source: draft-ietf-httpbis/02/p1-messaging.xml @ 377

Last change on this file since 377 was 219, checked in by julian.reschke@…, 12 years ago

Prepare for release of draft 02 on Monday, Feb 24.

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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 "02">
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;" day="24"/>
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           = %x20-7E | %x80-FF | LWS
890                 ; any OCTET except CTLs, but including LWS
891</artwork></figure>
892<t>
893   A CRLF is allowed in the definition of TEXT only as part of a header
894   field continuation. It is expected that the folding LWS will be
895   replaced with a single SP before interpretation of the TEXT value.
896</t>
897<t>
898   Hexadecimal numeric characters are used in several protocol elements.
899</t>
900<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
901  HEX            = "A" | "B" | "C" | "D" | "E" | "F"
902                 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
903</artwork></figure>
904<t>
905   Many HTTP/1.1 header field values consist of words separated by LWS
906   or special characters. These special characters &MUST; be in a quoted
907   string to be used within a parameter value (as defined in
908   <xref target="transfer.codings"/>).
909</t>
910<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="separators"/>
911  separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
912                 | "," | ";" | ":" | "\" | DQUOTE
913                 | "/" | "[" | "]" | "?" | "="
914                 | "{" | "}" | SP | HTAB
915                 
916  tchar          = "!" | "#" | "$" | "%" | "&amp;" | "'" | "*"
917                 | "+" | "-" | "." | "^" | "_" | "`" | "|" | "~"
918                 | DIGIT | ALPHA
919                 ; any CHAR except CTLs or separators
920                 
921  token          = 1*tchar
922</artwork></figure>
923<t>
924   Comments can be included in some HTTP header fields by surrounding
925   the comment text with parentheses. Comments are only allowed in
926   fields containing "comment" as part of their field value definition.
927   In all other fields, parentheses are considered part of the field
928   value.
929</t>
930<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
931  comment        = "(" *( ctext | quoted-pair | comment ) ")"
932  ctext          = &lt;any TEXT excluding "(" and ")"&gt;
933</artwork></figure>
934<t>
935   A string of text is parsed as a single word if it is quoted using
936   double-quote marks.
937</t>
938<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
939  quoted-string  = ( DQUOTE *(qdtext | quoted-pair ) DQUOTE )
940  qdtext         = &lt;any TEXT excluding DQUOTE and "\">
941</artwork></figure>
942<t>
943   The backslash character ("\") &MAY; be used as a single-character
944   quoting mechanism only within quoted-string and comment constructs.
945</t>
946<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
947  quoted-pair    = "\" CHAR
948</artwork></figure>
949</section>
950
951<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
952<t>
953  The ABNF rules below are defined in other parts:
954</t>
955<figure><!-- Part2--><artwork type="abnf2616">
956  request-header =  &lt;request-header, defined in &request-header-fields;&gt;
957  response-header = &lt;response-header, defined in &response-header-fields;&gt;
958</artwork></figure>
959<figure><!-- Part3--><artwork type="abnf2616">
960  accept-params   = &lt;accept-params, defined in &header-accept;&gt;
961  entity-body     = &lt;entity-body, defined in &entity-body;&gt;
962  entity-header   = &lt;entity-header, defined in &entity-header-fields;&gt;
963</artwork></figure>
964<figure><!-- Part6--><artwork type="abnf2616">
965  Cache-Control   = &lt;Cache-Control, defined in &header-pragma;&gt;
966  Pragma          = &lt;Pragma, defined in &header-pragma;&gt;
967  Warning         = &lt;Warning, defined in &header-warning;&gt;
968</artwork></figure>
969</section>
970
971</section>
972
973<section title="Protocol Parameters" anchor="protocol.parameters">
974
975<section title="HTTP Version" anchor="http.version">
976<t>
977   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
978   of the protocol. The protocol versioning policy is intended to allow
979   the sender to indicate the format of a message and its capacity for
980   understanding further HTTP communication, rather than the features
981   obtained via that communication. No change is made to the version
982   number for the addition of message components which do not affect
983   communication behavior or which only add to extensible field values.
984   The &lt;minor&gt; number is incremented when the changes made to the
985   protocol add features which do not change the general message parsing
986   algorithm, but which may add to the message semantics and imply
987   additional capabilities of the sender. The &lt;major&gt; number is
988   incremented when the format of a message within the protocol is
989   changed. See <xref target="RFC2145"/> for a fuller explanation.
990</t>
991<t>
992   The version of an HTTP message is indicated by an HTTP-Version field
993   in the first line of the message. HTTP-Version is case-sensitive.
994</t>
995<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
996  HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
997</artwork></figure>
998<t>
999   Note that the major and minor numbers &MUST; be treated as separate
1000   integers and that each &MAY; be incremented higher than a single digit.
1001   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
1002   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
1003   &MUST-NOT; be sent.
1004</t>
1005<t>
1006   An application that sends a request or response message that includes
1007   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
1008   with this specification. Applications that are at least conditionally
1009   compliant with this specification &SHOULD; use an HTTP-Version of
1010   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
1011   not compatible with HTTP/1.0. For more details on when to send
1012   specific HTTP-Version values, see <xref target="RFC2145"/>.
1013</t>
1014<t>
1015   The HTTP version of an application is the highest HTTP version for
1016   which the application is at least conditionally compliant.
1017</t>
1018<t>
1019   Proxy and gateway applications need to be careful when forwarding
1020   messages in protocol versions different from that of the application.
1021   Since the protocol version indicates the protocol capability of the
1022   sender, a proxy/gateway &MUST-NOT; send a message with a version
1023   indicator which is greater than its actual version. If a higher
1024   version request is received, the proxy/gateway &MUST; either downgrade
1025   the request version, or respond with an error, or switch to tunnel
1026   behavior.
1027</t>
1028<t>
1029   Due to interoperability problems with HTTP/1.0 proxies discovered
1030   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
1031   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
1032   they support. The proxy/gateway's response to that request &MUST; be in
1033   the same major version as the request.
1034</t>
1035<t>
1036  <list>
1037    <t>
1038      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1039      of header fields required or forbidden by the versions involved.
1040    </t>
1041  </list>
1042</t>
1043</section>
1044
1045<section title="Uniform Resource Identifiers" anchor="uri">
1046<t>
1047   URIs have been known by many names: WWW addresses, Universal Document
1048   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
1049   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
1050   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
1051   simply formatted strings which identify--via name, location, or any
1052   other characteristic--a resource.
1053</t>
1054
1055<section title="General Syntax" anchor="general.syntax">
1056<t>
1057   URIs in HTTP can be represented in absolute form or relative to some
1058   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1059   forms are differentiated by the fact that absolute URIs always begin
1060   with a scheme name followed by a colon. For definitive information on
1061   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1062   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1063   and <xref target="RFC1808"/>). This specification adopts the
1064   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
1065   "host", "abs_path", "query", and "authority" from that specification:
1066</t>
1067<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"/>
1068  absoluteURI   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1069  authority     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
1070  fragment      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
1071  path-absolute = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1072  port          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1073  query         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
1074  relativeURI   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
1075  uri-host      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1076</artwork></figure>
1077<t>
1078   HTTP does not place any a priori limit on the length of
1079   a URI. Servers &MUST; be able to handle the URI of any resource they
1080   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1081   provide GET-based forms that could generate such URIs. A server
1082   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1083   than the server can handle (see &status-414;).
1084</t>
1085<t>
1086  <list>
1087    <t>
1088      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1089      above 255 bytes, because some older client or proxy
1090      implementations might not properly support these lengths.
1091    </t>
1092  </list>
1093</t>
1094</section>
1095
1096<section title="http URL" anchor="http.url">
1097<t>
1098   The "http" scheme is used to locate network resources via the HTTP
1099   protocol. This section defines the scheme-specific syntax and
1100   semantics for http URLs.
1101</t>
1102<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
1103  http-URL = "http:" "//" uri-host [ ":" port ]
1104             [ path-absolute [ "?" query ]]
1105</artwork></figure>
1106<t>
1107   If the port is empty or not given, port 80 is assumed. The semantics
1108   are that the identified resource is located at the server listening
1109   for TCP connections on that port of that host, and the Request-URI
1110   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
1111   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1112   the path-absolute is not present in the URL, it &MUST; be given as "/" when
1113   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1114   receives a host name which is not a fully qualified domain name, it
1115   &MAY; add its domain to the host name it received. If a proxy receives
1116   a fully qualified domain name, the proxy &MUST-NOT; change the host
1117   name.
1118</t>
1119</section>
1120
1121<section title="URI Comparison" anchor="uri.comparison">
1122<t>
1123   When comparing two URIs to decide if they match or not, a client
1124   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1125   URIs, with these exceptions:
1126  <list style="symbols">
1127    <t>A port that is empty or not given is equivalent to the default
1128        port for that URI-reference;</t>
1129    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1130    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1131    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
1132  </list>
1133</t>
1134<t>
1135   Characters other than those in the "reserved" set (see
1136   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1137</t>
1138<t>
1139   For example, the following three URIs are equivalent:
1140</t>
1141<figure><artwork type="example">
1142   http://example.com:80/~smith/home.html
1143   http://EXAMPLE.com/%7Esmith/home.html
1144   http://EXAMPLE.com:/%7esmith/home.html
1145</artwork></figure>
1146</section>
1147</section>
1148
1149<section title="Date/Time Formats" anchor="date.time.formats">
1150<section title="Full Date" anchor="full.date">
1151<t>
1152   HTTP applications have historically allowed three different formats
1153   for the representation of date/time stamps:
1154</t>
1155<figure><artwork type="example">
1156   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1157   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1158   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1159</artwork></figure>
1160<t>
1161   The first format is preferred as an Internet standard and represents
1162   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1163   <xref target="RFC822"/>). The other formats are described here only for
1164   compatibility with obsolete implementations.
1165   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1166   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1167   only generate the RFC 1123 format for representing HTTP-date values
1168   in header fields. See <xref target="tolerant.applications"/> for further information.
1169</t>
1170<t><list><t>
1171      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1172      accepting date values that may have been sent by non-HTTP
1173      applications, as is sometimes the case when retrieving or posting
1174      messages via proxies/gateways to SMTP or NNTP.
1175</t></list></t>
1176<t>
1177   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1178   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1179   equal to UTC (Coordinated Universal Time). This is indicated in the
1180   first two formats by the inclusion of "GMT" as the three-letter
1181   abbreviation for time zone, and &MUST; be assumed when reading the
1182   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1183   additional LWS beyond that specifically included as SP in the
1184   grammar.
1185</t>
1186<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"/>
1187  HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1188  rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1189  rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1190  asctime-date = wkday SP date3 SP time SP 4DIGIT
1191  date1        = 2DIGIT SP month SP 4DIGIT
1192                 ; day month year (e.g., 02 Jun 1982)
1193  date2        = 2DIGIT "-" month "-" 2DIGIT
1194                 ; day-month-year (e.g., 02-Jun-82)
1195  date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1196                 ; month day (e.g., Jun  2)
1197  time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1198                 ; 00:00:00 - 23:59:59
1199  wkday        = "Mon" | "Tue" | "Wed"
1200               | "Thu" | "Fri" | "Sat" | "Sun"
1201  weekday      = "Monday" | "Tuesday" | "Wednesday"
1202               | "Thursday" | "Friday" | "Saturday" | "Sunday"
1203  month        = "Jan" | "Feb" | "Mar" | "Apr"
1204               | "May" | "Jun" | "Jul" | "Aug"
1205               | "Sep" | "Oct" | "Nov" | "Dec"
1206</artwork></figure>
1207<t>
1208      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1209      to their usage within the protocol stream. Clients and servers are
1210      not required to use these formats for user presentation, request
1211      logging, etc.
1212</t>
1213</section>
1214</section>
1215
1216<section title="Transfer Codings" anchor="transfer.codings">
1217<t>
1218   Transfer-coding values are used to indicate an encoding
1219   transformation that has been, can be, or may need to be applied to an
1220   entity-body in order to ensure "safe transport" through the network.
1221   This differs from a content coding in that the transfer-coding is a
1222   property of the message, not of the original entity.
1223</t>
1224<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1225  transfer-coding         = "chunked" | transfer-extension
1226  transfer-extension      = token *( ";" parameter )
1227</artwork></figure>
1228<t>
1229   Parameters are in  the form of attribute/value pairs.
1230</t>
1231<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"/>
1232  parameter               = attribute "=" value
1233  attribute               = token
1234  value                   = token | quoted-string
1235</artwork></figure>
1236<t>
1237   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1238   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1239   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1240</t>
1241<t>
1242   Whenever a transfer-coding is applied to a message-body, the set of
1243   transfer-codings &MUST; include "chunked", unless the message is
1244   terminated by closing the connection. When the "chunked" transfer-coding
1245   is used, it &MUST; be the last transfer-coding applied to the
1246   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1247   than once to a message-body. These rules allow the recipient to
1248   determine the transfer-length of the message (<xref target="message.length"/>).
1249</t>
1250<t>
1251   Transfer-codings are analogous to the Content-Transfer-Encoding
1252   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1253   binary data over a 7-bit transport service. However, safe transport
1254   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1255   the only unsafe characteristic of message-bodies is the difficulty in
1256   determining the exact body length (<xref target="message.length"/>), or the desire to
1257   encrypt data over a shared transport.
1258</t>
1259<t>
1260   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1261   transfer-coding value tokens. Initially, the registry contains the
1262   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1263   "gzip", "compress", and "deflate" (&content-codings;).
1264</t>
1265<t>
1266   New transfer-coding value tokens &SHOULD; be registered in the same way
1267   as new content-coding value tokens (&content-codings;).
1268</t>
1269<t>
1270   A server which receives an entity-body with a transfer-coding it does
1271   not understand &SHOULD; return 501 (Not Implemented), and close the
1272   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1273   client.
1274</t>
1275
1276<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1277<t>
1278   The chunked encoding modifies the body of a message in order to
1279   transfer it as a series of chunks, each with its own size indicator,
1280   followed by an &OPTIONAL; trailer containing entity-header fields. This
1281   allows dynamically produced content to be transferred along with the
1282   information necessary for the recipient to verify that it has
1283   received the full message.
