source: draft-ietf-httpbis/latest/p1-messaging.xml @ 360

Last change on this file since 360 was 360, checked in by julian.reschke@…, 11 years ago

simplify ABNF production by using WSP (related to #36)

  • Property svn:eol-style set to native
File size: 201.9 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY entity-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std" x:maturity-level="draft"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x='http://purl.org/net/xml2rfc/ext'>
56<front>
57
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
59
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email>fielding@gbiv.com</email>
73      <uri>http://roy.gbiv.com/</uri>
74    </address>
75  </author>
76
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email>jg@laptop.org</email>
88      <uri>http://www.laptop.org/</uri>
89    </address>
90  </author>
91 
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email>JeffMogul@acm.org</email>
104    </address>
105  </author>
106
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email>henrikn@microsoft.com</email>
118    </address>
119  </author>
120
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email>LMM@acm.org</email>
132      <uri>http://larry.masinter.net/</uri>
133    </address>
134  </author>
135 
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email>paulle@microsoft.com</email>
146    </address>
147  </author>
148   
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email>timbl@w3.org</email>
162      <uri>http://www.w3.org/People/Berners-Lee/</uri>
163    </address>
164  </author>
165
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email>ylafon@w3.org</email>
178      <uri>http://www.raubacapeu.net/people/yves/</uri>
179    </address>
180  </author>
181
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email>julian.reschke@greenbytes.de</email>       
193      <uri>http://greenbytes.de/tech/webdav/</uri>     
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
198
199<abstract>
200<t>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    The changes in this draft are summarized in <xref target="changes.since.04"/>.
224  </t>
225</note>
226</front>
227<middle>
228<section title="Introduction" anchor="introduction">
229<t>
230   The Hypertext Transfer Protocol (HTTP) is an application-level
231   protocol for distributed, collaborative, hypermedia information
232   systems. HTTP has been in use by the World-Wide Web global
233   information initiative since 1990. The first version of HTTP, commonly
234   referred to as HTTP/0.9, was a simple protocol for raw data transfer
235   across the Internet with only a single method and no metadata.
236   HTTP/1.0, as defined by <xref target="RFC1945"/>, improved
237   the protocol by allowing messages to be in the format of MIME-like
238   messages, containing metadata about the data transferred and
239   modifiers on the request/response semantics. However, HTTP/1.0 did
240   not sufficiently take into consideration the effects of hierarchical
241   proxies, caching, the need for persistent connections, or name-based
242   virtual hosts. In addition, the proliferation of incompletely-implemented
243   applications calling themselves "HTTP/1.0" necessitated a
244   protocol version change in order for two communicating applications
245   to determine each other's true capabilities.
246</t>
247<t>
248   This document is Part 1 of the seven-part specification that defines
249   the protocol referred to as "HTTP/1.1", obsoleting <xref target="RFC2616"/>.
250   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
251   requirements that enable reliable implementations and adding only
252   those new features that will either be safely ignored by an HTTP/1.0
253   recipient or only sent when communicating with a party advertising
254   compliance with HTTP/1.1.
255   Part 1 defines those aspects of HTTP/1.1 related to overall network
256   operation, message framing, interaction with transport protocols, and
257   URI schemes.
258</t>
259<t>
260   This document is currently disorganized in order to minimize the changes
261   between drafts and enable reviewers to see the smaller errata changes.
262   The next draft will reorganize the sections to better reflect the content.
263   In particular, the sections will be organized according to the typical
264   process of deciding when to use HTTP (URI schemes), overall network operation,
265   connection management, message framing, and generic message parsing.
266   The current mess reflects how widely dispersed these topics and associated
267   requirements had become in <xref target="RFC2616"/>.
268</t>
269
270<section title="Purpose" anchor="intro.purpose">
271<t>
272   Practical information systems require more functionality than simple
273   retrieval, including search, front-end update, and annotation. HTTP
274   allows an open-ended set of methods and headers that indicate the
275   purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
276   provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
277   (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
278   method is to be applied. Messages are passed in a format similar to
279   that used by Internet mail <xref target="RFC5322"/> as defined by the Multipurpose
280   Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
281</t>
282<t>
283   HTTP is also used as a generic protocol for communication between
284   user agents and proxies/gateways to other Internet systems, including
285   those supported by the SMTP <xref target="RFC2821"/>, NNTP <xref target="RFC3977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
286   and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
287   access to resources available from diverse applications.
288</t>
289</section>
290
291<section title="Requirements" anchor="intro.requirements">
292<t>
293   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
294   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
295   document are to be interpreted as described in <xref target="RFC2119"/>.
296</t>
297<t>
298   An implementation is not compliant if it fails to satisfy one or more
299   of the &MUST; or &REQUIRED; level requirements for the protocols it
300   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
301   level and all the &SHOULD; level requirements for its protocols is said
302   to be "unconditionally compliant"; one that satisfies all the &MUST;
303   level requirements but not all the &SHOULD; level requirements for its
304   protocols is said to be "conditionally compliant."
305</t>
306</section>
307
308<section title="Overall Operation" anchor="intro.overall.operation">
309<t>
310   HTTP is a request/response protocol. A client sends a
311   request to the server in the form of a request method, URI, and
312   protocol version, followed by a MIME-like message containing request
313   modifiers, client information, and possible body content over a
314   connection with a server. The server responds with a status line,
315   including the message's protocol version and a success or error code,
316   followed by a MIME-like message containing server information, entity
317   metainformation, and possible entity-body content. The relationship
318   between HTTP and MIME is described in &diff2045entity;.
319</t>
320<t>
321   Most HTTP communication is initiated by a user agent and consists of
322   a request to be applied to a resource on some origin server. In the
323   simplest case, this may be accomplished via a single connection (v)
324   between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327       request chain ------------------------&gt;
328    UA -------------------v------------------- O
329       &lt;----------------------- response chain
330</artwork></figure>
331<t>
332   A more complicated situation occurs when one or more intermediaries
333   are present in the request/response chain. There are three common
334   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
335   forwarding agent, receiving requests for a URI in its absolute form,
336   rewriting all or part of the message, and forwarding the reformatted
337   request toward the server identified by the URI. A gateway is a
338   receiving agent, acting as a layer above some other server(s) and, if
339   necessary, translating the requests to the underlying server's
340   protocol. A tunnel acts as a relay point between two connections
341   without changing the messages; tunnels are used when the
342   communication needs to pass through an intermediary (such as a
343   firewall) even when the intermediary cannot understand the contents
344   of the messages.
345</t>
346<figure><artwork type="drawing">
347       request chain --------------------------------------&gt;
348    UA -----v----- A -----v----- B -----v----- C -----v----- O
349       &lt;------------------------------------- response chain
350</artwork></figure>
351<t>
352   The figure above shows three intermediaries (A, B, and C) between the
353   user agent and origin server. A request or response message that
354   travels the whole chain will pass through four separate connections.
355   This distinction is important because some HTTP communication options
356   may apply only to the connection with the nearest, non-tunnel
357   neighbor, only to the end-points of the chain, or to all connections
358   along the chain. Although the diagram is linear, each participant may
359   be engaged in multiple, simultaneous communications. For example, B
360   may be receiving requests from many clients other than A, and/or
361   forwarding requests to servers other than C, at the same time that it
362   is handling A's request.
363</t>
364<t>
365   Any party to the communication which is not acting as a tunnel may
366   employ an internal cache for handling requests. The effect of a cache
367   is that the request/response chain is shortened if one of the
368   participants along the chain has a cached response applicable to that
369   request. The following illustrates the resulting chain if B has a
370   cached copy of an earlier response from O (via C) for a request which
371   has not been cached by UA or A.
372</t>
373<figure><artwork type="drawing">
374          request chain ----------&gt;
375       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
376          &lt;--------- response chain
377</artwork></figure>
378<t>
379   Not all responses are usefully cacheable, and some requests may
380   contain modifiers which place special requirements on cache behavior.
381   HTTP requirements for cache behavior and cacheable responses are
382   defined in &caching;.
383</t>
384<t>
385   In fact, there are a wide variety of architectures and configurations
386   of caches and proxies currently being experimented with or deployed
387   across the World Wide Web. These systems include national hierarchies
388   of proxy caches to save transoceanic bandwidth, systems that
389   broadcast or multicast cache entries, organizations that distribute
390   subsets of cached data via CD-ROM, and so on. HTTP systems are used
391   in corporate intranets over high-bandwidth links, and for access via
392   PDAs with low-power radio links and intermittent connectivity. The
393   goal of HTTP/1.1 is to support the wide diversity of configurations
394   already deployed while introducing protocol constructs that meet the
395   needs of those who build web applications that require high
396   reliability and, failing that, at least reliable indications of
397   failure.
398</t>
399<t>
400   HTTP communication usually takes place over TCP/IP connections. The
401   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
402   not preclude HTTP from being implemented on top of any other protocol
403   on the Internet, or on other networks. HTTP only presumes a reliable
404   transport; any protocol that provides such guarantees can be used;
405   the mapping of the HTTP/1.1 request and response structures onto the
406   transport data units of the protocol in question is outside the scope
407   of this specification.
408</t>
409<t>
410   In HTTP/1.0, most implementations used a new connection for each
411   request/response exchange. In HTTP/1.1, a connection may be used for
412   one or more request/response exchanges, although connections may be
413   closed for a variety of reasons (see <xref target="persistent.connections"/>).
414</t>
415</section>
416</section>
417
418<section title="Notational Conventions and Generic Grammar" anchor="notation">
419
420<section title="ABNF Extensions" anchor="notation.abnf">
421<t>
422   Two extensions to the ABNF rules of <xref target="RFC5234"/> are used to
423   improve readability.<cref>The current plan is to remove these extensions prior
424   to the last call draft.</cref>
425</t>
426
427<section title="#rule">
428  <t>
429    A construct "#" is defined, similar to "*", for defining lists of
430    elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
431    &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
432    (",") and &OPTIONAL; linear white space (LWS). This makes the usual
433    form of lists very easy; a rule such as
434    <figure><artwork type="example">
435 ( *<x:ref>LWS</x:ref> element *( *<x:ref>LWS</x:ref> "," *<x:ref>LWS</x:ref> element ))</artwork></figure>
436  </t>
437  <t>
438    can be shown as
439    <figure><artwork type="example">
440 1#element</artwork></figure>
441  </t>
442  <t>
443    Wherever this construct is used, null elements are allowed, but do
444    not contribute to the count of elements present. That is,
445    "(element), , (element) " is permitted, but counts as only two
446    elements. Therefore, where at least one element is required, at
447    least one non-null element &MUST; be present. Default values are 0
448    and infinity so that "#element" allows any number, including zero;
449    "1#element" requires at least one; and "1#2element" allows one or
450    two.
451  </t>
452</section>
453
454<section title="implied *LWS" anchor="implied.LWS">
455  <iref item="implied *LWS" primary="true"/>
456    <t>
457      The grammar described by this specification is word-based. Except
458      where noted otherwise, linear white space (LWS) can be included
459      between any two adjacent words (token or quoted-string), and
460      between adjacent words and separators, without changing the
461      interpretation of a field. At least one delimiter (LWS and/or
462      separators) &MUST; exist between any two tokens (for the definition
463      of "token" below), since they would otherwise be interpreted as a
464      single token.
465    </t>
466</section>
467</section>
468
469<section title="Basic Rules" anchor="basic.rules">
470<t anchor="core.rules">
471  <x:anchor-alias value="ALPHA"/>
472  <x:anchor-alias value="CHAR"/>
473  <x:anchor-alias value="CTL"/>
474  <x:anchor-alias value="CR"/>
475  <x:anchor-alias value="CRLF"/>
476  <x:anchor-alias value="DIGIT"/>
477  <x:anchor-alias value="DQUOTE"/>
478  <x:anchor-alias value="HEXDIG"/>
479  <x:anchor-alias value="HTAB"/>
480  <x:anchor-alias value="LF"/>
481  <x:anchor-alias value="OCTET"/>
482  <x:anchor-alias value="SP"/>
483  <x:anchor-alias value="WSP"/>
484   This specification uses the Augmented Backus-Naur Form (ABNF) notation
485   of <xref target="RFC5234"/>.  The following core rules are included by
486   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
487   ALPHA (letters), CHAR (any <xref target="USASCII"/> character,
488   excluding NUL), CR (carriage return), CRLF (CR LF), CTL (controls),
489   DIGIT (decimal 0-9), DQUOTE (double quote),
490   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab),
491   LF (line feed), OCTET (any 8-bit sequence of data), SP (space)
492   and WSP (white space).
493</t>
494<t anchor="rule.CRLF">
495  <x:anchor-alias value="CRLF"/>
496   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
497   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
498   tolerant applications). The end-of-line marker within an entity-body
499   is defined by its associated media type, as described in &media-types;.
500</t>
501<t anchor="rule.LWS">
502  <x:anchor-alias value="LWS"/>
503   HTTP/1.1 header field values can be folded onto multiple lines if the
504   continuation line begins with a space or horizontal tab. All linear
505   white space, including folding, has the same semantics as SP. A
506   recipient &MAY; replace any linear white space with a single SP before
507   interpreting the field value or forwarding the message downstream.
508</t>
509<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
510  <x:ref>LWS</x:ref>            = [<x:ref>CRLF</x:ref>] 1*<x:ref>WSP</x:ref>
511</artwork></figure>
512<t anchor="rule.whitespace">
513  <x:anchor-alias value="BWS"/>
514  <x:anchor-alias value="OWS"/>
515  <x:anchor-alias value="RWS"/>
516  <x:anchor-alias value="obs-fold"/>
517  <cref>Add prose introducing the different kinds of WS.</cref>
518<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OWS"/><iref primary="true" item="Grammar" subitem="RWS"/><iref primary="true" item="Grammar" subitem="BWS"/>
519  <x:ref>OWS</x:ref>            = 1*( [ obs-fold ] <x:ref>WSP</x:ref> )
520                 ; "optional" white space
521  <x:ref>RWS</x:ref>            = 1*( [ obs-fold ] <x:ref>WSP</x:ref> )
522                 ; "required" white space
523  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
524                 ; "bad" white space
525  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref>
526                 ; <cref>to be explained: do not produce</cref>
527</artwork></figure>
528</t>
529<t anchor="rule.TEXT">
530  <x:anchor-alias value="TEXT"/>
531   The TEXT rule is only used for descriptive field contents and values
532   that are not intended to be interpreted by the message parser. Words
533   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
534   <xref target="ISO-8859-1"/> only when encoded according to the rules of
535   <xref target="RFC2047"/>.
536</t>
537<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
538  <x:ref>TEXT</x:ref>           = %x20-7E / %x80-FF / <x:ref>LWS</x:ref>
539                 ; any <x:ref>OCTET</x:ref> except <x:ref>CTL</x:ref>s, but including <x:ref>LWS</x:ref>
540</artwork></figure>
541<t>
542   A CRLF is allowed in the definition of TEXT only as part of a header
543   field continuation. It is expected that the folding LWS will be
544   replaced with a single SP before interpretation of the TEXT value.
545</t>
546<t anchor="rule.token.separators">
547  <x:anchor-alias value="tchar"/>
548  <x:anchor-alias value="token"/>
549  <x:anchor-alias value="separators"/>
550   Many HTTP/1.1 header field values consist of words separated by LWS
551   or special characters. These special characters &MUST; be in a quoted
552   string to be used within a parameter value (as defined in
553   <xref target="transfer.codings"/>).
554</t>
555<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="separators"/>
556  <x:ref>separators</x:ref>     = "(" / ")" / "&lt;" / "&gt;" / "@"
557                 / "," / ";" / ":" / "\" / <x:ref>DQUOTE</x:ref>
558                 / "/" / "[" / "]" / "?" / "="
559                 / "{" / "}" / <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref>
560                 
561  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
562                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
563                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
564                 ; any <x:ref>CHAR</x:ref> except <x:ref>CTL</x:ref>s or <x:ref>separators</x:ref>
565                 
566  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
567</artwork></figure>
568<t anchor="rule.comment">
569  <x:anchor-alias value="comment"/>
570  <x:anchor-alias value="ctext"/>
571   Comments can be included in some HTTP header fields by surrounding
572   the comment text with parentheses. Comments are only allowed in
573   fields containing "comment" as part of their field value definition.
574   In all other fields, parentheses are considered part of the field
575   value.
576</t>
577<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
578  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
579  <x:ref>ctext</x:ref>          = &lt;any <x:ref>TEXT</x:ref> excluding "(" and ")"&gt;
580</artwork></figure>
581<t anchor="rule.quoted-string">
582  <x:anchor-alias value="quoted-string"/>
583  <x:anchor-alias value="qdtext"/>
584   A string of text is parsed as a single word if it is quoted using
585   double-quote marks.
586</t>
587<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
588  <x:ref>quoted-string</x:ref>  = ( <x:ref>DQUOTE</x:ref> *(<x:ref>qdtext</x:ref> / <x:ref>quoted-pair</x:ref> ) <x:ref>DQUOTE</x:ref> )
589  <x:ref>qdtext</x:ref>         = &lt;any <x:ref>TEXT</x:ref> excluding <x:ref>DQUOTE</x:ref> and "\">
590</artwork></figure>
591<t anchor="rule.quoted-pair">
592  <x:anchor-alias value="quoted-pair"/>
593  <x:anchor-alias value="quoted-text"/>
594   The backslash character ("\") &MAY; be used as a single-character
595   quoting mechanism only within quoted-string and comment constructs.
596</t>
597<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-text"/><iref primary="true" item="Grammar" subitem="quoted-pair"/>
598  <x:ref>quoted-text</x:ref>    = %x01-09 /
599                   %x0B-0C /
600                   %x0E-FF ; Characters excluding NUL, <x:ref>CR</x:ref> and <x:ref>LF</x:ref>
601  <x:ref>quoted-pair</x:ref>    = "\" <x:ref>quoted-text</x:ref>
602</artwork></figure>
603</section>
604
605<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
606  <x:anchor-alias value="request-header"/>
607  <x:anchor-alias value="response-header"/>
608  <x:anchor-alias value="accept-params"/>
609  <x:anchor-alias value="entity-body"/>
610  <x:anchor-alias value="entity-header"/>
611  <x:anchor-alias value="Cache-Control"/>
612  <x:anchor-alias value="Pragma"/>
613  <x:anchor-alias value="Warning"/>
614<t>
615  The ABNF rules below are defined in other parts:
616</t>
617<figure><!-- Part2--><artwork type="abnf2616">
618  <x:ref>request-header</x:ref>  = &lt;request-header, defined in &request-header-fields;&gt;
619  <x:ref>response-header</x:ref> = &lt;response-header, defined in &response-header-fields;&gt;
620</artwork></figure>
621<figure><!-- Part3--><artwork type="abnf2616">
622  <x:ref>accept-params</x:ref>   = &lt;accept-params, defined in &header-accept;&gt;
623  <x:ref>entity-body</x:ref>     = &lt;entity-body, defined in &entity-body;&gt;
624  <x:ref>entity-header</x:ref>   = &lt;entity-header, defined in &entity-header-fields;&gt;
625</artwork></figure>
626<figure><!-- Part6--><artwork type="abnf2616">
627  <x:ref>Cache-Control</x:ref>   = &lt;Cache-Control, defined in &header-pragma;&gt;
628  <x:ref>Pragma</x:ref>          = &lt;Pragma, defined in &header-pragma;&gt;
629  <x:ref>Warning</x:ref>         = &lt;Warning, defined in &header-warning;&gt;
630</artwork></figure>
631</section>
632
633</section>
634
635<section title="Protocol Parameters" anchor="protocol.parameters">
636
637<section title="HTTP Version" anchor="http.version">
638  <x:anchor-alias value="HTTP-Version"/>
639  <x:anchor-alias value="HTTP-Prot-Name"/>
640<t>
641   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
642   of the protocol. The protocol versioning policy is intended to allow
643   the sender to indicate the format of a message and its capacity for
644   understanding further HTTP communication, rather than the features
645   obtained via that communication. No change is made to the version
646   number for the addition of message components which do not affect
647   communication behavior or which only add to extensible field values.
648   The &lt;minor&gt; number is incremented when the changes made to the
649   protocol add features which do not change the general message parsing
650   algorithm, but which may add to the message semantics and imply
651   additional capabilities of the sender. The &lt;major&gt; number is
652   incremented when the format of a message within the protocol is
653   changed. See <xref target="RFC2145"/> for a fuller explanation.
