source: draft-ietf-httpbis/latest/p1-messaging.xml @ 403

Last change on this file since 403 was 403, checked in by julian.reschke@…, 11 years ago

restore RFC 2068 text describing how to proxy OPTIONS * (addresses #83)

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY entity-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std" x:maturity-level="draft"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x='http://purl.org/net/xml2rfc/ext'>
56<front>
57
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
59
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email>fielding@gbiv.com</email>
73      <uri>http://roy.gbiv.com/</uri>
74    </address>
75  </author>
76
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email>jg@laptop.org</email>
88      <uri>http://www.laptop.org/</uri>
89    </address>
90  </author>
91 
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email>JeffMogul@acm.org</email>
104    </address>
105  </author>
106
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email>henrikn@microsoft.com</email>
118    </address>
119  </author>
120
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email>LMM@acm.org</email>
132      <uri>http://larry.masinter.net/</uri>
133    </address>
134  </author>
135 
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email>paulle@microsoft.com</email>
146    </address>
147  </author>
148   
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email>timbl@w3.org</email>
162      <uri>http://www.w3.org/People/Berners-Lee/</uri>
163    </address>
164  </author>
165
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email>ylafon@w3.org</email>
178      <uri>http://www.raubacapeu.net/people/yves/</uri>
179    </address>
180  </author>
181
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email>julian.reschke@greenbytes.de</email>       
193      <uri>http://greenbytes.de/tech/webdav/</uri>     
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
198
199<abstract>
200<t>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    The changes in this draft are summarized in <xref target="changes.since.05"/>.
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   request/response protocol that uses extensible semantics and MIME-like
232   message payloads for flexible interaction with network-based hypertext
233   information systems. HTTP relies upon the Uniform Resource Identifier (URI)
234   standard <xref target="RFC3986"/> to indicate resource targets and
235   relationships between resources.
236   Messages are passed in a format similar to that used by Internet mail
237   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
238   (MIME) <xref target="RFC2045"/> (see &diff2045entity; for the differences
239   between HTTP and MIME messages).
240</t>
241<t>
242   HTTP is a generic interface protocol for informations systems. It is
243   designed to hide the details of how a service is implemented by presenting
244   a uniform interface to clients that is independent of the types of
245   resources provided. Likewise, servers do not need to be aware of each
246   client's purpose: an HTTP request can be considered in isolation rather
247   than being associated with a specific type of client or a predetermined
248   sequence of application steps. The result is a protocol that can be used
249   effectively in many different contexts and for which implementations can
250   evolve independently over time.
251</t>
252<t>
253   HTTP is also designed for use as a generic protocol for translating
254   communication to and from other Internet information systems, such as
255   USENET news services via NNTP <xref target="RFC3977"/>,
256   file services via FTP <xref target="RFC959"/>,
257   Gopher <xref target="RFC1436"/>, and WAIS <xref target="WAIS"/>.
258   HTTP proxies and gateways provide access to alternative information
259   services by translating their diverse protocols into a hypermedia
260   format that can be viewed and manipulated by clients in the same way
261   as HTTP services.
262</t>
263<t>
264   One consequence of HTTP flexibility is that we cannot define the protocol
265   in terms of how to implement it behind the interface. Instead, we are
266   limited to restricting the syntax of communication, defining the intent
267   of received communication, and the expected behavior of recipients. If
268   the communication is considered in isolation, then successful actions
269   should be reflected in the observable interface provided by servers.
270   However, since many clients are potentially acting in parallel and
271   perhaps at cross-purposes, it would be meaningless to require that such
272   behavior be observable.
273</t>
274<t>
275   This document is Part 1 of the seven-part specification of HTTP,
276   defining the protocol referred to as "HTTP/1.1" and obsoleting
277   <xref target="RFC2616"/>.
278   Part 1 defines the URI schemes specific to HTTP-based resources, overall
279   network operation, transport protocol connection management, and HTTP
280   message framing and forwarding requirements.
281   Our goal is to define all of the mechanisms necessary for HTTP message
282   handling that are independent of message semantics, thereby defining the
283   complete set of requirements for a message parser and transparent
284   message-forwarding intermediaries.
285</t>
286
287<section title="Requirements" anchor="intro.requirements">
288<t>
289   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
290   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
291   document are to be interpreted as described in <xref target="RFC2119"/>.
292</t>
293<t>
294   An implementation is not compliant if it fails to satisfy one or more
295   of the &MUST; or &REQUIRED; level requirements for the protocols it
296   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
297   level and all the &SHOULD; level requirements for its protocols is said
298   to be "unconditionally compliant"; one that satisfies all the &MUST;
299   level requirements but not all the &SHOULD; level requirements for its
300   protocols is said to be "conditionally compliant."
301</t>
302</section>
303
304<section title="Syntax Notation" anchor="notation">
305<iref primary="true" item="Grammar" subitem="ALPHA"/>
306<iref primary="true" item="Grammar" subitem="CR"/>
307<iref primary="true" item="Grammar" subitem="CRLF"/>
308<iref primary="true" item="Grammar" subitem="CTL"/>
309<iref primary="true" item="Grammar" subitem="DIGIT"/>
310<iref primary="true" item="Grammar" subitem="DQUOTE"/>
311<iref primary="true" item="Grammar" subitem="HEXDIG"/>
312<iref primary="true" item="Grammar" subitem="LF"/>
313<iref primary="true" item="Grammar" subitem="OCTET"/>
314<iref primary="true" item="Grammar" subitem="SP"/>
315<iref primary="true" item="Grammar" subitem="VCHAR"/>
316<iref primary="true" item="Grammar" subitem="WSP"/>
317<t anchor="core.rules">
318  <x:anchor-alias value="ALPHA"/>
319  <x:anchor-alias value="CTL"/>
320  <x:anchor-alias value="CR"/>
321  <x:anchor-alias value="CRLF"/>
322  <x:anchor-alias value="DIGIT"/>
323  <x:anchor-alias value="DQUOTE"/>
324  <x:anchor-alias value="HEXDIG"/>
325  <x:anchor-alias value="LF"/>
326  <x:anchor-alias value="OCTET"/>
327  <x:anchor-alias value="SP"/>
328  <x:anchor-alias value="VCHAR"/>
329  <x:anchor-alias value="WSP"/>
330   This specification uses the Augmented Backus-Naur Form (ABNF) notation
331   of <xref target="RFC5234"/>.  The following core rules are included by
332   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
333   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
334   DIGIT (decimal 0-9), DQUOTE (double quote),
335   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
336   OCTET (any 8-bit sequence of data), SP (space),
337   VCHAR (any visible <xref target="USASCII"/> character),
338   and WSP (whitespace).
339</t>
340
341<section title="ABNF Extension: #rule" anchor="notation.abnf">
342  <t>
343    One extension to the ABNF rules of <xref target="RFC5234"/> is used to
344    improve readability.
345  </t>
346  <t>
347    A construct "#" is defined, similar to "*", for defining lists of
348    elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
349    &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single comma
350    (",") and optional whitespace (OWS).   
351  </t>
352  <figure><preamble>
353    Thus,
354</preamble><artwork type="example">
355  1#element =&gt; element *( OWS "," OWS element )
356</artwork></figure>
357  <figure><preamble>
358    and:
359</preamble><artwork type="example">
360  #element =&gt; [ 1#element ]
361</artwork></figure>
362  <figure><preamble>
363    and for n &gt;= 1 and m &gt; 1:
364</preamble><artwork type="example">
365  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
366</artwork></figure>
367  <t>
368    For compatibility with legacy list rules, recipients &SHOULD; accept empty
369    list elements. In other words, consumers would follow the list productions:
370  </t>
371<figure><artwork type="example">
372#element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
373
3741#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
375</artwork></figure> 
376  <t>
377    <cref anchor="abnf.list">
378      At a later point of time, we may want to add an appendix containing
379      the whole ABNF, with the list rules expanded to strict RFC 5234
380      notation.
381    </cref>
382  </t>
383</section>
384
385<section title="Basic Rules" anchor="basic.rules">
386<t anchor="rule.CRLF">
387  <x:anchor-alias value="CRLF"/>
388   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
389   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
390   tolerant applications). The end-of-line marker within an entity-body
391   is defined by its associated media type, as described in &media-types;.
392</t>
393<t anchor="rule.LWS">
394   This specification uses three rules to denote the use of linear
395   whitespace: OWS (optional whitespace), RWS (required whitespace), and
396   BWS ("bad" whitespace).
397</t>
398<t>
399   The OWS rule is used where zero or more linear whitespace characters may
400   appear. OWS &SHOULD; either not be produced or be produced as a single SP
401   character. Multiple OWS characters that occur within field-content &SHOULD;
402   be replaced with a single SP before interpreting the field value or
403   forwarding the message downstream.
404</t>
405<t>
406   RWS is used when at least one linear whitespace character is required to
407   separate field tokens. RWS &SHOULD; be produced as a single SP character.
408   Multiple RWS characters that occur within field-content &SHOULD; be
409   replaced with a single SP before interpreting the field value or
410   forwarding the message downstream.
411</t>
412<t>
413   BWS is used where the grammar allows optional whitespace for historical
414   reasons but senders &SHOULD-NOT; produce it in messages. HTTP/1.1
415   recipients &MUST; accept such bad optional whitespace and remove it before
416   interpreting the field value or forwarding the message downstream.
417</t>
418<t anchor="rule.whitespace">
419  <x:anchor-alias value="BWS"/>
420  <x:anchor-alias value="OWS"/>
421  <x:anchor-alias value="RWS"/>
422  <x:anchor-alias value="obs-fold"/>
423</t>
424<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"/>
425  <x:ref>OWS</x:ref>            = *( [ obs-fold ] <x:ref>WSP</x:ref> )
426                 ; "optional" whitespace
427  <x:ref>RWS</x:ref>            = 1*( [ obs-fold ] <x:ref>WSP</x:ref> )
428                 ; "required" whitespace
429  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
430                 ; "bad" whitespace
431  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref>
432</artwork></figure>
433<t anchor="rule.token.separators">
434  <x:anchor-alias value="tchar"/>
435  <x:anchor-alias value="token"/>
436   Many HTTP/1.1 header field values consist of words separated by whitespace
437   or special characters. These special characters &MUST; be in a quoted
438   string to be used within a parameter value (as defined in
439   <xref target="transfer.codings"/>).
440</t>
441<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
442  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
443                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
444                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
445                 
446  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
447</artwork></figure>
448<t anchor="rule.quoted-string">
449  <x:anchor-alias value="quoted-string"/>
450  <x:anchor-alias value="qdtext"/>
451  <x:anchor-alias value="obs-text"/>
452   A string of text is parsed as a single word if it is quoted using
453   double-quote marks.
454</t>
455<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/><iref primary="true" item="Grammar" subitem="obs-text"/>
456  <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>
457  <x:ref>qdtext</x:ref>         = *( <x:ref>OWS</x:ref> / %x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref> )
458  <x:ref>obs-text</x:ref>       = %x80-FF
459</artwork></figure>
460<t anchor="rule.quoted-pair">
461  <x:anchor-alias value="quoted-pair"/>
462  <x:anchor-alias value="quoted-text"/>
463   The backslash character ("\") &MAY; be used as a single-character
464   quoting mechanism only within quoted-string and comment constructs.
465</t>
466<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-text"/><iref primary="true" item="Grammar" subitem="quoted-pair"/>
467  <x:ref>quoted-text</x:ref>    = %x01-09 /
468                   %x0B-0C /
469                   %x0E-FF ; Characters excluding NUL, <x:ref>CR</x:ref> and <x:ref>LF</x:ref>
470  <x:ref>quoted-pair</x:ref>    = "\" <x:ref>quoted-text</x:ref>
471</artwork></figure>
472</section>
473
474<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
475  <x:anchor-alias value="request-header"/>
476  <x:anchor-alias value="response-header"/>
477  <x:anchor-alias value="accept-params"/>
478  <x:anchor-alias value="entity-body"/>
479  <x:anchor-alias value="entity-header"/>
480  <x:anchor-alias value="Cache-Control"/>
481  <x:anchor-alias value="Pragma"/>
482  <x:anchor-alias value="Warning"/>
483<t>
484  The ABNF rules below are defined in other parts:
485</t>
486<figure><!-- Part2--><artwork type="abnf2616">
487  <x:ref>request-header</x:ref>  = &lt;request-header, defined in &request-header-fields;&gt;
488  <x:ref>response-header</x:ref> = &lt;response-header, defined in &response-header-fields;&gt;
489</artwork></figure>
490<figure><!-- Part3--><artwork type="abnf2616">
491  <x:ref>accept-params</x:ref>   = &lt;accept-params, defined in &header-accept;&gt;
492  <x:ref>entity-body</x:ref>     = &lt;entity-body, defined in &entity-body;&gt;
493  <x:ref>entity-header</x:ref>   = &lt;entity-header, defined in &entity-header-fields;&gt;
494</artwork></figure>
495<figure><!-- Part6--><artwork type="abnf2616">
496  <x:ref>Cache-Control</x:ref>   = &lt;Cache-Control, defined in &header-pragma;&gt;
497  <x:ref>Pragma</x:ref>          = &lt;Pragma, defined in &header-pragma;&gt;
498  <x:ref>Warning</x:ref>         = &lt;Warning, defined in &header-warning;&gt;
499</artwork></figure>
500</section>
501
502</section>
503</section>
504
505<section title="HTTP architecture" anchor="architecture">
506<t>
507   HTTP was created with a specific architecture in mind, the World Wide Web,
508   and has evolved over time to support the scalability needs of a worldwide
509   hypertext system. Much of that architecture is reflected in the terminology
510   and syntax productions used to define HTTP.
511</t>
512
513<section title="Uniform Resource Identifiers" anchor="uri">
514<t>
515   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
516   throughout HTTP as the means for identifying resources. URI references
517   are used to target requests, redirect responses, and define relationships.
518   HTTP does not limit what a resource may be; it merely defines an interface
519   that can be used to interact with a resource via HTTP. More information on
520   the scope of URIs and resources can be found in <xref target="RFC3986"/>.
521</t>
522  <x:anchor-alias value="URI"/>
523  <x:anchor-alias value="URI-reference"/>
524  <x:anchor-alias value="absolute-URI"/>
525  <x:anchor-alias value="relative-part"/>
526  <x:anchor-alias value="authority"/>
527  <x:anchor-alias value="fragment"/>
528  <x:anchor-alias value="path-abempty"/>
529  <x:anchor-alias value="path-absolute"/>
530  <x:anchor-alias value="port"/>
531  <x:anchor-alias value="query"/>
532  <x:anchor-alias value="uri-host"/>
533  <x:anchor-alias value="partial-URI"/>
534<t>
535   This specification adopts the definitions of "URI-reference",
536   "absolute-URI", "relative-part", "fragment", "port", "host",
537   "path-abempty", "path-absolute", "query", and "authority" from
538   <xref target="RFC3986"/>. In addition, we define a partial-URI rule for
539   protocol elements that allow a relative URI without a fragment.
540</t>
541<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"/><iref primary="true" item="Grammar" subitem="absolute-URI"/><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="uri-host"/>
542  <x:ref>URI</x:ref>           = &lt;URI, defined in <xref target="RFC3986" x:fmt="," x:sec="3"/>&gt;
543  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
544  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
545  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
546  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
547  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
548  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
549  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
550  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
551  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
552  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
553 
554  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
555</artwork></figure>
556<t>
557   Each protocol element in HTTP that allows a URI reference will indicate in
558   its ABNF production whether the element allows only a URI in absolute form
559   (absolute-URI), any relative reference (relative-ref), or some other subset
560   of the URI-reference grammar. Unless otherwise indicated, URI references
561   are parsed relative to the request target (the default base URI for both
562   the request and its corresponding response).
563</t>
564
565<section title="http URI scheme" anchor="http.uri">
566  <x:anchor-alias value="http-URI"/>
567  <iref item="http URI scheme" primary="true"/>
568  <iref item="URI scheme" subitem="http" primary="true"/>
569<t>
570   The "http" scheme is used to locate network resources via the HTTP
571   protocol. This section defines the syntax and semantics for identifiers
572   using the http or https URI schemes.
573</t>
574<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"/>
575  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
576</artwork></figure>
577<t>
578   If the port is empty or not given, port 80 is assumed. The semantics
579   are that the identified resource is located at the server listening
580   for TCP connections on that port of that host, and the request-target
581   for the resource is path-absolute (<xref target="request-target"/>). The use of IP addresses
582   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
583   the path-absolute is not present in the URL, it &MUST; be given as "/" when
584   used as a request-target for a resource (<xref target="request-target"/>). If a proxy
585   receives a host name which is not a fully qualified domain name, it
586   &MAY; add its domain to the host name it received. If a proxy receives
587   a fully qualified domain name, the proxy &MUST-NOT; change the host
588   name.
589</t>
590<t><list><t>
591  <iref item="https URI scheme"/>
592  <iref item="URI scheme" subitem="https"/>
593  <x:h>Note:</x:h> the "https" scheme is defined in <xref target="RFC2818"/>.
594</t></list></t>
595</section>
596
597<section title="URI Comparison" anchor="uri.comparison">
598<t>
599   When comparing two URIs to decide if they match or not, a client
600   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
601   URIs, with these exceptions:
602  <list style="symbols">
603    <t>A port that is empty or not given is equivalent to the default
604        port for that URI-reference;</t>
605    <t>Comparisons of host names &MUST; be case-insensitive;</t>
606    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
607    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
608  </list>
609</t>
610<t>
611   Characters other than those in the "reserved" set (see
612   <xref target="RFC3986" x:fmt="," x:sec="2.2"/>) are equivalent to their
613   ""%" <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding.
614</t>
615<t>
616   For example, the following three URIs are equivalent:
617</t>
618<figure><artwork type="example">
619   http://example.com:80/~smith/home.html
620   http://EXAMPLE.com/%7Esmith/home.html
621   http://EXAMPLE.com:/%7esmith/home.html
622</artwork></figure>
623</section>
624
625<section title="Scheme aliases considered harmful" anchor="scheme.aliases">
626<t>
627</t>
628</section>
629</section>
630
631<section title="Overall Operation" anchor="intro.overall.operation">
632<t>
633   HTTP is a request/response protocol. A client sends a
634   request to the server in the form of a request method, URI, and
635   protocol version, followed by a MIME-like message containing request
636   modifiers, client information, and possible body content over a
637   connection with a server. The server responds with a status line,
638   including the message's protocol version and a success or error code,
639   followed by a MIME-like message containing server information, entity
640   metainformation, and possible entity-body content.
641</t>
642<t>
643   Most HTTP communication is initiated by a user agent and consists of
644   a request to be applied to a resource on some origin server. In the
645   simplest case, this may be accomplished via a single connection (v)
646   between the user agent (UA) and the origin server (O).
647</t>
648<figure><artwork type="drawing">
649       request chain ------------------------&gt;
650    UA -------------------v------------------- O
651       &lt;----------------------- response chain
652</artwork></figure>
653<t>
654   A more complicated situation occurs when one or more intermediaries
655   are present in the request/response chain. There are three common
656   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
657   forwarding agent, receiving requests for a URI in its absolute form,
658   rewriting all or part of the message, and forwarding the reformatted
659   request toward the server identified by the URI. A gateway is a
660   receiving agent, acting as a layer above some other server(s) and, if
661   necessary, translating the requests to the underlying server's
662   protocol. A tunnel acts as a relay point between two connections
663   without changing the messages; tunnels are used when the
664   communication needs to pass through an intermediary (such as a
665   firewall) even when the intermediary cannot understand the contents
666   of the messages.
667</t>
668<figure><artwork type="drawing">
669       request chain --------------------------------------&gt;
670    UA -----v----- A -----v----- B -----v----- C -----v----- O
671       &lt;------------------------------------- response chain
672</artwork></figure>
673<t>
674   The figure above shows three intermediaries (A, B, and C) between the
675   user agent and origin server. A request or response message that
676   travels the whole chain will pass through four separate connections.
677   This distinction is important because some HTTP communication options
678   may apply only to the connection with the nearest, non-tunnel
679   neighbor, only to the end-points of the chain, or to all connections
680   along the chain. Although the diagram is linear, each participant may
681   be engaged in multiple, simultaneous communications. For example, B
682   may be receiving requests from many clients other than A, and/or
683   forwarding requests to servers other than C, at the same time that it
684   is handling A's request.
