coding has been
1420   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1421</t>
1422<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1423  message-body = entity-body
1424               | &lt;entity-body encoded as per Transfer-Encoding&gt;
1425</artwork></figure>
1426<t>
1427   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1428   applied by an application to ensure safe and proper transfer of the
1429   message. Transfer-Encoding is a property of the message, not of the
1430   entity, and thus &MAY; be added or removed by any application along the
1431   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1432   when certain transfer-codings may be used.)
1433</t>
1434<t>
1435   The rules for when a message-body is allowed in a message differ for
1436   requests and responses.
1437</t>
1438<t>
1439   The presence of a message-body in a request is signaled by the
1440   inclusion of a Content-Length or Transfer-Encoding header field in
1441   the request's message-headers. A message-body &MUST-NOT; be included in
1442   a request if the specification of the request method (&method;)
1443   does not allow sending an entity-body in requests. A server &SHOULD;
1444   read and forward a message-body on any request; if the request method
1445   does not include defined semantics for an entity-body, then the
1446   message-body &SHOULD; be ignored when handling the request.
1447</t>
1448<t>
1449   For response messages, whether or not a message-body is included with
1450   a message is dependent on both the request method and the response
1451   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1452   &MUST-NOT; include a message-body, even though the presence of entity-header
1453   fields might lead one to believe they do. All 1xx
1454   (informational), 204 (No Content), and 304 (Not Modified) responses
1455   &MUST-NOT; include a message-body. All other responses do include a
1456   message-body, although it &MAY; be of zero length.
1457</t>
1458</section>
1459
1460<section title="Message Length" anchor="message.length">
1461<t>
1462   The transfer-length of a message is the length of the message-body as
1463   it appears in the message; that is, after any transfer-codings have
1464   been applied. When a message-body is included with a message, the
1465   transfer-length of that body is determined by one of the following
1466   (in order of precedence):
1467</t>
1468<t>
1469  <list style="numbers">
1470    <x:lt><t>
1471     Any response message which "&MUST-NOT;" include a message-body (such
1472     as the 1xx, 204, and 304 responses and any response to a HEAD
1473     request) is always terminated by the first empty line after the
1474     header fields, regardless of the entity-header fields present in
1475     the message.
1476    </t></x:lt>
1477    <x:lt><t>
1478     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1479     is present, then the transfer-length is
1480     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1481     unless the message is terminated by closing the connection.
1482    </t></x:lt>
1483    <x:lt><t>
1484     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1485     decimal value in OCTETs represents both the entity-length and the
1486     transfer-length. The Content-Length header field &MUST-NOT; be sent
1487     if these two lengths are different (i.e., if a Transfer-Encoding
1488     header field is present). If a message is received with both a
1489     Transfer-Encoding header field and a Content-Length header field,
1490     the latter &MUST; be ignored.
1491    </t></x:lt>
1492    <x:lt><t>
1493     If the message uses the media type "multipart/byteranges", and the
1494     transfer-length is not otherwise specified, then this self-delimiting
1495     media type defines the transfer-length. This media type
1496     &MUST-NOT; be used unless the sender knows that the recipient can parse
1497     it; the presence in a request of a Range header with multiple byte-range
1498     specifiers from a 1.1 client implies that the client can parse
1499     multipart/byteranges responses.
1500    <list style="empty"><t>
1501       A range header might be forwarded by a 1.0 proxy that does not
1502       understand multipart/byteranges; in this case the server &MUST;
1503       delimit the message using methods defined in items 1, 3 or 5 of
1504       this section.
1505    </t></list>
1506    </t></x:lt>
1507    <x:lt><t>
1508     By the server closing the connection. (Closing the connection
1509     cannot be used to indicate the end of a request body, since that
1510     would leave no possibility for the server to send back a response.)
1511    </t></x:lt>
1512  </list>
1513</t>
1514<t>
1515   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1516   containing a message-body &MUST; include a valid Content-Length header
1517   field unless the server is known to be HTTP/1.1 compliant. If a
1518   request contains a message-body and a Content-Length is not given,
1519   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1520   determine the length of the message, or with 411 (Length Required) if
1521   it wishes to insist on receiving a valid Content-Length.
1522</t>
1523<t>
1524   All HTTP/1.1 applications that receive entities &MUST; accept the
1525   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1526   to be used for messages when the message length cannot be determined
1527   in advance.
1528</t>
1529<t>
1530   Messages &MUST-NOT; include both a Content-Length header field and a
1531   transfer-coding. If the message does include a
1532   transfer-coding, the Content-Length &MUST; be ignored.
1533</t>
1534<t>
1535   When a Content-Length is given in a message where a message-body is
1536   allowed, its field value &MUST; exactly match the number of OCTETs in
1537   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1538   invalid length is received and detected.
1539</t>
1540</section>
1541
1542<section title="General Header Fields" anchor="general.header.fields">
1543<t>
1544   There are a few header fields which have general applicability for
1545   both request and response messages, but which do not apply to the
1546   entity being transferred. These header fields apply only to the
1547   message being transmitted.
1548</t>
1549<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1550  general-header = Cache-Control            ; &header-cache-control;
1551                 | Connection               ; <xref target="header.connection"/>
1552                 | Date                     ; <xref target="header.date"/>
1553                 | Pragma                   ; &header-pragma;
1554                 | Trailer                  ; <xref target="header.trailer"/>
1555                 | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1556                 | Upgrade                  ; <xref target="header.upgrade"/>
1557                 | Via                      ; <xref target="header.via"/>
1558                 | Warning                  ; &header-warning;
1559</artwork></figure>
1560<t>
1561   General-header field names can be extended reliably only in
1562   combination with a change in the protocol version. However, new or
1563   experimental header fields may be given the semantics of general
1564   header fields if all parties in the communication recognize them to
1565   be general-header fields. Unrecognized header fields are treated as
1566   entity-header fields.
1567</t>
1568</section>
1569</section>
1570
1571<section title="Request" anchor="request">
1572<t>
1573   A request message from a client to a server includes, within the
1574   first line of that message, the method to be applied to the resource,
1575   the identifier of the resource, and the protocol version in use.
1576</t>
1577<!--                 Host                      ; should be moved here eventually -->
1578<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1579  Request       = Request-Line              ; <xref target="request-line"/>
1580                  *(( general-header        ; <xref target="general.header.fields"/>
1581                   | request-header         ; &request-header-fields;
1582                   | entity-header ) CRLF)  ; &entity-header-fields;
1583                  CRLF
1584                  [ message-body ]          ; <xref target="message.body"/>
1585</artwork></figure>
1586
1587<section title="Request-Line" anchor="request-line">
1588<t>
1589   The Request-Line begins with a method token, followed by the
1590   Request-URI and the protocol version, and ending with CRLF. The
1591   elements are separated by SP characters. No CR or LF is allowed
1592   except in the final CRLF sequence.
1593</t>
1594<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1595  Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1596</artwork></figure>
1597
1598<section title="Method" anchor="method">
1599<t>
1600   The Method  token indicates the method to be performed on the
1601   resource identified by the Request-URI. The method is case-sensitive.
1602</t>
1603<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1604  Method         = token
1605</artwork></figure>
1606</section>
1607
1608<section title="Request-URI" anchor="request-uri">
1609<t>
1610   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1611   identifies the resource upon which to apply the request.
1612</t>
1613<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1614  Request-URI    = "*"
1615                 | absoluteURI
1616                 | ( abs_path [ "?" query ] )
1617                 | authority
1618</artwork></figure>
1619<t>
1620   The four options for Request-URI are dependent on the nature of the
1621   request. The asterisk "*" means that the request does not apply to a
1622   particular resource, but to the server itself, and is only allowed
1623   when the method used does not necessarily apply to a resource. One
1624   example would be
1625</t>
1626<figure><artwork type="example">
1627    OPTIONS * HTTP/1.1
1628</artwork></figure>
1629<t>
1630   The absoluteURI form is &REQUIRED; when the request is being made to a
1631   proxy. The proxy is requested to forward the request or service it
1632   from a valid cache, and return the response. Note that the proxy &MAY;
1633   forward the request on to another proxy or directly to the server
1634   specified by the absoluteURI. In order to avoid request loops, a
1635   proxy &MUST; be able to recognize all of its server names, including
1636   any aliases, local variations, and the numeric IP address. An example
1637   Request-Line would be:
1638</t>
1639<figure><artwork type="example">
1640    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1641</artwork></figure>
1642<t>
1643   To allow for transition to absoluteURIs in all requests in future
1644   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1645   form in requests, even though HTTP/1.1 clients will only generate
1646   them in requests to proxies.
1647</t>
1648<t>
1649   The authority form is only used by the CONNECT method (&CONNECT;).
1650</t>
1651<t>
1652   The most common form of Request-URI is that used to identify a
1653   resource on an origin server or gateway. In this case the absolute
1654   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, abs_path) as
1655   the Request-URI, and the network location of the URI (authority) &MUST;
1656   be transmitted in a Host header field. For example, a client wishing
1657   to retrieve the resource above directly from the origin server would
1658   create a TCP connection to port 80 of the host "www.example.org" and send
1659   the lines:
1660</t>
1661<figure><artwork type="example">
1662    GET /pub/WWW/TheProject.html HTTP/1.1
1663    Host: www.example.org
1664</artwork></figure>
1665<t>
1666   followed by the remainder of the Request. Note that the absolute path
1667   cannot be empty; if none is present in the original URI, it &MUST; be
1668   given as "/" (the server root).
1669</t>
1670<t>
1671   The Request-URI is transmitted in the format specified in
1672   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1673   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1674   properly interpret the request. Servers &SHOULD; respond to invalid
1675   Request-URIs with an appropriate status code.
1676</t>
1677<t>
1678   A transparent proxy &MUST-NOT; rewrite the "abs_path" part of the
1679   received Request-URI when forwarding it to the next inbound server,
1680   except as noted above to replace a null abs_path with "/".
1681</t>
1682<t>
1683  <list><t>
1684      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1685      meaning of the request when the origin server is improperly using
1686      a non-reserved URI character for a reserved purpose.  Implementors
1687      should be aware that some pre-HTTP/1.1 proxies have been known to
1688      rewrite the Request-URI.
1689  </t></list>
1690</t>
1691</section>
1692</section>
1693
1694<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1695<t>
1696   The exact resource identified by an Internet request is determined by
1697   examining both the Request-URI and the Host header field.
1698</t>
1699<t>
1700   An origin server that does not allow resources to differ by the
1701   requested host &MAY; ignore the Host header field value when
1702   determining the resource identified by an HTTP/1.1 request. (But see
1703   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1704   for other requirements on Host support in HTTP/1.1.)
1705</t>
1706<t>
1707   An origin server that does differentiate resources based on the host
1708   requested (sometimes referred to as virtual hosts or vanity host
1709   names) &MUST; use the following rules for determining the requested
1710   resource on an HTTP/1.1 request:
1711  <list style="numbers">
1712    <t>If Request-URI is an absoluteURI, the host is part of the
1713     Request-URI. Any Host header field value in the request &MUST; be
1714     ignored.</t>
1715    <t>If the Request-URI is not an absoluteURI, and the request includes
1716     a Host header field, the host is determined by the Host header
1717     field value.</t>
1718    <t>If the host as determined by rule 1 or 2 is not a valid host on
1719     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1720  </list>
1721</t>
1722<t>
1723   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1724   attempt to use heuristics (e.g., examination of the URI path for
1725   something unique to a particular host) in order to determine what
1726   exact resource is being requested.
