source: draft-ietf-httpbis/latest/p1-messaging.xml @ 214

Last change on this file since 214 was 214, checked in by julian.reschke@…, 12 years ago

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

  • Property svn:eol-style set to native
File size: 182.6 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "February">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY entity-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x='http://purl.org/net/xml2rfc/ext'>
56<front>
57
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
59
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email>fielding@gbiv.com</email>
73      <uri>http://roy.gbiv.com/</uri>
74    </address>
75  </author>
76
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email>jg@laptop.org</email>
88      <uri>http://www.laptop.org/</uri>
89    </address>
90  </author>
91 
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email>JeffMogul@acm.org</email>
104    </address>
105  </author>
106
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email>henrikn@microsoft.com</email>
118    </address>
119  </author>
120
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email>LMM@acm.org</email>
132      <uri>http://larry.masinter.net/</uri>
133    </address>
134  </author>
135 
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email>paulle@microsoft.com</email>
146    </address>
147  </author>
148   
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email>timbl@w3.org</email>
162      <uri>http://www.w3.org/People/Berners-Lee/</uri>
163    </address>
164  </author>
165
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email>ylafon@w3.org</email>
178      <uri>http://www.raubacapeu.net/people/yves/</uri>
179    </address>
180  </author>
181
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email>julian.reschke@greenbytes.de</email>       
193      <uri>http://greenbytes.de/tech/webdav/</uri>     
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
198
199<abstract>
200<t>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    This draft incorporates those issue resolutions that were either
224    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
225    or which were agreed upon on the mailing list between October 2006 and
226    November 2007 (as published in "draft-lafon-rfc2616bis-03").
227  </t>
228</note>
229</front>
230<middle>
231<section title="Introduction" anchor="introduction">
232<t>
233   The Hypertext Transfer Protocol (HTTP) is an application-level
234   protocol for distributed, collaborative, hypermedia information
235   systems. HTTP has been in use by the World-Wide Web global
236   information initiative since 1990. The first version of HTTP, commonly
237   referred to as HTTP/0.9, was a simple protocol for raw data transfer
238   across the Internet with only a single method and no metadata.
239   HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
240   the protocol by allowing messages to be in the format of MIME-like
241   messages, containing metadata about the data transferred and
242   modifiers on the request/response semantics. However, HTTP/1.0 did
243   not sufficiently take into consideration the effects of hierarchical
244   proxies, caching, the need for persistent connections, or name-based
245   virtual hosts. In addition, the proliferation of incompletely-implemented
246   applications calling themselves "HTTP/1.0" necessitated a
247   protocol version change in order for two communicating applications
248   to determine each other's true capabilities.
249</t>
250<t>
251   This document is Part 1 of the seven-part specification that defines
252   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
253   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
254   requirements that enable reliable implementations and adding only
255   those new features that will either be safely ignored by an HTTP/1.0
256   recipient or only sent when communicating with a party advertising
257   compliance with HTTP/1.1.
258   Part 1 defines those aspects of HTTP/1.1 related to overall network
259   operation, message framing, interaction with transport protocols, and
260   URI schemes.
261</t>
262<t>
263   This document is currently disorganized in order to minimize the changes
264   between drafts and enable reviewers to see the smaller errata changes.
265   The next draft will reorganize the sections to better reflect the content.
266   In particular, the sections will be organized according to the typical
267   process of deciding when to use HTTP (URI schemes), overall network operation,
268   connection management, message framing, and generic message parsing.
269   The current mess reflects how widely dispersed these topics and associated
270   requirements had become in <xref target="RFC2616"/>.
271</t>
272
273<section title="Purpose" anchor="intro.purpose">
274<t>
275   Practical information systems require more functionality than simple
276   retrieval, including search, front-end update, and annotation. HTTP
277   allows an open-ended set of methods and headers that indicate the
278   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
279   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
280   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
281   method is to be applied. Messages are passed in a format similar to
282   that used by Internet mail <xref target="RFC2822"/> as defined by the Multipurpose
283   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
284</t>
285<t>
286   HTTP is also used as a generic protocol for communication between
287   user agents and proxies/gateways to other Internet systems, including
288   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
289   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
290   access to resources available from diverse applications.
291</t>
292</section>
293
294<section title="Requirements" anchor="intro.requirements">
295<t>
296   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
297   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
298   document are to be interpreted as described in <xref target="RFC2119"/>.
299</t>
300<t>
301   An implementation is not compliant if it fails to satisfy one or more
302   of the &MUST; or &REQUIRED; level requirements for the protocols it
303   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
304   level and all the &SHOULD; level requirements for its protocols is said
305   to be "unconditionally compliant"; one that satisfies all the &MUST;
306   level requirements but not all the &SHOULD; level requirements for its
307   protocols is said to be "conditionally compliant."
308</t>
309</section>
310
311<section title="Terminology" anchor="intro.terminology">
312<t>
313   This specification uses a number of terms to refer to the roles
314   played by participants in, and objects of, the HTTP communication.
315</t>
316<t>
317  <iref item="connection"/>
318  <x:dfn>connection</x:dfn>
319  <list>
320    <t>
321      A transport layer virtual circuit established between two programs
322      for the purpose of communication.
323    </t>
324  </list>
325</t>
326<t>
327  <iref item="message"/>
328  <x:dfn>message</x:dfn>
329  <list>
330    <t>
331      The basic unit of HTTP communication, consisting of a structured
332      sequence of octets matching the syntax defined in <xref target="http.message"/> and
333      transmitted via the connection.
334    </t>
335  </list>
336</t>
337<t>
338  <iref item="request"/>
339  <x:dfn>request</x:dfn>
340  <list>
341    <t>
342      An HTTP request message, as defined in <xref target="request"/>.
343    </t>
344  </list>
345</t>
346<t>
347  <iref item="response"/>
348  <x:dfn>response</x:dfn>
349  <list>
350    <t>
351      An HTTP response message, as defined in <xref target="response"/>.
352    </t>
353  </list>
354</t>
355<t>
356  <iref item="resource"/>
357  <x:dfn>resource</x:dfn>
358  <list>
359    <t>
360      A network data object or service that can be identified by a URI,
361      as defined in <xref target="uri"/>. Resources may be available in multiple
362      representations (e.g. multiple languages, data formats, size, and
363      resolutions) or vary in other ways.
364    </t>
365  </list>
366</t>
367<t>
368  <iref item="entity"/>
369  <x:dfn>entity</x:dfn>
370  <list>
371    <t>
372      The information transferred as the payload of a request or
373      response. An entity consists of metainformation in the form of
374      entity-header fields and content in the form of an entity-body, as
375      described in &entity;.
376    </t>
377  </list>
378</t>
379<t>
380  <iref item="representation"/>
381  <x:dfn>representation</x:dfn>
382  <list>
383    <t>
384      An entity included with a response that is subject to content
385      negotiation, as described in &content.negotiation;. There may exist multiple
386      representations associated with a particular response status.
387    </t>
388  </list>
389</t>
390<t>
391  <iref item="content negotiation"/>
392  <x:dfn>content negotiation</x:dfn>
393  <list>
394    <t>
395      The mechanism for selecting the appropriate representation when
396      servicing a request, as described in &content.negotiation;. The
397      representation of entities in any response can be negotiated
398      (including error responses).
399    </t>
400  </list>
401</t>
402<t>
403  <iref item="variant"/>
404  <x:dfn>variant</x:dfn>
405  <list>
406    <t>
407      A resource may have one, or more than one, representation(s)
408      associated with it at any given instant. Each of these
409      representations is termed a `variant'.  Use of the term `variant'
410      does not necessarily imply that the resource is subject to content
411      negotiation.
412    </t>
413  </list>
414</t>
415<t>
416  <iref item="client"/>
417  <x:dfn>client</x:dfn>
418  <list>
419    <t>
420      A program that establishes connections for the purpose of sending
421      requests.
422    </t>
423  </list>
424</t>
425<t>
426  <iref item="user agent"/>
427  <x:dfn>user agent</x:dfn>
428  <list>
429    <t>
430      The client which initiates a request. These are often browsers,
431      editors, spiders (web-traversing robots), or other end user tools.
432    </t>
433  </list>
434</t>
435<t>
436  <iref item="server"/>
437  <x:dfn>server</x:dfn>
438  <list>
439    <t>
440      An application program that accepts connections in order to
441      service requests by sending back responses. Any given program may
442      be capable of being both a client and a server; our use of these
443      terms refers only to the role being performed by the program for a
444      particular connection, rather than to the program's capabilities
445      in general. Likewise, any server may act as an origin server,
446      proxy, gateway, or tunnel, switching behavior based on the nature
447      of each request.
448    </t>
449  </list>
450</t>
451<t>
452  <iref item="origin server"/>
453  <x:dfn>origin server</x:dfn>
454  <list>
455    <t>
456      The server on which a given resource resides or is to be created.
457    </t>
458  </list>
459</t>
460<t>
461  <iref item="proxy"/>
462  <x:dfn>proxy</x:dfn>
463  <list>
464    <t>
465      An intermediary program which acts as both a server and a client
466      for the purpose of making requests on behalf of other clients.
467      Requests are serviced internally or by passing them on, with
468      possible translation, to other servers. A proxy &MUST; implement
469      both the client and server requirements of this specification. A
470      "transparent proxy" is a proxy that does not modify the request or
471      response beyond what is required for proxy authentication and
472      identification. A "non-transparent proxy" is a proxy that modifies
473      the request or response in order to provide some added service to
474      the user agent, such as group annotation services, media type
475      transformation, protocol reduction, or anonymity filtering. Except
476      where either transparent or non-transparent behavior is explicitly
477      stated, the HTTP proxy requirements apply to both types of
478      proxies.
479    </t>
480  </list>
481</t>
482<t>
483  <iref item="gateway"/>
484  <x:dfn>gateway</x:dfn>
485  <list>
486    <t>
487      A server which acts as an intermediary for some other server.
488      Unlike a proxy, a gateway receives requests as if it were the
489      origin server for the requested resource; the requesting client
490      may not be aware that it is communicating with a gateway.
491    </t>
492  </list>
493</t>
494<t>
495  <iref item="tunnel"/>
496  <x:dfn>tunnel</x:dfn>
497  <list>
498    <t>
499      An intermediary program which is acting as a blind relay between
500      two connections. Once active, a tunnel is not considered a party
501      to the HTTP communication, though the tunnel may have been
502      initiated by an HTTP request. The tunnel ceases to exist when both
503      ends of the relayed connections are closed.
504    </t>
505  </list>
506</t>
507<t>
508  <iref item="cache"/>
509  <x:dfn>cache</x:dfn>
510  <list>
511    <t>
512      A program's local store of response messages and the subsystem
513      that controls its message storage, retrieval, and deletion. A
514      cache stores cacheable responses in order to reduce the response
515      time and network bandwidth consumption on future, equivalent
516      requests. Any client or server may include a cache, though a cache
517      cannot be used by a server that is acting as a tunnel.
518    </t>
519  </list>
520</t>
521<t>
522  <iref item="cacheable"/>
523  <x:dfn>cacheable</x:dfn>
524  <list>
525    <t>
526      A response is cacheable if a cache is allowed to store a copy of
527      the response message for use in answering subsequent requests. The
528      rules for determining the cacheability of HTTP responses are
529      defined in &caching;. Even if a resource is cacheable, there may
530      be additional constraints on whether a cache can use the cached
531      copy for a particular request.
532    </t>
533  </list>
534</t>
535<t>
536  <iref item="upstream"/>
537  <iref item="downstream"/>
538  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
539  <list>
540    <t>
541      Upstream and downstream describe the flow of a message: all
542      messages flow from upstream to downstream.
543    </t>
544  </list>
545</t>
546<t>
547  <iref item="inbound"/>
548  <iref item="outbound"/>
549  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
550  <list>
551    <t>
552      Inbound and outbound refer to the request and response paths for
553      messages: "inbound" means "traveling toward the origin server",
554      and "outbound" means "traveling toward the user agent"
555    </t>
556  </list>
557</t>
558</section>
559
560<section title="Overall Operation" anchor="intro.overall.operation">
561<t>
562   HTTP is a request/response protocol. A client sends a
563   request to the server in the form of a request method, URI, and
564   protocol version, followed by a MIME-like message containing request
565   modifiers, client information, and possible body content over a
566   connection with a server. The server responds with a status line,
567   including the message's protocol version and a success or error code,
568   followed by a MIME-like message containing server information, entity
569   metainformation, and possible entity-body content. The relationship
570   between HTTP and MIME is described in &diff2045entity;.
571</t>
572<t>
573   Most HTTP communication is initiated by a user agent and consists of
574   a request to be applied to a resource on some origin server. In the
575   simplest case, this may be accomplished via a single connection (v)
576   between the user agent (UA) and the origin server (O).
577</t>
578<figure><artwork type="drawing">
579       request chain ------------------------&gt;
580    UA -------------------v------------------- O
581       &lt;----------------------- response chain
582</artwork></figure>
583<t>
584   A more complicated situation occurs when one or more intermediaries
585   are present in the request/response chain. There are three common
586   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
587   forwarding agent, receiving requests for a URI in its absolute form,
588   rewriting all or part of the message, and forwarding the reformatted
589   request toward the server identified by the URI. A gateway is a
590   receiving agent, acting as a layer above some other server(s) and, if
591   necessary, translating the requests to the underlying server's
592   protocol. A tunnel acts as a relay point between two connections
593   without changing the messages; tunnels are used when the
594   communication needs to pass through an intermediary (such as a
595   firewall) even when the intermediary cannot understand the contents
596   of the messages.
597</t>
598<figure><artwork type="drawing">
599       request chain --------------------------------------&gt;
600    UA -----v----- A -----v----- B -----v----- C -----v----- O
601       &lt;------------------------------------- response chain
602</artwork></figure>
603<t>
604   The figure above shows three intermediaries (A, B, and C) between the
605   user agent and origin server. A request or response message that
606   travels the whole chain will pass through four separate connections.
607   This distinction is important because some HTTP communication options
608   may apply only to the connection with the nearest, non-tunnel
609   neighbor, only to the end-points of the chain, or to all connections
610   along the chain. Although the diagram is linear, each participant may
611   be engaged in multiple, simultaneous communications. For example, B
612   may be receiving requests from many clients other than A, and/or
613   forwarding requests to servers other than C, at the same time that it
614   is handling A's request.
615</t>
616<t>
617   Any party to the communication which is not acting as a tunnel may
618   employ an internal cache for handling requests. The effect of a cache
619   is that the request/response chain is shortened if one of the
620   participants along the chain has a cached response applicable to that
621   request. The following illustrates the resulting chain if B has a
622   cached copy of an earlier response from O (via C) for a request which
623   has not been cached by UA or A.
624</t>
625<figure><artwork type="drawing">
626          request chain ----------&gt;
627       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
628          &lt;--------- response chain
629</artwork></figure>
630<t>
631   Not all responses are usefully cacheable, and some requests may
632   contain modifiers which place special requirements on cache behavior.
633   HTTP requirements for cache behavior and cacheable responses are
634   defined in &caching;.
635</t>
636<t>
637   In fact, there are a wide variety of architectures and configurations
638   of caches and proxies currently being experimented with or deployed
639   across the World Wide Web. These systems include national hierarchies
640   of proxy caches to save transoceanic bandwidth, systems that
641   broadcast or multicast cache entries, organizations that distribute
642   subsets of cached data via CD-ROM, and so on. HTTP systems are used
643   in corporate intranets over high-bandwidth links, and for access via
644   PDAs with low-power radio links and intermittent connectivity. The
645   goal of HTTP/1.1 is to support the wide diversity of configurations
646   already deployed while introducing protocol constructs that meet the
647   needs of those who build web applications that require high
648   reliability and, failing that, at least reliable indications of
649   failure.
650</t>
651<t>
652   HTTP communication usually takes place over TCP/IP connections. The
653   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
654   not preclude HTTP from being implemented on top of any other protocol
655   on the Internet, or on other networks. HTTP only presumes a reliable
656   transport; any protocol that provides such guarantees can be used;
657   the mapping of the HTTP/1.1 request and response structures onto the
658   transport data units of the protocol in question is outside the scope
659   of this specification.
660</t>
661<t>
662   In HTTP/1.0, most implementations used a new connection for each
663   request/response exchange. In HTTP/1.1, a connection may be used for
664   one or more request/response exchanges, although connections may be
665   closed for a variety of reasons (see <xref target="persistent.connections"/>).
666</t>
667</section>
668</section>
669
670<section title="Notational Conventions and Generic Grammar" anchor="notation">
671
672<section title="Augmented BNF" anchor="notation.abnf">
673<t>
674   All of the mechanisms specified in this document are described in
675   both prose and an augmented Backus-Naur Form (BNF) similar to that
676   used by <xref target="RFC822ABNF"/>. Implementors will need to be familiar with the
677   notation in order to understand this specification. The augmented BNF
678   includes the following constructs:
679</t>
680<t>
681   name = definition
682  <list>
683    <t>
684      The name of a rule is simply the name itself (without any
685      enclosing "&lt;" and "&gt;") and is separated from its definition by the
686      equal "=" character. White space is only significant in that
687      indentation of continuation lines is used to indicate a rule
688      definition that spans more than one line. Certain basic rules are
689      in uppercase, such as SP, LWS, HTAB, CRLF, DIGIT, ALPHA, etc. Angle
690      brackets are used within definitions whenever their presence will
691      facilitate discerning the use of rule names.