654</t>
655<t>
656   The version of an HTTP message is indicated by an HTTP-Version field
657   in the first line of the message. HTTP-Version is case-sensitive.
658</t>
659<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/>
660  <x:ref>HTTP-Version</x:ref>   = <x:ref>HTTP-Prot-Name</x:ref> "/" 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
661  <x:ref>HTTP-Prot-Name</x:ref> = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
662</artwork></figure>
663<t>
664   Note that the major and minor numbers &MUST; be treated as separate
665   integers and that each &MAY; be incremented higher than a single digit.
666   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
667   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
668   &MUST-NOT; be sent.
669</t>
670<t>
671   An application that sends a request or response message that includes
672   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
673   with this specification. Applications that are at least conditionally
674   compliant with this specification &SHOULD; use an HTTP-Version of
675   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
676   not compatible with HTTP/1.0. For more details on when to send
677   specific HTTP-Version values, see <xref target="RFC2145"/>.
678</t>
679<t>
680   The HTTP version of an application is the highest HTTP version for
681   which the application is at least conditionally compliant.
682</t>
683<t>
684   Proxy and gateway applications need to be careful when forwarding
685   messages in protocol versions different from that of the application.
686   Since the protocol version indicates the protocol capability of the
687   sender, a proxy/gateway &MUST-NOT; send a message with a version
688   indicator which is greater than its actual version. If a higher
689   version request is received, the proxy/gateway &MUST; either downgrade
690   the request version, or respond with an error, or switch to tunnel
691   behavior.
692</t>
693<t>
694   Due to interoperability problems with HTTP/1.0 proxies discovered
695   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
696   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
697   they support. The proxy/gateway's response to that request &MUST; be in
698   the same major version as the request.
699</t>
700<t>
701  <list>
702    <t>
703      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
704      of header fields required or forbidden by the versions involved.
705    </t>
706  </list>
707</t>
708</section>
709
710<section title="Uniform Resource Identifiers" anchor="uri">
711<t>
712   URIs have been known by many names: WWW addresses, Universal Document
713   Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
714   combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
715   <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
716   simply formatted strings which identify--via name, location, or any
717   other characteristic--a resource.
718</t>
719
720<section title="General Syntax" anchor="general.syntax">
721  <x:anchor-alias value="absoluteURI"/>
722  <x:anchor-alias value="authority"/>
723  <x:anchor-alias value="fragment"/>
724  <x:anchor-alias value="path-absolute"/>
725  <x:anchor-alias value="port"/>
726  <x:anchor-alias value="query"/>
727  <x:anchor-alias value="relativeURI"/>
728  <x:anchor-alias value="uri-host"/>
729<t>
730   URIs in HTTP can be represented in absolute form or relative to some
731   known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
732   forms are differentiated by the fact that absolute URIs always begin
733   with a scheme name followed by a colon. For definitive information on
734   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
735   Generic Syntax and Semantics," <xref target="RFC2396"/> (which replaces <xref target="RFC1738"/>
736   and <xref target="RFC1808"/>). This specification adopts the
737   definitions of "URI-reference", "absoluteURI", "fragment", "relativeURI", "port",
738   "host", "abs_path", "query", and "authority" from that specification:
739</t>
740<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="absoluteURI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="relativeURI"/><iref primary="true" item="Grammar" subitem="uri-host"/>
741  <x:ref>absoluteURI</x:ref>   = &lt;absoluteURI, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
742  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2"/>>
743  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC2396" x:fmt="," x:sec="4.1"/>>
744  <x:ref>path-absolute</x:ref> = &lt;abs_path, defined in <xref target="RFC2396" x:fmt="," x:sec="3"/>>
745  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
746  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC2396" x:fmt="," x:sec="3.4"/>>
747  <x:ref>relativeURI</x:ref>   = &lt;relativeURI, defined in <xref target="RFC2396" x:fmt="," x:sec="5"/>>
748  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC2396" x:fmt="," x:sec="3.2.2"/>>
749</artwork></figure>
750<t>
751   HTTP does not place any a priori limit on the length of
752   a URI. Servers &MUST; be able to handle the URI of any resource they
753   serve, and &SHOULD; be able to handle URIs of unbounded length if they
754   provide GET-based forms that could generate such URIs. A server
755   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
756   than the server can handle (see &status-414;).
757</t>
758<t>
759  <list>
760    <t>
761      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
762      above 255 bytes, because some older client or proxy
763      implementations might not properly support these lengths.
764    </t>
765  </list>
766</t>
767</section>
768
769<section title="http URL" anchor="http.url">
770  <x:anchor-alias value="http-URL"/>
771  <iref item="http URI scheme" primary="true"/>
772  <iref item="URI scheme" subitem="http" primary="true"/>
773<t>
774   The "http" scheme is used to locate network resources via the HTTP
775   protocol. This section defines the scheme-specific syntax and
776   semantics for http URLs.
777</t>
778<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URL"/>
779  <x:ref>http-URL</x:ref> = "http:" "//" <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ]
780             [ <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]]
781</artwork></figure>
782<t>
783   If the port is empty or not given, port 80 is assumed. The semantics
784   are that the identified resource is located at the server listening
785   for TCP connections on that port of that host, and the Request-URI
786   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
787   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
788   the path-absolute is not present in the URL, it &MUST; be given as "/" when
789   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
790   receives a host name which is not a fully qualified domain name, it
791   &MAY; add its domain to the host name it received. If a proxy receives
792   a fully qualified domain name, the proxy &MUST-NOT; change the host
793   name.
794</t>
795<t><list><t>
796  <iref item="https URI scheme"/>
797  <iref item="URI scheme" subitem="https"/>
798  <x:h>Note:</x:h> the "https" scheme is defined in <xref target="RFC2818"/>.
799</t></list></t>
800</section>
801
802<section title="URI Comparison" anchor="uri.comparison">
803<t>
804   When comparing two URIs to decide if they match or not, a client
805   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
806   URIs, with these exceptions:
807  <list style="symbols">
808    <t>A port that is empty or not given is equivalent to the default
809        port for that URI-reference;</t>
810    <t>Comparisons of host names &MUST; be case-insensitive;</t>
811    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
812    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
813  </list>
814</t>
815<t>
816   Characters other than those in the "reserved" set (see
817   <xref target="RFC2396" x:fmt="," x:sec="2.2"/>) are equivalent to their
818   ""%" <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding.
819</t>
820<t>
821   For example, the following three URIs are equivalent:
822</t>
823<figure><artwork type="example">
824   http://example.com:80/~smith/home.html
825   http://EXAMPLE.com/%7Esmith/home.html
826   http://EXAMPLE.com:/%7esmith/home.html
827</artwork></figure>
828</section>
829</section>
830
831<section title="Date/Time Formats" anchor="date.time.formats">
832<section title="Full Date" anchor="full.date">
833  <x:anchor-alias value="HTTP-date"/>
834  <x:anchor-alias value="obsolete-date"/>
835  <x:anchor-alias value="rfc1123-date"/>
836  <x:anchor-alias value="rfc850-date"/>
837  <x:anchor-alias value="asctime-date"/>
838  <x:anchor-alias value="date1"/>
839  <x:anchor-alias value="date2"/>
840  <x:anchor-alias value="date3"/>
841  <x:anchor-alias value="rfc1123-date"/>
842  <x:anchor-alias value="time"/>
843  <x:anchor-alias value="wkday"/>
844  <x:anchor-alias value="weekday"/>
845  <x:anchor-alias value="month"/>
846<t>
847   HTTP applications have historically allowed three different formats
848   for the representation of date/time stamps:
849</t>
850<figure><artwork type="example">
851   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
852   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
853   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
854</artwork></figure>
855<t>
856   The first format is preferred as an Internet standard and represents
857   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
858   <xref target="RFC822"/>). The other formats are described here only for
859   compatibility with obsolete implementations.
860   HTTP/1.1 clients and servers that parse the date value &MUST; accept
861   all three formats (for compatibility with HTTP/1.0), though they &MUST;
862   only generate the RFC 1123 format for representing HTTP-date values
863   in header fields. See <xref target="tolerant.applications"/> for further information.
864</t>
865<t><list><t>
866      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
867      accepting date values that may have been sent by non-HTTP
868      applications, as is sometimes the case when retrieving or posting
869      messages via proxies/gateways to SMTP or NNTP.
870</t></list></t>
871<t>
872   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
873   (GMT), without exception. For the purposes of HTTP, GMT is exactly
874   equal to UTC (Coordinated Universal Time). This is indicated in the
875   first two formats by the inclusion of "GMT" as the three-letter
876   abbreviation for time zone, and &MUST; be assumed when reading the
877   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
878   additional LWS beyond that specifically included as SP in the
879   grammar.
880</t>
881<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-date"/><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="obsolete-date"/><iref primary="true" item="Grammar" subitem="rfc850-date"/><iref primary="true" item="Grammar" subitem="asctime-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/><iref primary="true" item="Grammar" subitem="time"/><iref primary="true" item="Grammar" subitem="wkday"/><iref primary="true" item="Grammar" subitem="weekday"/><iref primary="true" item="Grammar" subitem="month"/>
882  <x:ref>HTTP-date</x:ref>    = <x:ref>rfc1123-date</x:ref> / <x:ref>obsolete-date</x:ref>
883  <x:ref>obsolete-date</x:ref> = <x:ref>rfc850-date</x:ref> / <x:ref>asctime-date</x:ref> 
884  <x:ref>rfc1123-date</x:ref> = <x:ref>wkday</x:ref> "," <x:ref>SP</x:ref> date1 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
885  <x:ref>rfc850-date</x:ref>  = <x:ref>weekday</x:ref> "," <x:ref>SP</x:ref> date2 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
886  <x:ref>asctime-date</x:ref> = <x:ref>wkday</x:ref> <x:ref>SP</x:ref> <x:ref>date3</x:ref> <x:ref>SP</x:ref> <x:ref>time</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
887  <x:ref>date1</x:ref>        = 2<x:ref>DIGIT</x:ref> <x:ref>SP</x:ref> <x:ref>month</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
888                 ; day month year (e.g., 02 Jun 1982)
889  <x:ref>date2</x:ref>        = 2<x:ref>DIGIT</x:ref> "-" <x:ref>month</x:ref> "-" 2<x:ref>DIGIT</x:ref>
890                 ; day-month-year (e.g., 02-Jun-82)
891  <x:ref>date3</x:ref>        = <x:ref>month</x:ref> <x:ref>SP</x:ref> ( 2<x:ref>DIGIT</x:ref> / ( <x:ref>SP</x:ref> 1<x:ref>DIGIT</x:ref> ))
892                 ; month day (e.g., Jun  2)
893  <x:ref>time</x:ref>         = 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref>
894                 ; 00:00:00 - 23:59:59
895  <x:ref>wkday</x:ref>        = s-Mon / s-Tue / s-Wed
896               / s-Thu / s-Fri / s-Sat / s-Sun
897  <x:ref>weekday</x:ref>      = l-Mon / l-Tue / l-Wed
898               / l-Thu / l-Fri / l-Sat / l-Sun
899  <x:ref>month</x:ref>        = s-Jan / s-Feb / s-Mar / s-Apr
900               / s-May / s-Jun / s-Jul / s-Aug
901               / s-Sep / s-Oct / s-Nov / s-Dec
902               
903  GMT   = <x:abnf-char-sequence>"GMT"</x:abnf-char-sequence> ; "GMT", case-sensitive
904
905  s-Mon = <x:abnf-char-sequence>"Mon"</x:abnf-char-sequence> ; "Mon", case-sensitive
906  s-Tue = <x:abnf-char-sequence>"Tue"</x:abnf-char-sequence> ; "Tue", case-sensitive
907  s-Wed = <x:abnf-char-sequence>"Wed"</x:abnf-char-sequence> ; "Wed", case-sensitive
908  s-Thu = <x:abnf-char-sequence>"Thu"</x:abnf-char-sequence> ; "Thu", case-sensitive
909  s-Fri = <x:abnf-char-sequence>"Fri"</x:abnf-char-sequence> ; "Fri", case-sensitive
910  s-Sat = <x:abnf-char-sequence>"Sat"</x:abnf-char-sequence> ; "Sat", case-sensitive
911  s-Sun = <x:abnf-char-sequence>"Sun"</x:abnf-char-sequence> ; "Sun", case-sensitive
912
913  l-Mon = <x:abnf-char-sequence>"Monday"</x:abnf-char-sequence>          ; "Monday", case-sensitive
914  l-Tue = <x:abnf-char-sequence>"Tuesday"</x:abnf-char-sequence>       ; "Tuesday", case-sensitive
915  l-Wed = <x:abnf-char-sequence>"Wednesday"</x:abnf-char-sequence> ; "Wednesday", case-sensitive
916  l-Thu = <x:abnf-char-sequence>"Thursday"</x:abnf-char-sequence>    ; "Thursday", case-sensitive
917  l-Fri = <x:abnf-char-sequence>"Friday"</x:abnf-char-sequence>          ; "Friday", case-sensitive
918  l-Sat = <x:abnf-char-sequence>"Saturday"</x:abnf-char-sequence>    ; "Saturday", case-sensitive
919  l-Sun = <x:abnf-char-sequence>"Sunday"</x:abnf-char-sequence>          ; "Sunday", case-sensitive
920
921  s-Jan = <x:abnf-char-sequence>"Jan"</x:abnf-char-sequence> ; "Jan", case-sensitive
922  s-Feb = <x:abnf-char-sequence>"Feb"</x:abnf-char-sequence> ; "Feb", case-sensitive
923  s-Mar = <x:abnf-char-sequence>"Mar"</x:abnf-char-sequence> ; "Mar", case-sensitive
924  s-Apr = <x:abnf-char-sequence>"Apr"</x:abnf-char-sequence> ; "Apr", case-sensitive
925  s-May = <x:abnf-char-sequence>"May"</x:abnf-char-sequence> ; "May", case-sensitive
926  s-Jun = <x:abnf-char-sequence>"Jun"</x:abnf-char-sequence> ; "Jun", case-sensitive
927  s-Jul = <x:abnf-char-sequence>"Jul"</x:abnf-char-sequence> ; "Jul", case-sensitive
928  s-Aug = <x:abnf-char-sequence>"Aug"</x:abnf-char-sequence> ; "Aug", case-sensitive
929  s-Sep = <x:abnf-char-sequence>"Sep"</x:abnf-char-sequence> ; "Sep", case-sensitive
930  s-Oct = <x:abnf-char-sequence>"Oct"</x:abnf-char-sequence> ; "Oct", case-sensitive
931  s-Nov = <x:abnf-char-sequence>"Nov"</x:abnf-char-sequence> ; "Nov", case-sensitive
932  s-Dec = <x:abnf-char-sequence>"Dec"</x:abnf-char-sequence> ; "Dec", case-sensitive
933</artwork></figure>
934<t>
935      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
936      to their usage within the protocol stream. Clients and servers are
937      not required to use these formats for user presentation, request
938      logging, etc.
939</t>
940</section>
941</section>
942
943<section title="Transfer Codings" anchor="transfer.codings">
944  <x:anchor-alias value="parameter"/>
945  <x:anchor-alias value="transfer-coding"/>
946  <x:anchor-alias value="transfer-extension"/>
947<t>
948   Transfer-coding values are used to indicate an encoding
949   transformation that has been, can be, or may need to be applied to an
950   entity-body in order to ensure "safe transport" through the network.
951   This differs from a content coding in that the transfer-coding is a
952   property of the message, not of the original entity.
953</t>
954<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
955  <x:ref>transfer-coding</x:ref>         = "chunked" / <x:ref>transfer-extension</x:ref>
956  <x:ref>transfer-extension</x:ref>      = <x:ref>token</x:ref> *( OWS ";" OWS <x:ref>parameter</x:ref> )
957</artwork></figure>
958<t anchor="rule.parameter">
959  <x:anchor-alias value="attribute"/>
960  <x:anchor-alias value="parameter"/>
961  <x:anchor-alias value="value"/>
962   Parameters are in  the form of attribute/value pairs.
963</t>
964<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"/>
965  <x:ref>parameter</x:ref>               = <x:ref>attribute</x:ref> BWS "=" BWS <x:ref>value</x:ref>
966  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
967  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
968</artwork></figure>
969<t>
970   All transfer-coding values are case-insensitive. HTTP/1.1 uses
971   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
972   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
973</t>
974<t>
975   Whenever a transfer-coding is applied to a message-body, the set of
976   transfer-codings &MUST; include "chunked", unless the message indicates it
977   is terminated by closing the connection. When the "chunked" transfer-coding
978   is used, it &MUST; be the last transfer-coding applied to the
979   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
980   than once to a message-body. These rules allow the recipient to
981   determine the transfer-length of the message (<xref target="message.length"/>).
982</t>
983<t>
984   Transfer-codings are analogous to the Content-Transfer-Encoding
985   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
986   binary data over a 7-bit transport service. However, safe transport
987   has a different focus for an 8bit-clean transfer protocol. In HTTP,
988   the only unsafe characteristic of message-bodies is the difficulty in
989   determining the exact body length (<xref target="message.length"/>), or the desire to
990   encrypt data over a shared transport.
991</t>
992<t>
993   The Internet Assigned Numbers Authority (IANA) acts as a registry for
994   transfer-coding value tokens. Initially, the registry contains the
995   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
996   "gzip", "compress", and "deflate" (&content-codings;).
997</t>
998<t>
999   New transfer-coding value tokens &SHOULD; be registered in the same way
1000   as new content-coding value tokens (&content-codings;).
1001</t>
1002<t>
1003   A server which receives an entity-body with a transfer-coding it does
1004   not understand &SHOULD; return 501 (Not Implemented), and close the
1005   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1006   client.
1007</t>
1008
1009<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1010  <x:anchor-alias value="chunk"/>
1011  <x:anchor-alias value="Chunked-Body"/>
1012  <x:anchor-alias value="chunk-data"/>
1013  <x:anchor-alias value="chunk-ext"/>
1014  <x:anchor-alias value="chunk-ext-name"/>
1015  <x:anchor-alias value="chunk-ext-val"/>
1016  <x:anchor-alias value="chunk-size"/>
1017  <x:anchor-alias value="last-chunk"/>
1018  <x:anchor-alias value="trailer-part"/>
1019<t>
1020   The chunked encoding modifies the body of a message in order to
1021   transfer it as a series of chunks, each with its own size indicator,
1022   followed by an &OPTIONAL; trailer containing entity-header fields. This
1023   allows dynamically produced content to be transferred along with the
1024   information necessary for the recipient to verify that it has
1025   received the full message.
1026</t>
1027<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-ext"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer-part"/>
1028  <x:ref>Chunked-Body</x:ref>   = *<x:ref>chunk</x:ref>
1029                   <x:ref>last-chunk</x:ref>
1030                   <x:ref>trailer-part</x:ref>
1031                   <x:ref>CRLF</x:ref>
1032 
1033  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1034                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1035  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1036  <x:ref>last-chunk</x:ref>     = 1*("0") *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1037 
1038  <x:ref>chunk-ext</x:ref>      = *( ";" *WSP <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] *WSP )
1039  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1040  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1041  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1042  <x:ref>trailer-part</x:ref>   = *(<x:ref>entity-header</x:ref> <x:ref>CRLF</x:ref>)
1043</artwork></figure>
1044<t>
1045   The chunk-size field is a string of hex digits indicating the size of
1046   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1047   zero, followed by the trailer, which is terminated by an empty line.
1048</t>
1049<t>
1050   The trailer allows the sender to include additional HTTP header
1051   fields at the end of the message. The Trailer header field can be
1052   used to indicate which header fields are included in a trailer (see
1053   <xref target="header.trailer"/>).
1054</t>
1055<t>
1056   A server using chunked transfer-coding in a response &MUST-NOT; use the
1057   trailer for any header fields unless at least one of the following is
1058   true:
1059  <list style="numbers">
1060    <t>the request included a TE header field that indicates "trailers" is
1061     acceptable in the transfer-coding of the  response, as described in
1062     <xref target="header.te"/>; or,</t>
1063
1064    <t>the server is the origin server for the response, the trailer
1065     fields consist entirely of optional metadata, and the recipient
1066     could use the message (in a manner acceptable to the origin server)
1067     without receiving this metadata.  In other words, the origin server
1068     is willing to accept the possibility that the trailer fields might
1069     be silently discarded along the path to the client.</t>
1070  </list>
1071</t>
1072<t>
1073   This requirement prevents an interoperability failure when the
1074   message is being received by an HTTP/1.1 (or later) proxy and
1075   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1076   compliance with the protocol would have necessitated a possibly
1077   infinite buffer on the proxy.
