source: draft-ietf-httpbis/latest/p1-messaging.xml @ 215

Last change on this file since 215 was 215, checked in by julian.reschke@…, 12 years ago

Rewrite "TEXT" rule, not using prose; relates to #36.

  • Property svn:eol-style set to native
File size: 182.7 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "February">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY entity-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x='http://purl.org/net/xml2rfc/ext'>
56<front>
57
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
59
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email>fielding@gbiv.com</email>
73      <uri>http://roy.gbiv.com/</uri>
74    </address>
75  </author>
76
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email>jg@laptop.org</email>
88      <uri>http://www.laptop.org/</uri>
89    </address>
90  </author>
91 
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email>JeffMogul@acm.org</email>
104    </address>
105  </author>
106
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email>henrikn@microsoft.com</email>
118    </address>
119  </author>
120
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email>LMM@acm.org</email>
132      <uri>http://larry.masinter.net/</uri>
133    </address>
134  </author>
135 
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email>paulle@microsoft.com</email>
146    </address>
147  </author>
148   
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email>timbl@w3.org</email>
162      <uri>http://www.w3.org/People/Berners-Lee/</uri>
163    </address>
164  </author>
165
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email>ylafon@w3.org</email>
178      <uri>http://www.raubacapeu.net/people/yves/</uri>
179    </address>
180  </author>
181
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email>julian.reschke@greenbytes.de</email>       
193      <uri>http://greenbytes.de/tech/webdav/</uri>     
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
198
199<abstract>
200<t>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    This draft incorporates those issue resolutions that were either
224    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
225    or which were agreed upon on the mailing list between October 2006 and
226    November 2007 (as published in "draft-lafon-rfc2616bis-03").
227  </t>
228</note>
229</front>
230<middle>
231<section title="Introduction" anchor="introduction">
232<t>
233   The Hypertext Transfer Protocol (HTTP) is an application-level
234   protocol for distributed, collaborative, hypermedia information
235   systems. HTTP has been in use by the World-Wide Web global
236   information initiative since 1990. The first version of HTTP, commonly
237   referred to as HTTP/0.9, was a simple protocol for raw data transfer
238   across the Internet with only a single method and no metadata.
239   HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
240   the protocol by allowing messages to be in the format of MIME-like
241   messages, containing metadata about the data transferred and
242   modifiers on the request/response semantics. However, HTTP/1.0 did
243   not sufficiently take into consideration the effects of hierarchical
244   proxies, caching, the need for persistent connections, or name-based
245   virtual hosts. In addition, the proliferation of incompletely-implemented
246   applications calling themselves "HTTP/1.0" necessitated a
247   protocol version change in order for two communicating applications
248   to determine each other's true capabilities.
249</t>
250<t>
251   This document is Part 1 of the seven-part specification that defines
252   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
253   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
254   requirements that enable reliable implementations and adding only
255   those new features that will either be safely ignored by an HTTP/1.0
256   recipient or only sent when communicating with a party advertising
257   compliance with HTTP/1.1.
258   Part 1 defines those aspects of HTTP/1.1 related to overall network
259   operation, message framing, interaction with transport protocols, and
260   URI schemes.
261</t>
262<t>
263   This document is currently disorganized in order to minimize the changes
264   between drafts and enable reviewers to see the smaller errata changes.
265   The next draft will reorganize the sections to better reflect the content.
266   In particular, the sections will be organized according to the typical
267   process of deciding when to use HTTP (URI schemes), overall network operation,
268   connection management, message framing, and generic message parsing.
269   The current mess reflects how widely dispersed these topics and associated
270   requirements had become in <xref target="RFC2616"/>.
271</t>
272
273<section title="Purpose" anchor="intro.purpose">
274<t>
275   Practical information systems require more functionality than simple
276   retrieval, including search, front-end update, and annotation. HTTP
277   allows an open-ended set of methods and headers that indicate the
278   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
279   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
280   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
281   method is to be applied. Messages are passed in a format similar to
282   that used by Internet mail <xref target="RFC2822"/> as defined by the Multipurpose
283   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
284</t>
285<t>
286   HTTP is also used as a generic protocol for communication between
287   user agents and proxies/gateways to other Internet systems, including
288   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
289   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
290   access to resources available from diverse applications.
291</t>
292</section>
293
294<section title="Requirements" anchor="intro.requirements">
295<t>
296   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
297   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
298   document are to be interpreted as described in <xref target="RFC2119"/>.
299</t>
300<t>
301   An implementation is not compliant if it fails to satisfy one or more
302   of the &MUST; or &REQUIRED; level requirements for the protocols it
303   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
304   level and all the &SHOULD; level requirements for its protocols is said
305   to be "unconditionally compliant"; one that satisfies all the &MUST;
306   level requirements but not all the &SHOULD; level requirements for its
307   protocols is said to be "conditionally compliant."
308</t>
309</section>
310
311<section title="Terminology" anchor="intro.terminology">
312<t>
313   This specification uses a number of terms to refer to the roles
314   played by participants in, and objects of, the HTTP communication.
315</t>
316<t>
317  <iref item="connection"/>
318  <x:dfn>connection</x:dfn>
319  <list>
320    <t>
321      A transport layer virtual circuit established between two programs
322      for the purpose of communication.
323    </t>
324  </list>
325</t>
326<t>
327  <iref item="message"/>
328  <x:dfn>message</x:dfn>
329  <list>
330    <t>
331      The basic unit of HTTP communication, consisting of a structured
332      sequence of octets matching the syntax defined in <xref target="http.message"/> and
333      transmitted via the connection.
334    </t>
335  </list>
336</t>
337<t>
338  <iref item="request"/>
339  <x:dfn>request</x:dfn>
340  <list>
341    <t>
342      An HTTP request message, as defined in <xref target="request"/>.
343    </t>
344  </list>
345</t>
346<t>
347  <iref item="response"/>
348  <x:dfn>response</x:dfn>
349  <list>
350    <t>
351      An HTTP response message, as defined in <xref target="response"/>.
352    </t>
353  </list>
354</t>
355<t>
356  <iref item="resource"/>
357  <x:dfn>resource</x:dfn>
358  <list>
359    <t>
360      A network data object or service that can be identified by a URI,
361      as defined in <xref target="uri"/>. Resources may be available in multiple
362      representations (e.g. multiple languages, data formats, size, and
363      resolutions) or vary in other ways.
364    </t>
365  </list>
366</t>
367<t>
368  <iref item="entity"/>
369  <x:dfn>entity</x:dfn>
370  <list>
371    <t>
372      The information transferred as the payload of a request or
373      response. An entity consists of metainformation in the form of
374      entity-header fields and content in the form of an entity-body, as
375      described in &entity;.
376    </t>
377  </list>
378</t>
379<t>
380  <iref item="representation"/>
381  <x:dfn>representation</x:dfn>
382  <list>
383    <t>
384      An entity included with a response that is subject to content
385      negotiation, as described in &content.negotiation;. There may exist multiple
386      representations associated with a particular response status.
387    </t>
388  </list>
389</t>
390<t>
391  <iref item="content negotiation"/>
392  <x:dfn>content negotiation</x:dfn>
393  <list>
394    <t>
395      The mechanism for selecting the appropriate representation when
396      servicing a request, as described in &content.negotiation;. The
397      representation of entities in any response can be negotiated
398      (including error responses).
399    </t>
400  </list>
401</t>
402<t>
403  <iref item="variant"/>
404  <x:dfn>variant</x:dfn>
405  <list>
406    <t>
407      A resource may have one, or more than one, representation(s)
408      associated with it at any given instant. Each of these
409      representations is termed a `variant'.  Use of the term `variant'
410      does not necessarily imply that the resource is subject to content
411      negotiation.
412    </t>
413  </list>
414</t>
415<t>
416  <iref item="client"/>
417  <x:dfn>client</x:dfn>
418  <list>
419    <t>
420      A program that establishes connections for the purpose of sending
421      requests.
422    </t>
423  </list>
424</t>
425<t>
426  <iref item="user agent"/>
427  <x:dfn>user agent</x:dfn>
428  <list>
429    <t>
430      The client which initiates a request. These are often browsers,
431      editors, spiders (web-traversing robots), or other end user tools.
432    </t>
433  </list>
434</t>
435<t>
436  <iref item="server"/>
437  <x:dfn>server</x:dfn>
438  <list>
439    <t>
440      An application program that accepts connections in order to
441      service requests by sending back responses. Any given program may
442      be capable of being both a client and a server; our use of these
443      terms refers only to the role being performed by the program for a
444      particular connection, rather than to the program's capabilities
445      in general. Likewise, any server may act as an origin server,
446      proxy, gateway, or tunnel, switching behavior based on the nature
447      of each request.
448    </t>
449  </list>
450</t>
451<t>
452  <iref item="origin server"/>
453  <x:dfn>origin server</x:dfn>
454  <list>
455    <t>
456      The server on which a given resource resides or is to be created.
457    </t>
458  </list>
459</t>
460<t>
461  <iref item="proxy"/>
462  <x:dfn>proxy</x:dfn>
463  <list>
464    <t>
465      An intermediary program which acts as both a server and a client
466      for the purpose of making requests on behalf of other clients.
467      Requests are serviced internally or by passing them on, with
468      possible translation, to other servers. A proxy &MUST; implement
469      both the client and server requirements of this specification. A
470      "transparent proxy" is a proxy that does not modify the request or
471      response beyond what is required for proxy authentication and
472      identification. A "non-transparent proxy" is a proxy that modifies
473      the request or response in order to provide some added service to
474      the user agent, such as group annotation services, media type
475      transformation, protocol reduction, or anonymity filtering. Except
476      where either transparent or non-transparent behavior is explicitly
477      stated, the HTTP proxy requirements apply to both types of
478      proxies.
479    </t>
480  </list>
481</t>
482<t>
483  <iref item="gateway"/>
484  <x:dfn>gateway</x:dfn>
485  <list>
486    <t>
487      A server which acts as an intermediary for some other server.
488      Unlike a proxy, a gateway receives requests as if it were the
489      origin server for the requested resource; the requesting client
490      may not be aware that it is communicating with a gateway.
491    </t>
492  </list>
493</t>
494<t>
495  <iref item="tunnel"/>
496  <x:dfn>tunnel</x:dfn>
497  <list>
498    <t>
499      An intermediary program which is acting as a blind relay between
500      two connections. Once active, a tunnel is not considered a party
501      to the HTTP communication, though the tunnel may have been
502      initiated by an HTTP request. The tunnel ceases to exist when both
503      ends of the relayed connections are closed.
504    </t>
505  </list>
506</t>
507<t>
508  <iref item="cache"/>
509  <x:dfn>cache</x:dfn>
510  <list>
511    <t>
512      A program's local store of response messages and the subsystem
513      that controls its message storage, retrieval, and deletion. A
514      cache stores cacheable responses in order to reduce the response
515      time and network bandwidth consumption on future, equivalent
516      requests. Any client or server may include a cache, though a cache
517      cannot be used by a server that is acting as a tunnel.
518    </t>
519  </list>
520</t>
521<t>
522  <iref item="cacheable"/>
523  <x:dfn>cacheable</x:dfn>
524  <list>
525    <t>
526      A response is cacheable if a cache is allowed to store a copy of
527      the response message for use in answering subsequent requests. The
528      rules for determining the cacheability of HTTP responses are
529      defined in &caching;. Even if a resource is cacheable, there may
530      be additional constraints on whether a cache can use the cached
531      copy for a particular request.
532    </t>
533  </list>
534</t>
535<t>
536  <iref item="upstream"/>
537  <iref item="downstream"/>
538  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
539  <list>
540    <t>
541      Upstream and downstream describe the flow of a message: all
542      messages flow from upstream to downstream.
543    </t>
544  </list>
545</t>
546<t>
547  <iref item="inbound"/>
548  <iref item="outbound"/>
549  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
550  <list>
551    <t>
552      Inbound and outbound refer to the request and response paths for
553      messages: "inbound" means "traveling toward the origin server",
554      and "outbound" means "traveling toward the user agent"
555    </t>
556  </list>
557</t>
558</section>
559
560<section title="Overall Operation" anchor="intro.overall.operation">
561<t>
562   HTTP is a request/response protocol. A client sends a
563   request to the server in the form of a request method, URI, and
564   protocol version, followed by a MIME-like message containing request
565   modifiers, client information, and possible body content over a
566   connection with a server. The server responds with a status line,
567   including the message's protocol version and a success or error code,
568   followed by a MIME-like message containing server information, entity
569   metainformation, and possible entity-body content. The relationship
570   between HTTP and MIME is described in &diff2045entity;.
571</t>
572<t>
573   Most HTTP communication is initiated by a user agent and consists of
574   a request to be applied to a resource on some origin server. In the
575   simplest case, this may be accomplished via a single connection (v)
576   between the user agent (UA) and the origin server (O).
577</t>
578<figure><artwork type="drawing">
579       request chain ------------------------&gt;
580    UA -------------------v------------------- O
581       &lt;----------------------- response chain
582</artwork></figure>
583<t>
584   A more complicated situation occurs when one or more intermediaries
585   are present in the request/response chain. There are three common
586   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
587   forwarding agent, receiving requests for a URI in its absolute form,
588   rewriting all or part of the message, and forwarding the reformatted
589   request toward the server identified by the URI. A gateway is a
590   receiving agent, acting as a layer above some other server(s) and, if
591   necessary, translating the requests to the underlying server's
592   protocol. A tunnel acts as a relay point between two connections
593   without changing the messages; tunnels are used when the
594   communication needs to pass through an intermediary (such as a
595   firewall) even when the intermediary cannot understand the contents
596   of the messages.
597</t>
598<figure><artwork type="drawing">
599       request chain --------------------------------------&gt;
600    UA -----v----- A -----v----- B -----v----- C -----v----- O
601       &lt;------------------------------------- response chain
602</artwork></figure>
603<t>
604   The figure above shows three intermediaries (A, B, and C) between the
605   user agent and origin server. A request or response message that
606   travels the whole chain will pass through four separate connections.
607   This distinction is important because some HTTP communication options
608   may apply only to the connection with the nearest, non-tunnel
609   neighbor, only to the end-points of the chain, or to all connections
610   along the chain. Although the diagram is linear, each participant may
611   be engaged in multiple, simultaneous communications. For example, B
612   may be receiving requests from many clients other than A, and/or
613   forwarding requests to servers other than C, at the same time that it
614   is handling A's request.
615</t>
616<t>
617   Any party to the communication which is not acting as a tunnel may
618   employ an internal cache for handling requests. The effect of a cache
619   is that the request/response chain is shortened if one of the
620   participants along the chain has a cached response applicable to that
621   request. The following illustrates the resulting chain if B has a
622   cached copy of an earlier response from O (via C) for a request which
623   has not been cached by UA or A.
624</t>
625<figure><artwork type="drawing">
626          request chain ----------&gt;
627       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
628          &lt;--------- response chain
629</artwork></figure>
630<t>
631   Not all responses are usefully cacheable, and some requests may
632   contain modifiers which place special requirements on cache behavior.
633   HTTP requirements for cache behavior and cacheable responses are
634   defined in &caching;.
635</t>
636<t>
637   In fact, there are a wide variety of architectures and configurations
638   of caches and proxies currently being experimented with or deployed
639   across the World Wide Web. These systems include national hierarchies
640   of proxy caches to save transoceanic bandwidth, systems that
641   broadcast or multicast cache entries, organizations that distribute
642   subsets of cached data via CD-ROM, and so on. HTTP systems are used
643   in corporate intranets over high-bandwidth links, and for access via
644   PDAs with low-power radio links and intermittent connectivity. The
645   goal of HTTP/1.1 is to support the wide diversity of configurations
646   already deployed while introducing protocol constructs that meet the
647   needs of those who build web applications that require high
648   reliability and, failing that, at least reliable indications of
649   failure.
650</t>
651<t>
652   HTTP communication usually takes place over TCP/IP connections. The
653   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
654   not preclude HTTP from being implemented on top of any other protocol
655   on the Internet, or on other networks. HTTP only presumes a reliable
656   transport; any protocol that provides such guarantees can be used;
657   the mapping of the HTTP/1.1 request and response structures onto the
658   transport data units of the protocol in question is outside the scope
659   of this specification.
660</t>
661<t>
662   In HTTP/1.0, most implementations used a new connection for each
663   request/response exchange. In HTTP/1.1, a connection may be used for
664   one or more request/response exchanges, although connections may be
665   closed for a variety of reasons (see <xref target="persistent.connections"/>).
666</t>
667</section>
668</section>
669
670<section title="Notational Conventions and Generic Grammar" anchor="notation">
671
672<section title="Augmented BNF" anchor="notation.abnf">
673<t>
674   All of the mechanisms specified in this document are described in
675   both prose and an augmented Backus-Naur Form (BNF) similar to that
676   used by <xref target="RFC822ABNF"/>. Implementors will need to be familiar with the
677   notation in order to understand this specification. The augmented BNF
678   includes the following constructs:
679</t>
680<t>
681   name = definition
682  <list>
683    <t>
684      The name of a rule is simply the name itself (without any
685      enclosing "&lt;" and "&gt;") and is separated from its definition by the
686      equal "=" character. White space is only significant in that
687      indentation of continuation lines is used to indicate a rule
688      definition that spans more than one line. Certain basic rules are
689      in uppercase, such as SP, LWS, HTAB, CRLF, DIGIT, ALPHA, etc. Angle
690      brackets are used within definitions whenever their presence will
691      facilitate discerning the use of rule names.