1727</t>
1728</section>
1729
1730</section>
1731
1732
1733<section title="Response" anchor="response">
1734<t>
1735   After receiving and interpreting a request message, a server responds
1736   with an HTTP response message.
1737</t>
1738<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1739  Response      = Status-Line               ; <xref target="status-line"/>
1740                  *(( general-header        ; <xref target="general.header.fields"/>
1741                   | response-header        ; &response-header-fields;
1742                   | entity-header ) CRLF)  ; &entity-header-fields;
1743                  CRLF
1744                  [ message-body ]          ; <xref target="message.body"/>
1745</artwork></figure>
1746
1747<section title="Status-Line" anchor="status-line">
1748<t>
1749   The first line of a Response message is the Status-Line, consisting
1750   of the protocol version followed by a numeric status code and its
1751   associated textual phrase, with each element separated by SP
1752   characters. No CR or LF is allowed except in the final CRLF sequence.
1753</t>
1754<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1755  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1756</artwork></figure>
1757
1758<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1759<t>
1760   The Status-Code element is a 3-digit integer result code of the
1761   attempt to understand and satisfy the request. These codes are fully
1762   defined in &status-codes;. The Reason-Phrase is intended to give a short
1763   textual description of the Status-Code. The Status-Code is intended
1764   for use by automata and the Reason-Phrase is intended for the human
1765   user. The client is not required to examine or display the Reason-Phrase.
1766</t>
1767<t>
1768   The first digit of the Status-Code defines the class of response. The
1769   last two digits do not have any categorization role. There are 5
1770   values for the first digit:
1771  <list style="symbols">
1772    <t>
1773      1xx: Informational - Request received, continuing process
1774    </t>
1775    <t>
1776      2xx: Success - The action was successfully received,
1777        understood, and accepted
1778    </t>
1779    <t>
1780      3xx: Redirection - Further action must be taken in order to
1781        complete the request
1782    </t>
1783    <t>
1784      4xx: Client Error - The request contains bad syntax or cannot
1785        be fulfilled
1786    </t>
1787    <t>
1788      5xx: Server Error - The server failed to fulfill an apparently
1789        valid request
1790    </t>
1791  </list>
1792</t>
1793<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"/>
1794  Status-Code    = 3DIGIT
1795  Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1796</artwork></figure>
1797</section>
1798</section>
1799
1800</section>
1801
1802
1803<section title="Connections" anchor="connections">
1804
1805<section title="Persistent Connections" anchor="persistent.connections">
1806
1807<section title="Purpose" anchor="persistent.purpose">
1808<t>
1809   Prior to persistent connections, a separate TCP connection was
1810   established to fetch each URL, increasing the load on HTTP servers
1811   and causing congestion on the Internet. The use of inline images and
1812   other associated data often require a client to make multiple
1813   requests of the same server in a short amount of time. Analysis of
1814   these performance problems and results from a prototype
1815   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1816   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1817   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1818   T/TCP <xref target="Tou1998"/>.
1819</t>
1820<t>
1821   Persistent HTTP connections have a number of advantages:
1822  <list style="symbols">
1823      <t>
1824        By opening and closing fewer TCP connections, CPU time is saved
1825        in routers and hosts (clients, servers, proxies, gateways,
1826        tunnels, or caches), and memory used for TCP protocol control
1827        blocks can be saved in hosts.
1828      </t>
1829      <t>
1830        HTTP requests and responses can be pipelined on a connection.
1831        Pipelining allows a client to make multiple requests without
1832        waiting for each response, allowing a single TCP connection to
1833        be used much more efficiently, with much lower elapsed time.
1834      </t>
1835      <t>
1836        Network congestion is reduced by reducing the number of packets
1837        caused by TCP opens, and by allowing TCP sufficient time to
1838        determine the congestion state of the network.
1839      </t>
1840      <t>
1841        Latency on subsequent requests is reduced since there is no time
1842        spent in TCP's connection opening handshake.
1843      </t>
1844      <t>
1845        HTTP can evolve more gracefully, since errors can be reported
1846        without the penalty of closing the TCP connection. Clients using
1847        future versions of HTTP might optimistically try a new feature,
1848        but if communicating with an older server, retry with old
1849        semantics after an error is reported.
1850      </t>
1851    </list>
1852</t>
1853<t>
1854   HTTP implementations &SHOULD; implement persistent connections.
1855</t>
1856</section>
1857
1858<section title="Overall Operation" anchor="persistent.overall">
1859<t>
1860   A significant difference between HTTP/1.1 and earlier versions of
1861   HTTP is that persistent connections are the default behavior of any
1862   HTTP connection. That is, unless otherwise indicated, the client
1863   &SHOULD; assume that the server will maintain a persistent connection,
1864   even after error responses from the server.
1865</t>
1866<t>
1867   Persistent connections provide a mechanism by which a client and a
1868   server can signal the close of a TCP connection. This signaling takes
1869   place using the Connection header field (<xref target="header.connection"/>). Once a close
1870   has been signaled, the client &MUST-NOT; send any more requests on that
1871   connection.
1872</t>
1873
1874<section title="Negotiation" anchor="persistent.negotiation">
1875<t>
1876   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1877   maintain a persistent connection unless a Connection header including
1878   the connection-token "close" was sent in the request. If the server
1879   chooses to close the connection immediately after sending the
1880   response, it &SHOULD; send a Connection header including the
1881   connection-token close.
1882</t>
1883<t>
1884   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1885   decide to keep it open based on whether the response from a server
1886   contains a Connection header with the connection-token close. In case
1887   the client does not want to maintain a connection for more than that
1888   request, it &SHOULD; send a Connection header including the
1889   connection-token close.
1890</t>
1891<t>
1892   If either the client or the server sends the close token in the
1893   Connection header, that request becomes the last one for the
1894   connection.
1895</t>
1896<t>
1897   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1898   maintained for HTTP versions less than 1.1 unless it is explicitly
1899   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1900   compatibility with HTTP/1.0 clients.
1901</t>
1902<t>
1903   In order to remain persistent, all messages on the connection &MUST;
1904   have a self-defined message length (i.e., one not defined by closure
1905   of the connection), as described in <xref target="message.length"/>.
1906</t>
1907</section>
1908
1909<section title="Pipelining" anchor="pipelining">
1910<t>
1911   A client that supports persistent connections &MAY; "pipeline" its
1912   requests (i.e., send multiple requests without waiting for each
1913   response). A server &MUST; send its responses to those requests in the
1914   same order that the requests were received.
1915</t>
1916<t>
1917   Clients which assume persistent connections and pipeline immediately
1918   after connection establishment &SHOULD; be prepared to retry their
1919   connection if the first pipelined attempt fails. If a client does
1920   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1921   persistent. Clients &MUST; also be prepared to resend their requests if
1922   the server closes the connection before sending all of the
1923   corresponding responses.
1924</t>
1925<t>
1926   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1927   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1928   premature termination of the transport connection could lead to
1929   indeterminate results. A client wishing to send a non-idempotent
1930   request &SHOULD; wait to send that request until it has received the
1931   response status for the previous request.
1932</t>
1933</section>
1934</section>
1935
1936<section title="Proxy Servers" anchor="persistent.proxy">
1937<t>
1938   It is especially important that proxies correctly implement the
1939   properties of the Connection header field as specified in <xref target="header.connection"/>.
1940</t>
1941<t>
1942   The proxy server &MUST; signal persistent connections separately with
1943   its clients and the origin servers (or other proxy servers) that it
1944   connects to. Each persistent connection applies to only one transport
1945   link.
1946</t>
1947<t>
1948   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1949   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
1950   discussion of the problems with the Keep-Alive header implemented by
1951   many HTTP/1.0 clients).
1952</t>
1953</section>
1954
1955<section title="Practical Considerations" anchor="persistent.practical">
1956<t>
1957   Servers will usually have some time-out value beyond which they will
1958   no longer maintain an inactive connection. Proxy servers might make
1959   this a higher value since it is likely that the client will be making
1960   more connections through the same server. The use of persistent
1961   connections places no requirements on the length (or existence) of
1962   this time-out for either the client or the server.
1963</t>
1964<t>
1965   When a client or server wishes to time-out it &SHOULD; issue a graceful
1966   close on the transport connection. Clients and servers &SHOULD; both
1967   constantly watch for the other side of the transport close, and
1968   respond to it as appropriate. If a client or server does not detect
1969   the other side's close promptly it could cause unnecessary resource
1970   drain on the network.
1971</t>
1972<t>
1973   A client, server, or proxy &MAY; close the transport connection at any
1974   time. For example, a client might have started to send a new request
1975   at the same time that the server has decided to close the "idle"
1976   connection. From the server's point of view, the connection is being
1977   closed while it was idle, but from the client's point of view, a
1978   request is in progress.
1979</t>
1980<t>
1981   This means that clients, servers, and proxies &MUST; be able to recover
1982   from asynchronous close events. Client software &SHOULD; reopen the
1983   transport connection and retransmit the aborted sequence of requests
1984   without user interaction so long as the request sequence is
1985   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1986   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1987   human operator the choice of retrying the request(s). Confirmation by
1988   user-agent software with semantic understanding of the application
1989   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
1990   be repeated if the second sequence of requests fails.
1991</t>
1992<t>
1993   Servers &SHOULD; always respond to at least one request per connection,
1994   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1995   middle of transmitting a response, unless a network or client failure
1996   is suspected.
1997</t>
1998<t>
1999   Clients that use persistent connections &SHOULD; limit the number of
2000   simultaneous connections that they maintain to a given server. A
2001   single-user client &SHOULD-NOT; maintain more than 2 connections with
2002   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2003   another server or proxy, where N is the number of simultaneously
2004   active users. These guidelines are intended to improve HTTP response
2005   times and avoid congestion.
2006</t>
2007</section>
2008</section>
2009
2010<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2011
2012<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2013<t>
2014   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2015   flow control mechanisms to resolve temporary overloads, rather than
2016   terminating connections with the expectation that clients will retry.
2017   The latter technique can exacerbate network congestion.
2018</t>
2019</section>
2020
2021<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2022<t>
2023   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2024   the network connection for an error status while it is transmitting
2025   the request. If the client sees an error status, it &SHOULD;
2026   immediately cease transmitting the body. If the body is being sent
2027   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2028   empty trailer &MAY; be used to prematurely mark the end of the message.
2029   If the body was preceded by a Content-Length header, the client &MUST;
2030   close the connection.
2031</t>
2032</section>
2033
2034<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2035<t>
2036   The purpose of the 100 (Continue) status (see &status-100;) is to
2037   allow a client that is sending a request message with a request body
2038   to determine if the origin server is willing to accept the request
2039   (based on the request headers) before the client sends the request
2040   body. In some cases, it might either be inappropriate or highly
2041   inefficient for the client to send the body if the server will reject
2042   the message without looking at the body.
2043</t>
2044<t>
2045   Requirements for HTTP/1.1 clients:
2046  <list style="symbols">
2047    <t>
2048        If a client will wait for a 100 (Continue) response before
2049        sending the request body, it &MUST; send an Expect request-header
2050        field (&header-expect;) with the "100-continue" expectation.
2051    </t>
2052    <t>
2053        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2054        with the "100-continue" expectation if it does not intend
2055        to send a request body.