1078</t>
1079<t>
1080   A process for decoding the "chunked" transfer-coding
1081   can be represented in pseudo-code as:
1082</t>
1083<figure><artwork type="code">
1084  length := 0
1085  read chunk-size, chunk-ext (if any) and CRLF
1086  while (chunk-size &gt; 0) {
1087     read chunk-data and CRLF
1088     append chunk-data to entity-body
1089     length := length + chunk-size
1090     read chunk-size and CRLF
1091  }
1092  read entity-header
1093  while (entity-header not empty) {
1094     append entity-header to existing header fields
1095     read entity-header
1096  }
1097  Content-Length := length
1098  Remove "chunked" from Transfer-Encoding
1099</artwork></figure>
1100<t>
1101   All HTTP/1.1 applications &MUST; be able to receive and decode the
1102   "chunked" transfer-coding, and &MUST; ignore chunk-ext extensions
1103   they do not understand.
1104</t>
1105</section>
1106</section>
1107
1108<section title="Product Tokens" anchor="product.tokens">
1109  <x:anchor-alias value="product"/>
1110  <x:anchor-alias value="product-version"/>
1111<t>
1112   Product tokens are used to allow communicating applications to
1113   identify themselves by software name and version. Most fields using
1114   product tokens also allow sub-products which form a significant part
1115   of the application to be listed, separated by white space. By
1116   convention, the products are listed in order of their significance
1117   for identifying the application.
1118</t>
1119<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1120  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
1121  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
1122</artwork></figure>
1123<t>
1124   Examples:
1125</t>
1126<figure><artwork type="example">
1127    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1128    Server: Apache/0.8.4
1129</artwork></figure>
1130<t>
1131   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1132   used for advertising or other non-essential information. Although any
1133   token character &MAY; appear in a product-version, this token &SHOULD;
1134   only be used for a version identifier (i.e., successive versions of
1135   the same product &SHOULD; only differ in the product-version portion of
1136   the product value).
1137</t>
1138</section>
1139
1140</section>
1141
1142<section title="HTTP Message" anchor="http.message">
1143
1144<section title="Message Types" anchor="message.types">
1145  <x:anchor-alias value="generic-message"/>
1146  <x:anchor-alias value="HTTP-message"/>
1147  <x:anchor-alias value="start-line"/>
1148<t>
1149   HTTP messages consist of requests from client to server and responses
1150   from server to client.
1151</t>
1152<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1153  <x:ref>HTTP-message</x:ref>   = <x:ref>Request</x:ref> / <x:ref>Response</x:ref>     ; HTTP/1.1 messages
1154</artwork></figure>
1155<t>
1156   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1157   message format of <xref target="RFC5322"/> for transferring entities (the payload
1158   of the message). Both types of message consist of a start-line, zero
1159   or more header fields (also known as "headers"), an empty line (i.e.,
1160   a line with nothing preceding the CRLF) indicating the end of the
1161   header fields, and possibly a message-body.
1162</t>
1163<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1164  <x:ref>generic-message</x:ref> = <x:ref>start-line</x:ref>
1165                    *(<x:ref>message-header</x:ref> <x:ref>CRLF</x:ref>)
1166                    <x:ref>CRLF</x:ref>
1167                    [ <x:ref>message-body</x:ref> ]
1168  <x:ref>start-line</x:ref>      = <x:ref>Request-Line</x:ref> / <x:ref>Status-Line</x:ref>
1169</artwork></figure>
1170<t>
1171   In the interest of robustness, servers &SHOULD; ignore any empty
1172   line(s) received where a Request-Line is expected. In other words, if
1173   the server is reading the protocol stream at the beginning of a
1174   message and receives a CRLF first, it should ignore the CRLF.
1175</t>
1176<t>
1177   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1178   after a POST request. To restate what is explicitly forbidden by the
1179   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1180   extra CRLF.
1181</t>
1182</section>
1183
1184<section title="Message Headers" anchor="message.headers">
1185  <x:anchor-alias value="field-content"/>
1186  <x:anchor-alias value="field-name"/>
1187  <x:anchor-alias value="field-value"/>
1188  <x:anchor-alias value="message-header"/>
1189<t>
1190   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1191   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1192   entity-header (&entity-header-fields;) fields, follow the same generic format as
1193   that given in <xref target="RFC5322" x:fmt="of" x:sec="2.1"/>. Each header field consists
1194   of a name followed by a colon (":") and the field value. Field names
1195   are case-insensitive. The field value &MAY; be preceded by any amount
1196   of LWS, though a single SP is preferred. Header fields can be
1197   extended over multiple lines by preceding each extra line with at
1198   least one SP or HTAB. Applications ought to follow "common form", where
1199   one is known or indicated, when generating HTTP constructs, since
1200   there might exist some implementations that fail to accept anything
1201   beyond the common forms.
1202</t>
1203<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"/>
1204  <x:ref>message-header</x:ref> = <x:ref>field-name</x:ref> ":" [ <x:ref>field-value</x:ref> ]
1205  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1206  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>LWS</x:ref> )
1207  <x:ref>field-content</x:ref>  = &lt;field content&gt;
1208                   ; the <x:ref>OCTET</x:ref>s making up the field-value
1209                   ; and consisting of either *<x:ref>TEXT</x:ref> or combinations
1210                   ; of <x:ref>token</x:ref>, <x:ref>separators</x:ref>, and <x:ref>quoted-string</x:ref>
1211</artwork></figure>
1212<t>
1213  <cref>whitespace between field-name and colon is an error and MUST NOT be accepted</cref>
1214</t>
1215<t>
1216   The field-content does not include any leading or trailing LWS:
1217   linear white space occurring before the first non-whitespace
1218   character of the field-value or after the last non-whitespace
1219   character of the field-value. Such leading or trailing LWS &MAY; be
1220   removed without changing the semantics of the field value. Any LWS
1221   that occurs between field-content &MAY; be replaced with a single SP
1222   before interpreting the field value or forwarding the message
1223   downstream.
1224</t>
1225<t>
1226   The order in which header fields with differing field names are
1227   received is not significant. However, it is "good practice" to send
1228   general-header fields first, followed by request-header or response-header
1229   fields, and ending with the entity-header fields.
1230</t>
1231<t>
1232   Multiple message-header fields with the same field-name &MAY; be
1233   present in a message if and only if the entire field-value for that
1234   header field is defined as a comma-separated list [i.e., #(values)].
1235   It &MUST; be possible to combine the multiple header fields into one
1236   "field-name: field-value" pair, without changing the semantics of the
1237   message, by appending each subsequent field-value to the first, each
1238   separated by a comma. The order in which header fields with the same
1239   field-name are received is therefore significant to the
1240   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1241   change the order of these field values when a message is forwarded.
1242</t>
1243<t>
1244  <list><t>
1245   <x:h>Note:</x:h> the "Set-Cookie" header as implemented in
1246   practice (as opposed to how it is specified in <xref target="RFC2109"/>)
1247   can occur multiple times, but does not use the list syntax, and thus cannot
1248   be combined into a single line. (See Appendix A.2.3 of <xref target="Kri2001"/>
1249   for details.) Also note that the Set-Cookie2 header specified in
1250   <xref target="RFC2965"/> does not share this problem.
1251  </t></list>
1252</t>
1253 
1254</section>
1255
1256<section title="Message Body" anchor="message.body">
1257  <x:anchor-alias value="message-body"/>
1258<t>
1259   The message-body (if any) of an HTTP message is used to carry the
1260   entity-body associated with the request or response. The message-body
1261   differs from the entity-body only when a transfer-coding has been
1262   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1263</t>
1264<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1265  <x:ref>message-body</x:ref> = <x:ref>entity-body</x:ref>
1266               / &lt;entity-body encoded as per <x:ref>Transfer-Encoding</x:ref>&gt;
1267</artwork></figure>
1268<t>
1269   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1270   applied by an application to ensure safe and proper transfer of the
1271   message. Transfer-Encoding is a property of the message, not of the
1272   entity, and thus &MAY; be added or removed by any application along the
1273   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1274   when certain transfer-codings may be used.)
1275</t>
1276<t>
1277   The rules for when a message-body is allowed in a message differ for
1278   requests and responses.
1279</t>
1280<t>
1281   The presence of a message-body in a request is signaled by the
1282   inclusion of a Content-Length or Transfer-Encoding header field in
1283   the request's message-headers. A message-body &MUST-NOT; be included in
1284   a request if the specification of the request method (&method;)
1285   explicitly disallows an entity-body in requests.
1286   When a request message contains both a message-body of non-zero
1287   length and a method that does not define any semantics for that
1288   request message-body, then an origin server &SHOULD; either ignore
1289   the message-body or respond with an appropriate error message
1290   (e.g., 413).  A proxy or gateway, when presented the same request,
1291   &SHOULD; either forward the request inbound with the message-body or
1292   ignore the message-body when determining a response.
1293</t>
1294<t>
1295   For response messages, whether or not a message-body is included with
1296   a message is dependent on both the request method and the response
1297   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1298   &MUST-NOT; include a message-body, even though the presence of entity-header
1299   fields might lead one to believe they do. All 1xx
1300   (informational), 204 (No Content), and 304 (Not Modified) responses
1301   &MUST-NOT; include a message-body. All other responses do include a
1302   message-body, although it &MAY; be of zero length.
1303</t>
1304</section>
1305
1306<section title="Message Length" anchor="message.length">
1307<t>
1308   The transfer-length of a message is the length of the message-body as
1309   it appears in the message; that is, after any transfer-codings have
1310   been applied. When a message-body is included with a message, the
1311   transfer-length of that body is determined by one of the following
1312   (in order of precedence):
1313</t>
1314<t>
1315  <list style="numbers">
1316    <x:lt><t>
1317     Any response message which "&MUST-NOT;" include a message-body (such
1318     as the 1xx, 204, and 304 responses and any response to a HEAD
1319     request) is always terminated by the first empty line after the
1320     header fields, regardless of the entity-header fields present in
1321     the message.
1322    </t></x:lt>
1323    <x:lt><t>
1324     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1325     is present and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
1326     is used, the transfer-length is defined by the use of this transfer-coding.
1327     If a Transfer-Encoding header field is present and the "chunked" transfer-coding
1328     is not present, the transfer-length is defined by the sender closing the connection.
1329    </t></x:lt>
1330    <x:lt><t>
1331     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1332     decimal value in OCTETs represents both the entity-length and the
1333     transfer-length. The Content-Length header field &MUST-NOT; be sent
1334     if these two lengths are different (i.e., if a Transfer-Encoding
1335     header field is present). If a message is received with both a
1336     Transfer-Encoding header field and a Content-Length header field,
1337     the latter &MUST; be ignored.
1338    </t></x:lt>
1339    <x:lt><t>
1340     If the message uses the media type "multipart/byteranges", and the
1341     transfer-length is not otherwise specified, then this self-delimiting
1342     media type defines the transfer-length. This media type
1343     &MUST-NOT; be used unless the sender knows that the recipient can parse
1344     it; the presence in a request of a Range header with multiple byte-range
1345     specifiers from a 1.1 client implies that the client can parse
1346     multipart/byteranges responses.
1347    <list style="empty"><t>
1348       A range header might be forwarded by a 1.0 proxy that does not
1349       understand multipart/byteranges; in this case the server &MUST;
1350       delimit the message using methods defined in items 1, 3 or 5 of
1351       this section.
1352    </t></list>
1353    </t></x:lt>
1354    <x:lt><t>
1355     By the server closing the connection. (Closing the connection
1356     cannot be used to indicate the end of a request body, since that
1357     would leave no possibility for the server to send back a response.)
1358    </t></x:lt>
1359  </list>
1360</t>
1361<t>
1362   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1363   containing a message-body &MUST; include a valid Content-Length header
1364   field unless the server is known to be HTTP/1.1 compliant. If a
1365   request contains a message-body and a Content-Length is not given,
1366   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1367   determine the length of the message, or with 411 (Length Required) if
1368   it wishes to insist on receiving a valid Content-Length.
1369</t>
1370<t>
1371   All HTTP/1.1 applications that receive entities &MUST; accept the
1372   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1373   to be used for messages when the message length cannot be determined
1374   in advance.
1375</t>
1376<t>
1377   Messages &MUST-NOT; include both a Content-Length header field and a
1378   transfer-coding. If the message does include a
1379   transfer-coding, the Content-Length &MUST; be ignored.
1380</t>
1381<t>
1382   When a Content-Length is given in a message where a message-body is
1383   allowed, its field value &MUST; exactly match the number of OCTETs in
1384   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1385   invalid length is received and detected.
1386</t>
1387</section>
1388
1389<section title="General Header Fields" anchor="general.header.fields">
1390  <x:anchor-alias value="general-header"/>
1391<t>
1392   There are a few header fields which have general applicability for
1393   both request and response messages, but which do not apply to the
1394   entity being transferred. These header fields apply only to the
1395   message being transmitted.
1396</t>
1397<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1398  <x:ref>general-header</x:ref> = <x:ref>Cache-Control</x:ref>            ; &header-cache-control;
1399                 / <x:ref>Connection</x:ref>               ; <xref target="header.connection"/>
1400                 / <x:ref>Date</x:ref>                     ; <xref target="header.date"/>
1401                 / <x:ref>Pragma</x:ref>                   ; &header-pragma;
1402                 / <x:ref>Trailer</x:ref>                  ; <xref target="header.trailer"/>
1403                 / <x:ref>Transfer-Encoding</x:ref>        ; <xref target="header.transfer-encoding"/>
1404                 / <x:ref>Upgrade</x:ref>                  ; <xref target="header.upgrade"/>
1405                 / <x:ref>Via</x:ref>                      ; <xref target="header.via"/>
1406                 / <x:ref>Warning</x:ref>                  ; &header-warning;
1407</artwork></figure>
1408<t>
1409   General-header field names can be extended reliably only in
1410   combination with a change in the protocol version. However, new or
1411   experimental header fields may be given the semantics of general
1412   header fields if all parties in the communication recognize them to
1413   be general-header fields. Unrecognized header fields are treated as
1414   entity-header fields.
1415</t>
1416</section>
1417</section>
1418
1419<section title="Request" anchor="request">
1420  <x:anchor-alias value="Request"/>
1421<t>
1422   A request message from a client to a server includes, within the
1423   first line of that message, the method to be applied to the resource,
1424   the identifier of the resource, and the protocol version in use.
1425</t>
1426<!--                 Host                      ; should be moved here eventually -->
1427<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1428  <x:ref>Request</x:ref>       = <x:ref>Request-Line</x:ref>              ; <xref target="request-line"/>
1429                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1430                   / <x:ref>request-header</x:ref>         ; &request-header-fields;
1431                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1432                  <x:ref>CRLF</x:ref>
1433                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1434</artwork></figure>
1435
1436<section title="Request-Line" anchor="request-line">
1437  <x:anchor-alias value="Request-Line"/>
1438<t>
1439   The Request-Line begins with a method token, followed by the
1440   Request-URI and the protocol version, and ending with CRLF. The
1441   elements are separated by SP characters. No CR or LF is allowed
1442   except in the final CRLF sequence.
1443</t>
1444<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1445  <x:ref>Request-Line</x:ref>   = <x:ref>Method</x:ref> <x:ref>SP</x:ref> <x:ref>Request-URI</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-Version</x:ref> <x:ref>CRLF</x:ref>
1446</artwork></figure>
1447
1448<section title="Method" anchor="method">
1449  <x:anchor-alias value="Method"/>
1450<t>
1451   The Method  token indicates the method to be performed on the
1452   resource identified by the Request-URI. The method is case-sensitive.
1453</t>
1454<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1455  <x:ref>Method</x:ref>         = <x:ref>token</x:ref>
1456</artwork></figure>
1457</section>
1458
1459<section title="Request-URI" anchor="request-uri">
1460  <x:anchor-alias value="Request-URI"/>
1461<t>
1462   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1463   identifies the resource upon which to apply the request.
1464</t>
1465<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1466  <x:ref>Request-URI</x:ref>    = "*"
1467                 / <x:ref>absoluteURI</x:ref>
1468                 / ( <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ] )
1469                 / <x:ref>authority</x:ref>
1470</artwork></figure>
1471<t>
1472   The four options for Request-URI are dependent on the nature of the
1473   request. The asterisk "*" means that the request does not apply to a
1474   particular resource, but to the server itself, and is only allowed
1475   when the method used does not necessarily apply to a resource. One
1476   example would be
1477</t>
1478<figure><artwork type="example">
1479    OPTIONS * HTTP/1.1
1480</artwork></figure>
1481<t>
1482   The absoluteURI form is &REQUIRED; when the request is being made to a
1483   proxy. The proxy is requested to forward the request or service it
1484   from a valid cache, and return the response. Note that the proxy &MAY;
1485   forward the request on to another proxy or directly to the server
1486   specified by the absoluteURI. In order to avoid request loops, a
1487   proxy &MUST; be able to recognize all of its server names, including
1488   any aliases, local variations, and the numeric IP address. An example
1489   Request-Line would be:
1490</t>
1491<figure><artwork type="example">
1492    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1493</artwork></figure>
1494<t>
1495   To allow for transition to absoluteURIs in all requests in future
1496   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
1497   form in requests, even though HTTP/1.1 clients will only generate
1498   them in requests to proxies.
1499</t>
1500<t>
1501   The authority form is only used by the CONNECT method (&CONNECT;).
1502</t>
1503<t>
1504   The most common form of Request-URI is that used to identify a
1505   resource on an origin server or gateway. In this case the absolute
1506   path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, path-absolute) as
1507   the Request-URI, and the network location of the URI (authority) &MUST;
1508   be transmitted in a Host header field. For example, a client wishing
1509   to retrieve the resource above directly from the origin server would
1510   create a TCP connection to port 80 of the host "www.example.org" and send
1511   the lines:
1512</t>
1513<figure><artwork type="example">
1514    GET /pub/WWW/TheProject.html HTTP/1.1
1515    Host: www.example.org
1516</artwork></figure>
1517<t>
1518   followed by the remainder of the Request. Note that the absolute path
1519   cannot be empty; if none is present in the original URI, it &MUST; be
1520   given as "/" (the server root).
1521</t>
1522<t>
1523   The Request-URI is transmitted in the format specified in
1524   <xref target="general.syntax"/>. If the Request-URI is encoded using the
1525   "% <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding
1526   (<xref target="RFC2396" x:fmt="," x:sec="2.4.1"/>), the origin server
1527   &MUST; decode the Request-URI in order to
1528   properly interpret the request. Servers &SHOULD; respond to invalid
1529   Request-URIs with an appropriate status code.
1530</t>
1531<t>
1532   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1533   received Request-URI when forwarding it to the next inbound server,
1534   except as noted above to replace a null path-absolute with "/".
1535</t>
1536<t>
1537  <list><t>
1538      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1539      meaning of the request when the origin server is improperly using
1540      a non-reserved URI character for a reserved purpose.  Implementors
1541      should be aware that some pre-HTTP/1.1 proxies have been known to
1542      rewrite the Request-URI.
1543  </t></list>
1544</t>
1545</section>
1546</section>
1547
1548<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1549<t>
1550   The exact resource identified by an Internet request is determined by
1551   examining both the Request-URI and the Host header field.
1552</t>
1553<t>
1554   An origin server that does not allow resources to differ by the
1555   requested host &MAY; ignore the Host header field value when
1556   determining the resource identified by an HTTP/1.1 request. (But see
1557   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1558   for other requirements on Host support in HTTP/1.1.)
1559</t>
1560<t>
1561   An origin server that does differentiate resources based on the host
1562   requested (sometimes referred to as virtual hosts or vanity host
1563   names) &MUST; use the following rules for determining the requested
1564   resource on an HTTP/1.1 request:
1565  <list style="numbers">
1566    <t>If Request-URI is an absoluteURI, the host is part of the
1567     Request-URI. Any Host header field value in the request &MUST; be
1568     ignored.</t>
1569    <t>If the Request-URI is not an absoluteURI, and the request includes
1570     a Host header field, the host is determined by the Host header
1571     field value.</t>
1572    <t>If the host as determined by rule 1 or 2 is not a valid host on
1573     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1574  </list>
1575</t>
1576<t>
1577   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1578   attempt to use heuristics (e.g., examination of the URI path for
1579   something unique to a particular host) in order to determine what
1580   exact resource is being requested.
