source: draft-ietf-httpbis/latest/p1-messaging.xml @ 94

Last change on this file since 94 was 94, checked in by ylafon@…, 12 years ago

added Yves Lafon in authors section

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1<?xml version="1.0" encoding="utf-8"?>
2<!DOCTYPE rfc [
3  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
4  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
5  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
6  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
7  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
8  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
9  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
10  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
11  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
12  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
13  <!ENTITY ID-VERSION "latest">
14  <!ENTITY ID-MONTH "December">
15  <!ENTITY ID-YEAR "2007">
16  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
17  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
46<rfc obsoletes="2068, 2616" category="std"
47     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
48     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
49<front>
50
51  <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
52
53  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
54    <organization abbrev="Day Software">Day Software</organization>
55    <address>
56      <postal>
57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
59        <region>CA</region>
60        <code>92660</code>
61        <country>USA</country>
62      </postal>
63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author initials="J." surname="Gettys" fullname="Jim Gettys">
71    <organization>One Laptop per Child</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@laptop.org</email>
81      <uri>http://www.laptop.org/</uri>
82    </address>
83  </author>
84 
85  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author initials="L." surname="Masinter" fullname="Larry Masinter">
115    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128 
129  <author initials="P." surname="Leach" fullname="Paul J. Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141   
142  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author initials="Y." surname="Lafon" fullname="Yves Lafon">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
176
177<abstract>
178<t>
179   The Hypertext Transfer Protocol (HTTP) is an application-level
180   protocol for distributed, collaborative, hypermedia information
181   systems. HTTP has been in use by the World Wide Web global information
182   initiative since 1990. This document is Part 1 of the seven-part specification
183   that defines the protocol referred to as "HTTP/1.1" and, taken together,
184   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
185   its associated terminology, defines the "http" and "https" Uniform
186   Resource Identifier (URI) schemes, defines the generic message syntax
187   and parsing requirements for HTTP message frames, and describes
188   general security concerns for implementations.
189</t>
190</abstract>
191
192<note title="Editorial Note (To be removed by RFC Editor)">
193  <t>
194    This version of the HTTP specification contains only minimal editorial
195    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
196    and authors' addresses).  All other changes are due to partitioning the
197    original into seven mostly independent parts.  The intent is for readers
198    of future drafts to able to use draft 00 as the basis for comparison
199    when the WG makes later changes to the specification text.  This draft
200    will shortly be followed by draft 01 (containing the first round of changes
201    that have already been agreed to on the mailing list). There is no point in
202    reviewing this draft other than to verify that the partitioning has been
203    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
204    will be the editors after draft 00 is submitted.
205  </t>
206  <t>
207    Discussion of this draft should take place on the HTTPBIS working group
208    mailing list (ietf-http-wg@w3.org). The current issues list is
209    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
210    and related documents (including fancy diffs) can be found at
211    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
212  </t>
213</note>
214</front>
215<middle>
216<section title="Introduction" anchor="introduction">
217<t>
218   This document will define aspects of HTTP related to overall network
219   operation, message framing, interaction with transport protocols, and
220   URI schemes. Right now it only includes the extracted relevant sections
221   of <xref target="RFC2616"/>.
222</t>
223<section title="Purpose" anchor="intro.purpose">
224<t>
225   The Hypertext Transfer Protocol (HTTP) is an application-level
226   protocol for distributed, collaborative, hypermedia information
227   systems. HTTP has been in use by the World-Wide Web global
228   information initiative since 1990. The first version of HTTP,
229   referred to as HTTP/0.9, was a simple protocol for raw data transfer
230   across the Internet. HTTP/1.0, as defined by RFC 1945 <xref target="RFC1945"/>, improved
231   the protocol by allowing messages to be in the format of MIME-like
232   messages, containing metainformation about the data transferred and
233   modifiers on the request/response semantics. However, HTTP/1.0 does
234   not sufficiently take into consideration the effects of hierarchical
235   proxies, caching, the need for persistent connections, or virtual
236   hosts. In addition, the proliferation of incompletely-implemented
237   applications calling themselves "HTTP/1.0" has necessitated a
238   protocol version change in order for two communicating applications
239   to determine each other's true capabilities.
240</t>
241<t>
242   This specification defines the protocol referred to as "HTTP/1.1".
243   This protocol includes more stringent requirements than HTTP/1.0 in
244   order to ensure reliable implementation of its features.
245</t>
246<t>
247   Practical information systems require more functionality than simple
248   retrieval, including search, front-end update, and annotation. HTTP
249   allows an open-ended set of methods and headers that indicate the
250   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
251   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
252   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
253   method is to be applied. Messages are passed in a format similar to
254   that used by Internet mail <xref target="RFC822"/> as defined by the Multipurpose
255   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
256</t>
257<t>
258   HTTP is also used as a generic protocol for communication between
259   user agents and proxies/gateways to other Internet systems, including
260   those supported by the SMTP <xref target="RFC821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
261   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
262   access to resources available from diverse applications.
263</t>
264</section>
265
266<section title="Requirements" anchor="intro.requirements">
267<t>
268   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
269   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
270   document are to be interpreted as described in RFC 2119 <xref target="RFC2119"/>.
271</t>
272<t>
273   An implementation is not compliant if it fails to satisfy one or more
274   of the &MUST; or &REQUIRED; level requirements for the protocols it
275   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
276   level and all the &SHOULD; level requirements for its protocols is said
277   to be "unconditionally compliant"; one that satisfies all the &MUST;
278   level requirements but not all the &SHOULD; level requirements for its
279   protocols is said to be "conditionally compliant."
280</t>
281</section>
282
283<section title="Terminology" anchor="intro.terminology">
284<t>
285   This specification uses a number of terms to refer to the roles
286   played by participants in, and objects of, the HTTP communication.
287</t>
288<t>
289  <iref item="connection"/>
290  <x:dfn>connection</x:dfn>
291  <list>
292    <t>
293      A transport layer virtual circuit established between two programs
294      for the purpose of communication.
295    </t>
296  </list>
297</t>
298<t>
299  <iref item="message"/>
300  <x:dfn>message</x:dfn>
301  <list>
302    <t>
303      The basic unit of HTTP communication, consisting of a structured
304      sequence of octets matching the syntax defined in <xref target="http.message"/> and
305      transmitted via the connection.
306    </t>
307  </list>
308</t>
309<t>
310  <iref item="request"/>
311  <x:dfn>request</x:dfn>
312  <list>
313    <t>
314      An HTTP request message, as defined in <xref target="request"/>.
315    </t>
316  </list>
317</t>
318<t>
319  <iref item="response"/>
320  <x:dfn>response</x:dfn>
321  <list>
322    <t>
323      An HTTP response message, as defined in <xref target="response"/>.
324    </t>
325  </list>
326</t>
327<t>
328  <iref item="resource"/>
329  <x:dfn>resource</x:dfn>
330  <list>
331    <t>
332      A network data object or service that can be identified by a URI,
333      as defined in <xref target="uri"/>. Resources may be available in multiple
334      representations (e.g. multiple languages, data formats, size, and
335      resolutions) or vary in other ways.
336    </t>
337  </list>
338</t>
339<t>
340  <iref item="entity"/>
341  <x:dfn>entity</x:dfn>
342  <list>
343    <t>
344      The information transferred as the payload of a request or
345      response. An entity consists of metainformation in the form of
346      entity-header fields and content in the form of an entity-body, as
347      described in &entity;.
348    </t>
349  </list>
350</t>
351<t>
352  <iref item="representation"/>
353  <x:dfn>representation</x:dfn>
354  <list>
355    <t>
356      An entity included with a response that is subject to content
357      negotiation, as described in &content.negotiation;. There may exist multiple
358      representations associated with a particular response status.
359    </t>
360  </list>
361</t>
362<t>
363  <iref item="content negotiation"/>
364  <x:dfn>content negotiation</x:dfn>
365  <list>
366    <t>
367      The mechanism for selecting the appropriate representation when
368      servicing a request, as described in &content.negotiation;. The
369      representation of entities in any response can be negotiated
370      (including error responses).
371    </t>
372  </list>
373</t>
374<t>
375  <iref item="variant"/>
376  <x:dfn>variant</x:dfn>
377  <list>
378    <t>
379      A resource may have one, or more than one, representation(s)
380      associated with it at any given instant. Each of these
381      representations is termed a `variant'.  Use of the term `variant'
382      does not necessarily imply that the resource is subject to content
383      negotiation.
384    </t>
385  </list>
386</t>
387<t>
388  <iref item="client"/>
389  <x:dfn>client</x:dfn>
390  <list>
391    <t>
392      A program that establishes connections for the purpose of sending
393      requests.
394    </t>
395  </list>
396</t>
397<t>
398  <iref item="user agent"/>
399  <x:dfn>user agent</x:dfn>
400  <list>
401    <t>
402      The client which initiates a request. These are often browsers,
403      editors, spiders (web-traversing robots), or other end user tools.
404    </t>
405  </list>
406</t>
407<t>
408  <iref item="server"/>
409  <x:dfn>server</x:dfn>
410  <list>
411    <t>
412      An application program that accepts connections in order to
413      service requests by sending back responses. Any given program may
414      be capable of being both a client and a server; our use of these
415      terms refers only to the role being performed by the program for a
416      particular connection, rather than to the program's capabilities
417      in general. Likewise, any server may act as an origin server,
418      proxy, gateway, or tunnel, switching behavior based on the nature
419      of each request.
420    </t>
421  </list>
422</t>
423<t>
424  <iref item="origin server"/>
425  <x:dfn>origin server</x:dfn>
426  <list>
427    <t>
428      The server on which a given resource resides or is to be created.
429    </t>
430  </list>
431</t>
432<t>
433  <iref item="proxy"/>
434  <x:dfn>proxy</x:dfn>
435  <list>
436    <t>
437      An intermediary program which acts as both a server and a client
438      for the purpose of making requests on behalf of other clients.
439      Requests are serviced internally or by passing them on, with
440      possible translation, to other servers. A proxy &MUST; implement
441      both the client and server requirements of this specification. A
442      "transparent proxy" is a proxy that does not modify the request or
443      response beyond what is required for proxy authentication and
444      identification. A "non-transparent proxy" is a proxy that modifies
445      the request or response in order to provide some added service to
446      the user agent, such as group annotation services, media type
447      transformation, protocol reduction, or anonymity filtering. Except
448      where either transparent or non-transparent behavior is explicitly
449      stated, the HTTP proxy requirements apply to both types of
450      proxies.
451    </t>
452  </list>
453</t>
454<t>
455  <iref item="gateway"/>
456  <x:dfn>gateway</x:dfn>
457  <list>
458    <t>
459      A server which acts as an intermediary for some other server.
460      Unlike a proxy, a gateway receives requests as if it were the
461      origin server for the requested resource; the requesting client
462      may not be aware that it is communicating with a gateway.
463    </t>
464  </list>
465</t>
466<t>
467  <iref item="tunnel"/>
468  <x:dfn>tunnel</x:dfn>
469  <list>
470    <t>
471      An intermediary program which is acting as a blind relay between
472      two connections. Once active, a tunnel is not considered a party
473      to the HTTP communication, though the tunnel may have been
474      initiated by an HTTP request. The tunnel ceases to exist when both
475      ends of the relayed connections are closed.
476    </t>
477  </list>
478</t>
479<t>
480  <iref item="cache"/>
481  <x:dfn>cache</x:dfn>
482  <list>
483    <t>
484      A program's local store of response messages and the subsystem
485      that controls its message storage, retrieval, and deletion. A
486      cache stores cacheable responses in order to reduce the response
487      time and network bandwidth consumption on future, equivalent
488      requests. Any client or server may include a cache, though a cache
489      cannot be used by a server that is acting as a tunnel.
490    </t>
491  </list>
492</t>
493<t>
494  <iref item="cacheable"/>
495  <x:dfn>cacheable</x:dfn>
496  <list>
497    <t>
498      A response is cacheable if a cache is allowed to store a copy of
499      the response message for use in answering subsequent requests. The
500      rules for determining the cacheability of HTTP responses are
501      defined in &caching;. Even if a resource is cacheable, there may
502      be additional constraints on whether a cache can use the cached
503      copy for a particular request.
504    </t>
505  </list>
506</t>
507<t>
508  <iref item="upstream"/>
509  <iref item="downstream"/>
510  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
511  <list>
512    <t>
513      Upstream and downstream describe the flow of a message: all
514      messages flow from upstream to downstream.
515    </t>
516  </list>
517</t>
518<t>
519  <iref item="inbound"/>
520  <iref item="outbound"/>
521  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
522  <list>
523    <t>
524      Inbound and outbound refer to the request and response paths for
525      messages: "inbound" means "traveling toward the origin server",
526      and "outbound" means "traveling toward the user agent"
527    </t>
528  </list>
529</t>
530</section>
531
532<section title="Overall Operation" anchor="intro.overall.operation">
533<t>
534   The HTTP protocol is a request/response protocol. A client sends a
535   request to the server in the form of a request method, URI, and
536   protocol version, followed by a MIME-like message containing request
537   modifiers, client information, and possible body content over a
538   connection with a server. The server responds with a status line,
539   including the message's protocol version and a success or error code,
540   followed by a MIME-like message containing server information, entity
541   metainformation, and possible entity-body content. The relationship
542   between HTTP and MIME is described in &diff2045entity;.
543</t>
544<t>
545   Most HTTP communication is initiated by a user agent and consists of
546   a request to be applied to a resource on some origin server. In the
547   simplest case, this may be accomplished via a single connection (v)
548   between the user agent (UA) and the origin server (O).
549</t>
550<figure><artwork type="drawing">
551       request chain ------------------------&gt;
552    UA -------------------v------------------- O
553       &lt;----------------------- response chain
554</artwork></figure>
555<t>
556   A more complicated situation occurs when one or more intermediaries
557   are present in the request/response chain. There are three common
558   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
559   forwarding agent, receiving requests for a URI in its absolute form,
560   rewriting all or part of the message, and forwarding the reformatted
561   request toward the server identified by the URI. A gateway is a
562   receiving agent, acting as a layer above some other server(s) and, if
563   necessary, translating the requests to the underlying server's
564   protocol. A tunnel acts as a relay point between two connections
565   without changing the messages; tunnels are used when the
566   communication needs to pass through an intermediary (such as a
567   firewall) even when the intermediary cannot understand the contents
568   of the messages.
569</t>
570<figure><artwork type="drawing">
571       request chain --------------------------------------&gt;
572    UA -----v----- A -----v----- B -----v----- C -----v----- O
573       &lt;------------------------------------- response chain
574</artwork></figure>
575<t>
576   The figure above shows three intermediaries (A, B, and C) between the
577   user agent and origin server. A request or response message that
578   travels the whole chain will pass through four separate connections.
579   This distinction is important because some HTTP communication options
580   may apply only to the connection with the nearest, non-tunnel
581   neighbor, only to the end-points of the chain, or to all connections
582   along the chain. Although the diagram is linear, each participant may
583   be engaged in multiple, simultaneous communications. For example, B
584   may be receiving requests from many clients other than A, and/or
585   forwarding requests to servers other than C, at the same time that it
586   is handling A's request.
587</t>
588<t>
589   Any party to the communication which is not acting as a tunnel may
590   employ an internal cache for handling requests. The effect of a cache
591   is that the request/response chain is shortened if one of the
592   participants along the chain has a cached response applicable to that
593   request. The following illustrates the resulting chain if B has a
594   cached copy of an earlier response from O (via C) for a request which
595   has not been cached by UA or A.
596</t>
597<figure><artwork type="drawing">
598          request chain ----------&gt;
599       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
600          &lt;--------- response chain
601</artwork></figure>
602<t>
603   Not all responses are usefully cacheable, and some requests may
604   contain modifiers which place special requirements on cache behavior.
605   HTTP requirements for cache behavior and cacheable responses are
606   defined in &caching;.
607</t>
608<t>
609   In fact, there are a wide variety of architectures and configurations
610   of caches and proxies currently being experimented with or deployed
611   across the World Wide Web. These systems include national hierarchies
612   of proxy caches to save transoceanic bandwidth, systems that
613   broadcast or multicast cache entries, organizations that distribute
614   subsets of cached data via CD-ROM, and so on. HTTP systems are used
615   in corporate intranets over high-bandwidth links, and for access via
616   PDAs with low-power radio links and intermittent connectivity. The
617   goal of HTTP/1.1 is to support the wide diversity of configurations
618   already deployed while introducing protocol constructs that meet the
619   needs of those who build web applications that require high
620   reliability and, failing that, at least reliable indications of
621   failure.
622</t>
623<t>
624   HTTP communication usually takes place over TCP/IP connections. The
625   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
626   not preclude HTTP from being implemented on top of any other protocol
627   on the Internet, or on other networks. HTTP only presumes a reliable
628   transport; any protocol that provides such guarantees can be used;
629   the mapping of the HTTP/1.1 request and response structures onto the
630   transport data units of the protocol in question is outside the scope
631   of this specification.
632</t>
633<t>
634   In HTTP/1.0, most implementations used a new connection for each
635   request/response exchange. In HTTP/1.1, a connection may be used for
636   one or more request/response exchanges, although connections may be
637   closed for a variety of reasons (see <xref target="persistent.connections"/>).