692    </t>
693  </list>
694</t>
695<t>
696   "literal"
697  <list>
698    <t>
699      Quotation marks surround literal text. Unless stated otherwise,
700      the text is case-insensitive.
701    </t>
702  </list>
703</t>
704<t>
705   rule1 | rule2
706  <list>
707    <t>
708      Elements separated by a bar ("|") are alternatives, e.g., "yes |
709      no" will accept yes or no.
710    </t>
711  </list>
712</t>
713<t>
714   (rule1 rule2)
715  <list>
716    <t>
717      Elements enclosed in parentheses are treated as a single element.
718      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
719      foo elem" and "elem bar elem".
720    </t>
721  </list>
722</t>
723<t>
724   *rule
725  <list>
726    <t>
727      The character "*" preceding an element indicates repetition. The
728      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
729      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
730      that "*(element)" allows any number, including zero; "1*element"
731      requires at least one; and "1*2element" allows one or two.
732    </t>
733  </list>
734</t>
735<t>
736   [rule]
737  <list>
738    <t>
739      Square brackets enclose optional elements; "[foo bar]" is
740      equivalent to "*1(foo bar)".
741    </t>
742  </list>
743</t>
744<t>
745   N rule
746  <list>
747    <t>
748      Specific repetition: "&lt;n&gt;(element)" is equivalent to
749      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
750      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
751      alphabetic characters.
752    </t>
753  </list>
754</t>
755<t>
756   #rule
757  <list>
758    <t>
759      A construct "#" is defined, similar to "*", for defining lists of
760      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
761      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
762      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
763      form of lists very easy; a rule such as
764    </t>
765    <t>
766         ( *LWS element *( *LWS "," *LWS element ))
767    </t>
768    <t>
769      can be shown as
770    </t>
771    <t>
772         1#element
773    </t>
774    <t>
775      Wherever this construct is used, null elements are allowed, but do
776      not contribute to the count of elements present. That is,
777      "(element), , (element) " is permitted, but counts as only two
778      elements. Therefore, where at least one element is required, at
779      least one non-null element &MUST; be present. Default values are 0
780      and infinity so that "#element" allows any number, including zero;
781      "1#element" requires at least one; and "1#2element" allows one or
782      two.
783    </t>
784  </list>
785</t>
786<t>
787   ; comment
788  <list>
789    <t>
790      A semi-colon, set off some distance to the right of rule text,
791      starts a comment that continues to the end of line. This is a
792      simple way of including useful notes in parallel with the
793      specifications.
794    </t>
795  </list>
796</t>
797<t>
798   implied *LWS
799  <list>
800    <t>
801      The grammar described by this specification is word-based. Except
802      where noted otherwise, linear white space (LWS) can be included
803      between any two adjacent words (token or quoted-string), and
804      between adjacent words and separators, without changing the
805      interpretation of a field. At least one delimiter (LWS and/or
806      separators) &MUST; exist between any two tokens (for the definition
807      of "token" below), since they would otherwise be interpreted as a
808      single token.
809    </t>
810  </list>
811</t>
812</section>
813
814<section title="Basic Rules" anchor="basic.rules">
815<x:anchor-alias value="OCTET"/>
816<x:anchor-alias value="CHAR"/>
817<x:anchor-alias value="ALPHA"/>
818<x:anchor-alias value="DIGIT"/>
819<x:anchor-alias value="CTL"/>
820<x:anchor-alias value="CR"/>
821<x:anchor-alias value="LF"/>
822<x:anchor-alias value="SP"/>
823<x:anchor-alias value="HTAB"/>
824<x:anchor-alias value="CRLF"/>
825<x:anchor-alias value="LWS"/>
826<x:anchor-alias value="TEXT"/>
827<x:anchor-alias value="HEX"/>
828<x:anchor-alias value="token"/>
829<x:anchor-alias value="separators"/>
830<x:anchor-alias value="comment"/>
831<x:anchor-alias value="ctext"/>
832<x:anchor-alias value="quoted-string"/>
833<x:anchor-alias value="qdtext"/>
834<x:anchor-alias value="quoted-pair"/>
835<t>
836   The following rules are used throughout this specification to
837   describe basic parsing constructs. The US-ASCII coded character set
838   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
839</t>
840<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><iref primary="true" item="Grammar" subitem="ALPHA"/><iref primary="true" item="Grammar" subitem="DIGIT"/><iref primary="true" item="Grammar" subitem="CTL"/><iref primary="true" item="Grammar" subitem="CR"/><iref primary="true" item="Grammar" subitem="LF"/><iref primary="true" item="Grammar" subitem="SP"/><iref primary="true" item="Grammar" subitem="HTAB"/><iref primary="true" item="Grammar" subitem="DQUOTE"/>
841  OCTET          = %x00-FF
842                   ; any 8-bit sequence of data
843  CHAR           = %x01-7F
844                   ; any US-ASCII character, excluding NUL
845  ALPHA          = %x41-5A | %x61-7A
846                   ; A-Z | a-z
847  DIGIT          = %x30-39
848                   ; any US-ASCII digit "0".."9"
849  CTL            = %x00-1F | %x7F
850                   ; (octets 0 - 31) and DEL (127)
851  CR             = %x0D
852                   ; US-ASCII CR, carriage return (13)
853  LF             = %x0A
854                   ; US-ASCII LF, linefeed (10)
855  SP             = %x20
856                   ; US-ASCII SP, space (32)
857  HTAB           = %x09
858                   ; US-ASCII HT, horizontal-tab (9)
859  DQUOTE         = %x22
860                   ; US-ASCII double-quote mark (34)
861</artwork></figure>
862<t>
863   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
864   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
865   tolerant applications). The end-of-line marker within an entity-body
866   is defined by its associated media type, as described in &media-types;.
867</t>
868<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
869  CRLF           = CR LF
870</artwork></figure>
871<t>
872   HTTP/1.1 header field values can be folded onto multiple lines if the
873   continuation line begins with a space or horizontal tab. All linear
874   white space, including folding, has the same semantics as SP. A
875   recipient &MAY; replace any linear white space with a single SP before
876   interpreting the field value or forwarding the message downstream.
877</t>
878<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
879  LWS            = [CRLF] 1*( SP | HTAB )
880</artwork></figure>
881<t>
882   The TEXT rule is only used for descriptive field contents and values
883   that are not intended to be interpreted by the message parser. Words
884   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
885   <xref target="ISO-8859-1"/> only when encoded according to the rules of
886   <xref target="RFC2047"/>.
887</t>
888<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
889  TEXT           = &lt;any OCTET except CTLs, but including LWS&gt;
890</artwork></figure>
891<t>
892   A CRLF is allowed in the definition of TEXT only as part of a header
893   field continuation. It is expected that the folding LWS will be
894   replaced with a single SP before interpretation of the TEXT value.
895</t>
896<t>
897   Hexadecimal numeric characters are used in several protocol elements.
898</t>
899<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
900  HEX            = "A" | "B" | "C" | "D" | "E" | "F"
901                 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
902</artwork></figure>
903<t>
904   Many HTTP/1.1 header field values consist of words separated by LWS
905   or special characters. These special characters &MUST; be in a quoted
906   string to be used within a parameter value (as defined in
907   <xref target="transfer.codings"/>).
908</t>
909<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="separators"/>
910  separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
911                 | "," | ";" | ":" | "\" | DQUOTE
912                 | "/" | "[" | "]" | "?" | "="
913                 | "{" | "}" | SP | HTAB
914                 
915  tchar          = "!" | "#" | "$" | "%" | "&amp;" | "'" | "*" | "+" | "-"
916                 | "." | "^" | "_" | "`" | "|" | "~" | DIGIT | ALPHA
917                 ; any CHAR except CTLs or separators
918                 
919  token          = 1*tchar
920</artwork></figure>
921<t>
922   Comments can be included in some HTTP header fields by surrounding
923   the comment text with parentheses. Comments are only allowed in
924   fields containing "comment" as part of their field value definition.
925   In all other fields, parentheses are considered part of the field
926   value.
927</t>
928<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
929  comment        = "(" *( ctext | quoted-pair | comment ) ")"
930  ctext          = &lt;any TEXT excluding "(" and ")"&gt;
931</artwork></figure>
932<t>
933   A string of text is parsed as a single word if it is quoted using
934   double-quote marks.
935</t>
936<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
937  quoted-string  = ( DQUOTE *(qdtext | quoted-pair ) DQUOTE )
938  qdtext         = &lt;any TEXT excluding DQUOTE and "\">
939</artwork></figure>
940<t>
941   The backslash character ("\") &MAY; be used as a single-character
942   quoting mechanism only within quoted-string and comment constructs.
943</t>
944<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
945  quoted-pair    = "\" CHAR
946</artwork></figure>
947</section>
948
949<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
950<t>
951  The ABNF rules below are defined in other parts:
952</t>
953<figure><!-- Part2--><artwork type="abnf2616">
954  request-header =  &lt;request-header, defined in &request-header-fields;&gt;
955  response-header = &lt;response-header, defined in &response-header-fields;&gt;
956</artwork></figure>
957<figure><!-- Part3--><artwork type="abnf2616">
958  accept-params   = &lt;accept-params, defined in &header-accept;&gt;
959  entity-body     = &lt;entity-body, defined in &entity-body;&gt;
960  entity-header   = &lt;entity-header, defined in &entity-header-fields;&gt;
961</artwork></figure>
962<figure><!-- Part6--><artwork type="abnf2616">
963  Cache-Control   = &lt;Cache-Control, defined in &header-pragma;&gt;
964  Pragma          = &lt;Pragma, defined in &header-pragma;&gt;
965  Warning         = &lt;Warning, defined in &header-warning;&gt;
966</artwork></figure>
967</section>
968
969</section>
970
971<section title="Protocol Parameters" anchor="protocol.parameters">
972
973<section title="HTTP Version" anchor="http.version">
974<t>
975   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
976   of the protocol. The protocol versioning policy is intended to allow
977   the sender to indicate the format of a message and its capacity for
978   understanding further HTTP communication, rather than the features
979   obtained via that communication. No change is made to the version
980   number for the addition of message components which do not affect
981   communication behavior or which only add to extensible field values.
982   The &lt;minor&gt; number is incremented when the changes made to the
983   protocol add features which do not change the general message parsing
984   algorithm, but which may add to the message semantics and imply
985   additional capabilities of the sender. The &lt;major&gt; number is
986   incremented when the format of a message within the protocol is
987   changed. See <xref target="RFC2145"/> for a fuller explanation.
988</t>
989<t>
990   The version of an HTTP message is indicated by an HTTP-Version field
991   in the first line of the message. HTTP-Version is case-sensitive.
992</t>
993<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
994  HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
995</artwork></figure>
996<t>
997   Note that the major and minor numbers &MUST; be treated as separate
998   integers and that each &MAY; be incremented higher than a single digit.
999   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
1000   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
1001   &MUST-NOT; be sent.
1002</t>
1003<t>
1004   An application that sends a request or response message that includes
1005   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
1006   with this specification. Applications that are at least conditionally
1007   compliant with this specification &SHOULD; use an HTTP-Version of
1008   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
1009   not compatible with HTTP/1.0. For more details on when to send
1010   specific HTTP-Version values, see <xref target="RFC2145"/>.
1011</t>
1012<t>
1013   The HTTP version of an application is the highest HTTP version for
1014   which the application is at least conditionally compliant.
1015</t>
1016<t>
1017   Proxy and gateway applications need to be careful when forwarding
1018   messages in protocol versions different from that of the application.
1019   Since the protocol version indicates the protocol capability of the
1020   sender, a proxy/gateway &MUST-NOT; send a message with a version
1021   indicator which is greater than its actual version. If a higher
1022   version request is received, the proxy/gateway &MUST; either downgrade
1023   the request version, or respond with an error, or switch to tunnel
1024   behavior.
1025</t>
1026<t>
1027   Due to interoperability problems with HTTP/1.0 proxies discovered
1028   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
1029   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
1030   they support. The proxy/gateway's response to that request &MUST; be in
1031   the same major version as the request.
1032</t>
1033<t>
1034  <list>
1035    <t>
1036      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1037      of header fields required or forbidden by the versions involved.
1038    </t>
1039  </list>
1040</t>
1041</section>
1042
1043<section title="Uniform Resource Identifiers" anchor="uri">
1044<t>
1045   URIs have been known by many names: WWW addresses, Universal Document
1046   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
1047   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
1048   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
1049   simply formatted strings which identify--via name, location, or any
1050   other characteristic--a resource.
1051</t>
1052
1053<section title="General Syntax" anchor="general.syntax">
1054<t>
1055   URIs in HTTP can be represented in absolute form or relative to some
1056   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
1057   forms are differentiated by the fact that absolute URIs always begin
1058   with a scheme name followed by a colon. For definitive information on
1059   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1060   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
1061   and <xref target="RFC1808"/>). This specification adopts the
1062   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
1063   "host", "abs_path", "query", and "authority" from that specification:
1064</t>
1065<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"/>
1066  absoluteURI   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1067  authority     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
1068  fragment      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
1069  path-absolute = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1070  port          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1071  query         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
1072  relativeURI   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
1073  uri-host      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1074</artwork></figure>
1075<t>
1076   HTTP does not place any a priori limit on the length of
1077   a URI. Servers &MUST; be able to handle the URI of any resource they
1078   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1079   provide GET-based forms that could generate such URIs. A server
1080   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1081   than the server can handle (see &status-414;).
1082</t>
1083<t>
1084  <list>
1085    <t>
1086      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1087      above 255 bytes, because some older client or proxy
1088      implementations might not properly support these lengths.
1089    </t>
1090  </list>
1091</t>
1092</section>
1093
1094<section title="http URL" anchor="http.url">
1095<t>
1096   The "http" scheme is used to locate network resources via the HTTP
1097   protocol. This section defines the scheme-specific syntax and
1098   semantics for http URLs.
1099</t>
1100<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
1101  http-URL = "http:" "//" uri-host [ ":" port ]
1102             [ path-absolute [ "?" query ]]
1103</artwork></figure>
1104<t>
1105   If the port is empty or not given, port 80 is assumed. The semantics
1106   are that the identified resource is located at the server listening
1107   for TCP connections on that port of that host, and the Request-URI
1108   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
1109   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1110   the path-absolute is not present in the URL, it &MUST; be given as "/" when
1111   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1112   receives a host name which is not a fully qualified domain name, it
1113   &MAY; add its domain to the host name it received. If a proxy receives
1114   a fully qualified domain name, the proxy &MUST-NOT; change the host
1115   name.
1116</t>
1117</section>
1118
1119<section title="URI Comparison" anchor="uri.comparison">
1120<t>
1121   When comparing two URIs to decide if they match or not, a client
1122   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1123   URIs, with these exceptions:
1124  <list style="symbols">
1125    <t>A port that is empty or not given is equivalent to the default
1126        port for that URI-reference;</t>
1127    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1128    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1129    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
1130  </list>
1131</t>
1132<t>
1133   Characters other than those in the "reserved" set (see
1134   <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1135</t>
1136<t>
1137   For example, the following three URIs are equivalent:
1138</t>
1139<figure><artwork type="example">
1140   http://example.com:80/~smith/home.html
1141   http://EXAMPLE.com/%7Esmith/home.html
1142   http://EXAMPLE.com:/%7esmith/home.html
1143</artwork></figure>
1144</section>
1145</section>
1146
1147<section title="Date/Time Formats" anchor="date.time.formats">
1148<section title="Full Date" anchor="full.date">
1149<t>
1150   HTTP applications have historically allowed three different formats
1151   for the representation of date/time stamps:
1152</t>
1153<figure><artwork type="example">
1154   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1155   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1156   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1157</artwork></figure>
1158<t>
1159   The first format is preferred as an Internet standard and represents
1160   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1161   <xref target="RFC822"/>). The other formats are described here only for
1162   compatibility with obsolete implementations.
1163   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1164   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1165   only generate the RFC 1123 format for representing HTTP-date values
1166   in header fields. See <xref target="tolerant.applications"/> for further information.
1167</t>
1168<t><list><t>
1169      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1170      accepting date values that may have been sent by non-HTTP
1171      applications, as is sometimes the case when retrieving or posting
1172      messages via proxies/gateways to SMTP or NNTP.
1173</t></list></t>
1174<t>
1175   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1176   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1177   equal to UTC (Coordinated Universal Time). This is indicated in the
1178   first two formats by the inclusion of "GMT" as the three-letter
1179   abbreviation for time zone, and &MUST; be assumed when reading the
1180   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1181   additional LWS beyond that specifically included as SP in the
1182   grammar.