1581</t>
1582</section>
1583
1584</section>
1585
1586
1587<section title="Response" anchor="response">
1588  <x:anchor-alias value="Response"/>
1589<t>
1590   After receiving and interpreting a request message, a server responds
1591   with an HTTP response message.
1592</t>
1593<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1594  <x:ref>Response</x:ref>      = <x:ref>Status-Line</x:ref>               ; <xref target="status-line"/>
1595                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1596                   / <x:ref>response-header</x:ref>        ; &response-header-fields;
1597                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1598                  <x:ref>CRLF</x:ref>
1599                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1600</artwork></figure>
1601
1602<section title="Status-Line" anchor="status-line">
1603  <x:anchor-alias value="Status-Line"/>
1604<t>
1605   The first line of a Response message is the Status-Line, consisting
1606   of the protocol version followed by a numeric status code and its
1607   associated textual phrase, with each element separated by SP
1608   characters. No CR or LF is allowed except in the final CRLF sequence.
1609</t>
1610<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1611  <x:ref>Status-Line</x:ref> = <x:ref>HTTP-Version</x:ref> <x:ref>SP</x:ref> <x:ref>Status-Code</x:ref> <x:ref>SP</x:ref> <x:ref>Reason-Phrase</x:ref> <x:ref>CRLF</x:ref>
1612</artwork></figure>
1613
1614<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1615  <x:anchor-alias value="Reason-Phrase"/>
1616  <x:anchor-alias value="Status-Code"/>
1617<t>
1618   The Status-Code element is a 3-digit integer result code of the
1619   attempt to understand and satisfy the request. These codes are fully
1620   defined in &status-codes;.  The Reason Phrase exists for the sole
1621   purpose of providing a textual description associated with the numeric
1622   status code, out of deference to earlier Internet application protocols
1623   that were more frequently used with interactive text clients.
1624   A client &SHOULD; ignore the content of the Reason Phrase.
1625</t>
1626<t>
1627   The first digit of the Status-Code defines the class of response. The
1628   last two digits do not have any categorization role. There are 5
1629   values for the first digit:
1630  <list style="symbols">
1631    <t>
1632      1xx: Informational - Request received, continuing process
1633    </t>
1634    <t>
1635      2xx: Success - The action was successfully received,
1636        understood, and accepted
1637    </t>
1638    <t>
1639      3xx: Redirection - Further action must be taken in order to
1640        complete the request
1641    </t>
1642    <t>
1643      4xx: Client Error - The request contains bad syntax or cannot
1644        be fulfilled
1645    </t>
1646    <t>
1647      5xx: Server Error - The server failed to fulfill an apparently
1648        valid request
1649    </t>
1650  </list>
1651</t>
1652<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"/>
1653  <x:ref>Status-Code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1654  <x:ref>Reason-Phrase</x:ref>  = *&lt;<x:ref>TEXT</x:ref>, excluding <x:ref>CR</x:ref>, <x:ref>LF</x:ref>&gt;
1655</artwork></figure>
1656</section>
1657</section>
1658
1659</section>
1660
1661
1662<section title="Connections" anchor="connections">
1663
1664<section title="Persistent Connections" anchor="persistent.connections">
1665
1666<section title="Purpose" anchor="persistent.purpose">
1667<t>
1668   Prior to persistent connections, a separate TCP connection was
1669   established to fetch each URL, increasing the load on HTTP servers
1670   and causing congestion on the Internet. The use of inline images and
1671   other associated data often require a client to make multiple
1672   requests of the same server in a short amount of time. Analysis of
1673   these performance problems and results from a prototype
1674   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1675   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1676   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1677   T/TCP <xref target="Tou1998"/>.
1678</t>
1679<t>
1680   Persistent HTTP connections have a number of advantages:
1681  <list style="symbols">
1682      <t>
1683        By opening and closing fewer TCP connections, CPU time is saved
1684        in routers and hosts (clients, servers, proxies, gateways,
1685        tunnels, or caches), and memory used for TCP protocol control
1686        blocks can be saved in hosts.
1687      </t>
1688      <t>
1689        HTTP requests and responses can be pipelined on a connection.
1690        Pipelining allows a client to make multiple requests without
1691        waiting for each response, allowing a single TCP connection to
1692        be used much more efficiently, with much lower elapsed time.
1693      </t>
1694      <t>
1695        Network congestion is reduced by reducing the number of packets
1696        caused by TCP opens, and by allowing TCP sufficient time to
1697        determine the congestion state of the network.
1698      </t>
1699      <t>
1700        Latency on subsequent requests is reduced since there is no time
1701        spent in TCP's connection opening handshake.
1702      </t>
1703      <t>
1704        HTTP can evolve more gracefully, since errors can be reported
1705        without the penalty of closing the TCP connection. Clients using
1706        future versions of HTTP might optimistically try a new feature,
1707        but if communicating with an older server, retry with old
1708        semantics after an error is reported.
1709      </t>
1710    </list>
1711</t>
1712<t>
1713   HTTP implementations &SHOULD; implement persistent connections.
1714</t>
1715</section>
1716
1717<section title="Overall Operation" anchor="persistent.overall">
1718<t>
1719   A significant difference between HTTP/1.1 and earlier versions of
1720   HTTP is that persistent connections are the default behavior of any
1721   HTTP connection. That is, unless otherwise indicated, the client
1722   &SHOULD; assume that the server will maintain a persistent connection,
1723   even after error responses from the server.
1724</t>
1725<t>
1726   Persistent connections provide a mechanism by which a client and a
1727   server can signal the close of a TCP connection. This signaling takes
1728   place using the Connection header field (<xref target="header.connection"/>). Once a close
1729   has been signaled, the client &MUST-NOT; send any more requests on that
1730   connection.
1731</t>
1732
1733<section title="Negotiation" anchor="persistent.negotiation">
1734<t>
1735   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1736   maintain a persistent connection unless a Connection header including
1737   the connection-token "close" was sent in the request. If the server
1738   chooses to close the connection immediately after sending the
1739   response, it &SHOULD; send a Connection header including the
1740   connection-token close.
1741</t>
1742<t>
1743   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1744   decide to keep it open based on whether the response from a server
1745   contains a Connection header with the connection-token close. In case
1746   the client does not want to maintain a connection for more than that
1747   request, it &SHOULD; send a Connection header including the
1748   connection-token close.
1749</t>
1750<t>
1751   If either the client or the server sends the close token in the
1752   Connection header, that request becomes the last one for the
1753   connection.
1754</t>
1755<t>
1756   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1757   maintained for HTTP versions less than 1.1 unless it is explicitly
1758   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1759   compatibility with HTTP/1.0 clients.
1760</t>
1761<t>
1762   In order to remain persistent, all messages on the connection &MUST;
1763   have a self-defined message length (i.e., one not defined by closure
1764   of the connection), as described in <xref target="message.length"/>.
1765</t>
1766</section>
1767
1768<section title="Pipelining" anchor="pipelining">
1769<t>
1770   A client that supports persistent connections &MAY; "pipeline" its
1771   requests (i.e., send multiple requests without waiting for each
1772   response). A server &MUST; send its responses to those requests in the
1773   same order that the requests were received.
1774</t>
1775<t>
1776   Clients which assume persistent connections and pipeline immediately
1777   after connection establishment &SHOULD; be prepared to retry their
1778   connection if the first pipelined attempt fails. If a client does
1779   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1780   persistent. Clients &MUST; also be prepared to resend their requests if
1781   the server closes the connection before sending all of the
1782   corresponding responses.
1783</t>
1784<t>
1785   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1786   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1787   premature termination of the transport connection could lead to
1788   indeterminate results. A client wishing to send a non-idempotent
1789   request &SHOULD; wait to send that request until it has received the
1790   response status for the previous request.
1791</t>
1792</section>
1793</section>
1794
1795<section title="Proxy Servers" anchor="persistent.proxy">
1796<t>
1797   It is especially important that proxies correctly implement the
1798   properties of the Connection header field as specified in <xref target="header.connection"/>.
1799</t>
1800<t>
1801   The proxy server &MUST; signal persistent connections separately with
1802   its clients and the origin servers (or other proxy servers) that it
1803   connects to. Each persistent connection applies to only one transport
1804   link.
1805</t>
1806<t>
1807   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1808   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
1809   discussion of the problems with the Keep-Alive header implemented by
1810   many HTTP/1.0 clients).
1811</t>
1812</section>
1813
1814<section title="Practical Considerations" anchor="persistent.practical">
1815<t>
1816   Servers will usually have some time-out value beyond which they will
1817   no longer maintain an inactive connection. Proxy servers might make
1818   this a higher value since it is likely that the client will be making
1819   more connections through the same server. The use of persistent
1820   connections places no requirements on the length (or existence) of
1821   this time-out for either the client or the server.
1822</t>
1823<t>
1824   When a client or server wishes to time-out it &SHOULD; issue a graceful
1825   close on the transport connection. Clients and servers &SHOULD; both
1826   constantly watch for the other side of the transport close, and
1827   respond to it as appropriate. If a client or server does not detect
1828   the other side's close promptly it could cause unnecessary resource
1829   drain on the network.
1830</t>
1831<t>
1832   A client, server, or proxy &MAY; close the transport connection at any
1833   time. For example, a client might have started to send a new request
1834   at the same time that the server has decided to close the "idle"
1835   connection. From the server's point of view, the connection is being
1836   closed while it was idle, but from the client's point of view, a
1837   request is in progress.
1838</t>
1839<t>
1840   This means that clients, servers, and proxies &MUST; be able to recover
1841   from asynchronous close events. Client software &SHOULD; reopen the
1842   transport connection and retransmit the aborted sequence of requests
1843   without user interaction so long as the request sequence is
1844   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1845   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1846   human operator the choice of retrying the request(s). Confirmation by
1847   user-agent software with semantic understanding of the application
1848   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
1849   be repeated if the second sequence of requests fails.
1850</t>
1851<t>
1852   Servers &SHOULD; always respond to at least one request per connection,
1853   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1854   middle of transmitting a response, unless a network or client failure
1855   is suspected.
1856</t>
1857<t>
1858   Clients that use persistent connections &SHOULD; limit the number of
1859   simultaneous connections that they maintain to a given server. A
1860   single-user client &SHOULD-NOT; maintain more than 2 connections with
1861   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1862   another server or proxy, where N is the number of simultaneously
1863   active users. These guidelines are intended to improve HTTP response
1864   times and avoid congestion.
1865</t>
1866</section>
1867</section>
1868
1869<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1870
1871<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1872<t>
1873   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1874   flow control mechanisms to resolve temporary overloads, rather than
1875   terminating connections with the expectation that clients will retry.
1876   The latter technique can exacerbate network congestion.
1877</t>
1878</section>
1879
1880<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
1881<t>
1882   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
1883   the network connection for an error status while it is transmitting
1884   the request. If the client sees an error status, it &SHOULD;
1885   immediately cease transmitting the body. If the body is being sent
1886   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
1887   empty trailer &MAY; be used to prematurely mark the end of the message.
1888   If the body was preceded by a Content-Length header, the client &MUST;
1889   close the connection.
1890</t>
1891</section>
1892
1893<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
1894<t>
1895   The purpose of the 100 (Continue) status (see &status-100;) is to
1896   allow a client that is sending a request message with a request body
1897   to determine if the origin server is willing to accept the request
1898   (based on the request headers) before the client sends the request
1899   body. In some cases, it might either be inappropriate or highly
1900   inefficient for the client to send the body if the server will reject
1901   the message without looking at the body.
1902</t>
1903<t>
1904   Requirements for HTTP/1.1 clients:
1905  <list style="symbols">
1906    <t>
1907        If a client will wait for a 100 (Continue) response before
1908        sending the request body, it &MUST; send an Expect request-header
1909        field (&header-expect;) with the "100-continue" expectation.
1910    </t>
1911    <t>
1912        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
1913        with the "100-continue" expectation if it does not intend
1914        to send a request body.
1915    </t>
1916  </list>
1917</t>
1918<t>
1919   Because of the presence of older implementations, the protocol allows
1920   ambiguous situations in which a client may send "Expect: 100-continue"
1921   without receiving either a 417 (Expectation Failed) status
1922   or a 100 (Continue) status. Therefore, when a client sends this
1923   header field to an origin server (possibly via a proxy) from which it
1924   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
1925   for an indefinite period before sending the request body.
1926</t>
1927<t>
1928   Requirements for HTTP/1.1 origin servers:
1929  <list style="symbols">
1930    <t> Upon receiving a request which includes an Expect request-header
1931        field with the "100-continue" expectation, an origin server &MUST;
1932        either respond with 100 (Continue) status and continue to read
1933        from the input stream, or respond with a final status code. The
1934        origin server &MUST-NOT; wait for the request body before sending
1935        the 100 (Continue) response. If it responds with a final status
1936        code, it &MAY; close the transport connection or it &MAY; continue
1937        to read and discard the rest of the request.  It &MUST-NOT;
1938        perform the requested method if it returns a final status code.
1939    </t>
1940    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
1941        the request message does not include an Expect request-header
1942        field with the "100-continue" expectation, and &MUST-NOT; send a
1943        100 (Continue) response if such a request comes from an HTTP/1.0
1944        (or earlier) client. There is an exception to this rule: for
1945        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
1946        status in response to an HTTP/1.1 PUT or POST request that does
1947        not include an Expect request-header field with the "100-continue"
1948        expectation. This exception, the purpose of which is
1949        to minimize any client processing delays associated with an
1950        undeclared wait for 100 (Continue) status, applies only to
1951        HTTP/1.1 requests, and not to requests with any other HTTP-version
1952        value.
1953    </t>
1954    <t> An origin server &MAY; omit a 100 (Continue) response if it has
1955        already received some or all of the request body for the
1956        corresponding request.
1957    </t>
1958    <t> An origin server that sends a 100 (Continue) response &MUST;
1959    ultimately send a final status code, once the request body is
1960        received and processed, unless it terminates the transport
1961        connection prematurely.
1962    </t>
1963    <t> If an origin server receives a request that does not include an
1964        Expect request-header field with the "100-continue" expectation,
1965        the request includes a request body, and the server responds
1966        with a final status code before reading the entire request body
1967        from the transport connection, then the server &SHOULD-NOT;  close
1968        the transport connection until it has read the entire request,
1969        or until the client closes the connection. Otherwise, the client
1970        might not reliably receive the response message. However, this
1971        requirement is not be construed as preventing a server from
1972        defending itself against denial-of-service attacks, or from
1973        badly broken client implementations.
1974      </t>
1975    </list>
1976</t>
1977<t>
1978   Requirements for HTTP/1.1 proxies:
1979  <list style="symbols">
1980    <t> If a proxy receives a request that includes an Expect request-header
1981        field with the "100-continue" expectation, and the proxy
1982        either knows that the next-hop server complies with HTTP/1.1 or
1983        higher, or does not know the HTTP version of the next-hop
1984        server, it &MUST; forward the request, including the Expect header
1985        field.
1986    </t>
1987    <t> If the proxy knows that the version of the next-hop server is
1988        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
1989        respond with a 417 (Expectation Failed) status.
1990    </t>
1991    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
1992        numbers received from recently-referenced next-hop servers.
1993    </t>
1994    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
1995        request message was received from an HTTP/1.0 (or earlier)
1996        client and did not include an Expect request-header field with
1997        the "100-continue" expectation. This requirement overrides the
1998        general rule for forwarding of 1xx responses (see &status-1xx;).
1999    </t>
2000  </list>
2001</t>
2002</section>
2003
2004<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2005<t>
2006   If an HTTP/1.1 client sends a request which includes a request body,
2007   but which does not include an Expect request-header field with the
2008   "100-continue" expectation, and if the client is not directly
2009   connected to an HTTP/1.1 origin server, and if the client sees the
2010   connection close before receiving any status from the server, the
2011   client &SHOULD; retry the request.  If the client does retry this
2012   request, it &MAY; use the following "binary exponential backoff"
2013   algorithm to be assured of obtaining a reliable response:
2014  <list style="numbers">
2015    <t>
2016      Initiate a new connection to the server
2017    </t>
2018    <t>
2019      Transmit the request-headers
2020    </t>
2021    <t>
2022      Initialize a variable R to the estimated round-trip time to the
2023         server (e.g., based on the time it took to establish the
2024         connection), or to a constant value of 5 seconds if the round-trip
2025         time is not available.
2026    </t>
2027    <t>
2028       Compute T = R * (2**N), where N is the number of previous
2029         retries of this request.
2030    </t>
2031    <t>
2032       Wait either for an error response from the server, or for T
2033         seconds (whichever comes first)
2034    </t>
2035    <t>
2036       If no error response is received, after T seconds transmit the
2037         body of the request.
2038    </t>
2039    <t>
2040       If client sees that the connection is closed prematurely,
2041         repeat from step 1 until the request is accepted, an error
2042         response is received, or the user becomes impatient and
2043         terminates the retry process.
2044    </t>
2045  </list>
2046</t>
2047<t>
2048   If at any point an error status is received, the client
2049  <list style="symbols">
2050      <t>&SHOULD-NOT;  continue and</t>
2051
2052      <t>&SHOULD; close the connection if it has not completed sending the
2053        request message.</t>
2054    </list>
2055</t>
2056</section>
2057</section>
2058</section>
2059
2060
2061<section title="Header Field Definitions" anchor="header.fields">
2062<t>
2063   This section defines the syntax and semantics of HTTP/1.1 header fields
2064   related to message framing and transport protocols.
2065</t>
2066<t>
2067   For entity-header fields, both sender and recipient refer to either the
2068   client or the server, depending on who sends and who receives the entity.
2069</t>
2070
2071<section title="Connection" anchor="header.connection">
2072  <iref primary="true" item="Connection header" x:for-anchor=""/>
2073  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2074  <x:anchor-alias value="Connection"/>
2075  <x:anchor-alias value="connection-token"/>
2076  <x:anchor-alias value="Connection-v"/>
2077<t>
2078   The general-header field "Connection" allows the sender to specify
2079   options that are desired for that particular connection and &MUST-NOT;
2080   be communicated by proxies over further connections.
2081</t>
2082<t>
2083   The Connection header's value has the following grammar:
2084</t>
2085<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="Connection-v"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2086  <x:ref>Connection</x:ref>       = "Connection" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Connection-v</x:ref>
2087  <x:ref>Connection-v</x:ref>     = 1#<x:ref>connection-token</x:ref>
2088  <x:ref>connection-token</x:ref> = <x:ref>token</x:ref>
2089</artwork></figure>
2090<t>
2091   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2092   message is forwarded and, for each connection-token in this field,
2093   remove any header field(s) from the message with the same name as the
2094   connection-token. Connection options are signaled by the presence of
2095   a connection-token in the Connection header field, not by any
2096   corresponding additional header field(s), since the additional header
2097   field may not be sent if there are no parameters associated with that
2098   connection option.
2099</t>
2100<t>
2101   Message headers listed in the Connection header &MUST-NOT; include
2102   end-to-end headers, such as Cache-Control.
2103</t>
2104<t>
2105   HTTP/1.1 defines the "close" connection option for the sender to
2106   signal that the connection will be closed after completion of the
2107   response. For example,
2108</t>
2109<figure><artwork type="example">
2110  Connection: close
2111</artwork></figure>
2112<t>
2113   in either the request or the response header fields indicates that
2114   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2115   after the current request/response is complete.
2116</t>
2117<t>
2118   An HTTP/1.1 client that does not support persistent connections &MUST;
2119   include the "close" connection option in every request message.
2120</t>
2121<t>
2122   An HTTP/1.1 server that does not support persistent connections &MUST;
2123   include the "close" connection option in every response message that
2124   does not have a 1xx (informational) status code.
2125</t>
2126<t>
2127   A system receiving an HTTP/1.0 (or lower-version) message that
2128   includes a Connection header &MUST;, for each connection-token in this
2129   field, remove and ignore any header field(s) from the message with
2130   the same name as the connection-token. This protects against mistaken
2131   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2132</t>
2133</section>
2134
2135<section title="Content-Length" anchor="header.content-length">
2136  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2137  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2138  <x:anchor-alias value="Content-Length"/>
2139  <x:anchor-alias value="Content-Length-v"/>
2140<t>
2141   The entity-header field "Content-Length" indicates the size of the
2142   entity-body, in decimal number of OCTETs, sent to the recipient or,
2143   in the case of the HEAD method, the size of the entity-body that
2144   would have been sent had the request been a GET.