2056    </t>
2057  </list>
2058</t>
2059<t>
2060   Because of the presence of older implementations, the protocol allows
2061   ambiguous situations in which a client may send "Expect: 100-continue"
2062   without receiving either a 417 (Expectation Failed) status
2063   or a 100 (Continue) status. Therefore, when a client sends this
2064   header field to an origin server (possibly via a proxy) from which it
2065   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2066   for an indefinite period before sending the request body.
2067</t>
2068<t>
2069   Requirements for HTTP/1.1 origin servers:
2070  <list style="symbols">
2071    <t> Upon receiving a request which includes an Expect request-header
2072        field with the "100-continue" expectation, an origin server &MUST;
2073        either respond with 100 (Continue) status and continue to read
2074        from the input stream, or respond with a final status code. The
2075        origin server &MUST-NOT; wait for the request body before sending
2076        the 100 (Continue) response. If it responds with a final status
2077        code, it &MAY; close the transport connection or it &MAY; continue
2078        to read and discard the rest of the request.  It &MUST-NOT;
2079        perform the requested method if it returns a final status code.
2080    </t>
2081    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2082        the request message does not include an Expect request-header
2083        field with the "100-continue" expectation, and &MUST-NOT; send a
2084        100 (Continue) response if such a request comes from an HTTP/1.0
2085        (or earlier) client. There is an exception to this rule: for
2086        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2087        status in response to an HTTP/1.1 PUT or POST request that does
2088        not include an Expect request-header field with the "100-continue"
2089        expectation. This exception, the purpose of which is
2090        to minimize any client processing delays associated with an
2091        undeclared wait for 100 (Continue) status, applies only to
2092        HTTP/1.1 requests, and not to requests with any other HTTP-version
2093        value.
2094    </t>
2095    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2096        already received some or all of the request body for the
2097        corresponding request.
2098    </t>
2099    <t> An origin server that sends a 100 (Continue) response &MUST;
2100    ultimately send a final status code, once the request body is
2101        received and processed, unless it terminates the transport
2102        connection prematurely.
2103    </t>
2104    <t> If an origin server receives a request that does not include an
2105        Expect request-header field with the "100-continue" expectation,
2106        the request includes a request body, and the server responds
2107        with a final status code before reading the entire request body
2108        from the transport connection, then the server &SHOULD-NOT;  close
2109        the transport connection until it has read the entire request,
2110        or until the client closes the connection. Otherwise, the client
2111        might not reliably receive the response message. However, this
2112        requirement is not be construed as preventing a server from
2113        defending itself against denial-of-service attacks, or from
2114        badly broken client implementations.
2115      </t>
2116    </list>
2117</t>
2118<t>
2119   Requirements for HTTP/1.1 proxies:
2120  <list style="symbols">
2121    <t> If a proxy receives a request that includes an Expect request-header
2122        field with the "100-continue" expectation, and the proxy
2123        either knows that the next-hop server complies with HTTP/1.1 or
2124        higher, or does not know the HTTP version of the next-hop
2125        server, it &MUST; forward the request, including the Expect header
2126        field.
2127    </t>
2128    <t> If the proxy knows that the version of the next-hop server is
2129        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2130        respond with a 417 (Expectation Failed) status.
2131    </t>
2132    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2133        numbers received from recently-referenced next-hop servers.
2134    </t>
2135    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2136        request message was received from an HTTP/1.0 (or earlier)
2137        client and did not include an Expect request-header field with
2138        the "100-continue" expectation. This requirement overrides the
2139        general rule for forwarding of 1xx responses (see &status-1xx;).
2140    </t>
2141  </list>
2142</t>
2143</section>
2144
2145<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2146<t>
2147   If an HTTP/1.1 client sends a request which includes a request body,
2148   but which does not include an Expect request-header field with the
2149   "100-continue" expectation, and if the client is not directly
2150   connected to an HTTP/1.1 origin server, and if the client sees the
2151   connection close before receiving any status from the server, the
2152   client &SHOULD; retry the request.  If the client does retry this
2153   request, it &MAY; use the following "binary exponential backoff"
2154   algorithm to be assured of obtaining a reliable response:
2155  <list style="numbers">
2156    <t>
2157      Initiate a new connection to the server
2158    </t>
2159    <t>
2160      Transmit the request-headers
2161    </t>
2162    <t>
2163      Initialize a variable R to the estimated round-trip time to the
2164         server (e.g., based on the time it took to establish the
2165         connection), or to a constant value of 5 seconds if the round-trip
2166         time is not available.
2167    </t>
2168    <t>
2169       Compute T = R * (2**N), where N is the number of previous
2170         retries of this request.
2171    </t>
2172    <t>
2173       Wait either for an error response from the server, or for T
2174         seconds (whichever comes first)
2175    </t>
2176    <t>
2177       If no error response is received, after T seconds transmit the
2178         body of the request.
2179    </t>
2180    <t>
2181       If client sees that the connection is closed prematurely,
2182         repeat from step 1 until the request is accepted, an error
2183         response is received, or the user becomes impatient and
2184         terminates the retry process.
2185    </t>
2186  </list>
2187</t>
2188<t>
2189   If at any point an error status is received, the client
2190  <list style="symbols">
2191      <t>&SHOULD-NOT;  continue and</t>
2192
2193      <t>&SHOULD; close the connection if it has not completed sending the
2194        request message.</t>
2195    </list>
2196</t>
2197</section>
2198</section>
2199</section>
2200
2201
2202<section title="Header Field Definitions" anchor="header.fields">
2203<t>
2204   This section defines the syntax and semantics of HTTP/1.1 header fields
2205   related to message framing and transport protocols.
2206</t>
2207<t>
2208   For entity-header fields, both sender and recipient refer to either the
2209   client or the server, depending on who sends and who receives the entity.
2210</t>
2211
2212<section title="Connection" anchor="header.connection">
2213  <iref primary="true" item="Connection header" x:for-anchor=""/>
2214  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2215<t>
2216   The Connection general-header field allows the sender to specify
2217   options that are desired for that particular connection and &MUST-NOT;
2218   be communicated by proxies over further connections.
2219</t>
2220<t>
2221   The Connection header has the following grammar:
2222</t>
2223<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2224  Connection = "Connection" ":" 1#(connection-token)
2225  connection-token  = token
2226</artwork></figure>
2227<t>
2228   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2229   message is forwarded and, for each connection-token in this field,
2230   remove any header field(s) from the message with the same name as the
2231   connection-token. Connection options are signaled by the presence of
2232   a connection-token in the Connection header field, not by any
2233   corresponding additional header field(s), since the additional header
2234   field may not be sent if there are no parameters associated with that
2235   connection option.
2236</t>
2237<t>
2238   Message headers listed in the Connection header &MUST-NOT; include
2239   end-to-end headers, such as Cache-Control.
2240</t>
2241<t>
2242   HTTP/1.1 defines the "close" connection option for the sender to
2243   signal that the connection will be closed after completion of the
2244   response. For example,
2245</t>
2246<figure><artwork type="example">
2247    Connection: close
2248</artwork></figure>
2249<t>
2250   in either the request or the response header fields indicates that
2251   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2252   after the current request/response is complete.
2253</t>
2254<t>
2255   An HTTP/1.1 client that does not support persistent connections &MUST;
2256   include the "close" connection option in every request message.
2257</t>
2258<t>
2259   An HTTP/1.1 server that does not support persistent connections &MUST;
2260   include the "close" connection option in every response message that
2261   does not have a 1xx (informational) status code.
2262</t>
2263<t>
2264   A system receiving an HTTP/1.0 (or lower-version) message that
2265   includes a Connection header &MUST;, for each connection-token in this
2266   field, remove and ignore any header field(s) from the message with
2267   the same name as the connection-token. This protects against mistaken
2268   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2269</t>
2270</section>
2271
2272<section title="Content-Length" anchor="header.content-length">
2273  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2274  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2275<t>
2276   The Content-Length entity-header field indicates the size of the
2277   entity-body, in decimal number of OCTETs, sent to the recipient or,
2278   in the case of the HEAD method, the size of the entity-body that
2279   would have been sent had the request been a GET.
2280</t>
2281<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2282  Content-Length    = "Content-Length" ":" 1*DIGIT
2283</artwork></figure>
2284<t>
2285   An example is
2286</t>
2287<figure><artwork type="example">
2288    Content-Length: 3495
2289</artwork></figure>
2290<t>
2291   Applications &SHOULD; use this field to indicate the transfer-length of
2292   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2293</t>
2294<t>
2295   Any Content-Length greater than or equal to zero is a valid value.
2296   <xref target="message.length"/> describes how to determine the length of a message-body
2297   if a Content-Length is not given.
2298</t>
2299<t>
2300   Note that the meaning of this field is significantly different from
2301   the corresponding definition in MIME, where it is an optional field
2302   used within the "message/external-body" content-type. In HTTP, it
2303   &SHOULD; be sent whenever the message's length can be determined prior
2304   to being transferred, unless this is prohibited by the rules in
2305   <xref target="message.length"/>.
2306</t>
2307</section>
2308
2309<section title="Date" anchor="header.date">
2310  <iref primary="true" item="Date header" x:for-anchor=""/>
2311  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2312<t>
2313   The Date general-header field represents the date and time at which
2314   the message was originated, having the same semantics as orig-date in
2315   <xref target="RFC2822" x:fmt="of" x:sec="3.6.1"/>. The field value is an HTTP-date, as described in <xref target="full.date"/>;
2316   it &MUST; be sent in rfc1123-date format.
2317</t>
2318<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2319  Date  = "Date" ":" HTTP-date
2320</artwork></figure>
2321<t>
2322   An example is
2323</t>
2324<figure><artwork type="example">
2325    Date: Tue, 15 Nov 1994 08:12:31 GMT
2326</artwork></figure>
2327<t>
2328   Origin servers &MUST; include a Date header field in all responses,
2329   except in these cases:
2330  <list style="numbers">
2331      <t>If the response status code is 100 (Continue) or 101 (Switching
2332         Protocols), the response &MAY; include a Date header field, at
2333         the server's option.</t>
2334
2335      <t>If the response status code conveys a server error, e.g. 500
2336         (Internal Server Error) or 503 (Service Unavailable), and it is
2337         inconvenient or impossible to generate a valid Date.</t>
2338
2339      <t>If the server does not have a clock that can provide a
2340         reasonable approximation of the current time, its responses
2341         &MUST-NOT; include a Date header field. In this case, the rules
2342         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2343  </list>
2344</t>
2345<t>
2346   A received message that does not have a Date header field &MUST; be
2347   assigned one by the recipient if the message will be cached by that
2348   recipient or gatewayed via a protocol which requires a Date. An HTTP
2349   implementation without a clock &MUST-NOT; cache responses without
2350   revalidating them on every use. An HTTP cache, especially a shared
2351   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2352   clock with a reliable external standard.
2353</t>
2354<t>
2355   Clients &SHOULD; only send a Date header field in messages that include
2356   an entity-body, as in the case of the PUT and POST requests, and even
2357   then it is optional. A client without a clock &MUST-NOT; send a Date
2358   header field in a request.
2359</t>
2360<t>
2361   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2362   time subsequent to the generation of the message. It &SHOULD; represent
2363   the best available approximation of the date and time of message
2364   generation, unless the implementation has no means of generating a
2365   reasonably accurate date and time. In theory, the date ought to
2366   represent the moment just before the entity is generated. In
2367   practice, the date can be generated at any time during the message
2368   origination without affecting its semantic value.