685</t>
686<t>
687   Any party to the communication which is not acting as a tunnel may
688   employ an internal cache for handling requests. The effect of a cache
689   is that the request/response chain is shortened if one of the
690   participants along the chain has a cached response applicable to that
691   request. The following illustrates the resulting chain if B has a
692   cached copy of an earlier response from O (via C) for a request which
693   has not been cached by UA or A.
694</t>
695<figure><artwork type="drawing">
696          request chain ----------&gt;
697       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
698          &lt;--------- response chain
699</artwork></figure>
700<t>
701   Not all responses are usefully cacheable, and some requests may
702   contain modifiers which place special requirements on cache behavior.
703   HTTP requirements for cache behavior and cacheable responses are
704   defined in &caching;.
705</t>
706<t>
707   In fact, there are a wide variety of architectures and configurations
708   of caches and proxies currently being experimented with or deployed
709   across the World Wide Web. These systems include national hierarchies
710   of proxy caches to save transoceanic bandwidth, systems that
711   broadcast or multicast cache entries, organizations that distribute
712   subsets of cached data via CD-ROM, and so on. HTTP systems are used
713   in corporate intranets over high-bandwidth links, and for access via
714   PDAs with low-power radio links and intermittent connectivity. The
715   goal of HTTP/1.1 is to support the wide diversity of configurations
716   already deployed while introducing protocol constructs that meet the
717   needs of those who build web applications that require high
718   reliability and, failing that, at least reliable indications of
719   failure.
720</t>
721<t>
722   HTTP communication usually takes place over TCP/IP connections. The
723   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
724   not preclude HTTP from being implemented on top of any other protocol
725   on the Internet, or on other networks. HTTP only presumes a reliable
726   transport; any protocol that provides such guarantees can be used;
727   the mapping of the HTTP/1.1 request and response structures onto the
728   transport data units of the protocol in question is outside the scope
729   of this specification.
730</t>
731<t>
732   In HTTP/1.0, most implementations used a new connection for each
733   request/response exchange. In HTTP/1.1, a connection may be used for
734   one or more request/response exchanges, although connections may be
735   closed for a variety of reasons (see <xref target="persistent.connections"/>).
736</t>
737</section>
738
739<section title="Use of HTTP for proxy communication" anchor="http.proxy">
740<t>
741   Configured to use HTTP to proxy HTTP or other protocols.
742</t>
743</section>
744<section title="Interception of HTTP for access control" anchor="http.intercept">
745<t>
746   Interception of HTTP traffic for initiating access control.
747</t>
748</section>
749<section title="Use of HTTP by other protocols" anchor="http.others">
750<t>
751   Profiles of HTTP defined by other protocol.
752   Extensions of HTTP like WebDAV.
753</t>
754</section>
755<section title="Use of HTTP by media type specification" anchor="http.media">
756<t>
757   Instructions on composing HTTP requests via hypertext formats.
758</t>
759</section>
760</section>
761
762<section title="Protocol Parameters" anchor="protocol.parameters">
763
764<section title="HTTP Version" anchor="http.version">
765  <x:anchor-alias value="HTTP-Version"/>
766  <x:anchor-alias value="HTTP-Prot-Name"/>
767<t>
768   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
769   of the protocol. The protocol versioning policy is intended to allow
770   the sender to indicate the format of a message and its capacity for
771   understanding further HTTP communication, rather than the features
772   obtained via that communication. No change is made to the version
773   number for the addition of message components which do not affect
774   communication behavior or which only add to extensible field values.
775   The &lt;minor&gt; number is incremented when the changes made to the
776   protocol add features which do not change the general message parsing
777   algorithm, but which may add to the message semantics and imply
778   additional capabilities of the sender. The &lt;major&gt; number is
779   incremented when the format of a message within the protocol is
780   changed. See <xref target="RFC2145"/> for a fuller explanation.
781</t>
782<t>
783   The version of an HTTP message is indicated by an HTTP-Version field
784   in the first line of the message. HTTP-Version is case-sensitive.
785</t>
786<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/>
787  <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>
788  <x:ref>HTTP-Prot-Name</x:ref> = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
789</artwork></figure>
790<t>
791   Note that the major and minor numbers &MUST; be treated as separate
792   integers and that each &MAY; be incremented higher than a single digit.
793   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
794   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
795   &MUST-NOT; be sent.
796</t>
797<t>
798   An application that sends a request or response message that includes
799   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
800   with this specification. Applications that are at least conditionally
801   compliant with this specification &SHOULD; use an HTTP-Version of
802   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
803   not compatible with HTTP/1.0. For more details on when to send
804   specific HTTP-Version values, see <xref target="RFC2145"/>.
805</t>
806<t>
807   The HTTP version of an application is the highest HTTP version for
808   which the application is at least conditionally compliant.
809</t>
810<t>
811   Proxy and gateway applications need to be careful when forwarding
812   messages in protocol versions different from that of the application.
813   Since the protocol version indicates the protocol capability of the
814   sender, a proxy/gateway &MUST-NOT; send a message with a version
815   indicator which is greater than its actual version. If a higher
816   version request is received, the proxy/gateway &MUST; either downgrade
817   the request version, or respond with an error, or switch to tunnel
818   behavior.
819</t>
820<t>
821   Due to interoperability problems with HTTP/1.0 proxies discovered
822   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
823   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
824   they support. The proxy/gateway's response to that request &MUST; be in
825   the same major version as the request.
826</t>
827<t>
828  <list>
829    <t>
830      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
831      of header fields required or forbidden by the versions involved.
832    </t>
833  </list>
834</t>
835</section>
836
837<section title="Date/Time Formats" anchor="date.time.formats">
838<section title="Full Date" anchor="full.date">
839  <x:anchor-alias value="HTTP-date"/>
840  <x:anchor-alias value="obsolete-date"/>
841  <x:anchor-alias value="rfc1123-date"/>
842  <x:anchor-alias value="rfc850-date"/>
843  <x:anchor-alias value="asctime-date"/>
844  <x:anchor-alias value="date1"/>
845  <x:anchor-alias value="date2"/>
846  <x:anchor-alias value="date3"/>
847  <x:anchor-alias value="rfc1123-date"/>
848  <x:anchor-alias value="time"/>
849  <x:anchor-alias value="wkday"/>
850  <x:anchor-alias value="weekday"/>
851  <x:anchor-alias value="month"/>
852<t>
853   HTTP applications have historically allowed three different formats
854   for the representation of date/time stamps:
855</t>
856<figure><artwork type="example">
857   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123
858   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
859   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
860</artwork></figure>
861<t>
862   The first format is preferred as an Internet standard and represents
863   a fixed-length subset of that defined by <xref target="RFC1123"/>. The
864   other formats are described here only for
865   compatibility with obsolete implementations.
866   HTTP/1.1 clients and servers that parse the date value &MUST; accept
867   all three formats (for compatibility with HTTP/1.0), though they &MUST;
868   only generate the RFC 1123 format for representing HTTP-date values
869   in header fields. See <xref target="tolerant.applications"/> for further information.
870</t>
871<t><list><t>
872      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
873      accepting date values that may have been sent by non-HTTP
874      applications, as is sometimes the case when retrieving or posting
875      messages via proxies/gateways to SMTP or NNTP.
876</t></list></t>
877<t>
878   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
879   (GMT), without exception. For the purposes of HTTP, GMT is exactly
880   equal to UTC (Coordinated Universal Time). This is indicated in the
881   first two formats by the inclusion of "GMT" as the three-letter
882   abbreviation for time zone, and &MUST; be assumed when reading the
883   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
884   additional whitespace beyond that specifically included as SP in the
885   grammar.
886</t>
887<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"/>
888  <x:ref>HTTP-date</x:ref>    = <x:ref>rfc1123-date</x:ref> / <x:ref>obsolete-date</x:ref>
889  <x:ref>obsolete-date</x:ref> = <x:ref>rfc850-date</x:ref> / <x:ref>asctime-date</x:ref> 
890  <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
891  <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
892  <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>
893  <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>
894                 ; day month year (e.g., 02 Jun 1982)
895  <x:ref>date2</x:ref>        = 2<x:ref>DIGIT</x:ref> "-" <x:ref>month</x:ref> "-" 2<x:ref>DIGIT</x:ref>
896                 ; day-month-year (e.g., 02-Jun-82)
897  <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> ))
898                 ; month day (e.g., Jun  2)
899  <x:ref>time</x:ref>         = 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref>
900                 ; 00:00:00 - 23:59:59
901  <x:ref>wkday</x:ref>        = s-Mon / s-Tue / s-Wed
902               / s-Thu / s-Fri / s-Sat / s-Sun
903  <x:ref>weekday</x:ref>      = l-Mon / l-Tue / l-Wed
904               / l-Thu / l-Fri / l-Sat / l-Sun
905  <x:ref>month</x:ref>        = s-Jan / s-Feb / s-Mar / s-Apr
906               / s-May / s-Jun / s-Jul / s-Aug
907               / s-Sep / s-Oct / s-Nov / s-Dec
908               
909  GMT   = <x:abnf-char-sequence>"GMT"</x:abnf-char-sequence> ; "GMT", case-sensitive
910
911  s-Mon = <x:abnf-char-sequence>"Mon"</x:abnf-char-sequence> ; "Mon", case-sensitive
912  s-Tue = <x:abnf-char-sequence>"Tue"</x:abnf-char-sequence> ; "Tue", case-sensitive
913  s-Wed = <x:abnf-char-sequence>"Wed"</x:abnf-char-sequence> ; "Wed", case-sensitive
914  s-Thu = <x:abnf-char-sequence>"Thu"</x:abnf-char-sequence> ; "Thu", case-sensitive
915  s-Fri = <x:abnf-char-sequence>"Fri"</x:abnf-char-sequence> ; "Fri", case-sensitive
916  s-Sat = <x:abnf-char-sequence>"Sat"</x:abnf-char-sequence> ; "Sat", case-sensitive
917  s-Sun = <x:abnf-char-sequence>"Sun"</x:abnf-char-sequence> ; "Sun", case-sensitive
918
919  l-Mon = <x:abnf-char-sequence>"Monday"</x:abnf-char-sequence>          ; "Monday", case-sensitive
920  l-Tue = <x:abnf-char-sequence>"Tuesday"</x:abnf-char-sequence>       ; "Tuesday", case-sensitive
921  l-Wed = <x:abnf-char-sequence>"Wednesday"</x:abnf-char-sequence> ; "Wednesday", case-sensitive
922  l-Thu = <x:abnf-char-sequence>"Thursday"</x:abnf-char-sequence>    ; "Thursday", case-sensitive
923  l-Fri = <x:abnf-char-sequence>"Friday"</x:abnf-char-sequence>          ; "Friday", case-sensitive
924  l-Sat = <x:abnf-char-sequence>"Saturday"</x:abnf-char-sequence>    ; "Saturday", case-sensitive
925  l-Sun = <x:abnf-char-sequence>"Sunday"</x:abnf-char-sequence>          ; "Sunday", case-sensitive
926
927  s-Jan = <x:abnf-char-sequence>"Jan"</x:abnf-char-sequence> ; "Jan", case-sensitive
928  s-Feb = <x:abnf-char-sequence>"Feb"</x:abnf-char-sequence> ; "Feb", case-sensitive
929  s-Mar = <x:abnf-char-sequence>"Mar"</x:abnf-char-sequence> ; "Mar", case-sensitive
930  s-Apr = <x:abnf-char-sequence>"Apr"</x:abnf-char-sequence> ; "Apr", case-sensitive
931  s-May = <x:abnf-char-sequence>"May"</x:abnf-char-sequence> ; "May", case-sensitive
932  s-Jun = <x:abnf-char-sequence>"Jun"</x:abnf-char-sequence> ; "Jun", case-sensitive
933  s-Jul = <x:abnf-char-sequence>"Jul"</x:abnf-char-sequence> ; "Jul", case-sensitive
934  s-Aug = <x:abnf-char-sequence>"Aug"</x:abnf-char-sequence> ; "Aug", case-sensitive
935  s-Sep = <x:abnf-char-sequence>"Sep"</x:abnf-char-sequence> ; "Sep", case-sensitive
936  s-Oct = <x:abnf-char-sequence>"Oct"</x:abnf-char-sequence> ; "Oct", case-sensitive
937  s-Nov = <x:abnf-char-sequence>"Nov"</x:abnf-char-sequence> ; "Nov", case-sensitive
938  s-Dec = <x:abnf-char-sequence>"Dec"</x:abnf-char-sequence> ; "Dec", case-sensitive
939</artwork></figure>
940<t>
941      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
942      to their usage within the protocol stream. Clients and servers are
943      not required to use these formats for user presentation, request
944      logging, etc.
945</t>
946</section>
947</section>
948
949<section title="Transfer Codings" anchor="transfer.codings">
950  <x:anchor-alias value="parameter"/>
951  <x:anchor-alias value="transfer-coding"/>
952  <x:anchor-alias value="transfer-extension"/>
953<t>
954   Transfer-coding values are used to indicate an encoding
955   transformation that has been, can be, or may need to be applied to an
956   entity-body in order to ensure "safe transport" through the network.
957   This differs from a content coding in that the transfer-coding is a
958   property of the message, not of the original entity.
959</t>
960<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
961  <x:ref>transfer-coding</x:ref>         = "chunked" / <x:ref>transfer-extension</x:ref>
962  <x:ref>transfer-extension</x:ref>      = <x:ref>token</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
963</artwork></figure>
964<t anchor="rule.parameter">
965  <x:anchor-alias value="attribute"/>
966  <x:anchor-alias value="parameter"/>
967  <x:anchor-alias value="value"/>
968   Parameters are in  the form of attribute/value pairs.
969</t>
970<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"/>
971  <x:ref>parameter</x:ref>               = <x:ref>attribute</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> <x:ref>value</x:ref>
972  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
973  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
974</artwork></figure>
975<t>
976   All transfer-coding values are case-insensitive. HTTP/1.1 uses
977   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
978   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
979</t>
980<t>
981   Whenever a transfer-coding is applied to a message-body, the set of
982   transfer-codings &MUST; include "chunked", unless the message indicates it
983   is terminated by closing the connection. When the "chunked" transfer-coding
984   is used, it &MUST; be the last transfer-coding applied to the
985   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
986   than once to a message-body. These rules allow the recipient to
987   determine the transfer-length of the message (<xref target="message.length"/>).
988</t>
989<t>
990   Transfer-codings are analogous to the Content-Transfer-Encoding
991   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
992   binary data over a 7-bit transport service. However, safe transport
993   has a different focus for an 8bit-clean transfer protocol. In HTTP,
994   the only unsafe characteristic of message-bodies is the difficulty in
995   determining the exact body length (<xref target="message.length"/>), or the desire to
996   encrypt data over a shared transport.
997</t>
998<t>
999   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1000   transfer-coding value tokens. Initially, the registry contains the
1001   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1002   "gzip", "compress", and "deflate" (&content-codings;).
1003</t>
1004<t>
1005   New transfer-coding value tokens &SHOULD; be registered in the same way
1006   as new content-coding value tokens (&content-codings;).
1007</t>
1008<t>
1009   A server which receives an entity-body with a transfer-coding it does
1010   not understand &SHOULD; return 501 (Not Implemented), and close the
1011   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1012   client.
1013</t>
1014
1015<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1016  <x:anchor-alias value="chunk"/>
1017  <x:anchor-alias value="Chunked-Body"/>
1018  <x:anchor-alias value="chunk-data"/>
1019  <x:anchor-alias value="chunk-ext"/>
1020  <x:anchor-alias value="chunk-ext-name"/>
1021  <x:anchor-alias value="chunk-ext-val"/>
1022  <x:anchor-alias value="chunk-size"/>
1023  <x:anchor-alias value="last-chunk"/>
1024  <x:anchor-alias value="trailer-part"/>
1025<t>
1026   The chunked encoding modifies the body of a message in order to
1027   transfer it as a series of chunks, each with its own size indicator,
1028   followed by an &OPTIONAL; trailer containing entity-header fields. This
1029   allows dynamically produced content to be transferred along with the
1030   information necessary for the recipient to verify that it has
1031   received the full message.
1032</t>
1033<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"/>
1034  <x:ref>Chunked-Body</x:ref>   = *<x:ref>chunk</x:ref>
1035                   <x:ref>last-chunk</x:ref>
1036                   <x:ref>trailer-part</x:ref>
1037                   <x:ref>CRLF</x:ref>
1038 
1039  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1040                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1041  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1042  <x:ref>last-chunk</x:ref>     = 1*("0") *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1043 
1044  <x:ref>chunk-ext</x:ref>      = *( ";" *WSP <x:ref>chunk-ext-name</x:ref>
1045                      [ "=" <x:ref>chunk-ext-val</x:ref> ] *WSP )
1046  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1047  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1048  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1049  <x:ref>trailer-part</x:ref>   = *(<x:ref>entity-header</x:ref> <x:ref>CRLF</x:ref>)
1050</artwork></figure>
1051<t>
1052   The chunk-size field is a string of hex digits indicating the size of
1053   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1054   zero, followed by the trailer, which is terminated by an empty line.
1055</t>
1056<t>
1057   The trailer allows the sender to include additional HTTP header
1058   fields at the end of the message. The Trailer header field can be
1059   used to indicate which header fields are included in a trailer (see
1060   <xref target="header.trailer"/>).
1061</t>
1062<t>
1063   A server using chunked transfer-coding in a response &MUST-NOT; use the
1064   trailer for any header fields unless at least one of the following is
1065   true:
1066  <list style="numbers">
1067    <t>the request included a TE header field that indicates "trailers" is
1068     acceptable in the transfer-coding of the  response, as described in
1069     <xref target="header.te"/>; or,</t>
1070
1071    <t>the server is the origin server for the response, the trailer
1072     fields consist entirely of optional metadata, and the recipient
1073     could use the message (in a manner acceptable to the origin server)
1074     without receiving this metadata.  In other words, the origin server
1075     is willing to accept the possibility that the trailer fields might
1076     be silently discarded along the path to the client.</t>
1077  </list>
1078</t>
1079<t>
1080   This requirement prevents an interoperability failure when the
1081   message is being received by an HTTP/1.1 (or later) proxy and
1082   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1083   compliance with the protocol would have necessitated a possibly
1084   infinite buffer on the proxy.
1085</t>
1086<t>
1087   A process for decoding the "chunked" transfer-coding
1088   can be represented in pseudo-code as:
1089</t>
1090<figure><artwork type="code">
1091  length := 0
1092  read chunk-size, chunk-ext (if any) and CRLF
1093  while (chunk-size &gt; 0) {
1094     read chunk-data and CRLF
1095     append chunk-data to entity-body
1096     length := length + chunk-size
1097     read chunk-size and CRLF
1098  }
1099  read entity-header
1100  while (entity-header not empty) {
1101     append entity-header to existing header fields
1102     read entity-header
1103  }
1104  Content-Length := length
1105  Remove "chunked" from Transfer-Encoding
1106</artwork></figure>
1107<t>
1108   All HTTP/1.1 applications &MUST; be able to receive and decode the
1109   "chunked" transfer-coding, and &MUST; ignore chunk-ext extensions
1110   they do not understand.
1111</t>
1112</section>
1113</section>
1114
1115<section title="Product Tokens" anchor="product.tokens">
1116  <x:anchor-alias value="product"/>
1117  <x:anchor-alias value="product-version"/>
1118<t>
1119   Product tokens are used to allow communicating applications to
1120   identify themselves by software name and version. Most fields using
1121   product tokens also allow sub-products which form a significant part
1122   of the application to be listed, separated by whitespace. By
1123   convention, the products are listed in order of their significance
1124   for identifying the application.
1125</t>
1126<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1127  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
1128  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
1129</artwork></figure>
1130<t>
1131   Examples:
1132</t>
1133<figure><artwork type="example">
1134    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1135    Server: Apache/0.8.4
1136</artwork></figure>
1137<t>
1138   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1139   used for advertising or other non-essential information. Although any
1140   token character &MAY; appear in a product-version, this token &SHOULD;
1141   only be used for a version identifier (i.e., successive versions of
1142   the same product &SHOULD; only differ in the product-version portion of
1143   the product value).
1144</t>
1145</section>
1146
1147</section>
1148
1149<section title="HTTP Message" anchor="http.message">
1150
1151<section title="Message Types" anchor="message.types">
1152  <x:anchor-alias value="generic-message"/>
1153  <x:anchor-alias value="HTTP-message"/>
1154  <x:anchor-alias value="start-line"/>
1155<t>
1156   HTTP messages consist of requests from client to server and responses
1157   from server to client.
