source: draft-ietf-httpbis/latest/p1-messaging.xml @ 624

Last change on this file since 624 was 624, checked in by fielding@…, 10 years ago

Move overall operation above URI definitions, closer to where
it is in 2616, since it reads better that way and defines
origin server. No content changed.

  • Property svn:eol-style set to native
File size: 215.3 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "July">
16  <!ENTITY ID-YEAR "2009">
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-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38]>
39<?rfc toc="yes" ?>
40<?rfc symrefs="yes" ?>
41<?rfc sortrefs="yes" ?>
42<?rfc compact="yes"?>
43<?rfc subcompact="no" ?>
44<?rfc linkmailto="no" ?>
45<?rfc editing="no" ?>
46<?rfc comments="yes"?>
47<?rfc inline="yes"?>
48<?rfc-ext allow-markup-in-artwork="yes" ?>
49<?rfc-ext include-references-in-index="yes" ?>
50<rfc obsoletes="2616" category="std" x:maturity-level="draft"
51     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
52     xmlns:x='http://purl.org/net/xml2rfc/ext'>
53<front>
54
55  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
56
57  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
58    <organization abbrev="Day Software">Day Software</organization>
59    <address>
60      <postal>
61        <street>23 Corporate Plaza DR, Suite 280</street>
62        <city>Newport Beach</city>
63        <region>CA</region>
64        <code>92660</code>
65        <country>USA</country>
66      </postal>
67      <phone>+1-949-706-5300</phone>
68      <facsimile>+1-949-706-5305</facsimile>
69      <email>fielding@gbiv.com</email>
70      <uri>http://roy.gbiv.com/</uri>
71    </address>
72  </author>
73
74  <author initials="J." surname="Gettys" fullname="Jim Gettys">
75    <organization>One Laptop per Child</organization>
76    <address>
77      <postal>
78        <street>21 Oak Knoll Road</street>
79        <city>Carlisle</city>
80        <region>MA</region>
81        <code>01741</code>
82        <country>USA</country>
83      </postal>
84      <email>jg@laptop.org</email>
85      <uri>http://www.laptop.org/</uri>
86    </address>
87  </author>
88 
89  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
90    <organization abbrev="HP">Hewlett-Packard Company</organization>
91    <address>
92      <postal>
93        <street>HP Labs, Large Scale Systems Group</street>
94        <street>1501 Page Mill Road, MS 1177</street>
95        <city>Palo Alto</city>
96        <region>CA</region>
97        <code>94304</code>
98        <country>USA</country>
99      </postal>
100      <email>JeffMogul@acm.org</email>
101    </address>
102  </author>
103
104  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
105    <organization abbrev="Microsoft">Microsoft Corporation</organization>
106    <address>
107      <postal>
108        <street>1 Microsoft Way</street>
109        <city>Redmond</city>
110        <region>WA</region>
111        <code>98052</code>
112        <country>USA</country>
113      </postal>
114      <email>henrikn@microsoft.com</email>
115    </address>
116  </author>
117
118  <author initials="L." surname="Masinter" fullname="Larry Masinter">
119    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
120    <address>
121      <postal>
122        <street>345 Park Ave</street>
123        <city>San Jose</city>
124        <region>CA</region>
125        <code>95110</code>
126        <country>USA</country>
127      </postal>
128      <email>LMM@acm.org</email>
129      <uri>http://larry.masinter.net/</uri>
130    </address>
131  </author>
132 
133  <author initials="P." surname="Leach" fullname="Paul J. Leach">
134    <organization abbrev="Microsoft">Microsoft Corporation</organization>
135    <address>
136      <postal>
137        <street>1 Microsoft Way</street>
138        <city>Redmond</city>
139        <region>WA</region>
140        <code>98052</code>
141      </postal>
142      <email>paulle@microsoft.com</email>
143    </address>
144  </author>
145   
146  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
147    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
148    <address>
149      <postal>
150        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
151        <street>The Stata Center, Building 32</street>
152        <street>32 Vassar Street</street>
153        <city>Cambridge</city>
154        <region>MA</region>
155        <code>02139</code>
156        <country>USA</country>
157      </postal>
158      <email>timbl@w3.org</email>
159      <uri>http://www.w3.org/People/Berners-Lee/</uri>
160    </address>
161  </author>
162
163  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
164    <organization abbrev="W3C">World Wide Web Consortium</organization>
165    <address>
166      <postal>
167        <street>W3C / ERCIM</street>
168        <street>2004, rte des Lucioles</street>
169        <city>Sophia-Antipolis</city>
170        <region>AM</region>
171        <code>06902</code>
172        <country>France</country>
173      </postal>
174      <email>ylafon@w3.org</email>
175      <uri>http://www.raubacapeu.net/people/yves/</uri>
176    </address>
177  </author>
178
179  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
180    <organization abbrev="greenbytes">greenbytes GmbH</organization>
181    <address>
182      <postal>
183        <street>Hafenweg 16</street>
184        <city>Muenster</city><region>NW</region><code>48155</code>
185        <country>Germany</country>
186      </postal>
187      <phone>+49 251 2807760</phone>
188      <facsimile>+49 251 2807761</facsimile>
189      <email>julian.reschke@greenbytes.de</email>
190      <uri>http://greenbytes.de/tech/webdav/</uri>
191    </address>
192  </author>
193
194  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
195  <workgroup>HTTPbis Working Group</workgroup>
196
197<abstract>
198<t>
199   The Hypertext Transfer Protocol (HTTP) is an application-level
200   protocol for distributed, collaborative, hypertext information
201   systems. HTTP has been in use by the World Wide Web global information
202   initiative since 1990. This document is Part 1 of the seven-part specification
203   that defines the protocol referred to as "HTTP/1.1" and, taken together,
204   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
205   its associated terminology, defines the "http" and "https" Uniform
206   Resource Identifier (URI) schemes, defines the generic message syntax
207   and parsing requirements for HTTP message frames, and describes
208   general security concerns for implementations.
209</t>
210</abstract>
211
212<note title="Editorial Note (To be removed by RFC Editor)">
213  <t>
214    Discussion of this draft should take place on the HTTPBIS working group
215    mailing list (ietf-http-wg@w3.org). The current issues list is
216    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
217    and related documents (including fancy diffs) can be found at
218    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
219  </t>
220  <t>
221    The changes in this draft are summarized in <xref target="changes.since.07"/>.
222  </t>
223</note>
224</front>
225<middle>
226<section title="Introduction" anchor="introduction">
227<t>
228   The Hypertext Transfer Protocol (HTTP) is an application-level
229   request/response protocol that uses extensible semantics and MIME-like
230   message payloads for flexible interaction with network-based hypertext
231   information systems. HTTP relies upon the Uniform Resource Identifier (URI)
232   standard <xref target="RFC3986"/> to indicate request targets and
233   relationships between resources.
234   Messages are passed in a format similar to that used by Internet mail
235   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
236   (MIME) <xref target="RFC2045"/> (see &diff2045entity; for the differences
237   between HTTP and MIME messages).
238</t>
239<t>
240   HTTP is a generic interface protocol for information systems. It is
241   designed to hide the details of how a service is implemented by presenting
242   a uniform interface to clients that is independent of the types of
243   resources provided. Likewise, servers do not need to be aware of each
244   client's purpose: an HTTP request can be considered in isolation rather
245   than being associated with a specific type of client or a predetermined
246   sequence of application steps. The result is a protocol that can be used
247   effectively in many different contexts and for which implementations can
248   evolve independently over time.
249</t>
250<t>
251   HTTP is also designed for use as a generic protocol for translating
252   communication to and from other Internet information systems.
253   HTTP proxies and gateways provide access to alternative information
254   services by translating their diverse protocols into a hypertext
255   format that can be viewed and manipulated by clients in the same way
256   as HTTP services.
257</t>
258<t>
259   One consequence of HTTP flexibility is that the protocol cannot be
260   defined in terms of what occurs behind the interface. Instead, we
261   are limited to defining the syntax of communication, the intent
262   of received communication, and the expected behavior of recipients.
263   If the communication is considered in isolation, then successful
264   actions should be reflected in corresponding changes to the
265   observable interface provided by servers. However, since multiple
266   clients may act in parallel and perhaps at cross-purposes, we
267   cannot require that such changes be observable beyond the scope
268   of a single response.
269</t>
270<t>
271   This document is Part 1 of the seven-part specification of HTTP,
272   defining the protocol referred to as "HTTP/1.1" and obsoleting
273   <xref target="RFC2616"/>.
274   Part 1 describes the architectural elements that are used or
275   referred to in HTTP, defines the "http" and "https" URI schemes,
276   describes overall network operation and connection management,
277   and defines HTTP message framing and forwarding requirements.
278   Our goal is to define all of the mechanisms necessary for HTTP message
279   handling that are independent of message semantics, thereby defining the
280   complete set of requirements for message parsers and
281   message-forwarding intermediaries.
282</t>
283
284<section title="Requirements" anchor="intro.requirements">
285<t>
286   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
287   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
288   document are to be interpreted as described in <xref target="RFC2119"/>.
289</t>
290<t>
291   An implementation is not compliant if it fails to satisfy one or more
292   of the &MUST; or &REQUIRED; level requirements for the protocols it
293   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
294   level and all the &SHOULD; level requirements for its protocols is said
295   to be "unconditionally compliant"; one that satisfies all the &MUST;
296   level requirements but not all the &SHOULD; level requirements for its
297   protocols is said to be "conditionally compliant."
298</t>
299</section>
300
301<section title="Syntax Notation" anchor="notation">
302<iref primary="true" item="Grammar" subitem="ALPHA"/>
303<iref primary="true" item="Grammar" subitem="CR"/>
304<iref primary="true" item="Grammar" subitem="CRLF"/>
305<iref primary="true" item="Grammar" subitem="CTL"/>
306<iref primary="true" item="Grammar" subitem="DIGIT"/>
307<iref primary="true" item="Grammar" subitem="DQUOTE"/>
308<iref primary="true" item="Grammar" subitem="HEXDIG"/>
309<iref primary="true" item="Grammar" subitem="LF"/>
310<iref primary="true" item="Grammar" subitem="OCTET"/>
311<iref primary="true" item="Grammar" subitem="SP"/>
312<iref primary="true" item="Grammar" subitem="VCHAR"/>
313<iref primary="true" item="Grammar" subitem="WSP"/>
314<t>
315   This specification uses the Augmented Backus-Naur Form (ABNF) notation
316   of <xref target="RFC5234"/>.
317</t>
318<t anchor="core.rules">
319  <x:anchor-alias value="ALPHA"/>
320  <x:anchor-alias value="CTL"/>
321  <x:anchor-alias value="CR"/>
322  <x:anchor-alias value="CRLF"/>
323  <x:anchor-alias value="DIGIT"/>
324  <x:anchor-alias value="DQUOTE"/>
325  <x:anchor-alias value="HEXDIG"/>
326  <x:anchor-alias value="LF"/>
327  <x:anchor-alias value="OCTET"/>
328  <x:anchor-alias value="SP"/>
329  <x:anchor-alias value="VCHAR"/>
330  <x:anchor-alias value="WSP"/>
331   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 
374  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
375</artwork></figure> 
376<t>
377  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
378  expanded as explained above.
379</t>
380</section>
381
382<section title="Basic Rules" anchor="basic.rules">
383<t anchor="rule.CRLF">
384  <x:anchor-alias value="CRLF"/>
385   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
386   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
387   tolerant applications). The end-of-line marker within an entity-body
388   is defined by its associated media type, as described in &media-types;.
389</t>
390<t anchor="rule.LWS">
391   This specification uses three rules to denote the use of linear
392   whitespace: OWS (optional whitespace), RWS (required whitespace), and
393   BWS ("bad" whitespace).
394</t>
395<t>
396   The OWS rule is used where zero or more linear whitespace characters may
397   appear. OWS &SHOULD; either not be produced or be produced as a single SP
398   character. Multiple OWS characters that occur within field-content &SHOULD;
399   be replaced with a single SP before interpreting the field value or
400   forwarding the message downstream.
401</t>
402<t>
403   RWS is used when at least one linear whitespace character is required to
404   separate field tokens. RWS &SHOULD; be produced as a single SP character.
405   Multiple RWS characters that occur within field-content &SHOULD; be
406   replaced with a single SP before interpreting the field value or
407   forwarding the message downstream.
408</t>
409<t>
410   BWS is used where the grammar allows optional whitespace for historical
411   reasons but senders &SHOULD-NOT; produce it in messages. HTTP/1.1
412   recipients &MUST; accept such bad optional whitespace and remove it before
413   interpreting the field value or forwarding the message downstream.
414</t>
415<t anchor="rule.whitespace">
416  <x:anchor-alias value="BWS"/>
417  <x:anchor-alias value="OWS"/>
418  <x:anchor-alias value="RWS"/>
419  <x:anchor-alias value="obs-fold"/>
420</t>
421<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"/>
422  <x:ref>OWS</x:ref>            = *( [ obs-fold ] <x:ref>WSP</x:ref> )
423                 ; "optional" whitespace
424  <x:ref>RWS</x:ref>            = 1*( [ obs-fold ] <x:ref>WSP</x:ref> )
425                 ; "required" whitespace
426  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
427                 ; "bad" whitespace
428  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref>
429                 ; see <xref target="message.headers"/>
430</artwork></figure>
431<t anchor="rule.token.separators">
432  <x:anchor-alias value="tchar"/>
433  <x:anchor-alias value="token"/>
434   Many HTTP/1.1 header field values consist of words separated by whitespace
435   or special characters. These special characters &MUST; be in a quoted
436   string to be used within a parameter value (as defined in
437   <xref target="transfer.codings"/>).
438</t>
439<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
440  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
441                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
442                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
443                 
444  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
445</artwork></figure>
446<t anchor="rule.quoted-string">
447  <x:anchor-alias value="quoted-string"/>
448  <x:anchor-alias value="qdtext"/>
449  <x:anchor-alias value="obs-text"/>
450   A string of text is parsed as a single word if it is quoted using
451   double-quote marks.
452</t>
453<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"/>
454  <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>
455  <x:ref>qdtext</x:ref>         = <x:ref>OWS</x:ref> / %x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
456                 ; <x:ref>OWS</x:ref> / &lt;<x:ref>VCHAR</x:ref> except <x:ref>DQUOTE</x:ref> and "\"&gt; / <x:ref>obs-text</x:ref> 
457  <x:ref>obs-text</x:ref>       = %x80-FF
458</artwork></figure>
459<t anchor="rule.quoted-pair">
460  <x:anchor-alias value="quoted-pair"/>
461  <x:anchor-alias value="quoted-text"/>
462   The backslash character ("\") &MAY; be used as a single-character
463   quoting mechanism only within quoted-string and comment constructs.
464</t>
465<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-text"/><iref primary="true" item="Grammar" subitem="quoted-pair"/>
466  <x:ref>quoted-text</x:ref>    = %x01-09 /
467                   %x0B-0C /
468                   %x0E-FF ; Characters excluding NUL, <x:ref>CR</x:ref> and <x:ref>LF</x:ref>
469  <x:ref>quoted-pair</x:ref>    = "\" <x:ref>quoted-text</x:ref>
470</artwork></figure>
471</section>
472
473<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
474  <x:anchor-alias value="request-header"/>
475  <x:anchor-alias value="response-header"/>
476  <x:anchor-alias value="entity-body"/>
477  <x:anchor-alias value="entity-header"/>
478  <x:anchor-alias value="Cache-Control"/>
479  <x:anchor-alias value="Pragma"/>
480  <x:anchor-alias value="Warning"/>
481<t>
482  The ABNF rules below are defined in other parts:
483</t>
484<figure><!-- Part2--><artwork type="abnf2616">
485  <x:ref>request-header</x:ref>  = &lt;request-header, defined in &request-header-fields;&gt;
486  <x:ref>response-header</x:ref> = &lt;response-header, defined in &response-header-fields;&gt;
487</artwork></figure>
488<figure><!-- Part3--><artwork type="abnf2616">
489  <x:ref>entity-body</x:ref>     = &lt;entity-body, defined in &entity-body;&gt;
490  <x:ref>entity-header</x:ref>   = &lt;entity-header, defined in &entity-header-fields;&gt;
491</artwork></figure>
492<figure><!-- Part6--><artwork type="abnf2616">
493  <x:ref>Cache-Control</x:ref>   = &lt;Cache-Control, defined in &header-pragma;&gt;
494  <x:ref>Pragma</x:ref>          = &lt;Pragma, defined in &header-pragma;&gt;
495  <x:ref>Warning</x:ref>         = &lt;Warning, defined in &header-warning;&gt;
496</artwork></figure>
497</section>
498
499</section>
500</section>
501
502<section title="HTTP architecture" anchor="architecture">
503<t>
504   HTTP was created for the World Wide Web architecture
505   and has evolved over time to support the scalability needs of a worldwide
506   hypertext system. Much of that architecture is reflected in the terminology
507   and syntax productions used to define HTTP.
508</t>
509
510<section title="Overall Operation" anchor="intro.overall.operation">
511<t>
512   HTTP is a request/response protocol. A client sends a
513   request to the server in the form of a request method, URI, and
514   protocol version, followed by a MIME-like message containing request
515   modifiers, client information, and possible body content over a
516   connection with a server. The server responds with a status line,
517   including the message's protocol version and a success or error code,
518   followed by a MIME-like message containing server information, entity
519   metadata, and possible entity-body content.
520</t>
521<t>
522   Most HTTP communication is initiated by a user agent and consists of
523   a request to be applied to a resource via the HTTP interface provided
524   by some origin server. In the
525   simplest case, this may be accomplished via a single connection (v)
526   between the user agent (UA) and the origin server (O).
527</t>
528<figure><artwork type="drawing">
529       request chain ------------------------&gt;
530    UA -------------------v------------------- O
531       &lt;----------------------- response chain
532</artwork></figure>
533<t>
534   A more complicated situation occurs when one or more intermediaries
535   are present in the request/response chain. There are three common
536   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
537   forwarding agent, receiving requests for a URI in its absolute form,
538   rewriting all or part of the message, and forwarding the reformatted
539   request toward the server identified by the URI. A gateway is a
540   receiving agent, acting as a layer above some other server(s) and, if
541   necessary, translating the requests to the underlying server's
542   protocol. A tunnel acts as a relay point between two connections
543   without changing the messages; tunnels are used when the
544   communication needs to pass through an intermediary (such as a
545   firewall) even when the intermediary cannot understand the contents
546   of the messages.
547</t>
548<figure><artwork type="drawing">
549       request chain --------------------------------------&gt;
550    UA -----v----- A -----v----- B -----v----- C -----v----- O
551       &lt;------------------------------------- response chain
552</artwork></figure>
553<t>
554   The figure above shows three intermediaries (A, B, and C) between the
555   user agent and origin server. A request or response message that
556   travels the whole chain will pass through four separate connections.
557   This distinction is important because some HTTP communication options
558   may apply only to the connection with the nearest, non-tunnel
559   neighbor, only to the end-points of the chain, or to all connections
560   along the chain. Although the diagram is linear, each participant may
561   be engaged in multiple, simultaneous communications. For example, B
562   may be receiving requests from many clients other than A, and/or
563   forwarding requests to servers other than C, at the same time that it
564   is handling A's request.
565</t>
566<t>
567   Any party to the communication which is not acting as a tunnel may
568   employ an internal cache for handling requests. The effect of a cache
569   is that the request/response chain is shortened if one of the
570   participants along the chain has a cached response applicable to that
571   request. The following illustrates the resulting chain if B has a
572   cached copy of an earlier response from O (via C) for a request which
573   has not been cached by UA or A.
574</t>
575<figure><artwork type="drawing">
576          request chain ----------&gt;
577       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
578          &lt;--------- response chain
579</artwork></figure>
580<t>
581   Not all responses are usefully cacheable, and some requests may
582   contain modifiers which place special requirements on cache behavior.
583   HTTP requirements for cache behavior and cacheable responses are
584   defined in &caching;.
585</t>
586<t>
587   In fact, there are a wide variety of architectures and configurations
588   of caches and proxies currently being experimented with or deployed
589   across the World Wide Web. These systems include national hierarchies
590   of proxy caches to save transoceanic bandwidth, systems that
591   broadcast or multicast cache entries, organizations that distribute
592   subsets of cached data via CD-ROM, and so on. HTTP systems are used
593   in corporate intranets over high-bandwidth links, and for access via
594   PDAs with low-power radio links and intermittent connectivity. The
595   goal of HTTP/1.1 is to support the wide diversity of configurations
596   already deployed while introducing protocol constructs that meet the
597   needs of those who build web applications that require high
598   reliability and, failing that, at least reliable indications of
599   failure.
600</t>
601<t>
602   HTTP communication usually takes place over TCP/IP connections. The
603   default port is TCP 80 (<eref target="http://www.iana.org/assignments/port-numbers"/>), but other ports can be used. This does
604   not preclude HTTP from being implemented on top of any other protocol
605   on the Internet, or on other networks. HTTP only presumes a reliable
606   transport; any protocol that provides such guarantees can be used;
607   the mapping of the HTTP/1.1 request and response structures onto the
608   transport data units of the protocol in question is outside the scope
609   of this specification.
610</t>
611<t>
612   In HTTP/1.0, most implementations used a new connection for each
613   request/response exchange. In HTTP/1.1, a connection may be used for
614   one or more request/response exchanges, although connections may be
615   closed for a variety of reasons (see <xref target="persistent.connections"/>).
616</t>
617</section>
618
619<section title="Uniform Resource Identifiers" anchor="uri">
620<iref primary="true" item="resource"/>
621<t>
622   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
623   throughout HTTP as the means for identifying resources. URI references
624   are used to target requests, indicate redirects, and define relationships.
625   HTTP does not limit what a resource may be; it merely defines an interface
626   that can be used to interact with a resource via HTTP. More information on
627   the scope of URIs and resources can be found in <xref target="RFC3986"/>.
628</t>
629  <x:anchor-alias value="URI"/>
630  <x:anchor-alias value="URI-reference"/>
631  <x:anchor-alias value="absolute-URI"/>
632  <x:anchor-alias value="relative-part"/>
633  <x:anchor-alias value="authority"/>
634  <x:anchor-alias value="fragment"/>
635  <x:anchor-alias value="path-abempty"/>
636  <x:anchor-alias value="path-absolute"/>
637  <x:anchor-alias value="port"/>
638  <x:anchor-alias value="query"/>
639  <x:anchor-alias value="uri-host"/>
640  <x:anchor-alias value="partial-URI"/>
641<t>
642   This specification adopts the definitions of "URI-reference",
643   "absolute-URI", "relative-part", "fragment", "port", "host",
644   "path-abempty", "path-absolute", "query", and "authority" from
645   <xref target="RFC3986"/>. In addition, we define a partial-URI rule for
646   protocol elements that allow a relative URI without a fragment.
