source: draft-ietf-httpbis/latest/p1-messaging.xml @ 335

Last change on this file since 335 was 335, checked in by julian.reschke@…, 11 years ago

reference RFC5234 instead of RFC822 for ABNF, but continue to extend it with implied LWS and #rule (related to #36)

  • Property svn:eol-style set to native
File size: 200.6 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 "November">
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" x:maturity-level="draft"
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://tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    The changes in this draft are summarized in <xref target="changes.since.02"/>.
224  </t>
225</note>
226</front>
227<middle>
228<section title="Introduction" anchor="introduction">
229<t>
230   The Hypertext Transfer Protocol (HTTP) is an application-level
231   protocol for distributed, collaborative, hypermedia information
232   systems. HTTP has been in use by the World-Wide Web global
233   information initiative since 1990. The first version of HTTP, commonly
234   referred to as HTTP/0.9, was a simple protocol for raw data transfer
235   across the Internet with only a single method and no metadata.
236   HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
237   the protocol by allowing messages to be in the format of MIME-like
238   messages, containing metadata about the data transferred and
239   modifiers on the request/response semantics. However, HTTP/1.0 did
240   not sufficiently take into consideration the effects of hierarchical
241   proxies, caching, the need for persistent connections, or name-based
242   virtual hosts. In addition, the proliferation of incompletely-implemented
243   applications calling themselves "HTTP/1.0" necessitated a
244   protocol version change in order for two communicating applications
245   to determine each other's true capabilities.
246</t>
247<t>
248   This document is Part 1 of the seven-part specification that defines
249   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
250   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
251   requirements that enable reliable implementations and adding only
252   those new features that will either be safely ignored by an HTTP/1.0
253   recipient or only sent when communicating with a party advertising
254   compliance with HTTP/1.1.
255   Part 1 defines those aspects of HTTP/1.1 related to overall network
256   operation, message framing, interaction with transport protocols, and
257   URI schemes.
258</t>
259<t>
260   This document is currently disorganized in order to minimize the changes
261   between drafts and enable reviewers to see the smaller errata changes.
262   The next draft will reorganize the sections to better reflect the content.
263   In particular, the sections will be organized according to the typical
264   process of deciding when to use HTTP (URI schemes), overall network operation,
265   connection management, message framing, and generic message parsing.
266   The current mess reflects how widely dispersed these topics and associated
267   requirements had become in <xref target="RFC2616"/>.
268</t>
269
270<section title="Purpose" anchor="intro.purpose">
271<t>
272   Practical information systems require more functionality than simple
273   retrieval, including search, front-end update, and annotation. HTTP
274   allows an open-ended set of methods and headers that indicate the
275   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
276   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
277   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
278   method is to be applied. Messages are passed in a format similar to
279   that used by Internet mail <xref target="RFC5322"/> as defined by the Multipurpose
280   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
281</t>
282<t>
283   HTTP is also used as a generic protocol for communication between
284   user agents and proxies/gateways to other Internet systems, including
285   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
286   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
287   access to resources available from diverse applications.
288</t>
289</section>
290
291<section title="Requirements" anchor="intro.requirements">
292<t>
293   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
294   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
295   document are to be interpreted as described in <xref target="RFC2119"/>.
296</t>
297<t>
298   An implementation is not compliant if it fails to satisfy one or more
299   of the &MUST; or &REQUIRED; level requirements for the protocols it
300   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
301   level and all the &SHOULD; level requirements for its protocols is said
302   to be "unconditionally compliant"; one that satisfies all the &MUST;
303   level requirements but not all the &SHOULD; level requirements for its
304   protocols is said to be "conditionally compliant."
305</t>
306</section>
307
308<section title="Terminology" anchor="intro.terminology">
309<t>
310   This specification uses a number of terms to refer to the roles
311   played by participants in, and objects of, the HTTP communication.
312</t>
313<t>
314  <iref item="connection"/>
315  <x:dfn>connection</x:dfn>
316  <list>
317    <t>
318      A transport layer virtual circuit established between two programs
319      for the purpose of communication.
320    </t>
321  </list>
322</t>
323<t>
324  <iref item="message"/>
325  <x:dfn>message</x:dfn>
326  <list>
327    <t>
328      The basic unit of HTTP communication, consisting of a structured
329      sequence of octets matching the syntax defined in <xref target="http.message"/> and
330      transmitted via the connection.
331    </t>
332  </list>
333</t>
334<t>
335  <iref item="request"/>
336  <x:dfn>request</x:dfn>
337  <list>
338    <t>
339      An HTTP request message, as defined in <xref target="request"/>.
340    </t>
341  </list>
342</t>
343<t>
344  <iref item="response"/>
345  <x:dfn>response</x:dfn>
346  <list>
347    <t>
348      An HTTP response message, as defined in <xref target="response"/>.
349    </t>
350  </list>
351</t>
352<t>
353  <iref item="resource"/>
354  <x:dfn>resource</x:dfn>
355  <list>
356    <t>
357      A network data object or service that can be identified by a URI,
358      as defined in <xref target="uri"/>. Resources may be available in multiple
359      representations (e.g. multiple languages, data formats, size, and
360      resolutions) or vary in other ways.
361    </t>
362  </list>
363</t>
364<t>
365  <iref item="entity"/>
366  <x:dfn>entity</x:dfn>
367  <list>
368    <t>
369      The information transferred as the payload of a request or
370      response. An entity consists of metainformation in the form of
371      entity-header fields and content in the form of an entity-body, as
372      described in &entity;.
373    </t>
374  </list>
375</t>
376<t>
377  <iref item="representation"/>
378  <x:dfn>representation</x:dfn>
379  <list>
380    <t>
381      An entity included with a response that is subject to content
382      negotiation, as described in &content.negotiation;. There may exist multiple
383      representations associated with a particular response status.
384    </t>
385  </list>
386</t>
387<t>
388  <iref item="content negotiation"/>
389  <x:dfn>content negotiation</x:dfn>
390  <list>
391    <t>
392      The mechanism for selecting the appropriate representation when
393      servicing a request, as described in &content.negotiation;. The
394      representation of entities in any response can be negotiated
395      (including error responses).
396    </t>
397  </list>
398</t>
399<t>
400  <iref item="variant"/>
401  <x:dfn>variant</x:dfn>
402  <list>
403    <t>
404      A resource may have one, or more than one, representation(s)
405      associated with it at any given instant. Each of these
406      representations is termed a `variant'.  Use of the term `variant'
407      does not necessarily imply that the resource is subject to content
408      negotiation.
409    </t>
410  </list>
411</t>
412<t>
413  <iref item="client"/>
414  <x:dfn>client</x:dfn>
415  <list>
416    <t>
417      A program that establishes connections for the purpose of sending
418      requests.
419    </t>
420  </list>
421</t>
422<t>
423  <iref item="user agent"/>
424  <x:dfn>user agent</x:dfn>
425  <list>
426    <t>
427      The client which initiates a request. These are often browsers,
428      editors, spiders (web-traversing robots), or other end user tools.
429    </t>
430  </list>
431</t>
432<t>
433  <iref item="server"/>
434  <x:dfn>server</x:dfn>
435  <list>
436    <t>
437      An application program that accepts connections in order to
438      service requests by sending back responses. Any given program may
439      be capable of being both a client and a server; our use of these
440      terms refers only to the role being performed by the program for a
441      particular connection, rather than to the program's capabilities
442      in general. Likewise, any server may act as an origin server,
443      proxy, gateway, or tunnel, switching behavior based on the nature
444      of each request.
445    </t>
446  </list>
447</t>
448<t>
449  <iref item="origin server"/>
450  <x:dfn>origin server</x:dfn>
451  <list>
452    <t>
453      The server on which a given resource resides or is to be created.
454    </t>
455  </list>
456</t>
457<t>
458  <iref item="proxy"/>
459  <x:dfn>proxy</x:dfn>
460  <list>
461    <t>
462      An intermediary program which acts as both a server and a client
463      for the purpose of making requests on behalf of other clients.
464      Requests are serviced internally or by passing them on, with
465      possible translation, to other servers. A proxy &MUST; implement
466      both the client and server requirements of this specification. A
467      "transparent proxy" is a proxy that does not modify the request or
468      response beyond what is required for proxy authentication and
469      identification. A "non-transparent proxy" is a proxy that modifies
470      the request or response in order to provide some added service to
471      the user agent, such as group annotation services, media type
472      transformation, protocol reduction, or anonymity filtering. Except
473      where either transparent or non-transparent behavior is explicitly
474      stated, the HTTP proxy requirements apply to both types of
475      proxies.
476    </t>
477  </list>
478</t>
479<t>
480  <iref item="gateway"/>
481  <x:dfn>gateway</x:dfn>
482  <list>
483    <t>
484      A server which acts as an intermediary for some other server.
485      Unlike a proxy, a gateway receives requests as if it were the
486      origin server for the requested resource; the requesting client
487      may not be aware that it is communicating with a gateway.
488    </t>
489  </list>
490</t>
491<t>
492  <iref item="tunnel"/>
493  <x:dfn>tunnel</x:dfn>
494  <list>
495    <t>
496      An intermediary program which is acting as a blind relay between
497      two connections. Once active, a tunnel is not considered a party
498      to the HTTP communication, though the tunnel may have been
499      initiated by an HTTP request. The tunnel ceases to exist when both
500      ends of the relayed connections are closed.
501    </t>
502  </list>
503</t>
504<t>
505  <iref item="cache"/>
506  <x:dfn>cache</x:dfn>
507  <list>
508    <t>
509      A program's local store of response messages and the subsystem
510      that controls its message storage, retrieval, and deletion. A
511      cache stores cacheable responses in order to reduce the response
512      time and network bandwidth consumption on future, equivalent
513      requests. Any client or server may include a cache, though a cache
514      cannot be used by a server that is acting as a tunnel.
515    </t>
516  </list>
517</t>
518<t>
519  <iref item="cacheable"/>
520  <x:dfn>cacheable</x:dfn>
521  <list>
522    <t>
523      A response is cacheable if a cache is allowed to store a copy of
524      the response message for use in answering subsequent requests. The
525      rules for determining the cacheability of HTTP responses are
526      defined in &caching;. Even if a resource is cacheable, there may
527      be additional constraints on whether a cache can use the cached
528      copy for a particular request.
529    </t>
530  </list>
531</t>
532<t>
533  <iref item="upstream"/>
534  <iref item="downstream"/>
535  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
536  <list>
537    <t>
538      Upstream and downstream describe the flow of a message: all
539      messages flow from upstream to downstream.
540    </t>
541  </list>
542</t>
543<t>
544  <iref item="inbound"/>
545  <iref item="outbound"/>
546  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
547  <list>
548    <t>
549      Inbound and outbound refer to the request and response paths for
550      messages: "inbound" means "traveling toward the origin server",
551      and "outbound" means "traveling toward the user agent"
552    </t>
553  </list>
554</t>
555</section>
556
557<section title="Overall Operation" anchor="intro.overall.operation">
558<t>
559   HTTP is a request/response protocol. A client sends a
560   request to the server in the form of a request method, URI, and
561   protocol version, followed by a MIME-like message containing request
562   modifiers, client information, and possible body content over a
563   connection with a server. The server responds with a status line,
564   including the message's protocol version and a success or error code,
565   followed by a MIME-like message containing server information, entity
566   metainformation, and possible entity-body content. The relationship
567   between HTTP and MIME is described in &diff2045entity;.
568</t>
569<t>
570   Most HTTP communication is initiated by a user agent and consists of
571   a request to be applied to a resource on some origin server. In the
572   simplest case, this may be accomplished via a single connection (v)
573   between the user agent (UA) and the origin server (O).
574</t>
575<figure><artwork type="drawing">
576       request chain ------------------------&gt;
577    UA -------------------v------------------- O
578       &lt;----------------------- response chain
579</artwork></figure>
580<t>
581   A more complicated situation occurs when one or more intermediaries
582   are present in the request/response chain. There are three common
583   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
584   forwarding agent, receiving requests for a URI in its absolute form,
585   rewriting all or part of the message, and forwarding the reformatted
586   request toward the server identified by the URI. A gateway is a
587   receiving agent, acting as a layer above some other server(s) and, if
588   necessary, translating the requests to the underlying server's
589   protocol. A tunnel acts as a relay point between two connections
590   without changing the messages; tunnels are used when the
591   communication needs to pass through an intermediary (such as a
592   firewall) even when the intermediary cannot understand the contents
593   of the messages.
594</t>
595<figure><artwork type="drawing">
596       request chain --------------------------------------&gt;
597    UA -----v----- A -----v----- B -----v----- C -----v----- O
598       &lt;------------------------------------- response chain
599</artwork></figure>
600<t>
601   The figure above shows three intermediaries (A, B, and C) between the
602   user agent and origin server. A request or response message that
603   travels the whole chain will pass through four separate connections.
604   This distinction is important because some HTTP communication options
605   may apply only to the connection with the nearest, non-tunnel
606   neighbor, only to the end-points of the chain, or to all connections
607   along the chain. Although the diagram is linear, each participant may
608   be engaged in multiple, simultaneous communications. For example, B
609   may be receiving requests from many clients other than A, and/or
610   forwarding requests to servers other than C, at the same time that it
611   is handling A's request.
612</t>
613<t>
614   Any party to the communication which is not acting as a tunnel may
615   employ an internal cache for handling requests. The effect of a cache
616   is that the request/response chain is shortened if one of the
617   participants along the chain has a cached response applicable to that
618   request. The following illustrates the resulting chain if B has a
619   cached copy of an earlier response from O (via C) for a request which
620   has not been cached by UA or A.
621</t>
622<figure><artwork type="drawing">
623          request chain ----------&gt;
624       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
625          &lt;--------- response chain
626</artwork></figure>
627<t>
628   Not all responses are usefully cacheable, and some requests may
629   contain modifiers which place special requirements on cache behavior.
630   HTTP requirements for cache behavior and cacheable responses are
631   defined in &caching;.
632</t>
633<t>
634   In fact, there are a wide variety of architectures and configurations
635   of caches and proxies currently being experimented with or deployed
636   across the World Wide Web. These systems include national hierarchies
637   of proxy caches to save transoceanic bandwidth, systems that
638   broadcast or multicast cache entries, organizations that distribute
639   subsets of cached data via CD-ROM, and so on. HTTP systems are used
640   in corporate intranets over high-bandwidth links, and for access via
641   PDAs with low-power radio links and intermittent connectivity. The
642   goal of HTTP/1.1 is to support the wide diversity of configurations
643   already deployed while introducing protocol constructs that meet the
644   needs of those who build web applications that require high
645   reliability and, failing that, at least reliable indications of
646   failure.
647</t>
648<t>
649   HTTP communication usually takes place over TCP/IP connections. The
650   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
651   not preclude HTTP from being implemented on top of any other protocol
652   on the Internet, or on other networks. HTTP only presumes a reliable
653   transport; any protocol that provides such guarantees can be used;
654   the mapping of the HTTP/1.1 request and response structures onto the
655   transport data units of the protocol in question is outside the scope
656   of this specification.
657</t>
658<t>
659   In HTTP/1.0, most implementations used a new connection for each
660   request/response exchange. In HTTP/1.1, a connection may be used for
661   one or more request/response exchanges, although connections may be
662   closed for a variety of reasons (see <xref target="persistent.connections"/>).
663</t>
664</section>
665</section>
666
667<section title="Notational Conventions and Generic Grammar" anchor="notation">
668
669<section title="Augmented BNF" anchor="notation.abnf">
670<t>
671   All of the mechanisms specified in this document are described in
672   both prose and an augmented Backus-Naur Form (ABNF) based on that
673   defined in <xref target="RFC5234"/>. Implementors will need to be
674   familiar with the notation in order to understand this specification. The
675   extensions to ABNF used in this specification are described below.
676</t>
677
678<section title="#rule">
679  <t>
680    A construct "#" is defined, similar to "*", for defining lists of
681    elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
682    &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
683    (",") and &OPTIONAL; linear white space (LWS). This makes the usual
684    form of lists very easy; a rule such as
685    <figure><artwork type="example">
686 ( *<x:ref>LWS</x:ref> element *( *<x:ref>LWS</x:ref> "," *<x:ref>LWS</x:ref> element ))</artwork></figure>
687  </t>
688  <t>
689    can be shown as
690    <figure><artwork type="example">
691 1#element</artwork></figure>
692  </t>
693  <t>
694    Wherever this construct is used, null elements are allowed, but do
695    not contribute to the count of elements present. That is,
696    "(element), , (element) " is permitted, but counts as only two
697    elements. Therefore, where at least one element is required, at
698    least one non-null element &MUST; be present. Default values are 0
699    and infinity so that "#element" allows any number, including zero;
700    "1#element" requires at least one; and "1#2element" allows one or
701    two.
702  </t>
703</section>
704
705<section title="implied *LWS" anchor="implied.LWS">
706  <iref item="implied *LWS" primary="true"/>
707    <t>
708      The grammar described by this specification is word-based. Except
709      where noted otherwise, linear white space (LWS) can be included
710      between any two adjacent words (token or quoted-string), and
711      between adjacent words and separators, without changing the
712      interpretation of a field. At least one delimiter (LWS and/or
713      separators) &MUST; exist between any two tokens (for the definition
714      of "token" below), since they would otherwise be interpreted as a
715      single token.
716    </t>
717</section>
718</section>
719
720<section title="Basic Rules" anchor="basic.rules">
721<t anchor="core.rules">
722  <x:anchor-alias value="OCTET"/>
723  <x:anchor-alias value="CHAR"/>
724  <x:anchor-alias value="ALPHA"/>
725  <x:anchor-alias value="DIGIT"/>
726  <x:anchor-alias value="CTL"/>
727  <x:anchor-alias value="CR"/>
728  <x:anchor-alias value="LF"/>
729  <x:anchor-alias value="SP"/>
730  <x:anchor-alias value="HTAB"/>
731  <x:anchor-alias value="DQUOTE"/>
732   The following rules are used throughout this specification to
733   describe basic parsing constructs. The US-ASCII coded character set
734   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
735</t>
736<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"/>
737  <x:ref>OCTET</x:ref>          = %x00-FF
738                   ; any 8-bit sequence of data
739  <x:ref>CHAR</x:ref>           = %x01-7F
740                   ; any US-ASCII character, excluding NUL
741  <x:ref>ALPHA</x:ref>          = %x41-5A / %x61-7A
742                   ; A-Z / a-z
743  <x:ref>DIGIT</x:ref>          = %x30-39
744                   ; any US-ASCII digit "0".."9"
745  <x:ref>CTL</x:ref>            = %x00-1F / %x7F
746                   ; (octets 0 - 31) and DEL (127)
747  <x:ref>CR</x:ref>             = %x0D
748                   ; US-ASCII CR, carriage return (13)
749  <x:ref>LF</x:ref>             = %x0A
750                   ; US-ASCII LF, linefeed (10)
751  <x:ref>SP</x:ref>             = %x20
752                   ; US-ASCII SP, space (32)
753  <x:ref>HTAB</x:ref>           = %x09
754                   ; US-ASCII HT, horizontal-tab (9)
755  <x:ref>DQUOTE</x:ref>         = %x22
756                   ; US-ASCII double-quote mark (34)
757</artwork></figure>
758<t anchor="rule.CRLF">
759  <x:anchor-alias value="CRLF"/>
760   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
761   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
762   tolerant applications). The end-of-line marker within an entity-body
763   is defined by its associated media type, as described in &media-types;.
764</t>
765<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
766  <x:ref>CRLF</x:ref>           = <x:ref>CR</x:ref> LF
767</artwork></figure>
768<t anchor="rule.LWS">
769  <x:anchor-alias value="LWS"/>
770   HTTP/1.1 header field values can be folded onto multiple lines if the
771   continuation line begins with a space or horizontal tab. All linear
772   white space, including folding, has the same semantics as SP. A
773   recipient &MAY; replace any linear white space with a single SP before
774   interpreting the field value or forwarding the message downstream.