638</t>
639</section>
640</section>
641
642<section title="Notational Conventions and Generic Grammar" anchor="notation">
643
644<section title="Augmented BNF" anchor="notation.abnf">
645<t>
646   All of the mechanisms specified in this document are described in
647   both prose and an augmented Backus-Naur Form (BNF) similar to that
648   used by RFC 822 <xref target="RFC822"/>. Implementors will need to be familiar with the
649   notation in order to understand this specification. The augmented BNF
650   includes the following constructs:
651</t>
652<t>
653   name = definition
654  <list>
655    <t>
656      The name of a rule is simply the name itself (without any
657      enclosing "&lt;" and "&gt;") and is separated from its definition by the
658      equal "=" character. White space is only significant in that
659      indentation of continuation lines is used to indicate a rule
660      definition that spans more than one line. Certain basic rules are
661      in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
662      brackets are used within definitions whenever their presence will
663      facilitate discerning the use of rule names.
664    </t>
665  </list>
666</t>
667<t>
668   "literal"
669  <list>
670    <t>
671      Quotation marks surround literal text. Unless stated otherwise,
672      the text is case-insensitive.
673    </t>
674  </list>
675</t>
676<t>
677   rule1 | rule2
678  <list>
679    <t>
680      Elements separated by a bar ("|") are alternatives, e.g., "yes |
681      no" will accept yes or no.
682    </t>
683  </list>
684</t>
685<t>
686   (rule1 rule2)
687  <list>
688    <t>
689      Elements enclosed in parentheses are treated as a single element.
690      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
691      foo elem" and "elem bar elem".
692    </t>
693  </list>
694</t>
695<t>
696   *rule
697  <list>
698    <t>
699      The character "*" preceding an element indicates repetition. The
700      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
701      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
702      that "*(element)" allows any number, including zero; "1*element"
703      requires at least one; and "1*2element" allows one or two.
704    </t>
705  </list>
706</t>
707<t>
708   [rule]
709  <list>
710    <t>
711      Square brackets enclose optional elements; "[foo bar]" is
712      equivalent to "*1(foo bar)".
713    </t>
714  </list>
715</t>
716<t>
717   N rule
718  <list>
719    <t>
720      Specific repetition: "&lt;n&gt;(element)" is equivalent to
721      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
722      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
723      alphabetic characters.
724    </t>
725  </list>
726</t>
727<t>
728   #rule
729  <list>
730    <t>
731      A construct "#" is defined, similar to "*", for defining lists of
732      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
733      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
734      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
735      form of lists very easy; a rule such as
736    </t>
737    <t>
738         ( *LWS element *( *LWS "," *LWS element ))
739    </t>
740    <t>
741      can be shown as
742    </t>
743    <t>
744         1#element
745    </t>
746    <t>
747      Wherever this construct is used, null elements are allowed, but do
748      not contribute to the count of elements present. That is,
749      "(element), , (element) " is permitted, but counts as only two
750      elements. Therefore, where at least one element is required, at
751      least one non-null element &MUST; be present. Default values are 0
752      and infinity so that "#element" allows any number, including zero;
753      "1#element" requires at least one; and "1#2element" allows one or
754      two.
755    </t>
756  </list>
757</t>
758<t>
759   ; comment
760  <list>
761    <t>
762      A semi-colon, set off some distance to the right of rule text,
763      starts a comment that continues to the end of line. This is a
764      simple way of including useful notes in parallel with the
765      specifications.
766    </t>
767  </list>
768</t>
769<t>
770   implied *LWS
771  <list>
772    <t>
773      The grammar described by this specification is word-based. Except
774      where noted otherwise, linear white space (LWS) can be included
775      between any two adjacent words (token or quoted-string), and
776      between adjacent words and separators, without changing the
777      interpretation of a field. At least one delimiter (LWS and/or
778      separators) &MUST; exist between any two tokens (for the definition
779      of "token" below), since they would otherwise be interpreted as a
780      single token.
781    </t>
782  </list>
783</t>
784</section>
785
786<section title="Basic Rules" anchor="basic.rules">
787<x:anchor-alias value="OCTET"/>
788<x:anchor-alias value="CHAR"/>
789<x:anchor-alias value="UPALPHA"/>
790<x:anchor-alias value="LOALPHA"/>
791<x:anchor-alias value="ALPHA"/>
792<x:anchor-alias value="DIGIT"/>
793<x:anchor-alias value="CTL"/>
794<x:anchor-alias value="CR"/>
795<x:anchor-alias value="LF"/>
796<x:anchor-alias value="SP"/>
797<x:anchor-alias value="HT"/>
798<x:anchor-alias value="CRLF"/>
799<x:anchor-alias value="LWS"/>
800<x:anchor-alias value="TEXT"/>
801<x:anchor-alias value="HEX"/>
802<x:anchor-alias value="token"/>
803<x:anchor-alias value="separators"/>
804<x:anchor-alias value="comment"/>
805<x:anchor-alias value="ctext"/>
806<x:anchor-alias value="quoted-string"/>
807<x:anchor-alias value="qdtext"/>
808<x:anchor-alias value="quoted-pair"/>
809<t>
810   The following rules are used throughout this specification to
811   describe basic parsing constructs. The US-ASCII coded character set
812   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
813</t>
814<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><iref primary="true" item="Grammar" subitem="UPALPHA"/><iref primary="true" item="Grammar" subitem="LOALPHA"/><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="HT"/>
815    OCTET          = &lt;any 8-bit sequence of data&gt;
816    CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
817    UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
818    LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
819    ALPHA          = UPALPHA | LOALPHA
820    DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
821    CTL            = &lt;any US-ASCII control character
822                     (octets 0 - 31) and DEL (127)&gt;
823    CR             = &lt;US-ASCII CR, carriage return (13)&gt;
824    LF             = &lt;US-ASCII LF, linefeed (10)&gt;
825    SP             = &lt;US-ASCII SP, space (32)&gt;
826    HT             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
827    &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
828</artwork></figure>
829<t>
830   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
831   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
832   tolerant applications). The end-of-line marker within an entity-body
833   is defined by its associated media type, as described in &payload;.
834</t>
835<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
836    CRLF           = CR LF
837</artwork></figure>
838<t>
839   HTTP/1.1 header field values can be folded onto multiple lines if the
840   continuation line begins with a space or horizontal tab. All linear
841   white space, including folding, has the same semantics as SP. A
842   recipient &MAY; replace any linear white space with a single SP before
843   interpreting the field value or forwarding the message downstream.
844</t>
845<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
846    LWS            = [CRLF] 1*( SP | HT )
847</artwork></figure>
848<t>
849   The TEXT rule is only used for descriptive field contents and values
850   that are not intended to be interpreted by the message parser. Words
851   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
852   <xref target="ISO-8859"/> only when encoded according to the rules of RFC 2047
853   <xref target="RFC2047"/>.
854</t>
855<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
856    TEXT           = &lt;any OCTET except CTLs,
857                     but including LWS&gt;
858</artwork></figure>
859<t>
860   A CRLF is allowed in the definition of TEXT only as part of a header
861   field continuation. It is expected that the folding LWS will be
862   replaced with a single SP before interpretation of the TEXT value.
863</t>
864<t>
865   Hexadecimal numeric characters are used in several protocol elements.
866</t>
867<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
868    HEX            = "A" | "B" | "C" | "D" | "E" | "F"
869                   | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
870</artwork></figure>
871<t>
872   Many HTTP/1.1 header field values consist of words separated by LWS
873   or special characters. These special characters &MUST; be in a quoted
874   string to be used within a parameter value (as defined in
875   <xref target="transfer.codings"/>).
876</t>
877<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
878    token          = 1*&lt;any CHAR except CTLs or separators&gt;
879    separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
880                   | "," | ";" | ":" | "\" | &lt;"&gt;
881                   | "/" | "[" | "]" | "?" | "="
882                   | "{" | "}" | SP | HT
883</artwork></figure>
884<t>
885   Comments can be included in some HTTP header fields by surrounding
886   the comment text with parentheses. Comments are only allowed in
887   fields containing "comment" as part of their field value definition.
888   In all other fields, parentheses are considered part of the field
889   value.
890</t>
891<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
892    comment        = "(" *( ctext | quoted-pair | comment ) ")"
893    ctext          = &lt;any TEXT excluding "(" and ")"&gt;
894</artwork></figure>
895<t>
896   A string of text is parsed as a single word if it is quoted using
897   double-quote marks.
898</t>
899<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
900    quoted-string  = ( &lt;"&gt; *(qdtext | quoted-pair ) &lt;"&gt; )
901    qdtext         = &lt;any TEXT except &lt;"&gt;&gt;
902</artwork></figure>
903<t>
904   The backslash character ("\") &MAY; be used as a single-character
905   quoting mechanism only within quoted-string and comment constructs.
906</t>
907<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
908    quoted-pair    = "\" CHAR
909</artwork></figure>
910</section>
911</section>
912
913<section title="Protocol Parameters" anchor="protocol.parameters">
914
915<section title="HTTP Version" anchor="http.version">
916<t>
917   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
918   of the protocol. The protocol versioning policy is intended to allow
919   the sender to indicate the format of a message and its capacity for
920   understanding further HTTP communication, rather than the features
921   obtained via that communication. No change is made to the version
922   number for the addition of message components which do not affect
923   communication behavior or which only add to extensible field values.
924   The &lt;minor&gt; number is incremented when the changes made to the
925   protocol add features which do not change the general message parsing
926   algorithm, but which may add to the message semantics and imply
927   additional capabilities of the sender. The &lt;major&gt; number is
928   incremented when the format of a message within the protocol is
929   changed. See RFC 2145 <xref target="RFC2145"/> for a fuller explanation.
930</t>
931<t>
932   The version of an HTTP message is indicated by an HTTP-Version field
933   in the first line of the message. HTTP-Version is case-sensitive.
934</t>
935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
936       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
937</artwork></figure>
938<t>
939   Note that the major and minor numbers &MUST; be treated as separate
940   integers and that each &MAY; be incremented higher than a single digit.
941   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
942   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
943   &MUST-NOT; be sent.
944</t>
945<t>
946   An application that sends a request or response message that includes
947   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
948   with this specification. Applications that are at least conditionally
949   compliant with this specification &SHOULD; use an HTTP-Version of
950   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
951   not compatible with HTTP/1.0. For more details on when to send
952   specific HTTP-Version values, see RFC 2145 <xref target="RFC2145"/>.
953</t>
954<t>
955   The HTTP version of an application is the highest HTTP version for
956   which the application is at least conditionally compliant.
957</t>
958<t>
959   Proxy and gateway applications need to be careful when forwarding
960   messages in protocol versions different from that of the application.
961   Since the protocol version indicates the protocol capability of the
962   sender, a proxy/gateway &MUST-NOT; send a message with a version
963   indicator which is greater than its actual version. If a higher
964   version request is received, the proxy/gateway &MUST; either downgrade
965   the request version, or respond with an error, or switch to tunnel
966   behavior.
967</t>
968<t>
969   Due to interoperability problems with HTTP/1.0 proxies discovered
970   since the publication of RFC 2068 <xref target="RFC2068"/>, caching proxies MUST, gateways
971   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
972   they support. The proxy/gateway's response to that request &MUST; be in
973   the same major version as the request.
974</t>
975<t>
976  <list>
977    <t>
978      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
979      of header fields required or forbidden by the versions involved.
980    </t>
981  </list>
982</t>
983</section>
984
985<section title="Uniform Resource Identifiers" anchor="uri">
986<t>
987   URIs have been known by many names: WWW addresses, Universal Document
988   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
989   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
990   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
991   simply formatted strings which identify--via name, location, or any
992   other characteristic--a resource.
993</t>
994
995<section title="General Syntax" anchor="general.syntax">
996<t>
997   URIs in HTTP can be represented in absolute form or relative to some
998   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
999   forms are differentiated by the fact that absolute URIs always begin
1000   with a scheme name followed by a colon. For definitive information on
1001   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
1002   Generic Syntax and Semantics," RFC 2396 <xref target="RFC2396"/> (which replaces RFCs
1003   1738 <xref target="RFC1738"/> and RFC 1808 <xref target="RFC1808"/>). This specification adopts the
1004   definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
1005   "host","abs_path", "rel_path", and "authority" from that
1006   specification.
1007</t>
1008<t>
1009   The HTTP protocol 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<t>
1029   The "http" scheme is used to locate network resources via the HTTP
1030   protocol. This section defines the scheme-specific syntax and
1031   semantics for http URLs.
1032</t>
1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http_URL"/>
1034http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
1035</artwork></figure>
1036<t>
1037   If the port is empty or not given, port 80 is assumed. The semantics
1038   are that the identified resource is located at the server listening
1039   for TCP connections on that port of that host, and the Request-URI
1040   for the resource is abs_path (<xref target="request-uri"/>). The use of IP addresses
1041   in URLs &SHOULD; be avoided whenever possible (see RFC 1900 <xref target="RFC1900"/>). If
1042   the abs_path is not present in the URL, it &MUST; be given as "/" when
1043   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1044
1045   receives a host name which is not a fully qualified domain name, it
1046   &MAY; add its domain to the host name it received. If a proxy receives
1047   a fully qualified domain name, the proxy &MUST-NOT; change the host
1048   name.
1049</t>
1050</section>
1051
1052<section title="URI Comparison" anchor="uri.comparison">
1053<t>
1054   When comparing two URIs to decide if they match or not, a client
1055   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1056   URIs, with these exceptions:
1057  <list style="symbols">
1058    <t>A port that is empty or not given is equivalent to the default
1059        port for that URI-reference;</t>
1060    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1061    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1062    <t>An empty abs_path is equivalent to an abs_path of "/".</t>
1063  </list>
1064</t>
1065<t>
1066   Characters other than those in the "reserved" set (see
1067   RFC 2396 <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1068</t>
1069<t>
1070   For example, the following three URIs are equivalent:
1071</t>
1072<figure><artwork type="example">
1073   http://example.com:80/~smith/home.html
1074   http://EXAMPLE.com/%7Esmith/home.html
1075   http://EXAMPLE.com:/%7esmith/home.html
1076</artwork></figure>
1077</section>
1078</section>
1079
1080<section title="Date/Time Formats" anchor="date.time.formats">
1081<section title="Full Date" anchor="full.date">
1082<t>
1083   HTTP applications have historically allowed three different formats
1084   for the representation of date/time stamps:
1085</t>
1086<figure><artwork type="example">
1087   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1088   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1089   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1090</artwork></figure>
1091<t>
1092   The first format is preferred as an Internet standard and represents
1093   a fixed-length subset of that defined by RFC 1123 <xref target="RFC1123"/> (an update to
1094   RFC 822 <xref target="RFC822"/>). The other formats are described here only for
1095   compatibility with obsolete implementations.
1096   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1097   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1098   only generate the RFC 1123 format for representing HTTP-date values
1099   in header fields. See <xref target="tolerant.applications"/> for further information.
1100</t>
1101<t><list><t>
1102      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1103      accepting date values that may have been sent by non-HTTP
1104      applications, as is sometimes the case when retrieving or posting
1105      messages via proxies/gateways to SMTP or NNTP.
1106</t></list></t>
1107<t>
1108   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1109   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1110   equal to UTC (Coordinated Universal Time). This is indicated in the
1111   first two formats by the inclusion of "GMT" as the three-letter
1112   abbreviation for time zone, and &MUST; be assumed when reading the
1113   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1114   additional LWS beyond that specifically included as SP in the
1115   grammar.
1116</t>
1117<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-date"/><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="rfc850-date"/><iref primary="true" item="Grammar" subitem="asctime-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/><iref primary="true" item="Grammar" subitem="time"/><iref primary="true" item="Grammar" subitem="wkday"/><iref primary="true" item="Grammar" subitem="weekday"/><iref primary="true" item="Grammar" subitem="month"/>
1118    HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1119    rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1120    rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1121    asctime-date = wkday SP date3 SP time SP 4DIGIT
1122    date1        = 2DIGIT SP month SP 4DIGIT
1123                   ; day month year (e.g., 02 Jun 1982)
1124    date2        = 2DIGIT "-" month "-" 2DIGIT
1125                   ; day-month-year (e.g., 02-Jun-82)
1126    date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1127                   ; month day (e.g., Jun  2)
1128    time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1129                   ; 00:00:00 - 23:59:59
1130    wkday        = "Mon" | "Tue" | "Wed"
1131                 | "Thu" | "Fri" | "Sat" | "Sun"
1132    weekday      = "Monday" | "Tuesday" | "Wednesday"
1133                 | "Thursday" | "Friday" | "Saturday" | "Sunday"
1134    month        = "Jan" | "Feb" | "Mar" | "Apr"
1135                 | "May" | "Jun" | "Jul" | "Aug"
1136                 | "Sep" | "Oct" | "Nov" | "Dec"
1137</artwork></figure>
1138<t>
1139      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1140      to their usage within the protocol stream. Clients and servers are
1141      not required to use these formats for user presentation, request
1142      logging, etc.
1143</t>
1144</section>
1145</section>
1146
1147<section title="Transfer Codings" anchor="transfer.codings">
1148<t>
1149   Transfer-coding values are used to indicate an encoding
1150   transformation that has been, can be, or may need to be applied to an
1151   entity-body in order to ensure "safe transport" through the network.
1152   This differs from a content coding in that the transfer-coding is a
1153   property of the message, not of the original entity.
1154</t>
1155<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1156    transfer-coding         = "chunked" | transfer-extension
1157    transfer-extension      = token *( ";" parameter )
1158</artwork></figure>
1159<t>
1160   Parameters are in  the form of attribute/value pairs.
1161</t>
1162<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"/>
1163    parameter               = attribute "=" value
1164    attribute               = token
1165    value                   = token | quoted-string
1166</artwork></figure>
1167<t>
1168   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1169   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1170   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1171</t>
1172<t>
1173   Whenever a transfer-coding is applied to a message-body, the set of
1174   transfer-codings &MUST; include "chunked", unless the message is
1175   terminated by closing the connection. When the "chunked" transfer-coding
1176   is used, it &MUST; be the last transfer-coding applied to the
1177   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1178   than once to a message-body. These rules allow the recipient to
1179   determine the transfer-length of the message (<xref target="message.length"/>).