2369</t>
2370
2371<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2372<t>
2373   Some origin server implementations might not have a clock available.
2374   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2375   values to a response, unless these values were associated
2376   with the resource by a system or user with a reliable clock. It &MAY;
2377   assign an Expires value that is known, at or before server
2378   configuration time, to be in the past (this allows "pre-expiration"
2379   of responses without storing separate Expires values for each
2380   resource).
2381</t>
2382</section>
2383</section>
2384
2385<section title="Host" anchor="header.host">
2386  <iref primary="true" item="Host header" x:for-anchor=""/>
2387  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2388<t>
2389   The Host request-header field specifies the Internet host and port
2390   number of the resource being requested, as obtained from the original
2391   URI given by the user or referring resource (generally an HTTP URL,
2392   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2393   the naming authority of the origin server or gateway given by the
2394   original URL. This allows the origin server or gateway to
2395   differentiate between internally-ambiguous URLs, such as the root "/"
2396   URL of a server for multiple host names on a single IP address.
2397</t>
2398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2399  Host = "Host" ":" host [ ":" port ] ; <xref target="http.url"/>
2400</artwork></figure>
2401<t>
2402   A "host" without any trailing port information implies the default
2403   port for the service requested (e.g., "80" for an HTTP URL). For
2404   example, a request on the origin server for
2405   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2406</t>
2407<figure><artwork type="example">
2408    GET /pub/WWW/ HTTP/1.1
2409    Host: www.example.org
2410</artwork></figure>
2411<t>
2412   A client &MUST; include a Host header field in all HTTP/1.1 request
2413   messages. If the requested URI does not include an Internet host
2414   name for the service being requested, then the Host header field &MUST;
2415   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2416   request message it forwards does contain an appropriate Host header
2417   field that identifies the service being requested by the proxy. All
2418   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2419   status code to any HTTP/1.1 request message which lacks a Host header
2420   field.
2421</t>
2422<t>
2423   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2424   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2425   for other requirements relating to Host.
2426</t>
2427</section>
2428
2429<section title="TE" anchor="header.te">
2430  <iref primary="true" item="TE header" x:for-anchor=""/>
2431  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2432<t>
2433   The TE request-header field indicates what extension transfer-codings
2434   it is willing to accept in the response and whether or not it is
2435   willing to accept trailer fields in a chunked transfer-coding. Its
2436   value may consist of the keyword "trailers" and/or a comma-separated
2437   list of extension transfer-coding names with optional accept
2438   parameters (as described in <xref target="transfer.codings"/>).
2439</t>
2440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2441  TE        = "TE" ":" #( t-codings )
2442  t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2443</artwork></figure>
2444<t>
2445   The presence of the keyword "trailers" indicates that the client is
2446   willing to accept trailer fields in a chunked transfer-coding, as
2447   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2448   transfer-coding values even though it does not itself represent a
2449   transfer-coding.
2450</t>
2451<t>
2452   Examples of its use are:
2453</t>
2454<figure><artwork type="example">
2455    TE: deflate
2456    TE:
2457    TE: trailers, deflate;q=0.5
2458</artwork></figure>
2459<t>
2460   The TE header field only applies to the immediate connection.
2461   Therefore, the keyword &MUST; be supplied within a Connection header
2462   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2463</t>
2464<t>
2465   A server tests whether a transfer-coding is acceptable, according to
2466   a TE field, using these rules:
2467  <list style="numbers">
2468    <x:lt>
2469      <t>The "chunked" transfer-coding is always acceptable. If the
2470         keyword "trailers" is listed, the client indicates that it is
2471         willing to accept trailer fields in the chunked response on
2472         behalf of itself and any downstream clients. The implication is
2473         that, if given, the client is stating that either all
2474         downstream clients are willing to accept trailer fields in the
2475         forwarded response, or that it will attempt to buffer the
2476         response on behalf of downstream recipients.
2477      </t><t>
2478         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2479         chunked response such that a client can be assured of buffering
2480         the entire response.</t>
2481    </x:lt>
2482    <x:lt>
2483      <t>If the transfer-coding being tested is one of the transfer-codings
2484         listed in the TE field, then it is acceptable unless it
2485         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2486         qvalue of 0 means "not acceptable.")</t>
2487    </x:lt>
2488    <x:lt>
2489      <t>If multiple transfer-codings are acceptable, then the
2490         acceptable transfer-coding with the highest non-zero qvalue is
2491         preferred.  The "chunked" transfer-coding always has a qvalue
2492         of 1.</t>
2493    </x:lt>
2494  </list>
2495</t>
2496<t>
2497   If the TE field-value is empty or if no TE field is present, the only
2498   transfer-coding  is "chunked". A message with no transfer-coding is
2499   always acceptable.
2500</t>
2501</section>
2502
2503<section title="Trailer" anchor="header.trailer">
2504  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2505  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2506<t>
2507   The Trailer general field value indicates that the given set of
2508   header fields is present in the trailer of a message encoded with
2509   chunked transfer-coding.
2510</t>
2511<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2512  Trailer  = "Trailer" ":" 1#field-name
2513</artwork></figure>
2514<t>
2515   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2516   message using chunked transfer-coding with a non-empty trailer. Doing
2517   so allows the recipient to know which header fields to expect in the
2518   trailer.
2519</t>
2520<t>
2521   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2522   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2523   trailer fields in a "chunked" transfer-coding.
2524</t>
2525<t>
2526   Message header fields listed in the Trailer header field &MUST-NOT;
2527   include the following header fields:
2528  <list style="symbols">
2529    <t>Transfer-Encoding</t>
2530    <t>Content-Length</t>
2531    <t>Trailer</t>
2532  </list>
2533</t>
2534</section>
2535
2536<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2537  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2538  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2539<t>
2540   The Transfer-Encoding general-header field indicates what (if any)
2541   type of transformation has been applied to the message body in order
2542   to safely transfer it between the sender and the recipient. This
2543   differs from the content-coding in that the transfer-coding is a
2544   property of the message, not of the entity.
2545</t>
2546<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2547  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2548</artwork></figure>
2549<t>
2550   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2551</t>
2552<figure><artwork type="example">
2553  Transfer-Encoding: chunked
2554</artwork></figure>
2555<t>
2556   If multiple encodings have been applied to an entity, the transfer-codings
2557   &MUST; be listed in the order in which they were applied.
2558   Additional information about the encoding parameters &MAY; be provided
2559   by other entity-header fields not defined by this specification.
2560</t>
2561<t>
2562   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2563   header.
2564</t>
2565</section>
2566
2567<section title="Upgrade" anchor="header.upgrade">
2568  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2569  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2570<t>
2571   The Upgrade general-header allows the client to specify what
2572   additional communication protocols it supports and would like to use
2573   if the server finds it appropriate to switch protocols. The server
2574   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2575   response to indicate which protocol(s) are being switched.
2576</t>
2577<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2578  Upgrade        = "Upgrade" ":" 1#product
2579</artwork></figure>
2580<t>
2581   For example,
2582</t>
2583<figure><artwork type="example">
2584    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2585</artwork></figure>
2586<t>
2587   The Upgrade header field is intended to provide a simple mechanism
2588   for transition from HTTP/1.1 to some other, incompatible protocol. It
2589   does so by allowing the client to advertise its desire to use another
2590   protocol, such as a later version of HTTP with a higher major version
2591   number, even though the current request has been made using HTTP/1.1.
2592   This eases the difficult transition between incompatible protocols by
2593   allowing the client to initiate a request in the more commonly
2594   supported protocol while indicating to the server that it would like
2595   to use a "better" protocol if available (where "better" is determined
2596   by the server, possibly according to the nature of the method and/or
2597   resource being requested).
2598</t>
2599<t>
2600   The Upgrade header field only applies to switching application-layer
2601   protocols upon the existing transport-layer connection. Upgrade
2602   cannot be used to insist on a protocol change; its acceptance and use
2603   by the server is optional. The capabilities and nature of the
2604   application-layer communication after the protocol change is entirely
2605   dependent upon the new protocol chosen, although the first action
2606   after changing the protocol &MUST; be a response to the initial HTTP
2607   request containing the Upgrade header field.
2608</t>
2609<t>
2610   The Upgrade header field only applies to the immediate connection.
2611   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2612   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2613   HTTP/1.1 message.
2614</t>
2615<t>
2616   The Upgrade header field cannot be used to indicate a switch to a
2617   protocol on a different connection. For that purpose, it is more
2618   appropriate to use a 301, 302, 303, or 305 redirection response.
2619</t>
2620<t>
2621   This specification only defines the protocol name "HTTP" for use by
2622   the family of Hypertext Transfer Protocols, as defined by the HTTP
2623   version rules of <xref target="http.version"/> and future updates to this
2624   specification. Any token can be used as a protocol name; however, it
2625   will only be useful if both the client and server associate the name
2626   with the same protocol.
2627</t>
2628</section>
2629
2630<section title="Via" anchor="header.via">
2631  <iref primary="true" item="Via header" x:for-anchor=""/>
2632  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2633<t>
2634   The Via general-header field &MUST; be used by gateways and proxies to
2635   indicate the intermediate protocols and recipients between the user
2636   agent and the server on requests, and between the origin server and
2637   the client on responses. It is analogous to the "Received" field of
2638   <xref target="RFC2822"/> and is intended to be used for tracking message forwards,
2639   avoiding request loops, and identifying the protocol capabilities of
2640   all senders along the request/response chain.
2641</t>
2642<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"/>
2643  Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2644  received-protocol = [ protocol-name "/" ] protocol-version
2645  protocol-name     = token
2646  protocol-version  = token
2647  received-by       = ( host [ ":" port ] ) | pseudonym
2648  pseudonym         = token
2649</artwork></figure>
2650<t>
2651   The received-protocol indicates the protocol version of the message
2652   received by the server or client along each segment of the
2653   request/response chain. The received-protocol version is appended to
2654   the Via field value when the message is forwarded so that information
2655   about the protocol capabilities of upstream applications remains
2656   visible to all recipients.
2657</t>
2658<t>
2659   The protocol-name is optional if and only if it would be "HTTP". The
2660   received-by field is normally the host and optional port number of a
2661   recipient server or client that subsequently forwarded the message.
2662   However, if the real host is considered to be sensitive information,
2663   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2664   be assumed to be the default port of the received-protocol.
2665</t>
2666<t>
2667   Multiple Via field values represents each proxy or gateway that has
2668   forwarded the message. Each recipient &MUST; append its information
2669   such that the end result is ordered according to the sequence of
2670   forwarding applications.
2671</t>
2672<t>
2673   Comments &MAY; be used in the Via header field to identify the software
2674   of the recipient proxy or gateway, analogous to the User-Agent and
2675   Server header fields. However, all comments in the Via field are
2676   optional and &MAY; be removed by any recipient prior to forwarding the
2677   message.
2678</t>
2679<t>
2680   For example, a request message could be sent from an HTTP/1.0 user
2681   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2682   forward the request to a public proxy at p.example.net, which completes
2683   the request by forwarding it to the origin server at www.example.com.
2684   The request received by www.example.com would then have the following
2685   Via header field:
2686</t>
2687<figure><artwork type="example">
2688    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2689</artwork></figure>
2690<t>
2691   Proxies and gateways used as a portal through a network firewall
2692   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2693   the firewall region. This information &SHOULD; only be propagated if
2694   explicitly enabled. If not enabled, the received-by host of any host
2695   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2696   for that host.