1158</t>
1159<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1160  <x:ref>HTTP-message</x:ref>   = <x:ref>Request</x:ref> / <x:ref>Response</x:ref>     ; HTTP/1.1 messages
1161</artwork></figure>
1162<t>
1163   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1164   message format of <xref target="RFC5322"/> for transferring entities (the payload
1165   of the message). Both types of message consist of a start-line, zero
1166   or more header fields (also known as "headers"), an empty line (i.e.,
1167   a line with nothing preceding the CRLF) indicating the end of the
1168   header fields, and possibly a message-body.
1169</t>
1170<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1171  <x:ref>generic-message</x:ref> = <x:ref>start-line</x:ref>
1172                    *(<x:ref>message-header</x:ref> <x:ref>CRLF</x:ref>)
1173                    <x:ref>CRLF</x:ref>
1174                    [ <x:ref>message-body</x:ref> ]
1175  <x:ref>start-line</x:ref>      = <x:ref>Request-Line</x:ref> / <x:ref>Status-Line</x:ref>
1176</artwork></figure>
1177<t>
1178   In the interest of robustness, servers &SHOULD; ignore any empty
1179   line(s) received where a Request-Line is expected. In other words, if
1180   the server is reading the protocol stream at the beginning of a
1181   message and receives a CRLF first, it should ignore the CRLF.
1182</t>
1183<t>
1184   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1185   after a POST request. To restate what is explicitly forbidden by the
1186   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1187   extra CRLF.
1188</t>
1189<t>
1190   Whitespace (WSP) &MUST-NOT; be sent between the start-line and the first
1191   header field. The presence of whitespace might be an attempt to trick a
1192   noncompliant implementation of HTTP into ignoring that field or processing
1193   the next line as a new request, either of which may result in security
1194   issues when implementations within the request chain interpret the
1195   same message differently. HTTP/1.1 servers &MUST; reject such a message
1196   with a 400 (Bad Request) response.
1197</t>
1198</section>
1199
1200<section title="Message Headers" anchor="message.headers">
1201  <x:anchor-alias value="field-content"/>
1202  <x:anchor-alias value="field-name"/>
1203  <x:anchor-alias value="field-value"/>
1204  <x:anchor-alias value="message-header"/>
1205<t>
1206   HTTP header fields follow the same general format as Internet messages in
1207   <xref target="RFC5322" x:fmt="of" x:sec="2.1"/>. Each header field consists
1208   of a name followed by a colon (":"), optional whitespace, and the field
1209   value. Field names are case-insensitive.
1210</t>
1211<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"/>
1212  <x:ref>message-header</x:ref> = <x:ref>field-name</x:ref> ":" OWS [ <x:ref>field-value</x:ref> ] OWS
1213  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1214  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>OWS</x:ref> )
1215  <x:ref>field-content</x:ref>  = *( <x:ref>WSP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1216</artwork></figure>
1217<t>
1218   Historically, HTTP has allowed field-content with text in the ISO-8859-1
1219   <xref target="ISO-8859-1"/> character encoding (allowing other character sets
1220   through use of <xref target="RFC2047"/> encoding). In practice, most HTTP
1221   header field-values use only a subset of the US-ASCII charset
1222   <xref target="USASCII"/>. Newly defined header fields &SHOULD; constrain
1223   their field-values to US-ASCII characters. Recipients &SHOULD; treat other
1224   (obs-text) octets in field-content as opaque data.
1225</t>
1226<t>
1227   No whitespace is allowed between the header field-name and colon. For
1228   security reasons, any request message received containing such whitespace
1229   &MUST; be rejected with a response code of 400 (Bad Request) and any such
1230   whitespace in a response message &MUST; be removed.
1231</t>
1232<t>
1233   The field value &MAY; be preceded by optional whitespace; a single SP is
1234   preferred. The field-value does not include any leading or trailing white
1235   space: OWS occurring before the first non-whitespace character of the
1236   field-value or after the last non-whitespace character of the field-value
1237   is ignored and &MAY; be removed without changing the meaning of the header
1238   field.
1239</t>
1240<t>
1241   Historically, HTTP header field values could be extended over multiple
1242   lines by preceding each extra line with at least one space or horizontal
1243   tab character (line folding). This specification deprecates such line
1244   folding except within the message/http media type
1245   (<xref target="internet.media.type.message.http"/>).
1246   HTTP/1.1 senders &MUST-NOT; produce messages that include line folding
1247   (i.e., that contain any field-content that matches the obs-fold rule) unless
1248   the message is intended for packaging within the message/http media type.
1249   HTTP/1.1 recipients &SHOULD; accept line folding and replace any embedded
1250   obs-fold whitespace with a single SP prior to interpreting the field value
1251   or forwarding the message downstream.
1252</t>
1253<t anchor="rule.comment">
1254  <x:anchor-alias value="comment"/>
1255  <x:anchor-alias value="ctext"/>
1256   Comments can be included in some HTTP header fields by surrounding
1257   the comment text with parentheses. Comments are only allowed in
1258   fields containing "comment" as part of their field value definition.
1259   In all other fields, parentheses are considered part of the field
1260   value.
1261</t>
1262<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1263  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1264  <x:ref>ctext</x:ref>          = *( <x:ref>OWS</x:ref> / %x21-27 / %x2A-7E / <x:ref>obs-text</x:ref> )
1265</artwork></figure>
1266<t>
1267   The order in which header fields with differing field names are
1268   received is not significant. However, it is "good practice" to send
1269   general-header fields first, followed by request-header or response-header
1270   fields, and ending with the entity-header fields.
1271</t>
1272<t>
1273   Multiple message-header fields with the same field-name &MAY; be
1274   present in a message if and only if the entire field-value for that
1275   header field is defined as a comma-separated list [i.e., #(values)].
1276   It &MUST; be possible to combine the multiple header fields into one
1277   "field-name: field-value" pair, without changing the semantics of the
1278   message, by appending each subsequent field-value to the first, each
1279   separated by a comma. The order in which header fields with the same
1280   field-name are received is therefore significant to the
1281   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1282   change the order of these field values when a message is forwarded.
1283</t>
1284<t>
1285  <list><t>
1286   <x:h>Note:</x:h> the "Set-Cookie" header as implemented in
1287   practice (as opposed to how it is specified in <xref target="RFC2109"/>)
1288   can occur multiple times, but does not use the list syntax, and thus cannot
1289   be combined into a single line. (See Appendix A.2.3 of <xref target="Kri2001"/>
1290   for details.) Also note that the Set-Cookie2 header specified in
1291   <xref target="RFC2965"/> does not share this problem.
1292  </t></list>
1293</t>
1294 
1295</section>
1296
1297<section title="Message Body" anchor="message.body">
1298  <x:anchor-alias value="message-body"/>
1299<t>
1300   The message-body (if any) of an HTTP message is used to carry the
1301   entity-body associated with the request or response. The message-body
1302   differs from the entity-body only when a transfer-coding has been
1303   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1304</t>
1305<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1306  <x:ref>message-body</x:ref> = <x:ref>entity-body</x:ref>
1307               / &lt;entity-body encoded as per <x:ref>Transfer-Encoding</x:ref>&gt;
1308</artwork></figure>
1309<t>
1310   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1311   applied by an application to ensure safe and proper transfer of the
1312   message. Transfer-Encoding is a property of the message, not of the
1313   entity, and thus &MAY; be added or removed by any application along the
1314   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1315   when certain transfer-codings may be used.)
1316</t>
1317<t>
1318   The rules for when a message-body is allowed in a message differ for
1319   requests and responses.
1320</t>
1321<t>
1322   The presence of a message-body in a request is signaled by the
1323   inclusion of a Content-Length or Transfer-Encoding header field in
1324   the request's message-headers. A message-body &MUST-NOT; be included in
1325   a request if the specification of the request method (&method;)
1326   explicitly disallows an entity-body in requests.
1327   When a request message contains both a message-body of non-zero
1328   length and a method that does not define any semantics for that
1329   request message-body, then an origin server &SHOULD; either ignore
1330   the message-body or respond with an appropriate error message
1331   (e.g., 413).  A proxy or gateway, when presented the same request,
1332   &SHOULD; either forward the request inbound with the message-body or
1333   ignore the message-body when determining a response.
1334</t>
1335<t>
1336   For response messages, whether or not a message-body is included with
1337   a message is dependent on both the request method and the response
1338   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1339   &MUST-NOT; include a message-body, even though the presence of entity-header
1340   fields might lead one to believe they do. All 1xx
1341   (informational), 204 (No Content), and 304 (Not Modified) responses
1342   &MUST-NOT; include a message-body. All other responses do include a
1343   message-body, although it &MAY; be of zero length.
1344</t>
1345</section>
1346
1347<section title="Message Length" anchor="message.length">
1348<t>
1349   The transfer-length of a message is the length of the message-body as
1350   it appears in the message; that is, after any transfer-codings have
1351   been applied. When a message-body is included with a message, the
1352   transfer-length of that body is determined by one of the following
1353   (in order of precedence):
1354</t>
1355<t>
1356  <list style="numbers">
1357    <x:lt><t>
1358     Any response message which "&MUST-NOT;" include a message-body (such
1359     as the 1xx, 204, and 304 responses and any response to a HEAD
1360     request) is always terminated by the first empty line after the
1361     header fields, regardless of the entity-header fields present in
1362     the message.
1363    </t></x:lt>
1364    <x:lt><t>
1365     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1366     is present and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
1367     is used, the transfer-length is defined by the use of this transfer-coding.
1368     If a Transfer-Encoding header field is present and the "chunked" transfer-coding
1369     is not present, the transfer-length is defined by the sender closing the connection.
1370    </t></x:lt>
1371    <x:lt><t>
1372     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1373     decimal value in OCTETs represents both the entity-length and the
1374     transfer-length. The Content-Length header field &MUST-NOT; be sent
1375     if these two lengths are different (i.e., if a Transfer-Encoding
1376     header field is present). If a message is received with both a
1377     Transfer-Encoding header field and a Content-Length header field,
1378     the latter &MUST; be ignored.
1379    </t></x:lt>
1380    <x:lt><t>
1381     If the message uses the media type "multipart/byteranges", and the
1382     transfer-length is not otherwise specified, then this self-delimiting
1383     media type defines the transfer-length. This media type
1384     &MUST-NOT; be used unless the sender knows that the recipient can parse
1385     it; the presence in a request of a Range header with multiple byte-range
1386     specifiers from a 1.1 client implies that the client can parse
1387     multipart/byteranges responses.
1388    <list style="empty"><t>
1389       A range header might be forwarded by a 1.0 proxy that does not
1390       understand multipart/byteranges; in this case the server &MUST;
1391       delimit the message using methods defined in items 1, 3 or 5 of
1392       this section.
1393    </t></list>
1394    </t></x:lt>
1395    <x:lt><t>
1396     By the server closing the connection. (Closing the connection
1397     cannot be used to indicate the end of a request body, since that
1398     would leave no possibility for the server to send back a response.)
1399    </t></x:lt>
1400  </list>
1401</t>
1402<t>
1403   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1404   containing a message-body &MUST; include a valid Content-Length header
1405   field unless the server is known to be HTTP/1.1 compliant. If a
1406   request contains a message-body and a Content-Length is not given,
1407   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1408   determine the length of the message, or with 411 (Length Required) if
1409   it wishes to insist on receiving a valid Content-Length.
1410</t>
1411<t>
1412   All HTTP/1.1 applications that receive entities &MUST; accept the
1413   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1414   to be used for messages when the message length cannot be determined
1415   in advance.
1416</t>
1417<t>
1418   Messages &MUST-NOT; include both a Content-Length header field and a
1419   transfer-coding. If the message does include a
1420   transfer-coding, the Content-Length &MUST; be ignored.
1421</t>
1422<t>
1423   When a Content-Length is given in a message where a message-body is
1424   allowed, its field value &MUST; exactly match the number of OCTETs in
1425   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1426   invalid length is received and detected.
1427</t>
1428</section>
1429
1430<section title="General Header Fields" anchor="general.header.fields">
1431  <x:anchor-alias value="general-header"/>
1432<t>
1433   There are a few header fields which have general applicability for
1434   both request and response messages, but which do not apply to the
1435   entity being transferred. These header fields apply only to the
1436   message being transmitted.
1437</t>
1438<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1439  <x:ref>general-header</x:ref> = <x:ref>Cache-Control</x:ref>            ; &header-cache-control;
1440                 / <x:ref>Connection</x:ref>               ; <xref target="header.connection"/>
1441                 / <x:ref>Date</x:ref>                     ; <xref target="header.date"/>
1442                 / <x:ref>Pragma</x:ref>                   ; &header-pragma;
1443                 / <x:ref>Trailer</x:ref>                  ; <xref target="header.trailer"/>
1444                 / <x:ref>Transfer-Encoding</x:ref>        ; <xref target="header.transfer-encoding"/>
1445                 / <x:ref>Upgrade</x:ref>                  ; <xref target="header.upgrade"/>
1446                 / <x:ref>Via</x:ref>                      ; <xref target="header.via"/>
1447                 / <x:ref>Warning</x:ref>                  ; &header-warning;
1448</artwork></figure>
1449<t>
1450   General-header field names can be extended reliably only in
1451   combination with a change in the protocol version. However, new or
1452   experimental header fields may be given the semantics of general
1453   header fields if all parties in the communication recognize them to
1454   be general-header fields. Unrecognized header fields are treated as
1455   entity-header fields.
1456</t>
1457</section>
1458</section>
1459
1460<section title="Request" anchor="request">
1461  <x:anchor-alias value="Request"/>
1462<t>
1463   A request message from a client to a server includes, within the
1464   first line of that message, the method to be applied to the resource,
1465   the identifier of the resource, and the protocol version in use.
1466</t>
1467<!--                 Host                      ; should be moved here eventually -->
1468<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1469  <x:ref>Request</x:ref>       = <x:ref>Request-Line</x:ref>              ; <xref target="request-line"/>
1470                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1471                   / <x:ref>request-header</x:ref>         ; &request-header-fields;
1472                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1473                  <x:ref>CRLF</x:ref>
1474                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1475</artwork></figure>
1476
1477<section title="Request-Line" anchor="request-line">
1478  <x:anchor-alias value="Request-Line"/>
1479<t>
1480   The Request-Line begins with a method token, followed by the
1481   request-target and the protocol version, and ending with CRLF. The
1482   elements are separated by SP characters. No CR or LF is allowed
1483   except in the final CRLF sequence.
1484</t>
1485<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1486  <x:ref>Request-Line</x:ref>   = <x:ref>Method</x:ref> <x:ref>SP</x:ref> <x:ref>request-target</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-Version</x:ref> <x:ref>CRLF</x:ref>
1487</artwork></figure>
1488
1489<section title="Method" anchor="method">
1490  <x:anchor-alias value="Method"/>
1491<t>
1492   The Method  token indicates the method to be performed on the
1493   resource identified by the request-target. The method is case-sensitive.
1494</t>
1495<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1496  <x:ref>Method</x:ref>         = <x:ref>token</x:ref>
1497</artwork></figure>
1498</section>
1499
1500<section title="request-target" anchor="request-target">
1501  <x:anchor-alias value="request-target"/>
1502<t>
1503   The request-target is a Uniform Resource Identifier (<xref target="uri"/>) and
1504   identifies the resource upon which to apply the request.
1505</t>
1506<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/>
1507  <x:ref>request-target</x:ref>    = "*"
1508                 / <x:ref>absolute-URI</x:ref>
1509                 / ( <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ] )
1510                 / <x:ref>authority</x:ref>
1511</artwork></figure>
1512<t>
1513   The four options for request-target are dependent on the nature of the
1514   request. The asterisk "*" means that the request does not apply to a
1515   particular resource, but to the server itself, and is only allowed
1516   when the method used does not necessarily apply to a resource. One
1517   example would be
1518</t>
1519<figure><artwork type="example">
1520  OPTIONS * HTTP/1.1
1521</artwork></figure>
1522<t>
1523   The absolute-URI form is &REQUIRED; when the request is being made to a
1524   proxy. The proxy is requested to forward the request or service it
1525   from a valid cache, and return the response. Note that the proxy &MAY;
1526   forward the request on to another proxy or directly to the server
1527   specified by the absolute-URI. In order to avoid request loops, a
1528   proxy &MUST; be able to recognize all of its server names, including
1529   any aliases, local variations, and the numeric IP address. An example
1530   Request-Line would be:
1531</t>
1532<figure><artwork type="example">
1533  GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1534</artwork></figure>
1535<t>
1536   To allow for transition to absolute-URIs in all requests in future
1537   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absolute-URI
1538   form in requests, even though HTTP/1.1 clients will only generate
1539   them in requests to proxies.
1540</t>
1541<t>
1542   The authority form is only used by the CONNECT method (&CONNECT;).
1543</t>
1544<t>
1545   The most common form of request-target is that used to identify a
1546   resource on an origin server or gateway. In this case the absolute
1547   path of the URI &MUST; be transmitted (see <xref target="http.uri"/>, path-absolute) as
1548   the request-target, and the network location of the URI (authority) &MUST;
1549   be transmitted in a Host header field. For example, a client wishing
1550   to retrieve the resource above directly from the origin server would
1551   create a TCP connection to port 80 of the host "www.example.org" and send
1552   the lines:
1553</t>
1554<figure><artwork type="example">
1555  GET /pub/WWW/TheProject.html HTTP/1.1
1556  Host: www.example.org
1557</artwork></figure>
1558<t>
1559   followed by the remainder of the Request. Note that the absolute path
1560   cannot be empty; if none is present in the original URI, it &MUST; be
1561   given as "/" (the server root).
1562</t>
1563<t>
1564   If a proxy receives a request without any path in the request-target and
1565   the method specified is capable of supporting the asterisk form of
1566   request-target, then the last proxy on the request chain &MUST; forward the
1567   request with "*" as the final request-target.
1568</t>
1569<figure><preamble>   
1570   For example, the request
1571</preamble><artwork type="example">
1572  OPTIONS http://www.example.org:8001 HTTP/1.1
1573</artwork></figure>
1574<figure><preamble>   
1575  would be forwarded by the proxy as
1576</preamble><artwork type="example">
1577  OPTIONS * HTTP/1.1
1578  Host: www.example.org:8001
1579</artwork>
1580<postamble>
1581   after connecting to port 8001 of host "www.example.org".
1582</postamble>
1583</figure>
1584<t>
1585   The request-target is transmitted in the format specified in
1586   <xref target="http.uri"/>. If the request-target is encoded using the
1587   "% <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding
1588   (<xref target="RFC3986" x:fmt="," x:sec="2.4"/>), the origin server
1589   &MUST; decode the request-target in order to
1590   properly interpret the request. Servers &SHOULD; respond to invalid
1591   request-targets with an appropriate status code.
1592</t>
1593<t>
1594   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1595   received request-target when forwarding it to the next inbound server,
1596   except as noted above to replace a null path-absolute with "/".
1597</t>
1598<t>
1599  <list><t>
1600      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1601      meaning of the request when the origin server is improperly using
1602      a non-reserved URI character for a reserved purpose.  Implementors
1603      should be aware that some pre-HTTP/1.1 proxies have been known to
1604      rewrite the request-target.
1605  </t></list>
1606</t>
1607<t>
1608   HTTP does not place a pre-defined limit on the length of a request-target.
1609   A server &MUST; be prepared to receive URIs of unbounded length and
1610   respond with the 414 (URI too long) status if the received
1611   request-target would be longer than the server wishes to handle
1612   (see &status-414;).
1613</t>
1614<t>
1615   Various ad-hoc limitations on request-target length are found in practice.
1616   It is &RECOMMENDED; that all HTTP senders and recipients support
1617   request-target lengths of 8000 or more OCTETs.
1618</t>
1619</section>
1620</section>
1621
1622<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1623<t>
1624   The exact resource identified by an Internet request is determined by
1625   examining both the request-target and the Host header field.
1626</t>
1627<t>
1628   An origin server that does not allow resources to differ by the
1629   requested host &MAY; ignore the Host header field value when
1630   determining the resource identified by an HTTP/1.1 request. (But see
1631   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1632   for other requirements on Host support in HTTP/1.1.)