775</t>
776<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
777  <x:ref>LWS</x:ref>            = [<x:ref>CRLF</x:ref>] 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
778</artwork></figure>
779<t anchor="rule.TEXT">
780  <x:anchor-alias value="TEXT"/>
781   The TEXT rule is only used for descriptive field contents and values
782   that are not intended to be interpreted by the message parser. Words
783   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
784   <xref target="ISO-8859-1"/> only when encoded according to the rules of
785   <xref target="RFC2047"/>.
786</t>
787<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
788  <x:ref>TEXT</x:ref>           = %x20-7E / %x80-FF / <x:ref>LWS</x:ref>
789                 ; any <x:ref>OCTET</x:ref> except <x:ref>CTL</x:ref>s, but including <x:ref>LWS</x:ref>
790</artwork></figure>
791<t>
792   A CRLF is allowed in the definition of TEXT only as part of a header
793   field continuation. It is expected that the folding LWS will be
794   replaced with a single SP before interpretation of the TEXT value.
795</t>
796<t anchor="rule.HEXDIG">
797  <x:anchor-alias value="HEXDIG"/>
798   Hexadecimal numeric characters are used in several protocol elements.
799</t>
800<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEXDIG"/>
801  <x:ref>HEXDIG</x:ref>         = "A" / "B" / "C" / "D" / "E" / "F"
802                 / "a" / "b" / "c" / "d" / "e" / "f" / <x:ref>DIGIT</x:ref>
803</artwork></figure>
804<t anchor="rule.token.separators">
805  <x:anchor-alias value="tchar"/>
806  <x:anchor-alias value="token"/>
807  <x:anchor-alias value="separators"/>
808   Many HTTP/1.1 header field values consist of words separated by LWS
809   or special characters. These special characters &MUST; be in a quoted
810   string to be used within a parameter value (as defined in
811   <xref target="transfer.codings"/>).
812</t>
813<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"/>
814  <x:ref>separators</x:ref>     = "(" / ")" / "&lt;" / "&gt;" / "@"
815                 / "," / ";" / ":" / "\" / <x:ref>DQUOTE</x:ref>
816                 / "/" / "[" / "]" / "?" / "="
817                 / "{" / "}" / <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref>
818                 
819  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
820                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
821                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
822                 ; any <x:ref>CHAR</x:ref> except <x:ref>CTL</x:ref>s or <x:ref>separators</x:ref>
823                 
824  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
825</artwork></figure>
826<t anchor="rule.comment">
827  <x:anchor-alias value="comment"/>
828  <x:anchor-alias value="ctext"/>
829   Comments can be included in some HTTP header fields by surrounding
830   the comment text with parentheses. Comments are only allowed in
831   fields containing "comment" as part of their field value definition.
832   In all other fields, parentheses are considered part of the field
833   value.
834</t>
835<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
836  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
837  <x:ref>ctext</x:ref>          = &lt;any <x:ref>TEXT</x:ref> excluding "(" and ")"&gt;
838</artwork></figure>
839<t anchor="rule.quoted-string">
840  <x:anchor-alias value="quoted-string"/>
841  <x:anchor-alias value="qdtext"/>
842   A string of text is parsed as a single word if it is quoted using
843   double-quote marks.
844</t>
845<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
846  <x:ref>quoted-string</x:ref>  = ( <x:ref>DQUOTE</x:ref> *(<x:ref>qdtext</x:ref> / <x:ref>quoted-pair</x:ref> ) <x:ref>DQUOTE</x:ref> )
847  <x:ref>qdtext</x:ref>         = &lt;any <x:ref>TEXT</x:ref> excluding <x:ref>DQUOTE</x:ref> and "\">
848</artwork></figure>
849<t anchor="rule.quoted-pair">
850  <x:anchor-alias value="quoted-pair"/>
851  <x:anchor-alias value="quoted-text"/>
852   The backslash character ("\") &MAY; be used as a single-character
853   quoting mechanism only within quoted-string and comment constructs.
854</t>
855<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-text"/><iref primary="true" item="Grammar" subitem="quoted-pair"/>
856  <x:ref>quoted-text</x:ref>    = %x01-09 /
857                   %x0B-0C /
858                   %x0E-FF ; Characters excluding NUL, <x:ref>CR</x:ref> and <x:ref>LF</x:ref>
859  <x:ref>quoted-pair</x:ref>    = "\" <x:ref>quoted-text</x:ref>
860</artwork></figure>
861</section>
862
863<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
864  <x:anchor-alias value="request-header"/>
865  <x:anchor-alias value="response-header"/>
866  <x:anchor-alias value="accept-params"/>
867  <x:anchor-alias value="entity-body"/>
868  <x:anchor-alias value="entity-header"/>
869  <x:anchor-alias value="Cache-Control"/>
870  <x:anchor-alias value="Pragma"/>
871  <x:anchor-alias value="Warning"/>
872<t>
873  The ABNF rules below are defined in other parts:
874</t>
875<figure><!-- Part2--><artwork type="abnf2616">
876  <x:ref>request-header</x:ref>  = &lt;request-header, defined in &request-header-fields;&gt;
877  <x:ref>response-header</x:ref> = &lt;response-header, defined in &response-header-fields;&gt;
878</artwork></figure>
879<figure><!-- Part3--><artwork type="abnf2616">
880  <x:ref>accept-params</x:ref>   = &lt;accept-params, defined in &header-accept;&gt;
881  <x:ref>entity-body</x:ref>     = &lt;entity-body, defined in &entity-body;&gt;
882  <x:ref>entity-header</x:ref>   = &lt;entity-header, defined in &entity-header-fields;&gt;
883</artwork></figure>
884<figure><!-- Part6--><artwork type="abnf2616">
885  <x:ref>Cache-Control</x:ref>   = &lt;Cache-Control, defined in &header-pragma;&gt;
886  <x:ref>Pragma</x:ref>          = &lt;Pragma, defined in &header-pragma;&gt;
887  <x:ref>Warning</x:ref>         = &lt;Warning, defined in &header-warning;&gt;
888</artwork></figure>
889</section>
890
891</section>
892
893<section title="Protocol Parameters" anchor="protocol.parameters">
894
895<section title="HTTP Version" anchor="http.version">
896  <x:anchor-alias value="HTTP-Version"/>
897  <x:anchor-alias value="HTTP-Prot-Name"/>
898<t>
899   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
900   of the protocol. The protocol versioning policy is intended to allow
901   the sender to indicate the format of a message and its capacity for
902   understanding further HTTP communication, rather than the features
903   obtained via that communication. No change is made to the version
904   number for the addition of message components which do not affect
905   communication behavior or which only add to extensible field values.
906   The &lt;minor&gt; number is incremented when the changes made to the
907   protocol add features which do not change the general message parsing
908   algorithm, but which may add to the message semantics and imply
909   additional capabilities of the sender. The &lt;major&gt; number is
910   incremented when the format of a message within the protocol is
911   changed. See <xref target="RFC2145"/> for a fuller explanation.
912</t>
913<t>
914   The version of an HTTP message is indicated by an HTTP-Version field
915   in the first line of the message. HTTP-Version is case-sensitive.
916</t>
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/>
918  <x:ref>HTTP-Version</x:ref>   = <x:ref>HTTP-Prot-Name</x:ref> "/" 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
919  <x:ref>HTTP-Prot-Name</x:ref> = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
920</artwork></figure>
921<t>
922   Note that the major and minor numbers &MUST; be treated as separate
923   integers and that each &MAY; be incremented higher than a single digit.
924   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
925   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
926   &MUST-NOT; be sent.
927</t>
928<t>
929   An application that sends a request or response message that includes
930   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
931   with this specification. Applications that are at least conditionally
932   compliant with this specification &SHOULD; use an HTTP-Version of
933   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
934   not compatible with HTTP/1.0. For more details on when to send
935   specific HTTP-Version values, see <xref target="RFC2145"/>.
936</t>
937<t>
938   The HTTP version of an application is the highest HTTP version for
939   which the application is at least conditionally compliant.
940</t>
941<t>
942   Proxy and gateway applications need to be careful when forwarding
943   messages in protocol versions different from that of the application.
944   Since the protocol version indicates the protocol capability of the
945   sender, a proxy/gateway &MUST-NOT; send a message with a version
946   indicator which is greater than its actual version. If a higher
947   version request is received, the proxy/gateway &MUST; either downgrade
948   the request version, or respond with an error, or switch to tunnel
949   behavior.
950</t>
951<t>
952   Due to interoperability problems with HTTP/1.0 proxies discovered
953   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
954   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
955   they support. The proxy/gateway's response to that request &MUST; be in
956   the same major version as the request.
957</t>
958<t>
959  <list>
960    <t>
961      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
962      of header fields required or forbidden by the versions involved.
963    </t>
964  </list>
965</t>
966</section>
967
968<section title="Uniform Resource Identifiers" anchor="uri">
969<t>
970   URIs have been known by many names: WWW addresses, Universal Document
971   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
972   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
973   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
974   simply formatted strings which identify--via name, location, or any
975   other characteristic--a resource.
976</t>
977
978<section title="General Syntax" anchor="general.syntax">
979  <x:anchor-alias value="absoluteURI"/>
980  <x:anchor-alias value="authority"/>
981  <x:anchor-alias value="fragment"/>
982  <x:anchor-alias value="path-absolute"/>
983  <x:anchor-alias value="port"/>
984  <x:anchor-alias value="query"/>
985  <x:anchor-alias value="relativeURI"/>
986  <x:anchor-alias value="uri-host"/>
987<t>
988   URIs in HTTP can be represented in absolute form or relative to some
989   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
990   forms are differentiated by the fact that absolute URIs always begin
991   with a scheme name followed by a colon. For definitive information on
992   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
993   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
994   and <xref target="RFC1808"/>). This specification adopts the
995   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
996   "host", "abs_path", "query", and "authority" from that specification:
997</t>
998<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"/>
999  <x:ref>absoluteURI</x:ref>   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1000  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
1001  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
1002  <x:ref>path-absolute</x:ref> = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
1003  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1004  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
1005  <x:ref>relativeURI</x:ref>   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
1006  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
1007</artwork></figure>
1008<t>
1009   HTTP does not place any a priori limit on the length of
1010   a URI. Servers &MUST; be able to handle the URI of any resource they
1011   serve, and &SHOULD; be able to handle URIs of unbounded length if they
1012   provide GET-based forms that could generate such URIs. A server
1013   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
1014   than the server can handle (see &status-414;).
1015</t>
1016<t>
1017  <list>
1018    <t>
1019      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1020      above 255 bytes, because some older client or proxy
1021      implementations might not properly support these lengths.
1022    </t>
1023  </list>
1024</t>
1025</section>
1026
1027<section title="http URL" anchor="http.url">
1028  <x:anchor-alias value="http-URL"/>
1029  <iref item="http URI scheme" primary="true"/>
1030  <iref item="URI scheme" subitem="http" primary="true"/>
1031<t>
1032   The "http" scheme is used to locate network resources via the HTTP
1033   protocol. This section defines the scheme-specific syntax and
1034   semantics for http URLs.
1035</t>
1036<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
1037  <x:ref>http-URL</x:ref> = "http:" "//" <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ]
1038             [ <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]]
1039</artwork></figure>
1040<t>
1041   If the port is empty or not given, port 80 is assumed. The semantics
1042   are that the identified resource is located at the server listening
1043   for TCP connections on that port of that host, and the Request-URI
1044   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
1045   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
1046   the path-absolute is not present in the URL, it &MUST; be given as "/" when
1047   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1048   receives a host name which is not a fully qualified domain name, it
1049   &MAY; add its domain to the host name it received. If a proxy receives
1050   a fully qualified domain name, the proxy &MUST-NOT; change the host
1051   name.
1052</t>
1053<t><list><t>
1054  <iref item="https URI scheme"/>
1055  <iref item="URI scheme" subitem="https"/>
1056  <x:h>Note:</x:h> the "https" scheme is defined in <xref target="RFC2818"/>.
1057</t></list></t>
1058</section>
1059
1060<section title="URI Comparison" anchor="uri.comparison">
1061<t>
1062   When comparing two URIs to decide if they match or not, a client
1063   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1064   URIs, with these exceptions:
1065  <list style="symbols">
1066    <t>A port that is empty or not given is equivalent to the default
1067        port for that URI-reference;</t>
1068    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1069    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1070    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
1071  </list>
1072</t>
1073<t>
1074   Characters other than those in the "reserved" set (see
1075   <xref target="RFC2396" x:fmt="," x:sec="2.2"/>) are equivalent to their
1076   ""%" <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding.
1077</t>
1078<t>
1079   For example, the following three URIs are equivalent:
1080</t>
1081<figure><artwork type="example">
1082   http://example.com:80/~smith/home.html
1083   http://EXAMPLE.com/%7Esmith/home.html
1084   http://EXAMPLE.com:/%7esmith/home.html
1085</artwork></figure>
1086</section>
1087</section>
1088
1089<section title="Date/Time Formats" anchor="date.time.formats">
1090<section title="Full Date" anchor="full.date">
1091  <x:anchor-alias value="HTTP-date"/>
1092  <x:anchor-alias value="obsolete-date"/>
1093  <x:anchor-alias value="rfc1123-date"/>
1094  <x:anchor-alias value="rfc850-date"/>
1095  <x:anchor-alias value="asctime-date"/>
1096  <x:anchor-alias value="date1"/>
1097  <x:anchor-alias value="date2"/>
1098  <x:anchor-alias value="date3"/>
1099  <x:anchor-alias value="rfc1123-date"/>
1100  <x:anchor-alias value="time"/>
1101  <x:anchor-alias value="wkday"/>
1102  <x:anchor-alias value="weekday"/>
1103  <x:anchor-alias value="month"/>
1104<t>
1105   HTTP applications have historically allowed three different formats
1106   for the representation of date/time stamps:
1107</t>
1108<figure><artwork type="example">
1109   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1110   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1111   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1112</artwork></figure>
1113<t>
1114   The first format is preferred as an Internet standard and represents
1115   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
1116   <xref target="RFC822"/>). The other formats are described here only for
1117   compatibility with obsolete implementations.
1118   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1119   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1120   only generate the RFC 1123 format for representing HTTP-date values
1121   in header fields. See <xref target="tolerant.applications"/> for further information.
1122</t>
1123<t><list><t>
1124      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1125      accepting date values that may have been sent by non-HTTP
1126      applications, as is sometimes the case when retrieving or posting
1127      messages via proxies/gateways to SMTP or NNTP.
1128</t></list></t>
1129<t>
1130   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1131   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1132   equal to UTC (Coordinated Universal Time). This is indicated in the
1133   first two formats by the inclusion of "GMT" as the three-letter
1134   abbreviation for time zone, and &MUST; be assumed when reading the
1135   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1136   additional LWS beyond that specifically included as SP in the
1137   grammar.
1138</t>
1139<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="obsolete-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"/>
1140  <x:ref>HTTP-date</x:ref>    = <x:ref>rfc1123-date</x:ref> / <x:ref>obsolete-date</x:ref>
1141  <x:ref>obsolete-date</x:ref> = <x:ref>rfc850-date</x:ref> / <x:ref>asctime-date</x:ref> 
1142  <x:ref>rfc1123-date</x:ref> = <x:ref>wkday</x:ref> "," <x:ref>SP</x:ref> date1 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
1143  <x:ref>rfc850-date</x:ref>  = <x:ref>weekday</x:ref> "," <x:ref>SP</x:ref> date2 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
1144  <x:ref>asctime-date</x:ref> = <x:ref>wkday</x:ref> <x:ref>SP</x:ref> <x:ref>date3</x:ref> <x:ref>SP</x:ref> <x:ref>time</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
1145  <x:ref>date1</x:ref>        = 2<x:ref>DIGIT</x:ref> <x:ref>SP</x:ref> <x:ref>month</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
1146                 ; day month year (e.g., 02 Jun 1982)
1147  <x:ref>date2</x:ref>        = 2<x:ref>DIGIT</x:ref> "-" <x:ref>month</x:ref> "-" 2<x:ref>DIGIT</x:ref>
1148                 ; day-month-year (e.g., 02-Jun-82)
1149  <x:ref>date3</x:ref>        = <x:ref>month</x:ref> <x:ref>SP</x:ref> ( 2<x:ref>DIGIT</x:ref> / ( <x:ref>SP</x:ref> 1<x:ref>DIGIT</x:ref> ))
1150                 ; month day (e.g., Jun  2)
1151  <x:ref>time</x:ref>         = 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref>
1152                 ; 00:00:00 - 23:59:59
1153  <x:ref>wkday</x:ref>        = s-Mon / s-Tue / s-Wed
1154               / s-Thu / s-Fri / s-Sat / s-Sun
1155  <x:ref>weekday</x:ref>      = l-Mon / l-Tue / l-Wed
1156               / l-Thu / l-Fri / l-Sat / l-Sun
1157  <x:ref>month</x:ref>        = s-Jan / s-Feb / s-Mar / s-Apr
1158               / s-May / s-Jun / s-Jul / s-Aug
1159               / s-Sep / s-Oct / s-Nov / s-Dec
1160               
1161  GMT   = <x:abnf-char-sequence>"GMT"</x:abnf-char-sequence> ; "GMT", case-sensitive
1162
1163  s-Mon = <x:abnf-char-sequence>"Mon"</x:abnf-char-sequence> ; "Mon", case-sensitive
1164  s-Tue = <x:abnf-char-sequence>"Tue"</x:abnf-char-sequence> ; "Tue", case-sensitive
1165  s-Wed = <x:abnf-char-sequence>"Wed"</x:abnf-char-sequence> ; "Wed", case-sensitive
1166  s-Thu = <x:abnf-char-sequence>"Thu"</x:abnf-char-sequence> ; "Thu", case-sensitive
1167  s-Fri = <x:abnf-char-sequence>"Fri"</x:abnf-char-sequence> ; "Fri", case-sensitive
1168  s-Sat = <x:abnf-char-sequence>"Sat"</x:abnf-char-sequence> ; "Sat", case-sensitive
1169  s-Sun = <x:abnf-char-sequence>"Sun"</x:abnf-char-sequence> ; "Sun", case-sensitive
1170
1171  l-Mon = <x:abnf-char-sequence>"Monday"</x:abnf-char-sequence>          ; "Monday", case-sensitive
1172  l-Tue = <x:abnf-char-sequence>"Tuesday"</x:abnf-char-sequence>       ; "Tuesday", case-sensitive
1173  l-Wed = <x:abnf-char-sequence>"Wednesday"</x:abnf-char-sequence> ; "Wednesday", case-sensitive
1174  l-Thu = <x:abnf-char-sequence>"Thursday"</x:abnf-char-sequence>    ; "Thursday", case-sensitive
1175  l-Fri = <x:abnf-char-sequence>"Friday"</x:abnf-char-sequence>          ; "Friday", case-sensitive
1176  l-Sat = <x:abnf-char-sequence>"Saturday"</x:abnf-char-sequence>    ; "Saturday", case-sensitive
1177  l-Sun = <x:abnf-char-sequence>"Sunday"</x:abnf-char-sequence>          ; "Sunday", case-sensitive
1178
1179  s-Jan = <x:abnf-char-sequence>"Jan"</x:abnf-char-sequence> ; "Jan", case-sensitive
1180  s-Feb = <x:abnf-char-sequence>"Feb"</x:abnf-char-sequence> ; "Feb", case-sensitive
1181  s-Mar = <x:abnf-char-sequence>"Mar"</x:abnf-char-sequence> ; "Mar", case-sensitive
1182  s-Apr = <x:abnf-char-sequence>"Apr"</x:abnf-char-sequence> ; "Apr", case-sensitive
1183  s-May = <x:abnf-char-sequence>"May"</x:abnf-char-sequence> ; "May", case-sensitive
1184  s-Jun = <x:abnf-char-sequence>"Jun"</x:abnf-char-sequence> ; "Jun", case-sensitive
1185  s-Jul = <x:abnf-char-sequence>"Jul"</x:abnf-char-sequence> ; "Jul", case-sensitive
1186  s-Aug = <x:abnf-char-sequence>"Aug"</x:abnf-char-sequence> ; "Aug", case-sensitive
1187  s-Sep = <x:abnf-char-sequence>"Sep"</x:abnf-char-sequence> ; "Sep", case-sensitive
1188  s-Oct = <x:abnf-char-sequence>"Oct"</x:abnf-char-sequence> ; "Oct", case-sensitive
1189  s-Nov = <x:abnf-char-sequence>"Nov"</x:abnf-char-sequence> ; "Nov", case-sensitive
1190  s-Dec = <x:abnf-char-sequence>"Dec"</x:abnf-char-sequence> ; "Dec", case-sensitive
1191</artwork></figure>
1192<t>
1193      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1194      to their usage within the protocol stream. Clients and servers are
1195      not required to use these formats for user presentation, request
1196      logging, etc.