2697</t>
2698<t>
2699   For organizations that have strong privacy requirements for hiding
2700   internal structures, a proxy &MAY; combine an ordered subsequence of
2701   Via header field entries with identical received-protocol values into
2702   a single such entry. For example,
2703</t>
2704<figure><artwork type="example">
2705    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2706</artwork></figure>
2707<t>
2708        could be collapsed to
2709</t>
2710<figure><artwork type="example">
2711    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2712</artwork></figure>
2713<t>
2714   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2715   under the same organizational control and the hosts have already been
2716   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2717   have different received-protocol values.
2718</t>
2719</section>
2720
2721</section>
2722
2723<section title="IANA Considerations" anchor="IANA.considerations">
2724<t>
2725   TBD.
2726</t>
2727</section>
2728
2729<section title="Security Considerations" anchor="security.considerations">
2730<t>
2731   This section is meant to inform application developers, information
2732   providers, and users of the security limitations in HTTP/1.1 as
2733   described by this document. The discussion does not include
2734   definitive solutions to the problems revealed, though it does make
2735   some suggestions for reducing security risks.
2736</t>
2737
2738<section title="Personal Information" anchor="personal.information">
2739<t>
2740   HTTP clients are often privy to large amounts of personal information
2741   (e.g. the user's name, location, mail address, passwords, encryption
2742   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2743   leakage of this information via the HTTP protocol to other sources.
2744   We very strongly recommend that a convenient interface be provided
2745   for the user to control dissemination of such information, and that
2746   designers and implementors be particularly careful in this area.
2747   History shows that errors in this area often create serious security
2748   and/or privacy problems and generate highly adverse publicity for the
2749   implementor's company.
2750</t>
2751</section>
2752
2753<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2754<t>
2755   A server is in the position to save personal data about a user's
2756   requests which might identify their reading patterns or subjects of
2757   interest. This information is clearly confidential in nature and its
2758   handling can be constrained by law in certain countries. People using
2759   the HTTP protocol to provide data are responsible for ensuring that
2760   such material is not distributed without the permission of any
2761   individuals that are identifiable by the published results.
2762</t>
2763</section>
2764
2765<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2766<t>
2767   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2768   the documents returned by HTTP requests to be only those that were
2769   intended by the server administrators. If an HTTP server translates
2770   HTTP URIs directly into file system calls, the server &MUST; take
2771   special care not to serve files that were not intended to be
2772   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2773   other operating systems use ".." as a path component to indicate a
2774   directory level above the current one. On such a system, an HTTP
2775   server &MUST; disallow any such construct in the Request-URI if it
2776   would otherwise allow access to a resource outside those intended to
2777   be accessible via the HTTP server. Similarly, files intended for
2778   reference only internally to the server (such as access control
2779   files, configuration files, and script code) &MUST; be protected from
2780   inappropriate retrieval, since they might contain sensitive
2781   information. Experience has shown that minor bugs in such HTTP server
2782   implementations have turned into security risks.
2783</t>
2784</section>
2785
2786<section title="DNS Spoofing" anchor="dns.spoofing">
2787<t>
2788   Clients using HTTP rely heavily on the Domain Name Service, and are
2789   thus generally prone to security attacks based on the deliberate
2790   mis-association of IP addresses and DNS names. Clients need to be
2791   cautious in assuming the continuing validity of an IP number/DNS name
2792   association.
2793</t>
2794<t>
2795   In particular, HTTP clients &SHOULD; rely on their name resolver for
2796   confirmation of an IP number/DNS name association, rather than
2797   caching the result of previous host name lookups. Many platforms
2798   already can cache host name lookups locally when appropriate, and
2799   they &SHOULD; be configured to do so. It is proper for these lookups to
2800   be cached, however, only when the TTL (Time To Live) information
2801   reported by the name server makes it likely that the cached
2802   information will remain useful.
2803</t>
2804<t>
2805   If HTTP clients cache the results of host name lookups in order to
2806   achieve a performance improvement, they &MUST; observe the TTL
2807   information reported by DNS.
2808</t>
2809<t>
2810   If HTTP clients do not observe this rule, they could be spoofed when
2811   a previously-accessed server's IP address changes. As network
2812   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2813   possibility of this form of attack will grow. Observing this
2814   requirement thus reduces this potential security vulnerability.
2815</t>
2816<t>
2817   This requirement also improves the load-balancing behavior of clients
2818   for replicated servers using the same DNS name and reduces the
2819   likelihood of a user's experiencing failure in accessing sites which
2820   use that strategy.
2821</t>
2822</section>
2823
2824<section title="Proxies and Caching" anchor="attack.proxies">
2825<t>
2826   By their very nature, HTTP proxies are men-in-the-middle, and
2827   represent an opportunity for man-in-the-middle attacks. Compromise of
2828   the systems on which the proxies run can result in serious security
2829   and privacy problems. Proxies have access to security-related
2830   information, personal information about individual users and
2831   organizations, and proprietary information belonging to users and
2832   content providers. A compromised proxy, or a proxy implemented or
2833   configured without regard to security and privacy considerations,
2834   might be used in the commission of a wide range of potential attacks.
2835</t>
2836<t>
2837   Proxy operators should protect the systems on which proxies run as
2838   they would protect any system that contains or transports sensitive
2839   information. In particular, log information gathered at proxies often
2840   contains highly sensitive personal information, and/or information
2841   about organizations. Log information should be carefully guarded, and
2842   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2843</t>
2844<t>
2845   Proxy implementors should consider the privacy and security
2846   implications of their design and coding decisions, and of the
2847   configuration options they provide to proxy operators (especially the
2848   default configuration).
2849</t>
2850<t>
2851   Users of a proxy need to be aware that they are no trustworthier than
2852   the people who run the proxy; HTTP itself cannot solve this problem.
2853</t>
2854<t>
2855   The judicious use of cryptography, when appropriate, may suffice to
2856   protect against a broad range of security and privacy attacks. Such
2857   cryptography is beyond the scope of the HTTP/1.1 specification.
2858</t>
2859</section>
2860
2861<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2862<t>
2863   They exist. They are hard to defend against. Research continues.
2864   Beware.
2865</t>
2866</section>
2867</section>
2868
2869<section title="Acknowledgments" anchor="ack">
2870<t>
2871   This specification makes heavy use of the augmented BNF and generic
2872   constructs defined by David H. Crocker for <xref target="RFC822ABNF"/>. Similarly, it
2873   reuses many of the definitions provided by Nathaniel Borenstein and
2874   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2875   specification will help reduce past confusion over the relationship
2876   between HTTP and Internet mail message formats.
2877</t>
2878<t>
2879   The HTTP protocol has evolved considerably over the years. It has
2880   benefited from a large and active developer community--the many
2881   people who have participated on the www-talk mailing list--and it is
2882   that community which has been most responsible for the success of
2883   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2884   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2885   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2886   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2887   VanHeyningen deserve special recognition for their efforts in
2888   defining early aspects of the protocol.
2889</t>
2890<t>
2891   This document has benefited greatly from the comments of all those
2892   participating in the HTTP-WG. In addition to those already mentioned,
2893   the following individuals have contributed to this specification:
2894</t>
2895<t>
2896   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2897   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2898   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2899   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2900   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2901   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2902   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2903   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2904   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2905   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2906   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2907   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2908   Josh Cohen.
2909</t>
2910<t>
2911   Thanks to the "cave men" of Palo Alto. You know who you are.
2912</t>
2913<t>
2914   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2915   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2916   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2917   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2918   Larry Masinter for their help. And thanks go particularly to Jeff
2919   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2920</t>
2921<t>
2922   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
2923   Frystyk implemented RFC 2068 early, and we wish to thank them for the
2924   discovery of many of the problems that this document attempts to
2925   rectify.
2926</t>
2927</section>
2928
2929</middle>
2930<back>
2931
2932<references title="Normative References">
2933
2934<reference anchor="ISO-8859-1">
2935  <front>
2936    <title>
2937     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2938    </title>
2939    <author>
2940      <organization>International Organization for Standardization</organization>
2941    </author>
2942    <date year="1998"/>
2943  </front>
2944  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2945</reference>
2946
2947<reference anchor="Part2">
2948  <front>
2949    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2950    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2951      <organization abbrev="Day Software">Day Software</organization>
2952      <address><email>fielding@gbiv.com</email></address>
2953    </author>
2954    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2955      <organization>One Laptop per Child</organization>
2956      <address><email>jg@laptop.org</email></address>
2957    </author>
2958    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2959      <organization abbrev="HP">Hewlett-Packard Company</organization>
2960      <address><email>JeffMogul@acm.org</email></address>
2961    </author>
2962    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2963      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2964      <address><email>henrikn@microsoft.com</email></address>
2965    </author>
2966    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2967      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2968      <address><email>LMM@acm.org</email></address>
2969    </author>
2970    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2971      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2972      <address><email>paulle@microsoft.com</email></address>
2973    </author>
2974    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2975      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2976      <address><email>timbl@w3.org</email></address>
2977    </author>
2978    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2979      <organization abbrev="W3C">World Wide Web Consortium</organization>
2980      <address><email>ylafon@w3.org</email></address>
2981    </author>
2982    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2983      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2984      <address><email>julian.reschke@greenbytes.de</email></address>
2985    </author>
2986    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2987  </front>
2988  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2989  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2990</reference>
2991
2992<reference anchor="Part3">
2993  <front>
2994    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2995    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2996      <organization abbrev="Day Software">Day Software</organization>
2997      <address><email>fielding@gbiv.com</email></address>
2998    </author>
2999    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3000      <organization>One Laptop per Child</organization>
3001      <address><email>jg@laptop.org</email></address>
3002    </author>
3003    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3004      <organization abbrev="HP">Hewlett-Packard Company</organization>
3005      <address><email>JeffMogul@acm.org</email></address>
3006    </author>
3007    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3008      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3009      <address><email>henrikn@microsoft.com</email></address>
3010    </author>
3011    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3012      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3013      <address><email>LMM@acm.org</email></address>
3014    </author>
3015    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3016      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3017      <address><email>paulle@microsoft.com</email></address>
3018    </author>
3019    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3020      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3021      <address><email>timbl@w3.org</email></address>
3022    </author>
3023    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3024      <organization abbrev="W3C">World Wide Web Consortium</organization>
3025      <address><email>ylafon@w3.org</email></address>
3026    </author>