2145</t>
2146<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/><iref primary="true" item="Grammar" subitem="Content-Length-v"/>
2147  <x:ref>Content-Length</x:ref>   = "Content-Length" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> 1*<x:ref>Content-Length-v</x:ref>
2148  <x:ref>Content-Length-v</x:ref> = 1*<x:ref>DIGIT</x:ref>
2149</artwork></figure>
2150<t>
2151   An example is
2152</t>
2153<figure><artwork type="example">
2154  Content-Length: 3495
2155</artwork></figure>
2156<t>
2157   Applications &SHOULD; use this field to indicate the transfer-length of
2158   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2159</t>
2160<t>
2161   Any Content-Length greater than or equal to zero is a valid value.
2162   <xref target="message.length"/> describes how to determine the length of a message-body
2163   if a Content-Length is not given.
2164</t>
2165<t>
2166   Note that the meaning of this field is significantly different from
2167   the corresponding definition in MIME, where it is an optional field
2168   used within the "message/external-body" content-type. In HTTP, it
2169   &SHOULD; be sent whenever the message's length can be determined prior
2170   to being transferred, unless this is prohibited by the rules in
2171   <xref target="message.length"/>.
2172</t>
2173</section>
2174
2175<section title="Date" anchor="header.date">
2176  <iref primary="true" item="Date header" x:for-anchor=""/>
2177  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2178  <x:anchor-alias value="Date"/>
2179  <x:anchor-alias value="Date-v"/>
2180<t>
2181   The general-header field "Date" represents the date and time at which
2182   the message was originated, having the same semantics as orig-date in
2183   <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>. The field value is an
2184   HTTP-date, as described in <xref target="full.date"/>;
2185   it &MUST; be sent in rfc1123-date format.
2186</t>
2187<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/><iref primary="true" item="Grammar" subitem="Date-v"/>
2188  <x:ref>Date</x:ref>   = "Date" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Date-v</x:ref>
2189  <x:ref>Date-v</x:ref> = <x:ref>HTTP-date</x:ref>
2190</artwork></figure>
2191<t>
2192   An example is
2193</t>
2194<figure><artwork type="example">
2195  Date: Tue, 15 Nov 1994 08:12:31 GMT
2196</artwork></figure>
2197<t>
2198   Origin servers &MUST; include a Date header field in all responses,
2199   except in these cases:
2200  <list style="numbers">
2201      <t>If the response status code is 100 (Continue) or 101 (Switching
2202         Protocols), the response &MAY; include a Date header field, at
2203         the server's option.</t>
2204
2205      <t>If the response status code conveys a server error, e.g. 500
2206         (Internal Server Error) or 503 (Service Unavailable), and it is
2207         inconvenient or impossible to generate a valid Date.</t>
2208
2209      <t>If the server does not have a clock that can provide a
2210         reasonable approximation of the current time, its responses
2211         &MUST-NOT; include a Date header field. In this case, the rules
2212         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2213  </list>
2214</t>
2215<t>
2216   A received message that does not have a Date header field &MUST; be
2217   assigned one by the recipient if the message will be cached by that
2218   recipient or gatewayed via a protocol which requires a Date. An HTTP
2219   implementation without a clock &MUST-NOT; cache responses without
2220   revalidating them on every use. An HTTP cache, especially a shared
2221   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2222   clock with a reliable external standard.
2223</t>
2224<t>
2225   Clients &SHOULD; only send a Date header field in messages that include
2226   an entity-body, as in the case of the PUT and POST requests, and even
2227   then it is optional. A client without a clock &MUST-NOT; send a Date
2228   header field in a request.
2229</t>
2230<t>
2231   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2232   time subsequent to the generation of the message. It &SHOULD; represent
2233   the best available approximation of the date and time of message
2234   generation, unless the implementation has no means of generating a
2235   reasonably accurate date and time. In theory, the date ought to
2236   represent the moment just before the entity is generated. In
2237   practice, the date can be generated at any time during the message
2238   origination without affecting its semantic value.
2239</t>
2240
2241<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2242<t>
2243   Some origin server implementations might not have a clock available.
2244   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2245   values to a response, unless these values were associated
2246   with the resource by a system or user with a reliable clock. It &MAY;
2247   assign an Expires value that is known, at or before server
2248   configuration time, to be in the past (this allows "pre-expiration"
2249   of responses without storing separate Expires values for each
2250   resource).
2251</t>
2252</section>
2253</section>
2254
2255<section title="Host" anchor="header.host">
2256  <iref primary="true" item="Host header" x:for-anchor=""/>
2257  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2258  <x:anchor-alias value="Host"/>
2259  <x:anchor-alias value="Host-v"/>
2260<t>
2261   The request-header field "Host" specifies the Internet host and port
2262   number of the resource being requested, as obtained from the original
2263   URI given by the user or referring resource (generally an HTTP URL,
2264   as described in <xref target="http.url"/>). The Host field value &MUST; represent
2265   the naming authority of the origin server or gateway given by the
2266   original URL. This allows the origin server or gateway to
2267   differentiate between internally-ambiguous URLs, such as the root "/"
2268   URL of a server for multiple host names on a single IP address.
2269</t>
2270<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/><iref primary="true" item="Grammar" subitem="Host-v"/>
2271  <x:ref>Host</x:ref>   = "Host" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Host-v</x:ref>
2272  <x:ref>Host-v</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.url"/>
2273</artwork></figure>
2274<t>
2275   A "host" without any trailing port information implies the default
2276   port for the service requested (e.g., "80" for an HTTP URL). For
2277   example, a request on the origin server for
2278   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2279</t>
2280<figure><artwork type="example">
2281  GET /pub/WWW/ HTTP/1.1
2282  Host: www.example.org
2283</artwork></figure>
2284<t>
2285   A client &MUST; include a Host header field in all HTTP/1.1 request
2286   messages. If the requested URI does not include an Internet host
2287   name for the service being requested, then the Host header field &MUST;
2288   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2289   request message it forwards does contain an appropriate Host header
2290   field that identifies the service being requested by the proxy. All
2291   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2292   status code to any HTTP/1.1 request message which lacks a Host header
2293   field.
2294</t>
2295<t>
2296   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2297   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2298   for other requirements relating to Host.
2299</t>
2300</section>
2301
2302<section title="TE" anchor="header.te">
2303  <iref primary="true" item="TE header" x:for-anchor=""/>
2304  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2305  <x:anchor-alias value="TE"/>
2306  <x:anchor-alias value="TE-v"/>
2307  <x:anchor-alias value="t-codings"/>
2308<t>
2309   The request-header field "TE" indicates what extension transfer-codings
2310   it is willing to accept in the response and whether or not it is
2311   willing to accept trailer fields in a chunked transfer-coding. Its
2312   value may consist of the keyword "trailers" and/or a comma-separated
2313   list of extension transfer-coding names with optional accept
2314   parameters (as described in <xref target="transfer.codings"/>).
2315</t>
2316<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="TE-v"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2317  <x:ref>TE</x:ref>        = "TE" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>TE-v</x:ref>
2318  <x:ref>TE-v</x:ref>      = #<x:ref>t-codings</x:ref>
2319  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-extension</x:ref> [ <x:ref>accept-params</x:ref> ] )
2320</artwork></figure>
2321<t>
2322   The presence of the keyword "trailers" indicates that the client is
2323   willing to accept trailer fields in a chunked transfer-coding, as
2324   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2325   transfer-coding values even though it does not itself represent a
2326   transfer-coding.
2327</t>
2328<t>
2329   Examples of its use are:
2330</t>
2331<figure><artwork type="example">
2332  TE: deflate
2333  TE:
2334  TE: trailers, deflate;q=0.5
2335</artwork></figure>
2336<t>
2337   The TE header field only applies to the immediate connection.
2338   Therefore, the keyword &MUST; be supplied within a Connection header
2339   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2340</t>
2341<t>
2342   A server tests whether a transfer-coding is acceptable, according to
2343   a TE field, using these rules:
2344  <list style="numbers">
2345    <x:lt>
2346      <t>The "chunked" transfer-coding is always acceptable. If the
2347         keyword "trailers" is listed, the client indicates that it is
2348         willing to accept trailer fields in the chunked response on
2349         behalf of itself and any downstream clients. The implication is
2350         that, if given, the client is stating that either all
2351         downstream clients are willing to accept trailer fields in the
2352         forwarded response, or that it will attempt to buffer the
2353         response on behalf of downstream recipients.
2354      </t><t>
2355         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2356         chunked response such that a client can be assured of buffering
2357         the entire response.</t>
2358    </x:lt>
2359    <x:lt>
2360      <t>If the transfer-coding being tested is one of the transfer-codings
2361         listed in the TE field, then it is acceptable unless it
2362         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2363         qvalue of 0 means "not acceptable.")</t>
2364    </x:lt>
2365    <x:lt>
2366      <t>If multiple transfer-codings are acceptable, then the
2367         acceptable transfer-coding with the highest non-zero qvalue is
2368         preferred.  The "chunked" transfer-coding always has a qvalue
2369         of 1.</t>
2370    </x:lt>
2371  </list>
2372</t>
2373<t>
2374   If the TE field-value is empty or if no TE field is present, the only
2375   transfer-coding  is "chunked". A message with no transfer-coding is
2376   always acceptable.
2377</t>
2378</section>
2379
2380<section title="Trailer" anchor="header.trailer">
2381  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2382  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2383  <x:anchor-alias value="Trailer"/>
2384  <x:anchor-alias value="Trailer-v"/>
2385<t>
2386   The general field "Trailer" indicates that the given set of
2387   header fields is present in the trailer of a message encoded with
2388   chunked transfer-coding.
2389</t>
2390<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="true" item="Grammar" subitem="Trailer-v"/>
2391  <x:ref>Trailer</x:ref>   = "Trailer" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Trailer-v</x:ref>
2392  <x:ref>Trailer-v</x:ref> = 1#<x:ref>field-name</x:ref>
2393</artwork></figure>
2394<t>
2395   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2396   message using chunked transfer-coding with a non-empty trailer. Doing
2397   so allows the recipient to know which header fields to expect in the
2398   trailer.
2399</t>
2400<t>
2401   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2402   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2403   trailer fields in a "chunked" transfer-coding.
2404</t>
2405<t>
2406   Message header fields listed in the Trailer header field &MUST-NOT;
2407   include the following header fields:
2408  <list style="symbols">
2409    <t>Transfer-Encoding</t>
2410    <t>Content-Length</t>
2411    <t>Trailer</t>
2412  </list>
2413</t>
2414</section>
2415
2416<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2417  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2418  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2419  <x:anchor-alias value="Transfer-Encoding"/>
2420  <x:anchor-alias value="Transfer-Encoding-v"/>
2421<t>
2422   The general-header "Transfer-Encoding" field indicates what (if any)
2423   type of transformation has been applied to the message body in order
2424   to safely transfer it between the sender and the recipient. This
2425   differs from the content-coding in that the transfer-coding is a
2426   property of the message, not of the entity.
2427</t>
2428<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/><iref primary="true" item="Grammar" subitem="Transfer-Encoding-v"/>
2429  <x:ref>Transfer-Encoding</x:ref>   = "Transfer-Encoding" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Transfer-Encoding-v</x:ref>
2430  <x:ref>Transfer-Encoding-v</x:ref> = 1#<x:ref>transfer-coding</x:ref>
2431</artwork></figure>
2432<t>
2433   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2434</t>
2435<figure><artwork type="example">
2436  Transfer-Encoding: chunked
2437</artwork></figure>
2438<t>
2439   If multiple encodings have been applied to an entity, the transfer-codings
2440   &MUST; be listed in the order in which they were applied.
2441   Additional information about the encoding parameters &MAY; be provided
2442   by other entity-header fields not defined by this specification.
2443</t>
2444<t>
2445   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2446   header.
2447</t>
2448</section>
2449
2450<section title="Upgrade" anchor="header.upgrade">
2451  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2452  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2453  <x:anchor-alias value="Upgrade"/>
2454  <x:anchor-alias value="Upgrade-v"/>
2455<t>
2456   The general-header "Upgrade" allows the client to specify what
2457   additional communication protocols it supports and would like to use
2458   if the server finds it appropriate to switch protocols. The server
2459   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2460   response to indicate which protocol(s) are being switched.
2461</t>
2462<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/><iref primary="true" item="Grammar" subitem="Upgrade-v"/>
2463  <x:ref>Upgrade</x:ref>   = "Upgrade" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Upgrade-v</x:ref>
2464  <x:ref>Upgrade-v</x:ref> = 1#<x:ref>product</x:ref>
2465</artwork></figure>
2466<t>
2467   For example,
2468</t>
2469<figure><artwork type="example">
2470  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2471</artwork></figure>
2472<t>
2473   The Upgrade header field is intended to provide a simple mechanism
2474   for transition from HTTP/1.1 to some other, incompatible protocol. It
2475   does so by allowing the client to advertise its desire to use another
2476   protocol, such as a later version of HTTP with a higher major version
2477   number, even though the current request has been made using HTTP/1.1.
2478   This eases the difficult transition between incompatible protocols by
2479   allowing the client to initiate a request in the more commonly
2480   supported protocol while indicating to the server that it would like
2481   to use a "better" protocol if available (where "better" is determined
2482   by the server, possibly according to the nature of the method and/or
2483   resource being requested).
2484</t>
2485<t>
2486   The Upgrade header field only applies to switching application-layer
2487   protocols upon the existing transport-layer connection. Upgrade
2488   cannot be used to insist on a protocol change; its acceptance and use
2489   by the server is optional. The capabilities and nature of the
2490   application-layer communication after the protocol change is entirely
2491   dependent upon the new protocol chosen, although the first action
2492   after changing the protocol &MUST; be a response to the initial HTTP
2493   request containing the Upgrade header field.
2494</t>
2495<t>
2496   The Upgrade header field only applies to the immediate connection.
2497   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2498   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2499   HTTP/1.1 message.
2500</t>
2501<t>
2502   The Upgrade header field cannot be used to indicate a switch to a
2503   protocol on a different connection. For that purpose, it is more
2504   appropriate to use a 301, 302, 303, or 305 redirection response.
2505</t>
2506<t>
2507   This specification only defines the protocol name "HTTP" for use by
2508   the family of Hypertext Transfer Protocols, as defined by the HTTP
2509   version rules of <xref target="http.version"/> and future updates to this
2510   specification. Any token can be used as a protocol name; however, it
2511   will only be useful if both the client and server associate the name
2512   with the same protocol.
2513</t>
2514</section>
2515
2516<section title="Via" anchor="header.via">
2517  <iref primary="true" item="Via header" x:for-anchor=""/>
2518  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2519  <x:anchor-alias value="protocol-name"/>
2520  <x:anchor-alias value="protocol-version"/>
2521  <x:anchor-alias value="pseudonym"/>
2522  <x:anchor-alias value="received-by"/>
2523  <x:anchor-alias value="received-protocol"/>
2524  <x:anchor-alias value="Via"/>
2525  <x:anchor-alias value="Via-v"/>
2526<t>
2527   The general-header field "Via" &MUST; be used by gateways and proxies to
2528   indicate the intermediate protocols and recipients between the user
2529   agent and the server on requests, and between the origin server and
2530   the client on responses. It is analogous to the "Received" field defined in
2531   <xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/> and is intended to be used for tracking message forwards,
2532   avoiding request loops, and identifying the protocol capabilities of
2533   all senders along the request/response chain.
2534</t>
2535<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Via"/><iref primary="true" item="Grammar" subitem="Via-v"/><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"/>
2536  <x:ref>Via</x:ref>               = "Via" <x:ref>BWS</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>Via-v</x:ref>
2537  <x:ref>Via-v</x:ref>             = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref> [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2538  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2539  <x:ref>protocol-name</x:ref>     = <x:ref>token</x:ref>
2540  <x:ref>protocol-version</x:ref>  = <x:ref>token</x:ref>
2541  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2542  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2543</artwork></figure>
2544<t>
2545   The received-protocol indicates the protocol version of the message
2546   received by the server or client along each segment of the
2547   request/response chain. The received-protocol version is appended to
2548   the Via field value when the message is forwarded so that information
2549   about the protocol capabilities of upstream applications remains
2550   visible to all recipients.
2551</t>
2552<t>
2553   The protocol-name is optional if and only if it would be "HTTP". The
2554   received-by field is normally the host and optional port number of a
2555   recipient server or client that subsequently forwarded the message.
2556   However, if the real host is considered to be sensitive information,
2557   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2558   be assumed to be the default port of the received-protocol.
2559</t>
2560<t>
2561   Multiple Via field values represents each proxy or gateway that has
2562   forwarded the message. Each recipient &MUST; append its information
2563   such that the end result is ordered according to the sequence of
2564   forwarding applications.
2565</t>
2566<t>
2567   Comments &MAY; be used in the Via header field to identify the software
2568   of the recipient proxy or gateway, analogous to the User-Agent and
2569   Server header fields. However, all comments in the Via field are
2570   optional and &MAY; be removed by any recipient prior to forwarding the
2571   message.
2572</t>
2573<t>
2574   For example, a request message could be sent from an HTTP/1.0 user
2575   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2576   forward the request to a public proxy at p.example.net, which completes
2577   the request by forwarding it to the origin server at www.example.com.
2578   The request received by www.example.com would then have the following
2579   Via header field:
2580</t>
2581<figure><artwork type="example">
2582  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2583</artwork></figure>
2584<t>
2585   Proxies and gateways used as a portal through a network firewall
2586   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2587   the firewall region. This information &SHOULD; only be propagated if
2588   explicitly enabled. If not enabled, the received-by host of any host
2589   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2590   for that host.
2591</t>
2592<t>
2593   For organizations that have strong privacy requirements for hiding
2594   internal structures, a proxy &MAY; combine an ordered subsequence of
2595   Via header field entries with identical received-protocol values into
2596   a single such entry. For example,
2597</t>
2598<figure><artwork type="example">
2599  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2600</artwork></figure>
2601<t>
2602        could be collapsed to
2603</t>
2604<figure><artwork type="example">
2605  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2606</artwork></figure>
2607<t>
2608   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2609   under the same organizational control and the hosts have already been
2610   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2611   have different received-protocol values.
2612</t>
2613</section>
2614
2615</section>
2616
2617<section title="IANA Considerations" anchor="IANA.considerations">
2618<section title="Message Header Registration" anchor="message.header.registration">
2619<t>
2620   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2621   with the permanent registrations below (see <xref target="RFC3864"/>):
2622</t>
2623<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2624<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2625   <ttcol>Header Field Name</ttcol>
2626   <ttcol>Protocol</ttcol>
2627   <ttcol>Status</ttcol>
2628   <ttcol>Reference</ttcol>
2629
2630   <c>Connection</c>
2631   <c>http</c>
2632   <c>standard</c>
2633   <c>
2634      <xref target="header.connection"/>
2635   </c>
2636   <c>Content-Length</c>
2637   <c>http</c>
2638   <c>standard</c>
2639   <c>
2640      <xref target="header.content-length"/>
2641   </c>
2642   <c>Date</c>
2643   <c>http</c>
2644   <c>standard</c>
2645   <c>
2646      <xref target="header.date"/>
2647   </c>
2648   <c>Host</c>
2649   <c>http</c>
2650   <c>standard</c>
2651   <c>
2652      <xref target="header.host"/>
2653   </c>
2654   <c>TE</c>
2655   <c>http</c>
2656   <c>standard</c>
2657   <c>
2658      <xref target="header.te"/>
2659   </c>
2660   <c>Trailer</c>
2661   <c>http</c>
2662   <c>standard</c>
2663   <c>
2664      <xref target="header.trailer"/>
2665   </c>
2666   <c>Transfer-Encoding</c>
2667   <c>http</c>
2668   <c>standard</c>
2669   <c>
2670      <xref target="header.transfer-encoding"/>
2671   </c>
2672   <c>Upgrade</c>
2673   <c>http</c>
2674   <c>standard</c>
2675   <c>
2676      <xref target="header.upgrade"/>
2677   </c>
2678   <c>Via</c>
2679   <c>http</c>
2680   <c>standard</c>
2681   <c>
2682      <xref target="header.via"/>
2683   </c>
2684</texttable>
2685<!--(END)-->
2686<t>
2687   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2688</t>
2689</section>
2690
2691<section title="URI Scheme Registration" anchor="uri.scheme.registration">
2692<t>
2693   The entry for the "http" URI Scheme in the registry located at
2694   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>
2695   should be updated to point to <xref target="http.url"/> of this document
2696   (see <xref target="RFC4395"/>).