1180</t>
1181<t>
1182   Transfer-codings are analogous to the Content-Transfer-Encoding
1183   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1184   binary data over a 7-bit transport service. However, safe transport
1185   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1186   the only unsafe characteristic of message-bodies is the difficulty in
1187   determining the exact body length (<xref target="message.length"/>), or the desire to
1188   encrypt data over a shared transport.
1189</t>
1190<t>
1191   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1192   transfer-coding value tokens. Initially, the registry contains the
1193   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1194   "gzip" (&payload;), "compress" (&payload;), and "deflate" (&payload;).
1195</t>
1196<t>
1197   New transfer-coding value tokens &SHOULD; be registered in the same way
1198   as new content-coding value tokens (&payload;).
1199</t>
1200<t>
1201   A server which receives an entity-body with a transfer-coding it does
1202   not understand &SHOULD; return 501 (Unimplemented), and close the
1203   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1204   client.
1205</t>
1206
1207<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1208<t>
1209   The chunked encoding modifies the body of a message in order to
1210   transfer it as a series of chunks, each with its own size indicator,
1211   followed by an &OPTIONAL; trailer containing entity-header fields. This
1212   allows dynamically produced content to be transferred along with the
1213   information necessary for the recipient to verify that it has
1214   received the full message.
1215</t>
1216<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"/>
1217    Chunked-Body   = *chunk
1218                     last-chunk
1219                     trailer
1220                     CRLF
1221
1222    chunk          = chunk-size [ chunk-extension ] CRLF
1223                     chunk-data CRLF
1224    chunk-size     = 1*HEX
1225    last-chunk     = 1*("0") [ chunk-extension ] CRLF
1226
1227    chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1228    chunk-ext-name = token
1229    chunk-ext-val  = token | quoted-string
1230    chunk-data     = chunk-size(OCTET)
1231    trailer        = *(entity-header CRLF)
1232</artwork></figure>
1233<t>
1234   The chunk-size field is a string of hex digits indicating the size of
1235   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1236   zero, followed by the trailer, which is terminated by an empty line.
1237</t>
1238<t>
1239   The trailer allows the sender to include additional HTTP header
1240   fields at the end of the message. The Trailer header field can be
1241   used to indicate which header fields are included in a trailer (see
1242   <xref target="header.trailer"/>).
1243</t>
1244<t>
1245   A server using chunked transfer-coding in a response &MUST-NOT; use the
1246   trailer for any header fields unless at least one of the following is
1247   true:
1248  <list style="numbers">
1249    <t>the request included a TE header field that indicates "trailers" is
1250     acceptable in the transfer-coding of the  response, as described in
1251     <xref target="header.te"/>; or,</t>
1252
1253    <t>the server is the origin server for the response, the trailer
1254     fields consist entirely of optional metadata, and the recipient
1255     could use the message (in a manner acceptable to the origin server)
1256     without receiving this metadata.  In other words, the origin server
1257     is willing to accept the possibility that the trailer fields might
1258     be silently discarded along the path to the client.</t>
1259  </list>
1260</t>
1261<t>
1262   This requirement prevents an interoperability failure when the
1263   message is being received by an HTTP/1.1 (or later) proxy and
1264   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1265   compliance with the protocol would have necessitated a possibly
1266   infinite buffer on the proxy.
1267</t>
1268<t>
1269   A process for decoding the "chunked" transfer-coding
1270   can be represented in pseudo-code as:
1271</t>
1272<figure><artwork type="code">
1273    length := 0
1274    read chunk-size, chunk-extension (if any) and CRLF
1275    while (chunk-size &gt; 0) {
1276       read chunk-data and CRLF
1277       append chunk-data to entity-body
1278       length := length + chunk-size
1279       read chunk-size and CRLF
1280    }
1281    read entity-header
1282    while (entity-header not empty) {
1283       append entity-header to existing header fields
1284       read entity-header
1285    }
1286    Content-Length := length
1287    Remove "chunked" from Transfer-Encoding
1288</artwork></figure>
1289<t>
1290   All HTTP/1.1 applications &MUST; be able to receive and decode the
1291   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1292   they do not understand.
1293</t>
1294</section>
1295</section>
1296
1297</section>
1298
1299<section title="HTTP Message" anchor="http.message">
1300
1301<section title="Message Types" anchor="message.types">
1302<t>
1303   HTTP messages consist of requests from client to server and responses
1304   from server to client.
1305</t>
1306<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1307    HTTP-message   = Request | Response     ; HTTP/1.1 messages
1308</artwork></figure>
1309<t>
1310   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1311   message format of RFC 822 <xref target="RFC822"/> for transferring entities (the payload
1312   of the message). Both types of message consist of a start-line, zero
1313   or more header fields (also known as "headers"), an empty line (i.e.,
1314   a line with nothing preceding the CRLF) indicating the end of the
1315   header fields, and possibly a message-body.
1316</t>
1317<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1318     generic-message = start-line
1319                       *(message-header CRLF)
1320                       CRLF
1321                       [ message-body ]
1322     start-line      = Request-Line | Status-Line
1323</artwork></figure>
1324<t>
1325   In the interest of robustness, servers &SHOULD; ignore any empty
1326   line(s) received where a Request-Line is expected. In other words, if
1327   the server is reading the protocol stream at the beginning of a
1328   message and receives a CRLF first, it should ignore the CRLF.
1329</t>
1330<t>
1331   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1332   after a POST request. To restate what is explicitly forbidden by the
1333   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1334   extra CRLF.
1335</t>
1336</section>
1337
1338<section title="Message Headers" anchor="message.headers">
1339<t>
1340   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1341   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1342   entity-header (&entity-header-fields;) fields, follow the same generic format as
1343   that given in <xref target="RFC822" x:fmt="sec" x:sec="3.1"/> of RFC 822 <xref target="RFC822"/>. Each header field consists
1344   of a name followed by a colon (":") and the field value. Field names
1345   are case-insensitive. The field value &MAY; be preceded by any amount
1346   of LWS, though a single SP is preferred. Header fields can be
1347   extended over multiple lines by preceding each extra line with at
1348   least one SP or HT. Applications ought to follow "common form", where
1349   one is known or indicated, when generating HTTP constructs, since
1350   there might exist some implementations that fail to accept anything
1351   beyond the common forms.
1352</t>
1353<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"/>
1354    message-header = field-name ":" [ field-value ]
1355    field-name     = token
1356    field-value    = *( field-content | LWS )
1357    field-content  = &lt;the OCTETs making up the field-value
1358                     and consisting of either *TEXT or combinations
1359                     of token, separators, and quoted-string&gt;
1360</artwork></figure>
1361<t>
1362   The field-content does not include any leading or trailing LWS:
1363   linear white space occurring before the first non-whitespace
1364   character of the field-value or after the last non-whitespace
1365   character of the field-value. Such leading or trailing LWS &MAY; be
1366   removed without changing the semantics of the field value. Any LWS
1367   that occurs between field-content &MAY; be replaced with a single SP
1368   before interpreting the field value or forwarding the message
1369   downstream.
1370</t>
1371<t>
1372   The order in which header fields with differing field names are
1373   received is not significant. However, it is "good practice" to send
1374   general-header fields first, followed by request-header or response-header
1375   fields, and ending with the entity-header fields.
1376</t>
1377<t>
1378   Multiple message-header fields with the same field-name &MAY; be
1379   present in a message if and only if the entire field-value for that
1380   header field is defined as a comma-separated list [i.e., #(values)].
1381   It &MUST; be possible to combine the multiple header fields into one
1382   "field-name: field-value" pair, without changing the semantics of the
1383   message, by appending each subsequent field-value to the first, each
1384   separated by a comma. The order in which header fields with the same
1385   field-name are received is therefore significant to the
1386   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1387   change the order of these field values when a message is forwarded.
1388</t>
1389</section>
1390
1391<section title="Message Body" anchor="message.body">
1392<t>
1393   The message-body (if any) of an HTTP message is used to carry the
1394   entity-body associated with the request or response. The message-body
1395   differs from the entity-body only when a transfer-coding has been
1396   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1397</t>
1398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1399    message-body = entity-body
1400                 | &lt;entity-body encoded as per Transfer-Encoding&gt;
1401</artwork></figure>
1402<t>
1403   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1404   applied by an application to ensure safe and proper transfer of the
1405   message. Transfer-Encoding is a property of the message, not of the
1406   entity, and thus &MAY; be added or removed by any application along the
1407   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1408   when certain transfer-codings may be used.)
1409</t>
1410<t>
1411   The rules for when a message-body is allowed in a message differ for
1412   requests and responses.
1413</t>
1414<t>
1415   The presence of a message-body in a request is signaled by the
1416   inclusion of a Content-Length or Transfer-Encoding header field in
1417   the request's message-headers. A message-body &MUST-NOT; be included in
1418   a request if the specification of the request method (&method;)
1419   does not allow sending an entity-body in requests. A server &SHOULD;
1420   read and forward a message-body on any request; if the request method
1421   does not include defined semantics for an entity-body, then the
1422   message-body &SHOULD; be ignored when handling the request.
1423</t>
1424<t>
1425   For response messages, whether or not a message-body is included with
1426   a message is dependent on both the request method and the response
1427   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1428   &MUST-NOT; include a message-body, even though the presence of entity-header
1429   fields might lead one to believe they do. All 1xx
1430   (informational), 204 (no content), and 304 (not modified) responses
1431   &MUST-NOT; include a message-body. All other responses do include a
1432   message-body, although it &MAY; be of zero length.
1433</t>
1434</section>
1435
1436<section title="Message Length" anchor="message.length">
1437<t>
1438   The transfer-length of a message is the length of the message-body as
1439   it appears in the message; that is, after any transfer-codings have
1440   been applied. When a message-body is included with a message, the
1441   transfer-length of that body is determined by one of the following
1442   (in order of precedence):
1443</t>
1444<t>
1445  <list style="numbers">
1446    <x:lt><t>
1447     Any response message which "&MUST-NOT;" include a message-body (such
1448     as the 1xx, 204, and 304 responses and any response to a HEAD
1449     request) is always terminated by the first empty line after the
1450     header fields, regardless of the entity-header fields present in
1451     the message.
1452    </t></x:lt>
1453    <x:lt><t>
1454     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1455     is present, then the transfer-length is
1456     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1457     unless the message is terminated by closing the connection.
1458    </t></x:lt>
1459    <x:lt><t>
1460     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1461     decimal value in OCTETs represents both the entity-length and the
1462     transfer-length. The Content-Length header field &MUST-NOT; be sent
1463     if these two lengths are different (i.e., if a Transfer-Encoding
1464     header field is present). If a message is received with both a
1465     Transfer-Encoding header field and a Content-Length header field,
1466     the latter &MUST; be ignored.
1467    </t></x:lt>
1468    <x:lt><t>
1469     If the message uses the media type "multipart/byteranges", and the
1470     transfer-length is not otherwise specified, then this self-delimiting
1471     media type defines the transfer-length. This media type
1472     &MUST-NOT; be used unless the sender knows that the recipient can parse
1473     it; the presence in a request of a Range header with multiple byte-range
1474     specifiers from a 1.1 client implies that the client can parse
1475     multipart/byteranges responses.
1476    <list style="empty"><t>
1477       A range header might be forwarded by a 1.0 proxy that does not
1478       understand multipart/byteranges; in this case the server &MUST;
1479       delimit the message using methods defined in items 1, 3 or 5 of
1480       this section.
1481    </t></list>
1482    </t></x:lt>
1483    <x:lt><t>
1484     By the server closing the connection. (Closing the connection
1485     cannot be used to indicate the end of a request body, since that
1486     would leave no possibility for the server to send back a response.)
1487    </t></x:lt>
1488  </list>
1489</t>
1490<t>
1491   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1492   containing a message-body &MUST; include a valid Content-Length header
1493   field unless the server is known to be HTTP/1.1 compliant. If a
1494   request contains a message-body and a Content-Length is not given,
1495   the server &SHOULD; respond with 400 (bad request) if it cannot
1496   determine the length of the message, or with 411 (length required) if
1497   it wishes to insist on receiving a valid Content-Length.
1498</t>
1499<t>
1500   All HTTP/1.1 applications that receive entities &MUST; accept the
1501   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1502   to be used for messages when the message length cannot be determined
1503   in advance.
1504</t>
1505<t>
1506   Messages &MUST-NOT; include both a Content-Length header field and a
1507   transfer-coding. If the message does include a
1508   transfer-coding, the Content-Length &MUST; be ignored.
1509</t>
1510<t>
1511   When a Content-Length is given in a message where a message-body is
1512   allowed, its field value &MUST; exactly match the number of OCTETs in
1513   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1514   invalid length is received and detected.
1515</t>
1516</section>
1517
1518<section title="General Header Fields" anchor="general.header.fields">
1519<t>
1520   There are a few header fields which have general applicability for
1521   both request and response messages, but which do not apply to the
1522   entity being transferred. These header fields apply only to the
1523   message being transmitted.
1524</t>
1525<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1526    general-header = Cache-Control            ; &header-cache-control;
1527                   | Connection               ; <xref target="header.connection"/>
1528                   | Date                     ; <xref target="header.date"/>
1529                   | Pragma                   ; &header-pragma;
1530                   | Trailer                  ; <xref target="header.trailer"/>
1531                   | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1532                   | Upgrade                  ; <xref target="header.upgrade"/>
1533                   | Via                      ; <xref target="header.via"/>
1534                   | Warning                  ; &header-warning;
1535</artwork></figure>
1536<t>
1537   General-header field names can be extended reliably only in
1538   combination with a change in the protocol version. However, new or
1539   experimental header fields may be given the semantics of general
1540   header fields if all parties in the communication recognize them to
1541   be general-header fields. Unrecognized header fields are treated as
1542   entity-header fields.
1543</t>
1544</section>
1545</section>
1546
1547<section title="Request" anchor="request">
1548<t>
1549   A request message from a client to a server includes, within the
1550   first line of that message, the method to be applied to the resource,
1551   the identifier of the resource, and the protocol version in use.
1552</t>
1553<!--                 Host                      ; should be moved here eventually -->
1554<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1555     Request       = Request-Line              ; <xref target="request-line"/>
1556                     *(( general-header        ; <xref target="general.header.fields"/>
1557                      | request-header         ; &request-header-fields;
1558                      | entity-header ) CRLF)  ; &entity-header-fields;
1559                     CRLF
1560                     [ message-body ]          ; <xref target="message.body"/>
1561</artwork></figure>
1562
1563<section title="Request-Line" anchor="request-line">
1564<t>
1565   The Request-Line begins with a method token, followed by the
1566   Request-URI and the protocol version, and ending with CRLF. The
1567   elements are separated by SP characters. No CR or LF is allowed
1568   except in the final CRLF sequence.
1569</t>
1570<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1571     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1572</artwork></figure>
1573
1574<section title="Method" anchor="method">
1575<t>
1576   The Method  token indicates the method to be performed on the
1577   resource identified by the Request-URI. The method is case-sensitive.
1578</t>
1579<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1580    Method         = token
1581</artwork></figure>
1582</section>
1583
1584<section title="Request-URI" anchor="request-uri">
1585<t>
1586   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1587   identifies the resource upon which to apply the request.
1588</t>
1589<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1590    Request-URI    = "*"
1591                   | absoluteURI
1592                   | ( abs_path [ "?" query ] )
1593                   | authority
1594</artwork></figure>
1595<t>
1596   The four options for Request-URI are dependent on the nature of the
1597   request. The asterisk "*" means that the request does not apply to a
1598   particular resource, but to the server itself, and is only allowed
1599   when the method used does not necessarily apply to a resource. One
1600   example would be
1601</t>
1602<figure><artwork type="example">
1603    OPTIONS * HTTP/1.1
1604</artwork></figure>
1605<t>
1606   The absoluteURI form is &REQUIRED; when the request is being made to a
1607   proxy. The proxy is requested to forward the request or service it
1608   from a valid cache, and return the response. Note that the proxy &MAY;
1609   forward the request on to another proxy or directly to the server
1610   specified by the absoluteURI. In order to avoid request loops, a
1611   proxy &MUST; be able to recognize all of its server names, including
1612   any aliases, local variations, and the numeric IP address. An example
1613   Request-Line would be:
1614</t>
1615<figure><artwork type="example">
1616    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1617</artwork></figure>
1618<t>
1619   To allow for transition to absoluteURIs in all requests in future
1620   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1621   form in requests, even though HTTP/1.1 clients will only generate
1622   them in requests to proxies.
1623</t>
1624<t>
1625   The authority form is only used by the CONNECT method (&CONNECT;).
1626</t>
1627<t>
1628   The most common form of Request-URI is that used to identify a
1629   resource on an origin server or gateway. In this case the absolute
1630   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, abs_path) as
1631   the Request-URI, and the network location of the URI (authority) &MUST;
1632   be transmitted in a Host header field. For example, a client wishing
1633   to retrieve the resource above directly from the origin server would
1634   create a TCP connection to port 80 of the host "www.example.org" and send
1635   the lines:
1636</t>
1637<figure><artwork type="example">
1638    GET /pub/WWW/TheProject.html HTTP/1.1
1639    Host: www.example.org
1640</artwork></figure>
1641<t>
1642   followed by the remainder of the Request. Note that the absolute path
1643   cannot be empty; if none is present in the original URI, it &MUST; be
1644   given as "/" (the server root).
