source: draft-ietf-httpbis/latest/p1-messaging.xml @ 212

Last change on this file since 212 was 212, checked in by julian.reschke@…, 12 years ago

Remove delta-seconds def from P1, just redefine it in P2 (revisits change [210]); relates to #36.

  • Property svn:eol-style set to native
File size: 182.3 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "February">
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-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x='http://purl.org/net/xml2rfc/ext'>
56<front>
57
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
59
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email>fielding@gbiv.com</email>
73      <uri>http://roy.gbiv.com/</uri>
74    </address>
75  </author>
76
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email>jg@laptop.org</email>
88      <uri>http://www.laptop.org/</uri>
89    </address>
90  </author>
91 
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email>JeffMogul@acm.org</email>
104    </address>
105  </author>
106
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email>henrikn@microsoft.com</email>
118    </address>
119  </author>
120
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email>LMM@acm.org</email>
132      <uri>http://larry.masinter.net/</uri>
133    </address>
134  </author>
135 
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email>paulle@microsoft.com</email>
146    </address>
147  </author>
148   
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email>timbl@w3.org</email>
162      <uri>http://www.w3.org/People/Berners-Lee/</uri>
163    </address>
164  </author>
165
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email>ylafon@w3.org</email>
178      <uri>http://www.raubacapeu.net/people/yves/</uri>
179    </address>
180  </author>
181
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email>julian.reschke@greenbytes.de</email>       
193      <uri>http://greenbytes.de/tech/webdav/</uri>     
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
198
199<abstract>
200<t>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    This draft incorporates those issue resolutions that were either
224    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
225    or which were agreed upon on the mailing list between October 2006 and
226    November 2007 (as published in "draft-lafon-rfc2616bis-03").
227  </t>
228</note>
229</front>
230<middle>
231<section title="Introduction" anchor="introduction">
232<t>
233   The Hypertext Transfer Protocol (HTTP) is an application-level
234   protocol for distributed, collaborative, hypermedia information
235   systems. HTTP has been in use by the World-Wide Web global
236   information initiative since 1990. The first version of HTTP, commonly
237   referred to as HTTP/0.9, was a simple protocol for raw data transfer
238   across the Internet with only a single method and no metadata.
239   HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
240   the protocol by allowing messages to be in the format of MIME-like
241   messages, containing metadata about the data transferred and
242   modifiers on the request/response semantics. However, HTTP/1.0 did
243   not sufficiently take into consideration the effects of hierarchical
244   proxies, caching, the need for persistent connections, or name-based
245   virtual hosts. In addition, the proliferation of incompletely-implemented
246   applications calling themselves "HTTP/1.0" necessitated a
247   protocol version change in order for two communicating applications
248   to determine each other's true capabilities.
249</t>
250<t>
251   This document is Part 1 of the seven-part specification that defines
252   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
253   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
254   requirements that enable reliable implementations and adding only
255   those new features that will either be safely ignored by an HTTP/1.0
256   recipient or only sent when communicating with a party advertising
257   compliance with HTTP/1.1.
258   Part 1 defines those aspects of HTTP/1.1 related to overall network
259   operation, message framing, interaction with transport protocols, and
260   URI schemes.
261</t>
262<t>
263   This document is currently disorganized in order to minimize the changes
264   between drafts and enable reviewers to see the smaller errata changes.
265   The next draft will reorganize the sections to better reflect the content.
266   In particular, the sections will be organized according to the typical
267   process of deciding when to use HTTP (URI schemes), overall network operation,
268   connection management, message framing, and generic message parsing.
269   The current mess reflects how widely dispersed these topics and associated
270   requirements had become in <xref target="RFC2616"/>.
271</t>
272
273<section title="Purpose" anchor="intro.purpose">
274<t>
275   Practical information systems require more functionality than simple
276   retrieval, including search, front-end update, and annotation. HTTP
277   allows an open-ended set of methods and headers that indicate the
278   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
279   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
280   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
281   method is to be applied. Messages are passed in a format similar to
282   that used by Internet mail <xref target="RFC2822"/> as defined by the Multipurpose
283   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
284</t>
285<t>
286   HTTP is also used as a generic protocol for communication between
287   user agents and proxies/gateways to other Internet systems, including
288   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
289   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
290   access to resources available from diverse applications.
291</t>
292</section>
293
294<section title="Requirements" anchor="intro.requirements">
295<t>
296   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
297   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
298   document are to be interpreted as described in <xref target="RFC2119"/>.
299</t>
300<t>
301   An implementation is not compliant if it fails to satisfy one or more
302   of the &MUST; or &REQUIRED; level requirements for the protocols it
303   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
304   level and all the &SHOULD; level requirements for its protocols is said
305   to be "unconditionally compliant"; one that satisfies all the &MUST;
306   level requirements but not all the &SHOULD; level requirements for its
307   protocols is said to be "conditionally compliant."
308</t>
309</section>
310
311<section title="Terminology" anchor="intro.terminology">
312<t>
313   This specification uses a number of terms to refer to the roles
314   played by participants in, and objects of, the HTTP communication.
315</t>
316<t>
317  <iref item="connection"/>
318  <x:dfn>connection</x:dfn>
319  <list>
320    <t>
321      A transport layer virtual circuit established between two programs
322      for the purpose of communication.
323    </t>
324  </list>
325</t>
326<t>
327  <iref item="message"/>
328  <x:dfn>message</x:dfn>
329  <list>
330    <t>
331      The basic unit of HTTP communication, consisting of a structured
332      sequence of octets matching the syntax defined in <xref target="http.message"/> and
333      transmitted via the connection.
334    </t>
335  </list>
336</t>
337<t>
338  <iref item="request"/>
339  <x:dfn>request</x:dfn>
340  <list>
341    <t>
342      An HTTP request message, as defined in <xref target="request"/>.
343    </t>
344  </list>
345</t>
346<t>
347  <iref item="response"/>
348  <x:dfn>response</x:dfn>
349  <list>
350    <t>
351      An HTTP response message, as defined in <xref target="response"/>.
352    </t>
353  </list>
354</t>
355<t>
356  <iref item="resource"/>
357  <x:dfn>resource</x:dfn>
358  <list>
359    <t>
360      A network data object or service that can be identified by a URI,
361      as defined in <xref target="uri"/>. Resources may be available in multiple
362      representations (e.g. multiple languages, data formats, size, and
363      resolutions) or vary in other ways.
364    </t>
365  </list>
366</t>
367<t>
368  <iref item="entity"/>
369  <x:dfn>entity</x:dfn>
370  <list>
371    <t>
372      The information transferred as the payload of a request or
373      response. An entity consists of metainformation in the form of
374      entity-header fields and content in the form of an entity-body, as
375      described in &entity;.
376    </t>
377  </list>
378</t>
379<t>
380  <iref item="representation"/>
381  <x:dfn>representation</x:dfn>
382  <list>
383    <t>
384      An entity included with a response that is subject to content
385      negotiation, as described in &content.negotiation;. There may exist multiple
386      representations associated with a particular response status.
387    </t>
388  </list>
389</t>
390<t>
391  <iref item="content negotiation"/>
392  <x:dfn>content negotiation</x:dfn>
393  <list>
394    <t>
395      The mechanism for selecting the appropriate representation when
396      servicing a request, as described in &content.negotiation;. The
397      representation of entities in any response can be negotiated
398      (including error responses).
399    </t>
400  </list>
401</t>
402<t>
403  <iref item="variant"/>
404  <x:dfn>variant</x:dfn>
405  <list>
406    <t>
407      A resource may have one, or more than one, representation(s)
408      associated with it at any given instant. Each of these
409      representations is termed a `variant'.  Use of the term `variant'
410      does not necessarily imply that the resource is subject to content
411      negotiation.
412    </t>
413  </list>
414</t>
415<t>
416  <iref item="client"/>
417  <x:dfn>client</x:dfn>
418  <list>
419    <t>
420      A program that establishes connections for the purpose of sending
421      requests.
422    </t>
423  </list>
424</t>
425<t>
426  <iref item="user agent"/>
427  <x:dfn>user agent</x:dfn>
428  <list>
429    <t>
430      The client which initiates a request. These are often browsers,
431      editors, spiders (web-traversing robots), or other end user tools.
432    </t>
433  </list>
434</t>
435<t>
436  <iref item="server"/>
437  <x:dfn>server</x:dfn>
438  <list>
439    <t>
440      An application program that accepts connections in order to
441      service requests by sending back responses. Any given program may
442      be capable of being both a client and a server; our use of these
443      terms refers only to the role being performed by the program for a
444      particular connection, rather than to the program's capabilities
445      in general. Likewise, any server may act as an origin server,
446      proxy, gateway, or tunnel, switching behavior based on the nature
447      of each request.
448    </t>
449  </list>
450</t>
451<t>
452  <iref item="origin server"/>
453  <x:dfn>origin server</x:dfn>
454  <list>
455    <t>
456      The server on which a given resource resides or is to be created.
457    </t>
458  </list>
459</t>
460<t>
461  <iref item="proxy"/>
462  <x:dfn>proxy</x:dfn>
463  <list>
464    <t>
465      An intermediary program which acts as both a server and a client
466      for the purpose of making requests on behalf of other clients.
467      Requests are serviced internally or by passing them on, with
468      possible translation, to other servers. A proxy &MUST; implement
469      both the client and server requirements of this specification. A
470      "transparent proxy" is a proxy that does not modify the request or
471      response beyond what is required for proxy authentication and
472      identification. A "non-transparent proxy" is a proxy that modifies
473      the request or response in order to provide some added service to
474      the user agent, such as group annotation services, media type
475      transformation, protocol reduction, or anonymity filtering. Except
476      where either transparent or non-transparent behavior is explicitly
477      stated, the HTTP proxy requirements apply to both types of
478      proxies.
479    </t>
480  </list>
481</t>
482<t>
483  <iref item="gateway"/>
484  <x:dfn>gateway</x:dfn>
485  <list>
486    <t>
487      A server which acts as an intermediary for some other server.
488      Unlike a proxy, a gateway receives requests as if it were the
489      origin server for the requested resource; the requesting client
490      may not be aware that it is communicating with a gateway.
491    </t>
492  </list>
493</t>
494<t>
495  <iref item="tunnel"/>
496  <x:dfn>tunnel</x:dfn>
497  <list>
498    <t>
499      An intermediary program which is acting as a blind relay between
500      two connections. Once active, a tunnel is not considered a party
501      to the HTTP communication, though the tunnel may have been
502      initiated by an HTTP request. The tunnel ceases to exist when both
503      ends of the relayed connections are closed.
504    </t>
505  </list>
506</t>
507<t>
508  <iref item="cache"/>
509  <x:dfn>cache</x:dfn>
510  <list>
511    <t>
512      A program's local store of response messages and the subsystem
513      that controls its message storage, retrieval, and deletion. A
514      cache stores cacheable responses in order to reduce the response
515      time and network bandwidth consumption on future, equivalent
516      requests. Any client or server may include a cache, though a cache
517      cannot be used by a server that is acting as a tunnel.
518    </t>
519  </list>
520</t>
521<t>
522  <iref item="cacheable"/>
523  <x:dfn>cacheable</x:dfn>
524  <list>
525    <t>
526      A response is cacheable if a cache is allowed to store a copy of
527      the response message for use in answering subsequent requests. The
528      rules for determining the cacheability of HTTP responses are
529      defined in &caching;. Even if a resource is cacheable, there may
530      be additional constraints on whether a cache can use the cached
531      copy for a particular request.
532    </t>
533  </list>
534</t>
535<t>
536  <iref item="upstream"/>
537  <iref item="downstream"/>
538  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
539  <list>
540    <t>
541      Upstream and downstream describe the flow of a message: all
542      messages flow from upstream to downstream.
543    </t>
544  </list>
545</t>
546<t>
547  <iref item="inbound"/>
548  <iref item="outbound"/>
549  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
550  <list>
551    <t>
552      Inbound and outbound refer to the request and response paths for
553      messages: "inbound" means "traveling toward the origin server",
554      and "outbound" means "traveling toward the user agent"
555    </t>
556  </list>
557</t>
558</section>
559
560<section title="Overall Operation" anchor="intro.overall.operation">
561<t>
562   HTTP is a request/response protocol. A client sends a
563   request to the server in the form of a request method, URI, and
564   protocol version, followed by a MIME-like message containing request
565   modifiers, client information, and possible body content over a
566   connection with a server. The server responds with a status line,
567   including the message's protocol version and a success or error code,
568   followed by a MIME-like message containing server information, entity
569   metainformation, and possible entity-body content. The relationship
570   between HTTP and MIME is described in &diff2045entity;.
571</t>
572<t>
573   Most HTTP communication is initiated by a user agent and consists of
574   a request to be applied to a resource on some origin server. In the
575   simplest case, this may be accomplished via a single connection (v)
576   between the user agent (UA) and the origin server (O).
577</t>
578<figure><artwork type="drawing">
579       request chain ------------------------&gt;
580    UA -------------------v------------------- O
581       &lt;----------------------- response chain
582</artwork></figure>
583<t>
584   A more complicated situation occurs when one or more intermediaries
585   are present in the request/response chain. There are three common
586   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
587   forwarding agent, receiving requests for a URI in its absolute form,
588   rewriting all or part of the message, and forwarding the reformatted
589   request toward the server identified by the URI. A gateway is a
590   receiving agent, acting as a layer above some other server(s) and, if
591   necessary, translating the requests to the underlying server's
592   protocol. A tunnel acts as a relay point between two connections
593   without changing the messages; tunnels are used when the
594   communication needs to pass through an intermediary (such as a
595   firewall) even when the intermediary cannot understand the contents
596   of the messages.
597</t>
598<figure><artwork type="drawing">
599       request chain --------------------------------------&gt;
600    UA -----v----- A -----v----- B -----v----- C -----v----- O
601       &lt;------------------------------------- response chain
602</artwork></figure>
603<t>
604   The figure above shows three intermediaries (A, B, and C) between the
605   user agent and origin server. A request or response message that
606   travels the whole chain will pass through four separate connections.
607   This distinction is important because some HTTP communication options
608   may apply only to the connection with the nearest, non-tunnel
609   neighbor, only to the end-points of the chain, or to all connections
610   along the chain. Although the diagram is linear, each participant may
611   be engaged in multiple, simultaneous communications. For example, B
612   may be receiving requests from many clients other than A, and/or
613   forwarding requests to servers other than C, at the same time that it
614   is handling A's request.
615</t>
616<t>
617   Any party to the communication which is not acting as a tunnel may
618   employ an internal cache for handling requests. The effect of a cache
619   is that the request/response chain is shortened if one of the
620   participants along the chain has a cached response applicable to that
621   request. The following illustrates the resulting chain if B has a
622   cached copy of an earlier response from O (via C) for a request which
623   has not been cached by UA or A.
624</t>
625<figure><artwork type="drawing">
626          request chain ----------&gt;
627       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
628          &lt;--------- response chain
629</artwork></figure>
630<t>
631   Not all responses are usefully cacheable, and some requests may
632   contain modifiers which place special requirements on cache behavior.
633   HTTP requirements for cache behavior and cacheable responses are
634   defined in &caching;.
635</t>
636<t>
637   In fact, there are a wide variety of architectures and configurations
638   of caches and proxies currently being experimented with or deployed
639   across the World Wide Web. These systems include national hierarchies
640   of proxy caches to save transoceanic bandwidth, systems that
641   broadcast or multicast cache entries, organizations that distribute
642   subsets of cached data via CD-ROM, and so on. HTTP systems are used
643   in corporate intranets over high-bandwidth links, and for access via
644   PDAs with low-power radio links and intermittent connectivity. The
645   goal of HTTP/1.1 is to support the wide diversity of configurations
646   already deployed while introducing protocol constructs that meet the
647   needs of those who build web applications that require high
648   reliability and, failing that, at least reliable indications of
649   failure.
650</t>
651<t>
652   HTTP communication usually takes place over TCP/IP connections. The
653   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
654   not preclude HTTP from being implemented on top of any other protocol
655   on the Internet, or on other networks. HTTP only presumes a reliable
656   transport; any protocol that provides such guarantees can be used;
657   the mapping of the HTTP/1.1 request and response structures onto the
658   transport data units of the protocol in question is outside the scope
659   of this specification.
660</t>
661<t>
662   In HTTP/1.0, most implementations used a new connection for each
663   request/response exchange. In HTTP/1.1, a connection may be used for
664   one or more request/response exchanges, although connections may be
665   closed for a variety of reasons (see <xref target="persistent.connections"/>).
666</t>
667</section>
668</section>
669
670<section title="Notational Conventions and Generic Grammar" anchor="notation">
671
672<section title="Augmented BNF" anchor="notation.abnf">
673<t>
674   All of the mechanisms specified in this document are described in
675   both prose and an augmented Backus-Naur Form (BNF) similar to that
676   used by <xref target="RFC822ABNF"/>. Implementors will need to be familiar with the
677   notation in order to understand this specification. The augmented BNF
678   includes the following constructs:
679</t>
680<t>
681   name = definition
682  <list>
683    <t>
684      The name of a rule is simply the name itself (without any
685      enclosing "&lt;" and "&gt;") and is separated from its definition by the
686      equal "=" character. White space is only significant in that
687      indentation of continuation lines is used to indicate a rule
688      definition that spans more than one line. Certain basic rules are
689      in uppercase, such as SP, LWS, HTAB, CRLF, DIGIT, ALPHA, etc. Angle
690      brackets are used within definitions whenever their presence will
691      facilitate discerning the use of rule names.