1633</t>
1634<t>
1635   An origin server that does differentiate resources based on the host
1636   requested (sometimes referred to as virtual hosts or vanity host
1637   names) &MUST; use the following rules for determining the requested
1638   resource on an HTTP/1.1 request:
1639  <list style="numbers">
1640    <t>If request-target is an absolute-URI, the host is part of the
1641     request-target. Any Host header field value in the request &MUST; be
1642     ignored.</t>
1643    <t>If the request-target is not an absolute-URI, and the request includes
1644     a Host header field, the host is determined by the Host header
1645     field value.</t>
1646    <t>If the host as determined by rule 1 or 2 is not a valid host on
1647     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1648  </list>
1649</t>
1650<t>
1651   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1652   attempt to use heuristics (e.g., examination of the URI path for
1653   something unique to a particular host) in order to determine what
1654   exact resource is being requested.
1655</t>
1656</section>
1657
1658</section>
1659
1660
1661<section title="Response" anchor="response">
1662  <x:anchor-alias value="Response"/>
1663<t>
1664   After receiving and interpreting a request message, a server responds
1665   with an HTTP response message.
1666</t>
1667<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1668  <x:ref>Response</x:ref>      = <x:ref>Status-Line</x:ref>               ; <xref target="status-line"/>
1669                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1670                   / <x:ref>response-header</x:ref>        ; &response-header-fields;
1671                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1672                  <x:ref>CRLF</x:ref>
1673                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1674</artwork></figure>
1675
1676<section title="Status-Line" anchor="status-line">
1677  <x:anchor-alias value="Status-Line"/>
1678<t>
1679   The first line of a Response message is the Status-Line, consisting
1680   of the protocol version followed by a numeric status code and its
1681   associated textual phrase, with each element separated by SP
1682   characters. No CR or LF is allowed except in the final CRLF sequence.
1683</t>
1684<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1685  <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>
1686</artwork></figure>
1687
1688<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1689  <x:anchor-alias value="Reason-Phrase"/>
1690  <x:anchor-alias value="Status-Code"/>
1691<t>
1692   The Status-Code element is a 3-digit integer result code of the
1693   attempt to understand and satisfy the request. These codes are fully
1694   defined in &status-codes;.  The Reason Phrase exists for the sole
1695   purpose of providing a textual description associated with the numeric
1696   status code, out of deference to earlier Internet application protocols
1697   that were more frequently used with interactive text clients.
1698   A client &SHOULD; ignore the content of the Reason Phrase.
1699</t>
1700<t>
1701   The first digit of the Status-Code defines the class of response. The
1702   last two digits do not have any categorization role. There are 5
1703   values for the first digit:
1704  <list style="symbols">
1705    <t>
1706      1xx: Informational - Request received, continuing process
1707    </t>
1708    <t>
1709      2xx: Success - The action was successfully received,
1710        understood, and accepted
1711    </t>
1712    <t>
1713      3xx: Redirection - Further action must be taken in order to
1714        complete the request
1715    </t>
1716    <t>
1717      4xx: Client Error - The request contains bad syntax or cannot
1718        be fulfilled
1719    </t>
1720    <t>
1721      5xx: Server Error - The server failed to fulfill an apparently
1722        valid request
1723    </t>
1724  </list>
1725</t>
1726<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"/>
1727  <x:ref>Status-Code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1728  <x:ref>Reason-Phrase</x:ref>  = *( <x:ref>WSP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1729</artwork></figure>
1730</section>
1731</section>
1732
1733</section>
1734
1735
1736<section title="Connections" anchor="connections">
1737
1738<section title="Persistent Connections" anchor="persistent.connections">
1739
1740<section title="Purpose" anchor="persistent.purpose">
1741<t>
1742   Prior to persistent connections, a separate TCP connection was
1743   established to fetch each URL, increasing the load on HTTP servers
1744   and causing congestion on the Internet. The use of inline images and
1745   other associated data often require a client to make multiple
1746   requests of the same server in a short amount of time. Analysis of
1747   these performance problems and results from a prototype
1748   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1749   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1750   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1751   T/TCP <xref target="Tou1998"/>.
1752</t>
1753<t>
1754   Persistent HTTP connections have a number of advantages:
1755  <list style="symbols">
1756      <t>
1757        By opening and closing fewer TCP connections, CPU time is saved
1758        in routers and hosts (clients, servers, proxies, gateways,
1759        tunnels, or caches), and memory used for TCP protocol control
1760        blocks can be saved in hosts.
1761      </t>
1762      <t>
1763        HTTP requests and responses can be pipelined on a connection.
1764        Pipelining allows a client to make multiple requests without
1765        waiting for each response, allowing a single TCP connection to
1766        be used much more efficiently, with much lower elapsed time.
1767      </t>
1768      <t>
1769        Network congestion is reduced by reducing the number of packets
1770        caused by TCP opens, and by allowing TCP sufficient time to
1771        determine the congestion state of the network.
1772      </t>
1773      <t>
1774        Latency on subsequent requests is reduced since there is no time
1775        spent in TCP's connection opening handshake.
1776      </t>
1777      <t>
1778        HTTP can evolve more gracefully, since errors can be reported
1779        without the penalty of closing the TCP connection. Clients using
1780        future versions of HTTP might optimistically try a new feature,
1781        but if communicating with an older server, retry with old
1782        semantics after an error is reported.
1783      </t>
1784    </list>
1785</t>
1786<t>
1787   HTTP implementations &SHOULD; implement persistent connections.
1788</t>
1789</section>
1790
1791<section title="Overall Operation" anchor="persistent.overall">
1792<t>
1793   A significant difference between HTTP/1.1 and earlier versions of
1794   HTTP is that persistent connections are the default behavior of any
1795   HTTP connection. That is, unless otherwise indicated, the client
1796   &SHOULD; assume that the server will maintain a persistent connection,
1797   even after error responses from the server.
1798</t>
1799<t>
1800   Persistent connections provide a mechanism by which a client and a
1801   server can signal the close of a TCP connection. This signaling takes
1802   place using the Connection header field (<xref target="header.connection"/>). Once a close
1803   has been signaled, the client &MUST-NOT; send any more requests on that
1804   connection.
1805</t>
1806
1807<section title="Negotiation" anchor="persistent.negotiation">
1808<t>
1809   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1810   maintain a persistent connection unless a Connection header including
1811   the connection-token "close" was sent in the request. If the server
1812   chooses to close the connection immediately after sending the
1813   response, it &SHOULD; send a Connection header including the
1814   connection-token close.
1815</t>
1816<t>
1817   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1818   decide to keep it open based on whether the response from a server
1819   contains a Connection header with the connection-token close. In case
1820   the client does not want to maintain a connection for more than that
1821   request, it &SHOULD; send a Connection header including the
1822   connection-token close.
1823</t>
1824<t>
1825   If either the client or the server sends the close token in the
1826   Connection header, that request becomes the last one for the
1827   connection.
1828</t>
1829<t>
1830   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1831   maintained for HTTP versions less than 1.1 unless it is explicitly
1832   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1833   compatibility with HTTP/1.0 clients.
1834</t>
1835<t>
1836   In order to remain persistent, all messages on the connection &MUST;
1837   have a self-defined message length (i.e., one not defined by closure
1838   of the connection), as described in <xref target="message.length"/>.
1839</t>
1840</section>
1841
1842<section title="Pipelining" anchor="pipelining">
1843<t>
1844   A client that supports persistent connections &MAY; "pipeline" its
1845   requests (i.e., send multiple requests without waiting for each
1846   response). A server &MUST; send its responses to those requests in the
1847   same order that the requests were received.
1848</t>
1849<t>
1850   Clients which assume persistent connections and pipeline immediately
1851   after connection establishment &SHOULD; be prepared to retry their
1852   connection if the first pipelined attempt fails. If a client does
1853   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1854   persistent. Clients &MUST; also be prepared to resend their requests if
1855   the server closes the connection before sending all of the
1856   corresponding responses.
1857</t>
1858<t>
1859   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1860   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1861   premature termination of the transport connection could lead to
1862   indeterminate results. A client wishing to send a non-idempotent
1863   request &SHOULD; wait to send that request until it has received the
1864   response status for the previous request.
1865</t>
1866</section>
1867</section>
1868
1869<section title="Proxy Servers" anchor="persistent.proxy">
1870<t>
1871   It is especially important that proxies correctly implement the
1872   properties of the Connection header field as specified in <xref target="header.connection"/>.
1873</t>
1874<t>
1875   The proxy server &MUST; signal persistent connections separately with
1876   its clients and the origin servers (or other proxy servers) that it
1877   connects to. Each persistent connection applies to only one transport
1878   link.
1879</t>
1880<t>
1881   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1882   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
1883   discussion of the problems with the Keep-Alive header implemented by
1884   many HTTP/1.0 clients).
1885</t>
1886</section>
1887
1888<section title="Practical Considerations" anchor="persistent.practical">
1889<t>
1890   Servers will usually have some time-out value beyond which they will
1891   no longer maintain an inactive connection. Proxy servers might make
1892   this a higher value since it is likely that the client will be making
1893   more connections through the same server. The use of persistent
1894   connections places no requirements on the length (or existence) of
1895   this time-out for either the client or the server.
1896</t>
1897<t>
1898   When a client or server wishes to time-out it &SHOULD; issue a graceful
1899   close on the transport connection. Clients and servers &SHOULD; both
1900   constantly watch for the other side of the transport close, and
1901   respond to it as appropriate. If a client or server does not detect
1902   the other side's close promptly it could cause unnecessary resource
1903   drain on the network.
1904</t>
1905<t>
1906   A client, server, or proxy &MAY; close the transport connection at any
1907   time. For example, a client might have started to send a new request
1908   at the same time that the server has decided to close the "idle"
1909   connection. From the server's point of view, the connection is being
1910   closed while it was idle, but from the client's point of view, a
1911   request is in progress.
1912</t>
1913<t>
1914   This means that clients, servers, and proxies &MUST; be able to recover
1915   from asynchronous close events. Client software &SHOULD; reopen the
1916   transport connection and retransmit the aborted sequence of requests
1917   without user interaction so long as the request sequence is
1918   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1919   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1920   human operator the choice of retrying the request(s). Confirmation by
1921   user-agent software with semantic understanding of the application
1922   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
1923   be repeated if the second sequence of requests fails.
1924</t>
1925<t>
1926   Servers &SHOULD; always respond to at least one request per connection,
1927   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1928   middle of transmitting a response, unless a network or client failure
1929   is suspected.
1930</t>
1931<t>
1932   Clients that use persistent connections &SHOULD; limit the number of
1933   simultaneous connections that they maintain to a given server. A
1934   single-user client &SHOULD-NOT; maintain more than 2 connections with
1935   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1936   another server or proxy, where N is the number of simultaneously
1937   active users. These guidelines are intended to improve HTTP response
1938   times and avoid congestion.
1939</t>
1940</section>
1941</section>
1942
1943<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1944
1945<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1946<t>
1947   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1948   flow control mechanisms to resolve temporary overloads, rather than
1949   terminating connections with the expectation that clients will retry.
1950   The latter technique can exacerbate network congestion.
1951</t>
1952</section>
1953
1954<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
1955<t>
1956   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
1957   the network connection for an error status while it is transmitting
1958   the request. If the client sees an error status, it &SHOULD;
1959   immediately cease transmitting the body. If the body is being sent
1960   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
1961   empty trailer &MAY; be used to prematurely mark the end of the message.
1962   If the body was preceded by a Content-Length header, the client &MUST;
1963   close the connection.
1964</t>
1965</section>
1966
1967<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
1968<t>
1969   The purpose of the 100 (Continue) status (see &status-100;) is to
1970   allow a client that is sending a request message with a request body
1971   to determine if the origin server is willing to accept the request
1972   (based on the request headers) before the client sends the request
1973   body. In some cases, it might either be inappropriate or highly
1974   inefficient for the client to send the body if the server will reject
1975   the message without looking at the body.
1976</t>
1977<t>
1978   Requirements for HTTP/1.1 clients:
1979  <list style="symbols">
1980    <t>
1981        If a client will wait for a 100 (Continue) response before
1982        sending the request body, it &MUST; send an Expect request-header
1983        field (&header-expect;) with the "100-continue" expectation.
1984    </t>
1985    <t>
1986        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
1987        with the "100-continue" expectation if it does not intend
1988        to send a request body.
1989    </t>
1990  </list>
1991</t>
1992<t>
1993   Because of the presence of older implementations, the protocol allows
1994   ambiguous situations in which a client may send "Expect: 100-continue"
1995   without receiving either a 417 (Expectation Failed) status
1996   or a 100 (Continue) status. Therefore, when a client sends this
1997   header field to an origin server (possibly via a proxy) from which it
1998   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
1999   for an indefinite period before sending the request body.
2000</t>
2001<t>
2002   Requirements for HTTP/1.1 origin servers:
2003  <list style="symbols">
2004    <t> Upon receiving a request which includes an Expect request-header
2005        field with the "100-continue" expectation, an origin server &MUST;
2006        either respond with 100 (Continue) status and continue to read
2007        from the input stream, or respond with a final status code. The
2008        origin server &MUST-NOT; wait for the request body before sending
2009        the 100 (Continue) response. If it responds with a final status
2010        code, it &MAY; close the transport connection or it &MAY; continue
2011        to read and discard the rest of the request.  It &MUST-NOT;
2012        perform the requested method if it returns a final status code.
2013    </t>
2014    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2015        the request message does not include an Expect request-header
2016        field with the "100-continue" expectation, and &MUST-NOT; send a
2017        100 (Continue) response if such a request comes from an HTTP/1.0
2018        (or earlier) client. There is an exception to this rule: for
2019        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2020        status in response to an HTTP/1.1 PUT or POST request that does
2021        not include an Expect request-header field with the "100-continue"
2022        expectation. This exception, the purpose of which is
2023        to minimize any client processing delays associated with an
2024        undeclared wait for 100 (Continue) status, applies only to
2025        HTTP/1.1 requests, and not to requests with any other HTTP-version
2026        value.
2027    </t>
2028    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2029        already received some or all of the request body for the
2030        corresponding request.
2031    </t>
2032    <t> An origin server that sends a 100 (Continue) response &MUST;
2033    ultimately send a final status code, once the request body is
2034        received and processed, unless it terminates the transport
2035        connection prematurely.
2036    </t>
2037    <t> If an origin server receives a request that does not include an
2038        Expect request-header field with the "100-continue" expectation,
2039        the request includes a request body, and the server responds
2040        with a final status code before reading the entire request body
2041        from the transport connection, then the server &SHOULD-NOT;  close
2042        the transport connection until it has read the entire request,
2043        or until the client closes the connection. Otherwise, the client
2044        might not reliably receive the response message. However, this
2045        requirement is not be construed as preventing a server from
2046        defending itself against denial-of-service attacks, or from
2047        badly broken client implementations.
2048      </t>
2049    </list>
2050</t>
2051<t>
2052   Requirements for HTTP/1.1 proxies:
2053  <list style="symbols">
2054    <t> If a proxy receives a request that includes an Expect request-header
2055        field with the "100-continue" expectation, and the proxy
2056        either knows that the next-hop server complies with HTTP/1.1 or
2057        higher, or does not know the HTTP version of the next-hop
2058        server, it &MUST; forward the request, including the Expect header
2059        field.
2060    </t>
2061    <t> If the proxy knows that the version of the next-hop server is
2062        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2063        respond with a 417 (Expectation Failed) status.
2064    </t>
2065    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2066        numbers received from recently-referenced next-hop servers.
2067    </t>
2068    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2069        request message was received from an HTTP/1.0 (or earlier)
2070        client and did not include an Expect request-header field with
2071        the "100-continue" expectation. This requirement overrides the
2072        general rule for forwarding of 1xx responses (see &status-1xx;).
2073    </t>
2074  </list>
2075</t>
2076</section>
2077
2078<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2079<t>
2080   If an HTTP/1.1 client sends a request which includes a request body,
2081   but which does not include an Expect request-header field with the
2082   "100-continue" expectation, and if the client is not directly
2083   connected to an HTTP/1.1 origin server, and if the client sees the
2084   connection close before receiving any status from the server, the
2085   client &SHOULD; retry the request.  If the client does retry this
2086   request, it &MAY; use the following "binary exponential backoff"
2087   algorithm to be assured of obtaining a reliable response:
2088  <list style="numbers">
2089    <t>
2090      Initiate a new connection to the server
2091    </t>
2092    <t>
2093      Transmit the request-headers
2094    </t>
2095    <t>
2096      Initialize a variable R to the estimated round-trip time to the
2097         server (e.g., based on the time it took to establish the
2098         connection), or to a constant value of 5 seconds if the round-trip
2099         time is not available.
2100    </t>
2101    <t>
2102       Compute T = R * (2**N), where N is the number of previous
2103         retries of this request.
2104    </t>
2105    <t>
2106       Wait either for an error response from the server, or for T
2107         seconds (whichever comes first)
2108    </t>
2109    <t>
2110       If no error response is received, after T seconds transmit the
2111         body of the request.
2112    </t>
2113    <t>
2114       If client sees that the connection is closed prematurely,
2115         repeat from step 1 until the request is accepted, an error
2116         response is received, or the user becomes impatient and
2117         terminates the retry process.
2118    </t>
2119  </list>
2120</t>
2121<t>
2122   If at any point an error status is received, the client
2123  <list style="symbols">
2124      <t>&SHOULD-NOT;  continue and</t>
2125
2126      <t>&SHOULD; close the connection if it has not completed sending the
2127        request message.</t>
2128    </list>
2129</t>
2130</section>
2131</section>
2132</section>
2133
2134
2135<section title="Header Field Definitions" anchor="header.fields">
2136<t>
2137   This section defines the syntax and semantics of HTTP/1.1 header fields
2138   related to message framing and transport protocols.
2139</t>
2140<t>
2141   For entity-header fields, both sender and recipient refer to either the
2142   client or the server, depending on who sends and who receives the entity.
2143</t>
2144
2145<section title="Connection" anchor="header.connection">
2146  <iref primary="true" item="Connection header" x:for-anchor=""/>
2147  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2148  <x:anchor-alias value="Connection"/>
2149  <x:anchor-alias value="connection-token"/>
2150  <x:anchor-alias value="Connection-v"/>
2151<t>
2152   The general-header field "Connection" allows the sender to specify
2153   options that are desired for that particular connection and &MUST-NOT;
2154   be communicated by proxies over further connections.
2155</t>
2156<t>
2157   The Connection header's value has the following grammar:
2158</t>
2159<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"/>
2160  <x:ref>Connection</x:ref>       = "Connection" ":" <x:ref>OWS</x:ref> <x:ref>Connection-v</x:ref>
2161  <x:ref>Connection-v</x:ref>     = 1#<x:ref>connection-token</x:ref>
2162  <x:ref>connection-token</x:ref> = <x:ref>token</x:ref>
2163</artwork></figure>
2164<t>
2165   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2166   message is forwarded and, for each connection-token in this field,
2167   remove any header field(s) from the message with the same name as the
2168   connection-token. Connection options are signaled by the presence of
2169   a connection-token in the Connection header field, not by any
2170   corresponding additional header field(s), since the additional header
2171   field may not be sent if there are no parameters associated with that
2172   connection option.
2173</t>
2174<t>
2175   Message headers listed in the Connection header &MUST-NOT; include
2176   end-to-end headers, such as Cache-Control.
2177</t>
2178<t>
2179   HTTP/1.1 defines the "close" connection option for the sender to
2180   signal that the connection will be closed after completion of the
2181   response. For example,
2182</t>
2183<figure><artwork type="example">
2184  Connection: close
2185</artwork></figure>
2186<t>
2187   in either the request or the response header fields indicates that
2188   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2189   after the current request/response is complete.
2190</t>
2191<t>
2192   An HTTP/1.1 client that does not support persistent connections &MUST;
2193   include the "close" connection option in every request message.
2194</t>
2195<t>
2196   An HTTP/1.1 server that does not support persistent connections &MUST;
2197   include the "close" connection option in every response message that
2198   does not have a 1xx (informational) status code.
2199</t>
2200<t>
2201   A system receiving an HTTP/1.0 (or lower-version) message that
2202   includes a Connection header &MUST;, for each connection-token in this
2203   field, remove and ignore any header field(s) from the message with
2204   the same name as the connection-token. This protects against mistaken
2205   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2206</t>
2207</section>
2208
2209<section title="Content-Length" anchor="header.content-length">
2210  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2211  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2212  <x:anchor-alias value="Content-Length"/>
2213  <x:anchor-alias value="Content-Length-v"/>
2214<t>
2215   The entity-header field "Content-Length" indicates the size of the
2216   entity-body, in decimal number of OCTETs, sent to the recipient or,
2217   in the case of the HEAD method, the size of the entity-body that
2218   would have been sent had the request been a GET.