1197</t>
1198</section>
1199</section>
1200
1201<section title="Transfer Codings" anchor="transfer.codings">
1202  <x:anchor-alias value="parameter"/>
1203  <x:anchor-alias value="transfer-coding"/>
1204  <x:anchor-alias value="transfer-extension"/>
1205<t>
1206   Transfer-coding values are used to indicate an encoding
1207   transformation that has been, can be, or may need to be applied to an
1208   entity-body in order to ensure "safe transport" through the network.
1209   This differs from a content coding in that the transfer-coding is a
1210   property of the message, not of the original entity.
1211</t>
1212<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1213  <x:ref>transfer-coding</x:ref>         = "chunked" / <x:ref>transfer-extension</x:ref>
1214  <x:ref>transfer-extension</x:ref>      = <x:ref>token</x:ref> *( ";" <x:ref>parameter</x:ref> )
1215</artwork></figure>
1216<t anchor="rule.parameter">
1217  <x:anchor-alias value="attribute"/>
1218  <x:anchor-alias value="parameter"/>
1219  <x:anchor-alias value="value"/>
1220   Parameters are in  the form of attribute/value pairs.
1221</t>
1222<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"/>
1223  <x:ref>parameter</x:ref>               = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
1224  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
1225  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1226</artwork></figure>
1227<t>
1228   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1229   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1230   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1231</t>
1232<t>
1233   Whenever a transfer-coding is applied to a message-body, the set of
1234   transfer-codings &MUST; include "chunked", unless the message indicates it
1235   is terminated by closing the connection. When the "chunked" transfer-coding
1236   is used, it &MUST; be the last transfer-coding applied to the
1237   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1238   than once to a message-body. These rules allow the recipient to
1239   determine the transfer-length of the message (<xref target="message.length"/>).
1240</t>
1241<t>
1242   Transfer-codings are analogous to the Content-Transfer-Encoding
1243   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1244   binary data over a 7-bit transport service. However, safe transport
1245   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1246   the only unsafe characteristic of message-bodies is the difficulty in
1247   determining the exact body length (<xref target="message.length"/>), or the desire to
1248   encrypt data over a shared transport.
1249</t>
1250<t>
1251   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1252   transfer-coding value tokens. Initially, the registry contains the
1253   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1254   "gzip", "compress", and "deflate" (&content-codings;).
1255</t>
1256<t>
1257   New transfer-coding value tokens &SHOULD; be registered in the same way
1258   as new content-coding value tokens (&content-codings;).
1259</t>
1260<t>
1261   A server which receives an entity-body with a transfer-coding it does
1262   not understand &SHOULD; return 501 (Not Implemented), and close the
1263   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1264   client.
1265</t>
1266
1267<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1268  <x:anchor-alias value="chunk"/>
1269  <x:anchor-alias value="Chunked-Body"/>
1270  <x:anchor-alias value="chunk-data"/>
1271  <x:anchor-alias value="chunk-extension"/>
1272  <x:anchor-alias value="chunk-ext-name"/>
1273  <x:anchor-alias value="chunk-ext-val"/>
1274  <x:anchor-alias value="chunk-size"/>
1275  <x:anchor-alias value="last-chunk"/>
1276  <x:anchor-alias value="trailer-part"/>
1277<t>
1278   The chunked encoding modifies the body of a message in order to
1279   transfer it as a series of chunks, each with its own size indicator,
1280   followed by an &OPTIONAL; trailer containing entity-header fields. This
1281   allows dynamically produced content to be transferred along with the
1282   information necessary for the recipient to verify that it has
1283   received the full message.
1284</t>
1285<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Chunked-Body"/><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="true" item="Grammar" subitem="chunk-extension"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer-part"/>
1286  <x:ref>Chunked-Body</x:ref>   = *<x:ref>chunk</x:ref>
1287                   <x:ref>last-chunk</x:ref>
1288                   <x:ref>trailer-part</x:ref>
1289                   <x:ref>CRLF</x:ref>
1290 
1291  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-extension</x:ref> ] <x:ref>CRLF</x:ref>
1292                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1293  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1294  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-extension</x:ref> ] <x:ref>CRLF</x:ref>
1295 
1296  <x:ref>chunk-extension</x:ref>= *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1297  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1298  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1299  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1300  <x:ref>trailer-part</x:ref>   = *(<x:ref>entity-header</x:ref> <x:ref>CRLF</x:ref>)
1301</artwork></figure>
1302<t>
1303   The chunk-size field is a string of hex digits indicating the size of
1304   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1305   zero, followed by the trailer, which is terminated by an empty line.
1306</t>
1307<t>
1308   The trailer allows the sender to include additional HTTP header
1309   fields at the end of the message. The Trailer header field can be
1310   used to indicate which header fields are included in a trailer (see
1311   <xref target="header.trailer"/>).
1312</t>
1313<t>
1314   A server using chunked transfer-coding in a response &MUST-NOT; use the
1315   trailer for any header fields unless at least one of the following is
1316   true:
1317  <list style="numbers">
1318    <t>the request included a TE header field that indicates "trailers" is
1319     acceptable in the transfer-coding of the  response, as described in
1320     <xref target="header.te"/>; or,</t>
1321
1322    <t>the server is the origin server for the response, the trailer
1323     fields consist entirely of optional metadata, and the recipient
1324     could use the message (in a manner acceptable to the origin server)
1325     without receiving this metadata.  In other words, the origin server
1326     is willing to accept the possibility that the trailer fields might
1327     be silently discarded along the path to the client.</t>
1328  </list>
1329</t>
1330<t>
1331   This requirement prevents an interoperability failure when the
1332   message is being received by an HTTP/1.1 (or later) proxy and
1333   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1334   compliance with the protocol would have necessitated a possibly
1335   infinite buffer on the proxy.
1336</t>
1337<t>
1338   A process for decoding the "chunked" transfer-coding
1339   can be represented in pseudo-code as:
1340</t>
1341<figure><artwork type="code">
1342    length := 0
1343    read chunk-size, chunk-extension (if any) and CRLF
1344    while (chunk-size &gt; 0) {
1345       read chunk-data and CRLF
1346       append chunk-data to entity-body
1347       length := length + chunk-size
1348       read chunk-size and CRLF
1349    }
1350    read entity-header
1351    while (entity-header not empty) {
1352       append entity-header to existing header fields
1353       read entity-header
1354    }
1355    Content-Length := length
1356    Remove "chunked" from Transfer-Encoding
1357</artwork></figure>
1358<t>
1359   All HTTP/1.1 applications &MUST; be able to receive and decode the
1360   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1361   they do not understand.
1362</t>
1363</section>
1364</section>
1365
1366<section title="Product Tokens" anchor="product.tokens">
1367  <x:anchor-alias value="product"/>
1368  <x:anchor-alias value="product-version"/>
1369<t>
1370   Product tokens are used to allow communicating applications to
1371   identify themselves by software name and version. Most fields using
1372   product tokens also allow sub-products which form a significant part
1373   of the application to be listed, separated by white space. By
1374   convention, the products are listed in order of their significance
1375   for identifying the application.
1376</t>
1377<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1378  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
1379  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
1380</artwork></figure>
1381<t>
1382   Examples:
1383</t>
1384<figure><artwork type="example">
1385    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1386    Server: Apache/0.8.4
1387</artwork></figure>
1388<t>
1389   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1390   used for advertising or other non-essential information. Although any
1391   token character &MAY; appear in a product-version, this token &SHOULD;
1392   only be used for a version identifier (i.e., successive versions of
1393   the same product &SHOULD; only differ in the product-version portion of
1394   the product value).
1395</t>
1396</section>
1397
1398</section>
1399
1400<section title="HTTP Message" anchor="http.message">
1401
1402<section title="Message Types" anchor="message.types">
1403  <x:anchor-alias value="generic-message"/>
1404  <x:anchor-alias value="HTTP-message"/>
1405  <x:anchor-alias value="start-line"/>
1406<t>
1407   HTTP messages consist of requests from client to server and responses
1408   from server to client.
1409</t>
1410<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1411  <x:ref>HTTP-message</x:ref>   = <x:ref>Request</x:ref> / <x:ref>Response</x:ref>     ; HTTP/1.1 messages
1412</artwork></figure>
1413<t>
1414   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1415   message format of <xref target="RFC5322"/> for transferring entities (the payload
1416   of the message). Both types of message consist of a start-line, zero
1417   or more header fields (also known as "headers"), an empty line (i.e.,
1418   a line with nothing preceding the CRLF) indicating the end of the
1419   header fields, and possibly a message-body.
1420</t>
1421<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1422  <x:ref>generic-message</x:ref> = <x:ref>start-line</x:ref>
1423                    *(<x:ref>message-header</x:ref> <x:ref>CRLF</x:ref>)
1424                    <x:ref>CRLF</x:ref>
1425                    [ <x:ref>message-body</x:ref> ]
1426  <x:ref>start-line</x:ref>      = <x:ref>Request-Line</x:ref> / <x:ref>Status-Line</x:ref>
1427</artwork></figure>
1428<t>
1429   In the interest of robustness, servers &SHOULD; ignore any empty
1430   line(s) received where a Request-Line is expected. In other words, if
1431   the server is reading the protocol stream at the beginning of a
1432   message and receives a CRLF first, it should ignore the CRLF.
1433</t>
1434<t>
1435   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1436   after a POST request. To restate what is explicitly forbidden by the
1437   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1438   extra CRLF.
1439</t>
1440</section>
1441
1442<section title="Message Headers" anchor="message.headers">
1443  <x:anchor-alias value="field-content"/>
1444  <x:anchor-alias value="field-name"/>
1445  <x:anchor-alias value="field-value"/>
1446  <x:anchor-alias value="message-header"/>
1447<t>
1448   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1449   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1450   entity-header (&entity-header-fields;) fields, follow the same generic format as
1451   that given in <xref target="RFC5322" x:fmt="of" x:sec="2.1"/>. Each header field consists
1452   of a name followed by a colon (":") and the field value. Field names
1453   are case-insensitive. The field value &MAY; be preceded by any amount
1454   of LWS, though a single SP is preferred. Header fields can be
1455   extended over multiple lines by preceding each extra line with at
1456   least one SP or HTAB. Applications ought to follow "common form", where
1457   one is known or indicated, when generating HTTP constructs, since
1458   there might exist some implementations that fail to accept anything
1459   beyond the common forms.
1460</t>
1461<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"/>
1462  <x:ref>message-header</x:ref> = <x:ref>field-name</x:ref> ":" [ <x:ref>field-value</x:ref> ]
1463  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1464  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>LWS</x:ref> )
1465  <x:ref>field-content</x:ref>  = &lt;field content&gt;
1466                   ; the <x:ref>OCTET</x:ref>s making up the field-value
1467                   ; and consisting of either *<x:ref>TEXT</x:ref> or combinations
1468                   ; of <x:ref>token</x:ref>, <x:ref>separators</x:ref>, and <x:ref>quoted-string</x:ref>
1469</artwork></figure>
1470<t>
1471   The field-content does not include any leading or trailing LWS:
1472   linear white space occurring before the first non-whitespace
1473   character of the field-value or after the last non-whitespace
1474   character of the field-value. Such leading or trailing LWS &MAY; be
1475   removed without changing the semantics of the field value. Any LWS
1476   that occurs between field-content &MAY; be replaced with a single SP
1477   before interpreting the field value or forwarding the message
1478   downstream.
1479</t>
1480<t>
1481   The order in which header fields with differing field names are
1482   received is not significant. However, it is "good practice" to send
1483   general-header fields first, followed by request-header or response-header
1484   fields, and ending with the entity-header fields.
1485</t>
1486<t>
1487   Multiple message-header fields with the same field-name &MAY; be
1488   present in a message if and only if the entire field-value for that
1489   header field is defined as a comma-separated list [i.e., #(values)].
1490   It &MUST; be possible to combine the multiple header fields into one
1491   "field-name: field-value" pair, without changing the semantics of the
1492   message, by appending each subsequent field-value to the first, each
1493   separated by a comma. The order in which header fields with the same
1494   field-name are received is therefore significant to the
1495   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1496   change the order of these field values when a message is forwarded.
1497</t>
1498<t>
1499  <list><t>
1500   <x:h>Note:</x:h> the "Set-Cookie" header as implemented in
1501   practice (as opposed to how it is specified in <xref target="RFC2109"/>)
1502   can occur multiple times, but does not use the list syntax, and thus cannot
1503   be combined into a single line. (See Appendix A.2.3 of <xref target="Kri2001"/>
1504   for details.) Also note that the Set-Cookie2 header specified in
1505   <xref target="RFC2965"/> does not share this problem.
1506  </t></list>
1507</t>
1508 
1509</section>
1510
1511<section title="Message Body" anchor="message.body">
1512  <x:anchor-alias value="message-body"/>
1513<t>
1514   The message-body (if any) of an HTTP message is used to carry the
1515   entity-body associated with the request or response. The message-body
1516   differs from the entity-body only when a transfer-coding has been
1517   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1518</t>
1519<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1520  <x:ref>message-body</x:ref> = <x:ref>entity-body</x:ref>
1521               / &lt;entity-body encoded as per <x:ref>Transfer-Encoding</x:ref>&gt;
1522</artwork></figure>
1523<t>
1524   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1525   applied by an application to ensure safe and proper transfer of the
1526   message. Transfer-Encoding is a property of the message, not of the
1527   entity, and thus &MAY; be added or removed by any application along the
1528   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1529   when certain transfer-codings may be used.)
1530</t>
1531<t>
1532   The rules for when a message-body is allowed in a message differ for
1533   requests and responses.
1534</t>
1535<t>
1536   The presence of a message-body in a request is signaled by the
1537   inclusion of a Content-Length or Transfer-Encoding header field in
1538   the request's message-headers. A message-body &MUST-NOT; be included in
1539   a request if the specification of the request method (&method;)
1540   explicitly disallows an entity-body in requests.
1541   When a request message contains both a message-body of non-zero
1542   length and a method that does not define any semantics for that
1543   request message-body, then an origin server &SHOULD; either ignore
1544   the message-body or respond with an appropriate error message
1545   (e.g., 413).  A proxy or gateway, when presented the same request,
1546   &SHOULD; either forward the request inbound with the message-body or
1547   ignore the message-body when determining a response.
1548</t>
1549<t>
1550   For response messages, whether or not a message-body is included with
1551   a message is dependent on both the request method and the response
1552   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1553   &MUST-NOT; include a message-body, even though the presence of entity-header
1554   fields might lead one to believe they do. All 1xx
1555   (informational), 204 (No Content), and 304 (Not Modified) responses
1556   &MUST-NOT; include a message-body. All other responses do include a
1557   message-body, although it &MAY; be of zero length.
1558</t>
1559</section>
1560
1561<section title="Message Length" anchor="message.length">
1562<t>
1563   The transfer-length of a message is the length of the message-body as
1564   it appears in the message; that is, after any transfer-codings have
1565   been applied. When a message-body is included with a message, the
1566   transfer-length of that body is determined by one of the following
1567   (in order of precedence):
1568</t>
1569<t>
1570  <list style="numbers">
1571    <x:lt><t>
1572     Any response message which "&MUST-NOT;" include a message-body (such
1573     as the 1xx, 204, and 304 responses and any response to a HEAD
1574     request) is always terminated by the first empty line after the
1575     header fields, regardless of the entity-header fields present in
1576     the message.
1577    </t></x:lt>
1578    <x:lt><t>
1579     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1580     is present and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
1581     is used, the transfer-length is defined by the use of this transfer-coding.
1582     If a Transfer-Encoding header field is present and the "chunked" transfer-coding
1583     is not present, the transfer-length is defined by the sender closing the connection.
1584    </t></x:lt>
1585    <x:lt><t>
1586     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1587     decimal value in OCTETs represents both the entity-length and the
1588     transfer-length. The Content-Length header field &MUST-NOT; be sent
1589     if these two lengths are different (i.e., if a Transfer-Encoding
1590     header field is present). If a message is received with both a
1591     Transfer-Encoding header field and a Content-Length header field,
1592     the latter &MUST; be ignored.
1593    </t></x:lt>
1594    <x:lt><t>
1595     If the message uses the media type "multipart/byteranges", and the
1596     transfer-length is not otherwise specified, then this self-delimiting
1597     media type defines the transfer-length. This media type
1598     &MUST-NOT; be used unless the sender knows that the recipient can parse
1599     it; the presence in a request of a Range header with multiple byte-range
1600     specifiers from a 1.1 client implies that the client can parse
1601     multipart/byteranges responses.
1602    <list style="empty"><t>
1603       A range header might be forwarded by a 1.0 proxy that does not
1604       understand multipart/byteranges; in this case the server &MUST;
1605       delimit the message using methods defined in items 1, 3 or 5 of
1606       this section.
1607    </t></list>
1608    </t></x:lt>
1609    <x:lt><t>
1610     By the server closing the connection. (Closing the connection
1611     cannot be used to indicate the end of a request body, since that
1612     would leave no possibility for the server to send back a response.)
1613    </t></x:lt>
1614  </list>
1615</t>
1616<t>
1617   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1618   containing a message-body &MUST; include a valid Content-Length header
1619   field unless the server is known to be HTTP/1.1 compliant. If a
1620   request contains a message-body and a Content-Length is not given,
1621   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1622   determine the length of the message, or with 411 (Length Required) if
1623   it wishes to insist on receiving a valid Content-Length.
1624</t>
1625<t>
1626   All HTTP/1.1 applications that receive entities &MUST; accept the
1627   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1628   to be used for messages when the message length cannot be determined
1629   in advance.
1630</t>
1631<t>
1632   Messages &MUST-NOT; include both a Content-Length header field and a
1633   transfer-coding. If the message does include a
1634   transfer-coding, the Content-Length &MUST; be ignored.
1635</t>
1636<t>
1637   When a Content-Length is given in a message where a message-body is
1638   allowed, its field value &MUST; exactly match the number of OCTETs in
1639   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1640   invalid length is received and detected.
1641</t>
1642</section>
1643
1644<section title="General Header Fields" anchor="general.header.fields">
1645  <x:anchor-alias value="general-header"/>
1646<t>
1647   There are a few header fields which have general applicability for
1648   both request and response messages, but which do not apply to the
1649   entity being transferred. These header fields apply only to the
1650   message being transmitted.