692    </t>
693  </list>
694</t>
695<t>
696   "literal"
697  <list>
698    <t>
699      Quotation marks surround literal text. Unless stated otherwise,
700      the text is case-insensitive.
701    </t>
702  </list>
703</t>
704<t>
705   rule1 | rule2
706  <list>
707    <t>
708      Elements separated by a bar ("|") are alternatives, e.g., "yes |
709      no" will accept yes or no.
710    </t>
711  </list>
712</t>
713<t>
714   (rule1 rule2)
715  <list>
716    <t>
717      Elements enclosed in parentheses are treated as a single element.
718      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
719      foo elem" and "elem bar elem".
720    </t>
721  </list>
722</t>
723<t>
724   *rule
725  <list>
726    <t>
727      The character "*" preceding an element indicates repetition. The
728      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
729      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
730      that "*(element)" allows any number, including zero; "1*element"
731      requires at least one; and "1*2element" allows one or two.
732    </t>
733  </list>
734</t>
735<t>
736   [rule]
737  <list>
738    <t>
739      Square brackets enclose optional elements; "[foo bar]" is
740      equivalent to "*1(foo bar)".
741    </t>
742  </list>
743</t>
744<t>
745   N rule
746  <list>
747    <t>
748      Specific repetition: "&lt;n&gt;(element)" is equivalent to
749      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
750      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
751      alphabetic characters.
752    </t>
753  </list>
754</t>
755<t>
756   #rule
757  <list>
758    <t>
759      A construct "#" is defined, similar to "*", for defining lists of
760      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
761      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
762      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
763      form of lists very easy; a rule such as
764    </t>
765    <t>
766         ( *LWS element *( *LWS "," *LWS element ))
767    </t>
768    <t>
769      can be shown as
770    </t>
771    <t>
772         1#element
773    </t>
774    <t>
775      Wherever this construct is used, null elements are allowed, but do
776      not contribute to the count of elements present. That is,
777      "(element), , (element) " is permitted, but counts as only two
778      elements. Therefore, where at least one element is required, at
779      least one non-null element &MUST; be present. Default values are 0
780      and infinity so that "#element" allows any number, including zero;
781      "1#element" requires at least one; and "1#2element" allows one or
782      two.
783    </t>
784  </list>
785</t>
786<t>
787   ; comment
788  <list>
789    <t>
790      A semi-colon, set off some distance to the right of rule text,
791      starts a comment that continues to the end of line. This is a
792      simple way of including useful notes in parallel with the
793      specifications.
794    </t>
795  </list>
796</t>
797<t>
798   implied *LWS
799  <list>
800    <t>
801      The grammar described by this specification is word-based. Except
802      where noted otherwise, linear white space (LWS) can be included
803      between any two adjacent words (token or quoted-string), and
804      between adjacent words and separators, without changing the
805      interpretation of a field. At least one delimiter (LWS and/or
806      separators) &MUST; exist between any two tokens (for the definition
807      of "token" below), since they would otherwise be interpreted as a
808      single token.
809    </t>
810  </list>
811</t>
812</section>
813
814<section title="Basic Rules" anchor="basic.rules">
815<x:anchor-alias value="OCTET"/>
816<x:anchor-alias value="CHAR"/>
817<x:anchor-alias value="ALPHA"/>
818<x:anchor-alias value="DIGIT"/>
819<x:anchor-alias value="CTL"/>
820<x:anchor-alias value="CR"/>
821<x:anchor-alias value="LF"/>
822<x:anchor-alias value="SP"/>
823<x:anchor-alias value="HTAB"/>
824<x:anchor-alias value="CRLF"/>
825<x:anchor-alias value="LWS"/>
826<x:anchor-alias value="TEXT"/>
827<x:anchor-alias value="HEX"/>
828<x:anchor-alias value="token"/>
829<x:anchor-alias value="separators"/>
830<x:anchor-alias value="comment"/>
831<x:anchor-alias value="ctext"/>
832<x:anchor-alias value="quoted-string"/>
833<x:anchor-alias value="qdtext"/>
834<x:anchor-alias value="quoted-pair"/>
835<t>
836   The following rules are used throughout this specification to
837   describe basic parsing constructs. The US-ASCII coded character set
838   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
839</t>
840<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><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"/>
841  OCTET          = %x00-FF
842                   ; any 8-bit sequence of data
843  CHAR           = %x01-7F
844                   ; any US-ASCII character, excluding NUL
845  ALPHA          = %x41-5A | %x61-7A
846                   ; A-Z | a-z
847  DIGIT          = %x30-39
848                   ; any US-ASCII digit "0".."9"
849  CTL            = %x00-1F | %x7F
850                   ; (octets 0 - 31) and DEL (127)
851  CR             = %x0D
852                   ; US-ASCII CR, carriage return (13)
853  LF             = %x0A
854                   ; US-ASCII LF, linefeed (10)
855  SP             = %x20
856                   ; US-ASCII SP, space (32)
857  HTAB           = %x09
858                   ; US-ASCII HT, horizontal-tab (9)
859  DQUOTE         = %x22
860                   ; US-ASCII double-quote mark (34)
861</artwork></figure>
862<t>
863   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
864   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
865   tolerant applications). The end-of-line marker within an entity-body
866   is defined by its associated media type, as described in &media-types;.
867</t>
868<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
869  CRLF           = CR LF
870</artwork></figure>
871<t>
872   HTTP/1.1 header field values can be folded onto multiple lines if the
873   continuation line begins with a space or horizontal tab. All linear
874   white space, including folding, has the same semantics as SP. A
875   recipient &MAY; replace any linear white space with a single SP before
876   interpreting the field value or forwarding the message downstream.
877</t>
878<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
879  LWS            = [CRLF] 1*( SP | HTAB )
880</artwork></figure>
881<t>
882   The TEXT rule is only used for descriptive field contents and values
883   that are not intended to be interpreted by the message parser. Words
884   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
885   <xref target="ISO-8859-1"/> only when encoded according to the rules of
886   <xref target="RFC2047"/>.
887</t>
888<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
889  TEXT           = &lt;any OCTET except CTLs, but including LWS&gt;
890</artwork></figure>
891<t>
892   A CRLF is allowed in the definition of TEXT only as part of a header
893   field continuation. It is expected that the folding LWS will be
894   replaced with a single SP before interpretation of the TEXT value.
895</t>
896<t>
897   Hexadecimal numeric characters are used in several protocol elements.
898</t>
899<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
900  HEX            = "A" | "B" | "C" | "D" | "E" | "F"
901                 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
902</artwork></figure>
903<t>
904   Many HTTP/1.1 header field values consist of words separated by LWS
905   or special characters. These special characters &MUST; be in a quoted
906   string to be used within a parameter value (as defined in
907   <xref target="transfer.codings"/>).
908</t>
909<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
910  token          = 1*&lt;any CHAR except CTLs or separators&gt;
911  separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
912                 | "," | ";" | ":" | "\" | DQUOTE
913                 | "/" | "[" | "]" | "?" | "="
914                 | "{" | "}" | SP | HTAB
915</artwork></figure>
916<t>
917   Comments can be included in some HTTP header fields by surrounding
918   the comment text with parentheses. Comments are only allowed in
919   fields containing "comment" as part of their field value definition.
920   In all other fields, parentheses are considered part of the field
921   value.
922</t>
923<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
924  comment        = "(" *( ctext | quoted-pair | comment ) ")"
925  ctext          = &lt;any TEXT excluding "(" and ")"&gt;
926</artwork></figure>
927<t>
928   A string of text is parsed as a single word if it is quoted using
929   double-quote marks.
930</t>
931<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
932  quoted-string  = ( DQUOTE *(qdtext | quoted-pair ) DQUOTE )
933  qdtext         = &lt;any TEXT excluding DQUOTE and "\">
934</artwork></figure>
935<t>
936   The backslash character ("\") &MAY; be used as a single-character
937   quoting mechanism only within quoted-string and comment constructs.
938</t>
939<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
940  quoted-pair    = "\" CHAR
941</artwork></figure>
942</section>
943
944<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
945<t>
946  The ABNF rules below are defined in other parts:
947</t>
948<figure><!-- Part2--><artwork type="abnf2616">
949  request-header =  &lt;request-header, defined in &request-header-fields;&gt;
950  response-header = &lt;response-header, defined in &response-header-fields;&gt;
951</artwork></figure>
952<figure><!-- Part3--><artwork type="abnf2616">
953  accept-params   = &lt;accept-params, defined in &header-accept;&gt;
954  entity-body     = &lt;entity-body, defined in &entity-body;&gt;
955  entity-header   = &lt;entity-header, defined in &entity-header-fields;&gt;
956</artwork></figure>
957<figure><!-- Part6--><artwork type="abnf2616">
958  Cache-Control   = &lt;Cache-Control, defined in &header-pragma;&gt;
959  Pragma          = &lt;Pragma, defined in &header-pragma;&gt;
960  Warning         = &lt;Warning, defined in &header-warning;&gt;
961</artwork></figure>
962</section>
963
964</section>
965
966<section title="Protocol Parameters" anchor="protocol.parameters">
967
968<section title="HTTP Version" anchor="http.version">
969<t>
970   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
971   of the protocol. The protocol versioning policy is intended to allow
972   the sender to indicate the format of a message and its capacity for
973   understanding further HTTP communication, rather than the features
974   obtained via that communication. No change is made to the version
975   number for the addition of message components which do not affect
976   communication behavior or which only add to extensible field values.
977   The &lt;minor&gt; number is incremented when the changes made to the
978   protocol add features which do not change the general message parsing
979   algorithm, but which may add to the message semantics and imply
980   additional capabilities of the sender. The &lt;major&gt; number is
981   incremented when the format of a message within the protocol is
982   changed. See <xref target="RFC2145"/> for a fuller explanation.
983</t>
984<t>
985   The version of an HTTP message is indicated by an HTTP-Version field
986   in the first line of the message. HTTP-Version is case-sensitive.
987</t>
988<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
989  HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
990</artwork></figure>
991<t>
992   Note that the major and minor numbers &MUST; be treated as separate
993   integers and that each &MAY; be incremented higher than a single digit.
994   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
995   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
996   &MUST-NOT; be sent.
997</t>
998<t>
999   An application that sends a request or response message that includes
1000   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
1001   with this specification. Applications that are at least conditionally
1002   compliant with this specification &SHOULD; use an HTTP-Version of
1003   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
1004   not compatible with HTTP/1.0. For more details on when to send
1005   specific HTTP-Version values, see <xref target="RFC2145"/>.
1006</t>
1007<t>
1008   The HTTP version of an application is the highest HTTP version for
1009   which the application is at least conditionally compliant.
1010</t>
1011<t>
1012   Proxy and gateway applications need to be careful when forwarding
1013   messages in protocol versions different from that of the application.
1014   Since the protocol version indicates the protocol capability of the
1015   sender, a proxy/gateway &MUST-NOT; send a message with a version
1016   indicator which is greater than its actual version. If a higher
1017   version request is received, the proxy/gateway &MUST; either downgrade
1018   the request version, or respond with an error, or switch to tunnel
1019   behavior.
1020</t>
1021<t>
1022   Due to interoperability problems with HTTP/1.0 proxies discovered
1023   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
1024   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
1025   they support. The proxy/gateway's response to that request &MUST; be in
1026   the same major version as the request.
1027</t>
1028<t>
1029  <list>
1030    <t>
1031      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1032      of header fields required or forbidden by the versions involved.
1033    </t>
1034  </list>
1035</t>
1036</section>
1037
1038<section title="Uniform Resource Identifiers" anchor="uri">
1039<t>
1040   URIs have been known by many names: WWW addresses, Universal Document
1041   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
1042   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
1043   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
1044   simply formatted strings which identify--via name, location, or any
1045   other characteristic--a resource.
1046</t>
1047
1048<section title="General Syntax" anchor="general.syntax">
1049<t>
1050   URIs in HTTP can be represented in absolute form or relative to some
1051   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1052   forms are differentiated by the fact that absolute URIs always begin
1053   with a scheme name followed by a colon. For definitive information on
1054   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1055   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1056   and <xref target="RFC1808"/>). This specification adopts the
1057   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
1058   "host", "abs_path", "query", and "authority" from that specification:
1059</t>
1060<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="absoluteURI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="relativeURI"/><iref primary="true" item="Grammar" subitem="uri-host"/>
1061  absoluteURI   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1062  authority     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
1063  fragment      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
1064  path-absolute = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1065  port          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1066  query         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
1067  relativeURI   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
1068  uri-host      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1069</artwork></figure>
1070<t>
1071   HTTP does not place any a priori limit on the length of
1072   a URI. Servers &MUST; be able to handle the URI of any resource they
1073   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1074   provide GET-based forms that could generate such URIs. A server
1075   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1076   than the server can handle (see &status-414;).
1077</t>
1078<t>
1079  <list>
1080    <t>
1081      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1082      above 255 bytes, because some older client or proxy
1083      implementations might not properly support these lengths.
1084    </t>
1085  </list>
1086</t>
1087</section>
1088
1089<section title="http URL" anchor="http.url">
1090<t>
1091   The "http" scheme is used to locate network resources via the HTTP
1092   protocol. This section defines the scheme-specific syntax and
1093   semantics for http URLs.
1094</t>
1095<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
1096  http-URL = "http:" "//" uri-host [ ":" port ]
1097             [ path-absolute [ "?" query ]]
1098</artwork></figure>
1099<t>
1100   If the port is empty or not given, port 80 is assumed. The semantics
1101   are that the identified resource is located at the server listening
1102   for TCP connections on that port of that host, and the Request-URI
1103   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
1104   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1105   the path-absolute is not present in the URL, it &MUST; be given as "/" when
1106   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1107   receives a host name which is not a fully qualified domain name, it
1108   &MAY; add its domain to the host name it received. If a proxy receives
1109   a fully qualified domain name, the proxy &MUST-NOT; change the host
1110   name.
1111</t>
1112</section>
1113
1114<section title="URI Comparison" anchor="uri.comparison">
1115<t>
1116   When comparing two URIs to decide if they match or not, a client
1117   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1118   URIs, with these exceptions:
1119  <list style="symbols">
1120    <t>A port that is empty or not given is equivalent to the default
1121        port for that URI-reference;</t>
1122    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1123    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1124    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
1125  </list>
1126</t>
1127<t>
1128   Characters other than those in the "reserved" set (see
1129   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1130</t>
1131<t>
1132   For example, the following three URIs are equivalent:
1133</t>
1134<figure><artwork type="example">
1135   http://example.com:80/~smith/home.html
1136   http://EXAMPLE.com/%7Esmith/home.html
1137   http://EXAMPLE.com:/%7esmith/home.html
1138</artwork></figure>
1139</section>
1140</section>
1141
1142<section title="Date/Time Formats" anchor="date.time.formats">
1143<section title="Full Date" anchor="full.date">
1144<t>
1145   HTTP applications have historically allowed three different formats
1146   for the representation of date/time stamps:
1147</t>
1148<figure><artwork type="example">
1149   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1150   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1151   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1152</artwork></figure>
1153<t>
1154   The first format is preferred as an Internet standard and represents
1155   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1156   <xref target="RFC822"/>). The other formats are described here only for
1157   compatibility with obsolete implementations.
1158   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1159   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1160   only generate the RFC 1123 format for representing HTTP-date values
1161   in header fields. See <xref target="tolerant.applications"/> for further information.
1162</t>
1163<t><list><t>
1164      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1165      accepting date values that may have been sent by non-HTTP
1166      applications, as is sometimes the case when retrieving or posting
1167      messages via proxies/gateways to SMTP or NNTP.
1168</t></list></t>
1169<t>
1170   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1171   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1172   equal to UTC (Coordinated Universal Time). This is indicated in the
1173   first two formats by the inclusion of "GMT" as the three-letter
1174   abbreviation for time zone, and &MUST; be assumed when reading the
1175   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1176   additional LWS beyond that specifically included as SP in the
1177   grammar.
1178</t>
1179<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"/>
1180  HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1181  rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1182  rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1183  asctime-date = wkday SP date3 SP time SP 4DIGIT
1184  date1        = 2DIGIT SP month SP 4DIGIT
1185                 ; day month year (e.g., 02 Jun 1982)
1186  date2        = 2DIGIT "-" month "-" 2DIGIT
1187                 ; day-month-year (e.g., 02-Jun-82)
1188  date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1189                 ; month day (e.g., Jun  2)
1190  time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1191                 ; 00:00:00 - 23:59:59
1192  wkday        = "Mon" | "Tue" | "Wed"
1193               | "Thu" | "Fri" | "Sat" | "Sun"
1194  weekday      = "Monday" | "Tuesday" | "Wednesday"
1195               | "Thursday" | "Friday" | "Saturday" | "Sunday"
1196  month        = "Jan" | "Feb" | "Mar" | "Apr"
1197               | "May" | "Jun" | "Jul" | "Aug"
1198               | "Sep" | "Oct" | "Nov" | "Dec"
1199</artwork></figure>
1200<t>
1201      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1202      to their usage within the protocol stream. Clients and servers are
1203      not required to use these formats for user presentation, request
1204      logging, etc.