1645</t>
1646<t>
1647   The Request-URI is transmitted in the format specified in
1648   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1649   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1650   properly interpret the request. Servers &SHOULD; respond to invalid
1651   Request-URIs with an appropriate status code.
1652</t>
1653<t>
1654   A transparent proxy &MUST-NOT; rewrite the "abs_path" part of the
1655   received Request-URI when forwarding it to the next inbound server,
1656   except as noted above to replace a null abs_path with "/".
1657</t>
1658<t>
1659  <list><t>
1660      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1661      meaning of the request when the origin server is improperly using
1662      a non-reserved URI character for a reserved purpose.  Implementors
1663      should be aware that some pre-HTTP/1.1 proxies have been known to
1664      rewrite the Request-URI.
1665  </t></list>
1666</t>
1667</section>
1668</section>
1669
1670<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1671<t>
1672   The exact resource identified by an Internet request is determined by
1673   examining both the Request-URI and the Host header field.
1674</t>
1675<t>
1676   An origin server that does not allow resources to differ by the
1677   requested host &MAY; ignore the Host header field value when
1678   determining the resource identified by an HTTP/1.1 request. (But see
1679   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1680   for other requirements on Host support in HTTP/1.1.)
1681</t>
1682<t>
1683   An origin server that does differentiate resources based on the host
1684   requested (sometimes referred to as virtual hosts or vanity host
1685   names) &MUST; use the following rules for determining the requested
1686   resource on an HTTP/1.1 request:
1687  <list style="numbers">
1688    <t>If Request-URI is an absoluteURI, the host is part of the
1689     Request-URI. Any Host header field value in the request &MUST; be
1690     ignored.</t>
1691    <t>If the Request-URI is not an absoluteURI, and the request includes
1692     a Host header field, the host is determined by the Host header
1693     field value.</t>
1694    <t>If the host as determined by rule 1 or 2 is not a valid host on
1695     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1696  </list>
1697</t>
1698<t>
1699   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1700   attempt to use heuristics (e.g., examination of the URI path for
1701   something unique to a particular host) in order to determine what
1702   exact resource is being requested.
1703</t>
1704</section>
1705
1706</section>
1707
1708
1709<section title="Response" anchor="response">
1710<t>
1711   After receiving and interpreting a request message, a server responds
1712   with an HTTP response message.
1713</t>
1714<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1715    Response      = Status-Line               ; <xref target="status-line"/>
1716                    *(( general-header        ; <xref target="general.header.fields"/>
1717                     | response-header        ; &response-header-fields;
1718                     | entity-header ) CRLF)  ; &entity-header-fields;
1719                    CRLF
1720                    [ message-body ]          ; <xref target="message.body"/>
1721</artwork></figure>
1722
1723<section title="Status-Line" anchor="status-line">
1724<t>
1725   The first line of a Response message is the Status-Line, consisting
1726   of the protocol version followed by a numeric status code and its
1727   associated textual phrase, with each element separated by SP
1728   characters. No CR or LF is allowed except in the final CRLF sequence.
1729</t>
1730<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1731    Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1732</artwork></figure>
1733
1734<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1735<t>
1736   The Status-Code element is a 3-digit integer result code of the
1737   attempt to understand and satisfy the request. These codes are fully
1738   defined in &status-codes;. The Reason-Phrase is intended to give a short
1739   textual description of the Status-Code. The Status-Code is intended
1740   for use by automata and the Reason-Phrase is intended for the human
1741   user. The client is not required to examine or display the Reason-Phrase.
1742</t>
1743<t>
1744   The first digit of the Status-Code defines the class of response. The
1745   last two digits do not have any categorization role. There are 5
1746   values for the first digit:
1747  <list style="symbols">
1748    <t>
1749      1xx: Informational - Request received, continuing process
1750    </t>
1751    <t>
1752      2xx: Success - The action was successfully received,
1753        understood, and accepted
1754    </t>
1755    <t>
1756      3xx: Redirection - Further action must be taken in order to
1757        complete the request
1758    </t>
1759    <t>
1760      4xx: Client Error - The request contains bad syntax or cannot
1761        be fulfilled
1762    </t>
1763    <t>
1764      5xx: Server Error - The server failed to fulfill an apparently
1765        valid request
1766    </t>
1767  </list>
1768</t>
1769<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"/>
1770   Status-Code    = 3DIGIT
1771   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1772</artwork></figure>
1773</section>
1774</section>
1775
1776</section>
1777
1778
1779<section title="Connections" anchor="connections">
1780
1781<section title="Persistent Connections" anchor="persistent.connections">
1782
1783<section title="Purpose" anchor="persistent.purpose">
1784<t>
1785   Prior to persistent connections, a separate TCP connection was
1786   established to fetch each URL, increasing the load on HTTP servers
1787   and causing congestion on the Internet. The use of inline images and
1788   other associated data often require a client to make multiple
1789   requests of the same server in a short amount of time. Analysis of
1790   these performance problems and results from a prototype
1791   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1792   measurements of actual HTTP/1.1 (RFC 2068) implementations show good
1793   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1794   T/TCP <xref target="Tou1998"/>.
1795</t>
1796<t>
1797   Persistent HTTP connections have a number of advantages:
1798  <list style="symbols">
1799      <t>
1800        By opening and closing fewer TCP connections, CPU time is saved
1801        in routers and hosts (clients, servers, proxies, gateways,
1802        tunnels, or caches), and memory used for TCP protocol control
1803        blocks can be saved in hosts.
1804      </t>
1805      <t>
1806        HTTP requests and responses can be pipelined on a connection.
1807        Pipelining allows a client to make multiple requests without
1808        waiting for each response, allowing a single TCP connection to
1809        be used much more efficiently, with much lower elapsed time.
1810      </t>
1811      <t>
1812        Network congestion is reduced by reducing the number of packets
1813        caused by TCP opens, and by allowing TCP sufficient time to
1814        determine the congestion state of the network.
1815      </t>
1816      <t>
1817        Latency on subsequent requests is reduced since there is no time
1818        spent in TCP's connection opening handshake.
1819      </t>
1820      <t>
1821        HTTP can evolve more gracefully, since errors can be reported
1822        without the penalty of closing the TCP connection. Clients using
1823        future versions of HTTP might optimistically try a new feature,
1824        but if communicating with an older server, retry with old
1825        semantics after an error is reported.
1826      </t>
1827    </list>
1828</t>
1829<t>
1830   HTTP implementations &SHOULD; implement persistent connections.
1831</t>
1832</section>
1833
1834<section title="Overall Operation" anchor="persistent.overall">
1835<t>
1836   A significant difference between HTTP/1.1 and earlier versions of
1837   HTTP is that persistent connections are the default behavior of any
1838   HTTP connection. That is, unless otherwise indicated, the client
1839   &SHOULD; assume that the server will maintain a persistent connection,
1840   even after error responses from the server.
1841</t>
1842<t>
1843   Persistent connections provide a mechanism by which a client and a
1844   server can signal the close of a TCP connection. This signaling takes
1845   place using the Connection header field (<xref target="header.connection"/>). Once a close
1846   has been signaled, the client &MUST-NOT; send any more requests on that
1847   connection.
1848</t>
1849
1850<section title="Negotiation" anchor="persistent.negotiation">
1851<t>
1852   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1853   maintain a persistent connection unless a Connection header including
1854   the connection-token "close" was sent in the request. If the server
1855   chooses to close the connection immediately after sending the
1856   response, it &SHOULD; send a Connection header including the
1857   connection-token close.
1858</t>
1859<t>
1860   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1861   decide to keep it open based on whether the response from a server
1862   contains a Connection header with the connection-token close. In case
1863   the client does not want to maintain a connection for more than that
1864   request, it &SHOULD; send a Connection header including the
1865   connection-token close.
1866</t>
1867<t>
1868   If either the client or the server sends the close token in the
1869   Connection header, that request becomes the last one for the
1870   connection.
1871</t>
1872<t>
1873   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1874   maintained for HTTP versions less than 1.1 unless it is explicitly
1875   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1876   compatibility with HTTP/1.0 clients.
1877</t>
1878<t>
1879   In order to remain persistent, all messages on the connection &MUST;
1880   have a self-defined message length (i.e., one not defined by closure
1881   of the connection), as described in <xref target="message.length"/>.
1882</t>
1883</section>
1884
1885<section title="Pipelining" anchor="pipelining">
1886<t>
1887   A client that supports persistent connections &MAY; "pipeline" its
1888   requests (i.e., send multiple requests without waiting for each
1889   response). A server &MUST; send its responses to those requests in the
1890   same order that the requests were received.
1891</t>
1892<t>
1893   Clients which assume persistent connections and pipeline immediately
1894   after connection establishment &SHOULD; be prepared to retry their
1895   connection if the first pipelined attempt fails. If a client does
1896   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1897   persistent. Clients &MUST; also be prepared to resend their requests if
1898   the server closes the connection before sending all of the
1899   corresponding responses.
1900</t>
1901<t>
1902   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1903   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1904   premature termination of the transport connection could lead to
1905   indeterminate results. A client wishing to send a non-idempotent
1906   request &SHOULD; wait to send that request until it has received the
1907   response status for the previous request.
1908</t>
1909</section>
1910</section>
1911
1912<section title="Proxy Servers" anchor="persistent.proxy">
1913<t>
1914   It is especially important that proxies correctly implement the
1915   properties of the Connection header field as specified in <xref target="header.connection"/>.
1916</t>
1917<t>
1918   The proxy server &MUST; signal persistent connections separately with
1919   its clients and the origin servers (or other proxy servers) that it
1920   connects to. Each persistent connection applies to only one transport
1921   link.
1922</t>
1923<t>
1924   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1925   with an HTTP/1.0 client (but see RFC 2068 <xref target="RFC2068"/> for information and
1926   discussion of the problems with the Keep-Alive header implemented by
1927   many HTTP/1.0 clients).
1928</t>
1929</section>
1930
1931<section title="Practical Considerations" anchor="persistent.practical">
1932<t>
1933   Servers will usually have some time-out value beyond which they will
1934   no longer maintain an inactive connection. Proxy servers might make
1935   this a higher value since it is likely that the client will be making
1936   more connections through the same server. The use of persistent
1937   connections places no requirements on the length (or existence) of
1938   this time-out for either the client or the server.
1939</t>
1940<t>
1941   When a client or server wishes to time-out it &SHOULD; issue a graceful
1942   close on the transport connection. Clients and servers &SHOULD; both
1943   constantly watch for the other side of the transport close, and
1944   respond to it as appropriate. If a client or server does not detect
1945   the other side's close promptly it could cause unnecessary resource
1946   drain on the network.
1947</t>
1948<t>
1949   A client, server, or proxy &MAY; close the transport connection at any
1950   time. For example, a client might have started to send a new request
1951   at the same time that the server has decided to close the "idle"
1952   connection. From the server's point of view, the connection is being
1953   closed while it was idle, but from the client's point of view, a
1954   request is in progress.
1955</t>
1956<t>
1957   This means that clients, servers, and proxies &MUST; be able to recover
1958   from asynchronous close events. Client software &SHOULD; reopen the
1959   transport connection and retransmit the aborted sequence of requests
1960   without user interaction so long as the request sequence is
1961   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1962   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1963   human operator the choice of retrying the request(s). Confirmation by
1964   user-agent software with semantic understanding of the application
1965   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT;
1966   be repeated if the second sequence of requests fails.
1967</t>
1968<t>
1969   Servers &SHOULD; always respond to at least one request per connection,
1970   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1971   middle of transmitting a response, unless a network or client failure
1972   is suspected.
1973</t>
1974<t>
1975   Clients that use persistent connections &SHOULD; limit the number of
1976   simultaneous connections that they maintain to a given server. A
1977   single-user client &SHOULD-NOT; maintain more than 2 connections with
1978   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1979   another server or proxy, where N is the number of simultaneously
1980   active users. These guidelines are intended to improve HTTP response
1981   times and avoid congestion.
1982</t>
1983</section>
1984</section>
1985
1986<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1987
1988<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1989<t>
1990   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1991   flow control mechanisms to resolve temporary overloads, rather than
1992   terminating connections with the expectation that clients will retry.
1993   The latter technique can exacerbate network congestion.
1994</t>
1995</section>
1996
1997<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
1998<t>
1999   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2000   the network connection for an error status while it is transmitting
2001   the request. If the client sees an error status, it &SHOULD;
2002   immediately cease transmitting the body. If the body is being sent
2003   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2004   empty trailer &MAY; be used to prematurely mark the end of the message.
2005   If the body was preceded by a Content-Length header, the client &MUST;
2006   close the connection.
2007</t>
2008</section>
2009
2010<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2011<t>
2012   The purpose of the 100 (Continue) status (see &status-100;) is to
2013   allow a client that is sending a request message with a request body
2014   to determine if the origin server is willing to accept the request
2015   (based on the request headers) before the client sends the request
2016   body. In some cases, it might either be inappropriate or highly
2017   inefficient for the client to send the body if the server will reject
2018   the message without looking at the body.
2019</t>
2020<t>
2021   Requirements for HTTP/1.1 clients:
2022  <list style="symbols">
2023    <t>
2024        If a client will wait for a 100 (Continue) response before
2025        sending the request body, it &MUST; send an Expect request-header
2026        field (&header-expect;) with the "100-continue" expectation.
2027    </t>
2028    <t>
2029        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2030        with the "100-continue" expectation if it does not intend
2031        to send a request body.
2032    </t>
2033  </list>
2034</t>
2035<t>
2036   Because of the presence of older implementations, the protocol allows
2037   ambiguous situations in which a client may send "Expect: 100-continue"
2038   without receiving either a 417 (Expectation Failed) status
2039   or a 100 (Continue) status. Therefore, when a client sends this
2040   header field to an origin server (possibly via a proxy) from which it
2041   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2042   for an indefinite period before sending the request body.
2043</t>
2044<t>
2045   Requirements for HTTP/1.1 origin servers:
2046  <list style="symbols">
2047    <t> Upon receiving a request which includes an Expect request-header
2048        field with the "100-continue" expectation, an origin server &MUST;
2049        either respond with 100 (Continue) status and continue to read
2050        from the input stream, or respond with a final status code. The
2051        origin server &MUST-NOT; wait for the request body before sending
2052        the 100 (Continue) response. If it responds with a final status
2053        code, it &MAY; close the transport connection or it &MAY; continue
2054        to read and discard the rest of the request.  It &MUST-NOT;
2055        perform the requested method if it returns a final status code.
2056    </t>
2057    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2058        the request message does not include an Expect request-header
2059        field with the "100-continue" expectation, and &MUST-NOT; send a
2060        100 (Continue) response if such a request comes from an HTTP/1.0
2061        (or earlier) client. There is an exception to this rule: for
2062        compatibility with RFC 2068, a server &MAY; send a 100 (Continue)
2063        status in response to an HTTP/1.1 PUT or POST request that does
2064        not include an Expect request-header field with the "100-continue"
2065        expectation. This exception, the purpose of which is
2066        to minimize any client processing delays associated with an
2067        undeclared wait for 100 (Continue) status, applies only to
2068        HTTP/1.1 requests, and not to requests with any other HTTP-version
2069        value.
2070    </t>
2071    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2072        already received some or all of the request body for the
2073        corresponding request.
2074    </t>
2075    <t> An origin server that sends a 100 (Continue) response &MUST;
2076    ultimately send a final status code, once the request body is
2077        received and processed, unless it terminates the transport
2078        connection prematurely.
2079    </t>
2080    <t> If an origin server receives a request that does not include an
2081        Expect request-header field with the "100-continue" expectation,
2082        the request includes a request body, and the server responds
2083        with a final status code before reading the entire request body
2084        from the transport connection, then the server &SHOULD-NOT;  close
2085        the transport connection until it has read the entire request,
2086        or until the client closes the connection. Otherwise, the client
2087        might not reliably receive the response message. However, this
2088        requirement is not be construed as preventing a server from
2089        defending itself against denial-of-service attacks, or from
2090        badly broken client implementations.
2091      </t>
2092    </list>
2093</t>
2094<t>
2095   Requirements for HTTP/1.1 proxies:
2096  <list style="symbols">
2097    <t> If a proxy receives a request that includes an Expect request-header
2098        field with the "100-continue" expectation, and the proxy
2099        either knows that the next-hop server complies with HTTP/1.1 or
2100        higher, or does not know the HTTP version of the next-hop
2101        server, it &MUST; forward the request, including the Expect header
2102        field.
2103    </t>
2104    <t> If the proxy knows that the version of the next-hop server is
2105        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2106        respond with a 417 (Expectation Failed) status.
2107    </t>
2108    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2109        numbers received from recently-referenced next-hop servers.
2110    </t>
2111    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2112        request message was received from an HTTP/1.0 (or earlier)
2113        client and did not include an Expect request-header field with
2114        the "100-continue" expectation. This requirement overrides the
2115        general rule for forwarding of 1xx responses (see &status-1xx;).
2116    </t>
2117  </list>
2118</t>
2119</section>
2120
2121<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2122<t>
2123   If an HTTP/1.1 client sends a request which includes a request body,
2124   but which does not include an Expect request-header field with the
2125   "100-continue" expectation, and if the client is not directly
2126   connected to an HTTP/1.1 origin server, and if the client sees the
2127   connection close before receiving any status from the server, the
2128   client &SHOULD; retry the request.  If the client does retry this
2129   request, it &MAY; use the following "binary exponential backoff"
2130   algorithm to be assured of obtaining a reliable response:
2131  <list style="numbers">
2132    <t>
2133      Initiate a new connection to the server
2134    </t>
2135    <t>
2136      Transmit the request-headers
2137    </t>
2138    <t>
2139      Initialize a variable R to the estimated round-trip time to the
2140         server (e.g., based on the time it took to establish the
2141         connection), or to a constant value of 5 seconds if the round-trip
2142         time is not available.