647</t>
648<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"/>
649  <x:ref>URI</x:ref>           = &lt;URI, defined in <xref target="RFC3986" x:fmt="," x:sec="3"/>&gt;
650  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
651  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
652  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
653  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
654  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
655  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
656  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
657  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
658  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
659  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
660 
661  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
662</artwork></figure>
663<t>
664   Each protocol element in HTTP that allows a URI reference will indicate in
665   its ABNF production whether the element allows only a URI in absolute form
666   (absolute-URI), any relative reference (relative-ref), or some other subset
667   of the URI-reference grammar. Unless otherwise indicated, URI references
668   are parsed relative to the request target (the default base URI for both
669   the request and its corresponding response).
670</t>
671
672<section title="http URI scheme" anchor="http.uri">
673  <x:anchor-alias value="http-URI"/>
674  <iref item="http URI scheme" primary="true"/>
675  <iref item="URI scheme" subitem="http" primary="true"/>
676<t>
677   The "http" URI scheme is hereby defined for the purpose of minting
678   identifiers according to their association with the hierarchical
679   namespace governed by a potential HTTP origin server listening for
680   TCP connections on a given port.
681   The HTTP server is identified via the generic syntax's
682   <x:ref>authority</x:ref> component, which includes a host
683   identifier and optional TCP port, and the remainder of the URI is
684   considered to be identifying data corresponding to a resource for
685   which that server might provide an HTTP interface.
686</t>
687<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"/>
688  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
689</artwork></figure>
690<t>
691   The host identifier within an <x:ref>authority</x:ref> component is
692   defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>.  If host is
693   provided as an IP literal or IPv4 address, then the HTTP server is any
694   listener on the indicated TCP port at that IP address. If host is a
695   registered name, then that name is considered an indirect identifier
696   and the recipient might use a name resolution service, such as DNS,
697   to find the address of a listener for that host.
698   The host &MUST-NOT; be empty; if an "http" URI is received with an
699   empty host, then it &MUST; be rejected as invalid.
700   If the port subcomponent is empty or not given, then TCP port 80 is
701   assumed (the default reserved port for WWW services).
702</t>
703<t>
704   Regardless of the form of host identifier, access to that host is not
705   implied by the mere presence of its name or address. The host may or may
706   not exist and, even when it does exist, may or may not be running an
707   HTTP server or listening to the indicated port. The "http" URI scheme
708   makes use of the delegated nature of Internet names and addresses to
709   establish a naming authority (whatever entity has the ability to place
710   an HTTP server at that Internet name or address) and allows that
711   authority to determine which names are valid and how they might be used.
712</t>
713<t>
714   When an "http" URI is used within a context that calls for access to the
715   indicated resource, a client &MAY; attempt access by resolving
716   the host to an IP address, establishing a TCP connection to that address
717   on the indicated port, and sending an HTTP request message to the server
718   containing the URI's identifying data as described in <xref target="request"/>.
719   If the server responds to that request with a non-interim HTTP response
720   message, as described in <xref target="response"/>, then that response
721   is considered an authoritative answer to the client's request.
722</t>
723<t>
724   Although HTTP is independent of the transport protocol, the "http"
725   scheme is specific to TCP-based services because the name delegation
726   process depends on TCP for establishing authority.
727   An HTTP service based on some other underlying connection protocol
728   would presumably be identified using a different URI scheme, just as
729   the "https" scheme (below) is used for servers that require an SSL/TLS
730   transport layer on a connection. Other protocols may also be used to
731   provide access to "http" identified resources --- it is only the
732   authoritative interface used for mapping the namespace that is
733   specific to TCP.
734</t>
735</section>
736
737<section title="https URI scheme" anchor="https.uri">
738   <x:anchor-alias value="https-URI"/>
739   <iref item="https URI scheme"/>
740   <iref item="URI scheme" subitem="https"/>
741<t>
742   The "https" URI scheme is hereby defined for the purpose of minting
743   identifiers according to their association with the hierarchical
744   namespace governed by a potential HTTP origin server listening for
745   SSL/TLS-secured connections on a given TCP port.
746   The host and port are determined in the same way
747   as for the "http" scheme, except that a default TCP port of 443
748   is assumed if the port subcomponent is empty or not given.
749</t>
750<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"/>
751  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
752</artwork></figure>
753<t>
754   The primary difference between the "http" and "https" schemes is
755   that interaction with the latter is required to be secured for
756   privacy through the use of strong encryption. The URI cannot be
757   sent in a request until the connection is secure. Likewise, the
758   default for caching is that each response that would be considered
759   "public" under the "http" scheme is instead treated as "private"
760   and thus not eligible for shared caching.
761</t>
762<t>
763   The process for authoritative access to an "https" identified
764   resource is defined in <xref target="RFC2818"/>.
765</t>
766</section>
767
768<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
769<t>
770   Since the "http" and "https" schemes conform to the URI generic syntax,
771   such URIs are normalized and compared according to the algorithm defined
772   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
773   described above for each scheme.
774</t>
775<t>
776   If the port is equal to the default port for a scheme, the normal
777   form is to elide the port subcomponent. Likewise, an empty path
778   component is equivalent to an absolute path of "/", so the normal
779   form is to provide a path of "/" instead. The scheme and host
780   are case-insensitive and normally provided in lowercase; all
781   other components are compared in a case-sensitive manner.
782   Characters other than those in the "reserved" set are equivalent
783   to their percent-encoded octets (see <xref target="RFC3986"
784   x:fmt="," x:sec="2.1"/>): the normal form is to not encode them.
785</t>
786<t>
787   For example, the following three URIs are equivalent:
788</t>
789<figure><artwork type="example">
790   http://example.com:80/~smith/home.html
791   http://EXAMPLE.com/%7Esmith/home.html
792   http://EXAMPLE.com:/%7esmith/home.html
793</artwork></figure>
794<t>
795   <cref>[[This paragraph does not belong here. --Roy]]</cref>
796   If path-abempty is the empty string (i.e., there is no slash "/"
797   path separator following the authority), then the "http" URI
798   &MUST; be given as "/" when
799   used as a request-target (<xref target="request-target"/>). If a proxy
800   receives a host name which is not a fully qualified domain name, it
801   &MAY; add its domain to the host name it received. If a proxy receives
802   a fully qualified domain name, the proxy &MUST-NOT; change the host
803   name.
804</t>
805</section>
806
807<section title="Scheme aliases considered harmful" anchor="scheme.aliases">
808<t>
809   <cref>TBS: describe why aliases like webcal are harmful.</cref>
810</t>
811</section>
812</section>
813
814<section title="Use of HTTP for proxy communication" anchor="http.proxy">
815<t>
816   <cref>TBD: Configured to use HTTP to proxy HTTP or other protocols.</cref>
817</t>
818</section>
819<section title="Interception of HTTP for access control" anchor="http.intercept">
820<t>
821   <cref>TBD: Interception of HTTP traffic for initiating access control.</cref>
822</t>
823</section>
824<section title="Use of HTTP by other protocols" anchor="http.others">
825<t>
826   <cref>TBD: Profiles of HTTP defined by other protocol.
827   Extensions of HTTP like WebDAV.</cref>
828</t>
829</section>
830<section title="Use of HTTP by media type specification" anchor="http.media">
831<t>
832   <cref>TBD: Instructions on composing HTTP requests via hypertext formats.</cref>
833</t>
834</section>
835</section>
836
837<section title="HTTP Message" anchor="http.message">
838
839<section title="Message Types" anchor="message.types">
840  <x:anchor-alias value="generic-message"/>
841  <x:anchor-alias value="HTTP-message"/>
842  <x:anchor-alias value="start-line"/>
843<t>
844   HTTP messages consist of requests from client to server and responses
845   from server to client.
846</t>
847<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
848  <x:ref>HTTP-message</x:ref>   = <x:ref>Request</x:ref> / <x:ref>Response</x:ref>     ; HTTP/1.1 messages
849</artwork></figure>
850<t>
851   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
852   message format of <xref target="RFC5322"/> for transferring entities (the payload
853   of the message). Both types of message consist of a start-line, zero
854   or more header fields (also known as "headers"), an empty line (i.e.,
855   a line with nothing preceding the CRLF) indicating the end of the
856   header fields, and possibly a message-body.
857</t>
858<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
859  <x:ref>generic-message</x:ref> = <x:ref>start-line</x:ref>
860                    *( <x:ref>message-header</x:ref> <x:ref>CRLF</x:ref> )
861                    <x:ref>CRLF</x:ref>
862                    [ <x:ref>message-body</x:ref> ]
863  <x:ref>start-line</x:ref>      = <x:ref>Request-Line</x:ref> / <x:ref>Status-Line</x:ref>
864</artwork></figure>
865<t>
866   In the interest of robustness, servers &SHOULD; ignore any empty
867   line(s) received where a Request-Line is expected. In other words, if
868   the server is reading the protocol stream at the beginning of a
869   message and receives a CRLF first, it should ignore the CRLF.
870</t>
871<t>
872   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
873   after a POST request. To restate what is explicitly forbidden by the
874   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
875   extra CRLF.
876</t>
877<t>
878   Whitespace (WSP) &MUST-NOT; be sent between the start-line and the first
879   header field. The presence of whitespace might be an attempt to trick a
880   noncompliant implementation of HTTP into ignoring that field or processing
881   the next line as a new request, either of which may result in security
882   issues when implementations within the request chain interpret the
883   same message differently. HTTP/1.1 servers &MUST; reject such a message
884   with a 400 (Bad Request) response.
885</t>
886</section>
887
888<section title="Message Headers" anchor="message.headers">
889  <x:anchor-alias value="field-content"/>
890  <x:anchor-alias value="field-name"/>
891  <x:anchor-alias value="field-value"/>
892  <x:anchor-alias value="message-header"/>
893<t>
894   HTTP header fields follow the same general format as Internet messages in
895   <xref target="RFC5322" x:fmt="of" x:sec="2.1"/>. Each header field consists
896   of a name followed by a colon (":"), optional whitespace, and the field
897   value. Field names are case-insensitive.
898</t>
899<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"/>
900  <x:ref>message-header</x:ref> = <x:ref>field-name</x:ref> ":" OWS [ <x:ref>field-value</x:ref> ] OWS
901  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
902  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>OWS</x:ref> )
903  <x:ref>field-content</x:ref>  = *( <x:ref>WSP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
904</artwork></figure>
905<t>
906   Historically, HTTP has allowed field-content with text in the ISO-8859-1
907   <xref target="ISO-8859-1"/> character encoding (allowing other character sets
908   through use of <xref target="RFC2047"/> encoding). In practice, most HTTP
909   header field-values use only a subset of the US-ASCII charset
910   <xref target="USASCII"/>. Newly defined header fields &SHOULD; constrain
911   their field-values to US-ASCII characters. Recipients &SHOULD; treat other
912   (obs-text) octets in field-content as opaque data.
913</t>
914<t>
915   No whitespace is allowed between the header field-name and colon. For
916   security reasons, any request message received containing such whitespace
917   &MUST; be rejected with a response code of 400 (Bad Request) and any such
918   whitespace in a response message &MUST; be removed.
919</t>
920<t>
921   The field value &MAY; be preceded by optional whitespace; a single SP is
922   preferred. The field-value does not include any leading or trailing white
923   space: OWS occurring before the first non-whitespace character of the
924   field-value or after the last non-whitespace character of the field-value
925   is ignored and &MAY; be removed without changing the meaning of the header
926   field.
927</t>
928<t>
929   Historically, HTTP header field values could be extended over multiple
930   lines by preceding each extra line with at least one space or horizontal
931   tab character (line folding). This specification deprecates such line
932   folding except within the message/http media type
933   (<xref target="internet.media.type.message.http"/>).
934   HTTP/1.1 senders &MUST-NOT; produce messages that include line folding
935   (i.e., that contain any field-content that matches the obs-fold rule) unless
936   the message is intended for packaging within the message/http media type.
937   HTTP/1.1 recipients &SHOULD; accept line folding and replace any embedded
938   obs-fold whitespace with a single SP prior to interpreting the field value
939   or forwarding the message downstream.
940</t>
941<t anchor="rule.comment">
942  <x:anchor-alias value="comment"/>
943  <x:anchor-alias value="ctext"/>
944   Comments can be included in some HTTP header fields by surrounding
945   the comment text with parentheses. Comments are only allowed in
946   fields containing "comment" as part of their field value definition.
947   In all other fields, parentheses are considered part of the field
948   value.
949</t>
950<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
951  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
952  <x:ref>ctext</x:ref>          = <x:ref>OWS</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
953                 ; <x:ref>OWS</x:ref> / &lt;<x:ref>VCHAR</x:ref> except "(", ")", and "\"&gt; / <x:ref>obs-text</x:ref>
954</artwork></figure>
955<t>
956   The order in which header fields with differing field names are
957   received is not significant. However, it is "good practice" to send
958   general-header fields first, followed by request-header or response-header
959   fields, and ending with the entity-header fields.
960</t>
961<t>
962   Multiple message-header fields with the same field-name &MAY; be
963   present in a message if and only if the entire field-value for that
964   header field is defined as a comma-separated list [i.e., #(values)].
965   It &MUST; be possible to combine the multiple header fields into one
966   "field-name: field-value" pair, without changing the semantics of the
967   message, by appending each subsequent field-value to the first, each
968   separated by a comma. The order in which header fields with the same
969   field-name are received is therefore significant to the
970   interpretation of the combined field value, and thus a proxy &MUST-NOT;
971   change the order of these field values when a message is forwarded.
972</t>
973<x:note>
974  <t>
975   <x:h>Note:</x:h> the "Set-Cookie" header as implemented in
976   practice (as opposed to how it is specified in <xref target="RFC2109"/>)
977   can occur multiple times, but does not use the list syntax, and thus cannot
978   be combined into a single line. (See Appendix A.2.3 of <xref target="Kri2001"/>
979   for details.) Also note that the Set-Cookie2 header specified in
980   <xref target="RFC2965"/> does not share this problem.
981  </t>
982</x:note>
983 
984</section>
985
986<section title="Message Body" anchor="message.body">
987  <x:anchor-alias value="message-body"/>
988<t>
989   The message-body (if any) of an HTTP message is used to carry the
990   entity-body associated with the request or response. The message-body
991   differs from the entity-body only when a transfer-coding has been
992   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
993</t>
994<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
995  <x:ref>message-body</x:ref> = <x:ref>entity-body</x:ref>
996               / &lt;entity-body encoded as per <x:ref>Transfer-Encoding</x:ref>&gt;
997</artwork></figure>
998<t>
999   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1000   applied by an application to ensure safe and proper transfer of the
1001   message. Transfer-Encoding is a property of the message, not of the
1002   entity, and thus &MAY; be added or removed by any application along the
1003   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1004   when certain transfer-codings may be used.)
1005</t>
1006<t>
1007   The rules for when a message-body is allowed in a message differ for
1008   requests and responses.
1009</t>
1010<t>
1011   The presence of a message-body in a request is signaled by the
1012   inclusion of a Content-Length or Transfer-Encoding header field in
1013   the request's message-headers.
1014   When a request message contains both a message-body of non-zero
1015   length and a method that does not define any semantics for that
1016   request message-body, then an origin server &SHOULD; either ignore
1017   the message-body or respond with an appropriate error message
1018   (e.g., 413).  A proxy or gateway, when presented the same request,
1019   &SHOULD; either forward the request inbound with the message-body or
1020   ignore the message-body when determining a response.
1021</t>
1022<t>
1023   For response messages, whether or not a message-body is included with
1024   a message is dependent on both the request method and the response
1025   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1026   &MUST-NOT; include a message-body, even though the presence of entity-header
1027   fields might lead one to believe they do. All 1xx
1028   (informational), 204 (No Content), and 304 (Not Modified) responses
1029   &MUST-NOT; include a message-body. All other responses do include a
1030   message-body, although it &MAY; be of zero length.
1031</t>
1032</section>
1033
1034<section title="Message Length" anchor="message.length">
1035<t>
1036   The transfer-length of a message is the length of the message-body as
1037   it appears in the message; that is, after any transfer-codings have
1038   been applied. When a message-body is included with a message, the
1039   transfer-length of that body is determined by one of the following
1040   (in order of precedence):
1041</t>
1042<t>
1043  <list style="numbers">
1044    <x:lt><t>
1045     Any response message which "&MUST-NOT;" include a message-body (such
1046     as the 1xx, 204, and 304 responses and any response to a HEAD
1047     request) is always terminated by the first empty line after the
1048     header fields, regardless of the entity-header fields present in
1049     the message.
1050    </t></x:lt>
1051    <x:lt><t>
1052     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1053     is present and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
1054     is used, the transfer-length is defined by the use of this transfer-coding.
1055     If a Transfer-Encoding header field is present and the "chunked" transfer-coding
1056     is not present, the transfer-length is defined by the sender closing the connection.
1057    </t></x:lt>
1058    <x:lt><t>
1059     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1060     value in OCTETs represents both the entity-length and the
1061     transfer-length. The Content-Length header field &MUST-NOT; be sent
1062     if these two lengths are different (i.e., if a Transfer-Encoding
1063     header field is present). If a message is received with both a
1064     Transfer-Encoding header field and a Content-Length header field,
1065     the latter &MUST; be ignored.
1066    </t></x:lt>
1067    <x:lt><t>
1068     If the message uses the media type "multipart/byteranges", and the
1069     transfer-length is not otherwise specified, then this self-delimiting
1070     media type defines the transfer-length. This media type
1071     &MUST-NOT; be used unless the sender knows that the recipient can parse
1072     it; the presence in a request of a Range header with multiple byte-range
1073     specifiers from a 1.1 client implies that the client can parse
1074     multipart/byteranges responses.
1075    <list style="empty"><t>
1076       A range header might be forwarded by a 1.0 proxy that does not
1077       understand multipart/byteranges; in this case the server &MUST;
1078       delimit the message using methods defined in items 1, 3 or 5 of
1079       this section.
1080    </t></list>
1081    </t></x:lt>
1082    <x:lt><t>
1083     By the server closing the connection. (Closing the connection
1084     cannot be used to indicate the end of a request body, since that
1085     would leave no possibility for the server to send back a response.)
1086    </t></x:lt>
1087  </list>
1088</t>
1089<t>
1090   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1091   containing a message-body &MUST; include a valid Content-Length header
1092   field unless the server is known to be HTTP/1.1 compliant. If a
1093   request contains a message-body and a Content-Length is not given,
1094   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1095   determine the length of the message, or with 411 (Length Required) if
1096   it wishes to insist on receiving a valid Content-Length.
1097</t>
1098<t>
1099   All HTTP/1.1 applications that receive entities &MUST; accept the
1100   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1101   to be used for messages when the message length cannot be determined
1102   in advance.
1103</t>
1104<t>
1105   Messages &MUST-NOT; include both a Content-Length header field and a
1106   transfer-coding. If the message does include a
1107   transfer-coding, the Content-Length &MUST; be ignored.
1108</t>
1109<t>
1110   When a Content-Length is given in a message where a message-body is
1111   allowed, its field value &MUST; exactly match the number of OCTETs in
1112   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1113   invalid length is received and detected.
1114</t>
1115</section>
1116
1117<section title="General Header Fields" anchor="general.header.fields">
1118  <x:anchor-alias value="general-header"/>
1119<t>
1120   There are a few header fields which have general applicability for
1121   both request and response messages, but which do not apply to the
1122   entity being transferred. These header fields apply only to the
1123   message being transmitted.
1124</t>
1125<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1126  <x:ref>general-header</x:ref> = <x:ref>Cache-Control</x:ref>            ; &header-cache-control;
1127                 / <x:ref>Connection</x:ref>               ; <xref target="header.connection"/>
1128                 / <x:ref>Date</x:ref>                     ; <xref target="header.date"/>
1129                 / <x:ref>Pragma</x:ref>                   ; &header-pragma;
1130                 / <x:ref>Trailer</x:ref>                  ; <xref target="header.trailer"/>
1131                 / <x:ref>Transfer-Encoding</x:ref>        ; <xref target="header.transfer-encoding"/>
1132                 / <x:ref>Upgrade</x:ref>                  ; <xref target="header.upgrade"/>
1133                 / <x:ref>Via</x:ref>                      ; <xref target="header.via"/>
1134                 / <x:ref>Warning</x:ref>                  ; &header-warning;
1135</artwork></figure>
1136<t>
1137   General-header field names can be extended reliably only in
1138   combination with a change in the protocol version. However, new or
1139   experimental header fields may be given the semantics of general
1140   header fields if all parties in the communication recognize them to
1141   be general-header fields. Unrecognized header fields are treated as
1142   entity-header fields.
1143</t>
1144</section>
1145</section>
1146
1147<section title="Request" anchor="request">
1148  <x:anchor-alias value="Request"/>
1149<t>
1150   A request message from a client to a server includes, within the
1151   first line of that message, the method to be applied to the resource,
1152   the identifier of the resource, and the protocol version in use.
1153</t>
1154<!--                 Host                      ; should be moved here eventually -->
1155<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1156  <x:ref>Request</x:ref>       = <x:ref>Request-Line</x:ref>              ; <xref target="request-line"/>
1157                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1158                   / <x:ref>request-header</x:ref>         ; &request-header-fields;
1159                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref> )  ; &entity-header-fields;
1160                  <x:ref>CRLF</x:ref>
1161                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1162</artwork></figure>
1163
1164<section title="Request-Line" anchor="request-line">
1165  <x:anchor-alias value="Request-Line"/>
1166<t>
1167   The Request-Line begins with a method token, followed by the
1168   request-target and the protocol version, and ending with CRLF. The
1169   elements are separated by SP characters. No CR or LF is allowed
1170   except in the final CRLF sequence.
1171</t>
1172<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1173  <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>
1174</artwork></figure>
1175
1176<section title="Method" anchor="method">
1177  <x:anchor-alias value="Method"/>
1178<t>
1179   The Method  token indicates the method to be performed on the
1180   resource identified by the request-target. The method is case-sensitive.
1181</t>
1182<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1183  <x:ref>Method</x:ref>         = <x:ref>token</x:ref>
1184</artwork></figure>
1185</section>
1186
1187<section title="request-target" anchor="request-target">
1188  <x:anchor-alias value="request-target"/>
1189<t>
1190   The request-target
1191   identifies the resource upon which to apply the request.
1192</t>
1193<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/>
1194  <x:ref>request-target</x:ref> = "*"
1195                 / <x:ref>absolute-URI</x:ref>
1196                 / ( <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ] )
1197                 / <x:ref>authority</x:ref>
1198</artwork></figure>
1199<t>
1200   The four options for request-target are dependent on the nature of the
1201   request. The asterisk "*" means that the request does not apply to a
1202   particular resource, but to the server itself, and is only allowed
1203   when the method used does not necessarily apply to a resource. One
1204   example would be
1205</t>
1206<figure><artwork type="example">
1207  OPTIONS * HTTP/1.1
1208</artwork></figure>
1209<t>
1210   The absolute-URI form is &REQUIRED; when the request is being made to a
1211   proxy. The proxy is requested to forward the request or service it
1212   from a valid cache, and return the response. Note that the proxy &MAY;
1213   forward the request on to another proxy or directly to the server
1214   specified by the absolute-URI. In order to avoid request loops, a
1215   proxy &MUST; be able to recognize all of its server names, including
1216   any aliases, local variations, and the numeric IP address. An example
1217   Request-Line would be:
1218</t>
1219<figure><artwork type="example">
1220  GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1221</artwork></figure>
1222<t>
1223   To allow for transition to absolute-URIs in all requests in future
1224   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absolute-URI
1225   form in requests, even though HTTP/1.1 clients will only generate
1226   them in requests to proxies.