1205</t>
1206</section>
1207</section>
1208
1209<section title="Transfer Codings" anchor="transfer.codings">
1210<t>
1211   Transfer-coding values are used to indicate an encoding
1212   transformation that has been, can be, or may need to be applied to an
1213   entity-body in order to ensure "safe transport" through the network.
1214   This differs from a content coding in that the transfer-coding is a
1215   property of the message, not of the original entity.
1216</t>
1217<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1218  transfer-coding         = "chunked" | transfer-extension
1219  transfer-extension      = token *( ";" parameter )
1220</artwork></figure>
1221<t>
1222   Parameters are in  the form of attribute/value pairs.
1223</t>
1224<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"/>
1225  parameter               = attribute "=" value
1226  attribute               = token
1227  value                   = token | quoted-string
1228</artwork></figure>
1229<t>
1230   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1231   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1232   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1233</t>
1234<t>
1235   Whenever a transfer-coding is applied to a message-body, the set of
1236   transfer-codings &MUST; include "chunked", unless the message is
1237   terminated by closing the connection. When the "chunked" transfer-coding
1238   is used, it &MUST; be the last transfer-coding applied to the
1239   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1240   than once to a message-body. These rules allow the recipient to
1241   determine the transfer-length of the message (<xref target="message.length"/>).
1242</t>
1243<t>
1244   Transfer-codings are analogous to the Content-Transfer-Encoding
1245   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1246   binary data over a 7-bit transport service. However, safe transport
1247   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1248   the only unsafe characteristic of message-bodies is the difficulty in
1249   determining the exact body length (<xref target="message.length"/>), or the desire to
1250   encrypt data over a shared transport.
1251</t>
1252<t>
1253   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1254   transfer-coding value tokens. Initially, the registry contains the
1255   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1256   "gzip", "compress", and "deflate" (&content-codings;).
1257</t>
1258<t>
1259   New transfer-coding value tokens &SHOULD; be registered in the same way
1260   as new content-coding value tokens (&content-codings;).
1261</t>
1262<t>
1263   A server which receives an entity-body with a transfer-coding it does
1264   not understand &SHOULD; return 501 (Not Implemented), and close the
1265   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1266   client.
1267</t>
1268
1269<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1270<t>
1271   The chunked encoding modifies the body of a message in order to
1272   transfer it as a series of chunks, each with its own size indicator,
1273   followed by an &OPTIONAL; trailer containing entity-header fields. This
1274   allows dynamically produced content to be transferred along with the
1275   information necessary for the recipient to verify that it has
1276   received the full message.
1277</t>
1278<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-part"/>
1279  Chunked-Body   = *chunk
1280                   last-chunk
1281                   trailer-part
1282                   CRLF
1283 
1284  chunk          = chunk-size [ chunk-extension ] CRLF
1285                   chunk-data CRLF
1286  chunk-size     = 1*HEX
1287  last-chunk     = 1*("0") [ chunk-extension ] CRLF
1288 
1289  chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1290  chunk-ext-name = token
1291  chunk-ext-val  = token | quoted-string
1292  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
1293  trailer-part   = *(entity-header CRLF)
1294</artwork></figure>
1295<t>
1296   The chunk-size field is a string of hex digits indicating the size of
1297   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1298   zero, followed by the trailer, which is terminated by an empty line.
1299</t>
1300<t>
1301   The trailer allows the sender to include additional HTTP header
1302   fields at the end of the message. The Trailer header field can be
1303   used to indicate which header fields are included in a trailer (see
1304   <xref target="header.trailer"/>).
1305</t>
1306<t>
1307   A server using chunked transfer-coding in a response &MUST-NOT; use the
1308   trailer for any header fields unless at least one of the following is
1309   true:
1310  <list style="numbers">
1311    <t>the request included a TE header field that indicates "trailers" is
1312     acceptable in the transfer-coding of the  response, as described in
1313     <xref target="header.te"/>; or,</t>
1314
1315    <t>the server is the origin server for the response, the trailer
1316     fields consist entirely of optional metadata, and the recipient
1317     could use the message (in a manner acceptable to the origin server)
1318     without receiving this metadata.  In other words, the origin server
1319     is willing to accept the possibility that the trailer fields might
1320     be silently discarded along the path to the client.</t>
1321  </list>
1322</t>
1323<t>
1324   This requirement prevents an interoperability failure when the
1325   message is being received by an HTTP/1.1 (or later) proxy and
1326   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1327   compliance with the protocol would have necessitated a possibly
1328   infinite buffer on the proxy.
1329</t>
1330<t>
1331   A process for decoding the "chunked" transfer-coding
1332   can be represented in pseudo-code as:
1333</t>
1334<figure><artwork type="code">
1335    length := 0
1336    read chunk-size, chunk-extension (if any) and CRLF
1337    while (chunk-size &gt; 0) {
1338       read chunk-data and CRLF
1339       append chunk-data to entity-body
1340       length := length + chunk-size
1341       read chunk-size and CRLF
1342    }
1343    read entity-header
1344    while (entity-header not empty) {
1345       append entity-header to existing header fields
1346       read entity-header
1347    }
1348    Content-Length := length
1349    Remove "chunked" from Transfer-Encoding
1350</artwork></figure>
1351<t>
1352   All HTTP/1.1 applications &MUST; be able to receive and decode the
1353   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1354   they do not understand.
1355</t>
1356</section>
1357</section>
1358
1359<section title="Product Tokens" anchor="product.tokens">
1360<t>
1361   Product tokens are used to allow communicating applications to
1362   identify themselves by software name and version. Most fields using
1363   product tokens also allow sub-products which form a significant part
1364   of the application to be listed, separated by white space. By
1365   convention, the products are listed in order of their significance
1366   for identifying the application.
1367</t>
1368<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1369  product         = token ["/" product-version]
1370  product-version = token
1371</artwork></figure>
1372<t>
1373   Examples:
1374</t>
1375<figure><artwork type="example">
1376    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1377    Server: Apache/0.8.4
1378</artwork></figure>
1379<t>
1380   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1381   used for advertising or other non-essential information. Although any
1382   token character &MAY; appear in a product-version, this token &SHOULD;
1383   only be used for a version identifier (i.e., successive versions of
1384   the same product &SHOULD; only differ in the product-version portion of
1385   the product value).
1386</t>
1387</section>
1388
1389</section>
1390
1391<section title="HTTP Message" anchor="http.message">
1392
1393<section title="Message Types" anchor="message.types">
1394<t>
1395   HTTP messages consist of requests from client to server and responses
1396   from server to client.
1397</t>
1398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1399  HTTP-message   = Request | Response     ; HTTP/1.1 messages
1400</artwork></figure>
1401<t>
1402   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1403   message format of <xref target="RFC2822"/> for transferring entities (the payload
1404   of the message). Both types of message consist of a start-line, zero
1405   or more header fields (also known as "headers"), an empty line (i.e.,
1406   a line with nothing preceding the CRLF) indicating the end of the
1407   header fields, and possibly a message-body.
1408</t>
1409<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1410  generic-message = start-line
1411                    *(message-header CRLF)
1412                    CRLF
1413                    [ message-body ]
1414  start-line      = Request-Line | Status-Line
1415</artwork></figure>
1416<t>
1417   In the interest of robustness, servers &SHOULD; ignore any empty
1418   line(s) received where a Request-Line is expected. In other words, if
1419   the server is reading the protocol stream at the beginning of a
1420   message and receives a CRLF first, it should ignore the CRLF.
1421</t>
1422<t>
1423   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1424   after a POST request. To restate what is explicitly forbidden by the
1425   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1426   extra CRLF.
1427</t>
1428</section>
1429
1430<section title="Message Headers" anchor="message.headers">
1431<t>
1432   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1433   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1434   entity-header (&entity-header-fields;) fields, follow the same generic format as
1435   that given in <xref target="RFC2822" x:fmt="of" x:sec="2.1"/>. Each header field consists
1436   of a name followed by a colon (":") and the field value. Field names
1437   are case-insensitive. The field value &MAY; be preceded by any amount
1438   of LWS, though a single SP is preferred. Header fields can be
1439   extended over multiple lines by preceding each extra line with at
1440   least one SP or HTAB. Applications ought to follow "common form", where
1441   one is known or indicated, when generating HTTP constructs, since
1442   there might exist some implementations that fail to accept anything
1443   beyond the common forms.
1444</t>
1445<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"/>
1446  message-header = field-name ":" [ field-value ]
1447  field-name     = token
1448  field-value    = *( field-content | LWS )
1449  field-content  = &lt;field content&gt;
1450                   ; the OCTETs making up the field-value
1451                   ; and consisting of either *TEXT or combinations
1452                   ; of token, separators, and quoted-string
1453</artwork></figure>
1454<t>
1455   The field-content does not include any leading or trailing LWS:
1456   linear white space occurring before the first non-whitespace
1457   character of the field-value or after the last non-whitespace
1458   character of the field-value. Such leading or trailing LWS &MAY; be
1459   removed without changing the semantics of the field value. Any LWS
1460   that occurs between field-content &MAY; be replaced with a single SP
1461   before interpreting the field value or forwarding the message
1462   downstream.
1463</t>
1464<t>
1465   The order in which header fields with differing field names are
1466   received is not significant. However, it is "good practice" to send
1467   general-header fields first, followed by request-header or response-header
1468   fields, and ending with the entity-header fields.
1469</t>
1470<t>
1471   Multiple message-header fields with the same field-name &MAY; be
1472   present in a message if and only if the entire field-value for that
1473   header field is defined as a comma-separated list [i.e., #(values)].
1474   It &MUST; be possible to combine the multiple header fields into one
1475   "field-name: field-value" pair, without changing the semantics of the
1476   message, by appending each subsequent field-value to the first, each
1477   separated by a comma. The order in which header fields with the same
1478   field-name are received is therefore significant to the
1479   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1480   change the order of these field values when a message is forwarded.
1481</t>
1482</section>
1483
1484<section title="Message Body" anchor="message.body">
1485<t>
1486   The message-body (if any) of an HTTP message is used to carry the
1487   entity-body associated with the request or response. The message-body
1488   differs from the entity-body only when a transfer-coding has been
1489   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1490</t>
1491<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1492  message-body = entity-body
1493               | &lt;entity-body encoded as per Transfer-Encoding&gt;
1494</artwork></figure>
1495<t>
1496   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1497   applied by an application to ensure safe and proper transfer of the
1498   message. Transfer-Encoding is a property of the message, not of the
1499   entity, and thus &MAY; be added or removed by any application along the
1500   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1501   when certain transfer-codings may be used.)
1502</t>
1503<t>
1504   The rules for when a message-body is allowed in a message differ for
1505   requests and responses.
1506</t>
1507<t>
1508   The presence of a message-body in a request is signaled by the
1509   inclusion of a Content-Length or Transfer-Encoding header field in
1510   the request's message-headers. A message-body &MUST-NOT; be included in
1511   a request if the specification of the request method (&method;)
1512   explicitly disallows an entity-body in requests.
1513   When a request message contains both a message-body of non-zero
1514   length and a method that does not define any semantics for that
1515   request message-body, then an origin server &SHOULD; either ignore
1516   the message-body or respond with an appropriate error message
1517   (e.g., 413).  A proxy or gateway, when presented the same request,
1518   &SHOULD; either forward the request inbound with the message-body or
1519   ignore the message-body when determining a response.
1520</t>
1521<t>
1522   For response messages, whether or not a message-body is included with
1523   a message is dependent on both the request method and the response
1524   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1525   &MUST-NOT; include a message-body, even though the presence of entity-header
1526   fields might lead one to believe they do. All 1xx
1527   (informational), 204 (No Content), and 304 (Not Modified) responses
1528   &MUST-NOT; include a message-body. All other responses do include a
1529   message-body, although it &MAY; be of zero length.
1530</t>
1531</section>
1532
1533<section title="Message Length" anchor="message.length">
1534<t>
1535   The transfer-length of a message is the length of the message-body as
1536   it appears in the message; that is, after any transfer-codings have
1537   been applied. When a message-body is included with a message, the
1538   transfer-length of that body is determined by one of the following
1539   (in order of precedence):
1540</t>
1541<t>
1542  <list style="numbers">
1543    <x:lt><t>
1544     Any response message which "&MUST-NOT;" include a message-body (such
1545     as the 1xx, 204, and 304 responses and any response to a HEAD
1546     request) is always terminated by the first empty line after the
1547     header fields, regardless of the entity-header fields present in
1548     the message.
1549    </t></x:lt>
1550    <x:lt><t>
1551     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1552     is present, then the transfer-length is
1553     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1554     unless the message is terminated by closing the connection.
1555    </t></x:lt>
1556    <x:lt><t>
1557     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1558     decimal value in OCTETs represents both the entity-length and the
1559     transfer-length. The Content-Length header field &MUST-NOT; be sent
1560     if these two lengths are different (i.e., if a Transfer-Encoding
1561     header field is present). If a message is received with both a
1562     Transfer-Encoding header field and a Content-Length header field,
1563     the latter &MUST; be ignored.
1564    </t></x:lt>
1565    <x:lt><t>
1566     If the message uses the media type "multipart/byteranges", and the
1567     transfer-length is not otherwise specified, then this self-delimiting
1568     media type defines the transfer-length. This media type
1569     &MUST-NOT; be used unless the sender knows that the recipient can parse
1570     it; the presence in a request of a Range header with multiple byte-range
1571     specifiers from a 1.1 client implies that the client can parse
1572     multipart/byteranges responses.
1573    <list style="empty"><t>
1574       A range header might be forwarded by a 1.0 proxy that does not
1575       understand multipart/byteranges; in this case the server &MUST;
1576       delimit the message using methods defined in items 1, 3 or 5 of
1577       this section.
1578    </t></list>
1579    </t></x:lt>
1580    <x:lt><t>
1581     By the server closing the connection. (Closing the connection
1582     cannot be used to indicate the end of a request body, since that
1583     would leave no possibility for the server to send back a response.)
1584    </t></x:lt>
1585  </list>
1586</t>
1587<t>
1588   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1589   containing a message-body &MUST; include a valid Content-Length header
1590   field unless the server is known to be HTTP/1.1 compliant. If a
1591   request contains a message-body and a Content-Length is not given,
1592   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1593   determine the length of the message, or with 411 (Length Required) if
1594   it wishes to insist on receiving a valid Content-Length.
1595</t>
1596<t>
1597   All HTTP/1.1 applications that receive entities &MUST; accept the
1598   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1599   to be used for messages when the message length cannot be determined
1600   in advance.
1601</t>
1602<t>
1603   Messages &MUST-NOT; include both a Content-Length header field and a
1604   transfer-coding. If the message does include a
1605   transfer-coding, the Content-Length &MUST; be ignored.
1606</t>
1607<t>
1608   When a Content-Length is given in a message where a message-body is
1609   allowed, its field value &MUST; exactly match the number of OCTETs in
1610   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1611   invalid length is received and detected.
1612</t>
1613</section>
1614
1615<section title="General Header Fields" anchor="general.header.fields">
1616<t>
1617   There are a few header fields which have general applicability for
1618   both request and response messages, but which do not apply to the
1619   entity being transferred. These header fields apply only to the
1620   message being transmitted.
1621</t>
1622<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1623  general-header = Cache-Control            ; &header-cache-control;
1624                 | Connection               ; <xref target="header.connection"/>
1625                 | Date                     ; <xref target="header.date"/>
1626                 | Pragma                   ; &header-pragma;
1627                 | Trailer                  ; <xref target="header.trailer"/>
1628                 | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1629                 | Upgrade                  ; <xref target="header.upgrade"/>
1630                 | Via                      ; <xref target="header.via"/>
1631                 | Warning                  ; &header-warning;
1632</artwork></figure>
1633<t>
1634   General-header field names can be extended reliably only in
1635   combination with a change in the protocol version. However, new or
1636   experimental header fields may be given the semantics of general
1637   header fields if all parties in the communication recognize them to
1638   be general-header fields. Unrecognized header fields are treated as
1639   entity-header fields.
1640</t>
1641</section>
1642</section>
1643
1644<section title="Request" anchor="request">
1645<t>
1646   A request message from a client to a server includes, within the
1647   first line of that message, the method to be applied to the resource,
1648   the identifier of the resource, and the protocol version in use.
1649</t>
1650<!--                 Host                      ; should be moved here eventually -->
1651<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1652  Request       = Request-Line              ; <xref target="request-line"/>
1653                  *(( general-header        ; <xref target="general.header.fields"/>
1654                   | request-header         ; &request-header-fields;
1655                   | entity-header ) CRLF)  ; &entity-header-fields;
1656                  CRLF
1657                  [ message-body ]          ; <xref target="message.body"/>
1658</artwork></figure>
1659
1660<section title="Request-Line" anchor="request-line">
1661<t>
1662   The Request-Line begins with a method token, followed by the
1663   Request-URI and the protocol version, and ending with CRLF. The
1664   elements are separated by SP characters. No CR or LF is allowed
1665   except in the final CRLF sequence.