2219</t>
2220<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/><iref primary="true" item="Grammar" subitem="Content-Length-v"/>
2221  <x:ref>Content-Length</x:ref>   = "Content-Length" ":" <x:ref>OWS</x:ref> 1*<x:ref>Content-Length-v</x:ref>
2222  <x:ref>Content-Length-v</x:ref> = 1*<x:ref>DIGIT</x:ref>
2223</artwork></figure>
2224<t>
2225   An example is
2226</t>
2227<figure><artwork type="example">
2228  Content-Length: 3495
2229</artwork></figure>
2230<t>
2231   Applications &SHOULD; use this field to indicate the transfer-length of
2232   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2233</t>
2234<t>
2235   Any Content-Length greater than or equal to zero is a valid value.
2236   <xref target="message.length"/> describes how to determine the length of a message-body
2237   if a Content-Length is not given.
2238</t>
2239<t>
2240   Note that the meaning of this field is significantly different from
2241   the corresponding definition in MIME, where it is an optional field
2242   used within the "message/external-body" content-type. In HTTP, it
2243   &SHOULD; be sent whenever the message's length can be determined prior
2244   to being transferred, unless this is prohibited by the rules in
2245   <xref target="message.length"/>.
2246</t>
2247</section>
2248
2249<section title="Date" anchor="header.date">
2250  <iref primary="true" item="Date header" x:for-anchor=""/>
2251  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2252  <x:anchor-alias value="Date"/>
2253  <x:anchor-alias value="Date-v"/>
2254<t>
2255   The general-header field "Date" represents the date and time at which
2256   the message was originated, having the same semantics as orig-date in
2257   <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>. The field value is an
2258   HTTP-date, as described in <xref target="full.date"/>;
2259   it &MUST; be sent in rfc1123-date format.
2260</t>
2261<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/><iref primary="true" item="Grammar" subitem="Date-v"/>
2262  <x:ref>Date</x:ref>   = "Date" ":" <x:ref>OWS</x:ref> <x:ref>Date-v</x:ref>
2263  <x:ref>Date-v</x:ref> = <x:ref>HTTP-date</x:ref>
2264</artwork></figure>
2265<t>
2266   An example is
2267</t>
2268<figure><artwork type="example">
2269  Date: Tue, 15 Nov 1994 08:12:31 GMT
2270</artwork></figure>
2271<t>
2272   Origin servers &MUST; include a Date header field in all responses,
2273   except in these cases:
2274  <list style="numbers">
2275      <t>If the response status code is 100 (Continue) or 101 (Switching
2276         Protocols), the response &MAY; include a Date header field, at
2277         the server's option.</t>
2278
2279      <t>If the response status code conveys a server error, e.g. 500
2280         (Internal Server Error) or 503 (Service Unavailable), and it is
2281         inconvenient or impossible to generate a valid Date.</t>
2282
2283      <t>If the server does not have a clock that can provide a
2284         reasonable approximation of the current time, its responses
2285         &MUST-NOT; include a Date header field. In this case, the rules
2286         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2287  </list>
2288</t>
2289<t>
2290   A received message that does not have a Date header field &MUST; be
2291   assigned one by the recipient if the message will be cached by that
2292   recipient or gatewayed via a protocol which requires a Date. An HTTP
2293   implementation without a clock &MUST-NOT; cache responses without
2294   revalidating them on every use. An HTTP cache, especially a shared
2295   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2296   clock with a reliable external standard.
2297</t>
2298<t>
2299   Clients &SHOULD; only send a Date header field in messages that include
2300   an entity-body, as in the case of the PUT and POST requests, and even
2301   then it is optional. A client without a clock &MUST-NOT; send a Date
2302   header field in a request.
2303</t>
2304<t>
2305   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2306   time subsequent to the generation of the message. It &SHOULD; represent
2307   the best available approximation of the date and time of message
2308   generation, unless the implementation has no means of generating a
2309   reasonably accurate date and time. In theory, the date ought to
2310   represent the moment just before the entity is generated. In
2311   practice, the date can be generated at any time during the message
2312   origination without affecting its semantic value.
2313</t>
2314
2315<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2316<t>
2317   Some origin server implementations might not have a clock available.
2318   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2319   values to a response, unless these values were associated
2320   with the resource by a system or user with a reliable clock. It &MAY;
2321   assign an Expires value that is known, at or before server
2322   configuration time, to be in the past (this allows "pre-expiration"
2323   of responses without storing separate Expires values for each
2324   resource).
2325</t>
2326</section>
2327</section>
2328
2329<section title="Host" anchor="header.host">
2330  <iref primary="true" item="Host header" x:for-anchor=""/>
2331  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2332  <x:anchor-alias value="Host"/>
2333  <x:anchor-alias value="Host-v"/>
2334<t>
2335   The request-header field "Host" specifies the Internet host and port
2336   number of the resource being requested, as obtained from the original
2337   URI given by the user or referring resource (generally an http URI,
2338   as described in <xref target="http.uri"/>). The Host field value &MUST; represent
2339   the naming authority of the origin server or gateway given by the
2340   original URL. This allows the origin server or gateway to
2341   differentiate between internally-ambiguous URLs, such as the root "/"
2342   URL of a server for multiple host names on a single IP address.
2343</t>
2344<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/><iref primary="true" item="Grammar" subitem="Host-v"/>
2345  <x:ref>Host</x:ref>   = "Host" ":" <x:ref>OWS</x:ref> <x:ref>Host-v</x:ref>
2346  <x:ref>Host-v</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2347</artwork></figure>
2348<t>
2349   A "host" without any trailing port information implies the default
2350   port for the service requested (e.g., "80" for an HTTP URL). For
2351   example, a request on the origin server for
2352   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2353</t>
2354<figure><artwork type="example">
2355  GET /pub/WWW/ HTTP/1.1
2356  Host: www.example.org
2357</artwork></figure>
2358<t>
2359   A client &MUST; include a Host header field in all HTTP/1.1 request
2360   messages. If the requested URI does not include an Internet host
2361   name for the service being requested, then the Host header field &MUST;
2362   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2363   request message it forwards does contain an appropriate Host header
2364   field that identifies the service being requested by the proxy. All
2365   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2366   status code to any HTTP/1.1 request message which lacks a Host header
2367   field.
2368</t>
2369<t>
2370   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2371   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2372   for other requirements relating to Host.
2373</t>
2374</section>
2375
2376<section title="TE" anchor="header.te">
2377  <iref primary="true" item="TE header" x:for-anchor=""/>
2378  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2379  <x:anchor-alias value="TE"/>
2380  <x:anchor-alias value="TE-v"/>
2381  <x:anchor-alias value="t-codings"/>
2382<t>
2383   The request-header field "TE" indicates what extension transfer-codings
2384   it is willing to accept in the response and whether or not it is
2385   willing to accept trailer fields in a chunked transfer-coding. Its
2386   value may consist of the keyword "trailers" and/or a comma-separated
2387   list of extension transfer-coding names with optional accept
2388   parameters (as described in <xref target="transfer.codings"/>).
2389</t>
2390<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"/>
2391  <x:ref>TE</x:ref>        = "TE" ":" <x:ref>OWS</x:ref> <x:ref>TE-v</x:ref>
2392  <x:ref>TE-v</x:ref>      = #<x:ref>t-codings</x:ref>
2393  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-extension</x:ref> [ <x:ref>accept-params</x:ref> ] )
2394</artwork></figure>
2395<t>
2396   The presence of the keyword "trailers" indicates that the client is
2397   willing to accept trailer fields in a chunked transfer-coding, as
2398   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2399   transfer-coding values even though it does not itself represent a
2400   transfer-coding.
2401</t>
2402<t>
2403   Examples of its use are:
2404</t>
2405<figure><artwork type="example">
2406  TE: deflate
2407  TE:
2408  TE: trailers, deflate;q=0.5
2409</artwork></figure>
2410<t>
2411   The TE header field only applies to the immediate connection.
2412   Therefore, the keyword &MUST; be supplied within a Connection header
2413   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2414</t>
2415<t>
2416   A server tests whether a transfer-coding is acceptable, according to
2417   a TE field, using these rules:
2418  <list style="numbers">
2419    <x:lt>
2420      <t>The "chunked" transfer-coding is always acceptable. If the
2421         keyword "trailers" is listed, the client indicates that it is
2422         willing to accept trailer fields in the chunked response on
2423         behalf of itself and any downstream clients. The implication is
2424         that, if given, the client is stating that either all
2425         downstream clients are willing to accept trailer fields in the
2426         forwarded response, or that it will attempt to buffer the
2427         response on behalf of downstream recipients.
2428      </t><t>
2429         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2430         chunked response such that a client can be assured of buffering
2431         the entire response.</t>
2432    </x:lt>
2433    <x:lt>
2434      <t>If the transfer-coding being tested is one of the transfer-codings
2435         listed in the TE field, then it is acceptable unless it
2436         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2437         qvalue of 0 means "not acceptable.")</t>
2438    </x:lt>
2439    <x:lt>
2440      <t>If multiple transfer-codings are acceptable, then the
2441         acceptable transfer-coding with the highest non-zero qvalue is
2442         preferred.  The "chunked" transfer-coding always has a qvalue
2443         of 1.</t>
2444    </x:lt>
2445  </list>
2446</t>
2447<t>
2448   If the TE field-value is empty or if no TE field is present, the only
2449   transfer-coding  is "chunked". A message with no transfer-coding is
2450   always acceptable.
2451</t>
2452</section>
2453
2454<section title="Trailer" anchor="header.trailer">
2455  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2456  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2457  <x:anchor-alias value="Trailer"/>
2458  <x:anchor-alias value="Trailer-v"/>
2459<t>
2460   The general field "Trailer" indicates that the given set of
2461   header fields is present in the trailer of a message encoded with
2462   chunked transfer-coding.
2463</t>
2464<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="true" item="Grammar" subitem="Trailer-v"/>
2465  <x:ref>Trailer</x:ref>   = "Trailer" ":" <x:ref>OWS</x:ref> <x:ref>Trailer-v</x:ref>
2466  <x:ref>Trailer-v</x:ref> = 1#<x:ref>field-name</x:ref>
2467</artwork></figure>
2468<t>
2469   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2470   message using chunked transfer-coding with a non-empty trailer. Doing
2471   so allows the recipient to know which header fields to expect in the
2472   trailer.
2473</t>
2474<t>
2475   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2476   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2477   trailer fields in a "chunked" transfer-coding.
2478</t>
2479<t>
2480   Message header fields listed in the Trailer header field &MUST-NOT;
2481   include the following header fields:
2482  <list style="symbols">
2483    <t>Transfer-Encoding</t>
2484    <t>Content-Length</t>
2485    <t>Trailer</t>
2486  </list>
2487</t>
2488</section>
2489
2490<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2491  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2492  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2493  <x:anchor-alias value="Transfer-Encoding"/>
2494  <x:anchor-alias value="Transfer-Encoding-v"/>
2495<t>
2496   The general-header "Transfer-Encoding" field indicates what (if any)
2497   type of transformation has been applied to the message body in order
2498   to safely transfer it between the sender and the recipient. This
2499   differs from the content-coding in that the transfer-coding is a
2500   property of the message, not of the entity.
2501</t>
2502<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/><iref primary="true" item="Grammar" subitem="Transfer-Encoding-v"/>
2503  <x:ref>Transfer-Encoding</x:ref>   = "Transfer-Encoding" ":" <x:ref>OWS</x:ref>
2504                        <x:ref>Transfer-Encoding-v</x:ref>
2505  <x:ref>Transfer-Encoding-v</x:ref> = 1#<x:ref>transfer-coding</x:ref>
2506</artwork></figure>
2507<t>
2508   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2509</t>
2510<figure><artwork type="example">
2511  Transfer-Encoding: chunked
2512</artwork></figure>
2513<t>
2514   If multiple encodings have been applied to an entity, the transfer-codings
2515   &MUST; be listed in the order in which they were applied.
2516   Additional information about the encoding parameters &MAY; be provided
2517   by other entity-header fields not defined by this specification.
2518</t>
2519<t>
2520   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2521   header.
2522</t>
2523</section>
2524
2525<section title="Upgrade" anchor="header.upgrade">
2526  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2527  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2528  <x:anchor-alias value="Upgrade"/>
2529  <x:anchor-alias value="Upgrade-v"/>
2530<t>
2531   The general-header "Upgrade" allows the client to specify what
2532   additional communication protocols it supports and would like to use
2533   if the server finds it appropriate to switch protocols. The server
2534   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2535   response to indicate which protocol(s) are being switched.
2536</t>
2537<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/><iref primary="true" item="Grammar" subitem="Upgrade-v"/>
2538  <x:ref>Upgrade</x:ref>   = "Upgrade" ":" <x:ref>OWS</x:ref> <x:ref>Upgrade-v</x:ref>
2539  <x:ref>Upgrade-v</x:ref> = 1#<x:ref>product</x:ref>
2540</artwork></figure>
2541<t>
2542   For example,
2543</t>
2544<figure><artwork type="example">
2545  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2546</artwork></figure>
2547<t>
2548   The Upgrade header field is intended to provide a simple mechanism
2549   for transition from HTTP/1.1 to some other, incompatible protocol. It
2550   does so by allowing the client to advertise its desire to use another
2551   protocol, such as a later version of HTTP with a higher major version
2552   number, even though the current request has been made using HTTP/1.1.
2553   This eases the difficult transition between incompatible protocols by
2554   allowing the client to initiate a request in the more commonly
2555   supported protocol while indicating to the server that it would like
2556   to use a "better" protocol if available (where "better" is determined
2557   by the server, possibly according to the nature of the method and/or
2558   resource being requested).
2559</t>
2560<t>
2561   The Upgrade header field only applies to switching application-layer
2562   protocols upon the existing transport-layer connection. Upgrade
2563   cannot be used to insist on a protocol change; its acceptance and use
2564   by the server is optional. The capabilities and nature of the
2565   application-layer communication after the protocol change is entirely
2566   dependent upon the new protocol chosen, although the first action
2567   after changing the protocol &MUST; be a response to the initial HTTP
2568   request containing the Upgrade header field.
2569</t>
2570<t>
2571   The Upgrade header field only applies to the immediate connection.
2572   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2573   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2574   HTTP/1.1 message.
2575</t>
2576<t>
2577   The Upgrade header field cannot be used to indicate a switch to a
2578   protocol on a different connection. For that purpose, it is more
2579   appropriate to use a 301, 302, 303, or 305 redirection response.
2580</t>
2581<t>
2582   This specification only defines the protocol name "HTTP" for use by
2583   the family of Hypertext Transfer Protocols, as defined by the HTTP
2584   version rules of <xref target="http.version"/> and future updates to this
2585   specification. Any token can be used as a protocol name; however, it
2586   will only be useful if both the client and server associate the name
2587   with the same protocol.
2588</t>
2589</section>
2590
2591<section title="Via" anchor="header.via">
2592  <iref primary="true" item="Via header" x:for-anchor=""/>
2593  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2594  <x:anchor-alias value="protocol-name"/>
2595  <x:anchor-alias value="protocol-version"/>
2596  <x:anchor-alias value="pseudonym"/>
2597  <x:anchor-alias value="received-by"/>
2598  <x:anchor-alias value="received-protocol"/>
2599  <x:anchor-alias value="Via"/>
2600  <x:anchor-alias value="Via-v"/>
2601<t>
2602   The general-header field "Via" &MUST; be used by gateways and proxies to
2603   indicate the intermediate protocols and recipients between the user
2604   agent and the server on requests, and between the origin server and
2605   the client on responses. It is analogous to the "Received" field defined in
2606   <xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/> and is intended to be used for tracking message forwards,
2607   avoiding request loops, and identifying the protocol capabilities of
2608   all senders along the request/response chain.
2609</t>
2610<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"/>
2611  <x:ref>Via</x:ref>               = "Via" ":" <x:ref>OWS</x:ref> <x:ref>Via-v</x:ref>
2612  <x:ref>Via-v</x:ref>             = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2613                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2614  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2615  <x:ref>protocol-name</x:ref>     = <x:ref>token</x:ref>
2616  <x:ref>protocol-version</x:ref>  = <x:ref>token</x:ref>
2617  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2618  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2619</artwork></figure>
2620<t>
2621   The received-protocol indicates the protocol version of the message
2622   received by the server or client along each segment of the
2623   request/response chain. The received-protocol version is appended to
2624   the Via field value when the message is forwarded so that information
2625   about the protocol capabilities of upstream applications remains
2626   visible to all recipients.
2627</t>
2628<t>
2629   The protocol-name is optional if and only if it would be "HTTP". The
2630   received-by field is normally the host and optional port number of a
2631   recipient server or client that subsequently forwarded the message.
2632   However, if the real host is considered to be sensitive information,
2633   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2634   be assumed to be the default port of the received-protocol.
2635</t>
2636<t>
2637   Multiple Via field values represents each proxy or gateway that has
2638   forwarded the message. Each recipient &MUST; append its information
2639   such that the end result is ordered according to the sequence of
2640   forwarding applications.
2641</t>
2642<t>
2643   Comments &MAY; be used in the Via header field to identify the software
2644   of the recipient proxy or gateway, analogous to the User-Agent and
2645   Server header fields. However, all comments in the Via field are
2646   optional and &MAY; be removed by any recipient prior to forwarding the
2647   message.
2648</t>
2649<t>
2650   For example, a request message could be sent from an HTTP/1.0 user
2651   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2652   forward the request to a public proxy at p.example.net, which completes
2653   the request by forwarding it to the origin server at www.example.com.
2654   The request received by www.example.com would then have the following
2655   Via header field:
2656</t>
2657<figure><artwork type="example">
2658  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2659</artwork></figure>
2660<t>
2661   Proxies and gateways used as a portal through a network firewall
2662   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2663   the firewall region. This information &SHOULD; only be propagated if
2664   explicitly enabled. If not enabled, the received-by host of any host
2665   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2666   for that host.
2667</t>
2668<t>
2669   For organizations that have strong privacy requirements for hiding
2670   internal structures, a proxy &MAY; combine an ordered subsequence of
2671   Via header field entries with identical received-protocol values into
2672   a single such entry. For example,
2673</t>
2674<figure><artwork type="example">
2675  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2676</artwork></figure>
2677<t>
2678        could be collapsed to
2679</t>
2680<figure><artwork type="example">
2681  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2682</artwork></figure>
2683<t>
2684   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2685   under the same organizational control and the hosts have already been
2686   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2687   have different received-protocol values.
2688</t>
2689</section>
2690
2691</section>
2692
2693<section title="IANA Considerations" anchor="IANA.considerations">
2694<section title="Message Header Registration" anchor="message.header.registration">
2695<t>
2696   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2697   with the permanent registrations below (see <xref target="RFC3864"/>):
2698</t>
2699<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2700<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2701   <ttcol>Header Field Name</ttcol>
2702   <ttcol>Protocol</ttcol>
2703   <ttcol>Status</ttcol>
2704   <ttcol>Reference</ttcol>
2705
2706   <c>Connection</c>
2707   <c>http</c>
2708   <c>standard</c>
2709   <c>
2710      <xref target="header.connection"/>
2711   </c>
2712   <c>Content-Length</c>
2713   <c>http</c>
2714   <c>standard</c>
2715   <c>
2716      <xref target="header.content-length"/>
2717   </c>
2718   <c>Date</c>
2719   <c>http</c>
2720   <c>standard</c>
2721   <c>
2722      <xref target="header.date"/>
2723   </c>
2724   <c>Host</c>
2725   <c>http</c>
2726   <c>standard</c>
2727   <c>
2728      <xref target="header.host"/>
2729   </c>
2730   <c>TE</c>
2731   <c>http</c>
2732   <c>standard</c>
2733   <c>
2734      <xref target="header.te"/>
2735   </c>
2736   <c>Trailer</c>
2737   <c>http</c>
2738   <c>standard</c>
2739   <c>
2740      <xref target="header.trailer"/>
2741   </c>
2742   <c>Transfer-Encoding</c>
2743   <c>http</c>
2744   <c>standard</c>
2745   <c>
2746      <xref target="header.transfer-encoding"/>
2747   </c>
2748   <c>Upgrade</c>
2749   <c>http</c>
2750   <c>standard</c>
2751   <c>
2752      <xref target="header.upgrade"/>
2753   </c>
2754   <c>Via</c>
2755   <c>http</c>
2756   <c>standard</c>
2757   <c>
2758      <xref target="header.via"/>
2759   </c>
2760</texttable>
2761<!--(END)-->
2762<t>
2763   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2764</t>
2765</section>
2766
2767<section title="URI Scheme Registration" anchor="uri.scheme.registration">
2768<t>
2769   The entry for the "http" URI Scheme in the registry located at
2770   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>
2771   should be updated to point to <xref target="http.uri"/> of this document
2772   (see <xref target="RFC4395"/>).