692    </t>
693  </list>
694</t>
695<t>
696   "literal"
697  <list>
698    <t>
699      Quotation marks surround literal text. Unless stated otherwise,
700      the text is case-insensitive.
701    </t>
702  </list>
703</t>
704<t>
705   rule1 | rule2
706  <list>
707    <t>
708      Elements separated by a bar ("|") are alternatives, e.g., "yes |
709      no" will accept yes or no.
710    </t>
711  </list>
712</t>
713<t>
714   (rule1 rule2)
715  <list>
716    <t>
717      Elements enclosed in parentheses are treated as a single element.
718      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
719      foo elem" and "elem bar elem".
720    </t>
721  </list>
722</t>
723<t>
724   *rule
725  <list>
726    <t>
727      The character "*" preceding an element indicates repetition. The
728      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
729      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
730      that "*(element)" allows any number, including zero; "1*element"
731      requires at least one; and "1*2element" allows one or two.
732    </t>
733  </list>
734</t>
735<t>
736   [rule]
737  <list>
738    <t>
739      Square brackets enclose optional elements; "[foo bar]" is
740      equivalent to "*1(foo bar)".
741    </t>
742  </list>
743</t>
744<t>
745   N rule
746  <list>
747    <t>
748      Specific repetition: "&lt;n&gt;(element)" is equivalent to
749      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
750      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
751      alphabetic characters.
752    </t>
753  </list>
754</t>
755<t>
756   #rule
757  <list>
758    <t>
759      A construct "#" is defined, similar to "*", for defining lists of
760      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
761      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
762      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
763      form of lists very easy; a rule such as
764    </t>
765    <t>
766         ( *LWS element *( *LWS "," *LWS element ))
767    </t>
768    <t>
769      can be shown as
770    </t>
771    <t>
772         1#element
773    </t>
774    <t>
775      Wherever this construct is used, null elements are allowed, but do
776      not contribute to the count of elements present. That is,
777      "(element), , (element) " is permitted, but counts as only two
778      elements. Therefore, where at least one element is required, at
779      least one non-null element &MUST; be present. Default values are 0
780      and infinity so that "#element" allows any number, including zero;
781      "1#element" requires at least one; and "1#2element" allows one or
782      two.
783    </t>
784  </list>
785</t>
786<t>
787   ; comment
788  <list>
789    <t>
790      A semi-colon, set off some distance to the right of rule text,
791      starts a comment that continues to the end of line. This is a
792      simple way of including useful notes in parallel with the
793      specifications.
794    </t>
795  </list>
796</t>
797<t>
798   implied *LWS
799  <list>
800    <t>
801      The grammar described by this specification is word-based. Except
802      where noted otherwise, linear white space (LWS) can be included
803      between any two adjacent words (token or quoted-string), and
804      between adjacent words and separators, without changing the
805      interpretation of a field. At least one delimiter (LWS and/or
806      separators) &MUST; exist between any two tokens (for the definition
807      of "token" below), since they would otherwise be interpreted as a
808      single token.
809    </t>
810  </list>
811</t>
812</section>
813
814<section title="Basic Rules" anchor="basic.rules">
815<x:anchor-alias value="OCTET"/>
816<x:anchor-alias value="CHAR"/>
817<x:anchor-alias value="ALPHA"/>
818<x:anchor-alias value="DIGIT"/>
819<x:anchor-alias value="CTL"/>
820<x:anchor-alias value="CR"/>
821<x:anchor-alias value="LF"/>
822<x:anchor-alias value="SP"/>
823<x:anchor-alias value="HTAB"/>
824<x:anchor-alias value="CRLF"/>
825<x:anchor-alias value="LWS"/>
826<x:anchor-alias value="TEXT"/>
827<x:anchor-alias value="HEX"/>
828<x:anchor-alias value="token"/>
829<x:anchor-alias value="separators"/>
830<x:anchor-alias value="comment"/>
831<x:anchor-alias value="ctext"/>
832<x:anchor-alias value="quoted-string"/>
833<x:anchor-alias value="qdtext"/>
834<x:anchor-alias value="quoted-pair"/>
835<t>
836   The following rules are used throughout this specification to
837   describe basic parsing constructs. The US-ASCII coded character set
838   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
839</t>
840<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><iref primary="true" item="Grammar" subitem="ALPHA"/><iref primary="true" item="Grammar" subitem="DIGIT"/><iref primary="true" item="Grammar" subitem="CTL"/><iref primary="true" item="Grammar" subitem="CR"/><iref primary="true" item="Grammar" subitem="LF"/><iref primary="true" item="Grammar" subitem="SP"/><iref primary="true" item="Grammar" subitem="HTAB"/><iref primary="true" item="Grammar" subitem="DQUOTE"/>
841  OCTET          = %x00-FF
842                   ; any 8-bit sequence of data
843  CHAR           = %x01-7F
844                   ; any US-ASCII character, excluding NUL
845  ALPHA          = %x41-5A | %x61-7A
846                   ; A-Z | a-z
847  DIGIT          = %x30-39
848                   ; any US-ASCII digit "0".."9"
849  CTL            = %x00-1F | %x7F
850                   ; (octets 0 - 31) and DEL (127)
851  CR             = %x0D
852                   ; US-ASCII CR, carriage return (13)
853  LF             = %x0A
854                   ; US-ASCII LF, linefeed (10)
855  SP             = %x20
856                   ; US-ASCII SP, space (32)
857  HTAB           = %x09
858                   ; US-ASCII HT, horizontal-tab (9)
859  DQUOTE         = %x22
860                   ; US-ASCII double-quote mark (34)
861</artwork></figure>
862<t>
863   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
864   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
865   tolerant applications). The end-of-line marker within an entity-body
866   is defined by its associated media type, as described in &media-types;.
867</t>
868<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
869  CRLF           = CR LF
870</artwork></figure>
871<t>
872   HTTP/1.1 header field values can be folded onto multiple lines if the
873   continuation line begins with a space or horizontal tab. All linear
874   white space, including folding, has the same semantics as SP. A
875   recipient &MAY; replace any linear white space with a single SP before
876   interpreting the field value or forwarding the message downstream.
877</t>
878<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
879  LWS            = [CRLF] 1*( SP | HTAB )
880</artwork></figure>
881<t>
882   The TEXT rule is only used for descriptive field contents and values
883   that are not intended to be interpreted by the message parser. Words
884   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
885   <xref target="ISO-8859-1"/> only when encoded according to the rules of
886   <xref target="RFC2047"/>.
887</t>
888<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
889  TEXT           = %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                 | "." | "^" | "_" | "`" | "|" | "~" | DIGIT | ALPHA
918                 ; any CHAR except CTLs or separators
919                 
920  token          = 1*tchar
921</artwork></figure>
922<t>
923   Comments can be included in some HTTP header fields by surrounding
924   the comment text with parentheses. Comments are only allowed in
925   fields containing "comment" as part of their field value definition.
926   In all other fields, parentheses are considered part of the field
927   value.
928</t>
929<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
930  comment        = "(" *( ctext | quoted-pair | comment ) ")"
931  ctext          = &lt;any TEXT excluding "(" and ")"&gt;
932</artwork></figure>
933<t>
934   A string of text is parsed as a single word if it is quoted using
935   double-quote marks.
936</t>
937<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
938  quoted-string  = ( DQUOTE *(qdtext | quoted-pair ) DQUOTE )
939  qdtext         = &lt;any TEXT excluding DQUOTE and "\">
940</artwork></figure>
941<t>
942   The backslash character ("\") &MAY; be used as a single-character
943   quoting mechanism only within quoted-string and comment constructs.
944</t>
945<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
946  quoted-pair    = "\" CHAR
947</artwork></figure>
948</section>
949
950<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
951<t>
952  The ABNF rules below are defined in other parts:
953</t>
954<figure><!-- Part2--><artwork type="abnf2616">
955  request-header =  &lt;request-header, defined in &request-header-fields;&gt;
956  response-header = &lt;response-header, defined in &response-header-fields;&gt;
957</artwork></figure>
958<figure><!-- Part3--><artwork type="abnf2616">
959  accept-params   = &lt;accept-params, defined in &header-accept;&gt;
960  entity-body     = &lt;entity-body, defined in &entity-body;&gt;
961  entity-header   = &lt;entity-header, defined in &entity-header-fields;&gt;
962</artwork></figure>
963<figure><!-- Part6--><artwork type="abnf2616">
964  Cache-Control   = &lt;Cache-Control, defined in &header-pragma;&gt;
965  Pragma          = &lt;Pragma, defined in &header-pragma;&gt;
966  Warning         = &lt;Warning, defined in &header-warning;&gt;
967</artwork></figure>
968</section>
969
970</section>
971
972<section title="Protocol Parameters" anchor="protocol.parameters">
973
974<section title="HTTP Version" anchor="http.version">
975<t>
976   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
977   of the protocol. The protocol versioning policy is intended to allow
978   the sender to indicate the format of a message and its capacity for
979   understanding further HTTP communication, rather than the features
980   obtained via that communication. No change is made to the version
981   number for the addition of message components which do not affect
982   communication behavior or which only add to extensible field values.
983   The &lt;minor&gt; number is incremented when the changes made to the
984   protocol add features which do not change the general message parsing
985   algorithm, but which may add to the message semantics and imply
986   additional capabilities of the sender. The &lt;major&gt; number is
987   incremented when the format of a message within the protocol is
988   changed. See <xref target="RFC2145"/> for a fuller explanation.
989</t>
990<t>
991   The version of an HTTP message is indicated by an HTTP-Version field
992   in the first line of the message. HTTP-Version is case-sensitive.
993</t>
994<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
995  HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
996</artwork></figure>
997<t>
998   Note that the major and minor numbers &MUST; be treated as separate
999   integers and that each &MAY; be incremented higher than a single digit.
1000   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
1001   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
1002   &MUST-NOT; be sent.
1003</t>
1004<t>
1005   An application that sends a request or response message that includes
1006   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
1007   with this specification. Applications that are at least conditionally
1008   compliant with this specification &SHOULD; use an HTTP-Version of
1009   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
1010   not compatible with HTTP/1.0. For more details on when to send
1011   specific HTTP-Version values, see <xref target="RFC2145"/>.
1012</t>
1013<t>
1014   The HTTP version of an application is the highest HTTP version for
1015   which the application is at least conditionally compliant.
1016</t>
1017<t>
1018   Proxy and gateway applications need to be careful when forwarding
1019   messages in protocol versions different from that of the application.
1020   Since the protocol version indicates the protocol capability of the
1021   sender, a proxy/gateway &MUST-NOT; send a message with a version
1022   indicator which is greater than its actual version. If a higher
1023   version request is received, the proxy/gateway &MUST; either downgrade
1024   the request version, or respond with an error, or switch to tunnel
1025   behavior.
1026</t>
1027<t>
1028   Due to interoperability problems with HTTP/1.0 proxies discovered
1029   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
1030   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
1031   they support. The proxy/gateway's response to that request &MUST; be in
1032   the same major version as the request.
1033</t>
1034<t>
1035  <list>
1036    <t>
1037      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1038      of header fields required or forbidden by the versions involved.
1039    </t>
1040  </list>
1041</t>
1042</section>
1043
1044<section title="Uniform Resource Identifiers" anchor="uri">
1045<t>
1046   URIs have been known by many names: WWW addresses, Universal Document
1047   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
1048   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
1049   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
1050   simply formatted strings which identify--via name, location, or any
1051   other characteristic--a resource.
1052</t>
1053
1054<section title="General Syntax" anchor="general.syntax">
1055<t>
1056   URIs in HTTP can be represented in absolute form or relative to some
1057   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1058   forms are differentiated by the fact that absolute URIs always begin
1059   with a scheme name followed by a colon. For definitive information on
1060   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1061   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1062   and <xref target="RFC1808"/>). This specification adopts the
1063   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
1064   "host", "abs_path", "query", and "authority" from that specification:
1065</t>
1066<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"/>
1067  absoluteURI   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1068  authority     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
1069  fragment      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
1070  path-absolute = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1071  port          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1072  query         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
1073  relativeURI   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
1074  uri-host      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1075</artwork></figure>
1076<t>
1077   HTTP does not place any a priori limit on the length of
1078   a URI. Servers &MUST; be able to handle the URI of any resource they
1079   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1080   provide GET-based forms that could generate such URIs. A server
1081   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1082   than the server can handle (see &status-414;).
1083</t>
1084<t>
1085  <list>
1086    <t>
1087      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1088      above 255 bytes, because some older client or proxy
1089      implementations might not properly support these lengths.
1090    </t>
1091  </list>
1092</t>
1093</section>
1094
1095<section title="http URL" anchor="http.url">
1096<t>
1097   The "http" scheme is used to locate network resources via the HTTP
1098   protocol. This section defines the scheme-specific syntax and
1099   semantics for http URLs.
1100</t>
1101<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
1102  http-URL = "http:" "//" uri-host [ ":" port ]
1103             [ path-absolute [ "?" query ]]
1104</artwork></figure>
1105<t>
1106   If the port is empty or not given, port 80 is assumed. The semantics
1107   are that the identified resource is located at the server listening
1108   for TCP connections on that port of that host, and the Request-URI
1109   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
1110   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1111   the path-absolute is not present in the URL, it &MUST; be given as "/" when
1112   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1113   receives a host name which is not a fully qualified domain name, it
1114   &MAY; add its domain to the host name it received. If a proxy receives
1115   a fully qualified domain name, the proxy &MUST-NOT; change the host
1116   name.
1117</t>
1118</section>
1119
1120<section title="URI Comparison" anchor="uri.comparison">
1121<t>
1122   When comparing two URIs to decide if they match or not, a client
1123   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1124   URIs, with these exceptions:
1125  <list style="symbols">
1126    <t>A port that is empty or not given is equivalent to the default
1127        port for that URI-reference;</t>
1128    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1129    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1130    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
1131  </list>
1132</t>
1133<t>
1134   Characters other than those in the "reserved" set (see
1135   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1136</t>
1137<t>
1138   For example, the following three URIs are equivalent:
1139</t>
1140<figure><artwork type="example">
1141   http://example.com:80/~smith/home.html
1142   http://EXAMPLE.com/%7Esmith/home.html
1143   http://EXAMPLE.com:/%7esmith/home.html
1144</artwork></figure>
1145</section>
1146</section>
1147
1148<section title="Date/Time Formats" anchor="date.time.formats">
1149<section title="Full Date" anchor="full.date">
1150<t>
1151   HTTP applications have historically allowed three different formats
1152   for the representation of date/time stamps:
1153</t>
1154<figure><artwork type="example">
1155   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1156   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1157   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1158</artwork></figure>
1159<t>
1160   The first format is preferred as an Internet standard and represents
1161   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1162   <xref target="RFC822"/>). The other formats are described here only for
1163   compatibility with obsolete implementations.
1164   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1165   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1166   only generate the RFC 1123 format for representing HTTP-date values
1167   in header fields. See <xref target="tolerant.applications"/> for further information.
1168</t>
1169<t><list><t>
1170      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1171      accepting date values that may have been sent by non-HTTP
1172      applications, as is sometimes the case when retrieving or posting
1173      messages via proxies/gateways to SMTP or NNTP.
1174</t></list></t>
1175<t>
1176   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1177   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1178   equal to UTC (Coordinated Universal Time). This is indicated in the
1179   first two formats by the inclusion of "GMT" as the three-letter
1180   abbreviation for time zone, and &MUST; be assumed when reading the
1181   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1182   additional LWS beyond that specifically included as SP in the
1183   grammar.
1184</t>
1185<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"/>
1186  HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1187  rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1188  rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1189  asctime-date = wkday SP date3 SP time SP 4DIGIT
1190  date1        = 2DIGIT SP month SP 4DIGIT
1191                 ; day month year (e.g., 02 Jun 1982)
1192  date2        = 2DIGIT "-" month "-" 2DIGIT
1193                 ; day-month-year (e.g., 02-Jun-82)
1194  date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1195                 ; month day (e.g., Jun  2)
1196  time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1197                 ; 00:00:00 - 23:59:59
1198  wkday        = "Mon" | "Tue" | "Wed"
1199               | "Thu" | "Fri" | "Sat" | "Sun"
1200  weekday      = "Monday" | "Tuesday" | "Wednesday"
1201               | "Thursday" | "Friday" | "Saturday" | "Sunday"
1202  month        = "Jan" | "Feb" | "Mar" | "Apr"
1203               | "May" | "Jun" | "Jul" | "Aug"
1204               | "Sep" | "Oct" | "Nov" | "Dec"
1205</artwork></figure>
1206<t>
1207      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1208      to their usage within the protocol stream. Clients and servers are
1209      not required to use these formats for user presentation, request
1210      logging, etc.