1666</t>
1667<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1668  Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1669</artwork></figure>
1670
1671<section title="Method" anchor="method">
1672<t>
1673   The Method  token indicates the method to be performed on the
1674   resource identified by the Request-URI. The method is case-sensitive.
1675</t>
1676<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1677  Method         = token
1678</artwork></figure>
1679</section>
1680
1681<section title="Request-URI" anchor="request-uri">
1682<t>
1683   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1684   identifies the resource upon which to apply the request.
1685</t>
1686<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1687  Request-URI    = "*"
1688                 | absoluteURI
1689                 | ( path-absolute [ "?" query ] )
1690                 | authority
1691</artwork></figure>
1692<t>
1693   The four options for Request-URI are dependent on the nature of the
1694   request. The asterisk "*" means that the request does not apply to a
1695   particular resource, but to the server itself, and is only allowed
1696   when the method used does not necessarily apply to a resource. One
1697   example would be
1698</t>
1699<figure><artwork type="example">
1700    OPTIONS * HTTP/1.1
1701</artwork></figure>
1702<t>
1703   The absoluteURI form is &REQUIRED; when the request is being made to a
1704   proxy. The proxy is requested to forward the request or service it
1705   from a valid cache, and return the response. Note that the proxy &MAY;
1706   forward the request on to another proxy or directly to the server
1707   specified by the absoluteURI. In order to avoid request loops, a
1708   proxy &MUST; be able to recognize all of its server names, including
1709   any aliases, local variations, and the numeric IP address. An example
1710   Request-Line would be:
1711</t>
1712<figure><artwork type="example">
1713    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1714</artwork></figure>
1715<t>
1716   To allow for transition to absoluteURIs in all requests in future
1717   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1718   form in requests, even though HTTP/1.1 clients will only generate
1719   them in requests to proxies.
1720</t>
1721<t>
1722   The authority form is only used by the CONNECT method (&CONNECT;).
1723</t>
1724<t>
1725   The most common form of Request-URI is that used to identify a
1726   resource on an origin server or gateway. In this case the absolute
1727   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1728   the Request-URI, and the network location of the URI (authority) &MUST;
1729   be transmitted in a Host header field. For example, a client wishing
1730   to retrieve the resource above directly from the origin server would
1731   create a TCP connection to port 80 of the host "www.example.org" and send
1732   the lines:
1733</t>
1734<figure><artwork type="example">
1735    GET /pub/WWW/TheProject.html HTTP/1.1
1736    Host: www.example.org
1737</artwork></figure>
1738<t>
1739   followed by the remainder of the Request. Note that the absolute path
1740   cannot be empty; if none is present in the original URI, it &MUST; be
1741   given as "/" (the server root).
1742</t>
1743<t>
1744   The Request-URI is transmitted in the format specified in
1745   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1746   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1747   properly interpret the request. Servers &SHOULD; respond to invalid
1748   Request-URIs with an appropriate status code.
1749</t>
1750<t>
1751   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1752   received Request-URI when forwarding it to the next inbound server,
1753   except as noted above to replace a null path-absolute with "/".
1754</t>
1755<t>
1756  <list><t>
1757      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1758      meaning of the request when the origin server is improperly using
1759      a non-reserved URI character for a reserved purpose.  Implementors
1760      should be aware that some pre-HTTP/1.1 proxies have been known to
1761      rewrite the Request-URI.
1762  </t></list>
1763</t>
1764</section>
1765</section>
1766
1767<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1768<t>
1769   The exact resource identified by an Internet request is determined by
1770   examining both the Request-URI and the Host header field.
1771</t>
1772<t>
1773   An origin server that does not allow resources to differ by the
1774   requested host &MAY; ignore the Host header field value when
1775   determining the resource identified by an HTTP/1.1 request. (But see
1776   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1777   for other requirements on Host support in HTTP/1.1.)
1778</t>
1779<t>
1780   An origin server that does differentiate resources based on the host
1781   requested (sometimes referred to as virtual hosts or vanity host
1782   names) &MUST; use the following rules for determining the requested
1783   resource on an HTTP/1.1 request:
1784  <list style="numbers">
1785    <t>If Request-URI is an absoluteURI, the host is part of the
1786     Request-URI. Any Host header field value in the request &MUST; be
1787     ignored.</t>
1788    <t>If the Request-URI is not an absoluteURI, and the request includes
1789     a Host header field, the host is determined by the Host header
1790     field value.</t>
1791    <t>If the host as determined by rule 1 or 2 is not a valid host on
1792     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1793  </list>
1794</t>
1795<t>
1796   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1797   attempt to use heuristics (e.g., examination of the URI path for
1798   something unique to a particular host) in order to determine what
1799   exact resource is being requested.
1800</t>
1801</section>
1802
1803</section>
1804
1805
1806<section title="Response" anchor="response">
1807<t>
1808   After receiving and interpreting a request message, a server responds
1809   with an HTTP response message.
1810</t>
1811<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1812  Response      = Status-Line               ; <xref target="status-line"/>
1813                  *(( general-header        ; <xref target="general.header.fields"/>
1814                   | response-header        ; &response-header-fields;
1815                   | entity-header ) CRLF)  ; &entity-header-fields;
1816                  CRLF
1817                  [ message-body ]          ; <xref target="message.body"/>
1818</artwork></figure>
1819
1820<section title="Status-Line" anchor="status-line">
1821<t>
1822   The first line of a Response message is the Status-Line, consisting
1823   of the protocol version followed by a numeric status code and its
1824   associated textual phrase, with each element separated by SP
1825   characters. No CR or LF is allowed except in the final CRLF sequence.
1826</t>
1827<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1828  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1829</artwork></figure>
1830
1831<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1832<t>
1833   The Status-Code element is a 3-digit integer result code of the
1834   attempt to understand and satisfy the request. These codes are fully
1835   defined in &status-codes;.  The Reason Phrase exists for the sole
1836   purpose of providing a textual description associated with the numeric
1837   status code, out of deference to earlier Internet application protocols
1838   that were more frequently used with interactive text clients.
1839   A client &SHOULD; ignore the content of the Reason Phrase.
1840</t>
1841<t>
1842   The first digit of the Status-Code defines the class of response. The
1843   last two digits do not have any categorization role. There are 5
1844   values for the first digit:
1845  <list style="symbols">
1846    <t>
1847      1xx: Informational - Request received, continuing process
1848    </t>
1849    <t>
1850      2xx: Success - The action was successfully received,
1851        understood, and accepted
1852    </t>
1853    <t>
1854      3xx: Redirection - Further action must be taken in order to
1855        complete the request
1856    </t>
1857    <t>
1858      4xx: Client Error - The request contains bad syntax or cannot
1859        be fulfilled
1860    </t>
1861    <t>
1862      5xx: Server Error - The server failed to fulfill an apparently
1863        valid request
1864    </t>
1865  </list>
1866</t>
1867<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"/>
1868  Status-Code    = 3DIGIT
1869  Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1870</artwork></figure>
1871</section>
1872</section>
1873
1874</section>
1875
1876
1877<section title="Connections" anchor="connections">
1878
1879<section title="Persistent Connections" anchor="persistent.connections">
1880
1881<section title="Purpose" anchor="persistent.purpose">
1882<t>
1883   Prior to persistent connections, a separate TCP connection was
1884   established to fetch each URL, increasing the load on HTTP servers
1885   and causing congestion on the Internet. The use of inline images and
1886   other associated data often require a client to make multiple
1887   requests of the same server in a short amount of time. Analysis of
1888   these performance problems and results from a prototype
1889   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1890   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1891   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1892   T/TCP <xref target="Tou1998"/>.
1893</t>
1894<t>
1895   Persistent HTTP connections have a number of advantages:
1896  <list style="symbols">
1897      <t>
1898        By opening and closing fewer TCP connections, CPU time is saved
1899        in routers and hosts (clients, servers, proxies, gateways,
1900        tunnels, or caches), and memory used for TCP protocol control
1901        blocks can be saved in hosts.
1902      </t>
1903      <t>
1904        HTTP requests and responses can be pipelined on a connection.
1905        Pipelining allows a client to make multiple requests without
1906        waiting for each response, allowing a single TCP connection to
1907        be used much more efficiently, with much lower elapsed time.
1908      </t>
1909      <t>
1910        Network congestion is reduced by reducing the number of packets
1911        caused by TCP opens, and by allowing TCP sufficient time to
1912        determine the congestion state of the network.
1913      </t>
1914      <t>
1915        Latency on subsequent requests is reduced since there is no time
1916        spent in TCP's connection opening handshake.
1917      </t>
1918      <t>
1919        HTTP can evolve more gracefully, since errors can be reported
1920        without the penalty of closing the TCP connection. Clients using
1921        future versions of HTTP might optimistically try a new feature,
1922        but if communicating with an older server, retry with old
1923        semantics after an error is reported.
1924      </t>
1925    </list>
1926</t>
1927<t>
1928   HTTP implementations &SHOULD; implement persistent connections.
1929</t>
1930</section>
1931
1932<section title="Overall Operation" anchor="persistent.overall">
1933<t>
1934   A significant difference between HTTP/1.1 and earlier versions of
1935   HTTP is that persistent connections are the default behavior of any
1936   HTTP connection. That is, unless otherwise indicated, the client
1937   &SHOULD; assume that the server will maintain a persistent connection,
1938   even after error responses from the server.
1939</t>
1940<t>
1941   Persistent connections provide a mechanism by which a client and a
1942   server can signal the close of a TCP connection. This signaling takes
1943   place using the Connection header field (<xref target="header.connection"/>). Once a close
1944   has been signaled, the client &MUST-NOT; send any more requests on that
1945   connection.
1946</t>
1947
1948<section title="Negotiation" anchor="persistent.negotiation">
1949<t>
1950   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1951   maintain a persistent connection unless a Connection header including
1952   the connection-token "close" was sent in the request. If the server
1953   chooses to close the connection immediately after sending the
1954   response, it &SHOULD; send a Connection header including the
1955   connection-token close.
1956</t>
1957<t>
1958   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1959   decide to keep it open based on whether the response from a server
1960   contains a Connection header with the connection-token close. In case
1961   the client does not want to maintain a connection for more than that
1962   request, it &SHOULD; send a Connection header including the
1963   connection-token close.
1964</t>
1965<t>
1966   If either the client or the server sends the close token in the
1967   Connection header, that request becomes the last one for the
1968   connection.
1969</t>
1970<t>
1971   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1972   maintained for HTTP versions less than 1.1 unless it is explicitly
1973   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1974   compatibility with HTTP/1.0 clients.
1975</t>
1976<t>
1977   In order to remain persistent, all messages on the connection &MUST;
1978   have a self-defined message length (i.e., one not defined by closure
1979   of the connection), as described in <xref target="message.length"/>.
1980</t>
1981</section>
1982
1983<section title="Pipelining" anchor="pipelining">
1984<t>
1985   A client that supports persistent connections &MAY; "pipeline" its
1986   requests (i.e., send multiple requests without waiting for each
1987   response). A server &MUST; send its responses to those requests in the
1988   same order that the requests were received.
1989</t>
1990<t>
1991   Clients which assume persistent connections and pipeline immediately
1992   after connection establishment &SHOULD; be prepared to retry their
1993   connection if the first pipelined attempt fails. If a client does
1994   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1995   persistent. Clients &MUST; also be prepared to resend their requests if
1996   the server closes the connection before sending all of the
1997   corresponding responses.
1998</t>
1999<t>
2000   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
2001   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
2002   premature termination of the transport connection could lead to
2003   indeterminate results. A client wishing to send a non-idempotent
2004   request &SHOULD; wait to send that request until it has received the
2005   response status for the previous request.
2006</t>
2007</section>
2008</section>
2009
2010<section title="Proxy Servers" anchor="persistent.proxy">
2011<t>
2012   It is especially important that proxies correctly implement the
2013   properties of the Connection header field as specified in <xref target="header.connection"/>.
2014</t>
2015<t>
2016   The proxy server &MUST; signal persistent connections separately with
2017   its clients and the origin servers (or other proxy servers) that it
2018   connects to. Each persistent connection applies to only one transport
2019   link.
2020</t>
2021<t>
2022   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2023   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
2024   discussion of the problems with the Keep-Alive header implemented by
2025   many HTTP/1.0 clients).
2026</t>
2027</section>
2028
2029<section title="Practical Considerations" anchor="persistent.practical">
2030<t>
2031   Servers will usually have some time-out value beyond which they will
2032   no longer maintain an inactive connection. Proxy servers might make
2033   this a higher value since it is likely that the client will be making
2034   more connections through the same server. The use of persistent
2035   connections places no requirements on the length (or existence) of
2036   this time-out for either the client or the server.
2037</t>
2038<t>
2039   When a client or server wishes to time-out it &SHOULD; issue a graceful
2040   close on the transport connection. Clients and servers &SHOULD; both
2041   constantly watch for the other side of the transport close, and
2042   respond to it as appropriate. If a client or server does not detect
2043   the other side's close promptly it could cause unnecessary resource
2044   drain on the network.
2045</t>
2046<t>
2047   A client, server, or proxy &MAY; close the transport connection at any
2048   time. For example, a client might have started to send a new request
2049   at the same time that the server has decided to close the "idle"
2050   connection. From the server's point of view, the connection is being
2051   closed while it was idle, but from the client's point of view, a
2052   request is in progress.
2053</t>
2054<t>
2055   This means that clients, servers, and proxies &MUST; be able to recover
2056   from asynchronous close events. Client software &SHOULD; reopen the
2057   transport connection and retransmit the aborted sequence of requests
2058   without user interaction so long as the request sequence is
2059   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2060   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2061   human operator the choice of retrying the request(s). Confirmation by
2062   user-agent software with semantic understanding of the application
2063   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2064   be repeated if the second sequence of requests fails.
2065</t>
2066<t>
2067   Servers &SHOULD; always respond to at least one request per connection,
2068   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2069   middle of transmitting a response, unless a network or client failure
2070   is suspected.
2071</t>
2072<t>
2073   Clients that use persistent connections &SHOULD; limit the number of
2074   simultaneous connections that they maintain to a given server. A
2075   single-user client &SHOULD-NOT; maintain more than 2 connections with
2076   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2077   another server or proxy, where N is the number of simultaneously
2078   active users. These guidelines are intended to improve HTTP response
2079   times and avoid congestion.
2080</t>
2081</section>
2082</section>
2083
2084<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2085
2086<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2087<t>
2088   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2089   flow control mechanisms to resolve temporary overloads, rather than
2090   terminating connections with the expectation that clients will retry.
2091   The latter technique can exacerbate network congestion.
2092</t>
2093</section>
2094
2095<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2096<t>
2097   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2098   the network connection for an error status while it is transmitting
2099   the request. If the client sees an error status, it &SHOULD;
2100   immediately cease transmitting the body. If the body is being sent
2101   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2102   empty trailer &MAY; be used to prematurely mark the end of the message.
2103   If the body was preceded by a Content-Length header, the client &MUST;
2104   close the connection.
2105</t>
2106</section>
2107
2108<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2109<t>
2110   The purpose of the 100 (Continue) status (see &status-100;) is to
2111   allow a client that is sending a request message with a request body
2112   to determine if the origin server is willing to accept the request
2113   (based on the request headers) before the client sends the request
2114   body. In some cases, it might either be inappropriate or highly
2115   inefficient for the client to send the body if the server will reject
2116   the message without looking at the body.
2117</t>
2118<t>
2119   Requirements for HTTP/1.1 clients:
2120  <list style="symbols">
2121    <t>
2122        If a client will wait for a 100 (Continue) response before
2123        sending the request body, it &MUST; send an Expect request-header
2124        field (&header-expect;) with the "100-continue" expectation.
2125    </t>
2126    <t>
2127        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2128        with the "100-continue" expectation if it does not intend
2129        to send a request body.
2130    </t>
2131  </list>
2132</t>
2133<t>
2134   Because of the presence of older implementations, the protocol allows
2135   ambiguous situations in which a client may send "Expect: 100-continue"
2136   without receiving either a 417 (Expectation Failed) status
2137   or a 100 (Continue) status. Therefore, when a client sends this
2138   header field to an origin server (possibly via a proxy) from which it
2139   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2140   for an indefinite period before sending the request body.
2141</t>
2142<t>
2143   Requirements for HTTP/1.1 origin servers:
2144  <list style="symbols">
2145    <t> Upon receiving a request which includes an Expect request-header
2146        field with the "100-continue" expectation, an origin server &MUST;
2147        either respond with 100 (Continue) status and continue to read
2148        from the input stream, or respond with a final status code. The
2149        origin server &MUST-NOT; wait for the request body before sending
2150        the 100 (Continue) response. If it responds with a final status
2151        code, it &MAY; close the transport connection or it &MAY; continue
2152        to read and discard the rest of the request.  It &MUST-NOT;
2153        perform the requested method if it returns a final status code.