1651</t>
1652<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1653  <x:ref>general-header</x:ref> = <x:ref>Cache-Control</x:ref>            ; &header-cache-control;
1654                 / <x:ref>Connection</x:ref>               ; <xref target="header.connection"/>
1655                 / <x:ref>Date</x:ref>                     ; <xref target="header.date"/>
1656                 / <x:ref>Pragma</x:ref>                   ; &header-pragma;
1657                 / <x:ref>Trailer</x:ref>                  ; <xref target="header.trailer"/>
1658                 / <x:ref>Transfer-Encoding</x:ref>        ; <xref target="header.transfer-encoding"/>
1659                 / <x:ref>Upgrade</x:ref>                  ; <xref target="header.upgrade"/>
1660                 / <x:ref>Via</x:ref>                      ; <xref target="header.via"/>
1661                 / <x:ref>Warning</x:ref>                  ; &header-warning;
1662</artwork></figure>
1663<t>
1664   General-header field names can be extended reliably only in
1665   combination with a change in the protocol version. However, new or
1666   experimental header fields may be given the semantics of general
1667   header fields if all parties in the communication recognize them to
1668   be general-header fields. Unrecognized header fields are treated as
1669   entity-header fields.
1670</t>
1671</section>
1672</section>
1673
1674<section title="Request" anchor="request">
1675  <x:anchor-alias value="Request"/>
1676<t>
1677   A request message from a client to a server includes, within the
1678   first line of that message, the method to be applied to the resource,
1679   the identifier of the resource, and the protocol version in use.
1680</t>
1681<!--                 Host                      ; should be moved here eventually -->
1682<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1683  <x:ref>Request</x:ref>       = <x:ref>Request-Line</x:ref>              ; <xref target="request-line"/>
1684                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1685                   / <x:ref>request-header</x:ref>         ; &request-header-fields;
1686                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1687                  <x:ref>CRLF</x:ref>
1688                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1689</artwork></figure>
1690
1691<section title="Request-Line" anchor="request-line">
1692  <x:anchor-alias value="Request-Line"/>
1693<t>
1694   The Request-Line begins with a method token, followed by the
1695   Request-URI and the protocol version, and ending with CRLF. The
1696   elements are separated by SP characters. No CR or LF is allowed
1697   except in the final CRLF sequence.
1698</t>
1699<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1700  <x:ref>Request-Line</x:ref>   = <x:ref>Method</x:ref> <x:ref>SP</x:ref> <x:ref>Request-URI</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-Version</x:ref> <x:ref>CRLF</x:ref>
1701</artwork></figure>
1702
1703<section title="Method" anchor="method">
1704  <x:anchor-alias value="Method"/>
1705<t>
1706   The Method  token indicates the method to be performed on the
1707   resource identified by the Request-URI. The method is case-sensitive.
1708</t>
1709<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1710  <x:ref>Method</x:ref>         = <x:ref>token</x:ref>
1711</artwork></figure>
1712</section>
1713
1714<section title="Request-URI" anchor="request-uri">
1715  <x:anchor-alias value="Request-URI"/>
1716<t>
1717   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1718   identifies the resource upon which to apply the request.
1719</t>
1720<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1721  <x:ref>Request-URI</x:ref>    = "*"
1722                 / <x:ref>absoluteURI</x:ref>
1723                 / ( <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ] )
1724                 / <x:ref>authority</x:ref>
1725</artwork></figure>
1726<t>
1727   The four options for Request-URI are dependent on the nature of the
1728   request. The asterisk "*" means that the request does not apply to a
1729   particular resource, but to the server itself, and is only allowed
1730   when the method used does not necessarily apply to a resource. One
1731   example would be
1732</t>
1733<figure><artwork type="example">
1734    OPTIONS * HTTP/1.1
1735</artwork></figure>
1736<t>
1737   The absoluteURI form is &REQUIRED; when the request is being made to a
1738   proxy. The proxy is requested to forward the request or service it
1739   from a valid cache, and return the response. Note that the proxy &MAY;
1740   forward the request on to another proxy or directly to the server
1741   specified by the absoluteURI. In order to avoid request loops, a
1742   proxy &MUST; be able to recognize all of its server names, including
1743   any aliases, local variations, and the numeric IP address. An example
1744   Request-Line would be:
1745</t>
1746<figure><artwork type="example">
1747    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1748</artwork></figure>
1749<t>
1750   To allow for transition to absoluteURIs in all requests in future
1751   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1752   form in requests, even though HTTP/1.1 clients will only generate
1753   them in requests to proxies.
1754</t>
1755<t>
1756   The authority form is only used by the CONNECT method (&CONNECT;).
1757</t>
1758<t>
1759   The most common form of Request-URI is that used to identify a
1760   resource on an origin server or gateway. In this case the absolute
1761   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1762   the Request-URI, and the network location of the URI (authority) &MUST;
1763   be transmitted in a Host header field. For example, a client wishing
1764   to retrieve the resource above directly from the origin server would
1765   create a TCP connection to port 80 of the host "www.example.org" and send
1766   the lines:
1767</t>
1768<figure><artwork type="example">
1769    GET /pub/WWW/TheProject.html HTTP/1.1
1770    Host: www.example.org
1771</artwork></figure>
1772<t>
1773   followed by the remainder of the Request. Note that the absolute path
1774   cannot be empty; if none is present in the original URI, it &MUST; be
1775   given as "/" (the server root).
1776</t>
1777<t>
1778   The Request-URI is transmitted in the format specified in
1779   <xref target="general.syntax"/>. If the Request-URI is encoded using the
1780   "% <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding
1781   (<xref target="RFC2396" x:fmt="," x:sec="2.4.1"/>), the origin server
1782   &MUST; decode the Request-URI in order to
1783   properly interpret the request. Servers &SHOULD; respond to invalid
1784   Request-URIs with an appropriate status code.
1785</t>
1786<t>
1787   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1788   received Request-URI when forwarding it to the next inbound server,
1789   except as noted above to replace a null path-absolute with "/".
1790</t>
1791<t>
1792  <list><t>
1793      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1794      meaning of the request when the origin server is improperly using
1795      a non-reserved URI character for a reserved purpose.  Implementors
1796      should be aware that some pre-HTTP/1.1 proxies have been known to
1797      rewrite the Request-URI.
1798  </t></list>
1799</t>
1800</section>
1801</section>
1802
1803<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1804<t>
1805   The exact resource identified by an Internet request is determined by
1806   examining both the Request-URI and the Host header field.
1807</t>
1808<t>
1809   An origin server that does not allow resources to differ by the
1810   requested host &MAY; ignore the Host header field value when
1811   determining the resource identified by an HTTP/1.1 request. (But see
1812   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1813   for other requirements on Host support in HTTP/1.1.)
1814</t>
1815<t>
1816   An origin server that does differentiate resources based on the host
1817   requested (sometimes referred to as virtual hosts or vanity host
1818   names) &MUST; use the following rules for determining the requested
1819   resource on an HTTP/1.1 request:
1820  <list style="numbers">
1821    <t>If Request-URI is an absoluteURI, the host is part of the
1822     Request-URI. Any Host header field value in the request &MUST; be
1823     ignored.</t>
1824    <t>If the Request-URI is not an absoluteURI, and the request includes
1825     a Host header field, the host is determined by the Host header
1826     field value.</t>
1827    <t>If the host as determined by rule 1 or 2 is not a valid host on
1828     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1829  </list>
1830</t>
1831<t>
1832   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1833   attempt to use heuristics (e.g., examination of the URI path for
1834   something unique to a particular host) in order to determine what
1835   exact resource is being requested.
1836</t>
1837</section>
1838
1839</section>
1840
1841
1842<section title="Response" anchor="response">
1843  <x:anchor-alias value="Response"/>
1844<t>
1845   After receiving and interpreting a request message, a server responds
1846   with an HTTP response message.
1847</t>
1848<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1849  <x:ref>Response</x:ref>      = <x:ref>Status-Line</x:ref>               ; <xref target="status-line"/>
1850                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1851                   / <x:ref>response-header</x:ref>        ; &response-header-fields;
1852                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1853                  <x:ref>CRLF</x:ref>
1854                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1855</artwork></figure>
1856
1857<section title="Status-Line" anchor="status-line">
1858  <x:anchor-alias value="Status-Line"/>
1859<t>
1860   The first line of a Response message is the Status-Line, consisting
1861   of the protocol version followed by a numeric status code and its
1862   associated textual phrase, with each element separated by SP
1863   characters. No CR or LF is allowed except in the final CRLF sequence.
1864</t>
1865<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1866  <x:ref>Status-Line</x:ref> = <x:ref>HTTP-Version</x:ref> <x:ref>SP</x:ref> <x:ref>Status-Code</x:ref> <x:ref>SP</x:ref> <x:ref>Reason-Phrase</x:ref> <x:ref>CRLF</x:ref>
1867</artwork></figure>
1868
1869<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1870  <x:anchor-alias value="Reason-Phrase"/>
1871  <x:anchor-alias value="Status-Code"/>
1872<t>
1873   The Status-Code element is a 3-digit integer result code of the
1874   attempt to understand and satisfy the request. These codes are fully
1875   defined in &status-codes;.  The Reason Phrase exists for the sole
1876   purpose of providing a textual description associated with the numeric
1877   status code, out of deference to earlier Internet application protocols
1878   that were more frequently used with interactive text clients.
1879   A client &SHOULD; ignore the content of the Reason Phrase.
1880</t>
1881<t>
1882   The first digit of the Status-Code defines the class of response. The
1883   last two digits do not have any categorization role. There are 5
1884   values for the first digit:
1885  <list style="symbols">
1886    <t>
1887      1xx: Informational - Request received, continuing process
1888    </t>
1889    <t>
1890      2xx: Success - The action was successfully received,
1891        understood, and accepted
1892    </t>
1893    <t>
1894      3xx: Redirection - Further action must be taken in order to
1895        complete the request
1896    </t>
1897    <t>
1898      4xx: Client Error - The request contains bad syntax or cannot
1899        be fulfilled
1900    </t>
1901    <t>
1902      5xx: Server Error - The server failed to fulfill an apparently
1903        valid request
1904    </t>
1905  </list>
1906</t>
1907<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"/>
1908  <x:ref>Status-Code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1909  <x:ref>Reason-Phrase</x:ref>  = *&lt;<x:ref>TEXT</x:ref>, excluding <x:ref>CR</x:ref>, <x:ref>LF</x:ref>&gt;
1910</artwork></figure>
1911</section>
1912</section>
1913
1914</section>
1915
1916
1917<section title="Connections" anchor="connections">
1918
1919<section title="Persistent Connections" anchor="persistent.connections">
1920
1921<section title="Purpose" anchor="persistent.purpose">
1922<t>
1923   Prior to persistent connections, a separate TCP connection was
1924   established to fetch each URL, increasing the load on HTTP servers
1925   and causing congestion on the Internet. The use of inline images and
1926   other associated data often require a client to make multiple
1927   requests of the same server in a short amount of time. Analysis of
1928   these performance problems and results from a prototype
1929   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1930   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1931   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1932   T/TCP <xref target="Tou1998"/>.
1933</t>
1934<t>
1935   Persistent HTTP connections have a number of advantages:
1936  <list style="symbols">
1937      <t>
1938        By opening and closing fewer TCP connections, CPU time is saved
1939        in routers and hosts (clients, servers, proxies, gateways,
1940        tunnels, or caches), and memory used for TCP protocol control
1941        blocks can be saved in hosts.
1942      </t>
1943      <t>
1944        HTTP requests and responses can be pipelined on a connection.
1945        Pipelining allows a client to make multiple requests without
1946        waiting for each response, allowing a single TCP connection to
1947        be used much more efficiently, with much lower elapsed time.
1948      </t>
1949      <t>
1950        Network congestion is reduced by reducing the number of packets
1951        caused by TCP opens, and by allowing TCP sufficient time to
1952        determine the congestion state of the network.
1953      </t>
1954      <t>
1955        Latency on subsequent requests is reduced since there is no time
1956        spent in TCP's connection opening handshake.
1957      </t>
1958      <t>
1959        HTTP can evolve more gracefully, since errors can be reported
1960        without the penalty of closing the TCP connection. Clients using
1961        future versions of HTTP might optimistically try a new feature,
1962        but if communicating with an older server, retry with old
1963        semantics after an error is reported.
1964      </t>
1965    </list>
1966</t>
1967<t>
1968   HTTP implementations &SHOULD; implement persistent connections.
1969</t>
1970</section>
1971
1972<section title="Overall Operation" anchor="persistent.overall">
1973<t>
1974   A significant difference between HTTP/1.1 and earlier versions of
1975   HTTP is that persistent connections are the default behavior of any
1976   HTTP connection. That is, unless otherwise indicated, the client
1977   &SHOULD; assume that the server will maintain a persistent connection,
1978   even after error responses from the server.
1979</t>
1980<t>
1981   Persistent connections provide a mechanism by which a client and a
1982   server can signal the close of a TCP connection. This signaling takes
1983   place using the Connection header field (<xref target="header.connection"/>). Once a close
1984   has been signaled, the client &MUST-NOT; send any more requests on that
1985   connection.
1986</t>
1987
1988<section title="Negotiation" anchor="persistent.negotiation">
1989<t>
1990   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1991   maintain a persistent connection unless a Connection header including
1992   the connection-token "close" was sent in the request. If the server
1993   chooses to close the connection immediately after sending the
1994   response, it &SHOULD; send a Connection header including the
1995   connection-token close.
1996</t>
1997<t>
1998   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1999   decide to keep it open based on whether the response from a server
2000   contains a Connection header with the connection-token close. In case
2001   the client does not want to maintain a connection for more than that
2002   request, it &SHOULD; send a Connection header including the
2003   connection-token close.
2004</t>
2005<t>
2006   If either the client or the server sends the close token in the
2007   Connection header, that request becomes the last one for the
2008   connection.
2009</t>
2010<t>
2011   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
2012   maintained for HTTP versions less than 1.1 unless it is explicitly
2013   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
2014   compatibility with HTTP/1.0 clients.
2015</t>
2016<t>
2017   In order to remain persistent, all messages on the connection &MUST;
2018   have a self-defined message length (i.e., one not defined by closure
2019   of the connection), as described in <xref target="message.length"/>.
2020</t>
2021</section>
2022
2023<section title="Pipelining" anchor="pipelining">
2024<t>
2025   A client that supports persistent connections &MAY; "pipeline" its
2026   requests (i.e., send multiple requests without waiting for each
2027   response). A server &MUST; send its responses to those requests in the
2028   same order that the requests were received.
2029</t>
2030<t>
2031   Clients which assume persistent connections and pipeline immediately
2032   after connection establishment &SHOULD; be prepared to retry their
2033   connection if the first pipelined attempt fails. If a client does
2034   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2035   persistent. Clients &MUST; also be prepared to resend their requests if
2036   the server closes the connection before sending all of the
2037   corresponding responses.
2038</t>
2039<t>
2040   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
2041   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
2042   premature termination of the transport connection could lead to
2043   indeterminate results. A client wishing to send a non-idempotent
2044   request &SHOULD; wait to send that request until it has received the
2045   response status for the previous request.
2046</t>
2047</section>
2048</section>
2049
2050<section title="Proxy Servers" anchor="persistent.proxy">
2051<t>
2052   It is especially important that proxies correctly implement the
2053   properties of the Connection header field as specified in <xref target="header.connection"/>.
2054</t>
2055<t>
2056   The proxy server &MUST; signal persistent connections separately with
2057   its clients and the origin servers (or other proxy servers) that it
2058   connects to. Each persistent connection applies to only one transport
2059   link.
2060</t>
2061<t>
2062   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2063   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
2064   discussion of the problems with the Keep-Alive header implemented by
2065   many HTTP/1.0 clients).
2066</t>
2067</section>
2068
2069<section title="Practical Considerations" anchor="persistent.practical">
2070<t>
2071   Servers will usually have some time-out value beyond which they will
2072   no longer maintain an inactive connection. Proxy servers might make
2073   this a higher value since it is likely that the client will be making
2074   more connections through the same server. The use of persistent
2075   connections places no requirements on the length (or existence) of
2076   this time-out for either the client or the server.
2077</t>
2078<t>
2079   When a client or server wishes to time-out it &SHOULD; issue a graceful
2080   close on the transport connection. Clients and servers &SHOULD; both
2081   constantly watch for the other side of the transport close, and
2082   respond to it as appropriate. If a client or server does not detect
2083   the other side's close promptly it could cause unnecessary resource
2084   drain on the network.
2085</t>
2086<t>
2087   A client, server, or proxy &MAY; close the transport connection at any
2088   time. For example, a client might have started to send a new request
2089   at the same time that the server has decided to close the "idle"
2090   connection. From the server's point of view, the connection is being
2091   closed while it was idle, but from the client's point of view, a
2092   request is in progress.
2093</t>
2094<t>
2095   This means that clients, servers, and proxies &MUST; be able to recover
2096   from asynchronous close events. Client software &SHOULD; reopen the
2097   transport connection and retransmit the aborted sequence of requests
2098   without user interaction so long as the request sequence is
2099   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2100   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2101   human operator the choice of retrying the request(s). Confirmation by
2102   user-agent software with semantic understanding of the application
2103   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2104   be repeated if the second sequence of requests fails.
2105</t>
2106<t>
2107   Servers &SHOULD; always respond to at least one request per connection,
2108   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2109   middle of transmitting a response, unless a network or client failure
2110   is suspected.
2111</t>
2112<t>
2113   Clients that use persistent connections &SHOULD; limit the number of
2114   simultaneous connections that they maintain to a given server. A
2115   single-user client &SHOULD-NOT; maintain more than 2 connections with
2116   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2117   another server or proxy, where N is the number of simultaneously
2118   active users. These guidelines are intended to improve HTTP response
2119   times and avoid congestion.
2120</t>
2121</section>
2122</section>
2123
2124<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2125
2126<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2127<t>
2128   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2129   flow control mechanisms to resolve temporary overloads, rather than
2130   terminating connections with the expectation that clients will retry.
2131   The latter technique can exacerbate network congestion.
2132</t>
2133</section>
2134
2135<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2136<t>
2137   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2138   the network connection for an error status while it is transmitting
2139   the request. If the client sees an error status, it &SHOULD;
2140   immediately cease transmitting the body. If the body is being sent
2141   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2142   empty trailer &MAY; be used to prematurely mark the end of the message.
2143   If the body was preceded by a Content-Length header, the client &MUST;
2144   close the connection.
2145</t>
2146</section>
2147
2148<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2149<t>
2150   The purpose of the 100 (Continue) status (see &status-100;) is to
2151   allow a client that is sending a request message with a request body
2152   to determine if the origin server is willing to accept the request
2153   (based on the request headers) before the client sends the request
2154   body. In some cases, it might either be inappropriate or highly
2155   inefficient for the client to send the body if the server will reject
2156   the message without looking at the body.
2157</t>
2158<t>
2159   Requirements for HTTP/1.1 clients:
2160  <list style="symbols">
2161    <t>
2162        If a client will wait for a 100 (Continue) response before
2163        sending the request body, it &MUST; send an Expect request-header
2164        field (&header-expect;) with the "100-continue" expectation.
2165    </t>
2166    <t>
2167        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2168        with the "100-continue" expectation if it does not intend
2169        to send a request body.
2170    </t>
2171  </list>
2172</t>
2173<t>
2174   Because of the presence of older implementations, the protocol allows
2175   ambiguous situations in which a client may send "Expect: 100-continue"
2176   without receiving either a 417 (Expectation Failed) status
2177   or a 100 (Continue) status. Therefore, when a client sends this
2178   header field to an origin server (possibly via a proxy) from which it
2179   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2180   for an indefinite period before sending the request body.
2181</t>
2182<t>
2183   Requirements for HTTP/1.1 origin servers:
2184  <list style="symbols">
2185    <t> Upon receiving a request which includes an Expect request-header
2186        field with the "100-continue" expectation, an origin server &MUST;
2187        either respond with 100 (Continue) status and continue to read
2188        from the input stream, or respond with a final status code. The
2189        origin server &MUST-NOT; wait for the request body before sending
2190        the 100 (Continue) response. If it responds with a final status
2191        code, it &MAY; close the transport connection or it &MAY; continue
2192        to read and discard the rest of the request.  It &MUST-NOT;
2193        perform the requested method if it returns a final status code.