1183</t>
1184<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"/>
1185  HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1186  rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1187  rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1188  asctime-date = wkday SP date3 SP time SP 4DIGIT
1189  date1        = 2DIGIT SP month SP 4DIGIT
1190                 ; day month year (e.g., 02 Jun 1982)
1191  date2        = 2DIGIT "-" month "-" 2DIGIT
1192                 ; day-month-year (e.g., 02-Jun-82)
1193  date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1194                 ; month day (e.g., Jun  2)
1195  time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1196                 ; 00:00:00 - 23:59:59
1197  wkday        = "Mon" | "Tue" | "Wed"
1198               | "Thu" | "Fri" | "Sat" | "Sun"
1199  weekday      = "Monday" | "Tuesday" | "Wednesday"
1200               | "Thursday" | "Friday" | "Saturday" | "Sunday"
1201  month        = "Jan" | "Feb" | "Mar" | "Apr"
1202               | "May" | "Jun" | "Jul" | "Aug"
1203               | "Sep" | "Oct" | "Nov" | "Dec"
1204</artwork></figure>
1205<t>
1206      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1207      to their usage within the protocol stream. Clients and servers are
1208      not required to use these formats for user presentation, request
1209      logging, etc.
1210</t>
1211</section>
1212</section>
1213
1214<section title="Transfer Codings" anchor="transfer.codings">
1215<t>
1216   Transfer-coding values are used to indicate an encoding
1217   transformation that has been, can be, or may need to be applied to an
1218   entity-body in order to ensure "safe transport" through the network.
1219   This differs from a content coding in that the transfer-coding is a
1220   property of the message, not of the original entity.
1221</t>
1222<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1223  transfer-coding         = "chunked" | transfer-extension
1224  transfer-extension      = token *( ";" parameter )
1225</artwork></figure>
1226<t>
1227   Parameters are in  the form of attribute/value pairs.
1228</t>
1229<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"/>
1230  parameter               = attribute "=" value
1231  attribute               = token
1232  value                   = token | quoted-string
1233</artwork></figure>
1234<t>
1235   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1236   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1237   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1238</t>
1239<t>
1240   Whenever a transfer-coding is applied to a message-body, the set of
1241   transfer-codings &MUST; include "chunked", unless the message is
1242   terminated by closing the connection. When the "chunked" transfer-coding
1243   is used, it &MUST; be the last transfer-coding applied to the
1244   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1245   than once to a message-body. These rules allow the recipient to
1246   determine the transfer-length of the message (<xref target="message.length"/>).
1247</t>
1248<t>
1249   Transfer-codings are analogous to the Content-Transfer-Encoding
1250   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1251   binary data over a 7-bit transport service. However, safe transport
1252   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1253   the only unsafe characteristic of message-bodies is the difficulty in
1254   determining the exact body length (<xref target="message.length"/>), or the desire to
1255   encrypt data over a shared transport.
1256</t>
1257<t>
1258   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1259   transfer-coding value tokens. Initially, the registry contains the
1260   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1261   "gzip", "compress", and "deflate" (&content-codings;).
1262</t>
1263<t>
1264   New transfer-coding value tokens &SHOULD; be registered in the same way
1265   as new content-coding value tokens (&content-codings;).
1266</t>
1267<t>
1268   A server which receives an entity-body with a transfer-coding it does
1269   not understand &SHOULD; return 501 (Not Implemented), and close the
1270   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1271   client.
1272</t>
1273
1274<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1275<t>
1276   The chunked encoding modifies the body of a message in order to
1277   transfer it as a series of chunks, each with its own size indicator,
1278   followed by an &OPTIONAL; trailer containing entity-header fields. This
1279   allows dynamically produced content to be transferred along with the
1280   information necessary for the recipient to verify that it has
1281   received the full message.
1282</t>
1283<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"/>
1284  Chunked-Body   = *chunk
1285                   last-chunk
1286                   trailer-part
1287                   CRLF
1288 
1289  chunk          = chunk-size [ chunk-extension ] CRLF
1290                   chunk-data CRLF
1291  chunk-size     = 1*HEX
1292  last-chunk     = 1*("0") [ chunk-extension ] CRLF
1293 
1294  chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1295  chunk-ext-name = token
1296  chunk-ext-val  = token | quoted-string
1297  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
1298  trailer-part   = *(entity-header CRLF)
1299</artwork></figure>
1300<t>
1301   The chunk-size field is a string of hex digits indicating the size of
1302   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1303   zero, followed by the trailer, which is terminated by an empty line.
1304</t>
1305<t>
1306   The trailer allows the sender to include additional HTTP header
1307   fields at the end of the message. The Trailer header field can be
1308   used to indicate which header fields are included in a trailer (see
1309   <xref target="header.trailer"/>).
1310</t>
1311<t>
1312   A server using chunked transfer-coding in a response &MUST-NOT; use the
1313   trailer for any header fields unless at least one of the following is
1314   true:
1315  <list style="numbers">
1316    <t>the request included a TE header field that indicates "trailers" is
1317     acceptable in the transfer-coding of the  response, as described in
1318     <xref target="header.te"/>; or,</t>
1319
1320    <t>the server is the origin server for the response, the trailer
1321     fields consist entirely of optional metadata, and the recipient
1322     could use the message (in a manner acceptable to the origin server)
1323     without receiving this metadata.  In other words, the origin server
1324     is willing to accept the possibility that the trailer fields might
1325     be silently discarded along the path to the client.</t>
1326  </list>
1327</t>
1328<t>
1329   This requirement prevents an interoperability failure when the
1330   message is being received by an HTTP/1.1 (or later) proxy and
1331   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1332   compliance with the protocol would have necessitated a possibly
1333   infinite buffer on the proxy.
1334</t>
1335<t>
1336   A process for decoding the "chunked" transfer-coding
1337   can be represented in pseudo-code as:
1338</t>
1339<figure><artwork type="code">
1340    length := 0
1341    read chunk-size, chunk-extension (if any) and CRLF
1342    while (chunk-size &gt; 0) {
1343       read chunk-data and CRLF
1344       append chunk-data to entity-body
1345       length := length + chunk-size
1346       read chunk-size and CRLF
1347    }
1348    read entity-header
1349    while (entity-header not empty) {
1350       append entity-header to existing header fields
1351       read entity-header
1352    }
1353    Content-Length := length
1354    Remove "chunked" from Transfer-Encoding
1355</artwork></figure>
1356<t>
1357   All HTTP/1.1 applications &MUST; be able to receive and decode the
1358   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1359   they do not understand.
1360</t>
1361</section>
1362</section>
1363
1364<section title="Product Tokens" anchor="product.tokens">
1365<t>
1366   Product tokens are used to allow communicating applications to
1367   identify themselves by software name and version. Most fields using
1368   product tokens also allow sub-products which form a significant part
1369   of the application to be listed, separated by white space. By
1370   convention, the products are listed in order of their significance
1371   for identifying the application.
1372</t>
1373<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1374  product         = token ["/" product-version]
1375  product-version = token
1376</artwork></figure>
1377<t>
1378   Examples:
1379</t>
1380<figure><artwork type="example">
1381    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1382    Server: Apache/0.8.4
1383</artwork></figure>
1384<t>
1385   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1386   used for advertising or other non-essential information. Although any
1387   token character &MAY; appear in a product-version, this token &SHOULD;
1388   only be used for a version identifier (i.e., successive versions of
1389   the same product &SHOULD; only differ in the product-version portion of
1390   the product value).
1391</t>
1392</section>
1393
1394</section>
1395
1396<section title="HTTP Message" anchor="http.message">
1397
1398<section title="Message Types" anchor="message.types">
1399<t>
1400   HTTP messages consist of requests from client to server and responses
1401   from server to client.
1402</t>
1403<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1404  HTTP-message   = Request | Response     ; HTTP/1.1 messages
1405</artwork></figure>
1406<t>
1407   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1408   message format of <xref target="RFC2822"/> for transferring entities (the payload
1409   of the message). Both types of message consist of a start-line, zero
1410   or more header fields (also known as "headers"), an empty line (i.e.,
1411   a line with nothing preceding the CRLF) indicating the end of the
1412   header fields, and possibly a message-body.
1413</t>
1414<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1415  generic-message = start-line
1416                    *(message-header CRLF)
1417                    CRLF
1418                    [ message-body ]
1419  start-line      = Request-Line | Status-Line
1420</artwork></figure>
1421<t>
1422   In the interest of robustness, servers &SHOULD; ignore any empty
1423   line(s) received where a Request-Line is expected. In other words, if
1424   the server is reading the protocol stream at the beginning of a
1425   message and receives a CRLF first, it should ignore the CRLF.
1426</t>
1427<t>
1428   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1429   after a POST request. To restate what is explicitly forbidden by the
1430   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1431   extra CRLF.
1432</t>
1433</section>
1434
1435<section title="Message Headers" anchor="message.headers">
1436<t>
1437   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1438   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1439   entity-header (&entity-header-fields;) fields, follow the same generic format as
1440   that given in <xref target="RFC2822" x:fmt="of" x:sec="2.1"/>. Each header field consists
1441   of a name followed by a colon (":") and the field value. Field names
1442   are case-insensitive. The field value &MAY; be preceded by any amount
1443   of LWS, though a single SP is preferred. Header fields can be
1444   extended over multiple lines by preceding each extra line with at
1445   least one SP or HTAB. Applications ought to follow "common form", where
1446   one is known or indicated, when generating HTTP constructs, since
1447   there might exist some implementations that fail to accept anything
1448   beyond the common forms.
1449</t>
1450<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"/>
1451  message-header = field-name ":" [ field-value ]
1452  field-name     = token
1453  field-value    = *( field-content | LWS )
1454  field-content  = &lt;field content&gt;
1455                   ; the OCTETs making up the field-value
1456                   ; and consisting of either *TEXT or combinations
1457                   ; of token, separators, and quoted-string
1458</artwork></figure>
1459<t>
1460   The field-content does not include any leading or trailing LWS:
1461   linear white space occurring before the first non-whitespace
1462   character of the field-value or after the last non-whitespace
1463   character of the field-value. Such leading or trailing LWS &MAY; be
1464   removed without changing the semantics of the field value. Any LWS
1465   that occurs between field-content &MAY; be replaced with a single SP
1466   before interpreting the field value or forwarding the message
1467   downstream.
1468</t>
1469<t>
1470   The order in which header fields with differing field names are
1471   received is not significant. However, it is "good practice" to send
1472   general-header fields first, followed by request-header or response-header
1473   fields, and ending with the entity-header fields.
1474</t>
1475<t>
1476   Multiple message-header fields with the same field-name &MAY; be
1477   present in a message if and only if the entire field-value for that
1478   header field is defined as a comma-separated list [i.e., #(values)].
1479   It &MUST; be possible to combine the multiple header fields into one
1480   "field-name: field-value" pair, without changing the semantics of the
1481   message, by appending each subsequent field-value to the first, each
1482   separated by a comma. The order in which header fields with the same
1483   field-name are received is therefore significant to the
1484   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1485   change the order of these field values when a message is forwarded.
1486</t>
1487</section>
1488
1489<section title="Message Body" anchor="message.body">
1490<t>
1491   The message-body (if any) of an HTTP message is used to carry the
1492   entity-body associated with the request or response. The message-body
1493   differs from the entity-body only when a transfer-coding has been
1494   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1495</t>
1496<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1497  message-body = entity-body
1498               | &lt;entity-body encoded as per Transfer-Encoding&gt;
1499</artwork></figure>
1500<t>
1501   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1502   applied by an application to ensure safe and proper transfer of the
1503   message. Transfer-Encoding is a property of the message, not of the
1504   entity, and thus &MAY; be added or removed by any application along the
1505   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1506   when certain transfer-codings may be used.)
1507</t>
1508<t>
1509   The rules for when a message-body is allowed in a message differ for
1510   requests and responses.
1511</t>
1512<t>
1513   The presence of a message-body in a request is signaled by the
1514   inclusion of a Content-Length or Transfer-Encoding header field in
1515   the request's message-headers. A message-body &MUST-NOT; be included in
1516   a request if the specification of the request method (&method;)
1517   explicitly disallows an entity-body in requests.
1518   When a request message contains both a message-body of non-zero
1519   length and a method that does not define any semantics for that
1520   request message-body, then an origin server &SHOULD; either ignore
1521   the message-body or respond with an appropriate error message
1522   (e.g., 413).  A proxy or gateway, when presented the same request,
1523   &SHOULD; either forward the request inbound with the message-body or
1524   ignore the message-body when determining a response.
1525</t>
1526<t>
1527   For response messages, whether or not a message-body is included with
1528   a message is dependent on both the request method and the response
1529   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1530   &MUST-NOT; include a message-body, even though the presence of entity-header
1531   fields might lead one to believe they do. All 1xx
1532   (informational), 204 (No Content), and 304 (Not Modified) responses
1533   &MUST-NOT; include a message-body. All other responses do include a
1534   message-body, although it &MAY; be of zero length.
1535</t>
1536</section>
1537
1538<section title="Message Length" anchor="message.length">
1539<t>
1540   The transfer-length of a message is the length of the message-body as
1541   it appears in the message; that is, after any transfer-codings have
1542   been applied. When a message-body is included with a message, the
1543   transfer-length of that body is determined by one of the following
1544   (in order of precedence):
1545</t>
1546<t>
1547  <list style="numbers">
1548    <x:lt><t>
1549     Any response message which "&MUST-NOT;" include a message-body (such
1550     as the 1xx, 204, and 304 responses and any response to a HEAD
1551     request) is always terminated by the first empty line after the
1552     header fields, regardless of the entity-header fields present in
1553     the message.
1554    </t></x:lt>
1555    <x:lt><t>
1556     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1557     is present, then the transfer-length is
1558     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1559     unless the message is terminated by closing the connection.
1560    </t></x:lt>
1561    <x:lt><t>
1562     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1563     decimal value in OCTETs represents both the entity-length and the
1564     transfer-length. The Content-Length header field &MUST-NOT; be sent
1565     if these two lengths are different (i.e., if a Transfer-Encoding
1566     header field is present). If a message is received with both a
1567     Transfer-Encoding header field and a Content-Length header field,
1568     the latter &MUST; be ignored.
1569    </t></x:lt>
1570    <x:lt><t>
1571     If the message uses the media type "multipart/byteranges", and the
1572     transfer-length is not otherwise specified, then this self-delimiting
1573     media type defines the transfer-length. This media type
1574     &MUST-NOT; be used unless the sender knows that the recipient can parse
1575     it; the presence in a request of a Range header with multiple byte-range
1576     specifiers from a 1.1 client implies that the client can parse
1577     multipart/byteranges responses.
1578    <list style="empty"><t>
1579       A range header might be forwarded by a 1.0 proxy that does not
1580       understand multipart/byteranges; in this case the server &MUST;
1581       delimit the message using methods defined in items 1, 3 or 5 of
1582       this section.
1583    </t></list>
1584    </t></x:lt>
1585    <x:lt><t>
1586     By the server closing the connection. (Closing the connection
1587     cannot be used to indicate the end of a request body, since that
1588     would leave no possibility for the server to send back a response.)
1589    </t></x:lt>
1590  </list>
1591</t>
1592<t>
1593   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1594   containing a message-body &MUST; include a valid Content-Length header
1595   field unless the server is known to be HTTP/1.1 compliant. If a
1596   request contains a message-body and a Content-Length is not given,
1597   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1598   determine the length of the message, or with 411 (Length Required) if
1599   it wishes to insist on receiving a valid Content-Length.
1600</t>
1601<t>
1602   All HTTP/1.1 applications that receive entities &MUST; accept the
1603   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1604   to be used for messages when the message length cannot be determined
1605   in advance.
1606</t>
1607<t>
1608   Messages &MUST-NOT; include both a Content-Length header field and a
1609   transfer-coding. If the message does include a
1610   transfer-coding, the Content-Length &MUST; be ignored.
1611</t>
1612<t>
1613   When a Content-Length is given in a message where a message-body is
1614   allowed, its field value &MUST; exactly match the number of OCTETs in
1615   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1616   invalid length is received and detected.
1617</t>
1618</section>
1619
1620<section title="General Header Fields" anchor="general.header.fields">
1621<t>
1622   There are a few header fields which have general applicability for
1623   both request and response messages, but which do not apply to the
1624   entity being transferred. These header fields apply only to the
1625   message being transmitted.
1626</t>
1627<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1628  general-header = Cache-Control            ; &header-cache-control;
1629                 | Connection               ; <xref target="header.connection"/>
1630                 | Date                     ; <xref target="header.date"/>
1631                 | Pragma                   ; &header-pragma;
1632                 | Trailer                  ; <xref target="header.trailer"/>
1633                 | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1634                 | Upgrade                  ; <xref target="header.upgrade"/>
1635                 | Via                      ; <xref target="header.via"/>
1636                 | Warning                  ; &header-warning;
1637</artwork></figure>
1638<t>
1639   General-header field names can be extended reliably only in
1640   combination with a change in the protocol version. However, new or
1641   experimental header fields may be given the semantics of general
1642   header fields if all parties in the communication recognize them to
1643   be general-header fields. Unrecognized header fields are treated as
1644   entity-header fields.
1645</t>
1646</section>
1647</section>
1648
1649<section title="Request" anchor="request">
1650<t>
1651   A request message from a client to a server includes, within the
1652   first line of that message, the method to be applied to the resource,
1653   the identifier of the resource, and the protocol version in use.