2773</t>
2774</section>
2775
2776<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
2777<t>
2778   This document serves as the specification for the Internet media types
2779   "message/http" and "application/http". The following is to be registered with
2780   IANA (see <xref target="RFC4288"/>).
2781</t>
2782<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
2783<iref item="Media Type" subitem="message/http" primary="true"/>
2784<iref item="message/http Media Type" primary="true"/>
2785<t>
2786   The message/http type can be used to enclose a single HTTP request or
2787   response message, provided that it obeys the MIME restrictions for all
2788   "message" types regarding line length and encodings.
2789</t>
2790<t>
2791  <list style="hanging" x:indent="12em">
2792    <t hangText="Type name:">
2793      message
2794    </t>
2795    <t hangText="Subtype name:">
2796      http
2797    </t>
2798    <t hangText="Required parameters:">
2799      none
2800    </t>
2801    <t hangText="Optional parameters:">
2802      version, msgtype
2803      <list style="hanging">
2804        <t hangText="version:">
2805          The HTTP-Version number of the enclosed message
2806          (e.g., "1.1"). If not present, the version can be
2807          determined from the first line of the body.
2808        </t>
2809        <t hangText="msgtype:">
2810          The message type -- "request" or "response". If not
2811          present, the type can be determined from the first
2812          line of the body.
2813        </t>
2814      </list>
2815    </t>
2816    <t hangText="Encoding considerations:">
2817      only "7bit", "8bit", or "binary" are permitted
2818    </t>
2819    <t hangText="Security considerations:">
2820      none
2821    </t>
2822    <t hangText="Interoperability considerations:">
2823      none
2824    </t>
2825    <t hangText="Published specification:">
2826      This specification (see <xref target="internet.media.type.message.http"/>).
2827    </t>
2828    <t hangText="Applications that use this media type:">
2829    </t>
2830    <t hangText="Additional information:">
2831      <list style="hanging">
2832        <t hangText="Magic number(s):">none</t>
2833        <t hangText="File extension(s):">none</t>
2834        <t hangText="Macintosh file type code(s):">none</t>
2835      </list>
2836    </t>
2837    <t hangText="Person and email address to contact for further information:">
2838      See Authors Section.
2839    </t>
2840                <t hangText="Intended usage:">
2841                  COMMON
2842    </t>
2843                <t hangText="Restrictions on usage:">
2844                  none
2845    </t>
2846    <t hangText="Author/Change controller:">
2847      IESG
2848    </t>
2849  </list>
2850</t>
2851</section>
2852<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
2853<iref item="Media Type" subitem="application/http" primary="true"/>
2854<iref item="application/http Media Type" primary="true"/>
2855<t>
2856   The application/http type can be used to enclose a pipeline of one or more
2857   HTTP request or response messages (not intermixed).
2858</t>
2859<t>
2860  <list style="hanging" x:indent="12em">
2861    <t hangText="Type name:">
2862      application
2863    </t>
2864    <t hangText="Subtype name:">
2865      http
2866    </t>
2867    <t hangText="Required parameters:">
2868      none
2869    </t>
2870    <t hangText="Optional parameters:">
2871      version, msgtype
2872      <list style="hanging">
2873        <t hangText="version:">
2874          The HTTP-Version number of the enclosed messages
2875          (e.g., "1.1"). If not present, the version can be
2876          determined from the first line of the body.
2877        </t>
2878        <t hangText="msgtype:">
2879          The message type -- "request" or "response". If not
2880          present, the type can be determined from the first
2881          line of the body.
2882        </t>
2883      </list>
2884    </t>
2885    <t hangText="Encoding considerations:">
2886      HTTP messages enclosed by this type
2887      are in "binary" format; use of an appropriate
2888      Content-Transfer-Encoding is required when
2889      transmitted via E-mail.
2890    </t>
2891    <t hangText="Security considerations:">
2892      none
2893    </t>
2894    <t hangText="Interoperability considerations:">
2895      none
2896    </t>
2897    <t hangText="Published specification:">
2898      This specification (see <xref target="internet.media.type.application.http"/>).
2899    </t>
2900    <t hangText="Applications that use this media type:">
2901    </t>
2902    <t hangText="Additional information:">
2903      <list style="hanging">
2904        <t hangText="Magic number(s):">none</t>
2905        <t hangText="File extension(s):">none</t>
2906        <t hangText="Macintosh file type code(s):">none</t>
2907      </list>
2908    </t>
2909    <t hangText="Person and email address to contact for further information:">
2910      See Authors Section.
2911    </t>
2912                <t hangText="Intended usage:">
2913                  COMMON
2914    </t>
2915                <t hangText="Restrictions on usage:">
2916                  none
2917    </t>
2918    <t hangText="Author/Change controller:">
2919      IESG
2920    </t>
2921  </list>
2922</t>
2923</section>
2924</section>
2925
2926</section>
2927
2928<section title="Security Considerations" anchor="security.considerations">
2929<t>
2930   This section is meant to inform application developers, information
2931   providers, and users of the security limitations in HTTP/1.1 as
2932   described by this document. The discussion does not include
2933   definitive solutions to the problems revealed, though it does make
2934   some suggestions for reducing security risks.
2935</t>
2936
2937<section title="Personal Information" anchor="personal.information">
2938<t>
2939   HTTP clients are often privy to large amounts of personal information
2940   (e.g. the user's name, location, mail address, passwords, encryption
2941   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2942   leakage of this information.
2943   We very strongly recommend that a convenient interface be provided
2944   for the user to control dissemination of such information, and that
2945   designers and implementors be particularly careful in this area.
2946   History shows that errors in this area often create serious security
2947   and/or privacy problems and generate highly adverse publicity for the
2948   implementor's company.
2949</t>
2950</section>
2951
2952<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2953<t>
2954   A server is in the position to save personal data about a user's
2955   requests which might identify their reading patterns or subjects of
2956   interest. This information is clearly confidential in nature and its
2957   handling can be constrained by law in certain countries. People using
2958   HTTP to provide data are responsible for ensuring that
2959   such material is not distributed without the permission of any
2960   individuals that are identifiable by the published results.
2961</t>
2962</section>
2963
2964<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2965<t>
2966   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2967   the documents returned by HTTP requests to be only those that were
2968   intended by the server administrators. If an HTTP server translates
2969   HTTP URIs directly into file system calls, the server &MUST; take
2970   special care not to serve files that were not intended to be
2971   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2972   other operating systems use ".." as a path component to indicate a
2973   directory level above the current one. On such a system, an HTTP
2974   server &MUST; disallow any such construct in the request-target if it
2975   would otherwise allow access to a resource outside those intended to
2976   be accessible via the HTTP server. Similarly, files intended for
2977   reference only internally to the server (such as access control
2978   files, configuration files, and script code) &MUST; be protected from
2979   inappropriate retrieval, since they might contain sensitive
2980   information. Experience has shown that minor bugs in such HTTP server
2981   implementations have turned into security risks.
2982</t>
2983</section>
2984
2985<section title="DNS Spoofing" anchor="dns.spoofing">
2986<t>
2987   Clients using HTTP rely heavily on the Domain Name Service, and are
2988   thus generally prone to security attacks based on the deliberate
2989   mis-association of IP addresses and DNS names. Clients need to be
2990   cautious in assuming the continuing validity of an IP number/DNS name
2991   association.
2992</t>
2993<t>
2994   In particular, HTTP clients &SHOULD; rely on their name resolver for
2995   confirmation of an IP number/DNS name association, rather than
2996   caching the result of previous host name lookups. Many platforms
2997   already can cache host name lookups locally when appropriate, and
2998   they &SHOULD; be configured to do so. It is proper for these lookups to
2999   be cached, however, only when the TTL (Time To Live) information
3000   reported by the name server makes it likely that the cached
3001   information will remain useful.
3002</t>
3003<t>
3004   If HTTP clients cache the results of host name lookups in order to
3005   achieve a performance improvement, they &MUST; observe the TTL
3006   information reported by DNS.
3007</t>
3008<t>
3009   If HTTP clients do not observe this rule, they could be spoofed when
3010   a previously-accessed server's IP address changes. As network
3011   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
3012   possibility of this form of attack will grow. Observing this
3013   requirement thus reduces this potential security vulnerability.
3014</t>
3015<t>
3016   This requirement also improves the load-balancing behavior of clients
3017   for replicated servers using the same DNS name and reduces the
3018   likelihood of a user's experiencing failure in accessing sites which
3019   use that strategy.
3020</t>
3021</section>
3022
3023<section title="Proxies and Caching" anchor="attack.proxies">
3024<t>
3025   By their very nature, HTTP proxies are men-in-the-middle, and
3026   represent an opportunity for man-in-the-middle attacks. Compromise of
3027   the systems on which the proxies run can result in serious security
3028   and privacy problems. Proxies have access to security-related
3029   information, personal information about individual users and
3030   organizations, and proprietary information belonging to users and
3031   content providers. A compromised proxy, or a proxy implemented or
3032   configured without regard to security and privacy considerations,
3033   might be used in the commission of a wide range of potential attacks.
3034</t>
3035<t>
3036   Proxy operators should protect the systems on which proxies run as
3037   they would protect any system that contains or transports sensitive
3038   information. In particular, log information gathered at proxies often
3039   contains highly sensitive personal information, and/or information
3040   about organizations. Log information should be carefully guarded, and
3041   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
3042</t>
3043<t>
3044   Proxy implementors should consider the privacy and security
3045   implications of their design and coding decisions, and of the
3046   configuration options they provide to proxy operators (especially the
3047   default configuration).
3048</t>
3049<t>
3050   Users of a proxy need to be aware that they are no trustworthier than
3051   the people who run the proxy; HTTP itself cannot solve this problem.
3052</t>
3053<t>
3054   The judicious use of cryptography, when appropriate, may suffice to
3055   protect against a broad range of security and privacy attacks. Such
3056   cryptography is beyond the scope of the HTTP/1.1 specification.
3057</t>
3058</section>
3059
3060<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
3061<t>
3062   They exist. They are hard to defend against. Research continues.
3063   Beware.
3064</t>
3065</section>
3066</section>
3067
3068<section title="Acknowledgments" anchor="ack">
3069<t>
3070   HTTP has evolved considerably over the years. It has
3071   benefited from a large and active developer community--the many
3072   people who have participated on the www-talk mailing list--and it is
3073   that community which has been most responsible for the success of
3074   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
3075   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
3076   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
3077   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
3078   VanHeyningen deserve special recognition for their efforts in
3079   defining early aspects of the protocol.
3080</t>
3081<t>
3082   This document has benefited greatly from the comments of all those
3083   participating in the HTTP-WG. In addition to those already mentioned,
3084   the following individuals have contributed to this specification:
3085</t>
3086<t>
3087   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
3088   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
3089   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
3090   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
3091   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
3092   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
3093   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
3094   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
3095   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
3096   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
3097   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
3098   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
3099   Josh Cohen.
3100</t>
3101<t>
3102   Thanks to the "cave men" of Palo Alto. You know who you are.
3103</t>
3104<t>
3105   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
3106   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
3107   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
3108   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
3109   Larry Masinter for their help. And thanks go particularly to Jeff
3110   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3111</t>
3112<t>
3113   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3114   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3115   discovery of many of the problems that this document attempts to
3116   rectify.
3117</t>
3118<t>
3119   This specification makes heavy use of the augmented BNF and generic
3120   constructs defined by David H. Crocker for <xref target="RFC5234"/>. Similarly, it
3121   reuses many of the definitions provided by Nathaniel Borenstein and
3122   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
3123   specification will help reduce past confusion over the relationship
3124   between HTTP and Internet mail message formats.
3125</t>
3126</section>
3127
3128</middle>
3129<back>
3130
3131<references title="Normative References">
3132
3133<reference anchor="ISO-8859-1">
3134  <front>
3135    <title>
3136     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3137    </title>
3138    <author>
3139      <organization>International Organization for Standardization</organization>
3140    </author>
3141    <date year="1998"/>
3142  </front>
3143  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3144</reference>
3145
3146<reference anchor="Part2">
3147  <front>
3148    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3149    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3150      <organization abbrev="Day Software">Day Software</organization>
3151      <address><email>fielding@gbiv.com</email></address>
3152    </author>
3153    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3154      <organization>One Laptop per Child</organization>
3155      <address><email>jg@laptop.org</email></address>
3156    </author>
3157    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3158      <organization abbrev="HP">Hewlett-Packard Company</organization>
3159      <address><email>JeffMogul@acm.org</email></address>
3160    </author>
3161    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3162      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3163      <address><email>henrikn@microsoft.com</email></address>
3164    </author>
3165    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3166      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3167      <address><email>LMM@acm.org</email></address>
3168    </author>
3169    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3170      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3171      <address><email>paulle@microsoft.com</email></address>
3172    </author>
3173    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3174      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3175      <address><email>timbl@w3.org</email></address>
3176    </author>
3177    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3178      <organization abbrev="W3C">World Wide Web Consortium</organization>
3179      <address><email>ylafon@w3.org</email></address>
3180    </author>
3181    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3182      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3183      <address><email>julian.reschke@greenbytes.de</email></address>
3184    </author>
3185    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3186  </front>
3187  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3188  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3189</reference>
3190
3191<reference anchor="Part3">
3192  <front>
3193    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3194    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3195      <organization abbrev="Day Software">Day Software</organization>
3196      <address><email>fielding@gbiv.com</email></address>
3197    </author>
3198    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3199      <organization>One Laptop per Child</organization>
3200      <address><email>jg@laptop.org</email></address>
3201    </author>
3202    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3203      <organization abbrev="HP">Hewlett-Packard Company</organization>
3204      <address><email>JeffMogul@acm.org</email></address>
3205    </author>
3206    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3207      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3208      <address><email>henrikn@microsoft.com</email></address>
3209    </author>
3210    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3211      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3212      <address><email>LMM@acm.org</email></address>
3213    </author>
3214    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3215      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3216      <address><email>paulle@microsoft.com</email></address>
3217    </author>
3218    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3219      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3220      <address><email>timbl@w3.org</email></address>
3221    </author>
3222    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3223      <organization abbrev="W3C">World Wide Web Consortium</organization>
3224      <address><email>ylafon@w3.org</email></address>
3225    </author>
3226    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3227      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3228      <address><email>julian.reschke@greenbytes.de</email></address>
3229    </author>
3230    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3231  </front>
3232  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3233  <x:source href="p3-payload.xml" basename="p3-payload"/>
3234</reference>
3235
3236<reference anchor="Part5">
3237  <front>
3238    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3239    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3240      <organization abbrev="Day Software">Day Software</organization>
3241      <address><email>fielding@gbiv.com</email></address>
3242    </author>
3243    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3244      <organization>One Laptop per Child</organization>
3245      <address><email>jg@laptop.org</email></address>
3246    </author>
3247    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3248      <organization abbrev="HP">Hewlett-Packard Company</organization>
3249      <address><email>JeffMogul@acm.org</email></address>
3250    </author>
3251    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3252      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3253      <address><email>henrikn@microsoft.com</email></address>
3254    </author>
3255    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3256      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3257      <address><email>LMM@acm.org</email></address>
3258    </author>
3259    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3260      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3261      <address><email>paulle@microsoft.com</email></address>
3262    </author>
3263    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3264      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3265      <address><email>timbl@w3.org</email></address>
3266    </author>
3267    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3268      <organization abbrev="W3C">World Wide Web Consortium</organization>
3269      <address><email>ylafon@w3.org</email></address>
3270    </author>
3271    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3272      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3273      <address><email>julian.reschke@greenbytes.de</email></address>
3274    </author>
3275    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3276  </front>
3277  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3278  <x:source href="p5-range.xml" basename="p5-range"/>
3279</reference>
3280
3281<reference anchor="Part6">
3282  <front>
3283    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3284    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3285      <organization abbrev="Day Software">Day Software</organization>
3286      <address><email>fielding@gbiv.com</email></address>
3287    </author>
3288    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3289      <organization>One Laptop per Child</organization>
3290      <address><email>jg@laptop.org</email></address>
3291    </author>
3292    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3293      <organization abbrev="HP">Hewlett-Packard Company</organization>
3294      <address><email>JeffMogul@acm.org</email></address>
3295    </author>
3296    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3297      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3298      <address><email>henrikn@microsoft.com</email></address>
3299    </author>
3300    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3301      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3302      <address><email>LMM@acm.org</email></address>
3303    </author>
3304    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3305      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3306      <address><email>paulle@microsoft.com</email></address>
3307    </author>
3308    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3309      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3310      <address><email>timbl@w3.org</email></address>
3311    </author>
3312    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3313      <organization abbrev="W3C">World Wide Web Consortium</organization>
3314      <address><email>ylafon@w3.org</email></address>
3315    </author>
3316    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3317      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3318      <address><email>julian.reschke@greenbytes.de</email></address>
3319    </author>
3320    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3321  </front>
3322  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3323  <x:source href="p6-cache.xml" basename="p6-cache"/>
3324</reference>
3325
3326<reference anchor="RFC5234">
3327  <front>
3328    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
3329    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
3330      <organization>Brandenburg InternetWorking</organization>
3331      <address>
3332      <postal>
3333      <street>675 Spruce Dr.</street>
3334      <city>Sunnyvale</city>
3335      <region>CA</region>
3336      <code>94086</code>
3337      <country>US</country></postal>
3338      <phone>+1.408.246.8253</phone>
3339      <email>dcrocker@bbiw.net</email></address> 
3340    </author>
3341    <author initials="P." surname="Overell" fullname="Paul Overell">
3342      <organization>THUS plc.</organization>
3343      <address>
3344      <postal>
3345      <street>1/2 Berkeley Square</street>
3346      <street>99 Berkely Street</street>
3347      <city>Glasgow</city>
3348      <code>G3 7HR</code>
3349      <country>UK</country></postal>
3350      <email>paul.overell@thus.net</email></address>
3351    </author>
3352    <date month="January" year="2008"/>
3353  </front>
3354  <seriesInfo name="STD" value="68"/>
3355  <seriesInfo name="RFC" value="5234"/>
3356</reference>
3357
3358<reference anchor="RFC2045">
3359  <front>
3360    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3361    <author initials="N." surname="Freed" fullname="Ned Freed">
3362      <organization>Innosoft International, Inc.</organization>
3363      <address><email>ned@innosoft.com</email></address>
3364    </author>
3365    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3366      <organization>First Virtual Holdings</organization>
3367      <address><email>nsb@nsb.fv.com</email></address>
3368    </author>
3369    <date month="November" year="1996"/>
3370  </front>
3371  <seriesInfo name="RFC" value="2045"/>
3372</reference>