2194    </t>
2195    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2196        the request message does not include an Expect request-header
2197        field with the "100-continue" expectation, and &MUST-NOT; send a
2198        100 (Continue) response if such a request comes from an HTTP/1.0
2199        (or earlier) client. There is an exception to this rule: for
2200        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2201        status in response to an HTTP/1.1 PUT or POST request that does
2202        not include an Expect request-header field with the "100-continue"
2203        expectation. This exception, the purpose of which is
2204        to minimize any client processing delays associated with an
2205        undeclared wait for 100 (Continue) status, applies only to
2206        HTTP/1.1 requests, and not to requests with any other HTTP-version
2207        value.
2208    </t>
2209    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2210        already received some or all of the request body for the
2211        corresponding request.
2212    </t>
2213    <t> An origin server that sends a 100 (Continue) response &MUST;
2214    ultimately send a final status code, once the request body is
2215        received and processed, unless it terminates the transport
2216        connection prematurely.
2217    </t>
2218    <t> If an origin server receives a request that does not include an
2219        Expect request-header field with the "100-continue" expectation,
2220        the request includes a request body, and the server responds
2221        with a final status code before reading the entire request body
2222        from the transport connection, then the server &SHOULD-NOT;  close
2223        the transport connection until it has read the entire request,
2224        or until the client closes the connection. Otherwise, the client
2225        might not reliably receive the response message. However, this
2226        requirement is not be construed as preventing a server from
2227        defending itself against denial-of-service attacks, or from
2228        badly broken client implementations.
2229      </t>
2230    </list>
2231</t>
2232<t>
2233   Requirements for HTTP/1.1 proxies:
2234  <list style="symbols">
2235    <t> If a proxy receives a request that includes an Expect request-header
2236        field with the "100-continue" expectation, and the proxy
2237        either knows that the next-hop server complies with HTTP/1.1 or
2238        higher, or does not know the HTTP version of the next-hop
2239        server, it &MUST; forward the request, including the Expect header
2240        field.
2241    </t>
2242    <t> If the proxy knows that the version of the next-hop server is
2243        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2244        respond with a 417 (Expectation Failed) status.
2245    </t>
2246    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2247        numbers received from recently-referenced next-hop servers.
2248    </t>
2249    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2250        request message was received from an HTTP/1.0 (or earlier)
2251        client and did not include an Expect request-header field with
2252        the "100-continue" expectation. This requirement overrides the
2253        general rule for forwarding of 1xx responses (see &status-1xx;).
2254    </t>
2255  </list>
2256</t>
2257</section>
2258
2259<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2260<t>
2261   If an HTTP/1.1 client sends a request which includes a request body,
2262   but which does not include an Expect request-header field with the
2263   "100-continue" expectation, and if the client is not directly
2264   connected to an HTTP/1.1 origin server, and if the client sees the
2265   connection close before receiving any status from the server, the
2266   client &SHOULD; retry the request.  If the client does retry this
2267   request, it &MAY; use the following "binary exponential backoff"
2268   algorithm to be assured of obtaining a reliable response:
2269  <list style="numbers">
2270    <t>
2271      Initiate a new connection to the server
2272    </t>
2273    <t>
2274      Transmit the request-headers
2275    </t>
2276    <t>
2277      Initialize a variable R to the estimated round-trip time to the
2278         server (e.g., based on the time it took to establish the
2279         connection), or to a constant value of 5 seconds if the round-trip
2280         time is not available.
2281    </t>
2282    <t>
2283       Compute T = R * (2**N), where N is the number of previous
2284         retries of this request.
2285    </t>
2286    <t>
2287       Wait either for an error response from the server, or for T
2288         seconds (whichever comes first)
2289    </t>
2290    <t>
2291       If no error response is received, after T seconds transmit the
2292         body of the request.
2293    </t>
2294    <t>
2295       If client sees that the connection is closed prematurely,
2296         repeat from step 1 until the request is accepted, an error
2297         response is received, or the user becomes impatient and
2298         terminates the retry process.
2299    </t>
2300  </list>
2301</t>
2302<t>
2303   If at any point an error status is received, the client
2304  <list style="symbols">
2305      <t>&SHOULD-NOT;  continue and</t>
2306
2307      <t>&SHOULD; close the connection if it has not completed sending the
2308        request message.</t>
2309    </list>
2310</t>
2311</section>
2312</section>
2313</section>
2314
2315
2316<section title="Header Field Definitions" anchor="header.fields">
2317<t>
2318   This section defines the syntax and semantics of HTTP/1.1 header fields
2319   related to message framing and transport protocols.
2320</t>
2321<t>
2322   For entity-header fields, both sender and recipient refer to either the
2323   client or the server, depending on who sends and who receives the entity.
2324</t>
2325
2326<section title="Connection" anchor="header.connection">
2327  <iref primary="true" item="Connection header" x:for-anchor=""/>
2328  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2329  <x:anchor-alias value="Connection"/>
2330  <x:anchor-alias value="connection-token"/>
2331<t>
2332   The Connection general-header field allows the sender to specify
2333   options that are desired for that particular connection and &MUST-NOT;
2334   be communicated by proxies over further connections.
2335</t>
2336<t>
2337   The Connection header has the following grammar:
2338</t>
2339<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2340  <x:ref>Connection</x:ref> = "Connection" ":" 1#(<x:ref>connection-token</x:ref>)
2341  <x:ref>connection-token</x:ref>  = <x:ref>token</x:ref>
2342</artwork></figure>
2343<t>
2344   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2345   message is forwarded and, for each connection-token in this field,
2346   remove any header field(s) from the message with the same name as the
2347   connection-token. Connection options are signaled by the presence of
2348   a connection-token in the Connection header field, not by any
2349   corresponding additional header field(s), since the additional header
2350   field may not be sent if there are no parameters associated with that
2351   connection option.
2352</t>
2353<t>
2354   Message headers listed in the Connection header &MUST-NOT; include
2355   end-to-end headers, such as Cache-Control.
2356</t>
2357<t>
2358   HTTP/1.1 defines the "close" connection option for the sender to
2359   signal that the connection will be closed after completion of the
2360   response. For example,
2361</t>
2362<figure><artwork type="example">
2363    Connection: close
2364</artwork></figure>
2365<t>
2366   in either the request or the response header fields indicates that
2367   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2368   after the current request/response is complete.
2369</t>
2370<t>
2371   An HTTP/1.1 client that does not support persistent connections &MUST;
2372   include the "close" connection option in every request message.
2373</t>
2374<t>
2375   An HTTP/1.1 server that does not support persistent connections &MUST;
2376   include the "close" connection option in every response message that
2377   does not have a 1xx (informational) status code.
2378</t>
2379<t>
2380   A system receiving an HTTP/1.0 (or lower-version) message that
2381   includes a Connection header &MUST;, for each connection-token in this
2382   field, remove and ignore any header field(s) from the message with
2383   the same name as the connection-token. This protects against mistaken
2384   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2385</t>
2386</section>
2387
2388<section title="Content-Length" anchor="header.content-length">
2389  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2390  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2391  <x:anchor-alias value="Content-Length"/>
2392<t>
2393   The Content-Length entity-header field indicates the size of the
2394   entity-body, in decimal number of OCTETs, sent to the recipient or,
2395   in the case of the HEAD method, the size of the entity-body that
2396   would have been sent had the request been a GET.
2397</t>
2398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2399  <x:ref>Content-Length</x:ref>    = "Content-Length" ":" 1*<x:ref>DIGIT</x:ref>
2400</artwork></figure>
2401<t>
2402   An example is
2403</t>
2404<figure><artwork type="example">
2405    Content-Length: 3495
2406</artwork></figure>
2407<t>
2408   Applications &SHOULD; use this field to indicate the transfer-length of
2409   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2410</t>
2411<t>
2412   Any Content-Length greater than or equal to zero is a valid value.
2413   <xref target="message.length"/> describes how to determine the length of a message-body
2414   if a Content-Length is not given.
2415</t>
2416<t>
2417   Note that the meaning of this field is significantly different from
2418   the corresponding definition in MIME, where it is an optional field
2419   used within the "message/external-body" content-type. In HTTP, it
2420   &SHOULD; be sent whenever the message's length can be determined prior
2421   to being transferred, unless this is prohibited by the rules in
2422   <xref target="message.length"/>.
2423</t>
2424</section>
2425
2426<section title="Date" anchor="header.date">
2427  <iref primary="true" item="Date header" x:for-anchor=""/>
2428  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2429  <x:anchor-alias value="Date"/>
2430<t>
2431   The Date general-header field represents the date and time at which
2432   the message was originated, having the same semantics as orig-date in
2433   <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>. The field value is an
2434   HTTP-date, as described in <xref target="full.date"/>;
2435   it &MUST; be sent in rfc1123-date format.
2436</t>
2437<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2438  <x:ref>Date</x:ref>  = "Date" ":" <x:ref>HTTP-date</x:ref>
2439</artwork></figure>
2440<t>
2441   An example is
2442</t>
2443<figure><artwork type="example">
2444    Date: Tue, 15 Nov 1994 08:12:31 GMT
2445</artwork></figure>
2446<t>
2447   Origin servers &MUST; include a Date header field in all responses,
2448   except in these cases:
2449  <list style="numbers">
2450      <t>If the response status code is 100 (Continue) or 101 (Switching
2451         Protocols), the response &MAY; include a Date header field, at
2452         the server's option.</t>
2453
2454      <t>If the response status code conveys a server error, e.g. 500
2455         (Internal Server Error) or 503 (Service Unavailable), and it is
2456         inconvenient or impossible to generate a valid Date.</t>
2457
2458      <t>If the server does not have a clock that can provide a
2459         reasonable approximation of the current time, its responses
2460         &MUST-NOT; include a Date header field. In this case, the rules
2461         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2462  </list>
2463</t>
2464<t>
2465   A received message that does not have a Date header field &MUST; be
2466   assigned one by the recipient if the message will be cached by that
2467   recipient or gatewayed via a protocol which requires a Date. An HTTP
2468   implementation without a clock &MUST-NOT; cache responses without
2469   revalidating them on every use. An HTTP cache, especially a shared
2470   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2471   clock with a reliable external standard.
2472</t>
2473<t>
2474   Clients &SHOULD; only send a Date header field in messages that include
2475   an entity-body, as in the case of the PUT and POST requests, and even
2476   then it is optional. A client without a clock &MUST-NOT; send a Date
2477   header field in a request.
2478</t>
2479<t>
2480   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2481   time subsequent to the generation of the message. It &SHOULD; represent
2482   the best available approximation of the date and time of message
2483   generation, unless the implementation has no means of generating a
2484   reasonably accurate date and time. In theory, the date ought to
2485   represent the moment just before the entity is generated. In
2486   practice, the date can be generated at any time during the message
2487   origination without affecting its semantic value.
2488</t>
2489
2490<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2491<t>
2492   Some origin server implementations might not have a clock available.
2493   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2494   values to a response, unless these values were associated
2495   with the resource by a system or user with a reliable clock. It &MAY;
2496   assign an Expires value that is known, at or before server
2497   configuration time, to be in the past (this allows "pre-expiration"
2498   of responses without storing separate Expires values for each
2499   resource).
2500</t>
2501</section>
2502</section>
2503
2504<section title="Host" anchor="header.host">
2505  <iref primary="true" item="Host header" x:for-anchor=""/>
2506  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2507  <x:anchor-alias value="Host"/>
2508<t>
2509   The Host request-header field specifies the Internet host and port
2510   number of the resource being requested, as obtained from the original
2511   URI given by the user or referring resource (generally an HTTP URL,
2512   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2513   the naming authority of the origin server or gateway given by the
2514   original URL. This allows the origin server or gateway to
2515   differentiate between internally-ambiguous URLs, such as the root "/"
2516   URL of a server for multiple host names on a single IP address.
2517</t>
2518<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2519  <x:ref>Host</x:ref> = "Host" ":" <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.url"/>
2520</artwork></figure>
2521<t>
2522   A "host" without any trailing port information implies the default
2523   port for the service requested (e.g., "80" for an HTTP URL). For
2524   example, a request on the origin server for
2525   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2526</t>
2527<figure><artwork type="example">
2528    GET /pub/WWW/ HTTP/1.1
2529    Host: www.example.org
2530</artwork></figure>
2531<t>
2532   A client &MUST; include a Host header field in all HTTP/1.1 request
2533   messages. If the requested URI does not include an Internet host
2534   name for the service being requested, then the Host header field &MUST;
2535   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2536   request message it forwards does contain an appropriate Host header
2537   field that identifies the service being requested by the proxy. All
2538   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2539   status code to any HTTP/1.1 request message which lacks a Host header
2540   field.
2541</t>
2542<t>
2543   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2544   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2545   for other requirements relating to Host.
2546</t>
2547</section>
2548
2549<section title="TE" anchor="header.te">
2550  <iref primary="true" item="TE header" x:for-anchor=""/>
2551  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2552  <x:anchor-alias value="TE"/>
2553  <x:anchor-alias value="t-codings"/>
2554<t>
2555   The TE request-header field indicates what extension transfer-codings
2556   it is willing to accept in the response and whether or not it is
2557   willing to accept trailer fields in a chunked transfer-coding. Its
2558   value may consist of the keyword "trailers" and/or a comma-separated
2559   list of extension transfer-coding names with optional accept
2560   parameters (as described in <xref target="transfer.codings"/>).
2561</t>
2562<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2563  <x:ref>TE</x:ref>        = "TE" ":" #( <x:ref>t-codings</x:ref> )
2564  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-extension</x:ref> [ <x:ref>accept-params</x:ref> ] )
2565</artwork></figure>
2566<t>
2567   The presence of the keyword "trailers" indicates that the client is
2568   willing to accept trailer fields in a chunked transfer-coding, as
2569   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2570   transfer-coding values even though it does not itself represent a
2571   transfer-coding.
2572</t>
2573<t>
2574   Examples of its use are:
2575</t>
2576<figure><artwork type="example">
2577    TE: deflate
2578    TE:
2579    TE: trailers, deflate;q=0.5
2580</artwork></figure>
2581<t>
2582   The TE header field only applies to the immediate connection.
2583   Therefore, the keyword &MUST; be supplied within a Connection header
2584   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2585</t>
2586<t>
2587   A server tests whether a transfer-coding is acceptable, according to
2588   a TE field, using these rules:
2589  <list style="numbers">
2590    <x:lt>
2591      <t>The "chunked" transfer-coding is always acceptable. If the
2592         keyword "trailers" is listed, the client indicates that it is
2593         willing to accept trailer fields in the chunked response on
2594         behalf of itself and any downstream clients. The implication is
2595         that, if given, the client is stating that either all
2596         downstream clients are willing to accept trailer fields in the
2597         forwarded response, or that it will attempt to buffer the
2598         response on behalf of downstream recipients.
2599      </t><t>
2600         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2601         chunked response such that a client can be assured of buffering
2602         the entire response.</t>
2603    </x:lt>
2604    <x:lt>
2605      <t>If the transfer-coding being tested is one of the transfer-codings
2606         listed in the TE field, then it is acceptable unless it
2607         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2608         qvalue of 0 means "not acceptable.")</t>
2609    </x:lt>
2610    <x:lt>
2611      <t>If multiple transfer-codings are acceptable, then the
2612         acceptable transfer-coding with the highest non-zero qvalue is
2613         preferred.  The "chunked" transfer-coding always has a qvalue
2614         of 1.</t>
2615    </x:lt>
2616  </list>
2617</t>
2618<t>
2619   If the TE field-value is empty or if no TE field is present, the only
2620   transfer-coding  is "chunked". A message with no transfer-coding is
2621   always acceptable.
2622</t>
2623</section>
2624
2625<section title="Trailer" anchor="header.trailer">
2626  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2627  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2628  <x:anchor-alias value="Trailer"/>
2629<t>
2630   The Trailer general field value indicates that the given set of
2631   header fields is present in the trailer of a message encoded with
2632   chunked transfer-coding.
2633</t>
2634<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2635  <x:ref>Trailer</x:ref>  = "Trailer" ":" 1#<x:ref>field-name</x:ref>
2636</artwork></figure>
2637<t>
2638   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2639   message using chunked transfer-coding with a non-empty trailer. Doing
2640   so allows the recipient to know which header fields to expect in the
2641   trailer.
2642</t>
2643<t>
2644   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2645   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2646   trailer fields in a "chunked" transfer-coding.
2647</t>
2648<t>
2649   Message header fields listed in the Trailer header field &MUST-NOT;
2650   include the following header fields:
2651  <list style="symbols">
2652    <t>Transfer-Encoding</t>
2653    <t>Content-Length</t>
2654    <t>Trailer</t>
2655  </list>
2656</t>
2657</section>
2658
2659<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2660  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2661  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2662  <x:anchor-alias value="Transfer-Encoding"/>
2663<t>
2664   The Transfer-Encoding general-header field indicates what (if any)
2665   type of transformation has been applied to the message body in order
2666   to safely transfer it between the sender and the recipient. This
2667   differs from the content-coding in that the transfer-coding is a
2668   property of the message, not of the entity.
2669</t>
2670<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2671  <x:ref>Transfer-Encoding</x:ref>       = "Transfer-Encoding" ":" 1#<x:ref>transfer-coding</x:ref>
2672</artwork></figure>
2673<t>
2674   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2675</t>
2676<figure><artwork type="example">
2677  Transfer-Encoding: chunked
2678</artwork></figure>
2679<t>
2680   If multiple encodings have been applied to an entity, the transfer-codings
2681   &MUST; be listed in the order in which they were applied.
2682   Additional information about the encoding parameters &MAY; be provided
2683   by other entity-header fields not defined by this specification.
2684</t>
2685<t>
2686   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2687   header.
2688</t>
2689</section>
2690
2691<section title="Upgrade" anchor="header.upgrade">
2692  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2693  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2694  <x:anchor-alias value="Upgrade"/>
2695<t>
2696   The Upgrade general-header allows the client to specify what
2697   additional communication protocols it supports and would like to use
2698   if the server finds it appropriate to switch protocols. The server
2699   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2700   response to indicate which protocol(s) are being switched.
2701</t>
2702<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2703  <x:ref>Upgrade</x:ref>        = "Upgrade" ":" 1#<x:ref>product</x:ref>
2704</artwork></figure>
2705<t>
2706   For example,
2707</t>
2708<figure><artwork type="example">
2709    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2710</artwork></figure>
2711<t>
2712   The Upgrade header field is intended to provide a simple mechanism
2713   for transition from HTTP/1.1 to some other, incompatible protocol. It
2714   does so by allowing the client to advertise its desire to use another
2715   protocol, such as a later version of HTTP with a higher major version
2716   number, even though the current request has been made using HTTP/1.1.
2717   This eases the difficult transition between incompatible protocols by
2718   allowing the client to initiate a request in the more commonly
2719   supported protocol while indicating to the server that it would like
2720   to use a "better" protocol if available (where "better" is determined
2721   by the server, possibly according to the nature of the method and/or
2722   resource being requested).
2723</t>
2724<t>
2725   The Upgrade header field only applies to switching application-layer
2726   protocols upon the existing transport-layer connection. Upgrade
2727   cannot be used to insist on a protocol change; its acceptance and use
2728   by the server is optional. The capabilities and nature of the
2729   application-layer communication after the protocol change is entirely
2730   dependent upon the new protocol chosen, although the first action
2731   after changing the protocol &MUST; be a response to the initial HTTP
2732   request containing the Upgrade header field.
2733</t>
2734<t>
2735   The Upgrade header field only applies to the immediate connection.
2736   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2737   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2738   HTTP/1.1 message.
