source: draft-ietf-httpbis/latest/p1-messaging.xml @ 84

Last change on this file since 84 was 84, checked in by fielding@…, 12 years ago

Resolve #48: remove unnecessary reference to RFC1123 for consistency with draft-lafon

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File size: 174.2 KB
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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  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
160
161<abstract>
162<t>
163   The Hypertext Transfer Protocol (HTTP) is an application-level
164   protocol for distributed, collaborative, hypermedia information
165   systems. HTTP has been in use by the World Wide Web global information
166   initiative since 1990. This document is Part 1 of the seven-part specification
167   that defines the protocol referred to as "HTTP/1.1" and, taken together,
168   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
169   its associated terminology, defines the "http" and "https" Uniform
170   Resource Identifier (URI) schemes, defines the generic message syntax
171   and parsing requirements for HTTP message frames, and describes
172   general security concerns for implementations.
173</t>
174</abstract>
175
176<note title="Editorial Note (To be removed by RFC Editor)">
177  <t>
178    This version of the HTTP specification contains only minimal editorial
179    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
180    and authors' addresses).  All other changes are due to partitioning the
181    original into seven mostly independent parts.  The intent is for readers
182    of future drafts to able to use draft 00 as the basis for comparison
183    when the WG makes later changes to the specification text.  This draft
184    will shortly be followed by draft 01 (containing the first round of changes
185    that have already been agreed to on the mailing list). There is no point in
186    reviewing this draft other than to verify that the partitioning has been
187    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
188    will be the editors after draft 00 is submitted.
189  </t>
190  <t>
191    Discussion of this draft should take place on the HTTPBIS working group
192    mailing list (ietf-http-wg@w3.org). The current issues list is
193    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
194    and related documents (including fancy diffs) can be found at
195    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
196  </t>
197</note>
198</front>
199<middle>
200<section title="Introduction" anchor="introduction">
201<t>
202   This document will define aspects of HTTP related to overall network
203   operation, message framing, interaction with transport protocols, and
204   URI schemes. Right now it only includes the extracted relevant sections
205   of <xref target="RFC2616"/>.
206</t>
207<section title="Purpose" anchor="intro.purpose">
208<t>
209   The Hypertext Transfer Protocol (HTTP) is an application-level
210   protocol for distributed, collaborative, hypermedia information
211   systems. HTTP has been in use by the World-Wide Web global
212   information initiative since 1990. The first version of HTTP,
213   referred to as HTTP/0.9, was a simple protocol for raw data transfer
214   across the Internet. HTTP/1.0, as defined by RFC 1945 <xref target="RFC1945"/>, improved
215   the protocol by allowing messages to be in the format of MIME-like
216   messages, containing metainformation about the data transferred and
217   modifiers on the request/response semantics. However, HTTP/1.0 does
218   not sufficiently take into consideration the effects of hierarchical
219   proxies, caching, the need for persistent connections, or virtual
220   hosts. In addition, the proliferation of incompletely-implemented
221   applications calling themselves "HTTP/1.0" has necessitated a
222   protocol version change in order for two communicating applications
223   to determine each other's true capabilities.
224</t>
225<t>
226   This specification defines the protocol referred to as "HTTP/1.1".
227   This protocol includes more stringent requirements than HTTP/1.0 in
228   order to ensure reliable implementation of its features.
229</t>
230<t>
231   Practical information systems require more functionality than simple
232   retrieval, including search, front-end update, and annotation. HTTP
233   allows an open-ended set of methods and headers that indicate the
234   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
235   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
236   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
237   method is to be applied. Messages are passed in a format similar to
238   that used by Internet mail <xref target="RFC822"/> as defined by the Multipurpose
239   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
240</t>
241<t>
242   HTTP is also used as a generic protocol for communication between
243   user agents and proxies/gateways to other Internet systems, including
244   those supported by the SMTP <xref target="RFC821"/>, NNTP <xref target="RFC977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
245   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
246   access to resources available from diverse applications.
247</t>
248</section>
249
250<section title="Requirements" anchor="intro.requirements">
251<t>
252   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
253   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
254   document are to be interpreted as described in RFC 2119 <xref target="RFC2119"/>.
255</t>
256<t>
257   An implementation is not compliant if it fails to satisfy one or more
258   of the &MUST; or &REQUIRED; level requirements for the protocols it
259   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
260   level and all the &SHOULD; level requirements for its protocols is said
261   to be "unconditionally compliant"; one that satisfies all the &MUST;
262   level requirements but not all the &SHOULD; level requirements for its
263   protocols is said to be "conditionally compliant."
264</t>
265</section>
266
267<section title="Terminology" anchor="intro.terminology">
268<t>
269   This specification uses a number of terms to refer to the roles
270   played by participants in, and objects of, the HTTP communication.
271</t>
272<t>
273  <iref item="connection"/>
274  <x:dfn>connection</x:dfn>
275  <list>
276    <t>
277      A transport layer virtual circuit established between two programs
278      for the purpose of communication.
279    </t>
280  </list>
281</t>
282<t>
283  <iref item="message"/>
284  <x:dfn>message</x:dfn>
285  <list>
286    <t>
287      The basic unit of HTTP communication, consisting of a structured
288      sequence of octets matching the syntax defined in <xref target="http.message"/> and
289      transmitted via the connection.
290    </t>
291  </list>
292</t>
293<t>
294  <iref item="request"/>
295  <x:dfn>request</x:dfn>
296  <list>
297    <t>
298      An HTTP request message, as defined in <xref target="request"/>.
299    </t>
300  </list>
301</t>
302<t>
303  <iref item="response"/>
304  <x:dfn>response</x:dfn>
305  <list>
306    <t>
307      An HTTP response message, as defined in <xref target="response"/>.
308    </t>
309  </list>
310</t>
311<t>
312  <iref item="resource"/>
313  <x:dfn>resource</x:dfn>
314  <list>
315    <t>
316      A network data object or service that can be identified by a URI,
317      as defined in <xref target="uri"/>. Resources may be available in multiple
318      representations (e.g. multiple languages, data formats, size, and
319      resolutions) or vary in other ways.
320    </t>
321  </list>
322</t>
323<t>
324  <iref item="entity"/>
325  <x:dfn>entity</x:dfn>
326  <list>
327    <t>
328      The information transferred as the payload of a request or
329      response. An entity consists of metainformation in the form of
330      entity-header fields and content in the form of an entity-body, as
331      described in &entity;.
332    </t>
333  </list>
334</t>
335<t>
336  <iref item="representation"/>
337  <x:dfn>representation</x:dfn>
338  <list>
339    <t>
340      An entity included with a response that is subject to content
341      negotiation, as described in &content.negotiation;. There may exist multiple
342      representations associated with a particular response status.
343    </t>
344  </list>
345</t>
346<t>
347  <iref item="content negotiation"/>
348  <x:dfn>content negotiation</x:dfn>
349  <list>
350    <t>
351      The mechanism for selecting the appropriate representation when
352      servicing a request, as described in &content.negotiation;. The
353      representation of entities in any response can be negotiated
354      (including error responses).
355    </t>
356  </list>
357</t>
358<t>
359  <iref item="variant"/>
360  <x:dfn>variant</x:dfn>
361  <list>
362    <t>
363      A resource may have one, or more than one, representation(s)
364      associated with it at any given instant. Each of these
365      representations is termed a `varriant'.  Use of the term `variant'
366      does not necessarily imply that the resource is subject to content
367      negotiation.
368    </t>
369  </list>
370</t>
371<t>
372  <iref item="client"/>
373  <x:dfn>client</x:dfn>
374  <list>
375    <t>
376      A program that establishes connections for the purpose of sending
377      requests.
378    </t>
379  </list>
380</t>
381<t>
382  <iref item="user agent"/>
383  <x:dfn>user agent</x:dfn>
384  <list>
385    <t>
386      The client which initiates a request. These are often browsers,
387      editors, spiders (web-traversing robots), or other end user tools.
388    </t>
389  </list>
390</t>
391<t>
392  <iref item="server"/>
393  <x:dfn>server</x:dfn>
394  <list>
395    <t>
396      An application program that accepts connections in order to
397      service requests by sending back responses. Any given program may
398      be capable of being both a client and a server; our use of these
399      terms refers only to the role being performed by the program for a
400      particular connection, rather than to the program's capabilities
401      in general. Likewise, any server may act as an origin server,
402      proxy, gateway, or tunnel, switching behavior based on the nature
403      of each request.
404    </t>
405  </list>
406</t>
407<t>
408  <iref item="origin server"/>
409  <x:dfn>origin server</x:dfn>
410  <list>
411    <t>
412      The server on which a given resource resides or is to be created.
413    </t>
414  </list>
415</t>
416<t>
417  <iref item="proxy"/>
418  <x:dfn>proxy</x:dfn>
419  <list>
420    <t>
421      An intermediary program which acts as both a server and a client
422      for the purpose of making requests on behalf of other clients.
423      Requests are serviced internally or by passing them on, with
424      possible translation, to other servers. A proxy &MUST; implement
425      both the client and server requirements of this specification. A
426      "transparent proxy" is a proxy that does not modify the request or
427      response beyond what is required for proxy authentication and
428      identification. A "non-transparent proxy" is a proxy that modifies
429      the request or response in order to provide some added service to
430      the user agent, such as group annotation services, media type
431      transformation, protocol reduction, or anonymity filtering. Except
432      where either transparent or non-transparent behavior is explicitly
433      stated, the HTTP proxy requirements apply to both types of
434      proxies.
435    </t>
436  </list>
437</t>
438<t>
439  <iref item="gateway"/>
440  <x:dfn>gateway</x:dfn>
441  <list>
442    <t>
443      A server which acts as an intermediary for some other server.
444      Unlike a proxy, a gateway receives requests as if it were the
445      origin server for the requested resource; the requesting client
446      may not be aware that it is communicating with a gateway.
447    </t>
448  </list>
449</t>
450<t>
451  <iref item="tunnel"/>
452  <x:dfn>tunnel</x:dfn>
453  <list>
454    <t>
455      An intermediary program which is acting as a blind relay between
456      two connections. Once active, a tunnel is not considered a party
457      to the HTTP communication, though the tunnel may have been
458      initiated by an HTTP request. The tunnel ceases to exist when both
459      ends of the relayed connections are closed.
460    </t>
461  </list>
462</t>
463<t>
464  <iref item="cache"/>
465  <x:dfn>cache</x:dfn>
466  <list>
467    <t>
468      A program's local store of response messages and the subsystem
469      that controls its message storage, retrieval, and deletion. A
470      cache stores cacheable responses in order to reduce the response
471      time and network bandwidth consumption on future, equivalent
472      requests. Any client or server may include a cache, though a cache
473      cannot be used by a server that is acting as a tunnel.
474    </t>
475  </list>
476</t>
477<t>
478  <iref item="cacheable"/>
479  <x:dfn>cacheable</x:dfn>
480  <list>
481    <t>
482      A response is cacheable if a cache is allowed to store a copy of
483      the response message for use in answering subsequent requests. The
484      rules for determining the cacheability of HTTP responses are
485      defined in &caching;. Even if a resource is cacheable, there may
486      be additional constraints on whether a cache can use the cached
487      copy for a particular request.
488    </t>
489  </list>
490</t>
491<t>
492  <iref item="upstream"/>
493  <iref item="downstream"/>
494  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
495  <list>
496    <t>
497      Upstream and downstream describe the flow of a message: all
498      messages flow from upstream to downstream.
499    </t>
500  </list>
501</t>
502<t>
503  <iref item="inbound"/>
504  <iref item="outbound"/>
505  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
506  <list>
507    <t>
508      Inbound and outbound refer to the request and response paths for
509      messages: "inbound" means "traveling toward the origin server",
510      and "outbound" means "traveling toward the user agent"
511    </t>
512  </list>
513</t>
514</section>
515
516<section title="Overall Operation" anchor="intro.overall.operation">
517<t>
518   The HTTP protocol is a request/response protocol. A client sends a
519   request to the server in the form of a request method, URI, and
520   protocol version, followed by a MIME-like message containing request
521   modifiers, client information, and possible body content over a
522   connection with a server. The server responds with a status line,
523   including the message's protocol version and a success or error code,
524   followed by a MIME-like message containing server information, entity
525   metainformation, and possible entity-body content. The relationship
526   between HTTP and MIME is described in &diff2045entity;.
527</t>
528<t>
529   Most HTTP communication is initiated by a user agent and consists of
530   a request to be applied to a resource on some origin server. In the
531   simplest case, this may be accomplished via a single connection (v)
532   between the user agent (UA) and the origin server (O).
533</t>
534<figure><artwork type="drawing">
535       request chain ------------------------&gt;
536    UA -------------------v------------------- O
537       &lt;----------------------- response chain
538</artwork></figure>
539<t>
540   A more complicated situation occurs when one or more intermediaries
541   are present in the request/response chain. There are three common
542   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
543   forwarding agent, receiving requests for a URI in its absolute form,
544   rewriting all or part of the message, and forwarding the reformatted
545   request toward the server identified by the URI. A gateway is a
546   receiving agent, acting as a layer above some other server(s) and, if
547   necessary, translating the requests to the underlying server's
548   protocol. A tunnel acts as a relay point between two connections
549   without changing the messages; tunnels are used when the
550   communication needs to pass through an intermediary (such as a
551   firewall) even when the intermediary cannot understand the contents
552   of the messages.
553</t>
554<figure><artwork type="drawing">
555       request chain --------------------------------------&gt;
556    UA -----v----- A -----v----- B -----v----- C -----v----- O
557       &lt;------------------------------------- response chain
558</artwork></figure>
559<t>
560   The figure above shows three intermediaries (A, B, and C) between the
561   user agent and origin server. A request or response message that
562   travels the whole chain will pass through four separate connections.
563   This distinction is important because some HTTP communication options
564   may apply only to the connection with the nearest, non-tunnel
565   neighbor, only to the end-points of the chain, or to all connections
566   along the chain. Although the diagram is linear, each participant may
567   be engaged in multiple, simultaneous communications. For example, B
568   may be receiving requests from many clients other than A, and/or
569   forwarding requests to servers other than C, at the same time that it
570   is handling A's request.
571</t>
572<t>
573   Any party to the communication which is not acting as a tunnel may
574   employ an internal cache for handling requests. The effect of a cache
575   is that the request/response chain is shortened if one of the
576   participants along the chain has a cached response applicable to that
577   request. The following illustrates the resulting chain if B has a
578   cached copy of an earlier response from O (via C) for a request which
579   has not been cached by UA or A.
580</t>
581<figure><artwork type="drawing">
582          request chain ----------&gt;
583       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
584          &lt;--------- response chain
585</artwork></figure>
586<t>
587   Not all responses are usefully cacheable, and some requests may
588   contain modifiers which place special requirements on cache behavior.
589   HTTP requirements for cache behavior and cacheable responses are
590   defined in &caching;.
591</t>
592<t>
593   In fact, there are a wide variety of architectures and configurations
594   of caches and proxies currently being experimented with or deployed
595   across the World Wide Web. These systems include national hierarchies
596   of proxy caches to save transoceanic bandwidth, systems that
597   broadcast or multicast cache entries, organizations that distribute
598   subsets of cached data via CD-ROM, and so on. HTTP systems are used
599   in corporate intranets over high-bandwidth links, and for access via
600   PDAs with low-power radio links and intermittent connectivity. The
601   goal of HTTP/1.1 is to support the wide diversity of configurations
602   already deployed while introducing protocol constructs that meet the
603   needs of those who build web applications that require high
604   reliability and, failing that, at least reliable indications of
605   failure.
606</t>
607<t>
608   HTTP communication usually takes place over TCP/IP connections. The
609   default port is TCP 80 <xref target="RFC1700"/>, but other ports can be used. This does
610   not preclude HTTP from being implemented on top of any other protocol
611   on the Internet, or on other networks. HTTP only presumes a reliable
612   transport; any protocol that provides such guarantees can be used;
613   the mapping of the HTTP/1.1 request and response structures onto the
614   transport data units of the protocol in question is outside the scope
615   of this specification.
616</t>
617<t>
618   In HTTP/1.0, most implementations used a new connection for each
619   request/response exchange. In HTTP/1.1, a connection may be used for
620   one or more request/response exchanges, although connections may be
621   closed for a variety of reasons (see <xref target="persistent.connections"/>).
622</t>
623</section>
624</section>
625
626<section title="Notational Conventions and Generic Grammar" anchor="notation">
627
628<section title="Augmented BNF" anchor="notation.abnf">
629<t>
630   All of the mechanisms specified in this document are described in
631   both prose and an augmented Backus-Naur Form (BNF) similar to that
632   used by RFC 822 <xref target="RFC822"/>. Implementors will need to be familiar with the
633   notation in order to understand this specification. The augmented BNF
634   includes the following constructs:
635</t>
636<t>
637   name = definition
638  <list>
639    <t>
640      The name of a rule is simply the name itself (without any
641      enclosing "&lt;" and "&gt;") and is separated from its definition by the
642      equal "=" character. White space is only significant in that
643      indentation of continuation lines is used to indicate a rule
644      definition that spans more than one line. Certain basic rules are
645      in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
646      brackets are used within definitions whenever their presence will
647      facilitate discerning the use of rule names.
648    </t>
649  </list>
650</t>
651<t>
652   "literal"
653  <list>
654    <t>
655      Quotation marks surround literal text. Unless stated otherwise,
656      the text is case-insensitive.
657    </t>
658  </list>
659</t>
660<t>
661   rule1 | rule2
662  <list>
663    <t>
664      Elements separated by a bar ("|") are alternatives, e.g., "yes |
665      no" will accept yes or no.