1227</t>
1228<t>
1229   The authority form is only used by the CONNECT method (&CONNECT;).
1230</t>
1231<t>
1232   The most common form of request-target is that used to identify a
1233   resource on an origin server or gateway. In this case the absolute
1234   path of the URI &MUST; be transmitted (see <xref target="http.uri"/>, path-absolute) as
1235   the request-target, and the network location of the URI (authority) &MUST;
1236   be transmitted in a Host header field. For example, a client wishing
1237   to retrieve the resource above directly from the origin server would
1238   create a TCP connection to port 80 of the host "www.example.org" and send
1239   the lines:
1240</t>
1241<figure><artwork type="example">
1242  GET /pub/WWW/TheProject.html HTTP/1.1
1243  Host: www.example.org
1244</artwork></figure>
1245<t>
1246   followed by the remainder of the Request. Note that the absolute path
1247   cannot be empty; if none is present in the original URI, it &MUST; be
1248   given as "/" (the server root).
1249</t>
1250<t>
1251   If a proxy receives a request without any path in the request-target and
1252   the method specified is capable of supporting the asterisk form of
1253   request-target, then the last proxy on the request chain &MUST; forward the
1254   request with "*" as the final request-target.
1255</t>
1256<figure><preamble>   
1257   For example, the request
1258</preamble><artwork type="example">
1259  OPTIONS http://www.example.org:8001 HTTP/1.1
1260</artwork></figure>
1261<figure><preamble>   
1262  would be forwarded by the proxy as
1263</preamble><artwork type="example">
1264  OPTIONS * HTTP/1.1
1265  Host: www.example.org:8001
1266</artwork>
1267<postamble>
1268   after connecting to port 8001 of host "www.example.org".
1269</postamble>
1270</figure>
1271<t>
1272   The request-target is transmitted in the format specified in
1273   <xref target="http.uri"/>. If the request-target is percent-encoded
1274   (<xref target="RFC3986" x:fmt="," x:sec="2.1"/>), the origin server
1275   &MUST; decode the request-target in order to
1276   properly interpret the request. Servers &SHOULD; respond to invalid
1277   request-targets with an appropriate status code.
1278</t>
1279<t>
1280   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1281   received request-target when forwarding it to the next inbound server,
1282   except as noted above to replace a null path-absolute with "/".
1283</t>
1284<x:note>
1285  <t>
1286    <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1287    meaning of the request when the origin server is improperly using
1288    a non-reserved URI character for a reserved purpose.  Implementors
1289    should be aware that some pre-HTTP/1.1 proxies have been known to
1290    rewrite the request-target.
1291  </t>
1292</x:note>
1293<t>
1294   HTTP does not place a pre-defined limit on the length of a request-target.
1295   A server &MUST; be prepared to receive URIs of unbounded length and
1296   respond with the 414 (URI Too Long) status if the received
1297   request-target would be longer than the server wishes to handle
1298   (see &status-414;).
1299</t>
1300<t>
1301   Various ad-hoc limitations on request-target length are found in practice.
1302   It is &RECOMMENDED; that all HTTP senders and recipients support
1303   request-target lengths of 8000 or more OCTETs.
1304</t>
1305</section>
1306</section>
1307
1308<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
1309<t>
1310   The exact resource identified by an Internet request is determined by
1311   examining both the request-target and the Host header field.
1312</t>
1313<t>
1314   An origin server that does not allow resources to differ by the
1315   requested host &MAY; ignore the Host header field value when
1316   determining the resource identified by an HTTP/1.1 request. (But see
1317   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
1318   for other requirements on Host support in HTTP/1.1.)
1319</t>
1320<t>
1321   An origin server that does differentiate resources based on the host
1322   requested (sometimes referred to as virtual hosts or vanity host
1323   names) &MUST; use the following rules for determining the requested
1324   resource on an HTTP/1.1 request:
1325  <list style="numbers">
1326    <t>If request-target is an absolute-URI, the host is part of the
1327     request-target. Any Host header field value in the request &MUST; be
1328     ignored.</t>
1329    <t>If the request-target is not an absolute-URI, and the request includes
1330     a Host header field, the host is determined by the Host header
1331     field value.</t>
1332    <t>If the host as determined by rule 1 or 2 is not a valid host on
1333     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1334  </list>
1335</t>
1336<t>
1337   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1338   attempt to use heuristics (e.g., examination of the URI path for
1339   something unique to a particular host) in order to determine what
1340   exact resource is being requested.
1341</t>
1342</section>
1343
1344</section>
1345
1346
1347<section title="Response" anchor="response">
1348  <x:anchor-alias value="Response"/>
1349<t>
1350   After receiving and interpreting a request message, a server responds
1351   with an HTTP response message.
1352</t>
1353<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1354  <x:ref>Response</x:ref>      = <x:ref>Status-Line</x:ref>               ; <xref target="status-line"/>
1355                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1356                   / <x:ref>response-header</x:ref>        ; &response-header-fields;
1357                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref> )  ; &entity-header-fields;
1358                  <x:ref>CRLF</x:ref>
1359                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1360</artwork></figure>
1361
1362<section title="Status-Line" anchor="status-line">
1363  <x:anchor-alias value="Status-Line"/>
1364<t>
1365   The first line of a Response message is the Status-Line, consisting
1366   of the protocol version followed by a numeric status code and its
1367   associated textual phrase, with each element separated by SP
1368   characters. No CR or LF is allowed except in the final CRLF sequence.
1369</t>
1370<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1371  <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>
1372</artwork></figure>
1373
1374<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1375  <x:anchor-alias value="Reason-Phrase"/>
1376  <x:anchor-alias value="Status-Code"/>
1377<t>
1378   The Status-Code element is a 3-digit integer result code of the
1379   attempt to understand and satisfy the request. These codes are fully
1380   defined in &status-codes;.  The Reason Phrase exists for the sole
1381   purpose of providing a textual description associated with the numeric
1382   status code, out of deference to earlier Internet application protocols
1383   that were more frequently used with interactive text clients.
1384   A client &SHOULD; ignore the content of the Reason Phrase.
1385</t>
1386<t>
1387   The first digit of the Status-Code defines the class of response. The
1388   last two digits do not have any categorization role. There are 5
1389   values for the first digit:
1390  <list style="symbols">
1391    <t>
1392      1xx: Informational - Request received, continuing process
1393    </t>
1394    <t>
1395      2xx: Success - The action was successfully received,
1396        understood, and accepted
1397    </t>
1398    <t>
1399      3xx: Redirection - Further action must be taken in order to
1400        complete the request
1401    </t>
1402    <t>
1403      4xx: Client Error - The request contains bad syntax or cannot
1404        be fulfilled
1405    </t>
1406    <t>
1407      5xx: Server Error - The server failed to fulfill an apparently
1408        valid request
1409    </t>
1410  </list>
1411</t>
1412<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"/>
1413  <x:ref>Status-Code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1414  <x:ref>Reason-Phrase</x:ref>  = *( <x:ref>WSP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1415</artwork></figure>
1416</section>
1417</section>
1418
1419</section>
1420
1421
1422<section title="Protocol Parameters" anchor="protocol.parameters">
1423
1424<section title="HTTP Version" anchor="http.version">
1425  <x:anchor-alias value="HTTP-Version"/>
1426  <x:anchor-alias value="HTTP-Prot-Name"/>
1427<t>
1428   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
1429   of the protocol. The protocol versioning policy is intended to allow
1430   the sender to indicate the format of a message and its capacity for
1431   understanding further HTTP communication, rather than the features
1432   obtained via that communication. No change is made to the version
1433   number for the addition of message components which do not affect
1434   communication behavior or which only add to extensible field values.
1435   The &lt;minor&gt; number is incremented when the changes made to the
1436   protocol add features which do not change the general message parsing
1437   algorithm, but which may add to the message semantics and imply
1438   additional capabilities of the sender. The &lt;major&gt; number is
1439   incremented when the format of a message within the protocol is
1440   changed. See <xref target="RFC2145"/> for a fuller explanation.
1441</t>
1442<t>
1443   The version of an HTTP message is indicated by an HTTP-Version field
1444   in the first line of the message. HTTP-Version is case-sensitive.
1445</t>
1446<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/>
1447  <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>
1448  <x:ref>HTTP-Prot-Name</x:ref> = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
1449</artwork></figure>
1450<t>
1451   Note that the major and minor numbers &MUST; be treated as separate
1452   integers and that each &MAY; be incremented higher than a single digit.
1453   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
1454   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
1455   &MUST-NOT; be sent.
1456</t>
1457<t>
1458   An application that sends a request or response message that includes
1459   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
1460   with this specification. Applications that are at least conditionally
1461   compliant with this specification &SHOULD; use an HTTP-Version of
1462   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
1463   not compatible with HTTP/1.0. For more details on when to send
1464   specific HTTP-Version values, see <xref target="RFC2145"/>.
1465</t>
1466<t>
1467   The HTTP version of an application is the highest HTTP version for
1468   which the application is at least conditionally compliant.
1469</t>
1470<t>
1471   Proxy and gateway applications need to be careful when forwarding
1472   messages in protocol versions different from that of the application.
1473   Since the protocol version indicates the protocol capability of the
1474   sender, a proxy/gateway &MUST-NOT; send a message with a version
1475   indicator which is greater than its actual version. If a higher
1476   version request is received, the proxy/gateway &MUST; either downgrade
1477   the request version, or respond with an error, or switch to tunnel
1478   behavior.
1479</t>
1480<t>
1481   Due to interoperability problems with HTTP/1.0 proxies discovered
1482   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
1483   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
1484   they support. The proxy/gateway's response to that request &MUST; be in
1485   the same major version as the request.
1486</t>
1487<x:note>
1488  <t>
1489    <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
1490    of header fields required or forbidden by the versions involved.
1491  </t>
1492</x:note>
1493</section>
1494
1495<section title="Date/Time Formats: Full Date" anchor="date.time.formats.full.date">
1496  <x:anchor-alias value="HTTP-date"/>
1497<t>
1498   HTTP applications have historically allowed three different formats
1499   for the representation of date/time stamps:
1500</t>
1501<figure><artwork type="example">
1502  Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123
1503  Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1504  Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1505</artwork></figure>
1506<t>
1507   The first format is preferred as an Internet standard and represents
1508   a fixed-length subset of that defined by <xref target="RFC1123"/>. The
1509   other formats are described here only for
1510   compatibility with obsolete implementations.
1511   HTTP/1.1 clients and servers that parse the date value &MUST; accept
1512   all three formats (for compatibility with HTTP/1.0), though they &MUST;
1513   only generate the RFC 1123 format for representing HTTP-date values
1514   in header fields. See <xref target="tolerant.applications"/> for further information.
1515</t>
1516<t>
1517   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
1518   (GMT), without exception. For the purposes of HTTP, GMT is exactly
1519   equal to UTC (Coordinated Universal Time). This is indicated in the
1520   first two formats by the inclusion of "GMT" as the three-letter
1521   abbreviation for time zone, and &MUST; be assumed when reading the
1522   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
1523   additional whitespace beyond that specifically included as SP in the
1524   grammar.
1525</t>
1526<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-date"/>
1527  <x:ref>HTTP-date</x:ref>    = <x:ref>rfc1123-date</x:ref> / <x:ref>obs-date</x:ref>
1528</artwork></figure>
1529<t anchor="preferred.date.format">
1530  <x:anchor-alias value="rfc1123-date"/>
1531  <x:anchor-alias value="time-of-day"/>
1532  <x:anchor-alias value="hour"/>
1533  <x:anchor-alias value="minute"/>
1534  <x:anchor-alias value="second"/>
1535  <x:anchor-alias value="day-name"/>
1536  <x:anchor-alias value="day"/>
1537  <x:anchor-alias value="month"/>
1538  <x:anchor-alias value="year"/>
1539  <x:anchor-alias value="GMT"/>
1540  Preferred format:
1541</t>
1542<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="time-of-day"/><iref primary="true" item="Grammar" subitem="hour"/><iref primary="true" item="Grammar" subitem="minute"/><iref primary="true" item="Grammar" subitem="second"/><iref primary="true" item="Grammar" subitem="day-name"/><iref primary="true" item="Grammar" subitem="day-name-l"/><iref primary="true" item="Grammar" subitem="day"/><iref primary="true" item="Grammar" subitem="month"/><iref primary="true" item="Grammar" subitem="year"/><iref primary="true" item="Grammar" subitem="GMT"/>
1543  <x:ref>rfc1123-date</x:ref> = <x:ref>day-name</x:ref> "," <x:ref>SP</x:ref> date1 <x:ref>SP</x:ref> <x:ref>time-of-day</x:ref> <x:ref>SP</x:ref> <x:ref>GMT</x:ref>
1544
1545  <x:ref>day-name</x:ref>     = <x:abnf-char-sequence>"Mon"</x:abnf-char-sequence> ; "Mon", case-sensitive
1546               / <x:abnf-char-sequence>"Tue"</x:abnf-char-sequence> ; "Tue", case-sensitive
1547               / <x:abnf-char-sequence>"Wed"</x:abnf-char-sequence> ; "Wed", case-sensitive
1548               / <x:abnf-char-sequence>"Thu"</x:abnf-char-sequence> ; "Thu", case-sensitive
1549               / <x:abnf-char-sequence>"Fri"</x:abnf-char-sequence> ; "Fri", case-sensitive
1550               / <x:abnf-char-sequence>"Sat"</x:abnf-char-sequence> ; "Sat", case-sensitive
1551               / <x:abnf-char-sequence>"Sun"</x:abnf-char-sequence> ; "Sun", case-sensitive
1552               
1553  <x:ref>date1</x:ref>        = <x:ref>day</x:ref> <x:ref>SP</x:ref> <x:ref>month</x:ref> <x:ref>SP</x:ref> <x:ref>year</x:ref>
1554               ; e.g., 02 Jun 1982
1555
1556  <x:ref>day</x:ref>          = 2<x:ref>DIGIT</x:ref>
1557  <x:ref>month</x:ref>        = <x:abnf-char-sequence>"Jan"</x:abnf-char-sequence> ; "Jan", case-sensitive
1558               / <x:abnf-char-sequence>"Feb"</x:abnf-char-sequence> ; "Feb", case-sensitive
1559               / <x:abnf-char-sequence>"Mar"</x:abnf-char-sequence> ; "Mar", case-sensitive
1560               / <x:abnf-char-sequence>"Apr"</x:abnf-char-sequence> ; "Apr", case-sensitive
1561               / <x:abnf-char-sequence>"May"</x:abnf-char-sequence> ; "May", case-sensitive
1562               / <x:abnf-char-sequence>"Jun"</x:abnf-char-sequence> ; "Jun", case-sensitive
1563               / <x:abnf-char-sequence>"Jul"</x:abnf-char-sequence> ; "Jul", case-sensitive
1564               / <x:abnf-char-sequence>"Aug"</x:abnf-char-sequence> ; "Aug", case-sensitive
1565               / <x:abnf-char-sequence>"Sep"</x:abnf-char-sequence> ; "Sep", case-sensitive
1566               / <x:abnf-char-sequence>"Oct"</x:abnf-char-sequence> ; "Oct", case-sensitive
1567               / <x:abnf-char-sequence>"Nov"</x:abnf-char-sequence> ; "Nov", case-sensitive
1568               / <x:abnf-char-sequence>"Dec"</x:abnf-char-sequence> ; "Dec", case-sensitive
1569  <x:ref>year</x:ref>         = 4<x:ref>DIGIT</x:ref>
1570
1571  <x:ref>GMT</x:ref>   = <x:abnf-char-sequence>"GMT"</x:abnf-char-sequence> ; "GMT", case-sensitive
1572
1573  <x:ref>time-of-day</x:ref>  = <x:ref>hour</x:ref> ":" <x:ref>minute</x:ref> ":" <x:ref>second</x:ref>
1574                 ; 00:00:00 - 23:59:59
1575                 
1576  <x:ref>hour</x:ref>         = 2<x:ref>DIGIT</x:ref>               
1577  <x:ref>minute</x:ref>       = 2<x:ref>DIGIT</x:ref>               
1578  <x:ref>second</x:ref>       = 2<x:ref>DIGIT</x:ref>               
1579</artwork></figure>
1580<t>
1581  The semantics of <x:ref>day-name</x:ref>, <x:ref>day</x:ref>,
1582  <x:ref>month</x:ref>, <x:ref>year</x:ref>, and <x:ref>time-of-day</x:ref> are the
1583  same as those defined for the RFC 5322 constructs
1584  with the corresponding name (<xref target="RFC5322" x:fmt="," x:sec="3.3"/>).
1585</t>
1586<t anchor="obsolete.date.formats">
1587  <x:anchor-alias value="obs-date"/>
1588  <x:anchor-alias value="rfc850-date"/>
1589  <x:anchor-alias value="asctime-date"/>
1590  <x:anchor-alias value="date1"/>
1591  <x:anchor-alias value="date2"/>
1592  <x:anchor-alias value="date3"/>
1593  <x:anchor-alias value="rfc1123-date"/>
1594  <x:anchor-alias value="day-name-l"/>
1595  Obsolete formats:
1596</t>
1597<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="obs-date"/>
1598  <x:ref>obs-date</x:ref>     = <x:ref>rfc850-date</x:ref> / <x:ref>asctime-date</x:ref> 
1599</artwork></figure>
1600<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="rfc850-date"/>
1601  <x:ref>rfc850-date</x:ref>  = <x:ref>day-name-l</x:ref> "," <x:ref>SP</x:ref> <x:ref>date2</x:ref> <x:ref>SP</x:ref> <x:ref>time-of-day</x:ref> <x:ref>SP</x:ref> <x:ref>GMT</x:ref>
1602  <x:ref>date2</x:ref>        = <x:ref>day</x:ref> "-" <x:ref>month</x:ref> "-" 2<x:ref>DIGIT</x:ref>
1603                 ; day-month-year (e.g., 02-Jun-82)
1604
1605  <x:ref>day-name-l</x:ref>   = <x:abnf-char-sequence>"Monday"</x:abnf-char-sequence> ; "Monday", case-sensitive
1606         / <x:abnf-char-sequence>"Tuesday"</x:abnf-char-sequence> ; "Tuesday", case-sensitive
1607         / <x:abnf-char-sequence>"Wednesday"</x:abnf-char-sequence> ; "Wednesday", case-sensitive
1608         / <x:abnf-char-sequence>"Thursday"</x:abnf-char-sequence> ; "Thursday", case-sensitive
1609         / <x:abnf-char-sequence>"Friday"</x:abnf-char-sequence> ; "Friday", case-sensitive
1610         / <x:abnf-char-sequence>"Saturday"</x:abnf-char-sequence> ; "Saturday", case-sensitive
1611         / <x:abnf-char-sequence>"Sunday"</x:abnf-char-sequence> ; "Sunday", case-sensitive
1612</artwork></figure>
1613<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="asctime-date"/>
1614  <x:ref>asctime-date</x:ref> = <x:ref>day-name</x:ref> <x:ref>SP</x:ref> <x:ref>date3</x:ref> <x:ref>SP</x:ref> <x:ref>time-of-day</x:ref> <x:ref>SP</x:ref> <x:ref>year</x:ref>
1615  <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> ))
1616                 ; month day (e.g., Jun  2)
1617</artwork></figure>
1618<x:note>
1619  <t>
1620    <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
1621    accepting date values that may have been sent by non-HTTP
1622    applications, as is sometimes the case when retrieving or posting
1623    messages via proxies/gateways to SMTP or NNTP.
1624  </t>
1625</x:note>
1626<x:note>
1627  <t>
1628    <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
1629    to their usage within the protocol stream. Clients and servers are
1630    not required to use these formats for user presentation, request
1631    logging, etc.
1632  </t>
1633</x:note>
1634</section>
1635
1636<section title="Transfer Codings" anchor="transfer.codings">
1637  <x:anchor-alias value="transfer-coding"/>
1638  <x:anchor-alias value="transfer-extension"/>
1639<t>
1640   Transfer-coding values are used to indicate an encoding
1641   transformation that has been, can be, or may need to be applied to an
1642   entity-body in order to ensure "safe transport" through the network.
1643   This differs from a content coding in that the transfer-coding is a
1644   property of the message, not of the original entity.
1645</t>
1646<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1647  <x:ref>transfer-coding</x:ref>         = "chunked" / <x:ref>transfer-extension</x:ref>
1648  <x:ref>transfer-extension</x:ref>      = <x:ref>token</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>transfer-parameter</x:ref> )
1649</artwork></figure>
1650<t anchor="rule.parameter">
1651  <x:anchor-alias value="attribute"/>
1652  <x:anchor-alias value="transfer-parameter"/>
1653  <x:anchor-alias value="value"/>
1654   Parameters are in  the form of attribute/value pairs.
1655</t>
1656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/>
1657  <x:ref>transfer-parameter</x:ref>      = <x:ref>attribute</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> <x:ref>value</x:ref>
1658  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
1659  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1660</artwork></figure>
1661<t>
1662   All transfer-coding values are case-insensitive. HTTP/1.1 uses
1663   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
1664   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1665</t>
1666<t>
1667   Whenever a transfer-coding is applied to a message-body, the set of
1668   transfer-codings &MUST; include "chunked", unless the message indicates it
1669   is terminated by closing the connection. When the "chunked" transfer-coding
1670   is used, it &MUST; be the last transfer-coding applied to the
1671   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
1672   than once to a message-body. These rules allow the recipient to
1673   determine the transfer-length of the message (<xref target="message.length"/>).
1674</t>
1675<t>
1676   Transfer-codings are analogous to the Content-Transfer-Encoding
1677   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
1678   binary data over a 7-bit transport service. However, safe transport
1679   has a different focus for an 8bit-clean transfer protocol. In HTTP,
1680   the only unsafe characteristic of message-bodies is the difficulty in
1681   determining the exact body length (<xref target="message.length"/>), or the desire to
1682   encrypt data over a shared transport.
1683</t>
1684<t>
1685   The Internet Assigned Numbers Authority (IANA) acts as a registry for
1686   transfer-coding value tokens. Initially, the registry contains the
1687   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
1688   "gzip", "compress", and "deflate" (&content-codings;).
1689</t>
1690<t>
1691   New transfer-coding value tokens &SHOULD; be registered in the same way
1692   as new content-coding value tokens (&content-codings;).