1211</t>
1212</section>
1213</section>
1214
1215<section title="Transfer Codings" anchor="transfer.codings">
1216<t>
1217   Transfer-coding values are used to indicate an encoding
1218   transformation that has been, can be, or may need to be applied to an
1219   entity-body in order to ensure "safe transport" through the network.
1220   This differs from a content coding in that the transfer-coding is a
1221   property of the message, not of the original entity.
1222</t>
1223<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1224  transfer-coding         = "chunked" | transfer-extension
1225  transfer-extension      = token *( ";" parameter )
1226</artwork></figure>
1227<t>
1228   Parameters are in  the form of attribute/value pairs.
1229</t>
1230<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"/>
1231  parameter               = attribute "=" value
1232  attribute               = token
1233  value                   = token | quoted-string
1234</artwork></figure>
1235<t>
1236   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1237   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1238   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1239</t>
1240<t>
1241   Whenever a transfer-coding is applied to a message-body, the set of
1242   transfer-codings &MUST; include "chunked", unless the message is
1243   terminated by closing the connection. When the "chunked" transfer-coding
1244   is used, it &MUST; be the last transfer-coding applied to the
1245   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1246   than once to a message-body. These rules allow the recipient to
1247   determine the transfer-length of the message (<xref target="message.length"/>).
1248</t>
1249<t>
1250   Transfer-codings are analogous to the Content-Transfer-Encoding
1251   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1252   binary data over a 7-bit transport service. However, safe transport
1253   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1254   the only unsafe characteristic of message-bodies is the difficulty in
1255   determining the exact body length (<xref target="message.length"/>), or the desire to
1256   encrypt data over a shared transport.
1257</t>
1258<t>
1259   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1260   transfer-coding value tokens. Initially, the registry contains the
1261   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1262   "gzip", "compress", and "deflate" (&content-codings;).
1263</t>
1264<t>
1265   New transfer-coding value tokens &SHOULD; be registered in the same way
1266   as new content-coding value tokens (&content-codings;).
1267</t>
1268<t>
1269   A server which receives an entity-body with a transfer-coding it does
1270   not understand &SHOULD; return 501 (Not Implemented), and close the
1271   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1272   client.
1273</t>
1274
1275<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1276<t>
1277   The chunked encoding modifies the body of a message in order to
1278   transfer it as a series of chunks, each with its own size indicator,
1279   followed by an &OPTIONAL; trailer containing entity-header fields. This
1280   allows dynamically produced content to be transferred along with the
1281   information necessary for the recipient to verify that it has
1282   received the full message.
1283</t>
1284<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"/>
1285  Chunked-Body   = *chunk
1286                   last-chunk
1287                   trailer-part
1288                   CRLF
1289 
1290  chunk          = chunk-size [ chunk-extension ] CRLF
1291                   chunk-data CRLF
1292  chunk-size     = 1*HEX
1293  last-chunk     = 1*("0") [ chunk-extension ] CRLF
1294 
1295  chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1296  chunk-ext-name = token
1297  chunk-ext-val  = token | quoted-string
1298  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
1299  trailer-part   = *(entity-header CRLF)
1300</artwork></figure>
1301<t>
1302   The chunk-size field is a string of hex digits indicating the size of
1303   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1304   zero, followed by the trailer, which is terminated by an empty line.
1305</t>
1306<t>
1307   The trailer allows the sender to include additional HTTP header
1308   fields at the end of the message. The Trailer header field can be
1309   used to indicate which header fields are included in a trailer (see
1310   <xref target="header.trailer"/>).
1311</t>
1312<t>
1313   A server using chunked transfer-coding in a response &MUST-NOT; use the
1314   trailer for any header fields unless at least one of the following is
1315   true:
1316  <list style="numbers">
1317    <t>the request included a TE header field that indicates "trailers" is
1318     acceptable in the transfer-coding of the  response, as described in
1319     <xref target="header.te"/>; or,</t>
1320
1321    <t>the server is the origin server for the response, the trailer
1322     fields consist entirely of optional metadata, and the recipient
1323     could use the message (in a manner acceptable to the origin server)
1324     without receiving this metadata.  In other words, the origin server
1325     is willing to accept the possibility that the trailer fields might
1326     be silently discarded along the path to the client.</t>
1327  </list>
1328</t>
1329<t>
1330   This requirement prevents an interoperability failure when the
1331   message is being received by an HTTP/1.1 (or later) proxy and
1332   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1333   compliance with the protocol would have necessitated a possibly
1334   infinite buffer on the proxy.
1335</t>
1336<t>
1337   A process for decoding the "chunked" transfer-coding
1338   can be represented in pseudo-code as:
1339</t>
1340<figure><artwork type="code">
1341    length := 0
1342    read chunk-size, chunk-extension (if any) and CRLF
1343    while (chunk-size &gt; 0) {
1344       read chunk-data and CRLF
1345       append chunk-data to entity-body
1346       length := length + chunk-size
1347       read chunk-size and CRLF
1348    }
1349    read entity-header
1350    while (entity-header not empty) {
1351       append entity-header to existing header fields
1352       read entity-header
1353    }
1354    Content-Length := length
1355    Remove "chunked" from Transfer-Encoding
1356</artwork></figure>
1357<t>
1358   All HTTP/1.1 applications &MUST; be able to receive and decode the
1359   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1360   they do not understand.
1361</t>
1362</section>
1363</section>
1364
1365<section title="Product Tokens" anchor="product.tokens">
1366<t>
1367   Product tokens are used to allow communicating applications to
1368   identify themselves by software name and version. Most fields using
1369   product tokens also allow sub-products which form a significant part
1370   of the application to be listed, separated by white space. By
1371   convention, the products are listed in order of their significance
1372   for identifying the application.
1373</t>
1374<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1375  product         = token ["/" product-version]
1376  product-version = token
1377</artwork></figure>
1378<t>
1379   Examples:
1380</t>
1381<figure><artwork type="example">
1382    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1383    Server: Apache/0.8.4
1384</artwork></figure>
1385<t>
1386   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1387   used for advertising or other non-essential information. Although any
1388   token character &MAY; appear in a product-version, this token &SHOULD;
1389   only be used for a version identifier (i.e., successive versions of
1390   the same product &SHOULD; only differ in the product-version portion of
1391   the product value).
1392</t>
1393</section>
1394
1395</section>
1396
1397<section title="HTTP Message" anchor="http.message">
1398
1399<section title="Message Types" anchor="message.types">
1400<t>
1401   HTTP messages consist of requests from client to server and responses
1402   from server to client.
1403</t>
1404<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1405  HTTP-message   = Request | Response     ; HTTP/1.1 messages
1406</artwork></figure>
1407<t>
1408   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1409   message format of <xref target="RFC2822"/> for transferring entities (the payload
1410   of the message). Both types of message consist of a start-line, zero
1411   or more header fields (also known as "headers"), an empty line (i.e.,
1412   a line with nothing preceding the CRLF) indicating the end of the
1413   header fields, and possibly a message-body.
1414</t>
1415<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1416  generic-message = start-line
1417                    *(message-header CRLF)
1418                    CRLF
1419                    [ message-body ]
1420  start-line      = Request-Line | Status-Line
1421</artwork></figure>
1422<t>
1423   In the interest of robustness, servers &SHOULD; ignore any empty
1424   line(s) received where a Request-Line is expected. In other words, if
1425   the server is reading the protocol stream at the beginning of a
1426   message and receives a CRLF first, it should ignore the CRLF.
1427</t>
1428<t>
1429   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1430   after a POST request. To restate what is explicitly forbidden by the
1431   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1432   extra CRLF.
1433</t>
1434</section>
1435
1436<section title="Message Headers" anchor="message.headers">
1437<t>
1438   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1439   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1440   entity-header (&entity-header-fields;) fields, follow the same generic format as
1441   that given in <xref target="RFC2822" x:fmt="of" x:sec="2.1"/>. Each header field consists
1442   of a name followed by a colon (":") and the field value. Field names
1443   are case-insensitive. The field value &MAY; be preceded by any amount
1444   of LWS, though a single SP is preferred. Header fields can be
1445   extended over multiple lines by preceding each extra line with at
1446   least one SP or HTAB. Applications ought to follow "common form", where
1447   one is known or indicated, when generating HTTP constructs, since
1448   there might exist some implementations that fail to accept anything
1449   beyond the common forms.
1450</t>
1451<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"/>
1452  message-header = field-name ":" [ field-value ]
1453  field-name     = token
1454  field-value    = *( field-content | LWS )
1455  field-content  = &lt;field content&gt;
1456                   ; the OCTETs making up the field-value
1457                   ; and consisting of either *TEXT or combinations
1458                   ; of token, separators, and quoted-string
1459</artwork></figure>
1460<t>
1461   The field-content does not include any leading or trailing LWS:
1462   linear white space occurring before the first non-whitespace
1463   character of the field-value or after the last non-whitespace
1464   character of the field-value. Such leading or trailing LWS &MAY; be
1465   removed without changing the semantics of the field value. Any LWS
1466   that occurs between field-content &MAY; be replaced with a single SP
1467   before interpreting the field value or forwarding the message
1468   downstream.
1469</t>
1470<t>
1471   The order in which header fields with differing field names are
1472   received is not significant. However, it is "good practice" to send
1473   general-header fields first, followed by request-header or response-header
1474   fields, and ending with the entity-header fields.
1475</t>
1476<t>
1477   Multiple message-header fields with the same field-name &MAY; be
1478   present in a message if and only if the entire field-value for that
1479   header field is defined as a comma-separated list [i.e., #(values)].
1480   It &MUST; be possible to combine the multiple header fields into one
1481   "field-name: field-value" pair, without changing the semantics of the
1482   message, by appending each subsequent field-value to the first, each
1483   separated by a comma. The order in which header fields with the same
1484   field-name are received is therefore significant to the
1485   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1486   change the order of these field values when a message is forwarded.
1487</t>
1488</section>
1489
1490<section title="Message Body" anchor="message.body">
1491<t>
1492   The message-body (if any) of an HTTP message is used to carry the
1493   entity-body associated with the request or response. The message-body
1494   differs from the entity-body only when a transfer-coding has been
1495   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1496</t>
1497<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1498  message-body = entity-body
1499               | &lt;entity-body encoded as per Transfer-Encoding&gt;
1500</artwork></figure>
1501<t>
1502   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1503   applied by an application to ensure safe and proper transfer of the
1504   message. Transfer-Encoding is a property of the message, not of the
1505   entity, and thus &MAY; be added or removed by any application along the
1506   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1507   when certain transfer-codings may be used.)
1508</t>
1509<t>
1510   The rules for when a message-body is allowed in a message differ for
1511   requests and responses.
1512</t>
1513<t>
1514   The presence of a message-body in a request is signaled by the
1515   inclusion of a Content-Length or Transfer-Encoding header field in
1516   the request's message-headers. A message-body &MUST-NOT; be included in
1517   a request if the specification of the request method (&method;)
1518   explicitly disallows an entity-body in requests.
1519   When a request message contains both a message-body of non-zero
1520   length and a method that does not define any semantics for that
1521   request message-body, then an origin server &SHOULD; either ignore
1522   the message-body or respond with an appropriate error message
1523   (e.g., 413).  A proxy or gateway, when presented the same request,
1524   &SHOULD; either forward the request inbound with the message-body or
1525   ignore the message-body when determining a response.
1526</t>
1527<t>
1528   For response messages, whether or not a message-body is included with
1529   a message is dependent on both the request method and the response
1530   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1531   &MUST-NOT; include a message-body, even though the presence of entity-header
1532   fields might lead one to believe they do. All 1xx
1533   (informational), 204 (No Content), and 304 (Not Modified) responses
1534   &MUST-NOT; include a message-body. All other responses do include a
1535   message-body, although it &MAY; be of zero length.
1536</t>
1537</section>
1538
1539<section title="Message Length" anchor="message.length">
1540<t>
1541   The transfer-length of a message is the length of the message-body as
1542   it appears in the message; that is, after any transfer-codings have
1543   been applied. When a message-body is included with a message, the
1544   transfer-length of that body is determined by one of the following
1545   (in order of precedence):
1546</t>
1547<t>
1548  <list style="numbers">
1549    <x:lt><t>
1550     Any response message which "&MUST-NOT;" include a message-body (such
1551     as the 1xx, 204, and 304 responses and any response to a HEAD
1552     request) is always terminated by the first empty line after the
1553     header fields, regardless of the entity-header fields present in
1554     the message.
1555    </t></x:lt>
1556    <x:lt><t>
1557     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1558     is present, then the transfer-length is
1559     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1560     unless the message is terminated by closing the connection.
1561    </t></x:lt>
1562    <x:lt><t>
1563     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1564     decimal value in OCTETs represents both the entity-length and the
1565     transfer-length. The Content-Length header field &MUST-NOT; be sent
1566     if these two lengths are different (i.e., if a Transfer-Encoding
1567     header field is present). If a message is received with both a
1568     Transfer-Encoding header field and a Content-Length header field,
1569     the latter &MUST; be ignored.
1570    </t></x:lt>
1571    <x:lt><t>
1572     If the message uses the media type "multipart/byteranges", and the
1573     transfer-length is not otherwise specified, then this self-delimiting
1574     media type defines the transfer-length. This media type
1575     &MUST-NOT; be used unless the sender knows that the recipient can parse
1576     it; the presence in a request of a Range header with multiple byte-range
1577     specifiers from a 1.1 client implies that the client can parse
1578     multipart/byteranges responses.
1579    <list style="empty"><t>
1580       A range header might be forwarded by a 1.0 proxy that does not
1581       understand multipart/byteranges; in this case the server &MUST;
1582       delimit the message using methods defined in items 1, 3 or 5 of
1583       this section.
1584    </t></list>
1585    </t></x:lt>
1586    <x:lt><t>
1587     By the server closing the connection. (Closing the connection
1588     cannot be used to indicate the end of a request body, since that
1589     would leave no possibility for the server to send back a response.)
1590    </t></x:lt>
1591  </list>
1592</t>
1593<t>
1594   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1595   containing a message-body &MUST; include a valid Content-Length header
1596   field unless the server is known to be HTTP/1.1 compliant. If a
1597   request contains a message-body and a Content-Length is not given,
1598   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1599   determine the length of the message, or with 411 (Length Required) if
1600   it wishes to insist on receiving a valid Content-Length.
1601</t>
1602<t>
1603   All HTTP/1.1 applications that receive entities &MUST; accept the
1604   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1605   to be used for messages when the message length cannot be determined
1606   in advance.
1607</t>
1608<t>
1609   Messages &MUST-NOT; include both a Content-Length header field and a
1610   transfer-coding. If the message does include a
1611   transfer-coding, the Content-Length &MUST; be ignored.
1612</t>
1613<t>
1614   When a Content-Length is given in a message where a message-body is
1615   allowed, its field value &MUST; exactly match the number of OCTETs in
1616   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1617   invalid length is received and detected.
1618</t>
1619</section>
1620
1621<section title="General Header Fields" anchor="general.header.fields">
1622<t>
1623   There are a few header fields which have general applicability for
1624   both request and response messages, but which do not apply to the
1625   entity being transferred. These header fields apply only to the
1626   message being transmitted.
1627</t>
1628<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1629  general-header = Cache-Control            ; &header-cache-control;
1630                 | Connection               ; <xref target="header.connection"/>
1631                 | Date                     ; <xref target="header.date"/>
1632                 | Pragma                   ; &header-pragma;
1633                 | Trailer                  ; <xref target="header.trailer"/>
1634                 | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1635                 | Upgrade                  ; <xref target="header.upgrade"/>
1636                 | Via                      ; <xref target="header.via"/>
1637                 | Warning                  ; &header-warning;
1638</artwork></figure>
1639<t>
1640   General-header field names can be extended reliably only in
1641   combination with a change in the protocol version. However, new or
1642   experimental header fields may be given the semantics of general
1643   header fields if all parties in the communication recognize them to
1644   be general-header fields. Unrecognized header fields are treated as
1645   entity-header fields.
1646</t>
1647</section>
1648</section>
1649
1650<section title="Request" anchor="request">
1651<t>
1652   A request message from a client to a server includes, within the
1653   first line of that message, the method to be applied to the resource,
1654   the identifier of the resource, and the protocol version in use.
1655</t>
1656<!--                 Host                      ; should be moved here eventually -->
1657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1658  Request       = Request-Line              ; <xref target="request-line"/>
1659                  *(( general-header        ; <xref target="general.header.fields"/>
1660                   | request-header         ; &request-header-fields;
1661                   | entity-header ) CRLF)  ; &entity-header-fields;
1662                  CRLF
1663                  [ message-body ]          ; <xref target="message.body"/>
1664</artwork></figure>
1665
1666<section title="Request-Line" anchor="request-line">
1667<t>
1668   The Request-Line begins with a method token, followed by the
1669   Request-URI and the protocol version, and ending with CRLF. The
1670   elements are separated by SP characters. No CR or LF is allowed
1671   except in the final CRLF sequence.