3027    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3028      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3029      <address><email>julian.reschke@greenbytes.de</email></address>
3030    </author>
3031    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3032  </front>
3033  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3034  <x:source href="p3-payload.xml" basename="p3-payload"/>
3035</reference>
3036
3037<reference anchor="Part5">
3038  <front>
3039    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3040    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3041      <organization abbrev="Day Software">Day Software</organization>
3042      <address><email>fielding@gbiv.com</email></address>
3043    </author>
3044    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3045      <organization>One Laptop per Child</organization>
3046      <address><email>jg@laptop.org</email></address>
3047    </author>
3048    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3049      <organization abbrev="HP">Hewlett-Packard Company</organization>
3050      <address><email>JeffMogul@acm.org</email></address>
3051    </author>
3052    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3053      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3054      <address><email>henrikn@microsoft.com</email></address>
3055    </author>
3056    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3057      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3058      <address><email>LMM@acm.org</email></address>
3059    </author>
3060    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3061      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3062      <address><email>paulle@microsoft.com</email></address>
3063    </author>
3064    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3065      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3066      <address><email>timbl@w3.org</email></address>
3067    </author>
3068    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3069      <organization abbrev="W3C">World Wide Web Consortium</organization>
3070      <address><email>ylafon@w3.org</email></address>
3071    </author>
3072    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3073      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3074      <address><email>julian.reschke@greenbytes.de</email></address>
3075    </author>
3076    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3077  </front>
3078  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3079  <x:source href="p5-range.xml" basename="p5-range"/>
3080</reference>
3081
3082<reference anchor="Part6">
3083  <front>
3084    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3085    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3086      <organization abbrev="Day Software">Day Software</organization>
3087      <address><email>fielding@gbiv.com</email></address>
3088    </author>
3089    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3090      <organization>One Laptop per Child</organization>
3091      <address><email>jg@laptop.org</email></address>
3092    </author>
3093    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3094      <organization abbrev="HP">Hewlett-Packard Company</organization>
3095      <address><email>JeffMogul@acm.org</email></address>
3096    </author>
3097    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3098      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3099      <address><email>henrikn@microsoft.com</email></address>
3100    </author>
3101    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3102      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3103      <address><email>LMM@acm.org</email></address>
3104    </author>
3105    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3106      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3107      <address><email>paulle@microsoft.com</email></address>
3108    </author>
3109    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3110      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3111      <address><email>timbl@w3.org</email></address>
3112    </author>
3113    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3114      <organization abbrev="W3C">World Wide Web Consortium</organization>
3115      <address><email>ylafon@w3.org</email></address>
3116    </author>
3117    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3118      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3119      <address><email>julian.reschke@greenbytes.de</email></address>
3120    </author>
3121    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3122  </front>
3123  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3124  <x:source href="p6-cache.xml" basename="p6-cache"/>
3125</reference>
3126
3127<reference anchor="RFC822ABNF">
3128  <front>
3129    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3130    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3131      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3132      <address><email>DCrocker@UDel-Relay</email></address>
3133    </author>
3134    <date month="August" day="13" year="1982"/>
3135  </front>
3136  <seriesInfo name="STD" value="11"/>
3137  <seriesInfo name="RFC" value="822"/>
3138</reference>
3139
3140<reference anchor="RFC2045">
3141  <front>
3142    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3143    <author initials="N." surname="Freed" fullname="Ned Freed">
3144      <organization>Innosoft International, Inc.</organization>
3145      <address><email>ned@innosoft.com</email></address>
3146    </author>
3147    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3148      <organization>First Virtual Holdings</organization>
3149      <address><email>nsb@nsb.fv.com</email></address>
3150    </author>
3151    <date month="November" year="1996"/>
3152  </front>
3153  <seriesInfo name="RFC" value="2045"/>
3154</reference>
3155
3156<reference anchor="RFC2047">
3157  <front>
3158    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3159    <author initials="K." surname="Moore" fullname="Keith Moore">
3160      <organization>University of Tennessee</organization>
3161      <address><email>moore@cs.utk.edu</email></address>
3162    </author>
3163    <date month="November" year="1996"/>
3164  </front>
3165  <seriesInfo name="RFC" value="2047"/>
3166</reference>
3167
3168<reference anchor="RFC2119">
3169  <front>
3170    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3171    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3172      <organization>Harvard University</organization>
3173      <address><email>sob@harvard.edu</email></address>
3174    </author>
3175    <date month="March" year="1997"/>
3176  </front>
3177  <seriesInfo name="BCP" value="14"/>
3178  <seriesInfo name="RFC" value="2119"/>
3179</reference>
3180
3181<reference anchor="RFC2396">
3182  <front>
3183    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3184    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3185      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3186      <address><email>timbl@w3.org</email></address>
3187    </author>
3188    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3189      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3190      <address><email>fielding@ics.uci.edu</email></address>
3191    </author>
3192    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3193      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3194      <address><email>masinter@parc.xerox.com</email></address>
3195    </author>
3196    <date month="August" year="1998"/>
3197  </front>
3198  <seriesInfo name="RFC" value="2396"/>
3199</reference>
3200
3201<reference anchor="RFC4288">
3202  <front>
3203    <title>Media Type Specifications and Registration Procedures</title>
3204    <author initials="N." surname="Freed" fullname="N. Freed">
3205      <organization>Sun Microsystems</organization>
3206      <address>
3207        <email>ned.freed@mrochek.com</email>
3208      </address>
3209    </author>
3210    <author initials="J." surname="Klensin" fullname="J. Klensin">
3211      <organization/>
3212      <address>
3213        <email>klensin+ietf@jck.com</email>
3214      </address>
3215    </author>
3216    <date year="2005" month="December"/>
3217  </front>
3218  <seriesInfo name="BCP" value="13"/>
3219  <seriesInfo name="RFC" value="4288"/>
3220</reference>
3221
3222<reference anchor="USASCII">
3223  <front>
3224    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3225    <author>
3226      <organization>American National Standards Institute</organization>
3227    </author>
3228    <date year="1986"/>
3229  </front>
3230  <seriesInfo name="ANSI" value="X3.4"/>
3231</reference>
3232
3233</references>
3234
3235<references title="Informative References">
3236
3237<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3238  <front>
3239    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3240    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3241      <organization/>
3242    </author>
3243    <author initials="J." surname="Gettys" fullname="J. Gettys">
3244      <organization/>
3245    </author>
3246    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3247      <organization/>
3248    </author>
3249    <author initials="H." surname="Lie" fullname="H. Lie">
3250      <organization/>
3251    </author>
3252    <author initials="C." surname="Lilley" fullname="C. Lilley">
3253      <organization/>
3254    </author>
3255    <date year="1997" month="September"/>
3256  </front>
3257  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3258</reference>
3259
3260<reference anchor="Pad1995">
3261  <front>
3262    <title>Improving HTTP Latency</title>
3263    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3264      <organization/>
3265    </author>
3266    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3267      <organization/>
3268    </author>
3269    <date year="1995" month="December"/>
3270  </front>
3271  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3272  <annotation>
3273    Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3274    which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3275  </annotation>
3276</reference>
3277
3278<reference anchor="RFC822">
3279  <front>
3280    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3281    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3282      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3283      <address><email>DCrocker@UDel-Relay</email></address>
3284    </author>
3285    <date month="August" day="13" year="1982"/>
3286  </front>
3287  <seriesInfo name="STD" value="11"/>
3288  <seriesInfo name="RFC" value="822"/>
3289</reference>
3290
3291<reference anchor="RFC959">
3292  <front>
3293    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3294    <author initials="J." surname="Postel" fullname="J. Postel">
3295      <organization>Information Sciences Institute (ISI)</organization>
3296    </author>
3297    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3298      <organization/>
3299    </author>
3300    <date month="October" year="1985"/>
3301  </front>
3302  <seriesInfo name="STD" value="9"/>
3303  <seriesInfo name="RFC" value="959"/>
3304</reference>
3305
3306<reference anchor="RFC1123">
3307  <front>
3308    <title>Requirements for Internet Hosts - Application and Support</title>
3309    <author initials="R." surname="Braden" fullname="Robert Braden">
3310      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3311      <address><email>Braden@ISI.EDU</email></address>
3312    </author>
3313    <date month="October" year="1989"/>
3314  </front>
3315  <seriesInfo name="STD" value="3"/>
3316  <seriesInfo name="RFC" value="1123"/>
3317</reference>
3318
3319<reference anchor="RFC1305">
3320  <front>
3321    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3322    <author initials="D." surname="Mills" fullname="David L. Mills">
3323      <organization>University of Delaware, Electrical Engineering Department</organization>
3324      <address><email>mills@udel.edu</email></address>
3325    </author>
3326    <date month="March" year="1992"/>
3327  </front>
3328  <seriesInfo name="RFC" value="1305"/>
3329</reference>
3330
3331<reference anchor="RFC1436">
3332  <front>
3333    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3334    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3335      <organization>University of Minnesota, Computer and Information Services</organization>
3336      <address><email>fxa@boombox.micro.umn.edu</email></address>
3337    </author>
3338    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3339      <organization>University of Minnesota, Computer and Information Services</organization>
3340      <address><email>mpm@boombox.micro.umn.edu</email></address>
3341    </author>
3342    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3343      <organization>University of Minnesota, Computer and Information Services</organization>
3344      <address><email>lindner@boombox.micro.umn.edu</email></address>
3345    </author>
3346    <author initials="D." surname="Johnson" fullname="David Johnson">
3347      <organization>University of Minnesota, Computer and Information Services</organization>
3348      <address><email>dmj@boombox.micro.umn.edu</email></address>
3349    </author>
3350    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3351      <organization>University of Minnesota, Computer and Information Services</organization>
3352      <address><email>daniel@boombox.micro.umn.edu</email></address>
3353    </author>
3354    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3355      <organization>University of Minnesota, Computer and Information Services</organization>
3356      <address><email>alberti@boombox.micro.umn.edu</email></address>
3357    </author>
3358    <date month="March" year="1993"/>
3359  </front>
3360  <seriesInfo name="RFC" value="1436"/>
3361</reference>
3362
3363<reference anchor="RFC1630">
3364  <front>
3365    <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>
3366    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3367      <organization>CERN, World-Wide Web project</organization>
3368      <address><email>timbl@info.cern.ch</email></address>
3369    </author>
3370    <date month="June" year="1994"/>
3371  </front>
3372  <seriesInfo name="RFC" value="1630"/>
3373</reference>
3374
3375<reference anchor="RFC1737">
3376  <front>
3377    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3378    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3379      <organization>Xerox Palo Alto Research Center</organization>
3380      <address><email>masinter@parc.xerox.com</email></address>
3381    </author>
3382    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3383      <organization>MIT Laboratory for Computer Science</organization>
3384      <address><email>sollins@lcs.mit.edu</email></address>
3385    </author>
3386    <date month="December" year="1994"/>
3387  </front>
3388  <seriesInfo name="RFC" value="1737"/>
3389</reference>
3390
3391<reference anchor="RFC1738">
3392  <front>
3393    <title>Uniform Resource Locators (URL)</title>
3394    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3395      <organization>CERN, World-Wide Web project</organization>