1284</t>
1285<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"/>
1286  Chunked-Body   = *chunk
1287                   last-chunk
1288                   trailer-part
1289                   CRLF
1290 
1291  chunk          = chunk-size [ chunk-extension ] CRLF
1292                   chunk-data CRLF
1293  chunk-size     = 1*HEX
1294  last-chunk     = 1*("0") [ chunk-extension ] CRLF
1295 
1296  chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1297  chunk-ext-name = token
1298  chunk-ext-val  = token | quoted-string
1299  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
1300  trailer-part   = *(entity-header CRLF)
1301</artwork></figure>
1302<t>
1303   The chunk-size field is a string of hex digits indicating the size of
1304   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1305   zero, followed by the trailer, which is terminated by an empty line.
1306</t>
1307<t>
1308   The trailer allows the sender to include additional HTTP header
1309   fields at the end of the message. The Trailer header field can be
1310   used to indicate which header fields are included in a trailer (see
1311   <xref target="header.trailer"/>).
1312</t>
1313<t>
1314   A server using chunked transfer-coding in a response &MUST-NOT; use the
1315   trailer for any header fields unless at least one of the following is
1316   true:
1317  <list style="numbers">
1318    <t>the request included a TE header field that indicates "trailers" is
1319     acceptable in the transfer-coding of the  response, as described in
1320     <xref target="header.te"/>; or,</t>
1321
1322    <t>the server is the origin server for the response, the trailer
1323     fields consist entirely of optional metadata, and the recipient
1324     could use the message (in a manner acceptable to the origin server)
1325     without receiving this metadata.  In other words, the origin server
1326     is willing to accept the possibility that the trailer fields might
1327     be silently discarded along the path to the client.</t>
1328  </list>
1329</t>
1330<t>
1331   This requirement prevents an interoperability failure when the
1332   message is being received by an HTTP/1.1 (or later) proxy and
1333   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1334   compliance with the protocol would have necessitated a possibly
1335   infinite buffer on the proxy.
1336</t>
1337<t>
1338   A process for decoding the "chunked" transfer-coding
1339   can be represented in pseudo-code as:
1340</t>
1341<figure><artwork type="code">
1342    length := 0
1343    read chunk-size, chunk-extension (if any) and CRLF
1344    while (chunk-size &gt; 0) {
1345       read chunk-data and CRLF
1346       append chunk-data to entity-body
1347       length := length + chunk-size
1348       read chunk-size and CRLF
1349    }
1350    read entity-header
1351    while (entity-header not empty) {
1352       append entity-header to existing header fields
1353       read entity-header
1354    }
1355    Content-Length := length
1356    Remove "chunked" from Transfer-Encoding
1357</artwork></figure>
1358<t>
1359   All HTTP/1.1 applications &MUST; be able to receive and decode the
1360   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1361   they do not understand.
1362</t>
1363</section>
1364</section>
1365
1366<section title="Product Tokens" anchor="product.tokens">
1367<t>
1368   Product tokens are used to allow communicating applications to
1369   identify themselves by software name and version. Most fields using
1370   product tokens also allow sub-products which form a significant part
1371   of the application to be listed, separated by white space. By
1372   convention, the products are listed in order of their significance
1373   for identifying the application.
1374</t>
1375<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1376  product         = token ["/" product-version]
1377  product-version = token
1378</artwork></figure>
1379<t>
1380   Examples:
1381</t>
1382<figure><artwork type="example">
1383    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1384    Server: Apache/0.8.4
1385</artwork></figure>
1386<t>
1387   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1388   used for advertising or other non-essential information. Although any
1389   token character &MAY; appear in a product-version, this token &SHOULD;
1390   only be used for a version identifier (i.e., successive versions of
1391   the same product &SHOULD; only differ in the product-version portion of
1392   the product value).
1393</t>
1394</section>
1395
1396</section>
1397
1398<section title="HTTP Message" anchor="http.message">
1399
1400<section title="Message Types" anchor="message.types">
1401<t>
1402   HTTP messages consist of requests from client to server and responses
1403   from server to client.
1404</t>
1405<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1406  HTTP-message   = Request | Response     ; HTTP/1.1 messages
1407</artwork></figure>
1408<t>
1409   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1410   message format of <xref target="RFC2822"/> for transferring entities (the payload
1411   of the message). Both types of message consist of a start-line, zero
1412   or more header fields (also known as "headers"), an empty line (i.e.,
1413   a line with nothing preceding the CRLF) indicating the end of the
1414   header fields, and possibly a message-body.
1415</t>
1416<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1417  generic-message = start-line
1418                    *(message-header CRLF)
1419                    CRLF
1420                    [ message-body ]
1421  start-line      = Request-Line | Status-Line
1422</artwork></figure>
1423<t>
1424   In the interest of robustness, servers &SHOULD; ignore any empty
1425   line(s) received where a Request-Line is expected. In other words, if
1426   the server is reading the protocol stream at the beginning of a
1427   message and receives a CRLF first, it should ignore the CRLF.
1428</t>
1429<t>
1430   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1431   after a POST request. To restate what is explicitly forbidden by the
1432   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1433   extra CRLF.
1434</t>
1435</section>
1436
1437<section title="Message Headers" anchor="message.headers">
1438<t>
1439   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1440   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1441   entity-header (&entity-header-fields;) fields, follow the same generic format as
1442   that given in <xref target="RFC2822" x:fmt="of" x:sec="2.1"/>. Each header field consists
1443   of a name followed by a colon (":") and the field value. Field names
1444   are case-insensitive. The field value &MAY; be preceded by any amount
1445   of LWS, though a single SP is preferred. Header fields can be
1446   extended over multiple lines by preceding each extra line with at
1447   least one SP or HTAB. Applications ought to follow "common form", where
1448   one is known or indicated, when generating HTTP constructs, since
1449   there might exist some implementations that fail to accept anything
1450   beyond the common forms.
1451</t>
1452<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"/>
1453  message-header = field-name ":" [ field-value ]
1454  field-name     = token
1455  field-value    = *( field-content | LWS )
1456  field-content  = &lt;field content&gt;
1457                   ; the OCTETs making up the field-value
1458                   ; and consisting of either *TEXT or combinations
1459                   ; of token, separators, and quoted-string
1460</artwork></figure>
1461<t>
1462   The field-content does not include any leading or trailing LWS:
1463   linear white space occurring before the first non-whitespace
1464   character of the field-value or after the last non-whitespace
1465   character of the field-value. Such leading or trailing LWS &MAY; be
1466   removed without changing the semantics of the field value. Any LWS
1467   that occurs between field-content &MAY; be replaced with a single SP
1468   before interpreting the field value or forwarding the message
1469   downstream.
1470</t>
1471<t>
1472   The order in which header fields with differing field names are
1473   received is not significant. However, it is "good practice" to send
1474   general-header fields first, followed by request-header or response-header
1475   fields, and ending with the entity-header fields.
1476</t>
1477<t>
1478   Multiple message-header fields with the same field-name &MAY; be
1479   present in a message if and only if the entire field-value for that
1480   header field is defined as a comma-separated list [i.e., #(values)].
1481   It &MUST; be possible to combine the multiple header fields into one
1482   "field-name: field-value" pair, without changing the semantics of the
1483   message, by appending each subsequent field-value to the first, each
1484   separated by a comma. The order in which header fields with the same
1485   field-name are received is therefore significant to the
1486   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1487   change the order of these field values when a message is forwarded.
1488</t>
1489</section>
1490
1491<section title="Message Body" anchor="message.body">
1492<t>
1493   The message-body (if any) of an HTTP message is used to carry the
1494   entity-body associated with the request or response. The message-body
1495   differs from the entity-body only when a transfer-coding has been
1496   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1497</t>
1498<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1499  message-body = entity-body
1500               | &lt;entity-body encoded as per Transfer-Encoding&gt;
1501</artwork></figure>
1502<t>
1503   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1504   applied by an application to ensure safe and proper transfer of the
1505   message. Transfer-Encoding is a property of the message, not of the
1506   entity, and thus &MAY; be added or removed by any application along the
1507   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1508   when certain transfer-codings may be used.)
1509</t>
1510<t>
1511   The rules for when a message-body is allowed in a message differ for
1512   requests and responses.
1513</t>
1514<t>
1515   The presence of a message-body in a request is signaled by the
1516   inclusion of a Content-Length or Transfer-Encoding header field in
1517   the request's message-headers. A message-body &MUST-NOT; be included in
1518   a request if the specification of the request method (&method;)
1519   explicitly disallows an entity-body in requests.
1520   When a request message contains both a message-body of non-zero
1521   length and a method that does not define any semantics for that
1522   request message-body, then an origin server &SHOULD; either ignore
1523   the message-body or respond with an appropriate error message
1524   (e.g., 413).  A proxy or gateway, when presented the same request,
1525   &SHOULD; either forward the request inbound with the message-body or
1526   ignore the message-body when determining a response.
1527</t>
1528<t>
1529   For response messages, whether or not a message-body is included with
1530   a message is dependent on both the request method and the response
1531   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1532   &MUST-NOT; include a message-body, even though the presence of entity-header
1533   fields might lead one to believe they do. All 1xx
1534   (informational), 204 (No Content), and 304 (Not Modified) responses
1535   &MUST-NOT; include a message-body. All other responses do include a
1536   message-body, although it &MAY; be of zero length.
1537</t>
1538</section>
1539
1540<section title="Message Length" anchor="message.length">
1541<t>
1542   The transfer-length of a message is the length of the message-body as
1543   it appears in the message; that is, after any transfer-codings have
1544   been applied. When a message-body is included with a message, the
1545   transfer-length of that body is determined by one of the following
1546   (in order of precedence):
1547</t>
1548<t>
1549  <list style="numbers">
1550    <x:lt><t>
1551     Any response message which "&MUST-NOT;" include a message-body (such
1552     as the 1xx, 204, and 304 responses and any response to a HEAD
1553     request) is always terminated by the first empty line after the
1554     header fields, regardless of the entity-header fields present in
1555     the message.
1556    </t></x:lt>
1557    <x:lt><t>
1558     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1559     is present, then the transfer-length is
1560     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1561     unless the message is terminated by closing the connection.
1562    </t></x:lt>
1563    <x:lt><t>
1564     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1565     decimal value in OCTETs represents both the entity-length and the
1566     transfer-length. The Content-Length header field &MUST-NOT; be sent
1567     if these two lengths are different (i.e., if a Transfer-Encoding
1568     header field is present). If a message is received with both a
1569     Transfer-Encoding header field and a Content-Length header field,
1570     the latter &MUST; be ignored.
1571    </t></x:lt>
1572    <x:lt><t>
1573     If the message uses the media type "multipart/byteranges", and the
1574     transfer-length is not otherwise specified, then this self-delimiting
1575     media type defines the transfer-length. This media type
1576     &MUST-NOT; be used unless the sender knows that the recipient can parse
1577     it; the presence in a request of a Range header with multiple byte-range
1578     specifiers from a 1.1 client implies that the client can parse
1579     multipart/byteranges responses.
1580    <list style="empty"><t>
1581       A range header might be forwarded by a 1.0 proxy that does not
1582       understand multipart/byteranges; in this case the server &MUST;
1583       delimit the message using methods defined in items 1, 3 or 5 of
1584       this section.
1585    </t></list>
1586    </t></x:lt>
1587    <x:lt><t>
1588     By the server closing the connection. (Closing the connection
1589     cannot be used to indicate the end of a request body, since that
1590     would leave no possibility for the server to send back a response.)
1591    </t></x:lt>
1592  </list>
1593</t>
1594<t>
1595   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1596   containing a message-body &MUST; include a valid Content-Length header
1597   field unless the server is known to be HTTP/1.1 compliant. If a
1598   request contains a message-body and a Content-Length is not given,
1599   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1600   determine the length of the message, or with 411 (Length Required) if
1601   it wishes to insist on receiving a valid Content-Length.
1602</t>
1603<t>
1604   All HTTP/1.1 applications that receive entities &MUST; accept the
1605   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1606   to be used for messages when the message length cannot be determined
1607   in advance.
1608</t>
1609<t>
1610   Messages &MUST-NOT; include both a Content-Length header field and a
1611   transfer-coding. If the message does include a
1612   transfer-coding, the Content-Length &MUST; be ignored.
1613</t>
1614<t>
1615   When a Content-Length is given in a message where a message-body is
1616   allowed, its field value &MUST; exactly match the number of OCTETs in
1617   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1618   invalid length is received and detected.
1619</t>
1620</section>
1621
1622<section title="General Header Fields" anchor="general.header.fields">
1623<t>
1624   There are a few header fields which have general applicability for
1625   both request and response messages, but which do not apply to the
1626   entity being transferred. These header fields apply only to the
1627   message being transmitted.
1628</t>
1629<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1630  general-header = Cache-Control            ; &header-cache-control;
1631                 | Connection               ; <xref target="header.connection"/>
1632                 | Date                     ; <xref target="header.date"/>
1633                 | Pragma                   ; &header-pragma;
1634                 | Trailer                  ; <xref target="header.trailer"/>
1635                 | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1636                 | Upgrade                  ; <xref target="header.upgrade"/>
1637                 | Via                      ; <xref target="header.via"/>
1638                 | Warning                  ; &header-warning;
1639</artwork></figure>
1640<t>
1641   General-header field names can be extended reliably only in
1642   combination with a change in the protocol version. However, new or
1643   experimental header fields may be given the semantics of general
1644   header fields if all parties in the communication recognize them to
1645   be general-header fields. Unrecognized header fields are treated as
1646   entity-header fields.
1647</t>
1648</section>
1649</section>
1650
1651<section title="Request" anchor="request">
1652<t>
1653   A request message from a client to a server includes, within the
1654   first line of that message, the method to be applied to the resource,
1655   the identifier of the resource, and the protocol version in use.
1656</t>
1657<!--                 Host                      ; should be moved here eventually -->
1658<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1659  Request       = Request-Line              ; <xref target="request-line"/>
1660                  *(( general-header        ; <xref target="general.header.fields"/>
1661                   | request-header         ; &request-header-fields;
1662                   | entity-header ) CRLF)  ; &entity-header-fields;
1663                  CRLF
1664                  [ message-body ]          ; <xref target="message.body"/>
1665</artwork></figure>
1666
1667<section title="Request-Line" anchor="request-line">
1668<t>
1669   The Request-Line begins with a method token, followed by the
1670   Request-URI and the protocol version, and ending with CRLF. The
1671   elements are separated by SP characters. No CR or LF is allowed
1672   except in the final CRLF sequence.