2143    </t>
2144    <t>
2145       Compute T = R * (2**N), where N is the number of previous
2146         retries of this request.
2147    </t>
2148    <t>
2149       Wait either for an error response from the server, or for T
2150         seconds (whichever comes first)
2151    </t>
2152    <t>
2153       If no error response is received, after T seconds transmit the
2154         body of the request.
2155    </t>
2156    <t>
2157       If client sees that the connection is closed prematurely,
2158         repeat from step 1 until the request is accepted, an error
2159         response is received, or the user becomes impatient and
2160         terminates the retry process.
2161    </t>
2162  </list>
2163</t>
2164<t>
2165   If at any point an error status is received, the client
2166  <list style="symbols">
2167      <t>&SHOULD-NOT;  continue and</t>
2168
2169      <t>&SHOULD; close the connection if it has not completed sending the
2170        request message.</t>
2171    </list>
2172</t>
2173</section>
2174</section>
2175</section>
2176
2177
2178<section title="Header Field Definitions" anchor="header.fields">
2179<t>
2180   This section defines the syntax and semantics of all standard
2181   HTTP/1.1 header fields. For entity-header fields, both sender and
2182   recipient refer to either the client or the server, depending on who
2183   sends and who receives the entity.
2184</t>
2185
2186<section title="Connection" anchor="header.connection">
2187  <iref primary="true" item="Connection header" x:for-anchor=""/>
2188  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2189<t>
2190   The Connection general-header field allows the sender to specify
2191   options that are desired for that particular connection and &MUST-NOT;
2192   be communicated by proxies over further connections.
2193</t>
2194<t>
2195   The Connection header has the following grammar:
2196</t>
2197<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2198    Connection = "Connection" ":" 1#(connection-token)
2199    connection-token  = token
2200</artwork></figure>
2201<t>
2202   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2203   message is forwarded and, for each connection-token in this field,
2204   remove any header field(s) from the message with the same name as the
2205   connection-token. Connection options are signaled by the presence of
2206   a connection-token in the Connection header field, not by any
2207   corresponding additional header field(s), since the additional header
2208   field may not be sent if there are no parameters associated with that
2209   connection option.
2210</t>
2211<t>
2212   Message headers listed in the Connection header &MUST-NOT; include
2213   end-to-end headers, such as Cache-Control.
2214</t>
2215<t>
2216   HTTP/1.1 defines the "close" connection option for the sender to
2217   signal that the connection will be closed after completion of the
2218   response. For example,
2219</t>
2220<figure><artwork type="example">
2221    Connection: close
2222</artwork></figure>
2223<t>
2224   in either the request or the response header fields indicates that
2225   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2226   after the current request/response is complete.
2227</t>
2228<t>
2229   An HTTP/1.1 client that does not support persistent connections &MUST;
2230   include the "close" connection option in every request message.
2231</t>
2232<t>
2233   An HTTP/1.1 server that does not support persistent connections &MUST;
2234   include the "close" connection option in every response message that
2235   does not have a 1xx (informational) status code.
2236</t>
2237<t>
2238   A system receiving an HTTP/1.0 (or lower-version) message that
2239   includes a Connection header MUST, for each connection-token in this
2240   field, remove and ignore any header field(s) from the message with
2241   the same name as the connection-token. This protects against mistaken
2242   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2243</t>
2244</section>
2245
2246<section title="Content-Length" anchor="header.content-length">
2247  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2248  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2249<t>
2250   The Content-Length entity-header field indicates the size of the
2251   entity-body, in decimal number of OCTETs, sent to the recipient or,
2252   in the case of the HEAD method, the size of the entity-body that
2253   would have been sent had the request been a GET.
2254</t>
2255<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2256    Content-Length    = "Content-Length" ":" 1*DIGIT
2257</artwork></figure>
2258<t>
2259   An example is
2260</t>
2261<figure><artwork type="example">
2262    Content-Length: 3495
2263</artwork></figure>
2264<t>
2265   Applications &SHOULD; use this field to indicate the transfer-length of
2266   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2267</t>
2268<t>
2269   Any Content-Length greater than or equal to zero is a valid value.
2270   <xref target="message.length"/> describes how to determine the length of a message-body
2271   if a Content-Length is not given.
2272</t>
2273<t>
2274   Note that the meaning of this field is significantly different from
2275   the corresponding definition in MIME, where it is an optional field
2276   used within the "message/external-body" content-type. In HTTP, it
2277   &SHOULD; be sent whenever the message's length can be determined prior
2278   to being transferred, unless this is prohibited by the rules in
2279   <xref target="message.length"/>.
2280</t>
2281</section>
2282
2283<section title="Date" anchor="header.date">
2284  <iref primary="true" item="Date header" x:for-anchor=""/>
2285  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2286<t>
2287   The Date general-header field represents the date and time at which
2288   the message was originated, having the same semantics as orig-date in
2289   RFC 822. The field value is an HTTP-date, as described in <xref target="full.date"/>;
2290   it &MUST; be sent in rfc1123-date format.
2291</t>
2292<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2293    Date  = "Date" ":" HTTP-date
2294</artwork></figure>
2295<t>
2296   An example is
2297</t>
2298<figure><artwork type="example">
2299    Date: Tue, 15 Nov 1994 08:12:31 GMT
2300</artwork></figure>
2301<t>
2302   Origin servers &MUST; include a Date header field in all responses,
2303   except in these cases:
2304  <list style="numbers">
2305      <t>If the response status code is 100 (Continue) or 101 (Switching
2306         Protocols), the response &MAY; include a Date header field, at
2307         the server's option.</t>
2308
2309      <t>If the response status code conveys a server error, e.g. 500
2310         (Internal Server Error) or 503 (Service Unavailable), and it is
2311         inconvenient or impossible to generate a valid Date.</t>
2312
2313      <t>If the server does not have a clock that can provide a
2314         reasonable approximation of the current time, its responses
2315         &MUST-NOT; include a Date header field. In this case, the rules
2316         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2317  </list>
2318</t>
2319<t>
2320   A received message that does not have a Date header field &MUST; be
2321   assigned one by the recipient if the message will be cached by that
2322   recipient or gatewayed via a protocol which requires a Date. An HTTP
2323   implementation without a clock &MUST-NOT; cache responses without
2324   revalidating them on every use. An HTTP cache, especially a shared
2325   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2326   clock with a reliable external standard.
2327</t>
2328<t>
2329   Clients &SHOULD; only send a Date header field in messages that include
2330   an entity-body, as in the case of the PUT and POST requests, and even
2331   then it is optional. A client without a clock &MUST-NOT; send a Date
2332   header field in a request.
2333</t>
2334<t>
2335   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2336   time subsequent to the generation of the message. It &SHOULD; represent
2337   the best available approximation of the date and time of message
2338   generation, unless the implementation has no means of generating a
2339   reasonably accurate date and time. In theory, the date ought to
2340   represent the moment just before the entity is generated. In
2341   practice, the date can be generated at any time during the message
2342   origination without affecting its semantic value.
2343</t>
2344
2345<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2346<t>
2347   Some origin server implementations might not have a clock available.
2348   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2349   values to a response, unless these values were associated
2350   with the resource by a system or user with a reliable clock. It &MAY;
2351   assign an Expires value that is known, at or before server
2352   configuration time, to be in the past (this allows "pre-expiration"
2353   of responses without storing separate Expires values for each
2354   resource).
2355</t>
2356</section>
2357</section>
2358
2359<section title="Host" anchor="header.host">
2360  <iref primary="true" item="Host header" x:for-anchor=""/>
2361  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2362<t>
2363   The Host request-header field specifies the Internet host and port
2364   number of the resource being requested, as obtained from the original
2365   URI given by the user or referring resource (generally an HTTP URL,
2366   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2367   the naming authority of the origin server or gateway given by the
2368   original URL. This allows the origin server or gateway to
2369   differentiate between internally-ambiguous URLs, such as the root "/"
2370   URL of a server for multiple host names on a single IP address.
2371</t>
2372<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2373    Host = "Host" ":" host [ ":" port ] ; <xref target="http.url"/>
2374</artwork></figure>
2375<t>
2376   A "host" without any trailing port information implies the default
2377   port for the service requested (e.g., "80" for an HTTP URL). For
2378   example, a request on the origin server for
2379   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2380</t>
2381<figure><artwork type="example">
2382    GET /pub/WWW/ HTTP/1.1
2383    Host: www.example.org
2384</artwork></figure>
2385<t>
2386   A client &MUST; include a Host header field in all HTTP/1.1 request
2387   messages . If the requested URI does not include an Internet host
2388   name for the service being requested, then the Host header field &MUST;
2389   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2390   request message it forwards does contain an appropriate Host header
2391   field that identifies the service being requested by the proxy. All
2392   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2393   status code to any HTTP/1.1 request message which lacks a Host header
2394   field.
2395</t>
2396<t>
2397   See sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2398   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2399   for other requirements relating to Host.
2400</t>
2401</section>
2402
2403<section title="TE" anchor="header.te">
2404  <iref primary="true" item="TE header" x:for-anchor=""/>
2405  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2406<t>
2407   The TE request-header field indicates what extension transfer-codings
2408   it is willing to accept in the response and whether or not it is
2409   willing to accept trailer fields in a chunked transfer-coding. Its
2410   value may consist of the keyword "trailers" and/or a comma-separated
2411   list of extension transfer-coding names with optional accept
2412   parameters (as described in <xref target="transfer.codings"/>).
2413</t>
2414<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2415    TE        = "TE" ":" #( t-codings )
2416    t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2417</artwork></figure>
2418<t>
2419   The presence of the keyword "trailers" indicates that the client is
2420   willing to accept trailer fields in a chunked transfer-coding, as
2421   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2422   transfer-coding values even though it does not itself represent a
2423   transfer-coding.
2424</t>
2425<t>
2426   Examples of its use are:
2427</t>
2428<figure><artwork type="example">
2429    TE: deflate
2430    TE:
2431    TE: trailers, deflate;q=0.5
2432</artwork></figure>
2433<t>
2434   The TE header field only applies to the immediate connection.
2435   Therefore, the keyword &MUST; be supplied within a Connection header
2436   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2437</t>
2438<t>
2439   A server tests whether a transfer-coding is acceptable, according to
2440   a TE field, using these rules:
2441  <list style="numbers">
2442    <x:lt>
2443      <t>The "chunked" transfer-coding is always acceptable. If the
2444         keyword "trailers" is listed, the client indicates that it is
2445         willing to accept trailer fields in the chunked response on
2446         behalf of itself and any downstream clients. The implication is
2447         that, if given, the client is stating that either all
2448         downstream clients are willing to accept trailer fields in the
2449         forwarded response, or that it will attempt to buffer the
2450         response on behalf of downstream recipients.
2451      </t><t>
2452         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2453         chunked response such that a client can be assured of buffering
2454         the entire response.</t>
2455    </x:lt>
2456    <x:lt>
2457      <t>If the transfer-coding being tested is one of the transfer-codings
2458         listed in the TE field, then it is acceptable unless it
2459         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2460         qvalue of 0 means "not acceptable.")</t>
2461    </x:lt>
2462    <x:lt>
2463      <t>If multiple transfer-codings are acceptable, then the
2464         acceptable transfer-coding with the highest non-zero qvalue is
2465         preferred.  The "chunked" transfer-coding always has a qvalue
2466         of 1.</t>
2467    </x:lt>
2468  </list>
2469</t>
2470<t>
2471   If the TE field-value is empty or if no TE field is present, the only
2472   transfer-coding  is "chunked". A message with no transfer-coding is
2473   always acceptable.
2474</t>
2475</section>
2476
2477<section title="Trailer" anchor="header.trailer">
2478  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2479  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2480<t>
2481   The Trailer general field value indicates that the given set of
2482   header fields is present in the trailer of a message encoded with
2483   chunked transfer-coding.
2484</t>
2485<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2486    Trailer  = "Trailer" ":" 1#field-name
2487</artwork></figure>
2488<t>
2489   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2490   message using chunked transfer-coding with a non-empty trailer. Doing
2491   so allows the recipient to know which header fields to expect in the
2492   trailer.
2493</t>
2494<t>
2495   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2496   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2497   trailer fields in a "chunked" transfer-coding.
2498</t>
2499<t>
2500   Message header fields listed in the Trailer header field &MUST-NOT;
2501   include the following header fields:
2502  <list style="symbols">
2503    <t>Transfer-Encoding</t>
2504    <t>Content-Length</t>
2505    <t>Trailer</t>
2506  </list>
2507</t>
2508</section>
2509
2510<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2511  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2512  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2513<t>
2514   The Transfer-Encoding general-header field indicates what (if any)
2515   type of transformation has been applied to the message body in order
2516   to safely transfer it between the sender and the recipient. This
2517   differs from the content-coding in that the transfer-coding is a
2518   property of the message, not of the entity.
2519</t>
2520<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2521  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2522</artwork></figure>
2523<t>
2524   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2525</t>
2526<figure><artwork type="example">
2527  Transfer-Encoding: chunked
2528</artwork></figure>
2529<t>
2530   If multiple encodings have been applied to an entity, the transfer-codings
2531   &MUST; be listed in the order in which they were applied.
2532   Additional information about the encoding parameters &MAY; be provided
2533   by other entity-header fields not defined by this specification.
2534</t>
2535<t>
2536   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2537   header.
2538</t>
2539</section>
2540
2541<section title="Upgrade" anchor="header.upgrade">
2542  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2543  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2544<t>
2545   The Upgrade general-header allows the client to specify what
2546   additional communication protocols it supports and would like to use
2547   if the server finds it appropriate to switch protocols. The server
2548   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2549   response to indicate which protocol(s) are being switched.
2550</t>
2551<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2552    Upgrade        = "Upgrade" ":" 1#product
2553</artwork></figure>
2554<t>
2555   For example,
2556</t>
2557<figure><artwork type="example">
2558    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2559</artwork></figure>
2560<t>
2561   The Upgrade header field is intended to provide a simple mechanism
2562   for transition from HTTP/1.1 to some other, incompatible protocol. It
2563   does so by allowing the client to advertise its desire to use another
2564   protocol, such as a later version of HTTP with a higher major version
2565   number, even though the current request has been made using HTTP/1.1.
2566   This eases the difficult transition between incompatible protocols by
2567   allowing the client to initiate a request in the more commonly
2568   supported protocol while indicating to the server that it would like
2569   to use a "better" protocol if available (where "better" is determined
2570   by the server, possibly according to the nature of the method and/or
2571   resource being requested).
2572</t>
2573<t>
2574   The Upgrade header field only applies to switching application-layer
2575   protocols upon the existing transport-layer connection. Upgrade
2576   cannot be used to insist on a protocol change; its acceptance and use
2577   by the server is optional. The capabilities and nature of the
2578   application-layer communication after the protocol change is entirely
2579   dependent upon the new protocol chosen, although the first action
2580   after changing the protocol &MUST; be a response to the initial HTTP
2581   request containing the Upgrade header field.
2582</t>
2583<t>
2584   The Upgrade header field only applies to the immediate connection.
2585   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2586   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2587   HTTP/1.1 message.
2588</t>
2589<t>
2590   The Upgrade header field cannot be used to indicate a switch to a
2591   protocol on a different connection. For that purpose, it is more
2592   appropriate to use a 301, 302, 303, or 305 redirection response.
2593</t>
2594<t>
2595   This specification only defines the protocol name "HTTP" for use by
2596   the family of Hypertext Transfer Protocols, as defined by the HTTP
2597   version rules of <xref target="http.version"/> and future updates to this
2598   specification. Any token can be used as a protocol name; however, it
2599   will only be useful if both the client and server associate the name
2600   with the same protocol.
2601</t>
2602</section>
2603
2604<section title="Via" anchor="header.via">
2605  <iref primary="true" item="Via header" x:for-anchor=""/>
2606  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2607<t>
2608   The Via general-header field &MUST; be used by gateways and proxies to
2609   indicate the intermediate protocols and recipients between the user
2610   agent and the server on requests, and between the origin server and
2611   the client on responses. It is analogous to the "Received" field of
2612   RFC 822 <xref target="RFC822"/> and is intended to be used for tracking message forwards,
2613   avoiding request loops, and identifying the protocol capabilities of
2614   all senders along the request/response chain.
2615</t>
2616<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"/>
2617   Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2618   received-protocol = [ protocol-name "/" ] protocol-version
2619   protocol-name     = token
2620   protocol-version  = token
2621   received-by       = ( host [ ":" port ] ) | pseudonym
2622   pseudonym         = token
2623</artwork></figure>
2624<t>
2625   The received-protocol indicates the protocol version of the message
2626   received by the server or client along each segment of the
2627   request/response chain. The received-protocol version is appended to
2628   the Via field value when the message is forwarded so that information
2629   about the protocol capabilities of upstream applications remains
2630   visible to all recipients.
2631</t>
2632<t>
2633   The protocol-name is optional if and only if it would be "HTTP". The
2634   received-by field is normally the host and optional port number of a
2635   recipient server or client that subsequently forwarded the message.
2636   However, if the real host is considered to be sensitive information,
2637   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2638   be assumed to be the default port of the received-protocol.