1693</t>
1694<t>
1695   A server which receives an entity-body with a transfer-coding it does
1696   not understand &SHOULD; return 501 (Not Implemented), and close the
1697   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
1698   client.
1699</t>
1700
1701<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
1702  <x:anchor-alias value="chunk"/>
1703  <x:anchor-alias value="Chunked-Body"/>
1704  <x:anchor-alias value="chunk-data"/>
1705  <x:anchor-alias value="chunk-ext"/>
1706  <x:anchor-alias value="chunk-ext-name"/>
1707  <x:anchor-alias value="chunk-ext-val"/>
1708  <x:anchor-alias value="chunk-size"/>
1709  <x:anchor-alias value="last-chunk"/>
1710  <x:anchor-alias value="trailer-part"/>
1711<t>
1712   The chunked encoding modifies the body of a message in order to
1713   transfer it as a series of chunks, each with its own size indicator,
1714   followed by an &OPTIONAL; trailer containing entity-header fields. This
1715   allows dynamically produced content to be transferred along with the
1716   information necessary for the recipient to verify that it has
1717   received the full message.
1718</t>
1719<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"/>
1720  <x:ref>Chunked-Body</x:ref>   = *<x:ref>chunk</x:ref>
1721                   <x:ref>last-chunk</x:ref>
1722                   <x:ref>trailer-part</x:ref>
1723                   <x:ref>CRLF</x:ref>
1724 
1725  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1726                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1727  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1728  <x:ref>last-chunk</x:ref>     = 1*("0") *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1729 
1730  <x:ref>chunk-ext</x:ref>      = *( ";" *WSP <x:ref>chunk-ext-name</x:ref>
1731                      [ "=" <x:ref>chunk-ext-val</x:ref> ] *WSP )
1732  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1733  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1734  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1735  <x:ref>trailer-part</x:ref>   = *( <x:ref>entity-header</x:ref> <x:ref>CRLF</x:ref> )
1736</artwork></figure>
1737<t>
1738   The chunk-size field is a string of hex digits indicating the size of
1739   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1740   zero, followed by the trailer, which is terminated by an empty line.
1741</t>
1742<t>
1743   The trailer allows the sender to include additional HTTP header
1744   fields at the end of the message. The Trailer header field can be
1745   used to indicate which header fields are included in a trailer (see
1746   <xref target="header.trailer"/>).
1747</t>
1748<t>
1749   A server using chunked transfer-coding in a response &MUST-NOT; use the
1750   trailer for any header fields unless at least one of the following is
1751   true:
1752  <list style="numbers">
1753    <t>the request included a TE header field that indicates "trailers" is
1754     acceptable in the transfer-coding of the  response, as described in
1755     <xref target="header.te"/>; or,</t>
1756
1757    <t>the server is the origin server for the response, the trailer
1758     fields consist entirely of optional metadata, and the recipient
1759     could use the message (in a manner acceptable to the origin server)
1760     without receiving this metadata.  In other words, the origin server
1761     is willing to accept the possibility that the trailer fields might
1762     be silently discarded along the path to the client.</t>
1763  </list>
1764</t>
1765<t>
1766   This requirement prevents an interoperability failure when the
1767   message is being received by an HTTP/1.1 (or later) proxy and
1768   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1769   compliance with the protocol would have necessitated a possibly
1770   infinite buffer on the proxy.
1771</t>
1772<t>
1773   A process for decoding the "chunked" transfer-coding
1774   can be represented in pseudo-code as:
1775</t>
1776<figure><artwork type="code">
1777  length := 0
1778  read chunk-size, chunk-ext (if any) and CRLF
1779  while (chunk-size &gt; 0) {
1780     read chunk-data and CRLF
1781     append chunk-data to entity-body
1782     length := length + chunk-size
1783     read chunk-size and CRLF
1784  }
1785  read entity-header
1786  while (entity-header not empty) {
1787     append entity-header to existing header fields
1788     read entity-header
1789  }
1790  Content-Length := length
1791  Remove "chunked" from Transfer-Encoding
1792</artwork></figure>
1793<t>
1794   All HTTP/1.1 applications &MUST; be able to receive and decode the
1795   "chunked" transfer-coding, and &MUST; ignore chunk-ext extensions
1796   they do not understand.
1797</t>
1798</section>
1799</section>
1800
1801<section title="Product Tokens" anchor="product.tokens">
1802  <x:anchor-alias value="product"/>
1803  <x:anchor-alias value="product-version"/>
1804<t>
1805   Product tokens are used to allow communicating applications to
1806   identify themselves by software name and version. Most fields using
1807   product tokens also allow sub-products which form a significant part
1808   of the application to be listed, separated by whitespace. By
1809   convention, the products are listed in order of their significance
1810   for identifying the application.
1811</t>
1812<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1813  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
1814  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
1815</artwork></figure>
1816<t>
1817   Examples:
1818</t>
1819<figure><artwork type="example">
1820  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1821  Server: Apache/0.8.4
1822</artwork></figure>
1823<t>
1824   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1825   used for advertising or other non-essential information. Although any
1826   token character &MAY; appear in a product-version, this token &SHOULD;
1827   only be used for a version identifier (i.e., successive versions of
1828   the same product &SHOULD; only differ in the product-version portion of
1829   the product value).
1830</t>
1831</section>
1832
1833<section title="Quality Values" anchor="quality.values">
1834  <x:anchor-alias value="qvalue"/>
1835<t>
1836   Both transfer codings (TE request header, <xref target="header.te"/>)
1837   and content negotiation (&content.negotiation;) use short "floating point"
1838   numbers to indicate the relative importance ("weight") of various
1839   negotiable parameters.  A weight is normalized to a real number in
1840   the range 0 through 1, where 0 is the minimum and 1 the maximum
1841   value. If a parameter has a quality value of 0, then content with
1842   this parameter is `not acceptable' for the client. HTTP/1.1
1843   applications &MUST-NOT; generate more than three digits after the
1844   decimal point. User configuration of these values &SHOULD; also be
1845   limited in this fashion.
1846</t>
1847<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="qvalue"/>
1848  <x:ref>qvalue</x:ref>         = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
1849                 / ( "1" [ "." 0*3("0") ] )
1850</artwork></figure>
1851<x:note>
1852  <t>
1853     <x:h>Note:</x:h> "Quality values" is a misnomer, since these values merely represent
1854     relative degradation in desired quality.
1855  </t>
1856</x:note>
1857</section>
1858
1859</section>
1860
1861<section title="Connections" anchor="connections">
1862
1863<section title="Persistent Connections" anchor="persistent.connections">
1864
1865<section title="Purpose" anchor="persistent.purpose">
1866<t>
1867   Prior to persistent connections, a separate TCP connection was
1868   established to fetch each URL, increasing the load on HTTP servers
1869   and causing congestion on the Internet. The use of inline images and
1870   other associated data often require a client to make multiple
1871   requests of the same server in a short amount of time. Analysis of
1872   these performance problems and results from a prototype
1873   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1874   measurements of actual HTTP/1.1 implementations show good
1875   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1876   T/TCP <xref target="Tou1998"/>.
1877</t>
1878<t>
1879   Persistent HTTP connections have a number of advantages:
1880  <list style="symbols">
1881      <t>
1882        By opening and closing fewer TCP connections, CPU time is saved
1883        in routers and hosts (clients, servers, proxies, gateways,
1884        tunnels, or caches), and memory used for TCP protocol control
1885        blocks can be saved in hosts.
1886      </t>
1887      <t>
1888        HTTP requests and responses can be pipelined on a connection.
1889        Pipelining allows a client to make multiple requests without
1890        waiting for each response, allowing a single TCP connection to
1891        be used much more efficiently, with much lower elapsed time.
1892      </t>
1893      <t>
1894        Network congestion is reduced by reducing the number of packets
1895        caused by TCP opens, and by allowing TCP sufficient time to
1896        determine the congestion state of the network.
1897      </t>
1898      <t>
1899        Latency on subsequent requests is reduced since there is no time
1900        spent in TCP's connection opening handshake.
1901      </t>
1902      <t>
1903        HTTP can evolve more gracefully, since errors can be reported
1904        without the penalty of closing the TCP connection. Clients using
1905        future versions of HTTP might optimistically try a new feature,
1906        but if communicating with an older server, retry with old
1907        semantics after an error is reported.
1908      </t>
1909    </list>
1910</t>
1911<t>
1912   HTTP implementations &SHOULD; implement persistent connections.
1913</t>
1914</section>
1915
1916<section title="Overall Operation" anchor="persistent.overall">
1917<t>
1918   A significant difference between HTTP/1.1 and earlier versions of
1919   HTTP is that persistent connections are the default behavior of any
1920   HTTP connection. That is, unless otherwise indicated, the client
1921   &SHOULD; assume that the server will maintain a persistent connection,
1922   even after error responses from the server.
1923</t>
1924<t>
1925   Persistent connections provide a mechanism by which a client and a
1926   server can signal the close of a TCP connection. This signaling takes
1927   place using the Connection header field (<xref target="header.connection"/>). Once a close
1928   has been signaled, the client &MUST-NOT; send any more requests on that
1929   connection.
1930</t>
1931
1932<section title="Negotiation" anchor="persistent.negotiation">
1933<t>
1934   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1935   maintain a persistent connection unless a Connection header including
1936   the connection-token "close" was sent in the request. If the server
1937   chooses to close the connection immediately after sending the
1938   response, it &SHOULD; send a Connection header including the
1939   connection-token close.
1940</t>
1941<t>
1942   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1943   decide to keep it open based on whether the response from a server
1944   contains a Connection header with the connection-token close. In case
1945   the client does not want to maintain a connection for more than that
1946   request, it &SHOULD; send a Connection header including the
1947   connection-token close.
1948</t>
1949<t>
1950   If either the client or the server sends the close token in the
1951   Connection header, that request becomes the last one for the
1952   connection.
1953</t>
1954<t>
1955   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1956   maintained for HTTP versions less than 1.1 unless it is explicitly
1957   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1958   compatibility with HTTP/1.0 clients.
1959</t>
1960<t>
1961   In order to remain persistent, all messages on the connection &MUST;
1962   have a self-defined message length (i.e., one not defined by closure
1963   of the connection), as described in <xref target="message.length"/>.
1964</t>
1965</section>
1966
1967<section title="Pipelining" anchor="pipelining">
1968<t>
1969   A client that supports persistent connections &MAY; "pipeline" its
1970   requests (i.e., send multiple requests without waiting for each
1971   response). A server &MUST; send its responses to those requests in the
1972   same order that the requests were received.
1973</t>
1974<t>
1975   Clients which assume persistent connections and pipeline immediately
1976   after connection establishment &SHOULD; be prepared to retry their
1977   connection if the first pipelined attempt fails. If a client does
1978   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1979   persistent. Clients &MUST; also be prepared to resend their requests if
1980   the server closes the connection before sending all of the
1981   corresponding responses.
1982</t>
1983<t>
1984   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1985   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1986   premature termination of the transport connection could lead to
1987   indeterminate results. A client wishing to send a non-idempotent
1988   request &SHOULD; wait to send that request until it has received the
1989   response status for the previous request.
1990</t>
1991</section>
1992</section>
1993
1994<section title="Proxy Servers" anchor="persistent.proxy">
1995<t>
1996   It is especially important that proxies correctly implement the
1997   properties of the Connection header field as specified in <xref target="header.connection"/>.
1998</t>
1999<t>
2000   The proxy server &MUST; signal persistent connections separately with
2001   its clients and the origin servers (or other proxy servers) that it
2002   connects to. Each persistent connection applies to only one transport
2003   link.
2004</t>
2005<t>
2006   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
2007   with an HTTP/1.0 client (but see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>
2008   for information and discussion of the problems with the Keep-Alive header
2009   implemented by many HTTP/1.0 clients).
2010</t>
2011</section>
2012
2013<section title="Practical Considerations" anchor="persistent.practical">
2014<t>
2015   Servers will usually have some time-out value beyond which they will
2016   no longer maintain an inactive connection. Proxy servers might make
2017   this a higher value since it is likely that the client will be making
2018   more connections through the same server. The use of persistent
2019   connections places no requirements on the length (or existence) of
2020   this time-out for either the client or the server.
2021</t>
2022<t>
2023   When a client or server wishes to time-out it &SHOULD; issue a graceful
2024   close on the transport connection. Clients and servers &SHOULD; both
2025   constantly watch for the other side of the transport close, and
2026   respond to it as appropriate. If a client or server does not detect
2027   the other side's close promptly it could cause unnecessary resource
2028   drain on the network.
2029</t>
2030<t>
2031   A client, server, or proxy &MAY; close the transport connection at any
2032   time. For example, a client might have started to send a new request
2033   at the same time that the server has decided to close the "idle"
2034   connection. From the server's point of view, the connection is being
2035   closed while it was idle, but from the client's point of view, a
2036   request is in progress.
2037</t>
2038<t>
2039   This means that clients, servers, and proxies &MUST; be able to recover
2040   from asynchronous close events. Client software &SHOULD; reopen the
2041   transport connection and retransmit the aborted sequence of requests
2042   without user interaction so long as the request sequence is
2043   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
2044   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2045   human operator the choice of retrying the request(s). Confirmation by
2046   user-agent software with semantic understanding of the application
2047   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2048   be repeated if the second sequence of requests fails.
2049</t>
2050<t>
2051   Servers &SHOULD; always respond to at least one request per connection,
2052   if at all possible. Servers &SHOULD-NOT;  close a connection in the
2053   middle of transmitting a response, unless a network or client failure
2054   is suspected.
2055</t>
2056<t>
2057   Clients that use persistent connections &SHOULD; limit the number of
2058   simultaneous connections that they maintain to a given server. A
2059   single-user client &SHOULD-NOT; maintain more than 2 connections with
2060   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
2061   another server or proxy, where N is the number of simultaneously
2062   active users. These guidelines are intended to improve HTTP response
2063   times and avoid congestion.
2064</t>
2065</section>
2066</section>
2067
2068<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
2069
2070<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
2071<t>
2072   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
2073   flow control mechanisms to resolve temporary overloads, rather than
2074   terminating connections with the expectation that clients will retry.
2075   The latter technique can exacerbate network congestion.
2076</t>
2077</section>
2078
2079<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
2080<t>
2081   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
2082   the network connection for an error status while it is transmitting
2083   the request. If the client sees an error status, it &SHOULD;
2084   immediately cease transmitting the body. If the body is being sent
2085   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
2086   empty trailer &MAY; be used to prematurely mark the end of the message.
2087   If the body was preceded by a Content-Length header, the client &MUST;
2088   close the connection.
2089</t>
2090</section>
2091
2092<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
2093<t>
2094   The purpose of the 100 (Continue) status (see &status-100;) is to
2095   allow a client that is sending a request message with a request body
2096   to determine if the origin server is willing to accept the request
2097   (based on the request headers) before the client sends the request
2098   body. In some cases, it might either be inappropriate or highly
2099   inefficient for the client to send the body if the server will reject
2100   the message without looking at the body.
2101</t>
2102<t>
2103   Requirements for HTTP/1.1 clients:
2104  <list style="symbols">
2105    <t>
2106        If a client will wait for a 100 (Continue) response before
2107        sending the request body, it &MUST; send an Expect request-header
2108        field (&header-expect;) with the "100-continue" expectation.
2109    </t>
2110    <t>
2111        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
2112        with the "100-continue" expectation if it does not intend
2113        to send a request body.
2114    </t>
2115  </list>
2116</t>
2117<t>
2118   Because of the presence of older implementations, the protocol allows
2119   ambiguous situations in which a client may send "Expect: 100-continue"
2120   without receiving either a 417 (Expectation Failed) status
2121   or a 100 (Continue) status. Therefore, when a client sends this
2122   header field to an origin server (possibly via a proxy) from which it
2123   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
2124   for an indefinite period before sending the request body.
2125</t>
2126<t>
2127   Requirements for HTTP/1.1 origin servers:
2128  <list style="symbols">
2129    <t> Upon receiving a request which includes an Expect request-header
2130        field with the "100-continue" expectation, an origin server &MUST;
2131        either respond with 100 (Continue) status and continue to read
2132        from the input stream, or respond with a final status code. The
2133        origin server &MUST-NOT; wait for the request body before sending
2134        the 100 (Continue) response. If it responds with a final status
2135        code, it &MAY; close the transport connection or it &MAY; continue
2136        to read and discard the rest of the request.  It &MUST-NOT;
2137        perform the requested method if it returns a final status code.
2138    </t>
2139    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
2140        the request message does not include an Expect request-header
2141        field with the "100-continue" expectation, and &MUST-NOT; send a
2142        100 (Continue) response if such a request comes from an HTTP/1.0
2143        (or earlier) client. There is an exception to this rule: for
2144        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
2145        status in response to an HTTP/1.1 PUT or POST request that does
2146        not include an Expect request-header field with the "100-continue"
2147        expectation. This exception, the purpose of which is
2148        to minimize any client processing delays associated with an
2149        undeclared wait for 100 (Continue) status, applies only to
2150        HTTP/1.1 requests, and not to requests with any other HTTP-version
2151        value.
2152    </t>
2153    <t> An origin server &MAY; omit a 100 (Continue) response if it has
2154        already received some or all of the request body for the
2155        corresponding request.
2156    </t>
2157    <t> An origin server that sends a 100 (Continue) response &MUST;
2158    ultimately send a final status code, once the request body is
2159        received and processed, unless it terminates the transport
2160        connection prematurely.
2161    </t>
2162    <t> If an origin server receives a request that does not include an
2163        Expect request-header field with the "100-continue" expectation,
2164        the request includes a request body, and the server responds
2165        with a final status code before reading the entire request body
2166        from the transport connection, then the server &SHOULD-NOT;  close
2167        the transport connection until it has read the entire request,
2168        or until the client closes the connection. Otherwise, the client
2169        might not reliably receive the response message. However, this
2170        requirement is not be construed as preventing a server from
2171        defending itself against denial-of-service attacks, or from
2172        badly broken client implementations.
2173      </t>
2174    </list>
2175</t>
2176<t>
2177   Requirements for HTTP/1.1 proxies:
2178  <list style="symbols">
2179    <t> If a proxy receives a request that includes an Expect request-header
2180        field with the "100-continue" expectation, and the proxy
2181        either knows that the next-hop server complies with HTTP/1.1 or
2182        higher, or does not know the HTTP version of the next-hop
2183        server, it &MUST; forward the request, including the Expect header
2184        field.
2185    </t>
2186    <t> If the proxy knows that the version of the next-hop server is
2187        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
2188        respond with a 417 (Expectation Failed) status.
2189    </t>
2190    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
2191        numbers received from recently-referenced next-hop servers.
2192    </t>
2193    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
2194        request message was received from an HTTP/1.0 (or earlier)
2195        client and did not include an Expect request-header field with
2196        the "100-continue" expectation. This requirement overrides the
2197        general rule for forwarding of 1xx responses (see &status-1xx;).
2198    </t>
2199  </list>
2200</t>
2201</section>
2202
2203<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
2204<t>
2205   If an HTTP/1.1 client sends a request which includes a request body,
2206   but which does not include an Expect request-header field with the
2207   "100-continue" expectation, and if the client is not directly
2208   connected to an HTTP/1.1 origin server, and if the client sees the
2209   connection close before receiving any status from the server, the
2210   client &SHOULD; retry the request.  If the client does retry this
2211   request, it &MAY; use the following "binary exponential backoff"
2212   algorithm to be assured of obtaining a reliable response:
2213  <list style="numbers">
2214    <t>
2215      Initiate a new connection to the server
2216    </t>
2217    <t>
2218      Transmit the request-headers
2219    </t>
2220    <t>
2221      Initialize a variable R to the estimated round-trip time to the
2222         server (e.g., based on the time it took to establish the
2223         connection), or to a constant value of 5 seconds if the round-trip
2224         time is not available.
2225    </t>
2226    <t>
2227       Compute T = R * (2**N), where N is the number of previous
2228         retries of this request.
2229    </t>
2230    <t>
2231       Wait either for an error response from the server, or for T
2232         seconds (whichever comes first)
2233    </t>
2234    <t>
2235       If no error response is received, after T seconds transmit the
2236         body of the request.
2237    </t>
2238    <t>
2239       If client sees that the connection is closed prematurely,
2240         repeat from step 1 until the request is accepted, an error
2241         response is received, or the user becomes impatient and
2242         terminates the retry process.
2243    </t>
2244  </list>
2245</t>
2246<t>
2247   If at any point an error status is received, the client
2248  <list style="symbols">
2249      <t>&SHOULD-NOT;  continue and</t>
2250
2251      <t>&SHOULD; close the connection if it has not completed sending the
2252        request message.</t>
2253    </list>
2254</t>
2255</section>
2256</section>
2257</section>
2258
2259
2260<section title="Header Field Definitions" anchor="header.fields">
2261<t>
2262   This section defines the syntax and semantics of HTTP/1.1 header fields
2263   related to message framing and transport protocols.
2264</t>
2265<t>
2266   For entity-header fields, both sender and recipient refer to either the
2267   client or the server, depending on who sends and who receives the entity.
2268</t>
2269
2270<section title="Connection" anchor="header.connection">
2271  <iref primary="true" item="Connection header" x:for-anchor=""/>
2272  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2273  <x:anchor-alias value="Connection"/>
2274  <x:anchor-alias value="connection-token"/>
2275  <x:anchor-alias value="Connection-v"/>
2276<t>
2277   The general-header field "Connection" allows the sender to specify
2278   options that are desired for that particular connection and &MUST-NOT;
2279   be communicated by proxies over further connections.
2280</t>
2281<t>
2282   The Connection header's value has the following grammar:
2283</t>
2284<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"/>
2285  <x:ref>Connection</x:ref>       = "Connection" ":" <x:ref>OWS</x:ref> <x:ref>Connection-v</x:ref>
2286  <x:ref>Connection-v</x:ref>     = 1#<x:ref>connection-token</x:ref>
2287  <x:ref>connection-token</x:ref> = <x:ref>token</x:ref>
2288</artwork></figure>
2289<t>
2290   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2291   message is forwarded and, for each connection-token in this field,
2292   remove any header field(s) from the message with the same name as the
2293   connection-token. Connection options are signaled by the presence of
2294   a connection-token in the Connection header field, not by any
2295   corresponding additional header field(s), since the additional header
2296   field may not be sent if there are no parameters associated with that
2297   connection option.
2298</t>
2299<t>
2300   Message headers listed in the Connection header &MUST-NOT; include
2301   end-to-end headers, such as Cache-Control.
2302</t>
2303<t>
2304   HTTP/1.1 defines the "close" connection option for the sender to
2305   signal that the connection will be closed after completion of the
2306   response. For example,
2307</t>
2308<figure><artwork type="example">
2309  Connection: close
2310</artwork></figure>
2311<t>
2312   in either the request or the response header fields indicates that
2313   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2314   after the current request/response is complete.