1673</t>
1674<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1675  Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1676</artwork></figure>
1677
1678<section title="Method" anchor="method">
1679<t>
1680   The Method  token indicates the method to be performed on the
1681   resource identified by the Request-URI. The method is case-sensitive.
1682</t>
1683<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1684  Method         = token
1685</artwork></figure>
1686</section>
1687
1688<section title="Request-URI" anchor="request-uri">
1689<t>
1690   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1691   identifies the resource upon which to apply the request.
1692</t>
1693<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1694  Request-URI    = "*"
1695                 | absoluteURI
1696                 | ( path-absolute [ "?" query ] )
1697                 | authority
1698</artwork></figure>
1699<t>
1700   The four options for Request-URI are dependent on the nature of the
1701   request. The asterisk "*" means that the request does not apply to a
1702   particular resource, but to the server itself, and is only allowed
1703   when the method used does not necessarily apply to a resource. One
1704   example would be
1705</t>
1706<figure><artwork type="example">
1707    OPTIONS * HTTP/1.1
1708</artwork></figure>
1709<t>
1710   The absoluteURI form is &REQUIRED; when the request is being made to a
1711   proxy. The proxy is requested to forward the request or service it
1712   from a valid cache, and return the response. Note that the proxy &MAY;
1713   forward the request on to another proxy or directly to the server
1714   specified by the absoluteURI. In order to avoid request loops, a
1715   proxy &MUST; be able to recognize all of its server names, including
1716   any aliases, local variations, and the numeric IP address. An example
1717   Request-Line would be:
1718</t>
1719<figure><artwork type="example">
1720    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1721</artwork></figure>
1722<t>
1723   To allow for transition to absoluteURIs in all requests in future
1724   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1725   form in requests, even though HTTP/1.1 clients will only generate
1726   them in requests to proxies.
1727</t>
1728<t>
1729   The authority form is only used by the CONNECT method (&CONNECT;).
1730</t>
1731<t>
1732   The most common form of Request-URI is that used to identify a
1733   resource on an origin server or gateway. In this case the absolute
1734   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1735   the Request-URI, and the network location of the URI (authority) &MUST;
1736   be transmitted in a Host header field. For example, a client wishing
1737   to retrieve the resource above directly from the origin server would
1738   create a TCP connection to port 80 of the host "www.example.org" and send
1739   the lines:
1740</t>
1741<figure><artwork type="example">
1742    GET /pub/WWW/TheProject.html HTTP/1.1
1743    Host: www.example.org
1744</artwork></figure>
1745<t>
1746   followed by the remainder of the Request. Note that the absolute path
1747   cannot be empty; if none is present in the original URI, it &MUST; be
1748   given as "/" (the server root).
1749</t>
1750<t>
1751   The Request-URI is transmitted in the format specified in
1752   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1753   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1754   properly interpret the request. Servers &SHOULD; respond to invalid
1755   Request-URIs with an appropriate status code.
1756</t>
1757<t>
1758   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1759   received Request-URI when forwarding it to the next inbound server,
1760   except as noted above to replace a null path-absolute with "/".
1761</t>
1762<t>
1763  <list><t>
1764      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1765      meaning of the request when the origin server is improperly using
1766      a non-reserved URI character for a reserved purpose.  Implementors
1767      should be aware that some pre-HTTP/1.1 proxies have been known to
1768      rewrite the Request-URI.
1769  </t></list>
1770</t>
1771</section>
1772</section>
1773
1774<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1775<t>
1776   The exact resource identified by an Internet request is determined by
1777   examining both the Request-URI and the Host header field.
1778</t>
1779<t>
1780   An origin server that does not allow resources to differ by the
1781   requested host &MAY; ignore the Host header field value when
1782   determining the resource identified by an HTTP/1.1 request. (But see
1783   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1784   for other requirements on Host support in HTTP/1.1.)
1785</t>
1786<t>
1787   An origin server that does differentiate resources based on the host
1788   requested (sometimes referred to as virtual hosts or vanity host
1789   names) &MUST; use the following rules for determining the requested
1790   resource on an HTTP/1.1 request:
1791  <list style="numbers">
1792    <t>If Request-URI is an absoluteURI, the host is part of the
1793     Request-URI. Any Host header field value in the request &MUST; be
1794     ignored.</t>
1795    <t>If the Request-URI is not an absoluteURI, and the request includes
1796     a Host header field, the host is determined by the Host header
1797     field value.</t>
1798    <t>If the host as determined by rule 1 or 2 is not a valid host on
1799     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1800  </list>
1801</t>
1802<t>
1803   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1804   attempt to use heuristics (e.g., examination of the URI path for
1805   something unique to a particular host) in order to determine what
1806   exact resource is being requested.
1807</t>
1808</section>
1809
1810</section>
1811
1812
1813<section title="Response" anchor="response">
1814<t>
1815   After receiving and interpreting a request message, a server responds
1816   with an HTTP response message.
1817</t>
1818<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1819  Response      = Status-Line               ; <xref target="status-line"/>
1820                  *(( general-header        ; <xref target="general.header.fields"/>
1821                   | response-header        ; &response-header-fields;
1822                   | entity-header ) CRLF)  ; &entity-header-fields;
1823                  CRLF
1824                  [ message-body ]          ; <xref target="message.body"/>
1825</artwork></figure>
1826
1827<section title="Status-Line" anchor="status-line">
1828<t>
1829   The first line of a Response message is the Status-Line, consisting
1830   of the protocol version followed by a numeric status code and its
1831   associated textual phrase, with each element separated by SP
1832   characters. No CR or LF is allowed except in the final CRLF sequence.
1833</t>
1834<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1835  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1836</artwork></figure>
1837
1838<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1839<t>
1840   The Status-Code element is a 3-digit integer result code of the
1841   attempt to understand and satisfy the request. These codes are fully
1842   defined in &status-codes;.  The Reason Phrase exists for the sole
1843   purpose of providing a textual description associated with the numeric
1844   status code, out of deference to earlier Internet application protocols
1845   that were more frequently used with interactive text clients.
1846   A client &SHOULD; ignore the content of the Reason Phrase.
1847</t>
1848<t>
1849   The first digit of the Status-Code defines the class of response. The
1850   last two digits do not have any categorization role. There are 5
1851   values for the first digit:
1852  <list style="symbols">
1853    <t>
1854      1xx: Informational - Request received, continuing process
1855    </t>
1856    <t>
1857      2xx: Success - The action was successfully received,
1858        understood, and accepted
1859    </t>
1860    <t>
1861      3xx: Redirection - Further action must be taken in order to
1862        complete the request
1863    </t>
1864    <t>
1865      4xx: Client Error - The request contains bad syntax or cannot
1866        be fulfilled
1867    </t>
1868    <t>
1869      5xx: Server Error - The server failed to fulfill an apparently
1870        valid request
1871    </t>
1872  </list>
1873</t>
1874<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"/>
1875  Status-Code    = 3DIGIT
1876  Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1877</artwork></figure>
1878</section>
1879</section>
1880
1881</section>
1882
1883
1884<section title="Connections" anchor="connections">
1885
1886<section title="Persistent Connections" anchor="persistent.connections">
1887
1888<section title="Purpose" anchor="persistent.purpose">
1889<t>
1890   Prior to persistent connections, a separate TCP connection was
1891   established to fetch each URL, increasing the load on HTTP servers
1892   and causing congestion on the Internet. The use of inline images and
1893   other associated data often require a client to make multiple
1894   requests of the same server in a short amount of time. Analysis of
1895   these performance problems and results from a prototype
1896   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1897   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1898   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1899   T/TCP <xref target="Tou1998"/>.
1900</t>
1901<t>
1902   Persistent HTTP connections have a number of advantages:
1903  <list style="symbols">
1904      <t>
1905        By opening and closing fewer TCP connections, CPU time is saved
1906        in routers and hosts (clients, servers, proxies, gateways,
1907        tunnels, or caches), and memory used for TCP protocol control
1908        blocks can be saved in hosts.
1909      </t>
1910      <t>
1911        HTTP requests and responses can be pipelined on a connection.
1912        Pipelining allows a client to make multiple requests without
1913        waiting for each response, allowing a single TCP connection to
1914        be used much more efficiently, with much lower elapsed time.
1915      </t>
1916      <t>
1917        Network congestion is reduced by reducing the number of packets
1918        caused by TCP opens, and by allowing TCP sufficient time to
1919        determine the congestion state of the network.
1920      </t>
1921      <t>
1922        Latency on subsequent requests is reduced since there is no time
1923        spent in TCP's connection opening handshake.
1924      </t>
1925      <t>
1926        HTTP can evolve more gracefully, since errors can be reported
1927        without the penalty of closing the TCP connection. Clients using
1928        future versions of HTTP might optimistically try a new feature,
1929        but if communicating with an older server, retry with old
1930        semantics after an error is reported.
1931      </t>
1932    </list>
1933</t>
1934<t>
1935   HTTP implementations &SHOULD; implement persistent connections.
1936</t>
1937</section>
1938
1939<section title="Overall Operation" anchor="persistent.overall">
1940<t>
1941   A significant difference between HTTP/1.1 and earlier versions of
1942   HTTP is that persistent connections are the default behavior of any
1943   HTTP connection. That is, unless otherwise indicated, the client
1944   &SHOULD; assume that the server will maintain a persistent connection,
1945   even after error responses from the server.
1946</t>
1947<t>
1948   Persistent connections provide a mechanism by which a client and a
1949   server can signal the close of a TCP connection. This signaling takes
1950   place using the Connection header field (<xref target="header.connection"/>). Once a close
1951   has been signaled, the client &MUST-NOT; send any more requests on that
1952   connection.
1953</t>
1954
1955<section title="Negotiation" anchor="persistent.negotiation">
1956<t>
1957   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1958   maintain a persistent connection unless a Connection header including
1959   the connection-token "close" was sent in the request. If the server
1960   chooses to close the connection immediately after sending the
1961   response, it &SHOULD; send a Connection header including the
1962   connection-token close.
1963</t>
1964<t>
1965   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1966   decide to keep it open based on whether the response from a server
1967   contains a Connection header with the connection-token close. In case
1968   the client does not want to maintain a connection for more than that
1969   request, it &SHOULD; send a Connection header including the
1970   connection-token close.
1971</t>
1972<t>
1973   If either the client or the server sends the close token in the
1974   Connection header, that request becomes the last one for the
1975   connection.
1976</t>
1977<t>
1978   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1979   maintained for HTTP versions less than 1.1 unless it is explicitly
1980   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1981   compatibility with HTTP/1.0 clients.
1982</t>
1983<t>
1984   In order to remain persistent, all messages on the connection &MUST;
1985   have a self-defined message length (i.e., one not defined by closure
1986   of the connection), as described in <xref target="message.length"/>.
1987</t>
1988</section>
1989
1990<section title="Pipelining" anchor="pipelining">
1991<t>
1992   A client that supports persistent connections &MAY; "pipeline" its
1993   requests (i.e., send multiple requests without waiting for each
1994   response). A server &MUST; send its responses to those requests in the
1995   same order that the requests were received.
1996</t>
1997<t>
1998   Clients which assume persistent connections and pipeline immediately
1999   after connection establishment &SHOULD; be prepared to retry their
2000   connection if the first pipelined attempt fails. If a client does
2001   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2002   persistent. Clients &MUST; also be prepared to resend their requests if
2003   the server closes the connection before sending all of the
2004   corresponding responses.
2005</t>
2006<t>
2007   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
2008   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
2009   premature termination of the transport connection could lead to
2010   indeterminate results. A client wishing to send a non-idempotent
2011   request &SHOULD; wait to send that request until it has received the
2012   response status for the previous request.
2013</t>
2014</section>
2015</section>
2016
2017<section title="Proxy Servers" anchor="persistent.proxy">
2018<t>
2019   It is especially important that proxies correctly implement the
2020   properties of the Connection header field as specified in <xref target="header.connection"/>.
2021</t>
2022<t>
2023   The proxy server &MUST; signal persistent connections separately with
2024   its clients and the origin servers (or other proxy servers) that it
2025   connects to. Each persistent connection applies to only one transport
2026   link.
2027</t>
2028<t>
2029   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2030   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
2031   discussion of the problems with the Keep-Alive header implemented by
2032   many HTTP/1.0 clients).
2033</t>
2034</section>
2035
2036<section title="Practical Considerations" anchor="persistent.practical">
2037<t>
2038   Servers will usually have some time-out value beyond which they will
2039   no longer maintain an inactive connection. Proxy servers might make
2040   this a higher value since it is likely that the client will be making
2041   more connections through the same server. The use of persistent
2042   connections places no requirements on the length (or existence) of
2043   this time-out for either the client or the server.
2044</t>
2045<t>
2046   When a client or server wishes to time-out it &SHOULD; issue a graceful
2047   close on the transport connection. Clients and servers &SHOULD; both
2048   constantly watch for the other side of the transport close, and
2049   respond to it as appropriate. If a client or server does not detect
2050   the other side's close promptly it could cause unnecessary resource
2051   drain on the network.
2052</t>
2053<t>
2054   A client, server, or proxy &MAY; close the transport connection at any
2055   time. For example, a client might have started to send a new request
2056   at the same time that the server has decided to close the "idle"
2057   connection. From the server's point of view, the connection is being
2058   closed while it was idle, but from the client's point of view, a
2059   request is in progress.
2060</t>
2061<t>
2062   This means that clients, servers, and proxies &MUST; be able to recover
2063   from asynchronous close events. Client software &SHOULD; reopen the
2064   transport connection and retransmit the aborted sequence of requests
2065   without user interaction so long as the request sequence is
2066   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2067   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2068   human operator the choice of retrying the request(s). Confirmation by
2069   user-agent software with semantic understanding of the application
2070   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2071   be repeated if the second sequence of requests fails.
2072</t>
2073<t>
2074   Servers &SHOULD; always respond to at least one request per connection,
2075   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2076   middle of transmitting a response, unless a network or client failure
2077   is suspected.
2078</t>
2079<t>
2080   Clients that use persistent connections &SHOULD; limit the number of
2081   simultaneous connections that they maintain to a given server. A
2082   single-user client &SHOULD-NOT; maintain more than 2 connections with
2083   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2084   another server or proxy, where N is the number of simultaneously
2085   active users. These guidelines are intended to improve HTTP response
2086   times and avoid congestion.