2639</t>
2640<t>
2641   Multiple Via field values represents each proxy or gateway that has
2642   forwarded the message. Each recipient &MUST; append its information
2643   such that the end result is ordered according to the sequence of
2644   forwarding applications.
2645</t>
2646<t>
2647   Comments &MAY; be used in the Via header field to identify the software
2648   of the recipient proxy or gateway, analogous to the User-Agent and
2649   Server header fields. However, all comments in the Via field are
2650   optional and &MAY; be removed by any recipient prior to forwarding the
2651   message.
2652</t>
2653<t>
2654   For example, a request message could be sent from an HTTP/1.0 user
2655   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2656   forward the request to a public proxy at p.example.net, which completes
2657   the request by forwarding it to the origin server at www.example.com.
2658   The request received by www.example.com would then have the following
2659   Via header field:
2660</t>
2661<figure><artwork type="example">
2662    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2663</artwork></figure>
2664<t>
2665   Proxies and gateways used as a portal through a network firewall
2666   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2667   the firewall region. This information &SHOULD; only be propagated if
2668   explicitly enabled. If not enabled, the received-by host of any host
2669   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2670   for that host.
2671</t>
2672<t>
2673   For organizations that have strong privacy requirements for hiding
2674   internal structures, a proxy &MAY; combine an ordered subsequence of
2675   Via header field entries with identical received-protocol values into
2676   a single such entry. For example,
2677</t>
2678<figure><artwork type="example">
2679    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2680</artwork></figure>
2681<t>
2682        could be collapsed to
2683</t>
2684<figure><artwork type="example">
2685    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2686</artwork></figure>
2687<t>
2688   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2689   under the same organizational control and the hosts have already been
2690   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2691   have different received-protocol values.
2692</t>
2693</section>
2694
2695</section>
2696
2697<section title="IANA Considerations" anchor="IANA.considerations">
2698<t>
2699   TBD.
2700</t>
2701</section>
2702
2703<section title="Security Considerations" anchor="security.considerations">
2704<t>
2705   This section is meant to inform application developers, information
2706   providers, and users of the security limitations in HTTP/1.1 as
2707   described by this document. The discussion does not include
2708   definitive solutions to the problems revealed, though it does make
2709   some suggestions for reducing security risks.
2710</t>
2711
2712<section title="Personal Information" anchor="personal.information">
2713<t>
2714   HTTP clients are often privy to large amounts of personal information
2715   (e.g. the user's name, location, mail address, passwords, encryption
2716   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2717   leakage of this information via the HTTP protocol to other sources.
2718   We very strongly recommend that a convenient interface be provided
2719   for the user to control dissemination of such information, and that
2720   designers and implementors be particularly careful in this area.
2721   History shows that errors in this area often create serious security
2722   and/or privacy problems and generate highly adverse publicity for the
2723   implementor's company.
2724</t>
2725</section>
2726
2727<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2728<t>
2729   A server is in the position to save personal data about a user's
2730   requests which might identify their reading patterns or subjects of
2731   interest. This information is clearly confidential in nature and its
2732   handling can be constrained by law in certain countries. People using
2733   the HTTP protocol to provide data are responsible for ensuring that
2734   such material is not distributed without the permission of any
2735   individuals that are identifiable by the published results.
2736</t>
2737</section>
2738
2739<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2740<t>
2741   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2742   the documents returned by HTTP requests to be only those that were
2743   intended by the server administrators. If an HTTP server translates
2744   HTTP URIs directly into file system calls, the server &MUST; take
2745   special care not to serve files that were not intended to be
2746   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2747   other operating systems use ".." as a path component to indicate a
2748   directory level above the current one. On such a system, an HTTP
2749   server &MUST; disallow any such construct in the Request-URI if it
2750   would otherwise allow access to a resource outside those intended to
2751   be accessible via the HTTP server. Similarly, files intended for
2752   reference only internally to the server (such as access control
2753   files, configuration files, and script code) &MUST; be protected from
2754   inappropriate retrieval, since they might contain sensitive
2755   information. Experience has shown that minor bugs in such HTTP server
2756   implementations have turned into security risks.
2757</t>
2758</section>
2759
2760<section title="DNS Spoofing" anchor="dns.spoofing">
2761<t>
2762   Clients using HTTP rely heavily on the Domain Name Service, and are
2763   thus generally prone to security attacks based on the deliberate
2764   mis-association of IP addresses and DNS names. Clients need to be
2765   cautious in assuming the continuing validity of an IP number/DNS name
2766   association.
2767</t>
2768<t>
2769   In particular, HTTP clients &SHOULD; rely on their name resolver for
2770   confirmation of an IP number/DNS name association, rather than
2771   caching the result of previous host name lookups. Many platforms
2772   already can cache host name lookups locally when appropriate, and
2773   they &SHOULD; be configured to do so. It is proper for these lookups to
2774   be cached, however, only when the TTL (Time To Live) information
2775   reported by the name server makes it likely that the cached
2776   information will remain useful.
2777</t>
2778<t>
2779   If HTTP clients cache the results of host name lookups in order to
2780   achieve a performance improvement, they &MUST; observe the TTL
2781   information reported by DNS.
2782</t>
2783<t>
2784   If HTTP clients do not observe this rule, they could be spoofed when
2785   a previously-accessed server's IP address changes. As network
2786   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2787   possibility of this form of attack will grow. Observing this
2788   requirement thus reduces this potential security vulnerability.
2789</t>
2790<t>
2791   This requirement also improves the load-balancing behavior of clients
2792   for replicated servers using the same DNS name and reduces the
2793   likelihood of a user's experiencing failure in accessing sites which
2794   use that strategy.
2795</t>
2796</section>
2797
2798<section title="Proxies and Caching" anchor="attack.proxies">
2799<t>
2800   By their very nature, HTTP proxies are men-in-the-middle, and
2801   represent an opportunity for man-in-the-middle attacks. Compromise of
2802   the systems on which the proxies run can result in serious security
2803   and privacy problems. Proxies have access to security-related
2804   information, personal information about individual users and
2805   organizations, and proprietary information belonging to users and
2806   content providers. A compromised proxy, or a proxy implemented or
2807   configured without regard to security and privacy considerations,
2808   might be used in the commission of a wide range of potential attacks.
2809</t>
2810<t>
2811   Proxy operators should protect the systems on which proxies run as
2812   they would protect any system that contains or transports sensitive
2813   information. In particular, log information gathered at proxies often
2814   contains highly sensitive personal information, and/or information
2815   about organizations. Log information should be carefully guarded, and
2816   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2817</t>
2818<t>
2819   Proxy implementors should consider the privacy and security
2820   implications of their design and coding decisions, and of the
2821   configuration options they provide to proxy operators (especially the
2822   default configuration).
2823</t>
2824<t>
2825   Users of a proxy need to be aware that they are no trustworthier than
2826   the people who run the proxy; HTTP itself cannot solve this problem.
2827</t>
2828<t>
2829   The judicious use of cryptography, when appropriate, may suffice to
2830   protect against a broad range of security and privacy attacks. Such
2831   cryptography is beyond the scope of the HTTP/1.1 specification.
2832</t>
2833</section>
2834
2835<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2836<t>
2837   They exist. They are hard to defend against. Research continues.
2838   Beware.
2839</t>
2840</section>
2841</section>
2842
2843<section title="Acknowledgments" anchor="ack">
2844<t>
2845   This specification makes heavy use of the augmented BNF and generic
2846   constructs defined by David H. Crocker for RFC 822 <xref target="RFC822"/>. Similarly, it
2847   reuses many of the definitions provided by Nathaniel Borenstein and
2848   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2849   specification will help reduce past confusion over the relationship
2850   between HTTP and Internet mail message formats.
2851</t>
2852<t>
2853   The HTTP protocol has evolved considerably over the years. It has
2854   benefited from a large and active developer community--the many
2855   people who have participated on the www-talk mailing list--and it is
2856   that community which has been most responsible for the success of
2857   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2858   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2859   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2860   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2861   VanHeyningen deserve special recognition for their efforts in
2862   defining early aspects of the protocol.
2863</t>
2864<t>
2865   This document has benefited greatly from the comments of all those
2866   participating in the HTTP-WG. In addition to those already mentioned,
2867   the following individuals have contributed to this specification:
2868</t>
2869<figure><artwork type="text/plain">
2870    Gary Adams                  Ross Patterson
2871    Harald Tveit Alvestrand     Albert Lunde
2872    Keith Ball                  John C. Mallery
2873    Brian Behlendorf            Jean-Philippe Martin-Flatin
2874    Paul Burchard               Mitra
2875    Maurizio Codogno            David Morris
2876    Mike Cowlishaw              Gavin Nicol
2877    Roman Czyborra              Bill Perry
2878    Michael A. Dolan            Jeffrey Perry
2879    David J. Fiander            Scott Powers
2880    Alan Freier                 Owen Rees
2881    Marc Hedlund                Luigi Rizzo
2882    Greg Herlihy                David Robinson
2883    Koen Holtman                Marc Salomon
2884    Alex Hopmann                Rich Salz
2885    Bob Jernigan                Allan M. Schiffman
2886    Shel Kaphan                 Jim Seidman
2887    Rohit Khare                 Chuck Shotton
2888    John Klensin                Eric W. Sink
2889    Martijn Koster              Simon E. Spero
2890    Alexei Kosut                Richard N. Taylor
2891    David M. Kristol            Robert S. Thau
2892    Daniel LaLiberte            Bill (BearHeart) Weinman
2893    Ben Laurie                  Francois Yergeau
2894    Paul J. Leach               Mary Ellen Zurko
2895    Daniel DuBois               Josh Cohen
2896</artwork></figure>
2897<t>
2898   Thanks to the "cave men" of Palo Alto. You know who you are.
2899</t>
2900<t>
2901   Jim Gettys (the current editor of this document) wishes particularly
2902   to thank Roy Fielding, the previous editor of this document, along
2903   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2904   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2905   Larry Masinter for their help. And thanks go particularly to Jeff
2906   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2907</t>
2908<t>
2909   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
2910   Frystyk implemented RFC 2068 early, and we wish to thank them for the
2911   discovery of many of the problems that this document attempts to
2912   rectify.
2913</t>
2914<t>
2915   Based on an XML translation of RFC 2616 by Julian Reschke.
2916</t>
2917</section>
2918
2919</middle>
2920<back>
2921<references>
2922
2923<reference anchor="Part2">
2924   <front>
2925      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2926      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2927         <organization abbrev="Day Software">Day Software</organization>
2928         <address><email>fielding@gbiv.com</email></address>
2929      </author>
2930      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2931         <organization>One Laptop per Child</organization>
2932         <address><email>jg@laptop.org</email></address>
2933      </author>
2934      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2935         <organization abbrev="HP">Hewlett-Packard Company</organization>
2936         <address><email>JeffMogul@acm.org</email></address>
2937      </author>
2938      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2939         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2940         <address><email>henrikn@microsoft.com</email></address>
2941      </author>
2942      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2943         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2944         <address><email>LMM@acm.org</email></address>
2945      </author>
2946      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2947         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2948         <address><email>paulle@microsoft.com</email></address>
2949      </author>
2950      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2951         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2952         <address><email>timbl@w3.org</email></address>
2953      </author>
2954      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2955   </front>
2956   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2957   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2958</reference>
2959
2960<reference anchor="Part3">
2961   <front>
2962      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2963      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2964         <organization abbrev="Day Software">Day Software</organization>
2965         <address><email>fielding@gbiv.com</email></address>
2966      </author>
2967      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2968         <organization>One Laptop per Child</organization>
2969         <address><email>jg@laptop.org</email></address>
2970      </author>
2971      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2972         <organization abbrev="HP">Hewlett-Packard Company</organization>
2973         <address><email>JeffMogul@acm.org</email></address>
2974      </author>
2975      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2976         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2977         <address><email>henrikn@microsoft.com</email></address>
2978      </author>
2979      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2980         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2981         <address><email>LMM@acm.org</email></address>
2982      </author>
2983      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2984         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2985         <address><email>paulle@microsoft.com</email></address>
2986      </author>
2987      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2988         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2989         <address><email>timbl@w3.org</email></address>
2990      </author>
2991      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2992   </front>
2993   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2994   <x:source href="p3-payload.xml" basename="p3-payload"/>
2995</reference>
2996
2997<reference anchor="Part6">
2998   <front>
2999      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3000      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3001         <organization abbrev="Day Software">Day Software</organization>
3002         <address><email>fielding@gbiv.com</email></address>
3003      </author>
3004      <author initials="J." surname="Gettys" fullname="Jim Gettys">
3005         <organization>One Laptop per Child</organization>
3006         <address><email>jg@laptop.org</email></address>
3007      </author>
3008      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3009         <organization abbrev="HP">Hewlett-Packard Company</organization>
3010         <address><email>JeffMogul@acm.org</email></address>
3011      </author>
3012      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3013         <organization abbrev="Microsoft">Microsoft Corporation</organization>
3014         <address><email>henrikn@microsoft.com</email></address>
3015      </author>
3016      <author initials="L." surname="Masinter" fullname="Larry Masinter">
3017         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3018         <address><email>LMM@acm.org</email></address>
3019      </author>
3020      <author initials="P." surname="Leach" fullname="Paul J. Leach">
3021         <organization abbrev="Microsoft">Microsoft Corporation</organization>
3022         <address><email>paulle@microsoft.com</email></address>
3023      </author>
3024      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3025         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3026         <address><email>timbl@w3.org</email></address>
3027      </author>
3028      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3029   </front>
3030   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3031   <x:source href="p6-cache.xml" basename="p6-cache"/>
3032</reference>
3033
3034<reference anchor="RFC2616">
3035   <front>
3036      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3037      <author initials="R." surname="Fielding" fullname="R. Fielding">
3038         <organization>University of California, Irvine</organization>
3039         <address><email>fielding@ics.uci.edu</email></address>
3040      </author>
3041      <author initials="J." surname="Gettys" fullname="J. Gettys">
3042         <organization>W3C</organization>
3043         <address><email>jg@w3.org</email></address>
3044      </author>
3045      <author initials="J." surname="Mogul" fullname="J. Mogul">
3046         <organization>Compaq Computer Corporation</organization>
3047         <address><email>mogul@wrl.dec.com</email></address>
3048      </author>
3049      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3050         <organization>MIT Laboratory for Computer Science</organization>
3051         <address><email>frystyk@w3.org</email></address>
3052      </author>
3053      <author initials="L." surname="Masinter" fullname="L. Masinter">
3054         <organization>Xerox Corporation</organization>
3055         <address><email>masinter@parc.xerox.com</email></address>
3056      </author>
3057      <author initials="P." surname="Leach" fullname="P. Leach">
3058         <organization>Microsoft Corporation</organization>
3059         <address><email>paulle@microsoft.com</email></address>
3060      </author>
3061      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3062         <organization>W3C</organization>
3063         <address><email>timbl@w3.org</email></address>
3064      </author>
3065      <date month="June" year="1999"/>
3066   </front>
3067   <seriesInfo name="RFC" value="2616"/>
3068</reference>
3069
3070<reference anchor="RFC1436">
3071<front>
3072<title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3073<author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3074<organization>University of Minnesota, Computer and Information Services</organization>
3075<address>
3076<postal>
3077<street>100 Union Street SE</street>
3078<street>Room 152 Shepherd Labs</street>
3079<city>Minneapolis</city>
3080<region>MN</region>
3081<code>55455</code>
3082<country>US</country></postal>
3083<phone>+1 612 625 1300</phone>
3084<email>fxa@boombox.micro.umn.edu</email></address></author>
3085<author initials="M." surname="McCahill" fullname="Mark McCahill">
3086<organization>University of Minnesota, Computer and Information Services</organization>
3087<address>
3088<postal>
3089<street>100 Union Street SE</street>
3090<street>Room 152 Shepherd Labs</street>
3091<city>Minneapolis</city>
3092<region>MN</region>
3093<code>55455</code>
3094<country>US</country></postal>
3095<phone>+1 612 625 1300</phone>
3096<email>mpm@boombox.micro.umn.edu</email></address></author>
3097<author initials="P." surname="Lindner" fullname="Paul Lindner">
3098<organization>University of Minnesota, Computer and Information Services</organization>
3099<address>
3100<postal>
3101<street>100 Union Street SE</street>
3102<street>Room 152 Shepherd Labs</street>
3103<city>Minneapolis</city>
3104<region>MN</region>
3105<code>55455</code>
3106<country>US</country></postal>
3107<phone>+1 612 625 1300</phone>
3108<email>lindner@boombox.micro.umn.edu</email></address></author>
3109<author initials="D." surname="Johnson" fullname="David Johnson">
3110<organization>University of Minnesota, Computer and Information Services</organization>
3111<address>
3112<postal>
3113<street>100 Union Street SE</street>
3114<street>Room 152 Shepherd Labs</street>
3115<city>Minneapolis</city>
3116<region>MN</region>
3117<code>55455</code>
3118<country>US</country></postal>
3119<phone>+1 612 625 1300</phone>
3120<email>dmj@boombox.micro.umn.edu</email></address></author>
3121<author initials="D." surname="Torrey" fullname="Daniel Torrey">
3122<organization>University of Minnesota, Computer and Information Services</organization>
3123<address>
3124<postal>
3125<street>100 Union Street SE</street>
3126<street>Room 152 Shepherd Labs</street>
3127<city>Minneapolis</city>
3128<region>MN</region>
3129<code>55455</code>
3130<country>US</country></postal>
3131<phone>+1 612 625 1300</phone>
3132<email>daniel@boombox.micro.umn.edu</email></address></author>
3133<author initials="B." surname="Alberti" fullname="Bob Alberti">