666    </t>
667  </list>
668</t>
669<t>
670   (rule1 rule2)
671  <list>
672    <t>
673      Elements enclosed in parentheses are treated as a single element.
674      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
675      foo elem" and "elem bar elem".
676    </t>
677  </list>
678</t>
679<t>
680   *rule
681  <list>
682    <t>
683      The character "*" preceding an element indicates repetition. The
684      full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
685      &lt;m&gt; occurrences of element. Default values are 0 and infinity so
686      that "*(element)" allows any number, including zero; "1*element"
687      requires at least one; and "1*2element" allows one or two.
688    </t>
689  </list>
690</t>
691<t>
692   [rule]
693  <list>
694    <t>
695      Square brackets enclose optional elements; "[foo bar]" is
696      equivalent to "*1(foo bar)".
697    </t>
698  </list>
699</t>
700<t>
701   N rule
702  <list>
703    <t>
704      Specific repetition: "&lt;n&gt;(element)" is equivalent to
705      "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
706      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
707      alphabetic characters.
708    </t>
709  </list>
710</t>
711<t>
712   #rule
713  <list>
714    <t>
715      A construct "#" is defined, similar to "*", for defining lists of
716      elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
717      &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
718      (",") and &OPTIONAL; linear white space (LWS). This makes the usual
719      form of lists very easy; a rule such as
720    </t>
721    <t>
722         ( *LWS element *( *LWS "," *LWS element ))
723    </t>
724    <t>
725      can be shown as
726    </t>
727    <t>
728         1#element
729    </t>
730    <t>
731      Wherever this construct is used, null elements are allowed, but do
732      not contribute to the count of elements present. That is,
733      "(element), , (element) " is permitted, but counts as only two
734      elements. Therefore, where at least one element is required, at
735      least one non-null element &MUST; be present. Default values are 0
736      and infinity so that "#element" allows any number, including zero;
737      "1#element" requires at least one; and "1#2element" allows one or
738      two.
739    </t>
740  </list>
741</t>
742<t>
743   ; comment
744  <list>
745    <t>
746      A semi-colon, set off some distance to the right of rule text,
747      starts a comment that continues to the end of line. This is a
748      simple way of including useful notes in parallel with the
749      specifications.
750    </t>
751  </list>
752</t>
753<t>
754   implied *LWS
755  <list>
756    <t>
757      The grammar described by this specification is word-based. Except
758      where noted otherwise, linear white space (LWS) can be included
759      between any two adjacent words (token or quoted-string), and
760      between adjacent words and separators, without changing the
761      interpretation of a field. At least one delimiter (LWS and/or
762      separators) &MUST; exist between any two tokens (for the definition
763      of "token" below), since they would otherwise be interpreted as a
764      single token.
765    </t>
766  </list>
767</t>
768</section>
769
770<section title="Basic Rules" anchor="basic.rules">
771<x:anchor-alias value="OCTET"/>
772<x:anchor-alias value="CHAR"/>
773<x:anchor-alias value="UPALPHA"/>
774<x:anchor-alias value="LOALPHA"/>
775<x:anchor-alias value="ALPHA"/>
776<x:anchor-alias value="DIGIT"/>
777<x:anchor-alias value="CTL"/>
778<x:anchor-alias value="CR"/>
779<x:anchor-alias value="LF"/>
780<x:anchor-alias value="SP"/>
781<x:anchor-alias value="HT"/>
782<x:anchor-alias value="CRLF"/>
783<x:anchor-alias value="LWS"/>
784<x:anchor-alias value="TEXT"/>
785<x:anchor-alias value="HEX"/>
786<x:anchor-alias value="token"/>
787<x:anchor-alias value="separators"/>
788<x:anchor-alias value="comment"/>
789<x:anchor-alias value="ctext"/>
790<x:anchor-alias value="quoted-string"/>
791<x:anchor-alias value="qdtext"/>
792<x:anchor-alias value="quoted-pair"/>
793<t>
794   The following rules are used throughout this specification to
795   describe basic parsing constructs. The US-ASCII coded character set
796   is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
797</t>
798<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"/>
799    OCTET          = &lt;any 8-bit sequence of data&gt;
800    CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
801    UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
802    LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
803    ALPHA          = UPALPHA | LOALPHA
804    DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
805    CTL            = &lt;any US-ASCII control character
806                     (octets 0 - 31) and DEL (127)&gt;
807    CR             = &lt;US-ASCII CR, carriage return (13)&gt;
808    LF             = &lt;US-ASCII LF, linefeed (10)&gt;
809    SP             = &lt;US-ASCII SP, space (32)&gt;
810    HT             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
811    &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
812</artwork></figure>
813<t>
814   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
815   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
816   tolerant applications). The end-of-line marker within an entity-body
817   is defined by its associated media type, as described in &payload;.
818</t>
819<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
820    CRLF           = CR LF
821</artwork></figure>
822<t>
823   HTTP/1.1 header field values can be folded onto multiple lines if the
824   continuation line begins with a space or horizontal tab. All linear
825   white space, including folding, has the same semantics as SP. A
826   recipient &MAY; replace any linear white space with a single SP before
827   interpreting the field value or forwarding the message downstream.
828</t>
829<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
830    LWS            = [CRLF] 1*( SP | HT )
831</artwork></figure>
832<t>
833   The TEXT rule is only used for descriptive field contents and values
834   that are not intended to be interpreted by the message parser. Words
835   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
836   <xref target="ISO-8859"/> only when encoded according to the rules of RFC 2047
837   <xref target="RFC2047"/>.
838</t>
839<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
840    TEXT           = &lt;any OCTET except CTLs,
841                     but including LWS&gt;
842</artwork></figure>
843<t>
844   A CRLF is allowed in the definition of TEXT only as part of a header
845   field continuation. It is expected that the folding LWS will be
846   replaced with a single SP before interpretation of the TEXT value.
847</t>
848<t>
849   Hexadecimal numeric characters are used in several protocol elements.
850</t>
851<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
852    HEX            = "A" | "B" | "C" | "D" | "E" | "F"
853                   | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
854</artwork></figure>
855<t>
856   Many HTTP/1.1 header field values consist of words separated by LWS
857   or special characters. These special characters &MUST; be in a quoted
858   string to be used within a parameter value (as defined in
859   <xref target="transfer.codings"/>).
860</t>
861<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
862    token          = 1*&lt;any CHAR except CTLs or separators&gt;
863    separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
864                   | "," | ";" | ":" | "\" | &lt;"&gt;
865                   | "/" | "[" | "]" | "?" | "="
866                   | "{" | "}" | SP | HT
867</artwork></figure>
868<t>
869   Comments can be included in some HTTP header fields by surrounding
870   the comment text with parentheses. Comments are only allowed in
871   fields containing "comment" as part of their field value definition.
872   In all other fields, parentheses are considered part of the field
873   value.
874</t>
875<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
876    comment        = "(" *( ctext | quoted-pair | comment ) ")"
877    ctext          = &lt;any TEXT excluding "(" and ")"&gt;
878</artwork></figure>
879<t>
880   A string of text is parsed as a single word if it is quoted using
881   double-quote marks.
882</t>
883<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
884    quoted-string  = ( &lt;"&gt; *(qdtext | quoted-pair ) &lt;"&gt; )
885    qdtext         = &lt;any TEXT except &lt;"&gt;&gt;
886</artwork></figure>
887<t>
888   The backslash character ("\") &MAY; be used as a single-character
889   quoting mechanism only within quoted-string and comment constructs.
890</t>
891<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
892    quoted-pair    = "\" CHAR
893</artwork></figure>
894</section>
895</section>
896
897<section title="Protocol Parameters" anchor="protocol.parameters">
898
899<section title="HTTP Version" anchor="http.version">
900<t>
901   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
902   of the protocol. The protocol versioning policy is intended to allow
903   the sender to indicate the format of a message and its capacity for
904   understanding further HTTP communication, rather than the features
905   obtained via that communication. No change is made to the version
906   number for the addition of message components which do not affect
907   communication behavior or which only add to extensible field values.
908   The &lt;minor&gt; number is incremented when the changes made to the
909   protocol add features which do not change the general message parsing
910   algorithm, but which may add to the message semantics and imply
911   additional capabilities of the sender. The &lt;major&gt; number is
912   incremented when the format of a message within the protocol is
913   changed. See RFC 2145 <xref target="RFC2145"/> for a fuller explanation.
914</t>
915<t>
916   The version of an HTTP message is indicated by an HTTP-Version field
917   in the first line of the message. HTTP-Version is case-sensitive.
918</t>
919<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
920       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
921</artwork></figure>
922<t>
923   Note that the major and minor numbers &MUST; be treated as separate
924   integers and that each &MAY; be incremented higher than a single digit.
925   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
926   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
927   &MUST-NOT; be sent.
928</t>
929<t>
930   An application that sends a request or response message that includes
931   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
932   with this specification. Applications that are at least conditionally
933   compliant with this specification &SHOULD; use an HTTP-Version of
934   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
935   not compatible with HTTP/1.0. For more details on when to send
936   specific HTTP-Version values, see RFC 2145 <xref target="RFC2145"/>.
937</t>
938<t>
939   The HTTP version of an application is the highest HTTP version for
940   which the application is at least conditionally compliant.
941</t>
942<t>
943   Proxy and gateway applications need to be careful when forwarding
944   messages in protocol versions different from that of the application.
945   Since the protocol version indicates the protocol capability of the
946   sender, a proxy/gateway &MUST-NOT; send a message with a version
947   indicator which is greater than its actual version. If a higher
948   version request is received, the proxy/gateway &MUST; either downgrade
949   the request version, or respond with an error, or switch to tunnel
950   behavior.
951</t>
952<t>
953   Due to interoperability problems with HTTP/1.0 proxies discovered
954   since the publication of RFC 2068 <xref target="RFC2068"/>, caching proxies MUST, gateways
955   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
956   they support. The proxy/gateway's response to that request &MUST; be in
957   the same major version as the request.
958</t>
959<t>
960  <list>
961    <t>
962      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
963      of header fields required or forbidden by the versions involved.
964    </t>
965  </list>
966</t>
967</section>
968
969<section title="Uniform Resource Identifiers" anchor="uri">
970<t>
971   URIs have been known by many names: WWW addresses, Universal Document
972   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
973   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
974   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
975   simply formatted strings which identify--via name, location, or any
976   other characteristic--a resource.
977</t>
978
979<section title="General Syntax" anchor="general.syntax">
980<t>
981   URIs in HTTP can be represented in absolute form or relative to some
982   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
983   forms are differentiated by the fact that absolute URIs always begin
984   with a scheme name followed by a colon. For definitive information on
985   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
986   Generic Syntax and Semantics," RFC 2396 <xref target="RFC2396"/> (which replaces RFCs
987   1738 <xref target="RFC1738"/> and RFC 1808 <xref target="RFC1808"/>). This specification adopts the
988   definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
989   "host","abs_path", "rel_path", and "authority" from that
990   specification.
991</t>
992<t>
993   The HTTP protocol does not place any a priori limit on the length of
994   a URI. Servers &MUST; be able to handle the URI of any resource they
995   serve, and &SHOULD; be able to handle URIs of unbounded length if they
996   provide GET-based forms that could generate such URIs. A server
997   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
998   than the server can handle (see &status-414;).
999</t>
1000<t>
1001  <list>
1002    <t>
1003      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
1004      above 255 bytes, because some older client or proxy
1005      implementations might not properly support these lengths.
1006    </t>
1007  </list>
1008</t>
1009</section>
1010
1011<section title="http URL" anchor="http.url">
1012<t>
1013   The "http" scheme is used to locate network resources via the HTTP
1014   protocol. This section defines the scheme-specific syntax and
1015   semantics for http URLs.
1016</t>
1017<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http_URL"/>
1018http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
1019</artwork></figure>
1020<t>
1021   If the port is empty or not given, port 80 is assumed. The semantics
1022   are that the identified resource is located at the server listening
1023   for TCP connections on that port of that host, and the Request-URI
1024   for the resource is abs_path (<xref target="request-uri"/>). The use of IP addresses
1025   in URLs &SHOULD; be avoided whenever possible (see RFC 1900 <xref target="RFC1900"/>). If
1026   the abs_path is not present in the URL, it &MUST; be given as "/" when
1027   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
1028
1029   receives a host name which is not a fully qualified domain name, it
1030   &MAY; add its domain to the host name it received. If a proxy receives
1031   a fully qualified domain name, the proxy &MUST-NOT; change the host
1032   name.
1033</t>
1034</section>
1035
1036<section title="URI Comparison" anchor="uri.comparison">
1037<t>
1038   When comparing two URIs to decide if they match or not, a client
1039   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
1040   URIs, with these exceptions:
1041  <list style="symbols">
1042    <t>A port that is empty or not given is equivalent to the default
1043        port for that URI-reference;</t>
1044    <t>Comparisons of host names &MUST; be case-insensitive;</t>
1045    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
1046    <t>An empty abs_path is equivalent to an abs_path of "/".</t>
1047  </list>
1048</t>
1049<t>
1050   Characters other than those in the "reserved" set (see
1051   RFC 2396 <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
1052</t>
1053<t>
1054   For example, the following three URIs are equivalent:
1055</t>
1056<figure><artwork type="example">
1057   http://abc.com:80/~smith/home.html
1058   http://ABC.com/%7Esmith/home.html
1059   http://ABC.com:/%7esmith/home.html
1060</artwork></figure>
1061</section>
1062</section>
1063
1064<section title="Date/Time Formats" anchor="date.time.formats">
1065<section title="Full Date" anchor="full.date">
1066<t>
1067   HTTP applications have historically allowed three different formats
1068   for the representation of date/time stamps:
1069</t>
1070<figure><artwork type="example">
1071   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1072   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1073   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1074</artwork></figure>
1075<t>
1076   The first format is preferred as an Internet standard and represents
1077   a fixed-length subset of that defined by RFC 1123 <xref target="RFC1123"/> (an update to
1078   RFC 822 <xref target="RFC822"/>). The other formats are described here only for
1079   compatibility with obsolete implementations.
1080   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1081   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1082   only generate the RFC 1123 format for representing HTTP-date values
1083   in header fields. See <xref target="tolerant.applications"/> for further information.
1084</t>
1085<t><list><t>
1086      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1087      accepting date values that may have been sent by non-HTTP
1088      applications, as is sometimes the case when retrieving or posting
1089      messages via proxies/gateways to SMTP or NNTP.
1090</t></list></t>
1091<t>
1092   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1093   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1094   equal to UTC (Coordinated Universal Time). This is indicated in the
1095   first two formats by the inclusion of "GMT" as the three-letter
1096   abbreviation for time zone, and &MUST; be assumed when reading the
1097   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1098   additional LWS beyond that specifically included as SP in the
1099   grammar.
1100</t>
1101<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"/>
1102    HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1103    rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1104    rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1105    asctime-date = wkday SP date3 SP time SP 4DIGIT
1106    date1        = 2DIGIT SP month SP 4DIGIT
1107                   ; day month year (e.g., 02 Jun 1982)
1108    date2        = 2DIGIT "-" month "-" 2DIGIT
1109                   ; day-month-year (e.g., 02-Jun-82)
1110    date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1111                   ; month day (e.g., Jun  2)
1112    time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1113                   ; 00:00:00 - 23:59:59
1114    wkday        = "Mon" | "Tue" | "Wed"
1115                 | "Thu" | "Fri" | "Sat" | "Sun"
1116    weekday      = "Monday" | "Tuesday" | "Wednesday"
1117                 | "Thursday" | "Friday" | "Saturday" | "Sunday"
1118    month        = "Jan" | "Feb" | "Mar" | "Apr"
1119                 | "May" | "Jun" | "Jul" | "Aug"
1120                 | "Sep" | "Oct" | "Nov" | "Dec"
1121</artwork></figure>
1122<t>
1123      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1124      to their usage within the protocol stream. Clients and servers are
1125      not required to use these formats for user presentation, request
1126      logging, etc.
1127</t>
1128</section>
1129</section>
1130
1131<section title="Transfer Codings" anchor="transfer.codings">
1132<t>
1133   Transfer-coding values are used to indicate an encoding
1134   transformation that has been, can be, or may need to be applied to an
1135   entity-body in order to ensure "safe transport" through the network.
1136   This differs from a content coding in that the transfer-coding is a
1137   property of the message, not of the original entity.
1138</t>
1139<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1140    transfer-coding         = "chunked" | transfer-extension
1141    transfer-extension      = token *( ";" parameter )
1142</artwork></figure>
1143<t>
1144   Parameters are in  the form of attribute/value pairs.
1145</t>
1146<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"/>
1147    parameter               = attribute "=" value
1148    attribute               = token
1149    value                   = token | quoted-string
1150</artwork></figure>
1151<t>
1152   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1153   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1154   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1155</t>
1156<t>
1157   Whenever a transfer-coding is applied to a message-body, the set of
1158   transfer-codings &MUST; include "chunked", unless the message is
1159   terminated by closing the connection. When the "chunked" transfer-coding
1160   is used, it &MUST; be the last transfer-coding applied to the
1161   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1162   than once to a message-body. These rules allow the recipient to
1163   determine the transfer-length of the message (<xref target="message.length"/>).
1164</t>
1165<t>
1166   Transfer-codings are analogous to the Content-Transfer-Encoding
1167   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1168   binary data over a 7-bit transport service. However, safe transport
1169   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1170   the only unsafe characteristic of message-bodies is the difficulty in
1171   determining the exact body length (<xref target="message.length"/>), or the desire to
1172   encrypt data over a shared transport.