2697</t>
2698</section>
2699
2700<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
2701<t>
2702   This document serves as the specification for the Internet media types
2703   "message/http" and "application/http". The following is to be registered with
2704   IANA (see <xref target="RFC4288"/>).
2705</t>
2706<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
2707<iref item="Media Type" subitem="message/http" primary="true"/>
2708<iref item="message/http Media Type" primary="true"/>
2709<t>
2710   The message/http type can be used to enclose a single HTTP request or
2711   response message, provided that it obeys the MIME restrictions for all
2712   "message" types regarding line length and encodings.
2713</t>
2714<t>
2715  <list style="hanging" x:indent="12em">
2716    <t hangText="Type name:">
2717      message
2718    </t>
2719    <t hangText="Subtype name:">
2720      http
2721    </t>
2722    <t hangText="Required parameters:">
2723      none
2724    </t>
2725    <t hangText="Optional parameters:">
2726      version, msgtype
2727      <list style="hanging">
2728        <t hangText="version:">
2729          The HTTP-Version number of the enclosed message
2730          (e.g., "1.1"). If not present, the version can be
2731          determined from the first line of the body.
2732        </t>
2733        <t hangText="msgtype:">
2734          The message type -- "request" or "response". If not
2735          present, the type can be determined from the first
2736          line of the body.
2737        </t>
2738      </list>
2739    </t>
2740    <t hangText="Encoding considerations:">
2741      only "7bit", "8bit", or "binary" are permitted
2742    </t>
2743    <t hangText="Security considerations:">
2744      none
2745    </t>
2746    <t hangText="Interoperability considerations:">
2747      none
2748    </t>
2749    <t hangText="Published specification:">
2750      This specification (see <xref target="internet.media.type.message.http"/>).
2751    </t>
2752    <t hangText="Applications that use this media type:">
2753    </t>
2754    <t hangText="Additional information:">
2755      <list style="hanging">
2756        <t hangText="Magic number(s):">none</t>
2757        <t hangText="File extension(s):">none</t>
2758        <t hangText="Macintosh file type code(s):">none</t>
2759      </list>
2760    </t>
2761    <t hangText="Person and email address to contact for further information:">
2762      See Authors Section.
2763    </t>
2764                <t hangText="Intended usage:">
2765                  COMMON
2766    </t>
2767                <t hangText="Restrictions on usage:">
2768                  none
2769    </t>
2770    <t hangText="Author/Change controller:">
2771      IESG
2772    </t>
2773  </list>
2774</t>
2775</section>
2776<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
2777<iref item="Media Type" subitem="application/http" primary="true"/>
2778<iref item="application/http Media Type" primary="true"/>
2779<t>
2780   The application/http type can be used to enclose a pipeline of one or more
2781   HTTP request or response messages (not intermixed).
2782</t>
2783<t>
2784  <list style="hanging" x:indent="12em">
2785    <t hangText="Type name:">
2786      application
2787    </t>
2788    <t hangText="Subtype name:">
2789      http
2790    </t>
2791    <t hangText="Required parameters:">
2792      none
2793    </t>
2794    <t hangText="Optional parameters:">
2795      version, msgtype
2796      <list style="hanging">
2797        <t hangText="version:">
2798          The HTTP-Version number of the enclosed messages
2799          (e.g., "1.1"). If not present, the version can be
2800          determined from the first line of the body.
2801        </t>
2802        <t hangText="msgtype:">
2803          The message type -- "request" or "response". If not
2804          present, the type can be determined from the first
2805          line of the body.
2806        </t>
2807      </list>
2808    </t>
2809    <t hangText="Encoding considerations:">
2810      HTTP messages enclosed by this type
2811      are in "binary" format; use of an appropriate
2812      Content-Transfer-Encoding is required when
2813      transmitted via E-mail.
2814    </t>
2815    <t hangText="Security considerations:">
2816      none
2817    </t>
2818    <t hangText="Interoperability considerations:">
2819      none
2820    </t>
2821    <t hangText="Published specification:">
2822      This specification (see <xref target="internet.media.type.application.http"/>).
2823    </t>
2824    <t hangText="Applications that use this media type:">
2825    </t>
2826    <t hangText="Additional information:">
2827      <list style="hanging">
2828        <t hangText="Magic number(s):">none</t>
2829        <t hangText="File extension(s):">none</t>
2830        <t hangText="Macintosh file type code(s):">none</t>
2831      </list>
2832    </t>
2833    <t hangText="Person and email address to contact for further information:">
2834      See Authors Section.
2835    </t>
2836                <t hangText="Intended usage:">
2837                  COMMON
2838    </t>
2839                <t hangText="Restrictions on usage:">
2840                  none
2841    </t>
2842    <t hangText="Author/Change controller:">
2843      IESG
2844    </t>
2845  </list>
2846</t>
2847</section>
2848</section>
2849
2850</section>
2851
2852<section title="Security Considerations" anchor="security.considerations">
2853<t>
2854   This section is meant to inform application developers, information
2855   providers, and users of the security limitations in HTTP/1.1 as
2856   described by this document. The discussion does not include
2857   definitive solutions to the problems revealed, though it does make
2858   some suggestions for reducing security risks.
2859</t>
2860
2861<section title="Personal Information" anchor="personal.information">
2862<t>
2863   HTTP clients are often privy to large amounts of personal information
2864   (e.g. the user's name, location, mail address, passwords, encryption
2865   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2866   leakage of this information.
2867   We very strongly recommend that a convenient interface be provided
2868   for the user to control dissemination of such information, and that
2869   designers and implementors be particularly careful in this area.
2870   History shows that errors in this area often create serious security
2871   and/or privacy problems and generate highly adverse publicity for the
2872   implementor's company.
2873</t>
2874</section>
2875
2876<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2877<t>
2878   A server is in the position to save personal data about a user's
2879   requests which might identify their reading patterns or subjects of
2880   interest. This information is clearly confidential in nature and its
2881   handling can be constrained by law in certain countries. People using
2882   HTTP to provide data are responsible for ensuring that
2883   such material is not distributed without the permission of any
2884   individuals that are identifiable by the published results.
2885</t>
2886</section>
2887
2888<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2889<t>
2890   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2891   the documents returned by HTTP requests to be only those that were
2892   intended by the server administrators. If an HTTP server translates
2893   HTTP URIs directly into file system calls, the server &MUST; take
2894   special care not to serve files that were not intended to be
2895   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2896   other operating systems use ".." as a path component to indicate a
2897   directory level above the current one. On such a system, an HTTP
2898   server &MUST; disallow any such construct in the Request-URI if it
2899   would otherwise allow access to a resource outside those intended to
2900   be accessible via the HTTP server. Similarly, files intended for
2901   reference only internally to the server (such as access control
2902   files, configuration files, and script code) &MUST; be protected from
2903   inappropriate retrieval, since they might contain sensitive
2904   information. Experience has shown that minor bugs in such HTTP server
2905   implementations have turned into security risks.
2906</t>
2907</section>
2908
2909<section title="DNS Spoofing" anchor="dns.spoofing">
2910<t>
2911   Clients using HTTP rely heavily on the Domain Name Service, and are
2912   thus generally prone to security attacks based on the deliberate
2913   mis-association of IP addresses and DNS names. Clients need to be
2914   cautious in assuming the continuing validity of an IP number/DNS name
2915   association.
2916</t>
2917<t>
2918   In particular, HTTP clients &SHOULD; rely on their name resolver for
2919   confirmation of an IP number/DNS name association, rather than
2920   caching the result of previous host name lookups. Many platforms
2921   already can cache host name lookups locally when appropriate, and
2922   they &SHOULD; be configured to do so. It is proper for these lookups to
2923   be cached, however, only when the TTL (Time To Live) information
2924   reported by the name server makes it likely that the cached
2925   information will remain useful.
2926</t>
2927<t>
2928   If HTTP clients cache the results of host name lookups in order to
2929   achieve a performance improvement, they &MUST; observe the TTL
2930   information reported by DNS.
2931</t>
2932<t>
2933   If HTTP clients do not observe this rule, they could be spoofed when
2934   a previously-accessed server's IP address changes. As network
2935   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2936   possibility of this form of attack will grow. Observing this
2937   requirement thus reduces this potential security vulnerability.
2938</t>
2939<t>
2940   This requirement also improves the load-balancing behavior of clients
2941   for replicated servers using the same DNS name and reduces the
2942   likelihood of a user's experiencing failure in accessing sites which
2943   use that strategy.
2944</t>
2945</section>
2946
2947<section title="Proxies and Caching" anchor="attack.proxies">
2948<t>
2949   By their very nature, HTTP proxies are men-in-the-middle, and
2950   represent an opportunity for man-in-the-middle attacks. Compromise of
2951   the systems on which the proxies run can result in serious security
2952   and privacy problems. Proxies have access to security-related
2953   information, personal information about individual users and
2954   organizations, and proprietary information belonging to users and
2955   content providers. A compromised proxy, or a proxy implemented or
2956   configured without regard to security and privacy considerations,
2957   might be used in the commission of a wide range of potential attacks.
2958</t>
2959<t>
2960   Proxy operators should protect the systems on which proxies run as
2961   they would protect any system that contains or transports sensitive
2962   information. In particular, log information gathered at proxies often
2963   contains highly sensitive personal information, and/or information
2964   about organizations. Log information should be carefully guarded, and
2965   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2966</t>
2967<t>
2968   Proxy implementors should consider the privacy and security
2969   implications of their design and coding decisions, and of the
2970   configuration options they provide to proxy operators (especially the
2971   default configuration).
2972</t>
2973<t>
2974   Users of a proxy need to be aware that they are no trustworthier than
2975   the people who run the proxy; HTTP itself cannot solve this problem.
2976</t>
2977<t>
2978   The judicious use of cryptography, when appropriate, may suffice to
2979   protect against a broad range of security and privacy attacks. Such
2980   cryptography is beyond the scope of the HTTP/1.1 specification.
2981</t>
2982</section>
2983
2984<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2985<t>
2986   They exist. They are hard to defend against. Research continues.
2987   Beware.
2988</t>
2989</section>
2990</section>
2991
2992<section title="Acknowledgments" anchor="ack">
2993<t>
2994   This specification makes heavy use of the augmented BNF and generic
2995   constructs defined by David H. Crocker for <xref target="RFC5234"/>. Similarly, it
2996   reuses many of the definitions provided by Nathaniel Borenstein and
2997   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
2998   specification will help reduce past confusion over the relationship
2999   between HTTP and Internet mail message formats.
3000</t>
3001<t>
3002   HTTP has evolved considerably over the years. It has
3003   benefited from a large and active developer community--the many
3004   people who have participated on the www-talk mailing list--and it is
3005   that community which has been most responsible for the success of
3006   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
3007   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
3008   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
3009   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
3010   VanHeyningen deserve special recognition for their efforts in
3011   defining early aspects of the protocol.
3012</t>
3013<t>
3014   This document has benefited greatly from the comments of all those
3015   participating in the HTTP-WG. In addition to those already mentioned,
3016   the following individuals have contributed to this specification:
3017</t>
3018<t>
3019   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
3020   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
3021   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
3022   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
3023   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
3024   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
3025   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
3026   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
3027   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
3028   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
3029   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
3030   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
3031   Josh Cohen.
3032</t>
3033<t>
3034   Thanks to the "cave men" of Palo Alto. You know who you are.
3035</t>
3036<t>
3037   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
3038   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
3039   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
3040   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
3041   Larry Masinter for their help. And thanks go particularly to Jeff
3042   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3043</t>
3044<t>
3045   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3046   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3047   discovery of many of the problems that this document attempts to
3048   rectify.
3049</t>
3050</section>
3051
3052</middle>
3053<back>
3054
3055<references title="Normative References">
3056
3057<reference anchor="ISO-8859-1">
3058  <front>
3059    <title>
3060     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3061    </title>
3062    <author>
3063      <organization>International Organization for Standardization</organization>
3064    </author>
3065    <date year="1998"/>
3066  </front>
3067  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3068</reference>
3069
3070<reference anchor="Part2">
3071  <front>
3072    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3073    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3074      <organization abbrev="Day Software">Day Software</organization>
3075      <address><email>fielding@gbiv.com</email></address>
3076    </author>
3077    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3078      <organization>One Laptop per Child</organization>
3079      <address><email>jg@laptop.org</email></address>
3080    </author>
3081    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3082      <organization abbrev="HP">Hewlett-Packard Company</organization>
3083      <address><email>JeffMogul@acm.org</email></address>
3084    </author>
3085    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3086      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3087      <address><email>henrikn@microsoft.com</email></address>
3088    </author>
3089    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3090      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3091      <address><email>LMM@acm.org</email></address>
3092    </author>
3093    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3094      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3095      <address><email>paulle@microsoft.com</email></address>
3096    </author>
3097    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3098      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3099      <address><email>timbl@w3.org</email></address>
3100    </author>
3101    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3102      <organization abbrev="W3C">World Wide Web Consortium</organization>
3103      <address><email>ylafon@w3.org</email></address>
3104    </author>
3105    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3106      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3107      <address><email>julian.reschke@greenbytes.de</email></address>
3108    </author>
3109    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3110  </front>
3111  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3112  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3113</reference>
3114
3115<reference anchor="Part3">
3116  <front>
3117    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3118    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3119      <organization abbrev="Day Software">Day Software</organization>
3120      <address><email>fielding@gbiv.com</email></address>
3121    </author>
3122    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3123      <organization>One Laptop per Child</organization>
3124      <address><email>jg@laptop.org</email></address>
3125    </author>
3126    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3127      <organization abbrev="HP">Hewlett-Packard Company</organization>
3128      <address><email>JeffMogul@acm.org</email></address>
3129    </author>
3130    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3131      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3132      <address><email>henrikn@microsoft.com</email></address>
3133    </author>
3134    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3135      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3136      <address><email>LMM@acm.org</email></address>
3137    </author>
3138    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3139      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3140      <address><email>paulle@microsoft.com</email></address>
3141    </author>
3142    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3143      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3144      <address><email>timbl@w3.org</email></address>
3145    </author>
3146    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3147      <organization abbrev="W3C">World Wide Web Consortium</organization>
3148      <address><email>ylafon@w3.org</email></address>
3149    </author>
3150    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3151      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3152      <address><email>julian.reschke@greenbytes.de</email></address>
3153    </author>
3154    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3155  </front>
3156  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3157  <x:source href="p3-payload.xml" basename="p3-payload"/>
3158</reference>
3159
3160<reference anchor="Part5">
3161  <front>
3162    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3163    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3164      <organization abbrev="Day Software">Day Software</organization>
3165      <address><email>fielding@gbiv.com</email></address>
3166    </author>
3167    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3168      <organization>One Laptop per Child</organization>
3169      <address><email>jg@laptop.org</email></address>
3170    </author>
3171    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3172      <organization abbrev="HP">Hewlett-Packard Company</organization>
3173      <address><email>JeffMogul@acm.org</email></address>
3174    </author>
3175    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3176      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3177      <address><email>henrikn@microsoft.com</email></address>
3178    </author>
3179    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3180      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3181      <address><email>LMM@acm.org</email></address>
3182    </author>
3183    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3184      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3185      <address><email>paulle@microsoft.com</email></address>
3186    </author>
3187    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3188      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3189      <address><email>timbl@w3.org</email></address>
3190    </author>
3191    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3192      <organization abbrev="W3C">World Wide Web Consortium</organization>
3193      <address><email>ylafon@w3.org</email></address>
3194    </author>
3195    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3196      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3197      <address><email>julian.reschke@greenbytes.de</email></address>
3198    </author>
3199    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3200  </front>
3201  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3202  <x:source href="p5-range.xml" basename="p5-range"/>
3203</reference>
3204
3205<reference anchor="Part6">
3206  <front>
3207    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3208    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3209      <organization abbrev="Day Software">Day Software</organization>
3210      <address><email>fielding@gbiv.com</email></address>
3211    </author>
3212    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3213      <organization>One Laptop per Child</organization>
3214      <address><email>jg@laptop.org</email></address>
3215    </author>
3216    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3217      <organization abbrev="HP">Hewlett-Packard Company</organization>
3218      <address><email>JeffMogul@acm.org</email></address>
3219    </author>
3220    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3221      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3222      <address><email>henrikn@microsoft.com</email></address>
3223    </author>
3224    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3225      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3226      <address><email>LMM@acm.org</email></address>
3227    </author>
3228    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3229      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3230      <address><email>paulle@microsoft.com</email></address>
3231    </author>
3232    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3233      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3234      <address><email>timbl@w3.org</email></address>
3235    </author>
3236    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3237      <organization abbrev="W3C">World Wide Web Consortium</organization>
3238      <address><email>ylafon@w3.org</email></address>
3239    </author>
3240    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3241      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3242      <address><email>julian.reschke@greenbytes.de</email></address>
3243    </author>
3244    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3245  </front>
3246  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3247  <x:source href="p6-cache.xml" basename="p6-cache"/>
3248</reference>
3249
3250<reference anchor="RFC5234">
3251  <front>
3252    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
3253    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
3254      <organization>Brandenburg InternetWorking</organization>
3255      <address>
3256      <postal>
3257      <street>675 Spruce Dr.</street>
3258      <city>Sunnyvale</city>
3259      <region>CA</region>
3260      <code>94086</code>
3261      <country>US</country></postal>
3262      <phone>+1.408.246.8253</phone>
3263      <email>dcrocker@bbiw.net</email></address> 
3264    </author>
3265    <author initials="P." surname="Overell" fullname="Paul Overell">
3266      <organization>THUS plc.</organization>
3267      <address>
3268      <postal>
3269      <street>1/2 Berkeley Square</street>
3270      <street>99 Berkely Street</street>
3271      <city>Glasgow</city>
3272      <code>G3 7HR</code>
3273      <country>UK</country></postal>
3274      <email>paul.overell@thus.net</email></address>
3275    </author>
3276    <date month="January" year="2008"/>
3277  </front>
3278  <seriesInfo name="STD" value="68"/>
3279  <seriesInfo name="RFC" value="5234"/>
3280</reference>
3281
3282<reference anchor="RFC2045">
3283  <front>
3284    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3285    <author initials="N." surname="Freed" fullname="Ned Freed">
3286      <organization>Innosoft International, Inc.</organization>
3287      <address><email>ned@innosoft.com</email></address>
3288    </author>
3289    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3290      <organization>First Virtual Holdings</organization>
3291      <address><email>nsb@nsb.fv.com</email></address>
3292    </author>
3293    <date month="November" year="1996"/>
3294  </front>
3295  <seriesInfo name="RFC" value="2045"/>
3296</reference>
3297
3298<reference anchor="RFC2047">
3299  <front>
3300    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3301    <author initials="K." surname="Moore" fullname="Keith Moore">
3302      <organization>University of Tennessee</organization>
3303      <address><email>moore@cs.utk.edu</email></address>
3304    </author>
3305    <date month="November" year="1996"/>
3306  </front>
3307  <seriesInfo name="RFC" value="2047"/>
3308</reference>
3309
3310<reference anchor="RFC2119">
3311  <front>