1672</t>
1673<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1674  Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1675</artwork></figure>
1676
1677<section title="Method" anchor="method">
1678<t>
1679   The Method  token indicates the method to be performed on the
1680   resource identified by the Request-URI. The method is case-sensitive.
1681</t>
1682<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1683  Method         = token
1684</artwork></figure>
1685</section>
1686
1687<section title="Request-URI" anchor="request-uri">
1688<t>
1689   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1690   identifies the resource upon which to apply the request.
1691</t>
1692<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1693  Request-URI    = "*"
1694                 | absoluteURI
1695                 | ( path-absolute [ "?" query ] )
1696                 | authority
1697</artwork></figure>
1698<t>
1699   The four options for Request-URI are dependent on the nature of the
1700   request. The asterisk "*" means that the request does not apply to a
1701   particular resource, but to the server itself, and is only allowed
1702   when the method used does not necessarily apply to a resource. One
1703   example would be
1704</t>
1705<figure><artwork type="example">
1706    OPTIONS * HTTP/1.1
1707</artwork></figure>
1708<t>
1709   The absoluteURI form is &REQUIRED; when the request is being made to a
1710   proxy. The proxy is requested to forward the request or service it
1711   from a valid cache, and return the response. Note that the proxy &MAY;
1712   forward the request on to another proxy or directly to the server
1713   specified by the absoluteURI. In order to avoid request loops, a
1714   proxy &MUST; be able to recognize all of its server names, including
1715   any aliases, local variations, and the numeric IP address. An example
1716   Request-Line would be:
1717</t>
1718<figure><artwork type="example">
1719    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1720</artwork></figure>
1721<t>
1722   To allow for transition to absoluteURIs in all requests in future
1723   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1724   form in requests, even though HTTP/1.1 clients will only generate
1725   them in requests to proxies.
1726</t>
1727<t>
1728   The authority form is only used by the CONNECT method (&CONNECT;).
1729</t>
1730<t>
1731   The most common form of Request-URI is that used to identify a
1732   resource on an origin server or gateway. In this case the absolute
1733   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1734   the Request-URI, and the network location of the URI (authority) &MUST;
1735   be transmitted in a Host header field. For example, a client wishing
1736   to retrieve the resource above directly from the origin server would
1737   create a TCP connection to port 80 of the host "www.example.org" and send
1738   the lines:
1739</t>
1740<figure><artwork type="example">
1741    GET /pub/WWW/TheProject.html HTTP/1.1
1742    Host: www.example.org
1743</artwork></figure>
1744<t>
1745   followed by the remainder of the Request. Note that the absolute path
1746   cannot be empty; if none is present in the original URI, it &MUST; be
1747   given as "/" (the server root).
1748</t>
1749<t>
1750   The Request-URI is transmitted in the format specified in
1751   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1752   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1753   properly interpret the request. Servers &SHOULD; respond to invalid
1754   Request-URIs with an appropriate status code.
1755</t>
1756<t>
1757   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1758   received Request-URI when forwarding it to the next inbound server,
1759   except as noted above to replace a null path-absolute with "/".
1760</t>
1761<t>
1762  <list><t>
1763      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1764      meaning of the request when the origin server is improperly using
1765      a non-reserved URI character for a reserved purpose.  Implementors
1766      should be aware that some pre-HTTP/1.1 proxies have been known to
1767      rewrite the Request-URI.
1768  </t></list>
1769</t>
1770</section>
1771</section>
1772
1773<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1774<t>
1775   The exact resource identified by an Internet request is determined by
1776   examining both the Request-URI and the Host header field.
1777</t>
1778<t>
1779   An origin server that does not allow resources to differ by the
1780   requested host &MAY; ignore the Host header field value when
1781   determining the resource identified by an HTTP/1.1 request. (But see
1782   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1783   for other requirements on Host support in HTTP/1.1.)
1784</t>
1785<t>
1786   An origin server that does differentiate resources based on the host
1787   requested (sometimes referred to as virtual hosts or vanity host
1788   names) &MUST; use the following rules for determining the requested
1789   resource on an HTTP/1.1 request:
1790  <list style="numbers">
1791    <t>If Request-URI is an absoluteURI, the host is part of the
1792     Request-URI. Any Host header field value in the request &MUST; be
1793     ignored.</t>
1794    <t>If the Request-URI is not an absoluteURI, and the request includes
1795     a Host header field, the host is determined by the Host header
1796     field value.</t>
1797    <t>If the host as determined by rule 1 or 2 is not a valid host on
1798     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1799  </list>
1800</t>
1801<t>
1802   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1803   attempt to use heuristics (e.g., examination of the URI path for
1804   something unique to a particular host) in order to determine what
1805   exact resource is being requested.
1806</t>
1807</section>
1808
1809</section>
1810
1811
1812<section title="Response" anchor="response">
1813<t>
1814   After receiving and interpreting a request message, a server responds
1815   with an HTTP response message.
1816</t>
1817<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1818  Response      = Status-Line               ; <xref target="status-line"/>
1819                  *(( general-header        ; <xref target="general.header.fields"/>
1820                   | response-header        ; &response-header-fields;
1821                   | entity-header ) CRLF)  ; &entity-header-fields;
1822                  CRLF
1823                  [ message-body ]          ; <xref target="message.body"/>
1824</artwork></figure>
1825
1826<section title="Status-Line" anchor="status-line">
1827<t>
1828   The first line of a Response message is the Status-Line, consisting
1829   of the protocol version followed by a numeric status code and its
1830   associated textual phrase, with each element separated by SP
1831   characters. No CR or LF is allowed except in the final CRLF sequence.
1832</t>
1833<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1834  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1835</artwork></figure>
1836
1837<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1838<t>
1839   The Status-Code element is a 3-digit integer result code of the
1840   attempt to understand and satisfy the request. These codes are fully
1841   defined in &status-codes;.  The Reason Phrase exists for the sole
1842   purpose of providing a textual description associated with the numeric
1843   status code, out of deference to earlier Internet application protocols
1844   that were more frequently used with interactive text clients.
1845   A client &SHOULD; ignore the content of the Reason Phrase.
1846</t>
1847<t>
1848   The first digit of the Status-Code defines the class of response. The
1849   last two digits do not have any categorization role. There are 5
1850   values for the first digit:
1851  <list style="symbols">
1852    <t>
1853      1xx: Informational - Request received, continuing process
1854    </t>
1855    <t>
1856      2xx: Success - The action was successfully received,
1857        understood, and accepted
1858    </t>
1859    <t>
1860      3xx: Redirection - Further action must be taken in order to
1861        complete the request
1862    </t>
1863    <t>
1864      4xx: Client Error - The request contains bad syntax or cannot
1865        be fulfilled
1866    </t>
1867    <t>
1868      5xx: Server Error - The server failed to fulfill an apparently
1869        valid request
1870    </t>
1871  </list>
1872</t>
1873<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"/>
1874  Status-Code    = 3DIGIT
1875  Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1876</artwork></figure>
1877</section>
1878</section>
1879
1880</section>
1881
1882
1883<section title="Connections" anchor="connections">
1884
1885<section title="Persistent Connections" anchor="persistent.connections">
1886
1887<section title="Purpose" anchor="persistent.purpose">
1888<t>
1889   Prior to persistent connections, a separate TCP connection was
1890   established to fetch each URL, increasing the load on HTTP servers
1891   and causing congestion on the Internet. The use of inline images and
1892   other associated data often require a client to make multiple
1893   requests of the same server in a short amount of time. Analysis of
1894   these performance problems and results from a prototype
1895   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1896   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1897   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1898   T/TCP <xref target="Tou1998"/>.
1899</t>
1900<t>
1901   Persistent HTTP connections have a number of advantages:
1902  <list style="symbols">
1903      <t>
1904        By opening and closing fewer TCP connections, CPU time is saved
1905        in routers and hosts (clients, servers, proxies, gateways,
1906        tunnels, or caches), and memory used for TCP protocol control
1907        blocks can be saved in hosts.
1908      </t>
1909      <t>
1910        HTTP requests and responses can be pipelined on a connection.
1911        Pipelining allows a client to make multiple requests without
1912        waiting for each response, allowing a single TCP connection to
1913        be used much more efficiently, with much lower elapsed time.
1914      </t>
1915      <t>
1916        Network congestion is reduced by reducing the number of packets
1917        caused by TCP opens, and by allowing TCP sufficient time to
1918        determine the congestion state of the network.
1919      </t>
1920      <t>
1921        Latency on subsequent requests is reduced since there is no time
1922        spent in TCP's connection opening handshake.
1923      </t>
1924      <t>
1925        HTTP can evolve more gracefully, since errors can be reported
1926        without the penalty of closing the TCP connection. Clients using
1927        future versions of HTTP might optimistically try a new feature,
1928        but if communicating with an older server, retry with old
1929        semantics after an error is reported.
1930      </t>
1931    </list>
1932</t>
1933<t>
1934   HTTP implementations &SHOULD; implement persistent connections.
1935</t>
1936</section>
1937
1938<section title="Overall Operation" anchor="persistent.overall">
1939<t>
1940   A significant difference between HTTP/1.1 and earlier versions of
1941   HTTP is that persistent connections are the default behavior of any
1942   HTTP connection. That is, unless otherwise indicated, the client
1943   &SHOULD; assume that the server will maintain a persistent connection,
1944   even after error responses from the server.
1945</t>
1946<t>
1947   Persistent connections provide a mechanism by which a client and a
1948   server can signal the close of a TCP connection. This signaling takes
1949   place using the Connection header field (<xref target="header.connection"/>). Once a close
1950   has been signaled, the client &MUST-NOT; send any more requests on that
1951   connection.
1952</t>
1953
1954<section title="Negotiation" anchor="persistent.negotiation">
1955<t>
1956   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1957   maintain a persistent connection unless a Connection header including
1958   the connection-token "close" was sent in the request. If the server
1959   chooses to close the connection immediately after sending the
1960   response, it &SHOULD; send a Connection header including the
1961   connection-token close.
1962</t>
1963<t>
1964   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1965   decide to keep it open based on whether the response from a server
1966   contains a Connection header with the connection-token close. In case
1967   the client does not want to maintain a connection for more than that
1968   request, it &SHOULD; send a Connection header including the
1969   connection-token close.
1970</t>
1971<t>
1972   If either the client or the server sends the close token in the
1973   Connection header, that request becomes the last one for the
1974   connection.
1975</t>
1976<t>
1977   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1978   maintained for HTTP versions less than 1.1 unless it is explicitly
1979   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1980   compatibility with HTTP/1.0 clients.
1981</t>
1982<t>
1983   In order to remain persistent, all messages on the connection &MUST;
1984   have a self-defined message length (i.e., one not defined by closure
1985   of the connection), as described in <xref target="message.length"/>.
1986</t>
1987</section>
1988
1989<section title="Pipelining" anchor="pipelining">
1990<t>
1991   A client that supports persistent connections &MAY; "pipeline" its
1992   requests (i.e., send multiple requests without waiting for each
1993   response). A server &MUST; send its responses to those requests in the
1994   same order that the requests were received.
1995</t>
1996<t>
1997   Clients which assume persistent connections and pipeline immediately
1998   after connection establishment &SHOULD; be prepared to retry their
1999   connection if the first pipelined attempt fails. If a client does
2000   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2001   persistent. Clients &MUST; also be prepared to resend their requests if
2002   the server closes the connection before sending all of the
2003   corresponding responses.
2004</t>
2005<t>
2006   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
2007   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
2008   premature termination of the transport connection could lead to
2009   indeterminate results. A client wishing to send a non-idempotent
2010   request &SHOULD; wait to send that request until it has received the
2011   response status for the previous request.
2012</t>
2013</section>
2014</section>
2015
2016<section title="Proxy Servers" anchor="persistent.proxy">
2017<t>
2018   It is especially important that proxies correctly implement the
2019   properties of the Connection header field as specified in <xref target="header.connection"/>.
2020</t>
2021<t>
2022   The proxy server &MUST; signal persistent connections separately with
2023   its clients and the origin servers (or other proxy servers) that it
2024   connects to. Each persistent connection applies to only one transport
2025   link.
2026</t>
2027<t>
2028   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2029   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
2030   discussion of the problems with the Keep-Alive header implemented by
2031   many HTTP/1.0 clients).
2032</t>
2033</section>
2034
2035<section title="Practical Considerations" anchor="persistent.practical">
2036<t>
2037   Servers will usually have some time-out value beyond which they will
2038   no longer maintain an inactive connection. Proxy servers might make
2039   this a higher value since it is likely that the client will be making
2040   more connections through the same server. The use of persistent
2041   connections places no requirements on the length (or existence) of
2042   this time-out for either the client or the server.
2043</t>
2044<t>
2045   When a client or server wishes to time-out it &SHOULD; issue a graceful
2046   close on the transport connection. Clients and servers &SHOULD; both
2047   constantly watch for the other side of the transport close, and
2048   respond to it as appropriate. If a client or server does not detect
2049   the other side's close promptly it could cause unnecessary resource
2050   drain on the network.
2051</t>
2052<t>
2053   A client, server, or proxy &MAY; close the transport connection at any
2054   time. For example, a client might have started to send a new request
2055   at the same time that the server has decided to close the "idle"
2056   connection. From the server's point of view, the connection is being
2057   closed while it was idle, but from the client's point of view, a
2058   request is in progress.
2059</t>
2060<t>
2061   This means that clients, servers, and proxies &MUST; be able to recover
2062   from asynchronous close events. Client software &SHOULD; reopen the
2063   transport connection and retransmit the aborted sequence of requests
2064   without user interaction so long as the request sequence is
2065   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2066   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2067   human operator the choice of retrying the request(s). Confirmation by
2068   user-agent software with semantic understanding of the application
2069   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2070   be repeated if the second sequence of requests fails.
2071</t>
2072<t>
2073   Servers &SHOULD; always respond to at least one request per connection,
2074   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2075   middle of transmitting a response, unless a network or client failure
2076   is suspected.
2077</t>
2078<t>
2079   Clients that use persistent connections &SHOULD; limit the number of
2080   simultaneous connections that they maintain to a given server. A
2081   single-user client &SHOULD-NOT; maintain more than 2 connections with
2082   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2083   another server or proxy, where N is the number of simultaneously
2084   active users. These guidelines are intended to improve HTTP response
2085   times and avoid congestion.
2086</t>
2087</section>
2088</section>
2089
2090<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2091
2092<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2093<t>
2094   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2095   flow control mechanisms to resolve temporary overloads, rather than
2096   terminating connections with the expectation that clients will retry.
2097   The latter technique can exacerbate network congestion.
2098</t>
2099</section>
2100
2101<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2102<t>
2103   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2104   the network connection for an error status while it is transmitting
2105   the request. If the client sees an error status, it &SHOULD;
2106   immediately cease transmitting the body. If the body is being sent
2107   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2108   empty trailer &MAY; be used to prematurely mark the end of the message.
2109   If the body was preceded by a Content-Length header, the client &MUST;
2110   close the connection.
2111</t>
2112</section>
2113
2114<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2115<t>
2116   The purpose of the 100 (Continue) status (see &status-100;) is to
2117   allow a client that is sending a request message with a request body
2118   to determine if the origin server is willing to accept the request
2119   (based on the request headers) before the client sends the request
2120   body. In some cases, it might either be inappropriate or highly
2121   inefficient for the client to send the body if the server will reject
2122   the message without looking at the body.
2123</t>
2124<t>
2125   Requirements for HTTP/1.1 clients:
2126  <list style="symbols">
2127    <t>
2128        If a client will wait for a 100 (Continue) response before
2129        sending the request body, it &MUST; send an Expect request-header
2130        field (&header-expect;) with the "100-continue" expectation.
2131    </t>
2132    <t>
2133        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2134        with the "100-continue" expectation if it does not intend
2135        to send a request body.
2136    </t>
2137  </list>
2138</t>
2139<t>
2140   Because of the presence of older implementations, the protocol allows
2141   ambiguous situations in which a client may send "Expect: 100-continue"
2142   without receiving either a 417 (Expectation Failed) status
2143   or a 100 (Continue) status. Therefore, when a client sends this
2144   header field to an origin server (possibly via a proxy) from which it
2145   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2146   for an indefinite period before sending the request body.
2147</t>
2148<t>
2149   Requirements for HTTP/1.1 origin servers:
2150  <list style="symbols">
2151    <t> Upon receiving a request which includes an Expect request-header
2152        field with the "100-continue" expectation, an origin server &MUST;
2153        either respond with 100 (Continue) status and continue to read
2154        from the input stream, or respond with a final status code. The
2155        origin server &MUST-NOT; wait for the request body before sending
2156        the 100 (Continue) response. If it responds with a final status
2157        code, it &MAY; close the transport connection or it &MAY; continue
2158        to read and discard the rest of the request.  It &MUST-NOT;
2159        perform the requested method if it returns a final status code.
2160    </t>
2161    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2162        the request message does not include an Expect request-header
2163        field with the "100-continue" expectation, and &MUST-NOT; send a
2164        100 (Continue) response if such a request comes from an HTTP/1.0
2165        (or earlier) client. There is an exception to this rule: for
2166        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2167        status in response to an HTTP/1.1 PUT or POST request that does
2168        not include an Expect request-header field with the "100-continue"
2169        expectation. This exception, the purpose of which is
2170        to minimize any client processing delays associated with an
2171        undeclared wait for 100 (Continue) status, applies only to
2172        HTTP/1.1 requests, and not to requests with any other HTTP-version
2173        value.