1654</t>
1655<!--                 Host                      ; should be moved here eventually -->
1656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1657  Request       = Request-Line              ; <xref target="request-line"/>
1658                  *(( general-header        ; <xref target="general.header.fields"/>
1659                   | request-header         ; &request-header-fields;
1660                   | entity-header ) CRLF)  ; &entity-header-fields;
1661                  CRLF
1662                  [ message-body ]          ; <xref target="message.body"/>
1663</artwork></figure>
1664
1665<section title="Request-Line" anchor="request-line">
1666<t>
1667   The Request-Line begins with a method token, followed by the
1668   Request-URI and the protocol version, and ending with CRLF. The
1669   elements are separated by SP characters. No CR or LF is allowed
1670   except in the final CRLF sequence.
1671</t>
1672<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1673  Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1674</artwork></figure>
1675
1676<section title="Method" anchor="method">
1677<t>
1678   The Method  token indicates the method to be performed on the
1679   resource identified by the Request-URI. The method is case-sensitive.
1680</t>
1681<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1682  Method         = token
1683</artwork></figure>
1684</section>
1685
1686<section title="Request-URI" anchor="request-uri">
1687<t>
1688   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1689   identifies the resource upon which to apply the request.
1690</t>
1691<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1692  Request-URI    = "*"
1693                 | absoluteURI
1694                 | ( path-absolute [ "?" query ] )
1695                 | authority
1696</artwork></figure>
1697<t>
1698   The four options for Request-URI are dependent on the nature of the
1699   request. The asterisk "*" means that the request does not apply to a
1700   particular resource, but to the server itself, and is only allowed
1701   when the method used does not necessarily apply to a resource. One
1702   example would be
1703</t>
1704<figure><artwork type="example">
1705    OPTIONS * HTTP/1.1
1706</artwork></figure>
1707<t>
1708   The absoluteURI form is &REQUIRED; when the request is being made to a
1709   proxy. The proxy is requested to forward the request or service it
1710   from a valid cache, and return the response. Note that the proxy &MAY;
1711   forward the request on to another proxy or directly to the server
1712   specified by the absoluteURI. In order to avoid request loops, a
1713   proxy &MUST; be able to recognize all of its server names, including
1714   any aliases, local variations, and the numeric IP address. An example
1715   Request-Line would be:
1716</t>
1717<figure><artwork type="example">
1718    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1719</artwork></figure>
1720<t>
1721   To allow for transition to absoluteURIs in all requests in future
1722   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1723   form in requests, even though HTTP/1.1 clients will only generate
1724   them in requests to proxies.
1725</t>
1726<t>
1727   The authority form is only used by the CONNECT method (&CONNECT;).
1728</t>
1729<t>
1730   The most common form of Request-URI is that used to identify a
1731   resource on an origin server or gateway. In this case the absolute
1732   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1733   the Request-URI, and the network location of the URI (authority) &MUST;
1734   be transmitted in a Host header field. For example, a client wishing
1735   to retrieve the resource above directly from the origin server would
1736   create a TCP connection to port 80 of the host "www.example.org" and send
1737   the lines:
1738</t>
1739<figure><artwork type="example">
1740    GET /pub/WWW/TheProject.html HTTP/1.1
1741    Host: www.example.org
1742</artwork></figure>
1743<t>
1744   followed by the remainder of the Request. Note that the absolute path
1745   cannot be empty; if none is present in the original URI, it &MUST; be
1746   given as "/" (the server root).
1747</t>
1748<t>
1749   The Request-URI is transmitted in the format specified in
1750   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1751   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1752   properly interpret the request. Servers &SHOULD; respond to invalid
1753   Request-URIs with an appropriate status code.
1754</t>
1755<t>
1756   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1757   received Request-URI when forwarding it to the next inbound server,
1758   except as noted above to replace a null path-absolute with "/".
1759</t>
1760<t>
1761  <list><t>
1762      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1763      meaning of the request when the origin server is improperly using
1764      a non-reserved URI character for a reserved purpose.  Implementors
1765      should be aware that some pre-HTTP/1.1 proxies have been known to
1766      rewrite the Request-URI.
1767  </t></list>
1768</t>
1769</section>
1770</section>
1771
1772<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1773<t>
1774   The exact resource identified by an Internet request is determined by
1775   examining both the Request-URI and the Host header field.
1776</t>
1777<t>
1778   An origin server that does not allow resources to differ by the
1779   requested host &MAY; ignore the Host header field value when
1780   determining the resource identified by an HTTP/1.1 request. (But see
1781   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1782   for other requirements on Host support in HTTP/1.1.)
1783</t>
1784<t>
1785   An origin server that does differentiate resources based on the host
1786   requested (sometimes referred to as virtual hosts or vanity host
1787   names) &MUST; use the following rules for determining the requested
1788   resource on an HTTP/1.1 request:
1789  <list style="numbers">
1790    <t>If Request-URI is an absoluteURI, the host is part of the
1791     Request-URI. Any Host header field value in the request &MUST; be
1792     ignored.</t>
1793    <t>If the Request-URI is not an absoluteURI, and the request includes
1794     a Host header field, the host is determined by the Host header
1795     field value.</t>
1796    <t>If the host as determined by rule 1 or 2 is not a valid host on
1797     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1798  </list>
1799</t>
1800<t>
1801   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1802   attempt to use heuristics (e.g., examination of the URI path for
1803   something unique to a particular host) in order to determine what
1804   exact resource is being requested.
1805</t>
1806</section>
1807
1808</section>
1809
1810
1811<section title="Response" anchor="response">
1812<t>
1813   After receiving and interpreting a request message, a server responds
1814   with an HTTP response message.
1815</t>
1816<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1817  Response      = Status-Line               ; <xref target="status-line"/>
1818                  *(( general-header        ; <xref target="general.header.fields"/>
1819                   | response-header        ; &response-header-fields;
1820                   | entity-header ) CRLF)  ; &entity-header-fields;
1821                  CRLF
1822                  [ message-body ]          ; <xref target="message.body"/>
1823</artwork></figure>
1824
1825<section title="Status-Line" anchor="status-line">
1826<t>
1827   The first line of a Response message is the Status-Line, consisting
1828   of the protocol version followed by a numeric status code and its
1829   associated textual phrase, with each element separated by SP
1830   characters. No CR or LF is allowed except in the final CRLF sequence.
1831</t>
1832<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1833  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1834</artwork></figure>
1835
1836<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1837<t>
1838   The Status-Code element is a 3-digit integer result code of the
1839   attempt to understand and satisfy the request. These codes are fully
1840   defined in &status-codes;.  The Reason Phrase exists for the sole
1841   purpose of providing a textual description associated with the numeric
1842   status code, out of deference to earlier Internet application protocols
1843   that were more frequently used with interactive text clients.
1844   A client &SHOULD; ignore the content of the Reason Phrase.
1845</t>
1846<t>
1847   The first digit of the Status-Code defines the class of response. The
1848   last two digits do not have any categorization role. There are 5
1849   values for the first digit:
1850  <list style="symbols">
1851    <t>
1852      1xx: Informational - Request received, continuing process
1853    </t>
1854    <t>
1855      2xx: Success - The action was successfully received,
1856        understood, and accepted
1857    </t>
1858    <t>
1859      3xx: Redirection - Further action must be taken in order to
1860        complete the request
1861    </t>
1862    <t>
1863      4xx: Client Error - The request contains bad syntax or cannot
1864        be fulfilled
1865    </t>
1866    <t>
1867      5xx: Server Error - The server failed to fulfill an apparently
1868        valid request
1869    </t>
1870  </list>
1871</t>
1872<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"/>
1873  Status-Code    = 3DIGIT
1874  Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1875</artwork></figure>
1876</section>
1877</section>
1878
1879</section>
1880
1881
1882<section title="Connections" anchor="connections">
1883
1884<section title="Persistent Connections" anchor="persistent.connections">
1885
1886<section title="Purpose" anchor="persistent.purpose">
1887<t>
1888   Prior to persistent connections, a separate TCP connection was
1889   established to fetch each URL, increasing the load on HTTP servers
1890   and causing congestion on the Internet. The use of inline images and
1891   other associated data often require a client to make multiple
1892   requests of the same server in a short amount of time. Analysis of
1893   these performance problems and results from a prototype
1894   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1895   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1896   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1897   T/TCP <xref target="Tou1998"/>.
1898</t>
1899<t>
1900   Persistent HTTP connections have a number of advantages:
1901  <list style="symbols">
1902      <t>
1903        By opening and closing fewer TCP connections, CPU time is saved
1904        in routers and hosts (clients, servers, proxies, gateways,
1905        tunnels, or caches), and memory used for TCP protocol control
1906        blocks can be saved in hosts.
1907      </t>
1908      <t>
1909        HTTP requests and responses can be pipelined on a connection.
1910        Pipelining allows a client to make multiple requests without
1911        waiting for each response, allowing a single TCP connection to
1912        be used much more efficiently, with much lower elapsed time.
1913      </t>
1914      <t>
1915        Network congestion is reduced by reducing the number of packets
1916        caused by TCP opens, and by allowing TCP sufficient time to
1917        determine the congestion state of the network.
1918      </t>
1919      <t>
1920        Latency on subsequent requests is reduced since there is no time
1921        spent in TCP's connection opening handshake.
1922      </t>
1923      <t>
1924        HTTP can evolve more gracefully, since errors can be reported
1925        without the penalty of closing the TCP connection. Clients using
1926        future versions of HTTP might optimistically try a new feature,
1927        but if communicating with an older server, retry with old
1928        semantics after an error is reported.
1929      </t>
1930    </list>
1931</t>
1932<t>
1933   HTTP implementations &SHOULD; implement persistent connections.
1934</t>
1935</section>
1936
1937<section title="Overall Operation" anchor="persistent.overall">
1938<t>
1939   A significant difference between HTTP/1.1 and earlier versions of
1940   HTTP is that persistent connections are the default behavior of any
1941   HTTP connection. That is, unless otherwise indicated, the client
1942   &SHOULD; assume that the server will maintain a persistent connection,
1943   even after error responses from the server.
1944</t>
1945<t>
1946   Persistent connections provide a mechanism by which a client and a
1947   server can signal the close of a TCP connection. This signaling takes
1948   place using the Connection header field (<xref target="header.connection"/>). Once a close
1949   has been signaled, the client &MUST-NOT; send any more requests on that
1950   connection.
1951</t>
1952
1953<section title="Negotiation" anchor="persistent.negotiation">
1954<t>
1955   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1956   maintain a persistent connection unless a Connection header including
1957   the connection-token "close" was sent in the request. If the server
1958   chooses to close the connection immediately after sending the
1959   response, it &SHOULD; send a Connection header including the
1960   connection-token close.
1961</t>
1962<t>
1963   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1964   decide to keep it open based on whether the response from a server
1965   contains a Connection header with the connection-token close. In case
1966   the client does not want to maintain a connection for more than that
1967   request, it &SHOULD; send a Connection header including the
1968   connection-token close.
1969</t>
1970<t>
1971   If either the client or the server sends the close token in the
1972   Connection header, that request becomes the last one for the
1973   connection.
1974</t>
1975<t>
1976   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1977   maintained for HTTP versions less than 1.1 unless it is explicitly
1978   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1979   compatibility with HTTP/1.0 clients.
1980</t>
1981<t>
1982   In order to remain persistent, all messages on the connection &MUST;
1983   have a self-defined message length (i.e., one not defined by closure
1984   of the connection), as described in <xref target="message.length"/>.
1985</t>
1986</section>
1987
1988<section title="Pipelining" anchor="pipelining">
1989<t>
1990   A client that supports persistent connections &MAY; "pipeline" its
1991   requests (i.e., send multiple requests without waiting for each
1992   response). A server &MUST; send its responses to those requests in the
1993   same order that the requests were received.
1994</t>
1995<t>
1996   Clients which assume persistent connections and pipeline immediately
1997   after connection establishment &SHOULD; be prepared to retry their
1998   connection if the first pipelined attempt fails. If a client does
1999   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2000   persistent. Clients &MUST; also be prepared to resend their requests if
2001   the server closes the connection before sending all of the
2002   corresponding responses.
2003</t>
2004<t>
2005   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
2006   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
2007   premature termination of the transport connection could lead to
2008   indeterminate results. A client wishing to send a non-idempotent
2009   request &SHOULD; wait to send that request until it has received the
2010   response status for the previous request.
2011</t>
2012</section>
2013</section>
2014
2015<section title="Proxy Servers" anchor="persistent.proxy">
2016<t>
2017   It is especially important that proxies correctly implement the
2018   properties of the Connection header field as specified in <xref target="header.connection"/>.
2019</t>
2020<t>
2021   The proxy server &MUST; signal persistent connections separately with
2022   its clients and the origin servers (or other proxy servers) that it
2023   connects to. Each persistent connection applies to only one transport
2024   link.
2025</t>
2026<t>
2027   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2028   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
2029   discussion of the problems with the Keep-Alive header implemented by
2030   many HTTP/1.0 clients).
2031</t>
2032</section>
2033
2034<section title="Practical Considerations" anchor="persistent.practical">
2035<t>
2036   Servers will usually have some time-out value beyond which they will
2037   no longer maintain an inactive connection. Proxy servers might make
2038   this a higher value since it is likely that the client will be making
2039   more connections through the same server. The use of persistent
2040   connections places no requirements on the length (or existence) of
2041   this time-out for either the client or the server.
2042</t>
2043<t>
2044   When a client or server wishes to time-out it &SHOULD; issue a graceful
2045   close on the transport connection. Clients and servers &SHOULD; both
2046   constantly watch for the other side of the transport close, and
2047   respond to it as appropriate. If a client or server does not detect
2048   the other side's close promptly it could cause unnecessary resource
2049   drain on the network.
2050</t>
2051<t>
2052   A client, server, or proxy &MAY; close the transport connection at any
2053   time. For example, a client might have started to send a new request
2054   at the same time that the server has decided to close the "idle"
2055   connection. From the server's point of view, the connection is being
2056   closed while it was idle, but from the client's point of view, a
2057   request is in progress.
2058</t>
2059<t>
2060   This means that clients, servers, and proxies &MUST; be able to recover
2061   from asynchronous close events. Client software &SHOULD; reopen the
2062   transport connection and retransmit the aborted sequence of requests
2063   without user interaction so long as the request sequence is
2064   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2065   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2066   human operator the choice of retrying the request(s). Confirmation by
2067   user-agent software with semantic understanding of the application
2068   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2069   be repeated if the second sequence of requests fails.
2070</t>
2071<t>
2072   Servers &SHOULD; always respond to at least one request per connection,
2073   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2074   middle of transmitting a response, unless a network or client failure
2075   is suspected.
2076</t>
2077<t>
2078   Clients that use persistent connections &SHOULD; limit the number of
2079   simultaneous connections that they maintain to a given server. A
2080   single-user client &SHOULD-NOT; maintain more than 2 connections with
2081   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2082   another server or proxy, where N is the number of simultaneously
2083   active users. These guidelines are intended to improve HTTP response
2084   times and avoid congestion.
2085</t>
2086</section>
2087</section>
2088
2089<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2090
2091<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2092<t>
2093   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2094   flow control mechanisms to resolve temporary overloads, rather than
2095   terminating connections with the expectation that clients will retry.
2096   The latter technique can exacerbate network congestion.
2097</t>
2098</section>
2099
2100<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2101<t>
2102   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2103   the network connection for an error status while it is transmitting
2104   the request. If the client sees an error status, it &SHOULD;
2105   immediately cease transmitting the body. If the body is being sent
2106   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2107   empty trailer &MAY; be used to prematurely mark the end of the message.
2108   If the body was preceded by a Content-Length header, the client &MUST;
2109   close the connection.
2110</t>
2111</section>
2112
2113<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2114<t>
2115   The purpose of the 100 (Continue) status (see &status-100;) is to
2116   allow a client that is sending a request message with a request body
2117   to determine if the origin server is willing to accept the request
2118   (based on the request headers) before the client sends the request
2119   body. In some cases, it might either be inappropriate or highly
2120   inefficient for the client to send the body if the server will reject
2121   the message without looking at the body.
2122</t>
2123<t>
2124   Requirements for HTTP/1.1 clients:
2125  <list style="symbols">
2126    <t>
2127        If a client will wait for a 100 (Continue) response before
2128        sending the request body, it &MUST; send an Expect request-header
2129        field (&header-expect;) with the "100-continue" expectation.
2130    </t>
2131    <t>
2132        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2133        with the "100-continue" expectation if it does not intend
2134        to send a request body.
2135    </t>
2136  </list>
2137</t>
2138<t>
2139   Because of the presence of older implementations, the protocol allows
2140   ambiguous situations in which a client may send "Expect: 100-continue"
2141   without receiving either a 417 (Expectation Failed) status
2142   or a 100 (Continue) status. Therefore, when a client sends this
2143   header field to an origin server (possibly via a proxy) from which it
2144   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2145   for an indefinite period before sending the request body.
2146</t>
2147<t>
2148   Requirements for HTTP/1.1 origin servers:
2149  <list style="symbols">
2150    <t> Upon receiving a request which includes an Expect request-header
2151        field with the "100-continue" expectation, an origin server &MUST;
2152        either respond with 100 (Continue) status and continue to read
2153        from the input stream, or respond with a final status code. The
2154        origin server &MUST-NOT; wait for the request body before sending
2155        the 100 (Continue) response. If it responds with a final status
2156        code, it &MAY; close the transport connection or it &MAY; continue
2157        to read and discard the rest of the request.  It &MUST-NOT;
2158        perform the requested method if it returns a final status code.