2087</t>
2088</section>
2089</section>
2090
2091<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2092
2093<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2094<t>
2095   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2096   flow control mechanisms to resolve temporary overloads, rather than
2097   terminating connections with the expectation that clients will retry.
2098   The latter technique can exacerbate network congestion.
2099</t>
2100</section>
2101
2102<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2103<t>
2104   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2105   the network connection for an error status while it is transmitting
2106   the request. If the client sees an error status, it &SHOULD;
2107   immediately cease transmitting the body. If the body is being sent
2108   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2109   empty trailer &MAY; be used to prematurely mark the end of the message.
2110   If the body was preceded by a Content-Length header, the client &MUST;
2111   close the connection.
2112</t>
2113</section>
2114
2115<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2116<t>
2117   The purpose of the 100 (Continue) status (see &status-100;) is to
2118   allow a client that is sending a request message with a request body
2119   to determine if the origin server is willing to accept the request
2120   (based on the request headers) before the client sends the request
2121   body. In some cases, it might either be inappropriate or highly
2122   inefficient for the client to send the body if the server will reject
2123   the message without looking at the body.
2124</t>
2125<t>
2126   Requirements for HTTP/1.1 clients:
2127  <list style="symbols">
2128    <t>
2129        If a client will wait for a 100 (Continue) response before
2130        sending the request body, it &MUST; send an Expect request-header
2131        field (&header-expect;) with the "100-continue" expectation.
2132    </t>
2133    <t>
2134        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2135        with the "100-continue" expectation if it does not intend
2136        to send a request body.
2137    </t>
2138  </list>
2139</t>
2140<t>
2141   Because of the presence of older implementations, the protocol allows
2142   ambiguous situations in which a client may send "Expect: 100-continue"
2143   without receiving either a 417 (Expectation Failed) status
2144   or a 100 (Continue) status. Therefore, when a client sends this
2145   header field to an origin server (possibly via a proxy) from which it
2146   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2147   for an indefinite period before sending the request body.
2148</t>
2149<t>
2150   Requirements for HTTP/1.1 origin servers:
2151  <list style="symbols">
2152    <t> Upon receiving a request which includes an Expect request-header
2153        field with the "100-continue" expectation, an origin server &MUST;
2154        either respond with 100 (Continue) status and continue to read
2155        from the input stream, or respond with a final status code. The
2156        origin server &MUST-NOT; wait for the request body before sending
2157        the 100 (Continue) response. If it responds with a final status
2158        code, it &MAY; close the transport connection or it &MAY; continue
2159        to read and discard the rest of the request.  It &MUST-NOT;
2160        perform the requested method if it returns a final status code.
2161    </t>
2162    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2163        the request message does not include an Expect request-header
2164        field with the "100-continue" expectation, and &MUST-NOT; send a
2165        100 (Continue) response if such a request comes from an HTTP/1.0
2166        (or earlier) client. There is an exception to this rule: for
2167        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2168        status in response to an HTTP/1.1 PUT or POST request that does
2169        not include an Expect request-header field with the "100-continue"
2170        expectation. This exception, the purpose of which is
2171        to minimize any client processing delays associated with an
2172        undeclared wait for 100 (Continue) status, applies only to
2173        HTTP/1.1 requests, and not to requests with any other HTTP-version
2174        value.
2175    </t>
2176    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2177        already received some or all of the request body for the
2178        corresponding request.
2179    </t>
2180    <t> An origin server that sends a 100 (Continue) response &MUST;
2181    ultimately send a final status code, once the request body is
2182        received and processed, unless it terminates the transport
2183        connection prematurely.
2184    </t>
2185    <t> If an origin server receives a request that does not include an
2186        Expect request-header field with the "100-continue" expectation,
2187        the request includes a request body, and the server responds
2188        with a final status code before reading the entire request body
2189        from the transport connection, then the server &SHOULD-NOT;  close
2190        the transport connection until it has read the entire request,
2191        or until the client closes the connection. Otherwise, the client
2192        might not reliably receive the response message. However, this
2193        requirement is not be construed as preventing a server from
2194        defending itself against denial-of-service attacks, or from
2195        badly broken client implementations.
2196      </t>
2197    </list>
2198</t>
2199<t>
2200   Requirements for HTTP/1.1 proxies:
2201  <list style="symbols">
2202    <t> If a proxy receives a request that includes an Expect request-header
2203        field with the "100-continue" expectation, and the proxy
2204        either knows that the next-hop server complies with HTTP/1.1 or
2205        higher, or does not know the HTTP version of the next-hop
2206        server, it &MUST; forward the request, including the Expect header
2207        field.
2208    </t>
2209    <t> If the proxy knows that the version of the next-hop server is
2210        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2211        respond with a 417 (Expectation Failed) status.
2212    </t>
2213    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2214        numbers received from recently-referenced next-hop servers.
2215    </t>
2216    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2217        request message was received from an HTTP/1.0 (or earlier)
2218        client and did not include an Expect request-header field with
2219        the "100-continue" expectation. This requirement overrides the
2220        general rule for forwarding of 1xx responses (see &status-1xx;).
2221    </t>
2222  </list>
2223</t>
2224</section>
2225
2226<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2227<t>
2228   If an HTTP/1.1 client sends a request which includes a request body,
2229   but which does not include an Expect request-header field with the
2230   "100-continue" expectation, and if the client is not directly
2231   connected to an HTTP/1.1 origin server, and if the client sees the
2232   connection close before receiving any status from the server, the
2233   client &SHOULD; retry the request.  If the client does retry this
2234   request, it &MAY; use the following "binary exponential backoff"
2235   algorithm to be assured of obtaining a reliable response:
2236  <list style="numbers">
2237    <t>
2238      Initiate a new connection to the server
2239    </t>
2240    <t>
2241      Transmit the request-headers
2242    </t>
2243    <t>
2244      Initialize a variable R to the estimated round-trip time to the
2245         server (e.g., based on the time it took to establish the
2246         connection), or to a constant value of 5 seconds if the round-trip
2247         time is not available.
2248    </t>
2249    <t>
2250       Compute T = R * (2**N), where N is the number of previous
2251         retries of this request.
2252    </t>
2253    <t>
2254       Wait either for an error response from the server, or for T
2255         seconds (whichever comes first)
2256    </t>
2257    <t>
2258       If no error response is received, after T seconds transmit the
2259         body of the request.
2260    </t>
2261    <t>
2262       If client sees that the connection is closed prematurely,
2263         repeat from step 1 until the request is accepted, an error
2264         response is received, or the user becomes impatient and
2265         terminates the retry process.
2266    </t>
2267  </list>
2268</t>
2269<t>
2270   If at any point an error status is received, the client
2271  <list style="symbols">
2272      <t>&SHOULD-NOT;  continue and</t>
2273
2274      <t>&SHOULD; close the connection if it has not completed sending the
2275        request message.</t>
2276    </list>
2277</t>
2278</section>
2279</section>
2280</section>
2281
2282
2283<section title="Header Field Definitions" anchor="header.fields">
2284<t>
2285   This section defines the syntax and semantics of HTTP/1.1 header fields
2286   related to message framing and transport protocols.
2287</t>
2288<t>
2289   For entity-header fields, both sender and recipient refer to either the
2290   client or the server, depending on who sends and who receives the entity.
2291</t>
2292
2293<section title="Connection" anchor="header.connection">
2294  <iref primary="true" item="Connection header" x:for-anchor=""/>
2295  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2296<t>
2297   The Connection general-header field allows the sender to specify
2298   options that are desired for that particular connection and &MUST-NOT;
2299   be communicated by proxies over further connections.
2300</t>
2301<t>
2302   The Connection header has the following grammar:
2303</t>
2304<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2305  Connection = "Connection" ":" 1#(connection-token)
2306  connection-token  = token
2307</artwork></figure>
2308<t>
2309   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2310   message is forwarded and, for each connection-token in this field,
2311   remove any header field(s) from the message with the same name as the
2312   connection-token. Connection options are signaled by the presence of
2313   a connection-token in the Connection header field, not by any
2314   corresponding additional header field(s), since the additional header
2315   field may not be sent if there are no parameters associated with that
2316   connection option.
2317</t>
2318<t>
2319   Message headers listed in the Connection header &MUST-NOT; include
2320   end-to-end headers, such as Cache-Control.
2321</t>
2322<t>
2323   HTTP/1.1 defines the "close" connection option for the sender to
2324   signal that the connection will be closed after completion of the
2325   response. For example,
2326</t>
2327<figure><artwork type="example">
2328    Connection: close
2329</artwork></figure>
2330<t>
2331   in either the request or the response header fields indicates that
2332   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2333   after the current request/response is complete.
2334</t>
2335<t>
2336   An HTTP/1.1 client that does not support persistent connections &MUST;
2337   include the "close" connection option in every request message.
2338</t>
2339<t>
2340   An HTTP/1.1 server that does not support persistent connections &MUST;
2341   include the "close" connection option in every response message that
2342   does not have a 1xx (informational) status code.
2343</t>
2344<t>
2345   A system receiving an HTTP/1.0 (or lower-version) message that
2346   includes a Connection header &MUST;, for each connection-token in this
2347   field, remove and ignore any header field(s) from the message with
2348   the same name as the connection-token. This protects against mistaken
2349   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2350</t>
2351</section>
2352
2353<section title="Content-Length" anchor="header.content-length">
2354  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2355  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2356<t>
2357   The Content-Length entity-header field indicates the size of the
2358   entity-body, in decimal number of OCTETs, sent to the recipient or,
2359   in the case of the HEAD method, the size of the entity-body that
2360   would have been sent had the request been a GET.
2361</t>
2362<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2363  Content-Length    = "Content-Length" ":" 1*DIGIT
2364</artwork></figure>
2365<t>
2366   An example is
2367</t>
2368<figure><artwork type="example">
2369    Content-Length: 3495
2370</artwork></figure>
2371<t>
2372   Applications &SHOULD; use this field to indicate the transfer-length of
2373   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2374</t>
2375<t>
2376   Any Content-Length greater than or equal to zero is a valid value.
2377   <xref target="message.length"/> describes how to determine the length of a message-body
2378   if a Content-Length is not given.
2379</t>
2380<t>
2381   Note that the meaning of this field is significantly different from
2382   the corresponding definition in MIME, where it is an optional field
2383   used within the "message/external-body" content-type. In HTTP, it
2384   &SHOULD; be sent whenever the message's length can be determined prior
2385   to being transferred, unless this is prohibited by the rules in
2386   <xref target="message.length"/>.
2387</t>
2388</section>
2389
2390<section title="Date" anchor="header.date">
2391  <iref primary="true" item="Date header" x:for-anchor=""/>
2392  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2393<t>
2394   The Date general-header field represents the date and time at which
2395   the message was originated, having the same semantics as orig-date in
2396   <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"/>;
2397   it &MUST; be sent in rfc1123-date format.
2398</t>
2399<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2400  Date  = "Date" ":" HTTP-date
2401</artwork></figure>
2402<t>
2403   An example is
2404</t>
2405<figure><artwork type="example">
2406    Date: Tue, 15 Nov 1994 08:12:31 GMT
2407</artwork></figure>
2408<t>
2409   Origin servers &MUST; include a Date header field in all responses,
2410   except in these cases:
2411  <list style="numbers">
2412      <t>If the response status code is 100 (Continue) or 101 (Switching
2413         Protocols), the response &MAY; include a Date header field, at
2414         the server's option.</t>
2415
2416      <t>If the response status code conveys a server error, e.g. 500
2417         (Internal Server Error) or 503 (Service Unavailable), and it is
2418         inconvenient or impossible to generate a valid Date.</t>
2419
2420      <t>If the server does not have a clock that can provide a
2421         reasonable approximation of the current time, its responses
2422         &MUST-NOT; include a Date header field. In this case, the rules
2423         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2424  </list>
2425</t>
2426<t>
2427   A received message that does not have a Date header field &MUST; be
2428   assigned one by the recipient if the message will be cached by that
2429   recipient or gatewayed via a protocol which requires a Date. An HTTP
2430   implementation without a clock &MUST-NOT; cache responses without
2431   revalidating them on every use. An HTTP cache, especially a shared
2432   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2433   clock with a reliable external standard.
2434</t>
2435<t>
2436   Clients &SHOULD; only send a Date header field in messages that include
2437   an entity-body, as in the case of the PUT and POST requests, and even
2438   then it is optional. A client without a clock &MUST-NOT; send a Date
2439   header field in a request.
2440</t>
2441<t>
2442   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2443   time subsequent to the generation of the message. It &SHOULD; represent
2444   the best available approximation of the date and time of message
2445   generation, unless the implementation has no means of generating a
2446   reasonably accurate date and time. In theory, the date ought to
2447   represent the moment just before the entity is generated. In
2448   practice, the date can be generated at any time during the message
2449   origination without affecting its semantic value.
2450</t>
2451
2452<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2453<t>
2454   Some origin server implementations might not have a clock available.
2455   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2456   values to a response, unless these values were associated
2457   with the resource by a system or user with a reliable clock. It &MAY;
2458   assign an Expires value that is known, at or before server
2459   configuration time, to be in the past (this allows "pre-expiration"
2460   of responses without storing separate Expires values for each
2461   resource).
2462</t>
2463</section>
2464</section>
2465
2466<section title="Host" anchor="header.host">
2467  <iref primary="true" item="Host header" x:for-anchor=""/>
2468  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2469<t>
2470   The Host request-header field specifies the Internet host and port
2471   number of the resource being requested, as obtained from the original
2472   URI given by the user or referring resource (generally an HTTP URL,
2473   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2474   the naming authority of the origin server or gateway given by the
2475   original URL. This allows the origin server or gateway to
2476   differentiate between internally-ambiguous URLs, such as the root "/"
2477   URL of a server for multiple host names on a single IP address.
2478</t>
2479<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2480  Host = "Host" ":" uri-host [ ":" port ] ; <xref target="http.url"/>
2481</artwork></figure>
2482<t>
2483   A "host" without any trailing port information implies the default
2484   port for the service requested (e.g., "80" for an HTTP URL). For
2485   example, a request on the origin server for
2486   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2487</t>
2488<figure><artwork type="example">
2489    GET /pub/WWW/ HTTP/1.1
2490    Host: www.example.org
2491</artwork></figure>
2492<t>
2493   A client &MUST; include a Host header field in all HTTP/1.1 request
2494   messages. If the requested URI does not include an Internet host
2495   name for the service being requested, then the Host header field &MUST;
2496   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2497   request message it forwards does contain an appropriate Host header
2498   field that identifies the service being requested by the proxy. All
2499   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2500   status code to any HTTP/1.1 request message which lacks a Host header
2501   field.