1173</t>
1174<t>
1175   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1176   transfer-coding value tokens. Initially, the registry contains the
1177   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>), "identity" (section
1178   3.6.2), "gzip" (&payload;), "compress" (&payload;), and "deflate"
1179   (&payload;).
1180</t>
1181<t>
1182   New transfer-coding value tokens &SHOULD; be registered in the same way
1183   as new content-coding value tokens (&payload;).
1184</t>
1185<t>
1186   A server which receives an entity-body with a transfer-coding it does
1187   not understand &SHOULD; return 501 (Unimplemented), and close the
1188   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1189   client.
1190</t>
1191
1192<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1193<t>
1194   The chunked encoding modifies the body of a message in order to
1195   transfer it as a series of chunks, each with its own size indicator,
1196   followed by an &OPTIONAL; trailer containing entity-header fields. This
1197   allows dynamically produced content to be transferred along with the
1198   information necessary for the recipient to verify that it has
1199   received the full message.
1200</t>
1201<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"/>
1202    Chunked-Body   = *chunk
1203                     last-chunk
1204                     trailer
1205                     CRLF
1206
1207    chunk          = chunk-size [ chunk-extension ] CRLF
1208                     chunk-data CRLF
1209    chunk-size     = 1*HEX
1210    last-chunk     = 1*("0") [ chunk-extension ] CRLF
1211
1212    chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1213    chunk-ext-name = token
1214    chunk-ext-val  = token | quoted-string
1215    chunk-data     = chunk-size(OCTET)
1216    trailer        = *(entity-header CRLF)
1217</artwork></figure>
1218<t>
1219   The chunk-size field is a string of hex digits indicating the size of
1220   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1221   zero, followed by the trailer, which is terminated by an empty line.
1222</t>
1223<t>
1224   The trailer allows the sender to include additional HTTP header
1225   fields at the end of the message. The Trailer header field can be
1226   used to indicate which header fields are included in a trailer (see
1227   <xref target="header.trailer"/>).
1228</t>
1229<t>
1230   A server using chunked transfer-coding in a response &MUST-NOT; use the
1231   trailer for any header fields unless at least one of the following is
1232   true:
1233  <list style="numbers">
1234    <t>the request included a TE header field that indicates "trailers" is
1235     acceptable in the transfer-coding of the  response, as described in
1236     <xref target="header.te"/>; or,</t>
1237
1238    <t>the server is the origin server for the response, the trailer
1239     fields consist entirely of optional metadata, and the recipient
1240     could use the message (in a manner acceptable to the origin server)
1241     without receiving this metadata.  In other words, the origin server
1242     is willing to accept the possibility that the trailer fields might
1243     be silently discarded along the path to the client.</t>
1244  </list>
1245</t>
1246<t>
1247   This requirement prevents an interoperability failure when the
1248   message is being received by an HTTP/1.1 (or later) proxy and
1249   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1250   compliance with the protocol would have necessitated a possibly
1251   infinite buffer on the proxy.
1252</t>
1253<t>
1254   A process for decoding the "chunked" transfer-coding
1255   can be represented in pseudo-code as:
1256</t>
1257<figure><artwork type="code">
1258    length := 0
1259    read chunk-size, chunk-extension (if any) and CRLF
1260    while (chunk-size &gt; 0) {
1261       read chunk-data and CRLF
1262       append chunk-data to entity-body
1263       length := length + chunk-size
1264       read chunk-size and CRLF
1265    }
1266    read entity-header
1267    while (entity-header not empty) {
1268       append entity-header to existing header fields
1269       read entity-header
1270    }
1271    Content-Length := length
1272    Remove "chunked" from Transfer-Encoding
1273</artwork></figure>
1274<t>
1275   All HTTP/1.1 applications &MUST; be able to receive and decode the
1276   "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
1277   they do not understand.
1278</t>
1279</section>
1280</section>
1281
1282</section>
1283
1284<section title="HTTP Message" anchor="http.message">
1285
1286<section title="Message Types" anchor="message.types">
1287<t>
1288   HTTP messages consist of requests from client to server and responses
1289   from server to client.
1290</t>
1291<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1292    HTTP-message   = Request | Response     ; HTTP/1.1 messages
1293</artwork></figure>
1294<t>
1295   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1296   message format of RFC 822 <xref target="RFC822"/> for transferring entities (the payload
1297   of the message). Both types of message consist of a start-line, zero
1298   or more header fields (also known as "headers"), an empty line (i.e.,
1299   a line with nothing preceding the CRLF) indicating the end of the
1300   header fields, and possibly a message-body.
1301</t>
1302<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1303     generic-message = start-line
1304                       *(message-header CRLF)
1305                       CRLF
1306                       [ message-body ]
1307     start-line      = Request-Line | Status-Line
1308</artwork></figure>
1309<t>
1310   In the interest of robustness, servers &SHOULD; ignore any empty
1311   line(s) received where a Request-Line is expected. In other words, if
1312   the server is reading the protocol stream at the beginning of a
1313   message and receives a CRLF first, it should ignore the CRLF.
1314</t>
1315<t>
1316   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1317   after a POST request. To restate what is explicitly forbidden by the
1318   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1319   extra CRLF.
1320</t>
1321</section>
1322
1323<section title="Message Headers" anchor="message.headers">
1324<t>
1325   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1326   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1327   entity-header (&entity-header-fields;) fields, follow the same generic format as
1328   that given in <xref target="RFC822" x:fmt="sec" x:sec="3.1"/> of RFC 822 <xref target="RFC822"/>. Each header field consists
1329   of a name followed by a colon (":") and the field value. Field names
1330   are case-insensitive. The field value &MAY; be preceded by any amount
1331   of LWS, though a single SP is preferred. Header fields can be
1332   extended over multiple lines by preceding each extra line with at
1333   least one SP or HT. Applications ought to follow "common form", where
1334   one is known or indicated, when generating HTTP constructs, since
1335   there might exist some implementations that fail to accept anything
1336   beyond the common forms.
1337</t>
1338<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"/>
1339    message-header = field-name ":" [ field-value ]
1340    field-name     = token
1341    field-value    = *( field-content | LWS )
1342    field-content  = &lt;the OCTETs making up the field-value
1343                     and consisting of either *TEXT or combinations
1344                     of token, separators, and quoted-string&gt;
1345</artwork></figure>
1346<t>
1347   The field-content does not include any leading or trailing LWS:
1348   linear white space occurring before the first non-whitespace
1349   character of the field-value or after the last non-whitespace
1350   character of the field-value. Such leading or trailing LWS &MAY; be
1351   removed without changing the semantics of the field value. Any LWS
1352   that occurs between field-content &MAY; be replaced with a single SP
1353   before interpreting the field value or forwarding the message
1354   downstream.
1355</t>
1356<t>
1357   The order in which header fields with differing field names are
1358   received is not significant. However, it is "good practice" to send
1359   general-header fields first, followed by request-header or response-header
1360   fields, and ending with the entity-header fields.
1361</t>
1362<t>
1363   Multiple message-header fields with the same field-name &MAY; be
1364   present in a message if and only if the entire field-value for that
1365   header field is defined as a comma-separated list [i.e., #(values)].
1366   It &MUST; be possible to combine the multiple header fields into one
1367   "field-name: field-value" pair, without changing the semantics of the
1368   message, by appending each subsequent field-value to the first, each
1369   separated by a comma. The order in which header fields with the same
1370   field-name are received is therefore significant to the
1371   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1372   change the order of these field values when a message is forwarded.
1373</t>
1374</section>
1375
1376<section title="Message Body" anchor="message.body">
1377<t>
1378   The message-body (if any) of an HTTP message is used to carry the
1379   entity-body associated with the request or response. The message-body
1380   differs from the entity-body only when a transfer-coding has been
1381   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1382</t>
1383<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1384    message-body = entity-body
1385                 | &lt;entity-body encoded as per Transfer-Encoding&gt;
1386</artwork></figure>
1387<t>
1388   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1389   applied by an application to ensure safe and proper transfer of the
1390   message. Transfer-Encoding is a property of the message, not of the
1391   entity, and thus &MAY; be added or removed by any application along the
1392   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1393   when certain transfer-codings may be used.)
1394</t>
1395<t>
1396   The rules for when a message-body is allowed in a message differ for
1397   requests and responses.
1398</t>
1399<t>
1400   The presence of a message-body in a request is signaled by the
1401   inclusion of a Content-Length or Transfer-Encoding header field in
1402   the request's message-headers. A message-body &MUST-NOT; be included in
1403   a request if the specification of the request method (&method;)
1404   does not allow sending an entity-body in requests. A server &SHOULD;
1405   read and forward a message-body on any request; if the request method
1406   does not include defined semantics for an entity-body, then the
1407   message-body &SHOULD; be ignored when handling the request.
1408</t>
1409<t>
1410   For response messages, whether or not a message-body is included with
1411   a message is dependent on both the request method and the response
1412   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1413   &MUST-NOT; include a message-body, even though the presence of entity-header
1414   fields might lead one to believe they do. All 1xx
1415   (informational), 204 (no content), and 304 (not modified) responses
1416   &MUST-NOT; include a message-body. All other responses do include a
1417   message-body, although it &MAY; be of zero length.
1418</t>
1419</section>
1420
1421<section title="Message Length" anchor="message.length">
1422<t>
1423   The transfer-length of a message is the length of the message-body as
1424   it appears in the message; that is, after any transfer-codings have
1425   been applied. When a message-body is included with a message, the
1426   transfer-length of that body is determined by one of the following
1427   (in order of precedence):
1428</t>
1429<t>
1430  <list style="numbers">
1431    <x:lt><t>
1432     Any response message which "&MUST-NOT;" include a message-body (such
1433     as the 1xx, 204, and 304 responses and any response to a HEAD
1434     request) is always terminated by the first empty line after the
1435     header fields, regardless of the entity-header fields present in
1436     the message.
1437    </t></x:lt>
1438    <x:lt><t>
1439     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>) is present and
1440     has any value other than "identity", then the transfer-length is
1441     defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
1442     unless the message is terminated by closing the connection.
1443    </t></x:lt>
1444    <x:lt><t>
1445     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1446     decimal value in OCTETs represents both the entity-length and the
1447     transfer-length. The Content-Length header field &MUST-NOT; be sent
1448     if these two lengths are different (i.e., if a Transfer-Encoding
1449     header field is present). If a message is received with both a
1450     Transfer-Encoding header field and a Content-Length header field,
1451     the latter &MUST; be ignored.
1452    </t></x:lt>
1453    <x:lt><t>
1454     If the message uses the media type "multipart/byteranges", and the
1455     transfer-length is not otherwise specified, then this self-delimiting
1456     media type defines the transfer-length. This media type
1457     &MUST-NOT; be used unless the sender knows that the recipient can parse
1458     it; the presence in a request of a Range header with multiple byte-range
1459     specifiers from a 1.1 client implies that the client can parse
1460     multipart/byteranges responses.
1461    <list style="empty"><t>
1462       A range header might be forwarded by a 1.0 proxy that does not
1463       understand multipart/byteranges; in this case the server &MUST;
1464       delimit the message using methods defined in items 1, 3 or 5 of
1465       this section.
1466    </t></list>
1467    </t></x:lt>
1468    <x:lt><t>
1469     By the server closing the connection. (Closing the connection
1470     cannot be used to indicate the end of a request body, since that
1471     would leave no possibility for the server to send back a response.)
1472    </t></x:lt>
1473  </list>
1474</t>
1475<t>
1476   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1477   containing a message-body &MUST; include a valid Content-Length header
1478   field unless the server is known to be HTTP/1.1 compliant. If a
1479   request contains a message-body and a Content-Length is not given,
1480   the server &SHOULD; respond with 400 (bad request) if it cannot
1481   determine the length of the message, or with 411 (length required) if
1482   it wishes to insist on receiving a valid Content-Length.
1483</t>
1484<t>
1485   All HTTP/1.1 applications that receive entities &MUST; accept the
1486   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1487   to be used for messages when the message length cannot be determined
1488   in advance.
1489</t>
1490<t>
1491   Messages &MUST-NOT; include both a Content-Length header field and a
1492   non-identity transfer-coding. If the message does include a non-identity
1493   transfer-coding, the Content-Length &MUST; be ignored.
1494</t>
1495<t>
1496   When a Content-Length is given in a message where a message-body is
1497   allowed, its field value &MUST; exactly match the number of OCTETs in
1498   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1499   invalid length is received and detected.
1500</t>
1501</section>
1502
1503<section title="General Header Fields" anchor="general.header.fields">
1504<t>
1505   There are a few header fields which have general applicability for
1506   both request and response messages, but which do not apply to the
1507   entity being transferred. These header fields apply only to the
1508   message being transmitted.
1509</t>
1510<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1511    general-header = Cache-Control            ; &header-cache-control;
1512                   | Connection               ; <xref target="header.connection"/>
1513                   | Date                     ; <xref target="header.date"/>
1514                   | Pragma                   ; &header-pragma;
1515                   | Trailer                  ; <xref target="header.trailer"/>
1516                   | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
1517                   | Upgrade                  ; <xref target="header.upgrade"/>
1518                   | Via                      ; <xref target="header.via"/>
1519                   | Warning                  ; &header-warning;
1520</artwork></figure>
1521<t>
1522   General-header field names can be extended reliably only in
1523   combination with a change in the protocol version. However, new or
1524   experimental header fields may be given the semantics of general
1525   header fields if all parties in the communication recognize them to
1526   be general-header fields. Unrecognized header fields are treated as
1527   entity-header fields.
1528</t>
1529</section>
1530</section>
1531
1532<section title="Request" anchor="request">
1533<t>
1534   A request message from a client to a server includes, within the
1535   first line of that message, the method to be applied to the resource,
1536   the identifier of the resource, and the protocol version in use.
1537</t>
1538<!--                 Host                      ; should be moved here eventually -->
1539<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1540     Request       = Request-Line              ; <xref target="request-line"/>
1541                     *(( general-header        ; <xref target="general.header.fields"/>
1542                      | request-header         ; &request-header-fields;
1543                      | entity-header ) CRLF)  ; &entity-header-fields;
1544                     CRLF
1545                     [ message-body ]          ; <xref target="message.body"/>
1546</artwork></figure>
1547
1548<section title="Request-Line" anchor="request-line">
1549<t>
1550   The Request-Line begins with a method token, followed by the
1551   Request-URI and the protocol version, and ending with CRLF. The
1552   elements are separated by SP characters. No CR or LF is allowed
1553   except in the final CRLF sequence.
1554</t>
1555<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1556     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1557</artwork></figure>
1558
1559<section title="Method" anchor="method">
1560<t>
1561   The Method  token indicates the method to be performed on the
1562   resource identified by the Request-URI. The method is case-sensitive.
1563</t>
1564<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1565    Method         = token
1566</artwork></figure>
1567</section>
1568
1569<section title="Request-URI" anchor="request-uri">
1570<t>
1571   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1572   identifies the resource upon which to apply the request.
1573</t>
1574<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1575    Request-URI    = "*"
1576                   | absoluteURI
1577                   | ( abs_path [ "?" query ] )
1578                   | authority
1579</artwork></figure>
1580<t>
1581   The four options for Request-URI are dependent on the nature of the
1582   request. The asterisk "*" means that the request does not apply to a
1583   particular resource, but to the server itself, and is only allowed
1584   when the method used does not necessarily apply to a resource. One
1585   example would be
1586</t>
1587<figure><artwork type="example">
1588    OPTIONS * HTTP/1.1
1589</artwork></figure>
1590<t>
1591   The absoluteURI form is &REQUIRED; when the request is being made to a
1592   proxy. The proxy is requested to forward the request or service it
1593   from a valid cache, and return the response. Note that the proxy &MAY;
1594   forward the request on to another proxy or directly to the server
1595   specified by the absoluteURI. In order to avoid request loops, a
1596   proxy &MUST; be able to recognize all of its server names, including
1597   any aliases, local variations, and the numeric IP address. An example
1598   Request-Line would be:
1599</t>
1600<figure><artwork type="example">
1601    GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
1602</artwork></figure>
1603<t>
1604   To allow for transition to absoluteURIs in all requests in future
1605   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1606   form in requests, even though HTTP/1.1 clients will only generate
1607   them in requests to proxies.
1608</t>
1609<t>
1610   The authority form is only used by the CONNECT method (&CONNECT;).
1611</t>
1612<t>
1613   The most common form of Request-URI is that used to identify a
1614   resource on an origin server or gateway. In this case the absolute
1615   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, abs_path) as
1616   the Request-URI, and the network location of the URI (authority) &MUST;
1617   be transmitted in a Host header field. For example, a client wishing
1618   to retrieve the resource above directly from the origin server would
1619   create a TCP connection to port 80 of the host "www.w3.org" and send
1620   the lines:
1621</t>
1622<figure><artwork type="example">
1623    GET /pub/WWW/TheProject.html HTTP/1.1
1624    Host: www.w3.org
1625</artwork></figure>
1626<t>
1627   followed by the remainder of the Request. Note that the absolute path
1628   cannot be empty; if none is present in the original URI, it &MUST; be
1629   given as "/" (the server root).
1630</t>
1631<t>
1632   The Request-URI is transmitted in the format specified in
1633   <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
1634   <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
1635   properly interpret the request. Servers &SHOULD; respond to invalid
1636   Request-URIs with an appropriate status code.
1637</t>
1638<t>
1639   A transparent proxy &MUST-NOT; rewrite the "abs_path" part of the
1640   received Request-URI when forwarding it to the next inbound server,
1641   except as noted above to replace a null abs_path with "/".
1642</t>
1643<t>
1644  <list><t>
1645      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1646      meaning of the request when the origin server is improperly using
1647      a non-reserved URI character for a reserved purpose.  Implementors
1648      should be aware that some pre-HTTP/1.1 proxies have been known to
1649      rewrite the Request-URI.