3373
3374<reference anchor="RFC2119">
3375  <front>
3376    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3377    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3378      <organization>Harvard University</organization>
3379      <address><email>sob@harvard.edu</email></address>
3380    </author>
3381    <date month="March" year="1997"/>
3382  </front>
3383  <seriesInfo name="BCP" value="14"/>
3384  <seriesInfo name="RFC" value="2119"/>
3385</reference>
3386
3387<reference anchor="RFC3986">
3388 <front>
3389  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
3390  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
3391    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3392    <address>
3393       <email>timbl@w3.org</email>
3394       <uri>http://www.w3.org/People/Berners-Lee/</uri>
3395    </address>
3396  </author>
3397  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
3398    <organization abbrev="Day Software">Day Software</organization>
3399    <address>
3400      <email>fielding@gbiv.com</email>
3401      <uri>http://roy.gbiv.com/</uri>
3402    </address>
3403  </author>
3404  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
3405    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
3406    <address>
3407      <email>LMM@acm.org</email>
3408      <uri>http://larry.masinter.net/</uri>
3409    </address>
3410  </author>
3411  <date month='January' year='2005'></date>
3412 </front>
3413 <seriesInfo name="RFC" value="3986"/>
3414 <seriesInfo name="STD" value="66"/>
3415</reference>
3416
3417<reference anchor="USASCII">
3418  <front>
3419    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3420    <author>
3421      <organization>American National Standards Institute</organization>
3422    </author>
3423    <date year="1986"/>
3424  </front>
3425  <seriesInfo name="ANSI" value="X3.4"/>
3426</reference>
3427
3428</references>
3429
3430<references title="Informative References">
3431
3432<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3433  <front>
3434    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3435    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3436      <organization/>
3437    </author>
3438    <author initials="J." surname="Gettys" fullname="J. Gettys">
3439      <organization/>
3440    </author>
3441    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3442      <organization/>
3443    </author>
3444    <author initials="H." surname="Lie" fullname="H. Lie">
3445      <organization/>
3446    </author>
3447    <author initials="C." surname="Lilley" fullname="C. Lilley">
3448      <organization/>
3449    </author>
3450    <date year="1997" month="September"/>
3451  </front>
3452  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3453</reference>
3454
3455<reference anchor="Pad1995" target="http://portal.acm.org/citation.cfm?id=219094">
3456  <front>
3457    <title>Improving HTTP Latency</title>
3458    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3459      <organization/>
3460    </author>
3461    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3462      <organization/>
3463    </author>
3464    <date year="1995" month="December"/>
3465  </front>
3466  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3467</reference>
3468
3469<reference anchor="RFC959">
3470  <front>
3471    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3472    <author initials="J." surname="Postel" fullname="J. Postel">
3473      <organization>Information Sciences Institute (ISI)</organization>
3474    </author>
3475    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3476      <organization/>
3477    </author>
3478    <date month="October" year="1985"/>
3479  </front>
3480  <seriesInfo name="STD" value="9"/>
3481  <seriesInfo name="RFC" value="959"/>
3482</reference>
3483
3484<reference anchor="RFC1123">
3485  <front>
3486    <title>Requirements for Internet Hosts - Application and Support</title>
3487    <author initials="R." surname="Braden" fullname="Robert Braden">
3488      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3489      <address><email>Braden@ISI.EDU</email></address>
3490    </author>
3491    <date month="October" year="1989"/>
3492  </front>
3493  <seriesInfo name="STD" value="3"/>
3494  <seriesInfo name="RFC" value="1123"/>
3495</reference>
3496
3497<reference anchor="RFC1305">
3498  <front>
3499    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3500    <author initials="D." surname="Mills" fullname="David L. Mills">
3501      <organization>University of Delaware, Electrical Engineering Department</organization>
3502      <address><email>mills@udel.edu</email></address>
3503    </author>
3504    <date month="March" year="1992"/>
3505  </front>
3506  <seriesInfo name="RFC" value="1305"/>
3507</reference>
3508
3509<reference anchor="RFC1436">
3510  <front>
3511    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3512    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3513      <organization>University of Minnesota, Computer and Information Services</organization>
3514      <address><email>fxa@boombox.micro.umn.edu</email></address>
3515    </author>
3516    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3517      <organization>University of Minnesota, Computer and Information Services</organization>
3518      <address><email>mpm@boombox.micro.umn.edu</email></address>
3519    </author>
3520    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3521      <organization>University of Minnesota, Computer and Information Services</organization>
3522      <address><email>lindner@boombox.micro.umn.edu</email></address>
3523    </author>
3524    <author initials="D." surname="Johnson" fullname="David Johnson">
3525      <organization>University of Minnesota, Computer and Information Services</organization>
3526      <address><email>dmj@boombox.micro.umn.edu</email></address>
3527    </author>
3528    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3529      <organization>University of Minnesota, Computer and Information Services</organization>
3530      <address><email>daniel@boombox.micro.umn.edu</email></address>
3531    </author>
3532    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3533      <organization>University of Minnesota, Computer and Information Services</organization>
3534      <address><email>alberti@boombox.micro.umn.edu</email></address>
3535    </author>
3536    <date month="March" year="1993"/>
3537  </front>
3538  <seriesInfo name="RFC" value="1436"/>
3539</reference>
3540
3541<reference anchor="RFC1900">
3542  <front>
3543    <title>Renumbering Needs Work</title>
3544    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3545      <organization>CERN, Computing and Networks Division</organization>
3546      <address><email>brian@dxcoms.cern.ch</email></address>
3547    </author>
3548    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3549      <organization>cisco Systems</organization>
3550      <address><email>yakov@cisco.com</email></address>
3551    </author>
3552    <date month="February" year="1996"/>
3553  </front>
3554  <seriesInfo name="RFC" value="1900"/>
3555</reference>
3556
3557<reference anchor="RFC1945">
3558  <front>
3559    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3560    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3561      <organization>MIT, Laboratory for Computer Science</organization>
3562      <address><email>timbl@w3.org</email></address>
3563    </author>
3564    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3565      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3566      <address><email>fielding@ics.uci.edu</email></address>
3567    </author>
3568    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3569      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3570      <address><email>frystyk@w3.org</email></address>
3571    </author>
3572    <date month="May" year="1996"/>
3573  </front>
3574  <seriesInfo name="RFC" value="1945"/>
3575</reference>
3576
3577<reference anchor="RFC2047">
3578  <front>
3579    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3580    <author initials="K." surname="Moore" fullname="Keith Moore">
3581      <organization>University of Tennessee</organization>
3582      <address><email>moore@cs.utk.edu</email></address>
3583    </author>
3584    <date month="November" year="1996"/>
3585  </front>
3586  <seriesInfo name="RFC" value="2047"/>
3587</reference>
3588
3589<reference anchor="RFC2068">
3590  <front>
3591    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3592    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3593      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3594      <address><email>fielding@ics.uci.edu</email></address>
3595    </author>
3596    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3597      <organization>MIT Laboratory for Computer Science</organization>
3598      <address><email>jg@w3.org</email></address>
3599    </author>
3600    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3601      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3602      <address><email>mogul@wrl.dec.com</email></address>
3603    </author>
3604    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3605      <organization>MIT Laboratory for Computer Science</organization>
3606      <address><email>frystyk@w3.org</email></address>
3607    </author>
3608    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3609      <organization>MIT Laboratory for Computer Science</organization>
3610      <address><email>timbl@w3.org</email></address>
3611    </author>
3612    <date month="January" year="1997"/>
3613  </front>
3614  <seriesInfo name="RFC" value="2068"/>
3615</reference>
3616
3617<reference anchor='RFC2109'>
3618  <front>
3619    <title>HTTP State Management Mechanism</title>
3620    <author initials='D.M.' surname='Kristol' fullname='David M. Kristol'>
3621      <organization>Bell Laboratories, Lucent Technologies</organization>
3622      <address><email>dmk@bell-labs.com</email></address>
3623    </author>
3624    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3625      <organization>Netscape Communications Corp.</organization>
3626      <address><email>montulli@netscape.com</email></address>
3627    </author>
3628    <date year='1997' month='February' />
3629  </front>
3630  <seriesInfo name='RFC' value='2109' />
3631</reference>
3632
3633<reference anchor="RFC2145">
3634  <front>
3635    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3636    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3637      <organization>Western Research Laboratory</organization>
3638      <address><email>mogul@wrl.dec.com</email></address>
3639    </author>
3640    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3641      <organization>Department of Information and Computer Science</organization>
3642      <address><email>fielding@ics.uci.edu</email></address>
3643    </author>
3644    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3645      <organization>MIT Laboratory for Computer Science</organization>
3646      <address><email>jg@w3.org</email></address>
3647    </author>
3648    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3649      <organization>W3 Consortium</organization>
3650      <address><email>frystyk@w3.org</email></address>
3651    </author>
3652    <date month="May" year="1997"/>
3653  </front>
3654  <seriesInfo name="RFC" value="2145"/>
3655</reference>
3656
3657<reference anchor="RFC2616">
3658  <front>
3659    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3660    <author initials="R." surname="Fielding" fullname="R. Fielding">
3661      <organization>University of California, Irvine</organization>
3662      <address><email>fielding@ics.uci.edu</email></address>
3663    </author>
3664    <author initials="J." surname="Gettys" fullname="J. Gettys">
3665      <organization>W3C</organization>
3666      <address><email>jg@w3.org</email></address>
3667    </author>
3668    <author initials="J." surname="Mogul" fullname="J. Mogul">
3669      <organization>Compaq Computer Corporation</organization>
3670      <address><email>mogul@wrl.dec.com</email></address>
3671    </author>
3672    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3673      <organization>MIT Laboratory for Computer Science</organization>
3674      <address><email>frystyk@w3.org</email></address>
3675    </author>
3676    <author initials="L." surname="Masinter" fullname="L. Masinter">
3677      <organization>Xerox Corporation</organization>
3678      <address><email>masinter@parc.xerox.com</email></address>
3679    </author>
3680    <author initials="P." surname="Leach" fullname="P. Leach">
3681      <organization>Microsoft Corporation</organization>
3682      <address><email>paulle@microsoft.com</email></address>
3683    </author>
3684    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3685      <organization>W3C</organization>
3686      <address><email>timbl@w3.org</email></address>
3687    </author>
3688    <date month="June" year="1999"/>
3689  </front>
3690  <seriesInfo name="RFC" value="2616"/>
3691</reference>
3692
3693<reference anchor='RFC2818'>
3694  <front>
3695    <title>HTTP Over TLS</title>
3696    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
3697      <organization>RTFM, Inc.</organization>
3698      <address><email>ekr@rtfm.com</email></address>
3699    </author>
3700    <date year='2000' month='May' />
3701  </front>
3702  <seriesInfo name='RFC' value='2818' />
3703</reference>
3704
3705<reference anchor='RFC2965'>
3706  <front>
3707    <title>HTTP State Management Mechanism</title>
3708    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
3709      <organization>Bell Laboratories, Lucent Technologies</organization>
3710      <address><email>dmk@bell-labs.com</email></address>
3711    </author>
3712    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3713      <organization>Epinions.com, Inc.</organization>
3714      <address><email>lou@montulli.org</email></address>
3715    </author>
3716    <date year='2000' month='October' />
3717  </front>
3718  <seriesInfo name='RFC' value='2965' />
3719</reference>
3720
3721<reference anchor='RFC3864'>
3722  <front>
3723    <title>Registration Procedures for Message Header Fields</title>
3724    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
3725      <organization>Nine by Nine</organization>
3726      <address><email>GK-IETF@ninebynine.org</email></address>
3727    </author>
3728    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
3729      <organization>BEA Systems</organization>
3730      <address><email>mnot@pobox.com</email></address>
3731    </author>
3732    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
3733      <organization>HP Labs</organization>
3734      <address><email>JeffMogul@acm.org</email></address>
3735    </author>
3736    <date year='2004' month='September' />
3737  </front>
3738  <seriesInfo name='BCP' value='90' />
3739  <seriesInfo name='RFC' value='3864' />
3740</reference>
3741
3742<reference anchor='RFC3977'>
3743  <front>
3744    <title>Network News Transfer Protocol (NNTP)</title>
3745    <author initials='C.' surname='Feather' fullname='C. Feather'>
3746      <organization>THUS plc</organization>
3747      <address><email>clive@demon.net</email></address>
3748    </author>
3749    <date year='2006' month='October' />
3750  </front>
3751  <seriesInfo name="RFC" value="3977"/>
3752</reference>
3753
3754<reference anchor="RFC4288">
3755  <front>
3756    <title>Media Type Specifications and Registration Procedures</title>
3757    <author initials="N." surname="Freed" fullname="N. Freed">
3758      <organization>Sun Microsystems</organization>
3759      <address>
3760        <email>ned.freed@mrochek.com</email>
3761      </address>
3762    </author>
3763    <author initials="J." surname="Klensin" fullname="J. Klensin">
3764      <organization/>
3765      <address>
3766        <email>klensin+ietf@jck.com</email>
3767      </address>
3768    </author>
3769    <date year="2005" month="December"/>
3770  </front>
3771  <seriesInfo name="BCP" value="13"/>
3772  <seriesInfo name="RFC" value="4288"/>
3773</reference>
3774
3775<reference anchor='RFC4395'>
3776  <front>
3777    <title>Guidelines and Registration Procedures for New URI Schemes</title>
3778    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
3779      <organization>AT&amp;T Laboratories</organization>
3780      <address>
3781        <email>tony+urireg@maillennium.att.com</email>
3782      </address>
3783    </author>
3784    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
3785      <organization>Qualcomm, Inc.</organization>
3786      <address>
3787        <email>hardie@qualcomm.com</email>
3788      </address>
3789    </author>
3790    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
3791      <organization>Adobe Systems</organization>
3792      <address>
3793        <email>LMM@acm.org</email>
3794      </address>
3795    </author>
3796    <date year='2006' month='February' />
3797  </front>
3798  <seriesInfo name='BCP' value='115' />
3799  <seriesInfo name='RFC' value='4395' />
3800</reference>
3801
3802<reference anchor="RFC5322">
3803  <front>
3804    <title>Internet Message Format</title>
3805    <author initials="P." surname="Resnick" fullname="P. Resnick">
3806      <organization>Qualcomm Incorporated</organization>
3807    </author>
3808    <date year="2008" month="October"/>
3809  </front> 
3810  <seriesInfo name="RFC" value="5322"/>
3811</reference>
3812
3813<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
3814  <front>
3815    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
3816    <author initials="D." surname="Kristol" fullname="David M. Kristol">
3817      <organization/>
3818    </author>
3819    <date year="2001" month="November"/>
3820  </front>
3821  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
3822</reference>
3823
3824<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3825  <front>
3826  <title>Analysis of HTTP Performance Problems</title>
3827  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3828    <organization/>
3829  </author>
3830  <date/>
3831  </front>
3832</reference>
3833
3834<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3835  <front>
3836  <title>Analysis of HTTP Performance</title>
3837  <author initials="J." surname="Touch" fullname="Joe Touch">
3838    <organization>USC/Information Sciences Institute</organization>
3839    <address><email>touch@isi.edu</email></address>
3840  </author>
3841  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3842    <organization>USC/Information Sciences Institute</organization>
3843    <address><email>johnh@isi.edu</email></address>
3844  </author>
3845  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3846    <organization>USC/Information Sciences Institute</organization>
3847    <address><email>katia@isi.edu</email></address>
3848  </author>
3849  <date year="1998" month="Aug"/>
3850  </front>
3851  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3852  <annotation>(original report dated Aug. 1996)</annotation>
3853</reference>
3854
3855<reference anchor="WAIS">
3856  <front>
3857    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3858    <author initials="F." surname="Davis" fullname="F. Davis">
3859      <organization>Thinking Machines Corporation</organization>
3860    </author>
3861    <author initials="B." surname="Kahle" fullname="B. Kahle">
3862      <organization>Thinking Machines Corporation</organization>
3863    </author>
3864    <author initials="H." surname="Morris" fullname="H. Morris">
3865      <organization>Thinking Machines Corporation</organization>
3866    </author>
3867    <author initials="J." surname="Salem" fullname="J. Salem">
3868      <organization>Thinking Machines Corporation</organization>
3869    </author>
3870    <author initials="T." surname="Shen" fullname="T. Shen">
3871      <organization>Thinking Machines Corporation</organization>
3872    </author>
3873    <author initials="R." surname="Wang" fullname="R. Wang">
3874      <organization>Thinking Machines Corporation</organization>
3875    </author>
3876    <author initials="J." surname="Sui" fullname="J. Sui">
3877      <organization>Thinking Machines Corporation</organization>
3878    </author>
3879    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3880      <organization>Thinking Machines Corporation</organization>
3881    </author>
3882    <date month="April" year="1990"/>
3883  </front>
3884  <seriesInfo name="Thinking Machines Corporation" value=""/>
3885</reference>
3886
3887</references>
3888
3889
3890<section title="Tolerant Applications" anchor="tolerant.applications">
3891<t>
3892   Although this document specifies the requirements for the generation
3893   of HTTP/1.1 messages, not all applications will be correct in their
3894   implementation. We therefore recommend that operational applications
3895   be tolerant of deviations whenever those deviations can be
3896   interpreted unambiguously.
3897</t>
3898<t>
3899   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3900   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3901   accept any amount of WSP characters between fields, even though
3902   only a single SP is required.
3903</t>
3904<t>
3905   The line terminator for message-header fields is the sequence CRLF.
3906   However, we recommend that applications, when parsing such headers,
3907   recognize a single LF as a line terminator and ignore the leading CR.
3908</t>
3909<t>
3910   The character set of an entity-body &SHOULD; be labeled as the lowest
3911   common denominator of the character codes used within that body, with
3912   the exception that not labeling the entity is preferred over labeling
3913   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3914</t>
3915<t>
3916   Additional rules for requirements on parsing and encoding of dates
3917   and other potential problems with date encodings include:
3918</t>
3919<t>
3920  <list style="symbols">
3921     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3922        which appears to be more than 50 years in the future is in fact
3923        in the past (this helps solve the "year 2000" problem).</t>
3924
3925     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3926        Expires date as earlier than the proper value, but &MUST-NOT;
3927        internally represent a parsed Expires date as later than the
3928        proper value.</t>
3929
3930     <t>All expiration-related calculations &MUST; be done in GMT. The
3931        local time zone &MUST-NOT; influence the calculation or comparison
3932        of an age or expiration time.</t>
3933
3934     <t>If an HTTP header incorrectly carries a date value with a time
3935        zone other than GMT, it &MUST; be converted into GMT using the
3936        most conservative possible conversion.</t>
3937  </list>
3938</t>
3939</section>
3940
3941<section title="Compatibility with Previous Versions" anchor="compatibility">
3942<t>
3943   HTTP has been in use by the World-Wide Web global information initiative
3944   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
3945   was a simple protocol for hypertext data transfer across the Internet
3946   with only a single method and no metadata.
3947   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
3948   methods and MIME-like messaging that could include metadata about the data
3949   transferred and modifiers on the request/response semantics. However,
3950   HTTP/1.0 did not sufficiently take into consideration the effects of
3951   hierarchical proxies, caching, the need for persistent connections, or
3952   name-based virtual hosts. The proliferation of incompletely-implemented
3953   applications calling themselves "HTTP/1.0" further necessitated a
3954   protocol version change in order for two communicating applications
3955   to determine each other's true capabilities.
3956</t>
3957<t>
3958   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
3959   requirements that enable reliable implementations, adding only
3960   those new features that will either be safely ignored by an HTTP/1.0
3961   recipient or only sent when communicating with a party advertising
3962   compliance with HTTP/1.1.
3963</t>
3964<t>
3965   It is beyond the scope of a protocol specification to mandate
3966   compliance with previous versions. HTTP/1.1 was deliberately
3967   designed, however, to make supporting previous versions easy. It is
3968   worth noting that, at the time of composing this specification
3969   (1996), we would expect commercial HTTP/1.1 servers to:
3970  <list style="symbols">
3971     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3972        1.1 requests;</t>
3973
3974     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3975        1.1;</t>
3976
3977     <t>respond appropriately with a message in the same major version
3978        used by the client.</t>
3979  </list>
3980</t>
3981<t>
3982   And we would expect HTTP/1.1 clients to:
3983  <list style="symbols">
3984     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3985        responses;</t>
3986
3987     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3988        1.1.</t>
3989  </list>
3990</t>
3991<t>
3992   For most implementations of HTTP/1.0, each connection is established
3993   by the client prior to the request and closed by the server after
3994   sending the response. Some implementations implement the Keep-Alive
3995   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3996</t>
3997
3998<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3999<t>
4000   This section summarizes major differences between versions HTTP/1.0
4001   and HTTP/1.1.
4002</t>
4003
4004<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">
4005<t>
4006   The requirements that clients and servers support the Host request-header,
4007   report an error if the Host request-header (<xref target="header.host"/>) is
4008   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4009   are among the most important changes defined by this
4010   specification.