2502</t>
2503<t>
2504   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2505   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2506   for other requirements relating to Host.
2507</t>
2508</section>
2509
2510<section title="TE" anchor="header.te">
2511  <iref primary="true" item="TE header" x:for-anchor=""/>
2512  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2513<t>
2514   The TE request-header field indicates what extension transfer-codings
2515   it is willing to accept in the response and whether or not it is
2516   willing to accept trailer fields in a chunked transfer-coding. Its
2517   value may consist of the keyword "trailers" and/or a comma-separated
2518   list of extension transfer-coding names with optional accept
2519   parameters (as described in <xref target="transfer.codings"/>).
2520</t>
2521<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2522  TE        = "TE" ":" #( t-codings )
2523  t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2524</artwork></figure>
2525<t>
2526   The presence of the keyword "trailers" indicates that the client is
2527   willing to accept trailer fields in a chunked transfer-coding, as
2528   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2529   transfer-coding values even though it does not itself represent a
2530   transfer-coding.
2531</t>
2532<t>
2533   Examples of its use are:
2534</t>
2535<figure><artwork type="example">
2536    TE: deflate
2537    TE:
2538    TE: trailers, deflate;q=0.5
2539</artwork></figure>
2540<t>
2541   The TE header field only applies to the immediate connection.
2542   Therefore, the keyword &MUST; be supplied within a Connection header
2543   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2544</t>
2545<t>
2546   A server tests whether a transfer-coding is acceptable, according to
2547   a TE field, using these rules:
2548  <list style="numbers">
2549    <x:lt>
2550      <t>The "chunked" transfer-coding is always acceptable. If the
2551         keyword "trailers" is listed, the client indicates that it is
2552         willing to accept trailer fields in the chunked response on
2553         behalf of itself and any downstream clients. The implication is
2554         that, if given, the client is stating that either all
2555         downstream clients are willing to accept trailer fields in the
2556         forwarded response, or that it will attempt to buffer the
2557         response on behalf of downstream recipients.
2558      </t><t>
2559         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2560         chunked response such that a client can be assured of buffering
2561         the entire response.</t>
2562    </x:lt>
2563    <x:lt>
2564      <t>If the transfer-coding being tested is one of the transfer-codings
2565         listed in the TE field, then it is acceptable unless it
2566         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2567         qvalue of 0 means "not acceptable.")</t>
2568    </x:lt>
2569    <x:lt>
2570      <t>If multiple transfer-codings are acceptable, then the
2571         acceptable transfer-coding with the highest non-zero qvalue is
2572         preferred.  The "chunked" transfer-coding always has a qvalue
2573         of 1.</t>
2574    </x:lt>
2575  </list>
2576</t>
2577<t>
2578   If the TE field-value is empty or if no TE field is present, the only
2579   transfer-coding  is "chunked". A message with no transfer-coding is
2580   always acceptable.
2581</t>
2582</section>
2583
2584<section title="Trailer" anchor="header.trailer">
2585  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2586  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2587<t>
2588   The Trailer general field value indicates that the given set of
2589   header fields is present in the trailer of a message encoded with
2590   chunked transfer-coding.
2591</t>
2592<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2593  Trailer  = "Trailer" ":" 1#field-name
2594</artwork></figure>
2595<t>
2596   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2597   message using chunked transfer-coding with a non-empty trailer. Doing
2598   so allows the recipient to know which header fields to expect in the
2599   trailer.
2600</t>
2601<t>
2602   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2603   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2604   trailer fields in a "chunked" transfer-coding.
2605</t>
2606<t>
2607   Message header fields listed in the Trailer header field &MUST-NOT;
2608   include the following header fields:
2609  <list style="symbols">
2610    <t>Transfer-Encoding</t>
2611    <t>Content-Length</t>
2612    <t>Trailer</t>
2613  </list>
2614</t>
2615</section>
2616
2617<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2618  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2619  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2620<t>
2621   The Transfer-Encoding general-header field indicates what (if any)
2622   type of transformation has been applied to the message body in order
2623   to safely transfer it between the sender and the recipient. This
2624   differs from the content-coding in that the transfer-coding is a
2625   property of the message, not of the entity.
2626</t>
2627<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2628  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2629</artwork></figure>
2630<t>
2631   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2632</t>
2633<figure><artwork type="example">
2634  Transfer-Encoding: chunked
2635</artwork></figure>
2636<t>
2637   If multiple encodings have been applied to an entity, the transfer-codings
2638   &MUST; be listed in the order in which they were applied.
2639   Additional information about the encoding parameters &MAY; be provided
2640   by other entity-header fields not defined by this specification.
2641</t>
2642<t>
2643   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2644   header.
2645</t>
2646</section>
2647
2648<section title="Upgrade" anchor="header.upgrade">
2649  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2650  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2651<t>
2652   The Upgrade general-header allows the client to specify what
2653   additional communication protocols it supports and would like to use
2654   if the server finds it appropriate to switch protocols. The server
2655   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2656   response to indicate which protocol(s) are being switched.
2657</t>
2658<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2659  Upgrade        = "Upgrade" ":" 1#product
2660</artwork></figure>
2661<t>
2662   For example,
2663</t>
2664<figure><artwork type="example">
2665    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2666</artwork></figure>
2667<t>
2668   The Upgrade header field is intended to provide a simple mechanism
2669   for transition from HTTP/1.1 to some other, incompatible protocol. It
2670   does so by allowing the client to advertise its desire to use another
2671   protocol, such as a later version of HTTP with a higher major version
2672   number, even though the current request has been made using HTTP/1.1.
2673   This eases the difficult transition between incompatible protocols by
2674   allowing the client to initiate a request in the more commonly
2675   supported protocol while indicating to the server that it would like
2676   to use a "better" protocol if available (where "better" is determined
2677   by the server, possibly according to the nature of the method and/or
2678   resource being requested).
2679</t>
2680<t>
2681   The Upgrade header field only applies to switching application-layer
2682   protocols upon the existing transport-layer connection. Upgrade
2683   cannot be used to insist on a protocol change; its acceptance and use
2684   by the server is optional. The capabilities and nature of the
2685   application-layer communication after the protocol change is entirely
2686   dependent upon the new protocol chosen, although the first action
2687   after changing the protocol &MUST; be a response to the initial HTTP
2688   request containing the Upgrade header field.
2689</t>
2690<t>
2691   The Upgrade header field only applies to the immediate connection.
2692   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2693   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2694   HTTP/1.1 message.
2695</t>
2696<t>
2697   The Upgrade header field cannot be used to indicate a switch to a
2698   protocol on a different connection. For that purpose, it is more
2699   appropriate to use a 301, 302, 303, or 305 redirection response.
2700</t>
2701<t>
2702   This specification only defines the protocol name "HTTP" for use by
2703   the family of Hypertext Transfer Protocols, as defined by the HTTP
2704   version rules of <xref target="http.version"/> and future updates to this
2705   specification. Any token can be used as a protocol name; however, it
2706   will only be useful if both the client and server associate the name
2707   with the same protocol.
2708</t>
2709</section>
2710
2711<section title="Via" anchor="header.via">
2712  <iref primary="true" item="Via header" x:for-anchor=""/>
2713  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2714<t>
2715   The Via general-header field &MUST; be used by gateways and proxies to
2716   indicate the intermediate protocols and recipients between the user
2717   agent and the server on requests, and between the origin server and
2718   the client on responses. It is analogous to the "Received" field of
2719   <xref target="RFC2822"/> and is intended to be used for tracking message forwards,
2720   avoiding request loops, and identifying the protocol capabilities of
2721   all senders along the request/response chain.
2722</t>
2723<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"/>
2724  Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2725  received-protocol = [ protocol-name "/" ] protocol-version
2726  protocol-name     = token
2727  protocol-version  = token
2728  received-by       = ( uri-host [ ":" port ] ) | pseudonym
2729  pseudonym         = token
2730</artwork></figure>
2731<t>
2732   The received-protocol indicates the protocol version of the message
2733   received by the server or client along each segment of the
2734   request/response chain. The received-protocol version is appended to
2735   the Via field value when the message is forwarded so that information
2736   about the protocol capabilities of upstream applications remains
2737   visible to all recipients.
2738</t>
2739<t>
2740   The protocol-name is optional if and only if it would be "HTTP". The
2741   received-by field is normally the host and optional port number of a
2742   recipient server or client that subsequently forwarded the message.
2743   However, if the real host is considered to be sensitive information,
2744   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2745   be assumed to be the default port of the received-protocol.
2746</t>
2747<t>
2748   Multiple Via field values represents each proxy or gateway that has
2749   forwarded the message. Each recipient &MUST; append its information
2750   such that the end result is ordered according to the sequence of
2751   forwarding applications.
2752</t>
2753<t>
2754   Comments &MAY; be used in the Via header field to identify the software
2755   of the recipient proxy or gateway, analogous to the User-Agent and
2756   Server header fields. However, all comments in the Via field are
2757   optional and &MAY; be removed by any recipient prior to forwarding the
2758   message.
2759</t>
2760<t>
2761   For example, a request message could be sent from an HTTP/1.0 user
2762   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2763   forward the request to a public proxy at p.example.net, which completes
2764   the request by forwarding it to the origin server at www.example.com.
2765   The request received by www.example.com would then have the following
2766   Via header field:
2767</t>
2768<figure><artwork type="example">
2769    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2770</artwork></figure>
2771<t>
2772   Proxies and gateways used as a portal through a network firewall
2773   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2774   the firewall region. This information &SHOULD; only be propagated if
2775   explicitly enabled. If not enabled, the received-by host of any host
2776   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2777   for that host.
2778</t>
2779<t>
2780   For organizations that have strong privacy requirements for hiding
2781   internal structures, a proxy &MAY; combine an ordered subsequence of
2782   Via header field entries with identical received-protocol values into
2783   a single such entry. For example,
2784</t>
2785<figure><artwork type="example">
2786    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2787</artwork></figure>
2788<t>
2789        could be collapsed to
2790</t>
2791<figure><artwork type="example">
2792    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2793</artwork></figure>
2794<t>
2795   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2796   under the same organizational control and the hosts have already been
2797   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2798   have different received-protocol values.
2799</t>
2800</section>
2801
2802</section>
2803
2804<section title="IANA Considerations" anchor="IANA.considerations">
2805<t>
2806   <cref>TBD.</cref>
2807</t>
2808</section>
2809
2810<section title="Security Considerations" anchor="security.considerations">
2811<t>
2812   This section is meant to inform application developers, information
2813   providers, and users of the security limitations in HTTP/1.1 as
2814   described by this document. The discussion does not include
2815   definitive solutions to the problems revealed, though it does make
2816   some suggestions for reducing security risks.
2817</t>
2818
2819<section title="Personal Information" anchor="personal.information">
2820<t>
2821   HTTP clients are often privy to large amounts of personal information
2822   (e.g. the user's name, location, mail address, passwords, encryption
2823   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2824   leakage of this information.
2825   We very strongly recommend that a convenient interface be provided
2826   for the user to control dissemination of such information, and that
2827   designers and implementors be particularly careful in this area.
2828   History shows that errors in this area often create serious security
2829   and/or privacy problems and generate highly adverse publicity for the
2830   implementor's company.
2831</t>
2832</section>
2833
2834<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2835<t>
2836   A server is in the position to save personal data about a user's
2837   requests which might identify their reading patterns or subjects of
2838   interest. This information is clearly confidential in nature and its
2839   handling can be constrained by law in certain countries. People using
2840   HTTP to provide data are responsible for ensuring that
2841   such material is not distributed without the permission of any
2842   individuals that are identifiable by the published results.
2843</t>
2844</section>
2845
2846<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2847<t>
2848   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2849   the documents returned by HTTP requests to be only those that were
2850   intended by the server administrators. If an HTTP server translates
2851   HTTP URIs directly into file system calls, the server &MUST; take
2852   special care not to serve files that were not intended to be
2853   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2854   other operating systems use ".." as a path component to indicate a
2855   directory level above the current one. On such a system, an HTTP
2856   server &MUST; disallow any such construct in the Request-URI if it
2857   would otherwise allow access to a resource outside those intended to
2858   be accessible via the HTTP server. Similarly, files intended for
2859   reference only internally to the server (such as access control
2860   files, configuration files, and script code) &MUST; be protected from
2861   inappropriate retrieval, since they might contain sensitive
2862   information. Experience has shown that minor bugs in such HTTP server
2863   implementations have turned into security risks.
2864</t>
2865</section>
2866
2867<section title="DNS Spoofing" anchor="dns.spoofing">
2868<t>
2869   Clients using HTTP rely heavily on the Domain Name Service, and are
2870   thus generally prone to security attacks based on the deliberate
2871   mis-association of IP addresses and DNS names. Clients need to be
2872   cautious in assuming the continuing validity of an IP number/DNS name
2873   association.
2874</t>
2875<t>
2876   In particular, HTTP clients &SHOULD; rely on their name resolver for
2877   confirmation of an IP number/DNS name association, rather than
2878   caching the result of previous host name lookups. Many platforms
2879   already can cache host name lookups locally when appropriate, and
2880   they &SHOULD; be configured to do so. It is proper for these lookups to
2881   be cached, however, only when the TTL (Time To Live) information
2882   reported by the name server makes it likely that the cached
2883   information will remain useful.
2884</t>
2885<t>
2886   If HTTP clients cache the results of host name lookups in order to
2887   achieve a performance improvement, they &MUST; observe the TTL
2888   information reported by DNS.
2889</t>
2890<t>
2891   If HTTP clients do not observe this rule, they could be spoofed when
2892   a previously-accessed server's IP address changes. As network
2893   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2894   possibility of this form of attack will grow. Observing this
2895   requirement thus reduces this potential security vulnerability.
2896</t>
2897<t>
2898   This requirement also improves the load-balancing behavior of clients
2899   for replicated servers using the same DNS name and reduces the
2900   likelihood of a user's experiencing failure in accessing sites which
2901   use that strategy.