1650  </t></list>
1651</t>
1652</section>
1653</section>
1654
1655<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1656<t>
1657   The exact resource identified by an Internet request is determined by
1658   examining both the Request-URI and the Host header field.
1659</t>
1660<t>
1661   An origin server that does not allow resources to differ by the
1662   requested host &MAY; ignore the Host header field value when
1663   determining the resource identified by an HTTP/1.1 request. (But see
1664   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1665   for other requirements on Host support in HTTP/1.1.)
1666</t>
1667<t>
1668   An origin server that does differentiate resources based on the host
1669   requested (sometimes referred to as virtual hosts or vanity host
1670   names) &MUST; use the following rules for determining the requested
1671   resource on an HTTP/1.1 request:
1672  <list style="numbers">
1673    <t>If Request-URI is an absoluteURI, the host is part of the
1674     Request-URI. Any Host header field value in the request &MUST; be
1675     ignored.</t>
1676    <t>If the Request-URI is not an absoluteURI, and the request includes
1677     a Host header field, the host is determined by the Host header
1678     field value.</t>
1679    <t>If the host as determined by rule 1 or 2 is not a valid host on
1680     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1681  </list>
1682</t>
1683<t>
1684   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1685   attempt to use heuristics (e.g., examination of the URI path for
1686   something unique to a particular host) in order to determine what
1687   exact resource is being requested.
1688</t>
1689</section>
1690
1691</section>
1692
1693
1694<section title="Response" anchor="response">
1695<t>
1696   After receiving and interpreting a request message, a server responds
1697   with an HTTP response message.
1698</t>
1699<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1700    Response      = Status-Line               ; <xref target="status-line"/>
1701                    *(( general-header        ; <xref target="general.header.fields"/>
1702                     | response-header        ; &response-header-fields;
1703                     | entity-header ) CRLF)  ; &entity-header-fields;
1704                    CRLF
1705                    [ message-body ]          ; <xref target="message.body"/>
1706</artwork></figure>
1707
1708<section title="Status-Line" anchor="status-line">
1709<t>
1710   The first line of a Response message is the Status-Line, consisting
1711   of the protocol version followed by a numeric status code and its
1712   associated textual phrase, with each element separated by SP
1713   characters. No CR or LF is allowed except in the final CRLF sequence.
1714</t>
1715<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1716    Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1717</artwork></figure>
1718
1719<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1720<t>
1721   The Status-Code element is a 3-digit integer result code of the
1722   attempt to understand and satisfy the request. These codes are fully
1723   defined in &status-codes;. The Reason-Phrase is intended to give a short
1724   textual description of the Status-Code. The Status-Code is intended
1725   for use by automata and the Reason-Phrase is intended for the human
1726   user. The client is not required to examine or display the Reason-Phrase.
1727</t>
1728<t>
1729   The first digit of the Status-Code defines the class of response. The
1730   last two digits do not have any categorization role. There are 5
1731   values for the first digit:
1732  <list style="symbols">
1733    <t>
1734      1xx: Informational - Request received, continuing process
1735    </t>
1736    <t>
1737      2xx: Success - The action was successfully received,
1738        understood, and accepted
1739    </t>
1740    <t>
1741      3xx: Redirection - Further action must be taken in order to
1742        complete the request
1743    </t>
1744    <t>
1745      4xx: Client Error - The request contains bad syntax or cannot
1746        be fulfilled
1747    </t>
1748    <t>
1749      5xx: Server Error - The server failed to fulfill an apparently
1750        valid request
1751    </t>
1752  </list>
1753</t>
1754<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"/>
1755   Status-Code    = 3DIGIT
1756   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1757</artwork></figure>
1758</section>
1759</section>
1760
1761</section>
1762
1763
1764<section title="Connections" anchor="connections">
1765
1766<section title="Persistent Connections" anchor="persistent.connections">
1767
1768<section title="Purpose" anchor="persistent.purpose">
1769<t>
1770   Prior to persistent connections, a separate TCP connection was
1771   established to fetch each URL, increasing the load on HTTP servers
1772   and causing congestion on the Internet. The use of inline images and
1773   other associated data often require a client to make multiple
1774   requests of the same server in a short amount of time. Analysis of
1775   these performance problems and results from a prototype
1776   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1777   measurements of actual HTTP/1.1 (RFC 2068) implementations show good
1778   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1779   T/TCP <xref target="Tou1998"/>.
1780</t>
1781<t>
1782   Persistent HTTP connections have a number of advantages:
1783  <list style="symbols">
1784      <t>
1785        By opening and closing fewer TCP connections, CPU time is saved
1786        in routers and hosts (clients, servers, proxies, gateways,
1787        tunnels, or caches), and memory used for TCP protocol control
1788        blocks can be saved in hosts.
1789      </t>
1790      <t>
1791        HTTP requests and responses can be pipelined on a connection.
1792        Pipelining allows a client to make multiple requests without
1793        waiting for each response, allowing a single TCP connection to
1794        be used much more efficiently, with much lower elapsed time.
1795      </t>
1796      <t>
1797        Network congestion is reduced by reducing the number of packets
1798        caused by TCP opens, and by allowing TCP sufficient time to
1799        determine the congestion state of the network.
1800      </t>
1801      <t>
1802        Latency on subsequent requests is reduced since there is no time
1803        spent in TCP's connection opening handshake.
1804      </t>
1805      <t>
1806        HTTP can evolve more gracefully, since errors can be reported
1807        without the penalty of closing the TCP connection. Clients using
1808        future versions of HTTP might optimistically try a new feature,
1809        but if communicating with an older server, retry with old
1810        semantics after an error is reported.
1811      </t>
1812    </list>
1813</t>
1814<t>
1815   HTTP implementations &SHOULD; implement persistent connections.
1816</t>
1817</section>
1818
1819<section title="Overall Operation" anchor="persistent.overall">
1820<t>
1821   A significant difference between HTTP/1.1 and earlier versions of
1822   HTTP is that persistent connections are the default behavior of any
1823   HTTP connection. That is, unless otherwise indicated, the client
1824   &SHOULD; assume that the server will maintain a persistent connection,
1825   even after error responses from the server.
1826</t>
1827<t>
1828   Persistent connections provide a mechanism by which a client and a
1829   server can signal the close of a TCP connection. This signaling takes
1830   place using the Connection header field (<xref target="header.connection"/>). Once a close
1831   has been signaled, the client &MUST-NOT; send any more requests on that
1832   connection.
1833</t>
1834
1835<section title="Negotiation" anchor="persistent.negotiation">
1836<t>
1837   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1838   maintain a persistent connection unless a Connection header including
1839   the connection-token "close" was sent in the request. If the server
1840   chooses to close the connection immediately after sending the
1841   response, it &SHOULD; send a Connection header including the
1842   connection-token close.
1843</t>
1844<t>
1845   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1846   decide to keep it open based on whether the response from a server
1847   contains a Connection header with the connection-token close. In case
1848   the client does not want to maintain a connection for more than that
1849   request, it &SHOULD; send a Connection header including the
1850   connection-token close.
1851</t>
1852<t>
1853   If either the client or the server sends the close token in the
1854   Connection header, that request becomes the last one for the
1855   connection.
1856</t>
1857<t>
1858   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1859   maintained for HTTP versions less than 1.1 unless it is explicitly
1860   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1861   compatibility with HTTP/1.0 clients.
1862</t>
1863<t>
1864   In order to remain persistent, all messages on the connection &MUST;
1865   have a self-defined message length (i.e., one not defined by closure
1866   of the connection), as described in <xref target="message.length"/>.
1867</t>
1868</section>
1869
1870<section title="Pipelining" anchor="pipelining">
1871<t>
1872   A client that supports persistent connections &MAY; "pipeline" its
1873   requests (i.e., send multiple requests without waiting for each
1874   response). A server &MUST; send its responses to those requests in the
1875   same order that the requests were received.
1876</t>
1877<t>
1878   Clients which assume persistent connections and pipeline immediately
1879   after connection establishment &SHOULD; be prepared to retry their
1880   connection if the first pipelined attempt fails. If a client does
1881   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1882   persistent. Clients &MUST; also be prepared to resend their requests if
1883   the server closes the connection before sending all of the
1884   corresponding responses.
1885</t>
1886<t>
1887   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1888   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1889   premature termination of the transport connection could lead to
1890   indeterminate results. A client wishing to send a non-idempotent
1891   request &SHOULD; wait to send that request until it has received the
1892   response status for the previous request.
1893</t>
1894</section>
1895</section>
1896
1897<section title="Proxy Servers" anchor="persistent.proxy">
1898<t>
1899   It is especially important that proxies correctly implement the
1900   properties of the Connection header field as specified in <xref target="header.connection"/>.
1901</t>
1902<t>
1903   The proxy server &MUST; signal persistent connections separately with
1904   its clients and the origin servers (or other proxy servers) that it
1905   connects to. Each persistent connection applies to only one transport
1906   link.
1907</t>
1908<t>
1909   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1910   with an HTTP/1.0 client (but see RFC 2068 <xref target="RFC2068"/> for information and
1911   discussion of the problems with the Keep-Alive header implemented by
1912   many HTTP/1.0 clients).
1913</t>
1914</section>
1915
1916<section title="Practical Considerations" anchor="persistent.practical">
1917<t>
1918   Servers will usually have some time-out value beyond which they will
1919   no longer maintain an inactive connection. Proxy servers might make
1920   this a higher value since it is likely that the client will be making
1921   more connections through the same server. The use of persistent
1922   connections places no requirements on the length (or existence) of
1923   this time-out for either the client or the server.
1924</t>
1925<t>
1926   When a client or server wishes to time-out it &SHOULD; issue a graceful
1927   close on the transport connection. Clients and servers &SHOULD; both
1928   constantly watch for the other side of the transport close, and
1929   respond to it as appropriate. If a client or server does not detect
1930   the other side's close promptly it could cause unnecessary resource
1931   drain on the network.
1932</t>
1933<t>
1934   A client, server, or proxy &MAY; close the transport connection at any
1935   time. For example, a client might have started to send a new request
1936   at the same time that the server has decided to close the "idle"
1937   connection. From the server's point of view, the connection is being
1938   closed while it was idle, but from the client's point of view, a
1939   request is in progress.
1940</t>
1941<t>
1942   This means that clients, servers, and proxies &MUST; be able to recover
1943   from asynchronous close events. Client software &SHOULD; reopen the
1944   transport connection and retransmit the aborted sequence of requests
1945   without user interaction so long as the request sequence is
1946   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1947   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1948   human operator the choice of retrying the request(s). Confirmation by
1949   user-agent software with semantic understanding of the application
1950   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
1951   be repeated if the second sequence of requests fails.
1952</t>
1953<t>
1954   Servers &SHOULD; always respond to at least one request per connection,
1955   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1956   middle of transmitting a response, unless a network or client failure
1957   is suspected.
1958</t>
1959<t>
1960   Clients that use persistent connections &SHOULD; limit the number of
1961   simultaneous connections that they maintain to a given server. A
1962   single-user client &SHOULD-NOT; maintain more than 2 connections with
1963   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1964   another server or proxy, where N is the number of simultaneously
1965   active users. These guidelines are intended to improve HTTP response
1966   times and avoid congestion.
1967</t>
1968</section>
1969</section>
1970
1971<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1972
1973<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1974<t>
1975   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1976   flow control mechanisms to resolve temporary overloads, rather than
1977   terminating connections with the expectation that clients will retry.
1978   The latter technique can exacerbate network congestion.
1979</t>
1980</section>
1981
1982<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
1983<t>
1984   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
1985   the network connection for an error status while it is transmitting
1986   the request. If the client sees an error status, it &SHOULD;
1987   immediately cease transmitting the body. If the body is being sent
1988   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
1989   empty trailer &MAY; be used to prematurely mark the end of the message.
1990   If the body was preceded by a Content-Length header, the client &MUST;
1991   close the connection.
1992</t>
1993</section>
1994
1995<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
1996<t>
1997   The purpose of the 100 (Continue) status (see &status-100;) is to
1998   allow a client that is sending a request message with a request body
1999   to determine if the origin server is willing to accept the request
2000   (based on the request headers) before the client sends the request
2001   body. In some cases, it might either be inappropriate or highly
2002   inefficient for the client to send the body if the server will reject
2003   the message without looking at the body.
2004</t>
2005<t>
2006   Requirements for HTTP/1.1 clients:
2007  <list style="symbols">
2008    <t>
2009        If a client will wait for a 100 (Continue) response before
2010        sending the request body, it &MUST; send an Expect request-header
2011        field (&header-expect;) with the "100-continue" expectation.
2012    </t>
2013    <t>
2014        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2015        with the "100-continue" expectation if it does not intend
2016        to send a request body.
2017    </t>
2018  </list>
2019</t>
2020<t>
2021   Because of the presence of older implementations, the protocol allows
2022   ambiguous situations in which a client may send "Expect: 100-continue"
2023   without receiving either a 417 (Expectation Failed) status
2024   or a 100 (Continue) status. Therefore, when a client sends this
2025   header field to an origin server (possibly via a proxy) from which it
2026   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2027   for an indefinite period before sending the request body.
2028</t>
2029<t>
2030   Requirements for HTTP/1.1 origin servers:
2031  <list style="symbols">
2032    <t> Upon receiving a request which includes an Expect request-header
2033        field with the "100-continue" expectation, an origin server &MUST;
2034        either respond with 100 (Continue) status and continue to read
2035        from the input stream, or respond with a final status code. The
2036        origin server &MUST-NOT; wait for the request body before sending
2037        the 100 (Continue) response. If it responds with a final status
2038        code, it &MAY; close the transport connection or it &MAY; continue
2039        to read and discard the rest of the request.  It &MUST-NOT;
2040        perform the requested method if it returns a final status code.
2041    </t>
2042    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2043        the request message does not include an Expect request-header
2044        field with the "100-continue" expectation, and &MUST-NOT; send a
2045        100 (Continue) response if such a request comes from an HTTP/1.0
2046        (or earlier) client. There is an exception to this rule: for
2047        compatibility with RFC 2068, a server &MAY; send a 100 (Continue)
2048        status in response to an HTTP/1.1 PUT or POST request that does
2049        not include an Expect request-header field with the "100-continue"
2050        expectation. This exception, the purpose of which is
2051        to minimize any client processing delays associated with an
2052        undeclared wait for 100 (Continue) status, applies only to
2053        HTTP/1.1 requests, and not to requests with any other HTTP-version
2054        value.
2055    </t>
2056    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2057        already received some or all of the request body for the
2058        corresponding request.
2059    </t>
2060    <t> An origin server that sends a 100 (Continue) response &MUST;
2061    ultimately send a final status code, once the request body is
2062        received and processed, unless it terminates the transport
2063        connection prematurely.
2064    </t>
2065    <t> If an origin server receives a request that does not include an
2066        Expect request-header field with the "100-continue" expectation,
2067        the request includes a request body, and the server responds
2068        with a final status code before reading the entire request body
2069        from the transport connection, then the server &SHOULD-NOT;  close
2070        the transport connection until it has read the entire request,
2071        or until the client closes the connection. Otherwise, the client
2072        might not reliably receive the response message. However, this
2073        requirement is not be construed as preventing a server from
2074        defending itself against denial-of-service attacks, or from
2075        badly broken client implementations.
2076      </t>
2077    </list>
2078</t>
2079<t>
2080   Requirements for HTTP/1.1 proxies:
2081  <list style="symbols">
2082    <t> If a proxy receives a request that includes an Expect request-header
2083        field with the "100-continue" expectation, and the proxy
2084        either knows that the next-hop server complies with HTTP/1.1 or
2085        higher, or does not know the HTTP version of the next-hop
2086        server, it &MUST; forward the request, including the Expect header
2087        field.
2088    </t>
2089    <t> If the proxy knows that the version of the next-hop server is
2090        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2091        respond with a 417 (Expectation Failed) status.
2092    </t>
2093    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2094        numbers received from recently-referenced next-hop servers.
2095    </t>
2096    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2097        request message was received from an HTTP/1.0 (or earlier)
2098        client and did not include an Expect request-header field with
2099        the "100-continue" expectation. This requirement overrides the
2100        general rule for forwarding of 1xx responses (see &status-1xx;).
2101    </t>
2102  </list>
2103</t>
2104</section>
2105
2106<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2107<t>
2108   If an HTTP/1.1 client sends a request which includes a request body,
2109   but which does not include an Expect request-header field with the
2110   "100-continue" expectation, and if the client is not directly
2111   connected to an HTTP/1.1 origin server, and if the client sees the
2112   connection close before receiving any status from the server, the
2113   client &SHOULD; retry the request.  If the client does retry this
2114   request, it &MAY; use the following "binary exponential backoff"
2115   algorithm to be assured of obtaining a reliable response:
2116  <list style="numbers">
2117    <t>
2118      Initiate a new connection to the server
2119    </t>
2120    <t>
2121      Transmit the request-headers
2122    </t>
2123    <t>
2124      Initialize a variable R to the estimated round-trip time to the
2125         server (e.g., based on the time it took to establish the
2126         connection), or to a constant value of 5 seconds if the round-trip
2127         time is not available.
2128    </t>
2129    <t>
2130       Compute T = R * (2**N), where N is the number of previous
2131         retries of this request.
2132    </t>
2133    <t>
2134       Wait either for an error response from the server, or for T
2135         seconds (whichever comes first)
2136    </t>
2137    <t>
2138       If no error response is received, after T seconds transmit the
2139         body of the request.