3396      <address><email>timbl@info.cern.ch</email></address>
3397    </author>
3398    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3399      <organization>Xerox PARC</organization>
3400      <address><email>masinter@parc.xerox.com</email></address>
3401    </author>
3402    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3403      <organization>University of Minnesota, Computer and Information Services</organization>
3404      <address><email>mpm@boombox.micro.umn.edu</email></address>
3405    </author>
3406    <date month="December" year="1994"/>
3407  </front>
3408  <seriesInfo name="RFC" value="1738"/>
3409</reference>
3410
3411<reference anchor="RFC1808">
3412  <front>
3413    <title>Relative Uniform Resource Locators</title>
3414    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3415      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3416      <address><email>fielding@ics.uci.edu</email></address>
3417    </author>
3418    <date month="June" year="1995"/>
3419  </front>
3420  <seriesInfo name="RFC" value="1808"/>
3421</reference>
3422
3423<reference anchor="RFC1900">
3424  <front>
3425    <title>Renumbering Needs Work</title>
3426    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3427      <organization>CERN, Computing and Networks Division</organization>
3428      <address><email>brian@dxcoms.cern.ch</email></address>
3429    </author>
3430    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3431      <organization>cisco Systems</organization>
3432      <address><email>yakov@cisco.com</email></address>
3433    </author>
3434    <date month="February" year="1996"/>
3435  </front>
3436  <seriesInfo name="RFC" value="1900"/>
3437</reference>
3438
3439<reference anchor="RFC1945">
3440  <front>
3441    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3442    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3443      <organization>MIT, Laboratory for Computer Science</organization>
3444      <address><email>timbl@w3.org</email></address>
3445    </author>
3446    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3447      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3448      <address><email>fielding@ics.uci.edu</email></address>
3449    </author>
3450    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3451      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3452      <address><email>frystyk@w3.org</email></address>
3453    </author>
3454    <date month="May" year="1996"/>
3455  </front>
3456  <seriesInfo name="RFC" value="1945"/>
3457</reference>
3458
3459<reference anchor="RFC2068">
3460  <front>
3461    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3462    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3463      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3464      <address><email>fielding@ics.uci.edu</email></address>
3465    </author>
3466    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3467      <organization>MIT Laboratory for Computer Science</organization>
3468      <address><email>jg@w3.org</email></address>
3469    </author>
3470    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3471      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3472      <address><email>mogul@wrl.dec.com</email></address>
3473    </author>
3474    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3475      <organization>MIT Laboratory for Computer Science</organization>
3476      <address><email>frystyk@w3.org</email></address>
3477    </author>
3478    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3479      <organization>MIT Laboratory for Computer Science</organization>
3480      <address><email>timbl@w3.org</email></address>
3481    </author>
3482    <date month="January" year="1997"/>
3483  </front>
3484  <seriesInfo name="RFC" value="2068"/>
3485</reference>
3486
3487<reference anchor="RFC2145">
3488  <front>
3489    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3490    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3491      <organization>Western Research Laboratory</organization>
3492      <address><email>mogul@wrl.dec.com</email></address>
3493    </author>
3494    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3495      <organization>Department of Information and Computer Science</organization>
3496      <address><email>fielding@ics.uci.edu</email></address>
3497    </author>
3498    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3499      <organization>MIT Laboratory for Computer Science</organization>
3500      <address><email>jg@w3.org</email></address>
3501    </author>
3502    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3503      <organization>W3 Consortium</organization>
3504      <address><email>frystyk@w3.org</email></address>
3505    </author>
3506    <date month="May" year="1997"/>
3507  </front>
3508  <seriesInfo name="RFC" value="2145"/>
3509</reference>
3510
3511<reference anchor="RFC2324">
3512  <front>
3513    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3514    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3515      <organization>Xerox Palo Alto Research Center</organization>
3516      <address><email>masinter@parc.xerox.com</email></address>
3517    </author>
3518    <date month="April" day="1" year="1998"/>
3519  </front>
3520  <seriesInfo name="RFC" value="2324"/>
3521</reference>
3522
3523<reference anchor="RFC2616">
3524  <front>
3525    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3526    <author initials="R." surname="Fielding" fullname="R. Fielding">
3527      <organization>University of California, Irvine</organization>
3528      <address><email>fielding@ics.uci.edu</email></address>
3529    </author>
3530    <author initials="J." surname="Gettys" fullname="J. Gettys">
3531      <organization>W3C</organization>
3532      <address><email>jg@w3.org</email></address>
3533    </author>
3534    <author initials="J." surname="Mogul" fullname="J. Mogul">
3535      <organization>Compaq Computer Corporation</organization>
3536      <address><email>mogul@wrl.dec.com</email></address>
3537    </author>
3538    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3539      <organization>MIT Laboratory for Computer Science</organization>
3540      <address><email>frystyk@w3.org</email></address>
3541    </author>
3542    <author initials="L." surname="Masinter" fullname="L. Masinter">
3543      <organization>Xerox Corporation</organization>
3544      <address><email>masinter@parc.xerox.com</email></address>
3545    </author>
3546    <author initials="P." surname="Leach" fullname="P. Leach">
3547      <organization>Microsoft Corporation</organization>
3548      <address><email>paulle@microsoft.com</email></address>
3549    </author>
3550    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3551      <organization>W3C</organization>
3552      <address><email>timbl@w3.org</email></address>
3553    </author>
3554    <date month="June" year="1999"/>
3555  </front>
3556  <seriesInfo name="RFC" value="2616"/>
3557</reference>
3558
3559<reference anchor="RFC2821">
3560  <front>
3561    <title>Simple Mail Transfer Protocol</title>
3562    <author initials="J." surname="Klensin" fullname="J. Klensin">
3563      <organization>AT&amp;T Laboratories</organization>
3564      <address><email>klensin@research.att.com</email></address>
3565    </author>
3566    <date year="2001" month="April"/>
3567  </front>
3568  <seriesInfo name="RFC" value="2821"/>
3569</reference>
3570
3571<reference anchor="RFC2822">
3572  <front>
3573    <title>Internet Message Format</title>
3574    <author initials="P." surname="Resnick" fullname="P. Resnick">
3575      <organization>QUALCOMM Incorporated</organization>
3576    </author>
3577    <date year="2001" month="April"/>
3578  </front> 
3579  <seriesInfo name="RFC" value="2822"/>
3580</reference>
3581
3582<reference anchor='RFC3977'>
3583  <front>
3584    <title>Network News Transfer Protocol (NNTP)</title>
3585    <author initials='C.' surname='Feather' fullname='C. Feather'>
3586      <organization>THUS plc</organization>
3587      <address><email>clive@demon.net</email></address>
3588    </author>
3589    <date year='2006' month='October' />
3590  </front>
3591  <seriesInfo name="RFC" value="3977"/>
3592</reference>
3593
3594<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3595  <front>
3596  <title>Analysis of HTTP Performance Problems</title>
3597  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3598    <organization/>
3599  </author>
3600  <date/>
3601  </front>
3602</reference>
3603
3604<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3605  <front>
3606  <title>Analysis of HTTP Performance</title>
3607  <author initials="J." surname="Touch" fullname="Joe Touch">
3608    <organization>USC/Information Sciences Institute</organization>
3609    <address><email>touch@isi.edu</email></address>
3610  </author>
3611  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3612    <organization>USC/Information Sciences Institute</organization>
3613    <address><email>johnh@isi.edu</email></address>
3614  </author>
3615  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3616    <organization>USC/Information Sciences Institute</organization>
3617    <address><email>katia@isi.edu</email></address>
3618  </author>
3619  <date year="1998" month="Aug"/>
3620  </front>
3621  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3622  <annotation>(original report dated Aug. 1996)</annotation>
3623</reference>
3624
3625<reference anchor="WAIS">
3626  <front>
3627    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3628    <author initials="F." surname="Davis" fullname="F. Davis">
3629      <organization>Thinking Machines Corporation</organization>
3630    </author>
3631    <author initials="B." surname="Kahle" fullname="B. Kahle">
3632      <organization>Thinking Machines Corporation</organization>
3633    </author>
3634    <author initials="H." surname="Morris" fullname="H. Morris">
3635      <organization>Thinking Machines Corporation</organization>
3636    </author>
3637    <author initials="J." surname="Salem" fullname="J. Salem">
3638      <organization>Thinking Machines Corporation</organization>
3639    </author>
3640    <author initials="T." surname="Shen" fullname="T. Shen">
3641      <organization>Thinking Machines Corporation</organization>
3642    </author>
3643    <author initials="R." surname="Wang" fullname="R. Wang">
3644      <organization>Thinking Machines Corporation</organization>
3645    </author>
3646    <author initials="J." surname="Sui" fullname="J. Sui">
3647      <organization>Thinking Machines Corporation</organization>
3648    </author>
3649    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3650      <organization>Thinking Machines Corporation</organization>
3651    </author>
3652    <date month="April" year="1990"/>
3653  </front>
3654  <seriesInfo name="Thinking Machines Corporation" value=""/>
3655</reference>
3656
3657</references>
3658
3659
3660<section title="Internet Media Types" anchor="internet.media.type.http">
3661<t>
3662   In addition to defining the HTTP/1.1 protocol, this document serves
3663   as the specification for the Internet media type "message/http" and
3664   "application/http". The following is to be registered with IANA <xref target="RFC4288"/>.
3665</t>
3666<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3667<iref item="Media Type" subitem="message/http" primary="true"/>
3668<iref item="message/http Media Type" primary="true"/>
3669<t>
3670   The message/http type can be used to enclose a single HTTP request or
3671   response message, provided that it obeys the MIME restrictions for all
3672   "message" types regarding line length and encodings.
3673</t>
3674<t>
3675  <list style="hanging" x:indent="12em">
3676    <t hangText="Type name:">
3677      message
3678    </t>
3679    <t hangText="Subtype name:">
3680      http
3681    </t>
3682    <t hangText="Required parameters:">
3683      none
3684    </t>
3685    <t hangText="Optional parameters:">
3686      version, msgtype
3687      <list style="hanging">
3688        <t hangText="version:">
3689          The HTTP-Version number of the enclosed message
3690          (e.g., "1.1"). If not present, the version can be
3691          determined from the first line of the body.
3692        </t>
3693        <t hangText="msgtype:">
3694          The message type -- "request" or "response". If not
3695          present, the type can be determined from the first
3696          line of the body.
3697        </t>
3698      </list>
3699    </t>
3700    <t hangText="Encoding considerations:">
3701      only "7bit", "8bit", or "binary" are permitted
3702    </t>
3703    <t hangText="Security considerations:">
3704      none
3705    </t>
3706    <t hangText="Interoperability considerations:">
3707      none
3708    </t>
3709    <t hangText="Published specification:">
3710      This specification (see <xref target="internet.media.type.message.http"/>).
3711    </t>
3712    <t hangText="Applications that use this media type:">
3713    </t>
3714    <t hangText="Additional information:">
3715      <list style="hanging">
3716        <t hangText="Magic number(s):">none</t>
3717        <t hangText="File extension(s):">none</t>
3718        <t hangText="Macintosh file type code(s):">none</t>
3719      </list>
3720    </t>
3721    <t hangText="Person and email address to contact for further information:">
3722      See Authors Section.
3723    </t>
3724                <t hangText="Intended usage:">
3725                  COMMON
3726    </t>
3727                <t hangText="Restrictions on usage:">
3728                  none
3729    </t>
3730    <t hangText="Author/Change controller:">
3731      IESG
3732    </t>
3733  </list>
3734</t>
3735</section>
3736<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3737<iref item="Media Type" subitem="application/http" primary="true"/>
3738<iref item="application/http Media Type" primary="true"/>
3739<t>
3740   The application/http type can be used to enclose a pipeline of one or more
3741   HTTP request or response messages (not intermixed).
3742</t>
3743<t>
3744  <list style="hanging" x:indent="12em">
3745    <t hangText="Type name:">
3746      application
3747    </t>
3748    <t hangText="Subtype name:">
3749      http
3750    </t>
3751    <t hangText="Required parameters:">
3752      none
3753    </t>
3754    <t hangText="Optional parameters:">
3755      version, msgtype
3756      <list style="hanging">
3757        <t hangText="version:">
3758          The HTTP-Version number of the enclosed messages
3759          (e.g., "1.1"). If not present, the version can be
3760          determined from the first line of the body.
3761        </t>
3762        <t hangText="msgtype:">
3763          The message type -- "request" or "response". If not
3764          present, the type can be determined from the first
3765          line of the body.
3766        </t>
3767      </list>
3768    </t>
3769    <t hangText="Encoding considerations:">
3770      HTTP messages enclosed by this type
3771      are in "binary" format; use of an appropriate
3772      Content-Transfer-Encoding is required when
3773      transmitted via E-mail.