2159    </t>
2160    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2161        the request message does not include an Expect request-header
2162        field with the "100-continue" expectation, and &MUST-NOT; send a
2163        100 (Continue) response if such a request comes from an HTTP/1.0
2164        (or earlier) client. There is an exception to this rule: for
2165        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2166        status in response to an HTTP/1.1 PUT or POST request that does
2167        not include an Expect request-header field with the "100-continue"
2168        expectation. This exception, the purpose of which is
2169        to minimize any client processing delays associated with an
2170        undeclared wait for 100 (Continue) status, applies only to
2171        HTTP/1.1 requests, and not to requests with any other HTTP-version
2172        value.
2173    </t>
2174    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2175        already received some or all of the request body for the
2176        corresponding request.
2177    </t>
2178    <t> An origin server that sends a 100 (Continue) response &MUST;
2179    ultimately send a final status code, once the request body is
2180        received and processed, unless it terminates the transport
2181        connection prematurely.
2182    </t>
2183    <t> If an origin server receives a request that does not include an
2184        Expect request-header field with the "100-continue" expectation,
2185        the request includes a request body, and the server responds
2186        with a final status code before reading the entire request body
2187        from the transport connection, then the server &SHOULD-NOT;  close
2188        the transport connection until it has read the entire request,
2189        or until the client closes the connection. Otherwise, the client
2190        might not reliably receive the response message. However, this
2191        requirement is not be construed as preventing a server from
2192        defending itself against denial-of-service attacks, or from
2193        badly broken client implementations.
2194      </t>
2195    </list>
2196</t>
2197<t>
2198   Requirements for HTTP/1.1 proxies:
2199  <list style="symbols">
2200    <t> If a proxy receives a request that includes an Expect request-header
2201        field with the "100-continue" expectation, and the proxy
2202        either knows that the next-hop server complies with HTTP/1.1 or
2203        higher, or does not know the HTTP version of the next-hop
2204        server, it &MUST; forward the request, including the Expect header
2205        field.
2206    </t>
2207    <t> If the proxy knows that the version of the next-hop server is
2208        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2209        respond with a 417 (Expectation Failed) status.
2210    </t>
2211    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2212        numbers received from recently-referenced next-hop servers.
2213    </t>
2214    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2215        request message was received from an HTTP/1.0 (or earlier)
2216        client and did not include an Expect request-header field with
2217        the "100-continue" expectation. This requirement overrides the
2218        general rule for forwarding of 1xx responses (see &status-1xx;).
2219    </t>
2220  </list>
2221</t>
2222</section>
2223
2224<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2225<t>
2226   If an HTTP/1.1 client sends a request which includes a request body,
2227   but which does not include an Expect request-header field with the
2228   "100-continue" expectation, and if the client is not directly
2229   connected to an HTTP/1.1 origin server, and if the client sees the
2230   connection close before receiving any status from the server, the
2231   client &SHOULD; retry the request.  If the client does retry this
2232   request, it &MAY; use the following "binary exponential backoff"
2233   algorithm to be assured of obtaining a reliable response:
2234  <list style="numbers">
2235    <t>
2236      Initiate a new connection to the server
2237    </t>
2238    <t>
2239      Transmit the request-headers
2240    </t>
2241    <t>
2242      Initialize a variable R to the estimated round-trip time to the
2243         server (e.g., based on the time it took to establish the
2244         connection), or to a constant value of 5 seconds if the round-trip
2245         time is not available.
2246    </t>
2247    <t>
2248       Compute T = R * (2**N), where N is the number of previous
2249         retries of this request.
2250    </t>
2251    <t>
2252       Wait either for an error response from the server, or for T
2253         seconds (whichever comes first)
2254    </t>
2255    <t>
2256       If no error response is received, after T seconds transmit the
2257         body of the request.
2258    </t>
2259    <t>
2260       If client sees that the connection is closed prematurely,
2261         repeat from step 1 until the request is accepted, an error
2262         response is received, or the user becomes impatient and
2263         terminates the retry process.
2264    </t>
2265  </list>
2266</t>
2267<t>
2268   If at any point an error status is received, the client
2269  <list style="symbols">
2270      <t>&SHOULD-NOT;  continue and</t>
2271
2272      <t>&SHOULD; close the connection if it has not completed sending the
2273        request message.</t>
2274    </list>
2275</t>
2276</section>
2277</section>
2278</section>
2279
2280
2281<section title="Header Field Definitions" anchor="header.fields">
2282<t>
2283   This section defines the syntax and semantics of HTTP/1.1 header fields
2284   related to message framing and transport protocols.
2285</t>
2286<t>
2287   For entity-header fields, both sender and recipient refer to either the
2288   client or the server, depending on who sends and who receives the entity.
2289</t>
2290
2291<section title="Connection" anchor="header.connection">
2292  <iref primary="true" item="Connection header" x:for-anchor=""/>
2293  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2294<t>
2295   The Connection general-header field allows the sender to specify
2296   options that are desired for that particular connection and &MUST-NOT;
2297   be communicated by proxies over further connections.
2298</t>
2299<t>
2300   The Connection header has the following grammar:
2301</t>
2302<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2303  Connection = "Connection" ":" 1#(connection-token)
2304  connection-token  = token
2305</artwork></figure>
2306<t>
2307   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2308   message is forwarded and, for each connection-token in this field,
2309   remove any header field(s) from the message with the same name as the
2310   connection-token. Connection options are signaled by the presence of
2311   a connection-token in the Connection header field, not by any
2312   corresponding additional header field(s), since the additional header
2313   field may not be sent if there are no parameters associated with that
2314   connection option.
2315</t>
2316<t>
2317   Message headers listed in the Connection header &MUST-NOT; include
2318   end-to-end headers, such as Cache-Control.
2319</t>
2320<t>
2321   HTTP/1.1 defines the "close" connection option for the sender to
2322   signal that the connection will be closed after completion of the
2323   response. For example,
2324</t>
2325<figure><artwork type="example">
2326    Connection: close
2327</artwork></figure>
2328<t>
2329   in either the request or the response header fields indicates that
2330   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2331   after the current request/response is complete.
2332</t>
2333<t>
2334   An HTTP/1.1 client that does not support persistent connections &MUST;
2335   include the "close" connection option in every request message.
2336</t>
2337<t>
2338   An HTTP/1.1 server that does not support persistent connections &MUST;
2339   include the "close" connection option in every response message that
2340   does not have a 1xx (informational) status code.
2341</t>
2342<t>
2343   A system receiving an HTTP/1.0 (or lower-version) message that
2344   includes a Connection header &MUST;, for each connection-token in this
2345   field, remove and ignore any header field(s) from the message with
2346   the same name as the connection-token. This protects against mistaken
2347   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2348</t>
2349</section>
2350
2351<section title="Content-Length" anchor="header.content-length">
2352  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2353  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2354<t>
2355   The Content-Length entity-header field indicates the size of the
2356   entity-body, in decimal number of OCTETs, sent to the recipient or,
2357   in the case of the HEAD method, the size of the entity-body that
2358   would have been sent had the request been a GET.
2359</t>
2360<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2361  Content-Length    = "Content-Length" ":" 1*DIGIT
2362</artwork></figure>
2363<t>
2364   An example is
2365</t>
2366<figure><artwork type="example">
2367    Content-Length: 3495
2368</artwork></figure>
2369<t>
2370   Applications &SHOULD; use this field to indicate the transfer-length of
2371   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2372</t>
2373<t>
2374   Any Content-Length greater than or equal to zero is a valid value.
2375   <xref target="message.length"/> describes how to determine the length of a message-body
2376   if a Content-Length is not given.
2377</t>
2378<t>
2379   Note that the meaning of this field is significantly different from
2380   the corresponding definition in MIME, where it is an optional field
2381   used within the "message/external-body" content-type. In HTTP, it
2382   &SHOULD; be sent whenever the message's length can be determined prior
2383   to being transferred, unless this is prohibited by the rules in
2384   <xref target="message.length"/>.
2385</t>
2386</section>
2387
2388<section title="Date" anchor="header.date">
2389  <iref primary="true" item="Date header" x:for-anchor=""/>
2390  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2391<t>
2392   The Date general-header field represents the date and time at which
2393   the message was originated, having the same semantics as orig-date in
2394   <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"/>;
2395   it &MUST; be sent in rfc1123-date format.
2396</t>
2397<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2398  Date  = "Date" ":" HTTP-date
2399</artwork></figure>
2400<t>
2401   An example is
2402</t>
2403<figure><artwork type="example">
2404    Date: Tue, 15 Nov 1994 08:12:31 GMT
2405</artwork></figure>
2406<t>
2407   Origin servers &MUST; include a Date header field in all responses,
2408   except in these cases:
2409  <list style="numbers">
2410      <t>If the response status code is 100 (Continue) or 101 (Switching
2411         Protocols), the response &MAY; include a Date header field, at
2412         the server's option.</t>
2413
2414      <t>If the response status code conveys a server error, e.g. 500
2415         (Internal Server Error) or 503 (Service Unavailable), and it is
2416         inconvenient or impossible to generate a valid Date.</t>
2417
2418      <t>If the server does not have a clock that can provide a
2419         reasonable approximation of the current time, its responses
2420         &MUST-NOT; include a Date header field. In this case, the rules
2421         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2422  </list>
2423</t>
2424<t>
2425   A received message that does not have a Date header field &MUST; be
2426   assigned one by the recipient if the message will be cached by that
2427   recipient or gatewayed via a protocol which requires a Date. An HTTP
2428   implementation without a clock &MUST-NOT; cache responses without
2429   revalidating them on every use. An HTTP cache, especially a shared
2430   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2431   clock with a reliable external standard.
2432</t>
2433<t>
2434   Clients &SHOULD; only send a Date header field in messages that include
2435   an entity-body, as in the case of the PUT and POST requests, and even
2436   then it is optional. A client without a clock &MUST-NOT; send a Date
2437   header field in a request.
2438</t>
2439<t>
2440   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2441   time subsequent to the generation of the message. It &SHOULD; represent
2442   the best available approximation of the date and time of message
2443   generation, unless the implementation has no means of generating a
2444   reasonably accurate date and time. In theory, the date ought to
2445   represent the moment just before the entity is generated. In
2446   practice, the date can be generated at any time during the message
2447   origination without affecting its semantic value.
2448</t>
2449
2450<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2451<t>
2452   Some origin server implementations might not have a clock available.
2453   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2454   values to a response, unless these values were associated
2455   with the resource by a system or user with a reliable clock. It &MAY;
2456   assign an Expires value that is known, at or before server
2457   configuration time, to be in the past (this allows "pre-expiration"
2458   of responses without storing separate Expires values for each
2459   resource).
2460</t>
2461</section>
2462</section>
2463
2464<section title="Host" anchor="header.host">
2465  <iref primary="true" item="Host header" x:for-anchor=""/>
2466  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2467<t>
2468   The Host request-header field specifies the Internet host and port
2469   number of the resource being requested, as obtained from the original
2470   URI given by the user or referring resource (generally an HTTP URL,
2471   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2472   the naming authority of the origin server or gateway given by the
2473   original URL. This allows the origin server or gateway to
2474   differentiate between internally-ambiguous URLs, such as the root "/"
2475   URL of a server for multiple host names on a single IP address.
2476</t>
2477<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2478  Host = "Host" ":" uri-host [ ":" port ] ; <xref target="http.url"/>
2479</artwork></figure>
2480<t>
2481   A "host" without any trailing port information implies the default
2482   port for the service requested (e.g., "80" for an HTTP URL). For
2483   example, a request on the origin server for
2484   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2485</t>
2486<figure><artwork type="example">
2487    GET /pub/WWW/ HTTP/1.1
2488    Host: www.example.org
2489</artwork></figure>
2490<t>
2491   A client &MUST; include a Host header field in all HTTP/1.1 request
2492   messages. If the requested URI does not include an Internet host
2493   name for the service being requested, then the Host header field &MUST;
2494   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2495   request message it forwards does contain an appropriate Host header
2496   field that identifies the service being requested by the proxy. All
2497   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2498   status code to any HTTP/1.1 request message which lacks a Host header
2499   field.
2500</t>
2501<t>
2502   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2503   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2504   for other requirements relating to Host.
2505</t>
2506</section>
2507
2508<section title="TE" anchor="header.te">
2509  <iref primary="true" item="TE header" x:for-anchor=""/>
2510  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2511<t>
2512   The TE request-header field indicates what extension transfer-codings
2513   it is willing to accept in the response and whether or not it is
2514   willing to accept trailer fields in a chunked transfer-coding. Its
2515   value may consist of the keyword "trailers" and/or a comma-separated
2516   list of extension transfer-coding names with optional accept
2517   parameters (as described in <xref target="transfer.codings"/>).
2518</t>
2519<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2520  TE        = "TE" ":" #( t-codings )
2521  t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2522</artwork></figure>
2523<t>
2524   The presence of the keyword "trailers" indicates that the client is
2525   willing to accept trailer fields in a chunked transfer-coding, as
2526   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2527   transfer-coding values even though it does not itself represent a
2528   transfer-coding.
2529</t>
2530<t>
2531   Examples of its use are:
2532</t>
2533<figure><artwork type="example">
2534    TE: deflate
2535    TE:
2536    TE: trailers, deflate;q=0.5
2537</artwork></figure>
2538<t>
2539   The TE header field only applies to the immediate connection.
2540   Therefore, the keyword &MUST; be supplied within a Connection header
2541   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2542</t>
2543<t>
2544   A server tests whether a transfer-coding is acceptable, according to
2545   a TE field, using these rules:
2546  <list style="numbers">
2547    <x:lt>
2548      <t>The "chunked" transfer-coding is always acceptable. If the
2549         keyword "trailers" is listed, the client indicates that it is
2550         willing to accept trailer fields in the chunked response on
2551         behalf of itself and any downstream clients. The implication is
2552         that, if given, the client is stating that either all
2553         downstream clients are willing to accept trailer fields in the
2554         forwarded response, or that it will attempt to buffer the
2555         response on behalf of downstream recipients.
2556      </t><t>
2557         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2558         chunked response such that a client can be assured of buffering
2559         the entire response.</t>
2560    </x:lt>
2561    <x:lt>
2562      <t>If the transfer-coding being tested is one of the transfer-codings
2563         listed in the TE field, then it is acceptable unless it
2564         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2565         qvalue of 0 means "not acceptable.")</t>
2566    </x:lt>
2567    <x:lt>
2568      <t>If multiple transfer-codings are acceptable, then the
2569         acceptable transfer-coding with the highest non-zero qvalue is
2570         preferred.  The "chunked" transfer-coding always has a qvalue
2571         of 1.</t>
2572    </x:lt>
2573  </list>
2574</t>
2575<t>
2576   If the TE field-value is empty or if no TE field is present, the only
2577   transfer-coding  is "chunked". A message with no transfer-coding is
2578   always acceptable.
2579</t>
2580</section>
2581
2582<section title="Trailer" anchor="header.trailer">
2583  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2584  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2585<t>
2586   The Trailer general field value indicates that the given set of
2587   header fields is present in the trailer of a message encoded with
2588   chunked transfer-coding.
2589</t>
2590<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2591  Trailer  = "Trailer" ":" 1#field-name
2592</artwork></figure>
2593<t>
2594   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2595   message using chunked transfer-coding with a non-empty trailer. Doing
2596   so allows the recipient to know which header fields to expect in the
2597   trailer.
2598</t>
2599<t>
2600   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2601   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2602   trailer fields in a "chunked" transfer-coding.
2603</t>
2604<t>
2605   Message header fields listed in the Trailer header field &MUST-NOT;
2606   include the following header fields:
2607  <list style="symbols">
2608    <t>Transfer-Encoding</t>
2609    <t>Content-Length</t>
2610    <t>Trailer</t>
2611  </list>
2612</t>
2613</section>
2614
2615<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2616  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2617  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2618<t>
2619   The Transfer-Encoding general-header field indicates what (if any)
2620   type of transformation has been applied to the message body in order
2621   to safely transfer it between the sender and the recipient. This
2622   differs from the content-coding in that the transfer-coding is a
2623   property of the message, not of the entity.
2624</t>
2625<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2626  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2627</artwork></figure>
2628<t>
2629   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2630</t>
2631<figure><artwork type="example">
2632  Transfer-Encoding: chunked
2633</artwork></figure>
2634<t>
2635   If multiple encodings have been applied to an entity, the transfer-codings
2636   &MUST; be listed in the order in which they were applied.
2637   Additional information about the encoding parameters &MAY; be provided
2638   by other entity-header fields not defined by this specification.
2639</t>
2640<t>
2641   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2642   header.
2643</t>
2644</section>
2645
2646<section title="Upgrade" anchor="header.upgrade">
2647  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2648  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2649<t>
2650   The Upgrade general-header allows the client to specify what
2651   additional communication protocols it supports and would like to use
2652   if the server finds it appropriate to switch protocols. The server
2653   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2654   response to indicate which protocol(s) are being switched.