2140    </t>
2141    <t>
2142       If client sees that the connection is closed prematurely,
2143         repeat from step 1 until the request is accepted, an error
2144         response is received, or the user becomes impatient and
2145         terminates the retry process.
2146    </t>
2147  </list>
2148</t>
2149<t>
2150   If at any point an error status is received, the client
2151  <list style="symbols">
2152      <t>&SHOULD-NOT;  continue and</t>
2153
2154      <t>&SHOULD; close the connection if it has not completed sending the
2155        request message.</t>
2156    </list>
2157</t>
2158</section>
2159</section>
2160</section>
2161
2162
2163<section title="Header Field Definitions" anchor="header.fields">
2164<t>
2165   This section defines the syntax and semantics of all standard
2166   HTTP/1.1 header fields. For entity-header fields, both sender and
2167   recipient refer to either the client or the server, depending on who
2168   sends and who receives the entity.
2169</t>
2170
2171<section title="Connection" anchor="header.connection">
2172  <iref primary="true" item="Connection header" x:for-anchor=""/>
2173  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2174<t>
2175   The Connection general-header field allows the sender to specify
2176   options that are desired for that particular connection and &MUST-NOT;
2177   be communicated by proxies over further connections.
2178</t>
2179<t>
2180   The Connection header has the following grammar:
2181</t>
2182<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2183    Connection = "Connection" ":" 1#(connection-token)
2184    connection-token  = token
2185</artwork></figure>
2186<t>
2187   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2188   message is forwarded and, for each connection-token in this field,
2189   remove any header field(s) from the message with the same name as the
2190   connection-token. Connection options are signaled by the presence of
2191   a connection-token in the Connection header field, not by any
2192   corresponding additional header field(s), since the additional header
2193   field may not be sent if there are no parameters associated with that
2194   connection option.
2195</t>
2196<t>
2197   Message headers listed in the Connection header &MUST-NOT; include
2198   end-to-end headers, such as Cache-Control.
2199</t>
2200<t>
2201   HTTP/1.1 defines the "close" connection option for the sender to
2202   signal that the connection will be closed after completion of the
2203   response. For example,
2204</t>
2205<figure><artwork type="example">
2206    Connection: close
2207</artwork></figure>
2208<t>
2209   in either the request or the response header fields indicates that
2210   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2211   after the current request/response is complete.
2212</t>
2213<t>
2214   HTTP/1.1 applications that do not support persistent connections &MUST;
2215   include the "close" connection option in every message.
2216</t>
2217<t>
2218   A system receiving an HTTP/1.0 (or lower-version) message that
2219   includes a Connection header MUST, for each connection-token in this
2220   field, remove and ignore any header field(s) from the message with
2221   the same name as the connection-token. This protects against mistaken
2222   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2223</t>
2224</section>
2225
2226<section title="Content-Length" anchor="header.content-length">
2227  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2228  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2229<t>
2230   The Content-Length entity-header field indicates the size of the
2231   entity-body, in decimal number of OCTETs, sent to the recipient or,
2232   in the case of the HEAD method, the size of the entity-body that
2233   would have been sent had the request been a GET.
2234</t>
2235<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
2236    Content-Length    = "Content-Length" ":" 1*DIGIT
2237</artwork></figure>
2238<t>
2239   An example is
2240</t>
2241<figure><artwork type="example">
2242    Content-Length: 3495
2243</artwork></figure>
2244<t>
2245   Applications &SHOULD; use this field to indicate the transfer-length of
2246   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2247</t>
2248<t>
2249   Any Content-Length greater than or equal to zero is a valid value.
2250   <xref target="message.length"/> describes how to determine the length of a message-body
2251   if a Content-Length is not given.
2252</t>
2253<t>
2254   Note that the meaning of this field is significantly different from
2255   the corresponding definition in MIME, where it is an optional field
2256   used within the "message/external-body" content-type. In HTTP, it
2257   &SHOULD; be sent whenever the message's length can be determined prior
2258   to being transferred, unless this is prohibited by the rules in
2259   <xref target="message.length"/>.
2260</t>
2261</section>
2262
2263<section title="Date" anchor="header.date">
2264  <iref primary="true" item="Date header" x:for-anchor=""/>
2265  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2266<t>
2267   The Date general-header field represents the date and time at which
2268   the message was originated, having the same semantics as orig-date in
2269   RFC 822. The field value is an HTTP-date, as described in <xref target="full.date"/>;
2270   it &MUST; be sent in rfc1123-date format.
2271</t>
2272<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
2273    Date  = "Date" ":" HTTP-date
2274</artwork></figure>
2275<t>
2276   An example is
2277</t>
2278<figure><artwork type="example">
2279    Date: Tue, 15 Nov 1994 08:12:31 GMT
2280</artwork></figure>
2281<t>
2282   Origin servers &MUST; include a Date header field in all responses,
2283   except in these cases:
2284  <list style="numbers">
2285      <t>If the response status code is 100 (Continue) or 101 (Switching
2286         Protocols), the response &MAY; include a Date header field, at
2287         the server's option.</t>
2288
2289      <t>If the response status code conveys a server error, e.g. 500
2290         (Internal Server Error) or 503 (Service Unavailable), and it is
2291         inconvenient or impossible to generate a valid Date.</t>
2292
2293      <t>If the server does not have a clock that can provide a
2294         reasonable approximation of the current time, its responses
2295         &MUST-NOT; include a Date header field. In this case, the rules
2296         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2297  </list>
2298</t>
2299<t>
2300   A received message that does not have a Date header field &MUST; be
2301   assigned one by the recipient if the message will be cached by that
2302   recipient or gatewayed via a protocol which requires a Date. An HTTP
2303   implementation without a clock &MUST-NOT; cache responses without
2304   revalidating them on every use. An HTTP cache, especially a shared
2305   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2306   clock with a reliable external standard.
2307</t>
2308<t>
2309   Clients &SHOULD; only send a Date header field in messages that include
2310   an entity-body, as in the case of the PUT and POST requests, and even
2311   then it is optional. A client without a clock &MUST-NOT; send a Date
2312   header field in a request.
2313</t>
2314<t>
2315   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2316   time subsequent to the generation of the message. It &SHOULD; represent
2317   the best available approximation of the date and time of message
2318   generation, unless the implementation has no means of generating a
2319   reasonably accurate date and time. In theory, the date ought to
2320   represent the moment just before the entity is generated. In
2321   practice, the date can be generated at any time during the message
2322   origination without affecting its semantic value.
2323</t>
2324
2325<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2326<t>
2327   Some origin server implementations might not have a clock available.
2328   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2329   values to a response, unless these values were associated
2330   with the resource by a system or user with a reliable clock. It &MAY;
2331   assign an Expires value that is known, at or before server
2332   configuration time, to be in the past (this allows "pre-expiration"
2333   of responses without storing separate Expires values for each
2334   resource).
2335</t>
2336</section>
2337</section>
2338
2339<section title="Host" anchor="header.host">
2340  <iref primary="true" item="Host header" x:for-anchor=""/>
2341  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2342<t>
2343   The Host request-header field specifies the Internet host and port
2344   number of the resource being requested, as obtained from the original
2345   URI given by the user or referring resource (generally an HTTP URL,
2346   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2347   the naming authority of the origin server or gateway given by the
2348   original URL. This allows the origin server or gateway to
2349   differentiate between internally-ambiguous URLs, such as the root "/"
2350   URL of a server for multiple host names on a single IP address.
2351</t>
2352<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2353    Host = "Host" ":" host [ ":" port ] ; <xref target="http.url"/>
2354</artwork></figure>
2355<t>
2356   A "host" without any trailing port information implies the default
2357   port for the service requested (e.g., "80" for an HTTP URL). For
2358   example, a request on the origin server for
2359   &lt;http://www.w3.org/pub/WWW/&gt; would properly include:
2360</t>
2361<figure><artwork type="example">
2362    GET /pub/WWW/ HTTP/1.1
2363    Host: www.w3.org
2364</artwork></figure>
2365<t>
2366   A client &MUST; include a Host header field in all HTTP/1.1 request
2367   messages . If the requested URI does not include an Internet host
2368   name for the service being requested, then the Host header field &MUST;
2369   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2370   request message it forwards does contain an appropriate Host header
2371   field that identifies the service being requested by the proxy. All
2372   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2373   status code to any HTTP/1.1 request message which lacks a Host header
2374   field.
2375</t>
2376<t>
2377   See sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2378   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2379   for other requirements relating to Host.
2380</t>
2381</section>
2382
2383<section title="TE" anchor="header.te">
2384  <iref primary="true" item="TE header" x:for-anchor=""/>
2385  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2386<t>
2387   The TE request-header field indicates what extension transfer-codings
2388   it is willing to accept in the response and whether or not it is
2389   willing to accept trailer fields in a chunked transfer-coding. Its
2390   value may consist of the keyword "trailers" and/or a comma-separated
2391   list of extension transfer-coding names with optional accept
2392   parameters (as described in <xref target="transfer.codings"/>).
2393</t>
2394<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2395    TE        = "TE" ":" #( t-codings )
2396    t-codings = "trailers" | ( transfer-extension [ accept-params ] )
2397</artwork></figure>
2398<t>
2399   The presence of the keyword "trailers" indicates that the client is
2400   willing to accept trailer fields in a chunked transfer-coding, as
2401   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2402   transfer-coding values even though it does not itself represent a
2403   transfer-coding.
2404</t>
2405<t>
2406   Examples of its use are:
2407</t>
2408<figure><artwork type="example">
2409    TE: deflate
2410    TE:
2411    TE: trailers, deflate;q=0.5
2412</artwork></figure>
2413<t>
2414   The TE header field only applies to the immediate connection.
2415   Therefore, the keyword &MUST; be supplied within a Connection header
2416   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2417</t>
2418<t>
2419   A server tests whether a transfer-coding is acceptable, according to
2420   a TE field, using these rules:
2421  <list style="numbers">
2422    <x:lt>
2423      <t>The "chunked" transfer-coding is always acceptable. If the
2424         keyword "trailers" is listed, the client indicates that it is
2425         willing to accept trailer fields in the chunked response on
2426         behalf of itself and any downstream clients. The implication is
2427         that, if given, the client is stating that either all
2428         downstream clients are willing to accept trailer fields in the
2429         forwarded response, or that it will attempt to buffer the
2430         response on behalf of downstream recipients.
2431      </t><t>
2432         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2433         chunked response such that a client can be assured of buffering
2434         the entire response.</t>
2435    </x:lt>
2436    <x:lt>
2437      <t>If the transfer-coding being tested is one of the transfer-codings
2438         listed in the TE field, then it is acceptable unless it
2439         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2440         qvalue of 0 means "not acceptable.")</t>
2441    </x:lt>
2442    <x:lt>
2443      <t>If multiple transfer-codings are acceptable, then the
2444         acceptable transfer-coding with the highest non-zero qvalue is
2445         preferred.  The "chunked" transfer-coding always has a qvalue
2446         of 1.</t>
2447    </x:lt>
2448  </list>
2449</t>
2450<t>
2451   If the TE field-value is empty or if no TE field is present, the only
2452   transfer-coding  is "chunked". A message with no transfer-coding is
2453   always acceptable.
2454</t>
2455</section>
2456
2457<section title="Trailer" anchor="header.trailer">
2458  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2459  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2460<t>
2461   The Trailer general field value indicates that the given set of
2462   header fields is present in the trailer of a message encoded with
2463   chunked transfer-coding.
2464</t>
2465<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2466    Trailer  = "Trailer" ":" 1#field-name
2467</artwork></figure>
2468<t>
2469   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2470   message using chunked transfer-coding with a non-empty trailer. Doing
2471   so allows the recipient to know which header fields to expect in the
2472   trailer.
2473</t>
2474<t>
2475   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2476   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2477   trailer fields in a "chunked" transfer-coding.
2478</t>
2479<t>
2480   Message header fields listed in the Trailer header field &MUST-NOT;
2481   include the following header fields:
2482  <list style="symbols">
2483    <t>Transfer-Encoding</t>
2484    <t>Content-Length</t>
2485    <t>Trailer</t>
2486  </list>
2487</t>
2488</section>
2489
2490<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2491  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2492  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2493<t>
2494   The Transfer-Encoding general-header field indicates what (if any)
2495   type of transformation has been applied to the message body in order
2496   to safely transfer it between the sender and the recipient. This
2497   differs from the content-coding in that the transfer-coding is a
2498   property of the message, not of the entity.
2499</t>
2500<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
2501  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
2502</artwork></figure>
2503<t>
2504   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2505</t>
2506<figure><artwork type="example">
2507  Transfer-Encoding: chunked
2508</artwork></figure>
2509<t>
2510   If multiple encodings have been applied to an entity, the transfer-codings
2511   &MUST; be listed in the order in which they were applied.
2512   Additional information about the encoding parameters &MAY; be provided
2513   by other entity-header fields not defined by this specification.
2514</t>
2515<t>
2516   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2517   header.
2518</t>
2519</section>
2520
2521<section title="Upgrade" anchor="header.upgrade">
2522  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2523  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2524<t>
2525   The Upgrade general-header allows the client to specify what
2526   additional communication protocols it supports and would like to use
2527   if the server finds it appropriate to switch protocols. The server
2528   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2529   response to indicate which protocol(s) are being switched.
2530</t>
2531<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
2532    Upgrade        = "Upgrade" ":" 1#product
2533</artwork></figure>
2534<t>
2535   For example,
2536</t>
2537<figure><artwork type="example">
2538    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2539</artwork></figure>
2540<t>
2541   The Upgrade header field is intended to provide a simple mechanism
2542   for transition from HTTP/1.1 to some other, incompatible protocol. It
2543   does so by allowing the client to advertise its desire to use another
2544   protocol, such as a later version of HTTP with a higher major version
2545   number, even though the current request has been made using HTTP/1.1.
2546   This eases the difficult transition between incompatible protocols by
2547   allowing the client to initiate a request in the more commonly
2548   supported protocol while indicating to the server that it would like
2549   to use a "better" protocol if available (where "better" is determined
2550   by the server, possibly according to the nature of the method and/or
2551   resource being requested).
2552</t>
2553<t>
2554   The Upgrade header field only applies to switching application-layer
2555   protocols upon the existing transport-layer connection. Upgrade
2556   cannot be used to insist on a protocol change; its acceptance and use
2557   by the server is optional. The capabilities and nature of the
2558   application-layer communication after the protocol change is entirely
2559   dependent upon the new protocol chosen, although the first action
2560   after changing the protocol &MUST; be a response to the initial HTTP
2561   request containing the Upgrade header field.
2562</t>
2563<t>
2564   The Upgrade header field only applies to the immediate connection.
2565   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2566   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2567   HTTP/1.1 message.
2568</t>
2569<t>
2570   The Upgrade header field cannot be used to indicate a switch to a
2571   protocol on a different connection. For that purpose, it is more
2572   appropriate to use a 301, 302, 303, or 305 redirection response.
2573</t>
2574<t>
2575   This specification only defines the protocol name "HTTP" for use by
2576   the family of Hypertext Transfer Protocols, as defined by the HTTP
2577   version rules of <xref target="http.version"/> and future updates to this
2578   specification. Any token can be used as a protocol name; however, it
2579   will only be useful if both the client and server associate the name
2580   with the same protocol.
2581</t>
2582</section>
2583
2584<section title="Via" anchor="header.via">
2585  <iref primary="true" item="Via header" x:for-anchor=""/>
2586  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2587<t>
2588   The Via general-header field &MUST; be used by gateways and proxies to
2589   indicate the intermediate protocols and recipients between the user
2590   agent and the server on requests, and between the origin server and
2591   the client on responses. It is analogous to the "Received" field of
2592   RFC 822 <xref target="RFC822"/> and is intended to be used for tracking message forwards,
2593   avoiding request loops, and identifying the protocol capabilities of
2594   all senders along the request/response chain.
2595</t>
2596<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"/>
2597   Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
2598   received-protocol = [ protocol-name "/" ] protocol-version
2599   protocol-name     = token
2600   protocol-version  = token
2601   received-by       = ( host [ ":" port ] ) | pseudonym
2602   pseudonym         = token
2603</artwork></figure>
2604<t>
2605   The received-protocol indicates the protocol version of the message
2606   received by the server or client along each segment of the
2607   request/response chain. The received-protocol version is appended to
2608   the Via field value when the message is forwarded so that information
2609   about the protocol capabilities of upstream applications remains
2610   visible to all recipients.
2611</t>
2612<t>
2613   The protocol-name is optional if and only if it would be "HTTP". The
2614   received-by field is normally the host and optional port number of a
2615   recipient server or client that subsequently forwarded the message.
2616   However, if the real host is considered to be sensitive information,
2617   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2618   be assumed to be the default port of the received-protocol.
2619</t>
2620<t>
2621   Multiple Via field values represents each proxy or gateway that has
2622   forwarded the message. Each recipient &MUST; append its information
2623   such that the end result is ordered according to the sequence of
2624   forwarding applications.
2625</t>
2626<t>
2627   Comments &MAY; be used in the Via header field to identify the software
2628   of the recipient proxy or gateway, analogous to the User-Agent and
2629   Server header fields. However, all comments in the Via field are
2630   optional and &MAY; be removed by any recipient prior to forwarding the
2631   message.
2632</t>
2633<t>
2634   For example, a request message could be sent from an HTTP/1.0 user
2635   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2636   forward the request to a public proxy at nowhere.com, which completes
2637   the request by forwarding it to the origin server at www.ics.uci.edu.