2902</t>
2903</section>
2904
2905<section title="Proxies and Caching" anchor="attack.proxies">
2906<t>
2907   By their very nature, HTTP proxies are men-in-the-middle, and
2908   represent an opportunity for man-in-the-middle attacks. Compromise of
2909   the systems on which the proxies run can result in serious security
2910   and privacy problems. Proxies have access to security-related
2911   information, personal information about individual users and
2912   organizations, and proprietary information belonging to users and
2913   content providers. A compromised proxy, or a proxy implemented or
2914   configured without regard to security and privacy considerations,
2915   might be used in the commission of a wide range of potential attacks.
2916</t>
2917<t>
2918   Proxy operators should protect the systems on which proxies run as
2919   they would protect any system that contains or transports sensitive
2920   information. In particular, log information gathered at proxies often
2921   contains highly sensitive personal information, and/or information
2922   about organizations. Log information should be carefully guarded, and
2923   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2924</t>
2925<t>
2926   Proxy implementors should consider the privacy and security
2927   implications of their design and coding decisions, and of the
2928   configuration options they provide to proxy operators (especially the
2929   default configuration).
2930</t>
2931<t>
2932   Users of a proxy need to be aware that they are no trustworthier than
2933   the people who run the proxy; HTTP itself cannot solve this problem.
2934</t>
2935<t>
2936   The judicious use of cryptography, when appropriate, may suffice to
2937   protect against a broad range of security and privacy attacks. Such
2938   cryptography is beyond the scope of the HTTP/1.1 specification.
2939</t>
2940</section>
2941
2942<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2943<t>
2944   They exist. They are hard to defend against. Research continues.
2945   Beware.
2946</t>
2947</section>
2948</section>
2949
2950<section title="Acknowledgments" anchor="ack">
2951<t>
2952   This specification makes heavy use of the augmented BNF and generic
2953   constructs defined by David H. Crocker for <xref target="RFC822ABNF"/>. Similarly, it
2954   reuses many of the definitions provided by Nathaniel Borenstein and
2955   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2956   specification will help reduce past confusion over the relationship
2957   between HTTP and Internet mail message formats.
2958</t>
2959<t>
2960   HTTP has evolved considerably over the years. It has
2961   benefited from a large and active developer community--the many
2962   people who have participated on the www-talk mailing list--and it is
2963   that community which has been most responsible for the success of
2964   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2965   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2966   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2967   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2968   VanHeyningen deserve special recognition for their efforts in
2969   defining early aspects of the protocol.
2970</t>
2971<t>
2972   This document has benefited greatly from the comments of all those
2973   participating in the HTTP-WG. In addition to those already mentioned,
2974   the following individuals have contributed to this specification:
2975</t>
2976<t>
2977   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2978   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2979   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2980   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2981   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2982   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2983   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2984   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2985   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2986   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2987   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2988   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2989   Josh Cohen.
2990</t>
2991<t>
2992   Thanks to the "cave men" of Palo Alto. You know who you are.
2993</t>
2994<t>
2995   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2996   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2997   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2998   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2999   Larry Masinter for their help. And thanks go particularly to Jeff
3000   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3001</t>
3002<t>
3003   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3004   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3005   discovery of many of the problems that this document attempts to
3006   rectify.
3007</t>
3008</section>
3009
3010</middle>
3011<back>
3012
3013<references title="Normative References">
3014
3015<reference anchor="ISO-8859-1">
3016  <front>
3017    <title>
3018     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3019    </title>
3020    <author>
3021      <organization>International Organization for Standardization</organization>
3022    </author>
3023    <date year="1998"/>
3024  </front>
3025  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3026</reference>
3027
3028<reference anchor="Part2">
3029  <front>
3030    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3031    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3032      <organization abbrev="Day Software">Day Software</organization>
3033      <address><email>fielding@gbiv.com</email></address>
3034    </author>
3035    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3036      <organization>One Laptop per Child</organization>
3037      <address><email>jg@laptop.org</email></address>
3038    </author>
3039    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3040      <organization abbrev="HP">Hewlett-Packard Company</organization>
3041      <address><email>JeffMogul@acm.org</email></address>
3042    </author>
3043    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3044      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3045      <address><email>henrikn@microsoft.com</email></address>
3046    </author>
3047    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3048      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3049      <address><email>LMM@acm.org</email></address>
3050    </author>
3051    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3052      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3053      <address><email>paulle@microsoft.com</email></address>
3054    </author>
3055    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3056      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3057      <address><email>timbl@w3.org</email></address>
3058    </author>
3059    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3060      <organization abbrev="W3C">World Wide Web Consortium</organization>
3061      <address><email>ylafon@w3.org</email></address>
3062    </author>
3063    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3064      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3065      <address><email>julian.reschke@greenbytes.de</email></address>
3066    </author>
3067    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3068  </front>
3069  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3070  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3071</reference>
3072
3073<reference anchor="Part3">
3074  <front>
3075    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3076    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3077      <organization abbrev="Day Software">Day Software</organization>
3078      <address><email>fielding@gbiv.com</email></address>
3079    </author>
3080    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3081      <organization>One Laptop per Child</organization>
3082      <address><email>jg@laptop.org</email></address>
3083    </author>
3084    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3085      <organization abbrev="HP">Hewlett-Packard Company</organization>
3086      <address><email>JeffMogul@acm.org</email></address>
3087    </author>
3088    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3089      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3090      <address><email>henrikn@microsoft.com</email></address>
3091    </author>
3092    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3093      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3094      <address><email>LMM@acm.org</email></address>
3095    </author>
3096    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3097      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3098      <address><email>paulle@microsoft.com</email></address>
3099    </author>
3100    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3101      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3102      <address><email>timbl@w3.org</email></address>
3103    </author>
3104    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3105      <organization abbrev="W3C">World Wide Web Consortium</organization>
3106      <address><email>ylafon@w3.org</email></address>
3107    </author>
3108    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3109      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3110      <address><email>julian.reschke@greenbytes.de</email></address>
3111    </author>
3112    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3113  </front>
3114  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3115  <x:source href="p3-payload.xml" basename="p3-payload"/>
3116</reference>
3117
3118<reference anchor="Part5">
3119  <front>
3120    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3121    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3122      <organization abbrev="Day Software">Day Software</organization>
3123      <address><email>fielding@gbiv.com</email></address>
3124    </author>
3125    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3126      <organization>One Laptop per Child</organization>
3127      <address><email>jg@laptop.org</email></address>
3128    </author>
3129    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3130      <organization abbrev="HP">Hewlett-Packard Company</organization>
3131      <address><email>JeffMogul@acm.org</email></address>
3132    </author>
3133    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3134      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3135      <address><email>henrikn@microsoft.com</email></address>
3136    </author>
3137    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3138      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3139      <address><email>LMM@acm.org</email></address>
3140    </author>
3141    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3142      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3143      <address><email>paulle@microsoft.com</email></address>
3144    </author>
3145    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3146      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3147      <address><email>timbl@w3.org</email></address>
3148    </author>
3149    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3150      <organization abbrev="W3C">World Wide Web Consortium</organization>
3151      <address><email>ylafon@w3.org</email></address>
3152    </author>
3153    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3154      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3155      <address><email>julian.reschke@greenbytes.de</email></address>
3156    </author>
3157    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3158  </front>
3159  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3160  <x:source href="p5-range.xml" basename="p5-range"/>
3161</reference>
3162
3163<reference anchor="Part6">
3164  <front>
3165    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3166    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3167      <organization abbrev="Day Software">Day Software</organization>
3168      <address><email>fielding@gbiv.com</email></address>
3169    </author>
3170    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3171      <organization>One Laptop per Child</organization>
3172      <address><email>jg@laptop.org</email></address>
3173    </author>
3174    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3175      <organization abbrev="HP">Hewlett-Packard Company</organization>
3176      <address><email>JeffMogul@acm.org</email></address>
3177    </author>
3178    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3179      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3180      <address><email>henrikn@microsoft.com</email></address>
3181    </author>
3182    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3183      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3184      <address><email>LMM@acm.org</email></address>
3185    </author>
3186    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3187      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3188      <address><email>paulle@microsoft.com</email></address>
3189    </author>
3190    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3191      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3192      <address><email>timbl@w3.org</email></address>
3193    </author>
3194    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3195      <organization abbrev="W3C">World Wide Web Consortium</organization>
3196      <address><email>ylafon@w3.org</email></address>
3197    </author>
3198    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3199      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3200      <address><email>julian.reschke@greenbytes.de</email></address>
3201    </author>
3202    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3203  </front>
3204  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3205  <x:source href="p6-cache.xml" basename="p6-cache"/>
3206</reference>
3207
3208<reference anchor="RFC822ABNF">
3209  <front>
3210    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3211    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3212      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3213      <address><email>DCrocker@UDel-Relay</email></address>
3214    </author>
3215    <date month="August" day="13" year="1982"/>
3216  </front>
3217  <seriesInfo name="STD" value="11"/>
3218  <seriesInfo name="RFC" value="822"/>
3219</reference>
3220
3221<reference anchor="RFC2045">
3222  <front>
3223    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3224    <author initials="N." surname="Freed" fullname="Ned Freed">
3225      <organization>Innosoft International, Inc.</organization>
3226      <address><email>ned@innosoft.com</email></address>
3227    </author>
3228    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3229      <organization>First Virtual Holdings</organization>
3230      <address><email>nsb@nsb.fv.com</email></address>
3231    </author>
3232    <date month="November" year="1996"/>
3233  </front>
3234  <seriesInfo name="RFC" value="2045"/>
3235</reference>
3236
3237<reference anchor="RFC2047">
3238  <front>
3239    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3240    <author initials="K." surname="Moore" fullname="Keith Moore">
3241      <organization>University of Tennessee</organization>
3242      <address><email>moore@cs.utk.edu</email></address>
3243    </author>
3244    <date month="November" year="1996"/>
3245  </front>
3246  <seriesInfo name="RFC" value="2047"/>
3247</reference>
3248
3249<reference anchor="RFC2119">
3250  <front>
3251    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3252    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3253      <organization>Harvard University</organization>
3254      <address><email>sob@harvard.edu</email></address>
3255    </author>
3256    <date month="March" year="1997"/>
3257  </front>
3258  <seriesInfo name="BCP" value="14"/>
3259  <seriesInfo name="RFC" value="2119"/>
3260</reference>
3261
3262<reference anchor="RFC2396">
3263  <front>
3264    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3265    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3266      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3267      <address><email>timbl@w3.org</email></address>
3268    </author>
3269    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3270      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3271      <address><email>fielding@ics.uci.edu</email></address>
3272    </author>
3273    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3274      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3275      <address><email>masinter@parc.xerox.com</email></address>
3276    </author>
3277    <date month="August" year="1998"/>
3278  </front>
3279  <seriesInfo name="RFC" value="2396"/>
3280</reference>
3281
3282<reference anchor="USASCII">
3283  <front>
3284    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3285    <author>
3286      <organization>American National Standards Institute</organization>
3287    </author>
3288    <date year="1986"/>
3289  </front>
3290  <seriesInfo name="ANSI" value="X3.4"/>
3291</reference>
3292
3293</references>
3294
3295<references title="Informative References">
3296
3297<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3298  <front>
3299    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3300    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3301      <organization/>
3302    </author>
3303    <author initials="J." surname="Gettys" fullname="J. Gettys">
3304      <organization/>
3305    </author>
3306    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3307      <organization/>
3308    </author>
3309    <author initials="H." surname="Lie" fullname="H. Lie">
3310      <organization/>
3311    </author>
3312    <author initials="C." surname="Lilley" fullname="C. Lilley">
3313      <organization/>
3314    </author>
3315    <date year="1997" month="September"/>
3316  </front>
3317  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3318</reference>
3319
3320<reference anchor="Pad1995">
3321  <front>
3322    <title>Improving HTTP Latency</title>
3323    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3324      <organization/>
3325    </author>
3326    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3327      <organization/>
3328    </author>
3329    <date year="1995" month="December"/>
3330  </front>
3331  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3332  <annotation>
3333    Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3334    which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3335  </annotation>
3336</reference>
3337
3338<reference anchor="RFC822">
3339  <front>
3340    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3341    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3342      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3343      <address><email>DCrocker@UDel-Relay</email></address>
3344    </author>
3345    <date month="August" day="13" year="1982"/>
3346  </front>
3347  <seriesInfo name="STD" value="11"/>
3348  <seriesInfo name="RFC" value="822"/>
3349</reference>
3350
3351<reference anchor="RFC959">
3352  <front>
3353    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3354    <author initials="J." surname="Postel" fullname="J. Postel">
3355      <organization>Information Sciences Institute (ISI)</organization>
3356    </author>