3312    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3313    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3314      <organization>Harvard University</organization>
3315      <address><email>sob@harvard.edu</email></address>
3316    </author>
3317    <date month="March" year="1997"/>
3318  </front>
3319  <seriesInfo name="BCP" value="14"/>
3320  <seriesInfo name="RFC" value="2119"/>
3321</reference>
3322
3323<reference anchor="RFC2396">
3324  <front>
3325    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
3326    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3327      <organization abbrev="MIT/LCS">World Wide Web Consortium</organization>
3328      <address><email>timbl@w3.org</email></address>
3329    </author>
3330    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3331      <organization abbrev="U.C. Irvine">Department of Information and Computer Science</organization>
3332      <address><email>fielding@ics.uci.edu</email></address>
3333    </author>
3334    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3335      <organization abbrev="Xerox Corporation">Xerox PARC</organization>
3336      <address><email>masinter@parc.xerox.com</email></address>
3337    </author>
3338    <date month="August" year="1998"/>
3339  </front>
3340  <seriesInfo name="RFC" value="2396"/>
3341</reference>
3342
3343<reference anchor="USASCII">
3344  <front>
3345    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3346    <author>
3347      <organization>American National Standards Institute</organization>
3348    </author>
3349    <date year="1986"/>
3350  </front>
3351  <seriesInfo name="ANSI" value="X3.4"/>
3352</reference>
3353
3354</references>
3355
3356<references title="Informative References">
3357
3358<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3359  <front>
3360    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3361    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3362      <organization/>
3363    </author>
3364    <author initials="J." surname="Gettys" fullname="J. Gettys">
3365      <organization/>
3366    </author>
3367    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3368      <organization/>
3369    </author>
3370    <author initials="H." surname="Lie" fullname="H. Lie">
3371      <organization/>
3372    </author>
3373    <author initials="C." surname="Lilley" fullname="C. Lilley">
3374      <organization/>
3375    </author>
3376    <date year="1997" month="September"/>
3377  </front>
3378  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3379</reference>
3380
3381<reference anchor="Pad1995" target="http://portal.acm.org/citation.cfm?id=219094">
3382  <front>
3383    <title>Improving HTTP Latency</title>
3384    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3385      <organization/>
3386    </author>
3387    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3388      <organization/>
3389    </author>
3390    <date year="1995" month="December"/>
3391  </front>
3392  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3393</reference>
3394
3395<reference anchor="RFC822">
3396  <front>
3397    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3398    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3399      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3400      <address><email>DCrocker@UDel-Relay</email></address>
3401    </author>
3402    <date month="August" day="13" year="1982"/>
3403  </front>
3404  <seriesInfo name="STD" value="11"/>
3405  <seriesInfo name="RFC" value="822"/>
3406</reference>
3407
3408<reference anchor="RFC959">
3409  <front>
3410    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3411    <author initials="J." surname="Postel" fullname="J. Postel">
3412      <organization>Information Sciences Institute (ISI)</organization>
3413    </author>
3414    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3415      <organization/>
3416    </author>
3417    <date month="October" year="1985"/>
3418  </front>
3419  <seriesInfo name="STD" value="9"/>
3420  <seriesInfo name="RFC" value="959"/>
3421</reference>
3422
3423<reference anchor="RFC1123">
3424  <front>
3425    <title>Requirements for Internet Hosts - Application and Support</title>
3426    <author initials="R." surname="Braden" fullname="Robert Braden">
3427      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3428      <address><email>Braden@ISI.EDU</email></address>
3429    </author>
3430    <date month="October" year="1989"/>
3431  </front>
3432  <seriesInfo name="STD" value="3"/>
3433  <seriesInfo name="RFC" value="1123"/>
3434</reference>
3435
3436<reference anchor="RFC1305">
3437  <front>
3438    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3439    <author initials="D." surname="Mills" fullname="David L. Mills">
3440      <organization>University of Delaware, Electrical Engineering Department</organization>
3441      <address><email>mills@udel.edu</email></address>
3442    </author>
3443    <date month="March" year="1992"/>
3444  </front>
3445  <seriesInfo name="RFC" value="1305"/>
3446</reference>
3447
3448<reference anchor="RFC1436">
3449  <front>
3450    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3451    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3452      <organization>University of Minnesota, Computer and Information Services</organization>
3453      <address><email>fxa@boombox.micro.umn.edu</email></address>
3454    </author>
3455    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3456      <organization>University of Minnesota, Computer and Information Services</organization>
3457      <address><email>mpm@boombox.micro.umn.edu</email></address>
3458    </author>
3459    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3460      <organization>University of Minnesota, Computer and Information Services</organization>
3461      <address><email>lindner@boombox.micro.umn.edu</email></address>
3462    </author>
3463    <author initials="D." surname="Johnson" fullname="David Johnson">
3464      <organization>University of Minnesota, Computer and Information Services</organization>
3465      <address><email>dmj@boombox.micro.umn.edu</email></address>
3466    </author>
3467    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3468      <organization>University of Minnesota, Computer and Information Services</organization>
3469      <address><email>daniel@boombox.micro.umn.edu</email></address>
3470    </author>
3471    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3472      <organization>University of Minnesota, Computer and Information Services</organization>
3473      <address><email>alberti@boombox.micro.umn.edu</email></address>
3474    </author>
3475    <date month="March" year="1993"/>
3476  </front>
3477  <seriesInfo name="RFC" value="1436"/>
3478</reference>
3479
3480<reference anchor="RFC1630">
3481  <front>
3482    <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>
3483    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3484      <organization>CERN, World-Wide Web project</organization>
3485      <address><email>timbl@info.cern.ch</email></address>
3486    </author>
3487    <date month="June" year="1994"/>
3488  </front>
3489  <seriesInfo name="RFC" value="1630"/>
3490</reference>
3491
3492<reference anchor="RFC1737">
3493  <front>
3494    <title abbrev="Requirements for Uniform Resource Names">Functional Requirements for Uniform Resource Names</title>
3495    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3496      <organization>Xerox Palo Alto Research Center</organization>
3497      <address><email>masinter@parc.xerox.com</email></address>
3498    </author>
3499    <author initials="K." surname="Sollins" fullname="Karen Sollins">
3500      <organization>MIT Laboratory for Computer Science</organization>
3501      <address><email>sollins@lcs.mit.edu</email></address>
3502    </author>
3503    <date month="December" year="1994"/>
3504  </front>
3505  <seriesInfo name="RFC" value="1737"/>
3506</reference>
3507
3508<reference anchor="RFC1738">
3509  <front>
3510    <title>Uniform Resource Locators (URL)</title>
3511    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3512      <organization>CERN, World-Wide Web project</organization>
3513      <address><email>timbl@info.cern.ch</email></address>
3514    </author>
3515    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3516      <organization>Xerox PARC</organization>
3517      <address><email>masinter@parc.xerox.com</email></address>
3518    </author>
3519    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3520      <organization>University of Minnesota, Computer and Information Services</organization>
3521      <address><email>mpm@boombox.micro.umn.edu</email></address>
3522    </author>
3523    <date month="December" year="1994"/>
3524  </front>
3525  <seriesInfo name="RFC" value="1738"/>
3526</reference>
3527
3528<reference anchor="RFC1808">
3529  <front>
3530    <title>Relative Uniform Resource Locators</title>
3531    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3532      <organization>University of California Irvine, Department of Information and Computer Science</organization>
3533      <address><email>fielding@ics.uci.edu</email></address>
3534    </author>
3535    <date month="June" year="1995"/>
3536  </front>
3537  <seriesInfo name="RFC" value="1808"/>
3538</reference>
3539
3540<reference anchor="RFC1900">
3541  <front>
3542    <title>Renumbering Needs Work</title>
3543    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3544      <organization>CERN, Computing and Networks Division</organization>
3545      <address><email>brian@dxcoms.cern.ch</email></address>
3546    </author>
3547    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3548      <organization>cisco Systems</organization>
3549      <address><email>yakov@cisco.com</email></address>
3550    </author>
3551    <date month="February" year="1996"/>
3552  </front>
3553  <seriesInfo name="RFC" value="1900"/>
3554</reference>
3555
3556<reference anchor="RFC1945">
3557  <front>
3558    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3559    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3560      <organization>MIT, Laboratory for Computer Science</organization>
3561      <address><email>timbl@w3.org</email></address>
3562    </author>
3563    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3564      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3565      <address><email>fielding@ics.uci.edu</email></address>
3566    </author>
3567    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3568      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3569      <address><email>frystyk@w3.org</email></address>
3570    </author>
3571    <date month="May" year="1996"/>
3572  </front>
3573  <seriesInfo name="RFC" value="1945"/>
3574</reference>
3575
3576<reference anchor="RFC2068">
3577  <front>
3578    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3579    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3580      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3581      <address><email>fielding@ics.uci.edu</email></address>
3582    </author>
3583    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3584      <organization>MIT Laboratory for Computer Science</organization>
3585      <address><email>jg@w3.org</email></address>
3586    </author>
3587    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3588      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3589      <address><email>mogul@wrl.dec.com</email></address>
3590    </author>
3591    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3592      <organization>MIT Laboratory for Computer Science</organization>
3593      <address><email>frystyk@w3.org</email></address>
3594    </author>
3595    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3596      <organization>MIT Laboratory for Computer Science</organization>
3597      <address><email>timbl@w3.org</email></address>
3598    </author>
3599    <date month="January" year="1997"/>
3600  </front>
3601  <seriesInfo name="RFC" value="2068"/>
3602</reference>
3603
3604<reference anchor='RFC2109'>
3605  <front>
3606    <title>HTTP State Management Mechanism</title>
3607    <author initials='D.M.' surname='Kristol' fullname='David M. Kristol'>
3608      <organization>Bell Laboratories, Lucent Technologies</organization>
3609      <address><email>dmk@bell-labs.com</email></address>
3610    </author>
3611    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3612      <organization>Netscape Communications Corp.</organization>
3613      <address><email>montulli@netscape.com</email></address>
3614    </author>
3615    <date year='1997' month='February' />
3616  </front>
3617  <seriesInfo name='RFC' value='2109' />
3618</reference>
3619
3620<reference anchor="RFC2145">
3621  <front>
3622    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3623    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3624      <organization>Western Research Laboratory</organization>
3625      <address><email>mogul@wrl.dec.com</email></address>
3626    </author>
3627    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3628      <organization>Department of Information and Computer Science</organization>
3629      <address><email>fielding@ics.uci.edu</email></address>
3630    </author>
3631    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3632      <organization>MIT Laboratory for Computer Science</organization>
3633      <address><email>jg@w3.org</email></address>
3634    </author>
3635    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3636      <organization>W3 Consortium</organization>
3637      <address><email>frystyk@w3.org</email></address>
3638    </author>
3639    <date month="May" year="1997"/>
3640  </front>
3641  <seriesInfo name="RFC" value="2145"/>
3642</reference>
3643
3644<reference anchor="RFC2324">
3645  <front>
3646    <title abbrev="HTCPCP/1.0">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</title>
3647    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3648      <organization>Xerox Palo Alto Research Center</organization>
3649      <address><email>masinter@parc.xerox.com</email></address>
3650    </author>
3651    <date month="April" day="1" year="1998"/>
3652  </front>
3653  <seriesInfo name="RFC" value="2324"/>
3654</reference>
3655
3656<reference anchor="RFC2616">
3657  <front>
3658    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3659    <author initials="R." surname="Fielding" fullname="R. Fielding">
3660      <organization>University of California, Irvine</organization>
3661      <address><email>fielding@ics.uci.edu</email></address>
3662    </author>
3663    <author initials="J." surname="Gettys" fullname="J. Gettys">
3664      <organization>W3C</organization>
3665      <address><email>jg@w3.org</email></address>
3666    </author>
3667    <author initials="J." surname="Mogul" fullname="J. Mogul">
3668      <organization>Compaq Computer Corporation</organization>
3669      <address><email>mogul@wrl.dec.com</email></address>
3670    </author>
3671    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3672      <organization>MIT Laboratory for Computer Science</organization>
3673      <address><email>frystyk@w3.org</email></address>
3674    </author>
3675    <author initials="L." surname="Masinter" fullname="L. Masinter">
3676      <organization>Xerox Corporation</organization>
3677      <address><email>masinter@parc.xerox.com</email></address>
3678    </author>
3679    <author initials="P." surname="Leach" fullname="P. Leach">
3680      <organization>Microsoft Corporation</organization>
3681      <address><email>paulle@microsoft.com</email></address>
3682    </author>
3683    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3684      <organization>W3C</organization>
3685      <address><email>timbl@w3.org</email></address>
3686    </author>
3687    <date month="June" year="1999"/>
3688  </front>
3689  <seriesInfo name="RFC" value="2616"/>
3690</reference>
3691
3692<reference anchor='RFC2818'>
3693  <front>
3694    <title>HTTP Over TLS</title>
3695    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
3696      <organization>RTFM, Inc.</organization>
3697      <address><email>ekr@rtfm.com</email></address>
3698    </author>
3699    <date year='2000' month='May' />
3700  </front>
3701  <seriesInfo name='RFC' value='2818' />
3702</reference>
3703
3704<reference anchor="RFC2821">
3705  <front>
3706    <title>Simple Mail Transfer Protocol</title>
3707    <author initials="J." surname="Klensin" fullname="J. Klensin">
3708      <organization>AT&amp;T Laboratories</organization>
3709      <address><email>klensin@research.att.com</email></address>
3710    </author>
3711    <date year="2001" month="April"/>
3712  </front>
3713  <seriesInfo name="RFC" value="2821"/>
3714</reference>
3715
3716<reference anchor='RFC2965'>
3717  <front>
3718    <title>HTTP State Management Mechanism</title>
3719    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
3720      <organization>Bell Laboratories, Lucent Technologies</organization>
3721      <address><email>dmk@bell-labs.com</email></address>
3722    </author>
3723    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3724      <organization>Epinions.com, Inc.</organization>
3725      <address><email>lou@montulli.org</email></address>
3726    </author>
3727    <date year='2000' month='October' />
3728  </front>
3729  <seriesInfo name='RFC' value='2965' />
3730</reference>
3731
3732<reference anchor='RFC3864'>
3733  <front>
3734    <title>Registration Procedures for Message Header Fields</title>
3735    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
3736      <organization>Nine by Nine</organization>
3737      <address><email>GK-IETF@ninebynine.org</email></address>
3738    </author>
3739    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
3740      <organization>BEA Systems</organization>
3741      <address><email>mnot@pobox.com</email></address>
3742    </author>
3743    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
3744      <organization>HP Labs</organization>
3745      <address><email>JeffMogul@acm.org</email></address>
3746    </author>
3747    <date year='2004' month='September' />
3748  </front>
3749  <seriesInfo name='BCP' value='90' />
3750  <seriesInfo name='RFC' value='3864' />
3751</reference>
3752
3753<reference anchor='RFC3977'>
3754  <front>
3755    <title>Network News Transfer Protocol (NNTP)</title>
3756    <author initials='C.' surname='Feather' fullname='C. Feather'>
3757      <organization>THUS plc</organization>
3758      <address><email>clive@demon.net</email></address>
3759    </author>
3760    <date year='2006' month='October' />
3761  </front>
3762  <seriesInfo name="RFC" value="3977"/>
3763</reference>
3764
3765<reference anchor="RFC4288">
3766  <front>
3767    <title>Media Type Specifications and Registration Procedures</title>
3768    <author initials="N." surname="Freed" fullname="N. Freed">
3769      <organization>Sun Microsystems</organization>
3770      <address>
3771        <email>ned.freed@mrochek.com</email>
3772      </address>
3773    </author>
3774    <author initials="J." surname="Klensin" fullname="J. Klensin">
3775      <organization/>
3776      <address>
3777        <email>klensin+ietf@jck.com</email>
3778      </address>
3779    </author>
3780    <date year="2005" month="December"/>
3781  </front>
3782  <seriesInfo name="BCP" value="13"/>
3783  <seriesInfo name="RFC" value="4288"/>
3784</reference>
3785
3786<reference anchor='RFC4395'>
3787  <front>
3788    <title>Guidelines and Registration Procedures for New URI Schemes</title>
3789    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
3790      <organization>AT&amp;T Laboratories</organization>
3791      <address>
3792        <email>tony+urireg@maillennium.att.com</email>
3793      </address>
3794    </author>
3795    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
3796      <organization>Qualcomm, Inc.</organization>
3797      <address>
3798        <email>hardie@qualcomm.com</email>
3799      </address>
3800    </author>
3801    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
3802      <organization>Adobe Systems</organization>
3803      <address>
3804        <email>LMM@acm.org</email>
3805      </address>
3806    </author>
3807    <date year='2006' month='February' />
3808  </front>
3809  <seriesInfo name='BCP' value='115' />
3810  <seriesInfo name='RFC' value='4395' />
3811</reference>
3812
3813<reference anchor="RFC5322">
3814  <front>
3815    <title>Internet Message Format</title>
3816    <author initials="P." surname="Resnick" fullname="P. Resnick">
3817      <organization>Qualcomm Incorporated</organization>
3818    </author>
3819    <date year="2008" month="October"/>
3820  </front> 
3821  <seriesInfo name="RFC" value="5322"/>
3822</reference>
3823
3824<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
3825  <front>
3826    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
3827    <author initials="D." surname="Kristol" fullname="David M. Kristol">
3828      <organization/>
3829    </author>
3830    <date year="2001" month="November"/>
3831  </front>
3832  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
3833</reference>
3834
3835<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3836  <front>
3837  <title>Analysis of HTTP Performance Problems</title>
3838  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3839    <organization/>
3840  </author>
3841  <date/>
3842  </front>
3843</reference>
3844
3845<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3846  <front>
3847  <title>Analysis of HTTP Performance</title>
3848  <author initials="J." surname="Touch" fullname="Joe Touch">
3849    <organization>USC/Information Sciences Institute</organization>
3850    <address><email>touch@isi.edu</email></address>
3851  </author>
3852  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3853    <organization>USC/Information Sciences Institute</organization>
3854    <address><email>johnh@isi.edu</email></address>
3855  </author>
3856  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3857    <organization>USC/Information Sciences Institute</organization>
3858    <address><email>katia@isi.edu</email></address>
3859  </author>
3860  <date year="1998" month="Aug"/>
3861  </front>
3862  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3863  <annotation>(original report dated Aug. 1996)</annotation>
3864</reference>
3865
3866<reference anchor="WAIS">
3867  <front>
3868    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3869    <author initials="F." surname="Davis" fullname="F. Davis">
3870      <organization>Thinking Machines Corporation</organization>
3871    </author>
3872    <author initials="B." surname="Kahle" fullname="B. Kahle">
3873      <organization>Thinking Machines Corporation</organization>
3874    </author>
3875    <author initials="H." surname="Morris" fullname="H. Morris">
3876      <organization>Thinking Machines Corporation</organization>
3877    </author>
3878    <author initials="J." surname="Salem" fullname="J. Salem">
3879      <organization>Thinking Machines Corporation</organization>
3880    </author>
3881    <author initials="T." surname="Shen" fullname="T. Shen">
3882      <organization>Thinking Machines Corporation</organization>
3883    </author>
3884    <author initials="R." surname="Wang" fullname="R. Wang">
3885      <organization>Thinking Machines Corporation</organization>
3886    </author>
3887    <author initials="J." surname="Sui" fullname="J. Sui">
3888      <organization>Thinking Machines Corporation</organization>
3889    </author>
3890    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3891      <organization>Thinking Machines Corporation</organization>
3892    </author>
3893    <date month="April" year="1990"/>
3894  </front>
3895  <seriesInfo name="Thinking Machines Corporation" value=""/>
3896</reference>
3897
3898</references>
3899
3900
3901<section title="Tolerant Applications" anchor="tolerant.applications">
3902<t>
3903   Although this document specifies the requirements for the generation
3904   of HTTP/1.1 messages, not all applications will be correct in their
3905   implementation. We therefore recommend that operational applications
3906   be tolerant of deviations whenever those deviations can be
3907   interpreted unambiguously.
3908</t>
3909<t>
3910   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3911   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3912   accept any amount of SP or HTAB characters between fields, even though
3913   only a single SP is required.
3914</t>
3915<t>
3916   The line terminator for message-header fields is the sequence CRLF.
3917   However, we recommend that applications, when parsing such headers,
3918   recognize a single LF as a line terminator and ignore the leading CR.
3919</t>
3920<t>
3921   The character set of an entity-body &SHOULD; be labeled as the lowest
3922   common denominator of the character codes used within that body, with
3923   the exception that not labeling the entity is preferred over labeling
3924   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3925</t>
3926<t>
3927   Additional rules for requirements on parsing and encoding of dates
3928   and other potential problems with date encodings include:
3929</t>
3930<t>
3931  <list style="symbols">
3932     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3933        which appears to be more than 50 years in the future is in fact
3934        in the past (this helps solve the "year 2000" problem).</t>
3935
3936     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3937        Expires date as earlier than the proper value, but &MUST-NOT;
3938        internally represent a parsed Expires date as later than the
3939        proper value.</t>
3940
3941     <t>All expiration-related calculations &MUST; be done in GMT. The
3942        local time zone &MUST-NOT; influence the calculation or comparison
3943        of an age or expiration time.</t>
3944
3945     <t>If an HTTP header incorrectly carries a date value with a time
3946        zone other than GMT, it &MUST; be converted into GMT using the
3947        most conservative possible conversion.</t>
3948  </list>
3949</t>
3950</section>
3951
3952<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
3953<t>
3954   HTTP/1.1 uses a restricted set of date formats (<xref target="full.date"/>) to
3955   simplify the process of date comparison. Proxies and gateways from
3956   other protocols &SHOULD; ensure that any Date header field present in a
3957   message conforms to one of the HTTP/1.1 formats and rewrite the date
3958   if necessary.