2638   The request received by www.ics.uci.edu would then have the following
2639   Via header field:
2640</t>
2641<figure><artwork type="example">
2642    Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
2643</artwork></figure>
2644<t>
2645   Proxies and gateways used as a portal through a network firewall
2646   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2647   the firewall region. This information &SHOULD; only be propagated if
2648   explicitly enabled. If not enabled, the received-by host of any host
2649   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2650   for that host.
2651</t>
2652<t>
2653   For organizations that have strong privacy requirements for hiding
2654   internal structures, a proxy &MAY; combine an ordered subsequence of
2655   Via header field entries with identical received-protocol values into
2656   a single such entry. For example,
2657</t>
2658<figure><artwork type="example">
2659    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2660</artwork></figure>
2661<t>
2662        could be collapsed to
2663</t>
2664<figure><artwork type="example">
2665    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2666</artwork></figure>
2667<t>
2668   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2669   under the same organizational control and the hosts have already been
2670   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2671   have different received-protocol values.
2672</t>
2673</section>
2674
2675</section>
2676
2677<section title="IANA Considerations" anchor="IANA.considerations">
2678<t>
2679   TBD.
2680</t>
2681</section>
2682
2683<section title="Security Considerations" anchor="security.considerations">
2684<t>
2685   This section is meant to inform application developers, information
2686   providers, and users of the security limitations in HTTP/1.1 as
2687   described by this document. The discussion does not include
2688   definitive solutions to the problems revealed, though it does make
2689   some suggestions for reducing security risks.
2690</t>
2691
2692<section title="Personal Information" anchor="personal.information">
2693<t>
2694   HTTP clients are often privy to large amounts of personal information
2695   (e.g. the user's name, location, mail address, passwords, encryption
2696   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2697   leakage of this information via the HTTP protocol to other sources.
2698   We very strongly recommend that a convenient interface be provided
2699   for the user to control dissemination of such information, and that
2700   designers and implementors be particularly careful in this area.
2701   History shows that errors in this area often create serious security
2702   and/or privacy problems and generate highly adverse publicity for the
2703   implementor's company.
2704</t>
2705</section>
2706
2707<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2708<t>
2709   A server is in the position to save personal data about a user's
2710   requests which might identify their reading patterns or subjects of
2711   interest. This information is clearly confidential in nature and its
2712   handling can be constrained by law in certain countries. People using
2713   the HTTP protocol to provide data are responsible for ensuring that
2714   such material is not distributed without the permission of any
2715   individuals that are identifiable by the published results.
2716</t>
2717</section>
2718
2719<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2720<t>
2721   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2722   the documents returned by HTTP requests to be only those that were
2723   intended by the server administrators. If an HTTP server translates
2724   HTTP URIs directly into file system calls, the server &MUST; take
2725   special care not to serve files that were not intended to be
2726   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2727   other operating systems use ".." as a path component to indicate a
2728   directory level above the current one. On such a system, an HTTP
2729   server &MUST; disallow any such construct in the Request-URI if it
2730   would otherwise allow access to a resource outside those intended to
2731   be accessible via the HTTP server. Similarly, files intended for
2732   reference only internally to the server (such as access control
2733   files, configuration files, and script code) &MUST; be protected from
2734   inappropriate retrieval, since they might contain sensitive
2735   information. Experience has shown that minor bugs in such HTTP server
2736   implementations have turned into security risks.
2737</t>
2738</section>
2739
2740<section title="DNS Spoofing" anchor="dns.spoofing">
2741<t>
2742   Clients using HTTP rely heavily on the Domain Name Service, and are
2743   thus generally prone to security attacks based on the deliberate
2744   mis-association of IP addresses and DNS names. Clients need to be
2745   cautious in assuming the continuing validity of an IP number/DNS name
2746   association.
2747</t>
2748<t>
2749   In particular, HTTP clients &SHOULD; rely on their name resolver for
2750   confirmation of an IP number/DNS name association, rather than
2751   caching the result of previous host name lookups. Many platforms
2752   already can cache host name lookups locally when appropriate, and
2753   they &SHOULD; be configured to do so. It is proper for these lookups to
2754   be cached, however, only when the TTL (Time To Live) information
2755   reported by the name server makes it likely that the cached
2756   information will remain useful.
2757</t>
2758<t>
2759   If HTTP clients cache the results of host name lookups in order to
2760   achieve a performance improvement, they &MUST; observe the TTL
2761   information reported by DNS.
2762</t>
2763<t>
2764   If HTTP clients do not observe this rule, they could be spoofed when
2765   a previously-accessed server's IP address changes. As network
2766   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2767   possibility of this form of attack will grow. Observing this
2768   requirement thus reduces this potential security vulnerability.
2769</t>
2770<t>
2771   This requirement also improves the load-balancing behavior of clients
2772   for replicated servers using the same DNS name and reduces the
2773   likelihood of a user's experiencing failure in accessing sites which
2774   use that strategy.
2775</t>
2776</section>
2777
2778<section title="Proxies and Caching" anchor="attack.proxies">
2779<t>
2780   By their very nature, HTTP proxies are men-in-the-middle, and
2781   represent an opportunity for man-in-the-middle attacks. Compromise of
2782   the systems on which the proxies run can result in serious security
2783   and privacy problems. Proxies have access to security-related
2784   information, personal information about individual users and
2785   organizations, and proprietary information belonging to users and
2786   content providers. A compromised proxy, or a proxy implemented or
2787   configured without regard to security and privacy considerations,
2788   might be used in the commission of a wide range of potential attacks.
2789</t>
2790<t>
2791   Proxy operators should protect the systems on which proxies run as
2792   they would protect any system that contains or transports sensitive
2793   information. In particular, log information gathered at proxies often
2794   contains highly sensitive personal information, and/or information
2795   about organizations. Log information should be carefully guarded, and
2796   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2797</t>
2798<t>
2799   Proxy implementors should consider the privacy and security
2800   implications of their design and coding decisions, and of the
2801   configuration options they provide to proxy operators (especially the
2802   default configuration).
2803</t>
2804<t>
2805   Users of a proxy need to be aware that they are no trustworthier than
2806   the people who run the proxy; HTTP itself cannot solve this problem.
2807</t>
2808<t>
2809   The judicious use of cryptography, when appropriate, may suffice to
2810   protect against a broad range of security and privacy attacks. Such
2811   cryptography is beyond the scope of the HTTP/1.1 specification.
2812</t>
2813</section>
2814
2815<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2816<t>
2817   They exist. They are hard to defend against. Research continues.
2818   Beware.
2819</t>
2820</section>
2821</section>
2822
2823<section title="Acknowledgments" anchor="ack">
2824<t>
2825   This specification makes heavy use of the augmented BNF and generic
2826   constructs defined by David H. Crocker for RFC 822 <xref target="RFC822"/>. Similarly, it
2827   reuses many of the definitions provided by Nathaniel Borenstein and
2828   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2829   specification will help reduce past confusion over the relationship
2830   between HTTP and Internet mail message formats.
2831</t>
2832<t>
2833   The HTTP protocol has evolved considerably over the years. It has
2834   benefited from a large and active developer community--the many
2835   people who have participated on the www-talk mailing list--and it is
2836   that community which has been most responsible for the success of
2837   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2838   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2839   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2840   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2841   VanHeyningen deserve special recognition for their efforts in
2842   defining early aspects of the protocol.
2843</t>
2844<t>
2845   This document has benefited greatly from the comments of all those
2846   participating in the HTTP-WG. In addition to those already mentioned,
2847   the following individuals have contributed to this specification:
2848</t>
2849<figure><artwork type="text/plain">
2850    Gary Adams                  Ross Patterson
2851    Harald Tveit Alvestrand     Albert Lunde
2852    Keith Ball                  John C. Mallery
2853    Brian Behlendorf            Jean-Philippe Martin-Flatin
2854    Paul Burchard               Mitra
2855    Maurizio Codogno            David Morris
2856    Mike Cowlishaw              Gavin Nicol
2857    Roman Czyborra              Bill Perry
2858    Michael A. Dolan            Jeffrey Perry
2859    David J. Fiander            Scott Powers
2860    Alan Freier                 Owen Rees
2861    Marc Hedlund                Luigi Rizzo
2862    Greg Herlihy                David Robinson
2863    Koen Holtman                Marc Salomon
2864    Alex Hopmann                Rich Salz
2865    Bob Jernigan                Allan M. Schiffman
2866    Shel Kaphan                 Jim Seidman
2867    Rohit Khare                 Chuck Shotton
2868    John Klensin                Eric W. Sink
2869    Martijn Koster              Simon E. Spero
2870    Alexei Kosut                Richard N. Taylor
2871    David M. Kristol            Robert S. Thau
2872    Daniel LaLiberte            Bill (BearHeart) Weinman
2873    Ben Laurie                  Francois Yergeau
2874    Paul J. Leach               Mary Ellen Zurko
2875    Daniel DuBois               Josh Cohen
2876</artwork></figure>
2877<t>
2878   Thanks to the "cave men" of Palo Alto. You know who you are.
2879</t>
2880<t>
2881   Jim Gettys (the current editor of this document) wishes particularly
2882   to thank Roy Fielding, the previous editor of this document, along
2883   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2884   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2885   Larry Masinter for their help. And thanks go particularly to Jeff
2886   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
2887</t>
2888<t>
2889   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
2890   Frystyk implemented RFC 2068 early, and we wish to thank them for the
2891   discovery of many of the problems that this document attempts to
2892   rectify.
2893</t>
2894<t>
2895   Based on an XML translation of RFC 2616 by Julian Reschke.
2896</t>
2897</section>
2898
2899</middle>
2900<back>
2901<references>
2902
2903<reference anchor="Part2">
2904   <front>
2905      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2906      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2907         <organization abbrev="Day Software">Day Software</organization>
2908         <address><email>fielding@gbiv.com</email></address>
2909      </author>
2910      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2911         <organization>One Laptop per Child</organization>
2912         <address><email>jg@laptop.org</email></address>
2913      </author>
2914      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2915         <organization abbrev="HP">Hewlett-Packard Company</organization>
2916         <address><email>JeffMogul@acm.org</email></address>
2917      </author>
2918      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2919         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2920         <address><email>henrikn@microsoft.com</email></address>
2921      </author>
2922      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2923         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2924         <address><email>LMM@acm.org</email></address>
2925      </author>
2926      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2927         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2928         <address><email>paulle@microsoft.com</email></address>
2929      </author>
2930      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2931         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2932         <address><email>timbl@w3.org</email></address>
2933      </author>
2934      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2935   </front>
2936   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2937   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2938</reference>
2939
2940<reference anchor="Part3">
2941   <front>
2942      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2943      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2944         <organization abbrev="Day Software">Day Software</organization>
2945         <address><email>fielding@gbiv.com</email></address>
2946      </author>
2947      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2948         <organization>One Laptop per Child</organization>
2949         <address><email>jg@laptop.org</email></address>
2950      </author>
2951      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2952         <organization abbrev="HP">Hewlett-Packard Company</organization>
2953         <address><email>JeffMogul@acm.org</email></address>
2954      </author>
2955      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2956         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2957         <address><email>henrikn@microsoft.com</email></address>
2958      </author>
2959      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2960         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2961         <address><email>LMM@acm.org</email></address>
2962      </author>
2963      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2964         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2965         <address><email>paulle@microsoft.com</email></address>
2966      </author>
2967      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2968         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2969         <address><email>timbl@w3.org</email></address>
2970      </author>
2971      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2972   </front>
2973   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2974   <x:source href="p3-payload.xml" basename="p3-payload"/>
2975</reference>
2976
2977<reference anchor="Part6">
2978   <front>
2979      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
2980      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2981         <organization abbrev="Day Software">Day Software</organization>
2982         <address><email>fielding@gbiv.com</email></address>
2983      </author>
2984      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2985         <organization>One Laptop per Child</organization>
2986         <address><email>jg@laptop.org</email></address>
2987      </author>
2988      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2989         <organization abbrev="HP">Hewlett-Packard Company</organization>
2990         <address><email>JeffMogul@acm.org</email></address>
2991      </author>
2992      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2993         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2994         <address><email>henrikn@microsoft.com</email></address>
2995      </author>
2996      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2997         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2998         <address><email>LMM@acm.org</email></address>
2999      </author>
3000      <author initials="P." surname="Leach" fullname="Paul J. Leach">
3001         <organization abbrev="Microsoft">Microsoft Corporation</organization>
3002         <address><email>paulle@microsoft.com</email></address>
3003      </author>
3004      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3005         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3006         <address><email>timbl@w3.org</email></address>
3007      </author>
3008      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3009   </front>
3010   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3011   <x:source href="p6-cache.xml" basename="p6-cache"/>
3012</reference>
3013
3014<reference anchor="RFC2616">
3015   <front>
3016      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3017      <author initials="R." surname="Fielding" fullname="R. Fielding">
3018         <organization>University of California, Irvine</organization>
3019         <address><email>fielding@ics.uci.edu</email></address>
3020      </author>
3021      <author initials="J." surname="Gettys" fullname="J. Gettys">
3022         <organization>W3C</organization>
3023         <address><email>jg@w3.org</email></address>
3024      </author>
3025      <author initials="J." surname="Mogul" fullname="J. Mogul">
3026         <organization>Compaq Computer Corporation</organization>
3027         <address><email>mogul@wrl.dec.com</email></address>
3028      </author>
3029      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3030         <organization>MIT Laboratory for Computer Science</organization>
3031         <address><email>frystyk@w3.org</email></address>
3032      </author>
3033      <author initials="L." surname="Masinter" fullname="L. Masinter">
3034         <organization>Xerox Corporation</organization>
3035         <address><email>masinter@parc.xerox.com</email></address>
3036      </author>
3037      <author initials="P." surname="Leach" fullname="P. Leach">
3038         <organization>Microsoft Corporation</organization>
3039         <address><email>paulle@microsoft.com</email></address>
3040      </author>
3041      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3042         <organization>W3C</organization>
3043         <address><email>timbl@w3.org</email></address>
3044      </author>