3357    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3358      <organization/>
3359    </author>
3360    <date month="October" year="1985"/>
3361  </front>
3362  <seriesInfo name="STD" value="9"/>
3363  <seriesInfo name="RFC" value="959"/>
3364</reference>
3365
3366<reference anchor="RFC1123">
3367  <front>
3368    <title>Requirements for Internet Hosts - Application and Support</title>
3369    <author initials="R." surname="Braden" fullname="Robert Braden">
3370      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3371      <address><email>Braden@ISI.EDU</email></address>
3372    </author>
3373    <date month="October" year="1989"/>
3374  </front>
3375  <seriesInfo name="STD" value="3"/>
3376  <seriesInfo name="RFC" value="1123"/>
3377</reference>
3378
3379<reference anchor="RFC1305">
3380  <front>
3381    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3382    <author initials="D." surname="Mills" fullname="David L. Mills">
3383      <organization>University of Delaware, Electrical Engineering Department</organization>
3384      <address><email>mills@udel.edu</email></address>
3385    </author>
3386    <date month="March" year="1992"/>
3387  </front>
3388  <seriesInfo name="RFC" value="1305"/>
3389</reference>
3390
3391<reference anchor="RFC1436">
3392  <front>
3393    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3394    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3395      <organization>University of Minnesota, Computer and Information Services</organization>
3396      <address><email>fxa@boombox.micro.umn.edu</email></address>
3397    </author>
3398    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3399      <organization>University of Minnesota, Computer and Information Services</organization>
3400      <address><email>mpm@boombox.micro.umn.edu</email></address>
3401    </author>
3402    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3403      <organization>University of Minnesota, Computer and Information Services</organization>
3404      <address><email>lindner@boombox.micro.umn.edu</email></address>
3405    </author>
3406    <author initials="D." surname="Johnson" fullname="David Johnson">
3407      <organization>University of Minnesota, Computer and Information Services</organization>
3408      <address><email>dmj@boombox.micro.umn.edu</email></address>
3409    </author>
3410    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3411      <organization>University of Minnesota, Computer and Information Services</organization>
3412      <address><email>daniel@boombox.micro.umn.edu</email></address>
3413    </author>
3414    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3415      <organization>University of Minnesota, Computer and Information Services</organization>
3416      <address><email>alberti@boombox.micro.umn.edu</email></address>
3417    </author>
3418    <date month="March" year="1993"/>
3419  </front>
3420  <seriesInfo name="RFC" value="1436"/>
3421</reference>
3422
3423<reference anchor="RFC1630">
3424  <front>
3425    <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>
3426    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3427      <organization>CERN, World-Wide Web project</organization>
3428      <address><email>timbl@info.cern.ch</email></address>
3429    </author>
3430    <date month="June" year="1994"/>
3431  </front>
3432  <seriesInfo name="RFC" value="1630"/>
3433</reference>
3434
3435<reference anchor="RFC1737">
3436  <front>
3437    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3438    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3439      <organization>Xerox Palo Alto Research Center</organization>
3440      <address><email>masinter@parc.xerox.com</email></address>
3441    </author>
3442    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3443      <organization>MIT Laboratory for Computer Science</organization>
3444      <address><email>sollins@lcs.mit.edu</email></address>
3445    </author>
3446    <date month="December" year="1994"/>
3447  </front>
3448  <seriesInfo name="RFC" value="1737"/>
3449</reference>
3450
3451<reference anchor="RFC1738">
3452  <front>
3453    <title>Uniform Resource Locators (URL)</title>
3454    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3455      <organization>CERN, World-Wide Web project</organization>
3456      <address><email>timbl@info.cern.ch</email></address>
3457    </author>
3458    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3459      <organization>Xerox PARC</organization>
3460      <address><email>masinter@parc.xerox.com</email></address>
3461    </author>
3462    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3463      <organization>University of Minnesota, Computer and Information Services</organization>
3464      <address><email>mpm@boombox.micro.umn.edu</email></address>
3465    </author>
3466    <date month="December" year="1994"/>
3467  </front>
3468  <seriesInfo name="RFC" value="1738"/>
3469</reference>
3470
3471<reference anchor="RFC1808">
3472  <front>
3473    <title>Relative Uniform Resource Locators</title>
3474    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3475      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3476      <address><email>fielding@ics.uci.edu</email></address>
3477    </author>
3478    <date month="June" year="1995"/>
3479  </front>
3480  <seriesInfo name="RFC" value="1808"/>
3481</reference>
3482
3483<reference anchor="RFC1900">
3484  <front>
3485    <title>Renumbering Needs Work</title>
3486    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3487      <organization>CERN, Computing and Networks Division</organization>
3488      <address><email>brian@dxcoms.cern.ch</email></address>
3489    </author>
3490    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3491      <organization>cisco Systems</organization>
3492      <address><email>yakov@cisco.com</email></address>
3493    </author>
3494    <date month="February" year="1996"/>
3495  </front>
3496  <seriesInfo name="RFC" value="1900"/>
3497</reference>
3498
3499<reference anchor="RFC1945">
3500  <front>
3501    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3502    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3503      <organization>MIT, Laboratory for Computer Science</organization>
3504      <address><email>timbl@w3.org</email></address>
3505    </author>
3506    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3507      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3508      <address><email>fielding@ics.uci.edu</email></address>
3509    </author>
3510    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3511      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3512      <address><email>frystyk@w3.org</email></address>
3513    </author>
3514    <date month="May" year="1996"/>
3515  </front>
3516  <seriesInfo name="RFC" value="1945"/>
3517</reference>
3518
3519<reference anchor="RFC2068">
3520  <front>
3521    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3522    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3523      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3524      <address><email>fielding@ics.uci.edu</email></address>
3525    </author>
3526    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3527      <organization>MIT Laboratory for Computer Science</organization>
3528      <address><email>jg@w3.org</email></address>
3529    </author>
3530    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3531      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3532      <address><email>mogul@wrl.dec.com</email></address>
3533    </author>
3534    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3535      <organization>MIT Laboratory for Computer Science</organization>
3536      <address><email>frystyk@w3.org</email></address>
3537    </author>
3538    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3539      <organization>MIT Laboratory for Computer Science</organization>
3540      <address><email>timbl@w3.org</email></address>
3541    </author>
3542    <date month="January" year="1997"/>
3543  </front>
3544  <seriesInfo name="RFC" value="2068"/>
3545</reference>
3546
3547<reference anchor="RFC2145">
3548  <front>
3549    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3550    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3551      <organization>Western Research Laboratory</organization>
3552      <address><email>mogul@wrl.dec.com</email></address>
3553    </author>
3554    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3555      <organization>Department of Information and Computer Science</organization>
3556      <address><email>fielding@ics.uci.edu</email></address>
3557    </author>
3558    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3559      <organization>MIT Laboratory for Computer Science</organization>
3560      <address><email>jg@w3.org</email></address>
3561    </author>
3562    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3563      <organization>W3 Consortium</organization>
3564      <address><email>frystyk@w3.org</email></address>
3565    </author>
3566    <date month="May" year="1997"/>
3567  </front>
3568  <seriesInfo name="RFC" value="2145"/>
3569</reference>
3570
3571<reference anchor="RFC2324">
3572  <front>
3573    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3574    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3575      <organization>Xerox Palo Alto Research Center</organization>
3576      <address><email>masinter@parc.xerox.com</email></address>
3577    </author>
3578    <date month="April" day="1" year="1998"/>
3579  </front>
3580  <seriesInfo name="RFC" value="2324"/>
3581</reference>
3582
3583<reference anchor="RFC2616">
3584  <front>
3585    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3586    <author initials="R." surname="Fielding" fullname="R. Fielding">
3587      <organization>University of California, Irvine</organization>
3588      <address><email>fielding@ics.uci.edu</email></address>
3589    </author>
3590    <author initials="J." surname="Gettys" fullname="J. Gettys">
3591      <organization>W3C</organization>
3592      <address><email>jg@w3.org</email></address>
3593    </author>
3594    <author initials="J." surname="Mogul" fullname="J. Mogul">
3595      <organization>Compaq Computer Corporation</organization>
3596      <address><email>mogul@wrl.dec.com</email></address>
3597    </author>
3598    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3599      <organization>MIT Laboratory for Computer Science</organization>
3600      <address><email>frystyk@w3.org</email></address>
3601    </author>
3602    <author initials="L." surname="Masinter" fullname="L. Masinter">
3603      <organization>Xerox Corporation</organization>
3604      <address><email>masinter@parc.xerox.com</email></address>
3605    </author>
3606    <author initials="P." surname="Leach" fullname="P. Leach">
3607      <organization>Microsoft Corporation</organization>
3608      <address><email>paulle@microsoft.com</email></address>
3609    </author>
3610    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3611      <organization>W3C</organization>
3612      <address><email>timbl@w3.org</email></address>
3613    </author>
3614    <date month="June" year="1999"/>
3615  </front>
3616  <seriesInfo name="RFC" value="2616"/>
3617</reference>
3618
3619<reference anchor="RFC2821">
3620  <front>
3621    <title>Simple Mail Transfer Protocol</title>
3622    <author initials="J." surname="Klensin" fullname="J. Klensin">
3623      <organization>AT&amp;T Laboratories</organization>
3624      <address><email>klensin@research.att.com</email></address>
3625    </author>
3626    <date year="2001" month="April"/>
3627  </front>
3628  <seriesInfo name="RFC" value="2821"/>
3629</reference>
3630
3631<reference anchor="RFC2822">
3632  <front>
3633    <title>Internet Message Format</title>
3634    <author initials="P." surname="Resnick" fullname="P. Resnick">
3635      <organization>QUALCOMM Incorporated</organization>
3636    </author>
3637    <date year="2001" month="April"/>
3638  </front> 
3639  <seriesInfo name="RFC" value="2822"/>
3640</reference>
3641
3642<reference anchor='RFC3977'>
3643  <front>
3644    <title>Network News Transfer Protocol (NNTP)</title>
3645    <author initials='C.' surname='Feather' fullname='C. Feather'>
3646      <organization>THUS plc</organization>
3647      <address><email>clive@demon.net</email></address>
3648    </author>
3649    <date year='2006' month='October' />
3650  </front>
3651  <seriesInfo name="RFC" value="3977"/>
3652</reference>
3653
3654<reference anchor="RFC4288">
3655  <front>
3656    <title>Media Type Specifications and Registration Procedures</title>
3657    <author initials="N." surname="Freed" fullname="N. Freed">
3658      <organization>Sun Microsystems</organization>
3659      <address>
3660        <email>ned.freed@mrochek.com</email>
3661      </address>
3662    </author>
3663    <author initials="J." surname="Klensin" fullname="J. Klensin">
3664      <organization/>
3665      <address>
3666        <email>klensin+ietf@jck.com</email>
3667      </address>
3668    </author>
3669    <date year="2005" month="December"/>
3670  </front>
3671  <seriesInfo name="BCP" value="13"/>
3672  <seriesInfo name="RFC" value="4288"/>
3673</reference>
3674
3675<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3676  <front>
3677  <title>Analysis of HTTP Performance Problems</title>
3678  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3679    <organization/>
3680  </author>
3681  <date/>
3682  </front>
3683</reference>
3684
3685<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3686  <front>
3687  <title>Analysis of HTTP Performance</title>
3688  <author initials="J." surname="Touch" fullname="Joe Touch">
3689    <organization>USC/Information Sciences Institute</organization>
3690    <address><email>touch@isi.edu</email></address>
3691  </author>
3692  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3693    <organization>USC/Information Sciences Institute</organization>
3694    <address><email>johnh@isi.edu</email></address>
3695  </author>
3696  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3697    <organization>USC/Information Sciences Institute</organization>
3698    <address><email>katia@isi.edu</email></address>
3699  </author>
3700  <date year="1998" month="Aug"/>
3701  </front>
3702  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3703  <annotation>(original report dated Aug. 1996)</annotation>
3704</reference>
3705
3706<reference anchor="WAIS">
3707  <front>
3708    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3709    <author initials="F." surname="Davis" fullname="F. Davis">
3710      <organization>Thinking Machines Corporation</organization>
3711    </author>
3712    <author initials="B." surname="Kahle" fullname="B. Kahle">
3713      <organization>Thinking Machines Corporation</organization>
3714    </author>
3715    <author initials="H." surname="Morris" fullname="H. Morris">
3716      <organization>Thinking Machines Corporation</organization>
3717    </author>
3718    <author initials="J." surname="Salem" fullname="J. Salem">
3719      <organization>Thinking Machines Corporation</organization>
3720    </author>
3721    <author initials="T." surname="Shen" fullname="T. Shen">
3722      <organization>Thinking Machines Corporation</organization>
3723    </author>
3724    <author initials="R." surname="Wang" fullname="R. Wang">
3725      <organization>Thinking Machines Corporation</organization>
3726    </author>
3727    <author initials="J." surname="Sui" fullname="J. Sui">
3728      <organization>Thinking Machines Corporation</organization>
3729    </author>
3730    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3731      <organization>Thinking Machines Corporation</organization>
3732    </author>
3733    <date month="April" year="1990"/>
3734  </front>
3735  <seriesInfo name="Thinking Machines Corporation" value=""/>
3736</reference>
3737
3738</references>
3739
3740
3741<section title="Internet Media Types" anchor="internet.media.type.http">
3742<t>
3743   In addition to defining HTTP/1.1, this document serves
3744   as the specification for the Internet media type "message/http" and
3745   "application/http". The following is to be registered with IANA <xref target="RFC4288"/>.
3746</t>
3747<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3748<iref item="Media Type" subitem="message/http" primary="true"/>
3749<iref item="message/http Media Type" primary="true"/>
3750<t>
3751   The message/http type can be used to enclose a single HTTP request or
3752   response message, provided that it obeys the MIME restrictions for all
3753   "message" types regarding line length and encodings.
3754</t>
3755<t>
3756  <list style="hanging" x:indent="12em">
3757    <t hangText="Type name:">
3758      message
3759    </t>
3760    <t hangText="Subtype name:">
3761      http
3762    </t>
3763    <t hangText="Required parameters:">
3764      none
3765    </t>
3766    <t hangText="Optional parameters:">
3767      version, msgtype
3768      <list style="hanging">
3769        <t hangText="version:">
3770          The HTTP-Version number of the enclosed message
3771          (e.g., "1.1"). If not present, the version can be
3772          determined from the first line of the body.
3773        </t>
3774        <t hangText="msgtype:">
3775          The message type -- "request" or "response". If not
3776          present, the type can be determined from the first
3777          line of the body.
3778        </t>
3779      </list>
3780    </t>
3781    <t hangText="Encoding considerations:">
3782      only "7bit", "8bit", or "binary" are permitted
3783    </t>
3784    <t hangText="Security considerations:">
3785      none
3786    </t>
3787    <t hangText="Interoperability considerations:">
3788      none
3789    </t>
3790    <t hangText="Published specification:">
3791      This specification (see <xref target="internet.media.type.message.http"/>).
3792    </t>
3793    <t hangText="Applications that use this media type:">
3794    </t>
3795    <t hangText="Additional information:">
3796      <list style="hanging">
3797        <t hangText="Magic number(s):">none</t>
3798        <t hangText="File extension(s):">none</t>
3799        <t hangText="Macintosh file type code(s):">none</t>
3800      </list>
3801    </t>
3802    <t hangText="Person and email address to contact for further information:">
3803      See Authors Section.
3804    </t>
3805                <t hangText="Intended usage:">
3806                  COMMON
3807    </t>
3808                <t hangText="Restrictions on usage:">
3809                  none
3810    </t>
3811    <t hangText="Author/Change controller:">
3812      IESG
3813    </t>
3814  </list>
3815</t>
3816</section>
3817<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3818<iref item="Media Type" subitem="application/http" primary="true"/>
3819<iref item="application/http Media Type" primary="true"/>
3820<t>
3821   The application/http type can be used to enclose a pipeline of one or more
3822   HTTP request or response messages (not intermixed).
3823</t>
3824<t>
3825  <list style="hanging" x:indent="12em">
3826    <t hangText="Type name:">
3827      application
3828    </t>
3829    <t hangText="Subtype name:">
3830      http
3831    </t>
3832    <t hangText="Required parameters:">
3833      none
3834    </t>
3835    <t hangText="Optional parameters:">
3836      version, msgtype
3837      <list style="hanging">
3838        <t hangText="version:">
3839          The HTTP-Version number of the enclosed messages
3840          (e.g., "1.1"). If not present, the version can be
3841          determined from the first line of the body.