2154    </t>
2155    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2156        the request message does not include an Expect request-header
2157        field with the "100-continue" expectation, and &MUST-NOT; send a
2158        100 (Continue) response if such a request comes from an HTTP/1.0
2159        (or earlier) client. There is an exception to this rule: for
2160        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2161        status in response to an HTTP/1.1 PUT or POST request that does
2162        not include an Expect request-header field with the "100-continue"
2163        expectation. This exception, the purpose of which is
2164        to minimize any client processing delays associated with an
2165        undeclared wait for 100 (Continue) status, applies only to
2166        HTTP/1.1 requests, and not to requests with any other HTTP-version
2167        value.
2168    </t>
2169    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2170        already received some or all of the request body for the
2171        corresponding request.
2172    </t>
2173    <t> An origin server that sends a 100 (Continue) response &MUST;
2174    ultimately send a final status code, once the request body is
2175        received and processed, unless it terminates the transport
2176        connection prematurely.
2177    </t>
2178    <t> If an origin server receives a request that does not include an
2179        Expect request-header field with the "100-continue" expectation,
2180        the request includes a request body, and the server responds
2181        with a final status code before reading the entire request body
2182        from the transport connection, then the server &SHOULD-NOT;  close
2183        the transport connection until it has read the entire request,
2184        or until the client closes the connection. Otherwise, the client
2185        might not reliably receive the response message. However, this
2186        requirement is not be construed as preventing a server from
2187        defending itself against denial-of-service attacks, or from
2188        badly broken client implementations.
2189      </t>
2190    </list>
2191</t>
2192<t>
2193   Requirements for HTTP/1.1 proxies:
2194  <list style="symbols">
2195    <t> If a proxy receives a request that includes an Expect request-header
2196        field with the "100-continue" expectation, and the proxy
2197        either knows that the next-hop server complies with HTTP/1.1 or
2198        higher, or does not know the HTTP version of the next-hop
2199        server, it &MUST; forward the request, including the Expect header
2200        field.
2201    </t>
2202    <t> If the proxy knows that the version of the next-hop server is
2203        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2204        respond with a 417 (Expectation Failed) status.
2205    </t>
2206    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2207        numbers received from recently-referenced next-hop servers.
2208    </t>
2209    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2210        request message was received from an HTTP/1.0 (or earlier)
2211        client and did not include an Expect request-header field with
2212        the "100-continue" expectation. This requirement overrides the
2213        general rule for forwarding of 1xx responses (see &status-1xx;).
2214    </t>
2215  </list>
2216</t>
2217</section>
2218
2219<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2220<t>
2221   If an HTTP/1.1 client sends a request which includes a request body,
2222   but which does not include an Expect request-header field with the
2223   "100-continue" expectation, and if the client is not directly
2224   connected to an HTTP/1.1 origin server, and if the client sees the
2225   connection close before receiving any status from the server, the
2226   client &SHOULD; retry the request.  If the client does retry this
2227   request, it &MAY; use the following "binary exponential backoff"
2228   algorithm to be assured of obtaining a reliable response:
2229  <list style="numbers">
2230    <t>
2231      Initiate a new connection to the server
2232    </t>
2233    <t>
2234      Transmit the request-headers
2235    </t>
2236    <t>
2237      Initialize a variable R to the estimated round-trip time to the
2238         server (e.g., based on the time it took to establish the
2239         connection), or to a constant value of 5 seconds if the round-trip
2240         time is not available.
2241    </t>
2242    <t>
2243       Compute T = R * (2**N), where N is the number of previous
2244         retries of this request.
2245    </t>
2246    <t>
2247       Wait either for an error response from the server, or for T
2248         seconds (whichever comes first)
2249    </t>
2250    <t>
2251       If no error response is received, after T seconds transmit the
2252         body of the request.
2253    </t>
2254    <t>
2255       If client sees that the connection is closed prematurely,
2256         repeat from step 1 until the request is accepted, an error
2257         response is received, or the user becomes impatient and
2258         terminates the retry process.
2259    </t>
2260  </list>
2261</t>
2262<t>
2263   If at any point an error status is received, the client
2264  <list style="symbols">
2265      <t>&SHOULD-NOT;  continue and</t>
2266
2267      <t>&SHOULD; close the connection if it has not completed sending the
2268        request message.</t>
2269    </list>
2270</t>
2271</section>
2272</section>
2273</section>
2274
2275
2276<section title="Header Field Definitions" anchor="header.fields">
2277<t>
2278   This section defines the syntax and semantics of HTTP/1.1 header fields
2279   related to message framing and transport protocols.
2280</t>
2281<t>
2282   For entity-header fields, both sender and recipient refer to either the
2283   client or the server, depending on who sends and who receives the entity.
2284</t>
2285
2286<section title="Connection" anchor="header.connection">
2287  <iref primary="true" item="Connection header" x:for-anchor=""/>
2288  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2289<t>
2290   The Connection general-header field allows the sender to specify
2291   options that are desired for that particular connection and &MUST-NOT;
2292   be communicated by proxies over further connections.
2293</t>
2294<t>
2295   The Connection header has the following grammar:
2296</t>
2297<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2298  Connection = "Connection" ":" 1#(connection-token)
2299  connection-token  = token
2300</artwork></figure>
2301<t>
2302   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2303   message is forwarded and, for each connection-token in this field,
2304   remove any header field(s) from the message with the same name as the
2305   connection-token. Connection options are signaled by the presence of
2306   a connection-token in the Connection header field, not by any
2307   corresponding additional header field(s), since the additional header
2308   field may not be sent if there are no parameters associated with that
2309   connection option.
2310</t>
2311<t>
2312   Message headers listed in the Connection header &MUST-NOT; include
2313   end-to-end headers, such as Cache-Control.
2314</t>
2315<t>
2316   HTTP/1.1 defines the "close" connection option for the sender to
2317   signal that the connection will be closed after completion of the
2318   response. For example,
2319</t>
2320<figure><artwork type="example">
2321    Connection: close
2322</artwork></figure>
2323<t>
2324   in either the request or the response header fields indicates that
2325   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2326   after the current request/response is complete.
2327</t>
2328<t>
2329   An HTTP/1.1 client that does not support persistent connections &MUST;
2330   include the "close" connection option in every request message.
2331</t>
2332<t>
2333   An HTTP/1.1 server that does not support persistent connections &MUST;
2334   include the "close" connection option in every response message that
2335   does not have a 1xx (informational) status code.
2336</t>
2337<t>
2338   A system receiving an HTTP/1.0 (or lower-version) message that
2339   includes a Connection header &MUST;, for each connection-token in this
2340   field, remove and ignore any header field(s) from the message with
2341   the same name as the connection-token. This protects against mistaken
2342   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2343</t>
2344</section>
2345
2346<section title="Content-Length" anchor="header.content-length">
2347  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2348  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2349<t>
2350   The Content-Length entity-header field indicates the size of the
2351   entity-body, in decimal number of OCTETs, sent to the recipient or,
2352   in the case of the HEAD method, the size of the entity-body that
2353   would have been sent had the request been a GET.
2354</t>
2355<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2356  Content-Length    = "Content-Length" ":" 1*DIGIT
2357</artwork></figure>
2358<t>
2359   An example is
2360</t>
2361<figure><artwork type="example">
2362    Content-Length: 3495
2363</artwork></figure>
2364<t>
2365   Applications &SHOULD; use this field to indicate the transfer-length of
2366   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2367</t>
2368<t>
2369   Any Content-Length greater than or equal to zero is a valid value.
2370   <xref target="message.length"/> describes how to determine the length of a message-body
2371   if a Content-Length is not given.
2372</t>
2373<t>
2374   Note that the meaning of this field is significantly different from
2375   the corresponding definition in MIME, where it is an optional field
2376   used within the "message/external-body" content-type. In HTTP, it
2377   &SHOULD; be sent whenever the message's length can be determined prior
2378   to being transferred, unless this is prohibited by the rules in
2379   <xref target="message.length"/>.
2380</t>
2381</section>
2382
2383<section title="Date" anchor="header.date">
2384  <iref primary="true" item="Date header" x:for-anchor=""/>
2385  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2386<t>
2387   The Date general-header field represents the date and time at which
2388   the message was originated, having the same semantics as orig-date in
2389   <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"/>;
2390   it &MUST; be sent in rfc1123-date format.
2391</t>
2392<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2393  Date  = "Date" ":" HTTP-date
2394</artwork></figure>
2395<t>
2396   An example is
2397</t>
2398<figure><artwork type="example">
2399    Date: Tue, 15 Nov 1994 08:12:31 GMT
2400</artwork></figure>
2401<t>
2402   Origin servers &MUST; include a Date header field in all responses,
2403   except in these cases:
2404  <list style="numbers">
2405      <t>If the response status code is 100 (Continue) or 101 (Switching
2406         Protocols), the response &MAY; include a Date header field, at
2407         the server's option.</t>
2408
2409      <t>If the response status code conveys a server error, e.g. 500
2410         (Internal Server Error) or 503 (Service Unavailable), and it is
2411         inconvenient or impossible to generate a valid Date.</t>
2412
2413      <t>If the server does not have a clock that can provide a
2414         reasonable approximation of the current time, its responses
2415         &MUST-NOT; include a Date header field. In this case, the rules
2416         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2417  </list>
2418</t>
2419<t>
2420   A received message that does not have a Date header field &MUST; be
2421   assigned one by the recipient if the message will be cached by that
2422   recipient or gatewayed via a protocol which requires a Date. An HTTP
2423   implementation without a clock &MUST-NOT; cache responses without
2424   revalidating them on every use. An HTTP cache, especially a shared
2425   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2426   clock with a reliable external standard.
2427</t>
2428<t>
2429   Clients &SHOULD; only send a Date header field in messages that include
2430   an entity-body, as in the case of the PUT and POST requests, and even
2431   then it is optional. A client without a clock &MUST-NOT; send a Date
2432   header field in a request.
2433</t>
2434<t>
2435   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2436   time subsequent to the generation of the message. It &SHOULD; represent
2437   the best available approximation of the date and time of message
2438   generation, unless the implementation has no means of generating a
2439   reasonably accurate date and time. In theory, the date ought to
2440   represent the moment just before the entity is generated. In
2441   practice, the date can be generated at any time during the message
2442   origination without affecting its semantic value.
2443</t>
2444
2445<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2446<t>
2447   Some origin server implementations might not have a clock available.
2448   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2449   values to a response, unless these values were associated
2450   with the resource by a system or user with a reliable clock. It &MAY;
2451   assign an Expires value that is known, at or before server
2452   configuration time, to be in the past (this allows "pre-expiration"
2453   of responses without storing separate Expires values for each
2454   resource).
2455</t>
2456</section>
2457</section>
2458
2459<section title="Host" anchor="header.host">
2460  <iref primary="true" item="Host header" x:for-anchor=""/>
2461  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2462<t>
2463   The Host request-header field specifies the Internet host and port
2464   number of the resource being requested, as obtained from the original
2465   URI given by the user or referring resource (generally an HTTP URL,
2466   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2467   the naming authority of the origin server or gateway given by the
2468   original URL. This allows the origin server or gateway to
2469   differentiate between internally-ambiguous URLs, such as the root "/"
2470   URL of a server for multiple host names on a single IP address.
2471</t>
2472<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2473  Host = "Host" ":" uri-host [ ":" port ] ; <xref target="http.url"/>
2474</artwork></figure>
2475<t>
2476   A "host" without any trailing port information implies the default
2477   port for the service requested (e.g., "80" for an HTTP URL). For
2478   example, a request on the origin server for
2479   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2480</t>
2481<figure><artwork type="example">
2482    GET /pub/WWW/ HTTP/1.1
2483    Host: www.example.org
2484</artwork></figure>
2485<t>
2486   A client &MUST; include a Host header field in all HTTP/1.1 request
2487   messages. If the requested URI does not include an Internet host
2488   name for the service being requested, then the Host header field &MUST;
2489   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2490   request message it forwards does contain an appropriate Host header
2491   field that identifies the service being requested by the proxy. All
2492   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2493   status code to any HTTP/1.1 request message which lacks a Host header
2494   field.
2495</t>
2496<t>
2497   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2498   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2499   for other requirements relating to Host.
2500</t>
2501</section>
2502
2503<section title="TE" anchor="header.te">
2504  <iref primary="true" item="TE header" x:for-anchor=""/>
2505  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2506<t>
2507   The TE request-header field indicates what extension transfer-codings
2508   it is willing to accept in the response and whether or not it is
2509   willing to accept trailer fields in a chunked transfer-coding. Its
2510   value may consist of the keyword "trailers" and/or a comma-separated
2511   list of extension transfer-coding names with optional accept
2512   parameters (as described in <xref target="transfer.codings"/>).
2513</t>
2514<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2515  TE        = "TE" ":" #( t-codings )
2516  t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2517</artwork></figure>
2518<t>
2519   The presence of the keyword "trailers" indicates that the client is
2520   willing to accept trailer fields in a chunked transfer-coding, as
2521   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2522   transfer-coding values even though it does not itself represent a
2523   transfer-coding.
2524</t>
2525<t>
2526   Examples of its use are:
2527</t>
2528<figure><artwork type="example">
2529    TE: deflate
2530    TE:
2531    TE: trailers, deflate;q=0.5
2532</artwork></figure>
2533<t>
2534   The TE header field only applies to the immediate connection.
2535   Therefore, the keyword &MUST; be supplied within a Connection header
2536   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2537</t>
2538<t>
2539   A server tests whether a transfer-coding is acceptable, according to
2540   a TE field, using these rules:
2541  <list style="numbers">
2542    <x:lt>
2543      <t>The "chunked" transfer-coding is always acceptable. If the
2544         keyword "trailers" is listed, the client indicates that it is
2545         willing to accept trailer fields in the chunked response on
2546         behalf of itself and any downstream clients. The implication is
2547         that, if given, the client is stating that either all
2548         downstream clients are willing to accept trailer fields in the
2549         forwarded response, or that it will attempt to buffer the
2550         response on behalf of downstream recipients.
2551      </t><t>
2552         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2553         chunked response such that a client can be assured of buffering
2554         the entire response.</t>
2555    </x:lt>
2556    <x:lt>
2557      <t>If the transfer-coding being tested is one of the transfer-codings
2558         listed in the TE field, then it is acceptable unless it
2559         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2560         qvalue of 0 means "not acceptable.")</t>
2561    </x:lt>
2562    <x:lt>
2563      <t>If multiple transfer-codings are acceptable, then the
2564         acceptable transfer-coding with the highest non-zero qvalue is
2565         preferred.  The "chunked" transfer-coding always has a qvalue
2566         of 1.</t>
2567    </x:lt>
2568  </list>
2569</t>
2570<t>
2571   If the TE field-value is empty or if no TE field is present, the only
2572   transfer-coding  is "chunked". A message with no transfer-coding is
2573   always acceptable.
2574</t>
2575</section>
2576
2577<section title="Trailer" anchor="header.trailer">
2578  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2579  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2580<t>
2581   The Trailer general field value indicates that the given set of
2582   header fields is present in the trailer of a message encoded with
2583   chunked transfer-coding.
2584</t>
2585<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2586  Trailer  = "Trailer" ":" 1#field-name
2587</artwork></figure>
2588<t>
2589   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2590   message using chunked transfer-coding with a non-empty trailer. Doing
2591   so allows the recipient to know which header fields to expect in the
2592   trailer.
2593</t>
2594<t>
2595   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2596   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2597   trailer fields in a "chunked" transfer-coding.
2598</t>
2599<t>
2600   Message header fields listed in the Trailer header field &MUST-NOT;
2601   include the following header fields:
2602  <list style="symbols">
2603    <t>Transfer-Encoding</t>
2604    <t>Content-Length</t>
2605    <t>Trailer</t>
2606  </list>
2607</t>
2608</section>
2609
2610<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2611  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2612  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2613<t>
2614   The Transfer-Encoding general-header field indicates what (if any)
2615   type of transformation has been applied to the message body in order
2616   to safely transfer it between the sender and the recipient. This
2617   differs from the content-coding in that the transfer-coding is a
2618   property of the message, not of the entity.
2619</t>
2620<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2621  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2622</artwork></figure>
2623<t>
2624   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2625</t>
2626<figure><artwork type="example">
2627  Transfer-Encoding: chunked
2628</artwork></figure>
2629<t>
2630   If multiple encodings have been applied to an entity, the transfer-codings
2631   &MUST; be listed in the order in which they were applied.
2632   Additional information about the encoding parameters &MAY; be provided
2633   by other entity-header fields not defined by this specification.
2634</t>
2635<t>
2636   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2637   header.
2638</t>
2639</section>
2640
2641<section title="Upgrade" anchor="header.upgrade">
2642  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2643  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2644<t>
2645   The Upgrade general-header allows the client to specify what
2646   additional communication protocols it supports and would like to use
2647   if the server finds it appropriate to switch protocols. The server
2648   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2649   response to indicate which protocol(s) are being switched.
2650</t>
2651<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2652  Upgrade        = "Upgrade" ":" 1#product
2653</artwork></figure>
2654<t>
2655   For example,
2656</t>
2657<figure><artwork type="example">
2658    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2659</artwork></figure>
2660<t>
2661   The Upgrade header field is intended to provide a simple mechanism
2662   for transition from HTTP/1.1 to some other, incompatible protocol. It
2663   does so by allowing the client to advertise its desire to use another
2664   protocol, such as a later version of HTTP with a higher major version
2665   number, even though the current request has been made using HTTP/1.1.