2739</t>
2740<t>
2741   The Upgrade header field cannot be used to indicate a switch to a
2742   protocol on a different connection. For that purpose, it is more
2743   appropriate to use a 301, 302, 303, or 305 redirection response.
2744</t>
2745<t>
2746   This specification only defines the protocol name "HTTP" for use by
2747   the family of Hypertext Transfer Protocols, as defined by the HTTP
2748   version rules of <xref target="http.version"/> and future updates to this
2749   specification. Any token can be used as a protocol name; however, it
2750   will only be useful if both the client and server associate the name
2751   with the same protocol.
2752</t>
2753</section>
2754
2755<section title="Via" anchor="header.via">
2756  <iref primary="true" item="Via header" x:for-anchor=""/>
2757  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2758  <x:anchor-alias value="protocol-name"/>
2759  <x:anchor-alias value="protocol-version"/>
2760  <x:anchor-alias value="pseudonym"/>
2761  <x:anchor-alias value="received-by"/>
2762  <x:anchor-alias value="received-protocol"/>
2763  <x:anchor-alias value="Via"/>
2764<t>
2765   The Via general-header field &MUST; be used by gateways and proxies to
2766   indicate the intermediate protocols and recipients between the user
2767   agent and the server on requests, and between the origin server and
2768   the client on responses. It is analogous to the "Received" field defined in
2769   <xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/> and is intended to be used for tracking message forwards,
2770   avoiding request loops, and identifying the protocol capabilities of
2771   all senders along the request/response chain.
2772</t>
2773<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"/>
2774  <x:ref>Via</x:ref> =  "Via" ":" 1#( <x:ref>received-protocol</x:ref> <x:ref>received-by</x:ref> [ <x:ref>comment</x:ref> ] )
2775  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2776  <x:ref>protocol-name</x:ref>     = <x:ref>token</x:ref>
2777  <x:ref>protocol-version</x:ref>  = <x:ref>token</x:ref>
2778  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2779  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2780</artwork></figure>
2781<t>
2782   The received-protocol indicates the protocol version of the message
2783   received by the server or client along each segment of the
2784   request/response chain. The received-protocol version is appended to
2785   the Via field value when the message is forwarded so that information
2786   about the protocol capabilities of upstream applications remains
2787   visible to all recipients.
2788</t>
2789<t>
2790   The protocol-name is optional if and only if it would be "HTTP". The
2791   received-by field is normally the host and optional port number of a
2792   recipient server or client that subsequently forwarded the message.
2793   However, if the real host is considered to be sensitive information,
2794   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2795   be assumed to be the default port of the received-protocol.
2796</t>
2797<t>
2798   Multiple Via field values represents each proxy or gateway that has
2799   forwarded the message. Each recipient &MUST; append its information
2800   such that the end result is ordered according to the sequence of
2801   forwarding applications.
2802</t>
2803<t>
2804   Comments &MAY; be used in the Via header field to identify the software
2805   of the recipient proxy or gateway, analogous to the User-Agent and
2806   Server header fields. However, all comments in the Via field are
2807   optional and &MAY; be removed by any recipient prior to forwarding the
2808   message.
2809</t>
2810<t>
2811   For example, a request message could be sent from an HTTP/1.0 user
2812   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2813   forward the request to a public proxy at p.example.net, which completes
2814   the request by forwarding it to the origin server at www.example.com.
2815   The request received by www.example.com would then have the following
2816   Via header field:
2817</t>
2818<figure><artwork type="example">
2819    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2820</artwork></figure>
2821<t>
2822   Proxies and gateways used as a portal through a network firewall
2823   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2824   the firewall region. This information &SHOULD; only be propagated if
2825   explicitly enabled. If not enabled, the received-by host of any host
2826   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2827   for that host.
2828</t>
2829<t>
2830   For organizations that have strong privacy requirements for hiding
2831   internal structures, a proxy &MAY; combine an ordered subsequence of
2832   Via header field entries with identical received-protocol values into
2833   a single such entry. For example,
2834</t>
2835<figure><artwork type="example">
2836    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2837</artwork></figure>
2838<t>
2839        could be collapsed to
2840</t>
2841<figure><artwork type="example">
2842    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2843</artwork></figure>
2844<t>
2845   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2846   under the same organizational control and the hosts have already been
2847   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2848   have different received-protocol values.
2849</t>
2850</section>
2851
2852</section>
2853
2854<section title="IANA Considerations" anchor="IANA.considerations">
2855<section title="Message Header Registration" anchor="message.header.registration">
2856<t>
2857   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2858   with the permanent registrations below (see <xref target="RFC3864"/>):
2859</t>
2860<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2861<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2862   <ttcol>Header Field Name</ttcol>
2863   <ttcol>Protocol</ttcol>
2864   <ttcol>Status</ttcol>
2865   <ttcol>Reference</ttcol>
2866
2867   <c>Connection</c>
2868   <c>http</c>
2869   <c>standard</c>
2870   <c>
2871      <xref target="header.connection"/>
2872   </c>
2873   <c>Content-Length</c>
2874   <c>http</c>
2875   <c>standard</c>
2876   <c>
2877      <xref target="header.content-length"/>
2878   </c>
2879   <c>Date</c>
2880   <c>http</c>
2881   <c>standard</c>
2882   <c>
2883      <xref target="header.date"/>
2884   </c>
2885   <c>Host</c>
2886   <c>http</c>
2887   <c>standard</c>
2888   <c>
2889      <xref target="header.host"/>
2890   </c>
2891   <c>TE</c>
2892   <c>http</c>
2893   <c>standard</c>
2894   <c>
2895      <xref target="header.te"/>
2896   </c>
2897   <c>Trailer</c>
2898   <c>http</c>
2899   <c>standard</c>
2900   <c>
2901      <xref target="header.trailer"/>
2902   </c>
2903   <c>Transfer-Encoding</c>
2904   <c>http</c>
2905   <c>standard</c>
2906   <c>
2907      <xref target="header.transfer-encoding"/>
2908   </c>
2909   <c>Upgrade</c>
2910   <c>http</c>
2911   <c>standard</c>
2912   <c>
2913      <xref target="header.upgrade"/>
2914   </c>
2915   <c>Via</c>
2916   <c>http</c>
2917   <c>standard</c>
2918   <c>
2919      <xref target="header.via"/>
2920   </c>
2921</texttable>
2922<!--(END)-->
2923<t>
2924   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2925</t>
2926</section>
2927
2928<section title="URI Scheme Registration" anchor="uri.scheme.registration">
2929<t>
2930   The entry for the "http" URI Scheme in the registry located at
2931   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>
2932   should be updated to point to <xref target="http.url"/> of this document
2933   (see <xref target="RFC4395"/>).
2934</t>
2935</section>
2936
2937<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
2938<t>
2939   This document serves as the specification for the Internet media types
2940   "message/http" and "application/http". The following is to be registered with
2941   IANA (see <xref target="RFC4288"/>).
2942</t>
2943<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
2944<iref item="Media Type" subitem="message/http" primary="true"/>
2945<iref item="message/http Media Type" primary="true"/>
2946<t>
2947   The message/http type can be used to enclose a single HTTP request or
2948   response message, provided that it obeys the MIME restrictions for all
2949   "message" types regarding line length and encodings.
2950</t>
2951<t>
2952  <list style="hanging" x:indent="12em">
2953    <t hangText="Type name:">
2954      message
2955    </t>
2956    <t hangText="Subtype name:">
2957      http
2958    </t>
2959    <t hangText="Required parameters:">
2960      none
2961    </t>
2962    <t hangText="Optional parameters:">
2963      version, msgtype
2964      <list style="hanging">
2965        <t hangText="version:">
2966          The HTTP-Version number of the enclosed message
2967          (e.g., "1.1"). If not present, the version can be
2968          determined from the first line of the body.
2969        </t>
2970        <t hangText="msgtype:">
2971          The message type -- "request" or "response". If not
2972          present, the type can be determined from the first
2973          line of the body.
2974        </t>
2975      </list>
2976    </t>
2977    <t hangText="Encoding considerations:">
2978      only "7bit", "8bit", or "binary" are permitted
2979    </t>
2980    <t hangText="Security considerations:">
2981      none
2982    </t>
2983    <t hangText="Interoperability considerations:">
2984      none
2985    </t>
2986    <t hangText="Published specification:">
2987      This specification (see <xref target="internet.media.type.message.http"/>).
2988    </t>
2989    <t hangText="Applications that use this media type:">
2990    </t>
2991    <t hangText="Additional information:">
2992      <list style="hanging">
2993        <t hangText="Magic number(s):">none</t>
2994        <t hangText="File extension(s):">none</t>
2995        <t hangText="Macintosh file type code(s):">none</t>
2996      </list>
2997    </t>
2998    <t hangText="Person and email address to contact for further information:">
2999      See Authors Section.
3000    </t>
3001                <t hangText="Intended usage:">
3002                  COMMON
3003    </t>
3004                <t hangText="Restrictions on usage:">
3005                  none
3006    </t>
3007    <t hangText="Author/Change controller:">
3008      IESG
3009    </t>
3010  </list>
3011</t>
3012</section>
3013<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3014<iref item="Media Type" subitem="application/http" primary="true"/>
3015<iref item="application/http Media Type" primary="true"/>
3016<t>
3017   The application/http type can be used to enclose a pipeline of one or more
3018   HTTP request or response messages (not intermixed).
3019</t>
3020<t>
3021  <list style="hanging" x:indent="12em">
3022    <t hangText="Type name:">
3023      application
3024    </t>
3025    <t hangText="Subtype name:">
3026      http
3027    </t>
3028    <t hangText="Required parameters:">
3029      none
3030    </t>
3031    <t hangText="Optional parameters:">
3032      version, msgtype
3033      <list style="hanging">
3034        <t hangText="version:">
3035          The HTTP-Version number of the enclosed messages
3036          (e.g., "1.1"). If not present, the version can be
3037          determined from the first line of the body.
3038        </t>
3039        <t hangText="msgtype:">
3040          The message type -- "request" or "response". If not
3041          present, the type can be determined from the first
3042          line of the body.
3043        </t>
3044      </list>
3045    </t>
3046    <t hangText="Encoding considerations:">
3047      HTTP messages enclosed by this type
3048      are in "binary" format; use of an appropriate
3049      Content-Transfer-Encoding is required when
3050      transmitted via E-mail.
3051    </t>
3052    <t hangText="Security considerations:">
3053      none
3054    </t>
3055    <t hangText="Interoperability considerations:">
3056      none
3057    </t>
3058    <t hangText="Published specification:">
3059      This specification (see <xref target="internet.media.type.application.http"/>).
3060    </t>
3061    <t hangText="Applications that use this media type:">
3062    </t>
3063    <t hangText="Additional information:">
3064      <list style="hanging">
3065        <t hangText="Magic number(s):">none</t>
3066        <t hangText="File extension(s):">none</t>
3067        <t hangText="Macintosh file type code(s):">none</t>
3068      </list>
3069    </t>
3070    <t hangText="Person and email address to contact for further information:">
3071      See Authors Section.
3072    </t>
3073                <t hangText="Intended usage:">
3074                  COMMON
3075    </t>
3076                <t hangText="Restrictions on usage:">
3077                  none
3078    </t>
3079    <t hangText="Author/Change controller:">
3080      IESG
3081    </t>
3082  </list>
3083</t>
3084</section>
3085</section>
3086
3087</section>
3088
3089<section title="Security Considerations" anchor="security.considerations">
3090<t>
3091   This section is meant to inform application developers, information
3092   providers, and users of the security limitations in HTTP/1.1 as
3093   described by this document. The discussion does not include
3094   definitive solutions to the problems revealed, though it does make
3095   some suggestions for reducing security risks.
3096</t>
3097
3098<section title="Personal Information" anchor="personal.information">
3099<t>
3100   HTTP clients are often privy to large amounts of personal information
3101   (e.g. the user's name, location, mail address, passwords, encryption
3102   keys, etc.), and &SHOULD; be very careful to prevent unintentional
3103   leakage of this information.
3104   We very strongly recommend that a convenient interface be provided
3105   for the user to control dissemination of such information, and that
3106   designers and implementors be particularly careful in this area.
3107   History shows that errors in this area often create serious security
3108   and/or privacy problems and generate highly adverse publicity for the
3109   implementor's company.
3110</t>
3111</section>
3112
3113<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3114<t>
3115   A server is in the position to save personal data about a user's
3116   requests which might identify their reading patterns or subjects of
3117   interest. This information is clearly confidential in nature and its
3118   handling can be constrained by law in certain countries. People using
3119   HTTP to provide data are responsible for ensuring that
3120   such material is not distributed without the permission of any
3121   individuals that are identifiable by the published results.
3122</t>
3123</section>
3124
3125<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3126<t>
3127   Implementations of HTTP origin servers &SHOULD; be careful to restrict
3128   the documents returned by HTTP requests to be only those that were
3129   intended by the server administrators. If an HTTP server translates
3130   HTTP URIs directly into file system calls, the server &MUST; take
3131   special care not to serve files that were not intended to be
3132   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3133   other operating systems use ".." as a path component to indicate a
3134   directory level above the current one. On such a system, an HTTP
3135   server &MUST; disallow any such construct in the Request-URI if it
3136   would otherwise allow access to a resource outside those intended to
3137   be accessible via the HTTP server. Similarly, files intended for
3138   reference only internally to the server (such as access control
3139   files, configuration files, and script code) &MUST; be protected from
3140   inappropriate retrieval, since they might contain sensitive
3141   information. Experience has shown that minor bugs in such HTTP server
3142   implementations have turned into security risks.
3143</t>
3144</section>
3145
3146<section title="DNS Spoofing" anchor="dns.spoofing">
3147<t>
3148   Clients using HTTP rely heavily on the Domain Name Service, and are
3149   thus generally prone to security attacks based on the deliberate
3150   mis-association of IP addresses and DNS names. Clients need to be
3151   cautious in assuming the continuing validity of an IP number/DNS name
3152   association.
3153</t>
3154<t>
3155   In particular, HTTP clients &SHOULD; rely on their name resolver for
3156   confirmation of an IP number/DNS name association, rather than
3157   caching the result of previous host name lookups. Many platforms
3158   already can cache host name lookups locally when appropriate, and
3159   they &SHOULD; be configured to do so. It is proper for these lookups to
3160   be cached, however, only when the TTL (Time To Live) information
3161   reported by the name server makes it likely that the cached
3162   information will remain useful.
3163</t>
3164<t>
3165   If HTTP clients cache the results of host name lookups in order to
3166   achieve a performance improvement, they &MUST; observe the TTL
3167   information reported by DNS.
3168</t>
3169<t>
3170   If HTTP clients do not observe this rule, they could be spoofed when
3171   a previously-accessed server's IP address changes. As network
3172   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
3173   possibility of this form of attack will grow. Observing this
3174   requirement thus reduces this potential security vulnerability.
3175</t>
3176<t>
3177   This requirement also improves the load-balancing behavior of clients
3178   for replicated servers using the same DNS name and reduces the
3179   likelihood of a user's experiencing failure in accessing sites which
3180   use that strategy.
3181</t>
3182</section>
3183
3184<section title="Proxies and Caching" anchor="attack.proxies">
3185<t>
3186   By their very nature, HTTP proxies are men-in-the-middle, and
3187   represent an opportunity for man-in-the-middle attacks. Compromise of
3188   the systems on which the proxies run can result in serious security
3189   and privacy problems. Proxies have access to security-related
3190   information, personal information about individual users and
3191   organizations, and proprietary information belonging to users and
3192   content providers. A compromised proxy, or a proxy implemented or
3193   configured without regard to security and privacy considerations,
3194   might be used in the commission of a wide range of potential attacks.
3195</t>
3196<t>
3197   Proxy operators should protect the systems on which proxies run as
3198   they would protect any system that contains or transports sensitive
3199   information. In particular, log information gathered at proxies often
3200   contains highly sensitive personal information, and/or information
3201   about organizations. Log information should be carefully guarded, and
3202   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
3203</t>
3204<t>
3205   Proxy implementors should consider the privacy and security
3206   implications of their design and coding decisions, and of the
3207   configuration options they provide to proxy operators (especially the
3208   default configuration).
3209</t>
3210<t>
3211   Users of a proxy need to be aware that they are no trustworthier than
3212   the people who run the proxy; HTTP itself cannot solve this problem.
3213</t>
3214<t>
3215   The judicious use of cryptography, when appropriate, may suffice to
3216   protect against a broad range of security and privacy attacks. Such
3217   cryptography is beyond the scope of the HTTP/1.1 specification.
3218</t>
3219</section>
3220
3221<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
3222<t>
3223   They exist. They are hard to defend against. Research continues.
3224   Beware.
3225</t>
3226</section>
3227</section>
3228
3229<section title="Acknowledgments" anchor="ack">
3230<t>
3231   This specification makes heavy use of the augmented BNF and generic
3232   constructs defined by David H. Crocker for <xref target="RFC5234"/>. Similarly, it
3233   reuses many of the definitions provided by Nathaniel Borenstein and
3234   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
3235   specification will help reduce past confusion over the relationship
3236   between HTTP and Internet mail message formats.
3237</t>
3238<t>
3239   HTTP has evolved considerably over the years. It has
3240   benefited from a large and active developer community--the many
3241   people who have participated on the www-talk mailing list--and it is
3242   that community which has been most responsible for the success of
3243   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
3244   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
3245   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
3246   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
3247   VanHeyningen deserve special recognition for their efforts in
3248   defining early aspects of the protocol.
3249</t>
3250<t>
3251   This document has benefited greatly from the comments of all those
3252   participating in the HTTP-WG. In addition to those already mentioned,
3253   the following individuals have contributed to this specification:
3254</t>
3255<t>
3256   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
3257   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
3258   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
3259   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
3260   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
3261   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
3262   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
3263   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
3264   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
3265   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
3266   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
3267   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
3268   Josh Cohen.
3269</t>
3270<t>
3271   Thanks to the "cave men" of Palo Alto. You know who you are.
3272</t>
3273<t>
3274   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
3275   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
3276   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
3277   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
3278   Larry Masinter for their help. And thanks go particularly to Jeff
3279   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3280</t>
3281<t>
3282   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3283   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3284   discovery of many of the problems that this document attempts to
3285   rectify.