3134<organization>University of Minnesota, Computer and Information Services</organization>
3135<address>
3136<postal>
3137<street>100 Union Street SE</street>
3138<street>Room 152 Shepherd Labs</street>
3139<city>Minneapolis</city>
3140<region>MN</region>
3141<code>55455</code>
3142<country>US</country></postal>
3143<phone>+1 612 625 1300</phone>
3144<email>alberti@boombox.micro.umn.edu</email></address></author>
3145<date month="March" year="1993"/>
3146</front>
3147<seriesInfo name="RFC" value="1436"/>
3148</reference>
3149
3150<reference anchor="RFC1630">
3151<front>
3152<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>
3153<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3154<organization>CERN, World-Wide Web project</organization>
3155<address>
3156<postal>
3157<street>1211 Geneva 23</street>
3158<city/>
3159<region/>
3160<code/>
3161<country>CH</country></postal>
3162<phone>+41 22 7673755</phone>
3163<facsimile>+41 22 7677155</facsimile>
3164<email>timbl@info.cern.ch</email></address></author>
3165<date month="June" year="1994"/></front>
3166<seriesInfo name="RFC" value="1630"/>
3167</reference>
3168
3169<reference anchor="RFC1738">
3170<front>
3171<title>Uniform Resource Locators (URL)</title>
3172<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3173<organization>CERN, World-Wide Web project</organization>
3174<address>
3175<postal>
3176<street>1211 Geneva 23</street>
3177<city/>
3178<region/>
3179<code/>
3180<country>CH</country></postal>
3181<phone>+41 22 7673755</phone>
3182<facsimile>+41 22 7677155</facsimile>
3183<email>timbl@info.cern.ch</email></address></author>
3184<author initials="L." surname="Masinter" fullname="Larry Masinter">
3185<organization>Xerox PARC</organization>
3186<address>
3187<postal>
3188<street>3333 Coyote Hill Road</street>
3189<city>Palo Alto</city>
3190<region>CA</region>
3191<code>94034</code>
3192<country>US</country></postal>
3193<phone>+1 415 812 4365</phone>
3194<facsimile>+1 415 812 4333</facsimile>
3195<email>masinter@parc.xerox.com</email></address></author>
3196<author initials="M." surname="McCahill" fullname="Mark McCahill">
3197<organization>University of Minnesota, Computer and Information Services</organization>
3198<address>
3199<postal>
3200<street>100 Union Street SE, Shepherd Labs</street>
3201<street>Room 152</street>
3202<city>Minneapolis</city>
3203<region>MN</region>
3204<code>55455</code>
3205<country>US</country></postal>
3206<phone>+1 612 625 1300</phone>
3207<email>mpm@boombox.micro.umn.edu</email></address></author>
3208<date month="December" year="1994"/>
3209</front>
3210<seriesInfo name="RFC" value="1738"/>
3211</reference>
3212
3213<reference anchor="RFC1945">
3214<front>
3215<title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3216<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3217<organization>MIT, Laboratory for Computer Science</organization>
3218<address>
3219<postal>
3220<street>545 Technology Square</street>
3221<city>Cambridge</city>
3222<region>MA</region>
3223<code>02139</code>
3224<country>US</country></postal>
3225<phone/>
3226<facsimile>+1 617 258 8682</facsimile>
3227<email>timbl@w3.org</email></address></author>
3228<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3229<organization>University of California, Irvine, Department of Information and Computer Science</organization>
3230<address>
3231<postal>
3232<street/>
3233<city>Irvine</city>
3234<region>CA</region>
3235<code>92717-3425</code>
3236<country>US</country></postal>
3237<phone/>
3238<facsimile>+1 714 824 4056</facsimile>
3239<email>fielding@ics.uci.edu</email></address></author>
3240<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3241<organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3242<address>
3243<postal>
3244<street>545 Technology Square</street>
3245<city>Cambridge</city>
3246<region>MA</region>
3247<code>02139</code>
3248<country>US</country></postal>
3249<phone/>
3250<facsimile>+1 617 258 8682</facsimile>
3251<email>frystyk@w3.org</email></address></author>
3252<date month="May" year="1996"/>
3253</front>
3254<seriesInfo name="RFC" value="1945"/>
3255</reference>
3256
3257<reference anchor="RFC2045">
3258<front>
3259<title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3260<author initials="N." surname="Freed" fullname="Ned Freed">
3261<organization>Innosoft International, Inc.</organization>
3262<address>
3263<postal>
3264<street>1050 East Garvey Avenue South</street>
3265<city>West Covina</city>
3266<region>CA</region>
3267<code>91790</code>
3268<country>US</country></postal>
3269<phone>+1 818 919 3600</phone>
3270<facsimile>+1 818 919 3614</facsimile>
3271<email>ned@innosoft.com</email></address></author>
3272<author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3273<organization>First Virtual Holdings</organization>
3274<address>
3275<postal>
3276<street>25 Washington Avenue</street>
3277<city>Morristown</city>
3278<region>NJ</region>
3279<code>07960</code>
3280<country>US</country></postal>
3281<phone>+1 201 540 8967</phone>
3282<facsimile>+1 201 993 3032</facsimile>
3283<email>nsb@nsb.fv.com</email></address></author>
3284<date month="November" year="1996"/>
3285</front>
3286<seriesInfo name="RFC" value="2045"/>
3287</reference>
3288
3289<reference anchor="RFC1123">
3290<front>
3291<title>Requirements for Internet Hosts - Application and Support</title>
3292<author initials="R." surname="Braden" fullname="Robert Braden">
3293<organization>University of Southern California (USC), Information Sciences Institute</organization>
3294<address>
3295<postal>
3296<street>4676 Admiralty Way</street>
3297<city>Marina del Rey</city>
3298<region>CA</region>
3299<code>90292-6695</code>
3300<country>US</country></postal>
3301<phone>+1 213 822 1511</phone>
3302<email>Braden@ISI.EDU</email></address></author>
3303<date month="October" year="1989"/></front>
3304<seriesInfo name="STD" value="3"/>
3305<seriesInfo name="RFC" value="1123"/>
3306</reference>
3307
3308<reference anchor="RFC822">
3309<front>
3310<title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3311<author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3312<organization>University of Delaware, Dept. of Electrical Engineering</organization>
3313<address>
3314<postal>
3315<street/>
3316<city>Newark</city>
3317<region>DE</region>
3318<code>19711</code>
3319<country>US</country></postal>
3320<email>DCrocker@UDel-Relay</email></address></author>
3321<date month="August" day="13" year="1982"/></front>
3322<seriesInfo name="STD" value="11"/>
3323<seriesInfo name="RFC" value="822"/>
3324</reference>
3325
3326<reference anchor="WAIS">
3327<front>
3328<title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3329<author initials="F." surname="Davis" fullname="F. Davis">
3330<organization>Thinking Machines Corporation</organization>
3331</author>
3332<author initials="B." surname="Kahle" fullname="B. Kahle">
3333<organization>Thinking Machines Corporation</organization>
3334</author>
3335<author initials="H." surname="Morris" fullname="H. Morris">
3336<organization>Thinking Machines Corporation</organization>
3337</author>
3338<author initials="J." surname="Salem" fullname="J. Salem">
3339<organization>Thinking Machines Corporation</organization>
3340</author>
3341<author initials="T." surname="Shen" fullname="T. Shen">
3342<organization>Thinking Machines Corporation</organization>
3343</author>
3344<author initials="R." surname="Wang" fullname="R. Wang">
3345<organization>Thinking Machines Corporation</organization>
3346</author>
3347<author initials="J." surname="Sui" fullname="J. Sui">
3348<organization>Thinking Machines Corporation</organization>
3349</author>
3350<author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3351<organization>Thinking Machines Corporation</organization>
3352</author>
3353<date month="April" year="1990"/></front>
3354<seriesInfo name="Thinking Machines Corporation" value=""/>
3355</reference>
3356
3357<reference anchor="RFC1808">
3358<front>
3359<title>Relative Uniform Resource Locators</title>
3360<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3361<organization>University of California Irvine, Department of Information and Computer Science</organization>
3362<address>
3363<postal>
3364<street/>
3365<city>Irvine</city>
3366<region>CA</region>
3367<code>92717-3425</code>
3368<country>US</country></postal>
3369<phone>+1 714 824 4049</phone>
3370<facsimile>+1 714 824 4056</facsimile>
3371<email>fielding@ics.uci.edu</email></address></author>
3372<date month="June" year="1995"/>
3373</front>
3374<seriesInfo name="RFC" value="1808"/>
3375</reference>
3376
3377<reference anchor='RFC3977'>
3378   <front>
3379      <title>Network News Transfer Protocol (NNTP)</title>
3380      <author initials='C.' surname='Feather' fullname='C. Feather'>
3381         <organization>THUS plc</organization>
3382         <address>
3383            <email>clive@demon.net</email>
3384         </address>
3385      </author>
3386      <date year='2006' month='October' />
3387   </front>
3388</reference>
3389
3390<reference anchor="RFC2047">
3391<front>
3392<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3393<author initials="K." surname="Moore" fullname="Keith Moore">
3394<organization>University of Tennessee</organization>
3395<address>
3396<postal>
3397<street>107 Ayres Hall</street>
3398<street>Knoxville TN 37996-1301</street></postal>
3399<email>moore@cs.utk.edu</email></address></author>
3400<date month="November" year="1996"/>
3401<area>Applications</area>
3402<keyword>Amercian Standard Code for Information Interchange</keyword>
3403<keyword>mail</keyword>
3404<keyword>multipurpose internet mail extensions</keyword>
3405</front>
3406<seriesInfo name="RFC" value="2047"/>
3407</reference>
3408
3409<reference anchor="RFC821">
3410<front>
3411<title>Simple Mail Transfer Protocol</title>
3412<author initials="J.B." surname="Postel" fullname="Jonathan B. Postel">
3413<organization>University of Southern California (USC)/Information Sciences Institute</organization>
3414<address>
3415<postal>
3416<street>4676 Admiralty Way</street>
3417<city>Marina del Rey</city>
3418<region>CA</region>
3419<code>90291</code>
3420<country>US</country></postal>
3421<phone>+1 213 822 1511</phone></address></author>
3422<date month="August" year="1982"/></front>
3423<seriesInfo name="STD" value="10"/>
3424<seriesInfo name="RFC" value="821"/>
3425</reference>
3426
3427<reference anchor="RFC4288">
3428  <front>
3429    <title>Media Type Specifications and Registration Procedures</title>
3430    <author initials="N." surname="Freed" fullname="N. Freed">
3431      <organization>Sun Microsystems</organization>
3432      <address>
3433        <email>ned.freed@mrochek.com</email>
3434      </address>
3435    </author>
3436    <author initials="J." surname="Klensin" fullname="J. Klensin">
3437      <organization/>
3438      <address>
3439        <email>klensin+ietf@jck.com</email>
3440      </address>
3441    </author>
3442    <date year="2005" month="December"/>
3443  </front>
3444  <seriesInfo name="BCP" value="13"/>
3445  <seriesInfo name="RFC" value="4288"/>
3446</reference>
3447
3448<reference anchor="RFC959">
3449<front>
3450<title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3451<author initials="J." surname="Postel" fullname="J. Postel">
3452<organization>Information Sciences Institute (ISI)</organization></author>
3453<author initials="J." surname="Reynolds" fullname="J. Reynolds">
3454<organization/></author>
3455<date month="October" year="1985"/></front>
3456<seriesInfo name="STD" value="9"/>
3457<seriesInfo name="RFC" value="959"/>
3458</reference>
3459
3460<reference anchor="RFC1737">
3461<front>
3462<title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3463<author initials="L." surname="Masinter" fullname="Larry Masinter">
3464<organization>Xerox Palo Alto Research Center</organization>
3465<address>
3466<postal>
3467<street>3333 Coyote Hill Road</street>
3468<city>Palo Alto</city>
3469<region>CA</region>
3470<code>94304</code>
3471<country>US</country></postal>
3472<phone>+1 415 812 4365</phone>
3473<facsimile>+1 415 812 4333</facsimile>
3474<email>masinter@parc.xerox.com</email></address></author>
3475<author initials="K." surname="Sollins" fullname="Karen Sollins">
3476<organization>MIT Laboratory for Computer Science</organization>
3477<address>
3478<postal>
3479<street>545 Technology Square</street>
3480<city>Cambridge</city>
3481<region>MA</region>
3482<code>02139</code>
3483<country>US</country></postal>
3484<phone>+1 617 253 2673</phone>
3485<email>sollins@lcs.mit.edu</email></address></author>
3486<date month="December" year="1994"/></front>
3487<seriesInfo name="RFC" value="1737"/>
3488</reference>
3489
3490<reference anchor="USASCII">
3491  <front>
3492    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3493    <author>
3494      <organization>American National Standards Institute</organization>
3495    </author>
3496    <date year="1986"/>
3497  </front>
3498  <seriesInfo name="ANSI" value="X3.4"/>
3499</reference>
3500
3501<reference anchor="ISO-8859">
3502<front>
3503<title>Information technology - 8-bit single byte coded graphic - character sets</title>
3504<author>
3505<organization>International Organization for Standardization</organization>
3506</author>
3507<date year="1987-1990"/>
3508</front>
3509<annotation>
3510  Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
3511  Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
3512  Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
3513  Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
3514  Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
3515  Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
3516  Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
3517  Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
3518  Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
3519</annotation>
3520</reference>
3521
3522<reference anchor="RFC1900">
3523<front>
3524<title>Renumbering Needs Work</title>
3525<author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3526<organization>CERN, Computing and Networks Division</organization>
3527<address>
3528<postal>
3529<street>1211 Geneva 23</street>
3530<country>CH</country></postal>
3531<phone>+41 22 7674967</phone>
3532<facsimile>+41 22 7677155</facsimile>
3533<email>brian@dxcoms.cern.ch</email></address></author>
3534<author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3535<organization>cisco Systems</organization>
3536<address>
3537<postal>
3538<street>170 West Tasman Drive</street>
3539<city>San Jose</city>
3540<region>CA</region>
3541<code>95134</code>
3542<country>US</country></postal>
3543<phone>+1 914 528 0090</phone>
3544<email>yakov@cisco.com</email></address></author>
3545<date month="February" year="1996"/>
3546</front>
3547<seriesInfo name="RFC" value="1900"/>
3548</reference>
3549
3550<reference anchor="Pad1995">
3551<front>
3552<title>Improving HTTP Latency</title>
3553<author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan"><organization/></author>
3554<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul"><organization/></author>
3555<date year="1995" month="Dec"/>
3556</front>
3557<seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3558<annotation>
3559  Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3560  which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3561</annotation>
3562</reference>
3563
3564<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3565<front>
3566<title>Analysis of HTTP Performance</title>
3567<author initials="J." surname="Touch" fullname="Joe Touch">
3568  <organization>USC/Information Sciences Institute</organization>
3569  <address><email>touch@isi.edu</email></address>
3570</author>
3571<author initials="J." surname="Heidemann" fullname="John Heidemann">
3572  <organization>USC/Information Sciences Institute</organization>
3573  <address><email>johnh@isi.edu</email></address>
3574</author>
3575<author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3576  <organization>USC/Information Sciences Institute</organization>
3577  <address><email>katia@isi.edu</email></address>
3578</author>
3579<date year="1998" month="Aug"/>
3580</front>
3581<seriesInfo name="ISI Research Report" value="ISI/RR-98-463 (original report dated Aug.1996)"/>
3582</reference>
3583
3584<reference anchor="RFC1305">
3585<front>
3586<title>Network Time Protocol (Version 3) Specification, Implementation</title>
3587<author initials="D." surname="Mills" fullname="David L. Mills">
3588<organization>University of Delaware, Electrical Engineering Department</organization>
3589<address>
3590<postal>
3591<street/>
3592<city>Newark</city>
3593<region>DE</region>
3594<code>19716</code>
3595<country>US</country></postal>
3596<phone>+1 302 451 8247</phone>
3597<email>mills@udel.edu</email></address></author>
3598<date month="March" year="1992"/>
3599<abstract>
3600<t>This document describes the Network Time Protocol (NTP), specifies its normal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons.</t></abstract></front>
3601<seriesInfo name="RFC" value="1305"/>
3602</reference>
3603
3604<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3605<front>
3606<title>Analysis of HTTP Performance Problems</title>
3607<author initials="S." surname="Spero" fullname="Simon E. Spero">
3608  <organization/>
3609</author>
3610<date/>
3611</front>
3612</reference>
3613
3614<reference anchor="RFC2068">
3615<front>
3616<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3617<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3618<organization>University of California, Irvine, Department of Information and Computer Science</organization>
3619<address>
3620<postal>
3621<street/>
3622<city>Irvine</city>
3623<region>CA</region>
3624<code>92717-3425</code>
3625<country>US</country></postal>
3626<facsimile>+1 714 824 4056</facsimile>
3627<email>fielding@ics.uci.edu</email></address></author>
3628<author initials="J." surname="Gettys" fullname="Jim Gettys">
3629<organization>MIT Laboratory for Computer Science</organization>
3630<address>
3631<postal>
3632<street>545 Technology Square</street>
3633<city>Cambridge</city>
3634<region>MA</region>
3635<code>02139</code>
3636<country>US</country></postal>
3637<facsimile>+1 617 258 8682</facsimile>
3638<email>jg@w3.org</email></address></author>
3639<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3640<organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3641<address>
3642<postal>
3643<street>250 University Avenue</street>
3644<city>Palo Alto</city>
3645<region>CA</region>
3646<code>94301</code>
3647<country>US</country></postal>
3648<email>mogul@wrl.dec.com</email></address></author>
3649<author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3650<organization>MIT Laboratory for Computer Science</organization>
3651<address>
3652<postal>
3653<street>545 Technology Square</street>
3654<city>Cambridge</city>
3655<region>MA</region>
3656<code>02139</code>
3657<country>US</country></postal>
3658<facsimile>+1 617 258 8682</facsimile>
3659<email>frystyk@w3.org</email></address></author>
3660<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3661<organization>MIT Laboratory for Computer Science</organization>
3662<address>
3663<postal>
3664<street>545 Technology Square</street>
3665<city>Cambridge</city>
3666<region>MA</region>
3667<code>02139</code>
3668<country>US</country></postal>
3669<facsimile>+1 617 258 8682</facsimile>
3670<email>timbl@w3.org</email></address></author>
3671<date month="January" year="1997"/>
3672<abstract>
3673<t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.</t>
3674<t>HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1".</t></abstract></front>
3675<seriesInfo name="RFC" value="2068"/>
3676</reference>
3677
3678<reference anchor="RFC2119">
3679<front>
3680<title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
3681<author initials="S." surname="Bradner" fullname="Scott Bradner">
3682<organization>Harvard University</organization>
3683<address>
3684<postal>
3685<street>1350 Mass. Ave.</street>
3686<street>Cambridge</street>
3687<street>MA 02138</street></postal>
3688<phone>- +1 617 495 3864</phone>
3689<email>-</email></address></author>
3690<date month="March" year="1997"/>
3691<area>General</area>
3692<keyword>keyword</keyword>
3693</front>
3694
3695<seriesInfo name="BCP" value="14"/>
3696<seriesInfo name="RFC" value="2119"/>
3697</reference>
3698
3699<reference anchor="RFC2145">
3700<front>
3701<title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3702<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3703<organization>Western Research Laboratory</organization>
3704<address>
3705<postal>
3706<street>Digital Equipment Corporation</street>
3707<street>250 University Avenue</street>
3708<street>Palo Alto</street>
3709<street>California</street>
3710<street>94305</street>
3711<country>USA</country></postal>
3712<email>mogul@wrl.dec.com</email></address></author>
3713<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3714<organization>Department of Information and Computer Science</organization>
3715<address>
3716<postal>
3717<street>University of California</street>
3718<street>Irvine</street>
3719<street>CA 92717-3425</street>
3720<country>USA</country></postal>
3721<facsimile>+1 (714) 824-4056</facsimile>
3722<email>fielding@ics.uci.edu</email></address></author>
3723<author initials="J." surname="Gettys" fullname="Jim Gettys">
3724<organization>MIT Laboratory for Computer Science</organization>
3725<address>
3726<postal>
3727<street>545 Technology Square</street>
3728<street>Cambridge</street>
3729<street>MA 02139</street>
3730<country>USA</country></postal>
3731<facsimile>+1 (617) 258 8682</facsimile>
3732<email>jg@w3.org</email></address></author>
3733<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3734<organization>W3 Consortium</organization>
3735<address>
3736<postal>
3737<street>MIT Laboratory for Computer Science</street>
3738<street>545 Technology Square</street>
3739<street>Cambridge</street>
3740<street>MA 02139</street>
3741<country>USA</country></postal>
3742<facsimile>+1 (617) 258 8682</facsimile>
3743<email>frystyk@w3.org</email></address></author>
3744<date month="May" year="1997"/>
3745<area>Applications</area>
3746<keyword>HTTP</keyword>
3747<keyword>hypertext transfer protocol</keyword>
3748<abstract>
3749<t>
3750   HTTP request and response messages include an HTTP protocol version
3751   number.  Some confusion exists concerning the proper use and
3752   interpretation of HTTP version numbers, and concerning
3753   interoperability of HTTP implementations of different protocol
3754   versions.  This document is an attempt to clarify the situation.  It
3755   is not a modification of the intended meaning of the existing
3756   HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention
3757   of the authors of those documents, and can be considered definitive
3758   when there is any ambiguity in those documents concerning HTTP
3759   version numbers, for all versions of HTTP.