3045      <date month="June" year="1999"/>
3046   </front>
3047   <seriesInfo name="RFC" value="2616"/>
3048</reference>
3049
3050<reference anchor="RFC1436">
3051<front>
3052<title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3053<author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3054<organization>University of Minnesota, Computer and Information Services</organization>
3055<address>
3056<postal>
3057<street>100 Union Street SE</street>
3058<street>Room 152 Shepherd Labs</street>
3059<city>Minneapolis</city>
3060<region>MN</region>
3061<code>55455</code>
3062<country>US</country></postal>
3063<phone>+1 612 625 1300</phone>
3064<email>fxa@boombox.micro.umn.edu</email></address></author>
3065<author initials="M." surname="McCahill" fullname="Mark McCahill">
3066<organization>University of Minnesota, Computer and Information Services</organization>
3067<address>
3068<postal>
3069<street>100 Union Street SE</street>
3070<street>Room 152 Shepherd Labs</street>
3071<city>Minneapolis</city>
3072<region>MN</region>
3073<code>55455</code>
3074<country>US</country></postal>
3075<phone>+1 612 625 1300</phone>
3076<email>mpm@boombox.micro.umn.edu</email></address></author>
3077<author initials="P." surname="Lindner" fullname="Paul Lindner">
3078<organization>University of Minnesota, Computer and Information Services</organization>
3079<address>
3080<postal>
3081<street>100 Union Street SE</street>
3082<street>Room 152 Shepherd Labs</street>
3083<city>Minneapolis</city>
3084<region>MN</region>
3085<code>55455</code>
3086<country>US</country></postal>
3087<phone>+1 612 625 1300</phone>
3088<email>lindner@boombox.micro.umn.edu</email></address></author>
3089<author initials="D." surname="Johnson" fullname="David Johnson">
3090<organization>University of Minnesota, Computer and Information Services</organization>
3091<address>
3092<postal>
3093<street>100 Union Street SE</street>
3094<street>Room 152 Shepherd Labs</street>
3095<city>Minneapolis</city>
3096<region>MN</region>
3097<code>55455</code>
3098<country>US</country></postal>
3099<phone>+1 612 625 1300</phone>
3100<email>dmj@boombox.micro.umn.edu</email></address></author>
3101<author initials="D." surname="Torrey" fullname="Daniel Torrey">
3102<organization>University of Minnesota, Computer and Information Services</organization>
3103<address>
3104<postal>
3105<street>100 Union Street SE</street>
3106<street>Room 152 Shepherd Labs</street>
3107<city>Minneapolis</city>
3108<region>MN</region>
3109<code>55455</code>
3110<country>US</country></postal>
3111<phone>+1 612 625 1300</phone>
3112<email>daniel@boombox.micro.umn.edu</email></address></author>
3113<author initials="B." surname="Alberti" fullname="Bob Alberti">
3114<organization>University of Minnesota, Computer and Information Services</organization>
3115<address>
3116<postal>
3117<street>100 Union Street SE</street>
3118<street>Room 152 Shepherd Labs</street>
3119<city>Minneapolis</city>
3120<region>MN</region>
3121<code>55455</code>
3122<country>US</country></postal>
3123<phone>+1 612 625 1300</phone>
3124<email>alberti@boombox.micro.umn.edu</email></address></author>
3125<date month="March" year="1993"/>
3126</front>
3127<seriesInfo name="RFC" value="1436"/>
3128</reference>
3129
3130<reference anchor="RFC1630">
3131<front>
3132<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>
3133<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3134<organization>CERN, World-Wide Web project</organization>
3135<address>
3136<postal>
3137<street>1211 Geneva 23</street>
3138<city/>
3139<region/>
3140<code/>
3141<country>CH</country></postal>
3142<phone>+41 22 7673755</phone>
3143<facsimile>+41 22 7677155</facsimile>
3144<email>timbl@info.cern.ch</email></address></author>
3145<date month="June" year="1994"/></front>
3146<seriesInfo name="RFC" value="1630"/>
3147</reference>
3148
3149<reference anchor="RFC1738">
3150<front>
3151<title>Uniform Resource Locators (URL)</title>
3152<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3153<organization>CERN, World-Wide Web project</organization>
3154<address>
3155<postal>
3156<street>1211 Geneva 23</street>
3157<city/>
3158<region/>
3159<code/>
3160<country>CH</country></postal>
3161<phone>+41 22 7673755</phone>
3162<facsimile>+41 22 7677155</facsimile>
3163<email>timbl@info.cern.ch</email></address></author>
3164<author initials="L." surname="Masinter" fullname="Larry Masinter">
3165<organization>Xerox PARC</organization>
3166<address>
3167<postal>
3168<street>3333 Coyote Hill Road</street>
3169<city>Palo Alto</city>
3170<region>CA</region>
3171<code>94034</code>
3172<country>US</country></postal>
3173<phone>+1 415 812 4365</phone>
3174<facsimile>+1 415 812 4333</facsimile>
3175<email>masinter@parc.xerox.com</email></address></author>
3176<author initials="M." surname="McCahill" fullname="Mark McCahill">
3177<organization>University of Minnesota, Computer and Information Services</organization>
3178<address>
3179<postal>
3180<street>100 Union Street SE, Shepherd Labs</street>
3181<street>Room 152</street>
3182<city>Minneapolis</city>
3183<region>MN</region>
3184<code>55455</code>
3185<country>US</country></postal>
3186<phone>+1 612 625 1300</phone>
3187<email>mpm@boombox.micro.umn.edu</email></address></author>
3188<date month="December" year="1994"/>
3189</front>
3190<seriesInfo name="RFC" value="1738"/>
3191</reference>
3192
3193<reference anchor="RFC1945">
3194<front>
3195<title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3196<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3197<organization>MIT, Laboratory for Computer Science</organization>
3198<address>
3199<postal>
3200<street>545 Technology Square</street>
3201<city>Cambridge</city>
3202<region>MA</region>
3203<code>02139</code>
3204<country>US</country></postal>
3205<phone/>
3206<facsimile>+1 617 258 8682</facsimile>
3207<email>timbl@w3.org</email></address></author>
3208<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3209<organization>University of California, Irvine, Department of Information and Computer Science</organization>
3210<address>
3211<postal>
3212<street/>
3213<city>Irvine</city>
3214<region>CA</region>
3215<code>92717-3425</code>
3216<country>US</country></postal>
3217<phone/>
3218<facsimile>+1 714 824 4056</facsimile>
3219<email>fielding@ics.uci.edu</email></address></author>
3220<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3221<organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3222<address>
3223<postal>
3224<street>545 Technology Square</street>
3225<city>Cambridge</city>
3226<region>MA</region>
3227<code>02139</code>
3228<country>US</country></postal>
3229<phone/>
3230<facsimile>+1 617 258 8682</facsimile>
3231<email>frystyk@w3.org</email></address></author>
3232<date month="May" year="1996"/>
3233</front>
3234<seriesInfo name="RFC" value="1945"/>
3235</reference>
3236
3237<reference anchor="RFC2045">
3238<front>
3239<title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3240<author initials="N." surname="Freed" fullname="Ned Freed">
3241<organization>Innosoft International, Inc.</organization>
3242<address>
3243<postal>
3244<street>1050 East Garvey Avenue South</street>
3245<city>West Covina</city>
3246<region>CA</region>
3247<code>91790</code>
3248<country>US</country></postal>
3249<phone>+1 818 919 3600</phone>
3250<facsimile>+1 818 919 3614</facsimile>
3251<email>ned@innosoft.com</email></address></author>
3252<author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3253<organization>First Virtual Holdings</organization>
3254<address>
3255<postal>
3256<street>25 Washington Avenue</street>
3257<city>Morristown</city>
3258<region>NJ</region>
3259<code>07960</code>
3260<country>US</country></postal>
3261<phone>+1 201 540 8967</phone>
3262<facsimile>+1 201 993 3032</facsimile>
3263<email>nsb@nsb.fv.com</email></address></author>
3264<date month="November" year="1996"/>
3265</front>
3266<seriesInfo name="RFC" value="2045"/>
3267</reference>
3268
3269<reference anchor="RFC1123">
3270<front>
3271<title>Requirements for Internet Hosts - Application and Support</title>
3272<author initials="R." surname="Braden" fullname="Robert Braden">
3273<organization>University of Southern California (USC), Information Sciences Institute</organization>
3274<address>
3275<postal>
3276<street>4676 Admiralty Way</street>
3277<city>Marina del Rey</city>
3278<region>CA</region>
3279<code>90292-6695</code>
3280<country>US</country></postal>
3281<phone>+1 213 822 1511</phone>
3282<email>Braden@ISI.EDU</email></address></author>
3283<date month="October" year="1989"/></front>
3284<seriesInfo name="STD" value="3"/>
3285<seriesInfo name="RFC" value="1123"/>
3286</reference>
3287
3288<reference anchor="RFC822">
3289<front>
3290<title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3291<author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3292<organization>University of Delaware, Dept. of Electrical Engineering</organization>
3293<address>
3294<postal>
3295<street/>
3296<city>Newark</city>
3297<region>DE</region>
3298<code>19711</code>
3299<country>US</country></postal>
3300<email>DCrocker@UDel-Relay</email></address></author>
3301<date month="August" day="13" year="1982"/></front>
3302<seriesInfo name="STD" value="11"/>
3303<seriesInfo name="RFC" value="822"/>
3304</reference>
3305
3306<reference anchor="WAIS">
3307<front>
3308<title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3309<author initials="F." surname="Davis" fullname="F. Davis">
3310<organization>Thinking Machines Corporation</organization>
3311</author>
3312<author initials="B." surname="Kahle" fullname="B. Kahle">
3313<organization>Thinking Machines Corporation</organization>
3314</author>
3315<author initials="H." surname="Morris" fullname="H. Morris">
3316<organization>Thinking Machines Corporation</organization>
3317</author>
3318<author initials="J." surname="Salem" fullname="J. Salem">
3319<organization>Thinking Machines Corporation</organization>
3320</author>
3321<author initials="T." surname="Shen" fullname="T. Shen">
3322<organization>Thinking Machines Corporation</organization>
3323</author>
3324<author initials="R." surname="Wang" fullname="R. Wang">
3325<organization>Thinking Machines Corporation</organization>
3326</author>
3327<author initials="J." surname="Sui" fullname="J. Sui">
3328<organization>Thinking Machines Corporation</organization>
3329</author>
3330<author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3331<organization>Thinking Machines Corporation</organization>
3332</author>
3333<date month="April" year="1990"/></front>
3334<seriesInfo name="Thinking Machines Corporation" value=""/>
3335</reference>
3336
3337<reference anchor="RFC1808">
3338<front>
3339<title>Relative Uniform Resource Locators</title>
3340<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3341<organization>University of California Irvine, Department of Information and Computer Science</organization>
3342<address>
3343<postal>
3344<street/>
3345<city>Irvine</city>
3346<region>CA</region>
3347<code>92717-3425</code>
3348<country>US</country></postal>
3349<phone>+1 714 824 4049</phone>
3350<facsimile>+1 714 824 4056</facsimile>
3351<email>fielding@ics.uci.edu</email></address></author>
3352<date month="June" year="1995"/>
3353</front>
3354<seriesInfo name="RFC" value="1808"/>
3355</reference>
3356
3357<reference anchor="RFC977">
3358<front>
3359<title abbrev="Network News Transfer Protocol">Network News Transfer Protocol</title>
3360<author initials="B." surname="Kantor" fullname="Brian Kantor">
3361<organization>University of California, San Diego (UCSD)</organization></author>
3362<author initials="P." surname="Lapsley" fullname="Phil Lapsley">
3363<organization>University of California, Berkeley (UCB)</organization></author>
3364<date month="February" year="1986"/></front>
3365<seriesInfo name="RFC" value="977"/>
3366</reference>
3367
3368<reference anchor="RFC2047">
3369<front>
3370<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3371<author initials="K." surname="Moore" fullname="Keith Moore">
3372<organization>University of Tennessee</organization>
3373<address>
3374<postal>
3375<street>107 Ayres Hall</street>
3376<street>Knoxville TN 37996-1301</street></postal>
3377<email>moore@cs.utk.edu</email></address></author>
3378<date month="November" year="1996"/>
3379<area>Applications</area>
3380<keyword>Amercian Standard Code for Information Interchange</keyword>
3381<keyword>mail</keyword>
3382<keyword>multipurpose internet mail extensions</keyword>
3383</front>
3384<seriesInfo name="RFC" value="2047"/>
3385</reference>
3386
3387<reference anchor="RFC821">
3388<front>
3389<title>Simple Mail Transfer Protocol</title>
3390<author initials="J.B." surname="Postel" fullname="Jonathan B. Postel">
3391<organization>University of Southern California (USC)/Information Sciences Institute</organization>
3392<address>
3393<postal>
3394<street>4676 Admiralty Way</street>
3395<city>Marina del Rey</city>
3396<region>CA</region>
3397<code>90291</code>
3398<country>US</country></postal>
3399<phone>+1 213 822 1511</phone></address></author>
3400<date month="August" year="1982"/></front>
3401<seriesInfo name="STD" value="10"/>
3402<seriesInfo name="RFC" value="821"/>
3403</reference>
3404
3405<reference anchor="RFC4288">
3406  <front>
3407    <title>Media Type Specifications and Registration Procedures</title>
3408    <author initials="N." surname="Freed" fullname="N. Freed">
3409      <organization>Sun Microsystems</organization>
3410      <address>
3411        <email>ned.freed@mrochek.com</email>
3412      </address>
3413    </author>
3414    <author initials="J." surname="Klensin" fullname="J. Klensin">
3415      <organization/>
3416      <address>
3417        <email>klensin+ietf@jck.com</email>
3418      </address>
3419    </author>
3420    <date year="2005" month="December"/>
3421  </front>
3422  <seriesInfo name="BCP" value="13"/>
3423  <seriesInfo name="RFC" value="4288"/>
3424</reference>
3425
3426<reference anchor="RFC959">
3427<front>
3428<title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3429<author initials="J." surname="Postel" fullname="J. Postel">
3430<organization>Information Sciences Institute (ISI)</organization></author>
3431<author initials="J." surname="Reynolds" fullname="J. Reynolds">
3432<organization/></author>
3433<date month="October" year="1985"/></front>
3434<seriesInfo name="STD" value="9"/>
3435<seriesInfo name="RFC" value="959"/>
3436</reference>
3437
3438<reference anchor="RFC1700">
3439<front>
3440<title abbrev="Assigned Numbers">Assigned Numbers</title>
3441<author initials="J." surname="Reynolds" fullname="Joyce K. Reynolds">
3442<organization>USC/Information Sciences Institute</organization>
3443<address>
3444<postal>
3445<street>4676 Admiralty Way</street>
3446<city>Marina del Rey</city>
3447<region>CA</region>
3448<code>90292-6695</code>
3449<country>US</country></postal>
3450<phone>+1 310 822 1511</phone>
3451<email>jkrey@isi.edu</email></address></author>
3452<author initials="J." surname="Postel" fullname="Jon Postel">
3453<organization>USC/Information Sciences Institute</organization>
3454<address>
3455<postal>
3456<street>4676 Admiralty Way</street>
3457<city>Marina del Rey</city>
3458<region>CA</region>
3459<code>90292-6695</code>
3460<country>US</country></postal>
3461<phone>+1 310 822 1511</phone>
3462<email>postel@isi.edu</email></address></author>
3463<date month="October" year="1994"/>
3464<abstract>
3465<t/></abstract></front>
3466<seriesInfo name="STD" value="2"/>
3467<seriesInfo name="RFC" value="1700"/>
3468</reference>
3469
3470<reference anchor="RFC1737">
3471<front>
3472<title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3473<author initials="L." surname="Masinter" fullname="Larry Masinter">
3474<organization>Xerox Palo Alto Research Center</organization>
3475<address>
3476<postal>
3477<street>3333 Coyote Hill Road</street>
3478<city>Palo Alto</city>
3479<region>CA</region>
3480<code>94304</code>
3481<country>US</country></postal>
3482<phone>+1 415 812 4365</phone>
3483<facsimile>+1 415 812 4333</facsimile>
3484<email>masinter@parc.xerox.com</email></address></author>
3485<author initials="K." surname="Sollins" fullname="Karen Sollins">
3486<organization>MIT Laboratory for Computer Science</organization>
3487<address>
3488<postal>
3489<street>545 Technology Square</street>
3490<city>Cambridge</city>
3491<region>MA</region>
3492<code>02139</code>
3493<country>US</country></postal>
3494<phone>+1 617 253 2673</phone>
3495<email>sollins@lcs.mit.edu</email></address></author>
3496<date month="December" year="1994"/></front>
3497<seriesInfo name="RFC" value="1737"/>
3498</reference>
3499
3500<reference anchor="USASCII">
3501  <front>
3502    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3503    <author>
3504      <organization>American National Standards Institute</organization>
3505    </author>
3506    <date year="1986"/>
3507  </front>
3508  <seriesInfo name="ANSI" value="X3.4"/>
3509</reference>
3510
3511<reference anchor="ISO-8859">
3512<front>
3513<title>Information technology - 8-bit single byte coded graphic - character sets</title>
3514<author>
3515<organization>International Organization for Standardization</organization>
3516</author>
3517<date year="1987-1990"/>
3518</front>
3519<annotation>
3520  Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
3521  Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
3522  Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
3523  Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
3524  Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
3525  Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
3526  Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
3527  Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
3528  Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
3529</annotation>
3530</reference>
3531
3532<reference anchor="RFC1900">
3533<front>
3534<title>Renumbering Needs Work</title>
3535<author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3536<organization>CERN, Computing and Networks Division</organization>
3537<address>
3538<postal>
3539<street>1211 Geneva 23</street>
3540<country>CH</country></postal>
3541<phone>+41 22 7674967</phone>
3542<facsimile>+41 22 7677155</facsimile>
3543<email>brian@dxcoms.cern.ch</email></address></author>
3544<author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3545<organization>cisco Systems</organization>
3546<address>
3547<postal>
3548<street>170 West Tasman Drive</street>
3549<city>San Jose</city>
3550<region>CA</region>
3551<code>95134</code>
3552<country>US</country></postal>
3553<phone>+1 914 528 0090</phone>
3554<email>yakov@cisco.com</email></address></author>
3555<date month="February" year="1996"/>
3556</front>
3557<seriesInfo name="RFC" value="1900"/>
3558</reference>
3559
3560<reference anchor="Pad1995">
3561<front>
3562<title>Improving HTTP Latency</title>
3563<author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan"><organization/></author>
3564<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul"><organization/></author>
3565<date year="1995" month="Dec"/>
3566</front>
3567<seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3568<annotation>
3569  Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994,
3570  which is available at <eref target="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html"/>.