2174    </t>
2175    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2176        already received some or all of the request body for the
2177        corresponding request.
2178    </t>
2179    <t> An origin server that sends a 100 (Continue) response &MUST;
2180    ultimately send a final status code, once the request body is
2181        received and processed, unless it terminates the transport
2182        connection prematurely.
2183    </t>
2184    <t> If an origin server receives a request that does not include an
2185        Expect request-header field with the "100-continue" expectation,
2186        the request includes a request body, and the server responds
2187        with a final status code before reading the entire request body
2188        from the transport connection, then the server &SHOULD-NOT;  close
2189        the transport connection until it has read the entire request,
2190        or until the client closes the connection. Otherwise, the client
2191        might not reliably receive the response message. However, this
2192        requirement is not be construed as preventing a server from
2193        defending itself against denial-of-service attacks, or from
2194        badly broken client implementations.
2195      </t>
2196    </list>
2197</t>
2198<t>
2199   Requirements for HTTP/1.1 proxies:
2200  <list style="symbols">
2201    <t> If a proxy receives a request that includes an Expect request-header
2202        field with the "100-continue" expectation, and the proxy
2203        either knows that the next-hop server complies with HTTP/1.1 or
2204        higher, or does not know the HTTP version of the next-hop
2205        server, it &MUST; forward the request, including the Expect header
2206        field.
2207    </t>
2208    <t> If the proxy knows that the version of the next-hop server is
2209        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2210        respond with a 417 (Expectation Failed) status.
2211    </t>
2212    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2213        numbers received from recently-referenced next-hop servers.
2214    </t>
2215    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2216        request message was received from an HTTP/1.0 (or earlier)
2217        client and did not include an Expect request-header field with
2218        the "100-continue" expectation. This requirement overrides the
2219        general rule for forwarding of 1xx responses (see &status-1xx;).
2220    </t>
2221  </list>
2222</t>
2223</section>
2224
2225<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2226<t>
2227   If an HTTP/1.1 client sends a request which includes a request body,
2228   but which does not include an Expect request-header field with the
2229   "100-continue" expectation, and if the client is not directly
2230   connected to an HTTP/1.1 origin server, and if the client sees the
2231   connection close before receiving any status from the server, the
2232   client &SHOULD; retry the request.  If the client does retry this
2233   request, it &MAY; use the following "binary exponential backoff"
2234   algorithm to be assured of obtaining a reliable response:
2235  <list style="numbers">
2236    <t>
2237      Initiate a new connection to the server
2238    </t>
2239    <t>
2240      Transmit the request-headers
2241    </t>
2242    <t>
2243      Initialize a variable R to the estimated round-trip time to the
2244         server (e.g., based on the time it took to establish the
2245         connection), or to a constant value of 5 seconds if the round-trip
2246         time is not available.
2247    </t>
2248    <t>
2249       Compute T = R * (2**N), where N is the number of previous
2250         retries of this request.
2251    </t>
2252    <t>
2253       Wait either for an error response from the server, or for T
2254         seconds (whichever comes first)
2255    </t>
2256    <t>
2257       If no error response is received, after T seconds transmit the
2258         body of the request.
2259    </t>
2260    <t>
2261       If client sees that the connection is closed prematurely,
2262         repeat from step 1 until the request is accepted, an error
2263         response is received, or the user becomes impatient and
2264         terminates the retry process.
2265    </t>
2266  </list>
2267</t>
2268<t>
2269   If at any point an error status is received, the client
2270  <list style="symbols">
2271      <t>&SHOULD-NOT;  continue and</t>
2272
2273      <t>&SHOULD; close the connection if it has not completed sending the
2274        request message.</t>
2275    </list>
2276</t>
2277</section>
2278</section>
2279</section>
2280
2281
2282<section title="Header Field Definitions" anchor="header.fields">
2283<t>
2284   This section defines the syntax and semantics of HTTP/1.1 header fields
2285   related to message framing and transport protocols.
2286</t>
2287<t>
2288   For entity-header fields, both sender and recipient refer to either the
2289   client or the server, depending on who sends and who receives the entity.
2290</t>
2291
2292<section title="Connection" anchor="header.connection">
2293  <iref primary="true" item="Connection header" x:for-anchor=""/>
2294  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2295<t>
2296   The Connection general-header field allows the sender to specify
2297   options that are desired for that particular connection and &MUST-NOT;
2298   be communicated by proxies over further connections.
2299</t>
2300<t>
2301   The Connection header has the following grammar:
2302</t>
2303<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2304  Connection = "Connection" ":" 1#(connection-token)
2305  connection-token  = token
2306</artwork></figure>
2307<t>
2308   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2309   message is forwarded and, for each connection-token in this field,
2310   remove any header field(s) from the message with the same name as the
2311   connection-token. Connection options are signaled by the presence of
2312   a connection-token in the Connection header field, not by any
2313   corresponding additional header field(s), since the additional header
2314   field may not be sent if there are no parameters associated with that
2315   connection option.
2316</t>
2317<t>
2318   Message headers listed in the Connection header &MUST-NOT; include
2319   end-to-end headers, such as Cache-Control.
2320</t>
2321<t>
2322   HTTP/1.1 defines the "close" connection option for the sender to
2323   signal that the connection will be closed after completion of the
2324   response. For example,
2325</t>
2326<figure><artwork type="example">
2327    Connection: close
2328</artwork></figure>
2329<t>
2330   in either the request or the response header fields indicates that
2331   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2332   after the current request/response is complete.
2333</t>
2334<t>
2335   An HTTP/1.1 client that does not support persistent connections &MUST;
2336   include the "close" connection option in every request message.
2337</t>
2338<t>
2339   An HTTP/1.1 server that does not support persistent connections &MUST;
2340   include the "close" connection option in every response message that
2341   does not have a 1xx (informational) status code.
2342</t>
2343<t>
2344   A system receiving an HTTP/1.0 (or lower-version) message that
2345   includes a Connection header &MUST;, for each connection-token in this
2346   field, remove and ignore any header field(s) from the message with
2347   the same name as the connection-token. This protects against mistaken
2348   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2349</t>
2350</section>
2351
2352<section title="Content-Length" anchor="header.content-length">
2353  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2354  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2355<t>
2356   The Content-Length entity-header field indicates the size of the
2357   entity-body, in decimal number of OCTETs, sent to the recipient or,
2358   in the case of the HEAD method, the size of the entity-body that
2359   would have been sent had the request been a GET.
2360</t>
2361<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2362  Content-Length    = "Content-Length" ":" 1*DIGIT
2363</artwork></figure>
2364<t>
2365   An example is
2366</t>
2367<figure><artwork type="example">
2368    Content-Length: 3495
2369</artwork></figure>
2370<t>
2371   Applications &SHOULD; use this field to indicate the transfer-length of
2372   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2373</t>
2374<t>
2375   Any Content-Length greater than or equal to zero is a valid value.
2376   <xref target="message.length"/> describes how to determine the length of a message-body
2377   if a Content-Length is not given.
2378</t>
2379<t>
2380   Note that the meaning of this field is significantly different from
2381   the corresponding definition in MIME, where it is an optional field
2382   used within the "message/external-body" content-type. In HTTP, it
2383   &SHOULD; be sent whenever the message's length can be determined prior
2384   to being transferred, unless this is prohibited by the rules in
2385   <xref target="message.length"/>.
2386</t>
2387</section>
2388
2389<section title="Date" anchor="header.date">
2390  <iref primary="true" item="Date header" x:for-anchor=""/>
2391  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2392<t>
2393   The Date general-header field represents the date and time at which
2394   the message was originated, having the same semantics as orig-date in
2395   <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"/>;
2396   it &MUST; be sent in rfc1123-date format.
2397</t>
2398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2399  Date  = "Date" ":" HTTP-date
2400</artwork></figure>
2401<t>
2402   An example is
2403</t>
2404<figure><artwork type="example">
2405    Date: Tue, 15 Nov 1994 08:12:31 GMT
2406</artwork></figure>
2407<t>
2408   Origin servers &MUST; include a Date header field in all responses,
2409   except in these cases:
2410  <list style="numbers">
2411      <t>If the response status code is 100 (Continue) or 101 (Switching
2412         Protocols), the response &MAY; include a Date header field, at
2413         the server's option.</t>
2414
2415      <t>If the response status code conveys a server error, e.g. 500
2416         (Internal Server Error) or 503 (Service Unavailable), and it is
2417         inconvenient or impossible to generate a valid Date.</t>
2418
2419      <t>If the server does not have a clock that can provide a
2420         reasonable approximation of the current time, its responses
2421         &MUST-NOT; include a Date header field. In this case, the rules
2422         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2423  </list>
2424</t>
2425<t>
2426   A received message that does not have a Date header field &MUST; be
2427   assigned one by the recipient if the message will be cached by that
2428   recipient or gatewayed via a protocol which requires a Date. An HTTP
2429   implementation without a clock &MUST-NOT; cache responses without
2430   revalidating them on every use. An HTTP cache, especially a shared
2431   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2432   clock with a reliable external standard.
2433</t>
2434<t>
2435   Clients &SHOULD; only send a Date header field in messages that include
2436   an entity-body, as in the case of the PUT and POST requests, and even
2437   then it is optional. A client without a clock &MUST-NOT; send a Date
2438   header field in a request.
2439</t>
2440<t>
2441   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2442   time subsequent to the generation of the message. It &SHOULD; represent
2443   the best available approximation of the date and time of message
2444   generation, unless the implementation has no means of generating a
2445   reasonably accurate date and time. In theory, the date ought to
2446   represent the moment just before the entity is generated. In
2447   practice, the date can be generated at any time during the message
2448   origination without affecting its semantic value.
2449</t>
2450
2451<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2452<t>
2453   Some origin server implementations might not have a clock available.
2454   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2455   values to a response, unless these values were associated
2456   with the resource by a system or user with a reliable clock. It &MAY;
2457   assign an Expires value that is known, at or before server
2458   configuration time, to be in the past (this allows "pre-expiration"
2459   of responses without storing separate Expires values for each
2460   resource).
2461</t>
2462</section>
2463</section>
2464
2465<section title="Host" anchor="header.host">
2466  <iref primary="true" item="Host header" x:for-anchor=""/>
2467  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2468<t>
2469   The Host request-header field specifies the Internet host and port
2470   number of the resource being requested, as obtained from the original
2471   URI given by the user or referring resource (generally an HTTP URL,
2472   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2473   the naming authority of the origin server or gateway given by the
2474   original URL. This allows the origin server or gateway to
2475   differentiate between internally-ambiguous URLs, such as the root "/"
2476   URL of a server for multiple host names on a single IP address.
2477</t>
2478<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2479  Host = "Host" ":" uri-host [ ":" port ] ; <xref target="http.url"/>
2480</artwork></figure>
2481<t>
2482   A "host" without any trailing port information implies the default
2483   port for the service requested (e.g., "80" for an HTTP URL). For
2484   example, a request on the origin server for
2485   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2486</t>
2487<figure><artwork type="example">
2488    GET /pub/WWW/ HTTP/1.1
2489    Host: www.example.org
2490</artwork></figure>
2491<t>
2492   A client &MUST; include a Host header field in all HTTP/1.1 request
2493   messages. If the requested URI does not include an Internet host
2494   name for the service being requested, then the Host header field &MUST;
2495   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2496   request message it forwards does contain an appropriate Host header
2497   field that identifies the service being requested by the proxy. All
2498   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2499   status code to any HTTP/1.1 request message which lacks a Host header
2500   field.
2501</t>
2502<t>
2503   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2504   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2505   for other requirements relating to Host.
2506</t>
2507</section>
2508
2509<section title="TE" anchor="header.te">
2510  <iref primary="true" item="TE header" x:for-anchor=""/>
2511  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2512<t>
2513   The TE request-header field indicates what extension transfer-codings
2514   it is willing to accept in the response and whether or not it is
2515   willing to accept trailer fields in a chunked transfer-coding. Its
2516   value may consist of the keyword "trailers" and/or a comma-separated
2517   list of extension transfer-coding names with optional accept
2518   parameters (as described in <xref target="transfer.codings"/>).
2519</t>
2520<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2521  TE        = "TE" ":" #( t-codings )
2522  t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2523</artwork></figure>
2524<t>
2525   The presence of the keyword "trailers" indicates that the client is
2526   willing to accept trailer fields in a chunked transfer-coding, as
2527   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2528   transfer-coding values even though it does not itself represent a
2529   transfer-coding.
2530</t>
2531<t>
2532   Examples of its use are:
2533</t>
2534<figure><artwork type="example">
2535    TE: deflate
2536    TE:
2537    TE: trailers, deflate;q=0.5
2538</artwork></figure>
2539<t>
2540   The TE header field only applies to the immediate connection.
2541   Therefore, the keyword &MUST; be supplied within a Connection header
2542   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2543</t>
2544<t>
2545   A server tests whether a transfer-coding is acceptable, according to
2546   a TE field, using these rules:
2547  <list style="numbers">
2548    <x:lt>
2549      <t>The "chunked" transfer-coding is always acceptable. If the
2550         keyword "trailers" is listed, the client indicates that it is
2551         willing to accept trailer fields in the chunked response on
2552         behalf of itself and any downstream clients. The implication is
2553         that, if given, the client is stating that either all
2554         downstream clients are willing to accept trailer fields in the
2555         forwarded response, or that it will attempt to buffer the
2556         response on behalf of downstream recipients.
2557      </t><t>
2558         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2559         chunked response such that a client can be assured of buffering
2560         the entire response.</t>
2561    </x:lt>
2562    <x:lt>
2563      <t>If the transfer-coding being tested is one of the transfer-codings
2564         listed in the TE field, then it is acceptable unless it
2565         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2566         qvalue of 0 means "not acceptable.")</t>
2567    </x:lt>
2568    <x:lt>
2569      <t>If multiple transfer-codings are acceptable, then the
2570         acceptable transfer-coding with the highest non-zero qvalue is
2571         preferred.  The "chunked" transfer-coding always has a qvalue
2572         of 1.</t>
2573    </x:lt>
2574  </list>
2575</t>
2576<t>
2577   If the TE field-value is empty or if no TE field is present, the only
2578   transfer-coding  is "chunked". A message with no transfer-coding is
2579   always acceptable.
2580</t>
2581</section>
2582
2583<section title="Trailer" anchor="header.trailer">
2584  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2585  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2586<t>
2587   The Trailer general field value indicates that the given set of
2588   header fields is present in the trailer of a message encoded with
2589   chunked transfer-coding.
2590</t>
2591<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2592  Trailer  = "Trailer" ":" 1#field-name
2593</artwork></figure>
2594<t>
2595   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2596   message using chunked transfer-coding with a non-empty trailer. Doing
2597   so allows the recipient to know which header fields to expect in the
2598   trailer.
2599</t>
2600<t>
2601   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2602   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2603   trailer fields in a "chunked" transfer-coding.
2604</t>
2605<t>
2606   Message header fields listed in the Trailer header field &MUST-NOT;
2607   include the following header fields:
2608  <list style="symbols">
2609    <t>Transfer-Encoding</t>
2610    <t>Content-Length</t>
2611    <t>Trailer</t>
2612  </list>
2613</t>
2614</section>
2615
2616<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2617  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2618  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2619<t>
2620   The Transfer-Encoding general-header field indicates what (if any)
2621   type of transformation has been applied to the message body in order
2622   to safely transfer it between the sender and the recipient. This
2623   differs from the content-coding in that the transfer-coding is a
2624   property of the message, not of the entity.
2625</t>
2626<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2627  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2628</artwork></figure>
2629<t>
2630   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2631</t>
2632<figure><artwork type="example">
2633  Transfer-Encoding: chunked
2634</artwork></figure>
2635<t>
2636   If multiple encodings have been applied to an entity, the transfer-codings
2637   &MUST; be listed in the order in which they were applied.
2638   Additional information about the encoding parameters &MAY; be provided
2639   by other entity-header fields not defined by this specification.
2640</t>
2641<t>
2642   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2643   header.
2644</t>
2645</section>
2646
2647<section title="Upgrade" anchor="header.upgrade">
2648  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2649  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2650<t>
2651   The Upgrade general-header allows the client to specify what
2652   additional communication protocols it supports and would like to use
2653   if the server finds it appropriate to switch protocols. The server
2654   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2655   response to indicate which protocol(s) are being switched.
2656</t>
2657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2658  Upgrade        = "Upgrade" ":" 1#product
2659</artwork></figure>
2660<t>
2661   For example,
2662</t>
2663<figure><artwork type="example">
2664    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2665</artwork></figure>
2666<t>
2667   The Upgrade header field is intended to provide a simple mechanism
2668   for transition from HTTP/1.1 to some other, incompatible protocol. It
2669   does so by allowing the client to advertise its desire to use another
2670   protocol, such as a later version of HTTP with a higher major version
2671   number, even though the current request has been made using HTTP/1.1.