3286</t>
3287</section>
3288
3289</middle>
3290<back>
3291
3292<references title="Normative References">
3293
3294<reference anchor="ISO-8859-1">
3295  <front>
3296    <title>
3297     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3298    </title>
3299    <author>
3300      <organization>International Organization for Standardization</organization>
3301    </author>
3302    <date year="1998"/>
3303  </front>
3304  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3305</reference>
3306
3307<reference anchor="Part2">
3308  <front>
3309    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3310    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3311      <organization abbrev="Day Software">Day Software</organization>
3312      <address><email>fielding@gbiv.com</email></address>
3313    </author>
3314    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3315      <organization>One Laptop per Child</organization>
3316      <address><email>jg@laptop.org</email></address>
3317    </author>
3318    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3319      <organization abbrev="HP">Hewlett-Packard Company</organization>
3320      <address><email>JeffMogul@acm.org</email></address>
3321    </author>
3322    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3323      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3324      <address><email>henrikn@microsoft.com</email></address>
3325    </author>
3326    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3327      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3328      <address><email>LMM@acm.org</email></address>
3329    </author>
3330    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3331      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3332      <address><email>paulle@microsoft.com</email></address>
3333    </author>
3334    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3335      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3336      <address><email>timbl@w3.org</email></address>
3337    </author>
3338    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3339      <organization abbrev="W3C">World Wide Web Consortium</organization>
3340      <address><email>ylafon@w3.org</email></address>
3341    </author>
3342    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3343      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3344      <address><email>julian.reschke@greenbytes.de</email></address>
3345    </author>
3346    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3347  </front>
3348  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3349  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3350</reference>
3351
3352<reference anchor="Part3">
3353  <front>
3354    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3355    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3356      <organization abbrev="Day Software">Day Software</organization>
3357      <address><email>fielding@gbiv.com</email></address>
3358    </author>
3359    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3360      <organization>One Laptop per Child</organization>
3361      <address><email>jg@laptop.org</email></address>
3362    </author>
3363    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3364      <organization abbrev="HP">Hewlett-Packard Company</organization>
3365      <address><email>JeffMogul@acm.org</email></address>
3366    </author>
3367    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3368      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3369      <address><email>henrikn@microsoft.com</email></address>
3370    </author>
3371    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3372      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3373      <address><email>LMM@acm.org</email></address>
3374    </author>
3375    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3376      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3377      <address><email>paulle@microsoft.com</email></address>
3378    </author>
3379    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3380      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3381      <address><email>timbl@w3.org</email></address>
3382    </author>
3383    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3384      <organization abbrev="W3C">World Wide Web Consortium</organization>
3385      <address><email>ylafon@w3.org</email></address>
3386    </author>
3387    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3388      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3389      <address><email>julian.reschke@greenbytes.de</email></address>
3390    </author>
3391    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3392  </front>
3393  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3394  <x:source href="p3-payload.xml" basename="p3-payload"/>
3395</reference>
3396
3397<reference anchor="Part5">
3398  <front>
3399    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3400    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3401      <organization abbrev="Day Software">Day Software</organization>
3402      <address><email>fielding@gbiv.com</email></address>
3403    </author>
3404    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3405      <organization>One Laptop per Child</organization>
3406      <address><email>jg@laptop.org</email></address>
3407    </author>
3408    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3409      <organization abbrev="HP">Hewlett-Packard Company</organization>
3410      <address><email>JeffMogul@acm.org</email></address>
3411    </author>
3412    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3413      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3414      <address><email>henrikn@microsoft.com</email></address>
3415    </author>
3416    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3417      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3418      <address><email>LMM@acm.org</email></address>
3419    </author>
3420    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3421      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3422      <address><email>paulle@microsoft.com</email></address>
3423    </author>
3424    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3425      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3426      <address><email>timbl@w3.org</email></address>
3427    </author>
3428    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3429      <organization abbrev="W3C">World Wide Web Consortium</organization>
3430      <address><email>ylafon@w3.org</email></address>
3431    </author>
3432    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3433      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3434      <address><email>julian.reschke@greenbytes.de</email></address>
3435    </author>
3436    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3437  </front>
3438  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3439  <x:source href="p5-range.xml" basename="p5-range"/>
3440</reference>
3441
3442<reference anchor="Part6">
3443  <front>
3444    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3445    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3446      <organization abbrev="Day Software">Day Software</organization>
3447      <address><email>fielding@gbiv.com</email></address>
3448    </author>
3449    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3450      <organization>One Laptop per Child</organization>
3451      <address><email>jg@laptop.org</email></address>
3452    </author>
3453    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3454      <organization abbrev="HP">Hewlett-Packard Company</organization>
3455      <address><email>JeffMogul@acm.org</email></address>
3456    </author>
3457    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3458      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3459      <address><email>henrikn@microsoft.com</email></address>
3460    </author>
3461    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3462      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3463      <address><email>LMM@acm.org</email></address>
3464    </author>
3465    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3466      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3467      <address><email>paulle@microsoft.com</email></address>
3468    </author>
3469    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3470      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3471      <address><email>timbl@w3.org</email></address>
3472    </author>
3473    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3474      <organization abbrev="W3C">World Wide Web Consortium</organization>
3475      <address><email>ylafon@w3.org</email></address>
3476    </author>
3477    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3478      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3479      <address><email>julian.reschke@greenbytes.de</email></address>
3480    </author>
3481    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3482  </front>
3483  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3484  <x:source href="p6-cache.xml" basename="p6-cache"/>
3485</reference>
3486
3487<reference anchor="RFC5234">
3488  <front>
3489    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
3490    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
3491      <organization>Brandenburg InternetWorking</organization>
3492      <address>
3493      <postal>
3494      <street>675 Spruce Dr.</street>
3495      <city>Sunnyvale</city>
3496      <region>CA</region>
3497      <code>94086</code>
3498      <country>US</country></postal>
3499      <phone>+1.408.246.8253</phone>
3500      <email>dcrocker@bbiw.net</email></address> 
3501    </author>
3502    <author initials="P." surname="Overell" fullname="Paul Overell">
3503      <organization>THUS plc.</organization>
3504      <address>
3505      <postal>
3506      <street>1/2 Berkeley Square</street>
3507      <street>99 Berkely Street</street>
3508      <city>Glasgow</city>
3509      <code>G3 7HR</code>
3510      <country>UK</country></postal>
3511      <email>paul.overell@thus.net</email></address>
3512    </author>
3513    <date month="January" year="2008"/>
3514  </front>
3515  <seriesInfo name="STD" value="68"/>
3516  <seriesInfo name="RFC" value="5234"/>
3517</reference>
3518
3519<reference anchor="RFC2045">
3520  <front>
3521    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3522    <author initials="N." surname="Freed" fullname="Ned Freed">
3523      <organization>Innosoft International, Inc.</organization>
3524      <address><email>ned@innosoft.com</email></address>
3525    </author>
3526    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3527      <organization>First Virtual Holdings</organization>
3528      <address><email>nsb@nsb.fv.com</email></address>
3529    </author>
3530    <date month="November" year="1996"/>
3531  </front>
3532  <seriesInfo name="RFC" value="2045"/>
3533</reference>
3534
3535<reference anchor="RFC2047">
3536  <front>
3537    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3538    <author initials="K." surname="Moore" fullname="Keith Moore">
3539      <organization>University of Tennessee</organization>
3540      <address><email>moore@cs.utk.edu</email></address>
3541    </author>
3542    <date month="November" year="1996"/>
3543  </front>
3544  <seriesInfo name="RFC" value="2047"/>
3545</reference>
3546
3547<reference anchor="RFC2119">
3548  <front>
3549    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3550    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3551      <organization>Harvard University</organization>
3552      <address><email>sob@harvard.edu</email></address>
3553    </author>
3554    <date month="March" year="1997"/>
3555  </front>
3556  <seriesInfo name="BCP" value="14"/>
3557  <seriesInfo name="RFC" value="2119"/>
3558</reference>
3559
3560<reference anchor="RFC2396">
3561  <front>
3562    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3563    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3564      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3565      <address><email>timbl@w3.org</email></address>
3566    </author>
3567    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3568      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3569      <address><email>fielding@ics.uci.edu</email></address>
3570    </author>
3571    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3572      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3573      <address><email>masinter@parc.xerox.com</email></address>
3574    </author>
3575    <date month="August" year="1998"/>
3576  </front>
3577  <seriesInfo name="RFC" value="2396"/>
3578</reference>
3579
3580<reference anchor="USASCII">
3581  <front>
3582    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3583    <author>
3584      <organization>American National Standards Institute</organization>
3585    </author>
3586    <date year="1986"/>
3587  </front>
3588  <seriesInfo name="ANSI" value="X3.4"/>
3589</reference>
3590
3591</references>
3592
3593<references title="Informative References">
3594
3595<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3596  <front>
3597    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3598    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3599      <organization/>
3600    </author>
3601    <author initials="J." surname="Gettys" fullname="J. Gettys">
3602      <organization/>
3603    </author>
3604    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3605      <organization/>
3606    </author>
3607    <author initials="H." surname="Lie" fullname="H. Lie">
3608      <organization/>
3609    </author>
3610    <author initials="C." surname="Lilley" fullname="C. Lilley">
3611      <organization/>
3612    </author>
3613    <date year="1997" month="September"/>
3614  </front>
3615  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3616</reference>
3617
3618<reference anchor="Pad1995" target="http://portal.acm.org/citation.cfm?id=219094">
3619  <front>
3620    <title>Improving HTTP Latency</title>
3621    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3622      <organization/>
3623    </author>
3624    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3625      <organization/>
3626    </author>
3627    <date year="1995" month="December"/>
3628  </front>
3629  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3630</reference>
3631
3632<reference anchor="RFC822">
3633  <front>
3634    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3635    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3636      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3637      <address><email>DCrocker@UDel-Relay</email></address>
3638    </author>
3639    <date month="August" day="13" year="1982"/>
3640  </front>
3641  <seriesInfo name="STD" value="11"/>
3642  <seriesInfo name="RFC" value="822"/>
3643</reference>
3644
3645<reference anchor="RFC959">
3646  <front>
3647    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3648    <author initials="J." surname="Postel" fullname="J. Postel">
3649      <organization>Information Sciences Institute (ISI)</organization>
3650    </author>
3651    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3652      <organization/>
3653    </author>
3654    <date month="October" year="1985"/>
3655  </front>
3656  <seriesInfo name="STD" value="9"/>
3657  <seriesInfo name="RFC" value="959"/>
3658</reference>
3659
3660<reference anchor="RFC1123">
3661  <front>
3662    <title>Requirements for Internet Hosts - Application and Support</title>
3663    <author initials="R." surname="Braden" fullname="Robert Braden">
3664      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3665      <address><email>Braden@ISI.EDU</email></address>
3666    </author>
3667    <date month="October" year="1989"/>
3668  </front>
3669  <seriesInfo name="STD" value="3"/>
3670  <seriesInfo name="RFC" value="1123"/>
3671</reference>
3672
3673<reference anchor="RFC1305">
3674  <front>
3675    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3676    <author initials="D." surname="Mills" fullname="David L. Mills">
3677      <organization>University of Delaware, Electrical Engineering Department</organization>
3678      <address><email>mills@udel.edu</email></address>
3679    </author>
3680    <date month="March" year="1992"/>
3681  </front>
3682  <seriesInfo name="RFC" value="1305"/>
3683</reference>
3684
3685<reference anchor="RFC1436">
3686  <front>
3687    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3688    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3689      <organization>University of Minnesota, Computer and Information Services</organization>
3690      <address><email>fxa@boombox.micro.umn.edu</email></address>
3691    </author>
3692    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3693      <organization>University of Minnesota, Computer and Information Services</organization>
3694      <address><email>mpm@boombox.micro.umn.edu</email></address>
3695    </author>
3696    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3697      <organization>University of Minnesota, Computer and Information Services</organization>
3698      <address><email>lindner@boombox.micro.umn.edu</email></address>
3699    </author>
3700    <author initials="D." surname="Johnson" fullname="David Johnson">
3701      <organization>University of Minnesota, Computer and Information Services</organization>
3702      <address><email>dmj@boombox.micro.umn.edu</email></address>
3703    </author>
3704    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3705      <organization>University of Minnesota, Computer and Information Services</organization>
3706      <address><email>daniel@boombox.micro.umn.edu</email></address>
3707    </author>
3708    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3709      <organization>University of Minnesota, Computer and Information Services</organization>
3710      <address><email>alberti@boombox.micro.umn.edu</email></address>
3711    </author>
3712    <date month="March" year="1993"/>
3713  </front>
3714  <seriesInfo name="RFC" value="1436"/>
3715</reference>
3716
3717<reference anchor="RFC1630">
3718  <front>
3719    <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>
3720    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3721      <organization>CERN, World-Wide Web project</organization>
3722      <address><email>timbl@info.cern.ch</email></address>
3723    </author>
3724    <date month="June" year="1994"/>
3725  </front>
3726  <seriesInfo name="RFC" value="1630"/>
3727</reference>
3728
3729<reference anchor="RFC1737">
3730  <front>
3731    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3732    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3733      <organization>Xerox Palo Alto Research Center</organization>
3734      <address><email>masinter@parc.xerox.com</email></address>
3735    </author>
3736    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3737      <organization>MIT Laboratory for Computer Science</organization>
3738      <address><email>sollins@lcs.mit.edu</email></address>
3739    </author>
3740    <date month="December" year="1994"/>
3741  </front>
3742  <seriesInfo name="RFC" value="1737"/>
3743</reference>
3744
3745<reference anchor="RFC1738">
3746  <front>
3747    <title>Uniform Resource Locators (URL)</title>
3748    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3749      <organization>CERN, World-Wide Web project</organization>
3750      <address><email>timbl@info.cern.ch</email></address>
3751    </author>
3752    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3753      <organization>Xerox PARC</organization>
3754      <address><email>masinter@parc.xerox.com</email></address>
3755    </author>
3756    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3757      <organization>University of Minnesota, Computer and Information Services</organization>
3758      <address><email>mpm@boombox.micro.umn.edu</email></address>
3759    </author>
3760    <date month="December" year="1994"/>
3761  </front>
3762  <seriesInfo name="RFC" value="1738"/>
3763</reference>
3764
3765<reference anchor="RFC1808">
3766  <front>
3767    <title>Relative Uniform Resource Locators</title>
3768    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3769      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3770      <address><email>fielding@ics.uci.edu</email></address>
3771    </author>
3772    <date month="June" year="1995"/>
3773  </front>
3774  <seriesInfo name="RFC" value="1808"/>
3775</reference>
3776
3777<reference anchor="RFC1900">
3778  <front>
3779    <title>Renumbering Needs Work</title>
3780    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3781      <organization>CERN, Computing and Networks Division</organization>
3782      <address><email>brian@dxcoms.cern.ch</email></address>
3783    </author>
3784    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3785      <organization>cisco Systems</organization>
3786      <address><email>yakov@cisco.com</email></address>
3787    </author>
3788    <date month="February" year="1996"/>
3789  </front>
3790  <seriesInfo name="RFC" value="1900"/>
3791</reference>
3792
3793<reference anchor="RFC1945">
3794  <front>
3795    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3796    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3797      <organization>MIT, Laboratory for Computer Science</organization>
3798      <address><email>timbl@w3.org</email></address>
3799    </author>
3800    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3801      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3802      <address><email>fielding@ics.uci.edu</email></address>
3803    </author>
3804    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3805      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3806      <address><email>frystyk@w3.org</email></address>
3807    </author>
3808    <date month="May" year="1996"/>
3809  </front>
3810  <seriesInfo name="RFC" value="1945"/>
3811</reference>
3812
3813<reference anchor="RFC2068">
3814  <front>
3815    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3816    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3817      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3818      <address><email>fielding@ics.uci.edu</email></address>
3819    </author>
3820    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3821      <organization>MIT Laboratory for Computer Science</organization>
3822      <address><email>jg@w3.org</email></address>
3823    </author>
3824    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3825      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3826      <address><email>mogul@wrl.dec.com</email></address>
3827    </author>
3828    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3829      <organization>MIT Laboratory for Computer Science</organization>
3830      <address><email>frystyk@w3.org</email></address>
3831    </author>
3832    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3833      <organization>MIT Laboratory for Computer Science</organization>
3834      <address><email>timbl@w3.org</email></address>
3835    </author>
3836    <date month="January" year="1997"/>
3837  </front>
3838  <seriesInfo name="RFC" value="2068"/>
3839</reference>
3840
3841<reference anchor='RFC2109'>
3842  <front>
3843    <title>HTTP State Management Mechanism</title>
3844    <author initials='D.M.' surname='Kristol' fullname='David M. Kristol'>
3845      <organization>Bell Laboratories, Lucent Technologies</organization>
3846      <address><email>dmk@bell-labs.com</email></address>
3847    </author>
3848    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3849      <organization>Netscape Communications Corp.</organization>
3850      <address><email>montulli@netscape.com</email></address>
3851    </author>
3852    <date year='1997' month='February' />
3853  </front>
3854  <seriesInfo name='RFC' value='2109' />
3855</reference>
3856
3857<reference anchor="RFC2145">
3858  <front>
3859    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3860    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3861      <organization>Western Research Laboratory</organization>
3862      <address><email>mogul@wrl.dec.com</email></address>
3863    </author>
3864    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3865      <organization>Department of Information and Computer Science</organization>
3866      <address><email>fielding@ics.uci.edu</email></address>
3867    </author>
3868    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3869      <organization>MIT Laboratory for Computer Science</organization>
3870      <address><email>jg@w3.org</email></address>
3871    </author>
3872    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3873      <organization>W3 Consortium</organization>
3874      <address><email>frystyk@w3.org</email></address>
3875    </author>
3876    <date month="May" year="1997"/>
3877  </front>
3878  <seriesInfo name="RFC" value="2145"/>
3879</reference>
3880
3881<reference anchor="RFC2324">
3882  <front>
3883    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3884    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3885      <organization>Xerox Palo Alto Research Center</organization>
3886      <address><email>masinter@parc.xerox.com</email></address>
3887    </author>
3888    <date month="April" day="1" year="1998"/>
3889  </front>
3890  <seriesInfo name="RFC" value="2324"/>
3891</reference>
3892
3893<reference anchor="RFC2616">
3894  <front>
3895    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3896    <author initials="R." surname="Fielding" fullname="R. Fielding">
3897      <organization>University of California, Irvine</organization>
3898      <address><email>fielding@ics.uci.edu</email></address>
3899    </author>
3900    <author initials="J." surname="Gettys" fullname="J. Gettys">
3901      <organization>W3C</organization>
3902      <address><email>jg@w3.org</email></address>
3903    </author>
3904    <author initials="J." surname="Mogul" fullname="J. Mogul">
3905      <organization>Compaq Computer Corporation</organization>
3906      <address><email>mogul@wrl.dec.com</email></address>
3907    </author>
3908    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3909      <organization>MIT Laboratory for Computer Science</organization>
3910      <address><email>frystyk@w3.org</email></address>
3911    </author>
3912    <author initials="L." surname="Masinter" fullname="L. Masinter">
3913      <organization>Xerox Corporation</organization>
3914      <address><email>masinter@parc.xerox.com</email></address>
3915    </author>
3916    <author initials="P." surname="Leach" fullname="P. Leach">
3917      <organization>Microsoft Corporation</organization>
3918      <address><email>paulle@microsoft.com</email></address>
3919    </author>
3920    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3921      <organization>W3C</organization>
3922      <address><email>timbl@w3.org</email></address>
3923    </author>
3924    <date month="June" year="1999"/>
3925  </front>
3926  <seriesInfo name="RFC" value="2616"/>
3927</reference>
3928
3929<reference anchor='RFC2818'>
3930  <front>
3931    <title>HTTP Over TLS</title>
3932    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
3933      <organization>RTFM, Inc.</organization>
3934      <address><email>ekr@rtfm.com</email></address>
3935    </author>
3936    <date year='2000' month='May' />
3937  </front>
3938  <seriesInfo name='RFC' value='2818' />
3939</reference>
3940
3941<reference anchor="RFC2821">
3942  <front>
3943    <title>Simple Mail Transfer Protocol</title>
3944    <author initials="J." surname="Klensin" fullname="J. Klensin">
3945      <organization>AT&amp;T Laboratories</organization>
3946      <address><email>klensin@research.att.com</email></address>
3947    </author>
3948    <date year="2001" month="April"/>
3949  </front>
3950  <seriesInfo name="RFC" value="2821"/>
3951</reference>
3952
3953<reference anchor='RFC2965'>
3954  <front>
3955    <title>HTTP State Management Mechanism</title>
3956    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
3957      <organization>Bell Laboratories, Lucent Technologies</organization>
3958      <address><email>dmk@bell-labs.com</email></address>
3959    </author>
3960    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3961      <organization>Epinions.com, Inc.</organization>
3962      <address><email>lou@montulli.org</email></address>
3963    </author>
3964    <date year='2000' month='October' />
3965  </front>
3966  <seriesInfo name='RFC' value='2965' />
3967</reference>
3968
3969<reference anchor='RFC3864'>
3970  <front>
3971    <title>Registration Procedures for Message Header Fields</title>
3972    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
3973      <organization>Nine by Nine</organization>
3974      <address><email>GK-IETF@ninebynine.org</email></address>
3975    </author>
3976    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
3977      <organization>BEA Systems</organization>
3978      <address><email>mnot@pobox.com</email></address>
3979    </author>
3980    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
3981      <organization>HP Labs</organization>
3982      <address><email>JeffMogul@acm.org</email></address>
3983    </author>
3984    <date year='2004' month='September' />
3985  </front>
3986  <seriesInfo name='BCP' value='90' />
3987  <seriesInfo name='RFC' value='3864' />
3988</reference>
3989
3990<reference anchor='RFC3977'>
3991  <front>
3992    <title>Network News Transfer Protocol (NNTP)</title>
3993    <author initials='C.' surname='Feather' fullname='C. Feather'>
3994      <organization>THUS plc</organization>
3995      <address><email>clive@demon.net</email></address>
3996    </author>
3997    <date year='2006' month='October' />
3998  </front>
3999  <seriesInfo name="RFC" value="3977"/>
4000</reference>
4001
4002<reference anchor="RFC4288">
4003  <front>
4004    <title>Media Type Specifications and Registration Procedures</title>
4005    <author initials="N." surname="Freed" fullname="N. Freed">
4006      <organization>Sun Microsystems</organization>
4007      <address>
4008        <email>ned.freed@mrochek.com</email>
4009      </address>
4010    </author>
4011    <author initials="J." surname="Klensin" fullname="J. Klensin">
4012      <organization/>
4013      <address>
4014        <email>klensin+ietf@jck.com</email>
4015      </address>
4016    </author>
4017    <date year="2005" month="December"/>
4018  </front>
4019  <seriesInfo name="BCP" value="13"/>
4020  <seriesInfo name="RFC" value="4288"/>
4021</reference>
4022
4023<reference anchor='RFC4395'>
4024  <front>
4025    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4026    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4027      <organization>AT&amp;T Laboratories</organization>
4028      <address>
4029        <email>tony+urireg@maillennium.att.com</email>
4030      </address>
4031    </author>
4032    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4033      <organization>Qualcomm, Inc.</organization>
4034      <address>
4035        <email>hardie@qualcomm.com</email>
4036      </address>
4037    </author>
4038    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4039      <organization>Adobe Systems</organization>
4040      <address>
4041        <email>LMM@acm.org</email>
4042      </address>
4043    </author>
4044    <date year='2006' month='February' />
4045  </front>
4046  <seriesInfo name='BCP' value='115' />
4047  <seriesInfo name='RFC' value='4395' />
4048</reference>
4049
4050<reference anchor="RFC5322">
4051  <front>
4052    <title>Internet Message Format</title>
4053    <author initials="P." surname="Resnick" fullname="P. Resnick">
4054      <organization>Qualcomm Incorporated</organization>
4055    </author>
4056    <date year="2008" month="October"/>
4057  </front> 
4058  <seriesInfo name="RFC" value="5322"/>
4059</reference>
4060
4061<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4062  <front>
4063    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4064    <author initials="D." surname="Kristol" fullname="David M. Kristol">
4065      <organization/>
4066    </author>
4067    <date year="2001" month="November"/>
4068  </front>
4069  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4070</reference>
4071
4072<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
4073  <front>
4074  <title>Analysis of HTTP Performance Problems</title>
4075  <author initials="S." surname="Spero" fullname="Simon E. Spero">
4076    <organization/>
4077  </author>
4078  <date/>
4079  </front>
4080</reference>
4081
4082<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
4083  <front>
4084  <title>Analysis of HTTP Performance</title>
4085  <author initials="J." surname="Touch" fullname="Joe Touch">
4086    <organization>USC/Information Sciences Institute</organization>
4087    <address><email>touch@isi.edu</email></address>
4088  </author>
4089  <author initials="J." surname="Heidemann" fullname="John Heidemann">
4090    <organization>USC/Information Sciences Institute</organization>
4091    <address><email>johnh@isi.edu</email></address>
4092  </author>
4093  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
4094    <organization>USC/Information Sciences Institute</organization>
4095    <address><email>katia@isi.edu</email></address>
4096  </author>
4097  <date year="1998" month="Aug"/>
4098  </front>
4099  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
4100  <annotation>(original report dated Aug. 1996)</annotation>
4101</reference>
4102
4103<reference anchor="WAIS">
4104  <front>
4105    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
4106    <author initials="F." surname="Davis" fullname="F. Davis">
4107      <organization>Thinking Machines Corporation</organization>
4108    </author>
4109    <author initials="B." surname="Kahle" fullname="B. Kahle">
4110      <organization>Thinking Machines Corporation</organization>
4111    </author>
4112    <author initials="H." surname="Morris" fullname="H. Morris">
4113      <organization>Thinking Machines Corporation</organization>
4114    </author>
4115    <author initials="J." surname="Salem" fullname="J. Salem">
4116      <organization>Thinking Machines Corporation</organization>
4117    </author>
4118    <author initials="T." surname="Shen" fullname="T. Shen">
4119      <organization>Thinking Machines Corporation</organization>
4120    </author>
4121    <author initials="R." surname="Wang" fullname="R. Wang">
4122      <organization>Thinking Machines Corporation</organization>
4123    </author>
4124    <author initials="J." surname="Sui" fullname="J. Sui">
4125      <organization>Thinking Machines Corporation</organization>
4126    </author>
4127    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
4128      <organization>Thinking Machines Corporation</organization>
4129    </author>
4130    <date month="April" year="1990"/>
4131  </front>
4132  <seriesInfo name="Thinking Machines Corporation" value=""/>
4133</reference>
4134
4135</references>
4136
4137
4138<section title="Tolerant Applications" anchor="tolerant.applications">
4139<t>
4140   Although this document specifies the requirements for the generation
4141   of HTTP/1.1 messages, not all applications will be correct in their
4142   implementation. We therefore recommend that operational applications
4143   be tolerant of deviations whenever those deviations can be
4144   interpreted unambiguously.