2315</t>
2316<t>
2317   An HTTP/1.1 client that does not support persistent connections &MUST;
2318   include the "close" connection option in every request message.
2319</t>
2320<t>
2321   An HTTP/1.1 server that does not support persistent connections &MUST;
2322   include the "close" connection option in every response message that
2323   does not have a 1xx (informational) status code.
2324</t>
2325<t>
2326   A system receiving an HTTP/1.0 (or lower-version) message that
2327   includes a Connection header &MUST;, for each connection-token in this
2328   field, remove and ignore any header field(s) from the message with
2329   the same name as the connection-token. This protects against mistaken
2330   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2331</t>
2332</section>
2333
2334<section title="Content-Length" anchor="header.content-length">
2335  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2336  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2337  <x:anchor-alias value="Content-Length"/>
2338  <x:anchor-alias value="Content-Length-v"/>
2339<t>
2340   The entity-header field "Content-Length" indicates the size of the
2341   entity-body, in number of OCTETs, sent to the recipient or,
2342   in the case of the HEAD method, the size of the entity-body that
2343   would have been sent had the request been a GET.
2344</t>
2345<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/><iref primary="true" item="Grammar" subitem="Content-Length-v"/>
2346  <x:ref>Content-Length</x:ref>   = "Content-Length" ":" <x:ref>OWS</x:ref> 1*<x:ref>Content-Length-v</x:ref>
2347  <x:ref>Content-Length-v</x:ref> = 1*<x:ref>DIGIT</x:ref>
2348</artwork></figure>
2349<t>
2350   An example is
2351</t>
2352<figure><artwork type="example">
2353  Content-Length: 3495
2354</artwork></figure>
2355<t>
2356   Applications &SHOULD; use this field to indicate the transfer-length of
2357   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2358</t>
2359<t>
2360   Any Content-Length greater than or equal to zero is a valid value.
2361   <xref target="message.length"/> describes how to determine the length of a message-body
2362   if a Content-Length is not given.
2363</t>
2364<t>
2365   Note that the meaning of this field is significantly different from
2366   the corresponding definition in MIME, where it is an optional field
2367   used within the "message/external-body" content-type. In HTTP, it
2368   &SHOULD; be sent whenever the message's length can be determined prior
2369   to being transferred, unless this is prohibited by the rules in
2370   <xref target="message.length"/>.
2371</t>
2372</section>
2373
2374<section title="Date" anchor="header.date">
2375  <iref primary="true" item="Date header" x:for-anchor=""/>
2376  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2377  <x:anchor-alias value="Date"/>
2378  <x:anchor-alias value="Date-v"/>
2379<t>
2380   The general-header field "Date" represents the date and time at which
2381   the message was originated, having the same semantics as orig-date in
2382   <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>. The field value is an
2383   HTTP-date, as described in <xref target="date.time.formats.full.date"/>;
2384   it &MUST; be sent in rfc1123-date format.
2385</t>
2386<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/><iref primary="true" item="Grammar" subitem="Date-v"/>
2387  <x:ref>Date</x:ref>   = "Date" ":" <x:ref>OWS</x:ref> <x:ref>Date-v</x:ref>
2388  <x:ref>Date-v</x:ref> = <x:ref>HTTP-date</x:ref>
2389</artwork></figure>
2390<t>
2391   An example is
2392</t>
2393<figure><artwork type="example">
2394  Date: Tue, 15 Nov 1994 08:12:31 GMT
2395</artwork></figure>
2396<t>
2397   Origin servers &MUST; include a Date header field in all responses,
2398   except in these cases:
2399  <list style="numbers">
2400      <t>If the response status code is 100 (Continue) or 101 (Switching
2401         Protocols), the response &MAY; include a Date header field, at
2402         the server's option.</t>
2403
2404      <t>If the response status code conveys a server error, e.g. 500
2405         (Internal Server Error) or 503 (Service Unavailable), and it is
2406         inconvenient or impossible to generate a valid Date.</t>
2407
2408      <t>If the server does not have a clock that can provide a
2409         reasonable approximation of the current time, its responses
2410         &MUST-NOT; include a Date header field. In this case, the rules
2411         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2412  </list>
2413</t>
2414<t>
2415   A received message that does not have a Date header field &MUST; be
2416   assigned one by the recipient if the message will be cached by that
2417   recipient or gatewayed via a protocol which requires a Date. An HTTP
2418   implementation without a clock &MUST-NOT; cache responses without
2419   revalidating them on every use. An HTTP cache, especially a shared
2420   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2421   clock with a reliable external standard.
2422</t>
2423<t>
2424   Clients &SHOULD; only send a Date header field in messages that include
2425   an entity-body, as in the case of the PUT and POST requests, and even
2426   then it is optional. A client without a clock &MUST-NOT; send a Date
2427   header field in a request.
2428</t>
2429<t>
2430   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2431   time subsequent to the generation of the message. It &SHOULD; represent
2432   the best available approximation of the date and time of message
2433   generation, unless the implementation has no means of generating a
2434   reasonably accurate date and time. In theory, the date ought to
2435   represent the moment just before the entity is generated. In
2436   practice, the date can be generated at any time during the message
2437   origination without affecting its semantic value.
2438</t>
2439
2440<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2441<t>
2442   Some origin server implementations might not have a clock available.
2443   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2444   values to a response, unless these values were associated
2445   with the resource by a system or user with a reliable clock. It &MAY;
2446   assign an Expires value that is known, at or before server
2447   configuration time, to be in the past (this allows "pre-expiration"
2448   of responses without storing separate Expires values for each
2449   resource).
2450</t>
2451</section>
2452</section>
2453
2454<section title="Host" anchor="header.host">
2455  <iref primary="true" item="Host header" x:for-anchor=""/>
2456  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2457  <x:anchor-alias value="Host"/>
2458  <x:anchor-alias value="Host-v"/>
2459<t>
2460   The request-header field "Host" specifies the Internet host and port
2461   number of the resource being requested, as obtained from the original
2462   URI given by the user or referring resource (generally an http URI,
2463   as described in <xref target="http.uri"/>). The Host field value &MUST; represent
2464   the naming authority of the origin server or gateway given by the
2465   original URL. This allows the origin server or gateway to
2466   differentiate between internally-ambiguous URLs, such as the root "/"
2467   URL of a server for multiple host names on a single IP address.
2468</t>
2469<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/><iref primary="true" item="Grammar" subitem="Host-v"/>
2470  <x:ref>Host</x:ref>   = "Host" ":" <x:ref>OWS</x:ref> <x:ref>Host-v</x:ref>
2471  <x:ref>Host-v</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2472</artwork></figure>
2473<t>
2474   A "host" without any trailing port information implies the default
2475   port for the service requested (e.g., "80" for an HTTP URL). For
2476   example, a request on the origin server for
2477   &lt;http://www.example.org/pub/WWW/&gt; would properly include:
2478</t>
2479<figure><artwork type="example">
2480  GET /pub/WWW/ HTTP/1.1
2481  Host: www.example.org
2482</artwork></figure>
2483<t>
2484   A client &MUST; include a Host header field in all HTTP/1.1 request
2485   messages. If the requested URI does not include an Internet host
2486   name for the service being requested, then the Host header field &MUST;
2487   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2488   request message it forwards does contain an appropriate Host header
2489   field that identifies the service being requested by the proxy. All
2490   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2491   status code to any HTTP/1.1 request message which lacks a Host header
2492   field.
2493</t>
2494<t>
2495   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
2496   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
2497   for other requirements relating to Host.
2498</t>
2499</section>
2500
2501<section title="TE" anchor="header.te">
2502  <iref primary="true" item="TE header" x:for-anchor=""/>
2503  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2504  <x:anchor-alias value="TE"/>
2505  <x:anchor-alias value="TE-v"/>
2506  <x:anchor-alias value="t-codings"/>
2507  <x:anchor-alias value="te-params"/>
2508  <x:anchor-alias value="te-ext"/>
2509<t>
2510   The request-header field "TE" indicates what extension transfer-codings
2511   it is willing to accept in the response and whether or not it is
2512   willing to accept trailer fields in a chunked transfer-coding. Its
2513   value may consist of the keyword "trailers" and/or a comma-separated
2514   list of extension transfer-coding names with optional accept
2515   parameters (as described in <xref target="transfer.codings"/>).
2516</t>
2517<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"/><iref primary="true" item="Grammar" subitem="te-params"/><iref primary="true" item="Grammar" subitem="te-ext"/>
2518  <x:ref>TE</x:ref>        = "TE" ":" <x:ref>OWS</x:ref> <x:ref>TE-v</x:ref>
2519  <x:ref>TE-v</x:ref>      = #<x:ref>t-codings</x:ref>
2520  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-extension</x:ref> [ <x:ref>te-params</x:ref> ] )
2521  <x:ref>te-params</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> *( <x:ref>te-ext</x:ref> )
2522  <x:ref>te-ext</x:ref>    = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
2523</artwork></figure>
2524<t>
2525   The presence of the keyword "trailers" indicates that the client is
2526   willing to accept trailer fields in a chunked transfer-coding, as
2527   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2528   transfer-coding values even though it does not itself represent a
2529   transfer-coding.
2530</t>
2531<t>
2532   Examples of its use are:
2533</t>
2534<figure><artwork type="example">
2535  TE: deflate
2536  TE:
2537  TE: trailers, deflate;q=0.5
2538</artwork></figure>
2539<t>
2540   The TE header field only applies to the immediate connection.
2541   Therefore, the keyword &MUST; be supplied within a Connection header
2542   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2543</t>
2544<t>
2545   A server tests whether a transfer-coding is acceptable, according to
2546   a TE field, using these rules:
2547  <list style="numbers">
2548    <x:lt>
2549      <t>The "chunked" transfer-coding is always acceptable. If the
2550         keyword "trailers" is listed, the client indicates that it is
2551         willing to accept trailer fields in the chunked response on
2552         behalf of itself and any downstream clients. The implication is
2553         that, if given, the client is stating that either all
2554         downstream clients are willing to accept trailer fields in the
2555         forwarded response, or that it will attempt to buffer the
2556         response on behalf of downstream recipients.
2557      </t><t>
2558         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2559         chunked response such that a client can be assured of buffering
2560         the entire response.</t>
2561    </x:lt>
2562    <x:lt>
2563      <t>If the transfer-coding being tested is one of the transfer-codings
2564         listed in the TE field, then it is acceptable unless it
2565         is accompanied by a qvalue of 0. (As defined in <xref target="quality.values"/>, a
2566         qvalue of 0 means "not acceptable.")</t>
2567    </x:lt>
2568    <x:lt>
2569      <t>If multiple transfer-codings are acceptable, then the
2570         acceptable transfer-coding with the highest non-zero qvalue is
2571         preferred.  The "chunked" transfer-coding always has a qvalue
2572         of 1.</t>
2573    </x:lt>
2574  </list>
2575</t>
2576<t>
2577   If the TE field-value is empty or if no TE field is present, the only
2578   transfer-coding is "chunked". A message with no transfer-coding is
2579   always acceptable.
2580</t>
2581</section>
2582
2583<section title="Trailer" anchor="header.trailer">
2584  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2585  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2586  <x:anchor-alias value="Trailer"/>
2587  <x:anchor-alias value="Trailer-v"/>
2588<t>
2589   The general field "Trailer" indicates that the given set of
2590   header fields is present in the trailer of a message encoded with
2591   chunked transfer-coding.
2592</t>
2593<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="true" item="Grammar" subitem="Trailer-v"/>
2594  <x:ref>Trailer</x:ref>   = "Trailer" ":" <x:ref>OWS</x:ref> <x:ref>Trailer-v</x:ref>
2595  <x:ref>Trailer-v</x:ref> = 1#<x:ref>field-name</x:ref>
2596</artwork></figure>
2597<t>
2598   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2599   message using chunked transfer-coding with a non-empty trailer. Doing
2600   so allows the recipient to know which header fields to expect in the
2601   trailer.
2602</t>
2603<t>
2604   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2605   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2606   trailer fields in a "chunked" transfer-coding.
2607</t>
2608<t>
2609   Message header fields listed in the Trailer header field &MUST-NOT;
2610   include the following header fields:
2611  <list style="symbols">
2612    <t>Transfer-Encoding</t>
2613    <t>Content-Length</t>
2614    <t>Trailer</t>
2615  </list>
2616</t>
2617</section>
2618
2619<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2620  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2621  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2622  <x:anchor-alias value="Transfer-Encoding"/>
2623  <x:anchor-alias value="Transfer-Encoding-v"/>
2624<t>
2625   The general-header "Transfer-Encoding" field indicates what (if any)
2626   type of transformation has been applied to the message body in order
2627   to safely transfer it between the sender and the recipient. This
2628   differs from the content-coding in that the transfer-coding is a
2629   property of the message, not of the entity.
2630</t>
2631<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/><iref primary="true" item="Grammar" subitem="Transfer-Encoding-v"/>
2632  <x:ref>Transfer-Encoding</x:ref>   = "Transfer-Encoding" ":" <x:ref>OWS</x:ref>
2633                        <x:ref>Transfer-Encoding-v</x:ref>
2634  <x:ref>Transfer-Encoding-v</x:ref> = 1#<x:ref>transfer-coding</x:ref>
2635</artwork></figure>
2636<t>
2637   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2638</t>
2639<figure><artwork type="example">
2640  Transfer-Encoding: chunked
2641</artwork></figure>
2642<t>
2643   If multiple encodings have been applied to an entity, the transfer-codings
2644   &MUST; be listed in the order in which they were applied.
2645   Additional information about the encoding parameters &MAY; be provided
2646   by other entity-header fields not defined by this specification.
2647</t>
2648<t>
2649   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2650   header.
2651</t>
2652</section>
2653
2654<section title="Upgrade" anchor="header.upgrade">
2655  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2656  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2657  <x:anchor-alias value="Upgrade"/>
2658  <x:anchor-alias value="Upgrade-v"/>
2659<t>
2660   The general-header "Upgrade" allows the client to specify what
2661   additional communication protocols it supports and would like to use
2662   if the server finds it appropriate to switch protocols. The server
2663   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2664   response to indicate which protocol(s) are being switched.
2665</t>
2666<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/><iref primary="true" item="Grammar" subitem="Upgrade-v"/>
2667  <x:ref>Upgrade</x:ref>   = "Upgrade" ":" <x:ref>OWS</x:ref> <x:ref>Upgrade-v</x:ref>
2668  <x:ref>Upgrade-v</x:ref> = 1#<x:ref>product</x:ref>
2669</artwork></figure>
2670<t>
2671   For example,
2672</t>
2673<figure><artwork type="example">
2674  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2675</artwork></figure>
2676<t>
2677   The Upgrade header field is intended to provide a simple mechanism
2678   for transition from HTTP/1.1 to some other, incompatible protocol. It
2679   does so by allowing the client to advertise its desire to use another
2680   protocol, such as a later version of HTTP with a higher major version
2681   number, even though the current request has been made using HTTP/1.1.
2682   This eases the difficult transition between incompatible protocols by
2683   allowing the client to initiate a request in the more commonly
2684   supported protocol while indicating to the server that it would like
2685   to use a "better" protocol if available (where "better" is determined
2686   by the server, possibly according to the nature of the method and/or
2687   resource being requested).
2688</t>
2689<t>
2690   The Upgrade header field only applies to switching application-layer
2691   protocols upon the existing transport-layer connection. Upgrade
2692   cannot be used to insist on a protocol change; its acceptance and use
2693   by the server is optional. The capabilities and nature of the
2694   application-layer communication after the protocol change is entirely
2695   dependent upon the new protocol chosen, although the first action
2696   after changing the protocol &MUST; be a response to the initial HTTP
2697   request containing the Upgrade header field.
2698</t>
2699<t>
2700   The Upgrade header field only applies to the immediate connection.
2701   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2702   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2703   HTTP/1.1 message.
2704</t>
2705<t>
2706   The Upgrade header field cannot be used to indicate a switch to a
2707   protocol on a different connection. For that purpose, it is more
2708   appropriate to use a 301, 302, 303, or 305 redirection response.
2709</t>
2710<t>
2711   This specification only defines the protocol name "HTTP" for use by
2712   the family of Hypertext Transfer Protocols, as defined by the HTTP
2713   version rules of <xref target="http.version"/> and future updates to this
2714   specification. Any token can be used as a protocol name; however, it
2715   will only be useful if both the client and server associate the name
2716   with the same protocol.
2717</t>
2718</section>
2719
2720<section title="Via" anchor="header.via">
2721  <iref primary="true" item="Via header" x:for-anchor=""/>
2722  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2723  <x:anchor-alias value="protocol-name"/>
2724  <x:anchor-alias value="protocol-version"/>
2725  <x:anchor-alias value="pseudonym"/>
2726  <x:anchor-alias value="received-by"/>
2727  <x:anchor-alias value="received-protocol"/>
2728  <x:anchor-alias value="Via"/>
2729  <x:anchor-alias value="Via-v"/>
2730<t>
2731   The general-header field "Via" &MUST; be used by gateways and proxies to
2732   indicate the intermediate protocols and recipients between the user
2733   agent and the server on requests, and between the origin server and
2734   the client on responses. It is analogous to the "Received" field defined in
2735   <xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/> and is intended to be used for tracking message forwards,
2736   avoiding request loops, and identifying the protocol capabilities of
2737   all senders along the request/response chain.
2738</t>
2739<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"/>
2740  <x:ref>Via</x:ref>               = "Via" ":" <x:ref>OWS</x:ref> <x:ref>Via-v</x:ref>
2741  <x:ref>Via-v</x:ref>             = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2742                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2743  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2744  <x:ref>protocol-name</x:ref>     = <x:ref>token</x:ref>
2745  <x:ref>protocol-version</x:ref>  = <x:ref>token</x:ref>
2746  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2747  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2748</artwork></figure>
2749<t>
2750   The received-protocol indicates the protocol version of the message
2751   received by the server or client along each segment of the
2752   request/response chain. The received-protocol version is appended to
2753   the Via field value when the message is forwarded so that information
2754   about the protocol capabilities of upstream applications remains
2755   visible to all recipients.
2756</t>
2757<t>
2758   The protocol-name is optional if and only if it would be "HTTP". The
2759   received-by field is normally the host and optional port number of a
2760   recipient server or client that subsequently forwarded the message.
2761   However, if the real host is considered to be sensitive information,
2762   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2763   be assumed to be the default port of the received-protocol.
2764</t>
2765<t>
2766   Multiple Via field values represents each proxy or gateway that has
2767   forwarded the message. Each recipient &MUST; append its information
2768   such that the end result is ordered according to the sequence of
2769   forwarding applications.
2770</t>
2771<t>
2772   Comments &MAY; be used in the Via header field to identify the software
2773   of the recipient proxy or gateway, analogous to the User-Agent and
2774   Server header fields. However, all comments in the Via field are
2775   optional and &MAY; be removed by any recipient prior to forwarding the
2776   message.
2777</t>
2778<t>
2779   For example, a request message could be sent from an HTTP/1.0 user
2780   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2781   forward the request to a public proxy at p.example.net, which completes
2782   the request by forwarding it to the origin server at www.example.com.
2783   The request received by www.example.com would then have the following
2784   Via header field:
2785</t>
2786<figure><artwork type="example">
2787  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2788</artwork></figure>
2789<t>
2790   Proxies and gateways used as a portal through a network firewall
2791   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2792   the firewall region. This information &SHOULD; only be propagated if
2793   explicitly enabled. If not enabled, the received-by host of any host
2794   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2795   for that host.
2796</t>
2797<t>
2798   For organizations that have strong privacy requirements for hiding
2799   internal structures, a proxy &MAY; combine an ordered subsequence of
2800   Via header field entries with identical received-protocol values into
2801   a single such entry. For example,
2802</t>
2803<figure><artwork type="example">
2804  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2805</artwork></figure>
2806<t>
2807        could be collapsed to
2808</t>
2809<figure><artwork type="example">
2810  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2811</artwork></figure>
2812<t>
2813   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2814   under the same organizational control and the hosts have already been
2815   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2816   have different received-protocol values.
2817</t>
2818</section>
2819
2820</section>
2821
2822<section title="IANA Considerations" anchor="IANA.considerations">
2823<section title="Message Header Registration" anchor="message.header.registration">
2824<t>
2825   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2826   with the permanent registrations below (see <xref target="RFC3864"/>):
2827</t>
2828<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2829<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2830   <ttcol>Header Field Name</ttcol>
2831   <ttcol>Protocol</ttcol>
2832   <ttcol>Status</ttcol>
2833   <ttcol>Reference</ttcol>
2834
2835   <c>Connection</c>
2836   <c>http</c>
2837   <c>standard</c>
2838   <c>
2839      <xref target="header.connection"/>
2840   </c>
2841   <c>Content-Length</c>
2842   <c>http</c>
2843   <c>standard</c>
2844   <c>
2845      <xref target="header.content-length"/>
2846   </c>
2847   <c>Date</c>
2848   <c>http</c>
2849   <c>standard</c>
2850   <c>
2851      <xref target="header.date"/>
2852   </c>
2853   <c>Host</c>
2854   <c>http</c>
2855   <c>standard</c>
2856   <c>
2857      <xref target="header.host"/>
2858   </c>
2859   <c>TE</c>
2860   <c>http</c>
2861   <c>standard</c>
2862   <c>
2863      <xref target="header.te"/>
2864   </c>
2865   <c>Trailer</c>
2866   <c>http</c>
2867   <c>standard</c>
2868   <c>
2869      <xref target="header.trailer"/>
2870   </c>
2871   <c>Transfer-Encoding</c>
2872   <c>http</c>
2873   <c>standard</c>
2874   <c>
2875      <xref target="header.transfer-encoding"/>
2876   </c>
2877   <c>Upgrade</c>
2878   <c>http</c>
2879   <c>standard</c>
2880   <c>
2881      <xref target="header.upgrade"/>
2882   </c>
2883   <c>Via</c>
2884   <c>http</c>
2885   <c>standard</c>
2886   <c>
2887      <xref target="header.via"/>
2888   </c>
2889</texttable>
2890<!--(END)-->
2891<t>
2892   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2893</t>
2894</section>
2895
2896<section title="URI Scheme Registration" anchor="uri.scheme.registration">
2897<t>
2898   The entry for the "http" URI Scheme in the registry located at
2899   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>
2900   should be updated to point to <xref target="http.uri"/> of this document
2901   (see <xref target="RFC4395"/>).
2902</t>
2903</section>
2904
2905<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
2906<t>
2907   This document serves as the specification for the Internet media types
2908   "message/http" and "application/http". The following is to be registered with
2909   IANA (see <xref target="RFC4288"/>).
2910</t>
2911<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
2912<iref item="Media Type" subitem="message/http" primary="true"/>
2913<iref item="message/http Media Type" primary="true"/>
2914<t>
2915   The message/http type can be used to enclose a single HTTP request or
2916   response message, provided that it obeys the MIME restrictions for all
2917   "message" types regarding line length and encodings.