2655</t>
2656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2657  Upgrade        = "Upgrade" ":" 1#product
2658</artwork></figure>
2659<t>
2660   For example,
2661</t>
2662<figure><artwork type="example">
2663    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2664</artwork></figure>
2665<t>
2666   The Upgrade header field is intended to provide a simple mechanism
2667   for transition from HTTP/1.1 to some other, incompatible protocol. It
2668   does so by allowing the client to advertise its desire to use another
2669   protocol, such as a later version of HTTP with a higher major version
2670   number, even though the current request has been made using HTTP/1.1.
2671   This eases the difficult transition between incompatible protocols by
2672   allowing the client to initiate a request in the more commonly
2673   supported protocol while indicating to the server that it would like
2674   to use a "better" protocol if available (where "better" is determined
2675   by the server, possibly according to the nature of the method and/or
2676   resource being requested).
2677</t>
2678<t>
2679   The Upgrade header field only applies to switching application-layer
2680   protocols upon the existing transport-layer connection. Upgrade
2681   cannot be used to insist on a protocol change; its acceptance and use
2682   by the server is optional. The capabilities and nature of the
2683   application-layer communication after the protocol change is entirely
2684   dependent upon the new protocol chosen, although the first action
2685   after changing the protocol &MUST; be a response to the initial HTTP
2686   request containing the Upgrade header field.
2687</t>
2688<t>
2689   The Upgrade header field only applies to the immediate connection.
2690   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2691   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2692   HTTP/1.1 message.
2693</t>
2694<t>
2695   The Upgrade header field cannot be used to indicate a switch to a
2696   protocol on a different connection. For that purpose, it is more
2697   appropriate to use a 301, 302, 303, or 305 redirection response.
2698</t>
2699<t>
2700   This specification only defines the protocol name "HTTP" for use by
2701   the family of Hypertext Transfer Protocols, as defined by the HTTP
2702   version rules of <xref target="http.version"/> and future updates to this
2703   specification. Any token can be used as a protocol name; however, it
2704   will only be useful if both the client and server associate the name
2705   with the same protocol.
2706</t>
2707</section>
2708
2709<section title="Via" anchor="header.via">
2710  <iref primary="true" item="Via header" x:for-anchor=""/>
2711  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2712<t>
2713   The Via general-header field &MUST; be used by gateways and proxies to
2714   indicate the intermediate protocols and recipients between the user
2715   agent and the server on requests, and between the origin server and
2716   the client on responses. It is analogous to the "Received" field of
2717   <xref target="RFC2822"/> and is intended to be used for tracking message forwards,
2718   avoiding request loops, and identifying the protocol capabilities of
2719   all senders along the request/response chain.
2720</t>
2721<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"/>
2722  Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2723  received-protocol = [ protocol-name "/" ] protocol-version
2724  protocol-name     = token
2725  protocol-version  = token
2726  received-by       = ( uri-host [ ":" port ] ) | pseudonym
2727  pseudonym         = token
2728</artwork></figure>
2729<t>
2730   The received-protocol indicates the protocol version of the message
2731   received by the server or client along each segment of the
2732   request/response chain. The received-protocol version is appended to
2733   the Via field value when the message is forwarded so that information
2734   about the protocol capabilities of upstream applications remains
2735   visible to all recipients.
2736</t>
2737<t>
2738   The protocol-name is optional if and only if it would be "HTTP". The
2739   received-by field is normally the host and optional port number of a
2740   recipient server or client that subsequently forwarded the message.
2741   However, if the real host is considered to be sensitive information,
2742   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2743   be assumed to be the default port of the received-protocol.
2744</t>
2745<t>
2746   Multiple Via field values represents each proxy or gateway that has
2747   forwarded the message. Each recipient &MUST; append its information
2748   such that the end result is ordered according to the sequence of
2749   forwarding applications.
2750</t>
2751<t>
2752   Comments &MAY; be used in the Via header field to identify the software
2753   of the recipient proxy or gateway, analogous to the User-Agent and
2754   Server header fields. However, all comments in the Via field are
2755   optional and &MAY; be removed by any recipient prior to forwarding the
2756   message.
2757</t>
2758<t>
2759   For example, a request message could be sent from an HTTP/1.0 user
2760   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2761   forward the request to a public proxy at p.example.net, which completes
2762   the request by forwarding it to the origin server at www.example.com.
2763   The request received by www.example.com would then have the following
2764   Via header field:
2765</t>
2766<figure><artwork type="example">
2767    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2768</artwork></figure>
2769<t>
2770   Proxies and gateways used as a portal through a network firewall
2771   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2772   the firewall region. This information &SHOULD; only be propagated if
2773   explicitly enabled. If not enabled, the received-by host of any host
2774   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2775   for that host.
2776</t>
2777<t>
2778   For organizations that have strong privacy requirements for hiding
2779   internal structures, a proxy &MAY; combine an ordered subsequence of
2780   Via header field entries with identical received-protocol values into
2781   a single such entry. For example,
2782</t>
2783<figure><artwork type="example">
2784    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2785</artwork></figure>
2786<t>
2787        could be collapsed to
2788</t>
2789<figure><artwork type="example">
2790    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2791</artwork></figure>
2792<t>
2793   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2794   under the same organizational control and the hosts have already been
2795   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2796   have different received-protocol values.
2797</t>
2798</section>
2799
2800</section>
2801
2802<section title="IANA Considerations" anchor="IANA.considerations">
2803<t>
2804   <cref>TBD.</cref>
2805</t>
2806</section>
2807
2808<section title="Security Considerations" anchor="security.considerations">
2809<t>
2810   This section is meant to inform application developers, information
2811   providers, and users of the security limitations in HTTP/1.1 as
2812   described by this document. The discussion does not include
2813   definitive solutions to the problems revealed, though it does make
2814   some suggestions for reducing security risks.
2815</t>
2816
2817<section title="Personal Information" anchor="personal.information">
2818<t>
2819   HTTP clients are often privy to large amounts of personal information
2820   (e.g. the user's name, location, mail address, passwords, encryption
2821   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2822   leakage of this information.
2823   We very strongly recommend that a convenient interface be provided
2824   for the user to control dissemination of such information, and that
2825   designers and implementors be particularly careful in this area.
2826   History shows that errors in this area often create serious security
2827   and/or privacy problems and generate highly adverse publicity for the
2828   implementor's company.
2829</t>
2830</section>
2831
2832<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2833<t>
2834   A server is in the position to save personal data about a user's
2835   requests which might identify their reading patterns or subjects of
2836   interest. This information is clearly confidential in nature and its
2837   handling can be constrained by law in certain countries. People using
2838   HTTP to provide data are responsible for ensuring that
2839   such material is not distributed without the permission of any
2840   individuals that are identifiable by the published results.
2841</t>
2842</section>
2843
2844<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2845<t>
2846   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2847   the documents returned by HTTP requests to be only those that were
2848   intended by the server administrators. If an HTTP server translates
2849   HTTP URIs directly into file system calls, the server &MUST; take
2850   special care not to serve files that were not intended to be
2851   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2852   other operating systems use ".." as a path component to indicate a
2853   directory level above the current one. On such a system, an HTTP
2854   server &MUST; disallow any such construct in the Request-URI if it
2855   would otherwise allow access to a resource outside those intended to
2856   be accessible via the HTTP server. Similarly, files intended for
2857   reference only internally to the server (such as access control
2858   files, configuration files, and script code) &MUST; be protected from
2859   inappropriate retrieval, since they might contain sensitive
2860   information. Experience has shown that minor bugs in such HTTP server
2861   implementations have turned into security risks.
2862</t>
2863</section>
2864
2865<section title="DNS Spoofing" anchor="dns.spoofing">
2866<t>
2867   Clients using HTTP rely heavily on the Domain Name Service, and are
2868   thus generally prone to security attacks based on the deliberate
2869   mis-association of IP addresses and DNS names. Clients need to be
2870   cautious in assuming the continuing validity of an IP number/DNS name
2871   association.
2872</t>
2873<t>
2874   In particular, HTTP clients &SHOULD; rely on their name resolver for
2875   confirmation of an IP number/DNS name association, rather than
2876   caching the result of previous host name lookups. Many platforms
2877   already can cache host name lookups locally when appropriate, and
2878   they &SHOULD; be configured to do so. It is proper for these lookups to
2879   be cached, however, only when the TTL (Time To Live) information
2880   reported by the name server makes it likely that the cached
2881   information will remain useful.
2882</t>
2883<t>
2884   If HTTP clients cache the results of host name lookups in order to
2885   achieve a performance improvement, they &MUST; observe the TTL
2886   information reported by DNS.
2887</t>
2888<t>
2889   If HTTP clients do not observe this rule, they could be spoofed when
2890   a previously-accessed server's IP address changes. As network
2891   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2892   possibility of this form of attack will grow. Observing this
2893   requirement thus reduces this potential security vulnerability.
2894</t>
2895<t>
2896   This requirement also improves the load-balancing behavior of clients
2897   for replicated servers using the same DNS name and reduces the
2898   likelihood of a user's experiencing failure in accessing sites which
2899   use that strategy.
2900</t>
2901</section>
2902
2903<section title="Proxies and Caching" anchor="attack.proxies">
2904<t>
2905   By their very nature, HTTP proxies are men-in-the-middle, and
2906   represent an opportunity for man-in-the-middle attacks. Compromise of
2907   the systems on which the proxies run can result in serious security
2908   and privacy problems. Proxies have access to security-related
2909   information, personal information about individual users and
2910   organizations, and proprietary information belonging to users and
2911   content providers. A compromised proxy, or a proxy implemented or
2912   configured without regard to security and privacy considerations,
2913   might be used in the commission of a wide range of potential attacks.
2914</t>
2915<t>
2916   Proxy operators should protect the systems on which proxies run as
2917   they would protect any system that contains or transports sensitive
2918   information. In particular, log information gathered at proxies often
2919   contains highly sensitive personal information, and/or information
2920   about organizations. Log information should be carefully guarded, and
2921   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2922</t>
2923<t>
2924   Proxy implementors should consider the privacy and security
2925   implications of their design and coding decisions, and of the
2926   configuration options they provide to proxy operators (especially the
2927   default configuration).
2928</t>
2929<t>
2930   Users of a proxy need to be aware that they are no trustworthier than
2931   the people who run the proxy; HTTP itself cannot solve this problem.
2932</t>
2933<t>
2934   The judicious use of cryptography, when appropriate, may suffice to
2935   protect against a broad range of security and privacy attacks. Such
2936   cryptography is beyond the scope of the HTTP/1.1 specification.
2937</t>
2938</section>
2939
2940<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2941<t>
2942   They exist. They are hard to defend against. Research continues.
2943   Beware.
2944</t>
2945</section>
2946</section>
2947
2948<section title="Acknowledgments" anchor="ack">
2949<t>
2950   This specification makes heavy use of the augmented BNF and generic
2951   constructs defined by David H. Crocker for <xref target="RFC822ABNF"/>. Similarly, it
2952   reuses many of the definitions provided by Nathaniel Borenstein and
2953   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2954   specification will help reduce past confusion over the relationship
2955   between HTTP and Internet mail message formats.
2956</t>
2957<t>
2958   HTTP has evolved considerably over the years. It has
2959   benefited from a large and active developer community--the many
2960   people who have participated on the www-talk mailing list--and it is
2961   that community which has been most responsible for the success of
2962   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2963   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2964   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2965   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2966   VanHeyningen deserve special recognition for their efforts in
2967   defining early aspects of the protocol.
2968</t>
2969<t>
2970   This document has benefited greatly from the comments of all those
2971   participating in the HTTP-WG. In addition to those already mentioned,
2972   the following individuals have contributed to this specification:
2973</t>
2974<t>
2975   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2976   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2977   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2978   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2979   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2980   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2981   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2982   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2983   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2984   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2985   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2986   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2987   Josh Cohen.
2988</t>
2989<t>
2990   Thanks to the "cave men" of Palo Alto. You know who you are.
2991</t>
2992<t>
2993   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2994   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2995   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2996   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2997   Larry Masinter for their help. And thanks go particularly to Jeff
2998   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2999</t>
3000<t>
3001   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3002   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3003   discovery of many of the problems that this document attempts to
3004   rectify.
3005</t>
3006</section>
3007
3008</middle>
3009<back>
3010
3011<references title="Normative References">
3012
3013<reference anchor="ISO-8859-1">
3014  <front>
3015    <title>
3016     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3017    </title>
3018    <author>
3019      <organization>International Organization for Standardization</organization>
3020    </author>
3021    <date year="1998"/>
3022  </front>
3023  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3024</reference>
3025
3026<reference anchor="Part2">
3027  <front>
3028    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3029    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3030      <organization abbrev="Day Software">Day Software</organization>
3031      <address><email>fielding@gbiv.com</email></address>
3032    </author>
3033    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3034      <organization>One Laptop per Child</organization>
3035      <address><email>jg@laptop.org</email></address>
3036    </author>
3037    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3038      <organization abbrev="HP">Hewlett-Packard Company</organization>
3039      <address><email>JeffMogul@acm.org</email></address>
3040    </author>
3041    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3042      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3043      <address><email>henrikn@microsoft.com</email></address>
3044    </author>
3045    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3046      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3047      <address><email>LMM@acm.org</email></address>
3048    </author>
3049    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3050      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3051      <address><email>paulle@microsoft.com</email></address>
3052    </author>
3053    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3054      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3055      <address><email>timbl@w3.org</email></address>
3056    </author>
3057    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3058      <organization abbrev="W3C">World Wide Web Consortium</organization>
3059      <address><email>ylafon@w3.org</email></address>
3060    </author>
3061    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3062      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3063      <address><email>julian.reschke@greenbytes.de</email></address>
3064    </author>
3065    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3066  </front>
3067  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3068  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3069</reference>
3070
3071<reference anchor="Part3">
3072  <front>
3073    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3074    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3075      <organization abbrev="Day Software">Day Software</organization>
3076      <address><email>fielding@gbiv.com</email></address>
3077    </author>
3078    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3079      <organization>One Laptop per Child</organization>
3080      <address><email>jg@laptop.org</email></address>
3081    </author>
3082    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3083      <organization abbrev="HP">Hewlett-Packard Company</organization>
3084      <address><email>JeffMogul@acm.org</email></address>
3085    </author>
3086    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3087      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3088      <address><email>henrikn@microsoft.com</email></address>
3089    </author>
3090    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3091      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3092      <address><email>LMM@acm.org</email></address>
3093    </author>
3094    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3095      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3096      <address><email>paulle@microsoft.com</email></address>
3097    </author>
3098    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3099      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3100      <address><email>timbl@w3.org</email></address>
3101    </author>
3102    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3103      <organization abbrev="W3C">World Wide Web Consortium</organization>
3104      <address><email>ylafon@w3.org</email></address>
3105    </author>
3106    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3107      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3108      <address><email>julian.reschke@greenbytes.de</email></address>
3109    </author>
3110    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3111  </front>
3112  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3113  <x:source href="p3-payload.xml" basename="p3-payload"/>
3114</reference>
3115
3116<reference anchor="Part5">
3117  <front>
3118    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3119    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3120      <organization abbrev="Day Software">Day Software</organization>
3121      <address><email>fielding@gbiv.com</email></address>
3122    </author>
3123    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3124      <organization>One Laptop per Child</organization>
3125      <address><email>jg@laptop.org</email></address>
3126    </author>
3127    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3128      <organization abbrev="HP">Hewlett-Packard Company</organization>
3129      <address><email>JeffMogul@acm.org</email></address>
3130    </author>
3131    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3132      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3133      <address><email>henrikn@microsoft.com</email></address>
3134    </author>
3135    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3136      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3137      <address><email>LMM@acm.org</email></address>
3138    </author>
3139    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3140      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3141      <address><email>paulle@microsoft.com</email></address>
3142    </author>
3143    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3144      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3145      <address><email>timbl@w3.org</email></address>
3146    </author>
3147    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3148      <organization abbrev="W3C">World Wide Web Consortium</organization>
3149      <address><email>ylafon@w3.org</email></address>
3150    </author>
3151    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3152      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3153      <address><email>julian.reschke@greenbytes.de</email></address>
3154    </author>
3155    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3156  </front>
3157  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3158  <x:source href="p5-range.xml" basename="p5-range"/>
3159</reference>
3160
3161<reference anchor="Part6">
3162  <front>