2666   This eases the difficult transition between incompatible protocols by
2667   allowing the client to initiate a request in the more commonly
2668   supported protocol while indicating to the server that it would like
2669   to use a "better" protocol if available (where "better" is determined
2670   by the server, possibly according to the nature of the method and/or
2671   resource being requested).
2672</t>
2673<t>
2674   The Upgrade header field only applies to switching application-layer
2675   protocols upon the existing transport-layer connection. Upgrade
2676   cannot be used to insist on a protocol change; its acceptance and use
2677   by the server is optional. The capabilities and nature of the
2678   application-layer communication after the protocol change is entirely
2679   dependent upon the new protocol chosen, although the first action
2680   after changing the protocol &MUST; be a response to the initial HTTP
2681   request containing the Upgrade header field.
2682</t>
2683<t>
2684   The Upgrade header field only applies to the immediate connection.
2685   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2686   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2687   HTTP/1.1 message.
2688</t>
2689<t>
2690   The Upgrade header field cannot be used to indicate a switch to a
2691   protocol on a different connection. For that purpose, it is more
2692   appropriate to use a 301, 302, 303, or 305 redirection response.
2693</t>
2694<t>
2695   This specification only defines the protocol name "HTTP" for use by
2696   the family of Hypertext Transfer Protocols, as defined by the HTTP
2697   version rules of <xref target="http.version"/> and future updates to this
2698   specification. Any token can be used as a protocol name; however, it
2699   will only be useful if both the client and server associate the name
2700   with the same protocol.
2701</t>
2702</section>
2703
2704<section title="Via" anchor="header.via">
2705  <iref primary="true" item="Via header" x:for-anchor=""/>
2706  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2707<t>
2708   The Via general-header field &MUST; be used by gateways and proxies to
2709   indicate the intermediate protocols and recipients between the user
2710   agent and the server on requests, and between the origin server and
2711   the client on responses. It is analogous to the "Received" field of
2712   <xref target="RFC2822"/> and is intended to be used for tracking message forwards,
2713   avoiding request loops, and identifying the protocol capabilities of
2714   all senders along the request/response chain.
2715</t>
2716<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"/>
2717  Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2718  received-protocol = [ protocol-name "/" ] protocol-version
2719  protocol-name     = token
2720  protocol-version  = token
2721  received-by       = ( uri-host [ ":" port ] ) | pseudonym
2722  pseudonym         = token
2723</artwork></figure>
2724<t>
2725   The received-protocol indicates the protocol version of the message
2726   received by the server or client along each segment of the
2727   request/response chain. The received-protocol version is appended to
2728   the Via field value when the message is forwarded so that information
2729   about the protocol capabilities of upstream applications remains
2730   visible to all recipients.
2731</t>
2732<t>
2733   The protocol-name is optional if and only if it would be "HTTP". The
2734   received-by field is normally the host and optional port number of a
2735   recipient server or client that subsequently forwarded the message.
2736   However, if the real host is considered to be sensitive information,
2737   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2738   be assumed to be the default port of the received-protocol.
2739</t>
2740<t>
2741   Multiple Via field values represents each proxy or gateway that has
2742   forwarded the message. Each recipient &MUST; append its information
2743   such that the end result is ordered according to the sequence of
2744   forwarding applications.
2745</t>
2746<t>
2747   Comments &MAY; be used in the Via header field to identify the software
2748   of the recipient proxy or gateway, analogous to the User-Agent and
2749   Server header fields. However, all comments in the Via field are
2750   optional and &MAY; be removed by any recipient prior to forwarding the
2751   message.
2752</t>
2753<t>
2754   For example, a request message could be sent from an HTTP/1.0 user
2755   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2756   forward the request to a public proxy at p.example.net, which completes
2757   the request by forwarding it to the origin server at www.example.com.
2758   The request received by www.example.com would then have the following
2759   Via header field:
2760</t>
2761<figure><artwork type="example">
2762    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2763</artwork></figure>
2764<t>
2765   Proxies and gateways used as a portal through a network firewall
2766   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2767   the firewall region. This information &SHOULD; only be propagated if
2768   explicitly enabled. If not enabled, the received-by host of any host
2769   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2770   for that host.
2771</t>
2772<t>
2773   For organizations that have strong privacy requirements for hiding
2774   internal structures, a proxy &MAY; combine an ordered subsequence of
2775   Via header field entries with identical received-protocol values into
2776   a single such entry. For example,
2777</t>
2778<figure><artwork type="example">
2779    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2780</artwork></figure>
2781<t>
2782        could be collapsed to
2783</t>
2784<figure><artwork type="example">
2785    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2786</artwork></figure>
2787<t>
2788   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2789   under the same organizational control and the hosts have already been
2790   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2791   have different received-protocol values.
2792</t>
2793</section>
2794
2795</section>
2796
2797<section title="IANA Considerations" anchor="IANA.considerations">
2798<t>
2799   <cref>TBD.</cref>
2800</t>
2801</section>
2802
2803<section title="Security Considerations" anchor="security.considerations">
2804<t>
2805   This section is meant to inform application developers, information
2806   providers, and users of the security limitations in HTTP/1.1 as
2807   described by this document. The discussion does not include
2808   definitive solutions to the problems revealed, though it does make
2809   some suggestions for reducing security risks.
2810</t>
2811
2812<section title="Personal Information" anchor="personal.information">
2813<t>
2814   HTTP clients are often privy to large amounts of personal information
2815   (e.g. the user's name, location, mail address, passwords, encryption
2816   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2817   leakage of this information.
2818   We very strongly recommend that a convenient interface be provided
2819   for the user to control dissemination of such information, and that
2820   designers and implementors be particularly careful in this area.
2821   History shows that errors in this area often create serious security
2822   and/or privacy problems and generate highly adverse publicity for the
2823   implementor's company.
2824</t>
2825</section>
2826
2827<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2828<t>
2829   A server is in the position to save personal data about a user's
2830   requests which might identify their reading patterns or subjects of
2831   interest. This information is clearly confidential in nature and its
2832   handling can be constrained by law in certain countries. People using
2833   HTTP to provide data are responsible for ensuring that
2834   such material is not distributed without the permission of any
2835   individuals that are identifiable by the published results.
2836</t>
2837</section>
2838
2839<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2840<t>
2841   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2842   the documents returned by HTTP requests to be only those that were
2843   intended by the server administrators. If an HTTP server translates
2844   HTTP URIs directly into file system calls, the server &MUST; take
2845   special care not to serve files that were not intended to be
2846   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2847   other operating systems use ".." as a path component to indicate a
2848   directory level above the current one. On such a system, an HTTP
2849   server &MUST; disallow any such construct in the Request-URI if it
2850   would otherwise allow access to a resource outside those intended to
2851   be accessible via the HTTP server. Similarly, files intended for
2852   reference only internally to the server (such as access control
2853   files, configuration files, and script code) &MUST; be protected from
2854   inappropriate retrieval, since they might contain sensitive
2855   information. Experience has shown that minor bugs in such HTTP server
2856   implementations have turned into security risks.
2857</t>
2858</section>
2859
2860<section title="DNS Spoofing" anchor="dns.spoofing">
2861<t>
2862   Clients using HTTP rely heavily on the Domain Name Service, and are
2863   thus generally prone to security attacks based on the deliberate
2864   mis-association of IP addresses and DNS names. Clients need to be
2865   cautious in assuming the continuing validity of an IP number/DNS name
2866   association.
2867</t>
2868<t>
2869   In particular, HTTP clients &SHOULD; rely on their name resolver for
2870   confirmation of an IP number/DNS name association, rather than
2871   caching the result of previous host name lookups. Many platforms
2872   already can cache host name lookups locally when appropriate, and
2873   they &SHOULD; be configured to do so. It is proper for these lookups to
2874   be cached, however, only when the TTL (Time To Live) information
2875   reported by the name server makes it likely that the cached
2876   information will remain useful.
2877</t>
2878<t>
2879   If HTTP clients cache the results of host name lookups in order to
2880   achieve a performance improvement, they &MUST; observe the TTL
2881   information reported by DNS.
2882</t>
2883<t>
2884   If HTTP clients do not observe this rule, they could be spoofed when
2885   a previously-accessed server's IP address changes. As network
2886   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2887   possibility of this form of attack will grow. Observing this
2888   requirement thus reduces this potential security vulnerability.
2889</t>
2890<t>
2891   This requirement also improves the load-balancing behavior of clients
2892   for replicated servers using the same DNS name and reduces the
2893   likelihood of a user's experiencing failure in accessing sites which
2894   use that strategy.
2895</t>
2896</section>
2897
2898<section title="Proxies and Caching" anchor="attack.proxies">
2899<t>
2900   By their very nature, HTTP proxies are men-in-the-middle, and
2901   represent an opportunity for man-in-the-middle attacks. Compromise of
2902   the systems on which the proxies run can result in serious security
2903   and privacy problems. Proxies have access to security-related
2904   information, personal information about individual users and
2905   organizations, and proprietary information belonging to users and
2906   content providers. A compromised proxy, or a proxy implemented or
2907   configured without regard to security and privacy considerations,
2908   might be used in the commission of a wide range of potential attacks.
2909</t>
2910<t>
2911   Proxy operators should protect the systems on which proxies run as
2912   they would protect any system that contains or transports sensitive
2913   information. In particular, log information gathered at proxies often
2914   contains highly sensitive personal information, and/or information
2915   about organizations. Log information should be carefully guarded, and
2916   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2917</t>
2918<t>
2919   Proxy implementors should consider the privacy and security
2920   implications of their design and coding decisions, and of the
2921   configuration options they provide to proxy operators (especially the
2922   default configuration).
2923</t>
2924<t>
2925   Users of a proxy need to be aware that they are no trustworthier than
2926   the people who run the proxy; HTTP itself cannot solve this problem.
2927</t>
2928<t>
2929   The judicious use of cryptography, when appropriate, may suffice to
2930   protect against a broad range of security and privacy attacks. Such
2931   cryptography is beyond the scope of the HTTP/1.1 specification.
2932</t>
2933</section>
2934
2935<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2936<t>
2937   They exist. They are hard to defend against. Research continues.
2938   Beware.
2939</t>
2940</section>
2941</section>
2942
2943<section title="Acknowledgments" anchor="ack">
2944<t>
2945   This specification makes heavy use of the augmented BNF and generic
2946   constructs defined by David H. Crocker for <xref target="RFC822ABNF"/>. Similarly, it
2947   reuses many of the definitions provided by Nathaniel Borenstein and
2948   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2949   specification will help reduce past confusion over the relationship
2950   between HTTP and Internet mail message formats.
2951</t>
2952<t>
2953   HTTP has evolved considerably over the years. It has
2954   benefited from a large and active developer community--the many
2955   people who have participated on the www-talk mailing list--and it is
2956   that community which has been most responsible for the success of
2957   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2958   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2959   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2960   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2961   VanHeyningen deserve special recognition for their efforts in
2962   defining early aspects of the protocol.
2963</t>
2964<t>
2965   This document has benefited greatly from the comments of all those
2966   participating in the HTTP-WG. In addition to those already mentioned,
2967   the following individuals have contributed to this specification:
2968</t>
2969<t>
2970   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2971   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2972   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2973   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2974   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2975   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2976   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2977   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2978   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2979   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2980   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2981   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2982   Josh Cohen.
2983</t>
2984<t>
2985   Thanks to the "cave men" of Palo Alto. You know who you are.
2986</t>
2987<t>
2988   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2989   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2990   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2991   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2992   Larry Masinter for their help. And thanks go particularly to Jeff
2993   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2994</t>
2995<t>
2996   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
2997   Frystyk implemented RFC 2068 early, and we wish to thank them for the
2998   discovery of many of the problems that this document attempts to
2999   rectify.
3000</t>
3001</section>
3002
3003</middle>
3004<back>
3005
3006<references title="Normative References">
3007
3008<reference anchor="ISO-8859-1">
3009  <front>
3010    <title>
3011     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3012    </title>
3013    <author>
3014      <organization>International Organization for Standardization</organization>
3015    </author>
3016    <date year="1998"/>
3017  </front>
3018  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3019</reference>
3020
3021<reference anchor="Part2">
3022  <front>
3023    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3024    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3025      <organization abbrev="Day Software">Day Software</organization>
3026      <address><email>fielding@gbiv.com</email></address>
3027    </author>
3028    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3029      <organization>One Laptop per Child</organization>
3030      <address><email>jg@laptop.org</email></address>
3031    </author>
3032    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3033      <organization abbrev="HP">Hewlett-Packard Company</organization>
3034      <address><email>JeffMogul@acm.org</email></address>
3035    </author>
3036    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3037      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3038      <address><email>henrikn@microsoft.com</email></address>
3039    </author>
3040    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3041      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3042      <address><email>LMM@acm.org</email></address>
3043    </author>
3044    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3045      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3046      <address><email>paulle@microsoft.com</email></address>
3047    </author>
3048    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3049      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3050      <address><email>timbl@w3.org</email></address>
3051    </author>
3052    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3053      <organization abbrev="W3C">World Wide Web Consortium</organization>
3054      <address><email>ylafon@w3.org</email></address>
3055    </author>
3056    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3057      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3058      <address><email>julian.reschke@greenbytes.de</email></address>
3059    </author>
3060    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3061  </front>
3062  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3063  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3064</reference>
3065
3066<reference anchor="Part3">
3067  <front>
3068    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3069    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3070      <organization abbrev="Day Software">Day Software</organization>
3071      <address><email>fielding@gbiv.com</email></address>
3072    </author>
3073    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3074      <organization>One Laptop per Child</organization>
3075      <address><email>jg@laptop.org</email></address>
3076    </author>
3077    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3078      <organization abbrev="HP">Hewlett-Packard Company</organization>
3079      <address><email>JeffMogul@acm.org</email></address>
3080    </author>
3081    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3082      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3083      <address><email>henrikn@microsoft.com</email></address>
3084    </author>
3085    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3086      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3087      <address><email>LMM@acm.org</email></address>
3088    </author>
3089    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3090      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3091      <address><email>paulle@microsoft.com</email></address>
3092    </author>
3093    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3094      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3095      <address><email>timbl@w3.org</email></address>
3096    </author>
3097    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3098      <organization abbrev="W3C">World Wide Web Consortium</organization>
3099      <address><email>ylafon@w3.org</email></address>
3100    </author>
3101    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3102      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3103      <address><email>julian.reschke@greenbytes.de</email></address>
3104    </author>
3105    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3106  </front>
3107  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3108  <x:source href="p3-payload.xml" basename="p3-payload"/>
3109</reference>
3110
3111<reference anchor="Part5">
3112  <front>
3113    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3114    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3115      <organization abbrev="Day Software">Day Software</organization>
3116      <address><email>fielding@gbiv.com</email></address>
3117    </author>
3118    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3119      <organization>One Laptop per Child</organization>
3120      <address><email>jg@laptop.org</email></address>
3121    </author>
3122    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3123      <organization abbrev="HP">Hewlett-Packard Company</organization>
3124      <address><email>JeffMogul@acm.org</email></address>
3125    </author>
3126    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3127      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3128      <address><email>henrikn@microsoft.com</email></address>
3129    </author>
3130    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3131      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3132      <address><email>LMM@acm.org</email></address>
3133    </author>
3134    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3135      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3136      <address><email>paulle@microsoft.com</email></address>
3137    </author>
3138    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3139      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3140      <address><email>timbl@w3.org</email></address>
3141    </author>
3142    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3143      <organization abbrev="W3C">World Wide Web Consortium</organization>
3144      <address><email>ylafon@w3.org</email></address>
3145    </author>
3146    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3147      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3148      <address><email>julian.reschke@greenbytes.de</email></address>
3149    </author>
3150    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3151  </front>
3152  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3153  <x:source href="p5-range.xml" basename="p5-range"/>
3154</reference>
3155
3156<reference anchor="Part6">
3157  <front>
3158    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3159    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3160      <organization abbrev="Day Software">Day Software</organization>
3161      <address><email>fielding@gbiv.com</email></address>
3162    </author>
3163    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3164      <organization>One Laptop per Child</organization>