2918</t>
2919<t>
2920  <list style="hanging" x:indent="12em">
2921    <t hangText="Type name:">
2922      message
2923    </t>
2924    <t hangText="Subtype name:">
2925      http
2926    </t>
2927    <t hangText="Required parameters:">
2928      none
2929    </t>
2930    <t hangText="Optional parameters:">
2931      version, msgtype
2932      <list style="hanging">
2933        <t hangText="version:">
2934          The HTTP-Version number of the enclosed message
2935          (e.g., "1.1"). If not present, the version can be
2936          determined from the first line of the body.
2937        </t>
2938        <t hangText="msgtype:">
2939          The message type -- "request" or "response". If not
2940          present, the type can be determined from the first
2941          line of the body.
2942        </t>
2943      </list>
2944    </t>
2945    <t hangText="Encoding considerations:">
2946      only "7bit", "8bit", or "binary" are permitted
2947    </t>
2948    <t hangText="Security considerations:">
2949      none
2950    </t>
2951    <t hangText="Interoperability considerations:">
2952      none
2953    </t>
2954    <t hangText="Published specification:">
2955      This specification (see <xref target="internet.media.type.message.http"/>).
2956    </t>
2957    <t hangText="Applications that use this media type:">
2958    </t>
2959    <t hangText="Additional information:">
2960      <list style="hanging">
2961        <t hangText="Magic number(s):">none</t>
2962        <t hangText="File extension(s):">none</t>
2963        <t hangText="Macintosh file type code(s):">none</t>
2964      </list>
2965    </t>
2966    <t hangText="Person and email address to contact for further information:">
2967      See Authors Section.
2968    </t>
2969    <t hangText="Intended usage:">
2970      COMMON
2971    </t>
2972    <t hangText="Restrictions on usage:">
2973      none
2974    </t>
2975    <t hangText="Author/Change controller:">
2976      IESG
2977    </t>
2978  </list>
2979</t>
2980</section>
2981<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
2982<iref item="Media Type" subitem="application/http" primary="true"/>
2983<iref item="application/http Media Type" primary="true"/>
2984<t>
2985   The application/http type can be used to enclose a pipeline of one or more
2986   HTTP request or response messages (not intermixed).
2987</t>
2988<t>
2989  <list style="hanging" x:indent="12em">
2990    <t hangText="Type name:">
2991      application
2992    </t>
2993    <t hangText="Subtype name:">
2994      http
2995    </t>
2996    <t hangText="Required parameters:">
2997      none
2998    </t>
2999    <t hangText="Optional parameters:">
3000      version, msgtype
3001      <list style="hanging">
3002        <t hangText="version:">
3003          The HTTP-Version number of the enclosed messages
3004          (e.g., "1.1"). If not present, the version can be
3005          determined from the first line of the body.
3006        </t>
3007        <t hangText="msgtype:">
3008          The message type -- "request" or "response". If not
3009          present, the type can be determined from the first
3010          line of the body.
3011        </t>
3012      </list>
3013    </t>
3014    <t hangText="Encoding considerations:">
3015      HTTP messages enclosed by this type
3016      are in "binary" format; use of an appropriate
3017      Content-Transfer-Encoding is required when
3018      transmitted via E-mail.
3019    </t>
3020    <t hangText="Security considerations:">
3021      none
3022    </t>
3023    <t hangText="Interoperability considerations:">
3024      none
3025    </t>
3026    <t hangText="Published specification:">
3027      This specification (see <xref target="internet.media.type.application.http"/>).
3028    </t>
3029    <t hangText="Applications that use this media type:">
3030    </t>
3031    <t hangText="Additional information:">
3032      <list style="hanging">
3033        <t hangText="Magic number(s):">none</t>
3034        <t hangText="File extension(s):">none</t>
3035        <t hangText="Macintosh file type code(s):">none</t>
3036      </list>
3037    </t>
3038    <t hangText="Person and email address to contact for further information:">
3039      See Authors Section.
3040    </t>
3041    <t hangText="Intended usage:">
3042      COMMON
3043    </t>
3044    <t hangText="Restrictions on usage:">
3045      none
3046    </t>
3047    <t hangText="Author/Change controller:">
3048      IESG
3049    </t>
3050  </list>
3051</t>
3052</section>
3053</section>
3054
3055</section>
3056
3057<section title="Security Considerations" anchor="security.considerations">
3058<t>
3059   This section is meant to inform application developers, information
3060   providers, and users of the security limitations in HTTP/1.1 as
3061   described by this document. The discussion does not include
3062   definitive solutions to the problems revealed, though it does make
3063   some suggestions for reducing security risks.
3064</t>
3065
3066<section title="Personal Information" anchor="personal.information">
3067<t>
3068   HTTP clients are often privy to large amounts of personal information
3069   (e.g. the user's name, location, mail address, passwords, encryption
3070   keys, etc.), and &SHOULD; be very careful to prevent unintentional
3071   leakage of this information.
3072   We very strongly recommend that a convenient interface be provided
3073   for the user to control dissemination of such information, and that
3074   designers and implementors be particularly careful in this area.
3075   History shows that errors in this area often create serious security
3076   and/or privacy problems and generate highly adverse publicity for the
3077   implementor's company.
3078</t>
3079</section>
3080
3081<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3082<t>
3083   A server is in the position to save personal data about a user's
3084   requests which might identify their reading patterns or subjects of
3085   interest. This information is clearly confidential in nature and its
3086   handling can be constrained by law in certain countries. People using
3087   HTTP to provide data are responsible for ensuring that
3088   such material is not distributed without the permission of any
3089   individuals that are identifiable by the published results.
3090</t>
3091</section>
3092
3093<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3094<t>
3095   Implementations of HTTP origin servers &SHOULD; be careful to restrict
3096   the documents returned by HTTP requests to be only those that were
3097   intended by the server administrators. If an HTTP server translates
3098   HTTP URIs directly into file system calls, the server &MUST; take
3099   special care not to serve files that were not intended to be
3100   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3101   other operating systems use ".." as a path component to indicate a
3102   directory level above the current one. On such a system, an HTTP
3103   server &MUST; disallow any such construct in the request-target if it
3104   would otherwise allow access to a resource outside those intended to
3105   be accessible via the HTTP server. Similarly, files intended for
3106   reference only internally to the server (such as access control
3107   files, configuration files, and script code) &MUST; be protected from
3108   inappropriate retrieval, since they might contain sensitive
3109   information. Experience has shown that minor bugs in such HTTP server
3110   implementations have turned into security risks.
3111</t>
3112</section>
3113
3114<section title="DNS Spoofing" anchor="dns.spoofing">
3115<t>
3116   Clients using HTTP rely heavily on the Domain Name Service, and are
3117   thus generally prone to security attacks based on the deliberate
3118   mis-association of IP addresses and DNS names. Clients need to be
3119   cautious in assuming the continuing validity of an IP number/DNS name
3120   association.
3121</t>
3122<t>
3123   In particular, HTTP clients &SHOULD; rely on their name resolver for
3124   confirmation of an IP number/DNS name association, rather than
3125   caching the result of previous host name lookups. Many platforms
3126   already can cache host name lookups locally when appropriate, and
3127   they &SHOULD; be configured to do so. It is proper for these lookups to
3128   be cached, however, only when the TTL (Time To Live) information
3129   reported by the name server makes it likely that the cached
3130   information will remain useful.
3131</t>
3132<t>
3133   If HTTP clients cache the results of host name lookups in order to
3134   achieve a performance improvement, they &MUST; observe the TTL
3135   information reported by DNS.
3136</t>
3137<t>
3138   If HTTP clients do not observe this rule, they could be spoofed when
3139   a previously-accessed server's IP address changes. As network
3140   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
3141   possibility of this form of attack will grow. Observing this
3142   requirement thus reduces this potential security vulnerability.
3143</t>
3144<t>
3145   This requirement also improves the load-balancing behavior of clients
3146   for replicated servers using the same DNS name and reduces the
3147   likelihood of a user's experiencing failure in accessing sites which
3148   use that strategy.
3149</t>
3150</section>
3151
3152<section title="Proxies and Caching" anchor="attack.proxies">
3153<t>
3154   By their very nature, HTTP proxies are men-in-the-middle, and
3155   represent an opportunity for man-in-the-middle attacks. Compromise of
3156   the systems on which the proxies run can result in serious security
3157   and privacy problems. Proxies have access to security-related
3158   information, personal information about individual users and
3159   organizations, and proprietary information belonging to users and
3160   content providers. A compromised proxy, or a proxy implemented or
3161   configured without regard to security and privacy considerations,
3162   might be used in the commission of a wide range of potential attacks.
3163</t>
3164<t>
3165   Proxy operators should protect the systems on which proxies run as
3166   they would protect any system that contains or transports sensitive
3167   information. In particular, log information gathered at proxies often
3168   contains highly sensitive personal information, and/or information
3169   about organizations. Log information should be carefully guarded, and
3170   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
3171</t>
3172<t>
3173   Proxy implementors should consider the privacy and security
3174   implications of their design and coding decisions, and of the
3175   configuration options they provide to proxy operators (especially the
3176   default configuration).
3177</t>
3178<t>
3179   Users of a proxy need to be aware that they are no trustworthier than
3180   the people who run the proxy; HTTP itself cannot solve this problem.
3181</t>
3182<t>
3183   The judicious use of cryptography, when appropriate, may suffice to
3184   protect against a broad range of security and privacy attacks. Such
3185   cryptography is beyond the scope of the HTTP/1.1 specification.
3186</t>
3187</section>
3188
3189<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
3190<t>
3191   They exist. They are hard to defend against. Research continues.
3192   Beware.
3193</t>
3194</section>
3195</section>
3196
3197<section title="Acknowledgments" anchor="ack">
3198<t>
3199   HTTP has evolved considerably over the years. It has
3200   benefited from a large and active developer community--the many
3201   people who have participated on the www-talk mailing list--and it is
3202   that community which has been most responsible for the success of
3203   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
3204   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
3205   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
3206   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
3207   VanHeyningen deserve special recognition for their efforts in
3208   defining early aspects of the protocol.
3209</t>
3210<t>
3211   This document has benefited greatly from the comments of all those
3212   participating in the HTTP-WG. In addition to those already mentioned,
3213   the following individuals have contributed to this specification:
3214</t>
3215<t>
3216   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
3217   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
3218   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
3219   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
3220   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
3221   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
3222   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
3223   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
3224   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
3225   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
3226   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
3227   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
3228   Josh Cohen.
3229</t>
3230<t>
3231   Thanks to the "cave men" of Palo Alto. You know who you are.
3232</t>
3233<t>
3234   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
3235   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
3236   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
3237   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
3238   Larry Masinter for their help. And thanks go particularly to Jeff
3239   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3240</t>
3241<t>
3242   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3243   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3244   discovery of many of the problems that this document attempts to
3245   rectify.
3246</t>
3247<t>
3248   This specification makes heavy use of the augmented BNF and generic
3249   constructs defined by David H. Crocker for <xref target="RFC5234"/>. Similarly, it
3250   reuses many of the definitions provided by Nathaniel Borenstein and
3251   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
3252   specification will help reduce past confusion over the relationship
3253   between HTTP and Internet mail message formats.
3254</t>
3255</section>
3256
3257</middle>
3258<back>
3259
3260<references title="Normative References">
3261
3262<reference anchor="ISO-8859-1">
3263  <front>
3264    <title>
3265     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3266    </title>
3267    <author>
3268      <organization>International Organization for Standardization</organization>
3269    </author>
3270    <date year="1998"/>
3271  </front>
3272  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3273</reference>
3274
3275<reference anchor="Part2">
3276  <front>
3277    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3278    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3279      <organization abbrev="Day Software">Day Software</organization>
3280      <address><email>fielding@gbiv.com</email></address>
3281    </author>
3282    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3283      <organization>One Laptop per Child</organization>
3284      <address><email>jg@laptop.org</email></address>
3285    </author>
3286    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3287      <organization abbrev="HP">Hewlett-Packard Company</organization>
3288      <address><email>JeffMogul@acm.org</email></address>
3289    </author>
3290    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3291      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3292      <address><email>henrikn@microsoft.com</email></address>
3293    </author>
3294    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3295      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3296      <address><email>LMM@acm.org</email></address>
3297    </author>
3298    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3299      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3300      <address><email>paulle@microsoft.com</email></address>
3301    </author>
3302    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3303      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3304      <address><email>timbl@w3.org</email></address>
3305    </author>
3306    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3307      <organization abbrev="W3C">World Wide Web Consortium</organization>
3308      <address><email>ylafon@w3.org</email></address>
3309    </author>
3310    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3311      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3312      <address><email>julian.reschke@greenbytes.de</email></address>
3313    </author>
3314    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3315  </front>
3316  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3317  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3318</reference>
3319
3320<reference anchor="Part3">
3321  <front>
3322    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3323    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3324      <organization abbrev="Day Software">Day Software</organization>
3325      <address><email>fielding@gbiv.com</email></address>
3326    </author>
3327    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3328      <organization>One Laptop per Child</organization>
3329      <address><email>jg@laptop.org</email></address>
3330    </author>
3331    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3332      <organization abbrev="HP">Hewlett-Packard Company</organization>
3333      <address><email>JeffMogul@acm.org</email></address>
3334    </author>
3335    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3336      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3337      <address><email>henrikn@microsoft.com</email></address>
3338    </author>
3339    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3340      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3341      <address><email>LMM@acm.org</email></address>
3342    </author>
3343    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3344      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3345      <address><email>paulle@microsoft.com</email></address>
3346    </author>
3347    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3348      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3349      <address><email>timbl@w3.org</email></address>
3350    </author>
3351    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3352      <organization abbrev="W3C">World Wide Web Consortium</organization>
3353      <address><email>ylafon@w3.org</email></address>
3354    </author>
3355    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3356      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3357      <address><email>julian.reschke@greenbytes.de</email></address>
3358    </author>
3359    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3360  </front>
3361  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3362  <x:source href="p3-payload.xml" basename="p3-payload"/>
3363</reference>
3364
3365<reference anchor="Part5">
3366  <front>
3367    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3368    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3369      <organization abbrev="Day Software">Day Software</organization>
3370      <address><email>fielding@gbiv.com</email></address>
3371    </author>
3372    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3373      <organization>One Laptop per Child</organization>
3374      <address><email>jg@laptop.org</email></address>
3375    </author>
3376    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3377      <organization abbrev="HP">Hewlett-Packard Company</organization>
3378      <address><email>JeffMogul@acm.org</email></address>
3379    </author>
3380    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3381      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3382      <address><email>henrikn@microsoft.com</email></address>
3383    </author>
3384    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3385      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3386      <address><email>LMM@acm.org</email></address>
3387    </author>
3388    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3389      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3390      <address><email>paulle@microsoft.com</email></address>
3391    </author>
3392    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3393      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3394      <address><email>timbl@w3.org</email></address>
3395    </author>
3396    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3397      <organization abbrev="W3C">World Wide Web Consortium</organization>
3398      <address><email>ylafon@w3.org</email></address>
3399    </author>
3400    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3401      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3402      <address><email>julian.reschke@greenbytes.de</email></address>
3403    </author>
3404    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3405  </front>
3406  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3407  <x:source href="p5-range.xml" basename="p5-range"/>
3408</reference>
3409
3410<reference anchor="Part6">
3411  <front>
3412    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3413    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3414      <organization abbrev="Day Software">Day Software</organization>
3415      <address><email>fielding@gbiv.com</email></address>
3416    </author>
3417    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3418      <organization>One Laptop per Child</organization>
3419      <address><email>jg@laptop.org</email></address>
3420    </author>
3421    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3422      <organization abbrev="HP">Hewlett-Packard Company</organization>
3423      <address><email>JeffMogul@acm.org</email></address>
3424    </author>
3425    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3426      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3427      <address><email>henrikn@microsoft.com</email></address>
3428    </author>
3429    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3430      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3431      <address><email>LMM@acm.org</email></address>
3432    </author>
3433    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3434      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3435      <address><email>paulle@microsoft.com</email></address>
3436    </author>
3437    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3438      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3439      <address><email>timbl@w3.org</email></address>
3440    </author>
3441    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3442      <organization abbrev="W3C">World Wide Web Consortium</organization>
3443      <address><email>ylafon@w3.org</email></address>
3444    </author>
3445    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
3446      <organization />
3447      <address><email>mnot@mnot.net</email></address>
3448    </author>
3449    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3450      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3451      <address><email>julian.reschke@greenbytes.de</email></address>
3452    </author>
3453    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3454  </front>
3455  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3456  <x:source href="p6-cache.xml" basename="p6-cache"/>
3457</reference>
3458
3459<reference anchor="RFC5234">
3460  <front>
3461    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
3462    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
3463      <organization>Brandenburg InternetWorking</organization>
3464      <address>
3465      <postal>
3466      <street>675 Spruce Dr.</street>
3467      <city>Sunnyvale</city>
3468      <region>CA</region>
3469      <code>94086</code>
3470      <country>US</country></postal>
3471      <phone>+1.408.246.8253</phone>
3472      <email>dcrocker@bbiw.net</email></address> 
3473    </author>
3474    <author initials="P." surname="Overell" fullname="Paul Overell">
3475      <organization>THUS plc.</organization>
3476      <address>
3477      <postal>
3478      <street>1/2 Berkeley Square</street>
3479      <street>99 Berkely Street</street>
3480      <city>Glasgow</city>
3481      <code>G3 7HR</code>
3482      <country>UK</country></postal>
3483      <email>paul.overell@thus.net</email></address>
3484    </author>
3485    <date month="January" year="2008"/>
3486  </front>
3487  <seriesInfo name="STD" value="68"/>
3488  <seriesInfo name="RFC" value="5234"/>
3489</reference>
3490
3491<reference anchor="RFC2119">
3492  <front>
3493    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3494    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3495      <organization>Harvard University</organization>
3496      <address><email>sob@harvard.edu</email></address>
3497    </author>
3498    <date month="March" year="1997"/>
3499  </front>
3500  <seriesInfo name="BCP" value="14"/>
3501  <seriesInfo name="RFC" value="2119"/>
3502</reference>
3503
3504<reference anchor="RFC3986">
3505 <front>
3506  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
3507  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
3508    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3509    <address>
3510       <email>timbl@w3.org</email>
3511       <uri>http://www.w3.org/People/Berners-Lee/</uri>
3512    </address>
3513  </author>
3514  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
3515    <organization abbrev="Day Software">Day Software</organization>
3516    <address>
3517      <email>fielding@gbiv.com</email>
3518      <uri>http://roy.gbiv.com/</uri>
3519    </address>
3520  </author>
3521  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
3522    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
3523    <address>
3524      <email>LMM@acm.org</email>
3525      <uri>http://larry.masinter.net/</uri>
3526    </address>
3527  </author>
3528  <date month='January' year='2005'></date>
3529 </front>
3530 <seriesInfo name="RFC" value="3986"/>
3531 <seriesInfo name="STD" value="66"/>
3532</reference>
3533
3534<reference anchor="USASCII">
3535  <front>
3536    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3537    <author>
3538      <organization>American National Standards Institute</organization>
3539    </author>
3540    <date year="1986"/>
3541  </front>
3542  <seriesInfo name="ANSI" value="X3.4"/>
3543</reference>
3544
3545</references>
3546
3547<references title="Informative References">
3548
3549<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
3550  <front>
3551    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3552    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3553      <organization/>
3554    </author>
3555    <author initials="J." surname="Gettys" fullname="J. Gettys">
3556      <organization/>
3557    </author>
3558    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3559      <organization/>
3560    </author>
3561    <author initials="H." surname="Lie" fullname="H. Lie">
3562      <organization/>
3563    </author>
3564    <author initials="C." surname="Lilley" fullname="C. Lilley">
3565      <organization/>
3566    </author>
3567    <date year="1997" month="September"/>
3568  </front>
3569  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3570</reference>
3571
3572<reference anchor="Pad1995" target="http://portal.acm.org/citation.cfm?id=219094">
3573  <front>
3574    <title>Improving HTTP Latency</title>
3575    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3576      <organization/>
3577    </author>
3578    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3579      <organization/>
3580    </author>
3581    <date year="1995" month="December"/>
3582  </front>
3583  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3584</reference>
3585
3586<reference anchor="RFC1123">
3587  <front>
3588    <title>Requirements for Internet Hosts - Application and Support</title>
3589    <author initials="R." surname="Braden" fullname="Robert Braden">
3590      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3591      <address><email>Braden@ISI.EDU</email></address>
3592    </author>
3593    <date month="October" year="1989"/>
3594  </front>
3595  <seriesInfo name="STD" value="3"/>
3596  <seriesInfo name="RFC" value="1123"/>
3597</reference>
3598
3599<reference anchor="RFC1305">
3600  <front>
3601    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3602    <author initials="D." surname="Mills" fullname="David L. Mills">
3603      <organization>University of Delaware, Electrical Engineering Department</organization>
3604      <address><email>mills@udel.edu</email></address>
3605    </author>
3606    <date month="March" year="1992"/>
3607  </front>
3608  <seriesInfo name="RFC" value="1305"/>
3609</reference>
3610
3611<reference anchor="RFC1900">
3612  <front>
3613    <title>Renumbering Needs Work</title>
3614    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3615      <organization>CERN, Computing and Networks Division</organization>
3616      <address><email>brian@dxcoms.cern.ch</email></address>
3617    </author>
3618    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3619      <organization>cisco Systems</organization>
3620      <address><email>yakov@cisco.com</email></address>
3621    </author>
3622    <date month="February" year="1996"/>
3623  </front>