4145</t>
4146<t>
4147   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
4148   tolerant when parsing the Request-Line. In particular, they &SHOULD;
4149   accept any amount of SP or HTAB characters between fields, even though
4150   only a single SP is required.
4151</t>
4152<t>
4153   The line terminator for message-header fields is the sequence CRLF.
4154   However, we recommend that applications, when parsing such headers,
4155   recognize a single LF as a line terminator and ignore the leading CR.
4156</t>
4157<t>
4158   The character set of an entity-body &SHOULD; be labeled as the lowest
4159   common denominator of the character codes used within that body, with
4160   the exception that not labeling the entity is preferred over labeling
4161   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
4162</t>
4163<t>
4164   Additional rules for requirements on parsing and encoding of dates
4165   and other potential problems with date encodings include:
4166</t>
4167<t>
4168  <list style="symbols">
4169     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
4170        which appears to be more than 50 years in the future is in fact
4171        in the past (this helps solve the "year 2000" problem).</t>
4172
4173     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
4174        Expires date as earlier than the proper value, but &MUST-NOT;
4175        internally represent a parsed Expires date as later than the
4176        proper value.</t>
4177
4178     <t>All expiration-related calculations &MUST; be done in GMT. The
4179        local time zone &MUST-NOT; influence the calculation or comparison
4180        of an age or expiration time.</t>
4181
4182     <t>If an HTTP header incorrectly carries a date value with a time
4183        zone other than GMT, it &MUST; be converted into GMT using the
4184        most conservative possible conversion.</t>
4185  </list>
4186</t>
4187</section>
4188
4189<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
4190<t>
4191   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
4192   simplify the process of date comparison. Proxies and gateways from
4193   other protocols &SHOULD; ensure that any Date header field present in a
4194   message conforms to one of the HTTP/1.1 formats and rewrite the date
4195   if necessary.
4196</t>
4197</section>
4198
4199<section title="Compatibility with Previous Versions" anchor="compatibility">
4200<t>
4201   It is beyond the scope of a protocol specification to mandate
4202   compliance with previous versions. HTTP/1.1 was deliberately
4203   designed, however, to make supporting previous versions easy. It is
4204   worth noting that, at the time of composing this specification
4205   (1996), we would expect commercial HTTP/1.1 servers to:
4206  <list style="symbols">
4207     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
4208        1.1 requests;</t>
4209
4210     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
4211        1.1;</t>
4212
4213     <t>respond appropriately with a message in the same major version
4214        used by the client.</t>
4215  </list>
4216</t>
4217<t>
4218   And we would expect HTTP/1.1 clients to:
4219  <list style="symbols">
4220     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
4221        responses;</t>
4222
4223     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
4224        1.1.</t>
4225  </list>
4226</t>
4227<t>
4228   For most implementations of HTTP/1.0, each connection is established
4229   by the client prior to the request and closed by the server after
4230   sending the response. Some implementations implement the Keep-Alive
4231   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
4232</t>
4233
4234<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4235<t>
4236   This section summarizes major differences between versions HTTP/1.0
4237   and HTTP/1.1.
4238</t>
4239
4240<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">
4241<t>
4242   The requirements that clients and servers support the Host request-header,
4243   report an error if the Host request-header (<xref target="header.host"/>) is
4244   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
4245   are among the most important changes defined by this
4246   specification.
4247</t>
4248<t>
4249   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4250   addresses and servers; there was no other established mechanism for
4251   distinguishing the intended server of a request than the IP address
4252   to which that request was directed. The changes outlined above will
4253   allow the Internet, once older HTTP clients are no longer common, to
4254   support multiple Web sites from a single IP address, greatly
4255   simplifying large operational Web servers, where allocation of many
4256   IP addresses to a single host has created serious problems. The
4257   Internet will also be able to recover the IP addresses that have been
4258   allocated for the sole purpose of allowing special-purpose domain
4259   names to be used in root-level HTTP URLs. Given the rate of growth of
4260   the Web, and the number of servers already deployed, it is extremely
4261   important that all implementations of HTTP (including updates to
4262   existing HTTP/1.0 applications) correctly implement these
4263   requirements:
4264  <list style="symbols">
4265     <t>Both clients and servers &MUST; support the Host request-header.</t>
4266
4267     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4268
4269     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4270        request does not include a Host request-header.</t>
4271
4272     <t>Servers &MUST; accept absolute URIs.</t>
4273  </list>
4274</t>
4275</section>
4276</section>
4277
4278<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4279<t>
4280   Some clients and servers might wish to be compatible with some
4281   previous implementations of persistent connections in HTTP/1.0
4282   clients and servers. Persistent connections in HTTP/1.0 are
4283   explicitly negotiated as they are not the default behavior. HTTP/1.0
4284   experimental implementations of persistent connections are faulty,
4285   and the new facilities in HTTP/1.1 are designed to rectify these
4286   problems. The problem was that some existing 1.0 clients may be
4287   sending Keep-Alive to a proxy server that doesn't understand
4288   Connection, which would then erroneously forward it to the next
4289   inbound server, which would establish the Keep-Alive connection and
4290   result in a hung HTTP/1.0 proxy waiting for the close on the
4291   response. The result is that HTTP/1.0 clients must be prevented from
4292   using Keep-Alive when talking to proxies.
4293</t>
4294<t>
4295   However, talking to proxies is the most important use of persistent
4296   connections, so that prohibition is clearly unacceptable. Therefore,
4297   we need some other mechanism for indicating a persistent connection
4298   is desired, which is safe to use even when talking to an old proxy
4299   that ignores Connection. Persistent connections are the default for
4300   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4301   declaring non-persistence. See <xref target="header.connection"/>.
4302</t>
4303<t>
4304   The original HTTP/1.0 form of persistent connections (the Connection:
4305   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4306</t>
4307</section>
4308
4309<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4310<t>
4311   This specification has been carefully audited to correct and
4312   disambiguate key word usage; RFC 2068 had many problems in respect to
4313   the conventions laid out in <xref target="RFC2119"/>.
4314</t>
4315<t>
4316   Transfer-coding and message lengths all interact in ways that
4317   required fixing exactly when chunked encoding is used (to allow for
4318   transfer encoding that may not be self delimiting); it was important
4319   to straighten out exactly how message lengths are computed. (Sections
4320   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4321   <xref target="header.content-length" format="counter"/>,
4322   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4323</t>
4324<t>
4325   The use and interpretation of HTTP version numbers has been clarified
4326   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4327   version they support to deal with problems discovered in HTTP/1.0
4328   implementations (<xref target="http.version"/>)
4329</t>
4330<t>
4331   Transfer-coding had significant problems, particularly with
4332   interactions with chunked encoding. The solution is that transfer-codings
4333   become as full fledged as content-codings. This involves
4334   adding an IANA registry for transfer-codings (separate from content
4335   codings), a new header field (TE) and enabling trailer headers in the
4336   future. Transfer encoding is a major performance benefit, so it was
4337   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4338   interoperability problem that could have occurred due to interactions
4339   between authentication trailers, chunked encoding and HTTP/1.0
4340   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4341   and <xref target="header.te" format="counter"/>)
4342</t>
4343</section>
4344
4345<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4346<t>
4347  The CHAR rule does not allow the NUL character anymore (this affects
4348  the comment and quoted-string rules).  Furthermore, the quoted-pair
4349  rule does not allow escaping NUL, CR or LF anymore.
4350  (<xref target="basic.rules"/>)
4351</t>
4352<t>
4353  Clarify that HTTP-Version is case sensitive.
4354  (<xref target="http.version"/>)
4355</t>
4356<t>
4357  Remove reference to non-existant identity transfer-coding value tokens.
4358  (Sections <xref format="counter" target="transfer.codings"/> and
4359  <xref format="counter" target="message.length"/>)
4360</t>
4361<t>
4362  Clarification that the chunk length does not include
4363  the count of the octets in the chunk header and trailer.
4364  (<xref target="chunked.transfer.encoding"/>)
4365</t>
4366<t>
4367  Fix BNF to add query, as the abs_path production in
4368  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4369  (<xref target="request-uri"/>)
4370</t>
4371<t>
4372  Clarify exactly when close connection options must be sent.
4373  (<xref target="header.connection"/>)
4374</t>
4375</section>
4376</section>
4377
4378<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
4379
4380<section title="Since RFC2616">
4381<t>
4382  Extracted relevant partitions from <xref target="RFC2616"/>.
4383</t>
4384</section>
4385
4386<section title="Since draft-ietf-httpbis-p1-messaging-00">
4387<t>
4388  Closed issues:
4389  <list style="symbols"> 
4390    <t>
4391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4392      "HTTP Version should be case sensitive"
4393      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4394    </t>
4395    <t>
4396      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4397      "'unsafe' characters"
4398      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4399    </t>
4400    <t>
4401      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4402      "Chunk Size Definition"
4403      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4404    </t>
4405    <t>
4406      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4407      "Message Length"
4408      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4409    </t>
4410    <t>
4411      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4412      "Media Type Registrations"
4413      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4414    </t>
4415    <t>
4416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4417      "URI includes query"
4418      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4419    </t>
4420    <t>
4421      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4422      "No close on 1xx responses"
4423      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4424    </t>
4425    <t>
4426      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4427      "Remove 'identity' token references"
4428      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4429    </t>
4430    <t>
4431      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4432      "Import query BNF"
4433    </t>
4434    <t>
4435      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4436      "qdtext BNF"
4437    </t>
4438    <t>
4439      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4440      "Normative and Informative references"
4441    </t>
4442    <t>
4443      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4444      "RFC2606 Compliance"
4445    </t>
4446    <t>
4447      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4448      "RFC977 reference"
4449    </t>
4450    <t>
4451      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4452      "RFC1700 references"
4453    </t>
4454    <t>
4455      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4456      "inconsistency in date format explanation"
4457    </t>
4458    <t>
4459      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4460      "Date reference typo"
4461    </t>
4462    <t>
4463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4464      "Informative references"
4465    </t>
4466    <t>
4467      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4468      "ISO-8859-1 Reference"
4469    </t>
4470    <t>
4471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4472      "Normative up-to-date references"
4473    </t>
4474  </list>
4475</t>
4476<t>
4477  Other changes:
4478  <list style="symbols"> 
4479    <t>
4480      Update media type registrations to use RFC4288 template.
4481    </t>
4482    <t>
4483      Use names of RFC4234 core rules DQUOTE and HTAB,
4484      fix broken ABNF for chunk-data
4485      (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4486    </t>
4487  </list>
4488</t>
4489</section>
4490
4491<section title="Since draft-ietf-httpbis-p1-messaging-01">
4492<t>
4493  Closed issues:
4494  <list style="symbols"> 
4495    <t>
4496      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4497      "Bodies on GET (and other) requests"
4498    </t>
4499    <t>
4500      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4501      "Updating to RFC4288"
4502    </t>
4503    <t>
4504      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4505      "Status Code and Reason Phrase"
4506    </t>
4507    <t>
4508      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4509      "rel_path not used"
4510    </t>
4511  </list>
4512</t>
4513<t>
4514  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4515  <list style="symbols"> 
4516    <t>
4517      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4518      "trailer-part").
4519    </t>
4520    <t>
4521      Avoid underscore character in rule names ("http_URL" ->
4522      "http-URL", "abs_path" -> "path-absolute").
4523    </t>
4524    <t>
4525      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4526      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4527      have to be updated when switching over to RFC3986.
4528    </t>
4529    <t>
4530      Synchronize core rules with RFC5234 (this includes a change to CHAR
4531      which now excludes NUL).
4532    </t>
4533    <t>
4534      Get rid of prose rules that span multiple lines.
4535    </t>
4536    <t>
4537      Get rid of unused rules LOALPHA and UPALPHA.
4538    </t>
4539    <t>
4540      Move "Product Tokens" section (back) into Part 1, as "token" is used
4541      in the definition of the Upgrade header.
4542    </t>
4543    <t>
4544      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4545    </t>
4546    <t>
4547      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4548    </t>
4549  </list>
4550</t>
4551</section>
4552
4553<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
4554<t>
4555  Closed issues:
4556  <list style="symbols"> 
4557    <t>
4558      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/51"/>:
4559      "HTTP-date vs. rfc1123-date"
4560    </t>
4561    <t>
4562      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/64"/>:
4563      "WS in quoted-pair"
4564    </t>
4565  </list>
4566</t>
4567<t>
4568  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
4569  <list style="symbols"> 
4570    <t>
4571      Reference RFC 3984, and update header registrations for headers defined
4572      in this document.
4573    </t>
4574  </list>
4575</t>
4576<t>
4577  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4578  <list style="symbols"> 
4579    <t>
4580      Replace string literals when the string really is case-sensitive (HTTP-Version).
4581    </t>
4582  </list>
4583</t>
4584</section>
4585
4586<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
4587<t>
4588  Closed issues:
4589  <list style="symbols"> 
4590    <t>
4591      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
4592      "Connection closing"
4593    </t>
4594    <t>
4595      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/97"/>:
4596      "Move registrations and registry information to IANA Considerations"
4597    </t>
4598    <t>
4599      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/120"/>:
4600      "need new URL for PAD1995 reference"
4601    </t>
4602    <t>
4603      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/127"/>:
4604      "IANA Considerations: update HTTP URI scheme registration"
4605    </t>
4606    <t>
4607      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
4608      "Cite HTTPS URI scheme definition"
4609    </t>
4610    <t>
4611      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/129"/>:
4612      "List-type headers vs Set-Cookie"
4613    </t>
4614  </list>
4615</t>
4616<t>
4617  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4618  <list style="symbols"> 
4619    <t>
4620      Replace string literals when the string really is case-sensitive (HTTP-Date).
4621    </t>
4622    <t>
4623      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
4624    </t>
4625  </list>
4626</t>
4627</section>
4628
4629<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
4630<t>
4631  Closed issues:
4632  <list style="symbols"> 
4633    <t>
4634      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
4635      "RFC 2822 is updated by RFC 5322"
4636    </t>
4637  </list>
4638</t>
4639<t>
4640  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4641  <list style="symbols"> 
4642    <t>
4643      Use "/" instead of "|" for alternatives.
4644    </t>
4645    <t>
4646      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
4647    </t>
4648  </list>
4649</t>
4650</section>
4651
4652</section>
4653
4654</back>
4655</rfc>
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