3163    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3164    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3165      <organization abbrev="Day Software">Day Software</organization>
3166      <address><email>fielding@gbiv.com</email></address>
3167    </author>
3168    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3169      <organization>One Laptop per Child</organization>
3170      <address><email>jg@laptop.org</email></address>
3171    </author>
3172    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3173      <organization abbrev="HP">Hewlett-Packard Company</organization>
3174      <address><email>JeffMogul@acm.org</email></address>
3175    </author>
3176    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3177      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3178      <address><email>henrikn@microsoft.com</email></address>
3179    </author>
3180    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3181      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3182      <address><email>LMM@acm.org</email></address>
3183    </author>
3184    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3185      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3186      <address><email>paulle@microsoft.com</email></address>
3187    </author>
3188    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3189      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3190      <address><email>timbl@w3.org</email></address>
3191    </author>
3192    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3193      <organization abbrev="W3C">World Wide Web Consortium</organization>
3194      <address><email>ylafon@w3.org</email></address>
3195    </author>
3196    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3197      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3198      <address><email>julian.reschke@greenbytes.de</email></address>
3199    </author>
3200    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3201  </front>
3202  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3203  <x:source href="p6-cache.xml" basename="p6-cache"/>
3204</reference>
3205
3206<reference anchor="RFC822ABNF">
3207  <front>
3208    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3209    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3210      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3211      <address><email>DCrocker@UDel-Relay</email></address>
3212    </author>
3213    <date month="August" day="13" year="1982"/>
3214  </front>
3215  <seriesInfo name="STD" value="11"/>
3216  <seriesInfo name="RFC" value="822"/>
3217</reference>
3218
3219<reference anchor="RFC2045">
3220  <front>
3221    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3222    <author initials="N." surname="Freed" fullname="Ned Freed">
3223      <organization>Innosoft International, Inc.</organization>
3224      <address><email>ned@innosoft.com</email></address>
3225    </author>
3226    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3227      <organization>First Virtual Holdings</organization>
3228      <address><email>nsb@nsb.fv.com</email></address>
3229    </author>
3230    <date month="November" year="1996"/>
3231  </front>
3232  <seriesInfo name="RFC" value="2045"/>
3233</reference>
3234
3235<reference anchor="RFC2047">
3236  <front>
3237    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3238    <author initials="K." surname="Moore" fullname="Keith Moore">
3239      <organization>University of Tennessee</organization>
3240      <address><email>moore@cs.utk.edu</email></address>
3241    </author>
3242    <date month="November" year="1996"/>
3243  </front>
3244  <seriesInfo name="RFC" value="2047"/>
3245</reference>
3246
3247<reference anchor="RFC2119">
3248  <front>
3249    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3250    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3251      <organization>Harvard University</organization>
3252      <address><email>sob@harvard.edu</email></address>
3253    </author>
3254    <date month="March" year="1997"/>
3255  </front>
3256  <seriesInfo name="BCP" value="14"/>
3257  <seriesInfo name="RFC" value="2119"/>
3258</reference>
3259
3260<reference anchor="RFC2396">
3261  <front>
3262    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3263    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3264      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3265      <address><email>timbl@w3.org</email></address>
3266    </author>
3267    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3268      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3269      <address><email>fielding@ics.uci.edu</email></address>
3270    </author>
3271    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3272      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3273      <address><email>masinter@parc.xerox.com</email></address>
3274    </author>
3275    <date month="August" year="1998"/>
3276  </front>
3277  <seriesInfo name="RFC" value="2396"/>
3278</reference>
3279
3280<reference anchor="USASCII">
3281  <front>
3282    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3283    <author>
3284      <organization>American National Standards Institute</organization>
3285    </author>
3286    <date year="1986"/>
3287  </front>
3288  <seriesInfo name="ANSI" value="X3.4"/>
3289</reference>
3290
3291</references>
3292
3293<references title="Informative References">
3294
3295<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3296  <front>
3297    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3298    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3299      <organization/>
3300    </author>
3301    <author initials="J." surname="Gettys" fullname="J. Gettys">
3302      <organization/>
3303    </author>
3304    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3305      <organization/>
3306    </author>
3307    <author initials="H." surname="Lie" fullname="H. Lie">
3308      <organization/>
3309    </author>
3310    <author initials="C." surname="Lilley" fullname="C. Lilley">
3311      <organization/>
3312    </author>
3313    <date year="1997" month="September"/>
3314  </front>
3315  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3316</reference>
3317
3318<reference anchor="Pad1995">
3319  <front>
3320    <title>Improving HTTP Latency</title>
3321    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3322      <organization/>
3323    </author>
3324    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3325      <organization/>
3326    </author>
3327    <date year="1995" month="December"/>
3328  </front>
3329  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3330  <annotation>
3331    Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3332    which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3333  </annotation>
3334</reference>
3335
3336<reference anchor="RFC822">
3337  <front>
3338    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3339    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3340      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3341      <address><email>DCrocker@UDel-Relay</email></address>
3342    </author>
3343    <date month="August" day="13" year="1982"/>
3344  </front>
3345  <seriesInfo name="STD" value="11"/>
3346  <seriesInfo name="RFC" value="822"/>
3347</reference>
3348
3349<reference anchor="RFC959">
3350  <front>
3351    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3352    <author initials="J." surname="Postel" fullname="J. Postel">
3353      <organization>Information Sciences Institute (ISI)</organization>
3354    </author>
3355    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3356      <organization/>
3357    </author>
3358    <date month="October" year="1985"/>
3359  </front>
3360  <seriesInfo name="STD" value="9"/>
3361  <seriesInfo name="RFC" value="959"/>
3362</reference>
3363
3364<reference anchor="RFC1123">
3365  <front>
3366    <title>Requirements for Internet Hosts - Application and Support</title>
3367    <author initials="R." surname="Braden" fullname="Robert Braden">
3368      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3369      <address><email>Braden@ISI.EDU</email></address>
3370    </author>
3371    <date month="October" year="1989"/>
3372  </front>
3373  <seriesInfo name="STD" value="3"/>
3374  <seriesInfo name="RFC" value="1123"/>
3375</reference>
3376
3377<reference anchor="RFC1305">
3378  <front>
3379    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3380    <author initials="D." surname="Mills" fullname="David L. Mills">
3381      <organization>University of Delaware, Electrical Engineering Department</organization>
3382      <address><email>mills@udel.edu</email></address>
3383    </author>
3384    <date month="March" year="1992"/>
3385  </front>
3386  <seriesInfo name="RFC" value="1305"/>
3387</reference>
3388
3389<reference anchor="RFC1436">
3390  <front>
3391    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3392    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3393      <organization>University of Minnesota, Computer and Information Services</organization>
3394      <address><email>fxa@boombox.micro.umn.edu</email></address>
3395    </author>
3396    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3397      <organization>University of Minnesota, Computer and Information Services</organization>
3398      <address><email>mpm@boombox.micro.umn.edu</email></address>
3399    </author>
3400    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3401      <organization>University of Minnesota, Computer and Information Services</organization>
3402      <address><email>lindner@boombox.micro.umn.edu</email></address>
3403    </author>
3404    <author initials="D." surname="Johnson" fullname="David Johnson">
3405      <organization>University of Minnesota, Computer and Information Services</organization>
3406      <address><email>dmj@boombox.micro.umn.edu</email></address>
3407    </author>
3408    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3409      <organization>University of Minnesota, Computer and Information Services</organization>
3410      <address><email>daniel@boombox.micro.umn.edu</email></address>
3411    </author>
3412    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3413      <organization>University of Minnesota, Computer and Information Services</organization>
3414      <address><email>alberti@boombox.micro.umn.edu</email></address>
3415    </author>
3416    <date month="March" year="1993"/>
3417  </front>
3418  <seriesInfo name="RFC" value="1436"/>
3419</reference>
3420
3421<reference anchor="RFC1630">
3422  <front>
3423    <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>
3424    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3425      <organization>CERN, World-Wide Web project</organization>
3426      <address><email>timbl@info.cern.ch</email></address>
3427    </author>
3428    <date month="June" year="1994"/>
3429  </front>
3430  <seriesInfo name="RFC" value="1630"/>
3431</reference>
3432
3433<reference anchor="RFC1737">
3434  <front>
3435    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3436    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3437      <organization>Xerox Palo Alto Research Center</organization>
3438      <address><email>masinter@parc.xerox.com</email></address>
3439    </author>
3440    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3441      <organization>MIT Laboratory for Computer Science</organization>
3442      <address><email>sollins@lcs.mit.edu</email></address>
3443    </author>
3444    <date month="December" year="1994"/>
3445  </front>
3446  <seriesInfo name="RFC" value="1737"/>
3447</reference>
3448
3449<reference anchor="RFC1738">
3450  <front>
3451    <title>Uniform Resource Locators (URL)</title>
3452    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3453      <organization>CERN, World-Wide Web project</organization>
3454      <address><email>timbl@info.cern.ch</email></address>
3455    </author>
3456    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3457      <organization>Xerox PARC</organization>
3458      <address><email>masinter@parc.xerox.com</email></address>
3459    </author>
3460    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3461      <organization>University of Minnesota, Computer and Information Services</organization>
3462      <address><email>mpm@boombox.micro.umn.edu</email></address>
3463    </author>
3464    <date month="December" year="1994"/>
3465  </front>
3466  <seriesInfo name="RFC" value="1738"/>
3467</reference>
3468
3469<reference anchor="RFC1808">
3470  <front>
3471    <title>Relative Uniform Resource Locators</title>
3472    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3473      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3474      <address><email>fielding@ics.uci.edu</email></address>
3475    </author>
3476    <date month="June" year="1995"/>
3477  </front>
3478  <seriesInfo name="RFC" value="1808"/>
3479</reference>
3480
3481<reference anchor="RFC1900">
3482  <front>
3483    <title>Renumbering Needs Work</title>
3484    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3485      <organization>CERN, Computing and Networks Division</organization>
3486      <address><email>brian@dxcoms.cern.ch</email></address>
3487    </author>
3488    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3489      <organization>cisco Systems</organization>
3490      <address><email>yakov@cisco.com</email></address>
3491    </author>
3492    <date month="February" year="1996"/>
3493  </front>
3494  <seriesInfo name="RFC" value="1900"/>
3495</reference>
3496
3497<reference anchor="RFC1945">
3498  <front>
3499    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3500    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3501      <organization>MIT, Laboratory for Computer Science</organization>
3502      <address><email>timbl@w3.org</email></address>
3503    </author>
3504    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3505      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3506      <address><email>fielding@ics.uci.edu</email></address>
3507    </author>
3508    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3509      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3510      <address><email>frystyk@w3.org</email></address>
3511    </author>
3512    <date month="May" year="1996"/>
3513  </front>
3514  <seriesInfo name="RFC" value="1945"/>
3515</reference>
3516
3517<reference anchor="RFC2068">
3518  <front>
3519    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3520    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3521      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3522      <address><email>fielding@ics.uci.edu</email></address>
3523    </author>
3524    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3525      <organization>MIT Laboratory for Computer Science</organization>
3526      <address><email>jg@w3.org</email></address>
3527    </author>
3528    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3529      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3530      <address><email>mogul@wrl.dec.com</email></address>
3531    </author>
3532    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3533      <organization>MIT Laboratory for Computer Science</organization>
3534      <address><email>frystyk@w3.org</email></address>
3535    </author>
3536    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3537      <organization>MIT Laboratory for Computer Science</organization>
3538      <address><email>timbl@w3.org</email></address>
3539    </author>
3540    <date month="January" year="1997"/>
3541  </front>
3542  <seriesInfo name="RFC" value="2068"/>
3543</reference>
3544
3545<reference anchor="RFC2145">
3546  <front>
3547    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3548    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3549      <organization>Western Research Laboratory</organization>
3550      <address><email>mogul@wrl.dec.com</email></address>
3551    </author>
3552    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3553      <organization>Department of Information and Computer Science</organization>
3554      <address><email>fielding@ics.uci.edu</email></address>
3555    </author>
3556    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3557      <organization>MIT Laboratory for Computer Science</organization>
3558      <address><email>jg@w3.org</email></address>
3559    </author>
3560    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3561      <organization>W3 Consortium</organization>
3562      <address><email>frystyk@w3.org</email></address>
3563    </author>
3564    <date month="May" year="1997"/>
3565  </front>
3566  <seriesInfo name="RFC" value="2145"/>
3567</reference>
3568
3569<reference anchor="RFC2324">
3570  <front>
3571    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3572    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3573      <organization>Xerox Palo Alto Research Center</organization>
3574      <address><email>masinter@parc.xerox.com</email></address>
3575    </author>
3576    <date month="April" day="1" year="1998"/>
3577  </front>
3578  <seriesInfo name="RFC" value="2324"/>
3579</reference>
3580
3581<reference anchor="RFC2616">
3582  <front>
3583    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3584    <author initials="R." surname="Fielding" fullname="R. Fielding">
3585      <organization>University of California, Irvine</organization>
3586      <address><email>fielding@ics.uci.edu</email></address>
3587    </author>
3588    <author initials="J." surname="Gettys" fullname="J. Gettys">
3589      <organization>W3C</organization>
3590      <address><email>jg@w3.org</email></address>
3591    </author>
3592    <author initials="J." surname="Mogul" fullname="J. Mogul">
3593      <organization>Compaq Computer Corporation</organization>
3594      <address><email>mogul@wrl.dec.com</email></address>
3595    </author>
3596    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3597      <organization>MIT Laboratory for Computer Science</organization>
3598      <address><email>frystyk@w3.org</email></address>
3599    </author>
3600    <author initials="L." surname="Masinter" fullname="L. Masinter">
3601      <organization>Xerox Corporation</organization>
3602      <address><email>masinter@parc.xerox.com</email></address>
3603    </author>
3604    <author initials="P." surname="Leach" fullname="P. Leach">
3605      <organization>Microsoft Corporation</organization>
3606      <address><email>paulle@microsoft.com</email></address>
3607    </author>
3608    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3609      <organization>W3C</organization>
3610      <address><email>timbl@w3.org</email></address>
3611    </author>
3612    <date month="June" year="1999"/>
3613  </front>
3614  <seriesInfo name="RFC" value="2616"/>
3615</reference>
3616
3617<reference anchor="RFC2821">
3618  <front>
3619    <title>Simple Mail Transfer Protocol</title>
3620    <author initials="J." surname="Klensin" fullname="J. Klensin">
3621      <organization>AT&amp;T Laboratories</organization>
3622      <address><email>klensin@research.att.com</email></address>
3623    </author>
3624    <date year="2001" month="April"/>
3625  </front>
3626  <seriesInfo name="RFC" value="2821"/>
3627</reference>
3628
3629<reference anchor="RFC2822">
3630  <front>
3631    <title>Internet Message Format</title>
3632    <author initials="P." surname="Resnick" fullname="P. Resnick">
3633      <organization>QUALCOMM Incorporated</organization>
3634    </author>
3635    <date year="2001" month="April"/>
3636  </front> 
3637  <seriesInfo name="RFC" value="2822"/>
3638</reference>
3639
3640<reference anchor='RFC3977'>
3641  <front>
3642    <title>Network News Transfer Protocol (NNTP)</title>
3643    <author initials='C.' surname='Feather' fullname='C. Feather'>
3644      <organization>THUS plc</organization>
3645      <address><email>clive@demon.net</email></address>
3646    </author>
3647    <date year='2006' month='October' />
3648  </front>
3649  <seriesInfo name="RFC" value="3977"/>
3650</reference>
3651
3652<reference anchor="RFC4288">
3653  <front>
3654    <title>Media Type Specifications and Registration Procedures</title>
3655    <author initials="N." surname="Freed" fullname="N. Freed">
3656      <organization>Sun Microsystems</organization>
3657      <address>
3658        <email>ned.freed@mrochek.com</email>
3659      </address>
3660    </author>
3661    <author initials="J." surname="Klensin" fullname="J. Klensin">
3662      <organization/>
3663      <address>
3664        <email>klensin+ietf@jck.com</email>
3665      </address>
3666    </author>
3667    <date year="2005" month="December"/>
3668  </front>
3669  <seriesInfo name="BCP" value="13"/>
3670  <seriesInfo name="RFC" value="4288"/>
3671</reference>
3672
3673<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3674  <front>
3675  <title>Analysis of HTTP Performance Problems</title>
3676  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3677    <organization/>
3678  </author>
3679  <date/>
3680  </front>
3681</reference>
3682
3683<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3684  <front>
3685  <title>Analysis of HTTP Performance</title>
3686  <author initials="J." surname="Touch" fullname="Joe Touch">
3687    <organization>USC/Information Sciences Institute</organization>
3688    <address><email>touch@isi.edu</email></address>
3689  </author>
3690  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3691    <organization>USC/Information Sciences Institute</organization>
3692    <address><email>johnh@isi.edu</email></address>
3693  </author>
3694  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3695    <organization>USC/Information Sciences Institute</organization>
3696    <address><email>katia@isi.edu</email></address>
3697  </author>
3698  <date year="1998" month="Aug"/>
3699  </front>
3700  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3701  <annotation>(original report dated Aug. 1996)</annotation>
3702</reference>
3703
3704<reference anchor="WAIS">
3705  <front>
3706    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3707    <author initials="F." surname="Davis" fullname="F. Davis">
3708      <organization>Thinking Machines Corporation</organization>
3709    </author>
3710    <author initials="B." surname="Kahle" fullname="B. Kahle">
3711      <organization>Thinking Machines Corporation</organization>
3712    </author>
3713    <author initials="H." surname="Morris" fullname="H. Morris">
3714      <organization>Thinking Machines Corporation</organization>
3715    </author>
3716    <author initials="J." surname="Salem" fullname="J. Salem">
3717      <organization>Thinking Machines Corporation</organization>
3718    </author>
3719    <author initials="T." surname="Shen" fullname="T. Shen">
3720      <organization>Thinking Machines Corporation</organization>
3721    </author>
3722    <author initials="R." surname="Wang" fullname="R. Wang">
3723      <organization>Thinking Machines Corporation</organization>
3724    </author>
3725    <author initials="J." surname="Sui" fullname="J. Sui">
3726      <organization>Thinking Machines Corporation</organization>
3727    </author>
3728    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3729      <organization>Thinking Machines Corporation</organization>
3730    </author>
3731    <date month="April" year="1990"/>
3732  </front>
3733  <seriesInfo name="Thinking Machines Corporation" value=""/>
3734</reference>
3735
3736</references>
3737
3738
3739<section title="Internet Media Types" anchor="internet.media.type.http">
3740<t>
3741   In addition to defining HTTP/1.1, this document serves
3742   as the specification for the Internet media type "message/http" and
3743   "application/http". The following is to be registered with IANA <xref target="RFC4288"/>.