3624  <seriesInfo name="RFC" value="1900"/>
3625</reference>
3626
3627<reference anchor="RFC1945">
3628  <front>
3629    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3630    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3631      <organization>MIT, Laboratory for Computer Science</organization>
3632      <address><email>timbl@w3.org</email></address>
3633    </author>
3634    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3635      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3636      <address><email>fielding@ics.uci.edu</email></address>
3637    </author>
3638    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3639      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3640      <address><email>frystyk@w3.org</email></address>
3641    </author>
3642    <date month="May" year="1996"/>
3643  </front>
3644  <seriesInfo name="RFC" value="1945"/>
3645</reference>
3646
3647<reference anchor="RFC2045">
3648  <front>
3649    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3650    <author initials="N." surname="Freed" fullname="Ned Freed">
3651      <organization>Innosoft International, Inc.</organization>
3652      <address><email>ned@innosoft.com</email></address>
3653    </author>
3654    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3655      <organization>First Virtual Holdings</organization>
3656      <address><email>nsb@nsb.fv.com</email></address>
3657    </author>
3658    <date month="November" year="1996"/>
3659  </front>
3660  <seriesInfo name="RFC" value="2045"/>
3661</reference>
3662
3663<reference anchor="RFC2047">
3664  <front>
3665    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3666    <author initials="K." surname="Moore" fullname="Keith Moore">
3667      <organization>University of Tennessee</organization>
3668      <address><email>moore@cs.utk.edu</email></address>
3669    </author>
3670    <date month="November" year="1996"/>
3671  </front>
3672  <seriesInfo name="RFC" value="2047"/>
3673</reference>
3674
3675<reference anchor="RFC2068">
3676  <front>
3677    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3678    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3679      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3680      <address><email>fielding@ics.uci.edu</email></address>
3681    </author>
3682    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3683      <organization>MIT Laboratory for Computer Science</organization>
3684      <address><email>jg@w3.org</email></address>
3685    </author>
3686    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3687      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3688      <address><email>mogul@wrl.dec.com</email></address>
3689    </author>
3690    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3691      <organization>MIT Laboratory for Computer Science</organization>
3692      <address><email>frystyk@w3.org</email></address>
3693    </author>
3694    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3695      <organization>MIT Laboratory for Computer Science</organization>
3696      <address><email>timbl@w3.org</email></address>
3697    </author>
3698    <date month="January" year="1997"/>
3699  </front>
3700  <seriesInfo name="RFC" value="2068"/>
3701</reference>
3702
3703<reference anchor='RFC2109'>
3704  <front>
3705    <title>HTTP State Management Mechanism</title>
3706    <author initials='D.M.' surname='Kristol' fullname='David M. Kristol'>
3707      <organization>Bell Laboratories, Lucent Technologies</organization>
3708      <address><email>dmk@bell-labs.com</email></address>
3709    </author>
3710    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3711      <organization>Netscape Communications Corp.</organization>
3712      <address><email>montulli@netscape.com</email></address>
3713    </author>
3714    <date year='1997' month='February' />
3715  </front>
3716  <seriesInfo name='RFC' value='2109' />
3717</reference>
3718
3719<reference anchor="RFC2145">
3720  <front>
3721    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3722    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3723      <organization>Western Research Laboratory</organization>
3724      <address><email>mogul@wrl.dec.com</email></address>
3725    </author>
3726    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3727      <organization>Department of Information and Computer Science</organization>
3728      <address><email>fielding@ics.uci.edu</email></address>
3729    </author>
3730    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3731      <organization>MIT Laboratory for Computer Science</organization>
3732      <address><email>jg@w3.org</email></address>
3733    </author>
3734    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3735      <organization>W3 Consortium</organization>
3736      <address><email>frystyk@w3.org</email></address>
3737    </author>
3738    <date month="May" year="1997"/>
3739  </front>
3740  <seriesInfo name="RFC" value="2145"/>
3741</reference>
3742
3743<reference anchor="RFC2616">
3744  <front>
3745    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3746    <author initials="R." surname="Fielding" fullname="R. Fielding">
3747      <organization>University of California, Irvine</organization>
3748      <address><email>fielding@ics.uci.edu</email></address>
3749    </author>
3750    <author initials="J." surname="Gettys" fullname="J. Gettys">
3751      <organization>W3C</organization>
3752      <address><email>jg@w3.org</email></address>
3753    </author>
3754    <author initials="J." surname="Mogul" fullname="J. Mogul">
3755      <organization>Compaq Computer Corporation</organization>
3756      <address><email>mogul@wrl.dec.com</email></address>
3757    </author>
3758    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3759      <organization>MIT Laboratory for Computer Science</organization>
3760      <address><email>frystyk@w3.org</email></address>
3761    </author>
3762    <author initials="L." surname="Masinter" fullname="L. Masinter">
3763      <organization>Xerox Corporation</organization>
3764      <address><email>masinter@parc.xerox.com</email></address>
3765    </author>
3766    <author initials="P." surname="Leach" fullname="P. Leach">
3767      <organization>Microsoft Corporation</organization>
3768      <address><email>paulle@microsoft.com</email></address>
3769    </author>
3770    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3771      <organization>W3C</organization>
3772      <address><email>timbl@w3.org</email></address>
3773    </author>
3774    <date month="June" year="1999"/>
3775  </front>
3776  <seriesInfo name="RFC" value="2616"/>
3777</reference>
3778
3779<reference anchor='RFC2818'>
3780  <front>
3781    <title>HTTP Over TLS</title>
3782    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
3783      <organization>RTFM, Inc.</organization>
3784      <address><email>ekr@rtfm.com</email></address>
3785    </author>
3786    <date year='2000' month='May' />
3787  </front>
3788  <seriesInfo name='RFC' value='2818' />
3789</reference>
3790
3791<reference anchor='RFC2965'>
3792  <front>
3793    <title>HTTP State Management Mechanism</title>
3794    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
3795      <organization>Bell Laboratories, Lucent Technologies</organization>
3796      <address><email>dmk@bell-labs.com</email></address>
3797    </author>
3798    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3799      <organization>Epinions.com, Inc.</organization>
3800      <address><email>lou@montulli.org</email></address>
3801    </author>
3802    <date year='2000' month='October' />
3803  </front>
3804  <seriesInfo name='RFC' value='2965' />
3805</reference>
3806
3807<reference anchor='RFC3864'>
3808  <front>
3809    <title>Registration Procedures for Message Header Fields</title>
3810    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
3811      <organization>Nine by Nine</organization>
3812      <address><email>GK-IETF@ninebynine.org</email></address>
3813    </author>
3814    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
3815      <organization>BEA Systems</organization>
3816      <address><email>mnot@pobox.com</email></address>
3817    </author>
3818    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
3819      <organization>HP Labs</organization>
3820      <address><email>JeffMogul@acm.org</email></address>
3821    </author>
3822    <date year='2004' month='September' />
3823  </front>
3824  <seriesInfo name='BCP' value='90' />
3825  <seriesInfo name='RFC' value='3864' />
3826</reference>
3827
3828<reference anchor="RFC4288">
3829  <front>
3830    <title>Media Type Specifications and Registration Procedures</title>
3831    <author initials="N." surname="Freed" fullname="N. Freed">
3832      <organization>Sun Microsystems</organization>
3833      <address>
3834        <email>ned.freed@mrochek.com</email>
3835      </address>
3836    </author>
3837    <author initials="J." surname="Klensin" fullname="J. Klensin">
3838      <organization/>
3839      <address>
3840        <email>klensin+ietf@jck.com</email>
3841      </address>
3842    </author>
3843    <date year="2005" month="December"/>
3844  </front>
3845  <seriesInfo name="BCP" value="13"/>
3846  <seriesInfo name="RFC" value="4288"/>
3847</reference>
3848
3849<reference anchor='RFC4395'>
3850  <front>
3851    <title>Guidelines and Registration Procedures for New URI Schemes</title>
3852    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
3853      <organization>AT&amp;T Laboratories</organization>
3854      <address>
3855        <email>tony+urireg@maillennium.att.com</email>
3856      </address>
3857    </author>
3858    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
3859      <organization>Qualcomm, Inc.</organization>
3860      <address>
3861        <email>hardie@qualcomm.com</email>
3862      </address>
3863    </author>
3864    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
3865      <organization>Adobe Systems</organization>
3866      <address>
3867        <email>LMM@acm.org</email>
3868      </address>
3869    </author>
3870    <date year='2006' month='February' />
3871  </front>
3872  <seriesInfo name='BCP' value='115' />
3873  <seriesInfo name='RFC' value='4395' />
3874</reference>
3875
3876<reference anchor="RFC5322">
3877  <front>
3878    <title>Internet Message Format</title>
3879    <author initials="P." surname="Resnick" fullname="P. Resnick">
3880      <organization>Qualcomm Incorporated</organization>
3881    </author>
3882    <date year="2008" month="October"/>
3883  </front> 
3884  <seriesInfo name="RFC" value="5322"/>
3885</reference>
3886
3887<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
3888  <front>
3889    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
3890    <author initials="D." surname="Kristol" fullname="David M. Kristol">
3891      <organization/>
3892    </author>
3893    <date year="2001" month="November"/>
3894  </front>
3895  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
3896</reference>
3897
3898<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
3899  <front>
3900  <title>Analysis of HTTP Performance Problems</title>
3901  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3902    <organization/>
3903  </author>
3904  <date/>
3905  </front>
3906</reference>
3907
3908<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
3909  <front>
3910  <title>Analysis of HTTP Performance</title>
3911  <author initials="J." surname="Touch" fullname="Joe Touch">
3912    <organization>USC/Information Sciences Institute</organization>
3913    <address><email>touch@isi.edu</email></address>
3914  </author>
3915  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3916    <organization>USC/Information Sciences Institute</organization>
3917    <address><email>johnh@isi.edu</email></address>
3918  </author>
3919  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3920    <organization>USC/Information Sciences Institute</organization>
3921    <address><email>katia@isi.edu</email></address>
3922  </author>
3923  <date year="1998" month="Aug"/>
3924  </front>
3925  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3926  <annotation>(original report dated Aug. 1996)</annotation>
3927</reference>
3928
3929</references>
3930
3931
3932<section title="Tolerant Applications" anchor="tolerant.applications">
3933<t>
3934   Although this document specifies the requirements for the generation
3935   of HTTP/1.1 messages, not all applications will be correct in their
3936   implementation. We therefore recommend that operational applications
3937   be tolerant of deviations whenever those deviations can be
3938   interpreted unambiguously.
3939</t>
3940<t>
3941   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3942   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3943   accept any amount of WSP characters between fields, even though
3944   only a single SP is required.
3945</t>
3946<t>
3947   The line terminator for message-header fields is the sequence CRLF.
3948   However, we recommend that applications, when parsing such headers,
3949   recognize a single LF as a line terminator and ignore the leading CR.
3950</t>
3951<t>
3952   The character set of an entity-body &SHOULD; be labeled as the lowest
3953   common denominator of the character codes used within that body, with
3954   the exception that not labeling the entity is preferred over labeling
3955   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3956</t>
3957<t>
3958   Additional rules for requirements on parsing and encoding of dates
3959   and other potential problems with date encodings include:
3960</t>
3961<t>
3962  <list style="symbols">
3963     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3964        which appears to be more than 50 years in the future is in fact
3965        in the past (this helps solve the "year 2000" problem).</t>
3966
3967     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3968        Expires date as earlier than the proper value, but &MUST-NOT;
3969        internally represent a parsed Expires date as later than the
3970        proper value.</t>
3971
3972     <t>All expiration-related calculations &MUST; be done in GMT. The
3973        local time zone &MUST-NOT; influence the calculation or comparison
3974        of an age or expiration time.</t>
3975
3976     <t>If an HTTP header incorrectly carries a date value with a time
3977        zone other than GMT, it &MUST; be converted into GMT using the
3978        most conservative possible conversion.</t>
3979  </list>
3980</t>
3981</section>
3982
3983<section title="Compatibility with Previous Versions" anchor="compatibility">
3984<t>
3985   HTTP has been in use by the World-Wide Web global information initiative
3986   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
3987   was a simple protocol for hypertext data transfer across the Internet
3988   with only a single method and no metadata.
3989   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
3990   methods and MIME-like messaging that could include metadata about the data
3991   transferred and modifiers on the request/response semantics. However,
3992   HTTP/1.0 did not sufficiently take into consideration the effects of
3993   hierarchical proxies, caching, the need for persistent connections, or
3994   name-based virtual hosts. The proliferation of incompletely-implemented
3995   applications calling themselves "HTTP/1.0" further necessitated a
3996   protocol version change in order for two communicating applications
3997   to determine each other's true capabilities.
3998</t>
3999<t>
4000   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4001   requirements that enable reliable implementations, adding only
4002   those new features that will either be safely ignored by an HTTP/1.0
4003   recipient or only sent when communicating with a party advertising
4004   compliance with HTTP/1.1.
4005</t>
4006<t>
4007   It is beyond the scope of a protocol specification to mandate
4008   compliance with previous versions. HTTP/1.1 was deliberately
4009   designed, however, to make supporting previous versions easy. It is
4010   worth noting that, at the time of composing this specification
4011   (1996), we would expect commercial HTTP/1.1 servers to:
4012  <list style="symbols">
4013     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
4014        1.1 requests;</t>
4015
4016     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
4017        1.1;</t>
4018
4019     <t>respond appropriately with a message in the same major version
4020        used by the client.</t>
4021  </list>
4022</t>
4023<t>
4024   And we would expect HTTP/1.1 clients to:
4025  <list style="symbols">
4026     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
4027        responses;</t>
4028
4029     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
4030        1.1.</t>
4031  </list>
4032</t>
4033<t>
4034   For most implementations of HTTP/1.0, each connection is established
4035   by the client prior to the request and closed by the server after
4036   sending the response. Some implementations implement the Keep-Alive
4037   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
4038</t>
4039
4040<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4041<t>
4042   This section summarizes major differences between versions HTTP/1.0
4043   and HTTP/1.1.
4044</t>
4045
4046<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">
4047<t>
4048   The requirements that clients and servers support the Host request-header,
4049   report an error if the Host request-header (<xref target="header.host"/>) is
4050   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4051   are among the most important changes defined by this
4052   specification.
4053</t>
4054<t>
4055   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4056   addresses and servers; there was no other established mechanism for
4057   distinguishing the intended server of a request than the IP address
4058   to which that request was directed. The changes outlined above will
4059   allow the Internet, once older HTTP clients are no longer common, to
4060   support multiple Web sites from a single IP address, greatly
4061   simplifying large operational Web servers, where allocation of many
4062   IP addresses to a single host has created serious problems. The
4063   Internet will also be able to recover the IP addresses that have been
4064   allocated for the sole purpose of allowing special-purpose domain
4065   names to be used in root-level HTTP URLs. Given the rate of growth of
4066   the Web, and the number of servers already deployed, it is extremely
4067   important that all implementations of HTTP (including updates to
4068   existing HTTP/1.0 applications) correctly implement these
4069   requirements:
4070  <list style="symbols">
4071     <t>Both clients and servers &MUST; support the Host request-header.</t>
4072
4073     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
4074
4075     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
4076        request does not include a Host request-header.</t>
4077
4078     <t>Servers &MUST; accept absolute URIs.</t>
4079  </list>
4080</t>
4081</section>
4082</section>
4083
4084<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4085<t>
4086   Some clients and servers might wish to be compatible with some
4087   previous implementations of persistent connections in HTTP/1.0
4088   clients and servers. Persistent connections in HTTP/1.0 are
4089   explicitly negotiated as they are not the default behavior. HTTP/1.0
4090   experimental implementations of persistent connections are faulty,
4091   and the new facilities in HTTP/1.1 are designed to rectify these
4092   problems. The problem was that some existing 1.0 clients may be
4093   sending Keep-Alive to a proxy server that doesn't understand
4094   Connection, which would then erroneously forward it to the next
4095   inbound server, which would establish the Keep-Alive connection and
4096   result in a hung HTTP/1.0 proxy waiting for the close on the
4097   response. The result is that HTTP/1.0 clients must be prevented from
4098   using Keep-Alive when talking to proxies.
4099</t>
4100<t>
4101   However, talking to proxies is the most important use of persistent
4102   connections, so that prohibition is clearly unacceptable. Therefore,
4103   we need some other mechanism for indicating a persistent connection
4104   is desired, which is safe to use even when talking to an old proxy
4105   that ignores Connection. Persistent connections are the default for
4106   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
4107   declaring non-persistence. See <xref target="header.connection"/>.
4108</t>
4109<t>
4110   The original HTTP/1.0 form of persistent connections (the Connection:
4111   Keep-Alive and Keep-Alive header) is documented in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
4112</t>
4113</section>
4114
4115<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
4116<t>
4117   This specification has been carefully audited to correct and
4118   disambiguate key word usage; RFC 2068 had many problems in respect to
4119   the conventions laid out in <xref target="RFC2119"/>.
4120</t>
4121<t>
4122   Transfer-coding and message lengths all interact in ways that
4123   required fixing exactly when chunked encoding is used (to allow for
4124   transfer encoding that may not be self delimiting); it was important
4125   to straighten out exactly how message lengths are computed. (Sections
4126   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
4127   <xref target="header.content-length" format="counter"/>,
4128   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4129</t>
4130<t>
4131   The use and interpretation of HTTP version numbers has been clarified
4132   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4133   version they support to deal with problems discovered in HTTP/1.0
4134   implementations (<xref target="http.version"/>)
4135</t>
4136<t>
4137   Quality Values of zero should indicate that "I don't want something"
4138   to allow clients to refuse a representation. (<xref target="quality.values"/>)
4139</t>
4140<t>
4141   Transfer-coding had significant problems, particularly with
4142   interactions with chunked encoding. The solution is that transfer-codings
4143   become as full fledged as content-codings. This involves
4144   adding an IANA registry for transfer-codings (separate from content
4145   codings), a new header field (TE) and enabling trailer headers in the
4146   future. Transfer encoding is a major performance benefit, so it was
4147   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4148   interoperability problem that could have occurred due to interactions
4149   between authentication trailers, chunked encoding and HTTP/1.0
4150   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4151   and <xref target="header.te" format="counter"/>)
4152</t>
4153</section>
4154
4155<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4156<t>
4157  Empty list elements in list productions have been deprecated.
4158  (<xref target="notation.abnf"/>)
4159</t>
4160<t>
4161  Rules about implicit linear whitespace between certain grammar productions
4162  have been removed; now it's only allowed when specifically pointed out
4163  in the ABNF. The NUL character is no longer allowed in comment and quoted-string
4164  text. The quoted-pair rule no longer allows escaping NUL, CR or LF.
4165  Non-ASCII content in header fields and reason phrase has been obsoleted and
4166  made opaque (the TEXT rule was removed)
4167  (<xref target="basic.rules"/>)
4168</t>
4169<t>
4170  Clarify that HTTP-Version is case sensitive.
4171  (<xref target="http.version"/>)
4172</t>
4173<t>
4174  Remove reference to non-existant identity transfer-coding value tokens.
4175  (Sections <xref format="counter" target="transfer.codings"/> and
4176  <xref format="counter" target="message.length"/>)
4177</t>
4178<t>
4179  Clarification that the chunk length does not include
4180  the count of the octets in the chunk header and trailer.
4181  (<xref target="chunked.transfer.encoding"/>)
4182</t>
4183<t>
4184  Require that invalid whitespace around field-names be rejected.
4185  (<xref target="message.headers"/>)
4186</t>
4187<t>
4188  Update use of abs_path production from RFC1808 to the path-absolute + query
4189  components of RFC3986.
4190  (<xref target="request-target"/>)
4191</t>
4192<t>
4193  Clarify exactly when close connection options must be sent.
4194  (<xref target="header.connection"/>)
4195</t>
4196</section>
4197</section>
4198
4199<section title="Terminology" anchor="terminology">
4200<t>
4201   This specification uses a number of terms to refer to the roles
4202   played by participants in, and objects of, the HTTP communication.
4203</t>
4204<t>
4205  <iref item="cache"/>
4206  <x:dfn>cache</x:dfn>
4207  <list>
4208    <t>
4209      A program's local store of response messages and the subsystem
4210      that controls its message storage, retrieval, and deletion. A
4211      cache stores cacheable responses in order to reduce the response
4212      time and network bandwidth consumption on future, equivalent
4213      requests. Any client or server may include a cache, though a cache
4214      cannot be used by a server that is acting as a tunnel.
4215    </t>
4216  </list>
4217</t>
4218<t>
4219  <iref item="cacheable"/>
4220  <x:dfn>cacheable</x:dfn>
4221  <list>
4222    <t>
4223      A response is cacheable if a cache is allowed to store a copy of
4224      the response message for use in answering subsequent requests. The
4225      rules for determining the cacheability of HTTP responses are
4226      defined in &caching;. Even if a resource is cacheable, there may
4227      be additional constraints on whether a cache can use the cached
4228      copy for a particular request.
4229    </t>
4230  </list>
4231</t>
4232<t>
4233  <iref item="client"/>
4234  <x:dfn>client</x:dfn>
4235  <list>
4236    <t>
4237      A program that establishes connections for the purpose of sending
4238      requests.
4239    </t>
4240  </list>
4241</t>
4242<t>
4243  <iref item="connection"/>
4244  <x:dfn>connection</x:dfn>
4245  <list>
4246    <t>
4247      A transport layer virtual circuit established between two programs
4248      for the purpose of communication.
4249    </t>
4250  </list>
4251</t>
4252<t>
4253  <iref item="content negotiation"/>
4254  <x:dfn>content negotiation</x:dfn>
4255  <list>
4256    <t>
4257      The mechanism for selecting the appropriate representation when
4258      servicing a request, as described in &content.negotiation;. The
4259      representation of entities in any response can be negotiated
4260      (including error responses).
4261    </t>
4262  </list>
4263</t>
4264<t>
4265  <iref item="entity"/>
4266  <x:dfn>entity</x:dfn>
4267  <list>
4268    <t>
4269      The information transferred as the payload of a request or
4270      response. An entity consists of metadata in the form of
4271      entity-header fields and content in the form of an entity-body, as
4272      described in &entity;.
4273    </t>
4274  </list>
4275</t>
4276<t>
4277  <iref item="gateway"/>
4278  <x:dfn>gateway</x:dfn>
4279  <list>
4280    <t>
4281      A server which acts as an intermediary for some other server.
4282      Unlike a proxy, a gateway receives requests as if it were the
4283      origin server for the requested resource; the requesting client
4284      may not be aware that it is communicating with a gateway.
4285    </t>
4286  </list>
4287</t>
4288<t>
4289  <iref item="inbound"/>
4290  <iref item="outbound"/>
4291  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
4292  <list>
4293    <t>
4294      Inbound and outbound refer to the request and response paths for
4295      messages: "inbound" means "traveling toward the origin server",
4296      and "outbound" means "traveling toward the user agent"
4297    </t>
4298  </list>
4299</t>
4300<t>
4301  <iref item="message"/>
4302  <x:dfn>message</x:dfn>
4303  <list>
4304    <t>
4305      The basic unit of HTTP communication, consisting of a structured
4306      sequence of octets matching the syntax defined in <xref target="http.message"/> and
4307      transmitted via the connection.
4308    </t>
4309  </list>
4310</t>
4311<t>
4312  <iref item="origin server"/>
4313  <x:dfn>origin server</x:dfn>
4314  <list>
4315    <t>
4316      The server on which a given resource resides or is to be created.
4317    </t>
4318  </list>
4319</t>
4320<t>
4321  <iref item="proxy"/>
4322  <x:dfn>proxy</x:dfn>
4323  <list>
4324    <t>
4325      An intermediary program which acts as both a server and a client
4326      for the purpose of making requests on behalf of other clients.
4327      Requests are serviced internally or by passing them on, with
4328      possible translation, to other servers. A proxy &MUST; implement
4329      both the client and server requirements of this specification. A
4330      "transparent proxy" is a proxy that does not modify the request or
4331      response beyond what is required for proxy authentication and
4332      identification. A "non-transparent proxy" is a proxy that modifies
4333      the request or response in order to provide some added service to
4334      the user agent, such as group annotation services, media type
4335      transformation, protocol reduction, or anonymity filtering. Except
4336      where either transparent or non-transparent behavior is explicitly
4337      stated, the HTTP proxy requirements apply to both types of
4338      proxies.