3165      <address><email>jg@laptop.org</email></address>
3166    </author>
3167    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3168      <organization abbrev="HP">Hewlett-Packard Company</organization>
3169      <address><email>JeffMogul@acm.org</email></address>
3170    </author>
3171    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3172      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3173      <address><email>henrikn@microsoft.com</email></address>
3174    </author>
3175    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3176      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3177      <address><email>LMM@acm.org</email></address>
3178    </author>
3179    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3180      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3181      <address><email>paulle@microsoft.com</email></address>
3182    </author>
3183    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3184      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3185      <address><email>timbl@w3.org</email></address>
3186    </author>
3187    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3188      <organization abbrev="W3C">World Wide Web Consortium</organization>
3189      <address><email>ylafon@w3.org</email></address>
3190    </author>
3191    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3192      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3193      <address><email>julian.reschke@greenbytes.de</email></address>
3194    </author>
3195    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3196  </front>
3197  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3198  <x:source href="p6-cache.xml" basename="p6-cache"/>
3199</reference>
3200
3201<reference anchor="RFC822ABNF">
3202  <front>
3203    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3204    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3205      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3206      <address><email>DCrocker@UDel-Relay</email></address>
3207    </author>
3208    <date month="August" day="13" year="1982"/>
3209  </front>
3210  <seriesInfo name="STD" value="11"/>
3211  <seriesInfo name="RFC" value="822"/>
3212</reference>
3213
3214<reference anchor="RFC2045">
3215  <front>
3216    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3217    <author initials="N." surname="Freed" fullname="Ned Freed">
3218      <organization>Innosoft International, Inc.</organization>
3219      <address><email>ned@innosoft.com</email></address>
3220    </author>
3221    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3222      <organization>First Virtual Holdings</organization>
3223      <address><email>nsb@nsb.fv.com</email></address>
3224    </author>
3225    <date month="November" year="1996"/>
3226  </front>
3227  <seriesInfo name="RFC" value="2045"/>
3228</reference>
3229
3230<reference anchor="RFC2047">
3231  <front>
3232    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3233    <author initials="K." surname="Moore" fullname="Keith Moore">
3234      <organization>University of Tennessee</organization>
3235      <address><email>moore@cs.utk.edu</email></address>
3236    </author>
3237    <date month="November" year="1996"/>
3238  </front>
3239  <seriesInfo name="RFC" value="2047"/>
3240</reference>
3241
3242<reference anchor="RFC2119">
3243  <front>
3244    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3245    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3246      <organization>Harvard University</organization>
3247      <address><email>sob@harvard.edu</email></address>
3248    </author>
3249    <date month="March" year="1997"/>
3250  </front>
3251  <seriesInfo name="BCP" value="14"/>
3252  <seriesInfo name="RFC" value="2119"/>
3253</reference>
3254
3255<reference anchor="RFC2396">
3256  <front>
3257    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3258    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3259      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3260      <address><email>timbl@w3.org</email></address>
3261    </author>
3262    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3263      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3264      <address><email>fielding@ics.uci.edu</email></address>
3265    </author>
3266    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3267      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3268      <address><email>masinter@parc.xerox.com</email></address>
3269    </author>
3270    <date month="August" year="1998"/>
3271  </front>
3272  <seriesInfo name="RFC" value="2396"/>
3273</reference>
3274
3275<reference anchor="USASCII">
3276  <front>
3277    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3278    <author>
3279      <organization>American National Standards Institute</organization>
3280    </author>
3281    <date year="1986"/>
3282  </front>
3283  <seriesInfo name="ANSI" value="X3.4"/>
3284</reference>
3285
3286</references>
3287
3288<references title="Informative References">
3289
3290<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3291  <front>
3292    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3293    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3294      <organization/>
3295    </author>
3296    <author initials="J." surname="Gettys" fullname="J. Gettys">
3297      <organization/>
3298    </author>
3299    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3300      <organization/>
3301    </author>
3302    <author initials="H." surname="Lie" fullname="H. Lie">
3303      <organization/>
3304    </author>
3305    <author initials="C." surname="Lilley" fullname="C. Lilley">
3306      <organization/>
3307    </author>
3308    <date year="1997" month="September"/>
3309  </front>
3310  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3311</reference>
3312
3313<reference anchor="Pad1995">
3314  <front>
3315    <title>Improving HTTP Latency</title>
3316    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3317      <organization/>
3318    </author>
3319    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3320      <organization/>
3321    </author>
3322    <date year="1995" month="December"/>
3323  </front>
3324  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3325  <annotation>
3326    Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3327    which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3328  </annotation>
3329</reference>
3330
3331<reference anchor="RFC822">
3332  <front>
3333    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3334    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3335      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3336      <address><email>DCrocker@UDel-Relay</email></address>
3337    </author>
3338    <date month="August" day="13" year="1982"/>
3339  </front>
3340  <seriesInfo name="STD" value="11"/>
3341  <seriesInfo name="RFC" value="822"/>
3342</reference>
3343
3344<reference anchor="RFC959">
3345  <front>
3346    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3347    <author initials="J." surname="Postel" fullname="J. Postel">
3348      <organization>Information Sciences Institute (ISI)</organization>
3349    </author>
3350    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3351      <organization/>
3352    </author>
3353    <date month="October" year="1985"/>
3354  </front>
3355  <seriesInfo name="STD" value="9"/>
3356  <seriesInfo name="RFC" value="959"/>
3357</reference>
3358
3359<reference anchor="RFC1123">
3360  <front>
3361    <title>Requirements for Internet Hosts - Application and Support</title>
3362    <author initials="R." surname="Braden" fullname="Robert Braden">
3363      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3364      <address><email>Braden@ISI.EDU</email></address>
3365    </author>
3366    <date month="October" year="1989"/>
3367  </front>
3368  <seriesInfo name="STD" value="3"/>
3369  <seriesInfo name="RFC" value="1123"/>
3370</reference>
3371
3372<reference anchor="RFC1305">
3373  <front>
3374    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3375    <author initials="D." surname="Mills" fullname="David L. Mills">
3376      <organization>University of Delaware, Electrical Engineering Department</organization>
3377      <address><email>mills@udel.edu</email></address>
3378    </author>
3379    <date month="March" year="1992"/>
3380  </front>
3381  <seriesInfo name="RFC" value="1305"/>
3382</reference>
3383
3384<reference anchor="RFC1436">
3385  <front>
3386    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3387    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3388      <organization>University of Minnesota, Computer and Information Services</organization>
3389      <address><email>fxa@boombox.micro.umn.edu</email></address>
3390    </author>
3391    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3392      <organization>University of Minnesota, Computer and Information Services</organization>
3393      <address><email>mpm@boombox.micro.umn.edu</email></address>
3394    </author>
3395    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3396      <organization>University of Minnesota, Computer and Information Services</organization>
3397      <address><email>lindner@boombox.micro.umn.edu</email></address>
3398    </author>
3399    <author initials="D." surname="Johnson" fullname="David Johnson">
3400      <organization>University of Minnesota, Computer and Information Services</organization>
3401      <address><email>dmj@boombox.micro.umn.edu</email></address>
3402    </author>
3403    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3404      <organization>University of Minnesota, Computer and Information Services</organization>
3405      <address><email>daniel@boombox.micro.umn.edu</email></address>
3406    </author>
3407    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3408      <organization>University of Minnesota, Computer and Information Services</organization>
3409      <address><email>alberti@boombox.micro.umn.edu</email></address>
3410    </author>
3411    <date month="March" year="1993"/>
3412  </front>
3413  <seriesInfo name="RFC" value="1436"/>
3414</reference>
3415
3416<reference anchor="RFC1630">
3417  <front>
3418    <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>
3419    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3420      <organization>CERN, World-Wide Web project</organization>
3421      <address><email>timbl@info.cern.ch</email></address>
3422    </author>
3423    <date month="June" year="1994"/>
3424  </front>
3425  <seriesInfo name="RFC" value="1630"/>
3426</reference>
3427
3428<reference anchor="RFC1737">
3429  <front>
3430    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3431    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3432      <organization>Xerox Palo Alto Research Center</organization>
3433      <address><email>masinter@parc.xerox.com</email></address>
3434    </author>
3435    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3436      <organization>MIT Laboratory for Computer Science</organization>
3437      <address><email>sollins@lcs.mit.edu</email></address>
3438    </author>
3439    <date month="December" year="1994"/>
3440  </front>
3441  <seriesInfo name="RFC" value="1737"/>
3442</reference>
3443
3444<reference anchor="RFC1738">
3445  <front>
3446    <title>Uniform Resource Locators (URL)</title>
3447    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3448      <organization>CERN, World-Wide Web project</organization>
3449      <address><email>timbl@info.cern.ch</email></address>
3450    </author>
3451    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3452      <organization>Xerox PARC</organization>
3453      <address><email>masinter@parc.xerox.com</email></address>
3454    </author>
3455    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3456      <organization>University of Minnesota, Computer and Information Services</organization>
3457      <address><email>mpm@boombox.micro.umn.edu</email></address>
3458    </author>
3459    <date month="December" year="1994"/>
3460  </front>
3461  <seriesInfo name="RFC" value="1738"/>
3462</reference>
3463
3464<reference anchor="RFC1808">
3465  <front>
3466    <title>Relative Uniform Resource Locators</title>
3467    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3468      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3469      <address><email>fielding@ics.uci.edu</email></address>
3470    </author>
3471    <date month="June" year="1995"/>
3472  </front>
3473  <seriesInfo name="RFC" value="1808"/>
3474</reference>
3475
3476<reference anchor="RFC1900">
3477  <front>
3478    <title>Renumbering Needs Work</title>
3479    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3480      <organization>CERN, Computing and Networks Division</organization>
3481      <address><email>brian@dxcoms.cern.ch</email></address>
3482    </author>
3483    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3484      <organization>cisco Systems</organization>
3485      <address><email>yakov@cisco.com</email></address>
3486    </author>
3487    <date month="February" year="1996"/>
3488  </front>
3489  <seriesInfo name="RFC" value="1900"/>
3490</reference>
3491
3492<reference anchor="RFC1945">
3493  <front>
3494    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3495    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3496      <organization>MIT, Laboratory for Computer Science</organization>
3497      <address><email>timbl@w3.org</email></address>
3498    </author>
3499    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3500      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3501      <address><email>fielding@ics.uci.edu</email></address>
3502    </author>
3503    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3504      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3505      <address><email>frystyk@w3.org</email></address>
3506    </author>
3507    <date month="May" year="1996"/>
3508  </front>
3509  <seriesInfo name="RFC" value="1945"/>
3510</reference>
3511
3512<reference anchor="RFC2068">
3513  <front>
3514    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3515    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3516      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3517      <address><email>fielding@ics.uci.edu</email></address>
3518    </author>
3519    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3520      <organization>MIT Laboratory for Computer Science</organization>
3521      <address><email>jg@w3.org</email></address>
3522    </author>
3523    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3524      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3525      <address><email>mogul@wrl.dec.com</email></address>
3526    </author>
3527    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3528      <organization>MIT Laboratory for Computer Science</organization>
3529      <address><email>frystyk@w3.org</email></address>
3530    </author>
3531    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3532      <organization>MIT Laboratory for Computer Science</organization>
3533      <address><email>timbl@w3.org</email></address>
3534    </author>
3535    <date month="January" year="1997"/>
3536  </front>
3537  <seriesInfo name="RFC" value="2068"/>
3538</reference>
3539
3540<reference anchor="RFC2145">
3541  <front>
3542    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3543    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3544      <organization>Western Research Laboratory</organization>
3545      <address><email>mogul@wrl.dec.com</email></address>
3546    </author>
3547    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3548      <organization>Department of Information and Computer Science</organization>
3549      <address><email>fielding@ics.uci.edu</email></address>
3550    </author>
3551    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3552      <organization>MIT Laboratory for Computer Science</organization>
3553      <address><email>jg@w3.org</email></address>
3554    </author>
3555    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3556      <organization>W3 Consortium</organization>
3557      <address><email>frystyk@w3.org</email></address>
3558    </author>
3559    <date month="May" year="1997"/>
3560  </front>
3561  <seriesInfo name="RFC" value="2145"/>
3562</reference>
3563
3564<reference anchor="RFC2324">
3565  <front>
3566    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3567    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3568      <organization>Xerox Palo Alto Research Center</organization>
3569      <address><email>masinter@parc.xerox.com</email></address>
3570    </author>
3571    <date month="April" day="1" year="1998"/>
3572  </front>
3573  <seriesInfo name="RFC" value="2324"/>
3574</reference>
3575
3576<reference anchor="RFC2616">
3577  <front>
3578    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3579    <author initials="R." surname="Fielding" fullname="R. Fielding">
3580      <organization>University of California, Irvine</organization>
3581      <address><email>fielding@ics.uci.edu</email></address>
3582    </author>
3583    <author initials="J." surname="Gettys" fullname="J. Gettys">
3584      <organization>W3C</organization>
3585      <address><email>jg@w3.org</email></address>
3586    </author>
3587    <author initials="J." surname="Mogul" fullname="J. Mogul">
3588      <organization>Compaq Computer Corporation</organization>
3589      <address><email>mogul@wrl.dec.com</email></address>
3590    </author>
3591    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3592      <organization>MIT Laboratory for Computer Science</organization>
3593      <address><email>frystyk@w3.org</email></address>
3594    </author>
3595    <author initials="L." surname="Masinter" fullname="L. Masinter">
3596      <organization>Xerox Corporation</organization>
3597      <address><email>masinter@parc.xerox.com</email></address>
3598    </author>
3599    <author initials="P." surname="Leach" fullname="P. Leach">
3600      <organization>Microsoft Corporation</organization>
3601      <address><email>paulle@microsoft.com</email></address>
3602    </author>
3603    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3604      <organization>W3C</organization>
3605      <address><email>timbl@w3.org</email></address>
3606    </author>
3607    <date month="June" year="1999"/>
3608  </front>
3609  <seriesInfo name="RFC" value="2616"/>
3610</reference>
3611
3612<reference anchor="RFC2821">
3613  <front>
3614    <title>Simple Mail Transfer Protocol</title>
3615    <author initials="J." surname="Klensin" fullname="J. Klensin">
3616      <organization>AT&amp;T Laboratories</organization>
3617      <address><email>klensin@research.att.com</email></address>
3618    </author>
3619    <date year="2001" month="April"/>
3620  </front>
3621  <seriesInfo name="RFC" value="2821"/>
3622</reference>
3623
3624<reference anchor="RFC2822">
3625  <front>
3626    <title>Internet Message Format</title>
3627    <author initials="P." surname="Resnick" fullname="P. Resnick">
3628      <organization>QUALCOMM Incorporated</organization>
3629    </author>
3630    <date year="2001" month="April"/>
3631  </front> 
3632  <seriesInfo name="RFC" value="2822"/>
3633</reference>
3634
3635<reference anchor='RFC3977'>
3636  <front>
3637    <title>Network News Transfer Protocol (NNTP)</title>
3638    <author initials='C.' surname='Feather' fullname='C. Feather'>
3639      <organization>THUS plc</organization>
3640      <address><email>clive@demon.net</email></address>
3641    </author>
3642    <date year='2006' month='October' />
3643  </front>
3644  <seriesInfo name="RFC" value="3977"/>
3645</reference>
3646
3647<reference anchor="RFC4288">
3648  <front>
3649    <title>Media Type Specifications and Registration Procedures</title>
3650    <author initials="N." surname="Freed" fullname="N. Freed">
3651      <organization>Sun Microsystems</organization>
3652      <address>
3653        <email>ned.freed@mrochek.com</email>
3654      </address>
3655    </author>
3656    <author initials="J." surname="Klensin" fullname="J. Klensin">
3657      <organization/>
3658      <address>
3659        <email>klensin+ietf@jck.com</email>
3660      </address>
3661    </author>
3662    <date year="2005" month="December"/>
3663  </front>
3664  <seriesInfo name="BCP" value="13"/>
3665  <seriesInfo name="RFC" value="4288"/>
3666</reference>
3667
3668<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3669  <front>
3670  <title>Analysis of HTTP Performance Problems</title>
3671  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3672    <organization/>
3673  </author>
3674  <date/>
3675  </front>
3676</reference>
3677
3678<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3679  <front>
3680  <title>Analysis of HTTP Performance</title>
3681  <author initials="J." surname="Touch" fullname="Joe Touch">
3682    <organization>USC/Information Sciences Institute</organization>
3683    <address><email>touch@isi.edu</email></address>
3684  </author>
3685  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3686    <organization>USC/Information Sciences Institute</organization>
3687    <address><email>johnh@isi.edu</email></address>
3688  </author>
3689  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3690    <organization>USC/Information Sciences Institute</organization>
3691    <address><email>katia@isi.edu</email></address>
3692  </author>
3693  <date year="1998" month="Aug"/>
3694  </front>
3695  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3696  <annotation>(original report dated Aug. 1996)</annotation>
3697</reference>
3698
3699<reference anchor="WAIS">
3700  <front>
3701    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3702    <author initials="F." surname="Davis" fullname="F. Davis">
3703      <organization>Thinking Machines Corporation</organization>
3704    </author>
3705    <author initials="B." surname="Kahle" fullname="B. Kahle">
3706      <organization>Thinking Machines Corporation</organization>
3707    </author>
3708    <author initials="H." surname="Morris" fullname="H. Morris">
3709      <organization>Thinking Machines Corporation</organization>
3710    </author>
3711    <author initials="J." surname="Salem" fullname="J. Salem">
3712      <organization>Thinking Machines Corporation</organization>
3713    </author>
3714    <author initials="T." surname="Shen" fullname="T. Shen">
3715      <organization>Thinking Machines Corporation</organization>
3716    </author>
3717    <author initials="R." surname="Wang" fullname="R. Wang">
3718      <organization>Thinking Machines Corporation</organization>
3719    </author>
3720    <author initials="J." surname="Sui" fullname="J. Sui">
3721      <organization>Thinking Machines Corporation</organization>
3722    </author>
3723    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3724      <organization>Thinking Machines Corporation</organization>
3725    </author>
3726    <date month="April" year="1990"/>
3727  </front>
3728  <seriesInfo name="Thinking Machines Corporation" value=""/>
3729</reference>
3730
3731</references>
3732
3733
3734<section title="Internet Media Types" anchor="internet.media.type.http">
3735<t>
3736   In addition to defining HTTP/1.1, this document serves
3737   as the specification for the Internet media type "message/http" and
3738   "application/http". The following is to be registered with IANA <xref target="RFC4288"/>.