3774    </t>
3775    <t hangText="Security considerations:">
3776      none
3777    </t>
3778    <t hangText="Interoperability considerations:">
3779      none
3780    </t>
3781    <t hangText="Published specification:">
3782      This specification (see <xref target="internet.media.type.application.http"/>).
3783    </t>
3784    <t hangText="Applications that use this media type:">
3785    </t>
3786    <t hangText="Additional information:">
3787      <list style="hanging">
3788        <t hangText="Magic number(s):">none</t>
3789        <t hangText="File extension(s):">none</t>
3790        <t hangText="Macintosh file type code(s):">none</t>
3791      </list>
3792    </t>
3793    <t hangText="Person and email address to contact for further information:">
3794      See Authors Section.
3795    </t>
3796                <t hangText="Intended usage:">
3797                  COMMON
3798    </t>
3799                <t hangText="Restrictions on usage:">
3800                  none
3801    </t>
3802    <t hangText="Author/Change controller:">
3803      IESG
3804    </t>
3805  </list>
3806</t>
3807</section>
3808</section>
3809
3810<section title="Tolerant Applications" anchor="tolerant.applications">
3811<t>
3812   Although this document specifies the requirements for the generation
3813   of HTTP/1.1 messages, not all applications will be correct in their
3814   implementation. We therefore recommend that operational applications
3815   be tolerant of deviations whenever those deviations can be
3816   interpreted unambiguously.
3817</t>
3818<t>
3819   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3820   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3821   accept any amount of SP or HTAB characters between fields, even though
3822   only a single SP is required.
3823</t>
3824<t>
3825   The line terminator for message-header fields is the sequence CRLF.
3826   However, we recommend that applications, when parsing such headers,
3827   recognize a single LF as a line terminator and ignore the leading CR.
3828</t>
3829<t>
3830   The character set of an entity-body &SHOULD; be labeled as the lowest
3831   common denominator of the character codes used within that body, with
3832   the exception that not labeling the entity is preferred over labeling
3833   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3834</t>
3835<t>
3836   Additional rules for requirements on parsing and encoding of dates
3837   and other potential problems with date encodings include:
3838</t>
3839<t>
3840  <list style="symbols">
3841     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3842        which appears to be more than 50 years in the future is in fact
3843        in the past (this helps solve the "year 2000" problem).</t>
3844
3845     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3846        Expires date as earlier than the proper value, but &MUST-NOT;
3847        internally represent a parsed Expires date as later than the
3848        proper value.</t>
3849
3850     <t>All expiration-related calculations &MUST; be done in GMT. The
3851        local time zone &MUST-NOT; influence the calculation or comparison
3852        of an age or expiration time.</t>
3853
3854     <t>If an HTTP header incorrectly carries a date value with a time
3855        zone other than GMT, it &MUST; be converted into GMT using the
3856        most conservative possible conversion.</t>
3857  </list>
3858</t>
3859</section>
3860
3861<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3862<t>
3863   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3864   simplify the process of date comparison. Proxies and gateways from
3865   other protocols &SHOULD; ensure that any Date header field present in a
3866   message conforms to one of the HTTP/1.1 formats and rewrite the date
3867   if necessary.
3868</t>
3869</section>
3870
3871<section title="Compatibility with Previous Versions" anchor="compatibility">
3872<t>
3873   It is beyond the scope of a protocol specification to mandate
3874   compliance with previous versions. HTTP/1.1 was deliberately
3875   designed, however, to make supporting previous versions easy. It is
3876   worth noting that, at the time of composing this specification
3877   (1996), we would expect commercial HTTP/1.1 servers to:
3878  <list style="symbols">
3879     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3880        1.1 requests;</t>
3881
3882     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3883        1.1;</t>
3884
3885     <t>respond appropriately with a message in the same major version
3886        used by the client.</t>
3887  </list>
3888</t>
3889<t>
3890   And we would expect HTTP/1.1 clients to:
3891  <list style="symbols">
3892     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3893        responses;</t>
3894
3895     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3896        1.1.</t>
3897  </list>
3898</t>
3899<t>
3900   For most implementations of HTTP/1.0, each connection is established
3901   by the client prior to the request and closed by the server after
3902   sending the response. Some implementations implement the Keep-Alive
3903   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3904</t>
3905
3906<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3907<t>
3908   This section summarizes major differences between versions HTTP/1.0
3909   and HTTP/1.1.
3910</t>
3911
3912<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">
3913<t>
3914   The requirements that clients and servers support the Host request-header,
3915   report an error if the Host request-header (<xref target="header.host"/>) is
3916   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3917   are among the most important changes defined by this
3918   specification.
3919</t>
3920<t>
3921   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
3922   addresses and servers; there was no other established mechanism for
3923   distinguishing the intended server of a request than the IP address
3924   to which that request was directed. The changes outlined above will
3925   allow the Internet, once older HTTP clients are no longer common, to
3926   support multiple Web sites from a single IP address, greatly
3927   simplifying large operational Web servers, where allocation of many
3928   IP addresses to a single host has created serious problems. The
3929   Internet will also be able to recover the IP addresses that have been
3930   allocated for the sole purpose of allowing special-purpose domain
3931   names to be used in root-level HTTP URLs. Given the rate of growth of
3932   the Web, and the number of servers already deployed, it is extremely
3933   important that all implementations of HTTP (including updates to
3934   existing HTTP/1.0 applications) correctly implement these
3935   requirements:
3936  <list style="symbols">
3937     <t>Both clients and servers &MUST; support the Host request-header.</t>
3938
3939     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
3940
3941     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
3942        request does not include a Host request-header.</t>
3943
3944     <t>Servers &MUST; accept absolute URIs.</t>
3945  </list>
3946</t>
3947</section>
3948</section>
3949
3950<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
3951<t>
3952   Some clients and servers might wish to be compatible with some
3953   previous implementations of persistent connections in HTTP/1.0
3954   clients and servers. Persistent connections in HTTP/1.0 are
3955   explicitly negotiated as they are not the default behavior. HTTP/1.0
3956   experimental implementations of persistent connections are faulty,
3957   and the new facilities in HTTP/1.1 are designed to rectify these
3958   problems. The problem was that some existing 1.0 clients may be
3959   sending Keep-Alive to a proxy server that doesn't understand
3960   Connection, which would then erroneously forward it to the next
3961   inbound server, which would establish the Keep-Alive connection and
3962   result in a hung HTTP/1.0 proxy waiting for the close on the
3963   response. The result is that HTTP/1.0 clients must be prevented from
3964   using Keep-Alive when talking to proxies.
3965</t>
3966<t>
3967   However, talking to proxies is the most important use of persistent
3968   connections, so that prohibition is clearly unacceptable. Therefore,
3969   we need some other mechanism for indicating a persistent connection
3970   is desired, which is safe to use even when talking to an old proxy
3971   that ignores Connection. Persistent connections are the default for
3972   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
3973   declaring non-persistence. See <xref target="header.connection"/>.
3974</t>
3975<t>
3976   The original HTTP/1.0 form of persistent connections (the Connection:
3977   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
3978</t>
3979</section>
3980
3981<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
3982<t>
3983   This specification has been carefully audited to correct and
3984   disambiguate key word usage; RFC 2068 had many problems in respect to
3985   the conventions laid out in <xref target="RFC2119"/>.
3986</t>
3987<t>
3988   Transfer-coding and message lengths all interact in ways that
3989   required fixing exactly when chunked encoding is used (to allow for
3990   transfer encoding that may not be self delimiting); it was important
3991   to straighten out exactly how message lengths are computed. (Sections
3992   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
3993   <xref target="header.content-length" format="counter"/>,
3994   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
3995</t>
3996<t>
3997   The use and interpretation of HTTP version numbers has been clarified
3998   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
3999   version they support to deal with problems discovered in HTTP/1.0
4000   implementations (<xref target="http.version"/>)
4001</t>
4002<t>
4003   Transfer-coding had significant problems, particularly with
4004   interactions with chunked encoding. The solution is that transfer-codings
4005   become as full fledged as content-codings. This involves
4006   adding an IANA registry for transfer-codings (separate from content
4007   codings), a new header field (TE) and enabling trailer headers in the
4008   future. Transfer encoding is a major performance benefit, so it was
4009   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4010   interoperability problem that could have occurred due to interactions
4011   between authentication trailers, chunked encoding and HTTP/1.0
4012   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4013   and <xref target="header.te" format="counter"/>)
4014</t>
4015</section>
4016
4017<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4018<t>
4019  Clarify that HTTP-Version is case sensitive.
4020  (<xref target="http.version"/>)
4021</t>
4022<t>
4023  Remove reference to non-existant identity transfer-coding value tokens.
4024  (Sections <xref format="counter" target="transfer.codings"/> and
4025  <xref format="counter" target="message.length"/>)
4026</t>
4027<t>
4028  Clarification that the chunk length does not include
4029  the count of the octets in the chunk header and trailer.
4030  (<xref target="chunked.transfer.encoding"/>)
4031</t>
4032<t>
4033  Fix BNF to add query, as the abs_path production in
4034  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4035  (<xref target="request-uri"/>)
4036</t>
4037<t>
4038  Clarify exactly when close connection options must be sent.
4039  (<xref target="header.connection"/>)
4040</t>
4041</section>
4042</section>
4043
4044<section title="Change Log (to be removed by RFC Editor before publication)">
4045
4046<section title="Since RFC2616">
4047<t>
4048  Extracted relevant partitions from <xref target="RFC2616"/>.
4049</t>
4050</section>
4051
4052<section title="Since draft-ietf-httpbis-p1-messaging-00">
4053<t>
4054  Closed issues:
4055  <list style="symbols"> 
4056    <t>
4057      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4058      "HTTP Version should be case sensitive"
4059      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4060    </t>
4061    <t>
4062      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4063      "'unsafe' characters"
4064      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4065    </t>
4066    <t>
4067      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4068      "Chunk Size Definition"
4069      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4070    </t>
4071    <t>
4072      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4073      "Message Length"
4074      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4075    </t>
4076    <t>
4077      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4078      "Media Type Registrations"
4079      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4080    </t>
4081    <t>
4082      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4083      "URI includes query"
4084      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4085    </t>
4086    <t>
4087      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4088      "No close on 1xx responses"
4089      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4090    </t>
4091    <t>
4092      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4093      "Remove 'identity' token references"
4094      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4095    </t>
4096    <t>
4097      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4098      "Import query BNF"
4099    </t>
4100    <t>
4101      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4102      "qdtext BNF"
4103    </t>
4104    <t>
4105      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4106      "Normative and Informative references"
4107    </t>
4108    <t>
4109      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4110      "RFC2606 Compliance"
4111    </t>
4112    <t>
4113      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4114      "RFC977 reference"
4115    </t>
4116    <t>
4117      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4118      "RFC1700 references"
4119    </t>
4120    <t>
4121      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4122      "inconsistency in date format explanation"
4123    </t>
4124    <t>
4125      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4126      "Date reference typo"
4127    </t>
4128    <t>
4129      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4130      "Informative references"
4131    </t>
4132    <t>
4133      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4134      "ISO-8859-1 Reference"
4135    </t>
4136    <t>
4137      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4138      "Normative up-to-date references"
4139    </t>
4140  </list>
4141</t>
4142<t>
4143  Other changes:
4144  <list style="symbols"> 
4145    <t>
4146      Update media type registrations to use RFC4288 template.
4147    </t>
4148    <t>
4149      Use names of RFC4234 core rules DQUOTE and HTAB,
4150      fix broken ABNF for chunk-data
4151      (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4152    </t>
4153  </list>
4154</t>
4155</section>
4156
4157</section>
4158
4159</back>
4160</rfc>
Note: See TracBrowser for help on using the repository browser.