2672   This eases the difficult transition between incompatible protocols by
2673   allowing the client to initiate a request in the more commonly
2674   supported protocol while indicating to the server that it would like
2675   to use a "better" protocol if available (where "better" is determined
2676   by the server, possibly according to the nature of the method and/or
2677   resource being requested).
2678</t>
2679<t>
2680   The Upgrade header field only applies to switching application-layer
2681   protocols upon the existing transport-layer connection. Upgrade
2682   cannot be used to insist on a protocol change; its acceptance and use
2683   by the server is optional. The capabilities and nature of the
2684   application-layer communication after the protocol change is entirely
2685   dependent upon the new protocol chosen, although the first action
2686   after changing the protocol &MUST; be a response to the initial HTTP
2687   request containing the Upgrade header field.
2688</t>
2689<t>
2690   The Upgrade header field only applies to the immediate connection.
2691   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2692   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2693   HTTP/1.1 message.
2694</t>
2695<t>
2696   The Upgrade header field cannot be used to indicate a switch to a
2697   protocol on a different connection. For that purpose, it is more
2698   appropriate to use a 301, 302, 303, or 305 redirection response.
2699</t>
2700<t>
2701   This specification only defines the protocol name "HTTP" for use by
2702   the family of Hypertext Transfer Protocols, as defined by the HTTP
2703   version rules of <xref target="http.version"/> and future updates to this
2704   specification. Any token can be used as a protocol name; however, it
2705   will only be useful if both the client and server associate the name
2706   with the same protocol.
2707</t>
2708</section>
2709
2710<section title="Via" anchor="header.via">
2711  <iref primary="true" item="Via header" x:for-anchor=""/>
2712  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2713<t>
2714   The Via general-header field &MUST; be used by gateways and proxies to
2715   indicate the intermediate protocols and recipients between the user
2716   agent and the server on requests, and between the origin server and
2717   the client on responses. It is analogous to the "Received" field of
2718   <xref target="RFC2822"/> and is intended to be used for tracking message forwards,
2719   avoiding request loops, and identifying the protocol capabilities of
2720   all senders along the request/response chain.
2721</t>
2722<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"/>
2723  Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2724  received-protocol = [ protocol-name "/" ] protocol-version
2725  protocol-name     = token
2726  protocol-version  = token
2727  received-by       = ( uri-host [ ":" port ] ) | pseudonym
2728  pseudonym         = token
2729</artwork></figure>
2730<t>
2731   The received-protocol indicates the protocol version of the message
2732   received by the server or client along each segment of the
2733   request/response chain. The received-protocol version is appended to
2734   the Via field value when the message is forwarded so that information
2735   about the protocol capabilities of upstream applications remains
2736   visible to all recipients.
2737</t>
2738<t>
2739   The protocol-name is optional if and only if it would be "HTTP". The
2740   received-by field is normally the host and optional port number of a
2741   recipient server or client that subsequently forwarded the message.
2742   However, if the real host is considered to be sensitive information,
2743   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2744   be assumed to be the default port of the received-protocol.
2745</t>
2746<t>
2747   Multiple Via field values represents each proxy or gateway that has
2748   forwarded the message. Each recipient &MUST; append its information
2749   such that the end result is ordered according to the sequence of
2750   forwarding applications.
2751</t>
2752<t>
2753   Comments &MAY; be used in the Via header field to identify the software
2754   of the recipient proxy or gateway, analogous to the User-Agent and
2755   Server header fields. However, all comments in the Via field are
2756   optional and &MAY; be removed by any recipient prior to forwarding the
2757   message.
2758</t>
2759<t>
2760   For example, a request message could be sent from an HTTP/1.0 user
2761   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2762   forward the request to a public proxy at p.example.net, which completes
2763   the request by forwarding it to the origin server at www.example.com.
2764   The request received by www.example.com would then have the following
2765   Via header field:
2766</t>
2767<figure><artwork type="example">
2768    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2769</artwork></figure>
2770<t>
2771   Proxies and gateways used as a portal through a network firewall
2772   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2773   the firewall region. This information &SHOULD; only be propagated if
2774   explicitly enabled. If not enabled, the received-by host of any host
2775   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2776   for that host.
2777</t>
2778<t>
2779   For organizations that have strong privacy requirements for hiding
2780   internal structures, a proxy &MAY; combine an ordered subsequence of
2781   Via header field entries with identical received-protocol values into
2782   a single such entry. For example,
2783</t>
2784<figure><artwork type="example">
2785    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2786</artwork></figure>
2787<t>
2788        could be collapsed to
2789</t>
2790<figure><artwork type="example">
2791    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2792</artwork></figure>
2793<t>
2794   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2795   under the same organizational control and the hosts have already been
2796   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2797   have different received-protocol values.
2798</t>
2799</section>
2800
2801</section>
2802
2803<section title="IANA Considerations" anchor="IANA.considerations">
2804<t>
2805   <cref>TBD.</cref>
2806</t>
2807</section>
2808
2809<section title="Security Considerations" anchor="security.considerations">
2810<t>
2811   This section is meant to inform application developers, information
2812   providers, and users of the security limitations in HTTP/1.1 as
2813   described by this document. The discussion does not include
2814   definitive solutions to the problems revealed, though it does make
2815   some suggestions for reducing security risks.
2816</t>
2817
2818<section title="Personal Information" anchor="personal.information">
2819<t>
2820   HTTP clients are often privy to large amounts of personal information
2821   (e.g. the user's name, location, mail address, passwords, encryption
2822   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2823   leakage of this information.
2824   We very strongly recommend that a convenient interface be provided
2825   for the user to control dissemination of such information, and that
2826   designers and implementors be particularly careful in this area.
2827   History shows that errors in this area often create serious security
2828   and/or privacy problems and generate highly adverse publicity for the
2829   implementor's company.
2830</t>
2831</section>
2832
2833<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2834<t>
2835   A server is in the position to save personal data about a user's
2836   requests which might identify their reading patterns or subjects of
2837   interest. This information is clearly confidential in nature and its
2838   handling can be constrained by law in certain countries. People using
2839   HTTP to provide data are responsible for ensuring that
2840   such material is not distributed without the permission of any
2841   individuals that are identifiable by the published results.
2842</t>
2843</section>
2844
2845<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2846<t>
2847   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2848   the documents returned by HTTP requests to be only those that were
2849   intended by the server administrators. If an HTTP server translates
2850   HTTP URIs directly into file system calls, the server &MUST; take
2851   special care not to serve files that were not intended to be
2852   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2853   other operating systems use ".." as a path component to indicate a
2854   directory level above the current one. On such a system, an HTTP
2855   server &MUST; disallow any such construct in the Request-URI if it
2856   would otherwise allow access to a resource outside those intended to
2857   be accessible via the HTTP server. Similarly, files intended for
2858   reference only internally to the server (such as access control
2859   files, configuration files, and script code) &MUST; be protected from
2860   inappropriate retrieval, since they might contain sensitive
2861   information. Experience has shown that minor bugs in such HTTP server
2862   implementations have turned into security risks.
2863</t>
2864</section>
2865
2866<section title="DNS Spoofing" anchor="dns.spoofing">
2867<t>
2868   Clients using HTTP rely heavily on the Domain Name Service, and are
2869   thus generally prone to security attacks based on the deliberate
2870   mis-association of IP addresses and DNS names. Clients need to be
2871   cautious in assuming the continuing validity of an IP number/DNS name
2872   association.
2873</t>
2874<t>
2875   In particular, HTTP clients &SHOULD; rely on their name resolver for
2876   confirmation of an IP number/DNS name association, rather than
2877   caching the result of previous host name lookups. Many platforms
2878   already can cache host name lookups locally when appropriate, and
2879   they &SHOULD; be configured to do so. It is proper for these lookups to
2880   be cached, however, only when the TTL (Time To Live) information
2881   reported by the name server makes it likely that the cached
2882   information will remain useful.
2883</t>
2884<t>
2885   If HTTP clients cache the results of host name lookups in order to
2886   achieve a performance improvement, they &MUST; observe the TTL
2887   information reported by DNS.
2888</t>
2889<t>
2890   If HTTP clients do not observe this rule, they could be spoofed when
2891   a previously-accessed server's IP address changes. As network
2892   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2893   possibility of this form of attack will grow. Observing this
2894   requirement thus reduces this potential security vulnerability.
2895</t>
2896<t>
2897   This requirement also improves the load-balancing behavior of clients
2898   for replicated servers using the same DNS name and reduces the
2899   likelihood of a user's experiencing failure in accessing sites which
2900   use that strategy.
2901</t>
2902</section>
2903
2904<section title="Proxies and Caching" anchor="attack.proxies">
2905<t>
2906   By their very nature, HTTP proxies are men-in-the-middle, and
2907   represent an opportunity for man-in-the-middle attacks. Compromise of
2908   the systems on which the proxies run can result in serious security
2909   and privacy problems. Proxies have access to security-related
2910   information, personal information about individual users and
2911   organizations, and proprietary information belonging to users and
2912   content providers. A compromised proxy, or a proxy implemented or
2913   configured without regard to security and privacy considerations,
2914   might be used in the commission of a wide range of potential attacks.
2915</t>
2916<t>
2917   Proxy operators should protect the systems on which proxies run as
2918   they would protect any system that contains or transports sensitive
2919   information. In particular, log information gathered at proxies often
2920   contains highly sensitive personal information, and/or information
2921   about organizations. Log information should be carefully guarded, and
2922   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2923</t>
2924<t>
2925   Proxy implementors should consider the privacy and security
2926   implications of their design and coding decisions, and of the
2927   configuration options they provide to proxy operators (especially the
2928   default configuration).
2929</t>
2930<t>
2931   Users of a proxy need to be aware that they are no trustworthier than
2932   the people who run the proxy; HTTP itself cannot solve this problem.
2933</t>
2934<t>
2935   The judicious use of cryptography, when appropriate, may suffice to
2936   protect against a broad range of security and privacy attacks. Such
2937   cryptography is beyond the scope of the HTTP/1.1 specification.
2938</t>
2939</section>
2940
2941<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2942<t>
2943   They exist. They are hard to defend against. Research continues.
2944   Beware.
2945</t>
2946</section>
2947</section>
2948
2949<section title="Acknowledgments" anchor="ack">
2950<t>
2951   This specification makes heavy use of the augmented BNF and generic
2952   constructs defined by David H. Crocker for <xref target="RFC822ABNF"/>. Similarly, it
2953   reuses many of the definitions provided by Nathaniel Borenstein and
2954   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2955   specification will help reduce past confusion over the relationship
2956   between HTTP and Internet mail message formats.
2957</t>
2958<t>
2959   HTTP has evolved considerably over the years. It has
2960   benefited from a large and active developer community--the many
2961   people who have participated on the www-talk mailing list--and it is
2962   that community which has been most responsible for the success of
2963   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2964   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2965   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2966   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2967   VanHeyningen deserve special recognition for their efforts in
2968   defining early aspects of the protocol.
2969</t>
2970<t>
2971   This document has benefited greatly from the comments of all those
2972   participating in the HTTP-WG. In addition to those already mentioned,
2973   the following individuals have contributed to this specification:
2974</t>
2975<t>
2976   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2977   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2978   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2979   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2980   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2981   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2982   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2983   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2984   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2985   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2986   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2987   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2988   Josh Cohen.
2989</t>
2990<t>
2991   Thanks to the "cave men" of Palo Alto. You know who you are.
2992</t>
2993<t>
2994   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2995   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2996   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2997   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2998   Larry Masinter for their help. And thanks go particularly to Jeff
2999   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3000</t>
3001<t>
3002   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3003   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3004   discovery of many of the problems that this document attempts to
3005   rectify.
3006</t>
3007</section>
3008
3009</middle>
3010<back>
3011
3012<references title="Normative References">
3013
3014<reference anchor="ISO-8859-1">
3015  <front>
3016    <title>
3017     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3018    </title>
3019    <author>
3020      <organization>International Organization for Standardization</organization>
3021    </author>
3022    <date year="1998"/>
3023  </front>
3024  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3025</reference>
3026
3027<reference anchor="Part2">
3028  <front>
3029    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3030    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3031      <organization abbrev="Day Software">Day Software</organization>
3032      <address><email>fielding@gbiv.com</email></address>
3033    </author>
3034    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3035      <organization>One Laptop per Child</organization>
3036      <address><email>jg@laptop.org</email></address>
3037    </author>
3038    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3039      <organization abbrev="HP">Hewlett-Packard Company</organization>
3040      <address><email>JeffMogul@acm.org</email></address>
3041    </author>
3042    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3043      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3044      <address><email>henrikn@microsoft.com</email></address>
3045    </author>
3046    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3047      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3048      <address><email>LMM@acm.org</email></address>
3049    </author>
3050    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3051      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3052      <address><email>paulle@microsoft.com</email></address>
3053    </author>
3054    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3055      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3056      <address><email>timbl@w3.org</email></address>
3057    </author>
3058    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3059      <organization abbrev="W3C">World Wide Web Consortium</organization>
3060      <address><email>ylafon@w3.org</email></address>
3061    </author>
3062    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3063      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3064      <address><email>julian.reschke@greenbytes.de</email></address>
3065    </author>
3066    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3067  </front>
3068  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3069  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3070</reference>
3071
3072<reference anchor="Part3">
3073  <front>
3074    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3075    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3076      <organization abbrev="Day Software">Day Software</organization>
3077      <address><email>fielding@gbiv.com</email></address>
3078    </author>
3079    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3080      <organization>One Laptop per Child</organization>
3081      <address><email>jg@laptop.org</email></address>
3082    </author>
3083    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3084      <organization abbrev="HP">Hewlett-Packard Company</organization>
3085      <address><email>JeffMogul@acm.org</email></address>
3086    </author>
3087    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3088      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3089      <address><email>henrikn@microsoft.com</email></address>
3090    </author>
3091    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3092      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3093      <address><email>LMM@acm.org</email></address>
3094    </author>
3095    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3096      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3097      <address><email>paulle@microsoft.com</email></address>
3098    </author>
3099    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3100      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3101      <address><email>timbl@w3.org</email></address>
3102    </author>
3103    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3104      <organization abbrev="W3C">World Wide Web Consortium</organization>
3105      <address><email>ylafon@w3.org</email></address>
3106    </author>
3107    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3108      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3109      <address><email>julian.reschke@greenbytes.de</email></address>
3110    </author>
3111    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3112  </front>
3113  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3114  <x:source href="p3-payload.xml" basename="p3-payload"/>
3115</reference>
3116
3117<reference anchor="Part5">
3118  <front>
3119    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3120    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3121      <organization abbrev="Day Software">Day Software</organization>
3122      <address><email>fielding@gbiv.com</email></address>
3123    </author>
3124    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3125      <organization>One Laptop per Child</organization>
3126      <address><email>jg@laptop.org</email></address>
3127    </author>
3128    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3129      <organization abbrev="HP">Hewlett-Packard Company</organization>
3130      <address><email>JeffMogul@acm.org</email></address>
3131    </author>
3132    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3133      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3134      <address><email>henrikn@microsoft.com</email></address>
3135    </author>
3136    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3137      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3138      <address><email>LMM@acm.org</email></address>
3139    </author>
3140    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3141      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3142      <address><email>paulle@microsoft.com</email></address>
3143    </author>
3144    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3145      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3146      <address><email>timbl@w3.org</email></address>
3147    </author>
3148    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3149      <organization abbrev="W3C">World Wide Web Consortium</organization>
3150      <address><email>ylafon@w3.org</email></address>
3151    </author>
3152    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3153      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3154      <address><email>julian.reschke@greenbytes.de</email></address>
3155    </author>
3156    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3157  </front>
3158  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3159  <x:source href="p5-range.xml" basename="p5-range"/>
3160</reference>
3161
3162<reference anchor="Part6">
3163  <front>
3164    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3165    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3166      <organization abbrev="Day Software">Day Software</organization>
3167      <address><email>fielding@gbiv.com</email></address>
3168    </author>
3169    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3170      <organization>One Laptop per Child</organization>