3842        </t>
3843        <t hangText="msgtype:">
3844          The message type -- "request" or "response". If not
3845          present, the type can be determined from the first
3846          line of the body.
3847        </t>
3848      </list>
3849    </t>
3850    <t hangText="Encoding considerations:">
3851      HTTP messages enclosed by this type
3852      are in "binary" format; use of an appropriate
3853      Content-Transfer-Encoding is required when
3854      transmitted via E-mail.
3855    </t>
3856    <t hangText="Security considerations:">
3857      none
3858    </t>
3859    <t hangText="Interoperability considerations:">
3860      none
3861    </t>
3862    <t hangText="Published specification:">
3863      This specification (see <xref target="internet.media.type.application.http"/>).
3864    </t>
3865    <t hangText="Applications that use this media type:">
3866    </t>
3867    <t hangText="Additional information:">
3868      <list style="hanging">
3869        <t hangText="Magic number(s):">none</t>
3870        <t hangText="File extension(s):">none</t>
3871        <t hangText="Macintosh file type code(s):">none</t>
3872      </list>
3873    </t>
3874    <t hangText="Person and email address to contact for further information:">
3875      See Authors Section.
3876    </t>
3877                <t hangText="Intended usage:">
3878                  COMMON
3879    </t>
3880                <t hangText="Restrictions on usage:">
3881                  none
3882    </t>
3883    <t hangText="Author/Change controller:">
3884      IESG
3885    </t>
3886  </list>
3887</t>
3888</section>
3889</section>
3890
3891<section title="Tolerant Applications" anchor="tolerant.applications">
3892<t>
3893   Although this document specifies the requirements for the generation
3894   of HTTP/1.1 messages, not all applications will be correct in their
3895   implementation. We therefore recommend that operational applications
3896   be tolerant of deviations whenever those deviations can be
3897   interpreted unambiguously.
3898</t>
3899<t>
3900   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3901   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3902   accept any amount of SP or HTAB characters between fields, even though
3903   only a single SP is required.
3904</t>
3905<t>
3906   The line terminator for message-header fields is the sequence CRLF.
3907   However, we recommend that applications, when parsing such headers,
3908   recognize a single LF as a line terminator and ignore the leading CR.
3909</t>
3910<t>
3911   The character set of an entity-body &SHOULD; be labeled as the lowest
3912   common denominator of the character codes used within that body, with
3913   the exception that not labeling the entity is preferred over labeling
3914   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3915</t>
3916<t>
3917   Additional rules for requirements on parsing and encoding of dates
3918   and other potential problems with date encodings include:
3919</t>
3920<t>
3921  <list style="symbols">
3922     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3923        which appears to be more than 50 years in the future is in fact
3924        in the past (this helps solve the "year 2000" problem).</t>
3925
3926     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3927        Expires date as earlier than the proper value, but &MUST-NOT;
3928        internally represent a parsed Expires date as later than the
3929        proper value.</t>
3930
3931     <t>All expiration-related calculations &MUST; be done in GMT. The
3932        local time zone &MUST-NOT; influence the calculation or comparison
3933        of an age or expiration time.</t>
3934
3935     <t>If an HTTP header incorrectly carries a date value with a time
3936        zone other than GMT, it &MUST; be converted into GMT using the
3937        most conservative possible conversion.</t>
3938  </list>
3939</t>
3940</section>
3941
3942<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3943<t>
3944   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3945   simplify the process of date comparison. Proxies and gateways from
3946   other protocols &SHOULD; ensure that any Date header field present in a
3947   message conforms to one of the HTTP/1.1 formats and rewrite the date
3948   if necessary.
3949</t>
3950</section>
3951
3952<section title="Compatibility with Previous Versions" anchor="compatibility">
3953<t>
3954   It is beyond the scope of a protocol specification to mandate
3955   compliance with previous versions. HTTP/1.1 was deliberately
3956   designed, however, to make supporting previous versions easy. It is
3957   worth noting that, at the time of composing this specification
3958   (1996), we would expect commercial HTTP/1.1 servers to:
3959  <list style="symbols">
3960     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3961        1.1 requests;</t>
3962
3963     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3964        1.1;</t>
3965
3966     <t>respond appropriately with a message in the same major version
3967        used by the client.</t>
3968  </list>
3969</t>
3970<t>
3971   And we would expect HTTP/1.1 clients to:
3972  <list style="symbols">
3973     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3974        responses;</t>
3975
3976     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3977        1.1.</t>
3978  </list>
3979</t>
3980<t>
3981   For most implementations of HTTP/1.0, each connection is established
3982   by the client prior to the request and closed by the server after
3983   sending the response. Some implementations implement the Keep-Alive
3984   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3985</t>
3986
3987<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3988<t>
3989   This section summarizes major differences between versions HTTP/1.0
3990   and HTTP/1.1.
3991</t>
3992
3993<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">
3994<t>
3995   The requirements that clients and servers support the Host request-header,
3996   report an error if the Host request-header (<xref target="header.host"/>) is
3997   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3998   are among the most important changes defined by this
3999   specification.
4000</t>
4001<t>
4002   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4003   addresses and servers; there was no other established mechanism for
4004   distinguishing the intended server of a request than the IP address
4005   to which that request was directed. The changes outlined above will
4006   allow the Internet, once older HTTP clients are no longer common, to
4007   support multiple Web sites from a single IP address, greatly
4008   simplifying large operational Web servers, where allocation of many
4009   IP addresses to a single host has created serious problems. The
4010   Internet will also be able to recover the IP addresses that have been
4011   allocated for the sole purpose of allowing special-purpose domain
4012   names to be used in root-level HTTP URLs. Given the rate of growth of
4013   the Web, and the number of servers already deployed, it is extremely
4014   important that all implementations of HTTP (including updates to
4015   existing HTTP/1.0 applications) correctly implement these
4016   requirements:
4017  <list style="symbols">
4018     <t>Both clients and servers &MUST; support the Host request-header.</t>
4019
4020     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4021
4022     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4023        request does not include a Host request-header.</t>
4024
4025     <t>Servers &MUST; accept absolute URIs.</t>
4026  </list>
4027</t>
4028</section>
4029</section>
4030
4031<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4032<t>
4033   Some clients and servers might wish to be compatible with some
4034   previous implementations of persistent connections in HTTP/1.0
4035   clients and servers. Persistent connections in HTTP/1.0 are
4036   explicitly negotiated as they are not the default behavior. HTTP/1.0
4037   experimental implementations of persistent connections are faulty,
4038   and the new facilities in HTTP/1.1 are designed to rectify these
4039   problems. The problem was that some existing 1.0 clients may be
4040   sending Keep-Alive to a proxy server that doesn't understand
4041   Connection, which would then erroneously forward it to the next
4042   inbound server, which would establish the Keep-Alive connection and
4043   result in a hung HTTP/1.0 proxy waiting for the close on the
4044   response. The result is that HTTP/1.0 clients must be prevented from
4045   using Keep-Alive when talking to proxies.
4046</t>
4047<t>
4048   However, talking to proxies is the most important use of persistent
4049   connections, so that prohibition is clearly unacceptable. Therefore,
4050   we need some other mechanism for indicating a persistent connection
4051   is desired, which is safe to use even when talking to an old proxy
4052   that ignores Connection. Persistent connections are the default for
4053   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4054   declaring non-persistence. See <xref target="header.connection"/>.
4055</t>
4056<t>
4057   The original HTTP/1.0 form of persistent connections (the Connection:
4058   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4059</t>
4060</section>
4061
4062<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4063<t>
4064   This specification has been carefully audited to correct and
4065   disambiguate key word usage; RFC 2068 had many problems in respect to
4066   the conventions laid out in <xref target="RFC2119"/>.
4067</t>
4068<t>
4069   Transfer-coding and message lengths all interact in ways that
4070   required fixing exactly when chunked encoding is used (to allow for
4071   transfer encoding that may not be self delimiting); it was important
4072   to straighten out exactly how message lengths are computed. (Sections
4073   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4074   <xref target="header.content-length" format="counter"/>,
4075   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4076</t>
4077<t>
4078   The use and interpretation of HTTP version numbers has been clarified
4079   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4080   version they support to deal with problems discovered in HTTP/1.0
4081   implementations (<xref target="http.version"/>)
4082</t>
4083<t>
4084   Transfer-coding had significant problems, particularly with
4085   interactions with chunked encoding. The solution is that transfer-codings
4086   become as full fledged as content-codings. This involves
4087   adding an IANA registry for transfer-codings (separate from content
4088   codings), a new header field (TE) and enabling trailer headers in the
4089   future. Transfer encoding is a major performance benefit, so it was
4090   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4091   interoperability problem that could have occurred due to interactions
4092   between authentication trailers, chunked encoding and HTTP/1.0
4093   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4094   and <xref target="header.te" format="counter"/>)
4095</t>
4096</section>
4097
4098<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4099<t>
4100  The CHAR rule does not allow the NUL character anymore (this affects
4101  the comment and quoted-string rules).
4102  (<xref target="basic.rules"/>)
4103</t>
4104<t>
4105  Clarify that HTTP-Version is case sensitive.
4106  (<xref target="http.version"/>)
4107</t>
4108<t>
4109  Remove reference to non-existant identity transfer-coding value tokens.
4110  (Sections <xref format="counter" target="transfer.codings"/> and
4111  <xref format="counter" target="message.length"/>)
4112</t>
4113<t>
4114  Clarification that the chunk length does not include
4115  the count of the octets in the chunk header and trailer.
4116  (<xref target="chunked.transfer.encoding"/>)
4117</t>
4118<t>
4119  Fix BNF to add query, as the abs_path production in
4120  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4121  (<xref target="request-uri"/>)
4122</t>
4123<t>
4124  Clarify exactly when close connection options must be sent.
4125  (<xref target="header.connection"/>)
4126</t>
4127</section>
4128</section>
4129
4130<section title="Change Log (to be removed by RFC Editor before publication)">
4131
4132<section title="Since RFC2616">
4133<t>
4134  Extracted relevant partitions from <xref target="RFC2616"/>.
4135</t>
4136</section>
4137
4138<section title="Since draft-ietf-httpbis-p1-messaging-00">
4139<t>
4140  Closed issues:
4141  <list style="symbols"> 
4142    <t>
4143      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4144      "HTTP Version should be case sensitive"
4145      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4146    </t>
4147    <t>
4148      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4149      "'unsafe' characters"
4150      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4151    </t>
4152    <t>
4153      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4154      "Chunk Size Definition"
4155      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4156    </t>
4157    <t>
4158      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4159      "Message Length"
4160      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4161    </t>
4162    <t>
4163      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4164      "Media Type Registrations"
4165      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4166    </t>
4167    <t>
4168      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4169      "URI includes query"
4170      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4171    </t>
4172    <t>
4173      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4174      "No close on 1xx responses"
4175      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4176    </t>
4177    <t>
4178      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4179      "Remove 'identity' token references"
4180      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4181    </t>
4182    <t>
4183      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4184      "Import query BNF"
4185    </t>
4186    <t>
4187      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4188      "qdtext BNF"
4189    </t>
4190    <t>
4191      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4192      "Normative and Informative references"
4193    </t>
4194    <t>
4195      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4196      "RFC2606 Compliance"
4197    </t>
4198    <t>
4199      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4200      "RFC977 reference"
4201    </t>
4202    <t>
4203      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4204      "RFC1700 references"
4205    </t>
4206    <t>
4207      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4208      "inconsistency in date format explanation"
4209    </t>
4210    <t>
4211      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4212      "Date reference typo"
4213    </t>
4214    <t>
4215      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4216      "Informative references"
4217    </t>
4218    <t>
4219      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4220      "ISO-8859-1 Reference"
4221    </t>
4222    <t>
4223      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4224      "Normative up-to-date references"
4225    </t>
4226  </list>
4227</t>
4228<t>
4229  Other changes:
4230  <list style="symbols"> 
4231    <t>
4232      Update media type registrations to use RFC4288 template.
4233    </t>
4234    <t>
4235      Use names of RFC4234 core rules DQUOTE and HTAB,
4236      fix broken ABNF for chunk-data
4237      (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4238    </t>
4239  </list>
4240</t>
4241</section>
4242
4243<section title="Since draft-ietf-httpbis-p1-messaging-01">
4244<t>
4245  Closed issues:
4246  <list style="symbols"> 
4247    <t>
4248      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4249      "Bodies on GET (and other) requests"
4250    </t>
4251    <t>
4252      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4253      "Updating to RFC4288"
4254    </t>
4255    <t>
4256      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4257      "Status Code and Reason Phrase"
4258    </t>
4259    <t>
4260      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4261      "rel_path not used"
4262    </t>
4263  </list>
4264</t>
4265<t>
4266  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4267  <list style="symbols"> 
4268    <t>
4269      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4270      "trailer-part").
4271    </t>
4272    <t>
4273      Avoid underscore character in rule names ("http_URL" ->
4274      "http-URL", "abs_path" -> "path-absolute").
4275    </t>
4276    <t>
4277      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4278      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4279      have to be updated when switching over to RFC3986.
4280    </t>
4281    <t>
4282      Synchronize core rules with RFC5234 (this includes a change to CHAR
4283      which now excludes NUL).
4284    </t>
4285    <t>
4286      Get rid of prose rules that span multiple lines.
4287    </t>
4288    <t>
4289      Get rid of unused rules LOALPHA and UPALPHA.
4290    </t>
4291    <t>
4292      Move "Product Tokens" section (back) into Part 1, as "token" is used
4293      in the definition of the Upgrade header.
4294    </t>
4295    <t>
4296      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4297    </t>
4298    <t>
4299      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4300    </t>
4301  </list>
4302</t>
4303</section>
4304
4305</section>
4306
4307</back>
4308</rfc>
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