3744</t>
3745<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3746<iref item="Media Type" subitem="message/http" primary="true"/>
3747<iref item="message/http Media Type" primary="true"/>
3748<t>
3749   The message/http type can be used to enclose a single HTTP request or
3750   response message, provided that it obeys the MIME restrictions for all
3751   "message" types regarding line length and encodings.
3752</t>
3753<t>
3754  <list style="hanging" x:indent="12em">
3755    <t hangText="Type name:">
3756      message
3757    </t>
3758    <t hangText="Subtype name:">
3759      http
3760    </t>
3761    <t hangText="Required parameters:">
3762      none
3763    </t>
3764    <t hangText="Optional parameters:">
3765      version, msgtype
3766      <list style="hanging">
3767        <t hangText="version:">
3768          The HTTP-Version number of the enclosed message
3769          (e.g., "1.1"). If not present, the version can be
3770          determined from the first line of the body.
3771        </t>
3772        <t hangText="msgtype:">
3773          The message type -- "request" or "response". If not
3774          present, the type can be determined from the first
3775          line of the body.
3776        </t>
3777      </list>
3778    </t>
3779    <t hangText="Encoding considerations:">
3780      only "7bit", "8bit", or "binary" are permitted
3781    </t>
3782    <t hangText="Security considerations:">
3783      none
3784    </t>
3785    <t hangText="Interoperability considerations:">
3786      none
3787    </t>
3788    <t hangText="Published specification:">
3789      This specification (see <xref target="internet.media.type.message.http"/>).
3790    </t>
3791    <t hangText="Applications that use this media type:">
3792    </t>
3793    <t hangText="Additional information:">
3794      <list style="hanging">
3795        <t hangText="Magic number(s):">none</t>
3796        <t hangText="File extension(s):">none</t>
3797        <t hangText="Macintosh file type code(s):">none</t>
3798      </list>
3799    </t>
3800    <t hangText="Person and email address to contact for further information:">
3801      See Authors Section.
3802    </t>
3803                <t hangText="Intended usage:">
3804                  COMMON
3805    </t>
3806                <t hangText="Restrictions on usage:">
3807                  none
3808    </t>
3809    <t hangText="Author/Change controller:">
3810      IESG
3811    </t>
3812  </list>
3813</t>
3814</section>
3815<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3816<iref item="Media Type" subitem="application/http" primary="true"/>
3817<iref item="application/http Media Type" primary="true"/>
3818<t>
3819   The application/http type can be used to enclose a pipeline of one or more
3820   HTTP request or response messages (not intermixed).
3821</t>
3822<t>
3823  <list style="hanging" x:indent="12em">
3824    <t hangText="Type name:">
3825      application
3826    </t>
3827    <t hangText="Subtype name:">
3828      http
3829    </t>
3830    <t hangText="Required parameters:">
3831      none
3832    </t>
3833    <t hangText="Optional parameters:">
3834      version, msgtype
3835      <list style="hanging">
3836        <t hangText="version:">
3837          The HTTP-Version number of the enclosed messages
3838          (e.g., "1.1"). If not present, the version can be
3839          determined from the first line of the body.
3840        </t>
3841        <t hangText="msgtype:">
3842          The message type -- "request" or "response". If not
3843          present, the type can be determined from the first
3844          line of the body.
3845        </t>
3846      </list>
3847    </t>
3848    <t hangText="Encoding considerations:">
3849      HTTP messages enclosed by this type
3850      are in "binary" format; use of an appropriate
3851      Content-Transfer-Encoding is required when
3852      transmitted via E-mail.
3853    </t>
3854    <t hangText="Security considerations:">
3855      none
3856    </t>
3857    <t hangText="Interoperability considerations:">
3858      none
3859    </t>
3860    <t hangText="Published specification:">
3861      This specification (see <xref target="internet.media.type.application.http"/>).
3862    </t>
3863    <t hangText="Applications that use this media type:">
3864    </t>
3865    <t hangText="Additional information:">
3866      <list style="hanging">
3867        <t hangText="Magic number(s):">none</t>
3868        <t hangText="File extension(s):">none</t>
3869        <t hangText="Macintosh file type code(s):">none</t>
3870      </list>
3871    </t>
3872    <t hangText="Person and email address to contact for further information:">
3873      See Authors Section.
3874    </t>
3875                <t hangText="Intended usage:">
3876                  COMMON
3877    </t>
3878                <t hangText="Restrictions on usage:">
3879                  none
3880    </t>
3881    <t hangText="Author/Change controller:">
3882      IESG
3883    </t>
3884  </list>
3885</t>
3886</section>
3887</section>
3888
3889<section title="Tolerant Applications" anchor="tolerant.applications">
3890<t>
3891   Although this document specifies the requirements for the generation
3892   of HTTP/1.1 messages, not all applications will be correct in their
3893   implementation. We therefore recommend that operational applications
3894   be tolerant of deviations whenever those deviations can be
3895   interpreted unambiguously.
3896</t>
3897<t>
3898   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3899   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3900   accept any amount of SP or HTAB characters between fields, even though
3901   only a single SP is required.
3902</t>
3903<t>
3904   The line terminator for message-header fields is the sequence CRLF.
3905   However, we recommend that applications, when parsing such headers,
3906   recognize a single LF as a line terminator and ignore the leading CR.
3907</t>
3908<t>
3909   The character set of an entity-body &SHOULD; be labeled as the lowest
3910   common denominator of the character codes used within that body, with
3911   the exception that not labeling the entity is preferred over labeling
3912   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3913</t>
3914<t>
3915   Additional rules for requirements on parsing and encoding of dates
3916   and other potential problems with date encodings include:
3917</t>
3918<t>
3919  <list style="symbols">
3920     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3921        which appears to be more than 50 years in the future is in fact
3922        in the past (this helps solve the "year 2000" problem).</t>
3923
3924     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3925        Expires date as earlier than the proper value, but &MUST-NOT;
3926        internally represent a parsed Expires date as later than the
3927        proper value.</t>
3928
3929     <t>All expiration-related calculations &MUST; be done in GMT. The
3930        local time zone &MUST-NOT; influence the calculation or comparison
3931        of an age or expiration time.</t>
3932
3933     <t>If an HTTP header incorrectly carries a date value with a time
3934        zone other than GMT, it &MUST; be converted into GMT using the
3935        most conservative possible conversion.</t>
3936  </list>
3937</t>
3938</section>
3939
3940<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3941<t>
3942   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3943   simplify the process of date comparison. Proxies and gateways from
3944   other protocols &SHOULD; ensure that any Date header field present in a
3945   message conforms to one of the HTTP/1.1 formats and rewrite the date
3946   if necessary.
3947</t>
3948</section>
3949
3950<section title="Compatibility with Previous Versions" anchor="compatibility">
3951<t>
3952   It is beyond the scope of a protocol specification to mandate
3953   compliance with previous versions. HTTP/1.1 was deliberately
3954   designed, however, to make supporting previous versions easy. It is
3955   worth noting that, at the time of composing this specification
3956   (1996), we would expect commercial HTTP/1.1 servers to:
3957  <list style="symbols">
3958     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3959        1.1 requests;</t>
3960
3961     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3962        1.1;</t>
3963
3964     <t>respond appropriately with a message in the same major version
3965        used by the client.</t>
3966  </list>
3967</t>
3968<t>
3969   And we would expect HTTP/1.1 clients to:
3970  <list style="symbols">
3971     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3972        responses;</t>
3973
3974     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3975        1.1.</t>
3976  </list>
3977</t>
3978<t>
3979   For most implementations of HTTP/1.0, each connection is established
3980   by the client prior to the request and closed by the server after
3981   sending the response. Some implementations implement the Keep-Alive
3982   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3983</t>
3984
3985<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3986<t>
3987   This section summarizes major differences between versions HTTP/1.0
3988   and HTTP/1.1.
3989</t>
3990
3991<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">
3992<t>
3993   The requirements that clients and servers support the Host request-header,
3994   report an error if the Host request-header (<xref target="header.host"/>) is
3995   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3996   are among the most important changes defined by this
3997   specification.
3998</t>
3999<t>
4000   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4001   addresses and servers; there was no other established mechanism for
4002   distinguishing the intended server of a request than the IP address
4003   to which that request was directed. The changes outlined above will
4004   allow the Internet, once older HTTP clients are no longer common, to
4005   support multiple Web sites from a single IP address, greatly
4006   simplifying large operational Web servers, where allocation of many
4007   IP addresses to a single host has created serious problems. The
4008   Internet will also be able to recover the IP addresses that have been
4009   allocated for the sole purpose of allowing special-purpose domain
4010   names to be used in root-level HTTP URLs. Given the rate of growth of
4011   the Web, and the number of servers already deployed, it is extremely
4012   important that all implementations of HTTP (including updates to
4013   existing HTTP/1.0 applications) correctly implement these
4014   requirements:
4015  <list style="symbols">
4016     <t>Both clients and servers &MUST; support the Host request-header.</t>
4017
4018     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4019
4020     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4021        request does not include a Host request-header.</t>
4022
4023     <t>Servers &MUST; accept absolute URIs.</t>
4024  </list>
4025</t>
4026</section>
4027</section>
4028
4029<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4030<t>
4031   Some clients and servers might wish to be compatible with some
4032   previous implementations of persistent connections in HTTP/1.0
4033   clients and servers. Persistent connections in HTTP/1.0 are
4034   explicitly negotiated as they are not the default behavior. HTTP/1.0
4035   experimental implementations of persistent connections are faulty,
4036   and the new facilities in HTTP/1.1 are designed to rectify these
4037   problems. The problem was that some existing 1.0 clients may be
4038   sending Keep-Alive to a proxy server that doesn't understand
4039   Connection, which would then erroneously forward it to the next
4040   inbound server, which would establish the Keep-Alive connection and
4041   result in a hung HTTP/1.0 proxy waiting for the close on the
4042   response. The result is that HTTP/1.0 clients must be prevented from
4043   using Keep-Alive when talking to proxies.
4044</t>
4045<t>
4046   However, talking to proxies is the most important use of persistent
4047   connections, so that prohibition is clearly unacceptable. Therefore,
4048   we need some other mechanism for indicating a persistent connection
4049   is desired, which is safe to use even when talking to an old proxy
4050   that ignores Connection. Persistent connections are the default for
4051   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4052   declaring non-persistence. See <xref target="header.connection"/>.
4053</t>
4054<t>
4055   The original HTTP/1.0 form of persistent connections (the Connection:
4056   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4057</t>
4058</section>
4059
4060<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4061<t>
4062   This specification has been carefully audited to correct and
4063   disambiguate key word usage; RFC 2068 had many problems in respect to
4064   the conventions laid out in <xref target="RFC2119"/>.
4065</t>
4066<t>
4067   Transfer-coding and message lengths all interact in ways that
4068   required fixing exactly when chunked encoding is used (to allow for
4069   transfer encoding that may not be self delimiting); it was important
4070   to straighten out exactly how message lengths are computed. (Sections
4071   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4072   <xref target="header.content-length" format="counter"/>,
4073   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4074</t>
4075<t>
4076   The use and interpretation of HTTP version numbers has been clarified
4077   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4078   version they support to deal with problems discovered in HTTP/1.0
4079   implementations (<xref target="http.version"/>)
4080</t>
4081<t>
4082   Transfer-coding had significant problems, particularly with
4083   interactions with chunked encoding. The solution is that transfer-codings
4084   become as full fledged as content-codings. This involves
4085   adding an IANA registry for transfer-codings (separate from content
4086   codings), a new header field (TE) and enabling trailer headers in the
4087   future. Transfer encoding is a major performance benefit, so it was
4088   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4089   interoperability problem that could have occurred due to interactions
4090   between authentication trailers, chunked encoding and HTTP/1.0
4091   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4092   and <xref target="header.te" format="counter"/>)
4093</t>
4094</section>
4095
4096<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4097<t>
4098  The CHAR rule does not allow the NUL character anymore (this affects
4099  the comment and quoted-string rules).
4100  (<xref target="basic.rules"/>)
4101</t>
4102<t>
4103  Clarify that HTTP-Version is case sensitive.
4104  (<xref target="http.version"/>)
4105</t>
4106<t>
4107  Remove reference to non-existant identity transfer-coding value tokens.
4108  (Sections <xref format="counter" target="transfer.codings"/> and
4109  <xref format="counter" target="message.length"/>)
4110</t>
4111<t>
4112  Clarification that the chunk length does not include
4113  the count of the octets in the chunk header and trailer.
4114  (<xref target="chunked.transfer.encoding"/>)
4115</t>
4116<t>
4117  Fix BNF to add query, as the abs_path production in
4118  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4119  (<xref target="request-uri"/>)
4120</t>
4121<t>
4122  Clarify exactly when close connection options must be sent.
4123  (<xref target="header.connection"/>)
4124</t>
4125</section>
4126</section>
4127
4128<section title="Change Log (to be removed by RFC Editor before publication)">
4129
4130<section title="Since RFC2616">
4131<t>
4132  Extracted relevant partitions from <xref target="RFC2616"/>.
4133</t>
4134</section>
4135
4136<section title="Since draft-ietf-httpbis-p1-messaging-00">
4137<t>
4138  Closed issues:
4139  <list style="symbols"> 
4140    <t>
4141      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4142      "HTTP Version should be case sensitive"
4143      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4144    </t>
4145    <t>
4146      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4147      "'unsafe' characters"
4148      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4149    </t>
4150    <t>
4151      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4152      "Chunk Size Definition"
4153      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4154    </t>
4155    <t>
4156      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4157      "Message Length"
4158      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4159    </t>
4160    <t>
4161      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4162      "Media Type Registrations"
4163      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4164    </t>
4165    <t>
4166      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4167      "URI includes query"
4168      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4169    </t>
4170    <t>
4171      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4172      "No close on 1xx responses"
4173      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4174    </t>
4175    <t>
4176      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4177      "Remove 'identity' token references"
4178      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4179    </t>
4180    <t>
4181      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4182      "Import query BNF"
4183    </t>
4184    <t>
4185      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4186      "qdtext BNF"
4187    </t>
4188    <t>
4189      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4190      "Normative and Informative references"
4191    </t>
4192    <t>
4193      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4194      "RFC2606 Compliance"
4195    </t>
4196    <t>
4197      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4198      "RFC977 reference"
4199    </t>
4200    <t>
4201      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4202      "RFC1700 references"
4203    </t>
4204    <t>
4205      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4206      "inconsistency in date format explanation"
4207    </t>
4208    <t>
4209      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4210      "Date reference typo"
4211    </t>
4212    <t>
4213      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4214      "Informative references"
4215    </t>
4216    <t>
4217      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4218      "ISO-8859-1 Reference"
4219    </t>
4220    <t>
4221      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4222      "Normative up-to-date references"
4223    </t>
4224  </list>
4225</t>
4226<t>
4227  Other changes:
4228  <list style="symbols"> 
4229    <t>
4230      Update media type registrations to use RFC4288 template.
4231    </t>
4232    <t>
4233      Use names of RFC4234 core rules DQUOTE and HTAB,
4234      fix broken ABNF for chunk-data
4235      (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4236    </t>
4237  </list>
4238</t>
4239</section>
4240
4241<section title="Since draft-ietf-httpbis-p1-messaging-01">
4242<t>
4243  Closed issues:
4244  <list style="symbols"> 
4245    <t>
4246      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4247      "Bodies on GET (and other) requests"
4248    </t>
4249    <t>
4250      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4251      "Updating to RFC4288"
4252    </t>
4253    <t>
4254      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4255      "Status Code and Reason Phrase"
4256    </t>
4257    <t>
4258      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4259      "rel_path not used"
4260    </t>
4261  </list>
4262</t>
4263<t>
4264  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4265  <list style="symbols"> 
4266    <t>
4267      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4268      "trailer-part").
4269    </t>
4270    <t>
4271      Avoid underscore character in rule names ("http_URL" ->
4272      "http-URL", "abs_path" -> "path-absolute").
4273    </t>
4274    <t>
4275      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4276      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4277      have to be updated when switching over to RFC3986.
4278    </t>
4279    <t>
4280      Synchronize core rules with RFC5234 (this includes a change to CHAR
4281      which now excludes NUL).
4282    </t>
4283    <t>
4284      Get rid of prose rules that span multiple lines.
4285    </t>
4286    <t>
4287      Get rid of unused rules LOALPHA and UPALPHA.
4288    </t>
4289    <t>
4290      Move "Product Tokens" section (back) into Part 1, as "token" is used
4291      in the definition of the Upgrade header.
4292    </t>
4293    <t>
4294      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4295    </t>
4296    <t>
4297      Rewrite prose rule "token" in terms of "tchar".
4298    </t>
4299  </list>
4300</t>
4301</section>
4302
4303</section>
4304
4305</back>
4306</rfc>
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