3171      <address><email>jg@laptop.org</email></address>
3172    </author>
3173    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3174      <organization abbrev="HP">Hewlett-Packard Company</organization>
3175      <address><email>JeffMogul@acm.org</email></address>
3176    </author>
3177    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3178      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3179      <address><email>henrikn@microsoft.com</email></address>
3180    </author>
3181    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3182      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3183      <address><email>LMM@acm.org</email></address>
3184    </author>
3185    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3186      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3187      <address><email>paulle@microsoft.com</email></address>
3188    </author>
3189    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3190      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3191      <address><email>timbl@w3.org</email></address>
3192    </author>
3193    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3194      <organization abbrev="W3C">World Wide Web Consortium</organization>
3195      <address><email>ylafon@w3.org</email></address>
3196    </author>
3197    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3198      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3199      <address><email>julian.reschke@greenbytes.de</email></address>
3200    </author>
3201    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3202  </front>
3203  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3204  <x:source href="p6-cache.xml" basename="p6-cache"/>
3205</reference>
3206
3207<reference anchor="RFC822ABNF">
3208  <front>
3209    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3210    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3211      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3212      <address><email>DCrocker@UDel-Relay</email></address>
3213    </author>
3214    <date month="August" day="13" year="1982"/>
3215  </front>
3216  <seriesInfo name="STD" value="11"/>
3217  <seriesInfo name="RFC" value="822"/>
3218</reference>
3219
3220<reference anchor="RFC2045">
3221  <front>
3222    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3223    <author initials="N." surname="Freed" fullname="Ned Freed">
3224      <organization>Innosoft International, Inc.</organization>
3225      <address><email>ned@innosoft.com</email></address>
3226    </author>
3227    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3228      <organization>First Virtual Holdings</organization>
3229      <address><email>nsb@nsb.fv.com</email></address>
3230    </author>
3231    <date month="November" year="1996"/>
3232  </front>
3233  <seriesInfo name="RFC" value="2045"/>
3234</reference>
3235
3236<reference anchor="RFC2047">
3237  <front>
3238    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3239    <author initials="K." surname="Moore" fullname="Keith Moore">
3240      <organization>University of Tennessee</organization>
3241      <address><email>moore@cs.utk.edu</email></address>
3242    </author>
3243    <date month="November" year="1996"/>
3244  </front>
3245  <seriesInfo name="RFC" value="2047"/>
3246</reference>
3247
3248<reference anchor="RFC2119">
3249  <front>
3250    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3251    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3252      <organization>Harvard University</organization>
3253      <address><email>sob@harvard.edu</email></address>
3254    </author>
3255    <date month="March" year="1997"/>
3256  </front>
3257  <seriesInfo name="BCP" value="14"/>
3258  <seriesInfo name="RFC" value="2119"/>
3259</reference>
3260
3261<reference anchor="RFC2396">
3262  <front>
3263    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3264    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3265      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3266      <address><email>timbl@w3.org</email></address>
3267    </author>
3268    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3269      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3270      <address><email>fielding@ics.uci.edu</email></address>
3271    </author>
3272    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3273      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3274      <address><email>masinter@parc.xerox.com</email></address>
3275    </author>
3276    <date month="August" year="1998"/>
3277  </front>
3278  <seriesInfo name="RFC" value="2396"/>
3279</reference>
3280
3281<reference anchor="USASCII">
3282  <front>
3283    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3284    <author>
3285      <organization>American National Standards Institute</organization>
3286    </author>
3287    <date year="1986"/>
3288  </front>
3289  <seriesInfo name="ANSI" value="X3.4"/>
3290</reference>
3291
3292</references>
3293
3294<references title="Informative References">
3295
3296<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3297  <front>
3298    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3299    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3300      <organization/>
3301    </author>
3302    <author initials="J." surname="Gettys" fullname="J. Gettys">
3303      <organization/>
3304    </author>
3305    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3306      <organization/>
3307    </author>
3308    <author initials="H." surname="Lie" fullname="H. Lie">
3309      <organization/>
3310    </author>
3311    <author initials="C." surname="Lilley" fullname="C. Lilley">
3312      <organization/>
3313    </author>
3314    <date year="1997" month="September"/>
3315  </front>
3316  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3317</reference>
3318
3319<reference anchor="Pad1995">
3320  <front>
3321    <title>Improving HTTP Latency</title>
3322    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3323      <organization/>
3324    </author>
3325    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3326      <organization/>
3327    </author>
3328    <date year="1995" month="December"/>
3329  </front>
3330  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3331  <annotation>
3332    Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3333    which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3334  </annotation>
3335</reference>
3336
3337<reference anchor="RFC822">
3338  <front>
3339    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3340    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3341      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3342      <address><email>DCrocker@UDel-Relay</email></address>
3343    </author>
3344    <date month="August" day="13" year="1982"/>
3345  </front>
3346  <seriesInfo name="STD" value="11"/>
3347  <seriesInfo name="RFC" value="822"/>
3348</reference>
3349
3350<reference anchor="RFC959">
3351  <front>
3352    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3353    <author initials="J." surname="Postel" fullname="J. Postel">
3354      <organization>Information Sciences Institute (ISI)</organization>
3355    </author>
3356    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3357      <organization/>
3358    </author>
3359    <date month="October" year="1985"/>
3360  </front>
3361  <seriesInfo name="STD" value="9"/>
3362  <seriesInfo name="RFC" value="959"/>
3363</reference>
3364
3365<reference anchor="RFC1123">
3366  <front>
3367    <title>Requirements for Internet Hosts - Application and Support</title>
3368    <author initials="R." surname="Braden" fullname="Robert Braden">
3369      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3370      <address><email>Braden@ISI.EDU</email></address>
3371    </author>
3372    <date month="October" year="1989"/>
3373  </front>
3374  <seriesInfo name="STD" value="3"/>
3375  <seriesInfo name="RFC" value="1123"/>
3376</reference>
3377
3378<reference anchor="RFC1305">
3379  <front>
3380    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3381    <author initials="D." surname="Mills" fullname="David L. Mills">
3382      <organization>University of Delaware, Electrical Engineering Department</organization>
3383      <address><email>mills@udel.edu</email></address>
3384    </author>
3385    <date month="March" year="1992"/>
3386  </front>
3387  <seriesInfo name="RFC" value="1305"/>
3388</reference>
3389
3390<reference anchor="RFC1436">
3391  <front>
3392    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3393    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3394      <organization>University of Minnesota, Computer and Information Services</organization>
3395      <address><email>fxa@boombox.micro.umn.edu</email></address>
3396    </author>
3397    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3398      <organization>University of Minnesota, Computer and Information Services</organization>
3399      <address><email>mpm@boombox.micro.umn.edu</email></address>
3400    </author>
3401    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3402      <organization>University of Minnesota, Computer and Information Services</organization>
3403      <address><email>lindner@boombox.micro.umn.edu</email></address>
3404    </author>
3405    <author initials="D." surname="Johnson" fullname="David Johnson">
3406      <organization>University of Minnesota, Computer and Information Services</organization>
3407      <address><email>dmj@boombox.micro.umn.edu</email></address>
3408    </author>
3409    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3410      <organization>University of Minnesota, Computer and Information Services</organization>
3411      <address><email>daniel@boombox.micro.umn.edu</email></address>
3412    </author>
3413    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3414      <organization>University of Minnesota, Computer and Information Services</organization>
3415      <address><email>alberti@boombox.micro.umn.edu</email></address>
3416    </author>
3417    <date month="March" year="1993"/>
3418  </front>
3419  <seriesInfo name="RFC" value="1436"/>
3420</reference>
3421
3422<reference anchor="RFC1630">
3423  <front>
3424    <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>
3425    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3426      <organization>CERN, World-Wide Web project</organization>
3427      <address><email>timbl@info.cern.ch</email></address>
3428    </author>
3429    <date month="June" year="1994"/>
3430  </front>
3431  <seriesInfo name="RFC" value="1630"/>
3432</reference>
3433
3434<reference anchor="RFC1737">
3435  <front>
3436    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3437    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3438      <organization>Xerox Palo Alto Research Center</organization>
3439      <address><email>masinter@parc.xerox.com</email></address>
3440    </author>
3441    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3442      <organization>MIT Laboratory for Computer Science</organization>
3443      <address><email>sollins@lcs.mit.edu</email></address>
3444    </author>
3445    <date month="December" year="1994"/>
3446  </front>
3447  <seriesInfo name="RFC" value="1737"/>
3448</reference>
3449
3450<reference anchor="RFC1738">
3451  <front>
3452    <title>Uniform Resource Locators (URL)</title>
3453    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3454      <organization>CERN, World-Wide Web project</organization>
3455      <address><email>timbl@info.cern.ch</email></address>
3456    </author>
3457    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3458      <organization>Xerox PARC</organization>
3459      <address><email>masinter@parc.xerox.com</email></address>
3460    </author>
3461    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3462      <organization>University of Minnesota, Computer and Information Services</organization>
3463      <address><email>mpm@boombox.micro.umn.edu</email></address>
3464    </author>
3465    <date month="December" year="1994"/>
3466  </front>
3467  <seriesInfo name="RFC" value="1738"/>
3468</reference>
3469
3470<reference anchor="RFC1808">
3471  <front>
3472    <title>Relative Uniform Resource Locators</title>
3473    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3474      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3475      <address><email>fielding@ics.uci.edu</email></address>
3476    </author>
3477    <date month="June" year="1995"/>
3478  </front>
3479  <seriesInfo name="RFC" value="1808"/>
3480</reference>
3481
3482<reference anchor="RFC1900">
3483  <front>
3484    <title>Renumbering Needs Work</title>
3485    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3486      <organization>CERN, Computing and Networks Division</organization>
3487      <address><email>brian@dxcoms.cern.ch</email></address>
3488    </author>
3489    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3490      <organization>cisco Systems</organization>
3491      <address><email>yakov@cisco.com</email></address>
3492    </author>
3493    <date month="February" year="1996"/>
3494  </front>
3495  <seriesInfo name="RFC" value="1900"/>
3496</reference>
3497
3498<reference anchor="RFC1945">
3499  <front>
3500    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3501    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3502      <organization>MIT, Laboratory for Computer Science</organization>
3503      <address><email>timbl@w3.org</email></address>
3504    </author>
3505    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3506      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3507      <address><email>fielding@ics.uci.edu</email></address>
3508    </author>
3509    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3510      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3511      <address><email>frystyk@w3.org</email></address>
3512    </author>
3513    <date month="May" year="1996"/>
3514  </front>
3515  <seriesInfo name="RFC" value="1945"/>
3516</reference>
3517
3518<reference anchor="RFC2068">
3519  <front>
3520    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3521    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3522      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3523      <address><email>fielding@ics.uci.edu</email></address>
3524    </author>
3525    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3526      <organization>MIT Laboratory for Computer Science</organization>
3527      <address><email>jg@w3.org</email></address>
3528    </author>
3529    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3530      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3531      <address><email>mogul@wrl.dec.com</email></address>
3532    </author>
3533    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3534      <organization>MIT Laboratory for Computer Science</organization>
3535      <address><email>frystyk@w3.org</email></address>
3536    </author>
3537    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3538      <organization>MIT Laboratory for Computer Science</organization>
3539      <address><email>timbl@w3.org</email></address>
3540    </author>
3541    <date month="January" year="1997"/>
3542  </front>
3543  <seriesInfo name="RFC" value="2068"/>
3544</reference>
3545
3546<reference anchor="RFC2145">
3547  <front>
3548    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3549    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3550      <organization>Western Research Laboratory</organization>
3551      <address><email>mogul@wrl.dec.com</email></address>
3552    </author>
3553    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3554      <organization>Department of Information and Computer Science</organization>
3555      <address><email>fielding@ics.uci.edu</email></address>
3556    </author>
3557    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3558      <organization>MIT Laboratory for Computer Science</organization>
3559      <address><email>jg@w3.org</email></address>
3560    </author>
3561    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3562      <organization>W3 Consortium</organization>
3563      <address><email>frystyk@w3.org</email></address>
3564    </author>
3565    <date month="May" year="1997"/>
3566  </front>
3567  <seriesInfo name="RFC" value="2145"/>
3568</reference>
3569
3570<reference anchor="RFC2324">
3571  <front>
3572    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3573    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3574      <organization>Xerox Palo Alto Research Center</organization>
3575      <address><email>masinter@parc.xerox.com</email></address>
3576    </author>
3577    <date month="April" day="1" year="1998"/>
3578  </front>
3579  <seriesInfo name="RFC" value="2324"/>
3580</reference>
3581
3582<reference anchor="RFC2616">
3583  <front>
3584    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3585    <author initials="R." surname="Fielding" fullname="R. Fielding">
3586      <organization>University of California, Irvine</organization>
3587      <address><email>fielding@ics.uci.edu</email></address>
3588    </author>
3589    <author initials="J." surname="Gettys" fullname="J. Gettys">
3590      <organization>W3C</organization>
3591      <address><email>jg@w3.org</email></address>
3592    </author>
3593    <author initials="J." surname="Mogul" fullname="J. Mogul">
3594      <organization>Compaq Computer Corporation</organization>
3595      <address><email>mogul@wrl.dec.com</email></address>
3596    </author>
3597    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3598      <organization>MIT Laboratory for Computer Science</organization>
3599      <address><email>frystyk@w3.org</email></address>
3600    </author>
3601    <author initials="L." surname="Masinter" fullname="L. Masinter">
3602      <organization>Xerox Corporation</organization>
3603      <address><email>masinter@parc.xerox.com</email></address>
3604    </author>
3605    <author initials="P." surname="Leach" fullname="P. Leach">
3606      <organization>Microsoft Corporation</organization>
3607      <address><email>paulle@microsoft.com</email></address>
3608    </author>
3609    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3610      <organization>W3C</organization>
3611      <address><email>timbl@w3.org</email></address>
3612    </author>
3613    <date month="June" year="1999"/>
3614  </front>
3615  <seriesInfo name="RFC" value="2616"/>
3616</reference>
3617
3618<reference anchor="RFC2821">
3619  <front>
3620    <title>Simple Mail Transfer Protocol</title>
3621    <author initials="J." surname="Klensin" fullname="J. Klensin">
3622      <organization>AT&amp;T Laboratories</organization>
3623      <address><email>klensin@research.att.com</email></address>
3624    </author>
3625    <date year="2001" month="April"/>
3626  </front>
3627  <seriesInfo name="RFC" value="2821"/>
3628</reference>
3629
3630<reference anchor="RFC2822">
3631  <front>
3632    <title>Internet Message Format</title>
3633    <author initials="P." surname="Resnick" fullname="P. Resnick">
3634      <organization>QUALCOMM Incorporated</organization>
3635    </author>
3636    <date year="2001" month="April"/>
3637  </front> 
3638  <seriesInfo name="RFC" value="2822"/>
3639</reference>
3640
3641<reference anchor='RFC3977'>
3642  <front>
3643    <title>Network News Transfer Protocol (NNTP)</title>
3644    <author initials='C.' surname='Feather' fullname='C. Feather'>
3645      <organization>THUS plc</organization>
3646      <address><email>clive@demon.net</email></address>
3647    </author>
3648    <date year='2006' month='October' />
3649  </front>
3650  <seriesInfo name="RFC" value="3977"/>
3651</reference>
3652
3653<reference anchor="RFC4288">
3654  <front>
3655    <title>Media Type Specifications and Registration Procedures</title>
3656    <author initials="N." surname="Freed" fullname="N. Freed">
3657      <organization>Sun Microsystems</organization>
3658      <address>
3659        <email>ned.freed@mrochek.com</email>
3660      </address>
3661    </author>
3662    <author initials="J." surname="Klensin" fullname="J. Klensin">
3663      <organization/>
3664      <address>
3665        <email>klensin+ietf@jck.com</email>
3666      </address>
3667    </author>
3668    <date year="2005" month="December"/>
3669  </front>
3670  <seriesInfo name="BCP" value="13"/>
3671  <seriesInfo name="RFC" value="4288"/>
3672</reference>
3673
3674<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3675  <front>
3676  <title>Analysis of HTTP Performance Problems</title>
3677  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3678    <organization/>
3679  </author>
3680  <date/>
3681  </front>
3682</reference>
3683
3684<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3685  <front>
3686  <title>Analysis of HTTP Performance</title>
3687  <author initials="J." surname="Touch" fullname="Joe Touch">
3688    <organization>USC/Information Sciences Institute</organization>
3689    <address><email>touch@isi.edu</email></address>
3690  </author>
3691  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3692    <organization>USC/Information Sciences Institute</organization>
3693    <address><email>johnh@isi.edu</email></address>
3694  </author>
3695  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3696    <organization>USC/Information Sciences Institute</organization>
3697    <address><email>katia@isi.edu</email></address>
3698  </author>
3699  <date year="1998" month="Aug"/>
3700  </front>
3701  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3702  <annotation>(original report dated Aug. 1996)</annotation>
3703</reference>
3704
3705<reference anchor="WAIS">
3706  <front>
3707    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3708    <author initials="F." surname="Davis" fullname="F. Davis">
3709      <organization>Thinking Machines Corporation</organization>
3710    </author>
3711    <author initials="B." surname="Kahle" fullname="B. Kahle">
3712      <organization>Thinking Machines Corporation</organization>
3713    </author>
3714    <author initials="H." surname="Morris" fullname="H. Morris">
3715      <organization>Thinking Machines Corporation</organization>
3716    </author>
3717    <author initials="J." surname="Salem" fullname="J. Salem">
3718      <organization>Thinking Machines Corporation</organization>
3719    </author>
3720    <author initials="T." surname="Shen" fullname="T. Shen">
3721      <organization>Thinking Machines Corporation</organization>
3722    </author>
3723    <author initials="R." surname="Wang" fullname="R. Wang">
3724      <organization>Thinking Machines Corporation</organization>
3725    </author>
3726    <author initials="J." surname="Sui" fullname="J. Sui">
3727      <organization>Thinking Machines Corporation</organization>
3728    </author>
3729    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3730      <organization>Thinking Machines Corporation</organization>
3731    </author>
3732    <date month="April" year="1990"/>
3733  </front>
3734  <seriesInfo name="Thinking Machines Corporation" value=""/>
3735</reference>
3736
3737</references>
3738
3739
3740<section title="Internet Media Types" anchor="internet.media.type.http">
3741<t>
3742   In addition to defining HTTP/1.1, this document serves
3743   as the specification for the Internet media type "message/http" and
3744   "application/http". The following is to be registered with IANA <xref target="RFC4288"/>.