3760</t></abstract></front>
3761<seriesInfo name="RFC" value="2145"/>
3762</reference>
3763
3764<reference anchor="Nie1997">
3765<front>
3766<title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3767<author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3768  <organization/>
3769</author>
3770<author initials="J." surname="Gettys" fullname="J. Gettys">
3771  <organization/>
3772</author>
3773<author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3774  <organization/>
3775</author>
3776<author initials="H." surname="Lie" fullname="H. Lie">
3777  <organization/>
3778</author>
3779<author initials="C." surname="Lilley" fullname="C. Lilley">
3780  <organization/>
3781</author>
3782<date year="1997" month="Sep"/>
3783</front>
3784<seriesInfo name="Proceedings of ACM SIGCOMM '97, Cannes France" value=""/>
3785</reference>
3786
3787<reference anchor="RFC2396">
3788<front>
3789<title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3790<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3791<organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3792<address>
3793<postal>
3794<street>MIT Laboratory for Computer Science, NE43-356</street>
3795<street>545 Technology Square</street>
3796<city>Cambridge</city>
3797<region>MA</region>
3798<code>02139</code></postal>
3799<facsimile>+1(617)258-8682</facsimile>
3800<email>timbl@w3.org</email></address></author>
3801<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3802<organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3803<address>
3804<postal>
3805<street>University of California, Irvine</street>
3806<city>Irvine</city>
3807<region>CA</region>
3808<code>92697-3425</code></postal>
3809<facsimile>+1(949)824-1715</facsimile>
3810<email>fielding@ics.uci.edu</email></address></author>
3811<author initials="L." surname="Masinter" fullname="Larry Masinter">
3812<organization abbrev="Xerox Corporation">Xerox PARC</organization>
3813<address>
3814<postal>
3815<street>3333 Coyote Hill Road</street>
3816<city>Palo Alto</city>
3817<region>CA</region>
3818<code>94034</code></postal>
3819<facsimile>+1(415)812-4333</facsimile>
3820<email>masinter@parc.xerox.com</email></address></author>
3821<date month="August" year="1998"/>
3822<area>Applications</area>
3823<keyword>uniform resource</keyword>
3824<keyword>URI</keyword>
3825</front>
3826<seriesInfo name="RFC" value="2396"/>
3827</reference>
3828
3829<reference anchor="RFC2324">
3830<front>
3831<title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3832<author initials="L." surname="Masinter" fullname="Larry Masinter">
3833<organization>Xerox Palo Alto Research Center</organization>
3834<address>
3835<postal>
3836<street>3333 Coyote Hill Road</street>
3837<city>Palo Alto</city>
3838<region>CA</region>
3839<code>94304</code></postal>
3840<email>masinter@parc.xerox.com</email></address></author>
3841<date month="April" day="1" year="1998"/>
3842<area>General</area>
3843<keyword>control protocol</keyword>
3844<keyword>coffee</keyword>
3845</front>
3846<seriesInfo name="RFC" value="2324"/>
3847</reference>
3848
3849</references>
3850
3851<section title="Internet Media Type message/http and application/http" anchor="internet.media.type.http">
3852<iref item="Media Type" subitem="message/http" primary="true"/>
3853<iref item="message/http Media Type" primary="true"/>
3854<iref item="Media Type" subitem="application/http" primary="true"/>
3855<iref item="application/http Media Type" primary="true"/>
3856<t>
3857   In addition to defining the HTTP/1.1 protocol, this document serves
3858   as the specification for the Internet media type "message/http" and
3859   "application/http". The message/http type can be used to enclose a
3860   single HTTP request or response message, provided that it obeys the
3861   MIME restrictions for all "message" types regarding line length and
3862   encodings. The application/http type can be used to enclose a
3863   pipeline of one or more HTTP request or response messages (not
3864   intermixed). The following is to be registered with IANA <xref target="RFC4288"/>.
3865</t>
3866<t>
3867  <list style="hanging" x:indent="12em">
3868    <t hangText="Media Type name:">
3869      message
3870    </t>
3871    <t hangText="Media subtype name:">
3872      http
3873    </t>
3874    <t hangText="Required parameters:">
3875      none
3876    </t>
3877    <t hangText="Optional parameters:">
3878      version, msgtype
3879      <list style="hanging">
3880        <t hangText="version:">
3881          The HTTP-Version number of the enclosed message
3882          (e.g., "1.1"). If not present, the version can be
3883          determined from the first line of the body.
3884        </t>
3885        <t hangText="msgtype:">
3886          The message type -- "request" or "response". If not
3887          present, the type can be determined from the first
3888          line of the body.
3889        </t>
3890      </list>
3891    </t>
3892    <t hangText="Encoding considerations:">
3893      only "7bit", "8bit", or "binary" are permitted
3894    </t>
3895    <t hangText="Security considerations:">
3896      none
3897    </t>
3898  </list>
3899</t>
3900<t>
3901  <list style="hanging" x:indent="12em">
3902    <t hangText="Media Type name:">
3903      application
3904    </t>
3905    <t hangText="Media subtype name:">
3906      http
3907    </t>
3908    <t hangText="Required parameters:">
3909      none
3910    </t>
3911    <t hangText="Optional parameters:">
3912      version, msgtype
3913      <list style="hanging">
3914        <t hangText="version:">
3915          The HTTP-Version number of the enclosed messages
3916          (e.g., "1.1"). If not present, the version can be
3917          determined from the first line of the body.
3918        </t>
3919        <t hangText="msgtype:">
3920          The message type -- "request" or "response". If not
3921          present, the type can be determined from the first
3922          line of the body.
3923        </t>
3924      </list>
3925    </t>
3926    <t hangText="Encoding considerations:">
3927      HTTP messages enclosed by this type
3928      are in "binary" format; use of an appropriate
3929      Content-Transfer-Encoding is required when
3930      transmitted via E-mail.
3931    </t>
3932    <t hangText="Security considerations:">
3933      none
3934    </t>
3935  </list>
3936</t>
3937</section>
3938
3939<section title="Tolerant Applications" anchor="tolerant.applications">
3940<t>
3941   Although this document specifies the requirements for the generation
3942   of HTTP/1.1 messages, not all applications will be correct in their
3943   implementation. We therefore recommend that operational applications
3944   be tolerant of deviations whenever those deviations can be
3945   interpreted unambiguously.
3946</t>
3947<t>
3948   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3949   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3950   accept any amount of SP or HT characters between fields, even though
3951   only a single SP is required.
3952</t>
3953<t>
3954   The line terminator for message-header fields is the sequence CRLF.
3955   However, we recommend that applications, when parsing such headers,
3956   recognize a single LF as a line terminator and ignore the leading CR.
3957</t>
3958<t>
3959   The character set of an entity-body &SHOULD; be labeled as the lowest
3960   common denominator of the character codes used within that body, with
3961   the exception that not labeling the entity is preferred over labeling
3962   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3963</t>
3964<t>
3965   Additional rules for requirements on parsing and encoding of dates
3966   and other potential problems with date encodings include:
3967</t>
3968<t>
3969  <list style="symbols">
3970     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3971        which appears to be more than 50 years in the future is in fact
3972        in the past (this helps solve the "year 2000" problem).</t>
3973
3974     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3975        Expires date as earlier than the proper value, but &MUST-NOT;
3976        internally represent a parsed Expires date as later than the
3977        proper value.</t>
3978
3979     <t>All expiration-related calculations &MUST; be done in GMT. The
3980        local time zone &MUST-NOT; influence the calculation or comparison
3981        of an age or expiration time.</t>
3982
3983     <t>If an HTTP header incorrectly carries a date value with a time
3984        zone other than GMT, it &MUST; be converted into GMT using the
3985        most conservative possible conversion.</t>
3986  </list>
3987</t>
3988</section>
3989
3990<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3991<t>
3992   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3993   simplify the process of date comparison. Proxies and gateways from
3994   other protocols &SHOULD; ensure that any Date header field present in a
3995   message conforms to one of the HTTP/1.1 formats and rewrite the date
3996   if necessary.
3997</t>
3998</section>
3999
4000<section title="Compatibility with Previous Versions" anchor="compatibility">
4001<t>
4002   It is beyond the scope of a protocol specification to mandate
4003   compliance with previous versions. HTTP/1.1 was deliberately
4004   designed, however, to make supporting previous versions easy. It is
4005   worth noting that, at the time of composing this specification
4006   (1996), we would expect commercial HTTP/1.1 servers to:
4007  <list style="symbols">
4008     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
4009        1.1 requests;</t>
4010
4011     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
4012        1.1;</t>
4013
4014     <t>respond appropriately with a message in the same major version
4015        used by the client.</t>
4016  </list>
4017</t>
4018<t>
4019   And we would expect HTTP/1.1 clients to:
4020  <list style="symbols">
4021     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
4022        responses;</t>
4023
4024     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
4025        1.1.</t>
4026  </list>
4027</t>
4028<t>
4029   For most implementations of HTTP/1.0, each connection is established
4030   by the client prior to the request and closed by the server after
4031   sending the response. Some implementations implement the Keep-Alive
4032   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="sec" target="RFC2068"/> of RFC
4033   2068 <xref target="RFC2068"/>.
4034</t>
4035
4036<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4037<t>
4038   This section summarizes major differences between versions HTTP/1.0
4039   and HTTP/1.1.
4040</t>
4041
4042<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">
4043<t>
4044   The requirements that clients and servers support the Host request-header,
4045   report an error if the Host request-header (<xref target="header.host"/>) is
4046   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
4047   are among the most important changes defined by this
4048   specification.
4049</t>
4050<t>
4051   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4052   addresses and servers; there was no other established mechanism for
4053   distinguishing the intended server of a request than the IP address
4054   to which that request was directed. The changes outlined above will
4055   allow the Internet, once older HTTP clients are no longer common, to
4056   support multiple Web sites from a single IP address, greatly
4057   simplifying large operational Web servers, where allocation of many
4058   IP addresses to a single host has created serious problems. The
4059   Internet will also be able to recover the IP addresses that have been
4060   allocated for the sole purpose of allowing special-purpose domain
4061   names to be used in root-level HTTP URLs. Given the rate of growth of
4062   the Web, and the number of servers already deployed, it is extremely
4063   important that all implementations of HTTP (including updates to
4064   existing HTTP/1.0 applications) correctly implement these
4065   requirements:
4066  <list style="symbols">
4067     <t>Both clients and servers &MUST; support the Host request-header.</t>
4068
4069     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4070
4071     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4072        request does not include a Host request-header.</t>
4073
4074     <t>Servers &MUST; accept absolute URIs.</t>
4075  </list>
4076</t>
4077</section>
4078</section>
4079
4080<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4081<t>
4082   Some clients and servers might wish to be compatible with some
4083   previous implementations of persistent connections in HTTP/1.0
4084   clients and servers. Persistent connections in HTTP/1.0 are
4085   explicitly negotiated as they are not the default behavior. HTTP/1.0
4086   experimental implementations of persistent connections are faulty,
4087   and the new facilities in HTTP/1.1 are designed to rectify these
4088   problems. The problem was that some existing 1.0 clients may be
4089   sending Keep-Alive to a proxy server that doesn't understand
4090   Connection, which would then erroneously forward it to the next
4091   inbound server, which would establish the Keep-Alive connection and
4092   result in a hung HTTP/1.0 proxy waiting for the close on the
4093   response. The result is that HTTP/1.0 clients must be prevented from
4094   using Keep-Alive when talking to proxies.
4095</t>
4096<t>
4097   However, talking to proxies is the most important use of persistent
4098   connections, so that prohibition is clearly unacceptable. Therefore,
4099   we need some other mechanism for indicating a persistent connection
4100   is desired, which is safe to use even when talking to an old proxy
4101   that ignores Connection. Persistent connections are the default for
4102   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4103   declaring non-persistence. See <xref target="header.connection"/>.
4104</t>
4105<t>
4106   The original HTTP/1.0 form of persistent connections (the Connection:
4107   Keep-Alive and Keep-Alive header) is documented in RFC 2068. <xref target="RFC2068"/>
4108</t>
4109</section>
4110
4111<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4112<t>
4113   This specification has been carefully audited to correct and
4114   disambiguate key word usage; RFC 2068 had many problems in respect to
4115   the conventions laid out in RFC 2119 <xref target="RFC2119"/>.
4116</t>
4117<t>
4118   Transfer-coding and message lengths all interact in ways that
4119   required fixing exactly when chunked encoding is used (to allow for
4120   transfer encoding that may not be self delimiting); it was important
4121   to straighten out exactly how message lengths are computed.
4122</t>
4123<t>
4124   The use and interpretation of HTTP version numbers has been clarified
4125   by RFC 2145. Require proxies to upgrade requests to highest protocol
4126   version they support to deal with problems discovered in HTTP/1.0
4127   implementations (<xref target="http.version"/>)
4128</t>
4129<t>
4130   Proxies should be able to add Content-Length when appropriate.
4131</t>
4132<t>
4133   Transfer-coding had significant problems, particularly with
4134   interactions with chunked encoding. The solution is that transfer-codings
4135   become as full fledged as content-codings. This involves
4136   adding an IANA registry for transfer-codings (separate from content
4137   codings), a new header field (TE) and enabling trailer headers in the
4138   future. Transfer encoding is a major performance benefit, so it was
4139   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4140   interoperability problem that could have occurred due to interactions
4141   between authentication trailers, chunked encoding and HTTP/1.0
4142   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4143   and <xref target="header.te" format="counter"/>)
4144</t>
4145</section>
4146</section>
4147</back>
4148</rfc>
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