3959</t>
3960</section>
3961
3962<section title="Compatibility with Previous Versions" anchor="compatibility">
3963<t>
3964   It is beyond the scope of a protocol specification to mandate
3965   compliance with previous versions. HTTP/1.1 was deliberately
3966   designed, however, to make supporting previous versions easy. It is
3967   worth noting that, at the time of composing this specification
3968   (1996), we would expect commercial HTTP/1.1 servers to:
3969  <list style="symbols">
3970     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3971        1.1 requests;</t>
3972
3973     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3974        1.1;</t>
3975
3976     <t>respond appropriately with a message in the same major version
3977        used by the client.</t>
3978  </list>
3979</t>
3980<t>
3981   And we would expect HTTP/1.1 clients to:
3982  <list style="symbols">
3983     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3984        responses;</t>
3985
3986     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3987        1.1.</t>
3988  </list>
3989</t>
3990<t>
3991   For most implementations of HTTP/1.0, each connection is established
3992   by the client prior to the request and closed by the server after
3993   sending the response. Some implementations implement the Keep-Alive
3994   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3995</t>
3996
3997<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3998<t>
3999   This section summarizes major differences between versions HTTP/1.0
4000   and HTTP/1.1.
4001</t>
4002
4003<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">
4004<t>
4005   The requirements that clients and servers support the Host request-header,
4006   report an error if the Host request-header (<xref target="header.host"/>) is
4007   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
4008   are among the most important changes defined by this
4009   specification.
4010</t>
4011<t>
4012   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4013   addresses and servers; there was no other established mechanism for
4014   distinguishing the intended server of a request than the IP address
4015   to which that request was directed. The changes outlined above will
4016   allow the Internet, once older HTTP clients are no longer common, to
4017   support multiple Web sites from a single IP address, greatly
4018   simplifying large operational Web servers, where allocation of many
4019   IP addresses to a single host has created serious problems. The
4020   Internet will also be able to recover the IP addresses that have been
4021   allocated for the sole purpose of allowing special-purpose domain
4022   names to be used in root-level HTTP URLs. Given the rate of growth of
4023   the Web, and the number of servers already deployed, it is extremely
4024   important that all implementations of HTTP (including updates to
4025   existing HTTP/1.0 applications) correctly implement these
4026   requirements:
4027  <list style="symbols">
4028     <t>Both clients and servers &MUST; support the Host request-header.</t>
4029
4030     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4031
4032     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4033        request does not include a Host request-header.</t>
4034
4035     <t>Servers &MUST; accept absolute URIs.</t>
4036  </list>
4037</t>
4038</section>
4039</section>
4040
4041<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4042<t>
4043   Some clients and servers might wish to be compatible with some
4044   previous implementations of persistent connections in HTTP/1.0
4045   clients and servers. Persistent connections in HTTP/1.0 are
4046   explicitly negotiated as they are not the default behavior. HTTP/1.0
4047   experimental implementations of persistent connections are faulty,
4048   and the new facilities in HTTP/1.1 are designed to rectify these
4049   problems. The problem was that some existing 1.0 clients may be
4050   sending Keep-Alive to a proxy server that doesn't understand
4051   Connection, which would then erroneously forward it to the next
4052   inbound server, which would establish the Keep-Alive connection and
4053   result in a hung HTTP/1.0 proxy waiting for the close on the
4054   response. The result is that HTTP/1.0 clients must be prevented from
4055   using Keep-Alive when talking to proxies.
4056</t>
4057<t>
4058   However, talking to proxies is the most important use of persistent
4059   connections, so that prohibition is clearly unacceptable. Therefore,
4060   we need some other mechanism for indicating a persistent connection
4061   is desired, which is safe to use even when talking to an old proxy
4062   that ignores Connection. Persistent connections are the default for
4063   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4064   declaring non-persistence. See <xref target="header.connection"/>.
4065</t>
4066<t>
4067   The original HTTP/1.0 form of persistent connections (the Connection:
4068   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4069</t>
4070</section>
4071
4072<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4073<t>
4074   This specification has been carefully audited to correct and
4075   disambiguate key word usage; RFC 2068 had many problems in respect to
4076   the conventions laid out in <xref target="RFC2119"/>.
4077</t>
4078<t>
4079   Transfer-coding and message lengths all interact in ways that
4080   required fixing exactly when chunked encoding is used (to allow for
4081   transfer encoding that may not be self delimiting); it was important
4082   to straighten out exactly how message lengths are computed. (Sections
4083   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4084   <xref target="header.content-length" format="counter"/>,
4085   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4086</t>
4087<t>
4088   The use and interpretation of HTTP version numbers has been clarified
4089   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4090   version they support to deal with problems discovered in HTTP/1.0
4091   implementations (<xref target="http.version"/>)
4092</t>
4093<t>
4094   Transfer-coding had significant problems, particularly with
4095   interactions with chunked encoding. The solution is that transfer-codings
4096   become as full fledged as content-codings. This involves
4097   adding an IANA registry for transfer-codings (separate from content
4098   codings), a new header field (TE) and enabling trailer headers in the
4099   future. Transfer encoding is a major performance benefit, so it was
4100   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4101   interoperability problem that could have occurred due to interactions
4102   between authentication trailers, chunked encoding and HTTP/1.0
4103   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4104   and <xref target="header.te" format="counter"/>)
4105</t>
4106</section>
4107
4108<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4109<t>
4110  The CHAR rule does not allow the NUL character anymore (this affects
4111  the comment and quoted-string rules).  Furthermore, the quoted-pair
4112  rule does not allow escaping NUL, CR or LF anymore.
4113  (<xref target="basic.rules"/>)
4114</t>
4115<t>
4116  Clarify that HTTP-Version is case sensitive.
4117  (<xref target="http.version"/>)
4118</t>
4119<t>
4120  Remove reference to non-existant identity transfer-coding value tokens.
4121  (Sections <xref format="counter" target="transfer.codings"/> and
4122  <xref format="counter" target="message.length"/>)
4123</t>
4124<t>
4125  Clarification that the chunk length does not include
4126  the count of the octets in the chunk header and trailer.
4127  (<xref target="chunked.transfer.encoding"/>)
4128</t>
4129<t>
4130  Fix BNF to add query, as the abs_path production in
4131  <xref x:sec="3" x:fmt="of" target="RFC2396"/> doesn't define it.
4132  (<xref target="request-uri"/>)
4133</t>
4134<t>
4135  Clarify exactly when close connection options must be sent.
4136  (<xref target="header.connection"/>)
4137</t>
4138</section>
4139</section>
4140
4141<section title="Terminology" anchor="terminology">
4142<t>
4143   This specification uses a number of terms to refer to the roles
4144   played by participants in, and objects of, the HTTP communication.
4145</t>
4146<t>
4147  <iref item="connection"/>
4148  <x:dfn>connection</x:dfn>
4149  <list>
4150    <t>
4151      A transport layer virtual circuit established between two programs
4152      for the purpose of communication.
4153    </t>
4154  </list>
4155</t>
4156<t>
4157  <iref item="message"/>
4158  <x:dfn>message</x:dfn>
4159  <list>
4160    <t>
4161      The basic unit of HTTP communication, consisting of a structured
4162      sequence of octets matching the syntax defined in <xref target="http.message"/> and
4163      transmitted via the connection.
4164    </t>
4165  </list>
4166</t>
4167<t>
4168  <iref item="request"/>
4169  <x:dfn>request</x:dfn>
4170  <list>
4171    <t>
4172      An HTTP request message, as defined in <xref target="request"/>.
4173    </t>
4174  </list>
4175</t>
4176<t>
4177  <iref item="response"/>
4178  <x:dfn>response</x:dfn>
4179  <list>
4180    <t>
4181      An HTTP response message, as defined in <xref target="response"/>.
4182    </t>
4183  </list>
4184</t>
4185<t>
4186  <iref item="resource"/>
4187  <x:dfn>resource</x:dfn>
4188  <list>
4189    <t>
4190      A network data object or service that can be identified by a URI,
4191      as defined in <xref target="uri"/>. Resources may be available in multiple
4192      representations (e.g. multiple languages, data formats, size, and
4193      resolutions) or vary in other ways.
4194    </t>
4195  </list>
4196</t>
4197<t>
4198  <iref item="entity"/>
4199  <x:dfn>entity</x:dfn>
4200  <list>
4201    <t>
4202      The information transferred as the payload of a request or
4203      response. An entity consists of metainformation in the form of
4204      entity-header fields and content in the form of an entity-body, as
4205      described in &entity;.
4206    </t>
4207  </list>
4208</t>
4209<t>
4210  <iref item="representation"/>
4211  <x:dfn>representation</x:dfn>
4212  <list>
4213    <t>
4214      An entity included with a response that is subject to content
4215      negotiation, as described in &content.negotiation;. There may exist multiple
4216      representations associated with a particular response status.
4217    </t>
4218  </list>
4219</t>
4220<t>
4221  <iref item="content negotiation"/>
4222  <x:dfn>content negotiation</x:dfn>
4223  <list>
4224    <t>
4225      The mechanism for selecting the appropriate representation when
4226      servicing a request, as described in &content.negotiation;. The
4227      representation of entities in any response can be negotiated
4228      (including error responses).
4229    </t>
4230  </list>
4231</t>
4232<t>
4233  <iref item="variant"/>
4234  <x:dfn>variant</x:dfn>
4235  <list>
4236    <t>
4237      A resource may have one, or more than one, representation(s)
4238      associated with it at any given instant. Each of these
4239      representations is termed a `variant'.  Use of the term `variant'
4240      does not necessarily imply that the resource is subject to content
4241      negotiation.
4242    </t>
4243  </list>
4244</t>
4245<t>
4246  <iref item="client"/>
4247  <x:dfn>client</x:dfn>
4248  <list>
4249    <t>
4250      A program that establishes connections for the purpose of sending
4251      requests.
4252    </t>
4253  </list>
4254</t>
4255<t>
4256  <iref item="user agent"/>
4257  <x:dfn>user agent</x:dfn>
4258  <list>
4259    <t>
4260      The client which initiates a request. These are often browsers,
4261      editors, spiders (web-traversing robots), or other end user tools.
4262    </t>
4263  </list>
4264</t>
4265<t>
4266  <iref item="server"/>
4267  <x:dfn>server</x:dfn>
4268  <list>
4269    <t>
4270      An application program that accepts connections in order to
4271      service requests by sending back responses. Any given program may
4272      be capable of being both a client and a server; our use of these
4273      terms refers only to the role being performed by the program for a
4274      particular connection, rather than to the program's capabilities
4275      in general. Likewise, any server may act as an origin server,
4276      proxy, gateway, or tunnel, switching behavior based on the nature
4277      of each request.
4278    </t>
4279  </list>
4280</t>
4281<t>
4282  <iref item="origin server"/>
4283  <x:dfn>origin server</x:dfn>
4284  <list>
4285    <t>
4286      The server on which a given resource resides or is to be created.
4287    </t>
4288  </list>
4289</t>
4290<t>
4291  <iref item="proxy"/>
4292  <x:dfn>proxy</x:dfn>
4293  <list>
4294    <t>
4295      An intermediary program which acts as both a server and a client
4296      for the purpose of making requests on behalf of other clients.
4297      Requests are serviced internally or by passing them on, with
4298      possible translation, to other servers. A proxy &MUST; implement
4299      both the client and server requirements of this specification. A
4300      "transparent proxy" is a proxy that does not modify the request or
4301      response beyond what is required for proxy authentication and
4302      identification. A "non-transparent proxy" is a proxy that modifies
4303      the request or response in order to provide some added service to
4304      the user agent, such as group annotation services, media type
4305      transformation, protocol reduction, or anonymity filtering. Except
4306      where either transparent or non-transparent behavior is explicitly
4307      stated, the HTTP proxy requirements apply to both types of
4308      proxies.
4309    </t>
4310  </list>
4311</t>
4312<t>
4313  <iref item="gateway"/>
4314  <x:dfn>gateway</x:dfn>
4315  <list>
4316    <t>
4317      A server which acts as an intermediary for some other server.
4318      Unlike a proxy, a gateway receives requests as if it were the
4319      origin server for the requested resource; the requesting client
4320      may not be aware that it is communicating with a gateway.
4321    </t>
4322  </list>
4323</t>
4324<t>
4325  <iref item="tunnel"/>
4326  <x:dfn>tunnel</x:dfn>
4327  <list>
4328    <t>
4329      An intermediary program which is acting as a blind relay between
4330      two connections. Once active, a tunnel is not considered a party
4331      to the HTTP communication, though the tunnel may have been
4332      initiated by an HTTP request. The tunnel ceases to exist when both
4333      ends of the relayed connections are closed.
4334    </t>
4335  </list>
4336</t>
4337<t>
4338  <iref item="cache"/>
4339  <x:dfn>cache</x:dfn>
4340  <list>
4341    <t>
4342      A program's local store of response messages and the subsystem
4343      that controls its message storage, retrieval, and deletion. A
4344      cache stores cacheable responses in order to reduce the response
4345      time and network bandwidth consumption on future, equivalent
4346      requests. Any client or server may include a cache, though a cache
4347      cannot be used by a server that is acting as a tunnel.
4348    </t>
4349  </list>
4350</t>
4351<t>
4352  <iref item="cacheable"/>
4353  <x:dfn>cacheable</x:dfn>
4354  <list>
4355    <t>
4356      A response is cacheable if a cache is allowed to store a copy of
4357      the response message for use in answering subsequent requests. The
4358      rules for determining the cacheability of HTTP responses are
4359      defined in &caching;. Even if a resource is cacheable, there may
4360      be additional constraints on whether a cache can use the cached
4361      copy for a particular request.
4362    </t>
4363  </list>
4364</t>
4365<t>
4366  <iref item="upstream"/>
4367  <iref item="downstream"/>
4368  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
4369  <list>
4370    <t>
4371      Upstream and downstream describe the flow of a message: all
4372      messages flow from upstream to downstream.
4373    </t>
4374  </list>
4375</t>
4376<t>
4377  <iref item="inbound"/>
4378  <iref item="outbound"/>
4379  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
4380  <list>
4381    <t>
4382      Inbound and outbound refer to the request and response paths for
4383      messages: "inbound" means "traveling toward the origin server",
4384      and "outbound" means "traveling toward the user agent"
4385    </t>
4386  </list>
4387</t>
4388</section>
4389
4390<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
4391
4392<section title="Since RFC2616">
4393<t>
4394  Extracted relevant partitions from <xref target="RFC2616"/>.
4395</t>
4396</section>
4397
4398<section title="Since draft-ietf-httpbis-p1-messaging-00">
4399<t>
4400  Closed issues:
4401  <list style="symbols"> 
4402    <t>
4403      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4404      "HTTP Version should be case sensitive"
4405      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4406    </t>
4407    <t>
4408      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4409      "'unsafe' characters"
4410      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4411    </t>
4412    <t>
4413      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4414      "Chunk Size Definition"
4415      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4416    </t>
4417    <t>
4418      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4419      "Message Length"
4420      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4421    </t>
4422    <t>
4423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4424      "Media Type Registrations"
4425      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4426    </t>
4427    <t>
4428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4429      "URI includes query"
4430      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4431    </t>
4432    <t>
4433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4434      "No close on 1xx responses"
4435      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4436    </t>
4437    <t>
4438      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4439      "Remove 'identity' token references"
4440      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4441    </t>
4442    <t>
4443      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4444      "Import query BNF"
4445    </t>
4446    <t>
4447      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4448      "qdtext BNF"
4449    </t>
4450    <t>
4451      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4452      "Normative and Informative references"
4453    </t>
4454    <t>
4455      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4456      "RFC2606 Compliance"
4457    </t>
4458    <t>
4459      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4460      "RFC977 reference"
4461    </t>
4462    <t>
4463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4464      "RFC1700 references"
4465    </t>
4466    <t>
4467      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4468      "inconsistency in date format explanation"
4469    </t>
4470    <t>
4471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4472      "Date reference typo"
4473    </t>
4474    <t>
4475      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4476      "Informative references"
4477    </t>
4478    <t>
4479      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4480      "ISO-8859-1 Reference"
4481    </t>
4482    <t>
4483      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4484      "Normative up-to-date references"
4485    </t>
4486  </list>
4487</t>
4488<t>
4489  Other changes:
4490  <list style="symbols"> 
4491    <t>
4492      Update media type registrations to use RFC4288 template.
4493    </t>
4494    <t>
4495      Use names of RFC4234 core rules DQUOTE and HTAB,
4496      fix broken ABNF for chunk-data
4497      (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4498    </t>
4499  </list>
4500</t>
4501</section>
4502
4503<section title="Since draft-ietf-httpbis-p1-messaging-01">
4504<t>
4505  Closed issues:
4506  <list style="symbols"> 
4507    <t>
4508      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4509      "Bodies on GET (and other) requests"
4510    </t>
4511    <t>
4512      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4513      "Updating to RFC4288"
4514    </t>
4515    <t>
4516      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4517      "Status Code and Reason Phrase"
4518    </t>
4519    <t>
4520      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4521      "rel_path not used"
4522    </t>
4523  </list>
4524</t>
4525<t>
4526  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4527  <list style="symbols"> 
4528    <t>
4529      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4530      "trailer-part").
4531    </t>
4532    <t>
4533      Avoid underscore character in rule names ("http_URL" ->
4534      "http-URL", "abs_path" -> "path-absolute").
4535    </t>
4536    <t>
4537      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4538      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4539      have to be updated when switching over to RFC3986.
4540    </t>
4541    <t>
4542      Synchronize core rules with RFC5234 (this includes a change to CHAR
4543      which now excludes NUL).
4544    </t>
4545    <t>
4546      Get rid of prose rules that span multiple lines.
4547    </t>
4548    <t>
4549      Get rid of unused rules LOALPHA and UPALPHA.
4550    </t>
4551    <t>
4552      Move "Product Tokens" section (back) into Part 1, as "token" is used
4553      in the definition of the Upgrade header.
4554    </t>
4555    <t>
4556      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4557    </t>
4558    <t>
4559      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4560    </t>
4561  </list>
4562</t>
4563</section>
4564
4565<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
4566<t>
4567  Closed issues:
4568  <list style="symbols"> 
4569    <t>
4570      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/51"/>:
4571      "HTTP-date vs. rfc1123-date"
4572    </t>
4573    <t>
4574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/64"/>:
4575      "WS in quoted-pair"
4576    </t>
4577  </list>
4578</t>
4579<t>
4580  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
4581  <list style="symbols"> 
4582    <t>
4583      Reference RFC 3984, and update header registrations for headers defined
4584      in this document.
4585    </t>
4586  </list>
4587</t>
4588<t>
4589  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4590  <list style="symbols"> 
4591    <t>
4592      Replace string literals when the string really is case-sensitive (HTTP-Version).
4593    </t>
4594  </list>
4595</t>
4596</section>
4597
4598<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
4599<t>
4600  Closed issues:
4601  <list style="symbols"> 
4602    <t>
4603      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
4604      "Connection closing"
4605    </t>
4606    <t>
4607      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/97"/>:
4608      "Move registrations and registry information to IANA Considerations"
4609    </t>
4610    <t>
4611      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/120"/>:
4612      "need new URL for PAD1995 reference"
4613    </t>
4614    <t>
4615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/127"/>:
4616      "IANA Considerations: update HTTP URI scheme registration"
4617    </t>
4618    <t>
4619      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
4620      "Cite HTTPS URI scheme definition"
4621    </t>
4622    <t>
4623      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/129"/>:
4624      "List-type headers vs Set-Cookie"
4625    </t>
4626  </list>
4627</t>
4628<t>
4629  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4630  <list style="symbols"> 
4631    <t>
4632      Replace string literals when the string really is case-sensitive (HTTP-Date).
4633    </t>
4634    <t>
4635      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
4636    </t>
4637  </list>
4638</t>
4639</section>
4640
4641<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
4642<t>
4643  Closed issues:
4644  <list style="symbols"> 
4645    <t>
4646      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
4647      "RFC 2822 is updated by RFC 5322"
4648    </t>
4649  </list>
4650</t>
4651<t>
4652  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4653  <list style="symbols"> 
4654    <t>
4655      Use "/" instead of "|" for alternatives.
4656    </t>
4657    <t>
4658      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
4659    </t>
4660    <t>
4661      Only reference RFC 5234's core rules.
4662    </t>
4663    <t>
4664      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
4665      whitespace ("OWS") and required whitespace ("RWS").
4666    </t>
4667    <t>
4668      Rewrite ABNFs to spell out whitespace rules, factor out
4669      header value format definitions.
4670    </t>
4671  </list>
4672</t>
4673</section>
4674
4675</section>
4676
4677</back>
4678</rfc>
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