3739</t>
3740<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3741<iref item="Media Type" subitem="message/http" primary="true"/>
3742<iref item="message/http Media Type" primary="true"/>
3743<t>
3744   The message/http type can be used to enclose a single HTTP request or
3745   response message, provided that it obeys the MIME restrictions for all
3746   "message" types regarding line length and encodings.
3747</t>
3748<t>
3749  <list style="hanging" x:indent="12em">
3750    <t hangText="Type name:">
3751      message
3752    </t>
3753    <t hangText="Subtype name:">
3754      http
3755    </t>
3756    <t hangText="Required parameters:">
3757      none
3758    </t>
3759    <t hangText="Optional parameters:">
3760      version, msgtype
3761      <list style="hanging">
3762        <t hangText="version:">
3763          The HTTP-Version number of the enclosed message
3764          (e.g., "1.1"). If not present, the version can be
3765          determined from the first line of the body.
3766        </t>
3767        <t hangText="msgtype:">
3768          The message type -- "request" or "response". If not
3769          present, the type can be determined from the first
3770          line of the body.
3771        </t>
3772      </list>
3773    </t>
3774    <t hangText="Encoding considerations:">
3775      only "7bit", "8bit", or "binary" are permitted
3776    </t>
3777    <t hangText="Security considerations:">
3778      none
3779    </t>
3780    <t hangText="Interoperability considerations:">
3781      none
3782    </t>
3783    <t hangText="Published specification:">
3784      This specification (see <xref target="internet.media.type.message.http"/>).
3785    </t>
3786    <t hangText="Applications that use this media type:">
3787    </t>
3788    <t hangText="Additional information:">
3789      <list style="hanging">
3790        <t hangText="Magic number(s):">none</t>
3791        <t hangText="File extension(s):">none</t>
3792        <t hangText="Macintosh file type code(s):">none</t>
3793      </list>
3794    </t>
3795    <t hangText="Person and email address to contact for further information:">
3796      See Authors Section.
3797    </t>
3798                <t hangText="Intended usage:">
3799                  COMMON
3800    </t>
3801                <t hangText="Restrictions on usage:">
3802                  none
3803    </t>
3804    <t hangText="Author/Change controller:">
3805      IESG
3806    </t>
3807  </list>
3808</t>
3809</section>
3810<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3811<iref item="Media Type" subitem="application/http" primary="true"/>
3812<iref item="application/http Media Type" primary="true"/>
3813<t>
3814   The application/http type can be used to enclose a pipeline of one or more
3815   HTTP request or response messages (not intermixed).
3816</t>
3817<t>
3818  <list style="hanging" x:indent="12em">
3819    <t hangText="Type name:">
3820      application
3821    </t>
3822    <t hangText="Subtype name:">
3823      http
3824    </t>
3825    <t hangText="Required parameters:">
3826      none
3827    </t>
3828    <t hangText="Optional parameters:">
3829      version, msgtype
3830      <list style="hanging">
3831        <t hangText="version:">
3832          The HTTP-Version number of the enclosed messages
3833          (e.g., "1.1"). If not present, the version can be
3834          determined from the first line of the body.
3835        </t>
3836        <t hangText="msgtype:">
3837          The message type -- "request" or "response". If not
3838          present, the type can be determined from the first
3839          line of the body.
3840        </t>
3841      </list>
3842    </t>
3843    <t hangText="Encoding considerations:">
3844      HTTP messages enclosed by this type
3845      are in "binary" format; use of an appropriate
3846      Content-Transfer-Encoding is required when
3847      transmitted via E-mail.
3848    </t>
3849    <t hangText="Security considerations:">
3850      none
3851    </t>
3852    <t hangText="Interoperability considerations:">
3853      none
3854    </t>
3855    <t hangText="Published specification:">
3856      This specification (see <xref target="internet.media.type.application.http"/>).
3857    </t>
3858    <t hangText="Applications that use this media type:">
3859    </t>
3860    <t hangText="Additional information:">
3861      <list style="hanging">
3862        <t hangText="Magic number(s):">none</t>
3863        <t hangText="File extension(s):">none</t>
3864        <t hangText="Macintosh file type code(s):">none</t>
3865      </list>
3866    </t>
3867    <t hangText="Person and email address to contact for further information:">
3868      See Authors Section.
3869    </t>
3870                <t hangText="Intended usage:">
3871                  COMMON
3872    </t>
3873                <t hangText="Restrictions on usage:">
3874                  none
3875    </t>
3876    <t hangText="Author/Change controller:">
3877      IESG
3878    </t>
3879  </list>
3880</t>
3881</section>
3882</section>
3883
3884<section title="Tolerant Applications" anchor="tolerant.applications">
3885<t>
3886   Although this document specifies the requirements for the generation
3887   of HTTP/1.1 messages, not all applications will be correct in their
3888   implementation. We therefore recommend that operational applications
3889   be tolerant of deviations whenever those deviations can be
3890   interpreted unambiguously.
3891</t>
3892<t>
3893   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3894   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3895   accept any amount of SP or HTAB characters between fields, even though
3896   only a single SP is required.
3897</t>
3898<t>
3899   The line terminator for message-header fields is the sequence CRLF.
3900   However, we recommend that applications, when parsing such headers,
3901   recognize a single LF as a line terminator and ignore the leading CR.
3902</t>
3903<t>
3904   The character set of an entity-body &SHOULD; be labeled as the lowest
3905   common denominator of the character codes used within that body, with
3906   the exception that not labeling the entity is preferred over labeling
3907   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3908</t>
3909<t>
3910   Additional rules for requirements on parsing and encoding of dates
3911   and other potential problems with date encodings include:
3912</t>
3913<t>
3914  <list style="symbols">
3915     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3916        which appears to be more than 50 years in the future is in fact
3917        in the past (this helps solve the "year 2000" problem).</t>
3918
3919     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3920        Expires date as earlier than the proper value, but &MUST-NOT;
3921        internally represent a parsed Expires date as later than the
3922        proper value.</t>
3923
3924     <t>All expiration-related calculations &MUST; be done in GMT. The
3925        local time zone &MUST-NOT; influence the calculation or comparison
3926        of an age or expiration time.</t>
3927
3928     <t>If an HTTP header incorrectly carries a date value with a time
3929        zone other than GMT, it &MUST; be converted into GMT using the
3930        most conservative possible conversion.</t>
3931  </list>
3932</t>
3933</section>
3934
3935<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3936<t>
3937   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3938   simplify the process of date comparison. Proxies and gateways from
3939   other protocols &SHOULD; ensure that any Date header field present in a
3940   message conforms to one of the HTTP/1.1 formats and rewrite the date
3941   if necessary.
3942</t>
3943</section>
3944
3945<section title="Compatibility with Previous Versions" anchor="compatibility">
3946<t>
3947   It is beyond the scope of a protocol specification to mandate
3948   compliance with previous versions. HTTP/1.1 was deliberately
3949   designed, however, to make supporting previous versions easy. It is
3950   worth noting that, at the time of composing this specification
3951   (1996), we would expect commercial HTTP/1.1 servers to:
3952  <list style="symbols">
3953     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3954        1.1 requests;</t>
3955
3956     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3957        1.1;</t>
3958
3959     <t>respond appropriately with a message in the same major version
3960        used by the client.</t>
3961  </list>
3962</t>
3963<t>
3964   And we would expect HTTP/1.1 clients to:
3965  <list style="symbols">
3966     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3967        responses;</t>
3968
3969     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3970        1.1.</t>
3971  </list>
3972</t>
3973<t>
3974   For most implementations of HTTP/1.0, each connection is established
3975   by the client prior to the request and closed by the server after
3976   sending the response. Some implementations implement the Keep-Alive
3977   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3978</t>
3979
3980<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3981<t>
3982   This section summarizes major differences between versions HTTP/1.0
3983   and HTTP/1.1.
3984</t>
3985
3986<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">
3987<t>
3988   The requirements that clients and servers support the Host request-header,
3989   report an error if the Host request-header (<xref target="header.host"/>) is
3990   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3991   are among the most important changes defined by this
3992   specification.
3993</t>
3994<t>
3995   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
3996   addresses and servers; there was no other established mechanism for
3997   distinguishing the intended server of a request than the IP address
3998   to which that request was directed. The changes outlined above will
3999   allow the Internet, once older HTTP clients are no longer common, to
4000   support multiple Web sites from a single IP address, greatly
4001   simplifying large operational Web servers, where allocation of many
4002   IP addresses to a single host has created serious problems. The
4003   Internet will also be able to recover the IP addresses that have been
4004   allocated for the sole purpose of allowing special-purpose domain
4005   names to be used in root-level HTTP URLs. Given the rate of growth of
4006   the Web, and the number of servers already deployed, it is extremely
4007   important that all implementations of HTTP (including updates to
4008   existing HTTP/1.0 applications) correctly implement these
4009   requirements:
4010  <list style="symbols">
4011     <t>Both clients and servers &MUST; support the Host request-header.</t>
4012
4013     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4014
4015     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4016        request does not include a Host request-header.</t>
4017
4018     <t>Servers &MUST; accept absolute URIs.</t>
4019  </list>
4020</t>
4021</section>
4022</section>
4023
4024<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4025<t>
4026   Some clients and servers might wish to be compatible with some
4027   previous implementations of persistent connections in HTTP/1.0
4028   clients and servers. Persistent connections in HTTP/1.0 are
4029   explicitly negotiated as they are not the default behavior. HTTP/1.0
4030   experimental implementations of persistent connections are faulty,
4031   and the new facilities in HTTP/1.1 are designed to rectify these
4032   problems. The problem was that some existing 1.0 clients may be
4033   sending Keep-Alive to a proxy server that doesn't understand
4034   Connection, which would then erroneously forward it to the next
4035   inbound server, which would establish the Keep-Alive connection and
4036   result in a hung HTTP/1.0 proxy waiting for the close on the
4037   response. The result is that HTTP/1.0 clients must be prevented from
4038   using Keep-Alive when talking to proxies.
4039</t>
4040<t>
4041   However, talking to proxies is the most important use of persistent
4042   connections, so that prohibition is clearly unacceptable. Therefore,
4043   we need some other mechanism for indicating a persistent connection
4044   is desired, which is safe to use even when talking to an old proxy
4045   that ignores Connection. Persistent connections are the default for
4046   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4047   declaring non-persistence. See <xref target="header.connection"/>.
4048</t>
4049<t>
4050   The original HTTP/1.0 form of persistent connections (the Connection:
4051   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4052</t>
4053</section>
4054
4055<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4056<t>
4057   This specification has been carefully audited to correct and
4058   disambiguate key word usage; RFC 2068 had many problems in respect to
4059   the conventions laid out in <xref target="RFC2119"/>.
4060</t>
4061<t>
4062   Transfer-coding and message lengths all interact in ways that
4063   required fixing exactly when chunked encoding is used (to allow for
4064   transfer encoding that may not be self delimiting); it was important
4065   to straighten out exactly how message lengths are computed. (Sections
4066   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4067   <xref target="header.content-length" format="counter"/>,
4068   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4069</t>
4070<t>
4071   The use and interpretation of HTTP version numbers has been clarified
4072   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4073   version they support to deal with problems discovered in HTTP/1.0
4074   implementations (<xref target="http.version"/>)
4075</t>
4076<t>
4077   Transfer-coding had significant problems, particularly with
4078   interactions with chunked encoding. The solution is that transfer-codings
4079   become as full fledged as content-codings. This involves
4080   adding an IANA registry for transfer-codings (separate from content
4081   codings), a new header field (TE) and enabling trailer headers in the
4082   future. Transfer encoding is a major performance benefit, so it was
4083   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4084   interoperability problem that could have occurred due to interactions
4085   between authentication trailers, chunked encoding and HTTP/1.0
4086   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4087   and <xref target="header.te" format="counter"/>)
4088</t>
4089</section>
4090
4091<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4092<t>
4093  The CHAR rule does not allow the NUL character anymore (this affects
4094  the comment and quoted-string rules).
4095  (<xref target="basic.rules"/>)
4096</t>
4097<t>
4098  Clarify that HTTP-Version is case sensitive.
4099  (<xref target="http.version"/>)
4100</t>
4101<t>
4102  Remove reference to non-existant identity transfer-coding value tokens.
4103  (Sections <xref format="counter" target="transfer.codings"/> and
4104  <xref format="counter" target="message.length"/>)
4105</t>
4106<t>
4107  Clarification that the chunk length does not include
4108  the count of the octets in the chunk header and trailer.
4109  (<xref target="chunked.transfer.encoding"/>)
4110</t>
4111<t>
4112  Fix BNF to add query, as the abs_path production in
4113  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4114  (<xref target="request-uri"/>)
4115</t>
4116<t>
4117  Clarify exactly when close connection options must be sent.
4118  (<xref target="header.connection"/>)
4119</t>
4120</section>
4121</section>
4122
4123<section title="Change Log (to be removed by RFC Editor before publication)">
4124
4125<section title="Since RFC2616">
4126<t>
4127  Extracted relevant partitions from <xref target="RFC2616"/>.
4128</t>
4129</section>
4130
4131<section title="Since draft-ietf-httpbis-p1-messaging-00">
4132<t>
4133  Closed issues:
4134  <list style="symbols"> 
4135    <t>
4136      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4137      "HTTP Version should be case sensitive"
4138      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4139    </t>
4140    <t>
4141      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4142      "'unsafe' characters"
4143      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4144    </t>
4145    <t>
4146      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4147      "Chunk Size Definition"
4148      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4149    </t>
4150    <t>
4151      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4152      "Message Length"
4153      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4154    </t>
4155    <t>
4156      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4157      "Media Type Registrations"
4158      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4159    </t>
4160    <t>
4161      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4162      "URI includes query"
4163      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4164    </t>
4165    <t>
4166      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4167      "No close on 1xx responses"
4168      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4169    </t>
4170    <t>
4171      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4172      "Remove 'identity' token references"
4173      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4174    </t>
4175    <t>
4176      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4177      "Import query BNF"
4178    </t>
4179    <t>
4180      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4181      "qdtext BNF"
4182    </t>
4183    <t>
4184      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4185      "Normative and Informative references"
4186    </t>
4187    <t>
4188      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4189      "RFC2606 Compliance"
4190    </t>
4191    <t>
4192      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4193      "RFC977 reference"
4194    </t>
4195    <t>
4196      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4197      "RFC1700 references"
4198    </t>
4199    <t>
4200      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4201      "inconsistency in date format explanation"
4202    </t>
4203    <t>
4204      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4205      "Date reference typo"
4206    </t>
4207    <t>
4208      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4209      "Informative references"
4210    </t>
4211    <t>
4212      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4213      "ISO-8859-1 Reference"
4214    </t>
4215    <t>
4216      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4217      "Normative up-to-date references"
4218    </t>
4219  </list>
4220</t>
4221<t>
4222  Other changes:
4223  <list style="symbols"> 
4224    <t>
4225      Update media type registrations to use RFC4288 template.
4226    </t>
4227    <t>
4228      Use names of RFC4234 core rules DQUOTE and HTAB,
4229      fix broken ABNF for chunk-data
4230      (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4231    </t>
4232  </list>
4233</t>
4234</section>
4235
4236<section title="Since draft-ietf-httpbis-p1-messaging-01">
4237<t>
4238  Closed issues:
4239  <list style="symbols"> 
4240    <t>
4241      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4242      "Bodies on GET (and other) requests"
4243    </t>
4244    <t>
4245      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4246      "Updating to RFC4288"
4247    </t>
4248    <t>
4249      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4250      "Status Code and Reason Phrase"
4251    </t>
4252    <t>
4253      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4254      "rel_path not used"
4255    </t>
4256  </list>
4257</t>
4258<t>
4259  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4260  <list style="symbols"> 
4261    <t>
4262      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4263      "trailer-part").
4264    </t>
4265    <t>
4266      Avoid underscore character in rule names ("http_URL" ->
4267      "http-URL", "abs_path" -> "path-absolute").
4268    </t>
4269    <t>
4270      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4271      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4272      have to be updated when switching over to RFC3986.
4273    </t>
4274    <t>
4275      Synchronize core rules with RFC5234 (this includes a change to CHAR
4276      which now excludes NUL).
4277    </t>
4278    <t>
4279      Get rid of prose rules that span multiple lines.
4280    </t>
4281    <t>
4282      Get rid of unused rules LOALPHA and UPALPHA.
4283    </t>
4284    <t>
4285      Move "Product Tokens" section (back) into Part 1, as "token" is used
4286      in the definition of the Upgrade header.
4287    </t>
4288    <t>
4289      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4290    </t>
4291  </list>
4292</t>
4293</section>
4294
4295</section>
4296
4297</back>
4298</rfc>
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