3745</t>
3746<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3747<iref item="Media Type" subitem="message/http" primary="true"/>
3748<iref item="message/http Media Type" primary="true"/>
3749<t>
3750   The message/http type can be used to enclose a single HTTP request or
3751   response message, provided that it obeys the MIME restrictions for all
3752   "message" types regarding line length and encodings.
3753</t>
3754<t>
3755  <list style="hanging" x:indent="12em">
3756    <t hangText="Type name:">
3757      message
3758    </t>
3759    <t hangText="Subtype name:">
3760      http
3761    </t>
3762    <t hangText="Required parameters:">
3763      none
3764    </t>
3765    <t hangText="Optional parameters:">
3766      version, msgtype
3767      <list style="hanging">
3768        <t hangText="version:">
3769          The HTTP-Version number of the enclosed message
3770          (e.g., "1.1"). If not present, the version can be
3771          determined from the first line of the body.
3772        </t>
3773        <t hangText="msgtype:">
3774          The message type -- "request" or "response". If not
3775          present, the type can be determined from the first
3776          line of the body.
3777        </t>
3778      </list>
3779    </t>
3780    <t hangText="Encoding considerations:">
3781      only "7bit", "8bit", or "binary" are permitted
3782    </t>
3783    <t hangText="Security considerations:">
3784      none
3785    </t>
3786    <t hangText="Interoperability considerations:">
3787      none
3788    </t>
3789    <t hangText="Published specification:">
3790      This specification (see <xref target="internet.media.type.message.http"/>).
3791    </t>
3792    <t hangText="Applications that use this media type:">
3793    </t>
3794    <t hangText="Additional information:">
3795      <list style="hanging">
3796        <t hangText="Magic number(s):">none</t>
3797        <t hangText="File extension(s):">none</t>
3798        <t hangText="Macintosh file type code(s):">none</t>
3799      </list>
3800    </t>
3801    <t hangText="Person and email address to contact for further information:">
3802      See Authors Section.
3803    </t>
3804                <t hangText="Intended usage:">
3805                  COMMON
3806    </t>
3807                <t hangText="Restrictions on usage:">
3808                  none
3809    </t>
3810    <t hangText="Author/Change controller:">
3811      IESG
3812    </t>
3813  </list>
3814</t>
3815</section>
3816<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3817<iref item="Media Type" subitem="application/http" primary="true"/>
3818<iref item="application/http Media Type" primary="true"/>
3819<t>
3820   The application/http type can be used to enclose a pipeline of one or more
3821   HTTP request or response messages (not intermixed).
3822</t>
3823<t>
3824  <list style="hanging" x:indent="12em">
3825    <t hangText="Type name:">
3826      application
3827    </t>
3828    <t hangText="Subtype name:">
3829      http
3830    </t>
3831    <t hangText="Required parameters:">
3832      none
3833    </t>
3834    <t hangText="Optional parameters:">
3835      version, msgtype
3836      <list style="hanging">
3837        <t hangText="version:">
3838          The HTTP-Version number of the enclosed messages
3839          (e.g., "1.1"). If not present, the version can be
3840          determined from the first line of the body.
3841        </t>
3842        <t hangText="msgtype:">
3843          The message type -- "request" or "response". If not
3844          present, the type can be determined from the first
3845          line of the body.
3846        </t>
3847      </list>
3848    </t>
3849    <t hangText="Encoding considerations:">
3850      HTTP messages enclosed by this type
3851      are in "binary" format; use of an appropriate
3852      Content-Transfer-Encoding is required when
3853      transmitted via E-mail.
3854    </t>
3855    <t hangText="Security considerations:">
3856      none
3857    </t>
3858    <t hangText="Interoperability considerations:">
3859      none
3860    </t>
3861    <t hangText="Published specification:">
3862      This specification (see <xref target="internet.media.type.application.http"/>).
3863    </t>
3864    <t hangText="Applications that use this media type:">
3865    </t>
3866    <t hangText="Additional information:">
3867      <list style="hanging">
3868        <t hangText="Magic number(s):">none</t>
3869        <t hangText="File extension(s):">none</t>
3870        <t hangText="Macintosh file type code(s):">none</t>
3871      </list>
3872    </t>
3873    <t hangText="Person and email address to contact for further information:">
3874      See Authors Section.
3875    </t>
3876                <t hangText="Intended usage:">
3877                  COMMON
3878    </t>
3879                <t hangText="Restrictions on usage:">
3880                  none
3881    </t>
3882    <t hangText="Author/Change controller:">
3883      IESG
3884    </t>
3885  </list>
3886</t>
3887</section>
3888</section>
3889
3890<section title="Tolerant Applications" anchor="tolerant.applications">
3891<t>
3892   Although this document specifies the requirements for the generation
3893   of HTTP/1.1 messages, not all applications will be correct in their
3894   implementation. We therefore recommend that operational applications
3895   be tolerant of deviations whenever those deviations can be
3896   interpreted unambiguously.
3897</t>
3898<t>
3899   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3900   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3901   accept any amount of SP or HTAB characters between fields, even though
3902   only a single SP is required.
3903</t>
3904<t>
3905   The line terminator for message-header fields is the sequence CRLF.
3906   However, we recommend that applications, when parsing such headers,
3907   recognize a single LF as a line terminator and ignore the leading CR.
3908</t>
3909<t>
3910   The character set of an entity-body &SHOULD; be labeled as the lowest
3911   common denominator of the character codes used within that body, with
3912   the exception that not labeling the entity is preferred over labeling
3913   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3914</t>
3915<t>
3916   Additional rules for requirements on parsing and encoding of dates
3917   and other potential problems with date encodings include:
3918</t>
3919<t>
3920  <list style="symbols">
3921     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3922        which appears to be more than 50 years in the future is in fact
3923        in the past (this helps solve the "year 2000" problem).</t>
3924
3925     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3926        Expires date as earlier than the proper value, but &MUST-NOT;
3927        internally represent a parsed Expires date as later than the
3928        proper value.</t>
3929
3930     <t>All expiration-related calculations &MUST; be done in GMT. The
3931        local time zone &MUST-NOT; influence the calculation or comparison
3932        of an age or expiration time.</t>
3933
3934     <t>If an HTTP header incorrectly carries a date value with a time
3935        zone other than GMT, it &MUST; be converted into GMT using the
3936        most conservative possible conversion.</t>
3937  </list>
3938</t>
3939</section>
3940
3941<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3942<t>
3943   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3944   simplify the process of date comparison. Proxies and gateways from
3945   other protocols &SHOULD; ensure that any Date header field present in a
3946   message conforms to one of the HTTP/1.1 formats and rewrite the date
3947   if necessary.
3948</t>
3949</section>
3950
3951<section title="Compatibility with Previous Versions" anchor="compatibility">
3952<t>
3953   It is beyond the scope of a protocol specification to mandate
3954   compliance with previous versions. HTTP/1.1 was deliberately
3955   designed, however, to make supporting previous versions easy. It is
3956   worth noting that, at the time of composing this specification
3957   (1996), we would expect commercial HTTP/1.1 servers to:
3958  <list style="symbols">
3959     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3960        1.1 requests;</t>
3961
3962     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3963        1.1;</t>
3964
3965     <t>respond appropriately with a message in the same major version
3966        used by the client.</t>
3967  </list>
3968</t>
3969<t>
3970   And we would expect HTTP/1.1 clients to:
3971  <list style="symbols">
3972     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3973        responses;</t>
3974
3975     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3976        1.1.</t>
3977  </list>
3978</t>
3979<t>
3980   For most implementations of HTTP/1.0, each connection is established
3981   by the client prior to the request and closed by the server after
3982   sending the response. Some implementations implement the Keep-Alive
3983   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3984</t>
3985
3986<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3987<t>
3988   This section summarizes major differences between versions HTTP/1.0
3989   and HTTP/1.1.
3990</t>
3991
3992<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">
3993<t>
3994   The requirements that clients and servers support the Host request-header,
3995   report an error if the Host request-header (<xref target="header.host"/>) is
3996   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3997   are among the most important changes defined by this
3998   specification.
3999</t>
4000<t>
4001   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4002   addresses and servers; there was no other established mechanism for
4003   distinguishing the intended server of a request than the IP address
4004   to which that request was directed. The changes outlined above will
4005   allow the Internet, once older HTTP clients are no longer common, to
4006   support multiple Web sites from a single IP address, greatly
4007   simplifying large operational Web servers, where allocation of many
4008   IP addresses to a single host has created serious problems. The
4009   Internet will also be able to recover the IP addresses that have been
4010   allocated for the sole purpose of allowing special-purpose domain
4011   names to be used in root-level HTTP URLs. Given the rate of growth of
4012   the Web, and the number of servers already deployed, it is extremely
4013   important that all implementations of HTTP (including updates to
4014   existing HTTP/1.0 applications) correctly implement these
4015   requirements:
4016  <list style="symbols">
4017     <t>Both clients and servers &MUST; support the Host request-header.</t>
4018
4019     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4020
4021     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4022        request does not include a Host request-header.</t>
4023
4024     <t>Servers &MUST; accept absolute URIs.</t>
4025  </list>
4026</t>
4027</section>
4028</section>
4029
4030<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4031<t>
4032   Some clients and servers might wish to be compatible with some
4033   previous implementations of persistent connections in HTTP/1.0
4034   clients and servers. Persistent connections in HTTP/1.0 are
4035   explicitly negotiated as they are not the default behavior. HTTP/1.0
4036   experimental implementations of persistent connections are faulty,
4037   and the new facilities in HTTP/1.1 are designed to rectify these
4038   problems. The problem was that some existing 1.0 clients may be
4039   sending Keep-Alive to a proxy server that doesn't understand
4040   Connection, which would then erroneously forward it to the next
4041   inbound server, which would establish the Keep-Alive connection and
4042   result in a hung HTTP/1.0 proxy waiting for the close on the
4043   response. The result is that HTTP/1.0 clients must be prevented from
4044   using Keep-Alive when talking to proxies.
4045</t>
4046<t>
4047   However, talking to proxies is the most important use of persistent
4048   connections, so that prohibition is clearly unacceptable. Therefore,
4049   we need some other mechanism for indicating a persistent connection
4050   is desired, which is safe to use even when talking to an old proxy
4051   that ignores Connection. Persistent connections are the default for
4052   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4053   declaring non-persistence. See <xref target="header.connection"/>.
4054</t>
4055<t>
4056   The original HTTP/1.0 form of persistent connections (the Connection:
4057   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4058</t>
4059</section>
4060
4061<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4062<t>
4063   This specification has been carefully audited to correct and
4064   disambiguate key word usage; RFC 2068 had many problems in respect to
4065   the conventions laid out in <xref target="RFC2119"/>.
4066</t>
4067<t>
4068   Transfer-coding and message lengths all interact in ways that
4069   required fixing exactly when chunked encoding is used (to allow for
4070   transfer encoding that may not be self delimiting); it was important
4071   to straighten out exactly how message lengths are computed. (Sections
4072   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4073   <xref target="header.content-length" format="counter"/>,
4074   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4075</t>
4076<t>
4077   The use and interpretation of HTTP version numbers has been clarified
4078   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4079   version they support to deal with problems discovered in HTTP/1.0
4080   implementations (<xref target="http.version"/>)
4081</t>
4082<t>
4083   Transfer-coding had significant problems, particularly with
4084   interactions with chunked encoding. The solution is that transfer-codings
4085   become as full fledged as content-codings. This involves
4086   adding an IANA registry for transfer-codings (separate from content
4087   codings), a new header field (TE) and enabling trailer headers in the
4088   future. Transfer encoding is a major performance benefit, so it was
4089   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4090   interoperability problem that could have occurred due to interactions
4091   between authentication trailers, chunked encoding and HTTP/1.0
4092   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4093   and <xref target="header.te" format="counter"/>)
4094</t>
4095</section>
4096
4097<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4098<t>
4099  The CHAR rule does not allow the NUL character anymore (this affects
4100  the comment and quoted-string rules).
4101  (<xref target="basic.rules"/>)
4102</t>
4103<t>
4104  Clarify that HTTP-Version is case sensitive.
4105  (<xref target="http.version"/>)
4106</t>
4107<t>
4108  Remove reference to non-existant identity transfer-coding value tokens.
4109  (Sections <xref format="counter" target="transfer.codings"/> and
4110  <xref format="counter" target="message.length"/>)
4111</t>
4112<t>
4113  Clarification that the chunk length does not include
4114  the count of the octets in the chunk header and trailer.
4115  (<xref target="chunked.transfer.encoding"/>)
4116</t>
4117<t>
4118  Fix BNF to add query, as the abs_path production in
4119  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4120  (<xref target="request-uri"/>)
4121</t>
4122<t>
4123  Clarify exactly when close connection options must be sent.
4124  (<xref target="header.connection"/>)
4125</t>
4126</section>
4127</section>
4128
4129<section title="Change Log (to be removed by RFC Editor before publication)">
4130
4131<section title="Since RFC2616">
4132<t>
4133  Extracted relevant partitions from <xref target="RFC2616"/>.
4134</t>
4135</section>
4136
4137<section title="Since draft-ietf-httpbis-p1-messaging-00">
4138<t>
4139  Closed issues:
4140  <list style="symbols"> 
4141    <t>
4142      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4143      "HTTP Version should be case sensitive"
4144      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4145    </t>
4146    <t>
4147      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4148      "'unsafe' characters"
4149      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4150    </t>
4151    <t>
4152      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4153      "Chunk Size Definition"
4154      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4155    </t>
4156    <t>
4157      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4158      "Message Length"
4159      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4160    </t>
4161    <t>
4162      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4163      "Media Type Registrations"
4164      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4165    </t>
4166    <t>
4167      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4168      "URI includes query"
4169      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4170    </t>
4171    <t>
4172      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4173      "No close on 1xx responses"
4174      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4175    </t>
4176    <t>
4177      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4178      "Remove 'identity' token references"
4179      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4180    </t>
4181    <t>
4182      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4183      "Import query BNF"
4184    </t>
4185    <t>
4186      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4187      "qdtext BNF"
4188    </t>
4189    <t>
4190      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4191      "Normative and Informative references"
4192    </t>
4193    <t>
4194      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4195      "RFC2606 Compliance"
4196    </t>
4197    <t>
4198      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4199      "RFC977 reference"
4200    </t>
4201    <t>
4202      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4203      "RFC1700 references"
4204    </t>
4205    <t>
4206      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4207      "inconsistency in date format explanation"
4208    </t>
4209    <t>
4210      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4211      "Date reference typo"
4212    </t>
4213    <t>
4214      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4215      "Informative references"
4216    </t>
4217    <t>
4218      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4219      "ISO-8859-1 Reference"
4220    </t>
4221    <t>
4222      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4223      "Normative up-to-date references"
4224    </t>
4225  </list>
4226</t>
4227<t>
4228  Other changes:
4229  <list style="symbols"> 
4230    <t>
4231      Update media type registrations to use RFC4288 template.
4232    </t>
4233    <t>
4234      Use names of RFC4234 core rules DQUOTE and HTAB,
4235      fix broken ABNF for chunk-data
4236      (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4237    </t>
4238  </list>
4239</t>
4240</section>
4241
4242<section title="Since draft-ietf-httpbis-p1-messaging-01">
4243<t>
4244  Closed issues:
4245  <list style="symbols"> 
4246    <t>
4247      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4248      "Bodies on GET (and other) requests"
4249    </t>
4250    <t>
4251      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4252      "Updating to RFC4288"
4253    </t>
4254    <t>
4255      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4256      "Status Code and Reason Phrase"
4257    </t>
4258    <t>
4259      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4260      "rel_path not used"
4261    </t>
4262  </list>
4263</t>
4264<t>
4265  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4266  <list style="symbols"> 
4267    <t>
4268      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4269      "trailer-part").
4270    </t>
4271    <t>
4272      Avoid underscore character in rule names ("http_URL" ->
4273      "http-URL", "abs_path" -> "path-absolute").
4274    </t>
4275    <t>
4276      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4277      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4278      have to be updated when switching over to RFC3986.
4279    </t>
4280    <t>
4281      Synchronize core rules with RFC5234 (this includes a change to CHAR
4282      which now excludes NUL).
4283    </t>
4284    <t>
4285      Get rid of prose rules that span multiple lines.
4286    </t>
4287    <t>
4288      Get rid of unused rules LOALPHA and UPALPHA.
4289    </t>
4290    <t>
4291      Move "Product Tokens" section (back) into Part 1, as "token" is used
4292      in the definition of the Upgrade header.
4293    </t>
4294    <t>
4295      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4296    </t>
4297    <t>
4298      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4299    </t>
4300  </list>
4301</t>
4302</section>
4303
4304</section>
4305
4306</back>
4307</rfc>
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