3571</annotation>
3572</reference>
3573
3574<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3575<front>
3576<title>Analysis of HTTP Performance</title>
3577<author initials="J." surname="Touch" fullname="Joe Touch">
3578  <organization>USC/Information Sciences Institute</organization>
3579  <address><email>touch@isi.edu</email></address>
3580</author>
3581<author initials="J." surname="Heidemann" fullname="John Heidemann">
3582  <organization>USC/Information Sciences Institute</organization>
3583  <address><email>johnh@isi.edu</email></address>
3584</author>
3585<author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3586  <organization>USC/Information Sciences Institute</organization>
3587  <address><email>katia@isi.edu</email></address>
3588</author>
3589<date year="1998" month="Aug"/>
3590</front>
3591<seriesInfo name="ISI Research Report" value="ISI/RR-98-463 (original report dated Aug.1996)"/>
3592</reference>
3593
3594<reference anchor="RFC1305">
3595<front>
3596<title>Network Time Protocol (Version 3) Specification, Implementation</title>
3597<author initials="D." surname="Mills" fullname="David L. Mills">
3598<organization>University of Delaware, Electrical Engineering Department</organization>
3599<address>
3600<postal>
3601<street/>
3602<city>Newark</city>
3603<region>DE</region>
3604<code>19716</code>
3605<country>US</country></postal>
3606<phone>+1 302 451 8247</phone>
3607<email>mills@udel.edu</email></address></author>
3608<date month="March" year="1992"/>
3609<abstract>
3610<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>
3611<seriesInfo name="RFC" value="1305"/>
3612</reference>
3613
3614<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3615<front>
3616<title>Analysis of HTTP Performance Problems</title>
3617<author initials="S." surname="Spero" fullname="Simon E. Spero">
3618  <organization/>
3619</author>
3620<date/>
3621</front>
3622</reference>
3623
3624<reference anchor="RFC2068">
3625<front>
3626<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3627<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3628<organization>University of California, Irvine, Department of Information and Computer Science</organization>
3629<address>
3630<postal>
3631<street/>
3632<city>Irvine</city>
3633<region>CA</region>
3634<code>92717-3425</code>
3635<country>US</country></postal>
3636<facsimile>+1 714 824 4056</facsimile>
3637<email>fielding@ics.uci.edu</email></address></author>
3638<author initials="J." surname="Gettys" fullname="Jim Gettys">
3639<organization>MIT Laboratory for Computer Science</organization>
3640<address>
3641<postal>
3642<street>545 Technology Square</street>
3643<city>Cambridge</city>
3644<region>MA</region>
3645<code>02139</code>
3646<country>US</country></postal>
3647<facsimile>+1 617 258 8682</facsimile>
3648<email>jg@w3.org</email></address></author>
3649<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3650<organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3651<address>
3652<postal>
3653<street>250 University Avenue</street>
3654<city>Palo Alto</city>
3655<region>CA</region>
3656<code>94301</code>
3657<country>US</country></postal>
3658<email>mogul@wrl.dec.com</email></address></author>
3659<author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3660<organization>MIT Laboratory for Computer Science</organization>
3661<address>
3662<postal>
3663<street>545 Technology Square</street>
3664<city>Cambridge</city>
3665<region>MA</region>
3666<code>02139</code>
3667<country>US</country></postal>
3668<facsimile>+1 617 258 8682</facsimile>
3669<email>frystyk@w3.org</email></address></author>
3670<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3671<organization>MIT Laboratory for Computer Science</organization>
3672<address>
3673<postal>
3674<street>545 Technology Square</street>
3675<city>Cambridge</city>
3676<region>MA</region>
3677<code>02139</code>
3678<country>US</country></postal>
3679<facsimile>+1 617 258 8682</facsimile>
3680<email>timbl@w3.org</email></address></author>
3681<date month="January" year="1997"/>
3682<abstract>
3683<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>
3684<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>
3685<seriesInfo name="RFC" value="2068"/>
3686</reference>
3687
3688<reference anchor="RFC2119">
3689<front>
3690<title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
3691<author initials="S." surname="Bradner" fullname="Scott Bradner">
3692<organization>Harvard University</organization>
3693<address>
3694<postal>
3695<street>1350 Mass. Ave.</street>
3696<street>Cambridge</street>
3697<street>MA 02138</street></postal>
3698<phone>- +1 617 495 3864</phone>
3699<email>-</email></address></author>
3700<date month="March" year="1997"/>
3701<area>General</area>
3702<keyword>keyword</keyword>
3703</front>
3704
3705<seriesInfo name="BCP" value="14"/>
3706<seriesInfo name="RFC" value="2119"/>
3707</reference>
3708
3709<reference anchor="RFC2145">
3710<front>
3711<title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3712<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3713<organization>Western Research Laboratory</organization>
3714<address>
3715<postal>
3716<street>Digital Equipment Corporation</street>
3717<street>250 University Avenue</street>
3718<street>Palo Alto</street>
3719<street>California</street>
3720<street>94305</street>
3721<country>USA</country></postal>
3722<email>mogul@wrl.dec.com</email></address></author>
3723<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3724<organization>Department of Information and Computer Science</organization>
3725<address>
3726<postal>
3727<street>University of California</street>
3728<street>Irvine</street>
3729<street>CA 92717-3425</street>
3730<country>USA</country></postal>
3731<facsimile>+1 (714) 824-4056</facsimile>
3732<email>fielding@ics.uci.edu</email></address></author>
3733<author initials="J." surname="Gettys" fullname="Jim Gettys">
3734<organization>MIT Laboratory for Computer Science</organization>
3735<address>
3736<postal>
3737<street>545 Technology Square</street>
3738<street>Cambridge</street>
3739<street>MA 02139</street>
3740<country>USA</country></postal>
3741<facsimile>+1 (617) 258 8682</facsimile>
3742<email>jg@w3.org</email></address></author>
3743<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3744<organization>W3 Consortium</organization>
3745<address>
3746<postal>
3747<street>MIT Laboratory for Computer Science</street>
3748<street>545 Technology Square</street>
3749<street>Cambridge</street>
3750<street>MA 02139</street>
3751<country>USA</country></postal>
3752<facsimile>+1 (617) 258 8682</facsimile>
3753<email>frystyk@w3.org</email></address></author>
3754<date month="May" year="1997"/>
3755<area>Applications</area>
3756<keyword>HTTP</keyword>
3757<keyword>hypertext transfer protocol</keyword>
3758<abstract>
3759<t>
3760   HTTP request and response messages include an HTTP protocol version
3761   number.  Some confusion exists concerning the proper use and
3762   interpretation of HTTP version numbers, and concerning
3763   interoperability of HTTP implementations of different protocol
3764   versions.  This document is an attempt to clarify the situation.  It
3765   is not a modification of the intended meaning of the existing
3766   HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention
3767   of the authors of those documents, and can be considered definitive
3768   when there is any ambiguity in those documents concerning HTTP
3769   version numbers, for all versions of HTTP.
3770</t></abstract></front>
3771<seriesInfo name="RFC" value="2145"/>
3772</reference>
3773
3774<reference anchor="Nie1997">
3775<front>
3776<title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3777<author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3778  <organization/>
3779</author>
3780<author initials="J." surname="Gettys" fullname="J. Gettys">
3781  <organization/>
3782</author>
3783<author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3784  <organization/>
3785</author>
3786<author initials="H." surname="Lie" fullname="H. Lie">
3787  <organization/>
3788</author>
3789<author initials="C." surname="Lilley" fullname="C. Lilley">
3790  <organization/>
3791</author>
3792<date year="1997" month="Sep"/>
3793</front>
3794<seriesInfo name="Proceedings of ACM SIGCOMM '97, Cannes France" value=""/>
3795</reference>
3796
3797<reference anchor="RFC2396">
3798<front>
3799<title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3800<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3801<organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3802<address>
3803<postal>
3804<street>MIT Laboratory for Computer Science, NE43-356</street>
3805<street>545 Technology Square</street>
3806<city>Cambridge</city>
3807<region>MA</region>
3808<code>02139</code></postal>
3809<facsimile>+1(617)258-8682</facsimile>
3810<email>timbl@w3.org</email></address></author>
3811<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3812<organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3813<address>
3814<postal>
3815<street>University of California, Irvine</street>
3816<city>Irvine</city>
3817<region>CA</region>
3818<code>92697-3425</code></postal>
3819<facsimile>+1(949)824-1715</facsimile>
3820<email>fielding@ics.uci.edu</email></address></author>
3821<author initials="L." surname="Masinter" fullname="Larry Masinter">
3822<organization abbrev="Xerox Corporation">Xerox PARC</organization>
3823<address>
3824<postal>
3825<street>3333 Coyote Hill Road</street>
3826<city>Palo Alto</city>
3827<region>CA</region>
3828<code>94034</code></postal>
3829<facsimile>+1(415)812-4333</facsimile>
3830<email>masinter@parc.xerox.com</email></address></author>
3831<date month="August" year="1998"/>
3832<area>Applications</area>
3833<keyword>uniform resource</keyword>
3834<keyword>URI</keyword>
3835</front>
3836<seriesInfo name="RFC" value="2396"/>
3837</reference>
3838
3839<reference anchor="RFC2324">
3840<front>
3841<title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3842<author initials="L." surname="Masinter" fullname="Larry Masinter">
3843<organization>Xerox Palo Alto Research Center</organization>
3844<address>
3845<postal>
3846<street>3333 Coyote Hill Road</street>
3847<city>Palo Alto</city>
3848<region>CA</region>
3849<code>94304</code></postal>
3850<email>masinter@parc.xerox.com</email></address></author>
3851<date month="April" day="1" year="1998"/>
3852<area>General</area>
3853<keyword>control protocol</keyword>
3854<keyword>coffee</keyword>
3855</front>
3856<seriesInfo name="RFC" value="2324"/>
3857</reference>
3858
3859</references>
3860
3861<section title="Internet Media Type message/http and application/http" anchor="internet.media.type.http">
3862<iref item="Media Type" subitem="message/http" primary="true"/>
3863<iref item="message/http Media Type" primary="true"/>
3864<iref item="Media Type" subitem="application/http" primary="true"/>
3865<iref item="application/http Media Type" primary="true"/>
3866<t>
3867   In addition to defining the HTTP/1.1 protocol, this document serves
3868   as the specification for the Internet media type "message/http" and
3869   "application/http". The message/http type can be used to enclose a
3870   single HTTP request or response message, provided that it obeys the
3871   MIME restrictions for all "message" types regarding line length and
3872   encodings. The application/http type can be used to enclose a
3873   pipeline of one or more HTTP request or response messages (not
3874   intermixed). The following is to be registered with IANA <xref target="RFC4288"/>.
3875</t>
3876<t>
3877  <list style="hanging" x:indent="12em">
3878    <t hangText="Media Type name:">
3879      message
3880    </t>
3881    <t hangText="Media subtype name:">
3882      http
3883    </t>
3884    <t hangText="Required parameters:">
3885      none
3886    </t>
3887    <t hangText="Optional parameters:">
3888      version, msgtype
3889      <list style="hanging">
3890        <t hangText="version:">
3891          The HTTP-Version number of the enclosed message
3892          (e.g., "1.1"). If not present, the version can be
3893          determined from the first line of the body.
3894        </t>
3895        <t hangText="msgtype:">
3896          The message type -- "request" or "response". If not
3897          present, the type can be determined from the first
3898          line of the body.
3899        </t>
3900      </list>
3901    </t>
3902    <t hangText="Encoding considerations:">
3903      only "7bit", "8bit", or "binary" are permitted
3904    </t>
3905    <t hangText="Security considerations:">
3906      none
3907    </t>
3908  </list>
3909</t>
3910<t>
3911  <list style="hanging" x:indent="12em">
3912    <t hangText="Media Type name:">
3913      application
3914    </t>
3915    <t hangText="Media subtype name:">
3916      http
3917    </t>
3918    <t hangText="Required parameters:">
3919      none
3920    </t>
3921    <t hangText="Optional parameters:">
3922      version, msgtype
3923      <list style="hanging">
3924        <t hangText="version:">
3925          The HTTP-Version number of the enclosed messages
3926          (e.g., "1.1"). If not present, the version can be
3927          determined from the first line of the body.
3928        </t>
3929        <t hangText="msgtype:">
3930          The message type -- "request" or "response". If not
3931          present, the type can be determined from the first
3932          line of the body.
3933        </t>
3934      </list>
3935    </t>
3936    <t hangText="Encoding considerations:">
3937      HTTP messages enclosed by this type
3938      are in "binary" format; use of an appropriate
3939      Content-Transfer-Encoding is required when
3940      transmitted via E-mail.
3941    </t>
3942    <t hangText="Security considerations:">
3943      none
3944    </t>
3945  </list>
3946</t>
3947</section>
3948
3949<section title="Tolerant Applications" anchor="tolerant.applications">
3950<t>
3951   Although this document specifies the requirements for the generation
3952   of HTTP/1.1 messages, not all applications will be correct in their
3953   implementation. We therefore recommend that operational applications
3954   be tolerant of deviations whenever those deviations can be
3955   interpreted unambiguously.
3956</t>
3957<t>
3958   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3959   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3960   accept any amount of SP or HT characters between fields, even though
3961   only a single SP is required.
3962</t>
3963<t>
3964   The line terminator for message-header fields is the sequence CRLF.
3965   However, we recommend that applications, when parsing such headers,
3966   recognize a single LF as a line terminator and ignore the leading CR.
3967</t>
3968<t>
3969   The character set of an entity-body &SHOULD; be labeled as the lowest
3970   common denominator of the character codes used within that body, with
3971   the exception that not labeling the entity is preferred over labeling
3972   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3973</t>
3974<t>
3975   Additional rules for requirements on parsing and encoding of dates
3976   and other potential problems with date encodings include:
3977</t>
3978<t>
3979  <list style="symbols">
3980     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3981        which appears to be more than 50 years in the future is in fact
3982        in the past (this helps solve the "year 2000" problem).</t>
3983
3984     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3985        Expires date as earlier than the proper value, but &MUST-NOT;
3986        internally represent a parsed Expires date as later than the
3987        proper value.</t>
3988
3989     <t>All expiration-related calculations &MUST; be done in GMT. The
3990        local time zone &MUST-NOT; influence the calculation or comparison
3991        of an age or expiration time.</t>
3992
3993     <t>If an HTTP header incorrectly carries a date value with a time
3994        zone other than GMT, it &MUST; be converted into GMT using the
3995        most conservative possible conversion.</t>
3996  </list>
3997</t>
3998</section>
3999
4000<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
4001<t>
4002   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
4003   simplify the process of date comparison. Proxies and gateways from
4004   other protocols &SHOULD; ensure that any Date header field present in a
4005   message conforms to one of the HTTP/1.1 formats and rewrite the date
4006   if necessary.
4007</t>
4008</section>
4009
4010<section title="Compatibility with Previous Versions" anchor="compatibility">
4011<t>
4012   It is beyond the scope of a protocol specification to mandate
4013   compliance with previous versions. HTTP/1.1 was deliberately
4014   designed, however, to make supporting previous versions easy. It is
4015   worth noting that, at the time of composing this specification
4016   (1996), we would expect commercial HTTP/1.1 servers to:
4017  <list style="symbols">
4018     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
4019        1.1 requests;</t>
4020
4021     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
4022        1.1;</t>
4023
4024     <t>respond appropriately with a message in the same major version
4025        used by the client.</t>
4026  </list>
4027</t>
4028<t>
4029   And we would expect HTTP/1.1 clients to:
4030  <list style="symbols">
4031     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
4032        responses;</t>
4033
4034     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
4035        1.1.</t>
4036  </list>
4037</t>
4038<t>
4039   For most implementations of HTTP/1.0, each connection is established
4040   by the client prior to the request and closed by the server after
4041   sending the response. Some implementations implement the Keep-Alive
4042   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="sec" target="RFC2068"/> of RFC
4043   2068 <xref target="RFC2068"/>.
4044</t>
4045
4046<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4047<t>
4048   This section summarizes major differences between versions HTTP/1.0
4049   and HTTP/1.1.
4050</t>
4051
4052<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">
4053<t>
4054   The requirements that clients and servers support the Host request-header,
4055   report an error if the Host request-header (<xref target="header.host"/>) is
4056   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
4057   are among the most important changes defined by this
4058   specification.
4059</t>
4060<t>
4061   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4062   addresses and servers; there was no other established mechanism for
4063   distinguishing the intended server of a request than the IP address
4064   to which that request was directed. The changes outlined above will
4065   allow the Internet, once older HTTP clients are no longer common, to
4066   support multiple Web sites from a single IP address, greatly
4067   simplifying large operational Web servers, where allocation of many
4068   IP addresses to a single host has created serious problems. The
4069   Internet will also be able to recover the IP addresses that have been
4070   allocated for the sole purpose of allowing special-purpose domain
4071   names to be used in root-level HTTP URLs. Given the rate of growth of
4072   the Web, and the number of servers already deployed, it is extremely
4073   important that all implementations of HTTP (including updates to
4074   existing HTTP/1.0 applications) correctly implement these
4075   requirements:
4076  <list style="symbols">
4077     <t>Both clients and servers &MUST; support the Host request-header.</t>
4078
4079     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4080
4081     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4082        request does not include a Host request-header.</t>
4083
4084     <t>Servers &MUST; accept absolute URIs.</t>
4085  </list>
4086</t>
4087</section>
4088</section>
4089
4090<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4091<t>
4092   Some clients and servers might wish to be compatible with some
4093   previous implementations of persistent connections in HTTP/1.0
4094   clients and servers. Persistent connections in HTTP/1.0 are
4095   explicitly negotiated as they are not the default behavior. HTTP/1.0
4096   experimental implementations of persistent connections are faulty,
4097   and the new facilities in HTTP/1.1 are designed to rectify these
4098   problems. The problem was that some existing 1.0 clients may be
4099   sending Keep-Alive to a proxy server that doesn't understand
4100   Connection, which would then erroneously forward it to the next
4101   inbound server, which would establish the Keep-Alive connection and
4102   result in a hung HTTP/1.0 proxy waiting for the close on the
4103   response. The result is that HTTP/1.0 clients must be prevented from
4104   using Keep-Alive when talking to proxies.
4105</t>
4106<t>
4107   However, talking to proxies is the most important use of persistent
4108   connections, so that prohibition is clearly unacceptable. Therefore,
4109   we need some other mechanism for indicating a persistent connection
4110   is desired, which is safe to use even when talking to an old proxy
4111   that ignores Connection. Persistent connections are the default for
4112   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4113   declaring non-persistence. See <xref target="header.connection"/>.
4114</t>
4115<t>
4116   The original HTTP/1.0 form of persistent connections (the Connection:
4117   Keep-Alive and Keep-Alive header) is documented in RFC 2068. <xref target="RFC2068"/>
4118</t>
4119</section>
4120
4121<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4122<t>
4123   This specification has been carefully audited to correct and
4124   disambiguate key word usage; RFC 2068 had many problems in respect to
4125   the conventions laid out in RFC 2119 <xref target="RFC2119"/>.
4126</t>
4127<t>
4128   Transfer-coding and message lengths all interact in ways that
4129   required fixing exactly when chunked encoding is used (to allow for
4130   transfer encoding that may not be self delimiting); it was important
4131   to straighten out exactly how message lengths are computed.
4132</t>
4133<t>
4134   The use and interpretation of HTTP version numbers has been clarified
4135   by RFC 2145. Require proxies to upgrade requests to highest protocol
4136   version they support to deal with problems discovered in HTTP/1.0
4137   implementations (<xref target="http.version"/>)
4138</t>
4139<t>
4140   Proxies should be able to add Content-Length when appropriate.
4141</t>
4142<t>
4143   Transfer-coding had significant problems, particularly with
4144   interactions with chunked encoding. The solution is that transfer-codings
4145   become as full fledged as content-codings. This involves
4146   adding an IANA registry for transfer-codings (separate from content
4147   codings), a new header field (TE) and enabling trailer headers in the
4148   future. Transfer encoding is a major performance benefit, so it was
4149   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4150   interoperability problem that could have occurred due to interactions
4151   between authentication trailers, chunked encoding and HTTP/1.0
4152   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4153   and <xref target="header.te" format="counter"/>)
4154</t>
4155</section>
4156</section>
4157</back>
4158</rfc>
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