4011</t>
4012<t>
4013   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4014   addresses and servers; there was no other established mechanism for
4015   distinguishing the intended server of a request than the IP address
4016   to which that request was directed. The changes outlined above will
4017   allow the Internet, once older HTTP clients are no longer common, to
4018   support multiple Web sites from a single IP address, greatly
4019   simplifying large operational Web servers, where allocation of many
4020   IP addresses to a single host has created serious problems. The
4021   Internet will also be able to recover the IP addresses that have been
4022   allocated for the sole purpose of allowing special-purpose domain
4023   names to be used in root-level HTTP URLs. Given the rate of growth of
4024   the Web, and the number of servers already deployed, it is extremely
4025   important that all implementations of HTTP (including updates to
4026   existing HTTP/1.0 applications) correctly implement these
4027   requirements:
4028  <list style="symbols">
4029     <t>Both clients and servers &MUST; support the Host request-header.</t>
4030
4031     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4032
4033     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4034        request does not include a Host request-header.</t>
4035
4036     <t>Servers &MUST; accept absolute URIs.</t>
4037  </list>
4038</t>
4039</section>
4040</section>
4041
4042<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4043<t>
4044   Some clients and servers might wish to be compatible with some
4045   previous implementations of persistent connections in HTTP/1.0
4046   clients and servers. Persistent connections in HTTP/1.0 are
4047   explicitly negotiated as they are not the default behavior. HTTP/1.0
4048   experimental implementations of persistent connections are faulty,
4049   and the new facilities in HTTP/1.1 are designed to rectify these
4050   problems. The problem was that some existing 1.0 clients may be
4051   sending Keep-Alive to a proxy server that doesn't understand
4052   Connection, which would then erroneously forward it to the next
4053   inbound server, which would establish the Keep-Alive connection and
4054   result in a hung HTTP/1.0 proxy waiting for the close on the
4055   response. The result is that HTTP/1.0 clients must be prevented from
4056   using Keep-Alive when talking to proxies.
4057</t>
4058<t>
4059   However, talking to proxies is the most important use of persistent
4060   connections, so that prohibition is clearly unacceptable. Therefore,
4061   we need some other mechanism for indicating a persistent connection
4062   is desired, which is safe to use even when talking to an old proxy
4063   that ignores Connection. Persistent connections are the default for
4064   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4065   declaring non-persistence. See <xref target="header.connection"/>.
4066</t>
4067<t>
4068   The original HTTP/1.0 form of persistent connections (the Connection:
4069   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
4070</t>
4071</section>
4072
4073<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4074<t>
4075   This specification has been carefully audited to correct and
4076   disambiguate key word usage; RFC 2068 had many problems in respect to
4077   the conventions laid out in <xref target="RFC2119"/>.
4078</t>
4079<t>
4080   Transfer-coding and message lengths all interact in ways that
4081   required fixing exactly when chunked encoding is used (to allow for
4082   transfer encoding that may not be self delimiting); it was important
4083   to straighten out exactly how message lengths are computed. (Sections
4084   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4085   <xref target="header.content-length" format="counter"/>,
4086   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4087</t>
4088<t>
4089   The use and interpretation of HTTP version numbers has been clarified
4090   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4091   version they support to deal with problems discovered in HTTP/1.0
4092   implementations (<xref target="http.version"/>)
4093</t>
4094<t>
4095   Transfer-coding had significant problems, particularly with
4096   interactions with chunked encoding. The solution is that transfer-codings
4097   become as full fledged as content-codings. This involves
4098   adding an IANA registry for transfer-codings (separate from content
4099   codings), a new header field (TE) and enabling trailer headers in the
4100   future. Transfer encoding is a major performance benefit, so it was
4101   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4102   interoperability problem that could have occurred due to interactions
4103   between authentication trailers, chunked encoding and HTTP/1.0
4104   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4105   and <xref target="header.te" format="counter"/>)
4106</t>
4107</section>
4108
4109<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4110<t>
4111  Empty list elements in list productions have been deprecated.
4112  (<xref target="notation.abnf"/>)
4113</t>
4114<t>
4115  Rules about implicit linear whitespace between certain grammar productions
4116  have been removed; now it's only allowed when specifically pointed out
4117  in the ABNF. The NUL character is no longer allowed in comment and quoted-string
4118  text. The quoted-pair rule no longer allows escaping NUL, CR or LF.
4119  Non-ASCII content in header fields and reason phrase has been obsoleted and
4120  made opaque (the TEXT rule was removed)
4121  (<xref target="basic.rules"/>)
4122</t>
4123<t>
4124  Clarify that HTTP-Version is case sensitive.
4125  (<xref target="http.version"/>)
4126</t>
4127<t>
4128  Remove reference to non-existant identity transfer-coding value tokens.
4129  (Sections <xref format="counter" target="transfer.codings"/> and
4130  <xref format="counter" target="message.length"/>)
4131</t>
4132<t>
4133  Clarification that the chunk length does not include
4134  the count of the octets in the chunk header and trailer.
4135  (<xref target="chunked.transfer.encoding"/>)
4136</t>
4137<t>
4138  Require that invalid whitespace around field-names be rejected.
4139  (<xref target="message.headers"/>)
4140</t>
4141<t>
4142  Update use of abs_path production from RFC1808 to the path-absolute + query
4143  components of RFC3986.
4144  (<xref target="request-target"/>)
4145</t>
4146<t>
4147  Clarify exactly when close connection options must be sent.
4148  (<xref target="header.connection"/>)
4149</t>
4150</section>
4151</section>
4152
4153<section title="Terminology" anchor="terminology">
4154<t>
4155   This specification uses a number of terms to refer to the roles
4156   played by participants in, and objects of, the HTTP communication.
4157</t>
4158<t>
4159  <iref item="connection"/>
4160  <x:dfn>connection</x:dfn>
4161  <list>
4162    <t>
4163      A transport layer virtual circuit established between two programs
4164      for the purpose of communication.
4165    </t>
4166  </list>
4167</t>
4168<t>
4169  <iref item="message"/>
4170  <x:dfn>message</x:dfn>
4171  <list>
4172    <t>
4173      The basic unit of HTTP communication, consisting of a structured
4174      sequence of octets matching the syntax defined in <xref target="http.message"/> and
4175      transmitted via the connection.
4176    </t>
4177  </list>
4178</t>
4179<t>
4180  <iref item="request"/>
4181  <x:dfn>request</x:dfn>
4182  <list>
4183    <t>
4184      An HTTP request message, as defined in <xref target="request"/>.
4185    </t>
4186  </list>
4187</t>
4188<t>
4189  <iref item="response"/>
4190  <x:dfn>response</x:dfn>
4191  <list>
4192    <t>
4193      An HTTP response message, as defined in <xref target="response"/>.
4194    </t>
4195  </list>
4196</t>
4197<t>
4198  <iref item="resource"/>
4199  <x:dfn>resource</x:dfn>
4200  <list>
4201    <t>
4202      A network data object or service that can be identified by a URI,
4203      as defined in <xref target="uri"/>. Resources may be available in multiple
4204      representations (e.g. multiple languages, data formats, size, and
4205      resolutions) or vary in other ways.
4206    </t>
4207  </list>
4208</t>
4209<t>
4210  <iref item="entity"/>
4211  <x:dfn>entity</x:dfn>
4212  <list>
4213    <t>
4214      The information transferred as the payload of a request or
4215      response. An entity consists of metainformation in the form of
4216      entity-header fields and content in the form of an entity-body, as
4217      described in &entity;.
4218    </t>
4219  </list>
4220</t>
4221<t>
4222  <iref item="representation"/>
4223  <x:dfn>representation</x:dfn>
4224  <list>
4225    <t>
4226      An entity included with a response that is subject to content
4227      negotiation, as described in &content.negotiation;. There may exist multiple
4228      representations associated with a particular response status.
4229    </t>
4230  </list>
4231</t>
4232<t>
4233  <iref item="content negotiation"/>
4234  <x:dfn>content negotiation</x:dfn>
4235  <list>
4236    <t>
4237      The mechanism for selecting the appropriate representation when
4238      servicing a request, as described in &content.negotiation;. The
4239      representation of entities in any response can be negotiated
4240      (including error responses).
4241    </t>
4242  </list>
4243</t>
4244<t>
4245  <iref item="variant"/>
4246  <x:dfn>variant</x:dfn>
4247  <list>
4248    <t>
4249      A resource may have one, or more than one, representation(s)
4250      associated with it at any given instant. Each of these
4251      representations is termed a `variant'.  Use of the term `variant'
4252      does not necessarily imply that the resource is subject to content
4253      negotiation.
4254    </t>
4255  </list>
4256</t>
4257<t>
4258  <iref item="client"/>
4259  <x:dfn>client</x:dfn>
4260  <list>
4261    <t>
4262      A program that establishes connections for the purpose of sending
4263      requests.
4264    </t>
4265  </list>
4266</t>
4267<t>
4268  <iref item="user agent"/>
4269  <x:dfn>user agent</x:dfn>
4270  <list>
4271    <t>
4272      The client which initiates a request. These are often browsers,
4273      editors, spiders (web-traversing robots), or other end user tools.
4274    </t>
4275  </list>
4276</t>
4277<t>
4278  <iref item="server"/>
4279  <x:dfn>server</x:dfn>
4280  <list>
4281    <t>
4282      An application program that accepts connections in order to
4283      service requests by sending back responses. Any given program may
4284      be capable of being both a client and a server; our use of these
4285      terms refers only to the role being performed by the program for a
4286      particular connection, rather than to the program's capabilities
4287      in general. Likewise, any server may act as an origin server,
4288      proxy, gateway, or tunnel, switching behavior based on the nature
4289      of each request.
4290    </t>
4291  </list>
4292</t>
4293<t>
4294  <iref item="origin server"/>
4295  <x:dfn>origin server</x:dfn>
4296  <list>
4297    <t>
4298      The server on which a given resource resides or is to be created.
4299    </t>
4300  </list>
4301</t>
4302<t>
4303  <iref item="proxy"/>
4304  <x:dfn>proxy</x:dfn>
4305  <list>
4306    <t>
4307      An intermediary program which acts as both a server and a client
4308      for the purpose of making requests on behalf of other clients.
4309      Requests are serviced internally or by passing them on, with
4310      possible translation, to other servers. A proxy &MUST; implement
4311      both the client and server requirements of this specification. A
4312      "transparent proxy" is a proxy that does not modify the request or
4313      response beyond what is required for proxy authentication and
4314      identification. A "non-transparent proxy" is a proxy that modifies
4315      the request or response in order to provide some added service to
4316      the user agent, such as group annotation services, media type
4317      transformation, protocol reduction, or anonymity filtering. Except
4318      where either transparent or non-transparent behavior is explicitly
4319      stated, the HTTP proxy requirements apply to both types of
4320      proxies.
4321    </t>
4322  </list>
4323</t>
4324<t>
4325  <iref item="gateway"/>
4326  <x:dfn>gateway</x:dfn>
4327  <list>
4328    <t>
4329      A server which acts as an intermediary for some other server.
4330      Unlike a proxy, a gateway receives requests as if it were the
4331      origin server for the requested resource; the requesting client
4332      may not be aware that it is communicating with a gateway.
4333    </t>
4334  </list>
4335</t>
4336<t>
4337  <iref item="tunnel"/>
4338  <x:dfn>tunnel</x:dfn>
4339  <list>
4340    <t>
4341      An intermediary program which is acting as a blind relay between
4342      two connections. Once active, a tunnel is not considered a party
4343      to the HTTP communication, though the tunnel may have been
4344      initiated by an HTTP request. The tunnel ceases to exist when both
4345      ends of the relayed connections are closed.
4346    </t>
4347  </list>
4348</t>
4349<t>
4350  <iref item="cache"/>
4351  <x:dfn>cache</x:dfn>
4352  <list>
4353    <t>
4354      A program's local store of response messages and the subsystem
4355      that controls its message storage, retrieval, and deletion. A
4356      cache stores cacheable responses in order to reduce the response
4357      time and network bandwidth consumption on future, equivalent
4358      requests. Any client or server may include a cache, though a cache
4359      cannot be used by a server that is acting as a tunnel.
4360    </t>
4361  </list>
4362</t>
4363<t>
4364  <iref item="cacheable"/>
4365  <x:dfn>cacheable</x:dfn>
4366  <list>
4367    <t>
4368      A response is cacheable if a cache is allowed to store a copy of
4369      the response message for use in answering subsequent requests. The
4370      rules for determining the cacheability of HTTP responses are
4371      defined in &caching;. Even if a resource is cacheable, there may
4372      be additional constraints on whether a cache can use the cached
4373      copy for a particular request.
4374    </t>
4375  </list>
4376</t>
4377<t>
4378  <iref item="upstream"/>
4379  <iref item="downstream"/>
4380  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
4381  <list>
4382    <t>
4383      Upstream and downstream describe the flow of a message: all
4384      messages flow from upstream to downstream.
4385    </t>
4386  </list>
4387</t>
4388<t>
4389  <iref item="inbound"/>
4390  <iref item="outbound"/>
4391  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
4392  <list>
4393    <t>
4394      Inbound and outbound refer to the request and response paths for
4395      messages: "inbound" means "traveling toward the origin server",
4396      and "outbound" means "traveling toward the user agent"
4397    </t>
4398  </list>
4399</t>
4400</section>
4401
4402<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
4403
4404<section title="Since RFC2616">
4405<t>
4406  Extracted relevant partitions from <xref target="RFC2616"/>.
4407</t>
4408</section>
4409
4410<section title="Since draft-ietf-httpbis-p1-messaging-00">
4411<t>
4412  Closed issues:
4413  <list style="symbols"> 
4414    <t>
4415      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4416      "HTTP Version should be case sensitive"
4417      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4418    </t>
4419    <t>
4420      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4421      "'unsafe' characters"
4422      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4423    </t>
4424    <t>
4425      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4426      "Chunk Size Definition"
4427      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4428    </t>
4429    <t>
4430      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4431      "Message Length"
4432      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4433    </t>
4434    <t>
4435      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4436      "Media Type Registrations"
4437      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4438    </t>
4439    <t>
4440      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4441      "URI includes query"
4442      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4443    </t>
4444    <t>
4445      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4446      "No close on 1xx responses"
4447      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4448    </t>
4449    <t>
4450      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4451      "Remove 'identity' token references"
4452      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4453    </t>
4454    <t>
4455      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4456      "Import query BNF"
4457    </t>
4458    <t>
4459      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4460      "qdtext BNF"
4461    </t>
4462    <t>
4463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4464      "Normative and Informative references"
4465    </t>
4466    <t>
4467      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4468      "RFC2606 Compliance"
4469    </t>
4470    <t>
4471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4472      "RFC977 reference"
4473    </t>
4474    <t>
4475      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4476      "RFC1700 references"
4477    </t>
4478    <t>
4479      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4480      "inconsistency in date format explanation"
4481    </t>
4482    <t>
4483      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4484      "Date reference typo"
4485    </t>
4486    <t>
4487      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4488      "Informative references"
4489    </t>
4490    <t>
4491      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4492      "ISO-8859-1 Reference"
4493    </t>
4494    <t>
4495      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4496      "Normative up-to-date references"
4497    </t>
4498  </list>
4499</t>
4500<t>
4501  Other changes:
4502  <list style="symbols"> 
4503    <t>
4504      Update media type registrations to use RFC4288 template.
4505    </t>
4506    <t>
4507      Use names of RFC4234 core rules DQUOTE and WSP,
4508      fix broken ABNF for chunk-data
4509      (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4510    </t>
4511  </list>
4512</t>
4513</section>
4514
4515<section title="Since draft-ietf-httpbis-p1-messaging-01">
4516<t>
4517  Closed issues:
4518  <list style="symbols"> 
4519    <t>
4520      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4521      "Bodies on GET (and other) requests"
4522    </t>
4523    <t>
4524      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4525      "Updating to RFC4288"
4526    </t>
4527    <t>
4528      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4529      "Status Code and Reason Phrase"
4530    </t>
4531    <t>
4532      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4533      "rel_path not used"
4534    </t>
4535  </list>
4536</t>
4537<t>
4538  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4539  <list style="symbols"> 
4540    <t>
4541      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4542      "trailer-part").
4543    </t>
4544    <t>
4545      Avoid underscore character in rule names ("http_URL" ->
4546      "http-URL", "abs_path" -> "path-absolute").
4547    </t>
4548    <t>
4549      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4550      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4551      have to be updated when switching over to RFC3986.
4552    </t>
4553    <t>
4554      Synchronize core rules with RFC5234.
4555    </t>
4556    <t>
4557      Get rid of prose rules that span multiple lines.
4558    </t>
4559    <t>
4560      Get rid of unused rules LOALPHA and UPALPHA.
4561    </t>
4562    <t>
4563      Move "Product Tokens" section (back) into Part 1, as "token" is used
4564      in the definition of the Upgrade header.
4565    </t>
4566    <t>
4567      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4568    </t>
4569    <t>
4570      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4571    </t>
4572  </list>
4573</t>
4574</section>
4575
4576<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
4577<t>
4578  Closed issues:
4579  <list style="symbols"> 
4580    <t>
4581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/51"/>:
4582      "HTTP-date vs. rfc1123-date"
4583    </t>
4584    <t>
4585      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/64"/>:
4586      "WS in quoted-pair"
4587    </t>
4588  </list>
4589</t>
4590<t>
4591  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
4592  <list style="symbols"> 
4593    <t>
4594      Reference RFC 3984, and update header registrations for headers defined
4595      in this document.
4596    </t>
4597  </list>
4598</t>
4599<t>
4600  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4601  <list style="symbols"> 
4602    <t>
4603      Replace string literals when the string really is case-sensitive (HTTP-Version).
4604    </t>
4605  </list>
4606</t>
4607</section>
4608
4609<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
4610<t>
4611  Closed issues:
4612  <list style="symbols"> 
4613    <t>
4614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
4615      "Connection closing"
4616    </t>
4617    <t>
4618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/97"/>:
4619      "Move registrations and registry information to IANA Considerations"
4620    </t>
4621    <t>
4622      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/120"/>:
4623      "need new URL for PAD1995 reference"
4624    </t>
4625    <t>
4626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/127"/>:
4627      "IANA Considerations: update HTTP URI scheme registration"
4628    </t>
4629    <t>
4630      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
4631      "Cite HTTPS URI scheme definition"
4632    </t>
4633    <t>
4634      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/129"/>:
4635      "List-type headers vs Set-Cookie"
4636    </t>
4637  </list>
4638</t>
4639<t>
4640  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4641  <list style="symbols"> 
4642    <t>
4643      Replace string literals when the string really is case-sensitive (HTTP-Date).
4644    </t>
4645    <t>
4646      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
4647    </t>
4648  </list>
4649</t>
4650</section>
4651
4652<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
4653<t>
4654  Closed issues:
4655  <list style="symbols"> 
4656    <t>
4657      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/34"/>:
4658      "Out-of-date reference for URIs"
4659    </t>
4660    <t>
4661      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
4662      "RFC 2822 is updated by RFC 5322"
4663    </t>
4664  </list>
4665</t>
4666<t>
4667  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4668  <list style="symbols"> 
4669    <t>
4670      Use "/" instead of "|" for alternatives.
4671    </t>
4672    <t>
4673      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
4674    </t>
4675    <t>
4676      Only reference RFC 5234's core rules.
4677    </t>
4678    <t>
4679      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
4680      whitespace ("OWS") and required whitespace ("RWS").
4681    </t>
4682    <t>
4683      Rewrite ABNFs to spell out whitespace rules, factor out
4684      header value format definitions.
4685    </t>
4686  </list>
4687</t>
4688</section>
4689
4690<section title="Since draft-ietf-httpbis-p1-messaging-05" anchor="changes.since.05">
4691<t>
4692  Closed issues:
4693  <list style="symbols"> 
4694    <t>
4695      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/30"/>:
4696      "Header LWS"
4697    </t>
4698    <t>
4699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/63"/>:
4700      "RFC2047 encoded words"
4701    </t>
4702    <t>
4703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/74"/>:
4704      "Character Encodings in TEXT"
4705    </t>
4706    <t>
4707      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/77"/>:
4708      "Line Folding"
4709    </t>
4710    <t>
4711      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/83"/>:
4712      "OPTIONS * and proxies"
4713    </t>
4714    <t>
4715      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/94"/>:
4716      "Reason-Phrase BNF"
4717    </t>
4718    <t>
4719      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/111"/>:
4720      "Use of TEXT"
4721    </t>
4722    <t>
4723      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
4724      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
4725    </t>
4726    <t>
4727      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/134"/>:
4728      "RFC822 reference left in discussion of date formats"
4729    </t>
4730  </list>
4731</t>
4732<t>
4733  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4734  <list style="symbols"> 
4735    <t>
4736      Rewrite definition of list rules, deprecate empty list elements.
4737    </t>
4738  </list>
4739</t>
4740<t>
4741  Other changes:
4742  <list style="symbols"> 
4743    <t>
4744      Rewrite introduction; add mostly new Architecture Section.
4745    </t>
4746  </list>
4747</t>
4748</section>
4749
4750</section>
4751
4752</back>
4753</rfc>
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