4339    </t>
4340  </list>
4341</t>
4342<t>
4343  <iref item="request"/>
4344  <x:dfn>request</x:dfn>
4345  <list>
4346    <t>
4347      An HTTP request message, as defined in <xref target="request"/>.
4348    </t>
4349  </list>
4350</t>
4351<t>
4352  <iref item="response"/>
4353  <x:dfn>response</x:dfn>
4354  <list>
4355    <t>
4356      An HTTP response message, as defined in <xref target="response"/>.
4357    </t>
4358  </list>
4359</t>
4360<t>
4361  <iref item="representation"/>
4362  <x:dfn>representation</x:dfn>
4363  <list>
4364    <t>
4365      An entity included with a response that is subject to content
4366      negotiation, as described in &content.negotiation;. There may exist multiple
4367      representations associated with a particular response status.
4368    </t>
4369  </list>
4370</t>
4371<t>
4372  <iref item="server"/>
4373  <x:dfn>server</x:dfn>
4374  <list>
4375    <t>
4376      An application program that accepts connections in order to
4377      service requests by sending back responses. Any given program may
4378      be capable of being both a client and a server; our use of these
4379      terms refers only to the role being performed by the program for a
4380      particular connection, rather than to the program's capabilities
4381      in general. Likewise, any server may act as an origin server,
4382      proxy, gateway, or tunnel, switching behavior based on the nature
4383      of each request.
4384    </t>
4385  </list>
4386</t>
4387<t>
4388  <iref item="tunnel"/>
4389  <x:dfn>tunnel</x:dfn>
4390  <list>
4391    <t>
4392      An intermediary program which is acting as a blind relay between
4393      two connections. Once active, a tunnel is not considered a party
4394      to the HTTP communication, though the tunnel may have been
4395      initiated by an HTTP request. The tunnel ceases to exist when both
4396      ends of the relayed connections are closed.
4397    </t>
4398  </list>
4399</t>
4400<t>
4401  <iref item="upstream"/>
4402  <iref item="downstream"/>
4403  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
4404  <list>
4405    <t>
4406      Upstream and downstream describe the flow of a message: all
4407      messages flow from upstream to downstream.
4408    </t>
4409  </list>
4410</t>
4411<t>
4412  <iref item="user agent"/>
4413  <x:dfn>user agent</x:dfn>
4414  <list>
4415    <t>
4416      The client which initiates a request. These are often browsers,
4417      editors, spiders (web-traversing robots), or other end user tools.
4418    </t>
4419  </list>
4420</t>
4421<t>
4422  <iref item="variant"/>
4423  <x:dfn>variant</x:dfn>
4424  <list>
4425    <t>
4426      A resource may have one, or more than one, representation(s)
4427      associated with it at any given instant. Each of these
4428      representations is termed a `variant'.  Use of the term `variant'
4429      does not necessarily imply that the resource is subject to content
4430      negotiation.
4431    </t>
4432  </list>
4433</t>
4434</section>
4435
4436<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4437<figure>
4438<artwork type="abnf" name="p1-messaging.parsed-abnf">
4439<x:ref>BWS</x:ref> = OWS
4440
4441<x:ref>Cache-Control</x:ref> = &lt;Cache-Control, defined in [Part6], Section 3.4&gt;
4442<x:ref>Chunked-Body</x:ref> = *chunk last-chunk trailer-part CRLF
4443<x:ref>Connection</x:ref> = "Connection:" OWS Connection-v
4444<x:ref>Connection-v</x:ref> = *( "," OWS ) connection-token *( OWS "," [ OWS
4445 connection-token ] )
4446<x:ref>Content-Length</x:ref> = "Content-Length:" OWS 1*Content-Length-v
4447<x:ref>Content-Length-v</x:ref> = 1*DIGIT
4448
4449<x:ref>Date</x:ref> = "Date:" OWS Date-v
4450<x:ref>Date-v</x:ref> = HTTP-date
4451
4452<x:ref>GMT</x:ref> = %x47.4D.54 ; GMT
4453
4454<x:ref>HTTP-Prot-Name</x:ref> = %x48.54.54.50 ; HTTP
4455<x:ref>HTTP-Version</x:ref> = HTTP-Prot-Name "/" 1*DIGIT "." 1*DIGIT
4456<x:ref>HTTP-date</x:ref> = rfc1123-date / obs-date
4457<x:ref>HTTP-message</x:ref> = Request / Response
4458<x:ref>Host</x:ref> = "Host:" OWS Host-v
4459<x:ref>Host-v</x:ref> = uri-host [ ":" port ]
4460
4461<x:ref>Method</x:ref> = token
4462
4463<x:ref>OWS</x:ref> = *( [ obs-fold ] WSP )
4464
4465<x:ref>Pragma</x:ref> = &lt;Pragma, defined in [Part6], Section 3.4&gt;
4466
4467<x:ref>RWS</x:ref> = 1*( [ obs-fold ] WSP )
4468<x:ref>Reason-Phrase</x:ref> = *( WSP / VCHAR / obs-text )
4469<x:ref>Request</x:ref> = Request-Line *( ( general-header / request-header /
4470 entity-header ) CRLF ) CRLF [ message-body ]
4471<x:ref>Request-Line</x:ref> = Method SP request-target SP HTTP-Version CRLF
4472<x:ref>Response</x:ref> = Status-Line *( ( general-header / response-header /
4473 entity-header ) CRLF ) CRLF [ message-body ]
4474
4475<x:ref>Status-Code</x:ref> = 3DIGIT
4476<x:ref>Status-Line</x:ref> = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
4477
4478<x:ref>TE</x:ref> = "TE:" OWS TE-v
4479<x:ref>TE-v</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
4480<x:ref>Trailer</x:ref> = "Trailer:" OWS Trailer-v
4481<x:ref>Trailer-v</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
4482<x:ref>Transfer-Encoding</x:ref> = "Transfer-Encoding:" OWS Transfer-Encoding-v
4483<x:ref>Transfer-Encoding-v</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
4484 transfer-coding ] )
4485
4486<x:ref>URI</x:ref> = &lt;URI, defined in [RFC3986], Section 3&gt;
4487<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
4488<x:ref>Upgrade</x:ref> = "Upgrade:" OWS Upgrade-v
4489<x:ref>Upgrade-v</x:ref> = *( "," OWS ) product *( OWS "," [ OWS product ] )
4490
4491<x:ref>Via</x:ref> = "Via:" OWS Via-v
4492<x:ref>Via-v</x:ref> = *( "," OWS ) received-protocol RWS received-by [ RWS comment
4493 ] *( OWS "," [ OWS received-protocol RWS received-by [ RWS comment ]
4494 ] )
4495
4496<x:ref>Warning</x:ref> = &lt;Warning, defined in [Part6], Section 3.6&gt;
4497
4498<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
4499<x:ref>asctime-date</x:ref> = day-name SP date3 SP time-of-day SP year
4500<x:ref>attribute</x:ref> = token
4501<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
4502
4503<x:ref>chunk</x:ref> = chunk-size *WSP [ chunk-ext ] CRLF chunk-data CRLF
4504<x:ref>chunk-data</x:ref> = 1*OCTET
4505<x:ref>chunk-ext</x:ref> = *( ";" *WSP chunk-ext-name [ "=" chunk-ext-val ] *WSP )
4506<x:ref>chunk-ext-name</x:ref> = token
4507<x:ref>chunk-ext-val</x:ref> = token / quoted-string
4508<x:ref>chunk-size</x:ref> = 1*HEXDIG
4509<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
4510<x:ref>connection-token</x:ref> = token
4511<x:ref>ctext</x:ref> = OWS / %x21-27 ; '!'-'''
4512 / %x2A-5B ; '*'-'['
4513 / %x5D-7E ; ']'-'~'
4514 / obs-text
4515
4516<x:ref>date1</x:ref> = day SP month SP year
4517<x:ref>date2</x:ref> = day "-" month "-" 2DIGIT
4518<x:ref>date3</x:ref> = month SP ( 2DIGIT / ( SP DIGIT ) )
4519<x:ref>day</x:ref> = 2DIGIT
4520<x:ref>day-name</x:ref> = %x4D.6F.6E ; Mon
4521 / %x54.75.65 ; Tue
4522 / %x57.65.64 ; Wed
4523 / %x54.68.75 ; Thu
4524 / %x46.72.69 ; Fri
4525 / %x53.61.74 ; Sat
4526 / %x53.75.6E ; Sun
4527<x:ref>day-name-l</x:ref> = %x4D.6F.6E.64.61.79 ; Monday
4528 / %x54.75.65.73.64.61.79 ; Tuesday
4529 / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
4530 / %x54.68.75.72.73.64.61.79 ; Thursday
4531 / %x46.72.69.64.61.79 ; Friday
4532 / %x53.61.74.75.72.64.61.79 ; Saturday
4533 / %x53.75.6E.64.61.79 ; Sunday
4534
4535<x:ref>entity-body</x:ref> = &lt;entity-body, defined in [Part3], Section 3.2&gt;
4536<x:ref>entity-header</x:ref> = &lt;entity-header, defined in [Part3], Section 3.1&gt;
4537
4538<x:ref>field-content</x:ref> = *( WSP / VCHAR / obs-text )
4539<x:ref>field-name</x:ref> = token
4540<x:ref>field-value</x:ref> = *( field-content / OWS )
4541<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
4542
4543<x:ref>general-header</x:ref> = Cache-Control / Connection / Date / Pragma / Trailer
4544 / Transfer-Encoding / Upgrade / Via / Warning
4545<x:ref>generic-message</x:ref> = start-line *( message-header CRLF ) CRLF [
4546 message-body ]
4547
4548<x:ref>hour</x:ref> = 2DIGIT
4549<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
4550<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
4551
4552<x:ref>last-chunk</x:ref> = 1*"0" *WSP [ chunk-ext ] CRLF
4553
4554<x:ref>message-body</x:ref> = entity-body /
4555 &lt;entity-body encoded as per Transfer-Encoding&gt;
4556<x:ref>message-header</x:ref> = field-name ":" OWS [ field-value ] OWS
4557<x:ref>minute</x:ref> = 2DIGIT
4558<x:ref>month</x:ref> = %x4A.61.6E ; Jan
4559 / %x46.65.62 ; Feb
4560 / %x4D.61.72 ; Mar
4561 / %x41.70.72 ; Apr
4562 / %x4D.61.79 ; May
4563 / %x4A.75.6E ; Jun
4564 / %x4A.75.6C ; Jul
4565 / %x41.75.67 ; Aug
4566 / %x53.65.70 ; Sep
4567 / %x4F.63.74 ; Oct
4568 / %x4E.6F.76 ; Nov
4569 / %x44.65.63 ; Dec
4570
4571<x:ref>obs-date</x:ref> = rfc850-date / asctime-date
4572<x:ref>obs-fold</x:ref> = CRLF
4573<x:ref>obs-text</x:ref> = %x80-FF
4574
4575<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
4576<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
4577<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
4578<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
4579<x:ref>product</x:ref> = token [ "/" product-version ]
4580<x:ref>product-version</x:ref> = token
4581<x:ref>protocol-name</x:ref> = token
4582<x:ref>protocol-version</x:ref> = token
4583<x:ref>pseudonym</x:ref> = token
4584
4585<x:ref>qdtext</x:ref> = OWS / "!" / %x23-5B ; '#'-'['
4586 / %x5D-7E ; ']'-'~'
4587 / obs-text
4588<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
4589<x:ref>quoted-pair</x:ref> = "\" quoted-text
4590<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
4591<x:ref>quoted-text</x:ref> = %x01-09 / %x0B-0C / %x0E-FF
4592<x:ref>qvalue</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
4593
4594<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
4595<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
4596<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
4597<x:ref>request-header</x:ref> = &lt;request-header, defined in [Part2], Section 3&gt;
4598<x:ref>request-target</x:ref> = "*" / absolute-URI / ( path-absolute [ "?" query ] )
4599 / authority
4600<x:ref>response-header</x:ref> = &lt;response-header, defined in [Part2], Section 5&gt;
4601<x:ref>rfc1123-date</x:ref> = day-name "," SP date1 SP time-of-day SP GMT
4602<x:ref>rfc850-date</x:ref> = day-name-l "," SP date2 SP time-of-day SP GMT
4603
4604<x:ref>second</x:ref> = 2DIGIT
4605<x:ref>start-line</x:ref> = Request-Line / Status-Line
4606
4607<x:ref>t-codings</x:ref> = "trailers" / ( transfer-extension [ te-params ] )
4608<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
4609 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
4610<x:ref>te-ext</x:ref> = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
4611<x:ref>te-params</x:ref> = OWS ";" OWS "q=" qvalue *te-ext
4612<x:ref>time-of-day</x:ref> = hour ":" minute ":" second
4613<x:ref>token</x:ref> = 1*tchar
4614<x:ref>trailer-part</x:ref> = *( entity-header CRLF )
4615<x:ref>transfer-coding</x:ref> = "chunked" / transfer-extension
4616<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
4617<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
4618
4619<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
4620
4621<x:ref>value</x:ref> = token / quoted-string
4622
4623<x:ref>year</x:ref> = 4DIGIT
4624</artwork>
4625</figure>
4626<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
4627; Chunked-Body defined but not used
4628; Content-Length defined but not used
4629; HTTP-message defined but not used
4630; Host defined but not used
4631; TE defined but not used
4632; URI defined but not used
4633; URI-reference defined but not used
4634; fragment defined but not used
4635; generic-message defined but not used
4636; http-URI defined but not used
4637; https-URI defined but not used
4638; partial-URI defined but not used
4639</artwork></figure></section>
4640
4641<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
4642
4643<section title="Since RFC2616">
4644<t>
4645  Extracted relevant partitions from <xref target="RFC2616"/>.
4646</t>
4647</section>
4648
4649<section title="Since draft-ietf-httpbis-p1-messaging-00">
4650<t>
4651  Closed issues:
4652  <list style="symbols"> 
4653    <t>
4654      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
4655      "HTTP Version should be case sensitive"
4656      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
4657    </t>
4658    <t>
4659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
4660      "'unsafe' characters"
4661      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
4662    </t>
4663    <t>
4664      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
4665      "Chunk Size Definition"
4666      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
4667    </t>
4668    <t>
4669      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
4670      "Message Length"
4671      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
4672    </t>
4673    <t>
4674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
4675      "Media Type Registrations"
4676      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
4677    </t>
4678    <t>
4679      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
4680      "URI includes query"
4681      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
4682    </t>
4683    <t>
4684      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
4685      "No close on 1xx responses"
4686      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
4687    </t>
4688    <t>
4689      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
4690      "Remove 'identity' token references"
4691      (<eref target="http://purl.org/NET/http-errata#identity"/>)
4692    </t>
4693    <t>
4694      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
4695      "Import query BNF"
4696    </t>
4697    <t>
4698      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
4699      "qdtext BNF"
4700    </t>
4701    <t>
4702      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
4703      "Normative and Informative references"
4704    </t>
4705    <t>
4706      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
4707      "RFC2606 Compliance"
4708    </t>
4709    <t>
4710      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
4711      "RFC977 reference"
4712    </t>
4713    <t>
4714      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
4715      "RFC1700 references"
4716    </t>
4717    <t>
4718      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
4719      "inconsistency in date format explanation"
4720    </t>
4721    <t>
4722      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
4723      "Date reference typo"
4724    </t>
4725    <t>
4726      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
4727      "Informative references"
4728    </t>
4729    <t>
4730      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
4731      "ISO-8859-1 Reference"
4732    </t>
4733    <t>
4734      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
4735      "Normative up-to-date references"
4736    </t>
4737  </list>
4738</t>
4739<t>
4740  Other changes:
4741  <list style="symbols"> 
4742    <t>
4743      Update media type registrations to use RFC4288 template.
4744    </t>
4745    <t>
4746      Use names of RFC4234 core rules DQUOTE and WSP,
4747      fix broken ABNF for chunk-data
4748      (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
4749    </t>
4750  </list>
4751</t>
4752</section>
4753
4754<section title="Since draft-ietf-httpbis-p1-messaging-01">
4755<t>
4756  Closed issues:
4757  <list style="symbols"> 
4758    <t>
4759      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
4760      "Bodies on GET (and other) requests"
4761    </t>
4762    <t>
4763      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
4764      "Updating to RFC4288"
4765    </t>
4766    <t>
4767      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
4768      "Status Code and Reason Phrase"
4769    </t>
4770    <t>
4771      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
4772      "rel_path not used"
4773    </t>
4774  </list>
4775</t>
4776<t>
4777  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4778  <list style="symbols"> 
4779    <t>
4780      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4781      "trailer-part").
4782    </t>
4783    <t>
4784      Avoid underscore character in rule names ("http_URL" ->
4785      "http-URL", "abs_path" -> "path-absolute").
4786    </t>
4787    <t>
4788      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4789      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4790      have to be updated when switching over to RFC3986.
4791    </t>
4792    <t>
4793      Synchronize core rules with RFC5234.
4794    </t>
4795    <t>
4796      Get rid of prose rules that span multiple lines.
4797    </t>
4798    <t>
4799      Get rid of unused rules LOALPHA and UPALPHA.
4800    </t>
4801    <t>
4802      Move "Product Tokens" section (back) into Part 1, as "token" is used
4803      in the definition of the Upgrade header.
4804    </t>
4805    <t>
4806      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4807    </t>
4808    <t>
4809      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4810    </t>
4811  </list>
4812</t>
4813</section>
4814
4815<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
4816<t>
4817  Closed issues:
4818  <list style="symbols"> 
4819    <t>
4820      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/51"/>:
4821      "HTTP-date vs. rfc1123-date"
4822    </t>
4823    <t>
4824      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/64"/>:
4825      "WS in quoted-pair"
4826    </t>
4827  </list>
4828</t>
4829<t>
4830  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
4831  <list style="symbols"> 
4832    <t>
4833      Reference RFC 3984, and update header registrations for headers defined
4834      in this document.
4835    </t>
4836  </list>
4837</t>
4838<t>
4839  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4840  <list style="symbols"> 
4841    <t>
4842      Replace string literals when the string really is case-sensitive (HTTP-Version).
4843    </t>
4844  </list>
4845</t>
4846</section>
4847
4848<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
4849<t>
4850  Closed issues:
4851  <list style="symbols"> 
4852    <t>
4853      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
4854      "Connection closing"
4855    </t>
4856    <t>
4857      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/97"/>:
4858      "Move registrations and registry information to IANA Considerations"
4859    </t>
4860    <t>
4861      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/120"/>:
4862      "need new URL for PAD1995 reference"
4863    </t>
4864    <t>
4865      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/127"/>:
4866      "IANA Considerations: update HTTP URI scheme registration"
4867    </t>
4868    <t>
4869      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
4870      "Cite HTTPS URI scheme definition"
4871    </t>
4872    <t>
4873      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/129"/>:
4874      "List-type headers vs Set-Cookie"
4875    </t>
4876  </list>
4877</t>
4878<t>
4879  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4880  <list style="symbols"> 
4881    <t>
4882      Replace string literals when the string really is case-sensitive (HTTP-Date).
4883    </t>
4884    <t>
4885      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
4886    </t>
4887  </list>
4888</t>
4889</section>
4890
4891<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
4892<t>
4893  Closed issues:
4894  <list style="symbols"> 
4895    <t>
4896      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/34"/>:
4897      "Out-of-date reference for URIs"
4898    </t>
4899    <t>
4900      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
4901      "RFC 2822 is updated by RFC 5322"
4902    </t>
4903  </list>
4904</t>
4905<t>
4906  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4907  <list style="symbols"> 
4908    <t>
4909      Use "/" instead of "|" for alternatives.
4910    </t>
4911    <t>
4912      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
4913    </t>
4914    <t>
4915      Only reference RFC 5234's core rules.
4916    </t>
4917    <t>
4918      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
4919      whitespace ("OWS") and required whitespace ("RWS").
4920    </t>
4921    <t>
4922      Rewrite ABNFs to spell out whitespace rules, factor out
4923      header value format definitions.
4924    </t>
4925  </list>
4926</t>
4927</section>
4928
4929<section title="Since draft-ietf-httpbis-p1-messaging-05" anchor="changes.since.05">
4930<t>
4931  Closed issues:
4932  <list style="symbols"> 
4933    <t>
4934      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/30"/>:
4935      "Header LWS"
4936    </t>
4937    <t>
4938      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/52"/>:
4939      "Sort 1.3 Terminology"
4940    </t>
4941    <t>
4942      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/63"/>:
4943      "RFC2047 encoded words"
4944    </t>
4945    <t>
4946      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/74"/>:
4947      "Character Encodings in TEXT"
4948    </t>
4949    <t>
4950      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/77"/>:
4951      "Line Folding"
4952    </t>
4953    <t>
4954      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/83"/>:
4955      "OPTIONS * and proxies"
4956    </t>
4957    <t>
4958      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/94"/>:
4959      "Reason-Phrase BNF"
4960    </t>
4961    <t>
4962      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/111"/>:
4963      "Use of TEXT"
4964    </t>
4965    <t>
4966      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
4967      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
4968    </t>
4969    <t>
4970      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/134"/>:
4971      "RFC822 reference left in discussion of date formats"
4972    </t>
4973  </list>
4974</t>
4975<t>
4976  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
4977  <list style="symbols"> 
4978    <t>
4979      Rewrite definition of list rules, deprecate empty list elements.
4980    </t>
4981    <t>
4982      Add appendix containing collected and expanded ABNF.
4983    </t>
4984  </list>
4985</t>
4986<t>
4987  Other changes:
4988  <list style="symbols"> 
4989    <t>
4990      Rewrite introduction; add mostly new Architecture Section.
4991    </t>
4992    <t>
4993      Move definition of quality values from Part 3 into Part 1;
4994      make TE request header grammar independent of accept-params (defined in Part 3).
4995    </t>
4996  </list>
4997</t>
4998</section>
4999
5000<section title="Since draft-ietf-httpbis-p1-messaging-06" anchor="changes.since.06">
5001<t>
5002  Closed issues:
5003  <list style="symbols"> 
5004    <t>
5005      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
5006      "base for numeric protocol elements"
5007    </t>
5008    <t>
5009      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/162"/>:
5010      "comment ABNF"
5011    </t>
5012  </list>
5013</t>
5014<t>
5015  Partly resolved issues:
5016  <list style="symbols"> 
5017    <t>
5018      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/88"/>:
5019      "205 Bodies" (took out language that implied that there may be
5020      methods for which a request body MUST NOT be included)
5021    </t>
5022    <t>
5023      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/163"/>:
5024      "editorial improvements around HTTP-date"
5025    </t>
5026  </list>
5027</t>
5028</section>
5029
5030<section title="Since draft-ietf-httpbis-p1-messaging-07" anchor="changes.since.07">
5031<t>
5032  None.
5033</t>
5034</section>
5035
5036</section>
5037
5038</back>
5039</rfc>
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